Micro and Nanopatterned Ion Exchange Membranes for Applications in Electrochemical Devices

Abstract

Topographical patterns on ion exchange membranes’ (IEMs’) surfaces have been show to improve the performance and energy efficiency in electrochemical energy conversion and water treatment processes. Substituting flat IEMs with patterned/profiled IEMs in electrodialysis reduces spacer channel mass transfer limitations by increasing the interfacial area between the liquid solution and membrane materials. Additionally, the extended surface of the profiled cation-exchange and anion-exchange membranes augment the spacer channel conductivity without obfuscating bulk liquid flow. Topographically patterned ion exchange membranes are fabricated using several methodologies that include photo-/soft-lithography, hot pressing, solution casting and 3D printing. The minimum lateral feature size that can be obtained by these methods is practically on the order of a few microns and potentially could be as low as few hundred nanometers. Block copolymer (BCP) lithography is a nanopatterning platform that generates lateral feature sizes of 5 to 50 nm. Thin films of diblock copolymers can self-assemble on a neutral layer resulting in lamella, cylinder, or sphere morphologies when the film is thermal or solvent vapor annealed. The morphology of the film primarily depends on volume fraction (fA, fB) of each block and interaction parameter between the blocks (χ). Poly(styrene-block-methylmethacrylate) (PS-b-PMMA) is one of the most studied block copolymer for BCP lithography because it form perpendicular aligned nanostructures via thermal annealing without topcoat layers to control the free surface interfacial energy. The etch contrast between the styrene and MMA blocks also makes it easier to transfer the pattern onto substrates – especially if sequential infiltration synthesis (SIS) is used.In this work, for the first time, a method to prepare nanopatterned IEMs using BCP lithography is reported. The SIS process post PS-b-PMMA self-assembly converts MMA into alumina creating organic-inorganic hard mask. The styrene block is removed via oxygen plasma in a reactive ion-etcher leaving behind a nanoporous alumina template on a silicon wafer. Sulfonated poly(arylene ether ether ketone) (SPEEK) is dropcasted onto the nanoporous alumina and the solvent is evaporated. Once releasing the SPEEK from the substrate, a nanopatterned cation-exchange membrane is attained. The electrochemical properties of the nanopatterned membrane such as conductivity, permselectivity and transference number are measured and compared to a flat, non-patterned membrane. The efficiency of these nanopatterned membranes for simple electrochemical devices, like a hydrogen pump, will also be presented.This work is supported from NSF Award # 1703307. Figure 1