WO2010132050A1

WO2010132050A1 - Pem fuel cell catalyst and diffusion layer structure for increased water storage capacity and improved cold start performance

Info

Publication number: WO2010132050A1
Application number: PCT/US2009/043747
Authority: WO
Inventors: Shivakumar Kameswaran; Rachid Zaffou; Takahisa Suzuki; Patrick L. Hagans; Ned E. Cipollini; Michael L. Perry; Cynthia Ann York
Original assignee: Toyota Motor Corp; UTC Power Corp
Current assignee: Toyota Motor Corp; UTC Power Corp
Priority date: 2009-05-13
Filing date: 2009-05-13
Publication date: 2010-11-18
Anticipated expiration: 2011-11-13

Abstract

Water produced during startup in sub-freezing conditions is accommodated by the various porous structures within the fuel cell. In one example, an interface between a catalyst layer and a gas diffusion layer includes a temperature responsive material that variably absorbs/desorbs water to temporarily increase water storage capacity at low temperatures. In another example, a catalyst layer with porosity of greater than 40% and contains hydrophilic membrane, a perfluorosulfonic acid material, with an equivalent weight of less than 1100 per equivalent weight gram. In yet another example, a gas diffusion layer having a carbon substrate treated with a modifier material to increase fuel cell water storage capacity under sub-freezing conditions.

Description

PA-0008056-WO

PEM FUEL CELL CATALYST AND DIFFUSION LAYER

STRUCTURE FOR INCREASED WATER STORAGE CAPACITY

AND IMPROVED COLD START PERFORMANCE

BACKGROUND

This disclosure relates to proton exchange membrane fuel cells. In particular, the disclosure relates to catalyst and diffusion layer structure and their characteristics that relate to water storage capacity during fuel cell cold start.

A proton exchange membrane fuel cell (PEMFC) includes multiple individual cells arranged in a stack. Each cell has an anode and a cathode on either side of a proton exchange membrane (PEM). A fuel, such as hydrogen, is supplied to the anode side of the PEM, and an oxidant or reactant, such as air, is supplied to the cathode side of the PEM. A porous catalyst layer is arranged on each of the anode and cathode sides adjacent to the PEM. Water is formed during the chemical reactions within the cathode electrode in the fuel cell. Under freezing conditions, the product water accumulates within various porous structures in the fuel cell. Water storage capacity (WSC) is used as a tool to evaluate water fill capacity of a fuel cell during cold start (< 0 ⁰C), which in turn is used as measure to investigate cold start performance. WSC is typically calculated from constant current or constant voltage isothermal experiments. Such experiments provide important information about fuel cell layers and various operational strategies critical to product water management during startup. For example, a catalyst layer having an ionomer with an equivalent weight of 1100, a thickness of 10 μm, a porosity of 40% and a volume fraction of 0.2 at a constant current density of 50 mA/cm² at a temperature of -2O⁰C has a water storage capacity that is filled in approximately 149 seconds of operation at fuel cell startup in sub-freezing temperatures when used with a PEM having the same equivalent weight and a thickness of 18 μm.

Ice can form from residual water remaining in the stack under sub-freezing temperatures, which is undesirable. The fuel cell typically includes various design PA-0008056-WO features to mitigate the unwanted effects of ice buildup. In addition, during startup from sub-freezing temperatures ice can form in a fuel cell, which can result in fuel cell failure.

More specifically, applying a current load to the stack in these conditions can lead to a continuous build up of ice that blocks the path of the reactant to the catalyst surface. The chemical reactions within the fuel cell will cease once the reactant flow has been blocked.

What is needed is a fuel cell in which the water can be accommodated such that when it freezes the ice does not obstruct the reactant flow during the transient conditions encountered during a cold start while the fuel cell temperature is below freezing.

SUMMARY

Water produced during startup in sub-freezing conditions is accommodated by the various porous structures within the fuel cell. In one example, an interface between a catalyst layer and a gas diffusion layer includes a temperature responsive material that variably absorbs/desorbs water to temporarily increase water storage capacity at low temperatures. In another example, a catalyst layer with porosity of greater than 40% and contains hydrophilic membrane, a perfluoro sulfonic acid material, with an equivalent weight of less than 1100 per equivalent weight gram. In yet another example, a gas diffusion layer having a carbon substrate treated with a modifier material to increase fuel cell water storage capacity under sub-freezing conditions. These and other features of the disclosure can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a highly schematic view of a proton exchange membrane fuel cell. Figure 2 is a schematic view of a catalyst layer having a material with a nonlinear water absorption isotherm.

Figure 3 is a schematic view of a gas diffusion layer having a material with a nonlinear water absorption isotherm.

Figure 4 schematically depicts a carbon substrate treated with a modifier material. PA-0008056-WO

DETAILED DESCRIPTION

A proton exchange membrane fuel cell (PEMFC) 10 is shown in Figure 1. The PEMFC 10 includes multiple individual cells 12 arranged in a stack. Typically dozens or more cells 12 are arranged adjacent to one another within the stack. Each cell 12 has an anode plate 16 and a cathode plate 18 on either side of a unitized electrode assembly (UEA) 14. The UEA 14 includes a proton exchange membrane (PEM) 30 arranged between catalyst layers 32. A gas diffusion layer (GDL) 34 is arranged adjacent to each catalyst layer 32 opposite the PEM 30. In one illustrated example, the GDL 34 includes a microporous layer (MiPL) 36 and a macroporous layer (MaPL) 38. The MiPL 36 includes pores that are smaller than the pores in the MaPL 38.

A fuel source 20 supplies fuel, such as hydrogen, to fuel flow fields 26 in the anode plate 16. An oxidant source 22, such as the surrounding environment, supplies an oxidant or reactant, such as air, with a pump 24 to a reactant flow field 28 in the cathode plate 18. The GDL 34 on the anode side evenly distributes the hydrogen from the fuel flow fields 26 to the catalyst 32 on the anode side. Similarly, the GDL 34 on the cathode side evenly distributes the oxygen from the reactant flow field 28 to the catalyst 32 on the cathode side. The anode catalyst separates the hydrogen to provide hydrogen ions, and the cathode catalyst separates the oxygen molecule into oxygen ions. The free electrons from the separated hydrogen atoms are used to power a load 40 and are returned to the catalyst 32 on the cathode side of the PEM 30. The hydrogen protons pass across the PEM 30 and react with the oxygen ions in the reactant and the returning electrons to form water and produce heat.

The water created during the chemical reactions within the fuel cell can form ice in sub-freezing temperatures. If enough ice forms, the reactant flow can become obstructed such that reactant is no longer supplied to the cathode catalyst, thus stopping the chemical reactions within the fuel cell 10. The fuel cell embodiments of this disclosure accommodate more water at fuel cell startup in sub-freezing temperatures than prior art fuel cell arrangements. As a result, the fuel cell is afforded additional time to PA-0008056-WO heat up to an above-freezing temperature, which prevents the reactant flow (through layers 32, 36, 38) from becoming entirely obstructed.

In one example, the fuel cell 10 includes a porous structure, such as the catalyst layer 32 or GDL 34, having a material 44 (Figures 2 and 3) with a non-linear water adsorption isotherm. In one example, the material 44 is arranged at an interface 42 between the catalyst layer 32 and the gas diffusion layer 34 on the cathode side, as shown in Figure 1. More particularly, the material 44 can be provided in the catalyst layer 32, for example, as shown in Figure 2. Alternatively or additionally, the material 44 can be provided in the MiPL 36, shown in Figure 3, which is typically constructed from carbon fibers.

In one example, the material 44 stores water without any significant change to its physical structure. The material 44 absorbs water below a first temperature and desorbs water at a second temperature that is greater than the first temperature. The first temperature is around the freezing point of water so that additional water is accommodated at sub-freezing temperatures that tend to create ice buildup. The second temperature is at a temperature above the freezing point of water so that this unneeded additional water storage capacity is eliminated.

Materials or morphologies can be used to form additional void space when cooled. For example, solids with relatively high coefficients of thermal expansion will contract significantly as they are cooled from the normal fuel cell operating temperature (e.g., ~ 80⁰C in the PEMFC) to sub-freezing temperatures (< 0⁰C). If the solid phases are constructed in a manner such that this contraction results in the creation of additional voids (e.g., between adjacent solid particles), this additional void space could provide a location to store liquid water in locations that do not hinder gas flow to the catalyst layer. Ideally, these low-temperature voids will be hydrophilic capillaries that can readily wick up excess water. During freeze ice will form in these voids, but it will not be detrimental to startup. On startup, the ice will melt and as the cell temperature continues to increase the water will be expelled as these voids close. This should not be detrimental to a cell that is designed to handle excess water during normal operation. PA-0008056-WO

In one example, the material 44 is a generally non-conductive zeolite. Zeolites are typically a naturally occurring mineral group that consists of a special crystalline structure that is porous but remains rigid in the presence of water. Some zeolites have extremely nonlinear adsorption isotherms to water. When heated they release water (and adsorb heat) and when cooled they will adsorb water and heat. By controlling the composition and structure of the zeolites one can design a material that absorbs and desorbs at the desired temperature, in one example between approximately 5°C (first temperature) and 65°C (second temperature).

A fuel cell with zeolites will take longer to freeze due to the heat of adsorption, which can be desirable. The fuel cell will also take longer to heat up, but may not be a disadvantage if the desorption occurs well above 0⁰C. Zeolites are not highly electrically conductive, so a desired location for these materials is in the UEA 14, for example, at the interface 42.

In another example arrangement for increased water storage capacity, the fuel cell 10 includes a hydrophilic (lower equivalent weight) membrane with characteristics that facilitate additional water storage over prior art, such as a PEM 30 or catalyst layer 32. The parameters given below relate to an example operating condition of approximately -20⁰C, ambient pressure, a current density of 50 mA/cm², an initial water content in the PEM 30 and catalyst layers 32 of 3, and an initial ice fraction of 0 in the pores of layers 32, 36 and 38.

PA-0008056-WO

Table I: Desired fuel cell parameters for increased water storage capacity and improved cold-start performance

The membrane equivalent weight in layers 30 and 32, and porosity of layer 32 on the cathode side have been found to have the greatest effect in increasing fuel cell water storage capacity. In one example, including a perfluoro sulfonic acid material, such as

NAFION, comprising an equivalent weight of less than 1100 per gram in layers 30 and

32 and having greater than 40% pores by volume in layer 32. The equivalent weight is the number of sulfonic acid groups in one gram of material. In one example, the perfluoro sulfonic acid material has an equivalent weight 800-1100 per gram.

The thickness of the layers 30 and 32 also has an impact upon increasing water storage capacity. In one example, the PEM 30 is approximately 10-35 μm thick, and in another example, at least 18 μm thick. In the case of a proton exchange membrane, the membrane is 18-35 μm thick, for example. In the case of a catalyst layer, the layer 32 is 5-16 μm thick, and in another example, at least 10 μm thick, for example. The volume fraction of perfluoro sulfonic acid has some ability to increase water storage capacity as well. In one example, the catalyst layer 32 includes a 0.3 volume fraction of perfluoro sulfonic acid material.

In one example, a catalyst layer 32 having an equivalent weight of 800, a thickness of 16 μm, a porosity of 60% and a volume fraction of 0.3 of the NAFION has a water storage capacity that is filled in approximately 398 seconds of operation at fuel cell PA-0008056-WO startup in sub-freezing temperatures (-20⁰C, ambient pressure, a current density of 50 mA/cm², an initial water content in the PEM 30 and catalyst layers 32 of 3, and an initial ice fraction of 0 in the pores of layers 32, 36 and 38) when used with a PEM having the same equivalent weight and a thickness of 25 μm. In another example, a catalyst layer 32 having an equivalent weight of 800, a thickness of 16 μm, a porosity of 50% and a volume fraction of 0.27 of the NAFION has a water storage capacity that is filled in approximately 348 seconds of operation at fuel cell startup in sub-freezing temperatures (-20⁰C, ambient pressure, a current density of 50 mA/cm², an initial water content in the PEM 30 and catalyst layers 32 of 3, and an initial ice fraction of 0 in the pores of layers 32, 36 and 38) when used with a PEM having the same equivalent weight and a thickness of 25 μm.

In yet another example arrangement for increased water storage capacity, the GDL 34 includes a carbon substrate having a first wicking capacity. In one example, the GDL contains carbon fibers with a carbonized phenolic binder. Referring to Figure 4, the carbon substrate 46 is coated with a modifier material 48 that provides the GDL 34 with a second wicking capacity that is greater than the first wicking capacity. The modifier increases the wicking capacity by creating more hydrophilic pores and/or smaller pores with better wicking due to capillary forces.

In one example, the carbon fibers of the MiPL or MaPL can be catalyzed to provide additional electrode surface area for oxygen reduction. The added surface area for oxygen reduction is associated with an additional volume of water storage capacity. For example, the MaPL is on the order of 175 μm thick and contains approximately 14 μL volume compared to the 0.5 μL of volume for a cathode catalyst layer. This is desirable whether product water is in the supercooled state or as ice. In another example, U 107 (available from Mitsubishi), which contains carbon fibers with a carbonized phenolic binder, was treated with an ink composed of 63 weight percent of 850 EW NAFION (DuPont DE2029) and 37 weight percent Pt/C (TKK 10V50E, 47w%Pt on VULCAN XC-72), on a dry weight basis. On a dry basis 1.4 mg/cm² of ink was added to U107, which is calculated to be about 0.5 μ coating on each PA-0008056-WO fiber. The ink was diluted with solvent so that it completely filled the GDL. The ink coated and bridged some of the fibers. The bridges were mainly perpendicular to the plane of the GDL, so their impact on performance is small, verified experimentally. Also, the coating was a lighter color within the GDL because of the Pt. The increased water storage capacity may simply be a result of small, hydrophilic pores introduced into the MiPL or MaPL that draw water away from the catalyst layer. This mechanism may only work if product water is in the supercooled state. The mechanism was further divided into two sub-mechanisms, water stored in meso-pores of the added catalyst layer and those stored in the nano-pores of the NAFION of the added catalyst layer.

In another example, an ionomer or other hydrophilic polymer, such as polystyrene sulfonic acid, is added to the MiPL or MaPL. Other polymers such as hydrocarbon analogs of NAFION or hydrocarbon membrane materials may be added. As an example, NAFION was added to U 107. The same amount and type of NAFION used in the ink was added in the same manner as the ink, described above. The increase in water storage capacity is about 60% of that observed with the UL ink even though the amount of NAFION is the same.

In another example, fibers of the MiPL or MaPL are coated and impregnated with materials having small, wettable pores, such as carbon black. Examples are inks made from Black Pearl 1000 carbon or Vulcan carbon (XC72), both available from Cabot. These may be immobilized with polyvinyl alcohol. Wettability may be induced by oxidation or surface treatment with tin and/or titanium hydroxide applied by a sol gel route. The GDL can also be coated with a polytetrafluoroethylene, such as TEFLON, which is mixed with carbon black to improve hydrophilicity. The concepts disclosed may be combined with other fuel cell water-management concepts taught elsewhere. For example, the disclosed concepts could be used in conjunction with a composite membrane that has improved water storage and transport properties, as taught in U.S. Patent Application 2009/0017344, assigned to the present

Assignee. PA-0008056-WO

Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.

Claims

PA-0008056-WOCLAIMS

1. A fuel cell comprising: a porous structure including a material having a non-linear adsorption isotherm that absorbs water below a first temperature and desorbs water at a second temperature that is greater than the first temperature.

2. The fuel cell according to claim 1, comprising a proton exchange membrane having a side that is in engagement with a catalyst layer; and a gas diffusion layer in engagement with the catalyst layer opposite the proton exchange membrane, the gas diffusion layer in fluid communication with a flow field; and the material arranged at an interface between the catalyst layer and the gas diffusion layer.

3. The fuel cell according to claim 2, wherein material is provided in the catalyst layer.

4. The fuel cell according to claim 2, wherein the gas diffusion layer includes a microporous layer and a macroporous layer in engagement with one another, the microporous layer have pores smaller than pores within the macroporous layer, the material provided in the microporous layer.

5. The fuel cell according to claim 2, wherein the material is a generally a non- conductive zeolite.

6. The fuel cell according to claim 1, wherein the first temperature is approximately the freezing point of water. PA-0008056-WO

7. A fuel cell comprising: a catalyst layer with greater than a 40% porosity that contains a hydrophilic ionomer including a perfluoro sulfonic acid material of equivalent weight of less than 1100 per equivalent weight gram.

8. The fuel cell according to claim 7, comprising a proton exchange membrane having a side that is in engagement with a catalyst layer; and a gas diffusion layer in engagement with the catalyst layer opposite the proton exchange membrane, the gas diffusion layer in fluid communication with a flow field, the hydrophilic membrane being at least one of the catalyst layer and proton exchange membrane.

9. The fuel cell according to claim 8, wherein the hydrophilic membrane is approximately 10-35 μm thick.

10. The fuel cell according to claim 9, wherein the proton exchange membrane includes the hydrophilic membrane, and the proton exchange membrane is 18-35 μm thick.

11. The fuel cell according to claim 9, wherein the catalyst layer includes the hydrophilic membrane, and the catalyst layer is 5-16 μm thick.

12. The fuel cell according to claim 7, wherein the perfluoro sulfonic acid material comprising an equivalent weight 800-1100 per equivalent weight gram.

13. The fuel cell according to claim 7, wherein the hydrophilic membrane includes 0.2-0.3 volume fraction of perfluoro sulfonic acid material. PA-0008056-WO

14. A fuel cell comprising: a proton exchange membrane having a side that is in engagement with a catalyst layer; and a gas diffusion layer arranged in engagement with the catalyst layer opposite the proton exchange membrane and in fluid communication with a flow field, the gas diffusion layer comprising a carbon substrate having a first wicking capacity, the carbon substrate coated with a modifier material that provides the gas diffusion layer with a second wicking capacity that is greater than the first wicking capacity.

15. The fuel cell according to claim 14, wherein the modifier material is a hydrophilic material including sulfonic acid groups.

16. The fuel cell according to claim 15, wherein the hydrophilic material includes polystyrene sulfonic acid.

17. The fuel cell according to claim 15, wherein the hydrophilic material includes perfluoro sulfonic acid.

18. The fuel cell according to claim 14, wherein the carbon substrate is a carbon fiber material having a carbonized phenolic binder.

19. The fuel cell according to claim 18, wherein the modifier material includes carbon black untreated or treated with polyvinyl alcohol and at least one of tin and titanium hydroxide.

20. The fuel cell according to claim 18, wherein the modifier material includes polytetrafluoroethylene mixed with carbon black.