Ion Selectivity of Intercalation Electrodes in Dual-Ion Electrolytes: Modeling Ion Selectivity as the Sum of Individual Cation Effects

Intercalation electrode materials reversibly insert cations into their lattices under applied potentials or currents, which can be used to perform electrochemical separations. Optimizing performance, however, remains challenging due to tradeoffs between selectivity and separation rate being influenced by multiple variables. This study developed a quantitative model to describe the current response and cation selectivity of an intercalation electrode in a binary cation solution during cyclic voltammetry. We hypothesized that current responses and selectivity could be calculated by summing individual ion contributions. Cyclic voltammograms were experimentally measured using nickel hexacyanoferrate electrodes in NaCl, KCl, or mixed solutions. Post-mortem electron probe micro-analysis quantified intercalated Na+ and K+ fractions. A one-dimensional finite element model incorporating the Nernst-Frumkin isotherm, Butler-Volmer kinetics, and ion diffusion was developed, parameterized with pure NaCl or KCl solutions, and validated against mixed solutions. The model accurately reproduced experimental cyclic voltammograms and ion partitioning behaviors at ionic strengths ⩾0.2 mol·l⁻¹ . However, at lower ionic strengths, significant discrepancies arose for reasons still unclear. Results indicate that modeling ion contributions individually effectively captures the electrochemical response of selective intercalation electrodes at sufficiently high ionic strengths.