
292 Fuel Cells
Figure7.4 shows an initial nonuniform distribution of element i in a medium
of j. Atoms of species i diffuse from the region of high concentration to the
region of low concentration and establish a more uniform concentration dis-
tribution of the species. Self-diffusion also takes place in a relatively pure
crystalline solid material controlled by a process known as vacancy mecha-
nism or the hopping process. The ion transport in crystalline electrolyte is con-
trolled by this vacancy diffusion or hopping diffusion mechanism. In this
(a)
(b)
Diffusion
E
a
E
o
FIGURE 7.4
Solid-state self-diffusion by hopping or vacancy mechanism.