Colloidal Synthesis Of Transition Metal Selenide And Phosphide For Electrocatalytical Oxygen Evolution And Hydrogen Evolution Reactions

Electrolysis of water is an effective,green approach to generate hydrogen with high purity.Electrocatalytic water splitting contains two half reactions:the hydrogen evolution reaction(HER)at the cathode,and the oxygen evolution reaction(OER)at the anode.The water splitting is an uphill reaction,which needs 237 kJ mol’1 of energy.In an electrochemical water splitting system,the external electric energy usually offers the energy to realize this reaction.The electrocatalysts can effectively accelerate the rate of electrode reaction,alter reaction mechanism,lower activation energy and reduce the overpotential.Recently,Pt is regarded as the most efficient catalyst for HER and RuO2 or IrO2 are the most active catalysts for OER.However,their high price and scarcity hinder their widespread application.Therefore,it is desirable to research highly efficient nanocatalysts with long-term stability.The performance of a nanocatalyst is in relation to its size,shape and electrical conductivity.Transition metal selenide andphosphide exhibit good electrical conductivity,corrosion resistance,outstanding stability and high mechanical strength,which is suitable for electrocatalysis.Hence,we have synthesized dendritic NiFeSe and two dimensional NiWSe nanosheets in organic solvent using a colloidal method.NiFeSe and NiWSe nanomaterials are applied to catalyze OER and HER in alkaline solution,respectively.In addition,metal phosphides are effective HER catalysts with hydrogenase-like catalytic mechanism.There are few noble metal phosphides prepared by colloidal synthesis.So we fabricated monodisperse Rh2P nanoparticles for HER in pH-universal electrolytes via a colloidal method.The experimental section of this dissertation is divided into three parts as follow.1.The NiFeSe nanocatalysts have been synthesized using a colloidal mehod.According to the procedure of adding Fe precursor,the synthesis is divided into two-step injection method and one-step method.One-step method is solvothermal reduction process in organic solvent using nickel aceylacetonate,iron aceylacetonate and selenium powder as precursors.In a two-step injection method,we first synthesized NiSe2 with nickel aceylacetonate and selenium in oleylamine,then quantitative carbonyl iron was injected to the as-synthesized NiSe2 solution to produce Fe doped NiSe2.The shape of NiFeSe nanocatalysts prepared in this two method are similar,which exhibit three dimensional nanodendrite morphology.This unique dendrite shape provides a large number of active sites,fast transportation of electrons and electrolytes,and facile release of the evolved O2 bubbles during the OER catalysis.The XPS results of NiFeSe catalyst reveal that there is strong electron interaction between Fe,Ni,and Se.It is convenient to adjust Fe doping content via varying the molar ratios of metal precursors.Ni1.12Fe0.49Se2 catalyst with best elemental composition exhibits excellent electrocatalytic activity and high durability toward the OER in 1 mol/L KOH solution,with an overpotential of 227 mV at a current density of 10 mA cm-2,which is,to the best of our knowledge,higher than most of the reported selenide-based electrocatalyst.The NiFeSe electrocatalyst after OER test is characterized by XPS,which reveals highly active Ni(Fe)OOH has formed on the surface of NiFeSe.So,as-synthesized NiFeSe is an effective precursor for catalyzing OER.2.We prepared NiWSe catalyst with the morphology of nanosheets using a colloidal synthesis.First,NiSe2 was synthesized by reacting nickel aceylacetonate and selenium powder in oleylamine.Then,quantitative amount of tungsten hexacarbonyl was added into the as-synthesized NiSe2 solution to form the NiWSe.Similarly,the doping W changed electronic structure of catalyst surface as well as morphology characteristic.Besides,W has enhanced the electrical conductivity of catalyst,which is crucial to HER catalytic performance.WNiSe catalysts with different compositions are obtained by changing the initial molar ratios of tungsten source and nickel source.All of the NiWSe catalysts exhibit much higher electrocatalytic performance toward HER in alkaline solution than pure NiSe2 and WSe2.Among them,Ni0.54W0.26Se nanosheets show the highest catalytic activity,with an overpotential of 162 mV to achieve 10 mA cm-2 and a small Tafel slope of 74 mV dec-1.This value of overpotential is smaller than most of the previous reported selenide-based catalysts toward HER in alkaline solution.3.We have prepared monodisperse Rh2P nanoparticles with an average size of 2.8 nm using a colloidal synthesis.And they shows superior catalytic activities for pH-universal HER.The Rh2P catalyst exhibits outstanding HER catalytic performance with overpotentials of 14,30,and 38 mV to deliver 10 mA cm-2 in 0.5 M H2SO4,1 M KOH and 1 M phosphate-buffered saline,respectively,exceeding almost all the documented electrocatalysts,including the commercial Pt and Rh.Density functional theory calculations reveal that the introduction of P into Rh can weaken the H adsorption strength of Rh2P to nearly zero,beneficial for boosting HER performance.Three different nanomaterials NiFeSe,NiWSe and Rh2P have been synthesized using a colloidal method.They are used for catalyzing OER and HER.The colloidal synthesis has the advantages of short reaction time,easy operation and controllable condition.More importantly,the solvent acts as surfactant in synthesis process,providing a lot of tiny micelles.The surfactant has great influence on properties of nanocrystals,such as stability,shape,crystalline structure and electronic properties.Thus,nanocrystals with different morphology can be conveniently fabricated in colloid synthesis method.