Electrochemical protonation/deprotonation of TiNb₂O₇ in protic ionic liquids

In recent years, there has been growing interest in rechargeable batteries utilizing protons or hydronium ions as charge carriers, driven by the rapid ionic conduction enabled by the proton-specific Grötthuss mechanism. However, the use of acidic aqueous electrolytes introduces side reactions, such as irreversible hydrogen evolution and the dissolution of active materials into the electrolyte, which are influenced by the reaction potential of the active materials. These challenges complicate the identification and development of active materials. While some combinations of Brønsted acids and bases may potentially compromise the advantages of the Grötthuss mechanism, this study successfully demonstrated the electrochemical protonation of TiNb₂O₇ using protic ionic liquids as electrolytes. Acetic acid (AcOH) and 1,1,1-trifluoro-N-((trifluoromethyl)sulfonyl)methanesulfonamide (HTFSA) were employed as Brønsted acids, while 1-methylimidazole (Im) and 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) were utilized as Brønsted bases. Irreversible hydrogen evolution was dominant in an aqueous buffer solution consisting of citric acid and trisodium citrate. The AcOH/DBU system showed negligible charge/discharge capacities within the cut-off potential range of -1.5 to +0.25 V. In contrast, AcOH/Im and HTFSA/Im systems exhibited reversible capacities of 61 and 55 mA h g^-1, respectively, during the first cycle. However, their Coulomb efficiencies were significantly low below 20%. Meanwhile, HTFSA/DBU, despite a lower reversible capacity of 40 mA h g^-1 (corresponding to H_0.5TiNb₂O₇), achieved a Coulomb efficiency exceeding 90%. Notably, it maintained an average Coulomb efficiency of 96% over 50 cycles without any capacity degradation.