The importance of OH− transport through anion exchange membrane in microbial electrolysis cells

In two-chamber microbial electrolysis cells (MECs) with anion exchange membranes (AEMs), a phosphate buffer solution (PBS) is typically used to avoid increases in catholyte pH as Nernst equation calculations indicate that high pHs adversely impact electrochemical performance. However, ion transport between the chambers will also impact performance, which is a factor not included in those calculations. To separate the impacts of pH and ion transport on MEC performance, a high molecular weight polymer buffer (PoB), which was retained in the catholyte due to its low AEM transport and cationic charge, was compared to PBS in MECs and abiotic electrochemical half cells (EHCs). In MECs, catholyte pH control was less important than ion transport. MEC tests using the PoB catholyte, which had a higher buffer capacity and thus maintained a lower catholye pH (<8), resulted in a 50% lower hydrogen production rate (HPR) than that obtained using PBS (HPR = 0.7 m³-H₂ m^?3 d^?1) where the catholyte rapidly increased to pH = 12. The main reason for the decreased performance using PoB was a lack of hydroxide ion transfer into the anolyte to balance pH. The anolyte pH in MECs rapidly decreased to 5.8 due to a lack of hydroxide ion transport, which inhibited current generation by the anode, whereas the pH was maintained at 6.8 using PBS. In abiotic tests in ECHs, where the cathode potential was set at ?1.2 V, the HPR was 133% higher using PoB than PBS due to catholyte pH control, as the anolyte pH was not a factor in the performance. These results show that maintaining charge transfer to control anolyte pH is more important than obtaining a more neutral pH catholyte.     

Hydrogen production rates with closely-spaced felt anodes and cathodes compared to brush anodes in two-chamber microbial electrolysis cells

Flat anodes placed close to the cathode or membrane to reduce distances between electrodes in microbial electrolysis cells (MECs) could be used to develop compact reactors, in contrast to microbial fuel cells (MFCs) where electrodes cannot be too close due to oxygen crossover from the cathode to the anode that reduces performance. Graphite fiber brush anodes are often used in MECs due to their proven performance in MFCs. However, brush anodes have not been directly compared to flat anodes in MECs, which are completely anaerobic, and therefore oxygen crossover is not a factor for felt or brush anodes. MEC performance was compared using flat felt or brush anodes in two-chamber, cubic type MECs operated in fed-batch mode, using acetate in a 50 mM phosphate buffer. Despite placement of felt anodes next to the membrane, MECs with felt anodes had a lower hydrogen gas production rate of 0.32 ± 0.02 m³-H₂/m³-d than brush anodes (0.38 ± 0.02 m³-H₂/m³-d). The main reason for the reduced performance was substrate-limited mass transfer to the felt anodes. To reduce mass transfer limitations, the felt anode electrolyte was stirred, which increased the hydrogen gas production rate to 0.41 ± 0.04 m³-H₂/m³-d. These results demonstrate brush electrodes can improve performance of bioelectrochemical reactors even under fully anaerobic conditions.     

Microbial electrolysis cell powered by an aluminum-air battery for hydrogen generation,in-situ coagulant production and wastewater treatment

Microbial electrolysis cells (MECs) are an efficient technology for generating hydrogen gas from organic matters, but an additional voltage is needed to overcome the thermodynamic barrier of the reaction. A combined system of MEC and the aluminum-air battery (Al-air battery) was designed for hydrogen generation, coagulant production and operated in an energy self-sufficient mode. The Al-air battery (28 mL) produced a voltage ranged from 0.58 V to 0.80 V, which powered an MEC (28 mL) to produce hydrogen. The hydrogen production rate reached 0.19 ± 0.01 m³ H₂/m³/d and 14.5 ± 0.9 mmol H₂/g COD. The total COD removal rate was 85 ± 1%, of which MEC obtained 75 ± 1% COD removal and 10 ± 1% COD removal was achieved by in-situ coagulating process. The microorganisms removal of MEC effluent was 97 ± 2% through ex-situ coagulating process. These results showed that the Al-air battery-MEC system can be operated in energy self-sufficient mode and recovered energy from wastewater with high quality effluent.     

Electrochemical evaluation of molybdenum disulfide as a catalyst for hydrogen evolution in microbial electrolysis cells

There is great interest in hydrogen evolution in bioelectrochemical systems, such as microbial electrolysis cells (MECs), but these systems require non-optimal near-neutral pH conditions and the use of low-cost, non-precious metal catalysts. Here we show that molybdenum disulfide (MoS₂) composite cathodes have electrochemical performance superior to stainless steel (SS) (currently the most promising low-cost, non-precious metal MEC catalyst) or Pt-based cathodes in phosphate or perchlorate electrolytes, yet they cost ∼4.5 times less than Pt-based composite cathodes. At current densities typical of many MECs (2-5 A/m²), the optimal surface density with MoS₂ particles on carbon cloth was 25 g/m², achieving 31 mV less hydrogen evolution overpotential than similarly constructed Pt cathodes in galvanostatic tests with a phosphate buffer. At higher current densities (8-10 A/m²) the MoS₂ catalyst had 82 mV less hydrogen evolution overpotential than the Pt-based catalyst. MoS₂ composite cathodes performed similarly to Pt cathodes in terms of current densities, hydrogen production rates and COD removal over several batch cycles in MEC reactors. These results show that MoS₂ can be used to substantially reduce the cost of cathodes used in MECs for hydrogen gas production.     

Hydrogen gas production with Ni,Ni–Co and Ni–Co–P electrodeposits as potential cathode catalyst by microbial electrolysis cells

The development of efficient and economical cathode, operating at ambient temperature and neutral pH is a crucial challenge for microbial electrolysis cell (MEC) to become commercialize hydrogen production technology. In the present work, eight different electrodes are prepared by the electroplating of Ni, Ni–Co and Ni–Co–P on two base metals i.e., Stainless Steel 316 and Copper separately to use as cathode in MEC. Electrodeposited cathode materials have been characterized by XRD, XPS, FESEM, EDX and linear voltammetry. The fabricated cathodes show higher corrosion stability with improved electro-catalytic performance for the hydrogen production in the MECs as compared to the bare cathodes (SS316 and Cu). Data obtained from linear voltammetry and MEC experiments show that developed cathode possess four times higher intrinsic catalytic activity in comparison to bare cathode. Electrodeposited cathodes are intensively examined in membrane-less MEC, operating under applied voltage of 0.6 V in batch mode at 30 ± 2 °C temperature, in neutral pH with acetate as substrate and activated sludge as inoculum. Ni–Co–P electrodeposit on Stainless Steel 316 cathode gives maximum hydrogen production rate of 4.2 ± 0.5 m³(H₂)m⁻³d⁻¹, columbic efficiencies 96.9 ± 2%, overall hydrogen recovery 90.3 ± 4%, overall energy efficiency 241.2 ± 5%, volumetric current density 310 ± 5 Am⁻³. The net energy recovery and COD removal are 4.25 kJ/gCOD and 61%, respectively. Prepared cathodes show stable performance for continuous 5 batch cycle operations in MEC.     

Hydrogen production in microbial electrolysis cells: Choice of catholyte

A catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs). Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffer solution (PBS) resulted in the highest hydrogen production rate of 0.237 ± 0.031 m³H₂/m³/d, followed by 0.171 ± 0.012 m³H₂/m³/d with a 134 mM NaCl solution and 0.171 ± 0.004 m³H₂/m³/d with the acidified water adjusted with sulfuric acid. The MEC with all catholytes achieved good organic removal efficiency, but the removal rate varied following the trend of the hydrogen production rate. The reuse of the catholyte for an extended period led to a decreasing hydrogen production rate, affected by the elevated pH. The cost of both the acidified water and the NaCl solution was much lower than the PBS, and therefore, they could be a better choice as an MEC catholyte with further consideration of cost reduction and chemical reuse/disposal.     

Hydrogen production by co-cultures of Clostridium butyricum and Rhodospeudomonas palustris: Optimization of yield using response surface methodology

Both dark and photo-fermentation can be used for biological hydrogen production; either performed separately, in two-stage systems, or in co-culture. A single stage process is less laborious and costly; however, the two types of microorganisms have different nutritional requirements requiring optimization of culture conditions. Here a response surface methodology (RSM) with a Box-Behnken design was used to optimize microorganism ratio and substrate and buffer concentrations, and to evaluate their interactive effects for maximization of hydrogen yield. Clostridium butyricum and Rhodopseudomonas palustris were grown on a potato starch/glucose base medium at 30 °C under continuous illumination (40 W m^?2 light intensity). The highest hydrogen yield, 6.4 ± 1.3 mol H₂/mol glucose, was obtained with a substrate concentration of 15 g/L, buffer concentration of 50 mM, and microorganism ratio of 3. The observed strong interaction between buffer and substrate concentration is most likely due to the need to optimize the pH for co-cultures.     

Optimization of catholyte concentration and anolyte pHs in two chamber microbial electrolysis cells

The hydrogen production rate in a microbial electrolysis cell (MEC) using a non-buffered saline catholyte (NaCl) can be optimized through proper control of the initial anolyte pH and catholyte NaCl concentration. The highest hydrogen yield of 3.3 ± 0.4 mol H₂/mole acetate and gas production rate of 2.2 ± 0.2 m³ H₂/m³/d were achieved here with an initial anolyte pH = 9 and catholyte NaCl concentration of 98 mM. Further increases in the salt concentration substantially reduced the anolyte pH to as low as 4.6, resulting in reduced MEC performance due to pH inhibition of exoelectrogens. Cathodic hydrogen recovery was high (r_cat > 90%) as hydrogen consumption by hydrogenotrophic methanogens was prevented by separating the anode and cathode chambers using a membrane. These results show that the MEC can be optimized for hydrogen production through proper choices in the concentration of a non-buffered saline catholyte and initial anolyte pH in two chamber MECs.     

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.     

An effective method for hydrogen production in a single-chamber microbial electrolysis by negative pressure control

In this study, we construct a scalable tubular single-chamber microbial electrolysis cell that using negative pressure (40.52 kPa) to enhance the hydrogen production. The impact of negative pressure on current production, hydrogen recovery, and microbial community of microbial electrolysis cells are investigated. Negative pressure could effectively enhance the hydrogen recovery and inhibit the growth of methanogens. Consequently, the microbial electrolysis cell operated under negative pressure achieves a maximum hydrogen production rate of 7.72 ± 0.06 L L^?1 d^?1, which is more than four times higher that of reactor running under normal pressure (1.51 ± 0.41 L L^?1 d^?1). Energy quantification shows that the electrical energy recovery under negative pressure is 146.98%, which is much higher than 95.00% under normal pressure. Therefore, negative pressure control is as effective for increasing hydrogen production and energy recovery in the scalable MEC, and has a great practical application prospect. However, negative pressure cannot knick out methanogens. Once negative pressure is removed, methanogens will quickly take over and after that applying negative pressure again can only partly inhibit methane production.     

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.     

Comparison of chemosynthetic and biological surfactants on accelerating hydrogen production from waste activated sludge in a short-cut fermentation-bioelectrochemical system

The effects of chemosynthetic and biological surfactants on accelerating hydrogen generation from waste activated sludge (WAS) is investigated in a short-cut fermentation-bioelectrochemical system. The specific experiments are conducted in a series of completely stirred tank reactors (CSTRs) and single-chamber microbial electrolysis cells (MECs). Results shows that rhamnolipid (RL) lead to a VFAs yield 1.16-fold and 3.63-fold higher over with sodium dodecylsulphate (SDS) and sodium dodecyl benzene sulfonate (SDBS) treatments in CSTRs on 72 h. By contrast, the corresponding conversion efficiency of methanogenesis is inhibited (0.18 ± 0.03% versus 1.89 ± 0.15% (SDS) and 6.63 ± 0.77% (SDBS)), which is beneficial for subsequent hydrogen production in MECs. The distribution of the acidogenesis metabolites is also affected by the types of surfactants, reflected on cascade changing of hydrogen production. Highest hydrogen yield is 12.90 mg H₂ g^?1 VSS in RL-MECs, which is larger than all values that have been reported for fermentation and single-chamber MECs. Current and electrochemical impedance spectroscopy clearly demonstrate the important role of RL treatment in electron/proton transfer and the internal resistance decrease. This study demonstrate the sustainability and attractiveness of WAS short-cut fermentation-elelctrohydrogenesis, providing a sound basis for sludge stabilization and bioenergy recovery.     

Performance optimization of microbial electrolysis cell (MEC) for palm oil mill effluent (POME) wastewater treatment and sustainable Bio-H2 production using response surface methodology (RSM)

Microbial electrolysis cells (MECs) are a new bio-electrochemical method for converting organic matter to hydrogen gas (H₂). Palm oil mill effluent (POME) is hazardous wastewater that is mostly formed during the crude oil extraction process in the palm oil industry. In the present study, POME was used in the MEC system for hydrogen generation as a feasible treatment technology. To enhance biohydrogen generation from POME in the MEC, an empirical model was generated using response surface methodology (RSM). A central composite design (CCD) was utilized to perform twenty experimental runs of MEC given three important variables, namely incubation temperature, initial pH, and influent dilution rate. Experimental results from CCD showed that an average value of 1.16 m³ H₂/m³ d for maximum hydrogen production rate (HPR) was produced. A second-order polynomial model was adjusted to the experimental results from CCD. The regression model showed that the quadratic term of all variables tested had a highly significant effect (P < 0.01) on maximum HPR as a defined response. The analysis of the empirical model revealed that the optimal conditions for maximum HPR were incubation temperature, initial pH, and influent dilution rate of 30.23 ^°C, 6.63, and 50.71%, respectively. Generated regression model predicted a maximum HPR of 1.1659 m³ H₂/m³ d could be generated under optimum conditions. Confirmation experimentation was conducted in the optimal conditions determined. Experimental results of the validation test showed that a maximum HPR of 1.1747 m³ H₂/m³ d was produced.     

Hydrogen evolution in microbial electrolysis cells treating landfill leachate: Dynamics of anodic biofilm

This study investigates the potential opportunities of hydrogen evolution treating landfill leachate in a set of two microbial electrolysis cells (MEC-1 and 2) under 30 °C and 17 ± 3 °C temperatures, respectively. The system achieved a projected current density of 1000–1200 mA m^?2 (MEC-1) and 530–755 mA m^?2 (MEC-2) coupled with low cost hydrogen production rate of 0.148 L La^?1 d^?1 (MEC-1) and 0.04 L La^?1 d^?1 (MEC-2) at an applied voltage of 1.0 V. Current generation led to a maximum COD oxidation of 73 ± 8% (MEC-1) and 65 ± 7% (MEC-2) with ≥100% energy recovery. The system also exhibited a high hydrogen recovery (66–95%), pure hydrogen yield (98%) and tremendous working stability during two months of operation. Electroactive microbes such as Pseudomonadaceae, Geobacteraceae and Comamonadaceae were found in anodophilic biofim, along with Rhodospirillaceae and Rhodocyclaceae, which could be involved in hydrogen production. These results demonstrated an energy-efficient approach for hydrogen production coupled with pollutants removal.     

Photohydrogen production from lactose and lactate by recombinant strains of Rhodobacter capsulatus: Modeling and optimization

Photofermentative hydrogen production from synthetic mixtures of lactose and lactate mimicking cheese whey was modeled and optimized using Design of Experiments and Response Surface Methodology. Five continuous parameters (light intensity, pH, lactose, lactate and glutamate concentrations) were studied as a function of buffer type (KPi or Borax) using two recombinant bacterial strains. For Rhodobacter capsulatus B10(lacZ), buffer type influenced the optimal parameter values but the optimal responses were similar in both buffers. In contrast, for R. capsulatus IR3(lacZ), responses were higher in Borax buffer than in KPi and were significantly higher than in strain B10(lacZ). Thus, the experimental optimized responses for specific volumetric H₂ production, volumetric H₂ production rate and substrate (lactose plus lactate) to H₂ conversion rate in Borax buffer, were 12,150 ml L^?1, 48.5 ml L^?1 h^?1 and 41.2%, respectively, for IR3(lacZ) compared to 6150 ml L^?1, 33.5 ml L^?1 h^?1 and 32.5%, respectively, for B10(lacZ).     

Novel nanostructured electrocatalysts for hydrogen evolution reaction in neutral and weak acidic solutions

Microbial electrolysis cells (MECs) provide an innovative bioelectrochemical approach for hydrogen production using microorganisms as biocatalysts. The development of cost-effective cathodes for near-neutral pH and ambient temperature conditions is the most critical challenge for the practical application of MEC technology. In this study, the electrocatalytic properties of electrodeposited onto carbon felt NiFe-, NiFeP- and NiFeCoP-nanostructures towards HER in neutral and weak acidic solutions were investigated. The voltage needed to initiate hydrogen production and the current production rates were estimated from obtained linear voltammograms. The developed composite materials possess much higher catalytic activity than bare carbon felt. The highest current production rate corresponding to 1.7 ± 0.1 m³H₂/day/m² was achieved with NiFeCoP/carbon felt electrodes. In addition, the applied modifications result in improvement of the corrosion resistance. The obtained results demonstrate that Ni-based nanomodified materials are promising electrocatalysts for HER in near-neutral electrolytes and could be applied as cathodes in MECs.     

Biological hydrogen production from sugar beet molasses by agar immobilized R. capsulatus in a panel photobioreactor

Biohydrogen production from sugar beet molasses was investigated by using agar immobilized R. capsulatus YO3. A panel photobioreactor (1.4 L) was employed for a long-term hydrogen production in both indoor and outdoor conditions. The impact of several initial molasses concentrations on hydrogen production, yield and productivity were assessed. Indoor studies revealed that initial sucrose concentration in molasses should be kept below 20 mM to prevent inhibition of hydrogen production. The highest hydrogen productivity of 0.64 ± 0.06 mmol H₂ L^?1 h^?1 and yield of 12.2 ± 1.5 mol H₂/mol sucrose were obtained in indoors throughout 20 days of operation. For outdoors, hydrogen production continued for 40 days including consecutive 10 rounds under natural outdoor conditions. In outdoor conditions, the maximum hydrogen productivity and yield were 0.79 ± 0.04 mmol H₂ L^?1 h^?1 and 5.2 ± 0.4 mol H₂/mol sucrose respectively. These results indicate that the proposed system is promising for biohydrogen production from molasses at large-scale natural conditions.     

Powering microbial electrolysis cells by electricity generation from simulated waste heat of anaerobic digesters using thermoelectric generators

Waste heat from anaerobic digesters can be converted to electricity by using thermoelectric generators (TEG). Herein, such energy was employed to power a microbial electrolysis cell (MEC) for producing hydrogen gas. Four TEG units could deliver a voltage of ~0.5 V, sufficient to drive the MEC that achieved a hydrogen production rate of 0.48 ± 0.13 m³ m⁻³ d⁻¹. This rate was further improved to 0.75 ± 0.05 m³ m⁻³ d⁻¹ when the temperature difference for TEG was increased from 18 to 28 °C. There was no significant difference between the TEG-powered MEC and power supply-supported MEC (at 0.6 V), in terms of current generation, hydrogen production, and organic removal. Ambient air was also studied as a cold-side source for TEG, although some challenges were encountered to maintain a large temperature difference. Those results will encourage further exploration of using TEG as a feasible power supply for sustainable MEC operation.     

Hydrogen production by a low-cost electrolyzer developed through the combination of alkaline water electrolysis and solar energy use

Electrolysis is a relatively simple process for obtaining hydrogen and can be combined with use of renewable energy sources, such as solar photovoltaic energy, for clean, sustainable gas production. This study designed a cylindrical electrolytic cell made of acrylic and 304 stainless steel electrodes to produce hydrogen. The electrolyte used was sodium hydroxide (NaOH 2–5 mol L^?1), and the direct current voltages applied were 2.0, 2.7, and 3.4 V. The maximum hydrogen production was achieved with 5.0 mol L^?1 NaOH and 3.4 V electric voltage. The system was connected to a photovoltaic panel of 20 W and exposed to solar radiation from 10 a.m. to 2 p.m. Approximately 2 L of hydrogen was produced within a period, and an average irradiance of 800.0 W m^?2 ± 60 W m^?2 was achieved. The system was stable throughout the tests.     

Microbial electrolysis cells for production of methane from CO2: long‐term performance and perspectives

A methane‐producing microbial electrolysis cell (MEC) is a technology to convert CO₂ into methane, using electricity as an energy source and microorganisms as the catalyst. A methane‐producing MEC provides the possibility to increase the fuel yield per hectare of land area, when the CO₂ produced in biofuel production processes is converted to additional fuel methane. Besides increasing fuel yield per hectare of land area, this also results in more efficient use of land area, water, and nutrients. In this research, the performance of a methane‐producing MEC was studied for 188 days in a flat‐plate MEC design. Methane production rate and energy efficiency of the methane‐producing MEC were investigated with time to elucidate the main bottlenecks limiting system performance. When using water as the electron donor at the anode during continuous operation, methane production rate was 0.006 m³/m³ per day at a cathode potential of ?0.55 V vs. normal hydrogen electrode with a coulombic efficiency of 23.1%. External electrical energy input was 73.5 kWh/m³ methane, resulting in a voltage efficiency of 13.4%. Consequently, overall energy efficiency was 3.1%. The maximum achieved energy efficiency was obtained in a yield test and was 51.3%. Analysis of internal resistance showed that in the short term, cathode and anode losses were dominant, but with time, also pH gradient and transport losses became more important. The results obtained in this study are used to discuss the possible contribution of methane‐producing MECs to increase the fuel yield per hectare of land area. Copyright © 2011 John Wiley & Sons, Ltd.