Nickel Oxide-Aerogel Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media: Experimental Approaches and Modeling-Assisted Strategies for Improving Performance and Durability

Abstract

Anion exchange membrane water electrolyzers (AEMWEs) represent an attractive technology for producing “green” hydrogen that enables operation on pure water using platinum group metal (PGM)-free electrocatalysts at both anode and cathode. Also, AEMWEs do not require the use of highly concentrated and corrosive alkaline electrolytes and PGM-based catalysts, which are the major drawbacks of the incumbent low-temperature liquid-alkaline and proton exchange membrane electrolyzers, respectively.1,2 In this context, the development of PGM-free electrocatalysts for oxygen evolution reaction (OER) in alkaline media has attracted considerable research interest. Among different types of transition metal-based oxides, Ni oxides doped with Fe have shown the highest OER activity in alkaline media.3,4 Recently, we have developed at Los Alamos National Laboratory (LANL) a series of Ni oxide-based aerogel materials that, primarily in combination with Fe in different proportions, have shown respectable OER performance in the electrochemical cell and at the AEMWE anode operating on either neat deionized water or with a supporting electrolyte, 0.1 M KOH or K2CO3.5 For application at the AEMWE anode, the catalyst integration into the electrode catalyst layer, i.e., combining the catalyst with anion exchange ionomer (AEI) and binding agents, is crucial to prevent catalyst layer delamination and to create a good catalyst/electrolyte interface, which in turn enables high OH- conductivity within the catalyst layer.6 This latter aspect is especially important for achieving high AEMWE performance in pure water operation. In this work, we investigate the impact of combining our Ni-Fe oxide aerogel catalysts with different AEIs (various backbone chemistries, OH- functional groups) and different binding agents (e.g., Nafion ionomer) on the AEMWE performance. We will show that full activation of the catalyst by phase transformation from the original Ni oxide-like structure to the active layered (oxy)hydroxide is essential for achieving high OER activity, and it can be influenced by the catalyst layer composition. By advanced characterization techniques such as high-resolution scanning transmission electron microscopy, X-ray absorption spectroscopy, and Mössbauer spectroscopy, we will shed light onto the phase transformation process that results in superior OER activity of these materials in alkaline media.Following our prior machine learning studies aimed at optimizing for the synthesis of oxygen reduction electrocatalysts,7,8 we will also show how to improve the synthesis of Ni oxide aerogel-based OER catalysts to maximize activity and stability. This work is further supported by density functional theory (DFT) modeling studies to better understand reaction mechanisms, active sites, and ultimately what role transition metal dopants (Fe and Co) play in modifying OER activity. Studies of in situ dissolution of these dopants using DFT-generated, phase-constrained Pourbaix diagrams9 will guide synthesis through understanding this likely materials degradation pathway. References H. A. Miller et al., Sustain. Energy Fuels, 4, 2114–2133 (2020).C. Santoro et al., ChemSusChem, 202200027 (2022).S. Fu et al., Nano Energy, 44, 319–326 (2018)D. Xu et al., ACS Catal., 9, 7–15 (2019).P. Zelenay and D. Myers, "ElectroCat (Electrocatalysis Consortium);” U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Program, 2022 Annual Merit Review and Peer Evaluation Meeting, June 6-8, 2022. https://www.hydrogen.energy.gov/pdfs/review22/fc160_myers_zelenay_2022_o.pdfL. Osmieri et al., J. Power Sources, 556, 232484 (2023).M. R. Karim et al., ACS Appl. Energy Mater., 3, 9083–9088 (2020).W. J. M. Kort-Kamp et al., J. Power Sources, 559 (2023).E. F. Holby, G. Wang, and P. Zelenay, ACS Catal., 10, 14527–14539 (2020).