Polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for the production of hydrogen from fluctuating renewable energies [1]. Due to the scarcity of iridium that is commonly used as anode catalysts for the oxygen evolution reaction (OER), low anode loadings have become increasingly important [2]. Some low iridium loaded electrodes come with the drawback of a low electrical conductivity, which can be mitigated if the adjacent porous transport layer (PTL) has a similarly fine pore structure as that of the carbon black based microporous layers (MPL) used in PEM fuel cells [3]. First developments of titanium MPLs for PEMWE were based on vacuum plasma spraying [4, 5]. In recent work, titanium MPLs have also been fabricated by powder sintering, however these MPLs are very thick (200-300 µm) [6] and thus are almost as thick as the complete PTL used in other work. In this work, we present a method to produce microporous layers for PEMWE anodes by a titanium powder sintering process. For this, we coat a titanium slurry on top of a commercial powder-sintered PTL substrate. The successive sintering step solidifies the MPL coating and forms a single component from the two layers. The effects of the sintering temperature on the surface morphology are investigated by scanning electron microscopy (SEM) top-view and cross-sectional imaging (see Figure 1). An analysis of the pore structure of the developed MPL/PTL composite through mercury intrusion porosimetry (MIP) reveals pore-sizes of approximately one order of magnitude below those of the PTL substrate. Electrochemical characterization is carried out in 5 cm2 PEMWE single-cells with low-iridium loadings (0.2 mg/cm2), measuring polarization curves up to 6 A/cm2 and performing electrochemical impedance spectroscopy (EIS) measurements. Our study demonstrates the benefits of MPLs for highly efficient PEM water electrolyzers with low-iridium loadings, as a 15% lowered HFR and a slightly improved iR-free cell voltage can be observed for our MPL compared to the pristine PTL substrate without MPL. Further analysis will be conducted to disentangle the contribution of the MPL (surface) morphology and of its surface properties to the observed performance benefits. 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."; Renew. Sust. Energ. Rev., 2018, 82, 2440-2454. [2] M. Bernt, A. Weiß, M. F. 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., 2020, 92, 31-39. [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., 2018, 165.5, F305-F314. [4] P. Lettenmeier, S. Kolb, F. Burggraf, A. S. Gago, K. A. Friedrich; „Towards developing a backing layer for proton exchange membrane electrolyzers”; J. Power Sources, 2016, 311, 153-158. [5] J. K. Lee, C. Lee, K. F. Fahy, P. J. Kim, J. M. LaManna, E. Baltic, D. L. Jacobson, D. S. Hussey, S. Stiber, A. S. Gago, K. A. Friedrich, A. Bazylak; “Spatially graded porous transport layers for gas evolving electrochemical energy conversion: High performance polymer electrolyte membrane electrolyzers”; Energy Convers. Manag, 2020, 226, 113545. [6] T. Schuler, J. M. Ciccone, B. Krentscher, F. Marone, C. Peter, T. J. Schmidt, F. N. Büchi; “Hierarchically structured porous transport layers for polymer electrolyte water electrolysis”; Adv. Energy Mater., 2020, 10.2, 1903216. Acknowledgements: This work was funded by the German Federal Ministry of Education and Research (BMBF) in the framework of the Kopernikus P2X project (funding number 03SFK2V0-2). Figure 1

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 mgIr/cm2 elelctrode to below ~0.05 mgIr/cm2 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 gIr/cm3 electrode), so that for iridium loadings below ~0.4 mgIr/cm2 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 cm2 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 IrOx 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).