Certain kinds of bacteria are adept at converting waste into useful energy. These microorganisms are presently being applied to the task, through an innovative technology known as a microbial fuel cell or MFC. An MFC can perform double duty, targeting electrons from waste streams and converting them into useful energy.
An MFC is a unique kind of battery—part electrochemical cell, part biological reactor. Typically, it contains two electrodes, separated by an ion exchange membrane. On the anode side, bacteria grow and proliferate, forming a dense cell aggregate known as a biofilm that adheres to the MFC’s anode. In the course of their microbial metabolism, the bacteria act as catalysts for converting the organic substrate into CO2, protons, and electrons.
Under natural conditions, many bacteria use oxygen as a final electron acceptor to produce water, but in the oxygen-free environment of the MFC, specialized bacteria that send the electrons to an insoluble electron acceptor, namely the MFC’s anode, dominate.
The anode-respiring bacteria are able to oxidize organic pollutants, such as those found in waste streams, and transfer the electrons to the anode. The scavenged electrons then flow through an electrical circuit, terminating at the MFC’s cathode, thus generating electricity. Ions are transported through the fuel cell’s ion membrane, to maintain electroneutrality, although the membrane is often excluded. The basic setup is pictured in Figure 1.
In an effort to refine the technology and address losses in MFC efficiency, a group of researchers looked at the oxygen reduction reaction at the MFC cathode. While it had earlier been speculated that efficiency loss at the cathode was due to a low availability of protons, the new research showed instead that the transport of hydroxide ions (OH-) away from the catalyst layer of the cathode and into the surrounding liquid largely governed cathode potential losses in the device.
“We found that the cathodes were limiting the power densities we can produce in these MFCs,” Popat says. “This is very surprising because, in chemical fuel cells, the same catalyst allows much greater power densities.”
A key to the disparity lies in the fact that MFC’s, unlike chemical fuel cells, must operate at neutral pH in the anode chamber to ensure optimum growth and activity of the microorganisms catalyzing the reactions. At the cathode, OH- ions cause a local increase in pH, due to a limiting rate of their transport. Further, every unit of pH increase at the cathode results in a loss of 59 millivolts of energy—the authors found that the local cathode pH could easily reach >12, representing a substantial loss.
To attempt to remedy this situation, the group conducted a detailed examination of transport properties at the cathode. An ion exchange binder contained in the cathode usually assists transport of ions to the surrounding electrolyte. Normally, this binder is made from a material called Nafion, which the authors explain is good for transporting positively charged cations like protons, but a poor conductor of negatively charged anions like the hydroxide ions that accumulate at the MFC cathode, or anionic buffer species, such as phosphates and bicarbonates, that help transport OH- ions.
An experimental polymer known as AS-4, which has high anion-exchange capacity, was substituted for Nafion as a cathode binder in the study. The modification ensured the efficient transport of hydroxide ions and improved the performance of the cathode. The study showed that OH- transport could be further enhanced by adjusting pH directly, though the addition of CO2 mixed with air as a buffer for the cathode catalyst.
In the future, MFC’s may be linked to municipal waste streams or sources of agricultural and animal waste, providing a sustainable system for waste treatment and energy production.
This story is reprinted from material from Arizona State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.