Functional role of single-atom catalysts in electrocatalytic hydrogen evolution: Current developments and future challenges

Single-atom catalysts (SACs) are currently regarded as the next generation of heterogeneous catalysts for electrochemical applications due to their distinct benefits of maximizing the efficiency of metal atom utilization and well-defined active centers. In this context, SACs for electrocatalytic hydrogen evolution reaction (HER) is a rapidly emerging field, and intense research has been devoted to rationally develop low-cost, high-performing, and durable SACs for HER. In the meantime, consistent efforts have also been made to gather insights on the geometric and electronic structures of SACs when anchored on appropriate support for HER and their subsequent structural transformation, active sites, and reaction mechanisms. Herein, we present a comprehensive and critical review that starts with an in-depth discussion on the three key aspects of SACs (stabilization, metal-support interaction, and coordination environment) and focuses on the significant contribution of support to favorably influence net HER efficiency. Further, we describe the importance of numerous support modification techniques along with concrete examples of the most frequently and widely used supports. We elaborate on various synthetic and in-situ (ex-situ) characterization techniques adapted in the field of HER electrocatalysis, discuss their activity trend, and highlight their understanding in terms of structure–activity-property relationships. Finally, the key challenges and prospects in employing SACs for electrocatalytic HER as well as guiding principles for fabricating intriguing SACs to acquire exceptional reactivity and stability have been underlined.