碳纳米材料修饰电极强化微生物燃料电池产电特性与机理
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摘要
环境与能源问题已然成为影响人类生存与发展的核心问题。近些年，微生物燃料电池（Microbial fuel cell, MFC）能够利用微生物催化直接将燃料的化学能转化为电能，是一种能实现污水净化同时产电的新技术，因此受到了广泛关注。
现阶段MFC输出功率密度偏低和构建成本过高限制了其在工程中的实际应用。随着反应器结构材料的优化，源于电极表面微生物催化反应的活化内阻逐渐成为制约MFC功率提升的关键；构建成本则主要来源于阴极用于催化氧气还原的贵金属，寻找廉价高效的化学催化剂，或者利用微生物代替化学催化剂形成生物阴极可以有效降低MFC的构建成本。
针对以上存在问题，本论文通过简单的方法实现对阳极电极的表面改性，提高微生物与阳极电极之间的电子转移速率，改善阳极性能；在阴极，开发出廉价高效的氧气还原复合催化剂用以替代贵金属Pt，或者采用生物阴极取代贵金属催化剂，筛选出廉价高效的适合生物阴极的电极材料以及利用简单高效方法修饰生物阴极材料，降低构建成本的同时提高MFC的性能，从而在材料制备和应用方面为实现MFC的大规模实际应用提供有价值的参考数据和切实可行的实施方案。得到如下主要结果与结论：
（1）通过层层自组装法制备了介孔碳修饰的碳纸电极，循环伏安法(Cyclicvoltammetry，CV)与电化学阻抗法（Electrochemical impedance spectroscopy，EIS）表明，相比于未经介孔碳修饰的碳纸，其具有更好的稳定的电化学性能,如增大了反应活化面积和电子转移速率，适宜应用于MFC阳极。以介孔碳修饰的碳纸作为阳极的MFC获得了237mW m-2功率密度，为未经介孔碳修饰的碳纸作为阳极的1.9倍（128mW m-2），同时期启动时间也缩短了68%，阳极极化损失也大大降低。
（2）原位合成的二氧化锰碳纳米管复合催化剂（‘in-situ MnO2/CNTs’），在中性缓冲溶液中对氧气有很好的还原催化活性。扫描电镜（Scanning electron microscope，SEM）显示MnO2充分均匀的修饰于CNTs表面，且在高强度超声后仍能稳定的附着在CNTs管壁。将‘in-situ MnO2/CNTs’修饰于碳纸电极上，应用于MFC阴极，最大输出功率密度达到了210mW m2，与Pt/C相当（229mW m2），表明‘in-situ MnO2/CNTs’可以作为一种廉价高效的氧气还原催化剂替代Pt/C而应用于MFC阴极。
（3）阴极材料的选择对生物阴极型MFC极为重要。主要探究了常用的几种材料石墨毡（Graphite felt，GF）、碳纸（Carbon paper）和不锈钢网（Stainless steel mesh，SSM）在MFC生物阴极中的应用比较，GF-MFC的电流密度和最大功率密度分别为350mA m2和109.5mW m2，高于CP-MFC（210mA m2and32.7mW m2）和SSM-MFC（18mA m2and3.1mW m2）。三种材料对应的MFC均获得了较高的COD去除率，GF-MFC获得了最大的库伦效率11.7%。三种材料中，石墨毡最适合于应用于双室型生物MFC中的阴极中。
（4）通过简单可扩大化的方法制得了CNTs修饰SSM电极CNTs-SSM。SEM显示CNTs均匀的分布在不锈钢网表面，形成了一个三维网状结构，其表面积也随之提高。经过高强度的超声，修饰在不锈钢网表面的CNTs也不会脱落，说明CNTs与不锈钢网的结合非常稳固。CNTs-SSM-MFC的最大功率密度达到147mW m2，是SSM-MFC（3mW m2）的49倍之多。CV测试证明生物阴极上的生物膜可以催化还原氧气，且CNTs-SSM生物阴极表现出氧气催化还原效率远远高于SSM生物阴极。SSM在生产过程中可以极为方面的调整期纤维直径、孔隙，这可以为其在MFC的实际应用提供进一步的优化。此外，SSM易折易弯曲的特性可以使得在MFC实际应用中根据需要构建相应结构形状的CNTs-SSM生物阴极，如立方形、圆柱形或者条状。
Energy and water supply are two of the biggest challenges facing humanity in thecoming decades. In comparison with conventional fuel cells and waste treatment equipment,microbial fuel cells (MFC) are promising clean energy sources for simultaneous wastetreatment while harvesting electricity, which have been greatly developed in recent years.
Currently, MFC is still in its infant and its future is filled with many challenges. Toeffectively apply MFC in commercial and practical application, challenges including lowerpower output and high cost have to be tackled first. As the optimization of material andconfiguration, ohmic resistance was sharply decreased. As a result, activation resistanceoriginated form the electrode reactions became the main limiting factor of the power outputimprovement. The high cost mainly resulted from the noble metal cathode, which used foroxygen reduction reaction. Developing an efficient and cost-effective cathodic electrocatalystor using biocathode instead of noble metal electrocatalyst was the solutions.
In this study, modification of the carbon anode material was used to increase the electrontransfer between the anode and the biocatalyst, thus improving anode performance. To lowerthe cost of cathodes and simultaneously improve oxygen reduction reaction, non-noble metalsand biocathode (self-regeneration, low cost, sustainability) have been investigated as cathodiccatalysts. For biocathode, several commonly used biocathode materials were tested andcompared. Moreover, modification of the biocathode was used to increase its performance.Some innovative findings have been found as follows:
(1) A novel mesoporous carbon (MC) modified carbon paper has been constructed usinglayer-by-layer self-assembly method and is used as anode in an air-cathode single-chambermicrobial fuel cell (MFC) for performance improvement. Using cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS), we have demonstrated that the MC modifiedelectrode exhibits a more favorable and stable electrochemical behavior, such as increasedactive surface area and enhanced electron-transfer rate, than that of the bare carbon paper. TheMFC equipped with MC modified carbon paper anode achieves considerably betterperformance than the one equipped with bare carbon paper anode: the maximum powerdensity is81%higher and the startup time is68%shorter. CV and EIS analysis confirm thatthe MC layer coated on the carbon paper promotes the electrochemical activity of the anodicbiofilm and decreases the charge transfer resistance from300to99. In addition, the anodeand cathode polarization curves reveal negligible difference in cathode potentials butsignificant difference in anode potentials, indicating that the MC modified anode other thanthe cathode was responsible for the performance improvement of MFC. In this paper, we have demonstrated the utilization of MC modified carbon paper to enhance the performance ofMFC.
(2) To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuelcells (MFC), carbon nanotubes (CNTs) coated with manganese dioxide using an in-situhydrothermal method (in-situ MnO2/CNTs) have been investigated for electrochemicaloxygen reduction reaction (ORR). Examination by transmission electron microscopy showsthat MnO2is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linearsweep voltammetry, we determine that the in-situ MnO2/CNTs are a better catalyst for theORR than CNTs that are simply mechanically mixed with MnO2powder, suggesting that thesurface coating of MnO2onto CNTs enhances their catalytic eactivity. Additionally, amaximum power density of210mW m2produced from the MFC with in-situ MnO2/CNTscathode is2.3times of that produced from the MFC using mechanically mixed MnO2/CNTs(93mW m2), and comparable to that of the MFC with a conventional Pt/C cathode (229mWm2). Electrochemical impedance spectroscopy analysis indicates that the uniform surfacedispersion of MnO2on the CNTs enhanced electron transfer of the ORR, resulting in higherMFC power output. The results of this study demonstrate that CNTs are an ideal catalystsupport for MnO2and that in-situ MnO2/CNTs offer a good alternative to Pt/C for practicalMFC applications.
(3) The choice of the cathode material is crucial for every bio-cathode microbial fuel cell(MFC) setup. The commonly used biocathode materials, Graphite felt (GF), carbon paper (CP)and stainless steel mesh (SSM) were compared and evaluated in terms of current density,power density, and polarization. The maximum current density and power density of the MFCwith GF-biocathode achieved350mA m2and109.5mW m2, which were higher than that ofthe MFC with CP-biocathode (210mA m2and32.7mW m2) and the MFC withSSM-biocathode (18mA m2and3.1mW m2). The polarization indicated that thebiocathode was the limiting factor for the three MFC reactors. Moreover, cyclic voltammetry(CV) showed that the microorganisms on the biocathode played a major role in oxygenreduction reaction (ORR) for GF-and CP-biocathode but SSM-biocathode. Electrochemicalimpedance spectroscopy suggested that GF biocathode performed better catalytic activitytowards ORR than that of CP-and SSM-biocathode, also supported by CV test. Additionally,the MFC with GF-biocathode had the highest Coulombic Efficiency. The results of this studydemonstrated GF was the most suitable biocathode for MFC application among the threetypes of materials when using anaerobic sludge as inoculums.
(4) A novel carbon nanotubes (CNTs) coated stainless steel mesh (SSM) electrode hasbeen fabricated by a simple and scalable process and is used as biocathode in microbial fuelcell (MFC) for performance improvement. Examination by scanning electron microscopeshows that CNTs are uniformly distributed over the surface of the SSM, thus forming athree-dimensional network structure. The MFC with CNT-SSM biocathode achieves highermaximum power density (147mW m2), which is49times larger than that (3mW m2)produced from the MFC with bare SSM biocathode. Moreover, cyclic voltammetry shows thatthe microorganisms on the CNTs-SSM biocathode plays a major role in oxygen reductionreaction (ORR), and the CNT-SSM biocathode performes better catalytic activity toward ORRthan that of SSM biocathode. Additionally, the MFC with CNTs-SSM biocathode has higherCoulombic Efficiency than that of MFC with bare SSM biocathode. In this study, wedemonstrate that the use of CNTs-SSM offers an effective mean to enhance the electricity ofbiocathode MFC.
现阶段MFC输出功率密度偏低和构建成本过高限制了其在工程中的实际应用。随着反应器结构材料的优化，源于电极表面微生物催化反应的活化内阻逐渐成为制约MFC功率提升的关键；构建成本则主要来源于阴极用于催化氧气还原的贵金属，寻找廉价高效的化学催化剂，或者利用微生物代替化学催化剂形成生物阴极可以有效降低MFC的构建成本。
针对以上存在问题，本论文通过简单的方法实现对阳极电极的表面改性，提高微生物与阳极电极之间的电子转移速率，改善阳极性能；在阴极，开发出廉价高效的氧气还原复合催化剂用以替代贵金属Pt，或者采用生物阴极取代贵金属催化剂，筛选出廉价高效的适合生物阴极的电极材料以及利用简单高效方法修饰生物阴极材料，降低构建成本的同时提高MFC的性能，从而在材料制备和应用方面为实现MFC的大规模实际应用提供有价值的参考数据和切实可行的实施方案。得到如下主要结果与结论：
（1）通过层层自组装法制备了介孔碳修饰的碳纸电极，循环伏安法(Cyclicvoltammetry，CV)与电化学阻抗法（Electrochemical impedance spectroscopy，EIS）表明，相比于未经介孔碳修饰的碳纸，其具有更好的稳定的电化学性能,如增大了反应活化面积和电子转移速率，适宜应用于MFC阳极。以介孔碳修饰的碳纸作为阳极的MFC获得了237mW m-2功率密度，为未经介孔碳修饰的碳纸作为阳极的1.9倍（128mW m-2），同时期启动时间也缩短了68%，阳极极化损失也大大降低。
（2）原位合成的二氧化锰碳纳米管复合催化剂（‘in-situ MnO2/CNTs’），在中性缓冲溶液中对氧气有很好的还原催化活性。扫描电镜（Scanning electron microscope，SEM）显示MnO2充分均匀的修饰于CNTs表面，且在高强度超声后仍能稳定的附着在CNTs管壁。将‘in-situ MnO2/CNTs’修饰于碳纸电极上，应用于MFC阴极，最大输出功率密度达到了210mW m2，与Pt/C相当（229mW m2），表明‘in-situ MnO2/CNTs’可以作为一种廉价高效的氧气还原催化剂替代Pt/C而应用于MFC阴极。
（3）阴极材料的选择对生物阴极型MFC极为重要。主要探究了常用的几种材料石墨毡（Graphite felt，GF）、碳纸（Carbon paper）和不锈钢网（Stainless steel mesh，SSM）在MFC生物阴极中的应用比较，GF-MFC的电流密度和最大功率密度分别为350mA m2和109.5mW m2，高于CP-MFC（210mA m2and32.7mW m2）和SSM-MFC（18mA m2and3.1mW m2）。三种材料对应的MFC均获得了较高的COD去除率，GF-MFC获得了最大的库伦效率11.7%。三种材料中，石墨毡最适合于应用于双室型生物MFC中的阴极中。
（4）通过简单可扩大化的方法制得了CNTs修饰SSM电极CNTs-SSM。SEM显示CNTs均匀的分布在不锈钢网表面，形成了一个三维网状结构，其表面积也随之提高。经过高强度的超声，修饰在不锈钢网表面的CNTs也不会脱落，说明CNTs与不锈钢网的结合非常稳固。CNTs-SSM-MFC的最大功率密度达到147mW m2，是SSM-MFC（3mW m2）的49倍之多。CV测试证明生物阴极上的生物膜可以催化还原氧气，且CNTs-SSM生物阴极表现出氧气催化还原效率远远高于SSM生物阴极。SSM在生产过程中可以极为方面的调整期纤维直径、孔隙，这可以为其在MFC的实际应用提供进一步的优化。此外，SSM易折易弯曲的特性可以使得在MFC实际应用中根据需要构建相应结构形状的CNTs-SSM生物阴极，如立方形、圆柱形或者条状。
Energy and water supply are two of the biggest challenges facing humanity in thecoming decades. In comparison with conventional fuel cells and waste treatment equipment,microbial fuel cells (MFC) are promising clean energy sources for simultaneous wastetreatment while harvesting electricity, which have been greatly developed in recent years.
Currently, MFC is still in its infant and its future is filled with many challenges. Toeffectively apply MFC in commercial and practical application, challenges including lowerpower output and high cost have to be tackled first. As the optimization of material andconfiguration, ohmic resistance was sharply decreased. As a result, activation resistanceoriginated form the electrode reactions became the main limiting factor of the power outputimprovement. The high cost mainly resulted from the noble metal cathode, which used foroxygen reduction reaction. Developing an efficient and cost-effective cathodic electrocatalystor using biocathode instead of noble metal electrocatalyst was the solutions.
In this study, modification of the carbon anode material was used to increase the electrontransfer between the anode and the biocatalyst, thus improving anode performance. To lowerthe cost of cathodes and simultaneously improve oxygen reduction reaction, non-noble metalsand biocathode (self-regeneration, low cost, sustainability) have been investigated as cathodiccatalysts. For biocathode, several commonly used biocathode materials were tested andcompared. Moreover, modification of the biocathode was used to increase its performance.Some innovative findings have been found as follows:
(1) A novel mesoporous carbon (MC) modified carbon paper has been constructed usinglayer-by-layer self-assembly method and is used as anode in an air-cathode single-chambermicrobial fuel cell (MFC) for performance improvement. Using cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS), we have demonstrated that the MC modifiedelectrode exhibits a more favorable and stable electrochemical behavior, such as increasedactive surface area and enhanced electron-transfer rate, than that of the bare carbon paper. TheMFC equipped with MC modified carbon paper anode achieves considerably betterperformance than the one equipped with bare carbon paper anode: the maximum powerdensity is81%higher and the startup time is68%shorter. CV and EIS analysis confirm thatthe MC layer coated on the carbon paper promotes the electrochemical activity of the anodicbiofilm and decreases the charge transfer resistance from300to99. In addition, the anodeand cathode polarization curves reveal negligible difference in cathode potentials butsignificant difference in anode potentials, indicating that the MC modified anode other thanthe cathode was responsible for the performance improvement of MFC. In this paper, we have demonstrated the utilization of MC modified carbon paper to enhance the performance ofMFC.
(2) To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuelcells (MFC), carbon nanotubes (CNTs) coated with manganese dioxide using an in-situhydrothermal method (in-situ MnO2/CNTs) have been investigated for electrochemicaloxygen reduction reaction (ORR). Examination by transmission electron microscopy showsthat MnO2is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linearsweep voltammetry, we determine that the in-situ MnO2/CNTs are a better catalyst for theORR than CNTs that are simply mechanically mixed with MnO2powder, suggesting that thesurface coating of MnO2onto CNTs enhances their catalytic eactivity. Additionally, amaximum power density of210mW m2produced from the MFC with in-situ MnO2/CNTscathode is2.3times of that produced from the MFC using mechanically mixed MnO2/CNTs(93mW m2), and comparable to that of the MFC with a conventional Pt/C cathode (229mWm2). Electrochemical impedance spectroscopy analysis indicates that the uniform surfacedispersion of MnO2on the CNTs enhanced electron transfer of the ORR, resulting in higherMFC power output. The results of this study demonstrate that CNTs are an ideal catalystsupport for MnO2and that in-situ MnO2/CNTs offer a good alternative to Pt/C for practicalMFC applications.
(3) The choice of the cathode material is crucial for every bio-cathode microbial fuel cell(MFC) setup. The commonly used biocathode materials, Graphite felt (GF), carbon paper (CP)and stainless steel mesh (SSM) were compared and evaluated in terms of current density,power density, and polarization. The maximum current density and power density of the MFCwith GF-biocathode achieved350mA m2and109.5mW m2, which were higher than that ofthe MFC with CP-biocathode (210mA m2and32.7mW m2) and the MFC withSSM-biocathode (18mA m2and3.1mW m2). The polarization indicated that thebiocathode was the limiting factor for the three MFC reactors. Moreover, cyclic voltammetry(CV) showed that the microorganisms on the biocathode played a major role in oxygenreduction reaction (ORR) for GF-and CP-biocathode but SSM-biocathode. Electrochemicalimpedance spectroscopy suggested that GF biocathode performed better catalytic activitytowards ORR than that of CP-and SSM-biocathode, also supported by CV test. Additionally,the MFC with GF-biocathode had the highest Coulombic Efficiency. The results of this studydemonstrated GF was the most suitable biocathode for MFC application among the threetypes of materials when using anaerobic sludge as inoculums.
(4) A novel carbon nanotubes (CNTs) coated stainless steel mesh (SSM) electrode hasbeen fabricated by a simple and scalable process and is used as biocathode in microbial fuelcell (MFC) for performance improvement. Examination by scanning electron microscopeshows that CNTs are uniformly distributed over the surface of the SSM, thus forming athree-dimensional network structure. The MFC with CNT-SSM biocathode achieves highermaximum power density (147mW m2), which is49times larger than that (3mW m2)produced from the MFC with bare SSM biocathode. Moreover, cyclic voltammetry shows thatthe microorganisms on the CNTs-SSM biocathode plays a major role in oxygen reductionreaction (ORR), and the CNT-SSM biocathode performes better catalytic activity toward ORRthan that of SSM biocathode. Additionally, the MFC with CNTs-SSM biocathode has higherCoulombic Efficiency than that of MFC with bare SSM biocathode. In this study, wedemonstrate that the use of CNTs-SSM offers an effective mean to enhance the electricity ofbiocathode MFC.
引文
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