【作者】 董恒；

【导师】 于宏兵；

【作者基本信息】 南开大学 ， 环境工程， 2013， 博士

【摘要】 随着我国经济的飞速发展,工业、农业、社会生活的用水量及废水排放量逐年增长,对污水处理行业提出了严峻的挑战。然而污水处理厂目前普遍采用的厌氧-好氧生物处理技术存在电耗高、污泥产量高、资源回收率低等问题,严重制约了该行业的发展。微生物燃料电池(Microbial fuel cells,MFCs)能够以有机污染物为电子供体,以氧气为电子受体,在净化污水的同时输出可利用的电能,从源头压缩了污泥产量,提高了污水资源回收率,是一种可持续的污水处理技术。空气阴极能够在无外力作用下直接从空气中吸取氧气,节省了曝气装置成本及其运行电耗,进一步提高了MFCs技术的经济性。空气阴极的气体扩散层与空气接触,需传输氧气同时防止电解液泄漏,催化层与电解液接触,内部需含有大量的氧气、质子和电子传输通道以及氧气还原所需的气-液-固三相界面。目前MFCs研究中普遍使用的空气阴极,其气体扩散层是由涂刷多层疏水性聚四氟乙烯(Polytetrafluoroethylene,PTFE)粘结剂并逐层高温烧结(340℃)制作而成,PTFE高温下融化、冷却后收缩的物理特性是构建气体传输孔道的基础；催化层是由涂刷Pt催化剂和亲水性的全氟磺酸聚四氟乙烯共聚物(Perfluorosulfonic acid-PTFE copolymer,Nafion)粘结剂制作而成,存在三相界面的“水淹”问题。该涂刷空气阴极不但有结构缺陷,而且成本昂贵,制作工艺繁琐,精确性差,严重限制了MFCs的大型化和实用性发展。因此,开发一种高性能、低成本、实用性强的新型空气阴极是本文的主要研究目标。首先建立了空气阴极的制作方法。以PTFE作为粘结剂分别构建催化层(含量较少)和气体扩散层(含量较多)。催化层中采用活性炭同时作为导电材料和催化剂,气体扩散层中添加导电炭黑增强导电性。采用先超声搅拌碳粉与粘结剂,再辊压成膜的方法制作催化膜和气体扩散膜,并在340℃下高温烧结建立孔道,然后与不锈钢网集流体一起辊压成空气阴极。重量称量和线性扫描伏安测试结果显示,辊压活性炭-PTFE空气阴极的电化学活性和电极制作的精确性均优于涂刷Pt-Nafion空气阴极。重点研究了辊压活性炭-PTFE催化层的三个影响因素,即活性炭与PTFE的比例、高温烧结工艺和表面活性剂含量。综合运用了扫描电镜、压汞测试、线性扫描伏安、Tafel曲线、交流阻抗以及MFC极化测试等多种研究手段,结果表明：过量的活性炭会阻碍氧气扩散,活性炭不足会阻碍质子和电子传递,本文获得的活性炭与PTFE的最佳比例为6：1(质量比)；催化层内PTFE的含量较少且被活性炭团聚体包围,不进行高温烧结增强了亲水性、增加了孔体积和机械强度,提高了质子与氧气的传输能力,使MFC的最大输出功率(MPD)和库伦效率(CE)分别提高了35%和18%；表面活性剂过少或过多会分别导致催化层“干涸”或“水淹”,与完全去除催化层的表面活性剂和添加非离子表面活性剂OP-10(8%,15ml·g-1活性炭)相比,保留PTFE乳液中5%的表面活性剂为催化层提供了更适的氧还原反应条件。进一步探讨了辊压活性炭-PTFE空气阴极催化氧还原的关键因素。利用旋转圆盘测试了两种活性炭粉末(AC1和AC2)、非活性炭粉末(XC-72)和Pt/C粉末催化氧气还原的电子转移数,然后使用PTFE粘结剂将碳粉辊压成催化层,Nafion粘结剂将Pt/C粉末涂刷成催化层,利用Tafel曲线测试四种空气阴极催化氧气还原的电子转移数,再结合表面形貌观察、催化膜与碳粉的孔结构分析、MFC的产电性能测试等研究手段,分析得到以下结论：疏水性PTFE粘结剂与辊压制作方法构建了催化膜和高温烧结的气体扩散膜的氧气传输孔道且孔径集中在6μm的大孔范围,这是关键因素之一。氧气沿此孔道进入催化层后,能够继续深入到活性炭大量的中孔和微孔内建立三相界面是另一关键因素,特别是孔结构集中分布在微孔范围的活性炭,能够抑制好氧生物膜污染,保护阳极的厌氧环境。研究了酸、碱处理活性炭对辊压活性炭-PTFE空气阴极性能的影响。分别使用5.6mol·L-1的HN03溶液和3mol·L-1的KOH溶液预处理活性炭,对处理前后活性炭的孔结构、伏安特性、内阻结构以及MFC的产电性能进行研究,发现使用3mol·L-1的KOH溶液在85℃下进行预处理,消除了活性炭原有的墨水瓶形状中孔结构,提高了氧气向微孔扩散的能力,将MFC的MPD提高了5%,是优化辊压活性炭-PTFE空气阴极的有效方法。考察了辊压活性炭-PTFE空气阴极的实用性。使用实际生活污水作为MFCs的底物对空气阴极进行180天的连续运行测试,记录MFCs产电量、生物膜污染过程、电极内阻结构变化,并对空气阴极的再生性进行了测试。结果显示,MFC对生活污水COD的去除率为90%,MPD为1185mW.m-2；生物膜污染仅在催化层表面发生,采用刮除生物膜的方法再生空气阴极,性能恢复到了初始水平的78%,减小再生周期有望进一步提高性能恢复率。与涂刷Pt-Nafion空气阴极相比,本文首创的辊压活性炭-PTFE空气阴极使MFC的MPD提高了142%(1355±26mW.m-2),成本压缩了32倍,单位空气阴极成本获得电能资本回收率提高了79倍,而且制作方法更简单、精确、适于再生。本工作为MFCs的实际应用开创了新前景,也为空气阴极的优化提供了指导方向。更多还原

【Abstract】 With the fast development of the national economy, the demands for water and subsequent wastewater discharge from industry, agriculture and social life are increasing every year, which has posed a great challenge to the sewage treatment industry. Unfortunately, the usual aerobic-anaerobic biotreatment technique is running in the most of the sewage treatment plants at the cost of high power consumption, huge sewage sludge and low recovery ratio. The fast growth is very hard for the sewage treatment industry unless clean production transformation is implemented. Microbial fuel cells (MFCs) can give the current by using the organic pollutants of the wastewater as the electron donor and by using the oxygen as the electron acceptor. Since the chemical energy contained in the organic pollutants is directly converted into the electric energy in MFCs, the productivity of the sewage sludge can be reduced at source thus the recovery ratio will be enhanced. So MFCs is a promising technique for the sewage treatment industry to forward its clean production transformation. The economics of MFCs are further enhanced owing to using the air-cathode. It has the capacity of absorbing oxygen from the atmosphere spontaneous, indicating that the aeration device is no longer needed and its electric consumption can be saved. Air-cathode contains a gas diffusion layer (GDL) toward the atmosphere to absorb and transfer oxygen as well as prevent electrolyte from leaking. It also contains a catalyst layer (CL) toward the electrolyte to provide sufficient transmission paths respectively for the oxygen, protons and electrons as well as areas for the establishment of the gas-liquid-solid three phase interfaces (TPIs). Currently, the brushed Pt-Nafion air-cathode is widely used in MFCs for the wastewater treatment study. Its GDL is prepared by brushing hydrophobic polytetrafluoroethylene (PTFE) binder for several layers and sintering treatment at340℃for each layer. Pores in the GDL for gas diffusion are formed basing on the physical properties of PTFE which melts at high temperature and then contracts during cooling. Its CL is made by brushing Pt/C catalyst and hydrophilic Nafion binder, however, the flooded TPIs by the electrolyte are easy to occur. Since its structure defects as well as high cost, troublesome and extensive manufacturing process, this brushing Pt-Nafion air-cathode would be a hindrance to the development of MFCs towards large-scale and practical application. Therefore, this work aimed to harvest a novel air-cathode with the features of high performance, low cost and practicable.The manufacture method for the air-cathode was established firstly. PTFE were used as the binder to construct the CL (minor constituent) and GDL (major constituent), respectively. The conductive carbon black was added in the GDL to improve the conductivity. The activated carbon (AC) used in the CL played the roles of conductor as well as the catalyst for ORR. Films of the CL and GDL were prepared by treating the carbon powder and binder with agitated ultrasonic firstly and then rolling the blend with mechanical roller. After being sintered at340℃for the gas pores formation, the prepared CL and GDL films were rolled together at two sides of the stainless steel mesh respectively to achieve the activated carbon-PTFE air-cathode (ACAC). The results from linear sweep voltammetry (LSV) measurements showed its obvious superiorities of electrochemical activity and reproducibility compared to the brushing Pt-Nafion air-cathode.Next, three important influences of the CL were studied, including the ratio of AC and PTFE, sintering treatment at high temperature and surfactant concentration. Comprehensive measurements involving scanning electron microscope, mercury porosimeter, LSV, tafel plot, electrochemical impedance spectroscopy as well as MFC polarization were adopted. The research for the ratio of AC and PTFE indicated that too much AC in the CL would restrict the oxygen diffusion. While, too little AC in the CL would hinder the protons and electrons migration. The optimum achieved in this paper was AC/PTFE=6. The investigation for the sintering treatment at high temperature showed that PTFE in CL should be enveloped by the AC aggregations. By avoiding the sintering treatment, the hydrophility of CL was enhanced, meanwhile, the inner pore volume and mechanical strength of CL were increased. It led the simultaneous improvement on the protons migration and oxygen diffusion. So that the increase by35%and18%respectively for the maximum power density (MPD) and coulombic efficiency (CE) of MFC were obtained. According to the study on the surfactant concentration, it was demonstrated that too little surfactant would make CL dry up while too much one would lead TPIs flooded. In contrast with eliminating surfactant completely from CL and adding OP-10nonionic surfactant (8%,15ml·g-1AC), retaining the surfactant from PTFE emulsion (5%) provided the best conditions for the oxygen reduction reaction (ORR) in the CL. The crucial factors for the catalysis of ACAC to the ORR were further investigated. The electron transfer number during ORR catalysed by two AC powders, non AC powder (XC-72) and Pt/C powder were measured using the rotating disk electrode. Then they were made to the CLs of the air-cathodes. Carbon powders were prepared with PTFE by the rolling method and Pt/C powder was prepared with Nafion by the brushing method. The electron transfer number during ORR catalysed by the four air-cathodes were measured using Tafel plot. Other measurements such as the surface morphology observation, pores analysis both for the films and carbon powders as well as MFC polarization were also implemented. The conclusions were achieved as follows:one of the crucial factors for the catalysis of ACAC to the ORR was using the hydrophobic PTFE binder combining with the rolling method to make the CL and GDL. It allowed the porous structure to be formed in the CL. The pore diameter in the CL and sintered GDL both concentrated at6μm which belongs to the macropores. The other crucial factor was oxygen in the macropores of CL could further diffuse into the AC particles through mesopores till the micropores to establish the TPIs. It was found that the ACs with enough and uniform micropores distribution were more beneficial for ACAC to prevent fouling from cathode biofilm and protect the anaerobic circumstances from oxygen diffusion.The effectiveness of AC modification by acid and alkali to ACAC were assessed. AC powders were modified respectively by5.6mol-L"1HNO3and3mol·L-1KOH. The changes concerning the porous structure of AC, voltage-current characteristic and internal resistance constitution in ACAC and its performance in MFCs were compared. It was found that being modified with3mol·L-1KOH at the temperature of85℃made the mesopores with’ink bottle’ shape vanished from AC and the MPD of MFC exhibited an increase by5since the oxygen diffusion was improved. AC modification by3mol·L-1KOH was proved an effective method for the ACAC optimization.At last, the practical applicability of ACAC in MFC for wastewater treatment were identified. With the domestic sewage as the substrates, the MFC reactor with ACAC operated for180days continuously. The power density output, biofilm fouling on ACAC and internal resistance constitution of ACAC were all recorded. The results showed that90%COD in the domestic sewage was degraded while the MPD of1185mW·m-2was achieved. The biofilm fouling occurred just on the interface of CL and electrolyte. The performance of ACAC recovered to78%of the initial level after it was regenerated via striking off the biofilm fouling. The recovery effect could be improved if the regenerating period is further shortened.Compared to the brushing Pt-Nafion air-cathode, the ACAC developed in this work enhanced the MPD of MFCs and its rate of electric energy recovery of capital by142%(1355±26mW·m-2) and79time respectively, while the cost of air-cathode manufacture was decreased by32times. Besides, the ACAC has advantages of simple manufacture process, high precision and good reproducibility over the brushing Pt-Nafion air-cathode. This work could open fresh vistas for the MFCs application in the sewage treatment industry and provide directions in the further optimization for the air-cathode.更多还原