Activated carbons (ACs) were treated with four concentrations of H3PO4 (0.5 M, 0.75 M, 1 M and 2 M) at 80 C, which was demonstrated to be a simple and effective method to enhance the performance of microbial fuel cells (MFCs). The maximum power densities (MPDs) were increased, and the MPD of AC-3 (treated with 1 M) was 954 +/- 36 mW/m(2), which was 34.8% higher than the control. Boehm titration and X-ray photoelectron spectroscopy demonstrated that the acidic surface functional groups (FGs) on AC, such as RCOOH, RCOOR' and ROH, were detrimental to the performance of MFCs. However, an elevated structural content of C=O and C-O (non-acidic FGs) was beneficial. It was also indicated that changes of resistance and pore structure were also associated with performance improvement. The enhanced performance of MFCs could be attributed to the combined influence of pore structure, resistance and chemical FGs on AC. (C) 2014 Elsevier B.V. All rights reserved.