Practical energy harvesting for microbial fuel cells: a review

Electricity generation and real oily wastewater treatment by Pseudomonas citronellolis 620C in a microbial fuel cell: pyocyanin production as electron shuttle

In the present study, the potential of Pseudomonas citronellolis 620C strain was evaluated, for the first time, to generate electricity in a standard, double chamber microbial fuel cell (MFC), with oily wastewater (OW) being the fuel at 43.625 mg/L initial chemical oxygen demand (COD). Both electrochemical and physicochemical results suggested that this P. citronellolis strain utilized efficiently the OW substrate and generated electricity in the MFC setup reaching 0.05 mW/m2 maximum power. COD removal was remarkable reaching 83.6 ± 0.1%, while qualitative and quantitative gas chromatography/mass spectrometry (GC/MS) analysis of the OW total petroleum and polycyclic aromatic hydrocarbons, and fatty acids revealed high degradation capacity. It was also determined that P. citronellolis 620C produced pyocyanin as electron shuttle in the anodic MFC chamber. To the authors' best knowledge, this is the first study showing (phenazine-based) pyocyanin production from a species other than P. aeruginosa and, also, the first time that P. citronellolis 620C has been shown to produce electricity in a MFC. The production of pyocyanin, in combination with the formation of biofilm in the MFC anode, as observed with scanning electron microscopy (SEM) analysis, makes this P. citronellolis strain an attractive and promising candidate for wider MFC applications.

A comprehensive review of microbial fuel cells considering materials, methods, structures, and microorganisms

Microbial fuel cells (MFCs) are promising for generating renewable energy from organic matter and efficient wastewater treatment. Ensuring their practical viability requires meticulous optimization and precise design. Among the critical components of MFCs, the membrane separator plays a pivotal role in segregating the anode and cathode chambers. Recent investigations have shed light on the potential benefits of membrane-less MFCs in enhancing power generation. However, it is crucial to recognize that such configurations can adversely impact the electrocatalytic activity of anode microorganisms due to increased substrate and oxygen penetration, leading to decreased coulombic efficiency. Therefore, when selecting a membrane for MFCs, it is essential to consider key factors such as internal resistance, substrate loss, biofouling, and oxygen diffusion. Addressing these considerations carefully allows researchers to advance the performance and efficiency of MFCs, facilitating their practical application in sustainable energy production and wastewater treatment. Accelerated substrate penetration could also lead to cathode clogging and bacterial inactivation, reducing the MFC's efficiency. Overall, the design and optimization of MFCs, including the selection and use of membranes, are vital for their practical application in renewable energy generation and wastewater treatment. Further research is necessary to overcome the challenges of MFCs without a membrane and to develop improved membrane materials for MFCs. This review article aims to compile comprehensive information about all constituents of the microbial fuel cell, providing practical insights for researchers examining various variables in microbial fuel cell research.

Performance analysis of microbial fuel cell - membrane bioreactor with reduced graphene oxide enhanced polypyrrole conductive ceramic membrane: Wastewater treatment, membrane fouling and microbial community under high salinity

The application of membrane bioreactor (MBR) in high salinity wastewater treatment was mainly hindered by membrane fouling. Microbial fuel cell (MFC)-MBR coupling system was established to alleviate membrane fouling and save energy. Reduced graphene oxide/polypyrrole ceramic membrane (rGO/PPy CM) with high conductivity and stability was innovatively placed in MFC-MBRs as both cathode and filter, with PPy CM, rGO/PPy CM and CM placed in other reactors. MFC-MBR (rGO/PPy) and MFC-MBR (PPy) achieved higher pollutant removal efficiencies (90.73 % and 90.45 % for TOC, 87.22 % and 86.56 % for NH4+-N, respectively) and superior anti-fouling performance (1.86 and 1.93 kPa/d for average membrane fouling rates) than both conventional MBRs (CMBRs). The stable voltage generation was around 287 and 242 mV, respectively. Through high throughput sequencing, electric field showed a positive correlation with the abundance and activity of most dominant phylum (Bacteroidetes, Chloroflexi, Actinobacteria, and Firmicutes) and functional genes (amoA, hao, narG, napA, nirK, norB, and nosZ), thereby improving pollutant removal efficiency. The higher conductivity of rGO/PPy CM resulted in enhanced electric field intensity, leading to superior performance of anti-fouling and pollutant removal. This study inventively explored the effects of conductive membrane property on electricity generation performance, microbial community, pollutant removal and membrane fouling, providing theoretical support for the selection of electrode materials in MFC-MBR.

Virtual Model Compound Approach for Calculating Redox Potentials of [Fe2S2]-Cys4 Centers in Proteins - Structure Quality Matters

The midpoint potential of the [Fe2S2]-Cys4-cluster in proteins is known to vary between -200 and -450 mV. This variation is caused by the different electrostatic environment of the cluster in the respective proteins. Continuum electrostatics can quantify the impact of the protein environment on the redox potential. Thus, if the redox potential of a [Fe2S2]-Cys4-cluster model compound in aqueous solution would be known, then redox potentials in various protein complexes could be calculated. However, [Fe2S2]-Cys4-cluster models are not water-soluble, and thus, their redox potential can not be measured in aqueous solution. To overcome this problem, we introduce a method that we call Virtual Model Compound Approach (VMCA) to extrapolate the model redox potential from known redox potentials of proteins. We carefully selected high-resolution structures for our analysis and divide them into a fit set, for fitting the model redox potential, and an independent test set, to check the validity of the model redox potential. However, from our analysis, we realized that the some structures can not be used as downloaded from the PDB but had to be re-refined in order to calculate reliable redox potentials. Because of the re-refinement, we were able to significantly reduce the standard deviation of our derived model redox potential for the [Fe2S2]-Cys4-cluster from 31 mV to 10 mV. As the model redox potential, we obtained -184 mV. This model redox potential can be used to analyze the redox behavior of [Fe2S2]-Cys4-clusters in larger protein complexes.

Sustainable circular biorefinery approach for novel building blocks and bioenergy production from algae using microbial fuel cell

The imminent need for transition to a circular biorefinery using microbial fuel cells (MFC), based on the valorization of renewable resources, will ameliorate the carbon footprint induced by industrialization. MFC catalyzed by bioelectrochemical process drew significant attention initially for its exceptional potential for integrated production of biochemicals and bioenergy. Nonetheless, the associated costly bioproduct production and slow microbial kinetics have constrained its commercialization. This review encompasses the potential and development of macroalgal biomass as a substrate in the MFC system for L-lactic acid (L-LA) and bioelectricity generation. Besides, an insight into the state-of-the-art technological advancement in the MFC system is also deliberated in detail. Investigations in recent years have shown that MFC developed with different anolyte enhances power density from several µW/m² up to 8160 mW/m². Further, this review provides a plausible picture of macroalgal-based L-LA and bioelectricity circular biorefinery in the MFC system for future research directions.

A Weak Electricigen-Based Bioelectrochemical Sensor for Real-Time Monitoring of Chemical Pollutants in Water

Electroactive microorganisms are now understood to be abundant across nature, though many are categorized as "weak electricigens" not suitable for reasonable power generation. We report the use of weak electricigens from the natural environment for rapid, real-time water quality monitoring. Using a variety of pesticides as model chemical pollutants, the bioelectrochemical sensor was responsive within minutes at all concentrations tested (0.05-2 ppm) and could be repreatedly used long-term. Due to the prevalence of electroactive microorganisms in the natural environment, such sensors could work in tandem with conventional monitoring methods and may be useful for detecting emerging contaminants.

Tailored hybrid microbial water disinfection system using sequentially assembled microbial fuel cells and an ultraviolet C light-emitting diode

An integrated ultraviolet C light-emitting diode (UV-C LED) water disinfection system activated by microbial fuel cells (MFCs) was developed, and optimized via electric circuit and device voltage profiling. The intensity of the renewable energy operated, self-powered UV-C LED for E. coli inactivation was calculated by bio-dosimetry to be 2.4  × 10-2 μW cm-2 using fluence-based rate constant (k) of ∼1.03 (±0.11) cm2/mJ to obtain the reduction equivalent fluence kinetics value. Finally, the first-order rate constant for E. coli inactivation during the tailored hybrid disinfection system was found to be 0.53 (±0.1) cm2/mJ by multiplying intensity with 1.09 (±0.1) × 10-5 s-1 derived from the linear regression of E. coli inactivation as a function of time. Furthermore, selected model microbial consisting of two bacteria (Salmonella sp. and Listeria sp.) and three viruses (MS2 bacteriophage, influenza A virus, and murine norovirus-1) were treated with UV-C LED irradiation under controlled experimental conditions to validate the disinfection efficiency of the system. Consequently, the required to achieve significant removal (i.e., >3-log; 99.9%) UV fluence and dose time were calculated to be 4-7 cm2/mJ and 54-76 h and 33-53 cm2/mJ and 400-622 h for model bacterial and viral, respectively. This study expands the applicability of microbial electrochemical system (MES) for microbial disinfection and could be utilized in future MFCs implementation studies for predicting and measuring the kinetics of microbial elimination using a tailored hybrid water treatment system.

Embracing the future of circular bio-enabled economy: unveiling the prospects of microbial fuel cells in achieving true sustainable energy

Sustainable development and energy security, highlighted by the United Nations Sustainable Development Goals (SDGs), necessitate the use of renewable and sustainable energy sources. However, upon careful evaluation of literature, we have discovered that many existing and emerging renewable energy systems (RESs) prioritize renewability over true sustainability. These systems not only suffer from performance inconsistencies and lack of scalability but also fall short in fully embodying the principles of sustainability and circular economy. To address this gap, we propose considering microbial fuel cells (MFCs) as a viable alternative and integral part of the renewable energy ecosystem. MFCs harness the omnipresence, abundance, and cost-effectiveness of their essential components, making them a promising candidate. Through our comprehensive analysis, we shed light on the limitations and advancements of this technology, which underscore the remarkable potential of MFCs to revolutionize our perception of clean, sustainable energy.

Customized Multichannel Measurement System for Microbial Fuel Cell Characterization

This work presents the development of an automatic and customized measuring system employing sigma-delta analog-to-digital converters and transimpedance amplifiers for precise measurements of voltage and current signals generated by microbial fuel cells (MFCs). The system can perform multi-step discharge protocols to accurately measure the power output of MFCs, and has been calibrated to ensure high precision and low noise measurements. One of the key features of the proposed measuring system is its ability to conduct long-term measurements with variable time steps. Moreover, it is portable and cost-effective, making it ideal for use in laboratories without sophisticated bench instrumentation. The system is expandable, ranging from 2 to 12 channels by adding dual-channel boards, which allows for testing of multiple MFCs simultaneously. The functionality of the system was tested using a six-channel setup, and the results demonstrated its ability to detect and distinguish current signals from different MFCs with varying output characteristics. The power measurements obtained using the system also allow for the determination of the output resistance of the MFCs being tested. Overall, the developed measuring system is a useful tool for characterizing the performance of MFCs, and can be helpful in the optimization and development of sustainable energy production technologies.

Electrochemical Energy Harvesting Using Microbial Active Matter

With the continuous growth in world population and economy, the global energy demand is increasing rapidly. Given that non-renewable energy sources will eventually deplete, there is increasing need for clean, alternative renewable energy sources, which will be inexpensive and involve minimum risk of environmental pollution. In this paper, harnessing the activity of cupric reductase NDH-2 enzyme present in Escherichia coli bacterial cells, we demonstrate a simple and efficient energy harvesting strategy within an electrochemical chamber without the requirement of any external fuels or force fields. The transduction of energy has been demonstrated with various strains of E. coli, indicating that this strategy could, in principle, be applicable for other microbial catalytic systems. We offer a simple mechanism of the energy transduction process considering the bacterial enzyme-mediated redox reaction occurring over the working electrode of the electrochemical cell. Also, the amount of energy generated has been found to be depending on the motility of bacteria within the experimental chamber, suggesting possible opportunities for developing microbial motility-controlled small scale power generators. Finally, we show that the Faradaic electrochemical energy harvested is large enough to power a commercial light emitting diode connected to an amplifier circuit. We expect the present study to generate sufficient interest within soft condensed matter and biophysics communities, and offer useful platforms for controlled energy generation at the small scales.

Valorization of wastewater to recover value-added products: A comprehensive insight and perspective on different technologies

In recent years, due to rapid globalization and urbanization, the demand for fuels, energy, water and nutrients has been continuously increasing. To meet the future need of the society, wastewater is a prominent and emerging source for resource recovery. It provides an opportunity to recover valuable resources in the form of energy, fertilizers, electricity, nutrients and other products. The aim of this review is to elaborate the scientific literature on the valorization of wastewater using wide range of treatment technologies and reduce the existing knowledge gap in the field of resource recovery and water reuse. Several versatile, resilient environmental techniques/technologies such as ion exchange, bioelectrochemical, adsorption, electrodialysis, solvent extraction, etc. are employed for the extraction of value-added products from waste matrices. Since the last two decades, valuable resources such as polyhydroxyalkanoate (PHA), matrix or polymers, cellulosic fibers, syngas, biodiesel, electricity, nitrogen, phosphorus, sulfur, enzymes and a wide range of platform chemicals have been recovered from wastewater. In this review, the aspects related to the persisting global water issues, the technologies used for the recovery of different products and/or by-products, economic sustainability of the technologies and the challenges encountered during the valorization of wastewater are discussed comprehensively.

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

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

High electrical energy harvesting performance of an integrated microbial fuel cell and low voltage booster-rectifier system treating domestic wastewater

To harvest directly usable electrical energy from real domestic wastewater, a new power management system (PMS), transistor-based low voltage boosters followed by a voltage rectifier (LVBR), was developed and tested for its energy harvesting performance. Three air-cathode MFCs were individually linked with LVBs, which were electrically stacked in parallel and then connected with a single voltage rectifier (MFC-LVBR). The MFC-LVBR system could increase V_MFCto 11.9 ± 0.6 V without voltage reversal, which was capable of charging a lithium-ion batteryand supercapacitor-based power banks. When the integrated MFC-LVBR system was linked with a lithium-ion battery, the highest normalized energy recovery (NER_COD) of 0.76 kWh/kg-COD (NER_volumeof 0.22 kWh/m³) was achieved with a minimal energy loss of 14.4%, whichwas much higher than those previously reported values.Furthermore, the electrical energy charged in the lithium-ion battery successfully powered a DC peristaltic pump requiring a minimum operating power of 0.46 W.

Synchronous bio-degradation and bio-electricity generation in a Microbial Fuel Cell with aged and fresh leachate from the identical subtropical area

Seasonal leachate from both sealed and operating landfill in the identical district were employed as the sole substrate in the Microbial Fuel Cell (MFC) to evaluate the power output performance and aqueous organic waste disposal. The electrical performance was characterized to study the power generation, while the Chemical Oxygen Demand (COD) removal ratio and Coulombic Efficiency (CE) were calculated to illustrate the substrate disposal effect. In addition, Scanning Electron Microscope (SEM) on the operated anode was conducted to preliminarily explain the microbial community difference, and the phylogenetic tree constructed on the cultivated microorganism was an insight into the dominant bacteria suitable for leachate degradation. It was found that the MFCs inoculated with seasonal leachate from both sealed and operating landfill could generate electricity successfully. Although the fresh leachate-inoculated MFCs had better electrical output performance (22.7-25.6 W/m³ versus 6.61-7.48 W/m³) and COD removal efficiency (55.8%∼61.7% versus 47.7%∼51.4%), the CEs were only 4.3%∼7.6%, which were lower than the aged leachate inoculated group (5.9%∼11.3%). Based on the SEM images and the phylogenetic tree of the operated anode, the composition impacts on the microbial community and power output performance were verified, which was instructive for the leachate disposal in the MFC.

Organic Waste Substrates for Bioenergy Production via Microbial Fuel Cells: A Key Point Review

High-energy consumption globally has raised questions about the low environmentally friendly and high-cost processes used until now for energy production. Microbial fuel cells (MFCs) may support alternative more economically and environmentally favorable ways of bioenergy production based on their advantage of using waste. MFCs work as bio-electrochemical devices that consume organic substrates in order for the electrogenic bacteria and/or enzyme cultures to produce electricity and simultaneously lower the environmental hazardous value of waste such as COD. The utilization of organic waste as fuels in MFCs has opened a new research path for testing a variety of by-products from several industry sectors. This review presents several organic waste substrates that can be employed as fuels in MFCs for bioenergy generation and the effect of their usage on power density, COD (chemical oxygen demand) removal, and Coulombic efficiency enhancement. Moreover, a demonstration and comparison of the different types of mixed waste regarding their efficiency for energy generation via MFCs are presented. Future perspectives for manufacturing and cost analysis plans can support scale-up processes fulfilling waste-treatment efficiency and energy-output densities.

Mainstreaming microfluidic microbial fuel cells: a biocompatible membrane grown in situ improves performance and versatility

A recent trend in microfluidic microbial fuel cells (MFCs) is to exclude a separation membrane, instead, relying on the physics of laminar flow to maintain isolation between anode and cathode compartments. To avoid solution crossover, the electrodes may be separated by distances of several millimeters, but this negatively affects the internal resistance and undermines a prime advantage of microscale MFCs. Therefore, we propose a facile method for in situ synthesis of a micromembrane that supports sub-millimeter electrode spacing. Membrane synthesis in situ reduces device fabrication complexity, and the proposed design avoids electrode contamination during its synthesis. Comparing results to a state-of-the-art membraneless MFC with 6 mm inter-electrode distances, the sub-millimeter membrane MFC under comparable flow conditions had an internal resistance that was 60% lower, power and current densities that were respectively 45% and 290% higher, and acetate conversion efficiencies that were 8 times higher. The enhanced flow stability provided stable operation under imbalanced flow conditions and delivered continuous increases to power density of up to 30% for flow rate increases of 100 times over baseline levels. As a result, maximum outputs obtained were 660 mW m^-1 and 3.5 A m^-1. These are the highest reported for microfluidic MFCs using pure culture bacteria, which advances the goal of competing with mainstream MFC formats.

Electrical mapping of thermoelectric power factor in WO3 thin film

With growing environmental awareness and considerable research investment in energy saving, the concept of energy harvesting has become a central topic in the field of materials science. The thermoelectric energy conversion, which is a classic physical phenomenon, has emerged as an indispensable thermal management technology. In addition to conventional experimental investigations of thermoelectric materials, seeking promising materials or structures using computer-based approaches such as machine learning has been considered to accelerate research in recent years. However, the tremendous experimental efforts required to evaluate materials may hinder us from reaping the benefits of the fast-developing computer technology. In this study, an electrical mapping of the thermoelectric power factor is performed in a wide temperature-carrier density regime. An ionic gating technique is applied to an oxide semiconductor WO3, systematically controlling the carrier density to induce a transition from an insulating to a metallic state. Upon electrically scanning the thermoelectric properties, it is demonstrated that the thermoelectric performance of WO3 is optimized at a highly degenerate metallic state. This approach is convenient and applicable to a variety of materials, thus prompting the development of novel functional materials with desirable thermoelectric properties.

Forward-Looking Roadmaps for Long-Term Continuous Water Quality Monitoring: Bottlenecks, Innovations, and Prospects in a Critical Review

Long-term continuous monitoring (LTCM) of water quality can bring far-reaching influences on water ecosystems by providing spatiotemporal data sets of diverse parameters and enabling operation of water and wastewater treatment processes in an energy-saving and cost-effective manner. However, current water monitoring technologies are deficient for long-term accuracy in data collection and processing capability. Inadequate LTCM data impedes water quality assessment and hinders the stakeholders and decision makers from foreseeing emerging problems and executing efficient control methodologies. To tackle this challenge, this review provides a forward-looking roadmap highlighting vital innovations toward LTCM, and elaborates on the impacts of LTCM through a three-hierarchy perspective: data, parameters, and systems. First, we demonstrate the critical needs and challenges of LTCM in natural resource water, drinking water, and wastewater systems, and differentiate LTCM from existing short-term and discrete monitoring techniques. We then elucidate three steps to achieve LTCM in water systems, consisting of data acquisition (water sensors), data processing (machine learning algorithms), and data application (with modeling and process control as two examples). Finally, we explore future opportunities of LTCM in four key domains, water, energy, sensing, and data, and underscore strategies to transfer scientific discoveries to general end-users.

Advances in the development of electrode materials for improving the reactor kinetics in microbial fuel cells

Microbial fuel cells (MFCs) are an emerging technology for converting organic waste into electricity, thus providing potential solution to energy crises along with eco-friendly wastewater treatment. The electrode properties and biocatalysts are the major factors affecting electricity production in MFC. The electrons generated during microbial metabolism are captured by the anode and transferred towards the cathode via an external circuit, causing the flow of electricity. This flow of electrons is greatly influenced by the electrode properties and thus, much effort has been made towards electrode modification to improve the MFC performance. Different semiconductors, nanostructured metal oxides and their composite materials have been used to modify the anode as they possess high specific surface area, good biocompatibility, chemical stability and conductive properties. The cathode materials have also been modified using metals like platinum and nano-composites for increasing the redox potential, electrical conductivity and surface area. Therefore, this paper reviews the recent developments in the modification of electrodes towards improving the power generation capacity of MFCs.

Recent progress in microbial fuel cells for industrial effluent treatment and energy generation: Fundamentals to scale-up application and challenges

Microbial fuel cells (MFCs) technology have the potential to decarbonize electricity generation and offer an eco-friendly route for treating a wide range of industrial effluents from power generation, petrochemical, tannery, brewery, dairy, textile, pulp/paper industries, and agro-industries. Despite successful laboratory-scale studies, several obstacles limit the MFC technology for real-world applications. This review article aimed to discuss the most recent state-of-the-art information on MFC architecture, design, components, electrode materials, and anodic exoelectrogens to enhance MFC performance and reduce cost. In addition, the article comprehensively reviewed the industrial effluent characteristics, integrating conventional technologies with MFCs for advanced resource recycling with a particular focus on the simultaneous bioelectricity generation and treatment of various industrial effluents. Finally, the article discussed the challenges, opportunities, and future perspectives for the large-scale applications of MFCs for sustainable industrial effluent management and energy recovery.

Bio-electrochemical frameworks governing microbial fuel cell performance: technical bottlenecks and proposed solutions

Microbial fuel cells (MFCs) are recognized as a future technology with a unique ability to exploit metabolic activities of living microorganisms for simultaneous conversion of chemical energy into electrical energy. This technology holds the promise to offer sustained innovations and continuous development towards many different applications and value-added production that extends beyond electricity generation, such as water desalination, wastewater treatment, heavy metal removal, bio-hydrogen production, volatile fatty acid production and biosensors. Despite these advantages, MFCs still face technical challenges in terms of low power and current density, limiting their use to powering only small-scale devices. Description of some of these challenges and their proposed solutions is demanded if MFCs are applied on a large or commercial scale. On the other hand, the slow oxygen reduction process (ORR) in the cathodic compartment is a major roadblock in the commercialization of fuel cells for energy conversion. Thus, the scope of this review article addresses the main technical challenges of MFC operation and provides different practical approaches based on different attempts reported over the years.

Sustainable operation requires addressing key MFC-bottleneck issues. Enhancing extracellular electron transfer is the key to elevated MFC performance.

Recent advancements in microbial fuel cells: A review on its electron transfer mechanisms, microbial community, types of substrates and design for bio-electrochemical treatment

The development in urbanization, growth in industrialization and deficiency in crude oil wealth has made to focus more for the renewable and also sustainable spotless energy resources. In the past two decades, the concepts of microbial fuel cell have caught more considerations among the scientific societies for the probability of converting, organic waste materials into bio-energy using microorganisms catalyzed anode, and enzymatic/microbial/abiotic/biotic cathode electro-chemical reactions. The added benefit with MFCs technology for waste water treatment is numerous bio-centered processes are available such as sulfate removal, denitrification, nitrification, removal of chemical oxygen demand and biological oxygen demand and heavy metals removal can be performed in the same MFC designed systems. The various factors intricate in MFC concepts in the direction of bioenergy production consists of maximum coulombic efficiency, power density and also the rate of removal of chemical oxygen demand which calculates the efficacy of the MFC unit. Even though the efficacy of MFCs in bioenergy production was initially quietly low, therefore to overcome these issues few modifications are incorporated in design and components of the MFC units, thereby functioning of the MFC unit have improvised the rate of bioenergy production to a substantial level by this means empowering application of MFC technology in numerous sectors including carbon capture, bio-hydrogen production, bioremediation, biosensors, desalination, and wastewater treatment. The present article reviews about the microbial community, types of substrates and information about the several designs of MFCs in an endeavor to get the better of practical difficulties of the MFC technology.

Green approach and strategies for wastewater treatment using bioelectrochemical systems: A critical review of fundamental concepts, applications, mechanism, and future trends

Millions of litters of multifarious wastewater are directly disposed into the environment annually to reduce the processing costs leading to eutrophication and destroying the clean water sources. The bioelectrochemical systems (BESs) have recently received significant attention from researchers due to their ability to convert waste into energy and their high efficiency of wastewater treatment. However, most of the performed researches of the BESs have focused on energy generation, which created a literature gap on the utilization of BESs for wastewater treatment. The review highlights this gap from various aspects, including the BESs trends, fundamentals, applications, and mechanisms. A different review approach has followed in the present work using a bibliometric review (BR) which defined the literature gap of BESs publications in the degradation process section and linked the systematic review (SR) with it to prove and review the finding systematically. The degradation mechanisms of the BESs have been illustrated comprehensively in the current work, and various suggestions have been provided for supporting future studies and cooperation.

Progress in microbial fuel cell technology for wastewater treatment and energy harvesting

The global energy crisis has stimulated the development of various forms of green energy technology such as microbial fuel cells (MFCs) that can be applied synergistically and simultaneously toward wastewater treatment and bioenergy generation. This is because electricigens in wastewater can act as catalysts for destroying organic pollutants to produce bioelectricity through bacterial metabolism. In this review, the factors affecting energy production are discussed to help optimize MFC processes with respect to design (e.g., single, double, stacked, up-flow, sediment, photosynthetic, and microbial electrolysis cells) and operational conditions/parameters (e.g., cell potential, microorganisms, substrate (in wastewater), pH, temperature, salinity, external resistance, and shear stress). The significance of electron transfer mechanisms and microbial metabolism is also described to pursue the maximum generation of power by MFCs. Technically, the generation of power by MFCs is still a significant challenge for real-world applications due to the difficulties in balancing between harvesting efficiency and upscaling of the system. This review summarizes various techniques used for MFC-based energy harvesting systems. This study aims to help narrow such gaps in their practical applications. Further, it is also expected to give insights into the upscaling of MFC technology while assisting environmental scientists to gain a better understanding on this energy harvesting approach.

Supercapacitive biofuel cells

Supercapacitive biofuel cells' (SBFCs) most recent advancements are herein disclosed. In conventional SBFCs the biocomponent is employed as the pseudocapacitive component, while in self-charging biodevices it also works as the biocatalyst. The performance of different types of SBFCs are summarized according to the categorization based on the biocatalyst employed: supercapacitive microbial fuel cells (s-MFCs), supercapacitive biophotovoltaics (SBPV) and supercapacitive enzymatic fuel cells (s-EFCs). SBFCs could be considered as promising 'alternative' energy devices (low-cost, environmentally friendly, and technically undemanding electric power sources etc.) being suitable for powering a new generation of miniaturized electronic applications.

Tuning of Electrode Surface for Enhanced Bacterial Adhesion and Reactions: A Review on Recent Approaches

The study of bacterial adhesion and its consequences has great significance in different fields such as marine science, renewable energy sectors, soil and plant ecology, food industry, and the biomedical field. Generally, the adverse effects of microbial surface interactions have attained wide visibility. However, herein, we present distinct approaches to highlight the beneficial aspects of microbial surface interactions for various applications rather than deal with the conventional negative aspects or prevention strategies. The surface microbial reactions can be tuned for useful biochemical or bio-electrochemical applications, which are otherwise unattainable through conventional routes. In this context, the present review is a comprehensive approach to highlight the basic principles and signature parameters that are responsible for the useful microbial-electrode interactions. It also proposes various surface tuning strategies, which are useful for tuning the electrode characteristics particularly suitable for the enhanced bacterial adhesion and reactions. The tuning of surface characteristics of electrodes is discussed with a special reference to the Microbial Fuel Cell as an example.

Bioelectrochemical anoxic ammonium nitrogen removal by an MFC driven single chamber microbial electrolysis cell

Nitrogen removal from wastewater is an indispensable but highly energy-demanding process, and thus more energy-saving treatment processes are required. Here, we investigated the performance of bioelectrochemical ammonium nitrogen (NH₄⁺-N) removal from real domestic wastewater without energy-intensive aeration by a single chamber microbial electrolysis cell (MEC) that was electrically powered by a double chamber microbial fuel cell (MFC). Anoxic NH₄⁺-N oxidation and total nitrogen (TN) removal rates were determined at various applied voltages (0-1.2 V), provided by the MFC. The MEC achieved a NH₄⁺-N oxidation rate of 151 ± 42 g NH₄⁺-N m^-3 d^-1 and TN removal rate of 95 ± 42 g-TN m^-3 d^-1 without aeration at the applied voltage of 0.8 V (the anode potential E_anode = +0.633 ± 0.218 V vs. SHE). These removal rates were much higher than the previously reported values and conventional biological nitrogen removal processes. Open and closed-circuit MEC batch experiments confirmed that anoxic NH₄⁺-N oxidation was an electrochemically mediated biological process (that is, an anode acted as an electron acceptor) and denitrification occurred simultaneously without NO₂^- and NO₃^- accumulation. Moreover, ex-situ¹⁵N tracer experiment and microbial community analysis revealed that anammox and heterotrophic denitrification mainly contributed to the TN removal. Thus, the bioelectrochemical anodic NH₄⁺-N oxidation was coupled with anammox and denitrification in this MFC-assisted MEC system. Taken together, our MFC-driven single chamber MEC could be a high rate energy-saving nitrogen removal process without external carbon and energy input and high energy-demanding aeration.

Microbial fuel cells: a comprehensive review for beginners

Microbial fuel cells (MFCs) have shown immense potential as a one-stop solution for three major sustainability issues confronting the world today-energy security, global warming and wastewater management. MFCs represent a cross-disciplinary platform for research at the confluence of the natural and engineering sciences. The diversity of variables influencing performance of MFCs has garnered research interest across varied scientific disciplines since the beginning of this century. The increasing number of research publications has made it necessary to keep track of work being carried out by research groups across the globe and consolidate significant findings on a regular basis. Review articles are often the nodal points for beginners who are required to undertake an exploratory survey of literature to identify a suitable research problem. This 'review of reviews' is a ready-reckoner that directs readers to relevant reviews and research articles reporting significant developments in MFC research in the last two decades. The article also highlights the areas needing research attention which when addressed could take this technology a few more steps closer to practical implementation.

Selenite and selenate removal in a permeable flow-through bioelectrochemical barrier

This study demonstrated the removal of selenite and selenate in flow-through permeable bioelectrochemical barriers (microbial electrolysis cells, MECs). The bioelectrochemical barriers consisted of cathode and anode electrode compartments filled with granular carbon or metallurgical coke. A voltage of 1.4 V was applied to the electrodes to enable the bioelectrochemical removal of selenium species. For comparison, a similarly designed permeable anaerobic biobarrier filled with granular carbon was operated without voltage. All biobarrier setups were fed with water containing up to 5,000 µg L^-1 of either selenite or selenate and 70 mg L^-1 of acetate as a source of organic carbon. Significant removal of selenite and selenate was observed in MEC experimental setups, reaching 99.5-99.8% over the course of the experiment, while in the anaerobic biobarrier the removal efficiency did not exceed 88%. By simultaneously operating several setups and changing operating parameters (selenium species, influent Se and acetate concentrations, etc.) we demonstrated enhanced removal of Se species under bioelectrochemical conditions.

Weak electricity stimulates biological nitrate removal of wastewater: Hypothesis and first evidences

Nitrate pollution in water is a worldwide health and environmental concern. Biological nitrate removal of wastewater is widely used countering eutrophication of water bodies; however it could be troublesome and expensive when influent carbon source is insufficient. Here we present a novel process, the microbial fuel cell (MFC)-resistance-type electrical stimulation denitrification process (RtESD) using microbial weak electricity originated from the wastewater, to enhance nitrate removal. Results show that the optimal nitrate dependent denitrification rate (0.027 mg N/L·h) and nitrate removal efficiency (98.1%) can be achieved; partial autotrophic denitrification was enhanced in RtESD under stimulation of 0.2 V of microbial weak electricity (MWE). Aromatic proteins also increased in the presence of 0.2 V MWE stimulation according to three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy profiles, indicating that electron transfer could be improved in the case of MWE stimulation. Furthermore, the microbial community structure and diversity analysis results demonstrated that MWE stimulation inhibited the heterotrophic denitrifying bacteria and activated the autotrophic denitrifying bacteria in RtESD. Two hypotheses, enhancement of electron transfer and improvement of microorganism activity, were proposed regarding to the MWE stimulated pathways. This study provided a promising method utilizing MWE derived from wastewater to improve the denitrification rate and removal efficiency of nitrate-containing wastewater treatment processes.

Fenton/Fenton-like processes with in-situ production of hydrogen peroxide/hydroxyl radical for degradation of emerging contaminants: Advances and prospects

Fenton processes based on the reaction between Fe²⁺ and H₂O₂ to produce hydroxyl radicals, have been widely studied and applied for the degradation of toxic organic contaminants in wastewater due to its high efficiency, mild condition and simple operation. However, H₂O₂ is usually added by bulk feeding, which suffers from the potential risks during the storage and transportation of H₂O₂ as well as its low utilization efficiency. Therefore, Fenton/Fenton-like processes with in-situ production of H₂O₂ have received increasing attention, in which H₂O₂ was in-situ produced through O₂ activation, then decomposed into hydroxyl radicals by Fenton catalysts. In this review, the in situ production of H₂O₂ for Fenton oxidation was introduced, the strategies for activation of O₂ to generate H₂O₂ were summarized, including chemical reduction, electro-catalysis and photo-catalysis, the influencing factors and the mechanisms of the in situ production and utilization of H₂O₂ in various Fenton/Fenton-like processes were analyzed and discussed, and the applications of these processes for the degradation of toxic organic contaminants were summarized. This review will deepen the understanding of the tacit cooperation between the in situ production and utilization of H₂O₂ in Fenton process, and provide the further insight into this promising process for degradation of emerging contaminants in industrial wastewater.

Light exposure interferes with electroactive biofilm enrichment and reduces extracellular electron transfer efficiency

Light plays a vital role in shaping the structure of natural biofilms, but the effect of light on electroactive biofilm (EAB) has not been systematically studied. Herein, the influence of light on the formation of EAB was investigated. The EAB grown in darkness was more electroactive (EAB-0) with a peak current of ∼4.5 A/m², which was 196 and 5556 times higher than EABs formed under light intensities of 600 (EAB-600) and 1200 lux (EAB-1200). A thin EAB (30 μm) with spherical pink aggregates was obtained after 13 days in the darkness, comparing to a dense and flat biofilm grown under light conditions. Although the biomass in EAB-1200 (38.5 ± 1.6 mg/L) was 3 times larger than that in EAB-0 (11.4 ± 1.8 mg/L), the degradation of substrate was not sufficient. EAB-0 contained 85% Geobacter species, while the Rhodopseudomonas species made up 66% and 75% of EAB-600 and EAB-1200, respectively. The polysaccharides produced by EAB-1200 was 4801 ± 253 mg/m², which were 2.3 times higher than 2073 ± 160 mg/m² of EAB-0, resulting in lower electro-conductivity of the extracellular polymeric substances (EPS) under light conditions. Our findings confirmed that the light exposure affected EAB performance by altering the microbial components, electron transfer capacity, and biofilm morphology, which can be used in predictions of the formation and properties of engineered EAB in outdoor environments.

Microfluidic Microbial Bioelectrochemical Systems: An Integrated Investigation Platform for a More Fundamental Understanding of Electroactive Bacterial Biofilms

It is the ambition of many researchers to finally be able to close in on the fundamental, coupled phenomena that occur during the formation and expression of electrocatalytic activity in electroactive biofilms. It is because of this desire to understand that bioelectrochemical systems (BESs) have been miniaturized into microBES by taking advantage of the worldwide development of microfluidics. Microfluidics tools applied to bioelectrochemistry permit even more fundamental studies of interactions and coupled phenomena occurring at the microscale, thanks, in particular, to the concomitant combination of electroanalysis, spectroscopic analytical techniques and real-time microscopy that is now possible. The analytical microsystem is therefore much better suited to the monitoring, not only of electroactive biofilm formation but also of the expression and disentangling of extracellular electron transfer (EET) catalytic mechanisms. This article reviews the details of the configurations of microfluidic BESs designed for selected objectives and their microfabrication techniques. Because the aim is to manipulate microvolumes and due to the high modularity of the experimental systems, the interfacial conditions between electrodes and electrolytes are perfectly controlled in terms of physicochemistry (pH, nutrients, chemical effectors, etc.) and hydrodynamics (shear, material transport, etc.). Most of the theoretical advances have been obtained thanks to work carried out using models of electroactive bacteria monocultures, mainly to simplify biological investigation systems. However, a huge virgin field of investigation still remains to be explored by taking advantage of the capacities of microfluidic BESs regarding the complexity and interactions of mixed electroactive biofilms.

High voltage generation from wastewater by microbial fuel cells equipped with a newly designed low voltage booster multiplier (LVBM)

Although microbial fuel cells (MFCs) can produce renewable energy from wastewater, the generated power is practically unusable. To extract usable power from an MFC fed with wastewater, we newly developed a low voltage booster multiplier (LVBM), which is composed of a self-oscillating LVB and multistage voltage multiplier circuits (VMCs). The low output MFC voltage (ca. 0.4 V) was successfully boosted up to 99 ± 2 V, which was the highest voltage that has been ever reported, without voltage reversal by connecting an LVB with 20-stage VMCs. Moreover, the boosted voltage (81 ± 1 V) was stably maintained for > 40 h even after disconnecting the LVBM from the MFC. The energy harvesting efficiency of LVBM was > 80% when an LVB with 4-stage VMCs was charged to 9.3 V. These results clearly suggest that the proposed LVBM system is an efficient and self-starting energy harvester and storage for low-power generating MFCs.

Scale-up and control the voltage of sediment microbial fuel cell for charging a cell phone

In this research, a power management system (PMS) has been developed to charge a cell phone battery based on sediment microbial fuel cells (SMFCs). The single SMFC produces a voltage of 1.16 V, which is too low for practical application. The voltage is increased by connecting several SMFCs in series or parallel, but the voltage reversal occurs when it is directly connected to the load. To prevent the voltage reversal, the super capacitor is first charged by the five different stack SMFCs and the charged super capacitor is used to provide the input power to a PMS. This PMS increases and regulates the input voltage of stack SMFCs up to 5.02 V for charging a cell phone battery. The charging and discharging times of the super capacitors have been investigated with five different stack SMFCs. In all five stack SMFCs, only module-5 provides power to PMS for long periods (13 min). Further, the cell phone battery is continuously charged using the two parallel-connected stack SMFCs similar to the module-5. The battery has been fully charged in 26 h using 72 SMFCs. The charged battery is used to perform for three purposes; voice calling, music playing and LED strip lighting. This study is informative for the application of SMFC in an off-grid location.

Operation mechanism of constructed wetland-microbial fuel cells for wastewater treatment and electricity generation: A review

Constructed wetland-microbial fuel cells (CWL-MFCs) are eco-friendly and sustainable technology, simultaneously implementing contaminant removal and electricity production. According to intensive research over the last five years, this review on the operation mechanism was conducted for in-depth understanding and application guidance of CWL-MFCs. The electrochemical mechanism based on anodic oxidation and cathodic reduction is the core for improved treatment in CWL-MFCs compared to CWLs. As the dominant bacterial community, the abundance and gene-expression patterns of electro-active bacteria responds to electrode potentials and contaminant loadings, further affecting operational efficiency of CWL-MFCs. Plants benefit COD and N removal by supplying oxygen for aerobic degradation and rhizosphere secretions for microorganisms. Multi-electrode configuration, carbon-based electrodes and rich porous substrates affect transfer resistance and bacterial communities. The possibilities of CWL-MFCs targeting at recalcitrant contaminants like flame retardants and interchain interactions among effect components need systematic research.

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P _max  = 13 mW; large: ESR = 4.2 Ω and P _max  = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm^-2) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm^-2 and 633 μW cm^-2, respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL^-1, 265 μW mL^-1 and 249 μW cm^-1, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g^-1-C- and 3270 μW g^-1-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.

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

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

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

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

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Apparent capacitance was boosted up by adding capacitive features.

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

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Series and parallel connections improved the galvanostatic discharges.

Microbial Fuel Cell-Based Biological Oxygen Demand Sensors for Monitoring Wastewater: State-of-the-Art and Practical Applications

Environmental pollution has been a continuous threat to sustainable development and global well-being. It has become a significant concern worldwide to combat the ecological crisis using low-cost innovative technologies. Biological oxygen demand (BOD) is a key indicator to comprehend the quality of water to guarantee environmental safety and human health; however, none of the present technologies are capable of online monitoring of the water at the source. Microbial fuel cells (MFC) are a promising technology for simultaneous power generation and wastewater treatment. MFCs have also been shown in fascinating applications to measure and detect the toxic pollutants present in wastewater. These are the bioreactors where exoelectrogenic microorganisms catalyze the conversion of the inherent chemical energy stored in organic compounds to electrical energy. Sensors employ energy conversion to measure BOD, which is considered an international index for the detection of organic material load present in wastewater. The MFC-based BOD sensors have gone through a wide range of advancement from mediator to mediator-less, double chamber to single-chamber, and large size to miniature. There have been detailed studies to improve the accuracy and reproducibility of the sensors for commercial applications. Additionally, multistage MFC-based BOD biosensors and miniature MFC-BOD sensors have also been ubiquitous in recent years. A considerable amount of work has been carried out to improve the performance of these devices by fabricating the proton exchange membranes and altering catalysts at the cathode. However, there remains a dearth for the fabrication of the devices in aspects like suitable microbes, proton exchange membranes, and cheaper catalysts for cathodes for effective real-time monitoring of wastewater. In this review, an extensive study has been carried out on various MFC-based BOD sensors. The efficiency and drawbacks associated with the different MFC-based BOD sensors have been critically evaluated, and future perspectives for their development have been investigated. The breadth of work compiled in this review will accelerate further research in MFC-based BOD biosensors. It will be of great importance to broad ranges of scientific research and industry.

An approach to the photocatalytic mechanism in the TiO2-nanomaterials microorganism interface for the control of infectious processes

The approach of this timely review considers the current literature that is focused on the interface nanostructure/cell-wall microorganism to understand the annihilation mechanism. Morphological studies use optical and electronic microscopes to determine the physical damage on the cell-wall and the possible cell lysis that confirms the viability and microorganism death. The key parameters of the tailoring the surface of the photoactive nanostructures such as the metal functionalization with bacteriostatic properties, hydrophilicity, textural porosity, morphology and the formation of heterojunction systems, can achieve the effective eradication of the microorganisms under natural conditions, ranging from practical to applications in environment, agriculture, and so on. However, to our knowledge, a comprehensive review of the microorganism/nanomaterial interface approach has rarely been conducted. The final remarks point the ideal photocatalytic way for the effective prevention/eradication of microorganisms, considering the resistance that the microorganism could develop without the appropriate regulatory aspects for human and ecosystem safety.

Enhanced Performance of Microbial Fuel Cells with Anodes from Ethylenediamine and Phenylenediamine Modified Graphite Felt

A microbial fuel cell (MFC) is a promising renewable energy option, which enables the effective and sustainable harvesting of electrical power due to bacterial activity and, at the same time, can also treat wastewater and utilise organic wastes or renewable biomass. However, the practical implementation of MFCs is limited and, therefore, it is important to improve their performance before they can be scaled up. The surface modification of anode material is one way to improve MFC performance by enhancing bacterial cell adhesion, cell viability and extracellular electron transfer. The modification of graphite felt (GF), used as an anode in MFCs, by electrochemical oxidation followed by the treatment with ethylenediamine or p-phenylenediamine in one-step short duration reactions with the aim of introducing amino groups on the surface of GF led to the enhancement of the overall performance characteristics of MFCs. The MFC with the anode from GF modified with p-phenylenediamine provided approx. 32% higher voltage than the control MFC with a bare GF anode, when electric circuits of the investigated MFCs were loaded with resistors of 659 Ω. Its surface power density was higher by approx. 1.75 times than that of the control. Decreasing temperature down to 0 °C resulted in just an approx. 30% reduction in voltage generated by the MFC with the anode from GF modified with p-phenylenediamine.