Modelling proton transfer in [HEIM][TFSI] ionic liquid

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, we model the proton transfer mechanism in 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide ([HEIM][TFSI]) IL by means of Neural Network Force Field simulations. The latter are combined with classi- cal polarizable molecular dynamics simulations to explore the structure and dynamics of the fully ionized system and Density Functional Theory calculations to estimate the energy barriers for the different proton transfer reactions. Our results show that proton transfer is indeed possible when doping the ionic liquid with an excess of deprotonated cations, but not with an excess of proto- nated anions. We highlight the importance of the formation of dimers between donor and acceptor species for the the reaction to occur, and we identify the main driving factor for the reaction to be the energy cost for reaching a suitable coordination environment and form such dimers, which is higher than that for the transfer reaction. • Proton transfer in [HEIM][TFSI] ionic liquid is studied using Neural Network Force Field simulations, complemented by polarisable molecular dynamics and DFT calculations. • Proton transfer occurs when doping the ionic liquid with excess deprotonated cations, but not with excess protonated anions. • Hydrogen bonding patterns enable proton transfer, particularly via the formation of [EIM]– [HEIM] + dimers.