Investigation Of High Performance Anodes And Proton Transfer Inside Anode-respiring Biofilms In MFCs

Microbial fuel cell（MFC）is a kind of green treatment technology that can continuously extract energy from organic waste and convert it into electrical current,involving in mass transfer,microbiology,electrochemistry,material science and environmental engineering.MFC has a great potential in simultaneous wastewater treatment and energy recovery,as well as H₂ production,desalination,contamination remediation,carbon dioxide conversion and biosensors.At present,the main challenge for MFC would be its low power density.The typical maximum power density is² to 3 W/m²,which is much lower than that in chemical fuel cell delivered on the order of 1 W/cm².The relatively low power density remains one of the main obstacles for its practical application.The anode surface characteristics,such as surface area,potential,biocompatibility,and surface roughness as well as anodic biofilm significantly affect the anodic reaction.So optimizing the anode and anodic biofilm is the key to improve the power density output of the whole MFC.The following two aspects are mainly studied in this dissertation aiming at improving MFC’s anode performance.Firstly,anode materials and structure have been optimized based on the latest nanomaterial technology by combining nanomaterials with three-dimensional macroporous electrodes.Secondly,the anodic biofilm plays an importent role in electricity generation of MFC.The microelectrode has been used to explore the proton transfer process inside the biofilm and also a method was recommended to estimate the thickness of biofilm.Based on the above purposes,the main work and results of this dissertation are summarized as follows:（1）A novel three-dimensional（3D）macroporous MFC anode was manufactured.Three-dimensional stainless steel fiber felts（SSFFs）decorated with activated carbon（AC）,carbon nanotube（CNT）,commercial graphene（GN）,polyaniline（PANI）and reduced graphene oxide（rGO）were developed by a simple impregnation approach.GN/SSFF and rGO/SSFF delivered a maximum power density of 2142 mW/m² and2393 mW/m² respectively,while CNT/SSFF-MFC achieved its maximum power density of 1280 mW/m² and the maximum power densities of AC/SSFF-MFC and PANI/SSFF-MFC were 560 mW/m² and 360 mW/m²,respectively.The experimental results proved that SSFF can not only serve as a macroporous and highly conductive scaffold,but also provide large surface for nanoparticle attachment.The nnoparticles on the scaffold improved the biocompatibility of the stainless steel electrode surface and provided high specific surface area for bacterial attachment.And also the macroporosity guaranteed bacterial internal colonization that could enhance electrode reaction.All these resulted in the high-performance MFC anode.Different amount of rGO（0.25,0.68 and 1.26 mg/cm²）on SSFF was investigated in MFCs.The results showed that MFC with a loading of rGO（0.68 mg/cm²）achieved a higher power density of 2393 mW/m².It demonstrated that there was an optimal rGO deposition existed for rGO electrode fabrication which can greatly improve the performance of anode and MFC as well.（2）The microelectrode test system was established.We quantified the spatial pH distributions（pH-depth profile）within an electricity-producing biofilm and its concentration boundary layer,as well as the bulk solution by employing pH microelectrode.The pH-depth profiles were sigmoid-shaped.It was found that pH values near the anode surface and in the bulk solution gradually decreased with the current density,and the pH difference between them increased.When the current density reached 10.23 A/m²,the pH difference was increased to 1.33.This means that the proton concentration near the electrode surface was almost 21.4-fold higher than that of the bulk layer.The buffer experiment demonstrated that under low buffer concentration conditions,the protons would not be effectively transported out of the biofilm.The pH value near the anode surface decreased to 4.91 under 25 mM phosphate buffer solution.（3）A method was recommended to estimate the thickness of biofilm.The thickness of biofilm and average pH inside biofilm was obtained by plotting the derivative of pH-depth profile.It was found that,when the current density increased to 10.71 A/m² from 1.27 A/m²,the biofilm thickness increased from 90μm to 325μm while average pH dropped from 7.29 to 5.88.This means that the detected free protons accumulated inside the Geobacter biofilm increased to 47-fold（1.68 pH units）.In conclusion,a novel 3D macroporous electrode was fabricated for high-performance MFC by coating the carbon nanoparticles on 3D stainless steel fiber felts.SSFF was applied in MFC for the first time.Next,a method for determining anodic biofilm thickness based on the proton concentration profiles was proposed.Then,the relationship between average pH of anodic biofilm and current density was investigated experimentally.