Single-atom sites (SASs) are commonly stabilized and influenced by neighboring atoms in the host; disclosing the structure-reactivity relationships of SASs in water electrolysis is one of the grand challenges originating from the tremendous wealth of support materials with complex structures. Through a multidisciplinary view of the design principles, synthesis strategies, characterization techniques, and theoretical analysis of structure-performance correlations, this timely Review is dedicated to summarizing the most recent progress in tailoring bond microenvironments on different supports and discussing the reaction pathways and performance advantages of different SAS structures for water electrolysis. The essence and mechanisms of how SAS structures influence electrocatalysis and the critical requirements for future developments are discussed. Finally, the challenges and perspectives are also provided to stimulate the practical, widespread utilization of SAS catalysts in water-splitting electrolyzers.
