Scientist Revs Up Power of Microbial Fuel Cells in Unexpected Ways
Scientists have boosted the power output of microbial fuel cells more than 10-fold by letting the bacteria congregate into a slimy matrix known as a biofilm. The research, led by microbiologist Derek Lovley of the University of Massachusetts Amherst, suggests that efficient technologies for generating electricity with microbes are much closer than anticipated. Lovley presented the results Wednesday in a plenary talk at the meeting of the Electrochemical Society in Denver.
A typical fuel cell converts fuels to electricity without the need for combustion and microbial fuel cells work the same way. They usually comprise two compartments, or cells, which are separated by an electrically insulating membrane. In one compartment, microorganisms pull electrons and protons from some sort of fuel鈥攕uch as waste organic matter. These protons and electrons are attracted to molecules in the second compartment鈥攗sually oxygen鈥攁nd will move towards those molecules. The protons do this by passing through the membrane. But the electrons can鈥檛 go through the membrane and so must travel via an alternate route鈥攁 wire, or electrode that connects the two compartments. It is this flow of electrons through the electrode that supplies power.
Microbial fuels cells harness the electron shuttling that occurs in the energy-making pathway of certain bacteria. In the energy-making pathway of most animals, electrons and protons are also shuttled about, and usually electrons are passed to oxygen brought in through the lungs. Early microbial fuel cells intercepted the bacteria鈥檚 electron shuttling with compounds called 鈥渕ediators,鈥� which would penetrate the bacteria, snatch electrons and then transfer them to the metal electrode. But the compounds typically used as mediators are often expensive and toxic. A more recent and efficient approach has been to use microbes that can pass electrons directly to a metal electrode.
These 鈥渕etal-reducing鈥� bacteria are ideal for fuel cells, says Lovley, especially species of Geobacter and Rhodoferax, microbes that evolved means to transfer electrons to metals in the surrounding environment. The microbes use thin wire-like growths, several cell lengths long, that extend from their cell membrane out into the environment. Many bacteria have these extended structures鈥攃alled pili鈥攖hey usually use the hair-like extensions to attach to other cells or surfaces. But Geobacter uses pili to transfer electrons onto iron in the surrounding soil. These so-called 鈥渕icrobial nanowires鈥� also seem to be critical for Geobacter to form a biofilm, says Lovley.
While investigating the microbes鈥� electron transfer mechanism, Lovely鈥檚 team created a mutant Geobacter that didn鈥檛 have the gene for making the pili, yet the microbes still produced electricity when placed in a fuel cell. The researchers suspected that a membrane protein that was part of the microbe鈥檚 energy-making pathway was also able to transfer electrons directly to the metal electrode.
鈥淭he microbes were lined up in a single, thin layer along the electrode,鈥� says Lovley. 鈥淚t seemed that either the nanowires or the membrane protein had to be in direct contact with the electrode for electron transfer to occur,鈥� he says.
But then the researchers tweaked their fuel cell so the second compartment could take as many electrons as the microbes could provide. To the scientists鈥� surprise, the power output increased dramatically and Geobacter began to grow on the electrode in a thick, sticky mass known as a biofilm. However, the mutant Geobacter that couldn鈥檛 make pili couldn鈥檛 congregate into a biofilm.
鈥淭he mutants produced electricity at a much slower rate鈥� says Lovley. 鈥淚t seems that Geobacter鈥檚 pili are essential for making a biofilm.鈥�
Many bacteria form biofilms鈥攖he gluey matrix of sugars serves to anchor free-floating microbes to various surfaces, such as teeth, a refrigerator drawer or rocks in a stream. Biofilms are usually the bane of those who encounter them鈥攖hey cause tooth decay, ruin the hulls of ships and can cause serious health problems when they glom onto medical implant devices such as catheters. But in this instance, the biofilm is a good thing, says Lovely. It seems to act as one big, slimy, conductive mat, allowing electrons to be transferred by bacteria that aren鈥檛 in direct contact with the electrode.
Further experiments by Lovley鈥檚 team confirmed that microbes in the center of the biofilm鈥攖oo far from the electrode to reach it themselves鈥攚ere transferring electrons at the same rate as microbes that were closer to the edge. It is a finding Lovley says he never would have predicted.
鈥淚t made sense that Geobacter would have to be in direct contact with the electrode to pass electrons,鈥� he says. 鈥淏ut now we have these big slime layers鈥攂ig red glops of Geobacter growing on the electrode鈥攁nd they are all passing electrons.鈥�
How the electrons are transferred through the gooey matrix isn鈥檛 clear, Lovley鈥檚 team is investigating that question now. But it does seem clear that improved methods for generating electricity via microbial fuel cells may be only a few slimy steps away.
Source: University of Massachusetts Amherst
