Nernstlagen

Nernstlagen, or the Nernst layer, is a term used in electrochemistry to describe the thin region of solution immediately adjacent to an electrode where the concentrations of reactive species vary from their values at the electrode surface to those in the bulk solution. The concept is named after Walther Nernst and is primarily a mass-transport phenomenon, distinguishing it from the electrical double layer that forms at the electrode surface. The Nernst layer represents the zone where diffusion (and, if present, convection) controls the transport of species to and from the electrode.

In a steady-state situation, the Nernstlagen has a finite thickness δ that characterizes the distance over which the concentration changes from the surface value Cs to the bulk value Cb. The net current is governed by the diffusive flux across this layer, J ≈ -D(dC/dx), which can be approximated as J ≈ D(Cb − Cs)/δ for a planar interface. If the electrode consumes species almost completely, Cs ≈ 0 and the current scales as I ≈ nFAD(Cb)/δ, illustrating how δ sets the mass-transport limitation.

The thickness and properties of the Nernstlagen depend on hydrodynamics and geometry. Stirring, quiescent conditions, or rotation (as in a rotating disk electrode) modify δ, and thus the limiting current. The Nernstlagen is often discussed in contrast with the nanoscale electrical double layer near the surface; together they describe the combined chemical and electrostatic environment that governs electrode kinetics.

Applications include analyzing diffusion-controlled currents in chronoamperometry and interpreting measurements under controlled mass transport. See also diffusion layer, Levich equation, and electrical double layer for related concepts.