Hydrogen ions (protons, H+) can accept reducing equivalents (conventionally represented as electrons, e-) generated either photosynthetically or by the oxidation of organic and inorganic substrates inside microbial cells:

2e — + 2H+ ^ H2

The terminal electron donor (e. g., reduced ferredoxin) could donate electrons to the anode of a battery. Protons could then, in the presence of O2, complete the electric circuit at the cathode by the reaction:

O2 + 4e — + 4H+ ^ 2H2O

thus forming a highly environmentally friendly source of electric power (a battery), fueled by microbial metabolic activity. That, in essence, is the definition of a microbial fuel cell (MFC).98-100

At its simplest, an MFC is a dual-chamber device with an electrolyte, a cation exchange membrane to separate anodic and cathodic compartments, a supply of O2 for the cathode, and an optional sparge of inert gas for the anode (figure 1.11). The transfer of electrons to the anode may be either direct (via unknown terminal electron donors on the cell surface) or employing redox-active “mediators” that can be reduced by the cells and reoxidized at the anode (e. g., neutral red reduced by hydrogenase).101,102 A wide spectrum of microbial species have been tested in MFC environments, usually anaerobes or facultative anaerobes chosen to function in the O2-deficient anode compartment. Recent examples include

• Immobilized cells of the yeast Hansenula anomala103

• A mixed microbial community of Proteobacterium, Azoarcus, and Desulfuromonas species with ethanol as the fuel source104

• Desulfitobacterium hafniense with humic acids or the humate analog anthraquinone-2,6-disulfonate added as an electron-carrying mediator with formic acid, H2, lactate, pyruvate, or ethanol as the fuel105

• E. coli in MFCs as power sources for implantable electronic devices106

The first use of the term “microbial fuel cell” appears to date from the early 1960s in studies of hydrocarbon-metabolizing Nocardia bacteria by research scientists of the Mobil Oil Company, but the basic concepts may date back 30 or even 50 years earlier.101 Developments of MFCs as commercial and industrial functional­ities are meth

y these systems is currently limited, primarily by high internal (ohmic) resistance, but improvements in system architecture might result in power generation that is more dependent on the bioenergetic capabilities of the

Cathodic compartment

FIGURE 7.11 Redox reactions occurring in an MFC: MED is the soluble mediator reduced by the microbial terminal electron donor at the microbial cell surface.

microorganisms.108 They are close to devices mobilized for the sets of science fiction films that call on “bioelectricity” in all its many conceptual forms (e. g., in the Matrix trilogy) and can easily be imagined in self-reliant ecosystems in deep space travel (Silent Running). But are they meaningful additions to the biofuels armory on planet Earth?

Creating a scalable architecture for MFCs is essential to provide large surface areas for oxygen reduction at the cathode and bacteria growth on the anode; a tubular ultrafiltration membrane with a conductive graphite coating and a nonprecious metal catalyst can be used to produce power in an MFC and is a promising architecture that is intrinsically scalable for creating larger systems.109 For the anodes, highly conduc­tive noncorrosive materials are needed that have a high specific surface area (i. e., surface area per volume) and an open structure to avoid biofouling; graphite fiber brush anodes have high surface areas and a porous structure can produce high power densities, qualities that make them ideal for scaling up MFC systems.110 The technol­ogy has long existed for very-large-scale microbial fermentations for bulk chemicals (e. g., citric acid and amino acids), with stirred tank volumes greater than 500,000 l being widely used; such structures could, in principle, be the “anodic” compartments of stationary MFC generators. Because rumen bacteria have been shown to generate electricity from cellulosic materials, potentially immense substrate supplies could be available for MFC arrays.111 Even greater flexibility can be designed, for example, coproducing H2 and ethanol production from glycerol-containing wastes discharged from a biodiesel fuel production plant with Enterobacter aerogenes in bioelectro­chemical cells with thionine as the exogenous electron transfer mediator.112