Understanding activity and selectivity of metal-nitrogen-doped carbon catalysts for electrochemical reduction of CO2

Abstract

Direct electrochemical reduction of CO₂ to fuels and chemicals using renewable electricity has attracted significant attention partly due to the fundamental challenges related to reactivity and selectivity, and partly due to its importance for industrial CO₂-consuming gas diffusion cathodes. Here, we present advances in the understanding of trends in the CO₂ to CO electrocatalysis of metal- and nitrogen-doped porous carbons containing catalytically active M-N _x moieties (M = Mn, Fe, Co, Ni, Cu). We investigate their intrinsic catalytic reactivity, CO turnover frequencies, CO faradaic efficiencies and demonstrate that Fe-N-C and especially Ni-N-C catalysts rival Au- and Ag-based catalysts. We model the catalytically active M-N _x moieties using density functional theory and correlate the theoretical binding energies with the experiments to give reactivity-selectivity descriptors. This gives an atomic-scale mechanistic understanding of potential-dependent CO and hydrocarbon selectivity from the M-N _x moieties and it provides predictive guidelines for the rational design of selective carbon-based CO₂ reduction catalysts.Inexpensive and selective electrocatalysts for CO₂ reduction hold promise for sustainable fuel production. Here, the authors report N-coordinated, non-noble metal-doped porous carbons as efficient and selective electrocatalysts for CO₂ to CO conversion.

Fig. 1

Visualization, porosity and illustration of the M–N–C catalyst. a Typical SEM image of the family of N-coordinated metal-doped (M–N–C) carbon electro-catalysts, scale bar=4 μm; b CO₂ physisorption isotherm (273 K); inset: the pore size distribution; c Materials model and a schematic local structure

Fig. 2

High-resolution XPS characterization. N-1s XPS core level region of a Co, b Mn, c Ni and d Fe doped M–N–C catalyst. The 2p_3/2 spectra of the corresponding metal peaks (Co-2p, Mn-2p, Ni-2p, Fe-2p) is shown in Supplementary Fig. 9

Fig. 3

CO₂ reduction reaction activities. Linear sweep voltammetry of a Mn–N–C, b Fe–N–C, c Co–N–C, d Ni–N–C and e Cu–N–C in CO₂-saturated 0.1 M KHCO₃ (solid lines) and in N₂-saturated 0.1 M KH₂PO₄/K₂HPO₄ (dashed lines) with a catalyst loading of 0.76 mg cm⁻² at 5 mV s⁻¹ in cathodic direction

Fig. 4

Catalytic performance and product analysis. a–c Faradaic Efficiencies (FE) vs. applied, IR-corrected electrode potential of a H₂, b CO and c CH₄. d Catalyst mass-normalized CO partial currents (mass activity) vs. applied potential for the five M–N–C catalysts compared to state-of-art Au catalysts (performance ranges of Au-nanoparticle and Au-nanowires are shown by filled areas^–. Lines to guide the eye. Conditions: 60 min at constant electrode potential in CO₂-saturated 0.1 M KHCO₃ with 0.76 mg cm⁻² M–N–C catalysts loading. Faradaic efficiencies and CO yields after 15 min are shown in Supplementary Fig. 11

Fig. 5

Experimental correlation to simulations. Experimental CO production turnover frequency (TOF) of the M–N–C catalysts vs. applied IR-corrected electrode potential (see Supplementary Equation 4). The catalytic reactivity trends a and reaction pathway b split into 3 potential regions with distinctly different rate-determining mechanistic features. Insets: Region 1: Low overpotentials, the experimental onset potentials of CO production (better seen on the log (CO TOF)—E plot in Supplementary Fig. 15) correlate with the binding energy of the reaction intermediate COOH* taken from Supplementary Fig. 14. Region 2: Intermediate over-potentials, CO production TOF at −0.6 V_RHE correlates with the free energy of adsorbed CO, CO* taken from Supplementary Fig. 14; Region 3: High overpotentials, free energy diagrams for the HER (dashed paths) and CO2RR (solid paths) at −0.8 V_RHE for each M–N–C catalyst. HER barriers are high for Ni and Cu, while CO2RR is downhill making these materials favorable CO producing catalysts