Analyzing the effect of nitrogen/sulfur groups' density ratio in porous carbons on the efficiency of CO2 electrochemical reduction

The catalytic activity for CO2 electrochemical conversion into CO on N-doped carbons has been previously suggested to be enhanced by introducing a sulfur co-dopant to the carbon matrix. In this study, nitrogen and sulfur-codoped porous carbons were synthesized by a high temperature treatment of commercial wood-based carbon impregnated with thiourea. By slightly changing synthesis conditions, three different carbons were obtained and used as electrocatalysts for reduction of CO2. Detailed analyses of chemistry and texture showed differences in the amounts of N and S heteroatoms, in the speciation of surface groups, and in porosity. The best preforming sample had the highest dispersion/lowest density of nitrogen-containing groups and the low dispersion/high density of thiophenic groups. The dependence of the Faradaic efficiency for CO reduction on the ratio of the density of nitrogen groups to that of sulfur showed a linear decrease with an increase in the ratio, supporting the previously suggested enhancing effect of the high dispersion of N-containing catalytic centers and their activation by sulfur co-doping on CO2 electroreduction. The results also indicated that the catalytic activity of pyridinic groups is more positively affected by sulfur co-doping than is that of quaternary nitrogen species.