A Critical Review on the Advancement of the Development of Low-Cost Membranes to Be Utilized in Microbial Fuel Cells

: Microbial fuel cells provide a promising solution for both generating electricity and treating wastewater at the same time. This review evaluated the effectiveness of using readily available earthen membranes, such as clayware and ceramics, in MFC systems. By conducting a comprehensive search of the Scopus database from 2015 to 2024, the study analyzed the performance of various earthen membranes, particularly in terms of wastewater treatment and energy production. Ceramic membranes were found to be the most effective, exhibiting superior power density, COD removal, and current density, with values of 229.12 ± 18.5 mW/m 2 , 98.41%, and 1535.0 ± 29 mW/m 2 , respectively. The review emphasizes the use of affordable resources like red soil, bentonite clay, CHI/MMT nanocomposites, and Kalporgan soil, which have proven to be effective in MFC applications. Incorporating earthen materials into the membrane construction of MFCs makes them more cost-effective and accessible.


Introduction
Water and energy scarcity are the critical and major global issues that need sustainable and economic solutions.In recent years, researchers have increasingly focused on developing effective and ecofriendly solutions to address these challenges [1].Nuclear and fossil fuels are non-renewable and have the greatest impact on total energy consumption.However, the use of fossil fuels is a major contributor to carbon dioxide emissions, which can lead to water pollution and global warming, posing significant risks to human life [2].Early detection of water quality issues is crucial to ensure the reliability of wastewater treatment facilities and drinking water sources.With freshwater scarcity and increasing pollution from industrial, agricultural, and domestic sources, it is essential to maintain treatment efficacy for the sake of public health and environmental protection.Currently, there are two critical challenges: energy scarcity and water depletion are the major issue worldwide [3].The first issue is made worse by over-dependence on limited energy supplies and growing energy needs, while the second is caused by fast industrialization and expanding consumption patterns.Addressing these challenges requires us to explore safe, eco-friendly energy alternatives for various sectors.Fossil fuel depletion, projected to occur by 2040-2042 for oil and gas and by 2112 for coal, highlights the urgent need for sustainable energy solutions [4].As a result, researchers are focusing on renewable energy sources with the goal of achieving environmental sustainability and economic viability to mitigate the global energy crisis and preserve water resources [5].
Water 2024, 16, 1597 2 of 21 Microbial fuel cells (MFCs) offer a unique solution to tackle some of the world's most pressing challenges, particularly in sustainable energy production and wastewater treatment [6].As renewable energy sources become increasingly crucial, MFCs harness the power of microorganisms to convert organic matter directly into electricity [7].This not only provides a renewable energy source but also reduces dependence on fossil fuels, mitigating greenhouse gas emissions and combating climate change.Additionally, MFCs offer a sustainable approach to wastewater treatment by simultaneously treating wastewater and generating electricity.This approach addresses two critical issues in tandem, and the technology can bring wastewater treatment closer to home, making it more accessible and affordable, especially in underserved areas [1,8].
Nafion is the most commonly used membrane in MFCs, but at the same time poses several challenges, such as high cost, biofouling, substrate losses, and oxygen leakage [9].Therefore, it is essential to explore alternative membranes to improve MFC efficiency and affordability.MFCs have two primary applications: sustainable electricity generation and wastewater treatment.Several variables impact MFC efficiency, including anode material selection, chemical media, proton exchange membranes, biocatalysts, and device arrangement [10].However, the practicality of using MFCs in industries is limited mainly by the cost of membrane materials, which limits their usability.The increased internal resistance of MFCs is another problem that affects their effectiveness.Capital costs are also significant, with electrode expenses (20-30%) for both anodes and cathodes [11] and membrane expenses (50-60%) [12] representing a considerable portion of the capital costs.Solving these problems can help us fully utilize MFCs in real-world applications and increase their effectiveness in wastewater treatment and sustainable energy generation [11].
Over the past few years, many researchers have worked on microbial fuel cells with positive outcomes.On 30 December 2023, a search was conducted on the ScienceDirect™ database using the keyword "microbial fuel cell," which yielded a total of 7302 papers the fields of "biotechnology applied microbiology" (5169) and "energy fuels" (1073), as shown in Figure 1.There has been a recent in-depth study of microbial fuel cells as a rising bioenergy source.Various extensive reviews have focused on different areas, such as electrode structures, designs and configurations, challenges, applications, and microbial community dynamics [13].
sustainable energy solutions [4].As a result, researchers are focusing on renewable energy sources with the goal of achieving environmental sustainability and economic viability to mitigate the global energy crisis and preserve water resources [5].
Microbial fuel cells (MFCs) offer a unique solution to tackle some of the world's most pressing challenges, particularly in sustainable energy production and wastewater treatment [6].As renewable energy sources become increasingly crucial, MFCs harness the power of microorganisms to convert organic matter directly into electricity [7].This not only provides a renewable energy source but also reduces dependence on fossil fuels, mitigating greenhouse gas emissions and combating climate change.Additionally, MFCs offer a sustainable approach to wastewater treatment by simultaneously treating wastewater and generating electricity.This approach addresses two critical issues in tandem, and the technology can bring wastewater treatment closer to home, making it more accessible and affordable, especially in underserved areas [1,8].
Nafion is the most commonly used membrane in MFCs, but at the same time poses several challenges, such as high cost, biofouling, substrate losses, and oxygen leakage [9].Therefore, it is essential to explore alternative membranes to improve MFC efficiency and affordability.MFCs have two primary applications: sustainable electricity generation and wastewater treatment.Several variables impact MFC efficiency, including anode material selection, chemical media, proton exchange membranes, biocatalysts, and device arrangement [10].However, the practicality of using MFCs in industries is limited mainly by the cost of membrane materials, which limits their usability.The increased internal resistance of MFCs is another problem that affects their effectiveness.Capital costs are also significant, with electrode expenses (20-30%) for both anodes and cathodes [11] and membrane expenses (50-60%) [12] representing a considerable portion of the capital costs.Solving these problems can help us fully utilize MFCs in real-world applications and increase their effectiveness in wastewater treatment and sustainable energy generation [11].
Over the past few years, many researchers have worked on microbial fuel cells with positive outcomes.On 30 December 2023, a search was conducted on the ScienceDirect™ database using the keyword "microbial fuel cell," which yielded a total of 7302 papers the fields of "biotechnology applied microbiology'' (5169) and "energy fuels" (1073), as shown in Figure 1.There has been a recent in-depth study of microbial fuel cells as a rising bioenergy source.Various extensive reviews have focused on different areas, such as electrode structures, designs and configurations, challenges, applications, and microbial community dynamics [13].

Fundamentals of MFCs
Microbial fuel cells transform wastewater biomass into a dependable source of electricity by utilizing the electrocatalytic qualities of bacteria.They comprise three essential parts, the anode, cathode, and a separator or membrane.Also known as microbial electrochemical cells, they are a practical and sustainable way to produce energy via a biological process [14].MFCs are available in a variety of configurations, but their fundamental design consists of two chambers joined by an ion exchange membrane: an anodic compartment and a cathodic compartment.Electrodes, substrates, an electrical circuit, and bacteria or microorganisms are all included within these chambers [15].Like conventional fuel cells, MFCs run on the idea of redox reactions; however, instead of utilizing expensive metal catalysts, they obtain their energy from live microbiological sources [16].Different organic sources are used to generate an electric current.Microorganisms break down organic materials to produce protons, electrons, and carbon dioxide, which are then used to generate energy [17].The presence of the anolyte medium affects the microbial fuel cell (MFC) reaction process, as shown in Equations ( 1)-(3) [18].

Anode reaction: CH3COOH + H2O
Cathode reaction: Overall reaction: In the overall process, the substrate is broken down into carbon dioxide and water, with the concurrent production of energy as a byproduct [19].By promoting the movement of electrons from the anode to the cathode in an external circuit, an MFC bioreactor produces electricity by extending the electrode reactions.Remarkably, two distinct applications may be served by an operation of the MFC process.Process variables are the membrane, electrodes in the design, and the MFC configuration, which all affect the performance of MFCs.Ion exchange membranes are essential for the building of MFCs, just like they are for fuel cells and batteries [20].

Fundamentals of MFCs
Microbial fuel cells transform wastewater biomass into a dependable source of electricity by utilizing the electrocatalytic qualities of bacteria.They comprise three essential parts, the anode, cathode, and a separator or membrane.Also known as microbial electrochemical cells, they are a practical and sustainable way to produce energy via a biological process [14].MFCs are available in a variety of configurations, but their fundamental design consists of two chambers joined by an ion exchange membrane: an anodic compartment and a cathodic compartment.Electrodes, substrates, an electrical circuit, and bacteria or microorganisms are all included within these chambers [15].Like conventional fuel cells, MFCs run on the idea of redox reactions; however, instead of utilizing expensive metal catalysts, they obtain their energy from live microbiological sources [16].Different organic sources are used to generate an electric current.Microorganisms break down organic materials to produce protons, electrons, and carbon dioxide, which are then used to generate energy [17].The presence of the anolyte medium affects the microbial fuel cell (MFC) reaction process, as shown in Equations ( 1)-(3) [18].

Anode reaction: CH
Cathode reaction: Overall reaction: In the overall process, the substrate is broken down into carbon dioxide and water, with the concurrent production of energy as a byproduct [19].By promoting the movement of electrons from the anode to the cathode in an external circuit, an MFC bioreactor produces electricity by extending the electrode reactions.Remarkably, two distinct applications may be served by an operation of the MFC process.Process variables are the membrane, electrodes in the design, and the MFC configuration, which all affect the performance of MFCs.Ion exchange membranes are essential for the building of MFCs, just like they are for fuel cells and batteries [20].

Configuration of MFCs
MFCs require proper design to work efficiently.MFCs are built according to a variety of architectural standards, and evaluation frequently focuses on elements such as power output, stability, durability, and Coulombic efficiency.Moreover, based on the quantity Water 2024, 16, 1597 4 of 21 of chambers or compartments, microbial fuel cells may be classified into two groups for optimization [16].

Dual Compartment of MFC
Dual chamber microbial fuel cells (D-MFCs) are among the most widely used and traditional varieties of microbial fuel cells.They are built with two chambers that vary in size and shape, including rectangular, U, and H shapes [21].An MFC's anode and cathode compartments are connected by an external circuit to facilitate electron flow and a salt bridge or PEM to transfer ions.Microorganisms develop in the anolyte or on the anode's surface in regular D-MFCs.Protons cross the membrane to reach the cathode while electrons are moved to the anode.Air or oxygen sparge or an electrical terminal electron acceptor are both present in the cathode chamber.For the D-MFCs to produce energy, the anaerobic anode must be maintained, and the cathode chamber conditions must be lowered so the separate operation of microbial metabolic processes and proton oxidation in D-MFCs results in better power densities.Nevertheless, they come with more complex structures due to the way the two chambers are constructed and separated.If the final material that receives electrons is oxygen, it also needs constant oxidation, primarily in the form of air spargers, as seen in Figure 3 [16,22].

Configuration of MFCs
MFCs require proper design to work efficiently.MFCs are built according to a variety of architectural standards, and evaluation frequently focuses on elements such as power output, stability, durability, and Coulombic efficiency.Moreover, based on the quantity of chambers or compartments, microbial fuel cells may be classified into two groups for optimization [16].

Dual Compartment of MFC
Dual chamber microbial fuel cells (D-MFCs) are among the most widely used and traditional varieties of microbial fuel cells.They are built with two chambers that vary in size and shape, including rectangular, U, and H shapes [21].An MFC's anode and cathode compartments are connected by an external circuit to facilitate electron flow and a salt bridge or PEM to transfer ions.Microorganisms develop in the anolyte or on the anode's surface in regular D-MFCs.Protons cross the membrane to reach the cathode while electrons are moved to the anode.Air or oxygen sparge or an electrical terminal electron acceptor are both present in the cathode chamber.For the D-MFCs to produce energy, the anaerobic anode must be maintained, and the cathode chamber conditions must be lowered so the separate operation of microbial metabolic processes and proton oxidation in D-MFCs results in better power densities.Nevertheless, they come with more complex structures due to the way the two chambers are constructed and separated.If the final material that receives electrons is oxygen, it also needs constant oxidation, primarily in the form of air spargers, as seen in Figure 3 [16,22].

Single Compartment MFCs
Oxygen serves as the last electron acceptor in most S-MFCs.In certain arrangements, the cathode is left open to the air while the membrane and cathode are firmly compressed together.Exoelectrogens produce electrons that move in the direction of the anode electrode while passing across the external circuit and arriving at the cathode [24,25].Protons in the electrolyte travel across the membrane at the same time and arrive at the cathode, where they assist in reducing oxygen levels in water.Anaerobic circumstances are the only ones in which exoelectrogens take place; hence, the anode compartment maintains an

Single Compartment MFCs
Oxygen serves as the last electron acceptor in most S-MFCs.In certain arrangements, the cathode is left open to the air while the membrane and cathode are firmly compressed together.Exoelectrogens produce electrons that move in the direction of the anode electrode while passing across the external circuit and arriving at the cathode [24,25].Protons in the electrolyte travel across the membrane at the same time and arrive at the cathode, where they assist in reducing oxygen levels in water.Anaerobic circumstances are the only ones in which exoelectrogens take place; hence, the anode compartment maintains an oxygen-free environment.S-MFCs are flexible and simple, and they come in a various of configurations, as shown in Figure 4 [16].

Electrodes
Microbial fuel cells rely on efficient anodes and cathodes to operate optimally.The selection of anode material is critical, with biocompatibility, electrical conductivity, and surface area being the top priorities.Biocompatible materials such as graphite or carbon cloth are essential for supporting the resident microbial communities and their metabolic activity [27,28].High electrical conductivity is crucial for efficient electron transport during microbial oxidation, which directly impacts the MFC's performance [29].Anodes with high surface area promote microbial attachment, while durability ensures long-term functionality [30].Further investigation is required to understand the interplay between the anode's catalytic activity and its compatibility with specific microbial consortia [31].
The cathode plays a complementary role in MFCs, facilitating the reduction reaction that balances the anode's oxidation.Efficient cathodes require high catalytic activity for the reduction of electron acceptors such as oxygen.Promising cathode materials include carbon-based materials, metals, and metal oxides [32].A large surface area is equally important for effective electron transfer and the reduction reaction.Durability is paramount for withstanding the harsh chemical environment within the cathode chamber.Moreover, good cathode conductivity ensures efficient electron flow from the anode, completing the MFC's electrical circuit [33,34].Our ongoing research endeavors to optimize these electrode properties, understand the underlying electrochemical kinetics, and explore novel electrode materials to amplify MFC performance across diverse applications.

Anode Reaction
In MFCs, selecting a proper coating for the electrodes is essential as it affects how interaction of bacteria with the anode.Additionally, it has a major effect on electron transport and processes involving protons that have a high reduction potential, especially on the cathode when interacting with materials such as oxygen.High conductivity, chemical stability in wastewater streams, strong biocompatibility with minimal toxicity to

Electrodes
Microbial fuel cells rely on efficient anodes and cathodes to operate optimally.The selection of anode material is critical, with biocompatibility, electrical conductivity, and surface area being the top priorities.Biocompatible materials such as graphite or carbon cloth are essential for supporting the resident microbial communities and their metabolic activity [27,28].High electrical conductivity is crucial for efficient electron transport during microbial oxidation, which directly impacts the MFC's performance [29].Anodes with high surface area promote microbial attachment, while durability ensures long-term functionality [30].Further investigation is required to understand the interplay between the anode's catalytic activity and its compatibility with specific microbial consortia [31].
The cathode plays a complementary role in MFCs, facilitating the reduction reaction that balances the anode's oxidation.Efficient cathodes require high catalytic activity for the reduction of electron acceptors such as oxygen.Promising cathode materials include carbonbased materials, metals, and metal oxides [32].A large surface area is equally important for effective electron transfer and the reduction reaction.Durability is paramount for withstanding the harsh chemical environment within the cathode chamber.Moreover, good cathode conductivity ensures efficient electron flow from the anode, completing the MFC's electrical circuit [33,34].Our ongoing research endeavors to optimize these electrode properties, understand the underlying electrochemical kinetics, and explore novel electrode materials to amplify MFC performance across diverse applications.

Anode Reaction
In MFCs, selecting a proper coating for the electrodes is essential as it affects how interaction of bacteria with the anode.Additionally, it has a major effect on electron transport and processes involving protons that have a high reduction potential, especially on the cathode when interacting with materials such as oxygen.High conductivity, chemical stability in wastewater streams, strong biocompatibility with minimal toxicity to bacteria, and large areas of the surface that help easily attach and spread bacteria on their surface are all desirable qualities in anode material, as shown in Figure 5 [35].It is also suggested that they be highly adaptive and stable at low temperatures and in the pH range of 5 to 7 [36].They ought to be immune to biofouling as well.It could be beneficial if the cost of manufacturing were low.In MFC applications, carbon is a particularly good anode material.It can be found in many different forms, including activated carbon, single-or multi-walled carbon nanotubes, carbon mesh, graphite granule brushes, graphite plates, carbon cloth, carbon fibers, surface-modified stainless steel, graphite plates or rods, as well as metallic anodes [37].However, since metal anodes are also common, selecting the appropriate anode material is crucial in avoiding metal corrosion, as shown in Equations ( 4) and ( 5) [18].
Water 2024, 16, x FOR PEER REVIEW 6 of 22 bacteria, and large areas of the surface that help easily attach and spread bacteria on their surface are all desirable qualities in anode material, as shown in Figure 5 [35].It is also suggested that they be highly adaptive and stable at low temperatures and in the pH range of 5 to 7 [36].They ought to be immune to biofouling as well.It could be beneficial if the cost of manufacturing were low.In MFC applications, carbon is a particularly good anode material.It can be found in many different forms, including activated carbon, single-or multi-walled carbon nanotubes, carbon mesh, graphite granule brushes, graphite plates, carbon cloth, carbon fibers, surface-modified stainless steel, graphite plates or rods, as well as metallic anodes [37].However, since metal anodes are also common, selecting the appropriate anode material is crucial in avoiding metal corrosion, as shown in Equations ( 4) and ( 5) [18].

Cathode Reaction
Protons are produced in the anode chamber and then travel to the cathode chamber through the PEM.Simultaneously, electrons generated at the anode site reach the cathode chamber via the external circuit [38].
This sequence of actions results in a steady current flow in the external circuit.The yield of the cathode reaction depends on various factors, such as the type and concentration of the oxidant, the availability of protons, the catalyst's performance, and the electrode's structure.Choosing the right catalyst, such as platinum [39], activated carbon, and other metal catalysts [40] such as (cobalt, titanium, and iron), is crucial for oxygen reduction in the cathode chamber [38,41].However, one of the major challenges in MFC technology is the low efficiency of the oxygen reduction reaction (ORR) that occurs at the cathode, as shown in Figure 6.The cathodic reaction is affected by various factors, such as cathode configuration (air-cathode or aqueous cathode), whether cathodes are biotic or

Cathode Reaction
Protons are produced in the anode chamber and then travel to the cathode chamber through the PEM.Simultaneously, electrons generated at the anode site reach the cathode chamber via the external circuit [38].
This sequence of actions results in a steady current flow in the external circuit.The yield of the cathode reaction depends on various factors, such as the type and concentration of the oxidant, the availability of protons, the catalyst's performance, and the electrode's structure.Choosing the right catalyst, such as platinum [39], activated carbon, and other metal catalysts [40] such as (cobalt, titanium, and iron), is crucial for oxygen reduction in the cathode chamber [38,41].However, one of the major challenges in MFC technology is the low efficiency of the oxygen reduction reaction (ORR) that occurs at the cathode, as shown in Figure 6.The cathodic reaction is affected by various factors, such as cathode configuration (air-cathode or aqueous cathode), whether cathodes are biotic or abiotic, electrode and catalyst materials, electrode dimensions, the cathode current collector, and catholytes.The air-cathode MFC design has many advantages over the aqueous cathode, provided that an appropriate gas diffusion layer prevents water leakage through the ceramic separator.Furthermore, using biotic cathodes that utilize microorganisms as a biocatalyst for ORR can improve the power performance of MFCs and simultaneously eliminate many toxic pollutants.Work has been carried out on aerobic cathodes to enhance the treatment quality.By entering the anodic effluent into the cathode chamber, researchers were able to improve the efficiency of the system [42,43].
Water 2024, 16, x FOR PEER REVIEW 7 of 22 abiotic, electrode and catalyst materials, electrode dimensions, the cathode current collector, and catholytes.The air-cathode MFC design has many advantages over the aqueous cathode, provided that an appropriate gas diffusion layer prevents water leakage through the ceramic separator.Furthermore, using biotic cathodes that utilize microorganisms as a biocatalyst for ORR can improve the power performance of MFCs and simultaneously eliminate many toxic pollutants.Work has been carried out on aerobic cathodes to enhance the treatment quality.By entering the anodic effluent into the cathode chamber, researchers were able to improve the efficiency of the system [42,43].

Oxygen Reduction Reaction
In ORR, various oxidants can be used as electron acceptors in the aqueous cathodes of microbial fuel cells.Oxygen is the ideal electron acceptor for cathode electrodes since it is readily available and cheap.ORR, which reduces oxygen molecules by taking up electrons from the electrode, is the primary reaction in an MFC cathode [45].
Several electron transfer pathways are involved in ORR, and they are contingent on the kind of catalyst employed at the cathode.Oxygen can be electro-reduced in two main approaches: these are the 2-electron pathway and the 4-electron pathway (shown in Figure 6) [46].Since a large overpotential of hydrogen peroxide can occur in the development of 2-electron paths, 4-electron paths are preferable.To evaluate the rate-determining step of the ORR, verify the initial adsorption of oxygen at the electrode catalyst's interface and its subsequent reduction to hydrogen peroxide and water.Depending on the kind of carbon utilized, there are two different ways in which oxygen reduction on carbon materials takes place in the context of non-Pt catalysts in electrolytic ORR.For example, a proposed mechanism describes the reduction of oxygen on an electrode constructed of glassy carbon [40], using the equations shown in ( 6)-( 13), 4-electron electro reduction of oxygen pathway, 2-electron electro reduction of oxygen pathway:

Oxygen Reduction Reaction
In ORR, various oxidants can be used as electron acceptors in the aqueous cathodes of microbial fuel cells.Oxygen is the ideal electron acceptor for cathode electrodes since it is readily available and cheap.ORR, which reduces oxygen molecules by taking up electrons from the electrode, is the primary reaction in an MFC cathode [45].
Several electron transfer pathways are involved in ORR, and they are contingent on the kind of catalyst employed at the cathode.Oxygen can be electro-reduced in two main approaches: these are the 2-electron pathway and the 4-electron pathway (shown in Figure 6) [46].Since a large overpotential of hydrogen peroxide can occur in the development of 2-electron paths, 4-electron paths are preferable.To evaluate the rate-determining step of the ORR, verify the initial adsorption of oxygen at the electrode catalyst's interface and its subsequent reduction to hydrogen peroxide and water.Depending on the kind of carbon utilized, there are two different ways in which oxygen reduction on carbon materials takes place in the context of non-Pt catalysts in electrolytic ORR.For example, a proposed mechanism describes the reduction of oxygen on an electrode constructed of glassy carbon [40], using the equations shown in ( 6)-( 13), 4-electron electro reduction of oxygen pathway, 2-electron electro reduction of oxygen pathway:

Membrane
To ensure the proper functioning of microbial fuel cells, it is important to use a membrane to separate the anode and cathode reactions [47].This membrane allows protons to move from the anode to the cathode while acting as a physical barrier between the two chambers and preventing the flow of oxygen from the cathode to the anaerobic anode chamber.MFC separators or membranes are typically made of polymers and ceramics and must possess specific characteristics [48], such as strong ionic conductivity, enduring stability over the long term, high proton conductivity, efficient mass transfer between the anaerobic anode and the oxygen-containing water in the cathode, low internal resistance, and good energy recovery [49].
There are several types of ion exchange membranes (IEMs), such as polymeric and ceramic membranes which allow interchange of both cations as well as anions [50].These membranes selectively allow ions with opposing charges to pass through while stopping ions with similar charges.The five categories of IEMs are cation exchange membranes (CEM), bipolar membranes (BPM), anion exchange membranes (AEM) [51], mosaic IEMs, and amphoteric IEMs [52].Protons and other cations can pass through a membrane and enter the cathode chamber via CEMs, which are sometimes referred to as PEMs.These create an overall negative group of functions on the membrane.Conversely, anion exchange membranes have positive charges such as carbonate or phosphate attached to facilitate proton transfer using proton carriers [45,53].

Types of Earthen Membrane
Continuous advancements are being made in the investigation of various types of ceramic membranes for use in microbial fuel cells.While ceramic membranes are currently preferred due to their superior mechanical strength, chemical resistance, and suitability for harsh MFC environments, earthen membranes offer a potentially attractive alternative.
Earthen membranes, composed of naturally occurring clays or earthen materials, boast significant advantages in terms of sustainability and affordability compared to commercially produced ceramic membranes.However, research on earthen membranes has fluctuated, likely due to concerns about their limitations, shown in Table 1.These limitations include lower mechanical strength, making them more susceptible to cracking, and potentially insufficient chemical resistance for the complex MFC environment [54].Despite these challenges, recent research suggests a renewed interest in earthen membranes for MFCs.This resurgence can be attributed to two key trends.Firstly, the growing emphasis on sustainable practices has encouraged the exploration of eco-friendly materials like natural clays for MFC applications.Secondly, the inherent affordability of earthen materials offers a significant economic advantage compared to ceramic membranes [23,67].
Further research and development efforts focused on exploring diverse earthen materials and developing earthen composite membranes with enhanced properties could unlock the full potential of this technology.By addressing the current limitations through targeted material science advancements, earthen membranes can evolve into a viable and sustainable alternative for MFC separator membranes.This would not only contribute to a more eco-friendly approach to MFC technology but also offer a cost-effective solution for wastewater treatment and electricity generation [68].

Earthen Membranes
Earthenware membranes made of natural clay or mud have been a reliable choice for a long time due to their easy availability and handling.They provide a sustainable and cost-effective option for construction, making them stand out from traditional materials.However, sand membranes, despite being overlooked, hold promise for potential improvements in microbial fuel cell applications.Although they are not as mechanically and chemically stable as engineered ceramics, their intrinsic porosity presents opportunities for facilitating ion exchange and mass transfer within MFC systems.
The use of sand membranes in such contexts is still in the early stages of exploration and needs to be investigated further and optimized (Table 1).A comprehensive review of literature, covering soil-based materials and ancient pottery, can yield valuable insights into the properties of these materials and their suitability for diverse applications.Despite the limited discussion regarding earthen membranes in MFCs, their potential justifies careful consideration and systematic study for future advancements in sustainable energy technologies [69][70][71][72].

Clayware Membranes
Clay, a type of soil rich in minerals and organic matter, has been used for pottery and construction for centuries.In MFCs, clay-based membranes are emerging as a costeffective and environmentally friendly option (Table 1).Clay's natural porosity and ion exchange capabilities make it ideal for crafting membranes tailored to MFCs.Its unique adsorption properties and sustainability make it a compelling alternative to traditional membranes.During manufacturing, clay can be shaped into a thin, porous layer that acts as a selective barrier.This membrane allows ions to pass through while blocking larger organic molecules.Optimizing this design could improve MFCs' electrochemical processes, boosting their efficiency and performance overall [73][74][75].

Ceramic Membranes
Ceramic materials have played pivotal roles in the progress of civilizations, appearing independently across diverse cultures to create similar objects.The production of "earthenware", characterized by its solid yet brittle structure, involves a process of extracting clay, mixing it with water, shaping it, sun-drying it, and finally, firing it in a kiln [76].
Ceramic membranes represent advanced materials with applications spanning separation, filtration, and purification in various industrial processes.Set apart from conventional polymeric membranes, as shown in Table 1, ceramic membranes are composed of inorganic materials such as alumina, zirconia, and more [77].A robust substrate supports a thin, porous layer crucial to the formation of ceramic membranes, wherein the filtration or separation process predominantly occurs.
Ceramics excel in demanding sectors like food and beverage, biotechnology, pharmaceuticals, and water treatment due to their remarkable attributes, including chemical stability, mechanical strength, and resistance to high temperatures [78].Notably, ceramic Water 2024, 16, 1597 10 of 21 membranes exhibit superior tolerance to corrosive chemical conditions, ensuring prolonged functionality even in harsh environments compared to polymer membranes [79].This durability translates into extended lifespans and reduced maintenance requirements.
Moreover, ceramic membranes thrive in processes involving elevated temperatures, thanks to their exceptional thermal stability [80].This feature proves particularly beneficial in applications like wastewater treatment, where higher temperatures enhance separation efficiency or facilitate the efficient treatment of certain toxins.Another significant advantage lies in the precise control of pore size offered by ceramic membranes.Manufacturers can tailor pore diameters to meet the specific separation needs of diverse applications, enhancing their adaptability and utility across various industrial processes [81].
Ceramic membranes are used in water treatment to filter and cleanse water and wastewater.They can eliminate particles, germs, and even viruses thanks to precisely calibrated porosity, ensuring that the water that is treated satisfies strict quality requirements.Additionally, the dairy and beverage sectors are using ceramic membranes more frequently to clarify and concentrate liquids [82].
Since ceramic membranes are so good at providing sterile filtration, their use in biotech and medical fields is growing.These membranes' durability makes it possible to undergo thorough cleaning and sterilizing operations, which are essential for preserving the integrity of delicate biological procedures [79,80].Ceramic membranes have many benefits, but they also have certain drawbacks.Compared to other polymer membranes, they are often more expensive to produce and need a more involved manufacturing procedure.Continued research and developments in manufacturing techniques aim to overcome these obstacles and increase the affordability and accessibility of ceramic membranes [79,83].
However, earthen membranes may have lower mechanical strength compared to their engineered counterparts, making them less robust and more susceptible to deformation.Clayware membranes, specifically processed to enhance mechanical strength, offer improved durability compared to earthen membranes.Ceramic membranes, with their inorganic composition and specialized manufacturing processes, boast the highest mechanical strength and durability among the three types.In terms of performance, clayware membranes may have porosity levels several times higher than earthen membranes, while ceramic membranes typically exhibit even higher porosity levels compared to both earthen and clayware membranes.Overall, while earthen membranes provide a cost-effective and accessible option for certain applications, clayware and ceramic membranes offer superior performance and durability, making them more suitable for demanding industrial processes and long-term use.

Ion Transport across Membranes and Its Characterization
Different analytical approaches listed in the available research, including proton transport, ion transport, oxygen transport, water uptake, and change in conductivity, were used to determine the transport of ions through the membranes [84].The next paragraphs provide a summary of each technique's inclusive description.

Mass Transport of Oxygen
The oxygen mass transfer coefficient in MFCs indicates whether the membranes can let or prevent oxygen from penetrating.Membranes play a critical role in preventing oxygen from diffusing from the cathode into the anode chamber.With an easily carried dissolved oxygen (D O ) probe, the oxygen coefficient for mass transfer may be established [85].The DO mass balance in the chamber is determined in a two-chamber system with total mixing and membrane-assisted chamber separation.
In the given expression, V is the chamber volume in liters; fo is the oxygen flux measured in (kg −3 /ms); A is the membrane area in (m −4 ); L t is the membrane thickness in (m −2 ) as provided by the manufacturer; Do is the diffusion coefficient expressed in (m −4 /s); x o is the saturating the oxygen concentration that exists in the aerated chamber measured in (kg −3 /L); and x is the dissolved oxygen (D O ) concentration in the anode chamber at a given time (t).This formulates the connection as follows: The oxygen mass transfer coefficient (Ko) and diffusion coefficient (Do) in a twochamber MFC system [51] may be calculated using the previously given formula, where Ko is (m/s).

Mass Transport of Proton
A dual-chamber system was employed in the experimental setting; the first chamber had pH-controlled deionized water, and the second chamber held a solution with a different pH.The systematic monitoring of pH variations at regular times in both chambers was carried out using two pH electrodes.This analytical methodology made it possible to assess the kinetics of proton transport across the membrane accurately.Using Equation ( 14), A two-chamber system's essential parameters are outlined in the formula that is provided: the liquid volume in the anode chamber (V) is shown in (m −6) , the projected membrane surface area (A) is expressed in (m −4 ), time (t) is expressed in (s), and the starting proton concentrations in the anodic and cathodic chambers are represented by C a and C c , respectively.The proton concentration in the cathode chamber at time t is represented by C p .The equation is also used to calculate the proton diffusion coefficient (D H , m −4 /s), where Lth is the membrane thickness (m −2 ) and K H is the proton mass transfer coefficient (m −2 /s) found in the previous calculation, as shown in Equation ( 15): This formulation has a key component in the comprehension and measurement of the dynamics of proton mass transfer in MFC devices [86].

Water Uptake
The water absorption capacity of membrane is assessed by putting it in demineralized water at 303 K for 24 h, allowing it to swell.Its weight, considering surface water removal, is then measured.Then, the membrane is dried for 15 h at 303 K, and its dry weight is recorded before immersing it in deionized water [87].Using the following formula, the water uptake capacity is given as a percentage: Water uptake capacity (%) = [(weight of expand membrane − weight of dried membrane)/weight of dried membrane] × 100

Ion Exchange Capacity
The ion exchange capacity (IEC) of a membrane shows whether charged it is as an outcome of its various functional groups.It represents the ions that are obtainable for exchange across the membrane and acts as a measure for the density of current [88].A technique called back titration can be used to measure the IEC of membranes.This means presenting the IEC as the membrane sample's entire charge divided by dry weight [51].IEC = total charge/dry weight (20) where the dry weight unit is grams (g).

Results and Discussion
As shown in Table 2, ceramic membrane configurations with a stainless-steel electrode in a dual chamber system proved to be the most efficient way to attain the maximum CD of 1422.22 ± 41.2 mW/m 2 and PD of 229.12 ± 18.5 mW/m 2 .The increased electron transfer kinetics were responsible for the membrane's observed K O rate of 9.1 × 10 −5 and K H rate of (222.73 ± 22.7) × 10 −3 .This configuration is ideal for the treatment of domestic wastewater [89].Secondly, CHI/MMT nanocomposites in a ceramic membrane displayed outstanding COD removal of 95.67%.The PD value of 119.58 ± 19.16 mW/m 2 and CD value of 869.44 ± 27.49 mW/m 2 were the greatest measured.These outcomes were found to be effectively achieved by both the modification of the nanocomposite and the usage of carbon cloth electrodes.Treatment of raw wastewater may be accomplished using this setup, which has a K O rate of 0.83 × 10 −4 [90].It was discovered that a bentonite clay-modified membrane worked well in a dual chamber for treating sewage.Effective electron transfer was made possible by the 60-day operation, which produced an acceptable PD of 15.38 mW/m 3 and a CD of 38.46 mW/m 2 [91].In the other method using graphite fiber as both the anode and cathode material, a ceramic membrane designed for sanitary sewage treatment attained a CD of 103 ± 7 mW/m 2 and a PD of 261 mW/m 2 .This system was shown to be successful when it was operated for an extended period of 210 days, treating mixed swine waste with a hydraulic retention time of 4 h [92].A mixed inoculum in a ceramic anaerobic treatment plant had the greatest COD removal rate of 96.6%.This arrangement is suitable for anaerobic treatment operations since it produced the maximum current density of 1535.0 ± 29 mW/m 2 when carbon brush electrodes and a dual chamber were used [93].The table presents a unique ceramic membrane combination that utilizes a dual-chamber layout of clay materials and a SUPER-MIX inoculum.A rate of 2.5 × 10 −5 oxygen transport was achieved with this arrangement.With a high power of 275 mW/m 2 attained, the COD removal was 91 ± 3.96%.Effective electron transfer and removal of pollutants were made possible by the SUPER-MIX inoculum, carbon-felt electrodes, and dual-chamber construction [94].This used a ceramic membrane made for treating effluent from rice mills, comprising a dual-chamber structure using soil with 30% silica and 20% w/w bentonite clay.The system achieved a K O rate of 11.35 × 10 −4 mW/m 2 and a K H rate of 3.64 × 10 −5 mW/m 2 .Effective electron transfer was facilitated using graphite plates and stainless-steel electrodes, and the system's COD removal of 71.3% was evidence of its success [95].A two-chamber setup used Kalporgan soil in different SiO 2 concentrations (0-30%) with carbon brushes and carbon clothes as electrodes, and reached a 769.23 mW/m 2 current density.This setup effectively removed pollutants and showed a significant 85% reduction of COD [66].This arrangement had a dual-chamber structure with stainless-steel electrodes and graphite plate, and used red soil that contained 20% bentonite.It accomplished a 52% elimination of COD, a K H rate of 6.55 × 10 −6 mW/m 2 , and a K O rate of 9.33 × 10 −4 mW/m 2 .Red soil treated with bentonite improved pollution removal and electron transfer rates [96].

Water Uptake Capacity
The water uptake capacity of microbial fuel cell separators is a crucial factor that affects their performance by facilitating proton transportation.Protons are generated as byproducts of microbial respiration, and they rely on cooperative OH covalent and hydrogen bonding dynamics to navigate through the membrane toward the cathode via the Grotthuss mechanism.This mechanism involves protons moving along water chains formed by hydrogen bonding, where initially, a proton interacts with a water molecule's oxygen, creating a hydrogen bond, and then transitioning into an OH covalent bond.This process enables proton transfer along the water chain until the terminal water molecules release the proton, thus completing the conduction process [108].
The water uptake capacity of various membranes significantly impacts their ability to facilitate proton transportation.According to Das et al. 2020, the G-5 membrane has an 18% water uptake, which is due to the hydrophilic nature of goethite within its matrix, and this increases its efficiency [102].On the other hand, Nafion membranes, known for their high water uptake (32%), demonstrate superior proton transport capabilities.Similarly, clay separators, with a 13% water uptake, contribute to proton transportation, albeit less efficiently compared to Nafion [12].
In addition, membranes with high water-holding capacities, such as the SMN membrane, show improved performance compared to counterparts like the SM membrane.Notably, the Nafion-117 membrane has the highest water-holding capacity among all membranes, which further enhances its proton transport capabilities [67].
In the context of ceramic membranes, which rely on pores for ion passage, water uptake is critical.Reports suggest that reducing silicone content in ceramic membranes enhances water absorption, resulting in a 64% improvement in power output due to heightened ionic conductivity and diminished ohmic resistance [61].Furthermore, the swelling ratio of membranes, which is influenced by additives like ACCS, also affects water uptake.While higher ACCS content initially increases swelling ratios, further augmentation beyond 5% ACCS fails to correspondingly enhance water uptake due to reduced pore volume.Therefore, membranes are cast with 5% ACCS based on preliminary studies to optimize both water uptake and overall performance [99].

Ion Exchange Capacity
The ion exchange capacity of a membrane is closely linked to its water retention ability.Usually, the higher the water retention capacity of a membrane, the higher its IEC.This happens because water helps in the exchange of ions, making it easier for the membrane to transport a larger number of ions.In addition, water retention is critical for the transportation of protons through membranes.Protons, produced in the anolyte due to bacterial activity, are moved with water through the membrane to the cathode.Essentially, membranes that can hold more water enable the transfer of a larger number of protons.To sum up, the higher the water retention capacity of a membrane, the higher the proton diffusion and water uptake.

Power Density
Gurjar and Behera conducted a study that showed a significant difference in power density between leachate and kitchen waste slurry treatments.Leachate exhibited a 20-fold higher power density compared to KW slurry treatments.The increase in substrate concentration positively influenced power density, which is consistent with observations from leachate treated MFC systems.However, the MFC subjected to higher organic loads (LW-5) showed a 35% decrease in power density compared to LW-4.This decrease was attributed to alternative metabolic pathways compromising exoelectrogen growth in the batch operation mode of the EMFC [96,109].
Gunaseelan et al. achieved a remarkable power density of 275 mW/m 2 using SUPER-MIX BPVs with a ceramic separator, which is comparable or superior to BPVs with heterogenic APBs and Nafion separators.This is contrary to previous studies that used mixed anoxygenic photosynthetic bacteria (APBs) and a Nafion-117 membrane [94].Raychaudhuri et al. compared separators containing 0% and 30% silica, achieving a maximum power density of 98.7 mW/m 2 when operated with real rice mill wastewater (Table 2) [95].
Obasi et al. reported a maximum power density of 82.4 mW/m 2 under optimized conditions.Cheraghipoor et al. observed a doubling of power density compared to the Nafion membrane by leaching Kalporgan soil to enhance ceramic membrane conductivity [105].Sarma and Mohanty noted significant improvements in power density and current density with acid-treated carbon fiber modification [91].
Pasternak et al. [60] reported superior power performance with ceramic materials, particularly earthenware and pyrophyllite with higher SiO 2 content.Neethu et al. achieved a remarkable maximum power density of 3.7 W/m 3 with MFC-ACCS/Clay, nearly twice that obtained using a Nafion 117 membrane [99].Lastly, Bagchi and Behera [97] reported a maximum power density of 142 mW/m 2 from a reactor with a separator containing 20% montmorillonite, which is comparable to the highest power generated in the current study at 100 mW/m 2 .

COD Removal
The provided dataset contains studies that assessed the effectiveness of microbial fuel cells in wastewater treatment by measuring COD removal efficiency.The range of COD removal percentages across these studies indicates a spectrum of treatment outcomes.The study conducted by Sarma and Mohanty reported the highest COD removal percentage of 98.41%, which is significantly higher than the values reported in other studies, such as the 96.6% COD removal efficiency reported by Cheraghipoor et al. and the 95.67% achieved by Yousefi et al.The disparities between these figures become apparent upon closer examination, with Sarma and Mohanty's efficiency being approximately 1.81 and 2.03 times higher than that of Cheraghipoor et al. and Yousefi et al., respectively.This comparison highlights the need to refine operational parameters in MFC systems to maximize treatment efficiency and underscores the potential for further advancements in wastewater treatment technologies through targeted research and development endeavors.

Coulombic Efficiency
Analyses of Coulombic efficiency in MFCs across various studies have revealed notable differences influenced by operational parameters, electrode configurations, and membrane materials.For instance, in the study by Cheraghipoor et al., a significant increase in CE, from 53% to 83%, was observed when shifting from an MFC using a Nafion membrane to one employing a ceramic membrane derived from leached soil [98].Similarly, investigations conducted by Das et al. showed higher CE values for specific MFC setups, with MFC-B achieving a CE of 10.2 ± 1.3%, representing a 34% improvement compared to MFC-N (7.6 ± 1.0%) and a 70% enhancement relative to MFC-A (6.0 ± 1.0%) [101].This trend persisted in studies like that of Das et al., where MFC-G5 exhibited the highest average CE of 23.74 ± 2.14%, surpassing MFC-N and MFC-C by approximately fivefold and 8.6%, respectively [102].Furthermore, findings from Obasi et al. showcased a CE of 15 ± 7.5% in an MFC operating under a 3 Ω external load [105].

Conclusions and Future Perspectives
Ceramics is a cost-effective material that works similarly to conventional membranes in wastewater treatment using microbial fuel cells.It can be used as a separator in MFCs because it carries both protons and ions.This review focused on how clayware, earthen, and ceramic membrane systems perform in MFCs for the treatment of wastewater and sustainable energy.
Various ceramic membranes are extensively studied for their role as separators in MFCs.Their properties, such as porosity, proton exchange, and electrical conductivity, have a significant impact on membrane performance.Compared to other materials, ceramic membranes have been found to exhibit superior outcomes in power density, current density, and COD removal.Materials such as red soil, Kalporgan soil, CHI/MMT nanocomposites, and bentonite clay have shown adaptability and long-term efficacy in treating diverse wastewaters.However, to optimize electron transfer dynamics and MFC efficiency, modifications like double-chamber setups, tailored inoculums, and specific electrode material selection are required.Despite complexities, ceramic-based membranes show promise in enhancing MFC efficiency, underlining their significance in wastewater treatment and sustainable energy generation.
Moreover, the properties of anode and cathode materials play a crucial role in MFC efficiency, with ceramic-based membranes showing significant promise in improving overall performance.When considering the cost of MFCs, dual-chamber configurations tend to be more costly than their single-chamber alternative.The membrane components contribute up to 60% of the overall cost of the MFCs.Therefore, it is important to explore single-chamber setups that use cost-effective ceramic-based membranes to improve the performance of the membrane.While this technology holds great promise, further study is required before its commercialization.

Figure 1 .
Figure 1.Number of published papers per year related to MFCs.Figure 1. Number of published papers per year related to MFCs.

Figure 1 .
Figure 1.Number of published papers per year related to MFCs.Figure 1. Number of published papers per year related to MFCs.This review aimed to evaluate the low-cost membrane development for microbial fuel cells over the past 8-9 years.It explored various MFC configurations, analyzed membrane characteristics, and examined how other factors influence their performance.The objective of this review was to identify the most suitable and cost-effective membrane options for MFC applications by assessing various membrane types based on factors such as energy recovery, current density, power density, and proton transfer efficiency.The study also

Figure 2 .
Figure 2. Different membranes per year related to MFCs.

Figure 2 .
Figure 2. Different membranes per year related to MFCs.
environment.S-MFCs are flexible and simple, and they come in a various of configurations, as shown in Figure4[16].

Table 1 .
Comparison of ceramic membrane properties.

Table 2 .
Overview of based on ceramic membrane MFC.