Effect of Cation on Proton Conductivity of a Non‑fluorinated Ionic Liquid

1‑Ethyl‑3‑methyl imidazolium hydrogensulfate (EMIHSO₄), a non‑fluorinated IL, was studied both as liquid and polymer electrolytes for electrochemical capacitors [1]. While EMIHSO₄exhibited a reasonable ionic conductivity, improvement in proton conductivity is possible by altering the structure of the cation, which warrants further investigation. The objectives of this study are twofold: to investigate the effect of cationic functional groups on the properties of the IL; and to select and characterize the IL system with optimum proton conductivity. 

                The electrolytes were prepared using ILs with different cations, namely 1‑ethyl‑3‑methyl imidazolium hydrogensulfate (EMIHSO₄), 1‑methylimidazolium hydrogensulfate (MIHSO₄), and imidazolium hydrogensulfate (ImHSO₄) in methanol solutions. For comparison, a solution of EMIHSO₄ with propylene carbonate (PC), an aprotic solvent, was prepared with the same concentration as those in methanol‑based electrolytes. To demonstrate the proton conductivity of the electrolytes, standard two‑electrode cells were assembled utilizing pseudocapacitive electrodes such as RuO₂and nanocarbon/polyoxometalate (POMs) composites [2].The cell performance were characterized using cyclic voltammtery (CV) and ac impedance spectroscopy. 

                RuO₂ exhibits pseudocapacitance via a coupled proton–electron transfer in a protic electrolyte. Figure 1 shows the CV profiles of the RuO₂‑based cells in all electrolytes. The ImHSO₄ and MIHSO₄‑based cells demonstrated similar CVs and the highest capacitance performance. Their higher capacitance compared to EMIHSO₄‑based cell indicates the higher proton conductivity of ImHSO₄ and MIHSO₄, which can contribute to the electrochemical reactions of RuO₂ electrodes. Due to the role of solvent on proton dissociation, the cell performance was superior using EMIHSO₄/MeOH electrolytes than that for EMIHSO₄/PC. 

                The proton conductivity of PILs was further examined on a carbon/POM electrode and compared to the bare carbon electrodes. POMs involve fast and reversible multielectron transfer reactions in proton containing electrolytes. For a better comparison, the respective carbon/POM cells were also tested in 0.5M H₂SO₄. The electrochemical performance of the carbon/POM‑based cells in the PIL electrolytes followed a similar trend as those for RuO₂‑based electrolytes. Figure 2 (a) shows the CV profiles of the carbon/POM‑based cells and the bare carbon in ImHSO₄/MeOH electrolyte. The increased capacitance of the carbon/POM cell over that for the bare carbon implies the proton conducting characteristic of ImHSO₄ electrolyte which participates in the redox reaction of the POM. Figure 2 (b) presents the capacitance ratio of the carbon/POM cells in the respective PILs to H₂SO₄ electrolyte. ImHSO₄‑based cells showed the highest capacitance ratio followed by MIHSO₄ and EMIHSO₄in both MeOH and PC solutions. Imidazoles consist of two nitrogen sites, and their protonated and unprotonated nitrogen functions may act as proton donor and acceptor in proton transfer reactions. As the nitrogen sites decrease in MI and EMI cations, the contribution from proton conduction becomes smaller. More details on optimizing the proton conductivity of these PILs for room temperature applications, especially polymer electrolytes will be presented.

References:

[1] S. Ketabi, Z. Le, and K. Lian, Electrochemical and Solid-State Letters, 15 (2), A19, 2012.

[2] G. Bajwa, M. Genovese, and K. Lian, ECS Journal of Solid State Science and Technology, 2 (10), M3046, 2013.

Figure 1. CVs of RuO₂ cells in ImHSO₄, MIHSO₄, EMIHSO₄ methanol‑based, and EMIHSO₄/PC electrolytes at a scan rate of 100 mV/s

Figure 2. (a) CVs of carbon/POM and bare carbon cells in ImHSO₄/MeOH; (b) capacitance ratio of carbon/POM cells in respective PILs to H₂SO₄ electrolyte (all tests at a scan rate of 100 mV/s)