1. Field of the Invention
The present invention relates to improvements of solid polymer fuel cells, otherwise referred to as solid polymer electrolyte or proton exchange membrane fuel cells, where these cells are operable at near ambient conditions of temperature and pressure and do not require external humidification.
2. Description of Related Art
Fuel cells are typically used as a source of primary power in remote areas where the usual sources of power are unavailable and where service and maintenance of equipment is limited. Fuel cells may also be used as an alternative power source in earth and space applications. Examples in this area are unattended communications repeaters, navigational aids, space flights and weather and oceanographic stations. For such applications, the power system must be self-contained and the reliability of the system is of maximum importance.
Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel and oxidant are stored external to the cell, which can generate power as long as the fuel and oxidant are supplied. A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte to produce electrons and protons in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Protons pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen gas or air, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte consuming the electrons circulated through the electrical circuit and the protons at the second electrode. The first electrode may alternatively be referred to as an oxidizing or as a fuel electrode, and the second electrode may alternatively be referred to as an oxidant or as a reducing electrode. The halfcell reactions at the two electrodes are, respectively, as follows: EQU H.sub.2 .fwdarw.2H.sup.+ +2e- EQU 1/2O.sub.2 +2H.sup.+ +2e-.fwdarw.H.sub.2 O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate halfcell reactions written above. A byproduct of the reaction is formed as well as some heat.
In practice, fuel cells are not operated as single units; rather, they are connected in a series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as a fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed with manifolds to the electrodes, and cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals, insulations, piping, and instrumentation. The stack, housing, and associated hardware make up the fuel cell module.
Fuel cells may be classified by the type of electrolyte, i.e., liquid or solid that they contain. A fuel cell using a solid electrolyte, such as a solid polymer referred to as a proton exchange membrane, operates best when the electrolyte membrane is kept moist with water because the membrane will not operate efficiently when it is dry. The membrane requires constant humidification during operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and oxygen, that pass by the membrane on each side of the membrane/electrodes assembly. The accessories required for humidification add instrumentation and hence weight to the fuel cell.
The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing a basic unit of fluorinated carbon chain and a sulphonic acid group. There may be variations in the molecular configurations of this membrane. One membrane commonly used as a fuel cell solid electrolyte is a well known perfluorocarbon material sold by E. I. DuPont de Nemours under the trademark "Nafion." Dow Chemicals Company has also developed proton exchange membranes that are still in the experimental stage. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified.
In addition to the humidification problem, the membrane contributes to the ohmic resistance within the cell itself and, in turn, lowers fuel cell performance. If the fuel cell must perform at a lower temperature and pressure and if the reaction kinetics of the two halfcell reactions shown above are slower, the use of less membrane as the electrolyte becomes imperative.
Cooling and humidification requirements increase the cost and complexity of the fuel cell, reducing its use as an alternative energy supply in many applications. Accordingly, there is a need for a fuel cell with no requirement for humidification which operates at near ambient conditions.