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

TL;DR

This paper develops a comprehensive theoretical model for Faradaically modulated redox active electrodes, enabling highly selective ion removal with optimized operation schemes, advancing electrochemical separation technologies.

Contribution

It introduces a general thermodynamics-based framework for competitive electrosorption involving multiple ions and surface redox species, including design criteria and operational strategies.

Findings

Target anions form self-sharpening reaction-diffusion wave fronts.

Proposed stop-flow scheme achieves near 100% purity in regeneration.

Model applies across various inlet ion concentration ratios.

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 anions 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.