Where Einstein Meets Edison

Microbial Fuel Cells: A Potential Platform for the New Decade

Microbial Fuel Cells: A Potential Platform for the New Decade

Sep 14, 2010

From the rise of social media and hybrid cars to the completion of the Human Genome Project, scientists, engineers, and entrepreneurs have made great strides over the last decade. We should be proud.

Yet, despite our successes, we solved so few of the developing world’s most pertinent problems. Nearly half of the world’s population, more than 3 billion people, lives without access to electricity. In addition, over a third of the world’s population lives without adequate sanitation facilities and a sixth lives without clean drinking water. These conditions are not only disastrous for socio-economic development, but create significant health hazards. Indoor air pollution resulting from the combustion of solid fuels used in cooking kills 1.5 million people a year worldwide, most of whom are children. To put this into perspective, this is nearly twice the number of deaths caused by malaria each year. In addition, water-related diseases are the leading cause of death for children worldwide. However, the lack of remote power generation and insufficient access to clean water in the developing world have something in common aside from their debilitating effects: a potential solution.

Microbial fuel cells (MFCs), a class of devices that convert organic matter such as sugars and other compounds directly into electricity, have the potential to revolutionize water treatment and remote power generation and could even serve as power sources for medical implants and microscale machines. Over the past several years, a surprisingly collaborative network of academic labs and startups, many of which are right here in Massachusetts, have begun to explore the commercial applications of this remarkable technology. While each product is at a different stage of development, ranging from pilot-scale plants for water remediation to brainstorming sessions for microscale power sources, the underlying technology is the same.

Microbial fuel cells function by harnessing the metabolic pathways of certain species of bacteria to catalyze the conversion of organic fuel to electrical energy. Their basic design consists of an anaerobic chamber containing the anode, bacteria, and organic compounds, an aerobic chamber containing the cathode, a proton-permeable membrane separating the chambers, and a wire connecting the anode to the cathode. This system produces electricity because the bacteria in the anode chamber are able to respire to an insoluble extracellular electron accepter, i.e., the anode, under anaerobic conditions. The protons created as a byproduct of this extracellular respiration pass through the permeable membrane, where they combine with oxygen and the electrons that travelled along the wire to cathode to produce water. Therefore, by placing a load on the wire that connects the anode and cathode, one is able to draw electric power from the system.

While this incredible phenomenon was first observed in 1911 by M.C. Potter, a professor of botany at the University of Durham, it was not until the mid-1980s that the efficiency of extracellular electron transfer in the anode became high enough to warrant sustained academic research. After a failed attempt by NASA in the 1960s to use microbial fuel cells as an energy-positive waste disposal system, H. Peter Benetto realized that one could add chemical mediators to assist the microorganisms in shuttling electrons to the anode. However, because of the cost and toxicity of these artificial mediators, commercial applications still remained out of reach. Then something truly remarkable happened.

In the early 2000s, several research groups, including Derek Lovley’s at UMASS Amherst, discovered that certain species of bacteria, and even whole genera such as Geobacter and Rhodoferax, could respire directly to the anode of an MFC. Not only did this eliminate the need for mediators, but the biofilms that these bacteria formed were significantly more efficient at transferring electrons to the anode. As it turned out, the bacteria were growing their own nanowires. These electrically conductive pilli, long chains of cytochrome-like proteins, allow sedimentary bacteria to transfer electrons amongst each other and respire to metals in their natural environment. More importantly however, by placing these bacteria in an MFC, one can draw electric current directly from their metabolism through these pilli.

Bruce Logan, the Kappa Professor of Environmental Engineering at Penn State University, was one of the first to see the commercial potential of microbial fuel cells after this discovery. Since 2004, he has been investigating the use of MFCs as an energy-positive wastewater treatment method. The idea behind this application is that the organic waste present in the water, whether sewage or industrial byproducts, is consumed by the MFC as fuel. This in turn powers the other components of the treatment system and could even sell power back to the grid. This idea has spawned numerous startups focused on revamping the agricultural, municipal, and industrial sectors of the multi-billion dollar wastewater treatment industry. While many of these ventures are gearing up to build pilot facilities in the coming year, IntAct Labs of Cambridge, MA stands out.

IntAct’s co-founder and CEO, Matt Silver, has been thinking about the commercial applications of microbial fuel cells for as long as anyone. He founded IntAct Labs in 2006 with funding from NASA as a research development company focused on proving feasibility and developing commercialization strategies for a broad array of bioelectric technologies. However, for the past several years IntAct Labs has focused on developing MFCs for the industrial wastewater market. Since this transition, it has received funding from the NSF and the U.S. Department of Agriculture as well as been a semi-finalist in the MIT 100K Entrepreneurship Competition and in the MIT Clean Energy Prize. In fact, IntAct won the Ignite Clean Energy Business Plan Competition this past November.  Dr. Silver credits IntAct’s recent success to their strategic planning, advanced computational models of MFCs, and innovative fuel cell design; although it probably couldn’t have hurt to have Bruce Logan on its advisory board. Currently, IntAct Labs is planning to have an energy neutral industrial wastewater remediation pilot plant operating later this year. While IntAct Labs is not focused on the developing world, their technology could theoretically be refined to purify water to potable levels. Even though such a system could also provide power to those living off the grid, the MFC platform supports a solution that is even closer to brining power to the people.

Originating from Peter Girguis’ lab at Harvard University, sediment-based microbial fuel cells currently offer a promising solution for remote power generation. Unlike the MFCs used in wastewater treatment, sedimentary MFCs are fueled by renewable organic compounds found in the soil and seabed. Girguis, whose academic research focuses on the physiology and biochemistry of deep-sea microbes, originally developed these systems as way to power remote sensing equipment for his research. He quickly realized that these devices could be used to power a wide variety of marine and terrestrial remote sensors and co-founded Trophos Energy in 2008 to bring these innovations to market. While this is likely to be a successful disruptive technology for remote sensing, it is more interesting to examine whether or not it can be used to power vital technologies such as lights and cell phones in developing countries.

Currently, all signs point to yes. Lebone, a social enterprise founded by five Harvard undergraduates in 2007, modified Prof. Girguis’ sedimentary microbial fuel cell under his guidance to create a sedimentary microbial battery. Using this technology as a batch process allowed them to create a 5-gallon portable bio-battery that could charge a cell phone or power a high-efficiency LED. With evidence that their technology was sound, they applied for and won a $200,000 Development Marketplace Grant from the World Bank. With part of this money, Lebone launched a pilot program in Namibia with 100 sediment MFCs that, if watered like a garden, could generate power continuously for several months. While currently focused on providing lighting and facilitating the rapid growth of Africa’s mobile phone market, Lebone’s technology serves as a proof of concept for the possibility of an MFC powered stove. While getting such a high power output from an MFC will be a technically challenging task, it is not even close to the most ambitious applications proposed for MFCs.

Beyond the immediate commercial applications of wastewater treatment and off-grid power generation, microbial fuel cells have been proposed as solutions to a wide variety of problems, including supplying power to medical implants and microscale devices. The idea behind powering medical devices is similar to that of the wastewater treatment system: the MFC would convert organic compounds, in this case blood glucose, into electricity to power the device. On the other hand, the reason one would need an MFC-based battery to power a microscale machine is akin to using an MFC to provide electricity in the developing world: one cannot currently get it any other way. The smallest electrochemical battery humans can produce is on the scale of millimeters, yet we have produced RFID tags an order of magnitude smaller. As we continue to miniaturize devices, we will need to power them in ways that conventional batteries cannot support, but MFC-based batteries could, in theory.

While it is easy to see the challenges that face these creative applications, such as proof of reliability for powering medical devices, there are real obstacles currently facing the implementation of microbial fuel cells for wastewater treatment and remote power generation. The most pertinent of these are proof of scalability, reliability, and consistently low current densities. MFCs for wastewater treatment have proven effective on scales up to tens of liters, yet to be implemented in the wastewater industry, they would need to function at a similar efficiency scaled up several orders of magnitude. While Matt Silver of IntAct Labs acknowledges the fact that power density decreases as the system increases in size, he believes that IntAct’s pilot plant will still be power-neutral. I, for one, am not yet convinced. In addition, one of the major downsides of using a living system is that it can contract and even die under certain circumstances. This inherent uncertainty makes using MFCs for wastewater treatment a somewhat risky proposition, although the biotech industry proves it is certainly not an unreasonable risk to take. It is important to note that while reliability is also important when examining MFCs for remote power generation, it is less of an issue, as there are no practical substitutes for the service they provide. Low current densities also limit the applications for which MFCs can be used. For wastewater treatment, this translates into the maximum size of the treatment system; for remote electricity generation, low power densities inhibit the type of devices that can be charged or powered. Gregory Stephanopoulos, professor of chemical engineering at MIT and a new entrant to the field of MFCs, believes that the low power densities are not caused by limits in bacterial metabolism, but by the electron transport from the bacteria to the anode. No matter the cause, it will be necessary to overcome these low current densities if MFCs are going to revolutionize water treatment and replace the need to burn solid fuels for cooking in the developing world.

Right now, microbial fuel cells are at an inflection point. Gateway applications that have been successful in the lab, such as wastewater treatment and remote power generation, are currently being tested at scale in the field. Many of the more advanced ideas and refinements to the gateway applications, such as purifying water to potable levels, are being explored in the lab. As a result, we will know within a few years whether or not this new platform can live up to its potential. Until then, I recommend paying close attention to MFCs, as we rarely get the opportunity to witness the deployment of a technology that combines immediate disruptive capability in the developed world while enabling significant social progress in the developing world.

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