Amorphous/Crystalline Heterostructured Nickel Phosphide Nanospheres for Electrocatalytic Water and Methanol Oxidation Reactions

Designing electrocatalysts for the water oxidation reaction (WOR) and the methanol oxidation reaction (MOR) using earth-abundant elements is crucial for the development of water electrolyzers and direct methanol fuel cells. We report the synthesis of nickel phosphide nanospheres with different crystallinities (Ni2P-H, Ni2P-L, and Ni-P-a) by reacting Ni-based metal–organic frameworks and P4 solvothermally. All the nickel phosphide nanospheres synthesized are active toward WOR and MOR, and Ni2P-L, with amorphous/crystalline heterostructures, shows the best electrocatalytic performance. For WOR, the overpotential at 10 mA cm–2 for Ni2P-L on Ni foam is 300 mV in 1 M KOH (loading 0.8 mgNi+P cm–2), and the Tafel slope is ∼86.4 mV dec–1. For MOR, Ni2P-L on carbon paper has the highest current density of 427.0 mA mg–1 at 1.74 VRHE in 1 M KOH + 0.5 M CH3OH. Structural characterizations and electrochemical kinetic studies suggest that the high electrocatalytic activity of Ni2P-L originates from the abundant electrochemically active sites exposed owing to the amorphous phase and the electron interaction between Ni and P, which tunes the WOR intermediate (OH*) adsorption energy. Conversion of phosphides to (oxy)hydroxides at the surface is also observed after long-term WOR.