A bacterial (microbial) fuel cell or biological fuel cell is a bio-electrochemical system that drives a current by using bacteria and mimicking bacterial interactions found in nature. A microbial fuel cell is a device that converts chemical energy to electrical energy by the action of microorganisms. These electrochemical cells are constructed using either a bioanode and/or a biocathode (WIKIPEDIA).
Approximate equations for producing electric energy:
1) On anode (-):
Oxidation of carbohydrate (CH2O)n, production of electrons:
(CH2O)n(aq) + nH2O(l) - 4ne- → nCO2(g) + 4nH+(aq)
2) On cathode (+):
Reduction of oxygen, consumtion of electrons:
O2(g) + 4H+(aq) + 4e- → 2H2O(l)
Microbes participate in the electron releasing process, but may also indirectly participate in the electron consumption process at the cathode. For example bacteria oxidize hydrocarbon to acid and in acid media around cathode the oxygen reduction potential becomes more positive.