Page 30 - Introduction to Transfer Phenomena in PEM Fuel Cells

Introduction to Hydrogen Technology 19
1.2.1.3.3. Aging
In the AFC cell, and in a closed cycle (without circulation), the
electrolyte is diluted following the incomplete removal of the formed water.
The electrolyte used is corrosive and it can attack the components with
which it is in contact.
1.2.1.4. Phosphoric acid fuel cell (PAFC)
The phosphoric acid fuel cell (PAFC) is one of the few commercially
available batteries. Hundreds of these batteries have been installed around
the world. Most of these provide power ranging from 50 to 200 KW, and
also 1 MW and 5 MW power plants have been built. The largest plant
installed to date provides 11 MW of alternating current (AC), corresponding
to a distribution network [ROS 99].

PAFCs have power generation efficiencies of 40%. Their operating
temperature is between 150 and 300°C. PAFCs are poor ion conductors at
low temperatures, and carbon monoxide (CO) tends to severely poison
platinum in the catalyst [HIR 98]. The chemical reactions that occur in
PAFCs are:

– anode:
+
H (g) ⎯⎯→ 2H (aq) 2e − [1.7]
+
2
– cathode:
1 O (g) 2H (aq) 2e ⎯⎯→ H O(liq) [1.8]
−
+
+
+
2 2 2
– overall reaction:
1 O(g) H (g) CO ⎯⎯→ H O(liq) CO [1.9]
+
+
+
2 2 2 2 2 2
Figure 1.8 shows the operating principle of a PAFC.
The operating temperature is between 180°C and 210°C. At low
temperatures, the electrolyte is not a good conductor and solidifies around
40°C. PAFC battery technology is the most mature in terms of development
and commercialization. Indeed, stationary installations, up to 50 MW, have
been set up. About 200 test facilities have been in operation worldwide, the