If a cell's limiting current is i_L = nFADCC/δ and both A and δ are doubled, what happens to i_L?

• A. Doubles
• B. Halves
• C. Remains the same
• D. Quadruples

Answer: (D)

Limiting current in a diffusion-controlled cell scales with the ratio A/δ (specifically i_L ∝ A D C / δ, with n and F as constants). If you double the active area and also double the diffusion-layer thickness, the two effects cancel: i_L' = n F (2A) D C / (2δ) = n F A D C / δ = i_L. So the limiting current remains unchanged. If only one parameter changed, you’d see a different outcome (doubling A would double i_L; doubling δ would halve i_L).