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INTRODUCTION TO CELLS: TERMINOLOGY AND BACKGROUND 289

Why do hydrogen fuel cells sometimes ‘dry up’?

Cells and half-cells

Hydrogen fuel cells promise to fuel prototype cars in the near future. We define such
a fuel cell as a machine for utilizing the energies of hydrogen and oxygen gases,
hitherto separated, to yield a usable electric current without combustion or explosion.
Unlike the simple batteries above, the oxygen and hydrogen gases fuelling these cells
are transported from large, high-pressure tanks outside the cell. The gases then feed
through separate pipes onto the opposing sides of a semi-permeable membrane (see
Figure 7.3), the two sides of which are coated with a thin layer of platinum metal,
and represent the anode and cathode of the fuel cell. This membrane helps explain
why such cells are often called PEM fuel cells, where the acronym stands for ‘proton
exchange membrane’.
When it reaches the polymer membrane, hydrogen gas is
oxidized at the negative side of the cell (drawn on the left of Figure The energy necessary
7.3), forming protons according to to cleave the H–H bond
is provided by the
+ − energy liberated when
H 2(g) −−→ 2H + 2e (Pt) (7.13)
forming the two H–Pt
The subscripted ‘Pt’ helps emphasize how the two electrons con- bonds after molecular
duct away from the membrane through the thin layer of platinum dissociation.

Load

− +

Hydrogen gas Oxygen gas

Permeable
polymer
membrane
Water

Thin electrodes
(layers of platinum)

Figure 7.3 A hydrogen–oxygen fuel cell. The water formed at the cathode on the right-hand side
of the cell condenses and collects at the bottom of the cell, and drains through a channel at the
bottom right-hand side