ИСТИНА

Effect of Specific Cation Adsorption on CO2 Electroreduction in Ionic Liquids Alexander V. Rudnev,*1,2 Kiran Kiran,1 Peter Broekmann*1 1Department of Chemistry and Biochemistry University of Bern Freiestrasse 3, 3012 Bern, Switzerland 2Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences Leninskii pr. 31, building 4, Moscow, 119071 Russia rudnev@phyche.ac.ru, peter.broekmann@dcb.unibe.ch Ionic liquids (ILs) have recently gained substantial attention as promising solvents for the capture and sequestration of CO2 and as reaction medium for its electrochemical conversion into valued-added products. It has been reported that ILs can co-catalyze the electrochemical CO2 reduction reaction (CO2RR), in particular when Ag or Ag-based materials are applied as the primary metallic electrocatalyst [1]. Despite several attempts to explain the co-catalytic effect of ILs in CO2RR, the mechanism is not yet fully understood. Recently, the co-catalytic effect was explained by stabilization of CO2RR intermediate, CO2 radical anion, with IL cations adsorbed on a Ag electrode surface [2]. In order to broaden this co-catalysis concept and to develop an in-depth understanding of the co-catalytic action of the IL, we present herein a comprehensive study on a number of electrode materials (Pt, Pd, Ag, Au, Bi, Pb, Sn, Ni, Mo, Fe, Zn, Cu, and glassy carbon) exposed to several ILs, differing in the chemical nature of their anions and cations (Fig. 1) [3]. The activity of the electrode/IL combinations towards CO2RR is examined on the basis of cyclic voltammetry (CV) data recorded in Ar- (blankets) and CO2-saturated ILs. The voltammetric experiments are further complemented by online gas chromatography analysis of the electrolysis products, demonstrating that CO is the main CO2RR product for most of the electrode/IL combinations chosen. Based on our analysis we identify three major classes of electrode materials differing in their synergistic interaction with the IL cations and the in resulting CO2RR activity: (i) non-active (CO2RR is negligible in the potential range of IL stability), (ii) moderately active (CO2RR is hindered and proceeds together with the cathodic degradation of ILs), and (iii) highly active (CO2RR proceeds with nearly 100% faradaic efficiency). We also demonstrate that IL cations have a strong effect on the CO2RR overpotential (Fig. 1a-c, compare data for [BMP][TFSI] and [BMIm][TFSI]). Moreover, the co-catalytic effect depends on the electrode material: [BMIm]+ promotes CO2RR on Ag and Sn more than [BMP]+, while the situation is opposite for a Au electrode (Fig. 1a-c). Therefore, co-catalytic effect of ILs in CO2RR is determined by specific IL cation adsorption, and only particular combinations of electrode materials and IL cations enhance the CO2RR activity. Figure 1. (Left) Cations and anions of ILs used in this work. (Right) Comparison of the CV fragments in CO2-saturated ILs for the (a) Ag, (b) Sn, and (c) Au electrodes. This work was supported by the CTI Swiss Competence Center for Energy Research (SCCER Heat and Electricity Storage). A.R. also acknowledges financial support from the Ministry of Science and Higher Education of the Russian Federation (Theme no. 00812019-0003). P.B. acknowledges financial support from the Swiss National Foundation (no. 200020_172507). [1] B. A. Rosen, A. Salehi-Khojin, M.R. Thorson, W. Zhu, D.T. Whipple, P.J.A. Kenis, R.I. Masel, Science 334 (2011) 643-644. [2] G.P.S. Lau, M. Schreier, D. Vasilyev, R. Scopelliti, M. Grätzel, P.J. Dyson, J. Amer. Chem. Soc. 138 (2016) 7820-7823. [3] A.V. Rudnev, K. Kiran, P. Broekmann, ChemElectroChem, 7 (2020) 1897-1903.