| With the rapid development of economy,the excessive emission of organic wastewater has led to increasingly serious environmental problems.At the same time,under the background of the depletion of traditional fossil energy,the treatment of organic wastewater needs to expend vast amounts of energy,which cause the energy problem becoming more serious.Therefore,the issue of significance confronting human beings is to seek a clean energy source and an efficient process for treating wastewater.Microbial fuel cells(MFCs)is a kind of new technology of bio-energy conversion technology,in which the electrochemically active bacteria attached on the anode can directly convert chemical energy into electricity energy by oxidation of organic matter in wastewater and simultaneous degrade wastewater.MFC becomes a promising practical application technology because it not only effectively controls the environmental pollution problems but also minimizes the present energy crisis.However,the working voltage of a typical MFC around 0.5 V hinders the practical application in wastewater treatment combine with driving an electrical appliance.In order to improve MFC performance,efforts have been made to form a MFC stack for enhancing the voltage/current output by connecting the individual cells in series/ parallel.Unfortunately,the voltage reversal probably occurred in MFC stack,which decreased their performance.Therefore,we constructed a MFC stack connected in series and then studied the reasons of voltage reversal.The results showed that the voltage reversal in MFC stack was attributed to the weak performance of unit MFC.To solve this problem,a new three-dimensional porous anode is proposed for improving anode performance;On the other hand,we proposed an effective operation method by decreasing external resistance value instead of a constant resistance to improve the anode performance of unit MFC,thereby enhancing the unit MFC and then improving the stack performance finally.The main results are summarized below:(1)A MFC stack in series consist of six unit MFC which started-up under different external resistance was constructed.The effect of weak performance of unit MFC on the performance of MFC stack and the reason of voltage reversal were investigated.It shows that with a low start-up external resistance,the superior MFC performance was obtained.During discharge process,the current output of the MFC stack,in which unit MFC was started up with different external resistance,was limited by the MFC(MFC-I)with weakest current output.When the discharge current of MFC stack exceeded the maximum current of the weakest unit MFC,the voltage reversal occurred and the anode potential of MFC-Irose sharply to 1.2 V vs.SHE.Therefore,the reason of voltage reversal in MFC stack was attributed to the sharply increased of anode potential of the weaker performance of unit MFC.Owing to the sharply increased of anode potential caused carbon corrosion,MFC-Iconverted from a galvanic cell to an electrochemical cell,resulting in a significance reduce of the stack performance.(2)A spongy three-dimensional porous anode electrode was proposed to improve the MFC performance.Under constant anode potential,the different anode electrode including carbon cloth and three-dimensional electrode(sponge-1、sponge-2)were started-up in the half call system.The effects of the different anode electrode on the biomass morphology and the electrochemical activities of the biofilm were investigated.The results showed that,compared to carbon cloth,the three-dimensional electrode(sponge-1、sponge-2)had the higher current density and the shorter started-up time.Due to the excellent stereoscopic open porous structure,both the transport of substance and bacterial adhesion onto solid surface increased at the three-dimensional electrode,therefore the formation of the more electrocatalytic activity sites,which lead to a sharp increase of electricity performance.(3)The improvement of the stack performance by operating unit MFC with variable resistance was proposed.Two unit MFCs with 50 Ω external resistances was started up and then was cultivated by changing external resistance followed by 50 Ω-25 Ω-10 Ω.The evolution of the unit MFC performance with variable resistance and the stack performance before and after cultivating process were compared.The result demonstrated that,decreasing the external resistance from 50 to 10 Ω,the internal resistance of biofilm decreased and its electrochemical activity increased,and the superior biofilm that facilitate the substrate and buffer supply formed.Therefore,the limited current of unit MFC improved and power overshoot phenomenon disappeared.Furthermore,compared to the MFC stack started with 50 Ω external resistances,maximum power of MFC stack operating with the variable resistance method was significantly improved and no voltage reversal was observed. |
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