【作者】 张培远；

【导师】 刘中良；

【作者基本信息】 北京工业大学 ， 热能工程， 2011， 博士

【摘要】 微生物燃料电池（MFC）涉及到微生物学、电化学、传热传质学、流体力学和水质工程学等多个学科,是典型的多学科交叉问题。目前,MFC的研究还处于实验阶段,主要是通过实验得到MFC的某些特性,缺乏系统深入的理论与实验。另外,功率太低是MFC技术的最大缺点（比燃料电池小三个数量级）。MFC的理论基础包括工程热力学、酶动力学和传质学等。本文对MFC进行了简单的理论分析,并且搭建了双室MFC试验台,从基础实验出发逐步深入地探索输出功率的影响因素。主要工作包括:基于酶动力学理论、化学反应理论和传质学理论分析了MFC工作性能的影响因素。为了便于分析,特以底物为葡萄糖和电子受体为氧气为例,根据酶动力学理论分析了化学反应速率与底物浓度之间的关系;根据化学反应理论分析了葡萄糖氧化反应进行的程度和反应速率;根据传质学理论建立了反应物传输的简单模型,并对一个实例进行了分析。发现,在相同条件下,与葡萄糖相比,氧气的质量传输是输出功率大小的限制性因素。另外,质子传输对MFC内阻的影响不容忽视。搭建了温度可调控的MFC实验台。首先,以开路电压为参数分析了MFC的启动情况;其次,详细分析了MFC运行时阳极液和阴极液pH值和电导率的变化规律;另外,还对MFC的产电过程和能量利用情况进行了分析和计算。通过定期测试MFC运行时阳极液和阴极液pH值和电导率,得出MFC运行时的变化规律。随着MFC的运行,阳极液的pH值和电导率呈现下降的趋势,阴极液的pH值和电导率呈现上升的趋势,阳极液和阴极液的平均电导率变化不大。实验和理论分析发现MFC理想的产电过程包括三个阶段:上升期输出电压的限制性因素是阳极电化学反应速率;稳定期输出电压的限制性因素是阳极电化学反应速率、阴极电化学反应速率或者质子的质量传输速率;下降期输出电压的限制性因素是反应物的质量传输速率。能量分析发现大部分葡萄糖被非产电菌消耗,用来发电的葡萄糖的比率很小。同时,能量利用效率很低。根据燃料电池的普通模型建立了适合MFC的模型,并结合电学知识推导出了MFC内阻的构成。然后,通过对实验数据的拟合,分析了各构成对总内阻的影响。另外,根据功率曲线分析出输出功率大小的限制性因素。MFC的内阻是与输出功率相关的一个重要参数,大多文献把功率最大时对应的内阻作为MFC的内阻,作为一个重要的参数来评价MFC性能的好坏。理论分析发现,总内阻由三部分构成:活化内阻、欧姆内阻和浓差内阻。本文设计了一个实验来估计三组分对总内阻的贡献,并且采用所建模型对实验数据进行拟合。结果发现尽管内阻在一定段的电流范围内基本上是一个常数,其还是随着电流变化而变化。电流较小时除外,总内阻的最大组分是浓差内阻。当电流较小时,活化内阻随着电流的增加减小的很快,最后,接近一个常数。在电流变化的整个过程中,欧姆内阻一直是一个常数。实验还揭示出增加极限电流和减小浓差损失对提高MFC的工作性能非常重要。分别固定外阻为无穷大、500Ω和100Ω,测试了MFC运行时阳极液和阴极液的pH值和电导率的变化、伏安曲线和输出电压,并对其进行了详细的比较分析。另外,还比较了不同外阻条件下MFC的能量利用情况。电流是决定功率高低的一个重要参数。如果想提高MFC的功率,必须采用减小外阻的方式来提高电路中的电流。本文把MFC工作时的外电阻分别固定为无穷大、500Ω和100Ω来进行比较分析。另外,从质量和能量角度对实验结果进行了详细分析。当MFC长期运行在低外阻条件下时,会引起溶液pH值和电导率的极大变化。当外阻固定为100Ω时,阴极液和阳极液的pH值最大相差0.82,电导率最大相差2.14 mS/cm;当外阻固定为500Ω时,pH值最大相差0.41,电导率最大相差1.55 mS/cm。另外,前者的极限电流为2.69 mA,而后者的极限电流为3.83 mA。这些说明减小MFC运行时阴极液和阳极液的pH值和电导率的变化以及改善低外阻条件下生成物和反应物的质量传输对提高MFC的性能至关重要。分别以铁氰化钾（K₃Fe（CN）₆, 1 g/L、2 g/l）和高锰酸钾（KMnO₄、1 g/l）作为电子受体,测试了MFC运行时阳极液和阴极液的pH值和电导率的变化规律、伏安曲线和输出电压,研究了电子受体对MFC的工作性能的影响。依据反应物的物性参数的比较和以前实验的总结,阴极的反应特性是限制MFC整体输出功率的瓶颈,为了提高MFC的输出功率和整体效能,需要侧重阴极方面的研究。本文在MFC的构型和运行条件都相同的基础上,分别采用铁氰化钾和高锰酸钾作为电子受体,测试MFC的pH值、电导率、输出电压和COD随时间的变化情况以及极化曲线。根据以上实验数据,对不同电子受体时MFC的性能进行了比较分析。结果发现,当高锰酸钾作为电子受体时,MFC的工作性能最好。然而,当高锰酸钾作为电子受体时,溶液电导率和pH值的变化幅度都非常大,这是由电路中大的电流引起的。另外,推导出了底物降解的公式。更多还原

【Abstract】 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.更多还原