Electric field derived from NiCo2S4-MoS2n-n heterojunction promoted electrocatalytic methanol oxidation assisted energy-saving hydrogen production.

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 (EF) 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.