A bioelectrochemical system (BES) can be operated in both "microbial fuel cell" (MFC) and "microbial electrolysis cell" (MEC) modes, in which power is delivered and invested respectively. To enhance the electric current production, a BES was operated in MFC mode first and a capacitor was used to collect power from the system. Then the charged capacitor discharged electrons to the system itself, switching into MEC mode. This alternate charging and discharging (ACD) mode helped the system produce 22-32% higher average current compared to an intermittent charging (IC) mode, in which the capacitor was first charged from an MFC and then discharged to a resistor, at 21.6 Ω external resistance, 3.3 F capacitance and 300 mV charging voltage. The effects of external resistance, capacitance and charging voltage on average current were studied. The average current reduced as the external resistance and charging voltage increased and was slightly affected by the capacitance. Acquisition of higher average current in the ACD mode was attributed to the shorter discharging time compared to the charging time, as well as a higher anode potential caused by discharging the capacitor. Results from circuit analysis and quantitatively calculation were consistent with the experimental observations.
"Two types of MFCs were constructed: a regular two-bottle MFC (designated as H-MFC)  and a graphite-packed MFC (designated as P-MFC) . As shown in Table 1, the H-MFC had a larger opencircuit voltage, a higher internal resistance but a lower internal capacitance than the P-MFC. "
[Show abstract][Hide abstract] ABSTRACT: Using microbial fuel cells (MFCs) to power a capacitive deionization (CDI) process enables simultaneous removal of salinity and organic matter in wastewater. The desalination performance of an MFC-CDI system can be influenced by not only the capacity of individual components but also the arrangement and operation of the MFC-CDI circuit. Five typical circuits (consisting of serial- or parallel-connected MFCs or CDIs) and two ion-desorption modes (short-circuit and reverse-voltage desorption) were compared. Results showed that the MFC-CDI system could be reasonably modeled (R2 > 0.967) by a first-order resistor–capacitor circuit. The optimal arrangement of the MFC-CDI circuit depended on the electrical characteristics of selected MFCs and CDIs as well as operating conditions. When the system was powered by two MFCs of a larger internal resistance (146 Ω), the highest salt removal after 60 min (m60; 7.5 mg) was achieved by paralleling the two MFCs; with MFCs of a smaller resistance (12 Ω) being used, the highest m60 (16.5 mg) was obtained when the two MFCs were connected in series. Further analysis revealed that the MFC's internal resistance and open-circuit voltage, along with the CDI's internal resistance and capacitance, were the chief factors affecting the charge transfer and accordingly desalination on the CDI electrodes.
"A charging and discharging circuit consisted of three parts: the MFC, two ultracapacitors (C 1 , C 2 ; capacitance: 2 F) and two external discharging resistors (R ex1 , R ex2 ). Four relay switches (S 1 –S 4 ) were used to achieve the function that the MFC charged the two capacitors alternately, while the uncharged one charged an external resistance (Liang et al., 2011) (Fig. S1 in supporting information). The preset time to alternate the switches was defined as t s . "
[Show abstract][Hide abstract] ABSTRACT: Capacitor is a storage device to harvest charge produced from microbial fuel cells (MFCs). In intermittent charging mode, the capacitor is charged by an MFC first, and then discharged through an external resistance. The charge harvested by capacitor is affected by the charging and discharging frequency. In the present study, the effect of the charging and discharging frequency on charge harvest was investigated. At the switching time (ts) of 100s, the average current over each time segment reached its maximum value (1.59mA) the earliest, higher than the other tested conditions, and the highest COD removal (63%) was also obtained, while the coulombic efficiency reached the highest of 67% at the ts of 400s. Results suggested that lower ts led to higher current output and COD removal, but appropriate ts should be selected in consideration of charge recovery efficiency.
"Moreover, the ACD mode employed a simpler external circuit, which may lead to a lower system cost than the IC mode. However, the electricity generation of the ACD-MFC was highly fluctuating – the crest-to-trough current difference in the ACD mode was nearly two times greater than that in the IC mode (Liang et al., 2011). For certain applications, extra devices and cost may be required to modulate the ACD-MFC's output current (or voltage). "
[Show abstract][Hide abstract] ABSTRACT: To enhance the MFC's denitrification performance, this study investigated three different external circuits/operation modes of the MFC: alternative charging and discharging (ACD), intermittent charging (IC) and constant external resistance (R). Results showed that the ACD and IC modes offered larger output currents as well as higher nitrate and COD removal rates than the steady R mode. The best performance was achieved with the ACD mode. At the initial [COD]=∼1200mg/L and [NO3(-)]=∼140mg/L, the ACD mode delivered an average power density of 0.91W/m(3), an average nitrate removal rate of 15.5mg/(Ld) and an average COD removal rate of 137mg/(Ld), 268%, 207% and 168% respectively greater than those by the R mode. The enhancement by the ACD and IC modes was more pronounced at lower nitrate and COD concentrations and/or with the lack of stirring of electrolyte solutions.
Jingya Shen, Yuliang Sun, Liping Huang, Jinhui Yang
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