(Invited) New Lithium-Conducting Ionic Liquids and Their Polymer Electrolytes

Abstract

In PEM Fuel Cells, Ionic liquids i.e. Proton-conducting Ionic Liquids, PILs, behave as proton-conducting materials to increase PEMFC operating temperature. Based on primary, secondary or tertiary ammoniums, they are ‘self-sufficient’ i.e. they don´t require any additional acids to perform the electrochemical reactions. Thus macroporous membranes have been successfully filled by PILs. Alternatively, PILs can be too associated to proton-conducting ionomers e.g. Nafion® or high performance based ionomers beforehand turned into their ammonium form in order to avoid molecular acid release. In the first case, high mechanical strength can be obtained while the swelling of PFSA ionomers by PILs increase markedly the proton-conductivity at the anhydrous state but strongly decrease the membrane mechanical strength. While PILs are self-sufficient in electrochemical conversion (FC), usual Li-ion battery ILs based on quaternary ammonium require conversely adding a lithium salt. The well-known advantages of ILs vs liquid solvents e.g. high Fp and high sub-ambient conductivities are partly compensated by the compulsory addition of the salt, which increases the viscosity and also the electrolyte cost. Oligoethers i.e. oligo(oxyethylene) or oligo-PEO) end-capped by 1, 2 or 3 salt moieties doesn’t match the performances of classical ionic liquids, in terms of sub-ambient conductivities but they both provide the lithium salt function and a solvating ability. This presentation will focus on new lithium-conducting oligomeric ionic liquids consisting of several oligoether chain lengths end-capped by organic perfluorinated anions. Their intrinsic ionic conductivities and those of related POE blends will be reported as well as their cationic transference numbers. Both classical ILs and oligomeric lithium-conducting ionic liquids degrading the electrolyte mechanical performances, they don´t allow thinning the polymer electrolyte films. A special attention will be paid to the reinforcement of ILs based polymer electrolytes.