Operando tracking of oxidation-state changes by coupling electrochemistry with time-resolved X-ray absorption spectroscopy demonstrated for water oxidation by a cobalt-based catalyst film

Chiara Pasquini,Stefan Loos,Katharina Klingan,Vadim Sikolenko,Diego Gonzalez-Flores,Luca D’Amario,Rodney D L Smith,Stefan Mebs,Paul Beyer,Michael Haumann,Petko Chernev,Ivelina Zaharieva,Holger Dau,Si Liu,Mohammad Reza Mohammadi,Shan Jiang,Paul Kubella

doi:10.1007/s00216-021-03515-0

Abstract

Transition metal oxides are promising electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), which is critical in electrochemical production of non-fossil fuels. The involvement of oxidation state changes of the metal in OER electrocatalysis is increasingly recognized in the literature. Tracing these oxidation states under operation conditions could provide relevant information for performance optimization and development of durable catalysts, but further methodical developments are needed. Here, we propose a strategy to use single-energy X-ray absorption spectroscopy for monitoring metal oxidation-state changes during OER operation with millisecond time resolution. The procedure to obtain time-resolved oxidation state values, using two calibration curves, is explained in detail. We demonstrate the significance of this approach as well as possible sources of data misinterpretation. We conclude that the combination of X-ray absorption spectroscopy with electrochemical techniques allows us to investigate the kinetics of redox transitions and to distinguish the catalytic current from the redox current. Tracking of the oxidation state changes of Co ions in electrodeposited oxide films during cyclic voltammetry in neutral pH electrolyte serves as a proof of principle.Graphical abstract

Highlights

The transition from fossil fuels towards renewable energy sources requires massive efforts in technological developments
Synthetic fuels are of high interest, with electrocatalytic water oxidation emerging as a critical key process [1]
For the Co oxide catalyst (CoCat) films investigated here, repeated cyclic voltammetry (CV) cycling before the start of the experiment reduces the impact of film dissolution on the X-ray fluorescence intensity to less than 10% of the changes resulting from potentialdependent oxidation state changes

Summary

Introduction

The transition from fossil fuels towards renewable energy sources requires massive efforts in technological developments. In this context, synthetic (non-fossil) fuels are of high interest, with electrocatalytic water oxidation emerging as a critical key process [1]. According to our current knowledge, during the catalytic splitting of H2O into protons, electrons, and molecular oxygen, catalysts accumulate oxidizing equivalents via changes in metal oxidation state [4,5,6,7,8,9,10]. The stored oxidizing equivalents are used to oxidize water molecules. The proposed reaction mechanism for OER on cobalt-based catalysts involves two proton-coupled electron transfer reactions, linked to CoII → CoIII → CoIV oxidation, before onset of the rate-limiting O–O bond formation [5, 7, 11, 12]

Methods

Results

Conclusion