BACKGROUND Microbial fuel cell (MFC) power production is limited by the cathodic oxygen reduction reaction (ORR). The quest for platinum-free materials for improving the cathodic ORR in MFCs is a challenging task. Birnessite-type MnO2/carbon cathodes in MFCs have been rarely reported so far. In this work, a birnessite/C cathode was tested in a MFC. RESULTS A birnessite/C cathode was synthesized (using a rapid, facile and low-cost method) and its structural and morphological characteristics were assessed. The ORR on such a cathode was investigated using linear sweep voltammetry in a catholyte of 0.8 mol L-1Na2SO4, pH 2 (same conditions as those of experimental MFCs). The current density values were higher than those for a plain C cathode. The volumetric power density improved from 224 to 6201 mW m(-3)on replacing the plain C cathode with the birnessite/C cathode in a MFC. Deltaproteobacteria were found at the C anode and they were associated with the high power densities obtained. CONCLUSIONS Birnessite/C is a promising cathodic electrode because its synthesis is straightforward, it is not expensive and it could promote the ORR and improve the power density in MFCs. (c) 2020 Society of Chemical Industry