# Microbial fuel cells How exoelectrogens perform extracellular electron transfer, what thermodynamics sets as the ceiling on cell voltage, and why internal resistance collapses power density. Bacteria can hand electrons straight to an electrode — but the biofilm that does it is such a poor conductor that power stays orders of magnitude below chemical fuel cells. Source: https://en.bioecon.ru/docs/bioenergy-climate/biofuels-bioenergy/microbial-fuel-cells-bioelectrochemical-systems/ Updated: 2026-09-07 A microbial fuel cell is a galvanic cell in which the anode replaces oxygen as the terminal electron acceptor of microbial respiration. Exoelectrogenic bacteria oxidise organic matter — wastewater, root exudate, soil humics — and export the electrons to an electrode; they flow through a load to a cathode where oxygen is reduced. The fuel is waste, the catalyst is a biofilm, and the circuit is the honest part of the design. ## Extracellular electron transfer The physical problem is awkward: respiration ends at a membrane-bound enzyme complex, while the electrode sits outside the cell. Exoelectrogens solved this two ways. *Geobacter* builds outer-membrane multiheme cytochromes and conductive protein filaments, so electrons are handed to the surface and conducted through the biofilm itself — cells buried far from the electrode still feed it through the layer. *Shewanella* secretes flavins that work as diffusing electron shuttles. A conducting biofilm is the enabling oddity: the anode is less a plate than a living wire, and start-up means letting a consortium self-select onto the electrode, which takes weeks. ## The thermodynamic ceiling Cell voltage cannot exceed the gap between the two redox couples. At neutral pH the acetate/bicarbonate pair sits near −0.29 V against the standard hydrogen electrode and the oxygen/water pair near +0.82 V, so even a perfect cell tops out around 1.1 V. Real cells open well below that and deliver 0.3–0.5 V under load. The bacterium also keeps its share: below the potential its own respiratory chain needs, it will not release electrons, so biology floor-limits the anode potential. And the fuel is pre-oxidised — waste organics carry far less energy per electron than hydrogen does, which is why the current is cheap but low-grade. ## Why power density collapses Power goes as the square of voltage divided by internal resistance, and resistance is where microbial cells lose to chemical ones. The electrolyte is dilute and ohmic; the proton-carrying membrane builds pH gradients; oxygen reduction at the cathode is kinetically sluggish without noble-metal catalyst; and the biofilm, for all its conductivity, adds resistance while limiting electrode area per reactor volume. Chemical fuel cells deliver on the order of a watt per square centimetre; microbial ones manage milliwatts — a gap of roughly three orders of magnitude. The gap is structural rather than an engineering shortfall, and it fixes the honest positioning: microbial fuel cells earn their keep as wastewater treatment that returns some power, not as power plants. Scaled down, the same trickle becomes an asset — see [bio-powered IoT charging](../bio-powered-iot-charging/); run in reverse, the process is [microbial electrosynthesis](../power-to-x-with-biological-step-microbial-electrosynthesis/).