Korrosion metallischer Bipolarplatten in Hochtemperatur-Polymerelektrolyt-Brennstoffzellen

With increasing energy consumption worldwide, fuel cell technology is growing in importance. In essence, fuel cells convert the chemical energy of an energy carrier (e.g., hydrogen) directly into electrical energy at high efficiency. The main object of this work comprises the investigation into the corrosion behavior of metallic materials for use as bipolar plate materials in high-temperature polymer electrolyte fuel cells. The advantages of metallic bipolar plates include i) cost-effective production by means of hydro-forming or die stamping processes, ii) weight-saving of up to 80 % and iii) reduction in the installation space of more than 50 % by using a material thickness of 0.1 mm, compared to 2 mm thick graphitic bipolar plates. However, the operating temperature (140 – 180 °C) and use of phosphoric acid-doped Polybenzimidazole electrolyte membrane also result in a corrosive environmental milieu that adversely affects long-term stability. The aim of this thesis is to elucidate the oxidation and passivation processes of metallic materials under dynamic fuel cell conditions on the one hand and, on the other, to show effective coating concepts. For this purpose, experiments were carried out in i) under-saturated; and ii) quasi-realistic conditions, as well as electrochemical measurements as a function of electrolyte volume, temperature and external electrical potential. At temperatures above 80 °C, Cr/Ni alloys form bilayered passive layers, which consist of an inner dense Cr2O3 layer and an outer porous metal phosphate layer. This leads to a strong increase in the electrical contact resistance at the bipolar plate/gas diffusion layer interface. In the potential window of around 0.2 – 1.2 V (vs. RHE), partially protective passive layers are formed that reduce the anodic corrosion current. Corrosion kinetics follow an exponential progression in accordance with the temperature-dependent Arrhenius equation. Carbon-based protective layers based on physical/chemical vapor deposition techniques show a higher resistance than ceramic metal nitrides, whereas layer quality and the homogeneity of the coating process play a decisive role. The results obtained show that the complex processes at the metal/coating/phosphoric acid interface can be investigated separately and may be transferred to the degradation of bipolar plates in high-temperature polymer electrolyte fuel cells