Cr(VI)-MFC of the double microbial fuel cell (d-MFC) arrangement could successfully complement the insufficient voltage and power needed to recover cadmium metal from Cd(II)-MFC, which operated as a redox-flow battery. It was also possible to drain electrical energy from the d-MFC by an additional passage. The highest maximum utilization power density (22.5 Wm(2)) of Cr(VI)-MFC, with the cathode optimized with sulfate buffer, was 11.3 times higher than the highest power density directly supplied to Cd(II)-MFC (2.0 Wm(2)). Cr(VI)-MFC could generate 3 times higher power with the additional passage than without it; and the current density for the former was 4.2 times higher than the latter at the same maximum power point (38.0 Am-2 vs. 9.0 Am-2). This boosting phenomenon could be explained by the Le Chatelier's principle, which addresses the rate of electron-hole pair formation that can be accelerated by quickly removing electrons generated by microorganisms. (C) 2014 Elsevier Ltd. All rights reserved.