Forward osmosis (FO)-microbial fuel cell (MFC) has been recently reported as an efficient system to treat waste-water and meanwhile recover energy, with better performance than conventional MFCs with cationic-exchange membranes (CEMs). However, the mechanism of FO membrane's superior performance remains unclear. This study compared the electrochemical performances of MFCs with FO membrane, CEM and anionic exchange membrane (AEM) as separators, and explored into the correlations between electricity generation and ion fluxes. The FO-MFC showed significantly lower internal resistance, due to higher salt accumulation and improved proton flux than other membrane systems. This led to an almost two-fold higher voltage in the FO-MFC than the CEM control. Unlike the CEM that encountered severe pH splitting due to suppressed proton diffusion by other competitive cations, the FO membrane favored an improved transport of protons in relative to other larger-sized cations attributed to its unique size-selectivity and the water flux as further driven force. The present work may have implications for the development and application of more efficient bioelectrochemical processes. (C) 2015 Elsevier B.V. All rights reserved.