Metal-free N, P-Codoped Carbon for Syngas Production with Tunable Composition via CO2 Electrolysis: Addressing the Competition Between CO2 Reduction and H2 Evolution.

Abstract

Electroreduction of carbon dioxide into value-added fine chemicals is a promising technique to realize the carbon cycle. Recently, metal-free heteroatom doped carbons are proposed as promising cost-effective electrocatalysts for CO2 reduction reaction (CO2RR). However, the lack of understanding of the active site prevents the realization of a high-performance electrocatalyst for the CO2RR. Herein, we synthesized metal-free N, P co-doped carbons (NPCs) for producing syngas, which is composed of H2 and CO, by CO2 electrolysis using inexpensive bio-based raw materials via simple pyrolysis. The syngas ratio (H2/CO) can be controlled within the high demand range (0.3-4) at low potentials using NPCs by tuning the N and P contents. In comparison with only N doping or P doping, N and P co-doping has a positive impact on improving CO2RR activity. Experimental analysis and density functional theoretical (DFT) calculations revealed that negatively charged C atoms adjacent to N and P atoms are the most favorable active sites for CO2-to-CO conversion compared to pyridinic N on N, P co-doped carbon. Introducing N atoms generates the preferable CO2 adsorption site, and P atoms contribute to decreasing the Gibbs free energy barrier for key *COOH intermediates adsorbed on the negatively charged C atoms.