Large Diffusion Coefficient of BMIM BF₄ in Porous Carbon Fibers

Confinement of ionic liquids in hydrophilic porous solid media has previously been shown to disrupt the lattice structures of the ionic liquids. An immobile ion layer adheres to the material surface, while the inner layer exhibits increased mobility. In this work, porous carbon fibers (PCF) synthesized from a polyacrylonitrile-block-polymethyl methacrylate (PAN-b-PMMA) block copolymer were used to study the dynamics of confined 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF₄). PCF contain mesoporous networks with unimodal pore size distributions. Elastic neutron scattering scans confirmed confinement in 13.6 nm diameter pores due to a lack of a freezing point between 20 K and 300 K. Quasi-elastic neutron scattering (QENS) was used to determine the diffusion coefficients of the bulk BMIM BF₄ and the confined BMIM BF₄. A seven-fold increase in diffusion coefficient was obtained for the confined BMIM BF₄ compared to the bulk. The tunability of the PCF offers opportunities for further work exploring the limits of confinement in the unique mesoporous networks of PCF.