Theory of Faradaically Modulated Redox Active Electrodes for Electrochemically Mediated Selective Adsorption Processes

Abstract

Electrochemically mediated selective adsorption is an emerging electrosorption technique that utilizes Faradaically enhanced redox active electrodes, which can adsorb ions not only electrostatically, but also electrochemically. The superb selectivity (>100) of this technique enables selective removal of toxic or high-value target ions under low energy consumption. Here, we develop a general theoretical framework to describe the competitive electrosorption phenomena involving multiple ions and surface-bound redox species. The model couples diffusion, convection and electromigration with competitive surface adsorption reaction kinetics, consistently derived from non-equilibrium thermodynamics. To optimize the selective removal of the target ions, design criteria were derived analytically from physically relevant dimensionless groups and time scales, where the propagation of the target anion’s concentration front is the limiting step. Detailed computational studies are reported for three case studies that cover a wide range of inlet concentration ratios between the competing ions. And in all three cases, target anions in the electrosorption cell forms a self-sharpening reaction-diffusion wave front. Based on the model, a three-step stop-flow operation scheme with a pure stripping solution of target anions is proposed that optimizes the ion adsorption performance and increases the purity of the regeneration stream to almost 100%, which is beneficial for downstream processing.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

“…This same framework was adapted to generate an equivalent circuit model to predict and validate the electroanalytical performance of intercalation materials . He et al further expanded the theory of electrosorption by redox-active materials by developing equivalent circuit models and a model based on coupled diffusion, convection, and electromigration with surface reaction kinetics. , Singh et al also presented a theory for CDI with porous electrodes comprising nanoparticles made of a redox-active intercalation material, and the authors described the dynamics of this system in terms of concentration of the product, distribution of intercalated ions, cell potential, and energy consumption . These models, however, neglect variations in ion concentration inside the particles and assume fast reactions with quasi-equilibrium adsorption isotherms.…”

Comparison of Ion Selectivity in Electrodialysis and Capacitive Deionization

“…Here, we will focus theoretically on Ca 2+ vs Na + , which in Figure showed major differences in achieved selectivity factor between the technology classes. Although important, theoretically analyzing monovalent–monovalent ion pairs, ,, or selectivity of CDI with intercalation or redox active electrodes − are outside the scope of this short review. Here, we develop theoretical expressions for of an ED membrane pair (Figure b) and a CDI cell consisting of porous activated carbon electrodes (Figure c) with macropores (Figure e) and micropores (Figure f).…”