Following the finding that power production in microbial fuel cells varies depending on the relative sizes of electrodes, we found that in double-chamber air-cathode MFCs working on lactate as substrate the surface area of anode does not need to be necessarily equal to cathode surface area. As the ratio of anode to cathode surface area decreased, impedance of substrate oxidation and electron transfer from bacteria to mediator increased while the impedance of electron transfer from mediator to electrode decreased. As a result, decreasing the ratio continuously led to more generated power but it passed a maximum value. The ratio of 0.6 for anode to cathode surface areas was found optimal in terms of power generation. The finding was supported by data obtained from EIS analysis, tafel plots, and polarization curves.

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