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

Sorting Out the Latest Advances in Separators and Pilot-Scale Microbial Electrochemical Systems for Wastewater Treatment: Concomitant Development, Practical Application, and Future Perspective

To date, dozens of pilot-scale microbial fuel cell (MFC) devices have been successfully developed worldwide for treating various types of wastewater. The availability and configurations of separators are determining factors for the economic feasibility, efficiency, sustainability, and operability of these devices. Thus, the concomitant advances between the separators and pilot-scale MFC configurations deserve further clarification. The analysis of separator configurations has shown that their evolution proceeds as follows: from ion-selective to ion-non-selective, from nonpermeable to permeable, and from abiotic to biotic. Meanwhile, their cost is decreasing and their availability is increasing. Notably, the novel MFCs configured with biotic separators are superior to those configured with abiotic separators in terms of wastewater treatment efficiency and capital cost. Herein, a highly comprehensive review of pilot-scale MFCs (>100 L) has been conducted, and we conclude that the intensive stack of the liquid cathode configuration is more advantageous when wastewater treatment is the highest priority. The use of permeable biotic separators ensures hydrodynamic continuity within the MFCs and simplifies reactor configuration and operation. In addition, a systemic comparison is conducted between pilot-scale MFC devices and conventional decentralized wastewater treatment processes. MFCs showed comparable cost, higher efficiency, long-term stability, and significant superiority in carbon emission reduction. The development of separators has greatly contributed to the availability and usability of MFCs, which will play an important role in various wastewater treatment scenarios in the future.

Investigation of microbial fuel cell performance based on the nickel thin film modified electrodes

Microbial fuel cells (MFCs) are a self-sustaining and environmentally friendly system for the simultaneous was tewater treatment and bioelectricity generation. The type and material of the electrode are critical factors that can influence the efficiency of this treatment process. In this study, graphite plates and carbon felt were modified through the electrodeposition of nickel followed by the formation of a biofilm, resulting in conductive bio-anode thin film electrodes with enhanced power generation capacity. The structural and morphological properties of the electrode surfaces were characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, elemental mapping, and field-emission scanning electron microscopy techniques. Maximum voltage, current density, and power generation were investigated using a dual-chamber MFC equipped with a Nafion 117 membrane and bio-nickel-doped carbon felt (bio-Ni@CF) and bio-nickel-doped graphite plate (bio-Ni@GP) electrodes under constant temperature conditions. The polarization and power curves obtained using different anode electrodes revealed that the maximum voltage, power and current density achieved with the bio-Ni@CF electrode were 468.0 mV, 130.72 mW/m2 and 760.0 mA/m2 respectively. Moreover, the modified electrodes demonstrated appropriate stability and resistance during successful runs. These results suggest that nickel-doped carbon-based electrodes can serve as suitable and stable supported catalysts and conductors for improving efficiency and increasing power generation in MFCs.

Examining current trends and future outlook of bio-electrochemical systems (BES) for nutrient conversion and recovery: an overview

Nutrient-rich waste streams from domestic and industrial sources and the increasing application of synthetic fertilizers have resulted in a huge-scale influx of reactive nitrogen and phosphorus in the environment. The higher concentrations of these pollutants induce eutrophication and foster degradation of aquatic biodiversity. Besides, phosphorus being non-renewable resource is under the risk of rapid depletion. Hence, recovery and reuse of the phosphorus and nitrogen are necessary. Over the years, nutrient recovery, low-carbon energy, and sustainable bioremediation of wastewater have received significant interest. The conventional wastewater treatment technologies have higher energy demand and nutrient removal entails a major cost in the treatment process. For these issues, bio-electrochemical system (BES) has been considered as sustainable and environment friendly wastewater treatment technologies that utilize the energy contained in the wastewater so as to recovery nutrients and purify wastewater. Therefore, this article comprehensively focuses and critically analyzes the potential sources of nutrients, working mechanism of BES, and different nutrient recovery strategies to unlock the upscaling opportunities. Also, economic analysis was done to understand the technical feasibility and potential market value of recovered nutrients. Hence, this review article will be useful in establishing waste management policies and framework along with development of advanced configurations with major emphasis on nutrient recovery rather than removal from the waste stream.

A New Reactor Concept for Single-Chamber Microbial Fuel Cells and Possible Anti-Fouling Strategies for Long-Term Operation

Microbial fuel cells are a promising technology for future wastewater treatment, as it allows cleaning and power generation simultaneously. The bottleneck of microbial fuel cells is often its cathodes because they determine the power output. Gas diffusion electrodes might overcome this bottleneck due to their low production costs and high oxygen reduction rates. However, biofilm formation on the gas diffusion electrodes reduces their performance over time. In this work, a new reactor design of the microbial fuel cell using rotating gas diffusion electrodes is presented. The biofilm growth on the electrode during operation was observed and its effect on the performance of the microbial fuel cell was examined. In addition, different antifouling strategies were investigated over a period of 80 days. It was found that already after 7 days of operation a complete biofilm had grown on an untreated gas diffusion electrode. However, this does not seem to affect the performance of the cells in the beginning. Differences in the performance of the reactors with and without an antifouling strategy only become apparent from day 15 onwards. The use of UV radiation and antibacterial membranes leads to the best results with maximum power densities of approx. 200 mW m^-2 while the untreated microbial fuel cell only achieves a maximum power density of approx. 20 mW m^-2.

Bioenergy Generation and Wastewater Purification with Li0.95Ta0.76Nb0.19Mg0.15O3 as New Air-Photocathode for MFCs

MFC is a promising technology that can be used for simultaneous electricity generation and wastewater treatment. Power energy generation of a ferroelectric cathodic ceramic, Li0.95Ta0.76Nb0.19Mg0.15O3 (LTNMg), has been measured in microbial fuel cells, integrating a single chamber fed by industrial wastewater (CODinitial = 471 mg L−1, and pHinitial = 7.24 at T = 27 °C). In this process, the mixed multicomponent oxide material has been prepared and characterized by XRD, PSD, TEM, and UV-Vis spectroscopy. The catalytic activity has been investigated by COD determination, analysis of heavy metals, and polarization measurement. The results show a high COD reduction efficiency, which reaches 95.70% after a working time of 168 h with a maximal power density of 228 mW m−2. In addition, the maximum value of generated voltage in the open-circuit potential (OCP) of this MFC configuration has been increased from 340 mV in the absence of a light source to 470 mV under irradiation, indicating the presence of a promoting photocatalytic effect of LTNMg, which improved the process of the cathodic electron transfer inside the MFC device.

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.

Use of activated carbon and camphor carbon as cathode and clay cup as proton exchange membrane in a microbial fuel cell for the bioenergy production from crude glycerol biodegradation

This work characterizes two alternative materials to substitute the most expensive microbial fuel cells (MFCs) components: proton exchange membrane (PEM) and cathode. Crude glycerol biodegradation was studied in MFCs using a clay cup as a PEM and activated carbon and camphor carbon mixture (CAC) as a cathode. The cathode performance was compared with Platinum on carbon cloth. Two clay cup single-chamber MFCs were operated with each cathode and fed with 2000 mg/L of crude glycerol. Electrochemical properties were characterized by linear sweep voltammetry, electrochemical impedance spectroscopy, and chronoamperometry. Biodegradation efficiencies were estimated with the chemical oxygen demand (COD) removal percentage. MFCs with CAC showed a maximum power density of 100 mW/m². This result was a 43.47% power response regarding MFCs with Platinum. COD removal efficiencies of 94% were achieved in 37 days for both cells. The Columbic efficiencies were 24.04% and 22.78% for the MFCs with Platinum and CAC. The economic analysis showed a cost of USD 9.97 for MFCs with CAC. This cost is five times lower than when using Platinum. MFCs utilizing clay cups and CAC showed an acceptable performance for the bioenergy production from crude glycerol biodegradation above all economic advantage in the cell cost.

Microbial Fuel Cell-Based Biosensor for Simultaneous Test of Sodium Acetate and Glucose in a Mixed Solution

Most microbial fuel cell (MFC) sensors only focus on the detection of mixed solutions with respect to the chemical oxygen demand (COD) or toxicity; however, the concentrations of the individual analytes in a mixed solution have rarely been studied. Herein, we developed two types of MFC sensors, adapted with sodium acetate (MFC-A) and glucose (MFC-B) as organic substrates in the startup period. An evident difference in the sensor sensitivities (the slope value of the linear-regression curve) was observed between MFC-A and MFC-B. MFC-A exhibited a superior performance compared with MFC-B in the detection of sodium acetate (4868.9 vs. 2202 mV/(g/L), respectively) and glucose (3895.5 vs. 3192.9 mV/(g/L), respectively). To further compare these two MFC sensors, the electrochemical performances were evaluated, and MFC-A exhibited a higher output voltage and power density (593.76 mV and 129.81 ± 4.10 mW/m², respectively) than MFC-B (484.08 mV and 116.21 ± 1.81 mW/m², respectively). Confocal laser scanning microscopy (CLSM) and microbial-community analysis were also performed, and the results showed a richer anode biomass of MFC-A in comparison with MFC-B. By utilizing the different sensitivities of the two MFC sensors towards sodium acetate and glucose, we proposed and verified a novel method for a simultaneous test on the individual concentrations of sodium acetate and glucose in a mixed solution. Linear equations of the two variables (concentrations of sodium acetate and glucose) were formulated. The linear equations were solved according to the output voltages of the two MFC sensors, and the solutions showed a satisfactory accuracy with regard to sodium acetate and glucose (relative error less than 20%).

Contribution of configurations, electrode and membrane materials, electron transfer mechanisms, and cost of components on the current and future development of microbial fuel cells

Microbial fuel cells (MFCs) are a technology that can be applied to both the wastewater treatment and bioenergy generation. This work discusses the contribution of improvements regarding the configurations, electrode materials, membrane materials, electron transfer mechanisms, and materials cost on the current and future development of MFCs. Analysis of the most recent scientific publications on the field denotes that dual-chamber MFCs configuration offers the greatest potential due to the excellent ability to be adapted to different operating environments. Carbon-based materials show the best performance, biocompatibility of carbon-brush anode favors the formation of the biofilm in a mixed consortium and in wastewater as a substrate resembles the conditions of real scenarios. Carbon-cloth cathode modified with nanotechnology favors the conductive properties of the electrode. Ceramic clay membranes emerge as an interesting low-cost membrane with a proton conductivity of 0.0817 S cm^-1, close to that obtained with the Nafion membrane. The use of nanotechnology in the electrodes also enhances electron transfer in MFCs. It increases the active sites at the anode and improves the interface with microorganisms. At the cathode, it favors its catalytic properties and the oxygen reduction reaction. These features together favor MFCs performance through energy production and substrate degradation with values above 2.0 W m^-2 and 90% respectively. All the recent advances in MFCs are gradually contributing to enable technological alternatives that, in addition to wastewater treatment, generate energy in a sustainable manner. It is important to continue the research efforts worldwide to make MFCs an available and affordable technology for industry and society.

Bio-membrane integrated systems for nitrogen recovery from wastewater in circular bioeconomy

Wastewater contains a significant amount of recoverable nitrogen. Hence, the recovery of nitrogen from wastewater can provide an option for generating some revenue by applying the captured nitrogen to producing bio-products, in order to minimize dangerous or environmental pollution consequences. The circular bio-economy can achieve greater environmental and economic sustainability through game-changing technological developments that will improve municipal wastewater management, where simultaneous nitrogen and energy recovery are required. Over the last decade, substantial efforts were undertaken concerning the recovery of nitrogen from wastewater. For example, bio-membrane integrated system (BMIS) which integrates biological process and membrane technology, has attracted considerable attention for recovering nitrogen from wastewater. In this review, current research on nitrogen recovery using the BMIS are compiled whilst the technologies are compared regarding their energy requirement, efficiencies, advantages and disadvantages. Moreover, the bio-products achieved in the nitrogen recovery system processes are summarized in this paper, and the directions for future research are suggested. Future research should consider the quality of recovered nitrogenous products, long-term performance of BMIS and economic feasibility of large-scale reactors. Nitrogen recovery should be addressed under the framework of a circular bio-economy.

The enhancement of microbial fuel cell performance by anodic bacterial community adaptation and cathodic mixed nickel–copper oxides on a graphene electrocatalyst

Background

Although microbial fuel cells (MFCs) represent a promising technology for capturing renewable energy from wastewater, their scaling-up is significantly limited by a slow-rate cathodic oxygen reduction reaction (ORR) and the development of a resilient anodic microbial community. In this study, mixed transition metal oxides of nickel and copper (Ni and Cu), supported on a graphene (G) (NiO–CuO/G) electrocatalyst, were synthesized and tested as a cost-effective cathode for ORR in MFCs. Electrochemical measurements of electrocatalyst were conducted using a rotating disk electrode (RDE) and linear sweep voltammetry (LSV) in a neutral electrolyte, and compared with a benchmark Pt/C catalyst. Furthermore, the long-term performance of the as-synthesized electrocatalyst was evaluated in a single-chamber MFC by measuring organic matter removal and polarization behavior. The successful enrichment of electroactive biofilm was also monitored using transmission electron microscopy and the Vitek2 compact system technique.

Results

When compared with the benchmark platinum cathode, the NiO–CuO/G electrocatalyst exhibited high selectivity toward ORR. The rotating disk electrode (RDE) experiments reveal that ORR proceeds via a 4-electron ORR mechanism. Furthermore, the NiO–CuO/G electrocatalyst also exhibited a high power density of 21.25 mW m−2 in an air-cathode MFC, which was slightly lower than that of Pt/C-based MFC (i.e., 50.4 mW m−2). Biochemical characterization of the most abundant bacteria on anodic biofilms identified four genera (i.e., Escherichia coli, Shewanella putrefaciens, Bacillus cereus, and Bacillus Thuringiensis/mycoides) that belonged to Gammaproteobacteria, and Firmicutesphyla.

Conclusions

This study demonstrates that the NiO–CuO/G cathode had an enhanced electrocatalytic activity toward ORR in a pH-neutral solution. This novel mixed transition metal oxide electrocatalyst could replace expensive Pt-based catalysts for MFC applications.

Activated carbon supported Fe–Cu–NC as an efficient cathode catalyst for a microbial fuel cell

Herein, the output power density produced by Fe–Cu–NC-x as the cathode catalyst of a MFC was higher than that of the AC control.