Cross-Reference to Related

[0001] This application is related to and claims priority benefits from U.S. Provisional Patent Application Serial No. 63/246,559 having a filing date of September 21, 2021, entitled “Flow Fields for Electrolyzers with Liquid Water Supplied to the Cathode”.

[0002] The ‘ 559 application is hereby incorporated by reference herein in its entirety.

Field of the Invention

[0003] The present invention relates generally to electrolyzers, and in particular to fluid flow channels and flow fields for electrolyzers, and electrolyzers incorporating such fluid flow channels and flow fields, and methods for operating such electrolyzers.

[0004] Although many electrolyzers are based on an alkaline (KOH) electrolyte, another option is to use a proton exchange membrane (PEM) as the electrolyte. In conventional PEM electrolysis, liquid 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.

[0005] Equation (1) shows the oxygen evolution reaction (OER) that occurs at the anode:

[0006] 2 H ₂ O (1) O ₂ (g) + 4H ⁺ (aq) + 4e’ (1) [0007] Equation (2) shows the hydrogen evolution reaction (HER) that occurs at the cathode:

[0008] 4H ⁺ (aq) + 4e’ 2H ₂ (g) (2)

[0009] 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. Increasing the amount of electrical current that is passed through the cell results in a corresponding increase in the generation of hydrogen and oxygen, and in the rate of consumption of water at the anode.

[0010] The electrolysis process is essentially the reverse of the process in a PEM fuel cell. A PEM electrolyzer cell can be 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, and FIG. 2B shows a simplified diagram of a fuel cell unit cell. The materials used in a PEM electrolyzer are generally different because carbon materials commonly used as catalyst supports, gas diffusion layers and flow field plates in fuel cells cannot generally 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) are often used instead for porous layers and flow field plates in PEM electrolyzers.

[0011] Multiple electrolyzer cells can be connected electrically either in series or in parallel (to get the desired output at a reasonable stack voltage) to form an electrolyzer stack. In addition to one or more electrolyzer stacks comprising end plates, bus plates and manifolds, and other system components, an electrolyzer system typically comprises 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). [0012] 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. 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.

[0013] In PEM fuel cells the gaseous reactants are generally supplied to the electrodes via channels formed in 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. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. In a stack, 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.

[0014] 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. Conventionally the reactant flow channels in fuel cell flow fields have a constant cross-section along their length. However, 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. For fuel cells operating on air as the oxidant, as the air flows along the cathode flow channel(s), 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.

[0015] In PEM electrolyzers the issues associated with 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. Although most conventional PEM electrolyzers operate with liquid water supplied to the anode, and either no fluid or a carrier gas (to facilitate removal of product hydrogen) supplied to the cathode, other modes of operation are sometimes used. Depending on the fluids that are supplied to the electrodes in a PEM electrolyzer, particular flow field or flow channel designs can be preferred and/or offer certain advantages.

Summary of the Invention

[0016] In some embodiments of a method of operating an electrolyzer assembly to generate hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells, with each unit cell comprising a proton exchange membrane interposed between an anode and a cathode. In at least some embodiments, there is a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet. In at least some embodiments, there is an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet. In at least some embodiments, the cross- sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet, and the method comprises applying an electric current to the electrolyzer assembly; supplying liquid water to the cathode via the cathode inlet and the cathode flow field; and supplying a humidified gas stream to the anode via the anode inlet and the anode flow field. [0017] In some embodiments, the method further comprises allowing water molecules to diffuse through the proton exchange membrane from the cathode to the anode; oxidizing the water molecules at the anode to produce protons, oxygen gas and electrons; allowing the protons to migrate through the proton exchange membrane from the anode to the cathode; reducing the protons at the cathode to produce hydrogen gas; and/or collecting the hydrogen gas from the cathode.

[0018] In some embodiments, the velocity of fluid flowing in the anode flow field increases along a least a portion of the length of the anode flow field between the anode inlet and the anode outlet.

[0019] In some embodiments of the method, the cross-sectional fluid flow area of the anode flow field decreases along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the cross-sectional fluid flow area of the anode flow field decreases monotonically along the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field layer comprises an anode flow field plate comprising at least one anode channel formed in the anode flow field plate, and the at least one anode channel has a cross-sectional area that decreases along at least a portion of the length of the anode channel. In some embodiments, the anode flow field layer comprises a porous material adjacent to the anode, and the anode flow field comprises passageways extending within the porous material. In some embodiments, the porosity of the porous material decreases monotonically from the anode inlet to the anode outlet. In some embodiments, the anode flow field layer comprises an anode plate and a porous material interposed between the anode and the anode plate, the porous material having two major surfaces and comprising an anode channel formed in one of the major surfaces. In some embodiments, the anode channel extends from the anode inlet to the anode outlet, and the cross- sectional area of the anode channel decreases monotonically from the anode inlet to the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 85% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 90% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 95% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along substantially the entire length of the anode flow field between the anode inlet and the anode outlet.

[0020] In some of the above-described methods for operating an electrolyzer assembly, the cross-sectional fluid flow area of the cathode flow field in the electrolyzer assembly also varies along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the velocity of fluid flowing in the cathode flow field increases along a least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field decreases along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field decreases monotonically along the length of the cathode flow field between the cathode inlet and the anode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 85% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 90% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 95% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field decreases along substantially the entire length of the cathode flow field between the cathode inlet and the cathode outlet. [0021] In some of the above-described embodiments of a method of operating an electrolyzer assembly, the humidified gas stream supplied to the anode is humidified air. In other embodiments it is another humidified gas stream, such as humidified nitrogen, argon, or helium.

[0022] In some embodiments of a method of operating an electrolyzer assembly, the relative humidity of the humidified gas stream is greater than 50% at the operating temperature of the electrolyzer assembly.

[0023] In some embodiments of a method of operating an electrolyzer assembly, the humified gas stream is supplied to the anode via a gas-to-gas humidifier upstream of the anode inlet.

[0024] In some embodiments of a method of operating an electrolyzer assembly to generate hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells, with each unit cell comprising a proton exchange membrane interposed between an anode and a cathode. In at least some embodiments, there is a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet. In at least some embodiments, there is an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet. In at least some embodiments, the cross- sectional fluid flow area of the anode and/or the cathode flow field varies along at least a portion of the length of the respective flow field between the respective inlet and the respective outlet, and the method comprises applying an electric current to the electrolyzer assembly; supplying liquid water to the cathode via the cathode inlet and the cathode flow field; and/or supplying a humidified gas stream to the anode via the anode inlet and the anode flow field.

[0025] In at least some embodiments of an electrolyzer assembly for generating hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells with each unit cell comprising a proton exchange membrane interposed between an anode and a cathode. In at least some embodiments, the assembly includes a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet for directing liquid water to the cathode via the cathode inlet and the cathode flow field, and for discharging hydrogen generated at the cathode via the cathode outlet. In at least some embodiments, the cathode inlet is fluidly connected to a liquid water supply. In at least some embodiments, the assembly also includes an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet for directing a humidified gas stream to the anode via the anode inlet and the anode flow field, and for discharging oxygen produced at the anode via the anode outlet, the anode inlet fluidly connected to a source of the humidified gas stream. In some embodiments, the cross-sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet.

[0026] In some embodiments of the electrolyzer assembly, the cross-sectional fluid flow area of the anode flow field decreases along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the cross-sectional fluid flow area of the anode flow field decreases monotonically along the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field layer comprises an anode flow field plate and the anode flow field comprises at least one anode channel formed in the anode flow field plate, and the at least one anode channel has a cross-sectional area that decreases along at least a portion of the length of the anode channel. In some embodiments, the anode flow field layer comprises a porous material adjacent to the anode, and the anode flow field comprises passageways extending within the porous material. In some embodiments, the porosity of the porous material decreases monotonically from the anode inlet to the anode outlet. In some embodiments, the anode flow field layer comprises an anode plate and a porous material interposed between the anode and the anode plate, the porous material having two major surfaces and comprising an anode channel formed in one of the major surfaces, the anode channel extending from the anode inlet to the anode outlet, and the cross-sectional area of the anode channel decreases monotonically from the anode inlet to the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 85% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 90% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along at least 95% of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the anode flow field has a fluid flow area that decreases along substantially the entire length of the anode flow field between the anode inlet and the anode outlet.

[0027] In some embodiments, the electrolyzer assembly further comprises a power supply configured to deliver electrical power to the electrolyzer assembly.

[0028] In some embodiments, the electrolyzer assembly further comprises a hydrogen containment vessel configured to collect hydrogen from the cathode flow field, the hydrogen generated by the electrolyzer assembly.

[0029] In some embodiments, the electrolyzer assembly further comprises a gas-togas humidifier for humidifying the gas stream supplied to the anode, the gas-to-gas humidifier on one side fluidly connected to supply the humidified gas stream to the anode inlet and on other side fluidly connected to receive a fluid stream from the anode outlet.

[0030] In some embodiments of the electrolyzer assembly, the cross-sectional fluid flow area of the cathode flow field also varies along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field decreases along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field decreases monotonically along the length of the cathode flow field between the cathode inlet and the anode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 85% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 90% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along at least 95% of the length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field has a fluid flow area that decreases along substantially the entire length of the cathode flow field between the cathode inlet and the cathode outlet. In some embodiments, the cathode flow field layer comprises a porous material adjacent to the cathode, and the cathode flow field comprises passageways extending within the porous material. In some embodiments, the porosity of the porous material decreases monotonically from the cathode inlet to the cathode outlet. In some embodiments, the cathode flow field layer comprises a cathode plate and a porous material interposed between the cathode and the cathode plate, the porous material having two major surfaces and comprising a cathode channel formed in one of the major surfaces, the cathode channel extending from the cathode inlet to the cathode outlet, and the cross- sectional area of the cathode channel decreases monotonically from cathode anode inlet to the cathode outlet.

[0031] In some embodiments of an electrolyzer assembly for generating hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells, with each unit cell comprising a proton exchange membrane interposed between an anode and a cathode. In some embodiments, the electrolyzer assembly includes a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet for directing liquid water to the cathode via the cathode inlet and the cathode flow field, and for discharging hydrogen generated at the cathode via the cathode outlet, the cathode inlet fluidly connected to a liquid water supply. In some embodiments, the electrolyzer assembly also includes an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet for directing a humidified gas stream to the anode via the anode inlet and the anode flow field, and for discharging oxygen produced at the anode via the anode outlet, the anode inlet fluidly connected to a source of the humidified gas stream. In some embodiments, the cross-sectional fluid flow area of the cathode flow field varies along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet, and the cross-sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet.

[0032] In some embodiments of a method of operating an electrolyzer assembly to generate hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells, with each unit cell comprising an anion exchange membrane interposed between an anode and a cathode. In some embodiments, the electrolyzer assembly includes a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet. In some embodiments, the electrolyzer assembly also includes an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet. In some embodiments, the cross-sectional fluid flow area of the cathode flow field varies along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet, and/or the cross-sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments, the method comprises applying an electric current to the electrolyzer assembly; supplying liquid water or water vapor to the cathode via the cathode inlet and the cathode flow field; and/or supplying liquid water or water vapor to the anode via the anode inlet and the anode flow field. In some embodiments, liquid water is supplied to the cathode via the cathode inlet and the cathode flow field, and/or a humidified gas stream is supplied to the anode supplying a humidified gas stream to the anode.

[0033] In embodiments of an electrolyzer assembly for generating hydrogen and oxygen from water, the electrolyzer assembly comprises a plurality of unit cells, with each unit cell comprising an anion exchange membrane interposed between an anode and a cathode. In some embodiments, the electrolyzer assembly includes a cathode flow field layer adjacent to the cathode, the cathode flow field layer defining a cathode flow field that fluidly connects a cathode inlet to a cathode outlet for directing liquid water or water vapor to the cathode via the cathode inlet and the cathode flow field, and for discharging hydrogen generated at the cathode via the cathode outlet. In some embodiments the cathode inlet is fluidly connected to a water supply. In some embodiments, the electrolyzer assembly also includes an anode flow field layer adjacent to the anode, the anode flow field layer defining an anode flow field that fluidly connects an anode inlet to an anode outlet for directing liquid water or water vapor to the cathode to the anode via the anode inlet and the anode flow field, and for discharging oxygen produced at the anode via the anode outlet. In at least some embodiments the anode inlet is fluidly connected to a source of water. In some embodiments, the cross-sectional fluid flow area of the cathode flow field varies along at least a portion of the length of the cathode flow field between the cathode inlet and the cathode outlet, and the cross-sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between the anode inlet and the anode outlet. In some embodiments the cathode inlet is fluidly connected to receive liquid water from the water supply, and/or the anode inlet is fluidly connected to a receive a humidified gas stream from the source of water.

Brief Description of the Drawings

[0034] FIG. 1 (Prior Art) is a schematic diagram of a PEM electrolyzer, illustrating a water electrolysis process in which liquid water is supplied to the anode. [0035] FIG. 2A (Prior Art) is a simplified diagram of a PEM electrolyzer unit cell showing a membrane electrode assembly (MEA) sandwiched between a pair of flow field plates.

[0036] FIG. 2B (Prior Art) is a simplified diagram of a PEM fuel cell unit cell, showing a membrane electrode assembly sandwiched between a pair of flow field plates.

[0037] FIG. 3 is a simplified schematic diagram of a PEM electrolyzer, illustrating a water electrolysis process in which liquid water is supplied to the cathode and humidified air is supplied to the anode.

[0038] FIG. 4 is an exploded schematic diagram of a PEM electrolyzer assembly in which the cross-sectional fluid flow area of an anode flow field and a cathode flow field decreases from a respective inlet to a corresponding outlet.

[0039] FIG. 5 A is a perspective view of a simplified representation of an electrolyzer flow field plate comprising a flow channel that decreases in depth, with constant width, along its length.

[0040] FIG. 5B is a perspective view of a simplified representation of an electrolyzer flow field plate comprising a flow channel that decreases exponentially in width, with constant depth, along its length.

[0041] FIG. 6 is a perspective view of a trapezoidal electrolyzer flow field plate comprising multiple flow channels that decrease exponentially in width along their length.

[0042] FIG. 7 is a perspective view of a simplified representation of a flow field plate with a serpentine flow channel, in which the channel width varies.

[0043] FIG. 8 A is a perspective view of a simplified representation of a flow field plate with a wavy flow channel, in which the channel width varies.

[0044] FIG. 8B is a perspective view of a simplified representation of a flow field plate with multiple wavy flow channels nested on the flow field plate. [0045] FIG. 9A (Prior Art) is a top view of a square flow field plate comprising a conventional serpentine flow field with three flow channels extending between a supply manifold opening and a discharge manifold opening.

[0046] FIG. 9B is a top view of a square flow field plate with a similar serpentine flow field to FIG. 9 A, but where the width of each serpentine flow channel decreases exponentially along its length.

[0047] FIG. 10A is a perspective view of 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.

[0048] FIG. 10B is a perspective view of 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.

[0049] FIG. 11 is a perspective view of 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.

[0050] FIG. 12 is a perspective view of 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.

[0051] FIG. 13 is a perspective view of 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. [0052] FIG. 14A (Prior Art) is a top view of a rectangular flow field plate comprising a multi-channel serpentine flow field extending between a supply and a discharge manifold opening.

[0053] FIG. 14B is a top view of 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.

[0054] FIG. 15 is a perspective view of a simplified representation of a flow field plate comprising a rectangular flow channel having a central rib with exponentially curved side walls.

[0055] FIG. 16A is a perspective view of 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 fluid flow direction.

[0056] FIG. 16B is a perspective view from the other end of the flow field plate of FIG. 16A.

[0057] FIG. 17 is a perspective view of 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 fluid flow direction.

[0058] FIG. 18 is a perspective view of 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 fluid flow direction.

[0059] FIG. 19 is a perspective view of a simplified representation of a flow filed plate illustrating an example where the flow channel width decreases in a stepwise, non-linear fashion in the fluid flow direction.

[0060] FIG. 20 is a perspective view of a simplified representation of a flow filed plate illustrating another example where the flow channel width decreases in a stepwise, non-linear fashion in the fluid flow direction. [0061] 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.

[0062] FIG. 22 is a simplified schematic diagram of an electrolyzer having an anion exchange membrane, illustrating a water electrolysis process in which liquid water is supplied to the cathode and a humidified gas stream is supplied to the anode.

[0063] FIG. 23 is an exploded schematic diagram of an electrolyzer assembly having an anion exchange membrane, in which the cross-sectional fluid flow area of an anode flow field and a cathode flow field decreases from a respective inlet to a corresponding outlet.

Detailed Description of Illustrative Embodiment(s)

[0064] Most conventional PEM electrolyzers operate with liquid water supplied to the anode, and either no fluid or a carrier gas supplied to the cathode. With liquid water supplied to the anode, mass transport issues can arise whereby oxygen gas generated at the anode impedes the access of liquid water to the electrocatalytic surface, resulting in increased mass transport overpotentials and an overall reduction in efficiency.

[0065] Another problematic issue that can arise in PEM electrolyzers is gas crossover through the membrane. The hydrogen and oxygen generated at the respective electrodes can permeate across the membrane (driven by the partial pressure gradient of the gases across the membrane), and mixtures of these gases at the electrodes can result. At the cathode, crossover-oxygen typically reacts with hydrogen at the platinum-based cathode catalyst to form water. At the anode, hydrogen and oxygen do not react at the catalyst that is typically used at the anode (e.g., iridium oxide). Therefore, potentially flammable mixtures of hydrogen in oxygen can occur at the anode. Hydrogen passing through the membrane to the anode can tend to reach or exceed this level, for example, if the oxygen production rate on the anode is too low (e.g., at low current densities) or the diffusion of hydrogen from the cathode is too high (e.g., thin membrane and/or high permeability). Crossover can also result in reduced electrolyzer efficiency due to faradaic losses, as energy supplied for hydrogen production is wasted when hydrogen crosses over from the cathode to the anode, and when hydrogen is lost due to reaction with oxygen at the cathode.

[0066] Gas crossover in PEM electrolyzers can be reduced by using a thicker membrane. Also, a hydrogen/oxygen recombination catalyst such as platinum or palladium can be introduced into the membrane, providing reaction sites for recombination of oxygen and hydrogen to form water within the membrane. However, to have the necessary amount of recombination catalyst and time for the recombination reaction to take place, it is still generally necessary to have a thick membrane (e.g., a thickness of 125 microns or higher for Nafion® 115 or the like). However, the use of thicker membranes introduces greater ohmic resistance and consequently results in lower efficiency operation of the electrolyzer.

[0067] For these and/or other reasons, some PEM electrolyzers are operated in a different manner. For example, rather than liquid water being supplied to the anode compartment, in some PEM electrolyzers liquid water is supplied to the cathode compartment, and humidified air is supplied to the anode compartment, as illustrated in a schematic diagram in FIG. 3. A direct electric current is applied, and water molecules pass through the membrane from the cathode compartment to the anode compartment where they are oxidized at the anode catalyst to produce protons, oxygen, and electrons. Protons move back through the membrane to the cathode where they are reduced at the cathode catalyst, generating hydrogen.

[0068] Operating a PEM electrolyzer in this manner, and with the pressure on the cathode side higher than the pressure on the anode side, can facilitate water transport from cathode to anode and result in an increased gas production rate from the electrolyzer. In at least some embodiments, the anode can be operated at pressure slightly above ambient pressure in order to overcome the pressure drop of flowing humidified air through the anode compartment. [0069] Using a thinner membrane can facilitate water transport from the cathode to the anode, resulting in a larger limiting current density, and increased hydrogen and oxygen gas generation. Use of a thinner membrane generally reduces the ohmic resistance of the cell which can increase the rate of hydrogen and oxygen generation for a given efficiency level.

[0070] However, use of a thinner membrane tends to increase gas crossover of hydrogen from the cathode to the anode, and oxygen from anode to cathode. As mentioned above, hydrogen crossing from the cathode to the anode and the resulting gas mixtures at the anode can be a problem. But the supply of humidified air to the anode generally mitigates this risk, as it can dilute the hydrogen passing from the cathode through the membrane to levels that are below the lower explosion limit. Furthermore, electrolyzers operated with humidified air supplied to the anode and liquid water supplied to the cathode tend to have lower formation of free radicals and a lower membrane degradation rate than conventional PEM electrolyzers. The combination of nitrogen and water vapor in the air provides a much lower partial pressure of oxygen at the anode (so less of a driver for oxygen crossover from anode to cathode) than in a conventional PEM electrolyzer. In addition, with this mode of operation the net water flux is from the cathode to the anode as opposed to a conventional PEM electrolyzer where the net water flux is from anode to cathode.

[0071] In conventional PEM electrolyzers in which liquid water is supplied to the anode, there is excess water at the anode, so there is generally not an issue with water concentration gradients at the anode. However, when an electrolyzer is operated with liquid water supplied to the cathode and humidified air supplied to the anode, it can be beneficial to consider and to mitigate water concentration gradients that can occur at the anode. For example, there may be a water concentration gradient between the anode inlet and outlet that occurs due to consumption of water crossing over from the cathode (and consumption of water from the humidified air supplied to the anode), as the water is converted to oxygen by the reaction at the anode. Also, the water vapor partial pressure (or concentration) at the anode can be further depleted as protons moving through the membrane from anode to cathode can transport water molecules along with them, by a phenomenon known as electroosmotic drag.

[0072] The particular flow field or flow channel used at the anode and/or cathode, in combination with this manner of operation, can be designed to reduce the effect of water concentration gradients at the anode and/or to improve performance of the electrolyzer. A flow field generally provides a fluid connection between an inlet port and a corresponding outlet port in an electrolyzer plate, and/or between an inlet header and corresponding outlet header in a stacked electrolyzer assembly. A flow field can include any or all of: one or more channels formed in a plate, pathways through porous media, interconnected pores, an open chamber or plenum region, and the like. As used herein, “an inlet port” can include one or more inlet ports, “an outlet port” can comprise one or more outlet ports, “an inlet header” can comprise one or more inlet headers, and/or “an outlet header” can comprise one or more outlet headers in a particular electrolyzer plate, unit cell, assembly, or stack.

[0073] In some embodiments of an electrolyzer, the electrolyzer cells are configured to operate with liquid water supplied to the cathode and a humidified gas stream (e.g., humidified air) supplied to the anode, as described above. In some such modes of operation, the humidified gas stream supplied to the anode can be saturated or supersaturated with water. In some modes of operation, the humidified gas stream supplied to the anode can have a relative humidity (RH) above 75% RH. In some modes of operation, the humidified gas stream supplied to the anode can have a relative humidity above 50% RH. In some modes of operation, the humidified gas stream supplied to the anode can have a relative humidity above 25% RH. In some modes of operation, the humidified gas stream supplied to the anode can have a relative humidity above 5% RH.

[0074] In some embodiments the humidified gas stream supplied to the anode is humidified air. In some embodiments the humidified gas stream supplied to the anode is humidified nitrogen, argon, helium, or another humidified carrier gas that is not reactive in the electrolyzer.

[0075] In some embodiments, the gas stream supplied to the anode is humidified using a gas-to-gas humidifier. In some embodiments, heat and moisture from the anode exhaust gas is transferred to the gas stream supplied to the anode using a gas- to-gas humidifier.

[0076] In some such modes of operation of an electrolyzer, it can be beneficial if the cross-sectional fluid flow area of the anode flow field varies along at least a portion of the length of the anode flow field between an anode inlet and an anode outlet. For example, in some embodiments it can be beneficial if the cross-sectional fluid flow area of the anode flow field decreases along at least a portion of the length of the anode flow field between an anode inlet and an anode outlet. In some embodiments, it can be beneficial if the cross-sectional fluid flow area of the anode flow field decreases in a linear manner along at least a portion of the length of the anode flow field between an anode inlet and an anode outlet. In some embodiments, it can be beneficial if the cross-sectional fluid flow area of the anode flow field decreases in a non-linear manner along at least a portion of the length of the anode flow field between an anode inlet and an anode outlet. In some embodiments, it can be beneficial if the cross-sectional fluid flow area of the anode flow field decreases according to an exponential function along at least a portion of the length of the anode flow field between an anode inlet and an anode outlet. In some embodiments, it can be beneficial if the cross-sectional fluid flow area of the anode flow field decreases monotonically between an anode inlet and an anode outlet. Improvements in electrolyzer performance can be obtained by incorporating a variation in flow field cross-sectional fluid flow area along only a portion of the length of the anode flow field. The performance improvements are not necessarily as great as if the variation is employed along the entire flow field length, but such flow field designs can in some cases provide most of the benefit. [0077] In some electrolyzers configured to operate with liquid water supplied to the cathode and a humidified gas stream, such as air, supplied to the anode, having a decreasing cross-sectional fluid flow area in the anode flow field between inlet and outlet can be used to provide a higher outlet velocity for a given flow rate at the inlet. For example, in some embodiments a higher outlet velocity can be achieved by providing an anode flow field having a decreasing cross-sectional fluid flow area rather than by increasing the compressor speed, and associated parasitic load, in the system. This can result in higher efficiency operation of the electrolyzer.

[0078] Having a decreasing cross-sectional fluid flow area in the anode flow field between inlet and outlet can also, in some embodiments, compensate for the consumption of water that occurs between the anode inlet and outlet during operation of the electrolyzer, as well as allowing for a lower rate of supply of humidified air to the anode inlet. The flow field can be designed and the electrolyzer operated so that the velocity of the fluid stream increases to compensate for reduced water vapor partial pressure. The variation in anode flow field cross-sectional fluid flow area can, in some embodiments, mitigate concentration polarization effects by providing uniform availability water vapor at the anode, supporting high current density operation. Furthermore, increasing velocity of the flow of fluid flow at the anode as it moves from the inlet to the outlet can facilitate the removal of product oxygen from the anode catalyst which can also help support high current density operation. Increasing the fluid flow velocity in the anode flow field by having a decreasing cross-sectional fluid flow area between inlet and outlet can, in some embodiments, reduce the residency time of hydrogen from cross over, which may allow for higher current density operation. In at least some embodiments, by designing the anode flow field with decreasing cross-sectional fluid flow area, in combination with selecting an appropriate flow rate of humidified air to the anode, it is possible to effectively tune the removal of oxygen and hydrogen from the anode independently from the liquid water supply. In this way, the concentration of the hydrogen-oxygen gas mixture at the anode can be kept below the flammability limit so that the possibility of formation of a flammable mixture occurring at the anode is avoided or at least reduced.

[0079] In some embodiments, instead of or as well as having a variation in the cross-sectional fluid flow area of the anode flow field between an anode inlet and an anode outlet, it can be beneficial if the cross-sectional fluid flow area of the cathode flow field varies along at least a portion of the length of the cathode flow field between a cathode inlet and a cathode outlet. For example, in some embodiments it can be beneficial if the cross-sectional fluid flow area of the cathode flow field decreases along at least a portion of the length of the cathode flow field between a cathode inlet and a cathode outlet. In at least some embodiments, this can result in an increasing velocity of the flow of liquid water as it moves from the inlet to the outlet at the cathode. In at least some embodiments, this can provide a driving force to evacuate produced hydrogen from the cathode compartment, limiting the permeation of hydrogen from cathode to anode and/or facilitating the evacuation of hydrogen from the cathode catalyst layer. In at least some embodiments, this can, in turn, allow for the use of higher operating currents to produce more hydrogen. For example, by appropriate combinations of flow field design and electrolyzer operation the liquid water velocity at the cathode can be tuned specifically for removing product hydrogen to optimize, or at least enhance, the interface between incoming reactant water and the membrane. In some embodiments it can be beneficial if the cross-sectional fluid flow area of the cathode flow field decreases in a linear manner along at least a portion of the length of the cathode flow field between a cathode inlet and a cathode outlet. In some embodiments it can be beneficial if the cross-sectional fluid flow area of the cathode flow field decreases in a non-linear manner along at least a portion of the length of the cathode flow field between a cathode inlet and a cathode outlet. In some embodiments it can be beneficial if the cross-sectional fluid flow area of the cathode flow field decreases according to an exponential function along at least a portion of the length of the cathode flow field between a cathode inlet and a cathode outlet. In some embodiments it can be beneficial if the cross-sectional fluid flow area of the cathode flow field decreases monotonically between a cathode inlet and a cathode outlet. In at least some embodiments, improvements in electrolyzer performance can be obtained by incorporating a variation in flow field cross- sectional fluid flow area along only a portion of the length of the cathode flow field. In some embodiments, the performance improvements are not necessarily as great as if the variation is employed along the entire flow field length, but such flow field designs can in some cases provide most of the benefit. In at least some embodiments, having a decreasing cross-sectional fluid flow area in the cathode flow field between inlet and outlet can be used to provide a higher outlet velocity for a given flow rate of liquid water at the inlet. Thus, the inlet pressure and parasitic load associated with the supply of liquid water can be reduced for a given outlet velocity, which can result in higher efficiency operation of the electrolyzer.

[0080] FIG. 4 is an exploded schematic diagram of a PEM electrolyzer in which the cross-sectional fluid flow area of the anode flow field and the cathode flow field decreases from a respective inlet to a corresponding outlet by a decrease in width. The illustrated cathode and anode flow field can each represent or comprise, for example, a single channel or a plurality of channels formed in a flow field plate or a porous layer, a plenum or chamber, and/or a porous transport layer, through which fluid flows from an inlet to an outlet during operation of the electrolyzer. The particular flow field fluidly connects the inlet(s) to the corresponding outlet(s).

[0081] In some embodiments, having a variation (such as a reduction) in the cross- sectional fluid flow area of the anode flow field between an anode inlet and an anode outlet and/or in the cross-sectional fluid flow area of the cathode flow field between a cathode inlet and a cathode outlet can assist with thermal management in the electrolyzer, for example, by providing uniform heat flux or by providing heat removal that is approximately matched with heat production in the electrolyzer. For example, by appropriate combinations of flow field design and electrolyzer operation, the liquid water velocity at the cathode can be tuned specifically for a desired temperature distribution across the electrolyzer cell. [0082] In some embodiments, decreasing the cross-sectional fluid flow area of the flow field can be achieved by varying the width and/or depth of flow field channels at the respective electrode. For varying channel widths, a co-flow arrangement of the anode and cathode fluids is often preferred. However, alternative designs such as using channels with a variation in depth allow for either co- or counter-flow arrangements. In some embodiments, counter- flow configurations with channels that vary in width can be achieved through non-symmetrical flow-field architectures or alternative channel designs (e.g., wavy, serpentine, dots) laid out in a rectilinear footprint.

[0083] Some examples of electrolyzers in which the cross-sectional fluid flow area of the anode and/or cathode flow field varies along at least a portion of the length of the flow field from an inlet to an outlet are described in reference to FIGS. 5-21 below.

[0084] In some 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. In at least some embodiments, the use of this type of flow field, particularly at the anode can improve performance and/or efficiency of operation of an electrolyzer assembly operating with liquid water supplied to the cathode and a humidified gas stream supplied to the anode.

[0085] In some embodiments, electrolyzer anode and cathode flow channels can be designed for substantially constant water velocity (or in some embodiments within ±10%) which maintains a substantially constant availability of water (or in some embodiments within ±10%) across the active area, for example, with substantially uniform humidity at the anode (or in some embodiments within ±10%) and substantially uniform water crossover from the cathode to the anode (or in some embodiments within ±10%).

[0086] In at least some embodiments, water availability is related to cell reaction performance. In at least some embodiments, more uniform water availability promotes more uniform current density. [0087] In some embodiments, of electrolyzer as described herein, liquid water is directed or pumped through the cathode flow field and passes through the membrane so that water vapor is distributed across the active area of the anode. In at least some embodiments, water vapor is also present in the humidified gas stream (e.g., air) that is supplied to the anode. However, as water vapor moves through the anode flow field it is consumed. Furthermore, each mole of water that is consumed is replaced by half a mole of oxygen. In at least some embodiments, several issues arise that can detrimentally affect the efficiency, lifetime and/or performance of the electrolyzer. For example, in some embodiments, as the water vapor is consumed, the amount of reactant being delivered per unit time varies across the active area of the cell. This can lead to non-uniformity in the current distribution. Conventional electrolyzers do not adequately address these issues.

[0088] FIG. 5 A is a simplified representation of electrolyzer anode flow field plate 100A comprising flow channel (or chamber) 110A that decreases in depth, with constant width, along its length. Referring to FIG. 5 A, the resulting channel 110A extends between water supply manifold opening 120 A 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.

[0089] In embodiments where there is a desire to reduce the thickness of the electrolyzer plates, it is generally desirable to keep 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, in at least some embodiments, 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, or in some embodiments to achieve water availability variation within ±10% along the length of the channel.

[0090] FIG. 5B is a simplified representation of electrolyzer anode flow field plate 100B comprising flow channel (or chamber) HOB that decreases in width along its length according to an exponential function. Referring to FIG. 5B, the resulting channel HOB extends between water supply manifold opening 120B and discharge manifold opening 130B and has a constant depth floor 112B with convexly curved side walls 114B that converge inwards from inlet to outlet. The convexly curved side walls 114B converge inwards towards outlet end 118B with inlet 116B 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. In some embodiments, one of the side walls could be straight and the other could be convexly curved. In some embodiments, one of the side walls could be concavely curved and the other could be convexly curved.

[0091] In some embodiments, it is preferable to vary the channel or chamber width. Flow field plates with channels of varying width are generally easier to manufacture than plates with a chamber or channels of varying depth, or a chamber or channels with a cross-sectional shape that varies along the channel length.

[0092] Referring to FIG. 6, multiple channels 210 having the channel profile shown in FIG. 5B can be applied to electrolyzer plate 200, to form electrolyzer anode flow field 222 extending between supply manifold opening 220 and discharge manifold opening 230. In the illustrated embodiment, electrolyzer anode flow field 222 is arrayed in a generally trapezoidal geometry to enable separating ribs 224 to have a relatively even width along their length.

[0093] In the illustrated embodiment, separator plate 200 includes partial ribs 226 located at the inlet of each channel 210. The partial ribs 226 serve to reduce the distance between separating ribs 224 and serve as a bridging structure for the adjacent membrane electrode assembly (not shown).

[0094] Embodiments in which the fluid flow channel width varies in some circumstances can be beneficial in enhancing the localized reactant and/or product flow velocity during electrolyzer operation thereby improving performance. In some embodiments, the inlet pressure to the channel can be reduced (relative to a channel of constant cross-sectional area). In at least some embodiments, this can lead to reduced parasitic loads and improved overall system efficiency. Furthermore, in some embodiments, 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:

[0095] (a) improved diffusivity of reactants for a more localized homogeneous concentration, increased access to the catalyst;

[0096] (b) more efficient removal of products from the cell;

[0097] (c) reduce inlet pressure;

[0098] (d) uniform heat flux.

[0099] In some embodiments, improved efficiency can be realized through a reduction in electrolyzer power input, an overall improvement in specific output of hydrogen, and/or an improvement in longevity from reduced and/or more uniform component wear.

[00100] Flow fields based on the description set forth above for the anode and/or cathode 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 (such as shown in FIG. 5B) 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). However, plates with channels where the depth profile changes are generally more challenging to fabricate.

[00101] 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 rectangular electrolyzer flow field plate 300 with 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. [00102] FIG. 8A shows rectangular electrolyzer flow field plate 400A with flow channel 410A extending between supply manifold opening 420 A and discharge manifold opening 430A, where the channel width is decreasing exponentially along its length. In FIG. 8A flow channel 410A is wavy. In FIG. 8A the amplitude of the path of the center-line of the flow channel 410A increases as the width of the channel decreases, so that the channel still occupies most of the width of the rectangular electrolyzer flow field plate 400A. Making the variable width channel serpentine or wavy, rather than straight, allows the channel to occupy a more rectangular shape making more efficient use of the surface area of the plate. FIGS. 7 and 8A show a single flow channel, however, 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).

[00103] FIG. 8B shows rectangular electrolyzer flow field plate 400B with multiple flow channels 410B (like flow channel 410A of FIG. 8 A repeated) extending between supply manifold opening 420B and discharge manifold opening 430B, arranged so that the channels nest together.

[00104] FIG. 9 A shows square electrolyzer flow field plate 500 A comprising a conventional (Prior Art) serpentine electrolyzer flow field with three flow channels 510A 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 510B decreases exponentially along its length as it extends from supply manifold opening 520B to discharge manifold opening 530B.

[00105] As discussed above, improvements in electrolyzer performance can be obtained by incorporating a variation in cross-sectional fluid flow area along only a portion of the length of the flow field. In at least some embodiments, the performance improvements are not necessarily as great as if the variation is employed along the entire flow field length, but such flow field designs can in some cases provide most of the benefit and can allow more efficient use of the plate area.

[00106] FIGS. 10A-12 show some examples where the flow channel width varies along just a portion of the length of the channel. FIG. 10A shows rectangular electrolyzer flow field plate 600A with flow channel 610A extending between supply manifold opening 620A and discharge manifold opening 630A. Similarly, FIG. 10B shows rectangular electrolyzer flow field plate 600B with flow channel 610B extending between supply manifold opening 620B and discharge manifold opening 630B. In FIG. 10A the flow channel width decreases exponentially for first portion 625A of the channel length (near the supply manifold) and is then constant for second portion 635A of the channel length (towards the discharge manifold). Conversely, in FIG. 10B the flow channel width is constant for first portion 625B and decreases exponentially for second portion 635B of the channel length.

[00107] In some cases it can be beneficial to incorporate a decrease in channel cross-sectional area along a first portion of the length of the channel and then incorporate an increase in channel cross-sectional area along a second portion of the length of the channel. FIG. 11 shows rectangular electrolyzer flow field plate 700 with flow channel 710 extending between supply manifold opening 720 and discharge manifold opening 730. The flow channel width decreases exponentially for first portion 725 of the channel length (near the supply manifold), and then increases so that the channel is flared for second portion 735 of the channel length.

[00108] FIG. 12 shows electrolyzer flow field plate 800 comprising two flow channels 810. The channels are initially serpentine with constant width in portion 825 near 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).

[00109] FIG. 13 shows an example of an electrolyzer flow field plate 900 comprising a flow channel 910 extending between a supply manifold opening 920 and a discharge manifold opening 930. The flow channel depth is constant along first portion 925 of the channel length and then decreases along second portion 935 of the length of flow channel 910.

[00110] In some embodiments, the electrolyzer flow channels can incorporate a variation in both width and depth along their entire length, or a portion of their length.

[00111] 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 rectangular flow field plate 1000 A 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 rectangular flow field plate 1000B in which the width of each channel 1010B decreases exponentially along a middle portion 1025B of the length of each channel.

[00112] It is also possible to take a conventional flow channel (for example, a channel with a rectangular and constant cross-sectional shape and area along its length) and incorporate a shaped rib, fillet, or other features within the volume of the original channel to reduce the channel cross-sectional area in a way that provides at least some of the desired benefits. FIG. 15 shows an example of electrolyzer flow field plate 1100 with single flow channel 1110 extending between supply manifold opening 1120 and discharge manifold opening 1130. Flow channel 1110 comprises 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.

[00113] FIGS. 16A and 16B show two different views of another example of flow field plate 1200 with single flow channel 1210 extending between supply manifold opening 1220 and discharge manifold opening 1230. Flow 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. [00114] In the examples described above, the cross-sectional fluid flow areas of the flow fields vary along at least a portion of the flow field length in a smooth and continuous fashion. However, performance benefits can also be obtained by using flow fields that incorporate discrete variations cross-sectional fluid flow area (e.g. in the flow channel dimensions). In other words, the characteristics of the flow field cross-sectional fluid flow area can be varied as a function of distance along the flow field in a stepwise or discontinuous fashion, but where the overall variation in cross-sectional fluid flow area trends with the desired smooth profile, either in fluctuations about the desired profile, or in discrete approximations of the desired profile. In some embodiments, 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 or unit cell geometries. In some of these embodiments, the outlet, or region near the outlet, is smaller or more constricted than the reactant inlet or inlet region. In some embodiments, flow fields can contain discrete features that obstruct fluid flow, where the density and/or size of those features increases in a fluid flow direction. An example of an electrolyzer flow field plate 1300 where the flow channels incorporate rib dots/raised columns 1350 is shown in FIG. 17. The density of rib dots/raised columns 1350 can increase in the fluid flow direction (indicated by the arrow). 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. 17, the channel is the entire active area and the rib dots (or other such features that obstruct fluid flow) are distributed across the active area in a varied density array. In some embodiments, the rib dots or other features can be incorporated into one or more separate channels. FIG. 18 is a simplified representation of electrolyzer flow field plate 1400 comprising wavy flow channel 1410 incorporating rib dots 1450, where the density of the rib dots increases in the fluid flow direction (indicated by the arrow).

[00115] In some embodiments, flow channel dimensions (for example, width or depth) can decrease in the fluid flow direction in a stepwise fashion. In some embodiments, the increments by which the dimensions change and the distance between the step-changes are selected so that the changes in channel dimensions in the fluid flow direction provide the desired benefits for a particular mode of operation. In some embodiments the increments by which the channel dimensions change can be the same along the channel length, and in other embodiments the increments can vary along the channel length. Similarly, in some embodiments 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.

[00116] FIGS. 19 and 20 illustrate examples where the channel width decreases in a stepwise, non-linear fashion in a fluid flow direction in accordance an exponential function. FIG. 19 is a simplified representation illustrating electrolyzer flow field plate 1500 where the width of flow channel 1510 decreases in a stepwise, nonlinear fashion in a reactant direction between supply manifold opening 1520 and discharge manifold opening 1530. FIG. 20 is a simplified representation illustrating electrolyzer flow field plate 1600 where the width of flow channel 1610 decreases in a stepwise, non-linear fashion in the fluid flow direction between supply manifold opening 1620 and discharge manifold opening 1630.

[00117] 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. Solid line 1710 represents changes in channel width and dashed line 1720 shows a smooth exponential variation in channel width.

[00118] In some embodiments, the porosity of the flow field varies based on the principles explained above. For example, in some embodiments, the porosity of one or more porous layers at the anode and/or the cathode in an electrolyzer assembly varies in the fluid flow direction. In some embodiments this change in porosity provides for, or contributes to, the cross-sectional fluid flow area of the flow field decreasing along at least a portion of the length of the flow field between the inlet and the outlet. In some examples, the dimensions of a flow field chamber or plenum or channel(s) containing a porous material varies in the fluid flow direction.

[00119] FIGS. 5A, 5B, 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 dimensions and/or characteristics are exaggerated for the purposes of clear illustration.

[00120] In the above-described embodiments, the dimensions and/or flow characteristics of the flow field vary along at least a portion of the length of the flow field. The variations can be continuous or discrete.

[00121] Flow fields with variations in cross-sectional fluid flow area as described herein can be used at either or both of the electrodes in a PEM electrolyzer assembly that is operated with liquid water supplied to the cathode and a humidified gas stream, such as air, supplied to the anode. However, as described above, they generally offer greater benefits when used at the anode. Also they can be used for some or all of the unit cells in a particular electrolyzer stack.

[00122] The open channel area versus the rib or landing area on an electrolyzer flow field plate can be selected to give sufficient electrical contact between the plates and the adjacent layers for efficient current delivery, while providing sufficient access of liquid water to cathode and water vapor to the anode to support the electrochemical reactions. In at least some embodiments, using a wider rib area (between flow channels) improves electrical connectivity and current delivery in an electrolyzer.

[00123] As used herein the “inlet” refers to the start of the flow field where reactant enters the electrode compartment; and “outlet” refers to the downstream end of the flow field where the fluid exits the electrode compartment.

[00124] Electrolyzer flow field plates can include flow channels or flow field designs as described above. Such plates can be made from suitable materials or combination of materials, and can be fabricated by suitable methods. Flow channels or passageways as described above can also be incorporated into other electrolyzer components. For example, such channels (or other ways of achieving variations in cross-sectional fluid flow area) can be incorporated into the gas diffusion layers, manifolds, or other components of the unit cell or stack. Further, electrolyzers and electrolyzer stacks can also incorporate these flow field plates and/or other components. The flow channels and flow field designs described herein have been found to be particularly advantageous in PEM electrolyzer assemblies and URFCs operating in electrolysis mode with liquid water supplied to the cathode and humidified air supplied to the anode, however they can be applied in other types of electrochemical devices. For example, they can be applied in electrolyzers that employ anion exchange membranes (AEM) as the electrolyte.

[00125] FIG. 22 is a simplified schematic diagram of an AEM electrolyzer illustrating a water electrolysis process in which liquid water is supplied to the cathode and a humidified gas stream is supplied to the anode. A direct electric current is applied, and water supplied to the cathode is reduced to produce hydrogen and hydroxide ions. The hydroxide ions pass through the AEM from the cathode to the anode where they are oxidized at the anode catalyst to produce water, oxygen, and electrons. In an AEM electrolyzer, supplying liquid water directly to the cathode, where it reacts to produce hydrogen, removes the transport limitation of water passing through the membrane electrolyte.

[00126] With this, or other modes of operation of an AEM electrolyzer, it can be advantageous to use flow fields at the anode and/or cathode in which the cross- sectional fluid flow area of the flow field varies along at least a portion of the length, for example, as described above in reference to various embodiments of PEM electrolyzers. FIG. 23 is an exploded schematic diagram of an AEM electrolyzer in which the cross-sectional fluid flow area of the anode flow field and the cathode flow field decreases from a respective inlet to a corresponding outlet by a decrease in width. As described above for embodiments of PEM electrolyzers, the illustrated cathode and anode flow field can each represent, for example, a single channel or a plurality of channels formed in a flow field plate or a porous layer, a plenum or chamber, and/or a porous transport layer, through which fluid flows from an inlet to an outlet during operation of the electrolyzer. The particular flow field fluidly connects the inlet(s) to the corresponding outlet(s).

[00127] As described above in reference to PEM electrolyzer, embodiments for configuring a cathode flow field in an AEM electrolyzer so that there is an increase in velocity towards the outlet facilitates removal of product hydrogen from the cathode catalyst, which can result in a higher limiting current density and increased hydrogen generation. In at least some embodiments, the cathode water velocity profile can also be tuned specifically for desired temperature distribution. In at least some embodiments, configuring an anode flow field in an AEM electrolyzer so that there is an increase in velocity towards the outlet can facilitate uniform removal of product water vapor and oxygen from the anode catalyst, and can mitigate mass transport limitations.

[00128] Where a component is referred to above, unless otherwise indicated, reference to that component should be interpreted as including as equivalents of that component and components which perform the function of the described component (namely, that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure, but which perform the function in the illustrated exemplary embodiments.

[00129] While particular embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications can be made by those skilled in the art without departing from the scope of the present disclosure, particularly in light of the foregoing teachings. For example, features from the embodiments described herein can be combined with features of other embodiments described herein to provide further embodiments. The changes and alternatives are considered within the spirit and scope of the present invention.