Junyeong An

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.

Coupling of anaerobic digester and microbial fuel cell for COD removal and ammonia recovery.

Microbial fuel cells (MFCs) were investigated for use in removing total ammonia nitrogen (TAN) and residual COD from effluent digested in an anaerobic digester (AD) fed with actual swine wastewater for 32 days in batch mode. Cumulative COD removal in the AD was as high as 59,647±2096 mg/L (80.5% removed), whereas TAN removal in the AD was negligible at 296±116 mg-N/L (5.8% removed), causing a decrease in the COD/TAN ratio from 14.5 to 3.0. In a subsequent MFC system, 77.5% of TAN was removed at 36 days, leading to an increase in COD/TAN ratio from 4.6 to 8.1. As a result, the COD in the anode was further reduced from 19,319±417 mg/L to 7519±554 mg/L (61.1% removed). From these results, removing the TAN in MFCs was found to increase the COD/TAN ratio, with the COD being further degraded.

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.

Interface resistances of anion exchange membranes in microbial fuel cells with low ionic strength.

The interface resistances between an anion exchange membrane (AEM) and the solution electrolyte were measured for low buffer (or ionic strength) of electrolytes typical of microbial fuel cells (MFCs). Three AEMs (AFN, AM-1, and ACS) having different properties were tested in a flat-plate MFC to which 5-mM acetate was fed to the anode and an air-saturated phosphate buffer (PB) solution was fed to the cathode. Current density achieved in the MFCs was correlated inversely with independently measured membrane-only resistances. However, the total interfacial resistances measured by current-voltage plots were approximately two orders higher than those of the membrane-only resistances, although membranes had the same order as with the membrane-only resistance. EIS spectra showed that the resistances from electric-double layer and diffusion boundary layer were the main resistances not the membranes resistance. The electric-double layer and diffusion boundary layer resistances of the AEMs were much larger in the 10 mM PB electrolyte, compared to 100 mM PB. EIS study also showed that the resistance of diffusion boundary layer decreased due to mechanical stirring. Therefore, the interface resistance that originates from the interaction between the membrane and the catholyte solution should be considered when designing and operating MFC processes with an AEM. The AEMs allowed transport of uncharged O(2) and acetate, but the current losses for both were low during normal MFC operation.

Multiphase electrode microbial fuel cell system that simultaneously converts organics coexisting in water and sediment phases into electricity.

Our challenge in this study was to harvest electricity from organics coexisting in two different phases (water and sediment) in an organics-contaminated benthic environment and to obtain increased current using a multiphase electrode microbial fuel cell (multiphase MFC). The multiphase MFC consisted of a floating electrode (FE), a midelectrode (ME), and a sediment electrode (SE) with no other components. The SE was embedded in sediment; the FE and ME were then overlaid in the water surface layer and in the middle of the water column of an aquarium, respectively. During continuous supply of organics at a COD loading rate of 94 mg of COD L(-1) day(-1) and after the cessation of organics being supplied at COD loading rates of 330 and 188 mg of COD L(-1) day(-1), the multiphase MFC showed the highest current production, as compared to the control MFCs [a floating-type MFC (FT-MFC) and two types of sediment MFCs (SMFC-A and SMFC-B)]. The total charges (in coulombs) of the multiphase MFC integrated from the currents, obtained under the three operating conditions mentioned above, were comparable to the sums of charges for the FT-MFC and SMFC. As a result, this study found that the multiphase MFC can (1) utilize organics in the sediment similarly to SMFCs, (2) use organics in the water phase similarly to FT-MFCs, and (3) obtain increased current analogous to the sum of an SMFC and a FT-MFC. Thus, it is thought that the multiphase MFC developed in this work could be suitable for use in water bodies being continuously or frequently contaminated with organic waste.

pH-dependent ammonia removal pathways in microbial fuel cell system.

In this work, ammonia removal paths in microbial fuel cells (MFCs) under different initial pH conditions (pH 7.0, 8.0, and 8.6) were investigated. At a neutral pH condition (pH 7.0), MFC used an electrical energy of 27.4% and removed 23.3% of total ammonia by electrochemical pathway for 192h. At the identical pH condition, 36.1% of the total ammonia was also removed by the biological path suspected to be biological ammonia oxidation process (e.g., Anammox). With the initial pH increased, the electrochemical removal efficiency decreased to less than 5.0%, while the biological removal efficiency highly increased to 61.8%. In this study, a neutral pH should be maintained in the anode to utilize MFCs for ammonia recovery via electrochemical pathways from wastewater stream.

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.