Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode and an oxidant being provided to a cathode, thereby generating a current flow between these electrodes. Such fuel cell assemblies are very useful and sought after due to their high efficiency, as compared to internal combustion fuel systems and the like. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products that are produced, such as water. In order to control the temperature within the fuel cell assembly, a coolant is provided to circulate about a fuel cell assembly, usually water. The use of reformed fuels within fuel cell assemblies makes them particularly sensitive to possible water contaminants.
Electrochemical fuel cell assemblies typically employ hydrogen as the fuel and oxygen as an oxidant where, as noted above, the reaction by-product is water. Such fuel cell assemblies employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the two electrodes formed of porous, electrically conductive sheet material--typically carbon fiber paper. The ion exchange membrane is also known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION.TM., and has a catalyst layer formed thereon at the membrane-electrode interface so as to promote the desired electrochemical reaction.
In operation, hydrogen fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous electrode material and reacts with the hydrogen ions and the electrons from the anode at the catalyst layer to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations: EQU Anode reaction: H.sub.2.fwdarw.2H.sup.+ +2e EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.fwdarw.H.sub.2 O
Conventional fuels cells have the ion exchange membrane positioned between two gas-permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite-polymer composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrodes, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
When flow channels are formed within these plates for the purposes of feeding either fuel or oxidant to the anode and cathode plates, they are referred to as fluid flow field plates. These plates may also function as water transfer plates in certain fuel cell configurations. When these plates simply overlay channels formed in the anode and cathode porous material, they are referred to as separator plates. Moreover, the plates may have formed therein reactant feed manifolds which are utilized for supplying fuel to the anode flow channels or, alternatively, oxidant to the cathode flow channels. They also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. Alternatively, the manifolds may be external to the fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748 issued to Kunz et al. and incorporated herein by reference in its entirety.
The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. Multiple electrically connected fuel cells consisting of two or more anode plate/membrane/cathode plate combinations are referred to as a fuel cell stack. A fuel cell stack is typically electrically connected in series.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing impure hydrogen produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like, into hydrogen. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. This conversion of hydrocarbons is generally accomplished through the use of a steam reformer. Reformed hydrocarbon fuels frequently contain quantities of ammonia, NH.sub.3, as well as significant quantities of carbon dioxide, CO.sub.2. These gases tend to dissolve and dissociate into the water which is provided to, and created within, the fuel cell assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in the coolant channels and manifold leading to degradation of fuel cell materials, as well as reducing the conductivity of the PEM and thereby r educing the efficiency of the fuel cell assembly as a whole.
As disclosed above, the anode and cathode plates provide coolant channels for the circulation of a water coolant, as well as for the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water so collected and circulated through a fuel cell assembly in the coolant channels is susceptible to water contamination and may therefore damage and impair the operation of the fuel cell assembly as the contaminated water circulates throughout the fuel cell assembly.
It is therefore necessary to provide a system which may protect the fuel cell assembly from water contamination, such as is described in commonly owned U.S. Pat. No. 4,344,850 issued to Grasso and incorporated herein by reference in its entirety. Grasso's system for treating the coolant supply of a fuel cell assembly, as illustrated in FIG. 1 of U.S. Pat. No. 4,344,850, utilizes a filter and demineralizer for purifying a portion of the coolant supplied to the fuel cell assembly. A deaerator is also utilized to process the condensed water obtained from a humidified cathode exit stream. As discussed in Grasso, the heat exchange occurring between the coolant stream and the body of the fuel cell assembly is accomplished according to commonly assigned U.S. Pat. No. 4,233,369 issued to Breault et al., incorporated herein by reference in its entirety. It is important to note that Grasso's coolant system does not provide for the cleansing of the coolant stream as a whole. This is due to the fact that the coolant conduits in Grasso, being fashioned from copper or the like, are not in diffusable communication with the body of the fuel cell assembly and as such, the coolant stream does not receive contamination from, inter alia, the CO.sub.2 or NH.sub.3 present in the reformed fuel stream. The burden of cleansing the coolant stream in Grasso is therefore born solely by the filter and demineralizer and results in greater wear on these components and hence greater repairs and replacements. Grasso also utilizes two distinct coolant pumps for circulating the coolant.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a fuel cell assembly with a water treatment system which overcomes the above-described drawbacks.