Cobalt - Iron phthalocyanine supported on carbide - Derived carbon as an excellent oxygen reduction reaction catalyst for microbial fuel cells

Construction of a stable S-scheme NiSnO3/g-C3N4 heterojunction on activated carbon fibre for the degradation of glyphosate in water under flow condition

Creating light-harvesting heterojunctions as a photocatalyst is critical for efficiently treating organics-laden wastewater. Yet the materials stabilization and limited reusability hinder their practical applications. In this study, an S-scheme heterojunction in the Sn-based perovskite and g-C3N4 (gCN) composite, supported on an activated carbon fiber (ACF) substrate, is developed for glyphosate (GLP) degradation under water under flow conditions. The reusable NiSnO3-gCN/ACF photocatalyst was synthesized using a simple wet impregnation and calcination method. The supported photocatalyst achieved 99% GLP-removal at 4 mL/min water flowrate and 1.25 g/m2 of photocatalyst loading in ACF. The photocatalyst showed a stable structure and repeat photocatalytic performance across 5 cycles despite prolonged visible light exposure under flow conditions. The materials stability is attributed to the effective dispersion of NiSnO3-gC3N4 in ACF, preventing the photocatalyst from elution in water flow. Radical trapping experiment revealed the superoxide and hydroxyl radicals as the primary reactive species in the GLP-degradation pathway. A plausible S-scheme mechanism was proposed for heterojunction formation, based on the high resolution deconvoluted spectra of X-ray photoelectron spectroscopy and the radical trapping experimental results. The inexpensive Sn-based perovskite synthesized in this study is indicated as an alternative to Ti-based perovskites for wastewater remediation application.

Highly Efficient Electrochemical Sensing of Acetaminophen by Cobalt Oxide-Embedded Nitrogen-Doped Hollow Carbon Spheres

With respect to sensor application investigations, hollow mesoporous carbon sphere-based materials of the spinel type of cobalt oxide (Co₃O₄) and heteroatom-doped materials are gaining popularity. In this contribution, dopamine hydrochloride (DA) and cobalt phthalocyanine (CoPc) precursors were employed to construct a highly homogeneous Co₃O₄-embedded N-doped hollow carbon sphere (Co₃O₄@NHCS) by a straightforward one-step polymerization procedure. The resulting Co₃O₄@NHCS materials may effectively tune the surface area, defect sites, and doping amount of N and Co elements by altering the loading amount of CoPc. The relatively high surface area, greater spherical wall thickness, enriched defect sites, and better extent of N and Co sites are all visible in the best 200 mg loaded Co₃O₄@NHCS-2 material. This leads to significant improvement in pyridine and graphitic N site concentrations, which offers exceptional electrochemical performance. Electrochemical analysis was used to study the electrocatalytic activity of Co₃O₄@NHCSs towards the sensing of pharmacologically active significant compounds (acetaminophen). Excellent sensor properties include the linear range (0.001-0.2 and 1.0-8.0 mM), sensitivity, limit of detection (0.07 and 0.11 μM), and selectivity in the modified Co₃O₄@NHCSs/GCE. The authentic sample (acetaminophen tablet) produces a satisfactory result when used practically.

Catalytic advancements in carbonaceous materials for bio-energy generation in microbial fuel cells: a review

Microbial fuel cells (MFCs) are a sustainable alternative for wastewater treatment and clean energy generation. The efficiency of the technology is dependent on the cathodic oxygen reduction reaction, where the sluggish reaction kinetics hampers its propensity. Carbonaceous materials with high electrical conductivity have been widely explored for oxygen reduction reaction (ORR) catalysts. Here, incorporating transition metal (TM) and heteroatom into carbon could further enhance the ORR activity and power generation in MFCs. Nitrogen (N)-doped carbons have also been a popular research hotspot due to abundant active sites formed, resulting in superior conductivity, stability, and catalytic activity over carbons. This review summarizes the progress in the carbon-based materials (primary focus on the cathode) for ORR and their utilization in MFCs. Furthermore, we discussed the conceptualization of MFCs and carbonaceous materials to instigate the ORR kinetics and power generation in MFC. Furthermore, prospects of carbon-based materials for actual application in bio-energy generation have been discussed. Carbonaceous catalysts and biomass-derived carbons exhibit good potential to replace precious Pt catalysts for ORR. M-N-C catalysts were found to be the most suitable catalysts. Electrocatalysts with MN_x sites are able to achieve excellent activity and high-power output by taking advantage of the active site exposure and rapid mass transfer rate. Moreover, the use of biomass-derived carbons/self-doped carbons could further reduce the overall cost of catalysts. It is anticipated that the research gaps and future perspectives discussed will show new avenues to develop excellent electrocatalysts for better performance and transformation of technology to industrial applications.

Fundamentals and recent progress in bioelectrochemical system-assisted biohythane production

Biohythane, a balanced mixture of 10%-30% v/v of hydrogen and 70%-90% v/v of methane, could be the backbone of an all-purpose future energy supply. Recently, bioelectrochemical systems (BES) became a new sensation among environmental biotechnology processes with the potential to sustainably generate biohythane. Therefore, to unleash its full potential for scaling up, researchers are consistently improving microbial metabolic pathways, novel reactors, and electrode designs. This review presents a detailed analysis of recently discovered fundamental mechanisms and science and engineering intervention of different strategies to improve the biohythane composition and production rate from BES. However, several milestones are to be achieved, for instance, improving electrode kinetics using efficient catalysts, engineered microbial communities, and improved reactor configurations, for commercializing this sustainable technology. Thus, a future perspective section is included to recommend novel research lines, mainly focusing on the microbial communities and the efficient electrocatalysts, to enhance reactor performance.

A critical review on microbe-electrode interactions towards heavy metal ion detection using microbial fuel cell technology

Implicit interaction of electroactive microbes with solid electrodes is an interesting phenomenon in nature, which supported development of bioelectrochemical systems (BESs), especially the microbial fuel cell (MFCs) for valorization of low-value waste streams into bioelectricity. Intriguingly, the metabolism of interacted microbes with electrode is affected by the microenvironment at electrodes, which influences the current response. For instance, when heavy metal ions (HMIs) are imposed in the medium, the current production decreases due to their intrinsic toxic effect. This event provides an immense opportunity to utilize MFC as a sensor to selectively detect HMIs in the environment, which has been explored vastly in recent decade. In this review, we have concisely discussed the microbial interaction with electrodes and mechanism of detection of HMIs using an MFC. Recent advancement in sensing elements and their application is elaborated with a future perspective section for follow-up research and development in this field.

Synergistically enhanced oxygen reduction reaction and oxytetracycline mineralization by FeCoO/GO modified cathode in microbial fuel cell

The widespread application of antibiotics have aroused serious pollution over the world. Microbial fuel cell (MFC) air cathode was able to simultaneously recover electricity and perform advanced oxidation of pollutions through electro-Fenton (EF). This study synthesized an iron‑cobalt oxide and graphene composite (FeCoO/GO), which possessed high electrochemical activity and ORR catalytic performance. The uniform decoration of FeCoO/GO in MFC air cathode distinctly increased the electricity generation (4.5 times higher than carbon felt) and oxytetracycline (OTC) degradation and detoxification (1.33 times higher). FeCoO/GO boosted the H₂O₂ generation from ORR (1.14 times higher than CF) and mineralization efficiency of OTC (2.63 times higher than CF). UPLC-QTOF-MS verified that OTC was degraded and mineralized through decarboxylation, demethylation, and carbon ring cleavage by the oxidation of ·OH. The enhanced degradation of OTC was not only benefited from the increased ORR catalytic performance, but also the excellent H₂O₂ catalytic activity by Fe and Co for ·OH generation. This study demonstrated an effective strategy by decorating FeCoO/GO in MFC air cathode for the synergistically enhanced ORR and OTC degradation and detoxification, giving promising guidance for the deep removal of antibiotic pollutants in the environment.

B-doped graphene quantum dots implanted into bimetallic organic framework as a highly active and robust cathodic catalyst in the microbial fuel cell

Developing efficient and durable oxygen reduction reaction (ORR) cathodic catalysts plays an essential role in application of microbial fuel cells (MFCs). Herein, the B-doped graphene quantum dots implanted into bimetallic organic framework (BGQDs/MOF-t) are fabricated by a facile electro-deposition. Results show that, the in-situ growth of FeCoMOF on nickel foam can effectively assist construction of nanoflowers with compact connections, thus improves the conductivity. More importantly, this nano-network can serve as the template for the implantation of BGQDs through powerful interface of M-O-C bonding, avoiding π-π rearrangement and providing efficient charge transfer and abundant edge active sites. Benefitting from the enhanced electrode/electrolyte transport interface, abundant catalytic sites and low charge transfer resistance, the BGQDs/MOF-15 exhibits excellent ORR activity, superior to commercial Pt/C catalyst. In the MFC with the BGQDs/MOF-15 cathode, the maximum power density of 703.55 mW m^-2 is achieved, which is 1.53 times of that of the Pt/C cathode. In addition, the BGQDs/MOF-15 cathode maintains great stability over 800 h, while that of Pt/C reduces to 61% of the initial voltage. This work opens new opportunities for developing efficient and durable MOF-derived ORR catalyst.

Degradation of high concentration starch and biocathode autotrophic denitrification using photo microbial fuel cell

In the study, a dual-chamber photo MFC was constructed with a photosynthetic bacteria consortium PB-Z and a heterotrophic nitrifier C16 as anode and cathode inoculant, respectively. The electron released from starch degradation in the anode by photosynthetic bacteria was transferred to the cathode, which was utilized by the nitrifying bacteria C16 to realize autotrophic denitrification. Lower resistance was more conducive to the electron transfer and pollutants removal. Comparing with natural light, continuous light greatly promoted starch degradation by the photosynthetic bacteria in the anode and the denitrification by the nitrifying bacteria in the cathode. Under continuous light and external resistance of 500 Ω, high concentration starch was degraded by photosynthetic bacteria PB-Z and the COD removal efficiency reached up to 88.45% within 12 d, and nitrate of 95.8% was removed within 4 d by autotrophic denitrification by heterotrophic nitrifier C16. The study provides some enlightenment and reference for the application of MFC in the field of wastewater treatment.

Critical insights into psychrophilic anaerobic digestion: Novel strategies for improving biogas production

Anaerobic digestion (AD) under psychrophilic temperature has only recently garnered deserved attention. In major parts of Europe, USA, Canada and Australia, climatic conditions are more suited for psychrophilic (<20 ℃) rather than mesophilic (35 - 37 ℃) and thermophilic (55 - 60 ℃) AD. Low temperature has adverse effects on important cellular processes which may render the cell biology inactive. Moreover, cold climate can also alter the physical and chemical properties of wastewater, thereby reducing the availability of substrate to microbes. Hence, the use of low temperature acclimated microbial biomass could overcome thermodynamic constraints and carry out flexible structural and conformational changes to proteins, membrane lipid composition, expression of cold-adapted enzymes through genotypic and phenotypic variations. Reduction in organic loading rate is beneficial to methane production under low temperatures. Moreover, modification in the design of existing reactors and the use of hybrid reactors have already demonstrated improved methane generation in the lab-scale. This review also discusses some novel strategies such as direct interspecies electron transfer (DIET), co-digestion of substrate, bioaugmentation, and bioelectrochemical system assisted AD which present promising prospects. While DIET can facilitate syntrophic electron exchange in diverse microbes, the addition of organic-rich co-substrate can help in maintaining suitable C/N ratio in the anaerobic digester which subsequently can enhance methane generation. Bioaugmentation with psychrophilic strains could reduce start-up time and ensure daily stable performance for wastewater treatment facilities at low temperatures. In addition to the technical discussion, the economic assessment and future outlook on psychrophilic AD are also highlighted.

An insight into the bioelectrochemical photoreduction of CO2 to value-added chemicals

Goal of sustainable carbon neutral economy can be achieved by designing an efficient CO₂ reduction system to generate biofuels, in particular, by mimicking the mechanism of natural photosynthesis using semiconducting nanomaterials interfaced with electroactive bacteria (EAB) in a photosynthetic microbial electrosynthesis (PMES) system. This review paper presents an overview of the recent advancements in the biohybrid photoanode and photocathode materials. We discuss the reaction mechanism observed at photoanode and photocathode to enhance our understanding on the solar driven MES. We extend the discussion by showcasing the potential activity of EABs toward high selectivity and production rates for desirable products by manipulating their genomic sequence. Additionally, the critical challenges associated in scaling up the PMES system including the strategies for diminution of reactive oxygen species, low solubility of CO₂ in the typical electrolytes, low selectivity of product species are presented along with the suggestions of alternative strategies to achieve economically viable generation of (bio)commodities.

Recent Progresses in Oxygen Reduction Reaction Electrocatalysts for Electrochemical Energy Applications

Abstract

Electrochemical energy storage systems such as fuel cells and metal–air batteries can be used as clean power sources for electric vehicles. In these systems, one necessary reaction at the cathode is the catalysis of oxygen reduction reaction (ORR), which is the rate-determining factor affecting overall system performance. Therefore, to increase the rate of ORR for enhanced system performances, efficient electrocatalysts are essential. And although ORR electrocatalysts have been intensively explored and developed, significant breakthroughs have yet been achieved in terms of catalytic activity, stability, cost and associated electrochemical system performance. Based on this, this review will comprehensively present the recent progresses of ORR electrocatalysts, including precious metal catalysts, non-precious metal catalysts, single-atom catalysts and metal-free catalysts. In addition, major technical challenges are analyzed and possible future research directions to overcome these challenges are proposed to facilitate further research and development toward practical application.

Graphic Abstract

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.

Multi-walled carbon nanotube and carbide-derived carbon supported metal phthalocyanines as cathode catalysts for microbial fuel cell applications

Metal phthalocyanine (CoPc and FePc) modified MWCNT or CDC materials were explored as superior cathode catalysts for MFC technology.