Changes in the structure of electrodeposited manganese oxide water oxidation catalysts revealed by in-operando Raman spectroscopy

Manganese oxides are promising water oxidation catalysts that have attracted great interest in the fuel cell industry. Previous studies have investigated differences in electrode activity based on the phases of MnO2 involved determining structures and composition ex-situ. Here we compare two preparations of MnO2 films, with differing catalytic activities as prepared, and observe morphological changes as a function of increasingly oxidizing potentials using in-operando Raman spectroscopy. We find that the Raman spectra of the films, both as-deposited and in solution, can be modelled as a binary mixture of amorphous manganese oxide and pure-phase cryptomelane that develop greater cryptomelane (alpha-MnO2) character as the oxidizing potential increases in 1M KOH. Analyzing the Raman spectra using classical least squares fitting, we determine that both films converge on a cryptomelane score of approximately 0.25; concurrently their catalytic activities improve. These results suggest that significant variations in electrocatalytic activity arise from short range ordering in the catalyst that is not easily determined using ex-situ structure determination. (C) 2019 Published by Elsevier Inc.