Study On Performance Enhancement Of Microbial Fuel Cells And Mass Transfer Characteristics Of Electrode Interface

Microbial fuel cell(MFC)is a promising bioelectrochemical device developed in the field of environment and energy in recent years that integrate wastewater treatment and energy recovery.MFC directly uses electroactive bacteria as biocatalyst to convert the chemical energy contained in wastewater into green electricity.It has broad application prospects in wastewater treatment,hydrogen production by electrolysis,desalination,microbial sensors,etc.However,the low energy density and high manufacturing cost are still two key issues that restrict MFC's practical application.In view of this,a large number of studies have been carried out.All these work focus on enhancing the mass transfer capacity between electrode catalyst and electron donor/electron acceptor,so as to effectively improve the power generation efficiency of MFC.This dissertation focused on these three aspects of developing electrode materials,enhancing mass transfer at the electrode interface and simplifing electrode preparation process.Firstly,electric double-layer capacitive(EDLC)material was used to modify the MFC anode to increase the adhesion of microorganisms and improve the electron transfer efficiency.Secondly,a dual functional organic compound was applied as additive/carbon source to optimize the anode environment and regulate the mass transfer in anode.Then,a three-dimensional(3D)metal fiber felt with abundant macropores was applied as the base material,and simple/low energy consumption methods were adopted to directly load metal/non-noble composite catalysts on metal fiber felt to fabricate air cathode.By improving the electrode structure and optimizing the combination of base material and catalyst,the three-phase interface(TPI)where oxygen reduction reaction(ORR)occurred was regulated,and the energy consumption was also reduced.Finally,a mathematical and physical model of air cathode containing aerobic biofilm was constructed to research the effect of biofilm and its thickness on the cathode performance.The main work of this dissertation are summarized as follows:Study on the modification of anode electrode.Super-capacitor activated-carbon carbon-nanotubes(SC-AC-CNTs)with large specific surface area and EDLC characteristics was first used to modify the anode to improve its low surface area,poor biocompatibility and weak electrochemical property.The results showed that SC-AC-CNTs modified carbon cloth(CC)significantly improved the electricity generation of MFC,which was much larger than that of SC-AC modified CC and the blank CC.The modification of SC-AC-CNTs obviously increased the specific surface area and electrochemical activity of CC;The high specific surface area and EDLC characteristics of SC-AC-CNTs synergistically promoted the adhesion of microorganisms on the anode surface,thus enhancing the electron transfer efficiency to anode.Study on the optimization of mass transfer in anode biofilm environment.Sodium citrate with the properties of p H adjustment and microbial degradability was applied as anolyte additive to regulate mass transfer in anode and enhance the bioelectricity generation.The p H adjustability helped to restrain the sharp decline of p H in the biofilm region caused by microbial metabolism.Adding an appropriate amount of sodium citrate(0.2 g L^-1)could effectively enhance the catalytic activity of bioanode,and also slow down its performance decline tendency as the operation cycle increases.The electroactive biofilm with sodium citrate addition had a more diverse and stable microbial community,which essentially enhanced the catalytic capacity of bioanode.Considering the excellent characters of sodium citrate,it was subsequently used as carbon source in MFC.Compared with the common carbon sources(sodium acetate and glucose),the MFC with sodium citrate as carbon source had higher power generation capacity,stronger electrochemical activity and smaller electron transfer impedance.And also,the protein content,population abundance and diversity in the anodic biofilm with sodium citrate as carbon source were significantly higher than those with sodium acetate and glucose.Study on the optimization of air cathode structure.Stainless steel fiber felt(SSFF)with abundant macropores was applied as the base material,and a one-step hydrothermal method was proposed for in-situ growing Pd nanocatalysts on SSFF(Pd-SSFF)to optimize the combination of catalyst layer and base material,while simplifying the preparation process of air cathode.The oxygen reduction activity of the achieved cathodes was effectively enhanced by filling conductive carbon black into the 3D pores of SSFF owing to the optimized TPI caused by the enhanced mass transfer(O₂ and OH^-)and the reduced catalyst water logging.At the same catalyst loading(0.5 mg cm^-2),the performance of Pd-SSFF cathode was comparable to that of Pt/C-CC cathode.To further reduce the energy consumption and avoid the use of noble metals,non-noble composite catalysts based on graphene and Mn O₂ were synthesized directly on SSFF by electrochemical and chemical methods to prepare an energy-efficient air cathode(r GO@Mn O₂-SSFF).The ORR ability of r GO@Mn O₂-SSFF cathode was greater than that of Pt/C-CC cathode,and the excellent performance was found to be due to the 3D framework-pore structure of SSFF which helped the prepared cathode possess larger electrochemical active area.Simulation study on the effect of biofilm on air cathode performance.A two-dimensional steady-state air cathode model that comprehensively considered the catalytic reaction,the biofilm growth kinetic and the internal mass transfer process was constructed to research the effect of biofilm and its thickness on the mass transfer and electricity generation of air cathode.The cathode performance predicted by the model was in good agreement with the experimental data.The existence of biofilm reduced the overall performance of air cathode.When the biofilm thickness was 0.5mm,the maximum power density of air cathode with biofilm decreased by about3.3%and the polarization overpotential increased by 8.7%compared to the air cathode without biofilm.As the biofilm thickness increases,the output current density of Pt/C catalyst layer continued to decrease,while the output current density of biofilm layer gradually increased.Since the electricity contribution of the biofilm layer was small,the total electricity output of air cathode maintained a continuous downward trend.By analyzing the mass distribution in Pt/C catalyst layer,it was concluded that the decrease of electricity generation was mainly due to the gradual accumulation of OH^-(products)and the continuous decrease of O₂(reactants),which were all caused by the growth of biofilm.