Electrochemical Impedance Spectroscopy on Metallic Materials for Bipolar Plate Application in HT-PEFC

High-temperature polymer electrolyte fuel cells (HT-PEFCs) operate in the range of 120 – 180 °C and currently employ phosphoric acid doped polybenzimidazole membranes. This in turn represents a quite aggressive environment for the components of a fuel cell. For this reason bipolar plates are presently made of graphitic composite materials. Indeed, they underlie moderate degradation due to corrosion phenomena, but otherwise they exhibit high mass and volume as well as a high effort of manufacturing. Therefore metallic materials come into play as promising candidates for bipolar plates. They provide some crucial advantages, like enhanced volumetric and gravimetric power density for stacks, increased ductility, high mechanical stability, high electronic and thermal conductivity and the capability of low-cost mass production. However, we still need further understanding of the electrochemical corrosion at the interface metal (bipolar plate) and electrolyte (phosphoric acid) at elevated temperatures.In the present work, we focus on the investigation of the fundamental corrosion mechanism of metals in phosphoric acid that depends strongly on passive layer formation of the respective metallic material. Using an electrochemical cell, which was specially designed for these experimental requirements, we analyzed various stainless steels and Ni-based alloys by means of steady-state voltammetry and impedance spectroscopy. Additionally, ex-situ characterizations using SEM, XPS and ICP-OES were carried out in order to investigate the metallic surface and the corresponding dissolved ion species in the electrolyte. A characteristic interfacial impedance as a function of applied potential illustrates the corrosion resistance due to passivation of the surface in the domain of 0.3 – 1.2 V (vs. SHE). The corrosion resistance of Ni-based alloys in the passive region has been found to be higher in comparison to ferrous stainless steels at 130 °C. This effect is attributed to the diminished charge transfer reaction in nickel oxides and phosphates in contrast to analogous iron specimen. Negative potentials and an excess of 1.2 V, which are quite destructive for the oxide layer, induce consequently low impedance values and rising corrosion rates. Notable is the lower charge transfer resistance at the free corrosion potential, which indicates a thinner passive layer