Metal-free Nanoporous Carbon as a Catalyst for Electrochemical Reduction of CO2 to CO and CH4.

S-doped and dual S,N-doped polymer-derived carbons were studied as electrocatalysts for the reduction of CO2. Higher Faradaic efficiencies for conversion to CO and CH4 were obtained for S,N-doped carbon than its S-doped counterpart. The former showed a maximum Faradaic efficiency of 11.3% for CO and 0.18% for CH4 formation. The S,N-nanoporous carbon was better at decreasing the overpotential of the reduction process. The pyridinic nitrogen groups were found to be actively participating in binding CO2. The quaternary nitrogen and thiophenic groups were also involved in the reduction process. It is proposed that the positively charged sites on the carbon atoms, adjacent to pyridinic nitrogen, stabilize the CO2(.-) and COOH* intermediates, promoting the formation of CO. The surface basicity of the catalysts improved the CO2 reduction selectivity when competing with H2 evolution. N2 adsorption measurements suggested that ultra-micropores enhance the reduction of CO2 to CH4.