Cation Co‐intercalation in Potassium Copper(II) Hexacyanoferrates

Abstract

The cation‐insertion solid state electrochemistry of a potassium copper(II) hexacyanoferrate in contact with LiClO4/DMSO, NaPF6/DMSO, and KPF6/DMSO electrolytes has been theoretically and experimentally studied using the voltammetry of immobilized particles methodology. Voltammetric data, combined with SEM/EDS analysis permit to determine a K0.876CuII1.328FeIII0.049[FeIII0.318FeII0.682(CN)6] stoichiometry for the synthesized solid. Separation of electronic and ionic contributions to Gibbs energy changes can be made based on cyclic voltammetric and open circuit potential measurements. These parameters can be combined to measure values of the Gibbs energy of cation‐independent electron transfer of 7.2±0.4 (K+), 7.1±0.5 (Na+) kJ mol−1, in close agreement with the expected independence of this parameter on the electrolyte cation. The reduction Fe(III) centers bound to cyano groups exhibit a cation‐dependent, essentially Nernstian character which can be described in terms of Na+ and K+ insertion/deinsertion while in the case of Li+ electrolytes there is significant co‐cation diffusion. Chronoamperometric data provide estimates of the diffusion coefficients of Na+, and K+ ions through the solid around 10−9 cm2 s−1.

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

“…On comparing this voltammetric pattern with that of Prussian blue, − one can attribute the couples R 1 /O 1 and R 2 /O 2 to the Fe­(III)/Fe­(II) interconversion associated with the oxidation of Fe­(II) centers bound to cyano groups and the reduction of the nonbonded Fe­(III) centers. As previously reported, both processes involve cation exchange between the solid and the electrolyte solution and proceed with a high degree of reversibility. The R 3 /O 3 couple can be assigned to the cation-assisted one-electron reduction of Cu­(II) centers.…”

“…Since the parent solid contains interchangeable K + ions, the most general situation in contact with a M + -containing electrolyte solution implies coinsertion and codeinsertion of K + and M + ions. Then, the electrochemical reduction processes can be represented as false{ normalK a Cu b II Fe c III [ Fe x III Fe ( 1 − x ) II ( CN ) 6 ] false} s + η normalM solv + + δ normalM solv + + ( δ + η ) normale − ⇌ false{ normalM δ normalK a Cu b II Fe c III [ Fe ( x − δ − η ) III Fe ( 1 − x + δ + η ) II ( CN ) 6 ] false} s …”

“…Since the parent solid contains interchangeable K + ions, the most general situation in contact with a M + -containing electrolyte solution implies coinsertion and codeinsertion of K + and M + ions. Then, the electrochemical reduction processes can be represented as respectively.…”

“…The model was tested by studying the Cu-centered reduction of a synthetic potassium–copper­(II) hexacyanoferrate (KCuHCF) in contact with LiClO 4 /DMSO, NaPF 6 /DMSO, and KPF 6 /DMSO electrolytes. The Fe-centered electrochemistry of the synthetic solid, of formula K 0.876 ­Cu II 1.328 ­Fe III 0.049 ­[Fe III 0.318 ­Fe II 0.682 ­(CN) 6 , was previously described with regard to the separation of electronic and ionic contributions to Gibbs energy changes and the analysis of cation coinsertion processes . Metal hexacyanoferrates are receiving considerable attention because of their use as cathodes in batteries. , Their electrochemistry has been widely studied, − with particular attention to thermochemical aspects. − Here, the voltammetry of the KCuHC was studied using microparticulate deposits of the solid transferred to paraffin-impregnated graphite electrodes using the voltammetry of immobilized particles (VIMP) − methodology.…”