Cobalt oxide/nanocarbon hybrid materials (graphene and carbon nanotube) are used as alternative cathode catalysts for oxygen reduction reaction in air-cathode microbial fuel cell (MFG) for the first time. Electrochemical results reveal that these hybrid materials exhibit high catalytic performance. In MFCs, the maximum power density of 469 +/- 17 mW m(-2) is achieved from the Co3O4/NCNT cathode, which is 5.3 times larger than that of the NCNT cathode. This value is competitive with those obtained using Pt/C (603 +/- 23 mW m(-2)). Therefore, Co3O4/NCNT nanocomposite is an efficient and cost-effective cathode catalyst for practical MFC applications. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved,