Electricity-mediated biological hydrogen production

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.

Circulation of anodic effluent to the cathode chamber for subsequent treatment of wastewater in photosynthetic microbial fuel cell with generation of bioelectricity and algal biomass

Synthetic wastewater containing 1500 mg L^-1 of COD was treated in the anode chamber for 5, 10, and 20 d. An anode chamber was conducted under anaerobic conditions with mixed culture bacteria inoculum attached to the anode. Anodic effluent was transferred to the cathode chamber for further treatment for 5, 10, and 20 d as the growth medium of Chlorella vulgaris. The microalgal photosynthesis process provided oxygen for the cathodic reaction. In 5 d of anodic hydraulic retention time (HRT), the effluent contained high COD, resulting in low power generation in the P-MFC due to the heterotrophic metabolism carried out by microalgae diminishing photosynthesis. However, high biomass productivity up to 0.649 g L^-1 d^-1 was obtained in the subsequent treatment of 5 d in the cathode chamber. An anodic HRT of 10 d resulted in higher power generation (0.0254 kWh kg^-1 COD), and higher COD removal efficiency up to 60%. A further 10 d treatment in the cathode chamber increased the COD removal efficiency up to 74%. Anode and cathode chambers combined removed 79% of NH₄⁺-N concentration from the original synthetic wastewater within 20 d. This study demonstrated that the anodic effluent of the P-MFC can be utilized in the cathode chamber as a growth medium for microalgae if conducted with appropriate HRT in the anode. P-MFC provides a promising sustainable solution for wastewater treatment while generating electricity and algal biomass as by-products.

Microbial electron uptake in microbial electrosynthesis: a mini-review

Microbial electron uptake (EU) is the biological capacity of microbes to accept electrons from electroconductive solid materials. EU has been leveraged for sustainable bioproduction strategies via microbial electrosynthesis (MES). MES often involves the reduction of carbon dioxide to multi-carbon molecules, with electrons derived from electrodes in a bioelectrochemical system. EU can be indirect or direct. Indirect EU-based MES uses electron mediators to transfer electrons to microbes. Although an excellent initial strategy, indirect EU requires higher electrical energy. In contrast, the direct supply of cathodic electrons to microbes (direct EU) is more sustainable and energy efficient. Nonetheless, low product formation due to low electron transfer rates during direct EU remains a major challenge. Compared to indirect EU, direct EU is less well-studied perhaps due to the more recent discovery of this microbial capability. This mini-review focuses on the recent advances and challenges of direct EU in relation to MES.

Electro-conversion of carbon dioxide (CO2) to low-carbon methane by bioelectromethanogenesis process in microbial electrolysis cells: The current status and future perspective

Given the aggravated greenhouse effect caused by CO₂ and the current energy shortage, CO₂ capture and reuse has been gaining ever-increasing concerns. Microbial Electrolysis Cells (MECs) has been considered to be a promising alternative to recycle CO₂ bioelectrochemically to low-carbon electrofuels such as CH₄ by combining electroactive microorganisms with electrochemical stimulation, enabling both CO₂ fixation and energy recovery. In spite of the numerous efforts dedicated in this field in recent years, there are still many problems that hinder CO₂ bioelectroconversion technique from the scaling-up and potential industrialization. This review comprehensively summarized the working principles, extracellular electron transfers behaviors, and the critical factors limiting the wide-spread utilization of CO₂ electromethanogenesis. Various characterization and electrochemical testing methods for helping to uncover the underlying mechanisms in CO₂ electromethanogenesis have been introduced. In addition, future research needs for pushing forward the development of MECs technology in real-world CO₂ fixation and recycling were elaborated.

Microbial nanowires and electroactive biofilms

Geobacter bacteria are the only microorganisms known to produce conductive appendages or pili to electronically connect cells to extracellular electron acceptors such as iron oxide minerals and uranium. The conductive pili also promote cell-cell aggregation and the formation of electroactive biofilms. The hallmark of these electroactive biofilms is electronic heterogeneity, mediated by coordinated interactions between the conductive pili and matrix-associated cytochromes. Collectively, the matrix-associated electron carriers discharge respiratory electrons from cells in multilayered biofilms to electron-accepting surfaces such as iron oxide coatings and electrodes poised at a metabolically oxidizable potential. The presence of pilus nanowires in the electroactive biofilms also promotes the immobilization and reduction of soluble metals, even when present at toxic concentrations. This review summarizes current knowledge about the composition of the electroactive biofilm matrix and the mechanisms that allow the wired Geobacter biofilms to generate electrical currents and participate in metal redox transformations.

A comprehensive comparison of five different carbon-based cathode materials in CO2 electromethanogenesis: Long-term performance, cell-electrode contact behaviors and extracellular electron transfer pathways

Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO₂. To optimize electromethanogenesis process, this study physically prepared and examined several carbon-based cathode materials: carbon stick (CS), CS twined by Ti wire (CS-Ti) or covered with carbon fiber (CS-CF), graphite felt (CS-GF) and carbon cloth (CS-CC). CS-GF electrode had constantly stable methane production (75.8 mL/L/d at -0.9 V vs. Ag/AgCl) while CS-CC showed a suppressed performance over time caused by the desposition of inorganic shell. Electrode material properties affected biofilms growth, cell-electrode contact behaviors and electron exchange. Methane formation with CS-CC biocathode was H₂-concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H₂ route. This study re-interpreted the application potentials of carbon-based materials in CO₂ electroreduction and electrofuel recovery, providing valuable guidance for materials' selection.

Effects of Applied Potential and Reactants to Hydrogen-Producing Biocathode in a Microbial Electrolysis Cell

Understanding the mechanism of electron transfer between the cathode and microorganisms in cathode biofilms in microbial electrolysis cells (MECs) for hydrogen production is important. In this study, biocathodes of MECs were successfully re-enriched and subjected to different operating parameters: applied potential, sulfate use and inorganic carbon consumption. It was hypothesized that biocathode catalytic activity would be affected by the applied potentials that initiate electron transfer. While inorganic carbon, in the form of bicarbonate, could be a main carbon source for biocathode growth, sulfate could be a terminal electron acceptor and thus reduced to elemental sulfurs. It was found that potentials more negative than -0.8 V (vs. standard hydrogen electrode) were required for hydrogen production by the biocathode. In additional, a maximum hydrogen production was observed at sulfate and bicarbonate concentrations of 288 and 610 mg/L respectively. Organic carbons were found in the cathode effluents, suggesting that microbial interactions probably happen between acetogens and sulfate reducing bacteria (SRB). The hydrogen-producing biocathode was sulfate-dependent and hydrogen production could be inhibited by excessive sulfate because more energy was directed to reduce sulfate (E° SO 4 2 - /H2S = -0.35 V) than proton (E° H+/H2 = -0.41 V). This resulted in a restriction to the hydrogen production when sulfate concentration was high. Domestic wastewaters contain low amounts of organic compounds and sulfate would be a better medium to enrich and maintain a hydrogen-producing biocathode dominated by SRB. Besides the risks of limited mass transport and precipitation caused by low potential, methane contamination in the hydrogen-rich environment was inevitable in the biocathode after long term operation due to methanogenic activities.

Microbial electrohydrogenesis linked to dark fermentation as integrated application for enhanced biohydrogen production: A review on process characteristics, experiences and lessons

Microbial electrohydrogenesis cells (MECs) are devices that have attracted significant attention from the scientific community to generate hydrogen gas electrochemically with the aid of exoelectrogen microorganisms. It has been demonstrated that MECs are capable to deal with the residual organic materials present in effluents generated along with dark fermentative hydrogen bioproduction (DF). Consequently, MECs stand as attractive post-treatment units to enhance the global H₂ yield as a part of a two-stage, integrated application (DF-MEC). In this review article, it is aimed (i) to assess results communicated in the relevant literature on cascade DF-MEC systems, (ii) describe the characteristics of each steps involved and (iii) discuss the experiences as well as the lessons in order to facilitate knowledge transfer and help the interested readers with the construction of more efficient coupled set-ups, leading eventually to the improvement of overall biohydrogen evolution performances.

On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Tracking bio-hydrogen-mediated production of commodity chemicals from carbon dioxide and renewable electricity

This study reveals that reduction of carbon dioxide (CO₂) to commodity chemicals can be functionally compartmentalized in bioelectrochemical systems. In the present example, a syntrophic consortium composed by H₂-producers (Rhodobacter sp.) in the biofilm is combined with carboxidotrophic Clostridium species, mainly found in the bulk liquid. The performance of these H₂-mediated electricity-driven systems could be tracked by the activity of a biological H₂ sensory protein identified at cathode potentials between -0.2V and -0.3V vs SHE. This seems to point out that such signal is not strain specific, but could be detected in any organism containing hydrogenases. Thus, the findings of this work open the door to the development of a biosensor application or soft sensors for monitoring such systems.

Enhanced microbial electrosynthesis by using defined co-cultures

Microbial uptake of free cathodic electrons presents a poorly understood aspect of microbial physiology. Uptake of cathodic electrons is particularly important in microbial electrosynthesis of sustainable fuel and chemical precursors using only CO₂ and electricity as carbon, electron and energy source. Typically, large overpotentials (200 to 400 mV) were reported to be required for cathodic electron uptake during electrosynthesis of, for example, methane and acetate, or low electrosynthesis rates were observed. To address these limitations and to explore conceptual alternatives, we studied defined co-cultures metabolizing cathodic electrons. The Fe(0)-corroding strain IS4 was used to catalyze the electron uptake reaction from the cathode forming molecular hydrogen as intermediate, and Methanococcus maripaludis and Acetobacterium woodii were used as model microorganisms for hydrogenotrophic synthesis of methane and acetate, respectively. The IS4-M. maripaludis co-cultures achieved electromethanogenesis rates of 0.1-0.14 μmol cm^-2 h^-1 at -400 mV vs standard hydrogen electrode and 0.6-0.9 μmol cm^-2 h^-1 at -500 mV. Co-cultures of strain IS4 and A. woodii formed acetate at rates of 0.21-0.23 μmol cm^-2 h^-1 at -400 mV and 0.57-0.74 μmol cm^-2 h^-1 at -500 mV. These data show that defined co-cultures coupling cathodic electron uptake with synthesis reactions via interspecies hydrogen transfer may lay the foundation for an engineering strategy for microbial electrosynthesis.

The biocathode of microbial electrochemical systems and microbially-influenced corrosion

The cathode reaction is one of the most important limiting factors in bioelectrochemical systems even with precious metal catalysts. Since aerobic bacteria have a much higher affinity for oxygen than any known abiotic cathode catalysts, the performance of a microbial fuel cell can be improved through the use of electrochemically-active oxygen-reducing bacteria acting as the cathode catalyst. These consume electrons available from the electrode to reduce the electron acceptors present, probably conserving energy for growth. Anaerobic bacteria reduce protons to hydrogen in microbial electrolysis cells (MECs). These aerobic and anaerobic bacterial activities resemble those catalyzing microbially-influenced corrosion (MIC). Sulfate-reducing bacteria and homoacetogens have been identified in MEC biocathodes. For sustainable operation, microbes in a biocathode should conserve energy during such electron-consuming reactions probably by similar mechanisms as those occurring in MIC. A novel hypothesis is proposed here which explains how energy can be conserved by microbes in MEC biocathodes.

Bio-electro degradation of azo-dye in a combined anaerobic–aerobic process along with energy recovery

Complete removal of reactive orange 16 in a microbial fuel cell coupled aerobic post-treatment process along with energy recovery.

Bioelectrochemical enhancement of anaerobic methanogenesis for high organic load rate wastewater treatment in a up-flow anaerobic sludge blanket (UASB) reactor

A coupling process of anaerobic methanogenesis and electromethanogenesis was proposed to treat high organic load rate (OLR) wastewater. During the start-up stage, acetate removal efficiency of the electric-biological reactor (R1) reached the maximization about 19 percentage points higher than that of the control anaerobic reactor without electrodes (R2), and CH4 production rate of R1 also increased about 24.9% at the same time, while additional electric input was 1/1.17 of the extra obtained energy from methane. Coulombic efficiency and current recorded showed that anodic oxidation contributed a dominant part in degrading acetate when the metabolism of methanogens was low during the start-up stage. Along with prolonging operating time, aceticlastic methanogenesis gradually replaced anodic oxidation to become the main pathway of degrading acetate. When the methanogens were inhibited under the acidic conditions, anodic oxidation began to become the main pathway of acetate decomposition again, which ensured the reactor to maintain a stable performance. FISH analysis confirmed that the electric field imposed could enrich the H2/H(+)-utilizing methanogens around the cathode to help for reducing the acidity. This study demonstrated that an anaerobic digester with a pair of electrodes inserted to form a coupling system could enhance methanogenesis and reduce adverse impacts.

Microbial electrolysis cells turning to be versatile technology: recent advances and future challenges

Microbial electrolysis cells (MECs) are an electricity-mediated microbial bioelectrochemical technology, which is originally developed for high-efficiency biological hydrogen production from waste streams. Compared to traditional biological technologies, MECs can overcome thermodynamic limitations and achieve high-yield hydrogen production from wide range of organic matters at relatively mild conditions. This approach greatly reduces the electric energy cost for hydrogen production in contrast to direct water electrolysis. In addition to hydrogen production, MECs may also support several energetically unfavorable biological/chemical reactions. This unique advantage of MECs has led to several alternative applications such as chemicals synthesis, recalcitrant pollutants removal, resources recovery, bioelectrochemical research platform and biosensors, which have greatly broaden the application scopes of MECs. MECs are becoming a versatile platform technology and offer a new solution for emerging environmental issues related to waste streams treatment and energy and resource recovery. Different from previous reviews that mainly focus on hydrogen production, this paper provides an up-to-date review of all the new applications of MECs and their resulting performance, current challenges and prospects of future.

Consolidated bioprocessing of AFEX-pretreated corn stover to ethanol and hydrogen in a microbial electrolysis cell

The consolidated bioprocessing (CBP) of corn stover pretreated via ammonia fiber expansion (AFEX-CS) into ethanol was investigated in a microbial electrolysis cell (MEC) driven by the exoelectrogen Geobacter sulfurreducens and the CBP bacterium Cellulomonas uda. C. uda was identified in a screening for its ethanologenic potential from AFEX-CS and for producing electron donors for G. sulfurreducens fermentatively. C. uda produced ethanol from AFEX-CS in MECs inoculated simultaneously or sequentially, with the concomitant conversion of the fermentation byproducts into electricity by G. sulfurreducens. The fermentation and electrical conversion efficiencies were high, but much of the AFEX-CS remained unhydrolyzed as nitrogen availability limited the growth of the CBP partner. Nitrogen supplementation stimulated the growth of C. uda, AFEX-CS hydrolysis and ethanologenesis. As a result, the synergistic activities of the CBP and exoelectrogen catalysts resulted in substantial energy recoveries from ethanologenesis alone (ca. 56%). The cogeneration of cathodic H(2) in the MEC further increased the energy recoveries to ca. 73%. This and the potential to optimize the activities of the microbial catalysts via culturing approaches and genetic engineering or adaptive evolution, make this platform attractive for the processing of agricultural wastes.

Electromicrobiology

Electromicrobiology deals with the interactions between microorganisms and electronic devices and with the novel electrical properties of microorganisms. A diversity of microorganisms can donate electrons to, or accept electrons from, electrodes without the addition of artificial electron shuttles. However, the mechanisms for microbe-electrode electron exchange have been seriously studied in only a few microorganisms. Shewanella oneidensis interacts with electrodes primarily via flavins that function as soluble electron shuttles. Geobacter sulfurreducens makes direct electrical contacts with electrodes via outer-surface, c-type cytochromes. G. sulfurreducens is also capable of long-range electron transport along pili, known as microbial nanowires, that have metallic-like conductivity similar to that previously described in synthetic conducting polymers. Pili networks confer conductivity to G. sulfurreducens biofilms, which function as a conducting polymer, with supercapacitor and transistor functionalities. Conductive microorganisms and/or their nanowires have a number of potential practical applications, but additional basic research will be necessary for rational optimization.

Forming microbial anodes under delayed polarisation modifies the electron transfer network and decreases the polarisation time required

Microbial anodes were formed from compost leachate on carbon cloth electrodes. The biofilms formed at the surface of electrodes kept at open circuit contained microorganisms that switched their metabolism towards electrode respiration in response to a few minutes of polarisation. When polarisation at -0.2 V/SCE (+0.04 V/SHE) was applied to a pre-established biofilm formed at open circuit (delayed polarisation), the bacteria developed an extracellular electron transport network that showed multiple redox systems, reaching 9.4 A/m(2) after only 3-9 days of polarisation. In contrast, when polarisation was applied from the beginning, bacteria developed a well-tuned extracellular electron transfer network concomitantly with their growth, but 36 days of polarisation were required to get current of the same order (6-8 A/m(2)). The difference in performance was attributed to the thinner, more heterogeneous structure of the biofilms obtained by delayed polarisation compared to the thick uniform structure obtained by full polarisation.

Bioelectrochemical systems: an outlook for practical applications

Bioelectrochemical systems (BESs) hold great promise for sustainable production of energy and chemicals. This review addresses the factors that are essential for practical application of BESs. First, we compare benefits (value of products and cleaning of wastewater) with costs (capital and operational costs). Based on this, we analyze the maximum internal resistance (in mΩ m(2) ) and current density that is required to make microbial fuel cells (MFCs) and hydrogen-producing microbial electrolysis cells (MECs) cost effective. We compare these maximum resistances to reported internal resistances and current densities with special focus on cathodic resistances. Whereas the current densities of MFCs still need to be increased considerably (i.e., internal resistance needs to be decreased), MECs are closer to application as their current densities can be increased by increasing the applied voltage. For MFCs, the production of high-value products in combination with electricity production and wastewater treatment is a promising route.

Enrichment of microbial electrolysis cell biocathodes from sediment microbial fuel cell bioanodes

Electron-accepting (electrotrophic) biocathodes were produced by first enriching graphite fiber brush electrodes as the anodes in sediment-type microbial fuel cells (sMFCs) using two different marine sediments and then electrically inverting the anodes to function as cathodes in two-chamber bioelectrochemical systems (BESs). Electron consumption occurred at set potentials of -439 mV and -539 mV (versus the potential of a standard hydrogen electrode) but not at -339 mV in minimal media lacking organic sources of energy. Results at these different potentials were consistent with separate linear sweep voltammetry (LSV) scans that indicated enhanced activity (current consumption) below only ca. -400 mV. MFC bioanodes not originally acclimated at a set potential produced electron-accepting (electrotrophic) biocathodes, but bioanodes operated at a set potential (+11 mV) did not. CO(2) was removed from cathode headspace, indicating that the electrotrophic biocathodes were autotrophic. Hydrogen gas generation, followed by loss of hydrogen gas and methane production in one sample, suggested hydrogenotrophic methanogenesis. There was abundant microbial growth in the biocathode chamber, as evidenced by an increase in turbidity and the presence of microorganisms on the cathode surface. Clone library analysis of 16S rRNA genes indicated prominent sequences most similar to those of Eubacterium limosum (Butyribacterium methylotrophicum), Desulfovibrio sp. A2, Rhodococcus opacus, and Gemmata obscuriglobus. Transfer of the suspension to sterile cathodes made of graphite plates, carbon rods, or carbon brushes in new BESs resulted in enhanced current after 4 days, demonstrating growth by these microbial communities on a variety of cathode substrates. This report provides a simple and effective method for enriching autotrophic electrotrophs by the use of sMFCs without the need for set potentials, followed by the use of potentials more negative than -400 mV.

pH, redox potential and local biofilm potential microenvironments within Geobacter sulfurreducens biofilms and their roles in electron transfer

The limitation of pH inside electrode-respiring biofilms is a well-known concept. However, little is known about how pH and redox potential are affected by increasing current inside biofilms respiring on electrodes. Quantifying the variations in pH and redox potential with increasing current is needed to determine how electron transfer is tied to proton transfer within the biofilm. In this research, we quantified pH and redox potential variations in electrode-respiring Geobacter sulfurreducens biofilms as a function of respiration rates, measured as current. We also characterized pH and redox potential at the counter electrode. We concluded that (1) pH continued to decrease in the biofilm through different growth phases, showing that the pH is not always a limiting factor in a biofilm and (2) decreasing pH and increasing redox potential at the biofilm electrode were associated only with the biofilm, demonstrating that G. sulfurreducens biofilms respire in a unique internal environment. Redox potential inside the biofilm was also compared to the local biofilm potential measured by a graphite microelectrode, where the tip of the microelectrode was allowed to acclimatize inside the biofilm.

Analysis of the microbial community of the biocathode of a hydrogen-producing microbial electrolysis cell

The microbial electrolysis cell (MEC) is a promising system for hydrogen production. Still, expensive catalysts such as platinum are needed for efficient hydrogen evolution at the cathode. Recently, the possibility to use a biocathode as an alternative for platinum was shown. The microorganisms involved in hydrogen evolution in such systems are not yet identified. We analyzed the microbial community of a mixed culture biocathode that was enriched in an MEC bioanode. This biocathode produced 1.1 A m⁻² and 0.63 m³ H₂ m⁻³ cathode liquid volume per day. The bacterial population consisted of 46% Proteobacteria, 25% Firmicutes, 17% Bacteroidetes, and 12% related to other phyla. The dominant ribotype belonged to the species Desulfovibrio vulgaris. The second major ribotype cluster constituted a novel taxonomic group at the genus level, clustering within uncultured Firmicutes. The third cluster belonged to uncultured Bacteroidetes and grouped in a taxonomic group from which only clones were described before; most of these clones originated from soil samples. The identified novel taxonomic groups developed under environmentally unusual conditions, and this may point to properties that have not been considered before. A pure culture of Desulfovibrio strain G11 inoculated in a cathode of an MEC led to a current development from 0.17 to 0.76 A m⁻² in 9 days, and hydrogen gas formation was observed. On the basis of the known characteristics of Desulfovibrio spp., including its ability to produce hydrogen, we propose a mechanism for hydrogen evolution through Desulfovibrio spp. in a biocathode system.

Redox and pH microenvironments within Shewanella oneidensis MR-1 biofilms reveal an electron transfer mechanism

The goal of this research was to quantify the variations in redox potential and pH in Shewanella oneidensis MR-1 biofilms respiring on electrodes. We grew S. oneidensis MR-1 on a graphite electrode, which was used to accept electrons for microbial respiration. We modified well-known redox and pH microelectrodes with a built-in reference electrode so that they could operate near polarized surfaces and quantified the redox potential and pH profiles in these biofilms. In addition, we used a ferri-/ferrocyanide redox system in which electrons were only transferred by mediated electron transfer to explain the observed redox potential profiles in biofilms. We found that regardless of the polarization potential of the biofilm electrode, the redox potential decreased toward the bottom of the biofilm. In a fully redox-mediated control system (ferri-/ferrocyanide redox system), the redox potential increased toward the bottom when the electrode was the electron acceptor. The opposite behavior of redox profiles in biofilms and the redox-controlled system is explained by S. oneidensis MR-1 biofilms not being redox-controlled when they respire on electrodes. The lack of a significant variation in pH implies that there is no proton transfer limitation in S. oneidensis MR-1 biofilms and that redox potential profiles are not caused by pH.

Electricity-assisted biological hydrogen production from acetate by Geobacter sulfurreducens

Geobacter sulfurreducens is a well-known current-producing microorganism in microbial fuel cells, and is able to use acetate and hydrogen as electron donor. We studied the functionality of G. sulfurreducens as biocatalyst for hydrogen formation at the cathode of a microbial electrolysis cell (MEC). Geobacter sulfurreducens was grown in the bioelectrode compartment of a MFC with acetate as the substrate and reduction of complexed Fe(III) at the counter electrode. After depletion of the acetate the electrode potential of the bioelectrode was decreased stepwise to -1.0 V vs Ag/AgCl reference. Production of negative current was observed, which increased in time, indicating that the bioelectrode was now acting as biocathode. Headspace analyses carried out at electrode potentials ranging from -0.8 to -1.0 V showed that hydrogen was produced, with higher rates at more negative cathode potentials. Subsequently, the metabolic properties of G. sulfurreducens for acetate oxidation at the anode and hydrogen production at the cathode were combined in one-compartment membraneless MECs operated at applied voltages of 0.8 and 0.65 V. After two days, current densities were 0.44 A m(-2) at 0.8 V applied voltage and 0.22 A m(-2) at 0.65 V, using flat-surface carbon electrodes for both anode and cathode. The cathodic hydrogen recovery ranged from 23% at 0.5 V applied voltage to 43% at 0.9 V.