Addition of reduced graphene oxide to an activated-carbon cathode increases electrical power generation of a microbial fuel cell by enhancing cathodic performance

Determination of operation parameters for magnesium-air fuel cell to recover nitrogen and phosphorus from wastewater

Recovering phosphorus (P) and nitrogen (N) from wastewater not only contributes to environmental protection but also aligns with sustainable development goals. This study employed a magnesium-air fuel cell (Mg-O2-FC) to extract P and N from wastewater in the form of struvite (MgNH4·6H2O), based on the removal efficiency of ammonia and phosphate, electricity generation capacity and struvite purity to determine the optimal operation parameters. These parameters included hydraulic retention time (HRT), service life of magnesium sheet, and precipitation discharge frequency. The results showed that the removal efficiency of ammonia from 0 to 4h was 55.99%, and that from 4 to 12h was only 15.74%. The phosphate removal efficiency in the initial cycle was 97.68% but decreased to 63.25% after 24h. The phosphate removal rate in 2 min increased by 145% when the precipitation discharge frequency increased from 4 h/time to 24 h/time. Consequently, the HRT, service life of the magnesium sheet, and precipitation discharge frequency were selected as 4 h, 24 h, and 24 h/time. These optimized conditions provide valuable insights for the practical implementation of Mg-O2-FC in recovering N and P from wastewater.

Development, performance evaluation, and kinetic studies of microbial fuel cell based auto dripping bioelectrochemical reactor (AutoDriBER) for urine treatment

A bioelectrochemical reactor is an assembly, which facilitates energy generation and resource recovery using electrochemically active microorganisms. To maximise energy production from wastewater in this bioreactor system special design is required. Therefore, in the present study, continuous flow auto dripping bioelectrochemical reactors (AutoDriBERs) were developed as a single and multi-electrode assembly for urine treatment. Further, their performance was assessed by connecting reactors in series and parallel arrangements. AutoDriBER configured in series connection showed the highest 93.64 ± 1.57% chemical oxygen demand removal rate with the 1.38 ± 0.64 V voltage and 2.54 W m-3 polarisation power density. The optimum flow rate for maximum voltage production was tested with various models i.e. the linear, exponential, Sweibull-1, and Sweibull-2 models to confirm voltage prediction and its validity. The Linear and exponential models were found best fitted for voltage production with R2 value of 0.999. These findings infer a novel approach toward optimisation of the complex, inexpensive and self-sufficient design for electricity generation from energy-rich urine wastewater in rural areas.

Improving oxygen reduction reaction by cobalt iron-layered double hydroxide layer on nickel-metal organic framework as cathode catalyst in microbial fuel cell

Cobalt Iron -layered double hydroxide (CoFe-LDH) nano sheets were attached to Nickel-metal organic frameworks (Ni-MOF) by utilizing hydrothermal reaction method, and CoFe-LDH@Ni-MOF was synthesized and worked as the cathode catalyst in microbial fuel cell. The surface of this composite material provided generous electrochemical active sites, consisting of wrinkled strips of CoFe-LDH adhering to a lamellar structure of Ni-MOF. In terms of the maximum output power density, CoFe-LDH@Ni-MOF as the catalyst was 211 mW/m2, 2.54 times higher than that of Ni-MOF (83 mW/m2), and it was stable at about 225 mV for 150 h. CoFe-LDH@Ni-MOF showed high oxygen reduction reaction capability and high specific surface area, and the electron transfer rate was accelerated. This work might set the stage for the development and utilization of fuel cell cathode catalysts.

Influence of carbon-based cathodes on biofilm composition and electrochemical performance in soil microbial fuel cells

Increasing energy demands and environmental pollution concerns press for sustainable and environmentally friendly technologies. Soil microbial fuel cell (SMFC) technology has great potential for carbon-neutral bioenergy generation and self-powered electrochemical bioremediation. In this study, an in-depth assessment on the effect of several carbon-based cathode materials on the electrochemical performance of SMFCs is provided for the first time. An innovative carbon nanofibers electrode doped with Fe (CNFFe) is used as cathode material in membrane-less SMFCs, and the performance of the resulting device is compared with SMFCs implementing either Pt-doped carbon cloth (PtC), carbon cloth, or graphite felt (GF) as the cathode. Electrochemical analyses are integrated with microbial analyses to assess the impact on both electrogenesis and microbial composition of the anodic and cathodic biofilm. The results show that CNFFe and PtC generate very stable performances, with a peak power density (with respect to the cathode geometric area) of 25.5 and 30.4 mW m^-2, respectively. The best electrochemical performance was obtained with GF, with a peak power density of 87.3 mW m^-2. Taxonomic profiling of the microbial communities revealed differences between anodic and cathodic communities. The anodes were predominantly enriched with Geobacter and Pseudomonas species, while cathodic communities were dominated by hydrogen-producing and hydrogenotrophic bacteria, indicating H₂ cycling as a possible electron transfer mechanism. The presence of nitrate-reducing bacteria, combined with the results of cyclic voltammograms, suggests microbial nitrate reduction occurred on GF cathodes. The results of this study can contribute to the development of effective SMFC design strategies for field implementation.

Enhancing extracellular electron transfer through selective enrichment of Geobacter with Fe@CN-modified carbon-based anode in microbial fuel cells

Microbial fuel cells (MFCs) have been demonstrated as a renewable energy strategy to efficiently recover chemical energy stored in wastewater into clean electricity, yet the limited power density limits their practical application. Here, Fe-doped carbon and nitrogen (Fe@CN) nanoparticles were synthesized by a direct pyrolysis process, which was further decorated to fabricate Fe@CN carbon paper anode. The modified Fe@CN anode with a higher electrochemically active surface area was not only benefit for the adhesion of electrochemically active microorganisms (EAMs) and extracellular electron transfer (EET) between the anode and EAMs but also selectively enriched Geobacter, a typical EAMs species. Accordingly, the MFCs with Fe@CN anode successfully achieved a highest voltage output of 792.76 mV and a prolonged stable voltage output of 300 h based on the mixed culture feeding with acetate. Most importantly, the electroactive biofilms on Fe@CN anode achieved more content ratio of proteins to polysaccharides (1.40) in extracellular polymeric substances for the balance between EET and cell protection under a harsh environment. This work demonstrated the feasibility of development on anode catalysts for the elaboration of the catalytic principle about interface modification, which may contribute to the practical application of MFC in energy generation and wastewater treatment.

In situ degradation of organic pollutants by novel solar cell equipped soil microbial fuel cell

The soil microbial fuel cell (SMFC) has been widely used for soil remediation for its low cost and being eco-friendly. But low degradation efficiency and high mass transfer resistance limit its performance. This study constructed a solar cell-soil microbial fuel cell (SC-SMFC) with different voltages, which use clean energy to improve system performance. At different voltages, 2.0-V system showed the best performance and the maximum output power increased by 330% compared with SMFC. Moreover, 2.0-V SC-SMFC showed the fastest phenol degradation rate of 14 μg·mL^-1·d^-1 at the concentration of 80 μg/mL, which was twice of SMFC. Further increasing the concentration to 320 μg/mL, the system showed extremely high concentration limit and degraded 90% within 19 days. Under this condition, SC-SMFC still showed excellent cycle stability, with the third-round degrading 90% phenol in 13 days. Finally, electrochemical impedance spectroscopy (EIS) mechanism study showed that solar cells can accelerate microbial metabolic process and reduce the internal resistance, in which the 2.0-V system was only 87% of SMFC. In conclusion, SC-SMFC provides a green, low-cost, and convenient method for in situ soil remediation in the future.

Rice husk-derived silicon nanostructured anode to enhance power generation in microbial fuel cell treating distillery wastewater

The present study aims to utilize rice husk as a source of silica to prepare rice husk derived silicon nanoparticles (RH-Si) and demonstrate its ability as an anode modifier in a two-chambered H-shaped microbial fuel cell (MFC). The silicon nanoparticles synthesized by magnesiothermal reduction process were spherical in shape and ranged in size from 15 to 60 nm. The anode modified with silicon nanoparticles of 0.50 mg cm^-2 recorded the maximum power and current density of 190.5 mW m^-2 and 1.5 A m^-2 corresponding to 7.6-fold and 3-fold increase as compared to the control . The modified anode also recorded a COD removal and coulombic efficiency of 74% and 49%, respectively in MFC operated with combined distillery and domestic wastewater at a HRT and OLR of 72 h and 59.2 gCOD L^-1 d^-1, respectively. The results evidence that RH derived silicon NPs are good anode modifiers and effective in enhancing bioelectricity generation and COD removal in MFCs.

Enhanced power generation, organics removal and water desalination in a microbial desalination cell (MDC) with flow electrodes

This study introduced a flow electrode microbial desalination cell (FE-MDC), which used activated carbon (AC) particles and carbon nanotubes (CNTs) as the electrode to promote electron harvesting. The recovered electricity energy (0.371 KWh/m³) and columbic efficiency (66.7 %) of the FE-MDC were over 2 times higher than those of the conventional MDC without the flow electrode. Consequently, the salt and COD removal efficiencies were enhanced to 77.8 % and 91.2 %, respectively. Electrochemical analysis implied that the charge transfer resistance of the system was reduced by the flow electrode. Electron accumulation and charging-discharging experiments proved that the flow electrode could accumulate electrons and transfer the electrons to the fixed anode. Bacterial community analysis indicated that the bacterial activity was improved by the flow electrode. The content of the exoelectrogen Pseudomonas increased from 5.0 % to 14.7 %, and Hydrogenophaga improved from 1.4 % to 5.9 %. Finally, a continuous operation mode of the FE-MDC was established, and the flow electrode slurry was returned to the anodic chamber for recirculated utilization. The voltage output, COD removal, and salt removal during the operation mode reached 610 mV, 78.8 %, and 76.1 %, respectively. This study proved that the flow electrode is a promising way to promote the practical application of MDC technology.

Applications of Nanomaterials in Microbial Fuel Cells: A Review

Microbial fuel cells (MFCs) are an environmentally friendly technology and a source of renewable energy. It is used to generate electrical energy from organic waste using bacteria, which is an effective technology in wastewater treatment. The anode and the cathode electrodes and proton exchange membranes (PEM) are important components affecting the performance and operation of MFC. Conventional materials used in the manufacture of electrodes and membranes are insufficient to improve the efficiency of MFC. The use of nanomaterials in the manufacture of the anode had a prominent effect in improving the performance in terms of increasing the surface area, increasing the transfer of electrons from the anode to the cathode, biocompatibility, and biofilm formation and improving the oxidation reactions of organic waste using bacteria. The use of nanomaterials in the manufacture of the cathode also showed the improvement of cathode reactions or oxygen reduction reactions (ORR). The PEM has a prominent role in separating the anode and the cathode in the MFC, transferring protons from the anode chamber to the cathode chamber while preventing the transfer of oxygen. Nanomaterials have been used in the manufacture of membrane components, which led to improving the chemical and physical properties of the membranes and increasing the transfer rates of protons, thus improving the performance and efficiency of MFC in generating electrical energy and improving wastewater treatment.

Polyaniline-Derived Nitrogen-Containing Carbon Nanostructures with Different Morphologies as Anode Modifier in Microbial Fuel Cells

Anode modification with carbon nanomaterials is an important strategy for the improvement of microbial fuel cell (MFC) performance. The presence of nitrogen in the carbon network, introduced as active nitrogen functional groups, is considered beneficial for anode modification. In this aim, nitrogen-containing carbon nanostructures (NCNs) with different morphologies were obtained via carbonization of polyaniline and were further investigated as anode modifiers in MFCs. The present study investigates the influence of NCN morphology on the changes in the anodic microbial community and MFC performance. Results show that the nanofibrillar morphology of NCNs is beneficial for the improvement of MFC performance, with a maximum power density of 40.4 mW/m2, 1.25 times higher than the anode modified with carbonized polyaniline with granular morphology and 2.15 times higher than MFC using the carbon cloth-anode. The nanofibrillar morphology, due to the well-defined individual nanofibers separated by microgaps and micropores and a better organization of the carbon network, leads to a larger specific surface area and higher conductivity, which can allow more efficient substrate transport and better bacterial colonization with greater relative abundances of Geobacter and Thermoanaerobacter, justifying the improvement of MFC performance.

Recovering phosphate and energy from anaerobic sludge digested wastewater with iron-air fuel cells: Two-chamber cell versus one-chamber cell

Anaerobic sludge digested (ASD) wastewater is widespread in wastewater treatment plants. Recovering phosphate from ASD wastewater not only removes pollutants but also solves the phosphorus deficiency problem. Iron-air fuel cells were chosen to recover phosphate and generate electricity from ASD wastewater. To optimize cell configuration, a two-chamber and a one-chamber iron-air fuel cell were set up. The phosphate removal efficiency, the vivianite yield and the electricity generation efficiency of the two fuel cells were evaluated. It turned out that the volumetric removal rate (VRR) of phosphate of the two-chamber cell was 11.60 mg P·L^-1·h^-1, which was about five times of that in the one-chamber cell. The phosphate recovery product vivianite was detected on the surface of the iron anodes and the calculated purities of the two-chamber fuel cell and one-chamber fuel cell were 90.6% and 58.7%, respectively. Considering the content and purity, the iron anode surface in the two-chamber fuel cell was the best point to recover phosphate. The proton exchange membrane (PEM) in the two-chamber fuel cell provided low pH conditions suitable for vivianite formation. Moreover, under the low pH condition, metal ions of Fe²⁺, Ca²⁺, Al³⁺ and so on were kept soluble, leading to a high conductivity. The high conductivity caused low internal resistance, which benefited the electricity generation. The total output electric power of the two-chamber fuel cell was 2.4 times that of the one-chamber fuel cell when treating 25 mL ASD wastewater (0.62 vs. 0.26 mW·h). Overall, the two-chamber fuel cell was the better choice for phosphate recovery and electricity generation from ASD wastewater. Further studies on the long-term operation of two-chamber fuel cells should be carried out.

Recycling urine for bioelectrochemical hydrogen production using a MoS2 nano carbon coated electrode in a microbial electrolysis cell

In this study, a novel molybdenum disulfide (MoS₂) nano-carbon (NC) coated cathode was developed for hydrogen production in a microbial electrolysis cell (MEC), while treating simulated urine with 2-6 times dilution (conductivity <20 mS cm^-1). MoS₂ nanoparticles were electrodeposited on the NC coated cathodes at -100, -150 and -200 μA cm^-2 and their performances were evaluated in the MEC. The chronopotentiometry (CP) tests showed the improved catalytic activity of MoS₂-NC cathodes with much lower cathode overpotential than non-MoS₂ coated electrodes. The MoS₂-NC200 cathode, electrodeposited at -200 μA cm^-2, showed the maximum hydrogen production rate of 0.152 ± 0.002 m³ H₂ m^-2 d^-1 at 0.9V of E_ap, which is comparable to the previously reported Pt electrodes. It was found that high solution conductivity over 20 mS cm^-1 (>600 mg L^-1 NH₃-N) can adversely affect the biofilm architecture and the bacterial activity at the anode of the MEC. Exoelectrogenic bacteria for this system at the anode were identified as Tissierella (Clostridia) and Bacteroidetes taxa. Maximum ammonia-nitrogen (NH₃-N) and phosphorus (PO₄ ^3--P) removal were 68.7 and 98.6%, respectively. This study showed that the newly fabricated MoS₂-NC cathode can be a cost-effective alternative to the Pt cathode for renewable bioelectrochemical hydrogen production from urine.

Gaseous isopropanol removal in a microbial fuel cell with deoxidizing anode: Performance, anode characteristics and microbial community

A deoxidizing packing material (DPM) with an encapsulated deoxidizing agent (DA) was developed to construct the packed anodes of a trickle-bed microbial fuel cell (TB-MFC) for treating waste gas. The encapsulated DA can consume O₂ in waste gas and increase the voltage output and power density (PD) of the constructed TB-MFC. The DPM effectively enables the circulating water in TB-MFC for maintaining a low level of dissolved oxygen for 80 h. The results revealed that when the concentration of isopropanol (IPA) in waste gas was 0.74 g/m³, the TB-MFC (DPM with DA) exhibited an IPA removal efficiency (RE) of up to 99.7%. When DPM with DA was used as the packing material of the TB-MFC (486.6 mW/m³), the PD was 2.54 times that obtained when using coke as the packing material (191.6 mW/m³). The next-generation sequencing results demonstrated that because the oxygen content of the MFC anode chamber decreased over time in the TB-MFC, the richness of anaerobic electrogens (Pseudoxanthomonas, Flavobacterium, and Ferruginibacter) in the packing materials was increased. These electrogens mainly attached to the DPM, and IPA-degraders appeared in the circulating water of the TB-MFC. This enabled the TB-MFC to simultaneously achieve a high voltage output and IPA RE.

Enhancement of isopropanol removal with concomitant power generation by microbial fuel cells: Optimization of deoxidizing composite anodes using response surface methodology

This study used a response surface method to develop a deoxidizing anode, which was introduced into microbial fuel cells (MFCs) to treat isopropanol (IPA) wastewater and waste gas. By embedding a deoxidizing agent (DA) into the anode of MFCs, a hypoxic environment can be created to enable anaerobic electrogens to be effectively attached to the anode surface and grow. Consequently, MFC power generation performance can be enhanced. The optimal coke and conductive carbon black ratio of an anode and percentage of DA added were 3.61 g/g and 3.15 %, respectively. The research design concurrently achieved the maximum deoxygenation efficiency (0.86 mg O₂/bead), minimum disintegration ratio (3.51 %), and minimum resistance (30.2 Ω). The regression model had high prediction power (R² > 0.93) for anode performance. As determined through multi-objective optimization, the results highly satisfied the target expectation (desirability = 0.82). The optimized deoxidizing anode was filled into an air-cathode MFC, which had a higher IPA removal efficiency (1.15-fold) and voltage output (1.24-fold) than an MFC filled with coke. The results for the trickling-bed MFC filled with a deoxidizing anode revealed that when the inlet concentration was 0.74 g/m³, the voltage output and power density were highest at 416.3 mV and 486.6 mW/m³, respectively. The deoxidizing anode developed has the potential to increase the MFC voltage output and the pollutant removal.

An overview in the development of cathode materials for the improvement in power generation of microbial fuel cells

Since the high cost and low power generation hinder the overall practical application of microbial fuel cells (MFCs), numerous attempts have been made in the field of cathode materials to enhance the electrical performance of MFCs because they directly catalyze the oxygen reduction reactions (ORR). To choose a proper cathode material, following principles such as ORR activity, conductivity, cost-efficiency, durability, surface area, and accessibility should be taken into consideration. In preparation of cathode materials, versatile materials have been chosen, synthesized, or modified to achieve an improvement in power generation of MFCs. The most widely applied cathode materials could be categorized into three classes, namely carbon-base materials, metal-based materials, and biocatalysts. This review summarizes the utilization, development, and the cost of cathode materials applied in MFCs and tries to highlight the effective modification methods of cathode materials which have helped in achieving enhanced power generation of MFCs in recent years.

Long-term electricity generation and denitrification performance of MFCs with different exchange membranes and electrode materials

Different biocathode electrode materials (graphite felt and carbon brush, GF and CB) and exchange membranes (proton exchange membrane and cation exchange membrane, PEM and CEM) were used in three microbial fuel cell (MFC) configurations operated for 300-days to investigate the power generation and the COD and N removal performance. Results showed no effect on the COD removal (all above 96%); however, the power generation (46.11 mW·h) and denitrification performance (68.0 ± 1.6%) of the MFC-B (GF + PEM) system were higher than those of the other systems (MFC-A: CB + PEM; MFC-C: CB + CEM) (P < 0.01), and the power generation and denitrification performance of all three systems decreased with time (P < 0.01). By analyzing the physicochemical properties of the exchange membrane and cathode electrode materials, the reasons that affect the power generation performance of the system were clarified. Furthermore, the increase in bioelectricity enhanced the electricity-related nitrification and denitrification reactions. The average 300-day unit denitrification cost of MFC-A was 4.2 and 6.3 times that of MFC-B and MFC-C, respectively. Comprehensive consideration of electricity generation, denitrification, and service life, combined with cost analysis and better selection of construction materials, provides a theoretical basis for the long-term stable operation and sustainable application of MFCs.

Recent progress of graphene based nanomaterials in bioelectrochemical systems

The application of graphene (Gr) to microbial fuel cells (MFCs) and microbial electrolysis cell (MECs) is considered a very promising approach in terms of enhancing their performance. The superior Gr properties of high electrical and thermal conductivities, along with: superior specific surface area, high electron mobility, and mechanical strength, are the key features that endorse this. Factors impeding the advancement of a microbial fuel cell into commercialization involve primarily the cost of their components, and their production on a small scale. Gr with such outstanding characteristics can help mitigate these challenges, when used as electrode material. The application of Gr as an anode material improves the efficiency of electron transfer and bacterial attachment. When used as a cathode material, it supports the oxygen reduction reaction. This investigation, presents a thorough analysis of the feasibility of Gr as an electrode material in both MFC and MEC applications - based on experimental results from the investigation. Current technological advancements in the implementation of Gr in MFC and MEC are also highlighted in this review. To summarise, the investigation exposes critical issues impeding the advancement of microbial fuel cells, and proposes possible solutions to mitigate these challenges.

CH4 control and associated microbial process from constructed wetland (CW) by microbial fuel cells (MFC)

Global warming is becoming more severe. We here proposed an innovative green technique aimed at reducing the CH₄ emissions from constructed wetlands (CWs) in which CH₄ is controlled by microbial fuel cells (MFCs). The results of our work indicated that CH₄ emissions from CWs could be controlled by operating MFC. The CH₄ fluxes significantly decreased in the MFC-CW (close circuit CC) compared with the control MFC-CW (open circuit OC). The bioelectricity generation and COD removal rates also differed in the two systems. The highest power density (0.27 W m^-3) and the lowest CH₄ emissions (4.7 mg m^-2 h^-1) were observed in the CC system. The plants' effects on the performance of the MFC-CWs were also investigated. The plant species had a profound impact on the CH₄ emissions and electricity production in MFC-CWs. The greatest CH₄ flux (9.5 mg m^-2 h^-1) was observed from the MFC-CW planted with Typha orientalis, while the CH₄ emissions from the MFC-CW planted with Cyperus alternifolius were reduced by 45%. Additional microbial processes were investigated. Quantitative real-time PCR (q-PCR) analysis indicated that the gene abundance of eubacterial 16 S rRNA, particulate methane monooxygenase (pmoA), and methyl coenzyme M reductase (mcrA) significantly differed for the control CW and MFC-CWs planted with different plants. In the CC systems, the mcrA genes in the anode were low, while the pmoA genes in the cathode were high. The operation of MFCs in CWs changed the exoelectrogenic and methanogenic community structures. Sequencing analysis indicated that phylotypes related to Geobacter, Bacteroides, and Desulfovibrio were specifically enriched in the CC systems. The results demonstrated that the operation of MFCs in the CWs resulted in the competition between the electrogenes and methanogenes, which resulted in distinctive microbial populations and biochemical processes that suppressed the CH₄ emissions from the CWs.

Reduced graphene oxide and biofilms as cathode catalysts to enhance energy and metal recovery in microbial fuel cell

In this study, reduced graphene oxide (rGO) was developed and employed as cathode catalyst in a membrane-less microbial fuel cell (MFC) to improve energy and metal (copper) recovery in combined with biofilms. Results showed that rGO-based cathode exhibited better characterizations in structure and electron transfer than graphene oxide (GO)-based cathode. The voltage with rGO was about 67% increased, and Cu²⁺ removal efficiency was 43% improved as compared to GO. Cu species on cathode demonstrated the favorable Cu²⁺ reduction to Cu with the catalysis of rGO. Moreover, microbial community analysis indicated that rGO-based cathode exhibited better biocompatibility for functional bacteria that related to electron transfer and Cu²⁺ resistance, such as Geobacter and Pseudomonas, demonstrating the interspecific synergism of microorganisms for efficient energy and copper recovery. It will be of important significance for the heavy metal and energy recovery from low concentrations wastewater by using microbial fuel cell.