This NSF award by the Chemical and Biological Separations program supports work by Professor Peter Pintauro at Vanderbilt University to develop new fabrication strategies and new nano-morphologies for anion-exchange, cation-exchange, and bipolar membranes. Membranes will be prepared by electrospinning simultaneously nanofibers of a functional ionomer (ion-exchange polymer) and an uncharged (inert) reinforcement polymer. The resulting dual fiber mat will be processed into a membrane with one of two phase-separated morphologies: (i) ionomer nanofibers surrounded by an uncharged polymer matrix or (ii) uncharged polymer nanofibers surrounded by charged ionomer. In these two nanofiber composite membrane designs, the function of the charged polymer (providing pathways for ion and solvent transport) is decoupled from that of the inert/uncharged polymeric material (providing mechanical strength to the membrane and restricting ionomer swelling). Thus, the use of embedded nanofiber networks represents a paradigm shift in the way membranes are made and the way membrane nano-morphology is controlled/manipulated. Experimental work will focus primarily on anion-exchange and bipolar membranes. The resulting films will have improved mechanical properties, higher ion fluxes and conductivity, and better separation selectivity. Applications for such membranes include electrodialytic salt separations, industrial electrochemical processes, bipolar membrane water splitting, and chemical sensors.
This project had three basic outcomes: (1) physical products: a new class of composite ion-exchange membranes were fabricated based on dual fiber electrospinning; certain films exhibited outstanding properties, (2) general knowledge: a better understanding of ionomer electrospinning, nanofiber composite membrane fabrication methods, and the inter-relationship between membrane morphology and macroscopic properties was obtained from the membrane fabrication and characterization experiments, and (3) student training: graduate and undergraduate students were trained in a unique combination of subject areas, i.e., polymer science, membrane science, nanofiber electrospinning, and electrochemistry. Cation-exchange, anion-exchange, and bipolar membranes were fabricated (and characterized) over the course of this project for use in hydrogen/air proton exchange membrane and alkaline fuel cells and for use in various electrodialysis separations, as related to wastewater treatment and the processing of industrial chemicals. In the project, the "forced assembly" of multiple electrospun polymer fibers was employed to generate a highly desirable phase separated membrane nano-morphology, where one electrospun polymer (a polymer with fixed ion charges) provides pathways for ion and solvent (water) transport and the second electrospun polymer (with no fixed charges) restricts ionomer swelling and imparts mechanical strength to the membrane. The method is an alternative to membrane preparation by polymer blending, copolymerization, or polymer crosslinking. The functional ionomer (the charged polymer) and the uncharged (inert) support polymer were electrospun simultaneously using two separate spinnerets. Suitable post-treatment converted the dual-fiber mat into a fully dense and defect-free membrane, while maintaining the nanofiber morphology of one polymer component. Membranes were made where: (i) ion-exchange polymer nanofibers are surrounded by an uncharged polymer matrix and (ii) uncharged polymer nanofibers are surrounded by the charged ionomer. Methods for fabricating such membranes were developed and optimized, the effects of varying processing parameters on the final membrane morphology were quantified, the properties of the final membranes were measured and contrasted with each other and with commercial materials, and fundamental insights were obtained from the experimental work in terms of membrane structure/function relationships. For cation-exchange membranes, perfluorosulfonic acid polymers from DuPont and 3M Company were used as the ionomer, whereas polysulfones with quaternary ammonium or imidazolium sites were utilized for the anion-exchange membranes. Polyphenylsulfone was used as the uncharged polymer for all membranes. Nanofiber composite membranes, of specific composition and structure, were made with outstanding properties: high ion transport (e.g., a near record hydroxide ion conductivity for some anion exchange membranes), moderate volumetric and lateral water swelling (which translates into improved durability in a fuel cell), and very good mechanical properties (in both the wet and dry states). The morphology and method of making the membranes was sufficiently innovative and unique that one invention disclosure and one U.S. patent application were filed on the nanofiber membrane technology. The broader impacts of the project outcomes are significant and noteworthy. Refereed journal papers and conference/invited presentations on the project results have helped to establish the embedded nanofiber mat design as an important new membrane morphology platform, where the final membranes can be used in a variety of different energy-related or separation applications. The graduate and undergraduate students funded on this grant acquired a unique and important set of experimental skills and general intellectual knowledge with regards to multi-fiber electrospinning and polymeric membrane fabrication. Graduate students also received training in communications skills via technical paper writing and the presentation of research results at engineering conferences. Whereas the focus of the project was to make fuel cell and electrodialysis membranes, the general nanofiber mat and membrane designs have utility in advanced drug delivery devices, supercapacitors, regenerative flow batteries, polymeric solar cells, and as a tissue engineering scaffold.