Research On Regulating Electrocatalytic Hydrolysis Performance Of NiS_x By Doping And Their Mechanisms

In recent years,energy shortages and environmental pollution have become increasingly prominent,therefore the development of clean energy has become urgent in order to alleviate the energy crisis.Hydrogen energy is considered to be a new type of energy that can be a perfect alternative to primary energy.Electrochemical decomposition of water for hydrogen production offers a promising option for the green and efficient production of high purity hydrogen.Researchers have found that transition metal chalcogenides（TMDs）are not only promising as cathode materials for hydrogen precipitation reactions（HER）,but also as anode materials for anodic reactions（OER）,i.e.bifunctional catalysts.The aim of this paper is to modulate the electronic structure of TMDs by using heterogeneous atom doping to increase hydrogen/oxygen active sites and thus enhance their electrocatalytic hydrolysis performance.The research contents of this paper are as follows:（1）A Ti,V bimetallic co-doped NiS_x nanowire structure directly grown in situ on nickel foam（Ti,V-NiS_x/NF）was successfully synthesized by a simple solvothermal method.During the solvothermal reaction,Ti and V atoms were successfully doped into the NiS/Ni₃S₂ heterojunction,denoted as Ti,V-NiS_x/NF.Electrochemical tests showed that Ti,V-NiS_x/NF exhibited an overpotential of 156 m V at a current density（j）of 10m A cm^-2 and a tafel slope of 100.4 m V dec^-1.The excellent electrochemical performance is attributed to the appropriate doping of heterogeneous atoms（Ti and V atoms）optimizing the electronic structure of NiS_x,improving the electrical conductivity and thus increasing its hydrogen precipitation reaction rate.On the other hand,the three-dimensional（3D）network structure of nickel foam has a larger specific surface area than the two-dimensional（2D）planar structure,which facilitates hydrogen precipitation.（2）The electrochemical performance of Ti,V-NiS_x/NF for OER catalyst materials showed that the overpotential of Ti,V-NiS_x/NF was only 24 m V and the Tafel slope was 64 m V dec^-1 at a current density of 10 m A cm^-2.The Faraday efficiency FE_O2 of oxygen in constant current polarization tests with Ni₃S₂/NF electrodes was found to be43%at a current density of 10 m A cm^-2 by gas chromatography.In contrast,no O₂generation was tested at the Ti,V-NiS_x/NF electrode under the same test conditions and test environment,suggesting that the Ti and V co-doping inhibited oxygen generation and that the ultra-low overpotential obtained from the polarization curve was not caused by O^2-generation of O₂,but by other side reactions occurring during the OER process.（3）SEM,Raman and XPS characterization of Ti,V-NiS_x/NF before and after the3h constant current polarization test were carried out to investigate the real reaction process during the OER.SEM observed a severe transformation of the surface microstructure after activation.The characteristic peaks of SO₄^2-and NiOOH were detected by Raman and XPS.The differential charge densities of Ti,V-NiS/Ni₃S₂ and NiS/Ni₃S₂ heterostructures were calculated in combination with DFT.The results showed that the electron cloud of NiS/Ni₃S₂ heterostructures redistributed after Ti and V co-doping and the Ni-S bond weakened,making S easier to leach.Gibbs free energy calculations and analysis indicate that the OER intermediates are more likely to combine with S to form SO₄^2-than to form O₂.These results suggest that the formation of ultra-low overpotentials is not the result of O^2-forming O₂,but rather the oxidation of S to SO₄^2-forming extremely low overpotentials.