Advances in fundamental and applied knowledge of catalytic water splitting and CO2 reduction by in situ x-ray absorption spectroscopies

Sustainable energy conversion and storage technologies are crucial for reducing fossil fuel dependency and achieving carbon neutrality for a clean environment and safe societies. Electrocatalytic O2 and H2 generations from water splitting, as well as CO2 reduction and capture, have achieved considerable advances toward addressing energy and environmental challenges. Although numerous electrocatalysts have demonstrated excellent activity and selectivity at the laboratory scale under controlled conditions, their structural evolution and active state transformation under realistic operational environments remain insufficiently understood. X-ray absorption spectroscopy (XAS) analysis provides a detailed information on the electronic structure and local coordination environment of catalysts. A more effective approach is to first establish the importance of studying structural evolution, as it is crucial for identifying accurate active species, clarifying reaction pathways, improving product selectivity, and evaluating catalyst stability. To address these challenges, in situ characterization techniques have been developed to investigate catalysts under realistic working environments. Among them, in situ XAS has emerged as a very powerful tool for probing the dynamic behavior of electrocatalysts, offering insights into their atomic structure, oxidation states, coordination environments, and electronic configurations, particularly in water splitting and CO2 reduction reaction. This review highlights the recent advances in ex situ and in situ XAS, combined with complementary analytical techniques, for elucidating catalyst transformations and catalytic mechanisms at various scales with an emphasis on the probing and monitoring of catalyst–electrolyte interactions. Moreover, a critical overview of recent advances in understanding the underlying mechanisms of water splitting and CO2 reduction is presented, emphasizing advanced in situ XAS characterization of these two key electrocatalytic reactions. Finally, perspectives are provided on emerging x-ray techniques that offer a comprehensive view of their roles in catalyst design, efficiency enhancement, and performance optimization.