Outline of microbial fuel cells technology and their significant developments, challenges, and prospects of oxygen reduction electrocatalysts

Abstract

The microbial fuel cells (MFCs) which demonstrates simultaneous production of electricity and wastewater treatment have been considered as one of the potential and greener energy production technology among the available bioelectrochemical systems. The air-cathode MFCs have gained additional benefits due to using air and avoiding any chemical substances as catholyte in the cathode chamber. The sluggish oxygen reduction reaction (ORR) kinetics at the cathode is one of the main obstacles to achieve high microbial fuel cell (MFC) performances. Platinum (Pt) is one of the most widely used efficient ORR electrocatalysts due to its high efficient and more stable in acidic media. Because of the high cost and easily poisoned nature of Pt, several attempts, such as a combination of Pt with other materials, and using non-precious metals and non-metals based electrocatalysts has been demonstrated. However, the efficient practical application of the MFC technology is not yet achieved mainly due to the slow ORR. Therefore, the review which draws attention to develop and choosing the suitable cathode materials should be urgent for the practical applications of the MFCs. In this review article, we present an overview of the present MFC technology, then some significant advancements of ORR electrocatalysts such as precious metals-based catalysts (very briefly), non-precious metals-based, non-metals and carbon-based, and biocatalysts with some significant remarks on the corresponding results for the MFC applications. Lastly, we also discussed the challenges and prospects of ORR electrocatalysts for the practical application of MFCs.

1 Introduction

The shrinking of non-renewable energy sources and environmental pollution are the main critical issues the world has been facing in recent years. Therefore, the research area of energy generation by the alternative renewable sources is increasing drastically. Converting organic or inorganic waste materials into useful products and energy can better address energy and environmental problems (Babanova et al., 2022; Díaz-Vázquez et al., 2022; Gargalo et al., 2022; Kamali et al., 2023). Bioelectrochemical systems (BES) can be an effective potential candidate for the waste materials conversion and energy production (Agudelo-Escobar et al., 2022). BES is one of the emerging bioengineering technologies which show the process of electrochemical reactions, such as the conversion of chemical energy into electrical energy or vice versa, in the presence of microorganisms or biomaterials (Zheng et al., 2020). In the BES, the microorganism can exchange electrons with the electrodes directly or indirectly via a chemical compound that acts as an electron carrier (E. Logan et al., 2006). The BES can be categorized based on the task as (i) microbial fuel cells (MFCs), (ii) plant MFCs (P-MFC), (iii) constructed wetlands MFCs (CW-MFC) (iv) benthic MFCs (B-MFC), (v) microbial electrolysis cells (MEC), (vi) anaerobic digestion coupled MEC (AD-MEC), (vii) microbial electrosynthesis cells (MES), (viii) microbial desalination cells (MDC), and (ix) microbial electro-Fenton (MEF) (Zou and He, 2018; Mier et al., 2021). MFCs and P-MFC will generate electricity from an organic substrate using microorganisms and living plants with microorganisms, respectively as a catalyst. In the CW-MFC, the electricity generation and wastewater treatment occurs in MFCs integrated with constructed wetlands. In B-MFC, the electricity will be generated using sea’s inorganic and organic matter in the presence of bacterial catalysis. In MEC, the hydrogen (H₂) can be produced in its cathode part by reducing H⁺ using small external voltage and the ammonia recovery from wastewater. By reducing carbon dioxide, the MES can generate value-added chemical products, such as formic acid, acetate, ethanol, and butanol. Besides, methane can be produced by the degradation of a substrate from the AD-MEC system. The MDC produces fresh water from seawater or brackish water with the help of self-produced electricity or applying the external electricity (Wang and Ren, 2013). In MEF, the electricity will be generated at the anode microbial chamber, and H₂O₂ will be generated at the cathode chamber (Mier et al., 2021). The BES has several advantages, such as the clean process, lower operational cost, flux of electrons, and the energy level can be adjusted and maintained constant. The electrical signal can be used for monitoring and high selectivity towards the target compounds. Besides, there are disadvantages of BES, such as the challenge of scale-up, the cathodic reaction in MFC may limit the anodic reaction, pH changes of the system may affect the electrochemical process and the formation of chlorine gas from marine environment (Daghio et al., 2017).

Among the above bioelectrochemical systems, microbial fuel cell (MFC) is considered one of the potential renewable energy devices where microorganisms used as biocatalysts to convert organic substrates into electricity (E. Logan et al., 2006; Slate et al., 2019; Priya et al., 2022; Abubackar et al., 2023). An MFC requires about three times less potential difference compared to a conventional electrochemical cell for the H₂ production (Rozendal et al., 2006). The simultaneous treatment of wastewater during the electricity generation and the removal of metals such as chromium, copper, nickel, and zinc has gained additional importance regarding the environmental benefits (Nancharaiah et al., 2015; Hidayat et al., 2022). There are several review articles available very recently which include the focusing on functional materials (Zhu Q. et al., 2022), graphene-based materials (Aiswaria et al., 2022), carbonaceous materials (Dhilllon et al., 2022), nanomaterials (Chen et al., 2022b; Kamali et al., 2022; Dey et al., 2023; Kausar et al., 2023), anodic modification materials (Ma et al., 2023), and different types of MFCs (Gupta et al., 2023). Because the sluggish oxygen reduction reaction (ORR) kinetics is one of the main obstacles, the review which draws attention to choosing the suitable cathode materials should be urgent for the practical applications of the MFC technology. Therefore, here we present an overview of the present MFC technology, then some significant advancements of ORR electrocatalysts such as precious metals-based catalysts (very briefly), non-precious metals-based, non-metals and carbon-based, and biocatalysts with some significant remarks on the corresponding results for the MFC applications. Lastly, we also discussed the challenges and prospects of ORR electrocatalysts for the practical application of MFCs.

2 Microbial fuel cells (MFCs)

In the early 20th century, Potter reported the first bioelectrochemical reaction to generate electricity with some live microbial cultures such as Escherichia coli and Saccharomyces spp. and Pt macro-electrodes in a battery style system (Potter M. C., 1911). Later in 1931, Cohen confirmed this reaction by generating 0.2 mA current and 35 V voltage with a stacked bacterial fuel cell arrangement (Cohen, 1931). Interestingly after a few years, in 1963, electricity generated using human waste during space flight was demonstrated by the National Aeronautics and Space Administration (NASA) space program (Canfield et al., 1963; Slate et al., 2019). Recently, the MFC technology has been one of the best emerging eco-friendly research areas, as can be witnessed from the extensive reports (Gude, 2016; Zhang Y. et al., 2019; Kaur et al., 2020; Huang et al., 2021; Munoz-Cupa et al., 2021). A typical MFC contains anodic and cathodic chambers, which are separated by a proton exchange membrane (PEM). In the anodic chamber, microbes degrade the organic substances and produce electrons, protons (H⁺), and carbon dioxide (CO₂). Then, the produced electrons and H⁺ are transported through an external circuit and PEM, respectively to the cathodic chamber and react with oxygen (O₂) to form water.

Typical electrode reactions of MFC with acetic acid (CH₃COOH) as a model organic substance are shown below:

The potentials of −0.300, 0.805, and 1.105 V (vs. NHE) correspond to the anode, cathode, and the cell, respectively, for the production of electricity from the above electrochemical cell reaction (Obileke et al., 2021).

The main classification of MFC includes double-chamber MFC, single-chamber MFC, and stacked MFC. The double chamber MFC will consist of two chambers separated by a cation exchange membrane (CEM) to separate the different electrolytes in the compartments and only to allow H⁺. The distance between the two electrodes and the small surface area of the membrane causes high internal resistance of these types of MFCs, hence limiting the output power density. Chemical substances like permanganate, ferricyanides, and dichromates act as oxidizing agents, and no catalysts are needed for cathodic reactions in double-chamber MFCs (Yu et al., 2017; Fang and Achal, 2019; Hidayat et al., 2022). Double-chamber MFCs are suitable for laboratory research as these are run in batch mode. Different double-chamber MFCs are available such as H-type MFC, cube-type MFC, flate-type MFC, tubular upflow MFC, and miniature MFC, which have decreased internal resistance and therefore increasing power generation(Kun et al., 2012). In a single chamber MFC, the cathode can be in direct contact with air and with the presence or absence of a membrane(Logan and Regan, 2006). The anode and cathode in a single-chamber MFCs are separated by a PEM in a single compartment and it is not necessary that the cathode part have to be filled with catholyte (electrolyte on the cathode part) when O₂ was used at the cathode. Single-chamber MFC is also called air-cathode MFC since the cathode is directly exposed to air. The single chamber MFC has emerged into several variants, such as cube-type MFC (Liu and Logan, 2004), horizontal tube-type MFC (Wang and Ren, 2013), side-arm bottle MFC (Logan et al., 2007), and upflow-type MFC (You et al., 2007). The single-chamber MFCs have several advantages over the double-chamber MFCs, such as cost-effective, simple construction, sustainable sources, and better performance. Directly harvesting electricity from the biodegradable organic substances using a single MFC is lower. This problem can be addressed by using another type of MFC called stacked MFCs, in which the MFCs are stacked in series or parallel or a combination of series and parallel (Aelterman et al., 2006; Oh and Logan, 2007; An et al., 2015; Santoro et al., 2019; Dziegielowski et al., 2021; Mukherjee et al., 2022). There are 4-module sedimentMFC (Prasad and Tripathi, 2021), 6-cell stacked MFC (Aelterman et al., 2006), bipolar type of stacked MFC (Shin et al., 2006), stacked MFCs bridged internally through an extra CEM (Liu et al., 2008) and the MFC stack assembled from two single MFCs (Oh and Logan, 2007).

3 Air-cathode MFC

The single-chamber or air-cathode MFCs demonstrated higher efficiency than double-chamber MFCs. However, the efficiency of air-cathode MFCs can be fine-tuned by controlling many factors such as substrates, inoculums, electrodes materials, PEMs, and operating conditions (Bagchi and Behera, 2019; Merino-Jimenez et al., 2019; Vilas Boas et al., 2019). The structures and the types of single-chamber MFCs were discussed earlier in Section 2.

3.1 Limitations of air-cathode MFCs performances

The limitations of the MFC technology for the industrial and social applications include using high-cost materials such as electrodes and proton exchange membranes (PEMs), low life spans, and low energy outputs.

3.1.1 Thermodynamic factors

Generally, the cathodic reactions take place in the aerobic or anaerobic conditions. The anaerobic cathodic reactions will have occurred in double-chamber MFCs where a chemical substance acts as an oxidizing agent. The aerobic condition usually occurs in the cathodic part of air-cathode MFCs. The potential generated from an MFC can be thermodynamically derived using the Nernst equation (Eq. 4) given below (Rismani-Yazdi et al., 2008),

Where, , R, T, n_e, F, and π are the standard cell potential (V), ideal gas constant (8.314 J/mol K), temperature (K), number of electrons transferred in the reaction, Faraday’s constant (96,485 C/mol), and the chemical activity of products divided by those of reactants, respectively.

The potential of an ideal MFC is always higher than the actual MFC because of irreversible losses such as activation losses, ohmic losses, and transport losses (Rabaey and Verstraete, 2005). The potential required for redox (oxidation-reduction) reactions to occur is the activation loss, also known as activation over potential. The overpotential is a limiting step, and this can be reduced by increasing operating temperature, efficient electrocatalysts, electrode surface area, the concentration of redox shuttles, etc. The electrical resistance between anode and cathode, solution and electrode interfaces, and the electrolyte and membrane interfaces cause ohmic losses. Ohmic losses can be avoided by using appropriate electrolyte and electrode materials which are having high electrical conductivity. Generally, at high current densities, mass transport losses occur due to the limited mass transport of species from or to the electrode. This limited mass transport causes product depletion or accumulation. These losses can be minimized by maintaining high bulk concentrations and distribution of oxidants such as O₂ at the cathode compartment (Oguz Koroglu et al., 2019).

3.1.2 Other factors

In addition to the above thermodynamic factors, several other factors such as biofouling, catalyst inactivation (if existing), and excessive biofilm growth are the main obstacles to the real-world applications of the technology (Sun M. et al., 2016; Li et al., 2023). The factors such as microbial electron transfer, supply of oxygen, fuel oxidation, circuit resistance, proton transfer via the PEM, pH, concentration, and reduction at the cathode influenced the MFCs’ performances (Woodward et al., 2010; Jatoi et al., 2021). The crossover of electron acceptors or organic compounds from the cathode compartment to the anode compartment and vice versa also decreases the efficiency of the MFCs (Harnisch et al., 2009; Winfield et al., 2013). The biofouling involves the generation of insulating materials such as polymeric and/or dead cells, which can separate the dynamic biofilm from the surface of the electrode or blockage the porous electrode surface. This results in the reduction of efficiently active sites of the electrodes and ultimately decrease the overall performance of MFCs (Amirul Islam et al., 2016; Sun M. et al., 2016; Blanchet et al., 2016).

4 Mechanisms of oxygen reduction reaction (ORR)

The ORR is an essential reaction in many systems, such as biological respiration, fuel cells, and metal-air batteries. In an aqueous solution, the ORR occurs mainly by two pathways which are a) direct four-electron transfer from O₂ to H₂O (Equations 5–9) and b) the two-electron transfer from O₂ to hydrogen peroxide (H₂O₂) (Equations 10–12). The one-electron transfer pathway from O₂ to superoxide (O^2-) could also occur in alkaline solutions and/or in non-aqueous aprotic solvents (Oguz Koroglu et al., 2019).

The ORR occurs in the cathodic part under ambient temperature and pH, involving complicated interfacial electron and mass transfer processes. The kinetics of the ORR is generally sluggish and result in a high overpotential at the cathode which decreases the MFC efficiency (Gao et al., 2020). Platinum (Pt) is one of the most widely used and the efficient ORR electrocatalysts. The ORR can occur through three possible mechanistic pathways such as dissociation of O₂, dissociation of OOH, and dissociation of H₂O₂, as shown in Figure 1 (Haile et al., 2020). The ORR in the presence of a Pt catalyst takes place predominately through a four-electron transfer pathway as shown below (Equations 13–17) (Si et al., 2014),

FIGURE 1

In the case of other electrocatalysts such as carbon based, transition metal oxides and hybrid nanomaterials may follow two-electron transfer or a combination of two- and four-electron transfer pathways. The pH (acidic or basic) also influences the ORR pathway as given below (Equations 18–25), In acidic pH,

In basic pH,

The formation of OH⁻ in the basic media accumulated at the cathode may result in a lower kinetic performance of ORR. Therefore, the effective elimination of OH⁻ at the catalyst active sites is a central challenge in the ORR. Besides, the functional groups on the surface of the catalyst (if present) can assist in the elimination of OH⁻ for ease of ORR (Hou et al., 2016). The electrocatalysts’ performance for the ORR is generally studied by rotating ring disc electrode (RRDE). The Koutecky-Levich equation can be used to measure the number of electron transfers in ORR as given below (Eq. 26),Where j, j_k and ɷ are the total current density, kinetic current density, and electrode rotation rate (rpm) respectively. The coefficient, B, can be obtained from the Levich equation (Eq. 27) and the slope of the Koutecky-Levich plots.Where n, F, D_O2, ν, and C _O2 are the number of electron transfer per O₂ molecule, Faraday constant (96,485 C/mol), the diffusion coefficient of O₂ in 0.1 M KOH (1.9 × 10⁻⁵ cm²/s), kinetic viscosity (0.01 cm²/s) and bulk concentration of O₂ (1.2 × 10⁻⁶ mol/cm³) respectively. The constant 0.2 in the above equation is assumed when the rotation speed is mentioned in rpm (Gautam and Verma, 2019).

5 Importance of ORR electrocatalysts

However, the infinite source of O₂ makes the air-cathode MFCs a potential eco-friendly technology; incomplete reduction of O₂ at the cathodic part produces some destructive reactive intermediates and free radicals. The performance of the MFC for the current generation depends on the ORR at the surface of the cathode, which is restricted by the activation barrier (Sawant et al., 2017; Liu et al., 2022). The large activation energy loss due to the sluggish ORR results in high overpotential at the cathode. The use of mediators, optimization of MFC operating conditions, and cathode modification with catalysts are adopted to enhance the ORR kinetics. Among these, using the ORR electrocatalysts is the most feasible way to facilitate the fast ORR kinetics (Kamali et al., 2022). The ORR electrocatalyst should possess excellent catalytic activity (including good electrical conductivity, high surface area, functional groups, and surface morphology), cost-effectiveness, and higher stability. Pt has been proven to be the most widely used conventional electrocatalyst for ORR activity due to the superior activity, and highly stable in acidic media when compared to other non-precious ORR electrocatalysts (Sui et al., 2017; Ma et al., 2020). The high cost and easily poisoned by anions such as carbon monoxide and sulphide limits its large-scale applications (Chaturvedi and Kundu, 2021). The fabrication cost of a Pt cathode is more than half of the total cost of a lab-scale MFC. Therefore, the research on cost-effective, efficient ORR electrocatalysts has become an attractive and vital research field (Chaturvedi and Kundu, 2021; Peera et al., 2021). There are several types of electrocatalysts have been reported to decrease and/or to replace the usage of Pt for the ORR activity which include Pt-based, non-precious metals-based, and carbon-based electrocatalysts (Chaturvedi and Kundu, 2021; Peera et al., 2021; Priyadarshini et al., 2021; Kamali et al., 2022).

6 Types of ORR electrocatalysts

The types of ORR electrocatalysts can be broadly categorized as (i) precious metals-based ORR electrocatalysts, (ii) non-precious metals-based ORR electrocatalysts, (iii) non-metals and/or carbon based ORR electrocatalysts and (iv) biocatalysts.

6.1 Precious metals-based ORR electrocatalysts

Due to the high-cost nature and aim toward the practical applications of the MFC technology, the precious metal-based catalysts are reviewed very briefly in this section. Pt is the most widely used and potential electrocatalyst for ORR applications. The Pt loaded on carbon cathode in MFC generated a maximum of 1,553 mW cm^-2 power with simultaneously treating piggery waste (Chandrasekhar and Ahn, 2017). The carbon paper electrode contains lower loading of Pt fabricated using an e-beam evaporation technique performed 2.5 times higher than the commercial Pt catalyst in MFC applications (Park et al., 2007). Similarly, the lower loading of Pt on carbon cloth using electrodeposition showed superior performances in the MFC applications (Yen et al., 2013). Zerrouki et al. demonstrated the Pt-PANI composite catalyst showed better ORR activity and a maximum power density of 1,510 mW cm^-2 with 88% of COD removal efficiency in MFC. The enhanced activity was ascribed to the presence of conducting polymer PANI, which enhanced the electron cloud on the catalyst (Zerrouki et al., 2022). The Pt-boron-nitride-carbon electrocatalyst recently showed superior stable MFC performances over 2 months with 936.31 mW cm^-2 of maximum power density. The superior performance was due to the availability of more and uniformly dispersed active sites as well as the strong coordination structure of Pt-N₄ (Shixuan et al., 2023).

The palladium (Pd) on the Si nanowire exhibited 84.5% methyl orange degradation with simultaneous generation of 0.119 W/m² of maximum output power density (Han et al., 2017). The Pd supported on stainless steel fiber felt prepared by simple water bath method having macropores showed a higher power density of 390.79 mW m⁻², comparable to the efficiency of conventional Pt/C ORR electrocatalyst (405.47 mW m⁻²) (Chen et al., 2019). Wang et al. demonstrated around 1.8 times enhanced MFC performance of 901 mW m^-2 maximum power density using Pd/GO-C catalyst than the other Pd catalysts. They used a novel method to prepare the Pd/GO-C catalyst by simply mixing GO, carbon block, and PdCl₂ as shown in Figure 2 (Wang et al., 2021). Remarkably, beyond 100 days, the MFC performance was demonstrated using sub-5nm Pd nanocrystals in FeN₃-Pd@NC NBs composite ORR catalyst. The active sites of Fe and Pd, and the structural properties of the catalyst resulted the higher MFC performances (Lin et al., 2022). The silver (Ag) based electrocatalysts show the good performances in the MFC applications (Dai et al., 2017). Ag₂O/Ag cathode showed stable maximum power output of 1.796 W m^-3 in MFC applications (Dai et al., 2017). The Ag performance in MFC was also enhanced by hybridizing with tungsten carbide, which showed better efficiency than the commercial Pt/C electrocatalyst (Gong et al., 2013). The Ag−Fe−N/C ORR catalyst derived from a zeolitic imidazole framework displayed a higher power density of 523 mW m⁻² than the commercial Pt/C (358 mW m⁻²) owing to the synergistic effects of Ag nanoparticles, Fe and N-doped porous carbon (Lai et al., 2022). Recently, Sun et al. reported CNFs-Ag/Fe based catalyst derived using metal organic framework material showed a higher maximum power density of 737.45 mW m⁻² than the commercial Pt/C (457.99 mW m⁻²), which was due to the incorporation of Fe in the catalyst (Sun et al., 2023).

FIGURE 2

The combination of different transition and/or non-precious metals with Pt and forming alloys provides high surface area by decreasing particle size and, thus, more active sites for ORR in MFCs. The Pt-Co alloy supported on carbon demonstrated more elevated and more stable MFC performances when compared with the commercial Pt/C ORR electrocatalyst due to the presence of Co with optimized composition (Yan et al., 2014b). The Pt-Ni alloy showed higher power generation efficiency of 0.637 W m⁻² in MFC than that of the Pt cathode of 0.180 W m⁻² due to its increased oxygen adsorption and reduction on the more active sites (Cetinkaya et al., 2015). The optimized Pt-Pd alloy coated on carbon paper via the electrodeposition method demonstrated 1,274 m Wm⁻² of maximum power density comparable to that of commercial Pt/C catalyst in air-cathode MFC application (Quan et al., 2015). The PtSn/C ORR electrocatalyst showed a maximum power density of 336 mW m⁻² in MFC application, while the commercial Pt/C catalyst showed lower efficiency of 307 mW m⁻². The modification of the electronic structure of the Pt by the introduction of Sn enhanced the MFC performances of the catalyst (Li et al., 2017). The Pt₃-Fe/C ORR electrocatalyst showed a 18% enhanced performance of 1,680 mW m⁻² power generation with outstanding durability compared with the commercial Pt/C (1,422 mW m⁻²) in MFC application (Yan et al., 2014a). The Pt coated onto carbon paper (CP) as cathode generated the maximum power density of 84.01 mW/m² in the MFC, which is approximately two times higher than neat CP (44.7 mW/m²). The performance of the MFC was then increased to about 147 mW/m² by incorporating carbon nanotube (CNT) with Pt electrocatalyst, which was attributed to the high surface area of the CNT(Halakoo et al., 2015). The summary of maximum power density, wastewater, and cell configuration with significant remarks corresponding to a few essential precious metals-based ORR electrocatalysts for the MFC applications are given in Table 1.

6.2 Non-precious metals-based ORR electrocatalysts

Several non-precious metals-based materials have been studied as the ORR electrocatalysts in air-cathode MFC. The main concern of using non-precious metals is to reduce the cost of electrocatalysts and aim for the future potential of scaling-up applications. The transition metal compounds were found to have higher activity as ORR electrocatalysts. Plentiful research articles displayed that properly designed non-precious metal-based ORR electrocatalysts can perform comparable to, or even higher than, the commercial Pt/C in MFC applications with high stability. The summary of maximum power density, wastewater, and cell configuration with important remarks corresponding to some significant non-precious metals-based ORR electrocatalysts for the MFC applications are given in Table 2.

The transition metal oxide ORR electrocatalysts such as Co₃O₄ (maximum power density of 75.12 mW m⁻² with 99.76% Cr(VI) removal efficiency) (Cheng et al., 2019), MnO₂ (maximum power density of 213 mW m⁻²) (Alireza et al., 2019) (maximum power density of 1,671 mW m⁻²) (Zhang et al., 2018b), V₂O₅ (maximum power density of 384 mW/m²) (Noori et al., 2017), TiO₂ nanotubes (15.16 mWm⁻²) (Yahia et al., 2016), TiO₂ nanoparticles (15.2 Wm⁻³) (Kumar A. et al., 2023), SnO₂ (Yap et al., 2023), etc., showed better activity in MFC applications. Among these metal oxides, manganese oxides (MnO_x) are a potential candidate for ORR due to their high chemical stability, low cost, and environmental benignity. However, they showed relatively less activity (Gao et al., 2020). The performance of MnOx was improved by adopting several ways, such as introducing oxygen vacancies, doping with other metals, combining with carbon-based materials, etc. The oxygen-deficient nest-like Cu_1.5Mn_1.5O₄ ORR electrocatalyst showed 1.53 times higher power generation of 1,928 mW m⁻² than that of the bare electrode. The enhanced activity with the four electron transfer oxygen reduction mechanism was due to the presence of oxygen deficient in the catalyst (Wang et al., 2016). The MnO₂/functionalized carbon nanotubes (f-CNT) exhibited a 86.6% of COD removal efficiency and a higher power density of 520 mW m⁻² than the individual components (Liew et al., 2015). The mixture of graphite, γ-MnO₂, and MoS₂ can achieve a higher maximum power density of up to 183 mW m⁻² due to the high surface area and porous structural properties of the catalyst(Jiang et al., 2017). Rout et al. reported that the higher charge transfer property of rGO was enhanced the power density of 5.06 W/m³ of MnO₂-reduced graphene oxide (rGO) ORR electrocatalysts than that of pure MnO₂ (3.96 W m⁻³) (Rout et al., 2018). The α-MnO₂ nanowires showed a power density of 111 mW m⁻², which was increased to 180 mW/m² when supported on carbon Vulcan which provides high surface area and surface structure (Majidi et al., 2019). The MnO₂@Co₃O₄ composite catalyst showed more than 2-fold higher maximum power generation efficiency than that of the individual components in MFC due to their multiple active sites, high surface area, and high conductivity (Chen et al., 2022a). The Cs₃PMo₁₂O₄₀ ORR electrocatalyst showed a maximum power density of 64.73 mW m⁻² with around 86% COD removal efficiency in MFC application which was higher than that of the bare graphite electrode. The higher efficiency of the catalyst was due to the mesoporous structure, good electrochemical active sites, electron and proton sink ability, and the presence of Mo (Rezaei et al., 2023). Likewise, the other mixed metal oxide CaFe_0.9Cu_0.1O₃ catalyst showed a higher maximum power density of 1,090 mW m⁻³ than the commercial Pt/C (970 mW m⁻³). The Fe³⁺ content was increased by introducing Cu in the catalyst which resulted in the higher MFC performances (Zhang H. et al., 2022). Besides, Pema et al. very recently reported BiFe_1−xLi_xO₃-graphene (G) composite as a low-cost catalyst. They achieved a higher ORR activity of 8.1 W/m³ compared with GO in MFC applications, with a more stable COD removal efficiency of 78.5%. The BiFe_1−xLi_xO₃-G composite favors mainly the four electron pathway for ORR in the single chamber MFC as shown in Figure 3 (Pema et al., 2023).

FIGURE 3

The ORR performance of several other metal oxide electrocatalysts was significantly enhanced by adopting some modifications, especially the formation of composites. The Co₃O₄/NiCo₂O₄ double-shelled nanocage ORR electrocatalyst showed a 104% higher maximum power density of 1,810 mW m⁻² than the control. This catalyst showed improved ORR activity due to its nanocage structure and the presence of Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ redox couples (Zhang et al., 2018a). The Fe₃O₄ supported nickel-iron layered double hydroxides (LDH) showed 34 times higher and more stable (for 110 h) power generation efficiency of about 211.40 mW/m² than the blank control components in the MFC. The enhanced efficiency was due to its high electrochemical active sites and the excellent conductivity of Fe₃O₄ (Jiang et al., 2020a). The CoNiAl-LDH@NiCo₂O₄ composite exhibited a stable output power density of 85.28 mW m⁻² for 93.66 h due to the hierarchical core-shell structure of the catalyst (Tajdid Khajeh et al., 2020). In another study, Jiang et al. displayed a remarkable stability and durability over 8 days with a maximum power density of 467.35 mW m^-2 by the NiFe-LDH@Co₃O₄ composite due to its rich active sites and high conductivity (Jiang et al., 2020b). Besides, Guo et al. demonstrated a 39% and 24% higher power density when employing the bimetallic FeMn₂ nanocatalysts than the plain AC cathode and Pt/C cathode, respectively due to the synergistic effect between Fe and Mn catalyst(Guo et al., 2019). The Co and Ni doped TiO₂ nanotubes (NTs) showed enhanced ORR activity and produced ∼104 mW m^-2 current density, which was higher than the commercial Pt/C catalyst. The higher performance was due to the higher specific surface area of TiO₂ NTs and active sites of the catalyst (Chaturvedi et al., 2022). Recently, a 3D flower-like metal organic material (3DHFLM), Zn/Co-S-3DHFLM showed enhanced ORR activity in MFC with 172.8 maximum power density and 93% COD removal efficiency 467.35 mW m^-2. The synergistic effect of the bimetallic active center and the sulfur has greatly enhanced the ORR activity of the Zn/Co-S-3DHFLM catalyst (Lu et al., 2023). Besides, the NiCo₂S₄/NiCo₂O₄@NSC electrocatalyst showed excellent ORR performances in MFC application which showed around 1.2 and 1.7 times the enhanced maximum power density of 831.74 mW m^-2 compared to the NiCo₂O₄@NSC, and N-C catalysts, respectively. The enhanced performance of the catalyst resulted from the synergistic effect between heteroatoms and metal species, optimized amount of N and S, and better active surface area (Dhillon and Kundu, 2023). Besides, the excellent properties such as unique Ping-pong chrysanthemum-like structure, pore size distribution and electrochemical active sites of N-MnO₂@NiAl-LDH ORR catalyst showed a maximum power density of 698 mW m^-2 (Xu et al., 2023). Gosh et al. reported the CeO₂-gC₃N₄ catalyst showed excellent ORR activity and a power density of 12.53 W m^-3 in MFC application. The high surface area, Ce³⁺ content, oxygen defects, and pyridinic N of the catalyst contributed to better MFC performances (Ghosh et al., 2023). Xie et al. reported a novel MXene@NiCoP ORR electrocatalyst and achieved a maximum power density of 732 W m^-3 in MFC application which was due to the presence of active NiCoP species on the MXene (Xie et al., 2023).

The carbon-based composite materials have attracted significant attention as ORR electrocatalysts in MFC applications due to their excellent physicochemical and electrical properties. Ge et al. reported the total resistance was reduced when doping activated carbon into the ortho-hexagon spinel nano Co₃O₄ which generated the maximum power density of 1,500 mW m⁻² in the MFC application, which was 97.36% and 41.24% higher when compared to the bare activated carbon and the commercial Co₃O₄ cathodes, respectively (Ge et al., 2015). Mecheri et al. demonstrated the oxygen adsorption and ORR rate was increased by increasing the ZrO₂ content on carbon which showed a maximum power generation efficiency of 600 mW m^-2, which was 15 times lower in cost as compared with the commercial Pt/C catalyst in the MFC application (Mecheri et al., 2016). Zhang et al. reported a Cu₂O doped activated carbon ORR electrocatalyst which showed a 59% higher power generation efficiency of 1,390 mW m^–2 than that of the bare activated carbon. The lattice (111) plane and surface oxygen defects of the catalyst was enhanced the ORR performances (Zhang et al., 2015). The CoFe₂O₄ (CFO) supported on nitrogen doped activated carbon (N-AC) performed 2.39 times higher maximum power density of 1770.8 mW m^–2 than pure activated carbon cathode catalyst in the MFC application, which was due to the synergistic effect between N-AC and CFO (Huang et al., 2017). The MnCo₂O₄ nanoparticles in the carbon block exhibited 545 mW m^–2 of maximum power density in MFC application which was comparable to the Pt/C catalyst (689 mW m^–2) and higher than the bare cathode (214 mW m^–2) (Hu et al., 2015). Ge et al. reported the activated carbon modified with NiCo₂O₄ exhibited a maximum power generation efficiency of 1730 mW m^–2, which was comparable with that of the commercial Pt/C catalyst in MFC applications (Ge et al., 2016). Noori et al. displayed the enhancement of the maximum power density of bare V₂O₅ of 384 mW/m² to 533 mW m^–2 by the incorporation of rGO (Noori et al., 2017). The MnO₂/TiO₂/g-C₃N₄ supported on granular activated carbon exhibited a maximum power density of 1,176.47 mW m^–3 while simultaneous efficient industrial wastewater treatment of 17.77 kg COD m^–3d^–1 COD removal capacity (Zhang and Liu, 2020). The iron phthalocyanine (FePc) and nitrogen-doped graphene oxide ORR electrocatalyst showed the better waste treatment and power generation performances (Mecheri et al., 2018). Liu et al. reported the Fe-N derived from FePc supported on activated carbon (AC) performed for MFC of 1,092 mW m^–2 maximum current density due to its porous structures and high N content (Liu et al., 2019). The Co and FePc supported in carbon derived from SiC generated 1.57 W m^–2 of power with up to 86% of COD removal efficiency of wastewater. The graphitic and high mesoporous properties of the catalyst promote the good four-electron ORR pathway (Noori and Verma, 2019). Huang et al. reported the CoFe₂O₄ NPs/N-doped AC composite electrocatalyst generated a maximum power density in MFC of 1770.8 mW m^–2, which was much greater than the AC of 741.5 mW m^–2 due to its large surface area and conductivity (Huang et al., 2017). Yang et al. reported Fe(III)-chitosan hydrogel derived Fe-N-C low-cost ORR electrocatalyst exhibited a 33% higher power generation efficiency of 2.4 W m^–2 than the AC control due to the improved ORR activity by higher electron transfer number of 3.4 (Yang et al., 2020b). The Fe, N co-doped graphene with CNTs generated a maximum power density of 1,210 mW m^–2 which was higher than the Pt catalyst performance of 1,080 mW m^–2 (Wang et al., 2018). Liang et al. achieved a maximum power density of 1,738 mW m^–-2 when using the N-doped carbon incorporated with cobalt (Co) nanoparticle ORR electrocatalyst, which was higher than the commercial Pt/C (1,203 mW m^–2). The higher performance of the catalyst was attributed to the high surface area and the porous carbon structure(Liang et al., 2020). The N and Co or Fe co-doped multi-walled carbon nanotubes (MWCNTs) exhibited an output power density of 5.1 W m⁻³ and 6 W m^–3, respectively which was due to the nitrogen-metal center active sites formation in the catalysts (Türk et al., 2018). The square-like cobalt oxide nanostructures on N-doped graphene showed a higher ORR activity and exhibited a 24.9% higher power density of 713.6 mW m⁻² than the commercial Pt/C (571.3 mW m^-2), which was due to the synergistic effect of N-G and Co nanostructures (Cao et al., 2016). Yang et al. reported a novel N-doped molybdenum sulfide combined with CNTs and carbon atoms (N-MoS₂/CNTs/C) composite showed a higher maximum power density of 987.4 mW m⁻² than that of commercial Pt/C of 601.96 mW m⁻² which was due to the excellent electrical properties of the CNT and N present in the catalyst (Yang et al., 2018). Jing et al. demonstrated that the porous structure, defects, heterojunctions, and N atoms in the Fe₃Se₄/FeSe heterojunctions in N-doped carbon achieved a higher 1,003 mW m⁻² power density with good stability for 105 days of operation of air cathode MFC (Jing et al., 2019). The Co₂P incorporated in a N-doped carbon nanoframework (Co₂PNC-NF) composite electrocatalyst showed 2001 mW m^–2 of maximum power density, which was 123% higher than the bare active carbon. The higher activity resulted from the double Co site of 001 face present in the catalyst (Lin et al., 2021). The NiCo alloy on N-doped carbon showed excellent ORR activity and showed a 2.16 times higher maximum power density of 2,325.60 mW m^–2 than that of the Pt/C catalyst which was due to the synergistic effect of NiCo active sites, and graphitic and pyridinic N (Huang et al., 2022). Similarly, a novel Co/Ni@GC/NCNTs/CNFs catalyst displayed a splendid performance than the commercial Pt/C in MFC which was due to the abundant active sites and 3D structure of the catalyst(Li J. et al., 2022). Besides, a 4.5 times higher maximum power density than that of bare carbon felt was obtained by FeCoO/Go composite owing to its higher ORR activity(Zheng et al., 2022). The Co nanoparticles on zeolite-GO showed a 306% higher maximum power density of 416.78 mW m^–2 than the Pt/C in the MFC application (Chaturvedi and Kundu, 2022). The N and S co-doped carbon based composite, Co₉S₈@HN/S-C, showed better ORR and COD removal activities in MFC application (Ding et al., 2022). Metal and N co-doped porous carbon, such as Fe-NpC (atomically dispersed Fe-N₄ moieties), showed superior performances than Pt/C in MFC (Wang et al., 2023b). Liang et al. reported Fe, N codoped carbon ORR electrocatalysts derived from different Fe ligands showed a maximum power density of 2,041 mW m^–2. The improved ORR and MFC activity of the catalysts was ascribed to the graphitization degree and different amounts of Fe²⁺ and Fe³⁺ of the catalysts (Liang et al., 2023). Similarly, single atom Fe sites in the N-C showed a higher MFC performance of 3,323 mW m^–2 than that of the Pt/C catalyst. The higher performance was due to the optimized Fe-N-C with porous structure and efficient electron transfer properties of the catalyst (Zhao et al., 2023). Long et al. reported CoFe-LDH on partially reduced GO (p-rGO) showed 30 times higher MFC performance than the blank due to more the active sites, structural and interlayer diffusion properties of the catalyst (Long et al., 2023). Remarkably, the fantastic ORR activity in the MFC applications of carbon based composites such as Cu₂O@Co/N-C (Chen et al., 2023), Fe-N-C (Kumar D. et al., 2023; Zhang et al., 2023), FeCoNi@N-C (Kaur Dhillon and Paban Kundu, 2023), Fe/Co-N-C (Liu et al., 2023; Zhuang et al., 2023), Co/CoS₂@N-CNF (Guo et al., 2023), Co/CoFe NAs@NCNFs (Wang et al., 2023a), etc., were witnessed in several recent reports. The maximum power density, wastewater, and cell configuration with important remarks corresponding to some significant non-precious carbon-based ORR electrocatalysts for the MFC applications are summarized in Table 3.

The metal organic framework (MOF) composites exhibited the excellent performances in MFCs (Priyadarshini et al., 2021). The iron-based MOF, Fe-t-MOF/PANI synthesized by a sustainable route using terephthalic acid monomer (t) derived from plastic waste showed a power density of 680 mW m^-2 (Kaur et al., 2021). The Ni-MOF-74, the another cost-effective ORR electrocatalyst, exhibited a maximum power density of 446 mW m^–2 with sulfamethoxazole degradation of 84% COD removal (Li S. et al., 2021). The Ni-catecholate-based MOF grown on NiCoAl-LDH/MWCNTs showed an enhanced maximum power density of 448.5 mW m^–2 in MFC, which also attributed to the high conductivity and more active sites (Chen et al., 2021). Cheng et al. reported the Co₃O₄-ZIF/Zn composite showed a highly stable power generation efficiency of 656.9 mW m^–2 for up to 30 days compared to the commercial Pt/C catalyst (Chang et al., 2021). The ZIF-67/CNFs showed higher power generation efficiency of 1.191 W m^–2, which was higher than that of Pt and CNF due to the porous structure and lower internal resistance of the catalyst (Jiang et al., 2021). Recently, Chen et al. reported the ZIF-67@Ti₃AlC₂/ZnAl-LDH, which exhibited a 2-fold higher maximum power density of 587 mW m^-2 than that of their components, owing to the multiple active sites, high electrical conductivity, and high surface area of the catalyst (Chen et al., 2022d). The covalent organic frameworks (COFs) based composites and their derivatives showed good efficiency in MFCs. For example, the cobalt oxides incorporated COF-derived carbon (Co/N-C-COPs) ORR electrocatalyst showed higher efficiency of 1,817 mW m⁻² than the Pt/C catalyst (1,622 mW m⁻²) in the MFC applications (Yang et al., 2020a). Recently, Chen et al. prepared a highly stable COF-300@NiAl-LDH/GO catalyst showed the enhanced ORR activity in MFC with a maximum power density of 481.69 mW m^–2. The high conductivity, catalytic activity, and high electron transfer properties of the catalyst improved the MFC performances of the catalyst(Chen et al., 2022c). Similarly, the MOF derivatives also demonstrated the excellent ORR performances in the MFC applications(Zhang Y. et al., 2018; Wang X. et al., 2022). Tang et al. reported the dual metal (Ni, Co) and N-doped carbon ORR electrocatalysts derived from MOF generated a higher maximum power density of 4,335.6 mW m⁻² with outstanding durability over 755 h. This excellent performance of the catalyst resulted from the high surface area and uniform distribution of N and metal species in the graphite structure(Tang et al., 2015). The zeolitic imidazolate framework-67 (ZIF-67) derived Co-N-C composite exhibited a maximum output power density of 399.7 mW m⁻² (Li J. C. et al., 2018). Xue et al. reported the Fe, Co, and N-doped carbon derived from ZIF-67 generated a higher maximum power density of 1769.95 mW m^-2 than the Pt/C catalyst of 1,410.3 mW m^–2. The higher performance was achieved by the optimum pyrolysis temperature of 900°C, which resulted from the higher graphitization and corrosion resistance along with the higher conductivity and active sites of the catalysts (Xue et al., 2020). The N-doped CNT-embedded Co nanoparticles derived from bimetallic MOF produced a much higher power density than Pt/C and 2.54 times higher than the pure AC. The N and metal combination with CNT enhanced the ORR activity, and the reduced total and charge transfer resistance resulted in the higher MFC performances (Zhang S. et al., 2019). The hierarchical porous Fe-N-C nanofibers developed using MOF and bacterial cellulose exhibited high power generation than the commercial Pt/C catalysts of 640.56 mW m⁻² with 66.6% COD removal performance which was attributed to its more active sites and porous structure (Li H. et al., 2021). in another report, Li et al. prepared ZIF-67-derived CoNi-LDH@CNFs, which exhibited long durability in MFC and showed a higher maximum power density of 1,390.37 mW m^–2 compared to the Pt/C. The higher four-electron transfer ORR was promoted by CoNi active sites and the nanoflower structure of the catalyst (Li H. et al., 2022). The MFC performance of the Fe-N-C catalyst prepared from Fe-doped ZIF-8 was optimized for cost-effective and stable activity using pyrolyzing at different temperature (Wang D. et al., 2022). Similarly, the Co_0.7Fe_0.3@Co-NC-1 catalyst derived from different MOF precursors showed good ORR activity with 2,486 mW m^–2 maximum power density due to the optimized electronic structure and porous carbon (Zhang X. et al., 2022). Recently, Huang et al. demonstrated the 1.45 times higher maximum power generation of 1974 mW m^-2 by Fe/Fe₃C/NC catalyst derived from MOF than the Pt/C catalyst due to the mesoporous structure and the presence of N in the catalyst (Huang et al., 2023). Ding et al. prepared Co encapsulated in hierarchical porous N-C (Co/HNC) derived from ZIF based MOF, which showed good ORR activity and a maximum power density of 1324 mW m^–2 in MFC application (Ding et al., 2023). Qin et al. reported a novel hybrid porous CoCu@N-CNFs ORR catalysts which showed a slightly higher maximum power density of 543 mW m^–2 than Pt/C due to the synergistic effect of CoCu alloy (Qin et al., 2023). Table 4 summarizes the maximum power density, wastewater, and cell configuration with important remarks corresponding to some significant non-precious MOFs- and COFs-based ORR electrocatalysts for the MFC applications.

6.3 Non-metals and/or carbon-based ORR electrocatalysts

The carbon-based materials without any metals have attracted scientists to discover the high-efficiency ORR electrocatalysts due to their chemical inertness, high electrical conductivity, high stability, high surface area and porosity, good mechanical properties, and inexpensive (Pötschke et al., 2022). The chemically modified carbon block (Vulcan XC-72R) showed the comparable ORR performance in the MFC applications (Duteanu et al., 2010; Yang et al., 2014). The nitrogen doped carbon block performed 2.2 times higher than the pure carbon block in the MFC applications (Kumar et al., 2017). The AC as ORR electrocatalyst on nickel foam as a current collector showed comparable power generation efficiency as that of commercial Pt/C in air-cathode MFC application. The cathode cost was very cheaper (1/30th) when compared with the commercial Pt/C electrocatalyst (Cheng and Wu, 2013). The chemically treated P-doped or non-doped AC showed enhanced performances in MFC applications (Chen Z. et al., 2014; Liu et al., 2015; Wang et al., 2017). Similarly, the low-cost N-doped carbon materials showed 11.3% higher(Feng et al., 2012) and comparable(Shi et al., 2012) performances as the commercial Pt/C catalyst in the MFC applications. The pretreated N-doped AC showed higher maximum power density in the air-cathode MFCs when compared with the commercial Pt/C and the pure AC (Zhang et al., 2014). The surface structural modification of AC also enhanced the ORR performance in MFC applications (Li et al., 2014; Liu et al., 2016). The porous structural modification of AC generated 2.4 times higher power density than that of the unmodified one in the air-cathode MFCs (Li et al., 2014). A large-scale MFC of 85 L was constructed with an AC-based air cathode of 0.62 m² exposed area and a graphite fiber-based anode of 5.1 cm diameter, 61 cm long. A maximum power density of 0.101 W m^–2 was achieved in static flow conditions. The maximum power density was further increased by recirculating the anolyte (domestic wastewater) over the electrodes in a diagonal direction by 17% to 0.118 W m^–2 at a hydraulic retention time (HRT) of 33 min (Rossi et al., 2019).

The graphite in the cathode activated using H₃PO₄ and HNO₃ generated 7.9 W m^-3 and 6.5 W m^-3, respectively of maximum power densities which were 2.4 and 1.8 times greater than that of the bare graphite in the MFC applications (Zhang et al., 2016). The porous structure and high crystallinity of graphite were proven to have a high surface area and, thus, better performance in MFC (Xing et al., 2017). The 3D graphite particle showed as a potential cathode for the generation of H₂O₂ in the MFC application with 84% COD removal efficiency (Fu et al., 2010; Chen J. Y. et al., 2014). Interestingly, an ultra-low cost MFC fabricated with pencil trace demonstrated excellent performance for portable applications of MFCs (Lee et al., 2016). The three dimensional (3D) graphene nanosheets exhibited a maximum power density of 2.059 W m^-2 in the MFC application which was higher than that of activated carbon (Santoro et al., 2017). Dong et al. reported the oxidized graphene in MFC application and achieved 131% more generation of H₂O₂ than that of pure graphene cathode which was due to the oxygen containing functional groups present in the catalyst (Dong et al., 2018).

The N is one of the most widely used heteroatoms doped with carbon to enhance the ORR activity due to its electronegativity, close atomic radius to carbon, and facilitate easy oxidant adsorption (Dhilllon et al., 2022). The N-doped carbon aerogel (CA) was developed for ORR activity, and its performance was further increased by activating the CA with KOH. The activation of KOH increased the surface area, hierarchically porous structure, reduced C-O-C and COOH, higher pyridinic N content, and decreased pyrrolic N content, thus the enhanced MFC performance than that of non-activated CA (Tian et al., 2018). In another report, Yang et al. demonstrated the improvement of active sites by N doping in the N-CA, which generated 1,048 mW m^–2 of power density in the MFC application, which was comparable with the efficiency of the commercial Pt/C (1,051 mW m^–2) catalyst (Yang et al., 2019). Wang et al. reported the N-doped carbon derived from isoreticular MOF-3 modified with g-C₃N₄, which showed a maximum output power density of 1,402.8 mW m^–2 in MFC application. This performance was higher than the Pt (1,292.8 mW m^–2) and attributed to the introduction of more active N and mesoporous structure when using g-C₃N₄ as the template for the catalyst synthesis (Wang et al., 2020).

The carbon materials derived from different organic precursors and/or biomass showed excellent performances in MFC applications. The porous AC derived from different sources such as Arhar stalks(Om Prakash et al., 2021), Bamboo(Yang et al., 2017), corncob (Li M. et al., 2018), and coconut shell (Sekhon et al., 2021) showed higher thermal stability and high surface area which results in better MFC performances. The other carbon materials derived from different biomass, such as cornstalk (Sun Y. et al., 2016), sewage sludge (Mian et al., 2019), bacterial cellulose (Wu et al., 2016), hemoglobin (Maruyama et al., 2007), etc., showed excellent ORR performance. Zhou et al. prepared the carbon nanofibers (CNFs) from spider silk exhibited 1800 mW m^–2 power density in the MFC, which was higher than the commercial Pt/C electrocatalyst (704 mW m^–2). The higher MFC performances were attributed to the increased surface area and more active sites by N and S doping in the catalyst (Zhou et al., 2016). The carbon derived from chitosan having high surface area by high temperature KOH activation showed an increased MFC performance of 1,435 mW m^–2, which was 101% higher than the pure AC in the medium of domestic wastewater and nutrient solution (1:1) (Liu et al., 2018). Linget al. demonstrated the MFC performance of the carbon material derived from chitosan was again enhanced by five times from 322.4–1,603.6 mW m^–2 by co-doping of N and P. The C-O bond, N, and P existence resulted in a large surface area, lower total resistance, more oxygen transfer, and abundant active sites in the catalyst, which enhanced the ORR and MFC performance (Liang et al., 2019). The P-doped carbon derived from cellulose showed 1,312 mW m⁻² of maximum power density in the air-cathode MFC application, which was three-fold higher than the pure carbon and also higher than that of Pt/C electrocatalyst (Liu et al., 2014). The N and F co-doped carbon black obtained from the polytetrafluoroethylene and BP-2000 mixture by pyrolysis under an ammonium atmosphere showed a maximum power density of 672 mW m^–2. At the same time, the commercial Pt/C achieved a lesser of 572 mW m^–2 (Meng et al., 2015). Ye et al. reported the carbon electrocatalyst obtained from the lotus leaf which showed a maximum power density of 511.5 mW m^–2. This hierarchically structured carbon catalyst was stable and showed four-electron transfer performance comparable to the commercial Pt/C catalyst due to the large surface area and porous structure (Ye et al., 2019). The N self-doped porous carbon derived from duckweed provided more active sites for ORR reaction in MFC, which resulted in 625.9 mW m^–2 of power density and higher stability than the commercial Pt/C electrocatalyst (Gong et al., 2020). Pepè Sciarria et al. reported the biochar obtained from Olive mill waste and pistachio nutshell which showed a high surface area and O and N functionalities. The MFC performance of the biochar was 271 mWm^–2 of maximum output power density, which was 15 times higher than the commercial carbon black (Pepè Sciarria et al., 2020). The N-doped carbon ORR electrocatalysts derived from pomelo peel showed MFC performance of 907.2 mW m^–2 power generation, which was comparable to the Pt/C (1,022.9 mW m^–2) electrocatalyst (Zhang et al., 2020). Carbon materials derived from watermelon as ORR catalysts showed the good performances in the MFC applications (Zhong et al., 2019; Jiang et al., 2022). The biochar materials derived from different sources such as microalgae, pomelo peel, and eggplant performed well as low-cost and durable materials compared to the commercial Pt/C (Chakraborty et al., 2020; Zhang et al., 2020; Zha et al., 2021; Wang K. et al., 2022; Dhanda et al., 2023). Zhu et al. developed the porous Co and N doped carbon using tea residue showed excellent MFC efficiencies, such as higher maximum power density than Pt/C with COD removal activity. The excellent activity was attributed to the pyridinic-N, and pore structure of the catalyst (Zhu H. et al., 2022). Yang et al. prepared N-doped biochar from microalgae residue by single-step pyrolysis method, which showed a maximum power density of 843.6 mW m⁻² and comparable MFC performances with commercial Pt/C. The optimized pyrolysis process resulted in porous structure and improved N content in the catalyst, which promoted the better four-electron pathway ORR in MFC (Yang et al., 2023). In addition, the activated carbon from areca nut husk demonstrated a maximum power density of 590 mW m^–2 with good COD removal activity. The better MFC performances was attributed to the porous structure, graphitic nature, and N content present in the catalyst (Subran et al., 2023). Several significant non-metals and/or carbon-based ORR electrocatalysts with their maximum power density, wastewater, cell configuration, and important remarks in the MFC applications are given in Table 5.

6.4 Biocatalysts

Besides exploring the chemically fabricated ORR electrocatalysts, several biomaterials, such as enzymes, living microbial cells, etc., are used as ORR catalysts at the cathode of MFCs (He and Angenent, 2006). The construction of the biocathode was done by growing electroactive bacteria on carbon felt materials or stainless steel mesh and demonstrated a maximum stable power generation of 12.3 μW cm⁻² (De Schamphelaire et al., 2010). The laccase immobilized on graphite cathode utilized in MFC showed enhanced decolorization of the organic contaminant and generated higher power density (Savizi et al., 2012). The microorganism on the CNT-deposited stainless steel mesh cathode played an essential role in enhancing the power generation efficiency in the MFC applications (Zhang et al., 2013). The cathode choosing was a limiting factor in the MFC application while using the biomaterials as the ORR electrocatalysts. In a comparative study, the graphite felt biocathode demonstrated the better power generation efficiency in the MFC application among graphite felt, carbon paper, and stainless steel mesh biocathodes (Zhang et al., 2012). The low-cost biocathode materials such as semicoke and activated carbon also showed better performance in MFC application, costing just 2.8% and 22.7% compared to graphite and carbon felt biocathode, respectively (Wei et al., 2011). Besides, the carbon materials such as graphite granules, activated carbon granules, and activated carbon powder in the cathodic part played a significant role to enhance the MFC performances. Among the above, activated carbon granules have greatly enhanced the biocathode’s efficiency by increasing the active microbes for ORR, which resulted in higher MFC performances (Tursun et al., 2016). The biocathode developed with CNT/chitosan exhibited 130% higher electricity generation efficiency when compared with the carbon paper biocathode in MFC application due to the decreased energy loss at the electrode/bacteria interface (Liu et al., 2011). The bacterial cellulose doped with Cu and P showed the maximum output current density of 1,177.31 mW m^–2, which was attributed to their more active sites of Cu and P present in the electrocatalyst(Li et al., 2019). Interestingly, Izadi et al. demonstrated highly enhanced power generation efficiency over gas diffusion biocathode consisting of iron-oxidizing bacteria (IOB) of 1.1 W m^–2 maximum power density compared to Pt catalyst in MFC application. The mechanism of the MFC process involves oxidation of Fe²⁺ by IOB and regeneration at the cathode as shown in Figure 4 (Izadi et al., 2019). Very recently, sarma et al. reported the Chlorella sorokiniana, and Philodendron erubescens in the cathodic part of the MFCs showed significantly increased power generation efficiency. The enhanced activity was attributed to the presence of C. sorokiniana, which improved the ORR rate and dissolved oxygen concentration (Sarma et al., 2023). The summary of several significant biomaterials-based ORR electrocatalysts with their maximum power density, wastewater, cell configuration, and essential remarks in the MFC applications are given in Table 6.

FIGURE 4

7 Durability of the ORR electrocatalysts

The long-term stability of the ORR electrocatalysts is a major challenging factor of the MFCs for real-time applications. The researchers have intensively focused on improving the durability along with the higher activity of the ORR electrocatalysts. Several methodologies, such as optimizing structural and chemical composition (Li J. C. et al., 2018), using support materials (Mecheri et al., 2018)., obtaining from other materials (e.g.,: derivatives of MOFs, COFs, and biomaterials) (Yang et al., 2020a), introducing interface with other components (Zhang and Liu, 2020), etc., are demonstrated improved stability and durability of the ORR electrocatalysts. Here are some significant achievements in improving the durability of the ORR electrocatalysts for MFC applications. The FePc stability was increased by less than 1% loss when the introduction of N-GO, which increased the OH group density, and the interaction between FePc and N-GO (Mecheri et al., 2018). Li et al. demonstrated the chemical composition and structural optimization of Co-N-C composite derived from MOF could achieve higher and more stable MFC performance for 1,200 h (Li J. C. et al., 2018). The activated carbon fiber on the stainless-steel ring electrode was used as an air cathode for a continuous-flow MFC system to treat azo dye wastewater for 400 days. This long-term efficiency was attributed to the abundance of microorganisms during the MFC operation. However, the removal efficiency remained stable, and the power output of 169 mW m^–2 decreased slowly on the 400th day (Long et al., 2019). The stability of Fe₃O₄@NiFe in MFC operation was enhanced by introducing LDH, which improved the rate capability, electroactive sites, and cycling stability (Jiang et al., 2020a). Yang et al. demonstrated a high stable MFC performance over 200 h when using COF-derived Co/N-C-COPs ORR electrocatalyst than the Pt/C catalyst, which was due to the high ORR activity and poison tolerance properties of the catalyst (Yang et al., 2020a). Zhang and Liu demonstrated a stable and durable MFC activity by a novel MnO₂/TiO₂/g-C₃N₄ ORR electrocatalyst when supported on granular activated carbon for more than 6 months which enhanced the ORR rate and decreased the activation energy of the reaction (Zhang and Liu, 2020). Zhang et al. demonstrated that the high degree of graphitization, good electrical conductivity, and higher specific surface area of the N-C OOR electrocatalyst derived from pomelo peel enhanced the long-term durability in MFC operation for 90 days (Zhang et al., 2020). The mesoporous structure, high N content, and abundant active sites of the Co₃O₄-ZIF/Zn composite catalyst played a significant role in stable performance for up to 30 days in the MFC operation compared to the commercial Pt/C catalyst (Chang et al., 2021). The improved stability of MnO₂@Co₃O₄ composite catalyst in MFC was achieved by tuning its structural properties, such as high surface area and high ion flow efficiency (Chen et al., 2022a). The CaFe_0.9Cu_0.1O₃ porous perovskite catalyst stability was increased due to the presence of Cu, which increased the amount of Fe³⁺ and resulted in the higher adsorption of oxygen (Zhang H. et al., 2022). Interestingly, N-doped partially graphitized carbon (NPGC) based composite, FeCo/Co/Co₂P/NPGC showed superior stable MFC performance over 90 days with a maximum power density of 997.74 mW m⁻², which was due to the different metallic active sites and N-doping of the catalyst (Xu et al., 2022). The combined action of Ti₃AlC₂, ZnAl-LDH, and ZIF-67 in the ZIF-67@Ti₃AlC₂/ZnAl-LDH ORR electrocatalyst improved the stability and durability in the MFC operation for 8 days (Chen et al., 2022d). The single-atom structure Phen-Fe MFC demonstrated highly stable and durable performance over 20 days due to the excellent ORR property of the catalyst (Liang et al., 2023). The stable performance of N-MnO₂@NiAl-LDH in the MFC application over 7 days was attributed to its excellent porosity, electrochemical active sites, and the presence of N and MnO₂ in the catalyst (Xu et al., 2023).The Co/HNC catalyst showed good stable ORR activity with a wide pH range owing to the strong interaction of Co and N-C, hierarchical porous structure, and presence of N in the catalyst (Ding et al., 2023). Therefore, the proper design and development of the ORR electrocatalyst for MFCs could still be explored using an appropriate methodology for efficient practical applications.

8 Challenges and prospects for the practical application

The design and development of highly efficient ORR electrocatalysts are one of the major challenging factors in achieving the practical applications of MFCs. However, Pt is a benchmark ORR electrocatalyst, and its high cost is challenging. In the case of precious metal-based ORR electrocatalysts such as precious metal-based alloys, composites, etc., again, their high cost comes into the picture of challenges. Thus, the alternative highly efficient ORR electrocatalysts contain significantly lower Pt content are should still be explored.

The most common challenge of non-precious metal-based ORR electrocatalysts is the leaching of corresponding metal ions during the reaction, which results in performance loss and environmental issues. The methods such as incorporating or forming composites with other suitable materials have been adopted to overcome the leaching issues. Still, the ORR electrocatalysts' efficiency is lower and involves complex synthetic processes.

Pure graphite or carbon as ORR electrocatalysts in MFCs are limited due to their low performance. Doping of metals or non-metals increased the above catalysts’ performances in the MFCs. However, the durability and the insight mechanism of ORR activity of the effect of doping should be further explored for a better understanding of the reaction. Besides, high N content in the graphite or carbon catalyst decreases the conductivity, which results in lowering the performance of the electrocatalyst.

The challenges involved in the biocatalysts are poor electron transfer and oxygen diffusion at the interface of the cathode and microbial metabolism, and growth utilizes organic carbon, which decreases the ORR performances. In addition to the above, the careful design and development of ORR electrocatalysts of any kind with suitable phase, crystalline, and surface morphological structure is the real challenge for the practical applications of MFCs.

Besides the performances of the air cathode MFC being quite good on the laboratory scale, the ORR electrocatalysts’ stability still needs to be achieved at the peak. Thus, more experimental and theoretical studies for the development of alternative cost-effective, highly efficient, and durable ORR electrocatalysts should be done for the real-time applications. The development of alternative efficient ORR electrocatalysts may be explored for the practical applications of MFCs by using several methodologies include, formation of potential heterojunction by integrating two or more different materials, the introduction of more multifunctional metals (e.g., Fe, Co, Ni, Cu, etc.), and or heteroatoms (e.g., C, N, P, F, etc.) active sites, porous structure, and surface defects, and preparation from inexpensive or waste materials.

9 Conclusion

In conclusion, the air-cathode MFC has been considered a potential technology due to the usage of air as a catholyte in the cathode chamber. The key hindrances to achieving better performances are the factors such as thermodynamic factors, which include activation losses, ohmic losses, and transport losses, and other factors, such as biofouling, catalyst inactivation, and excessive biofilm growth. The development of highly efficient and durable ORR electrocatalysts is one of the main challenges for their practical applications. The high cost of Pt is the main hindrance to large-scale applications. Several attempts have been made to develop alternative ORR electrocatalysts, such as the combination of Pt with other non-precious metals, carbon-based materials, and metal oxides. The non-precious metals based and non-metals and/or carbon based electrocatalysts were also extensively studied as potential ORR electrocatalysts in the MFCs. In addition to the above, the biocatalysts also showed good performances in the MFC applications. Based on the current review, metal-free ORR electrocatalysts such as carbon-based catalysts have been considered as one of the potential alternatives due to their excellent physicochemical properties. However, the stability and efficiency of the ORR electrocatalysts still need to improve for the practical applications of the MFCs. Besides, the quite good performances of the air-cathode MFCs on the lab-scale should be further extended to real-time applications by adopting several experimental and theoretical research.

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