V-Doping coupled H2O2 pre-oxidation synergistically promote NiCo-LDH for urea oxidation-assisted hydrogen production

Abstract

A NiCoV layered double hydroxide (LDH) nanosheet array structure grown on Ni foam was synthesized through V-doping coupled H₂O₂ pre-oxidation using a hydrothermal method. Systematic characterization proved that NiCoV_x-LDH/NF-y (x: 0.5, 1.0, and 1.5; y (the amount of H₂O₂ used): 50, 100, and 200) composites featured vertically interlaced LDH nanosheets with more highly valent metal species Ni³⁺ and Co³⁺. Especially, NiCoV_1.0-LDH/NF-100 demonstrated the best electrocatalytic performance for the urea oxidation reaction (UOR) and hydrogen evolution reaction (HER). Furthermore, the NiCoV_1.0-LDH nanosheet arrays were in situ grown on reduced graphene oxide (rGO) uniformly modified Ni foam (∼135 nm × 15 nm), only requiring a low potential of 1.33 V for the UOR and a low overpotential of 70 mV for the HER to offer 10 mA cm⁻². When it is used as both the cathode and anode in urea-assisted water electrolysis, the as-prepared array only requires 1.45 V to offer 10 mA cm⁻², outperforming many non-precious metal catalysts. In situ characterization and theoretical calculations showed that V-doping coupled H₂O₂ pre-oxidation can regulate the electronic environment of the catalyst via (1) producing more highly valent Ni³⁺ active sites and Co³⁺ auxiliary sites; (2) reducing the adsorption energy of urea at the Ni site in the UOR and simultaneously reducing the water adsorption energy and water dissociation energy in the HER, especially the active site of H* adsorption was shifted from Co sites to V sites, which optimized the Gibbs free energy of H* adsorption; and (3) reducing the band gap, thus accelerating the reaction kinetics of the UOR and HER. Additionally, the synergistic coupling between NiCoV-LDH and rGO further improved the electrocatalytic performance.