Page 323 - Physical chemistry understanding our chemical world

290 ELECTROCHEMISTRY

metal surrounding the electrolyte, and enter into the external circuit where they per-
form work. The platinum also catalyses the dissociation of diatomic H 2 gas to form
reactive H atoms.
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Once formed, the protons diffuse through the platinum layer and
The membrane, made enter deep into the layer of semi-permeable membrane. They travel
from a perfluorinated from the left-hand side of the membrane to its right extremity in
polymer bearing sul- response to a gradient in concentration. (Movement caused by a
phonic acid groups, is
concentration gradient will remind us of dye diffusing through a
known in the trade as saucer of water, as described on p. 129.)
Nafion TM .
At the positive side of the cell (drawn here on the right), oxygen
gas is reduced to oxide ions, according to
Note: the electron
−
count in Equations O 2(g) + 4e (Pt) −−→ 2O 2− (aq) (7.14)
(7.13) and (7.14)
should balance in The electrons necessary to effect the reduction of gaseous oxy-
reality gen come from the external circuit, and enter the oxygen half-cell
through the layer of platinum coating the cathode, thereby explain-
ing why the electrons in Equation (7.14) are subscripted with ‘Pt’.
The O 2− ions combine chemically with protons that have traversed the Nafion
membrane and form water, which collects at the foot of the cell.
Because the overall cell reaction is exothermic, the value of the cell emf decreases
with increasing temperature, so the temperature is generally kept relatively low at
◦
about 200 C. The cell emf is 1.23 V at this temperature.
One of the main advantages of this hydrogen fuel cell is the rapid
The rate of oxygen rate at which hydrogen is oxidized at the platinum surface. Most
reduction can be accel- of the cell’s operational difficulties relate to the oxygen side of the
erated by finely divid- device. Firstly, the reduction of gaseous oxygen in Equation (7.14)
ing the platinum cata- is relatively slow, so the rate at which the cell operates is somewhat
lyst, thereby increasing limited. But more serious is the way the cell requires a continual
its effective area. flow of gas, as below. The hydrogen half-cell comprises an elec-
trode couple because two redox states of the same material coexist
+
there (H 2 and H ). In a similar way, the oxygen half-cell also comprises an electrode
2−
couple, but this time of O 2 and O .
If the flow of oxygen falters, e.g. when the surface of the cathode is covered with
water, then no gaseous O 2 can reach the platinum outer layer. In response, firstly no
electrons are consumed to yield oxide ions, and secondly the right-hand side of the
cell ‘floods’ with the excess protons that have traversed the polymer membrane and
2−
not yet reacted with O . Furthermore, without the reduction of oxygen, there is no
redox couple at the cathode. The fuel cell ceases to operate, and can produce no more
electrical energy.
This simple example helps explain why a cell requires no fewer than two half-
cells. A half-cell on its own cannot exchange electrons, and cannot truly be termed
a cell.