Polarization (electrochemistry)

In electrochemistry, polarization is a collective term for certain mechanical side-effects (of an electrochemical process) by which isolating barriers develop at the interface between electrode and electrolyte. These side-effects influence the reaction mechanisms, as well as the chemical kinetics of corrosion and metal deposition. In a reaction we can displace the bonding electrons by attacking reagents. The electronic displacement in turn may be due to certain effects, some of which are permanent (inductive and mesomeric effects), and the others are temporary (electromeric effect). Those effects which are permanently operating in the molecule are known as polarization effects, and those effects which are brought into play by attacking reagent (and as the attacking reagent is removed, the electronic displacement disappears) are known as polarisability effects. The term 'polarization' derives from the early 19th-century discovery that electrolysis causes the elements in an electrolyte to be attracted towards one or the other pole i.e. the gasses were polarized towards the electrodes. Thus, initially 'polarization' was essentially a description of electrolysis itself, and in the context of electrochemical cells used to describe the effects on the electrolyte (which was then called "polarization liquid"). In time, as more electrochemical processes were invented, the term 'polarization' evolved to denote any (potentially undesirable) mechanical side-effects that occur at the interface between electrolyte and electrodes. These mechanical side-effects are: activation polarization: the accumulation of gasses (or other non-reagent products) at the interface between electrode and electrolyte. concentration polarization: uneven depletion of reagents in the electrolyte cause concentration gradients in boundary layers. Both effects isolate the electrode from the electrolyte, impeding reaction and charge transfer between the two. The immediate consequences of these barriers are: the reduction potential decreases, the reaction rate slows and eventually halts.