Ni-Rich Ni/NiO@C Nanocapsules Below 4 nm Constructed by Low-Temperature Graphitization of Self-Assembled Few-Layer Coordination Polymers toward Efficient Alkaline Hydrogen Evolution Electrocatalysis.

Low-cost and eco-friendly Ni/NiO heterojunctions have been theoretically proven to be the ideal candidate for stepwise electrocatalysis of alkaline hydrogen evolution reaction, attributed to the preferred OH adsorption by incompletely filled d orbitals of NiO phase and favorable H adsorption energy of Ni phase. Nevertheless, most Ni/NiO compounds reported so far fail to exhibit excellent catalytic activity, possibly due to the lack of efficient electron transport, limited interfacial active sites, and unregulated Ni ratios. To address the above bottlenecks, herein, the ultrasmall Ni/NiO@C nanocapsules (<5 nm) are directly constructed by graphitization of four-layer Ni-based coordination polymers at record low temperatures of 400 °C. Ascribed to the accelerated electron and mass transfer by the carbon nano-onions coated around Ni/NiO heterojunctions, the extreme rise in interfaces and Ni defects with t6 2ge1 g electronic configuration owed to the ultrasmall size, the Ni/NiO@C nanocapsules exhibit the highest catalytic activity and the lowest overpotential of η = 80 mV among various Ni/NiO materials (measured on the glassy carbon electrode). This work not only constructs an industrialized high-efficiency electrocatalyst toward alkaline HER, but also provides a novel strategy for the constant-scale preparation of multicomponent transition metals-based nanocrystals below 4 nm.