Ex situ testing of candidate bipolar plate materials for polymer electrolyte membrane (PEM) fuel cells and electrolyzers typically involves electrochemical polarization of the specimen in a three electrode cell. Relatively high potentials of between 1.5 V and 2.0 V vs RHE are commonly applied during such tests due to the widely held assumption that, during both start-up/shutdown and normal operation, the bipolar plate experiences the same potential as that of the nearest electrode. Here we present experimental and modelling evidence that the bipolar plate in an operating PEM fuel cell or electrolyzer actually sits at its natural open circuit potential due to the high resistivity of the aqueous phase in such devices, which effectively shields the material from the elevated potential at the electrode. The implications for reliable ex situ testing are discussed.
Electrochemical energy conversion devices, such as fuel cells and electrolyzers, are widely recognized as an essential component of the transition to a low carbon economy. Polymer electrolyte membrane (PEM) variants of both technologies show particular promise due to their high current density, fast start-up times and low gas crossover rates. The main barrier to widespread uptake of these devices is their capital cost, which includes both raw material and manufacturing costs. However, whilst much work has gone into reducing the cost of the catalyst layer, mainly by lowering the precious metal content, relatively little effort has focused on the bipolar plate, which makes up the bulk of the mass of a typical PEM fuel cell or electrolyzer stack.
Bipolar plates for PEM fuel cells and electrolyzers are subject to stringent material requirements due to their challenging operating environment. For metallic bipolar plates in particular, one of the most demanding aspects is the need to exhibit both low contact resistance and high corrosion resistance. This necessitates the development of novel surface treatments and/or coatings, which is an increasingly active area of research. Qualification of these materials is ideally carried out via long term testing in full stack configuration, but this is time consuming and expensive. As a result, ex situ testing under representative conditions is typically conducted as an initial screening step, before proceeding to single cell testing of the most promising materials, and finally stack testing of commercially viable options.
