Flower‐Like Nickel Phosphide Microballs Assembled by Nanoplates with Exposed High‐Energy (0 0 1) Facets: Efficient Electrocatalyst for the Hydrogen Evolution Reaction

Abstract

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.