Electrochemically assisted methane production in a biofilm reactor

Microbial electrolysis is a new technology for the production of value-added products, such as gaseous biofuels, from waste organic substrates. This study describes the performance of a methane-producing microbial electrolysis cell (MEC) operated at ambient temperature with a Geobacter sulfurreducens microbial bioanode and a methanogenic microbial biocathode. The cell was initially operated at a controlled cathode potential of −850 mV (vs. standard hydrogen electrode, SHE) in order to develop a methanogenic biofilm capable of reducing carbon dioxide to methane gas using abiotically produced hydrogen gas or directly the polarized electrode as electron donors. Subsequently, G. sulfurreducens was inoculated at the anode and the MEC was operated at a controlled anode potential of +500 mV, with acetate serving as electron donor. The rate of methane production at the cathode was found to be primarily limited by the acetate oxidation kinetics and in turn by G. sulfurreducens concentration at the anode of the MEC. Temperature had also a main impact on acetate oxidation kinetics, with an apparent activation energy of 58.1 kJ mol⁻¹.     

Evaluating the performance of microbial fuel cells powering electronic devices

A microbial fuel cell (MFC) is capable of powering an electronic device if we store the energy in an external storage device, such as a capacitor, and dispense that energy intermittently in bursts of high-power when needed. Therefore its performance needs to be evaluated using an energy-storing device such as a capacitor which can be charged and discharged rather than other evaluation techniques, such as continuous energy dissipation through a resistor. In this study, we develop a method of testing microbial fuel cell performance based on storing energy in a capacitor. When a capacitor is connected to a MFC it acts like a variable resistor and stores energy from the MFC at a variable rate. In practice the application of this method to testing microbial fuel cells is very challenging and time consuming; therefore we have custom-designed a microbial fuel cell tester (MFCT). The MFCT evaluates the performance of a MFC as a power source. It uses a capacitor as an energy storing device and waits until a desired amount of energy is stored then discharges the capacitor. The entire process is controlled using an analog-to-digital converter (ADC) board controlled by a custom-written computer program. The utility of our method and the MFCT is demonstrated using a laboratory microbial fuel cell (LMFC) and a sediment microbial fuel cell (SMFC). We determine (1) how frequently a MFC can charge a capacitor, (2) which electrode is current-limiting, (3) what capacitor value will allow the maximum harvested energy from a MFC, which is called the “optimum charging capacitor value,” and (4) what capacitor charging potential will harvest the maximum energy from a MFC, which is called the “optimum charging potential.” Using a LMFC we find that (1) the time needed to charge a 3-F capacitor from 0 to 500 mV is 108 min, (2) the optimum charging capacitor value is 3 F, and (3) the optimum charging potential is 300 mV. Using a SMFC we find that (1) the time needed to charge a 3-F capacitor from 0 to 500 mV is 5 min, (2) the optimum charging capacitor value is 3 F, and (3) the optimum charging potential is 500 mV. Our results demonstrate that the developed method and the MFCT can be used to evaluate and optimize energy harvesting when a MFC is used with a capacitor to power wireless sensors monitoring the environment.     

Anodic biofilm in single-chamber microbial fuel cells cultivated under different temperatures

The microorganisms in anodic biofilms of a microbial fuel cell (MFC) oxidize substrates to generate electrons, protons, and metabolic products. This study started up two single-chamber MFCs at different temperatures (25 °C for MFC A and 15 °C for MFC B); after successful startup, the cell temperatures were swapped. The MFC A had peak voltage at 540 mV at 25 °C, which was decreased rapidly as fed substrate was consumed. At 15 °C, the MFC A yielded a nearly constant voltage of 500 mV over complete feed cycle. Conversely, the MFC B produced higher maximum power than MFC A, and can deliver nearly constant voltage over the entire feed cycle at either 15 or 25 °C. Electrochemical analysis revealed that the MFC B had lower internal resistance than MFC A, with the former having much lower anodic resistance than the latter. Microbial analysis showed that the MFC started up at low temperatures had anodic biofilm enriched with psychrophilic bacteria Simplicispira psychrophila LMG 5408(T)[AF078755] and Geobacter psychrophilus P35(T)[AY653549]. This study suggests the strategy to promote the development of anodic biofilms at low temperatures that are capable of yielding electricity at constant voltage.     

Factors that influence the performance of two-chamber microbial fuel cell

The two-chamber microbial fuel cell (MFC) was operated in batch mode, using acclimated hydrogen-producing mixed bacteria as the anodic inoculum, artificial sucrose wastewater as the substrate (sucrose concentration 10.0 g/L). The performance of the MFC was analyzed at different anodic pH microenvironments, such as the initial pH of the anolyte of 8.57, 7.3, 7.0 and 6.0, respectively, while anodic pH-controlled of 7.3 and 7.0. It showed that the best performance was obtained when the MFC was carried out at anodic pH-controlled of 7.3. Taking the interaction of factors into consideration, we adopted response surface methodology (RSM) to investigate the effects of sucrose concentration, operating temperature and ferrous sulfate concentration on the performance of MFC. The optimum condition for maximum output voltage of the two-chamber MFC (external resistance 1000 Ω) was thus obtained.     

Stable and high energy generation by a strain of Bacillus subtilis in a microbial fuel cell

In this study, the Gram-positive aerobic bacterium Bacillus subtilis has for the first time been employed in a microbial fuel cell (MFC). A glucose-fed MFC with M9 minimal medium in the anode chamber was operated for 3 months, establishing a highly active MFC using filtered M9 medium as the catholyte, carbon cloth as the anode and a 20% platinum electrode as the cathode. The bioelectrical responses of the MFC were characterized by the circuit potential, measured at an average value of 370 mV. A potential of 115 mV appeared to characterize the maximum power produced from a polarization test was 1.05 mW cm⁻² at a resistance of 0.56 kΩ. In situ cyclic voltammograms with and without biofilm anodes were performed in the growth phase and showed that redox metabolites were produced, which varied with physiological status. Voltammograms obtained from a comparative study of broth, supernatant and resuspended bacterial cells revealed that the electrochemical activity in the anode chamber arose from the redox compounds in the supernatant. The results show that the microorganism B. subtilis is electrochemically active and that the electron transfer mechanism is mainly due to the excreted redox compounds (mediator) in the broth solution and not to the membrane-bound proteins.     

Selective inhibition of methanogens for the improvement of biohydrogen production in microbial electrolysis cells

The microbial electrolysis cell (MEC) is a promising technology for producing biohydrogen at greater yield than with conventional technology. However, during a run of an acetate-fed MEC at an applied voltage of 0.5 V, substantial amounts of substrate are consumed in undesirable methanogenesis. Therefore, in order to suppress the methanogens specifically without adversely affecting exoelectrogens, this study examined the effects of sudden changes in pH, temperature and air-exposure, as well as chemical inhibitors, such as 2-bromoethanesulfonate (BES) and lumazine on methanogenesis. An abrupt decrease in temperature and pH from 30 to 20 °C and 7 to 4.9, respectively, had no effect on methanogenesis. Exposing the anode biofilm to air was also ineffective in inhibiting specific methanogens because both methanogens and exoelectrogens were damaged by oxygen. However, an injection of BES (286 μM) reduced the methanogenic electron losses substantially from 36.4 ± 4.4 (= 145.8 ± 17.4 μmol-CH₄) to 2.5 ± 0.3% (= 10.2 ± 1.2 μmol-CH₄), which in turn improved the overall hydrogen efficiency (acetate to H₂) from 56.1 ± 5.7 to 80.1 ± 6.5% (= 3.2 mol-H₂/mol-acetate). Once after inhibited, the inhibitory influence was retained even after 10 batch cycles in the absence of further BES addition. In contrast to BES, methanogenesis was unaffected by lumazine, even at much higher concentrations. The installation of a Nafion membrane resulted in the production of high purity hydrogen at the cathode but hindered proton migration, which caused a serious pH imbalance between the anode and cathode compartments.     

Influence of catholyte pH and temperature on hydrogen production from acetate using a two chamber concentric tubular microbial electrolysis cell

Microbial electrolysis cells (MECs) could be integrated with dark fermentative hydrogen production to increase the overall system yield of hydrogen. The influence of catholyte pH on hydrogen production from MECs and associated parameters such as electrode potentials (vs Ag/AgCl), COD reduction, current density and quantity of acid needed to control pH in the cathode of an MEC were investigated. Acetate (10 mM, HRT 9 h, 24 °C, pH 7) was used as the substrate in a two chamber MEC operated at 600 mV and 850 mV applied voltage. The effect of catholyte pH on current density was more significant at an applied voltage of 600 mV than at 850 mV. The highest hydrogen production rate was obtained at 850 mV, pH 5 amounting to 200 cm³_stp/l_anode/day (coulombic efficiency 60%, cathodic hydrogen recovery 45%, H₂ yield 1.1 mol/mol acetate converted and a COD reduction of 30.5%). Within the range (18.5–49.4 °C) of temperatures tested, 30 °C was found to be optimal for hydrogen production in the system tested, with the performance of the reactor being reduced at higher temperatures. These results show that an optimum temperature (approximately 30 °C) exists for MEC and that lower pH in the cathode chamber improves hydrogen production and may be needed if potentials applied to MECs are to be minimised.     

Inoculation procedures and characterization of membrane electrode assemblies for microbial fuel cells

Membrane electrode assemblies were prepared following procedures adopted in the fabrication of polymer electrolyte membrane (PEM) fuel fells and used in microbial fuel cells (MFCs) with Shewanella oneidensis MR-1 as a single culture and sodium lactate as the electron donor. Improved inoculation procedures were developed and fuel cell performance with the biofilm density of microbes over the anode is discussed. A novel procedure to condition the membrane is also presented. Polarization measurements were carried out and power density plots were generated. Power density values of 300 mW m⁻² are typically obtained while a maximum value of 600 mW m⁻² is demonstrated indicating good performance for a single cell culture.     

Power generation of Shewanella oneidensis MR-1 microbial fuel cells in bamboo fermentation effluent

This study successfully demonstrates the recovery of energy from the effluent of hydrogen fermentation (EHF) by generating electrical power in batch dual-chamber microbial fuel cells (MFCs) inoculated with Shewanella oneidensis MR-1. The effluent obtained from the hydrogen fermentation process of pretreated liquid on Bambusa stenostachya Hack. bamboo which contained organic compounds such as acetate, lactate, and butyrate as carbon sources for Shewanella oneidensis MR-1 and other electro-active microorganisms. Two scenarios of the anolyte of MFC were considered. The first case comprises a supply of 10 mM of lactate in hydrogen fermentation wastewater while the second one is without lactate-supply. The power density and current density of these MFCs were determined to be 0.3–0.6 W/m² and 1.7–2.7 A/m², respectively. The highest voltage generating from MFC without lactate addition was 0.76 V while others were around 0.65 V. The percentage of COD removal on the effluent of hydrogen fermentation ranged from 75% to 83% after 8 operational days followed by the acclimation process. The differences in the impedance characteristics of these MFCs were analyzed by using EIS technique. The average thickness of biofilm formation on the anode electrode was from 7 μm to 23 μm which showed the enhanced electricity production of the MFC system. Moreover, the experimental results demonstrated that the performance of MFC without the lactate supply was better than the other one. Also, its lower substrate consumption efficiency was mentioned.     

Control of dual-chambered microbial fuel cell by anodic potential: Implications with sulfate reducing bacteria

Five dual chamber microbial fuel cell reactors were inoculated with a mixed culture of sulfate-reducing bacteria and fed with artificial wastewater containing lactate and sulfate. A negative poised anode potential enhanced the performance of this fuel cell while a positive poised anode potential or no anode potential had no effect on performance. The effect of this anode potential promoted microbial colonization on the anode surface (biofilm) thereby presenting an effective and successful way for the start-up of a sulfate reducing bacterial microbial fuel cell.     

Anodic potential on dual-chambered microbial fuel cell with sulphate reducing bacteria biofilm

Dual chamber microbial fuel cell reactors were inoculated with a mixed culture of sulfate-reducing bacteria with anode potential being the controlling parameter. A negative poised anode potential enhanced the performance of this fuel cell while a positive poised anode potential had adverse effects on cell performance. Negative anodic potential affects the biofilm characteristics, as evidenced by electrochemical analysis. Microbial community was changed accordingly.     

Improving microbial electrolysis stability using flow-through brush electrodes and monitoring anode potentials relative to thermodynamic minima

Graphite fiber brush electrodes are commonly used in microbial electrolysis cells (MECs) for simultaneous wastewater treatment and electrochemical hydrogen production. Previous brush anode designs for continuous flow systems were configured to have flow over an array of brush electrodes. Here we compared the performance of two systems, one with flow through a single smaller or larger brush anode to an MEC with multiple brushes. The single brush MECs had only a single large brush that had a diameter larger than the chamber height, so that the brush fibers were compressed to nearly (4.5 cm diameter) or completely (5.5 cm diameter) fill the 1.3 cm high anode chamber. To evaluate the time needed for acclimation of the anode potentials were continuously monitored for 138 d (4.5 cm brush) or 143 d (5.5 cm brush). The best performance was obtained using the 5.5 cm brush fibers with a volumetric current density of 554 ± 26 A/m³, compared to <400 A/m³ when using the smaller 4.5 cm brush or the multiple brush reactor. Full acclimation was shown by a consistent and low anode potential, for example by ?248 ± 8 mV (vs. a standard hydrogen electrode) for the 5.5 cm brush, which was only 31 ± 8 mV above the minimum estimated for acetate oxidation under standard biological conditions. These results show that brush compression into a smaller chamber can enhance MEC performance and produce anode potentials close the thermodynamic minima.     

Optimization and microbial community analysis for production of biohydrogen from palm oil mill effluent by thermophilic fermentative process

The optimum values of hydraulic retention time (HRT) and organic loading rate (OLR) of an anaerobic sequencing batch reactor (ASBR) for biohydrogen production from palm oil mill effluent (POME) under thermophilic conditions (60 °C) were investigated in order to achieve the maximum process stability. Microbial community structure dynamics in the ASBR was studied by denaturing gradient gel electrophoresis (DGGE) aiming at improved insight into the hydrogen fermentation microorganisms. The optimum values of 2-d HRT with an OLR of 60 gCOD l⁻¹ d⁻¹ gave a maximum hydrogen yield of 0.27 l H₂ g COD⁻¹ with a volumetric hydrogen production rate of 9.1 l H₂ l⁻¹ d⁻¹ (16.9 mmol l⁻¹ h⁻¹). The hydrogen content, total carbohydrate consumption, COD (chemical oxygen demand) removal and suspended solids removal were 55 ± 3.5%, 92 ± 3%, 57 ± 2.5% and 78 ± 2%, respectively. Acetic acid and butyric acid were the major soluble end-products. The microbial community structure was strongly dependent on the HRT and OLR. DGGE profiling illustrated that Thermoanaerobacterium spp., such as Thermoanaerobacterium thermosaccharolyticum and Thermoanaerobacterium bryantii, were dominant and probably played an important role in hydrogen production under the optimum conditions. The shift in the microbial community from a dominance of T. thermosaccharolyticum to a community where also Caloramator proteoclasticus constituted a major component occurred at suboptimal HRT (1 d) and OLR (80 gCOD l⁻¹ d⁻¹) conditions. The results showed that the hydrogen production performance was closely correlated with the bacterial community structure. This is the first report of a successful ASBR operation achieving a high hydrogen production rate from real wastewater (POME).     

Reduced internal resistance of microbial electrolysis cell (MEC) as factors of configuration and stuffing with granular activated carbon

With limited external applied voltage, the microbial electrolysis cell (MEC) could produce hydrogen by exoelectrogenic microorganisms. The present study revealed that a cubiod-shaped chamber effectively reduces the distance between electrodes and thereby reduces the internal resistance of the entire cell. With 0.6 V of applied voltage, the cuboid MEC had a columbic efficiency of 33.7%, much higher than that achieved in the H-shaped MEC test (ca. 15%) of comparable size. Filling the anode chamber with granular activated carbon further enhanced the columbic efficiency to 45%. The corresponding hydrogen conversion rate could reach 35%.     

Single medium microbial fuel cell: Stainless steel and graphite electrode materials select bacterial communities resulting in opposite electrocatalytic activities

A graphite electrode and a stainless steel electrode immersed in exactly the same medium and polarised at the same potential were colonised by different microbial biofilms. This difference in electroactive microbial population leads stainless steel and graphite to become a microbial cathode and a microbial anode respectively. The results demonstrated that the electrode material can drive the electrocatalytic property of the biofilm opening perspectives for designing single medium MFC.This new discovery led to of the first demonstration of a “single medium MFC.” Such a single medium MFC designed with a graphite anode connected to a stainless steel cathode, both buried in marine sediments, produced 280 mA m^?2 at a voltage of 0.3 V for more than 2 weeks.     

Influence of butyrate on fermentative hydrogen production and microbial community analysis

The effect of butyrate on hydrogen production and the potential mechanism were investigated by adding butyric acid into dark fermentative hydrogen production system at different concentrations at pH range of 5.5–7.0. The results showed that under all the tested pH from 5.5 to 7.0, the addition of butyric acid can inhibit the hydrogen production, and the inhibitory degree (from 10.5% to 100%) increased with the increase of butyric acid concentration and with the decrease of pH values, which suggested that the inhibition effect is highly associated with the concentration of undissociated acids. Substrate utilization rate and VFAs accumulation also decreased with the addition of butyric acid. The microbial community analysis revealed that butyrate addition can decrease the dominant position of hydrogen-producing microorganisms, such as Clostridium, and increase the proportion of other non-hydrogen-producing bacteria, including Pseudomonas, Klebsiella, Acinetobacter, and Bacillus.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

Characterization of energy losses in an upflow single-chamber microbial electrolysis cell

We characterized electrode energy losses and ohmic energy loss in an upflow, single-chamber microbial electrolysis cell (MEC) with no metal catalyst on the cathode. The MEC produced 0.57 m³-H₂/m³-d at an applied voltage of ∼1 V and achieved a cathodic conversion efficiency of 98% and a H₂ yield of 2.4 mol H₂/mol acetate. Eliminating the membrane lowered the ohmic energy loss to 0.005 V, and the pH energy loss became as small as 0.072 V. The lack of metal catalyst on the cathode led to a significant cathode energy loss of 0.56 V. The anode energy loss also was relatively large at 0.395 V, but this was artificial, due to the high positive anode potential, poised at +0.07 V (vs. the standard hydrogen electrode). The energy-conversion efficiency (ECE) was 75% in the single-chamber MEC when the energy input and outputs were compared directly as electrical energy. To achieve an energy benefit out of an MEC (i.e., an ECE >100%), the applied voltage must be less than 0.6 V with a cathodic conversion efficiency over 80%. An ECE of 180% could be achieved if the anode and cathode energy losses were reduced to 0.2 V each.     

Manipulating the hydrogen production from acetate in a microbial electrolysis cell-microbial fuel cell-coupled system

A biological hydrogen-producing system is configured through coupling an electricity-assisting microbial fuel cell (MFC) with a hydrogen-producing microbial electrolysis cell (MEC). The advantage of this biocatalyzed system is the in-situ utilization of the electric energy generated by an MFC for hydrogen production in an MEC without external power supply. In this study, it is demonstrated that the hydrogen production in such an MEC-MFC-coupled system can be manipulated through adjusting the power input on the MEC. The power input of the MEC is regulated by applying different loading resistors connected into the circuit in series. When the loading resistance changes from 10 Ω to 10 kΩ, the circuit current and volumetric hydrogen production rate varies in a range of 78 ± 12 to 9 ± 0 mA m⁻² and 2.9 ± 0.2 to 0.2 ± 0.0 mL L⁻¹ d⁻¹, respectively. The hydrogen recovery (RH₂), Coulombic efficiency (CE), and hydrogen yield (YH₂) decrease with the increase in loading resistance. Thereafter, in order to add power supply for hydrogen production in the MEC, additional one or two MFCs are introduced into this coupled system. When the MFCs are connected in series, the hydrogen production is significantly enhanced. In comparison, the parallel connection slightly reduces the hydrogen production. Connecting several MFCs in series is able to effectively increase power supply for hydrogen production, and has a potential to be used as a strategy to enhance hydrogen production in the MEC-MFC-coupled system from wastes.     

Pyrolyzing pyrite and microalgae for enhanced anode performance in microbial fuel cells

Developing low-cost and high-performance anodes is of great significance for wider applications of microbial fuel cells (MFCs). In this study, microalgae and pyrite were co-pyrolyzed (P/MC) and then coated on carbon felt (CF) with PTFE as a binder. P/MC modification resulted in increased electroactive surface area, superhydrophilicity and higher biocompatibility. Besides, the P/MC-CF anode reduced the charge transfer resistance from 35.1 Ω to 11.4 Ω. The highest output voltage and the maximum power density of the MFC equipped with the P/MC-CF anode were 657 mV and 1266.7 mW/m², respectively, which were much larger than that of the MFC with the CF anode (530 mV, 556.7 mW/m²). The P/MC-CF anode also displayed higher columbic efficiency (39.41%) than the CF anode (32.37%). This work suggests that pyrolyzing microalgae with pyrite is a promising method to enhance the performance of MFCs.