A nanofiber-reinforced composite proton exchange membrane (NFR-PEM), based on sulfonated polyether ether ketone (SPEEK) as a proton conductor, was prepared and studied for microbial electrolysis cells (MECs). The poor dimensional stability of conventional SPEEK membrane was overcome by embedding a polyimide nanofiber substrate in the centre of the polymer matrix, without sacrificing proton conductivity. This novel membrane showed lower gas and fuel crossovers, with higher proton conductivity, compared to a Nafion membrane. The installation of NFR-PEM into MECs improved hydrogen gas production at the cathode, with an overall hydrogen efficiency (e(-) to hydrogen) of 86.4 +/- 14.7%, as compared to 77.2 +/- 10.3% of Nafion. In particular, higher proton selectivity of the NFR-PEM remarkably mitigated the anodic pH decrease harmful for anode bacteria, by facilitating selective proton transfer rather than other competing cations, which led to overall improved hydrogen gas yield in the MEC as the NFR-PEM was installed. (C) 2014 Elsevier B.V. All rights reserved.