Page 27 - Introduction to Transfer Phenomena in PEM Fuel Cells

16 Introduction to Transfer Phenomena in PEM Fuel Cells
However, there are some disadvantages:
– high membrane cost;
– the membranes must be permanently maintained in a good state of
hydration to promote the transport of protons. Otherwise, there is a risk of
damage to the membrane;
– they are sensitive to carbon monoxide which poisons the catalytic sites.

1.2.1.2.3. Aging
In a PEMFC cell, components are subject to chemical or mechanical
aging, the main causes of which are:
– degradation of the membrane under the effect of temperature;
– loss of catalytic activity;

– heterogeneity of the materials used;
– moisture of the membrane not perfectly controlled.

1.2.1.3. Alkaline fuel cell (AFC)
Alkaline fuel cells (AFCs) have been used by NASA during space
missions; they can achieve power generation efficiencies of up to 70%. The
operating temperature of these batteries varies between 150 and 200°C. An
aqueous alkali potassium hydroxide solution embedded in a matrix serves as
an electrolyte. This is an advantageous configuration because the reaction at
the cathode is fast in an alkaline electrolyte, which allows a better
performance. Several manufacturers are examining ways to reduce the cost
and improve the operational flexibility of these fuel cells. Alkaline fuel cells
typically have powers of 300 watts to 5 kW [SPI 07]. The chemical reactions
that take place in an alkaline fuel cell are:
– anode:

−
2H (g) 4OH (aq) ⎯⎯→ 4H O(liq) 4e − [1.4]
+
+
2
2
– cathode:
−
−
+
+
O (g) 2H O(liq) 4e ⎯⎯→ 4OH (aq) [1.5]
2
2