Nafion/PVDF nanofiber composite membranes for regenerative hydrogen/bromine fuel cells

Abstract

Nanofiber composite proton exchange membranes were fabricated and their properties measured, for possible use in a regenerative hydrogen/bromine fuel cell. The membranes were prepared from dual nanofiber mats, composed of Nafion® perfluorosulfonic acid (PFSA) ionomer for proton transport and polyvinylidene fluoride (PVDF) for mechanical reinforcement. Two composite membranes structures containing Nafion volume fractions ranging from 0.30 to 0.65 were investigated: (1) Nafion nanofibers embedded in a PVDF matrix (N(fibers)/PVDF) and (2) PVDF nanofibers embedded in a Nafion matrix (N/PVDF(fibers)). The in-plane conductivity for films equilibrated in water and 2.0M HBr scaled linearly with Nafion volume fraction for both morphologies. The through-plane proton conductivity of N(fibers)/PVDF membranes in water was lower than that of N/PVDF(fibers) films with the same Nafion content for films with less than 55vol% Nafion, e.g., 0.03S/cm for N(fibers)/PVDF membrane vs. 0.04S/cm for N/PVDF(fibers) membrane at 40vol% Nafion. N(fibers)/PVDF membranes exhibited excellent Br2/Br3− barrier properties with a reasonable membrane resistance, e.g., a N(fibers)/PVDF membrane with 40vol% Nafion and a thickness of 48µm had the same area-specific-resistance as Nafion® 115 (0.13Ωcm2) but its steady-state Br2/Br3− crossover flux was 3.0 times lower than that of Nafion 115 (1.43×10−9mol/s/cm2vs. 4.28×10−9mol/s/cm2).