Particle Migration and Diffusion

The electrode is the final boundary for ionic migration. Here, some of the ions started, and here they meet a physical hindrance. The metal of an anode may furnish metal ions to the solution, crossing the interphase. Depletion or accumulation of matter and charge may occur at the electrodes, and chemical reactions may also occur. An electrode may also exchange electrons with neutral species (e.g., the reduction of dissolved oxygen at a cathode). The transport of neutral species in the hulk of the electrolyte is not by ionic migration but rather a diffusion transport process caused by a 

Interface a surface that forms the boundary between two materials (sharp transition). Interphase diffuse transition zone between two phases (e.g., solid/Uquid). 

Molecular diffusion is described by Pick s laws. Consider a simple system in the form of a compartment with unity width and height dimensions and a concentration gradient in the infinite length x direction. Pick s first law is 

Warburg solved this equation already in 1899, finding the concentration waves into the electrolyte at a distance from an AC-polarized electrode surface (cf. Section 7.9.3). 

Here is another example. Consider a diffusion process starting in a specified compartment with an infinitely thin (x-direction) band of solute, S [mole per unit width y and depth z], released in the middle of the compartment at x = 0 and t = 0. By solving Pick s second law under these boundary conditions, the concentration as a function of position x and time t is  

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