Biocatalyzed hydrogen production in a continuous flow microbial fuel cell with a gas phase cathode

A single liquid chamber microbial fuel cell (MFC) with a gas-collection compartment was continuously operated under electrically assisted conditions for hydrogen production. Graphite felt was used for anode construction, while the cathode was made of Pd/Pt coated Toray carbon fiber paper with a catalyst loading of 0.5 mg cm⁻². To achieve hydrogen production, the MFC was connected to a power supply and operated at voltages in a range of 0.5–1.3 V. Either acetate or glucose was used as a source of carbon. At an acetate load of 1.67 g (L_A d)⁻¹, the volumetric rate of hydrogen production reached 0.98 L_STP (L_A d)⁻¹ when a voltage of 1.16 V was applied. This corresponded to a hydrogen yield of 2 mol (mol-acetate)⁻¹ with a 50% conversion efficiency. Throughout the experiment, MFC efficiency was adversely affected by the metabolic activity of methanogenic microorganisms, which competed with exoelectrogenic microorganisms for the carbon source and consumed part of the hydrogen produced at the cathode.     

Polymer coatings as separator layers for microbial fuel cell cathodes

Membrane separators reduce oxygen flux from the cathode into the anolyte in microbial fuel cells (MFCs), but water accumulation and pH gradients between the separator and cathode reduces performance. Air cathodes were spray-coated (water-facing side) with anion exchange, cation exchange, and neutral polymer coatings of different thicknesses to incorporate the separator into the cathode. The anion exchange polymer coating resulted in greater power density (1167 ± 135 mW m⁻²) than a cation exchange coating (439 ± 2 mW m⁻²). This power output was similar to that produced by a Nafion-coated cathode (1114 ± 174 mW m⁻²), and slightly lower than the uncoated cathode (1384 ± 82 mW m⁻²). Thicker coatings reduced oxygen diffusion into the electrolyte and increased coulombic efficiency (CE = 56-64%) relative to an uncoated cathode (29 ± 8%), but decreased power production (255-574 mW m⁻²). Electrochemical characterization of the cathodes ex situ to the MFC showed that the cathodes with the lowest charge transfer resistance and the highest oxygen reduction activity produced the most power in MFC tests. The results on hydrophilic cathode separator layers revealed a trade off between power and CE. Cathodes coated with a thin coating of anion exchange polymer show promise for controlling oxygen transfer while minimally affecting power production.     

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 ± 89 mg L⁻¹) with brush anodes (360 ± 63 mV, 1000 Ω) than woven carbon mesh anodes (200 ± 81 mV) 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 ± 22 mW m⁻²), but with one separator the brush anode MFC power decreased to 130 ± 55 mW m⁻² after 114 days. Power densities in MFCs with mesh anodes were very low (<45 mW m⁻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.     

Biofilm formation and electricity generation of a microbial fuel cell started up under different external resistances

To investigate the effects of external resistance on the biofilm formation and electricity generation of microbial fuel cells (MFCs), active biomass, the content of extracellular polymeric substances (EPS) and the morphology and structure of the biofilms developed at 10, 50, 250 and 1000 Ω are characterized. It is demonstrated that the structure of biofilm plays a crucial role in the maximum power density and sustainable current generation of MFCs. The results show that the maximum power density of the MFCs increases from 0.93 ± 0.02 W m⁻² to 2.61 ± 0.18 W m⁻² when the external resistance decreases from 1000 to 50 Ω. However, on further decreasing the external resistance to 10 Ω, the maximum power density decreased to 1.25 ± 0.01 W m⁻² because of a less active biomass and higher EPS content in the biofilm. Additionally, the 10 Ω MFC shows a highest maximum sustainable current of 8.49 ± 0.19 A m⁻². This result can be attributed to the existence of void spaces beneficial for proton and buffer transport within the anode biofilm, which maintains a suitable microenvironment for electrochemically active microorganisms.     

Hydrogen production from cellulose in a two-stage process combining fermentation and electrohydrogenesis

A two-stage dark-fermentation and electrohydrogenesis process was used to convert the recalcitrant lignocellulosic materials into hydrogen gas at high yields and rates. Fermentation using Clostridium thermocellum produced 1.67 mol H₂/mol-glucose at a rate of 0.25 L H₂/L-d with a corn stover lignocellulose feed, and 1.64 mol H₂/mol-glucose and 1.65 L H₂/L-d with a cellobiose feed. The lignocelluose and cellobiose fermentation effluent consisted primarily of: acetic, lactic, succinic, and formic acids and ethanol. An additional 800 ± 290 mL H₂/g-COD was produced from a synthetic effluent with a wastewater inoculum (fermentation effluent inoculum; FEI) by electrohydrogensis using microbial electrolysis cells (MECs). Hydrogen yields were increased to 980 ± 110 mL H₂/g-COD with the synthetic effluent by combining in the inoculum samples from multiple microbial fuel cells (MFCs) each pre-acclimated to a single substrate (single substrate inocula; SSI). Hydrogen yields and production rates with SSI and the actual fermentation effluents were 980 ± 110 mL/g-COD and 1.11 ± 0.13 L/L-d (synthetic); 900 ± 140 mL/g-COD and 0.96 ± 0.16 L/L-d (cellobiose); and 750 ± 180 mL/g-COD and 1.00 ± 0.19 L/L-d (lignocellulose). A maximum hydrogen production rate of 1.11 ± 0.13 L H₂/L reactor/d was produced with synthetic effluent. Energy efficiencies based on electricity needed for the MEC using SSI were 270 ± 20% for the synthetic effluent, 230 ± 50% for lignocellulose effluent and 220 ± 30% for the cellobiose effluent. COD removals were ∼90% for the synthetic effluents, and 70–85% based on VFA removal (65% COD removal) with the cellobiose and lignocellulose effluent. The overall hydrogen yield was 9.95 mol-H₂/mol-glucose for the cellobiose. These results show that pre-acclimation of MFCs to single substrates improves performance with a complex mixture of substrates, and that high hydrogen yields and gas production rates can be achieved using a two-stage fermentation and MEC process.     

Power generation using carbon mesh cathodes with different diffusion layers in microbial fuel cells

An inexpensive carbon material, carbon mesh, was examined to replace the more expensive carbon cloth usually used to make cathodes in air-cathode microbial fuel cells (MFCs). Three different diffusion layers were tested using carbon mesh: poly(dimethylsiloxane) (PDMS), polytetrafluoroethylene (PTFE), and Goretex cloth. Carbon mesh with a mixture of PDMS and carbon black as a diffusion layer produced a maximum power density of 1355 ± 62 mW m⁻² (normalized to the projected cathode area), which was similar to that obtained with a carbon cloth cathode (1390 ± 72 mW m⁻²). Carbon mesh with a PTFE diffusion layer produced only a slightly lower (6.6%) maximum power density (1303 ± 48 mW m⁻²). The Coulombic efficiencies were a function of current density, with the highest value for the carbon mesh and PDMS (79%) larger than that for carbon cloth (63%). The cost of the carbon mesh cathode with PDMS/Carbon or PTFE (excluding catalyst and binder costs) is only 2.5% of the cost of the carbon cloth cathode. These results show that low cost carbon materials such as carbon mesh can be used as the cathode in an MFC without reducing the performance compared to more expensive carbon cloth.     

The use of double-sided cloth without diffusion layers as air-cathode in microbial fuel cells

The cost of electrode materials is one of the most important factors limiting the scale of microbial fuel cells (MFCs). In this study, a novel double-sided cloth (DC) without diffusion layer is using as air-cathode, which decreases the cost and simplifies electrode production process. Using Pt as catalyst, the maximum power density of MFC using DC cathode is 0.70 ± 0.02 W m⁻², which is similar to that obtained using carbon cloth (CC) cathodes (0.66 ± 0.01 W m⁻²). After running in stable status, the Coulombic efficiencies (CEs) (18 ± 1%) and COD removal rates (75 ± 3%) are almost the same as those of CC cathode with diffusion layers. Using carbon powder as catalyst on the DC cathode, the maximum powder density is 0.41 ± 0.01 W m⁻², with a COD removal rate of 66 ± 2% and a CE of 13.9 ± 0.5%. The total cost of cathode based on power output decreases as follows: CC with Pt (CC-Pt, 2652$ W⁻¹), DC with Pt (DC-Pt, 1007$ W⁻¹) and DC with carbon powder (DC-C, 22$ W⁻¹), showing that DC is an inexpensive and promising cathode material for future applications.     

Improved performance of microbial fuel cell using combination biocathode of graphite fiber brush and graphite granules

The efficiency and sustainability of microbial fuel cell (MFC) are heavily dependent on the cathode performance. We show here that the use of graphite fiber brush (GBF) together with graphite granules (GGs) as a basal material for biocathode (MFC reactor type R1) significantly improve the performance of a MFC compared with MFCs using GGs (MFC reactor type R2) or GFB (MFC reactor type R3) individually. Compared with R3, the use of the combination biocathode (R1) can shorten the start-up time by 53.75%, improve coulombic efficiencies (CEs) by 21.0 ± 2.7% at external resistance (R_EX) of 500 Ω, and increase maximum power densities by 38.2 ± 12.6%. Though the start-up time and open circuit voltage (OCV) of the reactor R2 are similar to R1, the CE (R_EX = 500 Ω) and maximum power density of R2 are 21.4 ± 1.7% and 38.2 ± 15.6% lower than that of R1. Fluorescence in situ hybridization (FISH) analyses indicate the bacteria on cathodes of R1 and R2 are richer than that of R3. Molecular taxonomic analyses reveal that the biofilm formed on the biocathode surface is dominated by strains belonging to Nitrobacter, Achromobacter, Acinetobacter, and Bacteroidetes. Combination of GFB and GGs as biocathode material in MFC is more efficient and can achieve sustainable electricity recovery from organic substances, which substantially increases the viability and sustainability of MFCs.     

A graphite-granule membrane-less tubular air-cathode microbial fuel cell for power generation under continuously operational conditions

An air-cathode microbial fuel cell (MFC) is an efficient and sustainable MFC configuration for recovering electrical energy from organic substances. In this paper, we developed a graphite-granule anode, tubular air-cathode MFC (GTMFC) capable of continuous electricity generation from glucose-based substrates. This GTMFC produced a maximum volumetric power of 50.2 W m⁻³ at current density of 216 A m⁻³ (R_EX = 22 Ω). Electrochemistry impedance spectroscopy (EIS) measurements demonstrated an overall internal resistance of 27 Ω, consisting of ohmic resistance of R_ohm = 13.8 Ω (51.1%), a charge-transfer resistance of R_c = 6.1 Ω (22.6%) and a diffusion resistance of R_d = 7.2 Ω (26.3%). Power generation with respect to initial chemical oxygen demand (COD) concentration was described well by an exponential saturation model. Recirculation was to found to have a significant effect on electrochemical performance at low COD concentrations, while such effect was absent at high COD concentrations. This study suggests a feasible and simple method to reduce internal resistance and improve power generation of sustainable air-cathode MFCs.     

Recycled tire crumb rubber anodes for sustainable power production in microbial fuel cells

One of the greatest challenges facing microbial fuel cells (MFCs) in large scale applications is the high cost of electrode material. We demonstrate here that recycled tire crumb rubber coated with graphite paint can be used instead of fine carbon materials as the MFC anode. The tire particles showed satisfactory conductivity after 2-4 layers of coating. The specific surface area of the coated rubber was over an order of magnitude greater than similar sized graphite granules. Power production in single chamber tire-anode air-cathode MFCs reached a maximum power density of 421 mW m⁻², with a coulombic efficiency (CE) of 25.1%. The control graphite granule MFC achieved higher power density (528 mW m⁻²) but lower CE (15.6%). The light weight of tire particle could reduce clogging and maintenance cost but posts challenges in conductive connection. The use of recycled material as the MFC anodes brings a new perspective to MFC design and application and carries significant economic and environmental benefit potentials.     

Microbial fuel cell cathode with dendrimer encapsulated Pt nanoparticles as catalyst

In this paper, we investigated the use of polyamidoamine (PAMAM) dendrimer-encapsulated platinum nanoparticles (Pt-DENs) as a promising type of cathode catalyst for air-cathode single chamber microbial fuel cells (SCMFCs). The Pt-DENs, prepared via template synthesis method, have uniform diameter distribution with size range of 3-5 nm. The Pt-DENs then loaded on to a carbon substrate. For comparison, we also electrodeposited Pt on carbon substrate. The calculation shows that the loading amount of Pt-DENs on carbon substrate is about 0.1 mg cm⁻², which is three times lower than that of the electrodeposited Pt (0.3 mg cm⁻²). By measuring batch experiments, the results show that Pt-DENs in air-cathode SCMFCs have a power density of 630 ± 5 mW m⁻² and a current density of 5200 ± 10 mA m⁻² (based on the projected anodic surface area), which is significantly better than electrodeposited Pt cathodes (power density: 275 ± 5 mW m⁻² and current density: 2050 ± 10 mA m⁻²). Additionally, Pt-DENs-based cathodes resulted in a higher power production with 129.1% as compared to cathode with electrodeposited Pt. This finding suggests that Pt-DENs in MFC cathodes is a better catalyst and has a lower loading amount than electrodeposited Pt, and may serve as a novel and alternative catalyst to previously used noble metals in MFC applications.     

Electricity generation by a baffle-chamber membraneless microbial fuel cell

Microbial fuel cells (MFCs) for organic waste and wastewater treatment represent innovative technologies for pollution control and energy generation. The research reported here considers the influence of reactor configurations designed to mitigate the impact of oxygen transport on electricity generation by a baffle-chamber membraneless MFC. The reactor was constructed to reduce mixing in the vicinity of the cathode and facilitate thick (>1 mm) biofilm formation on the cathode by adding anaerobic biomass/sludge (4330 ± 410 mg COD L⁻¹), resulting in an overall coulombic efficiency of more than 30% at glucose concentrations ranging from 96 to 960 mg COD L⁻¹, compared to previously reported efficiencies <10% in a completely mixed membraneless MFC. Efficiencies in the absence of anaerobic sludge dropped to 21.2 ± 3.7%, suggesting that the importance of pH buffering provided by the biomass in improving electron transport to the anode. However, the anaerobic sludge itself provided very limited power (approximately 0.3 mW m⁻²) and power generation was primarily associated with glucose degradation (e.g., 129 ± 15 mW m⁻²).     

Manganese dioxide-coated carbon nanotubes as an improved cathodic catalyst for oxygen reduction in a microbial fuel cell

To develop an efficient and cost-effective cathodic electrocatalyst for microbial fuel cells (MFCs), carbon nanotubes (CNTs) coated with manganese dioxide using an in situ hydrothermal method (in situ MnO₂/CNTs) have been investigated for electrochemical oxygen reduction reaction (ORR). Examination by transmission electron microscopy shows that MnO₂ is sufficiently and uniformly dispersed over the surfaces of the CNTs. Using linear sweep voltammetry, we determine that the in situ MnO₂/CNTs are a better catalyst for the ORR than CNTs that are simply mechanically mixed with MnO₂ powder, suggesting that the surface coating of MnO₂ onto CNTs enhances their catalytic activity. Additionally, a maximum power density of 210 mW m⁻² produced from the MFC with in situ MnO₂/CNTs cathode is 2.3 times of that produced from the MFC using mechanically mixed MnO₂/CNTs (93 mW m⁻²), and comparable to that of the MFC with a conventional Pt/C cathode (229 mW m⁻²). Electrochemical impedance spectroscopy analysis indicates that the uniform surface dispersion of MnO₂ on the CNTs enhanced electron transfer of the ORR, resulting in higher MFC power output. The results of this study demonstrate that CNTs are an ideal catalyst support for MnO₂ and that in situ MnO₂/CNTs offer a good alternative to Pt/C for practical MFC applications.     

Enhanced Coulombic efficiency and power density of air-cathode microbial fuel cells with an improved cell configuration

Single chamber air-cathode microbial fuel cells (MFCs) that lack a proton exchange membrane (PEM) hold a great promise for many practical applications due to their low operational cost, simple configuration and relative high power density. One of the great challenges for PEM-less MFC is that the Coulombic efficiency is much lower than those containing PEM. In this study, single-chamber PEM-less MFCs were adapted by applying a J-Cloth layer on the water-facing side of air cathode. Due to the significant reduction of oxygen diffusion by the J-Cloth, the MFCs with two-layers of J-Cloth demonstrated an over 100% increase in Coulombic efficiency in comparison with those without J-Cloth (71% versus 35%) at the same current density of 0.6 mA cm⁻². A new cell configuration, cloth electrode assembly (CEA), therefore, was designed by sandwiching the cloth between the anode and the cathode. Such an MFC configuration greatly reduced the internal resistance, resulting in a power density of 627 W m⁻³ when operated in fed-batch mode and 1010 W m⁻³ in continuous-flow mode, which is the highest reported power density for MFCs and more than 15 times higher than those reported for air-cathode MFCs using similar electrode materials. This study indicates that the Coulombic efficiency and power density of air-cathode MFCs can be improved significantly using an inexpensive cloth layer, which greatly increases the feasibility for the practical applications of MFCs.     

Improved performance of air-cathode single-chamber microbial fuel cell for wastewater treatment using microfiltration membranes and multiple sludge inoculation

Substantial optimization and cost reduction are required before microbial fuel cells (MFCs) can be practically applied. We show here the performance improvement of an air-cathode single-chamber MFC by using a microfiltration membrane (MFM) on the water-facing side of the cathode and using multiple aerobic sludge (AES), anaerobic sludge (ANS), and wetland sediment (WLS) as anodic inoculums. Batch test results show that the MFC with an MFM resulted in an approximately two-fold increase in maximum power density compared to the MFC with a proton exchange membrane (PEM). The Coulombic efficiency increased from 4.17% to 5.16% in comparison with the membrane-less MFC, without a significant negative effect on power generation and internal resistance. Overall performance of the MFC was also improved by using multiple sludge inoculums in the anode. The MFC inoculated with ANS + WLS produced the greatest maximal power density of 373 mW m⁻² with a substantially low internal resistance of 38 Ω. Higher power density with a decreased internal resistance was also achieved in MFC inoculated with ANS + AES and ANS + AES + WLS in comparison with those inoculated with only one sludge. The MFCs inoculated with AES + ANS achieved the highest Coulombic efficiency. Over 92% COD was removed from confectionery wastewater in all tested MFCs, regardless of the membrane or inoculum used.     

Integrating engineering design improvements with exoelectrogen enrichment process to increase power output from microbial fuel cells

Microbial fuel cells (MFC) hold promise as a green technology for bioenergy production. The challenge is to improve the engineering design while exploiting the ability of microbes to generate and transfer electrons directly to electrodes. A strategy using a combination of improved anode design and an enrichment process was formulated to improve power densities. The design was based on a flow-through anode with minimal dead volume and a high electrode surface area per unit volume. The strategy focused on promoting biofilm formation via a combination of forced flow through the anode, carbon limitation, and step-wise reduction of external resistance. The enrichment process resulted in development of exoelectrogenic biofilm communities dominated by Anaeromusa spp. This is the first report identifying organisms from the Veillonellaceae family in MFCs. The power density of the resulting MFC using a ferricyanide cathode reached 300 W m⁻³ net anode volume (3220 mW m⁻²), which is about a third of what is estimated to be necessary for commercial consideration. The operational stability of the MFC using high specific surface area electrodes was demonstrated by operating the MFC for a period of over four months.     

Simultaneous processes of electricity generation and ceftriaxone sodium degradation in an air-cathode single chamber microbial fuel cell

A single chamber microbial fuel cell (MFC) with an air-cathode is successfully demonstrated using glucose-ceftriaxone sodium mixtures or ceftriaxone sodium as fuel. Results show that the ceftriaxone sodium can be biodegraded and produce electricity simultaneously. Interestingly, these ceftriaxone sodium-glucose mixtures play an active role in production of electricity. The maximum power density is increased in comparison to 1000 mg L⁻¹ glucose (19 W m⁻³) by 495% for 50 mg L⁻¹ ceftriaxone sodium + 1000 mg L⁻¹ glucose (113 W m⁻³), while the maximum power density is 11 W m⁻³ using 50 mg L⁻¹ ceftriaxone sodium as the sole fuel. Moreover, ceftriaxone sodium biodegradation rate reaches 91% within 24 h using the MFC in comparison with 51% using the traditional anaerobic reactor. These results indicate that some toxic and bio-refractory organics such as antibiotic wastewater might be suitable resources for electricity generation using the MFC technology.     

Automatic control of load increases power and efficiency in a microbial fuel cell

Increasing power production and coulombic efficiency (CE) of microbial fuel cells (MFCs) is a common research ambition as the viability of the technology depends to some extent on these measures of performance. As MFCs are typically time varying systems, comparative studies of controlled and un-controlled external load impedance are needed to show if control affects the biocatalyst development and hence MFC performance. The application of logic based control of external load resistance is shown to increase the power generated by the MFC, when compared to an equivalent system which has a static resistive load. The controlled MFC generated 1600 ± 400 C, compared to 300 ± 10 C with an otherwise replicate fixed load MFC system. The use of a parsimonious gradient based control was able to increase the CE to within the range of 15.1-22.7%, while the CE for a 200 Ω statically loaded MFC lay in the range 3.3-3.7%. The controlled MFC improves the electrogenic anodic biofilm selection for power production, indicating that greater power and substrate conversion can be achieved by controlling load impedance. Load control ensured sustainable current demand, applied microbial selection pressures and provided near-optimal impedance for power transference, compared to the un-controlled system.     

Bioelectrochemical analyses of a thermophilic biocathode catalyzing sustainable hydrogen production

To achieve sustainable hydrogen production by microbial electrolysis cell (MEC) without precious metal catalysts, we examined the potential of thermophilic microorganisms as biocatalysts on the cathode of MEC. A biocathode was firstly developed in a single-chambered MEC operated at 55 °C and further analyzed in a two-chambered MEC. Linear sweep voltammetry showed that the biocathode had a reducing activity significantly higher than the control electrodes (bioanode or non-inoculated electrode). At the potential of −0.8 V vs. SHE, the thermophilic biocathode produced a current density of 1.28 ± 0.15 A m⁻² and an H₂ production rate of 376.5 ± 73.42 mmol day⁻¹ m⁻², which were around 10 times higher than those of the non-inoculated electrode, with the cathodic H₂ recovery of ca. 70%. The molecular-phylogenetic analysis of the bacteria on the biocathode indicated that the community was comprised of six phyla, in which Firmicutes was the most populated phylum (77% of the clones in the 16S rRNA library).     

A pilot-scale study on utilizing multi-anode/cathode microbial fuel cells (MAC MFCs) to enhance the power production in wastewater treatment

A new type of microbial fuel cell (MFC), multi-anode/cathode MFC (termed as MAC MFC) containing 12 anodes/cathodes were developed to harvest electric power treating domestic wastewater. The power density of MAC MFCs increased from 300 to 380 mW/m² at the range of the organic loading rates (0.19-0.66 kg/m³/day). MAC MFCs achieved 80% of contaminant removal at the hydraulic retention time (HRT) of 20 h but the contaminant removal deceased to 66% at the HRT of 5 h. In addition, metal-doped manganese dioxide (MnO₂) cathodes were developed to replace the costly platinum cathodes, and exhibited high power density. Cu-MnO₂ cathodes produced 465 mW/m² and Co-MnO₂ cathodes produced 500 mW/m². Due to the cathode fouling of the precipitation of calcium and sodium, a decrease in the power density (from 400 to 150 mW/m²) and an increase in internal resistance (R_in) (from 175 to 225 Ω) were observed in MAC MFCs.