Abstract:
A cathode for a fuel cell comprising a catalyst layer; a backing layer mounted to an aperture in a fuel chamber of said fuel cell; 1) wherein said catalyst layer is mounted to the backing layer on a face opposed to the aperture, so as to be in fluid communication with atmospheric oxygen in the case of microbial fuel cell; and 2) wherein said catalyst layer is mounted to the backing layer on a face opposed to the aperture, so as to be in fluid communication with water in the case of microbial electrolysis cell.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the manufacture and use of a fuel cell and having particular application to microbial fuel cells and microbial electrolysis cells. Specifically the invention relates to an arrangement and construction of a cathode for such a fuel cell. 
       BACKGROUND OF THE INVENTION 
       [0002]    Microbial fuel cells (MFCs) have recently been considered as an alternative and promising way for the direct conversion of the biochemical energy bound to wastewater into electricity without any intermediate step. 
         [0003]    A MFC is fundamentally an anaerobic process where bacteria grow in the absence of oxygen in a chamber containing an anode and form a biofilm that covers the anode. To generate electricity, bacteria in the anode chamber degrade organic matter (the influent/fuel) and transfer the electrons to the anode. These electrons pass through an external circuit producing current. The digestion of organic matters occurs in anaerobic environment so that the electrons are not consumed by electron acceptors such as oxygen. The transfer of these electrons occurs via bacterial respiratory enzymes. Protons, created at the anode to maintain a charge balance, migrate through the solution to a cathode where they combine with oxygen and the electrons produced at the anode to form water. Hence, the cathode has to be maintained under aerobic conditions. 
         [0004]    MFC can be of two types: 1) a two-chambered MFC, where the anode chamber is anaerobic and the cathode chamber is aerobic ( FIG. 1A ), or 2) a single-chambered MFC where both electrodes are placed in an anaerobic chamber, with one face of the cathode exposed to the air ( FIG. 1B ). A proton exchange membrane (PEM), aiming at facilitating the transfer of protons, usually separates the anode from the cathode. The potential difference between the respiratory enzyme and oxygen results in electricity generation. 
         [0005]    Microbial electrolysis cells (MECs) are, simply put, modified microbial fuel cells (MFCs) designed to produce hydrogen gas at the cathode. Like an MFC, the MEC is composed of one or two chambers containing an anode and a cathode. Bacteria on the anode catalyze the oxidation of organic substrates, producing electrons and protons. The electrons from the metabolic reactions travel through an external circuit towards the cathode. Protons are transferred to the cathode in the aqueous solution. At the cathode, the electrons and protons recombine to form hydrogen gas. To successfully produce hydrogen gas, an external voltage is applied to overcome thermodynamic barrier, as the direct production of hydrogen from the hydrolysis of organic compounds is not thermodynamically feasible. The hydrogen gas produced by MECs is of high purity compared to other biological hydrogen production processes, negates the use of expensive gas purification techniques. 
       SUMMARY OF THE INVENTION 
       [0006]    In a first aspect the invention provides a cathode for a microbial fuel cell comprising a backing layer mounted to an aperture in a fuel chamber of said microbial fuel cell; wherein a catalyst layer is mounted to the backing layer on a face opposed to the aperture, so as to be in fluid communication with atmospheric oxygen. 
         [0007]    In one embodiment, the cathode may include a piece of conductive material, such as but not limited to carbon paper, conductive plastic polymers, steel, graphite granules, graphite fibers, reticulated vitreous carbon, or any non-conductive material coated with a conductive paint, on which are applied the catalyst layer. 
         [0008]    In a second aspect, the invention provides a method for generating electricity, said method including the steps of: providing a microbial fuel cell (MFC) having a cathode, said cathode including a catalyst layer and a backing layer mounted to an aperture in a fuel chamber of said microbial fuel cell; said catalyst layer mounted to the backing layer on a face opposed to the aperture, so as to be in fluid communication with atmospheric oxygen; providing fuel to an anode of said MFC; generating electrons passing through said anode to an external circuit coupling the anode and cathode; communicating resulting protons to said cathode. 
         [0009]    In a third aspect, the invention provides a cathode for a microbial electrolysis cell comprising a catalyst layer, a backing layer mounted to an aperture in a fuel chamber of said microbial electrolysis cell; wherein said catalyst layer is mounted to the backing layer on a face opposed to the aperture, so as to be in fluid communication with either a gas or a water of minimal contaminants. 
         [0010]    In one embodiment, the current generated by the MFC may be a function of the activity of the micro-organisms contained in the microbial fuel cell. This may be a measure of the amount of organic matter available in the fuel and/or of the presence of toxic substances in the fuel. To this end a microbial fuel cell may be used as a biosensor to monitor the concentration of organics/toxicants in the fuel. 
         [0011]    An MFC according to one aspect of the invention provides a new type of cathode for single-chambered air-cathode MFC wherein the catalyst layer is applied to the side of the cathode exposed to the air on top of a backing layer. The catalyst layer can then be protected by one or more gas diffusion layers. By providing a catalyst layer that is facing the air, the arrangement may become more efficient through closer and better contact with the atmospheric oxygen, leading to better performance. Further, by separating the catalysts from the biomass/influent/fuel, the catalysts remain uncontaminated, and so may also increase performance and functional life of the MFC. 
         [0012]    An MEC according to a further aspect of the invention provides a cathode for double-chambered MEC wherein the catalyst layer is applied to the side of the cathode exposed to either a gas (i.e., absolute absence of oxygen) or water of minimal contaminants. By providing a catalyst layer that is facing either a gas (i.e., absolute absence of oxygen) or water having minimal contaminants, the separation of the catalysts from the biomass/influent/fuel, the catalysts remain uncontaminated, and may also increase performance and functional life of the MEC. 
         [0013]    The invention provides for an arrangement whereby the carbon cloth and/or backing layer isolates the catalyst layer from contamination by the fuel/biomass within the anode chamber of the microbial fuel cell. 
         [0014]    The backing layer provides support for the catalyst layer and allows the diffusion of protons to the catalyst. Such a backing layer may seal an aperture in the anode chamber of the MFC so as to prevent the flow of biomass, such as wastewater from the chamber. The backing layer may also provide mechanical support for the catalyst layer. It may be made of Nafion or polytetrafluoroethylene (PTFE) which may also be mixed with carbon black. 
         [0015]    An advantage provided by embodiments of the present invention may include higher electricity generation in the case of the MFC; and higher hydrogen production in the case of MEC. 
         [0016]    The influent/fuel may include (i) any biodegradable organics in a solution; (ii) biofuel (defined as a substrate derived from the biomass), (iii) wastewater (low strength, such as domestic wastewater or high strength, such as industrial wastewater), (iv) organic solid waste mixed with liquid (such as wastewater) to become liquid waste, and (v) pretreated forms of items (ii) to (iv). 
         [0017]    The present invention may also use any organics dissolved in solution as the influent for the purpose of 1) generating electricity in the case of the MFC, and (2) generating hydrogen in the case of MEC. 
         [0018]    For MFC, the extent of electricity thus generated may be used to monitor the presence and/or concentration of toxicants, and concentration of organic matter present in the said solution. 
         [0019]    Possible applications of the present invention related to MFC may include (i) wastewater treatment plants, (ii) power plants for electricity generation from any solutions that contain organics (intentionally or unintentionally produced), (iii) any locations where wastewater or waste are generated and required to be treated, (iv) as a biosensor to monitor the concentration of organics in solution, the current generated by the fuel cell being proportional to the concentration of organics, and (v) as a biosensor to monitor the presence and concentration of toxicants in solution, the current generated by the fuel cell being dependent on microbial activity and/or inhibition by toxicants. 
         [0020]    In the case of MFC, the gas diffusion layer (GDL) may provide pathways for reaction between atmospheric O 2  and water. This GDL may be made of polytetrafluoroethylene (PTFE) mixed with carbon black. Deposition methods may include brushing, air brushing, sputtering. Additional GDLs made of pure PTFE may be added on top of the first GDL. 
         [0021]    The catalyst layer is where the reaction takes place. The nature of the catalyst may include Cobalt, Platinum and alloys thereof. Deposition methods may include but are not limited to brushing, air brushing, sputtering. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]    It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention. 
           [0023]      FIG. 1A  is a schematic of a two chamber microbial fuel cell according to the prior art; 
           [0024]      FIG. 1B  is a schematic view of a single chamber microbial fuel cell having the catalyst facing the biomass within the chamber, according to the prior art; 
           [0025]      FIG. 2A  is a schematic view of a microbial fuel cell according to one embodiment of the present invention; 
           [0026]      FIG. 2B  is a schematic view of a microbial electrolysis cell according to one embodiment of the present invention; 
           [0027]      FIG. 3  is a graphical representation of the performance of a microbial fuel cell according to the present invention as compared to the prior art and; 
           [0028]      FIG. 4  is a graphical representation of the performance of a microbial fuel cell according to the present invention as compared to the prior art. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]      FIGS. 1A and 1B  show different microbial fuel cells (MFC) according to the prior art. 
         [0030]    With respect to  FIG. 1A , shown is a two chamber MFC  5  having an anode chamber  10  and cathode chamber  20 . The anode chamber  10  is characterized by being anaerobic so as to prevent released electrons being consumed by oxygen feeding into the chamber  20 . Within the chamber is a supply of biomass, such as wastewater, within the chamber  10 . The organic material within the wastewater then forms a biofilm  40  on the anode  15  which on degradation of the influent/fuel  45  produces electrons. The anode  15  is connected to an external circuit  35  through which the consequential current flows to the cathode  25  within the cathode chamber  20 . Thus, the fuel  45  supplied to the anode chamber  10  is oxidized  50  producing a flow of protons  55  through a proton exchange membrane (PEM)  30  whereby the protons combine with oxygen  57  within the cathode chamber  20  to form water  58 . 
         [0031]      FIG. 1B  shows an alternative arrangement of an MFC  60  whereby a single chamber  70  exists being the anode chamber  70 , again being anaerobic to prevent oxygen entering the chamber. The cathode chamber of  FIG. 1A  is replaced by an external cathode  65 , having a catalyst layer attached to the MFC, and so facing the biomass within the chamber  70 . The protons  75  combine at the site of the cathode  65  with atmospheric oxygen  80  to produce water  85 . Again a PEM  90  is provided to separate the cathode  65  from the anode chamber  70 . 
         [0032]    The present invention provides an alternative arrangement of the above single chamber MFC. As shown in  FIG. 2A , in one embodiment of the present invention, the cathode  110  includes a sheet of carbon cloth  115  situated at the wastewater/air interface  141  of a single-chambered MFC  100 . In the following example, the cathode working surface area equaled 64 cm 2 . In this case, the carbon cloth acts as a separation layer isolating the catalyst layer from the influent/fuel. Accordingly, the catalyst layer is directed in a direction away from the aperture. For clarity, this will be called an air-facing catalyst, as compared to an influent/fuel, or water, facing catalyst, as shown in  FIG. 1B . 
         [0033]    In a further aspect, the present invention provides a double chamber microbial electrolysis cell. One embodiment of such a microbial electrolysis cell is provided in  FIG. 2B , which shows a two chamber MEC  1000  having an anode chamber  1100  and cathode chamber  1200 . The anode chamber  1100  is characterized by being anaerobic. Within the chamber  1100  is a supply of biomass, such as wastewater. The organic material within the wastewater forms a biofilm  1300  on the anode  1400  which on degradation of the influent/fuel  1500  produces electrons. The anode  1400  is connected to an external circuit  1600  through which the consequential current flows to the cathode  1700 . An external voltage  1900  is applied to overcome thermodynamic barrier, as the direct production of hydrogen from the hydrolysis of organic compounds is not thermodynamically feasible. Thus, the fuel  1500  supplied to the anode chamber  1100  is oxidized  2000  producing a flow of protons  2100  towards the cathode  1700  to form hydrogen  2200  in the cathode chamber  1200 . The medium in the cathode chamber  1200  may be either a gas such as hydrogen or nitrogen in the absolute absence of oxygen or water with minimal contaminants, so as to maintain an uncontaminated catalyst. 
         [0034]    As shown in  FIG. 2B , in one embodiment of the present invention for MEC application, the cathode  1700  shall have a similar structure to that of  FIG. 2A . In this case, the carbon cloth acts as a separation layer isolating the catalyst layer from the influent/fuel. Accordingly, the catalyst layer is directed in a direction away from the aperture or anode chamber  1100  facing either an inert gas or a water of minimal contaminants in the cathode chamber  1200 . 
         [0035]    The fabrication of a cathode  110  or  1700  similar to that shown in  FIGS. 2A and 2B  may include the following successive steps: 
         [0036]    Teflonization by soaking carbon cloth  115  in 30 wt % PTFE suspension in water. The carbon cloth  115  is dried in oven at 100° C. and then weighed to determine the Teflon content  115  so as to achieve a target  115  in the range of 15-20 wt %. The teflonized carbon cloth  115  is then transferred to an oven and heated to 350° C. at 4.5° C./min to gradually remove the dispersant agent present in PTFE and to melt the PTFE particles at 350° C. for 30 minutes. 
         [0037]    Next is provided an application of a backing Layer (BL)  120  made of Nafion and carbon black (VXC72-R) and Nafion particles. The weight ratio of the carbon black to Nafion is 2:1. Carbon black is mixed with distilled water and ethanol (1:1 v/v). Nafion solution (5%) is added to the carbon black ink and stirred to make a homogeneous dispersion. The mixture is ultrasonicated for 30 min. The diffusion ink is applied to one side of the carbon cloth  115  using an air brush. The gas diffusion electrodes are baked in oven at 130° C. for 30 min. Typical loading of the diffusion ink is around 3.6 mg/cm 2 . 
         [0038]    Application of the catalyst layer  125  includes Cobalt being applied at a load of 0.1 mg cm 2  by sputter-deposition using a Denton Vacuum Discovery TM 18 Deposition System. 
         [0039]    In the case of an MFC, application of a GDL  130  made of PTFE on top of the catalyst layer  125 . The GDL  130  for the electrodes comprises carbon black (VXC72-R) and PTFE particles. The weight ratio of the carbon black to PTFE is 3:2. The carbon black is treated in an ultrasonicator in a mixture of distilled water and ethanol (1:2 vol. ratio) and the PTFE is stirred in distilled water. The PTFE solution is added to the carbon black ink and the mixture is ultrasonicated for 30 min. The diffusion ink is applied to one side of the carbon electrode using an air brush. Typical loading of the diffusion ink is around 3.6 mg/cm 2 . The gas diffusion electrode is dried in oven at 100° C. The gas diffusion electrode is sintered at 350° C. under the same conditions as the heat treatment of the Teflonized carbon electrodes. The sheet of carbon cloth is then ready to be incorporated in a single-chambered MFC for test. 
         [0040]    The MFC  100  used for tests and illustrated in  FIG. 2 , had dimensions 120×120×30 mm and the working volume of the MFC  100  was 85 mL. It was incorporated with channels that provided a serpentine pathway for the wastewater. The flow channels were sandwiched between a sheet of anode and a sheet of cathode and the distance between the anode and cathode was set to 2 cm. Anode and cathode surface areas were similar and each equaled 64 cm 2 . The anode was made of non-wet-proofed plain carbon cloth and was applied against a piece of acrylic to ensure anaerobic conditions. 
         [0041]    Inoculation was done using bacteria naturally present in domestic wastewater. This inoculation step was performed in a batch-mode, the MFC being refilled with fresh domestic wastewater when the voltage dropped below 50 mV. The inoculation step was considered achieved when the profiles of voltage generation showed similar pattern for at least three consecutive batches. The MFC  100  operation was then switched to continuous mode using sodium acetate as substrate. Sodium acetate was pumped continuously using a peristaltic pump at a flow rate of 0.3 mL min −1  into the bottom of the MFC and was allowed to flow through channels directly between an anode and a cathode in an upflow mode. The loading rate was 16 Kg acetate m −3  d −1  and the hydraulic retention time equaled 4.7 hrs. 
         [0042]    Comparison of performance obtained with the new type of cathode  145  and with a conventional cathode  150  built in the same way except that the catalyst layer faces the wastewater is displayed in  FIG. 3 . Cell potential is in average higher with the present invention by 40 mV (i.e. an increase of ≈15%) 
         [0043]    These improved results are confirmed by polarization curves obtained at different external resistances by using a resistance box and displayed in  FIG. 4  for the new type of cathode  115  and the conventional cathode  150 . 
         [0044]    The level of teflonization of the carbon cloth can be modified. 
         [0045]    The carbon cloth  115  can be replaced by any other type of conductive material, such as but not limited to carbon paper, conductive plastic polymers, steel, graphite granules, graphite fibers, reticulated vitreous carbon, or any non conductive material coated with a conductive paint. 
         [0046]    The nature of the BL and GDL may be changed to other materials (e.g. PTFE, Nafion, others). 
         [0047]    Additional GDLs can be added on top of the catalyst layer to increase cathode hydrophobicity and prevent leakage. Such GDLs can be made typically but not only of pure PTFE. 
         [0048]    The nature and the load of the catalyst on the cathode can be changed (e.g. Pt, Co, Alloys, others). 
         [0049]    In further embodiments of the present invention, the cathode  110  may omit the BL  120  and GDL  130  so as to have the catalyst layer  125  applied directly to the carbon cloth  115 . A key feature of the present invention is ensuring the catalyst layer is not contaminated by the waste water within the anode chamber and hence, the separation by the BL  120 . The omission of a PEM allows higher power output. This is due to reduced internal resistance of the MFC and higher cathode potential. The omission of a PEM adds further advantage through the omission of a construction step and so saving materials and costs. 
         [0050]    The embodiment shown in  FIG. 2  has further advantage in replacing the platinum catalyst with a cobalt alloy which combined with the less complex construction of the MFC  100  which may provide significant savings in construction costs at little or no sacrifice as compared to more complex and expensive construction. Further, advantages of such an air facing cathode compared to having the catalyst layer facing the waste water, is demonstrated by the performance shown in  FIGS. 3 and 4 .