A brief description of various electrolyte cells of interest

Polymer Electrolyte Fuel Cell (PEFC): The electrolyte in this fuel cell is an ion exchange
membrane (fluorinated sulfonic acid polymer or other similar polymers) that is an excellent proton
conductor. The only liquid in this fuel cell is water; thus, corrosion problems are minimal. Water
management in the membrane is critical for efficient performance; the fuel cell must operate under
conditions where the byproduct water does not evaporate faster than it is produced because the
membrane must be hydrated. Because of the limitation on the operating temperature imposed by
the polymer, usually less than 120° C, and because of problems with water balance, an H2-rich gas
with minimal or no CO (a poison at low temperature) is used. Higher catalysts loading (Pt in
most cases) than those used in PAFCs is required in both the anode and cathode.
Alkaline Fuel Cell (AFC): The electrolyte in this fuel cell is concentrated (85 wt%) KOH in fuel
cells operated at high temperature (~250° C), or less concentrated (35-50 wt%) KOH for lower
temperature (<120° C) operation. The electrolyte is retained in a matrix (usually asbestos), and a
wide range of electrocatalysts can be used (e.g., Ni, Ag, metal oxides, spinels, and noble metals).
The fuel supply is limited to non-reactive constituents except for hydrogen. CO is a poison, and
CO2 will react with the KOH to form K2CO3, thus altering the electrolyte. Even the small amount of CO2 in air must be considered with the alkaline cell.
Phosphoric Acid Fuel Cell (PAFC): Concentrated to 100% phosphoric acid is used for the
electrolyte in this fuel cell, which operates at 150 to 220° C. At lower temperatures, phosphoric
acid is a poor ionic conductor, and CO poisoning of the Pt electrocatalyst in the anode becomes
severe. The relative stability of concentrated phosphoric acid is high compared to other common
acids; consequently the PAFC is capable of operating at the high end of the acid temperature
range (100 to 220° C). In addition, the use of concentrated acid (100%) minimizes the water
vapor pressure so water management in the cell is not difficult. The matrix universally used to
retain the acid is silicon carbide (1), and the electrocatalyst in both the anode and cathode is Pt.
Molten Carbonate Fuel Cell (MCFC): The electrolyte in this fuel cell is usually a combination
of alkali carbonates or combination (Na and K), which is retained in a ceramic matrix of LiAlO2.
The fuel cell operates at 600 to 700° C where the alkali carbonates form a highly conductive
molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures
in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to promote reaction. Noble
metals are not required.
Solid Oxide Fuel Cell (SOFC): The electrolyte in this fuel cell is a solid, nonporous metal oxide,
usually Y2O3-stabilized ZrO2. The cell operates at 650 to 1000° C where ionic conduction by
oxygen ions takes place. Typically, the anode is Co-ZrO2 or Ni-ZrO2 cermet, and the cathode is
Sr-doped LaMnO3.