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 Nalion (R) 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 2.0 M HBr scaled linearly with Narion 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 Narion content for films with less than 55 vol% Nafion, e.g., 0.03 S/cm for N(fibers)/PVDF membrane vs. 0.04S/cm for N/PVDF(fibers) membrane at 40 vol% Nafion. N(fibers)/PVDF membranes exhibited excellent Br-2/Br-3(-) barrier properties with a reasonable membrane resistance, e.g., a N(fibers)/PVDF membrane with 40 vol% Nation and a thickness of 48 pm had the same area-specific-resistance as Nafion (R) 115 (0.13 Omega cm(2)) but its steady-state Br-2/Br-3(-) crossover flux was 3.0 times lower than that of Nafion 115 (1.43 x 10(-9) mol/s/cm(2) vs. 4.28 x 10(-9) mol/s/cm(2)). (C) 2015 Elsevier B.V. All rights reserved.