【作者】 张亮；

【导师】 朱恂； 李俊；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 众所周知，能源问题和环境问题是21世纪人类面临的两大难题，它们严重制约着人类的生存和社会的可持续发展。传统的化石能源日益趋近于枯竭，而且环境问题尤其是水体污染日益突出。此外传统污水处理行业一直是“高投入、零产出”。因此，寻求清洁的新能源和新的污水处理工艺是当务之急。在此背景下，微生物燃料电池（Microbial Fuel Cell，MFC），一种可以将废水中有机物所蕴含的能量进行回收利用的新型可再生能源装置，以其处理污水的同时回收电能这一独特的优势应运而生，这为环境保护、提高能源利用率和发展可持续能源都具有重大意义。近些年来MFC技术发展很快，其功率密度从0.1mW m-2被提升到6800mWm-2，但这还不足以面向实际应用。因此，为了进一步提升MFC性能，研究者们对MFC性能影响因素进行了大量实验研究。影响MFC性能的因素很多，其中最重要的因素之一就是MFC中物质传输尤其是质子的传输。研究者表明阳极生物膜内质子的传输是MFC性能的限制性因素，而且阴极pH对空气阴极的性能影响很大。随着MFC的运行，较差的质子传输效果导致MFC阴、阳极之间的质子梯度不断增大，极大地限制了MFC的性能。同时，面临未来实际应用，需要对MFC进行放大化研究从而提高MFC的功率，而且从经济和环保角度而言，质子交换膜和磷酸缓冲液不再适用于未来放大化的MFC。针对以上问题，本文从工程热物理学科角度出发，立足于强化物质传输和提升MFC性能，设计了多种结构的MFC反应器，针对不同传质类型的MFC传输特性及性能特性进行了研究。研究内容主要分为三个部分：（1）采用电极阵列电极对矩形MFC进行放大化研究，并研究了不同阵列排列形式（叉排或顺排）对升级MFC启动、性能、电流分布和污水处理效果的影响，同时还揭示了放大化MFC阳极存在的电流分布不均现象；构建了三合一膜电极式MFC，并研究了阳极不同传质形式对其启动和性能的影响；（2）构建基于对流扩散传质的平板式MFC，研究了不同外接电阻启动条件下MFC启动特性、阳极生物膜成膜及物质传输特性和产电特性；构建了平板式MFC串联电堆并研究了电堆的性能及限制性因素，针对串联电堆存在的子电池反极现象提出了改善措施；（3）构建了通流式MFC，研究了其质子传输特性和性能特性，并对其阳极生物膜传输特性进行数值模拟；构建空气阴极通流式MFC，研究了电解液流量、底物浓度、电解液离子强度和阴极圆孔柱结构对空气阴极通流式MFC性能的影响；构造了漂浮式空气阴极环流MFC，研究了无缓冲液下环流MFC运行的可行性及循环流速对MFC质子传输、性能及污水处理效果的影响。主要研究成果如下：1)研究了阵列电极排列方式对升级MFC启动及性能的影响。研究结果表明：与采用顺排电极阵列的MFC相比而言，采用叉排电极阵列的MFC不但启动速度较快，启动完成后最高功率密度（23.8W m-3）要高24.6%；叉排和顺排电极阵列方式的MFC均出现明显的阳极电流分布不均现象，但是电极阵列采用叉排的MFC阳极电流分布不均匀程度稍小；序批方式下，两种排列方式的MFC COD去除率相近似，均可高达81%以上，但是电极采用叉排方式排列的MFC库伦效率较高。2)揭示了放大化后MFC阳极电流密度分布不均现象。实验结果表明，升级电极阵列MFC中，阳极电流沿着阴、阳极两电极距离的方向出现不均匀分布现象：距离阴极电极越远的阳极电极对电池的总电流贡献越小，而且其不均匀分布程度随着电池的电流增加而加剧。分析研究表明，这主要是由于阳极各部分电极与阴极电极不同的距离导致其欧姆内阻分布均匀，在启动阶段致使阳极各部分电极上形成的阳极生物膜不均匀。在产电过程中，欧姆内阻和阳极生物膜分布不均最终导致了阳极电流分布不均。针对电流分布不均，可采用增加阳极电解液的COD浓度或者离子强度来减小MFC阳极电流分布不均的程度。3)研究了阳极传质形式对三合一矩形MFC性能的影响。研究表明，由于PEM膜面积增加和阴、阳极电极间间距小导致其最高功率密度（2149.0mW m-2）高于实验室前期H型MFC（310mW m-2）和矩形MFC（745mW m-2）。与采用大腔室（扩散传质）的MFC相比，阳极采用蛇形流道（对流扩散传质）的三合一矩形MFC不但启动速度较快，而且最大功率密度要高24.5%。4)研究了不同外接电阻启动条件下MFC启动特性、阳极生物膜成膜特性和性能特性。研究结果表明，采用较小的电阻启动，启动过程中电流较大，但启动速度较慢；启动过程中的能量获得不同会导致MFC阳极生物膜中活性生物量和EPS成分含量的不同，从而导致生物膜的结构有所不同。采用较小外阻启动的MFC阳极生物膜具有较大的生物量和较大的EPS含量，呈现出较厚生物膜厚度，最终导致生物膜电化学活性较高，MFC最大功率密度也较大；然而，当启动外阻降低到过小值时，MFC阳极生物膜中EPS含量剧增，然而活性生物量反而减少。同时，生物膜的多孔隙结构有利于物质的传输，但也导致了生物膜导电性的降低。这最终导致了采用过小外阻启动的MFC虽然具有较大的电流密度，但是其最大功率密度较低。5)研究了物质传输对平板式MFC阳极生物膜成膜及性能的影响。研究结果表明，流场板结构使底物分布在槽道处，这导致阳极生物膜主要分布在与槽道相对应的碳布表面。同时，较厚的生物膜阻碍了物质向碳布内侧的传输，导致生物膜仅分布在碳布电极表面；采用蛇形流道MFC的最大功率密度随着阳极底物流速的增加先急剧增加后基本维持不变，随着阳极底物浓度的增加先急剧增加后逐步减小，阳极采用交指流场后由于较佳的物质传输致使MFC性能提高14.8%；由于交指流道较佳的传质和平板式电池结构较小的内阻，致使阴极采用交指流道的MFC的最大功率密度相对于H型MFC大大提高，其电池性能随阴极电子受体浓度的增加而增加，而几乎不受阴极水力停留时间的影响。6)研究了平板式MFC串联电堆的性能。平板式MFC串联堆在电压高达2.11V时到达最高功率密度（2226mW m-2）。然而，在较大电流时发生的子电池电压反极现象限制了串联堆功率密度的进一步增加；适度增加反极电池阴阳极电解液流量可促使电堆性能大幅度提高；采用混联方式运行可提高电堆的可运行的最大电流，一定程度上避免了子电池电压反极现象，从而提升了其性能；移除电压反极的子电池并不能有效地避免反极现象的发生；反接反极电池反而会进一步加剧反极电池的反极，致使电堆性能更低。7)构建新型质子传输方式的通流式MFC并研究了其质子传输及性能特性。研究结果表明，由于纺织物的可渗透性，在流动的情况下，强化了通流式MFC中质子从阳极到阴极的传输，从而大幅提升了MFC性能；而且，一定流量范围内，增加电解液流量会强化质子传输，导致MFC性能的提高。8)通流式MFC阳极生物膜传输特性进行数值模拟。将模拟结果与实验数据进行了比较，在小电流范围内二者基本吻合，然而在大电流然而在大电流下模拟值要高于实验值，这主要是模拟中没有考虑pH对生物膜反应动力学的影响等因素造成的。模拟结果表明，MFC阳极生物膜内电势和电流呈现一维分布，电势和电流均随着距离阳极电极板距离的增大而减小。阳极生物膜内乙酸钠浓度和pH随着阳极电极板垂直距离的增加而增加，且沿着流动方向逐步降低。随着阳极电势的增加，MFC阳极电流增加，生物膜内乙酸钠浓度和pH降低而且其分布不均匀程度增加。9)研究了空气阴极通流式MFC的性能特性。研究结果表明，在电流约为2.5mA时到达最大功率密度（约为622mW m-2），空气阴极较大的活化损失是其性能的限制性因素；在一定流量范围内，其性能随着电解液流量的增加先增加，当增加到一定程度后性能不再增加；在一定范围内，其性能随着阳极电解液COD和缓冲液浓度的增加而增加；空气阴极圆柱采用圆孔阵列结构时比采用沿着流动方向的直槽结构时MFC获得的性能要高28.6%。10)构造了漂浮式空气阴极环流式MFC，研究了采用阳极电解液循环运行方式替代磷酸缓冲液的可行性。研究结果表明，与有磷酸缓冲液条件运行下相比，无缓冲液条件下MFC（50外阻下）输出功率要低27%，最大性能要低9.7%，但是其库伦效率却要高64.2%，这表明此种运行方式在无磷酸缓冲液下时可行的，而且具有较大的应用前景；当增加电解液流量，氢离子传输明显增强，导致MFC性能和库伦效率提高；然而，当进一步增大电解液流量，由于过多的氧气传输到阳极室，这导致性能和库伦效率的降低。当电解液流量为0.35ml min-1时无磷酸缓冲液条件下MFC获得最高的最大性能为1.32mW），最大库伦效率为16.6%。更多还原

【Abstract】 It is well known that, energy shortage and environment pollution as two majorproblems, severely limit existence and sustainable development of human society in the21st century. The traditional fossil energy is drying up and brings a large amount ofcarbon dioxide emission which may lead to the Greenhouse Effect. In addition, theproblem of environment pollution, especially water pollution, becomes more and moreserious and the existing wastewater treatment process has been a “high input and zerooutput”. Therefore, how to seek a new and clean energy and a new process for treatingwastewater around global environment has been the pressing matter of the moment.Fortunately, Microbial Fuel Cells (MFCs) technology as promising alternativepower sources with the unique capability of simultaneous wastewater treatment andelectricity generation provides a practicable way for solving the above problems.Although experiencing a significant development in recent years, the powergeneration is still insufficient for the practical applications. In order to make a furtherimprovement in MFC performance, efforts have been made to investigate the affectingfactors of MFC performance. Among these factors, one of the key factors is masstransfer especially proton transfer in MFC. The previous studies reported that protontransfer inner biofilm was one of important limited factors for MFC and pH had asignificant effect on cathode performance. With a long-term run, the increasing pHgradient between anode and cathode chambers resulted from the poor proton transfer ofProton Exchange Membrane (PEM) would largely decrease MFC performance.Meanwhile, scale-up of MFC is an important consideration for future practicalapplication while the expensive PEM and un-eco-friendly phosphate buffer is notsuitable to use any longer.In this study, in order to enhance mass transfer and then improve MFCperformance, MFC reactors with different mass transfer pattern were constructed andthe performance and transport characteristics were invested. Firstly, based on “H” typeMFC with small PEM, rectangular MFC with a large PEM was constructed to enhanceproton transfer from anode to cathode. For scale-up, graphite rods arrays with inline andstaggered arrangement were used as the electrodes to constructed two liter-scale MFCs.The effects of electrode array pattern on MFC performance were investigated and theanodic nonuniform current distribution was found as well. Moreover, membrane electrode assembly typed MFC was constructed by decreasing the electrode distances toimprove power output and the effects of anodic mass transfer pattern on its performancewere also investigated. By using a convection mass transfer way, Flat Plate MicrobialFuel Cells (FPMFCs) with serpentine flow fields were constructed to enhance protontransfer inner biofilm. The effects of external resistance on the startup process, biofilmformation and electricity generation were investigated. Based on FPMFC with highpower density, a stacking microbial fuel cell with serpentine flow field in series wasconstructed and several possible measures of improving its performance were tested.Besides, to investigate the effects of proton transfer on power generation, twocontinuous-flow tubular MFCs using PEM (MFC-PEM) and textile separator (MFC-S)were operated under different anolytes (with and without buffer) and differentcatholytes (K3[Fe(CN)6] and KMnO4). The factors for the performance improvement ofMFC-S were discussed and the effects of flow rates on proton transfer and MFC powergeneration was investigated as well. For future practical application, a tubularair-cathode was used in the continuous-flow tubular MFC and the effects of anolyteflow rate, COD concentration, ionic strength and the structure of cathode hole on itsperformance were investigated. In order to make a feasible study on operating MFCunder buffer-less condition, an anolyte recirculation design strategy as an alternative tophosphate buffer in single-chamber air-cathode MFCs was proposed to enhance protontransfer while avoiding phosphate release into the environment. Two MFCs with afloating air-cathode were operated under either buffer (MFC-B) or buffer-less (MFC-BL)condition in a recirculation mode. The feasibility of this eco-friendly way and theeffects of flow rates on proton transfer, power generation and wastewater treatmentwere investigated. The main results are summarized below:1) Effect of electrode array pattern on liter-scale MFC with electrode arrays wasinvestigated. MFC with staggered electrode array (MFC-S) had a faster startup withhigher voltage output compared with MFC with inline electrode array (MFC-I).Moreover, the maximum power density (23.8W m-3) of MFC-S was approximately onequarter higher than that of MFC-I (19.1W m-3) due to the structure-induced better masstransfer of staggered array. No noticeable difference in anodic current maldistributionbetween the two MFCs was observed at a similar cell current. For a batch feeding mode,compared with MFC-I, MFC-S had a slightly higher COD removal efficiency (84.3%)but much higher coulombic efficiency (82.3%) with a nonuniform segment CEdistribution. 2) Anodic ununiform current distribution was observed in a liter-scale MFC. It isdemonstrated that the electrode spacing between the anode segment and cathodesignificantly influenced the ohmic resistance and the biomass content of each segment,further affected the anodic current distribution. A significant current maldistribution wasfound in MFC-EA, especially at high currents. The further the anode segment was awayfrom the cathode, the smaller the segment current generation contributed to the totalcurrent. Consequently, a suitable MFC structure with short and equidistant electrodespacing will be a necessary consideration for large-scale MFC design. Moreover, for thetested MFC-EA, improvement on the current maldistribution was achieved by feedingthe anolyte with a COD concentration of1000mg COD L-1or with0.2M KCl.3) Effect of anodic mass transfer pattern on the performance of membraneelectrode assembly typed MFC was studied. MFC with serpentine flow field in anode(MFC-2) had a faster startup process and a higher voltage output due to the better masstransfer of anolyte during the startup period compared with MFC with bulk chamber inanode (MFC-1). After startup, the cyclic voltammetry tests were showed that MFC-2biofilm had a higher electrochemical active behavior as the better mass transfer ofserpentine flow filed. The above results lead to a24.5%increase in maximal powerdensity of MFC-2(2676.2mW m-2) compared with MFC-1(2149.0mW m-2).4) Effect of external resistance on biofilm formation and electricity generationwas investigated. It is demonstrated that a suitable biofilm structure plays a crucial rolein the maximum power density and stable current generation of the MFCs. It is alsofound that the maximum power density of the microbial fuel cells (MFC) increasedfrom0.93W m-2to2.43W m-2when the external resistance decreases from1000to50, which may due to the increasing active biomass and thickness of biofilm. However,on further decreasing the external resistance to10, the maximum power densitydecrease to1.24W m-2because of a less active biomass and higher EPS content in thebiofilm. Additionally, the10MFC shows a highest maximum stable current of6.55Am-2. This result can be attributed to the existence of void spaces beneficial for protonand buffer transport within the anode biofilm, which maintains a suitablemicroenvironment for electrochemically active microorganisms.5) Effect of mass transfer on the biofilm formation and performance of FPMFCwas investigated. The results showed that, main anodic biofilm developed on the part ofcarbon cloth surface near the channel of flow field. Minor biofilm was observed both atthe carbon cloth surface near the rib of flow field and inner carbon cloth due to lacking of substrate for bacterial growth. FPMFC voltage dropped sharply under high currentdensity and the reason was attributed to mass transfer limitation in anode. Themaximum power density of the FPMFC increased with increasing flow rate andconcentration of the influent substrate. The performance of MFC using an interdigitatedflow field in anode was higher than that of FPMFC under the same operationalconditions due to better mass transfer process induced by the interdigitated flow field.Using triiodide anion complex（I3-）as cathodic electron acceptor, microbial fuel cellusing an interdigitated flow field in cathode (IMFC) had a higher steady cell voltageafter incubation and a larger maximal power density compared with “H” type MFC.This could be attributed to the lower internal electrical resistance and the better masstransfer property of IMFC. In addition, it was found that the maximal power density ofthe IMFC increased with the I3-concentrations and showed no relationship withhydraulic retention time.6) The performance of stacking MFC in series with serpentine flow fields wasstudied. The experimental results showed that a maximal power density of2226m Wm-2was observed at a high voltage of2.11V. A cell with low performance (MFC-R) inMFC stack would present a reversed voltage at a certain current density and then resultin a low stack cell, which was the main limitation of further improving MFC stackperformance. Although not improving the voltage reverse, an increase in the flow rate ofthe anolyte and catholyte in MFC-R result in a significantly improved power output ofMFC stack. By using a hybrid connection, the voltage reverse would be avoided,resulting in a largely increased stack performance. However, the voltage reverse cannotbe improved by removing the MFC-R and even got worse after switching it into reversein the series circuit.7) Comparative studies on proton transfer and electricity generation in twocontinuous-flow tubular MFCs using PEM (MFC-PEM) and textile separator (MFC-S)were performed experimentally. The results showed that, using K3[Fe(CN)6] catholyte,similar startup processes and minor difference on performance were observed in the twoMFCs due to minor effect of pH on cathode. However, after using KMnO4cathode,MFC-S had a significant performance improvement while a decrease in MFC-PEMperformance was observed under both buffer and buffer-less conditions resulting fromthe significant effects of catholyte pH. The main contributors for MFC-S highperformance were the enhanced proton transfer and the increased catholyte conductivityby the sequential anode-cathode flow. It was found that proton transfer can be significantly enhanced by increasing the electrolyte flow rate, largely improving MFC-Sperformance.8) A numerical simulation of mass transport in anodic biofilm based on thetubular MFC with a sequential anode-cathode flow was developed to predict MFCperformance and substance distribution in the biofilm. The simulation results basicallyagree with the experimental data at a low MFC current while the modeling values werehigher than the experimental ones at high MFC current due to lacking of the significantpH effects in Monod-Nernst equation. The numerical results indicate that the localpotential and current in biofilm decreased with increasing distance from the electrodesurface. Substrate concentration and pH increased with the increasing vertical distancefrom the electrode surface and decreased along the flow direction. Increasing anodeelectrode potential would lead to an increase in current output and drop in substrateconcentration and pH. Meanwhile, increasing nonuniform distribution in substrateconcentration and pH in the biofilm was observed after increasing the anode electrodepotential.9) Performance of air-cathode MFC with a sequential anode-cathode flow wasstudied. The results showed that MFC had a maximum power (622mW m-2) at thecurrent of2.5mA and the poor air-cathode performance limited performanceimprovement. Increasing anolyte flow rate would firstly promote power generation andthen maintain the similar level. Increasing anolyte COD concentration and bufferconcentration within a range would improve MFC power generation. MFC usingcircular hole arrays in cathode support had a higher performance than MFC usingstraight channel along the flow direction in cathode support.10) An anolyte recirculation design strategy as an alternative to phosphate bufferwas investigated in single-chamber air-cathode MFCs. It was demonstrated thatMFC-BL operated with a50external resistance in recirculation mode (1.0ml min-1)had a27%lower power (9.7%lower maximal power) but a64%higher coulombicefficiency (CE) compared with MFC-B, suggesting a feasible approach for future MFCapplication. With increased recirculation rates, MFC-B showed a decreased voltageoutput, batch time, and CE resulting from increased oxygen transfer into the anode. InMFC-BL, increasing the flow rate within a low range significantly enhanced protontransfer, resulting in a higher voltage output, a longer batch time, and a higher CE.Above this range, increased flow rates also decreased the batch time and CE ofMFC-BL due to excess oxygen transfer into the anode. MFC-BL showed a maximal power of1.32mW and CE of16.6%at a flow rate of0.35ml min-1.更多还原