Ohmic contact interface in the self-surface reconstructed Ni4Mo/MoO2 heterostructure for achieving effective alkaline electrocatalytic water splitting

Abstract

Designing a bifunctional and cost-effective electrocatalyst for overall electrocatalytic water-splitting with high energy conversion efficiency poses a significant challenge that researchers are attempting to address. We have proposed an electrochemical mechanism featuring a junction-based electrode that enhances electrochemical activity during the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Our precursor Ni₄Mo/MoO₂ electrocatalyst employs a metal/n-type semiconductor (ohmic junction), which, upon undergoing electrochemical reactions, in alkaline pH along with molybdenum dissolution, transforms into NiOOH/MoO₂ (p–n junction) in the OER region and Ni(OH)₂/MoO₂ (p–n junction) in the HER region. The ohmic junction is advantageous for both HER and OER when the work function of the n-type semiconductor surpasses that of the metal, as this configuration reduces resistance at the junction or interface. Likewise, the formation of the p–n junction during electrochemical reactions facilitates enhanced electrochemical activity. The Ni₄Mo/MoO₂ demonstrates an overpotential (η) of 89 mV at 100 mA cm⁻² for HER and an η of 321 mV at 100 mA cm⁻² for OER. A two-electrode setup exhibited an average potential of 2.48 V (iR-uncompensated) at a high current density of 500 mA cm⁻² for 250 h, underscoring the electrode's durability. Consequently, the introduced ohmic junction and p–n junction-based electrocatalysts offer an effective strategy for developing bifunctional electrocatalysts.