Research progress in the application of metal organic frameworks and its complex in microbial fuel cells: Present status,opportunities, future directions and prospects

Metal organic frameworks (MOFs) could greatly improve the power generation and degradation performance of microbial fuel cells (MFCs). MOFs and their compound derivatives played key role in cathode, anode and proton exchange membrane of MFCs, which greatly promoted the power generation of MFC and the degradation efficiency of various pollutants. However, MOFs were still possessed some defects, such as complex synthesis process, difficult regulation, instability, etc. Moreover, the application of MFC was limited in low power density, system internal resistance, microbial consumption, etc. Which further limited the degradation of pollutants by MFC. The existing problems and various improvement schemes of MOFs for MFCs were further summarized, which would provide references for promoting the application of MOFs materials in MFC system. It was expected to enhance the application of MOFs materials and promote the performance of MFC.     

Electrophoretically fabricated nickel/nickel oxides as cost effective nanocatalysts for the oxygen reduction reaction in air-cathode microbial fuel cell

The high cost and limited availability of cathode catalyst materials (most commonly Pt) prevent the large-scale practical application of microbial fuel cells (MFCs). In this study, unique Pt group metal-free (PGM-free) nanocatalysts were fabricated using a simple and cost-effective technique called electrophoretic deposition (EPD) to create a high catalytic oxygen reduction reaction rate (ORR) on the cathode surface of MFCs. Among the tested PGM-free catalysts (Ni, Co, and Cd-based), a maximum power density of 1630.7 mW m⁻² was achieved based on nickel nanoparticles. This value was 400% greater than that obtained using a commercial Pt catalyst under the same conditions. This result was due to the uniform deposition of a thin layer of Ni/NiO_x nanoparticles on the cathode, which improved electrical conductivity, catalytic activity, and long-term stability while reducing electron transfer resistance. The fabricated PGM-free catalysts significantly improved MFC performance and accelerated ORR induced by the novel layered morphology of metal/metal oxide nanoparticles.     

FeNx nanoparticles embedded in three-dimensional N-doped carbon nanofibers as efficient oxygen reduction reaction electrocatalyst for microbial fuel cells in alkaline and neutral media

Herein, an approach is reported for the fabrication of 3D carbon nanofibers (CNFs) wrapped by carbon nanotubes (CNT) with graphitic carbon-encased FeNx nanoparticles originated from metal–organic frameworks (MOFs). It is found that Fe-FeNx@N-CNT/CNFs exhibits outstanding catalytic activity towards ORR, whose half-wave potential are 0.89 V and 0.87 V in alkaline and neutral environments, respectively, much higher than MOF-based catalysts reported so far and commercial Pt/C. When the obtained cathode catalysts are loaded in MFCs for power generation test, the experimental consequences show that the Fe-FeNx@N-CNT/CNFs cathode exhibits a supernal power density of 742.26 mW·m^?2 and output current density of 3241 mA·m^?2 which are comparable to Pt/C. The splendid ORR catalytic performance is mainly attributable to the three-dimensional structure of carbon nanofibers and the active sites of Fe-Nx. These result in a higher graphitization degree beneficial for electronic mobility, high specific surface area, benign mesoporous nanostructure and excellent mass transfer capability. The strategy provides a new scheme to devise and research Fe-Nx electrocatalysts with MOF-based for the conversion of clean and environment-friendly energy.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

N-doping modification by plasma treatment in polyacrylonitrile derived carbon-based nanofibers for Oxygen Reduction Reaction

This work aims at providing a simple and effective method to optimize and improve the nitrogen-based defects into intrinsically N-doped carbon-based electrospun nanofibers (CNFs) for achieving highly active catalysts for the oxygen reduction reaction (ORR). To reach this goal, plasma treatments are investigated as an effective method to tune the distribution of the N-doping sites that are vital to improve the ORR catalytic activity of the material. Morphological, physical, chemical as well as electrochemical characterizations were performed on plasma-treated CNFs to demonstrate the effectiveness of such treatments to improve the catalytic behavior of CNFs toward ORR. Finally, Microbial Fuel Cells (MFCs) were selected to test the plasma-treated nanofibers as the cathode catalyst layer demonstrating an increase of the overall performance of the MFCs with respect to the devices using the Pt-based reference catalyst.     

Increased power output from micro porous layer (MPL) cathode microbial fuel cells (MFC)

Microbial fuel cells are bio-electrochemical transducers that utilise microorganisms to generate electricity, through the oxidation of organic matter. They consist of a negative anode and a positive cathode, separated by an ion selective membrane. The key to improve power, in open-to-air cathode MFCs, is the efficient utilisation of oxygen, by using high surface area materials and effective gas diffusion. This study investigated the effect of single micro porous layers, used as the coating on various electrode substrata, on the performance of small-scale MFCs. Furthermore, 2 of the modified small-scale (6.25 mL) MFCs were implemented as the power source for the TI Chronos digital wristwatch, thus successfully substituting the 3 V button cell, at least for the duration of the experiment.     

Enhanced bioelectricity generation and cathodic oxygen reduction of air breathing microbial fuel cells based on MoS2 decorated carbon nanotube

Increasing efforts have been devoted to enhancing the cathode activity towards oxygen reduction and improve power generation of air breathing microbial fuel cells. Exploring non-precious metal and highly active cathodic catalyst plays a key role in improving cathode performance. Our work aims to investigate the electrocatalyst behavior and power output of the single-chamber MFC equipped with carbon nanotubes hybridized molybdenum disulfide nanocomposites (CNT/MoS₂) cathode. MoS₂ nanosheets embedded into the CNTs network structure is synthesized by a facile hydrothermal method. The CNT/MoS₂-MFC achieves a maximum power density of 53.0 mW m⁻², which is much higher than those MFCs with pure CNTs (21.4 mW m⁻²) or solely MoS₂ (14.4 mW m⁻²) cathode. The oxygen reduction reaction (ORR) test also demonstrates a promoted electrocatalytic activity of synthesized material, which may be attributed to the special interlaced structure and abundant oxygen chemisorption sites of CNT/MoS₂. Such CNTs-based noble-metal-free catalyst presents a new approach to the application of MFCs cathode materials.     

Bimetallic zeolitic imidazole framework derived Co@NC materials as oxygen reduction reaction catalysts application for microbial fuel cells

Microbial fuel cells (MFCs), a promising future energy conversion technology, play a significant role in the area of sustainable and renewable energy. In air-cathode MFCs, the catalytic activity for oxygen reduction reaction (ORR) of cathode electrocatalyst is the key factor to the performance of MFCs. Development of efficient and economical ORR electrocatalysts is an important step for the wide application of MFCs. Herein, Co wrapped carbon nanotubes (CNTs) N-doped nanoporous carbon materials (Co@NC-Co_xZn_y) are constructed via a facile zinc-assisted growth pyrolytic approach of bimetallic zeolitic imidazole frameworks (BMZIFs)-derived strategy. They are directly prepared via carbonization of the precursor Co_xZn_y-BMZIFs. During the pyrolysis process, the evaporation of zinc plays critical role in the in-situ growth of CNTs. For instance, the optimal catalyst, Co@NC-Co₁Zn₃, exhibits excellent ORR performance activity and stability with on-set potential (E_on-set) of 0.830 V (vs. RHE) and diffusion-limited current density (j_L) of 6.706 mA cm^?2, which is superior to the benchmark catalyst of commercial 20 wt% Pt/C. Additionally, Co@NC-Co₁Zn₃ displays four-electron pathway, long-term stability and better resistance to methanol tolerance. The MFC with Co@NC-Co₁Zn₃ cathode shows a maximum power density of 1039 mW m^?2, and outperforms the MFC with commercial 20 wt% Pt/C catalyst (678 mW m^?2). This work paved the way for exploring cost-effective, superior performance non-precious metal-based catalysts for air-cathode MFCs.     

The fabrication of silane modified graphene oxide supported Ni–Co bimetallic electrocatalysts: A catalytic system for superior oxygen reduction in microbial fuel cells

Microbial fuel cells (MFCs) exploit the ability of microorganisms to generate clean energy from organic pollutants in wastewater. However, the poor cathode performance and the use of the expensive rare metal platinum as a catalyst limit their application and scalability. In this study, we have synthesised a Ni–Co/GO nanocomposite and applied it as a potential cathode catalyst to single-chamber MFCs. To improve the performance of a Ni–Co-based hybrid nanocomposite, the support of graphene oxide (GO) is covalently modified with γ-amino propyl tri-ethoxy silane (APTES) through a silane modification reaction. The physical and chemical properties of the synthesised materials are characterised with Fourier transform infrared (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) techniques. A microscopic study has shown that metal nanoparticles are distributed uniformly on the MGO matrix. The electrocatalytic activity of the synthesised hybrid nanocatalysts is analysed for oxygen reduction reaction (ORR). A cyclic voltammetry experiment has shown that the Ni–Co/MGO catalyst exhibits a higher reduction peak current value and a higher positive onset potential than the Ni–Co/GO catalyst and Pt/C catalyst, indicating an enhanced ORR activity of the Ni–Co/MGO catalyst. Ni–Co/MGO also exhibits the highest initial current of 0.116 mA in the chronoamperometry test, which decreases to 0.049 mA after 16000 s. The electrochemical results demonstrate that the synthesised Ni–Co/MGO catalyst has a higher electrocatalytic activity and higher stability than the state-of-the-art Pt/C catalyst. More importantly, a MFC with Ni–Co/MGO as a cathode catalyst shows the maximum power density of 1003.18 mWm⁻², which is much higher than in the case of the Ni–Co/GO catalyst (889.6 mWm⁻²) and approximately 2.1 times higher than that of the state-of-the-art Pt/C (483.48 mWm⁻²). Consequently, the Ni–Co/MGO nanocomposite also shows the highest open circuit voltage of 0.857 V among the other studied catalysts. Moreover, the Ni–Co/MGO catalyst has a lower biofouling level than a commercial 10 wt% Pt/C catalyst, which shows that the synthesised cathode catalyst is superior in terms of stability, overall performance and usage. These results suggest that the newly developed Ni–Co/MGO catalyst can be applied as a potential substitute for the Pt/C cathode catalyst for the practical application of MFCs.     

Porous Co3O4 decorated nitrogen-doped graphene electrocatalysts for efficient bioelectricity generation in MFCs

High-performance, low-cost, and robust oxygen reduction reaction (ORR) catalysts have played a very crucial role in the development of microbial fuel cells (MFCs). Herein, A novel in-situ Co₃O₄ nanoparticles (NPs) modified nitrogen-doped graphene with three-dimensional porous structure (3D GN-Co₃O₄) has been successfully synthesized and employed as an efficient ORR catalyst in MFCs. Benefiting from 3D porous architecture feature, highly intrinsic conductivity and synergistic effect between nitrogen-doped graphene and Co₃O₄ NPs, the 3D GN-Co₃O₄ as a cathode catalyst in alkaline condition realizes significantly enhanced electrochemical performance and outstanding cycling stability. Furthermore, the self-assembly of MFCs based on the 3D GN-Co₃O₄ cathode offers a high power density of 578 ± 10 mW m^?2, which is even comparable to the commercial Pt/C.     

Fe and N co-doped carbon derived from melamine resin capsuled biomass as efficient oxygen reduction catalyst for air-cathode microbial fuel cells

Cathode oxygen reduction reaction (ORR) performance is crucial for power generation of microbial fuel cells (MFCs). The current study provides a novel strategy to prepare Fe/N-doped carbon (Fe/N/C) catalyst for MFCs cathode through high temperature pyrolyzing of biomass capsuling melamine resin polymer. The obtained Fe/N/C can effectively enhance activity, selectivity and stability toward 4 e^– ORR in pH neutral solution. Single chamber MFC with Fe/N/C air cathode produces maximum power density of 1166 mW m⁻², which is 140% higher than AC cathode. The improved performance of Fe/N/C can be attributed to the involvement of nitrogen and iron species. The excellent stability can be attributed to the preferential structure of the catalyst. The moderate porosity of the catalyst facilitates mass transfer of oxygen and protons and prevents water flooding of triple-phase boundary where ORR occurs. The biomass particles encapsulated in the catalyst act as skeletons, which prevents catalyst collapse and agglomeration.     

An overview of electrode materials in microbial fuel cells

Electrode materials play an important role in the performance (e.g., power output) and cost of microbial fuel cells (MFCs), which use bacteria as the catalysts to oxidize organic (inorganic) matter and convert chemical energy into electricity. In this paper, the recent progress of anode/cathode materials and filling materials as three-dimensional electrodes for MFCs has been systematically reviewed, resulting in comprehensive insights into the characteristics, options, modifications, and evaluations of the electrode materials and their effects on different actual wastewater treatment. Some existing problems of electrode materials in current MFCs are summarized, and outlooks for future development are also suggested.     

Three-dimensional flower-like Co-based metal organic frameworks as efficient multifunctional catalysts towards oxygen reduction,oxygen evolution and hydrogen evolution reactions

Design and development of cost-efficient multifunctional three-dimensional (3D) metal organic frameworks (MOFs) towards oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are very significant for green energy devices. Herein, a scalable one-pot solvothermal method is developed to obtain a series of multifunctional 3D flower-like MOFs. In addition, systematic studies are also conducted on the effects of various metal cations and N-containing ligands on the structures, compositions, and multifunctional performance of the obtained MOFs. As a result, 3D flower-like Co-MOFs using Co²⁺ as a metal cation and 2,2’:6′,2″-terpyridine as a N-containing ligand exhibit the highest multifunctional performance towards ORR, OER and HER. The scalable method provides a new prospect to design and develop other MOFs-based multifunctional catalysts.     

Polypyrrole/carbon black composite as a novel oxygen reduction catalyst for microbial fuel cells

A polypyrrole/carbon black (Ppy/C) composite has been employed as an electrocatalyst for the oxygen reduction reaction (ORR) in an air-cathode microbial fuel cell (MFC). The electrocatalytic activity of the Ppy/C is evaluated toward the oxygen reduction using cyclic voltammogram and linear sweep voltammogram methods. In comparison with that at the carbon black electrode, the peak potential of the ORR at the Pp/C electrode shifts by approximate 260 mV towards positive potential, demonstrating the electrocatalytic activity of Ppy toward ORR. Additionally, the results of the MFC experiments show that the Ppy/C is well suitable to fully substitute the traditional cathode materials in MFCs. The maximum power density of 401.8 mW m⁻² obtained from the MFC with a Ppy/C cathode is higher than that of 90.9 mW m⁻² with a carbon black cathode and 336.6 mW m⁻² with a non-pyrolysed FePc cathode. Although the power output with a Ppy/C cathode is lower than that with a commercial Pt cathode, the power per cost of a Ppy/C cathode is 15 times greater than that of a Pt cathode. Thus, the Ppy/C can be a good alternative to Pt in MFCs due to the economic advantage.     

Biomass-derived N-doped porous activated carbon as a high-performance and cost-effective pH-universal oxygen reduction catalyst in fuel cell

Nitrogen (N) doped porous activated carbons (TGC-T) derived from tofu gel are prepared through a facile, economic and eco-friendly method. The as-prepared TGC-900 possesses high specific surface area (651.78 m² g⁻¹) and homogeneous doping N (Content of N: 5.52 at.%). Reasonably, TGC-900 exhibits excellent oxygen reduction reaction (ORR) activity, stability and methanol resistance in neutral, alkaline and acidic medium. Moreover, TGC-900 also shows outstanding ORR performance in the application of microbial fuel cell (MFC) with the highest output voltage (544 ± 6 mV) and maximum power density (977 ± 32 mW m⁻²). Inspiringly, four single-chamber air cathode MFCs (AC-MFCs) in series can drive a light-emitting diode (LED) to work is firstly reported which further provides a more intuitively method to evaluate the performance of generating electricity for MFCs. Thus, the high performance and cost-effective ORR catalyst TGC-900 is expected to apply in the field of fuel cells.     

Performance optimization of microbial fuel cells using carbonaceous monolithic air-cathodes

Bamboo charcoal tube (BCT) has been demonstrated to be the air-cathode for microbial fuel cells (MFCs). To enhance the performance of MFCs with BCT cathodes, we herein focus on optimizing BCT diameters and doping with nitrogen and phosphorus elements to improve the physical and chemical properties of BCT cathodes. Scanning electron microscope (SEM) analyses indicated that the cathode with 35 mm inner diameter had relatively ideal pore size distribution. Polarization curves, electrochemical impedance spectroscopy, and cyclic voltammetry results suggested that the cathode (35 mm inner diameter) delivered the highest power density of 7.5 ± 0.6 W m⁻³ among the cathodes with different diameters (27, 31, 35 and 39 mm). It was mainly because of its high oxygen reduction reaction performance, low cathode resistance, and beneficial effects on anode biofilm. In addition, rotating disk electrode (RDE) tests revealed that nitrogen and phosphorus doping could significantly improve the ORR catalytic activity of cathode by around 25% in the limiting current density.     

Recent progress in carbon nanotubes support materials for Pt-based cathode catalysts in PEM fuel cells

Support materials have a significant impact on catalytic activity, stability, and performance of catalysts toward the oxygen reduction reaction (ORR). The properties of carbon-based materials have made them an excellent alternative for use as support for nanosized catalysts. Recently, carbon nanotubes (CNTs) have been explored as catalyst support materials, and their properties make them a promissory alternative. Furthermore, catalysts supported on CNTs exhibit higher resistance to electrochemical oxidation, better catalytic performance, and higher durability than catalysts supported on carbon black. In recent years, CNTs have acquired great relevance as catalysts support materials for ORR in acid media. This review addresses the most relevant studies on CNTs modification using methods such as functionalization, doping, and hybrid supports (CNTs-metal oxide) used as supports for Pt-based cathode catalysts in proton exchange membrane fuel cells.     

Recent advances in covalent organic framework (COF) nanotextures with band engineering for stimulating solar hydrogen production: A comprehensive review

Due to the continuous consumption of fossil fuels, natural reserves are depleting and it has been earnest need for developing new sources of energy. Among the several solar energy conversion techniques, photocatalytic hydrogen (H₂) generation is regarded as one of the most promising routes. Till date, several metal-based semiconductor materials have been investigated, however, H₂ generation is not substantial with the notion of sustainable development. Current research trends show the growing interest in advanced and metal free photocatalyst materials such as covalent organic frameworks (COFs) due to their several benefits such as crystalline porous polymers with pre-designed architectures, large surface area, exceptional stability, and ease of molecular functionalization. By combining COFs with other functional materials, composites may be created that display unique characteristics that exceed those of the separate components. This work provides a comprehensive development on COFs as a photocatalysts and their composites/hybrids for photocatalytic hydrogen generation with a focus on visible-light irradiation. To reduce the dependency on novel metals and overcome the drawbacks of individual material, the creation of composite materials based on covalent-organic frameworks (COFs) are systematically discussed. In addition, advantages in terms of performance, stability, durability of composites/hybrids COFs for photocatalytic hydrogen production in reference to traditional catalysts are investigated. Different composites such as metals loading, morphological development, band engineering, and heterojunctions are systematically discussed. Finally, challenges and opportunities associated with constructing COF-based catalysts as future research prospective for chemistry and materials science are highlighted.     

Electricity and catholyte production from ceramic MFCs treating urine

The use of ceramics as low cost membrane materials for Microbial Fuel Cells (MFCs) has gained increasing interest, due to improved performance levels in terms of power and catholyte production. The catholyte production in ceramic MFCs can be attributed to a combination of water or hydrogen peroxide formation from the oxygen reduction reaction in the cathode, water diffusion and electroosmotic drag through the ion exchange membrane. This study aims to evaluate, for the first time, the effect of ceramic wall/membrane thickness, in terms of power, as well as catholyte production from MFCs using urine as a feedstock. Cylindrical MFCs were assembled with fine fire clay of different thicknesses (2.5, 5 and 10 mm) as structural and membrane materials. The power generated increased when the membrane thickness decreased, reaching 2.1 ± 0.19 mW per single MFC (2.5 mm), which was 50% higher than that from the MFCs with the thickest membrane (10 mm). The amount of catholyte collected also decreased with the wall thickness, whereas the pH increased. Evidence shows that the catholyte composition varies with the wall thickness of the ceramic membrane. The possibility of producing different quality of catholyte from urine opens a new field of study in water reuse and resource recovery for practical implementation.     

Transition metal phthalocyanine-modified shungite-based cathode catalysts for alkaline membrane fuel cell

The alkaline anion exchange membrane fuel cell (AEMFC) is one of the green solutions for the growing need for energy conversion technologies. For the first time, we propose a natural shungite based non-precious metal catalyst (NPMC) as an alternative cathode catalyst to Pt-based materials for AEMFCs application. The Co and Fe phthalocyanine (Pc)-modified shungite materials were prepared via pyrolysis and used for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) studies. The most promising ORR performance was observed in alkaline media for FePc-modified and acid-leached shungite-based NPMC material. The catalysts were also evaluated as cathode materials in a single cell AEMFC and peak power densities of 232 and 234 mW cm^?2 at 60 °C using H₂ and O₂ gases at 100% RH were observed for CoPc- and FePc-modified acid-treated materials, respectively.