Question

how does diffusion, migration, and convention affect the concentration polarization of a redox molecule and current density?

Answer 1

Diffusion it is the motion of a species ubder the influence of chemical potential gradient.Flux at the electrode may be written as: J = - D(dC/dx)x=0 (35) D is the diffusion coefficient of the reactant.The Nernst diffusion layer Thin layer that lies intermediate bounds the electrodes and the bulk)A thin layer of electrolyte (thickness δ) in contact with the electrode surface - ion transfer controlled solely by diffusion. • Concentration of species - maintained constant outside this layer (in the “bulk”) by convective transfer.

Assume - linear conc. gradient in the Nernst diffusion layer • Eqn. 35 can be rewritten as:

J = [D/δ][C* -C(x=0)] (36) δ - thickness of the Nernst diffusion layer C* - bulk concentration of the reactant

The mass transfer coefficient • Difficult to obtain precisely values of δ • Therefore δ is grouped along with D to yield a new constant

k m = D/ δ (37) k m is called the mass transfer coefficient (units: cm/s)