Investigating transport properties of protic ionic liquids
Maleen MIDDENDORF1, Monika SCHÖNHOFF1
1Institute of Physical Chemistry, WWU Münster, Münster, Germany
Protic ionic liquds (PILS) are a subclass of ionic liquids and have some properties in common with them, such as low flammability and volatility. Since they are synthesized by a neutralization reaction of a Brønsted acid and a Brønsted base, they also have a proton available for a proton conductivity. This property opens up the possibility of using PILs as a part of an electrolyte for fuel cell applications.
One way to increase the proton conduction is to strengthen the hydrogen bond network, which should enable a structural transport. This can be done by using non-stoichiometric PILs that contain an excess of acid or base and thus add proton donor and acceptor sides, respectively.
This work focuses on stoichiometric and non-stoichiometric triethylamine (TEA) based PILs with different anions, the latter containing an excess of acid. Various NMR techniques, such as Pulsed Field Gradient (PFG) NMR or electrophoretic NMR (eNMR), as well as impedance spectroscopy, enables the investigation of transport properties.
The results show that a minor amount of neutral acid is still present also in the stoichiometric PILs, which can be seen by a separate NMR signal for the proton of the hydroxyl-group. Further NMR experiments indicate strong ion-ion correlations within the electrolyte, resulting in low inverse Haven ratios.
Electrophoretic mobility data from eNMR measurements provide the drift velocity and direction in an electric field for different species. For example, an increased negative mobility of the OH-proton of the neutral acid, which signifies a drift towards the positive electrode, hints towards strong correlations between the anion and the neutral acid. Since in some PILs this mobility is even larger than that of the anion, positive correlations between anion and cation can also be assumed. When the acidic proton of the cation is considered, the mobility is in good agreement with that of the CH-signals of the triethylamine suggesting a well-defined vehicular transport.
In summary, the transport properties in the PILs studied here are defined by strong correlations between the individual species, so that no separate transport of the acidic proton can be observed.