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

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.