Microbial fuel cells (MFCs) are facing several technological challenges before they can be considered as reliable energy sources. Although several feasible inocula, materials and catalysts have been employed to produce energy, the design of a MFC should be done under realistic conditions: abundant and economic feedstock. In this study, two different MFC designs (parallel plate and tank reactors) are tested with non-catalyzed. carbon electrodes and natural inocula. In both approaches cathodic oxygen reduction is performed on two different non-catalyzed carbon materials: carbon fabric and reticulate vitreous carbon. This study shows that power and current densities can be boosted by systematically decreasing the catholyte resistance (by additions of NaCl or Na2SO4) and dissolved oxygen concentration. In the parallel plate cell configuration, a mixed culture coming from sludge wastewater was used and power outputs up to 73 mW m(-2) (2867 mW m(-3)) coupled to 187 mA m(-2) (anode surface area), were achieved. In the Sediment MFC cell configuration, lagoon sediment was used as both organic source of energy and natural supply of bacteria. Under this approach, the concentration of the organic matter is limited but it is demonstrated that bacteria can be adapted to degrade acetate. Power outputs up to 3.9 mW m(-2) normalized to the anodic electrode footprint area, coupled to 13 mA m(-2) were achieved. (C) 2011 Elsevier Ltd. All rights reserved.