Influence of Ionomer Content in IrO2/TiO2 Electrodes on PEM Water Electrolyser Performance

Abstract

PEM water electrolysers are promising candidates for energy storage in combination with renewable energy sources. At the moment, a large-scale application is still hindered by the high capital costs associated with PEM electrolysis [1, 2]. One attempt to overcome this problem is to increase the H2 output by operating an electrolyser at current densities much higher than the values typically reported in literature (1-2 A cm-2). Recent publications have shown that current densities of 5 A cm-2 and higher are possible [3, 4]. However for such high current densities the overpotential increases, leading to a lower overall efficiency. Therefore a careful analysis of the voltage losses is necessary to identify how parameters like catalyst loading, electrode thickness, and ionomer content influence the performance of the electrolyser and how the MEAs can be modified to minimize the overpotential. Previous studies have shown a significant influence of ionomer content on the electrolyser performance [5, 6]. This was attributed to changes in the catalyst/ionomer interfacial resistance and or catalyst layer resistance. However a complete understanding of the effect, especially for current densities above 1.5 A cm-2 is still missing. In this study, MEAs based on a carbon-supported platinum catalyst (Pt/C) for the hydrogen evolution reaction (HER) and an IrO2/TiO2 catalyst (Umicore) for the oxygen evolution reaction (OER) were fabricated with different anode ionomer loadings. Polarization curves were recorded for current densities up to 6 A cm-2. The best performance was obtained for an ionomer content of 11.6 wt% (relative to total mass of electrode). The MEAs were analyzed via cross-sectional SEM imaging to determine the electrode thickness. This allows an estimation of the ionomer volume fraction in the electrode which can then be related to an effective proton transport resistance in the electrode according to Liu et al. [7]. The proton transport resistance, along with the ohmic resistance determined by impedance spectroscopy and kinetic losses obtained from Tafel plot analysis is used to model the voltage losses of the electrolyser MEAs. It is shown that while the proton transport resistance decreases for higher ionomer loadings, additional losses occur which can be attributed to mass transport and electronic conduction resistances. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies). Seed-funding by the Bavarian State Ministry of Education and Culture, Science and Art through the Munich School of Engineering in the framework of the “Energy Valley Bavaria” project, as well as technical support by the TUM chemistry department workshop and S. Koynov, is gratefully acknowledged.