Voltage reversal is a critical issue for serially stacking microbial fuel cells (MFCs). It occurs when current density in stacked MFCs increases over critical current density (jcritical). In this ...study, we clearly show that no voltage reversal occurs in stacked MFCs if current density is maintained below jcritical where the anode and the cathode potential in an inferior unit become identical, with an external resistance placed between individual MFCs. We define threshold resistance (Rthreshold) that enables current density below jcritical in stacked MFCs, and demonstrate the validity of Rthreshold theoretically and experimentally. Voltage reversal is controlled in the stacked MFC equipped with Rthreshold by which the current density in the stacked MFC is kept below jcritical. In comparison, a stacked MFC without Rthreshold faces voltage reversal over jcritical. Energy loss in Rthreshold is comparable to energy loss with other voltage control methods, such as passive or active methods. However, the Rthreshold approach is a simple, inexpensive way of controlling voltage reversal, especially for small MFCs (<50 mL).
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•Voltage reversal occurred above critical current density (jcritical) in a stacked MFC.•A threshold resistance (Rthreshold) was determined with jcritical in the stacked MFC.•Voltage reversal was successfully controlled with Rthreshold in the stacked MFC.•Power loss in Rthreshold accounted for 20% of the maximum power density in the stacked MFC.
Voltage reversal in stacked microbial fuel cells (MFCs) is a significant challenge that must be addressed, and the information on its definite cause and occurrence process is still obscure. In this ...work, we first demonstrated that different anodic reaction rates caused voltage reversal in a stacked MFC. Sluggish reaction rates on the anode in unit 1 of the stacked MFC resulted in a significantly increased anode overpotential of up to 0.132 V, as compared to negligible anode overpotential (0.0247 V) in unit 2. This work clearly verified the process of voltage reversal in the stacked MFC. As the current was gradually increased in the stacked MFC, the voltage in the stacked unit 1 decreased to 0 V prior to that of the stacked unit 2. Then, when the voltage in unit 1 became 0 V, it was converted from a galvanic cell to an electrochemical cell powered by unit 2. We found that the stacked unit 2 provided electrical energy for the stacked unit 1 as a power supply. Finally, the anode potential of the stacked unit 1 significantly increased over cathode potential as current increased further, which caused voltage reversal in unit 1. Voltage reversal occurs in stacked MFCs as a result of non‐spontaneous anode overpotential in a unit MFC that has sluggish anode kinetics compared to the other unit MFCs.
Role reversal: Sluggish kinetics on the anode can cause voltage reversal in stacked microbial fuel vells (MFCs). Non‐spontaneous anode overpotential in a unit MFC that has sluggish anode kinetics compared to the other unit MFCs switches the sluggish MFC to a microbial electrochemical cell mode powered by the other rapid units in the MFCs. The anode potential significantly increases over cathode potential as current increases, resulting in voltage reversal.
The power overshoot generated by electron depletion in microbial fuel cells (MFCs) was characterized in this study. Various causes of power overshoot, identified in previous studies, are discussed in ...terms of their plausible contributions to electron depletion. We found that power overshoot occurred if the anodic overpotential generated by electron depletion exceeded the cathodic overpotential. The introduction of assistance current from anode connections, which ameliorated the electron depletion in the MFCs, immediately eliminated the power overshoot. As a result, if the electron production at the anode exceeded electron reduction at the cathode, a power overshoot was not generated. The results revealed that introducing assistance current supplied from an additional anode to the limited anode eliminated power overshoot. The power overshoot is not generated by kinetic limitation at the cathode; it is only generated by the kinetic limitation at the anode. The mechanism underlying power overshoot should be considered in the design of MFCs to improve reliability, particularly in scaled‐up plant applications. The proposed technique is more practical than previously proposed methods.
A stop to the overshoot: Power overshoot is a crucial issue occurring during discharge tests that are performed to determine the viability of microbial fuel cells. All previously identified causes of power overshoot share the common feature of electron depletion. A method for controlling electron depletion based on assistance current is proposed and eliminates the power overshoot.
We developed an innovative strategy to address the inhibition of anode-respiring bacteria due to voltage reversal in serially stacked microbial fuel cells by inducing cathodic voltage reversal and ...H2O2 production. When platinum-coated carbon (Pt/C) cathodes were employed (stacked MFCPt/C) and the MFC was operated with acetate medium, the last unit (MFC 4) caused a voltage reversal of −0.8 V with a substantial anode overpotential of 1.22 V. After replacing the Pt/C cathode with a Pt-free carbon gas diffusion electrode in MFC 4, an electrode overpotential, approximately 0.5 V, was shifted from the anode to the cathode, inducing cathodic voltage reversal. Under cathodic voltage reversal, MFC 4 generated H2O2 at a production rate of 117 mg H2O2/m2-h. Hence, under cathodic voltage reversal induced by Pt-free cathodes, due to less anode polarization, the anode-respiring activity can largely be sustained in a stacked MFC that treats organic wastewater consistently and the quality of treated wastewater may be improved with energy-efficient and on-site generated H2O2.
•A sluggish MFC caused voltage reversal with significant anode overpotential.•Replacing Pt-coated cathode with carbon gas diffusion electrode induced cathodic voltage reversal.•Cathodic voltage reversal shifted 0.5 V of anode overpotential to the cathode.•H2O2 was produced in the stacked MFC at cathodic voltage reversal.
► We investigated the relation of anode-embedding depth with the power output of SMFC. ► As the anode depth was increased, the internal resistances of SMCs were increased. ► Nevertheless, the SMFC ...performances were increased as the anode depth was increased. ► The anode potential could be a primary parameter for determining the anode depth.
Five rigid graphite plates were embedded in evenly divided sections of sediment, ranging from 2cm (A1) to 10cm (A5) below the top sediment layer. The maximum power and current of the MFCs increased in depth order; however, despite the increase in the internal resistance, the power and current density of the A5 MFC were 2.2 and 3.5times higher, respectively, than those of the A1 MFC. In addition, the anode open circuit potentials (OCPs) of the sediment microbial fuel cells (SMFCs) became more negative with sediment depth. Based on these results, it could be then concluded that as the anode-embedding depth increases, that the anode environment is thermodynamically and kinetically favorable to anodophiles or electrophiles. Therefore, the anode-embedding depth should be considered an important parameter that determines the performance of SMFCs, and we posit that the anode potential could be one indicator for selecting the anode-embedding depth.
Particulate matter (PM) pollution is a crucial environmental issue. Considering its adverse health impacts, especially on children’s immune systems, Korean regulations require annual PM2.5 ...measurements in daycare centers. Therefore, we developed a low-cost PM2.5 sensor calibration model for measuring the indoor PM concentrations in daycare centers using long short-term memory (LSTM) algorithms. Moreover, we trained the model to predict the PM2.5 based on temperature and humidity, and optimized its hyperparameters. The model achieved a high accuracy and outperformed traditional calibration methods. The optimal lookback period was 76, which led to a high calibration performance with root mean and mean squared errors, a coefficient of determination, and mean absolute errors of 3.57 and 12.745, 0.962, and 2.7, respectively. The LSTM model demonstrated a better calibration performance than those of the linear (r2 = 0.57) and multiple (r2 = 0.75) linear regression models. The developed calibration model provided precise short-term measurement values for the optimal management of indoor PM concentrations. This methodology can be applied to similar environments to obtain new learning and hyper-parameters. Our results will aid in improving the accuracy of low-cost sensors for measuring indoor PM concentrations, thereby providing cost-effective solutions for enhancing children’s health and well-being in daycare centers and other multiuse facilities.
•A pilot microbial electrolysis cell (MEC) was tested for H2O2 production.•Passive air diffusion to a carbon electrode successfully produced H2O2.•The highest H2O2 conversion was only 7.2%.•Catholyte ...pH over 11 can mitigate H2O2 loss in MECs.
A pilot-scale dual-chamber microbial electrolysis cell (MEC) equipped with a carbon gas-diffusion cathode was evaluated for H2O2 production using acetate medium as the electron donor. To assess the effect of cathodic pH on H2O2 yield, the MEC was tested with an anion exchange membrane (AEM) and a cation exchange membrane (CEM), respectively. The maximum current density reached 0.94–0.96 A/m2 in the MEC at applied voltage of 0.35–1.9 V, regardless of membranes. The highest H2O2 conversion efficiency was only 7.2 ± 0.09% for the CEM-MEC. This low conversion would be due to further H2O2 reduction to H2O on the cathode or H2O2 decomposition in bulk liquid. This low H2O2 conversion indicates that large-scale MECs are not ideal for production of concentrated H2O2 but could be useful for a sustainable in-situ oxidation process in wastewater treatment.
Differences in internal resistances or operational conditions that affect the current between series-connected MFC units are known to cause voltage reversal. In this work, we proved that voltage ...reversal does not happen when MFCs produce an identical maximum current (i.e., limiting current), even though their internal resistances may differ. Here, two MFCs having an internal resistance difference of 206 Ω produced an almost identical maximum current of 0.4 mA in non-stacked mode. When the MFCs were connected in series, there was no voltage reversal; the voltage at the maximum current of 0.37 mA ranged from 1 mV to 3 mV. This result clearly indicates that differences of internal resistances or operational conditions are not an essential prerequisite for occurrences of voltage reversal in stacked MFCs, and that the maximum current of MFCs may be a direct indicator for predicting voltage reversal occurrences prior to the series connection of MFCs.
Significance of the maximum current in series-connected MFCs: voltage reversal does not occur when the maximum current is the same in MFCs although the internal resistances of individual MFCs may differ from each other; the maximum current of an inferior MFC is a direct indicator for predicting voltage reversal occurrences prior to the series connection of MFCs. Display omitted
•Differences of internal resistances are known as the cause of voltage reversal.•Internal resistances for an identical maximum current were different in MFCs.•Voltage reversal did not happen when MFCs produced an identical maximum current.•Thus, differences of internal resistances could not be the cause of voltage reversal.•The maximum current of MFCs could be an indicator for voltage reversal.
A new architecture for a membraneless and single-chambered microbial fuel cell (MFC) which has a unique bipolar plate-electrode assembly (BEA) design was demonstrated. The maximum power of MFC units ...connected in series (denoted as a stacked MFC) was up to 22.8±0.13mW/m2 for 0.946±0.003V working voltage, which is 2.5 times higher than the averaged maximum power density of the non-stacked MFC units. The power density in the stacked MFC using BEA was comparable to the stacked MFC using electric wire. These results demonstrate that BEAs having air-exposed cathodes can potentially be used in the stacking of membraneless single-chambered MFCs. In addition, we confirmed that the current in the stacked mode flowed faster than the non-stacked mode due to voltage increase by series connection, and the poorest of the stacked units quickly faced current depletion at higher external resistance than the non-stacked mode, leading to voltage reversal. These results imply that stacked MFC units require a relatively large current capacity in order to prevent high voltage reversal at high current region. To increase total current capacity and prevent voltage reversal of stacked MFC units, we suggested series/parallel-integrated MFC module system for scaling-up. This new concept could likely allow the application of MFC technology to be extended to various wastewater treatment processes or plants.
•MFC stack using BEA was demonstrated.•The maximum power density of the stacked MFC using BEA was 22.8mW/m2 for 0.946V.•The performance of the stacked MFC with BEA was comparable to that using wires.•The poorest of the stacked units quickly faced current depletion due to the increased voltage.•This led to voltage reversal in the poorest unit.
We report a methodology for enhancing the mass transfer at the anode electrode of sediment microbial fuel cells (SMFCs), by employing a fabric baffle to create a separate water-layer for installing ...the anode electrode in sediment. The maximum power in an SMFC with the anode installed in the separate water-layer (SMFC-wFB) was improved by factor of 6.6 compared to an SMFC having the anode embedded in the sediment (SMFC-woFB). The maximum current density in the SMFC-wFB was also 3.9 times higher (220.46 mA/m2) than for the SMFC-woFB. We found that the increased performance in the SMFC-wFB was due to the improved mass transfer rate of organic matter obtained by employing the water-layer during anode installation in the sediment layer. Acetate injection tests revealed that the SMFC-wFB could be applied to natural water bodies in which there is frequent organic contamination, based on the acetate flux from the cathode to the anode.
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