MFCs (microbial fuel cells) are an emerging technology for simultaneous treatment of wastewater and energy recovery. These devices exploit microbial metabolism to generate electricity from organic matter. The separator is a critical factor in the design of MFCs as it plays a crucial role in the transport of protons from the anode to the cathode, affecting the performance of the cell. It is thus of interest to develop a method to predict the behavior of a separator before being used in MFCs. The present work proposes a new method based on spectroscopy to calculate the internal resistance of several PIMs (polymer inclusion membranes) based on ILs (ionic liquids) and predict their behavior as novel proton exchange membranes in MFCs. Four types of PIMs based on three different groups of ionic liquids were prepared and electrochemically characterized: Methyltrioctyl ammonium chloride, [OMIM+][Cl-], 1-methyl-3-octylimidazolium hexafluorophosphate, [OMIM][PF6-](]), Tri-butylmethylphosphonium methylsulphate, [P-4,P-4,P-4,P-1+1]MeSO4-], and Triisobutyl-(methyl)-phosphonium tosylate, [P-I4,I4,I4,1(+)][TOS-], some of whichwere patented by our research group to be used as separator in MFCs (P201330453). Finally, the PIMs were evaluated in MFCs for energy production and wastewater treatment and compared with Naflon (R) 117. The results show that the [P-I4,P-I4,P-I4,P-11[TOS-]-based membrane outperformed the rest of separators in terms of power output. (C) 2015 Elsevier Ltd. All rights reserved.