Volcano Trend in Electrocatalytic CO₂ Reduction Activity over Atomically Dispersed Metal Sites on Nitrogen-Doped Carbon

Abstract

The development of catalysts for electrochemical reduction of carbon dioxide (eCO 2 RR) with high activity and selectivity remains a grand challenge to render the technology useable. As promising candidates, metal−nitrogen−carbon (MNC) catalysts with metal atoms present as atomically dispersed metal−N x moieties (MN x , M = Mn, Fe, Co, Ni, and Cu) were investigated as model catalysts. The distinct activity for CO formation observed along the series of catalysts is attributed to the nature of the transition metal in MN x moieties because of otherwise similar composition, structure, and morphology of the carbon matrix. We identify a volcano trend between their activity toward CO formation and the nature of the transition metal in MN x sites, with Fe and/or Co at the top of the volcano, depending on the electrochemical potential. Regarding selectivity, FeNC, NiNC, and MnNC had Faradaic efficiency for CO >80%. To correctly model the active sites in operando conditions, experimental operando X-ray absorption near edge structure spectroscopy was performed to follow changes in the metal oxidation state with electrochemical potential. Co and Mn did not change the oxidation state with potential, whereas Fe and Ni were partially reduced and Cu largely reduced to Cu(0). Computational models then led to the identification of M 2+ N 4 −H 2 O as the most active centers in FeNC and CoNC, whereas Ni 1+ N 4 was predicted as the most active one in NiNC at the considered potentials of −0.5 and −0.6 V versus the reversible hydrogen electrode. The experimental activity and selectivity could be rationalized from our density functional theory results, identifying in particular the difference between the binding energies for CO 2 * − and H* as a descriptor of selectivity toward CO. This indepth understanding of the activity and selectivity based on the speciation of the metals for eCO 2 RR over atomically dispersed MN x sites provides guidelines for the rational design of MNC catalysts toward eCO 2 RR for their application in highperformance devices.

Optimizing Electron Densities of Ni‐N‐C Complexes by Hybrid Coordination for Efficient Electrocatalytic CO2 Reduction

“…Recently,t he influencing factorso fM -N-C catalysts have been explored, such as the type of transition metal,v alence states, and coordination unsaturation effects. [36][37][38][39][40][41] Even though the concentration of Cspecies is always greater than 95 at %i nt he resultingM -N-C composites, most studies have focused only on the effect of as mall amount of Nw hile ignoring the contribution of Ct oa ctive sites. Indeed, it has been recently reported in the literature that MÀNa nd MÀCb onds coexist in the M-N-C catalysts, [16][17][18][19][42][43][44] however,t he roles of Na nd Ca toms in the active sites of M-N-C complexes are still unclear.T herefore, as ystematic investigation of the activity trends and catalytic mechanisms of N-and C-coordinated metal complexes through ac ombination of theoretical calculations and experimental validation is essential foracomprehensive and indepth understanding of M-N-C electrocatalysts.…”

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

“…Instead, CO formation is favored, with only as mall amount of formica cid/formate. [60] As shown by Mçller et al, [45] an Ni-N-C electrocatalyst can achieveacurrent density and faradaic efficiencyf or CO generation of 220 mA cm À2 and 80 %, respectively,i n1 m KHCO 3 at E = À1.1 Vv ersusR HE with a liquid electrolyte and gas-diffusion electrode. This paper was highlighted, as it is, to date, the most successful implementation of an M-N-C electrocatalyst in an industrial-like flow-cell or electrolyzer for the CO 2 RR.…”

“…This was experimentally confirmed by Ju et al [75] and Varela et al, [76] who evidenced that, on Fe-N-C, the CO 2 !CO ads step shows anon-Nernstian pH dependence and, by DFT calculationsd ecorrelating the CO 2 !CO 2 À ,ads and CO 2 À ,ads !COOH ads steps,h ighlighted that the ET as the uphill step in free energy for M-N-C electrocatalysts (M = Mn, Fe, Co, Ni, Cu). [60] The free energy for the CO 2 RR first step differs for each metal element [42,45,51,52,54,57,58,69] (Figure 3A). For the early 3d transition metals (e.g.,C r, Mn, Fe, and Co), as light increase in free energy is required to achieve the CO 2 !COOH ads step (0.24-0.45 eV), whereas Ni and CuN 4 requirea pproximately 1.6 eV.…”

Hydrogen Evolution Reaction - From Single Crystal to Single Atom Catalysts

“…Schematic representation of crucial hydrogen evolution steps in protic acidic medium on an extended (111) surface of an fcc metal (b) and a typical [166] SAHC (Pd atoms supported on heptazinic carbon nitride) (c). Coordination environments of successful single atom HER catalyst described in the literature [167][168][169][170][171][172][173].…”