Mechanism for Interplay between Electron and Ionic Fluxes in K_hFe_k[Fe(CN)₆]_l·mH₂O Compounds

Abstract

This paper develops a framework for the interpretation of ionic insertion/deinsertion reactions in an aqueous environment taking place in transition-metal hexacyanoferrates of the general formula K(h)[Fe(2+) (CN)(6)](l).mH(2)O, also called Prussian Blue. Three different processes were fully separated in the electrochemistry of these films. It was clearly identified that one of these electrochemical processes involves the insertion/deinsertion of H(3)O(+) (hydrated protons) through the channels of the K(h)[Fe(2+) (CN)(6)](l).mH(2)O structure to reach the film electroneutrality during the electron transfer between Everitt's Salt and Prussian Blue. The other electrochemical processes involve K(+) or H(+) (proton) exchange through the water crystalline structure existing in the channels of the K(h)[Fe(2+)(CN)(6)](l).mH(2)O structure.

Redox switching of Prussian blue thin films investigated by ac-electrogravimetry

“…This difficulty of water transport at low pH is probably due to the fact that PB is an ionic crystalline solid, that is, a rigid structure [51] Obviously, this differs greatly from polymer electroactive film where the solvent molecules may contribute to the mass change during redox switching [52], particularly for polypyrrole [53] and polyaniline [38].…”

Electronic Perspective on the Electrochemistry of Prussian Blue Films

“…As commented above, these bands allow Fe͑II͒ low-spin -CN-Fe͑III͒ high-spin and Fe͑III͒ low-spin -CN-Fe͑II͒ high-spin electroactive sites to be monitored during the PB ES voltammetric scan. It is important to indicate that counterions of electrochemical reactions of PB films interact with the crystalline structure into a specific structural site 21 and as a result, the electrochemical reaction of each chromophore or each electroactive site may involve only a counterion having appropriate dimensions to fill these sites. Figure 2 shows that the derivative of the voltabsommetric curves ͑dA/dt͒ at 686 and 380 nm have a maximum at 0.17 and 0.14 V, respectively.…”

“…[11][12][13][14] In particular, the PB electromagnetic properties can also be tailored by external conditions, such as the magnetic field, [15][16][17] light, 15,18,19 and by electrochemical methods. 18,[20][21][22][23][24] X-ray studies showed that the PB three-dimensional ͑3D͒ structure is face-centered cubic 25 with a general formula equal to Fe 4 ͓Fe͑CN͒ 6 ͔ 3 ·mH 2 O. Trivalent and divalent iron ions are in highand low-spin sites, Fe͑III͒ high-spin and Fe͑II͒ low-spin , respectively. 26 Enclosed in this structure, high-and low-spin sites are both octahedral and surrounded by -NC and -CN units, respectively.…”

“…35 During the voltammetric scan between the soluble PB and ES forms, changes of oxidation states of trivalent iron ions are accompanied by the exchange of counterions to reach the film electroneutrality. 20,21,29,[36][37][38][39][40] In potassium salt solutions, these counterions are potassium cations, hydrated protons, and protons. Furthermore, a structural changeover occurs during this scan within a very narrow potential range.…”

“…8 The main goal of this work is to distinguish the electrochemical reactions of the different electroactive Fe sites that compose the PB structure during the PB ES voltammetric scan. This study is based directly on new spectroscopic measurements and data processing associated with previous electrochemical 20,29,41 and electrogravimetric measurements 7,8,21,23,24,39,40 that allowed the counterions exchanged during this scan to be distinguished according to the applied potential. Previous structural results 33,34 allowed the electroactive Fe sites to be specified.…”