Electroreduction of CO2 in a Non-aqueous Electrolyte-The Generic Role of Acetonitrile

Abstract

Transition metal carbides, especially Mo₂C, are praised to be efficient electrocatalysts to reduce CO₂ to valuable hydrocarbons. However, on Mo₂C in an aqueous electrolyte, exclusively the competing hydrogen evolution reaction takes place, and this discrepancy to theory was traced back to the formation of a thin oxide layer at the electrode surface. Here, we study the CO₂ reduction activity at Mo₂C in a non-aqueous electrolyte to avoid such passivation and to determine products and the CO₂ reduction reaction pathway. We find a tendency of CO₂ to reduce to carbon monoxide. This process is inevitably coupled with the decomposition of acetonitrile to a 3-aminocrotonitrile anion. Furthermore, a unique behavior of the non-aqueous acetonitrile electrolyte is found, where the electrolyte, instead of the electrocatalyst, governs the catalytic selectivity of the CO₂ reduction. This is evidenced by in situ electrochemical infrared spectroscopy on different electrocatalysts as well as by density functional theory calculations.

Figure 1

(a) CVs of Mo₂C recorded in Ar-purged (red) and CO₂-saturated (black) acetonitrile with 0.1 M TBAPF₆. Scan rate: 50 mV/s. (b) EC-IRRA spectra for the CO₂ reduction at Mo₂C in CO₂-saturated acetonitrile with 0.1 M TBAPF₆. The spectra were recorded in cathodic direction (step potential: bottom to top). The spectra show the consumption of CO₂ (gray box) and residual water (blue box) in the electrolyte and the formation of carbonate/bicarbonate species (orange box) due to water reduction. The reference potential was at −1.0 V_Fc/Fc+ (see the Supporting Information for details). All the non-highlighted bands are associated with TBAPF₆ or acetonitrile since they also appear in the reference spectrum (Figure S2). (c) Enlarged view of the CO₂ region to distinguish the dissolved CO₂ band (2344 cm^–1) from the R- and P-branches of gaseous CO₂ (2360 and 2331 cm^–1).

Figure 2

(a) Top view of the C-rich Mo₂C (110) surface, showing the employed (1 × 1) and (2 × 2) surface unit cells as white rectangles for the calculation of the small and large size adsorbates, respectively. (b) Most stable adsorbate configurations on C-rich Mo₂C (110). Offset-corrected DFT vibrational frequencies in acetonitrile are given at each C–O or C–C bond (unit: cm^–1). Due to the small energy differences, the frequencies of the second most stable adsorption configuration are also provided in parentheses for C₂O₂ and CO₃. Large green spheres: Mo; gray spheres: C; red spheres: O; and white spheres: H.

Figure 3

EC-IRRA spectra at Mo₂C in CO₂-saturated acetonitrile with 0.1 M TBAPF₆. The reaction of the 3-aminocrotonitrile anion with CO₂ (reaction 3, red boxes) takes place simultaneously with the disproportionation reaction of CO₂ to dissolved CO and carbonate (reaction 4, blue and black boxes) at potentials below −2.4 V_Fc/Fc+. The reference spectrum was at −1.0 V_Fc/Fc+.

Figure 4

(a) Illustration and theoretically calculated wavenumbers of the (I) 3-aminocrotonitrile anion, (II) 3-aminocrotonitrile, (III) carboxylated acetonitrile, and (IV) carboxylated 3-aminocrotonitrile anion. Offset-corrected DFT vibrational frequencies (Table S4) are given for each species (unit: cm^–1). Blue atoms: N; gray atoms: C; red atoms: O; and white atoms: H. (b) Enlarged view of the EC-IRRA spectra (extracted from Figures 3 and S5) in the wavenumber region between 2200 and 2000 cm^–1. The theoretically calculated and experimentally measured wavenumbers align and prove the proposed reaction 3 (see Figure 3).

Figure 5

EC-IRRA spectra (a) before and (b) after the decomposition of acetonitrile for the CO₂ reduction in the same electrolyte (acetonitrile + 0.1 M TBAPF₆) at different electrodes. Mo (top, black), Mo₂C (middle, red), and GC (bottom, green) were chosen as electrocatalysts. The comparison of all spectra, especially the identical band formation, emphasizes the central role of acetonitrile in the CO₂ reduction selectivity. The potential for all reference spectra was −1.0 V_Fc/Fc+.