A review of metal separator plate materials suitable for automotive PEM fuel cells

Abstract

In this paper we review the range of materials which have been studied for use as separator plates in automotive PEM fuel cells, with particular emphasis on metals. For commercial application separator plates must be resistant to corrosion, durable and offer low contact resistance in a fuel cell stack. Graphite and carbon composite materials have been widely used, as they offer durability and give reliable performance. For portable and transport applications, new materials offering reduced cost, weight and volume are needed. Metal plates may offer a compact, low cost alternative to graphite and carbon based composites. However in the aggressive fuel cell environment, corrosion of metal plates can significantly effect fuel cell performance while passivation can also lead to increased ohmic losses. The only metal plate material studied in the literature which meets the performance targets for contact resistance and corrosion is gold coated stainless steel. New corrosion resistant coatings are being developed in order to address these significant issues and this review paper evaluates them in the context of the DOE targets established for 2010.

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Nuria de las Heras studies the development and evaluation of metal bipolar plates for polymer electrolyte membrane (PEM) fuel cells. She has developed experimental techniques for characterising the corrosion and contact resistance characteristics of plate materials.

E. P. L. Roberts

Edward Roberts studies the design of electrochemical processes for energy and environmental applications. He has worked with industrial collaborators on the development of new metal plate materials for fuel cells. He has shown how membrane materials can be optimised for direct methanol fuel cells. He is co-inventor of the Arvia Process™ for wastewater treatment.

Richard Langton led the PEMcoat group at INEOS Technologies Ltd who developed and manufactured coated metal plates for PEM fuel cells.

David Hodgson has developed electrode materials and electrocatalysts for a wide variety of applications and is the inventor of the PEMcoat materials, developing it into a business.

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Broader context

Fuel cells are widely considered to be a sustainable energy conversion system and are a key technology for the development of a hydrogen economy. Low temperature (<100 °C) fuel cells, which use a polymer membrane to separate the electrodes, have been undergoing rapid development for mobile applications and in particular for the transport sector. Currently the cost of fuel cell systems is too high for them to be able to compete with conventional systems such as the internal combustion engine. One of the important challenges is the development of materials for electrode plates which are able to offer long term durability, reduced volume, lightweight and minimum manufacturing costs. These plates make up a very significant part of the weight and cost of a fuel cell system. In this paper the range of available plate materials is reviewed, with particular emphasis on metal plate materials which have the potential to offer significantly reduced weight and volume. Currently no metal plate material has been demonstrated to deliver the required performance at a competitive cost. However stainless steel plates coated with a thin corrosion resistant layer have the potential to achieve these requirements.

1 Introduction

Polymer electrolyte membrane fuel cells (PEMFC) have been undergoing rapid development in the last few years for portable, mobile applications and in particular for the transport sector.^1,2 PEMFCs are relatively compact, have a high power density, rapid start up and operate at low temperatures (60–120 °C). These characteristics make PEMFCs very attractive as a power source for vehicles in comparison with other fuel cell systems. However, there are many technological challenges to overcome before mass production and commercialisation of this technology becomes a reality. One of these challenges is the development of materials for bipolar separator plates able to offer long term durability, reduced volume, lightweight and minimum manufacturing costs. In this paper the range of bipolar plate materials are reviewed, with particular emphasis on metal plate materials which have the potential to offer reduced weight and volume.

2 Separator plates for automotive fuel cells

The bipolar separator plates are one of the most important components of a PEMFC stack, as they distribute reactant gases to the gas diffusion electrodes, allow removal of water and heat, and provide mechanical support and electrical connection between the cells in the stack. Bipolar plates typically account for 80–85% of the weight and most of the volume of fuel cell stacks.³ Estimates of the cost contribution of bipolar plates vary widely with variations in the cost evaluation methodology, differences in material and processing costs and improvements in the technology. Estimates of the cost contribution of bipolar plates to overall stack costs include values from as low as 5%⁴ to as high as 90%⁵ (although this seems to be based on stainless steel plates of thickness 2–4 mm), with many intermediate values (15%,⁵ 29%,⁶ 45%,³ 68%⁷). In spite of this variation, it is clear that low cost materials are needed which do not compromise the fuel cell stack performance and durability.

The bipolar plates of a PEMFC must be able to operate in an acidic environment of pH 3–5 in the presence of SO₄²⁻, NO₃⁻, Cl⁻, F⁻ ions arising from the Nafion membrane while in direct contact with oxidizing gases such as pure O₂ or air on the cathode side, and reducing gases such as H₂ on the anode side. The PEMFC separator plates are also typically exposed to potentials of 0 to 1000 mV versusRHE by contact with the electrodes, and potentials as high as 1400 mV versusRHE can occur at the cathode.⁸ These working conditions greatly condition the selection of materials and determine the characteristics of an ideal separator plate material which should incorporate excellent corrosion resistance, minimum electrical resistivity and good mechanical strength. Additional constraints influencing material selection for bipolar plates arise from reliability and durability aspects, with a suggested maximum cell performance degradation of 0.1% over 1000 h required for projected operational lifetimes of 5000 h in transport applications.⁹ Furthermore, it should be possible to mass produce separator plates by integrating suitable flow fields, manifolds and seals at a low cost to enable PEMFCs to compete with the internal combustion engine. For transport applications the requirement to reduce cell cost and volume without compromising fuel cell performance is crucial; in the USA, cost and power density targets for automobiles have been quoted as $10 kW⁻¹ and 1 kW dm⁻³ respectively.⁵ The US Department of Energy (DOE) has established specific targets for separator plates in PEMFC stacks for automotive applications to be achieved by 2010 (see Table 1).

Table 1 Technical targets for 2010 for PEMFC bipolar plates to be used in automotive applications, set by the US Department of Energy^10,11

DOE Technical Targets: Bipolar Plates
Cost/$ kW^−1	< 10
System weight/kg kW^−1	< 1
H2 permeation flux/cm^3 s^−1 cm^−2	< 2 × 10^−6
Corrosion/µA cm^−2	< 16
Electrical conductivity/S cm^−1	> 100
Durability with cycling/h	> 5000
Interfacial contact resistance at 140 N cm^−2/mΩ cm^2	< 10

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In this review, materials currently used and being developed will be considered in the context of the DOE targets described in Table 1. The conductivity of materials can be measured by standard techniques, but the other key properties are associated with the fuel cell environment. Literature values of contact resistance between plate and gas diffusion layer (GDL) materials can vary significantly. This can be due to variations in any pretreatment and the GDL material used, but the most significant factor is the compression force. The DOE target in Table 1 is based on a standard technique with a compression force of 140 N m⁻². In this review we will only compare contact resistance values made at this compression force. Measurements of corrosion rate and durability are typically carried out under a wide range of different conditions, which can make comparisons difficult.

Materials used for separator plates in PEMFC to date fall into three main categories: graphite, composites and metals (Fig. 1).

Fig. 1 Materials used for bipolar separator plates for PEMFC.

3 Graphite

Traditionally PEMFC bipolar plates have been constructed from graphite, both natural and synthetic, as it is resistant to corrosion in the fuel cell environment and has high electrical conductivity. Many studies have shown the benefits of using graphitic plates; Davies and co-authors¹² presented long term data from Poco™ graphite plates tested in a single fuel cell with insignificant interfacial ohmnic losses relative to uncoated stainless steel and titanium plates; Cleghorn et al.¹³ also operated a single PEMFC continuously for three years with minimal power losses using Poco™ graphite plates. Although graphitic plates offer an adequate performance in stacks showing minimum degradation over time, graphite lacks mechanical strength increasing the size of the fuel cell stack and therefore limiting the volumetric power density. In addition, its high production costs remain a major problem with currently machined graphite plates accounting for 60% of the stack cost. For this reason graphite is unsuitable for automotive applications involving high volume manufacturing.¹⁴

4 Composites

Composites are an alternative to graphite offering a reduction in weight and an improvement in manufacture as they can be extruded or molded to any shape reducing manufacturing time and cost, and consequently they have gained much attention in the last few years. Cunningham stated that the cost of molding a polymer composite bipolar plate is 10–20 times less expensive than machining.¹⁵ Composite materials can be classified as carbon–carbon composites and carbon–polymer composites.

Besmann¹⁶ proposed carbon–carbon composite plates prepared by slurry molding a chopped carbon fiber–phenolic resin preform, followed by sealing the material with chemically vapour infiltrated carbon, resulting in a hermetic graphitic carbon system. These materials have good conductivity (200–300 S cm⁻¹) and adequate corrosion resistance however the complexity of the production process leads to high costs of fabrication. Hentall et al.² recommended exfoliated graphite (Grafoil™) as an ideal plate material for PEMFC as it showed a significant increase in performance than standard graphite materials. This enhanced performance is due to its compressibility, enabling the material to form an intimate contact with the MEA thus minimizing interfacial resistance. This material is ideally suited for production as it is inexpensive and simple to cut into foils but it also suffers from impact damage and its compressibility could deter the manufacture of multiple cell stacks.

Carbon–polymer composite materials have been developed as an alternative to carbon–carbon composites. They consist of a polymer resin (thermosetting or thermoplastic), used as a binder and a conductive filler such as graphite, metal coated graphite or low melting metal alloys. A high loading of filler is required in order to increase the electrical conductivity, while insufficient binder will lead to poor mechanical properties. Compression or injection molding is used to generate the bipolar plates and the cost is reduced because the machining process is removed. This composite route has been the approach of SGL Technologies GmbH who presented a promising composite comprised of polypropylene and phenolic bonded graphite, which can be injection molded, shows good electrical conductivity, is thermally stable above 100 °C and can be produced cost effectively.¹⁷ Composite materials using liquid crystal polymer with a carbon content of around 85% have recently been developed.¹⁸ These materials have the advantage that they can be injection molded.

Until recently, carbon–polymer composite materials offered low bulk conductivity relative to carbon–carbon or graphite plates. In addition, these materials often exhibit lower through-plane conductivity than in-plane. Recently nanocomposite materials have been reported using fillers such as carbon nanotubes, graphitic powders and silver coated glass^19,20 which can offer improved mechanical properties and in particular, increased conductivity. Laminated materials have also been developed which can improve the through-plane conductivity.²¹

Los Alamos National Laboratory has developed a metal-based composite bipolar plate based on porous graphite, polycarbonate plastic and stainless steel.²² Producing porous graphitic plates is not as expensive as non porous graphitic plates where the impermeability is provided by the steel and the polycarbonate. The stainless steel also provides rigidity to the structure while the polycarbonate contributes to the moldability and corrosion resistance. The layered plate appears to be a good alternative from stability and cost standpoints.

The main disadvantage of composite materials is their relatively low conductivity (typically 50–200 S cm⁻¹ in-plane, 20–50 S cm⁻¹ through-plane) compared to graphite (>500 S cm⁻¹) and metal (>10 000 S cm⁻¹) plates. In addition they offer lower thermal conductivity and limited operating temperature range.

5 Metals

Metals offer some advantages over graphite and composite materials as they can be manufactured as thin sheets at a lower cost. In addition, the electrical conductivity of metallic materials (e.g.iron alloys >5 × 10⁶ S cm⁻¹, titanium >2 × 10⁶ S cm⁻¹) is superior to graphite (∼10³ S cm⁻¹) and composites (1–300 S cm⁻¹).⁹ Metallic plates also offer lower permeation rates for H₂ than its competitors, in the order of 10⁻¹² cm³ s⁻¹ cm⁻² for SS316 plates compared to values of 10⁻² cm³ s⁻¹ cm⁻² for graphite plates (Table 2).¹⁰

Table 2 Comparison of properties of graphite and SS316¹⁰

	Graphite	SS316
Cost/$ kg^−1	75	7
Density/g cm^−3	2.25	8.02
Plate thickness (mm) required to meet the DOE target of $10 per kW (assuming 0.6 W cm^−2)	0.35	1.0
Electrical resistivity/Ω cm 10^−6	6000	73
Corrosion current/mA cm^−2	< 0.01	< 0.1
Gas permeability/cm^3 s^−1 cm^−2	10^−2–10^−6	<10^−12
Thickness of plate for same weight/mm	2.5–4	0.16
Modulus of elasticity/GPa	10	193
Tensile strength/MPa	15.85	515
Thermal conductivity/W m^−1K^−1	23.9	16.3

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The primary cost of bipolar plates is the material cost. The material costs for a stainless steel plate, taking into account the bulk cost, density (Table 2) and typical plate thickness are significantly less than a graphite plate. It is possible to estimate the maximum viable plate thickness based on the DOE target plate cost of $10 per kW assuming a power density of 0.6 W cm⁻². On this basis the bipolar plate material cost will exceed $10 per kW for a graphite thickness of only 0.4 mm. For stainless steel plates the raw material costs would be less than $10 per kW for a plate thickness of 1 mm. Of course in practice it would also be necessary to account for the processing and production costs.

5.1 Stainless steel

Austenitic stainless steel. Numerous uncoated metals have been studied as bipolar plate materials, and in particular austenitic stainless steels have received considerable attention due to their relatively low cost, suitability for mass production and the wide variety of commercially available alloys. The benefits of using stainless steel are low cost, high strength and the ease of incorporating a flow field via stamping or embossing. In addition, because they can be produced as thin sheets (0.2–1 mm), they offer significant improvement in power to volume and power to weight ratios (Table 2). By using stainless steel bipolar plates, cost estimates published in the Fuel Cells for Transportation Report 1998, show that a 70 kW PEMFC stack with a cost of less than $20 kW⁻¹ could be achieved,²³ below the DOE target for a fuel cell set at $30 kW⁻¹. Consequently stainless steel 316L has been identified as a very promising candidate for transport fuel cell applications.²⁴

However, the performance of stainless steels as PEMFC plates is limited due to its inherent characteristics. Stainless steels typically contain small quantities of chromium, which forms a thin layer of oxide (Cr₂O₃) on the steel surface, inhibiting corrosion. The passive layer, however also leads to increased contact resistance between the plates and the gas diffusion layer. In a fuel cell, the contact resistance can become significant, particularly at the cathode²⁵ where the oxidising environment leads to a thicker passive layer.

In addition to the issue of contact resistance, corrosion can lead to the release of metal ions into the fuel cell which can lead to degradation of performance.^26,27 While the presence of metal ions could influence the electrocatlayst performance, the main effect of these ions is on the membrane. Metal ions tend to exchange with protons in the membrane, binding with the associated SO₃ sites and reducing the membrane conductivity.²⁶ High valence chromium, iron and nickel ions have been found to cause the most significant effects. In addition, Cu²⁺ and Fe²⁺ ions can catalyse the degradation of the membrane by peroxide species generated at the cathode.²⁷

Experiments carried out by Wind et al.²⁸ in single cells at 75 °C, with a Nafion™ 115 membrane, revealed a voltage drop of up to 300 mV after 500 h of operation at 0.7 A cm⁻² by using untreated stainless steel 316L. Mallant et al.²⁹ found that PEMFC using stainless steel 316L bipolar plates showed about 10% degradation in performance over 1000 h. Furthermore, Tran et al.³⁰ Hentall et al.,² Makkus et al.,³¹Liet al.³² and Pozio et al.³³ concluded that bare stainless steel SS316 is not suitable for application as a PEMFC plate material.

Research into steel alloys has shown that the elemental composition of these alloys and the nature of the surface have a significant influence on the fuel cell performance and durability. Lee et al.,³⁴ discussed the importance of surface roughness that creates small potential differences in the PEMFC electrochemical environment leading to a more rapid dissolution of the metallic ions. The authors proposed an electrochemical process to smooth the surface roughness of SS316 improving the corrosion resistance and chemical stability of the plates. Davies et al.¹² studied the composition of the superficial passive film on several types of austenitic stainless steel aiming to find the relation between the electrical resistance and the oxide film and found that the passive film thickness, determined by Auger Electron Spectroscopy analysis, decreased according to the sequence 321 > 304 > 347 > 316 > 310 > 904L > Incoloy 800 > Inconel 601, findings that corresponded to the trend observed in interfacial contact resistance. Despite exhibiting reduced interfacial resistance Incoloy and Inconel are prohibitively expensive to be used at large volumes.

In a later study, Davies et al.³⁵ presented results from long term tests (over 3000 h) of alloys 904L and 310, showing that these alloys can produced higher fuel cell performance than 316. By means of AES, he showed that the oxide film is thicker in stainless steel containing less Ni and Cr and that increased performance is related to the decreased thickness of the oxide film. A thicker oxide film may offer a longer anti corrosion life but the surface conductivity will be lower, consequently a thin but dense oxide film is preferred. Wang et al.³⁶ concluded that stainless steel 349™ exhibited the best behaviour over other stainless steel alloys such as 904L, 317L, 316L for both the cathode and anode environment.

Ferritic stainless steel. These materials have been considered as bipolar plate materials as they have very little or no nickel thus decreasing the total cost and reducing the possibility of ions leaching within the stack. Wang and Turner²⁵ evaluated the performance of some ferritic steels by measuring the interfacial contact resistance and the electrochemical corrosion resistance of each candidate in a solution simulating the PEMFC environment. Polarization results in the anode environment ranked the ferritic alloys in the order of AISI446 > AISI444 > AISI436 > AISI434 > AISI441 and in the cathode environment as AISI446 > AISI444 > AISI436 > AISI441 > AISI434, suggesting AISI446 is the best candidate among the materials tested. The higher Cr, Ni and Mo content of this alloy may be the cause of the improved corrosion resistance.

The corrosion rate of austenitic/ferritic Duplex 2205 stainless steel was investigated by Wang et al.³⁷ in a solution of 1M H₂SO₄ + 2ppm F⁻ at 70 °C purged either with H₂ or air to simulate the environment of a PEMFC plate. This material was found to have a corrosion rate similar to austenitic 349™ and ferritic AISI446.

Iron based amorphous alloys. In an effort to improve the corrosion vulnerability of iron based alloys, Kim³⁸ investigated iron based amorphous alloys such as Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆ and a newly developed Fe₅₀Cr₁₈Mo₈Al₂Y₂C₁₄B₆, by testing them in a simulated PEMFC environment of 1M H₂SO₄ + 2ppm F⁻ at 80 °C. The results indicated a significant improvement in corrosion resistance in comparison to stainless steel 316L of identical Cr content. Another interesting fact from this investigation was that although these amorphous alloys presented higher bulk electrical resistivity than their crystalline counterparts due to the reduced mobility of the electrons, their interfacial resistance at stack assembly conditions (i.e. at a compaction force of 180 N cm⁻²) differed only slightly from the values offered by SS316L and graphite.

Titanium . Titanium has also been considered as a material for PEMFC applications as it provides adequate mechanical strength, high corrosion resistance and is 40% lighter than stainless steel. Unfortunately, titanium tends to develop very low conductivity surface passive films, significantly increasing ohmic losses within the stack. Davies et al.¹² determined that the growth of the surface resistivity from the insulating oxide film on titanium plates in a PEMFC, grew from 32 mΩ cm² to 250 mΩ cm² after 1300 h of operation, offering a much lower fuel cell performance compared to the use of uncoated stainless steel 316 plates.

Titanium must be therefore protected with coatings to preserve its conductivity. Hodgson et al.³⁹ described the use of titanium coated with PEMcoat™ FC5, a proprietary coating developed by INEOS Chlor, showing that the contact resistance of this material is similar to graphite and far superior to uncoated stainless steel 316. Using this material, lifetimes in excess of 8000 h were achieved, with fuel cell power densities in a single cell of 1.8 kW dm⁻³ and >1 kW kg⁻¹. Moore et al.⁴⁰ also tested titanium coated with PEMcoat™ FC5 for over 10 000 h with no significant drop in performance observed.

Overall titanium is a relatively expensive material better suited for aerospace applications rather than low cost automotive use.⁴¹

Aluminium alloys. Aluminium has the advantage of low cost compared to other bipolar plate materials such as graphite (Table 3). However, aluminium has the same tendency as stainless steel and titanium to develop a superficial oxide film which lowers the fuel cell power output. Consequently aluminium has always been used in combination with other metals or coatings. Frangini and Masci⁴³ proposed an intermetallic alloy coating formed by FeAl deposited on stainless steel 316L plates for PEMFC applications. A Japanese patent application⁴⁴ disclosed the production of PEMFC bipolar plates made of aluminium or its alloys by molding, etching and plating a film of a noble metal on the surface of the plate.

Table 3 Material costs for high volumes of graphite and aluminium⁴²

Material	Cost/US$ kg^−1	Density/g cm^−3	Plate thickness for $10 kW^−1 material cost/mm
Graphite	105	1.79	0.25
Aluminium	8.8	2.7	2.5

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Nevertheless, some researchers have expressed concerns about the stability of aluminium as a bipolar plate material. Hentall et al.² stated that aluminium had proved reactive even when coated with gold.

Nickel alloys. Nickel is a widely available material, relatively inexpensive, which exhibits excellent ductility and manufacturability. Woodman et al.⁴¹ evaluated phosphorous nickel as a potential PEMFC plate material, concluding that it offered very low corrosion rates of <15 µm year⁻¹ (DOE targets <95 µm year⁻¹) when tested in an acidic solution representative of PEMFC operating conditions.

Shanian and Savadogo⁴⁵ reported the Ni–50Cr alloy as a very stable and electrically conductive material, with minimum corrosion rates and low electrical resistivity values when compared to other stainless steels alloys (Table 4), although cost could be an issue.

Table 4 Cost, corrosion data, resistivity and gas permeability values for materials used as plates in PEMFC.⁴⁵

	Cost/C$ kg^−1	Corrosion rate/yr^−1	Electrical resistivity/µΩ cm	H2 permeability
316	5.089	0.081	71	5.1
316L	5.184	0.081	69	2.2
304	5.99	0.081	77	5.4
317L	7.142	0.23	74	5.3
310	10.83	0.081	80	5.4
446	4.954	0.105	65	0.69
444	5.53	0.105	57	0.69
436	5.69	0.105	55	0.69
434	5.76	0.105	62	0.69
Ni–50Cr	10.37	0.005	40	4.2
TiN	34.56	0.061	60.3	0.32
AuAl	50	2	3.9	160

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Copper alloys. Good electrical and thermal conductivity are the main advantages of copper alloys over conventional materials. Recently, Reddy and Nikam^46–48 have studied the copper–beryllium alloy C-17200 in a simulated PEMFC environment and they found corrosion rates of 0.3 µA cm⁻² for this alloy, which is within the DOE corrosion rate target (Table 1). Further experiments to study the performance of this alloy in a single prototype fuel cell, showed maximum current density and cell resistance values comparable to SS316, with no corrosion products observed on the MEA after 100 h although long term stability of such alloy over 5000 h to meet DOE targets is yet to be demonstrated.³⁶

5.2 Coatings

To overcome oxide formation and ion dissolution for metal plate materials such as stainless steel, aluminium and titanium, various protective coatings have been employed. These coatings aim to be impermeable to the reactant gases and chemically inert, providing low contact resistance and offering good corrosion resistance in order to withstand the aggressive fuel cell environment. In many cases the integrity of the coating can be a key issue, as cracks or pinoles can lead to accelerated corrosion of the substrate. The various coatings that have been reported in the literature are discussed below.

Conducting polymers. McClure et al.⁴⁹ proposed the use of two conducting polymers: polypyrrole (PPY) and polyaniline (PANI), electrochemically deposited onto 304 stainless steel. Corrosion current densities of bare stainless, PPY and PANI coated steel were found to be 10, 0.1 and 0.01 µA cm⁻² respectively, indicating that the polymer coatings provided corrosion resistance for the stainless steel. Conducting polymers have also been investigated on mild steel,⁵⁰iron,⁵¹ aluminium and aluminium alloys,^52–54copper and silver.⁵⁵ Although corrosion resistance could be increased with these polymer-coated plates, electrical conductivities remain below the DOE targets with measured values of contact resistance of around 1.3 mΩ cm² at 100 N cm⁻², which is 10 times higher than the value for graphite of 0.1 mΩ cm² at the same compaction force.⁴⁹

Noble metals. The use of a thin coating of a noble metal on a low cost substrate is an attractive option for obvious reasons. However, differences in thermal expansion coefficients can lead to cracking of the coating.⁴¹ Furthermore, imperfections in the coating (pinholes or cracks) are likely to lead to accelerated corrosion due to the differences in the electrode potential of the metals.

In spite of this concern, gold, which is a chemically stable and very conductive material (45 × 10⁶ S cm⁻¹), has been successfully used as a coating for PEMFC bipolar plates. It has been found to decrease the contact resistance of passive metals such as stainless steel, aluminium, titanium and their alloys. Lu and Wang⁵⁶ used stainless steel bipolar plates with a gold coating of 0.5 µm in direct methanol fuel cells and obtained excellent results. Hentall et al.² reported a superior performance of gold coated stainless steel 316L plates compared to that of Poco™ graphite when being tested in a single PEMFC. Wind et al.²⁸ has also reported no deterioration of the cell voltage from gold coated stainless steel 316L plates after 1000 h of operation, with no difference in performance compared to PEMFC using graphite plates. Several patent applications have claimed low corrosion currents from gold coated stainless steel with thicknesses in the range of 10–20 nm.^57,58 Hornung and Kappelt⁵⁹ found that gold coated nickel bipolar plates offered contact resistance values low enough (2 mΩ cm² at 21 bar) to perform in fuel cell applications and achieving the DOE target for contact resistance (<10 mΩ cm² at 140 N cm⁻²).

Hentall et al.² reported similar initial performance of PEMFC using gold coated aluminium and graphite plates (1.2 A cm⁻² at 0.5 V) although the performance of the cells using the gold coated aluminium plates deteriorated very quickly. It was observed that some of the gold layer lifted from the plate and the membrane became contaminated with aluminium. Woodman et al.⁴¹ also studied aluminium plates coated with gold by electrodeposition, with thicknesses of 1 to 10 µm. The performance of these plates was equivalent to that of graphite when pulse current deposition was used but poorer performance was obtained when continuous current deposition was employed.

All the reported studies carried out with gold coatings showed that a very thin layer of gold is sufficient to prevent corrosion and to improve electrical conductivity. However, despite the fact that gold coatings offer excellent results when performing in the fuel cell environment, its high price is a major disadvantage for economies of mass production. Based on a power density of 0.6 W cm⁻², the material cost of the gold coating alone amounts to more than the DOE target of $10 per kW for a coating thickness of 0.4 µm. It is clear that the very thin (of order 10–50 nm) coatings are most likely to offer a viable material.

Nitrides. The corrosion resistance of steels can be improved by forming a protective metal–nitride layer. Cho and co-workers¹⁴ compared the performance of graphite plates, bare 316 stainless steel and 316 coated with a 1 µm layer of TiN in a single fuel cell at a voltage of 0.6 V, and obtained initial current densities of 996, 796 and 896 mA cm⁻² respectively. These results show that the TiN layer improved the performance of the fuel cell compared to uncoated 316 stainless steel plates but they also found that the lifetime was significantly improved, although less than that of graphite. Similar work was conducted by Hentall et al.² who confirmed that coating 316L steel with titanium nitride reduced the long term voltage decay rate almost to that of graphite. Tian et al.⁶⁰ also reported the benefits of coating stainless steel 316L with a layer of TiN, which improved the surface stability and conductivity of SS316L.

The feasibility of using thermally nitride coated AISI 446 was studied by Wang et al.⁶¹ The nitrification process created a modification of the native oxide layer increasing the corrosion resistance, however the interfacial contact resistance was 40 mΩ cm² at 150 N cm⁻², above the DOE targets of 10 mΩ cm² at 140 N cm⁻² (Table 1).

Brady et al.⁶² at the Oak Ridge National Laboratory have recently developed a thermal nitridation process to form pinhole free CrN/CrN₂ coatings on a Ni–Cr alloy. The process is achieved by diffusing nitrogen into the substrate and has the advantage that the layer does not peel off mechanically. The result is a chromium nitride surface layer which exhibits excellent corrosion resistance, little metal dissolution and negligible contact resistance increase over the course of a 4100 h exposure in a 80 °C sulfuric acid environment. In the same line of research, Brady et al.⁶³ studied the nitration of several nickel chromium alloys such as Ni–50Cr and Ni–30Cr with promising results. These alloys are however very expensive for PEM applications and research is being directed towards developing Fe–Ni bases that can form similar protective chromium nitride surfaces.

Carbides and borides. Carbides and borides offer a combination of good corrosion resistance and high electrical conductivity. Transition metal carbides and borides are ‘metallic ceramics’ with many desirable properties such as excellent electrical conductivity, low rates of diffusion of impurities, hardness, good resistance to mechanical wear and low rates of corrosion.^64,65 The use of molded titanium carbide bipolar plates in PEMFCs had been described by Laconti in a patent application⁶⁶ which suggested that long term mechanical and thermodynamic stability could be achieved in acidic media at high anodic potentials. This patent also claimed how plates constructed from these materials had good electrical conductivity and could be produced economically. Another patent application⁶⁷ described the use of carbides such as TiC, ZrC, HfC and WC, sputtered onto stainless steel 316L for stable PEMFC bipolar plates.

Similarly, sputtered coatings of CrB₂ have been reported by Dahm et al.,⁶⁸ who observed excellent wear and corrosion resistance. There has also been recent work on ZrB₂ thin films obtained by chemical vapour deposition.⁶⁹

Another interesting boride alloy NiCoB has been investigated by Gamboa et al.⁷⁰ They studied the corrosion behaviour of a NiCoB coating on carbon steel 1018, aluminium A6061 and stainless steel 304 in simulated PEMFC conditions of 0.5M H₂SO₄ at 60 °C using polarization curves, LPR, EIS and OCP measurements. In all cases the corrosion resistance of NiCoB coated alloys was superior to the uncoated alloy, with up to two orders of magnitude improvement for the carbon steel and one order of magnitude for the stainless steel.

Futhermore, researchers at Sumitomo Ltd⁷¹ have developed and patented stainless steel bipolar plates with metallic inclusions of carbide and boride protruding through the outer surface of the passive film for use in PEMFC, claiming superior electrical conductivity.

Conducting oxides. Conducting oxides can also be considered as candidates for bipolar plates in PEMFC applications as they offer corrosion protection and high conductivity. Wang et al.⁷² have studied SnO²:F deposited on stainless steel, a material used in solar cells. Elhamid et al. revealed in a patent application⁷³ that silicon dioxide and titanium oxide coatings have shown to possess hydrophilic properties which represent a desirable property improving the performance of the stack.

Diamond like carbon (DLC). Another material used as a coating for bipolar plates is diamond like carbon (DLC), which had been deposited in stainless steel 316L substrates offering a combination of wear resistance, chemical inertness and good metal adhesion properties.⁷⁴ Similar results were reported in a Japanese patent application⁷⁵ that mentioned excellent corrosion resistance for bipolar separator plates composed of a carbon layer with diamond like structure. Huang et al.⁷⁶ also investigated the influence of DLC as bipolar plate material by studying the performance in a single PEMFC of aluminium 5052 alloy, PVD coated with a DLC film and concluding that such plates performed better than graphite but the cell life was shorter.

Although these materials may offer durability, they add significant cost to prepare and increase contact resistance respect to the bare substrate. The cost of diamond coatings has been estimated^77,78 to be around $10 cm⁻², around three orders of magnitude higher than the DOE target.

Other coatings. La Conti et al.⁷⁹ tested the corrosion behaviour of some refractory materials such as Ta, Hf, Nb and Zr in a simulated fuel cell media of 10% H₂SO₄ 80 °C, claiming that they all exhibited satisfactory corrosion resistance and durability over a wide range of voltages. Niobium was the material used by General Electric in the 60s in bipolar plates due to its extraordinary corrosion qualities although this was later replaced by graphite to reduce cost.⁸⁰ Furthermore, Gladczuk⁸¹ studied tantalum deposited onto steel and aluminium by dc magnetron sputtering, to be used as bipolar plates for PEMFC with promising results. An alternative approach described by Quet al.⁸² was the addition of small amounts (<0.5 wt%) of rare earth metals such as yttrium or lanthanum in order to improve the surface conductivity of stainless steel by decreasing the rate of formation of the surface oxide passive layer.

Huang et al.¹¹ evaluated SS316 coated with Zr, ZrN, ZrNb and ZrNAu. Experimental results showed that only the Zr/SS316 satisfied the corrosion DOE goals at cathode conditions and only the ZrNAu/SS316 satisfied the DOE contact resistance targets. Despite their benefits one of the main disadvantages of these materials for commercial applications is their relatively high cost.²⁶

6 Conclusions

Of the DOE targets in Table 1, the corrosion rate, durability and contact resistance are the key performance parameters for bipolar plate materials. Table 4 shows these properties for all of the materials discussed in this paper for which data is available, compared against the DOE targets. The contact resistance data in Table 4 were all reported at a pressure of 140 N cm⁻². The conditions used for corrosion testing varied and the data should therefore be considered to be indicative only. Measurements were typically carried out in sulfuric acid solution (0.1 to 1 M) and in some cases a few ppm of fluoride ions was added. Although the conditions do vary slightly, the corrosion data shown in Table 5 is useful in giving an indicative comparison of alternative materials relative to the DOE targets.

Table 5 Key properties of bipolar plate materials compared to the DOE target. Bold indicates materials which do meet the target. Italic indicates materials which do not meet the target. All contact resistance data were reported at a pressure of 140 N cm⁻². Conditions for the corrosion measurements varied and figures are therefore indicative only

Plate material			Corrosion		Durability	Contact resistance	References
a indicates that there is insufficient or no data available.
DOE targets 2010^(6,35)			<16 µA cm^−2	<95 µm year^−1	5000 h (<10% drop in power)	<10 mΩ cm^2 at 140 N cm^2	10,46
Graphite		Poco graphite	<10 µA cm^−2	<15 µm year^−1	2.9% after 1300 h ∼2% after 5000 h (mainly due to membrane degradation )	15 mΩ cm^2	10,12,41,13
Metals	Austenitic SS	SS316	100–500 µA cm^−2	<100 µm year^−1	10% after 1000 h^a	50–150 mΩ cm ^2	12,29,25,35,36,41,49,60
		SS304	10 µA cm^−2		^a	100 mΩ cm^2	12,49
		SS310	^a	^a	1.5% after 1300 h^a	42 mΩ cm^2	12
		SS317	20–40 µA cm^−2		^a	147 mΩ cm^2	36
		SS321	^a	^a	^a	190 mΩ cm^2	12
		SS347	^a	^a	^a	100 mΩ cm^2	12
		904L	10–20 µA cm^−2		^a	40–135 mΩ cm ^2	12,36
		349™	10 µA cm^−2		^a	110 mΩ cm^2	36
	Ferritic SS	AISI434	100–200 µA cm^−2		^a	150 mΩ cm^2	25
		AISI436	20–60 µA cm^−2		^a	100 mΩ cm^2	25
		AISI441	60–300 µA cm^−2		^a	100 mΩ cm^2	25
		AISI444	20–50 µA cm^−2		^a	100 mΩ cm^2	25
		AISI446	8–20 µA cm^−2		^a	200 mΩ cm^2	25
		Duplex 2205	^a	^a	^a	130 mΩ cm ^2	84
		Incoloy 800	^a	^a	^a	37 mΩ cm ^2	12
		Inconel 601	^a	^a	^a	20 mΩ cm^2	12
	Amorph. alloys	Fe41Cr18Mo14Y2C15B6N4	5–36 µA cm^−2		^a	10 mΩ cm^2	38
		Fe50Cr18Mo8Y2C14B6Al2	100–182 µA cm ^−2		^a	15mΩ cm ^2	38
	Titanium	Ti		<100 µm year^−1	32% after 1300 h^a	50 mΩ cm^2	12,41
	Aluminium	Al		250 µm year^−1	^a	^a	41
	Aluminium alloy	Al 5052		1100 µm year^−1	^a	^a	85
	Nickel alloys	P–Ni		<30 µm year^−1	^a	^a	41
		Ni–50Cr	^a	^a	^a	50 mΩ cm^2	61
	Copper alloys	C-17200	0.3 µA cm^−2		^a	^a	47
Coatings	Polymers	PPY/304	1 µA cm^−2		^a	800 mΩ cm^2	49
		PANI/304	0.1 µA cm^−2		^a	800 mΩ cm^2	49
	Gold	316L/Au	5.65 µA cm^−2		0% after 1000 h^a	5 mΩ cm^2 (>10 nm Au)	11,28
	Nitrides	Ni–50Cr/CrNCr2N	^a	^a	^a	<20 mΩ cm^2	61
		SS349™/CrNCr2N	^a	^a	^a	10 mΩ cm^2	61
		SS446/CrNCr2N	^a	^a	^a	6–40 mΩ cm ^2	61,84
		SS316L/CrNCr2N	<10 µA cm^−2		^a	10 mΩ cm^2	60
		SS316/TiN	^a	^a	11% after 1000 h (stack)^a	40 mΩ cm^2	14
	Refractory metals	Zr/SS316 & ZrN/SS316	^a	^a	^a	150–1000 mΩ cm^2	11

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Little data is available regarding the durability of materials. Graphite and gold coated electrodes have been shown to meet the DOE target.^13,83 However, none of the other metal plate materials have been tested for the 5000 h timescale specified in the DOE targets.

Only a few materials meet the target for contact resistance and of these, the only material which has been shown to meet the corrosion target is gold coated SS316. The aggressive fuel cell environment severely restricts the choice of materials for PEMFC bipolar plates. It is likely only carbon–polymer composites and coated metal systems will meet in the future the long term cost requirements needed for automotive fuel cells.

Companies and institutions such as The Oak Ridge National Laboratory, Da Lian Institute of Physical Chemistry, Denora, Intelligent Energy Ltd, Sarnoff Corporation and GenCell Corporation have made substantial efforts towards developing metallic bipolar plates,²⁶ while polymer composites bipolar plates are being sold by companies such as DuPont, H₂ Economy, ICM Plastics, NedStack, etc evidence that both routes are developing new materials and maturing. Although significant advances have been made to date, there is still a need for further research and development in order to overcome the significant deficiencies of currently available materials.

References

1. G. J. K. Acres, J. Power Sources, 2001, 100, 60–66.
2. P. L. Hentall, J. B. Lakeman, G. O. Mepsted, P. L. Adcock and J. M. Moore, J. Power Sources, 1999, 80, 235–241.
3. H. Tsuchiya and O. Kobayashi, Int. J. Hydrogen Energy, 2004, 29, 985–990.
4. E. Carlsson, P. Kopf, J. Sinha, S. Sriramul and Y. S. Yang, Fuel Cell Seminar, 2005, Palm Springs, CA.
5. I. Bar-On, R. Kirchain and R. Roth, J. Power Sources, 2002, 109, 71–75.
6. F. D. Lomax Jr, B. D. James, G. N. Baum and C. E. Thomas, Detailed Manufacturing Cost Estimates for Polymer Electrolyte Membrane (PEM) Fuel Cells for Light Duty Vehicles, Directed Technologies Inc.Arlington, VA 1998.
7. K. S. Jeong and B. S. Oh, J. Power Sources, 2002, 105, 58–61.
8. C. A. Reiser, L. Bregoli, T. W. Patterson, J. S. Yi, J. D. L. Yang, M. L. Perry and T. D. Jarvi, Electrochem. Solid State Lett., 2005, 8, A273–A276.
9. B. C. H. Steele and A. Heinzel, Nature, 2001, 414, 345–352.
10. B. R. Padhy and R. G. Reddy, J. Power Sources, 2006, 153, 125–129.
11. W. Yoon, X. Huang, P. Fazzino, K. L. Reifsnider and M. A. Akkaoui, J. Power Sources, 2008, 179, 265–273.
12. D. P. Davies, P. L. Adcock, M. T. Turpin and S. J. Rowen, J. Appl. Electrochem., 2000, 30, 101.
13. S. J. C. Cleghorn, D. K. Mayfield, D. A. Moore, J. C. Moore, G. Rusch, T. W. Sherman, N. T. Sisofo and U. Beuscher, J. Power Sources, 2006, 158, 446–454.
14. E. A. Cho, I. H. Oh, U. S. Jeon, S. A. Hong and S. G. Kang, J. Power Sources, 2005, 142, 177–183.
15. B. Cunningham and D. G. Baird, J. Mater. Chem., 2006, 16, 4385–4388.
16. T. M. Besmann, J. W. Klett, J. J. Henry and E. Lara-Curzio, J. Electrochem. Soc., 2000, 147(11), 4083–4086.
17. A. Müller, P. Kauranen, A. von Ganski and B. Hell, J. Power Sources, 2006, 154, 467–471.
18. S. H. Liao, C. Y. Yen, C. H. Hung, C. C. Weng, M. C. Tsai, Y. F. Lin, C. C. M. Ma, C. Pan and A. Su, J. Mater. Chem., 2008, 18, 3993–4002.
19. M. G. Miller, J. M. Keith, J. A. King, B. J. Edwards and N. Klinkenberg, Polym. Compos., 2006, 27, 388–394.
20. S. Bouatia, F. Mighri and M. Bousmina, Fuel Cells, 2008, 8, 120–128.
21. B. D. Cunningham and D. G. Baird, J. Power Sources, 2007, 168, 418–425.
22. A. Hermann, T. Chaudhuri and P. Spagnol, Int. J. Hydrogen Energy, 2005, 30, 1297–1302.
23. J. Adams, Fuel cells for transportation programme, US DOE's Fuel Cells for Transportation Programme. Contractors Annual Progress Report, 1998.
24. W. M. Yang, S. K. Chou and C. Shu, J. Power Sources, 2007, 164, 549–554.
25. H. Wang and and J. A. Turner, J. Power Sources, 2004, 128, 193–200.
26. L. Ma, S. Warthesen and D. A. Shores, J. New Mater. Electrochem. Syst., 2000, 3, 221–228.
27. A. B. LaConti, M. Hamdan and R. C. McDonaldin Handbook of Fuel Cells - Fundamentals, Technology and Applications, ed. W. Vielstich, H. A. Gasteiger and A. Lamm, Wiley, New York, 2004, vol. 3, 647–662.
28. J. Wind, R. Späh, W. Kaiser and G. Böhm, J. Power Sources, 2002, 105, 256–260.
29. R. K. A. M. Mallant, F. G. H. Koene, C. W. G. Verhoeve and A. Ruiter, Fuel Cell Seminar, San Diego, CA, 1994.
30. D. T. Tran, O. A. Velev, I. J. Kakwan, S. Gamburzev, F. Simoneaux, T. R. Lalk and S. Srinivasan, Electrochem. Soc. Abstr. 190th Fall Meeting, San Antonio, TX, 1996.
31. R. C. Makkus, A. H. H. Janssen, F. A. de Brujin and R. K. A. M. Mallant, J. Power Sources, 2000, 86, 274–282.
32. M. C. Li, C. L. Zeng, S. Z. Luo, J. N. Shen, H. C. Lin and C. N. Cao, Electrochim. Acta, 2003, 48, 1735–1741.
33. A. Pozio, R. F. Silva, M. de Francesco and L. Giorgi, Electrochim. Acta, 2003, 48, 1543–1549.
34. S. J. Lee, J. J. Lai and C. H. Huang, J. Power Sources, 2005, 145, 362–368.
35. D. P. Davies, P. L. Adcock, M. Turpin and S. J. Rowen, J. Power Sources, 2000, 86, 237–242.
36. H. L. Wang, M. A. Sweikart and J. A. Turner, J. Power Sources, 2003, 115, 243–251.
37. H. L. Wang, G. Teeter and J. A. Turner, J. Electrochem. Soc., 2005, 152(3), B99–B104.
38. E. Fleury, J. Jayaraj, Y. C. Kim, H. K. Seok, K. Y. Kim and K. B. Kim, J. Power Sources, 2006, 159, 34–37.
39. D. R. Hodgson, B. May, P. L. Adcock and D. P. Davies, J. Power Sources, 2001, 96, 233–235.
40. J. M. Moore, J. B. Lakeman and G. O. Mepsted, J. Power Sources, 2002, 106, 16–20.
41. A. S. Woodman, E. B. Anderson, K. D. Jayne and M. C. Kimble, American Electroplaters and Surface Finishers Society, AESF SUR/FIN ′99 Proc., 1999, 6, 21–24.
42. H. Tawfik, Y. Hung and D. Mahajan, J. Power Sources, 2007, 163, 755–767.
43. S. Frangini and A. Masci, Surf. Coat. Technol., 2004, 184, 31–39.
44. H. Shimada and K. Ebihara, Jpn. Pat., JP2005302669-A, 2005.
45. A. Shanian and O. Savadogo, J. Power Sources, 2006, 159, 1095–1104.
46. R. G. Reddy and V. V. Nikam, J. Power Sources, 2005, 152, 146–155.
47. V. V. Nikam and R. G. Reddy, Int. J. Hydrogen Energy, 2006, 31, 1863–1873.
48. R. G. Reddy and V. V. Nikam, Electrochim. Acta, 2006, 51, 6338–6345.
49. S. Joseph, J. C. McClure, R. Chianelli, P. Pich and P. J. Sebastian, Int. J. Hydrogen Energy, 2005, 30, 1339–1344.
50. T. Tüken, G. Arslan, B. Yazini and M. Erbil, Corros. Sci., 2004, 46, 2743–2754.
51. J. E. P. da Silva, S. I. C. de Torresi and R. M. Torresi, Corros. Sci., 2005, 47, 811–822.
52. K. Uehara, T. Ichikawa, T. Serikawa, S. Yoshikawa, S. Ehara and M. Tsunooka, Thin Solid Films, 1998, 322, 198–205.
53. D. E. Tallman, Y. Pae and G. P. Bierwagen, Corrosion, 2000, 56, 401–410.
54. D. E. Tallman, C. Vang, D. D. Wallace and G. P. Bierwagen, J. Electrochem. Soc., 2002, 149, C173–C179.
55. V. Brusic, M. Angelopulos and T. Graham, J. Electrochem. Soc., 1997, 144, 436–442.
56. G. Q. Lu and C. Y. Wang, J. Power Sources, 2005, 144, 141–145.
57. M. Utsunomiya, M. Tsuji, T. Kuwayama and T. Ohtani, US Pat., US7,214,440, Honda Motors Co, 2004.
58. G. Vyas, Y. T. Cheng, M. H. Abd Elhamid and Y. M. Mikhail, US Pat., US6,866,958, 2003.
59. R. Hornung and G. Kappelt, J. Power Sources, 1998, 72, 20–21.
60. R. J. Tian, J. C. Sun and L. Wang, Int. J. Hydrogen Energy, 2006, 31, 1874–1878.
61. H. Wang, M. P. Brady, G. Teeter and J. A. Turner, J. Power Sources, 2004, 138, 86.
62. M. P. Brady, B. Yang, H. Wang, J. A. Turner, K. L. More, M. Wilson and F. Garzon, J. Min. Metals. Mater. Soc., 2006, 58, 50–57.
63. M. P. Brady, B. Yang, P. F. Tortorelly and K. L. More, FY2005 Progress Report, DOE Hydrogen Program, 2005.
64. J. Castaing and P. Costa. in Boron and Refractory Borides. ed. V. I. Matkovich, Springer, Berlin, 1977.
65. C. Mitterer, J. Solid State Chem., 1997, 133, 279–291.
66. A. B. LaConti, A. E. Griffith, C. C. Cropley and J. A. Kosek, US Pat., US6,083,641, 2000.
67. J. K. Lee, H. J. Kweon and J. W. Suh, Jpn. Pat. Appl., JP2006156386, 2006.
68. K. L. Dahm, L. R. Jordan, J. Haase and P. A. Dearnley, Surf. Coat. Technol., 1998, 109, 413–418.
69. S. Jayaraman, E. J. Klein, Y. Yang, D. Y. Kim, G. S. Girolami and J. R. Abelson, J. Vac. Sci. Technol., A, 2005, 23, 631–633.
70. S. A. Gamboa, J. G. Gonzalez-Rodriguez, E. Valenzuela, B. Campillo, P. J. Sebastian and A. Reyes-Rojas, Electrochim. Acta, 2006, 51, 4045–4051.
71. Y. Tarutani, T. Doi, A. Seki and S. Fukuta, US Pat., US6,379,476, 2002.
72. H. Wang, J. A. Turner, X. Li and R. Bhattacharya, J. Power Sources, 2007, 171, 567–574.
73. M. H. A. Elhamid, G. Vyas, Y. T. Cheng and R. H. Blunk, Wld. Pat. Appl., WO2006055146, 2006.
74. M. M. Morshed, B. P. McNamara, D. C. Cameron and M. S. J. Hashmi, J. Mater. Process. Technol., 2003, 141, 127–131.
75. T. Iijima, M. Okada and K. Kubomura. Jpn. Pat., JP2005093172, 2005.
76. S. J. Lee, C. H. Huang and Y. P. Chen, J. Mater. Process. Technol., 2003, 140, 688–693.
77. J. V. Busch and J. P. Dismukes, Diamond Relat. Mater., 1994, 3, 295–302.
78. I. A. Martorell, W. D. Partlow, R. M. Young, J. J. Schreurs and H. E. Saunders, Diamond Relat. Mater., 1999, 8, 29–36.
79. A. B. LaConti, A. R. Fragala and J. R. Boyack, J. Electrochem. Soc., 1977, 124, C132–C132.
80. P. Costamagna and S. Srinivasan, J. Power Sources, 2001, 102, 253–269.
81. L. Gladczuk, C. Joshi, A. Patel, J. Guiheen, Z. Iqbal and M. Sosnowski, Mater. Res. Soc. Symp. Proc., 2003, 756, 423–428.
82. W. Qu, L. Jian, D. G. Ivey and J. M. Hill, J. Power Sources, 2006, 157, 335–350.
83. S. Yoshioka, A. Yoshimura, H. Fukumoto, O. Hiroi and H. Yoshiyasu, J. Power Sources, 2005, 144, 146–151.
84. M. S. Wilson, C. Zawadzinski, S. Moller-Holst, D. N. Busick, F. A. Uribe and T. Zawodzinski, Proc. 1999 US DOE Hydrogen Program Annu. Rev., 1999.
85. S. J. Lee, C. H. Huang, Y. P. Chen and Y. M. Chen, J. Fuel Cell Sci. Technol., 2005, 2, 290–294.

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