(Invited) Design, Performance Characterization, and Durability of an Iridium-Based OER Catalyst for PEM Water Electrolysis (2022)

One of the building blocks to transition to a fully renewable energy supply is the utilization of hydrogen as a replacement of fossil fuels and as a chemical energy storage/carrier medium. This requires the economical and sustainable generation of hydrogen by water electrolysis, whereby proton exchange membrane (PEM) water electrolyzers would enable much higher power densities compared to conventional electrolyzers based on liquid alkaline electrolytes [1]. However, one of the short-comings of PEM water electrolyzers (PEMWEs) is the need for expensive and resource-limited iridium based catalysts for the oxygen evolution reaction (OER), so that the large-scale global deployment of PEMWEs would require a substantial reduction of the iridium loading from currently ~1-2 mg Ir /cm 2 elelctrode to below ~0.05 mg Ir /cm 2 elelctrode [2]. In this contribution, we will discuss the technical challenge to reduce the iridium loading using currently employed iridium catalysts, which is related to the high iridium packing density in the electrode (in units of g Ir /cm 3 electrode ), so that for iridium loadings below ~0.4 mg Ir /cm 2 the electrode becomes too thin to allow for a homogenous electrode with sufficient in-plane electrical conductivity [3]. We will then present a catalyst concept that results in much lower iridium packing densities and that thus enables lower iridium loadings [4]. While such a catalyst exhibits a lower electrical conductivity than a currently employed benchmark catalyst, this drawback can be mitigated by utilizing porous transport layers at the anode that have a highly conductive coating [4]. The long-term stability of this novel type of iridium based OER catalyst will be examined in a 30 cm 2 active area short-stack over ~3700 h; comparing the evolution of the OER mass activity and of the high frequency resistance corrected cell voltage with that of a benchmark catalyst that is evaluated in the same short-stack, which allows for mechanistic insights into the observed degradation rates [5]. References: [1] A. Buttler, H. Spliethoff; "Current status of water electrolysis for energy storage, grid balancing and sector coupling via power-to-gas and power-to-liquids: A review"; Renewable and Sustainable Energy Reviews 82 (2018) 2440. [2] M. Bernt, A. Weiß, M. Fathi Tovini, H. El-Sayed, C. Schramm, J. Schröter, C. Gebauer, H. A. Gasteiger; "Current Challenges in Catalyst Development for PEM Water Electrolyzers"; Chem. Ing. Tech. 92 (2020) 31. [3] M. Bernt, A. Siebel, H. A. Gasteiger; "Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings"; J. Electrochem. Soc. 165 (2018) F305. [4] M. Bernt, C. Schramm, J. Schröter, C. Gebauer, J. Byrknes, C. Eickes, H. A. Gasteiger; "Effect of the IrO x Conductivity on the Anode Electrode/Porous Transport Layer Interfacial Resistance in PEM Water Electrolyzers"; J. Electrochem. Soc. 168 (2021) 084513. [5] M. Möckl, M. Ernst, M. Kornherr, F. Allebrod, M. Bernt, J. Byrknes, C. Eickes, C. Gebauer, A. Moskovtseva, H. A. Gasteiger; "Durability investigation and benchmarking of a novel iridium catalyst in a PEM water electrolyzer at low iridium loading"; manuscript to be submitted. Acknowledgements: This work was conducted within the framework of the Kopernikus P2X project funded by the German Federal Ministry of Education and Research (BMBF).