GB2494175A

GB2494175A - Bipolar electrode for fuel cell stacks

Info

Publication number: GB2494175A
Application number: GB1115136.2A
Authority: GB
Inventors: Jun Cai; Chun Wang; Muzhong Shen
Original assignee: Individual
Current assignee: Individual
Priority date: 2011-09-01
Filing date: 2011-09-01
Publication date: 2013-03-06
Also published as: CN202917582U; CN102903935A; CN102903935B; GB201115136D0

Abstract

A bipolar electrode (6) includes an electron conducting substrate (6c), two mass diffusion layers (6b) and (6d) for facilitating mass transportation which are attached to the surfaces of the substrate, and two catalyst layers (6a) and (6e) for catalyzing electrochemical reactions. The electron conducting substrate is permeable to gases and liquids so that the mass transportation may be processed inside the electrode. The mass diffusion layer is a hydrophilic or hydrophobic porous structure which may transport liquid and stop gas accessing the catalyst layer or vice versa. The catalyst layer is made by applying catalyst particles with ionomer binders on the top surface of gas diffusion layer by spread, or brush, or print. A fuel cell stack may include one or more bipolar electrodes which combine with polymer electrolyte membranes as repeating units to take the place of conventional units of MEAs and bipolar plates.

Description

Bipolar electrode for fuel ccli stacks

1. Field of Invention

This invention relates to a fuel cell stack for generating electrical energy. More particularly, the invention relates to a bipolar electrode which is a part of a repeating unit applied in a fuel cell sack without the need to use a bipolar plate.

2. Background Tnformation (Description of the Related Art) Fuel cells are elcctrochemical devices which directly convert the chemical energy of a fuel to electrical energy, and typically employ an anode where the fuel is oxidized, a cathode where the oxidant is reduced, and an electrolyte layer for transporting electroactive ions. In conventional proton exchange membrane fuel cells (PEMFCs) or direct methanol fuel cells DMFCs), a proton exchange membrane (also known as ion exchange membrane) which is used as the electrolyte layer, the anode and cathode may be bonded or sealed to form a single integral unit know as the membrane electrode assembly (MEA). Figure 1 shows the schematic structure of an MEA. The MEA is further interposed between two fluid flow plates to form a unit fuel cell. The plates allow reactants access to the anode and cathode of the MBA, and act as current collectors of the unit fuel cell. Figure 2 shows the schematic structure of a unit fuel cell.

Generally the output voltage of a unit fuel cell is limited [11. For example, the output voltage of a PEM fuel cell is usually less than IV. The output voltage of a DMFC fuel cell is usually less than 0.7 V. In practice, a fuel cell stack is necessary by connecting unit cells in series to satisfy the power demand and voltage requirement of practical devices. The output voltage of the fuel cell stack increase with the number of its unit cells. A conventional fuel cell stack is made with a plurality of MEAs and a plurality of bipolar plates which are interposed between two fluid flow plates. The bipolar plates and the fluid flow plates play the rule of current collectors, mass flow conduits and brackets of the fuel cell stack, Figure 3 shows the schematic structure of a fuel cell stack with two MEAs connected with a bipolar plate.

Bipolar plates contain an array of channels formed in the surface of the plate contacting an MEA, which function as the flow field. These are usually made of graphite composites or metals with high corrosion resistances [2]. The requirements of materials for bipolar plates are: good electrical conductivity, high thermal conductivity, high chemical and corrosion resistance, mechanical stability toward compression forces, and impermeable to gases and liquids. This causes the manufacturing costs of bipolar plates to be high and represents about a third of the overall cost of a fuel cell stack [3], Besides, bipolar plates also take up a majority of the volume and weight of a fuel cell stack, Bipo'ar plates are by weight, volume, and cost one of the most significant parts of a fuel cell stack [4, 5].

3. Summary of the invention

To reduce the cost, weight and volume of a fuel cell stack, the present invention proposes a bipolar electrode which is a part of a repeating unit applied in a fuel cell stack. The bipolar electrode is made with applying anode catalyst and cathode catalyst on surface of each side of an electron conducting substrate which may facilitate mass transportation.

The fuel cell stack uses repeating units of bipolar electrodes and electrolyte layers to take the place of conventional units of MEAs and bipolar plates which will greatly reduce the weight. volume, and cost.

The fuel cell stack has at least one bipolar electrode.

Preferably. the electron conducting substrate of the bipolar electrode has porous structure and chemical and corrosion resistance.

Preferably, mass diffusion layers are applied on each side of the electron conducting substrate.

Preferably, a proton exchange membrane or an ion exchange membrane is used as the electrolyte layer to combine with a hi polar electrode to make the repeating unit of fuel cell stack.

Preferably. liquid fuel, for example, methanol solution is used as the fuel of the fuel cell stack.

4. Introduction to drawings

An example of the invention will now be described by referring to the accompanying drawings: Figure 1 shows the schematic structure of a conventional MEA; and Figure 2 shows a conventional unit fuel cell; and Figure 3 shows a conventional fuel cell stack with a bipolar plate; and Figure 4A shows the structure of a single-polar electrode (cathode); and Figure 4B shows the structure of a bipolar electrode; and Figure 4C shows the structure of a single-polar electrode (anode); and Figure 5 shows a ffiel cell stack with a bipolar electrode.

5. Description of reference numbers and signs

I: Polymer electrolyte membrane, 2: Electrode (anode), 3: Electrode (cathode), 4: Fluid flow plate 5: Bipolar plate 6: Bipolar electrode 1: Single-polar electrode (cathode) 8: Single-polar electrode (anode) 9: Electrode bracket 10: Mass flow conduit 2a: Electrode (anode) catalyst layer 2b: Electrode (anode) mass diffusion layer 2c: Electrode (anode) substrate 3a: Electrode (cathode) catalyst layer 3b: Electrode (cathode) mass diffusion layer 3c: Electrode (cathode) substrate 4a: Mass flow channel in fluid flow plate 5a: Mass flow channel in bipolar plate 6a: Electrode (anode) catalyst layer 6b: Electrode (anode) mass diffusion layer 6c: Electrode substrate 6d: Electrode (cathode) mass diffusion layer 6e: Electrode (cathode) catalyst layer

6. Detailed description of the invention

Figure 4B shows an exemplary cross section diagram of a bipolar electrode (6) of the present invention. The electrode substrate 6c) is made of electron conducting materials which preferably have a porous structure with chemical and corrosion resistance. For example, carbon, graphite, or metal and metal alloys with chemical and corrosion resistance may be used as the material of the electrode substrate. The electrode substrate is permeable to gases and liquids. For example, a piece of carbon paper with 1 mm thickness may be used as the electrode substrate.

For enhancing the effect of mass transportation, the mass diffusion layers (6b) and (ôd) may be applied on the surfaces of each side of the substrate (6c). The mass diffusion layer is an electron conducting micro-porous structure which increases the specific area of the electrode, For example, a mass diffusion layer can be made by applying carbon black with polymer binder on the surface of the electrode substrate by spraying or brushing or printing An example of polymer binder is polytetrafluoroethylene (PTFE); another example of polymer binder is perfluorinated ionomer. The particle size of the carbon black may be around 30 nm and the thickness of the gas difftsion layer may be around 0.2 mcii. For effectively moving side products of the electrochemical reaction away from electrodes, the mass diffusion layers may be made of hydrophobic or hydrophilic according to specific fuel cell requirements. For example, in a direct methanol fuel cell stack, the electrode (anode) mass diffusion layer Gb) is hydrophilic which inclines to push gas bubbles out of the porous structure when the polymer binder is perfluorinated ionomer; the electrode (cathode) mass diffusion layer (Gd) is hydrophobic which inclines to push water solution out of the porous structure when the polymer binder is PTFE.

The electrode catalyst layers (Ga) and (6e) are made by applying catalyst particles with ionomer binder on the top surface of the mass diffusion layer by spraying or brushing or printing. An example of cathode catalyst is Pt particles distributed on the surface of carbon black. An example of anode catalyst is Pt/Ru alloy particles distributed on the surface of carbon black. An example of the ionomer binder is perfluorinated ionomer which may supply electrolyte for three-phase zone of the electrode to facilitate relevant electrochemical reactions [6]. The thickness of the catalyst layer may bc around 5 pm. The catalyst layers may be made from selective catalysts which only facilitate anode chemical reactions or cathode chemical reactions.

An example of selective anode catalyst and selective cathode catalyst is alloy of RhSe and Co/Fe porphoiyns respectively. Other examples of selective catalysts may be made from the group 4 and 5 metals [7].

Figure 4A and Figure 4C show the structure of single-polar electrodes of a fuel cell stack. In a fuel cell stack, there are only two single-polar electrodes which are cathode (7) and anode (8). The substrate of a single-polar electrode (Gc) is same as the substrate of bipolar electrode. In comparison to the bipolar electrode, there is only one gas diffusion layer and one catalyst layer on the substrate of the single-polar electrode. The mass diffusion layer (6b) and catalyst layer (Ga) of anode (8) are same as the anode mass diffusion layer and anode catalyst layer on the left side of bipolar electrode (b) as shown in figure 4B. The mass diffusion layer (Gd) and catalyst layer (6e) of cathode (7) are same as the cathode mass diffusion layer and cathode catalyst layer on the right side of bipolar electrode (6) as shown in figure 4W The present invention of the bipolar electrode is applied in a fuel cell stack without using a bipolar plate. Figure 5 shows an example of a fuel cell stack made with one bipolar electrode. In the fuel cell stack, one bipolar electrode (6) and two polymer electrolyte membrane (I) are interposed between an anode (8) and a cathode (7) which is held by electrode brackets (9). Compared with the conventional fuel cell stack in figure 3, the fuel cell stack with present invention will reduce the weight, volume, and cost dramatically. The more units of bipolar electrodes and polymer electrolyte membranes which are used in a fuel cell stack, the higher the export voltage; and the more striking is the advantage of present invention.

h comparison to the conventional fuel cell stack, which uses mass flow channels of fluid flow plate (4a) and bipolar plate (5a) to transport reactants to the anode and cathode of the MEA separately, the fuel cell stack with present invention transports electrochemical reactants inside porous electrodes (6c) through mass flow conduit (10) as shown in figure 5. As an example of DMFC fuel cell stack with present invention, anode mass diffusion layer (6b) is made of hydrophilic porous structure which may hold a methanol solution to form a liquid film and transport the methanol solution to the anode catalyst layer with help of capillarity; and cathode mass diffusion layer (6d) is made of hydrophobic porous structure which may stop the methanol solution and facilitate air to access the cathode catalyst layer. Another example of fuel cell stack with present invention is to use selective catalysts in the anode and cathode catalyst layers, When the fuel and air are transported inside the porous electrodes (6c) and difThsed to the catalyst layers 6a and Ge, the electrochemical reactions occur on the anode and cathode respectively and selectively with help of selective catalysts.

7. Reference [1] C. Karimi, J.J. Baschuk, X. Li. Peifoimance analysis and optimization of PEM fuel cell stacks using flow network approach, Journal of Power Sources 147 (2005) 162-17 7 [2] H. Tawfik, Y. Hung, D. Mahajan, Metal bipolar plates for PEM fuel cell A review. Journal of Power Sources 163 (2007) 755-767 [3] Brent cunningham* and Donald 0. Baird, The development of economical bipolar plates for fuel cell, Journal of Materials Chemistry, 2006, 16,4385-4388 [4] 1, Bar-On, R. Kirchain and R. Roth, Technical Cost Analysis of PEM Fuel Cells, J. Power Sources, 2002, 109, 71-75.

[5] E. Middleman, W. Kout, B. Vogelaar, J. Lenssen and E. de Waal. Bipolar Plates for PEM Fuel Cells, J. Power Sources, 2003, 118. 44-46.

[6] M. Watanabe. NI. Tozawa and S. Motoo, A gas diffusion electrode for oxygen reduction working at 100% utilization of catalyst clusters, J. Electroanalytical Chemistry and Interfacial Electrochemistry. 183(1985) p39 1-394 [7] K. Otal, Y. Ohgi. K-D. Nam, K. Matsuzawa. S. Mitsushima and A. Ishihara, Development of group 4 and 5 metal oxide-based cathodes for polymer electrolyte fuel cell. Journal of Power Sources 196 (2011) 5256 -5263

Claims

<claim-text>8. Claims 1. A bipolar electrode which is a part of repeating units applied in a fuel cell stack, is made by applying an anode catalyst layer and a cathode catalyst layer on the surface of each side of a piece of electron conducting aibstrate.</claim-text> <claim-text>2. The fuel cell stack according to claim 1, in which the stack is composed of an anode electrode, a cathode electrode and repeating units of bipolar electrodes and proton exchange membranes with at least one bipolar electrode.</claim-text> <claim-text>3. The electron conducting substrate according to claim 1, in which the substrate has chemical and corrosion resistance, and is permeable to gases and liquids.</claim-text> <claim-text>4. The electron conducting substrate according to claim 1, in which the gas diffusion layers are made on the surface of each side of the substrate.</claim-text> <claim-text>5. The gas difthsion layer according to claim 4, in which the layer is a hydrophilic or hydrophobic porous structure.</claim-text> <claim-text>6. The anode catalyst layer and the cathode catalyst layer according to claim 1.in which catalyst particles with ionomer binders are applied on the top surface of gas diffusion layer by spread, or brush, or print.</claim-text> <claim-text>7. The anode catalyst layer according to claim I, in which the catalyst has or only has electrocatalytic activity to oxidation reaction.</claim-text> <claim-text>8. The cathode catalyst layer according to claim 1, in which the catalyst has or only has electrocatalytic activity to reduction reaction.</claim-text>