Selective CO₂ Reduction to CO in Water using Earth-Abundant Metal and Nitrogen-Doped Carbon Electrocatalysts

Abstract

Earth-abundant transition metal (Fe, Co, or Ni) and nitrogen-doped porous carbon electrocatalysts (M-N-C, where M denotes the metal) were synthesized from cheap precursors via silica-templated pyrolysis. The effect of the material composition and structure (i.e., porosity, nitrogen doping, metal identity, and oxygen functionalization) on the activity for the electrochemical CO2 reduction reaction (CO2RR) was investigated. The metal-free N-C exhibits a high selectivity but low activity for CO2RR. Incorporation of the Fe and Ni, but not Co, sites in the N-C material is able to significantly enhance the activity. The general selectivity order for CO2-to-CO conversion in water is found to be Ni > Fe ≫ Co with respect to the metal in M-N-C, while the activity follows Ni, Fe ≫ Co. Notably, the Ni-doped carbon exhibits a high selectivity with a faradaic efficiency of 93% for CO production. Tafel analysis shows a change of the rate-determining step as the metal overtakes the role of the nitrogen as the most active site. Recording the X-ray photoelectron spectra and extended X-ray absorption fine structure demonstrates that the metals are atomically dispersed in the carbon matrix, most likely coordinated to four nitrogen atoms and with carbon atoms serving as a second coordination shell. Presumably, the carbon atoms in the second coordination shell of the metal sites in M-N-C significantly affect the CO2RR activity because the opposite reactivity order is found for carbon supported metal meso-tetraphenylporphyrin complexes. From a better understanding of the relationship between the CO2RR activity and the material structure, it becomes possible to rationally design high-performance porous carbon electrocatalysts involving earth-abundant metals for CO2 valorization.

Engineering Local Coordination Environments of Atomically Dispersed and Heteroatom‐Coordinated Single Metal Site Electrocatalysts for Clean Energy‐Conversion

“…Both the nature of the metal site and the corresponding coordination environment can affect the adsorption energies of *COOH, *CO, and CO on MN 4 , thus determining their activity and selectivity during the CO 2 RR. A combination of theory and experimental results suggests that the CO 2 RR activity of MN 4 sites follows the trend Ni ≥ Fe > Co . Recently, five MN x catalysts (M = Ni, Fe, Co Mn, and Cu) were investigated by Ju et al At low overpotentials, the first electron transfer to generate a surface adsorbed *COOH species is usually the rate‐determining step for CO evolution.…”

Optimizing FeNC Materials as Electrocatalysts for the CO2 Reduction Reaction: Heat‐Treatment Temperature, Structure and Performance Correlations

“…Recently, carbon-based solid catalyst materials doped with nitrogen and nitrogen-coordinated transition metals (MÀ NÀ C) have emerged as a selective and cost-efficient alternative to noble metal catalysts for the direct electrochemical reduction of CO 2 to CO. [13][14][15][16][17][18] Such materials have been widely studied as an alternative to Pt catalyst for the Oxygen Reduction Reaction (ORR) in cathode's fuel cells. [19][20][21][22] To explore the possibility of using MÀ NÀ C catalyst for the CO2RR Tripkovik et al, performed Density Functional Theory calculations (DFT) on porphyrin-like metal-functionalized graphene structures and found that these materials should in fact be active in the reduction of CO 2 .…”

Metal–Nitrogen–Carbon Electrocatalysts for CO2 Reduction towards Syngas Generation

“…Figure B–D shows the performance of several M‐N‐C electrocatalysts for the CO 2 RR in CO, along with their faradaic efficiencies for CO generation and the ideal overpotentials to achieve syngas formation (Figure B). Fe‐N‐C electrocatalysts exhibit the highest CO formation activity and selectivity at low overpotentials ( E =−0.3 to −0.5 V vs. RHE), similar to N‐C, although N‐C catalysts exhibit a lower overall current, whereas Ni‐N‐C electrocatalysts dominate regarding selectivity/activity at higher overpotentials. Because of their more thermodynamically favorable nature toward the HER, Co‐N‐C electrocatalysts, driven by the activity of their CoN 4 sites, exhibit a lower selectivity for CO production throughout the entire potential range.…”

“…For example, most CO 2 RR characterizations are carried out in KHCO 3 solution (phosphate‐buffered), as CO 2 replenishment at the electrocatalyst interface is easier; this results in enhanced faradaic efficiency for CO 2 RR with respect to HER . Also, Hu et al . observed FE CO =50–90 % for Fe‐N‐C electrocatalysts in 0.5 m KHCO 3 , and Wu et al .…”