Journal of Cleaner Production 18 (2010) S105eS111
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Hydrogen gas production in a microbial electrolysis cell by electrohydrogenesis Nathan Wrana a, Richard Sparling b, Nazim Cicek a, David B. Levin a, * a b
Department of Biosystems Engineering, University of Manitoba, Winnipeg MB, Canada R3T 5V6 Department of Microbiology, University of Manitoba, Winnipeg MB, Canada R3T 5V6
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 February 2010 Received in revised form 3 June 2010 Accepted 12 June 2010 Available online 1 July 2010
Electrohydrogenesis is a bio-electrochemical process where organic material is microbially oxidized to protons and electrons, which in turn are reduced to form hydrogen gas (H2). The reactor in which these reactions occur is termed a microbial electrolysis cell (MEC). The microorganisms that colonize the anode are known as electricigens and behave as biological catalysts, signiﬁcantly reducing the energy required to drive this process. Electricigens are capable of complete substrate degradation, leading to very high cathodic H2 recovery efﬁciencies from sources previously considered organic waste. In this short review, the origination of the bio-electrochemical system (BES) is introduced, mechanisms for electron transfer between microbe and electrode are discussed, the challenges these electrochemical systems face are presented, and ﬁnally an overview of current MEC systems and their respective performance is evaluated. Electrohydrogenesis has established itself as a promising technology for sustainable H2 production from renewable sources. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Biohydrogen Microbial electrolysis cell Electrohydrogenesis Electricigen Renewable
1. Introduction Microbial electrolysis cells (MECs) are electrochemical devices that microbial oxidize organic material at the anode, producing carbon dioxide (CO2), electrons (e), and protons (Hþ). In the presence of a catalyst, electrons and protons are chemically reduced at the cathode to form hydrogen gas (H2). This process is termed electrohydrogenesis (Fig. 1). Electrons are shuttled from one electrode to the other through an external electric circuit while protons diffuse through the electrolyte to the cathode. Because current production is very small, the voltage drop (Vdrop) across an external resistor (Rext) is measured and the current is calculated according to Ohm’s law. This is not a spontaneous reaction. Under standard temperature and pressure conditions (STP: 25 C, 1 bar, [1 M], adjusted to pH 7.0), the reduction potential, Eo0 , for H2 production from Hþ and e is very low at 0.414 V (versus Normal Hydrogen Electrode e NHE). However, with the exception of glucose (Logan, 2008), the reduction potential of the anodic half-reaction is usually much higher. In practice, this potential difference is even greater due to resistance in the system and polarization at the electrodes. Consequently, in order to drive electrons from the anode to the cathode, energy from an external power source must be supplied to the cell.
* Corresponding author. Tel.: þ1 204 4747429; fax þ1 204 4747523. E-mail address: [email protected]
(D.B. Levin). 0959-6526/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jclepro.2010.06.018
The focus of this paper is to review the fundamental concepts of MEC technology. Investigation into the various mechanisms of electron transfer via microbeeelectrode interactions will be explored in detail. The electrochemical challenges MEC technology faces will be discussed, outlining key areas where improvements can be made. Finally, an overview of the major MEC studies will be examined. First, however, a brief look into history of MEC technology and its evolution from bio-electrochemical systems (BES) is explored. 2. History of the bio-electrochemical system The ﬁrst connection between electricity and biology was made in 1791 by Luigi Galvani, who discovered that severed frog’s legs contract every time the muscle and nerve endings are connected to a static electricity generator (Kipnis, 1987; Bullen et al., 2006). Around the same time, the concept of the fuel cell was developed by W.R. Grove who successfully produced an electric current and water by combining hydrogen gas with oxygen (Grove, 1839). Surprisingly, the ﬁrst correlation between a half cell and microorganisms was not made until 1911 when M.C. Potter observed small amounts of electricity could be derived from microbial communities, introducing the possibility that bacteria were capable of generating an electric current (Potter, 1911; Bullen et al., 2006). This discovery led to the BES in which anodic and/or cathodic reactions are biologically catalyzed by microorganisms. When electricity is produced in a BES, the term microbial fuel cell (MFC) is used. However, the term MEC is employed when electricity is consumed
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to acceptors outside of the cell (Allen and Bennetto, 1993). However, the breakthrough came with the discovery of the electricigen, bacteria capable of direct extracellular electron transfer to fuel cell anodes through the complete oxidation of organic compounds (Debabov, 2008; Lovley and Nevin, 2008). It has been known for almost 100 years that electrode-reducing microorganisms can transfer electrons via direct contact with the anode or through the synthesis of chemical mediators, both exogenous and endogenous. However, it was observed for the ﬁrst time that organic compounds were completely oxidized to CO2 with all available electrons converted to current (Bond et al., 2002). 3. Electrogenic bacteria
Fig. 1. Principles of operation of a microbial electrolysis cell (MEC). Microbial oxidation of organic material at the anode, e.g. acetate oxidation, liberates electrons (e) and protons (Hþ), which undergo chemical reduction at the cathode to form H2. The reduction potential at the anode is higher than at the cathode and thus a small voltage must be supplied to drive this non-spontaneous reaction.
to drive electrochemical reactions (Clauwaert et al., 2008). In the context of this paper, BESs apply speciﬁcally to microbial catalysis and not isolated enzymatic catalysis when assisting redox reactions in electrochemical cells (Bullen et al., 2006; He and Angenent, 2006; Rabaey et al., 2007). Electricity production has been and remains the primary driving force behind the development of BES technology. In 1931, B. Cohen conducted the ﬁrst bacteria-electrode interaction studies by examining overall potentiometric intensities of chemical reactions during bacterial growth (Cohen, 1931; Berk and Canﬁeld, 1964). He demonstrated bacterial cultures are electrical half-cells that can perform work. With the introduction of artiﬁcial electron mediators (potassium ferricyanide or benzoquinone), Cohen was able to build the ﬁrst bacterial battery, producing a voltage of 35 V when six cells were connected in electrical series. During the late 1950s and throughout the1960s, interest in converting organic material into electrical energy surfaced. Initiated by the US space program, MFCs were seen as potential waste disposal units that could generate power during space missions (Shukla et al., 2004; Bullen et al., 2006). Despite an improved understanding of the electrical connections between electrodes and microorganisms as well as the introduction of potentially new applications, it was determined that current could not be produced at a consistent rate or in quantities large enough to be a viable source of electrical energy (Cohen, 1931; Lewis, 1966; Shukla et al., 2004; He and Angenent, 2006; Lovley and Nevin, 2008). A revived interest in electricity production from microbes came in the 1990’s. Apart from the search for new, environmentally responsible sources of energy, studies showed increases in power densities using exogenous chemical mediators to deliver electrons
Taxonomic proﬁles of electrode-reducing microbial communities from numerous MFC systems have been reported (Logan and Regan, 2006; Jung and Regan, 2007; Aelterman et al., 2008; Clauwaert et al., 2008; Chae et al., 2009). The bacterial communities that develop show great diversity and typically depend on the enrichment conditions used to colonize the electrode surface (Lovley, 2008b). However, amongst this group of electroderespiring microorganisms, bacteria from the phylum Proteobacteria dominate anode communities. According to an eight system comparative study conducted by Aelterman et al. (2008), 64% of the anode population belonged to the class of a-, b-, g-, or d-Proteobacteria, the most studied of these belonging to the families of Shewanella and Geobacteraceae (Debabov, 2008). The complete genome of Shewanella oneidensis was sequenced in 2002 (Heidelberg et al., 2002) and subsequently that of Geobacter sulfurreducens in 2003 (Methe et al., 2003); both organisms will serve as excellent models to elucidate the mechanisms of electron transfer between microorganism and electrode. 3.1. Mechanisms for microbeeelectrode electron transfer Understanding the principles of electron transfer between a microorganism and an electrode are essential in optimizing the current generated by any BES. Closer investigation into these mechanisms could not only inﬂuence material selection used to improve the electrical connection between the bacteria and the electrode but also manipulate the surface design of the electrode to facilitate electron delivery (Lovley and Nevin, 2008). Although the exact mechanisms for electron transfer are not completely understood, three methods have been proposed (Fig. 2): i) long-range electron transfer via electron shuttles, ii) direct electron transfer via outer-surface c-type cytochromes, and iii) long-range electron transfer via conductive pili or “microbial nanowires”. 3.1.1. Electron shuttles Certain microorganisms produce soluble exogenous mediators that shuttle electrons from cells to insoluble compounds via diffusion. This phenomenon was ﬁrst identiﬁed in mutant species of Shewanella putrifaciens (Newman and Kolter, 2000) and later demonstrated in MFCs inoculated with S. oneidensis MR-1 (Lanthier et al., 2008). Results showed that as many as half of the S. oneidensis cells were planktonic, suggesting substrate oxidation and concomitant current generation were coupled to long-range electron shuttles. Various techniques used to detect redox-active molecules eventually identiﬁed riboﬂavin secretion as the mechanism for extracellular electron transfer in Shewanella sp. (Lovley, 2008b; Marsili et al., 2008; von Canstein et al., 2008). This mechanism for electron transfer has been observed in other bacteria, such as Geothrix fermentans (Bond and Lovley, 2005). Unfortunately, despite the advantages of long-distance interaction with an electrode, electron shuttles are energetically taxing and may not be the
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targeted based on high transcript levels during growth on an electrode as the sole electron acceptor (Holmes et al., 2006). Deletion of omcS signiﬁcantly reduced current production, which was restored after re-expression of the gene in trans on a plasmid. However, current inhibition was only temporary following the deletion of omcE. Over time, G. sulfurreducens found alternative routes to transfer electrons to the anode surface (Lovley and Nevin, 2008). Results from this study outline the signiﬁcance of outer membrane c-type cytochromes in direct electron transfer to an electrode. However, some organisms capable of reducing Fe(III) oxides lack c-cytochromes (Lovley et al., 1995). In addition, electron transport proteins attached directly to the outer membrane of the cell cannot explain how thick bioﬁlms up to 75 mm (Lovley, 2008a) develop along anode surfaces as reported in many MFC studies (Bond and Lovley, 2003; Reguera et al., 2006; Nevin et al., 2008). Further investigation using G. sulfurreducens led to the discovery of electrically conductive pili known as “nanowires” (Reguera et al., 2005).
Fig. 2. Proposed mechanisms for electron transfer to the anode of an MEC. i) Longrange electron transfer via electron shuttles (yellow hexagon), ii) direct electron transfer via outer-surface c-type cytochromes (red circles), and iii) long-range electron transfer via conductive pili or “microbial nanowires” (orange rods).
most desirable system for the bacterium (Mahadevan et al., 2006; Lovley and Nevin, 2008). 3.1.2. Direct contact via outer membrane c-type cytochromes In contrast to synthesized shuttles that indirectly transfer electrons to an electrode, direct contact between the cell and anode is also possible through outer membrane c-type cytochromes. In the same study conducted by Lanthier et al. (2008), the other half of S. oneidensis cells that were not planktonic were attached to the surface of the anode. This suggests that multiple strategies for electron transfer may exist. In order to differentiate between direct and indirect mineral reduction pathways, further investigation is required. Nanoporous glass beads deposited with Fe(III) (hydr) oxide were used to measure iron reduction by S. oneidensis MR-1 both indirectly and as a bioﬁlm (Lies et al., 2005). Results from strains with mutations in cyma and omcB identiﬁed a potential reduction process in which cytoplasmic c-type cytochrome CymA is required for both direct and indirect mineral reduction whereas outer membrane c-type cytochrome OmcB is not necessary for indirect iron reduction (Lies et al., 2005). It has been proposed that CymA functions as the terminal electron acceptor in the electron transport chain of S. oneidensis MR-1 whereas OmcB acts as a direct electrical contact between the microbe and the electrode surface (Debabov, 2008). Gene deletion studies in G. sulfurreducens further highlight the importance outer membrane c-type cytochromes have as direct electrical contacts between microbe and electrode. Two genes that encode outer membrane cytochromes, omcS and omcE, were
3.1.3. Electron transfer via microbial nanowires According to Reguera (2009), the nanowires of G. sulfurreducens are protein ﬁlaments composed of the repeated single unit PilA. They are classiﬁed as type IV pili but differ from convention due to their size (4e5 nm in diameter and 20 mm in length) and function. Unlike other pili that aid in cell motility or adhesion to solid surfaces, nanowires are electrically conductive conduits that enable microbial cell-cell communication. They are responsible for maximizing bioﬁlm health by coordinating a cooperative electronic community, aggregating and interconnecting cells into a network capable of effectively distributing and dissipating electrons. Nanowires are responsible for high current and power production in MFCs, enabling active participation from cells located not only on the surface of the electrode but also at the outer boundaries of the bioﬁlm. A study by Reguera et al. (2005) investigated the relationship between the nanowires of G. sulfurreducens and soluble and insoluble electron acceptors. Deletion of the pilA gene inhibited the production of pili and the reduction of insoluble Fe(III) oxides. The mutant was still able to reduce the soluble electron acceptors fumarate and Fe(III) citrate. Introducing a copy of the pilA gene in trans restored pili production and the ability to reduce Fe(III) oxides . Further studies revealed that PilA may have a structural role in bioﬁlm formation (Reguera et al., 2007). However, it was observed that pilA-deﬁcient mutants of G. sulfurreducens still formed bioﬁlms along the surface of a graphite anode that was not connected in electrical series to a cathode, using fumarate as the terminal electron acceptor (Lovley and Nevin, 2008). These ﬁndings suggest that although nanowires may not be necessary for bioﬁlm growth, they are required for high-current production in BESs with G. sulfurreducens. 4. Electrochemical challenges in microbial electrolysis cells Microbial electrolysis cells are much more complicated than typical electrochemical cells. The extent to which electrons are generated and transferred to an anode will strongly inﬂuence system performance. Energy losses in the form of overpotentials and ohmic resistances greatly affect the overall efﬁciency of the system as well. In this section, the thermodynamics of MEC reactions are discussed from an electrochemical context, focusing on ohmic losses, activation overpotentials, concentration overpotentials, and coulombic losses. 4.1. Thermodynamics Hydrogen gas generation in an MEC is not a spontaneous reaction. It requires the addition of energy to overcome the
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thermodynamic limitations set by the chemical reactions at the electrodes and the potential losses within the system (described below). It is possible to calculate the overall cell potential, expressed as an electromotive force (emf), by evaluating the potential difference between the cathode and the anode:
Eemf ¼ ECat EAn
The upper limits of the MEC are set by the half-reactions at the electrodes. These values can be calculated using the Nernst equation, which determines the equilibrium reduction potential at the cathode and anode: o Eelectrode ¼ Eelectrode
RT ½reductionp ln nF ½oxidationr
where R is the universal gas constant, 8.31447 J mol1 K1, T is the absolute temperature, n is the number of electrons transferred during the reaction, and F is Farraday’s constant, 96,485 C/mol. According to IUPAC convention, all reactions are reported as a reduction potential: oxidized þ e /reduced. Therefore, the reaction quotient is the ratio of products (reduced species) to reactants (oxidized species) raised to their respective stoichiometric coefﬁcients. o (V), is referenced to The standard reduction potential, Eelectrode the normal hydrogen electrode (NHE), which has a potential of 0.000 V under standard conditions (298 K, 1 bar, 1 M). However, because the MEC is a biological system, the NHE potential is adjusted to a pH of 7.0. Therefore, the corrected NHE potential becomes:
2Hþ þ 2e /H2
RT H o= E ¼ Eo ln h 2i2 ðH2 =Hþ Þ nF Hþ
8:31$298:15 1 o= E ¼ 0þ ln 2 ¼ 0:414 V ðH2 =Hþ Þ 2$9:65$104 107
Depending on the substrate consumed at the anode by the bacteria and the operating conditions of the MEC, the theoretical total cell potential will vary. The various chemical reactions that may occur are not presented here however there are many excellent review papers and books that compare the standard potentials against theoretical potentials of typical BESs (Bagotsky, 2005; Logan et al., 2006; Logan, 2008; Hamelers et al., 2010). 4.2. Energy losses Calculating the thermodynamics of a system determines the theoretical limitation under a given set of operating conditions. For an MEC, this is the minimum amount of energy required to generate H2. In practice, voltage requirements are considerably higher due to overpotentials at the anode and cathode and ohmic losses within the system. Actual energy requirements, Ecell, follow the equation:
Ecell ¼ Eemf P
hc þ IRU
4.2.1. Ohmic voltage losses Conduction of electric current in any electrochemical cell can be electronic or ionic (Bagotsky, 2005). In MECs, ohmic voltage loss is determined by resistance to electron ﬂow through electrical conductors (electrodes and external circuitry) and resistance to ion ﬂow through ionic conductors (electrolyte and proton exchange membrane e providing one exists) (Logan et al., 2006; Clauwaert et al., 2008). According to Ohm’s law, ohmic losses are independent of current. Reducing electrode spacing, increasing electrolyte conductivity, and selecting electrode and membrane material with low resistivity are options necessary to manage voltage losses and increase system performance. 4.2.2. Electrode overpotentials The minimum cell potential (i.e. the smallest amount of energy that must be supplied to the cell) is generated under open circuit voltage (OCV) conditions, where inﬁnite resistance and zero current are experienced. However, current ﬂow gives rise to overpotentials, where shifts in potential away from the equilibrium value are observed. The extent of polarization at any one of the two electrodes depends on the current density and the nature of the reaction but is independent of the other electrode and the processes occurring there (Bagotsky, 2005). Therefore, each electrode is treated individually when studying this phenomenon. Overpotentials in MECs are classiﬁed according to: i) activation losses, ii) coulombic losses, and iii) concentration losses (Logan et al., 2006). In order to overcome the activation energy of a redox reaction, additional energy is required. The transfer of electrons to or from a substance reacting at the electrode surface results in activation losses. At the anode, activation overpotentials highly depend on the electron transfer mechanism between microbe and electrode, i.e. electron shuttles, c-type cytochromes, or nanowires, and are most signiﬁcant during low-current situations. According to the Tafel plot, the rate of activation polarization rates decrease as current densities increase, thus strategies to minimize its impact should focus on improving catalyst reaction kinetics, increasing operating temperatures, and increasing reaction surface areas (Logan et al., 2006; Clauwaert et al., 2008; Logan, 2008). Coulombic efﬁciency is deﬁned as the amount of electrons recovered as current versus the amount of electrons available in the substrate for H2 production. Bacterial metabolic losses are responsible for signiﬁcant increases in coulombic losses. The amount of energy that can be used for bacterial growth is regulated by the potential difference between the substrate and electrode (terminal electron acceptor). Lowering this reduction potential lowers the amount of energy required to drive H2 production but also limits the energy gain for the bacteria. A balance between bacterial energy gain and electrode potential is required for optimal performance. During current ﬂow, the surface concentrations of the substances involved in the reaction change relative to the bulk concentrations in solution. Concentration polarization is observed when the supply or elimination of each substance is limited by poor mass transfer kinetics (Bagotsky, 2005). Substrate ﬂux to the bioﬁlm, diffusional gradients resulting from improper mixing, or unbalanced ratios of oxidized to reduced species at the electrode surface are all contributing factors that will result in a potential shift away from the electrode’s equilibrium position (Logan et al., 2006; Clauwaert et al., 2008).
ha and j hc j are the sum of the overpotentials at the where anode and cathode respectively and IRU is the sum of all ohmic losses within the system (Logan et al., 2006).
5. Overview of MEC systems The number of studies investigating H2 generation via electrohydrogenesis is rapidly increasing. High H2 yields and recovery
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efﬁciencies can be achieved using energy sources that were once considered waste, such as fermentation end-products and wastewater streams. An overview of the major MEC systems that have been published is summarized in Table 1. Experiments were conducted at varying applied voltages and the performance of each reactor was compared in terms of H2 yields and coulombic efﬁciencies. For the purpose of consistency, all papers selected in this evaluation were MECs inoculated with mixed cultures and used acetate as the sole carbon source. It should be noted that acetate is typically used as a carbon source because it is a preferred substrate by electricigens, particularly G. sulfurreducens. This statement is supported by high H2 recovery yields (91%) and production rates (1.1 m3-H2 m3 reactor day1) by MECs fed with acetic acid only (Cheng and Logan, 2007). In addition, the removal of acetate has potential industrial applications for MEC technology, which include the treatment of wastewater, landﬁll lecheate, and fermentation waste. Because the production of acetate is the highest H2-yielding pathway in dark fermentation (4 mol-H2/mol glucose), it is an ideal candidate for the optimized production of H2 from the simultaneous fermentation of cellulosic material and electrohydrogenesis of its end-products (Lu et al., 2009). In practice, however, acetate generally does not exist alone and is typically found mixed with other organic acids. Studies have tested other substrates, including butyric acid, lactic acid, propionic acid, and valeric acid (Cheng and Logan, 2007) as well as diluted carbon sources found in wastewaters (Ditzig et al., 2007; Wagner et al., 2009). Results showed that overall H2 recovery efﬁciencies were much lower when wastewater was used as a substrate (10e28%) versus acetate-fed MECs. The ﬁrst group to evaluate H2 production in an MEC was Liu et al. (2005), who developed two different reactors adapted from previous work on MFCs (Liu and Logan, 2004; Oh et al., 2004). Both systems had an anode chamber and a cathode chamber separated at the middle by a cation exchange membrane (CEM). The anodes were constructed from carbon cloth and the cathodes were made from carbon paper loaded with a platinum catalyst (0.5 mg Pt/cm2). The ﬁrst system was an H-cell conﬁguration where two connected Wheaton bottles with working chamber volumes of 200 ml each
(CE ¼ 60%; Eap ¼ 0.25 V). produced 2.2 mol-H2/mol-acetate The second reactor was a cube design with a 28 ml hole bored though the middle and capped at both ends. At an applied voltage of 0.45 V and a coulombic efﬁciency of 65%, an average of 2.4 molH2/mol-acetate was generated. Having proven electrohydrogenesis in an MEC was feasible, the next stage was to maximize H2 output. The focus of ensuing research projects has been to test various reactor conﬁgurations and select for improved materials of construction, i.e. electrodes and membranes, to optimize reactor performance. 5.1. Electrode selection Electrodes should be highly conductive, non-corrosive, possess a high speciﬁc surface area, should be non-fouling, inexpensive, easy to fabricate, and scalable (Logan, 2008). Additionally, bacteria must be able to attach to its surface to achieve good electrical connection. Carbon and graphite meet many of these properties. Not only are these materials suitable for bacterial growth, but their high conductivity and low cost have also made them excellent candidates for MEC systems. As a result, the majority of experiments have implemented carbon or graphite electrodes, particularly at the anode (Liu et al., 2005; Cheng and Logan, 2007; Rozendal et al., 2007; Call and Logan, 2008; Hu et al., 2008). Several studies focused on increasing the surface area of the anode in order to improve biomass development and concomitant current densities. Cheng and Logan (2007) ﬁlled the anode chamber of a two-chamber MEC with graphite granules, increasing total electrode surface area to 528 cm2 (assuming an average particle size of 4 mm). At an applied voltage of 0.6 V and coulombic efﬁciency of 88%, 3.5 mol-H2/mol-acetate was generated. Coulombic efﬁciencies further increased to 92% with the use of a carbon brush anode treated with ammonia (Call and Logan, 2008), with H2 yields and applied voltages similar to Cheng and Logan (2007). However, this system used a single-chambered design lacking the use of a membrane. For the ﬁrst time, the necessity of having a membrane to promote Hþ ﬂow through the electrolyte was put into question.
Table 1 Overview of various MEC architectures and system performances reported in literature. All studies used mixed consortia as inocula and acetate as the sole substrate. Overall H2 production ðYH2 ;th Þ and coulombic efﬁciency (CE), measured at speciﬁc applied voltages (Eap), were compared to reactor volume and electrode and membrane material and size. Reference
Liu et al. (2005) Bottle Liu et al. (2005) Cube Rozendal et al. (2006) Rozendal et al. (2007) Cheng and Logan (2007) Call and Logan (2008) Hu et al. (2008)
Applied voltage Eap (V)
Overall H2 yield YH2 ;th (%)
Coulombic efﬁciency CE (%)
Total liquid volume (L)
CEM (Naﬁon 117)
C-paper (0.5 mg Pt/cm2)
CEM (Naﬁon 117)
CEM (Naﬁon 117) AEM (Fumasep FAB)
452 (400)b 400
452 (400)b 400
Direct contact w/ CEM Direct contact w/membrane
7 Ti-mesh (5 mg Pt/cm2) MEA Ti-mesh (5 mg Pt/cm2) C-cloth (0.5 mg Pt/cm2) C-cloth (0.5 mg Pt/cm2) C-cloth (0.5 mg Pt/cm2)
Electrode spacing (cm)
YH2 ;th is calculated based on the number of moles H2 produced versus theoretical amount of H2 that can be extracted form substrate (i.e. 4 mol-H2/mol-acetate). CEM cation exchange membrane, AEM anion exchange membrane, C carbon, G graphite, Ti titanium, MEA membrane electrode assembly. n/a Indicates data was not available in the respective papers. a Referenced from Logan (2008). b Surface area of electrode submerged in electrolyte. c Referenced from Logan et al. (2008).
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5.2. Membrane selection The importance of a membrane is to regulate the ionic ﬂux between the anode and cathode. The concentration and chemical ﬂux of Hþ is a particularly important factor to control since low migration rates or inhibiting preferential conductance of other charged species could dramatically affect the pH balance of the overall system (Logan, 2008). The majority of studies use CEMs, or more speciﬁcally, proton exchange membranes (Naﬁon 117), which selectively permeate Hþ. Rozendal et al. (2007) studied H2 production using both a CEM and anion exchange membrane (AEM). A single-chambered conﬁguration was tested for the ﬁrst time by replacing the cathode chamber with a membrane electrode assembly (MEA). The cylindrical anode was constructed from graphite felt and was placed in direct contact with the MEA to limit Hþ diffusion through the electrolyte. At an applied voltage of 1.0 V, H2 yields and coulombic efﬁciencies were approximately the same despite membrane selection, producing 0.9 mol-H2/mol-acetate at a coulombic efﬁciency of 23%. As described previously by Call and Logan (2008), the use of a membrane is not necessary for the production of H2. This was again demonstrated in a single-chamber membrane-free MEC designed by Hu et al. (2008). Using a carbon cloth and carbon cloth loaded with platinum (0.5 mg Pt/cm2) for the anode and cathode respectively, an applied voltage of 0.6 V produced 2.5 mol-H2/molacetate at a coulombic efﬁciency of 75%. 5.3. Mixed consortia versus pure culture studies The vast majority of MEC studies have used mixed inocula enriched from sediment or wastewater MFCs, although pure studies have been tested using G. sulfurreducens only (Call et al., 2009). Results demonstrated similar H2 production rates and recovery yields for both conﬁgurations, which is in agreement with taxonomic proﬁling studies which suggest the majority of anode populations are enriched with microorganisms from the Geobacter sp. (see section 3 on electrogenic bacteria). Advantages of using mixed cultures within MECs include: i) increased versatility with respect to substrate utilization, ii) increased system robustness due to biological diversity, and iii) practicality e wastewater treatment, for example, will never be a pure process. However, one major disadvantage to using consortia is the undesired selection for methanogenic bacteria (Clauwaert and Verstraete, 2009; Wagner et al., 2009). Methanogens combine the CO2 produced by electricigens with the H2 generated via electrohydrogenesis to produce methane (CH4), ultimately lowering H2 yields. This problem can be solved with the insertion of a membrane to separate the anodic and cathodic reactions. Unfortunately, membranes increase the ohmic resistance within the cell and pH gradients will develop along the membrane surface, reducing system performance. 6. Concluding remarks The idea of exploiting microorganisms to catalyze reactions in an electrochemical cell is by no means novel. The use of electricigens to produce H2 from organic matter using electrolysis is. Since the ﬁrst published paper in 2005, electrohydrogenesis within MECs has received signiﬁcant attention. This is apparent with over 100 papers now published on the subject in only 5 years. The MEC has established itself as a promising technology for sustainable H2 production from renewable sources, providing the energy required to drive this process comes from a clean source (Logan et al., 2008). Rapid advancements leading to the commercialization of this technology are continually being made. Expensive cathode catalysts represent a major bottleneck during scale-up. However, studies have
shown that the replacement of platinum with low-cost stainless steel and nickel alloys can be made with no loss in performance (Selembo et al., 2009). Development of microbial biocathodes has also successfully catalyzed H2 production, although reported cathodic H2 recovery yields (21%) and maximum production rates (0.04 m3-H2 m3 reactor day1) have been too low for any practical application (Jeremiasse et al., 2010). Even the scale-up from bench experiments to pilot plant testing is now a reality at the Napa Wine Company in Oakville, CA, USA (Logan, 2010). Yet, despite recent progress, many issues must be addressed before electrohydrogenesis is considered a mature technology. High-current densities must be generated with minimal electrode overpotentials and ohmic losses before high H2 production rates can be achieved. To overcome these challenges, several strategies have been employed. Appropriate selection pressure has been applied to anodic bioﬁlms to enhance microbeeelectrode electron transfer. Enriching consortia in an MEC may select for organisms with the enhanced capacity to interact electrochemically with electrodes. This approach was implemented in MFCs where a variant of G. sulfurreducens was selected with improved current production abilities (Yi et al., 2009). Electrode overpotentials and ohmic losses have been reduced to achieve a positive energy balance. The energy produced by the generation of H2 must be greater than the energy applied to drive the reaction (i.e. the sum of all energy losses). Recently, electrode and ohmic energy losses were characterized in a single-chamber upﬂow MEC (Lee and Rittmann, 2010). It was determined that at a cathodic energy recovery of at least 80%, the applied voltage must be lower than 0.6 V to achieve an energy beneﬁt from an MEC. The lack of a membrane in this study signiﬁcantly reduced ohmic energy losses to 0.005 V. However, the decision to remove the membrane increases the risk of concomitant CH4 production and H2 consumption. Therefore, other optimization strategies have focused on membrane design and use to eliminate methanogenic competition and reduce energy losses associated with membrane pH gradients (Rozendal et al., 2007; Logan, 2008). The generation of H2 by electrohydrogenesis is a ﬁeld that is experiencing continued growth and development. Despite the number of challenges that must be overcome before MEC technology is considered an economically viable option, the world is beginning to recognize that sustainable H2 may one day be a reality. Clearly, the future for MEC technology looks very promising. Acknowledgments This work was supported by funds provided by the Natural Sciences and Engineering Research Council of Canada’s (NSERC) Hydrogen Canada (H2CAN) Strategic Network. References Aelterman, P., Rabaey, K., Schamphelaire, L.D., Clauwaert, P., Boon, N., Verstraete, W., 2008. Microbial fuel cells as an engineered ecosystem. In: Wall, J.D., Harwood, C.S., Demain, A.L. (Eds.), Bioenergy. ASM Press, Washington, D.C., pp. 307e322. Allen, R.M., Bennetto, H.P., 1993. Microbial fuel-cells: electricity production from carbohydrates. Appl. Biochem. Biotechnol. 39/40 (1), 27e40. Bagotsky, V.S., 2005. Fundamentals of Electrochemistry, vol. 2. John Wiley & Sons, Inc., New Jersey. Berk, R.S., Canﬁeld, J.H., 1964. Bioelectrochemical energy conversion. Appl. Microbiol. 12, 10e12. Bond, D.R., Holmes, D.E., Tender, L.M., Lovley, D.R., 2002. Electrode-reducing microorganisms that harvest energy from marine sediments. Science 295 (5554), 483e485. Bond, D.R., Lovley, D.R., 2003. Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69 (3), 1548e1555. Bond, D.R., Lovley, D.R., 2005. Evidence for involvement of an electron shuttle in electricity generation by Geothrix fermentans. Appl. Environ. Microbiol. 71 (4), 2186e2189.
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