Fuel Cells - Page 2/3
Phosphoric acid fuel cells use liquid phosphoric acid as an electrolyte the acid is contained in a Teflon-bonded silicon carbide matrix and porous carbon electrodes containing a platinum catalyst. The chemical reactions that take place in the cell are shown in the diagram to the right. The phosphoric acid fuel cell (PAFC) is considered the "first generation" of modern fuel cells. It is one of the most mature cell types and the first to be used commercially, with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses.
PAFCs are more tolerant of
impurities in the reformate than PEM cells, which are easily "poisoned"
by carbon monoxide carbon monoxide binds to the platinum catalyst at the
anode, decreasing the fuel cell's efficiency. They are 85 percent
efficient when used for the cogeneration of electricity and heat, but
less efficient at generating electricity alone (37 to 42 percent). This
is only slightly more efficient than combustion-based power plants,
which typically operate at 33 to 35 percent efficiency. PAFCs are also
less powerful than other fuel cells, given the same weight and volume.
As a result, these fuel cells are typically large and
Most fuel cells are powered by
hydrogen, which can be fed to the fuel cell system directly or can be
generated within the fuel cell system by reforming hydrogen-rich fuels
such as methanol, ethanol, and hydrocarbon fuels. Direct methanol fuel
cells (DMFCs), however, are powered by pure methanol, which is mixed
with steam and fed directly to the fuel cell anode.
Alkaline fuel cells (AFCs) were one
of the first fuel cell technologies developed, and they were the first
type widely used in the U.S. space program to produce electrical energy
and water onboard spacecraft. These fuel cells use a solution of
potassium hydroxide in water as the electrolyte and can use a variety of
non-precious metals as a catalyst at the anode and cathode.
High-temperature AFCs operate at temperatures between 100ºC and 250ºC
(212ºF and 482ºF). However, more-recent AFC designs operate at lower
temperatures of roughly 23ºC to 70ºC (74ºF to 158ºF).