Harnessing the heterostructure linkage of NiFe-layered double hydroxide and Ni₃S₄ for overall urea electrolysis

Empowering non-noble metal-based electrocatalysts is essential to meet the global energy demands for cost-effective green hydrogen production and efficient wastewater treatment through water-splitting applications. Transition metal-based hydroxides and sulfides have gained immense research interest in the scientific community owing to their excellent activity in hydrogen and oxygen evolution reactions (HER/OER). Herein, we report a rational design of heterostructure composed of nickel iron layered double hydroxide (NiFe-LDH) and polydymite-phase nickel sulphide (Ni₃S₄) to drive efficient electrocatalytic urea electrolysis. This work systematically studied the effect of NiFe-LDH-Ni₃S₄ heterostructures (NF:NS-X) with varying mass loadings of Ni₃S₄. The strategic addition of Ni₃S₄ phase on NiFe-LDH is found to create a compressive strain in NiFe-LDH together with enhanced hydroxyl vacancy. Furthermore, the X-ray photoelectronic studies and electron paramagnetic resonance unravelled a significant interfacial electron transfer from Ni₃S₄ to NiFe-LDH, which modulates the Ni and Fe valencies to facilitate superior reaction kinetics. The optimal catalyst, NF:NS-40 achieved the urea oxidation reaction (UOR) at potential of 1.34 V and HER with a low overpotential 119 mV (at 10 mA/cm2). The excellent reaction kinetics of NF:NS-40 are further supported with the lower Tafel slopes of 83 mV/dec (UOR) and 139 mV/dev (HER). When employed as a bifunctional catalyst in a membrane free electrolyzer, NF:NS-40 achieves a cell potential of only 1.48 V (at 10 mA/cm2) in overall urea electrolysis, demonstrating prolonged stability and significant energy savings.