Under what condition does the limiting current i_L occur?

• A. Mass transport becomes rate-limiting
• B. Activation energy is minimized
• C. Temperature increases
• D. Potential equals equilibrium

Answer: (A)

At the heart of a limiting current is mass transport. The limiting current i_L appears when the rate at which reactant can be delivered to the electrode surface is the bottleneck, not how fast the surface reaction itself can proceed. When you push the potential far enough, the surface reaction would want to go faster, but the supply of reactant to the surface can’t keep up, so the current levels off at i_L. This diffusion (and possibly convection or migration under different conditions) sets the plateau value, independent of further increases in driving force.

Activation energy being small would speed the surface reaction, which can help if transport isn’t limiting, but it doesn’t create a plateau by itself. Raising temperature can increase both diffusion and kinetics, shifting i_L somewhat, but again the defining condition for i_L is that mass transport is the limiting step. At equilibrium potential, there is no net current, so a limiting current isn’t established.