Atomic precision unleashed: Single-atom catalysts revolutionizing electrocatalytic hydrogen evolution reaction

The hydrogen evolution reaction (HER) is a key electrochemical process for sustainable hydrogen production, but its sluggish kinetics require highly efficient electrocatalysts. Single-atom catalysts (SACs) have emerged as a promising class of HER electrocatalysts owing to their maximum atomic utilization, well-defined active sites, tunable coordination environments, and exceptional catalytic performance. Despite significant progress in this field, existing reviews mainly focus on catalyst development and performance, with limited emphasis on mechanistic insights catalyst design strategies, descriptor-based understanding, stability, and practical implementation. To address this gap, this review provides a critical discussion on the recent advances in both noble-metal-based and noble-metal-free SACs for HER, highlighting the evolution of catalyst design from precious-metal to earth-abundant SACs together with their coordination environments, support materials, and catalytic performance. The review systematically discusses HER mechanisms, compares the key catalytic descriptors governing HER activity, and summarizes recent strategies including coordination engineering, defect engineering, and metal-support interaction engineering for improving the activity and stability of SACs. Furthermore, descriptor-based catalyst design, supported by density functional theory (DFT) calculations, is discussed to explain the physical significance of key catalytic descriptors and their roles in guiding the rational design of high-performance HER electrocatalysts. Finally, the review highlights the current challenges associated with catalyst stability, scalable synthesis, and practical device integration, while outlining future research directions, including industrial HER evaluation, membrane-electrode assembly integration, scalable catalyst manufacturing, and advanced materials engineering toward the development of highly efficient, durable, and economically viable SACs for sustainable hydrogen production.