What is a diffusion layer in electrochemical cells and why is its thickness important?

• A. The region near the electrode where concentration gradients exist; thickness controls mass transport and hence the limiting current and response time.
• B. A thick insulating layer that prevents charge transfer.
• C. A layer where the solution is completely mixed with no gradients.
• D. A physical boundary that separates electrolyte from the electrode.

Answer: (A)

The diffusion layer is the region of the solution right next to the electrode where concentrations of reacting species vary with distance from the surface because the electrode is consuming or producing them. Its thickness matters because it sets how far the surface-initiated concentration changes extend into the bulk and, therefore, how easily species can be transported to or away from the electrode by diffusion. In diffusion-limited conditions, the current depends on the rate of this diffusion flux, which is inversely related to the layer thickness. A thicker diffusion layer means slower mass transport, a smaller limiting current, and a longer time for the system to respond to changes in potential. A thinner layer enhances transport, increases the limiting current, and yields a faster response.