Study On Hydrogen Evolution Activity Of Metal Carbides Regulated By Surface Non-Metal Atoms

Energy is the foundation of social development and economic growth.However,the approaching depletion of traditional fossil fuel pushes us to develop new and renewable energy sources.Hydrogen is an environmentally friendly and sustainable energy source.Hydrogen production from electrolytic water has attracted wide attention due to its simplicity,low pollution,high efficiency and high hydrogen purity.The catalysts used in hydrogen evolution reaction（HER）need to have high catalytic activity,high electronic conductivity,large enough catalytic activity area and good electrochemical stability.Therefore,noble metals are widely used in the study of electrolytic water.Noble metal platinum-based catalysts have excellent activity for hydrogen production from electrolyzed water in acidic electrolyte,but these noble metals have small crustal reserves and are expensive,and do not use the practical application and industrial production of hydrogen production from electrolyzed water.Therefore,the development of efficient and stable non-noble metal catalysts has become the key to water electrolysis technology.Transition metal carbides have platinum-like surface electronic structure and catalytic performance,and are considered as potential candidates to replace precious metal Pt.Generally,the methods to improve the activity of electrocatalysts mainly include increasing the number of active sites of catalysts and improving the intrinsic activity of unit active sites.In this paper,the hydrogen evolution activity of transition metal carbides was optimized by surface regulation strategy of nonmetallic atoms（C,N,P）.Through chemical vapor deposition technology,non-metallic hydrides are mainly decomposed and deposited on the surface of transition metal carrier,so that non-metallic atoms and metal atoms on the surface of the carrier form a strong electron coupling interaction,and then develop epitaxial graphene and transition metal catalysts modified on the surface of non-metallic nitrogen and phosphorus atoms.This strategy improves the intrinsic activity of the active site of the catalyst,and we deeply studies the geometric electronic structure and catalytic reaction mechanism of the catalyst.The work is mainly carried out from the following three aspects.（1）In order to solve the problem that carbon stacking hinders the exposure of active sites in the high temperature synthesis of metal carbides,we epitaxially grown graphene heterostructures with lower layers on V₈C₇ substrates by vapor deposition at lower temperatures and atmospheric pressures.The experimental results show that there is a strong interaction between the graphene heterostructure covered by partial coating and V₈C₇,which optimizes the hydrogen adsorption free energy of V atom and enhances its corrosion resistance in strong acidic and alkaline solution.The synthesized V₈C₇NMs/GR catalyst shows high HER activity with a low overpotential at 10 m A cm^-2（156m V in 1 M KOH and 52 m V in 0.5 M H₂SO₄）,a small Tafel slope(89.4 m V dec^-1 in 1 M KOH and 48.3 m V dec^-1 in 0.5 M H₂SO₄),and preferable durability in both acidic and alkaline media.（2）Based on the problem that non-metallic heteroatom doping may destroy the structural stability of the catalyst and lead to the loss of its inherent catalytic activity,we propose a surface nitrogen atom modification strategy to control the surface N modification of V₈C₇and monolayer graphene heterostructures at low temperatures by a simplified pressure auxiliary device.While ensuring the structural stability of vanadium carbide,the electronic structure of the surface-active sites of V₈C₇ was regulated,which significantly improved the hydrophilicity of the heterostructure and optimized the alkaline hydrogen evolution reaction.Owing to the N introduced by electronically coupling with V atoms,the V sites can effectively reduce the kinetic energy barrier for water adsorption and dissociation.The existence of multi-regional synergistic catalysis in N-modified G/V₈C₇,proved by experimental observations and density functional theory calculations,reveals that the accumulation of electrons on epitaxial graphene enable it to serve as a source of sites for H^*adsorption and the subsequent reaction with an adjacent H₂O.Combining the two kinds of methods,the developed N_0.08-G/V₈C₇ displays an exceptional alkaline hydrogen evolution capability.In the 1 M KOH electrolyte,the overpotential of N-G/V₈C₇ is only 62 m V,and the Tafel slope is 60.8 m V dec^-1,which is much higher than that of the previously studied V₈C₇ NMs/GR catalyst.（3）Based on the simple and efficient control method of surface modification of non-metallic atoms,we use the strong electronegativity of non-metallic atoms,which can be electronically coupled with the metal atoms on the surface of the matrix to regulate the electrocatalytic activity of the matrix.In addition,a simplified pressure auxiliary device was successfully used to control the pressure,temperature and atmosphere of the reaction,and the non-metallic phosphorus single atoms were loaded on the hexagonal Mo₂C exposed on the single crystal（001）plane.The structure,mechanism,catalytic performance and structure-activity relationship of SAP-Mo₂C-CS were also studied.The results show that single-atom P forms stable structure on Mo₂C surface by hybridization with Mo atomic orbital.Because of the bidirectional regulation of single P atoms,P bonds with the surrounding Mo atoms to form a localized electronic state,which shows that the free energy of hydrogen adsorption(ΔG_H*)is close to zero.In acidic electrolyte,SAP-Mo₂C-CS showed comparable catalytic activity to noble metal Pt,with a lower overpotential of only 36 m V and a Tafel slope of 38.1 m V dec^-1,which was much better than the reported metal carbide hydrogen evolution catalyst.