Practical Application Of Microbial Fuel Cells Research Articles

Silver/iron oxide/graphitic carbon composites as bacteriostatic catalysts for enhancing oxygen reduction in microbial fuel cells

Biofilms from anode heterotrophic bacteria are inevitably formed over cathodic catalytic sites, limiting the performances of single-chamber microbial fuel cells (MFCs). Graphitic carbon (GC) – based nano silver/iron oxide (AgNPs/Fe3O4/GC) composites are prepared from waste pomelo skin and used as antibacterial oxygen reduction catalysts for MFCs. AgNPs and Fe3O4 are introduced in situ into the composites by one-step carbothermal reduction, enhancing their conductivity and catalytic activity. To investigate the effects of Fe species on the antibacterial and catalytic properties, AgNPs/Fe3O4/GC is washed with sulfuric acid (1 mol L−1) for 0.5 h, 1 h, and 5 h and marked as AgNPs/Fe3O4/GC-x (x = 0.5 h, 1 h and 5 h, respectively). A maximum power density of 1712 ± 35 mW m−2 is obtained by AgNPs/Fe3O4/GC-1 h, which declines by 4.12% after 17 cycles. Under catalysis of all AgNP-containing catalysts, oxygen reduction reaction (ORR) proceeds via the 4e− pathway, and no toxic effects to anode microorganisms result from inhibiting the cathodic biofilm overgrowth. With the exception of AgNPs/Fe3O4/GC-5 h, the AgNPs-containing composites exhibit remarkable power output and coulombic efficiency through lowering proton transfer resistance and air-cathode biofouling. This study provides a perspective for the practical application of MFCs using these efficient antibacterial ORR catalysts.

Synergistic effect of titanium dioxide nanocrystal/reduced graphene oxide hybrid on enhancement of microbial electrocatalysis

A small sized TiO2 nanocrystal (∼10 nm)/reduced graphene oxide (TiO2/rGO) hybrid is synthesized through a sol–gel process for hybrid TiO2/GO followed by solvothermal reduction of GO to rGO and is further used as a microbial fuel cell (MFC) anode. The strong synergistic effect from a large surface area produced by uniformly deposited TiO2 nanocrystals, good hydrophilicity of TiO2 nanocrystals and superior conductivity of rGO leads to significantly improved electrocatalysis. In particular, a direct electrochemistry is realized by generating endogenous flavins from a large amount of microbes grown on the highly biocompatible TiO2 nanocrystals to mediate fast electron transfer between microbes and conductive rGO for a high performance anode. The TiO2/rGO hybrid anode delivers a maximum power density of 3169 mW m−2 in Shewanella putrefaciens CN32 MFC, which is much large than that of the conventional carbon cloth anode and reported TiO2/carbon hybrid anode, thus offering great potential for practical applications of MFC. This work is for the first time to report that the synergistic effect from tailoring the physical structure to achieve small sized TiO2 nanocrystals while rationally designing chemistry to introduce highly conductive rGO and superior biocompatible TiO2 is able to significantly boost the MFC performance.

Graphene-Supported Silver Nanoparticles for pH-Neutral Electrocatalytic Oxygen Reduction

Developing an electrocatalyst with desired activity and affordable cost for oxygen reduction reaction (ORR) of microbial fuel cell (MFC) remains a key challenge for practical application of MFC in wastewater treatment. In order to find an economic replacement of Pt-based catalysts while maintaining comparable catalytic efficiency, an electrocatalyst of graphene-supported silver nanoparticles (AgNPs/rGO) was prepared via a facile coreduction and its activity toward ORR in pH-neutral MFC was examined. It has been demonstrated that one-pot aqueous coreduction yielded high-quality AgNPs/rGO catalyst, as revealed by X-ray diffraction and transmission electron microscope. Interestingly, the XPS profiles also indicated the presence of oxygen-containing groups on graphene surface, which provided nuclei to form AgNPs. The resultant AgNPs/rGO catalyst displayed good ORR catalytic activity under pH-neutral condition in cyclic voltammogram and its selectivity of four-electron reduction was verified by rotating disk electrode (RDE) measurement. Moreover, AgNPs/rGO could deliver power generation and sustainability comparable to those of commercial Pt/C in a double-chamber MFC. Thus, we have demonstrated, for the first time, that graphene sheets may provide an alternative way for preparation of Ag nanocomposite catalyst and AgNPs when loaded onto graphene surface can function as a promising replacement of Pt-based catalysts under pH-neutral condition. Since Ag is less expensive and more resistant to poisons than Pt, AgNPs/rGO has better potential to be applied to MFC for recovering energy during wastewater treatment.

A phosphorus-free anolyte to enhance coulombic efficiency of microbial fuel cells

In this study, a phosphorus-free anolyte is prepared by using bicarbonate to replace phosphate buffer for application in two chamber microbial fuel cells (MFCs). Optical density test and Bradford protein assay shows that this phosphorus-free anolyte effectively inhibits the growth and reproduction of microorganisms suspended in the solution and greatly reduces the suspended cell mass. As a result, it considerably enhances the coulombic efficiency (CE) of MFCs. When the acetate concentration is 11 mM, the CE of the MFC using the pH 7 phosphate-containing anolyte is 9.7% and the CE with the pH 8.3 phosphate-containing anolyte is 9.1%, while the CE of the MFC using the phosphorus-free anolyte (pH 8.3) achieves 26.6%. This study demonstrates that this phosphorus-free anolyte holds the potential to enhance the feasibility for practical applications of MFCs.

Enhancement of bioelectricity generation by cofactor manipulation in microbial fuel cell

Microbial fuel cells (MFCs) are promising for harnessing bioenergy from various organic wastes. However, low electricity power output (EPT) is one of the major bottlenecks in the practical application of MFCs. In this study, EPT improvement by cofactor manipulation was explored in the Pseudomonas aeruginosa-inoculated MFCs. By overexpression of nadE (NAD synthetase gene), the availability of the intracellular cofactor pool (NAD(H/+)) significantly increased, and delivered approximately three times higher power output than the original strain (increased from 10.86μW/cm2 to 40.13μW/cm2). The nadE overexpression strain showed about a onefold decrease in charge transfer resistance and higher electrochemical activity than the original strain, which should underlie the power output improvement. Furthermore, cyclic voltammetry, HPLC, and LC–MS analysis showed that the concentration of the electron shuttle (pyocyanin) increased approximately 1.5 fold upon nadE overexpression, which was responsible for the enhanced electrochemical activity. Thus, the results substantiated that the manipulation of intracellular cofactor could be an efficient approach to improve the EPT of MFCs, and implied metabolic engineering is of great potential for EPT improvement.

Enhancement of bioelectricity generation by manipulation of the electron shuttles synthesis pathway in microbial fuel cells

Microbial fuel cells (MFCs) are promising for generating bioenergy and treating organic waste simultaneously. However, low extracellular electron transfer (EET) efficiency between electrogens and anodes remains one of the major bottlenecks in practical applications of MFCs. In this paper, pyocyanin (PYO) synthesis pathway was manipulated to improve the EET efficiency in Pseudomonas aeruginosa-inoculated MFCs. By overexpression of phzM (methyltransferase encoding gene), the maximum power density of P. aeruginosa-phzM-inoculated MFC was enhanced to 166.68 μW/cm(2), which was four folds of the original strain. In addition, the phzM overexpression strain exhibited an increase of 1.6 folds in PYO production and about a onefold decrease in the total internal resistance than the original strain, which should underlie the enhancement of the EET efficiency and the electricity power output (EPT). On the basis of these results, the manipulation of electron shuttles synthesis pathways could be an efficient approach to improve the EPT of MFCs.

A Highly Efficient Catalyst toward Oxygen Reduction Reaction in Neutral Media for Microbial Fuel Cells

A nanocomposite of cobaltosic oxide and nitrogen-doped graphene (Co3O4/N-G) was prepared by the facile hydrothermal method. Morphology characterizations show that the Co3O4 nanoparticles with crystalline spinel structure are uniformly dispersed on the nitrogen-doped graphene nanosheets, and the graphene weight fraction in Co3O4/N-G composite is estimated to be ∼20%. Meanwhile, electrochemical measurements reveal that the as-prepared Co3O4/N-G nanocomposite exhibits a high catalytic activity and long-term stability in neutral electrolyte. Moreover, the use of Co3O4/N-G as cathode catalyst for oxygen reduction in microbial fuel cells (MFCs) to produce electricity was also investigated. The obtained maximum power density was 1340 ± 10 mW m–2, which was as high as almost four times that of the plain cathode (340 ± 10 mW m–2), and only slightly lower than that of a commercial Pt/C catalyst (1470 ± 10 mW m–2). All the results prove that a Co3O4/N-G hybrid can be a good alternative to platinum catalysts for practical MFC applications.

Macroporous and Monolithic Anode Based on Polyaniline Hybridized Three-Dimensional Graphene for High-Performance Microbial Fuel Cells

Microbial fuel cell (MFC) is of great interest as a promising green energy source to harvest electricity from various organic matters. However, low bacterial loading capacity and low extracellular electron transfer efficiency between the bacteria and the anode often limit the practical applications of MFC. In this work, a macroporous and monolithic MFC anode based on polyaniline hybridized three-dimensional (3D) graphene is demonstrated. It outperforms the planar carbon electrode because of its abilities to three-dimensionally interface with bacterial biofilm, facilitate electron transfer, and provide multiplexed and highly conductive pathways. This study adds a new dimension to the MFC anode design as well as to the emerging graphene applications.

Functionally Stable and Phylogenetically Diverse Microbial Enrichments from Microbial Fuel Cells during Wastewater Treatment

Microbial fuel cells (MFCs) are devices that exploit microorganisms as biocatalysts to recover energy from organic matter in the form of electricity. One of the goals of MFC research is to develop the technology for cost-effective wastewater treatment. However, before practical MFC applications are implemented it is important to gain fundamental knowledge about long-term system performance, reproducibility, and the formation and maintenance of functionally-stable microbial communities. Here we report findings from a MFC operated for over 300 days using only primary clarifier effluent collected from a municipal wastewater treatment plant as the microbial resource and substrate. The system was operated in a repeat-batch mode, where the reactor solution was replaced once every two weeks with new primary effluent that consisted of different microbial and chemical compositions with every batch exchange. The turbidity of the primary clarifier effluent solution notably decreased, and 97% of biological oxygen demand (BOD) was removed after an 8–13 day residence time for each batch cycle. On average, the limiting current density was 1000 mA/m2, the maximum power density was 13 mW/m2, and coulombic efficiency was 25%. Interestingly, the electrochemical performance and BOD removal rates were very reproducible throughout MFC operation regardless of the sample variability associated with each wastewater exchange. While MFC performance was very reproducible, the phylogenetic analyses of anode-associated electricity-generating biofilms showed that the microbial populations temporally fluctuated and maintained a high biodiversity throughout the year-long experiment. These results suggest that MFC communities are both self-selecting and self-optimizing, thereby able to develop and maintain functional stability regardless of fluctuations in carbon source(s) and regular introduction of microbial competitors. These results contribute significantly toward the practical application of MFC systems for long-term wastewater treatment as well as demonstrating MFC technology as a useful device to enrich for functionally stable microbial populations.

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO 2/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO 2 is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO 2/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO 2 powder, suggesting that the surface coating of MnO 2 onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m −2 produced from the MFC with in situ MnO 2/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO 2/CNTs (93 mW m −2), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m −2). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO 2 on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO 2 and that in situ MnO 2/CNTs offer a good alternative to Pt/C for practical MFC applications.

Recent advances in the separators for microbial fuel cells

Separator plays an important role in microbial fuel cells (MFCs). Despite of the rapid development of separators in recent years, there are remaining barriers such as proton transfer limitation and oxygen leakage, which increase the internal resistance and decrease the MFC performance, and thus limit the practical application of MFCs. In this review, various separator materials, including cation exchange membrane, anion exchange membrane, bipolar membrane, microfiltration membrane, ultrafiltration membranes, porous fabrics, glass fibers, J-Cloth and salt bridge, are systematically compared. In addition, recent progresses in separator configuration, especially the development of separator electrode assemblies, are summarized. The advances in separator materials and configurations have opened up new promises to overcome these limitations, but challenges remain for the practical application. Here, an outlook for future development and scaling-up of MFC separators is presented and some suggestions are highlighted.

Enhanced Coulombic efficiency and power density of air-cathode microbial fuel cells with an improved cell configuration

Single chamber air-cathode microbial fuel cells (MFCs) that lack a proton exchange membrane (PEM) hold a great promise for many practical applications due to their low operational cost, simple configuration and relative high power density. One of the great challenges for PEM-less MFC is that the Coulombic efficiency is much lower than those containing PEM. In this study, single-chamber PEM-less MFCs were adapted by applying a J-Cloth layer on the water-facing side of air cathode. Due to the significant reduction of oxygen diffusion by the J-Cloth, the MFCs with two-layers of J-Cloth demonstrated an over 100% increase in Coulombic efficiency in comparison with those without J-Cloth (71% versus 35%) at the same current density of 0.6 mA cm −2. A new cell configuration, cloth electrode assembly (CEA), therefore, was designed by sandwiching the cloth between the anode and the cathode. Such an MFC configuration greatly reduced the internal resistance, resulting in a power density of 627 W m −3 when operated in fed-batch mode and 1010 W m −3 in continuous-flow mode, which is the highest reported power density for MFCs and more than 15 times higher than those reported for air-cathode MFCs using similar electrode materials. This study indicates that the Coulombic efficiency and power density of air-cathode MFCs can be improved significantly using an inexpensive cloth layer, which greatly increases the feasibility for the practical applications of MFCs.