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

Abstract

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 (CxHy) nor alcohol or aldehyde (CxHyOz) 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.

Real‐Time Carbon Monoxide Detection using a Rotating Gold Ring Electrode: A Feasibility Study

“…To demonstrate the practicality of using an Au ring detector, we have attempted a direct CO measurement during a CO 2 RR using a CO‐producing nitrogen‐doped carbon nanotube (NCNT, nitrogen content=3.7 at%) disc electrode. Metal‐free NCNT with a rich quaternary nitrogen content is well‐known to behave as a CO/H 2 producing catalyst system, [40–41] and has been validated in our previous report (XPS of the NCNT used is provided in Figure S6, SI) [41–42] . Figure 3 shows the SCV measurements using a RRDE equipped with either a Pt or an Au ring electrode.…”

“…Metal-free NCNT with a rich quaternary nitrogen content is well-known to behave as a CO/H 2 producing catalyst system, [40][41] and has been validated in our previous report (XPS of the NCNT used is provided in Figure S6, SI). [41][42] Figure 3 shows the SCV measurements using a RRDE equipped with either a Pt or an Au ring electrode. It is worth mentioning that we have adopted a wire mounting method to reduce the interference of bubble scrubbing to achieve signals at a better quality.…”

Metal‐Free Fluorine‐Doped Carbon Electrocatalyst for CO2 Reduction Outcompeting Hydrogen Evolution

“…DFT calculations were conducted based on the structure model in Figure 4a to investigate the possible role of fluorine doping played in this progress.A ccording to that the formation of adsorbed COOH* is ak ey step and the initial reduction barrier for CO 2 -to-CO reduction, CO generation could be inhibited by the high Gibbs free energy of COOH* adsorption, [5,13] becoming the RDS for CO 2 -to-CO conversion, as calculation results show in Figure 4b.T he lowest Gibbs free energy barrier of 0.64 eV for COOH* adsorption occurred at the forth carbon atom (short for C4, the red atom in Figure 4a)near the CF 2 bonds,followed by 1.49 eV on C1 just besides the CF 2 .T he nineth carbon atom (C9 in Figure 4b)located on the metal position of CF showed the third lowest free energy barrier for COOH*. On the contrary,t he Gibbs free energy for COOH* adsorption on carbon atoms in PC without fluorine doping exhibited the highest value.I n contrast, the high Gibbs free energy for COOH* adsorption on the neutral carbon atoms in PC requires an extra energy input to overcome the barrier,corresponding to aT afel slope of more than 120 mV dec À1 and only 1.6 %C Og eneration at À0.9 VonPC, which is consistent with previous studies on the dependence of overpotential for key intermediate form- Angewandte Chemie Zuschriften ation.…”

“…[4] Among these metal-free carbon catalysts,v ery few match the best level of noble metals. [5] Thebest of these catalysts,such as graphene,carbon nanotubes,and nanodiamond, are still of high costs and their synthesis is complicated. On the other hand, almost all the metal-free ECDRR catalysts with performance comparable to noble metals are dependent on nitrogen and boron replacing the carbon atom in lattices,a sw ell as sulfur on the edges,w hich have been known to be critical for the enhanced ECDRR process,b yi nducing active sites of high activity to ECDRR.…”

Rational Control of Topological Defects in Porous Carbon for High‐Efficiency Carbon Dioxide Conversion

“…The conversion of CO 2 to COOH* is considered to be the rate‐limiting step of CO 2 reduction. [ 31 ] As depicted in Figure 5b and Figures S21–S24 (Supporting Information), the edge C sites (C1) in Vac, Pen, N/Pen1, and N/Pen2 display lower free energy paths for the conversion of CO 2 to COOH* (Δ G COOH* ) than those of C2 and C3 sites, which suggest the higher catalytic activity of edge carbon species. The Δ G COOH* values of C1 sites in Pen (0.28 eV), N/Pen1 (1.11 eV), and N/Pen2 (0.96 eV) models are considerably lower than that of the Vac model (1.67 eV), indicating that the pentagonal structure possesses the higher catalytic activity.…”