Fuel cells have the potential to become an important energy tool for management, but their technical and economic practicality must be enhanced (Inamuddin et al., 2017). The increasing energy demand and the negative environmental implications of power production from irreversible fossil fuel usage have sparked scientific interest in using biotechnology to harvest energy from various organic waste sources. Converting such renewable organic matter using MFCs has been widely regarded as the most promising source of renewable energy among other microbial technologies (Xia et al., 2018). The technology allows for direct electricity harvesting from organics via anodic microbial catalysis, as shown in Figure 2, and has been utilized to extract electrical energy from organic sewage (Zhao et al., 2017). Energy conversion using MFCs appeared to be a promising technique for the remediation of environmental toxins in terms of efficient removal of different organic chemicals and energy recovery (Zhang Q. et al., 2018). As a result, it is a viable long-term solution to the energy and environmental crises (Saravanan, 2018). Even from organic wastes beyond their bioelectricity production, it is also possible to produce different chemicals (e.g., methane and methanol) using novel microbes through fermentation reactions (Dhiman et al., 2018). Thus, to recover energy from methane, MFCs, on the other hand, can reduce dissolved methane emissions from anaerobic effluents (Chen and Smith, 2018). To fulfill such a requirement, researchers sometimes immobilize microbes on carbon nanotubes (CNTs) to improve the electrode/anodic performance because nanotubes have a long-term electrochemical property effect, which helps enhance power output (Zhang Y. et al., 2018). Other processes such as hydrolysis and acidogenesis can also improve wastewater treatment performance and power generation simultaneously in MFCs by reducing internal losses, i.e., by minimizing the internal resistance of the cell (López Zavala et al., 2018).

MFC technology has piqued the interest of scientists over the last half a century due to its ability to convert organic waste directly into electricity using microbially catalyzed anodic and microbial/enzymatic/abiotic cathodic electrochemical reactions under moderate conditions (Santoro et al., 2017). Using bacterial metabolism, the cell device can produce alternating energy from a wide variety of organic substrates under these conditions (Liu et al., 2017). The generated current is mostly created by the metabolic activities of microorganisms. It would be beneficial to learn more about microbial systems’ ability to optimize energy extraction methods in MFCs. The most likely use of this technology appears to be large-scale coupling of such a device with treating wastewater (Krieg et al., 2017). As a result, MFCs have risen to prominence in recent years as a novel technique for converting chemical energy, especially from carbon-rich organic matter, into power generation through a sequence of energy metabolisms mediated by electrochemically active microorganisms (Chandrasekhar and Ahn, 2017). The effluents in the anodic chamber is digested by microorganisms, generating electrons (e-), which are received by the anode and transmitted to the cathode via load resistance in a standard MFC principle and setup, as shown in Figure 3. At the same time, the released protons (H⁺) created during the metabolism reach the cathode via proton exchange membranes, which are H⁺ exchangers. An electron acceptor accepts e⁻ and H⁺ at the cathode, completing the circuit and providing electricity (Capodaglio et al., 2013; Deval et al., 2017). Ion membranes—either cation or anion exchange membranes—are widely used in microbial fuel cells (Jiang S. et al., 2018). Nafion is a commercially available fuel cell ion exchange membrane; however, it has drawbacks such as low conductivity at low humidity and low-stability at high temperatures (Peo-nio-liclo, 2010). In addition to reduced cost, the advancements of electroactive-based carbon nanotube membranes are reviewed in detail (Liu et al., 2020).

For example, the bacterial microorganism Chlorella vulgaris (Huarachi-Olivera et al., 2018), which is a chlorophyte microalga, has been used to cleanse wastewater and create oxygen at the cathode of MFCs, accumulating algal biomass and creating power in bioindustrial settings. In addition to anolytes, the bioelectrochemical performance of MFCs was examined using various catholytes, such as phosphate buffer and anode effluents, in the presence or absence of C. vulgaris (Commault et al., 2017). The presence of exoelectrogenic microorganisms increases the performance of MFCs, which can be further improved by using a nanocomposite cathode catalyst (Kodali et al., 2018). Since ancient times, biomass, a sustainable fuel derived from biological materials, including plant and animal waste, has been utilized as the major energy source (Hoa et al., 2017). Due to enhanced efficiency levels in terms of energy and catholyte production, the use of ceramics as low-cost membrane materials for MFCs is gaining traction. Water or hydrogen peroxide creation from the oxygen reduction reaction in the cathode, water diffusion, and electroosmotic drag through the ion exchange membrane contribute to catholyte production in ceramic MFCs, as shown in Figure 4 (Gajda et al., 2020). Thus, the ceramic membrane’s efficient performance shows its appropriateness as a feasible and economical choice for speeding up the scale-up process and expanding the use of MFC technology.

There is currently a scarcity of data on the elements that influence the power generation of MFCs that use soil organic matter, sewage sludge, marine sediment, garden compost, industrial and home waste, and animal waste as fuel sources (Abbas et al., 2018; On et al., 2017). Integrating all elemental issues in the bioprocesses may be able to boost the overall yield and efficiency of the process. Especially, using an integrated system, energy recovery from water hyacinth can be improved even today (Varanasi et al., 2018). Furthermore, municipal solid waste is employed as a valuable power generator for global energy recovery (Abdel-Shafy and Mansour, 2018). Conversion of environmentally friendly municipal solid wastes into fuels and chemicals is also beneficial to the society (Matsakas et al., 2017). However, enhancing power generation in low-light settings and efficiently collecting electrons from photosynthetic bacteria on the electrode are also key difficulties in the development of a functional photosynthetic microbial fuel cell (PMFC). To address these issues, an anode was developed in a dual-chambered PMFC with an abiotic cathode by casting a nanocomposite matrix that assisted biofilm development of the photo-catalyst Synechococcus sp., surging the bacterial photosystems (PS I and PS II) with suitable light (λ_{650–750 nm}) at a broad excitation spectrum (λ_{350–644 nm}) through fluorescence resonance energy transfer (Kaushik et al., 2017). Researchers have established that an adequate microbial population structure and metabolic activity of the electrode biofilm communities are required for satisfying the performance of microbial electrochemical cells using this conversion technology (Liu et al., 2017).

In air-cathode single-chamber MFCs, electrode improvement and optimization are also ongoing (Wang Y. et al., 2018). Extracellular electron transfer of surface active microbes (Li F. et al., 2018) as well as precise electron carriers events of electron rich microbial species for electron transport functionality (e.g., biofilm formation, electron shuttles, swarming motility, dye decolorization, and bioelectricity generation) to MFCs aids in the generation of more stable and reliable power (Ng et al., 2017). Due to such benefits, MFCs have a lot of practical applications. Some of them are bioremediation and wastewater treatment, removal and recovery of heavy metals, constructed wasteland management, water desalination, biophotovoltaics, biosensors as alternate power tools, and biochemical production via microbial electrosynthesis (Mounia and Renouvelables, 2016). The detailed comparison of MFC performances from previous works using different anodes is clearly summarized and shown in Table 1.

Specific combinatorial biobased remediation, including bioremediation, and other approaches are highly effective for sustainable power generation from waste and reduction of heavy metal loads (ROY et al., 2018). Several possible enzymes have been found from various microorganisms that degrade complex chemicals by biodegradation, biostimulation, or bioaugmentation processes, according to recent advances in the environmental remediation of environmental contaminants and their environmental effects (Gaur et al., 2018). As a result, significant progress has been achieved in optimizing MFC performance for both renewable bioenergy generation and environmental remediation. As a result, a concentrated effort has been made to improve MFC performance by improving the reactor structure and electrode architecture, the addition of redox-active electron-donating mediators, biofilm acclimation, feed nutrient correction, and other parameters (Li M. et al., 2018). Significant effort has been taken for the remediation of organically polluted soil, leading to the development of numerous remediation technologies, including physical, bioremediation, and chemical remediation (Zhang H. et al., 2017). Organic-metal co-contamination is also evolving as a major environmental concern worldwide. Such contamination is more difficult to treat than individual contamination because of its great difference in chemical courses and the remediation mechanism of organic and metal pollutants (Chen F. et al., 2017).

The use of heterogeneous catalysts to promote the efficiency (Yuan et al., 2016; Khater et al., 2022) and sustainability of reactions using water as a solvent and the use of solid support (metal nanomaterials and bimetallic and polymetallic nanoparticles; Nascimento et al., 2018) have shown increased stability, reactivity, selectivity, and reusability. An electron-rich polypyrrole hybrid with mercaptoacetic acid (PPy-MAA) composite, which is known to be a very efficient adsorbent in Ag⁺ removal from an aqueous environment in the cleanup of metal-contaminated water sources, was used for this purpose. In the absence of an additional reducing agent, in situ reduction of Ag⁺ cations to Ag⁰ nanoparticles (NPs) was achieved, as well as the silver nanocomposites (AgNCs), which were studied using several optical and electron spectroscopic techniques (Giri et al., 2017). Catalytic reduction of organic pollutants was also achieved using a biosynthesized Ag/C/Fe₃O₄ nanocomposite made from red water and Caesalpinia gilliesii flower extract (Rostami-Vartooni et al., 2018). In addition to silver nanomaterials, biogenic gold nanoparticles can also modify the anode electrode in the presence of the microbial community for enhanced MFC performance (Wu et al., 2018b). However, both silver and gold are too expensive. Hence, they are not recommended for use as a common electrode for MFC devices at the larger scale. Therefore, this review highlights and discusses the behavior and the way of synthesis of some inexpensive nanocomposite electrode materials.

Nanomaterial-based technologies are gaining traction as a viable approach for removing toxins and completing the environmental cleanup (Wang D. et al., 2018). Synthesizing multifunctional, stable, and regeneratable composite material for cleanup is always seen as a long-term solution for contaminant degradation (Farooq et al., 2018). Such technical aspects have environmental advantages over lignocellulose-based carbon materials (Suganya and Senthil Kumar, 2018). Conventional waste treatment from acid black liquor using the electro-fenton process (Buftia et al., 2018) and an alternative sorbent (activated carbon from kraft lignin) from anthracite (AC_COAL) or coconut (AC_BIO) was evaluated using black liquor in the paper pulp industrial process for in situ contaminated sediment remediation (Gustafsson et al., 2017). However, the remediation is not much effective using the sorption method in the presence of active carbon materials. Hence, in this review, we briefly visualize that the listed metal–polymer combinations differ from carbon and its composites for their application.

One of the most significant obstacles in MFC technology for full-scale application is power output limitation (Gajda et al., 2018). Colloidal magnetic nanocomposite-based iron oxide (Fe₃O₄, γ-Fe₂O₃) (Natarov et al., 2018) and its composites (Fe₂O₃-in-CNT) (Guo et al., 2021) have attracted considerable attention due to a wide range of their applications. Biofilm formation and enrichment of exoelectrogenic bacteria are influenced by iron and iron-doped metal oxide nanoparticle coatings in the anode to boost current production of microbial electrochemical cells. This type of coating has high hydrophilicity and a unique electric characteristic (Muthukumar et al., 2019). For example, bacteria isolated from the soil of rice paddy fields can generate renewable energy (electricity) due to their exoelectrogenic nature (Fakhirruddin et al., 2018). The reaction of microbial populations in anode biofilms to increase concentrations of ferrous iron was more sensitive, according to principal component analysis.

The bulk of the anode biofilm communities of MFCs belongs to Geobacter, which was not the case for the cathode biofilm populations (Liu et al., 2017). A liquid-catalyzed fuel cell is a redox flow fuel cell that converts carbohydrates straight to power. Ferric chloride (FeCl₃) was employed as the major catalyst to boost efficiency (Xu et al., 2017).

Anode modification using Fe₃O₄–PPy composite-modified anodes is most attractive and cost-effective to increase MFC performance. Fe₃O₄–PPy composite-modified carbon felt anodes, according to the paper, may effectively improve the power production capacity and sewage treatment efficiency of MFCs. Compared to the unmodified anode, the steady-state current density and chemical oxygen demand removal rate were increased by 59.5 and 95.3%, respectively (Fan and Xi, 2021). When this composite was transformed with the addition of MnO₂, however, the surface of the anode was altered, resulting in kinetic activities. Because of their large surface area and abundance of electron-donating functional groups and heteroatoms, they have a high surface area. As a result, the PPy-encapsulated MnO₂, Fe₂O₃, and MnO₂–Fe₂O₃ nanocomposite materials act as promising MFC anode electrodes with good electrochemical operations and power densities, according to the findings (Prakash et al., 2020).

In general, immobilization of metal or metal oxide catalysts on the surface of a pure polymer or a functionalized polymer could result in conducting polymer-based nanohybrids (CPNHs) with better catalytic performance and durability. Metal oxides have a lot of surface area and/or porous architectures; hence, they have a lot of synergistic effects with conducting polymers. As a result, CPNHs can provide a stable, environmentally acceptable bio/electro-catalyst with improved catalytic activity and electron-transfer rate. Therefore, blending or direct deposition between CPNHs and metal or metal oxide nanomaterials as anode/cathode and membrane applications in fuel cells (either in direct alcohol fuel cells or microbials) is common today, as shown in Figure 9. The detailed understanding of the electrode materials’ features, changes, and performances is thoroughly reviewed (Ghosh et al., 2020).

It is vital to use catalysts on the cathode in order to improve MFC performance in real wastewater. Potential electrochemical catalysts for MFCs have to be widely available and should be low-cost materials. Manganese dioxide (MnO₂) was also thought to be one of the most effective materials for MFC catalysts because of its inexpensive cost, nontoxic and ecologically benign nature, excellent chemical stability, and catalytic activity (Jiang et al., 2017; Dessie et al., 2020). To enhance electrochemical activities in MFCs, MnO₂ electrodeposition on PPy-coated SS presents a promising technology for bioelectricity generation and wastewater treatment (Phonsa et al., 2018). As a result, using in situ chemical polymerization processes to sandwich MnO₂ in PPy provides an alternate viable solution for high performance cells, as shown in Figure 10B. This is due to the acquisition of higher electron-storage capacity property of MnO₂/PPy composite-modified electrodes, which is clearly shown in Figure 10A. The electron transfer mechanism on MnO₂/PPy composite-modified anodes is also shown in Figure 10C. As a result of sandwiching MnO₂ in PPy by in situ chemical polymerization, the MnO₂/PPy composite-modified anode surface has more microorganisms, according to the results. Manganese ions play an intermediary role in electron transfer and transfers electrons from bacteria to the anode in this case. As a result, the “mosaic” structure causes electrons to reach the anode more efficiently in the redox process due to 1) increased electronic transmission channels between microbes and the anode due to microstructure improvement, 2) manganese ions acting as an electron transfer intermediary to accelerate the electron transfer rate, and 3) the composites’ pseudo-capacitance property, which improves the anode antipolarization performance.