Non-stoichiometric protic ionic liquids

Protic ionic liquids (PILs) have a set of properties that make them interesting candidates for many applications including energy storage and conversion, gas absorption, biological applications and organic synthesis. Compared to stoichiometric (neat) PILs, non-stoichiometric PILs are formed by the combination of neat PILs with an excess of the (precursor) acids or bases, which is generally ignored by many researchers and there are only limited studies. Non-stoichiometric PILs have a variety of specific physicochemical properties, which are likely caused by the unique interaction between neat salts and excess acids or bases and in some cases by particular anion structures. In this first review of non-stoichiometric PILs, we briefly discuss the history of PIL development and how non-stoichiometric PILs differ in properties. An analysis of the physicochemical properties of non-stoichiometric PILs is presented and their application in various fields is discussed. Finally, we give a prospect on the development of non-stoichiometric PILs including conceivable future challenges.