Fortifying Fuel Cell Technology
Scientists have known for nearly a century that certain bacteria can convert organic material into electricity. Only recently though, have microbiologists and engineers worked to exploit this phenomenon in the development of microbial fuel cells for powering environmental monitoring devices and treating wastewater.
Although fuel cells come in various shapes, sizes, chemistries and operating temperatures among other things, running these devices requires a set of universal components: a fuel source, two electrodes (an anode and a cathode), an electrolyte and a catalyst. The fuel source, hydrogen for example, reacts with a catalyst resulting in the release of electrons and the formation of positively-charged hydrogen atoms called protons. While the electrons travel through the anode to an external circuit where they are used to do useful work, the protons migrate through an electrolyte, which is a kind of permeable solid or liquid. The electrons and protons eventually meet up again at the cathode. Here, these particles react with oxygen to form water, which is drained from the fuel cell.
Today, research is aimed at manipulating these components to maximize efficiency, power output and practicality. In the past few years, one branch of fuel cell research gaining attention is the work being done by microbiologists and environmental engineers to design a device that uses microbes to convert the chemical energy stored in organic fuels like wastewater, organic debris or renewable biomass into electricity. At this time, microbial fuel cells don't produce a lot of power-some just about enough to illuminate a dozen or so 60-watt light bulbs-but efficiencies and power outputs keep improving and interest in the technology is growing.
The Right Bugs
"Although we talk about it being a young field, it's been known for 100 years that you could get electricity from microbial cultures," says Derek Lovley, a microbiologist at the University of Massachusetts in Amherst. "The big breakthrough occurred five or six years ago when we realized that with the right microorganisms this process could be a lot more efficient than had previously been considered." Those "right" microbes turned out to be anaerobic electricity-generating organisms called electricigens or electrogens. In their natural environments such as marine sediments, these bacteria, for example, species of the genus Geobacter, make the energy they need to survive by breaking down organic matter and transferring the resulting electrons to other substrates like iron minerals. "To these bacteria, the solid surface of the anode looks a lot like the solid surface of iron oxide so even though there's no evolutionary pressure on them to make electricity, it's a fortuitous reaction," Lovley says. "This was a big jump in fuel cell research because it made it apparent that you could be much more efficient at converting fuels," he says. In fact, more than 90 percent of the electrons available in an organic fuel source can be converted into electricity by these bacteria.
In a typical H-type microbial fuel cell, bacteria (represented by the orange circle) form a biofilm on the surface of the anode. Organic matter is added and the microbes break this biomass down to carbon dioxide (CO2), protons (H+) and electrons (e-). The electrons are transferred directly to the anode and flow through the circuit while the protons pass through a selective membrane to a second chamber. Here, oxygen combines with the protons and electrons at the cathode to make water.
Lovley's team of researchers mainly study the biology of the process, in other words, how the microbes do what they do. These microbiologists have discovered that when you engineer a better fuel cell to get more power, a thick, multi-layered growth of bacteria, called a biofilm, grows on the surface of the electrode. Since most of the cells in the biofilm are no longer in direct contact with the anode, Lovley discovered that the microbes transfer electrons using special hair-like structures. "It looks like electron transfer is highly dependent on appendages that appear to be electrically conductive and able to promote long-range electron transfer through the biofilm," he explains. Lovley's research group is currently evolving strains of Geobacter that produce more power. In addition, they have practical projects and funding from Toyota Motor Corp. and the National Science Foundation to develop the technology to one day power a car or mobile electronics like cell phones. However, a more immediate application of microbial fuel cells is for powering electronic sensors.
Scientists at the Naval Research Laboratory in Washington, D.C., under the direction of Leonard Tender, have deployed a weather buoy in the Potomac River that exploits the electricigens that naturally reside in the river-bottom sediments. The buoy is powered by what the team calls a benthic unattended generator (BUG) or sediment fuel cell. The BUG consists of an anode, which is buried in anaerobic sediments, and a cathode, which floats in the overlying water. Electricigens attach to the anode and convert organic material in the sediments to electrons and carbon dioxide. The electrons produce a current, which powers a device floating on the surface that measures air temperature, air pressure, relative humidity and water temperature, and transmits the information by a radio transmitter to a receiver in a nearby building.
In addition to powering sensing gadgets, MFCs are being developed and scaled-up for the treatment of wastewater and the generation of electricity as a byproduct. In this type of MFC, wastewater is flushed through an oxygen-free compartment that holds an anode. Bacteria in the water attach to the anode and strip electrons from the organic wastes in the fluid. The electrons run the circuit while the protons pass through the electrolyte to the cathode where they meet with oxygen to form clean water.
Hong Liu, an environmental engineer at Oregon State University, recently reported in the Journal of Power Sources a method for improving the power output of MFCs for this purpose. "One of the greatest challenges in the development of microbial fuel cells is that the internal resistance is really high and limits power generation," Liu explains. "Our study reduces resistance by significantly reducing the distance between the anode and the cathode." To do this, Liu and her team sandwiched a piece of cloth between the two electrodes, which effectively brought the anode and cathode closer together.
Because 5 percent of the electricity used in the United States is consumed in water and wastewater treatment facilities, implementing MFCs in these plants would reduce the cost of operation. "If you look at wastewater treatment, this is an area where we spend money and use energy," says Bruce Logan, a former postdoctoral advisor of Liu's and an environmental engineer at Penn State University. "If we can install a technology that just saves money, then it's making money. We don't have to make it pay for itself, we just have to make it better than what people are currently using."
Liu says scaling-up these systems for use in domestic water treatment is a long-term goal, however, and the more immediate need is to develop pilot-scale reactors for industrial locations like food-processing facilities or in remote parts of the world that lack central waste treatment facilities.
Another approach to improving the power output of MFCs is to develop new anodes and cathodes with greater surface area for the reactions to take place, Logan explains. "We recently published a couple of papers showing that you could use what look like bottle brushes-graphite fibers sitting in a metal core-that provide a very high surface area for bacteria to grow and transfer electrons to the electrode," he says. Logan's team is now working on developing tubular cathodes with a similar high surface area to volume.
The big hurdle to commercializing MFCs is drumming up investors and interest in scaling-up the technology and designing demonstration-size reactors, Logan says. "Once we do those demonstrations, then it can move forward to commercialization."
A group at the University of Queensland in Australia recently received government and industry support to build a pilot-scale MFC at Foster's brewery in Yatala, Queensland. "While most researchers make tiny reactors, we have been building larger systems and investing much time in making them work really well," says Korneel Rabaey, a postdoctoral research fellow with the university's Advanced Wastewater Management Center. The reactor consists of 12 modules; each one is a 3-meter-high tube with carbon brushes on the inside that serve as the anode. The wall of the tube is a membrane that facilitates the transport of electrons to the outside of the cylinder, which consists of cathode-carbon brushes clamped to a stainless steel mesh. The goal of the pilot facility is to remove at least 5 kilograms of organics per cubic meter of reactor volume per day. "Depending on this removal, we can achieve power production of up to 500 watts continuously," Rabaey explains. "But power is always the secondary target. In this first phase we want to clean the wastewater. MFCs clean wastewater in an energy efficient way and without generating much sludge. That is where the real benefit is."
Jessica Ebert is a Biomass Magazine staff writer. Reach her at firstname.lastname@example.org or (701) 746-8385.