Microporous Transport Layers Facilitating Low Iridium Loadings in Polymer Electrolyte Water Electrolyzers

Abstract

Minimizing iridium content in the anodic catalyst layer (CL) of polymer electrolyte water electrolyzers (PEWE) is essential for facilitating the needed rapid scale-up of hydrogen production capacity. However, reducing the iridium loading inevitably produces thinner CLs (if not changing the catalyst itself). This can have severe effects on performance and stability, mainly due to the losses in the connectivity of the 3D structure. Microporous layers (MPLs) have the potential to improve the performance and catalyst layer utilization of PEWE cells[1], especially at low Ir-loadings since they minimize the in-plane transport length of the required species (i.e. electric and fluidic transport).In this work, we characterized the 3D structure and transport properties of typical IrO2/TiO2 core-shell PEWE catalyst layers[2] by synchrotron-based ptychographic X-ray laminography (PyXL)[3]. Furthermore, we fabricated CLs at different Ir-loadings between 2.5 – 0.1 mgIr cm-2 depositing them either on the membrane i.e. catalyst-coated membranes (CCM) or on the MPL/PTL i.e. porous transport electrode (PTE). We tested the electrodes in PEWE cells using novel MPLs and standard commercial porous transport layers (PTLs) and compared their electrochemical performance and overpotentials. We observe that ultra-low iridium loadings (0.1 mgIr cm-2) with only relatively small setbacks in performance are possible when using Pt-coated MPLs. Moreover, we correlate the electrochemical results with the 3D structure of the CL by PyXL and the MPL/CL interface by lab-based X-ray tomographic microscopy (Figure 1). Finally, we will discuss the challenges of fabricating low-loaded anodic CLs in CCM and PTE configuration and the advantages of using an MPL in both scenarios.[1] T. Schuler, J. M. Ciccone, B. Krentscher, F. Marone, C. Peter, T. J. Schmidt, F. N. Büchi, Adv. Energy Mater. 2020, 10, 1903216.[2] S. De Angelis, T. Schuler, M. Sabharwal, M. Holler, M. Guizar-Sicairos, E. Müller, F. N. Büchi, Sci Rep 2023, 13, 4280.[3] M. Holler, M. Odstrcil, M. Guizar-Sicairos, M. Lebugle, E. Müller, S. Finizio, G. Tinti, C. David, J. Zusman, W. Unglaub, O. Bunk, J. Raabe, A. F. J. Levi, G. Aeppli, Nature Electronics 2019, 2, 464.Figure 1. Surface renderings from XTM/PyXL and SEM images of porous transport electrodes (PTE, left) and catalyst-coated membranes (CCM, right) of selected samples used in this work. Figure 1