A membrane-integrated proton exchange membrane fuel cell that enables in situ fermentation of sugar to ethanol, diffusion-driven separation of ethanol, and its catalytic oxidation in a single continuous process is reported. The fuel cell consists of a fermentation chamber coupled to a direct ethanol fuel cell. The anode and fermentation chambers are separated by a reverse osmosis (RO) membrane. Ethanol generated from fermented biomass in the fermentation chamber diffuses through the RO membrane into a glucose solution contained in the DEFC anode chamber. The glucose solution is osmotically neutral to the biomass solution in the fermentation chamber preventing the anode chamber from drying out. The fuel cell sustains > 1.3 mW cm(-2) at 47 degrees C with high discharge capacity. No separate purification or dilution is necessary, resulting in an efficient and portable system for direct conversion of fermenting biomass to electricity. (C) 2015 Elsevier Ltd. All rights reserved.