External CO₂ and water supplies for enhancing electrical power generation of air-cathode microbial fuel cells

Alkalization on the cathode electrode limits the electrical power generation of air-cathode microbial fuel cells (MFCs), and thus external proton supply to the cathode electrode is essential to enhance the electrical power generation. In this study, the effects of external CO₂ and water supplies to the cathode electrode on the electrical power generation were investigated, and then the relative contributions of CO₂ and water supplies to the total proton consumption were experimentally evaluated. The CO₂ supply decreased the cathode pH and consequently increased the power generation. Carbonate dissolution was the main proton source under ambient air conditions, which provides about 67% of total protons consumed for the cathode reaction. It is also critical to adequately control the water content on the cathode electrode of air-cathode MFCs because the carbonate dissolution was highly dependent on water content. On the basis of these experimental results, the power density was increased by 400% (143.0 ± 3.5 mW/m² to 575.0 ± 36.0 mW/m²) by supplying a humid gas containing 50% CO₂ to the cathode chamber. This study demonstrates that the simultaneous CO₂ and water supplies to the cathode electrode were effective to increase the electrical power generation of air-cathode MFCs for the first time.