What X‐Ray Absorption Spectroscopy Can Tell Us About the Active State of Earth‐Abundant Electrocatalysts for the Oxygen Evolution Reaction**

Abstract

Implementation of chemical energy storage for a sustainable energy supply requires the rational improvement of electrocatalyst materials, for which their nature under reaction conditions needs to be revealed. 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) is very insightful, yet still holds untapped potential. Herein, we concisely introduce EC and XAS, providing the necessary framework to discuss changes that electrocatalytic materials undergo during preparation and storage, during immersion in an electrolyte, as well as during application of potentials, showing Mn oxides as examples. We conclude with a summary of how EC and XAS are currently combined to elucidate active states, as well as an outlook on opportunities to understand the mechanisms of electrocatalysis using combined operando EC-XAS experiments.

A Comparison of Photodeposited RuOx for Alkaline Water Electrolysis

“…Initial oxidation state changes are expected, given the catalyst is being submerged in an alkaline electrolyte and small potentials are applied. 68 However, the drastic changes of the Ru 2eh UV are beyond expectation; rather it may be a demonstration of the instability of the amorphous RuO x phase. Evidence of this instability is also seen in the XPS and chronopotentiometry results.…”

Upgrading the Detection of Electrocatalyst Degradation During the Oxygen Evolution Reaction

“…Many electrocatalysts were likely discarded prematurely due to inappropriate electrolyte composition or experimental apparatuses.  The identification of the suspected process of degradation can likely be achieved in a well-designed postmortem investigation with the caveat that the degradation of the material may occur before electrocatalysis, [45] e.g., a reaction with an additive during sample preparation. [46] Understanding the mechanism of degradation needs an operando or in situ experiment, particularly those with long duration.…”

“…[46] Understanding the mechanism of degradation needs an operando or in situ experiment, particularly those with long duration. [45] Mechanistic insight is a key prerequisite for the knowledgeguided mitigation of electrocatalyst degradation, e.g. by optimizing the composition.…”

Operando X-ray characterization of interfacial charge transfer and structural rearrangements

“…The application of operando XAS measurements in electrocatalysts 34,72 , solar energy materials, 73 and lithium-ion batteries 35 have been discussed is recent reviews, therefore only selected examples will be discussed briefly.…”