Nonstoichiometric Triazolium Protic Ionic Liquids for All-Organic Batteries

Nonstoichiometric protic ionic liquids (NSPILs) are efficient electrolytes for protic electrochemical devices such as the all-organic 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 all-organic proton battery. The NSPILs were based on either 1,2,4-triazole 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.