Alkaline
fuel cells were one of the first modern fuel cells to be developed at
the beginning of the 1960's. They were the first type widely used in the
US
space programme to provide on-board electric power
for the Apollo space shuttle. The electrolyte is concentrated (85 wt%)
potassium hydroxide (KOH) in fuel cells operating at high temperatures (above
250°C), and less concentrated (35-50 wt%) KOH for lower temperature
operation. Recently developed systems can run at temperatures below
100°C.
Oxygen reduction in alkaline environments is
much faster than in acidic conditions, as in the PEMFC. Thus,
it is possible to use a wide range of cathode electrocatalysts, e.g., Ni, Ag, metal
oxides, spinels, in addition to noble metals. Inexpensive catalyst materials
such as Raney Nickel or Raney Silver
were investigated in the early stages of the AFC's history. The electrode
reactions are as shown below:
Anode
oxidation of hydrogen:
H2
+ 2OH-
2H2O + 2e-
Cathode
reduction of oxygen:
½O2
+ H2O + 2e-
2OH-
Total
AFC reaction:
H2 + ½O2
H2O
Instead
of protons as in PAFC and PEMFC, hydroxyl ions (OH-) are the conductive ions in
AFC. The fuel supply is limited to very pure hydrogen and oxygen. Carbon
monoxide is a catalyst poison, and carbon dioxide will react with the
electrolyte (KOH) to form K2CO3 (solid salts), thus altering the
electrolyte. Even small amounts of CO2 in the air are damaging to the
alkaline cell. The carbonates are formed according to the following
reaction equation:
CO2
+ 2OH-
(CO3)2- + H2O
In the first space AFC systems,
the KOH electrolyte was retained in a matrix. Further development led to a
circulating electrolyte, which provides a better barrier against gas
leakage, increases the lifetime of the electrolyte and the liquid can also
be used to cool the stack. Desirable attributes of the AFC include its
excellent performance with hydrogen and oxygen due to its active cathode
kinetics. In space applications efficiencies up to 60% have been reached.
This is much higher than has been possible with other fuel cell types.
Karl
Kordesch AFC car (hybride).
Karl
Kordesch AFC car inside.
Recently
the interest in AFC has dropped considerably. Practical problems with
cost-effective removal of CO2 in the cathode air feed has limited the use
of AFC systems to special applications like spacecraft and submarines. A
British company, Zetek, focussed on AFC in automobile applications for
some time, see pictures below. In 2002 the activity was stopped due to
lack of funding.