Electric field derived from NiCo2S4-MoS2 n-n heterojunction promoted electrocatalytic methanol oxidation assisted energy-saving hydrogen production

Abstract

Water electrolysis coupling methanol oxidation is an energy-efficient hydrogen generation technology, while obtaining high-value formic acid, however, superior catalysts are often required to achieve efficient hydrogen production. Herein, the n-n heterojunction (NiCo₂S₄-MoS₂) composed of NiCo₂S₄ nanoparticles uniformly dispersed on ultrathin MoS₂ nanosheet arrays has been prepared through hydrothermal-pyrolysis process. At a current density (j) of 10 mA cm⁻², the potentials of NiCo₂S₄-MoS₂ for methanol oxidation reaction (MOR) and hydrogen evolution reaction (HER) are respectively 1.282 V and −36 mV with very high selectivity for formic acid. Importantly, under the same conditions, by using the n-n heterojunction as electrodes, the voltage (1.355 V) of the methanol electrolysis is 216 mV lower than that of the water electrolysis. Meanwhile, at higher j of 100 mA cm⁻², the voltage of the methanol electrolysis is only 6.8 % higher than the original after 150 h chronopotentiometry (CP) test. The outstanding performance of NiCo₂S₄-MoS₂ exceeds that previously reported in most literature, which is primarily attributed to the fact that the built-in electric field (BEF) derived from the Fermi level (E_F) difference between NiCo₂S₄ and MoS₂, accelerates the charge transfer, optimizes the electronic structure of the heterojunctions, and thus improving the electrical conductivity. Furthermore, MoS₂ nanosheet arrays with large specific surface area provide a fast charge/mass transfer channel, uniformly dispersed NiCo₂S₄ and defect sites produce abundant active sites. And the superior hydrophilicity and aerophobicity of the heterojunction surface accelerate the reaction kinetics. Finally, density functional theory (DFT) calculations show that the formation of the heterojunction optimizes the electron density and d-band center of composites, thus enhancing H* adsorption and CH₃OH dehydrogenation kinetics.