Cathode Performance And Mass Transfer Enhancement Of Microbial Fuel Cell With Air-cathode Based On Microalgae Derived Biocarbon

The serious pollution of water resources and energy shortage are two important issues for humankind.In order to solve these problems simultaneously,microbial fuel cell?MFC?technology has been proposed.MFC is one of the bioelectrochemical energy conversion devices,which can degrade organic matter in wastewater under the catalysis of the anode biofilm to reduce water pollution and generate additional electric power to alleviate energy crisis,simultaneously.MFC has many advantages such as mild reaction condition,stable and reliable operation,clean and efficient utilization,and environmentally friendly features.MFCs generally can be divided into two kinds of structure:the single-chamber MFC and the double-chamber MFC.Single-chamber MFCs have the advantages of simple structure and easy processing compared to double-chamber MFCs.In addition,single-chamber MFCs can run without the use of proton exchange membrane,reducing the cost and mass transfer resistance,effectively preventing the anodic acidification and eliminating the risk that cathode liquid flow into the anode chamber.The air-cathodes are usually used in single-chamber MFCs,primarily because the oxygen in the air is cheap and readily available,and there are no additional system operating costs.Therefore,the catalytic properties of oxygen reduction reaction?ORR?and the transport of substances and charges in the air-cathode play a crucial role for the electricity generation performance of MFCs.ORR consists of two reaction pathways:one is the production of water through a four-electron pathway;the other is the production of hydrogen peroxide through a two-electron pathway.The direct reduction of oxygen molecules to water is more preferable for the ORR process due to the four-electron route possessing a much lower energy loss and a lower overpotential,which is conducive to further strengthening the MFC performance.At present,platinum nanoparticle supported on high surface area carbon?Pt/C?is the most common catalyst for ORR.However,Pt-based catalysts are not suitable to the real application of MFCs because of its high cost,limited reserves,low specific surface area and poisoning by microbial metabolites in MFCs,which affect the large-scale application of MFCs and long-term operating conditions of the MFC electricity generation performance.In view of the shortcomings of the traditional noble metal ORR catalysts,high-performance carbonaceous nitrogen self-doping ORR catalysts,Fe-N-C ORR catalysts and size controllable three-dimensional tubular air-cathode were prepared from three aspects:reducing the cost of ORR catalysts,enhancing the ORR catalyst activity and producing an air-cathode that is conducive to mass transport.The physicochemical characterization of the catalysts and air-cathodes were investigated.Experiments and theoretical studies were carried out on the mass transfer properties and the electricity generation performance of the MFCs.The main contents of this study include:?1?Nitrogen self-doping carbonaceous ORR catalyst was prepared in one-step method using Chlorella pyrenoidosa as the precursor.The influences of carbonization temperature on the pore structure,graphitization degree and heteroatom doping of the catalyst were studied.Then,the mass transfer characteristics and power generation performance of the catalyst in MFC were studied;?2?Fe/N-C ORR catalysts derived from Chlorella pyrenoidosa were prepared by doping iron using hydrothermal or direct carbonization method.The influences of valence and concentration of iron element on the pore structure and surface defects of the Fe/N-C catalysts were investigated.The effect of heteroatom doping on the electricity generation performance of MFCs with different catalysts was also investigated.?3?A MFC with dimensionally controllable three-dimensional tubular air-cathode was constructed.The effects of different preparation conditions on the pore structure and ORR performance of tubular air-cathodes were studied,and the influence of pore structure of tubular air-cathodes on the electricity generation performance was further investigated.The main results are as follows:1)Chlorella pyrenoidosa,a kind of natural biomass enriched in nitrogen,can be used as a precursor for the preparation of nitrogen self-doping ORR catalysts.The carbonization temperature has a great influence on the ORR performance of the catalysts.The main conclusions are as follows:With the increase of carbonization temperature,the degree of graphitization of catalysts gradually increases,the conductivity of catalysts gradually increases,and nitrogen doping content decreases.But the proportions of Pyridinic-N and Graphitic-N increases as the carbonization temperature increases.The specific surface area of the catalyst first increases and then decreases as the carbonization temperature increases.This result is mainly due to the reason that high temperature causes more micropores switch to macropores.Based on the above factors,the ORR performance of catalysts appears to increase first and then decrease with carbonization temperature increasing.The catalyst?CP900?prepared at900^oC shows the best ORR performance.The maximum power density of the MFC loaded with the CP900 is 2068±30 mW·m^-2,which is 13%higher than that of the MFC with the same loading Pt/C(1826±37 mW·m^-2).2)The Fe-N-C ORR catalyst was prepared by direct carbonization method with different valence iron salts?FeCl₂ and FeCl₃?and Chlorella pyrenoidosa.It is found that the addition of iron reduces the doping content of nitrogen and the valence of iron has an effect on the species of nitrogen-containing functional groups.Fe³⁺is more conducive to the formation of pyridinic-N functional groups.In addition,the valence of iron has an influence on the pore structure and the degree of surface defects of catalysts.CP-FeCl₃ obtained a higher maximum power density of 2358.37±90 mW m^-2 in MFC.3)Doping the iron element during the hydrothermal treatment can further enhance the ORR performance of the catalysts.The valence of iron has an important influence on the ORR performance of the catalysts during the hydrothermal treatment.The experimental results show that the method of adding Fe²⁺during hydrothermal treatment can form embedded iron oxide nanoparticles as the ORR active site.At the same time,Fe²⁺can form more micropores and mesopores on the surface of the catalysts,which can be increase the electrochemical reaction area and strengthen the mass transport in the catalytic layer of the electrode.The catalysts were tested by rotating ring disk electrode in 50 mM phosphate buffer solution?PBS?and CP-HT-FeCl₂ exhibited the highest ORR catalytic activity.In MFC operating conditions,the maximum power density of MFC loaded with CP-HT-FeCl₂ is 2739.8±160 mW m^-2,which is 50%higher than that of MFC loaded with Pt/C catalyst as a same loading(1826±37 mW m^-2).4)The influence of Fe²⁺concentration in the hydrothermal treatment on the iron doping amount of the catalysts was studied.It is found that the doping content of iron increase first and then decrease as the Fe²⁺concentration increase.In addition,the concentration of Fe²⁺has an effect on the surface morphology,pore structure,and iron-containing functional groups of the catalysts.The experiment also find that the doping content of iron and the air-cathode electricity generation performance are in direct proportion to each other,indicating that the iron-containing functional group is the decisive factor in promoting ORR process in the air-cathodes.5)The size-controllable tubular air-cathode is prepared using the agar gel characteristics and freeze-drying pore-forming method.The MFC electricity generation performance is enhanced by enhancing the tubular air-cathode ORR activity and enhancing mass transport in the tubular air-cathode.The obtained tubular air-cathode has a rich interconnected pore structure,ensuring a good mass transport efficiency within the electrode.In addition,the ORR performance of tubular air-cathode prepared by Fe³⁺hydrothermal treatment and direct carbonation is compared.It is found that Fe³⁺hydrothermal treatment can promote the formation of defects on the air-cathode surface and carbon particles on the carbon skeleton.The tubular air-cathode micropores,mesopores and macropore pore volumes have all improved.In addition,the doping of iron makes CPT-HT-Fe obtain a higher content of pyridinic-N and graphitic-N.All of the above factors lead to a higher ORR activity of CPT-HT-Fe.The MFC with CPT-HT-Fe obtained the highest maximum power density value of 112.5 mW m^-3,which is higher than that of Pt/C(81.5 mW m^-3).It shows that CPT-HT-Fe has broad application prospects.