Different microbial electrochemical technologies are being developed for many diverse applications, including wastewater treatment, biofuel production, water desalination, remote power sources, and biosensors. Current and energy densities will always be limited relative to batteries and chemical fuel cells, but these technologies have other advantages based on the self-sustaining nature of the microorganisms that can donate or accept electrons from an electrode, the range of fuels that can be used, and versatility in the chemicals that can be produced. The high cost of membranes will likely limit applications of microbial electrochemical technologies that might require a membrane. For microbial fuel cells, which do not need a membrane, questions about whether larger-scale systems can produce power densities similar to those obtained in laboratory-scale systems remain. It is shown here that configuration and fuel (pure chemicals in laboratory media vs actual wastewaters) remain the key factors in power production, rather than the scale of the application. Systems must be scaled up through careful consideration of electrode spacing and packing per unit volume of the reactor.

Activated carbon (AC) is a low‐cost and effective catalyst for oxygen reduction in air cathodes of microbial fuel cells (MFCs), but its performance must be maintained over time. AC was modified by three methods: 1) pyrolysis with iron ethylenediaminetetraacetic acid (AC‐Fe), 2) heat treatment (AC‐heat), and 3) mixing with carbon black (AC‐CB). The maximum power densities after one month with these AC cathodes were 35 % higher with AC‐Fe (1410±50 mW m−2) and AC‐heat (1400±20 mW m−2), and 16 % higher with AC‐CB (1210±30 mW m−2) than for plain AC (1040±20 mW m−2), versus 1270±50 mW m−2 for a Pt control. After 16 months, the Pt cathodes produced only 250±10 mW m−2. However, the AC‐heat and AC‐CB cathodes still produced 960–970 mW m−2, whereas plain AC produced 860±60 mW m−2. The performance of the AC cathodes was restored to >85 % of the initial maximum power densities by cleaning with a weak acid solution. Based on cost considerations among the AC materials, AC‐CB appears to be the best choice for long‐term performance.

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