Electrocatalytic hydrogen and oxygen evolution reactions: Role of two-dimensional layered materials and their composites

Electrocatalytic hydrogen and oxygen evolution reactions (HER & OER) are the fundamental key processes of the overall water splitting towards sustainable hydrogen production. The reaction efficiency of these processes are majorly controlled by the characteristics (structure and properties) of the crucial components such as the elec-trocatalyst as well as the electrolyte system. In particular, the detailed investigation of the structure-property relationship of electrocatalyst along with reaction mechanism and electrokinetics supporting through compu-tational calculations would boost the design and development of high-performance next-generation electro-catalytic systems. The electrocatalysts with higher electrical conductivity and more exposed active sites always exhibit better performance than the nonporous bulk materials. The transition-metal based 2D layered materials are highly suitable for this purpose due to their tunable interlayer spacing, edge-active sites and better electrical conductivity. In this review, the development of non-precious transition metal-based 2D layered materials including transition metal dichalcogenides, layered-double hydroxides, MXenes, and graphitic carbon nitride towards the HER & OER activities is discussed with very recent examples. The synthesis and electrocatalytic performance of these 2D layered materials and their composites are summarized along with the computational studies. Finally, the present challenges in the design of high-performance electrocatalysts together with the future perspectives are detailed in this review.