Flower-Like Nickel Phosphide Microballs Assembled by Nanoplates with Exposed High-Energy (001) Facets: Efficient Electrocatalyst for the Hydrogen Evolution Reaction

The fabrication of low-cost and earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) over a broad pH range is attractive. In this work, a facile precursor route is developed to synthesize flower-like nickel phosphide microballs with a diameter of approximately 12 mu m. With a controlled phosphorization temperature, flower-like nickel phosphide microballs with different crystalline structures (Ni5P4 and Ni2P) were obtained easily. Flower-like Ni5P4 microballs possessed two advantageous features for enhanced HER: fast vectorial electron transfer path along the building block nanoplates and enhanced inherent catalytic activity of each active site for high-energy (001) facets. The flower-like Ni5P4 microballs electrocatalyst thus displayed excellent activity for the HER with a low overpotential (eta) of 35.4mV to reach current densities of 10 mAcm(-2) and a small Tafel slope of 48 mVdec(-1) in acid solution. In addition, it showed excellent activity in 1m KOH with eta = 47 mV at 10 mAcm(-2). DFT studies indicated that the free energy of hydrogen adsorbed on the Ni site of Ni5P4 was 0.152 eV, which is smaller than that of the Ni site of Ni2P (0.182 eV). Therefore, flower-like Ni5P4 microballs exhibited better HER activity than Ni2P, which is consistent with our HER data. This hierarchical structure with exposed high-energy (001) facets paves the way to design and synthesize low-cost, high-performance catalysts for the HER.