The activation ηact and the concentration ηconc overpotentials are electrode phenomena and
are respectively associated with the energy required to initiate a charge transfer and caused
by concentration differences between the bulk solution and the electrode surface; in addition,
the ohmic ηohm and ionic ηionic losses also alter the stack voltage. The ohmic losses ηohm
occur in the electrodes, the bipolar plates and the collector plates and the ionic losses ηionic
occur in the electrolytes and the membranes. But these overpotentials are seldom found in the
literature and often applicable only to peculiar conditions. Therefore, an equivalent resistance
is introduced instead: