Mass Transfer Properties And Performance Enhancement Of Microalgae Biomass Derived Carbon Air Cathode In Microbial Fuel Cell

In recent years,with the development of industrial society,energy is increasingly scarce and water pollution is growing serious,so it is extremely urgent to exploit and utilize environmentally friendly renewable energy.As a new bio-electrochemical energy conversion technology,microbial fuel cell（MFC）can generate electricity while degrading organic matter in wastewater,and has unique environmental effects.Thus,it has attracted much attention and research.In particular,the membrane-less air cathodes established a solid ground for the practical application of MFCs because of their simplified design,automatic replenishment of freely available oxygen in air and no extra energy requirement.However,the performance of the air cathodes was still limited due to the sluggish kinetics of oxygen reduction reaction（ORR）.Currently,the research on improving the performance of air cathode mainly focuses on two aspects.First,high catalytic activity of ORR catalysts is important in the neutral p H conditions suitable for survival of microbe.The efficient cathode catalyst can not only reduce the activation energy of the reaction,promote the ORR to the four-electron（4e^-）pathway with faster reaction rate,but also enhance the kinetics,strengthen the performance of cathode,and improve the output power of MFC.Platinum（Pt）and its alloys were widely considered as the most effective catalysts that favor a 4e^-pathway for ORR in the neutral electrolyte solution.However,Pt and its alloy catalysts are expensive and susceptible to the poisoning species such as S^2-,NH4⁺and phosphate that commonly present in the MFC relevant conditions.To mitigate these problems,previous studies employed carbon materials doped with heteroatoms（e.g S,B,N,and P）as the ORR catalyst for MFC because of the favorable 4e^-transfer and the high tolerance to the poisoning species in wastewater.Among these carbon catalysts,the self-doped ORR catalysts derived from the cheap,renewable and naturally abundant biomass materials have gained a growing attention by virtue of their unique instinct porous structure,good accessibility,and their capability of avoiding the use of the toxic,environmentally harmful dopants.However,the specific surface area of the carbon catalysts prepared by direct carbonization is not high enough,the pore distribution is simple,and the electrocatalytic activity is still low,which limit its application in MFC.Besides the catalytic activity of ORR catalysts,the architecture of the air cathode,which determines the accessibility and utilization of the catalysts,also greatly affect the cathode performance.An MFC air cathode typically consisted of several layers,including a waterproof polytetrafluoroethylene（PTFE）layer,a porous gas diffusion layer（GDL）and a catalyst layer（CL）supported on conductive macroporous substrate.The air cathode was conventionally prepared by hot pressing or rolling pressing method.In these methods,however,binders such as Nafion and PTFE were required during the fabrication to ensure a sufficient reactant supply and product removal to/from the electrode as well as a sufficient mechanical strength of the electrode.But the use of binders not only increases the ohmic resistance of the electrodes,but also covers the catalyst surface and blocks the pathway for oxygen and ion transfer,which significantly restricts the performance of MFCs.In view of the weakness of the materials and structure of the air cathode of MFC,based on the electrochemical catalysis and transport enhancement of biomass carbon cathode,the ORR catalytic activity,material transport characteristics and MFC performance were studied.The main contents of this study include:（1）Chlorella pyrenoidosa was used as the raw materials of carbon catalyst,Mg5（OH）2（CO3）4 and Zn Cl2 were served as hard templates,and the hierarchically porous carbon catalyst was prepared by direct carbonization.The effects of different hard templates on the physical properties and ORR catalytic activity of the catalysts were studied.In addition,the power generation performances of different catalysts in MFC were tested.（2）The preparation method of the monolithic binder-free air cathode was proposed.The influences of the proportion of deionized water during the preparation process on the micro-morphology,pore distribution,oxygen diffusion and ORR catalytic activity of the cathode were studied.Furthermore,the mass transfer characteristic and power generation of the MFC assembled with different cathodes were analyzed.（3）On the basis of the monolithic air cathode,Fe³⁺hydrothermal treatment was used to realize the doping of heteroatom and the further enrichment of pore structure.The effects of Fe³⁺concentration on the morphology,pore distribution and heteroatom doping of the cathode were compared,and the effects of Fe³⁺on power generation of the different cathodes in MFC were studied.The main conclusions are as follows:1)With Chlorella pyrenoidosa as the precursor of the carbon catalyst as well as Mg5（OH）2（CO3）4 and Zn Cl2 as double templates,the hierarchically porous carbon catalyst with high catalytic activity can be carbonized directly.The Mg salt contributes to the formation of macropores in the catalyst.Zn Cl2 can form macropores and mesopores on the carbon skeleton.The double templates are helpful to the doping of more nitrogen elements and the formation of graphitic-N in the catalyst,lead to the formation of more effective active sites and the enhancement of the ORR catalytic activity of the catalyst.The ORR reaction of CP-M-Z with double templates mainly involves 4e^-pathway,and it shows a higher limiting current density than Pt/C in RRDE test.In addition,the electrochemical active area of CP-M-Z is much larger that of CP-M and CP-Z with single template respectively.The performance of the different air cathode MFCs with the same catalyst load was tested,and the result show that CP-M-Z in MFC achieved a maximum power density of 2288±30 m W m^-2,which is much higher than that of Pt/C cathode MFC(1403±75 m W m^-2).Furthermore,compared with the carbon catalyst reported in other literatures,CP-M-Z shows a leading level of maximum power density with the condition of extremely low load;meanwhile,its mass specific power is far higher than other carbon catalysts,which is in a leading position.2)In this paper,a new type of monolithic binder-free air cathode is proposed.Chlorella pyrenoidosa was used as the precursor of carbon catalyst;the tubular air cathode was formed by agar gel;deionized water was utilized as solvent and pore forming agent.During the freeze-drying process,the pores of the cathode were enriched by the sublimation of ice crystals.It is found that the proportion of deionized water plays an important role in the pore distribution of the cathode.The appropriate proportion of deionized water not only increases the specific surface area of the cathode,but also enriches the pore structure and strengthens the mass transfer efficient in the cathode.3)Compared with other tubular air cathode MFC,CP-TAC-50 achieved the highest maximum power density of 106.34±5.9 W m^-3,33.2%higher than Pt/C cathode MFC(79.84±4.1 W m^-3),and also higher than other water content CP-TAC cathode MFCs.This is mainly due to CP-TAC-50 has more developed pore structure and abundant active sites.The synergistic effect of macropores,micropores and mesopores in CP-TAC-50 promotes the efficient transport of oxygen and electrolyte in the cathode.4)After further Fe³⁺hydrothermal treatment of the tubular air cathode prepared by direct carbonization,the Fe doping amounts and the ratio of graphitic-N in the cathode increased significantly,providing more catalytic sites for ORR.The maximum power density of 195.1 W m^-3was obtained in MFC for the tubular air cathode after hydrothermal treatment with the optimal concentration of Fe³⁺（1 M）,which is higher that of air cathode MFC treated with other concentrations of Fe³⁺.The experimental results show that the tubular air cathode has high ORR performance and has a broad prospect.