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Abstract:
Urea-assisted electrolysis boosts hydrogen production by substituting the sluggish oxygen evolution reaction (OER) with the energetically favorable urea oxidation reaction (UOR), thereby lowering energy consumption. Rational heterojunction engineering modulates charge distribution and generates abundant active sites, facilitating urea adsorption and C─N bond cleavage. Herein, we report a facile electrodeposition strategy to construct g-C3N4/CoMoS2 hybrid electrocatalysts. The built-in electric field at the heterojunction creates electrophilic regions on g-C3N4 and nucleophilic regions on CoMoS2, selectively activating urea and promoting rapid bond cleavage. Anchoring g-C3N4 onto CoMoS2 enables remarkable bifunctional activity toward both UOR and HER, achieving potentials of 1.27 V vs. RHE in 1 m KOH + 0.33 m urea and -80 mV vs. RHE in 1 m KOH at 10 mA cm−2, respectively. Density functional theory (DFT) calculations reveal that interfacial electron transfer enriches CoMoS2 with electrons and depletes g-C3N4, enhancing charge transfer, optimizing urea adsorption, and lowering reaction energy barriers. Notably, the g-C3N4/CoMoS2//g-C3N4/CoMoS2 cell delivers 10 mA cm−2 at 1.34 V with excellent stability, demonstrating superior efficiency. This work provides a rational framework for designing efficient, energy-saving urea-assisted hydrogen production systems and reveals how intrinsic electric fields can precisely control charge distribution during catalysis.

Keywords:
built-in electric field, heterojunction, hydrogen production, urea oxidation

Abstract:
The anodic substitution of a sluggish oxygen evolution reaction with a more energy-saving hydrazine oxidation reaction has the potential to greatly reduce energy consumption for hydrogen production. However, the underlying mechanism of the hydrazine oxidation reaction remains ambiguous, and the existing hydrazine splitting generally requires an external power source to drive the anodic and cathodic reactions, which is not suitable for outdoor applications. In this study, we have developed a heterostructure sulfide-based catalyst that effectively catalyzes both hydrazine oxidation and hydrogen evolution reactions. Through in situ Raman spectroscopy, we have confirmed that the breakage of the nitrogen-nitrogen single bond is a pathway for the hydrazine oxidation reaction. The enhanced electrocatalytic performance is attributed to the increased active sites and accelerated electron transfer within the heterostructures, which reduced the energy barrier, thereby enabling the fabricated electrolyzer using the g-C3N4/Ni(CN)2@NiS catalyst to deliver 200 mA cm−2 with a low voltage of 0.31 V. The assembled electrolyzer can be powered by a g-C3N4/Ni(CN)2@NiS anode-equipped direct hydrazine fuel cell, achieving self-powered hydrogen production with faradaic efficiency of more than 97 %.

Keywords:
Functional electrod, Carbon nitride, Hydrogen generation, Hydrazine