Hydrogen PEFCs solid polymer electrolyte

For alkaline electrolytes, the oxidizer reduction reaction (ORR) kinetics are more efficient than acid-based electrolytes (e.g., PEFC, PAFC). Many space appUcations utiUze pure oxygen and hydrogen for chemical propulsion, so the AFC was well suited as an APU. However, the alkaline electrolyte suffers an intolerance to even small fractions of carbon dioxide (CO2) found in air which react to form potassium carbonate (K2CO3) in the electrolyte, gravely reducing performance over time. For terrestrial applications, CO2 poisoning has limited lifetime of AFC systems to well below that required for commercial application, and filtration of CO2 has proven too expensive for practical use. Due to this limitation, relatively little commercial development of the AFC beyond space applications has been realized. Some recent development of alkaline-based solid polymer electrolytes is underway, however. The AFC is discussed in greater detail in Chapter 7. 

Throughout this textbook, the PEFC has been emphasized because it is a likely candidate for power replacement for portable, auxiliary, stationary, and automotive power systems. As shown in Figure 6.1, the PEFC class of fuel cells includes the hydrogen, direct methanol, direct alcohol, and other fuel cell systems utilizing a solid polymer electrolyte. While the higher temperature molten carbonate and solid oxide fuel cell systems are well-suited for steady power systems, only the low-temperature PEFC offers the rapid startup and lower operating temperatures (20-90°C) required for transient operation of portable, reserve, and automotive power applications. 

The purpose of the present review is to summarize the current status of fundamental models for fuel cell engineering and indicate where this burgeoning field is heading. By choice, this review is limited to hydrogen/air polymer electrolyte fuel cells (PEFCs), direct methanol fuel cells (DMFCs), and solid oxide fuel cells (SOFCs). Also, the review does not include microscopic, first-principle modeling of fuel cell materials, such as proton conducting membranes and catalyst surfaces. For good overviews of the latter fields, the reader can turn to Kreuer, Paddison, and Koper, for example. 

There exist a variety of fuel cells. For practical reasons, fuel cells are classified by the type of electrolyte employed. The following names and abbreviations are frequently used in publications alkaline fuel cells (AFC), molten carbonate fuel cells (MCFC), phosphoric acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and proton exchange membrane fuel cells (PEMFC). Among different types of fuel cells under development today, the PEMFC, also called polymer electrolyte membrane fuel cells (PEFC), is considered as a potential future power source due to its unique characteristics [1-3]. The PEMFC consists of an anode where hydrogen oxidation takes place, a cathode where oxygen reduction occurs, and an electrolyte membrane that permits the transfer of protons from anode to cathode. PEMFC operates at low temperature that allows rapid start-up. Furthermore, with the absence of corrosive cell constituents, the use of the exotic materials required in other fuel cell types is not required [4]. 

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