Optimization of the cathode catalyst is critical to the study of microbial fuel cells (MFCs). By using the open circuit voltage and power density as evaluation standards, this study focused on the use of polyaniline (PANI)/multi-walled carbon nanotube (MWNT) composites as cathode catalysts for the replacement of platinum (Pt) in an air-cathode MFC, which was fed with synthetic wastewater. Scanning electron microscopy and linear scan voltammogram methods were used to evaluate the morphology and electrocatalytic activity of cathodes. A maximum power density of 476 mW/m(2) was obtained with a 75% wt PANI/MWNT composite cathode, which was higher than the maximum power density of 367 mW/m(2) obtained with a pure MWNT cathode but lower than the maximum power density of 541 mW/m(2) obtained with a Pt/C cathode. Thus, the use of PANI/MWNT composites may be a suitable alternative to a Pt/C catalyst in MFCs. PANI/MWNT composites were initially used as cathodic catalysts to replace Pt/C catalysts, which enhanced the power generation of MFCs and substantially reduced their cost. Copyright (C) 2014 John Wiley & Sons, Ltd.