WO2025005383A1 - Fuel cell-water electrolyzer hybrid system based on oxidation reaction sharing - Google Patents

Abstract

The present invention presents a structure and circuit of a cell, wherein a fuel cell is used as an auxiliary device for water electrolysis in order to reduce the overvoltage required for a water electrolysis reaction, with a circuit designed to maintain the open circuit potential due to the absence of current flow, while an oxidation reaction in the fuel cell can be applied as a counter reaction to a hydrogen evolution reaction in water electrolysis. In this hybrid fuel cell-water electrolysis system presented in the present patent, the fuel cell and the water electrolysis share the same oxidation reaction to decrease the voltage difference from the hydrogen evolution reaction, thereby reducing the overvoltage for hydrogen evolution. The use of the system presented in the present patent enables the production of hydrogen at a voltage lower than a theoretical voltage of 1.23 V of a water electrolyzer by using an oxygen oxidation reaction as a counter reaction.

Description

Fuel cell-water electrolysis hybrid system based on oxidation reaction sharing

The present invention is characterized by lowering the overvoltage required for hydrogen generation in a water electrolysis device by sharing an oxidation reaction in a fuel cell (fuel cell/galvanic cell) and a water electrolyzer (water electrolyzer/electrolytic cell) compared to the theoretical overvoltage required when using the water electrolyzer alone.

The overvoltage for hydrogen generation can be lowered to a large value by introducing a 3 compartment cell (H-cell or zero gap cell) for sharing the oxidation reaction and configuring a circuit for sharing the oxidation reaction.

Oxidation reactions that can be shared across a 3-compartment cell must have a standard reduction potential (SRP) between the hydrogen redox reaction (0 V vs. RHE) and the oxygen redox reaction (1.23 V vs. RHE) with respect to the reversible hydrogen electrode (RHE).

The present invention enables sharing of a specific oxidation reaction in a three-compartment cell by allowing the standard reduction voltage of a specific oxidation reaction to exist between the hydrogen oxidation/reduction reaction and the oxygen oxidation/reduction reaction, thereby lowering the overvoltage of a water electrolysis reaction for hydrogen generation, thereby efficiently producing hydrogen.

Hydrogen is considered a new energy source that can replace fossil fuels as an environmentally friendly energy storage medium with high energy density. In addition, hydrogen has high efficiency because it can be converted directly from chemical energy to electrical energy in one step through a fuel cell. Ideally, such hydrogen should be produced through a water electrolysis system using electrical energy generated through renewable energy, but it has been produced as a byproduct of petrochemical processes to date. The reason why hydrogen production through water electrolysis reaction has not yet been activated is because the efficiency is low due to high overvoltage and the durability of the water electrolysis catalyst has not yet been secured.

The electrolysis reaction is a non-spontaneous reaction that occurs when voltage is applied from the outside, and is called an electrolytic cell. When voltage is applied to the cell from the outside, an oxygen evolution reaction (oxygen oxidation reaction) occurs at the anode through an oxidation reaction to generate oxygen, and a hydrogen evolution reaction (hydrogen reduction reaction) occurs at the cathode through a reduction reaction, resulting in the production of environmentally friendly hydrogen. In the electrolysis system, the hydrogen evolution reaction to generate hydrogen must occur in parallel with the oxygen evolution reaction, which is an oxidation reaction. However, the oxygen evolution reaction is not only the rate determining step (RDS) in the overall reaction, but also reduces the durability of the electrode due to the high oxidation voltage. In addition, expensive precious metals such as iridium and ruthenium are used for the oxygen evolution reaction, and platinum is used for the hydrogen evolution reaction. In order to produce hydrogen that can replace fossil fuels through electrolysis, the high cost problem caused by the use of precious metal catalysts must also be solved. In the case of platinum, which is highly active in the hydrogen evolution reaction, the minimum usage amount has been greatly reduced thanks to many studies, but in the case of iridium and ruthenium for the oxygen evolution reaction, the amount is still relatively large due to durability issues caused by the harsh oxidation conditions due to high overvoltage, and there are still many issues to be resolved before commercialization.

The theoretical voltage of the oxygen evolution reaction is 1.23 V vs. RHE, but in reality, a higher voltage than the theoretical voltage must be applied to cause the oxygen evolution reaction due to the overvoltage. This causes a problem of reduced electrode durability, and a general method to solve this problem is to develop a catalyst with higher activity and durability. However, this method has not yet achieved practical levels of activity and durability. Another method to lower the overvoltage for hydrogen evolution is to use another oxidation reaction instead of the oxygen oxidation reaction as the counterpart reaction of the hydrogen evolution reaction. Representative reactions that can replace the oxygen oxidation reaction include urea oxidation and methanol oxidation. In the case of urea electrolysis, if anion exchange membrane is utilized, the hydrogen evolution reaction (H ₂ O → H ₂ + OH ^- ) reaction occurs at the reduction electrode (cathode), and the urea oxidation reaction (urea oxidation reaction: CO(NH ₂ ) ₂ + 6OH ^- → N ₂ (g) + CO ₂ (g) + 5H ₂ O (l)) occurs at the oxidation electrode (anode), in which the OH ^_ ion that has crossed through the alkaline electrolyte (or alkaline electrolyte membrane) reacts with urea to generate hydrogen. Theoretically, since the urea oxidation reaction has a standard reduction potential of 0.338 V, the overvoltage for the hydrogen evolution reaction should be about 0.4 V as the on-set potential. However, due to the reaction characteristics of the Ni catalyst, which is the most active in the urea oxidation reaction (since the oxidation of Ni must precede), the on-set potential of the actual reaction has a similar voltage to the hydrogen evolution reaction, in which the oxygen evolution reaction proceeds as a counter reaction.

The purpose of the present invention is to propose a practical water electrolysis system in which the standard reduction voltage is between 0 V vs. RHE and 1.2 V vs. RHE and the oxidation reaction that can be commonly used in both fuel cells and water electrolysis reactions is shared between fuel cells and water electrolysis cells, thereby lowering the overvoltage for hydrogen production in water electrolysis.

This concept can be utilized in both acidic and alkaline conditions according to the sharing reaction, and can produce hydrogen with low overvoltage and low energy, while at the same time improving durability by excluding the oxygen evolution reaction that requires high oxidation voltage from the electrolytic cell. In the case of electrode sharing, the fuel cell plays a supporting role to the water electrolytic cell.

The present invention proposes a system that can generate hydrogen with less external energy by lowering the overvoltage of the hydrogen generation reaction with the help of the direct urea fuel cell, which is a spontaneous reaction, by utilizing a direct urea fuel cell (DUFC) that uses urea oxidation as an oxidation reaction and oxygen reduction reaction as a reduction reaction, and by also using the urea oxidation reaction of the direct urea fuel cell as an oxidation reaction of urea electrolysis (reaction sharing).

According to one embodiment of the present invention, a fuel cell-water electrolysis cell hybrid system is provided, which includes an oxygen reduction reaction (ORR) electrode, a hydrogen evolution reaction (HER) electrode, and a shared oxidation reaction electrode between the electrodes.

The fuel cell-water electrolysis hybrid system proposed in the present invention enables oxidation reaction sharing if any arbitrary oxidation reaction generates H ⁺ in an acid and uses ^OH- in an alkaline, and exists between 0 V, which is a standard reduction potential for hydrogen oxidation/reduction reactions, and 1.23 V, which is a standard reduction voltage for oxygen oxidation/reduction reactions, with respect to RHE. Therefore, the shared oxidation reaction of the fuel cell-water electrolysis hybrid system is characterized in that the standard reduction voltage has a value between 0 V (vs. RHE) and 1.23 V (vs. RHE) with respect to the reversible hydrogen electrode (RHE).

The oxidation reaction that can be used as a shared reaction of the above fuel cell and water electrolytic cell must have a standard reduction voltage of the reaction between 0 V and 1.23 V with respect to RHE, and in order to be shared under acidic conditions, protons (H ⁺ ) must be generated as a result of the oxidation reaction, and in order to be shared under basic conditions, hydroxide ions (OH ^- ) must be utilized during the oxidation reaction.

The above shared oxidation reaction may be any one of a urea oxidation reaction, a methanol oxidation reaction, or a formic acid oxidation reaction, but is not limited thereto. Specifically, examples that can be used as the shared reaction of the fuel cell and the water electrolytic cell include a methanol oxidation reaction (CH ₃ OH + H ₂ O → 6H ⁺ + 6e ^- + CO ₂ ) in an acidic environment, a methanol oxidation reaction (CH ₃ OH + 6OH ^- → CO ₂ + 5H ₂ O + 6e ^- ) in an alkaline environment, and a urea oxidation reaction (urea oxidation: CO(NH ₂ ) ₂ + 6OH ^- → N ₂ + CO ₂ + 5H ₂ O + 6e ^- ). In addition, a formic acid oxidation reaction can also be applied, and all reactions that satisfy the above conditions are possible.

The above fuel cell or water electrolysis cell structure may be any one of an H-cell, a zero-gap cell, and a flow cell that can share the shared oxidation reaction electrode, but is not limited thereto.

The fuel cell-water electrolysis cell hybrid system may include three separate compartments, each of which may be separated by an ion exchange membrane electrolyte. The electrodes may be positioned in each of the three separate compartments, each of which may be separated by an ion exchange membrane electrolyte.

In the above fuel cell and water electrolysis cell, the electrolyte may be a liquid electrolyte, a membrane electrolyte or a solid electrolyte, and the oxidation reaction shared in the fuel cell and water electrolysis cell may use a cation (hydrogen ion) exchange membrane for the reaction that produces hydrogen ions, and may use an anion (hydroxide ion) exchange membrane for the reaction that uses (consumes) hydroxide ions.

As electrodes for the above-mentioned sharing reaction and catalysts for the oxygen reduction reaction (ORR) of fuel cells and the hydrogen evolution reaction (HER) of water electrolysis cells, Pt, Ru, Ir, Au, Pd, Ni, Fe, Co, Mo and alloys of two or more thereof can be used.

The above fuel cell and water electrolyzer may be a polymer fuel cell and a polymer water electrolyzer or a solid oxide fuel cell and a solid oxide water electrolyzer. The hydrogen production system through the above-mentioned sharing reaction can be applied to a solid oxide fuel cell - solid oxide water electrolysis operated at high temperature in addition to a polymer electrolyte fuel cell and polymer electrolyte water electrolysis operated at room temperature under the same concept.

The above polymer fuel cell and polymer water electrolysis cell include a polymer membrane, and the polymer membrane may be a proton exchange membrane or anion exchange membrane. Specifically, the 3 compartments cell is composed of 3 compartments and 2 membranes, and the two membranes separate the shared oxidation reaction electrode compartment and the oxygen reduction reaction compartment (fuel cell) and the shared oxidation reaction electrode compartment and the hydrogen evolution reaction (water electrolysis cell) compartment, and a proton exchange membrane is used under acidic conditions and an alkaline exchange membrane is used under basic conditions.

The structure and circuit diagram of a 3-compartment cell for sharing the above oxidation reaction can be divided into a case where the structure of the 3-compartment cell shares reactants but has an insulator so that the flow of electrons is not shared, and a case where the structure of the 3-compartment cell shares both the flow of reactants and electrons.

If the 3 compartment cell structure shares both the flow of reactants and electrons, the shared oxidation reaction electrode must be connected to the power supply, and the oxygen reduction electrode of the fuel cell must be connected to the hydrogen evolution electrode/power supply cathode of the water electrolysis cell to form a closed circuit, and a variable resistor or a resistance so high that almost no current flows between them is installed.

When an insulator is included in the shared oxidation reaction electrode of a fuel cell-water electrolysis hybrid system, a circuit can be configured in which the water electrolysis hydrogen generation electrode - fuel cell shared oxidation electrode - fuel cell oxygen reduction electrode - water electrolysis shared oxidation electrode are connected in series.

The above insulator is characterized by insulating the fuel cell shared oxidation reaction electrode and the water electrolysis shared oxidation reaction electrode while allowing sharing of the reactant of the shared oxidation reaction electrode. Materials that can be used as the insulator include materials that are resistant to strong acidic and basic conditions, such as Viton or Teflon, and polymers or inorganic materials that are resistant to acids and bases are also possible.

The above-mentioned water electrolysis hydrogen generation electrode - fuel cell shared oxidation electrode may further include a power supply device connected to the wire.

Unlike the above embodiment, when the shared oxidation reaction electrode of the fuel cell-water electrolysis hybrid system does not include an insulator, a circuit can be configured to directly connect the fuel cell oxygen reduction electrode and the hydrogen generation electrode/power supply cathode of the water electrolysis. A high resistance and a diode can be further included in the wire connecting the fuel cell oxygen reduction electrode and the power supply cathode.

According to one embodiment of the present invention, a system for efficiently producing hydrogen can be implemented by solving the problem of high overvoltage for hydrogen generation in a water electrolysis cell by sharing an oxidation reaction in a fuel cell and a water electrolysis cell.

As a result, the present invention solves the problem of catalyst development in existing studies to improve the activity and durability of hydrogen-evolution reaction electrolysis cells by simply presenting a system, and the fuel cell reaction, which is a spontaneous reaction through oxidation reaction sharing, supports the overall reaction, and by thermodynamically controlling the voltage together with a power supply, hydrogen generation can be realized with low overvoltage.

Figure 1 shows the core concept of the present invention, which is that in order for a shared oxidation reaction to be shared between a fuel cell and a water electrolytic cell, the standard reduction voltage of the reaction must be between the oxygen evolution reaction (oxygen oxidation reaction) and the hydrogen evolution reaction (hydrogen ion reduction reaction). The urea oxidation reaction (UOR) and methanol oxidation reaction (MOR) in an alkaline electrolyte and the methanol oxidation reaction (MOR) in an acid electrolyte are shown as examples.

Fig. 2(a) shows the circuit configuration of a 3-compartment cell in which, when an alkaline electrolyte (anion exchange membrane) is used, hydroxide ions generated at the oxygen reduction electrode and the hydrogen evolution electrode are provided to the shared oxidation reaction electrode through the anion exchange membrane, so that the fuel cell and water electrolysis reactions can occur simultaneously. The shared oxidation reaction can occur when the standard reduction voltage exists between the hydrogen oxidation/reduction voltage (0 V vs. RHE) and the oxygen oxidation/reduction voltage (1.23 V vs. RHE). When an acidic electrolyte (acid electrolyte, proton exchange membrane) is used in the 3-compartment cell, hydrogen ions generated in the shared oxidation reaction can be provided to the oxygen reduction reaction and the hydrogen evolution reaction, so that the fuel cell and water electrolysis reactions can occur simultaneously.

Figure 2(b) shows a circuit diagram in which a fuel cell and a water electrolysis cell are connected in series with an insulator installed in the shared oxidation reaction compartment in a 3 compartment cell. Although a 3 compartment cell is used, the effect of a series connection can be achieved by utilizing the membrane and circuit in the oxidation reaction shared electrode.

Figure 3 shows the implementation of the oxidation reaction shared electrode presented in Figure 2(a) using a 3 compartment H-cell and an alkaline exchange membrane.

Figure 4 shows the results of the voltage applied from the power supply and the current representing hydrogen production for the HER reaction in the case of (a) urea oxidation and (b) methanol oxidation as the shared oxidation reaction in the 3 compartment H-cell implemented in Figure 3.

The above objects, other objects, features and advantages of the present invention will be easily understood through the following preferred embodiments related to the attached materials. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. Rather, the embodiments introduced herein are provided so that the disclosed contents can be thorough and complete and so that the spirit of the present invention can be sufficiently conveyed to those skilled in the art.

In this specification, it should be understood that terms such as “include” or “have” are intended to specify the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

Unless otherwise specified, all numbers, values, and/or expressions expressing components and reaction conditions used in the specification are to be understood as being modified in all instances by the term "about" because these numbers are approximations that inherently reflect, among other things, the various uncertainties of measurement that arise in obtaining such values. Furthermore, whenever a numerical range is disclosed herein, such range is continuous and includes every value from the minimum value to the maximum value inclusive, unless otherwise indicated. Furthermore, whenever such a range refers to an integer, every integer from the minimum value to the maximum value inclusive, unless otherwise indicated, is included.

Experimental example

To verify the concept of the present invention, a hydrogen generation reaction was implemented by operating a 3 compartment H-cell using urea oxidation reaction and methanol oxidation as shared oxidation reactions.

1. Experimental method

1) Preparation of 3 compartments cell

The 3 compartments cell was implemented using an H-cell, and as shown in Fig. 2(a) and Fig. 3, 1) oxygen reduction (ORR) electrode, 2) urea oxidation reaction (UOR) or methanol oxidation reaction (MOR) electrode, and 3) hydrogen evolution (HER) electrode were implemented in each compartment, and FAA50 anion exchange membranes were positioned between electrodes 1) and 2) and between electrodes 2) and 3).

Pt mesh was used for the oxygen reduction electrode (electrode 1) and the hydrogen evolution electrode (electrode 3). 2) Ni catalyst was used as the urea oxidation reaction electrode, and PtRu/C catalyst was used as the methanol oxidation reaction electrode. 1 M KOH solution was used as the electrolyte for each cell, and 0.3 M urea/1 M KOH electrolyte was used as the urea oxidation reaction electrode, and 3 M methanol/1 M KOH was used as the reactant and electrolyte for the methanol oxidation reaction electrode.

2) Circuit configuration

In order to configure a circuit like Fig. 2(a), 2) the element water oxidation reaction electrode was connected to the (+) electrode of the power supply, and 3) the hydrogen generation electrode was connected to the (-) electrode of the power supply. 1) The oxygen reduction electrode of the fuel cell was connected to the (-) electrode of 3) the hydrogen generation electrode/power supply using a wire, and a 10 M ohm resistor and diode were connected to the wire.

3) Hydrogen generation reaction experiment

The current and voltage were measured using a potentiostat connected to a 3 compartments cell as a power supply. The working electrode was connected to the UOR or MOR reaction, and the counter and reference electrodes were connected to the HER electrode. The entire control was performed through the current, and the voltage change was observed while the current was scanned at a rate of 0 mA to 0.2 mA/s. At the same time, the hydrogen produced from the HER was also confirmed.

2. Experimental Results

1) Hydrogen production through elemental water oxidation reaction sharing

Fig. 4(a) shows the results of measuring the HER reaction by linear sweep voltammetry with the urea water oxidation reaction (UOR) as a shared reaction after installing a 3-compartment H-cell as in Fig. 3 with the circuit configuration of Fig. 2(a). The on-set potential for HER starts below 1.4 V, and as the current increases, the voltage reaches a minimum value of 1.27 V and then increases again. The on-set voltage and minimum voltage for HER have the lowest values among those reported for HER in urea water-electrolyte cells so far.

The reason why the oxygen evolution reaction for the HER reaction does not fall below the theoretical value of 1.23 V when applying the shared oxidation reaction suggested in the concept of this patent and using it as a corresponding reaction is thought to be because the standard reduction voltage of UOR is 0.338 V, but a high voltage of 1.4 V or higher is required for the Ni catalyst used as a catalyst for UOR to become Ni oxide active for UOR.

2) Hydrogen production through methanol oxidation reaction sharing

Figure 4(b) shows the results of measuring the HER reaction by linear sweep voltammetry with the methanol oxidation reaction (MOR) as a shared reaction after installing a 3-compartment H-cell as in Figure 3 with the circuit configuration of Figure 2(a).

The on-set potential for HER starts below 0.35 V and increases again after reaching a minimum value of 0.077 V as the current increases. 0.077 V is a value close to the thermodynamic potential of 0.04 V for methanol oxidation reaction. This value indicates that in a fuel cell-water electrolysis hybrid system, the fuel cell thermodynamically supports water electrolysis, allowing water electrolysis to start near the theoretical value when MOR is used as a counterpart reaction for the hydrogen evolution reaction. The on-set voltage and minimum voltage obtained by covalently synthesizing methanol oxidation reaction are the lowest values among the methanol-water electrolysis voltages reported for HER so far.

The present invention described above is not limited to the above-described embodiments, as various substitutions and changes can be made without departing from the technical spirit of the present invention by a person having ordinary skill in the art to which the present invention pertains.

<National Research and Development Project that Supported This Invention>

This patent application is the result of the mid-career researcher support project conducted with the support of the National Research Foundation of Korea and the funds of the Ministry of Science and ICT (Project No. 2021R1A2C2003397).

Claims (12)

Oxygen reduction reaction (ORR) electrode; Hydrogen Evolution Reaction (HER) electrode; and A fuel cell-water electrolysis cell hybrid system comprising a shared oxidation reaction electrode between the above electrodes. In the first paragraph, A fuel cell-water electrolysis cell hybrid system, wherein the oxidation reaction has a standard reduction voltage of 0 V (vs. RHE) to 1.23 V (vs. RHE) with respect to a reversible hydrogen electrode (RHE). In the first paragraph, A fuel cell-water electrolysis cell hybrid system, characterized in that the oxidation reaction is any one of a urea oxidation reaction, a methanol oxidation reaction, or a formic acid oxidation reaction. In the first paragraph, A fuel cell-water electrolysis cell hybrid system, characterized in that the fuel cell or water electrolysis cell structure is any one of an H-cell, a zero-gap cell, and a flow cell capable of sharing the shared oxidation reaction electrode. In the first paragraph, A fuel cell-water electrolysis cell hybrid system, wherein the electrodes are each located in three separate compartments, and each compartment is separated by an ion exchange membrane electrolyte. In the first paragraph, A fuel cell-water electrolyzer hybrid system, characterized in that the fuel cell and water electrolyzer are a polymer fuel cell and a polymer water electrolyzer, or a solid oxide fuel cell and a solid oxide water electrolyzer. In Article 6, A fuel cell-water electrolysis cell hybrid system, characterized in that the polymer fuel cell and polymer water electrolysis cell include a polymer membrane, and the polymer membrane is a proton exchange membrane or an anion exchange membrane. In the first paragraph, A fuel cell-water electrolysis hybrid system characterized in that, when the shared oxidation reaction electrode includes an insulator, a circuit can be configured in which a water electrolysis hydrogen generation electrode - a fuel cell shared oxidation electrode - a fuel cell oxygen reduction electrode - a water electrolysis shared oxidation electrode are connected in series. In Article 8, A fuel cell-water electrolysis hybrid system, characterized in that the insulator insulates the fuel cell shared oxidation reaction electrode and the water electrolysis shared oxidation reaction electrode while allowing sharing of reactants of the shared oxidation reaction electrode. In Article 8, A fuel cell-water electrolysis cell hybrid system further characterized by including a power supply device on a wire connecting the above-mentioned water electrolysis hydrogen generation electrode and the fuel cell shared oxidation electrode. In the first paragraph, A fuel cell-water electrolysis cell hybrid system characterized in that a circuit can be configured to directly connect a fuel cell oxygen reduction electrode and a water electrolysis hydrogen generation electrode/power supply cathode when the shared oxidation reaction electrode does not include an insulator. In Article 11, A fuel cell-water electrolysis cell hybrid system characterized by further including a high resistance and a diode in the wire connecting the fuel cell oxygen reduction electrode and the power supply device cathode.

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