Location: Bioenergy ResearchTitle: Carbon Dioxide Addition to Microbial Fuel Cell Cathodes Maintains Sustainable Catholyte pH and Improves Anolyte pH, Alkalinity, and Conductivity Author
Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2010
Publication Date: 3/1/2010
Citation: Fornero, J.J., Rosenbaum, M., Cotta, M.A., Angenent, L.T. 2010. Carbon Dioxide Addition to Microbial Fuel Cell Cathodes Maintains Sustainable Catholyte pH and Improves Anolyte pH, Alkalinity, and Conductivity. Environmental Science and Technology. 44(7):2728-2734. Interpretive Summary: Interest in the production of energy from agricultural resources has increased markedly in recent years in response to concerns over the high cost and limited supply of petroleum. Bioelectrochemical systems (BES) such as microbial fuel cells are gaining importance as a means of producing power from a wide variety of feedstocks, but improvements in efficiencies of these systems are needed if they are to gain widespread acceptance. During typical operation, pH imbalances develop in BESs due to proton-generating oxidation reactions in the anode chamber and hydroxide-ion-generating reduction reactions in the cathode chamber. Until now, workers added unsustainable buffers to reduce the pH difference between the anode and cathode because the pH imbalance contributes to potential losses and, therefore, power losses. Here, we report that adding carbon dioxide (CO2) gas to the cathode, creates a CO2/bicarbonate buffered system. CO2 addition resulted in a stabilization of pH and a 152% increase in steady-state power density. These findings will be useful in advancing the BESs toward practical application.
Technical Abstract: Bioelectrochemical system (BES) pH imbalances develop due to anodic proton-generating oxidation reactions and cathodic hydroxide-ion-generating reduction reactions. Until now, workers added unsustainable buffers to reduce the pH difference between the anode and cathode because the pH imbalance contributes to BES potential losses and, therefore, power losses. Here, we report that adding carbon dioxide (CO2) gas to the cathode, which creates a CO2/bicarbonate buffered catholyte system, can diminish microbial fuel cell (MFC) pH imbalances. We operated an air-cathode and catholyte-cathode MFC side-by-side. For the air-cathode MFC, CO2 addition resulted in a stable catholyte film pH of 6.61 ± 0.12 and a 152% increase in steady-state power density. By adding CO2 to the catholyte-cathode system, we sustained a steady catholyte pH (pH = 5.94 ± 0.02) and a low pH imbalance (delta pH = 0.65 ± 0.18) over a 2-week period, without external salt buffer addition. With the migration of bicarbonate ions (with an anion-exchange membrane), we increased the anolyte pH (delta pH = 0.39 ± 0.31), total alkalinity (494 ± 6 to 582 ± 6 as mg CaCO3/L), and conductivity (1.53 ± 0.49 to 2.16 ± 0.03 mS) relative to the feed properties. We also verified with a phosphate-buffered catholyte MFC that our reaction rates were limited mainly by the reactor configuration rather than limitations due to the bicarbonate buffer. However, we predict that with an enhanced configuration, the concentration of bicarbonate must be increased to optimize the CO2/bicarbonate system.