Keywords

1 Introduction

An eco-friendly and green environment is one of the necessities for human beings to live a healthy life, but due to discharge of industrial effluents containing irorganic and organic pollutants into water bodies, results in water contamination which has adverse effects on human beings and other aquatic life [1,2,3]. These crises are well countered by the microbial fuel cell approach due to its unique properties of achieving energy and wastewater treatments [4]. The microbial fuel cells (MFCs) approach is an innovative step in research to convert toxic chemicals into non-toxic chemicals and convert chemical outputs to electrical outputs in the form of energies by using various catalysts (e.g., bacteria) present in wastewater [5, 6]. There have been significant advancements in wastewater remediations and power outputs. MFCs have yet to be commercialized due to their low energy generation and removal efficiency. Low energy consumption or removal efficiency can be caused by a variety of factors, including the utilization of low-graded materials as working electrodes or material cost issues. Because it offers the essential surface area for bacterial proliferation, the working electrode seems to be the most critical section of MFCs. These bacteria produce electrons and protons and transferred it to the anode. Fabrication of anode materials for MFCs functioning, on the other hand, remains difficult. Recently, there has been a surge of interest in electrode configurations, materials, and design that results in steadily increased performance of MFCs [7]. Electrode grading materials should have a few general features to face high-performance criteria, including high conductivity, comparable biocompatibility, stable thermal temperature, chemical and electrical stability, mechanical strength, and an expanded surface region. MFCs use a variety of electrode graded materials, however, these graded materials have several limitations that are unsuitable for industrial usage [2, 8]. According to a prior study, electrode modification has emerged as a novel point in the field of MFCs for achieving an expansion of surface regions, bacterial adherence, and have the ability of electron transference

In MFCs, the electrode is known as the cathode and anode in which anode plays a vital role to transfer bacteria toward cathode to generate electricity. The materials used in the fabrication of anodes have some limitations and till now, not applied at large scale. In literature, El Mekawy [9] et al. mention that anode is the crucial part of MFCs fabrication approaches. As in our knowledge that many researchers started their study on graphene derivatives as modifiers or enhancers to provide a good performance of MFCs on electrode surfaces either as anode or cathode. From one of the previous reports [9], the authors come to the conclusion that graphene derivatives based or carbon-based materials electrodes as cathode or anode are shown as superior and emerging materials for electrodes in MFCs. These materials provide a new dimensions to the researchers working in same area due to cost-effective and efficient materials [10]. Nowadays, the most usable graphene derivative, graphene oxide, is easily fabricated through industrial and domestic waste materials. Moreover, the issue of corrosion or the influence of toxic bacteria on MFCs is resolved by using polymer layers or metal oxide layers with graphene derivatives. The modifications of polymers and metallic composites not only resolved the corrosion issues, but also increase the working performance and efficiency of the electrode in MFCs [11, 12]. It increased the conductivity, biocompatibility, and stability between bacteria and electrodes either as anode or cathode. That’s why it is an up-to-date approach to modify electrodes with graphene or carbon-based derivatives to achieve better performance. In this chapter, we reviewed the different types of electrode materials with their unique properties of surface modifications, sizes, designs, electricity generation, and inoculation sources. To discuss the significance of biomass wastes as an emerging materials, many ideas and sources based on electrode fabrications have been summarized in this chapter. Furthermore, the effects of electrodes on wastewater remediation and energy processing are discussed, together with new difficulties and prospects.

2 Essential Properties of Electrode Materials

It is foremost and essential step in MFCs to investigate the unique properties of electrode materials in terms of achieving steady electron mobility, electrochemical efficiency, and bacterial adhesions between system and electrode materials [13]. Some of the unique properties which helps us in this matter is mentioned in this chapter to understand the reproducibility of working of MFCs.

2.1 Conductivity of Material

It is an important aspect of electrode because the electrons due to bacterial adhesions travel from negative terminal to positive terminal via the channel of outer circuit. As in literature, the electrode material is in charge of allowing electrons to flow and enhance their speed [14, 15]. The more highly conductive materials are more helpful in resisting the bulk solutions resistance and increasing the electron transfer rate [16]. To improve electron transfer, lower the interfacial impedance between substrate and electrode as mentioned in the literature [17, 18]. Before constructing the electrodes for MFCs, the electrical conductivity of materials is typically investigated.

2.2 Physiological Properties

The surface regions of the electrode severely affects energy generation in MFCs [19,20,21]. Because resistivity of electrodes depends on ohmic losses in a MFCs, increasing its surface area is the given suggestions in reports to minimize resistance power. More active sites are gained with an expanded surface regions for bacterial colonization and improves the efficiency of the electrode kinetics. Microorganisms, for example, Geobacter species, E. coli, Pseudomonas species, and others were immobilized efficiently on the active regions of electrodes, allowing for suitable electron transfer [20]. Because biological responses occur on the active regions of the electrodes, the surface regions has a significant impact on MFC performance [22].

2.3 Material Biocompatibility

The anode electrode's biocompatibility is critical in MFC operations since it comes into direct contact with microscopic organisms and their respiration cycles. Many materials utilized as electrodes in MFCs, such as silver, gold, and copper, are not considered biocompatible due to their corrosive nature [23,24,25]. The poisonous nature of such compounds can prevent bacterial development during MFC operation, resulting in lower energy generation.

2.4 Stability and Durability

In case of any research-based system, the stability and durability are one of the most significant points for your research. Various environmental factors affect the stability and durability of electrode. The decomposition, corrosion, and swelling are caused due to interaction of electrode with environment that affects its stability and duration of working performance [23, 26, 27]. Thus, the use of more preferable material as an electrode makes your system more durable and stable for good performance.

2.5 Cost and Access of Material

The cost and access of material is a key factor to approach your work without stress and also opens the feasible ways for other researchers. And moreover, the cost of electrode also has chance to provide MFCs system with easy and cheap approach as compared to expensive and heavy approaches. In present time, carbon derived graphene based composites have been widely used due to easily available and low cost. The expensive metal composites such as gold, silver, platinum are also replaced with inexpensive bimetal composites such as ZnO, Fe2O3, etc. [28, 29].

3 Electrode Materials

Removal efficiency of pollutants and energy production have been optimized to investigate the effectiveness of electrode materials. The electrode materials, as mentioned above, has some of the basic properties such as high stability and conductivity, more compatible than other materials. For this purpose, we investigated some electrode materials in two categories as electrode material as anode and electrode material as cathode.

3.1 Electrode Materials as Anode

From previous studies, it becomes clear that there are various types of materials used to fabricate anode of MFCs in terms of large surface area to increase the extracellular efficiency of electron transfer via biofilm. Moreover, the anode materials have more significancy because it is useful for metabolic rates in oxidizing organic wastes by anaerobic microorganisms [30,31,32]. It is notable that the kinds and concentration of bacteria have great effect on power density of MFCs, but now, it is also proven that the anode materials have also significant feature for MFCs to work better. Thus, fabrication of the anode materials through various chemical modifications, must be taken an account in the future to enhance the capacity of anode. Some of the generally used sources are composite materials, allotropes of carbon, conducting polymers, or metal or metal oxides, which seem to have significant value to be worth materials for anode fabrication.

3.2 Carbon-Graded Materials

Nowadays, carbon-derived materials are gaining more attention due to their unique properties such as low cost, chemical and mechanical stability, high electron transfer kinetics, biocompatibility, and highly conductive in nature. By studying the recent literature, it is well known that different types of carbon-graded materials like graphite, carbon nanotubes, fullerenes, carbon nanorods, carbon cloth paper, carbon fiber, reticulated vitreous carbon, glassy carbon, and carbon quantum dots have been investigated. The latest carbon-based materials which are now an emerging class of carbon allotropes are graphene and its derivatives.

Carbon paper, brushes, rods, felt, fabric, meshes, and other carbon-graded materials are normally utilized materials in MFCs. A carbon mesh is somewhat more affordable than other carbon structures, as indicated by Wang et al. [33] and it likewise has a higher current thickness. On the other hand, modification of carbon meshes with alkali or gas give good results. Therefore, no untreated material presently conveys a more powerful thickness. Borsje et al. [34] investigated the functioning of single carbon granules as capacitive bioanodes. Charge stockpiling execution and current creation via solitary carbon granule was utilized to decide the outcomes. The bioanode stored the charge in the form of two-fold sandwich. To assess the undiscovered capability of granular bioanodes, scientists utilized granular and initiated graphite carbon granules. In contrast with Ag/AgCl anodes, single enacted carbon-graded granules create 0.6 mA at 300 mV. Capacitive granules produce 1.3 times extra electricity as compared to graphite granules at the lower surface regions [35,36,37]. Li et al. [38] investigated granule-activated carbon, which delivered twice more energy than the customary carbon materials. According to the findings, granule-actuated carbon could be a viable alternative for anode preparation. Carbon cloth/sheets are flexible and allow bacteria to grow on their surface. It is, however, much more expensive at larger scales [39]. Actuated carbon cloth has an expanded surface region and suitable adsorption capacity for the expulsion of sulphide in electrochemical oxidation at the anode. Wang et al. [40] arranged carbon cloth that provides an increment of the current effectiveness of 2777.7 mW/m2. Doped with nitrogen gas, the carbon cloth produced high power production, and it could be valuable for future researches. Likewise, graphite is one of the regular forms utilized for an electrode in MFCs. Graphite is known as a crystalline allotrope of carbon with Sp2 hybridization. MFCs use graphite as an anode because of its good conductivity and long-term stability. For the production of electrodes, different forms of graphite are effectively used [41,42,43]. Ter-Heijne et al. [44] observed the raw form of carbon for the electrode in MFC rather than flat forms, which showed higher current density. But they have a low surface region and high cost which makes this material inadequate for commercial use in the production of energy. The graphite brush as the best model for electrodes with the best performance to be used as anodes in MRCs for improved energy generation and toxic pollution removal was reported by Lowy et al. [45]. Yazdi et al. [46] later reported that the rate of bacterial colonization on the electrode’s surface is proportional to the anode's surface area. In another study, Zhang et al. [47] found a category of graphite brushes within the range of sizes. Little brushes can deliver more energy output than bigger ones. 1771 mW/m2 small value of power density is also reported in the Cassava mill by graphitic brushes in wastewater remediation [48, 49]. Bacteria feed on organic material and flourish in environments with lots of carbon because of its increased particular surface area [50]. Yasri et al. [51] created an efficient anode material by doping graphite with calcium sulphide to promote bacterial interaction with the active regions of electrodes. In the modern period, graphene, a newly developing carbon allotrope (found in a 2D hexagonal lattice), has earned a lot of interest. With its emerging features of outstanding conductivity and mechanical and thermal strength, graphene is an ideal material for electrode construction. When compared to graphite materials, graphene possesses a nonlinear and better diamagnetism. Graphene and its derivatives, on the other hand, are still being studied as anodes in MFCs [52]. Graphene has been synthesized using a variety of processes. Commercially available graphene is expensive, whereas graphene made from waste materials is less expensive [53,54,55,56] Due to its more energy generation relative to other typical carbons, graphene as an anodic material enables high scale functioning for MFCs. Graphene-based electrodes have better electrode efficiency as anodes than conventional carbon-based electrodes [57]. During MFC operations, graphene has non-toxic impacts on bacterial growth. As a result, by modifying or combining it with conductive polymers and metals, drawbacks of other materials, like copper, can be reduced [58, 59]. Modified carbon allotropes have the potential to revolutionize wastewater treatment and energy.

3.3 Natural Biomass Source as Anode

The properties of the electrode materials vary significantly in terms of physical, chemical, and biological nature. The electrodes require electrical, and specifically microbial, compatibility with specific bacteria strains to affect the movement of electrons, just as surface opposition of electrodes [60]. Nonetheless, electrode materials, fabrication, and processing have been recently becoming a popular and up-and-coming research area. MFCs use waste materials for construction. Changing waste materials into worthy and valuable materials is time-consuming and somehow effective in contrast with commercial materials in a few features [61]. Cheng et al. [62] researched a waste-inferred decreased graphene (rGO) composite for anodes to accomplish more powerful outcomes as far as energy age and wastewater treatment by means of MFCs. Utilizing dried eucalyptus leaves as waste material, the rGO was prepared successfully. Later, rGO/gold nanoparticle nanocomposites were fabricated by layering for the manufacturing of biocompatible anodes. The electrode prepared in this study has a higher surface roughness, which facilitates bacterial colonization. Gold nanoparticles are considered as a highly electroactive agent which tranfer the electrons and produces electricity at negative terminal. Singh et al. [63] prepared an effective electrode for MFCs using carbon nanoparticles derived from candle soot. The candle sediment was disseminated on the outer layer of a hardened steel circle, which permitted the carbon nanoparticles to be utilized as cathodes straightforwardly. The consequences of the electrical, physical, and compound portrayal of an anode’s mechanical, chemical and electrical strength are just as progressively permeable qualities. The production of carbon nanoparticle electrodes from candle soot is reusable, budget-friendly, robust, and dependable. Bose et al. [64] have also used biomass to manufacture a bioenergy active carbon cathode via MFCs. This was one of a kind method of generating electricity and treating water that had no negative environmental consequences. Platinum is commonly utilized as an impetus for oxygen decrease at the terminal of the cathode. In terms of reliability, functionality, and prices, the authors examined the effectiveness of actuated carbon derived from sugar cane waste. At different temperatures for 60 min, this useless material followed the carbonization process. Electrodes derived from various biomass sources are considered as an alternative for the treatment of pollutants from wastewater with electricity generation simultaneously. As we know that in MFCs, there have been only a small number of publications on the source of biomass anodes. That’s why the concept of reusability of biomass is a viable substitute for enhancing MFC's working efficiency with no high costs. Graphene and its derivatives can easily be produced by numerous methods like chemical vapor deposition, arc detection, epitaxial growth, scotch tape, electrochemical synthesis, reduction of GO/rGO, exfoliation, confined self-assembly, and Hummer's method. Its favorable points over other methods made it the most important and promising method. This is an eco-friendly method, for example, without producing harmful gases during processing, with a structured product, and with a larger output supplied. Hung et al. recently used [65] a coffee-based renewable waste anode in MFCs to expand the power thickness. The authors have transformed waste material into precious carbonized materials and have used it to lessen squander from the environment as an anodic material in MFC. The energy density achieved was 3800 mW/m2, much higher than traditional techniques. In our vicinity, various types of waste materials cause serious dangers. Therefore, the use of biomass waste materials as valuable materials is a positive approach. In Hummer's process, however, various useless materials are carbonized to obtain fine carbonated powder materials affected by argon gas at 1050 °C. The graphic powder is treated to obtain graphene oxide with the oxidizing agent KMnO4/H2O2. Fabricated graphene oxide can also be used to manufacture the graphene oxide material in anode-shaped electrodes with polymer binders like nafion, polyethyleneimine, and polylactic acid [66]. The graphene oxide synthesized can be utilized as positive or negative terminal material, however, its use as the anode is preferable, as previously stated. This type of modification may enhance the materials efficiency and reduces the expenses. The use of composites synthesized for low-cost use with metal oxides such as CuO/GO, ZnO/GO, etc., is an optimal way to deal with various difficulties. Table 1 summarizes the electrodes produced in recent years using natural biomass resources.

Table 1 List of electrodes synthesized using natural waste resource for MFCs
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3.4 Metal/metal Oxide-Sourced Materials

Different materials were utilized to fabricate metal/metal oxides based anode–cathode, but consumption restricts the utilization of metal-sourced terminals, especially for MFC anodes. Metals are commonly penetrable than carbon-graded materials because of their capacity to work with proficient electron stream [76]. While each metal has exceptional properties, not all metals are reasonable for cathode creation because of the noncorrosive necessities of the interaction. Also, certain metals repress bacterial bonds. For instance, in contrast with other carbon-graded materials, for example, graphite and graphene, non-destructive tempered steel materials don't have a powerful thickness. Overall, the smooth surfaces of metals are not helpful for bacterial grip. Predefined non-destructive materials, like tempered steel, can't accomplish higher energy thickness than materials dependent on carbon. At the anode chamber, stainless steel had a power density of 23 mW/m2 [77]. An anode-based stainless-steel grid increased the relative current density of a single electrode of graphite [78, 79]. Silver, platinum, gold, and titanium are ideal anode metals. While noble metal-based anode electrodes contribute to the reduction of interior obstruction in MFCs, their significant expense and poor bacterial grip block their far and widespread use in MFC operation [24, 80]. Platinum and titanium are commonly suitable as catalysts to enhance electrode performance [81]. Moreover, commercialization of some of pure metal-based anodes in MFCs have some limitation due to their high expense. The reactivity of metallic nanoparticles and transition metals is comparable to precious metals, altogether decreasing obstruction and working on microscopic organisms’ connection to surfaces. Additionally, nanometallic particles offer an excellent opportunity to diminish the impact of harmfulness on bacterial cells [82]. These issues are mitigated by coating metal/metal oxide nanoparticles (Ag, ZnO, etc.) with comparable materials such as carbon-graded or polymers.

3.5 Polymer Composite Material

Various conductive polymers, namely polypyyrole, polyindoles, polythiophene, polycarbazoles, polyaniline, polyadenines, etc., were used in terms of highly conductive materials at anode surfaces on the basis of their efficient conductive properties [83,84,85]. The combination of carbon-based materials and conductive polymers produce very efficient and good results. As shown in previous reports, the polyaniline-modified carbon cloth produced more power production than unmodified carbon cloth [86]. In another report, one of the most important conductive polymers, polypyyrole with the layer of carbon paper, showed a 452 mW/m2 power output [87]. To our knowledge, Polypyyrole can enter bacterial cell membranes and transport electrons via metabolic pathway easily [88]. Thus, polymer composites combined with different materials, similar to carbon-graded materials and metals, significantly further develop anode productivity. For example, Dumitru et al. [89] investigated two polymers such as polypyyrole and polyaniline with CNTs as a nanocomposite anode. Due to their synergistic effect, CNTs and conducting polymer nanocomposites perform justifiably well enough in electrochemical applications [90]. The use of conductive polymers (especially polyaniline and polycarbazole) with metal oxide composites could significantly improve MFC performance [91,92,93] But despite more researches, there is little exertion that has been made to plan polymeric composite-based MFC electrodes. Figure 1 depicts common electrodes such as conductive polymer, metal, and carbon electrodes.

Fig. 1
Thirteen images of commonly used electrodes labeled a to m. They are carbon paper, carbon cloth, carbon fiber, reticulated vitrified carbon, carbon mesh, graphitic granular, carbon brushes, graphite rod, polycrystalline graphite, carbon felt, platinum mesh, different metal electrode strips, and conductive polymer-based strips, respectively.

Adapted from reference [25] with MDPI permission

List of commonly used electrodes: a carbon paper, b carbon cloth, c carbon fiber, d reticulated vitrified carbon, e carbon mesh, f graphitic granular, g carbon brushes, h graphite rod, i polycrystalline graphite, j carbon felt, k platinum mesh, l different metal electrode strips, and m conductive polymer-based strips.

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4 Electrode Materials as Cathode

Despite the anode (negative terminal), the cathode (positive terminal) material has also a significant place in the functioning of MFCs. Nowadays, the most widespread material for the cathode is carbon-based, but their features like size, model, and efficiency for cathode materials are challenging as compared to anode material [94,95,96,97]. The mostly reported anode materials are also used as cathode material. Due to deprived catalyst activity, reactions to reduce substrate commonly occur in the cathode section, reducing MFC performance [98,99,100,101]. The cathode terminals can be derived as with catalyst or without catalyst. The main distinction between these setups is the spark. Platinum and titanium are the most commonly used catalysts. A terminal named air cathode is directly influenced by oxygen [102]. The setup has drawn attention for its lack of aeration, functional simplicity, and appropriate electrode design. An air cathode can significantly expand the energy effectiveness through MFCs [103, 104]. Aqueous air cathodes use conductive materials like platinum meshes and carbon felt, cloth, and fiber to form electrodes. The catalyst is sandwiched with aqueous regions in low oxygen contact [105]. As an air cathode, carbon-derived forms are the most ideal conductive material. Catalysts (platinum, copper, etc.) are fixed to electrodes using binders [106, 107]. Poly(tetrafluoroethylene) and perfluorosulfonic acid are popular binders (nafion). Zhang et al. [108] compared the performance of articulated carbon and its derivatives as cathode utilizing poly(tetrafluoroethylene) for binding. In the presence of Pt as a catalyst, articulated carbon outperforms carbon cloth (1220 mW/m2) in terms of power density. So articulated carbon seems to be a good cathode material substitute for the fabrication of a positive terminal. Zhao et al. [109] employed catalyst Pt combined carbon derived as a motivating factor. According to the latest findings, this catalyst has a power efficiency of 1.2 W/m3. Cu is a preferable catalyst to Pt at lower temperatures due to its sustainable power. Under normal conditions, Pt is considered better and more generally known catalyst than other metals [110]. As a result, the materials utilized in the fabrication of positive and negative terminals (cathode/anode) can be performed as oxygen reduction catalysts. Due to their low overpotential, gold and platinum are considered potential catalysts, but their expensive cost makes them unsuitable [76, 111, 112]. Transition metals are cosidered as a alternative materials to fabricate potential electrode due to high stability, affordable and avoid any disruption in the microbial fuel system. Composite materials, known as molybdenum and carbide, perform well, but stainless steel and nickel alloys outperform them all [113]. Nanocomposites, on the other hand, are less expensive and provide a significant chance to boost MFC efficiency (for example, Ni and palladium nanoparticles/nanomaterials) [114]. In comparison to conventional materials, nanomaterials have an expanded surface region, superior electrochemical functioning, and stronger thermal and mechanical durability [115]. To improve the oxygen reduction reaction, a recent trend involves modifying the electrode using additional materials. According to the literature, fresh materials must be studied in order to improve the feasibility of electrodes, particularly anodes. Utilization of high-graded materials for anodes, for example, graphene and its derivatives with metal oxides, could usher in a major shift in the MFC area. The preferable composites are GO/Ag, GO/Fe2O3, GO/ZnO, GO/chitosan, and GO/TiO2, all of which have a significant influence on power outputs. In addition, Table 2 lists the many types of classic carbon-graded materials, composite-based, metal/metal oxides, and Carbon-based + Polymer composite that can be utilized as electrodes (anodes and cathodes).

Table 2 Some reported materials used as electrodes in microbial fuel cells
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5 Influence of Electrodes (Cathode/Anode/) in MFCs

In the presence of a biocatalyst, the electrode (anode/cathode) is a critical component due to its unique feature of assisting in the remediation of hazardous agents and generation of energy during MFC operations. During the respiration process of bacteria, the bacteria are cooperated with the electrode region to create protons and electrons. As seen in Fig. 2, the electrode provides enough surface region for bacteria to proliferate and oxidize. The performance of the anode as compared to the cathode provides MFCs with high electric production, wastewater bioremediation, and compactable economic features.

Fig. 2
A diagram illustrates the functioning of an electrode. On the left, it illustrates glycolysis with an arrow towards N A D H and electron. It also depicts the Krebs cycle with an arrow that points to an electron via N A D H 2 or F A D H 2 and N A D positive or F A D positive. On the right, it has an anode and cathode circuit.

Adapted from reference [25] with MDPI permission

Functioning of the electrode in MFCs.

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6 Influence of Electrode (Anode/Cathode) on Removal of Pollutants

MFCs are thought to be a particularly efficient prospective use for wastewater bioremediation. Many traditional wastewater treatment technologies have been described, but they all have significant limitations such as high prices, being difficult to run, the possibility of self-toxicity, and being unstable in terms of ecosystem safety [51]. Fossil fuel industrial wastewater, scum wastewater, aquaculture wastewater, cassava mill wastewater, food processing waste, dairy wastewater, crop residues, and surgical cotton waste, are all examples of wastewater that could benefit from the MFC approach [151]. Organic agents are oxidized to generate electrons and protons in the chamber of the anode via exoelectrogens, thereby destroying the hazardous organic pollutants in water [152, 153]. Protons were transmitted directly to the cathode or via membrane sources, while electrons were transported via the outer circuit. The electrodes’ functioning efficiency is crucial to this procedure. The electrodes offer bacteria a surface area for respiration and growth, making it easier for electrons and protons to be transferred to the negative chamber through bacteria and ultimately to the positive chamber.

Zhang et al. [154] investigated the suppression of two elements with the implementation of electricity utilizing vanadium-sourced water with waste as an electron acceptor in dual terminal microbial fuel cells. V(V) and Cr (VI) are primary metals found in vanadium-sourced effluent, both of which are highly hazardous and abundant. Qiu et al. [155] reported vanadium based biocathode and got 60% fatality rate through MFC in presence of Dysgonomonas and Klebsiella. The power density of MFCs after seven days of operation with a 200 mg/L starting concentration of anaerobic sludge was 529 12 mW/m2. Jiang et al. [156] looked at wastewater from the oil sands process to see if MFCs could create electricity while also treating oil sand tailings. The MFCs cleaned various heavy metals from wastewater derived from the oil sands process with constant energy production and with good outputs of efficiencies percentages. The removal efficiency was somehow low due to the usage of carbon derivatives as cathode and anode. For a variety of reasons, the carbon fiber felt outperformed the carbon cloth, but graded anode and surface region available to bacteria were essential. Habibul et al. [157] utilized a graphite manufactured anode to research electro kinetic biosorption of heavy metals from disturbed soils, in order to improve the anode's quality. However, research into the breakdown of particularly harmful metals such as cadmium, lead, and mercury is scarce. Bacteria require high-quality anode materials to digest harmful elements from the water supply. Similarly, the researchers utilized various MFC used anodic chamber agents to decolorize the organic dyes that were damaging the ecosystem. Fang et al. [158] investigated the potential of MFCs which are made of activated carbon and a cathode built of stainless-steel mesh to process azo dye. The decolorization rate was high due to articulated carbon serving as an anode. To decolorize methyl orange from anaerobic sludge, Kawale et al. [159] utilized a graphitic rod as an anode to decolorize methyl orange from anaerobic sludge. Both decolorization and energy output were significantly influenced by the electrode. However, several researchers used MFCs with various anode materials to remove organic contaminants. Kabutey et al. [160] utilized a macrophyte cathode silt microbial power module to research the evacuation of natural impurities and energy age from metropolitan waterway dregs. Carbon fiber was utilized as both cathode and anode terminals in this investigation, with an expulsion proficiency of 28.2%. Microorganisms like Euryarchaeota and Proteobacteria were unable to separate phosphorus due to its acidic nature and inefficient ability of cathode used in this work. Marks et al. [161] investigated the functioning of MFCs in anoxic surroundings and found that they could remove 22% of nitrate from anaerobic sludge. The cathode and anode electrodes in this experiment were graphite plates. According to an exhaustive literature study, the authors determined that different types of anode materials are utilized under different situations since MFC functioning is influenced by a variety of parameters. One of the most significant functions of an anode is to give bacteria sufficient surface area for respiration while also assisting them in carrying electrons from colonization of bacterium to the cathode via an outer circuit. As a result, it has been shown that employing a high-graded anode will yield superior outcomes with less environmental constraints. Many difficulties that cause disruption during processing, such as long-term stability, might be addressed using this high-quality material. To reduce metal corrosion, we may fabricate anode more efficiently with the help of conductive material and high surface regions materials like composite materials, graphene, and its derivatives, containing metal/metal oxides. As a result, in order to get better outcomes for remediation purposes, the anode should be unique and efficient.

7 Influence of Electrode (Anode/Cathode) on Energy Production

MFCs as an innovative approach opened new avenues in the domain of ecosystem pollution and its safe elimination. MFCs generate energy from various organic waste materials using microorganisms as exoelectrogens [162, 163]. In Single chamber MFCs, 3D terminal materials and the improved anodes are utilized for the generation of energy. [128]. Many materials and operating parameters were regularly modified at the start, making it difficult to pinpoint the aspects that helped improve the present generation over traditional approaches. In view of the progression of this framework, more consideration is currently needed to produce a more prominent electrical yield [164, 165]. The electrode is straightforwardly connected to the creation of power. The production of energy rises as the electrode's strength and conductivity improve. Wang et al. [96] additionally demonstrated proficiency of carbon felt as an anode within sight of platinum in form of impetus, but force creation was exceptionally low. Zhang et al. [154] utilized the incorporated adsorption method to separate chromium from anaerobic assimilation ooze and had the option to accomplish a current force of 343 mV. Utilizing engineered arrangements, Liu et al. [166] explored MFC execution utilizing carbon fabric as both terminals within sight of Fe/Ni/actuated carbon as an impetus and delivered remarkable energy yield. To improve the material, Santoro et al. [131] utilized graphite brushes within sight of Pt impetus SMFCs to accomplish a high energy yield. In the wake of utilizing local wastewater as an inoculum source, a force thickness of 1280 mW/m2 was reached. The performance of the electrodes determines the amount of energy produced. Carbon felt, for example, has a lower surface area and conductive efficiency than graphite-based materials. As a result, graphite-based materials produce multiple times the results of carbon-derived items. Nguyen et al. [167] reported a novel method to develop a high quality anode’s material. Zhang et al. [168] recently published a paper describing the outstanding electrochemical presentation of MFCs with an allotropic form of carbon named graphene oxide for electrode enhancing performance. When contrasted with other carbon-based materials, graphene oxide further developed electron transport and created more energy. Therefore, graphene is preferable and encouraging material for the fabrication of electrode (anode/cathode) in MFCs.

Natural assets, on the other hand, are used in current research for anode fabrication since they are practical and elite materials when contrasted with manufactured materials. Yang et al. [169] found that banana strips and underwater wetland dregs, which were utilized as an inoculum hotspot for MFC activity, straightforwardly created energy. Accordingly, utilizing regular materials as terminals (anodes/cathode) is a viable answer for tending to introduce difficulties and orchestrating excellent anode materials, like GO and derivatives modified with metal oxides. The attributes of the anode can be improved by joining GO composites with metal oxides. Anodes made of ZnO/GO, Fe2O3/GO, and CuO/GO are generally utilized in MFCs to acquire high power execution. From the last few decades, low-cost anode and cathode materials have been developed for the removal of toxic pollutants with energy generation in MFCs system (Table 3).

Table 3 Influence of the electrode (anode/cathode) on removal efficiency and energy production through MFCs
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8 Challenges and Future Recommendations

Regardless of the multitude of advancements in MFCs, mainstream researchers actually face numerous difficulties and issues as far as power age and aqueous treatment. It must be evidently quick advancement in designing MFCs as productive and preferable. Besides, reactors of various plans have already been presented, such as one and two-fold chambers, film less, H-shape, and rounded MFCs [191, 192]. Basically, the primary objective of all improvements is to accomplish commonsense execution of MFCs for remediation purposes at a business level. The principal segment in MFCs is the anode, that additionally dependable somewhat for their financial and functional capability. There are a few challenges related to electrode (anode/cathode) that have reduced the use of MFCs on a modern level:

  1. 1.

    The electrode components are crucial for the monetary province of MFCs. Thusly, removing costs for materials is a significant issue for executions in MFC applications. To resolve this matter, we ought to consider the waste material sources and converted them into carbonized structures that can be furthermore used as terminal material in a couple of constructions, similar to posts, brushes, bars, and plates. Nevertheless, one more technique is the improvement of composites with metals and utilizing polymers to fabricate them more compelling at an insignificant cost [193].

  2. 2.

    During the creation of an electrode, the cover is indispensable for assembling material in its ideal shape. To confirm the assurance of an astoundingly essential factor for researchers to develop materials to make it firmer and steadier. It is alluring to find more sensible and spending plan agreeable folios for the terminal (anode/cathodes).

  3. 3.

    The size and setup are imperative viewpoints in the creation of electrodes. The surface area of electrode play significant role in bacterial growth and electron transferred from negative to positive terminal in MFCs [194].

  4. 4.

    Adjustment of the electrode has made critical redesigns regards to power age and the bioremediation of wastewater. So that, material parts and fitting standards stay dim. Researchers ought to discover more authentic parts for preferable adjustments.

  5. 5.

    One flaw is the long-term steadiness of electrodes at the bulk level. At present, for the genaration of energy, nobody has yet explored the strength of electrode for long term use [195, 196]. Steadiness is a significant issue that resists MFCs functioning at a mechanical scale. Accordingly, scientists should carry on tracking a compelling manufacturing procedure for electrodes while remembering the strength factor for electrode materials. An exceptionally steady fastener like nafion or polysulfides can be utilized to tie the graphene derivatives to keep up with long-term steadiness.

9 Conclusion

The impacts of electrode (cathode/anode) in MFCs were summarized in this chapter. Carbon-graded materials, conductive polymers, composite-based materials, and metal/metal oxide-based materials have all been proposed as electrode materials in MFCs. The adhesion of bacteria and the growth of biofilm are major areas of progress in the development of electrodes. To achieve higher biofilm densities, significant effort has been put into expanding the surface area of electrode materials. As indicated in this chapter, there are a variety of different materials proposed for use as anodes or cathodes. Notwithstanding, there is as yet a critical hole in the advancement of conceivable cathode materials. A terminal (anode/cathode) in MFCs can be made of incredibly spongy and conductive materials like metallic composites and 3D graphene. During long haul MFCs activity, cathode materials should be amazingly steady in wastewater. These qualities make a terminal more significant on a modern scale when it stays stable for quite a while. A terminal material should thusly have a huge pore size to forestall issues in the bioremediation of wastewater applications from being discouraged. The utilization of MFCs is mainly depend on the material expense and surface modification of electrode. Cheap and accessible materials and effective methods should, therefore, be introduced in the MFCs applications industry for metallic or polymer nanocomposite or carbon-based electrons. In future, the testing of upscaling of resource anodes should be a key effort. It is vital to develop an electrode/diaphragm collection for excellent membrane assembly for practical use. But the anode efficiency available is still not enough to be used on a business level. Further studies should focus on the use and optimization of waste material to fabricate electrodes.