【作者】 杨伟；

【导师】 李俊；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2018， 博士

【摘要】 微生物燃料电池（MFC）是一种利用电化学活性细菌催化氧化有机物从而产生电能的装置。近年来MFC在污水处理方面得到了广泛的关注和研究。其中,单室空气阴极由于电极结构简单、直接利用空气中的氧气作为电子受体、阴极产物无污染等优势得到了重点的研究。目前,针对空气阴极MFC的研究主要集中于解决阴极氧还原（ORR）速率较慢、性能不佳等问题。研究从早期的贵金属催化剂发展到现在碳质催化剂,电池性能已经得到了明显提升;然而,使用碳催化剂的碳基空气阴极电催化活性依然较低,MFC功率输出远无法满足实际应用需求;另一方面,为了获得可观的阴极性能,研究中常采用极高载量的阴极碳催化剂,导致阴极催化层厚度较厚,严重制约了阴极中反应物和产物的传输,限制了廉价的碳基电极应用。本文针对碳基电极由于氧气和H⁺（OH^-）供给不佳所导致的电化学性能限制问题,立足于碳基阴极的电化学催化和传输强化,并结合电化学相关理论,对单室碳基空气阴极MFC的电化学特性和传输机理展开研究。主要内容包括:（1）通过考察传统Pt/C和3种碳质催化剂（生物炭、氮掺杂碳和金属/氮掺杂碳）在典型MFC操作条件下(高S^2-和NH₄⁺浓度)的稳定性和抗毒化特性,明确了碳质ORR催化剂在MFC操作条件下的适用性。（2）针对传统阴极Pt/C催化剂成本较高、稳定性较差和易被毒化等特点,分别构建了基于生物炭、氮掺杂碳凝胶和钴/氮掺杂碳纳米管的碳基空气阴极,并研究了其在MFC中的性能特性。（3）利用ZnO在高温条件下易于蒸发的特点以及纳米结构ZnO和石墨烯的抗菌特性,通过改善阴极催化层孔隙结构构建了具有强化离子传输、增加活性位点暴露和强抗菌能力的高性能空气阴极。（4）构建了基于瓦楞纸的无粘合剂空气阴极。通过材料和电化学表征,获得了该类阴极的性能特性。并基于无粘合剂思想构建了一体式空气阴极MFC,表征了其氧气传递、气液界面分布、电极微观结构及其表面化学特性,并测试了该一体式阴极MFC的极化特性、产电特性、COD去除率和库伦效率等参数。（5）构建了具有孔隙梯度结构的碳催化层空气阴极,并基于此建立了二维数学模型。通过模拟孔隙梯度催化层内部的氧气和OH^-浓度场,揭示了孔隙梯度催化层对强化催化层内氧气和OH^-传输特性以及阴极性能的影响规律。得到的主要结论如下:（1）不同碳基催化剂的稳定性和抗毒化研究发现:在NH₄⁺和S^2-离子存在的情况下,碳质催化剂都表现出了比商业Pt/C催化剂更好的抗毒化能力。其中S^2-对ORR动力学的影响更为显著。碳催化剂在不同浓度NH₄⁺和S^2-离子存在的条件下进行8 h恒电位放电后,均表现出了比Pt/C更优越的稳定性。（2）以生物炭催化剂制备载量为50 mg cm^-2的空气阴极在MFC中获得了1056±38 mW m^-2的最大功率密度;进一步增加生物炭催化剂N、P掺杂量,电池最大功率密度提高了65%(1719±82 mW m^-2)。以氮富集碳凝胶制备载量为2 mg cm^-2的空气阴极在MFC中获得了与Pt/C相当的功率密度(1087 mW m^-2),说明高含氮的碳材料与生物炭相比,具有更优异的电子传递特性,从而促进了电极性能的提升。以钴/氮共掺杂碳纳米碳制备了载量为2 mg cm^-2的空气阴极,其最大功率密度较使用Pt/C催化剂的MFC性能高16.7%。（3）Co/Zn摩尔比对碳基催化剂的电催化活性和孔隙结构进行调控的研究中发现,当Co/Zn摩尔比为1/1,热解温度为800°C时,催化剂的电化学活性面积和离子扩散系数最大;同时,该催化剂能抑制希瓦氏菌（S.oneidensis）生物膜的成膜;以催化剂载量为1 mg cm^-2的空气阴极在MFC中实现了773 mW m^-2的功率密度输出,略高于Pt/C的功率密度输出(744 mW m^-2);阴极生物量测试表明,该阴极表面生物膜的生物量为3.62±1.74 mg cm^-2,远低于Pt/C阴极的11.41±2.05 mg cm^-2,证明该催化剂具有较好的生物膜抑制特性,因此在持续运行过程中表现出了比Pt/C电极更优越的稳定性。（4）基于瓦楞纸构建了无粘合剂空气阴极。制备参数优化研究发现,700°C的热解温度可获得最佳性能,且阴极ORR主要以4e^-途径进行;基于上述阴极的MFC最大功率密度为830±15 mW m^-2。基于一体式三维阴极MFC的优化研究发现,内径为35 mm的管式阴极具有丰富的孔隙结构、最佳的性能和最小的内阻。根据阴极优化结果,研究发现沿着溶液侧-气体侧方向电解液的润湿性逐渐降低,在阴极内部大部分区域形成了ORR的气-液-固三相界面,而在阴极表面边缘附近形成用于气体传输的疏水孔道。通过对其氧扩散传输的测量发现,该阴极具有4.5×10^-5 cm s^-1的氧传质系数。通过构建三维一体式阴极MFC实现了40.4±1.5 W m^-3的最大功率密度输出、90.1±1.4%的COD去除率和55.8±1.0%的库伦效率。（5）通过对具有孔隙梯度催化层MFC阴极的数值模拟研究发现,该催化层结构能有效强化电化学反应过程中的OH^-和O₂传输,从而降低了阴极电化学反应过程中的浓差过电位,强化阴极性能。在MFC中,具有孔隙梯度催化层阴极的MFC最大功率密度可达1781±92 mW m^-2,远高于没有孔隙梯度催化层的MFC(1183±205 mW m^-2)。更多还原

【Abstract】 Microbial fuel cell（MFC）is a device that can catalyze the oxidation of organic matter and generate electrical energy using electrochemically active bacteria on the anode.This technology has received wide attentions in terms of wastewater treatment in recent years.Among different types of MFCs,single-chamber air cathode MFCs have been extensively studied,due to their simple electrode structure,direct use of oxygen in the air as electron acceptors,and non-contamination of cathode products.At present,researches on air cathode MFC mainly focuses on solving the problems of slow cathode oxygen reduction reaction（ORR）rate and insufficient performance.Many efforts have been dovoted to the development of carbonaceous catalysts,compared to the previous researches on precious metal catalysts.However,the electrocatalytic activity of air cathode using carbon catalysts is still insuficient,and the MFC power output is far from the requirement of practical application.On the other hand,in order to obtain a considerable cathode performance,the cathode was usually fabricated using a high loading rate of carbon catalysts,resulting in a thicker catalyst layer and hence a worse mass transfer performance.Considering the low electrochemical performance and insufficient mass transfer in carbon-based electrodes,researches are carried out to enhance the electrochemical and transport performance of carbon-based air cathode.The main contents include:（1）The stability and poison tolerance of carbonaceous catalyst layer（eg.biomass derived carbon,nitrogen-doped carbon and metal/nitrogen-doped carbon）were investigated to verify their feasibilirty under MFC relevant conditions(high S^2-and NH₄⁺concentration).（2）Considering the high cost,poor stability and low poison tolerance of traditional Pt/C catalysts,carbon-based air cathodes were fabricated based on biomass derived carbon,nitrogen-doped carbon and cobalt/nitrogen-doped carbon nanotubes,respectively;their physicochemical properties,electrochemcial performance,and the power density of MFCs using these cathodes was studied.（3）An air cathode with an enhanced ion transport and antibacterial property was constructed.In this research,the ZnO was applied as pore former to improve the pore structure and surface area,and thus improve the ion transport and the exposure of active sites.Meanwhile,the nanocomposite of ZnO/graphene was used for inhibition of the biofilm due to its antibacterial property.（4）The binder free air cathode MFC was constructed and the performance of as prepared electrode was optimized through preparation approaches and preparation parameters.Also,the physicochemical properties were characterized based on material chemistry and electrochemistry.According to the design of binder free electrode,the monilithic air cathode was proposed for MFCs,and the oxygen transfer performance,gas-liquid interface,microstructure and surface chemistry was characterized.Furthermore,the polarization curves,electricity generation,COD removal rate and the coulomb efficiency of MFCs were tested to evaluate the cell performance.（5）An air cathode with a pore-gradient catalyst layer was constructed and a two-dimensional mathematical model was established based on this cathode.The oxygen and OH^-concentration distribution inside the catalytic layer were analyzed through numerical calculation,aiming to reveal the influence of the pore-gradient catalytic layer on the mass transfer and the cathode performance.The main achivements obtained are as follows:1)Studies on the stability and poison tolerance of different catalysts revealed that carbonaceous catalysts showed a better poison tolerance and stability than commercial Pt/C catalyst in the presence of NH₄⁺and S^2-.Notably,the S^2-has a much more signicficant effect on the kinetics of ORR than NH₄⁺.Also,the carbon catalysts showed superior stability to Pt/C after 8 hours’operation at a constant potential in the presence of NH₄⁺and S^2-.2)The research found that the air cathode with 50 mg cm^-2 loading rate of biomass-derived carbon obtained the maximum power density of 1056±38 mW m^-2 in MFCs;with the further increase of N and P doping,the maximum power density increased by 65%(1719±82 mW m^-2).The nitrogen-enriched carbon aerogel air cathode with a loading rate of 2 mg cm^-2 delivered the maximum power density of 1087mW m^-2,which is similar to that of Pt/C,indicating that the nitrogen-enriched carbon material has a better performance to faciliate the electron transfer than biomass derived carbon.The air cathode MFC with 2 mg cm^-2 loading of cobalt/nitrogen co-doped carbon namotube achieved a 16.7%higher power output than that of the Pt/C cathode,demonstrating the the cobalt/nitrogen co-doped carbon namotube can further improve the cathode performance,compared to the nitrogen enriched carbon aerogel.3)The research found that the electrocatalytic activity and pore structure of carbon-based catalysts can be controlled by Co/Zn.The catalyst obtained the highest electrochemical active area and ion diffusion coefficiency with the Co/Zn molar ratio of1/1 and the pyrolysis temperature of 800°C,meanwhile,the as-prepared catalyst was also found to exhibit an excellent antibacterial property to S.Oneidensis.The air cathode MFC using as preparede catalyst achieved a power density output of 773 mW m^-2,slightly higher than that of Pt/C(744 mW m^-2).Also,it is found that the as prepared cathode had a biomass of 3.62±1.74 mg cm^-2,which was much lower than the Pt/C cathode of 11.41±2.05 mg cm^-2,showing a good biofilm inhibition and stability of cathode performance.4)Based on the binder free air cathode,the electrode was optimized through different preparation routes and the preparation temperatures.It was found that the electrode obtained the best performance with a nearly 4e^-pathway of ORR at a preparation temperature of 700°C,and the prepared cathode obtained a maximum power density of 830±15 mW m^-2 in MFCs.Based on the numerical simulation study of the porosity-gradient catalytic layer,it is found that the catalyst layer structure can effectively enhance the OH^-and O₂ transfer,reducing the concentration overpotential,and thereby enhancing the cathode performance during the electrochemical reaction.The MFC equipped with the cathode using pore-gradient catalytic layer achieved a maximum power density of 1781±92 mW m^-2,which is much higher than that of 1183±205 mW m^-22 without a gradient catalytic layer.5)Based on the optimization of monolithic three-dimensional cathode,it is found that the tubular cathode with an inner diameter of 35 mm had a richer pore structure,higher performance and lower internal resistance than other cathodes.According to the optimized results,it is shown that the wettability of the electrolyte decreases gradually from solution to air side of cathodes.It is also observed that the three-phase interface of oxygen reduction is formed in the most areas of the cathode,while gas phase is only existed near the edge of the cathode surface.The results indicated that as prepared cathode had an oxygen mass transfer coefficient of 4.5×10^-5 cm s^-1,a maximum power density of 40.4±1.5 W m^-3,a COD removal rate of 90.1±1.4%and a coulombic efficiency of 55.8±1.0%in MFCs.更多还原