Microalgal Microbial Fuel Cells: A Comprehensive Review of Mechanisms and Electrochemical Performance

Abstract

Microbial Fuel Cells (MFCs) are an emerging technology enabling electricity generation from the oxidation of biodegradable substrates by exoelectrogenic microorganisms. The use of microalgae in Microbial Fuel Cells (mMFCs) presents significant advantages such as their simultaneous contribution to the reduction in operational energy, CO₂ capture, value-added compound production, and the endogenous supply of organic matter—through the decay biomass—to generate electrical current with coupled wastewater treatment. To achieve the desired electrical and wastewater performance, it is crucial to optimize the architecture, electrode and membrane characteristics, and operational conditions such as light intensity, CO₂ and nutrient availability, pH, and algae strains used in the mMFCs. This optimization can be aided by mathematical models, with the goal of achieving efficient large-scale operation. This review provides a comprehensive overview of the advances in Microbial Fuel Cells with microalgae, highlighting their electron transfer mechanisms, evaluating strategies to enhance their efficiency and their potential applications.

1. Introduction

In recent decades, economic and demographic growth has posed significant challenges for our society, including water and energy scarcity, the generation of large amounts of waste, and environmental degradation [1]. Currently, fossil fuels supply over 80% of the world’s energy demand. However, projections indicate that the reliance on fossil sources will decrease to 73% by 2030 and further to 40% by 2050 [2,3]. Thus, it is important to develop new renewable and carbon-neutral energy sources, based on raw materials such as biomass, wastewater, and even the conversion of CO₂, to obtain chemicals and energy-dense molecules [4,5]. It is estimated that biomass derived from agro-industrial processes globally accumulates to over 2 trillion tons per year [6]. Additionally, global wastewater generation is estimated at 359.4 billion m³ per year, with only 188 billion m³ per year being treated [7]. The combination of energy generation with waste management would be a very convenient option. This is because it would allow us to treat the waste at the same time that the chemical energy contained in it is valorized [8].

Organic wastes, including food waste, municipal, agricultural, and industrial wastewaters, pose challenges in disposal due to conventional energy-intensive processes generating significant sludge [9,10,11]. Nowadays, various technologies exist for the valorization of organic wastes, including incineration, pyrolysis, gasification, and anaerobic digestion, each with its own advantages and drawbacks [10,12]. Conventional wastewater treatment methods, like activated sludge treatment, are energy-intensive (energy requirements of about 0.3–0.6 kWh/m³), presenting significant greenhouse gas (GHG) emissions (about 0.9 kg CO₂/m³) [11]. Given current economic and environmental considerations, there is a pressing need for environmentally friendly and energy-efficient processes, especially those that can extract value from organic waste streams [13,14]. In this sense, novel approaches, like Microbial Fuel Cell (MFC) technology, a bioelectrochemical system (BES), offer potential for electricity recovery from wastewater with significant environmental and economic advantages (the net worth would be $0.10/kg-COD) [15,16].

The BES is a revolutionary technology that employs exoelectrogenic and/or electrotrophic microorganisms as biocatalysts within an electrochemical circuit to convert organic matter into electrical energy or to convert electrical energy into value-added organic chemicals using other organic molecules as raw materials, including the conversion of CO₂ to produce useful products such as hydrogen, butanol, etc. [17,18]. Exoelectrogenic microorganisms donate electrons to an anode during anaerobic oxidation of organic substrates, while electrotrophic organisms can consume electrons from a cathode using processes like electrofermentation of organic matter and bioelectrosynthesis using CO₂ reduction [19,20]. Based on the aforementioned, BESs include MFCs for electricity generation from chemical reactions and microbial electrolysis cells (MECs) for value-added chemical production consuming electricity [21,22,23,24]. They offer additional benefits such as wastewater treatment, nutrient recovery, and removal of recalcitrant compounds [25,26]. Photosynthetic microorganisms can also be employed in BESs, as electron donors in MFC bio-anodes for electricity production, as a source of oxygen in the cathode of an MFC, replacing external aeration, or as electron acceptors in a biocathode forming biofilms for various biosynthesis reactions [8,27,28]. Overall, BESs are able to integrate electricity generation, wastewater treatment, and value-added products’ production while reducing costs and generating less sludge compared to conventional methods [29,30]. In Figure 1, a basic representation of a BES device is presented.

The study of BESs started in 1911, when Potter came to the conclusion that it was possible to release electrical energy from the decomposition of biodegradable substrates by microbial cultures [31]. The research history of BESs is characterized by its discontinuity. BESs gained relevance in the 1960s when researchers from the National Aeronautics Space Administration (NASA) space program considered the technology to obtain electricity from organic wastes in spaceships; however, other emerging technologies such as photovoltaic energy eclipsed the interest in BESs [32]. Between the 1980s and 2000s, some research works were published (on average, three publications per year) focused on the study of BESs and MFCs; however, it was only in the 2000s that research on this technology emerged, going from around 10 publications per year in 2000 to more than 1500 in 2024. The exponential growth in publications observed since 2005 has helped facilitate the increase in the power density significantly (1–1000 mW m⁻²) by means of electrode material modifications, microbial culture selection, metabolic mechanism understanding and cell architecture design; however, there are still challenges before full-scale implementation of the technology can be carried out for industrial or commercial application of BESs [29,33].

Unfortunately, the use of photosynthetic microorganisms or microalgae as biocatalysts in MFCs, usually called Microalgae Microbial Fuel Cells (mMFCs), is more recent as can be seen in Figure 2. During the last years, the number of publications in the topic mMFC reached around 40 per year, a value significantly lower than those obtained by conventional MFCs based on other cathodic catalysts.

Among BESs, MFCs are devices capturing great interest because of their applications in wastewater treatment, versatility and wide-ranging applications and studies. Additionally, the coupling of microalgae in MFCs could present several advantages that make mMFCs an interesting option.

In this context, the present review offers a comprehensive analysis of the current state and future prospects of the mMFCs. Following the introductory section, the broader landscape of BES configurations will be contextualized. The review then delves into the core of mMFCs, detailing key aspects such as the fundamental role of microalgae, their integration with MFC technology, and the critical influence of environmental factors like light, CO₂, and trace minerals on mMFC performance. The main components, operational mechanisms, and behaviors of mMFCs are also examined in light of factors affecting them, including operational variables, design variables, and electrochemical characteristics, to enhance the mMFC efficiency.

Additionally, the review discusses the most relevant research topics, including system configurations, materials, electron transfer mechanisms, and scale-up considerations. Looking towards the future, the review addresses improvements in geometry, electrodes, membranes, pH, harvesting techniques, strain selection, and the importance of modeling and metabolic research. This comprehensive analysis highlights the potential applications of mMFC systems in electricity generation, wastewater treatment, and CO₂ capture, addressing critical global challenges. Furthermore, the manuscript explores the integration of mMFCs with ecological and industrial systems, proposing pathways for practical deployment. This work outlines strategies to enhance their efficiency, providing a solid foundation for future research and technological advancement.

2. Bioelectrochemical Systems’ Configurations

BESs, also referred to as Microbial Electrochemical Technology (MET), are composed of two electrodes, an anode and a cathode, connected by an external electric circuit. In the anode, exoelectrogenic microorganisms act as biocatalysts, transferring electrons to the electrode through the anaerobic oxidation of organic substrates. Meanwhile, the cathodic chamber acts as a sink for the electrons received from the anode. When both anodic and cathodic reactions occur spontaneously, the system is referred to as MFC. However, when the reactions are not spontaneous, and require energy consumption, the devices are named microbial electrolysis cells (MECs).

Table 1. Characteristics of the different types of BESs.

Type of BES	Function	Anodic Chamber	Cathodic Chamber	Microorganism	Substrate	Electron Acceptor	Comments
MFCs (Microbial Fuel Cells)	Electricity generation, organic matter removal	Anaerobic oxidation of organic matter	Aerobic reduction reactions	Shewanella, Geobacter, activated sludge	Domestic/industrial wastewater with biodegradable organic matter	O2, K3Fe(CN)6, NaOCl, other oxidizing compounds	Maximum current density: 390 A/m2. Research focuses on improving materials and configurations.
Microbial electrolysis cells (MECs)	Allow electrolysis of water, synthesis of H2, H2O2, CH4, NaOH, struvite	Anaerobic oxidation of organic matter	Anaerobic, hydrogen generation	Electrogenic at the anode	Biodegradable wastewater	H+, CO2, organics	Requires external power for water electrolysis. pH and aeration control are critical (1000 L produces 0.19 ± 0.04 L H2/L/day).
Microbial Desalination Cells (MDCs)	Electricity generation, desalination	Anaerobic oxidation of biodegradable substrates	Aerobic reduction reactions	Electrogenic microorganisms in the anode	Domestic/industrial wastewater	O2, K3Fe(CN)6, NaOCl, other oxidizing compounds	Coupled desalination with energy generation.
Microbial electrosynthesis (MES)	Production of valuable compounds (acetate, etc.) using CO2	Anaerobic oxidation of organic matter	Reduction in the final electron acceptor to produce value-added compounds	Electrogenic at anode; electrotrophic microorganism, reducing, methanogen bacteria at the cathode	CO2	CO2	Requires external power. Challenges include product selectivity and low reaction kinetics.
MFC	Electricity generation, carbon dioxide capture	Anaerobic oxidation of organic matter	Aerobic reduction	Photosynthetic organism	Organic matter	O2, K3Fe(CN)6, NaOCl, other oxidizing compounds	Combines electricity generation with carbon capture.
Sediment Microbial Fuel Cells (SMFCs) or benthic MFCs	Energy harvesting from natural organic matter	Anaerobic in sediments	Aerobic	Natural microorganism in aquatic sediments in anode	Organic matter	H+	Low-cost technology for pollutant removal.
Plant-Type Microbial Fuel Cells (PMFCs)	Electricity generation and pollutant removal	Anaerobic in soil	Aerobic	Soil microorganisms interacting with the rhizosphere	Plant-excreted carbohydrates	H⁺	Plants convert sunlight to electricity via rhizosphere processes.
Constructed Wetlands (CWMFC)	Pollutant treatment, energy generation	Anaerobic in soil	Aerobic	Electrogenic microorganisms in soil	Wastewater	H⁺	Significant contribution to wastewater treatment.
mMFCs	Electricity generation, value-added products, carbon dioxide capture	Anaerobic	Aerobic	Microalgae in cathodic chamber	Organic matter	H+	Microalgae play a key role in power generation.

describes several characteristics of the different types of BESs, such as configurations, anodic and cathodic materials, type of microorganisms used, substrates, electron acceptors, among others. These BESs are also classified according to their configuration and application as shown in Figure 3:

• Microbial Fuel Cells (MFCs) can generate electricity by performing oxidation and reduction reactions; usually, the reduction reaction is based on the oxygen reduction to water due to its high redox potential [34,35]. A typical MFC consists of an anodic and a cathodic chamber, separated by a proton exchange membrane (PEM). In the anodic chamber, microorganisms metabolize organic substrates to produce electrons and protons. The redox potential difference between the electron donor, usually an organic substrate at the anode, and the electron acceptor, typically oxygen at the cathode, causes electrons to move to the surface of the anode and then flow to the cathode through an external electrical circuit. Meanwhile, protons migrate to the cathode through the PEM [36]. In the cathode chamber, electrons and protons combine to reduce oxygen to water [37]. MFCs are among the most studied types of BES. For instance, a single-chamber MFC with a carbon brush anode and a platinum (Pt) cathode was used with a 1:1 ratio of wood to domestic wastewater [38]. The results were significant, showing an 85% removal of Chemical Oxygen Demand (COD), an 18% Coulombic Efficiency (CE), and a power density (PD) of 360 mW m⁻² with a current density (CD) of 1.3 A m⁻². Additionally, this setup allowed for a comprehensive analysis of biological diversity within the system [38]. Double-chamber MFCs utilized rice husk charcoal electrodes in wastewater from poultry droppings. This configuration achieved a PD of 287 mW m⁻² and a CD of 0.51 A m⁻², with a 40% reduction in COD. The use of new materials and real wastewater further validated the practical applications of this setup [39].
• Microalgal Microbial Fuel Cells (mMFCs) could be described as a type of MFC that uses microalgae as a biocatalyst in the anodic or cathodic chamber. These devices will be discussed in detail in the next sections.
• Microbial Electrosynthesis Cells (MES) can reduce chemical compounds to generate different value-added chemicals. In this configuration, the anodic oxidation reaction is similar to the MFC’s; however, the cathodic reaction in a MEC requires additional energy to drive the non-spontaneous overall reaction to obtain the desired products. Several studies on MES systems demonstrated the versatility of this technology. For example, a two-chamber MES with polished graphite plate electrodes and 35 mM acetate at −0.8 V showed that sulfate-reducing bacteria could enhance electron transfer within the cathode biofilm, leading to a symbiotic effect among the bacteria [40]. A two-chambered planar carbon felt electrode setup with 2.7 mM acetic acid at −0.8 V led to higher specialization within the biofilm, which improved acetate generation and demonstrated an important symbiotic relationship with bacteria [41].
• Microbial electrolysis cells (MECs) can produce hydrogen gas and/or enrich metabolites through the electrolysis of water with externally applied voltage [15]. MECs have demonstrated potential in both single- and two-chamber configurations. For instance, a single-chamber MEC with a carbon fiber brush anode and a carbon cloth cathode coated with platinum (Pt) achieved hydrogen production of 38 L H₂ m⁻² d⁻¹ and a COD removal efficiency of 44.92%. The system also reached a power density of 0.012 A at 0.9 V and a CE of 75.60% [42]. In a two-chamber MEC, a Pt-deposited carbon cathode and a carbon felt anode resulted in hydrogen production of 4.3 L H₂ d⁻¹, with a COD removal efficiency of 61.5% [43].

Other significant BES configurations include the following:

• Sediment Microbial Fuel Cells (SMFCs), also known as benthic MFCs, generate energy by reducing oxygen in the water column and oxidizing organic substrates in anoxic sediments. These systems typically feature a cathode placed in the oxygen-rich water above and an anode embedded in the anaerobic sediments below, such as those found in paddy soils and benthic environments. Detailed construction and operational guidelines are described in the literature [33,44].
• Plant-Type Microbial Fuel Cells (PMFCs) feature an anode located in the soil, in contact with the plant rhizosphere, while the cathode is exposed to atmospheric oxygen in order to facilitate the reduction reactions [45].
• Microbial Desalination Cells (MDCs) utilize bioenergy from the oxidation of organic wastes to achieve desalination by integrating electrodialysis into MFCs. Unlike traditional electrodialysis, the desalination process in an MDC offers relevant energy benefits, as it does not require external energy input for ion separation [46]. MDCs have been investigated in both laboratory and pilot-scale setups. For example, a study using a carbon brush anode and carbon cathode demonstrated a notable desalination rate of 31.5 mg L⁻¹ h⁻¹, achieving a 90% removal of organic compounds and 75% removal of ammonium (NH₄) [47]. In a pilot-scale study using a 10 L MDC, the system achieved a removal rate of 154 mg L⁻¹ h⁻¹, showcasing the scalability and effectiveness of MDC technology in real-world applications [48].
• Constructed Wetlands Microbial Fuel Cells (CWMFCs) combine constructed wetlands and MFC technology to control water pollution with coupled energy generation. The effect of the anode material on system performance has been studied. The results indicated an 18% COD retention and a power density of 1730 mW m⁻² (346 mW m⁻³), underscoring the importance of selecting appropriate anode materials to enhance the efficiency and output of CWMFC systems [49,50,51,52,53].

From Table 1, it is possible to analyze the functional diversity, microbial communities, and electron transfer mechanisms of the different type of BESs. MFCs and MDCs primarily focus on electricity generation and wastewater treatment, while MECs and MES are designed for the synthesis of valuable compounds, requiring external energy input. Systems like SMFCs, PMFCs, and CWMFCs integrate natural ecosystems for sustainable energy harvesting and bioremediation. The inclusion of microalgae in mMFCs offers additional benefits, such as CO₂ capture and value-added product generation, distinguishing them from other BESs. However, each system faces specific challenges, including electron transfer limitations, substrate availability, and efficiency constraints, which require further optimization to enhance scalability and performance.

Other applications of BESs are related to the cathode and the ability of microorganisms to catalyze several cathodic reactions such as the oxygen reduction to water, the proton reduction to hydrogen, and the nitrate reduction to nitrogen gas, among others, generating value-added products [37,54,55,56]. BESs could take advantage of the current flow not only for power generation, but also to reduce organic or inorganic compounds in the cathodic chamber, using them as electron acceptors, and thus obtain reduced chemicals with high value added. Benefits of BESs in wastewater treatment include offering economic advantages by generating less sludge compared to traditional aerobic methods and converting pollutants directly into electrical energy or chemicals. This makes BESs particularly suitable for areas with limited electrical infrastructure. In the literature, it has been described that the complete oxidation of organics in municipal wastewater can generate a theoretical electricity supply of around 1.9 kWh m⁻³ wastewater (assuming typical COD concentration of domestic wastewater as 0.5 kg m⁻³, and energy content of 3.6 MWh kg⁻¹ COD), and direct nutrient recovery for fertilizer applications can save up to 0.79 kWh m⁻³ [26,57]. Anyway, additional efforts are needed to enhance energy production in BESs by developing cost-effective materials, increasing current density through the use of more efficient electrogenic bacterial strains, consortiums, or genetic engineering, and producing high-value compounds like acetate, methanol, butyrate, butanol, etc. In the literature, recent economic evaluations have shown returns surpassing those of conventional processes, highlighting the potential for viable and profitable applications [58,59].

3. Microalgae Microbial Fuel Cell (mMFC)

3.1. Microalgae

Microalgae are small, single-celled, photosynthetic organisms that can be either prokaryotic or eukaryotic. The microalgae are capable of producing a broad range of commercial compounds, such as proteins, ω-3 fatty acids, pigments, cosmetics, different value-added chemical products, as well as third-generation biofuels [60]. The growth of microalgae is influenced by various factors, including light, temperature, nutrient availability, and pH levels [61,62,63].

Microalgae cultures possess several interesting characteristics that make them highly applicable in biotechnological processes. The ability to grow in different kinds of waters, removing pollutants from wastewater, consuming CO₂ as a carbon source and thus mitigating GHG emissions (6.24 kg of microalgae biomass per m³ CO₂), their ability to generate value-added products [64,65], their tolerance to a wide variety of environmental conditions, the high biomass productivity per unit area, high photosynthetic efficiency and fast growth, among others, make the microalgae very interesting biocatalysts mainly when dealing with wastewaters [66,67].

Algae-based CO₂ capture surpasses terrestrial sequestration by one to two orders of magnitude. Algae play a crucial role in generating about 75% of the atmosphere’s oxygen, only depending on the access to CO₂ and light [61]. The use of microalgae for biofuel production represents one of the most direct applications, given their high lipid content (over 30% by weight) and their capacity to CO₂ capture [65].

One of the most crucial factors influencing the growth and makeup of microalgae biomass (fatty acid and pigment profiles) is light (quality, intensity, and dark/light regimes). Light intensity influences photosynthesis, CO₂ removal rates, biomass concentration, and overall growth rates [68]. Generally, increasing light intensity boosts the growth rate of microalgae [69]. However, excessively high light intensities can lead to photo-inhibition. Conversely, low light intensities may restrict growth and increase chlorophyll content. It is well documented that cultivation under stress conditions, such as very high or low light intensities, can alter the composition to favor lipid accumulation, depending on the algal species and its ability to capture CO₂ under high light conditions [70].

Nutrients play a critical role in the growth and composition of biomass [61,62]. The lipid production in microalgae is determined by the species’ inherent capacity (genotype/phenotype) and abiotic factors such as wavelength, light intensity, and the composition of the culture medium (primarily carbon source, nitrogen, and phosphorus) [71]. Carbon is the primary component of biomass, comprising 50% by weight. The concentration of CO₂ in the air is 380 ppm, and the dry weight composition of algae requires 1.8 kg of CO₂ per kg of biomass. For microalgae culture, additional carbon is supplied through aeration, enriched air, or organic sources during photosynthesis to form various organic metabolites [71,72,73]. Optimal CO₂ concentration values vary not only among different microalgae strains but also for the same strain under slightly different conditions.

3.2. Microalgae and MFC

In a conventional MFC, the oxygen is reduced at the cathodic surface consuming the electrons flowing from the anode through the external circuit. Because of that, oxygen availability used to be a limiting factor in the cathodic performance, mainly due to the cost of aeration and the diffusion dynamics [37,74]. A cathodic microalgae culture, supplemented with CO₂ and nutrients, can be utilized to produce oxygen and other valuable compounds such as biomass and lipids. Moreover, employing biotic cathodes can enhance power outputs (from approximately 10 mW m⁻² to over 1 W m⁻²) [75] and lower aeration costs by replacing mechanical aeration systems with algal oxygen supply [76], which can also help reduce CO₂ emissions [21,55,77,78]. Integrating photosynthesis with MFCs can be achieved through various configurations to facilitate the conversion of sunlight into electricity within the system [30,74].

Among various algae strains, Chlorella vulgaris has been extensively researched as a biocathode microorganism due to its ability to increase dissolved oxygen concentration and the oxygen reduction rate. This is attributed to its ease of cultivation, high growth rates, and lipid composition. The literature reports a maximum power generation of 38 mW m⁻² achieved with a carbon nanotube coating on the cathode, highlighting the importance of cathode coating for oxygen production [79]. Additionally, the algal biomass generated on the biocathode can be used as an endogenous carbon energy source for MFC anodes. This strategy could lead to a fully self-sustaining system [78,80,81] with high power densities of up to 1926 mW m⁻² [82]. Data presented in

Table 2. Configurations of mMFC systems.

Microalgae Specie (Cathode)	MFC Configuration	Electrode	Maximum Power Density	Highlights	Reference
Synechocystis + lagoon microorganism	Single chamber	Carbon cloths	1.3 mW m−2	Positive response to light	[83]
Chlorella vulgaris	Double chamber	Glassy graphite rods	2.7 mW m−2	Successful removal of CO2	[84]
Spirulina platensis	Single chamber	Platinum electrodes	6.5 mW m−2	Higher power densities under non illuminated conditions	[85]
Chlorella vulgaris	Double chamber	Carbon cloths	13.5 mW m−2	Polarization resistance more significant at cathode	[86]
Chlorella vulgaris	Double chamber	Plain graphite	62.7 mW m−2	Increasing light intensity (96 µEm2s−1) enhanced power density by 600%	[61]
Anaerobic mixed consortia	Double chamber	Graphite plate	57 mW m−2	Spring season yields 50 times greater power densities	[87]
Chlorella vulgaris	Double chamber	Plain graphite	1926 mW m−2	Closed loop system. High COD	[82]
Lagoon community	Double chamber	Carbon fiber	61 µW	Closed loop system	[88]
Green algae collected from a local water pond (mixture of algae and bacteria)	Tubular Coupled mMFC	Anode: carbon brush Cathode: carbon cloth	0.205 kWh m−3	Couple of mMFC and membrane photobioreactor	[89]
Mixed algae consortia culture	H-type membrane-less mMFC	Graphite rods	-	Potential application to increase biomass production in algal-based treatment systems	[90]
Chlorella sp. and bacteria	Double chamber	Anode: carbon brush Cathode: stainless steal	6.4 mW m−2	Novel configuration	[91]
Spirulina sp.	Double chamber	Carbon cloths	0.8–1 W m−2	Anode removes COD and cathode recovers minerals in wastewater treatment of	[92]
Chlorella vulgaris	Single chamber	Carbon fibers	6.46 mW m−2	real dye textile wastewater. Novel MFC configuration, microalgae biofilm exposed to air	[93]
Chlorella vulgaris	Dual chamber	Graphite plates	327.67 mW m−2	Operation for 32 days with C. vulgaris and bacterial community for bioelectricity generation and bioremediation of oils	[94]
Golenkinia sp. SDEC-16	Tubular	Carbon cloth	0.57 kWhm−3	Tertiary treatment of kitchen waste	[95]
Scenedesmus acutus	H-type	Anode: Carbon cloth Cathode: Pt-based gas diffusion electrode	400 mW m−3	Use of a membrane based on polybenzimidazole, with better energy performances, cost and sustainability than Nafion.	[96]
mixed consortium of microalgae collected from lily pond	Cubic dual chamber	Plain graphite plates	3.33 mW m−2	Treatment of ghee manufacturing wastewater	[97]
Chlorella vulgaris	Tubular, polyethylene bags	Graphite felt	1200 mW m−3	10 L outdoor mMFC, low-cost materials	[98]
Chlorella sp.	Earthen pots dual-chamber mMFC	Anode: stainless steel mesh; cathode: carbon felt	1.78 W m−2	Microalgae is used as substrate in anode and as live-culture in the cathode	[99]
Chlorella pyrenoidosa	Dual chamber	Graphite electrode	1600 mW m−2	Treatment of a real batik wastewater	[100]
Chlorella vulgaris	Dual chamber	Carbon fiber cloth	1114 mW m−2	anodic effluent of the mMFC can be utilized in the cathode chamber as a growth medium for microalgae with recirculation	[101]
Chlorella sp.	Dual chamber coupled to a tubular photobioreactor	Graphite rod	2.8 W m−3	Treatment of real dairy wastewater. Influence of light/dark cycles were studied	[102]
Escherichia coli + Desmodesmus subspicatus	Dual chamber	Graphite electrode	-	The combination of bacteria and microalgae in the mMFC system showed promising outcomes in both generating bioelectricity and bioremediating nutrients in wastewater treatment	[103]
Saccharomyces cerevisiae + Spirulina platensis	Dual chamber	Graphite rod	18.30 mW m−2	Optimizing mMFC performance through adjustments in waste and yeast concentrations shows significant potential for enhancing efficiency	[104]
Microalgae +ammonia oxidizing bacteria+ denitrifying bacteria	Dual chamber	Carbon cloth	2.809 mW m−2	Integration of nitritation into the cathodic compartment, improving bioelectricity generation	[105]
Nostoc sp. + Enterobacter aerogenes	Dual chamber	Carbon plate	168 mW m−2	Bioelectricity generation using dairy wastewater as the substrate	[106]
Chlorella vulgaris + Spirulina platensis	Dual chamber	Carbon rod	323.477 mW m−2	The microalgae consortium improves performance	[107]
Cladophora sp.	Membrane-less Microbial Fuel Cells	Stainless steel and platinized titanium	619.1 mW m−2	Cladophora sp. has significant potential	[108]
Scenedesmus sp.	Dual chamber	Graphite rods	34.2 mW m−2	Domestic wastewater and sugar industry wastewater used as anolyte	[109]
Scenedesmus obliquus	PBR-MFC-SOC hybrid system	-	474 mW m−2	Fixation of CO2 and collecting bioelectricity	[110]
Spirulina sp.	Dual chamber	Anode: Copper Cathode: zinc	584.45 mW cm−2	Pepper residues and Spirulina sp. for large-scale bioelectricity generation	[111]
Chlorella sp. (closer to sorokiniana)	Dual chamber	Anode: stainless steel mesh Cathode: graphite rod	34.88 mW m−2	Use of spent engine oil and cellulosic waste	[112]
Chlorella sorokiniana	Dual chamber	Graphite sheet	47.57 mW m−3	Promising for sustainable bioelectricity generation and wastewater treatment	[113]
Chlorella sp.	Dual chamber	Graphite rods	30.1 W m−2	Removes H2S and CO2 from biogas	[114]
Scenedesmus sp.	Dual chamber	Graphite rods	42 mW m−2	Hydrogen production	[115]

and in Figure 4 indicate that Chlorella spp. is the most frequently used microalgae species, accounting for 42.9% of the total, followed by microalgae consortia at 22.9%. Spirulina spp. and Scenedesmus spp. each representing 11.4%, with other species making up the remaining 11.4%.

Different reactor configurations can be implemented, such as photosynthetic microalgae column [116], membrane algae reactors [89], photobioreactors (PBRs) [117], etc. These reactors can function as cathodic half-cells and be coupled with existing microbial processes such as yeast fermentation at bioethanol plants or wastewater treatment plants. Fermentative processes serve as anodic half-cells in creating coupled MFCs. Integrating MFCs into a bioethanol plant can generate some of the power needed for bioethanol production. Additionally, the microalgae species Chlorella vulgaris produces oil that can be used as a byproduct in biodiesel production. To make this integrated system economically viable, it is essential to determine the necessary design specifications of PBR cathodes [37,55,118].

One of the earliest reports on mMFCs studied the biomass production of Chlorella vulgaris as a biological electron acceptor at the cathode while reducing CO₂ to biomass using a mediator with low bioelectricity generation [84,119]. Subsequently, the use of CO₂ in an aqueous system as an electron acceptor was investigated. Microalgae were inoculated at the cathode without mediators, resulting in the production of phospholipids and electricity comparable to that obtained by ferricyanide-based MFCs [120].

Given that mechanical aeration in MFCs constitutes about 50% of the total operating cost [77], utilizing microalgae for oxygen production emerges as a crucial approach in the application of mMFCs. The cathode significantly affects MFC performance due to the oxygen reduction reaction (ORR) influencing the onset potential (V) value [121]. However, photosynthesis is not a continuous process and depends on the photoperiod, limiting power production to the light phase reaction [122]. Despite this, the production of oxygen, which depends on the microalgae in the cathodic chamber, is generally sufficient even when variations in electricity production occur [62].

The mMFC technology still requires additional investigation, as the highest power reported for an MFC (5850 mW m⁻²) is three orders of magnitude lower than that produced by conventional abiotic fuel cells [123,124]. While the main advantage of mMFCs over MFCs is that the replacement of external aeration for the cathodic reaction by microalgae produced oxygen [125], the integration of microalgae in the cathode chamber offers additional benefits such as production of lipids and biomass, which can be used to make biofuels and other chemicals with added value. Algae also aid in the oxygen production and the CO₂ capture, which enhances MFC efficiency and reduces greenhouse gas emissions [64,65,126].

Further research is necessary to improve the performance of MFC photosynthetic systems, particularly in terms of biological catalysts, reactors, and electrodes coupled with MFCs. A better understanding of this technology and the successful practical application of MFCs are required [86]. Using mMFCs as a platform for biofuel/biochemical production (extracted from cultivated microalgal cells) would be financially beneficial. The organics present in wastewater and the residue remaining after algal extraction can potentially be utilized as anodic feeds in MFCs. To evaluate the economic feasibility of using MFCs or individual microalgae units, a detailed techno-economic analysis that considers factors such as land and water usage and greenhouse gas emissions is crucial [62]. Another challenge is researching genetic engineering to obtain better microalgae strains with higher extracellular electron transfer abilities. This is important when considering the use of microalgae at the anode and cathode [127]. The most important parameters in mMFC research will be discussed in the next sections.

3.3. Light Effect on mMFC

Since light is the primary energy source for photosynthesis, it is crucial for the microalgae of the mMFCs to receive adequate light distribution to perform photosynthesis [96,128]. Parameters associated with lighting, such as light irradiation, dark/light regime, and wavelength, have been studied in mMFCs [96]. This is because these factors influence chlorophyll formation, the photosynthesis process, and stomatal opening in microalgae cells [129]. Increasing light intensity from 26 to 96 µE m⁻² s⁻¹ in a biocathode with Chlorella vulgaris results in approximately a six-fold increase in power production. The operation of the MFC with the Chlorella vulgaris biocathode was enhanced using intermittent illumination. However, continuous illumination could reduce the lifespan of the microalgae [62]. Operating an mMFC during the spring, with a mixed microalgal culture, showed higher bioelectrogenic activity (57.0 mW m⁻²) compared to summer (1.1 mW m⁻²) when temperature and radiation were inhibitory for the algal culture. This demonstrated that in the microalgae biocathode, dissolved oxygen levels depend on the photosynthetic activity of the microalgae, which is related to factors such as temperature and light intensity [87]. Studies under different light irradiation conditions reveal that increased current outputs during light periods are associated with photosynthetic processes at the cathode. The in situ oxygen production by microalgae in mMFCs enhances the kinetics of cathodic reactions by reducing mass transport limitations for the ORR [130]. Rapid growth of microalgae can hinder light penetration in the photic zone of the cathodic chamber, halting growth [96]. Additionally, seasonal variations in light intensity impact oxygen production, affecting bioelectricity generation in MFCs [77].

The behavior of voltage peaks related to the dark/light phase and dissolved oxygen has been extensively explained in the literature [74]. Microalgal photosynthetic activity during light conditions increases the cathodic oxygen concentration, which increases the current exerted [90]. Conversely, under dark conditions, algae consume the available dissolved oxygen. As a result, the oxygen content at the cathode diminishes, leading to insufficient oxygen for reduction processes and a consequent drop in output current [131]. This dynamic of oxygen production and consumption causes peaks and valleys in the current generation trend [78].

The mMFC with Chlorella vulgaris has shown power generation peaks during the day, with varying dissolved oxygen levels in the catholyte according to daily cycles. Dissolved oxygen (DO) levels were higher during the day (4.5 mg L⁻¹) due to photosynthetic activity, while night-time levels decreased (1.2 mg L⁻¹) because of respiratory oxygen consumption [132]. Chlorella vulgaris was studied in a double-chamber mMFC with plain graphite electrodes, finding that increasing light intensity (96 µE m² s⁻¹) enhanced power density by 600% [61].

The impact of light intensity on mMFCs has been investigated for Chlorella vulgaris. An increase in continuous light intensity from 26 to 96 µE m² s⁻¹ resulted in approximately a six-fold increase in power production. This enhancement is likely due to higher light intensity promoting greater photosynthetic activity and oxygen production. Additionally, the concentration of microalgal biomass was positively affected by high light intensity, leading to a growth rate up to ten times higher than that achieved under lower light intensity [61]. The results indicate that the increased current outputs during photoperiods are associated with photosynthetic processes. The in situ bio-oxygen produced reduces mass transport limitations for the ORR, thereby improving the kinetics of the cathodic semi-reaction, which is typically a rate-limiting step in MFCs [130]. This improvement is particularly significant for MFCs utilizing easily biodegradable substrates, such as acetate [101].

3.4. CO₂ and Trace Minerals Effects on mMFC

Under stress conditions, microalgae increase its lipid, carbohydrate, and pigment production [62,133]. The growth mode (autotrophic or mixotrophic) and biochemical composition of microalgae in mMFCs are influenced by cathodic CO₂ availability, as well as the carbon source and concentration, which affect power generation [77,96]. In the literature, the use of wastewater as fed to algae cultures has been described. However, its dark color and potentially harmful contaminants can sometimes inhibit their growth, either primary wastewater treatment or dilution being necessary [134]. In the literature, Polontalo et al. (2021) investigated the use of batik clothes wastewater (100% v) as anolyte in an mMFC with Chlorella pyrenoidosa at the cathode, achieving a COD removal of 48.5% and a maximum voltage of 0.322 V [100]. Bolognesi et al. (2021) employed dairy wastewater in the anolyte of a coupled mMFC with Chlorella sp. in the cathode, resulting in a COD removal of 99% and a power production of 2.5 ± 0.4 W m⁻³. This demonstrated the system’s efficiency in treating real wastewater and highlighted the benefits of oxygen provided by algae in improving the overall energy balance [102]. Recent research works have studied consortia of microalgae and other microorganisms, such as yeasts [104] and bacteria [105], to enhance electrical energy production through the treatment of different substrates, including wastewater [103], dairy wastewater [106], domestic wastewater [109], pepper residues [111], spent engine oil, and cellulosic waste [112]. Additionally, MFCs have been studied for their potential to fix CO₂ and remove H₂S and CO₂ from biogas [114]. These applications could be very interesting, mainly when dealing with organic wastes. However, there are few studies on substrates from agricultural activities, mainly carried out in countries like Colombia, Brazil, and other nations with significant agricultural industries [135].

3.5. mMFC Configurations

Figure 5 illustrates the four most common mMFC configurations [136,137,138]. In a coupled mMFC, a bioanodic MFC is connected to a PBR, where CO₂ is pumped directly from the MFC to the PBR. A single-chamber mMFC employs microorganisms capable of transferring electrons directly to the anode. In some cases, prokaryotic microalgae can act as electron donors at night and oxygen producers during the day. The most common mMFC design is the dual-chamber mMFC, which includes an anodic chamber with heterotrophic electrogenic bacteria and a cathodic chamber with microalgae, separated by a PEM. A photosynthetic sedimentary MFC (PSMFC) features an anode placed within a sediment layer covered with sand and a cathode compartment filled with a microalgal culture medium, leveraging natural electro-potential differences in a lagoon [37]. In single-chamber mMFCs, photosynthetic microbes such as S. plantesis, Golenkinia sp. [139], mixed cultures [83], or Chlorella sp. with mediators like methylene blue [140] can shuttle electrons to the electrode, serving as both electron donors and acceptors. This membrane-less configuration is cost-effective and features an aerobic cathode, and some microalgae species can adopt a mixotrophic metabolism, increasing the lipid content [37,139]. Single-chamber configurations represent 8% of the studies in Table 2. Dual-chamber mMFCs are the most widely used configuration, accounting for 72% of the studies according to Table 2 and Figure 4. Generally, the dual-chamber configuration can be efficiently initiated through a three-stage process involving the separate production of bacteria and microalgae cultures, replacing the mechanical aeration system with a microalgae culture and finally adjusting the light dosage to promote photosynthesis [133]. Coupled mMFCs, which represent 12% of the configurations, according to Table 2 and Figure 4, use an MFC connected to a PBR, allowing for dual productive processes. An example includes ethanol production by yeast in the anodic chamber and microalgae biomass production in the cathodic chamber [81,116]. In PSMFCs, oxygen mass transfer at the cathode is generally more efficient compared to other types of BESs. This efficiency is primarily due to the sufficient oxygen concentration produced by algae surrounding the cathode electrode. However, it is important to note that the high density of dissolved solids in sediments may still limit oxygen mass transfer in PSMFCs, potentially impacting their overall efficiency [83,131].

In the literature, a novel single-chamber mMFC configuration with a C. vulgaris biofilm exposed to air was utilized to treat real dye textile wastewater [93]. A double-chamber mMFC with graphite plate electrodes operated for 32 days with C. vulgaris and a bacterial community, showing promising results in both bioelectricity production and oil bioremediation [94]. Tubular polyethylene bags with graphite felt electrodes were used in a 10 L outdoor mMFC, demonstrating the feasibility of using low-cost materials to achieve high power densities (1200 mW m⁻³) with Chlorella vulgaris [98]. mMFCs can be applied as closed systems, for example, where microalgae are used as substrate in the anode and as live culture in the cathode [99]. Additionally, the anodic effluent of the mMFC can be utilized in the cathode chamber as a growth medium for microalgae with recirculation [101].

3.6. Electrodic Materials and Membranes

Electrodes are essential components of MFCs because they facilitate microbial attachment, growth, and biofilm formation, as well as serve as electron acceptors and donors at the end of the extracellular electron transfer process. Key features of electrodes include the following: (i) high biocompatibility, specific surface, and electroactive area to support more microorganisms and electron transfer; (ii) excellent electrical conductivity and low internal resistance to minimize power loss and enable rapid electron transfer; (iii) high mechanical strength and chemical stability to withstand complex conditions; and (iv) affordability and easy availability [141]. Carbonaceous materials are the most widely used electrode material in mMFCs (84% of the electrodes presented in Table 2). It is available in various forms such as cloth, paper, mesh, foam, graphite, felt, or reticulated vitreous carbon (RVC), and in configurations like planar, bars, packed, or brush [142,143]. Carbon is frequently used as an anode material due to its chemical stability in microbial inoculum, high conductivity, high specific surface area, good biocompatibility, and relatively low cost [142,143,144,145,146]. Larger surface areas can be achieved by using compact materials such as RVC or brush-type configurations. Metals like tungsten, titanium, and stainless steel can also be used as anode materials due to their high conductivity (compared to carbon) and low corrosivity (compared to other metals), but their cost is a limiting factor [144]. Research on developing composite materials for anodes is ongoing, with several studies using carbon-based electrodes combined with metals such as Fe, Al, Mn, or Ni [144]. Modifying the anode surface through ammonia, acid, or thermal treatment and electrochemical oxidation can also be attractive to increase bacterial adhesion and power density [147]. In mMFCs, oxygen serves as the electron acceptor at the cathode, making the ORR a limiting factor due to the low solubility of oxygen in electrolytes. This reaction occurs at the interface of three phases (liquid–gas and solid, water–oxygen–electrode), making the cathodic process the limiting element in an mMFC and affecting the MFC performance [121,145]. Often, a catalyst is required to increase the rate of oxygen reduction. The most commonly used cathode material is carbon paper, doped with a Pt catalyst on one side at 0.1 mg cm⁻² to reduce costs. Additionally, noble-metal-free catalysts, such as pyrolyzed iron(II) phthalocyanine, have been proposed for use in MFC cathodes [146,148].

Most MFC designs require the separation of anode and cathode compartments. mMFCs can use various types of separators, such as salt bridges, anion exchange membranes, and cation exchange membranes (CEMs), or even operate without a separator (membrane-less) [149,150]. The cost of separators can be a significant issue, as they account for nearly 40% of the total MFC cost [141]. Naturally separated systems, like sediment MFCs or single-compartment mMFCs, are exceptions. Separators should be impermeable to chemicals like oxygen, ferricyanide, other ions, or organic matter used as substrates, while being permeable to protons, and having long-term stability, low cost, and resistance to fouling [144]. The most commonly used proton exchange membrane (PEM) is Nafion, produced by Dupont Co. (Wilmington, DE, USA) [151].

There is no clear correlation between power density and microalgae species, mMFC configuration or electrode material (Figure 6). Notable variability within and between groups could be highlighted for microalgae species. Chlorella spp. and Scenedesmus spp. exhibit particularly broad ranges of power densities, suggesting that under certain conditions, they can achieve very high power outputs, while consortia display more moderate values with a few high outliers. Dual-chamber configuration, as the most studied one, has a wide range of power densities, also presenting the higher power densities. Single-chamber configurations have fewer data points, primarily in the lower range, while the “others” category includes some very high values, highlighting that less conventional setups can still achieve substantial power outputs. For example, 474 mW m⁻² in a novel self-breathing oxygen-consuming carbon felt SOC was integrated into the PBR-MFC to optimize O₂ concentration distribution and separate microalgae/bacteria [110].

The highest power densities observed were reached by Chlorella sp. in a dual-chamber configuration and with carbon electrodes. Carbon electrodes are versatile in their use and efficiency, and other materials have potential applications. In the literature, it has been reported that a power density of 1926 mW m⁻² was reached using dead microalgae biomass (a potential pollution vector in streams) as a substrate for anaerobic sludge at anode and CO₂ generated at the anode to grow freshwater microalgae at the cathode, demonstrating that a synergetic relationship between biomass degradation and microalgae cultivation [82]. A power density of 1600 mW m⁻² was reached using batik wastewater as anolyte and adding yeast to increase mMFC’s ability to degrade components of the wastewater [100]. A power density of 1114 mW m⁻² was reached using the anodic effluent in the cathode chamber as a growth medium for microalgae with an optimization of hydraulic retention time in the anode [101]. These three studies have in common the use of non-conventional wastewater and an integrated approach of the anodic and cathodic phenomenon, reinforcing the importance of optimizing multiple variables for enhanced performance.

3.7. Electron Transfer Mechanisms

The production of electricity in mMFCs depends on the redox potential of the electron acceptor availability [62]. The mechanisms of electron uptake for microalgae at the cathode are not yet fully understood [87]. Figure 7 presents four possible mechanisms for cathodic reactions in mMFCs [77,84,152]:

• Direct CO₂ reduction: Carbon dioxide is reduced without the involvement of microalgae or their metabolic products.
• Direct electron transfer from cathode to algae: Algae directly receive electrons from the cathode, with CO₂ acting as the electron acceptor. In this process, microorganisms may participate in reactions involving cytochrome C and indirect transfer mediators such as pyrroloquinolinequinone (PQQ) [153].
• Mediator-assisted electron transfer: This can occur with endogenous mediators produced by the microalgae or when using external mediators. In the literature, it has been proposed that when a mediator is utilized in the cathode compartment for electron transfer, electrons travel from the anode to the catholyte, where the oxidized mediator is reduced. The mediator then enters the microalgal cells, releasing its electrons, and returning to its oxidized state. The cultivated microalgae consume these transferred electrons in their metabolic pathways, enabling the conversion of CO₂ into biomass and oxygen. The oxidized mediator is released back into the liquid bulk, beginning the cycle anew [37,55,84,154]. For example, C. vulgaris cultivated in a cathodic chamber acts as a final electron acceptor through electron transfer mediated by methylene blue and thionine blue [116]. In some cases, extracellular electron transfer may be supported by autonomous mediators, such as c-type cytochromes found in the outer membrane of algal cells [77].
• Oxygen reduction or indirect electron transfer: This occurs due to the reduction of oxygen produced during photosynthesis. Photosynthetic oxygenation of the cathode can happen in two ways: microalgae can be grown in separate photobioreactors, with oxygen transferred to the cathode chamber, or the algae can grow directly in the cathode chamber, generating oxygen in situ [154].

The mMFC technology is based on the natural lagooning process that occurs when wastewater accumulates in ponds or basins [63]. In this process, a synergistic effect between heterotrophic microorganisms and algae takes place. Algae produce oxygen through photosynthesis, while heterotrophic microorganisms consume the organic matter in the water, utilizing the oxygen produced by the algae [37]. This integration allows algae to use solar energy and CO₂, making MFCs more environmentally and economically sustainable. The general biochemical reactions that occur during illumination are presented in Equation (1) for the anode and Equation (2) for the cathode [37,67,96].

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6+6{\mathrm{H}}_2\mathrm{O}\to 6\mathrm{C}{\mathrm{O}}_2+24{\mathrm{H}}^{+}+24e^{-}$$

(1)

$$6\mathrm{C}{\mathrm{O}}_2+24{\mathrm{H}}^{+}+24e^{-}\to \mathrm{C}{\mathrm{H}}_{1.71}{\mathrm{O}}_{0.4}{\mathrm{N}}_{0.15}{\mathrm{P}}_{0.002}\left(microalgalbiomass\right)+3{\mathrm{O}}_2$$

(2)

During the light phase, algae perform photosynthesis producing oxygen and resulting in the highest electricity production in MFCs. In contrast, during the dark phase, when dissolved oxygen levels are at their lowest, electricity production suggests that nitrates or sulfates serve as electron acceptors [21]. Oxygenic photosynthesis involves three membrane-bound protein complexes that sequentially transport electrons from water to NADP⁺. Small, mobile molecules called plastoquinones and plastocyanins carry electrons between these large protein complexes over relatively long distances, playing a vital role in electron transfer during photosynthetic energy conversion. Investigating this mechanism under different conditions may help elucidate microalgal processes at the biocathode [87].

Although there is not enough information to elucidate mechanisms of microalgae interaction with the cathode, recent studies have suggested an association of Reactive Oxygen Species (ROS) levels, such as H₂O₂, with electric stimulation and cellular lipid accumulation [155,156]. When H₂O₂ was applied to Dunaliella salina, the concentration of the oxidative stress marker, malondialdehyde, increases as an indicator of membrane peroxidation implying an increase in ROS levels. Microalgae can modify its photosynthetic system under stress, resulting in a decrease in the gene expression of various proteins forming up the photosystem complexes I and II. As a result of decreased photosynthesis rate, overall anabolic reaction flux is severely constrained. In this context, algae cells may favor storage of energetic molecules, such as lipids under stress that could be caused by the interaction with electrons in the cathode [157]. The electrical treatment of E. gracilis by the application of direct current power supply at different current intensities in a thin platinum plate electrode induced the generation of ROS, indicating response to stress, stimulating the paramylon production by enhancing the glucose uptakes and by increasing the stress level [158]. Figure 8 explains a possible mechanism of interaction of C. sorokiniana with the cathode of an mMFC [159,160].

After identifying the metabolic pathways and key enzymes in the microalgae–electrode interaction, genetic engineering has the potential to improve lipid, biomass productivity and energy generation in mMFCs [161]. Metabolic engineering has been applied in microalgae production technology in several lines:

• Improve carbon fixation through the modification of key enzymes such as RUbisco, SBPase, pseudoheptulose 1.7 bisphosphatase and Phosphoenolpyruvate carboxylase [162].
• Improving the use of light, by the modification and regulation of the synthesis of antenna pigments [162] and modification of plastoquinone (PQ) involved in the coupling of protons and electrons in electron transfer [9].
• Improving downstream processes, by auto flocculation through the expression of glycoproteins located in the extracellular matrix [162,163].
• Modify specific enzymes of the fatty acid synthesis pathway by the overexpression of enzymes [164] such as Acetyl-CoA [72,161], diacylglycerol acyl transferase (DGAT) [162].
• Blocking competitive pathways such as carbohydrate synthesis to enhance lipid accumulation in microalgae [165,166].

In this sense, research on genetically modified microalgae for enhanced electron transfer would provide valuable insights into the future development of mMFC technology.

3.8. Scale-Up and Economic Feasibility

The MFC designs must be scalable to enhance wastewater treatment capacity and improve cell performance in terms of electricity generation. The most commonly employed scaling strategy involves the installation of multiple units connected in series in a modular configuration [167,168,169], as output power increases linearly with the number of units [170,171]. Using this approach, working volumes of up to 850 L have been achieved, reaching power densities of 0.043 W/m² [172].

The main challenges associated with MFC scaling include increased limitations in substrate transfer to the cells and electron transfer to the anode, establishing an appropriate electrode area-to-cell-volume ratio, pH control, developing large-scale materials without significantly increasing costs, and maintaining a stable microbial community in large-scale systems [171].

Economic feasibility studies on MFC scaling have shown that capital costs can be up to 30 times higher than those of conventional wastewater treatment, with membrane and electrode costs accounting for up to 80% of these expenses [173,174]. Carbon anodes with a metallic core and stainless steel cathodes with nickel foam have been recommended due to their low cost [171].

Although knowledge on MFC scaling has been increasing in recent years, studies on the scaling of Microalgal Microbial Fuel Cells (mMFCs) remain very limited [125]. A 10 L-scale study employing polyethylene bags for the cathodic chamber, clay slabs for the anodic chamber and as a membrane, graphite electrodes, and phosphate rock as a phosphorus source reported a microalgal biomass productivity of 0.307 kg m⁻³ d⁻¹ and an energy generation of 11.53 kWh m⁻³ with a total cost of $11,225 [98].

4. Perspectives in mMFC

In order to reach full-scale implementation of mMFC, some limitations must be overcome such as stability in anodic and cathodic reaction, voltage generation, investment cost, feasibility of scale-up, selection of microalgal strains, lightening, nutrients, pH gradient, high energy demand for harvesting, modeling and metabolic research [62,77,84,96,128,130,133,134,175]. Although mMFCs are an eco-friendly and sustainable approach, there are still bottlenecks related to its performance that should be solved; these are presented below.

4.1. Geometry

The mMFCs need a large surface area, compared with conventional wastewater treatment, to receive the required light for microalgae photosynthesis, resulting in higher investments and operational costs [62,77,96,134]. Additionally, the mMFC configuration can be improved to minimize the loss of CO₂ when transported from the anode to the cathode chamber [128].

4.2. Electrodes and Cathodic Reaction

The mMFCs have relatively low voltage generation and unreliable electricity production. The cathodic ORR is oxygen-limited due to DO production constraints in the light phase, increasing internal resistance and reducing power density [101]. This limitation can be avoided by using redox mediators which facilitate electron consumption. Unfortunately, these chemicals are unsustainable, making them unviable [176].

In order to ensure high performance in the biocathodes of the mMFC, an efficient oxygen diffusion to the electrode is required [77].

The generation of energy in mMFC is reduced when increasing the cathodic biofilm thickness, both at the electrode site and at the current collector, generating phenomena of catalyst poisoning and biofouling [96,134].

When using an easily biodegradable substrate at the anode, such as acetate, the global electron transfer efficiency is limited by the cathodic process. Recent research has shown that electron transfer efficiency is lower in mMFCs compared to MFCs, as it depends on light availability and algal respiration. For instance, when actual wastewater is used as the anodic substrate, it reduces the electrons released by substrate oxidation. This, in turn, diminishes the limiting influence of microalgal metabolism on cathodic activity [101].

4.3. Physicochemical Influence of Membrane and pH

Bio-incrustations in the membrane are responsible for the low rate of proton transfer. Consequently, to maintain continuous operation, periodic membrane changes are necessary, which raises the cost of the process [134]. Crossover of substances through membrane [124] and their high cost are also challenges in mMFC application. Further research is required to identify a low-cost membrane that offers minimal oxygen and substrate diffusion while ensuring high proton conduction [128].

In double-chamber mMFC, the pH membrane gradient lowers output power and cell voltage. This restriction results from nonspecific proton exchange across the membrane, acid formation at the anode, and alkalinity formation at the cathode [96,128,133,134]. The use of buffer systems is an alternative; however, it would increase the operational cost [77].

4.4. Microalgae Harvesting and Scale-Up

The high cost of structural arrangements and the energy consumption for the cultivation and harvesting of microalgae are some of the drawbacks of mMFCs [128,134]. Developing new sustainable treatment techniques, such as integrated mMFCs–photobioreactors, could enhance biomass recovery and produce value-added products from algal biomass [77].

The system must balance the rates of CO₂, oxygen, and biomass production and consumption, and controlling these fluxes is difficult for full-scale operation. mMFCs have not yet been properly deployed on a full scale. Considering the limitation of the light source, mMFC applications may require the installation of an anodic chamber as well as a large-scale microalgal cultivation. Reducing costs of CO₂ supply and aeration for mMFCs would also be very interesting for the full implementation of the technology [62,77].

4.5. Selection of Microalgal Strains

Scenedesmus sp., Chlorella sp., and Chlamydomonas reinharditi are the most commonly used species in mMFCs [96,134]. However, it is crucial to continue researching the selection of algal strains suitable for use in both the anodic (as substrate) and cathodic compartments to develop a closed mMFC system [77]. Additionally, characterizing the biochemical composition of microalgae, such as their carbohydrate and lipid profiles, and their metabolite production is essential in evaluating their potential for power generation and integrated systems that produce high-value products [134].

4.6. Modeling and Metabolic Research

Understanding and controlling the mechanisms of algae-based cathodic processes, such as lipid accumulation and biomass synthesis, are crucial for improving power generation, and biomass recovery in mMFCs [77]. Methods such as flux balance analysis (FBA) are powerful tools that provide details about metabolic fluxes, identifying those that are more active or highly correlated with the process under study. Metabolic flux analysis can be used to identify pathways that can be manipulated for optimal results in terms of electricity generation, biomass and metabolite productions. By analyzing stoichiometric models of the reaction network, the theory of reaction kinetics, and obtaining experimental measurements of metabolite concentration and mass balances, metabolic fluxes for each metabolite can be determined. Modifying different pathways will change the levels of NADH and FADH₂ and, consequently, alter the power output in an MFC [145].

5. Conclusions

BESs have seen significant advancements in the past decade due to their emerging role as sustainable technologies for concurrent electricity production, wastewater treatment, and CO₂ fixation. BESs also offer excellent prospects for the clean and efficient production of high-value fuels and chemicals using microorganisms. A noteworthy type of BES is the mMFC, which generates energy by exploiting the capture of CO₂ for biomass and lipid production. Building upon the extensive comparison of mMFC configurations and parameters presented throughout this review, it becomes evident that achieving significantly enhanced efficiency in mMFC technology requires a multifaceted approach. Key areas of focus include increasing power densities, reducing internal resistance, enhancing current density, and producing high-value products. In this sense, the application of unconventional wastewater and the development of technologies that integrate anodic and cathodic systems could be a strategy in future research in the frame of the biorefinery approach. Other important research areas are development of carbon-based electrodes with metal catalyzers that maximize surface area and conductivity; optimization of dual camera configurations that minimize internal resistance; selection of microalgae strains with high oxygen production and improved electron transfer capacity; and meticulously control of operational parameters like light, CO₂ and nutrient specific for each strain. Additionally, understanding the mechanisms of electron transfer, developing novel reactor designs and architectures, and scaling up reactor volume and operation time are crucial. Despite the significance and efforts to advance mMFC technology, limitations still exist that hinder its widespread use, such as low Coulombic Efficiency and a limited understanding of the bioelectrochemical mechanisms behind electron transfer between algae and electrodes. It is vital to comprehend the conditions provided by mMFCs concerning electron capture, lipid synthesis, and accumulation. New studies should utilize modern techniques to explore the complex world of metabolism, aiming to enhance our understanding of the metabolic characteristics of microalgae, with the goal of promoting lipid formation in mMFCs.