Scale-up studies of Microbial Fuel Cells are required before practical application comes into sight. We studied an MFC with a surface area of 0.5 m(2) and a volume of 5 L Ferric iron (Fe(3+)) was used as the electron acceptor to improve cathode performance. MFC performance increased in time as a combined result of microbial growth at the bio-anode, increase in iron concentration from 1 g L(-1) to 6g L(-1), and increased activity of the iron oxidizers to regenerate ferric iron. Finally, a power density of 2.0 Wm(-2) (200W m(-3)) was obtained. Analysis of internal resistances showed that anode resistance decreased from 109 to 7 m Omega m(2), while cathode resistance decreased from 939 to 85 m Omega m(2). The cathode was the main limiting factor, contributing to 58% of the total internal resistance. Maximum energy efficiency of the MFC was 41%. (C) 2011 Elsevier B.V. All rights reserved.