Redox mediator as cathode modifier for enhanced degradation of azo dye in a sequential dual chamber microbial fuel cell-aerobic treatment process

The electron transfer from cathode to azo dye Acid Blue 29 (AB29) using thionine (TH) and anthraquinone-2-sulfonate (AQS) redox mediators were investigated in dual chamber microbial fuel cells (DCMFCs). More than 90% of color was removed using electropolymerized TH (192 h) and AQS (264 h) cathodes. Chemical oxygen demand (COD) removal after anaerobic treatment in cathode chamber of TH-MFC, AQS-MFC and unmodified-MFC were 76.6 ± 1.7, 70.8 ± 2.5 and 18.3 ± 2.9%, respectively, which increased to 85.4 ± 1.5, 79.8 ± 3 and 20.6 ± 2.1%, respectively, after aerobic post treatment. Gas chromatography–mass spectrometry (GC–MS) investigations revealed the formation of aromatic amines in DCMFCs which were further degraded into low molecular-weight products in the aerobic post treatment. Electrochemical impedance spectroscopic (EIS) analysis showed lowest charge transfer resistance of TH-cathode which increased the electrochemical reactions and electron transfer rates. These results indicated that AB29 can be efficiently degraded by utilizing modified cathode based DCMFC-aerobic post treatment process along with bioelectricity generation.     

Study on synergistic mechanism of PANDAN modification,current and electroactive biofilms on Congo red decolorization in microbial fuel cells

Effect on microbial fuel cells (MFCs) for decolorization of Congo red by Poly (aniline-1,8-diaminonaphthalene) (PANDAN) modification, current and electroactive biofilms (EABs) is investigated. With the synergism of the three factors: PANDAN modification, current and EABs (A2 reactor), the COD removal and decolorization rate significantly increase to 88% and 97%, as well as the Congo red is thoroughly degraded. The decolorization performance comparison and Redundancy analysis (RDA) results indicate that the EABs take more responsibility (contribute ~ 50%) for the decolorization, rather than the modification and current. Therefore, the effects and mechanism of PANDAN modification and current on EABs are further revealed by the Confocal Scanning Laser Microscopy (CSLM) and high-throughput sequencing analysis. The effects of current (anodic dynamical microenvironment), material adsorption, and electron transfer mediating act comprehensively on the thickness, viability, EPS and microbial community of the EABs, among which the relationship is discussed in depth by hierarchically comparison, with “independent and additional effects”. It is demonstrated that the modification, current and EABs present the synergistic effect and promote each other in the performance of the decolorization MFC.     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC applications.     

Unveiling characteristics of dye-bearing microbial fuel cells for energy and materials recycling: Redox mediators

This study disclosed why and how some decolorized intermediates (e.g., 2-aminophenol) could act as electron-shuttling mediator(s) to enhance the capabilities of reductive decolorization and bioelectricity generation. It also selected several model auxochrome-containing compounds structurally associated to 2AP to explore how chemical structure influenced the feasibility of possible electron shuttles for power producing capabilities in microbial fuel cells (MFCs). The selection criteria of electron-shuttling mediators were suggested for optimal reductive decolorization and bioelectricity generation in MFCs for practical application.     

Enhanced sludge stabilization coupled with microbial fuel cells (MFCs)

This study presents research results on electricity production from waste activated sludge using MFCs during stabilization process. Different MFC configurations equipped with various electrodes were used. Voltage measurements were continuously done during 35 days of MFC operation. Experimental results showed that bioelectricity generation was linked to volatile solids (VS) and protein reductions as a fraction of extracellular polymeric substances (EPS). Double chamber MFC reactor equipped with graphite electrodes had better power and current densities as 312.98 mW/m² and 39.07 μA/cm² while single chamber MFC equipped with titanium electrodes revealed better power and current densities as 97.60 mW/m² and 17.63 μA/cm², respectively. Molecular results indicated that power outputs of MFCs effected by diverse microbial communities in anode biofilms. Although organic matter degradation is reported as 35%–55% VS reduction for digesters, this research provided a promising approach for sludge stabilization with enhanced degrading of organic matters up to 75% by using MFCs.     

Impact of salinity on cathode catalyst performance in microbial fuel cells (MFCs)

Several alternative cathode catalysts have been proposed for microbial fuel cells (MFCs), but effects of salinity (sodium chloride) on catalyst performance, separate from those of conductivity on internal resistance, have not been previously examined. Three different types of cathode materials were tested here with increasingly saline solutions using single-chamber, air-cathode MFCs. The best MFC performance was obtained using a Co catalyst (cobalt tetramethoxyphenyl porphyrin; CoTMPP), with power increasing by 24 ± 1% to 1062 ± 9 mW/m² (normalized to the projected cathode surface area) when 250 mM NaCl (final conductivity of 31.3 mS/cm) was added (initial conductivity of 7.5 mS/cm). This power density was 25 ± 1% higher than that achieved with Pt on carbon cloth, and 27 ± 1% more than that produced using an activated carbon/nickel mesh (AC) cathode in the highest salinity solution. Linear sweep voltammetry (LSV) was used to separate changes in performance due to solution conductivity from those produced by reductions in ohmic resistance with the higher conductivity solutions. The potential of the cathode with CoTMPP increased by 17–20 mV in LSVs when the NaCl addition was increased from 0 to 250 mM independent of solution conductivity changes. Increases in current were observed with salinity increases in LSVs for AC, but not for Pt cathodes. Cathodes with CoTMPP had increased catalytic activity at higher salt concentrations in cyclic voltammograms compared to Pt and AC. These results suggest that special consideration should be given to the type of catalyst used with more saline wastewaters. While Pt oxygen reduction activity is reduced, CoTMPP cathode performance will be improved at higher salt concentrations expected for wastewaters containing seawater.     

Long-term effect of carbon nanotubes on electrochemical properties and microbial community of electrochemically active biofilms in microbial fuel cells

Carbon nanotubes (CNTs) have been widely exploited to improve anodic performance, but information is needed on their long-term stability for improvement. Herein, we prepared a novel CNTs-modified graphite felt (CNTs-GF) by a simple and scalable process and evaluated its long-term performance using anaerobic sludge as inoculum. the MFC with CNTs-GF yielded a sustained enhancement of power output, increasing from 1.93 ± 0.09 W m^?2 after 1 month to 2.10 ± 0.05 W m^?2 after 3 months and reaching 2.00 ± 0.10 W m^?2 after 13 month, indicating the enhancement in electricity generation by the CNTs was not declined over one year. However, the bare GF showed a declining tendency of performance during 13 months. The long-term enhancement can be explained by the facts that the CNTs-GF was beneficial to electrochemically active biofilms (EABs) growth and interacted better with EABs and increased the extracellular electron transfer. Community analysis showed an increase in Geobacter in response to CNTs modification. These results demonstrated that CNTs modification could sustain a superior long-term enhancement in MFC performance.     

Power generation of microbial fuel cells (MFCs) with low cathodic platinum loading

This study aims at investigating the effects of platinum (Pt) loadings on the cathodic reactions in Single Chamber Microbial Fuel Cells (SCMFCs) and developing cost-effective MFC operational protocols. The power generation of SCMFCs was examined with different Pt loadings (0.005–1 mgPt/cm²) on cathodes. The results showed that the power generation of the SCMFCs with 0.5–1 mgPt/cm² were the highest in the tests, decreased 10–15% at 0.01–0.25 mgPt/cm², and decreased further 10–15% at 0.005 mgPt/cm². The SCMFCs with Pt-free cathode (graphite) had the lowest power generation. In addition, the power generation of SCMFCs with different Pt loadings were compared in raw wastewater (Chemical oxygen demand (COD): 0.36 g/L) and wastewater enriched with sodium acetate (COD: 2.95 g/L). The solution conductivity in SCMFCs decreased with the degradation of organic substrates. Daily polarization curves (V–I) showed a decrease in current generation and an increase in ohmic losses over the operational period (8 days). The SCMFCs (with 0.5–1 mgPt/cm² at cathode) fed with wastewater and sodium acetate (NaOAc) reached the highest power generation (786 mW/m²), while the SCMFCs (with 0.5–1 mgPt/cm² at cathode) fed only with wastewater obtained the lower power generation (81 mW/m²). The study demonstrated that lowering the Pt loadings in two magnitude orders (1 to 0.01, 0.5 to 0.005 mgPt/cm²) only reduced the power generation of 15–30%, and this reduction of the power generation become less substantial with the decrease in the solution conductivity of SCMFCs.     

Carbon nanotube modified air-cathodes for electricity production in microbial fuel cells

The use of air-cathodes in microbial fuel cells (MFCs) has been considered sustainable for large scale applications, but the performance of most current designs is limited by the low efficiency of the three-phase oxygen reduction on the cathode surface. In this study we developed carbon nanotube (CNT) modified air-cathodes to create a 3-D electrode network for increasing surface area, supporting more efficient catalytic reaction, and reducing the kinetic resistance. Compared with traditional carbon cloth cathodes, all nanotube modified cathodes showed higher performance in electrochemical response and power generation in MFCs. Reactors using carbon nanotube mat cathodes showed the maximum power density of 329 mW m⁻²; more than twice that of the peak power obtained with carbon cloth cathodes (151 mW m⁻²). The addition of Pt catalysts significantly increased the current densities of all cathodes, with the maximum power density obtained using the Pt/carbon nanotube mat cathode at 1118 mW m⁻². The stable maximum power density obtained from other nanotube coated cathodes varied from 174 mW m⁻² to 522 mW m⁻². Scanning electron micrographs showed the presence of conductive carbon nanotube networks on the CNT modified cathodes that provide more efficient oxygen reduction.     

Performance of plug flow microbial fuel cell (PF-MFC) and complete mixing microbial fuel cell (CM-MFC) for wastewater treatment and power generation

Two flow patterns (plug flow (PF) and complete mixing (CM)) of microbial fuel cells (MFCs) with multiple anodes–cathodes were compared in continuous flow mode for wastewater treatment and power generation. The results indicated that PF-MFCs had higher power generation and columbic efficiency (CE) than CM-MFCs, and the power generation varied along with the flow pathway in the PF-MFCs. The gradient of substrate concentrations along the PF-MFCs was the driving force for the power generation. In contrast, the CM-MFCs had higher wastewater removal efficiency than PF-MFCs, but had lower power conversion efficiency and power generation. This work demonstrated that MFC configuration is a key factor for enhancing power generation and wastewater treatment.     

Synthesis of polypyrrole (PPy) based porous N-doped carbon nanotubes (N-CNTs) as catalyst support for PEM fuel cells

In this study, it was aimed to synthesize catalytically active, high surface area carbon nanotubes (CNTs) by means of nitrogen doping (N-doping). The synthesized nitrogen doped carbon nanotubes (N-CNTs) were used as Pt catalyst support in order to improve oxygen reduction reaction (ORR) kinetics at the cathode electrode in PEM fuel cell. Polypyrrole (PPy) was served as both carbon and nitrogen source and FeCl₃ solution was used as oxidizing agent in the synthesis procedure of N-CNTs. Chemical activation of the materials was made with potassium hydroxide (KOH) solution during 12 and 18 h time periods. It was considered that activation period is of great importance on the properties of the synthesized PPy based N-CNTs. 12 h activated N-CNTs gave higher surface area (1607.2 m²/g) and smaller micropore volume (0.355 cm³/g) in comparison to 18 h activated N-CNTs having smaller surface area (1170.7 m²/g) and higher micropore volume (0.383 cm³/g). PEM fuel cell performance results showed that 12 h activated N-CNTs are better catalyst supports than 18 h activated N-CNTs for Pt nanoparticle decoration.     

Ceramic-based microbial fuel cells (MFCs): A review

Recently, porous ceramic and clayware membranes have been widely used in microbial fuel cells (MFCs) as separators. Chemical, thermal and mechanical stability, low-cost and many other advantages of ceramic membranes make them an appropriate substitute for expensive polymeric ion exchange membranes. Moreover, good power performances in short and long-term periods were observed using ceramic membranes. In this review, we attempted to gather and assort all the experiments which applied ceramic or other earthenware membranes as the separator of MFCs. The effects of physical and chemical properties of ceramic membranes on the power efficiency of MFCs as well as scale-up challenges and future aspects were also studied.     

Recycled tire crumb rubber anodes for sustainable power production in microbial fuel cells

One of the greatest challenges facing microbial fuel cells (MFCs) in large scale applications is the high cost of electrode material. We demonstrate here that recycled tire crumb rubber coated with graphite paint can be used instead of fine carbon materials as the MFC anode. The tire particles showed satisfactory conductivity after 2-4 layers of coating. The specific surface area of the coated rubber was over an order of magnitude greater than similar sized graphite granules. Power production in single chamber tire-anode air-cathode MFCs reached a maximum power density of 421 mW m⁻², with a coulombic efficiency (CE) of 25.1%. The control graphite granule MFC achieved higher power density (528 mW m⁻²) but lower CE (15.6%). The light weight of tire particle could reduce clogging and maintenance cost but posts challenges in conductive connection. The use of recycled material as the MFC anodes brings a new perspective to MFC design and application and carries significant economic and environmental benefit potentials.     

Facile reconstruction of microbial fuel cell (MFC) anode with enhanced exoelectrogens selection for intensified electricity generation

The present work emphasized on the enhancement of microbial fuel cell (MFC) anode through the utilization of conductive polymer. The conductive polymer, poly(3,4-ethylenedioxythiophene) (PEDOT) was coated with varied concentrations onto graphite felt base anodes. The findings demonstrated that the optimum loading of 2.5 mg/cm² recorded maximum current density of 3.5 A/m² and coulombic efficiency of 51%. Higher loading of PEDOT enhanced the electrochemical characteristics of the anodes but exhibited unfavorable functionality. The charge transfer resistance of the modified anodes, R_a decreased significantly compared to the control anode after biofilm formation. The successful application of palm oil mill effluent (POME) wastewater as substrate indicates that the optimum anode was effective in degrading high organic wastewater. Exoelectrogens were found to be distributed mainly on the anodic biofilm. The microbial diversity of the anodes varied greatly from the inoculum and Geobacter was identified as the prevailing exoelectrogen responsible for the power generation.     

Bioenergy production from glycerol in hydrogen producing bioreactors (HPBs) and microbial fuel cells (MFCs)

The supply of glycerol has increased substantially in recent years as a by-product of biodiesel production. To explore the value of glycerol for further application, the conversion of glycerol to bioenergy (hydrogen and electricity) was investigated using Hydrogen Producing Bioreactors (HPBs) and Microbial Fuel Cells (MFCs). Pure-glycerol and the glycerol from biodiesel waste stream were compared as the substrates for bioenergy production. In terms of hydrogen production, the yields of hydrogen and 1,3-propanediol at a pure-glycerol concentration of 3 g/L were 0.20 mol/mol glycerol and 0.46 mol/glycerol, respectively. With glucose as the co-metabolism substrate at the ratio of 3:1 (glycerol:glucose), the yields of hydrogen and 1,3-propanediol from glycerol significantly increased to 0.37 mol/mol glycerol and 0.65 mol/glycerol, respectively. The glycerol from biodiesel waste stream had good hydrogen yields (0.17-0.18 mol H₂/mole glycerol), which was comparable with the pure-glycerol. In terms of power generation in MFCs, pure-glycerol was examined at concentrations of 0.5-5 g/L with the highest power density of 4579 mW/m³ obtained at a concentration of 2 g/L. The power densities from the biodiesel waste glycerol were 1614-2324 mW/m³, which were likely caused by the adverse effects of impurities on electrode materials. An economic analysis indicates that with the annual waste stream of 70 million gallons of glycerol, the expected values generated from HPBs and MFCs were $311 and $98 million, respectively.     

Electricity generation and COD removal of microbial fuel cells (MFCs) operated with alkaline substrates

The characteristics of electricity generation and COD removal of dual-chamber microbial fuel cells (MFCs) operated with alkaline substrates were studied. Substrates with constant pH of either 7 or 9 as well as varying pH in a cycle of 7-8-9-8-7 were used. MFCs operated with these substrates were denoted as MFC-pH7, MFC-pH9 and MFC-pHV, respectively. The experimental results indicate that the MFC-pHV can generate the highest performance of 2554 ± 159 mW/m². Cyclic voltammetry (CV), active biomass and electrochemical impedance spectroscopy (EIS) measurements were conducted and these results suggested that the MFC-pHV had the highest electrochemical activity per unit biomass and the lowest internal resistance, which together contributed to the improved power output of the MFC-pHV. In addition, compared with the other two MFCs operated at fixed pH values, the COD removal efficiency of the MFC-pHV was improved due to the stronger adaptation to the varying pH-environment.     

Flame synthesis of carbon nanostructures on stainless steel anodes for use in microbial fuel cells

Microbial fuel cells (MFCs) offer a promising alternative energy technology, but suffer from low power densities which hinder their practical applicability. In order to improve anodic power density, we deposited carbon nanostructures (CNSs) on an otherwise plain stainless steel mesh (SS-M) anode. Using a flame synthesis method that did not require pretreatment of SS-M substrates, we were able to produce these novel CNS-enhanced SS-M (CNS-M) anodes quickly (in a matter of minutes) and inexpensively, without the added costs of chemical pretreatments. During fed batch experiments with biomass from anaerobic digesters in single-chamber MFCs, the median power densities (based on the projected anodic surface area) were 2.9 mW m⁻² and 187 mW m⁻² for MFCs with SS-M and CNS-M anodes, respectively. The addition of CNSs to a plain SS-M anode via flame deposition therefore resulted in a 60-fold increase in the median power production. The combination of CNSs and metallic current collectors holds considerable promise for power production in MFCs.     

A dual photoelectrode-based double-chambered microbial fuel cell applied for simultaneous COD and Cr (VI) reduction in wastewater

Cerium oxide (CeO₂) and cuprous oxide (Cu₂O) were used for the first time as photoanode and photocathode, respectively, in a microbial fuel cell (MFC) for simultaneous reduction of chemical oxygen demand (COD) and Cr(VI) in wastewater. The photoelectrodes, viz. Photoanode and photocathode were separately prepared by impregnating activated carbon fiber (ACF) with the respective metal oxide nanoparticles, followed by growing carbon nanofibers (CNFs) on the ACF substrate using catalytic chemical vapor deposition. The MFC, operated under visible light irradiation, showed reduction in COD and Cr(VI) by approximately 94 and 97%, respectively. The MFC also generated high bioelectricity with a current density of ~6918 mA/m² and a power density of ~1107 mW/m². The enhanced performance of the MFC developed in this study was attributed to the combined effects of the metal oxide photocatalysts, the graphitic CNFs, and the microporous ACF substrate. The MFC based on the inexpensive transition metal oxides-based photoelectrodes developed in this study has a potential to be used at a large scale for treating the industrial aqueous effluents co-contaminated with organics and toxic Cr(VI).     

Poly (acrylamide-co-acrylonitrile) hydrogel-derived iron-doped carbon foam electrocatalyst for enhancing oxygen reduction reaction in microbial fuel cell

Developing advanced non-precious metal catalysts for oxygen reduction reaction (ORR) is critical for microbial fuel cells (MFCs). Fe–N–C catalysts are considered the best successor to platinum-based catalysts for ORR. Herein, we have synthesized environmental friendly, cost-effective Fe–N-doped carbon foam catalyst [Fe-embedded poly (acrylamide-co-acrylonitrile) hydrogel-based carbon foam(Fe@Am-co-An/CF)] by using Fe-embedded poly (Am-co-An) hydrogel for MFCs. Poly(Am-co-An) hydrogel is used as a carbon and nitrogen precursor. The synthesized catalysts are characterized by FTIR, SEM, TEM, XRD and XPS. Furthermore, four different catalysts based on different ratios of the metal such as Fe@Am-co-An/CF (1:22), Fe@Am-co-An/CF (2:22), Fe@Am-co-An/CF (3:22), and Am-co-An/CF have been prepared. Results indicate that the Fe@Am-co-An/CF (2:22) catalyst exhibits the highest power density (736.06 mWm^?2 at the current density of 1132.04 mAm^?2) compared to the other catalysts. The results of CV, LSV, EIS, and chronoamperometry indicate that Fe@Am-co-An/CF (2:22) is the most promising catalyst for ORR activity in MFCs.     

Power recovery with multi-anode/cathode microbial fuel cells suitable for future large-scale applications

Multi-anode/cathode microbial fuel cells (MFCs) incorporate multiple MFCs into a single unit, which maintain high power generation at a low cost and small space occupation for the scale-up MFC systems. The power production of multi-anode/cathode MFCs was similar to the total power production of multiple single-anode/cathode MFCs. The power density of a 4-anode/cathode MFC was 1184 mW/m³, which was 3.2 times as that of a single-anode/cathode MFC (350 mW/m³). The effect of chemical oxygen demand (COD) was studied as the preliminary factor affecting the MFC performance. The power density of MFCs increased with COD concentrations. Multi-anode/cathode MFCs exhibited higher power generation efficiencies than single-anode/cathode MFCs at high CODs. The power output of the 4-anode/cathode MFCs kept increasing from 200 mW/m³ to 1200 mW/m³ as COD increased from 500 mg/L to 3000 mg/L, while the single-anode/cathode MFC showed no increase in the power output at CODs above 1000 mg/L. In addition, the internal resistance (R_in) exhibited strong dependence on COD and electrode distance. The R_in decreased at high CODs and short electrode distances. The tests indicated that the multi-anode/cathode configuration efficiently enhanced the power generation.