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PEFCs use a polymeric membrane as an electrolyte, such as Nafion 117 polymer, analogous in acidity to the electrolyte in the automotive battery, but dimensionally fixed. This simplifies sealing in the production process and provides both cell and stack longevity. PEFCs can produce high power density at temperatures below 100°C, allowing fast start-ups and immediate response to changes in the demand for power. They are ideally suited to transportation and smaller stationary applications. One of the earliest applications of PE fuel cells was the General Electrical-built 1 kW Gemini power plant used on the Gemini spacecraft built in the early 1960's. The performance and life of the Gemini fuel cells were limited to the membrane used at the time. Since then, cell performances and size have increased significantly. A schematic description of the components in an PEFC is shown here:
It consists of two porous electrodes at which the energy conversion process takes place. Hydrogen is supplied to the anode while oxygen from the air is supplied to the cathode. Hydrogen molecules give up electrons to the electrode producing electric current across an external load. In the oxidation process, hydrogen is converted into protons or positively charged hydrogen ions. When the electrons, drawn back by the voltage, arrive at the oxidant electrode, they are removed from the electrode by oxygen molecules, producing negatively charged ions that react with hydrogen ions to form water molecules.
In this Figure, the membrane, or electrolyte, allows the positively charged hydrogen ions to migrate from the anode to the cathode where they react with the negative oxygen ions to produce the product of this reaction, water. The individual PEFCs can be stacked up to create the power necessary for an application as illustrated in Figure 13. The MARK V stack in Figure 13 was, in the 1990s, the benchmark for power demanding applications such as urban buses or other electric vehicles.
PEFCs are of interest to automobile manufacturers as future mini-power plants (about 50 kW) for electric vehicles. In addition, their quick-start capability, ruggedness, and potentially low cost make them attractive for stationary distributed power applications, including remote off-grid applications. PE cells are targeted for use in continuous premium-power service and as small peaking generators in retail markets. Although their operating efficiency is expected to be somewhat lower than that of commercial PAFCs, experts believe PE fuel cell generators could achieve installed costs below $1000/kW for continuous premium-power service and below $500/kW as peaking units. Meeting such cost targets would make them competitive as standby, backup power sources or as distributed peaking capacity.
