| The characteristic of electricity production and simultaneous wastewater treatment from microbial fuel cells(MFCs) has drawn much research attention recently. MFC is a promising technology in which microbial oxidation of organic matter and biomass can directly convert chemical energy into electricity energy. However, low power density has been one of the bottlenecks for the MFC application. The anode performances play a crucial role on the overall performance of the MFC. Among many anode performance-limiting issues, the anode material is a one of the significant factor, which can affect bacterial growth, electron transfer and substrate supply. Therefore, the high-performance anode materials are highly desired.In this study, electrochemical oxidation as a simple, quick and cost-effective anode modification method was proposed to modify the carbon cloth, mechanism of the accelerated electron transfer by the pre-treatment was investigated. Aimed at the change of the alkaline wastewater p H in MFC practical applications, the cultivation of the bacteria in varying p H condition was considered to the suitable treatment of the alkaline wastewater and the electrochemical activity of the biofilm, active biomass, the internal resistance and power generation of the MFC were studied. Based on the poor substrate supply and product removal in the inner space of carbon-brush anode, the spiral anode rotation was used to enhance the mass transport of substrate as well as products and the effect of the rotation carbon-brush anode on the substrate diffusion resistance, the electrochemical activity of the biofilm and MFC performance were investigated. The recycle of carbon-brush by the method of high-temperature carbonization and alkaline treatment was researched. Moreover, in order to overcome the small pores blocking in the inner space of carbon-brush anode, the equally spaced stainless steel mesh anode was constructed in a tubular MFC and the effect of varying concentrations and rates of substrate on the COD removal efficiency and performance of the MFC were studied. Due to high special surface area, strong biocompatibility and adsorption capacity of the bamboo charcoal, the performance of the MFC with tubular bamboo charcoal anode was investigated and scale-up of the bamboo charcoal tube anode was demonstrated by employing a bundle of tubular bamboo charcoal. Subsequently, the inner diameter and length of the bamboo charcoal anode were optimized in terms of the electrochemical activity of the biofilm, substrate transfer, the stabilization during a long term and the biofilm distribution. Based on the hierarchical porous structure of loofah sponge, the performance of the MFC equipping with natural loofah sponge anode decorated by carbon black was investigated. The effect of the pre-treatment method of carbon black on the electrochemical active surface area, the morphology of the biofilm as well as the internal resistance and performance of the MFC was studied. The main results are summarized below:(1) The performances of two-chamber microbial fuel cells(MFCs) with surface-modified carbon cloth anodes by four methods are compared including soaking in aqueous ammonia(CC-A), electrolysis in phosphate buffer(CC-P), electrolysis in nitric acid(CC-N) and electrolysis in nitric acid followed by soaking in aqueous ammonia(CC-NA). It is found that performances of all these modified anodes are better than that of the bare one. The MFC with the anode modified by CC-NA yield the maximum power density of 3.20 ± 0.05 W m-2, which is 58%, 36%, 35% and 28% higher than those of the MFC with untreated anode(CC-C, 2.01 ± 0.02 W m-2), and treated by the CC-A(2.35 ± 0.15 W m-2), CC-N(2.38 ± 0.02 W m-2) and CC-P(2.50 ± 0.08 W m-2) methods, respectively. The active biomass and EIS analysis for the anodes indicated that the highest power output of the CC-NA MFC may primarily attributed to the enhanced electron transfer resulting from the existence of quinoid groups on the CC-NA surface, rather than the biomass effect. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopic analyses of the CC-NA anode further demonstrate that the formation of quinoid groups contributes to the improvement in the MFC performance by accelerating the electron transfer between electrochemically active bacteria(EAB) and the anode surface.(2) The characteristics of electricity generation and COD removal of dual-chamber microbial fuel cells(MFCs) operated with alkaline substrates were studied. Substrates with constant p H of either 7 or 9 as well as varying p H in a cycle of 7-8-9-8-7 were used. MFCs operated with these substrates were denoted as MFC-p H7, MFC-p H9 and MFC-p HV, respectively. The experimental results indicate that the MFC-p HV can generate the highest performance of 2554 ± 159 m W m-2. Cyclic voltammetry(CV), active biomass and electrochemical impedance spectroscopy(EIS) measurements were conducted and these results suggested that the MFC-p HV had the highest electrochemical activity per unit biomass and the lowest internal resistance, which together contributed to the improved power output of the MFC-p HV. In addition, compared with the other two MFCs operated at fixed p H values, the COD removal efficiency of the MFC-p HV was improved due to the stronger adaptation to the varying p H-environment.(3) A novel method was proposed to improve the power output of microbial fuel cells(MFCs) by rotating the carbon-brush anode. The MFC with a rotating anode produced a peak power density of 210 ± 3 W m-3 and a maximum current density of 945 ± 43 A m-3, 1.4 and 2.7 times higher than those of the non-rotating case, respectively. The difference of the electrochemical impedance spectroscopy and cyclic voltammetry before and after anode rotation clearly suggested that the mass transfer to the spiral space was enhanced by the rotating anode. Furthermore, Tafel plots analysis also revealed that the rotating anode can improve the electrochemical activity of the biofilm.(4) Aimed at the recycle of carbon-brush anode, the two methods of high-temperature carbonization and alkaline treatment were adopted to treat the used carbon-brush anode. The effect of the different treatment of the used anode on the MFC performance was investigated. The results suggested that high-temperature carbonization was an effective method to recycle carbon-brush anode for MFC engineering application. Compared with the situ carbon-brush, the carbon-brush anode treated by high-temperature carbonization resulted in the reduction of start-up time and charge transfer resistance as well as the improvement of the MFC performance and COD removal efficiency.(5) The equally spaced stainless steel mesh was used to anode which enhanced the use efficiency of the biofilm on the anode surface and overcame the limited development of the biofilm in the inner space of carbon-brush anode due to the small pores. Compared with carbon brush anode, the performance of the MFC outfitted with the stainless steel mesh anode having the same dimension was improved. Moreover, the MFC equipping with the stainless steel mesh with holes further enhanced the substrate transfer in the inner space of anode and boosted active biomass on the anode surface, resulting in the improvement of the MFC performance and wastewater treatment.(6) The bamboo charcoal tube was proposed as a novel anode substrate by carbonizing the natural bamboo. Its surface functional groups, biocompatibility and internal resistance are thoroughly investigated. Performance of the MFCs with a conventional graphite tube anode and a bamboo charcoal tube anode is also compared. The results indicate that the tubular bamboo charcoal anode exhibits advantages over the graphite tube anode in terms of rougher surface, superior biocompatibility and smaller total internal resistance. Moreover, the X-ray photoelectron spectroscopy(XPS) analysis for the bamboo charcoal reveals that the introduced C–N bonds facilitate the electron transfer between the biofilm and electrodes. As a result, the MFC with a bamboo charcoal tube anode achieves a 50% improvement in the maximum power density over the graphite tube case. Furthermore, scale-up of the bamboo charcoal tube anode is demonstrated by employing a bundle of tubular bamboo charcoal to reach higher power output.(7) The effect of inner diameter of bamboo charcoal tube anodes was experimentally investigated on the microbial fuel cells(MFCs) performance to obtain an optimal structure. After successful star-up, bamboo charcoal tube anodes with different inner diameters(1mm, 1.5 mm, 2 mm and 3 mm in inner diameter were named as MFC-D1, MFC-D1.5, MFC-D2 and MFC-D3) resulted in various voltage output. However, MFC-D2 and MFC-D3 still kept stable output while the MFC-D1 and MFC-D1.5 performances had a significant drop for a long-term operation(after operation for 30 days). Scanning electron microscope and electrochemical impedance spectroscopy results indicated that the reduction in the powder density for MFC-D1 and MFC-D1.5 attributed to a compact and thicker biofilm on the anode surface leading to the increased the internal resistance of MFCs. Furthermore, compared with other anodes, the highest power density(3303 W m-3) for MFC-D2 suggested that the tubular bamboo charcoal with 2 mm in diameter was more suitable for electricity generation.(8) A new composite anode material fabricated by depositing carbon black on the loofah sponge matrix surface was proposed. Carbon black is oxidized in nitric acid(HNO3) and hydrogen peroxide(H2O2), respectively, to increase electrochemical properties before starting deposition. Cyclic voltammetry(CV) in the potassium ferrocyanide solution demonstrate better electrochemical performance of the H2O2 treated carbon black-loofah sponge composite than that of others. The MFC anode decorated with the H2O2 treated carbon black achieve the maximum power density of 61.7 ± 0.6 W m-3, which is higher than one decorated with the HNO3 treated or untreated carbon black. Scanning electron microscopy(SEM) observation show a compact bacterial attachment on the H2O2 treated carbon black surface. Results of electrochemical impedance spectroscopy(EIS) and Tafel plots indicate a faster electron transfer rate between the H2O2 pre-treatment of carbon black and electrochemically active bacteria(EAB). Therefore, using the MFC outfitted with the H2O2 treated carbon black–loofah sponge anode enhances the MFC performance. |
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