Abstract:
A fuel cell system running on direct neat methanol. Back diffusion of water from the cathode to the anode is sufficiently high so that water is not accumulated at the cathode, thereby leading to fuel cell systems without the need for a pump system to remove circulate water from the cathode to the anode. Other embodiments are described and claimed.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/879,257, filed 8 Jan. 2007. 
     
    
     GOVERNMENT INTEREST 
       [0002]    The claimed invention was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain. 
     
    
     FIELD 
       [0003]    The present invention relates to fuel cells. 
       BACKGROUND 
       [0004]      FIG. 1  illustrates a high-level system diagram of a prior art direct methanol fuel cell operating on an aqueous feed of methanol. Neat (100%) methanol is stored in container  102  and then diluted with water to a concentration of 2-3% before it is introduced into fuel cell stack  104 . The methanol fuel solution is re-circulated, indicated by the loop comprising flow line  106 , gas and liquid separator  108 , radiator  110  and bypass  112 , pump  114 , mixer  116 , methanol sensor  118 , and cold-start heater  120 . Methanol is added to the solution by way of pump  114  and mixer  116  as needed to maintain the required concentration delivered to anode  122 . The fuel solution entering anode  122  is accurately monitored and controlled using methanol sensor  118 . 
         [0005]    The water used for this dilution is gathered at cathode  124 , flows through flow line  126  to sump tank  128 , and is pumped by sump pump  130  to gas and liquid separator  108 . Carbon dioxide is generated at and removed from anode  122 , as indicated by flow line  132 . 
         [0006]    Diluting methanol, collecting and circulating water, circulating fuel, and controlling concentration entail the use of several pumps and control systems, with their resulting use of electrical energy, and add to the size and cost of the fuel cell system. These auxiliary components may constitute about 50% of the overall volume and mass of present state-of-art direct methanol fuel cell systems, and may contribute to at least 50% of the parasitic energy consumption. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a prior art fuel cell system. 
           [0008]      FIG. 2  illustrates a fuel cell according to an embodiment. 
           [0009]      FIG. 3  illustrates a fuel cell system according to an embodiment. 
           [0010]      FIG. 4  illustrates dependence of current density upon membrane thickness for an embodiment. 
           [0011]      FIG. 5  illustrates an anode electrode according to an embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0012]    In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments. 
         [0013]    These letters patent teach the design of a direct methanol fuel cell in which back diffusion of water from the cathode to the anode in a methanol fuel stack is high enough so that water need not be collected at the cathode, and neat methanol may be provided to the anode without the need for dilution. By not requiring the gathering of water at the cathode, or the dilution of neat methanol, it is expected that embodiments may have reduced parasitic energy losses, higher power density, and higher reliability at reduced cost, compared to the prior art direct methanol fuel cell system of  FIG. 1 . 
         [0014]      FIG. 2  illustrates an embodiment fuel cell  200 , and some of the processes involved. During fuel cell operation neat methanol (CH 3 OH) is oxidized at anode  202  to protons and carbon dioxide, and oxygen (O 2 ) is reduced to water at cathode  204 . As protons migrate from anode  202  to cathode  204  by way of membrane  206 , water is transported with the protons (H + ), as indicated by arrow  208 . This process is termed electro-osmotic drag, in which water molecules associated with the protons are dragged across membrane  206  in the direction of ionic movement. 
         [0015]    Water is consumed at anode  202  by the oxidation of methanol, and water is produced at cathode  204  by reduction of oxygen. Air flowing across cathode  204  evaporates some of this water. The remaining water generated at cathode  204 , as well as the water arriving at cathode  204  due to electro-osmotic drag, is brought to anode  202  by to back diffusion, as indicated by arrows  208  and  210 . 
         [0016]    It is useful to understand the factors that govern the rates of various water transport processes. The rate of consumption of water at anode  202  is determined by the current density, and for some embodiments, one mole of water is consumed for every 6 Faradays of electricity. The rate of migration of water from anode  202  to cathode  204  by electro-osmotic drag is determined by the current density and the drag coefficient. The some embodiments, the drag coefficient is about 3 molecules of water per proton. The rate of production of water at cathode  204  by reduction is also determined by the current density, and for some embodiments is given by 0.5 mole of water for every Faraday of electricity. 
         [0017]    The flow rate of air over cathode  204 , the temperature of fuel cell  200 , and the absolute humidity of air at the specified temperature determine the rate of evaporative loss. The concentration gradient of water between anode  202  and cathode  204 , and the diffusion coefficient of water in the membrane-electrode composite ( 206 ) determine the rate of back-diffusion. Accordingly, the rate of back diffusion may be increased by increasing the concentration gradient for water, and this process may be relatively independent of the current density. 
         [0018]    Thus, the rate of back diffusion to move water from cathode  204  to anode  202  that has not been removed due to evaporation may be achieved by appropriately choosing the concentration of methanol, the porosity of the electrodes in anode  202  and cathode  204 , the thickness of membrane  206 , the operating temperature of fuel cell  200 , and the stoichiometric rate of airflow over cathode  204 . For any given set of conditions for the foregoing variables, there will be a current density at which water balance will be achieved. Water balance means that the back diffusion rate and evaporative rate at cathode  204  are such that water does not accumulate at cathode  204 . 
         [0019]    Calculations have shown that when about 1 molar (3%) methanol solutions are circulated past anode  202 , the rate of back diffusion is inadequate to transport the water for a practical value of current density, and the current density at which water balance is achieved is quite low to be of practical value for most applications. However, when neat (100%) methanol is used, the back diffusion process alone is expected to achieve the water balance at a current density of 170 mA/cm 2  (mill-Ampere per square cm). This latter value of current density is expected to be in the range useful for practical applications. 
         [0020]    More specifically, calculations show that for a membrane thickness of 0.0635 cm, a stoic rate for air flow over cathode  204  of 3.0, a drag coefficient of 3.0 water molecules for each proton (H + ) transported by electro-osmotic drag, a temperature of 45° C., and a diffusion coefficient for water of 8.16*10 −6  cm 2 /sec; water balance is achieved for a methanol concentration of 3% at a current density of only 7 mA/cm 2 , but water balance is achieved using neat methanol according to an embodiment at a useful current density of 170 mA/cm 2 . 
         [0021]    Thus, the above description teaches that by using neat methanol and designing the fuel cell such that water is returned to the anode by back diffusion from the cathode, a practical current density may be achieved, and mechanical means for collecting and returning water may be avoided.  FIG. 3  illustrates an embodiment, where neat methanol stored in container  302  is pumped by pump  304  to delivery system  306 . Delivery system  306  provides neat methanol directly to anode  202 , and fuel cell  300  is designed so that the back diffusion of water is sufficient so that water need not be removed at cathode  204 , other than by evaporation. An embodiment for delivery system  306  is described later. 
         [0022]    For some embodiments, membrane  206  may comprise Nafion. Nafion is a sulfonated tetrafluorethylene copolymer, and is a registered trademark of E. I. Du Pont de Nemours and Company, a corporation of Delaware. For such embodiments, membrane  206  is hydrophilic, and may for some embodiments allow water retention of up to about 40% of the membrane mass. Also, for some embodiments, water and carbon dioxide is produced by direct reaction of methanol with oxygen at anode  306  or cathode  204 , so that an adequate supply of water may be produced. 
         [0023]    If anode  202 , cathode  204 , or both are not capable of sustaining desired current densities due to slow catalysis or mass transport of reactants, then the electrode structures for anode  202  or cathode  204 , and the thickness of membrane  206 , may be adjusted so that an acceptable current density value may be achieved. An example is illustrated by  FIG. 4 . 
         [0024]      FIG. 4  illustrates a functional relationship between fuel cell current density (mA/cm 2 ) and membrane thickness (cm) for the following fuel cell parameters: a stoic air flow rate of 3.0; a drag coefficient of 3.0 water molecules per proton; and a diffusion coefficient for water of 8.16*10 −6  cm 2 /sec. As noted in  FIG. 4 , reducing the membrane thickness may lead to an increase in current density. For example, whereas a membrane thickness of 0.2 cm provides for a current density of about 46 mA/cm 2 , reducing the membrane thickness to 0.06 cm provides for a current density of about 170 mA/cm 2 . 
         [0025]    The presence of an uncontrolled excess of neat methanol at anode  202  may result in swelling of membrane  206 , which may lead to permanent damage of the membrane-electrode assembly. Accordingly, for some embodiments, the delivery rate of methanol to anode  202  should be such that only a relatively small quantity of methanol reaches the anode electrode. Furthermore, for some embodiments, the entire quantity of neat methanol that is delivered to the anode electrode should be utilized within the electrode structure, but the neat methanol should not reach the surface of membrane  206  in any significant quantity. 
         [0026]    Full utilization of neat methanol at the anode electrode may be achieved if the electrode structure is modified to be porous and thick, so that the residence time for methanol is adequate for complete consumption in the body of the electrode structure. For some embodiments, such a porous electrode should have enough ionomer material to form conducting paths for the protons and water, but have the enough tortousity to assure a high residence time. 
         [0027]    An ionomer is a polyelectrolyte comprising copolymers. For some embodiments, the electrode structure may have layers of varying ionomer content so that the desired level of utilization may be achieved. The thickness, layer design, and porosity of the electrodes may depend on the delivery rate.  FIG. 5  illustrates in a simplified pictorial form anode electrode  502  and membrane  503 . Anode electrode  502  comprises carbonaceous substrate  504  and electrocatalyst layer  506 . Ionomer material is impregnated into carbonaceous substrate  504  to form varying layers of ionomer material  508 ,  510 ,  512 , and  514 . 
         [0028]    For some embodiments, the optimization of the electrode structure should be done in conjunction with the delivery method for the neat methanol. For some embodiments, delivery system  306  for delivering the neat methanol to anode  202  may be an aerosol delivery system as described in U.S. Pat. No. 6,440,594. As another example, for some embodiments, delivery system  306  may include a diffusion barrier of sufficient thickness. 
         [0029]    For some embodiments, the use of neat methanol at anode  202  and a back diffusion that provides water balance may involve the use of a modified fuel cell stack design that incorporates methods not only for fuel delivery but also for heat removal. For some embodiments, circulating feed  308  may be used, so that excess heat may be removed from anode  202  by way of radiator  310 . Heat may also be removed by evaporative cooling on cathode  204 . For some embodiments without circulating feed  308 , heat loss due to evaporation at cathode  204  may be augmented with heat removal by way of cooling fins  312  on the fuel stack. The design of such fins may depend on the power level and other resources available for cooling. 
         [0030]    Various modifications may be made to the described embodiments without departing from the scope of the invention as claimed below.