A brief overview of fuel cells

Types of fuel cells

There are many types of fuel cells under development for use in transportation, stationary power, or portable power. Fuel cells are typically distinguished by the electrolyte that is used and will fall into two broad category types: low temperature and high temperature.

The temperature issue: Operating temperature is a critical characteristic for transportation fuel cells due to crashworthiness and safety concerns, as well as the need for quick start and thermal insulation requirements.

In general, high temperature fuel cells are more efficient than low temperature ones in generating electrical energy. They provide high temperature waste heat which increases their efficiency. While this is a benefit in stationary cogeneration applications, it presents a problem for transportation applications.

Low temperature have quicker start-up times, compact volume and lower weight compared to high temperature fuel cells. Importantly, crashworthiness requires a low temperature system. As a result, most transportation fuel cell developments have been of the low temperature variety.

The following is a brief description of each fuel cell type, its operating characteristics, and its applicability for transportation use.

See our Links Page for where to find more detailed technical information on fuel cell types.

Proton exchange membrane fuel cells (PEM)

A majority of transportation fuel cell development is focused on PEM fuel cells, as they offer a number of transportation-friendly characteristics. They operate at low temperature and feature high power density, quick start-up, rapid response to varying loads and low operating criteria. PEM fuel cells also use inexpensive manufacturing materials (plastic membrane). Disadvantages are lower efficiency levels and low tolerance for carbon monoxide contamination.

Phosphoric acid fuel cells (PAFC)

Phosphoric acid fuel cells (PAFCs) are the most commercially developed fuel cell type. They also operate at a low temperature (around 200¡C), utilize widely available phosphoric acid as the electrolyte, and have a slightly higher tolerance for carbon monoxide than other fuel cell types. They are disadvantageous for transportation use, however, as they require a warm up period before energy is generated and are large and heavy. However, PAFCs are being used in stationary applications, such as hospitals, schools and utility power plants.

Direct methanol fuel cells (DMFC)

Direct methanol fuel cells use methanol directly as the source for hydrogen. Operating temperatures of direct methanol fuel cells are in the same range as PEM fuel cells Ð 50 to 100¡C (122 to 212¡F). These fuel cells are still in an early development stage, but could prove a less expensive technology as they eliminate the fuel reformation process.

Alkaline fuel cells (AFC)

Alkaline fuel cells (AFCs) are some of the most developed of the fuel cell technologies, as thye were used in the Gemini-Apollo space program to produce electrical energy and drinking water. AFCs have the advantage of being built from relatively inexpensive components, but their low tolerance for carbon dioxide contamination requires pure hydrogen and oxygen supplies. This poses a large barrier for use of AFCs in transportation, although it is expected that AFCs will continue to be utilized in the space program.

Solid oxide fuel cells (SOFC)

Solid oxide fuel cells (SOFCs) operate between 800¡C and 1000¡C, have a good tolerance for fuel impurities, and use ceramic as an electrolyte, which reduces some problems associated with liquid electrolytes such as corrosion. As with other types of high temperature fuel cells, transportation applications will be limited to the heavy-duty sector due to its size and warm-up requirements. The SOFC is one of the least developed fuel cell technologies, but shows promising potential.

Molten carbonate fuel cells (MCFC)

Molten carbonate fuel cells operate at 650¡C, which gives this fuel cell the advantage of being able to internally reform hydrocarbons, such as natural gas and petroleum-based fuels. However, the high operating temperatures cause corrosion problems and requires the use of costly platinum metals for fuel cell construction. MCFC technology promises high efficiency and is being developed for stationary applications.

Regenerative fuel cells (RFC)

Regenerative fuel cells separate water into hydrogen and oxygen by a solar-powered electrolyser. Hydrogen and oxygen are fed into regenerative fuel cells, generating electricity, heat and water. Water is then recirculated back to the electrolyser of the regenerative fuel cell and the process repeats.

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