Low-cost nanowired α-MnO2/C as an ORR catalyst in air-cathode microbial fuel cell

Hydrothermal synthesis of tungsten oxide photo/electrocatalysts: precursor-driven morphological tailoring and electrochemical performance for hydrogen evolution and oxygen reduction reaction application

A hydrothermal approach was adopted to synthesize tungsten oxide (WO3) nanocatalysts with tailored morphology, using oxalic acid (H2C2O4) and hydrochloric acid (HCl) as precursors. This precursor-driven method yielded two distinct WO3 catalysts with unique structural and functional properties, viz. rod-shaped WO3-ox and nanoflower-shaped WO3-h. Characterization by FESEM and XRD revealed variations in morphology and crystallite size, contributing to their specialized catalytic applications. UV-Vis spectroscopy confirmed strong UV absorption by WO3-ox at 283.57 nm with an optical band gap of 2.86 eV, making it ideal for photocatalytic activities. Electrochemical analysis demonstrated that WO3-ox effectively drives the hydrogen evolution reaction (HER), while WO3-h is more suitable for the oxygen reduction reaction (ORR), an essential process in microbial fuel cells (MFCs). In practical applications, WO3-ox achieved an 83.9% degradation efficiency of methylene blue (MB) within 3 h, validating its high photocatalytic efficacy for wastewater treatment. Meanwhile, WO3-h, utilized as a cathode catalyst in MFCs, significantly enhanced system performance, elevating chemical oxygen demand (COD) removal efficiency to 78.7% and improving coulombic efficiency by 3%. These findings underscore the potential of precursor-driven hydrothermal synthesis for optimizing WO3 catalysts tailored for energy and environmental applications, specifically in hydrogen production and sustainable wastewater treatment systems.

Applications of Nanomaterials in Microbial Fuel Cells: A Review

Microbial fuel cells (MFCs) are an environmentally friendly technology and a source of renewable energy. It is used to generate electrical energy from organic waste using bacteria, which is an effective technology in wastewater treatment. The anode and the cathode electrodes and proton exchange membranes (PEM) are important components affecting the performance and operation of MFC. Conventional materials used in the manufacture of electrodes and membranes are insufficient to improve the efficiency of MFC. The use of nanomaterials in the manufacture of the anode had a prominent effect in improving the performance in terms of increasing the surface area, increasing the transfer of electrons from the anode to the cathode, biocompatibility, and biofilm formation and improving the oxidation reactions of organic waste using bacteria. The use of nanomaterials in the manufacture of the cathode also showed the improvement of cathode reactions or oxygen reduction reactions (ORR). The PEM has a prominent role in separating the anode and the cathode in the MFC, transferring protons from the anode chamber to the cathode chamber while preventing the transfer of oxygen. Nanomaterials have been used in the manufacture of membrane components, which led to improving the chemical and physical properties of the membranes and increasing the transfer rates of protons, thus improving the performance and efficiency of MFC in generating electrical energy and improving wastewater treatment.

Synthesis and study of CuNiTiO3 as an ORR electrocatalyst to enhance microbial fuel cell efficiency

Microbial fuel cells (MFCs) have the capability of simultaneous sewage treatment and electricity generation. Modifying the cathode electrode enhances their efficiency. In this study, NiTiO₃ and CuNiTiO₃ were synthesized for practical application as cathode catalysts in a dual-chamber MFC and the performance of the modified cathodes was evaluated against a bare graphite electrode. SEM images showed that the particle sizes were mostly in the range of 40-120 and 20-80 nm for NiTiO₃ and CuNiTiO₃, respectively. According to AFM results, CuNiTiO₃ presented a higher surface roughness than NiTiO₃. MFC using CuNiTiO₃/G electrode with a reduction potential value of -0.27 V (vs. SCE) and a power density of 62.18 mW m^-2 showed better oxygen reduction reaction (ORR) activity compared with NiTiO₃/G and the bare graphite. MFC using CuNiTiO₃ cathode also showed the highest values in terms of chemical oxygen demand (COD) removal (75%) and the calculated coulombic efficiency (CE, 10%). The results obtained in this study, introduce CuNiTiO₃ as a promising electrocatalyst for further improvement of the cathodic reactions in MFC applications.

A Review of Recent Advances in Microbial Fuel Cells: Preparation, Operation, and Application

The microbial fuel cell has been considered a promising alternative to traditional fossil energy. It has great potential in energy production, waste management, and biomass valorization. However, it has several technical issues, such as low power generation efficiency and operational stability. These issues limit the scale-up and commercialization of MFC systems. This review presents the latest progress in microbial community selection and genetic engineering techniques for enhancing microbial electricity production. The summary of substrate selection covers defined substrates and some inexpensive complex substrates, such as wastewater and lignocellulosic biomass materials. In addition, it also includes electrode modification, electron transfer mediator selection, and optimization of operating conditions. The applications of MFC systems introduced in this review involve wastewater treatment, production of value-added products, and biosensors. This review focuses on the crucial process of microbial fuel cells from preparation to application and provides an outlook for their future development.

ZrP2O7 as a Cathodic Material in Single-Chamber MFC for Bioenergy Production

The present work is the first investigation of the electrocatalytic performances of ZrP₂O₇ as a cathode in a single-chamber Microbial Fuel Cell (MFC) for the conversion of chemical energy from wastewater to bioelectricity. This catalyst was prepared by a coprecipitation method, then characterized by X-ray diffraction analysis (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), ultraviolet-visible-near-infrared spectrophotometry (UV-Vis-NIR), and cyclic voltammetry analyses. The acid-basic characteristics of the surface were probed by using 2-butanol decomposition. The conversion of 2-butanol occurs essentially through the dehydrating reaction, indicating the predominantly acidic character of the solid. The electrochemical test shows that the studied cathode material is electroactive. In addition, the ZrP₂O₇ in the MFC configuration exhibited high performance in terms of bioelectricity generation, giving a maximum output power density of around 449 mW m^-2; moreover, it was active for wastewater treatment, reducing the chemical oxygen demand (COD) charge to 50% after three days of reaction.

Prussian blue analogues derived electrocatalyst with multicatalytic centers for boosting oxygen reduction reaction in the wide pH range

Due to the complex of oxygen reduction reaction (ORR), designing catalysts with multicatalytic centers is considered as a promising way for boosting the ORR. Herein, a multicatalytic centers electrocatalyst Fe₃C/Mn₃O₄ encased by N-doped graphitic layers (FeMn PDA-900) is synthesized using iron manganese Prussian blue analogues and dopamine as the precursor. It exhibits a half-wave potential (E_1/2) of 0.86 V for ORR and yields of H₂O₂ lower than 5% in 0.1 M KOH. Moreover, the prepared catalyst has also shown high catalytic ORR performance in both acidic and neutral electrolyte solutions, which exhibits the potential application in both the proton exchange membrane fuel cell and the microbial electrolysis cell. It is found that the good performance can be well explained by proton-coupled electron transfer mechanism due to the multicatalytic centers from Fe-N_x, Fe₃C and Mn₃O₄ for providing enough active sites at the same time and the N-doped graphitic layers as a bridge for facilitating the electron transfer between the interfaces of Fe₃C/Mn₃O₄ nanoparticles, which paves the way for protons and electrons transfer simultaneously and rapidly, and thus lowing the energy barrier and facilitating the ORR process. Therefore, FeMn PDA-900 is a promising candidate to replace precious metal-based ORR electrocatalysts at the whole pH range.

Magnetized manganese-doped watermelon rind biochar as a novel low-cost catalyst for improving oxygen reduction reaction in microbial fuel cells

Microbial fuel cells (MFCs) are promising equipment for water treatment and power generation. The catalyst used in the oxygen reduction reaction (ORR) at the cathode is a critical factor for efficacy of MFCs. Therefore, it is important to develop cost-effective cathode catalysts to enhance application of MFCs. In the current study, a novel cathode catalyst was developed, which was annealed with watermelon rind as raw material and transition metals including iron, and manganese were introduced. The 700Mn/Fe@WRC catalyst, which was annealed at 700 °C, exhibited excellent electrochemical performance. The high relative content of pyridine nitrogen caused by the inherent nitrogen element of the watermelon rind and the high content of iron and manganese elements introduced resulted in increase in electrochemical surface area to 657.6 m²/g. The number of electrons transferred ORR was 3.96, indicating that ORR occurs through a four-electron pathway. The maximum power density of MFCs was 399.3 ± 7.4 mW/m² with a fitting total internal resistance of 15.242 Ω, and the removal efficiency of COD was 97.1 ± 1.2%. The cost of the 700Mn/Fe@WRC catalyst was approximately 0.15 $/g, which is significantly lower compared with Pt/C (33.0 $/g). Experimental verification showed that the 700Mn/Fe@WRC prepared using the economical watermelon rind biochar (WRC) is an excellent substitute for non-precious metal catalysts used in MFCs.

Carbon Nanotube/Pt Cathode Nanocomposite Electrode in Microbial Fuel Cells for Wastewater Treatment and Bioenergy Production

In this paper, we reported the fabrication, characterization, and application of carbon nanotube (CNT)-platinum nanocomposite as a novel generation of cathode catalyst in microbial fuel cells (MFCs) for sustainable energy production and wastewater treatment. The efficiency of the carbon nanocomposites was compared by platinum (Pt), which is the most effective and common cathode catalyst. This nanocomposite is utilized to benefit from the catalytic properties of CNTs and reduce the amount of required Pt, as it is an expensive catalyst. The CNT/Pt nanocomposites were synthesized via a chemical reduction technique and the electrodes were characterized by field emission scanning electron microscopy, electronic dispersive X-Ray analysis, and transmission electron microscopy. The nanocomposites were applied as cathode catalysts in the MFC to obtain polarization curve and coulombic efficiency (CE) results. The catalytic properties of electrodes were tested by linear sweep voltammetry. The CNT/Pt at the concentration of 0.3 mg/cm2 had the highest performance in terms of CE (47.16%), internal resistance (551 Ω), COD removal (88.9%), and power generation (143 mW/m2). In contrast, for the electrode with 0.5 mg/L of Pt catalyst, CE, internal resistance, COD removal, and power generation were 19%, 810 Ω, 96%, and 84.1 mW/m2, respectively. So, it has been found that carbon nanocomposite cathode electrodes had better performance for sustainable clean energy production and COD removal by MFC.

Characterization and Electrocatalytic Performance of Molasses Derived Co-Doped (P, N) and Tri-Doped (Si, P, N) Carbon for the ORR

There is a growing need to develop sustainable electrocatalysts to facilitate the reduction of molecular oxygen that occurs at the cathode in fuel cells, due to the excessive cost and limited availability of precious metal-based catalysts. This study reports the synthesis and characterization of phosphorus and nitrogen co-doped carbon (PNDC) and silicon, phosphorus, and nitrogen tri-doped carbon (SiPNDC) electrocatalysts derived from molasses. This robust microwave-assisted synthesis approach is used to develop a low cost and environmentally friendly carbon with high surface area for application in fuel cells. Co-doped PNDC as well as tri-doped SiPNDC showed Brunauer–Emmet–Teller (BET) surface areas of 437 and 426 m2 g−1, respectively, with well-developed porosity. However, examination of X-ray photoelectron spectroscopy (XPS) data revealed significant alteration in the doping elemental composition among both samples. The results obtained using rotating disk electrode (RDE) measurements show that tri-doped SiPNDC achieves much closer to a 4-electron process than co-doped PNDC. Detailed analysis of experimental results acquired from rotating ring disk electrode (RRDE) studies indicates that there is a negligible amount of peroxide formation during ORR, further confirming the direct-electron transfer pathway results obtained from RDE. Furthermore, SiPNDC shows stable oxygen reduction reaction (ORR) performance over 2500 cycles, making this material a promising electrocatalyst for fuel cell applications.

α-FeOOH nanowires loaded on carbon paper anodes improve the performance of microbial fuel cells

Nanowires synthesized from metal oxides exhibit better conductivity than nanoparticles due to their greater aspect ratio which means that they can transmit electrons over longer distances; in addition, they are also more widely available than pili because their synthesis is not affected by the bacteria themselves. However, there is still little research on the application of metal oxides nanowires to enhance power generation of microbial fuel cells (MFC). In this study, a simple hydrothermal synthesis method was adopted to synthesize α-FeOOH nanowires on carbon paper (α-FeOOH-NWs), which serve as an anode to explore the mechanism of power generation enhancement of MFC. Characterization results reveal α-FeOOH-NWs on carbon paper are approximately 30-50 nm in diameter, with goethite structure. Electrochemical test results indicate that α-FeOOH nanowires could enhance the electrochemical activity of carbon paper and reduce the electron transfer resistance (R_ct). Furthermore, α-FeOOH-NWs made the power density of MFC 3.2 times of the control device. SEM result demonstrates that nanowires are beneficial to the formation of biofilms and increase biomass on the electrode surface. Our results demonstrate that nanowires not only improve the electrochemical activity and conductivity of carbon paper but also facilitate the formation of biofilms and increase the biomass of the anode surface. These two mechanisms work together to boost extracellular electron transfer and power generation efficiency of MFC with α-FeOOH-NWs. Our study provides further evidence for the electrical conductivity of metal nanowires, promoting their potential applications in electricity generation such as MFC or other energy development fields.

An overview in the development of cathode materials for the improvement in power generation of microbial fuel cells

Since the high cost and low power generation hinder the overall practical application of microbial fuel cells (MFCs), numerous attempts have been made in the field of cathode materials to enhance the electrical performance of MFCs because they directly catalyze the oxygen reduction reactions (ORR). To choose a proper cathode material, following principles such as ORR activity, conductivity, cost-efficiency, durability, surface area, and accessibility should be taken into consideration. In preparation of cathode materials, versatile materials have been chosen, synthesized, or modified to achieve an improvement in power generation of MFCs. The most widely applied cathode materials could be categorized into three classes, namely carbon-base materials, metal-based materials, and biocatalysts. This review summarizes the utilization, development, and the cost of cathode materials applied in MFCs and tries to highlight the effective modification methods of cathode materials which have helped in achieving enhanced power generation of MFCs in recent years.

A highly sustainable hydrothermal synthesized MnO2 as cathodic catalyst in solar photocatalytic fuel cell

A unidirectional flow solar photocatalytic fuel cell (PFC) was successfully developed for the first time to offer alternative for electricity generation and simultaneous wastewater treatment. This study was focused on the synthesis of α-, δ- and β-MnO₂ by wet chemical hydrothermal method for application as the cathodic catalyst in PFC. The crystallographic evolution was performed by varying the ratios of KMnO₄ to MnSO₄. The mechanism of the PFC with the MnO₂/C as cathode was also discussed. Results showed that the catalytic activity of MnO₂/C cathode was mainly predominated by their crystallographic structures which included Mn-O bond strength and tunnel size, following order of α- > δ- > β-MnO₂/C. Interestingly, it was discovered that the specific surface areas (S_BET) of different crystal phases did not give an impact on the PFC performance. However, the P_max could be significantly influenced by the micropore surface area (S_micro) in the comparison among α-MnO₂. Furthermore, the morphological transformation carried out by altering the hydrothermal duration demonstrated that the nanowire α-M3(24 h)/C with 1:1 ratio of KMnO₄ and MnSO₄ yielded excellent PFC performance with a P_max of 2.8680 μW cm^-2 and the lowest R_int of 700 Ω.

Cu/S-Occupation Bifunctional Oxygen Catalysts for Advanced Rechargeable Zinc-Air Batteries

The design and synthesis of low-cost and highly efficient bifunctional catalysts is an inevitable path for rechargeable zinc-air batteries (rZABs). In this work, double-carbon co-supported Co-based oxide with the Cu and S substitutions are synthesized by a one-step hydrothermal method and formed a unique honeycomb structure. As expected, the (Cu, Co)₃OS₃@CNT-C₃N₄ exhibits high oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity with low overpotential (0.86 V), high power density (215 mW cm^-2), and long-term discharge stability (115 h). The (Cu, Co)₃OS₃@CNT-C₃N₄-based rZAB also shows a stronger charge-discharge durability with a very low voltage gap of merely 0.5 V than that of Pt/C+RuO₂. The high catalytic performances are attributed to these following reasons: (i) the porous morphology and hierarchical structure with plentiful "catalytic buffer", which accelerates the mass transfer; (ii) a high-speed electronic transmission network established by C₃N₄ and carbon nanotube (CNT), enhancing the conductivity; (iii) the strong synergistic effect between (Cu, Co)₃OS₃@CNT and C₃N₄, which improves the kinetics of ORR/OER; and (iv) the controllable occupation of Cu ions and S ions, which effectively regulates the CoO₆ surface and increases the active site density. This work not only offers a promising ORR/OER electrode for rZAB but also provides a new pathway to understand the improvement mechanism for catalysts by the bi-ion substitutions.

Combined metal/air fuel cell and electrocoagulation process: Energy generation, flocs production and pollutant removal

Electrocoagulation (EC) which is characterized by in-situ generation of highly absorbable hydroxide flocs, is an environmentally friendly process for treating heavy metal ions and toxic organic wastewater. In order to decrease EC's energy consumption, a combined metal/air FC-EC process which contains two successive parts: metal/air fuel cell (FC) and electrocoagulation (EC) was studied with the consideration of hydroxide flocs production, pollutant removal and energy generation analysis. For the combined iron/air FC-EC process, the porous nickel cathode which has a good performance in high polarization zone was selected as the ideal air cathode. It was found that iron/air FC-EC with acid electrolyte condition has a high energy generation (as high as 20% EC energy consumption). The energy generation increases with iron/air FC time. Also energy generation increases with wastewater's conductivity. Beside the energy generation, the iron/air fuel cell generate extra coagulants Fe²⁺ for the subsequent EC process. The coagulants generated from iron/air FC and EC process together have further spontaneous hydrolysis reactions with the OH^- to form hydroxide flocs, which are beneficial for a rapid adsorption and pollutant trapping. Compared with EC process, iron/air (or Al/air) FC-EC process shows lower energy consumption and high removal efficiency for treating acid wastewater.

Study of a Thin Film Aluminum-Air Battery

A thin film aluminum-air battery has been constructed using a commercial grade Al-6061 plate as anode electrode, an air-breathing carbon cloth carrying an electrocatalyst as cathode electrode, and a thin porous paper soaked with aqueous KOH as electrolyte. This type of battery demonstrates a promising behavior under ambient conditions of 20 °C temperature and around 40% humidity. It presents good electric characteristics when plain nanoparticulate carbon (carbon black) is used as electrocatalyst but it is highly improved when MnO2 particles are mixed with carbon black. Thus, the open-circuit voltage was 1.35 V, the short-circuit current density 50 mA cm−2, and the maximum power density 20 mW cm−2 in the absence of MnO2 and increased to 1.45 V, 60 mA cm−2, and 28 mW cm−2, respectively, in the presence of MnO2. The corresponding maximum energy yield during battery discharge was 4.9 mWh cm−2 in the absence of MnO2 and increased to 5.5 mWh cm−2 in the presence of MnO2. In the second case, battery discharge lasted longer under the same discharge conditions. The superiority of the MnO2-containing electrocatalyst is justified by electrode electrochemical characterization data demonstrating reduction reactions at higher potential and charge transfer with much smaller resistance.

Carbon-Based Electrocatalysts Derived From Biomass for Oxygen Reduction Reaction: A Minireview

Oxygen reduction reaction (ORR) electrocatalysts derived from biomass have become one of the research focuses in hetero-catalysis due to their low cost, high performance, and reproducibility properties. Related researches are of great significance for the development of next-generation fuel cells and metal-air batteries. Herein, the preparation methods of various biomass-derived catalysts and their performance in alkaline, neutral, and acidic media are summarized. This review clarifies the research progress of biomass carbon-based electrocatalysts for ORR in acidic, alkaline and neutral media, and discusses the future development trends. This minireview can give us an important enlightenment to practical application in the future.

Investigation and Improvement of Scalable Oxygen Reducing Cathodes for Microbial Fuel Cells by Spray Coating

This contribution describes the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). The increase in coating quality led to a strong increase in MFC performance in terms of peak power density and long-term stability. This more uniform coating was realized by an airbrush coating method for applying a self-developed polymeric solution containing different catalysts (MnO2, MoS2, Co3O4). In addition to the possible automation of the presented coating, this method did not require a calcination step. A cathode coated with catalysts, for instance, MnO2/MoS2 (weight ratio 2:1), by airbrush method reached a peak and long-term power density of 320 and 200–240 mW/m2, respectively, in a two-chamber MFC. The long-term performance was approximately three times higher than a cathode with the same catalyst system but coated with the former paintbrush method on a smaller cathode surface area. This extraordinary increase in MFC performance confirmed the high impact of catalyst coating quality, which could be stronger than variations in catalyst concentration and composition, as well as in cathode surface area.

Biofouling effects on the performance of microbial fuel cells and recent advances in biotechnological and chemical strategies for mitigation

The occurrence of biofouling in MFC can cause severe problems such as hindering proton transfer and increasing the ohmic and charge transfer resistance of cathodes, which results in a rapid decline in performance of MFC. This is one of the main reasons why scaling-up of MFCs has not yet been successfully accomplished. The present review article is a wide-ranging attempt to provide insights to the biofouling mechanisms on surfaces of MFC, mainly on proton exchange membranes and cathodes, and their effects on performance of MFC based on theoretical and practical evidence. Various biofouling mitigation techniques for membranes are discussed, including preparation of antifouling composite membranes, modification of the physical and chemical properties of existing membranes, and coating with antifouling agents. For cathodes of MFC, use of Ag nanoparticles, Ag-based composite nanoparticles, and antifouling chemicals is outlined in considerable detail. Finally, prospective techniques for mitigation of biofouling are discussed, which have not been given much previous attention in the field of MFC research. This article will help to enhance understanding of the severity of biofouling issues in MFCs and provides up-to-date solutions. It will be beneficial for scientific communities for further strengthening MFC research and will also help in progressing this cutting-edge technology to scale-up, using the most efficient methods as described here.

One-pot achievement of MnO2/Fe2O3 nanocomposites for the oxygen reduction reaction with enhanced catalytic activity

MnO₂/Fe₂O₃ nanocomposites were achieved in one-pot followed by high-temperature treatment, which presented excellent electrocatalytic activity for the oxygen reduction reaction.

Azadirachta indica leaf-extract-assisted synthesis of CoO–NiO mixed metal oxide for application in a microbial fuel cell as a cathode catalyst

In this study, a highly efficient yet low-cost mixed transition metal oxide of Ni and Co as CoO–NiO was synthesized using Azadirachta indica leaf extract as a bioactive chelating agent.