| Optimizing energy structure,promoting energy supply-side reform and establishing multi-supply system are the main goal of building green,low-carbon,high-efficiency and sustainable energy supplies during the 13th Five-Year Plan period.Developing efficient catalysts is the key to address the energy and environmental concerns.Due to the unique geometry and electronic structures,single-atom catalysts as an emerging and novel catalysts have shown outstanding activity,selectivity and stability during energy conversion such as industrial catalysis and electrochemical reactions.In addition,by virtue of the highly homogeneous active sites,single-atom catalysts can be used as an ideal model for the investigation of catalytic reaction mechanism.Therefore,single-atom catalysts have become one of research hotspot and frontier in the field of heterogeneous catalysis.However,it has been generally reported that many catalysts would reconstructed under working conditions,which maybe induced by several physical and chemical effects,such as the temperature,applied voltage or adsorbates.Meanwhile,it is difficult to capture these key intermediates by ex-situ characterizations.These limitations hinder the further understanding the corresponding reactive mechanism.Therefore,capturing the reactive intermediates under operating conditions is the prerequisite for the rational design of new catalysts.In order to address these scientific challenges,in the thesis,the synthesis and application of single-atom catalysts in energy conversion are taken as the starting point.Understanding the well-defined structure-activity relationship is crucial for the rational guidance and design of efficient low-cost catalysts.Hence,we monitored the local atomic and electronic structures of the active sites and captured the key reactive intermediates at the atomic scale by using cutting-edge Operando synchrotron techniques.The corresponding contents were summarized as follow:1.Identifying the active sites of Co single-atom catalyst at atomic scale during HER process by using Operando XAFS technique.The highly homogeneous cobalt-based single-atom catalyst was obtained by wet chemical method and achived high-efficiency alkaline HER activity,reaching 10 mA cm-2 current density by just applying a low overpotential of 89 mV.Moreover,the as-obtained Co1/CPN possesses outstandingly intrinsic activity,delivering TOF of 5.98 H2 s-1 at an overpotential of 100 mV.We overcome the difficulties of the low concentration of active sites at the interface of catalyst and electrolyte and the dynamic change of structure that induced by the applied voltage.As a result,identification of the real structure and dynamic evolution of the active sites of cobalt-based single-atom catalyst during the electrocatalytic HER by Operando XAFS.Our results reveal the formation of a high-valence"HO-Co1-N2" moiety by the binding between isolated Co1-N4 sites with electrolyte hydroxide.In fact,this moiety is the real active site for HER,which will continue to adsorb water molecules driven by the voltage to form the reaction intermediate "H2O-(HO-Co1-N2)".The energy barrier for water dissociation can be effectively reduced by synergetic effect of single Co site with coordinated nitrogen,thus achieving efficient electrocatalytic water splitting for hydrogen production.2.Combing Operando XAFS and in situ FTIR,the dynamic working mechanism of Ru single-atom catalyst and the key intermediates were investigated during acid OER process.The atomically dispersed ruthenium-based single-atom catalyst(Ru-N-C)was synthesized by facile wet chemistry.Operando XAFS and SR-FTIR reveal the oxygen atom pre-adsorption on "Ru1-N4" site in the form of "O-Ru1-N4" under working voltage.The further DFT calculation demonstrated that such structure can effectively promote the transfer of charge from the Ru atom to the adsorbed O atom,and avoid the strong adsorption of the reactants,thus significantly reducing the required overpotential for the OER.As a result,the Ru-N-C single-atom catalyst exhibits excellent OER activity with a low overpotential of 267 mV at the current density of 10 mA cm-2 for continuous acid operation of 30 h,which is closed to those best electrocatalysts reported in acid OER.3.Development of low-cost and high-efficient cobalt oxide nanoclusters OER electrocatalyst.Benefitting from the strong confinement effect of phosphorus-doped carbon nitride,the cobalt oxide nanoclusters with a size of about 1.5 nm were synthesized(CoOx/PCN).XAFS results reveal that there are large of oxygen vacancies on the surface,such unsaturated coordination and interfacial coupling could affect activate the intrinsic activity of cobalt oxide nanoclusters and thus boosting OER activity.The resulting electrocatalyst delivers high OER activity,reaching 10 mA cm-2 by applying a low overpotential of 253 mV for continuous operation of 10 h.Moreover,the CoOx clusters exhibit high intrinsic activity,delivering turnover frequencies of 1.69 O2 s-1 at an overpotential of 300 mV,which are comparable to those of benchmark OER catalysts. |
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