
48 Fuel Cells
eld strength E, and on conductor cross section A. In electrochemistry, for
current, a conductor dimension-independent parameter current density i
(unit: A/cm
2
) is used. The conductivity σ is expressed as the ratio of the cur-
rent I to the electric eld strength E
σ = I/E. (2.22)
The reciprocal of conductivity (1/conductivity) is resistivity (ρ). Thus, resis-
tivity is an index of difculty of ow of electric current.
σ = 1/ρ (2.23)
In metals, the electrons are the carrier of the electrical charge, and in an
electrolyte solution, the ions carry the electrical charge. In a NaCl electrolyte
solution, sodium ions (Na
+
) and chlori