Amorphous/Crystalline Heterostructured Nickel Phosphide Nanospheres for Electrocatalytic Water and Methanol Oxidation Reactions
Chao Wang, Le Chai, Xian Cui, Zixiang Zhou, Shuling Liu
Abstract
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.