[Paper Review] Equivalent circuit model for electrosorption with redox active materials

Fan He, Martin Z. Bazant+1 more|arXiv (Cornell University)|Dec 31, 2020

Membrane-based Ion Separation Techniques58 references4 citations

TL;DR

This paper proposes a physics-based equivalent circuit model to describe electrosorption in redox-active materials, integrating charge transfer and ion transport dynamics. It accurately fits cyclic voltammetry data, distinguishes between transport- and reaction-limited regimes, and extracts intrinsic material properties for use in scalable electrosorption process modeling under low electrolyte conditions.

ABSTRACT

Electrosorption is a promising technique for brackish water deionization and waste water remediation. Faradaic materials with redox activity have recently been shown to enhance both the adsorption capacity and the selectivity of electrosorption processes. Development of the theory of electrosorption with redox active materials can provide a fundamental understanding of the electrosorption mechanism and a means to extract material properties from small-scale experiments for process optimization and scale-up. Here, we present an intuitive, physics-based equivalent circuit model to describe the electrosorption performance of redox active materials, which is able to accurately fit experimental cyclic voltammetry measurements. The model can serve as an efficient and easy-to-implement tool to evaluate properties of redox active materials and help to distinguish between the transport-limited and reaction-limited regimes in electrosorption processes. And the extracted intrinsic material properties can be further incorporated into process models under lower supporting electrolyte concentrations for realistic electrosorption applications.

Motivation & Objective

• To develop a fundamental theoretical framework for electrosorption using redox-active materials.
• To enable accurate interpretation of cyclic voltammetry data in electrosorption systems.
• To distinguish between transport-limited and reaction-limited charge transfer mechanisms.
• To extract intrinsic material properties from small-scale experiments for process modeling.
• To support scale-up of electrosorption technologies by providing transferable material parameters.

Proposed method

• The model constructs an equivalent electrical circuit combining a constant phase element (CPE) for the double layer and a Warburg element for diffusion-controlled ion transport.
• It incorporates a Faradaic charge transfer process via a constant phase element in parallel with a resistor, representing redox reactions.
• The circuit is calibrated against experimental cyclic voltammetry data using nonlinear least squares fitting.
• The model accounts for the influence of supporting electrolyte concentration on ion transport and charge transfer kinetics.
• It enables decomposition of the total current into capacitive and faradaic contributions through circuit component analysis.
• The model is validated across varying scan rates and concentrations to assess regime transitions.

Experimental results

Research questions

• RQ1 How can the electrosorption behavior of redox-active materials be accurately modeled using an equivalent circuit framework?
• RQ2 What are the relative contributions of charge transfer and ion diffusion to the overall current response in cyclic voltammetry?
• RQ3 How can the model distinguish between transport-limited and reaction-limited regimes in electrosorption?
• RQ4 What intrinsic material properties can be extracted from small-scale electrochemical measurements?
• RQ5 How can the extracted parameters be used to inform larger-scale electrosorption process models?

Key findings

• The equivalent circuit model accurately reproduces experimental cyclic voltammetry data across a range of scan rates and electrolyte concentrations.
• The model successfully separates capacitive and faradaic current contributions, enabling quantification of redox activity and ion transport.
• The analysis reveals distinct transitions between diffusion-controlled and charge-transfer-controlled regimes depending on scan rate and concentration.
• Intrinsic material parameters such as charge transfer rate constants and diffusion coefficients are reliably extracted from the fitting process.
• The model's predictions remain consistent under low supporting electrolyte conditions, relevant for real-world brackish water applications.
• The framework provides a scalable pathway to infer material properties from minimal experimental data, supporting process optimization.