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.     

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.     

Innovative microbial fuel cell for electricity production from anaerobic reactors

A submersible microbial fuel cell (SMFC) was developed by immersing an anode electrode and a cathode chamber in an anaerobic reactor. Domestic wastewater was used as the medium and the inoculum in the experiments. The SMFC could successfully generate a stable voltage of 0.428 ± 0.003 V with a fixed 470 Ω resistor from acetate. From the polarization test, the maximum power density of 204 mW m⁻² was obtained at current density of 595 mA m⁻² (external resistance = 180 Ω). The power generation showed a saturation-type relationship as a function of wastewater strength, with a maximum power density (P_max) of 218 mW m⁻² and a saturation constant (K_s) of 244 mg L⁻¹. The main limitations for achieving higher electricity production in the SMFC were identified as the high internal resistance at the electrolyte and the inefficient electron transfer at the cathode electrode. As the current increased, a large portion of voltage drop was caused by the ohmic (electrolyte) resistance of the medium present between two electrodes, although the two electrodes were closely positioned (about 3 cm distance; internal resistance = 35 ± 2 Ω). The open circuit potential (0.393 V vs. a standard hydrogen electrode) of the cathode was much smaller than the theoretical value (0.804 V). Besides, the short circuit potential of the cathode electrode decreased during the power generation in the SMFC. These results demonstrate that the SMFC could successfully generate electricity from wastewater, and has a great potential for electricity production from existing anaerobic reactors or other anaerobic environments such as sediments. The advantage of the SMFC is that no special anaerobic chamber (anode chamber) is needed, as existing anaerobic reactors can be used, where the cathode chamber and anode electrode are immersed.     

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).     

Scale-up of membrane-free single-chamber microbial fuel cells

Scale-up of microbial fuel cells (MFCs) will require a better understanding of the effects of reactor architecture and operation mode on volumetric power densities. We compared the performance of a smaller MFC (SMFC, 28 mL) with a larger MFC (LMFC, 520 mL) in fed-batch mode. The SMFC produced 14 W m⁻³, consistent with previous reports for this reactor with an electrode spacing of 4 cm. The LMFC produced 16 W m⁻³, resulting from the lower average electrode spacing (2.6 cm) and the higher anode surface area per volume (150 m² m⁻³ vs. 25 m² m⁻³ for the SMFC). The effect of the larger anode surface area on power was shown to be relatively insignificant by adding graphite granules or using graphite fiber brushes in the LMFC anode chamber. Although the granules and graphite brushes increased the surface area by factors of 6 and 56, respectively, the maximum power density in the LMFC was only increased by 8% and 4%. In contrast, increasing the ionic strength of the LMFC from 100 to 300 mM using NaCl increased the power density by 25% to 20 W m⁻³. When the LMFC was operated in continuous flow mode, a maximum power density of 22 W m⁻³ was generated at a hydraulic retention time of 11.3 h. Although a thick biofilm was developed on the cathode surface in this reactor, the cathode potentials were not significantly affected at current densities <1.0 mA cm⁻². These results demonstrate that power output can be maintained during reactor scale-up; increasing the anode surface area and biofilm formation on the cathode do not greatly affect reactor performance, and that electrode spacing is a key design factor in maximizing power generation.     

Electrodeposition of nickel particles on a gas diffusion cathode for hydrogen production in a microbial electrolysis cell

Gas diffusion cathodes with electrodeposited nickel (Ni) particles have been developed and tested for hydrogen production in a continuous flow microbial electrolysis cell (MEC). A high catalytic activity of electrodeposited Ni particles in such a MEC was obtained without a proton exchange membrane, i.e. under direct cathode exposure to anodic liquid. Co-electrodeposition of Pt and Ni particles did not improve any further hydrogen production. The maximum hydrogen production rate was 5.4 L/L_R/day, corresponding to Ni loads between 0.2 and 0.4 mg cm⁻². Continuous MEC operation demonstrated stable hydrogen production for over one month. Owing to the fast hydrogen transport through the cathodic gas diffusion layer, the loss of hydrogen production to methanogenic activity was minimal, generally with less than 5% methane in the off-gas. Overall, gas diffusion cathodes with electrodeposited Ni particles demonstrated excellent stability for hydrogen production compared to expensive Pt cathodes.     

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.     

Anaerobes unleashed: Aerobic fuel cells of Geobacter sulfurreducens

One of the limitations of power generation with microbial fuel cells is that the anode must typically be maintained under anaerobic conditions. When oxygen is present in the anode chamber microorganisms oxidize the fuel with the reduction of oxygen rather than electron transfer to the anode. A system in which fuel is provided from within a graphite anode and diffuses out to the outer surface of the anode was designed to overcome these limitations. A biofilm of Geobacter sulfurreducens strain KN400, pregrown on the surface of a graphite electrode in a traditional two-chambered system with an anaerobic anode chamber and acetate as an external fuel source, produced current just as well under aerobic conditions when acetate was provided via diffusion from an internal concentrated acetate solution. No acetate was detectable in the external medium. In contrast, aerobic systems in which acetate was provided in the external medium completely failed within 48 h. Internally fed anodes colonized by a strain of KN400 adapted to grow at marine salinities produced current in aerobic seawater as well as an anaerobic anode system. The ability to generate current with an anode under aerobic conditions increases the potential applications and design options for microbial fuel cells.     

Electrolyte effects on hydrogen evolution and solution resistance in microbial electrolysis cells

Protonated weak acids commonly used in microbial electrolysis cell (MEC) solutions can affect the hydrogen evolution reaction (HER) through weak acid catalysis, and by lowering solution resistance between the anode and the cathode. Weak acid catalysis of the HER with protonated phosphate, acetate, and carbonate electrolyte species improved MEC performance by lowering the cathode's overpotential by up to 0.30 V at pH 5, compared to sodium chloride electrolytes. Deprotonation of weak acids into charged species at higher pHs improved MEC performance primarily by increasing the electrolyte's conductivity and therefore decreasing the solution resistance between electrodes. The potential contributions from weak acid catalysis and solution resistance were compared to determine whether a reactor would operate more efficiently at lower pH because of the HER, or at higher pH because of solution resistance. Phosphate and acetate electrolytes allowed the MEC to operate more efficiently under more acidic conditions (pH 5). Carbonate electrolytes increased performance from pH 5 to 9 due to a relatively large increases in conductivity. These results demonstrate that specific buffers can substantially contribute to MEC performance through both reduction in cathode overpotential and solution resistance.     

Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells

Hydrogen represents a promising clean fuel for future applications. The biocathode of a two-chambered microbial electrolysis cell (biotic MEC) was studied and compared with an abiotic cathode (abiotic MEC) in order to assess the influence of naturally selected microorganisms for hydrogen production in a wide range of cathode potentials (from −400 to −1800 mV vs SHE). Hydrogen production in both MECs increased when cathode potential was decreased. Microorganisms present in the biotic MEC were identified as Hoeflea sp. and Aquiflexum sp. Supplied energy was utilized more efficiently in the biotic MEC than in the abiotic, obtaining higher hydrogen production respect to energy consumption. At −1000 mV biotic MEC produced 0.89 ± 0.10 m³ H₂ d⁻¹ m⁻³NCC (Net Cathodic Compartment) at a minimum operational cost of 3.2 USD kg⁻¹ H₂. This cost is lower than the estimated market value for hydrogen (6 USD kg⁻¹ H₂).     

An analysis of the performance of an anaerobic dual anode-chambered microbial fuel cell

The performance of a dual anode-chambered microbial fuel cell (MFC) inoculated with Shewanella oneidesis MR-1 was evaluated. This reactor was constructed by incorporating two anode chambers flanking a shared air cathode chamber in an electrically parallel, geometrically stacked arrangement. The device was shown to have the same maximum power density (approximately 24 W m⁻³, normalized by the anode volume) as a single anode-, single cathode-chambered MFC. The dual anode-chambered unit generated a maximum current of 3.66 mA (at 50 Ω), twice the value of 1.69 mA (at 100 Ω) for the single anode-chambered device at approximately the same volumetric current density. Increasing the Pt-coated cathode surface area by 100% (12 to 24 cm²) had no significant effect on the power generation of the dual anode-chambered MFC, indicating that the performance of the device was limited by the anode. The medium recirculation rate and substrate concentration in the anode were varied to determine their effect on the anode-limited power density. At the highest recirculation rate, 5 ml min⁻¹, the power density was about 25% higher than at the lowest recirculation rate, 1 ml min⁻¹. The dependence of the power density on the lactate concentration showed saturation kinetics with a half-saturation constant K_s on the order of 4.4 mM.     

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.     

Hydrogen production in single-chamber tubular microbial electrolysis cells using non-precious-metal catalysts

Platinum has excellent catalytic capabilities and is commonly used as cathode catalyst in microbial electrolysis cells (MECs). Its high cost, however, limits the practical applications of MECs. In this study, precious-metal-free cathodes were developed by electrodepositing NiMo and NiW on a carbon-fiber-weaved cloth material and evaluated in electrochemical cells and tubular MECs with cloth electrode assemblies (CEA). While similar performances were observed in electrochemical cells, NiMo cathode exhibited better performances than NiW cathode in MECs. At an applied voltage of 0.6 V, the MECs with NiMo cathode accomplished a hydrogen production rate of 2.0 m³/day/m³ at current density of 270 A/m³ (12 A/m²), which was 33% higher than that of the NiW MECs and slightly lower than that of the MECs with Pt catalyst (2.3 m³/day/m³). At an applied voltage of 0.4 V, the energy efficiencies based on the electrical energy input reached 240% for the NiMo MECs. These results demonstrated the great potential of using carbon cloth with Ni-alloy catalysts as a cathode material for MECs. The enhanced MEC performances also demonstrate the scale-up potential of the CEA structure, which can significantly reduce the electrode spacing and lower the internal resistance of MECs, thus increasing the hydrogen production rate.     

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.     

Comparison of microbial electrolysis cells operated with added voltage or by setting the anode potential

Hydrogen production in a microbial electrolysis cell (MEC) can be achieved by either setting the anode potential with a potentiostat, or by adding voltage to the circuit with a power source. In batch tests the largest total gas production (46 ± 3 mL), lowest energy input (2.3 ± 0.3 kWh/m³ of H₂ generated), and best overall energy recovery (?_E+S = 58 ± 6%) was achieved at a set anode potential of E_An = −0.2 V (vs Ag/AgCl), compared to set potentials of −0.4 V, 0 V and 0.2 V, or an added voltage of E_ap = 0.6 V. Gas production was 1.4 times higher with E_An = −0.2 V than with E_ap = 0.6 V. Methane production was also reduced at set anode potentials of −0.2 V and higher than the other operating conditions. Continuous flow operation of the MECs at the optimum condition of E_An = −0.2 V initially maintained stable hydrogen gas production, with 68% H₂ and 21% CH₄, but after 39 days the gas composition shifted to 55% H₂ and 34% CH₄. Methane production was not primarily anode-associated, as methane was reduced to low levels by placing the anode into a new MEC housing. These results suggest that MEC performance can be optimized in terms of hydrogen production rates and gas composition by setting an anode potential of −0.2 V, but that methanogen proliferation must be better controlled on non-anodic surfaces.     

Current generation in microbial electrolysis cells with addition of amorphous ferric hydroxide,Tween 80, or DNA

Iron-oxide nanoparticles and the Tween 80 have previously been shown to improve power generation in microbial fuel cells (MFCs), presumably by improving electron transfer from the bacteria to the anode. We examined whether several chemicals would affect current production in single-chamber microbial electrolysis cells (MECs), where hydrogen gas is produced at the cathode, using mixed cultures and Geobacter sulfurreducens. Tween 80 did not increase the current. Fe(OH)₃ addition increased the maximum current density of both the mixed cultures (from 6.1 ± 0.9 A/m² to 8.8 ± 0.3 A/m²) and pure cultures (from 4.8 ± 0.5 A/m² to 7.4 ± 1.1 A/m²). Improved current production was sustained even after iron was no longer added to the medium. It was demonstrated that increased current resulted from improved cathode performance. Analysis using electrochemical impedance spectroscopy (EIS) showed that the iron primarily reduced the diffusion resistances of the cathodes, and scanning electron microscopy (SEM) images showed the formation of highly porous structures on the cathode. The addition of DNA also did not improve MEC or MFC performance. These results demonstrated that among these treatments only Fe(OH)₃ addition was a viable method for enhancing current densities in MECs, primarily by improving cathode performance.     

Optimization of culture conditions and electricity generation using Geobacter sulfurreducens in a dual-chambered microbial fuel-cell

The promise of generating electricity from the oxidation of organic substances using metal-reducing bacteria is drawing attention as an alternate form of bio-technology with positive environmental implications. In this study, we examined various experimental factors to obtain the maximum power output in a dual-chamber mediator-less microbial fuel-cell (MFC) using Geobacter sulfurreducens and acetate as an electron donor in a semi-continuous mode. The G. sulfurreducens culture conditions were optimized in a nutrient buffer containing 20 mM of acetate and 50 mM of fumarate at pH 6.8 and 30 °C. For use in the MFC system, electrodes were made with carbon paper (area: 11.5 cm²) and spaced 1.5 cm apart. Once the MFC was inoculated with the pre-cultured G. sulfurreducens in the anode chamber and while air was continuously sparged to the cathode chamber, the cells produced electricity stably over 60 days with the regular addition of 20 mM acetate, generating the maximum power density of 7 mW/m² with a 5000 Ω load. The current output was significantly increased, by 1.6 times after 20 days of incubation under the same experimental conditions, when the carbon-paper anode was coated with carbon nanotubes.     

Development of a tubular microbial fuel cell (MFC) employing a membrane electrode assembly cathode

Tubular microbial fuel cells (MFC) with air cathode might be amenable to scale-up but with increasing volume a mechanically robust, cost-effective cathode structure is required. Membrane electrode assemblies (MEA) are investigated in a tubular MFC using cost-effective cation (CEM) or anion (AEM) exchange membrane. The MEA fabrication mechanically combines a cathode electrode with the membrane between a perforated cylindrical polypropylene shell and tube. Hydrogel application between membrane and cathode increases cathode potential by ∼100 mV over a 0–5.5 mA range in a CEM-MEA. Consequently, 6.1 W m⁻³ based on reactor liquid volume (200 cm³) are generated compared with 5 W m⁻³ without hydrogel. Cathode potential is also improved in AEM-MEA using hydrogel. Electrochemical Impedance Spectroscopy (EIS) to compare MEA's performance suggests reduced impedance and enhanced membrane–cathode contact area when using hydrogel. The maximum coulombic efficiency observed with CEM-MEA is 71% and 63% with AEM-MEA. Water loss through the membrane varies with external load resistance, indicating that total charge transfer in the MFC is related to electro-osmotic drag of water through the membrane. The MEA developed here has been shown to be mechanically robust, operating for more than six month at this scale without problem.     

Ni foam cathode enables high volumetric H2 production in a microbial electrolysis cell

Valuable, “green” H₂ can be produced with a microbial electrolysis cell (MEC). To achieve a high volumetric production rate of high purity H₂, a continuous flow MEC with an anion exchange membrane, a flow through bioanode and a flow through Ni foam cathode was constructed. At an electrical energy input of 2.6 kWh m⁻³ H₂ (applied cell voltage: 1.00 V), this MEC was able to produce over 50 m³ H₂ m⁻³ MEC d⁻¹ (22.8 ± 0.1 A m⁻²). The MEC had a low cathode overpotential compared to an MEC with Pt-based cathode, because of the high specific surface area of Ni foam (128 m² m⁻² projected area). The MEC performance however, decreased during 32 days of operation due to an increase in anode and cathode overpotentials. Scaling likely caused the increase in anode overpotential, but it remained unclear what caused the increase in cathode overpotential.