Voltage reversal during microbial fuel cell stack operation

Microbial fuel cells (MFC) can be used to directly generate electricity from organic matter, but the voltage produced by a single reactor is only similar to 0.5 V. Voltage can be increased by stacking cells, i.e. by linking individual reactors in series, as is commonly done with hydrogen fuel cells, to provide a higher voltage output. A two-cell air-cathode MFC stack tested here produced a working voltage of 0.9 V (external load 500 2) and had an open circuit voltage (OCV) of 1.3 V when operated in fed batch mode under substrate-sufficient conditions. When multiple cells are stacked together, however, charge reversal can result in the reverse polarity of one or more cells and a loss of power generation. We investigated the causes of charge reversal and the impact of prolonged reversal on power generation using a two air-cathode MFCs stack. When voltage began to decline at the end of a fed batch cycle, we observed voltage reversal with one cell producing a working voltage of 0.6 V, and the other cell having a reversed voltage of -0.58 V, producing only a minimal stack voltage of 0.02 V. The reason for the voltage reversal was shown to be fuel starvation, resulting in a loss of bacterial activity. Voltage reversal adversely affected bacteria on the anode of the affected cell, as shown by a relative decrease in cell performance following a cycle of starvation (no feeding). The control of voltage reversal will be crucial for long-term operation of MFCs in series. Rapid feeding of a cell can restore positive voltage generation, but the long-term impact of charge reversal will be inactivation of bacteria and it will require that the affected cells be short-circuited to maintain stack power production. A better understanding of the long term effects of voltage reversal on power generation by MFC stacks is needed in order to efficiently increase voltage production by using stacked MFC systems. (C) 2007 Elsevier B.V. All rights reserved.