Interfacial Electron Transfer of Ni2P–NiP2 Polymorphs Inducing Enhanced Electrochemical Properties

Heterointerface engineering can be used to develop excellent catalysts through electronic coupling effects between different components or phases. As one kind of promising Pt-free electrocatalysts for hydrogen evolution reaction (HER), pure-phased metal phosphide exhibits the unfavorable factor of strong or weak H*-adsorption performance. Here, 6 nm wall-thick Ni₂ P-NiP₂ hollow nanoparticle polymorphs combining metallic Ni₂ P and metalloid NiP₂ with observable heterointerfaces are synthesized. It shows excellent catalytic performance toward the HER, requiring an overpotential of 59.7 mV to achieve 10 mA cm^-2 with a Tafel slope of 58.8 mV dec^-1 . Density functional theory calculations verify electrons' transfer from P to Ni at the heterointerfaces, which decreases the absolute value of H* adsorption energy and simultaneously enhance electronic conductivity. That is, the heterojunctions balance the metallic Ni₂ P and the metalloid NiP₂ to form an optimized phosphide polymorph catalyst for the HER. Furthermore, this polymorph combination is used with NiFe-LDH nanosheets to form an alkaline electrolyzer. It shows highly desirable electrochemical properties, which can reach 10 mA cm^-2 in 1 m KOH at 1.48 V and be driven by an AAA battery with a nominal voltage of 1.5 V. The work about interfacial charge transfer might provide an insight into designing excellent polymorph catalysts.