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High overpotential and soaring prices of the cathode electrode are the bottlenecks for the development of microbial electrolysis technology for hydrogen production. In this study, a novel one-step electrodeposition method has been attempted to fabricate electrodeposited cathodes in situ growth of Ni–Co–S, Ni–S, Co–S catalyst on nickel foam (NF) to reduce the overpotential of electrodes. Finally, a uniform nanosheet with a high specific surface area and more active sites is formed on the NF surface, resulting in a lower overpotential than plain NF. At 0.8 V, the Co–S/NF cathode produces a favorable 42% increase in hydrogen yield (0.68 m3·m−3·d−1), 40% upsurge in current density (10.6 mA/cm3) and 39% rise of cathodic recovery rate (58.0 ± 3.2%) than bare NF, followed by Ni–Co–S/NF and Ni–S/NF cathode. All the electrodeposited electrodes demonstrate enhanced current density and reduced electron losses, thereby achieving efficient hydrogen production. These innovative varieties of electrodes are highly advantageous as they are relatively inexpensive and easy to manufacture with great potential in reducing costs and further real time application in large scale. • The overpotential of nickel foam is greatly reduced by Ni/Co/S electrodeposition. • High hydrogen yield is achieved by Co–S/NF electrode in neutral condition. • Electrodeposited electrodes show high current density and low electron losses in MEC. [ABSTRACT FROM AUTHOR]

In comparison to precious metal catalyst especially Platinum (Pt), nickel foam (NF) owned cheap cost and unique three-dimensional (3D) structure, however, it was scarcely applied as cathode material in microbial electrolysis cell (MEC) as the intrinsic laggard electrochemical activity for hydrogen recovery. In this study, a self-assembly 3D nickel foam-graphene (NF-G) cathode was fabricated by facile hydrothermal approach for hydrogen evolution in MECs. Electrochemical analysis (linear scan voltammetry and electrochemical impedance spectroscopy) revealed the improved electrochemical activity and effective mass diffusion after coating with graphene. NF-G as cathode in MEC showed a significant enhancement in hydrogen production rate compared with nickel foam at a variety of biases. Noticeably, NF-G showed a comparable averaged hydrogen production rate (1.31±0.07 mL H 2 mL −1 reactor d −1 ) to Platinum/carbon (Pt/C) (1.32±0.07 mL H 2 mL −1 reactor d −1 ) at 0.8 V. Profitable energy recovery could be achieved by NF-G cathode at higher applied voltage, which performed the best hydrogen yield of 3.27±0.16 mol H 2 mol −1 acetate at 0.8 V and highest energy efficiency of 185.92±6.48% at 0.6 V. [ABSTRACT FROM AUTHOR]

Abstract: The short-chain fatty acids (SCFAs) accumulated in waste activated sludge (WAS) fermentation was adopted as an alternative extra carbon source for biohydrogen production in microbial electrolysis cells (MECs). WAS was pretreated by bi-frequency ultrasonic and the highest SCFAs were accumulated at 3rd day. Three groups of tests were conducted in single chamber MECs for H2 production under different SCOD concentrations. SCOD removals were up to 60% at diluted influent, but reduced to 50% at original concentration. Highest H2 yield was 1.2 mL H2/mg COD at 2-fold dilution with 155% energy efficiency. Results showed that >90% of acetate and ∼90% of propionate were effectively converted to hydrogen, and next were n-butyrate and n-valerate (at dilutions), but <20% of iso-butyrate and iso-valerate were converted. The overall biohydrogen recovery in this study was 120 ml H2/g VSS/d. This work shows a possibility of cascade utilization of WAS fermentation liquid and H2 generation in MEC. [Copyright &y& Elsevier]

Abstract: 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 0.6V 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%. [ABSTRACT FROM AUTHOR]

Abstract: In order to optimize operations of microbial electrolysis cell (MEC) for hydrogen production, microbial anode potential (MAP) was analyzed as a function of factors in biofilm anode system, including pH, substrate and applied voltage. The results in “H” shape reactor showed that MAP reflected the information when any factor became limiting for hydrogen production. Commonly, hydrogen generation started around anode potential of −250 mV to −300 mV. While, higher current density and higher hydrogen rate were obtained when MAP went down to −400 mV or even lower in this study. Biofilm anode could work normally between pH 6.5 and 7.0, while the lowest anode potential appeared around 6.8–7.0. However, when pH was lower 6.0 or substrate concentration was less than 50 mg L−1 in anode chamber, MAP went up to −300 mV or above, leading to hydrogen reduction. Applied voltage did not affect MAP much during the process of hydrogen production. Anode potential analysis also showed that planktonic bacteria in suspended solution presented positive effects on biofilm anode system and they contributed to enhance electron transfer by reducing internal resistance and lowering minimum voltage needed for hydrogen production to some extent. [ABSTRACT FROM AUTHOR]

Abstract: Methane production occurs during hydrogen gas generation in microbial electrolysis cells (MECs), particularly when single chamber systems are used which do not keep gases, generated at the cathode, separate from the anode. Few studies have examined the factors contributing to methane gas generation or the main pathway in MECs. It is shown here that methane generation is primarily associated with current generation and hydrogenotrophic methanogenesis and not substrate (acetate). Little methane gas was generated in the initial reaction time (<12h) in a fed batch MEC when acetate concentrations were high. Most methane was produced at the end of a batch cycle when hydrogen and carbon dioxide gases were present at the greatest concentrations. Increasing the cycle time from 24 to 72h resulted in complete consumption of hydrogen gas in the headspace (applied voltage of 0.7V) with methane production. High applied voltages reduced methane production. Little methane (<4%) accumulated in the gas phase at an applied voltage of 0.6–0.9V over a typical 24h cycle. However, when the applied voltage was decreased to 0.4V, there was a greater production of methane than hydrogen gas due to low current densities and long cycle times. The lack of significant hydrogen production from acetate was also supported by Coulombic efficiencies that were all around 90%, indicating electron flow was not altered by changes in methane production. These results demonstrate that methane production in single chamber MECs is primarily associated with current generation and hydrogen gas production, and not acetoclastic methanogenesis. Methane generation will therefore be difficult to control in mixed culture MECs that produce high concentrations of hydrogen gas. By keeping cycle times short, and using higher applied voltages (≥0.6V), it is possible to reduce methane gas concentrations (<4%) but not eliminate methanogenesis in MECs. [Copyright &y& Elsevier]

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 H 2 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. [ABSTRACT FROM AUTHOR]

Quorum sensing has been widely applied to enhance the energy recovery of bioelectrochemical system as a sustainable pathway to enhance communication between cells and electrodes. However, how signalling molecules (acyl-homoserine lactones, AHLs) regulate the microbial community to improve hydrogen generation in microbial electrolysis cells (MECs) is not well understood, especially the subsequent influence on interspecies relationships among not only electrode-respiring bacteria but also hydrogen scavengers. Understanding AHL regulation in a complicated and actual biofilm system will be valuable for future applications of microbial electrochemical technology. Herein, we added short-chain AHLs (3OC6) to regulate the biofilm community on bio-electrodes in MECs. As a result, hydrogen yields were enhanced with AHL addition, increasing by 5.57%, 38.68%, and 81.82% with varied external voltages (0.8 V, 0.6 V, and 0.4 V, respectively). Accordingly, overall reactor performance was enhanced, including coulombic efficiency, electron recovery efficiency, and energy efficiency. Based on an electrochemical impedance spectra analysis, the structured biofilm under simple nutrient conditions (acetate) showed a lower internal resistance with AHL addition, indicating that the microbial communities were altered to enhance electron transfer between the biofilm and electrode. The change in the cathodic microbial structure with more electrochemically active bacteria and fewer hydrogen scavengers could contribute to a higher electron recovery and hydrogen yield with AHL addition. The regulation of the microbial community structure by AHLs represents a potential strategy to enhance electron transfer and hydrogen generation in bioelectrochemical systems. [ABSTRACT FROM AUTHOR]

Free nitrous acid (FNA) has been recently reported to be an effective and eco-friendly inactivator for waste activated sludge (WAS), while the limited decomposition of the extracellular polymeric substance (EPS) matrix hampers resource recovery from WAS. This work employed peracetic acid (PAA) to assist FNA and explored the contribution of co-pretreatment to hydrogen recovery in a prefermentation-bioelectrolysis cascading system. The results showed that co-pretreatment led to approximately 8.8% and 20.4% increases in the exfoliation of particulate proteins and carbohydrates, respectively, from tightly bound EPS (TB-EPS) over that of sole FNA pretreatment. Electron paramagnetic resonance analysis verified that the synergistic effect of FNA, PAA and various generated free radicals was the essential process. This effect further promoted the accumulation of volatile fatty acids (VFAs) after 96 h of prefermentation, and the peak concentration in co-pretreated WAS (AD-FPWAS) was approximately 2.5-fold that in sole FNA-pretreated WAS (AD-FWAS). Subsequently, the cascading utilization of organics in the bioelectrolysis step contributed to efficient hydrogen generation. A total of 10.8 ± 0.3 mg H 2 /g VSS was harvested in microbial electrolysis cells (MECs) fed with AD-FPWAS, while 6.2 ± 0.1 mg H 2 /g VSS was obtained from AD-FWAS. X-ray photoelectron spectroscopy (XPS) revealed the effective decomposition of the phospholipid bilayer in the cytomembrane and the transformation of macromolecular organics into VFAs and hydrogen in the cascading system. Further microbial community analysis demonstrated that co-pretreatment enhanced the accumulation of functional consortia, including anaerobic fermentative bacteria (AFB, 28.1%), e.g., Macellibacteroides (6.3%) and Sedimentibacter (6.9%), and electrochemically active bacteria (EAB, 57.0%), e.g., Geobacter (39.0%) and Pseudomonas (13.6%), in the prefermentation and MEC steps, respectively. The possible synergetic and competitive relationships among AFB, EAB, homo-acetogens, nitrate-reducing bacteria and methanogens were explored by molecular ecological network analysis. From an environmental and economic perspective, this promising FNA and PAA co-pretreatment approach provides new insight for energy recovery from WAS biorefineries. Image 1 • PAA, FNA and their intermediates led to effective EPS decomposition. • PAA + FNA pretreatment boosted 2.5-fold VFAs and 1.7-fold H 2 recovery versus sole FNA. • XPS confirmed the destruction of phospholipid bilayer and organics transformation. • 28% AFB in prefermentation and 57% EAB in bioelectrolysis were enriched. • Synergetic relationships of AFB, EAB, homoacetogen, NRB and methanogen were explored. [ABSTRACT FROM AUTHOR]

To investigate the influence of antibiotics on microbial interactions in a biofilm community, we set up eight replicate reactors of microbial electrolysis cell (MEC) and applied a broad-spectrum antibiotic florfenical (FLO) as an environmental disturbance. According to the results, exposure to FLO resulted in degradation of reactor performance. The MEC could also rebound back to the comparably stable state at a certain time which exhibited a great resilience ability in response to antibiotic disturbance. The FLO perturbation showed a significant influence on the electroactive biofilms (EABs) with a distinct reformation of the community structure. Network analysis revealed that microbial interactions in the biofilms after full recovery became much closer, with a rapid increase in the positive interactions between the predominant genus Geobacter and other microorganisms as compared to the stage before FLO disturbance. Moreover, the keystone species in the networks after full recovery possessed more connections between Geobacter and potential synergistic species. Our results demonstrated that FLO, with broad-spectrum antibacterial ability, could restructure the EABs with more positive interactions for hydrogen production. This study demonstrated the response mechanisms of the MECs to the antibiotic disturbance, providing a scientific reference for the rapid development of this biotechnology to treat wastewater containing antibiotics. Image 1 • Reactor performance could successfully recover in response to the FLO disturbance. • FLO restructured the microbial structure and interaction networks to be more complex. • More positive interactions among the functional species facilitated the performance recovery. • FLO promoted the synergistic effects between exoelectrogens and fermenters. [ABSTRACT FROM AUTHOR]

Methane production enhanced by electrode respiring microorganisms is a recently developing technology for bioelectrolysis assisted anaerobic digestion. The direct interspecies electron transfer between electrodes and microbe has been recognized as the core mechanism for methanogenesis in this coupled system. However, the detailed mechanisms of methane electrosynthesis on cathodes need in-depth investigation. This work is to evaluate the effects of hydrogen evolution capacity of cathode on methane production and biocathode community that developed on commonly used cathode materials (copper, nickel and stainless steel). Integrated reactors with nickel cathode exhibited a maximum methane yield of 59.2 mL CH 4 /gVSS with a higher current density of 9 A/m2 compared to copper electrode and stainless-steel electrode. About 40% increase of methane production rate is achieved in the coupled reactors with nickel electrode compared to traditional anaerobic digestion technique. Analysis of microbial community structure reveals that the bioelectrolysis cathode persuades the whole system towards the hydrogenotrophic methanogenic pathway. A methanogenic system derived from Methanobacterium is formed on metal electrodes, which simultaneously utilizes both electrons and hydrogen to produce methane. However, higher hydrogen evolution reaction capacity leads to higher electrochemical contribution for methane synthesis, which results in an improved conversion efficiency. • High efficiency CO 2 -reducing biocathodes can formed on metal electrodes. • Higher HER capacity of cathode means higher contribution for methanogenesis. • Electrodes stimulate the sludge towards hydrogenotrophic methanogenic pathway. • The cathode community structure can be affected by the material properties. [ABSTRACT FROM AUTHOR]