Impact of initial biofilm growth on the anode impedance of microbial fuel cells

Electrochemical impedance spectroscopy (EIS) was used to study the behavior of a microbial fuel cell (MFC) during initial biofilm growth in an acetate-fed, two-chamber MFC system with ferricyanide in the cathode. EIS experiments were performed both on the full cell (between cathode and anode) as well as on individual electrodes. The Nyquist plots of the EIS data were fitted with an equivalent electrical circuit to estimate the contributions of various intrinsic resistances to the overall internal MFC impedance. During initial development of the anode biofilm, the anode polarization resistance was found to decrease by over 70% at open circuit and by over 45% at 27 microA/cm(2), and a simultaneous increase in power density by about 120% was observed. The exchange current density for the bio-electrochemical reaction on the anode was estimated to be in the range of 40-60 nA/cm(2) for an immature biofilm after 5 days of closed circuit operation, which increased to around 182 nA/cm(2) after more than 3 weeks of operation and stable performance in an identical parallel system. The polarization resistance of the anode was 30-40 times higher than that of the ferricyanide cathode for the conditions tested, even with an established biofilm. For a two-chamber MFC system with a Nafion 117 membrane and an inter-electrode spacing of 15 cm, the membrane and electrolyte solution dominate the ohmic resistance and contribute to over 95% of the MFC internal impedance. Detailed EIS analyses provide new insights into the dominant kinetic resistance of the anode bio-electrochemical reaction and its influence on the overall power output of the MFC system, even in the high internal resistance system used in this study. These results suggest that new strategies to address this kinetic constraint of the anode bio-electrochemical reactions are needed to complement the reduction of ohmic resistance in modern designs.     

Microbial fuel cells demonstrate high coulombic efficiency applicable for water remediation

Microbial fuel cells (MFCs) convert biomass into electricity by the metabolic activity of microorganisms and are also used for remediation and water treatment. Power output was compared for a dual chambered membrane MFC using either E. coli or two Yamuna river samples, Yamuna (before the Sangam region)--slow flow (sample 1) and Sangam region--fast flow (sample 2). E. coli and the two river water samples 1 and 2 gave a maximum voltage of 779, 463 and 415 mV respectively. Using E. coli the maximum power density obtained with a 100 omega resistor was 220.66 mW/cm2 and the highest power generated 6068.41 mW. The results demonstrate E. coli, river sample 1 and river sample 2 have a comparable coulombic efficiency of 85.2, 71 and 77% respectively when using 0.4% sucrose as substrate. The decrease in chemical oxidative demand of all river water samples using MFC technology demonstrates efficient remediation of inland water.     

Increased sustainable electricity generation in up-flow air-cathode microbial fuel cells

Sustainable electricity was generated from glucose in up-flow air-cathode microbial fuel cells (MFCs) with carbon cloth cathode and carbon granular anode. Plastic sieves rather than membrane were used to separate the anode and cathode. Based on 1g/l glucose as substrate, a maximum volumetric power density of 25+/-4 W/m(3) (89 A/m(3)) was obtained for the MFC with a sieve area of 30 cm(2) and 49+/-3 W/m(3) (215 A/m(3)) for the MFC with a sieve area of 60 cm(2). The increased power density with larger sieve area was mainly due to the decrease of internal resistance according to the electrochemistry impedance spectroscopy analysis. Increasing the sieve area from 30 cm(2) to 60 cm(2) resulted in a decrease of overall internal resistance from 41 ohm to 27.5 ohm and a decrease of ohmic resistance from 24.3 ohm to 14 ohm. While increasing operational recirculation ratio (RR) decreased internal resistance and increased power output at low substrate concentration, the effect of RR on cell performance was negligible at higher substrate concentration.     

Electricity production in membrane-less microbial fuel cell fed with livestock organic solid waste

Two different MFC configurations designed for handling solid wastes as a feedstock were evaluated in batch mode: a single compartment combined membrane-electrodes (SCME) design; and a twin-compartment brush-type anode electrodes (TBE) design (reversed T-shape MFC with two-air cathode) without a proton exchange membrane (PEM). Cattle manure was tested as a model livestock organic solid waste feedstock. Under steady conditions, voltage of 0.38 V was recorded with an external resistance of 470 Ω. When digested anaerobic sludge was used as the seed in the SCME design, a maximum power density of 36.6 mW/m² was recorded. When hydrogen-generating bacteria (HGB) were used as the seed used in the TBE design, a higher power density of 67 mW/m² was recorded.     

Effect of vitamins and cell constructions on the activity of microbial fuel cell battery

Construction of efficient performance of microbial fuel cells (MFCs) requires certain practical considerations. In the single chamber microbial fuel cell, there is no border between the anode and the cathode, thus the diffusion of the dissolved oxygen has a contrary effect on the anodic respiration and this leads to the inhibition of the direct electron transfer from the biofilm to the anodic surface. Here, a fed-batch single chambered microbial fuel cells are constructed with different distances 3 and 6?cm (anode- cathode spacing), while keeping the working volume is constant. The performance of each MFC is individually evaluated under the effects of vitamins & minerals with acetate as a fed load. The maximum open circuit potential during testing the 3 and 6?cm microbial fuel cells is about 946 and 791?mV respectively. By decreasing the distance between the anode and the cathode from 6 to 3?cm, the power density is decreased from 108.3?mW?m^?2 to 24.5?mW?m^?2. Thus, the short distance in membrane-less MFC weakened the cathode and inhibited the anodic respiration which affects the overall performance of the MFC efficiency. The system is displayed a maximum potential of 564 and 791?mV in absence & presence of vitamins respectively. Eventually, the overall functions of the acetate single chamber microbial fuel cell can be improved by the addition of vitamins & minerals and increasing the distance between the cathode and the anode.     

Fabrication of stainless steel mesh gas diffusion electrode for power generation in microbial fuel cell

This study reports the fabrication of a new membrane electrode assembly by using stainless steel mesh (SSM) as raw material and its effectiveness as gas diffusion electrode (GDE) for electrochemical oxygen reduction in microbial fuel cell (MFC). Based on feeding glucose (0.5 g L(-1)) substrate to a single-chambered MFC, power generation using SSM-based GDE was increased with the decrease of polytetrafluoroethylene (PTFE) content applied during fabrication, reaching the optimum power density of 951.6 mW m(-2) at 20% PTFE. Repeatable cell voltage of 0.51 V (external resistance of 400 Ω) and maximum power density of 951.6 mW m(-2) produced for the MFC with SSM-based GDE are comparable to that of 0.52 V and 972.6 mW m(-2), respectively obtained for the MFC containing typical carbon cloth (CC)-made GDE. Besides, Coulombic efficiency (CE) is found higher for GDE (SSM or CC) with membrane assembly than without, which results preliminarily from the mitigation of Coulombic loss being associated with oxygen diffusion and substrate crossover. This study demonstrates that with its good electrical conductivity and much lower cost, the SSM-made GDE suggests a promising alternative as efficient and more economically viable material to conventional typical carbon for power production from biomass in MFC.     

Continuous flowing membraneless microbial fuel cells with separated electrode chambers

Microbial fuel cell (MFC) is an emerging technology in the energy and environment field. Its application is limited due to its high cost caused by the utilization of membranes and noble metal catalysts. In this paper, a membraneless MFC, with separated electrode chambers, was designed. The two separated chambers are connected via a channel and the continuous electrolyte flow from anode to cathode drives proton transfer. The proton mass transfer coefficiency in this MFC is 0.9086 cm/s, which is higher than reported MFCs with membranes, such as J-cloth and glass fiber. The maximum output voltage is 160.7 mV, with 1000 Ω resistor. Its peak power density is 24.33 mW/m3. SCOD removal efficiency can reach 90.45% via this MFC. If the connection between the two electrode chambers is blocked, the performance of MFC will decrease severely. All the above results prove the feasibility and advantages of this special MFC model.     

A comparison of membranes and enrichment strategies for microbial fuel cells

The external resistance is perhaps the easiest way to influence the operation of a microbial fuel cell (MFC). In this paper, three enrichment strategies, whereby the external resistance was fixed at: (1) a high value in order to maximize the cell voltage (U strategy); (2) a low value in order to maximize the current (I strategy); and (3) a value equal to the internal resistance of the MFC to maximize the power output (P strategy), were investigated. The I strategy resulted in increased maximum power generation and the likely reason is that electron transfer was facilitated under low external resistance, which in turn, favored the development of an electrochemically active biofilm. This experiment was conducted in a single-chamber MFC system equipped with a membrane electrode assembly, and a comparison of the performance achieved by five different membranes is also provided. Selemion was found to be a suitable alternative to Nafion.     

Evaluation of microbial fuel cell operation using algae as an oxygen supplier: carbon paper cathode vs. carbon brush cathode

Microbial fuel cell (MFC) and its cathode performances were compared with use of carbon fiber brush and plain carbon paper cathode electrodes in algae aeration. The MFC having carbon fiber brush cathode exhibited a voltage of 0.21 ± 0.01 V (1,000 Ω) with a cathode potential of around ?0.14 ± 0.01 V in algal aeration, whereas MFC with plain carbon paper cathode resulted in a voltage of 0.06 ± 0.005 V with a cathode potential of ?0.39 ± 0.01 V. During polarizations, MFC equipped with carbon fiber brush cathode showed a maximum power density of 30 mW/m², whereas the MFC equipped with plain carbon paper showed a power density of 4.6 mW/m². In algae aeration, the internal resistance with carbon fiber brush cathode was 804 Ω and with plain carbon paper it was 1,210 Ω. The peak currents of MFC operation with carbon fiber brush and plain carbon paper cathodes were ?31 mA and ?850 µA, respectively.     

Biochemical evaluation of bioelectricity production process from anaerobic wastewater treatment in a single chambered microbial fuel cell (MFC) employing glass wool membrane

Biochemical functioning of single chambered microbial fuel cell (MFC) using glass wool as proton exchange membrane (PEM) operated with selectively enriched acidogenic mixed culture was evaluated in terms of bioelectricity production and wastewater treatment. Performance of MFC was studied at two different organic/substrate loading rates (OLR) (2.64 and 3.54 kg COD/m(3)) and operating pH 6 and 7 using non-coated plain graphite electrodes (mediatorless anode; air cathode). Applied OLR in association with operating pH showed marked influence on the power output and substrate degradation efficiency. Higher current density was observed at acidophilic conditions [pH 6; 98.13 mA/m(2) (2.64 kg COD/m(3)-day; 100 Omega) and 111.29 mA/m(2) (3.54 kg COD/m(3)-day; 100 Omega)] rather than neutral conditions [pH 7; 100.52 mA/m(2) (2.64 kg COD/m(3)-day; 100 Omega) and 98.13 mA/m(2) (3.54 kg COD/m(3)-day; 100 Omega)]. On the contrary, effective substrate degradation was observed at neutral pH. MFC performance was evaluated employing polarization curve, impedance analysis, cell potential, Coulombic efficiency and bioprocess monitoring. Sustainable power yield was calculated at stable cell potential.     

原子力显微镜观测血卟啉单甲醚对细菌光动力杀伤作用

[目的]探讨血卟啉单甲醚(Hematoporyrin monomethyl Ether,HMME)对革兰氏阳性(G )、阴性(G-)菌的光动力杀伤作用.[方法]通过平板菌落计数法和原子力显微镜(AFM),观察细菌与HMME作用前后形貌的变化.[结论]当HMME浓度为50 μg/mL,可见光(光功密度为200 mW/cm2)光照30min时90%以上的金黄色葡萄球菌(Staphyrlococcus aureus)能被杀死,无光照时对S.aureus杀灭效果显著.同等条件下,无论光照还是无光照,HMME对大肠杆菌(E.coli)无明显的杀伤作用.AFM图像显示,S.aureus细菌表面破坏严重,完全碎裂成鱼鳞状的片状堆积.对HMME作用后的E.coli扫描可见,菌体原来光滑的表面变成网格状的裂纹排列.[讨论]HMME对G 有明显的光失活效应,而对G-效果不明显.AFM的超微图像显示HMME对细菌细胞的攻击位点主要在细胞膜上.AFM为我们研究光敏剂对细菌的光动力损伤作用机制的可视化提供了依据.     

Influence of anodic biofilm growth on bioelectricity production in single chambered mediatorless microbial fuel cell using mixed anaerobic consortia

The effect of anodic biofilm growth and extent of its coverage on the anodic surface of a single chambered mediatorless microbial fuel cell (MFC) was evaluated for bioelectricity generation using designed synthetic wastewater (DSW) and chemical wastewater (CW) as substrates and anaerobic mixed consortia as biocatalyst. Three MFCs (plain graphite electrodes, air cathode, Nafion membrane) were operated separately with variable biofilm coverage [control; anode surface coverage (ASC), 0%], partially developed biofilm [PDB; ASC approximately 44%; 90 days] and fully developed biofilm [FDB; ASC approximately 96%; 180 days] under acidophilic conditions (pH 6) at room temperature. The study depicted the effectiveness of anodic biofilm formation in enhancing the extracellular electron transfer in the absence of mediators. Higher specific power production [29mW/kg COD(R) (CW and DSW)], specific energy yield [100.46J/kg VSS (CW)], specific power yield [0.245W/kg VSS (DSW); 0.282W/kg VSS (CW)] and substrate removal efficiency of 66.07% (substrate degradation rate, 0.903kgCOD/m(3)-day) along with effective functioning fuel cell at relatively higher resistance [4.5kOmega (DSW); 14.9kOmega (CW)] correspond to sustainable power [0.008mW (DSW); 0.021mW (CW)] and effective electron discharge (at higher resistance) and recovery (Coulomb efficiency; 27.03%) were observed especially with FDB operation. Cyclic voltammetry analysis documented six-fold increment in energy output from control (1.812mJ) to PDB (10.666mJ) operations and about eight-fold increment in energy from PDB to FDB (86.856mJ). Biofilm configured MFC was shown to have the potential to selectively support the growth of electrogenic bacteria with robust characteristics, capable of generating higher power yields along with substrate degradation especially operated with characteristically complex wastewaters as substrates.     

Increased power generation from primary sludge by a submersible microbial fuel cell and optimum operational conditions

Microbial fuel cells (MFCs) have received attention as a promising renewable energy technology for waste treatment and energy recovery. We tested a submersible MFC with an innovative design capable of generating a stable voltage of 0.250 ± 0.008 V (with a fixed 470 Ω resistor) directly from primary sludge. In a polarization test, the maximum power density was 0.18 W/m² at a current density of 0.8 A/m² with an external resistor of 300 Ω. The anodic solution of the primary sludge needs to be adjusted to a pH 7 for high power generation. The modified primary sludge with an added phosphate buffer prolonged the current generation and increased the power density by 7 and 1.5 times, respectively, in comparison with raw primary sludge. These findings suggest that energy recovery from primary sludge can be maximized using an advanced MFC system with optimum conditions.     

Importance of temperature and anodic medium composition on microbial fuel cell (MFC) performance

The performance of a microbial fuel cell (MFC) was investigated at different temperatures and anodic media. A lag phase of 30 h occurred at 30 degrees C which was half that at room temperature (22 degrees C). The maximum power density at 30 degrees C was 70 mW/m(2) and at 22 degrees C was 43 mW/m(2). At 15 degrees C, no successful operation was observed even after several loadings for a long period of operation. Maximum power density of 320 mW/m(2) was obtained with wastewater medium containing phosphate buffer (conductivity: 11.8 mS/cm), which was approx. 4 times higher than the value without phosphate additions (2.89 mS/cm).     

Electricity generation and microbial community analysis of alcohol powered microbial fuel cells

Two different microbial fuel cell (MFC) configurations were investigated for electricity production from ethanol and methanol: a two-chambered, aqueous-cathode MFC; and a single-chamber direct-air cathode MFC. Electricity was generated in the two-chamber system at a maximum power density typical of this system (40+/-2 mW/m2) and a Coulombic efficiency (CE) ranging from 42% to 61% using ethanol. When bacteria were transferred into a single-chamber MFC known to produce higher power densities with different substrates, the maximum power density increased to 488+/-12 mW/m2 (CE = 10%) with ethanol. The voltage generated exhibited saturation kinetics as a function of ethanol concentration in the two-chambered MFC, with a half-saturation constant (Ks) of 4.86 mM. Methanol was also examined as a possible substrate, but it did not result in appreciable electricity generation. Analysis of the anode biofilm and suspension from a two-chamber MFC with ethanol using 16S rDNA-based techniques indicated that bacteria with sequences similar to Proteobacterium Core-1 (33.3% of clone library sequences), Azoarcus sp. (17.4%), and Desulfuromonas sp. M76 (15.9%) were significant members of the anode chamber community. These results indicate that ethanol can be used for sustained electricity generation at room temperature using bacteria on the anode in a MFC.     

Comparison of membrane- and cloth-cathode assembly for scalable microbial fuel cells: Construction,performance and cost

The aim of this study is to compare the performance of different membrane cathode assembly (MCA) and cloth-cathode assembly (CCA) in air-chamber microbial fuel cells (MFCs) and provide an optimum cathode configuration for MFC scaling up. Two MCAs were prepared by hot-pressing carbon cloth containing cathodic catalyst to anion exchange membrane (AEM) and cation exchange membrane (CEM), respectively. A CCA was built by coating GORE-TEX^® cloth with a mixture of nickel-based conductive paint and cathodic catalyst. Under the fed-batch mode using brewery wastewater, the MFCs were compared with respect to power production, coulombic efficiency, COD removal, internal resistance and material cost. The experimental results show that CCA is a more favorable alternative than MCAs due to its easier preparation, higher maximum power density and COD removal, and lower internal resistance and cost. The optimum cathode assembly of CCA is cost-effective and mechanically robust enough to meet the important requirements for MFC scalability.     

Air-cathode structure optimization in separator-coupled microbial fuel cells

Microbial fuel cells (MFC) with 30% wet-proofed air cathodes have previously been optimized to have 4 diffusion layers (DLs) in order to limit oxygen transfer into the anode chamber and optimize performance. Newer MFC designs that allow close electrode spacing have a separator that can also reduce oxygen transfer into the anode chamber, and there are many types of carbon wet-proofed materials available. Additional analysis of conditions that optimize performance is therefore needed for separator-coupled MFCs in terms of the number of DLs and the percent of wet proofing used for the cathode. The number of DLs on a 50% wet-proofed carbon cloth cathode significantly affected MFC performance, with the maximum power density decreasing from 1427 to 855 mW/m² for 1–4 DLs. A commonly used cathode (30% wet-proofed, 4 DLs) produced a maximum power density (988 mW/m²) that was 31% less than that produced by the 50% wet-proofed cathode (1 DL). It was shown that the cathode performance with different materials and numbers of DLs was directly related to conditions that increased oxygen transfer. The coulombic efficiency (CE) was more affected by the current density than the oxygen transfer coefficient for the cathode. MFCs with the 50% wet-proofed cathode (2 DLs) had a CE of >84% (6.8 A/m²), which was substantially larger than that previously obtained using carbon cloth air-cathodes lacking separators. These results demonstrate that MFCs constructed with separators should have the minimum number of DLs that prevent water leakage and maximize oxygen transfer to the cathode.     

Effect of formation of biofilms and chemical scale on the cathode electrode on the performance of a continuous two-chamber microbial fuel cell

A two-chamber MFC system was operated continuously for more than 500 days to evaluate effects of biofilm and chemical scale formation on the cathode electrode on power generation. A stable power density of 0.57 W/m² was attained after 200 days operation. However, the power density decreased drastically to 0.2 W/m² after the cathodic biofilm and chemical scale were removed. As the cathodic biofilm and chemical scale partially accumulated on the cathode, the power density gradually recovered with time. Microbial community structure of the cathodic biofilm was analyzed based on 16S rRNA clone libraries. The clones closely related to Xanthomonadaceae bacterium and Xanthomonas sp. in the Gammaproteobacteria subdivision were most frequently retrieved from the cathodic biofilm. Results of the SEM-EDX analysis revealed that the cation species (Na⁺ and Ca²⁺) were main constituents of chemical scale, indicating that these cations diffused from the anode chamber through the Nafion membrane. However, an excess accumulation of the biofilm and chemical scale on the cathode exhibited adverse effects on the power generation due to a decrease in the active cathode surface area and an increase in diffusion resistance for oxygen. Thus, it is important to properly control the formation of chemical scale and biofilm on the cathode during long-term operation.     

Simultaneous Congo red decolorization and electricity generation in air-cathode single-chamber microbial fuel cell with different microfiltration, ultrafiltration and proton exchange membranes

Different microfiltration membrane (MFM), proton exchange membrane (PEM) and ultrafiltration membranes (UFMs) with different molecular cutoff weights of 1 K (UFM-1K), 5 K (UFM-5K) and 10 K (UFM-10K) were incorporated into air-cathode single-chamber microbial fuel cells (MFCs) which were explored for simultaneous azo dye decolorization and electricity generation to investigate the effect of membrane on the performance of the MFC. Batch test results showed that the MFC with an UFM-1K produced the highest power density of 324 mW/m² coupled with an enhanced coulombic efficiency compared to MFM. The MFC with UMF-10K achieved the fastest decolorization rate (4.77 mg/L h), followed by MFM (3.61 mg/L h), UFM-5K (2.38 mg/L h), UFM-1K (2.02 mg/L h) and PEM (1.72 mg/L h). These results demonstrated the possibility of using various membranes in the system described here, and showed that UFM-1K was the best one based on the consideration of both cost and performance.     

Effects of biofouling on ion transport through cation exchange membranes and microbial fuel cell performance

This study examines the effects of biofouling on the electrochemical properties of cation exchange membranes (CEMs), such as membrane electrical resistance (MER), specific proton conductivity (SC), and ion transport number (t₊), in addition to on microbial fuel cell (MFC) performance. CEM biofouling using a 15.5 ± 4.6 μm biofilm was found to slightly increase the MER from 15.65 Ω cm² (fresh Nafion) to 19.1 Ω cm², whereas an increase of almost two times was achieved when the electrolyte was changed from deionized water to an anolyte containing a high cation concentration supporting bacterial growth. The simple physical cleaning of CEMs had little effect on the Coulombic efficiency (CE), whereas replacing a biofouled CEM with new one resulted in considerable increase of up to 59.3%, compared to 45.1% for a biofouled membrane. These results clearly suggest the internal resistance increase of MFC was mainly caused by the sulfonate functional groups of CEM being occupied with cations contained in the anolyte, rather than biofouling itself.