Two common causes of “polarization” are activation and concentration effects. They are different limits on reaction rate. In CP work, confusing them leads to bad interpretation of potentials and current response.
Activation polarization (surface reaction limited)
- Limitation is the charge-transfer step at the metal/electrolyte interface.
- Think: the surface reaction itself is “slow,” even if reactants are available.
- Often shows a Tafel-type relationship: small potential changes can strongly change current.
Concentration polarization (mass transport limited)
- Limitation is getting reactants to the surface (or removing products).
- Think: the reaction would be fast, but the surface region runs low on reactant (e.g., oxygen).
- Can lead to a limiting current behavior: adding more driving force doesn’t increase current much because reactant supply is the bottleneck.
Simple “field” intuition
- If response seems capped by environment (oxygen availability, stagnant electrolyte), suspect concentration effects.
- If response is strongly tied to surface kinetics/material condition, suspect activation effects.