Amorphous‐Crystalline CoNiS Heterojunctions with Optimized Interfacial Charge Transfer for Highly Efficient Oxygen Evolution and Urea Oxidation

ABSTRACT Controlling catalyst crystallinity is crucial yet challenging in electrocatalysis. Herein, ZIF‐67‐derived cobalt–nickel sulfide (CoNiS) with continuously tunable crystallinity from amorphous to semi‐crystalline and fully crystalline was synthesized by controlling sulfurization time. The semi‐crystalline CoNiS (a/c‐CoNiS, ∼35% crystallinity) delivered optimal oxygen evolution reaction (OER) and urea oxidation reaction (UOR) activity, requiring only 292 mV and 1.379 V to reach 100 mA cm −2 , respectively, outperforming both amorphous and crystalline counterparts. Operando Raman spectroscopy and density functional theory calculations revealed that abundant defect interfaces in the semi‐crystalline state facilitate in situ formation of a CoNiS/CoNiOOH active heterointerface, which optimizes the adsorption of oxygen‐containing intermediates and accelerates electron transfer. A dual‐electrode electrolyzer employing a/c‐CoNiS achieved 100 mA cm −2 for overall urea splitting at 1.76 V and for overall water splitting at 1.95 V, while demonstrating stability exceeding 120 h at a current density of 500 mA cm −2 . This work establishes a clear structure‐activity relationship for crystallinity engineering in transition metal sulfide electrocatalysts.