Transport Properties of Iridium Oxide Catalyst Layers for Proton Exchange Membrane Water Electrolyzers

Abstract

One of the current challenges of proton exchange membrane (PEM) water electrolyzers (PEMWE) is to reduce the high costs associated with iridium as the anode catalyst material. In commercial PEMWE membrane electrode assemblies (MEAs), loadings of iridium are as high as 2 to 3 mgIr/cm2. However, for PEMWE systems to match global energy demand, the Ir-specific power density must increase by ~ 50-fold [1, 2]. This can be achieved by decreasing iridium loadings and increasing the utilization of iridium. Previous work has determined that homogeneity of anode catalyst layers is generally favorable and even necessary for good electrochemical performance at lower iridium loadings [1-3]. However, gaining a better understanding of the important micro- and nano-scale features of iridium oxide (IrO2) catalyst layers is still needed for informing fabrication methods and achieving higher Ir-specific power density operation. Our previous work used nano-scale X-ray computed tomography (nano-CT) and plasma-focused ion beam cross sectioning with scanning-electron microscopy (pFIB-SEM) to characterize several IrO2 catalyst layers varying in coating methods and steps along the fabrication and testing lifetime, finding differences in morphological properties particularly of the pore size distributions. Our current work expands on this image-based characterization to elucidate transport properties between catalyst layers fabricated with different coating methods and iridium loadings. Using modeling and simulation techniques, effective transport properties for pore and solid phases such as oxygen diffusivity, conductance, and tortuosity will be examined using the high-resolution image-based data. The results presented in this work will help elucidate the transport phenomena through the IrO2 catalyst layer and the relationship between fabrication, morphology, and performance.This conference presentation was developed based upon funding from the Alliance for Sustainable Energy, LLC, Managing and Operating Contractor for the National Renewable Energy Laboratory for the U.S. Department of Energy.[1] Bernt, M., Siebel, A., & Gasteiger, H. A. (2018). Analysis of Voltage Losses in PEM Water Electrolyzers with Low Platinum Group Metal Loadings. Journal of The Electrochemical Society, 165(5), F305–F314. https://doi.org/10.1149/2.0641805jes[2] Kulkarni, D., Huynh, A., Satjaritanun, P., O’Brien, M., Shimpalee, S., Parkinson, D., Shevchenko, P., DeCarlo, F., Danilovic, N., Ayers, K. E., Capuano, C., & Zenyuk, I. v. (2022). Elucidating effects of catalyst loadings and porous transport layer morphologies on operation of proton exchange membrane water electrolyzers. Applied Catalysis B: Environmental, 308. https://doi.org/10.1016/j.apcatb.2022.121213[3] Taie, Z., Peng, X., Kulkarni, D., Zenyuk, I. v., Weber, A. Z., Hagen, C., & Danilovic, N. (2020). Pathway to Complete Energy Sector Decarbonization with Available Iridium Resources using Ultralow Loaded Water Electrolyzers. ACS Applied Materials and Interfaces, 12(47), 52701–52712. https://doi.org/10.1021/acsami.0c15687