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

Highlights: • Heating rate and washing affected the porosity and chemistry of N/S-carbon. • FE for CO₂ reduction increased upon N and S-doping. • FE did not directly depend on the porosity and surface chemistry of N/S- carbons. • Relative dispersion of N/S groups on the surface strongly affected CO₂ 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.