Performance and microbial ecology of air-cathode microbial fuel cells with layered electrode assemblies

Microbial fuel cells (MFCs) can be built with layered electrode assemblies, where the anode, proton exchange membrane (PEM), and cathode are pressed into a single unit. We studied the performance and microbial community structure of MFCs with layered assemblies, addressing the effect of materials and oxygen crossover on the community structure. Four MFCs with layered assemblies were constructed using Nafion or Ultrex PEMs and a plain carbon cloth electrode or a cathode with an oxygen-resistant polytetrafluoroethylene diffusion layer. The MFC with Nafion PEM and cathode diffusion layer achieved the highest power density, 381 mW/m² (20 W/m³). The rates of oxygen diffusion from cathode to anode were three times higher in the MFCs with plain cathodes compared to those with diffusion-layer cathodes. Microsensor studies revealed little accumulation of oxygen within the anode cloth. However, the abundance of bacteria known to use oxygen as an electron acceptor, but not known to have exoelectrogenic activity, was greater in MFCs with plain cathodes. The MFCs with diffusion-layer cathodes had high abundance of exoelectrogenic bacteria within the genus Geobacter. This work suggests that cathode materials can significantly influence oxygen crossover and the relative abundance of exoelectrogenic bacteria on the anode, while PEM materials have little influence on anode community structure. Our results show that oxygen crossover can significantly decrease the performance of air-cathode MFCs with layered assemblies, and therefore limiting crossover may be of particular importance for these types of MFCs.     

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 nitrogen addition on the performance of microbial fuel cell anodes

Carbon cloth anodes were modified with 4(N,N-dimethylamino)benzene diazonium tetrafluoroborate to increase nitrogen-containing functional groups at the anode surface in order to test whether the performance of microbial fuel cells (MFCs) could be improved by controllably modifying the anode surface chemistry. Anodes with the lowest extent of functionalization, based on a nitrogen/carbon ratio of 0.7 as measured by XPS, achieved the highest power density of 938 mW/m². This power density was 24% greater than an untreated anode, and similar to that obtained with an ammonia gas treatment previously shown to increase power. Increasing the nitrogen/carbon ratio to 3.8, however, decreased the power density to 707 mW/m². These results demonstrate that a small amount of nitrogen functionalization on the carbon cloth material is sufficient to enhance MFC performance, likely as a result of promoting bacterial adhesion to the surface without adversely affecting microbial viability or electron transfer to the surface.     

Improving the power generation of microbial fuel cells by modifying the anode with single-wall carbon nanohorns

Objectives

To increase the power generation of microbial fuel cells (MFCs), anode modification with carbon materials (activated carbon, carbon nanotubes, and carbon nanohorns) was investigated.

Results

Maximum power densities of a stainless-steel anode MFC with a non-modified electrode (SS-MFC), an activated carbon-modified electrode (AC-MFC), a carbon nanotube-modified electrode (CNT-MFC) and a carbon nanohorn-modified electrode (CNH-MFC) were 72, 244, 261 and 327 mW m^?2, respectively. The total polarization resistance measured by electrochemical impedance spectroscopy were 3610 Ω for SS-MFC, 283 Ω for AC-MFC, 231 Ω for CNTs-MFC, and 136 Ω for CNHs-MFC, consistent with the anode resistances obtained by fitting the anode polarization curves.

Conclusions

Single-wall carbon nanohorns are better than activated carbon and carbon nanotubes as a new anode modification material for improving anode performance.

     

Improved performance of membrane free single-chamber air-cathode microbial fuel cells with nitric acid and ethylenediamine surface modified activated carbon fiber felt anodes

Surface modifications of anode materials are important for enhancing power generation of microbial fuel cell (MFC). Membrane free single-chamber air-cathode MFCs, MFC-A and MFC-N, were constructed using activated carbon fiber felt (ACF) anodes treated by nitric acid and ethylenediamine (EDA), respectively. Experimental results showed that the start-up time to achieve the maximum voltages for the MFC-A and MFC-N was shortened by 45% and 51%, respectively as compared to that for MFC-AT equipped with an unmodified anode. Moreover, the power output of MFCs with modified anodes was significantly improved. In comparison with MFC-AT which had a maximum power density of 1304 mW/m², the MFC-N achieved a maximum power density of 1641 mW/m². The nitric acid-treated anode in MFC-A increased the power density by 58% reaching 2066 mW/m². XPS analysis of the treated and untreated anode materials indicated that the power enhancement was attributable to the changes of surface functional groups.     

Open circuit versus closed circuit enrichment of anodic biofilms in MFC: effect on performance and anodic communities

The influence of various carbon anodes; graphite, sponge, paper, cloth, felt, fiber, foam and reticulated vitreous carbon (RVC); on microbial fuel cell (MFC) performance is reported. The feed was brewery wastewater diluted in domestic wastewater. Biofilms were grown at open circuit or under an external load. Microbial diversity was analysed as a function of current and anode material. The bacterial community formed at open circuit was influenced by the anode material. However at closed circuit its role in determining the bacterial consortia formed was less important than the passage of current. The rate and extent of organic matter removal were similar for all materials: over 95% under closed circuit. The biofilm in MFCs working at open circuit and in the control reactors, increased COD removal by up to a factor of nine compared with that for baseline reactors. The average voltage output was 0.6 V at closed circuit, with an external resistor of 300 kΩ and 0.75 V at open circuit for all materials except RVC. The poor performance of this material might be related to the surface area available and concentration polarizations caused by the morphology of the material and the structure of the biofilm. Peak power varied from 1.3 mW m^?2 for RVC to 568 mW m^?2 for graphite with biofilm grown at closed circuit.     

Increasing power generation for scaling up single-chamber air cathode microbial fuel cells

Scaling up microbial fuel cells (MFCs) requires a better understanding the importance of the different factors such as electrode surface area and reactor geometry relative to solution conditions such as conductivity and substrate concentration. It is shown here that the substrate concentration has significant effect on anode but not cathode performance, while the solution conductivity has a significant effect on the cathode but not the anode. The cathode surface area is always important for increasing power. Doubling the cathode size can increase power by 62% with domestic wastewater, but doubling the anode size increases power by 12%. Volumetric power density was shown to be a linear function of cathode specific surface area (ratio of cathode surface area to reactor volume), but the impact of cathode size on power generation depended on the substrate strength (COD) and conductivity. These results demonstrate the cathode specific surface area is the most critical factor for scaling-up MFCs to obtain high power densities.     

Electrochemical surface modification of carbon mesh anode to improve the performance of air-cathode microbial fuel cells

A convenient and promising alternative to surface modification of carbon mesh anode was fulfilled by electrochemical oxidation in the electrolyte of nitric acid or ammonium nitrate at ambient temperature. It was confirmed that such an anode modification method was low cost and effective not only in improving the efficiency of power generation in microbial fuel cells (MFCs) for synthetic wastewater treatment, but also helping to reduce the period for MFCs start-up. The MFCs with anode modification in electrolyte of nitric acid performed the best, achieving a Coulombic efficiency enhancement of 71 %. As characterized, the electrochemical modification resulted in the decrease of the anode potential and internal resistance but the increase of current response and nitrogen-containing and oxygen-containing functional groups on the carbon surface, which might contribute to the enhancement on the performances of MFCs.     

Power production enhancement with a polyaniline modified anode in microbial fuel cells

In this paper, an approach of improving power generation of microbial fuel cells (MFCs) by using a HSO(4)(-) doped polyaniline modified carbon cloth anode was reported. The modification of carbon cloth anode was accomplished by electrochemical polymerization of aniline in 5% H(2)SO(4) solution. A dual-chamber MFC reactor with the modified anode achieved a maximum power density of 5.16 Wm(-3), an internal resistance of 90 Ω, and a start-up time of 4 days, which was respectively 2.66 times higher, 65.5% lower, and 33.3% shorter than the corresponding values of the MFC with unmodified anode. Evidence from X-ray photoelectron spectroscopy and scanning electron microscopy results proved that the formation of biofilm on the anode surface could prevent the HSO(4)(-) doped polyaniline to be de-doped, and the results from electrochemical tests confirmed that the electrochemical activity of the modified anode was enhanced significantly after inoculation. Charge transfer was facilitated by polyaniline modification. All the results indicated that the polyaniline modification on the anode was an efficient approach of improving the performance of MFCs.     

The effect of physico-chemically immobilized methylene blue and neutral red on the anode of microbial fuel cell

A fast and cost effective immobilization of electron carriers, methylene blue (MB) and neutral red (NR) by pH shift was proposed to improve bioanodic performance. The adsorption of mediators onto the carbon cloth anode was verified using cyclic voltammogram (CV) and the effect of the immobilized mediators on acclimation, power density, and acetate removal of MFCs was investigated. A peak power density of P _max(MB) = 11.3 W/m³ was achieved over days 110 ∼ 120, as compared to P _max(Control) = 5.4 W/m³ and P _max(NR) = 3.1 W/m³ for the treated anode after 15 sequential fed-batch operations. The VFA removal rates however were similar for all MFC systems, ranging from 82 to 87%. It could be suggested that the increase in power density for the MB treated electrode resulted from an enhanced electron transport from exo-electrogenic bacteria. MB may also have a selective effect on the bacterial community during the start-up stage, increasing the voltage production and acetate removal from day 1 to 16. However, MFC with NR treated anode produced an initial voltage under 100 mV, with lower coulombic efficiency (CE). NR exhibited less favourable mediator molecule binding to the electrode surface, when subject to pH driven physico-chemical immobilization.     

Granular activated carbon single-chamber microbial fuel cells (GAC-SCMFCs): A design suitable for large-scale wastewater treatment processes

As an emerging biotechnology capable of removing contaminants and producing electricity, microbial fuel cells (MFCs) hold a promising future in wastewater treatment. However, several main problems, including the high internal resistance (R_in), low power output, expensive material, and complicated configuration have severely hindered the large-scale application of MFCs. The study targeted these challenges by developing a novel MFC system, granular activated carbon single-chamber MFC, termed as GAC-SCMFC. The batch tests showed that GAC was a good substitute for carbon cloth and GAC-SCMFCs generated high and stable power outputs compared with the traditional two-chamber MFCs (2CMFCs). Critical operational parameters (i.e. wastewater substrate concentrations, GAC amount, electrode distance) affecting the performance of GAC-SCMFCs were examined at different levels. The results showed that the R_in gradually decreased from 60 Ω to 45 Ω and the power output increased from 0.2 W/m³ to 1.2 W/m³ when the substrate concentrations increased from 100 mg/L to 850 mg/L. However, at high concentrations of 1000–1500 mg/L, the power output leveled off. The R_in of MFCs decreased 50% when the electrode distance was reduced from 7.5 cm to 1 cm. The highest power was achieved at the electrode distance of 2 cm. The power generation increased with more GAC being added in MFCs due to the higher amount of biomass attached. Finally, the multi-anode GAC-SCMFCs were developed to effectively collect the electrons generated in the GAC bed. The results showed that the current was split among the multiple anodes, and the cathode was the limiting factor in the power production of GAC-SCMFCs.     

Granular activated carbon single-chamber microbial fuel cells (GAC-SCMFCs): A design suitable for large-scale wastewater treatment processes

As an emerging biotechnology capable of removing contaminants and producing electricity, microbial fuel cells (MFCs) hold a promising future in wastewater treatment. However, several main problems, including the high internal resistance (R_in), low power output, expensive material, and complicated configuration have severely hindered the large-scale application of MFCs. The study targeted these challenges by developing a novel MFC system, granular activated carbon single-chamber MFC, termed as GAC-SCMFC. The batch tests showed that GAC was a good substitute for carbon cloth and GAC-SCMFCs generated high and stable power outputs compared with the traditional two-chamber MFCs (2CMFCs). Critical operational parameters (i.e. wastewater substrate concentrations, GAC amount, electrode distance) affecting the performance of GAC-SCMFCs were examined at different levels. The results showed that the R_in gradually decreased from 60 Ω to 45 Ω and the power output increased from 0.2 W/m³ to 1.2 W/m³ when the substrate concentrations increased from 100 mg/L to 850 mg/L. However, at high concentrations of 1000–1500 mg/L, the power output leveled off. The R_in of MFCs decreased 50% when the electrode distance was reduced from 7.5 cm to 1 cm. The highest power was achieved at the electrode distance of 2 cm. The power generation increased with more GAC being added in MFCs due to the higher amount of biomass attached. Finally, the multi-anode GAC-SCMFCs were developed to effectively collect the electrons generated in the GAC bed. The results showed that the current was split among the multiple anodes, and the cathode was the limiting factor in the power production of GAC-SCMFCs.     

Layer-by-layer construction of graphene-based microbial fuel cell for improved power generation and methyl orange removal

Development of highly efficient anode is critical for enhancing the power output of microbial fuel cells (MFCs). The aim of this work is to investigate whether modification of carbon paper (CP) anode with graphene (GR) via layer-by-layer assembly technique is an effective approach to promote the electricity generation and methyl orange removal in MFCs. Using cyclic voltammetry and electrochemical impedance spectroscopy, the GR/CP electrode exhibited better electrochemical behavior. Scanning electron microscopy results revealed that the surface roughness of GR/CP increased, which was favorable for more bacteria to attach to the anode surface. The MFCs equipped with GR/CP anode achieved a stable maximum power density of 368 mW m^?2 under 1,000 Ω external resistance and a start time for the initial maximum voltage of 180 h, which were, respectively, 51 % higher and 31 % shorter than the corresponding values of the MFCs with blank anode. The anode and cathode polarization curves revealed negligible difference in cathode potentials but obviously difference in anode potentials, indicating that the GR-modified anode other than the cathode was responsible for the performance improvement of MFC. Meanwhile, compared with MFCs with blank anode, 11 % higher decolorization efficiency and 16 % higher the chemical oxygen demand removal rate were achieved in MFC with GR-modified anode during electricity generation. This study might provide an effective way to modify the anode for enhanced electricity generation and efficient removal of azo dye in MFCs.     

石墨烯-多壁碳纳米管复合材料协同刃天青修饰阳极对微生物燃料电池性能的影响

【目的】利用石墨烯与多壁碳纳米管复合材料协同刃天青修饰微生物燃料电池(Microbial fuel cell,MFC)阳极,提高MFC的运行性能。【方法】以碳布为基底,采用滴涂法分别制备了刃天青/碳布(R/CC)、刃天青+石墨烯/碳布(R+GNS/CC)、刃天青+多壁碳纳米管/碳布(R+MWCNT/CC)、刃天青+石墨烯+多壁碳纳米管/碳布(R+GNS+MWCNT/CC)四种阳极材料。【结果】在降解高氯酸盐的过程中,与刃天青/碳布(最高输出电压54 m V)相比,刃天青+石墨烯/碳布、刃天青+多壁碳纳米管/碳布和刃天青+石墨烯+多壁碳纳米管/碳布阳极MFC最高输出电压分别为87、145、275 m V,分别提高了61.11%、168.52%、409.26%;高氯酸盐的还原速率也分别提高了59.1%、89.7%、147.3%。4种阳极的电化学交流阻抗(EIS)和塔菲尔(Tafel)测试发现,与刃天青/碳布阳极相比,刃天青+石墨烯/碳布、刃天青+多壁碳纳米管/碳布阳极活化内阻减小,电极反应速率提高,但刃天青+石墨烯+多壁碳纳米管/碳布阳极的活化内阻更小,电极反应速率更快,同时4种阳极附着微生物胞外聚合物(EPS)分析表明,修饰过的阳极附着微生物数量增加,多糖减少,R+GNS+MWCNT/CC阳极变化最大,更有利于微生物传递电子。【结论】石墨烯、多壁碳纳米管复合材料协同刃天青修饰MFC阳极可以减小活化内阻从而加快电子传递,进而提高MFC的性能。     

Evaluation of procedures to acclimate a microbial fuel cell for electricity production

A microbial fuel cell (MFC) is a relatively new type of fixed film bioreactor for wastewater treatment, and the most effective methods for inoculation are not well understood. Various techniques to enrich electrochemically active bacteria on an electrode were therefore studied using anaerobic sewage sludge in a two-chambered MFC. With a porous carbon paper anode electrode, 8 mW/m² of power was generated within 50 h with a Coulombic efficiency (CE) of 40%. When an iron oxide-coated electrode was used, the power and the CE reached 30 mW/m² and 80%, respectively. A methanogen inhibitor (2-bromoethanesulfonate) increased the CE to 70%. Bacteria in sludge were enriched by serial transfer using a ferric iron medium, but when this enrichment was used in a MFC the power was lower (2 mW/m²) than that obtained with the original inoculum. By applying biofilm scraped from the anode of a working MFC to a new anode electrode, the maximum power was increased to 40 mW/m². When a second anode was introduced into an operating MFC the acclimation time was not reduced and the total power did not increase. These results suggest that these active inoculating techniques could increase the effectiveness of enrichment, and that start up is most successful when the biofilm is harvested from the anode of an existing MFC and applied to the new anode.     

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.     

A Flexible and Ultrahigh Energy Density Capacitor via Enhancing Surface/Interface of Carbon Cloth Supported Colloids

Pseudocapacitors are now reaching the energy density limits set by the surface redox reaction of their electrode materials, requiring new cation paradigms for a fast cation Faradaic reaction with high capacitance. In this work, a flexible and ultrahigh energy density capacitor is reported via enhancing surface/interface of active colloids and supported carbon cloth. A flexible asymmetrical capacitor assembled with Ni²⁺ colloidal cathode and Fe³⁺ colloidal anode displays a high energy density of 353 W h kg^?1 at the power density of 2250 W kg^?1, outperforming recent reported pseudocapacitors, and shows superior cycling stability after 10 000 charge–discharge cycles at current density of 30 A g^?1. This work demonstrates that the optimized surface/interface of carbon cloth and colloids can lead to the enhancement of both stability and activity of colloidal electrode.     

Effect of conductive polymers coated anode on the performance of microbial fuel cells (MFCs) and its biodiversity analysis

Conductive polymer, one of the most attractive electrode materials, has been applied to coat anode of MFC to improve its performance recently. In this paper, two conductive polymer materials, polyaniline (PANI) and poly(aniline-co-o-aminophenol) (PAOA) were used to modify carbon felt anode and physical and chemical properties of the modified anodes were studied. The power output and biodiversity of modified anodes, along with unmodified carbon anode were compared in two-chamber MFCs. Results showed that the maximum power density of PANI and PAOA MFC could reach 27.4 mW/m(2) and 23.8 mW/m(2), comparing with unmodified MFC, increased by 35% and 18% separately. Low temperature caused greatly decrease of the maximum voltage by 70% and reduced the sorts of bacteria on anodes in the three MFCs. Anode biofilm analysis showed different bacteria enrichment: a larger mount of bacteria and higher biodiversity were found on the two modified anodes than on the unmodified one. For PANI anode, the two predominant bacteria were phylogenetically closely related to Hippea maritima and an uncultured clone MEC_Bicarb_Ac-008; for PAOA, Clostridiales showed more enrichment. Compare PAOA with PANI, the former introduced phenolic hydroxyl group by copolymerization o-aminophenol with aniline, which led to a different microbial community and the mechanism of group effect was proposed.     

Two‐Step Activated Carbon Cloth with Oxygen‐Rich Functional Groups as a High‐Performance Additive‐Free Air Electrode for Flexible Zinc–Air Batteries

A flexible air electrode (FAE) with both high oxygen electrocatalytic activity and excellent flexibility is the key to the performance of various flexible devices, such as Zn–air batteries. A facile two‐step method, mild acid oxidation followed by air calcination that directly activates commercial carbon cloth (CC) to generate uniform nanoporous and super hydrophilic surface structures with optimized oxygen‐rich functional groups and an enhanced surface area, is presented here. Impressively, this two‐step activated CC (CC‐AC) exhibits superior oxygen electrocatalytic activity and durability, outperforming the oxygen‐doped carbon materials reported to date. Especially, CC‐AC delivers an oxygen evolution reaction (OER) overpotential of 360 mV at 10 mA cm^?2 in 1 m KOH, which is among the best performances of metal‐free OER electrocatalysts. The practical application of CC‐AC is presented via its use as an FAE in a flexible rechargeable Zn–air battery. The bendable battery achieves a high open circuit voltage of 1.37 V, a remarkable peak power density of 52.3 mW cm^?3 at 77.5 mA cm^?3, good cycling performance with a small charge–discharge voltage gap of 0.98 V and high flexibility. This study provides a new approach to the design and construction of high‐performance self‐supported metal‐free electrodes.     

Metagenomic Analyses Reveal the Involvement of Syntrophic Consortia in Methanol/Electricity Conversion in Microbial Fuel Cells

Methanol is widely used in industrial processes, and as such, is discharged in large quantities in wastewater. Microbial fuel cells (MFCs) have the potential to recover electric energy from organic pollutants in wastewater; however, the use of MFCs to generate electricity from methanol has not been reported. In the present study, we developed single-chamber MFCs that generated electricity from methanol at the maximum power density of 220 mW m⁻² (based on the projected area of the anode). In order to reveal how microbes generate electricity from methanol, pyrosequencing of 16S rRNA-gene amplicons and Illumina shotgun sequencing of metagenome were conducted. The pyrosequencing detected in abundance Dysgonomonas, Sporomusa, and Desulfovibrio in the electrolyte and anode and cathode biofilms, while Geobacter was detected only in the anode biofilm. Based on known physiological properties of these bacteria, it is considered that Sporomusa converts methanol into acetate, which is then utilized by Geobacter to generate electricity. This speculation is supported by results of shotgun metagenomics of the anode-biofilm microbes, which reconstructed relevant catabolic pathways in these bacteria. These results suggest that methanol is anaerobically catabolized by syntrophic bacterial consortia with electrodes as electron acceptors.