Non-uniform ion adsorption of the scaled-up electrode in ESIX system: Numerical analysis

Abstract

Electrochemically switched ion exchange (ESIX) technology is an efficient and environmentally friendly method for extracting lithium ions from salt lakes. The non-uniformity of ion adsorption is a potential problem that needs to be addressed during the scaled-up ESIX devices, yet there are few relevant reports so far. In this study, a three-dimensional model for extracting lithium ions from solution in the ESIX system was established. The Butler-Volmer equation with the introduction of the electroactive site concentration (ESC) was used to describe the electrochemical adsorption reaction of Li⁺ by the electroactive material coating, and the Nernst-Planck equation was used to describe the ion transport in the solution. Using this model, it was discovered that there is a non-uniform phenomenon in the lithium adsorption capacity during the electrode scaling-up process. It was found that the degree of non-uniformity of lithium ion adsorption first increases and then decreases with the adsorption time. Moreover, the larger the scaling-up ratio, the higher the degree of non-uniformity. By analyzing the temporal and spatial distributions of the working voltage and current density, it was found that the non-steady-state and spatial non-uniformity of the current density were the direct causes of the non-uniform adsorption of lithium ions. An equivalent circuit diagram of the ESIX system was established, and the cause of non-uniform adsorption was explained from the perspective of the impedance characteristics of the system. Finally, two measures to optimize the system impedance and improve the adsorption uniformity were proposed.