M-N-C-based single-atom catalysts for H2, O2 & CO2 electrocatalysis: activity descriptors, active sites identification, challenges and prospects

Owing to the atomic economy and exceptional electronic properties, single-atom catalysts (SACs) are considered as the emerging frontier for electrocatalytic activation of small molecules like H2, O2, and CO2 in energy sectors. However, an ideal catalyst is essentially required to understand the nature of active sites and reaction pathways for subsequent progress in electrocatalysis. In this review, we have highlighted (i) theories and hypothesis related to activity descriptors for metal-nitrogen-carbon (M-N-C)-based SACs, (ii) recent advancements on electrocatalytic reactions involving H2, O2, & CO2 molecules using M-N-C-based SACs, (iii) the techniques like in situ/operando XAS, SEM/TEM, FT-IR, and operando time-of-flight MS for characterization of M-N-C-based SACs, (iv) various approaches, including defects and heteroatoms engineering, metal-support interaction, alloying, molecular and spatial confinement based strategies to design and achieve highly stabilized M-N-C-based SACs (v) challenges and future prospective. We believe that this review will receive the attention of readers and researchers working in the field of electrocatalysis and materials sciences for further development on SACs and their applications.