During an electrochemical reaction, reactants need to be transported from the bulk solution to the catalytic surface, while products need to be removed from the electrode surface to the bulk solution. If the rate of this mass transport is slower than the rate of the electrochemical reaction, a concentration gradient develops. This gradient causes a difference in concentration between the bulk and the electrode surface, which in turn results in a concentration overpotential. This is particularly significant in reactions involving gas evolution or highly reactive intermediates.
