Abstract text (incl. figure legends and references)

The transition to a carbon-neutral energy sector is linked with the implementation of energy storage solutions. The need for energy storage is due to the fluctuation and intermittency of renewable energy sources. Hydrogen, as an energy carrier produced from electrochemical water splitting, is a promising candidate as a storage solution. Proton exchange membrane water electrolyzers (PEMWEs) are suited to produce hydrogen in a renewable energy-based energy infrastructure due to their flexibility, rapid system response, and scalability.

The membrane electrode assembly of PEMWEs consists either of a catalyst-coated membrane (CCM) or a porous transport electrode (PTE) configuration. In the latter, the catalyst ink is directly coated onto the porous substrate, typically a titanium fiber or sintered titanium powder porous transport layer (PTL) on the anode side.

To date, the CCM configuration is prevalent, but the PTE configuration is a promising alternative concept for PEMWEs.[1] Its different fabrication approach allows the usage of a wider variety of membranes and can reduce the number of processing steps in manufacturing compared to the fabrication of CCMs. However, the PTE design has higher kinetic overpotentials due to a worse connection between catalyst particles and the membrane.[2] The less efficient catalyst utilization originates from the deep infiltration of the catalyst ink into the PTL. We could prove this catalyst distribution and the associated worse catalyst utilization in our recently submitted paper.[3]

Further improvements in PTEs are necessary to make them competitive against CCMs. One essential objective is to understand the whole structure of PTEs to elucidate the transport-activity nexus of these electrodes. Therefore, we used a combination of different tomographic methods to image the structure of the titanium fiber substrate, the IrO₂ catalyst layer, and its catalyst distribution. X-ray tomography and focused ion beam scanning electron microscopy (FIB-SEM) were employed to resolve the whole electrode on the µm- and nm-scale for the first time. Based on the reconstructions, we calculated the structural and transport parameters of the PTE. The analysis revealed the distribution and thickness of the catalyst layer and the grain and pore size distributions on the different length scales. Our correlative microscopy approach revealed that future PTE designs should aim to use a PTL with a porous blocking layer or a micro-porous layer.

[1] M. Bühler, F. Hegge, P. Holzapfel, M. Bierling, M. Suermann, S. Vierrath, S. Thiele, Journal of Materials Chemistry A 2019, 7, 26984.

[2] D. Kulkarni, A. Huynh, P. Satjaritanun, M. O"Brien, S. Shimpalee, D. Parkinson, P. Shevchenko, F. DeCarlo, N. Danilovic, K. E. Ayers, C. Capuano, I. V. Zenyuk, Applied Catalysis B: Environmental 2022, 308, 121213.

[3] M. Bierling, D. McLaughlin, B. Mayerhöfer, S. Thiele, Towards understanding catalyst layer deposition processes and distribution in anodic porous transport electrodes in proton exchange membrane water electrolyzers (submitted).