Coralloid Co₂P₂O₇ Nanocrystals Encapsulated by Thin Carbon Shells for Enhanced Electrochemical Water Oxidation

Core-shell nanohybrids containing cheap inorganic nanocrystals and nanocarbon shells are promising electrocatalysts for water splitting or other renewable energy options. Despite that great progress has been achieved, biomimetic synthesis of metal phosphates@nanocarbon core-shell nanohybrids remains a challenge, and their use for electrocatalytic oxygen evolution reaction (OER) has not been explored. In this paper, novel nanohybrids composed of coralloid Co₂P₂O₇ nanocrystal cores and thin porous nanocarbon shells are synthesized by combination of the structural merits of supramolecular polymer gels and a controllable thermal conversion technique, i.e., temperature programmable annealing of presynthesized supramolecular polymer gels that contain cobalt salt and phytic acid under a proper gas atmosphere. Electrocatalytic tests in alkaline solution show that such nanohybrids exhibit greatly enhanced electrocatalytic OER performance compared with that of Co₂P₂O₇ nanostructure. At a current density of 10 mA cm^-2, their overpotential is 0.397 V, which is much lower than that of Co₂P₂O₇ nanostructures, amorphous Co-Pi nanomaterials, Co(PO₃)₂ nanosheets, Pt/C, and some reported OER catalysts, and close to that of commercial IrO₂. Most importantly, both of their current density at the overpotential over 0.40 V and durability are superior to those of IrO₂ catalyst. As revealed by a series of spectroscopic and electrochemical analyses, their enhanced electrocatalytic performance results from the presence of thin porous nanocarbon shells, which not only improve interfacial electron penetration or transfer dynamics but also vary the coordination environment and increase the number of active 5-coordinated Co²⁺ sites in Co₂P₂O₇ cores.