| Due to the environmental deterioration and the increasingly serious energy crisis,the development of sustainable and clean energy is of great significance to human society.Since hydrogen gas is considered to be an attractive alternative to fossil fuels resulting from its excellent combustion performance,high calorific value,and zeropollution,electrochemical water splitting becomes a promising renewable energy conversion route to produce pure hydrogen.Nevertheless,water splitting still needs to overcome the large overpotential generated by the hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)occurring at the cathode and anode,respectively,so it is of great importance to develop efficient electrocatalysts for water splitting.Recently,it is possible to obtain higher current densities at lower energy consumption by replacing OER with oxidation reactions that are more readily to occur,such as the oxidation of urea,methanol,ethanol and hydrazine hydrate.In the past few years,onedimensional(1D)nanomaterials have been widely developed as efficient water splitting electrocatalysts because of their small size,large specific surface area,and high aspect ratio,and their ability to transfer electrons and mass along a controlled direction.Electrospinning technology has attracted much attention as one of the most convenient methods for preparing fibrous or tubular nanomaterials,and has been flexibly applied to the field of energy conversion and storage.This thesis focuses on the preparation and electrocatalytic properties of 1D nanofibers,and modulates their morphology,chemical composition and electronic structure by means of post-treatment such as hightemperature carbonization or calcination,solvothermal reaction and electrochemical deposition,so that the catalysts would exhibit more excellent catalytic performances in electrocatalytic water splitting and urea-assisted water splitting.The specific research of this thesis is as follows:1.Electrospinning technology combined with different post-treatment methods is used to prepare metal sulfide-based nanofibers/nanotubes.Resulting from the surface reconstruction and the fabrication of heterostructures,the electrocatalytic water splitting properties are greatly enhanced.Herein,we carried out two main parts of work.(a)We present a template-directed hydrothermal reaction by using electrospun nanofibers to prepare hierarchical Ni S nanotubes(NTs)composed of tight vertical nanosheets.Then,the Fe(III)/Ni S NTs were prepared by a post-treatment of ion adsorption remaining a unique 1D hollow structure assembled with vertical nanosheets on the surface.Owing to the reconstruction of Fe(III)/Ni S NTs to generate active Ni(Fe)OOH as electrochemical sites,the prepared hierarchical catalyst displays a wonderful OER activity in alkaline medium,which requires a low overpotential of 264 m V at the current density of 10 m A cm-2 and it can be maintained at a continuous voltage for over 70 h.Besides,the unique 1D hollow nanostructure provides a more convenient way for electron and mass transfer,leading to excellent redox chemical performance and ion transport property.Furthermore,the hierarchical tubular structure assembled with vertical nanosheets enable abundant electrochemically active sites,also resulting in the excellent electrochemical properties.(b)Ni Fe-carbon nanofibers(CNFs)decorated with Fe-doped Ni S2 crystals((Ni,Fe)S2/Ni Fe-CNFs)have been prepared via an electrospinning,calcination and vulcanization process.The doping of Fe element modulates the electronic structure of the Ni S2,endowing with more exposed active sites,which is beneficial to enhance the electrochemical activity.The remaining nanoscale Ni Fe alloys distributed within the interior and on the surface of the CNFs can not only improve their electrical conductivity but also bring the synergistic effect with(Ni,Fe)S2 to promote the OER performance.(Ni,Fe)S2/Ni Fe-CNFs catalyst exhibits a low overpotential of 263 m V to deliver the current density of 10 m A cm-2 in 1 M KOH.Furthermore,a two-electrode electrolyzer constructed by using(Ni,Fe)S2/Ni Fe-CNFs as the anode and Pt/C as the cathode presents a low cell voltage of 1.54 V at 10 m A cm-2 in alkaline electrolyte,illustrating its promising potential toward overall water splitting.2.The price of Ru is much lower than that of Pt,and metallic Ru and Ru O2 are good HER and OER catalysts,respectively.Partially oxidized Ru nanoparticlesembedded 1D carbon nanofibers(Ru O2/Ru-CNFs)have been prepared via an electrospinning technology combined with high-temperature carbonization method.Subsequently,after a short oxidative calcination process in air,Ru nanoparticles are partially oxidized with partial retention of the carbon substrate to produce Ru O2/RuCNFs.Owing to the configuration of the heterostructure between Ru O2 and metallic Ru as well as the favorable electrical conductivity of CNFs,the Ru O2/Ru-CNFs exhibit both superior OER and HER activities,showing the overpotentials of 203 and 21 m V to reach 10 m A cm-2 and excellent durability in 1 M KOH.Therefore,Ru O2/Ru-CNFs as bifunctional catalyst is used as anode and cathode for assembling the overall water splitting electrolyzer,which only requires a voltage of 1.452 V to reach the current density of 10 m A cm-2,much better than standard commercial Ru O2||Pt/C electrolyzer.3.Two types of bifunctional electrocatalysts with both HER and UOR properties have been developed and assembled into an electrocatalytic overall water splitting assisted by urea oxidation,showing a low cell voltage.The urea-assisted water splitting provided in this work not only significantly reduces the energy consumption of hydrogen production,but also provides an opportunity for the purification of urea-rich wastewater.The following two aspects are included:(a)Ni-CNFs are prepared via an electrospinning combined with high-temperature carbonization process.After that,β-Ni S-coated Ni-CNFs(Ni S@Ni-CNFs)are prepared through an electrodeposition combined with hydrothermal vulcanization.Finally,Ni S@Ni-CNFs are loaded with low content of Pt nanoparticles via an adsorption process and H2 reduction pathway to achieve Pt-Ni S@Ni-CNFs.The excellent HER catalytic activity of Pt-Ni S@Ni-CNFs was mainly derived from that the partial conversion of b-Ni S to Ni3S2 at cathodic potential,which synergistically promoted the catalytic effect with the integration of Pt nanoparticles.The mass activity of the PtNi S@Ni-CNFs can reach 1209.2 A g-1 at an overpotential of 150 m V,which is 2.74 times as high as that of commercial Pt/C catalyst.In addition,the prepared Pt-Ni S@NiCNFs also possess excellent UOR performance.Assembling the Pt-Ni S@Ni-CNFs modified carbon paper as electrodes into a urea-assisted water splitting electrolyzer,a voltage of only 1.44 V is required to achieve a current density of 10 m A cm-2,which proves its promising application in the field of low-energy hydrogen production.(b)Pt-Ni(OH)2@Ni-CNFs and Pt@Ni-CNFs have been prepared via a one-step electrodeposition route using the same conductive Ni-CNFs as the substrate.The excellent UOR activity of Pt-Ni(OH)2@Ni-CNFs is attributed to the successful formation of the heterostructure between Pt nanoparticles and a-Ni(OH)2,which can effectively enhance the rate of urea oxidation.In addition,the superior HER catalytic performance of Pt@Ni-CNFs results from the synergistic interaction between Pt nanoparticles and metallic Ni in the Ni-CNFs substrate.Similarly,the urea-assisted water splitting electrolyzer assembled by using Pt-Ni(OH)2@Ni-CNFs and Pt@NiCNFs as the anode and cathode,respectively,can achieve a current density of 10 m A cm-2 with a voltage of only 1.4 V.This study indicates that our catalysts have promising applications in reducing energy consumption for water electrolysis and urea pollution control. |
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