Abstract:
A microbial electrolysis cell having a brush anode is described. A method of producing products, such as hydrogen, at the cathode of the microbial electrolysis cell is also provided. The microbial electrolysis cell is configured in a cylindrical shape having an anode, cathode and anion exchange membrane all disposed concentrically. A brush anode spirally wound around the outside of the cylindrical microbial electrolysis cell is described. The method may include sparging the anode and/or cathode with air in some cases. In addition, CO 2 -containing gas may be injected into a cathode chamber to reduce pH is some cases.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/616,893, filed Mar. 28, 2012, entitled “MICROBIAL ELECTROLYSIS CELLS AND METHODS FOR THE PRODUCTION OF CHEMICAL PRODUCTS”, which is incorporated herein by reference in its entirety. 
     
    
     FIELD 
       [0002]    This disclosure relates to the field of Microbial Electrolysis Cells (MEGs), including MECs that may be utilized to produce useful chemical products, such as hydrogen gas or caustic soda. 
       BACKGROUND 
       [0003]    Wastewater from industrial and domestic sources typically contains dissolved organics that need to be removed before the water can be reused. Traditionally, this has been done by aerobic biological treatment. However, this treatment method requires aeration, which consumes large amounts of energy and thus it is an energy intensive process. 
         [0004]    Although wastewater is typically thought of as a nuisance, it is being increasingly recognized as a resource for the production of energy, fuels, and chemicals. While anaerobic digestion has already developed into a mature technology for conversion of wastewater organics to the energetic gas methane (CH 4 ), not all of the energy extracted from wastewater is available for use, as there are significant losses associated with the conversion of methane to easily usable energy forms such as electricity. 
         [0005]    Recently, a new technology referred to as Microbial Electrolysis Cells (MEC) has gained significant attention with respect to sustainable wastewater treatment. This technology relies on specialized bacteria called anode-respiring bacteria (ARB) that oxidize wastewater organics and transfer electrons thus extracted to an anode. These electrons move through a circuit to a cathode, where water (H 2 O) is reduced to produce hydrogen (H 2 ) gas by applying additional voltage. The MEC is especially attractive since the produced H 2  gas has higher energetic value than CH 4  gas, can be readily converted to useful electrical power using chemical fuel cells, and is a major feedstock to the chemical and petrochemical industries. Other chemical products may be produced, such as caustic soda (NaOH), which is useful in the manufacture of pulp and paper, textiles, soaps and detergents. 
       SUMMARY 
       [0006]    A general schematic illustration of an MEC is shown in  FIG. 1 . The MEC  100  includes an anode chamber  112  and a cathode chamber  114  that are separated by a membrane  106 . An anode  102  is disposed in the anode chamber  112 . The anode  102  includes anode-respiring bacteria  108  disposed on the surface of the anode  102 . A cathode  104  is disposed in the cathode chamber  114 . In operation, organics dissolved within an anode solution  118  (e.g., wastewater) are oxidized by the anode-respiring bacteria  108 . The oxidized organics release electrons which are transported to the anode  102 . Concurrently, H 2 O in the cathode solution  120  is reduced to H 2  at the cathode  104 , and hydroxyl ions (OH − ) are transported through the membrane  106  to the anode  102 . The reactions may be driven by a power source  110  (e.g., a DC voltage power source) that is operatively connected (e.g., electrically) to the anode  102  and the cathode  104 . 
         [0007]    Practical application of MECs hinges on the ability to achieve high current densities within the MEC at low applied voltages, so that high rates of wastewater treatment and H 2  production can be obtained with reduced MEC energy input. Theoretically, a potential of about 0.14 V must be applied to the MEC to produce H 2 ; however, practical applied voltages in laboratory systems are greater, since losses of potential occur in MECs due to activation losses at both the anode and the cathode, and Ohmic losses occur that depend upon the nature of the electrolyte used to separate the electrodes, as well as on the separation distance between the electrodes. 
         [0008]    ARB produce maximum current densities in the range of 10 A/m 2  (amps per square meter of the anode), with anode-potential losses of 0.1 to 0.2 V. This loss of potential is the energy that ARB derive for their growth, and thus is unavoidable. For the cathodic H 2 -evolution reaction, various metal catalysts can be used to decrease losses of potential, and the activation loss for the reaction depends on the nature of the metal catalyst. 
         [0009]    The conductivity of the anode and cathode solutions and the nature of the membrane used to separate the two electrodes govern the Ohmic losses, which scale linearly with the separation distance between the anode and the cathode. Wastewaters a re typically of poor ionic conductivity; thus, it is important to place the anode and the cathode very close to each other to minimize Ohmic losses. Anion exchange membranes (AEM) may be a better alternative to cation exchange membranes (OEM) for separating the anode and the cathode, because of their low resistance, e.g., to ion transport. In principle, the membrane could be entirely excluded, but removing the membrane requires increasing the distance between the anode and the cathode to avoid H 2  short-circuiting, thus increasing Ohmic losses. 
         [0010]    Thus, in one embodiment, a microbial electrolysis cell is provided. The microbial electrolysis cell includes a high surface-area brush anode, a cathode, a power source operatively connected to the brush anode and the cathode, an anion exchange membrane separating the brush anode from the cathode and a reaction chamber containing the brush anode, the cathode and the anion exchange membrane. The anion exchange membrane operatively separates the reaction chamber into an anode chamber containing the brush anode and a cathode chamber containing the cathode. The brush anode and the cathode are separated by a separation distance that is not greater than about 1.5 cm. 
         [0011]    According to one configuration, the brush anode comprises a plurality of brush fibers emanating from a conductive wire disposed approximately through the center of the anode, where the brush fibers comprise carbon fibers and the conductive wire comprises stainless steel. In another configuration, the cathode comprises a metal foam. For example, the metal foam may be characterized as having a thickness of not greater than about 5 mm. The cathode may also be characterized as comprising nickel metal foam. 
         [0012]    In another configuration, the separation distance between the brush anode and the cathode is not greater than about 1.25 cm. In another characterization, the brush anode has a thickness of not greater than about 2.0 cm. In yet another characterization, the brush anode has a length, where the brush anode is in direct physical contact with the anion exchange membrane along at least a portion of its length. In another characterization, the cathode is in direct physical contact with the anion exchange membrane. 
         [0013]    In one configuration, the brush anode, the cathode and the anion exchange membrane comprise substantially cylindrical bodies that are disposed in a substantially concentric relationship in a cylindrical electrolysis cell. In one characterization, the anode is disposed on the outside of the cylindrical electrolysis cell. In another characterization, the anode comprises a brush anode that is wound into a cylindrical body. In another characterization, the anode having a brush comprising a plurality of brush fibers emanating from a conductive wire, wherein the brush anode is spirally wound into a cylindrical body. 
         [0014]    In another embodiment, a multi-module microbial electrolysis cell treatment apparatus is provided. The apparatus includes a treatment tank having an inlet for influent and an outlet for effluent, and a plurality of cylindrical microbial electrolysis cells operatively disposed within the treatment tank. The microbial electrolysis cells comprise a high surface-area brush anode, a cathode, a power source operatively connected to the brush anode and the cathode and an anion exchange membrane separating the brush anode from the cathode. Advantageously, the brush anode, the cathode and the anion exchange membrane comprise substantially cylindrical bodies that are disposed in a substantially concentric relationship to form the cylindrical microbial electrolysis cells, where the anion exchange membrane operatively separates the influent from the effluent in the treatment tank. 
         [0015]    In one characterization of the apparatus, the brush anodes are disposed on the outside of the cylindrical electrolysis cells. 
         [0016]    In yet another embodiment, a method for the production of H 2  from a fluid stream comprising organic matter in a microbial electrolysis cell is provided. The method comprises the steps of providing at least one microbial electrolysis cell, the cell comprising an anode disposed in an anode chamber, a cathode disposed in a cathode chamber, a power source operatively connected to the anode and the cathode, and an anion exchange membrane separating the anode and the cathode. The fluid stream is contacted with anode-respiring bacteria disposed on the anode, and the cathode is contacted with a cathode solution comprising H 2 O. A gaseous composition comprising at least 98% H 2  may be removed from the cathode chamber. 
         [0017]    In one characterization, the fluid stream comprises a sodium salt. In another characterization, caustic soda is extracted from the cathode chamber. In another characterization, the method includes periodically sparging the anode chamber with air, and/or periodically sparging the cathode chamber with air. The method may also include the step of injecting a CO 2 -containing gas into the cathode chamber to reduce the pH of the cathode solution. 
         [0018]    In another characterization, the at least one microbial electrolysis cell comprises a cylindrical microbial electrolysis cell, wherein the anode, the cathode and the anion exchange membrane comprise substantially cylindrical bodies that are disposed in a substantially concentric relationship to form the cylindrical electrolysis cell, and wherein the anion exchange membranes operatively separate the anode chamber from the cathode chamber. In yet another characterization, the brush anodes are disposed on the outside of the cylindrical electrolysis cell whereby the anion exchange membranes operatively separates a single anode chamber from a plurality of cathode chambers configured within the plurality of cylindrical electrolysis cells. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  illustrates a schematic representation of a microbial electrolysis cell (MEC). 
           [0020]      FIG. 2  illustrates a schematic representation of a MEC. 
           [0021]      FIG. 3  illustrates a schematic view of an anode, a cathode and a membrane separating the anode and cathode. 
           [0022]      FIG. 4  illustrates a schematic view of an anode, a cathode and a membrane separating the anode and cathode. 
           [0023]      FIG. 5  illustrates a side (e.g. front) view of brush anode. 
           [0024]      FIG. 6  illustrates a top view of and a perspective view of a MEC. 
           [0025]      FIG. 7  illustrates the comparative Ohmic losses of several different anion exchange membranes used in a MEC. 
           [0026]      FIG. 8  illustrates a top view of a multi-module MEC tank. 
       
    
    
     DESCRIPTION 
       [0027]    Disclosed herein are designs for a MEC that may achieve high current densities at relatively low applied voltages. The MEC may produce, at the cathode, a high purity H 2  gas stream (e.g., greater than 98% H 2 ) that can be directly used in energy-conversion applications or as a chemical precursor. Other useful chemical products may also be produced, such as caustic soda (NaOH). 
         [0028]    Microbial electrolysis cell (MEC), as used herein, utilizes electrons produced at the anode for the purpose of generating chemical products at the cathode and not as a significant power source external to the MEC, such as is the case with a microbial fuel cell. 
         [0029]      FIG. 2  illustrates a schematic representation of a MEC. The MEC includes a reaction chamber  226  that is operatively separated into an anode chamber  212  and a cathode chamber  214  by a membrane  206 . The anode chamber  212  includes an anode  202  (e.g., a brush anode) disposed therein and the cathode chamber  214  includes a cathode  204  disposed therein. The anode chamber  212  and cathode chamber  214 , and hence the anode  202  and the cathode  204 , are separated by a membrane  206  (e.g., an anion exchange membrane). In particular, the membrane  206  may be of such a nature and may be configured within the reaction chamber  226  such that reaction products from chambers  212  and  214  cannot intermix. 
         [0030]    The anode  202  may include anode-respiring bacteria  208  (e.g.,  G. sulfurreducens ) disposed thereon. The anode  202  may be a three-dimensional, high surface area anode and in one embodiment is a brush anode. For example, the anode  202  may be a carbon brush anode. The anode  202  may also include a conductive wire  216  (e.g., a titanium or stainless steel conductor wire) to facilitate connection of the anode  202  to the power source  210  for the transport of electrons. 
         [0031]    The cathode  204  may be a mesh cathode (e.g., a stainless steel mesh cathode) or the cathode  204  may comprise a metal foam (e.g., a nickel metal foam). The cathode may advantageously be very thin, such as having a thickness of not greater than about 5 mm, such as not greater than about 2 mm, and even not greater than about 1 mm. 
         [0032]    As illustrated in  FIG. 2 , the electrolysis cell  200  comprises an anode  202 , a cathode  204 , and a membrane  206  that comprise substantially planar bodies that are disposed in a substantially co-planar relationship. During operation of the cell, an anode solution  218  may be disposed within the anode chamber  212  and a cathode solution  220  may be disposed within the cathode chamber  214 . For example, the anode solution  218  may include organic matter (e.g., dissolved organic matter), such as a waste water stream (e.g., a standard domestic waste water). In this regard, the organics may be oxidized at the anode  202  by the anode-respiring bacteria  208 , and an oxidized carbon gas species (e.g., CO 2 ) may be removed from the anode chamber  212  at an anode chamber outlet  222 . Concurrently, a cathode solution  220  may be disposed within the cathode chamber  214 , where the cathode solution is comprised mainly of H 2 O. Thus, the H 2 O may be reduced at the cathode  204  and a gaseous composition comprising H 2  may be withdrawn from the cathode liquid chamber  214  at the cathode chamber outlet  224 . 
         [0033]      FIGS. 3 and 4  illustrate schematic side views of a brush anode separated from a cathode by a membrane. Referring to  FIG. 3 , the brush anode  302  comprises a plurality of carbon brush fibers  317  emanating from a conductive wire  316  disposed approximately through the center of the anode  302  and substantially parallel to the surfaces of a membrane  306  and a cathode  304 . The brush anode may advantageously have a length (L) of at least about 5 cm such as at least about 10 cm. 
         [0034]    Referring now to  FIG. 4 , a portion of a brush anode  402 , a membrane  406 , and a cathode  404  are illustrated. The brush anode includes carbon brush fibers  417  and anode-respiring bacteria  408  disposed on the carbon brush fibers  417  throughout the thickness (T) of the brush anode  402 . The thickness of the brush anode (e.g., the diameter) may be relatively small, to reduce Ohmic losses in the MEC. For example, the brush anode  402  may have a thickness of not greater than about 2.0 cm, such as not greater than about 1.5 cm, such as not greater than about 1.0 cm and even not greater than about 0.75 cm. 
         [0035]    Further, the separation distance (G) between the tips of the carbon brush fibers  417  (e.g., the tips adjacent the membrane  406 ) and the surface of the cathode  404  (e.g., the surface adjacent to the membrane  406 ) may be significantly reduced. In one characterization, the separation distance (G) is not greater than about 1.25 cm, such as not greater than about 1.0 cm, not greater than about 0.9 cm, not greater than about 0.8 cm, not greater than about 0.7 cm, not greater than about 0.6 cm, not greater than about 0.5 cm, not greater than about 0.4 cm, and even not greater than about 0.3 cm. In a further characterization, the anode  402 , the cathode  404 , or both may be in physical contact with the membrane  406  along at least a portion of the length thereof. In this manner, the thickness of the membrane  406  will be the separation distance (G) between the anode and cathode. In one aspect, the thickness of the membrane is not greater than about 5 mm, such as not greater than about 1 mm. 
         [0036]      FIGS. 5 and 6  illustrate various facets of a MEC design according to another embodiment. The MEC illustrated in  FIGS. 5 and 6  may advantageously reduce Ohmic losses, may produce high purity H 2  (e.g., &gt;99% pure H 2 ), and/or may have high volumetric current densities and H 2  production rates. This MEC advantageously utilizes cylindrical bodies (e.g., cylindrical bodies of the anode, cathode and membrane) that are disposed in substantially concentric relationship to form a cylindrical electrolysis cell and increase the efficiencies of the MEC. 
         [0037]    In the MEC  600  illustrated in  FIG. 6 , the brush anode  602  may be wound around (e.g., onto) the cylindrical assembly of a membrane  606  and a cathode  602  along its length. The length of the cylindrical assembly  600  may be selected as per the requirements for treatment and H 2 -production performance. 
         [0038]    The AEM  606  is rolled over a cylindrical stainless steel mesh that acts as the cathode  604 . A 316-grade stainless steel mesh may be used for the cathode  604  in the present MEC design, but any grade stainless steel may be used as long as it has appreciable nickel content. Carbon cloths coated with nickel or other metal catalyst powders can also be used in the MEC  600 . 
         [0039]    Other materials that are comparable in cost to stainless steel, may be used for the cathode  604 . For example, a nickel mesh or a metal foam (e.g., a nickel metal foam) may be used in the form of hollow cylinders. This nickel mesh or metal foam should be relatively thin to minimize Ohmic losses, and may advantageously have a thickness of not greater than about 5 mm, such as not greater than about 2 mm, or even not greater than about 1 mm. 
         [0040]    As is discussed with respect to  FIGS. 3 and 4 , the MEC  600  may utilize a relatively short separation distance between the anode  602  and the cathode  604  to reduce Ohmic losses. As used herein, the separation distance is measured from outer surface to outer surface, e.g., from the tips of the anode brush to the surface of the cathode mesh. See  FIG. 4 . The separation distance may be not greater than about 1.5 cm, such as not greater than about 1.25 cm, or even not greater than about 1.0 cm. In one characterization, the separation distance is approximately 0.5 cm. For a 100-mM PBS (phosphate buffer solution), which has conductivity of 14 mS/cm, a distance of 3 results in an Ohmic loss of 0.42 V at 20 A/m , while the Ohmic loss in the disclosed 
         [0041]    MEC configuration having 0.5 cm separation distance may be reduced to as low as 0.07 V. 
         [0042]    As discussed above, an AEM may advantageously be used in the MEC design to separate the anode and the cathode. When a membrane (e.g., an AEM) is used to separate the anode and the cathode, a pH gradient develops and results in additional losses of potential. While a pH gradient exists when using an AEM, the cathode pH is lower and additional methods to mitigate the cathode pH (e.g., CO 2  addition to the cathode) may also be utilized. 
         [0043]    An AEM sold under the trade name AMI-7001 (Membranes International, Ringwood N.J., USA) is believed to be the most widely used AEM in MECs. AMI-7001 is a strong base anion exchange membrane utilizing quaternary ammonium as a functional group. However, is has been discovered that this membrane introduces too much loss in an MEC due to its resistance to ion flow. Five AEMs are evaluated for the present MEC design. The resistance that each provides to ion flow in 100-mM phosphate buffer solution (PBS) is illustrated in  FIG. 7 . As it gives the best performance, the A201 membrane (Tokuyama Corp., Tokyo, JP) may advantageously be utilized in the disclosed MEC design. This membrane is a hydrocarbon polymer membrane containing quaternary ammonium moieties. 
         [0044]    The MECs disclosed herein may provide high operating efficiencies when used to treat fluid streams containing organic matter (e.g., wastewater fluid streams) and produce a gas composition comprising H 2  gas. More specifically, relatively low applied voltages may be utilized to provide relatively high current densities in the cell. In one aspect, the applied voltage (i.e., across the anode and the cathode) is not greater than about 1.2 V, and the resulting volumetric current density is at least about 500 A/m 3 , such as at least about 600 A/m 3  or even at least about 750 A/m 3 . 
         [0045]    The present MEC designs may provide other important advantages. One is the ability to collect pure H 2  at the cathode. In one aspect, the gas composition withdrawn from the cathode chamber comprises at least 95% H 2 , such as at least 98% H 2 , at least 98.5% H 2 , at least 99% H 2 , at least 99.5% H 2  or even at least 99.9% H 2 . A second advantage is that the membrane creates a significant resistance to diffusion of H 2  across to the anode, and this reduces H 2  loss to biological activity at the anode. Scaled-up MECs not containing a membrane have shown poor H 2  recovery caused by extensive CH 4  formation as a result of H 2  consumption by methanogens. The MEC designs disclosed herein may substantially preclude this problem. In one aspect, the gas composition withdrawn from the cathode chamber comprises substantially no CH 4  (e.g., not greater than 0.1% CH 4 ). Methane production, however, may still occur at the anode, from acetate (Ac − ) and H 2  produced from fermentation reactions, and this diverts electrons away from electrical current, and ultimately decreases H 2  production. Thus, periodical sparging of the anode chamber with air or other O 2 -containing gas may be implemented, for operation at large scale. This curbs the growth of methanogens, as they are severely inhibited by O 2 , while dominant ARB such as those belonging to the  Geobacter  genus are reported to tolerate O 2  to certain extent. In the unlikely event of methanogenesis at the cathode, a similar strategy could also be applied to the cathode to limit methane formation. 
         [0046]    While a single MEC module is described above, a multi-module apparatus may be assembled to achieve wastewater treatment at flows typically generated by industries or municipalities. Scaling may be achieved by using multiple modules.  FIG. 8  illustrates a design of a multi-module MEC treatment tank  850  that is capable of a high volumetric treatment rate. The tank  850  may consist of several modules  800  of the anode, membrane and cathode configuration described above (e.g.,  FIG. 6 ). The height and diameter of the individual modules  800 , and the distance between any two modules can be selected as per the requirement for treatment performance, as long as within a single module  800  the effective distances between the anode and the cathode are maintained as is disclosed above. 
         [0047]    In addition to high purity hydrogen, caustic soda (NaOH) may be produced in the current MEC designs. The present MEC designs will allow the production of caustic soda at high rates, but with lower applied voltages. Caustic soda is produced in MECs via the formation of OH −  at the cathode from the water reduction reaction, and the transfer of Na +  from the anode to the cathode through a cation exchange membrane. Although an AEM is typically used in the MECs disclosed herein, caustic soda can still be produced if NaCl is added to the cathode solution. In this case, the Cl −  ions move from the cathode to the anode, leaving behind Na +  that combine with OH − . The MECs disclosed herein reduces Ohmic losses, thus allowing for faster ion movement and thus higher rates of caustic production at a given applied voltage. 
         [0048]    In summary, the MECs disclosed herein and the methods disclosed herein advantageously enable a scalable technology that may be optimized for the low-energy treatment of organic materials such as in wastewaters. The wastewaters may be domestic wastewaters or may be industrial wastewaters such as those that are common to the food and beverage processing industries. For example, the wastewater may be an industrial wastewater from a potato treatment plant (e.g., in the manufacture of potato snacks), breweries, wineries, confectioneries, dairies, fruit processing plants, frozen dinner product plants, soy product plants, grain processing plants and pulp and paper manufacturing plants. The methods may use about 70% less electricity and produce about 80% less solid sludge as compared to aeration methods for the treatment of wastewater, while delivering identical or improved treatment performance. Hydrogen generated by the method may be used as a versatile commodity chemical or as a carbon-free energy source. Caustic soda as a byproduct may also be used in many food and beverage production plants. The MEC is advantageously low-maintenance, energy efficient and may be assembled without the use of exotic materials. The MECs may advantageously provide a high-surface area for bacterial growth, translating into efficient contaminant breakdown in the wastewater stream. The MECs are easily scaled to treat high volumes of wastewater. 
         [0049]    While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.