A Theoretical And Experimental Study Of Microbial Fuel Cell Energy Characteristics

Microbial fuel cell (MFC) is a typical interdisciplinary problem which mainly relates to microbiology, electrochemistry, heat and mass transfer, fluid mechanics, and water quality engineering. At present, the study on MFCs is still in experimental stage. The characteristics of MFCs are obtained mainly from experimental observations and there is a lack of systematic and profound experimental and theoretical investigations. In addition, low power output is the biggest problem of the MFC technology (three orders of magnitude smaller compared with a normal fuel cell). The theoretical basis of MFCs involves engineering thermodynamics, enzyme kinetics and mass transfer, et al. In this article, a simple theoretical analysis is given, a double-chamber MFC experimental rig is built and the factors affecting power output are investigated through a series of experiments. The main work are summarized as follows:The factors affecting the MFC performances were analyzed from enzyme kinetics, chemical reaction theory and mass transfer. In order to facilitate the analysis, glucose is used as the substrate and oxygen as the electron acceptor. The relationship between substrate concentration and chemical reaction rate was analyzed based on enzyme kinetics; the extent of glucose oxidation reaction and reaction rate were also analyzed based on chemical reaction theory; A simple model of reactant transfer was established according to mass transfer theory, which was used to analyze a case of other experimental results, and found that under the same conditions, the mass transfer of oxygen is the limiting factor of the MFC power output compared to glucose. In addition, the impact of proton transfer on the MFC internal resistance can not be ignored.A temperature-adjustable MFC experimental rig was built. Firstly, the start-up situation of the MFC was analyzed using open-circuit voltage as the parameter; secondly, the pH and ionic conductivity variation of the anode and cathode solution during the MFC operation was analyzed in detail. In addition, producing electricity process and energy utilization were analyzed and calculated in detail. The pH and ionic conductivity variation of the anode and cathode solution were obtained by testing them regularly. The pH and ionic conductivity of the anode solution decreased, while that of the cathode solution increased with the MFC operation, and the average ionic conductivity of which changed slightly. Electricity generation of the MFC ideally includes three phases. At the ascending phase, the rate of anodic electrochemical reaction is the limiting factor of the voltage; at the stationary phase, the rate of anodic electrochemical reaction, the rate of the proton mass transfer or the rate of cathode electrochemical reaction is the limiting factor of the voltage; at the declining phase, the rate of the reactant mass transfer becomes the limiting factor of the voltage. According to energy analysis, most glucose was consumed by other microorganisms. A small amount of glucose was used to product electricity. Meanwhile, energy efficiency is very low.The model being suitable for an MFC was obtained based on the general fuel cell model, and the various components of the total internal resistance were analyzed combined with electrical knowledge. Then, the impact of the various components on the total internal resistance was analyzed by using the model to fit experimental data. In addition, the limiting factor of the power output was found out according to the power curve. The internal resistance of a MFC is closely related to the power output. The particular internal resistance that results in the largest power output is used as an important parameter to evaluate MFC performances in most literatures. According to theoretical analysis, the total internal resistance consists of three parts, activation loss internal resistance (AIR), ohmic loss internal resistance (OIR) and concentration loss internal resistance (CIR). The experimental investigations were completed to estimate the contributions of these three components to the internal resistance, and the model was used to fit the experimental data. The result shows as follows: the internal resistance is found to vary with electric current, although it is almost a constant for the current is within a certain region. The largest component of the internal resistance is CIR except for small currents. The AIR decreases quickly for small current and reduces its decreasing rate as the current increases and approaches to a constant. The OIR is constant over the whole current range. The experiments also disclose that increasing the limiting current and reducing the concentration loss are both important for improving the MFC performance.The pH and ionic conductivity of the anode and cathode solutions, the polarization curve and the voltage were tested and compared in detail when the external resistance was fixed at infinity, 500Ωand 100Ω, respectively. In addition, the energy utilization was also compared under the different external resistance condition. The current is an important parameter in determining the power output, and which must be increased by reducing the external resistance in order to increase the power output. The MFC performance was analyzed when the external resistance was fixed at infinity, 500Ωand 100Ω. In addition, experimental results were analyzed in detail from the perspective of mass and energy. The solution pH and ionic conductivity changed greatly when the MFC was operated at low external resistance in a long time. When the external resistance was fixed at 100Ω, the maximum pH difference of the cathode and anode solution was 0.82, and the maximum ionic conductivity difference of which was 2.14 mS/cm; When the external resistance was fixed at 500Ω, the maximum pH difference of the cathode and anode solution was 0.41, and the maximum ionic conductivity difference of which was 1.55 mS/cm. In addition, limiting current of the former was 2.69 mA, while that of the latter was 3.83 mA. These indicate that reducing the changes of the solution pH and ionic conductivity during the MFC operation and improving the mass transfer of the reactants and products at low external resistance are very important to enhance the MFC performance.The pH and ionic conductivity of the anode and cathode solutions, the polarization curve and the voltage were tested when the electron acceptor was Potassium ferricyanide (K₃Fe(CN)₆, 1 g/L, 2 g/l), potassium permanganate (KMnO₄, 1 g/l). And the impact of electron acceptor on the MFC performance was studied. Based on the comparison of reactant's physical parameters and the summary of previous experiments, the characteristics of the cathode reaction are the bottleneck that limiting the MFC power output. In order to improve the performance or power output of the MFC, the research should focus on the cathode. Potassium ferricyanide and potassium permanganate were used as the electron acceptor in the same MFC at the same operating conditions, and the variation of the pH, ionic conductivity, voltage output and COD with time as well as polarization curve were tested in this paper. Based on the above experimental data, the MFC performance at different electron acceptors was compared. Results show as follows: the MFC performance was the best when potassium permanganate was used as the electron acceptor. However, the pH and ionic conductivity changed largely too, and that was caused by a great current. In addition, the equation used to describe the substrate degradation was deduced.