Boosting the electrocatalytic activity on CoN/Ni₃N heterojunction by Ce modification for efficient water splitting

The design of highly efficient and low-cost bifunctional electrocatalysts for overall water splitting (OWS) is essential to realize a sustainable hydrogen economy. Heterojunction engineering can realize synergistic catalysis by coupling distinct active components, while the low activity at the interfaces needs further improvement by introducing elaborate elements. Herein, we constructed a Ce-modified CoN/Ni₃N heterojunction catalyst via sequential electrodeposition and nitridation. The introduction of redox-active Ce modulates the interfacial electronic configuration and creates a high density of nitrogen vacancies (N_v), substantially boosting the bifunctional catalytic performance of Ce-CoN/Ni₃N. The optimal catalyst requires overpotentials of only 27 mV and 267 mV to deliver 10 mA cm^-2 for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH. A two-electrode flow electrolyzer equipped with Ce-CoN/Ni₃N as bifunctional electrodes achieves a high industrial current density of 500 mA cm^-2 at a cell voltage of 2.04 V and maintains stable operation for 500 h. Mechanistic investigations reveal that Ce incorporation elevates the oxidation states of Ni and Co and promotes the generation of N_v, which accelerates water dissociation and HER kinetics. Meanwhile, Ce incorporation facilitates surface reconstruction from β-NiOOH to the more active γ-NiOOH phase and activates lattice oxygen, enabling cooperative adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM) pathways that collectively enhance OER kinetics. This study reveals that heterointerface engineering offers an effective and feasible strategy for creating highly active bifunctional electrocatalysts toward practical water electrolysis.