NOVELTY - A bio-fuel cell system (10) has cathode compartment (14) containing cathode (22), anode compartment (16) containing anode (20), and bioreactor (26). Bioreactor comprises microorganisms, pump for pumping fluid containing oxygen and carbon dioxide into bioreactor, and pump for pumping fluid containing ferric ions. Bioreactor is connected with compartment (14) to circulate solution containing ferrous ions and protons.    USE - For generating electricity.    ADVANTAGE - The bio-fuel cell system has high overall efficiency, and does not produce carbon dioxide while generating electricity using hydrogen as fuel.    DETAILED DESCRIPTION - A bio-fuel cell system (10) has a cathode compartment (14) containing a cathode (22), an anode compartment (16) containing an anode (20), and a bioreactor (26). The bioreactor comprises chemolithotrophic microorganisms, a pump for pumping a fluid containing oxygen and carbon dioxide into the bioreactor, and a pump for pumping a fluid containing ferric ions (Fe3+) into the cathode compartment. Compartments (14, 16) are separated by a proton exchange membrane (18). The aqueous solution containing ferric ions is circulated in the compartment (14) and ferric ions are reduced to ferrous ions at the cathode. Fuel containing hydrogen constituent is pumped into the compartment (16). Electrochemical oxidation of fuel in compartment (16) produces electrons and protons. The protons formed by oxidation of hydrogen pass through the proton exchange membrane into compartment (14). The bioreactor is connected with cathode compartment, to circulate aqueous solution containing ferrous ions and protons. Ferrous ions are oxidized by the microorganisms to ferric ions in aerobic oxidation reaction, in the bioreactor. Electrical power is obtained by making electrical connection between load, and anode and cathode. An INDEPENDENT CLAIM is included for method for generating electricity, which involves reducing redox couple (I) contained in cathode to redox couple (II) in lower oxidation state, in the cathode compartment, producing protons and electrons by oxidizing fuel in the anode compartment, and oxidizing redox couple (II) in lower oxidation state back to higher oxidation state by the microorganisms, in the presence of oxygen.    DESCRIPTION OF DRAWING(S) - The figure shows the diagrammatic representation of the bio-fuel cell system.    bio-fuel cell system (10)    cathode compartment (14)    anode compartment (16)    proton exchange membrane (18)    anode (20)    cathode (22)    bioreactor (26)