Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell systems have also been proposed for use in vehicles as a replacement for internal combustion engines. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, hydrogen (H2) is supplied to the anode and oxygen (O2) is supplied to the cathode. In some systems, the source of the hydrogen is reformate and the source of the oxygen (O2) is air.
In a first half-cell reaction, dissociation of the hydrogen (H2) at the anode generates hydrogen protons (H+) and electrons (e−). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane while the electrons flow through an electrical load that is connected across the membrane. In a second half-cell reaction, oxygen (O2) at the cathode reacts with protons (H+), and electrons (e−) are taken up to form water (H2O).
To operate efficiently and to produce the maximum amount of electricity, the fuel cell must be properly humidified. To achieve the proper humidity range, the hydrogen stream and the oxygen stream are typically humidified by one of several methods known in the art. Conventional humidity control methods generally fail to sufficiently control the humidity of the hydrogen and the oxygen streams to the fuel cell. Providing too much humidity to the fuel cell blocks the reacting gases from accessing the catalyst thereby impeding the electrochemical reaction between the hydrogen and the oxygen and reducing the production of electricity. Providing too little humidity to the fuel cell restricts or limits the proton transportation required for reaction within the fuel cell and can also physically damage the fuel cell.
In some conventional fuel cell systems, the oxygen stream that is provided to the fuel cell is humidified as much as possible given the temperature of the oxygen and the humidifying water. These fuel cell systems are concerned with the prevention of dry oxygen that can potentially damage the fuel cell stack. The fuel cell systems are not concerned with the overly moist oxygen stream because it will not damage the fuel cell stack. While preventing damage, these fuel cell systems have less than optimum performance because the overly moist oxygen stream is not optimal for fuel cell performance. Therefore, a fuel cell system that prevents both a dry oxygen stream to the fuel cell and an overly moist oxygen stream to the fuel cell would be desirable.