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Article updated on 21/01/20 by Kerry Taylor-Smith
The continuing climate change crisis means that research into sources of alternative renewable energy generation is a top priority. Researchers at the Biodesign Institute at the Arizona State University have used the tiniest organisms on the planet – bacteria – as a viable option to make electricity.
In a 2008 study featured in the journal Biotechnology and Bioengineering, lead author Andrew Kato Marcus and colleagues César Torres and Bruce Rittmann gained critical insights that could lead to commercialization of a promising microbial fuel cell (MFC) technology, which could reduce the reliance on fossil fuels.
Any Kind of Waste can be Used to Generate Electricity
“We can use any kind of waste, such as sewage or pig manure, and the microbial fuel cell will generate electrical energy,” says Marcus, a civil and environmental engineering graduate student, and member of the institute’s Center for Environmental Biotechnology.
Microbial Fuel Cell and Water-Based Organic Fuels
Unlike conventional fuel cells that rely on hydrogen gas as a fuel source, the microbial fuel cell can handle a variety of water-based organic fuels.
“There is a lot of biomass out there that we look at simply as energy stored in the wrong place,” says Rittmann, the director of the center.
“We can take this waste, keeping it in its normal liquid form but allowing the bacteria to convert the energy value to our society’s most useful form, electricity. They get food while we get electricity.”
Bacteria have such a rich diversity that researchers can find a bacterium that can handle almost any waste compound in their daily diet. By linking bacterial metabolism directly with electricity production, the MFC eliminates the extra steps necessary in other fuel cell technologies.
“We like to work with bacteria because bacteria provide a cheap source of electricity,” Marcus says.
How Do Microbial Fuel Cells Work?
There are many types of MFC reactors and research teams throughout the world. However, all reactors share the same operating principles. All MFCs have a pair of battery-like terminals: an anode and cathode electrode. The electrodes are connected by an external circuit and an electrolyte solution to help conduct electricity. The difference in voltage between the anode and cathode, along with the electron flow in the circuit, generates electrical power.
In the first step of the MFC, an anode-respiring bacterium breaks down the organic waste to carbon dioxide and transfers the electrons released to the anode. Next, the electrons travel from the anode through an external circuit to generate electrical energy. Finally, the electrons complete the circuit by traveling to the cathode, where they are taken up by oxygen and hydrogen ions to form water.
What is the Matrix?
“We knew that the MFC process is relatively stable, but one of the biggest questions is: How do the bacteria get the electrons to the anode?” Marcus says.
The bacteria depend on the anode for life. The bacteria at the anode “breathe” the anode, much like people breathe air, by transferring electrons to the anode. Because bacteria use the anode in their metabolism, they strategically position themselves on the anode surface to form a bacterial community called a biofilm.
Bacteria in the biofilm produce a matrix of material so that they stick to the anode. The sticky biofilm matrix is rich with material that can potentially transport electrons; it is made up of a complex of extracellular proteins, sugars, and bacterial cells. The matrix also has been shown to contain tiny conductive nanowires that could help stimulate electron conduction.
“Our numerical model develops and supports the idea that the bacterial matrix is conductive,” Marcus says.
In electronics, conductors most commonly are made up of materials such as copper that make it easier for a current to flow through.
“In a conductive matrix, the movement of electrons is driven by the change in the electrical potential,” Marcus says.
Like a waterfall, the resulting voltage drop in the electrical potential pushes the flow of electrons.
The Relationship Between the Biofilm Matrix and Anode
The treatment of the biofilm matrix as a conductor allowed the team to describe the transport of electrons driven by the gradient in the electrical potential. The relationship between the biofilm matrix and the anode could now be described by a standard equation for an electrical circuit: Ohm’s law.
Within the MFC is a complex ecosystem where bacteria are living within a self-generated matrix that conducts the electrons.
“The whole biofilm is acting like the anode itself, a living electrode,” Marcus says. “This is why we call it the ‘biofilm anode’.”
Life at the Jolt
The concept of the “biofilm anode” allowed the team to describe the transport of electrons from bacteria to the electrode and the electrical potential gradient. The importance of electrical potential is well-known in a traditional fuel cell, but its relevance to bacterial metabolism has been less clear. The next important concept the team had to develop ways to understand the response of bacteria to the electrical potential within the biofilm matrix.
The Growth and Metabolism of Bacteria
Bacteria will grow as long as there is an abundant supply of nutrients. Jacques Monod, one of the founding fathers of molecular biology, developed an equation to describe this relationship. While the team recognized the importance of the Monod equation for bacteria bathed in a rich nutrient broth, the challenge was to apply the Monod equation to the anode, which is solid.
Previous studies have shown that the rate of bacterial metabolism at the anode increases when the electrical potential of the anode increases. The researchers could now think of the electrical potential as fulfilling the same role as a bacterial nutrient broth. The team recognized that the electrical potential is equivalent to the concentration of electrons – and the electrons are precisely what the bacteria transfer to the anode.
Rate of Bacterial Metabolism and Concentration of Electrons
Equipped with this key insight, the team developed a new model, the Nernst-Monod equation, to describe the rate of bacterial metabolism in response to the “concentration of electrons” or the electrical potential.
Promise Meeting Potential
In their model, the team identified three crucial variables to controlling an MFC: the amount of waste material (fuel), the accumulation of biomass on the anode and the electrical potential in the biofilm anode. The third factor is a novel concept in MFC research.
“Modeling the potential in the biofilm anode, we now have a handle on how the MFC is working and why,” Marcus says. “We can predict how much voltage we get and how to maximize the power output by tweaking the various factors.”
For example, he says, the team has shown that the biofilm produces more current when the biofilm thickness is at a happy medium, not too thick or thin.
“If the biofilm is too thick, the electrons have to travel too far to get to the anode,” he says. “On the other hand, if the biofilm is too thin, it has too few bacteria to extract the electrons rapidly from the fuel.”
To harvest the benefits of MFCs, the research team is using its innovative model to optimize performance and power output. The project, which has been funded by NASA and industrial partners OpenCEL and NZLegacy, lays out the framework for MFC research and development to pursue the commercialization of the technology.
MFC development has been limited by low power generation, expensive electrode materials and the inability to scale up to industrially relevant capacities. However, advanced electrode materials, such as 2D nanomaterials, could move the field forward.
Source: Arizona State University
Last Update 20th January 2008
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