International Journal of Hydrogen Energy 30 (2005) 1297 1302 www.elsevier.com/locate/ijhydene

Bipolar plates for PEM fuel cells: A reviewAllen Hermanna, , Tapas Chaudhuria, b , Priscila Spagnolba Department of Physics, University of Colorado, Boulder, CO 80309-0390, USA b National Renewable Energy Laboratory, Golden, CO 80401, USA

Available online 2 June 2005

Abstract Bipolar plates (BPs) are a key component of proton exchange membrane (PEM) fuel cells with multifunctional character. They uniformly distribute fuel gas and air, conduct electrical current from cell to cell, remove heat from the active area, and prevent leakage of gases and coolant. BPs also signicantly contribute to the volume, weight and cost of PEM fuel cell stacks. Hence, there are vigorous efforts worldwide to nd suitable materials for BPs. The materials include non-porous graphite, coated metallic sheets, polymer composites, etc. This paper reviews various types of materials, in use and proposed, for BPs and critically examines their physical and chemical properties. 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1. Introduction Fuel cells are expected to play a major role in the economy of this century and for the foreseeable future [1]. It is anticipated that the development and deployment of economical and reliable fuel cells would usher in the sustainable hydrogen age [2]. The polymer electrolyte membrane fuel cell (PEMFC) is a cell of choice for future automotive propulsion applications, in part because of its modestly low operation temperature (< 100 C) [3]. The PEMFC system is an energy system that can convert hydrogen and oxygen (or air) to electricity with water as the only byproduct, and hence is also of great interest [4] from an environmental point of view. A signicant part of the PEMFC fuel cell stack is the bipolar plates (BPs) [5], which account for about 80% of total weight and 45% of stack cost [6]. They are designed to accomplish many functions, such as distribute reactants uniformly over the active areas, remove heat from the active areas, carry current from cell to cell and prevent leakage of reactants and coolant. Furthermore, the plates must be of Corresponding author.

inexpensive, lightweight materials and must be easily (and inexpensively) manufactured. Efforts are underway to develop BP materials that satisfy these demands. The main materials studied to date include [7] electro graphite, sheet metal (coated and uncoated) and graphite polymer composites. In a recent review, Mehta and Cooper [7] analyzed the design and manufacturing of proton exchange membrane (PEM) fuel cells for vehicular application and also discussed the engineering aspects of BP materials. Earlier, Borup and Vanderborgh [5] briey reviewed different BP materials. While conducting detailed design analysis of BP for PEM fuel cells, Cooper [8] also elucidated a number of properties desirable in a BP material. Multifaceted characteristics of BP and amalgamation of different physical and chemical properties have brought to light a host of new materials. This review provides a current overview of BP materials and their characteristics.

2. BPs and their material properties A typical PEM fuel cell (Fig. 1) essentially consists of an anode backing, membrane/electrode assembly (MEA) and

E-mail address: Allen.hermann@colorado.edu (A. Hermann).

0360-3199/$30.00 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2005.04.016

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A. Hermann et al. / International Journal of Hydrogen Energy 30 (2005) 1297 1302

Hydrogen flow field

eAir and water out

Bipolar Plate (Anode)

Bipolar Plate (Cathode)

2H2

4H + 4e

+

-

O2 + 4H + 4e

+

-

2H2O

Hydrogen outlet

Air (oxygen) flow field

Anode backing Membrane/ Electrode Assembly

Cathode backing

Fig. 1. Schematic diagram of a polymer electrolyte membrane fuel cell.

cathode backing sandwiched between two BPs (anode and cathode) [9]. The BPs have the following functions to perform: (1) (2) (3) (4) (5) To To To To To distribute the fuel and oxidant within the cell. facilitate water management within the cell. separate individual cells in the stack. carry current away from the cell. facilitate heat management.

The materials investigated so far can be broadly classied as: 1. Non-metal: non-porous graphite/electrographite. 2. Metals: non-coated and coated. 3. Composites: polymercarbon and polymermetal. The detailed classication of BP materials is shown in Fig. 2.

Hence, the materials that BPs are made of would have different physical and chemical properties related to each function as listed in Table 1. Other important physical properties of BP materials are: Coefcient of thermal expansion. Density. Hydrophobicity. For a material to qualify for BP, Borup and Vanderborgh [5] and Mehta and Cooper [7] have suggested the following criteria/properties: Electrical conductivity: plate resistance < 0.01 cm2 . Thermal conductivity: as high as possible. Hydrogen/gas permeability: < 104 cm3 /s cm2 . Corrosion resistance: corrosion rate < 0.016 mA/cm2 . Compressive strength: > 22 lb/in2 . Density: < 5 gm/cm2 .

3. Non-porous graphite Traditionally, the most commonly used BP material is graphite, both natural as well as synthetic, because it has excellent chemical stability to survive the fuel cell environment. It also has very low resistivity, resulting in highest electrochemical power output. However, it is fraught with problems of high cost, low mechanical strength and the need for machining to form the ow channels [10]. Hence, the search is on for better alternatives as described below.

4. Metals Metals, as sheets, are potential candidates for BP material since they have good mechanical stability, electrical conductivity and thermal conductivity and can be easily stamped

A. Hermann et al. / International Journal of Hydrogen Energy 30 (2005) 1297 1302 Table 1 Physical and chemical properties of BP materials Functions Distribution and management of fuel and oxidants and residual gases and liquids Conduct electrical current Facilitate heat management Separate the individual cells in the stack Physical or chemical properties

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H2 permeability (dry, non-porous plates), bubble pressure (wet, porous plates), corrosion resistance Electrical conductivity Thermal conductivity Compressive strength

Bipolar plates

Non-metal

Non-porous graphite plates

Stainless Steel *Austenitic *Ferritic

Non-coated

Metals

Metal plates

Coated

Bases *Aluminum *Titanium *Nickel *Stainless Steel

*Graphite, polycarbonate, stainless steel Metal based

Coatings Carbon-based *Graphite *Conductive polymer *Diamond-like carbon *Self-assembled monopolymers Metal-based *Noble metals *Carbides *Nitrides

Composites

Carbon based

Resin Thermoplastics *Poly(vinylidene fluoride) *Polypropylene *Polyethylene Thermosets *Epoxy resin *Phenolic resins *Furan resin *Vinyl ester

Filler *Carbon/graphite *Carbon black *Coke-graphite

Fiber *Carbon/graphite *Cellulose *Cotton flock

Fig. 2. Classication of materials for BPs used in PEM fuel cells.

to desired shape to accommodate the ow channels. However, as BPs are exposed to an operating environment with a pH of 23 at temperatures of around 80 C, metal plates are prone to corrosion or dissolution. The dissolved metal ions may lead to poisoning of PEM membrane and hence lowering of ionic conductivity. Moreover, a corrosion layer on the surface of a BP increases the electrical resistance and decreases the output of the cell. In the light of these issues, two types of metallic plates have been studied, non-coated and coated with a protective layer. Metals investigated include aluminum, stainless steel, titanium and nickel. 4.1. Non-coated Stainless steels (SSs) are the only material in this category to have received considerable attention due to their

relatively high strength, high chemical stability, low gas permeability, wide range of alloy choice, and applicability to mass production and low cost [5]. Major concerns have been extent of corrosion (and its products) and the contact resistance of the surface passivation lm. Candidate SSs have been tested and used as BPs by different authors [1113], which showed that corrosion rate is low and PEM cell output is stable for thousands of hours. Recent studies by Wang et al. [14,15] at the National Renewable Energy Laboratory, Golden (USA) show that SS is a strong candidate for BP material. They have found that both austenitic (349TM ) and ferritic (AISI446) SS with high Cr content can be suitable for BP, although AISI446 requires some improvement in contact resistance.

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Table 2 Coating materials for metallic BPs Coatings Base plate materials

product yet due to issues pertaining to corrosion and contact resistance.

5. Polymer composite platesAl Conductive polymer [5] Diamond-like carbon [5] Gold [16,17] Graphite foil [20] Graphite topcoat [20] Indium tin oxide [21] Lead oxide [21] Organic monopolymer [5] Silicon carbide [21] Titaniumaluminum [21] Titanium nitride [21] Oxides [18] Chromium nitride [19] NS, not specied. NS NS SS Ti Ni

NS

Ni/Cr

4.2. Coated Aluminum, stainless steel, titanium and nickel are considered as possible alternative materials for BP in PEM fuel cells. To avoid corrosion, metallic BPs are coated with protective coating layers. Coatings should be conductive and adhere to the base metal without exposing it [5]. Further the coefcient of thermal expansion of the base metal and the coating should be as close as possible to eliminate the formation of micro-pores and micro-cracks in coatings due to unequal expansion [16]. Two types of coatings, carbon-based and metal-based, have been investigated [5,7,16]. Carbon-based coatings include graphite, conductive polymer, diamond-like carbon and organic self-assembled monopolymers. Noble metals, metal nitrides and metal carbides are some of the metal-based coatings. Table 2 presents different types of BP coatings investigated. Recently, there has been keen interest in the use of coated SS (316L) as BPs [17,18]. Wind et al. [17] have shown that the performance of BP made from gold-coated SS 316L is similar to that of graphite. Lee et al. [18] have suggested an electrochemical surface treatment of SS 316L to deposit a suitable coating. Coatings of metallic BP have not been sufciently viable due to pinhole defects. To make defect-free coatings, Brady et al. [19] have recently developed a preferential thermal nitridation process to form pinhole free CrN/Cr 2 N coating on a model NiCr alloy, which show promise with excellent corrosion resistance and negligible contact resistance. However, application of this type of coating on stainless or any other steel is yet to be demonstrated. The above reveals that even though there has been much effort to develop SS (non-coated and coated) bipolar plates, mostly for vehicular applications, there is no commercial

Polymer composites offer alternative routes to BP materials. Composites are lightweight and can be molded into any shape and size, which makes them attractive for PEM fuel stacks. Composite plates can be categorized as metalor carbon-based. Los Alamos National Laboratory has developed a metalbased composite BP [22] based on porous graphite, polycarbonate plastic and stainless steel. Since producing porous graphite plates is not as time consuming or expensive as producing non-porous graphite plates, it can be used while the impermeability is provided by the stainless steel and polycarbonate parts. Stainless steel also provides rigidity to the structure while the graphite resists corrosion. The polycarbonate provides chemical resistance and can be molded to any shape to provide for gaskets and manifolding. The layered plate appears to be a very good alternative from stability and cost standpoints. There has been extensive study on the use of carbonbased composite bipolar plates in fuel cells. Table 3 presents some of the notable ones. As shown, carbon composite bipolar plates have been made using thermoplastic [polypropylene, polyethylene, poly(vinylidene uoride)] or thermosetting resins (phenolics, epoxies and vinyl esters) with llers and with or without ber reinforcement. Recent literature [3336] shows some interesting progress in graphitepolymer composites. Middelman et al. [33] reported the development of conductive composite material with excellent properties and a process for large-scale production of BP. Cho et al. [34] have developed carbon composite BPs whose long-term performances are comparable with graphite. Injection molding of low-cost BP based on a thermoplastic and a carbon compound was reported by Heinzel et al. [35]. Kuan et al. [36] have developed vinyl estergraphite composite BPs by a bulk-moulding compound process with properties similar to graphite plates. Polymergraphite composite BPs are now available in the market and sold by companies, such as DuPont, H2 Economy, ICM Plastics, NedStack, etc. which shows that this route is maturing.

6. Discussion Prospective materials for BPs of PEM fuel cells have been reviewed. From the utilitarian point of view, the benets and drawbacks of each type of BP material are summarized in Table 4. For graphite/carbon composites, the materials are reaching a maturity stage, which is evident by the fact that BPs are now available commercially. However, the electrical

A. Hermann et al. / International Journal of Hydrogen Energy 30 (2005) 1297 1302 Table 3 Different types of carbon composite materials used in BPs Resin Poly(vinylidene uoride) Poly(vinylidene uoride) Polypropylene Mixture of epoxy resin and amine hardener Phenyl-aldehyde resol or novolac Phenyl-aldehyde resol or novolac Reichhold 24-655 phenolic resin Phenol resin or furan resin Phenolic resin Vinyl ester Vinyl ester [36] Refs. [2336]. Filler Carbon/graphite particles Carbon/graphite particles Carbon black, graphite powder Graphite powder Graphite powder Coke-graphite particles Graphite powder Graphite powder Fiber

1301

Carbon/graphite bers

Graphite bers Cellulose bers (not rayon and cellulose acetate) Cellulose bers (not rayon and cellulose acetate) Carbon ber (PAN-based) Cotton ock (graphite/carbon, glass, cotton and polymers)

Graphite powder Graphite powder

Table 4 Characteristics of different types of bipolar plates Graphite (machined) Benets Stability Low specic density Low contact resistance with electrodes High corrosion resistance Drawbacks Expensive to machine Brittle Thick Metal Graphite composites

High thermal conductivity Recyclable Consistency of product

Lower contact resistance High corrosion resistance

Needs coating Membrane poisoning Formation of insulating surface oxide

Low bulk electrical conductivity

conductivities of these composites are still below the target. For example, Middelman et al. [33] reported about 20 S/cm, which is below the target of > 100 S/cm [7]. DuPonts BPs (T8 series) [37] have a bulk conductivity of 2533 S/cm. Recent interest has been on low-cost production of BPs with consistent and acceptable physical and chemical properties. The use of metals as BPs is important for the vehicular industry. Metals offer lower thickness and weight and improved electrical and thermal conductivities. The use of SS would make it cost effective. However, it is clear from the investigations that non-coated SS is still having the problem of a surface-insulating layer. A thin coating of a chemically stable and electrically conducting lm is required before it can be used as BPs. There is signicant ongoing research on both coated metal plates and polymergraphite composites. Even though there has been signicant progress in both the materials, they are yet to attain the long-term reliability and power densities achievable by non-porous graphite. Economics, manufac-

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