Ionic liquids’s Co-catalytic Effect on Electrochemical CO2 Reduction Mediated by Model Molecular Catalysts
Résumé
The use of Ionic Liquids (ILs) has proven to enhance carbon dioxide (CO2) electroreduction, while their
inherent conductivity and large electrochemical windows has sparked investigation in their use as a
solvent and/or supporting electrolyte.1 However, the high viscosity values of ILs that hinder mass transfer
and cause lower currents2 deterred us from using ILs as the solvent in the present study, in which we
opted instead for binary mixtures of ILs and acetonitrile. While the role of ILs in heterogeneous catalysis
has been extensively addressed, little study has been devoted to the elucidation of their role in the context
of molecular electrocatalysis for CO2 reduction.3
In this study we evaluate the role of ILs in the case of CO2 electroreduction by a molecular catalyst and
for this purpose we selected the well-established [Re(CO)3bpyCl] complex, also known as Lehn’s
catalyst, because of its high selectivity for CO production, as well as its fully elaborated mechanism. To
this effect, a series of 5 ILs were chosen, belonging to the imidazolium and pyrrolidinium cation families.
Each of those ILs, at a concentration of 0.5M in acetonitrile was studied in presence of the rhenium
catalyst initially by cyclic voltammetry and subsequently by controlled potential electrolysis experiments.
According to our results, the presence of the IL under CO2 catalytic conditions was enough to cause a
shift in the CO2 electroreduction overpotential of about 330mV, proving the existence of a co-catalystic
role of the IL for the system, though this effect was less pronounced when an external proton source was
added.4 Moreover, a mechanistic explanation of the IL effect is proposed, evoking the electrostatic
stabilization of the negatively-charged intermediates of the catalyst by the IL cations and in the case of
imidazolium cations, stabilization through π-π stacking, a characteristic that could explain their superior
performance.
References:
(1) Sánchez-Sánchez, C. M.; Encyclopedia of Interfacial Chemistry : Surface Science and
Electrochemistry; Wandelt, K., Ed.; Elsevier, 2018, p 539–551.
(2) Grills, D. C.; Matsubara, Y.; Kuwahara, Y.; Golisz, S. R.; Kurtz, D. A.; Mello, B. A. J. Phys.
Chem. Lett. 2014, 5 (11), 2033–2038.
(3) Choi, J.; Benedetti, T. M.; Jalili, R.; Walker, A.; Wallace, G. G.; Officer, D. L. Chem. - Eur. J.
2016, 22 (40), 14158–14161.
(4) Vichou, E.; Li, Y.; Gomez-Mingot, M.; Fontecave, M.; Sánchez-Sánchez, C.M. J. Phys. Chem.
C, 2020, 124, 43, 23764–23772