Highly exposed atomic Fe–N active sites within carbon nanorods towards electrocatalytic reduction of CO2 to CO

Single-Atom Catalysts for the Electro-Reduction of CO2 to Syngas with a Tunable CO/H2 Ratio: A Review

“…Among single-atom catalysts for the CO 2 RR to CO, Fe-based single-atom catalysts have been also recently explored as single atoms dispersed on N-doped carbon supports. Fe SACs can exhibit high selectivity towards CO, with a maximum achievable CO FE of about 98% at −0.68 vs. RHE [75], comparable to the values achieved with Co-based and Ni-based SACs. However, unlike them, Fe-N-C electrocatalysts exhibit the highest CO selectivity at low overpotentials, from approximately −0.43 V vs. RHE to -0.68 V vs. RHE.…”

“…In particular, the maximum CO FE of C-AFC©ZIF-8 is 93.0% at −0.43 V vs. RHE (versus the CO FE of 84% at the same potential for C-AFC@ZIF-8) and the total current density reaches its maximum value of around 23 mA/cm 2 at −0.85 V vs. RHE (versus the value of around 12 mA/cm 2 at the same potential for C-AFC@ZIF-8) and at the maximum iron content of 1.47%, suggesting the importance of the Fe-N active sites for the catalysis of CO 2 RR. Wu et al [75] synthesized an Fe-N-C catalyst with highly exposed Fe-N x active sites through pyrolysis of a 3D sea-urchin-like FeOOH-polyaniline precursor, which also guaranteed large specific surface and electrochemically active surface areas. The prepared catalyst achieved a high CO FE of 95% with CO partial current density of 1.9 mA/cm 2 at an overpotential of 530 mV.…”

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

“…The Hemin precursor is chosen mainly because, in addition to containing the metal precursor Fe, it has carboxyl groups that are doped in the polyaniline chains through an electrostatic interaction, thus allowing a highly uniform dispersion of single atoms in the carbon structure, this is carried out by means of a heat treatment at a temperature of 700 • C. Within the chemical composition of the synthesized catalyst, it is found that during the treatment the majority of Fe 3+ species are reduced to Fe 2+ species, an atomic content of Fe equal to 0.86% was obtained, the single atoms are coordinated with Fe-N 4 sites, and the nitrogen species present were Pyridinic N > Pyrrolic N > Graphitic N > Oxidized N. According to theoretical studies, the graphitic N nitrogen species are the indicated species to accompany the Fe-N 4 active sites due to the fact that it presents a moderate binding force with the intermediates CHOO* and CO* in comparison with the other Pyridinic N and Pyrrolic N species. Wu and co-authors [135] also used polyaniline along with sea urchin-like FeOOH as an intermediate compound to obtain carbon nanorods with N atoms from using urea as a precursor. This carbonaceous matrix allows a high exposure of active Fe-Nx sites, for which a high FE of 95% and a low over potential of 530 mV were obtained.…”

Designing Undercoordinated Ni–Nx and Fe–Nx on Holey Graphene for Electrochemical CO2 Conversion to Syngas

“…For instance, Ni atoms coordinated with N (denoted as Ni–N x ) have displayed Faradaic efficiencies (FEs) for CO of up to 100%. − Among these sites, Ni–N 4 has attracted particular attention in Ni SACs, with recent quantum mechanics calculations revealing Ni–N 4 as the highest performing site for CO 2 RR, over other ascribed sites such as Ni–N 2 C 2 and Ni–N 3 C 1. SACs reporting Fe–N 4 active sites generally achieve a lower FE CO compared to Ni SACs, particularly at higher potentials of −1.0 to −1.2 V vs RHE. − Recently, we have revealed that undercoordinated (or unsaturated) M–N x sites (consisting of a decrease in the M–N x coordination) lower the free energy barrier for the adsorption of CO 2 radicals, increasing catalytic activity over conventional M–N 4 sites . In contrast, it has been found that Fe atoms undercoordinated with C are highly active toward HER; albeit, their activity for CO 2 RR remains unexplored .…”