Equilibrium Structure, Hydrogen Bonding and Proton Conductivity in Half-Neutralised Diamine Ionic Liquids.

Recent experiments on proton conducting ionic liquids point to half-neutralised diamine-triflate salts as promising candidates for applications in power generation and energy conversion electrochemical devices. Structural and dynamical properties of the simplest among these compounds are investigated by a combination of density-functional theory (DFT) and molecular dynamics (MD) simulation based on an empirical force field. Three different cations have been considered, consisting of a pair of amine-ammonium terminations joined by a short aliphatic segment -(CH$_2$)$_n$- with $n=2$, $3$ and $4$. First, the ground state structure, vibrational eigenstates and hydrogen-bonding properties of single ions, neutral ion pairs, small neutral aggregates of up to eight ions and molecularly-thin hydrogen bonded wires have been investigated by DFT computations. Second, structural and dynamical properties of homogeneous liquid and amorphous phases are investigated by MD simulations over the temperature range $200$ K $\leq T \leq 440$ K. Structure factors, radial distribution functions, diffusion coefficient and electrical conductivity are computed and discussed, highlighting the inherent structural heterogeneity of these compounds. The core investigation, however, is the characterisation of connected paths consisting of cation chains that could support proton transport via a Grotthuss-type mechanism. Since simulations are carried out using a force field of fixed bonding topology, this analysis is based on the equilibrium structure only, using geometrical criteria to identify potential paths for proton conduction. Paths of connected cations can reach a length of 80 cations and 30 \AA\ , provided bridging oxygen atoms from triflate anions are taken into account. The effects of water contamination at the 1 \% weight concentration on structure, dynamics and paths for proton transport are discussed.