Decisive Intermediates Responsible for the Carbonaceous Products of CO₂ Electro-reduction on Nitrogen-Doped sp² Nanocarbon Catalysts in NaHCO₃ Aqueous Electrolyte

Nitrogen-doped sp² nanocarbon materials have been considered promising catalysts for CO₂ electro-reduction. However, a fundamental understanding about product selectivity and the reaction routes is still lacking. In this report, the reaction mechanism on nitrogen-doped sp² nanocarbon materials is resolved by clarifying the authentic origin of the carbonaceous products: CO and HCO₂⁻. Two carbon-reduction pathways are identified based on a series of comparative studies by using differential electrochemical mass spectrometry and in situ CO adsorption experiments: a dominant pathway leading to CO from CO₂ and a secondary pathway leading to HCO₂⁻ from HCO₃⁻. Neither hydrocarbon (C_xH_y) nor alcohol or aldehyde (C_xH_yO_z) were detected in the reduction of CO₂. However, CO, which is generally regarded as an intermediate to be transformed into these products on metal catalysts, can undoubtedly be produced and adsorbed on nitrogen-doped sp² nanocarbon catalysts during the reaction.