The present invention relates to a catalyst layer for fuel cells, especially PEM fuel cells, in which a solid polymer is used as the electrolyte.
Fuel cells convert a fuel with an oxidizing agent, physically separated from one another, into electricity, heat and water at two electrodes. Hydrogen or a hydrogen-rich gas can be used as the fuel and oxygen or air as the oxidizing agent. The energy conversion process in the fuel cell is distinguished by particularly high efficiency. For this reason, fuel cells in combination with electric motors are gaining increasing importance as an alternative to conventional combustion engines.
The so-called polymer electrolyte fuel cell (PEM fuel cell) is suitable for use as an energy converter in motor vehicles thanks to its compact construction, its power density and its high efficiency.
The PEM fuel cell consists of a stacked arrangement of membrane electrode assemblies (MEAs), between which bipolar plates are arranged for the supply of gas and current conduction. A membrane electrode assembly consists of a polymer electrolyte membrane which is provided on both sides with catalyst layers. One of the catalyst layers acts as an anode for the oxidation of hydrogen and the second catalyst layer as a cathode for the reduction of oxygen. So-called gas diffusion structures made of carbon fiber paper or nonwoven carbon fabric, which, as a result of their high porosity of up to 75%, allow the reaction gases good access to the catalyst layers and permit good discharge of the cell current, are placed on the catalyst layers.
The two catalyst layers in a membrane electrode assembly, i.e. anode and cathode, contain so-called electrocatalysts which catalytically support the respective reaction (oxidation of hydrogen or reduction of oxygen). The metals of the platinum group of The Periodic Table of Elements are preferably used as catalytically active components. For the most part, so-called support catalysts are used, in which the catalytically active platinum group metals are applied in a highly disperse form on to the surface of a conductive support material. The average crystallite size of the precious metal particles is approximately between 1 and 10 nm. Fine particle size carbon blacks have proved suitable as support materials. Depending on the area of application, anode and cathode layers can be constructed in the same way or can contain different electrocatalysts.
The polymer electrolyte membrane in a PEM fuel cell consists of proton-conducting polymer materials. These materials are also referred to below as ionomers for short. A tetrafluoroethylene-fluorovinyl ether copolymer with acid functions, especially sulfonic acid groups, is preferably used. A material of this type is marketed for example by E. I. du Pont with the trade name NAFION.RTM.. However, other, especially fluorine-free, ionomer materials such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles may also be used.
For the widespread commercial use of PEM fuel cells in motor vehicles, further improvement in the electrochemical cell performance and a marked reduction in the system costs, which are largely caused by the platinum group metals required, is necessary. To reduce the costs per kilowatt of installed capacity, the loading of the electrodes in a fuel cell with the platinum group metals must be reduced. To this end, the electrocatalysts or the catalyst layers must be further improved and the catalytically active precious metal particles must be utilized more effectively.
Essential for the effectiveness of a catalyst layer is the formation of the so-called three-phase zones, in which the catalytically active precious metal particles fixed on the support, the polymer electrolyte and the reaction gas are in direct contact.
U.S. Pat. No. 4,876,115 describes a process for treating a porous gas diffusion electrode which has a catalyst load with platinum applied on to carbon particles of less than 0.5 mg Pt/cm.sup.2. The electrode is impregnated with a solution of an ionomer. As a result, the surfaces of the carbon particles are coated with the ionomer.
In U.S. Pat. No. 5,234,777 a membrane electrode assembly consisting of a polymer electrolyte membrane and catalyst layers and porous gas diffusion structures on both sides is proposed. The catalyst layers consist of a platinum catalyst (platinum on a carbon support) and a proton-conducting ionomer. The thickness of the catalyst layers is less than 10 .mu.m. The platinum catalyst is evenly dispersed in the proton-conducting ionomer. The platinum load of the catalyst layers is less than 0.35 mg/cm.sup.2.
To produce the membrane electrode assembly according to this patent specification, two processes are described (protocol I and protocol II). According to protocol I, the platinum catalyst is dispersed in an alcoholic solution of the ionomer. This dispersion, generally also referred to as ink, is applied on to a support film of PTFE (polytetrafluoroethylene), dried and laminated on to the opposite sides of the polymer electrolyte membrane by heat pressing. According to protocol II the polymer electrolyte membrane is directly coated with an ink consisting of platinum catalyst and ionomer solution. The applied coat is dried at a minimum of 150.degree. C.
The electrode layers according to U.S. Pat. No. 5,234,777 are distinguished by a homogeneous distribution of the catalyst in the ionomer. A deliberate construction of three-phase zones, and thus a better utilization of the catalyst used, is not possible by this process.
U.S. Pat. No. 5,084,144 describes the production of a gas diffusion electrode with an increased number of three-phase zones and thus improved electrocatalytic activity. To produce the gas diffusion electrode, an arrangement consisting of a layer of an electrically conductive support material on a hydrophobic gas diffusion structure is taken as the starting point. The layer is impregnated with a solution of an ionomer and then introduced into an electrolytic bath with precious metal ions, which are then deposited by short pulses of current in the form of crystallites with diameters of less than 10 nanometers. According to this patent specification, the catalytically active precious metal particles are therefore introduced into the catalyst layer by a subsequent electrochemical process.
The disadvantage of this process is that, although contact of the platinum catalyst with the ionomer is guaranteed, the access of the reactive gases is not sufficiently taken into account. This leads to gas transport problems, particularly with high current densities.
The contact of the electrocatalyst with the system of pores in the catalyst layer for reactive gases can allegedly be deliberately improved by a process according to German application DE 19502622 A1. According to this process, an inorganic compound of a precious metal is crystallized out in the system of pores of an uncatalyzed gas diffusion electrode and then reduced under an electrolyte while a gas is being fed in. The uncatalyzed gas diffusion electrode consists, for example, of a layer of activated carbon bonded with PTFE. According to this process too, the catalytically active precious metal particles are introduced into the catalyst layer in a separate process step. The process requires a final reduction of the precious metal compounds.
The process according to DE 19502622 A1 was developed for gas diffusion electrodes in fuel cells with liquid electrolytes. The process is not suitable for polymer electrolyte fuel cells, since a solid, and thus stationary, polymer electrolyte is present in this case, which cannot be used for the deliberate formation of the three-phase zones in accordance with the above process.
Another process for the production of a gas diffusion electrode is described in DE 4417403 A1. According to this document, a flat base material for a gas diffusion electrode is first formed from a mixture of a carbon powder and a fluorinated resin powder and calcined at 350.degree. C. One side of the flat base material is coated with a solution of a platinum group metal salt in a complexing organic solvent and dried. This formed material is then calcined again at 250-380.degree. C. in a protective gas atmosphere.
The process according to DE 4417403 A1 also has a separate process step for the introduction of the precious metal particles into the catalyst layer. Because calcination has to be carried out twice, it is very time-consuming and expensive. No real increase in the proportion of three-phase zones is achieved, since the process cannot be carried out in the presence of ionomer, since this would be thermally damaged during the calcination. Only after the last calcination is the electrode placed on a polymer membrane coated with a NAFION.RTM. liquid and heat-pressed with this at 130.degree. C.
A general disadvantage of the known membrane electrode assemblies and of the processes for the production thereof or for the production of gas diffusion electrodes is that the electrocatalyst (generally a precious metal on a carbon support) either has to be prepared from a precious metal compound and a support material in a previous production process or has to be introduced into the catalyst coating afterwards. These additional steps increase the costs of a PEM fuel cell system.
An object of the present invention was therefore to provide a catalyst layer for polymer electrolyte fuel cells which exhibits better electrocatalytic activity than known catalyst layers and can be produced in a simple and inexpensive process. Another object of the invention is to enable the preparation of an ink for the production of the catalyst layer and the production process itself, and the gas diffusion electrodes and membrane electrode assemblies produced thereby.