Response Of Anodic Biofilm And Performance Of A Plate Microbial Fuel Cells To Variable Current Conditions

A microbial fuel cells (MFC) is a kind of energy tranformation device byconverting the chemical energy in wastewater into electricity using microorganisms ascatalysts. It has drawn much attention due to its unique advantage of treatingwastewater and simultaneously producing electricity. However, the power density ofMFCs is too low to meet the needs of practical application. The performance of MFCsis influenced by many factors such as the reactor structure, anode materials, biofilm,electron transfer mechanism and operating conditions. Compared with chemical fuelcells, response of biofilm and performance of a microbial fuel cell to variable conditionsis relatively slower. The variation of operating conditions plays an important role in themass transfer in the anode biofilm as well as the cell performance. Therefore, it isessencial to investigate the respone of anode biofilm and performance of MFCs underdifferent operating conditions.In the present study, operating currents of MFCs were controlled by a constantcurrent source. The response of biofilm structure and performance of MFCs to variableoperating conditions were studied, including different constant currents, step currentsand reverse currents. The main results of this paper are listed as follows:(1) The biofilm structure and performance of MFCs under different constantcurrents were studied after start-up. A constant current of0.75,4,7.5,9mA wereapplied on the four MFCs for10days, respectively, after successful start-up with a100external resistance. It was found that the electrochemical activity of biofilm increasedand anodic resistance decreased as the constant current increased. The result offluorescence microscope also showed that the protein content of the biofilm rose withan increase in the operating current. Compared with the MFC operated at100external resistance all the time, a significant increase in the maxmum power density andcurrent density was obtained by the MEC operated with9mA constant current, reaching2.6±0.2W·m-2and11.3±0.3A·m-2, respectively. It can be seen from high-resolutionSEM images that much micropores were found on the surface of biofilm when operatedat constant current of9mA. It was expected that these micropores benifited the masstransport of the substrate and product within the biofilm, leading to an increase in theoxidation rate of substrate and consequently the improved performance of the MFC.(2) Based on the aforementioned results, when the MFC was operated under a high current for a long time, the reverse phenomenon usually occurred in the MFC.Therefore, the response of anode biofilm and performance of MFCs to step currentmode was investigated. In this experiment, the current varied from7.5mA to0.75mAor from7.5mA to open circuit condition. It was observed that compared with MFCoperated at a constant current of7.5mA, a smaller electrochemical activity of biofilmas well as active biomass for MFC was obtained in these two step current conditions.Moreover, the MFC produced the maximum power density of2.0W·m-2, which wasabout15%lower than that of the MFC operated at7.5mA constant current (2.3W·m-2),.Therefore, this result confirmed that MFC operating at a continuous high current cankeep high biofilm activity and superior performance.(3) When the MFC undergoing reverse condition for a long time, theelectrochemical activity and power density of MFC would deteriorate significantly dueto the complete deciduous biofilm. Therefore, the reponse of anode biofilm andperformance to the reverse current was also studied. The experimental results showedthat the maximum power density of the MFC decreased gradually with an increase inthe duration of the reverse current. When the MFC was operated under the reversecurrent for more than12h, the maximum power density reached almost zero. Theperformance of the MFC, which had undergone the reverse current for10min, wouldrecover to the same value as that before reverse. However, for MFCs having sufferedfrom the reverse current for longer than1h, their performances could only recover to85%of their original power densities. The test results of CV and biofilm dry weightindicated that the deteriorated performance could be attributed to the slightly reducedactivity of anode biofilm and the decreased biofilm dry weight for MFCs with durationof the reverse current for more than1h.