A naturally rich, low cost electrocatalyst of spinel CoMn2O4 doped reduced graphene oxides (CoMn2O4/rGO) is prepared as oxygen reduction reaction (ORR) electrocatalyst. The CoMn2O4 nanoparticles (similar to 10 nm) disperse uniformly on the rGO nanosheets via in-situ hydrothermal synthesis, endowing the CoMn2O4/rGO electrocatalyst a large Brunauer-Emmet-Teller surface area (78.4 m(2) g(-1)) and abundant Co3+/Co2+, Mn3+/Mn2+ and Mn4+/Mn3+ electroactive sites, resulting in faster electron transport and improved electrochemical activity for ORR. The mechanism of oxygen reduction for CoMn2O4/rGO catalyst proceeds through both two-electron and four-electron pathways while the former pathway dominates according to linear sweep voltammetry analysis. These nanocomposites are doped on graphite felt to act as air-cathode in a double-chambered microbial fuel cell (MFC). Benefiting from the more positive reduction potential, higher electron conductivity and reduced charge-transfer resistance, the MFCs using GF-CoMn2O4/rGO air-cathodes exhibit much better cell performance than those using GF-CoMn2O4 and bare GF air-cathodes. The MFC using GF-CoMn2O4/rGO-8.0 reaches the maximum power output of 361 mW m(-2) and open circuit potential of 0.822 V. The excellent catalytic activity makes GF-CoMn2O4/rGO a promising air-cathode for practical applications of MFC. (C) 2018 Published by Elsevier Ltd.