In Situ Synthesis Of Nickel Sulfide-and Selenide-based Hybrid Electrocatalysts And Their Electrocatalytic Performance For Oxygen Evolution Reaction

Water electrolysis can effectively make use of intermittent electricity generated by renewable energy and rich source of water on the earth,with low carbon emittion in the meantime,which makes it as an ideal and clean technology for hydrogen production.However,cathodic and anodic overpotentials,especially for a slow four-electron transfer oxygen evolution reaction?OER?,severly hinder the enhancement of hydrogen production efficiency by water electrolysis.It has been demonstrated that some nickel sulfides and selenides exhibit high electrocatalytic activity for OER in alkaline media,which makes it possible to successfully substitude for noble metal oxides?RuO₂?IrO₂?as OER catalysts.In this dissertation,solvothermal method and template-oriented method have been utilized to in situ synthesize highly-active nickel sulfide-and selenide-based OER electrocatalysts based on nickel foam.The synthetic routes have been designed in two ways:controllable synthesis of highly-active crystal phase and hybrid design with functional nanostructures.By means of various physical characterizations and electrocatalytic measurements,growth mechanism of sulfide-and selenide-based highly-active phases and unique nanostructures and their internal connections with electrocatalytic performance have been studied in detail,which would provide theoretical references for furture synthesis of highly-active OER electrocatalysts.?1?An in situ electrochemical oxidation pretreatment?ISEO?on nickel foam substrate has been adopted to change valency of partial surface metallic Ni?oxidized to Ni?OH?₂?,which effectively facilitates formation of NiS_x/NF?Ox?hybrid phase structure?composed of Ni₃S₂ and?-NiS?.Compared with normal Ni₃S₂ electrocatalysts,NiS_x/NF?Ox?exhibts higher electrocatalytic activity with a lower OER overpotential???of 128 mV to achieve the current density of 10 mA·cm^-2 and a smaller Tafel slope of 93 mV·dec^-1.It can be speculated that?-NiS phase with pyramid structure is the main reason of improved OER acitvity;?2?A chemical pretreatment on nickel foam has been utilized to promote growth of?-Ni?OH?₂ nanosheets on surface of metallic Ni,which has realized phase transformation from NiSe nanorods to NiSe₂ pyramids compared with the sample based on nickel foam without any pretreatment.Because of high proportion of Ni²⁺and an advantage of NiSe₂crystal structure to produce active Ni³⁺,NiSe₂/NF?Ox?has demonstrated higher current density and smaller onset potential?¹.54 V vs.RHE?for OER;?3?An ISEO treatment on NiSe nanopillars in basic media has been used to obtain amorphous NiOOH thin film on the surface,preparing novel hyacinth-like NiSe@NiOOH core-shell nanostructures.The hybrid system has made fully use of advantages of synergitic effects among nickel foam,NiSe and NiOOH,the real active sites for OER,largely increasing charge-transfer rate and exhibiting evidently improved OER electrocatalytic activity;?4?A two-step solvothermal method has been adoped to achieve NiSe-Ni_xS_y polycrystalline nanorod structure.Additionally,an in situ electrochemical activation in basic media has been combined to grow amorphous Ni?OH?₂ activating the electrocatalyst.Activated NiSe-Ni_xS_y/NF has been proved to demonstrate high electrocatalytic acitivity,which may owe to doping of Ni_xS_y phases enhancing charge-transfer rate and the growth of Ni?OH?₂ facilitating generation of active sites for OER.