Alloys that form conductive and passivating oxides for proton exchange membrane fuel cell bipolar plates
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During the operation of proton exchange membrane (PEM) fuel cells, a high-resistance oxide is often formed on the cathode surface of base metal bipolar plates. Over time, this corrosion mechanism leads to a drop in fuel cell efficiency and potentially to complete failure. To address this problem, we have developed alloys capable of forming oxides that are both conductive and chemically stable under PEM fuel cell operating conditions. Five alloys of titanium with tantalum or niobium were investigated. The oxides were formed on the alloys by cyclic voltammetry in solutions mimicking the cathode- and anode-side environment of a PEM fuel cell. The oxides of all tested alloys had lower surface resistance than the oxide of pure titanium. We also investigated the chemical durability of Ti–Nb and Ti–Ta alloys in more concentrated solutions beyond those typically found in PEM fuel cells. The oxide films formed on Ti–Nb and Ti–Ta alloys remained conductive and chemically stable in these concentrated solutions. The stability of the oxide films was evaluated; Ti alloys having 3% Ta and Nb were identified as potential candidates for bipolar plate materials.
I. INTRODUCTION
The proton exchange membrane fuel cell (PEMFC) utilizes a solid polymer electrolyte from the fluoroethylene family, on each side of which Pt-catalyzed porous electrodes are mounted. To increase output power, individual fuel cells are connected in series using bipolar plates. The functions of each bipolar plate include connecting the individual fuel cells, distributing fuel gas and oxygen over the surfaces of the anode and the cathoderespectively, and conducting the electrical current from the anode of one cell to the cathode of the next.1,2 Metals are attractive materials for bipolar plates2,4,5,6 as they are good electrical conductors and usually not porous. In addition, metals are relatively easy to machine or stamp, which allows the bipolar plates to be thin, thus decreasing the overall cost of the PEMFC stack.4 However, the PEMFC environment is corrosive for all but the noble metals as water vapor, oxygen, and heat coexist in a PEMFC. Furthermore, during long-term operation of a PEMFC, small concentrations of HF and H2SO4 leach out of the fluoroethylene membrane, thus making the environment even more corrosive. This environment
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Address all correspondence to this author. email: [email protected] DOI: 10.1557/JMR.2004.0216 J. Mater. Res., Vol. 19, No. 6, Jun 2004
leads to either pitting corrosion or the formation of a high-resistance passivating layer on the cathode and/or anode side of a metal bipolar plate. A high-resistance passivating layer interferes with electron conduction from the bipolar plate resulting in a drop in fuel cell efficiency over time. To date, titanium and stainless steels have been studied as candidates for metal bipolar plate materials.3,4,5,6 To prevent corrosion of the stainless steels or the formation of highly resistive TiO2 on the surface of titanium, research has primarily focused on developing polymer or noble-met
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