Patent Publication Number: US-4317789-A

Title: Method of making thin porous strips for fuel cell electrodes

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of continuously manufacturing thin porous conductive strips by a calendering technique. 
     It also relates to the product obtained using the method and in particular to the use of the product as an electrode for a fuel cell. 
     The invention aims to produce continuous porous strips at an industrial rate, which strips may include a plurality of thin layers from about a few microns to a few hundreds of microns thick simply by using a calender and feeding powdered materials into it. 
     The invention also aims to produce thin homogeneous layers of uniform porosity in which there are no fractures and which are self supporting. 
     Proposals have already been made to produce strips by calendering powdered material based on carbon powder or on a metal powder with a binder. 
     However, in most cases, it is necessary to associate a support such as a mesh or a fabric with these materials, the mesh or fabric being embedded after calendering in the layer formed; this causes anisotropy, which may be detrimental to the uses envisaged. 
     Further, the powdered material is generally conveyed by a conveyor belt which also passes through the calender due to the fact that in general, the layer formed is not self supporting. Such a layer must then undergo heat treatment so as to consolidate it. 
     In the majority of cases, unless particular precautions are taken requiring the use of complex apparatus for feeding the calender, there occurs a binding phenomenon which results from the mutual adherence of the grains during calendering, which adherence prevents the calender from being fed uniformly and results in lack of homogeneity in the layer produced. 
     Lastly, this manufacturing method generally entails the use of pore-forming products in the powdered material for obtaining suitable porosity, and the removal of said pore-forming products after calendering may be detrimental to the uses envisaged. 
     Preferred applications of the present invention make it possible to mitigate the disadvantages described hereinabove. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of continuously manufacturing strips using a calender, the strips including at least one thin, porous, conductive layer and being suitable for forming the electrodes of fuel cells, the method comprising: 
     mixing an aqueous dispersion of carbon powder with an emulsion of binder; 
     precipitating the binder onto the powder; 
     drying, and then crushing the precipitate; and 
     while still crushing the precipitate, wetting it with a liquid suitable for lubricating the precipitate and preventing grains of the precipitate from adhering to the rolls of the calender, whereby a wetted powder is obtained which is free from any pore-forming product and is capable of being calendered without a support; 
     said wetted powder being finally passed through the calender, without a support, to obtain a continuous strip. 
     The lack of a pore-forming substance and the fact that a support is not used during calendering make it possible to obtain a very homogeneous thin layer which is free from fractures. 
     Advantageously, to cause homogeneous precipitation of the binder on the carbon powder, both the aqueous dispersion and the binder emulsion are very diluted to obtain as homogeneous a mixture as possible of these two ingredients before precipitation. 
     Thus, the aqueous dispersion of carbon powder may include 10 to 50 g/l of carbon according to the kind of carbon used, the carbon being optionally coated with a catalyst such as platinum, for example. Both carbon black and active carbon can be used. 
     After dispersion, a degassing operation should be carried out so as to remove any air bubbles from the surface of the carbon grains. 
    
    
     DETAILED DESCRIPTION 
     A polytetrafluoroethylene binder in an emulsion which includes about 100 to 120 g of dry extract per liter is preferably used. Said emulsion may be formed by diluting a more concentrated emulsion which may contain e.g. 400 to 600 g of polytetrafluoroethylene per kilogram of emulsion. 
     The binder emulsion may contain stabilizing agents which are then removed after the precipitation process by simply washing in water. 
     The carbon dispersion is mixed with the binder emulsion at a controlled temperature of less than 18° C., preferably between 12° and 15° C. so as to avoid spontaneous precipitation. Likewise, with this aim in view, the pH of the dispersion and of the emulsion are balanced before mixing. 
     Precipitation is then started either by raising the temperature, e.g. to about 25° to 30° C., or by adding cations, e.g. by adding dilute hydrochloric acid. 
     After precipitation (and possible washing in water if stabilizing agents have been added to the emulsion), the mixture is dried and then dry crushed, preferably in a knife crusher. 
     After a crushing period and while crushing continues, the mixture is wetted with a liquid intended to prevent the grains from adhering to the rolls of the calender, while lubricating the grains. 
     The liquid is chosen from the group which includes cyclohexanone, tetralin, decalin, and white spirit. 
     Thus, a white spirit can be used whose boiling point lies between about 140° and 160° C. 
     By way of example, the mixture can be wetted using 15 to 20 cc of liquid for 30 to 50 g of solid product. 
     After this operation, the powdered product is placed in a hopper which feeds the calender directly without using a supporting substrate. After calendering at ambient temperature, a porous, self-supporting conductive strip is thereby produced. 
     The binder can also be constituted by polyvinyl chloride, but in that case, cyclohexanone must not be used as a wetting liquid since it dissolves polyvinyl chloride. 
     A predetermined porosity is imparted to the thin layer by adjusting in particular the discharge of powder into the calender and the rotation speed of the calender rolls. 
     The thickness of the strip can be adjusted simply by adjusting the distance between the calender rolls. 
     Practically, thicknesses from a few microns to a few hundreds of microns can be obtained. 
     The method of the invention allows thin porous multilayer strips to be produced by cold calendering together several layers obtained according to the above-described method. 
     Continuous manufacture of a strip with two layers is possible, one layer being a barrier layer which includes carbon and a binder and the other layer being a catalytic layer which includes carbon coated with a catalyst and with a binder; the proportion by weight of binder in each layer, in particular polytetrafluoroethylene, may lie between 20 and 99%, with that of carbon lying between 80 and 1%. 
     EXAMPLE 
     A particular example of the invention is described by way of illustration. The description relates to the production of a two-layer electrode for use in a hydrogen-air fuel cell. 
     The electrode includes a barrier layer and a catalytic layer. 
     The barrier layer is conductive and provides electron transfer from the catalytic layer to the collector of the cell, while the permeability of the barrier layer to reagent gases (hydrogen or air) allow the catalytic layer to be reached by diffusion at a low feed pressure. 
     Further, due to the fact that it is hydrophobic, the barrier layer makes it possible to locate the liquid-gas interface within the active liquid. 
     The conductivity of the active or catalytic layer allows electron transfer from the reaction zones towards the collector via the barrier layer, with the porosity and the thickness of the active or catalytic layer ensuring ion diffusion towards or away from the reaction zones as well as bringing in the reagents. 
     Of course, the active layer acts as a catalyst to the electro-chemical process. 
     Such layers are obtained as follows. 
     In a first instance, to prepare the catalytic layer, firstly, 120 g of carbon with 20% platinum are dispersed in 4.5 liters of twice distilled water while stirring and at a temperature of about 12° C., then the above suspension is carefully degassed. 
     Secondly, 487 g of a PTFE emulsion with 37% of dry extract, known in the trade as &#34;Soreflon&#34;, are diluted in 3 liters of twice distilled water at a temperature of 12° to 15° C. &#34;Soreflon&#34; PTFE emulsion is sold by Produits Chimiques Ugine Kuhlmann, PCUK and is a dispersion of polytetrafluoroethylene particles in water, the average size of the particles being about 0.25 micron and their density being about 2.16 to 2.25. 
     Then, the catalyst dispersion is poured into the PTFE emulsion, at a temperature of less than 15° C. while continuously stirring to homogenize the mixture suitably while preventing the mixture from agglomerating. The binder is then precipitated by adding dilute hydrochloric acid, the precipitation time being about 10 minutes. The precipitate is filtered and dried in an oven at 80° C. for 10 hours. The dried precipitate is then crushed, and during crushing it is slightly wetted with a liquid such as cyclohexanone, tetralin, decalin, or white spirit. 
     The purpose of such a liquid is to prevent the grains from adhering to the rolls of the calender by lubricating the grains and thus counteracting the binding phenomenon which results in the mutual adherence of said grains. Further, such a liquid makes it possible to adjust final porosity between given limits. 
     In the case of polytetrafluoroethylene, cyclohexanone is advantageously used. 
     Thereafter, the powder thus wetted is fed into the calender to obtain a thin strip as previously described. 
     In a second step the barrier layer is prepared as described hereinafter. 
     90 g of &#34;Vulcan XC72&#34; carbon are dispersed in 2 liters of twice distilled water while stirring, then the suspension is degassed. &#34;Vulcan XC72&#34; carbon is manufactured by Cabot Company; the average particle diameter is about 30.10 microns; the specific surface area is 254 m 2  /gm; oil absorption (DBP) is 178 cm 3  /100 gm; and pH is 7.5. Also, an emulsion of 568 g of Soreflon in 2 liters of twice distilled water is prepared at a temperature of 12° to 15° C. The dispersion and the suspension are then mixed together as in the case of the catalytic layer. 
     A precipitate is then formed by raising the temperature above 26° C. for about 25 minutes. 
     The precipitate is dried in an oven at 80° C. for 24 hours and is then crushed and wetted as in the case of the catalytic layer. 
     Then, the powder thus obtained is in its turn fed into the calender and another thin strip is obtained. 
     The two strips thus obtained are calendered together so as to produce the two-layer electrode which is then dried and possibly heat-treated to modify its hydrophobic quality. The electrode thus prepared includes 30% of Vulcan XC 72 and 70% of PTFE for the barrier layer, and 40% of catalyst and 60% of PTFE for the catalytic layer. 
     Such two-layer electrodes can easily be integrated into filter-press type fuel cells e.g. of the type described in U.S. Pat. No. 4,002,493, for &#34;A fuel cell structure and system, more particularly for a carbon fuel and atmospheric air.&#34; In such cells the current can be collected by means of collectors with points or lines of contact a few millimeters apart, e.g. by a corrugated bipolar collector. Generally, such a collector can be made by any conductive material; advantageously it can be formed by a sheet of plastic material that is impregnated with conductive fibres, in particular carbon fibres. The contact between the collector and the electrode is provided either by pressure or, preferably, by welding or by glueing with a conductive glue, preferably an epoxy resin impregnated with carbon. 
     Further, the Applicant was surprised to find that such a way of drawing current from electrodes such as those produced in accordance with the method of the invention which have a high proportion of PTFE allows current densities to be obtained which are of the order of 300 mA/cm 2  for hydrogen and 200 mA/cm 2  for air for relatively low over-voltages which, for example, do not exceed 200 millivolts. 
     The table below gives a summary of the physical characteristics of such electrodes. 
     
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Thickness  ρ/thickness                                                
                     average ρ                                        
                               Apparent                                   
                                      Porosity                            
(micrometers)                                                             
           (ohms)    ohm . cm  density                                    
                                      %                                   
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Barrier 180    50        0.9     1.25   42                                
layer                                                                     
Active   60    3200      1.9     1.36   43                                
layer                                                                     
Electrode                                                                 
        240    50        1.2     --                                       
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