Recent Advances in the Development of Noble Metal-Free Cathode Catalysts for Microbial Fuel Cell Technologies

Abstract

The accelerating growth of the global population and the depletion of conventional energy resources have intensified the dual challenges of water scarcity and sustainable energy production. Microbial fuel cells (MFCs) have emerged as a promising bioelectrochemical technology capable of simultaneously treating wastewater and generating renewable electricity. Their performance is strongly dependent on electrode materials, particularly cathodes, which govern the kinetics of the oxygen reduction reaction (ORR) and overall energy conversion efficiency. Therefore, in order to improve the electro-kinetics of ORR, it is necessary to use catalysts with specific catalytic properties. An ideal catalyst for ORR must combine fast kinetics, high conductivity, high durability, and cost-effectiveness. Although platinum-based electrodes remain the most efficient ORR catalysts, their scarcity and prohibitive cost are hindering their commercialization. Therefore, research has focused on viable alternatives, such as metal oxides, perovskites, heterojunction composites, and emerging carbon-based materials, paving the way toward highly effective energy conversion and industrial-scale implementation of MFCs.

1. Introduction

The exponential growth of the global population, expected to reach approximately 9.7 billion by 2050 and 10.9 billion in 2100, up from an estimated 8.2 billion in 2025 [1], exerts significant pressure on natural resources and accelerates their depletion. In this context, modern society faces two major challenges that determine its future: (i) the energy crisis and (ii) environmental pollution. To address these current challenges, a rapid transition to clean and sustainable energy sources is essential. Moreover, optimized water management in energy systems and the development of advanced wastewater treatment techniques will be essential to achieve balance and resilience between energy and the environment [2]. These intertwined challenges highlight the necessity of innovative technologies capable of simultaneously addressing water purification and renewable energy generation.

Microbial fuel cells (MFCs) have emerged as one of the most promising bioelectrochemical systems to meet these dual demands. By harnessing the metabolic activity of electroactive microorganisms, MFCs convert the chemical energy stored in organic substrates directly into electrical energy, while simultaneously degrading pollutants in wastewater. This dual functionality, energy recovery and wastewater treatment, positions MFCs as a strategic technology for sustainable resource management [3,4,5,6]. Structurally, MFCs consist of an anode and a cathode connected through an external circuit and separated by a specific membrane. In the anode chamber, microorganisms oxidize organic matter, releasing electrons (e⁻) and protons (H⁺). Electrons flow through the circuit to the cathode, while protons migrate across the separator, enabling the oxygen reduction reaction (ORR) at the cathode to form water (H₂O). When such electron acceptors are absent in an MFC, the microorganisms transport the electrons to the anode surface, enabling electricity to be generated [7,8,9,10].

Despite their promising potential, MFCs face significant challenges in terms of efficiency, scalability, and economic viability [4,11]. Among these, the cathode remains the most limiting component due to sluggish ORR kinetics, which require high activation energy (498 kJ/mol) to break the O=O bond [12]. Moreover, the ORR in MFCs is severely hindered by their neutral pH and low operating temperature. Unlike conventional fuel cells, these mild conditions restrict proton availability, forcing the reaction toward slower, less efficient pathways, such as the two-electron reduction to peroxide [13]. To overcome these constraints, it is necessary to use ORR catalysts with specific catalytic properties that reduce the activation energy of oxygen reactivity while decreasing the binding energy of the O=O bond, and remarkably improve the electro-kinetics of ORR. Noble metals, particularly Pt, Rh, and Pd, are the most active systems reported so far for the ORR process [8,14,15]. The exceptional catalytic performance of these Pt-group metals fundamentally arises from their optimal binding energy with key oxygen-containing intermediates, such as *OOH, *OH, and *O [16]. This characteristic behavior is illustrated by a volcano plot (Figure 1), which correlates ORR activity with the calculated free energy (ΔE) of adsorbed intermediates on various metal surfaces. The peak of the volcano represents the ideal binding strength; intermediates should adsorb neither too weakly, which would hinder activation, nor too strongly, which would impede product desorption. Pt’s position near the top of the volcano plot confirms its superior activity for ORR [17]. Nevertheless, their prohibitive cost, scarcity, and susceptibility to long-term degradation under operational conditions present critical barriers to the scalable deployment in MFCs [18]. Consequently, recent research has focused on developing alternative cathode materials that combine affordability, durability, and high catalytic activity for ORR [19,20], such as carbon graphite [21], metal oxides [22], metals [23], and perovskites [24], which have all been explored as potential substitutes, offering tunable electronic structures, oxygen vacancy engineering, and synergistic interfacial effects that enhance ORR kinetics. Furthermore, hybrid systems integrating photocatalysis with MFCs have been proposed to improve pollutant degradation and energy recovery, leveraging both solar energy and microbial metabolism [25]. These innovations underscore the importance of material science in advancing MFC technology, particularly in the design of cathodes that can overcome kinetic barriers and deliver high-effective energy conversion.

This review is structured to provide an overview of recent advances in noble metal-free cathode catalysts development for ORR in MFCs, with comprehensive comparison across the full spectrum of emerging catalyst classes. Section 1 introduces the different MFC configurations, establishing the technological context for electrode design. Section 2 focuses specifically on cathode catalysts, highlighting recent advances in metal oxides, perovskite, and heterojunction composites. Section 3 explores the mechanistic pathways of the ORR within MFCs, emphasizing the fundamental electrochemical processes that govern system performance. Finally, the concluding section outlines future perspectives and research directions, with particular attention to material innovation and scalability.

2. MFC Configurations

Several MFC designs have been implemented so far, and all require two different phases or at least two different physically separated compartments to be connected: one for the anode, and the other for the cathode [8]. In dual-chamber MFCs (DCMFCs), the anode and cathode compartments are physically separated by a specific membrane, most commonly a proton exchange membrane (PEM), which facilitates proton transfer while preventing mixing of the electrolytes. In some designs, a simple salt bridge is employed as an alternative separator [26]. The most widely adopted laboratory-scale DCMFC is the H-type device, consisting of two reactors connected by a tube containing a separator (Figure 2a) [27]. This architecture maintains strict anaerobic conditions in the anodic compartment, minimizing oxygen diffusion that could inhibit exoelectrogenic bacteria. Oxygen typically serves as the oxidizing agent at the cathode due to its availability in air, high oxidation potential, and formation of water as a benign byproduct. To enhance oxygen transport to the cathode surface, air can be bubbled into the electrolyte of an aqueous cathode compartment [28,29]. However, the low solubility of oxygen in water (approximately 1 mM at room temperature under 1 atm of pure oxygen and only 0.28 mM at equilibrium with air) severely limits mass transfer and constitutes a major performance bottleneck in aqueous-cathode systems. This constraint often results in higher internal resistance, lower power densities, and reduced overall efficiency, confining DCMFCs largely to fundamental research applications despite their advantages in controlled environments.

To overcome these oxygen-related limitations, air cathodes that expose one side directly to the atmosphere have been developed, enabling passive oxygen diffusion without the need for active aeration. Due to their simpler, more economical design, reduced internal resistance, and improved practical implementation characteristics, including lower construction costs and easier scalability, many researchers have recently transitioned to single-chamber MFCs (SCMFCs) [30,31,32,33,34]. In SCMFCs (Figure 2b), the anode and cathode are housed within a single compartment, with the cathode typically configured as an air-cathode exposed to ambient air. This configuration eliminates the need for a separate catholyte and often omits or minimizes the use of a PEM, further decreasing costs and complexity while enhancing proton diffusion and oxygen reduction kinetics at the cathode surface. Consequently, SCMFC systems frequently achieve higher power outputs compared to traditional DCMFC setups as demonstrated by a study of Kim et al. [35], who evaluated both configurations under similar conditions; their results revealed a maximum power density (MPD) of 40 mW m⁻² and 488 mW m⁻² for DCMFC and SCMFC, respectively. This makes SCMFC more suitable for potential real-world applications such as integrated wastewater treatment with energy recovery. Other factors can affect the MFC’s performance in power generation and wastewater treatment such as operating conditions, substrate type, microorganisms, membrane type, and electrode materials [27,36,37]. It is essential to note that the materials used at both the anode and cathode determine the rate of reaction at the electrodes, which is the key step in the operating mode of an MFC [4,11]. The rate of electron transfer in MFCs is proportional to the number of viable electroactive microorganisms attached to the anode surface. Meanwhile, a larger surface area can accommodate more microbial adhesion and thus facilitate greater total current output. Moreover, performance is determined not merely by the total geometric area, but by the catalytically active surface area, the specific regions where microorganisms establish effective electrochemical contact, and the physicochemical heterogeneity of the electrode interface, which can significantly affect biofilm formation, electron transfer kinetics, and overall system efficiency. The catalytic performance of the materials used at the cathode is the most limiting factor at this electrode [12]; ongoing research continues to optimize cathode catalysts to address challenges related to ORR kinetics and long-term stability, paving the way for more efficient and commercially viable MFC technologies.

3. Cathode Materials

ORR efficiency in the cathodic MFC compartment is typically associated with four key parameters: (i) the concentration of the electron acceptor, (ii) the availability of protons from the anode, (iii) the performance of the catalyst, and (iv) the electrode composition. Carbon- and graphite-based materials are most commonly used as cathodes. However, the ORR kinetics at these conventional electrodes are sluggish, necessitating the integration of electrocatalysts to enhance their performance. Noble metals, particularly Pt, are widely used as cathode catalysts in MFCs. Nevertheless, their practical application is hindered by critical drawbacks, including high cost, limited availability, and catalyst poisoning. Therefore, extensive research efforts have been directed toward the development of noble metal-free catalysts, aiming to provide cost-effective, durable, and efficient alternatives for practical MFC applications [38].

3.1. Metal Oxides

Metal oxides have emerged as promising cathode catalysts in MFCs due to their intrinsic redox activity, structural stability, and cost-effectiveness compared to noble metals such as Pt. Transition metal oxides, including manganese dioxide (MnO₂) [39], cobalt oxide (Co₃O₄) [40], and iron oxides (Fe₂O₃ [41], Fe₃O₄ [42]), exhibit multiple oxidation states that facilitate electron transfer during the ORR. Xia et al. [43] revealed that the MPD produced by a cobalt oxide cathode (1540 mW m⁻²) was 13% higher than that from Pt/C cathodes (1360 mW m⁻²), due to the presence of Co²⁺ and Co³⁺ species on the catalyst’s surface. TiO₂ has been proven to be an effective ORR catalyst for improving the MFC’s performance. Jaswal et al. [44] explored the influence of a TiO₂ nanoparticle-modified cathode catalyst in DCMFC. The results indicated a power output of 162.5 mW m⁻², accompanied by significant wastewater treatment efficiency evaluated in terms of chemical oxygen demand (COD) removal efficiency, which was 72.3%. Such activity was attributed to the presence of interfacial oxygen vacancy active sites on the modified cathode surface where electrons are trapped in oxygen vacancies and subsequently localize to the neighboring Ti sites, resulting in the formation of Ti³⁺ species at low-coordinated surface positions. This makes the nanoparticle more oxygen-deficient, resulting in the adsorption of more oxygen from the catholyte. A higher redox potential of O₂ then favors its reaction with electrons transferred from the anode, leading to a higher cathode reduction potential and higher power generation efficiency. Bhowmick et al. [45] investigated bismuth-doped TiO₂ (Bi-TiO₂) as a photocathode component in a DCMFC. The obtained MPD was 224 mW m⁻², significantly higher than that achieved using a Pt cathode catalyst (194 mW m⁻²). The performance of the Bi-TiO₂ photocatalyst was attributed to its reduced band gap (2.80 eV), enabling efficient visible-light absorption, coupled with Bi³⁺–O³⁻ orbital hybridization in Bi₂O₃, which facilitates the spatial separation of photogenerated electron–hole pairs, thereby significantly suppressing their recombination and enhancing the ORR activity. Furthermore, the COD removal efficiency was 89% owing to the enhanced ORR performance, which facilitated the rapid scavenging of electrons and protons from the anode chamber and thereby contributed to maintaining a stable microenvironment at the anode. Wang et al. [46] explored the use of 2 wt% Cu/TiO₂ as photocathode catalysts for DCMFC, which yielded a remarkable MPD of 312 mW m⁻² and achieved 99% degradation efficiency of the methyl orange pollutant. These outstanding performances were correlated with its high specific surface area (70.42 m² g⁻¹), superior charge separation efficiency, and enhanced electrochemical activity. Chen et al. [47] investigated MnO₂@Co₃O₄, which provided an MPD of 475 mW m⁻², 2.24 times higher than that of Co₃O₄ (212 mW m⁻²) and 2.63 times higher than that of the MnO₂ (180 mW m⁻²). Such activity is attributed to the rod-like structure of MnO₂, which can effectively improve the ion flow efficiency and reduce the transfer resistance, coupled with the point-like structure of Co₃O₄, which can increase the specific surface area of the complex, providing more active sites. Other metal oxides such as NiCo₂O₄ nanoplatelets [48], V₂O₅ [49], SnO₂ [50], CuO [51], Mn-doped ZnO [52], NiMn₂O₄ [53], ZrP₂O₇ [54], and Ni₂V₂O₇ [55] highlight that the transition metal oxides are a cost-effective and feasible active substitute for conventional costly Pt-based cathode catalysts. Nevertheless, their conductivity is low because of the nature of the oxides; it is therefore necessary to improve the conductivity and reduce the resistance of the materials.

3.2. Perovskites

Other researchers have developed efficient perovskite (ABO₃) cathode catalysts, owing to their tunable electronic structure, enhanced catalytic behavior toward the ORR, and relatively low cost compared to noble metals. Their activity is primarily governed by several intrinsic properties: (i) the presence of mixed valence states at the B-site transition metal cations (e.g., Mn, Co, Fe, Ni), which facilitate electron transfer and redox cycling; (ii) an adjustable oxygen vacancy concentration, enhancing ionic conductivity and promoting oxygen adsorption and dissociation; and (iii) high structural stability under aqueous and electrochemical conditions. Furthermore, the ability to tailor the A-site cation (e.g., La, Sr, Ca) allows for fine control of electronic conductivity and ORR kinetics, making perovskites versatile candidates for cathodic applications. In a study by Dai et al. [56], a CaFe_0–7Zn_0–3O₃ perovskite was used as an electrocatalyst with a carbon cloth cathode in an SCMFC, achieving a remarkable efficiency, with an MPD of 892.10 mW m⁻³ (normalized to the reactor volume) with excellent stability over 160 h of operation. This performance is attributed to the excellent ORR kinetics, which are due to the synergy between Zn and Fe that enhances the formation of oxygen vacancies, which increase oxygen unavailability rates and conductivity, as well as the formation of a mixed valence state of Fe²⁺/Fe³⁺, which helps to stabilize the catalytic activity of perovskite. Nourbakhsh et al. [57] tested LaCoO₃, LaMnO₃, and LaCo_0.5Mn_0.5O₃ perovskite nanoparticles as cathode catalysts on carbon cloth in a DCMFC. LaMnO₃ provided an MPD greater than 13.9 mW m⁻². The electrochemical performance of perovskites can be classified in the following order: LaCoO₃ < LaCo_0.5Mn_0.5O₃ < LaMnO₃. Dong and his group studied the La_0.4Ca_0.6Co_0.9Fe_0.1O₃/carbon (LCCF/C) cathode in an SCMFC; a power density of 405 mW m⁻² was obtained [58]. The perovskite is a type of structure that provides oxygen vacancies, facilitating oxygen mobility and reducing biofilm coverage. As a result, the LCCF/C catalyst behaved in a cycling experiment with much higher performance: after 15 cycles, it had the lowest rate of decay of the open-circuit voltage (2%) and MPD (15%) relative to those of C (22%, 69%) and Pt/C (4%, 17%) cathodes, respectively. In a neutral operating medium, lanthanum-based perovskites displayed a lower internal resistance, which is favorable for the ORR process, as the required activation energy decreases [58]. The participation of transition metal cations and their valence largely affect the ORR and electrocatalytic activity of perovskite-type oxides. In a study by Bai et al. [59], the perovskite La_xSr_1-xCoO₃ was introduced as the cathode compound in an anaerobic fluidized bed MFC. A comparative study between La_0.7Sr_0.3CoO₃, La_0.3Sr_0.5CoO₃, and La_0.8Sr_0.2CoO₃ was carried out, which showed that La_0.7Sr_0.3CoO₃ attained the best lanthanum and strontium concentrations in perovskite, as it exhibited the most effective electrocatalytic activity, with a power output of 104.59 mW m⁻² and an open-circuit voltage of 594.0 mV. Representative perovskite cathode catalyst formulations tested for MFCs are recapitulated in

Table 1. Performance of different perovskite materials used as MFC cathode catalysts.

Catalyst	Matrix	Reactor Type and Scale	Inoculum Source	MPD (mW m−2)	OCV (mV)	Ref.
LaCoO3	Carbon cloth	DCMFC (350 mL)	Pure culture of Shewanella	6.99	613.84	[57]
LaMnO3	Carbon cloth	DCMFC (350 mL)	Pure culture of Shewanella	13.91	656.24	[57]
LaCo0.5Mn0.5O3	Carbon cloth	DCMFC (450 mL)	Pure culture of Shewanella	8.78	634.30	[57]
La0.4Ca0.6Co0.9Fe0.1O3	Carbon mesh	SCMFC (28 mL)	Domestic wastewater	405	530	[58]
LaFeO3	Carbon cloth	SCMFC (100 mL)	Mature MFC unit	726.43	–	[60]

(–) indicates that the value was not reported in the original study.

.

The group of Touach et al. evaluated the photoelectrocatalytic performance of carbon cloth coated with ferroelectric perovskite materials for the construction of cathodes for SCMFCs. In the absence and the presence of a UV–visible light condition, BaTiO₃ [61], LiTaO₃ [62], LiNbO₃ [63], Li_1-xTa_1-xW_xO₃ [64], Li_0.95Ta_0.76Nb_0.19Mg_0.15O₃ [65], (Li_0.95Cu_0.15)Ta_0.76Nb_0.19O₃ [66], LiTa_0.5Nb_0.5O₃/gC₃N₄ [67], and Li_0.95Ta_0.57Nb_0.38Mg_0.15O₃ [68] were tested. The photoelectrocatalytic activity results obtained for these cathodes are summarized in

Table 2. Typical perovskites listed by their performance as photocatalysts on carbon cloth cathodes for MFCs.

Catalyst	Reactor Type and Scale	Inoculum Source	MPD (mW m−2)	OCV (mV)	CODr (%)	Ref.
BaTiO3	SCMFC (250 mL)	Domestic wastewater	498	387	90	[61]
Li0.95Ta0.76Nb0.19Mg0.15O3	SCMFC (250 mL)	Industrial wastewater	228	470	95.74	[65]
Li0.95Ta0.57Nb0.38Mg0.15O3	SCMFC (250 mL)	Domestic wastewater	764	680	75	[68]
Li0.9Mn0.05TaO3	SCMFC (250 mL)	Industrial wastewater	370	350	91	[71]
3 wt% BiFeO3/ZnO	DCMFC (80 mL)	Sewage wastewater	1301	922	–	[70]

(–) indicates that the value was not reported in the original study.

, highlighting the photocatalytic activity of perovskite materials for ORR. This performance was attributed to the spontaneous polarization characteristic of ferroelectric materials, often linked to the displacement of transition cations relative to oxygen anions, promoting charge transport to catalytic interfaces. Ferroelectric/electrode interfaces typically create charged defects that modify redox activity and enhance electrochemical reactivity [61]. Thus, the presence of positively charged surfaces in perovskites promotes the ORR. Other perovskites such as NiTiO₃ [69] and BiFeO₃ [70] are also investigated as cathode photocatalysts for MFCs.

3.3. Metal–Organic Frameworks (MOFs) and Polyoxometalates (POMs)

In recent years, MOFs and their derivatives have emerged as a major research focus owing to their exceptional structural and functional properties. MOFs are crystalline materials with two- or three-dimensional framework architectures constructed from metal ions or clusters bridged by organic ligands. They possess remarkable features, including extremely high surface areas, tunable porosity, diverse ligand chemistry, versatile functionalization, and outstanding chemical and thermal stability, which have fueled their rapid development and widespread interest [72]. In a recent study carried out by Lan et al. [73], a hybrid material, Cu-MOF@Fe-MOF, produced a maximum power density of 231.2 mW m⁻² and demonstrated high stability and durability, maintaining a potential of 238 mV for 150 h, highlighting the performance of MOF as a catalyst for the ORR. Additionally, Noori et al. [74] also investigated MOFs with NH₂-UiO-66(Zr/Ni) as a cathode catalyst to enhance the ORR in MFCs. NH₂-UiO-66(Zr/Ni) significantly improved electricity production. The MFC equipped with this bimetallic MOF catalyst achieved a significant improvement in power generation, reaching a power density of 800 mW m⁻². This efficiency was notably higher than that achieved with other tested electrocatalysts, including a standard 10% Pt-C catalyst. The substantial increase in MPD confirms the efficacy of NH₂-UiO-66(Zr/Ni) in improving the energy recovery process. Due to their unsaturated metal ion active sites, MOFs exhibit intrinsic electrocatalytic activity, highlighting their potential as ORR catalysts [75].

POM complexes represent a rich and important class of inorganic molecular compounds with unique electronic versatility due to their dual electronic and acid–base properties, allowing them to be both electrocatalysts and electron reservoirs [76,77]. The compositional flexibility and the presence of large numbers of redox active sites make these molecular metal oxide clusters excellent candidates for ORR catalysts. Kegging-type heteropolyacids have been reported as effective ORR catalysts [78]. H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀ exhibited notable performance in terms of both bioelectricity production and wastewater treatment, achieving maximum power densities of 497 and 514 mW m⁻², along with COD removal efficiencies of 84.66% and 73.6%, respectively. Rezaei et al. [79] investigated the electrocatalytic performance of a cesium phosphomolybdate (Cs₃PMo₁₂O₄₀) on graphite in a DCMFC system. The results showed an MPD of 64.73 mW m⁻² and a COD removal efficiency of 86.4%. Furthermore, lacunary POM salts, including Cs₅PMo₁₁FeO₃₉ and Cs₅PMo₁₁CoO₃₉, were also studied. Cs₅PMo₁₁CoO₃₉ catalysts demonstrated an MPD of 418.15 mW m⁻² and a COD removal efficiency of up to 97% after 96 h of MFC operation under UV–Vis irradiation [80]. These studies confirm that Keggin-type POMs can be promising ORR catalysts for MFCs.

Table 3. Representative materials of efficient cathode catalysts for ORR in MFCs.

Catalyst	Matrix	Reactor Type and Scale	Inoculum Source	MPD (mW m−2)	OCV (mV)	CODr (%)	Ref.
H3PMo12O40	Carbon cloth	SCMFC (250 mL)	Domestic wastewater	514	638	84.66	[78]
H3PW12O40	Carbon cloth	SCMFC (250 mL)	Domestic wastewater	497	790	73.9	[78]
Cs5PMo11FeO39	Carbon cloth	SCMFC (250 mL)	Domestic wastewater	163.87	504	83.92	[80]
Cs5PMo11CoO39	Carbon cloth	SCMFC (250 mL)	Domestic wastewater	119.62	470	75.9	[80]
Cs3PMo12O40	Graphite	DCMFC (190 mL)	Activated sludge	64.73	260	86.4	[79]
UiO-66 (Zr-MOF)	Carbon felt	SCMFC (150 mL)	MFC anode sludge	131.2	891.1	67	[81]
Cu3(BTC)2	Stainless steel mesh	SCMFC (28 mL)	Domestic wastewater	1772	–	–	[82]
Ni-MOF-74	Graphene oxide	SCMFC (28 mL)	Bacterial solution	446	500	84	[83]
ZIF-67@MoS2	Stainless steel mesh	SCMFC (28 mL)	–	302.5	420	–	[84]

(–) indicates that the value was not reported in the original study.

displays the performance of different MOFs and POMs used as cathode compounds in SCMFCs.

3.4. Heterojunction Composites

Non-precious metals such as Fe, Mn, and Cu and heteroatom-doped carbon-based materials were examined to replace noble metals used as catalysts for the ORR in MFCs. Li et al. [23] investigated transition metal composites (N@C, CuN@C, MnN@C, and CoN@C) as ORR electrocatalysts to enhance power generation in an SCMFC. The CoN@C cathode was able to achieve an MPD of 1202.3 mW m⁻², which is 1.09 times higher compared to that of commercial Pt/C (1104 mW m⁻²), demonstrating that this material is an efficient cathode electrocatalyst for replacing valuable Pt/C in MFC applications. Kodali et al. [85] compared the performance of Fe-, Co-, Ni-, and Mn-aminoantipyrine (AAPyr) ORR catalysts with that of an activated carbon (AC) cathode. The findings revealed that the electrocatalytic activity of the materials can be classified in the following order: Fe-AAPyr > Co-AAPyr > Ni-AAPyr > Mn-AAPyr > AC. Cathode catalysts based on multiwall CNTs impregnated with nitrogen, Co, and Fe were tested by Turk et al. [86], and the results obtained showed excellent electrocatalytic activity, with maximum power densities of 5.1 W m⁻³ and 6 W m⁻³ for Co-N-CNT and Fe-N-CNT, respectively. This performance was associated with the formation of active nitrogen–metal centers. Wang et al. [87] developed metal-free cathode catalysts based on nitrogen-modified carbon (CN) powder pyrolyzed at different temperatures, which showed a distinct cost–effectiveness ratio. An MPD of 371 mW m⁻² was achieved for the MFC operating with CN-800 with a COD removal of 77.2%. The MFC demonstrated excellent stability over 580 h of operation and efficiency. A reduced graphene oxide–copper sulfide–zinc sulfide hybrid nanocomposite (rGO-CuS-ZnS) was investigated by the team of Mahalingam et al. [88] for its effectiveness as a cathode catalyst in an SCMFC. This cathode showed an MPD of 1692 mW/m². Bhowmick et al. [89] tested a cathode constructed from TiO₂/activated carbon irradiated with UV-A light, which exhibited an MPD of 494 mW m⁻² and a COD removal efficiency of 95%. Karthick and Haribabu [90] reported a polypyrrole/molybdenum oxide (Ppy/MoO₂) composite on carbon cloth as a cathode for SCMFC that yielded a power density of 0.63 W m⁻². Ali et al. [91] tested a graphite felt modified with iron sulfide wrapped in reduced graphene oxide (FeS@rGO) as a cathode electrocatalyst in a DCMFC. The MFC showed an MPD of 154 mW m⁻² as well as a COD and Cr(VI) removal efficiency of 72% and 100%, respectively. This electrochemical performance was explained by the high conductivity, low internal resistance, and improved reaction kinetics of FeS@rGo nanocomposites. Papiya et al. [92] employed Co/Al₂O₃-rGO as the cathode electrocatalyst in an SCMFC. Co nanoparticles with varying percentages were incorporated into a mixture of alumina and graphene oxide. Four different weight percentages of Co were investigated, with the material containing 80 wt% Co found to have the highest power density of 548.19 mW m⁻² and COD removal efficiency of 91.48%. In addition, PANI@Fe/NC was evaluated by Dhillon and Kundu [93] as a novel and cost-effective cathode catalyst for MFCs. They demonstrated that this catalyst reached a high MPD of 637.53 mW m⁻², which was 36.25% greater than that of a conventional 10 wt% Pt/C catalyst (467.92 mW m⁻²) under identical operating conditions. A Cu-N/C@Cu-2 catalyst derived from ZIF-8 exhibited an MPD of 581 ± 13 mW m⁻², outperforming commercial Pt/C catalysts (499 ± 13 mW m⁻²) owing to the improved conductivity and more active sites of Cu-N_x as demonstrated by Lai et al. [94]. Zhang et al. [95] investigated a new Fe-N-C catalyst based on pomelo peel derived-carbon (PPC), demonstrating an MPD of 184 mW m⁻² and effective wastewater treatment, achieving 86.6% and 99.0% in COD and Congo Red removal, respectively. This performance was attributed to graphitized structures, pyridine-N and graphitic-N, which enhanced the conductivity of PPC, as well as the presence of Fe₃C nanocrystals, Fe-N_x active sites, and the rod-like CNTs. Liu et al. [96] showed that the use of a biochar and conductive carbon black (CCB) composite has very promising applications as an efficient and cost-effective cathode catalyst material. By promoting the growth of electrogenic and degrading bacteria, the optimized biochar/CCB catalyst enhanced the MPD to 269.5 mW m⁻³, which was 2.47 times higher than that of carbon cloth (109.1 mW m⁻³). The performance of cobalt oxide nanoparticles supported on nitrogen-doped carbon nanotubes (Co/N-CNT) was also evaluated [97]. This nanocomposite exhibited high electrocatalytic activity, with an MPD of 1260 mW m⁻², 16.6% higher compared to Pt/Carbon (1080 mW m⁻²) due to the formation of high-efficiency Co-N active sites that facilitated the ORR kinetics. Furthermore, magnesium cobaltite (MgCo₂O₄) coated on nitrogen-doped carbon (NC-700) was investigated as a cathode catalyst [98]. The MFC using MgCo₂O₄/NC-700 as the electrocatalyst for the cathode showed a remarkable power output of 873.81 mW m⁻² accompanied by the highest COD removal efficiency of 82.92%. This performance was significantly superior to that of other catalysts; it was 59.56% higher than that of an MFC using Pt/C (547.65 mW m⁻²) and 216.05% higher than that of one using a Co/NC-700 catalyst (276.48 mW m⁻²). Some MFC cathode electrocatalysts using heterojunction composites are summarized in

Table 4. Comparison of the performance of different heterojunction composites used as cathode catalysts for MFCs.

Catalyst	Matrix	Reactor Type and Scale	Inoculum Source	MPD (mW m−2)	OCV (mV)	Ref.
CoN@C	Carbon cloth	SCMFC (350 mL)	Anodic mixture from dual MF	1202.3	715	[23]
Ppy/MoO2	Carbon cloth	SCMFC (300 mL)	Shewanella putrefaciens	630	671	[90]
FeS@rGO	Graphite felt	DCMFC (140 mL)	MFC anodic effluent	154	–	[91]
Cu-N/C@Cu-2	Carbon cloth	SCMFC (28 mL)	Anaerobic sludge	581	703	[94]
Fe-N-C	Carbon cloth	SCMFC (252 mL)	Wastewater sludge	184	–	[95]
Co/Al2O3-rGO	Carbon cloth	SCMFC (150 mL)	MFC anode consortium	548.19	633	[92]
PANI@Fe/NC	Stainless steel mesh	SCMFC (50 mL)	Activated sludge	637.53	609	[93]
Co/N-CNT	Teflonized carbon cloth	SCMFC (28 mL)	Anaerobic digester sludge	1260	–	[97]
MgCo2O4/NC-700	Stainless steel mesh	SCMFC (50 mL)	Activated sludge	873.81	702	[98]
NiO/CNT	Carbon cloth	SCMFC (6.28 mL)	Sludge supernatant	670	772	[99]

(–) indicates that the value was not reported in the original study.

.

4. ORR Mechanistic Pathways in MFC

In an MFC system, the efficiency of the cathodic process is decisive for the overall performance of the system. At the anode, microorganisms oxidize carbon-based organic matter, converting biochemical energy into a flux of electrons. These electrons must then be transferred to the cathode, where the ORR takes place. The ORR is responsible for accepting the electrons and reducing oxygen to water, thereby closing the electrochemical cycle and sustaining continuous current generation. Because the ORR is intrinsically sluggish and often represents the rate-limiting step, its kinetics must be carefully controlled. Mechanistic investigations are essential to identify the reaction pathways, intermediate species, and rate-determining steps that influence electron transfer efficiency [13,100].

Typically, the ORR process occurs through two pathways: either a direct four-electron reduction or an indirect 2 + 2 electron reduction pathway. In an acidic environment, the mechanism involves the formation of two water molecules (Equation (1)), whereas in an alkaline medium, it implies the production of four OH⁻ ions (Equation (2)). Achieving a direct four-electron reduction is crucial for maintaining high energy output [24,101].

Acidic electrolyte 4e⁻ pathway:

O₂ + 4H⁺ + 4e⁻ → 2H₂O E₀ = +1.229 V_SHE

(1)

Alkaline electrolyte 4e⁻ pathway:

O₂ + 2H₂O + 4e⁻→ 4OH⁻ E₀ = +0.401 V_SHE

(2)

Reaction 2 can be further divided into single-electron reaction steps. However, the specific reaction pathway depends on the electrolyte pH and the catalyst used. There are two main mechanistic routes for the ORR. The first one consists of a dissociative pathway implying the breakage of the O–O bond as O₂ adsorbs onto two metal active sites, preventing the formation of a peroxide intermediate. The energy required to break the O–O bond in oxygen molecules is high [102], making the dissociative pathway less likely for most catalysts [103,104]. The second route is an associative pathway; in this mechanism, the O₂ molecule adsorbs onto a single metal site, forming a peroxide intermediate (Equation (3)), which makes breaking the O–O bond easier [102,105]. In alkaline media, hydrogen peroxide exists as HO₂⁻ [106].

O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻ E₀ = −0.065 V_SHE

(3)

The HO₂⁻ intermediate can then undergo electrochemical reactions on the catalyst surface (Equation (4)) or desorb from the catalyst. Additionally, the HO₂⁻ can also interact with the electrolyte or at the surface of the catalyst (Equation (5)). In both cases, only two electrons are transferred, leading to lower energy efficiency compared to the four-electron pathway.

HO₂⁻ + H₂O + 2e⁻ → 3 OH⁻ E₀ = +0.867 V_SHE

(4)

2HO₂⁻ → 2OH⁻ + O₂ E₀ = −0.07 V_SHE

(5)

More electrons can be transferred if the O₂ formed by HO₂⁻ chemical disproportionation (Equation (5)) undergoes further reduction via Equation (3). A rotating ring disk electrode (RRDE) is used in an analytical electrochemical technique that can measure the number of electrons transferred by detecting the amount of hydroperoxide at the ring electrode [107]. This method allows the determination of whether two or four electrons have been transferred. However, the RRDE cannot distinguish whether four electrons have been transferred through the dissociative or associative pathways (i.e., with or without the formation of HO₂⁻ intermediates). An overview of the possible reaction paths examined to date is provided in Figure 3.

Although acidic media provide a higher thermodynamic potential (E₀ = +1.229 V_SHE) and faster intrinsic ORR kinetics than alkaline media, alkaline conditions are often favored in MFCs due to enhanced catalyst stability and microbial compatibility. Nevertheless, controlling the kinetics of the ORR through mechanistic investigations is crucial. A detailed understanding of the reaction mechanism allows researchers to identify limiting steps, optimize catalyst design, and minimize energy losses. These insights are essential for improving the durability, efficiency, and economic viability of MFC technology.

5. Conclusions and Outlooks

As a new high-effective energy conversion system, the electrical efficiency of an MFC is fundamentally governed by the reaction kinetics at both the anode and the cathode. Therefore, employing high-performance catalysts to accelerate the ORR at the cathode represents a pivotal strategy for enhancing the overall electrical output and efficiency of MFCs. This review provides an updated overview of recent advances in ORR noble metal-free catalysts and their development for MFCs, including transition metal oxides, perovskites, metal–organic frameworks, polyoxometalates, and heterojunction structures. Although significant progress has been made in electricity production from wastewater biomass conversion using these ORR catalysts, several issues remain unresolved. Metal oxides have a wide range of sources, low prices, and good stability and catalytic performance, but their conductivity is slightly insufficient. Additionally, perovskite catalysts have tunable properties and low cost, but their main limitations are a lack of long-term stability and scalability. The MOF has a large specific surface area and porosity, which exposes metal catalytic sites to a greater extent, but makes it prone to aggregation, leading to a decrease in catalytic performance. POMs offer cost-effectiveness, multifunctionality, and tunability of redox properties, but susceptibility to degradation or leaching in aqueous environments restricts their long-term durability and practical use. These limitations highlight the need for further research and innovation, paving the way for future opportunities to optimize and implement MFCs toward eventual commercialization. Future research could target quantitatively comparable performance and rigorous benchmarking, especially long-term stability, to enable practical scaling. Particular focus should be placed on developing hybrid catalysts for MFCs to improve ORR kinetics at the cathode, reduce overpotentials, enhance electron transfer, and lower costs compared to conventional Pt-based catalysts.

Furthermore, several aspects must be improved in order to see the implementation of MFCs in industrial environments, i.e., the use of efficient and inexpensive cathodic materials as well as the improvement of the configuration and the expansion of a high-efficiency cell. The crucial key here is to achieve high-effective energy conversion for sustainable use by developing large-volume MFC-based devices or a stack of MFCs, or even deploy systems allowing their insertion into existing installations.