To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO(2)/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO(2) is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO(2)/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO(2) powder, suggesting that the surface coating of MnO(2) onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m(-2) produced from the MFC with in situ MnO(2)/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO(2)/CNTs (93 mW m(-2)), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m(-2)). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO(2) on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO(2) and that in situ MnO(2)/CNTs offer a good alternative to Pt/C for practical MFC applications. (C) 2011 Elsevier B.V. All rights reserved.