Proton transfer in [HEIM][TFSI] ionic liquid: insights from polarisable and neural network simulations

Protic ionic liquids, PILs, are promising materials for energy storage applications, in part due to their ability to decouple proton transport from ion diffusion. In this work, the proton transfer mechanism in 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide ([HEIM][TFSI]) IL was studied by means of three different computational approaches. Classical polarizable molecular dynamics simulations were used to explore the structure and dynamics of the fully ionized system, while Density Functional Theory calculations were carried out to estimate the energy barriers for the different proton transfer reactions. Finally, the proton transfer was explicitly studied by means of Neural Network Force Field simulations. Our results show that this reaction is indeed possible when doping the IL with an excess of deprotonated cations, and highlight the importance of the formation of dimers between donor and acceptor species for the proton transfer to occur. The main driving factor for the reaction was found to be the energy cost for reaching a suitable coordination environment and form such dimers, which is higher than that for the transfer reaction.