Gradient Built-in Electric Fields in Janus Catalysts Govern Hydrogen Evolution via pH-Dependent Mechanisms

Hydrogen evolution reaction is essential for the hydrogen economy but is limited by sluggish kinetics, largely due to the difficulty in precisely tuning the electronic structure of the electrocatalysts. The built-in electric field (BEF) caused by the functional interface generated by loading noble metals on transition metal derivatives can effectively break the above limitation. However, the effects of the BEF have primarily been explored through qualitative or theoretical calculations, lacking quantitative experimental insight into the underlying mechanisms. In this work, we design a series of Janus catalysts by anchoring trace amounts of Pt onto crystalline or amorphous transition-metal phosphides, enabling gradient modulation of the BEF from 0.1 to 1.77 eV and enhancing HER activity in both acidic and alkaline media via distinct mechanisms. Crystalline CoP/Pt, with the weakest BEF, shows a lower energy barrier for *H spillover from Pt to Co. It requires only 10 mV to reach 10 mA cm–2 in 0.5 M H2SO4, with a Tafel slope of 22.35 mV dec–1. In contrast, amorphous NiP/Pt, exhibiting the strongest BEF, lowers the energy barrier for water dissociation. It delivers 10 mA cm–2 at 12 mV in 1 M KOH, with a Tafel slope of 20.81 mV dec–1.