Effect of electrode spacing on the performance of microbial fuel cells with a honeycomb flow straightener

Microbial fuel cells (MFCs) are bioelectrochemical transducers that can be used to produce electrical power under the activity of microbes during the wastewater treatment processes. In the present study, the electrode spacing was considered as a parameter to investigate the influence on the performance of MFCs. The electrode spacing was defined as the distance of the anode electrode plate to the polymer exchange membrane in the MFCs. Three values were set at 0.0, 3.0, and 6.0 cm, respectively. In addition, a flow device, like a honeycomb type flow straightener, was introduced and implemented in the anode chamber for creating a uniform flow. The inner diameter of the honeycomb was 0.7 cm. Results showed that a higher limiting current density with 4108.7 mA/m² and a lower resistance with 2.51 Ω can be found in the case of the 0.0 cm electrode spacing. These results also indicated that the shorter electrode spacing with flow straightener devices would improve the performance of MFCs, leading to lower internal resistance and higher power density. In addition, the scanning electron microscopy was employed to analyze the biofilm thickness for MFCs with different electrode spacing. It was also found that the biofilm thickness with 0 cm electrode spacing was larger than the other two cases, leading to a lower internal resistance in the MFCs.     

Effect of cathode electron-receiver on the performance of microbial fuel cells

Performance of cathode electron receivers has direct effect on the voltage and power density of MFC. This paper explored the electrical performance of MFC with potassium permanganate, ferricyanide solution and dissolved oxygen (DO) as cathode electron receivers. The results showed that the internal resistance of MFC with DO depends on catalyst and is higher than that of MFC with potassium permanganate and potassium ferricyanide solution. The maximum volume power density is 4.35 W/m³, and the smallest internal resistance is only about 54 Ω. In case of DO, the internal resistance and power density is different depending on the catalyst and is not too much related to the membranes.     

Effect of coal based pyrolysis gases on the performance of solid oxide direct carbon fuel cells

Solid oxide direct carbon fuel cells (SO-DCFCs) offer a promising way to convert common carbon materials such as coal and biomass to electricity efficiently. It is inevitable that these carbon fuels decompose during SO-DCFCs operation and release pyrolysis gases, which may affect the cell performance and change the anode reaction pathway. In this study, three coals and an activated carbon are used to investigate the role of pyrolysis gases in SO-DCFCs. The cell polarization curve and durability performance are measured for all raw fuels and a pyrolysis pretreated coal at temperatures ranging from 1023 to 1123 K. Mass spectrometry is employed to analyze pyrolysis gases under open circuit operation. The results show that all raw fuels have good electrochemical performance at 1123 K. The cell power density is greatly improved by the presence of pyrolysis gases (mainly H₂ and CO) in the anode chamber. And the cell durability performance is largely dependent on both the amounts of active pyrolysis gases and the carbon gasification reactivity. It is demonstrated that the dominant electrochemical reactions are the electrooxidation of gaseous fuels, namely pyrolysis gases in the initial stage of discharge, then CO produced by carbon gasification. The direct electrooxidation of carbon is less significant than that of gaseous fuels.     

Enhanced performance of microbial fuel cells by electrospinning carbon nanofibers hybrid carbon nanotubes composite anode

A novel carboxylated multiwalled carbon nanotubes/carbon nanofibers (CNTs/CNFs) composite electrode was fabricated by electrospinning. Heat pressing process was applied to improve the interconnection of fiber aggregates, mechanical stability and reduce the contact resistance. Optimal dose of carbon nanotubes was selected to fabricate the anode in microbial fuel cells after comparing with plain electrospinning CNFs anode and commercial carbon felt (CF) anode. As a result, the optimal anode delivered a maximum power density of 362 ± 20 mW m⁻², which is 110%, 122% higher than that of carbon nanofibers and carbon felt anodes. Cyclic voltammograms, Tafel and electrochemical impedance spectroscopy tests also verified that the prepared electrode has largest catalytic current (148 μA cm⁻²) and exchange current density i₀ (6.3 × 10⁻⁵ A cm⁻²), as well as smallest internal resistance (∼40 Ω). The as-prepared anode exhibited a better conductivity, excellent biocompatibility, good hydrophilicity and superior electrocatalytic activity, which was not only beneficial to the attachment and reproduction of microorganisms, but also promoted extracellular electron transfer between bacteria cells and the anode. This result shows that electrospinning has a promising perspective in fabricating high performance electrodes for microbial fuel cells.     

Analysis of carbon fiber brush loading in anodes on startup and performance of microbial fuel cells

Flat carbon anodes placed near a cathode in a microbial fuel cell (MFC) are adversely affected by oxygen crossover, but graphite fiber brush anodes placed near the cathode produce high power densities. The impact of the brush size and electrode spacing was examined by varying the distance of the brush end from the cathode and solution conductivity in multiple MFCs. The startup time was increased from 8 ± 1 days with full brushes (all buffer concentrations) to 13 days (50 mM), 14 days (25 mM) and 21 days (8 mM) when 75% of the brush anode was removed. When MFCs were all first acclimated with a full brush, up to 65% of the brush material could be removed without appreciably altering maximum power. Electrochemical impedance spectroscopy (EIS) showed that the main source of internal resistance (IR) was diffusion resistance, which together with solution resistance reached 100 Ω. The IR using EIS compared well with that obtained using the polarization data slope method, indicating no major components of IR were missed. These results show that using full brush anodes avoids adverse effects of oxygen crossover during startup, although brushes are much larger than needed to sustain high power.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

Design and research progress of nano materials in cathode catalysts of microbial fuel cells: A review

With the advantages of clean, efficient and energy-saving, microbial fuel cells (MFCs) were characterized with perfect significance in the field of degrading environmental pollutants and generating electricity meanwhile. The cathode materials affected the activity of oxygen reduction reaction (ORR), and affected the power generation performance for MFCs. There were many kinds of nano materials played an important role in the field of cathode catalysis. The advantages of metal and non-metal composites were easy to obtain and low cost; layered double hydroxide (LDH) was easy to control and compound, and could be fully realized functionalization; metal organic frameworks (MOFs) were widely used since their porosity, high specific surface area and high activity; covalent organic frameworks (COFs) were low density and easy to be modified, so as to modify and realize functionalization; MXene was an excellent two-dimensional material, which could provide more channels for the movement of ions. The nano materials formed by the composite of various materials combined the advantages of various materials and played key role in improving ORR performance of MFCs.     

Investigation of fabrication of gas diffusion substrate for proton exchange membrane fuel cells

The gas diffusion substrate (GDS) is essential in the proton exchange membrane fuel cells. Its fabrication techniques affect the performance significantly and are worthy of investigation. In this study, a manufacturing process of the GDS is proposed to understand the formation process of GDS and promote its structure and performance more pertinently. Different states during the preparation process, raw carbon paper, pre-curing, curing, carbonation, and graphitization, are characterized and measured. Experimental and numerical methods are employed to determine the relationships between microstructure, transport, and mechanical performance variation with the fabricating processes. The results show that its porosity, average pore size, and effective diffusivity decrease first and increase after curing. These parameters after graphitization are lower than that of the carbon paper (CP). The electrical resistivity increases dramatically while pre-curing and decreases gradually after curing, carbonation, and graphitization, and it is much reduced after graphitization. Moreover, mechanical measurement results show that both the picks of tensile strength and flexural modulus occur after curing. Its tensile strength shows little change after graphitization compared to the initial paper's. In contrast, the flexural modulus is improved significantly.     

Study on synergistic mechanism of PANDAN modification,current and electroactive biofilms on Congo red decolorization in microbial fuel cells

Effect on microbial fuel cells (MFCs) for decolorization of Congo red by Poly (aniline-1,8-diaminonaphthalene) (PANDAN) modification, current and electroactive biofilms (EABs) is investigated. With the synergism of the three factors: PANDAN modification, current and EABs (A2 reactor), the COD removal and decolorization rate significantly increase to 88% and 97%, as well as the Congo red is thoroughly degraded. The decolorization performance comparison and Redundancy analysis (RDA) results indicate that the EABs take more responsibility (contribute ~ 50%) for the decolorization, rather than the modification and current. Therefore, the effects and mechanism of PANDAN modification and current on EABs are further revealed by the Confocal Scanning Laser Microscopy (CSLM) and high-throughput sequencing analysis. The effects of current (anodic dynamical microenvironment), material adsorption, and electron transfer mediating act comprehensively on the thickness, viability, EPS and microbial community of the EABs, among which the relationship is discussed in depth by hierarchically comparison, with “independent and additional effects”. It is demonstrated that the modification, current and EABs present the synergistic effect and promote each other in the performance of the decolorization MFC.     

High performance cathode based on carbon fiber felt for magnesium-air fuel cells

A series of carbon fiber felt/PTFE based gas diffusion layers (GDL) for Mg-air fuel cells were prepared by a simple method of immersing carbon fiber felt in PTFE suspension. Critical properties of the as-prepared GDL, including the surface morphology, electronic resistivity, porosity and gas permeability, have been characterized to investigate the effect of PTFE suspension concentration and PTFE content on the properties of the GDL. The micrographs indicated that the PTFE was homogenously dispersed on the carbon fiber felts and showed structure with a microporous layer. The as-prepared GDL exhibited good mechanical property, high electronic conductivity, sufficient water repellency and high gas permeability. Compared with the Mg-air fuel cell with a traditional carbon powder based cathode, the performance and the stability of Mg-air fuel cell with the carbon fiber felt based GDL are improved significantly.     

Investigation of detailed kinetic scheme performance on modelling of turbulent non-premixed sooting flames

This paper presents the results of an application of a first-order conditional moment closure (CMC) approach coupled with a semi-empirical soot model to investigate the effect of various detailed combustion chemistry schemes on soot formation and destruction in turbulent non-premixed flames.A two-equation soot model representing soot particle nucleation,growth,coagulation and oxidation,was incorporated into the CMC model.The turbulent flow-field of both flames is described using the Favre-averaged fluid-flow equations,applying a standard k-ε turbulence model.A number of five reaction kinetic mechanisms having 50-100 species and 200-1000 elementary reactions called ABF,Miller-Bowman,GRI-Mech3.0,Warnatz,and Qin were employed to study the effect of combustion chemistry schemes on soot predictions.The results showed that of various kinetic schemes being studied,each yields similar accuracy in temperature prediction when compared with experimental data.With respect to soot prediction,the kinetic scheme containing benzene elementary reactions tends to result in a better prediction on soot concentrations in comparison to those contain no benzene elementary reactions.Among five kinetic mechanisms being studied,the Qin combustion scheme mechanism turned to yield the best prediction on both flame temperature and soot levels.     

Electrochemical performance of different carbon fuels on a hybrid direct carbon fuel cell

In this work, three processed carbon fuels including activated carbon, carbon black and graphite have been employed to investigate influence of the chemical and physical properties of carbon on the HDCFC performance in different anode atmospheres at 650–800 °C. The results reveal that the electrochemical activity is strongly dependent on crystalline structure, thermal stability and textural properties of carbon fuels. The activated carbon samples demonstrate a better performance with a peak power density of 326 mW cm^?2 in CO₂ at 750 °C, compared to 147 and 59 mW cm^?2 with carbon black and graphite samples, respectively. Compared to the ohmic resistance, the polarization resistance plays a more dominated role in the cell performance. When replacing N₂ by CO₂ purge gas, the power density is the strongly temperature dependent due to the Boudouard reaction.     

阴极电子受体对微生物燃料电池性能的影响

以双室型微生物燃料电池为试验装置,比较铁氰化钾、重铬酸钾、高锰酸钾作为阴极电子受体时微生物燃料电池的电压和功率输出。结果表明,高锰酸钾与重铬酸钾混合电子受体对微生物燃料电池性能的提高没有显著效果,不如两者的单独表现;高锰酸钾对应的最高输出电压可达1 160 mV,但很不稳定,会很快下降到600 mV左右,在实际应用中有一定障碍;在酸性条件(pH=3.0)下,重铬酸钾的开路电压为1 081.2 mV,最大输出功率密度为35.1 W/m3,电池内阻为170.27Ω,而且表现稳定,是理想的阴极电子受体。     

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO₂ gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high F_Ar. Polarization performance of the chars under high F_Ar was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO₂. Thermal pretreated AC displays the best durability performance for its stable and moderate CO₂ gasification rate. Additionally, the coal ash does not affect the cell performance significantly.     

Effects of periodically alternating temperatures on performance of single-chamber microbial fuel cells

Practical applications of microbial fuel cells (MFCs) for wastewater treatment are usually operated over a wide range of temperature, especially day–night temperature difference. Here, MFCs at alternating temperatures were compared with those at constant temperatures. MFCs at 6/18 °C reached a steady-state voltage of 0.41 ± 0.05 V at 6 °C and 0.36 ± 0.04 V at 18 °C, which were lower than that of MFCs at 18/30 °C (0.42 ± 0.01 V at 18 °C and 0.47 ± 0.02 V at 30 °C). MFCs at 18/30 °C produced the highest power density of 2169 ± 82 mW m⁻² at 30 °C, even higher than that of MFCs at constant temperature 30 °C. Moreover, MFCs at 6/18 °C and 18/30 °C obtained a comparable coulombic efficiencies (94.6 ± 5.2%, 83.2 ± 4.1%, respectively) compared with MFCs at constant temperatures (86.3 ± 7.3% at 18 °C and 84.1 ± 5.5% at 30 °C). These results demonstrate that MFCs could be successfully adapted for use under day–night temperature difference conditions.     

Effect of hydrogen producing mixed culture on performance of microbial fuel cells

This study examined the influence of H₂-producing mixed cultures on improving power generation using air-cathode microbial fuel cells (MFCs) inoculated with heat-treated anaerobic sludge. The MFCs installed with graphite brush anode generated higher power than the MFCs with carbon cloth anode, regardless heat treatment of anaerobic sludge. When the graphite brush anode-MFCs were inoculated selectively with H₂-producing bacteria by heat treatment, power production was not improved (about 490 mW/m²) in batch mode operation, but for slightly increased in carbon cloth anode-MFCs (from 0.16 to 2.0 mW/m²). Although H⁺/H₂ produced from H₂-producing bacteria can contribute to the performance of MFCs, suspended biomass did not affect the power density or potential, but the Coulombic efficiency (CE) increased. A batch test shows that propionate and acetate were used effectively for electricity generation, whereas butyrate made a minor contribution. H₂-producing mixed cultures do not affect the improvement in power generation and seed sludge, regardless of the pretreatment, can be used directly for the MFC performance.     

Small boreholes embedded in the sediment layers make big difference in performance of sediment microbial fuel cells: Bioelectricity generation and microbial community

The practical applications of sediment microbial fuel cells (SMFCs) are limited by their low power densities. In this work, a novel SMFC configuration with a cylindrical borehole embedded in the sediment layer is proposed with expectations of reducing internal resistance, enhancing mass transfer, and accordingly increasing power density. Two types of boreholes with same diameter of 10 cm, but different depths of 3 cm and 6 cm are constructed in SMFCs (SMFC-3 and SMFC-6). Results demonstrate that SMFC-3 produces the highest maximum power density (65.6 mW/m²), which is 25.5% and 65.6% higher than that in SMFC-6 (52.3 mW/m²) and the control SMFC (SMFC-C, 39.4 mW/m²), respectively. The improved power performance in SMFC-3 is mainly due to the greatly reduced internal resistance. Compared to SMFC-6, the higher power density in SMFC-3 is also due to the relatively low overlying water pH values, providing suitable pH condition for cathodic reactions. Microbial community analyses demonstrate that Alphaproteobacteria and Gammaproteobacteria are major contributors to the bioelectricity, and that electroactive species enriched on the top and bottom sides of anodes are significantly different. Generally, embedding a small borehole into the sediment layer is an easy-to-implement and cost-effective strategy for improving the power performance of SMFCs.     

Long-term effect of carbon nanotubes on electrochemical properties and microbial community of electrochemically active biofilms in microbial fuel cells

Carbon nanotubes (CNTs) have been widely exploited to improve anodic performance, but information is needed on their long-term stability for improvement. Herein, we prepared a novel CNTs-modified graphite felt (CNTs-GF) by a simple and scalable process and evaluated its long-term performance using anaerobic sludge as inoculum. the MFC with CNTs-GF yielded a sustained enhancement of power output, increasing from 1.93 ± 0.09 W m^?2 after 1 month to 2.10 ± 0.05 W m^?2 after 3 months and reaching 2.00 ± 0.10 W m^?2 after 13 month, indicating the enhancement in electricity generation by the CNTs was not declined over one year. However, the bare GF showed a declining tendency of performance during 13 months. The long-term enhancement can be explained by the facts that the CNTs-GF was beneficial to electrochemically active biofilms (EABs) growth and interacted better with EABs and increased the extracellular electron transfer. Community analysis showed an increase in Geobacter in response to CNTs modification. These results demonstrated that CNTs modification could sustain a superior long-term enhancement in MFC performance.     

Enhanced bioelectricity generation and cathodic oxygen reduction of air breathing microbial fuel cells based on MoS2 decorated carbon nanotube

Increasing efforts have been devoted to enhancing the cathode activity towards oxygen reduction and improve power generation of air breathing microbial fuel cells. Exploring non-precious metal and highly active cathodic catalyst plays a key role in improving cathode performance. Our work aims to investigate the electrocatalyst behavior and power output of the single-chamber MFC equipped with carbon nanotubes hybridized molybdenum disulfide nanocomposites (CNT/MoS₂) cathode. MoS₂ nanosheets embedded into the CNTs network structure is synthesized by a facile hydrothermal method. The CNT/MoS₂-MFC achieves a maximum power density of 53.0 mW m⁻², which is much higher than those MFCs with pure CNTs (21.4 mW m⁻²) or solely MoS₂ (14.4 mW m⁻²) cathode. The oxygen reduction reaction (ORR) test also demonstrates a promoted electrocatalytic activity of synthesized material, which may be attributed to the special interlaced structure and abundant oxygen chemisorption sites of CNT/MoS₂. Such CNTs-based noble-metal-free catalyst presents a new approach to the application of MFCs cathode materials.