Ni³⁺-Rich Ni/NiO_x@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_ad adsorption by incompletely filled d orbitals of NiO phase and favorable H_ad 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 Niⁿ⁺ ratios. To address the above bottlenecks, herein, the ultrasmall Ni/NiO_x@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_x heterojunctions, the extreme rise in interfaces and Ni³⁺ defects with t6 2ge1 g electronic configuration owed to the ultrasmall size, the Ni/NiO_x@C nanocapsules exhibit the highest catalytic activity and the lowest overpotential of η₁₀ = 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.