Microbial Fuel Cell Stack Research Articles

Control of voltage reversal in serially stacked microbial fuel cells through manipulating current: Significance of critical current density

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).

Pluggable microbial fuel cell stacks for septic wastewater treatment and electricity production

Septic tanks and other decentralized wastewater treatment systems play an important role in protecting public health and water resource for remote or developing communities. Current septic systems do not have energy production capability, yet such feature can be very valuable for areas lack access to electricity. Here we present an easy-to-operate microbial fuel cell (MFC) stack that consists a common base and multiple pluggable units, which can be connected in either series or parallel for electricity generation during waste treatment in septic tanks. Lab studies showed such easy configuration obtained a power density of 142±6.71mWm−2 when 3 units are connected in parallel, and preliminary calculation indicates that a system that costs approximately US $25 can power a 6-watt LED light for 4h per day with great improvement potential. Detailed electrochemical characterizations provide insights on system internal loss and technology advancement needed.

Dynamic electrical reconfiguration for improved capacitor charging in microbial fuel cell stacks

A microbial fuel cell (MFC) is a bioelectrochemical device that uses anaerobic bacteria to convert chemical energy locked in biomass into small amounts of electricity. One viable way of increasing energy extraction is by stacking multiple MFC units and exploiting the available electrical configurations for increasing the current or stepping up the voltage. The present study illustrates how a real-time electrical reconfiguration of MFCs in a stack, halves the time required to charge a capacitor (load) and achieves 35% higher current generation compared to a fixed electrical configuration. This is accomplished by progressively switching in-parallel elements to in-series units in the stack, thus maintaining an optimum potential difference between the stack and the capacitor, which in turn allows for a higher energy transfer.

Controlling for peak power extraction from microbial fuel cells can increase stack voltage and avoid cell reversal

Microbial fuel cells (MFCs) are bioelectrochemical systems which can degrade organic materials and are increasingly seen as potential contributors to low carbon technologies, particularly in energy recovery from and treatment of wastewaters. The theoretical maximum open circuit voltage from MFCs lies in the region of 1.1 V, but is reduced substantially by overvoltage losses. Practical use of the power requires stacking or other means to increase voltage. Series stacking of MFCs with typically encountered variability in operating conditions and performance raises the risk of cell reversal, which diminishes overall power performance. A novel strategy of MFC subsystem series connectivity along with maximum power point tracking (MPPT) generates increased power from individual MFCs whilst eliminating cell reversal. MFCs fed with lower concentrations of substrate experienced voltage reversal when connected in normal series connection with one common load, but when MFCs and loads together were connected in series, the underperforming cell is effectively bypassed and maximum power is made available. It is concluded that stack voltage may be increased and cell reversal avoided using the hybrid connectivity along with MPPT. This approach may be suitable for stacked MFC operations in the event that large scale arrays/modules are deployed in treating real wastewaters.

Electricity generation from rice straw using a microbial fuel cell

This study demonstrated electricity generation from rice straw without pretreatment in a two-chambered microbial fuel cell (MFC) inoculated with a mixed culture of cellulose-degrading bacteria (CDB). The power density reached 145 mW/m2 with an initial rice straw concentration of 1 g/L; while the coulombic efficiencies (CEs) ranged from 54.3 to 45.3%, corresponding to initial rice straw concentrations of 0.5–1 g/L. Stackable MFCs in series and parallel produced an open circuit voltage of 2.17 and 0.723 V, respectively, using hexacyanoferrate as the catholyte. The maximum power for serial connection of three stacked MFCs was 490 mW/m2 (0.5 mA). In parallelly stacked MFCs, the current levels were approximately 3-fold (1.5 mA) higher than those produced from the serial connection. These results demonstrated that electricity can be produced from rice straw by exploiting CDB as the biocatalyst. Thus, this method provides a promising way to utilize rice straw for bioenergy production.

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±0.13mW/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.

The first self-sustainable microbial fuel cell stack

This study reports for the first time on the development of a self-sustainable microbial fuel cell stack capable of self-maintenance (feeding, hydration, sensing & reporting). Furthermore, the stack system is producing excess energy, which can be used for improved functionality. The self-maintenance is performed by the stack powering single and multi-channel peristaltic pumps.

Long-term evaluation of a 10-liter serpentine-type microbial fuel cell stack treating brewery wastewater

A 10-liter serpentine-type microbial fuel cell (MFC) stack was constructed by extending 40 tubular air-cathode MFC units in a 3-D alignment pattern. When operated in series and fed with brewery wastewater, the stack produced an open circuit voltage of 23.0V and a maximum power density of 4.1W/m3 (at 0.7A/m3). During long-term performance (180days), electrochemical tests were conducted to explore the reasons for deterioration in performance of the stack system. Cyclic voltammetric measurements suggested that the cathodes, not the anodes, were responsible for the decrease in performance over time. After the cathode surface was rinsed with water, the power density produced by the stack system fully recovered instantaneously, due to the decrease in cathode alkalization and increase in humidity of the cathode side. This study provided an optimal configuration of a MFC stack for MFC scale-up towards large-scale applications.

Conversion of a substrate carbon source to formic acid for carbon dioxide emission reduction utilizing series-stacked microbial fuel cells

Carbon dioxide (CO2) is now known to be a major cause of global warming, and the reduction of its atmospheric concentration has therefore become a critical issue. The amount of CO2 discharged from the degradation process of waste organic substances is both tremendous and neglected. Here, an environmentally friendly method to achieve the fixation of the CO2 released from the degradation of waste carbonaceous substances has been investigated. A series-connected microbial fuel cell (MFC) stack is used to electro-reduce the CO2 to formic acid in situ utilizing the electricity generated from the degradation of the carbonaceous substances in the anodic chambers of the MFC stack. Consequently, formic acid at 4.27 mg L−1 h−1 is produced, and a Faraday efficiency of 64.8% is achieved. This technology will be helpful in recycling and reusing the carbon in wastewaters or wastes in the absence of external energy input, thereby promoting the virtuous cycle of carbon.

Bio-inspired chemical hydrogen storage and discharge as a source of electrical energy

Reversible bio-inspired chemical hydrogen storage systems accumulate electrical energy in the form of electrons and proton ions located on biomolecules or bio-like storage molecules. Electro-active biomolecules (EAB) in Yeast media show such behavior: 2e− + 2H+ + EAB (aq) + ⇆ EABH/H (aq) + , also electro-active Methylene Blue (MB): 2e− + 2H+ + MB (aq) + ⇆ MBH/H (aq) + . The power characteristics of microbial fuel cell stacks equipped with such bio-inspired hydrogen storage systems were examined. E. coli cultures charged these bio-inspired separate chemical hydrogen storage units up to E = 0.50 ± 0.06 V; cell potentials increased proportionally in serial double, triple, and quadruple hydrogen storage stacks up to E OCV = 1.98 V; the maximum power densities that were obtained improved proportionally with stack length by an increment of 1.4. The bio-inspired chemical hydrogen storage principle is of great interest for application in low-cost batteries that store renewable energy.

A novel microbial fuel cell stack for continuous production of clean energy

Production of sustainable and clean energy through oxidation of biodegradable materials was carried out in a novel stack of microbial fuel cells (MFCs). Saccharomyces cerevisiae as an active biocatalyst was used for power generation. The novel stack of MFCs consist of four units was fabricated and operated in continuous mode. Pure glucose as substrate was used with concentration of 30 g l−1 along with 200 μmol l−1 of natural red (NR) as a mediator in the anode and 400 μmol l−1 of potassium permanganate as oxidizing agent in the cathode. Polarimetry technique was employed to analyze the single cell as well as stack electrical performance. Performance of the MFCs stack was evaluated with respect to amount of electricity generation. Maximum current and power generation in the stack of MFC were 6447 mA.m−2 and 2003 mW.m−2, respectively. Columbic efficiency of 22 percent was achieved at parallel connection. At the end of process, image of the outer surface of graphite electrode was taken by Atomic Force Microscope at magnification of 5000. The high electrical performance of MFCs was attributed to the uniform growth of microorganism on the graphite surface which was confirmed by the obtained images.

Scalable microbial fuel cell (MFC) stack for continuous real wastewater treatment

A tubular air-cathode microbial fuel cell (MFC) stack with high scalability and low material cost was constructed and the ability of simultaneous real wastewater treatment and bioelectricity generation was investigated under continuous flow mode. At the two organic loading rates (ORLs) tested (1.2 and 4.9 kg COD/m 3 d), five non-Pt MFCs connected in series and parallel circuit modes treating swine wastewater can enable an increase of the voltage and the current. The parallel stack retained high power output and the series connection underwent energy loss due to the substrate cross-conduction effect. With continuous electricity production, the parallel stack achieved 83.8% of COD removal and 90.8% of NH 4 + -N removal at 1.2 kg COD/m 3 d, and 77.1% COD removal and 80.7% NH 4 + -N removal at 4.9 kg COD/m 3 d. The MFC stack system in this study was demonstrated to be able to treat real wastewater with the added benefit of harvesting electricity energy.

Voltage Reversal during Stacking Microbial Fuel Cells with or without Diodes

Stacking microbial fuel cells (MFCs) in series can provide higher voltage; however, voltage reversal (VR) adversely affects performance of the stacked MFCs. In this paper, diodes are introduced into three stacked MFCs so as to investigate the VR behavior and offer a diodes-based explanation of the VR. The results show that VR occurs in the different stacked MFCs systems. VR of the stacked MFCs connected with forward diodes (～0.37 V) happens in a similar pattern as that without diodes (～0.80 V). However, it only happens at the end of a cycle. This can be analyzed that the resistance of the diodes consume a part of the flowing electrons and the speed of potential changes of the electrodes slows down. Differently, in the stacked MFCs connected with reverse diodes, the voltages of each unit MFC approximately equal their open circuit voltages (～0.75 V), and VR happens in the unit MFC with reverse diodes at the cathode end only. It implies that the imbalanced consumption of electrons in unit MFCs and the potential changes of specific electrode directly result in VR.

Comparative analysis of different polymer materials for the construction of microbial fuel cell stacks

Comparative analysis of different polymer materials for the construction of microbial fuel cell stacks

Analysis and Improvement of a Scaled-Up and Stacked Microbial Fuel Cell

Scaling up microbial fuel cells (MFCs) is inevitable when power outputs have to be obtained that can power electrical devices other than small sensors. This research has used a bipolar plate MFC stack of four cells with a total working volume of 20 L and a total membrane surface area of 2 m(2). The cathode limited MFC performance due to oxygen reduction rate and cell reversal. Furthermore, residence time distribution curves showed that bending membranes resulted in flow paths through which the catholyte could flow from inlet to outlet, while leaving the reactants unconverted. The cathode was improved by decreasing the pH, purging pure oxygen, and increasing the flow rate, which resulted in a 13-fold power density increase to 144 W m(-3) and a volumetric resistivity of only 1.2 mOmega m(3) per cell. Both results are major achievements compared to results currently published for laboratory and scaled-up MFCs. When designing a scaled-up MFC, it is important to ensure optimal contact between electrodes and substrate and to minimize the distances between electrodes.

Substrate cross-conduction effect on the performance of serially connected microbial fuel cell stack

This study presented a new design of scalable, air-cathode microbial fuel cell (MFC) stack that shared a common fuel feed passage. As two individual cells were electrically connected in series by metal wires and hydraulically joined by conductive substrate flow, the performance degradation phenomenon was observed. The open circuit voltage (OCV) and low current behavior of stacked MFC were lower than should be expected. This energy loss was proposed to be a consequence of parasitic current flow due to the substrate cross-conduction effect and can be likely minimized through controlling the distance between the anode electrodes or/and the cross-sectional area of substrate flow. The unique and simple water distribution system of the tubular MFC stack would contribute to the further scale-up and implementation of MFC technologies, especially for wastewater treatment.

A novel configuration of microbial fuel cell stack bridged internally through an extra cation exchange membrane

This paper reports a novel configuration of stacked microbial fuel cells (MFCs) bridged internally through an extra cation exchange membrane (CEM). The MFC stack (MFC(stack)), assembled from two single MFCs (MFC(single)), resulted in double voltage output and half optimal external resistance. COD removal rate was increased from 32.4% to 54.5%. The performance improvement could be attributed to the smaller internal resistance and enhanced cations transfer. A result from a half cell study further confirmed the important role of the extra CEM. This study also demonstrated MFCs where the anode and cathode were sandwiched between two CEMs possessed significantly high power outputs.

Voltage reversal during microbial fuel cell stack operation

Microbial fuel cells (MFC) can be used to directly generate electricity from organic matter, but the voltage produced by a single reactor is only ∼0.5 V. Voltage can be increased by stacking cells, i.e. by linking individual reactors in series, as is commonly done with hydrogen fuel cells, to provide a higher voltage output. A two-cell air-cathode MFC stack tested here produced a working voltage of 0.9 V (external load 500 Ω) and had an open circuit voltage (OCV) of 1.3 V when operated in fed batch mode under substrate-sufficient conditions. When multiple cells are stacked together, however, charge reversal can result in the reverse polarity of one or more cells and a loss of power generation. We investigated the causes of charge reversal and the impact of prolonged reversal on power generation using a two air-cathode MFCs stack. When voltage began to decline at the end of a fed batch cycle, we observed voltage reversal with one cell producing a working voltage of 0.6 V, and the other cell having a reversed voltage of −0.58 V, producing only a minimal stack voltage of 0.02 V. The reason for the voltage reversal was shown to be fuel starvation, resulting in a loss of bacterial activity. Voltage reversal adversely affected bacteria on the anode of the affected cell, as shown by a relative decrease in cell performance following a cycle of starvation (no feeding). The control of voltage reversal will be crucial for long-term operation of MFCs in series. Rapid feeding of a cell can restore positive voltage generation, but the long-term impact of charge reversal will be inactivation of bacteria and it will require that the affected cells be short-circuited to maintain stack power production. A better understanding of the long term effects of voltage reversal on power generation by MFC stacks is needed in order to efficiently increase voltage production by using stacked MFC systems.

Continuous Electricity Generation at High Voltages and Currents Using Stacked Microbial Fuel Cells

Connecting several microbial fuel cell (MFC) units in series or parallel can increase voltage and current; the effect on the microbial electricity generation was as yet unknown. Six individual continuous MFC units in a stacked configuration produced a maximum hourly averaged power output of 258 W m(-3) using a hexacyanoferrate cathode. The connection of the 6 MFC units in series and parallel enabled an increase of the voltages (2.02 V at 228 W m(-3)) and the currents (255 mA at 248 W m(-3)), while retaining high power outputs. During the connection in series, the individual MFC voltages diverged due to microbial limitations at increasing currents. With time, the initial microbial community decreased in diversity and Gram-positive species became dominant. The shift of the microbial community accompanied a tripling of the short time power output of the individual MFCs from 73 W m(-3) to 275 W m(-3), a decrease of the mass transfer limitations and a lowering of the MFC internal resistance from 6.5 +/- 1.0 to 3.9 +/- 0.5 omega. This study demonstrates a clear relation between the electrochemical performance and the microbial composition of MFCs and further substantiates the potential to generate useful energy by means of MFCs.