Brush Anode Research Articles

Combined carbon mesh and small graphite fiber brush anodes to enhance and stabilize power generation in microbial fuel cells treating domestic wastewater

Abstract Microbial fuel cells (MFCs) need to have a compact architecture, but power generation using low strength domestic wastewater is unstable for closely-spaced electrode designs using thin anodes (flat mesh or small diameter graphite fiber brushes) due to oxygen crossover from the cathode. A composite anode configuration was developed to improve performance, by joining the mesh and brushes together, with the mesh used to block oxygen crossover to the brushes, and the brushes used to stabilize mesh potentials. In small, fed-batch MFCs (28 mL), the composite anode produced 20% higher power densities than MFCs using only brushes, and 150% power densities compared to carbon mesh anodes. In continuous flow tests at short hydraulic retention times (HRTs, 2 or 4 h) using larger MFCs (100 mL), composite anodes had stable performance, while brush anode MFCs exhibited power overshoot in polarization tests. Both configurations exhibited power overshoot at a longer HRT of 8 h due to lower effluent CODs. The use of composite anodes reduced biomass growth on the cathode (1.9 ± 0.2 mg) compared to only brushes (3.1 ± 0.3 mg), and increased coulombic efficiencies, demonstrating that they successfully reduced oxygen contamination of the anode and the bio-fouling of cathode.

Potassium permanganate as an electron receiver in a microbial fuel cell

ABSTRACTMicrobial fuel cells with air as a cathode electron receiver are simple systems but they need expensive catalysts. In comparison to microbial fuel cells with oxygen as an electron receiver, microbial fuel cells with potassium permanganate produce higher voltage. In this study, electrical performance of a microbial fuel cells containing anaerobic sludge and potassium permanganate as an oxidizing agent was investigated. Glucose (1 g/l) was used as a carbon and energy source. The maximum power density and current density at the maximum power density were 93.13 mW/m2 and 0.030 mA/cm2 with respect to a potassium permanganate concentration of 400 µM. It is observed that the maximum power density increased to 114.00 mW/m2 using an acid-heat treated carbon brush anode. Also, chemical oxygen demand removal was 51% when the microbial fuel cells was operated using 400 µM of potassium permanganate.

Evaluation of organic matter removal and electricity generation by using integrated microbial fuel cells for wastewater treatment

A floating all-in-one type of microbial fuel cell (Fa-MFC) that allows simple operation and installation in existing wastewater reservoirs for decomposition of organic matter was designed. A prototype cell was constructed by fixing a tubular floater to an assembly composed of a proton-exchange membrane and an air-cathode. To compare anode materials, carbon-cloth anodes or carbon-brush anodes were included in the assembly. The fabricated assemblies were floated in 1-L beakers filled with acetate medium. Both reactors removed acetate at a rate of 133–181 mg/L/d. The Fa-MFC quipped with brush anodes generated a 1.7-fold higher maximum power density (197 mW/m2-cathode area) than did that with cloth anodes (119 mW/m2-cathode area). To evaluate the performance of the Fa-MFCs on more realistic substrates, artificial wastewater, containing peptone and meat extract, was placed in a 2-L beaker, and the Fa-MFC with brush anodes was floated in the beaker. The Fa-MFC removed the chemical oxygen demand of the wastewater at a rate of 465–1029 mg/L/d, and generated a maximum power density of 152 mW/m2-cathode area. When the Fa-MFC was fed with actual livestock wastewater, the biological oxygen demand of the wastewater was removed at a rate of 45–119 mg/L/d, with electricity generation of 95 mW/m2-cathode area. Bacteria related to Geobacter sulfurreducens were predominantly detected in the anode biofilm, as deduced from the analysis of the 16S rRNA gene sequence.

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.

Obtaining microbial communities with exoelectrogenic activity from anaerobic sludge using a simplified procedure

AbstractBACKGROUNDThe microbial fuel cell (MFC) technology transforms the chemical energy present in substrates into electricity. Starting‐up these systems, i.e. enriching the anodic community in exoelectrogenic bacteria, is a lengthy process or requires expensive equipment.RESULTSAn easy and low‐cost procedure based on a sediment MFC was developed to select microbial communities with exoelectrogenic activity from the anaerobic sludge of a waste water treatment plant (WWTP). The configuration was based on a simple vessel working as a single chamber MFC with a cathode of stainless steel wool in the liquid surface and a submerged graphite fibre brush as anode. In 30 days of operation, a biofilm with remarkable exoelectrogenic activity was grown on the anode of the MFC. This graphite fibre brush anode was able to supply 0.9 W m‐2 when working in an air‐cathode MFC (AC‐MFC) for 45 days.CONCLUSIONThe procedure presented was demonstrated to be a successful, low‐cost and low‐maintenance procedure to obtain exoelectrogenic activity and had performances comparable with other more costly and complex inoculation procedures. The Sed‐MFC does not require a potentiostat, external aeration, stirring, membranes or an enriched inoculum in the exoelectrogenic biomass. © 2013 Society of Chemical Industry

Evaluation of multi-brush anode systems in microbial fuel cells

The packing density of anodes in microbial fuel cells (MFCs) was examined here using four different graphite fiber brush anode configurations. The impact of anodes on performance was studied in terms of carbon fiber length (brush diameter), the number of brushes connected in parallel, and the wire current collector gage. MFCs with different numbers of brushes (one, three or six) set perpendicular to the cathode all produced similar power densities (1200 ± 40 mW/m(2)) and coulombic efficiencies (60% ± 5%). Reducing the number of brushes by either disconnecting or removing them reduced power, demonstrating the importance of anode projected area covering the cathode, and therefore the need to match electrode projected areas to maintain high performance. Multi-brush reactors had the same COD removal as single-brush systems (>90%). The use of smaller Ti wire gages did not affect power generation, which will enable the use of less metal, reducing material costs.

Effects of carbon brush anode size and loading on microbial fuel cell performance in batch and continuous mode

Larger scale microbial fuel cells (MFCs) require compact architectures to efficiently treat wastewater. We examined how anode-brush diameter, number of anodes, and electrode spacing affected the performance of the MFCs operated in fed-batch and continuous flow mode. All anodes were initially tested with the brush core set at the same distance from the cathode. In fed-batch mode, the configuration with three larger brushes (25 mm diameter) produced 80% more power (1240 mW m−2) than reactors with eight smaller brushes (8 mm) (690 mW m−2). The higher power production by the larger brushes was due to more negative and stable anode potentials than the smaller brushes. The same general result was obtained in continuous flow operation, although power densities were reduced. However, by moving the center of the smaller brushes closer to the cathode (from 16.5 to 8 mm), power substantially increased from 690 to 1030 mW m−2 in fed batch mode. In continuous flow mode, power increased from 280 to 1020 mW m−2, resulting in more power production from the smaller brushes than the larger brushes (540 mW m−2). These results show that multi-electrode MFCs can be optimized by selecting smaller anodes, placed as close as possible to the cathode.

Effects of brush lengths and fiber loadings on the performance of microbial fuel cells using graphite fiber brush anodes

An alternative method for fabricating graphite fiber brush (GFB) electrodes was proposed. Two series of GFB electrodes with different lengths (L) and loaded fiber masses (m) were fabricated. The effects of m/L on the biomass distribution, active biomass content, electrochemical behavior and MFC performance were investigated. For the electrodes with a similar m but different L, substrate supply within the interior of GFB electrodes improved with L, leading to higher biomass content and consequently the improved performance. However, a complex trend was found for the electrodes with different m and similar L, due to the opposing trends of substrate supply and actual functional area for electrochemically active bacteria with m. Furthermore, m-normalized biomass content and power density of the GFB electrodes increased with decreasing of m/L ratio due to the improved graphite fiber utilization until 0.014 g mm−1, below which they remained constant since the utilization of graphite fibers plateaued.

Altering Anode Thickness To Improve Power Production in Microbial Fuel Cells with Different Electrode Distances

A better understanding of how anode and separator physical properties affect power production is needed to improve energy and power production by microbial fuel cells (MFCs). Oxygen crossover from the cathode can limit power production by bacteria on the anode when using closely spaced electrodes [separator electrode assembly (SEA)]. Thick graphite fiber brush anodes, as opposed to thin carbon cloth, and separators have previously been examined as methods to reduce the impact of oxygen crossover on power generation. We examined here whether the thickness of the anode could be an important factor in reducing the effect of oxygen crossover on power production, because bacteria deep in the electrode could better maintain anaerobic conditions. Carbon felt anodes with three different thicknesses were examined to see the effects of thicker anodes in two configurations: widely spaced electrodes and SEA. Power increased with anode thickness, with maximum power densities (604 mW/m2, 0.32 cm; 764 mW/m2, 0.64 cm; an...

Application of PAN-Based Carbon Fibers as Brush Anode in Microbial Fuel Cells for Higher Electricity Production

PAN-based carbon fiber was used to prepare brush anode, and its electrochemical performance was investigated in a tubular microbial fuel cells. Experimental results showed that electricity production can be enhanced by using brush anode, with maximum current (up to 25 mA) much higher than that of reported microbial fuel cells using other types of carbon materials. Heat treatment is demonstrated to be an effective method to further increase the performance of the brush anode, not only reducing the start-up time of the microbial fuel cells, but also increasing the duration of electricity production and polarization properties.

Effect of PH on the Performance of the Anode in Microbial Fuel Cells

Microbial fuel cells with microbial brush anode and ferricyanide-cathode which could discharge at current up to 350 mA were constructed, and the effect of anodic pH on the performance of the microbial anode was investigated in the present study. Anodic pH was found to decrease significantly， from 6.23 to 4.35, when the MFC was operated to discharge at high current levels, which in turn reduced the performance of the microbial anode. No obvious pH change was observed during MFC discharge at lower current level (~12 mA), meaning that proton production is mainly related to the electrochemical oxidization of the organic fuels, rather than to the microbial degradation in the anode chamber. Results presented herein indicate that effective approach should be employed to control the anodic pH close to neutral conditions when optimizing the power output of microbial fuel cells, especially at high discharge current.

Domestic wastewater treatment using multi-electrode continuous flow MFCs with a separator electrode assembly design

Treatment of domestic wastewater using microbial fuel cells (MFCs) will require reactors with multiple electrodes, but this presents unique challenges under continuous flow conditions due to large changes in the chemical oxygen demand (COD) concentration within the reactor. Domestic wastewater treatment was examined using a single-chamber MFC (130mL) with multiple graphite fiber brush anodes wired together and a single air cathode (cathode specific area of 27m(2)/m(3)). In fed-batch operation, where the COD concentration was spatially uniform in the reactor but changed over time, the maximum current density was 148 ± 8mA/m(2) (1,000Ω), the maximum power density was 120mW/m(2), and the overall COD removal was >90%. However, in continuous flow operation (8h hydraulic retention time, HRT), there was a 57% change in the COD concentration across the reactor (influent versus effluent) and the current density was only 20 ± 13mA/m(2). Two approaches were used to increase performance under continuous flow conditions. First, the anodes were separately wired to the cathode, which increased the current density to 55 ± 15mA/m(2). Second, two MFCs were hydraulically connected in series (each with half the original HRT) to avoid large changes in COD among the anodes in the same reactor. The second approach improved current density to 73 ± 13mA/m(2). These results show that current generation from wastewaters in MFCs with multiple anodes, under continuous flow conditions, can be improved using multiple reactors in series, as this minimizes changes in COD in each reactor.

A multi-electrode continuous flow microbial fuel cell with separator electrode assembly design

Scaling up microbial fuel cells (MFCs) requires the development of compact reactors with multiple electrodes. A scalable single chamber MFC (130 mL), with multiple graphite fiber brush anodes and a single air-cathode cathode chamber (27 m2/m3), was designed with a separator electrode assembly (SEA) to minimize electrode spacing. The maximum voltage produced in fed-batch operation was 0.65 V (1,000 Ω) with a textile separator, compared to only 0.18 V with a glass fiber separator due to short-circuiting by anode bristles through this separator with the cathode. The maximum power density was 975 mW/m2, with an overall chemical oxygen demand (COD) removal of >90% and a maximum coulombic efficiency (CE) of 53% (50 Ω resistor). When the reactor was switched to continuous flow operation at a hydraulic retention time (HRT) of 8 h, the cell voltage was 0.21 ± 0.04 V, with a very high CE = 85%. Voltage was reduced to 0.13 ± 0.03 V at a longer HRT = 16 h due to a lower average COD concentration, and the CE (80%) decreased slightly with increased oxygen intrusion into the reactor per amount of COD removed. Total internal resistance was 33 Ω, with a solution resistance of 2 Ω. These results show that the SEA type MFC can produce stable power and a high CE, making it useful for future continuous flow treatment using actual wastewaters.

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.

Performance of two different types of anodes in membrane electrode assembly microbial fuel cells for power generation from domestic wastewater

Graphite fiber brush electrodes provide high surface areas for exoelectrogenic bacteria in microbial fuel cells (MFCs), but the cylindrical brush format limits more compact reactor designs. To enable MFC designs with closer electrode spacing, brush anodes were pressed up against a separator (placed between the electrodes) to reduce the volume occupied by the brush. Higher maximum voltages were produced using domestic wastewater (COD=390±89mgL−1) with brush anodes (360±63mV, 1000Ω) than woven carbon mesh anodes (200±81mV) with one or two separators. Maximum power densities were similar for brush anode reactors with one or two separators after 30 days (220±1.2 and 240±22mWm−2), but with one separator the brush anode MFC power decreased to 130±55mWm−2 after 114 days. Power densities in MFCs with mesh anodes were very low (<45mWm−2). Brush anodes MFCs had higher COD removals (80±3%) than carbon mesh MFCs (58±7%), but similar Coulombic efficiencies (8.6±2.9% brush; 7.8±7.1% mesh). These results show that compact (hemispherical) brush anodes can produce higher power and more effective domestic wastewater treatment than flat mesh anodes in MFCs.

High hydrogen production rate of microbial electrolysis cell (MEC) with reduced electrode spacing

Practical applications of microbial electrolysis cells (MECs) require high hydrogen production rates and a compact reactor. These goals can be achieved by reducing electrode spacing but high surface area anodes are needed. The brush anode MEC with electrode spacing of 2 cm had a higher hydrogen production rate and energy efficiency than an MEC with a flat cathode and a 1-cm electrode spacing. The maximum hydrogen production rate with a 2 cm electrode spacing was 17.8 m(3)/m(3)d at an applied voltage of E(ap)=1 V. Reducing electrode spacing increased hydrogen production rates at the lower applied voltages, but not at the higher (>0.6 V) applied voltages. These results demonstrate that reducing electrode spacing can increase hydrogen production rate, but that the closest electrode spacing do not necessarily produce the highest possible hydrogen production rates.

Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate

Most microbial electrolysis cells (MECs) contain only a single set of electrodes. In order to examine the scalability of a multiple-electrode design, we constructed a 2.5 L MEC containing 8 separate electrode pairs made of graphite fiber brush anodes pre-acclimated for current generation using acetate, and 304 stainless steel mesh cathodes (64 m 2/m 3). Under continuous flow conditions and a one day hydraulic retention time, the maximum current was 181 mA (1.18 A/m 2, cathode surface area; 74 A/m 3) within three days of operation. The maximum hydrogen production (day 3) was 0.53 L/L-d, reaching an energy efficiency relative to electrical energy input of η E = 144%. Current production remained relatively steady (days 3–18), but the gas composition dramatically shifted over time. By day 16, there was little H 2 gas recovered and methane production increased from 0.049 L/L-d (day 3) to 0.118 L/L-d. When considering the energy value of both hydrogen and methane, efficiency relative to electrical input remained above 100% until near the end of the experiment (day 17) when only methane gas was being produced. Our results show that MECs can be scaled up primarily based on cathode surface area, but that hydrogen can be completely consumed in a continuous flow system unless methanogens can be completely eliminated from the system.

Scalable air cathode microbial fuel cells using glass fiber separators, plastic mesh supporters, and graphite fiber brush anodes

The combined use of brush anodes and glass fiber (GF1) separators, and plastic mesh supporters were used here for the first time to create a scalable microbial fuel cell architecture. Separators prevented short circuiting of closely-spaced electrodes, and cathode supporters were used to avoid water gaps between the separator and cathode that can reduce power production. The maximum power density with a separator and supporter and a single cathode was 75 ± 1 W/m(3). Removing the separator decreased power by 8%. Adding a second cathode increased power to 154 ± 1 W/m(3). Current was increased by connecting two MFCs connected in parallel. These results show that brush anodes, combined with a glass fiber separator and a plastic mesh supporter, produce a useful MFC architecture that is inherently scalable due to good insulation between the electrodes and a compact architecture.

Treatment of biodiesel production wastes with simultaneous electricity generation using a single-chamber microbial fuel cell

Biodiesel production through transesterification of lipids generates large quantity of biodiesel waste (BW) containing mainly glycerin. BW can be treated in various ways including distillation to produce glycerin, use as substrate for fermentative propanediol production and discharge as wastes. This study examined microbial fuel cells (MFCs) to treat BW with simultaneous electricity generation. The maximum power density using BW was 487 ± 28 mW/m(2) cathode (1.5A/m(2) cathode) with 50mM phosphate buffer solution (PBS) as the electrolyte, which was comparable with 533 ± 14 mW/m(2) cathode obtained from MFCs fed with glycerin medium (COD 1400 mg/L). The power density increased from 778 ± 67 mW/m(2) cathode using carbon cloth to 1310 ± 15 mW/m(2) cathode using carbon brush as anode in 200 mM PBS electrolyte. The power density was further increased to 2110 ± 68 mW/m(2) cathode using the heat-treated carbon brush anode. Coulombic efficiencies (CEs) increased from 8.8 ± 0.6% with carbon cloth anode to 10.4 ± 0.9% and 18.7 ± 0.9% with carbon brush anode and heat-treated carbon brush anode, respectively.

Treatment of carbon fiber brush anodes for improving power generation in air–cathode microbial fuel cells

Carbon brush electrodes have been used to provide high surface areas for bacterial growth and high power densities in microbial fuel cells (MFCs). A high-temperature ammonia gas treatment has been used to enhance power generation, but less energy-intensive methods are needed for treating these electrodes in practice. Three different treatment methods are examined here for enhancing power generation of carbon fiber brushes: acid soaking (CF-A), heating (CF-H), and a combination of both processes (CF-AH). The combined heat and acid treatment improve power production to 1370 mW m −2, which is 34% larger than the untreated control (CF-C, 1020 mW m −2). This power density is 25% higher than using only acid treatment (1100 mW m −2) and 7% higher than that using only heat treatment (1280 mW m −2). XPS analysis of the treated and untreated anode materials indicates that power increases are related to higher N1s/C1s ratios and a lower C–O composition. These findings demonstrate efficient and simple methods for improving power generation using graphite fiber brushes, and provide insight into reasons for improving performance that may help to further increase power through other graphite fiber modifications.