Advancements on sustainable microbial fuel cells and their future prospects: A review

Potential application of bioelectrochemical systems in cold environments

Globally, more than half of the world's regions and populations inhabit psychrophilic and seasonally cold environments. Lower temperatures can inhibit the metabolic activity of microorganisms, thereby restricting the application of traditional biological treatment technologies. Bioelectrochemical systems (BES), which combine electrochemistry and biocatalysis, can enhance the resistance of microorganisms to unfavorable environments through electrical stimulation, thus showing promising applications in low-temperature environments. In this review, we focus on the potential application of BES in such environments, given the relatively limited research in this area due to temperature limitations. We select microbial fuel cells (MFC), microbial electrolytic cells (MEC), and microbial electrosynthesis cells (MES) as the objects of analysis and compare their operational mechanisms and application fields. MFC mainly utilizes the redox potential of microorganisms during substance metabolism to generate electricity, while MEC and MES promote the degradation of refractory substances by augmenting the electrode potential with an applied voltage. Subsequently, we summarize and discuss the application of these three types of BES in low-temperature environments. MFC can be employed for environmental remediation as well as for biosensors to monitor environmental quality, while MEC and MES are primarily intended for hydrogen and methane production. Additionally, we explore the influencing factors for the application of BES in low-temperature environments, including operational parameters, electrodes and membranes, external voltage, oxygen intervention, and reaction devices. Finally, the technical, economic, and environmental feasibility analyses reveal that the application of BES in low-temperature environments has great potential for development.

Key issues to consider toward an efficient constructed wetland-microbial fuel cell: the idea and the reality

The research on constructed wetland (CW) and microbial fuel cell (MFC) has been separate studies worldwide with crucial achievements being made in both fields. Due to environmentally friendly feature (of CW) and rich microbial population and excellent electrode catalytic activity (of MFC), CW and MFC have their own anticipated application prospect in wastewater purification and biological electricity generation. More significantly, the idea of embedding MFC into CW to form CW-MFC expands the scope for both of them and this has received much interest in recent years due to its striking features of sewage treatment efficiency, electricity generation, sustainability, and environmental friendliness. The increasing interest and the lack of soul of CW-MFC emerging to the new researchers reflect the need to recall the idea and summarize its development with regard to achieving its reality via some key issues This forms the basis of the paper. The paper also includes how to enhance the efficiency of electricity generation and supplement energy consumption, the degradation of emerging pollutants, and the degradation mechanism as well as the potential joint application of CW-MFC with other treatment technique. A mass of CW-MFC design parameters has been synthesized from the literature. Challenges and potential directions of CW-MFC in the future are prospected. It is expected that the paper can serve as a linkage for bridging knowledge gaps for further studies of CW-MFC.

Effects of municipal waste compost on microbial biodiversity and energy production in terrestrial microbial fuel cells

Microbial Fuel Cells (MFCs) transform organic matter into electricity through microbial electrochemical reactions catalysed on anodic and cathodic half-cells. Terrestrial MFCs (TMFCs) are a bioelectrochemical system for bioelectricity production as well as soil remediation. In TMFCs, the soil is the ion-exchange electrolyte, whereas a biofilm on the anode oxidises organic matter through electroactive bacteria. Little is known of the overall microbial community composition in a TMFC, which impedes complete exploitation of the potential to generate energy in different soil types. In this context, an experiment was performed to reveal the prokaryotic community structure in single chamber TMFCs with soil in the presence and absence of a municipal waste compost (3% w/v). The microbial community was assessed on the anode and cathode and in bulk soil at the end of the experiment (54 days). Moreover, TMFC electrical performance (voltage and power) was also evaluated over the experimental period, varying the external resistance to improve performance. Compost stimulated soil microbial activity, in line with a general increase in voltage and power. Significant differences were observed in the microbial communities between initial soil conditions and TMFCs, and between the anode, cathode and bulk soil in the presence of the compost. Several electroactive genera (Bacillus, Fulvivirga, Burkholdeira and Geobacter) were found at the anode in the presence of compost. Overall, the use of municipal waste compost significantly increased the performance of the MFCs in terms of electrical power and voltage generated, not least thanks to the selective pressure towards electroactive bacteria on the anode.

Simultaneous naphthalene degradation and electricity production in a biowaste-powered microbial fuel cell

Naphthalene is a very common and hazardous environmental pollutant, and its biodegradation has received serious attention. As demonstrated in this study, naphthalene-contaminated wastewater can be biodegraded using a microbial fuel cell (MFC). Furthermore, the potential of MFC for electricity generation appears to be a promising technology to meet energy demands other than those produced from fossil fuels. Nowadays, efforts are being made to improve the overall performance of MFC by integrating biowaste materials for anode fabrication. In this study, palm kernel shell waste was used to produce palm kernel shell-derived graphene oxide (PKS-GO) and palm kernel shell-derived reduced graphene oxide (PKS-rGO), which were then fabricated into anode electrodes to improve the system's electron mobilization and transport. The MFC configuration with the PKS-rGO anode demonstrated greater energy production potential, with a maximum power density of 35.11 mW/m2 and a current density of 101.76 mA/m2, compared to the PKS-GO anode, which achieved a maximum power density of 17.85 mW/m2 and a current density of 72.56 mA/m2. Furthermore, there is simultaneous naphthalene biodegradation with energy production, where the biodegradation efficiency of naphthalene with PKS-rGO and PKS-GO is 85.5%, and 79.7%, respectively. In addition, the specific capacitance determined from the cyclic voltammetry curve revealed a value for PKS-rGO of 2.23 × 10-4 F/g, which is also higher than the value for PKS-GO (1.57 × 10-4 F/g) on the last day of operation. Anodic microbial analysis shows that electrogens thrive in the MFC process. Finally, a comparison with previous literature and the future prospects of the study are also presented.

Iron/iron carbide coupled with S, N co-doped porous carbon as effective oxygen reduction reaction catalyst for microbial fuel cells

As a novel energy device, microbial fuel cells (MFCs) have attracted much attention for their dual functions of electricity generation and sewage treatment. However, the sluggish oxygen reduction reaction (ORR) kinetic on the cathode have hindered the practical application of MFCs. In this work, metallic organic framework derived carbon framework co-doped by Fe, S, N tri-elements was used as alternative electrocatalyst to the conventional Pt/C cathode catalyst in pH-universal electrolytes. The amount of thiosemicarbazide from 0.3 to 3 g determined the surface chemical property, and therefore the ORR activity of FeSNC catalysts. The sulfur/nitrogen doping and Fe/Fe₃C embedded in carbon shell was characterized by X-ray photoelectron spectroscopy and transmission electron microscopy. The synergy of iron salt and thiosemicarbazide contributed to the improvement of nitrogen and sulfur doping. Sulfur atoms were successfully doped into the carbon matrix and formed a certain amount of thiophene- and oxidized-sulfur. The optimal FeSNC-3 catalyst synthesized with 1.5 g of thiosemicarbazide exhibited the highest ORR activity with a positive half wave potential of 0.866 V in alkaline and 0.691 V (vs. Reversible Hydrogen Electrode) in neutral electrolyte, which both outperformed the commercial Pt/C catalyst. However, as the amount of thiosemicarbazide surpassed 1.5 g, the catalytic performance of FeSNC-4 was lowered, and this could be assigned to the decreased defects and low specific surface area. The excellent ORR performance in neutral medium urged FeSNC-3 as good cathode catalyst in single chambered MFC (SCMFC). It showed the highest maximum power density of 2126 ± 100 mW m^-2, excellent output stability of 8.14% decline in 550 h, chemical oxygen demand removal of 90.7 ± 1.6% and coulombic efficiency of 12.5 ± 1.1%, all superior to those of benchmark SCMFC-Pt/C (1637 ± 35 mW m^-2, 15.4%, 88.9 ± 0.9%, and 10.2 ± 1.1%). These outstanding results were associated to the large specific surface area and synergistic interaction of multiple active sites, like Fe/Fe₃C, Fe-N₄, pyridinic N, graphite N and thiophene-S.

Bioelectrochemical stability improvement by Ce-N modified carbon-based cathode in high-salt stress and mechanism research

Although microbial fuel cells (MFCs) have potential for high-salt wastewater treatment, their application is limited by poor salt tolerance, deactivation and unstable catalytic performance. This study designed Ce-C, N-C, and Ce-N modified activated carbon (Ce-N-C) based on the catalytic mechanism and salt tolerance performance of Ce and N elements to address these limitations. With activated carbon (AC) as the control, this study analyzed the stability of the four cathodes under different salinity environments using norfloxacin (NOR) as a probe to assess the effect of cathodes and salinity on MFC degradation performance. After three months, comparing with other three cathodes, the Ce-N-C cathode demonstrated superior and stable electrochemical and power generation performance. In particular, the advantages of Ce-N-C in high-salt (600 mM NaCl) environment is more significant than no-salt or low-salt. The potential of Ce-N-C-End at current density of 0 was 14.0% higher than AC-End, and the power density of the MFC with Ce-N-C cathode was 105.7 mW/m², which was 3.1 times higher than AC. Also, the stability of NOR removal under the function of Ce-N-C improved with the increase of NaCl concentration or operation time. The CeO₂(111) crystal form, N-Ce-O bond and pyridine N might be the key factors in improving the catalytic performance and salt tolerance of the Ce-N modified carbon-based cathode using XPS and XRD analysis.

Polymer-Based Conductive Nanocomposites for the Development of Bioanodes Using Membrane-Bound Enzyme Systems of Bacteria Gluconobacter oxydans in Biofuel Cells

The development of biofuel cells (BFCs) currently has high potential since these devices can be used as alternative energy sources. This work studies promising materials for biomaterial immobilization in bioelectrochemical devices based on a comparative analysis of the energy characteristics (generated potential, internal resistance, power) of biofuel cells. Bioanodes are formed by the immobilization of membrane-bound enzyme systems of Gluconobacter oxydans VKM V-1280 bacteria containing pyrroloquinolinquinone-dependent dehydrogenases into hydrogels of polymer-based composites with carbon nanotubes. Natural and synthetic polymers are used as matrices, and multi-walled carbon nanotubes oxidized in hydrogen peroxide vapor (MWCNTox) are used as fillers. The intensity ratio of two characteristic peaks associated with the presence of atoms C in the sp³ and sp² hybridization for the pristine and oxidized materials is 0.933 and 0.766, respectively. This proves a reduced degree of MWCNTox defectiveness compared to the pristine nanotubes. MWCNTox in the bioanode composites significantly improve the energy characteristics of the BFCs. Chitosan hydrogel in composition with MWCNTox is the most promising material for biocatalyst immobilization for the development of bioelectrochemical systems. The maximum power density was 1.39 × 10^-5 W/mm², which is 2 times higher than the power of BFCs based on other polymer nanocomposites.

Tricks and tracks in waste management with a special focus on municipal landfill leachate: Leads and obstacles

Landfilling is the most widely used disposal method for municipal solid waste around the world. The main disadvantage of this strategy is formation of leachate, among other aspects. Landfill leachate contains highly toxic and bio-refractory substances that are detrimental to the environment and human health. Hence, the risk(s) of discharging potentially harmful landfill leachate into the environment need to be assessed and measured in order to make effective choices about landfill leachate management and treatment. In view of this, the present review aims to investigate (a) how landfill leachate is perceived as an emerging concern, and (b) the stakeholders' mid- to long-term policy priorities for implementing technological and integrative solutions to reduce the harmful effects of landfill leachate. Because traditional methods alone have been reported ineffective, and in response to emerging contaminants and stringent regulations, new effective and integrated leachate treatments have been developed. This study gives a forward-thinking of the accomplishments and challenges in landfill leachate treatment during the last decade. It also provides a comprehensive compilation of the formation and characterization of landfill leachate, the geo-environmental challenges that it raises, as well as the resource recovery and industrial linkage associated with it in order to provide an insight into its sustainable management.

Critical Review on Nanomaterials for Enhancing Bioconversion and Bioremediation of Agricultural Wastes and Wastewater

Anaerobic digestion (AD), microalgae cultivation, and microbial fuel cells (MFCs) are the major biological processes to convert organic solid wastes and wastewater in the agricultural industry into biofuels, biopower, various biochemical and fertilizer products, and meanwhile, recycle water. Various nanomaterials including nano zero valent irons (nZVIs), metal oxide nanoparticles (NPs), carbon-based and multicompound nanomaterials have been studied to improve the economics and environmental sustainability of those biological processes by increasing their conversion efficiency and the quality of products, and minimizing the negative impacts of hazardous materials in the wastes. This review article presented the structures, functionalities and applications of various nanomaterials that have been studied to improve the performance of AD, microalgae cultivation, and MFCs for recycling and valorizing agricultural solid wastes and wastewater. The review also discussed the methods that have been studied to improve the performance of those nanomaterials for their applications in those biological processes.