Cathodic Performance Improvement And Transport Characteristics Of Microbial Fuel Cells

MFCs are considerd as a promising alternative power source due to its unique capability of simultaneous waste water treatment and electricity generation and its prospect of reducing the operating fees of wastewater treatment.Although the performance of MFCs has been recently improved significantly, it is still orders-of-magnitude lower than that of chemical fuel cells. It is found that the cathodic loss, especially the cathodic activation loss, is the one of the most important factors dominate the MFC performance. Hitherto, oxygen is the most widely used cathodic electron acceptor in MFCs due to its unlimited availability in the environment and high redox potential. However, the unfavorable reaction kinetics of oxygen reduction on the surface of carbon electrodes and the low mass transfer coefficient of gaseous oxygen in catholyte severely hamper its application. In addition to oxygen, many soluble electron acceptors with high reduction rate on carbon electrodes have been also adopted in MFCs, such as hydrogen peroxide, ferricyanide, and permanganate due to theirs high redox potentials. However, the application of these cathodic electron acceptors in MFC is also limited because they can not be regenerated under the condition of without additional power input.In this paper, two types of MFCs with two novel soluble electron acceptors (potassium persulfate and triiodide ion) were proposed. The MFC using potassium persulfate as electron acceptor can treat oxidative and organic wastewater at the same time, whereas the MFC with triiodide is a new system that the cathodic electron acceptor can be regenerated without additional energy input, except free sunlight illumulation. The main results are summerized below:â‘ It was found that the electricity generation of MFC can be mainly attributed to the biofilms attached on the anode, rather than the suspended microorganism cells in the anolyte.â‘¡The maximum power density of MFCs with fresh K2S2O8 solution and K3[Fe(CN)6] solution as electron acceptor was 83.9 mW m^-2 and166.7 mW m^-2, respectively. After 2 days operation under an external resistance 1000 ohm, the maximum power density of the MFC using K3[Fe(CN)6] solution as the cathodic acceptor droped slightly, while almost two times increase in the maximum power density was observed in the case of the K2S2O8 used MFC. By comparing the results of the cathode and anode polarization curves before and after operation, it was found that the overall cell performance improvement was resulted from the increased cathode performance. It was also demonstrated that the enhanced cathode performance can be mainly attributed to the decrease of the pH value in the cathode chamber as a consequence of the hydrolysis of the persulfate ion in aqueous solution.â‘¢MFC performance increased with the increasing K2S2O8 concentration and the decreasing pH value. While the concentration of H⁺ and K⁺ was too high, the MFC performance could also be reduced as the K⁺ and H⁺ in the cathode chamber could penetrate the proton exchange membrane to anode chamber and caused unfavorable growth condition for microorganism.â‘£A renewable MFC cathode system was proposed and construted by incorporating I₃^-/I- redox couple into cathode. It was demonstrated that I₃^- could be regenerated by the reaction between oxygen and I- under sunlight, which significantly improved the MFC performance. The extended operation of the MFC showed that I₃^- is capable of supporting the MFC for long-term electricity generation.⑤Linear sweep voltammetry showed that a current density of 4.2 mA cm^-2 can be obtained from the electroreduction of I₃^-. This value was approximately twice that of K3[Fe(CN)6] and was indepent of the pH of the electrolyte. It was also found that the mass transfer of I₃^- was the main factor influencing the MFC performance. The effect of regeneration condition, such as the pH of the KI solution, KI concentration, oxygen flow rate and the light intensity, on the I₃^- yield and MFC performance was also investigated. A sufficient supply of I₃^- can be achieved when the concentration of an air-saturated KI solution was greater than 0.2 M and its pH as around 2.0, under an irradiation higher than 300 mW cm^-2.â‘¥The effect of startup mode on the MFC performance was also investigated.It was found that the MFC startup time was shorter and the maximum power density using continuous startup mode was twice that of MFC using batch startup mode. It was observed that the current density and voltage could drop severely with the decrease of external resistance due to the severe I₃^- transfer limitation under high current densities. We also demonstrated that this situation can be relieved by stirring the catholyte or using the electron acceptor with high concentrations.⑦The MFC performance can be improved by changing MFC structure and using suitable electron acceptor. It was found that the performance of the flat plate MFC with serpentine field was twice higher than that of MFC using interdigited field; the performance of the rectangular MFC was higher than that of the H-type MFC and flat plate MFC; and the MFC performance with I₃^- as electron acceptor was higher than that of MFC used K3[Fe(CN)6] and oxygen. The maximum power density of rectangular MFC with 20 mM I₃^- as electron acceptror is 2000 mW m^-2.