The electrochemical reduction of CO 2 to CO in ionic liquids and ionic-liquid/water mixtures has received considerable attention due to recent claims of extraordinarily high energy efficiencies. We report here a study of CO 2 electroreduction on Au in an [EMIM]BF 4 /H 2 O mixture (18% mol/mol) combining cyclic voltammetry and surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS). The onset of the reduction current in the CV coincides with a decrease of the interfacial CO 2 concentration, but the appearance of adsorbed CO (CO ad ) is slightly delayed, as CO must probably first reach a minimum concentration at the interface. Comparisons with spectra collected in the absence of CO 2 and in CO-saturated electrolyte reveal that the structure of the double layer at negative potentials is different when CO 2 is present (probably due to the formation of CO ad ) and allow us to assign the main band in the spectra to CO adsorbed linearly on Au (CO L ), with a smaller band corresponding to bridge-bonded CO (CO B ). The CO bands show a large inhomogeneous broadening and are considerably broader than those typically observed in aqueous electrolytes. While both CO L and CO B can be observed in the CO adlayer generated by the electroreduction of CO 2 , only a single, even broader band, at a frequency characteristic of CO L is seen in CO-saturated solutions. We attribute this to the lower coverage of the adlayer formed upon reduction of CO 2 , which leads to a lower degree of dipole−dipole coupling. Upon reversing the direction of the sweep in the CV, the intensity of the CO bands continues increasing for as long as a reduction current flows but starts decreasing at more positive potentials due to CO desorption from the surface.

The strong effect of the electrolyte cation on the activity and selectivity of the CO2 reduction reaction (CO2RR) can only be understood and controlled if the cation's effect on the interfacial potential distribution is known. Using CO (the key intermediate in the CO2RR) adsorbed on Pt as a probe molecule, and combining IR spectroscopy, capacitance measurements and ab initio molecular dynamics, we show that the cation size determines the location of the outer Helmholtz plane, whereby smaller cations increase not just the polarisation but, most importantly, the polarizability of adsorbed CO (COad) and the accumulation of electronic density on the oxygen atom of COad. This strongly affects its adsorption energy, the degree of hydrogen bonding of interfacial water to COad and the degree of polarisation of water molecules in the cation's solvation shell, all of which can deeply affect the subsequent steps of the CO2RR.

Energetically efficient electrochemical reduction of CO2 would offer the possibility of storing electricity from renewables in the form of fuels and other valuable chemicals. It may also help mitigate the increase of atmospheric CO2 associated with global warming. However, the process suffers from a low energy efficiency due to the large overpotentials required. In aqueous electrolytes, the competing hydrogen evolution reaction also decreases the faradaic efficiency (which contributes to the low energy efficiency of the process). Recent claims of high faradaic efficiency and low overpotentials for the reduction of CO2 in room temperature ionic liquids (RTILs) and RTIL-water mixtures have spurred considerable research. Here, we offer a critical review of those claims and of recent work aimed at understanding the details of this important reaction in these non-conventional electrolytes.

We report a combined computational and experimental work aimed at estimating the equilibrium potential for the electroreduction of CO 2 to CO 2 − (widely accepted to be a crucial and overpotential-determining step) and at providing an alternative view on the reason behind the lower overpotential for CO 2 reduction in imidazolium-based ionic liquid/water mixtures. To begin with, we obtained an 80 ps ab-initio molecular dynamics trajectory of the CO 2 − solvation structures in an 18% EMIM−BF 4 / water mixture, which delivered no evidence of interaction between EMIM + and CO 2 − . Next, using the Fc + /Fc couple as the nonaqueous reference, we calculated the equilibrium potential of the CO 2 /CO 2 − couple in the mixture and aligned it with the aqueous SHE scale, proving that the equilibrium potential of CO 2 /CO 2 − in the mixture is about 0.3 V less negative than in the aqueous medium. We then looked for the origin of this catalytic effect by comparing the computed vibrational spectra with experimental Fourier transform infrared spectra. This revealed the presence of two water populations in the mixture, namely, bulk-like water and water in the vicinity of EMIM−BF 4 . Finally, we compared the hydrogen bonding interactions between the CO 2 − radical and H 2 O molecules in water and in the mixture, which showed that stabilization of CO 2 − by water molecules in the EMIM−BF 4 /water mixture is stronger than in the aqueous medium. This suggests that water in EMIM−BF 4 /water mixtures could be responsible for the low overpotential reported in these kinds of electrolytes.