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 Ni₂P–NiP₂ hollow nanoparticle polymorphs combining metallic Ni₂P 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 Ni₂P 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.