Ultra-thin carbon layer encapsulated NiCoP coralline-like catalysts for efficient overall water electrolysis

Bimetallic phosphides exhibit excellent catalytic performance for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) because the adsorption/desorption energies of reactants are optimized by charge redistributions among metals and phosphorus. However, their wide application is limited due to unsatisfactory stability, inappropriate reaction intermediates' adsorption/desorption capacities, and a lack of activation study. Herein, a coralline-like nano-composite catalyst, NC@NiCoP@NF, is synthesized through coupling a bimetallic phosphide (NiCoP) and N-doped carbon layer (NC layer) on a Ni foam. The hierarchical structure and carbon coating result in modified electronic structures, improved adsorption/desorption capacities of intermediates, abundant active sites, and well-defined electron/electrolyte pathways, thus exhibiting optimal performance to catalyze the HER, the OER, and water electrolysis. The analysis after a long-term reaction gives an insight into the surface activation process of the catalyst, in which NiCoP evolves to hydroxide (HER) and oxyhydroxide (OER) to form new heterostructures (NiCoP/M-OH or NiCoP/MOOH) on surfaces. Density functional theory (DFT) calculations prove that the interaction between NiCoP and M-OH in the new heterostructure reaction system greatly improves the HER performance through promoting electronic conductivity, lowering the adsorption/dissociation energy barrier for H2O molecules, and optimizing the free energy of hydrogen adsorption (Delta GH*). This paper reports innovative methods to design and develop advanced high-performance catalysts for renewable energy generation. Carbon encapsulation and coralline-like structures improve the electronic structures and number of active sites, respectively. Surface activation promotes the synergy between NiCoP and M-OH to optimize adsorption/desorption abilities for intermediates.