Protic and aprotic bis(oxalato)borate based ionic liquids as electrolytes for electrical double layer capacitors

In this study, we report about the synthesis of two fluorine-free ionic liquids (ILs), the protic 1-methylimidazolium bis(oxalato)borate (1-MimBOB) and the aprotic 1-butyl-2,3-dimethylimidazolium bis(oxalato)borate (BDMimBOB) and about their use as electrolytes for electric double-layer capacitors (EDLCs). We showed that the electrolytes 1.5 M 1-MimBOB in water and 1.5 M BDMimBOB in acetonitrile exhibited good solubility, high ionic conductivity and high thermal stability. EDLC containing protic IL can operate at 1.2 V, while those containing the organic electrolyte can operate at 2.9 V. EDLCs containing these electrolytes display ideal capacitive behaviour, excellent rate capability and promising capacitance retention after 200 hours of floating at their maximum operating voltage. The devices also achieved energy densities up to 23.3 Wh kg⁻¹, highlighting BOB-based ILs as sustainable, fluorine-free electrolytes for next-generation high-performance EDLCs. This article is part of the theme issue 'Electrolytes within the domain of electrochemistry and electrochemical energy storage'.