"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 energythe 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.
The study represents the first comprehensive analysis of cathode limitations in MFC's and will further the development of these systems through refinement of materials and operating
|Contact: Joseph Caspermeyer|
Arizona State University