Fuel cell MEA with combined metal gas diffusion layer and microporous layer

Abstract

The invention describes a membrane electrode assembly for use as a transport layer in polymer electrolyte fuel cells, the assembly comprising a porous metal gas diffusion layer (GDL) (20) and a catalyst layer (40) with a microporous layer (MPL) (30) interposed between them, the MPL (30) being constructed to fill the pores of the GDL (20) and coat the surface thereof.

Claims

1. A membrane electrode assembly comprising a bipolar plate comprising a series of ribs and channels, a porous metal gas diffusion layer (GDL) having a tortuosity of less than 1.5, and a catalyst layer, with a microporous layer (MPL) interposed between the GDL and the catalyst layer, wherein the porous metal gas diffusion layer comprises a repeating pattern of holes and bridges, the widths of the individual holes and bridges being defined by the following equations (1)-3):
W.sub.H2W.sub.B(1)
2(W.sub.H+W.sub.B)=L.sub.C+L.sub.R(2)
L.sub.CL.sub.R(3) and wherein W.sub.H represents the width of the holes and W.sub.B represents the width of the bridges of the metal GDL and L.sub.C represents the width of the channels and L.sub.R represents the width of the ribs of the bipolar plate of the electrode respectively.

2. A membrane electrode assembly according to claim 1 wherein the metal GDL and bipolar plate include an alignment aid in the form of one or more complimentary engaging and mating formations, which when in register ensure correct alignment of the metal GDL with the bipolar plate.

3. A membrane electrode assembly according to claim 2 wherein the complimentary engaging and mating formations comprise a pin and a hole for receiving the pin.

4. A membrane electrode assembly according to claim 2 wherein the alignment aid is located within the active area of the membrane electrode assembly.

5. A membrane electrode assembly according to claim 2 wherein the complementary engaging and mating formations are located on abutting surface of the bridges of the metal gas diffusion layer and the ribs of the bipolar plate.

6. A membrane electrode assembly according to claim 5 wherein the complementary engaging and mating formations comprise ridges on one surface and complimentary grooves on the other surface in abutment therewith, wherein the formations are orientated at right angles to the channels of the bipolar plate.

7. A polymer electrolyte fuel cell including a membrane electrode assembly according to claim 1.

8. A membrane electrode assembly according to claim 3 wherein the pin is located on the bridge area of the metal GDL and the corresponding hole is located on the rib area of the bipolar plate.

Description

EMBODIMENTS OF THE INVENTION

(1) Embodiments of the invention are described below with reference to the accompanying drawings and examples:

(2) FIG. 1: is a cross-sectional schematic of fuel cell bipolar plate, metal GDL and catalyst layer

(3) FIG. 2: is a cross-sectional schematic of fuel cell bipolar plate, metal GDL, carbon black MPL on the surface and in the holes of metal GDL, and catalyst layer according to the invention;

(4) FIG. 3: is a cross-sectional schematic of fuel cell bipolar plate, metal GDL, flake based MPL on the surface and in the holes of metal GDL, a second carbon black MPL on the flake based MPL, and catalyst layer, according to the invention;

(5) FIG. 4: is a cross-sectional schematic of fuel cell bipolar plate, metal GDL, 3 layer MPL design consisting of (i) larger particle MPL in metal GDL holes, (ii) flake based MPL on surface of metal GDL and (iii) carbon black MPL on flake based MPL, and catalyst layer, according to the invention;

(6) FIG. 5: is a schematic of hole and bridge dimensions of the metal GDL as presented in equations (1)-(3), according to the invention;

(7) FIG. 6: is a schematic showing enhanced rigidity of the metal GDL as a result of ridges on the metal GDL and corresponding grooves on the ribs of the bipolar plate, according to the invention;

(8) FIG. 7: is a cross section of a prior art pores in metal GDL's;

(9) FIG. 8: Is an illustration of a prior Art metal GDL of U.S. Pat. No. 7,785,748 B2;

(10) FIG. 9: Is a schematic showing the difference in effectiveness between circular and-non circular pores during wetting phase;

(11) FIG. 10: Is a graph illustrating some of the problems faced with the prior art GDL of US778574 B2

(12) FIG. 11; is a schematic depicting best and worst case scenarios in aligning the holes in the GDL of the invention with the land/channel design of the bipolar plate;

(13) FIG. 12; is a schematic of a conventional GDL as known in the art, illustrating the reduced diffusivity of the gas to the catalyst layer;

(14) FIG. 13; is a schematic illustrating the effect of no GDL on diffusion;

(15) FIG. 14; is a schematic illustrating the effect of a thinner Carbon GDL layer on gas diffusion; and

(16) FIG. 15: is a Table (Table 1) summarizing fuel cell performance and high frequency resistance (HFR) of different metal GDL-MPL designs

(17) In FIG. 1, the basic construction of a membrane electrode assembly (MEA) is shown to comprise a fuel cell bipolar plate (10), metal GDL (20) and catalyst layer (40). The catalyst layer is on the surface of the metal GDL. The reactant gases are supplied through the gas channels (12) of the bipolar plate (10) and then diffuse through the holes or pores (21) of the metal GDL. Similar to problem 1 and 2 of the Zhang et al. invention, water can accumulate in the hole (21) of the metal GDL and not all reactant gas is accessible to the catalyst layer (40) as a result of the bridge (22) of the metal GDL. Furthermore the ribs (11) of the bipolar plate can fully or partially cover a pore (21) of the metal GDL making it difficult for reactant gas to enter the hole or for liquid water to leave the hole and enter the channel of the bipolar plate.

(18) FIG. 2 illustrates an improvement on the construction of the MEA as shown in FIG. 1, according to the invention and which addresses the issue of water accumulation. A MPL (30) was inserted between the catalyst layer (40) and the metal GDL (20). The area of the MPL (31) prevents accumulation of water at the interface of the catalyst layer and metal GDL and also enhances diffusion of gasses to the catalyst layer under the bridge (22) area of the metal GDL. To further prevent water accumulation the holes of metal GDL were also filled with the MPL material (32). The MPL in this case is a standard MPL design, made of carbon-black (CB) powder and polytetrafluoroethylene (PTFE). The CB/PTFE MPL forms nano-sized and hydrophobic pores. The pores prevent the growth of large liquid water droplets and limit the liquid water to small finger like particles as has been illustrated in prior art.

(19) The MPL is typically applied onto a standard carbon GDL in the form of ink containing water, CB and PTFE. The GDL/MPL is then sintered for 30 min at 350 C. The viscosity of the MPL ink is such that the dense fibers of GDL prevent the ink from penetrating into the GDL resulting in a layer on the surface of the GDL. If the same MPL ink is used in the case of the metal GDL, the ink will penetrate through the holes of the metal GDL (21). To fill the holes of the metal GDL and subsequently form a layer on the metal GDL, the viscosity of the MPL ink must be increased using a viscous agent like polyvinyl alcohol (PVA) resulting in an ink viscosity in the range of around 1000 cP10000 cP. The viscous agent should be soluble in water and have decomposition temperature below 400 C. The glass transition temperature of PTFE is 320 C.340 C., and the decomposition temperature of PTFE is approximately 420 C. The viscous MPL is coated on one side of metal using a doctor blade technique and by applying pressure during the application process. The pressure is needed to force the ink into the holes of the metal GDL.

(20) Turning now to FIG. 3, a further improvement in the MEA construction is shown which enhances gas diffusion under the bridges (22) of the metal GDL. In this form of the invention, an anisotropic flake based MPL (130) is coated on the surface of the metal GDL and also made to fill the pores of the metal GDL. A conventional CB MPL is then coated on the surface of the anisotropic MPL. The anisotropic MPL is composed of flake like shaped particles and PTFE and enhances gas diffusion due to its horizontal layered structure. The flake material can be carbon, silver or titanium flakes. During the application of the flake based MPL, the orientation of the flakes may become disordered due to the sheer stress of the doctor blade. To maintain the horizontal layered structure the flake MPL should be first be coated as a high viscous ink similar to the CB MPL and then again as a lower viscous ink in the range of 10 cP1000 cP. The first coating forces the flake material into the metal GDL holes and the second coating at lower viscosity ensures the horizontal structure of the flakes is maintained in the final MPL layer (131). The dimensions of the flakes are in the micron range and as result the flake based MPL will have pores in the micron range as well. The larger pores compared to a conventional CB MPL can lead to larger water particles forming in the MPL. This is undesirable and can prevent reactant gases reaching the catalyst layer. To prevent this formation of larger liquid water particles a CB MPL (133) is coated onto the flake based MPL such that the CB MPL is in contact with the catalyst layer. In situations where the flake material is silver or titanium, the CB MPL prevents electrochemical corrosion of the metal flakes by acting as a barrier and preventing direct contact of the flakes with the catalyst layer.

(21) FIG. 4 shows a modification to the MPL design of FIG. 3 which permits improved gas diffusion through the metal GDL. The modification results in larger pores in the MPL which enhances gas diffusion horizontally. In this form of the invention, a larger particle MPL (232) was deposited in the holes of the metal GDL. The result is a 3-layer MPL design comprising a CB MPL next to the catalyst layer (233), an anisotropic flake based MPL on top of the metal GDL (231) and a larger particle MPL (232) in the holes of the metal GDL. The CB MPL prevents formation of large liquid water particles; the flake based MPL enhances gas diffusion horizontally under the bridge of the metal MPL, and the larger particle MPL enhances vertical gas diffusion through the holes of the MPL as a result of its larger pores. The relative pore sizes of the MPLs are CB MPL<flake MPL<larger particle MPL. The material for the larger particle MPL may be carbon, titanium or silver with particle diameter in range of 1 m to 10 m.

(22) In FIG. 5, the gas diffusion from the bipolar plate to the catalyst is addressed by modifying the relative dimensions of the metal GDL and bipolar plates. A specific arrangement of rectangular holes in the metal GDL is provided which results in improved gas diffusion and more uniform gas supply to the catalyst layer.

(23) In FIG. 5, the contact areas of the land and rib of the bipolar plate on the metal GDL, referred to as Under Channel and Under Rib respectively. W.sub.H and W.sub.B represent the width of the hole and the width of the bridge of the metal GDL whilst L.sub.C and L.sub.R represent width of the channel and the width of the rib respectively. If these dimensions follow equations (1)-(3) then two thirds of the area of some of the holes (21) of the metal GDL will be exposed to the channel area (12) of the bipolar plate (10) and the remaining third of these holes will be covered by the rib (11) of the bipolar plate (10). If the metal GDL and bipolar plate are then aligned correctly a repeating pattern is created such that there is one hole (X) completely exposed to the channel next to the hole (Y) which is two thirds exposed to the channel area. These dimensions on the metal GDL and bipolar plate lead to better gas diffusion and more uniform gas supply to the catalyst layer.

(24) Referring back to FIG. 2, and for better alignment of the metal GDL and the bipolar plate, the metal GDL (20) should have at least one pin (50) which registers with a hole (60) in the bipolar plate (10). The pin (50) should be located on the bridge area (22) of the metal GDL and the hole (60) should be on the rib area (11) of the bipolar plate (10). The location of the pin and hole should be within the active area of the MEA so that there is no effect on the gasket and sealing system.

(25) FIG. 6 shows how the compression pressure distribution onto the catalyst layer can be improved. The metal GDL can be made to have ridges (23) on top of the bridge areas. The ridges (23) on the metal GDL (20) should be at right angles to the channels of the bipolar plate (10). With this configuration the ridges enhance the rigidity of the metal GDL resulting in a more uniform pressure distribution from the bipolar plate onto the catalyst layer. More uniform pressure reduces the contact resistances between the catalyst layer, CB MPL, flake based MPL and larger particle MPL. To improve contact between the metal MPL with the ridges and the bipolar plate, the bipolar plate (10) should have grooves (13) on the rib area which align with the ridges on the metal GDL. This design can however result in the ridges trapping liquid water in the channels of the bipolar plate. To mitigate this, the fuel cell should be configured such that the channels are lying vertically to ensure that gas flows downwards and liquid water removal is assisted by gravity.

EXAMPLES

(26) The following examples illustrate the construction and testing of the various embodiments of the invention with the results of the testing being provided in Table 1 of FIG. 15.

(27) Preparation of Catalyst Coated Membrane (CCM) and Membrane Electrode Assembly (MEA)

(28) Catalyst ink was first prepared by mixing 40 wt % platinum on carbon catalyst (HySA-V40, Mintek, South Africa) Nafion solution (5 wt % ionomer, Ion Power, United States), isopropanol (Sigma-Aldrich) and ultrapure water. The ratio of carbon to ionomer in the ink was 0.65 and the solids content of the ink was 20 wt %. The mixed ink was spray coated onto 125 micron Teflon sheets using a USI Prism 300 ultrasonic spray coater. The coated Teflon sheets were then dried in an oven at 80 C. for 3 hours. The Pt loadings on the coated substrate was 0.4 mg/cm.sup.2 Following the drying step two coated Teflon sheets were hot-pressed on either side of Nafion-XL membrane (Ion Power, United States) using a manual Carver hot-press at 135 C. and 10 MPa for 10 min. The resultant CCM with an active area of 15 cm.sup.2 (3 cm5 cm) was then combined with different GDL-MPL cases as described in the various examples to follow. A fresh CCM was used for each GDL-MPL case.

(29) Fuel Cell Testing

(30) MEAs were evaluated in a single fuel cell set up consisting of bipolar plates and endplates. The bipolar plates had channels of length 50 mm, depth 0.1 mm and width 0.2 mm, and ribs of length 50 mm, height 0.1 mm, and width 0.1 mm. The active area of the bipolar plates was gold-plated resulting in a 1 m thick gold layer on the bipolar plate. The fuel cell was operated at a cell temperature of 80 C., ambient pressure, hydrogen/air at constant flowrates of 0.5 NL/min and 1.0 NL/min respectively and a relative humidity of 100%.

(31) Polarization curve and 1 kHz high frequency resistance (HFR) measurements were measured using a FuelCon test station. Table 1 of FIG. 15 summarizes the cell voltage at 1 A cm.sup.2 and the HFR for the following different examples.

Example 1

(32) Commercial GDL/MPL

(33) A CCM was combined with a commercial GDL with MPL (TGP-H60, Toray, Japan). The GDL is a carbon fiber based paper material and the MPL is made of carbon black and PTFE.

Example 2

(34) Metal GDL with CB MPL on the Surface of the Metal GDL

(35) A CCM was combined with a metal GDL with a CB MPL on the surface of the metal GDL. The metal GDL was manufactured by Meltec Corp. (Japan) and consisted of circular holes of diameter 70 m and bridges between the holes of width 30 m. The holes were chemically etched on a 30 m thick stainless steel (SS316L) foil and gold-plated to leave a 1 m thick gold layer on the surface of the stainless steel. The CB MPL was coated by a doctor blade technique on the surface of the metal GDL using a low viscous MPL ink. The low viscosity ensures the MPL is only on the surface of the metal GDL and not in the holes. The MPL ink was prepared using CB powder (acetylene black from Sigma-Aldrich), PTFE emulsion (Fuel Cell Earth), surfactant (TRITON X-114 from Sigma-Aldrich) and ultrapure water in a weight ratio of 4:1:8:80. Following coating of the MPL, the metal GDL with MPL was sintered at 350 C. for 1 hour.

Example 3

(36) Metal GDL with CB MPL on the Surface and in the Holes of the Metal GDL

(37) A CCM was combined with a metal GDL with a CB MPL on the surface and in the holes of the metal GDL. The CB MPL was coated on the surface of the metal GDL and forced into the holes of metal GDL using the doctor blade technique with a viscous MPL ink and by applying a 20 kg load on the doctor blade. The MPL ink was prepared using CB powder (acetylene black from Sigma-Aldrich), PTFE emulsion (Fuel Cell Earth), surfactant (TRITON X-114 from Sigma-Aldrich), PVP (Sigma-Aldrich) and ultrapure water in a weight ratio of 4:1:8:4:4. Following coating of the MPL, the metal GDL with MPL was sintered at 420 C. for 1 hour.

Example 4

(38) Metal GDL with Flake Based MPL on the Surface and in the Holes of the Metal GDL and Second CB MPL on the Flake Based MPL

(39) A CCM was combined with a metal GDL with a flake based MPL on the surface and in the holes of the metal GDL and a CB MPL on the surface of the flake based MPL. The flake material was silver and the flake MPL was coated on the surface of the metal GDL and forced into the holes of metal GDL using the doctor blade technique with a viscous MPL ink and by applying a 20 kg load on the doctor blade. The MPL ink was prepared using silver flakes (10 m diameter and 1 m thickness from Sigma-Aldrich), PTFE emulsion (Fuel Cell Earth), PVP (Sigma-Aldrich) and ultrapure water in a weight ratio of 20:1:8:4. Surfactant was not required for silver flakes due to the large particle size. Following coating of the MPL, the metal GDL with MPL was sintered at 420 C. for 1 hour.

Example 5

(40) Metal GDL with MPL Design as Per Example 4 and Metal GDL and Bipolar Dimensions as Per Equations (1)-(3)

(41) A CCM was combined with a metal GDL with new dimensions and with an MPL design as per example 4. The metal GDL was manufactured by Meltec Corp. (Japan) and consisted of rectangular shaped holes with dimensions of 100 m50 m and bridges between the holes of width 50 m. The arrangement of the holes is shown in FIG. 5. The holes were chemically etched on a 30 m thick stainless steel (SS316L) foil and gold-plated to leave a 1 m thick gold layer on the surface of the stainless steel. A silver flake MPL and CB MPL were added as per example 4.

Example 6

(42) Metal GDL as Per Example 5 with Added Ridges on the Metal GDL for Higher Rigidity

(43) A CCM was combined with a metal GDL as per example 5 with additional ridges and with an MPL design as per example 4. The metal GDL was manufactured in the same way as per example 5 except that prior to the gold plating step, another stainless steel (SS316L) foil of 50 m thickness was welded onto the metal GDL using a diffusion welding technique. The combined stainless steel sheet was then chemical etched from the flat side leaving ridges of height 50 m and width 25 m. The metal GDL with ridges was then gold plated as per examples 2-5. A silver flake MPL and CB MPL were added as per example 4. The surface of the ribs of bipolar plates was also chemically etched for the testing of this example. The etching resulted in grooves of depth 50 m on the bipolar plate. The grooves on the bipolar plate were then aligned to the ridges of the metal GDL as shown in FIG. 6.

(44) Results

(45) The results are summarized in Table 1. Example 1 showed no mass transfer limitations and flooding (relatively high voltage at 1 A cm.sup.2) but a high HFR. The narrow ribs on the bipolar plate enhanced the gas diffusion under the ribs reducing mass transfer limitations. However the narrow ribs also result in less contact of the bipolar plate with the carbon GDL increasing the contact resistance.

(46) Examples 2 and 3 show similar HFR but example 2 shows a much lower voltage at 1 A cm.sup.2. The similar HFR results from very similar metal GDL-MPL designs. The HFR is lower than example 1 as the contact between metal GDLs and the narrow ribs of the bipolar plate is better than for carbon GDLs. The low voltage for example 2 indicates severe mass transfer limitations as a result of flooding. The different between example 2 and 3 is the presence of CB MPL in the holes of the metal GDL. In the absence of the CB MPL in the holes as in example 2, liquid water will accumulate in the holes leading to flooding. For example 3 the voltage at 1 A cm.sup.2 is much better than in example 2 indicating that the CB prevents liquid water build up in the holes.

(47) Example 4 showed a lower HFR and higher voltage than examples 1-3. This is because the silver flake based MPL has a higher conductivity than the CB MPL. The improved voltage is as a result of the horizontal arrangement of the silver flakes. The horizontal arrangement enhances gas diffusion especially under the bridge areas of the metal GDL.

(48) Example 5 shows a very similar result to example 4. The rectangular holes of the metal GDL with larger openings as used in example 5 should result in better gas diffusion but this does not show up in the voltage at 1 A cm.sup.2. The slightly higher HFR in example 5 is because of larger holes in the metal GDL and therefore less contact of the metal GDL area with the ribs of the bipolar plate.

(49) Example 6 shows a much lower HFR than example 5. The ridges help to uniformly distribute the compression pressure onto the active area and as a result this will reduce the contact resistances between catalyst layer, MPL, metal GDL and bipolar plate.