ecs transactions [ecs 219th ecs meeting - montreal, qc, canada (may 1 - may 6, 2011)] -...

Electrochemical stabilization of Prussian Blue films in NH4Cl aqueous medium

J. Agrisuelasa,c, C. Delgadoc, C. Gabriellia,b, J. J. García-Jareñoc, H. Perrota,b, F. Vicentec

a CNRS, UPR 15 du CNRS, Laboratoire Interfaces et Systèmes Electrochimiques-LISE .b UPMC, Université Pierre et Marie Curie-Paris 6, Laboratoire Interfaces et Systèmes

Electrochimiques, 75005-Paris, France. c Departament de Química Física, Universitat de València. 46100-Burjassot, Spain.

Summary

Derivative voltabsommetric scans in near-UV/visible-NIR electromagnetic range together with nano-electrogravimetric measurements during the stabilization process of Prussian Blue in NH4Cl aqueous solution yields information about the localization of the into the crystalline structure of this material. It is proposed that the ammonium ion is placed into the vacancies of the framework that affect to the motional resistance. The simultaneous measure of current, mass, motional resistance, and near-UV-vi-N IR spectra allows to explain the differences observed between the Prussian Blue films placed in a solution of NH4Cl salt regarding the behaviour showed in KCl aqueous solution.

Introduction

Prussian Blue, PB, also known as ferric ferrocyanide, is one of the most studied hexacyanometallates due to their easiness to form controlled quality films on the surface of very different kind of modified electrodes. It is well known that their electrochemical processes are accompanied by color changes at different wavelengths and also by an ion exchange between the film and the solution to keep the film electroneutrality. Anions incorporated from the salt solutions in the synthesis of the Prussian Blue can found as traces that affect to the lifetime of devices that can be built with these material [1,2]. However, alkaline cations placed into the crystal structure of the material play a more decisive role than anions, owing to they can form part of the self crystallographic network [3,4].

Prussian Blue films freshly deposited following the procedure described by Itaya et al. [5] are in the “insoluble form” which contents alkaline cations at trace level only. After a few successive voltammetric cycles around the Prussian Blue (PB) ⇄ Everitt's Salt (ES) system, a structural change takes place (named stabilization process) and these films undergo into the “soluble form. This structural change has been described in the past decades as the loss of a part of inner high spin iron atoms replaced by potassium cations. However, recent X-ray experiences suggest that the soluble and insoluble forms

ECS Transactions, 35 (10) 53-61 (2011)10.1149/1.3640404 © The Electrochemical Society

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are very similar and that the main difference may be the presence of alkali cations such as the potassium into the crystalline structure of the “soluble form” [4]. Moreover, the water molecules have been found in two different positions in the face-centered cubic lattice structure of PB: the coordinated water in vacancies and the uncoordinated water in interstitial positions.

. The present work is focused on the analysis of the stabilization of galvanostatic-deposited films of Prussian Blue on gold electrode when the counter cation that enters into the framework is ammonium. It was used cesium and potassium alternatively for comparative purposes. These films were synthesized from Fe [(CN) 6K3] and FeCl3 in HCl aqueous solutions following the Itaya’ procedure [5]. This study was carry out by means simultaneous measurements of the resonant frequency of quartz and the motional resistance of the Butterworth-Van Dyke (BVD) equivalent circuit model by means EQCM device together with the instantaneous and in situ record of the optical intensity reflected in the near-UV/visible-NIR electromagnetic range during successive cyclic voltammograms.

The analysis of the mass/electrical charge ratio allows active species that take part in the reaction to be identified by the calculation of the molar mass of the species exchanged between the reaction substrate and the working solution [6]. For EQCM electrodes, it is also possible to follow in situ changes in the motional resistance which is related to mechanical and magnetic properties of the Prussian Blue [7]. Working with high reflectance gold electrode in the EQCM also allows us to record the changes of reflectance of the PB during the voltammograms. Thus, spectroscopic techniques provide information about the chromophore sites during the electrochemical process. Particularly interesting is the band centered at 672 nm since it is associated with Fe(II)low spin-CN-Fe(III)high spin structural units that compose to a large extend the PB rigid skeleton. This bans is associated to the electron charge transfer from the Fe (II) atoms surrounded by –CN units to the Fe(III) atoms surrounded by –NC unities [8]. Furthermore, this band and their electrochromic efficiency remain unaltered during the transformation of insoluble PB to soluble PB in the stabilization process if the work outer medium is a aqueous KCl solution due to the fact that a potassium ion is placed into each ferrocyanide vacancy [9].

Experimental

Insoluble PB films were electrochemically deposited by immersion in 0.02 M K3Fe(CN)6 (analytical reagent, Scharlau), 0.02 M FeCl3 (sigma), and 0.01 M HCl (Analytical reagent, Scharlau) and 0.01 M HCl (R.P. Normapur) solution. A controlled cathodic current of 40 µA.cm-2 was applied for 150 s to obtain the insoluble PB films. The soluble form was generated by cycling the system between 0.6 V (PB form) and -0.2 V (ES form) in 0.5 M KCl (Scharlau RA), 0.5 M NH4Cl (Scharlau RA), 0.5 M (CsCl Scharlau RA) with pH = 3.0 and at 20 mV s-1 at room temperature. The voltammetric cycles were carried out in a typical electrochemical three-electrode cell. A high surface mesh of Pt/Ir was used as counter electrode. An Autolab potentiostat-galvanostat (POSTAT302) was used in the voltammetric measurements, whereas an EQCM (RQCM, Maxtek Inc.) was used to record the motional resistance and changes of frequencies of quartz resonance. A photodiode (silicon PIN photodiode/OSD5.8-7Q)

ECS Transactions, 35 (10) 53-61 (2011)



was used , which gave a current proportional to the received luminous intensity in spectroscopic measurements ( with a spectrophotometer StellarNet SL1 + EPP2000) in the Near-UV/visible-near-IR electromagnetic region. This intensity was transformed in an analog potential (home made). The potential was converted in an apparent absorbance. The working electrode was a high reflectance gold/quartz-crystal electrode (AT-cut quartz crystal, 6 MH, Matel-Fordahl, France, which allowed the simultaneous measurement of current, mass (from frequencies changes), motional resistance and reflectance (apparent absorbance).

Results and discussion

It can be deduced that the transformation of the PB freshly generated (named insoluble Prussian Blue in literature) to the soluble stabilized form (named soluble Prussian Blue) in NH4Cl aqueous medium is less drastic than in presence of potassium ion [8], since the shape of voltammograms slightly changes in NH4Cl aqueous medium.

-0.2 0.0 0.2 0.4 0.6

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Figure 1. Voltammograms of Prussian Blue.a) 1st Cyclic voltammogram in NH4Cl 0.5 M of fresly synthetized Prussian Blue. b) 9th voltammogram in 0.5 M NH4Cl. c) 1st voltammogram in 0.5 M KCl of fresly synthetized Prussian Blue.d) 9th voltammogram in KCl 0.5 M. Scan rate: 20 mV.s-1. Voltammograms stars in the reduction sense.

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During this process of stabilization of the PB film in the NH4Cl solution, the voltammograms do not change appreciably in successive cycles, only slightly decreases the charge transferred. If the system is cycled around the range of potentials that correspond to Prussian Blue (blue) � Everitt’s salt (colorless) it is observed (Figure 1 and Table 1) that the shape of the voltammograms remains practically unaffected in the NH4Cl solution, while peaks turns narrower in KCl solution. If compare the directly measured magnitudes on the respective voltammograms, it is easy to conclude that the ammonium ion is more retained into the material that the potassium one. Perhaps it is the cause that the shape of the voltammograms does not changes during, the stabilization process in NH4Cl aqueous medium in spite that ammonium enters into the film.

Table 1. Magnitudes directly measured on voltammograms of Figure 1: peak potential Ep, peak current intensity ip, Charge transferred in the reduction scan qc, half-peak width potential w1/2, apparent formal potential, Eº’, peak to peak potential Epp

Cycle Epc (mV)

ipc (mA)

w1/2 c (mV)

qc (mC)

Epa (mV)

ipa (mA)

w1/2 a (mV)

Eo’ (mV)

Epp (mV)

NH4Cl 0.5 M

1st 205 -0.43 116 -5.3 314 0.29 266 260 109 9 th 215 -0.41 142 -4.4 318 0.29 271 266 103

KCl 0.5M

1st 214 -0.49 157 -5.5 245 0.49 119 223 31 9 th 213 -0.76 66 -4.8 250 0.61 82 232 37

The PB deposited on the Au electrode studied in NH4Cl solutions changes in mass similarly as occurs if the counter ion is the potassium [9] (Fig.2). The overall mass decreases until it reaches a constant value before that 10th cycle characteristic of the stabilized Prussian Blue form. However, there are some details than shows an important and specific role of the entrance of ammonium into the film, that requires an more detailed analysis.

The F(dm/dq) function yields a apparent molar mass of the counter ion smaller than the expected 18 g.mol-1 for the ammonium molecule (Figure 3 a) and b)). Then, the anodic peak involucres the participation other species such as water molecules and hydrogen ions for balancing the electrical charge during the oxidation scan as occurs also in the KCl potassium medium at some potentials. However, in presence of potassium ions yields F(dm/dq) values close to 40 g.mol-1 at the peak potentials as correspond to the direct participation of this cation (Figure 3 c) and d)). This singular behavior of the ammonium denotes a complicated process where the water molecules also participated.




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Figure 2. Electrical current, mass, apparent dA/dt at 690 nm during successive cycles of stabilization. a) In 0.5M NH4Cl 0.5 M and b) in 0.5M KCl aqueous solutions.




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Figure 3. Instantaneous mass/charge, F(dm/dq).a) 1st Cyclic in NH4Cl 0.5 M of freshly synthesized Prussian Blue. b) 9th cycle in 0.5 M NH4Cl. c) 1st cycle in 0.5 M KCl of fresly synthetized Prussian Blue.d) 9th cycle in KCl 0.5 M. Scan rate: 20 mV.s-1. Voltammograms stars in the reduction sense

Another important fact if compared with the potassium participation case is the great increase of the motional resistance during the first cathodic scan (Fig. 4). This means that the consistence of the film increases due the entrance of the ammonium ion for forming part of the compound. Initially, the films contain water molecules into the vacancies and also in the zeolitic channels of the crystalline cell unity. During the first cathodic scan, the ammonium enters to the vacancies of the crystalline cell unity for yielding a solid more rigid than the freshly-prepared Prussian Blue. Due to the he ammonium ion remains into the film and the anodic peak proves wider, together with the fact that the peak to peak potential Epp is higher than in the case of the PB stabilized in potassium ion (table 1), it is inferred that part of the molecules of ammonium remains into the film as occurs in the case of the alkaline countercations, but more strongly retained.

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Figure 4. Evolution of the motional resistance during the successive potential cycles of stabilization of Prussian Blue. a) in 0.5 M KCl b) in 0.5 M NH4Cl, c) in 0.5 M CsCl.

As we can see in Figure 4, the motional resistance evolves during the successive potential cycles in different manner depending on the type of cation exchanged in the stabilization process (Figure 4). From a structural perspective, the motional resistance measured from the BVD electrical circuit has several different contributions: One, due to the electrode support itself ( 0

mR ). Another associated with the interface Au / film

( /Au filmmR ). Another, associated with mechanical and magnetic properties as the film

( filmmR ) and a fourth overlaped contribution associated with the film/solution interfacial

region ( /film solutionmR ).

filmmR depends on the structure and inner composition of the film, whilst

/film solutionmR Presumably, 0

mR remains constant in each experiment carried out in this work. However, secondary processes such as, for example, the Au oxidation or the Fe oxide formation, or perhaps little differences in the manufacture of the electrode ensembles, causes loss of reproducibility between different series of experiences. We can also assume that /Au film

mR / depends on the charge transfer at the Au / film interface but

should be of a lower order of magnitude than filmmR . Then, from these assumptions a first

approximation can be written:

0 / / 0 / /.Au film film film solution film film solution film film solution

m m m m m m m m m mR R R R R R R R const R R= + + + ≈ + + ≈ + +




Where /film solutionmR includes the contributions due to the friction between the film and the

medium together the insertion phenomenon through this interface region. This friction between both phases is directly related with the viscosity of the solution

Unfortunately, the motional resistance includes almost all these contributions. However, from a qualitative perspective, the evolution of the value of this magnitude in the stabilization process by applying successive cyclic voltammograms demonstrates the important role of counterion in the stabilization of Prussian Blue. Motional resistance increases in the first sscan of reduction. Also, two little peaks are observed in each cycle in KCl an NH4Cl solutions, at the potentials range near to peak potentials when the entrance or exit of ions is maxima.

Ammonium molecules into the vacancies yields enough flexibility to the stable Prussian Blue structure to show two little maxima of the motional resistance during the latest voltammograms (Fig. 4) . This structural changes associated to the maxima of these magnitude are modulated by means the electrochemical perturbation. This suggest the possibility for controlling the optical, electrical, and magnetic proprieties of the stabilized form in the way of to controll the ratio of the Fe(III)/Fe(II) centers together with the inner composition of the films by means of electrochemical techniques at room temperature as was proposed for the Prussian Blue stabilized in potassium chloride solutions [9]. Both maxima are associated to the reduction and oxidation processes of Prussian Blue, since as it occurs in the case of potassium counterion, it causes a loss of the resonant energy of the quartz due to important role of the inner and outer water molecules in the coupled ion entrance/exit processes. We can show the absence of both maxima in the case of the experience in aqueous CsCl solution, since the cesium ion is an ion without a solvated layer. Furthermore, the motional resistance increases when the Cs is incorporated into the film during the successive voltammetric cycles due to a clear loss of flexibility of the material.

The structural switch that it is put in evidence by observing the �R values at determinate potentials is explained by the composition of the vacancies of the stabilized material. An interesting hypothesis is that the Fe(III) centers near the vacancies form Fe[(CN)5NH3] complexes in spite Fe[(CN)5H2O] complexes as it is postulated for the Prussian Blue stabilized in potassium chloride solutions. Independently where the counterions are placed into the crystalline structure, the evolution of the motion resistance depends on the type of counter cation.

Spectroscopic behavior of Prussian Blue in the Visible-Near IR range was analyzed in the past by Robin [10] who proposed two main spectroscopic bands centered at 380 nm, 690 nm and another forbidden band centered near at 1000 nm. The transition at 690 nm is related to the electronic charge transfer from _( )low spinFe II atoms surrounded by cyanide units to _( )high spinFe III atoms, so monitoring the amount of

( ) ( )low spin high spinFe II CN Fe III− − electronic states (change of color from blue to colorless). On the other hand, the transition detected at 380 nm is related to the electronic charge transfer from _( )high spinFe II atoms to _( )low spinFe III atoms so

monitoring the amount of ( ) ( )high spin low spinFe II CN Fe III− − (ionic trapping states). Then, this behavior is very similar to that of soluble PB form when the potassium is the counterion, but in this case, the vacancies contents ammonium in place of potassium. This behavior supports the idea that the ion ammonium plays a multiple role: it can act




as a counterion and also as a chelating agent into the vacancies in substitution of water molecules. In spite of these promising results obtained by means of spectroscopic and EQCM in situ techniques, a further effort is needed for obtaining a best structural picture of this electrochromic material.

Conclusions

Ammonium ions play apparently a role similar to the potassium as counterion in the Prussian Blue (blue) � Everitt’s salt (colorless) process. However, the evolution of the motional resistance during the successive potential cycles depends on the type of counterion. In spite that the presence of ammonium hinders the flexibility of the material if compared with the presence of potassium, the framework of chromophores sites that absorbs at 690 nm remains unaltered. This is consistent with the possibility that they are placed into the vacancies of the crystalline framework during the successive voltammograms of stabilization. This cation insertion affects to the degree of rigidity of the material.

Acknowledgements

Part of this work was supported by FEDER-CICyT project CTQ2010-21133/BQU. J. Agrisuelas acknowledges his position to the Generalitat Valenciana.

References

1) A. Roig, J. Navarro, R. Tamarit, F. Vicente. J. Electroanal. Chem., 360 (1993) 55-69.

2) A. Roig, J. Navarro, J.J. García, F. Vicente. Electrochim Acta, 39 (1994) 437-442. 3) J.J. García-Jareño, A. , D. Benito, J. Navarro-Laboulais, F. Vicente. International Journal of Inorganic Materials (Solid State Sciences), 1 (1999), 343. 4) P.R. Bueno, F.F. Ferreira, D. Gimenez-Romero, G.O,Setti, R.C. Faria, C. Gabrielli, H. Perrot, J.J. Garcia-Jareno, F. Vicente, J. Phys. Chem. C 112 (2008)13264. 5) K. Itaya, T. Ataka, S. Toshima, J. Am. Chem. Soc 104 (1982) 4767. 6) J. Agrisuelas, C. Gabrielli, J.J. García-Jareño, D. Giménez-Romero, H. Perrot, F. Vicente, J. Electrochem. Soc. 154 (2007) F134. 7) J. Agrisuelas, J.J. García-Jareño, D. Giménez, F. Vicente, J. Phys. Chem. C, 112 (2008) 20099. 8) J. Agrisuelas, P.R. Bueno, F.F. Ferreira, C. Gabrielli, J.J. Garcia-Jareño, D. Gimenez-Romero, H. Perrot, F. Vicente, J. Electrochem. Soc., 156 (2009) P 74. 9) J. Agrisuelas, J.J. García-Jareño, D. Giménez-Romero, F. Vicente, J. Electrochem. Soc. 156 (2009) P149. 10) M.B. Robin, Inorg. Chem. 1 (1962) 337.




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