Transitions and Instabilities in Imperfect Ion-Selective Membranes

Area of Science:

• Physical Chemistry
• Electrochemistry
• Membrane Science

Background:

• Ion-selective membranes are crucial in various electrochemical applications.
• Understanding current-voltage (I-V) characteristics, including limiting and overlimiting regimes, is essential for optimizing membrane performance.
• Fluid flow and membrane properties significantly impact ion transport phenomena.

Purpose of the Study:

• To numerically investigate current modes and transitions in ion-selective membranes under fluid flow.
• To analyze the influence of membrane properties (fixed charge density) and electrolyte concentration on current regimes.
• To explore the electrokinetic instability leading to overlimiting currents.

Main Methods:

• A three-layer composite model (electrolyte-membrane-electrolyte) was employed.
• The Nernst-Planck-Poisson-Stokes system for electrolytes and the Darcy-Brinkman approach for the membrane were utilized.
• Quasi-spectral methods with Chebyshev polynomials resolved thin Debye and Darcy layers; linear stability analysis and direct numerical simulations were performed.

Main Results:

• Imperfect membranes with high electrolyte concentrations can exhibit a direct transition from underlimiting to overlimiting currents, bypassing the limiting current.
• The transition to overlimiting currents is monotonic for low-concentration electrolytes and oscillatory for high-concentration electrolytes.
• Fluid velocities within the membrane, though small, significantly influence the nature and transition to overlimiting regimes.

Conclusions:

• A detailed map of bifurcations, transitions, and regimes was constructed based on fixed membrane charge and Darcy number.
• The study highlights the complex interplay between fluid flow, membrane properties, and electrolyte concentration in determining membrane electrochemistry.
• Findings provide critical insights for designing and optimizing ion-selective membranes for specific applications.