200 Fuel Cells
charge transfer occurs. In general, depletion of the reactant affects the fuel
cell performance by increasing the cell overpotential in two ways: (i) accord-
ing to the Nernst equation, the fuel cell reversible potential decreases when
the reactant concentration at the interface is lower than the bulk concentra-
tion; (ii) the reaction activation loss increases with decrease in the reactant
concentration at the interface relative to the bulk concentration.
Convection refers to the transport of the reactant or product species by bulk
uid motion driven by natural or applied mechanical forces. The natural con-
vection limitations are due to convective transport caused by differences in
densities as a result of temperature or concentra ...