Recent Trends in the Use of Electrode Materials for Microbial Fuel Cells Accentuating the Potential of Photosynthetic Cyanobacteria and Microalgae: A Review

Abstract

As of 2024, approximately 81.5% of global energy consumption is still derived from non-renewable fossil fuels, such as coal, oil, and natural gas. This highlights the urgent need to transition to alternative energy sources amid the escalating climate crisis. Cyanobacteria and microalgae have emerged as promising biocatalysts in microbial fuel cells (MFCs) for eco-friendly energy production, owing to their photosynthetic abilities and resilience in regard to various environmental conditions. This review explores the potential of cyanobacteria and microalgae to drive bioelectricity generation via metabolic and extracellular electron transfer processes, leveraging their ability to fix carbon and nitrogen, while thriving in challenging environments. Bioengineering and electrode design advances are integrated to enhance the electron transfer efficacy and constancy of cyanobacteria-based MFCs. This approach addresses the growing demand for carbon-neutral energy and can be applied to wastewater treatment and bioremediation scenarios. By synergizing biological innovation with sustainable engineering techniques, this review establishes cyanobacteria and microalgal-driven MFCs as a scalable and eco-friendly platform for next-generation energy systems. The findings lay the groundwork for further exploration of the role of cyanobacteria and microalgae in bridging the gap between renewable energy production and environmental stewardship.

1. Introduction

In recent years, the process of photosynthesis has emerged as a promising route for sustainable energy production, offering environmentally friendly alternatives to fossil fuels, like hydrogen, bioelectricity, and other biofuels. This natural process involves the transformation of solar energy into chemical energy, which is then utilized for biomass generation. However, the efficiency of this type of energy conversion depends on the species of the photosynthetic organisms involved. For example, while plants generally exhibit conversion efficiencies of between 4.6% and 6%, microalgae demonstrate superior performance, with efficiencies reaching up to 9% [1]. Algae also offer several benefits in comparison to higher plants, such as rapid growth, seasonal availability, the ability to grow on non-arable land, and they do not compete with food crops, making them ideal candidates for biomass-based energy production. Typically, photosynthesis begins with light-induced excitation of electrons and concludes with carbon fixation in the form of carbohydrates, lipids, or proteins, with oxygen being a primary byproduct. Cyanobacteria, better known as blue–green algae, are a diverse group of photosynthetic microorganisms that play an important role in global biogeochemical cycles. They are found in various environments, from freshwater to marine ecosystems [1]. These bacteria are among the oldest known living organisms on Earth, which have been characterized by their ability to convert light energy into chemical energy through photosynthesis. Their unique biochemical pathways enable nitrogen fixation from the atmosphere, enhancing the surrounding ecosystem and contributing to soil fertility. Additionally, their remarkable adaptability allows them to survive in harsh environments, including those with high salinity, temperature fluctuations, and low light intensity. Thus, they play an important role in maintaining ecosystems around the world [2]. Beyond their ecological importance, cyanobacteria have gained recognition as a sustainable resource for biofuel production, thanks to their ability to photosynthesize. Many strains of cyanobacteria can produce biohydrogen, methane, and lipids, which can be converted into valuable biofuels, offering an environmentally friendly alternative to fossil fuels. Additionally, researchers have explored their potential in regard to bioremediation due to their efficiency in decomposing pollutants and absorbing excess nutrients from contaminated water [3]. Its ability to treat wastewater and capture carbon also underscores its versatility [4]. Recent research has also shown applications in regard to the synthesis of high-value biochemicals, including pigments, antioxidants, and pharmaceuticals, making cyanobacteria a promising platform for biotechnological and industrial applications [5].

A microbial fuel cell (MFC) is a bio-electrochemical system that uses the metabolic activity of microorganisms to generate electricity. MFCs work by harnessing the electrons released during microbial degradation of organic substrates. These electrons are transferred to the anode electrode. This creates electricity that can be used to produce energy or for other uses [6]. The process is inherently sustainable and environmentally friendly. MFCs also work with renewable organic matter, such as wastewater, agricultural residues, or atmospheric gases, such as CO₂. MFCs hold significant promise in regard to applications such as wastewater treatment, remote sensing, and even renewable energy production [7]. Algae and photosynthetic cyanobacteria use photosynthetic MFCs to capture solar energy and produce electricity. When microorganisms absorb solar energy, they start a chain of processes that divide water and produce protons (H⁺ ions), electrons, and oxygen. By adding an anodic chamber, which is isolated from the cathodic chamber by a semipermeable membrane that is selective for hydrogen ions, photosynthetic organisms absorb the energy from this series of reactions. Electrons generated by microbial photosynthetic activity move from an exterior connection, from an anodic chamber to a cathodic chamber. At the reductive electrode, also known as the cathode, the electrons combine with protons and oxygen to create water. This type of MFC also has an abundance of potential for converting solar energy into electrical energy [8]. This effort has led to the development of photosynthetic MFCs, which involve the microorganisms being in constant contact with a single electrode, while a light source is used at the electrode. The combination of bacteria and light in photosynthetic MFCs increases the cell’s voltage, increasing the power generated. A wide range of photosynthetic microorganisms, known as anodic microbes, have been employed in photosynthetic MFCs. These typically include cyanobacteria, which are blue–green algae, that obtain their energy from photosynthesis [9]. The integration of cyanobacteria into MFCs is a promising area of research that seeks to combine the advantages of photosynthetic energy conversion with bio-electrochemical power generation. Cyanobacteria are capable of generating electrons via light-driven processes, which can be harvested in MFCs in illuminated conditions. This unique attribute allows cyanobacteria to serve as an efficient source of electrons for the anode in MFCs, thus providing a renewable and continuous flow of energy when exposed to light [10]. In illuminated conditions, the photosynthetic activity of cyanobacteria generates high-potential electrons that can be transferred to the anode through direct or mediated electron transfer mechanisms, making them an ideal candidate for hybrid MFC configurations [11]. Additionally, cyanobacteria can perform nitrogen fixation, which contributes to the sustainable operation of the MFC system by improving the nutrient dynamics of the microbial community [12].

2. MFCs and Their Variations

MFCs are a form of bio-electrochemical device that use the aid of microbes to convert chemical energy (found in the substrate, as organic or inorganic substances) into electrical energy [13]. A distinctive MFC consists of a couple of chambers, namely the anode and cathode, and it is connected by a proton exchange membrane (PEM). The PEM is useful for the proton transfer from the anode to the cathode. The connection between the cathode and the anode is made of copper wires or titanium wires. The desired microorganisms, which generate protons, electrons, and carbon dioxide through biological oxidation processes and oxygen reduction, are supplied in the anode portion of the MFC, along with the organic substrates. The microorganisms in the anode break down the substrates, catalyzing the production of protons and electrons. [14]. The electrons produced are transferred to the anode with the help of cytochromes or some redox-active proteins and are passed to the cathode via an electrical circuit. The reduction of electrons occurs at the cathode, where an electron acceptor (O₂ or ferricyanide) is provided. The electrons combine with protons and oxygen to form water at the cathode. Catalysts like platinum can facilitate this process [15].

The reaction at the anode: (1) glucose as the substrate and (2) acetate as the substrate:

C₆H₁₂O₆ + 6H₂O → CO₂⁺ HCO₃⁻ + 8H⁺ + 8e⁻

(1)

CH₃COO⁻ + 2H₂O → CO₂ + 7H⁺ + 8e⁻

(2)

The reaction at the cathode (3):

O₂ + 4e⁻ + 4H⁺ → 2H₂O

(3)

Along with the generation of electricity, MFCs can help in treating complex organic waste and natural organic matter, contributing to wastewater treatment [16].

2.1. Microbial Fuel Cells with a Single Compartment

The architecture of single-chambered microbial fuel cells (MFCs) is both simple and cost effective. In these systems, the anode and cathode electrodes are integrated within a single chamber, eliminating the need for cation membranes [17]. Various studies have explored the different types of configurations that can be utilized in single-chamber MFCs [18]. This design features only the anode chamber, while the cathode is exposed to the surrounding atmosphere. Consequently, there is no requirement for additional aeration or oxygen supply, as the cathode chamber is directly connected to the air [19]. Figure 1 shows the operation of a single-chambered MFC. The major drawback of single-chambered MFCs is that there is no membrane to separate the two electrodes, which results in reduced power output and issues concerning microbial activity (e.g., competition for substrates).

2.2. Double-Chambered Microbial Fuel Cells

This system consists of anodic and cathodic compartments, separated by a salt bridge or membrane facilitating proton exchange. The cathode chamber is filled with fresh water, oxygen, and an electrode, while the anode chamber contains substrates, such as glucose or acetate, along with its electrode [20]. Common materials for electrodes include graphite, carbon paper, graphite fiber brushes, copper, stainless steel mesh, and carbon cloth. To maintain anaerobic conditions in the anode compartment, a continuous supply of nitrogen is necessary. The H-type MFC represents the basic design within this category, utilizing carbon paper for both the cathode and anode electrodes, with the cathode being treated with a platinum catalyst. Two sections of the fuel cells were constructed using microorganism sediment obtained from the inoculum. The microorganisms were preserved at 4 °C, after being cultivated in a mineral salt medium for future use. Proton buildup in the anode chamber, membrane fouling, instability, and the requirement for continuous cathode chamber aeration are some of the drawbacks of double-chambered MFCs. Furthermore, they can be expensive and inefficient, particularly for large-scale applications. The operation of a double-chambered MFC is depicted in Figure 2.

2.3. Continuous Mode (CFP-MFC)

In regard to the continuous mode operation of an MFC, a peristaltic pump is used to provide a continuous supply of substrate to the FP-MFC, achieving a hydraulic retention time of 24 h. This feed system was designed to regulate the inflow and outflow within the anodic chamber, while a silicone sealant was utilized to secure the effluent port. The feed and effluent streams were introduced and extracted, respectively, from the top and bottom of the anodic chamber to enhance substrate consumption and promote optimal mixing. A potentiostat was directly connected to both the anode and cathode, thereby ensuring that the units functioned at their maximum power output [21]. Three distinct MFCs featuring a two-part diffusion gas cathode design were each operated in three distinct modes: batch (MFC-BM), semi-continuous (MFC-SCM), and continuous (MFC-CM) mode. The electron losses, along with the mass transfer limitations of the MFC-BM, account for the low power density (PD, 1.31 ± 1.75 mW/m²) exhibited by that mode; however, the MFC-SCM (19.06 ± 2.01 mW/m²) and MFC-CM (15.53 ± 2.51 mW/m²) showed an increase in the PD. The energy conversion efficiency (ECE) was also higher during the MFC-CM in comparison to the MFC-BM and MFC-SCM, even though the MFC-SCM had a higher power output. The stacking method was then applied using DFE in continuous mode, wherein adequate power generation and a relatively high treatment efficiency were achieved. The major challenge faced by double-chambered MFCs is its complex construction, which may cause excessive fouling and difficulties in regard to proton accumulation.

2.4. MFCs Using a Flat Plate (FP-MFC)

In the feed batch mode of operation for a flat-plate MFC (FP-MFC), the external resistance can be adjusted between the polarization curves, executed at a minimum of 100 kV and a maximum of 1 mV. Before the occurrence of an external load step change, a stable output voltage was attained. Polarization curves facilitated the calculation of the ideal maximum power output required for the external resistance of the load (1 mV) for optimal power production [22]. A membrane is positioned vertically between the anode and cathode, which are positioned adjacent to one another in the flat-plate MFC. This configuration reduces the distance between the anode and cathode and minimizes the transport resistance. The product of the internal resistances and the volume did decrease, but the product of the internal resistances and the membrane surface area did not change. The anode was segmented into three sections at different depths, allowing for the assessment of the internal resistances at those depths. Due to the limitations of the substrate or mass transfer, the anodic resistance of the flat-plate MFC peaks when a chemical cathode is utilized. To overcome the substrate limitations that arise when the rhizosphere matures, the plant either increases its exudation, enhances the conversion of exudates into energy, or utilizes other rhizome deposits like dead root debris, which may boost the substrate availability in the FP-MFC. By adjusting the plant growth medium to encourage exudation, it may be possible to achieve higher exudation rates. Notably, since most roots were found at the middle and bottom level and those anodes typically generated less electricity than those at the top, it would be interesting for future studies to explore ways to further reduce the anode height [23]. In comparison with a sediment MFC, the flat-plate MFC results in lower output, and it is expensive too.

2.5. Sediment Microbial Fuel Cell

Sediment microbial fuel cells (SMFCs) are bio-electrochemical systems that generate power by oxidizing reduced chemicals, such as sulfides or organic carbon molecules, found in anoxic sediments. These systems were maintained at room temperature, while operating in sequencing batch mode. A new anodic solution was introduced whenever the closed-circuit voltage reductions over 24 h were less than 10 mV. To preserve an anaerobic microenvironment, the anodic solution was sparged with N₂ gas for two minutes, before the addition of the cathode buffer solution. The polarizations were executed following the third batch cycle, after the SMFC power density and substrate removal remained stable over two batch cycles. Additionally, two more SMFC reactors were inoculated with the mixed anaerobic consortia from the SMFC during its stable operation and were utilized to investigate the electron transport properties [24]. The ability of SMFCs to remove sulfide indicates their potential effectiveness in cleaning up sediments impacted by point source organic matter loading, such as that occurring beneath open-pen aquaculture operations. However, for SMFCs to be a viable solution for environmental contamination, they must be able to remove sulfide on an appropriate scale for the situation. SMFCs were kept at room temperature and operated in sequencing batch mode. A new anodic solution was added when the closed-circuit voltage reached a certain threshold. To maintain an anaerobic environment, the anodic solution was sparged with N₂ gas for two minutes, before adding the cathode buffer solution. Polarizations were carried out after the third batch cycle, once the SMFC power density and substrate removal had stabilized for two batch cycles. Additionally, two more SMFC reactors were inoculated with mixed anaerobic consortia from the SMFC during its stable operation to explore the electron transport properties [25]. Essentially, SMFCs operate similarly to MFCs, where microorganisms in the sediment oxidize organic molecules, transferring the generated electrons to the anode, and initiating the process of pollutant removal or energy production [26,27]. The oxygen-rich aqueous phase acts as the cathodic medium in SMFCs, while the organic-rich sediment serves as the anodic medium. Consequently, the anode is placed at a specific depth in the sediment, with the cathode located above it, in aerobic conditions [28]. The silt is teeming with a variety of microorganisms, including electrogenic bacteria, making it naturally nutrient rich due to the decomposition of plants and other settling organisms. In the SMFC system, aerobic bacteria can also be found in the sediment, acting as oxygen filters. These microorganisms are abundant in nutrients from decaying plants and settling creatures. While operating in sequencing batch mode, the SMFCs were maintained at room temperature. If the closed-circuit voltage dropped below 10 mV over 24 h, a new anodic solution was introduced. Before adding the cathode buffer solution, the anodic solution was sparged with N₂ gas for two minutes to preserve the anaerobic environment. After two batch cycles wherein the SMFC’s power density and substrate removal remained stable, polarizations were conducted following the third batch cycle. Additionally, during stable operation, mixed anaerobic consortia from the SMFC were inoculated into two other SMFC reactors to investigate their electron transport properties [25]. Even though sediment MFCs can produce an increased voltage, it may face difficulties such as anode biofilm corrosion and voltage reversal, which may lead to reduced output.

2.6. Stacking of Microbial Fuel Cells

Numerous MFCs are interconnected in parallel or in a series. These are connected, facilitating substantial output or current generation. A stacked MFC was created by combining six distinct MFCs that function continuously. Both compartments were constructed from graphite particles. A rod composed of graphite was utilized to connect the exterior, and the graphite particles afford the bacteria with the most extensive surface area possible for electron transfer [29]. It is possible to stack several biological fuel cell units in either an electrical or hydraulic configuration. Different MFCs can be coupled in parallel or in a series, depending on particular uses. Significant output or current generation is possible with this configuration. Six distinct continuously operating MFCs were combined to create a stacked MFC. Graphite particles were used to construct both compartments. A graphite rod was used to connect the exterior, while the graphite particles provided the bacteria with a large surface area for efficient electron transfer [30]. In an electrically stacked arrangement, the electrodes are connected in series or in parallel to boost the voltage or current output from the cells [31]. By connecting several individual cells in a series, the voltages are combined, and each MFC generates its own current. To enhance fuel flow between the anodic and cathodic depths and maintain consistent redox potential gradients within the biofilms, the test units featured six elongated supports at certain cathodic depths. The larger unit, referred to as M5–21, was a scaled-up version of M0.25–6 in regard to both the x and y dimensions. The M5–21 stack comprised of 20 MFCs that were electrically connected in parallel and were supported by 21 vertical ceramic supports. Each MFC was twice the length of M0.25–6, while maintaining the same height and thickness. The final dimensions of each cathode sub-unit were 21 × 3.8 cm, with a thickness of 2 mm. Each ceramic support was flanked by a pair of anodes and cathodes, linked together by stainless steel wire [32]. The stacking MFC has the potential to produce increased power output, but due to electrochemical degradation, energy loss may occur.

2.7. Upward Flow Fuel Cells

This form of fuel cell is comprised of polyacrylic plastic constructed in a tubular form, which is 10 cm in diameter and 100 cm in height in terms of its dimensions. The ports designated for the samples were positioned along the structure. A sequence of glass wool and bead layers was employed between the anode and cathode, as well as throughout the length of the reactor. Platinum wire, exhibiting a resistance of 10 Ω, connected the electrodes to an external circuit [33]. The ascending trajectory MFCs were equipped with aerators that served to oxygenate the cathode layer, where the electrodes were affixed. The primary advantage of this design is its elimination of the need for a proton exchange membrane, enabling its continuous operation, which effectively reduces the costs. The main disadvantage of this method is that it produces less energy in comparison to that generated by wastewater. The lab-scale setup maintained volatile fatty acid concentrations around 40 mg/L and achieved soluble chemical oxygen demand (COD) removal rates exceeding 90%, indicating effective wastewater treatment. Impedance spectroscopy analysis, derived from fitting experimental data into an analogous circuit, revealed an overall internal resistance of 17.13 Ω at a volumetric loading rate of 3.40 kg COD/(m³ day). This internal resistance comprises of diffusion resistance (1.46 Ω), charge transfer resistance (7.05 Ω), and electrolyte resistance (8.62 Ω), with electrolyte resistance being the most significant across the entire loading rate range. Additionally, impedance spectroscopy indicated that both the cathodic and anodic charge transfer resistances were major limiting factors. To enhance the power output further, we need to optimize the reactor layout to reduce electrolyte resistance [34]. An upward flow MFC has certain benefits while using wastewater as the substrate, such as increased recirculation of fluid, which results in enhanced COD removal, easy scalability, and a high loading rate; hence, it is suitable for wastewater treatment and also energy recovery.

3. Methods of Electron Transfer

MFC technology is dependent on the transfer of electrons from the electricigenic respiration chain to the electrode for bioenergy harvesting. The electron transfer mechanism of microbes cannot be classified as a biological process. Even though the mechanism is not yet fully understood, numerous electron transport strategies for electrodes have been suggested. These mechanisms can be classified into two basic types: a mechanism involving direct contact between the cell surface and the electrode to facilitate direct electron transfer (DET), and indirect electron transfer mediators [35].

3.1. Direct Electron Transfer (DET)

In regard to direct electron transfer, electron transfer occurs through physical contact between the cell’s outer membrane and the anode. Electricigens form conductive biofilms, or electrically conductive nanowires (like pili and flagella), on the surface of the anode [36]. Electron transfer occurs with the help of cytochrome in the outer membrane and transmembrane electron transport proteins or nanowires via contact between them rather than through diffusional electron mediators [37]. Multi-heme proteins, known as c-type cytochromes, are quite resistant to chemical changes and have a redox potential range of over 1 volt [38]. As a result of such advantages, it is clear that c-type cytochromes are crucial in moving electrons from the inside of the cell to an external terminal electron acceptor [39]. Nanowires connected to these cytochromes allow electron transfer to take place, even when direct cell–electrode contact is absent. DET can also be established via the formation of conductive pili (nanowires) in exoelectrogenic microorganisms, which enable the bacteria to transfer electrons to a solid electron acceptor [40]. These cytochromes interact with the anode directly or via conductive pili (nanowires), enabling transfer to (TEAs), even when they are not in direct contact with the cell [41]. While pili and flagella are homopolymers, nanowires consist of cytochromes and outer membrane proteins that enable long-range electron transport [42]. Direct electron transfer is preferred when using MFCs for effective current generation. One of the challenges of direct electron transfer is that the active sites of the electron transport proteins are usually embedded within the proteins, leading to a low electron transfer rate [43]. The only bacteria that have been discovered to have bacterial nanowires known to be electrochemically active are Shewanella and Geobacter, which transfer the electrons away from the cell and move and operate within a fuel cell [44]. The direct electron transfer method using pili/nanowires, flagella, and biofilms is shown in Figure 3.

The only bacteria in the first monolayer are in direct contact with the anode surface and engage in electron transfer, which is an undesirable result of the DET requirement that the cytochrome of the bacterial outer membrane makes physical contact with the anode. Accordingly, the idea of conductive biofilms was developed as a result of the discovery of EET occurring despite the formation of thick biofilms on the anode [45]. The Geobacteraceae family and allied microbes that are enriched with wastewater sludge are mainly linked to these biofilms, which have strong electrical conductivity [45]. Cable bacteria are multicellular filamentous bacteria found in freshwater and marine sediments, which have been shown to transfer electrons over centimeter-scale distances, enabling their survival in nutrient-scarce environments [46]. Thus, cable bacteria may survive in low-nutrient conditions by using electron donors and acceptors that are separated and far away from each other [47]. According to Winaikij et al. [48], cyclic voltammetry at a low scan rate (LSCV), electron impedance spectroscopy (EIS), Raman microscopy, and environmental scanning electron microscopy (ESEM) are some of the methods used to examine direct electron transmission in exoelectrogenic biofilms [39].

3.2. Indirect or Mediated Electron Transfer (I)

Many microorganisms cannot perform DET due to their inability to directly contact the electrode surface or the absence of the required cellular mechanisms. Instead, they may use a small soluble mediator to facilitate indirect electron transfer. This mediator can cross bacterial membranes, collect electrons produced by the metabolic activities of electricigens, and deliver them to the anode of an MFC [35]. Initially, the presence of electron mediators in an electrode during MFC operational procedures was thought to be a prerequisite [49]. They can originate from electricigens or be supplemented from outside the anodic chamber. There are several bacterial species that are known to be able to produce self-mediators, such as phenazine [50] and pyocyanin [51] etc. However, the advantage of the potential difference between the redox proteins and the mediators, in this case, is that it affects the efficiency of the electron transfer process to a large extent [35]. A few chemical molecules that create prospective possibilities for promoting the performance of electron transfer are anthracene-dione, thionine [35], including neutral red [52], humic acid [35,53], along with riboflavin [54], and methylene blue [35]. Adding synthetic mediators does not always work well; sometimes they have low current densities and can damage the electrodes being utilized, which will stop them from growing properly. Furthermore, adding mediators continuously is not always possible and could even be bad for the environment. Therefore, in the absence of outside mediators, it is better to use a microorganism that can transmit electrons [35]. A redox carrier often serves as a shuttle to move electrons to the terminal electron acceptor during mediated electron transfer (MET). This redox mediator in MFCs delivers electrons to the anode, becoming oxidized during the process, after gaining electrons from the bacterial cells, leaving them in a reduced state [39]. The next phase of electron transfer can then be started in regard to the oxidized form. According to Ieropoulos et al. [55], a good mediator should have the following qualities: (1) non-toxicity to bacteria; (2) ease of passage across the bacterial cell membrane; (3) a sufficiently positive redox potential to promote electron transfer; (4) good solubility in the anolyte; and (5) cost effectiveness and marketability. There are two types of redox mediators: endogenous (produced by the bacterium) and exogenous (compounds introduced from outside). Typically, endogenous mediators are metabolites that cells produce [39]. The indirect means of electron transfer is shown in Figure 4.

4. Design of Electrodes

The use of a fast prototype method for fabricating pillar-structured electrodes has enabled the analysis of the structure–activity link and highlighted the importance of prioritizing light flux in electrode design. Interestingly, despite having a lower cell loading and electroactive surface area, micropillar electrodes were found to have higher photocurrents than inverse-opal electrodes. This is primarily due to their noticeably higher light penetration [56]. Meanwhile, opaque carbon-based electrodes are gaining attention due to their scalability, ease of modification, and favorable surface chemistry, making them suitable for MFC applications [57]. The choice of biofilms and the electron flow between the microorganism and the electron acceptor are both strongly influenced by the anode material. Polymers like polypyrene (Py), poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) and carbon-based compounds are mostly used due to their high electrical conductivity and biocompatibility [58]. PEDOT/PSS is used for flexible and stretchable electronics, and has been subject to various modifications, strategies, and applications. The electrodes used in these MFCs are used to transport the electrons, which were released by microorganisms. A list of commonly used anode and cathode materials for MFCs are provided in Figure 5 and Figure 6.

4.1. Oxide Electrodes

Transparent vacuum deposition techniques are used to emit organic semiconductor films composed of indium, tin, zinc, or cadmium oxides, which are known as conducting oxide (TCO) anodes. These films have a low resistance of r < 1 10³ U cm and a high transmittance rating of T > 85%, which means that they are biocompatible and show long-term stability [57]. An ideal anode material must be biocompatible, chemically inert, cost effective, and possess high conductivity, along with a surface favorable for cell attachment. The low surface roughness of metal electrodes is one possible drawback that could restrict living cells’ ability to adhere to the surface. The challenge of designing metal electrodes to produce materials with distinct pores wherein cells can proliferate is a second drawback [58].

4.2. Metallic Electrodes

Metals such as stainless steel, silver, nickel, platinum, and gold are widely used as electrodes, due to their excellent mechanical strength, electrical conductivity, and stability as current collectors. These materials may naturally have a polished or oxidized surface, irrespective of the environment they are in [59]. Thus far, the metallic electrodes used as anodes in bio-photoelectrochemical systems (BPVs) have demonstrated respectable conductivity, stability, and biocompatibility [60]. However, their limitations include the inconsistent and limited photocurrent generated by cyanobacterial biofilms and the low biocompatibility of unmodified metal surfaces. The low biocompatibility of unmodified metals reduces the power output generated by bio-anodes, which is a significant drawback, despite the fact that they are comparatively inexpensive. The low surface roughness of metal electrodes, which reduces the surface area available for living cell adhesion, is another disadvantage for microbial growth that is restricted to the external environment. Carbonaceous materials, including brushes, cloth, paper, felt, and reticulated carbon, are commonly utilized to support microbial growth [60].

4.3. Pt-/CNT Electrode

Graphite and other plain carbon materials are often insufficient cathodes due to their poor oxygen reduction reaction (ORR) efficiency. To address this, a catalyst layer, commonly platinum, is applied to enhance ORR performance, due to its low overpotential and its large surface area [20]. Carbon nanotubes (CNTs), as allotropes of carbon, offer exceptional electrical conductivity, chemical stability, biocompatibility, and a high specific surface area. CNT-based materials support strong microbial growth, attachment, and biofilm formation [60]. While CNTs improve the hydrophilicity and electrochemical performance of electrodes, they also present challenges, such as biotoxicity, a relatively high cost, and potential physical damage to microbial cells due to their sharp edges [61]. However, the primary disadvantages of using anodes made of natural materials are their low electrical conductivity and durability. Excellent electrical conductivity that increases the catalytic activity and the simplicity of system scaling are all features of metal-based an-ode materials.

4.4. Graphene Electrodes

Compared to graphite, graphene has more robust mechanical strength, chemical stability, and electrical conductivity. These properties have made it a popular subject of several investigations. Since commercial graphene is very costly, there is always a demand for a less expensive substitute. The carbonization of graphene in powder form proved to be an ambient cost-effective substitute for commercial graphene products. Furthermore, the surface area available for microbial growth was enhanced by graphene powder made from waste [62]. Graphene is a noteworthy carbon material that can be used in electrochemical energy storage systems because of its exceptional chemical stability, high electrical conductivity, and robust surface characteristics [63]. Graphene’s hydrophobic nature, however, makes it incompatible with polymer matrices, leading to aggregation. To address this problem, graphene is often chemically, electrochemically, or π–π modified. Functionalization helps improve its dispersion and compatibility with polymer chains, often using organic compounds. For example, melamine acts as both a carbon and heteroatom source at the same time, which has led to a boom in the production of carbon materials.

4.5. Natural Electrodes

Derived activated carbon outperformed commercially available activated carbon in terms of its output current density, with a range of up to 3927 mW/m². An increased power density was observed that was appropriate to the distribution of the pore sizes, which aided in the rapid growth of microorganisms and improved the electron transfer and surface adherence. According to the study, using activated carbon material made from coffee and kitchen waste as an anode over an extended period resulted in a current production of 2000 mW/m² over 100 h. Furthermore, as an activating reagent, the concentration of potassium hydroxide (KOH) was crucial [64]. In regard to an MFC, the electrodes are selected according to their biological and physical characteristics. Organic matter added to natural waste products can be an excellent substitute for commercial carbon-based materials when looking for inexpensive carbon-based materials. Although raw garbage is cheap and widely accessible, it needs to be prepared so that it works with the system. Once prepared, these materials offer biocompatibility, availability, and cost benefits, although with longer preparation times [64].

4.6. Advancement of Electrodes

Consequently, improvements in the electrical conductivity, surface area, and microbial affinity must continue to be the main goals of electrode research in regard to MFCs. Currently, one of the primary factors preventing MFC systems from becoming widely available for purchase is the high cost of electrode materials. In the future, this issue will be resolved by producing high-performance electrode materials from waste and renewable natural feedstocks [65]. Moreover, 3D technology provides stability in regard to a range of substrate settings and enhances the electrical performance of electrodes; thus, electrode modification must also concentrate on other physical and chemical characteristics. P-MFC electrodes benefit from electrodes that allow light to penetrate the anode chamber, supporting the phototrophic biofilm. It has been claimed that p-MFC designs should incorporate graphite plates, rods, foils, and carbon fabric for light and microbial interactions [66].

Table 1. Maximum power density obtained using carbon-based, metallic, and composite electrodes in MFCs.

Type of Material	Anode	Cathode	Power Density	Reference
Carbon-based material	Carbon cloth	Carbon cloth	679.7 mW/m2	[67]
	Graphene	Carbon cloth	2850 mW/m2	[68]
	Graphene coating on carbon cloth	Carbon cloth	52.5 mW/m2	[69]
	Graphene nanosheet coating on carbon paper	Carbon cloth	610 mW/m2	[70]
	Graphene oxide	Carbon paper	102 mW/m2	[71]
	Glassy carbon	Carbon cloth	1905 mW/m2	[72]
	Carbon felt	Carbon fiber felt	784 mW/m2	[73]
	Carbon mesh	Carbon mesh	893 mW/m2	[74]
	Activated carbon	Activated carbon	36.39 mW/m2	[75]
	Graphite rod	Graphite rod	6.73 mW/m2	[76]
	Carbon fiber paper	Carbon fiber paper	124 mW/m2	[77]
	Graphite rod	Carbon cloth	93 mW/m2	[78]
	Carbon paper	Carbon cloth	46 mW/m2	[79]
	Carbon cloth	Carbon cloth	13 mW/m2	[80]
	Graphite plate	Graphite fiber brush	68.4 W/m3	[81]
Metal	Stainless steel	Stainless steel	23 mW/m2	[82]
Metal and metal oxide	Titanium/titanium dioxide	Platinum mesh	2317 mW/m3	[83]
Composite	Polyaniline networks applied to graphene nanoribbons coated on carbon paper	Carbon paper	856 mW/m2	[84]
	N-doped graphene nanosheets on carbon cloth	Carbon cloth	1008 mW/m2	[85]
	Graphene powder/polytetrafluoroethylene on carbon cloth	Carbon cloth	0.329 mW/m2	[86]
	Polypyrrole/graphene oxide	Carbon felt	1326 mW/m2	[87]
	Graphene/Au composite	Carbon paper	508 mW/m2	[88]
	Graphite plates	Platinum meshes	1410 mW/m2	[89]
	Polypyrrole coating on carbon cloth	Granular activated carbon	5 W/m3	[90]
	Carbon paper	Platinum-coated carbon paper	70.8 mW/m2	[91]

presents a comparative analysis of the maximum power density obtained using carbon-based, metallic, and composite electrodes in microbial fuel cells.

5. Material Fabrication

5.1. Nickel Metallic Thin

A coating was applied to the carbon felt and graphite electrode surfaces using the electroplating or cathodic electrodeposition technique. Prior to coating, the CF and GP electrodes were cleaned via soaking in acetone solution for 20 min and 0.1 M HCl for 15 min, then rinsing thoroughly with distilled water to remove surface contaminants. An undivided homemade reactor, with a metallic nickel plate as the anode, a graphite plate and carbon felt as the cathode, and nickel sulfate (NiSO₄·6H₂O) as the supporting electrolyte in an aqueous solution made of boric acid (H₃BO₃) at room temperature, was used to perform the electroplating process. The prepared nickel-coated carbon felt (Ni@CF) and nickel-coated graphite plate (Ni@GP) electrodes were immersed in a microorganism solution for a month during which multiple loadings took place. This process was used to produce bio-Ni@CF and bio-Ni@GP electrodes. As previously stated, mixed active microorganisms from anaerobic sludge were utilized to supply the microbial communities and create a biofilm layer. Nickel catalyzes to enhance the electrodes’ electroactive qualities and conductivity [92].

5.2. Nanocoated Electrodes

To create a modified carbon electrode using electrocatalyst powder, the carbon surface was first polished using soft emery paper and then rinsed with ethanol and double-distilled water. An electrocatalyst ink was prepared by ultrasonically dispersing 20 mg of electrocatalyst powder in a mixture consisting of 1 mL isopropyl alcohol, 1 mL double-distilled water, and 1 mL of 0.5 wt% Nafion solution, for 20 min [93].

5.3. Granular Activated Carbon (GAC)

GAC is an inexpensive and biocompatible material. However, its high porosity and low electrical conductivity restrict electron movement and impair its electrochemical performance. To increase its conductivity and reduce its porosity, this material requires additional modification [94]. Three single carbon granules were examined in [60] as bio-anodes in MFCs to produce energy for two large-surface area GACs and one small-surface area graphite granule. It was demonstrated that compared to standard graphite bio-anodes, GAC bio-anodes generated 1.3% to 2% more energy. To determine the crucial factors affecting the granular system’s performance, more investigations are necessary.

5.4. Immobilization of the Electrode

Using a dip-coating approach, a cellulose acetate membrane was applied to the functionalized electrode by immersing it in a 5% w/v (Mr ≈ 61,000, 40% acetyl groups, Fluka) cellulose acetone/THF 60:40 solution. The electrode was coated using a dip coater (immersion speed 1.5 cms⁻¹) and allowed to dry for eighteen hours at room temperature. Following drying, the electrode was placed in a sterile YPD solution (10 gL⁻¹ peptone, 20 gL⁻¹ yeast extract, and 2% glucose) to cultivate the immobilized cells [95].

5.5. Electrodeposition Using MnO₂

MnO₂ was electrochemically deposited on an SS mesh, with and without carbon nanostructures, to assess the impact of the MnO₂ catalyst on the cathode-side resistance to the O₂ reduction reaction. A 0.1 M Na₂SO₄ aqueous solution is used and the SS-mesh electrodes, with or without GNWs, are used as the anode, and a graphite electrode as the cathode. The electrodes are spaced approximately 6 cm apart. The experiment was conducted using the same apparatus as the MFCs setup. A thin coating of MnO₂ was electrode deposited onto the sample surface, after 0.5 cm³ of a 0.2 M MnSO₄·H₂O solution was introduced dropwise to the electrolyte through a cathode hole, while 1 mA·cm⁻² was applied for two minutes [96].

5.6. Pyrolysis

The most popular technique for creating an anode is direct pyrolysis of biomass precursors at high temperatures, which allows the biomass to be converted into porous carbon, with self-incorporating heteroatoms [97]. However, the pyrolysis process’s high energy input requirement invariably increases the anode preparation cost. For instance, the pyrolysis of sewage sludge at 900 °C produced a carbon monolith anode with a maximum power density of 486 ± 18 mW/m², which was higher than that of a conventional graphite plate anode.

5.7. Hummer’s Approach

Hummer’s method is a chemical approach for synthesizing graphene oxide (GO) from graphite. It involves oxidizing graphite using potassium permanganate in concentrated sulfuric acid, often in the presence of sodium nitrate. In regard to this method, graphite powder is mixed with sodium nitrate and sulfuric acid and stirred for 30 min. According to stoichiometry, potassium permanganate was then gradually added to the same solution, while being carefully stirred, while keeping the temperature below 20 °C using an ice bath, due to the exothermic nature of the reaction. The color’s transition from violet to brownish violet indicates progress. Then, demineralized water is added carefully, while heating the solution to around 95 °C [98].

5.8. Surface–Bacteria Interaction

Biofilm formation on electrode surfaces is influenced by the electrode’s physicochemical properties. Several aspects, including their growth, electron transport, metabolic activity, and formation dynamics, have been the subject of biofilm research. It is generally acknowledged that the formation of biofilm typically entails the following stages: (i) bacterial attachment to the electrode surface; (ii) monolayer and multilayer bacterial formation; (iii) polymeric scaffold formation; and, lastly, (iv) biofilm maturation that involves the formation of a three-dimensional structure. Among these, the attachment and formation of monolayer bacteria are the critical steps that determine the subsequent development of the biofilm [18]. Chemical oxidation is a quick and economical method that involves adding particular functional groups and morphologies to the anode surface to promote charge transfer and bacterial adhesion. During the interfacial interaction between electrolytes and electrodes in a single-chamber air cathode MFC, the electrolyte and catholyte are in direct contact. Microorganisms can readily colonize the surface of the biocarbon-based catholyte and produce a thick biofilm because of its excellent biocompatibility. Furthermore, the biofilm’s extracellular secretions may alter the catholytes’ physicochemical characteristics and impair its ORR activity. The development of biofilm on the surface of activated carbon-based catholytes has been demonstrated to hinder the movement of hydroxide ions from the cathode to the anode.

5.9. Interactions Using Cyclic Voltagrams

Cyclic voltammetry (CV) is a valuable technique for analyzing electron transfer processes within biofilms. However, it primarily captures the activity of biofilms in close proximity to the electrode, as potential gradients within the biofilm can limit the detection depth [99].

6. Photosynthetic Microbial Fuel Cells

Algae, plants, and certain microorganisms possess an inherent capability to harness light energy through a process known as photosynthesis. These biological photoautotrophic organisms can convert solar energy into electrical energy when integrated into photoelectrochemical microbial fuel cells [100]. The term “photosynthetic microbial fuel cells” (MFCs) refers specifically to those MFCs that utilize photosynthetic light-absorbing organisms [101]. Algae and photosynthetic microorganisms use photosynthesis within these systems to absorb solar radiation and produce electricity. These microorganisms split water molecules and produce protons (H⁺ ions), electrons, and oxygen as a result of a sequence of metabolic reactions that follow their absorption of solar energy. The photosynthetic activity of these microbes generates electrons within the anodic chamber, which then travel through an external circuit to the cathodic chamber.

6.1. Whole-Cell Fabrication

Fungi, bacteria, algae, plants, and other organisms are employed as biocatalysts in whole cells. These biocatalysts make use of isolated photosynthetic protein–chlorophyll complexes, such as PSII or PSI, or thylakoid membranes, whose anode and photosynthetic components do not have an electron transfer mediator. The formation of a pseudo-biofilm allows the direct and unmediated passage of electrons to the electrode. Moreover, they can be employed to construct semi-artificial Z-scheme architectures that attract electrons to the anode by combining with metal complexes [102].

6.2. Plant Microbial Fuel Cells

This system facilitates the conversion of solar energy into environmentally friendly electricity through the utilization of bacteria and plants. Since plant microbial fuel cells (PMFCs) get their energy directly from plants, they are classified as mediator-free systems. This ability is seen in a variety of plant species, including rice, algae, tomatoes, lupines, and grasses like reed sweetgrass and cod. A significant advantage of this particular type of microbial fuel cell is its ability to harness in situ energy generation, thereby producing power from living plants [103]. It is noteworthy that nearly 60% of the carbon captured during the photosynthetic process can be recovered as energy through PMFC technology.

6.3. Cyanobacteria and Microalgae Use in Microbial Fuel Cells

Some species of cyanobacteria, such as Anabaena, Nostoc, Chroococcales, Synechococcus, Lyngbya, Leptolyngbya, and Synechocystis, and microalgae, such as Chlorella sp., Scenedesmus, and Desmodesmus, have been studied for their ability to produce electricity in MFCs because of their photosynthetic properties [104]. These organisms harness light energy to produce electrons, which can be relocated to the anode, powering the MFC. Synechocystis sp. PCC 6803 as a photosynthetic organism is mainly dependent on the light source, and by using genetic and chemical inhibition, namely the splitting up of water molecules through PS-II (Photosystem-II), produces electrons [105]. Anabaena variabilis is the initial photosynthetic material in microbial fuel cells made of Anabaena, which include 2-hydroxy-1,4-naphthoquinone as an electron mediator. Anabaena cells that have been subject to around 200 h of cultivation are more effective in photosynthetic microbial fuel cells than those that have been subject to 100 h of cultivation [106]. A list of photosynthetic microorganisms, including the electrode material used to generate power, is presented in Table 1. The thylakoid membranes of eukaryotic photosynthetic microorganisms are compartmentalized in regard to the chloroplast. They can, however, conduct light-induced EET by avoiding this barrier, which does not feature in MFCs involving cyanobacteria. Dunaliella salina and Chlorella sp. are two of the most frequently used microalgal species in BPECs (

Table 2. Maximum power density obtained using photosynthetic microorganisms in MFCs.

Organism	Electrode Material	Power Density	References
Calothrix	Polypyrrole/carbon fabric	6 mW/m2	[108]
Nostoc	Polypyrrole/carbon fabric	1.2 mW/m2	[108]
Pseudanabaena limnetica	Stainless steel	1.2 × 10−7 mW/m2	[109]
Synechococcus sp. PCC 6803	Indium tin oxide/polyethylene terephthalate	10 mW/m2	[110]
Synechocystis	(a) Nanoporous indium tin oxide/fluorine tin oxide (b) Indium tin oxide/nanoporous indium tin oxide	3.770 mW/m2 0.630 mW/m2	[111]
Synechococcus	Carbon fiber	10.3 mW/m2	[108]
Oscillatoria sp.	Graphite plate	32.5 ± 0.5 mW/m2	[112]
Scenedesmus sp.	Graphite plate	28.5 ± 0.3 mW/m2	[112]
Scenedesmus quadricauda SDEC-8	Carbon cloth cathode with titanium	0.094 kWh per m3	[113]
Scenedesmus obliquus	Platinum-coated carbon paper	153 mW/m2	[114]
Synechococcus	Graphite electrodes	0.0956 W/m2	[115]
Synechococcus leopoliensis	Black acrylic as the cathode and carbon fiber veil as the anode	42,500 mW/m3	[116]
Anabaena ambigua	Carbon felt	63.84 mW/m2	[117]
Chroococcus sp.	Carbon cloth	467.55 mW/m2	[118]
Scenedesmus obliquus	Graphite rod	1.94 mW/m2	[119]
Synechococcus	Carbon felt	183 mW/m2	[120]
Synechococcus	Carbon felt	10 mW/m2	[121]
Spirulina platensis	Gold mesh as an anode and a graphite carbon cloth as a cathode	10 mW/m2	[122]
Scenedesmus obliquus	Carbon paper	102 mW/m2	[123]
Chlorella pyrenoidosa	Graphite/carbon electrodes	30.15 mW/m2	[124]
Chlorella vulgaris	The anode was made of carbon fiber brushes and the cathode was carbon felt (Pt catalyst)	187 mW/m2	[125]
Chlorella vulgaris	Carbon cloths with 10% Teflon	13.5 mW/m2	[126]
Chlorella vulgaris	Carbon nanotube	38 mW/m2	[127]
Chlorella vulgaris	Carbon paper	68 ± 5 mW/m2	[128]
Chlorella sp. G29-5	Carbon cloth	505.6 mW/m2	[129]
Chlorella vulgaris	Graphite sheet as the anode and a stainless-steel grid as the cathode	19.8 mW/m2	[130]
Chlorella sp.	Stainless steel mesh as the anode and carbon felt as the cathode	54.48 mW/m2	[131]
Desmodesmus sp. A8	Graphite felt	99.09 mW/m2	[132]
Chlorella vulgaris	Graphite plate	34.2 mW/m2	[133]
Chlorella sorokiniana	Carbon felt	213 mW/m2	[134]
Chlorella sorokiniana	Carbon felt	24.09 mW/m2	[135]
Spirulina platensis	Graphite rod	14.47 ± 0.7 mW/m2	[136]
Chlorella vulgaris	Carbon graphite	0.6 mW/m2	[137]

) [107,108].

7. Electrogenic Bacteria Use in Microbial Fuel Cells

Electrogenic bacteria are a critical element in MFCs, since they can transfer electrons directly to the electrode during the oxidation of organic substrates. These microorganisms typically reside in the anode chamber, where they oxidize organic compounds, and, hence, they generate the electrons that power the cell. The most common electrogenic bacteria utilized in MFCs include:

Geobacter sulfurreducens: An extensively studied bacterium known for its ability to transfer electrons directly to the anode via conductive pili, also known as nanowires, making it a crucial organism in numerous MFC designs. The presence of pili, tiny hair-like appendages, can make direct contact with the anode, causing the transfer of electrons [138]. Shewanella oneidensis: This species is prominent due to its electron transfer capabilities, predominantly in anaerobic conditions, and is often used in MFCs for its high efficacy in reducing metal ions. Similar to Geobacter sp., Shewanella can also produce nanowires or conductive pili for the transfer of electrons. This structure is mainly composed of protein molecules, and, hence, has high conductivity [138]. Desulfobulbus propionicus: This bacterium is a commonly studied organism in regard to the sulfur cycle. This sulfate-reducing bacteria can catalyze hydrogen evolution and oxidation. However, biofilm formation in this context decreased the overpotential in regard to the evolution of hydrogen and oxidation by around 0.2 V [139]. Pseudomonas aeruginosa and Bacillus: These bacteria are uncommon in this context like Geobacter/Shewanella, but they can transfer electrons to electrodes. These organisms can mediate the transfer of electrons through naturally secreted redox mediators. Certain bacteria can produce soluble redox molecules, which can act as shuttle molecules, that transport electrons from cells to the anode [140]. Enterococcus and Escherichia: These bacteria are unable to transport electrons directly, but they can do so through the addition of artificial redox chemicals. These molecules function as an artificial electron shuttle system, and these substances speed up the electron transfer rate [8]. Cyanobacteria and microalgae can perform oxygenic photosynthesis, and some cyanobacteria can perform nitrogen fixation. Due to the fact that these organisms can utilize a wide variety of substrates, including wastewater, their ability to grow in much smaller places (unlike plants which need a large amount of arable land), and because they can be used in the MFC system as a pure inoculum or in mixed culture, these microalgal/cyanobacterial MFCs will be subject to ongoing development in regard to their use in renewable energy production.

Fusion of Photosynthetic and Non-Photosynthetic Organisms

The integration of several organisms into an electric production consortium has been proposed as a novel way to enhance the performance of BPECs using a H-shaped configuration with microalgae. Both the cathode and anode chamber contain synthetic wastewater that contains microorganisms separated by a PEM. Acetic acid produced by bacterial cells can be oxidized at the anode. These chambers are parted by (PEM) acetic acid, ref. [141] released by the bacterial cells, for oxidation at the anode to release CO₂ and H⁺. When microalgae are exposed to light, their electrogenic activity produces O₂, which the cathode then reduces with the H⁺ to create H₂O. These electrons are harnessed from several thylakoids, cyanobacteria, algae, or plants. Bio-hybrid system proof-of-concept mediators ([Fe(CN)₆]³) and (DCBQ) are frequently utilized.

8. The Microbe’s Genetic Modification

Research on the precise process of extracellular electron transfer in cyanobacteria and algae is still ongoing, but experts agree that it most likely happens as a result of the production of a tiny, soluble redox mediator [142]. To boost photocurrents, several recent pieces of research have looked for and eliminated biological obstacles that prevent this endogenous mediator from being exported. Wey et al. [143] achieved a four-fold increase in the photocurrent through the genetic deprivation of the exopolysaccharide matrix, which was ascribed to both the increased permeability of the small molecules and increased biofilm formation. Kusama et al. [144] achieved an order-of-magnitude increase in the photocurrent by genetically removing the outer membrane. Kusama et al. demonstrated the impact of the genetic deprivation of the outer membrane, both in regard to the achievement of a significant increase in photocurrent generation and for the generation of new insights into the endogenous mechanism of extracellular electron transport in Synechocystis [144].

9. Synthetic Biology Approach to Improved Energy Harvest

By enabling precise genetic and metabolic engineering of microbial hosts, including bacteria, cyanobacteria, and microalgae, synthetic biology has become a potent tool for improving EET efficiency in MFCs [145]. The main approach in terms of synthetic biology to improve energy harvest is to improve the metabolism of microbes. For example, Shewanella cannot utilize glycerol, but once engineered, it can take up glycerol [146]. E. coli has been metabolically altered to activate the TCA cycle for the oxidation of glycerol at the anode of the MFC [147]. Clostridium ljungdahlii has been engineered to increase the concentration of NADH, which improves the performance of MFCs [148]. When combined with electrochemical analysis and systems biology, these methods offer a framework for creating a robust microbial chassis that can produce bioelectricity with high efficiency and concurrently form bioproducts in regard to MFC systems [149].

10. Practical and Technical Challenges to Implementing Microbial Fuel Cells on a Large Scale

MFCs are a new technology that takes advantage of bacteria to produce electricity. Although there have been frequent investigations into these technologies, increasing the implementation of MFCs into a system that can feasibly be commercialized presents a greater challenge. A substantial contribution to this challenge includes them not producing enough power for larger systems. To lessen this issue, we need to improve the efficiency of electron movement and also advance bioelectrode component systems. The choice of material for the electrode is of paramount importance. Carbon electrodes provide more opportunities for practical use; however, they are not durable enough [150]. Metal-based electrodes may provide a faster system; however, they can be less stable [150]. The use of composite electrodes can provide a much firmer surface and they are more inert, but they are much more sensitive and may be toxic to the microbes used [151,152]. There is also the challenge of these MFCs not sustaining sufficient performance consistency over time. Biofouling by organisms occurs on their surfaces and can limit their performance, not to mention the structural edge effects of the setting in which microbial communities occur. For large systems, changes in the conditions, including in regard to the pH, temperature, and nutrients, are factors related to engineering controls that will need to be considered to engineer sustainable operating conditions. Additionally, creating MFCs that can be easily maintained to benefit their adaptability or use will require completely different types of designs, suitable for current energy infrastructures and wastewater facilities that are seamless, cost effective, and sustainable, according to the same principles. These challenges will require a multidisciplinary effort in regard to the relevant materials, biology, and engineering to collectively address the operational challenges involving MFC technologies. Figure 7 depicts the merits and demerits of commonly used electrode materials in MFCs.

11. Comparative Analysis of Power Generation in Different MFCs

Power generation in MFCs varies widely, in accordance with the device configuration and operational settings. Single-chamber MFCs have a maximum power density of 113.8 mW/m² and have the beneficial features of low technology requirements and a low cost, although they have lower coulombic efficiency, due to oxygen diffusion into the anode chamber [153]. The design of dual-chamber MFCs provides a greater power density of 382.4 mW/m², but they are more costly because of the poor separation between the anode and cathode chambers and the frequent need for cathode replacement [153]. Power densities in the range from 120 to 270 mW/m² are produced by treating wastewater through the use of continuous-flow MFCs, as well as being simple to maintain and design. Stacked MFCs can significantly increase the power output by stacking cells in series or in parallel, but these configurations can result in charge reversal and voltage instability that damages the electrodes [154]. Sediment MFCs produce a relatively low power density of 6–32.18 mW/m², yet they are cost effective and ideal for remote locations, despite their high internal resistance and poor wastewater treatment capability [155]. Finally, upward-flow MFCs are on par with continuous-flow systems in terms of their power density production of 254 mW/m², while effectively treating wastewater. This continuous-flow MFC has been shown to be capable of removing around 95% of phenol from wastewater [156].

12. Applications for MFCs

MFCs have a wide range of applications, including bioelectricity production. Electrogens, the microorganisms used in MFCs, can convert organic compounds into electrical energy, making them a key source of bioelectricity generation. There is a wide variety of microorganisms, including bacteria, cyanobacteria, micro and macroalgae, and even some plants that have been used in MFCs for the generation of electricity. MFCs are also used in the production of biohydrogen, which can be used as a third-generation biofuel [19]. The use of a wider range of substrates, including wastewater, makes MFCs an important factor in wastewater treatment and bioremediation. A variety of wastewater, such as domestic waste and industrial waste, etc., can be used as a substrate, and it significantly reduces the COD of water. In the case of bioremediation, the reduction in recalcitrant compounds takes place, as there is a large number of non-degradable chemicals in water; these MFCs are used as an in situ method of bioremediation. MFCs act as a biosensor in the environment, by monitoring environmental pollution, acting as a BOD sensor, etc. The microorganisms capture carbon during metabolic processes and are used in carbon sequestration [157,158]. Various applications of MFCs are presented in Figure 8.

13. Future Perspectives

Photosynthetic microalgae in the field of MFCs represent a noteworthy advancement. MFCs involve bacteria or algae that supply electrons extracellularly to the cathode to generate bioelectricity. When microorganisms oxidize substrates, CO₂ is released at the anode and directed toward the cathode, where it is utilized by microalgae. Thus, free oxygen is released as a terminal electron acceptor, while the living microalgal cells consume CO₂ for photosynthesis. Due to their reliance on sunlight for their survival, photosynthetic microalgae in the field of MFC power production are greatly influenced by the wavelength, intensity, and quantity of light. Using microbial metabolism in the presence of light, photosynthetic MFCs transform wastewater’s chemical energy into electrical energy in anaerobic conditions. Thus, MFCs have the potential to significantly reduce carbon footprints. Even though the technology is still in its infancy, the use of photosynthetic microbial fuel cells has made significant strides in the direction of achieving a sustainable environment by competing with conventional process-specific, energy-intensive technologies. In addition, the ecology, metabolism, and extracellular electron transport of MFCs are covered in depth in the literature. The optimization of the light source, regulating parameters, and bottlenecks is of great importance for the operation of MFCs based on photosynthetic bacteria. The energy from the sun can be used by photosynthetic microbial fuel cells to produce electricity. Because they employ microalgae at the cathode, they can be utilized both indoors and outdoors and may be more cost effective than traditional MFCs. This is especially the case in regard to the treatment of wastewater. Additionally, they can create biopolymers that can be employed as nanoadsorbents to extract contaminants from wastewater [9]. Algal biomass is a promising feedstock for extracting biofuels, which is mainly important for biopharmaceutical products. Biomass, especially from microalgae, represents a sustainable bioresource for various applications in food, nutraceuticals, pharmaceuticals, feed, and other bio-based products. Also, we think that the following research gaps and bottlenecks are playing a major role in regard to this future research direction. Microbial fuel cells (MFCs) that employ cyanobacteria and microalgae are becoming viable options for generating sustainable bioelectricity, since they may facilitate this process in an environmentally favorable manner. However, a lack of studies in some areas has resulted in a number of practical difficulties. Understanding the mechanisms underlying extracellular electron transport in photoautotrophic bacteria is a significant challenge that could improve the efficiency of such systems and power output. Furthermore, little is known about how different environmental and operational conditions affect the development and stability of photosynthetic biofilms. Algal MFCs function poorly due to a number of additional challenges, including a low current density, inconsistent light absorption, and high oxygen near the anode. Furthermore, outside of controlled laboratory settings, these systems’ scalability and economic viability have not been thoroughly evaluated [9]. Promising areas that require more research include concentrating on improving the electron transfer from cyanobacterial and algal strains, creating sophisticated bioelectrodes with nanomaterials instead of the use of biocompatible materials only, and combining these systems with bioremediation for waste management and optimal energy utilization. It would be easier to move from addressing real-world problems to finding workable answers if uniform guidelines and extensive multi-day investigations were established.

14. Conclusions

In conclusion, MFCs represent a promising frontier in sustainable energy production, harnessing the power of nature’s smallest organisms to generate electricity. From the diverse array of MFC types to the intricate selection of electrode materials, this technology showcases the ingenuity of bioengineering. The fascinating methods of electron transfer, be it direct, mediated, or shuttle based, highlight the complex interplay between microorganisms and electrodes, pushing the boundaries of our understanding of microbial metabolism. The versatility of MFCs is further exemplified by the wide range of microorganisms employed, from robust bacteria to the photosynthetic prowess of cyanobacteria and microalgae. Each group brings its own unique strengths, offering tailored solutions for different applications and environments. Bacterial MFCs, with their rapid metabolism and adaptability, continue to be the workhorses of this technology. Meanwhile, integrating photosynthetic microorganisms opens up exciting possibilities for solar-powered bioelectricity generation, potentially revolutionizing our approach to renewable energy. As we stand on the cusp of a new era in clean energy, MFCs offer a glimpse into a future where the line between biology and technology becomes increasingly blurred. The ongoing research and development in this field not only promises more efficient and scalable energy solutions, but will also deepen our appreciation for the untapped potential of the microbial world. With continued innovation and interdisciplinary collaboration, MFCs will surely become a cornerstone of our sustainable energy landscape, turning the invisible power of microbes into a visible force for positive change in our world.