WO2025160572A2

WO2025160572A2 - Alkali metal fuel cells, and related systems and methods

Info

Publication number: WO2025160572A2
Application number: PCT/US2025/013250
Authority: WO
Inventors: Yet-Ming Chiang; Karen SUGANO; Sunil Rohit MAIR; Saahir GANTI-AGRAWAL
Original assignee: Massachusetts Institute of Technology
Current assignee: Massachusetts Institute of Technology
Priority date: 2024-01-27
Filing date: 2025-01-27
Publication date: 2025-07-31
Anticipated expiration: 2026-07-27
Also published as: WO2025160572A3

Abstract

Alkali metal fuel cells, and related systems and methods, are generally described. The description herein comprises materials, designs, and methods of use for an electrical power system using a metal comprising sodium metal as an energy carrier or fuel in an electrochemical reactor wherein the reactant comprises oxygen and comprises materials and designs for an electrolytic reactor producing at least a metal from a metal chloride.

Description

ALKALI METAL FUEL CELLS, AND RELATED SYSTEMS AND METHODS

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 63/625,957, filed January 27, 2024, and entitled “REDOX POWER SOURCE AND ASSOCIATED MATERIALS AND SYSTEMS,” which is hereby incorporated herein by reference in its entirety for all purposes.

GOVERNMENT SPONSORSHIP

The invention was made with Government support. The Government has certain rights in the invention.

TECHNICAL FIELD

Alkali metal fuel cells, and related systems and methods, are generally described.

BACKGROUND

Alkali metal-air chemistries may have the requisite theoretical energy, but as a battery, none has reached the performance and cost metrics needed for electric aviation, or indeed of any commercial application.

SUMMARY

This Summary introduces a selection of concepts in simplified form that are described further below in the Detailed Description. This Summary neither identifies key nor essential features, nor limits the scope, of the claimed subject matter.

Certain aspects relate to alkali metal fuel cells. In some embodiments, the alkali metal fuel cell comprises a cathode comprising a cathodic reactant comprising gaseous water; and an anode comprising an anodic reactant comprising a liquid alkali metal; wherein the alkali metal fuel cell is configured to produce a discharge product; wherein the discharge product comprises an alkali metal hydroxide, and wherein at least a portion of the alkali metal hydroxide is in the form of a liquid solution.

In some embodiments, the alkali metal fuel cell comprises a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; an anode comprising an anodic reactant comprising a liquid alkali metal; and a solid electrolyte; wherein the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull.

In some embodiments, the alkali metal fuel cell comprises a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 5 centimeters.

In some embodiments, the alkali metal fuel cell comprises a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the alkali metal fuel cell is configured such that the liquid alkali metal is not replenished during its period of operation.

Certain aspects relate to methods. In some embodiments, the method comprises discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous water; and an anode comprising an anodic reactant comprising a liquid alkali metal; wherein the alkali metal fuel cell produces a discharge product comprising an alkali metal hydroxide during the discharging; and wherein at least a portion of the alkali metal hydroxide is in the form of a liquid solution.

In some embodiments, the method comprises discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; an anode comprising an anodic reactant comprising a liquid alkali metal; and a solid electrolyte; wherein the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull; and wherein, during the discharging, a discharge product exits the cathode in a direction substantially parallel to the direction of gravitational pull.

In some embodiments, the method comprises discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 10 centimeters. In some embodiments, the method comprises discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the liquid alkali metal is not replenished during the discharging.

One aspect of the disclosure herein is a sodium metal-based power system comprising

(a) a sodium-air fuel cell, wherein the fuel cell uses liquid sodium and produces solid Na2O2;

(b) a solid-electrolyte electrochemical cell using NaCl-AlCh to produce sodium metal; and

(c) a sodium metal storage and handling system comprising petroleum or silicone oil.

In one embodiment of the disclosed system, the fuel cell comprises Na, Na 0”- alumina, sputtered gold, and 1 atm O2.

In one embodiment of the disclosed system, the fuel cell comprises a 2-dimensional cathode that comprises metal or carbon films, sintered cermets, or mixed ionic-electronic conductors (MIEC) that is adherent to a solid electrolyte.

The following Detailed Description references the accompanying drawings which form a part this application, and which show, by way of illustration, specific example implementations. Other implementations may be made without departing from the scope of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures:

FIG. 1 is the sodium-air fuel cell system disclosed herein, according to some embodiments.

FIG. 2 shows the disclosed sodium-air fuel cell system can address close to 80% of all aircraft departures and over 30% of current total jet fuel consumption and its associated emissions, according to some embodiments. FIG. 3 is a Ragone plot that shows that the disclosed Na-air cell can reach pulse power density of 3000 W/kg and continuous-discharge energy density of 1500 Wh/kg, while retaining the oxygen onboard just reaches the FOA targets, according to some embodiments.

FIG. 4 is a schematic of a cell (FIG. 4A), a lab-scale cell (FIG. 4B); and a Multilayer stack (FIG. 4C), according to some embodiments.

FIG. 5A shows an intermittent galvanostatic discharge for solid electrolyte Na- Air cell, sputtered gold cathode. FIG. 5B shows Na2O2 discharge product can form under or over the cathode, according to some embodiments. FIG. 5C shows non-limiting embodiments of an oxygen/air electrode.

FIG. 6A is a configuration of a Downs cell and FIG. 6B is a configuration of a disclosed solid electrolyte cell, according to some embodiments.

FIG. 7 is a representation of the swappable approach with ground-based Na-metal production, storage, and handling with the reloadable/swappable Na-air cell, according to some embodiments.

FIG. 8 demonstrates a fast reaction of Na-air discharge product, according to some embodiments. FIG. 8A shows crystalline NaOH formed immediately upon exposure of Na_xO_y reacts with ambient water within <2 min losing crystallinity. FIG. 8B shows that after Ih exposure, the discharge product had reacted with ambient CO2 forming Na2CO3’H2O.

FIG. 8C shows that Na2CO3’H2O crystals were observed via SEM.

FIG. 9A, FIG. 9B, and FIG. 9C are, in accordance with some embodiments, a sodium-gas test cell.

FIG. 10 is a plot of the voltage versus the throughput for various conditions.

FIG. 11 shows x-ray diffraction patterns for inlet gas streams of dry O2 vs. 100% humidity O2.

FIG. 12 shows scanning electron microscope images of the discharge product at different stages of evolution, showing that it is possible to control the morphology of the discharge product by varying humidity, in accordance with some embodiments.

FIG. 13 shows design principles for an MIEC-based oxygen/air electrode, according to some embodiments.

FIG. 14 shows cathode materials for sodium-air fuel cells, in accordance with some embodiments.

FIG. 15 shows a process of making an MIEC cathode and building a cell, in accordance with some embodiments. FIG. 16A and FIG. 16B show electrochemical test results for a Na-air fuel cell with an MIEC cathode comprising a composite of Nao.?Mn02/Super P carbon/PVDF, in accordance with some embodiments.

FIG. 17 shows SEM images of the top surface of an MIEC electrode before and after discharging.

FIG. 18A and FIG. 18B show cross-sectional SEM images and elemental maps confirming the formation of a Na_xO_y layer on the cathode after discharging.

FIG. 19A and FIG. 19B show Raman spectra confirming Na COa formation in the discharge product after air exposure.

FIG. 20 shows a cermet design for an MIEC cathode comprising a solid electrolyte phase, in accordance with some embodiments.

FIG. 21 shows a process for making a cermet electrode, in accordance with some embodiments.

FIG. 22 shows a process for making a cermet electrode, in accordance with some embodiments.

FIG. 23A plots the current density versus areal capacity for various lithium and sodium comparators compared to an alkali metal fuel cell in accordance with embodiments disclosed herein. FIG. 23B plots the power density versus energy density for various lithium and sodium comparators compared to an alkali metal fuel cell in accordance with embodiments disclosed herein.

FIG. 24A shows a solid state pellet fixture fuel cell and a liquid tray fixture fuel cell, in accordance with some embodiments.

FIG. 24B shows an H-cell, in accordance with some embodiments.

FIG. 25A shows GITT discharge data obtained using a sodium cell configuration in accordance with FIG. 24 A.

FIG. 25B plots cell overpotential, taken as the difference between cell voltage at the end of a galvanostatic segment and the OCV, shown against water activity.

FIG. 25C and FIG. 25D show FIB cross-section images of sodium cell stacks after discharging 0.98 mAh/cm² (9 pm thick Na metal) in dry oxygen (FIG. 25C) and 26 mAh/cm² (240 pm thick Na metal) in 12% (FIG. 25D).

FIG. 26A and FIG. 26B are time-series x-ray diffraction plots.

FIG. 26C plots temperature versus weight percent NaOH (%).

FIG. 27A plots voltage versus throughput under various conditions. FIG. 27B plots voltage versus current density at various temperatures. FIG. 27C plots DC Area Specific Resistance versus throughput under various conditions. FIG. 27D is a photo of an H-cell design, in accordance with some embodiments.

FIG. 28 plots the first discharge pulse followed by a rest at varying operating temperatures, while holding the bubbler temperature constant.

FIG. 29 shows GITT cycling data for two planar cells, one cycled at 1 mA/cm² (cell A) and the other at 2 mA/cm² (cell B).

FIG. 30A shows an FIB cross-section image of a cell post-mortem after passing 9um of Na for the dry conditions. FIG. 30B shows an FIB cross-section image of a cell postmortem after passing 250um of Na for the humidified conditions. FIG. 30C plots voltage versus throughput from the dry and humidified conditions of FIGs. 30A and 30B. FIG. 30D shows schematics of the initial, dry, and humidified cells.

FIGs. 31A and 3 IB show the Raman spectra measured during discharge, separated by pulse number as well as the mode (discharge versus rest), where FIG. 3 IB is zoomed in to 3500-3700cm^-1.

FIGs. 32A and 32B are planview SEM images of a cell after it has been discharged for 24mAh/cm² with a pulsed discharge of 2mA/cm², 105 ’C operation, 50’ C bubbler. FIG. 32A was taken after the cell had been assembled and exposed to air. The planview image after washing is shown in FIG. 32B. EDS results are shown in FIG. 32C.

FIG. 33 shows the temperature dependence of the ionic conductivity of Na-/?” alumina measured with electrochemical impedance spectroscopy.

FIGs. 34A and 34B show a first discharge pulse followed by rest at varying cell and bubbler temperatures (15 minutes at ImA/cm² followed by a 5 minute rest with a 50nm sputtered gold film cathode). FIG. 34A uses a fixed bubbler temperature of 25 °C. FIG. 34B uses a fixed cell temperature of 100 °C.

FIGs. 35A and 35B show EDS mapping of FIB cross-sections following discharge in dry (FIG. 35A) and humidified conditions (FIG. 35B).

FIG. 36 shows an experimental setup.

FIG. 37 shows the results of a deliquescence study.

FIG. 38 plots mass versus time for NaOH.

FIG. 39 is an alkali metal fuel cell, in accordance with some embodiments.

FIG. 40 is a system comprising an optional electrochemical cell, an optional alkali metal storage and handling system, and an alkali metal fuel cell, in accordance with some embodiments. FIG. 41 shows a liquid tray fixture fuel cell comprising a separator, in accordance with some embodiments.

FIG. 42 shows an H-cell fuel cell comprising a separator, in accordance with some embodiments.

FIG. 43 is a schematic of a fuel cell comprising a separator and/or inclined separator, and a drain and/or collection system, in accordance with some embodiments.

FIG. 44 is a schematic of a Multilayer stack comprising fuel cells wherein one or more of the fuel cells comprise a separator, according to some embodiments.

FIG. 45A and 45B are schematics of an alkali fuel cell comprising a cathode that is not bonded to the solid electrolyte, in accordance with some embodiments.

FIG. 46 shows the voltage, current, and power for an alkali metal fuel cell comprising a cathode that is not bonded to the solid electrolyte, in accordance with some embodiments.

DETAILED DESCRIPTION

Alkali metal fuel cells, and related systems and methods, are generally described. In some embodiments, the alkali metal fuel cell comprises a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide. In certain embodiments, the alkali metal fuel cell comprises an anode comprising an anodic reactant comprising a liquid alkali metal (e.g., liquid sodium metal) (e.g., a layer of liquid alkali metal). In some cases, the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 10 centimeters. In certain embodiments, the alkali metal fuel cell comprises a solid electrolyte. In some embodiments, the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull.

In some cases, the alkali metal fuel cell is configured to produce a discharge product. In certain instances, the discharge product comprises a condensed phase discharge product. For example, in certain instances, the discharge product comprises an alkali metal hydroxide (e.g., sodium hydroxide) and at least a portion of the alkali metal hydroxide is in the form of a liquid solution. According to some embodiments, the alkali metal fuel cell is configured such that at least a portion of the discharge product is removed from the alkali metal fuel cell during operation of the alkali metal fuel cell. In certain instances, the fuel cell is configured such that the liquid alkali metal is not replenished during its period of operation. Certain embodiments are related to alkali metal fuel cells. Non-limiting examples of such alkali metal fuel cells are shown in FIGs. 1, 4A-4C, 7, 9, 13, 24A, 24B, 39, and 41-44.

In some embodiments, the alkali metal fuel cell comprises a cathode (e.g., any cathode disclosed herein). For example, as shown in FIG. 39, in some cases, alkali metal fuel cell 100 comprises cathode 103. In certain embodiments, the cathode comprises a cathodic reactant and/or solids that aid in electron or ion transfer. For example, in some cases, the cathode comprises a cathodic reactant. According to certain embodiments, the cathode comprises a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide. For example, in some cases, the cathode comprises a cathodic reactant comprising gaseous water. In certain instances, the cathode comprises a 2- dimensional cathode. In certain cases, the cathode comprises metal or carbon films, sintered cermets, gas diffusion electrodes, and/or mixed ionic-electronic conductors (MIEC).

In some embodiments, the alkali metal fuel cell comprises an anode (e.g., any anode disclosed herein). For example, as shown in FIG. 39, in some cases, alkali metal fuel cell 100 comprises anode 101. In certain embodiments, the anode comprises an anodic reactant and/or solids that aid in electron or ion transfer. For example, in some cases, the anode comprises an anodic reactant. According to certain embodiments, the anode comprises an anodic reactant comprising a liquid alkali metal (e.g., liquid sodium metal). For example, in some cases, the anode comprises an anodic reactant comprising a layer of liquid alkali metal (e.g., liquid sodium metal).

The layer of liquid alkali metal may have a suitable thickness. For example, in certain embodiments, the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter, greater than or equal to 2 millimeters, greater than or equal to 3 millimeters, greater than or equal to 4 millimeters, greater than or equal to 5 millimeters, greater than or equal to 6 millimeters, greater than or equal to 7 millimeters, greater than or equal to 8 millimeters, greater than or equal to 9 millimeters, greater than or equal to 1 centimeter, greater than or equal to 1.1 centimeters, greater than or equal to 1.2 centimeters, greater than or equal to 1.3 centimeters, greater than or equal to 1.4 centimeters, greater than or equal to 1.5 centimeters, greater than or equal to 2.0 centimeters, greater than or equal to 2.5 centimeters, greater than or equal to 3.0 centimeters, greater than or equal to 3.5 centimeters, or greater than or equal to 4.0 centimeters. In some embodiments, the layer of liquid alkali metal has a thickness of less than or equal to 10 centimeters, less than or equal to 9 centimeters, less than or equal to 8 centimeters, less than or equal to 7 centimeters, less than or equal to 6 centimeters, less than or equal to 5 centimeters, less than or equal to 4.8 centimeters, less than or equal to 4.5 centimeters, less than or equal to 4.3 centimeters, less than or equal to 4.0 centimeters, less than or equal to 3.8 centimeters, less than or equal to 3.5 centimeters, less than or equal to 3.3 centimeters, less than or equal to 3.0 centimeters, less than or equal to 2.8 centimeters, less than or equal to 2.5 centimeters, less than or equal to 2.3 centimeters, less than or equal to 2.0 centimeters, less than or equal to 1.8 centimeters, less than or equal to 1.5 centimeters, less than or equal to 1.3 centimeters, less than or equal to 1.0 centimeters, or less than or equal to 5 millimeters. Combinations of these ranges are also possible (e.g., greater than or equal to 1 millimeter and less than or equal to 10 centimeters, greater than or equal to 1 millimeter and less than or equal to 5 centimeters, or greater than or equal to 1 millimeter and less than or equal to 2 centimeters).

According to certain embodiments, the alkali metal fuel cell is configured such that the liquid alkali metal is not replenished during its period of operation. For example, in some cases, the liquid alkali metal layer is sufficiently thick (e.g., a thickness disclosed herein) that the alkali metal is sufficient for the entire period of operation. In certain instances, the period of operation is the intended duration of use of the alkali metal fuel cell. For example, in the case of a vehicle, the period of operation is the intended travel time (e.g., flight time), in some instances. That is, in some cases, the liquid alkali metal is not replenished during the intended travel time (e.g. flight time) of a vehicle (e.g., aviation vehicle).

In some embodiments, the period of operation is greater than or equal to 10 minutes, greater than or equal to 30 minutes, greater than or equal to 1 hour, greater than or equal to 2 hour, greater than or equal to 3 hours, greater than or equal to 5 hours, greater than or equal to 7 hours, or greater than or equal to 10 hours. In certain embodiments, the period of operation is less than or equal to 1 month, less than or equal to 3 weeks, less than or equal to 2 weeks, less than or equal to 1 week, less than or equal to 5 days, less than or equal to 3 days, less than or equal to 48 hours, less than or equal to 24 hours, less than or equal to 20 hours, less than or equal to 18 hours, less than or equal to 16 hours, less than or equal to 14 hours, less than or equal to 12 hours, less than or equal to 10 hours, or less than or equal to 8 hours. Combinations of these ranges are also possible (e.g., greater than or equal to 10 minutes and less than or equal to 1 month, greater than or equal to 10 minutes and less than or equal to 24 hours, or greater than or equal to 3 hours and less than or equal to 18 hours).

In accordance with certain embodiments, the alkali metal fuel cell is configured such that the liquid alkali metal is replenished continuously or intermittently during its period of operation. In some embodiments, the alkali metal fuel cell comprises a solid electrolyte (e.g., any solid electrolyte disclosed herein). For example, as shown in FIG. 39, in some cases, alkali metal fuel cell 100 comprises optional solid electrolyte 102. In certain embodiments, the solid electrolyte is below the anode and/or the cathode is below the solid electrolyte in the direction of gravitational pull. For example, in some cases, the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull. For example, as shown in FIG. 39, in some cases, optional solid electrolyte 102 is below anode 101 and cathode 103 is below optional solid electrolyte 102 in the direction of gravitational pull. In some embodiments, a discharge product forms on the outside of the cathode. For example, as shown in FIG. 39, in some cases, optional discharge product 104 forms on the outside of cathode 103. In accordance with certain embodiments, the alkali metal fuel cell is configured such that a discharge product exits the cathode in a direction substantially parallel (e.g., within 45 degrees, within 30 degrees, within 15 degrees, or within 5 degrees of parallel, or parallel) to the direction of gravitational pull. For example, as shown in FIG. 39, in some cases, cathode 103 is configured such that optional discharge product 104 exits cathode 103 in a direction substantially parallel to the direction of gravitational pull. In some embodiments, the discharge product is convected substantially normal to the direction of gravitational pull during or after exiting the cathode, for example, for the purpose of collecting said discharge product.

In certain embodiments, the cathode is adherent to the solid electrolyte. In other embodiments, the cathode and solid electrolyte are at least partially separated. In some embodiments, a space between the cathode and the solid electrolyte is at least partially filled by a condensed phase that comprises a discharge product of the fuel cell.

In some embodiments, the cathode is not bonded to the solid electrolyte. For example, as shown in FIGs. 45A-45B, in some cases, the cathode is not bonded to the solid electrolyte. For example, in some embodiments, the alkali metal fuel cell comprises a liquid alkali metal anode, a solid electrolyte, and a cathode (e.g., air cathode) that is not bonded to the solid electrolyte. In this embodiment, the liquid discharge product may act as a catholyte, providing interfacial contact between the cathode (e.g., air cathode) and the solid electrolyte. In some cases, this liquid discharge product has a high (>100 mS/cm) conductivity for alkali metal cations and anions such as, but not limited to, hydroxides. Thus, in certain instances, discharge products form in the catholyte solution, where metal cations meet the oxygenbased anions. In certain embodiments, this liquid-phase discharge product is removed by in-plane flow to the edge of the cathode (e.g., air cathode), using the hydrostatic pressure that results from producing a liquid in the confined space between the cathode (e.g., air cathode) and the solid electrolyte. In some embodiments, gravitational pull is used to further facilitate liquid removal. In certain embodiments, the liquid discharge product is removed through channels in the air cathode, which enables the liquid to leave the cell in the out of plane direction. According to some embodiments, the discharge product removal is augmented by a pump. As shown in FIGs. 45A-45B, in some cases, the cathode is not bonded to the solid electrolyte, and the discharge product forms in between the cathode and the solid electrolyte. As shown in FIG. 45A, in certain cases, the discharge product is removed via in-plane flow, while, as shown in FIG. 45B, in some instances, the discharge product is removed via out-ofplane flow.

In certain embodiments, the solid electrolyte has an anode-facing surface. In some cases, the anode-facing surface comprises a coating. In some cases, the coating comprises a composition that is wetted by the anodic reactant. In certain instances, the coating comprises tin, silver, gold, and/or carbon. Without wishing to be bound by theory, it is believed that the solid electrolyte having an anode-facing surface comprising a coating comprising tin, silver, gold, and/or carbon promotes alkali metal (e.g., sodium metal) wetting of the anode-facing surface and/or provides more uniform electrical contact between the anode-facing surface and the alkali metal (e.g., sodium metal), in some embodiments.

According to some embodiments, the alkali metal fuel cell is configured to produce a discharge product (e.g., any discharge product disclosed herein). For example, as shown in FIG. 39, in some cases, alkali metal fuel cell 100 is configured to produce optional discharge product 104. In some cases, the discharge product comprises an alkali metal hydroxide, an alkali metal oxide, an alkali metal peroxide, an alkali metal carbonate, an alkali metal bicarbonate, an alkali metal oxalate, an alkali metal peroxyoxylate, and/or an alkali metal halide. For example, in certain embodiments, the discharge product comprises an alkali metal hydroxide. In some cases, the alkali metal hydroxide comprises sodium hydroxide. In some cases, at least a portion (e.g., at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or all) of the discharge product is in the form of a liquid solution. For example, in certain instances, the discharge product comprises an alkali metal hydroxide, such as sodium hydroxide, and at least a portion (e.g., at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or all) of the alkali metal hydroxide, such as sodium hydroxide, is in the form of a liquid solution. In certain instances, the liquid solution comprises an aqueous solution. In certain embodiments, the alkali metal fuel cell is configured such that at least a portion (e.g., at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or all) of the discharge product is removed from the alkali metal fuel cell during operation of the alkali metal fuel cell. In some cases, the discharge product is removed intermittently or continuously.

In accordance with certain embodiments, the alkali metal fuel cell is configured to produce the liquid solution at suitable operating temperatures. For example, in some embodiments, the alkali metal fuel cell is configured to produce the liquid solution at an operating temperature of greater than or equal to 98 °C, greater than or equal to 100 °C, greater than or equal to 105 °C, greater than or equal to 110 °C, greater than or equal to 115 °C, greater than or equal to 120 °C, greater than or equal to 125 °C, greater than or equal to 130 °C, greater than or equal to 135 °C, greater than or equal to 140 °C, greater than or equal to 145 °C, greater than or equal to 150 °C, greater than or equal to 160 °C, greater than or equal to 170 °C, greater than or equal to 180 °C, or greater than or equal to 190 °C. In certain cases, the alkali metal fuel cell is configured to produce the liquid solution at an operating temperature of less than or equal to 323 °C, less than or equal to 320 °C, less than or equal to 310 °C, less than or equal to 300 °C, less than or equal to 290 °C, less than or equal to 280 °C, less than or equal to 270 °C, less than or equal to 260 °C, less than or equal to 250 °C, less than or equal to 240 °C, less than or equal to 230 °C, less than or equal to 220 °C, less than or equal to 210 °C, less than or equal to 200 °C, less than or equal to 190 °C, less than or equal to 180 °C, less than or equal to 170 °C, less than or equal to 160 °C, less than or equal to 150 °C, less than or equal to 145 °C, less than or equal to 140 °C, less than or equal to 135 °C, less than or equal to 130 °C, less than or equal to 125 °C, less than or equal to 120 °C, less than or equal to 115 °C, less than or equal to 110 °C, or less than or equal to 105 °C. Combinations of these ranges are also possible (e.g., greater than or equal to 98 °C and less than or equal to 323 °C, greater than or equal to 98 °C and less than or equal to 200 °C, or greater than or equal to 100 °C and less than or equal to 150 °C). In some embodiments, the alkali metal fuel cell is configured to produce the liquid solution at an operating temperature less than, equal to, or greater than, the melting point of the anodic reactant.

In some embodiments, the anodic reactant comprises an alkali metal reactant. In certain cases, the anodic reactant has a melting point lower than the alkali metal reactant when present as a substantially pure metal. For example, in certain instances, a sodium metal fuel cell comprises an anodic reactant comprising a sodium-potassium alloy having a melting point lower than that of sodium metal, such as a melting point below about 20°C. In certain embodiments, the anodic reactant is a solid at the temperature or temperatures present in the fuel cell during operation.

In accordance with some embodiments, the alkali metal fuel cell is configured to produce the liquid solution at a suitable water partial pressure. For example, in certain embodiments, the alkali metal fuel cell is configured to produce the liquid solution at a water partial pressure of greater than or equal to 0.03 atm, greater than or equal to 0.05 atm, greater than or equal to 0.07 atm, greater than or equal to 0.1 atm, greater than or equal to 0.12 atm, greater than or equal to 0.15 atm, greater than or equal to 0.2 atm, greater than or equal to 0.3 atm, greater than or equal to 0.4 atm, greater than or equal to 0.5 atm, greater than or equal to 0.6 atm, greater than or equal to 0.7 atm, greater than or equal to 0.8 atm, greater than or equal to 0.9 atm, greater than or equal to 1.0 atm, greater than or equal to 1.1 atm, greater than or equal to 1.2 atm, greater than or equal to 1.3 atm, greater than or equal to 1.4 atm, greater than or equal to 1.5 atm, or greater than or equal to 1.7 atm. In some embodiments, the alkali metal fuel cell is configured to produce the liquid solution at a water partial pressure of less than or equal to 5 atm, less than or equal to 4.8 atm, less than or equal to 4.5 atm, less than or equal to 4.3 atm, less than or equal to 4.0 atm, less than or equal to 3.8 atm, less than or equal to 3.5 atm, less than or equal to 3.3 atm, less than or equal to 3.0 atm, less than or equal to 2.8 atm, less than or equal to 2.5 atm, less than or equal to 2.3 atm, less than or equal to 2.0 atm, less than or equal to 1.8 atm, less than or equal to 1.5 atm, less than or equal to 1.3 atm, less than or equal to 1.0 atm, less than or equal to 0.8 atm, or less than or equal to 0.5 atm. Combinations of these ranges are also possible (e.g., greater than or equal to 0.03 atm and less than or equal to 5 atm, greater than or equal to 0.1 atm and less than or equal to 3.0 atm, or greater than or equal to 1.2 atm and less than or equal to 2.0 atm).

In certain embodiments, the alkali metal fuel cell further comprises a separator. Nonlimiting examples of alkali metal fuel cells comprising a separator are shown in FIGs. 41-44. For example, as shown in FIGs. 41-44, in some cases, the alkali metal fuel cell comprises separator 300.

In some cases, the separator is permeable to the liquid solution (e.g., aqueous solution). For example, in certain instances, at least 50 vol%, at least 75 vol%, at least 90 vol%, or all of the liquid solution can pass through the separator. In some embodiments, the separator is impermeable to the alkali metal (e.g., sodium metal). For example, in certain embodiments, less than 50 vol%, less than 25 vol%, less than 10 vol%, or none of the alkali metal can pass through the separator. In accordance with some embodiments, the separator is permeable to the liquid solution and impermeable to the alkali metal. Without wishing to be bound by theory, it is believed that, in some embodiments, having a separator permeable to the liquid solution and impermeable to the alkali metal reduces contact between the alkali metal and the liquid solution in the instance of a rupture of the alkali metal fuel cell, which would otherwise allow crossover of the sodium metal.

It is appreciated that reactions between an alkali metal (e.g., sodium metal) and aqueous solutions may generate hydrogen, which may be combustible under certain conditions. Accordingly, in some embodiments the alkali metal fuel cell includes materials and/or designs that separate alkali metal (e.g., sodium metal) from an aqueous discharge product in the event of a cell membrane failure or leakage of alkali metal (e.g., sodium metal) from the anode side of the cell to the cathode side of the cell. Such a separator may be used anywhere in the alkali metal fuel cell where such separation may be desired.

It is appreciated that alkali metal (e.g., sodium metal) has a high surface tension and may not wet certain materials, including certain ceramics, metals, and polymers, while aqueous solutions may have a relatively low surface tension and wet certain materials that alkali metal (e.g., sodium metal) does not. Accordingly, in some embodiments, the separator comprises a material that is not wetted by alkali metal (e.g., sodium metal) and is wetted by aqueous solutions, such as alkaline hydroxide solutions, such as sodium hydroxide solutions. As used herein, “wetting” means a contact angle of a droplet of said alkali metal (e.g., sodium metal), which may be solid or liquid, or said aqueous solution, when placed on said separator material, that is less than about 90 degrees (e.g., less than or equal to 80 degrees, less than or equal to 70 degrees, less than or equal to 60 degrees, less than or equal to 50 degrees, less than or equal to 40 degrees, less than or equal to 30 degrees, or less than or equal to 20 degrees). In some embodiments, wetting of the separator material by said aqueous solution includes the instance where the contact angle is about zero degrees, and spreading of the liquid on the separator material occurs.

In some embodiments, a mechanical separation of said alkali metal (e.g., sodium metal) and said aqueous solution is produced. For example, in some instances, a mechanical separation of said alkali metal (e.g., sodium metal) and said aqueous solution is produced using a separator comprising a material that is not wetted by alkali metal (e.g., sodium metal) and is wetted by aqueous solutions. A non-limiting example, in accordance with certain embodiments, is the placement of a mesh or screen of such material on the cathode side of the fuel cell such that alkali metal (e.g., sodium metal) is prevented from passing through the mesh or screen due to its high surface tension and non-wetting nature, while any aqueous solution present can pass through, thereby achieving separation. In certain embodiments, the design of the separator mesh or screen varies depending on operating parameters such as the pressure exerted on the alkali metal (e.g., sodium metal), the contact angle of the alkali metal (e.g., sodium metal) and aqueous solution on the separator, and/or the flow rate of aqueous solution away from the separator. In some embodiments, the mesh or screen has perforations with a minimum dimension of greater than or equal to 0.1 mm (e.g., greater than or equal to 0.3 mm, greater than or equal to 0.5 mm, greater than or equal to 0.7 mm greater than or equal to 1 mm, greater than or equal to 2 mm, or greater than or equal to 3 mm) and less than or equal to 5 mm (e.g., less than or equal to 5 mm, less than or equal to 4.5 mm, less than or equal to 4 mm, less than or equal to 3 mm, less than or equal to 2 mm, or less than or equal to 1 mm) (combinations of these ranges are also possible). In some embodiments, the separator is inclined. For example, in certain cases, the separator is inclined to cause any alkali metal (e.g., sodium metal) that is caught by it to be diverted to a drain and/or collection system.

In some embodiments, the alkali metal fuel cell further comprises a drain and/or collection system for the liquid solution (e.g., aqueous solution). In certain instances, the separator is positioned upstream of the drain and/or collection system. For example, as shown in FIG. 43, in some cases, the alkali metal fuel cell comprises separator 300 and/or inclined separator 301, and drain and/or collection system 302.

According to certain embodiments, the power density of the alkali metal fuel cell increases during operation. Without wishing to be bound by theory, it is believed that the power density of the alkali metal fuel cell increases during operation because the mass of the alkali metal fuel cell decreases during operation as the discharge product is discharged and/or removed, in some embodiments.

Certain embodiments relate to systems. Non-limiting examples of such systems are shown in FIGs. 1, 4C, 7, 40, and 44.

In some embodiments, the system comprises an alkali metal fuel cell (e.g., any alkali metal fuel cell disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises alkali metal fuel cell 100. In accordance with certain embodiments, the system comprises multiple alkali metal fuel cells. For example, as shown in FIG. 4C and FIG. 44, in some cases, the system comprises multiple alkali metal fuel cells. In some embodiments, the system comprises multiple alkali fuel cells electrically connected in series, resulting in a net voltage which is the sum of the voltages of the cells. In certain embodiments, the net voltage is greater than or equal to 2 volts (e.g., greater than or equal to 5 volts, greater than or equal to 10 volts, greater than or equal to 25 volts, greater than or equal to 50 volts, greater than or equal to 100 volts, greater than or equal to 200 volts, greater than or equal to 300 volts, greater than or equal to 400 volts, or greater than or equal to 500 volts) and less than or equal to 1000 volts (e.g., less than or equal 950 volts, less than or equal 900 volts, less than or equal 800 volts, less than or equal 700 volts, less than or equal 600 volts, less than or equal 500 volts, less than or equal 250 volts, or less than or equal 100 volts) (combinations of these ranges are also possible). In certain embodiments, the system comprises multiple alkali fuel cells electrically connected in parallel, resulting in a net current which is the sum of the currents of the cells.

According to some embodiments, the system comprises a suitable number of cells connected in series and parallel configuration. For example, in certain embodiments, the system comprises greater than or equal to 2, greater than or equal to 10, greater than or equal to 50, greater than or equal to 100, greater than or equal to 200, or greater than or equal to 500 alkali metal fuel cells. According to some embodiments, the system comprises less than or equal to 1000, less than or equal to 600, less than or equal to 400, less than or equal to 300, less than or equal to 200, less than or equal to 100, less than or equal to 50, less than or equal to 20, or less than or equal to 10 alkali metal fuel cells. Combinations of these ranges are also possible (e.g., greater than or equal to 2 and less than or equal to 1000, greater than or equal to 2 and less than or equal to 20, greater than or equal to 50 and less than or equal to 100, or greater than or equal to 2 and less than or equal to 10).

In some instances, the multiple alkali metal fuel cells may be the same or different. For example, in some cases, the multiple alkali metal fuel cells are each the same. In other cases, the multiple alkali metal fuel cells are each different. In yet other cases, some of the multiple alkali metal fuel cells are the same and some are different.

According to certain embodiments, the multiple alkali metal fuel cells are stacked. For example, in some instances, the multiple alkali metal fuel cells are stacked substantially parallel (e.g., within 45 degrees, within 30 degrees, within 15 degrees, or within 5 degrees of parallel, or parallel) to the direction of gravitational pull.

In accordance with some embodiments, the system comprises an electrochemical cell (e.g., any electrochemical cell disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises optional electrochemical cell 202. For example, in certain instances, the system comprises an electrochemical cell (e.g., any electrochemical cell disclosed herein) and an alkali metal fuel cell (e.g., any alkali metal fuel cell disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises optional electrochemical cell 202 and alkali metal fuel cell 100. In certain embodiments, the electrochemical cell comprises a solid-electrolyte electrochemical cell. In some cases, the solid-electrolyte electrochemical cell comprises an alkali metal-conducting solid electrolyte. For example, as shown in FIG. 40, in some cases, optional electrochemical cell 202 comprises alkali metal-conducting solid electrolyte 203. For example, in some embodiments, the solid electrolyte conducts the same alkali metal (e.g., sodium) as comprised within the anode of the alkali metal fuel cell (e.g., sodium).

In certain embodiments, the solid-electrolyte electrochemical cell is configured to produce the alkali metal (e.g., sodium metal). For example, in some instances, the solidelectrolyte electrochemical cell is configured to produce the alkali metal (e.g., sodium metal) from an alkali metal salt (e.g., NaCl). In certain cases, the solid-electrolyte electrochemical cell is configured to produce the alkali metal (e.g., sodium metal) from an alkali metal salt combined with another chloride salt (e.g., XCl-ZCl_y, wherein X is the alkali metal, such as sodium, Z is another metal, and y is a number, such as a whole number from 1-10). In some embodiments, Z lowers the melting point of the chloride mixture compared to XC1 alone. In some embodiments, Z is Al or Ca. According to some embodiments, y is 3. For example, in certain embodiments, ZCl_y is AlCh or CaCh. For example, in some cases, the solidelectrolyte electrochemical cell is configured to produce sodium metal from NaCl combined with AlCh. In some embodiments, the alkali metal produced (e.g., sodium metal) is transported to and/or used in the alkali metal fuel cell (e.g., in the anode).

In accordance with certain embodiments, the system comprises an alkali metal storage and handling system (e.g., any storage and handling system disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises optional alkali metal storage and handling system 201. In some embodiments, the system comprises an alkali metal storage and handling system (e.g., any storage and handling system disclosed herein) and an alkali metal fuel cell (e.g., any alkali metal fuel cell disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises optional alkali metal storage and handling system 201 and alkali metal fuel cell 100. For example, in certain instances, the system comprises an alkali metal storage and handling system (e.g., any storage and handling system disclosed herein) and an alkali metal fuel cell (e.g., any alkali metal fuel cell disclosed herein) and an electrochemical cell (e.g. , any electrochemical cell disclosed herein). For example, as shown in FIG. 40, in some cases, system 200 comprises optional alkali metal storage and handling system 201 and alkali metal fuel cell 100 and optional electrochemical cell 202. In some cases, the alkali metal storage and handling system comprises petroleum or silicone oil. According to some embodiments, the alkali metal (e.g., in the alkali metal fuel cell, of the alkali metal storage and handling system, and/or in the electrochemical cell) comprises lithium, sodium, potassium, rubidium, cesium, and/or francium. For example, in certain embodiments, the alkali metal (e.g., in the alkali metal fuel cell, of the alkali metal storage and handling system, and/or in the electrochemical cell) comprises lithium, sodium, and/or potassium. As another example, in some instances, the alkali metal (e.g., in the alkali metal fuel cell, of the alkali metal storage and handling system, and/or in the electrochemical cell) comprises sodium. In certain embodiments, the alkali metal is metallic. For example, in some embodiments, the sodium is sodium metal (i.e., metallic sodium).

As used herein, “metallic” metals are metals having an oxidation state of zero. For example, pure sodium metal in a zero oxidation state would be metallic sodium. Sodium that is part of sodium chloride salt, however, would not be metallic sodium because sodium in that form has an oxidation state of +1. Metal elements in metallic form are also referred to herein as that element followed by “metal.” For example, metallic sodium is also referred to herein as “sodium metal.”

According to some embodiments, the alkali metal is solid and/or liquid. For example, in some cases, the alkali metal is liquid, such as molten alkali metal. For example, in certain instances, the alkali metal is liquid sodium metal, such as molten sodium metal.

In certain cases, the alkali metal is the same or different throughout the system (e.g., in the alkali metal fuel cell, of the alkali metal storage and handling system, and/or in the electrochemical cell). For example, in some cases, the alkali metal (e.g., sodium) is the same throughout the system (e.g., in the alkali metal fuel cell, of the alkali metal storage and handling system, and/or in the electrochemical cell).

Certain embodiments relate to methods (e.g., any methods disclosed herein). In some embodiments, the method comprises discharging an alkali metal fuel cell (e.g., any alkali metal fuel cell disclosed herein) to produce an electric current. In some cases, the method comprises producing a discharge product (e.g., any discharge product disclosed herein, such as sodium hydroxide). In certain instances, during the discharging, the discharge product exits the cathode in a direction substantially parallel (e.g., within 45 degrees, within 30 degrees, within 15 degrees, or within 5 degrees of parallel, or parallel) to the direction of gravitational pull. In accordance with certain embodiments, the method comprises removing at least a portion (e.g., at least 10%, at least 25%, at least 50%, at least 75%, at least 90%, or all) of the discharge product from the alkali metal fuel cell during operation. According to some embodiments, the liquid alkali metal is not replenished during the discharging. For example, in certain embodiments, the method comprises discharging an alkali metal fuel cell for the intended duration of use (e.g., the intended travel time of a vehicle) of the alkali metal fuel cell without replenishing the liquid alkali metal.

In accordance with certain embodiments, the method comprises replenishing the alkali metal in the alkali metal fuel cell continuously or intermittently during its period of operation.

In accordance with some embodiments, the method comprises using the discharge product and/or a downstream product thereof to capture and/or sequester atmospheric carbon dioxide and/or to decrease the acidity of a body of water. For example, in some cases, the discharge product is sodium hydroxide and the method comprises using the sodium hydroxide to capture atmospheric carbon dioxide. As another example, in certain instances, the discharge product is sodium hydroxide and the method comprises converting the sodium hydroxide to a downstream product. In some cases, the downstream product is sodium carbonate and/or sodium bicarbonate. In certain instances, the sodium carbonate and/or sodium bicarbonate is enriched in carbon dioxide compared to the discharge product prior to exposure to carbon dioxide. For example, in some embodiments, using the sodium hydroxide to capture atmospheric carbon dioxide results in formation of sodium carbonate, which can optionally be further converted to sodium bicarbonate. In certain embodiments, the method comprises using the sodium bicarbonate to decrease the acidity of a body of water.

In certain embodiments, the method comprises using a discharge product comprising sodium hydroxide to capture carbon dioxide from the atmosphere, or from a source including but not limited to the product of a combustion process. In some embodiments, said combustion process comprises the combustion of coal or a hydrocarbon. In some embodiments, the carbon dioxide results from the decomposition of a carbonate mineral, including but not limited to limestone. Processes yielding carbon dioxide which may be captured by the method in accordance with some embodiments include but are not limited to combustion of fuels to produce energy, cement production, steel production, or ammonia production.

In some embodiments, an objective is to deliver a power source that offers specific energy >1000Wh/kg at a continuous power density of >500 W/kg and an electricity cost of <$0.30/kWh. In other embodiments, an objective is to deliver a power source that offers specific energy >1000Wh/kg at a continuous power density of >100 W/kg and an electricity cost of <$0.20/kWh. In still other embodiments, an objective is to deliver a power source that offers specific energy >1000Wh/kg at a continuous power density of <100 W/kg and an electricity cost of <$0.15/kWh.

According to certain embodiments, the technology is a sodium- air fuel cell that uses a solid electrolyte and consumes liquid sodium, producing a solid discharge product that is intermittently or continuously removed from the exterior of the air cathode. In some cases, the re-usable power pack can be continuously or intermittently refueled with sodium metal. In certain instances, alongside the power cell, subsystems for low-cost sodium production from NaCl and for the safe storage and handling of sodium metal are provided, providing an end-to-end system for using sodium metal as a high energy density energy carrier.

High-energy density metal-air batteries are frequently more efficient when discharging than when charging. Notably, thirty years of R&D have failed to produce a commercial rechargeable Li-air battery. However, discharge energy densities have been demonstrated which may project to systems meeting the present targets, in some instances. In accordance with certain embodiments, for a use-once fuel cell approach, sodium is used due to its low cost and high abundance. In accordance with certain embodiments, for a fuel cell approach wherein a metal is consumed but not recharged in the same device, sodium is used due to its low cost and high abundance. In addition, valorization of chlorine from Na metal production, and the discharge products of the Na-air cell, can further reduce the cost of delivered electricity, perform carbon capture and storage (CCS) functions, and/or supply valuable NaOH to other markets, in accordance with certain embodiments.

In certain embodiments, the sodium- air cell features a refillable sodium metal anode and a removable discharge product. For example, described herein is a sodium-air cell that features a refillable sodium metal anode and a removable solid discharge product, in accordance with some embodiments. In some cases, the strategy includes identifying suitable Na-ion solid electrolytes and cathode materials, integrating these with low-cost interconnect and housing materials, and performing technoeconomic analyses to identify pathways to the cost targets. Described herein, in accordance with certain embodiments, is a solid-electrolyte based electrolysis cell for the production of Na metal and Ch gas from molten chlorides, including but not limited to NaCI-AICh. For example, described herein, in accordance with certain embodiments, is a solid-electrolyte based electrolysis cell for the production of Na metal and Ch gas from molten chlorides, beginning with NaCI-AICh. In certain embodiments, the disclosed sodium air cell comprises an electrochemical cell for lower energy consumption and direct purification of input salts, to lower the cost of Na metal production to <$0.30/kg-Na and delivering high energy density electricity from the Na-air cell at an LCOS of <$0.30/kWh. For example, the disclosed sodium air cell comprises a Downs cell for lower energy consumption and direct purification of input salts, to lower the cost of Na metal production to <$0.30/kg-Na and delivering high energy density electricity from the Na-air cell at an LCOS of <$0.30/kWh, in some embodiments. Also disclosed is a buoyancy-based system for safe storage, handling and delivery of Na metal using non- reactive liquids with density in between that of solid and liquid Na, in certain embodiments.

In certain embodiments, an onboard power system providing >1,000 Wh/kg and >1000 Wh/L can address close to 80% of all aircraft departures and over 30% of current total jet fuel consumption and its associated emissions (FIG. 2). Metal-air chemistries may have the requisite theoretical energy, but as a battery, none has reached the performance and cost metrics needed for electric aviation, or indeed of any commercial application. In accordance with some embodiments, the disclosed system (FIG. 1) combines several insights: a) Metalair discharge reactions are much more efficient than charge reactions; b) Those that use the cation as the working ion (Li-air and Na-air) form discharge product on the exterior of the air electrode, from which it can be removed; c) Sodium uniquely has a cost entitlement low enough to meet the LCOS cost targets of many applications without recharging; and d) Discharge products of the Na-air cell as well as chlorinated co-products from Na metal production can be valorized to further reduce LCOS.

In accordance with certain embodiments, the proposed sodium metal-based power system combines three subsystems shown in FIG. 1. In certain embodiments, the first subsystem is a sodium-air fuel cell that uses liquid Na, a solid electrolyte, and has a removable discharge product. For example, in some embodiments, the first subsystem is a sodium-air fuel cell that uses liquid Na, a solid electrolyte, and has a removable solid discharge product. Using liquid sodium avoids well-known failure modes of solid metal anodes, in some cases. In certain embodiments, discarding the discharge product in flight reduces mass and increases energy density. The Ragone plot in FIG. 3 shows that a Na-air cell of such design can reach pulse power density of 3000 W/kg and continuous-discharge energy density of 1500 Wh/kg. Retaining the oxygen onboard in a discharge product may also allow these targets to be reached, in certain instances. For example, the Ragone plot in FIG. 3 shows that a Na-air cell of this design can reach pulse power density of 3000 W/kg and continuous-discharge energy density of 1500 Wh/kg, while retaining the oxygen onboard just reaches the FOA targets, in certain instances. Note that, in certain embodiments, a IMWh flight produces only 0.27 m³ of an exemplary discharge product, Na2O2, which is a negligible amount when distributed over a 100-200-mile flight path. For example, note that, in some embodiments, a IMWh flight produces only 0.27 m³ of Na2<32, a negligible amount when distributed over a 100-200-mile flight path. According to some embodiments, when exposed to humidity and atmospheric CO2, an exemplary discharge product Na_xO_y spontaneously converts to sodium bicarbonate (NaHCCh), capturing 0.92 tonnes CO2 per MWh in addition to the averted fossil fuel emissions. For example, according to certain embodiments, when exposed to humidity and atmospheric CO2, Na_xO_y spontaneously converts to sodium bicarbonate (NaHCCF), capturing 0.92 tonnes CO2 per MWh in addition to the averted fossil fuel emissions. If discharged into the ocean, the sodium bicarbonate has the further benefit of deacidification, in some instances. According to some embodiments, a flight of ~10h consumes -lOmrn of Na (capacity ~lAh/cm²) and does not require continuous delivery of Na. For example, according to certain embodiments, a flight of ~10h consumes -lOmrn of Na (capacity ~lAh/cm²) and does not require continuous delivery of Na, but removal of the discharge product is necessary for energy density and to maintain power. In certain instances, multi-cell modules as in FIG. 4C can be rapidly swapped to recharge, and the depleted modules refilled with Na metal (the only consumable of the reaction which needs to be provided) offline. For example, in some instances, multi-cell modules as in FIG. 4C can be rapidly swapped to recharge, and the depleted modules refilled with Na metal (the only consumable) offline. For Na metal at a cost of $0.30/kg-Na, the LCOS is $0.18/kWh, in some cases. Monetization of discharge products can further reduce this cost, in certain instances. For example, in some cases, at $100/tonne CO2 price, CCS via sodium bicarbonate reduces LCOS by $0.08/kWh. As another example, in certain instances, an alternative coproduct, high purity NaOH, would reduce LCOS by $0.16/kWh at current market price of ~$400/tonne (while lowering energy density). In accordance with some embodiments, the second subsystem is a solid-electrolyte electrochemical cell (FIG. 1) for low-cost electrolytic production of Na metal from metal chlorides. Na metal may be made electrolytically from NaCl-CaCh eutectic melts using the Downs cell operating at an overpotential of 3.5V (7.1V total). The solid electrolyte cell, according to certain embodiments, may operate as low as 4.5V, use a lower-melting eutectic such as NaCl-AlCL, and produce higher purity Na at a target cost of $0.30/kg-Na. For example, the solid electrolyte cell, according to some embodiments of the invention, may operate at 4.5V, use a lower-melting eutectic such as NaCl-AlCL, and produce higher purity Na at a target cost of $0.30/kg-Na. Monetization of the CI2 co-product has the potential to reduce Na metal cost by $0.46/kg-Na, in some cases. According to certain embodiments, the third subsystem is a buoyancy-based sodium metal storage and handling system. Petroleum or silicone oils with density intermediate between that of solid sodium (>0.95 g/cm³) when below the melting point of 98°C and greater than that of liquid sodium above the melting point (<0.93 g/cm³) will store the solid metal safely under oil while floating the liquid metal for refilling of Na-air modules, in accordance with some embodiments. In certain cases, these three subsystems may be spatially co-located or located separately and may be operated simultaneously or at different times. Each subsystem may further comprise additional subsystems, in some cases.

In some embodiments, the proposed technology may provide electrical storage with high gravimetric or volumetric energy density and high continuous or pulse power at affordable cost. For example, in certain embodiments, the proposed technology may provide electrical storage with breakthrough energy density and high pulse power at affordable cost. In some embodiments, sodium metal as an energy carrier in the Na-air cell format may enable widespread electrification of difficult-to-decarbonize transportation sectors including but not limited to aviation, locomotion, maritime shipping, and long-haul trucking. The technology has the potential to deliver a large reduction in greenhouse gas emissions from different modes of transportation, in some cases. The proposed fuel cell system also has potential applications beyond transportation, including stationary or mobile storage for a wide variety of applications including commercial and industrial, data centers, residential, military, and disaster relief, or temporary field operations, in certain instances. For example, the proposed fuel cell system also has potential applications beyond transportation, including stationary or mobile storage for a wide variety of applications including commercial and industrial, data centers, residential, military, and disaster relief, in certain instances.

In accordance with some embodiments, innovations in the proposed technology include:

• Use of sodium metal as a transportable flowable high energy density energy carrier;

• A fuel cell designed to use a liquid metal as the fuel, have a removable discharge product, and valorize the discharge product (e.g., a fuel cell designed to use a liquid metal as the fuel, have a removable solid discharge product, and valorize the discharge product);

• Simple refueling of discharged cells with a single consumable (e.g., Na metal);

• Novel solid-electrolyte cell for Na metal production from feedstocks comprising NaCl operating at low overpotential (-4.5V cell) and simultaneously purifying feedstock (e.g., novel solid-electrolyte cell for Na metal production from NaCl operating at low overpotential (-4.5V cell) and simultaneously purifying feedstock); and/or

• Novel buoyancy-based systems to store solid Na metal safely under oil as a solid and to deliver it as liquid Na metal at modest temperature (~100°C).

In some instances, the proposed Na-air cell has higher energy density at ambient pressure (1600 Wh/L) than does compressed hydrogen at 690 bar and 15°C (1250 Wh/L).

The Ragone curve in FIG. 3 provides power and energy metrics, based on a cell model that includes temperature-dependent cell voltage and solid electrolyte conductivity, cathode overpotential, and masses of Na metal, solid electrolyte, air cathode, and endplates in accordance with some embodiments. The Ragone curve is then obtained by varying the current density, in some instances. In some embodiments, for the parameters given in the FIG. 3 legend, a specific power of 500 W/kg requires a current density of 334 mA/cm² and a specific power of 1500 W/kg requires 1.1 A/cm². At these current densities, the corresponding specific energy exceeds 1000 Wh/kg by 40-55%, in some cases. For example, at these current densities, the corresponding specific energy exceeds the minimum 1000 Wh/kg by 40-55%, in some cases. This model is an optimization tool for cell design, in some embodiments. See Table 1.

Table 1. Proposed Target (in accordance with some embodiments) and FOA Category A for Various Parameters

In one embodiment, the disclosed sodium-air cell features a refillable sodium metal anode and a removable discharge product. In some embodiments, the sodium- air cell comprises a Na-ion conducting solid electrolyte with high ionic conductivity and stability against sodium in the operating temperature range of -50°C to 200°C, which comprises temperatures where the metal comprising the electrode may be solid or liquid. For example, sodium metal has a melting temperature of about 98°C at 1 atm pressure. In another embodiment, said sodium-air cell comprises a gas cathode from which the discharge product (a sodium oxide/hydroxide/carbonate) can be removed. These components may be assembled into a unit that meets desired energy density and power metrics, in accordance with certain embodiments. In another embodiment, low-cost materials are used for the current collectors and housing of the refuelable sodium-air cell.

Achieving 1000 Wh/kg in the sodium-air battery is possible utilizing a sodium thickness of about 10 mm corresponding to an areal capacity of ~lAh/cm², producing an equivalent discharge product thickness of ~2 cm (at 50% solids packing density), in some cases. For example, achieving 1000 Wh/kg in the sodium-air battery requires utilizing a sodium thickness of ~10 mm corresponding to an areal capacity of ~lAh/cm², and an equivalent discharge product thickness of ~2 cm (at 50% solids packing density), in some cases. Concurrently, to achieve, e.g., 500 W/kg continuous power requires a current density of 0.3A/cm², in certain instances. To achieve 18s peak power of 1500 W/kg requires a current density of ~lA/cm², in some instances. This duration is short enough that discharge product removal is probably not necessary during that step, in certain cases. For example, this duration is short enough that discharge product removal during the peak power pulse is not necessary, in certain cases. According to some embodiments, the sodium metal-solid electrolyte interface has adequately fast transport for these requirements. For example according to certain embodiments, the sodium metal- solid electrolyte interface is selected to have adequately fast transport for these requirements. In accordance with certain embodiments, liquid sodium used with P” alumina has stripping capacities > lOAh/cm² at 1 A/cm² and used with NaSICON has stripping capacity of at least 10Ah/cm² at O.lA/cm². In some embodiments, continuous sodium stripping is demonstrated at >100mA/cm² for >lAh/cm² at 100-200°C with an average overpotential <0.2V, excluding separator resistance, in a symmetric cell format.

According to some embodiments, at the air cathode, Na-air cells have three possible discharge sodium oxide products each resulting in a characteristic operating voltage - Na2<D (Eceii = 1.95V), Na2<D2 (E_ceii = 2.33V), and NaCh (E_ceii = 2.27V). While each can meet the energy density goals, in certain embodiments, sodium peroxide is most commonly formed. In some embodiments, candidate solid electrolytes include Na P”-alumina and NaSICON, both of which have conductivity > 10'² S/cm at 100°C and are available commercially. For example, in some embodiments, candidate solid electrolytes include Na P”-alumina and NaSICON, both of which have conductivity > 10'² S/cm at 100°C and are sold commercially.

In some embodiments, unlike rechargeable metal-air batteries, the gas cathode does not need to store the discharge product, thereby reducing the need for porous gas diffusion electrodes of large thickness. In certain embodiments, unlike rechargeable metal-air, the cathode does not need to store the discharge product, thereby eliminating the need for thick, porous gas diffusion electrodes. In some cases, a 2-dimensional cathode that is adherent to the solid electrolyte may be used, including metal or carbon films, sintered cermets, or mixed ionic-electronic conductors (MIEC). In certain instances, cathodes that are only electronically conductive will form Na_xO_y at the three-phase boundary between the solid electrolyte, conductor, and gas phase, while MIECs can form Na_xO_y at their surface as well. According to some embodiments, cathodes comprise continuous and patterned cathodes of both types, using materials including but not limited to noble metals, lanthanum strontium manganate (LSRM) and mixed ionic-electronic conductors (MIECs) such as NaCoCE, sodium polyanionic compounds that are the sodiated analogs of lithium transition metal phosphates such as LiFePCU and LiMnPCU, and Na-NMCs, which are sodium-substituted analogs of lithium battery cathode compounds such as LiCoCh and lithiated nickel-manganese-cobalt (NMC) compounds such as NMC111, NMC523, and NMC811, the numbers referring to the relative proportions of Ni, Mn and Co. According to some embodiments, cathodes comprise continuous and patterned cathodes of both types, using materials such as noble metals, lanthanum strontium manganate (LSRM) and mixed ionic-electronic conductors (MIECs) such as NaCoO2 and Na-NMCs, which are sodium-substituted analogs of lithium battery cathode compounds such as LiCoCE and lithiated nickel-manganese-cobalt (NMC) compounds such as NMC111, NMC523, and NMC811, the numbers referring to the relative proportions of Ni, Mn and Co. According to certain embodiments of the disclosure herein, sodium-conducting MIECs may comprise any sodium-ion battery active electrode compound or its derivative, or any sodium-metal battery cathode or its derivative.

According to some embodiments of the invention, the gas composition may be controlled to obtain desired Na_xO_y discharge products or related hydroxides and/or carbonates. For example, in some instances, water vapor in the gas stream may cause the formation of NaOH or NaOH dissolved in aqueous solution, and carbon dioxide in the gas stream may cause formation of sodium carbonate or bicarbonate. In certain cases, removal of the discharge product may also comprise control of gas flow velocity or flow patterns. Control of the discharge product and its removal may maximize power as well as energy of the sodium- air fuel cell, in accordance with certain embodiments.

Some embodiments of the invention comprise a solid-electrolyte molten salt electrolysis cell for the production of Na metal and Ch gas. Currently, sodium metal may be produced electrolytically in a Downs cell (FIG. 6A), from a NaCl-CaCh eutectic melt in which the overall cell reaction is 2NaCl(i) — 2Na(i) + Ch(_g). Addition of calcium chloride allows a decrease in the operating temperature of such a cell to -590 °C. Both positive and negative electrodes are in contact with the molten salt, and density differences are used to separate the molten salt, sodium metal, and chlorine gas. Any impurities in the melt with a greater reduction potential than sodium or oxidation potential than chlorine can contaminate the products, so the incoming salts need to be of high purity. And, despite having a lower reduction potential than sodium, calcium contaminates the sodium to -1% concentration, requiring further purification. These additional process steps contribute to cost, such that Na metal sells for $2-3/kg-Na despite the electrical energy consumption being only $0.50/kg-Na (7.1V cell, $0.05/kWh electricity price). For example, these additional process steps contribute to cost, such that Na metal sells for ~$3/kg-Na despite the electrical energy consumption being only $0.50/kg-Na (7.1V cell, $0.05/kWh electricity price).

Instead, in accordance with some embodiments, an advantageous electrolytic cell configuration uses a solid-state, largely single-ion conductor to separate the molten salt mixture from the liquid sodium (FIG. 6B). For example, instead, in accordance with some embodiments, an advantageous electrolytic cell configuration uses a solid-state, single-ion conductor to separate the molten salt mixture from the liquid sodium (FIG. 6B). This simultaneously reduces the purity requirement for the incoming salt and removes the need for post-electrolysis purification, in some cases. In certain embodiments, a broader pool of molten salt mixtures becomes usable, including cations with greater reduction potentials than sodium and lower melting point than NaCl-CaCh. At high current density, using NaCI- AICI3 with a Na-/?” alumina separator and producing high-purity sodium at 300 °C, an anode reaction A1CU' — AICI3 + 2 Ch + e’ may result in the deposition of aluminum chloride, fouling the graphite electrode. According to some embodiments of the invention, such potential issues are mitigated with temperature and salt composition control (i.e., operating with NaCl-rich liquid) and active convection, amongst other enhancements. In some embodiments, the operating voltage is accordingly reduced from 7.1V to 4.5V (IV overpotential), where the electricity cost to produce Na metal is $0.29/kg-Na and the LCOS reaches $0.18/kWh.

In some embodiments, a buoyancy-based approach for safe storage, handling, and delivery of sodium metal comprises the use of petroleum or silicone oils with density in between that of solid and liquid sodium as a storage medium. Accordingly, in certain embodiments, solid sodium will remain safely immersed, but upon warming to above the sodium melting point (98°C), the solid sodium melts to form liquid sodium that floats on the oil and may be delivered to a container or transport mechanism that supplies the sodium-air fuel cell.

Additional embodiments and examples:

Battery components and design according to some embodiments:

• A cell comprising a sodium metal anode (liquid or solid), a Na-ion conducting electrolyte, and a gas electrode, in some embodiments. Examples of gas electrodes include metal or carbon films, sintered cermets, or mixed ionic-electronic conductors (MIEC), in certain cases. A cell comprising a sodium metal anode (liquid or solid), a Na-ion conducting electrolyte, and an air electrode, in some embodiments. Examples of air electrodes include metal or carbon films, sintered cermets, or mixed ionic- electronic conductors (MIEC), in certain cases. o In some instances, metals include Au, Pt, Fe, Cu, Ni, Sn, Mo, Cr, Ti. o In certain embodiments, metal and carbon films can be deposited by the following methods (sputtering, screen-printing, controlled vapor deposition, evaporation, electroplating). The films can be conformal or heterogeneous, (i.e. grids achieved via screen-printing), according to some embodiments. o In some cases, cermet refers to a composite material consisting of an electronic conductor and an ionic conductor. In certain embodiments, the Na- ion conductor can comprise the following: (Na beta- alumina, NaSICON). In some embodiments, the electronic conductor could be a metal (Fe, V, Cr, Zr, Ni, Cu, Al, Sn), an electronically conductive oxide (FC3O4, Lai-_xSr_xMnO3), or other conductors (carbon black, graphite). o In certain embodiments, mixed-ionic-electronic conductors include Na-ion cathodes (NaCrCh, Nao.vCoCh, Nao.44Mn02, Nao.vMnCh, NaCoPC , NaNiPC , NaFePC , NaMnPC , Prussian blue, Prussian white). o According to some embodiments, cermets and MIECs are deposited onto the electrolyte by any of the following methods (spray-casting, drop-casting, screen-printing, tape-casting, sputtering). In certain instances, binders including PVDF, PEO... (e.g., PVDF or PEO) can be added to the mixture to improve adhesion. Deposited films can be sintered, with and without the use of a press, in accordance with some embodiments. o In certain cases, surface-treated Na-ion electrolytes can also be an MIEC (oxygen-deficient Na beta alumina, transition-metal doped Na-ion conductors).

• Cell fixture design: o In some embodiments, single cell design comprises a sodium metal anode (liquid or solid), a Na-ion conducting electrolyte in the form of a tray to house sodium metal, and an air electrode sandwiched between a perforated current collector which allows for air flow, which can be stacked to increase voltage (through series connections of the cells) and/or capacity (through parallel connections of the cells). o In certain cases, the thickness dimensions of the components to reach lOOOWh/kg are 0.5mm for the electrolyte, 0.6mm for the end plates, and 10mm for the sodium.

In accordance with some embodiments, a sodium-gas test cell is shown in FIGs. 9A- 9C.

Non-limiting embodiments of the oxygen/air electrode are shown in FIG. 5C.

In some embodiments, the gas stream for the sodium- air cell (see FIGs. 9A-9C) comprises oxygen and water. FIGs. 10, 8A, 8B, 11, and 12 show that a gas stream that is humidified changes the composition and structure of the discharge product, facilitating its removal and improving the discharge performance of the cell. As shown in FIG. 10, lower overpotential was observed upon switching from dry, static O2 gas at the cathode to 100% humidity, flowing (30ml/min) O2. As shown in FIGs. 8A and 8B, different discharge products were observed when comparing inlet gas streams of dry O2 vs. 100% humidity O2. In dry O2, the x-ray diffraction pattern initially showed NaOH phase, but evolved to a pattern with almost no detectable crystalline peaks within 4 minutes, showing the formation of a liquid solution from the crystalline NaOH. Within 1 hour, crystalline Na2CO3-H2O formed, illustrating capture of carbon dioxide from ambient air. In FIG. 8B, the data labeled “after discharge” was obtained from the cathode after the cell was subjected to a current density of 0.5mA/cm², and reached a charge capacity of 4.89 mAh/cm², at 100’C in static O2 atmosphere. The data labeled “after discharge, exposure in air” was obtained under the conditions 0.2mA/cm², 8.3 mAh/cm², 50°C, static O2. In the experiments in FIG. 8B, 7 pm of NaOH formed per 1 mAh/cm² of capacity.

As shown in X-ray diffraction patterns in FIG. 11, different discharge products were observed for inlet gas streams of dry O2 vs. 100% humidity O2. In the latter case, NaOH- H2O discharge product evolved to Na COi within 30 minutes, showing that carbon dioxide was captured from the ambient air.

In some embodiments, the gas electrode for the sodium-air cell comprises a mixed ionic-electronic conductor (MIEC). In some embodiments, the ionic conduction is of oxygen ions. FIGs. 13-22 illustrate specific compositions and embodiments of the MIEC and its beneficial use, in accordance with some embodiments.

FIG. 14 shows cathode materials for sodium-air fuel cells, in accordance with some embodiments. In some embodiments, the MIEC comprises Nao.?Mn02, Nao.44Mn02, Nao.?Co02, NaNiFeMnO2, or NaNio.5Mn1.5O4. In certain embodiments, Nao.?Mn02 can be used as MIEC.

FIG. 15 shows a process of making an MIEC cathode and building a cell, in accordance with some embodiments.

FIGs. 16A-16B show electrochemical test results for Na-air fuel cell with an MIEC cathode comprising a composite of Nao.?Mn02/Super P Carbon/PVDF, in accordance with some embodiments. Both FIG. 16A and FIG. 16B showed initial OCV at 2.4 V followed by a plateau at 2.33 V. The formation of NaxOy occurred at 2.33 V. FIG. 17 shows SEM images of the top surface of an MIEC electrode before and after discharging of the sodium-air fuel cell, in which the needle morphology after discharging showed the formation of a discharge product and the elemental analysis of the surface before and after discharge showed an enrichment of Na relative to Mn.

FIGs. 18A-18B show cross-sectional SEM images and elemental maps confirming the formation of a Na_xO_y layer on the electrode after discharging.

FIGs. 19A-19B show Raman spectra confirming Na COa formation in the discharge product after air exposure.

FIG. 20 shows a cermet design for an MIEC cathode comprising a solid electrolyte phase to provide ionic conductivity and a metallic phase to provide electronic conductivity.

FIG. 21 shows a process for making a cermet electrode, in accordance with some embodiments. In some embodiments, the metallic phase comprises Fe, Cr, V, Zr, W, or Mo.

FIG. 22 shows an MIEC cermet cathode comprising vanadium metal and sodium |3” alumina applied to a solid electrolyte sheet.

The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention.

EXAMPLE 1

A solid-electrolyte test cell (FIGs. 4A and 4B) was constructed with configuration (sequence of components from one side of the cell to the other): Na metal | Na P”-alumina | sputtered gold | 1 atm O2. An open circuit potential of -2.33V vs. Na/Na⁺ was observed indicating that the main discharge product was Na2O2. An important finding was that even with a thin sputtered metal cathode, it was possible to induce formation of the discharge product on the exterior of the cathode. FIG. 27 A shows pulsed discharge curves at 25 and 50 °C where the latter reached 8 mAh/cm². Cross-sections of such cells after cycling show cases where the Na2O2 discharge product formed between the gold layer and solid electrolyte and on the exterior of the gold layer (FIG. 27B). Surprisingly, the thicker gold layer (250 nm) produced the latter, which is desirable for discharge product removal.

EXAMPLE 2

Alkali-air batteries may be limited by poor rechargeability and low power densities, due to the formation of stable electrically insulating oxide discharge products which may passivate the air electrode surface and may not easily be decomposed. This example presents a novel sodium-humidified-oxygen fuel cell, which uses a molten sodium anode as a liquid metal fuel that can be oxidized to form a sodium hydroxide discharge product at the cathode. Under sufficiently humid conditions, this sodium hydroxide absorbs enough moisture from the input gas stream to form a liquid solution, which facilitates the removal of the discharge product from the cathode, thus preventing passivation of the cathode and allowing continuous operation of the fuel cell. Along with the use of a solid-state electrolyte, this cell chemistry can be used, in some cases, to make a “refillable primary” battery or a “metal-air fuel cell”, in which cell recharging is avoided by refilling the anode with fresh molten sodium, and the sodium hydroxide discharge products are removed and may be beneficially used, for example as a caustic, or for the capture of carbon dioxide. Said sodium hydroxide discharge products may be converted to sodium carbonate by exposure to carbon dioxide, or may be further converted to sodium bicarbonate upon exposure to water, said sodium carbonate or sodium bicarbonate being beneficially used for various purposes including as a reagent for raising the pH of water bodies.

Initial lab scale tests demonstrated a stable open circuit voltage and working voltage for upwards of 30 mAh/cm² of discharge capacity, at a current density of 2 mA/cm² and with an overpotential <0.5V. In total, the cell demonstrated 94 mAh/cm² of discharge capacity at a current density of 2 mA/cm².

Electric power systems that can deliver high gravimetric and volumetric energy at low cost have the potential to decarbonize hard-to-abate transportation sectors such as aviation, maritime, rail transport, and long-haul trucking. Such systems could also provide stationary or transportable electric power for applications such as natural disaster relief. Technical and commercial success of metal-air electrochemical couples has been elusive, in part due to the difficulty of achieving rechargeability without using pure oxygen as a reactant, which limits the discharge products to metal oxides, as opposed to more stable metal hydroxides or carbonates. The additional balance of plant necessary to provide purified oxygen can erase any system-level energy density advantage over alternatives such as lithium-ion batteries.

In this example, we show that a fuel cell based on sodium metal and air can simultaneously meet the energy density and continuous operation requirements of hard-to- abate transportation (e.g., >1 kWh/kg specific energy at cell level), at a low cost of delivered electricity comparable to that of liquid fuels, while producing a sodium oxide or hydroxide discharge product which may, in an open-system configuration, serve to capture carbon dioxide, further decarbonizing the mode of transportation. The proposed design uses liquid sodium metal (melting point 98°C) to feed a fuel cell incorporating a sodium-ion conducting solid electrolyte (here, Na-/?” alumina), and an air cathode from which the discharge product is continuously removed. Techno-economic analysis shows the proposed fuel cell system is capable of delivering high energy density electricity at < 0.20 USD/kWh, a comparable cost to jet fuel today. The economics of electricity production may be further improved if the value of either atmospheric CO2 removal or the co-products are realized.

Metal-air electrochemical cells may have two fundamentally different designs: 1) systems in which the electrolyte is an anion conductor (e.g., alkaline electrolyte) and the discharge product (metal hydroxide or oxide) forms at the metal negative electrode, such as zinc-air, aluminum- air, and iron-air batteries; and 2) systems in which the electrolyte is a cation conductor (e.g., non-aqueous or solid-state Li and Na conductors) and the discharge product forms at the exterior of the air electrode. The second type may be the basis for a metal-air fuel cell in which the metal is continuously fed as a fluid, and the discharge product is continuously removed, in accordance with some embodiments. This example focuses on sodium metal for its combination of high crustal abundance, low cost, and high energy density, although the concepts can be applied to other metals. To facilitate discharge product removal, the input oxygen or air stream was humidified in order to form NaOH as the primary discharge product. Furthermore, this discharge product was deliquesced to concentrated sodium hydroxide liquid solutions (>50 wt% NaOH) at moderate operating temperatures (100-150°C) and water partial pressures (>~0.1 atm), allowing easy removal as a liquid and facilitating continuous discharge of the fuel cell. In this example, galvanostatic discharge was demonstrated for >150 operating hours and a cumulative area capacity > 240 mAh/cm² (about 100 times that of a typical Li-ion cell). With increasing discharge duration, the energy density of the system asymptotically approached that of the fuel itself. This example demonstrates an open-loop mode of operation whereby the NaOH-water discharge product spontaneously captured ambient carbon dioxide forming sodium carbonate. Subsequent conversion to sodium bicarbonate could be utilized to de-acidify water bodies.

Electrochemical Cell Design and Testing:

Two laboratory electrochemical cell designs were utilized (FIGs. 24A and 24B), an H-cell configuration with a vertically oriented solid electrolyte membrane (FIG. 24B), and a horizontally-oriented cell with a planar solid electrolyte in the form of a tray or sheet (FIG. 24A). In each design, the solid electrolyte separated the sodium metal chamber from an air cathode to which a flowing gas of controlled composition was provided. Both cell designs were configurations which with appropriate sealing and manifolding could be the basis for multi-cell stacks such as that schematized in FIG. 4C, in accordance with some embodiments. The Na-/?” alumina used here has a sodium ion conductivity of 3.3 mS/cm at room temperature and follows an Arrhenius relationship with temperature, which was confirmed experimentally in symmetric cells with platinum blocking electrodes (FIG. 33). During discharge of the fuel cell, the anode half-cell reaction was: Na(i) = Na⁺ + e". The liquid sodium - Na-/?” alumina interface operates at 300-350°C. At the lower temperatures used in the present example, 100-250°C, it was confirmed experimentally that the liquid sodium-solid electrolyte interface has low impedance. A thin tin film (~50 nm thick) was sputtered on the Na-/?” alumina to further improve its wetting by liquid sodium. The application of tin lowered the contact angle between molten sodium and Na-/?” alumina from 125° to 50° at 125°C.

With a Na metal anode, the open-circuit voltage (OCV) was equivalent to the air cathode potential with respect to Na/Na⁺. At the cathode, several half-cell reactions are possible, and reaction products can form as solids or as dissolved species in an alkaline solution. The reference potentials for the solid case assume that the product formed is an anhydrous, crystalline solid, while the reference potential for the solution case assumes that the product is present in a 1 M aqueous solution:

2Na⁺ + O₂ + 2 e~ = Na₂O₂ (E_s°_olid = 2.33 7)

Na⁺ + O₂ + e~ = NaO₂ (E_s°_olid = 2.287, E_s°_oln = 2.47)

4Na⁺ + H₂O + O₂ + 4e“ = INaOH (E_s°_olid = 2.71 V, E_s°_oln = 3.11 7 )

2Na⁺ + H₂O + O₂ + 2e~ NaOH + NaOOH E_s°_oln = 2.63 7) 4Na⁺ + 2CO₂ + 0₂ + 4 e" = 2Na₂CO₃ E_s°_olid = 3.37 7)

In addition, if the cathode active sites are flooded or otherwise blocked from oxygen access, the anoxic Na-water reaction is also possible:

Na⁺ + H₂O + e~ = NaOH + H₂ (E_s°_olid = 1.467, E_s°_oln = 1.887)

Li-air and Na-air batteries are most reversible when the discharge product is the corresponding oxide or peroxide rather than the more-stable hydroxide or carbonate. For a fuel cell where rechargeability is not required, the higher cell voltage associated with a more stable discharge product was attractive, and the selection criteria fell to which discharge product was most easily removed in order to enable continuous operation of the fuel cell. The most stable sodium salt in air atmosphere (e.g., with ~400ppm CO2) is sodium carbonate, Na2COa. At relative humidities and temperatures found in most places on Earth, one can also form NaOH, NaOH hydrate, or NaOH- water solutions. The hygroscopicity of sodium hydroxide means that it can deliquesce to form liquid solutions over a wide range of temperatures and humidities. The behavior of the air cathode was studied under a wide range of conditions in order to understand the half-cell reaction and identify operating conditions which minimize overpotential and maximize operational stability.

Initial experiments were conducted using a sputtered gold film of -400 nm thickness as the cathode-active layer because gold is stable at the high pH of the discharge products. Subsequent experiments were carried out using gas-diffusion layer (GDL) electrodes comprising a nickel foam. Using a cell of the configuration in FIG. 24A, discharge results at 50°C with and without humidification of an oxygen gas stream are shown in FIG. 25A. The experiments followed a galvanostatic intermittent titration test (GITT) protocol in which a constant current was applied for 15 min, followed by a 5 min rest to allow the cell voltage to relax towards the open-circuit voltage (OCV), followed by an electrochemical impedance spectroscopy (EIS) scan. Thus, the upper bound of the curves traces the OCV as the cell is discharged, while the lower bound traces the working voltage of the cell, with the horizontal axis being discharge capacity normalized to electrode area, mAh/cm². In this example, the cells were discharged to a capacity about three times that of a typical lithium-ion battery (-3 mAh/cm²). It was seen that a flowing, humidified oxygen stream produced the highest OCV and working voltage as well as the lowest polarization (voltage gap between the OCV and working voltage). To quantify the dependence of overpotential on water activity, the temperature of the electrochemical cell was varied between 50°C and 150°C (i.e., spanning the sodium metal melting point, 98°C) and the intake water activity, a, was varied over a factor of about 200 (using a water bubbler held at 25°C to 80°C). The results in FIG. 25B show that the overpotential decreased with increasing water activity, reaching a relatively constant value above a ~ 0.1 (raw data appear in FIGs. 34A-34B). This behavior was then correlated with changes in the morphology and phase state of the discharge product. Focused ion beam (FIB) cross-sections of cells discharged at 2 mA/cm² with dry and 12% water oxygen streams are shown in FIGs. 25C and 25D, respectively (discharge data appear in FIG. 30C). For dry oxygen, the discharge product was found to form between the solid electrolyte and the gold film, disrupting the film, whereas for humidified oxygen, the discharge product formed on the exterior of the gold cathode, causing the film to remain adhered to the solid electrolyte even after thirty times higher capacity had been passed, compared to the dry oxygen case. For a cell discharged in humidified oxygen, scanning electron microscopy of the cathode surface before and after removal of the discharge product with water showed that the gold film remained largely intact and adherant to the solid electrolyte.

FIG. 25A shows GITT discharge data obtained using a sodium cell configuration in accordance with FIG. 24A, measured at 0.2 mA/cm² current density for solid sodium (50°C) under three inlet gas conditions- static oxygen at 0% RH, static oxygen at 100% RH, and continuous oxygen flow at 100% RH. The initial value of OCV was higher at 2.5 V for humidified oxygen compared to 2.4 V for dry oxygen, as was the average working voltage, 2.4 V vs 1.2 V, respectively.

FIG. 25B plots the cell overpotential, taken as the difference between cell voltage at the end of a galvanostatic segment and the OCV, shown against water activity. Increasing water activity above a ~ 0.1 led to a nearly constant overpotential.

FIG. 25C and FIG. 25D show FIB cross-section images of sodium cell stacks after discharging 0.98 mAh/cm² (9 pm thick Na metal) in dry oxygen (FIG. 25C) and 26 mAh/cm² (240 pm thick Na metal) in 12% (FIG. 25D). The sodium electrode was liquid during the test (T = 105 °C). For dry oxygen the discharge product formed underneath the gold film, whereas for humidified oxygen the discharge product formed at the exterior of the gold film.

The nature of the discharge product formed under humid conditions and its reaction with ambient air was studied using X-ray diffraction. A cell discharged at 50°C to 9 mAh/cm² of sodium capacity was cooled to room temperature, purged with argon, and disassembled in an argon-filled glove box. Using an air-sensitive sample holder, the first diffraction scan was conducted under argon atmosphere, and subsequent scans were obtained upon exposing the sample to ambient air. As shown in FIG. 26A, the predominant crystalline phase was initially NaOH-FUO. While this phase was solid at the 50°C cell test temperature, according to the NaOH-FhO phase diagram, FIG. 26C, at room temperature where the X-ray experiments are conducted, a slight water excess would produce a co-existing liquid. Accordingly, within 3 minutes of air exposure, the NaOH-FhO diffraction peaks disappeared almost completely, consistent with the initial composition at NaOH-H O moving to the left into the single phase liquid field in FIG. 26C. After about 10 minutes of air exposure, diffraction peaks for Na COa were observable, FIG. 26B, which then grew until saturating in intensity after about 50 min. Thus, the results showed that under humid conditions the discharge product of the sodium-air cell was initially rich in NaOH but subsequently absorbed atmospheric water, causing deliquescence, then absorbed carbon dioxide (assumed to be present at ~400ppm concentration), causing sodium carbonate to crystallize within minutes.

FIG. 26A and FIG. 26B are time-series x-ray diffraction plots. FIG. 26C plots temperature versus weight percent NaOH (%). NaOH facilitated substantial boiling point elevation, with liquid solution being stable at temperatures as high as 150 °C. The constant partial pressure curves (system pressure was 1 atm) showed that under a partial pressure of 0.12 (dew point of 50 °C), NaOH will deliquesce and remain as a single-phase liquid, up to temperatures around 120 °C. This demonstrates that it is possible to form a liquid NaOH discharge product at temperatures where Na metal is molten (> 97 °C). This can facilitate low anodic and cathodic overpotentials.

Further analysis was conducted to determine the equilibrium composition of the discharge product as a function of temperature and water activity. NaOH is capable of absorbing enough moisture from its environment to form a liquid NaOH-H2O solution, defined as deliquescence. Water absorption occurs when the ambient water vapor pressure is higher than the vapor pressure of the NaOH-H2O mixture, and can continue until the water partial pressures are at equilibrium. As shown by the phase diagram in FIG. 26C, for a wide range of NaOH-H2O compositions and temperature, the corresponding composition falls in a phase field containing a liquid. However, deliquescence is not maintained indefinitely if the composition and temperature fall in the liquid-NaOH (s) two-phase regime, because the vapor pressure of the liquid phase in the two-phase equilibrium is higher than the ambient water vapor pressure. Under a flow of gas at constant humidity, the liquid phase will eventually dry out forming dehydrated NaOH. By Henry’s Law, the water vapor pressure of a solution is the product of the water activity in solution and the vapor pressure of pure water Pvap^{1 =} equilibrium Na0H-H20 composition as a function of temperature and water activity was determined. Results, plotted as curves of constant water activity in FIG. 26C, show that above the melting point of sodium metal (98°C), a substantial liquid fraction can be achieved at relatively low water partial pressure, P(H2<D). For example, at P(H20) = 0.12 atm (50°C dew point), the equilibrium composition is a single-phase liquid of about 70% NaOH concentration at temperature of 105°C. The calculated results were experimentally verified by heating solutions of known initial concentration under fixed P(H2O) until the liquidus curve was crossed and solid NaOH precipitated, and by measuring water uptake by NaOH using thermogravimetric analysis.

Fuel Cell Performance:

These results were used to design fuel cell experiments in which both the sodium and the discharge product are liquid. Operating temperatures were above the melting point of sodium such that the discharge product, even if initially solid, would deliquesce to a singlephase liquid under operating conditions, facilitating removal of discharge product as a liquid from the air cathode via wetting and gravitational flow. Results from an H-cell (FIG. 24B) with a gold film cathode and a gas diffusion electrode (GDE) are shown in FIG. 27A.

FIG. 27A plots voltage versus throughput under various conditions. FIG. 27B plots voltage versus current density at various temperatures. FIG. 27C plots DC Area Specific Resistance versus throughput under various conditions. FIG. 27D is a photo of an H-cell design.

Unlike batteries, fuel cells by design can consume fuel without limit, and therefore asymptotically approach the energy density of the fuel at long operating durations.

These observations of the discharge product provide understanding of the electrochemical reactions at the air cathode and facilitate design of the fuel cell for continuous removal of the discharge product. From the GITT curves in FIG. 25A, the cell OCV was initially ~ 2.55 V for all gas compositions but equilibrated to ~ 2.4V quickly in dry oxygen and more slowly for static humidified oxygen. While in FIG. 25A the OCV remained at 2.5V for flowing humidified oxygen, this was only seen at low current density (e.g., 0.2 mA/cm²) and low total Na throughput. With continued discharge, and at higher current densities, a further decrease in OCV to 2.15-2.05 V was observed; see for example FIGs. 27A and 27B. These three characteristic OCV values were seen in the absence of CO2, so Na2COa formation was excluded as a possible reaction. Possible electrochemically formed products were NaO2, Na2O2, NaOH, NaOOH, and NaOH-H O solution. Gold exhibits orientation-dependent ORR catalysis, with the (100) planes being known to be selective for 4-electron ORR forming NaOH, which has a potential of 2.64 V at 100°C, while the (110) and (111) planes are selective for the 2-electron ORR reaction forming Na2O2, which has a potential of 2.23 V at 100°C. Since the sputtered gold electrode used in these experiments was polycrystalline, it is possible that the initially observed OCV of 2.55 V was a mixed potential between those for the 2-electron and 4-electron reactions. The OCV of 2.4 V, occurring after some discharge had occurred, was likely the result of aqueous NaO2 formation, given that superoxide (O2') is the reaction intermediate for the 2-electron ORR in highly concentrated NaOH solutions. After substantial discharge and deliquescence had occurred, flooding of the cathode made Na-water reactions more likely.

Regardless of the exact electrochemical mechanism, only NaOH was observed in these ex-situ XRD measurements, since products like Na2O2, NaO2, and NaOOH would chemically react to form NaOH.

Overpotential vs. water activity

The open circuit potential and working voltage both decreased with increasing temperature, but the rate of decrease of the working voltage was higher than that of the OCV, implying an increase in overpotential with temperature. Increasing the operating temperature increased the conductivity of the electrolyte; however, it also decreased the activity of H2O due to the increase in saturation vapor pressure of H2O at higher temperatures. FIG. 25B plots the overpotential, defined as the difference between the voltage at the end of the discharge pulse and that of the resting pulse as a function of water activity. It was observed that the overpotential could be reduced either by decreasing the operating temperature or increasing the bubbler temperature. The overpotential decreased with increased water activity initially, but reached a plateau at higher activity (a~ 0.1).

This example presents three lab-scale sodium-air cell designs, which are distinguished by the morphology of the anode and solid-state electrolyte. The “planar cell” used a 24-mm diameter, 1mm thick sodium beta-alumina solid electrolyte (BASE), with a 0.5 cm² sodium foil as the anode and a 50 um-thick copper foil as the anode-side current collector. The “sodium tray cell” used a BASE one-ended tube as the electrolyte filled with sodium metal, and a metal pin connected to the cell fixture at one end protruding into the sodium metal to act as the anode-side current collector. The “sodium H cell” used a glass H cell fixture with the same planar BASE electrolyte as the planar cell, and like the sodium tray cell the H cell had a steel wire current-collector inserted into the molten sodium anode. BASE was chosen due to its commercial availability in different geometries, and all BASE components were purchased from lonotec. All cells utilized a 400-nm thick sputtered gold film as the cathode, made using a Cressington sputter coater inside of an Ar-filled glovebox. A 99.9% Au target from Ted Pella was used with a 40 mA sputtering current and a 120 sec deposition time (or a 20 mA/cm² current and a 60 sec deposition time).

For the planar cell and the tray cell, lithium-air battery fixtures purchased from MTI were used. In the case of the tray cell, the fixture was modified to include an additional O- ring to help seal the BASE tray to the fixture wall, and the metal pin to provide electrical connection to the sodium. Prior to assembly, solid electrolytes were heat-treated in an Ar glovebox at 1000 °C for 1 hr to volatilize any sodium carbonate species that formed on the pellet surface. For the planar cell, a 0.5 cm² Na foil of either 0.5 mm or 1 mm thickness was punched out and attached to the electrolyte using the copper foil current collector and an adhesive polymer ring for sealing. For the tray cell, liquid Na was dropped onto the inside of the BASE tray and allowed to solidify. For the H cell, the glass fixture was preheated and molten sodium was poured into it. The planar electrolyte with the sputtered Au film was clamped into place with a steel mesh on the air side acting as a current collector.

All cells were tested inside of a furnace (Espec/MTPYamato) in humidified oxygen. Oxygen gas flowed through an MTI mass flow controller, then through a heated bubbler to provide a certain water vapor pressure to the cells. The OCV and potentiostatic EIS scans (Biologic, 1 MHz to 100 mHz) were conducted on the cells before and after introducing humidified O2. After introducing humidified O2, the cells were ramped up to operating temperature (100-105 °C). Cells were cycled using a GITT protocol (15 minutes of constant current discharge at either 1 mA/cm² or 2 mA/cm², followed by a 5 min rest period and a potentio static EIS scan with the same parameters), until failure. SEM was performed on the cell surface after cycling, and the cathode-electrolyte-discharge product layers were characterized using XRD (Smartlab) and FIB-SEM (Helios).

Planar Cell GITT results

FIG. 29 shows GITT cycling data for two planar cells, one cycled at 1 mA/cm² (cell A) and the other at 2 mA/cm² (cell B). In both cases, the cell cycled until nearly all Na at the anode side was consumed. Cell A had a 0.5 mm-thick Na layer, which gives a total theoretical capacity of 54.2 mAh/cm², and cell B had a 1 mm-thick Na layer, giving a theoretical capacity of 108.4 mAh/cm². Thus, cell A used 79% of its theoretical capacity, while cell B used 88% of its theoretical capacity - the remaining capacity was lost due to leakage of water from the cathode side, which reacted with the sodium to reduce the amount available for operation. In cell A, the OCV during rest periods was roughly stable around 2.4V for the first 15 mAh/cm², after which the OCV equilibrated to a new plateau around 2.1V. In cell B, these two OCV plateaus were also seen, though the OCV switched from 2.4V to 2.1V within the first mAh/cm² of capacity passed.

Impact of Dry vs. Humidified Gas

FIG. 25A shows a GITT discharge data at 0.2mA/cm² under three inlet gas conditions- static flow at 0% RH, static flow at 100% RH, and continuous flow at 100% RH. The initial OCV increased from 2.4V under dry conditions to 2.5V under humidified conditions. In addition, the average working voltage increased from 1.2V under static, dry O2 atmosphere to 2.4V in flowing, 100% RH O2.

SEM/FIB of discharge product in fuel cells discharged in dry vs humidified conditions

To understand the role of humidity on discharge product formation, two cells were built, one with a dry O2 gas inlet stream and one with a humidified (12% H2O) O2 inlet stream. As discussed in the previous section, the addition of humidity lowered the overpotential. FIG. 30A shows an FIB cross-section image of a cell post-mortem after passing 9um of Na for the dry conditions. FIG. 30B shows an FIB cross-section image of a cell post-mortem after passing 250um of Na for the humidified conditions. FIG. 30C plots voltage versus throughput from the dry and humidified conditions of FIGs. 30A and 30B. FIG. 30D shows schematics of the initial, dry, and humidified cells. In dry conditions, the discharge product formed underneath the gold, displacing and in some areas rupturing the gold cathode layer. However, under humidified conditions, even after passing 30 times the amount of throughput, the cathode layer remained adhered to the surface. The discharge product initially formed at the triple phase boundary, where there was a discontinuity in the gold film. Under dry conditions, the discharge product continued to grow from that nucleation site, eventually rupturing the cathode and severing the electronic path. However, in humidified conditions, the discharge product could dissolve and leave the triple phase boundary site, keeping the cathode intact. Since the solubility and diffusivity of O2 in high- concentration NaOH solution is extremely low, the reaction will occur at the solid-liquid-gas interface, thus potentially facilitating reaction products to form away from the cathodeelectrolyte interface. This may explain the lower overpotential observed in humidified cells.

Understanding Reaction Mechanisms

There are multiple sodium-oxygen reaction products (NaO2, Na2O2, etc.); NaOH is also possible when moisture is present, and Na2COa is possible when carbon/CO2 is present. Under some of the testing conditions, CO2 was not in the gas stream and the cathode did not contain carbon, so the potential products that were formed electrochemically were NaO2, Na2O2, NaOH, and NaOOH (NaOH-H2O decomposes at 65 °C to form an NaOH-H2O solution, as seen in the phase diagram, so it was not considered). Several experimental configurations were studied in order to understand the cathode reaction mechanisms: a sputtered Au thin film electrode, an Au mesh electrode, a Pt mesh electrode, and a Pt/C microporous gas diffusion electrode. Studies with the Au thin film electrode showed that the OCV prior to cycling was around 2.55 V, which then stabilized to 2.4 V after cycling began, and eventually fell to a voltage between 2.05 - 2.15 V. An OCV of 2.4 V after some throughput had passed was likely the result of aqueous NaO2 formation - forming the O2' (superoxide) ion is the reaction intermediate for the 2-electron ORR.

Cyclic voltammetry on the Au mesh and Pt mesh electrodes showed that Pt (which is selective for the 4-electron ORR) had one oxidation and one reduction peak, suggesting a singular electrochemical reaction product, while Au (which is a non-selective catalyst capable of 2 and 4 electron ORR at different crystallographic facets) exhibited two oxidative peaks, which suggested that multiple reaction products could be generated. The reduction current in Au was much larger than its corresponding oxidation current, which suggested that the electrochemical reaction product was chemically reacting with the environment, so that there was less product available for the reverse reaction (i.e., an EC-type reaction). This aligns with the possibility that the O2' ion was the electrochemical reaction product, and that aqueous NaCh quickly converted into NaOH. Since the thin film Au electrode was polycrystalline (and was thus non- selective) it is possible that the initial OCV observed before any throughput (2.55 V) was a mixed potential between the OCVs of the 2-electron and 4-electron reactions (2.23V for the 2-electron formation of solid Na2©2 at 100 °C, and 2.64V for the 4-electron formation of solid NaOH at 100 °C). Once the cell has passed substantial throughput, flooding of the gold surface could cause the Na-water reaction to become more likely, and thus a mixed potential between the Na-water reaction (1.88V at RT) and NaO2 formation (2.4V at RT) could be observed.

Regardless of the exact electrochemical mechanism, only NaOH was observed in the ex-situ XRD measurements, since products like Na2O2, NaO2, and NaOOH would chemically react to form NaOH. Thus, in-situ Raman spectroscopy was conducted to identify reaction products during constant current and open-circuit operation.

In-situ Raman for understanding discharge product formation mechanism

To elucidate the reaction mechanism of discharge product formation, an in-situ electrochemical Raman cell was built to identify the cathode side products during discharge. A sputtered gold film (400 nm) was used as the cathode. Before discharging, the fixture was filled with O2 flowed through a 25°C bubbler (3 % water vapor). The cell was discharged for 15 minutes at 0.25mA/cm² followed by 7 minutes at rest at an operating temperature of 25°C. A lower current density was used to prevent voiding of solid Na. Raman measurements were taken every minute during the experiment. The Raman spectrum peaks for the possible products have been reported in past Na-air literature — NaCE with a peak at 1156 cm'¹, Na2<32 with two peaks at 735 and 791 cm'¹, Na2COa at 1078 cm'¹, and NaOH at 3638 cm'¹. In addition, liquid water has a broad peak between 1580 and 1640 cm'¹ from -OH bending modes, as well as between 3240 and 3620 cm'¹ from -OH stretching modes. Increasing the concentration of NaOH in aqueous solutions causes further broadening of these water peaks in addition to the increase in its characteristic peak. FIGs. 31A and 3 IB show the Raman spectra measured during discharge, separated by pulse number as well as the mode (discharge versus rest), where FIG. 3 IB is zoomed in to 3500-3700cm^-1. Initially, no peaks were detected — however, by the end of the first discharge pulse, a peak formed at 3623 cm'¹, presumably NaOH, and a steady increase in the background intensity was also observed. During the second discharge pulse, there was a rapid increase in the background intensity and the NaOH peak. In the third discharge pulse, there continued to be an increase in the NaOH peak, but the background intensity no longer changed. During the rest cycles, both the background intensity and the NaOH peak decreased slightly. Since no Na02 nor Na2O2 peaks were observed over the course of the experiment, and the relative NaOH intensity remained constant during the rest, either NaOH was the primary discharge product, or any other discharge product reacted away quickly enough that its steady- state concentration was undetectable. In addition, the three cycles potentially captured the three phases of the reaction mechanism — the generation of solid NaOH, the deliquescence of NaOH (resulting in the sharp rise in the background intensity) and the continued increase in weight percentage of NaOH in the deliquesced solution.

Demonstration of discharge product removal

FIGs. 32A and 32B are planview SEM images of a cell after it had been discharged for 24mAh/cm² with a pulsed discharge of 2mA/cm², 105 °C operation, 50°C bubbler temperature. FIG. 32A was taken after the cell had been assembled and exposed to air. The surface was covered in sheets of a low-Z phase. With EDS, these sheets were shown to have high atomic percentages of Na, C, and O (FIG. 32C) - the low Z-phase sheets were the residual discharge product left on the cathode surface. After imaging, the same cell was washed with water to remove the discharge product- the planview image after washing is shown in FIG. 32B. The sheets were no longer present due to its dissolution with water. A conformal Au cathode layer remained, highlighting the cell’s ability to reset with the addition of water.

Understanding and Modeling the Role of Humidity on Cell Performance

Ex-situ XRD measurements demonstrated that the primary discharge product observed at the cathode was NaOH. NaOH was very hygroscopic, to the point that it demonstrated deliquescence - under the right humidity conditions, NaOH is capable of absorbing enough moisture from its environment to form a liquid NaOH-H2O solution. While solid NaOH is an electronic insulator, a liquid NaOH solution can facilitate charge transfer due to the presence of dissociated ions, which prevents the formation of discharge products from rapidly increasing the ohmic resistance between the cathode and the current collector. Additionally, the formation of a liquid NaOH solution ensures that the discharge product does not block the cathode active sites from forming further discharge products, which can facilitate steady-state operation.

Deliquescence occurs when the water vapor pressure over the NaOH is lower than the ambient water vapor pressure, thus providing a driving force for the uptake of water from the environment. Equilibrium is achieved when the vapor pressure of the NaOH solution (L) equals the ambient vapor pressure. By Henry’s Law, the water vapor pressure of a solution is the product of the water activity in solution, and the vapor pressure of pure water. Thus, ambient _ _nL °

PH₂O ~ PH₂O — ^UH₂OPH₂O

Given a model for the activities in an Na0H-H20 solution, it should be possible to predict the equilibrium water content for an NaOH solution as a function of ambient vapor pressure and temperature. Combining these predictions with the Na0H-H20 phase diagram will show which temperatures and ambient vapor pressures will yield an Na0H-H20 composition that is in the single-phase liquid regime of the phase diagram.

The temperature versus composition curves for NaOH under varying pH2Os were calculated. These curves show that under constant pH2O, increasing temperature will result in higher concentration NaOH solutions, until the curves cross the liquid phase boundary, and solid NaOH begins to precipitate out of solution. This predicted behavior was validated by observing the appearance of NaOH at constant pH2O with varying temperatures, and by measuring water uptake by NaOH through thermogravimetric analysis.

Conclusion

In this example, we demonstrated a sodium-air fuel cell concept that achieved the highest areal capacity and power density ever demonstrated. High current density operation with low overpotential and large metal anode loadings were achieved by the deliquescence of the NaOH discharge product, which facilitated the formation of a liquid NaOH solution at temperatures well above the melting point of Na metal, thus eliminating voiding issues on the anode side, and electrode clogging/passivation on the cathode side. The abundance of Na metal means that it is feasible to operate this cell in a discharge-only mode and either dispose of or collect the discharge that is produced.

Concept schematic in multi-stack configuration FIG. 4C shows a conceptual drawing of a multi-stack module based on single stack in series and parallel, in accordance with some embodiments.

Ionic conductivity of Na- /I” alumina

Symmetric cells were built to measure the ionic conductivity of Na-/?” alumina. Roughly 400 nm of platinum was deposited on both sides of the solid-state-electrolyte, and was sandwiched between two pieces of copper foil, and pressed between two stainless steel plates loaded with springs. Electrochemical impedance spectroscopy was used to measure the bulk resistance of Na-/?” alumina. All measurements were conducted in the glovebox at <0.1 ppm O2 level and <0.1 ppm H2O level.

FIG. 33 shows the temperature dependence of the ionic conductivity of Na-/?” alumina measured with electrochemical impedance spectroscopy. The conductivity follows an Arrhenius relationship, where the log of GT scales linearly with the inverse of temperature.

Overpotential vs. water activity

FIGs. 34A-34B plot the first discharge pulse followed by a rest at varying cell and bubbler temperatures, which was used to calculate the overpotential as a function of water activity in FIG. 25B. In FIG. 34A, the cell temperature was varied while the bubbler temperature was held constant, and in FIG. 34B, the bubbler temperature was varied while the cell temperature was held constant. The overpotential was calculated as the difference between the voltage at the end of the discharge pulse and that of the resting pulse. The activity at each condition was calculated as follows:

The overpotential decreased with water activity, which can be lowered either by decreasing the cell temperature or increasing the bubbler temperature (see Table 2).

FIGs. 34A and 34B show a first discharge pulse followed by rest at varying cell and bubbler temperatures (15 minutes at ImA/cm² followed by a 5 minute rest with a 50nm sputtered gold film cathode). FIG. 34A uses a fixed bubbler temperature of 25 °C. FIG. 34B uses a fixed cell temperature of 100 °C. Table 2: Activity of water at different cell and bubbler temperatures

FIB cross-section comparison of dry vs. humidified inlet gas stream

FIG. 30B shows a GITT comparison between dry and humidified inlet gas stream (15 minutes at 2 mA/cm², followed by a 5 minute rest) using a 400 nm sputtered gold film cathode. The working voltage was lower when using a dry inlet stream compared to a humidified inlet stream.

FIGs. 35A and 35B show EDS mapping of FIB cross-sections following discharge in dry (FIG. 35A) and humidified conditions (FIG. 35B). Deliquescence

Two sets of experiments were conducted to study deliquescence. First, some anhydrous NaOH powder (stored in an Ar-filled glovebox) was transferred into a gas washing bottle which was connected to a source of humidified Ar (Ar bubbled through room temperature water). The NaOH powder was placed on a hot plate, and the hot plate temperature was varied. It was observed whether the NaOH powder had transformed into a liquid solution, and vice versa. Using Ar instead of atmospheric air prevents the NaOH from being converted into Na2CO3. The experimental setup is shown in FIG. 36.

The results from this study are shown in FIG. 37. These images suggest that the onset of deliquescence (when the equilibrium NaOH concentration crosses into the liquid phase of the phase diagram), occurs around 90°C under 0.03 atm of water vapor (the saturation vapor pressure at 25C). However, due to the qualitative nature of this study, and since there was the risk of forming Na2COa over the long timescales needed to observe deliquescence and recrystallization, the deliquescence was further validated using TGA.

The TGA experiment involved placing some NaOH powder in a platinum pan and quickly ramping up the temperature to 150°C and holding under flowing humidified Ar. Even though the Ar was humidified, at 150°C the equilibrium NaOH concentration was far from the single-phase regime, and thus the NaOH dehydrated. After the weight started stabilizing, the temperature was brought down to 100°C, and the weight continued stabilizing, since 100°C is also too high of a temperature for NaOH to absorb moisture (under those humidity conditions). Once the weight had finally stabilized, the temperature was decreased and held at various lower temperatures, and the stabilized weights were used to determine the water content, assuming that the weight after the 100°C isothermal hold gives the mass of the NaOH in the system. The mass vs time data is shown in FIG. 38.

Using the data in FIG. 38, the NaOH weight percentage was calculated at 80°C, 85°C, 90°C, and 95°C, as shown in Table 3.

Table 3: Temperature versus NaOH wt% from thermogravimetric analysis (TGA)

This experimental data demonstrates NaOH deliquescence behavior. EXAMPLE 3

A continuously operating sodium-oxygen fuel cell was developed that achieved an energy density of 1000 Wh/kg, a power density of 170 W/kg, and a capacity of 400 mAh/cm², at a form factor of 0.5 cm² with an operating current of 100 mA/cm² and an operating voltage of 1.93V. The fuel cell had an H-cell configuration (see FIG. 24B). The anode comprised an anodic reactant comprising liquid sodium metal. The cathode was a gas diffusion cathode comprising a porous PTFE film, a Ni foam substrate, and a microporous layer adorned with catalyst. Linear sweep voltammetry results suggested that this design is capable of achieving a current density of 330 mW/cm², which would result in a power of 0.35 W/cm².

Visual confirmation showed formation of a liquid discharge product under oxygen at 120 °C with a partial water pressure of 0.46 atm. The liquid discharge product flowed down the cell and settled in the cathode chamber.

EXAMPLE 4

A continuously operating sodium-air fuel cell was developed that discharged at 80 mA/cm² with an average voltage of 1.32V for 2360 mAh/cm² (1188 Wh/kg, 40 W/kg, capacity equivalent to 2.1 cm thickness of Na). The fuel cell had a liquid tray fixture configuration (see FIG. 24A) and was operated at 110 °C using an air flow with a partial water pressure of 0.46 atm. The fuel cell achieved an energy density of 1188 Wh/kg, a capacity of 2360 mAh/cm², and a power density of 40 W/kg, at a form factor of 0.12 cm².

EXAMPLE 5

A Na-air fuel cell, in which the discharge product forms in between the solid electrolyte and air cathode, was developed. The discharge product was removed from the fuel cell via in-plane flow to the edge of the air electrode. The fuel cell operated at 120 °C, using a liquid Na anode, a sodium-beta-alumina solid-state electrolyte tray, and an gas diffusion cathode. The tray was oriented so that the Na anode was depleted in the direction of gravity. The voltage, current, and power for this fuel cell are shown in FIG. 46. It should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific implementations described above. The specific implementations described above are disclosed as examples only.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of’ or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc.

As used herein, unless indicated to the contrary, all percentages disclosed herein that refer to relative amounts are weight percentages. In the claims, as well as in the specification above, all transitional phrases such as

“comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to.

Only the transitional phrases “consisting of’ and “consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

CLAIMS What is claimed is:

1. An alkali metal fuel cell, comprising: a cathode comprising a cathodic reactant comprising gaseous water; and an anode comprising an anodic reactant comprising a liquid alkali metal; wherein the alkali metal fuel cell is configured to produce a discharge product; wherein the discharge product comprises an alkali metal hydroxide, and wherein at least a portion of the alkali metal hydroxide is in the form of a liquid solution.

2. An alkali metal fuel cell, comprising: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; an anode comprising an anodic reactant comprising a liquid alkali metal; and a solid electrolyte; wherein the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull.

3. An alkali metal fuel cell, comprising: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 5 centimeters.

4. An alkali metal fuel cell, comprising: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the alkali metal fuel cell is configured such that the liquid alkali metal is not replenished during its period of operation.

5. A method, comprising, discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous water; and an anode comprising an anodic reactant comprising a liquid alkali metal; wherein the alkali metal fuel cell produces a discharge product comprising an alkali metal hydroxide during the discharging; and wherein at least a portion of the alkali metal hydroxide is in the form of a liquid solution.

6. A method, comprising: discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; an anode comprising an anodic reactant comprising a liquid alkali metal; and a solid electrolyte; wherein the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull; and wherein, during the discharging, a discharge product exits the cathode in a direction substantially parallel to the direction of gravitational pull.

7. A method, comprising: discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 10 centimeters.

8. A method, comprising: discharging an alkali metal fuel cell to produce an electric current, wherein the alkali metal fuel cell comprises: a cathode comprising a cathodic reactant comprising gaseous oxygen, gaseous water, and/or gaseous carbon dioxide; and an anode comprising an anodic reactant comprising a layer of liquid alkali metal; wherein the liquid alkali metal is not replenished during the discharging.

9. A method, comprising discharging the alkali metal fuel cell of any preceding claim to produce an electric current.

10. The alkali metal fuel cell and/or method of any preceding claim, wherein: the cathodic reactant comprises gaseous water; the alkali metal fuel cell is configured to produce a discharge product; the discharge product comprises an alkali metal hydroxide, and at least a portion of the alkali metal hydroxide is in the form of a liquid solution.

11. The alkali metal fuel cell and/or method of any preceding claim, wherein: the alkali metal fuel cell comprises a solid electrolyte; and the solid electrolyte is below the anode and the cathode is below the solid electrolyte in the direction of gravitational pull.

12. The alkali metal fuel cell and/or method of any preceding claim, wherein: the anodic reactant comprises a layer of the liquid alkali metal; and the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 5 centimeters.

13. The alkali metal fuel cell and/or method of any preceding claim, wherein: the anodic reactant comprises a layer of the liquid alkali metal; and the alkali metal fuel cell is configured such that the liquid alkali metal is not replenished during its period of operation.

14. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal hydroxide comprises sodium hydroxide.

15. The alkali metal fuel cell and/or method of any preceding claim, wherein: the alkali metal fuel cell further comprises a separator; and the alkali metal fuel cell further comprises a drain and/or collection system for the liquid solution; wherein the separator is permeable to the liquid solution and impermeable to the alkali metal; and wherein the separator is positioned upstream of the drain and/or collection system.

16. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured to produce the liquid solution at an operating temperature of greater than or equal to 98 °C and less than or equal to 323 °C.

17. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured to produce the liquid solution at an operating temperature of greater than or equal to 98 °C and less than or equal to 200 °C.

18. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured to produce the liquid solution at a water partial pressure of greater than or equal to 0.03 atm and less than or equal to 5 atm.

19. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured to produce the liquid solution at a water partial pressure of greater than or equal to 0.12 atm and less than or equal to 2 atm.

20. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured such that a discharge product exits the cathode in a direction substantially parallel to the direction of gravitational pull.

21. The alkali metal fuel cell and/or method of any preceding claim, wherein the solid electrolyte has an anode-facing surface, and wherein the anode-facing surface comprises a coating comprising tin, silver, gold, and/or carbon.

22. The alkali metal fuel cell and/or method of any preceding claim, wherein the layer of liquid alkali metal has a thickness of greater than or equal to 1 millimeter and less than or equal to 2 centimeters.

23. The method of any preceding claim, wherein the method comprises removing at least a portion of the discharge product from the alkali metal fuel cell during operation.

24. The method of any preceding claim, wherein the method comprises using the discharge product and/or a downstream product thereof to capture atmospheric carbon dioxide and/or decrease the acidity of a body of water.

25. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured such that the liquid alkali metal is replenished continuously or intermittently during its period of operation.

26. The alkali metal fuel cell and/or method of any preceding claim, wherein the alkali metal fuel cell is configured such that at least a portion of the discharge product is removed from the alkali metal fuel cell during operation of the alkali metal fuel cell.

27. The alkali metal fuel cell and/or method of any preceding claim, wherein the cathode comprises metal or carbon films, sintered cermets, gas diffusion electrodes, and/or mixed ionic-electronic conductors (MIEC).

28. The alkali metal fuel cell and/or method of any preceding claim, wherein the power density of the alkali metal fuel cell increases during operation.

29. A system comprising multiple alkali metal fuel cells according to any preceding claim, stacked substantially parallel to the direction of gravitational pull.

30. A system comprising: the alkali metal fuel cell according to any preceding claim; and a solid-electrolyte electrochemical cell, comprising an alkali metal-conducting solid electrolyte; wherein the solid-electrolyte electrochemical cell is configured to produce the alkali metal from XCl-AlCh, wherein X is the alkali metal.

31. A system comprising: the alkali metal fuel cell according to any preceding claim; and an alkali metal storage and handling system comprising petroleum or silicone oil.

32. The alkali metal fuel cell and/or system and/or method of any preceding claim, wherein the alkali metal comprises sodium.

33. A sodium metal-based power system comprising a. a sodium-air fuel cell, wherein the fuel cell uses liquid sodium and produces solid Na2<32; b. a solid-electrolyte electrochemical cell using NaCl-AlCh to produce sodium metal; and c. a sodium metal storage and handling system comprising petroleum or silicone oil.

34. The system of claim 33, wherein the fuel cell comprises Na, Na P”-alumina, sputtered gold, and 1 atm O2.

35. The system of claim 33, wherein the fuel cell comprises a 2-dimensional cathode that comprises metal or carbon films, sintered cermets, or mixed ionic-electronic conductors (MIEC) that is adherent to a solid electrolyte.