A comprehensive review of microbial electrolysis cells (MEC) reactor designs and configurations for sustainable hydrogen gas production

Microbial electrolysis cell (MEC): Reactor configurations, recent advances and strategies in biohydrogen production

Hydrogen gas can be produced by electrohydrogenesis in microbial electrolysis cells (MECs) at greater yields than fermentation and at greater energy efficiencies than water electrolysis. It has been assumed that a membrane is needed in an MEC to avoid hydrogen losses due to bacterial consumption of the product gas. However, high cathodic hydrogen recoveries (78 +/- 1% to 96 +/- 1%) were achieved in an MEC despite the absence of a membrane between the electrodes (applied voltages of 0.3 < E(ap) < 0.8 V; 7.5 mS/cm solution conductivity). Through the use of a membrane-less system, a graphite fiber brush anode, and close electrode spacing, hydrogen production rates reached a maximum of 3.12 +/- 0.02 m3 H2/m3 reactor per day (292 +/- 1 A/m3) at an applied voltage of E(ap) = 0.8 V. This production rate is more than double that obtained in previous MEC studies. The energy efficiency relative to the electrical input decreased with applied voltage from 406 +/- 6% (E(ap) = 0.3 V) to 194 +/- 2% (E(ap) = 0.8 V). Overall energy efficiency relative to both E(ap) and energy of the substrate averaged 78 +/- 4%, with a maximum of 86 +/- 2% (1.02 +/- 0.05 m3 H2/m3 day, E(ap) = 0.4 V). At E(ap) = 0.2 V, the hydrogen recovery substantially decreased, and methane concentrations increased from an average of 1.9 +/- 1.3% (E(ap) = 0.3-0.8 V) to 28 +/- 0% of the gas, due to the long cycle time of the reactor. Increasing the solution conductivity to 20 mS/ cm increased hydrogen production rates for E(ap) = 0.3-0.6 V, but consistent reactor performance could not be obtained in the high conductivity solution at E(ap) > 0.6 V. These results demonstrate that high hydrogen recovery and production rates are possible in a single chamber MEC without a membrane, potentially reducing the costs of these systems and allowing for new and simpler designs.

BackgroundWhile microbial electrolysis cells (MECs) can simultaneously produce bioelectrochemical hydrogen and treat wastewater, they consume considerable energy to overcome the unfavorable thermodynamics, which is not sustainable and economically feasible in practical applications. This study presents a proof-of-concept system in which hydrogen can be produced in an MEC powered by theoretically predicated energy from pressure-retarded osmosis (PRO). The system consists of a PRO unit that extracts high-quality water and generates electricity from water osmosis, and an MEC for organic removal and hydrogen production. The feasibility of the system was demonstrated using simulated PRO performance (in terms of energy production and effluent quality) and experimental MEC results (e.g., hydrogen production and organic removal).ResultsThe PRO and MEC models were proven to be valid. The model predicted that the PRO unit could produce 485 mL of clean water and 579 J of energy with 600 mL of draw solution (0.8 M of NaCl). The amount of the predicated energy was applied to the MEC by a power supply, which drove the MEC to remove 93.7 % of the organic compounds and produce 32.8 mL of H2 experimentally. Increasing the PRO influent volume and draw concentration could produce more energy for the MEC operation, and correspondingly increase the MEC hydraulic retention time (HRT) and total hydrogen production. The models predicted that at an external voltage of 0.9 V, the MEC energy consumption reached the maximum PRO energy production. With a higher external voltage, the MEC energy consumption would exceed the PRO energy production, leading to negative effects on both organic removal and hydrogen production.ConclusionsThe PRO-MEC system holds great promise in addressing water-energy nexus through organic removal, hydrogen production, and water recovery: (1) the PRO unit can reduce the volume of wastewater and extract clean water; (2) the PRO effluents can be further treated by the MEC; and (3) the osmotic energy harvested from the PRO unit can be applied to the MEC for sustainable bioelectrochemical hydrogen production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0305-0) contains supplementary material, which is available to authorized users.

Author(s): Liu, C; Liu, W; Wang, A; Zhou, J | Abstract: © 2015, Science Press. All right reserved. Microbial electrolysis cells (MECs) has been recently developed as a new technology for hydrogen production. The organics are degraded by exoelectrogens in anode biofilm and transport electrons directly to anode, while hydrogen is produced by combining electrons and protons on the surface of Pt-catalyzing cathode under a small external voltage between anode and cathode. Municipal waste water was used as the same inoculum to enrich functional communities in 15 single chamber MEC reactors. Various gas production (hydrogen and methane) was maintained over 1 month using glucose as the sole carbon source in phosphate buffer solution (50 mmol·L-1, pH=7.0). The highest hydrogen production rate was up to (3.9±0.6) mol H2/mol glucose with conversion rate of 32.2% for high-H2generation MECs. The highest methane conversion rate was 48.4% in low-H2generation MECs. A 48 h low pH shock was put into anode biofilm and MEC performances were recovered to their functions in 10~15 d. The microbial diversities increased and gas production rates were changed after low pH shock. Hydrogen yield was reduced by 1.8 mol H2/mol glucose in high-H2generation MECs, while the methane yield increased by 0.4 mol CH4/mol glucose. Based on Geochip analysis, cytochrome C genes were mostly enriched in high-H2yield MECs. Thus functional genes were still recovered dominantly after low pH shock and it supported the recovery of electron transport. The carbon degradation genes were substantially changed among anodic communities in most MECs. The functional genes of labile carbon degradation and methane production were significantly changed after microbial community recovery. The carbon degradation gene structure was more significantly changed in high-H2yield MECs than high-methane yield MECs. The methane increase and hydrogen decrease matched well with their stoichiometric relationship.

Microbial electrolysis cell (MEC) is one of the promising and cutting-edge technologies for generating hydrogen from wastewater through biodegradation of organic waste by exoelectrogenic microbes. In the MECs, the operational parameters, such as applied voltage (Eap), anode surface area, anode-cathode distance, and N2/CO2 volume ratio have a significant impact on the hydrogen yield and production. In the present study, to enhance current and hydrogen production of MEC, the effects of key operational conditions on the MEC performance were extensively investigated. The optimal operating condition for hydrogen production in MECs was determined as: the optimum applied voltage of 1.1 V, anode surface area of 94 (cm2), anode-cathode distance of 1.5 (cm), and a N2/CO2 volume ratio of 4:1. With these optimum conditions, the maximum H2 volume, current density and hydrogen production rate (HPR) of MEC could be reached to 270.09 mL, 314.01 ± 2.81 A/m3, and 4.25 ± 0.55 m3 H2 /m3 d, respectively. The results obtained in this study imply that a systematic investigation of the key operational variables is an effective strategy to maximize the hydrogen production in single-chamber MECs.

A novel tubular microbial electrolysis cell for high rate hydrogen production

Enhancing energy efficiency and H2 production in lab-scale dual chamber microbial electrolysis cells: A focus on catholyte composition and voltage losses

Scalable membrane-less microbial electrolysis cell with multiple compact electrode assemblies for high performance hydrogen production

Performance of a microbial electrolysis cell (MEC) for hydrogen production with a new process for the biofilm formation

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

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

BACKGROUND: Microbial electrolysis cell (MEC) has been considered as a promising new technology for the production of bio‐hydrogen from renewable biomass, but low‐cost alternatives to typical cathode material (platinum) are needed. In this study, CNTs‐based electrode alternatives to Pt were examined in a single‐chamber membrane‐free MEC. To the best of our knowledge, the use of carbon nanotube as the MEC cathode catalyst has not been reported so far.RESULTS: For all cathodes, hydrogen production rates increased in response to increase in voltage and the highest hydrogen production was achieved at 0.9 V. At an applied voltage of 0.9 V, MECs with Pt/MWNT cathodes obtained a hydrogen production rate of 1.42 m3 m−3 day−1 with a current density of 192 A m−3, a coulombic efficiency of 94%, a cathodic hydrogen recovery of 65%, and electrical energy efficiency based on electricity input of 126%.CONCLUSIONS: The Pt/MWNT cathode developed demonstrated better electrocatalytic activity than the MWNT cathode and achieved performance comparable with the Pt cathode in terms of hydrogen production rate. These results demonstrate the great potential of using carbon cloth with CNTs‐based electrodes as a cathode material for MECs. Copyright © 2012 Society of Chemical Industry

The impact of anode acclimation strategy on microbial electrolysis cell treating hydrogen fermentation effluent

Microbial electrolysis cells (MECs) can electrochemically produce green hydrogen from waste streams. Although MECs are excellent options for implementing an energy recovery process while treating wastewater, the cathode side still hinders practical applications. Platinum (Pt) has been the top reference for hydrogen evolution reaction (HER) in MECs, however, Pt is not practical due to the high capital cost. Inexpensive nickel-loaded activated carbon (Ni/AC) cathodes were recently developed for replacing Pt in MECs and have shown comparable performance to Pt. This dissertation aims to breakthrough the current cathode challenges regarding the catalytic activity, manufacturing, and operational concept toward a cost-competitive scaling-up process of MECs. First, the electroactivity of the Ni/AC cathode was improved by increasing the oxygen (16.9%) and nitrogen (124%) containing species on the AC surface using nitric acid oxidation. The acid-treated AC (t-AC) showed 21% enhanced wettability which reduced the charge transfer resistance (33%) and ohmic resistance (6.7%) of the cathode. Ni/t-AC achieved 84% higher hydrogen production rates (0.35±0.02 L-H2/L-d) than the control (pristine AC). Second, increases in polyvinylidene fluoride binder loading (60%), during the manufacturing process, demonstrated to increase by 47% the hydrogen production rates in MECs, yet all cathodes showed a decline in electroactivities (≤9.2%) after MEC cycles. It is unclear how the binder content will impact the performance on a large scale. Third, to overcome this drawback, we have examined a novel binder-free flowable cathode. Ni/AC powders were suspended in a buffering solution as a cathode with no electrode fabrication processes. The Ni/AC flow cathode with higher Ni content and minimum Ni/AC loading (4 Ni-atom% and 0.125 wt-AC%, noted as Nix/ACY, X is nickel content, and Y is powder loading. Hereafter: Ni4AC0.125) demonstrated the highest catalytic activities (─0.86 V vs. Ag/AgCl at ─10 A/m2) and Faradaic response (1.6 F/g) among Ni/AC flow cathodes tested. This result indicates that pseudo-capacitive behavior toward Faradaic reactions can be promoted by increasing Ni loadings on AC particles. The MEC with Ni4AC0.125 flow cathode produced comparable hydrogen production rates (1.62±0.15 L-H2/Lreactor-d) to the Pt control (1.64±0.09 L-H2/L-d), and 40% higher than the blank (without Ni/AC, 1.29±0.02 L-H2/L-d). There was a 10% increase in hydrogen production rates with the lowest carbon black (CB) blending (CB: 0.06 wt.%) in a Ni2/AC0.125 flow cathode but hydrogen production rates were not further improved as CB content increased. The new Ni/AC flow cathodes showed ~5 times higher hydrogen production rates than previous stationary Ni/AC cathodes. To

Enhancing hydrogen production with Ni–P coated nickel foam as cathode catalyst in single chamber microbial electrolysis cells