Mass Transfer And Performance Enhancement Of Carbon Based Air Cathode In Microbial Fuel Cell

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.