Activation Of Nitrogen And Sulfur Co-doped Graphene For Electrochemical Water Splitting And Catalytic Oxidation

Energy dilemma and environmental issue are two most urgent problems to be dealt with.Electro-chemical water splitting and advanced oxidation process（AOPs）are efficient methods to solve these two problems.Graphene-based catalysts,as efficient metal-free catalysts,have attracted significant interests since they are low cost and environmentally friendly.Graphene-based materials have been proved to be of remarkable catalytic efficiency towards electrochemical water splitting and peroxymonosulfate（PMS）based AOPs,moreover,it has no risk of pollutant to water systems.Heteroatoms doping process and porous creating process could enhance the catalytic activity of graphene to a large extent,which facilitates the large-scale application of it.In this work,nitrogen and sulfur co-doped graphene（N,S-G）is synthesized,then chemical and physical methods are employed to involve a porous structure in N,S-G.The obtained materials are used as the catalysts towards electrochemical water splitting and advanced oxidation process（AOPs）,exploring the effect of the heteroatoms doping process and pore creating process.In the first part,nitrogen and sulfur co-doped graphene（N,S-G）is synthesized via a one-pot calcination with dopamine hydrochloride（DA）and mercaptoethanol（ME）as the nitrogen and sulfur sources.Subsequently,N,S-G is activated by ZnCl₂ to enlarge the specific surface areas to construct a porous structure（a-N,S-G）.The specific surface area（SSA）of the materials is enlarged from 35.5 m²/g to554.4 m²/g.The catalytic activity of a-N,S-G can be optimized by adjusting the mass ratio of ZnCl₂and N,S-G.While the mass ratio of ZnCl₂ and N,S-G is 7.5,the a-N,S-G performed a best catalytic activity.In the OER process,the overpotential of a-N,S-G is only 330 mV vs.RHE at 10 mA·cm^-2in 1 M KOH,which surpasses the most reported carbon catalysts.In the HER process,-10 mA·cm^-2 can be achieved at an overpotential of 0.29 V vs.RHE in 1 M KOH and 0.31 V vs.RHE in 0.5 M H₂SO₄.A new turnover frequencies（TOF）calculation method is involved to analyze the reactivity of specific active sites of carbo-catalyst including both heteroatoms and structural defects,thus enhancing the catalytic activity of the doped graphene.In the second part of this work,CO₂ activated N and S co-doped graphene（a-N,S-G-2）are synthesis in CO₂ atmosphere.a-N,S-G-2 are used as the catalysts towards peroxymonosulfate（PMS）based AOPs.Compared with N-G,S-G,N,S-G,a-N,S-G-2 performs an enhanced catalytic activity in phenol degradation process,attaining 100%phenol oxidation in 60 minutes with a reaction rate constant of 0.34 min^-1 when the concentration of catalyst is 100 mg/L and the concentration of phenol is 10 ppm.The radical quenching experiment,linear sweep voltammetry（LSV）curves and density function theory（DFT）calculation are employed to analyze the PMS activation mechanism in different systems.The results indicate that the involvement of secondary S atoms and the pore-creating process will change the PMS activation pathway,which lead to different reactive oxygen species（ROS）,thus effluence the catalytic activity of the as prepared material.In these two parts of work,we have proved that the catalytic activity of the N,S-G towards electro-catalytic water splitting and advanced oxidation process could be further enhanced by the involvement porous structure.The catalysts are low cost and environmentally friendly,which ensure their good application prospects.Further,the specific active sites and reaction mechanisms are analyzed in this work,providing new strategies for the design of high-performance graphene catalysts.