Abstract:
An improved direct liquid-feed fuel cell having a solid membrane electrolyte for electrochemical reactions of an organic fuel. Catalyst utilization and catalyst/membrane interface improvements are disclosed. Specifically, the catalyst layer is applied directly onto the membrane electrolyte.

Description:
ORIGIN OF INVENTION 
     The invention described herein 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 title. 
    
    
     FIELD 
     This disclosure generally relates to organic fuel cells and in particular liquid feed organic fuel cells and the manufacturing thereof. 
     BACKGROUND 
     Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells use renewable fuels such as methanol; typical products from the electrochemical reactions are mostly carbon dioxide and water. Fuel cells can be an attractive alternative to the combustion of fossil fuels. 
     In the past, fuel cells used reformers to convert methanol into hydrogen gas for use by the fuel cells. Direct oxidation fuel cells offer considerable weight and volume advantage over the indirect reformer fuel cells. However, initial direct oxidation models used a strong acid electrolyte which caused corrosion, degradation of catalyst, and other problems that compromise efficiency. Problems associated with such conventional direct liquid-feed cells are well recognized in the art. 
     Jet Propulsion Laboratory (JPL) developed an improved direct liquid-feed cell using solid-state membrane electrolyte. The JPL fuel cell eliminates the use of liquid acidic and alkaline electrolyte and therefore solves many problems in the conventional fuel cells. The subject matter of this improvement is described in U.S. Pat. No. 5,599,638, U.S. patent application Ser. No. 08/569,452 filed Dec. 8, 1995, now U.S. Pat. No. 5,773,162 and U.S. patent application Ser. No. 08/827,319, filed Mar. 26, 1997, now U.S. Pat. No. 5,945,231 the disclosures of which are herewith incorporated by reference to the extent necessary for proper understanding. 
     FIG. 1 illustrates a typical structure  100  of a JPL fuel cell with an anode  120 , a solid polymer proton-conducting cation-exchange electrolyte membrane  110 , and a cathode  130  enclosed in housing  102 . An anode  120  is formed on a first surface of the membrane  110  with a first catalyst for electro-oxidation and a cathode  130  is formed on a second surface thereof opposing the first surface with a second catalyst for electro-reduction. The anode  120 , membrane  110 , and the cathode  130  are hot press bonded together to form a composite multi-layer structure called the membrane electrode assembly (MEA). An electrical load  140  is connected to the anode  120  and cathode  130  for electrical power output. 
     The membrane  110  divides the fuel cell  100  into a first section  122  on the side of the anode  120  and a second section  132  on the side of the cathode  130 . A feeding port  124  in the first section  122  is connected to a fuel feed system  126 . A gas outlet  127  is deployed in the first section  122  to release gas therein and a liquid outlet  128  leads to a fuel re-circulation system  129  to recycle the fuel back to the fuel feed system  126 . In the second section  132  of the cell  100 , an air or oxygen supply  136  (e.g., an air compressor) supplies oxygen to the cathode  130  through a gas feed port  134 . Water and used air/oxygen are expelled from the cell through an output port  138 . 
     During operation, a mixture of an organic fuel (e.g., methanol) and water is fed into the first section  122  of the cell  100  while oxygen gas is fed into the second section  132 . Electrochemical reactions happen simultaneously at both the anode  120  and cathode  130 , thus generating electrical power. For example, when methanol is used as the fuel, the electro-oxidation of methanol at the anode  120  can be represented by: 
     
       
         Anode: CH 3 OH+H 2 O→CO 2 +6H + +6e −   
       
     
     and the electro-reduction of oxygen at the cathode  130  can be represented by: 
     
       
         Cathode: O 2 +4H + +4e − →2H 2 O 
       
     
     Thus, the protons generated at the anode  120  traverse the membrane  110  to the cathode  130  and are consumed by the reduction reaction therein while the electrons generated at the anode  120  migrate to the cathode  130  through the electrical load  140 . This generates an electrical current from the cathode  130  to the anode  120 . The overall cell reaction is: 
     
       
         Cell: CH 3 OH+1.5O 2 →CO 2 +2H 2 O 
       
     
     The energy generated by JPL&#39;s direct feed fuel cell and the advantages of using a solid electrolyte membrane fostered further research. Efforts are targeted toward improving manufacturing efficiency while achieving better performance at reduced cost. 
     Prior art for preparing methanol fuel cell&#39;s membrane electrode assembly, as disclosed in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452, involves the formation of catalyst layers on a porous carbon substrate which is then mounted on either side of a solid electrolyte membrane. Although considerable energy output has been achieved at high catalyst loading levels, there may be significant performance limitations associated with this process. 
     In some resulting catalyst layers, at least fifty percent of the catalyst gets impregnated deep in the pores of the carbon substrate. Hence, the impregnated catalyst are inaccessible for electrochemical reaction and are essentially wasted. Some prior art methods of preparing membrane electrode assemblies for direct methanol fuel cells employ excessive catalyst. Improved techniques of catalyst application may help reduce the amount of catalyst necessary for attaining the desired performance levels. Reduction of the use of expensive catalyst and more efficient catalyst utilization are improvements that may propel this technology toward commercialization. 
     Another obstacle to desired performance levels is inadequate catalyst/membrane interface. A large area of electrochemically active interface between the carbon-paper coated catalyst layer and the membrane is usually desired for attaining maximum energy output by a particular fuel cell. There are some prior art methods that rely on heat and pressure for membrane electrode assembly fabrication. Since the catalyst layer is kept relatively dry after application of the catalyst onto the carbon paper substrate, the interface formed between the catalyst layer and the membrane is usually not optimum. Improved methods to enhance the area of electrochemical contact at the catalyst layer/membrane interface are desired for attaining high performance levels. 
     SUMMARY 
     The inventors disclose a direct feed fuel cell that can be manufactured efficiently while producing better performance at a reduced cost. This direct feed fuel cell features improved catalyst utilization and enhanced catalyst layer/membrane interface. 
     A process of catalyst application is presented. Instead of coating catalyst layers onto a support substrate, the catalyst mixture is applied directly onto the membrane. This method involves pre-treatment of the membrane in swelling agents, direct application of catalyst mixture onto the pre-treated membrane, and subsequent slow evaporation of the catalyst coating. Direct application of catalyst onto the membrane reduces catalyst waste due to impregnation of the catalyst into the support substrate. 
     The direct coating process also improves interfacial contact. Softening the membrane by pre-swelling and the proximity of the uniformly deposited catalyst layer to the membrane enhance the interfacial contact area formed between the electrode and the membrane. 
     This method of directly applying catalyst layers on the membrane offers very high catalyst utilization and improved catalyst/membrane interface. Laboratory tests reveal that at low catalyst loading levels, e.g. 2-3 mg/cm 2 , a fifty percent increase in power density can be achieved using this method. Such results demonstrate significant improvements in fuel cell performance by depositing the catalyst directly on the membrane. 
     In an effort to bring this innovation closer to efficient mass production, the present inventors further developed a direct spray deposition process. The spray deposition process produces uniformly thin catalyst layers and has the flexibility of producing well-defined multiple thin layers of different composition. The catalyst ink is adjusted to a sprayable composition and is spray deposited onto a pre-treated membrane; during the spraying process the coating is dried using warm air guns. The inventors disclose sprayer apparatuses adapted for catalyst or slurry deposition. These sprayer apparatuses produce slow and fine sprays without nozzle clogging. The new sprayer designs allow uniform controlled deposition of multiple thin layers at low rates and without wastage of catalyst material. 
     These features save time, allows attainment of high power densities at low catalyst loading levels, and are amenable to efficient production of fuel cells. This technology may be useful for portable power applications in the range of 50 Watts to 5000 Watts, such as military communications, emergency power, and vehicle power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiment thereof, as illustrated in the accompanying drawings, in which: 
     FIG. 1 shows a typical direct liquid-feed fuel cell having a solid-state membrane electrolyte; 
     FIGS. 2A-2C show direct pour deposition process; 
     FIG. 3 show direct spray deposition; 
     FIG. 4A shows sprayer  1 ; 
     FIG. 4B shows sprayer  2 ; 
     FIGS. 5A-5B are graphs illustrating performance of MEA utilizing direct pour deposition; 
     FIG. 6 is a graph illustrating performance of MEA utilizing direct spray deposition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The membrane electrode assembly (MEA) is a component of the direct methanol fuel cell that has been continuously advanced at Jet Propulsion Laboratory. FIG. 1 illustrates the typical JPL fuel cell. The present specification describes improvements in the fabrication of the membrane electrode assembly. In summary, the technique includes: 1) Pre-treatment of the membrane with swelling agents; 2) Preparation of the catalyst ink especially formulated for the mode of application; 3) Application of the catalyst layer on the membrane in a defined area and subsequent drying of the catalyst layer on the membrane; 4) Hot pressing of the porous current collection substrate on both sides of the catalyst coated membrane to form a membrane electrode assembly. This technique is described in detail herein. 
     1. Pre-treatment of the Membrane 
     The NAFION™ membrane which is conditioned in water has been found to wrinkle and deform when the catalyst ink is brought in contact with the membrane. The catalyst ink includes catalyst, aliphatic alcohols, and dissolved NAFION™ ionomer. Since the solvent of the catalyst contains alcohol, a membrane soaked in pure water will wrinkle upon contact with the catalyst. As a result, a non-uniform catalyst layer may be formed. 
     Some approaches in the past have applied the catalyst on the membrane by starting from a precursor of the membrane. These previous methods involve several processing steps before the final acid form of NAFION™ can be obtained. These methods are laborious and multiple steps can be detrimental to the catalyst itself. 
     The membrane conditioned in water is allowed to soak in a water-alcohol mixture. Solutions of 10%-90% isopropanol in water can be used. In a preferred embodiment, the membrane is soaked in a 50% isopropanol solution for 24 hours. Similar effective results could be obtained over a wide range of compositions having at least 10% isopropanol and the rest being mostly water. Methanol and other aliphatic alcohols can also be used instead of isopropanol. 
     Pre-treatment of the membrane reduces wrinkling during catalyst contact since the membrane and the catalyst layer have similar solvent compositions. Pre-treatment also improves catalyst bonding to the membrane. After pre-treatment, the membrane becomes very soft. The catalyst layer integrates with the membrane more readily during the drying stages and in the hot pressing phase. 
     The pre-treated membrane is stored wet. When the catalyst ink is ready to be applied onto the membrane, the membrane is then held in a non-corrodible frame to prevent contamination of the NAFION™ membrane. 
     2. Preparation of the Catalyst Ink 
     The catalyst layer is formulated from an ink consisting of a selected catalyst material, polytetrafluoroethylene and perfluorovinylether sulfonic acid such as NAFION™ by DUPONT™, and polytetrafluoroethylene, e.g. TEFLON™, mixed together in appropriate proportions. The ink mixture preferably includes approximately 150 mg of catalyst, which can include platinum and/or platinum-ruthenium catalyst, 0.7-1.4 g of 5% NAFION™ ionomer solution, and 0.2-0.4 g of a TEFLON™ emulsion such as PTFE-30 diluted to 11% in solids. The solvent includes water and isopropanol. Preferable ink compositions use very little or none of the TEFLON™ additive. This ink is separately prepared with platinum for the cathode, and platinum-ruthenium for the anode. Inks using other catalyst can also be formulated in this manner. 
     The mixture formed from the foregoing constituents is mixed by an ultrasonic bath. A viscous ink results after the mixing. The viscosity of this ink is adjusted to the specific mode of application. An ink prepared for direct pour deposition is more viscous than an ink prepared for direct spray deposition. A sprayable composition is prepared by adding appropriate amounts of water and isopropanol. 
     The amount of ink needed is dependent on the catalyst loading area of electrode desired. In a preferred embodiment, a loading level of 2-3 mg/cm 2  of the catalyst is used for the direct pour deposition process. In the direct spray deposition process, a loading level of 1-2 mg/cm 2  of the catalyst is used. 
     3. Direct Catalyst Application Onto the Membrane and Subsequent Drying Techniques 
     Direct Pour Deposition 
     FIG. 2A shows a pre-treated membrane  210  spread on a fine absorbent lint-free tissue  220  while the membrane  210  is still wet. After this, the membrane  210  is held in position by a frame  230  as shown in FIG. 2B. A catalyst coating  240  is poured and spread over a defined area  250  of the membrane  210 . The spreading can be accomplished using a glass rod  260 . The membrane  210  is kept on a flat surface to ensure that the poured coating  240  evenly coats the membrane  210  surface and uniform thickness results. The entire membrane  210  and the coating  240  is sealed off in a polyethylene bag  270  with very small orifices  280  for the escape of moisture/alcohols during slow evaporation of the ink as shown in FIG.  2 C. 
     This controlled evaporation of the ink allows slow evaporation which produces a uniform, crack-free coating. After 24-48 hours the coating is dry. The membrane is recovered and taken through a hot pressing process. 
     Direct Spray Deposition 
     The pre-treated membrane  210  is held in a non-corrodible frame  230  as shown in FIG.  3 . This ensures that the membrane dimensions are not altered during spraying and drying of the catalyst layers. Once the membrane  210  is pre-treated and held in the frame  230 , the membrane  210  is sprayed. 
     The pre-treated membrane  210  is held in a frame  230 , e.g. a rotating dual cut-out mounting jig. A sprayer applicator  310  is fixed at a predetermined distance. Two blower/heat guns  320 ,  330  are fixed at either side of the frame  230 . 
     The performance requirement for sprayers for precious metal catalyst inks can be very demanding if the quality of the resulting spray coating is to be satisfactory. Some of desirable features include the following: The platinum-containing catalyst ink can be very expensive. Therefore, the unit should be capable of handling very small volumes of spray solution. The unit should be able to spray directly on a desired area without wastage of material. Preferably, the unit produces very fine droplet sizes, a fine mist. The desired unit is also capable of very low velocity mist transport. The unit should be able to maintain a continuous spray without nozzle clogging. Nozzle and container should be chemically stable to the constituents of the ink. The unit should also be easy to clean and should be protected from contamination. 
     Two sprayer devices, sprayer  1  and sprayer  2 , are respectively shown in FIG.  4 A and FIG.  4 B. These sprayers may be used for other applications such as coating other materials. These materials include, but are not limited to, battery electrodes, surface finishing materials, corrosion inhibitors, coloring layers, and masking layers. 
     FIG. 4A illustrates sprayer  1 . Sprayer  1  has a material reservoir chamber  410  for storing catalyst materials  415  prior to dispensing. A venturi feed tube  420  is set inside the material reservoir chamber  410  with one end  425  of the feed tube perpendicular to the bottom  430  of the reservoir and in contact with the catalyst material  415 . An air supply  435  is connected to one side of the venturi feed tube  420  by a first conduit  440 . A misting sphere  445  is connected to the other side of the venturi feed tube  420  by a second conduit  450 . The misting sphere  445  provides the atomization of the sprayed particles. The misting sphere  445  also helps to control the spray rate. An exit stem  460  is coupled to the material reservoir chamber  410 . The exit stem  460  length is tuned for fine mist adjustment. The diameter of the exit stem opening is 3 mm. 
     Sprayer  2 , shown in FIG. 4B, has a material reservoir chamber  410  for storing catalyst materials  415  prior to dispensing. A venturi feed tube  420  is positioned within the material reservoir chamber  410 , the venturi feed tube  420  has one end  425  in contact with the catalyst materials  415 . An air supply  435  is attached to the material reservoir chamber  410  via an inlet conduit  470 . The venturi feed tube  420  sends the catalyst material  415  from the material reservoir chamber  410  to the misting sphere  445 . The misting sphere  445  is connected to an exit tube  480 . The exit tube  480  arrangement eliminates the loss of catalyst materials  415 . 
     In both sprayer devices, air from a pressure vessel is used as the spray vehicle. The rates of air flow are controlled to regulate the spray characteristics. Both devices incorporate a venturi feed tube. A venturi feed tube is a short tube with a tapering constriction in the middle that causes an increase in the velocity of flow of a fluid and a corresponding decrease in fluid pressure. This tube is used for creating a suction. The sprayer devices are capable of producing a low velocity spray that deposits an uniform mono-layer of catalyst onto the pre-treated membrane. 
     The sprayable catalysts ink is transferred into one of the sprayers. The spray from that sprayer is directed to the open area of the pre-treated wet membrane where the coating is desired. The distance between the sprayer and the membrane is so adjusted that the sprayed material does not dry out before it reaches the membrane. This assures that the spray deposited material bonds to the membrane and forms an electrochemically active interface. After a thin layer has been deposited, the coating is still very moist. 
     The moist spray coated area is then allowed to dry by directing a stream of air on the coated surface. The rate of air flow is adjusted such that it is enough to cause drying without causing the deposit to crack or dislodge from the surface. Warm air, 40-60 degrees Celsius, can be used to enhance the rate of drying. The passage of air across the surface of the coating prevents cracking and ensures formation of a uniform coating. 
     After the coating has reached the desired level of dryness, additional layers may be applied by the same procedure including consecutive spraying and drying steps. This process is called alternative-side spraying. The layers are applied on the anode side and the cathode side of the membrane alternatively to minimize any stresses due to the coating process. In a preferred embodiment, alternative side coating is accomplished by rotating the frame  230  after each application so that the spray applicator  310  can deposit catalyst on the other side of the membrane  210 . A single coat can deposit as little as 0.1 mg/cm 2  of catalyst material to the surface. The process is repeated until approximately 1-2 mg/cm 2  of catalyst is loaded onto the membrane. 
     4. Membrane Electrode Assembly 
     After coating the desired amount of catalysts on both sides of the membrane, the coated membrane is released from the frame and is ready for hot pressing with support substrates, preferably carbon paper supports. Before hot pressing, the carbon paper supports are prepared. The anode paper is plain TGPH-090 or 060 paper which is manufactured by Toray Inc. The cathode is also the same type of paper except it is taken through a standard teflonization process described in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452 (Patent Pending). The degree of teflonization of the paper on the cathode is 5% in a preferred embodiment. However, the degree of teflonization can be varied from 5%-20% to obtain enhanced air electrode performance. 
     The coated membrane is sandwiched between the anode and cathode supports and held in the press for 10 minutes under a pressure that can vary from 500 psi-1500 psi. For papers that are thin, such as the TGPH-060 (six millimeters thick), the preferred pressures are close to 500 psi. With thicker papers the optimum pressures are as high as 1250 psi. 
     After 10 minutes of pressure, heating is commenced. The heat is slowly ramped up to about 145° C. The slow ramping up should take place over 25-30 minutes, with the last 5 minutes of heating being a time of temperature stabilization. The heat is switched off, but the pressure is maintained. The press is rapidly cooled using circulating water while the pressure is maintained. On cooling to about 60° C., the membrane-electrode assembly is removed from the press and stored in water in a sealed plastic bag. 
     Electrical Performance Evaluation 
     Electrical performance evaluation is carried out in a standard laboratory setup which allows circulation of methanol solutions past the anode and air/oxygen across the cathode. The current-voltage performance of a fuel cell at 90° C. is evaluated. 
     FIG. 5A illustrates the performance enhancement of a fuel cell in which the membrane electrode assembly is fabricated by a direct pour deposition process compared to prior JPL fuel cell models as described in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452 (Patent Pending). The current performance is 0.5 V at 300 mA/cm 2  on air at 2.5 atm at 90° C. Under similar conditions, the performance on oxygen is 0.55 V at 300 mA/cm 2 . Prior art devices, by contrast, produces 0.45 V at 300 mA/cm 2  on air. 
     The improvement is even better represented by the increase in peak power density as shown in FIG.  5 B. The peak power density has been significantly increased from 160 mW/cm 2  to 210 mW/cm 2 . This means that the fuel cell stacks operating with this new performance level would be 25% lower in weight and volume compared to the prior art. 
     FIG. 6 illustrates the performance of a fuel cell in which the membrane electrode assembly is fabricated by a direct spray deposition process. The performance of fuel cells using these MEAs is compared with the performance of those produced by the prior art as described in U.S. Pat. No. 5,599,638 and U.S. patent application Ser. No. 08/569,452 (Patent Pending). Direct spray deposition process uses half the amount of catalyst (1-2 mg/cm 2  as opposed to 4 mg/cm 2  used in the prior art) and delivers comparable performance levels as earlier technologies. This demonstrates the higher utilization levels attained by the new technology. These results are produced using a method that is readily adapted to lower cost mass production. Although only a few embodiments have been described in detail above, those having ordinary skill in the art will certainly understand that many modifications are possible in the preferred embodiment without departing from the teachings thereof. 
     All such modifications are intended to be encompassed within the following claims.