Push-pull electronic effects of three-component two-heterojunction electrocatalyst for directional carrier transport in bifunctional water electrolysis

Abstract

Developing earth-abundant electrocatalysts with accelerated kinetics for water splitting is pivotal yet challenging due to the sluggish nature of multi-electron transfer processes. Herein, we report the strategic design of a hierarchical ternary heterostructure, Ni₃(PO₄)₂/NiCo₂S₄/NiCo(OH)₂ (NiPi/NCS/NCOH), based on a "push-pull" electronic modulation mechanism. By integrating electron-donating sulfide (“push”) and electron-withdrawing phosphate groups (“pull”), we tune the electronic density at the metal centers. Mott-Schottky analysis confirms the establishment of a dual n-n-p heterojunction, which induces a built-in electric field that drives directional carrier transport, channelling holes to the NCOH component for the oxygen evolution reaction (OER) and electrons to the NCS component for the hydrogen evolution reaction (HER). Consequently, this optimised interface delivers improved bifunctional performance in alkaline media as compared to its bare counterparts, achieving low overpotentials of 290 mV for OER and 95 mV for HER to reach 10 mA cm^-2, accompanied by better kinetics (Tafel slopes of 65 and 70 mV dec^-1, respectively for OER and HER). The hierarchical assembly significantly expands the electrochemically active surface area (C_dl of 35.2 μF cm^-2) and enhances intrinsic activity, yielding a 3.7-fold improvement in turnover frequency (0.87 s^-1) over the bare counterpart. Furthermore, Distribution of Relaxation Time (DRT) analysis reveals a shortened relaxation time (τ ≈ 0.1s), validating that the dual-junction design effectively minimizes energy barriers and accelerates interfacial charge transfer kinetics.