Flower-Like Nickel Phosphide Microballs Assembled by Nanoplates with Exposed High-Energy (0 0 1) 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 μm. With a controlled phosphorization temperature, flower-like nickel phosphide microballs with different crystalline structures (Ni5 P4 and Ni2 P) were obtained easily. Flower-like Ni5 P4 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 (0 0 1) facets. The flower-like Ni5 P4 microballs electrocatalyst thus displayed excellent activity for the HER with a low overpotential (η) of 35.4 mV to reach current densities of 10 mA cm-2 and a small Tafel slope of 48 mV dec-1 in acid solution. In addition, it showed excellent activity in 1 m KOH with η=47 mV at 10 mA cm-2 . DFT studies indicated that the free energy of hydrogen adsorbed on the Ni site of Ni5 P4 was 0.152 eV, which is smaller than that of the Ni site of Ni2 P (0.182 eV). Therefore, flower-like Ni5 P4 microballs exhibited better HER activity than Ni2 P, which is consistent with our HER data. This hierarchical structure with exposed high-energy (0 0 1) facets paves the way to design and synthesize low-cost, high-performance catalysts for the HER.