PGM-Free Catalysts and Electrodes for Anion Exchange Membrane Water Electrolyzers

Abstract

Recent progress in the development of anion exchange membranes (AEMs) with improved performance and durability has opened the way for the application of the AEM-based electrolyzers in low-temperature water electrolysis (LTWE),1 an important technology for producing “green” hydrogen.2 AEM-LTWEs can potentially operate on pure water, i.e., without highly concentrated and corrosive supporting electrolyte, and they allow for replacement of electrocatalysts based on platinum group metals (PGMs) with PGM-free ones, thus addressing the main drawbacks of the liquid-alkaline (LA) and proton exchange membrane (PEM) electrolyzers.3 Consequently, the development of PGM-free electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media is of primary importance for the deployment of AEM-LTWEs that has attracted significant attention of researchers.4–6 Besides improving the catalytic activity, the integration of PGM-free HER and OER electrocatalysts into electrodes for operation in AEM electrolyzers is crucial to achieving satisfactory electrolyzer performance and making them competitive with the LA and PEM systems.7,8 In this work, we measured electrocatalytic activity of a series of OER and HER catalysts in a three-electrode cell and then implemented these catalysts in electrodes for testing in an AEM electrolyzer. We investigated different classes of OER catalysts, including commercial IrO2 (a PGM ORR benchmark), LaxSr1-xCoO3-δ oxides, Ni-Fe nanofoam oxides, Ni-Fe aerogel-derived oxides, and MOF-derived Co oxides. In the HER-catalyst part of the study, we compared a commercial PtRu/C (a PGM HER benchmark) with an aerogel NiMo/C catalyst. Catalysts and electrodes before and after testing were characterized by XRD, SEM, EDS, and XPS.In addition to exploring different catalysts, we investigated the impact of several fabrication variables such as the ink deposition method, amount of ionomer, incorporation of a binding agent, and the type of anode porous transport layer on performance. The tests were carried in an electrolyzer operating with pure water and two electrolyte solutions, 0.1 M KOH and 1% K2CO3. The results show that, in addition to the OER and HER electrocatalytic activity, the electrode fabrication is an important factor affecting AEM electrolyzer performance, especially in the pure-water operation mode, in which case assuring an effective transport of the OH– ions within the catalyst layer is especially challenging. References Y. S. Kim, ACS Appl. Polym. Mater. (2021).C. Santoro et al., ChemSusChem, 202200027 (2022).H. A. Miller et al., Sustain. Energy Fuels, 4, 2114–2133 (2020).D. Xu et al., ACS Catal., 9, 7–15 (2019).H. Shi et al., Adv. Funct. Mater., 2102285, 1–10 (2021).H. Doan et al., J. Electrochem. Soc., 168, 084501 (2021).N. U. Hassan, M. Mandal, B. Zulevi, P. A. Kohl, and W. E. Mustain, Electrochim. Acta, 409, 140001 (2022).G. A. Lindquist et al., ACS Appl. Mater. Interfaces (2021).