The use of inexpensive and high-performance cathode materials is important for constructing large-scale microbial fuel cells (MFCs) for wastewater treatment and bioelectricity production. We show that the air-breathing MFC with a MnO2 (68%)/CNT and polymethylphenyl siloxane (PMPS) coated-stainless steel mesh cathode delivers a maximum power density of 2676mWm−2 (normalized to the cathode surface area) or 86Wm−3 (normalized to the anode chamber volume). The cathode performance is found to be highly replied on the percentage of MnO2 in the as-prepared MnO2/CNT nanocomposites, in which the birnessite-type MnO2 is uniformly formed on the exterior CNT surfaces, as revealed by the scanning electron microscopy (SEM) and X-ray diffraction (XRD) results. Furthermore, it is found that PMPS coated onto the mesh electrode offers the advantages of low cost, easy handling and low water loss and exhibits improved cathode performance as compared to polydimethyl siloxane (PDMS). These findings suggest that the cathode materials, MnO2/CNT and PMPS in MFCs can function well as the electrocatalysts and the water-repellent gas-diffusion layer, respectively.