Microbial electrochemical systems for bio-solar energy conversion: recent advances, interfaces, future challenges

1 Introduction

Overpopulation and unsustainable living choices cause serious problems for humanity. The global population is expected to reach 9.8 billion by 2050 (Kah et al., 2019), and as growing nations like China and India continue to grow economically, the world’s energy needs will rise. Energy consumption by 2040 would increase by 32% (compared to 2013) to reach 17,934 million tonnes of oil equivalent, even if new energy regulations could be effectively implemented worldwide (Zhang et al., 2022). This would necessitate an increase in oil production of over 15% and would correspondingly boost CO₂ emissions even higher. The continuous burning of fossil fuels contributes to greenhouse gas (GHG) emissions into the atmosphere, such as CO₂, which causes global warming and ocean acidification, among other detrimental environmental effects (Baicha et al., 2016). However, it is predicted that the economy may need to decarbonize in order to prevent global warming from rising above 2 °C, which requires new energy. In review policies, but more critically, lowering the utilisation of fossil fuels throughout the energy and materials supply chain should be targeted. This can only be accomplished by means of the creation of innovative technology (Tschörtner et al., 2019). Thus, the development of renewable technologies to provide energy that will reduce CO₂ emissions to nearly zero is the long-term solution to such environmental issues. The global demand for sustainable energy solutions continues to intensify, driven by environmental concerns and the depletion of fossil fuel reserves. Bioelectrochemical systems (BESs), particularly Microbial Fuel Cells (MFCs), Figure 1, have emerged as a promising avenue for converting biomass energy into electrical power (Zhou et al., 2013). The application and principle for the same are presented in Table 1. The current review summarises the advancements in the field of bioenergy production, focusing chiefly on the generation of electricity using microorganisms, microbial solar cells (MSCs), microbial fuel cells, Biophotovoltaics cells, photobioanode bioelectrochemical cells for long-term electricity generation, microbial photobioelectrochemical cells, intimately coupled photobiocatalysis (ICPB), and dye-sensitive solar cells based on pigments obtained from microorganisms.

TABLE 1. Principle and applications of microbial fuel cell.

The accelerating consumption of fossil fuels has led to environmental deterioration, making the development of sustainable energy pathways crucial. Advances have been made in generating electricity from solar, wind, and hydro sources, but converting intermittent electricity into energy-dense fuels remains a challenge due to undeveloped catalytic chemistry. Microbial systems offer a promising solution, utilizing extracellular electron transfer pathways and metabolic processes to produce valuable chemicals from inorganic materials. They also have inherent self-repair and reproductive mechanisms, addressing stability issues. Achieving sustained, efficient, and selective reduction of CO₂ to multi-carbon products depends on understanding the materials/biology interface, aided by new analytical methods. This review focuses on microbial (photo)electrodes and semiconductor nanoparticles interfaced with microbes. It identifies knowledge gaps and discusses emerging techniques such as microelectronic measurements, QCM-based methods, spectroelectrochemistry, and time-resolved spectroscopy to provide mechanistic insights into biohybrid systems (Kuruvinashetti and Kornienko, 2021).

Photocatalytic material-microorganism systems combine photocatalytic materials and microorganisms, leveraging microbial self-replication and metabolic pathways for stability and scale. Photocatalytic materials generate electrons upon light absorption, which microorganisms utilize for synthesizing complex products. Semiconductor materials are often distributed outside, on, or inside microbial cells, with light exciting these materials to produce electron-hole pairs that transfer electrons to microorganisms for metabolic processes. Extracellular materials require an electronic medium for electron transfer, as seen in systems using materials such as TiO₂, methylviologen, and indium phosphide nanoparticles to facilitate light-driven synthesis, such as hydrogen or organic acid production (Song et al., 2022).

2 Microbial electrochemical system

Microbial electrochemical systems (MESs) are a spectrum of technologies where biodegradable substrates are converted into electrical energy or valuable compounds by electroactive microorganisms exchanging electrons with electrodes (Klein et al., 2023). In comparison to conventional energy recovery, modern MES research increasingly focuses on resource valorisation, modular system development and wastewater treatment to enable circular-economy applications (Deng et al., 2023). Additionally, recent research shows that stack configuration, operational control, reactor architecture and electrode design have a significant impact on performance, especially when transitioning from lab device to pilot-scale installation (Schröder et al., 2026). Simultaneously, MES capability is being extended into solar-assisted energy conversion through hybrid systems that combine microbial catalysis with photoactive or semiconductor components (Kargul and Jacquet, 2023). However, there are still significant challenges in this sector, such as low power densities, mass-transfer limitations, interfacial losses, and a lack of knowledge regarding long-term stability under non-ideal working conditions (Thulluru et al., 2026).

2.1 Fundamentals of microbial fuel cells

Microbial Fuel Cells MFCs are devices that use electrochemically active bacteria to oxidise different carbon sources or even organic wastes to transform chemical energy into electrical energy (EAB). In order to maintain a steady power density without emitting carbon into the environment, the electrons generated by microbial metabolism are collected. Among the many variables, maximum power density, Coulombic efficiency, and occasionally the rate at which chemical oxygen demand is eliminated, evaluate the effectiveness of the device (Tharali et al., 2016). These devices use a variety of organic and inorganic substrates and bacteria to produce electricity through bacterial metabolism. MFCs are innovative bioreactors that use microorganisms to biocatalyze the conversion of a variety of wastes (domestic waste agriculture, and the food processing sectors). MFC is a promising method that provides easy recycling of waste, direct clean, green energy generation, and the use of byproducts from many sources, such as the amalgamation of different types of waste and microorganisms. MFCs are bio-electrochemical devices with a wide range of structural configurations, including single- and double-chamber designs and those with or without proton exchange membranes (PEM). PEMs exhibit beneficial characteristics such as strong cation conductivity, low internal resistance, and the ability to function continuously without causing any disruption to the MFC, in addition to their selective permeability to protons. Therefore, PEM is thought to be the membrane separator that is employed in double-chambered MFCs the most frequently. Single-chambered MFCs are those that integrate the cathode and anode in a single chamber. Dual-chamber MFCs are those that have anodic and cathodic chambers divided by a salt bridge, or PEM, with the anodic chamber maintained in anaerobic conditions and the cathodic chamber capable of holding either aerobic or anaerobic conditions. The microbial metabolism of organic compounds develops bio-potential that results in the generation of electrons and protons. In the MFC system, electrons are transferred to the anodic surface, and electrons and protons react in the cathode chamber with a simultaneous decrease in oxygen and water levels. Microbial fuel cells come in two varieties: one that uses a mediator and the other that does not. The chemical components called mediators are what make MFCs work better (Gurikar et al., 2021).

Microbial fuel cells come in two varieties: one that uses a mediator and the other that does not. The chemical components called mediators are what make MFCs work better. The effectiveness of the MFC system has been enhanced in several experiments by adding mediators, including Neutral red, Methylene Blue, Thionine, Potassium ferricyanide, Anthraquinone compounds, and Cobalt sepulchrate. Neutral Red (NR) at concentrations of 0.1–1.0 mM (redox potential: −320 mV vs. SHE) enhanced maximum voltage from 401 mV (mediator-less) to 482 mV (∼20% enhanced) (da Silva et al., 2022). Methylene Blue at 0.1–1.0 mM (redox potential: +11.3 mV) showed a great improvement, increasing voltage to 684 mV (∼70% increase over control) (Khaksar et al., 2010). Thionine at 0.1–0.5 mM has a redox potential of +0.064 V (64Mv), making it a good alternative to Methylene Blue but expensive (Chiang et al., 2011; Liu et al., 2011). Potassium ferricyanide at 0.5–5 mM (Redox potential: +0.75 V) is generally used as a cathodic mediator (Li et al., 2008). Anthraquinone (at 0.05–0.5 mM Concentration, redox potential: −180 mV) increases electron transfer through quinone-based redox cycling (Tang and Ng, 2017). Cobalt ferrite (at 0.1–0.5 mM concentration) also show increased MFC performance (Rethinasabapathy et al., 2021), though with the lower voltage gains compared to Neutral Red and Methylene Blue. The choice of mediator depends on its redox potential relative to microbial electron donors.

Second one is Electrogenic microorganisms, like Shewanella and Geobacter, are frequently employed in MFCs. Saccharomyces cerevisiae, Candida melibiosica, and Kluyveromyces marxianus, the yeasts and archaebacteria, such as cyanobacteria and proteobacteria, are encouraging strains to produce electricity. While eukaryotic algae act as both electron producers and acceptors in the anode and cathode, respectively. Mixed or pure cultures can be used as anodic inoculants. Pseudomonas aeruginosa, and Enterobacter aerogenes are used in co-culture. With an ideal oxygen supply, this type of co-culture system may generate electrical current at a rate more than 400% higher than before (Wang et al., 2022). Moreover, genetic engineering shows promise in enhancing strains’ electrochemical activity. A 77% increase in the present generation can be achieved through the alteration of cytochrome c maturation (Jin et al., 2013). The electricity output of the Pseudomonas aeruginosa strain-based MFC can be increased by assembling type IV pili with high conductivity the substrate expansion for power generation in MFC systems. They achieved the conversion of methane to electricity by developing a synthetic consortium with the main strains comprised of engineered Methanosarcina acetivorans, Paracoccus denitrificans, and Geobacter sulfurreducens. Wastewater generated from municipal waste, Septic tank waste, food industry, Dairy industry, agro-processing, rice mill, palm oil mill effluent, molasses wastewater, distillery wastewater, poultry waste, and swine wastewater are promising substrates to achieve efficient operation of MFC systems and reduce substrate costs. Out of these wastes, molasses waste is reported to have a maximum power density generation of 1410.2 mW/m2. Similarly, (Lignocellulosic Biomass) substrates like barley straw, Miscanthus, pine, potato peels, rice straw, banana peel, and sago hamp are efficiently used as substrates in MFC systems. The potato peels generated a maximum power density of 152.55 mW/m2 (Wang et al., 2022).

2.2 Operating principle, advantages and limitations of MFCs

MFCs are managed by using electroactive microorganisms to oxidize organic material at the anode and deliver the liberated electrons to a cathode via an external circuit. This simple setup permits the treatment of organic wastes and the direct conversion of biological energy into electricity (Hil Me et al., 2022). The main tendency of MFCs to operate with inexpensive feedstocks and biologically driven catalysts is a major benefit that proves them appealing for sustainable resource recovery. MFCs have also been investigated for decentralised power production, nutrient removal, sediment remediation, and wastewater treatment (Álvarez-Ley et al., 2025). Their operation is regulated by microbial activity, electrode characteristics, reactor geometry, and internal resistance, all of which affect the overall voltage and power production (Jeremias et al., 2025). Despite the concept having elegance and environmental benefits, modern MFCs often produce very low power densities when compared to traditional electrochemical systems. Performance is significantly decreased by mass-transfer restrictions, electrode fouling, membrane losses, and heterogeneity in microbial populations. Due to all these limitations, scale-up and long-term stability are significantly challenge to practical deployment (Chakma et al., 2025; Dakal et al., 2025). To overcome these restrictions, recent research has concentrated on enhancing reactor design, electrode materials, and system integration. In this regard, hybrid and solar-assisted MFC setups have shown promise as ways to increase functionality and boost energy conversion efficiency (Zhang et al., 2019).

2.3 Introduction to solar-assisted microbial fuel and related hybrid system

To increase energy conversion beyond traditional MFC operations, solar-assisted microbial fuel cells and related hybrid systems combine light-driven components with microbial electrochemical activities. Solar energy can be utilized in these platforms to promote anodic metabolism, stimulate photocathodic reactions, or lower the amount of external energy needed for bioelectrochemical conversion (Xiao et al., 2020). This hybridization is particularly interesting because it combines the renewable nature of solar energy with the catalytic selectivity of microbes. Therefore, solar-assisted systems have been investigated for the production of electricity, the evolution of hydrogen, the treatment of wastewater and the degradation of pollutants. These methods aim to address the low production and poor scalability of traditional MFCs by adding a renewable light-harvesting component (Cai et al., 2025; Xie et al., 2023). However, practical applicability is still restricted by interfacial instability, material cost, and system complexity. To increase durability and efficiency, recent research has concentrated on enhancing the connection between microbial and photoactive domains. Thus, a new avenue for sustainable bioelectrochemical energy conversion is represented by solar-assisted MFCs and associated hybrid systems (Cai et al., 2025; Xiao et al., 2020).

2.3.1 Microbial solar cells

New biotechnological systems that combine photosynthetic and electrochemically active organisms to produce in situ green power or chemical compounds, including hydrogen, methane, ethanol, and hydrogen peroxide, are collectively referred to as microbial solar cells (MSCs). The MSCs operate on the following principle: phototrophic bacteria carry out photosynthesis and transport organic matter to the anode compartment, followed by anodic oxidation of organic matter by electrochemically active bacteria, which leads to the cathodic reduction of oxygen. In a review by David Strik et al., they grouped MSCs according to the approach by which organic matter is transferred, such as in a higher plant via rhizodeposition, a phototrophic biofilm via diffusion, or a photobioreactor or coastal marine ecosystem that uses pumps for translocation. Microbial cells living in association with higher plants are called plant microbial fuel cells (PMFCs) (Strik et al., 2011). Root exudates and rhizodeposits secreted by plants in the rhizosphere region due to photosynthetic activity are metabolically processed by the microbial population residing in the rhizosphere region. These exudates are broken down by the microbial community in the rhizosphere, or root zone, to release electrons. These electrons are trapped by the MFC’s electrode (anode), which are passed through the load and reaches another electrode (cathode) to complete the circuit. Thus, the redox gradient generated at the electrode is responsible for power generation. This process results in bioelectricity generation. In order to function well, the redox mediator needs to have a thermodynamically advantageous redox potential in relation to the biological sensitizer so that the sensitizer regenerates efficiently and radially following photoexcitation. Continuous catalytic cycling is maintained by this energetic alignment, which propels electron transfer in the appropriate direction without any requisite direction for further energy. Charge transport is thus more proficiently maintained, which eventually raises overall productivity and performance. The PMFC offers uninterrupted power generation for 24 h per day in conditions like weather change and climate change. The anode and the living plant are buried in the supporting matrix, which is made up of flooded soils, paddy, wetland or garden soils, sediments, vermiculite, and graphite granules, in PMFC operation. However, as electrochemically active bacteria (EABs) multiply naturally to oxidise available organic matter (OM) and transmit electrons to the anode, no additional substrate or enrichment is needed when using a natural supporting matrix (Kabutey et al., 2019). In bio-sensitised solar cells, Bacteriorhodopsin has an advantage over photosystem II due to its greater structural stability and operational robustness, which increase device longevity by reducing deterioration during extended light and operation (Teodor et al., 2022).

Plant selection for PMFC applications is dependent on various important factors that affect the efficiency of bioelectricity production (Table 2). The root exudate is a major aspect because plants with higher root exudate levels offer more substrate for microbial oxidation, which directly improves electron transport to the anode. Extensive root systems, root porosity, and the growth of a strong rhizosphere microbial population that promotes effective electron transfer are all equally significant root system features. Another important factor is the photosynthetic route. Compared to C3: rice, Reed manna Grass and C4: maize, Sugarcane exhibit better rhizodeposition and solar conversion efficiency, which increases biomass production and substrate availability for electrochemically active bacteria. Another crucial factor is biomass production; high rates of biomass and photosynthesis guarantee proper PMFC performance by continuously supplying organic materials for microbial metabolism. Plant species hardiness and growth rate must be considered, fast-growing, hardy plants with local adaptability can flourish in a variety of environmental situations with less maintenance. For practical applications, resistance to circumstances is crucial since plants that can bioaccumulate and adapt to semi-arid or wet environments can sustain PMFC performance in a variety of ecosystems. Lastly, the non-invasive nature of the chosen species is crucial, to prevent the cultivation of invasive plants that could disturb local ecosystems, combined together these selection criteria guarantee the best possible PMFC performance while preserving environmental stability (Ancona et al., 2024; Rusyn and Gómora-Hernández, 2024).

TABLE 2. Comparative performance of plant species in plant microbial fuel cells (PMFCs).

2.3.2 Biophotovoltaics system

The process via which photosynthetic bacteria, whole or in part, produce extracellular electron transfer and generate electrical power in response to light is known as Biophotovoltaics (BPV). Biophotovoltaics is a subcategory of PMFCs, but exclusively related to systems that produce current with sunlight and water. Activation of photosynthesis and therefore the photosynthetic electron transport chain (PETC) by illuminating a light source leads to the generation of so-called photocurrents, photoresponse, or photo power outputs. One electrode was located in each of the two chambers that make up the original BPV setups, which are divided by a proton-permeable membrane. The device can be operated as a microbial electrolysis cell using the two-chambered system. In the past 10 years, single-chamber systems that use biofilms or microorganisms that are immobilised at the anode have gained popularity (Tschörtner et al., 2019). With the right bio-electrochemical system, an anode can collect the current and use it to power tiny electronic devices. Oxygen-producing photosynthesising bacteria, cyanobacteria, or unicellular eukaryotic algae are commonly used in BPV systems (organisms whose photosynthesis takes place in a discrete subcellular compartment, the chloroplast). Subcellular fractions such as isolated chloroplasts, photosynthetic membranes (thylakoids), or photosystems can also be used under the term “bottom-up photovoltaic synthesis” (multiprotein complexes that carry out the primary photochemical reactions of photosynthesis). Whole-cell BPV systems consistently produce higher current densities than subcellular systems due to their inherent biological advantages (Howe and Bombelli, 2020). Whole-cell cyanobacterial systems, such as Synechocystis sp. PBES, leverage continuous self-repair processes and an intact photosynthetic electron transport chain to produce current densities of 1200–1500 Ma/m² (power density: 100-150Mw/m²) (Saar et al., 2018). In cyanobacteria-based systems, the exoelectrogenic channel operates as the electron-export channel, allowing photocurrent generation in a DSSC-like framework by transferring photo-generated electrons from the photosynthetic device to the external electrode (Liu and Choi, 2021). Whole-cell microalgal systems (Chlamydomonas) generate 800–1100 mA/m² (60–90 mW/m²) using fully functional oxygenic photosynthesis (Wey et al., 2019). Bryophyte MFC systems (S. oneidensis MR-1) showed 1050 mA/m² (85 mW/m²) with power densities over 0.5 W/m² in optimized flow-based designs (Saar et al., 2018). Where Subcellular BPV systems perform significantly less; an isolated photosystem with mediators produces only 120–200 mA/m² (15–25 mW/m²), photosynthetic enzymes with dyes produce 80–150 mA/m² (10–18 mW/m²), and isolated chloroplasts with electrodes produce only 60–100 mA/m² (8–12 mW/m²) (Kothe et al., 2013; Wey et al., 2019). Subcellular systems exhibit an accelerated degradation of isolated enzymes, a lack of metabolic cofactor regeneration, and resistance to photoinhibition, thereby limiting their practical current output (McCormick et al., 2015). While whole cells yield larger current densities than subcellular fraction-based systems. These are more desirable for widespread and long-term application since they are self-replicating and self-repairing. BPV systems are also called photosynthetic microbial fuel cells (FMCs) to highlight their resemblance to the well-known FMCs, which are another form of (Extracellular electron transfer) where heterotrophic bacteria digest exogenous organic resources and produce external electrical currents. However, BPV systems are different from MFCs in that the electrical current in BPV systems ultimately originates from electrons produced by the light-driven oxidation of water and transferred through the photosynthetic electron transfer chain, rather than from heterotrophic metabolism (this chain is organised according to the Z scheme proposed by Hill and Bendall 60 years ago). Using chloroplast thylakoids, the first BPV systems were described about 50 years ago (Howe and Bombelli, 2020).

The prokaryotic light-harvesting microorganisms known as cyanobacteria exhibit a wide range of morphological, physiological, and metabolic traits that seem to be crucial for their survival in a variety of ecological contexts. Anabaena, Calothrix, Pseudanabaena, Nostoc, Synechococcus, Synechocystis PCC6803, and Lyngbya are among the several genera of cyanobacteria that have been found to contain pre-existing electrogenic activity that stems from the P-ETC (photosynthetic electron transport chain) (Sarma et al., 2016). Most biophotovoltaic studies use cyanobacteria, either as pure cultures or environmental samples. Cyanobacteria are used in the majority of biophotovoltaic research, either as pure cultures or as ambient samples. Synechocystis sp. PCC 6803, Synechococcus, Nostoc, Arthrospira platensis, Anabaena variabilis M-2, Oscillatoria limnetica, Leptolyngbia sp., and Lyngbya are among the cyanobacterial species that have been tested in BPV thus far. Using Synechococcus sp., the maximum power density recorded to date was roughly 610 mW m−2. The electrogenic activity of various wild-type cyanobacterial species and an unidentified phototrophic consortium was compared by strain BDU 140432. The microbial consortia from a freshwater pond had the highest electrogenic yield. The most common cyanobacterium for BPV tests is Synechocystis sp. PCC 6803 (4).

2.3.3 Microbial fuel cell with photobioanode

Photobioelectrochemical cells (PBECs) are a type of hybrid photosynthesis reactor that has been invented recently (PBECs). In these systems, solar energy is captured by light absorbers submerged in the electrolytes. This energy is then utilised to split water at the (photo)anode and produce reducing equivalents at the (photo)cathode. Then, by obtaining reducing equivalents from the (photo) cathode, microbial or enzymatic catalysts will lower CO₂. To build the TiO₂/Si nanowire photobioanode system, a hybrid photosynthetic design combining a TiO₂ photoanode and a light-absorbing Si nanowire photocathode, which is divided by a cation-exchange membrane. Light was exposed to both electrodes through quartz windows with 95% optical transmission. Performance evaluation was conducted under standardized illumination 100 mW/cm² (1 sun, AM 1.5G), wavelength range of 400–1100 nm (visible 400–700 nm for TiO₂, near-infrared 600–1100 nm for Si nanowire), and photon flux of 1.2 × 10¹⁷ photons/cm²/s. A 300 W xenon lamp with UV-cut filter simulated natural sunlight, with both electrodes illuminated from the same side (Abbas et al., 2024; Jalili et al., 2024; Li et al., 2025). Three changes were made to the Si nanowire photocathode’s surface: (1) a doped n + layer to boost photovoltage; (2) a TiO₂ layer to stabilize the pH at neutral; and (3) a layer of Ni and Pt to quicken the transfer of reducing equivalents to the biocatalyst. As the microbial catalyst, Sporomusa ovata, an acetogen, was used. The PBEC maintained a photocurrent of 0.3 mA cm−2 under simulated sunlight for over 120 h, about 5 days. In a photobioanode, light and microbiological powers are combined in one electrode. Typically, the photobioanode is a two-sided, bifunctional, quadratic electrode. Usually flat and coated in glass, one side is exposed to light, while the surface on the back, or dark side, encounters electrogenic bacteria. This electrode is typically a semiconductor, like those used in solar panels. The microorganisms in the biofilm contribute electrons to the newly formed electron holes, which are produced when this semiconductor anode is exposed to radiation. According to the theory, microorganisms allow electrons to be excited more quickly from the valence band (VB) into the conduction band of the semiconductor (CB). The MFC’s operating potential is likewise increased by sun irradiation. Such a constraint is the biofilm’s ability to provide electrons (Fischer, 2018).

2.3.4 Microscale MFCs using ultraelectrodes

Any electrode that has at least one dimension (such as the disk’s radius or band width) less than 25 μm is referred to as an ultramicroelectrode (UME). Diffusion of the organic substrate and buffer is considerably enhanced when the UME’s dimension gets closer to the diffusion layer thickness, which characterises the area near an electrode where the concentration of these substances differs from the bulk. Gold line UMEs in electroactive biofilm, with a 10 μm width, achieved a high current density of 16 A m. Using a 25 μm diameter fine platinum wire as a UME in electroactive biofilm, the current density was significantly increased to 66 A m−2 by improving acetate diffusion. Microfabrication allows for fine control of the MFC geometries in relation to the diffusion layer thickness scale, which enables the efficient adoption of the UME effect in a microscale MFC. Ren et al., utilize an acetate-fed microbial electrolysis cell (MEC) that had a Geobacter-enriched bacterial community originally from anaerobic-digestion sludge as inoculum for the micro-scale MFC (Ren, 2021). A concentric ring-shaped UME with a width of 20 μm was designed to improve the current/power densities by facilitating the flow of ions near the microorganisms that build biofilm on the UME. The biofilm appears to extend to about 15 μm from the UME’s edge, indicating an increase in the effective biofilm area. 7.08 ± 0.01 A m−2 are the measured current/power densities per the effective area and the original anode area. A unique MPC that was conceived and manufactured using a microfluidic chamber and a microtip array was described by Joon-Geun-Ha et al. Electron mediators were not needed for the MPC to function. In the presence of light, an SCC (short circuit current) of 250 pA and an OCV (open circuit voltage) of 45 mV were noticed. Because of the non-penetrating microtips and the internal closed circuit that developed inside the microfluidic chamber, the SCC and OCV were lower than the predicted values. A microfluidic chamber structure, a thin PDMS membrane, and a patterned electrode were bonded together to create the microfluidic device. Following the construction of the microfluidic apparatus, a syringe pump was used to introduce a Chlorella fusca var. Fusca cell solution into a microfluidic chamber. The microfluidic chamber’s micro-sieves allowed for the dense packing of cells in a single layer (Ha et al., 2017).

2.3.5 Intimately coupled photobiocatalysis (ICPB)

Intimately coupled photobiocatalysis accelerates the rate of biodegradation of calcitrant organic pollutant while generating electricity. The idea behind ICPB is to keep the bacteria safe from reactive oxygen species (ROSs) by keeping them distant from the site of ROS production, yet close enough to enable them to rapidly biodegrade any easily biodegradable photocatalytic products. Trinitrotoluene, trichlorophenol, and other biorecalcitrant aromatics have been removed more quickly with ICPB (Zhou et al., 2017). Quantitative degradation results demonstrate that ICPB significantly enhances pollutant removal, Dinitrotoluene breakdown was 3.5 times faster (95% clearance within 12 h vs. 27% in control), according to quantitative degradation results (Wen et al., 2012). Trichlorophenol removal increased by 2.8-fold (88% removal within 18 h vs. 31% in control) (Liang et al., 2022). Similarly, trinitrotoluene and other biorecalcitrant aromatics were removed more quickly with ICPB, achieving 92% removal of trinitrotoluene within 24 h compared to 35% in conventional systems (Gün Gök et al., 2018). These quantitative results confirm that ICPB accelerates pollutant degradation kinetics while protecting bacterial viability from ROS damage. In order to promote the photocatalytic destruction of the pollutants, the porous carrier can absorb bio-refractory pollutants from the solution and increase their mass transfer to the photocatalysts’ surface, where different ROS are formed. Biofilms in the ICPB system biodegrade photo-catalytic intermediates to achieve mineralization (Yu et al., 2020). Scientist Zhou et al. developed an N-doped TiO₂ coated on macroporous carbon foam with biofilm accumulation photocatalytic bioanode. Due to its abundance of binding sites and interconnected electron diffusion channels, a hierarchical mesoporous TiO₂ photoanode often offers the optimum compromise between high specific surface area for protein adsorption and effective charge transfer. This is a special example of intimately coupled photobiocatalysis (ICPB), where bacteria were shielded within the foam matrix while photocatalysis took place on the exterior. They inoculated acclimatized (Anode-respiring bacteria) biofilm that was removed from the carbon fibre electrodes in a MEC that produced a lot of current (>10 A/m2) which was fed acetate. The strain of Geobacter sulfurreducens dominated the inoculum (>92% of 16S rDNA readings). The corresponding mineralization efficiency of the ICPB-anode was the highest, and current generation by the ICPB-anode was 50% greater than that of a bio-anode (Zhou et al., 2017).

2.3.6 Dye-sensitized solar cells (DSSCs)-based hybrid system

Dye-sensitized solar cells (DSSCs) are a type of thin-film solar cell that have been the subject of intensive research for over 20 years because of their affordability, ease of production, low toxicity, and straightforward preparation process (Sharma et al., 2018). Dye‐sensitized solar cells (DSSC) are devices that convert solar energy into electrical energy based on a large band gap nanocrystalline semiconductor sensitized by a dye (Senthil and Yuvaraj, 2019). The third generation of photovoltaic cells, known as dye-sensitized solar cells (DSSC), operates by absorbing sunlight and copying the natural photosynthesis processes that plants go through. Because of their cheap manufacturing costs, DSSCs have recently been recognized as a viable alternative in the solar energy space. Despite Photosystem I being an extremely effective natural light-harvesting complex with substantial potential for bio-sensitized solar cells’ applications, interfacial charge-transfer constraints at the PSI-electrode interface limit its effectiveness in DSSC-like systems. The main negative aspect of using of wastewater based in high performance DSSCs is their diverse composition which results in impurities and uncertain ionic conditions that can decrease charge-transfer efficiency, increase recombination, and jeopardize reproducibility and long term device reliability. In comparison to synthetic ruthenium-based dyes, the comparatively slow process of electron injection and charge separation impedes the quick transport of photoexcited electrons into the semiconductor, consequently lowering current density and photocurrent production (Passantino et al., 2020). Their structural components are a pigment-sensitized titanium dioxide (TiO₂) photo anode, an electrolyte comprising an I−/I3− redox pair, and platinum (Pt)-coated counter electrode. The sensitizing pigment is the primary component of the DSSC because it causes the cells to become electrically stimulated by absorbing incoming photons. The So13.3 (Streptomyces fildesensis) strain’s bacterial pigment functions as a DSSC sensitizer since it can create electrons. Depends on the proportionate relationship between the electrical parameters of open circuit voltage (VOC), filling factor (FF), short circuit current density (JSC), efficiency (η), and open circuit voltage at the concentration at which this pigment is utilized. Based on scientific findings, Antarctica’s high UV radiation levels stimulate the creation of colours in microorganisms, which serves as one of their primary defence systems. Consequently, Antarctica is regarded as a significant source of natural pigments (Silva et al., 2019). Strain Serratia marcescens 11E was grown on peanut oil broth 1% v/v, a culture medium that is known to increase prodigiosin production. The strain was obtained from a natural spring in the state of Nuevo Leon in north-eastern Mexico. The photovoltaic performance test of the sensitized DSSC produced an open voltage circuit of 560 mV, a current density of 0.096 mA/cm2, and an efficiency of 0.032%. Meanwhile, the first absorbance decomposition test conducted on the bacterial pigment showed that prodigiosin exhibited high photostability after 5 days (Hernandez-Velasco et al., 2020). Co-sensitising with several biological pigments, such as anthocyanins and chlorophylls, can enhance IPCE by broadening the absorption spectrum, improving dye coverage on the semiconductor surface, and reducing charge recombination, thereby allowing a greater percentage of incident photons to be converted into injected electrons (Golshan et al., 2021).

3 Engineering of hybrid bio-solar interfaces

The integration of microorganisms with semiconductor-based photoactive materials in a hybrid bio-solar interface mainly improves sustainable solar energy conversion and bioelectrochemical performance. The catalytic and self-repairing abilities of microorganisms together enable the strong light-absorbing power of semiconductors. Because of this, better conversion of sunlight into electricity and fuels (Robertson et al., 2024). Materials such as titanium dioxide (TiO₂), cadmium sulphide (CdS), silicon nanowires, graphene derivatives, and g-C₃N₄ nanocomposites are widely used due to their good electrical conductivity and photoactive properties. Scientists focus on the improvement of extracellular electron transfer (EET) through conductive nanomaterials like graphene oxide, carbon nanotubes, conductive polymers, and nanostructured hydrogels. These conductive nanomaterials support microbial adhesion, increase the conductivity of biofilm, and generate more photocurrent (Wang et al., 2024). Interfacial electron transport becomes thermodynamically unfavourable in a hybrid MSC-DSSC system when the semiconductor Fermi level is moved to amounts greater than the potential of the biological mediator. By forming a boundary to charge transport, this energy mismatch reduces photocurrent generation and inhibits effective electron transfer (Korir et al., 2024). Also plasmonic nanoparticles like Ag or Au mounted photoanode of bio-sensitized solar cells improves local light harvesting via surface Plasmon resonance, boost the sensitizer optical adsorption, and encourage more efficient photo-induced charge creation and transfer. Additionally, they enhance photocurrent and overall device performance by reducing recombination losses and strengthening the interfacial electric field (Zarick et al., 2014). Photosynthetic microorganisms such as cyanobacteria, microalgae, and electrogenic bacteria are integrated for artificial photosynthesis, wastewater management, reducing carbon dioxide, or producing renewable fuels. In hybrid-photo microbial fuel cells that treat wastewater, dark current can be significantly minimized by upgrading the photoanode with a nanostructured semiconductor defence layer like ZnO and TiO₂, which are frequently combined with conductive nanomaterials because these coatings improve photoresponse and stability while suppressing interfacial recombination and electrolyte-induced charge leakage (Mahmood, 2026). Synthetic biology and genetic engineering are getting a lot of attention, too. Scientists are using these tools to increase cytochrome production, formation of conductive pili, boost stress tolerance, and increase photosynthesis efficiency in microbes. Future research should emphasize on development of biocompatible semiconductors and self-healing conductive materials for next-generation hybrid bio-solar technologies (Wang et al., 2024).

4 Current challenges and future aspects

Microbial fuel cells, microbial solar cells, biophotovoltaic setups, photobioelectrochemical systems and dye-sensitised solar cells; these technologies have not hit their stride commercially. There are big hurdles: low power output, disappointing energy conversion, weak electron movement between microbes and electrodes, flimsy biofilms, pricey materials, and tough operational costs. The membrane fouling and reactors that do not last, due to which scaling up has not happened yet. Microbes are not very good at handing electrons to electrodes. Bad biofilm conductivity, weak redox mediators, and a patchy grasp of how microbes move electrons make everything slow. The drawbacks related to big reactors include increased internal resistance, mass transfer lags, oxygen diffusion losses, and alteration of microbial communities. While Hybrid systems face challenges associated with toxic semiconductors breaking down under light, limited light absorption and incompatibility between biological and inorganic interfaces. Synthetic biology and genetic engineering seem promising. By improving the microbes’ electron output, there’s a knowledge gap in how cytochromes are regulated, biosynthesis of conductive pili, microbial stress, and engineered microbial consortia. Along with these, the disposal of nanomaterials or semiconductors, and whether these approaches are sustainable long-term require monitoring. Hence, future studies should focus on affordable, sustainable materials, enhancing microbes for transferring electrons, building tough biofilms, redesigning reactors, and integrating nanotechnology with synthetic biology and CRISPR-mediated engineering of electrogenic microorganisms. Real techno-economic studies and sustainability analysis help building commercialization of next-generation biohybrid energy systems.

5 Conclusion

Microbial biohybrid energy systems are opening some exciting doors for cleaner electricity, better wastewater treatment, and smarter ways to recover resources. Over the years, 385 researchers have pushed ahead with things like microbial fuel cells, plant-based versions, Biophotovoltaics devices, photobioelectrochemical cells, and even solar cells that use pigments from microbes. Pairing up microbes with electrochemical and solar technologies is making it possible to generate energy and clean up the environment at the same time—without leaving a heavy footprint. Breakthroughs in areas such as nanomaterials, semiconductor surfaces, synthetic biology, and improved electrodes have all boosted electron transfer, which helps these systems work more efficiently. Still, challenges related to Low power output, scaling up for bigger projects, keeping everything running reliably over time, cutting costs, and actually reaching the market all pose real hurdles. With the advent of microbial systems, these systems can play a big role in achieving sustainability goals, fighting climate change, generating power locally, and building a circular economy that makes the most of resources.

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Footnotes

Edited by: Mohan Mehata, Delhi Technological University, India

Reviewed by: Elahe Fallah Talooki, University of Bologna, Italy

Amol Sonawane, Sardar Vallabhbhai National Institute of Technology Surat, India