The Studies On Performance And Mechanism Of CO₂ Reduction Catalyzed By Noble Metal-free Carbon-based Nano-electrode

With the development of industrialization and urban modernization,the atmospheric carbon dioxide?CO₂?concentration was increased due to the burning of fossil fuels and automobile exhausting.CO₂ as a major component of greenhouse gases has caused a serious impact on the global climate and environment.Recently,in order to remit the global challenge delivered by excessive emission of CO₂,the topic of effective conversion of CO₂ into the value-added chemicals by electrochemical reduction reaction has become a target of research.It can not only decrease the emission of CO₂,but also can suitably solve the problem of energy shortages.Despite the certain efforts have been devoted into studying the electroreduction of CO₂?CO₂ER?,it still faces the requirement of developing highly-efficient,low-cost,and stable electrocatalysts to enhance the conversion of CO₂.As the previous reports,the noble metal and noble metal oxides based materials were used to be applied for CO₂ER,however,the practical application of this materials were highly hindered due to the high-cost and scarcity of noble metal.Hence,the carbon materials supported transition metal have been become one of the most effective electrocatalysts to replace the noble metal based one for highly-efficient CO₂ER reaction.In this work,we developed a series of composite materials that composed of carbon materials and transition metals for highly-efficient CO₂ER into carbon monoxide?CO?.Based on the results of Aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy?AC HAADF-STEM?and X-ray adsorption spectroscopy?XAS?,combined with the density functional theory?DFT?calculations,the active sites and catalytic reaction mechanisms were revealed.The main research contained the following aspects:?1?We designed a composite electrocatalyst?Ni@NCNTs?with nitrogen doped carbon nanotubes?NCNTs?confined the nickel nanoparticles?Ni NPs?to form the‘confinement effect'for highly enhancing the performance of CO₂ER into CO.The Ni NPs were directly confined inside the NCNTs via a one-step pyrolysis strategy.The‘confinement effect'of NCNTs highly boosting the performance of Ni@NCNTs for CO₂ER,in which the onset potential was-0.3 V,Tafel slope was 97 m V dec^-1,and a maximum CO selectivity of 99.1%was achieved at-0.8 V,superior to the pure NCNTs with 2.8 times.DFT calculations confirmed that the desorption of*CO intermediate was the rate-determining step during the electroreduction reaction of CO₂ into CO;the‘confinement effect'of NCNTs highly weakened the binding strength between the*CO intermediate and Ni NPs,thus accelerating the desorption of*CO from the surface of Ni NPs to form the CO gas,boosting the reaction kinetics of CO₂ER reaction.?2?we reported a highly efficient CO₂ER electrocatalyst composed of coordinatively unsaturated single copper?Cu?atom coordinated with nitrogen atoms sites anchored into graphene matrix?Cu-N₂/GN?.The Cu-N₂/GN was synthesized via a one-step in-situ pyrolysis of graphene with chlorophyllin and dicyandiamide under nitrogen atmosphere at 900oC.Benefitting from the unsaturated coordination environment and atomic dispersion,the Cu-N₂/GN exhibited a high CO₂ER activity and selectivity for CO production with an onset potential of-0.33 V and the maximum CO Faradaic efficiency of 81%at a low potential of-0.5 V,superior to the previously reported atomically dispersed Cu-N anchored on carbon based electrocatalytic materials.Experimental results manifested the highly exposed and atomically dispersed Cu-N₂active sites in graphene framework where the Cu species were coordinated by two N atoms.DFT calculations demonstrated that the optimized reaction free energy for Cu-N₂sites to capture CO₂ promotes the adsorption of CO₂ molecules on Cu-N₂ sites;meanwhile,the short bond lengths of Cu-N₂ sites accelerated the electron transfer from Cu-N₂ sites to*CO₂,thus efficiently boosting the*COOH intermediate generation and CO₂ER performance.A designed rechargeable Zn-CO₂ battery composed of anode of Zn foil and cathode of Cu-N₂/GN nanosheets delivered a peak power density of 0.6 m W cm^-2,and the charge process of battery could be driven by natural solar energy.