Carbon Dioxide Addition to Microbial Fuel Cell Cathodes Maintains Sustainable Catholyte pH and Improves Anolyte pH, Alkalinity, and Conductivity

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 in contrast to the CO₂/carbonate buffered catholyte system by Torres, Lee, and Rittmann [Environ. Sci. Technol.2008, 42, 8773]. We operated an air-cathode and liquid-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 liquid-cathode system, we sustained a steady catholyte pH (pH = 5.94 ± 0.02) and a low pH imbalance (ΔpH = 0.65 ± 0.18) over a 2-week period without external salt buffer addition. By migrating bicarbonate ions from the cathode to the anode (with an anion-exchange membrane), we increased the anolyte pH (ΔpH = 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/cm) relative to the feed properties. We also verified with a phosphate-buffered MFC that our reaction rates were limited mainly by the reactor configuration rather than limitations due to the bicarbonate buffer.