Carbon-based electrodes for photo-bio-electrocatalytic microbial fuel and electrolysis cells: advances and perspectives

Abstract

The growing demand for sustainable energy and effective wastewater treatment has propelled the advancement of bio-electrochemical systems (BESs), particularly microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). These systems integrate bioelectricity generation with organic and inorganic pollutant degradation, offering a sustainable solution for environmental remediation. However, challenges such as high overpotential, reliance on noble metal electrodes, and inconsistent performance have necessitated innovative improvements. The incorporation of photocatalysis into BESs has led to the development of photo-bio-electrochemical systems (PBESs), including photo-microbial fuel cells (PMFCs) and photo-microbial electrolysis cells (PMECs), which leverage optical energy to enhance efficiency. Carbon-based electrode materials, owing to their high porosity, conductivity, and biocompatibility, have emerged as ideal candidates for improving PBES performance. Advanced carbon nanostructures, such as graphene, carbon nanotubes, and metal-graphitic carbon nitride composites, have demonstrated superior photocatalytic properties, promoting enhanced charge separation, CO₂ reduction, hydrogen production, and wastewater treatment. PBES integrating light-activated semiconductor materials with BESs, further amplify pollutant degradation and energy conversion efficiency. Despite significant progress, optimizing electrode materials and improving charge transport remain key challenges for scalable and cost-effective deployment. This review highlights the latest advancements in carbon-based electrodes for PBESs, detailing their mechanisms, photocatalytic properties, and future prospects in sustainable energy production and environmental remediation. By addressing existing material limitations and exploring novel photocatalytic enhancements, this work aims to contribute to the development of next-generation PBESs, fostering circular economy practices and carbon-neutral energy solutions.

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Ankita Chaurasiya

Ankita Chaurasiya is a PhD student in the Industrial Waste Utilization, Nano and Biomaterials Department at CSIR-Advanced Materials and Processes Research Institute, Bhopal. She holds an M.Sc. degree in Chemical Science from the University of Allahabad, India. Her research interests focus on the synthesis of graphitic noble electrodes for CO2 CCU (Carbon Capture and Utilization) via a bio-electrocatalytic pathway.

Yashmeen Budania

Yashmeen Budania is a PhD scholar in the Industrial Waste Utilization, Nano and Biomaterials Department at CSIR-Advanced Materials and Processes Research Institute (AMPRI), Bhopal. She has an M.Sc. degree in Chemical Science from Punjabi University, Patiala. Her research interests include wastewater treatment, bio-electrochemical systems, electrocatalysis and energy generation.

Goldy Shah

Dr Goldy Shah is the Research and Development Manager at a Biomedical Waste Incineration plant. She completed her PhD in 2022 at the Centre for Rural Development and Technology, Indian Institute of Technology (IIT) Delhi. Her research expertise encompass bioenergy, CO2 capture, hydrogen production, adsorption, electrochemistry, and life cycle assessment. Dr Shah has contributed significantly to academia, with seven research articles and three book chapters published to date.

Aradhana Mishra

Dr Aradhana Mishra, currently a Senior Principal Scientist in the Microbial Technology Division at CSIR-NBRI, Lucknow, joined as a senior scientist in January 2013. Her research focuses on bio-inspired nano-material synthesis and plant-microbe interactions under stress conditions. She has received the Women Scientist Fellowship Award (2007, 2011), the prestigious Women Scientist Award (2018) by The Biotech Research Society, India, and the Excellence in Research Award (2020) by SVWS. Dr Mishra has completed several major projects, authored 76 SCI journal articles and 14 book chapters, and holds several patents. She has also earned multiple awards for best research papers by CSIR.

Shiv Singh

Dr Shiv Singh is a Senior Scientist at CSIR-AMPRI, Bhopal, India. He obtained his PhD in Chemical Engineering from IIT Kanpur in 2015. His expertise lie in synthesizing novel carbon-based nanomaterials, including CNFs, CNTs, CNPs, Graphene, and C-dots. With around 60 high-impact SCI publications, his research focuses on developing electrode materials for CO2 bio-electrochemical reduction, bio-energy, and hydrogen generation. Dr Singh is actively involved in scientific communities, serving on editorial and advisory boards, including RSC Materials Horizons, Springer Nano-Micro Letters, Wiley's Energy & Environmental Materials, Carbon Neutralization, and Scientific Reports. His work contributes to sustainable energy solutions.

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Wider impact

Photo-bio-electrochemical systems (PBESs), including photo-microbial fuel cells (PMFCs) and photo-microbial electrolysis cells (PMECs), offer sustainable solutions to global challenges of energy scarcity, environmental pollution, and wastewater management. By integrating photocatalysis with renewable energy generation and wastewater treatment, PBESs contribute to circular economy principles, enabling resource recovery while reducing carbon emissions. These systems uniquely combine solar energy utilization with microbial electrochemical processes to produce green electricity, hydrogen, and clean water, positioning them as transformative technologies for sustainable urban development and environmental remediation. Photocatalysis plays a pivotal role in enhancing the efficiency of PBESs by harnessing solar energy for improved degradation of recalcitrant pollutants and enhanced electron transfer processes. Carbon-based electrodes, known for their excellent conductivity, affordability, and environmental compatibility, are particularly effective in boosting photocatalytic performance. They enhance power output, improve coulombic efficiency, and support the degradation of complex compounds in wastewater, thereby optimizing the efficiency and scalability of microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). Despite these advantages, challenges such as high overpotentials, membrane biofouling, oxygen diffusivity, and instability of electrode materials hinder the large-scale application of PBESs. Addressing these challenges requires continued research in advanced photocatalytic materials, innovative catalytic systems, and cost-effective membrane technologies. Interdisciplinary collaboration among material scientists, environmental engineers, and energy specialists is crucial to overcoming these limitations. Furthermore, strategic investments and supportive policies are needed to accelerate the commercialization of PBESs. Integrating these systems into industrial and municipal wastewater treatment facilities can revolutionize sustainable urban infrastructure by enabling energy-positive water treatment plants. Additionally, their application in decentralized energy systems and remote environmental monitoring presents new opportunities for off-grid energy solutions, especially in resource-constrained regions. As the world transitions towards renewable energy systems, PBESs hold significant potential to contribute to global sustainability goals by bridging the gap between environmental management, renewable energy generation, and photocatalysis. By maximizing resource recovery, enhancing pollutant degradation, and minimizing environmental impact, these technologies are poised to play a pivotal role in achieving a circular economy and ensuring a sustainable and resilient energy future.

1. Introduction

The transition toward sustainability and a circular economy presents multifaceted challenges, particularly in energy production and environmental management. The widespread dependence on fossil fuels has significantly contributed to climate change, urging the need for cleaner, renewable energy sources.¹ Simultaneously, rapid industrialization, urbanization, and population growth have intensified environmental concerns, leading to excessive wastewater generation, landfill accumulation, and severe water pollution.² The improper disposal of organic, inorganic, and toxic waste not only endangers human health but also disrupts ecological balance, underscoring the critical interconnection between energy security and environmental sustainability. Addressing these challenges requires innovative and integrative approaches that not only mitigate pollution but also harness untapped resources for energy production.^3–7 Conventional wastewater treatment methods primarily focus on pollutant degradation but fail to utilize the chemical energy inherent in wastewater. As a result, bio-electrochemical systems (BESs) have emerged as a promising technology capable of addressing both wastewater remediation and sustainable energy recovery.^8–11 These systems leverage microbial catalysis to convert organic and inorganic pollutants into electricity and valuable bio-based products with minimal energy input, making them an environmentally friendly alternative to traditional treatment processes.^12,13 BESs encompass a broad range of technologies, including microbial fuel cells (MFCs), microbial electrolysis cells (MECs), enzymatic fuel cells, microbial solar cells, and plant microbial fuel cells, each offering distinct advantages in bioenergy production and waste treatment.^14,15 Among these, MFCs have garnered significant attention due to their dual functionality of wastewater treatment and bioelectricity generation. Extensive research on MFC components, such as electrode materials, cell architecture, membrane selection, and electron transport mechanisms, has sought to address key limitations, including high operational costs and low energy efficiency.¹⁶ To overcome these barriers, researchers have integrated photocatalysis with MFCs, enabling enhanced electron transfer and facilitating hydrogen (H₂) production as a clean and renewable energy source without generating harmful byproducts.

In addition, MECs have emerged as an advanced BES variant with the ability to convert carbon dioxide (CO₂), a major greenhouse gas and primary byproduct of fossil fuel combustion, into high-value biofuels and chemicals. Through microbial metabolism and external electrical input, MECs efficiently produce hydrogen, formate, acetate, methane, butyrate, and various alcohols, offering a low-energy, carbon-neutral pathway for biofuel synthesis. This capability positions MECs as a viable solution for carbon capture, resource recovery, and sustainable bioenergy generation.^17–22

To further enhance the performance of BESs, advanced oxidation processes (AOPs) have been incorporated into these systems. AOPs are known for their ability to decompose a wide range of organic pollutants by generating highly reactive species such as hydroxyl radicals (˙OH), holes (h⁺), and other reactive oxygen species (ROS). The integration of AOPs into BESs enhances pollutant degradation, accelerates organic matter breakdown, and improves overall system efficiency. This hybrid approach not only maximizes wastewater purification but also optimizes energy recovery, making it a promising strategy for next-generation bioelectrochemical technologies.^1,23,24 With continued advancements in materials science, electrode engineering, and system optimization, BESs hold immense potential for addressing pressing global challenges related to wastewater management, renewable energy production, and carbon mitigation. Future research should focus on improving scalability, cost-effectiveness, and operational efficiency to facilitate real-world applications of these sustainable technologies. The integration of BESs with complementary strategies such as photocatalysis and AOPs further strengthens their role as a key enabler of green energy and environmental sustainability.⁶

The shift toward sustainable, environmentally friendly, and cost-effective electricity generation has become a global priority. According to the International Energy Agency, the reliance on fossil fuels for electricity production has been declining since 2019, coinciding with the increasing adoption of renewable energy sources.²⁵ Projections indicate that solar photovoltaic technology will become the dominant electricity source before 2040, and by 2050, renewable sources are expected to account for 70% of global electricity generation. Among these, green hydrogen has emerged as a key energy carrier due to its environmentally benign nature. Produced through electrolysis powered by renewable energy sources such as solar, wind, and hydroelectric power, green hydrogen offers a sustainable alternative that emits no greenhouse gases during its production or use.²⁶ In pursuit of this objective, MECs have been explored as a promising technology for biohydrogen production. A hybrid system integrating an MFC with a bio-photo-electrochemical cell (BPEC), where the BPEC photocathode is illuminated with visible light and the MFC supplies the electrolysis voltage, has been demonstrated for efficient hydrogen generation.²⁷ Furthermore, MECs have shown remarkable potential for CO₂ reduction, utilizing photo-electrodes as either the anode, cathode, or both, to drive conversion processes.²⁸ Another variation, the photo-MECs (PMECs), leverages the synergistic interaction between a semiconductor-based photocathode and a bio-anode to simultaneously extract energy from organic pollutants and sunlight.²⁹ Compared to photosynthesis and dark fermentation, MECs offer superior hydrogen production efficiency due to their higher yield and lower energy input.²⁷ A critical aspect in advancing MEC technology lies in the development of efficient electrode materials for CO₂ reduction and hydrogen evolution. As a central component, the cathode must exhibit high biocompatibility, surface area, conductivity, and cost-effectiveness.^18,30 Carbon-based materials meet these criteria, offering diverse structural variations such as traditional carbon materials (activated carbon (AC), carbon cloth, graphite, carbon brush), nanocarbon materials (graphene, carbon nanotubes (CNT)), and metal carbide–carbon composites, many of which exhibit photo-electrochemical properties essential for MEC applications. While metal-based catalysts typically enhance hydrogen evolution rates, their high cost and potential environmental impact due to metal dissolution have limited their widespread adoption in MECs.^18,27

Despite significant progress, there remains a need for a comprehensive review focusing specifically on carbon-based photocatalytic materials for BES. This review aims to provide researchers with an in-depth analysis of the role of carbon-based electrodes in CO₂ reduction and hydrogen production under visible and ultraviolet irradiation along with bioenergy generation. By outlining the fundamental principles, reaction mechanisms, and relevant chemical equations, this review seeks to serve as a foundation for advancing research in this field. As the world transitions toward carbon-neutral energy sources and embraces the principles of a circular economy, BESs have emerged as a promising solution. A long series of fabrication methods of electrodes is also reported for BESs, as shown in Table 1, highlighting the advantages and drawbacks of the respective methods.

Table 1 Widely used carbon-based electrode fabrication methods highlighting the advantages and drawbacks of the respective methods in BESs

System	Electrode	Fabrication method	Pros	Cons	Ref.
Where CF represents carbon felt, GO is graphene oxide, rGO is reduced GO, ACF is activated carbon fiber, and CNF is carbon nanofiber.
MFC	Waste-extracted biochar	Thermochemical activation	Economical, simple and highly effective in enriching the physiochemical properties	Need of high temperature and chemicals (NaOH, KOH)	31
MFC	Graphene/TiO2/CF	Spray coating	Simple, fast and efficient to obtain a uniform distribution	Proper surface preparation and high temperature is needed	32 and 33
MEC	3D-Synthetic biofilm/carbon cloth	3D-bioprinting	Increase the surface area for the growth of biofilms and enhance the efficiency of MES	It is challenging to utilize the conventional technique to precisely fabricate the electrode with geometrical complexity	34 and 35
MEC	MXene-coated CF	Dip coating	Low cost and simple adjustment of layer thickness	Slow process, high risk of uneven coating	34 and 36
MEC	Mn/CF	Electrodeposition	Develops a uniform and pristine film without any contamination of other elements	Need of expensive equipment and is a time-consuming process	37
MEC	rGO-TEPA/carbon cloth	Self-assembly method	Simultaneous reduction of GO to rGO and formation of rGO-biofilm hybrid electrode	Only individual graphene-based electrodes can be fabricated	38
MFC	Ni-ACF/CNF	Chemical vapour deposition	Highly conformal and high-purity coating can be performed	Slow deposition rate, expensive equipment and high temperature needed	4
MFC	Au/carbon paper composite	Sputtering	Deposit thin, uniform, smooth and durable films.	Requires expensive vacuum systems.	39

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However, their widespread implementation is hindered by challenges related to material cost, durability, and stability. Carbon-based materials are recognised as cost-effective, abundant, durable and biocompatible compared to other classes of materials (metal, metal–oxide, conductive polymer, ceramic, and covalent organic frameworks (COFs) or metal organic frameworks (MOFs)-based) as shown in a summary table illustrating the advantages and limitations of various types of materials in BESs (Table 2). To address these limitations, research is increasingly focused on developing carbon-based electrocatalysts that are abundant, cost-effective, and exhibit inherent photocatalytic properties. This review highlights innovations in electrode materials aimed at enhancing the sustainability and performance of photo-enhanced MFCs and MECs. Reflecting this concern, Fig. 1 illustrates advancements in carbon-based electrodes for photo-MFCs (PMFCs) and PMECs over the past five years.

Table 2 A summary table of various types of materials including carbon-based and other classes of materials (metal oxides, conducting polymers, doped ceramic, COFs/MOFs) with their advantages and limitations in BESs

Electrode material	Advantages	Limitation	Ref.
Where EET is the abbreviation for extracellular electron transfer.
Graphene-based	2D-carbon materials provide a larger surface area for bacterial attachment and extracellular electron transfer	High cost and mass production are challenging	40
CNT	1D material amplifies EET processes more than bare glassy carbon electrodes	Less efficient for EET processes than 2D material	41
Carbon cloth	Highly porous and biocompatible	Low catalytic activity, requires modifications for improved performance	42
Biochar	Cost-effective and easily available	Physical or chemical treatments are needed to activate it to use it as an electrode material	31
Metal-based	Improves power generation and electron transfer kinetics	Dissolution, corrosion and some have antimicrobial properties	43
Metal oxide-based	Boost both conductivity and mechanical strength	Dissolution, metal particle agglomeration and corrosion in electrolyte solutions	44
Heteroatom doped-metal oxide/CNF/ACF	Heteroatoms are favorable to the EET between the anode and cytochrome-c on the outer membranes of bacterial cells	In some cases efficiency of post-doping is low and diverse while in the case of direct doping the pore size and porosity of the catalyst are difficult to be precisely controlled	8
Ceramic-based (fluorine doped TiO2 ceramic)	The algae-based PMFCs with the ceramic anodes are attested to be 16 times more powerful than the best performing carbon-based electrode	Power curves displayed the overshoot phenomenon, which points to a system underperformance	45
Conducting polymer-based (PEDOT:PSS-modified anode)	Improves conductivity and bacterial adhesion due to synergy between constituents	The enrichment of specific microbial cultures for industrial-scale plants is technically challenging	46
MOFs/COFs-based (PANI@COF-CNT)	High surface area and tunable porosity, enhance EETs and provide more sites for microbial activity	Expensive and complex synthesis process, sometimes its application is narrowed in BESs due to low conductivity and stability	47

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Fig. 1 In the last five years, research regarding carbon-based electrodes in PMFCs and PMECs was analysed through Scopus in terms of (a) Journals (displaying journal name, no. of documents in respective journal), (b) publication kind (exhibiting no., percentage) and (c) indulged countries (exhibiting no. of documents with respect to the country).

As research in catalyst innovations and development continues to progress, there is a growing interest in translating laboratory-scale breakthroughs into industrial-scale implementations. Despite rapid advancements in photo-electrocatalysis in recent years, its large-scale application remains underdeveloped, facing numerous critical challenges. At the laboratory scale, catalyst synthesis benefits from precise control over reaction conditions, solvent systems, stoichiometric ratios, and other parameters often resulting in high-performance materials with excellent photo-electrocatalytic activity. However, the transition to industrial-scale production reveals significant obstacles. The key issue lies in the dependence of photo-electrocatalytic properties on the exposure of active sites, which requires precise regulation of morphology, crystal formation, compositional order, and defect concentration parameters that are difficult to control on a larger scale. Although effective, techniques like atomic layer deposition, magnetron sputtering, and galvanic replacement are costly and yield limited quantities of catalyst materials. Furthermore, these techniques often lack universality and are optimized for specific catalyst types. Another critical challenge is the high cost of raw materials. Many high-performance catalysts rely on precious metals such as Pt, Pd, and Ru, or rare earth elements like La and Ce, which are expensive and limit the feasibility of large-scale production. Therefore, the development of more economical and sustainable alternatives is essential. Catalyst stability also plays a crucial role in determining suitability for scale-up. Enhancing the lifetime of catalysts reduces the replacement frequency, thereby improving productivity and cost-efficiency.⁴⁸ This article highlights the potential of low-cost catalyst materials and synthesis methods for industrial-level photo-electrocatalysis. It also presents the current challenges and future directions for scalable, cost-effective, and sustainable catalyst technologies. This review provides a comprehensive overview of the theoretical foundations and practical applications of photocatalysis and AOPs, with a focus on PBESs, including PMFCs and PMECs. It begins by outlining the fundamentals of photocatalysis, incorporating both natural and artificial photosynthesis, and then explores the underlying mechanisms involved in wastewater treatment, CO₂ reduction, and H₂ generation. The manuscript further discusses the significance and recent developments of carbon-based electrode materials in photoelectrocatalytic processes, again emphasizing their roles in wastewater treatment, CO₂ reduction, and H₂ generation, and expanding into emerging trends and applications. The final section addresses research challenges, future opportunities, and key perspectives within the intersection of PBESs and carbon materials. While previous studies have reviewed electrode materials in electrochemical processes related to wastewater treatment or, more rarely, CO₂ reduction and H₂ generation, their focus tends to be broader covering general electrode roles, nanostructured materials for AOPs, anodic materials for electrochemical oxidation, or general developments in electrode technology.⁴⁹ This review uniquely explores the potential of carbon-based materials as feasible and promising photocatalysts in PBESs, a topic that remains underexplored in current literature. The novelty of the review lies in its interdisciplinary approach and focus on integrating PMFCs and PMECs, specifically targeting environmental applications such as wastewater treatment, CO₂ reduction, and hydrogen generation. This review sets itself apart by centering on recent advancements in electrode materials and their novel applications across the key areas of photocatalysis, MFCs, and MECs. The insights presented here aim to guide researchers in both experimental and industrial contexts, paving the way for scalable innovations in photocatalytic fuel systems.

2. Mechanism of wastewater treatment via photocatalysis and energy recovery

Solar energy is the most abundant renewable resource available on earth, encompassing visible, infrared, and ultraviolet radiation. Photocatalysis is a redox-driven process facilitated by photon-induced activation, offering a sustainable and energy-efficient approach to wastewater treatment. This technique is particularly advantageous due to its ability to reduce activation energy while utilizing environmentally friendly precursors significantly. The efficiency of photocatalysis is governed by the intrinsic properties of photocatalytic materials, which exhibit distinct band alignments characterized by electronic discontinuities. A diagrammatic display exhibiting the principle of photocatalysis along with its mechanism is shown in Fig. 2. Photocatalytic materials typically comprise a valence band (VB) with lower energy and a conduction band (CB) with higher energy. The energy gap between these bands, known as the forbidden band (FB) or band gap, has a zero-energy state density where electron transitions do not naturally occur.^50,51 When the surface of a photosensitive material is irradiated with light of photon energy equal to or greater than the FB gap, electrons are excited from the VB to the CB, generating electron–hole (e⁻/h⁺) pairs. In brief, photocatalysis begins when a photocatalyst absorbs light energy, exciting valence electrons to the conduction band and generating e⁻/h⁺ pairs. These charge carriers drive surface redox reactions essential for pollutant degradation and energy conversion. In the absence of an external field, random charge recombination reduces efficiency. However, an applied electric field enhances charge separation, forming electron and hole currents that move in opposite directions but contribute additively to overall conductivity. This mechanism is key to improving photocatalytic performance in environmental and energy applications. These charge carriers play a crucial role in pollutant degradation through two primary reaction pathways. In the oxidation pathway, photogenerated holes (h⁺) either directly oxidize organic contaminants adsorbed on the surface of the photocatalyst or react with water molecules to generate hydroxyl radicals (˙OH), which exhibit strong oxidizing potential for the breakdown of complex pollutants. In the reduction pathway, photogenerated electrons (e⁻) interact with dissolved oxygen, forming superoxide radicals (˙O₂⁻) with high oxidative capacity,⁵² further enhancing the degradation of organic and inorganic pollutants. The integration of photocatalysis into wastewater treatment systems offers an efficient and sustainable method for degrading a broad spectrum of pollutants, making it a key strategy in advancing green wastewater remediation technologies.⁵³

h⁺ + OH⁻ → ˙OH

O₂ + e⁻ → ˙OH₂⁻

2O₂⁻ + 2H⁺ → H₂O₂ + O₂

H₂O₂ + e⁻ → ˙OH + OH⁻

Fig. 2 Scheme representing the photocatalysis principle and primary mechanism.

Photocatalysis often requires an external bias to enhance performance, as the recombination of photogenerated e⁻/h⁺ pairs due to electrostatic attraction significantly reduces quantum efficiency. This necessity a bias voltage that aligns well with the bioelectricity generated by MFCs, enabling a synergistic integration of both systems. A model of bioelectricity generation in PMFCs is shown in Fig. 3. In photocatalytic fuel cells (PFCs), ROS facilitate the degradation of organic matter at the photoanode while the generated electrons are transferred to the cathode, contributing to electricity generation. This dual-functionality of simultaneous pollutant degradation and energy conversion renders PFCs a promising solution for wastewater treatment. Recent advancements have demonstrated the feasibility of PFCs in efficiently eliminating both persistent and emerging organic contaminants, making them a revolutionary technology for sustainable wastewater management.^23,54,55 Photocatalysts, materials capable of absorbing photons and generating excited states, play a crucial role in photocatalytic systems. These materials exhibit desirable properties such as high optical transmittance, mechanical durability, thermal stability, and hydrophobicity, making them suitable for applications in photovoltaics, supercapacitors, electrochemical water splitting, gas sensors, and self-cleaning surfaces. Over the past decade, significant progress has been made in developing nanostructured photocatalysts with enhanced light absorption, charge separation, and catalytic efficiency. Novel inorganic, molecular, and hybrid organic–inorganic materials have been engineered to optimize oxygen evolution and facilitate efficient water splitting for hydrogen production.⁵⁶ To assess the true efficiency of photocatalytic devices, it is imperative to evaluate their performance without the aid of sacrificial agents, which are often costly and impractical for large-scale applications. Instead, utilizing abundant natural resources such as seawater could provide a sustainable approach for real-world deployment. Despite the rapid expansion of photovoltaic technologies, large-scale solar-driven water-splitting modules remain an ongoing research challenge.^57,58 Environmental pollution and energy shortages are two of the most pressing issues in modern society, driving the development of high-performance, eco-friendly photocatalysts for air and water purification. Semiconductor-based photocatalysis has gained significant attention due to its broad applications in energy and environmental remediation, including water splitting, CO₂ reduction, and organic pollutant degradation.⁵⁹ The degradation of organic contaminants via semiconductor photocatalysis involves the generation of photogenerated charge carriers, their migration to the catalyst surface, and subsequent redox reactions with adsorbed pollutants. Developing novel photocatalysts with high visible-light absorption (∼48% of sunlight) and superior photodegradation capabilities remains a key research focus, offering a sustainable pathway for addressing global environmental and energy challenges.⁶⁰ The development of visible-light-sensitive photocatalysts for efficient utilization of abundant solar and indoor light has emerged as a crucial research direction in the field of photocatalysis. Extensive studies have been conducted to enhance the photocatalytic activity of materials under visible-light irradiation. Efforts have primarily focused on oxides and composite oxides, including TiO₂, N-doped TiO₂, Ag–TiO₂, MnO₂, Nb₂O₅, and Fe₃O₄/Fe₂O₃, as well as sulfides and composite sulfides such as CdS, ZnS, MoS₂/MoS₃, and Cu₂ZnSnS₄. These materials have demonstrated promising potential in improving light absorption and facilitating photocatalytic reactions for various environmental and energy applications.⁶¹ The Kubelka–Munk model establishes a relationship between the absorption coefficient (α) and the bandgap energy, allowing for the determination of the semiconductor's bandgap using the Tauc equation:

(αhv)m = A(hv − E_g)

where h is Planck's constant, v is the photon frequency, E_g is the band gap energy and A is a constant, respectively. The value of m is determined by the transition characteristics in the semiconductor (m = 1/2 for indirect transition and m = 2 for direct transition).⁶² The band edge position of the VB and CB of the semiconductor can be calculated according to the following empirical formula:

E_VB = X − E_e + 0.5E_g

E_CB = E_VB − E_g

where E_VB represents valence band potential/energy and E_CB, conduction band potential/energy. X is the absolute electronegativity of the semiconductor. E_e represents the energy of free electrons on the hydrogen scale.^63,64

Fig. 3 Bioenergy generation on a dual photoelectrode MFC.

3. Natural and artificial photosynthesis

Natural photosynthesis, which drives chemical reactions using optical energy, serves as an inspiration for photocatalysis. Both processes involve light absorption, the generation of photoexcited charge carriers (electrons and holes), and subsequent surface reactions, as shown in Fig. 4A. In natural photosynthesis, a “Z-scheme” mechanism (Fig. 4B) operates within plant chloroplasts, primarily through photosystem I (PSI) and photosystem II (PSII). When sunlight irradiates plant leaves, chlorophyll molecules in PSII absorb photons at a wavelength of 680 nm (P680), utilizing the energy for the photolysis of water, leading to oxygen evolution. The electrons released are transferred to PSI, where chlorophylls absorb photons at 700 nm (P700), further exciting the electrons. These high-energy electrons reduce nicotinamide adenine dinucleotide phosphate (NADP⁺) to NADPH, which subsequently drives CO₂ reduction through the Calvin cycle.⁵¹ The proton gradient established during this process also contributes to ATP synthesis, sustaining biological energy conversion. Although photosynthesis serves as a model for photocatalysis, fundamental thermodynamic differences exist between the two processes. Photocatalytic reactions are thermodynamically favourable, exergonic (ΔG < 0) and proceed via catalytic acceleration, whereas photosynthetic reactions are endergonic (ΔG > 0), as illustrated in Fig. 4C, requiring photonic energy input. Additionally, photosynthetic systems must suppress reverse reactions to maintain free-energy storage, a distinction that sets them apart from conventional photocatalysts.⁶⁵ An ideal photocatalyst must exhibit an appropriate bandgap to efficiently harness a broad spectrum of light while ensuring effective charge separation to minimize e⁻/h⁺ recombination. Furthermore, photocatalysts used in photoelectrochemical systems must demonstrate stability against corrosion in acidic or alkaline electrolytes and withstand degradation caused by photogenerated holes. The development of visible-light-sensitive photocatalysts has thus, become a key research focus to enhance solar energy conversion efficiency and broaden the applicability of photocatalytic systems for environmental and energy-related applications.^66,67

Fig. 4 (A) Schematic of charge transfer route and CO₂ conversion in nature photosynthesis via Z-scheme; (B) mechanistic steps involved on an artificial photocatalyst: (a) light absorption, (b) charge transfer, (c) redox reactions, (d) adsorption, desorption and mass diffusion of chemical species and (e) charge recombination resulting in the generation of heat and light; (C) thermodynamics of photosynthesis and photocatalysis.

4. Pursuit of photocatalysis in carbon electrode-based photo-electrochemical systems

Photo-electrochemical systems or cells (PESs or PECs) combine the principles of photocatalysis and electrochemistry to enable the direct conversion of solar energy into storable chemical energy.⁶⁸ These systems involve an electrochemical reaction catalyzed by a photocatalyst, while PMEC systems integrate both photocatalysts and biocatalysts and incorporate an external electrical bias, which plays a crucial role in extending the charge carrier to enhance efficiency. Unlike conventional particulate photocatalysts, PESs incorporate an external electrical bias, which plays a crucial role in extending charge carrier lifetimes, reducing e⁻/h⁺ recombination, and facilitating more efficient redox reactions. The external bias also allows for the utilization of earth-abundant and cost-effective materials that may not independently drive complex reactions like water splitting and CO₂ reduction.⁵⁵

First and foremost, selecting renewable and environmentally benign raw materials is essential to reduce the dependence on fossil-derived resources and promoting a more sustainable production paradigm. In parallel, minimising the use of hazardous substances and prioritising non-toxic or low-toxicity alternatives in catalyst synthesis significantly mitigates environmental and health-related concerns. Second, it is crucial to design synthesis pathways that emphasize atom economy, reduce waste generation, and streamline material and energy usage. Third, adopting efficient waste treatment and disposal strategies, alongside recycling and reusing by-products, contributes to pollution reduction and resource conservation. Fourth, integrating energy-efficient processes, particularly those powered by renewable energy sources, into catalyst production can substantially decrease greenhouse gas emissions and overall energy consumption. Among all these considerations, the activity and selectivity of photo-electrocatalysts remain paramount, as they directly influence the efficiency and viability of various catalytic reactions. Addressing these sustainability challenges can be effectively achieved by using carbon-based photo-electrocatalysts. These materials represent a promising class of catalysts due to their tunable surface chemistry, structural stability, cost-effectiveness, and high electrochemical activity. Additionally, carbon-based materials are highly modifiable, experimentally versatile, and possess excellent durability, making them ideal candidates for applications in waste treatment and renewable energy generation. Their inherent properties not only support current performance demands but also offer substantial potential for further optimization and innovation.⁴⁸

Carbon-based nanomaterials, including graphite, diamond, graphene, CNTs, and fullerenes, have gained significant attention in PEC applications, particularly for hydrogen production through water splitting. Graphene, with its high specific surface area, superior electron mobility, optical transparency, and excellent electrical and thermal conductivity, stands out as a highly efficient material for photocatalytic and PEC applications. Graphene derivatives such as graphene quantum dots (GQDs), GO, and rGO further enhance PEC performance due to their tunable electronic properties and surface functionalization capabilities. Despite certain carbon-based materials lacking intrinsic photocatalytic properties, strategies such as surface modification, heterostructure construction, and elemental doping have been employed to enhance their activity.^69–71

In recent years, PEC technology has shown immense promise for sustainable energy conversion, particularly in the production of green hydrogen via water splitting and the reduction of CO₂ into valuable chemical feedstocks. The integration of nanostructured carbon materials with PESs is expected to drive further advancements, improving efficiency, scalability, and commercial viability for future clean energy applications. Continued research into material optimization and system design will be instrumental in developing next-generation PEC technologies for sustainable environmental and energy solutions.⁷²

5. Carbon-based photocatalytic materials for PMEC and PMFC applications

Recent advancements in photocatalysis have highlighted the significance of carbon-based materials in enhancing semiconductor performance for visible light-driven applications. Carbonaceous-supported semiconductors have garnered considerable interest due to their ability to modulate the band gap of conventional photocatalysts, improving charge separation and visible light absorption. By integrating bulk semiconductors with carbon-based materials such as AC, graphene, CNTs, CNFs and fullerenes, researchers have successfully developed highly efficient photocatalytic systems for applications in PMECs and PMFCs.⁶⁷ Nanocarbon/semiconductor hybrids, including CNTs, GO, and graphene, have demonstrated superior performance in CO₂ reduction compared to traditional metal oxides.⁷³

In particular, CNT-based composites, such as MoS₂/CNT hybrids, exhibit improved photocatalytic efficiency and stability due to enhanced charge separation and extended light absorption into the visible range (>400 nm). Similarly, CdS/MoS₂/CNT composites have demonstrated significantly higher photocurrent densities—approximately 12 times greater than pristine CdS, attributed to the role of CNTs as co-catalysts and charge transfer facilitators.⁷⁴ Bismuth vanadate, a promising photoanode material, suffers from limited charge separation and transport properties. However, incorporating carbon nanomaterials, such as rGO and graphitic carbon nitride (g-C₃N₄ or gCN), enhances light absorption and surface porosity, thereby improving photocatalytic efficiency.⁷⁵ gCN-based composites, particularly those engineered via defect modulation and heterojunction formation, have gained attention for their ability to enhance charge separation and conductivity. Transforming g-C₃N₄ from nanoplates to nanorods increases photocatalytic activity and photocurrent response under visible light by 1.5 and 2 times, respectively. Additionally, heteroatom doping (e.g., C, P-doped g-C₃N₄) has demonstrated a 9.7-fold enhancement in H₂ production via photocatalytic water splitting.^76–80

Carbon quantum dots (CQDs) have emerged as a new class of photocatalysts owing to their low toxicity, cost-effectiveness, long photoluminescence lifetimes, and resistance to photobleaching. Compared to metal-based counterparts, their exceptional photostability makes them suitable candidates for sustainable photocatalysis. Furthermore, the functionalisation of g-C₃N₄ with porphyrins has shown enhanced charge transfer and hydrogen evolution efficiency, while porphyrin/rGO nanocomposites have exhibited superior electron transfer and high photocatalytic activity under visible light irradiation.^67,81

Overall, carbon-based photocatalysts offer significant potential for advancing PESs, particularly in wastewater treatment, hydrogen production, and CO₂ reduction. Ongoing research in band gap engineering, surface functionalization, and nanostructured materials is expected to improve photocatalytic performance further and enable scalable, cost-effective applications in renewable energy and environmental remediation.

6. Application of carbon-based photo-electrodes in MECs

MECs have been measured as a key route for biological H₂ generation and CO₂ reduction with carbon-based materials as they significantly contribute to EET processes, which have an insightful role in the microbial oxidation reaction. Carbon-based materials hold the features of low cost, rich natural profusion, good thermal and chemical stability, adjustable surface properties and spatial organization, and with respect to EET, carbon structures have the most profound effect; however, carbon materials must have good conductivity, biocompatibility and anticorrosive ability. Carbon materials with macroporous structures (>2 μm) provide a conducive environment for bacterial colonization within the anode, whereas microporous architectures facilitate the diffusion of endogenous electron transfer mediators like riboflavin, thereby enhancing EET efficiency. One-dimensional carbon materials, such as CNTs, are acknowledged for their ability to amplify EET processes, significantly boosting the performance of BESs. Notably, a CNT-modified glassy carbon electrode (GCE) exhibited an 82-fold increase in current generation compared to an unmodified GCE in electrochemical cells involving Shewanella oneidensis. The planar architecture of two-dimensional (2D) carbon materials provides a larger surface area for bacterial attachment and EET compared to one-dimensional (1D) structures. Graphene serves as a prime example, with multi-walled carbon nanotube (MWCNT)-derived graphene oxide nanoribbons (GONRs) recognized as 2D carbon materials that retain the conductive, pili-like attributes of MWCNTs. These materials offer an extensive electrochemically active surface akin to graphene, thereby enhancing the efficiency of EET processes in MFC anodes. The porous structures of 3D (three-dimensional) carbon materials can offer penetration of bacterial cells into the interior surface of the anode and accelerate the mass transfer of fuel and the diffusion of the self-secreted mediator. A 3D, two-scale porous CNT–textile, developed via conformally coating CNTs on a textile electrode, was proposed where the macroscale porous structure of the textile delivered open 3D channels for the internal colonization of bacterial cells and proficient substrate transport; however the microscale porous CNT layer intensely interacted with bacterial cells to hasten the EET.⁴¹ Modification in pure carbon material provides significant steps for BES performance as hydrophilic defects created by doping of heteroatoms are favourable to the EET between the anode and cytochrome c on the outer membranes of bacterial cells. 3D macroporous carbon foams (NMCFs) with different N-doped states have been proposed wherein DFT calculations revealed that the pyrrolic-N was the key component to promote the EET. In another study, it was recognised that the performance of MFCs with an N-doped CNTs/rGO anode was superior to those of MFCs with N-CNTs, N-rGO, and CNTs/rGO anodes, confirming that both the fabrication of composites and the doping of N were effective for enriching the bacterial biofilm and to adsorb flavin for improving EET efficiency, respectively. The noble metal nanoparticles/carbon material composite favors bacterial attachment and enhances the EET efficiency between the anode and bacterial cells because of their good biocompatibility and excellent electrocatalytic activity. However, the high cost of these noble metal nanoparticles will limit their application. In a recent study, the NiO-N-CNF/ACB electrode has been recognised to facilitate improved contact with wastewater and foster the development of a thick biofilm of electroactive bacteria that facilitates extracellular electron transfer at the anode for wastewater treatment.⁸ SnO₂-decorated CNT anodes enhance EET via electrostatic interactions with cytochrome c. Metal sulfides and carbides, like FeS₂/rGO, improve microbial film interaction and EET efficiency. Carbon/conducting polymer composites, such as PPy/GO, boost BES's performance with 37% higher cell voltage, improved conductivity, and bacterial adhesion due to PPy-GO synergy.⁴¹ The above mentioned studies highlight the influence of carbon structures, including pure carbon in 1D, 2D, and 3D forms, and modified carbon materials such as heteroatom-doped, metal/heteroatom-doped carbon and carbon-based composites towards EET.

The carbon structure also plays a crucial role in extracellular electron uptake, especially in CO₂ reduction on a photo-biocathode. In the rGO–SiO₂–TiO₂ heterojunction system, the introduction of rGO into the SiO₂–TiO₂ photocatalyst, was revealed as an electron transfer facilitator, assisted by its π–π conjugation effect. This mediation remarkably elevates the electron transfer rate between SiO₂–TiO₂ and rGO, enhancing electron transfer to hydrogen and direct transfer to microbes.⁸² PMECs represent an advanced class of MECs that incorporate photocatalysts at the anode, cathode, or both electrodes to facilitate semi-artificial photosynthesis.^83,84 These hybrid systems leverage both biocatalysts (e.g., enzymes, microbes) and synthetic materials (e.g., semiconductors, electrodes) to drive complex and endergonic chemical reactions.⁸⁵ Biocatalysts contribute to electrochemical transformations by enhancing selectivity and efficiency, whereas synthetic materials aid in immobilization, light absorption, charge transfer, and product separation. The integration of microbes and enzymes with electrodes provides a viable alternative to noble-metal-based catalytic processes, reducing dependency on scarce and expensive materials. Light, a sustainable source of renewable energy in PMECs, facilitates generation of charge carriers, which requires the carbon-based material to be photoactive, and after illumination of light, the subsequent steps are light absorption, charge transfer, redox reactions, adsorption, desorption and mass diffusion of chemical species and charge recombination, with all the sequential processes depicted in Fig. 4 for a typical particulate photocatalyst. TiO₂ and SiO₂ are widely used semiconductors. Incorporation of rGO into the SiO₂–TiO₂ semiconductor was recognized as a crucial element in enhancing its photocatalytic performance in a PMEC system, which is attributed to its large specific surface area. This modification broadened the light absorption range, thereby increasing the UV-Vis absorption intensity and providing additional active sites for photocatalytic reactions. Eventually, the interaction between rGO and light leads to an increased generation of charge carriers.⁸² Despite their promise, the practical implementation of photocatalysts in MECs presents several challenges. A significant issue arises from the generation of ROS, which inhibits hydrogen production at neutral pH (pH ≈ 7) and poses toxicity risks to microbial communities. Additionally, certain photocatalysts contain metal ions that may dissolve into the system, leading to contamination and performance degradation.⁵⁴ To mitigate these effects, strategies such as controlling metal ion dissolution and employing carbon-based photocatalytic electrodes have been explored. In photo-electrochemical CO₂ reduction applications, Cu₂O nanowires have been investigated as photoanodes. However, their susceptibility to photo-corrosion, primarily due to self-reduction by photo-generated electrons, presents a major limitation.⁸⁶ Tailoring the electrolyte conditions has been proposed as a potential strategy to alleviate photo-corrosion and enhance the stability of these materials. Addressing these challenges through material optimization and system design improvements is crucial for advancing PMEC technology toward scalable and sustainable applications. The development of efficient PMECs represents a promising step toward sustainable energy production and environmental remediation. However, addressing the limitations associated with photocatalyst stability, biocompatibility, and charge transfer efficiency remains crucial for advancing PMEC technology. Future research should focus on optimizing electrode materials, improving bioelectrode integration strategies, and designing scalable systems that can operate efficiently under real-world conditions. Through these advancements, PMECs have the potential to revolutionize hydrogen production, CO₂ conversion, and wastewater treatment in an energy-efficient and environment-friendly manner.

6.1. MECs for CO₂ reduction: bioelectrode functionality, challenges and mechanism

Nature has evolved an efficient mechanism for CO₂ reduction through photosynthesis, wherein chlorophyll-containing chloroplasts in plants and algae utilize sunlight and water to convert atmospheric CO₂ into carbohydrates. However, CO₂ reduction in MECs presents a viable alternative with distinct advantages, as it operates independently of sunlight and photosynthetic organisms while offering high efficiency in product formation. MECs are typically designed as two-compartment systems consisting of an anodic and a cathodic chamber. In conventional MECs, the anode is usually abiotic and facilitates water electrolysis, whereas the cathode is biotic and ropes CO₂ reduction.^87,88 MECs enable the electrochemical conversion of CO₂ into a range of valuable products, including formate, acetate, alcohols, organic acids, and methane, by utilizing microbial biocatalysts under a low external voltage supply. This technology not only facilitates carbon capture and utilization, but also integrates wastewater treatment, making it a sustainable and energy-efficient approach for environmental remediation. The products generated in MEC-driven electro-CO₂ reduction are largely influenced by the type of microorganisms used, their metabolic pathways, and the magnitude of the applied voltage. Specific microbial strains catalyze targeted electrochemical reactions, leading to selective production of formate, acetate, or higher-value compounds such as ethanol and butanol. The dual functionality of MEC-driven electro-CO₂ reduction and wastewater treatment makes them a promising strategy for mitigating greenhouse gas emissions, while simultaneously recovering biofuels and valuable organic compounds. Further optimization of microbial consortia, electrode materials, and operational parameters will be crucial in enhancing MEC performance for scalable applications in carbon-neutral energy production and environmental sustainability.⁸⁹ However, MECs incorporating dual bioelectrodes – both anodic and cathodic, offer distinct advantages over conventional designs. In contrast to a chemical anode, a biotic anode can generate electrons at a lower potential, thereby reducing the external power input required for water electrolysis.^18,90,91 In this system, microbes in the anodic compartment oxidize organic matter present in wastewater, generating electrons and protons. The electrons are transferred to the cathode via an external circuit, while protons migrate through the electrolyte. The anodic chamber hosts electrogenic microbes that facilitate electron transfer by metabolizing organic compounds. Conversely, the cathodic compartment harbors electrotrophic microbes that accept electrons from the anode to catalyze CO₂ reduction into value-added products. EET from electrodes to microbes is a crucial mechanism driving the electrocatalytic conversion of CO₂. The two widely recognized EET pathways are indirect electron transfer (IET) and direct electron transfer (DET). IET involves mediators such as neutral red that shuttle electrons, whereas DET relies on biofilms or membrane-bound redox enzymes or outer-membrane cytochromes that facilitate direct electron transfer via physical contact with the electrode.¹⁸ The choice of the electron transfer mechanism is dictated by the intended end product and operational conditions, influencing microbial selection and genetic engineering approaches.⁹² CO₂ reduction at the cathode occurs via bio-electrochemical pathways, requiring a low applied voltage (0.2–0.8 V). Depending on the extent of electron and proton transfer, these reactions can proceed through two-, four-, six-, eight-, or twelve-electron pathways. The microbial species and their specific metabolic pathways dictate the selection of reduction mechanisms in the cathodic compartment. Table 3 provides a list of major reactions for value-added products, along with their corresponding standard reduction potentials (E₀) in volts versus the standard hydrogen electrode (SHE) at 1 atm pressure, pH = 7 and 25 °C.^93–95 One of the most widely studied pathways for CO₂ reduction to acetate is the reductive acetyl-CoA pathway (Wood–Ljungdahl pathway). This process involves the reduction of one CO₂ molecule to a methyl group and another to carbon monoxide, which then combines with coenzyme A to form acetyl-CoA.⁵⁴ Subsequently, acetyl-CoA is converted into acetate via an eight-electron, eight-proton reaction:

2CO₂ + 4H₂ → CH₃COOH + 2H₂O

Table 3 Major reactions studied in MECs for CO₂ reduction along with their electrode potential

Electrochemical reaction	E 0 vs. SHE(V)
All the values of the above table have been taken from the literature.^93–95
CO2 + e^− → CO2˙^−	−1.85
CO2(g) + 2H^+ + 2e^− → HCOOH(aq)	−0.61
CO2(g) + 2H^+ + 2e^− → CO(g) + H2O(l)	−0.52
CO2(g) + 4H^+ + 4e^− → HCHO(aq) + H2O(l)	−0.48
CO2(g) + 6H^+ + 6e^− → CH3OH(aq) + 4H2O(l)	−0.38
2CO2(g) + 12H^+ + 12e^− → C2H4(g) + 4H2O(l)	−0.34
CO2(g) + 8H^+ + 8e^− → CH3COOH(aq) + 2H2O(l)	−0.28
CO2(g) + 8H^+ + 8e^− → CH4(g) + 2H2O(l)	−0.24
2H^+ + 2e^− → H2(g)	−0.41

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Methanogenesis is another prominent metabolic route, following the Wolfe cycle, in which hydrogenotrophic methanogens convert CO₂ into methane. Hydrogen molecules (H₂) serve as electron donors, facilitating CO₂ reduction through hydrogen-mediated electron transfer, thereby enhancing methane production rates.^96,97 The overall reaction carried out by the Wolfe cycle for CO₂ reduction is:

CO₂ + 4H₂ → CH₄ + 2H₂O (ΔG° = −130.7 kJ mol⁻¹)

MECs provide a superior alternative to photocatalytic and electrocatalytic CO₂ reduction due to their ability to harness microbial activity for electrochemical reactions at low applied voltages (0.2–0.8 V). Additionally, their integration with wastewater treatment presents a promising “waste-to-value” approach. However, several challenges hinder their efficiency, including poor biocompatibility between electrodes and microbes and low electron transfer efficiency at the cathode. To address these limitations, researchers have explored novel carbonaceous cathodes, such as prussian blue nanocube-modified carbon felt (PBNC-CF), which function as artificial electron mediators. Studies have demonstrated that PBNC-CF-based cathodes achieved twice the biofilm thickness compared to unmodified carbon felt, leading to enhanced volumetric acetate production (0.20 ± 0.01 g L⁻¹ day⁻¹) in a 10 L MES.⁹⁸ These advancements underscore the potential of engineered carbon-based electrodes in enhancing MEC's performance for sustainable CO₂ reduction and biofuel production.

6.2. Electro-CO₂-reduction in PMEC

PMECs represent a more advanced approach to CO₂ reduction compared to MECs, (Fig. 5) owing to the synergistic interaction between semiconductors/photocatalysts and electrodes. This synergy allows PMECs to harness energy simultaneously from organic pollutants and sunlight, enhancing overall efficiency. Semiconductors serve as prevalent photocatalysts due to their tunable conductivity. However, for optimal integration into MEC systems, photocatalysts should possess specific properties, including a narrow bandgap to minimize energy requirements, excellent biocompatibility to ensure coexistence with biocatalysts, and resistance to corrosion and photo corrosion. The preference for carbon-based materials arises from their stability, as metal-based photocatalysts may undergo dissolution in the electrolyte. Additionally, minimal or no production of reactive oxygen species is essential to mitigate potential adverse effects on microbial growth and ensure long-term operational stability.^99–101 In PMEC systems, commonly employed semiconductor configurations include p-n heterojunctions and Z-scheme heterojunctions. The former facilitates efficient charge separation and broader solar spectrum absorption, while the latter supports effective charge transfer, analogous to natural photosynthesis, and offers high redox potential. These attributes render them highly versatile for photocatalytic applications.⁵⁴

Fig. 5 A schematic diagram of a photo-microbial electrolysis cell (PMEC) illustrating the CO₂ reduction.

A dual-photoelectrode PMEC has been proposed, employing BiVO₄–RuO₂–IrO₂ on a Ti mesh as the photoanode and ZIF-67/g-C₃N₄ on carbon felt as the photocathode to facilitate CO₂ reduction to acetate. In this system, the anode remains abiotic, whereas the cathode is enriched with a mixed microbial culture. The dual-photoelectrode configuration enables the simultaneous generation of e⁻/h⁺ pairs upon light irradiation, improving charge transfer efficiency. The electrons generated at the photoanode compensate for the holes at the photocathode, enhancing system compatibility. To achieve water oxidation at the abiotic photoanode, its conduction band edge potential must be more negative than the H⁺/H₂ redox pair, promoting efficient electron transport from the anode to the cathode. Simultaneously, the valence band edge potential should be sufficiently positive relative to the O₂/H₂O redox potential. The BiVO₄–RuO₂–IrO₂/Ti photoanode, where RuO₂–IrO₂/Ti acts as a co-catalyst and supports the fabrication of BiVO₄, facilitates water oxidation, leading to proton and electron generation along with oxygen evolution. Electrons migrate toward the cathode, while holes generated on BiVO₄ (bandgap: 2.4–2.5 eV) upon visible light irradiation help overcome kinetic barriers associated with water oxidation.¹⁰² This process effectively prevents charge carrier recombination, ensuring enhanced performance. Both photoexcited electrons and those generated via water oxidation are transported through an external circuit to the cathode, while protons migrate through a proton exchange membrane. In the cathodic compartment, ZIF-67/g-C₃N₄ serves as the photocathode. ZIF-67, a metal–organic framework with high porosity and chemical stability, enhances CO₂ adsorption, ensuring a consistent substrate supply. Meanwhile, g-C₃N₄ (bandgap ∼2.7 eV) contributes to efficient photogenerated electron–hole separation, a high specific surface area due to its intrinsic layered structure, and an extended π-conjugated system akin to graphene. Upon visible light irradiation, holes generated on g-C₃N₄ are compensated by electrons transferred from the anode, while photogenerated electrons from the photoanode are either directly transferred to microbes or mediated by H₂, acting as an electron shuttle. This process leads to acetate production, achieving a yield of 6.93 g L⁻¹.¹⁰² Further, a mesoporous rGO–SiO₂–TiO₂ (Z-scheme heterojunction) nanocomposite has been reported (Fig. 6) for efficient carbon dioxide reduction and high-performance supercapacitor applications. The incorporation of SiO₂–TiO₂ nanocomposites into rGO significantly improves charge transport, overall conductivity, electrode–electrolyte interaction, and reaction kinetics. When utilized as a cathode in a MEC with cultivated anaerobic bacteria, this system demonstrated 2.1 times higher photocurrents compared to a bare carbon felt electrode, achieving an acetate production rate of 3.21 mM day⁻¹ with a coulombic efficiency of 78% and a current density of 2.7 A m⁻².⁸² Another study investigated a photo-assisted MEC employing Serratia marcescens Q1 as an electrotrophic microorganism and a photocathode composed of graphite felt coated with g-C₃N₄ and decorated with WO₃–MoO₃ nanoparticle mixtures. The Z-scheme heterojunction of WO₃/MoO₃/g-C₃N₄ facilitated bicarbonate (HCO₃⁻) reduction to acetate, yielding a photocurrent 4.8 times higher than that recorded with a g-C₃N₄ electrode alone and 2.1 times greater than that of WO₃/MoO₃ species. Upon light irradiation, the Z-scheme heterojunction generated long-lived e⁻/h⁺ pairs, where conduction band electrons of g-C₃N₄ reduced protons to H₂, which was subsequently bio-assimilated by microbes for acetate production, while holes were neutralized by electrons transferred from the anodic chamber.¹⁰³ For enhanced acetate synthesis from bicarbonate (HCO₃⁻) in photo-assisted microbial electrosynthesis (MES), a Z-scheme Ag₃PO₄/g-C₃N₄ photocatalytic heterojunction was proposed. When coupled with the non-photosynthetic bacterium Serratia marcescens Q1, this system achieved an acetate production rate of 5.4 mM day⁻¹, with a coulombic efficiency of 93% at a current density of 3.3 A m⁻².¹⁰⁴ Similarly, an innovative CuO/g-C₃N₄/rGO photocatalyst-based cathode, when combined with a mixed microbial culture, facilitated CO₂ reduction, producing acetate at a rate of 0.27 g L⁻¹ day⁻¹ at a potential of −0.9 V (versus Ag/AgCl). The inclusion of rGO in CuO/g-C₃N₄ improved light absorption capacity and enhanced photogenerated electron–hole separation and migration efficiency compared to CuO/g-C₃N₄ alone.¹⁰⁵ A Z-scheme heterojunction, α-Fe₂O₃/g-C₃N₄, was proposed as a photocathode for a photo-assisted MEC, where the system exhibited enhanced photogenerated electron–hole separation under visible light, achieving an acetate production rate of 0.33 g L⁻¹ day⁻¹ from CO₂ reduction, which was three times higher than that of a carbon felt cathode.¹⁰⁶ Additionally, a CoP-Fe₂O₃/C₃N₄ photocathode was developed for the reduction of CO₂ into polyhydroxybutyrate (PHB) in an MES system utilizing Ralstonia eutropha as a biocatalyst. This photocathode integration facilitated charge carrier separation, providing additional reducing power for CO₂ conversion. The system yielded 87.54 mg L⁻¹ of PHB at −1.05 V, approximately three times higher than that obtained with a carbon felt electrode. At −0.9 V, PHB production further increased to 142.20 mg L⁻¹.¹⁰⁷ Recent advancements have demonstrated that PBESs can serve as efficient platforms for H₂ generation utilizing carbon-based photocatalysts in conjunction with biocatalysts. In another study, a PBES was designed for CO₂ reduction using a carbonaceous material as a supportive base, integrating NCW-g-C₃N₄/ACF and g-C₃N₄/ACF as bioanodes, with Escherichia coli derived from wastewater acting as a biocatalyst. This system synthesized approximately 12.8 mM of formate per day under visible light irradiation. The 2D structure of g-C₃N₄ in the biocathode facilitated efficient charge carrier transport between the bacteria and substrate, while ACF served as a supportive material, offering high biocompatibility, electrical conductivity, and surface area. In the photoanode, g-C₃N₄, a metal-free semiconductor with a bandgap of ∼2.7 eV, exhibited a high charge recombination rate, whereas Ni or Co metal tungstate possessed a relatively higher bandgap and lower charge recombination rate. The integration of these materials resulted in the formation of a Z-scheme heterojunction, effectively mitigating the limitations of each component.²⁸ Overall, the photocatalyst plays a crucial role in generating highly reactive excited electrons, facilitating redox reactions, while the biocatalyst efficiently utilizes these electrons to produce value-added products according to their metabolic pathways. The integration of advanced photocatalytic materials and microbial electrocatalysis in PMECs underscores their potential for sustainable CO₂ conversion and bioenergy generation, addressing critical environmental challenges through innovative technological solutions.

Fig. 6 Photo-reduction mechanism of CO₂ to acetate using a mixed culture via the utilization of an rGO-doped SiO₂–TiO₂ heterojunction in MES. Reprinted with permission.⁸² Copyright 2023, Elsevier.

6.3. Mechanism of hydrogen production in MECs

MECs are gaining attention as a sustainable and efficient technology for biohydrogen production, serving as a clean energy source while simultaneously treating wastewater. Several factors influence hydrogen production in MECs, which can be categorized into physical (electrode material and membrane type), biological (microbial species and electron transfer pathways), and operational (substrate type, applied voltage, pH, and temperature) parameters.¹⁰⁸ In MECs, biohydrogen is produced through a bio-electrochemical process facilitated by electrogenic microorganisms in the anodic chamber. These microbes oxidize organic matter from wastewater, generating electrons, protons, and CO₂. The electrons flow through an external circuit to the cathode, where they combine with protons to form molecular hydrogen. The electrochemical reactions governing this process are as follows:

Anodic chamber:

CH₃COO⁻ + 4H₂O → 2HCO₃⁻ + 9H⁺ + 8e⁻

Cathodic chamber:

8H⁺ + 8e⁻ → 4H₂

Overall reaction:

CH₃COO⁻ + 4H₂O → 2HCO₃⁻ + H⁺ + 4H₂

Acetate is considered as the most efficient substrate for microbial oxidation in MECs, leading to enhanced hydrogen production. Under standard biological conditions, the anodic potential (E_an) is reported as −0.279 V, while the cathodic potential (E_cat) is approximately −0.414 V. The equilibrium potential (E_eq) for the reaction is calculated as −0.14 V. However, to drive hydrogen evolution effectively, an external voltage between 0.2–1.0 V is typically applied due to electrode overpotentials in real MEC systems.¹⁰⁸ Despite these applied voltage requirements, hydrogen production in MECs is preferred over conventional electrochemical water splitting. This is because traditional electrolysis requires a significantly higher applied potential (>+1.23 V) under standard conditions (25 °C, 1 atm), regardless of the electrolyte used.¹⁰⁹ The lower energy input required in MECs makes them a promising alternative for energy-efficient biohydrogen generation while simultaneously mitigating wastewater pollution.

6.4. Hydrogen production in PMECs

PMECs have emerged as a promising technology for H₂ production, simultaneous dye degradation, wastewater treatment, and the removal of hazardous metal ions. Fig. 7 depicts the synergistic interaction between microbial biocatalysts and the optical energy within PMEC systems, offering a significant advantage over conventional MECs for H₂ generation. Recent research has focused on improving H₂ production by developing advanced photoelectrodes as a typical PMEC setup as illustrated in Fig. 8A with enhanced biocompatibility, narrow bandgap, corrosion resistance, and reduced electron–hole recombination rate, all of which contribute to the superior performance of PMECs compared to MECs. Among various photocatalysts, TiO₂ has been widely studied due to its stability and high photocatalytic activity. However, its wide bandgap (∼3.2 eV) restricts visible light absorption, limiting its practical application in PMECs. Additionally, metal-based photocatalysts often face challenges related to dissolution and corrosion in electrolyte solutions. To overcome these limitations, carbon-based materials have gained popularity due to their superior electrical conductivity, high surface area, and chemical stability. Several carbon-based materials, such as graphitic carbon, rGO, CNTs, graphene, CNFs, and graphite, have been extensively utilized in photoelectrochemical and electrocatalytic applications, including H₂ production.^110–112 A study demonstrated H₂ production in a double-chambered PMEC system employing CeO₂–rGO/carbon film as both the photoanode and cathode, using wastewater as the microbial source in the anodic chamber. Under an applied voltage of 1.0 V (vs. Ag/AgCl) and visible light irradiation, this system achieved H₂ generation at a rate of ∼5 m³ m⁻³ day⁻¹, while simultaneously reducing the chemical oxygen demand (COD) of wastewater by 95%. In contrast, under dark conditions, the H₂ production rate dropped to 3.44 m³ m⁻³ day⁻¹, highlighting the essential role of photoanode activity in improving efficiency. The photoelectrochemical performance of the CeO₂–rGO composite can be attributed to the O species in CeO₂, which actively participate in redox reactions. The interaction between CeO₂ and carbon-based dopants (e.g., rGO) enhances the formation of e⁻/h⁺ pairs, which act as effective oxidizing agents (for organic matter degradation and COD reduction) and reducing agents (for H₂ evolution). Additionally, the incorporation of rGO into CeO₂ introduces structural defects and impurities, leading to an improved visible light absorption capability and enhanced charge separation at the electrode surface. Notably, the integration of rGO with CeO₂ reduces the bandgap from ∼3.2 eV to 2.85 eV, significantly boosting the photocatalytic efficiency of CeO₂–rGO-based photoelectrodes. During light irradiation, photoexcited electrons are efficiently captured by Ce ions in the CB, resulting in an accumulation of negative charge, increasing the number of active sites for H₂ generation. This mechanism explains the superior photo-electrochemical H₂ production performance of CeO₂–rGO-based PMECs, making them a highly effective alternative for sustainable bio-H₂ generation and wastewater remediation.^70,113 A distinctive photo-bioelectrochemical mechanism is depicted in Fig. 8B for H₂ production.

Fig. 7 A schematic sketch of the PMEC demonstrating hydrogen production.

Fig. 8 (A) H₂ generation in a PMEC. Reprinted with permission.¹¹⁶ Copyright 2017, ACS. (B) Schematic diagram of photogenerated electron–hole pair transfer and H₂ production in a typical PMEC with a bioanode and photocathode. (C) Illustration of electron transfer on the anode-microbe interface. (D) Mechanistic sketch of electron transfer and H₂ production in an AgInS₂/In₂S₃@E. coli semiconductor-biohybrid system. Reprinted with permission.¹¹⁴ Copyright 2021, Elsevier.

The mechanism of generation of charge carriers, and their fate and interaction at the bio-electrode interface can be explained more efficaciously using the example of a carbon-based photo-bio electrode either as a photo-bioanode (Fig. 8C) or photo-biocathode (Fig. 6 and 8D). In the case of photo-biocathodes, in a PMEC, light illumination results in the generation of e⁻/h⁺ pairs, e⁻ migrate to the cathode via an external circuit and meanwhile, there is manifestation of a microbial oxidation reaction that also results in the generation of e⁻ (acetate oxidation needs only ∼–0.3 V potential whereas oxidation of H₂O demands ∼0.8 V potential to generate e⁻), and these e⁻ are responsible for compensation of h⁺ in the valence band of the photocatalyst. Thus, the photocatalyst facilitates an additional electron source, and heightens the system's overall efficiency, surpassing the performance of MECs.^114,115

In the case of photo-biocathode, enlightenment excites the e⁻ to the conduction band, leaving behind h⁺ in the valence band. These photogenerated electrons are transported to microbes to facilitate the metabolic reduction reaction through their conductive pilli, membrane protein (cytochrome c) or exogenous electron shuttles, and occasionally H₂ act as a mediator to transfer electrons between microbes and electrodes. Photogenerated holes also play a significant role as in the rGO–SiO₂–TiO₂ photo-bio-cathode it was revealed that h⁺ serves as an additional driving force that effectively traps the electrons in the external circuit, resulting in increased current generation, in the presence of KI as a hole scavenger, a reduction in acetate production rate from 3.2 ± 0.16 mM day⁻¹ to 2.10 ± 0.1 mM day⁻¹ was observed. Besides the synergistic relation between microbes (waste) and light, the carbon-based photoactive electrodes offer a better environment and stage for their growth and survival, so in cutting-edge research, there is inciting competition to develop PMECs with carbon-based electrodes.⁸²

In a separate study, a single-chambered PMEC system utilizing a ZnFe₂O₄/g-C₃N₄ photocathode and graphite felt as the anode material demonstrated the exclusive production of H₂ without the generation of CH₄ and CO₂, achieving a production rate of 1.70 ± 0.04 m³ m⁻³ day⁻¹. The integration of ZnFe₂O₄ (bandgap: 1.90 eV) with the carbonaceous material g-C₃N₄ (bandgap: 2.70 eV) effectively reduced the overall bandgap of the photocathode and minimized charge recombination.

Additionally, a single-chambered MEC incorporating a dual bioelectrode configuration was reported, where ZnFe₂O₄/g-C₃N₄ cathodes facilitated efficient and exclusive H₂ production at a rate of 0.11 ± 0.01 m³ m⁻² day⁻¹ and 1.70 ± 0.04 m³ m⁻³ day⁻¹. This system exhibited a solar-to-hydrogen conversion efficiency of 4.08 ± 0.17% and an energy efficiency relative to electrical input of 233 ± 5%. The structured ZnFe₂O₄/g-C₃N₄ cathodes demonstrated significantly higher photocurrents compared to the individual components (4.3-fold increase over g-C₃N₄ and 3.3-fold over ZnFe₂O₄). Furthermore, after four operational cycles, the system exhibited negligible Fe and Zn leaching, indicating the cathode's structural stability and long-term durability.¹¹⁷ The UV-Vis DRS spectra of ZnFe₂O₄, g-C₃N₄, and ZnFe₂O₄/g-C₃N₄ revealed distinct absorption edges corresponding to their intrinsic E_g. Specifically, g-C₃N₄ exhibited an absorption edge at approximately 450 nm, aligning with its reported E_g of 2.70 eV, while ZnFe₂O₄ showed broad absorption within the 350–650 nm range, consistent with its E_g of 1.90 eV. The Mott–Schottky plots of ZnFe₂O₄ and g-C₃N₄ demonstrated positive slopes in the linear regions, confirming their n-type semiconducting nature. The E_CB determined from these plots was −0.35 eV for ZnFe₂O₄ and −1.2 eV for g-C₃N₄, whereas the corresponding E_VB values were reported as 1.55 eV and 1.50 eV, respectively. These findings indicate the formation of an n–n heterojunction at the photocathode interface. Upon light irradiation, the photo-excited electrons migrated from g-C₃N₄ to ZnFe₂O₄, driven by the more negative E_CB of g-C₃N₄, whereas the holes moved in the opposite direction, from the more positive E_VB of ZnFe₂O₄ to g-C₃N₄. This directional charge transfer significantly enhanced charge separation efficiency, minimized electron–hole recombination, and improved the photocatalytic activity of the system. MEC was measured as a key route for biological H₂ generation, although PMEC can be recognized as a more advanced version as a comparative study was reported for H₂ evolution in the system with a bioanode made of carbon fiber brush and when Pt as a cathode is used at 0 bias conditions in a MEC, it resulted in almost zero current density while at 0.8 V bias, it only produced 1 mA cm⁻² current density; on the other hand, the same bioanode with GaInP₂–TiO₂–MoS_x as a photocathode in a PMEC recorded a >10 mA cm⁻² current density.¹¹⁶

7. Application of carbon-based photo-electrodes in MFCs

7.1. Bioelectrode functionality, challenges and mechanism of MFCs

Carbon-based electrodes are the most widely used in MFC research due to their optimal electrical conductivity, biocompatibility, chemical stability, and surface area, all of which are critical for efficient electron transfer and microbial attachment. However, before discussing advancements in electrode materials and their role in enhancing MFC performance, it is essential to understand the underlying mechanism governing MFC operation.^8,9 MFCs, as a type of BES, utilize microbes either naturally present or artificially introduced into wastewater—to facilitate bioremediation while simultaneously generating electricity.¹¹⁸ Microbes oxidize organic matter from wastewater at the anode, breaking it down into simpler components, including electrons and protons.^10,119 The generated electrons, through an external circuit, migrate toward the cathode, while the protons travel in the opposite direction through a cation exchange membrane (CEM) or salt bridge, completing the circuit and generating current flow. Upon reaching the cathode, the electrons participate in the oxygen reduction reaction (ORR) by binding with an oxidant, typically oxygen, and the protons received from the anolyte via the CEM, leading to water formation.^9,120 Despite the promising applications of MFCs, several challenges hinder large-scale implementation, including high costs, non-sustainable membranes, and expensive catalysts.^3,5 Additionally, the power output of MFCs remains relatively low compared to conventional energy technologies, limiting their widespread deployment. In recent years, integrating photocatalytic energy sources with MFCs has gained attraction, allowing for enhanced voltage generation to support photo-assisted degradation processes.⁴ This hybrid approach leverages the synergistic interaction between MFC-driven bioelectricity and photocatalysis, improving both energy efficiency and pollutant removal performance.⁷¹

7.2. Enhancement of electron transfer and power output with photocatalytic carbon-based electrodes

Integrating microbial technology with photoelectrochemical techniques in PMFCs harnesses the synergistic interplay between microbial metabolism and photoelectrocatalysis, enabling the conversion of chemical energy from complex organic fuels or biomass into environmentally friendly electrical energy. The design of PMFCs necessitates a comprehensive evaluation of the oxidation–reduction potentials of the anodic microorganisms alongside the electrocatalytic reaction potentials of the cathode or the energy level positions of the semiconductor materials as exhibited in Fig. 9. This ensures an optimal energy alignment between the (photo)biological anode and the (photo)electrocatalytic cathode, thereby facilitating a directional electron transfer process. During the microbial metabolic process at the anode, electrons and protons are generated concomitantly with the biodegradation of organic compounds. The electrons migrate to the cathode via an external circuit, whereas protons traverse the PEM to reach the cathodic chamber, where they collectively engage in the cathodic reduction reaction. This electron transfer mechanism minimizes the recombination of photogenerated e⁻/h⁺ pairs during photocatalytic reactions, thereby improving the photocatalytic efficiency of the cathode. This hybrid system enables efficient bioelectricity generation while simultaneously promoting synergistic photoelectrocatalytic pollutant degradation.¹²¹

Fig. 9 Schematic representation of the typical MFC principle with inclusion of photocatalysis at either/both ends, photo-bioanode and photocathode.

The diversity and resilience of microbial communities, characterized by their high reproductive capacities and ability to thrive under mild environmental conditions, make them ideal for sustained, long-term operation in PMFCs. Adequate illumination is a crucial environmental factor for optimal PMFC performance. Unlike photo-driven PFCs, where operation is limited by light availability, the use of electrogenic microorganisms on anodes ensures the stable operation of PMFCs even in low-light or dark conditions. When photocatalysts are incorporated into the cathode of an MFC, hole–electron pairs are generated upon light irradiation. The photogenerated holes interact with electrons arriving from the anodic chamber, while the photogenerated electrons readily react with electron acceptor species at the cathode. This mechanism effectively suppresses the recombination of e⁻/h⁺ pairs, thereby improving photocatalytic efficiency. Moreover, a major limitation in MFCs is the high cathodic overpotential and slow reaction kinetics of cathodic reduction reactions, which restrict large-scale application. The introduction of a cathodic photocatalyst not only enhances pollutant degradation (e.g., dye removal) but also accelerates cathodic reaction kinetics, thereby overcoming the high cathodic overpotential. Photocatalytic processes have the capability to break down complex organic compounds into simpler, less harmful molecules, which can then be biodegraded more efficiently by microorganisms. It has been demonstrated that photocatalytic oxidation serves as an effective pre-treatment strategy before biodegradation, as it enhances the bioavailability of recalcitrant pollutants. Although photodegradation is generally more efficient and rapid than biodegradation for stubborn contaminants, it often leads to partial degradation rather than complete mineralization.^122,123 Additionally, the rapid recombination of photogenerated e⁻/h⁺ pairs limits the overall photodegradation efficiency. Therefore, the integration of photocatalysis with MFCs has gained significant attention, as it combines microbial metabolism with photocatalytic processes for enhanced wastewater treatment and bioenergy generation. In PMFC configurations, photocatalysts can be incorporated either as a photoanode or photocathode. Most reported studies have focused on dual-chamber configurations, where bioanodes are coupled with photocathodes, or biocathodes are combined with photoanodes. This arrangement ensures an efficient electron transfer pathway, enhanced degradation of pollutants, and improved bioelectricity generation, making PMFCs a promising hybrid system for sustainable energy production and environmental remediation.¹²⁴

7.3. Bioenergy generation on carbon-based photoelectrode MFCs

Several doping strategies have been employed to enhance the light absorption properties of GO-based photocatalysts, extending their photoresponsiveness from the UV region to the visible spectrum. Metal-ion doping plays a crucial role in modifying the electronic band structure of semiconductors, thereby improving their photocatalytic efficiency, charge transport kinetics, and overall stability. One such novel GO-based nanocomposite (Fe/GTiP) was synthesized via a simple and scalable fabrication process and subsequently utilized as an anode in photo-assisted PFCs. This Fe/GTiP-modified anode exhibited superior degradation of rhodamine B (RhB), coupled with simultaneous bioelectricity generation due to its enhanced charge separation efficiency and high surface area.¹²⁵ Recently, a PFC was realised to treat tannery and shale gas wastewater with simultaneous S²⁻ removal and H₂O₂ production. The used graphite cathode displayed great catalytic activity for H₂O₂ production.¹²⁶ A Z-scheme photocatalytic system utilizing g-C₃N₄/iron (Fe⁰)-doped TiO₂ (1%) as the anodic catalyst and WO₃ as the cathodic catalyst demonstrated 98% RhB degradation under 50 W visible light irradiation.¹²⁷ Similarly, bismuth vanadate (BiVO₄)/WO₃ photoanodes coupled with manganese dioxide (MnO₂)/GO cathodes were successfully integrated for efficient RhB degradation, further emphasizing the effectiveness of heterojunction-based materials in photocatalytic applications.¹²⁸ Researchers are focussed on the development of a 2e⁻ catalyst for the ORR worthy of generating scaled-up industrial level production of compounds with economic ease. Carbon-based electrocatalysts have shown for ages great prospects for 2e⁻ ORR, plus abundance and inexpensiveness in materials and modifications.¹²⁹ To further mitigate electron–hole recombination and expand the visible-light response range, a g-C₃N₄/WO₃/TiO₂/Ti photoanode was developed. This system achieved a maximum RhB degradation efficiency of 87.7%, attributed to the synergistic co-doping of TiO₂/Ti with g-C₃N₄ and WO₃. This co-doping approach significantly enhanced the light absorption capacity, improved charge carrier mobility, and increased overall photocatalytic efficiency, making it a promising candidate for photo-assisted PFC applications.¹³⁰ Recent advancements in photo-assisted PFCs have led to the development of g-C₃N₄/BiOI/Ti photoanode-based systems, which exhibited enhanced RhB degradation efficiency due to synergistic charge transfer mechanisms at the heterojunction interfaces. Another study by Liu et al. introduced a multifunctional PFC system that integrated titanium dioxide nanotube arrays (TNA) as a photoanode and graphite as a photocathode. This system successfully facilitated the reductive treatment of hexavalent chromium (Cr(VI)) pollutants, while simultaneously enabling oxidative decomposition of organic contaminants and efficient bioelectricity generation.¹³¹ To further improve pollutant degradation efficiency and energy production, a hybrid photo-assisted microbial fuel cell-electro-Fenton system was developed. This system employed a WO₃/W photoanode and an iron-carbon (Fe@Fe₂O₃/carbon felt) cathode within an undivided chamber, enabling enhanced electron–hole separation and radical-driven oxidation reactions. The integration of electro-Fenton chemistry significantly boosted the organic degradation efficiency, outperforming conventional single-process photocatalysis or electro-Fenton systems.¹³² A recent study exhibited coupling of photocatalytic oxidation and bio trickling filter-MFCs, which degraded aniline wastewater. The used g-C₃N₄ reduced the start-up time of the cell effectively.¹³³

The research community has increasingly focused on carbonaceous electrode materials, particularly bamboo charcoal (BC), due to its high specific surface area, exceptional electrochemical stability, cost-effectiveness, and superior ORR activity. A multi-cathode PFC system, employing a TiO₂ photoanode and a BC/Ti cathode, demonstrated superior methyl orange (MO) degradation efficiency and higher bioelectricity generation compared to conventional single-cathode PFC designs.^134,135 A research study gave excellent results regarding degradation of Ciprofloxacin by a Zn–Al mixed metal-oxide (MMO)/rGO nano composite under visible light, which was 3 and 4.6 times greater than zinc–aluminium layered double hydroxide and zinc–aluminium MMO, respectively. It was attributed to response to broad range of visible light that they showed such great efficiency. Upon further comparison with TiO₂, nanocomposites of rGO–Fe₃O₄/TiO₂ showed 98.99% degradation in only 55 minutes of visible light irradiation, while TiO₂ degraded 67% of the same in 120 minutes. Such increment of the result was wheeled by greater surface area of Fe₃O₄ and rGO in the nano composites, which provides TiO₂ with more active sites for catalysis.¹³⁶ For the degradation of reactive green 19 (RG19), a membraneless PFC system was developed using a zinc oxide/carbon (ZnO/C) photoanode and a platinum/carbon (Pt/C) cathode. This setup was designed to evaluate the impact of dissolved oxygen (DO) concentrations on dye degradation and bioelectricity production.¹³⁷ Experimental results revealed that higher DO levels significantly enhanced the degradation efficiency of diazo dyes and boosted electricity output. In a parallel study, ZnO was immobilized onto a Zn-based photoanode, paired with a Pt-loaded carbon cathode, where DO played a crucial role in prolonging e⁻/h⁺ pair separation in ZnO. This process led to improved degradation efficiency of RG19.¹³⁸ Notably, the degradation rate of RG19 was found to be higher than that of acid orange 7, primarily due to the presence of electron-donating triazine (–NH–) functional groups in RG19, which facilitated its oxidative decomposition. Moreover, an integrated PFC system incorporating periodate activation was designed using a rGO/TiO₂ nanotube (NT) photoanode and a polypyrrole (Ppy)/MWCNT cathode. This system achieved an unprecedented maximum acid red 14 degradation efficiency of 99%, with citric acid employed as an effective co-catalyst to enhance electron transfer dynamics.¹³⁹ A PFC system incorporating a nanostructured BiVO₄ film and an Au-decorated, commercially available buried junction silicon photoanode was constructed. This novel system exhibited high-efficiency energy conversion and accelerated MO degradation. The introduction of a small amount of sulfite along with sufficient oxygen in the reaction chamber further enhanced bioelectricity generation and organic pollutant degradation, demonstrating the feasibility of integrating advanced photocatalytic-electrochemical approaches in MFC technology.¹⁴⁰ These findings underscore the immense potential of carbon-based PMFCs in achieving dual-purpose applications of bioenergy generation and wastewater treatment. A study exhibited improved electron utilization upon using bi-anodes in MFCs having TiO₂/Fe₂O₃ as a photoanode and another, as a bioanode integrated with a solar cell.¹⁴¹ The microbial oxidation and photo-electrocatalysis were significantly accelerated with dual-anode MFC to carry out enhanced electron transfer to the external circuit.¹⁴² The incorporation of doping strategies, hybrid catalyst architectures, and advanced carbon-based electrode materials has led to significant enhancements in photocatalytic efficiency, charge transport properties, and overall system stability. Moving forward, future research should focus on integrating nanostructured materials, optimizing charge transfer mechanisms, and developing scalable designs that facilitate real-world deployment of bioelectrochemical energy systems. By synergistically combining microbial electrochemical activity with advanced photocatalytic processes, PMFCs offer a sustainable pathway for clean energy production and environmental remediation.^123,124

8. Comparative analysis

Carbon-based photoelectrodes have emerged as a promising alternative to noble metal-based counterparts due to their abundant availability, cost-effectiveness, low toxicity, and excellent chemical stability. These materials, including graphene, CNTs, rGO, and CQDs, exhibit remarkable corrosion resistance and structural adaptability, making them attractive for photo-electrochemical applications. However, their efficiency in the HER remains lower than that of noble metals, primarily due to intrinsic limitations in charge transport and catalytic activity. While noble metals such as platinum and gold possess superior electrocatalytic properties with low overpotentials, carbon-based materials generally require strategic modifications to enhance their hydrogen production efficiency. The high charge recombination rates in pristine carbon materials hinder their ability to sustain prolonged catalytic activity, leading to suboptimal HER performance. Additionally, their optical absorption capacity, although tunable, remains lower than plasmonic metal-based systems, limiting their effectiveness in utilizing the full solar spectrum. In this regard, Table 4 highlights the efficiency of various cathode materials in hybrid PMECs. Metal-based cathodes, such as Si-nanoarray and GaInP₂–TiO₂–MoS_x, demonstrate coulombic efficiencies of 90% and >93%, respectively. Meanwhile, carbon cloth-based cathodes incorporating methanogens and chitosan exhibit slightly higher coulombic efficiencies of 95.2 ± 1.8% and 94.4%, emphasizing the advantageous role of carbon-based electrodes. Additionally, the data confirms the superiority of PMECs over MECs, as PMECs utilizing carbon cloth as the cathode achieve a faradaic efficiency of 95.2 ± 1.8%, surpassing the 86 ± 1.0% efficiency observed with rGO/WO₃/carbon cloth cathodes in MEC systems. Researchers have developed strategies to overcome these challenges, including heterostructure formation, transition metal doping, surface functionalization, and hybridization with semiconductors. Coupling carbon-based photoelectrodes with materials such as titanium dioxide, bismuth vanadate, and graphitic carbon nitride has improved charge separation and extended light absorption to the visible range. Furthermore, doping with elements like iron, cobalt, or nickel introduces active catalytic sites, while hybridization with plasmonic nanoparticles enhances light harvesting and facilitates electron transfer. These approaches significantly improve the overall efficiency of carbon-based electrodes for hydrogen evolution. Despite these advancements, achieving efficiency comparable to noble metals remains a challenge. Continued research on optimizing carbon-based material's electronic structure, defect engineering, and surface chemistry is necessary to enhance their catalytic performance further. With further innovations, carbon-based photoelectrodes could evolve into a viable alternative for sustainable and scalable hydrogen production technologies.²⁷

Table 4 Comparison of various microbial electrochemical systems with metal oxides, conducting polymers, and doped ceramics

Material type	Cathode	Microbial inoculum	Electrode fabrication method	System (MEC/MFC/PEC/PMEC)	Product	Applied potential (V)	Power density (W m^−2)/current density (A m^−2)	Stability	COD removal	Coulombic efficiency/faradaic efficiency (%)	Ref.
Metal-based	Si-nanoarray	Sporomusa ovata	—	Photo-biohybrid system	Acetate	0.2	3	200 h	—	90	143
Carbon-based	Carbon cloth	Methanogen	Hydrothermal	Hybrid PMEC	CH4	−0.3	0.35	90 h	—	95.2 ± 1.8%	144
Carbon-based	Carbon cloth	E. coli	Hydrothermal	MFC	ORR	—	1.964	—	—	—	145
Metal oxide-based	GaInP2–TiO2–MoSx	Anaerobic sludge	Vapour phase epitaxy, reductive electrodeposition	PMEC	H2	0, 0.8	4.2, ∼100	24 h	—	>93	116
Doped ceramic-based	rGO/WO3/carbon cloth	Mixed culture	Electrospinning	MEC	Acetate	−0.5	13.56 ± 0.5	—	—	86 ± 1.0	146
Conducting polymer-based	Carbon cloth	Chitosan	SILAR method	Hybrid PMEC	CH4	−0.7	0.375	—	—	94.4	147
Metal oxide-based	LaCoO3	Shewanella species	Sol–gel method	MFC	ORR	—	0.0139	—	—	—	148
Metal oxide-based	Co0.5Cu0.5 Bi0.1Fe1.9O4	Synthetic wastewater+ sludge	Sol–gel auto-combustion	MFC	ORR	—	11.44	50 cycles	—	21.4	149
Conducting polymer-based	Nickel-phthalocyanine/MnO2	Mixed culture	Hydrothermal	MFC	ORR	—	8.02 ± 0.38	—	80.3 ± 3.8	27.9	150

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8.1. Performance of MECs

The integration of photocatalysts with biocatalysts in MECs has been demonstrated to significantly enhance system performance, particularly in terms of power density and hydrogen production efficiency. This synergistic combination optimizes electron transfer processes by leveraging the metabolic activity of electrogenic microorganisms alongside the photogenerated charge carriers from semiconductor electrodes. The overall efficiency of such hybrid systems is governed by several critical factors, including the band gap energy of the semiconductor, which dictates the absorption and utilization of light energy. A narrow but optimal band gap facilitates effective photoexcitation while minimizing electron–hole recombination losses. Additionally, the biocompatibility of semiconductor materials plays a crucial role in sustaining microbial viability and promoting stable long-term bioelectrocatalysis. Charge carrier recombination rates are another determining factor, as rapid recombination limits the availability of free electrons for cathodic reactions, thereby reducing overall efficiency. Strategies such as heterojunction formation, surface passivation, and elemental doping have been employed to mitigate these losses and enhance charge separation. Optical transparency of semiconductor electrodes also influences light penetration and subsequent photocatalytic activity, impacting the overall energy conversion efficiency. By optimizing these key parameters, the integration of photocatalysts with biocatalysts in MECs presents a promising approach to improve bioelectrochemical hydrogen production, offering an efficient and sustainable route toward clean energy generation. Fig. 10 illustrates the benefits of PMECs over conventional MECs for CO₂ reduction and H₂ production. Table 5 and Fig. 11 are such examples, representing the comparative performance of PMECs for CO₂ electro-reduction and H₂ generation. g-C₃N₄ is regarded as a promising carbon-based photoelectrocatalyst in MECs for both H₂ production and CO₂ reduction. Its advantages include an appropriate band gap of 2.7 eV, ease of synthesis, high stability, safety, and non-toxicity. However, its practical application is significantly constrained by its rapid charge carrier recombination and limited utilization of visible light. To overcome these limitations and enhance the photoelectrocatalytic performance of g-C₃N₄, various strategies such as heteroatom doping, surface engineering, and functionalization have been explored. In PMECs, the redox process is accelerated by the excitation of semiconductor photocatalysts (e.g., Ni foam/g-C₃N₄/BiOBr, MoS₂/Cu₂O, and CdS/g-C₃N₄) deposited on either the anode or cathode. Upon exposure to sunlight, these materials generate photoexcited electrons, which facilitate reductive processes such as hydrogen evolution or organic pollutant reduction, while simultaneously producing oxidative holes that drive organic oxidation reactions. Despite their efficiency, the widespread application of these semiconductor materials is hindered by the presence of toxic metals such as cadmium, copper, and nickel, which pose environmental risks due to potential leaching and complete dissolution over prolonged operation. Therefore, future advancements in PMECs should focus on the development of environmentally friendly semiconductor photocatalysts that maintain high efficiency while minimizing ecological and health hazards.²⁷

Fig. 10 Advantages of PMECs for organic and inorganic pollutant degradation and sustainable energy production.

Table 5 Comparative performance of PMECs for CO₂ electro-reduction and H₂ generation

Cathode/photocathode	Anode/photoanode	Band gap of photoelectrode (eV)	Potential (vs. Ag/AgCl)	Biocatalyst	Product	Production rate	Coulombic efficiency/faradaic efficiency(%)	Current density(A m^−2)	Ref.
rGO–SiO2–TiO2	Carbon felt	—	—	Mixed culture	Acetate from inorganic carbon	3.21 mM day^−1	78	2.7	82
WO3/MoO3/g-C3N4	Carbon rod	2.56	−1.3	Serratia marcescens	Acetate from inorganic carbon	3.12 ± 0.20 mM day^−1	73 ± 4%	2.5 ± 0.3	103
CuO/g-C3N4/rGO	Ti-mesh coated with Ru and Ir	2.21	—	Mixed culture	Acetate	0.27 g L^−1 day^−1	—	14.8	105
ZIF-67/g-C3N4 on carbon felt	BiVO4–RuO2–IrO2 on Ti mesh	1.95, 2.12	−0.9	Mixed culture	Acetate	0.46 g L^−1 day^−1	—	—	102
Ag3PO4/g-C3N4	Carbon rod	2.3	−1.3	Serratia marcescens	Acetate from inorganic carbon	5.4 mM day^−1	93	3.3	104
α-Fe2O3/g-C3N4	Ru/Ir–Ti mesh	—	−0.9	Mixed culture	Acetate	0.33 g L^−1 day^−1	—	−13.5	106
CuO/g-C3N4	Ru/Ir–Ti mesh	—	—	Mixed culture	Acetate	0.16 g L^−1 day^−1	—	−114	105
CoP–Fe2O3/C3N4	Ru/Ir–Ti mesh	—	−0.9	Ralstonia eutropha	Polyhydroxybutyrate	0.02 g L^−1 day^−1	—	—	107
Graphitic-C3N4/ACF	NCW-gCN/ACF	∼2.80 eV, ∼2.66 eV	−1.0	E. coli	Formate	12.8 mM day^−1	∼57	0.64	28
CDs/g-C3N4	Graphite felt	—	0.8	—	H2	0.1138 m^3 m^−3 day^−1	—	—	111
CeO2–rGO/carbon film	CeO2–rGO/carbon film	—	1	Mixed culture	H2	5 m^3 m^−3 day^−1	95	—	70
g-C3N4/CQDs/BiOBr	Pt/C-carbon brush	∼2.74	0.8	Mixed culture	H2	0.02967 L L^−1 day^−1	—	∼10	151
ZnFe2O4/g-C3N4	Graphite felt	—	0.6	Mixed culture	H2	1.70 ± 0.04 m^3 m^−3 day^−1	—	—	117

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Fig. 11 A 3D-barchart showing the yield of acetate, formate, PHB and H₂ in a PMEC system with carbon-based photoelectrodes.

8.2. Performance of MFCs

In recent years, significant research has focused on developing MFCs for potential commercialization. However, most of these studies have concentrated on lab-scale experiments, with few addressing the scaling up of MFCs to field-scale implementation.

Scaling up often reveals that the power output does not increase proportionally with the volume of the anodic chamber or the electrode surface area. The primary limitations in larger MFCs arise from electrochemical thermodynamics, electrode reaction kinetics, and mass transport constraints, which restrict the system's power production. High overpotentials and reduced coulombic efficiency limit the scalability of a single MFC. Therefore, electrode materials with low-cost catalysts are needed to reduce activation overpotentials and improve coulombic efficiency, making MFCs more viable for field applications. MFCs offer notable advantages, such as non-conventional green electricity generation, powering underwater sensors, remote sensing, biological oxygen demand monitoring, hydrogen production, and wastewater treatment. However, several challenges persist, including membrane biofouling, pH splitting, oxygen diffusivity, membrane internal resistance, production costs, and lower power output. To enhance the power output and make MFCs more practical, it is necessary to minimize activation overpotentials at both the anode and cathode by developing more efficient yet cost-effective electrode materials.^8,9 Additionally, reducing ohmic losses due to electron flow resistance at the electrodes and ion flow resistance in the membrane is essential. Strategies such as decreasing electrode spacing, using more conductive electrode materials with coatings, and selecting ion-conductive membranes with lower resistance can help address these losses.¹⁵² Replacing costly membrane separators with ceramic separators modified with cation exchangers to improve proton conductivity could also reduce costs.¹⁵³ Commercializing MFCs will depend on using catalyzed electrodes that support higher current densities, thus enabling larger anodic chamber volumes and maximizing energy extraction while treating wastewater.^8,9Table 6 is an account of the comparative performance of different PMFCs. Recent advancements in nanomaterials have led to innovative hetero-nanostructures for solar water-splitting, with photocurrents ranging from microamperes to milliamperes per square centimeter. This progress has been widely reviewed, with a focus on fabrication techniques and improvements in large-bandgap semiconductors like TiO₂ and ZnO, which show strong performance in solar water-splitting. New systems, such as oxynitrides, tantalates, and niobates, also show promise.¹⁵⁴ Nowadays, photo-bioelectrofenton systems are mostly prevalent in batch-mode, but it is continuous-mode which makes up for much more stability in electron utilization when applied in practical industrial conditions. Previous studies have taken up organic loading rate (OLR) and hydraulic retention time (HRT) as two major parameters to keep note of electron flow in continuous flow mode systems. In a bi-chambered system, pharmaceutical wastewater was continuously flowed into the anodic chamber through a peristaltic pump and the operated effluent, treated wastewater was sent to be a clarifier, again continuously, where after the settlement of sludge by gravity, the suspended liquid was transported to the cathodic chamber where it took part in reaction. This operated system showed OLR of 0.58 g L⁻¹ day⁻¹, HRT equalling 83.33 h, COD removal efficiency of 93% and coulombic efficiency equal to 5.36%.¹⁵⁵ In another study, a 20 L reactor was set up in continuous mode to incur industrial-level research where methylene blue (MB) was degraded from operated wastewater with a decolorization efficiency of 99% at 28 h HRT.¹⁵⁶ This study exhibited a scaled-up industrial level continuous-flow mode wastewater treatment system for real world application successfully. A study displayed a continuously fed bio-electric Fenton system for the decomposition of metoprolol with the rate of 77%, 92%, and 95% with respect to 2, 4, and 6 h HRT, respectively.¹⁵⁷ This research clearly showed how hydroxyl radicals generated during the reaction get cleansed out through the effluent at shorter HRT in a continuous-flow system, while as the HRT is significantly increased, they ought to react and take part in the reaction. The above-mentioned research about continuous mode systems demonstrated a variety of pros over batch mode systems indicating better coulometric efficiency and higher operational efficiency with excellent stability and great ease in operability, plus control over microbial metabolism, and repeatability at the economic level.¹⁵⁸ MFCs present a sustainable approach to wastewater treatment, which is typically energy-intensive and costly. Traditional treatment methods do not generate revenue or add value to the treated water. Sustainable wastewater treatment processes are essential to addressing energy scarcity, resource depletion, and pollution. These processes aim for minimal resource consumption, neutral energy operation, consistent treatment performance, high-quality effluent for reuse, balanced investment, strong social equity, and low environmental impact.^123,124 MFCs and MECs offer a cost-effective alternative by utilizing microorganisms as biocatalysts for organic matter oxidation and electron transfer to the anode for bioelectricity production. The performance of MFCs is influenced by electrochemical reactions at the anode and cathode, where methane or hydrogen may be produced, depending on the requirements. Carbon-based materials such as graphite and CNTs are commonly used due to their affordability, conductivity, and abundance.⁹ CNTs, despite issues with biotoxicity and biodegradability, have excellent mechanical, thermal, and electrical properties.¹⁷⁰ Molybdenum and stainless steel are also used as anode materials, with molybdenum exhibiting high durability and corrosion resistance and stainless steel offering a high nickel content.¹⁷¹ For example, stainless-steel brushes used as cathodes have achieved hydrogen production rates of 1.7 m³ m⁻³ day⁻¹, with an efficiency of 84%.¹⁷² One of the main limitations of MFCs is the reduction of molecular oxygen at the cathode, which increases potential losses. Despite the use of various catalysts at the cathode, oxygen reduction remains a key bottleneck. This issue can be addressed by increasing the cathode surface area and employing biological catalysts. The industrial scalability of MFCs for energy production and wastewater treatment is hindered by electrode material instability. Further research is needed to couple energy-harvesting systems with MFCs, potentially allowing for voltage storage and enhanced output during wastewater treatment. Cost reduction, improved harvesting efficiency, and design optimization are crucial for the sustainable development of MFCs, which could be integrated into self-powered energy-harvesting systems.^8,9,122,123 MFCs being an impactful resource of green life are highly applicative and performative and the same is exhibited in Fig. 12.

Table 6 Comparative performance of different PMFCs

Anode/photoanode	Cathode/photocathode	Performance	Application	Ref.
Where Pmax represents maximum power density, Jsc – short circuit current density, Voc – open circuit voltage, J – current density,TOC is total organic carbon and RB5 is commonly abbreviated for reactive black 5 dye.
ZnO/carbon felt	Pt/C	P max = 2.34 μW cm^−2, Jsc = 0.0069 mA cm^−2, Voc = 1000 mV	Degradation of Azo dye RB5 (13.6% at anode and 8.7% at cathode)	159
Carbon felt	Bi–TiO2/Vulcan AC/carbon-felt	P max = 224 mW m^−2, COD removal efficiency = 78–89%	Visible light irradiation photocatalytic activity	160
Fe-graphene oxide-titanium phosphate	ZnIn2S4	90% removal of rhodamine B at pH 1, cell voltage = 0.4 V	Degradation of rhodamine B	125
NiFe layered double oxide/TiO2	Carbon black	P max = 169 μW cm^−2, Jsc = 1093 μA cm^−2, Voc = 0.78 V	Electricity generation with hydrogen peroxide production	161
Ag–TiO2	Pt/C on carbon cloth	P max = 1.85 W m^−2, COD removal efficiency = 14.8%	Coulombic efficiency of 9.4% generates electrical power from the remediation of wastewater	162
WO3/W	Fe@Fe2O3/carbon felt	J sc = 0.59 mA cm^−2, Pmax = 0.34 mW cm^−2	Wastewater decontamination and electricity generation	132
CdS–ZnS–TiO2	Pt black on carbon black coated carbon paper	P max = 1.01 mW cm^−2, Jsc = 2.1 mA cm^−2, Voc = 1.07 V	Simultaneous organic compound degradation and electricity generation	163
Stainless steel/α-Fe2O3/carbon	Carbon felt disk	J = 46,500 mA m^−2	Photo-electrochemical interaction for high current output	164
N-doped graphene aerogel	Cu2O/Au nanowire	P max = 2849.9 mW m^−2, J = 6500 mA m^−2	Efficient power output	165
Graphite rod	Co–N@MoS2/C	LEV degradation rate = 4.9 × 10^−2 min^−1, H2 evolution rate = 3.1 × 10^−3 m^3 m^−3 min^−1, J = 60 000 mA m^−2	Levofloxacin remediation and energy catalysis	166
Carbon brush	Carbon cloth	CO2 fixation rate = 1292.8 mg L^−1 day^−1, Pmax = 5.94 W m^−3, J = 441.75 mA m^−2	CO2 fixation and bioenergy generation	167
Ag3PO4-coated carbon cloth + TiN coated carbon cloth	Carbon brush	P max = 2.90 W m^−2, COD removal efficiency = 83.21%, TOC removal efficiency = 72.47%	Synergistic effect of photo- and bio-degradation resulted in decolourization efficiency of 95.8% MB	168
Graphite plate	AgBr/ZnO-modified graphite	P max = 53.8 mW m^−2 upon visible light irradiation and 32.5 mW m^−2 in dark conditions	AgBr/ZnO-modified graphite resulted in 61% RB5 dye degradation over 72 h compared to bare graphite output of 11.74%	169

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Fig. 12 As an innovative solution, MFCs promise to lead us toward a greener and more efficient future.

9. Challenges and future perspectives of PMECs and PMFCs

This article emphasizes the focused attention on carbon-based electrodes in PBESs, particularly in PMFCs and PMECs, an area that has received limited comprehensive coverage in existing literature. It contains highlighted recent advancements in nanomaterial development, such as CNTs, and their potential integration to enhance the scalability, sustainability, and efficiency of PBESs. The interdisciplinary approach of this review stresses on integrating perspectives from environmental engineering, material science, and energy systems to provide a holistic understanding of current challenges and future prospects in the field.

Despite substantial progress in the development of microbial fuel cells and microbial electrolysis cells for wastewater treatment and bioenergy generation, several challenges still hinder their commercialization and large-scale implementation. A major limitation is their poor scalability, as the power output does not increase proportionally with the expansion of the anodic chamber volume or the electrode surface area. This issue primarily arises from constraints related to electrochemical thermodynamics, the kinetics of electrode reactions, and mass transport, all of which restrict overall system performance.

To develop MFCs and MECs for field-scale applications, it is necessary to overcome these limitations and improve energy efficiency. One of the key issues is the presence of high energy losses and low conversion efficiency, which directly reduce the electrical output and operational performance of these systems. Elevated energy losses at both the anode and cathode lead to decreased efficiency, while low conversion efficiency limits the ability of the system to generate electricity from the oxidation of organic matter. Therefore, identifying low-cost, high-performance electrode materials with efficient catalytic properties is essential to minimize energy losses and enhance the system's viability in real-world applications.

Another major challenge is the internal resistance within the system. Electrical resistance at the electrodes and ion transport resistance through the membrane reduce the overall voltage output. Addressing this issue involves decreasing the distance between electrodes, improving the conductivity of electrode materials through surface modifications, and selecting membrane materials with superior ion transport properties. Modified ceramic membranes incorporating cation exchangers have shown potential as affordable alternatives to traditional membranes by enhancing ion conductivity and reducing system costs. However, challenges such as membrane fouling, variations in acidity and alkalinity, oxygen diffusion, and internal resistance remain critical barriers to performance, particularly in large-scale applications.

Mitigating these issues requires the design of improved membranes that resist biological fouling, better control of acidity and alkalinity, and enhanced oxygen transport mechanisms. Despite these obstacles, microbial fuel cells and microbial electrolysis cells hold substantial promise in a wide range of applications, including decentralized electricity generation, wastewater treatment, power supply for remote sensors, monitoring of biological oxygen demand, and hydrogen gas production. Continued innovation in electrode materials and catalytic systems is expected to broaden the applicability of these technologies, especially in decentralized and environmentally sustainable wastewater treatment systems. A promising approach is the development of photoelectrochemical biohybrid systems, which integrate semiconductor materials with biological elements such as enzymes or whole-cell microorganisms. In these systems, light energy generates charge carriers within the semiconductor. The resulting electrons are transferred to the biological components, initiating biochemical reactions. These systems offer the potential to simultaneously treat pollutants and generate energy by leveraging solar energy and biologically driven processes. For instance, recent developments have demonstrated the effectiveness of combining carbon cloth biocathodes with titanium dioxide photoanodes to achieve direct electron-driven conversion of carbon dioxide to methane, with an energy efficiency of approximately 95% and minimal energy losses, as shown in Fig. 13. However, titanium dioxide's wide energy band gap restricts its ability to absorb light to ultraviolet wavelengths and makes it susceptible to chemical degradation under light exposure.¹⁷³ To address these limitations, graphitic carbon has emerged as a promising material. It is abundant, chemically stable, and can be engineered to work with semiconductors that have lower energy band gaps (tandem strategies). This combination improves light absorption and charge transfer while reducing degradation, making carbon-based materials essential in the development of robust and efficient photoelectrochemical biohybrid systems. Scaling up microbial fuel cells and microbial electrolysis cells from laboratory or pilot setups to industrial applications presents several technical and economic challenges. Once again, the power output does not scale proportionally with the size of the anodic chamber or the electrode surface area due to the same underlying limitations in transport, kinetics, and thermodynamics. Cost is a significant hurdle as well, and using expensive materials like platinum and palladium in photo-bioelectrochemical systems is not feasible for large-scale deployment. Developing affordable, durable, and high-performance electrode materials is, therefore, a top priority. While carbon-based nanomaterials such as graphite and carbon nanotubes offer good electrical conductivity and affordability, concerns regarding their long-term stability and potential biological toxicity must be addressed. In microbial electrolysis cells, the biological process that produces methane competes with hydrogen gas production, reducing efficiency.¹⁴⁴ Operational conditions and the type of organic material used influence this competition. Additional operational challenges such as membrane fouling, oxygen diffusion limitations, and internal resistance further impact system efficiency and scalability. From an industrial perspective, these factors contribute to higher costs and increased complexity.

Fig. 13 A Microbial-photoelectrochemical hybrid system depicting mechanism (A) and performance (B) outlining up to 96% faradaic efficiency for methane production via direct electron transfer-driven CO₂ reduction. Reprinted with permission.¹⁴⁴ Copyright 2018, Elsevier.

Integrating MFCs and MECs with renewable energy sources, such as solar or wind power, could significantly improve their sustainability by reducing reliance on external electricity. For instance, photocatalysis along with wastewater treatment plants is found to utilize 1.4 US$ per hour at 10 mA cm⁻² current density, which is far less than conventional systems. However, such integration requires the development of efficient energy management systems and energy storage technologies to ensure reliable operation. Moreover, such commercial projects make use of noble metals, whose processing gives negative eco impact.¹⁷⁴ Long-term stability and durability are crucial for practical use. Electrode degradation due to corrosion, fouling, or mechanical stress remains a significant barrier. Materials like stainless steel and molybdenum offer promise due to their corrosion resistance and mechanical strength. Additionally, strategies to prevent biological fouling on electrode surfaces can help extend system lifespan and improve performance. The transition from laboratory research to real-world industrial use will depend on the development of scalable, low-cost methods for synthesizing, modifying, and fabricating electrode materials. One promising approach involves the use of biomass-derived materials, which offer high surface area, good electrical properties, and biological compatibility for microbial attachment and electron transfer. Combining microbial fuel cells with renewable energy systems could allow for simultaneous hydrogen production and wastewater treatment, an important step toward realizing circular economy. However, the cost of photovoltaic components and the lack of effective energy storage solutions must be resolved to make this hybrid approach feasible. In conclusion, while microbial fuel cells and microbial electrolysis cells face numerous challenges, their potential for sustainable energy production and wastewater treatment makes them a compelling area of research. Addressing the technical, economic, and environmental issues outlined above is essential to fully realize the promise of these technologies on an industrial scale.

10. Conclusion

This review has underscored the critical role of carbon-based electrodes in enhancing the performance of PBESs, especially in PMFCs and PMECs. Innovations in carbon-based photocatalytic materials have demonstrated significant improvements in system efficiency, sustainability, and scalability. With properties such as high conductivity, abundance, and cost-effectiveness, carbon-based electrodes have emerged as key enablers for addressing major technical bottlenecks, including high overpotentials, low coulombic efficiency, and ohmic losses.

Despite these advances, challenges such as membrane biofouling, pH splitting, oxygen diffusivity, electrode instability, and cathodic oxygen reduction remain. This review also identifies the need for durable electrode materials and low-cost, scalable synthesis processes suitable for real-world applications. There has also been discussion referring to the potential of nanomaterials, energy-harvesting systems, and emerging membrane technologies as avenues for overcoming these hurdles.

By synthesizing knowledge across multiple disciplines, this review offers a comprehensive roadmap for advancing PBESs. It also serves to guide future experimental and industrial efforts aimed at scaling up photocatalytic fuel systems for wastewater treatment and sustainable energy production. We hope that this work will contribute meaningfully to the ongoing discourse and inspire new strategies in the pursuit of global renewable energy and environmental goals.

Author contributions

Ankita Chaurasiya, Yashmeen Budania, Goldy Shah and Aradhana Mishra: formal analysis, investigation, writing – original draft preparation, data curation, software. Shiv Singh: conceptualization, methodology, validation, resources, writing – review & editing, funding acquisition, supervision.

Conflicts of interest

There is no conflict of interest.

Data availability

No data were used to write the review.

Acknowledgements

SS and AC extend their gratitude to the Department of Biotechnology, New Delhi, India (BT/PR51527/BCE/8/1825/2023, GAP-141) and the Department of Science and Technology (DST) for financial support through the DST/INSPIRE/03/2023/004044, IF230322, respectively. They sincerely appreciate the generous grants that facilitated the execution of this review. Special thanks are also extended to the Director of CSIR-Advanced Materials and Processes Research Institute, Bhopal, for his invaluable support in offering the necessary infrastructural facilities for the successful completion of this work.

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Footnote

† Authors contributed equally.

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