Carbon Materials Confining Single Atoms For Electrocatalytic CO₂ Reduction

Carbon materials confining single atoms have shown excellent performance for electrocatalytic CO2 reduction to CO,which can be categorized into carbon materials confining single metal atoms and carbon materials confining single non-metal atoms.The active sites and reaction mechanisms of electrocatalytic CO2 reduction over carbon materials confining single atoms remain elusive due to the complicated coordination structures,which are necessary to perform in-depth research.In the thesis,the reaction mechanisms of electrocatalytic CO2 reduction over(Metal-N4)Me-N4 sites have been studied using transition metal phthalocyanine(MePc)with well-defined atomic structure.Furthermore,for the active structure of Fe-N4,carbon foam confining single iron atoms has been prepared via an approach of template-assisted pyrolysis.The modulations of coordination environments for the catalytic performance have been studied.In addition,for the carbon materials confining single non-metal atoms,the active sites of N-doped carbon foam for electrocatalytic CO2 reduction to CO have been unveiled.The main results are summarized as following:(1)The reaction mechanisms on Me-N4 sites for electrocatalytic CO2 reduction have been explored by using well-defined MePc as model catalyst.Theoretical calculations reveal that cobalt phthalocyanine exhibits the optimum activity for CO2 reduction to CO compared to other metal phthalocyanines,due to the moderate binding energys of intermediates on the Co site which accommodates the*COOH formation and the*CO desorption.It is further confirmed by electrochemical experiments,where cobalt phthalocyanine delivers the best performance,with a maximal CO Faradaic efficiency reaching 99%,and maintains the stable performance for 60 hours.(2)A multiscale carbon foam confining single iron atoms has been prepared with the assistant of SiO2 template.The pore-enriched environment of carbon foam at the macro-scale facilitates the diffusion of reactants and products.The graphene nanosheets at the nano-scale promote the charge transfer during the reaction.The single iron atoms confined in carbon matrix at the atomic-scale provide the active sites for electrocatalytic CO2 reduction.The optimized catalyst achieves a CO Faradaic efficiency of 94.9%at a potential of-0.5 V vs.RHE.Furthermore,the performance can be maintained for 60 hours.(3)N-doped carbon foams with controllable N types(graphitic N,pyridinic N,pyrrolic N,and N-oxide)and contents have been synthesized through a template-assisted pyrolysis method.The catalyst with only graphitic N dopants exhibits the optimum activity with a Faradaic efficiency of 95%for CO2 reduction to CO at a potential of-0.5 V vs.RHE.The excellent activity can be stably maintained for 80 hours.The controlled experiments and theoretical calculations demonstrate that graphitic N-induced active carbon atoms are more selective for CO2 reduction to CO against hydrogen evolution,while the case of pyridinic N-modified carbon atoms is just the opposite.