New Variant of Electrochemical Intercalation Isotherm: Analysis of Instability Region Dependence on Electrolyte Concentration

Abstract

Electroactive redox materials, encompassing redox polymers, metal−organic frameworks, and intercalation materials, play an important role in a wide range of emerging applications. However, many theoretical issues concerning quantitative thermodynamic description of such materials remain unclear so far. Existing variants of electrochemical intercalation isotherms (namely, the Langmuir−Frumkin isotherm, which often applies to redox polymers and electroactive solid materials, and the reformulated isotherm, accounting for the phase character of the potential distribution within the material) do not explain the dependence of redox materials' voltammetric responses on the electrolyte concentration. In addition to that, studies show that the reformulated isotherm has a higher value of the so-called shortrange interaction parameter at which phase separation starts and asymmetric single-phase regions in comparison to the Langmuir− Frumkin isotherm. A simple modification, accounting for the possibility of both positive and negative ions of the electrolyte solution to intercalate the electroactive redox material, leads to a new variant of electrochemical intercalation isotherm, which includes Langmuir−Frumkin and reformulated isotherms as limiting cases. The new model explains the dependence of redox materials' voltammetric responses on the electrolyte concentration. Theory predicts the dependence of short-range interaction parameters at which phase separation starts on the electrolyte composition. Thus, the new electrochemical intercalation isotherm shows the dependence of its unstable, metastable, and stable regions on the electrolyte concentration and ion distribution coefficients. It opens up a possibility to tune material characteristics such as sizes of single-phase/two-phase regions. In some particular cases, it is possible to induce or eliminate the phase transition by changing the concentration of the electrolyte. We believe that studying such properties may help in future realization of new smart materials with superior characteristics for electrochemical energy storage, use in chemical sensors, medical applications, and so forth.

Thermodynamic model for voltammetric responses in conducting redox polymers

“…Many redox polymers demonstrate narrow voltammetric peaks, which can be attributed to interspecies interaction. 90,91 The hybrid conducting redox polymers considered in this work are no exception: from Fig. 2 one can see that mid-widths of all TEMPO-related CV peaks are less than 130 mV, which strictly points to attractive short-range interactions between redoxcenters (details are given below in the chapter ''Approximation of experimental CV curves of conducting redox polymer films with the derived model'').…”

“…This kind of approach has been widely used previously. 70,75,90,91 We should underline here that despite the fact that the capacity of electric double layers is considered to be negligibly small in comparison to the faradaic capacity of the bulk of the polymer material, the potential drop caused by EDL at both current collector/polymer film and polymer film/solution interfaces plays a substantial role in the developed model (see Fig. 1).…”

“…A further decrease in a 0 value (below À4) may lead to a phase transition during charge/discharge of the polymer material. 90 A phase transition introduces irreversibility into voltammetric responses and, thus, such effects are not in scope of this work, which focuses on quasi-equilibrium responses.…”

Open-Circuit Potential Drift in Intercalation Electrodes: Role of Charge Redistribution in a Prussian Blue Analog

“…4 are discussed in section S2 of the supplementary text. [36][37][38] Collectively, the potential drift profiles from Figs. 3b and 4 indicated that there were no significant contributions from electrode dissolution, faradaic processes, or other experimental anomalies.…”

Logistic Approximation to Ion-Intercalation Electrochemistry: Application to Potassium–Copper(II) Hexacyanoferrate in DMSO Electrolytes

“…Electrochemical ion intercalation plays an essential role in the study of batteries . Theoretical modeling of these processes has received considerable attention − in particular concerning their voltammetric monitoring. − The thermochemical aspects with separation of ion and electron transport contributions, − the influence of solvation in aqueous and nonaqueous alectrolytes, − and analysis of different electroinsertion regimes and phase transitions eventually accompanying it have been the focus of much of the recent research. , …”