Control of voltage reversal in serially stacked microbial fuel cells through manipulating current: Significance of critical current density

• Voltage reversal occurred above critical current density (jcritical) in a stacked MFC.
• A threshold resistance (Rthreshold) was determined with jcritical in the stacked MFC.
• Voltage reversal was successfully controlled with Rthreshold in the stacked MFC.
• Power loss in Rthreshold accounted for 20% of the maximum power density in the stacked MFC.

Voltage reversal is a critical issue for serially stacking microbial fuel cells (MFCs). It occurs when current density in stacked MFCs increases over critical current density (jcritical). In this study, we clearly show that no voltage reversal occurs in stacked MFCs if current density is maintained below jcritical where the anode and the cathode potential in an inferior unit become identical, with an external resistance placed between individual MFCs. We define threshold resistance (Rthreshold) that enables current density below jcritical in stacked MFCs, and demonstrate the validity of Rthreshold theoretically and experimentally. Voltage reversal is controlled in the stacked MFC equipped with Rthreshold by which the current density in the stacked MFC is kept below jcritical. In comparison, a stacked MFC without Rthreshold faces voltage reversal over jcritical. Energy loss in Rthreshold is comparable to energy loss with other voltage control methods, such as passive or active methods. However, the Rthreshold approach is a simple, inexpensive way of controlling voltage reversal, especially for small MFCs (<50 mL).

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