Performance and Durability of Iridium Oxide Catalysts to Enable Large Scale Deployment of Proton Exchange Membrane Water Electrolyzers

Abstract

To meet cost targets for gigawatt-scale deployment of proton exchange membrane water electrolyzers (PEMWEs), catalyst, which is iridium (Ir) use needs to be reduced by about factor of ten to below 0.1 mg/cm2. However, catalyst layer structure for low IrOx catalyst loadings can suffer from poor in-plane electric conductivity and particle-to-particle connectivity. To study low catalyst loadings a rigorous methodology is needed, where catalyst layer inhomogeneities can be identified before extensive activity and durability studies. Here, we present a framework for testing low loading IrOx catalysts within membrane electrode assembly (MEA), where loading study was conducted using two Ishifuku catalysts having different structures: amorphous and crystalline. By varying catalyst loading, electrochemical descriptors were obtained such as mass specific activity, exchange current density, specific surface charge and others. The change in voltage in kinetic region between various catalyst loadings corresponded to Tafel slope, indicating that the catalyst surface area reduction was the major reason for the activity loss at lower loadings. The amorphous catalyst showed higher activity across all the loadings and current densities, lower Tafel slope, higher surface charge and about 5 times higher mass activity and exchange current density compared to crystalline catalyst. The physical characterization of the two catalysts through x-ray photoemission spectroscopy (XPS), Raman spectroscopy, BET analysis, x-ray diffraction (XRD), electron imaging reveals the morphological and surface characteristics for each catalyst. Amorphous Ishifuku catalyst is confirmed to have amorphous structure with high population of µ1 oxygen species on the surface that are believed to be responsible for high oxygen evolution reaction (OER) rates. Overall, here we show that loading study is a necessary first step for catalyst layer characterization with low catalyst loadings. Furthermore, we extend the study for 750 hours durability operation and evaluate crystalline and amorphous catalysts at the end of life. Amorphous catalyst structure shows transformation into crystalline, where exact mechanisms for such transformation are not yet well understood. This transformation occurs continuously within these 750 hours of testing, starting as soon as within first hours. The transformation is tracked with both cyclic voltammetry peaks shift and also periodic evaluation with XRD. This work highlights the importance of understanding structure to property to performance relation of evaluated catalysts to enable durable operation of water electrolyzers.

Biography

CBE Professor at University of California Irvine (UC Irvine) Iryna Zenyuk holds a BS (2008) in mechanical engineering from the New York University Tandon School of Engineering. She continued her studies at Carnegie Mellon University, where she earned MS (2011) and PhD (2013) in mechanical engineering. Zenyuk did her postdoctoral fellowship at Lawrence Berkeley National Laboratory in Electrochemical Technologies Group.

At UC Irvine, Zenyuk’s group works on enabling clean energy solutions, sustainable cement manufacturing and critical materials recovery by researching hydrogen fuel-cells, and various types of electrolyzers. Zenyuk works on design strategy encompassing novel materials, diagnostic tools and device-level testing to understand durability and feasibility of novel designs to be translated to industrial setting.

She is a recipient of the NSF CAREER award (2017), Interpore society Fraunhofer Award for Young Researchers (2017), Research Corporation for Science Advancement, Scialog Fellow in Advanced Energy Storage (2017-2019), Electrochemical Society (ECS) Toyota Young Investigator Award (2018), UCI Samueli School of Engineering Early Career Faculty Excellence in Research Award (2019) and ECS Energy Technology Division Srinivasan Young Investigator Award (2021), UCI Beal Applied Innovations Early Career Innovator of the Year (2021), UCI Samueli School of Engineering Mid Career Faculty Excellence in Research Award (2022). Prof. Zenyuk published over 150 journal publications and delivered more than 150 invited presentations on topics of energy conversion and storage.