Synthesis And Performance Of Transition Metal/Double Doped Graphene For Electrocatalytic Water Splitting

Hydrogen energy is the most promising alternative to traditional counterparts because of its high energy density,abundant energy resource and wide applications.Compared with traditional steam reforming,electrochemical water splitting is an environmentally friendly,simple and effective technology to produce sustainable hydrogen energy.Electrochemical water splitting is the conversion of electrical energy into chemical energy,which includes hydrogen evolution reaction（HER）on the cathode and oxygen evolution reaction（OER）on the anode.As well known,electrocatalysts are the core part for the development of electrochemical water splitting.Therefore,the exploration of highly efficient electrocatalysts plays a crucial role in the electrochemical water splitting.Transition metals（eg.transition metal oxides and transition phosphides）have attracted increasing attention,owing to lower cost,more abundant and better toxicity resistance.However,due to the low conductivity,the electrocatalytic performance is poor.Binary-doped graphene has attracted increasing attention,owing to its excellent electrical conductivity,chemical stability and large specific surface area.To overcome these shortcomings,the transition metal oxides were combined with the binary-doped graphene,aiming to form a strong interaction among metal-heteroatom-carbon atoms to improve the electrocatalytic performance.The specific work is as follows:Ⅰ:In this part,the rod-shaped nickel/nickel oxide/cobalt oxide composite materials supported by sulfur and nitrogen co-doped graphene（Ni/Ni O/Co O@SNG）were synthesized by a simple hydrothermal-calcining technique using surfactant（sodium dodecyl sulfate）as template,nickel nitrate and cobalt nitrateurea as nickel source and cobalt source,sulfur and nitrogen co-doped graphene（SNG）as substrate,urea as the hydrolysis controlling agent.In order to use P sites with moderate electronegativity to serve as Lewis base sites,the Co P/Ni Co P@SNG-400 composite materials were prepared by using sodium hypophosphite as the phosphorus source（400 mg）through low-temperature phosphorization method.The results indicated that Co P/Ni Co P@SCN-400 showed high activity for the OER and HER,which owned overpotentials of 313and 221 m V at a current density of 10 m A cm^-2 and Tafer slopes of 128.34 and 102.75m V dec^-1in 1 M KOH.In order to adjust the 3d-orbital electron filling degree of metal centers to improve the alkaline HER activity by doping highly electronegative anions,the three-dimensional rod-shaped Se-Co P/Ni Co P@SNG-800 composite materials were prepared by low-temperature phosphorization and selenization method using sodium hypophosphite as the phosphorus source,selenium powder as selenium source（800 mg,20 mg）.The results indicated that Co P/Ni Co P@SNG-800 showed high activity for the HER,which owned an overpotential of 139 m V at a current density of 10 m A cm^-2 and a Tafer slope of 118.41 m V dec^-1in 1 M KOH.Ⅱ:In this part,calcination-hydrothermal method was used to prepare Ni/Ni O@BNG composite materials supported on the thin layer of g-C₃N₄（Ni/Ni O@BNG-CN）using nickel nitrate as nickel source,dicyandiamide as precursor and boron and nitrogen co-doped graphene（BNG）as substrate.When the doping temperature was 900℃,the nickel salts were 5%,the dicyandiamide content was 50mg,and the thermal exfoliating time was 3 h,the best OER performance could be obtained.The Ni/Ni O@BNG-CN obtained a Tafer slope of 118.59 m V dec^-1 along with an overpotential of 346 m V to drive a current density of 10 m A cm^-2 for OER in 1 M KOH electrolyte.The results indicated that the binary-doped graphene could be more helpful to adsorb O/OH^-to improve OER performance comparing with a mono-doped graphene.Simultaneously,the content of B atom,edged N（pyrindinic-N and pyrollic-N）sites and graphitic-N in the graphene matrix could be effectively adjusted to improve the OER activity by changing the doping temperature.Secondly,the formation of Ni-N-C metal active centers could facilitate the OER process.Moreover,the tightly stacked and wrinkled thin-layer structure could effectively prevent the aggregation of Ni/Ni O nanoparticles,and expose more active sites,thereby increase OER activity.Finally,the synergistic effect between Ni/Ni O nanoparticles,thin-layer CN and defect-rich BNG was an important factor in the improvement of OER performance.