Bongkyu Kim

Scaling-Up Microbial Fuel Cells: Configuration and Potential Drop Phenomenon at Series Connection of Unit Cells in Shared Anolyte

To scale-up microbial fuel cells (MFCs), installing multiple unit cells in a common reactor has been proposed; however, there has been a serious potential drop when connecting unit cells in series. To determine the source of the loss, a basic stack-MFC (BS-MFC) has been devised, and the results show that the phenomenon is due to ions on the anode electrode traveling through the electrolyte to be reduced at the cathode connected in series. As calculated by means of the percentage potential drop, the degree of potential drop decreased with an increase in the unit-cell distance. When the distance was increased from 1 to 8 cm, the percentage potential drop in BS-MFC1 decreased from 46.76 ± 0.90 to 45.08 ± 0.70 % and in BS-MFC2 from 46.41 ± 0.95 to 43.82 ± 2.23 %. As the p-value of the t-test was lower than 0.05, the difference was considered significant; however, if the unit cells are installed far enough from each other to avoid the potential drop phenomenon, the system will be less dense, consequently reducing the ratio of electrode area per volume of anode compartment and decreasing the power density of the system. Finally, this study suggests design criteria for scaling-up MFC systems: Multiple-electrode-installed MFCs are modularized, and the unit cells are connected in series across the module (connecting each unit cell does not share the anolyte).

Comparison in performance of sediment microbial fuel cells according to depth of embedded anode.

Five rigid graphite plates were embedded in evenly divided sections of sediment, ranging from 2 cm (A1) to 10 cm (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 3.5 times 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.

New architecture for modulization of membraneless and single-chambered microbial fuel cell using a bipolar plate-electrode assembly (BEA)

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±0.13 mW/m(2) for 0.946±0.003 V 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.

Bioelectronic platforms for optimal bio-anode of bio-electrochemical systems: From nano- to macro scopes.

The current trend of bio-electrochemical systems is to improve strategies related to their applicability and potential for scaling-up. To date, literature has suggested strategies, but the proposal of correlations between each research field remains insufficient. This review paper provides a correlation based on platform techniques, referred to as bio-electronics platforms (BEPs). These BEPs consist of three platforms divided by scope scale: nano-, micro-, and macro-BEPs. In the nano-BEP, several types of electron transfer mechanisms used by electrochemically active bacteria are discussed. In the micro-BEP, factors affecting the formation of conductive biofilms and transport of electrons in the conductive biofilm are investigated. In the macro-BEP, electrodes and separators in bio-anode are debated in terms of real applications, and a scale-up strategy is discussed. Overall, the challenges of each BEP are highlighted, and potential solutions are suggested. In addition, future research directions are provided and research ideas proposed to develop research interest.

Performance variation according to anode-embedded orientation in a sediment microbial fuel cell employing a chessboard-like hundred-piece anode.

The effect of two different anode-embedding orientations, lengthwise- and widthwise-embedded anodes was explored, on the performance of sediment microbial fuel cells (SMFCs) using a chessboard anode. The maximum current densities and power densities in SMFCs having lengthwise-embedded anodes (SLA1-SLA10) varied from 38.2mA/m(2) to 121mA/m(2) and from 5.5mW/m(2) to 20mW/m(2). In comparison, the maximum current densities and maximum power densities in SMFCs having anodes widthwise-embedded between 0cm to 8cm (SWA2-SWA5) increased from 82mA/m(2) to 140mA/m(2) and from 14.7mW/m(2) to 31.1mW/m(2) as the anode depth became deeper. Although there was a difference in the performance among SWA5-SWA10, it was considered negligible. Hence, it is concluded that it is important to embed anodes widthwise at the specific anode depths, in order to improve of SMFC performance. Chessboard anode used in this work could be a good option for the determination of optimal anode depths.

Development of anode zone using dual-anode system to reduce organic matter crossover in membraneless microbial fuel cells

To prevent the occurrence of the organic crossover in membraneless microbial fuel cells (ML-MFCs), dual-anode MFC (DA-MFC) was designed from multi-anode concept to ensure anode zone. The anode zone addressed increase the utilization of organic matter in ML-MFCs, as the result, the organic crossover was prevented and performance of MFCs were enhanced. The maximum power of the DA-MFC was 0.46mW, which is about 1.56 times higher than the ML-MFC (0.29mW). Furthermore, the DA-MFC had advantage in correlation of organic substance concentration and dissolved oxygen concentration, and even electric over-potential. In addition, in terms of cathode fouling, the DA-MFC showed clearer surface. Hence, the anode zone should be considered in the advanced ML-MFC for practically use in wastewater treatment process, and also for scale-up of MFCs.

Elimination of Power Overshoot at Bioanode through Assistance Current in Microbial Fuel Cells

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.

Increased Power in Sediment Microbial Fuel Cell: Facilitated Mass Transfer via a Water-Layer Anode Embedded in Sediment

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.

Self-recoverable voltage reversal in stacked microbial fuel cells due to biofilm capacitance

In order to assess the effects of biofilm capacitance on self-recovering voltage reversals, the restored current is determined and compared with the measured biofilm capacitance by analyzing the results of electrochemical impedance spectroscopy. This comparison demonstrates that self-recovering voltage reversals are caused by temporary damage to, and the recovery of, biofilm capacitance which arises due to the ability of redox enzymes in the electron transfer system to temporarily store electrons. Thus, the development of procedures for voltage reversal control and for the maintenance of serially connected microbial fuel cells (MFCs) should take into account such temporary voltage reversal phenomenon. This discovery and characterization of self-recovering voltage reversals is expected to be practically useful to enhance the reliability of MFCs to be scaled up and implemented in practical systems.

Elimination of voltage reversal in multiple membrane electrode assembly installed microbial fuel cells (mMEA-MFCs) stacking system by resistor control

Voltage reversal (VR) in series connection of multiple membrane electrode assembly installed microbial fuel cells (mMEA-MFC) is eliminated by manipulating the resistor control. Discharge test results collected from two mMEA-MFCs initially operated (designated as P1 and P2) confirm that the performance of P2 exceeds that of P1. Thus, driving P1 and P2 as serially stacked MFCs generate the VR in P1. Controlling the inserted resistor adjust the current production of P2 to maintain balance with P1, and the VR in P1 is eliminated in the operation of stacking mode. Thus, manipulating the internal resistance provide an applicable approach to suppress VR in the stacking of mMEA-MFCs system.