Insertion of Cesium in Prussian Blue Films
Authors: Vicente Francisco , Agrisuelas Jeronimo , Cuenca Alejandro , Roig Antoni Francesc , García Jose , San Matías Ana , Alfonso Gema 
Published: February 2020

Hexacyanometallate-like materials have raised intense interests because of their electrocatalytic, electrochromic, ion-exchange, ion-sensing or photomagnetic properties. Nano-electrogravimetric measures during the electrochemical stabilization process of Prussian Blue (PB) in CsCl aqueous solution yields interesting information about the insertion role of the Cs into the crystalline structure trough the interfacial regions [1]. In addition to the intrinsic scientific interest of this topic, many works have suggested that hexacyanoferrate compounds can be used as precipitants for cesium removal from aqueous radioactive wastes [2].
This work was carried out by means of simultaneous measurements of resonant frequency of quartz as well as the motional resistance of the Butterworth-Van Dyke equivalent circuit model measured by means of a an electrochemical quartz crystal microbalance (R-EQCM) during successive cyclic voltammograms. The simultaneous measure of current, mass and motional resistance magnitudes allows to explain the differences between the electrochemical cation insertion processes in a solution of CsCl salt with respect to the cation insertion observed in KCl aqueous solutions [3,4]. Accordingly, the main goal of this work was to study in depth the different redox processes which involve the PB electrochemistry during the PB«ES switch, where ES is the reduced form (Everitt’s salt).
Derivative voltabsommetric scans together with nano-electrogravimetric results allow the different electrochemical processes to be distinguished during voltammetric scans. Cesium, proton and hydrated proton counter-ions involved in the PB electrochemistry are related here to the electrochemical reactions of specific Fe-sites. Cesium counterions show two different sites for their insertion: one located into the crystalline framework and other into ferrocyanide vacancies
PB deposits were voltammetric cycled or only immersed in CsCl aqueous solutions. In both cases, their respective motional resistances R increase about of 300 ohm. Simultaneously, a decrease in the resonance frequency is also recorded, suggesting that in both cases a spontaneous entrance of Cs should be considered. Then, two different types of cesium exchange are postulated: one associated to the electrochemical reaction during the first scan, and another spontaneous non-stoichiometric entrance where the Cs is located inside the inner water cluster. This means that a part of the water molecules are expelled from the cluster and consequently the film losses electrical conductance as well as their mechanical rigidity increases.
It is proposed that the Cs is retained into the interstitial water cluster placed between the framework formed by means Fe(II)low spin-CN-Fe(III)high spin structural units. Then, there are two different ways for entering the cesium to the structural zeolite-building of Prussian Blue: CsCl enters chemically by an spontaneous substitution of inner water molecules of the interstitial water cluster and, also its could enters as counter-ion Cs+ during the first reduction scan of the electrochemical stabilization process of the PB.
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[2]C.Thammawong,P. Opaprakasit, P. Tangboribonrat,.P, Sreerunothai. PB-coated magnetic nanoparticles for removal of cesium from contaminated environment. Journal of Nanoparticles Research 15 (2013) 1689.
[3] J. Garcia-Jareño, J. Navarro-Laboulais, A. Sanmatias, F. Vicente. The correlation between electrochemical CsCl aqueous solutionimpedance spectra and voltammograms of PB films in aqueous NH4Cl and CsCl., Electrochim. Acta, 43 (1998) 1045-1052.
[4] J. Agrisuelas, J.J. García-Jareño, C. Moreno-Guerrero, A. Roig, F. Vicente, Identification of electroactive sites in Prussian Yellow films, Electrochimica Acta. 113 (2013) 825–833.
ACKNOWLEDGEMENTS
Part of this work was supported by FEDER-CICyT CTQ2015-71794-R