Hydrogen Production In Microbial Electrolysis Cells Research Articles

Trends and perspectives of microbial electrolysis cell technology for ultimate green hydrogen production

Currently, gray hydrogen and blue hydrogen are widely recognized as renewable energy, but in reality, they are made from fossil fuels. The most important task to achieve the hydrogen-based society is the development of economic green hydrogen production technology. Microbial electrolysis cell (MEC) is a next-generation energy-producing wastewater treatment technology that treats renewable organic wastewater and simultaneously produces the ultimate green hydrogen. For hydrogen production in MFC, it is necessary to input electrical energy into MEC. However, that energy is all covered by the energy produced by the MEC. Therefore, hydrogen production in MEC can be defined as the ultimate green hydrogen. This review contains an in-depth summary and analysis of the principles and feasibility of MEC technology, the composition and shape of MEC, electrode materials, and practical application cases in various types of wastewaters. Furthermore, compatibility and scalability with other environmental systems were reviewed at the pilot scale. Based on this, the technical limitations of MEC were diagnosed and future research directions for the practical application of MEC technology were suggested.

High current density via direct electron transfer by hyperthermophilic archaeon, Geoglobus acetivorans, in microbial electrolysis cells operated at 80 °C

Utilization of hyperthermophilic electro-active microorganisms in microbial electrolysis cells (MECs) that are used for hydrogen production from organic wastes offers significant advantages, such as increased reaction rate and enhanced degradation of insoluble materials. However, only a limited number of hyperthermophilic bioelectrochemical systems have been investigated so far. This study is the first to illustrate hydrogen production in hyperthermophilic MECs with a maximum rate of 0.57 ± 0.06 m3 H2/m3d, where an iron reducing archaeon, Geoglobus acetivorans, was used as inoculum. In fact, this is the first study to report that G. acetivorans, as the fourth hyperthermophilic electro-active archaeon. In single chamber MECs operated at 80 °C with a set potential of 0.7 V, a peak current density of 1.53 ± 0.24 A/m2 has been attained and this is the highest record of current produced by pure culture hyperthermophilic microorganisms. Turnover cyclic voltammetry curve illustrated a sigmoidal shape (midpoint of −0.40 V vs. Ag/AgCl), and together with linear relation of scan rate and peak anodic current, proves the biofilm attachment to the anode and its capability of direct electron transfer. Along with simple substrate (acetate), G. acetivorans effectively utilized dark fermentation effluent for hydrogen production in MECs.

Impact of surface area and current generation of microbial electrolysis cell electrodes inserted into anaerobic digesters

The integration of microbial electrolysis cells (MECs) and anaerobic digestion (AD) has recently been pursued to improve methane production through a combined system (AD-MEC). Analysis of 24 different experimental results published in 13 different studies showed that the only significant correlation between increased methane generation rate and other parameters was the surface area provided by the electrodes per volume of reactor, or the S/V ratio (R2 = 0.63, p-value < 0.01). Other possible relationships between hydrogen production in MECs, and methane enhancement in AD-MECs, were examined as a function of applied voltage and current density using data from previous studies. For the MECs, there was a high correlation between current density and hydrogen production rate. There was no significant relationship between methane production rate and current density in AD-MECs. A comparison of other factors between MECs and AD-MECs showed large differences in substrate concentration, substrate type (single substrates or wastewaters), and volumetric current densities. Adding electrodes increased methane production on average by 65%, but surface area was a more important contributing factor in AD-MECs than current generation. Based on our analysis of the results in these previous studies, increased area and conductive surfaces provided by the electrodes can have beneficial effects for methane production other than hydrogen gas production by increasing biomass retention and stimulating electrical syntrophy between microorganisms.

Alkaline thermal pretreatment of waste activated sludge for enhanced hydrogen production in microbial electrolysis cells

Resource utilization of waste activated sludge (WAS) has become a mainstream development direction. Alkaline thermal pretreatment (TPT) was found to greatly promote the bioaccessibility and biodegradability of the sludge. The organic matter including soluble chemical oxygen demand (SCOD), soluble carbohydrate, soluble protein and volatile fatty acids (VFAs) after low temperature (90 °C) pretreatment was 4.8%–65.9% higher than that after high temperature (180 °C) pretreatment. These increasements could be contributed by the alkaline treatment condition and the longer treatment time. The alkaline condition reduced the resistance of cell wall to the temperature. The pretreatment time at 90 °C was two times of that at 180 °C, allowing more organic matter to be released. But the total energy consumption of low temperature pretreatment (2580.7 kJ/L) was 30.5% lower than that of high temperature pretreatment (3711.8 kJ/L). The sludge fermentation liquid (SFL) was then employed as the substrate in microbial electrolysis cells (MECs), and the utilization efficiency of acetic acid was the highest (74.9%–83.2%). The hydrogen yield using low temperature pretreated sludge was 0.44 m3/(m3·d), which was higher than that of using high temperature pretreated sludge (0.31 m3/(m3·d)). These results suggested that alkaline TPT at 90 °C was an effective way to hydrolyze sludge and further enhance hydrogen production in MECs.

Activated carbon-Fe catalyst modification on stainless steel cathode affects hydrogen production in microbial electrolysis cell

Microbial Electrolysis Cell (MEC) can be used to produce hydrogen gas from organic matter contained in wastewater. However, at the cathode of MECs, hydrogen production may be limited by methanogenesis wherein CO2 and hydrogen protons react to form methane and water. In this study, activated carbon (AC)-Fe was used as a catalyst coated onto SS mesh 304 cathode. AC-Fe/SS was chosen for its high surface area, good activity, and stability. The combination of AC-Fe on SS was expected to increase hydrogen production in MECs. Adsorption and phase inversion were chosen to coat AC-Fe mixture on SS. The research was carried out in a 100 mL MEC reactor with an operating time of 258 h. The produced hydrogen was analyzed for its purity by GC-TCD. Voltage measurements were carried out using a digital multimeter. Additionally, bacterial growth was analyzed by spectrophotometer. The highest fraction of hydrogen gas production was 60% without catalyst (uncoated) over only 0.08% with AC-Fe/SS. The highest value of optical density for bacterial growth was 0.611 for AC-Fe/SS, higher than 0.427 without catalyst. The highest current density was 99.11 mA/m2 for AC-Fe/SS and 59.52 mA/m2 without catalyst. The results suggested AC-Fe/SS coating allows for increased bacterial growth and voltage generation at the cost of an adverse effect on hydrogen gas production.

Enhanced hydrogen production by mevastatin in microbial electrolysis cells

Mevastatin is one of the common pharmaceuticals found in wastewaters, and its biodegradation is still an environmental problem due to its recalcitrant properties. Although various biological, chemical and physical methods have been investigated for the removal of mevastin, microbial electrolysis cells (MECs) have not been examined, yet. MECs are new generation biotechnological tools used in hydrogen energy production. In this study, the effects of mevastatin on hydrogen production in MECs were investigated for the first time. MECs produced hydrogen gas in the presence of mevastatin, and an increase in hydrogen production was observed. Over 90% of the mevastatin were removed in MECs. These results also showed that the fermentation process associated with carbon dioxide production can favor hydrogen production with mevastatin. In conclusion, the efficiency of hydrogen production in microbial cells can be increased by the use of wastewaters contaminated with mevastatin.

Effect of gas atmosphere on hydrogen production in microbial electrolysis cells

Inert gas is often used in the deoxygenation of microbial electrolysis cells (MECs) to maintain growth and viability of anaerobes. However, the effects of the gas atmosphere on hydrogen production and microbial community of MECs are often neglected. Here, the performances and biofilm microbiomes of MECs pre-sparged with different gases were compared. MECs pre-sparged with argon gas (Ar) yielded more hydrogen (3.73 ± 0.13 mol-H2/mol-acetate) and a higher hydrogen production rate (2.99 ± 0.17 L-H2/L-reactor-day) than MECs pre-sparged with N2 (3.41 ± 0.13 mol-H2/mol-acetate and 2.27 ± 0.28 L-H2/L-reactor-day, respectively). Microbiome analysis indicated that the relative abundance of Geobacter increased from 59.25% to 77.79% when the gas atmosphere in MECs shifted from N2 to Ar. Hydrogen production may have been catalyzed by nitrogenase from Geobacter and photosynthetic bacteria in MECs pre-sparged with Ar. These findings suggested that the gas atmosphere substantially influences the microbiome of anode biofilms and Ar sparging is most effective for enhancing hydrogen production in MECs.

Competitive exclusion in a DAE model for microbial electrolysis cells

Microbial electrolysis cells (MECs) are devices that employ electroactive bacteria to perform extracellular electron transfer, enabling hydrogen generation from biodegradable substrates. In our previous work, we developed and analyzed a differential-algebraic equation (DAE) model for MECs. The model resembles a chemostat or continuous stirred tank reactor (CSTR). It consists of ordinary differential equations for concentrations of substrate, microorganisms, and an extracellular mediator involved in electron transfer. There is also an algebraic constraint for electric current and hydrogen production. Our goal is to determine the outcome of competition between methanogenic archaea and electroactive bacteria, because only the latter contribute to electric current and the resulting hydrogen production. We investigate asymptotic stability in two industrially relevant versions of the model. An important aspect of many chemostat models is the principle of competitive exclusion. This states that only microbes which grow at the lowest substrate concentration will survive as t → ∞.We show that if methanogens can grow at the lowest substrate concentration, then the equilibrium corresponding to competitive exclusion by methanogens is globally asymptotically stable. The analogous result for electroactive bacteria is not necessarily true. In fact we show that local asymptotic stability of competitive exclusion by electroactive bacteria is not guaranteed, even in a simplified version of the model. In this case, even if electroactive bacteria can grow at the lowest substrate concentration, a few additional conditions are required to guarantee local asymptotic stability. We provide numerical simulations supporting these arguments. Our results suggest operating conditions that are most conducive to success of electroactive bacteria and the resulting current and hydrogen production in MECs. This will help identify when producing methane or electricity and hydrogen is favored.

Nickel powder blended activated carbon cathodes for hydrogen production in microbial electrolysis cells

Although pure Ni catalysts can achieve a hydrogen production rate similar to Pt in microbial electrolysis cells (MECs), a reduction in the amount of Ni used is needed to reduce the cost. In this study, nickel powder (pNi) was blended with activated carbon (AC) to reduce the mass of Ni used, while improving catalytic activity for the hydrogen evolution reaction (HER) by increasing the active surface area. Ni powder blended AC cathodes (AC-pNi) were fabricated at different nickel powder loadings (4.8, 19, 46 mg/cm2 with AC and 77 mg/cm2 without AC as control). AC-pNi4.8 (Ni loading: 4.8 mg/cm2) produced higher hydrogen production rates (0.38 ± 0.04 L-H2/L-d) than pNi77 (0.28 ± 0.02 L-H2/L-d) with a 16 times less Ni loading. Cathodic hydrogen recovery of using the AC-pNi4.8 (98 ± 5%) was also higher than pNi77 (82 ± 4%), indicating catalytic activities were improved by AC blending. Nickel dissolution into the catholyte after completion of each cycle was negligible for AC-pNi4.8 (<0.2 mg/L), while Ni dissolution was detected for pNi77 (5–10 mg/L). These results indicate that AC blending with Ni powder can improve hydrogen production in MECs while minimizing the amount of Ni in the cathode.

Factors Affecting the Effectiveness of Bioelectrochemical System Applications: Data Synthesis and Meta-Analysis

Microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are promising bioelectrochemical systems (BESs) for simultaneous wastewater treatment and energy/resource recovery. Unlike conventional fuel cells that are based on stable chemical reactions, these BESs are sensitive to environmental and operating conditions, such as temperature, pH, external resistance, etc. Substrate type, electrode material, and reactor configuration are also important factors affecting power generation in MFCs and hydrogen production in MECs. In order to discuss the influence of these above factors on the performance of MFCs and MECs, this study analyzes published data via data synthesis and meta-analysis. The results revealed that domestic wastewater would be more suitable for treatment using MFCs or MECs, due to their lower toxicity for anode biofilms compared to swine wastewater and landfill leachate. The optimal temperature was 25–35 °C, optimal pH was 6–7, and optimal external resistance was 100–1000 Ω. Although systems using carbon cloth as the electrodes demonstrated better performance (due to carbon cloth’s large surface area for microbial growth), the high prices of this material and other existing carbonaceous materials make it inappropriate for practical applications. To scale up and commercialize MFCs and MECs in the future, enhanced system performance and stability are needed, and could be possibly achieved with improved system designs.

Optimal interval of periodic polarity reversal under automated control for maximizing hydrogen production in microbial electrolysis cells

Microbial electrolysis cells (MECs) are a promising approach for producing hydrogen gas from low-grade substrates with low energy consumption. However, pH increase in a cathode due to proton reduction and thus the need for buffering this pH increase remains a challenge for MEC operation. In this study, a previously reported operational strategy for pH buffer - periodic polarity reversal (PPR) was further studied by developing and applying an automatically control system. The effect of PPR interval on the hydrogen production was investigated and the optimal PPR interval was determined. With an optimal PPR interval of 40 min, the MEC had a significantly low pH increase rate of 0.0085 min−1 in its cathodes, and this resulted in the highest current density of 1.58 ± 0.02 A m−2, Coulombic efficiency of 130.3 ± 1.8%, hydrogen production rate of 1.65 ± 0.01 m3 H2 m−3d−1, overall hydrogen recovery of 75.9 ± 0.4%, and energy efficiency relative to the substrate input of 140.8 ± 1.4%. Further analysis suggested that this optimal value of PPR interval was affected by both reaction time and hydrogen supply. When the PPR interval increased from 10 min to 40 min, a longer reaction time helped produce more protons and thus generated a stronger buffer capacity. Beyond 40 min, the mass transfer of the dissolved hydrogen gas could become a limiting factor, leading to a weaker buffer capacity with a longer PPR interval. Those findings have provided an effective pH control strategy with a convenient control system for maximizing hydrogen production in MECs.

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

The impact of different anode acclimation methods for enhancing hydrogen production in microbial electrolysis cell (MEC) was investigated in this study. The anodes were first acclimated in microbial fuel cells using acetate, butyrate and corn stalk fermentation effluent (CSFE) as substrate before moving into MECs, respectively. Subsequently, CSFE was used as feedstock in all the three MECs. The maximum hydrogen yield with the anode pre-acclimated with butyrate (5.21±0.24L H2/L CSFE) was higher than that pre-acclimated with acetate (4.22±0.19L H2/L CSFE) and CSFE (4.55±0.14L H2/L CSFE). The current density (480±11A/m3) and hydrogen production rate (4.52±0.13m3/m3/d) with the anode pre-acclimated with butyrate were also higher that another two reactors. These results demonstrated that the anode biofilm pre-acclimated with butyrate has significant advantages in CSFE treatment and could improve the performance of hydrogen production in MEC.

Enhancing hydrogen production in microbial electrolysis cells by in situ hydrogen oxidation for self-buffering pH through periodic polarity reversal

Successful pH control plays a key role in hydrogen production in microbial electrolysis cells (MECs). Herein, periodic polarity reversal (PPR) is applied to a dual-cathode MEC and achieves the enhanced hydrogen production. The MEC with PPR produces 1.3 ± 0.1 m3 H2 m−3d−1 with 50-mM NaCl as the catholyte, much higher than 0.9 ± 0.1 m3 H2 m−3d−1 from the MEC with dual-working cathodes or 0.8 ± 0.1 m3 H2 m−3d−1 from the MEC with one working cathode. Such enhancement benefits from a slower increase in the catholyte pH, for example, it takes 15.3 h to increase the 10-mM NaCl pH from 7.00 to 12.00 in the MEC with PPR, 1.7–3.6 times that of the MECs without PPR, which is due to the decrease in the catholyte pH of the reversed cathode during PPR. The potential of the reversed electrode is more positive than the anode, suggesting that the reversed electrode acts as a second anode electrode using residue hydrogen gas as an electron source. Thus, a mechanism of in situ oxidation of hydrogen gas for pH buffering is proposed and discussed. These findings have provided a simple but effective pH control strategy for enhancing hydrogen production in MECs.

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.

Hydrogen gas production with an electroformed Ni mesh cathode catalysts in a single-chamber microbial electrolysis cell (MEC)

Microbial electrolysis cells (MECs) are generally regarded as a promising future technology for manufacturing green hydrogen from organic material present in wastewaters and other renewable energy sources. However, the development of inexpensive and high-efficient cathode catalyst is the most critical challenge for MECs to become a commercialized H2 production technology. In this study, a non-noble metal electroformed Ni mesh cathode alternatives to typical cathode material (Pt/CC) was intensively examined in a single-chamber membrane-free MEC. To the best of our knowledge, the use of electroformed Ni mesh as the MEC cathode catalyst has not been reported so far. The MEC was operated in fed-batch mode and the performance of the Ni mesh cathode was compared with that of Pt/CC cathode in terms of columbic efficiencies (75 ± 4% vs. 72.7 ± 1%), overall hydrogen recovery (89.3 ± 4% vs. 90.9 ± 3%), overall energy efficiency (62.9 ± 5% vs. 69.1 ± 2%), the maximum volumetric hydrogen production rate (4.18 ± 1 m3 H2/m3 d vs. 4.25 ± 1 m3 H2/m3 d), volumetric current density (312 ± 9 A/m3 vs. 314 ± 5 A/m3). The obtained results in this study highlight the great potential of using the electroformed Ni mesh catalysts as a viable cathode material for hydrogen production in MECs.

Suppression of methanogenesis for hydrogen production in single-chamber microbial electrolysis cells using various antibiotics

Methanogens can utilize the hydrogen produced in microbial electrolysis cells (MECs), thereby decreasing the hydrogen generation efficiency. However, various antibiotics have previously been shown to inhibit methanogenesis. In the present study antibiotics, including neomycin sulfate, 2-bromoethane sulfonate, 2-chloroethane sulfonate, 8-aza-hypoxanthine, were examined to determine if hydrogen production could be improved through inhibition of methanogenesis but not hydrogen production in MECs. 1.1mM neomycin sulfate inhibited both methane and hydrogen production while 2-chloroethane sulfonate (20mM), 2-bromoethane sulfonate (20mM), and 8-aza-hypoxanthine (3.6mM) can inhibited methane generation and with concurrent increases in hydrogen production. Our results indicated that adding select antibiotics to the mixed species community in MECs could be a suitable method to enhance hydrogen production efficiency.

Recovery of flakey cobalt from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells

Flakey cobalt was successfully recovered from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells (MECs). At applied voltages of 0.3–0.5 V, the yields of 0.81 mol Co/mol COD and 1.21 ± 0.03–1.49 ± 0.11 mol H2/mol COD were achieved while the energy efficiency relative to the electrical input was 22.5 ± 0.1–43.2 ± 0.7% (cobalt) and 170 ± 12–262 ± 7% (hydrogen), and the overall energy efficiency relative to both the electrical input and the energy of the anodic substrate averaged 9.4% (cobalt) and 62.8% (hydrogen). Cathode accumulated flakey crystals were verified as cobalt using both a scanning electron microscope capable of energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction analysis (XRD). Dominant bacteria on the anodes and known as exoelectrogens or recalcitrant substance degraders included Geobacter uraniireducens, Comamonas nitrativorans, uncultured Geobacter sp., Acidovorax caeni, Pseudorhodoferax caeni, and Diaphorobacter nitroreducens. The evidence of influence factors including applied voltage, pH, solution conductivity, temperature and type of buffer can contribute to improving understanding of and optimizing cobalt recovery with simultaneous hydrogen production in MECs.