379910 Ionic Conductivity and Gas Permeability of Polymerized Ionic Liquid Block Copolymer Membranes for Energy Applications
Polymer membranes for many energy applications, such as solar-to-hydrogen fuel production, require ionic conductivity while acting as gas diffusion barriers. We have synthesized a diblock copolymer consisting of poly(styrene-block-(4-(2-methacrylamidoethyl)-imidazolium trifluoroacetate) by treating poly(styrene-block-histamine methacrylamide) (PS-b-PHMA) with trifluoroacetic acid. The PS block serves as the structural support while the imidazolium derivative is an ion-conducting polymerized ionic liquid (PIL). Small angle X-ray scattering and transmission electron microscopy demonstrate that the block copolymer self-assembles into well-ordered nanostructures, with lamellae and hexagonally packed cylindrical morphologies. The ionic conductivities of the PS-b-PHMA materials were as high as 2 x 10-4 S/cm while an order of magnitude increase in conductivity was observed upon conversion to PS-b-PIL. The ionic conductivity of the PS-b-PIL increased by a factor of ~4 up to 1.2 x 10-3 S/cm as the PIL domain size increased from 20 to 40 nm. These insights allow for the rational design of high performance ion conducting membranes with even greater conductivities via precise morphological control.
Oxygen diffusion measurements were also performed on these materials as well as Nafion, the canonical cation exchange membrane, using a transient electrochemical reduction method. We provide evidence for dual-mode diffusion whereby distinct diffusion coefficients are observed for oxygen moving through the hydrophilic channels and fluorocarbon matrix. These diffusive modes can be tuned by thermal annealing which changes both the percent crystallinity and crystallite size (determined by WAXS and DSC). Thus, the oxygen permeation can be decreased by more than a factor of 2 in a facile manner. Our results provide key insight into the balance of ionic conductivity and gas diffusion in membranes for energy applications.