Nano Lett. A new composite textile promotes microbial colonization as seen at low magnification in the way a biofilm wraps around each fiber (left). High magnification (right) reveals that the microbes are tethered intimately to the material via hairlike nanowire structures (pili).

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A porous composite textile made from carbon nanotubes and polymer fibers can be fashioned into a new type of electrode that could boost the performance of microbial fuel cells (MFCs), according to Stanford University researchers (Nano Lett., DOI: 10.1021/nl103905t).

MFCs, which are at a pilot-plant scale but not yet commercially available, are bacteria-driven electrochemical devices capable of cleaning up wastewater that contains organic matter. The microbes use the organic matter as fuel while simultaneously generating power by extracting electrical energy from controlled chemical reactions. The study demonstrates a procedure for preparing custom materials to build the cells that outperform those commercially available.

Other fuel cells, such as polymer electrolyte membrane fuel cells—the ones widely studied and popularized for powering automobiles—oxidize hydrogen at an anode. The liberated electrons provide electrical power as they flow through an external circuit to a cathode, where they reduce an oxidant—typically oxygen. The overall process, which produces water, is facilitated by the presence of platinum or other catalysts.

MFCs operate in much the same way with the exception that biomass or organic matter constitutes the fuel. And rather than relying on precious metals, MFCs use "exoelectrogenic" bacteria as biocatalysts and exploit the microbes' metabolic machinery for decomposing the fuel—their food. The defining property of exoelectrogens is their ability to transfer electrons through their plasma membranes via biological nanowires to an anode.

A key challenge in designing MFCs has been finding a high-performance anode material. Ideally, the material should be sufficiently porous to support bacterial colonization, mechanically robust, and electrically conductive to facilitate electron transfer between microbial films and the anode surface.

Researchers have experimented with various types of porous carbon-based cloths, papers, and other materials. But the pores in anodes made from those materials can quickly become clogged due to initial microbial growth, which hampers further colonization and thereby limits fuel-cell performance.

In an effort to overcome that problem, the Stanford team, which includes Yi Cui, Craig S. Riddle, Xing Xie, and coworkers, prepared a highly porous, conductive, biocompatible material by intimately blending carbon nanotubes with a holey textile made of randomly intertwined 20-μm-diameter polyester fibers. The team reports that anodes made from that composite material feature an open three-dimensional structure ideal for internal microbial colonization.

Tests show that MFCs equipped with the composite textile anodes outperform ones made with commercial woven carbon cloth anodes, which, as a result of their relatively small pores, support biofilm growth only on exterior anode surfaces. Specifically, the new MFCs generated a maximum current density that is 157% higher and a maximum power density that is 68% higher than traditional MFCs.

"This paper is a nice contribution to the literature on microbial fuel cells," says Pennsylvania State University's Bruce E. Logan, one of the field's pioneers. Logan goes on to say that being electrically conductive isn't the only prerequisite for an anode material. "The bacteria must be able to bind to the material, and it needs to have high porosity," he says, pointing out the strengths of the Stanford textile. Sounding a note of caution, Logan comments that the potential for scale-up will depend strongly on the cost of the new material, which has not yet been reported.