Design,Preparation And Characterization Of M-N-C Catalyst With High Density Of Active Sites For Carbon Dioxide Reduction Reaction

The electrochemical reduction of CO₂（CO₂RR）to high value-added products has received extensive attention as it is considered as a solution to cope with global warming and increasing energy demands.CO₂ needs to get more electrons in the reduction process to be converted into chemical fuel,resulting in slow reduction of its reaction.Thus,development of highly efficient catalysts is of crucial significance.In the early stage of related research,Au and Ag catalysts were found to have good catalytic activity and selectivity for CO₂RR,but they are limited by the expensive price as precious metals.Nowadays,the development of low-cost,highly efficient non-precious metal catalysts has become a hotspot.Transition metal and nitrogen co-doped carbon（M-N-C）materials have been recently considered as promising substitutes for noble metal catalysts due to their low cost,high specific surface area,tunable structure,and unique electronic feature.The Fe-N-C was identified to obviously decrease the overpotential of CO₂RR,but there is great room for further improvement.In this research,we develop a strategy to prepare the Fe-N-C catalysts with high density of active sites for efficient CO₂ electroreduction,employing SiO₂ as protecting shell to prevent aggregation of isolated Fe atoms and volatilization of N-containing species during the formation of Fe-N-C materials at high temperature.The research mainly includes the following two aspects:1、Using pyrrole as the nitrogen source and potassium ferricyanide as the iron source.The Fe（CN）₆^3-dissolved in the solution can be assembled into PPy network through the electrostatic interaction because of the positively charged groups of PPy chain,and then the outer layer is coated with SiO₂ protecting shell.The Fe-N-C catalyst has an iron content of 3.9 wt.%and a nitrogen content of 7.82 at.%.The FE_CO of the Fe-N-C reaches over 90%with a high j_total of 12.8 mA cm^-2 at-0.49 V（RHE）in CO₂-saturated 0.5 mol·L^-1 KHCO₃ solution,which is much better than most of the Fe-N-C catalysts reported to date.The poisoning experiment and selective chemical modification verify that both Fe atom and ortho-C of pyridinic N are conducive to improving the CO₂RR activity.Theoretical calculations demonstrate that amorphous Fe clusters can dramatically lower CO bonding strength,thus facilitating the CO₂RR activity of Fe-N_x active sites.2、Using pyrrole as the nitrogen source,Ferric chloride hexahydrate as the iron source and Potassium thiocyanate as the sulfur source to synthesize Sulfur-containing precursor.The sulfur content in the catalyst was improved by the confinement of the SiO₂ protecting shell.The Fe-S/N-C double heteroatom doping catalyst was synthesized,and the sulfur content was 1.73 at.%.After doping with sulfur atoms,the FE_CO of Fe-S/N-C increased to 96.3%.The M-N-C catalysts prepared in the oresent research have good catalytic activity and selectivity.At the same time,the exploration of the active site expected to serve as a reference for the design and synthesis of novel M-N-C catalysts.