Electrospun Nafion/PVDF Blended Nanofiber Membranes for Regenerative H2/Br2 fuel Cells

Abstract

Development of energy storage systems is required to provide stable/continuous electrical energy from intermittent renewable energy sources such as wind and solar. The Hydrogen/Bromine (H2/Br2) regenerative fuel cell is being considered for such applications because of its fast H2 and Br2 electrochemical reaction kinetics and high round-trip efficiency [1]. This type of fuel cell utilizes a membrane-electrode-assembly with a cation-exchange membrane. Although commercial Nafion® is typically used here, its bromine species (Br-, Br2, Br3 -) crossover is high, causing significant columbic losses in the cell and degradation of the platinum catalyst in the hydrogen electrode. The desired cation-exchange membrane would exhibit high proton conductivity, with low bromine species permeability. Electrospinning has been used to fabricate nanofiber composite fuel cell membranes, as an alternative to commercially available films and conventional films from solution-cast polymer blends and copolymers. Pintauro and co-workers demonstrated two different nanofiber electrospinning strategies to fabricate composite fuel cell membranes for hydrogen/air fuel cells: (1) electrospun polyelectrolyte fibers that were impregnated with an uncharged reinforcing polymer matrix [2,3] and (2) dual nanofiber electrospun mats, where the ionomer and uncharged reinforcing polymer are electrospun separately but simultaneously, followed by fiber mat processing to generate one of two distinct membrane morphologies: (i) an interconnected network of ionomer nanofibers embedded in an uncharged polymer reinforcing polymer matrix and (ii) a network of uncharged reinforcing polymer nanofibers surrounded by ionomer. The resultant electrospun composite membranes were shown to have excellent properties for use in a hydrogen/air fuel cell, with improved mechanical properties and lower membrane water swelling than a neat ionomer film, while maintaining a low membrane sheet (areal) resistance [3]. Nanofiber composite membranes from dual fiber mats have also been fabricated and tested for H2/Br2 fuel cells, where the membranes were composed of Nafion and either polyphenylsulfone or poly(vinylidene fluoride) as the uncharged reinforcing polymer [4,5]. For both membrane systems, there was a significant reduction in bromine species crossover, as compared to commercial Nafion®117, with no increase in membrane sheet resistance. Recently, we have developed a new nanofiber electrospinning strategy, where a single solution of a binary polymer mixture consisting of ionomer (i.e., Nafion) and reinforcing polymer, poly(vinylidene fluoride) (PVDF), is electrospun, followed by hot-pressing the resultant nanofiber mat (Figure 1a) into a dense and defect-free membrane. In a further modification of this membrane preparation strategy, we have electrospun Nafion/PVDF blended fibers onto a target drum that is rotating at very high RPM and obtained mats with highly aligned fibers (Figure 1b). To create a membrane, the aligned fiber mats were stacked in multiples layers (with fibers of successive mats rotated 90o) and then hot-pressed, followed by ionomer annealing. Our presentation will focus on the fabrication and characterization of the electrospun Nafion/PVDF membranes from random and aligned nanofiber mats, for use in H2/Br2fuel cells. The membranes will be compared to solution cast Nafion/PVDF blended films and to commercial Nafion 212 in terms of morphology, proton conductivity, swelling, bromine species permeability and mechanical characteristics.