Vittorio de Nora Award Address) Analysis of the Catalyst Requirements with Regards to Catalyst Structure and Catalyst Durability Studies for PEM Water Electrolysis

Proton exchange membrane (PEM) based water electrolyzers are a promising technology for the large-scale production of hydrogen from renewable energy. The most efficient and durable catalysts for PEM water electrolysis are platinum to catalyze the hydrogen evolution reaction (HER) and iridium oxide to catalyze the oxygen evolution reaction (OER). While the currently rather high noble metal loadings (≈0.3-0.5 mgPt/cm2 and ≈1-3 mgIr/cm2) have only a minor impact on the overall water electrolyzer system cost [1], the large-scale global implementation of PEM water electrolysis would require a substantial reduction of the noble metal loadings due to the limited availability of Pt and Ir. This is easy to achieve for the Pt cathode catalyst, owing to its fast HER kinetics and the fact that a high dispersion of Pt is readily achieved with carbon-supported Pt (Pt/C). However, lowering the Ir loading below ≈0.5 mgIr/cm2 is not possible with currently available Ir catalysts (Ir black, nano-IrO2, or IrO2 supported on a TiO2 substrate), as it results in too thin, non-contiguous electrode layers with poor in-plane conductivity, owing to their low iridium packing density of ≈2.3 gIr/cm3electrode (this is in contrast to ≈0.05-0.10 gPt/cm3electrode for a typical Pt/C catalyst) [2]. Thus, to reach the long-term target of an Ir-specific power density of ≤0.01 gIr/kW [2], novel catalysts with a lower iridium packing density are required, which could be achieved, e.g., by supporting iridium or iridium oxide nanoparticles on conductive supports or by other approaches [3]. A detailed analysis leading to this conclusion will be discussed in this presentation.The development and implementation of such novel catalysts also requires the evaluation of their long-term stability. Conventionally, this is either done by measurements in liquid electrolyte, typically using the rotating disk electrode (RDE) technique, or testing in an actual PEM electrolyzer. Unfortunately, recent studies revealed that the RDE based approach does not yield reliable catalyst durability data, since for yet unknown reasons, the catalyst life-times obtained by the RDE method are three to four orders of magnitude lower than what is observed in PEM electrolyzers [3, 4]. On the other hand, catalyst durability testing in a PEM electrolyzer requires very long measurement times, so that accelerated durability test protocols are necessary [5]. These aspects will also be discussed.References:[1] K. E. Ayers, N. Danilovic, R. Ouimet, M. Carmo, B. Pivovar, M. Bornstein; Annual Review of Chemical and Biomolecular Engineering 10 (2019) 219.[2] M. Bernt, A. Siebel, H. A. Gasteiger; J. Electrochem. Soc. 165 (2018) F305.[3] M. Bernt, A. Weis, M. F. Tovini, H. El-Sayed, C. Schramm, J. Schroter, C. Gebauer, H. A. Gasteiger; Chem. Ing. Tech 92 (2020); https://doi.org/10.1002/cite.201900101.[4] H. A. El-Sayed, A. Weis, L. F. Olbrich, G. P. Putro, H. A. Gasteiger; J. Electrochem. Soc. 166 (2019) F458.[5] A. Weis, A. Siebel, M. Bernt, T. H. Shen, V. Tileli, H. A. Gasteiger; J. Electrochem. Soc. 166 (2019) F487.