Over-limiting currents and deionization "shocks" in current-induced polarization: local-equilibrium analysis

Area of Science:

• Electrochemistry
• Physical Chemistry
• Materials Science

Background:

• Concentration polarization at interfaces of ion-exchange media is crucial for electrochemical systems.
• Understanding "leaky" (non-ideally perm-selective) media is essential for accurate modeling.
• Previous studies focused on deionization shock propagation dynamics.

Purpose of the Study:

• To theoretically investigate current-induced concentration polarization dynamics in leaky ion-exchange media.
• To analyze the behavior under galvanostatic conditions with negligible volume flow.
• To provide model-independent results and explore novel features of deionization shocks.

Main Methods:

• Analysis based on local thermodynamic equilibrium and local electric neutrality.
• Derivation of model-independent results for stationary and transient states.
• Numerical solution of a 1D partial differential equation (PDE) using a fine-pore model.

Main Results:

• Model-independent results obtained in quadratures for stationary state and approximate scaling-form for transient response.
• Formulation of results in terms of phenomenological properties: ion transport numbers, salt diffusion permeability, and chemical capacity.
• Identification of deionization shock patterns, revealing pronounced effects at intermediate charge concentration to salt concentration ratios and high voltages.

Conclusions:

• The study confirms propagating deionization shocks but highlights their dependence on specific conditions.
• Novel features of voltage drop and interface salt concentration evolution are detailed.
• The applicability of current models is limited to early stages at high steady-state voltages.