Specific Types of Fuel Cells
There are a number of different types of fuel cells, each with its pros and cons. A fuel cell's operating temperature has a major bearing on its performance and its suitable applications. In general, small fuel cell stacks need to operate at relatively low temperatures and large fuel stacks should run at much higher temperatures. Since small stacks have a large surface area to volume ratio, it is difficult to keep them heated to several hundred degrees. Due to radiative and conductive losses, a substantial amount of their electric output may be needed to maintain the high temperature. On the other side of the scale, large fuel cell stacks that need to operate at low temperature can have difficulty radiating away enough of the heat produced along with the electricity. Putting waste heat to work in a cogeneration process can increase the efficiency of large stacks operating at high temperature. The availability of certain catalysts and fuels can also make one type of fuel cell favorable over another. In order to be successful on a commercial scale, the fuel cell industry must provide the right type of fuel cell to meet the demands of the application and the economic market.
This downloadable table lists the more common varieties of fuel cells that are currently in use and show strong promise for the future. The actual performance of a fuel cell system depends on its operating conditions, and may vary from the laboratory values listed herein.
Polymer Electrolyte Membrane Fuel Cell (PEMFC)
Commonly
known as a proton exchange membrane fuel cell (PEM), this is one of the
most promising types of fuel cells for widespread use. The electrolyte
is an advanced polymer membrane that conducts electrons from the anode
to the cathode. One of the most common types of membrane is Nafion, which
looks like a sheet of plastic and is manufactured by the DuPont Corporation.
PEMs are very responsive to changes in electrical load, making them well
suited to light-duty transportation, household electrical generation,
and electrical demands in space. Since they operate as fairly low temperatures,
they can start up quickly. As researchers find ways to reduce the amount
of platinum catalyst the PEMs require, they are becoming less and less
expensive.
Direct Methanol
fuel cell (DMFC)
The
direct methanol fuel cell draws hydrogen from liquid methanol, eliminating
the need for an on-board fuel reformer. The DMFC, which makes uses of
a polymeric electrolyte, has an operating temperature of 80°C, and
has an experimental efficiency of only 25%. Its conducting ion is a proton,
and platiunium is usually used as the fuel cell catalyst. The direct methanol
fuel cell has a reported power density of 20 mW/cm3, which
would make the DMFC useful in transportation, remote power, and standby
power applications. The electrochemical challenges associated with the
DMFC include issues with methanol electrocatalysis and anode poisoning.
AFCs
were developed by NASA for use in the Gemini program and subsequent missions,
including the Space Shuttle. They are highly efficient and produce potable
water as a by-product. The alkaline potassium hydroxide electrolyte they
employ is extremely expensive and is susceptible to poisoning by carbon
dioxide, which is present in many fuel streams. AFCs are therefore suitable
for specialized niche markets.
Phosphoric Acid
fuel cell (PAFC)
PAFCs are the most commercially developed type of fuel cell available today. They operate at a medium temperature of about 200°C, which makes them well-suited to fairly large electrical demands. They have been used in hotels, hospitals, office buildings and buses. Employing the waste heat in a cogeneration cycle can further increase the efficiency of stationary units to over 80%. Even though they employ a platinum catalyst, PAFCs can use impure reformed hydrogen.
Molten Carbonate
fuel cell (MCFC)
MCFCs are a developing technology using a molten carbonate-salt-impregnated ceramic matrix as the electrolyte. They operate at high temperatures and are best suited to stationary large electrical generating needs. Due to their high operating temperature, cogeneration can boost total efficiency past 85%.
Solid oxide fuel cells make use of a thin layer of zirconium oxide as
a ceramic electrolyte, with a lanthanium manganate cathode and a nickel-zirconium
anode. The conducting ion of the SOFC is O2- and the electrocatalyst
is usually nickel or Perovskites. Reformed hydrogen and carbon monoxide
as well as methane can be used as fuels. The solid oxide fuel cell has
a power density of 240 mW/cm3. These characteristics will make
the SOFC useful in high-powered applications like industrial power supplies
and electrical generators, as it can achieve an efficiency of 45%. SOFCs
would not be well suited for vehicles and other smaller uses because they
require an operating temperature of at least 1000°C. The component
layers are expensive to fabricate and require different electrolytes for
lower temperature operation.