Study On Low Cost And High Performance Air Cathode Basing On Activated Carbon In Microbial Fuel Cells

With the fast development of the national economy, the demands for water and subsequent wastewater discharge from industry, agriculture and social life are increasing every year, which has posed a great challenge to the sewage treatment industry. Unfortunately, the usual aerobic-anaerobic biotreatment technique is running in the most of the sewage treatment plants at the cost of high power consumption, huge sewage sludge and low recovery ratio. The fast growth is very hard for the sewage treatment industry unless clean production transformation is implemented. Microbial fuel cells (MFCs) can give the current by using the organic pollutants of the wastewater as the electron donor and by using the oxygen as the electron acceptor. Since the chemical energy contained in the organic pollutants is directly converted into the electric energy in MFCs, the productivity of the sewage sludge can be reduced at source thus the recovery ratio will be enhanced. So MFCs is a promising technique for the sewage treatment industry to forward its clean production transformation. The economics of MFCs are further enhanced owing to using the air-cathode. It has the capacity of absorbing oxygen from the atmosphere spontaneous, indicating that the aeration device is no longer needed and its electric consumption can be saved. Air-cathode contains a gas diffusion layer (GDL) toward the atmosphere to absorb and transfer oxygen as well as prevent electrolyte from leaking. It also contains a catalyst layer (CL) toward the electrolyte to provide sufficient transmission paths respectively for the oxygen, protons and electrons as well as areas for the establishment of the gas-liquid-solid three phase interfaces (TPIs). Currently, the brushed Pt-Nafion air-cathode is widely used in MFCs for the wastewater treatment study. Its GDL is prepared by brushing hydrophobic polytetrafluoroethylene (PTFE) binder for several layers and sintering treatment at340℃for each layer. Pores in the GDL for gas diffusion are formed basing on the physical properties of PTFE which melts at high temperature and then contracts during cooling. Its CL is made by brushing Pt/C catalyst and hydrophilic Nafion binder, however, the flooded TPIs by the electrolyte are easy to occur. Since its structure defects as well as high cost, troublesome and extensive manufacturing process, this brushing Pt-Nafion air-cathode would be a hindrance to the development of MFCs towards large-scale and practical application. Therefore, this work aimed to harvest a novel air-cathode with the features of high performance, low cost and practicable.The manufacture method for the air-cathode was established firstly. PTFE were used as the binder to construct the CL (minor constituent) and GDL (major constituent), respectively. The conductive carbon black was added in the GDL to improve the conductivity. The activated carbon (AC) used in the CL played the roles of conductor as well as the catalyst for ORR. Films of the CL and GDL were prepared by treating the carbon powder and binder with agitated ultrasonic firstly and then rolling the blend with mechanical roller. After being sintered at340℃for the gas pores formation, the prepared CL and GDL films were rolled together at two sides of the stainless steel mesh respectively to achieve the activated carbon-PTFE air-cathode (ACAC). The results from linear sweep voltammetry (LSV) measurements showed its obvious superiorities of electrochemical activity and reproducibility compared to the brushing Pt-Nafion air-cathode.Next, three important influences of the CL were studied, including the ratio of AC and PTFE, sintering treatment at high temperature and surfactant concentration. Comprehensive measurements involving scanning electron microscope, mercury porosimeter, LSV, tafel plot, electrochemical impedance spectroscopy as well as MFC polarization were adopted. The research for the ratio of AC and PTFE indicated that too much AC in the CL would restrict the oxygen diffusion. While, too little AC in the CL would hinder the protons and electrons migration. The optimum achieved in this paper was AC/PTFE=6. The investigation for the sintering treatment at high temperature showed that PTFE in CL should be enveloped by the AC aggregations. By avoiding the sintering treatment, the hydrophility of CL was enhanced, meanwhile, the inner pore volume and mechanical strength of CL were increased. It led the simultaneous improvement on the protons migration and oxygen diffusion. So that the increase by35%and18%respectively for the maximum power density (MPD) and coulombic efficiency (CE) of MFC were obtained. According to the study on the surfactant concentration, it was demonstrated that too little surfactant would make CL dry up while too much one would lead TPIs flooded. In contrast with eliminating surfactant completely from CL and adding OP-10nonionic surfactant (8%,15ml·g-1AC), retaining the surfactant from PTFE emulsion (5%) provided the best conditions for the oxygen reduction reaction (ORR) in the CL. The crucial factors for the catalysis of ACAC to the ORR were further investigated. The electron transfer number during ORR catalysed by two AC powders, non AC powder (XC-72) and Pt/C powder were measured using the rotating disk electrode. Then they were made to the CLs of the air-cathodes. Carbon powders were prepared with PTFE by the rolling method and Pt/C powder was prepared with Nafion by the brushing method. The electron transfer number during ORR catalysed by the four air-cathodes were measured using Tafel plot. Other measurements such as the surface morphology observation, pores analysis both for the films and carbon powders as well as MFC polarization were also implemented. The conclusions were achieved as follows:one of the crucial factors for the catalysis of ACAC to the ORR was using the hydrophobic PTFE binder combining with the rolling method to make the CL and GDL. It allowed the porous structure to be formed in the CL. The pore diameter in the CL and sintered GDL both concentrated at6μm which belongs to the macropores. The other crucial factor was oxygen in the macropores of CL could further diffuse into the AC particles through mesopores till the micropores to establish the TPIs. It was found that the ACs with enough and uniform micropores distribution were more beneficial for ACAC to prevent fouling from cathode biofilm and protect the anaerobic circumstances from oxygen diffusion.The effectiveness of AC modification by acid and alkali to ACAC were assessed. AC powders were modified respectively by5.6mol-L"1HNO3and3mol·L-1KOH. The changes concerning the porous structure of AC, voltage-current characteristic and internal resistance constitution in ACAC and its performance in MFCs were compared. It was found that being modified with3mol·L-1KOH at the temperature of85℃made the mesopores with’ink bottle’ shape vanished from AC and the MPD of MFC exhibited an increase by5since the oxygen diffusion was improved. AC modification by3mol·L-1KOH was proved an effective method for the ACAC optimization.At last, the practical applicability of ACAC in MFC for wastewater treatment were identified. With the domestic sewage as the substrates, the MFC reactor with ACAC operated for180days continuously. The power density output, biofilm fouling on ACAC and internal resistance constitution of ACAC were all recorded. The results showed that90%COD in the domestic sewage was degraded while the MPD of1185mW·m-2was achieved. The biofilm fouling occurred just on the interface of CL and electrolyte. The performance of ACAC recovered to78%of the initial level after it was regenerated via striking off the biofilm fouling. The recovery effect could be improved if the regenerating period is further shortened.Compared to the brushing Pt-Nafion air-cathode, the ACAC developed in this work enhanced the MPD of MFCs and its rate of electric energy recovery of capital by142%(1355±26mW·m-2) and79time respectively, while the cost of air-cathode manufacture was decreased by32times. Besides, the ACAC has advantages of simple manufacture process, high precision and good reproducibility over the brushing Pt-Nafion air-cathode. This work could open fresh vistas for the MFCs application in the sewage treatment industry and provide directions in the further optimization for the air-cathode.