Preparation Of Transition Metal Nitrides And Their Application In Water Splitting

While the use of fossil energy promotes social progress,it also brings several environmental problems which cannot be ignored:acid rain,smog,greenhousephenomenon and large-scale environmental pollution have been frequently reported recently;in addition,globally limited reserves and increasing demand have led to steady increase in the price of fossil energy.Therefore,it is imperative to develop clean and renewable 'green' energy.Among many candidates,hydrogen energy is an ideal choice,because it is of high energy density,environmentally friendly,non-polluting,and abundant in Earth.Electrocatalytic water splitting is the most promising method for large-scale hydrogen production in the future.At present,the biggest obstacle which affects its application is the electrothermical conversion caused by the overpotential.It is highly desirable to fully understand the mechanism of electrocatalytic water splitting and design catalysts with better performance and lower costs.Noble metal-based electrocatalysts usually possess excellent catalytic properties for water splitting.However,these materials are expensive and unsuitable for large-scale application.Much cheaper transition metals of 4th and 5th periods and their derivtives have been extensively studied as alternatives to noble metals,despite of their low activities and instability for water splitting.Compared to alloys,oxides,carbides and sulfides,transition metal nitrides generally have good conductivity and electrical/chemical stability.The incorporation of N atoms can adjust the d-band configuration of the transition metal to an energy structure close to that of the noble metal.The above characteristics make transition metal nitrides very attractive for use in electrocatalytic water splitting.This thesis is aimed to develop new,high-performance electrocatalyst for water splitting based on cheap transition metals.We prepared transition metal nitride materials with specific morphology on nickel foam substrates by using new synthetic methods including in situ grown of active components on electrodes,synegestic effect of bimetallic alloy and promotion of heteroatoms.Their preparation conditions and performance in electrocatalytic water decomposition were throughly studied.The thesis mainly includes the following two aspects:1.Nickel cobalt oxide nanorods with a vertical and uniform distribution was hydrothermally in-situ grown on nickel foam(Nifoam@NiCoO2).After drying,Nifoam@NiCoO2 was put in a tube furnace and heated in a flowing NH3 atmosphere for nitridation to obtain nickel cobalt nitride nanorods on nickel foam(Nifoam@Co3N-Ni3N).Nifoam@Co3N-Ni3N exhibits excellent hydrogen evolution reaction(HER)performance in 1.0 M KOH with low overpotential of 110 mV at the current density of 10 mA/cm2.The overpotential can maintain for more than 25 h under such a current density.The results show that the incorporation of Co and N promotes the HER activity of Ni.2.Nickel molybdenum oxide nanorods with a vertical and uniform distribution was hydrothermally in-situ grown on nickel foam(Nifoam@NiMoO4).After drying,Nifoam@NiMoO4 was put in a tube furnace and heated in a flowing NH3 atmosphere for nitridation to obtain Ni-doped nickel-molybdenum nitride nanorods on nickel foam(Nifoam@Ni-Ni0.2Mo0.8N).Nifoam@Ni-Nio.2Mo0.8N exhibits excellent HER and OER performance in 1.0 M KOH with low overpotentials of 15 mV and 218 mV at the current density of 10 mA/cm2,which is better than the commercial benchmark materials Pt/C and RuO2 with a same load quality.Electrolytic cell with Nifoam@Ni-Ni0.2Mo0.8N used as both anode and cathode requires a cell potential of merely 1.49 V to achieve the current density of 10 mA/cm2 and this cell potential may maintain more than 110 h.The excellent performance of the electrode could be attributed to the formation of highly conductive,corrosion-and oxidation-resistant metal nitrides and the synergetic effect between intimately interconnected,electrochemically active nickel molybdenum nitride and Ni or NiO nanoparticles.This study shows that the use of transition metal nitrides in combination of nanostructured heterojunctions of multiple active components enables one to develop highly stable and efficient water electrolyzers without precious metals.The preparative strategy used in this work could be applied to devise new electrocatalysts.