In Situ/Operando Characterization Techniques to Probe the Electrochemical Reactions for Energy Conversion

The water‐splitting reaction, including the hydrogen and oxygen evolution reactions, as well as the electrochemical oxygen and CO 2 reduction reactions offer promising solutions to address the global energy scarcity and the associated environmental issues. However, the lack of deep insight into the reaction mechanisms and clear identification of the catalytic active sites hinder any breakthrough for the development of efficient electrocatalysts with high performance and durability. Operando characterization techniques allowing in situ monitoring the surface oxidation state and local atomic‐structure transformation are capable of probing the active sites and promoting the fundamental understanding of the reaction mechanism in these systems. Herein, the recent applications of various operando characterization techniques in identifying the active sites and capturing the geometric structure, oxidation state, and local atomic‐structure evolution of the catalysts during water electrolysis and O 2 /CO 2 electroreduction are thoroughly summarized. The challenges and outlook in developing operando techniques to further extend the understanding of the underlying mechanism during electrochemical energy‐conversion reactions are discussed.