Page 290 - Biofuels Refining and Performance

Fuel Cells 269

Electrolyte. The PAFC uses 100% concentrated phosphoric acid (H PO )
3
4
as an electrolyte. The electrolyte assembly is a 0.1- to 0.2-mm-thick matrix
made of silicon carbide particles held together with a small amount of
PTFE. The pores of the matrix retain the electrolyte (phosphoric acid)
by capillary action. At lower temperatures, H PO is a poor ionic con-
4
3
ductor and CO poisoning of the Pt electrocatalyst in the anode can
PO over long periods,
become severe. There will be some loss of H 3 4
depending upon the operating conditions. Hence, as a general rule, suf-
ficient acid reserve is kept in the matrix at the beginning.
Electrode. The PAFC (similar to a PEMFC) uses gas diffusion electrodes.
Platinum or platinum alloys are used as the catalyst at both electrodes.
In the mid-1960s, the conventional porous electrodes were PTFE-bonded
2
Pt black, and the loadings of Pt were about 9 mg/cm . In recent years,
Pt supported on carbon black has replaced Pt black in porous PTFE-
bonded electrode structures. Pt loading has also dramatically reduced
2 2
to about 0.25 mg Pt/cm in the anode and about 0.50 mg Pt/cm in the
cathode. The porous electrodes used in a PAFC consist of a mixture of
the electrocatalyst supported on carbon black and a polymeric binder to
bind the carbon black particles together to form an integral structure.
A porous carbon paper substrate provides structural support for the
electrocatalyst layer and also acts as the current collector. The com-
posite structure consisting of a carbon black/binder layer onto the carbon
paper substrate forms a three-phase interface, with the electrolyte on
one side and the reactant gases on the other side of the carbon paper.
The stack consists of a repeating arrangement of a bipolar plate, the
anode, electrolyte matrix, and cathode.

Hardware. A bipolar plate separates the individual cells and electri-
cally connects them in a series in a fuel cell stack. A bipolar plate has a
multifunction design; it has to separate the reactant gases in the adja-
cent cells in the stack, so it must be impermeable to reactant gases; it
must transmit electrons to the next cell (series connection), so it has to
be electrically conducting; and it must be heat conducting for proper heat
transfer and thermal management of the fuel cell stack. In some designs,
gas channels are also provided on the bipolar plates to feed reactant
gases to the porous electrodes and to remove the reaction products.
Bipolar plates should have very low porosity so as to minimize phosphoric
acid absorption. These plates must be stable and corrosion-resistant in
the PAFC environment. Bipolar plates are usually made of graphite–
resin mixtures that are carbonized and heat treated to 2700 C to
increase corrosion resistance. For 100-kW and larger power generation
systems, water cooling has to be used and cooling channels are provided
in the bipolar plates to cool the stack.