What X-ray absorption spectroscopy can tell us about the active state of earth-abundant electrocatalysts for the oxygen evolution reaction

Abstract

Chemical energy storage is an attractive solution to secure a sustainable energy supply. It requires an electrocatalyst to be implemented efficiently. In order to rationally improve the electrocatalyst materials and thereby the reaction efficiency, one must reveal the nature of the electrocatalyst under reaction conditions, i.e., its active state. For a better understanding of earth-abundant metal oxides as electrocatalysts for the oxygen evolution reaction (OER), the combination of electrochemical (EC) methods and X-ray absorption spectroscopy (XAS) has been very insightful and still holds untapped potential. Herein, we concisely introduce the basics of EC and XAS and provide the necessary framework to discuss changes that electrocatalytic materials undergo, presenting manganese oxides as examples. Such changes may occur during preparation and storage, during immersion in an electrolyte, as well as during application of potentials without or with catalytic reactions. We conclude with a concise summary of how EC and XAS are currently combined to elucidate the active state as well as an outlook on future opportunities to understand the mechanisms of electrocatalysis using combined operando EC-XAS experiments.

Design of 2D nanomaterials (photo-)electrocatalysts for biomass valorization coupled with H2 production

“…In this context, a clear understanding of catalyst interfacial processes such as dynamic chemical/structural/electronic evolution, formation/interaction of key electroactive intermediates, and dominant reaction pathways, is required to rationalize the selection of design strategies and to drive the knowledge-based development of next-generation biomass valorization/HER electrocatalysts. Combination of (photo-)electrochemical and cutting-edge in situ characterization techniques under operation can offer valuable insights at atomic-level to this end, leading to emergence of a wide range of in situ microscopic and spectroscopic methods, such as surface interrogation-scanning electrochemical microscopy (SI-SECM) [115][116][117] , transmission electron microscopy (TEM) 118 , Fourier transform infrared spectroscopy (FTIR), 59,119,120 vibrational sum-frequency generation (vSFG), [121][122][123] Raman spectroscopy, [124][125][126][127] X-ray absorption spectroscopy (XAS), [128][129][130] X-ray diffraction (XRD), 131,132 and Xray photoelectron spectroscopy (XPS). 133,134 In addition to these landmarks, facilitating the use of machine-learning methods for theoretical calculation is versatile approach for in-depth kinetic modeling of the focused catalyst interfaces, which can be used to predict the reaction mechanism by calculating the energy levels of the electrons in the catalytic material and the interactions between the electrons and the ions in the electrolyte.…”

A comparison of photodeposited RuOx for alkaline water electrolysis

“…of 0.2 and 1.2 were observed for Ru NIR Cl and Ru NIR 2eh , respectively. Initial oxidation state changes are expected, given the catalyst is being submerged in an alkaline electrolyte and small potentials are applied 67. However, the drastic changes of the Ru UV 2eh are beyond expected, rather it may be a demonstration of the instability of the amorphous RuO x phase.…”