Novel insight from in-/ex-situ investigation toward intercalated water in high-performance oxygen evolution electrocatalysts and their mechanisms

Hydrogen is a green energy source with zero carbon emissions and renewable properties. Green hydrogen, produced via water electrolysis, can efficiently harness excess renewable energy during peak periods, making it a key player in grid stabilization through processes like power-to-gas. Consequently, there is a pressing need to develop catalysts with high activity, stability, and cost-effectiveness in the energy sector. The development of electrochemical (EC) water splitting, a promising path to meet alternative energy demands, however, is hindered by two main challenges that persist in water splitting: the anodic oxygen evolution reaction (OER) catalysts still need lower overpotential, and they must have enough stability with high catalytic activity. Both factors determine water electrolysis reactions' overall energy consumption and commercialization potential. This article introduces several significant parameters for assessing catalyst performance; three primary OER mechanisms are also briefly reviewed. Thereby, numerous electrocatalysts for OER are categorized by their composition and morphology. Furthermore, we generalize a common phenomenon that occurs at the surface of catalysts during the OER process. It is deduced that the surface transformation from as-prepared to an activated state, that is, the surface-environment change of the active site due to redox-induced dissolution and re-deposition, plays a critical role in OER. On the other hand, generating a layered structure leads to the accommodation of intercalated water molecules, which may enhance the activity and robustness via hydrogen bonds and dominate the absorption energy of oxygen species on the metal site. Lastly, we enumerate several in-/ex-situ methodologies that can discriminate the real active sites of the bulk, near-surface region, interface, and intermediates adsorbed on the electrocatalyst surface. Further investigation is needed to unveil the interaction between active sites and embedded water molecules in EC catalytic processes. This paper provides a novel perspective of the intercalated water for future development of OER electrocatalysts, simultaneously considering performance and stability.