Air-cathode Microbial Fuel Cells Research Articles

Enhancing the Proton Exchange Membrane in Tubular Air-Cathode Microbial Fuel Cells through a Hydrophobic Polymer Coating on a Hydrogel.

The usage time of air-cathode microbial fuel cells (MFCs) is significantly influenced by the moisture content within the proton exchange membrane (PEM). Therefore, enhancing the water retention capability of the PEM by applying a hydrophobic polymer coating to its surface has extended the PEM's usage time by three times and increased MFCs' operational duration by 66%. Moreover, the hydrophobic nature of the polymer coating reduces contamination on the PEM and prevents anode liquid from permeating into the air cathode. Towards the end of MFC operation, the internal resistance of the MFC is reduced by 45%. The polymer coating effectively maintained the oxygen reduction reaction activity in the cathode. The polymer coating's ability to restrict oxygen transmembrane diffusion is demonstrated by experimental data showing a significant decrease in oxygen diffusion coefficient due to its presence. The degradation efficiency of the chemical oxygen demand from 16% to 35% increased by a factor of one.

Assessing the Performance of Continuous-Flow Microbial Fuel Cells and Membrane Electrode Assembly with Electrodeposited Mn Oxide Catalyst

Microbial fuel cells (MFCs) are considered promising energy sources whereby chemical energy is converted into electricity via bioelectrochemical reactions utilizing microorganisms. Several factors affect MFC performance, including cathodic reduction of oxygen, electrode materials, cell internal and external resistances, and cell design. This work describes the effect of the catalyst coating in the air-cathode membrane electrode assembly (MEA) for a microbial fuel cell (MFC) prepared via electrodeposition of manganese oxide. The characterization of the synthesized air-cathode MFC, operating in a continuous mode, was made via electrochemical impedance spectroscopy (EIS) analyses for the determination of the intrinsic properties of the electrode that are crucial for scalability purposes. EIS analysis of the MFCs and of the MEA reveals that the anode and cathode contribute to polarization resistance by about 85% and 15%, respectively, confirming the high catalytic activity of the Mn-based air cathode. The maximum power density of the Mn-based cathode is about 20% higher than that recorded using a Pt/C electrode.

Carbon-coated Fe3O4@cotton fiber biomass carbon for the oxygen reduction reaction in air cathode microbial fuel cells

Iron-based oxide catalysts on biomass carbon fibers can jointly use their respective advantages of chemical composition and structure to develop catalysts for the oxygen reduction reaction (ORR). In this study, carbon-coated Fe3O4@cotton fiber biomass carbon (C@Fe3O4@CFBC) was prepared as an ORR catalyst through hydrothermal growth and calcination treatment for air cathode microbial fuel cells (MFCs). The carbon coating layer enhanced the O2 adsorption energy, decreased the overpotential, and promoted surface activation of the Fe3O4 catalyst. The carbon coating layer can modulate the surface electronic structure of the Fe3O4 catalyst with C–O–Fe bonding and improve the electrical conductivity to accumulate electrons on the Fe3O4 surface to enhance the ORR performance. The CFBC supported Fe3O4 nanoparticles to prevent severe aggregation and expose more effective surface-active sites. The C@Fe3O4@CFBC catalyst achieved a high positive half-wave potential (0.801 V) and low Tafel slope (73.7 mV·dec−1) compared with the Pt/C catalyst (0.877 V and 94.3 mV·dec−1). The assembled air cathode MFCs achieved a desirable maximum power density (1059 ± 16 mW·m−2) based on 1266 ± 94 mW·m−2 of Pt/C MFC devices. This work provides a strategy for designing highly active non-precious metal catalysts for air cathode MFCs.

Application of biomimetically synthesized silver nanoparticles as cathode catalyst, quorum-quencher, and anti-biofouling agent for the performance boosting of microbial fuel cell

A microbial fuel cell (MFC) is a cutting-edge bioelectrochemical technology, which demonstrates power and other valuables recovery while treating wastewater by cultivating electroactive microbes. However, rampant biofilm growth over the cathode surface of air cathode MFC exacerbates the oxidation-reduction reaction rate, triggering a dip in the overall performance of MFC. In this sense, biosynthesized silver nanoparticles (AgNPs) have garnered a plethora of potential applications as cathode catalysts as well as anti-biofouling agent for MFCs without harming nature. The MFC equipped with the mixture of aloe vera and algae (@5 mg/cm2) synthesized AgNPs on cathode generated a maximum power density of 66.5 mW/m2 and chemical oxygen demand removal efficiency of 85.2%, which was ca. 5.6 times and 1.2 times higher compared to control MFC operated without any catalyst on cathode. Thus, this investigation paves the way for using eco-amiable, low-cost bioderived organic compounds to assist MFC in achieving high power output and other valuables with minimal reliance on chemicals.

Graphitized mango seed as an effective 3D anode in batch and continuous mode microbial fuel cells for sustainable wastewater treatment and power generation.

Herein, we explored the utilization of graphitized mango seeds as 3D-packed anodes in microbial fuel cells (MFCs) powered by sewage wastewater. Mango seeds were graphitized at different temperatures (800 °C, 900 °C, 1000 °C, and 1100 °C) and their effectiveness as anodes was evaluated. Surface morphology analysis indicated that the proposed anode was characterized by layered branches and micro-sized deep holes, facilitating enhanced biofilm formation and microorganism attachment. Maximum power densities achieved in the MFCs utilizing the mango seed-packed anodes graphitized at 1100 °C and 1000 °C were 2170.8 ± 90 and 1350.6 ± 125 mW m-2, respectively. Furthermore, the weight of the graphitized seed anode demonstrated a positive correlation with the generated power density and cell potential. Specifically, MFCs fabricated with 9 g and 6 g anodes achieved maximum power densities of 2170.8 ± 90 and 1800.5 ± 40 mW m-2, respectively. A continuous mode air cathode MFC employing the proposed graphitized mango anode prepared at 1100 °C and operated at a flow rate of 2 L h-1 generated a stable current density of approximately 12 A m-2 after 15 hours of operation, maintaining its stability for 75 hours. Furthermore, a chemical oxygen demand (COD) removal efficiency of 85% was achieved in an assembled continuous mode MFC. Considering that the proposed MFC was driven by sewage wastewater without the addition of external microorganisms, atmospheric oxygen was used as the electron acceptor through an air cathode mode, agricultural biomass waste was employed for the preparation of the anode, and a higher power density was achieved (2170.8 mW m-2) compared to reported values; it is evident that the proposed graphitized mango seed anode exhibits high efficiency for application in MFCs.

Multi-anode enhanced the bioelectricity generation in air-cathode microbial fuel cells towards energy self-sustaining wastewater treatment

Microbial fuel cells (MFCs) hold considerable promise for harnessing the substantial energy resources present in wastewater. However, their practical application in wastewater treatment is limited by inadequate removal of organic matter and inefficient power recovery. Previous studies have investigated aeration as a method to enhance the removal of organic matter, but this method is energy-intensive. To address this issue, this study proposed using MFC-recovered bioelectricity for aeration, thereby mitigating the associated expenses. An air-cathode MFC with multi-anode was constructed and optimized to maximize electricity supply for aeration. Carbon-felt anodes were chosen as the most effective anode configuration, due to the high abundance of electroactive bacteria and genes observed in the biofilm generated on their surface. By incorporating six carbon felt anodes, the MFC achieved a 1.7 and 1.1 fold enhancement in the maximum power and current density, respectively. The optimized MFC unit achieved a stable current density of 0.32 A/m2 and achieved COD removal of 60% in the long-term operation of 140 days in a 50 L reactor. In a reactor scaled up to 1600 L, 72 MFCs successfully powered a mini air pump work for 10 s after an 81-s charging period. The intermittent aeration resulted in partial increases in DO concentrations to 0.03–3.5 mg/L, which is expected to promote the removal of nitrogen compounds by the nitrification-anammox process. These groundbreaking results lay the foundation for self-sustaining wastewater treatment technologies.

Polyacrylic co-maleic acid as an anti-scaling binder for air–cathode microbial fuel cell: An oxygen reduction reaction perspective

Polydimethylsiloxane (PDMS) is a binder commonly used in air–cathode MFC fabrication. Herein, the performance of poly acrylic co-maleic acid (PMA) as a binder and anti-scaling agent for cathode was evaluated for application in an air–cathode MFC. Two different cathode catalysts were tested for applicability in MFC, viz., (a) Vulcan-XC: with PMA (MFC-1), with PDMS (MFC-3), and (b) Nitrogen-doped carbon catalyst (NDCP) with PDMS (MFC-2) and with PMA (MFC-4). The PMA, as an alternative to costly PDMS, prevented scale formation on the cathode surface and helped to sustain the performance of MFC-3 and MFC-4 for over 30 days. As evidenced by the cyclic voltammetry and electrochemical impedance spectroscopy, MFC-4 (NDCP and PMA) showed the best cathodic current density owing to the core–shell structure formed by nitrogen and carbon atoms in NDCP. In the future, using PMA can prevent scale formation over the (air) cathode surface and deliver a stable performance of MFC without any need for costly binder agents, such as PDMS.

Impact of liquid velocity and stacking modes on the performance of anaerobic fluidized bed microbial fuel cell

A stacked anaerobic fluidized bed microbial fuel cell (AFB-MFC) consisting of 10 individual air-cathode MFCs was constructed to measure its power generation performance at different liquid velocities and stacking modes. Open-circuit voltages produced in parallel and series connection modes were observed to reach the peak value of 617.6 mV and 5.3 V, respectively, at the liquid velocity of 7.5 mm/s. In series mode, the stacked AFB-MFC had the highest power density of 3440.1 mW/m2. The chemical oxygen demand removal rate of the stacked AFB-MFC under series connection reached up to 96.87%, slightly higher than that under the parallel condition. The results illustrated that a proper liquid velocity can effectively improve the mass transfer efficiency of the stacked AFB-MFC, while mitigating the internal resistance. By putting the stacked MFCs in series connection mode, voltage losses are evident, indicating that high internal resistances lead to the generation of a parasitic cross current.

Reduced graphene oxide-supported palladium oxide-MOx for improving the performance of air-cathode microbial fuel cells: Influence of the Sn, Ce, Zn, and Fe precursors

Over the past few decades, microbial fuel cells (MFCs) have attracted significant interest within the realm of renewable energy technologies owing to their capability to generate electricity from low-grade substrates. However, their commercialization and scaling-up are impeded by significant obstacles, including high capital costs, limited durability, and sluggish oxygen reduction reaction (ORR). Here, we demonstrate an environmentally-friendly approach for developing highly efficient electrocatalysts consisting of palladium oxide (PdO) and different transition metal oxides (i.e., tin (IV) oxide (SnO2), ceric oxide (CeO2), iron (III) oxide (Fe2O3), and zinc oxide (ZnO)) embedded within reduced graphene oxide (rGO) framework for improving the efficiency of ORR in neutral medium and MFCs. Among the as-prepared electrocatalysts, the PdO–SnO2/rGO possesses the highest ORR electrochemical catalytic activity in a neutral medium (pH ∼ 7.2) following a four-electron ORR pathway, with a current density slightly lower than that of benchmark Pt/C electrocatalyst. The assessment of MFC performance is conducted by employing activated sludge as the inoculum and glucose as the sole electron donor in the anode chamber of single-chamber, air-cathode MFCs. Consistent with electrochemical performance, the maximum power output of an MFC equipped with PdO–SnO2/rGO as an air cathode is significantly higher than other tested MFCs, reaching 84.6 ± 2.8 mW m−2, which represents ∼ 91 % of the power output of an MFC with Pt/C air cathode. Our results reveal that the incorporation of PdO into transition metal reduced graphene oxide framework has the potential to facilitate their long-term application as air cathodes in MFCs that possess easy fabrication, relatively affordable cost, and outstanding electrochemical catalytic oxygen reduction capability, for enhancing electricity generation from wastewater.

Enhanced performance of microbial fuel cells using electrochemically treated carbon felt anode

The microbial fuel cell (MFC) technology is emerging as an effective technology for wastewater treatment to remove and detect many pollutants and simultaneously generate power. The anode is an essential component for bacterial attachment and extracellular electron transfer (EET). Thus, anode performance is critical for improving an MFC system's overall performance. Materials that are electronically conductive and provide favorable conditions for creating biofilm are good candidates for the anode. Carbon paper and carbon felt are the most used anode materials. However, the characteristics of these materials can be modified to increase their performance. This paper presents a modification of the conventional carbon anode by electrolyzing the carbon felt in a phosphate buffer and an experimental evaluation of the impact of the treatment. The tests were performed using an air cathode MFC, electrogenic bacteria Shewanella Baltica 20 fed on lactose-rich L-B. The treatment with phosphate buffer generates functional groups on the surface of the carbon felt, which establishes robust biofilm on the anode and offers lower charge-transfer resistance compared to the untreated carbon felt. This facilitates electron transfer from exoelectrogens to the anode, increasing the current density and power density output. Three times higher power density was observed in the cell with a modified anode than in an untreated carbon-felt anode.

A Nanofiber-Based Gas Diffusion Layer for Improved Performance in Air Cathode Microbial Fuel Cells.

This work investigates a new nanostructured gas diffusion layer (nano-GDL) to improve the performance of air cathode single-chamber microbial fuel cells (a-SCMFCs). The new nano-GDLs improve the direct oxygen reduction reaction by exploiting the best qualities of nanofibers from electrospinning in terms of high surface-area-to-volume ratio, high porosity, and laser-based processing to promote adhesion. By electrospinning, nano-GDLs were fabricated directly by collecting two nanofiber mats on the same carbon-based electrode, acting as the substrate. Each layer was designed with a specific function: water-resistant, oxygen-permeable polyvinylidene-difluoride (PVDF) nanofibers served as a barrier to prevent water-based electrolyte leakage, while an inner layer of cellulose nanofibers was added to promote oxygen diffusion towards the catalytic sites. The maximum current density obtained for a-SCMFCs with the new nano-GDLs is 132.2 ± 10.8 mA m-2, and it doubles the current density obtained with standard PTFE-based GDL (58.5 ± 2.4 mA m-2) used as reference material. The energy recovery (EF) factor, i.e., the ratio of the power output to the inner volume of the device, was then used to evaluate the overall performance of a-SCMFCs. a-SCMFCs with nano-GDL provided an EF value of 60.83 mJ m-3, one order of magnitude higher than the value of 3.92 mJ m-3 obtained with standard GDL.

Vermiculite nanosheet augmented novel proton exchange membrane for microbial fuel cell

Microbial fuel cell (MFC) is promoted as an eco-friendly technology for wastewater treatment. However, the use of synthetic polymeric membrane separators raises concerns about the sustainability of MFC, mandating efforts to develop sustainable polymeric membrane with high protonic transfer for MFC assembly. Low-cost and eco-friendly polymer matrix of agar-polyvinyl alcohol (AP) augmented with exfoliated vermiculite nanosheets (Vm NSs) are used to prepare membrane separators for MFC. A simple and facile method for synthesis of AP membrane without and with Vm NSs concentration of 0.25 mg/mL (0.25 Vm), 0.5 mg/mL (0.5 Vm) and 1 mg/mL (1.0 Vm) is adopted in aqueous phase. The proton mass transfer coefficients of prepared membranes (AP, 0.25 Vm, 0.5 Vm and 1.0 Vm) are respectively 94.4, 134.46, 216.26 and 197.69 % higher than commercially available Nafion membrane. These membranes are tested in three different configurations of MFC: aerated cathode, air cathode and anaerobic dye cathode MFC. Highest current densities of 1515 and 949 mA/m3 are obtained with 1.0 Vm membrane in aerated and air cathode-MFC respectively while Nafion membrane could produce 623 and 405 mA/m3. In anaerobic dye cathode MFC, 0.5 Vm membrane is able to achieve the highest current density of 935 mA/m3 along with 71 % methyl red decolorization while Nafion membrane produced highest current density of 104 mA/m3 with 59 % decolorization. Vm NSs augmentation enhances the properties of base AP membrane which can be conveniently used as an alternative to expensive Nafion membrane in the desired configuration of MFC.

Outline of microbial fuel cells technology and their significant developments, challenges, and prospects of oxygen reduction electrocatalysts

The microbial fuel cells (MFCs) which demonstrates simultaneous production of electricity and wastewater treatment have been considered as one of the potential and greener energy production technology among the available bioelectrochemical systems. The air-cathode MFCs have gained additional benefits due to using air and avoiding any chemical substances as catholyte in the cathode chamber. The sluggish oxygen reduction reaction (ORR) kinetics at the cathode is one of the main obstacles to achieve high microbial fuel cell (MFC) performances. Platinum (Pt) is one of the most widely used efficient ORR electrocatalysts due to its high efficient and more stable in acidic media. Because of the high cost and easily poisoned nature of Pt, several attempts, such as a combination of Pt with other materials, and using non-precious metals and non-metals based electrocatalysts has been demonstrated. However, the efficient practical application of the MFC technology is not yet achieved mainly due to the slow ORR. Therefore, the review which draws attention to develop and choosing the suitable cathode materials should be urgent for the practical applications of the MFCs. In this review article, we present an overview of the present MFC technology, then some significant advancements of ORR electrocatalysts such as precious metals-based catalysts (very briefly), non-precious metals-based, non-metals and carbon-based, and biocatalysts with some significant remarks on the corresponding results for the MFC applications. Lastly, we also discussed the challenges and prospects of ORR electrocatalysts for the practical application of MFCs.

Architecture Optimization of a Single-Chamber Air-Cathode MFC by Increasing the Number of Cathode Electrodes

The aim of this study was the optimization of a single-chamber microbial fuel cell (MFC) architecture, by increasing the number of cathode electrodes. An air cathode single chamber MFC with a modifiable lid and bottom was operated with 4 and 6 Gore-Tex MnO2 cathode electrodes. The anode consisted of graphite granules. It was found that the increase from 4 (total cathodic surface area of 160 cm2) to 6 (total cathodic surface area of 240 cm2) electrodes resulted in an increase of the maximum current and the maximum power output of the cell by approximately 72% and 129%, respectively. Additionally, by increasing the cathodic surface area the internal resistance (Rint) of the unit decreased by approximately 19%. The organic removal from the substrate was not affected by the addition of the new electrodes while it was high across all cases studied (chemical oxygen demand (COD) removal > 89%). The average coulombic efficiency (CE) during the 4-electrode operation was 14.3%, while the corresponding efficiency for 6-electrode operation was 18.5%.

Scaling up 3D-printed urine microbial fuel cells with the membrane electrode assembly

Urine microbial fuel cells (MFCs) show potential for urine treatment to achieve simultaneous organic matter removal and electricity production. However, factors affecting the performance of urine MFCs during scaling up remain unclear. This study develops a 3D-printed air-cathode urine MFC with the membrane electrode assembly (MEA). After scale-up in parallel, unstable anode potential is responsible for the open circuit voltage (OCV) difference among various units, which leads to the energy loss in the system. For systems connected in series, unstable cathode potential mainly contributes to the voltage reversal, resulting in large power loss. Electrochemical impedance spectroscopy analysis indicates dramatically increased ohmic and polarization resistance in both serial and parallel systems. Bode plot further demonstrates after parallel connection, an obvious capacitance effect is observed, which provides evidence to illustrate the phenomenon of non-Faradaic current. Compared to serially connected systems, the capacitor of power management system can harvest more energy from systems connected in parallel. The microbial community analysis demonstrates Geobacter is the most dominant anodic electroactive bacteria both before and after scale-up. Overall, these results show the different strategic improvement should be executed while utilizing parallel connection (focusing on anode) or serial connection (focusing on cathode) to scale up urine MFCs.

Effects of chemical oxygen demand/nitrogen on electrochemical performances and denitrification efficiency in single-chamber microbial fuel cells: Insights from electron transfer and bacterial communities

The electrochemical performances and denitrification efficiency of microbial fuel cells (MFCs) are often limited by chemical oxygen demand/nitrogen (COD/N) of wastewater. To overcome this limitation, single-chamber air cathode MFCs with varying COD/N (16/1, 8/1, and 4/1) were established to investigate their electrochemical performances, denitrification efficiency, and bacterial communities. The optimal COD/N for maximizing electricity generation and denitrification efficiency was 8/1, as supported by the greatest corrected coulomb efficiency (13.6%) and electron transfer rate (2.36 C/h for electricity generation, 39.77 C/h for denitrification). As COD/N decreased, the electrochemically active genus Geobacter was replaced by the denitrifying genera Un._f_Burkholderiaceae, Dechlorosoma, and Petrimonas. These results indicated that the efficiency of electricity generation and denitrification was not solely determined by the abundance of electrochemically active and denitrifying bacteria. The presence of a faster electron transfer pathway, possibly direct interspecies electron transfer, enhanced simultaneous electricity generation and denitrification in MFCs with COD/N of 8/1.

Bioelectrogeneic performance of air-cathode microbial fuel cells with diesel contaminants

Bioremediation of diesel contaminants and then harnessing the bioenergy could be another cost‐effective and eco‐friendly solution to hydrocarbons soil contaminants. Four series of parallel circuit air–cathode MFCs (100 mL working volume) were operated under different diesel concentrations using acclimated bacterial consortium from diesel contaminated soil. Maximum current (Imax = 43.11 mA) under applied potential was recorded using fuel cell (DMFC-31DMFC-3 = 50mlL−1 diesel concentration in microbial fuel cells.1) fed with 50 mlL−1 diesel (substrate), whereas it was 10 mA in DMFC-122DMFC-1 = 0mlL−1 diesel concentration in microbial fuel cells. (0 mlL−1), Imax of 9.6 mA in DMFC-23DMFC-2 = 10mlL−1 diesel concentration in microbial fuel cells.3(10 mlL−1) while in DMFC-444DMFC-4 = 100mlL−1 diesel concentration in microbial fuel cells. (100 mlL−1) Imax of 18.47 mA was recorded. Cyclic voltammogram and CLSM clearly indicated the effect of different diesel concentrations on the electrogenic performance of anode respiring bacteria (ARB). Six less explored, culturable species of ARB enriched bacteria on anodic biofilms in DMFC-3 were procured and characterized on molecular basis (16S rRNA gene) as Paenibacillus sp. (MN173851), Bacillus toyonensis (MN173853), Ochrobactrum sp. (MN173857), Bacillus sp. (MN173854), Bacillus methylotrophicus (MN173856), and Bacillus licheniformis (MN173855). Bioelectrochemical activity of Bacillus toyonensis (MN173853) was typically monitored for the first time in MFC reactors. Among the tested organisms, Bacillus sp., was found to have greatest biodegradation capability with rate of 88% followed by Paenibacillus sp. 85%, Ochrobactrum sp. 79% by fast decolourization of redox indicator. The present study verified the presence and suitability of ARB under the influence of diesel in MFCs.

Spatial stratification of cathodic biofilm on nitrogen removal in the microbial fuel cell under different carbon/nitrogen ratios

The aim of this study was to investigate nitrogen removal in the microbial fuel cell (MFC) related to spatial stratification of bacterial community in cathodic biofilm under different carbon/nitrogen ratios. Single-chamber air-cathode MFC was tested under different ratio of COD/N (i.e., 40, 16, and 10), respectively. With the freezing microtome method, matured cathodic biofilms after 13 operation cycles were sliced into 5 layers (layer 1 close to anolyte and layer 5 close to cathode) with a thickness of 20–30 μm each layer. Results showed that high removals of ammonia and total nitrogen were achieved with stratification of bacterial communities in the cathodic biofilms of MFC. The relative abundance of SM1A02 was in a range of 14.7 % – 26.5 % in all the layers under COD/N of 40, but only 0.5 % in layers of 1 and 2 under COD/N of 10. Paracoccus had the relative abundance higher than 30 % and lower than 1.5 % in all the layers under COD/N of 10 and 40, respectively. The functional genes including AMO, nxrA, nxrB, narG, nirS and norC were upregulated under low COD/N ratio, indicating that the nitrification and denitrification were enhanced in the cathodic biofilms. The freezing microtome combined with high throughput sequencing of biofilms provided a useful way to characterize the spatial stratification of bacterial community in the nitrogen removal.

Nano-flower like CoFe-layered double hydroxide@reduced graphene oxide with efficient oxygen reduction reaction for high-power air-cathode microbial fuel cells

Microbial fuel cells (MFCs) are “self-sufficient” renewable energy sources that utilize the redox processes inherent in microbial metabolism to generate an electrical potential. However, the cathodic region of these fuel cells typically utilizes an oxygen reduction reaction (ORR) operating at high overpotential and slow reaction kinetics significantly, restricting the power generation capacity of MFCs. Herein, to improve the cathodic reduction efficiency, we synthesized CoFe-layered double hydroxides (LDH) on partially-reduced graphene oxide (p-rGO) via electrostatic interaction. In contrast to pure CoFe-LDH, CoFe-LDH@p-rGO composite possessed a three-dimensional uniform porous nanoflower-like morphology, giving the catalyst a high surface area for efficient catalysis. The proportions of CoFe-LDH to p-rGO were optimized to minimize layer stacking through strong π-π bond and van der Waals force, maximizing the overal electron transfer numbers n (3.66), with the lowest charge transfer resistance Rct (38.5 Ω). As a result, this novel catalyst combined the rich Co/Fe metal center catalytic active sites and interlayer structure of CoFe-LDH, with the high electrical conductivity of p-rGO to accelerate the electron transfer rate of ORR. Air-cathode microbial fuel cell (ACMFC) utilizing CoFe-LDH@p-rGO catalyst showed comparable output voltage of 423 mV to Pt/C catalyst, and a comparably high-power density of 204 mW m−2. This work provides a new path towards development of nanostructured non-noble metal catalysts for the optimization of oxygen reduction reaction in high-power ACMFCs.

Microbial fuel cell: Interplay of energy production, wastewater treatment, toxicity assessment with hydraulic retention time

Microbial fuel cell (MFC) operation under similar conditions to conventional methods will support the use of this technology in large-scale wastewater treatment. The operation of scaled-up air-cathode MFC (2 L) fed with synthetic wastewater (similar to domestic) in a continuous flow was evaluated using three different hydraulic retention times (HRT), 12, 8, and 4 h. We found that electricity generation and wastewater treatment could be enhanced under an HRT of 12 h. Additionally, the longer HRT led to greater coulombic efficiency (5.44%) than MFC operating under 8 h and 4 h, 2.23 and 1.12%, respectively. However, due to the anaerobic condition, the MFC was unable to remove nutrients. Furthermore, an acute toxicity test with Lactuca sativa revealed that MFC could reduce wastewater toxicity. These outcomes demonstrated that scaled-up MFC could be operated as a primary effluent treatment and transform a wastewater treatment plant (WWTP) into a renewable energy producer.