| Microbial fuel cell (MFC) is a low energy-consuming wastewater treatment technology which can purify wastewater and simultaneously convert its chemical energy into electrical energy. In recent years, it has become one hot topic in the environment field due to its excellent theoretical research value and application prospect.In this study, based on the electrode in packing-type microbial fuel cell (PMFC), the contact resistance between activated carbons and seven kinds of electricity-collecting materials was measured. Besides, electrochemical impedance spectroscopy (EIS) was compared with constant current method on the evaluation of electrode ohmic resistance. To study the factors on conductivity of the packed electrode, ionic conductivity in the solution and electronic conductivity from different electrodes were investigated.In addition, ohmic resistance of PMFC was examined through measuring electric potential distribution of the packing-type electrode and the aqueous solution. Meanwhile, the external pressure was used as an operating condition to evaluated the electricity generation performance and the electric potential distribution of PMFC.The PMFC was designed in a total volume of25L, and titanium mesh was used to collect current more effectively in PMFC and the power productions of PMFCs with different current collecting methods (outside, inside and bilateral) were compared. Regarding to the influence of water flow direction (up-flow, down-flow and cross flow), the distinction of performances of different layers in PFMC was also contrasted as well as the contact resistance between electrode and collector under a range of depth.The results included:(1) The contact resistance between the activated carbon and different sizes of stainless steel mesh was38.8Ω,51.6Ω,57.7Ω and77.2Ω, respectively corresponding to the sizes including60mesh x0.15mm,30mesh x0.3mm,80mesh x0.12mm, and10mesh x05mm. We demonstrated that both the mesh and the wire diameter have an impact on contact resistance, and the stainless steel (60mesh x0.15mm) yielded the minimum contact resistance with activated carbon. Furthermore, graphite and titanium sheet were also used as electricity-collecting materials, and the contact resistance with activated carbon were26.15Ω and62.58Ω respectively. It was believed that the electron could be effectively collected and the ohmic resistance might be reduced when,the graphite was used as electrode current collector.(2) It was verified that EIS could not be used to measure the ohmic resistance loss during the real charge transfer process in a circuit system with no stable connectivity or the presence of the electron conduction and ionic conduction. However, for a connective circuit system, when there was only electron conduction in the system, EIS and the constant current method could be exploited to measure the ohmic resistance.(3) The main obstacles that limited the ion migration in PMFC were the characteristics of packed electrode and external pressure. The activated carbon yielded a higher capacitance (46.7F) than the graphite (5.48F). When the external pressure was increased from0N to10N, the unit ion mobility of activated carbon decreased from17.330/cm to0.29Ω/cm (minimum). However, there was no change for graphite with a constant unit ion mobility (29.54Ω/cm).It was seen that the activated carbon used as electricity-collecting materials had advantages on the ion migration.(4) The part of electricity production in the anode of PMFC was mainly centralized in the areas closed to the ion exchange membrane. The ion transport resistance accounted for the main part of ohmic resistance in PMFC. Moreover, for equal conditions, electric potential of the packing-type electrode and the aqueous solution were affected by the outer current. Larger the outer current would have a greater electrical potential gradient.(5) The maximum power densities of the three methods (outside, inside and two sides connection) were9W/m3,13W/m3and20W/m3, respectively. Stratified connection obtained5%～16%higher average current density than parallel operation in the PMFC. It is noted that the water flow direction and external pressure contributed to the distinction of electricity generation performances of different layers in PFMC. The performance of the sub layer was significantly influenced, while those of the upper and middle layers showed much smaller change. The pressure between the electrode and the mesh was believed to be the key factor to affect the performance of the electricity generation in PMFCs. |
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