Evaluation of Electrode and Solution Area-Based Resistances Enables Quantitative Comparisons of Factors Impacting Microbial Fuel Cell Performance

Preliminary evaluation of electricity recovery from palm oil mill effluent by anion exchange microbial fuel cell

This study assessed the viability of an anion-exchange microbial fuel cell (MFC) for extracting electricity from palm oil mill effluent (POME), a major pollutant in palm-oil producing regions due to increasing demand. The MFC incorporated a tubular membrane electrode assembly (MEA) with an air core, featuring a carbon-painted carbon-cloth cathode, an anion exchange membrane (AEM), and a nonwoven graphite fabric (NWGF) anode. An additional carbon brush (CB) anode was placed adjacent to the tubular MEA. The MFC operated under semi-batch conditions with POME replacement every 7 days. Results showed superior performance of the AEM, with the highest power density (Pmax) observed in POME-treated MFCs. Current and power density increased with CB addition; the best chemical oxygen demand (COD) removal efficiency reached 73 %, decreasing from 1249 to 332 mg/L with three CBs. The Pmax was 0.18 W/m-2(-|-) with 1000 mg/L COD and three CBs, dropping to 0.0031 W/m-2(-|-) without CB and at 410 mg/L COD. Anode resistance, calculated using organic matter supplementation, COD, and anode surface area, decreased with increased COD or surface area, improving electricity production. AEM and CB compatibility synergistically enhanced MFC performance, offering potential for POME wastewater treatment and energy recovery.

Hydrogen production in microbial electrolysis cells with biocathodes

Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe-electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production.

Enhancing proton transport in polyvinylidenedifluoride membranes and reducing biofouling for improved hydrogen production in microbial electrolysis cells

Hydrophilic porous membranes, exemplified by polyvinylidene fluoride (PVDF) membranes, have demonstrated significant potential for replacing ion exchange membranes in microbial electrolysis cells (MECs). Membrane fouling remains a major challenge in MECs, impeding proton transport and consequently limiting hydrogen production. This study aims to investigate a synergistic antifouling strategy for PVDF membrane through the incorporation of a coating composed of polydopamine (PDA), polyethyleneimine (PEI), and silver nanoparticles (AgNPs). The PDA-PEI-Ag@PVDF membrane not only effectively mitigates fouling through steric and electrostatic repulsion forces, but also amplifies ion transport by facilitating water diffusion and electromigration. The PDA-PEI-Ag@PVDF membrane exhibited a reduced membrane resistance of 1.01 mΩ m2 and PDA-PEI-Ag modifying PVDF membrane was found to be effective in enhancing the proton transportation of PVDF membrane. Therefore, the enhanced hydrogen production rate of 2.65 ± 0.02 m3/m3/d was achieved in PDA-PEI-Ag@PVDF-MECs.

Fabrication of a Molybdenum Dioxide/Multi-Walled Carbon Nanotubes Nanocomposite as an Anodic Modification Material for High-Performance Microbial Fuel Cells

A nanocomposite of multi-walled carbon nanotubes (MWCNTs) decorated with molybdenum dioxide (MoO2) nanoparticles is fabricated through the reduction of phosphomolybdic acid hydrate on functionalized MWCNTs in a hydrogen-argon (10%) atmosphere in a tube furnace. The MoO2/MWCNTs composite is proposed as an anodic modification material for microbial fuel cells (MFCs). MWCNTs have outstanding physical and chemical peculiarities, with functionalized MWCNTs having substantially large electroactive areas. In addition, combined with the exceptional properties of MoO2 nanoparticles, the synergistic advantages of functionalized MWCNTs and MoO2 nanoparticles give a MoO2/MWCNTs anode a large electroactive area, excellent electronic conductivity, enhanced extracellular electron transfer capacity, and improved nutrient transfer capability. Finally, the power harvesting of an MFC with the MoO2/MWCNTs anode is improved, with the MFC showing long-term repeatability of voltage and current density outputs. This exploratory research advances the fundamental application of anodic modification to MFCs, simultaneously providing valuable guidance for the use of carbon-based transition metal oxide nanomaterials in high-performance MFCs.

Elucidating the impacts of cobalt (II) ions on extracellular electron transfer and pollutant degradation by anodic biofilms in bioelectrochemical systems during industrial wastewater treatment

Electrogenic biofilms in bioelectrochemical systems (BES) are critical in wastewater treatment. Industrial effluents often contain cobalt (Co2+); however, its impact on biofilms is unknown. This study investigated how increasing Co2+ concentrations (0-30 mg/L) affect BES biofilm community dynamics, extracellular polymeric substances, microbial metabolism, electron transfer gene expression, and electrochemical performance. The research revealed that as Co2+ concentrations increased, power generation progressively declined, from 345.43 ± 4.07 mW/m2 at 0 mg/L to 160.51 ± 0.86 mW/m2 at 30 mg/L Co2+. However, 5 mg/L Co2+ had less effect. The Co2+ removal efficiency in the reactors fed with 5 and 10 mg/L concentrations exceeded 99% and 94%, respectively. However, at 20 and 30 mg/L, the removal efficiency decreased substantially, likely because of reduced biofilm viability. FTIR indicated the participation of biofilm functional groups in Co2+ uptake. XPS revealed Co2+ presence in biofilms as CoO and Co(OH)2, indicating precipitation also aided removal. Cyclic voltammetry and electrochemical impedance spectroscopy tests revealed that 5 mg/L Co2+ had little impact on the electrocatalytic activity, while higher concentrations impaired it. Furthermore, at a concentration of 5 mg/L Co2+, there was an increase in the proportion of the genus Anaeromusa-Anaeroarcus, while the genus Geobacter declined at all tested Co2+ concentrations. Additionally, higher concentrations of Co2+ suppressed the expression of extracellular electron transfer genes but increased the expression of Co2+-resistance genes. Overall, this study establishes how Co2+ impacts electrogenic biofilm composition, function, and treatment efficacy, laying the groundwork for the optimized application of BES in remediating Co2+-contaminated wastewater.

Biotechnology to reduce logistics burden and promote environmental stewardship for Air Force civil engineering requirements

This review provides discussion of advances in biotechnology with specific application to civil engineering requirements for airfield and airbase operations. The broad objectives are soil stabilization, waste management, and environmental protection. The biotechnology focal areas address (1) treatment of soil and sand by biomineralization and biopolymer addition, (2) reduction of solid organic waste by anaerobic digestion, (3) application of microbes and higher plants for biological processing of contaminated wastewater, and (4) use of indigenous materials for airbase construction and repair. The consideration of these methods in military operating scenarios, including austere environments, involves comparison with conventional techniques. All four focal areas potentially reduce logistics burden, increase environmental sustainability, and may provide energy source, or energy-neutral practices that benefit military operations.

The restricted mass transfer inside the anode pore channel affects the electroactive biofilms formation, community composition and the power production in microbial electrochemical systems

Porous anodes improve system performance in microbial electrochemical systems by increasing the specific surface area for electroactive bacteria. In this study, multilayer anodes with different pore diameters were constructed to assess the impact of pore size and depth on anode performance. This layered structure makes detecting electroactive biofilms more accessible layer by layer, which is the first study to examine electroactive biofilms' molecular biology and electrochemical properties at different depths in pores with varied pore sizes. The millimeter-scale pores inside the bioanode have a limited effect in increasing power. The larger the pore diameter, the higher the maximum power density (Pmax) obtained. The Pmax of anodes with 4 mm pore (1.91 ± 0.15 W m-2) was 1.4 times higher than that of the non-perforated (1.37 ± 0.07 W m-2) and 0.5 mm pore anodes (1.39 ± 0.04 W m-2). Electricigens can colonize into pore channels for at least 10 mm with a pore diameter ≥3 mm and current densities >0.05 A m-2. However, in the pores channel with 0.5 mm diameter, electricigens can only colonize to a depth of 2 mm. The biofilm thickness, electricity output, metabolic activity, and biocommunity changed with pore depth and were restricted by the limited mass transfer. The Geobacter sp. was the dominant species in inter-pore biofilms, with 43.8 %-78.6 % in abundance and decreased in quantity as pore depth increased. The inter-pore biofilms on the outer layer contributed a current density of 0.17 ± 0.003 A m-2, while that of the inner layer was only 0.02 ± 0.01 A m-2. Further studies found that the pore edge mass transfer effect can contribute up to 75 % of the current. The mass transfer process at the pore edge region could be a multidirectional mass transfer rather than a pore channel mass transfer.

Treatment of Organic and Sulfate/Sulfide Contaminated Wastewater and Bioelectricity Generation by Sulfate-Reducing Bioreactor Coupling with Sulfide-Oxidizing Fuel Cell

A wastewater treatment system has been established based on sulfate-reducing and sulfide-oxidizing processes for treating organic wastewater containing high sulfate/sulfide. The influence of COD/SO42- ratio and hydraulic retention time (HRT) on removal efficiencies of sulfate, COD, sulfide and electricity generation was investigated. The continuous operation of the treatment system was carried out for 63 days with the optimum COD/SO42- ratio and HRT. The result showed that the COD and sulfate removal efficiencies were stable, reaching 94.8 ± 0.6 and 93.0 ± 1.3% during the operation. A power density level of 18.0 ± 1.6 mW/m2 was obtained with a sulfide removal efficiency of 93.0 ± 1.2%. However, the sulfide removal efficiency and power density decreased gradually after 45 days. The results from scanning electron microscopy (SEM) with an energy dispersive X-ray (EDX) show that sulfur accumulated on the anode, which could explain the decline in sulfide oxidation and electricity generation. This study provides a promising treatment system to scale up for its actual applications in this type of wastewater.

Investigating the Performance of Lithium-Doped Bismuth Ferrite [BiFe1−xLixO3]-Graphene Nanocomposites as Cathode Catalyst for the Improved Power Output in Microbial Fuel Cells

In this study, multifunctional lithium-doped bismuth ferrite [BiFe1−xLixO3]-graphene nanocomposites (x = 0.00, 0.02, 0.04, 0.06) were synthesized by a sol-gel and ultrasonication assisted chemical reduction method. X-ray diffraction and FESEM electron microscopy techniques disclosed the nanocomposite phase and nanocrystalline nature of [BiFe1−xLixO3]-graphene nanocomposites. The FESEM images and the EDX elemental mapping revealed the characteristic integration of BiFe1−xLixO3 nanoparticles (with an average size of 95 nm) onto the 2D graphene layers. The Raman spectra of the [BiFe1−xLixO3]-graphene nanocomposites evidenced the BiFe1−xLixO3 and graphene nanostructures in the synthesized nanocomposites. The photocatalytic performances of the synthesized nanocomposites were assessed for ciprofloxacin (CIP) photooxidation under UV-visible light illumination. The photocatalytic efficiencies of [BiFe1−xLixO3]-graphene nanocomposites were measured to be 42%, 47%, 43%, and 10%, for x = 0.00, 0.02, 0.04, 0.06, respectively, within 120 min illumination, whereas the pure BiFeO3 nanoparticles were 21.0%. BiFe1−xLixO3 nanoparticles blended with graphene were explored as cathode material and tested in a microbial fuel cell (MFC). The linear sweep voltammetry (LSV) analysis showed that the high surface area of BiFeO3 was attributed to efficient oxygen reduction reaction (ORR) activity. The increasing loading rates of (0.5–2.5 mg/cm2) [BiFe1−xLixO3]-graphene composite on the cathode surface showed increasing power output, with 2.5 and 2 mg/cm2 achieving the maximum volumetric power density of 8.2 W/m3 and 8.1 W/m3, respectively. The electrochemical impedance spectroscopy (EIS) analysis showed that among the different loading rates used in this study, BiFeO3, with a loading rate of 2.5 mg/cm2, showed the lowest charge transfer resistance (Rct). The study results showed the potential of [BiFe1−xLixO3]-graphene composite as a cost-effective alternative for field-scale MFC applications.

Impact of reactor configuration on pilot-scale microbial fuel cell performance

Different microbial fuel cell (MFC) configurations have been successfully operated at pilot-scale levels (>100 L) to demonstrate electricity generation while accomplishing domestic or industrial wastewater treatment. Two cathode configurations have been primarily used based on either oxygen transfer by aeration of a liquid catholyte or direct oxygen transfer using air-cathodes. Analysis of several pilot-scale MFCs showed that air-cathode MFCs outperformed liquid catholyte reactors based on power density, producing 233% larger area-normalized power densities and 181% higher volumetric power densities. Reactors with higher electrode packing densities improved performance by enabling larger power production while minimizing the reactor footprint. Despite producing more power than the liquid catholyte MFCs, and reducing energy consumption for catholyte aeration, pilot MFCs based on air-cathode configuration failed to produce effluents with chemical oxygen demand (COD) levels low enough to meet typical threshold for discharge. Therefore, additional treatment would be required to further reduce the organic matter in the effluent to levels suitable for discharge. Scaling up MFCs must incorporate designs that can minimize electrode and solution resistances to maximize power and enable efficient wastewater treatment.

Bioelectrochemical systems-based metal removal and recovery from wastewater and polluted soil: Key factors, development, and perspective

Bioelectrochemical systems (BES) are considered efficient and sustainable technologies for bioenergy generation and simultaneously removal/recovery metal (loid)s from soil and wastewater. However, several current challenges of BES-based metal removal and recovery, especially concentrating target metals from complex contaminated wastewater or soil and their economic feasibility of engineering applications. This review summarized the applications of BES-based metal removal and recovery systems from wastewater and contaminated soil and evaluated their performances on electricity generation and metal removal/recovery efficiency. In addition, an in depth review of several key parameters (BES configurations, electrodes, catalysts, metal concentration, pH value, substrate categories, etc.) of BES-based metal removal and recovery was carried out to facilitate a deep understanding of their development and to suggest strategies for scaling up their specific application fields. Finally, the future intervention on multifunctional BES to improve their performances of mental removal and recovery were revealed.

Quantification of Internal Resistance Contributions of Sediment Microbial Fuel Cells Using Petroleum-Contaminated Sediment Enriched with Kerosene

Anaerobic biodegradation of petroleum-contaminated sediments can be accomplished by a sediment microbial fuel cell (SMFC), but the recovered energy is very low (~4 mW m−2). This is due to a high internal resistance (Ri) that develops in the SMFC. The evaluation of the main experimental parameters that contribute to Ri is essential for developing a feasible SMFC design and this task is normally performed by electrochemical impedance spectroscopy (EIS). A faster and easier alternative procedure to EIS is to fit the SMFC polarization curve to an electrochemical model. From there, the main resistance contributions to Ri are partitioned. This enables the development of a useful procedure for attaining a low SMFC Ri while improving its power output. In this study, the carbon-anode surface was increased, the biodegradation activity of the indigenous populations was improved (by the biostimulation method, i.e., the addition of kerosene), the oxygen reduction reaction was catalyzed, and a 0.8 M Na2SO4 solution was used as a catholyte at pH 2. As a result, the initial SMFC Ri was minimized 20 times, and its power output was boosted 47 times. For a given microbial fuel cell (MFC), the main resistance contributions to Ri, evaluated by the electrochemical model, were compared with their corresponding experimental results obtained by the EIS technique. Such a validation is also discussed herein.

A light-enhanced α-FeOOH nanowires/polyaniline anode for improved electricity generation performance in microbial fuel cells

Low power density and poor anode performance seriously limit the potential of practical application of microbial fuel cell (MFC). Utilizing solar energy by developing photoanode is one of the effective pathways to improve the performance of MFC. Here solar energy harvesting was integrated into MFC to achieved the comprehensive utilization of multiple energy sources. A hybrid MFC photoanode (α-FeOOH-NWs/PANI anode) was constructed by loading polyaniline (PANI) and α-FeOOH nanowires (α-FeOOH-NWs) on carbon paper through electro-polymerization synthesis method. Compared with clean carbon paper, nanowires and PANI increased the surface roughness of the electrode, which facilitated the biofilm formation. The electrochemical and photoelectric analysis demonstrated that PANI introduced new electroactive groups and reduced the charge transfer resistance, exhibiting excellent electrochemical and photoelectric activites. The MFC with the α-FeOOH-NWs/PANI photoanode had higher voltage output and power density under light illumination, with the power density of 1.95 W/m² under light, which was 1.4 times higher than that without light. The hybrid α-FeOOH-NWs/PANI photoanode enhanced the separation efficiency of photogenerated electron-hole pairs, thereby improving the photoelectric response capability and generating a high photocurrent. Our research provided a new concept for the combination of solar energy harvesting and MFCs, yielding an overall enhancement of electricity eneration performance in MFC.

Pilot scale microbial fuel cells using air cathodes for producing electricity while treating wastewater

Microbial fuel cells (MFCs) can generate electrical energy from the oxidation of the organic matter, but they must be demonstrated at large scales, treat real wastewaters, and show the required performance needed at a site to provide a path forward for this technology. Previous pilot-scale studies of MFC technology have relied on systems with aerated catholytes, which limited energy recovery due to the energy consumed by pumping air into the catholyte. In the present study, we developed, deployed, and tested an 850 L (1400 L total liquid volume) air-cathode MFC treating domestic-type wastewater at a centralized wastewater treatment facility. The wastewater was processed over a hydraulic retention time (HRT) of 12 h through a sequence of 17 brush anode modules (11 m² total projected anode area) and 16 cathode modules, each constructed using two air-cathodes (0.6 m² each, total cathode area of 20 m²) with the air side facing each other to allow passive air flow. The MFC effluent was further treated in a biofilter (BF) to decrease the organic matter content. The field test was conducted for over six months to fully characterize the electrochemical and wastewater treatment performance. Wastewater quality as well as electrical energy production were routinely monitored. The power produced over six months by the MFC averaged 0.46 ± 0.35 W (0.043 W m^-2 normalized to the cross-sectional area of an anode) at a current of 1.54 ± 0.90 A with a coulombic efficiency of 9%. Approximately 49 ± 15 % of the chemical oxygen demand (COD) was removed in the MFC alone as well as a large amount of the biochemical oxygen demand (BOD₅) (70%) and total suspended solid (TSS) (48%). In the combined MFC/BF process, up to 91 ± 6 % of the COD and 91 % of the BOD₅ were removed as well as certain bacteria (E. coli, 98.9%; fecal coliforms, 99.1%). The average effluent concentration of nitrate was 1.6 ± 2.4 mg L^-1, nitrite was 0.17 ± 0.24 mg L^-1 and ammonia was 0.4 ± 1.0 mg L^-1. The pilot scale reactor presented here is the largest air-cathode MFC ever tested, generating electrical power while treating wastewater.

Boosting microfluidic microbial fuel cells performance via investigating electron transfer mechanisms, metal-based electrodes, and magnetic field effect

The presented paper fundamentally investigates the influence of different electron transfer mechanisms, various metal-based electrodes, and a static magnetic field on the overall performance of microfluidic microbial fuel cells (MFCs) for the first time to improve the generated bioelectricity. To do so, as the anode of microfluidic MFCs, zinc, aluminum, tin, copper, and nickel were thoroughly investigated. Two types of bacteria, Escherichia coli and Shewanella oneidensis MR-1, were used as biocatalysts to compare the different electron transfer mechanisms. Interaction between the anode and microorganisms was assessed. Finally, the potential of applying a static magnetic field to maximize the generated power was evaluated. For zinc anode, the maximum open circuit potential, current density, and power density of 1.39 V, 138,181 mA m^-2 and 35,294 mW m^-2 were obtained, respectively. The produced current density is at least 445% better than the values obtained in previously published studies so far. The microfluidic MFCs were successfully used to power ultraviolet light-emitting diodes (UV-LEDs) for medical and clinical applications to elucidate their application as micro-sized power generators for implantable medical devices.

Effect of Ion Selectivity on Current Production in Sewage Microbial Fuel Cell Separators

This study compared the performance of two microbial fuel cells (MFCs) equipped with separators of anion or cation exchange membranes (AEMs or CEMs) for sewage wastewater treatment. Under chemostat feeding of sewage wastewater (hydraulic retention time of approximately 7 h and polarization via an external resistance of 1 Ω), the MFCs with AEM (MFCAEM) generated a maximum current that was 4-5 times greater than that generated by the MFC with CEM (MFCCEM). The high current in the MFCAEM was attributed to the approximately neutral pH of its cathode, in contrast to the extremely high pH of the MFCCEM cathode. Due to the elimination of the pH imbalance, the cathode resistance for the MFCAEM (13-19 Ω·m2) was lower than that for the MFCCEM (41-44 Ω·m2). The membrane resistance measured as the Cl- mobility of AEMs for the MFCAEM operated for 35, 583, and 768 days showed an increase with operation time and depth, and this increase contributed minimally to the cathode resistance of the MFCAEM. These results indicate the advantage of the AEM over the CEM for air-cathode MFCs. The membrane resistance may increase when the AEM is applied in large-scale MFCs on a meter scale for extended periods.

Use of Onion Waste as Fuel for the Generation of Bioelectricity

The enormous environmental problems that arise from organic waste have increased due to the significant population increase worldwide. Microbial fuel cells provide a novel solution for the use of waste as fuel for electricity generation. In this investigation, onion waste was used, and managed to generate maximum peaks of 4.459 ± 0.0608 mA and 0.991 ± 0.02 V of current and voltage, respectively. The conductivity values increased rapidly to 179,987 ± 2859 mS/cm, while the optimal pH in which the most significant current was generated was 6968 ± 0.286, and the ° Brix values decreased rapidly due to the degradation of organic matter. The microbial fuel cells showed a low internal resistance (154,389 ± 5228 Ω), with a power density of 595.69 ± 15.05 mW/cm2 at a current density of 6.02 A/cm2; these values are higher than those reported by other authors in the literature. The diffractogram spectra of the onion debris from FTIR show a decrease in the most intense peaks, compared to the initial ones with the final ones. It was possible to identify the species Pseudomona eruginosa, Acinetobacter bereziniae, Stenotrophomonas maltophilia, and Yarrowia lipolytica adhered to the anode electrode at the end of the monitoring using the molecular technique.

Vapor-Fed Cathode Microbial Electrolysis Cells with Closely Spaced Electrodes Enables Greatly Improved Performance

Hydrogen can be electrochemically produced in microbial electrolysis cells (MECs) by current generated from bacterial anodes with a small added voltage. MECs typically use a liquid catholyte containing a buffer or salts. However, anions in these catholytes result in charge being balanced predominantly by ions other than hydroxide or protons, leading to anode acidification. To enhance only hydroxide ion transport to the anode, we developed a novel vapor-fed MEC configuration lacking a catholyte with closely spaced electrodes and an anion exchange membrane to limit the acidification. This MEC design produced a record-high sustained current density of 43.1 ± 0.6 A/m² and a H₂ production rate of 72 ± 2 LH₂/L-d (cell voltage of 0.79 ± 0.00 V). There was minimal impact on MEC performance of increased acetate concentrations, solution conductivity, or anolyte buffer capacity at applied voltages up to 1.1 V, as shown by a nearly constant internal resistance of only 6.8 ± 0.3 mΩ m². At applied external voltages >1.1 V, the buffer capacity impacted performance, with current densities increasing from 28.5 ± 0.6 A/m² (20 mM phosphate buffer solution (PBS)) to 51 ± 1 A/m² (100 mM PBS). These results show that a vapor-fed MEC can produce higher and more stable performance than liquid-fed cathodes by enhancing transport of hydroxide ions to the anode.

Comparison of different chemical treatments of brush and flat carbon electrodes to improve performance of microbial fuel cells

Anodes in microbial fuel cells (MFCs) can be chemically treated to improve performance but the impact of treatment on power generation has not been examined for different electrode base materials. Brush or flat anodes were chemically treated and then compared in identical two-chambered MFCs using the electrode potential slope (EPS) analysis to quantify the anode resistances. Flat carbon cloth anodes modified with carbon nanotubes (CNTs) produced 1.42 ± 0.06 W m^-2, which was 3.2 times more power than the base material (0.44 ± 0.00 W m^-2), but less than the 2.35 ± 0.1 W m^-2 produced using plain graphite fiber brush anodes. An EPS analysis showed that there was a 90% decrease in the anode resistances of the CNT-treated carbon cloth and a 5% decrease of WO₃ nanoparticle-treated brushes compared to unmodified controls. Certain chemical treatments can therefore improve performance of flat anodes, but plain brush anodes achieved the highest power densities.

Comparative evaluation of fibrous artificial carbons and bamboo charcoal in terms of recovery of current from sewage wastewater

In this study, two fibrous carbon anodes (namely, pleated non-woven graphite (PNWG) and carbon brush (CB) made from artificial carbon) and bamboo charcoal (BC) were evaluated for current recovery from sewage wastewater. When these anodes were polarized at 0.2 V vs. Ag/AgCl in sewage wastewater, CB produced a maximum current of 2.9 A/m². This exceeded that produced by PNWG (1.5 A/m²) and BC (1.4 A/m²). The accumulative charge recovery achieved with CB was superior to those achieved with the other two (1.6- and 2.2-fold higher than that with PNWG and BC, respectively). During the cyclic voltammetry analysis, CB demonstrated the highest catalytic current with maximum potential in the range of -0.6 to 0.4 V vs. Ag/AgCl and the smallest anode resistance (0.20 Ωm²). Direct cell counting revealed that the fibrous anodes (CB and PNWG) attached most of the cells in the anodes (80%), whereas BC did not. In contrast, the proportion of Geobacter species, a representative electrogenic microorganism in the total bacteria, was observed to be similar among the three anodes (4.4-5.8%). The tubular microbial fuel cell (ø 5.0 cm) equipped with an air-chamber core wrapped with an anion exchange membrane (AEM) and the CB delivered a current of 1.8 A/m². This is higher than those reported in the existing literature for the same microbial fuel cell (MFC) configuration. This indicates that the alteration of the anode from planar to brush can contribute toward improving the current recovery through the air-cathode-AEM-MFC. The BC needs improvement to have more specific surface area, whereas it showed superiority in cost efficiency considering material and processing.

Advanced microbial fuel cell for waste water treatment-a review

Petroleum, coal, and natural gas reservoir were depleting continuously due to an increase in industrialization, which enforced study to identify alternative sources. The next option is the renewable resources which are most important for energy purpose coupled with environmental problem reduction. Microbial fuel cells (MFCs) have become a promising approach to generate cleaner and more sustainable electrical energy. The involvement of various disciplines had been contributing to enhancing the performance of the MFCs. This review covers the performance of MFC along with different wastewater as a substrate in terms of treatment efficiencies as well as for energy generation. Apart from this, effect of various parameters and use of different nanomaterials for performance of MFC were also studied. From the current study, it proves that the use of microbial fuel cell along with the use of nanomaterials could be the waste and energy-related problem-solving approach. MFC could be better in performances based on optimized process parameters for handling any wastewater from industrial process.

Modeling and optimization strategies towards performance enhancement of microbial fuel cells

Considering the complexity associated with bioelectrochemical processes, the performance of a microbial fuel cell (MFC) is governed by input operating parameters. For scaled-up applications, a MFC system needs to be modeled from engineering perspectives in terms of optimum operating conditions to get higher performance and energy recovery. Several conceptual numerical models to advanced computational simulation approaches have been developed to represent simple-form of a complex MFC system. Application of mathematical and computation models are explored to establish the relationship between operating input-variables and power output. The present review discusses about the complexity of system, modeling strategies used and reality of such modeling for scaling-up applications of MFCs. Additionally, the selection of an appropriate mathematical model reduces the computational duration and provides better understanding of the system process. It also explores the possibility and progress towards commercialization of MFCs and thus the need of development of model-based optimization and process-control approaches.

Impact of external resistance acclimation on charge transfer and diffusion resistance in bench-scale microbial fuel cells

Reducing the external resistance (R_ext) for microbial fuel cell (MFC) acclimation can substantially alter the anode performance in terms of charge transfer (R_CT), diffusion (R_d) and total anode resistance (R_An). Electrochemical impedance spectroscopy (EIS) was used to quantify anode impedance at different set potentials. Reducing R_ext from 50 Ω to 20 Ω during acclimation reduced R_CT by 31% (from 6.12 ± 0.09 mΩ m² to 4.21 ± 0.03 mΩ m²) and R_d by 18% (from 3.4 ± 0.2 mΩ m² to 2.8 ± 0.1 mΩ m²) at a set anode potential of -115 mV during EIS. Overall R_An decreased by 27%, to 5.13 ± 0.02 mΩ m² for acclimation at 20 Ω, enabling the anode to achieve 38% higher current densities of 29 ± 1 A m^-2. The results show a clear dependence of acclimation procedures and external resistance on kinetic and diffusion components of anode impedance that can impact overall bioelectrochemical performance.

Diffusion-layer-free air cathode based on ionic conductive hydrogel for microbial fuel cells

High hydraulic pressure in air-cathode microbial fuel cells (MFCs) can lead to severe cathodic water leakage and power reduction, thereby hindering the practical applications of MFCs. In this study, an alternative air cathode without a diffusion layer was developed using a cross-linked hydrogel, oxidized konjac glucomannan/2-hydroxypropytrimethyl ammonium chloride chitosan (OKH), for ion bridging. The cathode was placed horizontally to avoid hydraulic pressure on its surface. Ion transportation was sustained with a minimal OKH hydrogel loading of 10 mg/cm². A maximum power density of 1.0 ± 0.04 W/m² was achieved, which was only slightly lower than the 1.28 ± 0.02 W/m² of common air cathodes. Moreover, the cost of the OKH hydrogel is only $0.12/m², which can reduce ~85% of the cathode cost without using the advanced polyvinylidene fluoride diffusion layer. Therefore, the development of this new diffusion-layer-free air cathode using conductive ionic hydrogel provides a low-cost strategy for stable MFC operation, thereby demonstrating great potential for practical applications of MFC technology.

Impact of cathodic electron acceptor on microbial fuel cell internal resistance

Ferricyanide is often used in microbial fuel cells (MFCs) to avoid oxygen intrusion that occurs with air cathodes. However, MFC internal resistances using ferricyanide can be larger than those with air cathodes even though ferricyanide results in higher power densities. Using a graphite fiber brush cathode and a ferricyanide catholyte (FC-B) the internal resistance was 62 ± 4 mΩ m², with 84 ± 8 mΩ m² obtained using ferricyanide and a flat carbon paper cathode (FC-F) and only 51 ± 1 mΩ m² using a 70% porosity air cathode (A-70). The FC-B MFCs produced the highest maximum power density of all configurations examined: 2.46 ± 0.26 W/m², compared to 1.33 ± 0.14 W/m² for the A-70 MFCs. The electrode potential slope (EPS) analysis method showed that electrode resistances were similar for ferricyanide and air-cathode MFCs, and that higher power was due to the larger experimental working potential (500 ± 12 mV) of ferricyanide compared to the air cathode (233 ± 5 mV).

Air-breathing cathode self-powered supercapacitive microbial fuel cell with human urine as electrolyte

In this work, a membraneless microbial fuel cell (MFC) with an empty volume of 1.5 mL, fed continuously with hydrolysed urine, was tested in supercapacitive mode (SC-MFC). In order to enhance the power output, a double strategy was used: i) a double cathode was added leading to a decrease in the equivalent series resistance (ESR); ii) the apparent capacitance was boosted up by adding capacitive features on the anode electrode. Galvanostatic (GLV) discharges were performed at different discharge currents. The results showed that both strategies were successful obtaining a maximum power output of 1.59 ± 0.01 mW (1.06 ± 0.01 mW mL⁻¹) at pulse time of 0.01 s and 0.57 ± 0.01 mW (0.38 ± 0.01 mW mL⁻¹) at pulse time of 2 s. The highest energy delivered at i_pulse equal to 2 mA was 3.3 ± 0.1 mJ. The best performing SC-MFCs were then connected in series and parallel and tested through GLV discharges. As the power output was similar, the connection in parallel allowed to roughly doubling the current produced. Durability tests over ≈5.6 days showed certain stability despite a light overall decrease.

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Air-breathing microbial fuel cell was tested in supercapacitive mode.

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A double cathode addition lead to a decrease in ohmic resistance.

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Apparent capacitance was boosted up by adding capacitive features.

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Maximum power output of 1.59 mW (1.06 mW mL⁻¹) was reached at t_pulse 0.01s.

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Series and parallel connections improved the galvanostatic discharges.

A review on the contribution of electron flow in electroactive wetlands: Electricity generation and enhanced wastewater treatment

In less than a decade, bioelectrochemical systems/microbial fuel cell integrated constructed wetlands (electroactive wetlands) have gained a considerable amount of attention due to enhanced wastewater treatment and electricity generation. The enhancement in treatment has majorly emanated from the electron transfer or flow, particularly in anaerobic regions. However, the chemistry associated with electron transfer is complex to understand in electroactive wetlands. The electroactive wetlands accommodate diverse microbial community in which each microbe set their own potential to further participate in electron transfer. The conductive materials/electrodes in electroactive wetlands also contain some potential, due to which, several conflicts occur between microbes and electrode, and results in inadequate electron transfer or involvement of some other reaction mechanisms. Still, there is a considerable research gap in understanding of electron transfer between electrode-anode and cathode in electroactive wetlands. Additionally, the interaction of microbes with the electrodes and understanding of mass transfer is also essential to further understand the electron recovery. This review mainly deals with the electron transfer mechanism and its role in pollutant removal and electricity generation in electroactive wetlands.

Quantitative evaluation of effects of different cathode materials on performance in Cd(II)-reduced microbial electrolysis cells

Three materials including stainless steel woven mesh (SSM), nickel foam (NF) and carbon cloth (CC) were conducted as cathode in Cd(II)-reduced microbial electrolysis cells (MECs), respectively. By using electrode potential slope (EPS) method, the experimental open circuit potentials of three cathodes were similar, while the SSM cathode showed the smallest resistance (6 ± 1 mΩ m²), following by NF cathode (18 ± 2 mΩ m²) and CC cathode (32 ± 5 mΩ m²). These values were analyzed to predicte higher current density and more positive cathode potential in the MEC with SSM cathode under subsequent operating conditions. Electrochemical performance was more likely to be limited by current density than cathode potential. Accordingly, the MEC with SSM cathode obtained better system performance than that with other cathodes. This study further expands the application of EPS method that quantitatively evaluating and effectively selecting cathode materials for better system performance in Cd(II)-reduced MECs.

Deteriorated biofilm-forming capacity and electroactivity of Shewanella oneidnsis MR-1 induced by insertion sequence (IS) elements

Shewanella oneidensis MR-1, a model species of exoelectrogenic bacteria (EEB), has been widely applied in bioelectrochemical systems. Biofilms of EEB grown on electrodes are essential in governing the current output and power density of bioelectrochemical systems. The MR-1 genome is exceptionally dynamic due to the existence of a large number of insertion sequence (IS) elements. However, to date, the impacts of IS elements on the biofilm-forming capacity of EEB and performance of bioelectrochemical systems remain unrevealed. Herein, we isolated a non-motile mutant (NMM) with biofilm-deficient phenotype from MR-1. We found that the insertion of an ISSod2 element into the flrA (encoding the master regulator for flagella synthesis and assembly) of MR-1 resulted in the non-motile and biofilm-deficient phenotypes in NMM cells. Notably, such a variant was readily confused with the wild-type strain because there were no obvious differences in growth rates and colonial morphologies between the two strains. However, the reduced biofilm formation on the electrodes and the deteriorated performances of bioelectrochemical systems and Cr(VI) immobilization for the strain NMM were observed. Given the wide distribution of IS elements in EEB, appropriate cultivation and preservation conditions should be adopted to reduce the likelihood that IS elements-mediated mutation occurs in EEB. These findings reveal the negative impacts of IS elements on the biofilm-forming capacity of EEB and performance of bioelectrochemical systems and suggest that great attention should be given to the actual physiological states of EEB before their applications.

Highly efficient anaerobic co-degradation of complex persistent polycyclic aromatic hydrocarbons by a bioelectrochemical system

Polycyclic aromatic hydrocarbons (PAHs) that undergo long-distance migration and have strong biological toxicity are a great threat to the health of ecosystems. In this study, the biodegradation characteristics and combined effects of mixed PAHs in bioelectrochemical systems (BESs) were studied. The results showed that, compared with a mono-carbon source, low-molecular-weight PAHs (LMW PAHs)-naphthalene (NAP) served as the co-substrate to promote the degradation of phenanthrene (PHE) and pyrene (PYR). The maximum degradation rates of PHE and PYR were 89.20% and 51.40% at 0.2500 mg/L in NAP-PHE and NAP-PYR at the degradation time of 120 h, respectively. Intermediate products were also detected, which indicated that the appending of relatively LMW PAHs had different effects on the metabolism of high-molecular-weight PAHs (HMW PAHs). The microbe species under different substrates (NAP-B, PHE-B, PYR-B, NAP-PHE, NAP-PYR, PHE-PYR) are highly similar, although the structure of the microbial community changed on the anode in the BES. In this study, the degradation regularity of mixed PAHs in BES was studied and provided theoretical guidance for the effective co-degradation of PAHs in the environment.

Balancing Water Dissociation and Current Densities To Enable Sustainable Hydrogen Production with Bipolar Membranes in Microbial Electrolysis Cells

Hydrogen production using two-chamber microbial electrolysis cells (MECs) is usually adversely impacted by a rapid rise in catholyte pH because of proton consumption for the hydrogen evolution reaction. While using a bipolar membrane (BPM) will maintain a more constant electrolyte pH, the large voltage loss across this membrane reduces performance. To overcome these limitations, we used an acidic catholyte to compensate for the potential loss incurred by using a BPM. A hydrogen production rate of 1.2 ± 0.7 L-H₂/L/d (j_max = 10 ± 0.4 A/m²) was obtained using a Pt cathode and BPM with a pH difference (ΔpH = 6.1) between the two chambers. This production rate was 2.8 times greater than that of a conventional MEC with an anion exchange membrane (AEM, 0.43 ± 0.1 L-H₂/L/d, j_max = 6.5 ± 0.3 A/m²). The catholyte pH gradually increased to 11 ± 0.3 over 9 days using the BPM and Pt/C, which decreased current production (j_max = 2.5 ± 0.3 A/m²). However, this performance was much better than that obtained using an AEM as the catholyte pH increased to 10 ± 0.4 after just one day. The use of an activated carbon cathode with the BPM enabled stable performance over a longer period of 12 days, although it reduced the hydrogen production rate (0.45 ± 0.1 L-H₂/L/d).

Application of phase-pure nickel phosphide nanoparticles as cathode catalysts for hydrogen production in microbial electrolysis cells

Transition metal phosphide catalysts such as nickel phosphide (Ni₂P) have shown excellent activities for the hydrogen evolution reaction, but they have primarily been studied in strongly acidic or alkaline electrolytes. In microbial electrolysis cells (MECs), however, the electrolyte is usually a neutral pH to support the bacteria. Carbon-supported phase-pure Ni₂P nanoparticle catalysts (Ni₂P/C) were synthesized using solution-phase methods and their performance was compared to Pt/C and Ni/C catalysts in MECs. The Ni₂P/C produced a similar quantity of hydrogen over a 24 h cycle (0.29 ± 0.04 L-H₂/L-reactor) as that obtained using Pt/C (0.32 ± 0.03 L-H₂/L) or Ni/C (0.29 ± 0.02 L-H₂/L). The mass normalized current density of the Ni₂P/C was 14 times higher than that of the Ni/C, and the Ni₂P/C exhibited stable performance over 11 days. Ni₂P/C may therefore be a useful alternative to Pt/C or other Ni-based catalysts in MECs due to its chemical stability over time.

Impact of cleaning procedures on restoring cathode performance for microbial fuel cells treating domestic wastewater

Degradation of cathode performance over time is one of the major drawbacks in applications of microbial fuel cells (MFCs) for wastewater treatment. Over a two month period the resistance of air cathodes (R_Ct) with a polyvinylidene fluoride (PVDF) diffusion layer increased of 111% from 70 ± 10 mΩ m² to 148 ± 32 mΩ m². Soaking the cathodes in hydrochloric acid (100 mM HCl) restored cathode performance to R_Ct = 74 ± 17 mΩ m². Steam, ethanol, or sodium hydroxide treatment produced only a small change in performance, and slightly increased R_Ct. With a polytetrafluoroethylene (PTFE) diffusion layer on the cathodes, R_Ct increased from 54 ± 14 mΩ m² to 342 ± 142 mΩ m² after two months of operation. The acid concentration was critical for effectiveness in cleaning, as HCl (100 mM) decreased R_Ct to 28 ± 8 mΩ m². A lower concentration of HCl (<1 mM) showed no improvement, and vinegar (5% acetic acid) produced 48 ± 4 mΩ m².

Applying the electrode potential slope method as a tool to quantitatively evaluate the performance of individual microbial electrolysis cell components

Improving the design of microbial electrolysis cells (MECs) requires better identification of the specific factors that limit performance. The contributions of the electrodes, solution, and membrane to internal resistance were quantified here using the newly-developed electrode potential slope (EPS) method. The largest portion of total internal resistance (120 ± 0 mΩ m²) was associated with the carbon felt anode (71 ± 5 mΩ m², 59% of total), likely due to substrate and ion mass transfer limitations arising from stagnant fluid conditions and placement of the electrode against the anion exchange membrane. The anode resistance was followed by the solution (25 mΩ m²) and cathode (18 ± 2 mΩ m²) resistances, and a negligible membrane resistance. Wide adoption and application of the EPS method will enable direct comparison between the performance of the components of MECs with different solution characteristics, electrode size and spacing, reactor architecture, and operating conditions.

Azadirachta indica leaf-extract-assisted synthesis of CoO–NiO mixed metal oxide for application in a microbial fuel cell as a cathode catalyst

In this study, a highly efficient yet low-cost mixed transition metal oxide of Ni and Co as CoO–NiO was synthesized using Azadirachta indica leaf extract as a bioactive chelating agent.