Synthesis of hyperbranched Co-Ni-P nanocrystals and their splitting degree dependent HER performances

Abstract Hyperbranched transition metal phosphides (TMPs) nanocrystals are an interesting class of materials, which can be used as an efficient catalyst for hydrogen evolution reaction (HER). However, big challenge remains for the preparation of ternary hyperbranched TMPs, and relationship between their HER performances and the splitting degree is also not well understood. Here, hyperbranched Co-Ni-P nanocrystals with different splitting degrees were prepared via a novel strategy, and their alkaline HER performances were studied. During the synthesis process, hyperbranched Co2P nanocrystals were used as template, and Ni precursors were injected into the reaction solution at different holding stage. When the Ni precursor was injected earlier, the podgy particles with low splitting degree would be obtained. Conversely, delaying the injection of Ni precursor would lead to high splitting degree. The formation process of ternary phase was also studied. After Ni precursor was injected, a Co-Ni-P amorphous layer was formed on the surface of the Co2P particles, and then the Ni atoms diffused into the Co2P phase to form Co-Ni-P nanocrystals. Electrochemical performances tests show that the splitting degree has great impact on HER performances. The higher splitting degree usually leads to larger electrochemical surface area, and higher catalytic activity can be achieved. The sheaf-like Co-Ni-P nanocrystals demonstrate a small overpotential of 119 mV at current density of 10 mA cm−2, and good durability over 23 h in 1 M KOH solution. We believe that this work can help understand the relationship between the HER performances of hyperbranched TMPs nanocrystals and their splitting degree, and provide a synthetic strategy for the preparation of highly efficient TMPs catalysts.