Role of Cations in Electrochemical Reactivity and Combustion Performance of Room-Temperature Ionic Liquid (RTIL) Fuels

Room-temperature ionic liquid (RTIL) fuels, characterized by negligible vapor pressure and high thermal stability, are generally considered nonflammable. However, they can be rendered combustible electrochemically, enabling on-demand ignition and extinction for safer fuel applications. This study investigates the role of cations in the electrochemical reactivity and combustion performance of nitrate anion-based RTILs, including 1-butyl-3-methylimidazolium nitrate ([BMIM] + [NO 3 ] − ), 1-(2′-hydroxylethyl)-3-methylimidazolium nitrate ([HyMIM] + [NO 3 ] − ), and ethylammonium nitrate ([EA] + [NO 3 ] − ). Electrochemical measurements reveal that reactivity is strongly governed by ion transport, with [EA] + [NO 3 ] − exhibiting higher reactivity due to its higher conductivity, which is influenced by its smaller size, higher ion mobility, and higher charge carrier concentration. However, the combustion performance does not directly correlate with reactivity. By analyzing the combustion power output, it has been demonstrated that [BMIM] + [NO 3 ] − outperforms [EA] + [NO 3 ] −, highlighting the importance of intrinsic energy density. In operando mass spectrometry reveals that −OH functionalization in the imidazolium ring promotes a different electrochemical decomposition pathway for [HyMIM] + [NO 3 ] − . These findings demonstrate that the electrochemical combustion performance arises from a balance between ion transport and fuel energy density. This work establishes key structure–property–function relationships, providing design guidelines for tunable and safer electrochemical combustion systems incorporating RTIL fuels.