Fabrication Of Nitrogen-doped Carbon Catalysts For Efficient Electrocatalytic Reduction Of Carbon Dioxide

The rapid growing combustion of fossil fuels causes energy crisis and significant accumulation of CO₂,resulting in negatively irreversible damage on the planet.Electrochemical reduction of CO₂ has been gradually regarded as the sustainable solution to mitigate these issues,which realizing carbon cycle by catalyzing CO₂ to various value-added fuels or chemicals under the action of renewable energy.Metal-free carbon materials are substantially applied as cathodic catalysts to realize efficient electroreduction of CO₂.However,how to improve their catalytic performances remains a big challenge.The electrochemical activities of carbon catalysts largely depend on their structures,conductivities as well as the nitrogen contents and types.Based on the above discussion,several kinds of carbon materials were fabricated as the catalysts for electroreduction of CO₂,and the relationship of structures and catalytic activities was discussed in this thesis.The main contents are as follows:（1）The hybrid nitrogen-doped carbon nanofibers were fabricated through an electrospinning process from a precursor solution containing quinoline-polymerized pitch（NP）and polyacrylonitrile（PAN）,and the effects of the chemical composition on the catalytic activities were discussed by adjusting the raw material composition and carbonization temperature.The resulting hybrid CNFs,specifically the mass ratio was 1:1 and carbonization temperature reached 1000 ^oC,showed the onset over potential of 90 mV for electrochemical reduction of CO₂ and could convert CO₂ into CO with FE of 60.9%at the over potential of590 mV,meanwhile,the FE of H₂ reached 36.4%,and the very proportion of CO and H₂could be used to produce high grade C1 compound by Fischer-Tropsch synthesis,Tafel analysis was performed to certify the rate-limiting single-electron transfer to CO₂(Tafel slope was about 118 mV dec^-1).The fast electron and proton transmission which could be ascribed to the interconnected 3 dimensional structure of nanofibers was proved to favor the excellent performance,and high pyridinic N-doping contents could provide abundant active sites on the basis of the comparative relationship of heteroatomic configuration and performance.（2）The nitrogen-doped porous carbons were synthesized by combining the hard-template and in-situ activation method using NP as precursor and Nano-CaCO₃ as template and activating agent.The obtained porous carbon,specifically the mass ratio was 1:5and carbonization temperature reached 950 ^oC,showed the onset over potential of 140 mV and could convert CO₂ into CO with a high FE of 81.2%at the low over potential of 540 mV,which showed excellent catalytic performance and high product selectivity,and the long-term performance at over potential of 540 mV for 10 h demonstrated that the catalyst had excellent durability,Tafel slope was calculated to be consistent with the rate-limiting single-electron transfer process,and the double layer capacitance indicated that the obtained porous carbon possessed a large electrochemically active surface area,and the highly opened porous structure not only facilitated the electron and proton transfer but also provided exposed active sites,which could further promote electrocatalysis reaction.