Nonstoichiometric Triazolium Protic Ionic Liquids for All-Organic Batteries

Abstract

Nonstoichiometric protic ionic liquids (NSPILs) are efficient electrolytes for protic electrochemical devices such as the allorganic proton battery, which has been suggested as a sustainable approach to energy storage. NSPILs contain a mixture of proton donors and acceptors and are ideal for this purpose due to their high proton conductivity, high electrochemical stability, low cost, and ease of synthesis. However, the electrolyte proton activity must be controlled carefully in these devices since it greatly influences the kinetics and energetics of the electrode redox reactions and, hence, also impacts battery device performance. In this study, specific NSPILs were designed and evaluated as electrolytes for the allorganic proton battery. The NSPILs were based on either 1,2,4triazole or 1-methyl-1,2,4-triazole partially protonated with bis-(trifluoromethylsulfonyl)imide (TFSI) to produce a range of NSPILs with different degrees of protonation. Both types of NSPIL investigated here exhibited a maximum conductivity of 1.2 S/cm (at 120 and 70 °C, respectively), and the eutectic composition of 1-methyl-1,2,4-triazolium TFSI also had high conductivity at 25 °C (24.9 mS/cm), superior to, e.g., imidazolium TFSI NSPILs. Pulsed field gradient NMR in conjunction with electrochemical impedance spectroscopy showed that the conductivity originated mainly from vehicle diffusion and proton hopping. Quinone functionalized poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes exhibited reversible, fast, and stable redox conversion in these electrolytes, and a model is suggested to determine proton activities of NSPILs based on the quinone formal potential. An all-organic proton battery cell was assembled to demonstrate the usefulness of these electrolytes in devices. Fast and complete redox conversion with a cell potential of 0.45 V was demonstrated, even up to scan rates corresponding to 140 C. Compared to the pyridine based electrolytes used for the all-organic proton battery up until now, the present electrolytes display several advantages including lower melting point, lower toxicity, and compatibility with plastic materials.

A Comparative Review of Electrolytes for Organic‐Material‐Based Energy‐Storage Devices Employing Solid Electrodes and Redox Fluids

“…Different from the common approach of employing ILs as a “solvent” for the conducting salt, Karlsson et al. reported the use of nonstoichiometric protic ILs as electrolytes for completely metal‐free all‐organic proton batteries . In this case, molecules such as 1,2,4‐triazole or 1‐methyl‐1,2,4‐triazole act as a proton acceptor and their protonated derivatives, which are counterbalanced by the TFSI − anion, serve as a proton donor.…”

Rocking-Chair Proton Batteries with Conducting Redox Polymer Active Materials and Protic Ionic Liquid Electrolytes

“…The acidity and relative fraction of the protonated (acid) and the unprotonated (base) heterocycle moieties can be used to tune both the kinetics and the energetics of proton-coupled redox reactions. 37 …”

A Rocking-Chair Proton Battery with Conducting Redox Polymer Active Materials and a Protic Ionic Liquid Electrolyte

“…[19][20][21][22][23] By replacing the water solvent it should therefore be possible to increase the stability of proton batteries by suppressing the Michael addition reaction. This could be done in organic protic electrolytes [24] or using nonstoichiometric protic ionic liquids, [25] which also could allow a potential window over 1.23 V. [26][27][28] We have previously shown that the use of non-aqueous solvents significantly extends the possibility to tune quinone redox potentials by substitution, since in aqueous solution, specific interactions with water molecules counteract the effect of the substituents. [29] Finally, protic ionic liquids may provide a solution for redox-active materials that show poor wettability in water electrolyte.…”

“…The acidity and relative fraction of the protonated (acid) and the unprotonated (base) heterocycle moieties can be used to tune both the kinetics and the energetics of proton coupled redox reactions. [25] Herein, a 1-methyl-1,2,4-triazole (MeTri) based ionic liquid (pKa = 3.2) was used as electrolyte together with conducting redox polymer (CRP) electrode materials. Capacity-carrying quinizarin (1,4-dihydroxyanthraquinone, Qz) and NQ pendants covalently attached onto a conducting polymer backbone were used as cathode and anode, respectively.…”