Synthesis and Electrocatalytic Performance of Nickel(Cobalt)-based Sulfide(Phosphide)catalysts Toward Electrochemical Water Splitting

Author:Lu Shi Yu

Supervisor:li chang ming

Database:Doctor

Degree Year:2019

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Pages:139

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At present,due to the rapid depletion of fossil fuels,human beings are facing two major worldwide problems:1)environmental pollution and deterioration caused by the burning of such fossil fuels.2)energy shortage caused by the fast exhausting fossil fuels.They could greatly limit the development and survival of society and mankind if the problems cannot conqued as soon as possible.It is imperative to innovate and develop clean,sustainable and high energy density alternative energy.Among various sustainable energy sources discovered and developed up to date,hydrogen energy is considered as one of the most potential clean energy sources in the future due to its advantages of high energy density,wide distribution of resources,easy storage,pollution-free combustion and renewable nature.Electrolysis of water is a method of converting electrical energy directly into chemical energy,which can be flexibly produced by other renewable energy sources such as wind,tidal,solar,etc.,and can be stored in the form of chemical energy.In this way,electrolytic water technology can also be used as a transit system for other renewable energy sources by converting to hydrogen for storage.Water electrochemically spliting reaction can be divided into hydrogen evolution reaction(HER)at the cathode and oxygen evolution reaction(OER)at the anode.However,both the cathode and the anode,especially the OER encounter high polarization thus requiring highly efficient and stable electrocatalysts to greatly reduce the energy consuption.Pt and its alloys are considered to be the most active electrocatalysts in hydrogen evolution.Ir,Ru and their alloys are the best electrocatalysts for oxygen evolution.Nevertheless,they are precious,expensive and scarce in stock to inhibit large-scale productions and practical applications of such electrocatalysts.Therefore,the design and development of less expensive,highly catalytically active and stable non-noble metal catalysts have become the focus of the research attention.In this thesis,highly efficient,less expensive and stable electrolytic water catalysts were designed and synthesized by using nanoscience to accomplish pore structure regulation,morphology control,conductivity improvement and surface electronic state adjustment,and the corresponding mechanisms were thoughtfully investigated.This thesis work mainly includes four works:firstly,we prepared nickel phosphate nanoporous hollow spheres by co-precipitation to regulate morphology for enhanced electrocatalytic performance of oxygen evolution.Secondly,we calcined nickel phosphate nanoporous hollow spheres prepared in the previous chapter at high temperature in a reducing atmosphere to prepare oxygen-deficient nickel phosphate porous hollow spheres,of which the morphology and surface defect were jointly tailored to offer double-function catalysts.Then,in order to tune the surface state of the catalyst with doped elements,carbon cloth was used as a carrier to support the bifunctional catalyst Co0.84Sn0.16P CC.Finally,we used nickel mesh as the conductive carrier to load the high-efficiency hydrogen evolution catalyst Ni3Sn2S2@Ni3S2 NF,of which the morphology and surface electronic states were delicately tuned.The detail content is as follows:1.To improve the electrocatalytic performance of the catalyst by the catalyst morphology and pore structure,the hollow porous nickel phosphate nanoparticles were prepared by facile room temperature precipitation reaction,and the growth process and mechanism of the nanoparticles were explored.The OER catalytic properties of different carbon composite porous hollow nickel phosphate nanosphere catalysts were studied in the electrochemical testing process.The optimal catalyst had a potential of 330 mV at a current density of 10mA cm-2 and a tafel slope of 76.5 mV dec-1.Its performance is similar to that of commercial RuO2,and no significant attenuation is observed during continuous OER electrolysis for 20 hours.The conductive carbon improves the overall conductivity of the catalyst and the porous and hollow structure enhances the charge transfer rate of nickel phosphate in the catalytic process,while increasing the contact area between the electrolyte and the catalyst.2.By controlling the reduction temperature and retaining the pore structure and morphology of the material reported in the previous chapter,a large number of O vacancies were introduced into the hollow porous nano-spheres to prepare the oxygen-rich vacancy nickel phosphine nano-porous hollow spheres,and the effect of the reduction temperature on the pore structure change and the material morphology was explored.The performance of nickel phosphite nanoporous hollow spheres containing oxygen vacancy was much better than that of NaH2PO2 catalyst.The optimal HER catalyst had an overpotential of only 93 mV at the potential density of 10 mA cm-2 with excellent electrochemical stability.3.Doping additional elements into metal or non-metal based catalysts to modify and adjust the overall physical and chemical properties as well as electronic structure of the catalyst can jointly improve the catalyst activity,which is also an effective and simple method.However,it should be noted that these transition metal elements themselves have catalytic activity,and the enhancement mechanism can be attributed to the synergistic effect of doped elements and catalyst bodies.Electrocatalysts doped with transition metal elements without catalytic activity have not been synthesized,and the mechanism of hydrogen evolution is not yet clear.In view of this,we successfully doped Sn,a 3d transition metal with no catalytic activity,into CoP nanowires with regulation of the Sn content.The experimental results show that proper addition of Sn not only increases the dispersion of col-xsnxp on the substrate,but also enhances the catalytic activity of Co ion sites in CoP.The optimal Co0.84Sn0.16P CC shows excellent HER and OER activity.In HER,when the current density reached 10 mA cm-2,the overpotential was 60.3 mV,and the tafel slope was 40.8 mV dec-1.In OER,the overpotential was 360 mV when the current density reached 50 mA cm-2 and the corresponding tafer slope was 82.8 mV dec-1.4.After tailor the morphology and surface state,the adsorption of intermediates on the electrocatalyst is affected the performance of the catalyst is greatly improved.We synthesized a new material with uniform dispersion and relatively high density of Ni3Sn2S2 quantum dots anchored on ultra-thin Ni3S2 nanosheets(Ni3Sn2S2@Ni3S2)as a highly efficient non-noble metal hydrogen precipitation electrocatalyst under alkaline environment.The nanostructured catalyst exhibits high catalytic activity similar to that of noble metals(Pt/C)at low current densities,and even exceeds that of noble metals(Pt/C)at high current densities.Through the analysis of experimental results and theoretical calculation,it is concluded that the main reason for the improvement of the catalyst performance is due to more catalytic sites exposed and the weakening of h-s bond caused by atomic recombination.The introduction of Sn atom causes the electron delocalization of the unpaired nia-d orbital in the crystal of Ni3Sn2S2,making the d orbital center of the material closer to the Fermi level.At the same time,it also results in electron delocalization of the sn-p orbital and prevent the formation of h-s bond hybridization on the catalyst surface.It can optimize and regulate the adsorption and desorption of H atoms on the electrocatalyst surface.