Preparation And Properties Of Nanocomposites Based On Graphene Hydrogel For Hydrogen Evolution Reaction

Hydrogen is one of the promising alternatives to fossil fuels due to its high energy density and environmentally friendly.Among the main hydrogen production pathways,water electrolysis is the most proming method because it is possible to produce hydrogen in a sustainable way.As the hydrogen evolution catalysts for electrochemical water splitting,unitary materials are hard to achieve excellent electrochemical performance due to their intrinsic defects.Therefore,nanocomposites which combine the superiorities of varied materials become a research hotspot.In this study,graphene-based nanocomposites were prepared through hydrothermal and heat treatment using reduced graphene oxide(RGO)gel as substrate material to improve their electrochemical performance.The main research results are summarized as follows:(1)The RGO hydrogel prepared by hydrothermal method was thoroughly mixed with different masses of melamine(C3N3(NH2)3)under ultrasonic condition.Next,the as-prepared hybrid was carbonized at 550 ℃ and RGO/g-C3N4 composite was obtained.The electrochemical measurement in 0.5 M H2SO4 displays that the onset overpotential of RGO/g-C3N4 composite only 110 mV and the Tafel slope was 91 mV dec-1,the overpotential needed to achieve the current density of 10 mA cm-2 was 245 mV,which are lower than those of RGO,g-C3N4,indicate a high HER electrocatalytic ability of composite.The durability of the RGO/g-C3N4 composite was examined by continuous CV scanning.No change in the polarization curve of RGO/g-C3N4 composite after 2000 cycles is observed,suggesting the stable HER electrocatalysis of the material in acid.(2)Graphite oxide was prepared using a modified Hummers method,followed by thoroughly mixing the GO and g-C3N4 nanosheets under ultrasonic condition then the RGO/g-C3N4 composite was obtained by hydrothermal method.Afterwards,CoS2 nanoparticles were loaded on the surface of RGO/g-C3N4 by a hydrothermal process.After freeze drying,RGO/g-C3N4/CoS2 composite was obtained.The research result shows that the composite exhibits excellent electrochemical performance when the mass ratio of g-C3N4 to CoCl2 6H2O was 1:10.The assembled three-dimensional hierarchically structured RGO/g-C3N4/CoS2 composite has a high electrocatalytic ability for HER with low onset overpotential of only 70 mV and a Tafel slope of 41 mV dec-1,achieved current density of 10 mA cm-2 at overpotential of 175 mV in 0.5 M H2SO4.The CV test suggest the long-term stability of the RGO/g-C3N4/CoS2 composite.(3)The RGO hydrogel was ultrasonic dispersed in 1 M HC1 with a certain amount of anliline monomers.Subsequently,the mixture was stirred for 12 h with ammonium persulfate as initiator and the RGO/PANI was obtained.Secondly,the RGO/PANI was immersed in cobalt nitrate solution for 4 h.The RGO/PANI-Co hybrid was carbonized at a given temperature in N2 atmosphere.Finally,the carbonized products were soaked in 0.5 M H2SO4 for 12 h to remove unstable cobalt species and the RGO/Co-C-N composite was obtained.Electrochemical characterizations in 0.5 M H2SO4 show that the RGO/Co-C-N composite has a high electrocatalytic ability for HER with low onset overpotential of only 80 mV,an overpotential of 235 mV of 10 mA cm-2.The durability of the RGO/Co-C-N composite was examined by continuous CV scanning,no obvious decay of the activity was observed after 2000 cycles,suggesting a high catalytic durability of the RGO/Co-C-N electrocatalyst.