Application of biocathode in microbial fuel cells: cell performance and microbial community

Instead of the utilization of artificial redox mediators or other catalysts, a biocathode has been applied in a two-chamber microbial fuel cell in this study, and the cell performance and microbial community were analyzed. After a 2-month startup, the microorganisms of each compartment in microbial fuel cell were well developed, and the output of microbial fuel cell increased and became stable gradually, in terms of electricity generation. At 20 ml/min flow rate of the cathodic influent, the maximum power density reached 19.53 W/m3, while the corresponding current and cell voltage were 15.36 mA and 223 mV at an external resistor of 14.9 Omega, respectively. With the development of microorganisms in both compartments, the internal resistance decreased from initial 40.2 to 14.0 Omega, too. Microbial community analysis demonstrated that five major groups of the clones were categorized among those 26 clone types derived from the cathode microorganisms. Betaproteobacteria was the most abundant division with 50.0% (37 of 74) of the sequenced clones in the cathode compartment, followed by 21.6% (16 of 74) Bacteroidetes, 9.5% (7 of 74) Alphaproteobacteria, 8.1% (6 of 74) Chlorobi, 4.1% (3 of 74) Deltaproteobacteria, 4.1% (3 of 74) Actinobacteria, and 2.6% (2 of 74) Gammaproteobacteria.     

Activity and stability of pyrolyzed iron ethylenediaminetetraacetic acid as cathode catalyst in microbial fuel cells

A low-cost and effective iron-chelated catalyst was developed as an electrocatalyst for the oxygen reduction reaction (ORR) in microbial fuel cells (MFCs). The catalyst was prepared by pyrolyzing carbon mixed iron-chelated ethylenediaminetetraacetic acid (PFeEDTA/C) in an argon atmosphere. Cyclic voltammetry measurements showed that PFeEDTA/C had a high catalytic activity for ORR. The MFC with a PFeEDTA/C cathode produced a maximum power density of 1122 mW/m², which was close to that with a Pt/C cathode (1166 mW/m²). The PFeEDTA/C was stable during an operation period of 31 days. Based on X-ray diffraction and X-ray photoelectron spectroscopy measurements, quaternary-N modified with iron might be the active site for the oxygen reduction reaction. The total cost of a PFeEDTA/C catalyst was much lower than that of a Pt catalyst. Thus, PFeEDTA/C can be a good alternative to Pt in MFC practical applications.     

Long-term cathode performance and the microbial communities that develop in microbial fuel cells fed different fermentation endproducts

To better understand how cathode performance and substrates affected communities that evolved in these reactors over long periods of time, microbial fuel cells were operated for more than 1 year with individual endproducts of lignocellulose fermentation (acetic acid, formic acid, lactic acid, succinic acid, or ethanol). Large variations in reactor performance were primarily due to the specific substrates, with power densities ranging from 835 ± 21 to 62 ± 1 mW/m³. Cathodes performance degraded over time, as shown by an increase in power of up to 26% when the cathode biofilm was removed, and 118% using new cathodes. Communities that developed on the anodes included exoelectrogenic families, such as Rhodobacteraceae, Geobacteraceae, and Peptococcaceae, with the Deltaproteobacteria dominating most reactors. Pelobacter propionicus was the predominant member in reactors fed acetic acid, and it was abundant in several other MFCs. These results provide valuable insights into the effects of long-term MFC operation on reactor performance.     

光合细菌在微生物燃料电池中的应用研究进展

微生物燃料电池(Microbial fuel cells,MFCs)降解污染物的同时产生电能,受到广泛关注。光合细菌在MFCs领域的应用实现了污水处理、CO2捕捉、光电转换等多重功能,并显示出了良好的产电特性。本文根据光合细菌在MFCs中所起作用的不同对其产电机理进行评述,并在此基础上分析了光照对光合细菌型MFCs产电性能的影响;针对当前研究的不足与面临的问题,提出了今后光合细菌在MFCs领域的应用前景与发展方向。     

Recent progress in electrodes for microbial fuel cells

The performance and cost of electrodes are the most important aspects in the design of microbial fuel cell (MFC) reactors. A wide range of electrode materials and configurations have been tested and developed in recent years to improve MFC performance and lower material cost. As well, anodic electrode surface modifications have been widely used to improve bacterial adhesion and electron transfer from bacteria to the electrode surface. In this paper, a review of recent advances in electrode material and a configuration of both the anode and cathode in MFCs are provided. The advantages and drawbacks of these electrodes, in terms of their conductivity, surface properties, biocompatibility, and cost are analyzed, and the modification methods for the anodic electrode are summarized. Finally, to achieve improvements and the commercial use of MFCs, the challenges and prospects of future electrode development are briefly discussed.     

Sustainable wastewater treatment: How might microbial fuel cells contribute

The need for cost-effective low-energy wastewater treatment has never been greater. Clean water for our expanding and predominantly urban global population will be expensive to deliver, eats into our diminishing carbon-based energy reserves and consequently contributes to green house gases in the atmosphere and climate change. Thus every potential cost and energy cutting measure for wastewater treatment should be explored. Microbial fuel cells (MFCs) could potentially yield such savings but, to achieve this, requires significant advances in our understanding in a few critical areas and in our designs of the overall systems. Here we review the research which might accelerate our progress towards sustainable wastewater treatment using MFCs: system control and modelling and the understanding of the ecology of the microbial communities that catalyse the generation of electricity.     

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.     

Electrocatalytic activity of anodic biofilm responses to pH changes in microbial fuel cells

This study investigates the effects of anodic pH on electricity generation in microbial fuel cells (MFCs) and the intrinsic reasons behind them. In a two-chamber MFC, the maximum power density is 1170 ± 58 mW m⁻² at pH 9.0, which is 29% and 89% higher than those working at pH 7.0 and 5.0, respectively. Electrochemical measurements reveal that pH affects the electron transfer kinetics of anodic biofilms. The apparent electron transfer rate constant (k_app) and exchange current density (i₀) are greater whereas the charge transfer resistance (R_ct) is smaller at pH 9.0 than at other conditions. Scanning electron microscopy verifies that alkaline conditions benefit biofilm formation in MFCs. These results demonstrate that electrochemical interactions between bacteria and electrodes in MFCs are greatly enhanced under alkaline conditions, which can be one of the important reasons for the improved MFC output.     

Glycerol degradation in single-chamber microbial fuel cells

Glycerol degradation with electricity production by a pure culture of Bacillus subtilis in a single-chamber air cathode of microbial fuel cell (MFC) has been demonstrated. Steady state polarization curves indicated a maximum power density of 0.06 mW/cm² with an optimal external resistance of 390Ω. Analysis of the effect of pH on MFC performance demonstrated that electricity generation was sustained over a long period of time under neutral to alkaline conditions. Cyclic voltammetry exhibited the increasing electrochemical activity with the increase of pH of 7, 8 and 9. Voltammetric studies also demonstrated that a two-electron transfer mechanism was occurring in the reactor. The low Coulombic efficiency of 23.08% could be attributed to the loss of electrons for various activities other than electricity generation. This study describes an application of glycerol that could contribute to transformation of the biodiesel industry to a more environmentally friendly microbial fuel cell-based technology.     

微生物燃料电池对污染物的强化降解及其机理综述

产电和污染物降解是微生物燃料电池(Microbial Fuel Cells,MFCs)的两个基本功能,也是MFCs作为一种新型的环境治理和能源技术最具吸引力的优势。大量的研究已表明:相对于一般厌氧生物降解技术,MFCs具有更高效的废弃物、废水或污染物降解的能力。解析MFCs强化污染物降解的机理对于进一步优化MFCs的性能具有重要的指导意义,也可以为MFCs在实际环境中的原位应用提供理论支持。本文在综述MFCs强化污染物降解研究报道的基础上,从MFCs中微生物群落的代谢模式、生物膜的活性以及MFCs对局部氧化还原环境的影响等方面为MFCs强化污染物降解的功能提供可能的理论依据,并对MFCs在污染物降解方面的几个可能的发展方向进行展望,为不同学科背景的相关研究者提供参考。     

Simultaneous organics removal and bio-electrochemical denitrification in microbial fuel cells

Simultaneous organics removal and bio-electrochemical denitrification using a microbial fuel cell (MFC) reactor were investigated in this study. The electrons produced as a result of the microbial oxidation of glucose in the anodic chamber were transferred to the anode, which then flowed to the cathode in the cathodic chamber through a wire, where microorganisms used the transferred electrons to reduce the nitrate. The highest power output obtained on the MFCs was 1.7 mW/m(2) at a current density of 15 mA/m(2). The maximum volumetric nitrate removal rate was 0.084 mg NO(3)(-)-N cm(-2) (electrode surface area) day(-1). The coulombic efficiency was about 7%, which demonstrated that a substantial fraction of substrate was lost without current generation.     

Flow shear stress applied in self-buffered microbial fuel cells

The development of renewable and clean energy has been the priority of the global research field due to the urgent effects of climate change. Microbial fuel cell (MFC) is recognized as a sustainable approach to simultaneously generate power and treat wastewater through the employment of microorganisms as catalyst. The use of buffer solution in the wastewater treatment makes the commercial application of MFCs challenging due to their environmental impact and high costs. This work uses rotational motion to generate the flow stress in the anode chamber of the MFCs to enhance biofilm growth and mass transfer that leads to an overall performance improvement of the system. The effects on pH, chemical oxygen demand (COD), and power density were evaluated under rotational speeds of the magnetic stirrer from 0 to 640 rpm. The influence of the stirrer was then assessed utilizing the same parameters specified for scenarios with and without buffer. The results reveal that at 480 rpm of stirring speed, the pH value was neutral with a maximum COD removal of 82 % for bufferless and 93 % for buffered scenarios. In addition, for bufferless operation at 480 rpm yielded a power density of 402 mWm⁻². The results of the flow stress analysis for bufferless and buffered MFCs are beneficial for the commercialization and future development of the system for wastewater treatment applications.     

Landfill leachate treatment with microbial fuel cells; scale-up through plurality

Three Microbial Fuel Cells (MFCs) were fluidically connected in series, with a single feed-line going into the 1st column through the 2nd column and finally as a single outflow coming from the 3rd column. Provision was also made for re-circulation in a loop (the outflow from the 3rd column becoming the feed-line into the 1st column) in order to extend the hydraulic retention time (HRT) on treatment of landfill leachate. The effect of increasing the electrode surface area was also studied whilst the columns were (fluidically) connected in series. An increase in the electrode surface area from 360 to 1080 cm² increased the power output by 118% for C2, 151% for C3 and 264% for C1. COD and BOD₅ removal efficiencies also increased by 137% for C1, 279% for C2 and 182% for C3 and 63% for C1, 161% for C2 and 159% for C3, respectively. The system when configured into a loop was able to remove 79% of COD and 82% of BOD₅ after 4 days. These high levels of removal efficiency demonstrate the MFC system’s ability to treat leachate with the added benefit of generating energy.     

Electricity generation and microbial community changes in microbial fuel cells packed with different anodic materials

Four materials, carbon felt cube (CFC), granular graphite (GG), granular activated carbon (GAC) and granular semicoke (GS) were tested as packed anodic materials to seek a potentially practical material for microbial fuel cells (MFCs). The microbial community and its correlation with the electricity generation performance of MFCs were explored. The maximum power density was found in GAC, followed by CFC, GG and GS. In GAC and CFC packed MFCs, Geobacter was the dominating genus, while Azospira was the most populous group in GG. Results further indicated that GAC was the most favorable for Geobacter adherence and growth, and the maximum power densities had positive correlation with the total biomass and the relative abundance of Geobacter, but without apparent correlation with the microbial diversity. Due to the low content of Geobacter in GS, power generated in this system may be attributed to other microorganisms such as Synergistes, Bacteroidetes and Castellaniella.     

Use of inexpensive semicoke and activated carbon as biocathode in microbial fuel cells

In this study, two inexpensive semicoke and activated carbon packed bed biocathode were developed for oxygen reduction in microbial fuel cells (MFCs). These two materials were compared with two commonly used biocathode materials graphite and carbon felt in terms of material characteristic, power density, biomass density and price-performance ratio. MFCs with semicoke and activated carbon biocathode produced a maximum power density of 20.1 W/m3 (normalized liquid volume in cathodic compartment) and 24.3 W/m3, respectively, compared to 14.1 and 17.1 W/m3 obtained by MFCs with graphite and carbon felt biocathode, respectively. The bacteria attached on biocathode played a major role in oxygen reduction for all the materials investigated. The material cost per Watt produced for semicoke and activated carbon biocathode is only 2.8% and 22.7% of that for graphite biocathode, respectively. These two inexpensive carbon materials, especially semicoke, are very cost-effective biocathode materials for future large scale MFCs.     

Analysis of microbial diversity in oligotrophic microbial fuel cells using 16S rDNA sequences

Molecular ecological techniques were applied to analyze the bacterial diversity of two oligotrophic microbial fuel cells (MFCs) enriched using river water or artificial wastewater (AWW) as fuel. Denaturing gradient gel electrophoresis (DGGE) of the PCR amplified 16S rDNA showed that different microbial communities were present in the two MFCs and these were different from the river sediment used to initiate the enrichment. Nearly complete 16S rDNA was amplified and sequenced. Over 80% of the clones were Proteobacteria. Betaproteobacteria were the dominant clones (46.2%) in MFCs fed with river water, and about 64.4% of the clones in MFCs fed with AWW were Alphaproteobacteria. Actinobacteria were found only in the MFC fed with AWW, and Deltaproteobacteria, Acidobacteria, Chloroflexi and Verrucomicrobia in the MFC fed with river water. Many clones were related to uncultured bacteria, some with homology less than 95%, indicating that many novel bacteria were enriched in the oligotrophic MFCs.     

Bioelectricity enhancement via overexpression of quorum sensing system in Pseudomonas aeruginosa-inoculated microbial fuel cells

Low electron transfer efficiency from bacteria to electrodes remains one of the major bottlenecks that limit industrial applications of microbial fuel cells (MFCs). Elucidating biological mechanism of the electron transfer processes is of great help in improving the efficiency of MFCs. Here, we reported that Pseudomonas aeruginosa could use different electron shuttles in a MFC under different quorum sensing (QS) expression patterns. An electron shuttle (rather than phenazines) with a high mid-point potential of 0.20 V (vs. Ag/AgCl–KCl saturated electrode) was found to be the dominating shuttle in a wild-type P. aeruginosa strain. Strikingly, upon genetic overexpression of rhl QS system in this wild-type strain, the electron shuttle was substituted by phenazines (pyocyanin and phenazine-1-carboxylate, with a low mid-point potential of −0.17 V and −0.28 V, respectively), which directly resulted in an increase of about 1.6 times of the maximum current of the rhl overexpressed strain over the wild-type strain. Our result implied that manipulating electron transfer pathways to improve MFCs’ efficiency could be achieved by rewiring gene regulatory circuits, thus synthetic biology strategies would be adopted.     

Autotrophic nitrite removal in the cathode of microbial fuel cells

Nitrification to nitrite (nitritation process) followed by reduction to dinitrogen gas decreases the energy demand and the carbon requirements of the overall process of nitrogen removal. This work studies autotrophic nitrite removal in the cathode of microbial fuel cells (MFCs). Special attention was paid to determining whether nitrite is used as the electron acceptor by exoelectrogenic bacteria (biologic reaction) or by graphite electrodes (abiotic reaction). The results demonstrated that, after a nitrate pulse at the cathode, nitrite was initially accumulated; subsequently, nitrite was removed. Nitrite and nitrate can be used interchangeably as an electron acceptor by exoelectrogenic bacteria for nitrogen reduction from wastewater while producing bioelectricity. However, if oxygen is present in the cathode chamber, nitrite is oxidised via biological or electrochemical processes. The identification of a dominant bacterial member similar to Oligotropha carboxidovorans confirms that autotrophic denitrification is the main metabolism mechanism in the cathode of an MFC.     

Effect of external resistance on bacterial diversity and metabolism in cellulose-fed microbial fuel cells

External resistance affects the performance of microbial fuel cells (MFCs) by controlling the flow of electrons from the anode to the cathode. The purpose of this study was to determine the effect of external resistance on bacterial diversity and metabolism in MFCs. Four external resistances (20, 249, 480, and 1000 Ω) were tested by operating parallel MFCs independently at constant circuit loads for 10 weeks. A maximum power density of 66 mW m⁻² was achieved by the 20 Ω MFCs, while the MFCs with 249, 480, and 1000 Ω external resistances produced 57.5, 27, and 47 mW m⁻², respectively. Denaturing gradient gel electrophoresis analysis of partial 16S rRNA genes showed clear differences between the planktonic and anode-attached populations at various external resistances. Concentrations of short chain fatty acids were higher in MFCs with larger circuit loads, suggesting that fermentative metabolism dominated over anaerobic respiration using the anode as the final electron acceptor.     

Power generation using spinel manganese-cobalt oxide as a cathode catalyst for microbial fuel cell applications

This study focused on the use of spinel manganese-cobalt (Mn-Co) oxide, prepared by a solid state reaction, as a cathode catalyst to replace platinum in microbial fuel cells (MFCs) applications. Spinel Mn-Co oxides, with an Mn/Co atomic ratios of 0.5, 1, and 2, were prepared and examined in an air cathode MFCs which was fed with a molasses-laden synthetic wastewater and operated in batch mode. Among the three Mn-Co oxide cathodes and after 300 h of operation, the Mn-Co oxide catalyst with Mn/Co atomic ratio of 2 (MnCo-2) exhibited the highest power generation 113 mW/m2 at cell potential of 279 mV, which were lower than those for the Pt catalyst (148 mW/m2 and 325 mV, respectively). This study indicated that using spinel Mn-Co oxide to replace platinum as a cathodic catalyst enhances power generation, increases contaminant removal, and substantially reduces the cost of MFCs.