Studies of the corrosion of materials for bipolar plates in fuel cells

Abstract

Studies were undertaken to investigate the corrosion processes of materials for metal bipolar plates in fuel cells based on a poly(benzimidazole) membrane filled with H3PO4. In addition to mild steel, stainless steel, nickel, electroplated Ni and Ni/graphite, the focus was on Ni-Cr alloys and the influence of composition on the rate of corrosion. The medium chosen was phosphoric acid pH 2 at 353 K and a potential of + 100 mV was considered in order to mimic the conditions at an oxygen cathode in a fuel cell. Stainless steel showed good corrosion resistance but was considered too expensive. Mild steel corroded rapidly and although a Ni electroplate lowered the corrosion rate its performance was insufficient. The graphite in the coating was necessary to reduce the contact resistance of the bipolar plate with the electrocatalyst layer. The replacement of the Ni layer by a NiCr layer was therefore studied. The quality of Ni and Ni/C electroplates was determined by AC impedance in H2SO4 at 293K. Nyquist plots showed semi-circles from which the corrosion resistance was easily determined. Layers with thickness > 3 mm showed corrosion resistances similar to bulk Ni but those < 1 mm showed very high corrosion rates indicating pits and corrosion of the underlying mild steel. The corrosion of the Ni-Cr alloys was investigated using (a) commercial, bulk materials and (b) a high throughput technique whereby a wedge modification of physical vapour deposition was used to produce 10 x 10 arrays of electrodes with variable composition – the voltammetry at all 100 electrodes was then recorded simultaneously. Small additions of Cr to Ni led to a large drop in the rate of corrosion but it is shown that the microstructure of the alloy, determined by electron microscopy and X-ray diffraction, can be as important as the composition

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