Understanding the Phase Equilibrium and Kinetics of Electrochemically Driven Phase Transition in CoOxHy during Electrocatalytic Reactions

Abstract

Water splitting is one of the most promising methods for mass production of green hydrogen (H2), with the oxygen evolution reaction (OER) being the current main kinetic bottleneck. Cobalt (oxy-)hydroxides are among the most active electrocatalysts for OER in alkaline electrolytes free from rare-earth metals. However, identifying the active phase of electrocatalysts under operational OER conditions is often difficult, which largely impedes the design of cobalt-based electrocatalysts with improved performance and durability. This lack of understanding is partially contributed by the difficulties in operando characterizations on the details of electrochemically driven phase transitions. In this work, we combine operando Raman spectroscopy and operando optical characterization to investigate the phase equilibrium and kinetics of the phase transition in CoOxHy driven by applied electrochemical driving force. We found an irreversible phase transition during the first electrochemical test, after which phase transition became fully reversible in each following cycle. We further established the relationship between kinetic parameters of the reversible phase transition and applied potential by using the time-resolved operando optical method. Our work provides a precise approach toward a better understanding of the electrochemically driven phase transition in OER electrocatalysts.