Classifications

• • 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
  • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
• • 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
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous 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
  • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
  • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
  • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
  • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
• • 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
  • H01M8/10—Fuel cells with solid electrolytes
  • H01M2008/1095—Fuel cells with polymeric electrolytes
• • 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 to a method of operating fuel cell stacks, in particular, operating solid polymer fuel cell stacks under low pressure and low power operating conditions.
• Electrochemical fuel cells convert fuel and oxidant into electricity.
• Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly that includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth.
• the membrane electrode assembly comprises a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction.
• the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit.
• a number of membrane electrode assemblies are electrically coupled in series to form a fuel cell stack having a desired power output.
• the membrane electrode assembly is typically interposed between two electrically conductive bipolar flow field plates or separator plates wherein the bipolar flow field plates may comprise polymeric, carbonaceous, graphitic, or metallic materials. These bipolar flow field plates act as current collectors, provide support for the electrodes, and provide passages for the reactants and products.
• Such bipolar flow field plates may comprise flow fields to direct the flow of the fuel and oxidant reactant fluids to the anode and cathode electrodes of the MEA, respectively, and to remove excess reactant gases and reaction products, such as water formed during fuel cell operation.
• a certain amount of pressure is typically needed to deliver reactant fluids to the fuel cell and to run other fuel cell system components, all of which governs the operating pressure of the fuel cell.
• the reactant streams are typically pressurized to an operating pressure by means of a compressor, pump, blower, fan, or the like.
• a lower operating pressure is desirable in order to lower the amount of parasitic power that is needed to pressurize the anode and cathode reactant streams.
• a high efficiency blower or fan is desirable to pressurize the reactant streams because it consumes a lower amount of parasitic power than compressors and pumps.
• most commercially available blowers and fans pressurize the reactants to significantly lower pressures than conventional fuel cells, for example, up to 0.21 barg, thereby undesirably limiting the highest operating pressure.
• water is produced at the cathode, which may condense into water droplets within the catalyst layer, within the gas diffusion layer, in the reactant flow fields, or surfaces thereof.
• An excess of water droplets is undesirable because the water droplets contribute to unstable performance (for example, water "flooding" in the anode and/or cathode), and may cause nonuniform reactant fluid flow and reactant starvation.
• the anode and cathode flow field geometry may be designed to have a high pressure drop to passively enhance the flow of reactant fluids through the fuel cell and the removal of reactant product fluids, for example, water, out of the fuel cell.
• reactant product fluids for example, water
• pressure drop in the flow fields is usually minimized, for example, to as low as 150 mbar, particularly for low pressure fuel cells.
• Temperatures may be controlled by heaters or by heating process air, by flowing air through just the anode to be cooled by vaporization, by flowing air through both anodes and the cathodes, the cathode's air being warmer, or by bleeding H 2 into cathodes momentarily.
• these methods consume extra parasitic power in order to prevent water from excessively flooding the cathode or to evaporate excess water, thereby preventing/recovering fuel cell performance loss due to cathode flooding.
• a method for operating a low pressure drop fuel cell stack with improved performance stability at low pressure and low power operating conditions, wherein each fuel cell of the fuel cell stack comprises an anode flow field plate, a cathode flow field plate and a membrane electrode assembly, such that during low pressure and low power operation, the cathode flow field plate of each fuel cell has a higher heat transfer rate than the anode flow field plate of the same fuel cell.
• a heat transfer rate of a cathode flow field plate of each fuel cell is greater than the heat transfer rate of an anode flow field plate of the same fuel cell.
• the reactants are delivered to the anode flow field plate and the cathode flow field plate of each fuel cell by means of a blower or fan.
• the cathode flow field plate material has a higher thermal conductivity than the anode flow field plate material, for example, by using different materials for the anode and cathode flow field plates.
• the web thickness of the anode flow field plate is made greater than the web thickness of the cathode flow field plate to ensure a greater heat transfer rate in the cathode flow field plate than in the anode flow field plate.
• the distance of the bottom of the anode flow fields to the parallel edge of the coolant flow fields is greater than the distance of the bottom of the cathode flow fields to the opposite edge of the coolant flow fields of the bipolar flow field plate.
• Figure 1 is a cross-sectional diagram of a bipolar flow field plate with low pressure drop anode and cathode flow fields.
• Figure 2 is a cross-sectional diagram of a low pressure drop fuel cell and fuel cell stack.
• Figure 3 is a cell voltage vs. cell position diagram showing the average performance of each cell in a first 10-cell stack using a first set of anode flow field plates.
• Figure 4 is a cell voltage vs. cell position diagram showing the average performance of each cell in a second 10-cell stack using a second set of anode flow field plates.
• the invention is characterized by a method of operating a fuel cell stack at a low pressure (for example, less than 0.21 barg) and low power (for example, less than 0.5 A/cm 2 ), wherein the performance of each fuel cell in the fuel cell stack is stabilized by establishing different heat transfer rates for the anode and cathode flow field plates of each fuel cell, such that the heat transfer rate of the cathode flow field plate of each fuel cell is greater than the heat transfer rate of the anode flow field plate of the same fuel cell.
• Figure 1 shows the cross-section of an exemplary bipolar flow field plate
• anode flow fields 8 and cathode flow fields 12 should have a low pressure drop, preferably less than 150 mbar.
• Anode flow field plate 4 and cathode flow field plate 6 may be adhesively joined together around a peripheral edge thereof (not shown) such that a second surface 18 of anode flow field plate 4 faces and contacts second surface 20 of cathode flow field plate 6.
• anode flow field plate 4 and cathode flow field plate 6 are not adhesively joined together.
• At least one of the second surfaces of the anode flow field plate and the cathode flow field plate may further comprise at least one coolant flow field.
• At least one coolant flow field for circulating a coolant fluid is typically employed for fuel cells to remove heat from the reactants in the anode and cathode flow fields, thereby keeping the fuel cell at an optimum operating temperature.
• the coolant fluid helps distribute heat evenly throughout the fuel cell to prevent hot spots from forming therein, which may damage components of the membrane electrode assembly (hereinafter referred to as "MEA").
• cathode flow field plate 6 further comprises coolant flow fields 22 on a second surface 20 of cathode flow field plate 6.
• the second surfaces of both the anode and cathode flow field plates may comprise coolant channels (not shown).
• the heat transfer rate of cathode flow field plate 6 is preferably greater than the heat transfer rate of anode flow field plate 4 during operation.
• oxidant is delivered to cathode flow fields 12 by means of a blower or fan (not shown)
• fuel is delivered to anode flow fields 8 by means of a blower or fan (not shown) (which may optionally go through a reformer prior to entering the anode flow fields 8)
• a coolant fluid is flowing in coolant flow fields 22.
• the amount of heat conducted or removed from the oxidant in cathode flow fields 12 to the coolant fluid in coolant flow fields 22 is greater than the amount of heat conducted or removed from the fuel in anode flow fields 8 to the coolant fluid in coolant flow fields 22.
• creating different heat transfer rates for the anode and cathode flow field plate of each fuel cell is of particular importance for low pressure and low power operation, for example, less than about 0.21 barg and less than about 0.5 A/cm 2 , respectively, in order to reduce parasitic power loss and to increase fuel efficiency.
• Preferably, only the stoichiometrically required amount of fuel is supplied to the anode flow fields because this reduces the amount of power needed to run the blowers and/or fans that deliver the fuel and improves fuel utilization, thereby improving fuel efficiency.
• this approach minimizes the amount of fuel being delivered to the anode flow fields, particularly if the fuel is 100% hydrogen and the temperature of the fuel cell is not particularly high (because voltage is high at low power), which may cause an inadequate flow velocity in the anode flow fields and too low of a fuel cell temperature to sufficiently remove water that has migrated from the cathode to the anode.
• One way of achieving different heat transfer rates for the anode flow field plate and the cathode flow field plate of each fuel cell during low pressure and low power operation is to use materials of different thermal conductivities for each plate, such that the thermal conductivity of cathode flow field plate 6 is greater than the thermal conductivity of anode flow field plate 4.
• materials of different thermal conductivities for each plate such that the thermal conductivity of cathode flow field plate 6 is greater than the thermal conductivity of anode flow field plate 4.
• various blends of materials such as carbon, graphite, metal and/or polymer, may be used to obtain the desired thermal conductivity of the anode and cathode flow field plates.
• different resins may be used for each of the flow field plates to vary its thermal conductivity.
• layered plate structures with different materials for each layer of the plate may also be used to control the thermal conductivity and/or heat transfer rate of each flow field plate during operation, such as incorporating a metallic layer to increase thermal conductivity or incorporating a relatively thermally insulating layer to decrease thermal conductivity, or using metallic plates with different coatings for the anode and cathode flow field plates.
• the amount of resin on one surface of the plate may be higher than an opposite surface of the flow field plate.
• materials with anisotropic thermal properties may also be used as part of the layered structure to obtain and/or control the desired heat transfer rate of each flow field plate.
• an anisotropic material is expanded graphite; its in-plane thermal conductivity is orders of magnitude greater than its through-plane thermal conductivity.
• the coefficient of thermal expansion for each of the plate materials should not be so different as to create large thermal stresses in the bipolar flow field plate and the fuel cells.
• the web thickness is defined as the cross-sectional distance of the bottom of a reactant flow field on the first surface of the flow field plate to the opposing second surface of the same flow field plate.
• the web thickness is the distance from the bottom of the reactant fluid flow field to a bottom of the coolant flow field.
• anode plate web thickness 34 of anode flow field plate 4 is the distance from surface 36 of anode flow field 8 to second surface 18 of anode flow field plate 4.
• cathode plate web thickness 35 is the distance from surface 38 of cathode flow field 12 to surface 40 of coolant flow field 22.
• a bipolar flow field plate 2 is formed by contacting second surface 18 of anode flow field plate 4 with second surface 20 of cathode flow field plate 8. An embodiment of the present method is discussed in reference to Figure 2.
• Figure 2 shows fuel cell stack 42 comprising two fuel cells 30 and 30-1.
• Fuel cell 30 comprises anode flow field plate 4, cathode flow field plate 6 and MEA 32, wherein MEA 32 comprises anode electrode 24, cathode electrode 26, and membrane 28, and further-comprising anode flow fields 8, cathode flow fields 12, and coolant flow fields 22.
• Adjacent fuel cell 30-1 similarly comprises anode flow field plate 4-1, cathode flow field plate 6-1 and MEA 32-1 having anode electrode 24-1, cathode electrode 26-1, and membrane 28-1, and further comprising anode flow fields 8-1, cathode flow fields 12-1, and coolant flow fields 22-1.
• a coolant fluid is circulated in coolant flow fields 22 on second surface 20 of cathode flow field plate 6, the coolant fluid being in contact with the second surface 18-1 of anode flow field plate 4-1 of fuel cell 30-1 to evenly remove and/or distribute heat within fuel cells 30 and 30-1.
• the temperature of the reactant fluid in anode flow fields 8-1 in contact with anode electrode 24-1 is different than the temperature of the reactant fluid in cathode flow fields 12 in contact with cathode electrode 26 due to the difference in relative plate web thicknesses (e.g., anode web thickness 34 is great than cathode web thickness 35).
• the reactant fluid in the anode flow fields is maintained at a higher temperature than the reactant fluid in the cathode flow fields in order to reject more heat from the reactant in the cathode flow field than from the reactant to the anode flow field.
• the heat transfer rate of the cathode flow field plate is greater than the heat transfer rate of the adjacent anode flow field plate by the means previously described and/or other methods known in the art for passively inducing different heat transfer rates for the anode and the cathode flow field plates.
• fuel cell stack 42 may be formed by stacking fuel cell 30 next to an adjacent fuel cell 30-1 such that second surface 20 of cathode flow field plate 6 of fuel cell 30 is in contact with second surface 18-1 of anode flow field plate 4-1 of adjacent fuel cell 30-1.
• coolant flow fields 22 and 22- 1 are formed on the second surface of cathode flow field plates 6 and 6-1, respectively.
• cathode flow field plate 6 of fuel cell 30 comprises coolant flow fields 22 so that a coolant fluid may flow between cathode flow field plate 6 of fuel cell 30 and anode flow field plate 4-1 of adjacent fuel cell 30-1.
• the heat transfer rate of the cathode flow field plate of each fuel cell is greater than the heat transfer rate of the adjacent anode flow field plate of an adjacent fuel cell, as discussed above.
• one or both of the second surfaces of anode flow field plates 4 and 4-1 and cathode flow field plates 6 and 6-1 may comprise coolant flow fields 22 and 22-1, respectively.
• no coolant flow fields may be present on the second surface of either anode flow field plate 4 or cathode flow field plate 6.
• bipolar flow field plate 2 further comprises an additional coolant plate disposed between second surface 18-1 of anode flow field plate 4-1 and second surface 20 of cathode flow field plate 6, and coolant flow fields are formed on the coolant plate (not shown).
• the coolant flow field plate may be such that during low pressure and low power operation, it produces a higher heat transfer rate in the cathode flow field plate than the anode flow field plate (e.g., the amount of heat removed from the cathode flow field plate is greater than the amount of heat removed from the anode flow field plate) by using different materials with different thermal conductivities and/or by orienting the coolant flow fields such that they are closer to the cathode flow fields than the anode flow fields and/or other methods known in the art for passively inducing different heat transfer rates for the anode and the cathode flow field plates.
• Two 10-cell fuel cell stacks were tested under the following conditions: diluted fuel (74% hydrogen, 20% carbon dioxide, 6% nitrogen) was supplied to the anode at a pressure of 17.2 kPag, a humidification temperature of 57 0 C and a stoichiometry of 1.25, while air was supplied to the cathode at a pressure of 10.7 kPag, a humidif ⁇ cation temperature of 57 0 C and a stoichiometry of 2.0.
• the anode flow fields of both stacks had a pressure drop of 120 mbar while the cathode flow fields of both stacks had a pressure drop of 100 mbar.
• the anode flow field plate web thickness of the anode flow field plates in the first stack was 1.88-millimeters, while the anode flow field plate web thickness of the anode flow field plates in the second stack was 3.6- millimeters. Both stacks were operated at 0.285A/cm 2 for about 15 minutes.
• Figure 3 shows the average performance of each fuel cell in the first 10- cell stack comprising 1.88-millimeter web thickness anode flow field plates.
• the average performance was unstable and had a large cell-to-cell voltage variability, greater than 50 mV difference between the best performing cell and the worst performing cell.
• the average performance at 0.285A/cm was 544mV.
• Figure 4 shows the average performance of each fuel cell in the second 10-cell stack comprising 3.6-millimeter web thickness anode flow field plates. Performance was stable and had a much lower cell-to-cell voltage variability, less than 17 mV difference between the best performing cell and the worst performing cell. The average performance at 0.285A/cm 2 was 722mV, which was significantly better than the first 10-cell stack at only 544mV.
• a reduction in water back-diffusion from the cathode to the anode was verified by collecting water that was condensed from a water knockout at the anode outlet of the fuel cell.
• the same two 10-cell fuel cell stacks were operated for 8 hours at 0.221 A/cm 2 .
• a total of 15.4 grams/hour of water was collected from the first 10-cell fuel cell stack while a total of only 1.2 grams/hour of water was collected from the second 10-cell fuel cell stack, thus illustrating the significant influence of anode flow field plate web thickness on water back-diffusion from the cathode to the anode and the reduction in anode flooding with low pressure drop anode flow fields.

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• 2005
  • 2005-11-18 CA CA002629087A patent/CA2629087A1/en not_active Abandoned
  • 2005-11-18 JP JP2008541137A patent/JP2009516351A/ja active Pending
  • 2005-11-18 EP EP05851816A patent/EP1961061A1/de not_active Withdrawn
  • 2005-11-18 WO PCT/US2005/041863 patent/WO2007058657A1/en active Application Filing

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