Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
  • C25B1/04—Hydrogen or oxygen by electrolysis of water
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/02—Hydrogen or oxygen
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/02—Electrodes; Manufacture thereof not otherwise provided for characterised by shape or form
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B15/00—Operating or servicing cells
  • C25B15/08—Supplying or removing reactants or electrolytes; Regeneration of electrolytes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/17—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof
  • C25B9/19—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms
  • C25B9/23—Cells comprising dimensionally-stable non-movable electrodes; Assemblies of constructional parts thereof with diaphragms comprising ion-exchange membranes in or on which electrode material is embedded
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/70—Assemblies comprising two or more cells
  • C25B9/73—Assemblies comprising two or more cells of the filter-press type
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0258—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant
  • H01M8/026—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant characterised by grooves, e.g. their pitch or depth
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
  • H01M8/184—Regeneration by electrochemical means
  • H01M8/186—Regeneration by electrochemical means by electrolytic decomposition of the electrolytic solution or the formed water product
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M16/00—Structural combinations of different types of electrochemical generators
  • H01M16/003—Structural combinations of different types of electrochemical generators of fuel cells with other electrochemical devices, e.g. capacitors, electrolysers
  • H01M16/006—Structural combinations of different types of electrochemical generators of fuel cells with other electrochemical devices, e.g. capacitors, electrolysers of fuel cells with rechargeable batteries
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0247—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the form
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04746—Pressure; Flow
  • H01M8/04753—Pressure; Flow of fuel cell reactants
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04746—Pressure; Flow
  • H01M8/04768—Pressure; Flow of the coolant
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
  • H01M8/04298—Processes for controlling fuel cells or fuel cell systems
  • H01M8/04694—Processes for controlling fuel cells or fuel cell systems characterised by variables to be controlled
  • H01M8/04955—Shut-off or shut-down of fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/36—Hydrogen production from non-carbon containing sources, e.g. by water electrolysis
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates generally to electrolyzers, and in particular to fluid flow channels and flow fields for an electrolyzer, and electrolyzers and electrolyzer stacks incorporating such fluid flow channels and flow fields.
• PEM proton exchange membrane
• water is supplied to the anode and is split into oxygen, protons and electrons by applying a DC voltage.
• Protons pass through the polymer electrolyte membrane and combine with electrons at the cathode to form hydrogen; thus oxygen is produced at the anode, and hydrogen is produced at the cathode as illustrated in a schematic diagram in FIG. 1. It is important that the hydrogen and oxygen, which evolve at the surfaces of the respective electrodes, are kept separate and do not mix.
• the electrolysis process is essentially the reverse of the process in a PEM fuel cell.
• a PEM electrolyzer cell can be very similar in structure to a PEM fuel cell, with a polymer membrane sandwiched between a pair of porous electrodes and flow field plates.
• FIG. 2A shows a simplified diagram of an electrolyzer unit cell
• FIG. 2B shows a simplified diagram of a fuel cell unit cell.
• the materials used in a PEM electrolyzer are generally different because the carbon materials commonly used as catalyst supports, gas diffusion layers and flow field plates in fuel cells cannot be used on the oxygen side of a PEM electrolyzer due to corrosion.
• Metallic components for example, tantalum, niobium, titanium, or stainless steel plated with such metals
• PEM porous layers and flow field plates
• the catalyst is typically platinum or a platinum alloy, and is designed to operate in the presence of liquid water.
• an electrolyzer system will typically comprise a power supply, a voltage regulator, water purification and supply equipment including a circulation pump, water-gas separators for hydrogen and optionally oxygen, a thermal management system, controls and instrumentation, and equipment for storage and subsequent dispensing of the product gas(es).
• a fuel cell system can be combined with an electrolyzer system, so that a renewable energy source can be used to power an electrolyzer to generate hydrogen and oxygen which can be stored, and then subsequently used as reactants for a fuel cell to produce electric power.
• a combined electrolyzer/fuel cell system is illustrated in FIG. 3 A.
• Efforts are presently underway to develop a unitized stack that could serve as both fuel cell and electrolyzer.
• Such a device has been referred to as a "reversible fuel cell” or a "unitized regenerative fuel cell” (URFC).
• a PEM URFC stack delivers power when operated as a fuel cell using hydrogen as the fuel, and either air or oxygen as the oxidant, and generates hydrogen and oxygen when operated as an electrolysis cell.
• FIG. 3B A URFC system is illustrated in FIG. 3B.
• Design of the individual cells and cell components for a URFC should address the distinctly different operating conditions occurring during each mode of operation. For exam' pie, the oxygen/air electrode potential is quite different in one mode versus the other.
• humidified, gaseous reactants are generally required along with rapid removal of the heat and water produced, while in the electrolysis mode, liquid water is required as the reactant at one electrode, with rapid removal of the product oxygen at the anode and hydrogen at the cathode.
• the balance of plant supporting the PEM URFC is designed to handle product water in the fuel cell mode, maintain the thermal balance within the fuel cell (cooling plates are typically used to remove excess heat when the fuel cell is producing power), deliver clean reactants, and produce regulated power.
• Balance of plant issues for a URFC include design of the thermal management system (because operation in the electrolysis mode is slightly endothermic), and collection of the product hydrogen and optionally oxygen.
• a PEM electrolyzer In a PEM electrolyzer the issues associated with liquid reactant supply and gaseous product removal are somewhat different to those in a PEM fuel cell, where hydrogen and a gaseous oxidant (for example, air) are typically supplied to the anode and cathode respectively, and water is produced at the cathode.
• a gaseous oxidant for example, air
• the gaseous reactants are generally supplied to the electrodes via channels formed in the flow field plates.
• a typical reactant fluid flow field plate has at least one channel through which a reactant stream flows.
• the fluid flow field is typically integrated with the separator plate by locating a plurality of open-faced channels on one or both faces of the separator plate. The open-faced channels face an electrode, where the reactants are electrochemically converted.
• separator plates are provided on each of the anode and cathode sides.
• bipolar plates are generally used between adjacent cells; these bipolar plates generally have flow fields on both sides of the plate.
• the plates act as current collectors and provide structural support for the electrodes.
• the flow field used at both the anode and the cathode can have an important influence on fuel cell performance, and much work has been done on the optimization of flow field designs for PEM fuel cells.
• the reactant flow channels in fuel cell flow fields have a constant cross-section along their length.
• U.S. Patent No. 6,686,082 (which is hereby incorporated by reference herein in its entirety) describes fuel cell embodiments in which the fuel flow channels have a cross-sectional area that decreases linearly in the flow direction.
• the oxygen content in the air stream tends to be depleted and the air pressure tends to drop, resulting in reduced performance in the fuel cell.
• U.S. Patent No. 7,838, 169 (which is hereby incorporated by reference herein in its entirety) describes improved cathode flow field channels that can be used to achieve substantially constant oxygen availability along the channel.
• flow fields that are preferred for fuel cells are not necessarily the same as those that are preferred for electrolyzers, the Applicants have discovered that flow fields where the channel cross-sectional area varies along the length of the channel, particularly at the oxygen electrode, can offer advantages in electrolyzers, as well as in URFCs that can operate in both fuel cell and electrolyzer modes.
• An electrolyzer assembly for generating hydrogen and oxygen from water comprises one or more unit cells.
• the unit cells each comprise a membrane electrode assembly comprising a proton exchange membrane interposed between an anode and a cathode; a cathode flow field plate adjacent to the cathode; and an anode flow field plate adjacent to the anode.
• the cathode flow field plate optionally has at least one cathode channel formed therein, for carrying away hydrogen produced at the cathode during operation of the electrolyzer assembly.
• the anode flow field plate has at least one anode channel formed therein for directing water in contact with the anode during operation of the electrolyzer assembly.
• the at least one anode channel has a cross-sectional area that varies along at least a portion of the length of the anode channel.
• anode channel cross-sectional area can be varied by a variation in at least one of the channel width, channel depth and channel shape.
• the channel cross-sectional area varies (such as, by variations in dimensions of width, depth or shape) along substantially the entire length of the anode channel.
• the depth of the anode channel is substantially constant, and the width of the anode channel decreases along at least a portion of the channel length in a direction of reactant (water) flow along the channel. In some embodiments, the depth of the anode channel is substantially constant, and the width of the anode channel decreases along at least a portion of the channel length in a direction of reactant flow according to a natural exponential function. In some embodiments, the depth of the anode channel is substantially constant, and the width of the anode channel at a selected lengthwise position along the channel portion is proportional to a natural exponential function of the selected lengthwise position.
• the width of the anode channel is substantially constant for a portion of the channel length and the channel width varies along another portion of the channel length.
• the depth of the anode channel is substantially constant, and the width of the anode channel varies as a function of distance along the portion of the channel length such that: where W(x) is the anode channel width at lengthwise position x; x is a selected position along the channel length; D is the channel depth; v constant flow velocity; ST H 0 is water stoichiometry; k H2 Q is flow rate coefficient for water; i d is the total channel current; and L is the channel length.
• the width of the anode channel is substantially constant, and the depth of the anode channel decreases along at least a portion of the channel length in a direction of reactant flow along the channel.
• the channel depth can, for example, decrease substantially linearly along at least a portion of the channel length in a direction of reactant flow.
• the channel depth can, for example, vary as a function of distance along the portion of the channel length such that: where D(x) is the anode channel depth at lengthwise position x; x is a selected position along the channel length; ST H 0 is water stoichiometry; k H 0 is flow rate coefficient for water; i d is current density; and L is the channel length.
• the electrolyzer assembly can comprise a plurality of the unit cells arranged in a stack.
• the electrolyzer assembly is configured to also operate as a fuel cell to generate electric power and water when oxygen and hydrogen are supplied to the anodes and cathodes.
• a unitized regenerative fuel cell assembly is configured to operate both as an electrolyzer to produce hydrogen and oxygen from water, and as a fuel cell to produce electric power from hydrogen and oxygen.
• the unitized regenerative fuel cell comprises one or more unit cells.
• the unit cells each comprise a membrane electrode assembly that comprises a proton exchange membrane interposed between a first electrode and a second electrode; a first flow field plate adjacent to the first electrode, the flow field plate comprising at least one oxygen- side channel for directing a first fluid stream in contact with the adjacent first electrode.
• the at least one oxygen-side channel has a length and a cross-sectional area that varies along at least a portion of the channel length (for example, as for the anode channel in the electrolyzer assembly embodiments described above).
• Each unit cell optionally further comprises a second flow field plate adjacent to the second electrode, the flow field plate comprising at least one hydrogen-side channel.
• the hydrogen-side channel can, for example, be used for directing a second fluid stream in contact with the adjacent second electrode (for example, delivering hydrogen to the anode during fuel cell operation) and for carrying away hydrogen produced at the second electrode (the cathode, during electrolyzer operation).
• the unitized regenerative fuel cell assembly can comprise a plurality of the unit cells arranged in a stack.
• the at least one unit cell is connected to a source of electrical power and the at least one oxygen-side channel is fluidly connected to a water supply for flowing reactant water through the at least one oxygen-side channel.
• the at least one oxygen-side channel is fluidly connected to receive an oxygen- containing reactant stream
• the at least one hydrogen-side channel is fluidly connected to receive a hydrogen-containing reactant stream
• the at least one unit cell is connected to an electrical load, to provide electric current between the first and second electrodes (anodes and cathodes).
• the depth of the at least one oxygen-side channel is substantially constant, and the width of the at least one oxygen-side channel decreases along at least a portion of the channel length (in a direction of water reactant flow during electrolyzer operation, and in a direction of oxygen-containing reactant stream flow during fuel cell operation).
• the variation in width can be, for example, as described above for the various embodiments of an anode channel of an electrolyzer assembly.
• Embodiments of the above-described electrolyzer assembly, or unitized regenerative fuel cell assembly can further comprise one or more of the following: a water supply fluidly coupled, via a valve, to deliver water to the oxygen-side channels; a power supply switchably connected to deliver electrical power to the electrolyzer assembly or unitized regenerative fuel cell assembly; a hydrogen containment vessel fluidly coupled, via a valve, to the cathode flow field plates to collect hydrogen generated by the electrolyzer assembly or unitized regenerative fuel cell assembly; an oxygen containment vessel fluidly coupled, via a valve, to the cathode flow field plates to collect oxygen generated by the electrolyzer assembly or unitized regenerative fuel cell assembly.
• FIG. 1 (Prior Art) is a schematic diagram of an electrolyzer, showing the reactions occurring in a water electrolysis process.
• FIG. 2A is a simplified diagram of an electrolyzer unit cell showing a membrane electrode assembly sandwiched between a pair of flow field plates.
• FIG. 2B is a simplified diagram of a fuel cell unit cell, showing a membrane electrode assembly sandwiched between a pair of flow field plates.
• FIG. 3 A is a simplified diagram of a combined electrolyzer/fuel cell system with separate stacks for the fuel cell and the electrolyzer.
• FIG. 3B (Prior Art) is a simplified diagram of a unitized regenerative fuel cell (URFC) system.
• URFC unitized regenerative fuel cell
• FIG. 4A is a simplified representation of an electrolyzer flow field plate comprising a flow channel that decreases in depth, with constant width, along its length.
• FIG. 4B is a simplified representation of an electrolyzer flow field plate comprising a flow channel that decreases exponentially in width, with constant depth, along its length.
• FIG. 5 shows a trapezoidal electrolyzer flow field plate comprising multiple flow channels that decrease exponentially in width along their length.
• FIG. 6A is a graph showing fluid flow velocity along electrolyzer anode flow channels with two different profiles, modeled for an electrolyzer operating at a higher reactant water stoichiometry than in FIG. 6B.
• FIG. 6B is a graph showing fluid flow velocity along electrolyzer anode flow channels with two different profiles, modeled for an electrolyzer operating at a lower reactant water stoichiometry than in FIG. 6A.
• FIG. 7 is a simplified representation showing an example of how a serpentine flow channel, which the channel width varies, can be applied to a rectangular flow field plate.
• FIG. 8 A is a simplified representation showing an example of how a wavy flow channel, in which the channel width varies, can be applied to a rectangular flow field plate.
• FIG. 8B is a simplified representation showing an example of how multiple wavy flow channels can be nested on a rectangular flow field plate.
• FIG. 9A shows a square flow field plate comprising a conventional serpentine flow field with 3 flow channels extending between a supply manifold opening and a discharge manifold opening.
• FIG. 9B shows a similar serpentine flow field to FIG. 9A, but where the width of each serpentine flow channel decreases exponentially along its length.
• FIG. 1 OA is a simplified representation of a flow field plate comprising a flow channel that decreases exponentially in width for a first portion of the channel length and is then constant for a second portion of the channel.
• FIG. 1 OB is a simplified representation of a flow field plate comprising a flow channel that is constant in width a first portion of the channel length and decreases exponentially for a second portion of the channel length.
• FIG. 11 is a simplified representation of a flow field plate comprising a flow channel that decreases exponentially in width for a first portion of the channel length, and then flares with increasing channel width along a second portion of the channel length.
• FIG. 12 is a simplified representation of a flow field plate comprising two flow channels that are serpentine with constant width for a first portion of the channel length, and then the channel width decreases exponentially for a second portion of the channel length.
• FIG. 13 is a simplified representation of a flow field plate comprising a flow channel in which the channel depth is constant along a first portion of the channel length and then decreases along second a portion of the channel length.
• FIG. 14A shows a rectangular flow field plate comprising a multi-channel serpentine flow field extending between a supply and a discharge manifold opening.
• FIG. 14B shows a modification to the flow field plate of FIG. 14 A, in which the width of each channel decreases exponentially along a middle portion of the length of each channel.
• FIG. 15 is a simplified representation of a flow field plate comprising a substantially rectangular flow channel having a central rib with exponentially curved side walls.
• FIG. 16A is a simplified representation of a flow field plate comprising a flow channel that has a conventional rectangular cross-section at one end and is gradually filleted to reduce its cross-section towards the other end, in the reactant flow direction.
• FIG. 16B is an alternative view of the flow field plate of FIG. 16A.
• FIG. 17 is a simplified representation of a flow field plate comprising a rectangular flow channel incorporating rib dots, where density of the rib dots increases in the reactant flow direction.
• FIG. 18 is a simplified representation of a flow field plate comprising a wavy flow channel incorporating rib dots, where density of the rib dots increases in the reactant flow direction.
• FIG. 19 is a simplified representation illustrating an example where the flow channel width decreases in a stepwise, non-linear fashion in the reactant flow direction.
• FIG. 20 is a simplified representation illustrating another example where the flow channel width decreases in a stepwise, non-linear fashion in the reactant flow direction.
• FIG. 21 is a graphical representation illustrating how stepwise or discrete changes in channel width can be used to approximate a smooth exponential change in channel width.
• FIG. 22 is a block diagram of an electrolyzer/regenerative fuel cell system. Detailed Description of Preferred Embodiments)
• An electrolyzer assembly includes a flow field plate comprising at least one channel, wherein the cross-sectional area of the channel varies along at least a portion of the channel length.
• the channel width decreases in the direction of reactant flow along at least a portion of the length of the channel according to a natural exponential function.
• electrolyzer anode can improve performance and/or efficiency of operation of an electrolyzer assembly.
• Water availability is related to cell reaction performance.
• water is directed or pumped through a flow field in order to distribute the water across the active area of the anode.
• water moves through the flow field it is consumed.
• each mole of water that is consumed is replaced by half a mole of oxygen.
• Several issues arise that can detrimentally affect the efficiency and/or performance of the electrolyzer. For example, as the water is consumed the velocity of water flowing down the channel will tend to decrease. The result is that the amount of reactant being delivered per unit time varies across the active area of the cell.
• the product oxygen that is evolved at the anode tends to form bubbles in the flow field. This can impede access of reactant water to the anode catalyst sites.
• the channel cross-sectional area varies with the decreasing volumetric flow rate of water according to the following equation:
• i d being the nominal current density on the plate.
• the total current can also be rewritten as a product of the current density and
• the total area which is the width function integrated over the length of the channel.
• the water flow rate coefficient can be calculated as follows: where the quantsities of n H 0 and n e are constants resulting from the chemical reactions for the electrolysis of water, and p H and M(H 2 0 'J are the density and molecular mass of water, respectively.
• an electrolyzer anode channel profile can be designed on this basis, to achieve substantially constant or uniform reactant water availability. If the channel width is held constant, and the integrals are appropriately evaluated, then equation (6) can be rearranged to solve for the channel depth, D(x) :
• the depth profile is a linear function of x.
• FIG. 4A is a simplified representation of an electrolyzer anode flow field plate 100A comprising a flow channel 1 10A that decreases in depth, with constant width, along its length.
• a channel profile can be defined by solving for D(x) in equation (9) at each position x along the length of the channel, given a specified operating reactant water stoichiometry and channel length J, and assuming a constant channel width.
• the resulting channel 1 10A extends between water supply manifold opening 120A and discharge manifold opening 130A, and has a linearly decreasing depth floor 112A from inlet 116A to outlet 118A, with straight (parallel) side walls 114A.
• the depth of the channel shallow. Therefore, instead of varying the depth of the channel, which would require a sufficiently thick plate to accommodate the deepest part of the channel, it can be preferred to keep the channel depth constant (D) and to vary the width of the channel to achieve substantially uniform water availability along the length of the channel.
• D channel depth constant
• the channel width can be expressed as follows:
• the resulting channel profile has an exponentially decreasing width.
• FIG. 4B is a simplified representation of an electrolyzer anode flow field plate 100B comprising a flow channel H OB that decreases in width along its length according to an exponential function.
• a channel profile can be defined by solving for W(x) in equation (15) at each position x along the length of the channel, given a specified design reactant water stoichiometry, channel length J, total current draw (or current density and active area) and assuming a flat channel floor (constant depth, D).
• the resulting channel HOB extends between water supply manifold opening 120B and discharge manifold opening 130B, and has a constant depth floor 1 12B with convexly curved side walls 1 14B that converge inwards from inlet to outlet.
• the walls 1 14B converge inwards towards an outlet end 1 18B with an inlet 1 16B having the largest width and the channel profile delineating at a diminishing rate. That is, the channel width decreases exponentially along the length of the channel from the inlet to the outlet according to the equation (15). It would be possible for one of the side walls to be straight and the other to be convexly curved.
• Flow field plates with channels of varying width are generally easier to manufacture than plates with channels of varying depth, or channels with a cross-sectional shape that varies along the channel length.
• multiple channels 210 having the channel profile shown in FIG. 4B can be applied to an electrolyzer plate 200, to form an electrolyzer anode flow field 222 extending between a water supply manifold opening 220 and discharge manifold opening 230.
• the flow field 222 is arrayed in a generally trapezoidal geometry to enable separating ribs 224 to have a relatively even width along their length.
• the separator plate 20 includes partial ribs 26 located at the inlet of each channel 10.
• the partial ribs 26 serve to reduce the distance between channel side walls 14, and serves as a bridging structure for the adjacent membrane electrode assembly (not shown).
• Embodiments in which the fluid flow channel width varies in an exponential manner in can in some circumstances be beneficial in enhancing the localized reactant and/or product flow velocity during electrolyzer operation thereby improving performance.
• the pressure drop along the channel can be reduced (relative to a channel of constant cross-sectional area). This can lead to reduced parasitic loads improved overall system efficiency.
• the variation in channel width can be designed to adjust or control the localized residency time of the gaseous products in the channels, in some circumstances allowing some or all of the following:
• k Q can be written a way that is similar to k H , o-
• An electrolyzer can be thermally controlled during operation in various ways. Sometimes the reactant water supplied to the anode is also used to maintain the temperature of the
• the graph shown in FIG. 6A illustrates the fluid flow velocities, as generated by the model, for an electrolyzer in which the reactant water is also used for thermal management.
• Plot A shows the velocity for a channel with a constant profile
• plot B shows the velocity for a compensated channel.
• the graph shown in FIG. 6B illustrates the fluid flow velocities, as generated by the model, for an electrolyzer in which the reactant water stream is separate from the coolant water stream supplied to the electrolyzer.
• Plot X shows the velocity for a channel with a constant profile
• plot Y solid line
• the compensated channel produces a far larger increase in fluid flow velocity down the channel.
• the compensated channel increases velocity 4.86 times the inlet velocity, whereas the constant profile channel only achieves 1.62 times the inlet velocity.
• the compensated channel multiplies the inlet velocity 1,244 times, versus 414 times for the constant profile channel. In both cases, however, the advantage in the ratio of inlet velocity to outlet velocity can be written as:
• channels with an exponentially varying width can be used to provide substantially constant oxygen availability, significantly improving the uniformity of current density and increasing fuel cell
• a standard flow field can only provide between 0.8 and 1.35 kW at peak power in fuel cell mode, and can require 50% to 28%) more active area to produce 1.7 kW, leading to a more expensive URFC.
• the channels can be substantially straight from inlet to outlet, or can be wavy or serpentine. Generally for electrolysis applications shorter channels are preferred, but for URFC the channel profile and path can be a compromise between what is preferred for fuel cell operation and what is preferred for electrolyzer operation.
• Flow fields based on the equations and description set forth above for the oxygen electrode (anode) of an electrolyzer are more likely to be adopted if they can be accommodated within conventional flow field plate geometries and into conventional electrochemical stack architectures (which typically have rectangular flow field plates).
• Flow channels where the depth profile changes along the length of the channel can be accommodated by using an existing flow field design (pattern) and merely altering the depth profile of the channels along their length (keeping the channel width and ribs the same as in the original flow field design).
• plates with channels where the depth profile changes are generally more challenging to fabricate. They also result in a need for thicker plates, in order to accommodate the deepest part of the channel, leading to decreased stack power density and higher cost.
• FIGS. 7-9 show some examples of ways in which flow fields where the flow channel width varies, can be applied to a rectangular electrolyzer flow field plate.
• FIG. 7 shows a rectangular electrolyzer flow field plate 300 with a serpentine channel 310 where the channel width is decreasing exponentially as it zigzags across the plate between supply manifold opening 320 and discharge manifold opening 330.
• FIG. 8A shows a rectangular electrolyzer reactant flow field plate 400 A with a wavy channel 41 OA extending between reactant supply manifold opening 420A and discharge manifold opening 430A, where the channel width is decreasing exponentially along its length.
• FIG. 7 shows a rectangular electrolyzer flow field plate 300 with a serpentine channel 310 where the channel width is decreasing exponentially as it zigzags across the plate between supply manifold opening 320 and discharge manifold opening 330.
• FIG. 8A shows a rectangular electrolyzer reactant flow field plate 400 A with a wavy channel 41 OA extending between reactant
• FIGS. 7 and 8 A show a single flow channel, however, it is apparent that such channels can be repeated or arrayed across a rectangular plate so that a large portion of the plate area can be active area (for example, so that a large portion of the plate surface is covered in channels, with a large open channel area exposed to the adjacent electrode or MEA).
• FIG. 7 and 8 A show a single flow channel, however, it is apparent that such channels can be repeated or arrayed across a rectangular plate so that a large portion of the plate area can be active area (for example, so that a large portion of the plate surface is covered in channels, with a large open channel area exposed to the adjacent electrode or MEA).
• FIG. 8B shows a rectangular electrolyzer flow field plate 400B with multiple flow channels 410B (like flow channel 41 OA of FIG. 8 A repeated) extending between reactant supply manifold opening 420B and discharge manifold opening 430B, arranged so that the channels nest together.
• FIG. 9A shows a square electrolyzer flow field plate 500A comprising a conventional (Prior Art) serpentine electrolyzer flow field with three flow channels 51 OA extending between supply manifold opening 520A and discharge manifold opening 530A.
• FIG. 9B shows a similar serpentine electrolyzer flow field plate 500B, but where the width of each serpentine channel 51 OB decreases exponentially along its length as it extends from supply manifold opening 520B to discharge manifold opening 530B.
• FIGS. 10-12 show some examples where the flow channel width varies along just a portion of the length of the channel.
• FIG. 10A shows a rectangular electrolyzer flow field plate 600 A with a flow channel 61 OA extending between reactant supply manifold opening 620A and discharge manifold opening 63 OA.
• FIG. 10B shows a rectangular electrolyzer flow field plate 600B with a flow channel 610B extending between reactant supply manifold opening 620B and discharge manifold opening 630B.
• the flow channel width decreases
• the flow channel width is constant for a first portion 625A and decreases exponentially for a second portion 635B of the channel length.
• FIG. 11 shows a rectangular electrolyzer flow field plate 700 with a flow channel 710 extending between reactant supply manifold opening 720 and discharge manifold opening 730.
• the flow channel width decreases exponentially for a first portion 725 of the channel length (near the supply manifold), and is then increases so that the channel is flared for a second portion 735 of the channel length.
• FIG. 12 shows an electrolyzer flow field plate 800 comprising two flow channels 810.
• the channels are initially serpentine with constant width in portion 825 near the reactant supply manifold opening 820, and then, after abruptly increasing, the channel width decreases exponentially for a second portion 835 of the channel length (towards discharge manifold opening 830).
• FIG. 13 shows an example of an electrolyzer flow field plate 900 comprising a flow channel 910 extending between a reactant supply manifold opening 920 and a discharge manifold opening 930.
• the flow channel depth is constant along a first portion 925 of the channel length and then decreases along a second portion 935 of the length of the channel 910.
• the electrolyzer flow channels can incorporate a variation in both width and depth along their entire length, or a portion of their length.
• FIGS. 14A and 14B illustrate how an existing flow field design can be readily modified to incorporate an exponential variation in channel width along a portion of the length of the flow channels.
• FIG. 14A (Prior Art) shows a rectangular flow field plate 1000A comprising a fairly complex serpentine flow field with multiple serpentine channels 1010A extending between a supply and a discharge manifold opening.
• FIG. 14B shows a modification in which the width of each channel 1010B decreases exponentially along a middle portion 1025B of the length of each channel.
• FIG. 15 shows an example of an electrolyzer flow field plate 1100 with a single flow channel 1110 extending between a supply manifold opening 1120 and a discharge manifold opening 1130.
• the channel 1110 comprises a central rib 1140 with exponentially curved side walls. The rib splits the flow channel 1110 in two and effectively reduces its width gradually along most of its length.
• 16A and 16B show two different views of another example of a flow field plate 1200 with a single flow channel 1210 extending between a supply manifold opening 1220 and a discharge manifold opening 1230.
• the channel 1210 is of a conventional rectangular cross-section at one end 1225, and is gradually filleted to reduce its cross-section towards the other end 1235.
• the flow channel dimensions vary along at least a portion of the channel length in a smooth and continuous fashion.
• performance benefits can also be obtained by using flow channels that incorporate discrete variations.
• the characteristics of the channel can be varied as a function of distance along the channel in a stepwise or discontinuous fashion, but where the overall variation trends the smooth desired profile, either in fluctuations about the calculated profile, or in discrete approximations of the desired profile.
• This approach can be used to achieve at least some of the performance benefits, and can provide some options for improved flow fields that are easier to fabricate or to incorporate into existing plate geometries.
• the outlet, or region near the outlet is smaller or more constricted than the reactant inlet or inlet region.
• the channels can contain discrete features that obstruct reactant flow, where the density and/or size of those features increases in a reactant flow direction.
• the density of the rib dots 1350 can increase in the reactant (water) flow direction (indicated by the arrow) in accordance with the e flow equations.
• Such features can be as high as the channel is deep (so that they touch the adjacent electrode) or can obstruct only part of the channel depth. In the example illustrated in FIG.
• FIG. 18 is a simplified representation of an electrolyzer flow field plate 1400 comprising a wavy flow channel 1410 incorporating rib dots 1450, where the density of the rib dots increases in a reactant flow direction (indicated by the arrow).
• the flow channel dimensions can decrease in the reactant flow direction in a stepwise fashion.
• the increments by which the channel dimensions change can be the same along the channel length, and in other embodiments it can vary along the channel length.
• the distance between (or frequency of) the step-changes in channel dimensions can be the same along the channel length, and in other embodiments it can vary along the channel length.
• FIGS. 19 and 20 illustrate examples where the channel width decreases in a stepwise, nonlinear fashion in a reactant flow direction in accordance an exponential function.
• FIG. 19 is a simplified representation illustrating an example electrolyzer flow field plate 1500 where the width of flow channel 1510 decreases in a stepwise, nonlinear fashion in a reactant direction between a reactant supply manifold opening 1520 and a discharge manifold opening 1530.
• FIG. 20 is a simplified representation illustrating another example electrolyzer flow field plate 1600 where the width of flow channel 1610 decreases in a stepwise, non-linear fashion in the reactant flow direction between a supply manifold opening 1620 and a discharge manifold opening 1630.
• FIG. 21 is a graphical representation 1700 illustrating how stepwise or discrete changes in channel width can be used to approximate a smooth exponential change in channel width.
• the solid line 1710 represents changes in channel width and the dashed line 1720 shows a smooth exponential variation in channel width.
• FIG. 22 is a block diagram illustrating an example of an electrolyzer/regenerative fuel cell system 1800 comprising a multi-cell stack 1810. Each unit cell in the stack can comprise components and flow channels such as, for example, those described above.
• System 1800 also comprises a power supply 1825, which can be connected by closing switch 1820, to deliver electrical power to stack 1810 when stack 1810 is to be operated in electrolyzer mode to generate hydrogen and oxygen.
• Power supply 1825 can comprise, for example, an electricity grid, an energy storage device, or a renewable source of electric power such as a photovoltaic cell or a wind turbine.
• valve system 1840 When system 1800 is to be operated in electrolyzer mode, water is supplied to flow channels within stack 1810 from a water supply 1830 via a valve system 1840 which can comprise multiple valves for controlling the supply of fluids (reactants and products) to and from stack 1810. Water can be supplied as both a reactant and a coolant to flow channels adjacent the oxygen-side electrodes (not shown in FIG. 22) in stack 1810; or water can be supplied as a reactant to flow channels adjacent the oxygen-side electrodes, and optionally to separate cooling channels (not shown in FIG. 22) in stack 1810.
• System 1800 also comprises a hydrogen containment vessel 1850 selectively fluidly coupleable, via valve system 1840, to collect hydrogen generated during electrolyzer operation of stack 1810.
• System 1800 further comprises an oxygen containment vessel 1860 selectively fluidly coupleable, via valve system 1840, to collect oxygen generated during electrolyzer operation of stack 1810.
• System 1800 can also be configured so that stack 1810 operates as a fuel cell to generate electric power which can power electrical load 1870 when switch 1875 is closed (and switch 1820 is open).
• hydrogen can be supplied to stack 1810 from hydrogen containment vessel 1850 which is selectively fluidly coupleable to supply hydrogen to stack 1810, via valve system 1840.
• oxygen can be supplied to stack 1810 from oxygen containment vessel 1850 which is also selectively fluidly coupleable to supply oxygen to stack 1810, via valve system 1840.
• air can be supplied to stack 1810 as the oxidant, via another oxidant supply subsystem (not shown in FIG. 22).
• water can optionally be supplied as a coolant to cooling channels (not shown in FIG.
• valve system 1840 in stack 1810 via valve system 1840.
• Product water generated during fuel cell operation can optionally be directed to water supply 1830 via valve system 1840.
• a controller 1880 can operate the valve system 1840 to provide reactants and coolant to and collect products from stack 1810, as appropriate, during fuel cell and electrolyzer operation.
• Controller 1880 can also close and open switches 1820 and 1875, as appropriate, for fuel cell and electrolyzer operation. Controller 1880 can also configure stack 1810 for operation alternatively in fuel cell mode and electrolyzer mode.
• System 1800 is one embodiment of a system comprising a regenerative fuel cell/electrolyzer stack according to various embodiments of the present invention. Other systems can exclude some of the components shown in system 1800, or include additional components.
• FIGS. 4A, 4B, 7, 8A, 8B, 10A, 10B, 11, 12, 13, 15, 16A, 16B, 17, 18, 19 and 20 are simplified drawings, in which the size of the flow channel and the manifold openings, and variations in channel dimensions and/or characteristics are exaggerated for the purposes of clear illustration.
• the dimensions and/or flow characteristics of the flow channel vary along at least a portion of the channel length.
• the variations can be continuous or discrete.
• flow channels with variations in cross-sectional area as described herein can be used at either or both of the electrodes in an electrolyzer or URFC assembly.
• they generally offer greater benefits when used at the oxygen-side electrode (which is the anode for an electrolyzer, and the cathode during fuel cell operation of a URFC).
• they can be used for some or all of the unit cells in a particular electrolyzer or URFC stack.
• the open channel area versus the rib or landing area on a reactant flow field plate is generally selected to give sufficient electrical contact between the plates and the adjacent MEAs for efficient current delivery, while providing sufficient water access to the electrolyzer anode to support the electrochemical reactions.
• Using a wider rib area (between flow channels) improves electrical connectivity and current delivery in an electrolyzer.
• outlet refers to either the start of the flow channel where reactant enters the channel, or the start of a region where the channel characteristics vary as a function of channel length as described herein; and “outlet” refers to either the downstream end of the channel, or the end of a region over which channel characteristics vary as a function of channel length as described herein.
• the present invention includes electrolyzer flow field plates comprising any of the reactant flow channels or flow field designs described above. Such plates can be made from any suitable material or combination of materials, and can be fabricated by any suitable method.
• the present invention also includes other electrolyzer components incorporating flow channels or passageways as described herein. For example, such channels could be incorporated into the gas diffusion layers, manifolds, or other components of the unit cell or stack. Further, the present invention includes electrolyzers and electrolyzer stacks

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