Iron-Group Metal Sulfides Integrated With Heteroatom-Doped Vertically Oriented Graphene Nanosheets As Composite Electrocatalysts For Electrocatalytic Hydrogen

The global demand for energy is increasing.The extensive use of traditional fossil energy has caused environmental pollution and other problems.With the depletion of traditional fossil energy,people need to find a clean and sustainable energy.Hydrogen energy is considered to be the most promising green and sustainable energy.Among many hydrogen production processes,water electrolysis has attracted much attention because of its advantages of no pollutants and high hydrogen production concentration.However,in the actual process of water electrolysis,the actual voltage is higher than the theoretical voltage.People study electrocatalysts to reduce the actual voltage in the process of water electrolysis.At present,noble metal catalysts Pt and Ir O₂/Ru O₂ are the most efficient catalysts for electrolytic water hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.Nevertheless,their large-scale application is limited due to the high price and low storage of noble metals.It is very important to develop an economical,rich and high catalytic performance electrocatalyst.In this paper,iron group metal sulfides（IMSs）and heteroatom doped vertical graphene were used to prepare composite catalysts for overall water splitting.The main research contents of this paper were as follows:（1）Nitrogen doped vertical oriented graphene（NVG）/iron group metal sulfides composite catalysts（IMSs/NVG）were prepared by heating reflux method and one-step electrodeposition method.The structure,composition and alloy particle size of IMSs/NVG composite catalysts were analyzed by various characterization techniques.The results showed that nitrogen doped vertical graphene on IMSs/NVG was evenly wrapped by IMSs alloy particles,nitrogen was successfully doped into graphene,and nitrogen doped graphene increased the load of alloy particles;The effects of the structure and composition of IMSs/NVG composite catalysts on the catalytic performance were studied by electrochemical measurements.The results showed that the three metals catalyst FeCoNiS/NVG with vertical structure had the best catalytic activity.In 1.0 M KOH electrolyte;After that,the sample composition and phase structure of FeCoNiS/NVG composite catalyst after stability were analyzed by X-ray diffraction and X-ray photoelectron spectroscopy.It was found that IMSs themselves were the catalytic active sites for HER.During OER,IMSs were transformed into iron-group metal oxides,and iron-group metal oxides were the actual catalytic active sites;The catalytic performance of FeCoNiS/NVG composite catalyst was superior to most of the previously reported non-noble metal bifunctional electrocatalysts.（2）In order to further improve the catalytic performance of IMSs,nitrogen doped FeCoNiS nanoparticles on N,S-co-doped vertical graphene（N-FeCoNiS/SVG）composite catalyst was prepared by one-step electrodeposition and in-situ synthesis,and N-FeCoNiS/SVG with uniform particles distribution and vertical structure was obtained;The changes of composition and electronic structure of FeCoNiS/SVG doped with nitrogen were studied by characterization techniques.The results showed that the electronic structure of FeCoNiS/SVG was changed after nitrogen doping.The electronegativity of nitrogen was stronger than that of sulfur.The electrons of metals and S around N were close to N,which increased the electron density of N and reduced the electron density of metals and S.The HER and OER properties of N-FeCoNiS/SVG material were studied by electrochemical measurements,and the effect of the change of electronic structure of N-FeCoNiS/SVG on the catalytic performance was studied.The results showed that N-FeCoNiS/SVG had the best catalytic activity..Nitrogen incorporation into FeCoNiS/SVG fundamentally improved the catalytic performance of IMSs by adjusting the electronic structure.Further,the electronic structure and composition of N-FeCoNiS/SVG composite catalyst after stability test were studied by Characterization techniques.It was found that IMSs were the actual active sites for HER,and IMSs were transformed into iron-group metal oxides as the actual catalytic active sites for OER.N-FeCoNiS/SVG composite catalyst had better catalytic performance than FeCoNiS/SVG composite catalyst,FeCoNiS/NVG composite catalyst and most other reported catalysts.