Comparison of electrochemical performances and microbial community structures of two photosynthetic microbial fuel cells

Developing a plant microbial fuel cell by planting water spinach in a hanging-submerged plant pot system

To plant crops (especially dry crops such as water spinach) with concomitant electricity recovery, a hanging-submerged-plant-pot system (HSPP) is developed. The HSPP consists of a soil pot (anodic) partially submerged under the water surface of a cathode tank. The microbial communities changed with conditions were also investigated. It was found that with chemical fertilizers the closed-circuit voltage (CCV, with 1 kΩ) was stable (approximately 250 mV) within 28 d; however, without fertilizer, the water spinach could adjust to the environment to obtain a better power output (approximately 3 mW m-2) at day 28. The microbial-community analyses revealed that the Pseudomonas sp. was the only exoeletrogens found in the anode pots. Using a secondary design of HSPP, for a better water-level adjustment, the maximum power output of each plant was found to be approximately 27.1 mW m-2. During operation, high temperature resulted in low oxygen solubility, and low CCV as well. At this time, it is yet to be concluded whether the submerged water level significantly affects electricity generation.

Deciphering the differentiated performance on electricity generation and COD degradation by Rhodopseudomonas-dominated bioanode in light or dark

Anoxygenic phototrophic bacteria is a promising catalyst for constructing bioanode, but the mixed culture with non-photosynthetic bacteria is inevitable in an open environment application. In this study, a Rhodopseudomonas-dominated mixed culture with other electrogenic bacteria was investigated for deciphering the differentiated performance on electricity generation in light or dark conditions. The kinetic study showed that reaction rate of OM degradation was 9 times higher than that under dark condition, demonstrating that OM degradation was enhanced by photosynthesis. However, CE under light condition was lower. It indicated that part of OM was used to provide hydrogen donors for the fixation of CO2 or hydrogen production in photosynthesis, decreasing the OM used for electron transfer. In addition, higher COD concentration was not conducive to electricity generation. EIS analysis demonstrated that higher OM concentration would increase Rct to hinder the transfer of electrons from bacteria to the electrode. Indirect and direct electron transfer were revealed by CV analysis for light and dark biofilm, respectively, and nanowires were also observed by SEM graphs, further revealing the differentiate performance. Microbial community analysis demonstrated Rhodopseudomonas was dominated in light and decreased in dark, but Geobacter increased apparently from light to dark, resulting in different power generation performance. The findings revealed the differentiated performance on electricity generation and pollutant removal by mixed culture of phototrophic bacteria in light or dark, which will improve the power generation from photo-microbial fuel cells.

Photo-assisted microbial fuel cell systems: critical review of scientific rationale and recent advances in system development

Bioelectrochemical systems such as microbial fuel cells (MFCs) have gained extensive attention due to their abilities to simultaneously treat wastewater and generate renewable energy resources. Recently, to boost the system performance, the photoelectrode has been incorporated into MFCs for effectively exploiting the synergistic interaction between light and microorganisms, and the resultant device is known as photo-assisted microbial fuel cells (photo-MFCs). Combined with the metabolic reaction of organic compounds by microorganisms, photo-MFCs are capable of simultaneously converting both chemical energy and light energy into electricity. This article aims to systematically review the recent advances in photo-MFCs, including the introduction of specific photosynthetic microorganisms used in photo-MFCs followed by the discussion of the fundamentals and configurations of photo-MFCs. Moreover, the materials used for photoelectrodes and their fabrication approaches are also explored. This review has shown that the innovative strategy of utilizing photoelectrodes in photo-MFCs is promising and further studies are warranted to strengthen the system stability under long-term operation for advancing practical application.

Influence of Light Color on Power Generation and Microalgae Growth in Photosynthetic Microbial Fuel Cell with Chlorella Vulgaris Microalgae as Bio-Cathode

Photosynthetic microbial fuel cell (PMFC) is an environmentally friendly sustainable technique for simultaneous wastewater treatment and power recovery. PMFC utilizes the microalgae to generate oxygen by photosynthesis process in the biocathode. Light sources and intensities have direct effect on chlorophyll pigment formation, photosynthesis processes and microalgae growth. In this study, Chlorella vulgaris was utilized as biocathode in PMFC fed with actual slaughterhouse wastewater. The biocathode was illuminated with florescent light as well as yellow, red and blue LED lights with light intensities of 67.46, 47.03, 26.18 and 4.70 µmol/m.s, respectively. Power output and microalgae growth were considered in evaluating the PMFC performance. Results demonstrated that the highest power output was 217.04 mW/m² generated under florescent light compared to 28.41, 171.08, and 21.65 mW/m² observed under yellow, red and blue LEDs, respectively. Additionally, statistical analysis was performed using fifth-degree polynomial model which fitted well the experimental data with a determination coefficient (R²) > 0.97. The results reflected a high confidence level in depicting the growth mechanism of Chlorella vulgaris under lighting sources with different light colors and intensities.

Anode Modification with Fe2O3 Affects the Anode Microbiome and Improves Energy Generation in Microbial Fuel Cells Powered by Wastewater

This study investigated how anode electrode modification with iron affects the microbiome and electricity generation of microbial fuel cells (MFCs) fed with municipal wastewater. Doses of 0.0 (control), 0.05, 0.1, 0.2, and 0.4 g Fe2O3 per the total anode electrode area were tested. Fe2O3 doses from 0.05 to 0.2 g improved electricity generation; with a dose of 0.10 g Fe2O3, the cell power was highest (1.39 mW/m2), and the internal resistance was lowest (184.9 Ω). Although acetate was the main source of organics in the municipal wastewater, propionic and valeric acids predominated in the outflows from all MFCs. In addition, Fe-modification stimulated the growth of the extracellular polymer producers Zoogloea sp. and Acidovorax sp., which favored biofilm formation. Electrogenic Geobacter sp. had the highest percent abundance in the anode of the control MFC, which generated the least electricity. However, with 0.05 and 0.10 g Fe2O3 doses, Pseudomonas sp., Oscillochloris sp., and Rhizobium sp. predominated in the anode microbiomes, and with 0.2 and 0.4 g doses, the electrogens Dechloromonas sp. and Desulfobacter sp. predominated. This is the first study to holistically examine how different amounts of Fe on the anode affect electricity generation, the microbiome, and metabolic products in the outflow of MFCs fed with synthetic municipal wastewater.

General aspects and novel PEMss in microbial fuel cell technology: A review

The current scenario of energy production is mostly shifted towards sustainable renewable energy sources. Other than the energy production from natural resources such as sun, wind and water, microbial fuel cell system (MFC) has earned attraction in recent times. These microbial fuel cell systems are bioelectrochemical cell that possesses a unique ability to generate power as well as treats wastewater simultaneously. In this paper, an overview of the microbial fuel cell system and the effect of significant components on the performance of microbial fuel cell systems are reviewed. Firstly, the importance of the MFC system in power generation, its components, the working principle and various configurations of the MFC were briefly introduced. Biofilm plays a major role in the MFC system. Thus the importance of bio film, bio film formation and characterization techniques are summarised. Further, the review mainly addresses the mechanism of conventional and novel membrane materials on the performance of the MFC system. In addition, special emphasis on ceramic-based materials in the MFC system is presented. Finally, recent applications of the MFC systems are discussed.

Prospects and trends in bioelectrochemical systems: Transitioning from CO2 towards a low-carbon circular bioeconomy

Resource scarcity and climate change are the most quested topics in view of environmental sustainability. CO2 sequestration through bioelectrochemical systems is an attractive option for fostering bioeconomy development upon several value-added products generation. This review details the state-of-the-art of bioelectrochemical systems for resource recovery from CO2 along with various biocatalysts capable of utilizing CO2. Two bioprocesses (photo-electrosynthesis and chemolithoelectrosynthesis) were discussed projecting their potential for biobased economy development from CO2. Significance of adopting circular strategies for efficient resource recycling, intensifying product value, integrations/interlinking of multiple process chains for the development of circular bioeconomy were discussed. Existing constrains as well as outlook for near establishment of circular bioeconomy from CO2 is presented by weighing fore-sighted plans with current actions. Need for developing CO2-based circular bioeconomy via innovative business models by analyzing social, technical, environmental and product related aspects are also discussed providing a roadmap of gaps to pursue for attaining practicality.

Exploring indigenous freshwater chlorophytes in integrated biophotovoltaic system for simultaneous wastewater treatment, heavy metal biosorption, CO2 biofixation and biodiesel generation

The study explored the combined photosynthetic activities of two green microalgal species, Tetradesmus obliquus and Tetradesmus reginae, on an integrated biophotovoltaic (BPV) platform for simultaneous wastewater treatment, toxic metal biosorption, carbon biofixation, bioelectricity generation and biodiesel production. The experimental setup comprised of a dual-chambered BPV with copper anode surrounded by T. obliquus in BG11 media, and copper cathode with T. reginae in municipal wastewater separated by Nafion 117 membrane. The study reported a maximum power density of 0.344 Wm^-2 at a cell potential of 0.415 V with external resistance of 1000 Ω and 0.3268 V maximum open-circuit voltage. The wastewater electrical conductivity and pH increased from 583 ± 22 to 2035 ± 29.31 mS/cm and 7.403 ± 0.174 to 8.263 ± 0.055 respectively, signifying increased photosynthetic and electrochemical activities. Residual nitrogen, phosphorus, chemical oxygen demand, arsenic, cadmium, chromium and lead removal efficiencies by T. reginae were 100%, 80.68%, 71.91%, 47.6%, 88.82%, 71.24% and 92.96%, respectively. T. reginae accumulated maximum biomass of 0.605 ± 0.033 g/L with a CO₂ biosequestration rate of 0.166 ± 0.010 gCO₂/L/day and 42.40 ± 1.166% lipid content. Methyl palmitate, methyl undecanoate and 13-octadecenoic acid with relative abundances of 37.24%, 24.80% and 12.02%, respectively were confirmed.

Co, N co-doped hierarchical porous carbon as efficient cathode electrocatalyst and its impact on microbial community of anode biofilm in microbial fuel cell

The exploration of low-cost, long-term stable, and highly electrochemically active cathode catalysts is important for the practical application of microbial fuel cell (MFC). In this work, a series of the 3D hierarchical porous Co-N-C (3DHP Co-N-C) materials are designed and synthesized by a metal-organic framework ZIF-67 as a precursor and SiO₂ sphere of different sizes as the hard template. The 3DHP Co-N-C-2 with 129 nm macropore exhibits excellent ORR performance in 0.1 M KOH solution with a half-wave potential of 0.80 V vs. RHE and superior durability than Pt/C (20%) due to the specific macropore-mesopore-micropore structure that exposes a large number of active sites and accelerates the electrolyte transport and oxygen diffusion. The MFC with 3DHP Co-N-C-2 as the cathode catalysts shows excellent performance with a maximum power density of 426.9±7.87 mW m^-2 and favorable durability after 50 d of operation. In addition, 16s rDNA results reveal the presence of different dominant electrogenic bacteria and different abundance of important non-electrogenic bacteria in the anode biofilm in MFCs using cathode catalysts with different ORR activity. And 3DHP Co-N-C-2 was found to be beneficial to the synergistic effect of electrogenic and non-electrogenic bacteria. This study explores electrocatalysts in terms of both electrocatalytic activity and anode microorganisms, providing new and comprehensive insights into the power generation of MFC.

Recent progress in microbial fuel cells for industrial effluent treatment and energy generation: Fundamentals to scale-up application and challenges

Microbial fuel cells (MFCs) technology have the potential to decarbonize electricity generation and offer an eco-friendly route for treating a wide range of industrial effluents from power generation, petrochemical, tannery, brewery, dairy, textile, pulp/paper industries, and agro-industries. Despite successful laboratory-scale studies, several obstacles limit the MFC technology for real-world applications. This review article aimed to discuss the most recent state-of-the-art information on MFC architecture, design, components, electrode materials, and anodic exoelectrogens to enhance MFC performance and reduce cost. In addition, the article comprehensively reviewed the industrial effluent characteristics, integrating conventional technologies with MFCs for advanced resource recycling with a particular focus on the simultaneous bioelectricity generation and treatment of various industrial effluents. Finally, the article discussed the challenges, opportunities, and future perspectives for the large-scale applications of MFCs for sustainable industrial effluent management and energy recovery.

Blending of microbial inocula: An effective strategy for performance enhancement of clayware Biophotovoltaics microbial fuel cells

Performance of clayware Biophotovoltaics (BPVs) with three variants of inocula namely anoxygenic photosynthetic bacteria (APB) rich Effective microbes (EM), Up-flow anaerobic sludge blanket reactor (UASB) sludge, SUPER-MIX the blend of EM and UASB inoculum were evaluated on the basis of electrical output and pollutant removal. SUPER-MIX inocula with microbial community comprising of 28.42% APB and 71.58% of other microbes resulted in peak power density of 275 mW/m², 69.3 ± 1.74% Coulombic efficiency and 91 ± 3.96% organic matter removal. The higher performance of the SUPER-MIX than EM and UASB inocula was due to the syntrophic associations of the various APBs and other heterogenous microorganisms in perfect blend which improved biocatalytic electron transfer, electro-kinetic activities with higher redox current and bio-capacitance. The promising performance of clayware BPVs with SUPER-MIX inocula indicate the possibility of BPVs to move towards the scale-up process to minimize the investment towards pure culture by effective blending strategies of inocula.

Enhanced performance and degradation of wastewater in microbial fuel cells using titanium dioxide nanowire photocathodes

This paper explores the decolorization of dye wastewaters and electricity generation using dual-chamber microbial fuel cells (MFCs) with titanium dioxide nanowire (TiO2 NW) photocathodes. TiO2 NW cathodes under ultraviolet light are observed to enhance the reduction of azo dye Active Red 30 (AR 30) and electricity generation. The analysis of electrochemical impedance spectra (EIS) indicates acceleration of the electron transfer processes of photoelectrode reduction by the photocatalysis of TiO2 NWs, with polarization resistance of the photocathode being 10.45 Ω under light irradiation from 294 Ω in the dark. Ultraviolet-visible light spectroscopy shows that the maximum degradation of the MFCs is 78.1%; the azo bond of AR 30 may be cleaved by photoelectrons generated by light irradiation of the illuminated TiO2 NW photocathode. The electricity produced by microbial fuel cells (MFCs) is expected to enhance the reductive decolorization of the azo dye AR 30 solution.

The imidacloprid remediation, soil fertility enhancement and microbial community change in soil by Rhodopseudomonas capsulata using effluent as carbon source

The effects of Rhodopseudomonas capsulata (R. capsulata) in the treated effluent of soybean processing wastewater (SPW) on the remediation of imidacloprid in soil, soil fertility, and the microbial community structure in soil were studied. Compared with the control group, with the addition of effluent containing R. capsulata, imidacloprid was effectively removed, soil fertility was enhanced, and the microbial community structure was improved. Molecular analysis indicated that imidacloprid could exert induction effects on expression of cpm gene and regulation effects on the synthesis of cytochrome P450 monooxygenases (P450) by activating HKs gene in two-component system (TCS). For R. capsulata, this induction process required 1 day. The synthesis of P450 occurred 1 day after inoculation, because R. capsulata are a type of archaea and imidacloprid is an environmental stress. Before expression of the cpm gene and synthesis of P450, R. capsulata need a period of time to adapt to external imidacloprid stimulation. However, the lack of organic matter in the soil cannot sustain R. capsulata growth for more than 1 day. In four groups with added effluent, the remaining organic matter in the effluent provided a sufficient carbon source and energy for R. capsulata. Five days later, the microbial community structure was improved by R. capsulata in the soil. The new technique could be used to remediate imidacloprid, enhance soil fertility, treat SPW and realize the recycling and reuse of wastewater and R. capsulata cells.

Simultaneous energy harvest and nitrogen removal using a supercapacitor microbial fuel cell

The insufficient removal of pollutants and bioelectricity production have become a bottleneck for high-concentration saline wastewater treatment through microbial fuel cell (MFC) technology. Herein, a novel supercapacitor MFC (SC-MFC) was constructed with carbon nanofibers composite electrodes to investigate pollutant removal ability, power generation, and electrochemical properties using real landfill leachate. The possible extracellular electron transfer and nitrogen element conversion pathways in the bioanode were also analyzed. Results showed that the SC-MFC had higher pollutant removal rates (COD: 59.4 ± 1.2%; NH₄⁺-N: 78.2 ± 1.6%; and TN: 77.8 ± 1.2%), smaller internal impedance R_t (∼6 Ω), higher exchange current density i₀ (2.1 × 10^-4 A cm^-2), and a larger catalytic current j₀ (704 μA cm^-2) with 60% leachate than those with 10% and 20% leachate, resulting in a power output of 298 ± 22 mW m^-2. Ammonium could be incorporated by chemoautotrophic bacteria to produce organic compounds that could be further utilized by heterotrophs to generate power when biodegradable organic matters are depleted. Three conversion pathways of nitrogen might be involved, including NH₄⁺ diffusion from anode to cathode chamber, nitrification, and the denitrification process. Additionally, cyclic voltammetry tests showed that both the direct electron transfer (DET) and the mediator electron transfer in bioanode were involved and dominated by DET. The microbial analysis revealed that the bioanode was dominated by salt-tolerant denitrifying bacteria (38.5%), which was deduced to be the key functional microorganism. The electrochemically active bacteria decreased significantly from 61.7% to 4% over three stages of leachate treatment. Overall, the SC-MFC has demonstrated the potential for wastewater treatment along with energy harvesting and provides a new avenue toward sustainable leachate management.

Microbial Structure and Energy Generation in Microbial Fuel Cells Powered with Waste Anaerobic Digestate

Development of economical and environment-friendly Microbial Fuel Cells (MFCs) technology should be associated with waste management. However, current knowledge regarding microbiological bases of electricity production from complex waste substrates is insufficient. In the following study, microbial composition and electricity generation were investigated in MFCs powered with waste volatile fatty acids (VFAs) from anaerobic digestion of primary sludge. Two anode sizes were tested, resulting in organic loading rates (OLRs) of 69.12 and 36.21 mg chemical oxygen demand (COD)/(g MLSS∙d) in MFC1 and MFC2, respectively. Time of MFC operation affected the microbial structure and the use of waste VFAs promoted microbial diversity. High abundance of Deftia sp. and Methanobacterium sp. characterized start-up period in MFCs. During stable operation, higher OLR in MFC1 favored growth of exoelectrogens from Rhodopseudomonas sp. (13.2%) resulting in a higher and more stable electricity production in comparison with MFC2. At a lower OLR in MFC2, the percentage of exoelectrogens in biomass decreased, while the abundance of genera Leucobacter, Frigoribacterium and Phenylobacterium increased. In turn, this efficiently decomposed complex organic substances, favoring high and stable COD removal (over 85%). Independent of the anode size, Clostridium sp. and exoelectrogens belonging to genera Desulfobulbus and Acinetobacter were abundant in MFCs powered with waste VFAs.

Toward enhanced performance of integrated photo-bioelectrochemical systems: Taxa and functions in bacteria-algae communities

An integrated photo-bioelectrochemical (IPB) system uses microalgae in the cathode of a microbial fuel cell to achieve higher electricity generation and nutrient removal from wastewater. Using multivariate analysis and surveys of IPB studies, this paper identifies key algal and bacterial taxa and discusses their functions critical for IPB performance. Unicellular algae with high photosynthetic oxygen production and biofilm formation can enhance IPB energy production. Diverse bacterial taxa achieve nitrogen transformations and can improve total nitrogen removal. Understanding bacteria-algae interactions via quorum sensing in the IPB cathode may potentially aid in boosting system performance. Future advances in development of IPBs for wastewater treatment will benefit from interdisciplinary collaboration in analysis of microbial community functions.

New insights in photosynthetic microbial fuel cell using anoxygenic phototrophic bacteria

Anoxygenic phototrophic bacteria (APB) pay a key role in biogeochemical cycles, and it can convert light energy to chemical energy by photosynthesis process. Photosynthetic microbial fuel cell (photo-MFC) is regarded as a promising energy-harvesting technology, which is also applied to environment treatment in recent years. The previous studies show that photo-MFC with APB have higher power putout than other bioelectrochemical systems. However, photo-MFC with APB is not reviewed due to some limited factors in the development process. In this review, photo-MFC with APB is treated according to its electron transfer pathways, the current understanding, APB strains, application, influence of substrates, and economic assessment. Meanwhile, knowledge of photosynthesis components and electron transfer pathways of APB is crucial for developing new energy and easing the serious energy crisis. Moreover, some new insights (the optimization of light source and self-sustaining bioelectricity generation) are proposed for the future explorations.