Protic ionic liquids with N-chloroalkyl functionalized cations as electrolytes for carbon-based electrochemical capacitors

This study reports on the implementation of protic ionic liquids (PILs) with N-chloroalkyl functionalized cations and [TFSI] anion as electrolytes for activated carbon (AC) based electrochemical capacitors (ECs). The electrochemical stability of PILs themselves and risk of corrosive behavior in contact with stainless steel (SSt) and aluminum (Al), as prospective current collectors materials, were investigated. Similar negative stability limits were proven for both materials, yet advantageous passivation of Al under anodic polarization was evidenced. Afterwards, the electrochemical stability was evaluated on porous AC electrodes by potential window opening measurements. For all PILs, it evidenced almost identical performance and stability on AC-electrode under positive polarization owing to the common [TFSI] anion. Under negative polarization, depending on the electron withdrawing effect of the N-chloroalkyl group on the N-H bond, two types of PIL behavior followed the region of EDL charging: hydrogen chemisorption onto AC or direct electrolyte decomposition. In the former case, the reduction potential of the protonated amine during cathodic sweep, as well as the desorption peak location during the anodic one were proven to be dependent on pK(a) of the protonated amine. The stability data were used to interpret the voltage limits of symmetric two-electrode cells implementing electrodes made from the same carbon. Finally, the performance of ECs based on the PILs with N-chloroalkyl functionalized cations was determined. (C) 2017 Elsevier Ltd. All rights reserved.