Design And Reaction Mechanism Studies Of Electrocatalysts Via Synchrotron-Based X-ray Absorption Spectroscopy

The ever-increasing consumption of fossil fuels has caused severe energy shortage and environmental pollution.Developing clean energy is an effective pathway to mitigate the current energy and environmental problems and has great research significance.Among various strategies for clean energy production,electrocatalytic conversion of CO2 and H2O to value-added chemicals,such as CO and H2,is regarded as one of the promising routes.Noble metal-based materials as conventional catalysts have exhibited superior catalytic performance,but the high cost and low reserves severely impede their large-scale applications.Therefore,how to reduce the utilization of noble metal and achieve the robust catalytic activity of noble metal-based catalysts is in the frontier.Meanwhile,earth-abundant transition metal-based materials are considered as the promising candidates to noble metals,but the deficient active sites and low intrinsic activity hamper their overall catalytic performance.Therefore,further development of highly efficient transition metal-based catalysts should be concerned.Undoubtedly,the rational design of catalysts should be considered based on the precise identification of catalytically active sites and the in-depth understanding of dynamic reaction processes.Directing at the key scientific questions in the electrocatalysts for water and CO2 activation,this dissertation focuses on the material design and controllable synthesis,and the correlation exploration between local structure and activity of catalysts by means of the high-resolution X-ray absorption spectroscopy(XAS).Combining the theoretical calculations with in situ XAS,the dynamic catalytic processes of electrocatalysts are probed,meanwhile the true active phase and atomic structure under working conditions are revealed.These findings will provide the experimental and technical bases for further design of efficient electrocatalysts.The main research contents and results of this dissertation are listed in the following:1 The design of Ni-based oxidation electrocatalyst and its structure reconstructionThe layered Ni(OH)2 catalysts with different nickel vacancies are prepared through a controlled alkalization precipitation method.The relative concentrations of nickel vacancy in catalysts are explored through synchrotron-based X-ray absorption fine structure(XAFS)spectroscopy,positron annihilation spectroscopy,etc.Electrochemical measurements and DFT calculations demonstrate that the nickel vacancy would promote the reconstruction of pristine catalysts to form active phase,thereby boosting activity of oxygen evolution reaction(OER).2 The design of Co-based OER electrocatalyst and the identification of true active sites after reconstructionA new Co2(OH)3Cl precatalyst for OER is prepared and it can achieve almost complete reconstruction during electrochemical process by means of the etching of lattice anion.In-situ XAFS and DFT calculations demonstrate that the exchange of lattice Cl-with OH-in electrolyte holds the key to promoting the reconstruction process and identify that the main OER activity is from the reconstruction-induced coordinately unsaturated active sites.3 The design of carbon-supported Co/Ni co-existed single-atom electrocatalysts and their electrocatalytic CO2 reductionThe Co/Ni-containing carbonaceous single-atom catalyst is prepared through a coordination-assisted pyrolysis method.The obtained catalyst can efficiently produce synthesis gas with adjustable CO/H2 ratios due to the different catalytic selectivity of different metal sites(Co or Ni).Upon the ratio regulation of loaded metals,the catalyst can perform both high current density and wide synthesis gas ratios.Finally,the XAFS characterizations,combining DFT calculations,demonstrate the single-atom structure of catalyst and reveal the interaction between reaction intermediates and different single metal sites.4 The design of carbon-supported Pd single-atom catalyst and its electrochemical CO2 reductionThe catalyst consisting of single Pd atoms coordinated into nitrogen-doped carbon is prepared and it can achieve efficient electrochemical conversion of CO2 to CO.In-situ XAFS analysis reveals that the Pd-N4 sites should be the true active sites,and DFT calculations further demonstrate that this Pd-N4 sites would promote the stability of adsorbed intermediates during CO2 reduction.5 The structure study of a Ni5P4 catalyst incorporating single-atomic Ru sites and its electrocatalytic water splitting under alkaline conditionThe stabilizing of metal ions with metal vacancies is carried out to obtain a Ni5P4 catalyst with single-atomic Ru sites doping.The synchrotron-based XAFS analysis,combining DFT calculations,reveals the localized structure polarization on the interfaces,and the structure polarization can bring electron-rich Ru sites to efficient catalyze water dissociation.More interestingly,the Ru-related interstitial sites can boost the transfer of produced species from Ru sites and release Ru sites efficiently,thereby achieving the efficient water splitting under alkaline condition.