Abstract
Efficient charge transport has been observed in iodine-doped, iodide-based room-temperature ionic liquids, yielding high ionic conductivity. To elucidate preferred mechanistic pathways for the iodide ()-to-triiodide () exchange reactions, we have performed 10 ns reactive molecular-dynamics calculations in the liquid state for 1-butyl-3-methylimidazolium iodide ([BMIM][I]) at 450 to 750 K. Energy-barrier distributions for the iodine-swapping process were determined as a function of temperature, employing a charge-reassignment scheme drawn in part from electronic-structure calculations. Bond-exchange events were observed with rate-determining energy barriers ranging from ~0.19 to 0.23 ± 0.06 eV at 750 and 450 K, respectively, with an approximately Arrhenius temperature dependence for iodine self-diffusivity and reaction kinetics, although diffusion dominates/limits the bond-exchange events. This charge transfer is not dissimilar in energetics to those in solid-state superionic conductors.
Highlights
Room-temperature ionic liquids (RTILs) are molten salts with low volatility, comprising of molecular cations and anions having a melting point below 100 ◦C
High ionic conductivity constitutes a particular desideratum, leveraged in dye-sensitised solar cells (DSSCs) so that a redox electrolyte leads to ongoing charge renewal via electron-hole transfer from the dye to the electrolyte, whilst injected electrons travel through the semiconductor into the external circuit, until these charge carriers close the circuit by recombining at the cathode [7]
Molecular-dynamics (MD) simulations were performed with this ReaxFF/charge-setting scheme for 10 ns at 450, 550, 650 and 750 K in the NVT ensemble with Nosé-Hoover thermostat (τ = 0.25 ps) on the respective MM-based molecular dynamics (MD) configurations with the corresponding time-averaged densities
Summary
Room-temperature ionic liquids (RTILs) are molten salts with low volatility, comprising of molecular cations and anions having a melting point below 100 ◦C. RTILs’ outstanding charge-transport efficiency upon iodine addition to iodide-based systems [11,12,13,14,15,16,17], as compared with viscous systems, appear predicated upon a bond-exchange mechanism [17], which is redolent of, in large part, the Grotthuss mechanism for protons’ diffusion in water [18]. This has been advanced as an explanation of molten polyiodides’ high electrical conductivity [19,20]. Wachter et al have presented observations of non-Stokesian charge-transport behaviour in binary mixtures of 1-methyl-3-propylimidazolium iodide ([MPIM][I]) and 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) [21]; Bentley et al gauged charge transfer in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulphonyl)imide ([C2MIM][NTf2]) as consistent with a Grotthuss-style bond-exchange processes [22]
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