Nano-electrocataysts with A Critical Content of Platinum for Hydrogen Evolution Reaction in Acidic Solution and the Mechanism Study

Author:Guo Rui Zuo

Supervisor:li ze lin

Database:Doctor

Degree Year:2019

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Pages:115

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With the development of society,the energy crisis is an important issue which should be faced.The research and development of new clean alternative energy replacing fossil fuels is imperative.Hydrogen energy is a new kind of renewable and clean energy.Electrocatalytic decomposition of water is an important hydrogen generation technology.Pt and Pt-based catalyst have excellent electrocatalytic activity for hydrogen evolution reaction(HER).The preparation of high-efficiency catalysts which only contains a small amount of Pt is important for HER.In this dissertation,several AuPt alloy nanomaterials with a critical content of Pt were prepared by using different methods.The electrocatalytic activity of HER for these nanomaterials were tested in acidic solution.Formic acid was selected as the probe molecule and the electrocatalytic oxidation activity was used to proved that highly dispersed Pt existed in catalyst.According to the electrocatalytic activity of HER,theoretical calculation were performed for Au95Pt5(Ⅲ)model.A new HER mechanism on the surface of AuPt alloy electrode with a critical content of Pt(about 5 at.%)was proposed.The main contents are as follows:1.A small amount of highly dispersed Pt atoms was alloyed on the suface of nanoporous gold film(NPGF)electrode by using in situ electrochemical cyclic voltammetry(CV)scanning in H2SO4 soulution.The prepared electrode was named NPGF-t-Pt electrode.In this methed,the Pt source was Pt counter electrode.Pt on the surface of counter electrode could be slightly oxidized and dissolved in sulfuric acid solution through anodic polarization.The anodic dissolved trace Pt could be reduced and modified on working electrode to obtain NPGF-t-Pt electrode by during CV scanning.The modified amount of Pt atoms on the surface of NPGF electrode can be controlled by varying the cycle number of CV scanning.When the Pt content on NPGF-t-Pt electrode was about 5 at.%,the electrocatalytic performance of NPGF-t-Pt for HER in acidic solution was close to NPGF-m-Pt which was prepared by loading massice amount of Pt to cover the surface of NPGF electrode completely.The Tafel slope was 33 mV dec?1.NPGF-t-Pt electrode also shown excellent HER stability.2.AuPt alloy sols with different Au/Pt atom ratio were obtained by one-step reduction synthesis in aqueous solution,using sodium citrate as reducing agent and stabilizer.Au95Pt5 nanoparticles having different diameters were obtained by controlling the added amount of sodium citrate.The AuPt alloy sol was modified on the surface of mooth glassy carbon(GC)electrode and then measured in acidic solution for HER.It was found that Au95Pt5/GC whose particle size was about 14.2 nm shown the best catalytic activity of HER.When the content amount of Pt in AuPt alloy sol was 5 at.%,the HER catalytic activity Au95Pt5/GC was close to Pt/GC electrode.When the current density was 10 mA cm?2,the overpotential was?31 mV(vs.RHE).The Tafel slope was 33 mV dec?1.This result further demonstrates that the preparation cost of catalyst could be reduced by reducing the Pt content in catalyst.3.Reduced graphene oxide(rGO)supported well-dispersed AuPt alloy nanoparticles(Au95Pt5 NPs/rGO)was prepared by a one-step hydrothermal method without using other reducing agent.Under high temperature and high pressure,redox reaction occurred between the oxygen containing functional groups on GO and AuCl4?and PtCl62?.During the redox process,GO could translate to rGO and AuCl4?and PtCl62?were reduced to AuPt NPs.The electrocatalytic activity of HER on Au95Pt5 NPs/rGO were tested in H2SO4 solution.Au95Pt5 NPs/rGO displayed excellent catalytic performance for HER.Tafel slope was 29mV dec?1.When the current density was 10 mA cm?2,the overpotential was?35 mV(vs.RHE).Au95Pt5 NPs/rGO showed well HER stability and durability.4.A hierarchically porous nitrogen-doped rGO loaded with high dispersion AuPt alloy nanoparticles(Au95Pt5-PNrGO)catalyst was prepared by using Zeolite imidazolide framework(Zn-ZIF)as a dispersant and template for the first time.Gold(Ⅲ)imidazolium aurate salt(AuCl3-Im)and PtCl62?could enter into the pore structure of ZIFs by solvent effect and then graphene oxide was coated on the surface of ZIFs.During the electrochemically reducing process in H2SO4 solution,Zn-ZIF was firstly dissolved and AuCl3-Im and PtCl62?adsorbed in its pore structure was simultaneously releases.The released AuCl3-Im and PtCl62?were electrochemically reduced to AuPt alloy nanoparticles and dispersed on the surface of the hierarchically porous rGO which was formed after the dissolution of Zn-ZIF.In this process,ZIFs have three main effects:ZIFs was used as a porous template to encapsulate AuCl3-Im and PtCl62?in order to form uniform and highly dispersed AuPt alloy nanoparticles;ZIFs was also used as a soluble template to form graded porous rGO after dissolution;The N source in Au95Pt5-PNrGO was mainly from the Im released from ZIFs.Compared with commercial 20%Pt/C,Au95Pt5-PNrGO had better electrocatalysis activity of HER in acidic solution.When the current density was 10 mA cm?2,the overpotential was?22 mV(vs.RHE).Tafel slope was 18 mV dec?1.Au95Pt5-PNrGO displayed excellent HER stability.The electron effect of nitrogen doping rGO,the pore effect of graded porous structure and the Au base effect in AuPt alloy were all beneficial to enhance the electrocatalysis activity of HER.5.When the Pt content in the AuPt alloy nanoparticles was about 5at.%,the prepared catalyst showed excellent electrocatalysis activity of HER closed to pure Pt.The AuPt(Ⅲ)model was constructed by replacing one Au atom with one Pt atom on the surface of Au(Ⅲ)model.The adsorption energy of Hads on Au or Pt atom was calculate by using density functional theory.The HER mechanism on the suface of AuPt alloy was preliminarily proposed.The HER mechanism on AuPt alloy was different from pure Pt.Besides the classical HER elementary reaction in acidic solutions,A double Volmer reaction would occured on the isolated Pt atom in AuPt alloy.The supersaturated adsorbed Hadsds could overflow from Pt to adjacent Au atoms and diffuse on Au substrate.Since the adsorption energy of Hads on Au atom was weak,the Hads on Au atom was very easily desorbed to form H2.The double Volmer reaction on Pt atom and the Tafel reaction on the adjacent Au-Pt/Au-Au atoms were beneficial to improve the electrocatalysis activity of HER.The new HER mechanism on the single platinum atom-gold support catalyst with critical content Pt provides a new idea for the study of the reaction mechanism of single platinum atom-gold support catalyst.