Mario Berrettoni

Electrolyte-cation-dependent coloring, electrochromism and thermochromism of cobalt(II) hexacyanoferrate(III, II) films

Abstract The redox behavior and color of cobalt hexacyanoferrate films depend on the nature of the counter-cations which are sorbed from the aqueous supporting electrolyte into the system during reduction. Whereas cobalt(II) hexacyanoferrate(II) is olive-brown in the presence of hydrated K+ or Cs+ ions, a green color is produced upon incorporation of larger cations (hydrated Na+ or Li+). The “normal” system CoII2FeII(CN)6 · nH2O, which is free of counter-cations, is deep green. These color changes are coreelated with the thermochromic transformation of K2CoIIFeII(CN)6 · nH2O from olive-brown to green upon heating above 61°C. It seems that, at elevated temperatures, K2CoIIFeII(CN)6 undergoes reorganization to the solid solution CoII2FeII(CN)6 · K4FeII(CN)6 which contains the green “normal” phase. An analogous solid solution is presumably also formed upon exposure of K2CoIIFeII(CN)6 film to electrolytes containing larger hydrated Li+ or Na+ cations. The voltammetric behavior in these electrolytes is consistent with the existence of such structures. The color of cobalt(II) hexacyanoferrate(II) is linked to the extent of aquation of the interstitial Co(II) ion, which is likely to be less hydrated in “green” systems (existing at higher temperatures or as “normal” phases free of structural hydrated counter-cations). In potassium electrolyte, cobalt(II) hexacyanoferrate(II) is electrochromic and becomes purple-brown upon oxidation.

Electrochemical preparation and characterization of electrodes modified with mixed hexacyanoferrates of nickel and palladium.

Mixed nickel/palladium hexacyanoferrates have been prepared both as thin films and bulk precipitates (powders) attached to electrode surfaces. The mixed material does not seem to be a simple mixture of hexacyanoferrates of nickel and palladium, and it shows unique voltammetric and electrochromic characteristics when compared with the respective single-metal hexacyanoferrates. Electrodeposition of a mixed film is achieved by potential cycling in the solution for modification containing nickel(II), palladium(II) and hexacyanoferrate(III). It comes from elemental analysis that, in general, the stoichiometric ratios of nickel to palladium in mixed metal hexacyanoferrate films reflect relative concentrations of Pd(II) and Ni(II) in the solutions for modification. In the case of the films that have been electrodeposited from the solutions containing palladium ions in amounts lower or comparable with those of nickel ions, the mechanism of film growth seems to involve formation of nickel hexacyanoferrate during negative potential scans followed by simultaneous insertion of palladium ions as countercations into the system. In such cases, palladium ions tend to substitute potassium countercations at interstitial positions in the electrodeposited nickel hexacyanoferrate microstructures. We have determined the following stoichiometric formula, K1.74−2yPdIIyNiII1.13[FeII(CN)6] (where y<0.72) for such films. At higher molar fractions of palladium in solutions for modification, the formation of a mixed phase of nickel/palladium hexacyanoferrate (in which both nickel(II) and palladium(II) are nitrogen-coordinated within the cyanometallate lattice) is expected. This seems to be more probable than simple codeposition of separate palladium hexacyanoferrate and nickel hexacyanoferrate microstructures during the film growth. Mixed (composite) nickel/palladium hexacyanoferrate films show long-term stability as well as promising charge storage and transport capabilities during voltammetric potential cycling. Well-defined and reversible cyclic voltammetric responses have been obtained in lithium, sodium and potassium electrolytes.

Electrochemical characterisation of Ni/AlX hydrotalcites and their electrocatalytic behaviour

Abstract Some ‘conductive’ hydrotalcites containing nickel as divalent cation and Cl − , SO 4 2− or CO 3 2− as interlayer anion, have been synthesised and membranised with different types of polymeric matrices to modify glassy carbon electrodes. The mechanism responsible of the conductive properties has been deeply investigated by voltammetric techniques using both a stationary and a rotating disk electrode, demonstrating that the interlayer anion does not affect significantly the electrochemical behaviour of the material. The electrocatalytic properties have been also studied pointing out the key role of the steric hindrance of the oxidisable substrate. In particular, mono- and polyhydric compounds have been taken into account. As to the electrocatalytic efficiency, the nature of the interlayer anion and, hence, the dimension of the interlayer spacing, is important in determining the sensitivity of the measurement since it can affect the analyte diffusion inside the hydrotalcite structure.

XAS and electrochemical characterization of lithiated high surface area V2O5 aerogels

Abstract V 2 O 5 aerogel (ARG) has been recently proposed as cathode material for rechargeable lithium batteries. Such a material is amorphous and consists of a highly interconnected solid network with a surface area up to 450 m 2 /g, and a specific pore volume as much as 2.3 cm 3 /g. In a previous paper, it was shown that up to 4 equivalents of lithium per mole of V 2 O 5 aerogel can be inserted by means of chemical or electrochemical lithiation. In the present work, the lithium composition range has been extended. By chemical lithiation (CL) a composition Li 5.8 V 2 O 5 , the highest ever reported for any vanadium oxide host, was achieved. The equilibrium open circuit voltage (OCV)–composition curve of the chemically lithiated aerogel samples showed a wide plateau extending up to 5.8 equivalents of lithium per mole of V 2 O 5 . The surprisingly high OCV has been correlated with the characteristic morphology and structure of the aerogel material by means of X-ray diffraction and absorption and XPS spectroscopies.

Study of amorphous and crystalline Li1+xV3O8 by FTIR, XAS and electrochemical techniques

Abstract Crystalline and amorphous Li 1+ x V 3 O 8 have been compared by electrochemical and spectroscopic investigations. In particular, the behavior of the two materials in cyclic voltammetric experiments at very low scan rate and the Li + diffusion coefficient have been studied. The cyclic voltammetry of crystalline Li 1+ x V 3 O 8 shows the existence of well defined site energies, whilst in the amorphous form the sites have a rather broad energy distribution. Higher D Li + values have been measured in amorphous Li 1+ x V 3 O 8 , this being in agreement with its higher rate capability. The different electrochemical behavior of the two forms have been explained on the basis of structural information obtained by FTIR and X-ray absorption spectroscopy (XAS). These have shown, in particular, that the V-O distances are more homogeneous in the amorphous form and that a partial amorphization occurs in the crystalline form upon Li + intercalation.

Countercation‐Sensitive Electrochromism of Cobalt Hexacyanoferrate Films

Cobalt(II) hexacyanoferrate(III,II) a system analogous to prussian blue, is a unique electrochromic material: its color is not only dependent on the oxidation potential, but also on the nature of the countercations sorbed from electrolyte during reduction. The electrodeposition of cobalt hexacyanoferrate thin films, their voltammetric behavior and spectroelectrochemical identity are reported here in potassium and sodium electrolytes. The oxidized film is purple brown in both electrolytes, but following reduction, the system turns olive-brown in 1 M KCl and becomes green in 1 M NaCl.

XAS and electrochemical characterization of lithium intercalated V2O5 xerogels

Abstract The use of sol-gel processes in the preparation of cathode materials is of growing interest because of their ease and flexibility. The electrochemical properties, e.g. the rate of lithium intercalation, appear to depend on the morphology of the thin-film vanadium oxide xerogels that can be changed by modifying the preparation. In this context, in order to extend the study to bulk materials, xerogel powder samples with surface areas in the range 2–5 m 2 /g have been prepared from pure vanadium pentoxide hydrogels, or in the form of composites, from carbon powder added to hydrogels. The electrochemical properties have been correlated with the morphological and structural changes induced by the presence of carbon using X-ray and XAS spectroscopy.

Spectroelectrochemical characterization of cobalt hexacyanoferrate films in potassium salt electrolyte

Cobalt(II) hexacyanoferrate(III, II) films show reversible electrochromic behavior in potassium salt electrolyte. Gold-covered foil was used as a conductive, optically transparent substrate onto which cobalt hexacyanoferrate films were deposited by a slow coagulation method. Both voltammetric and spectroelectrochemical results are consistent with the existence of two cobalt hexacyanoferrate forms, presumably KCoII1.5[FeII(CN)6] and K2CoII[FeII(CN)6]. The separate nature of the redox reactions of these forms is clearly evident from spectroelectrochemical measurements, particularly from voltabsorptometry, which involves monitoring of the time-derivative signal of absorbance as a function of the linearly scanned potential. Combination of voltabsorptometry with voltammetry allows determinations of molar absorptivity and film loading, and it permits changes of concentration of colored redox centers vs the applied potential to be monitored.

Hybrid Metal Cyanometallates Electrochemical Charging and Spectrochemical Identity of Heteronuclear Nickel/Cobalt Hexacyanoferrate

Hybrid metal nickel/cobalt hexacyanoferrate (Ni/CoHCNFe) has been prepared in the form of thin films on electrode surfaces and as bulk precipitates (powders). This heteronuclear metal hexacyanoferrate cannot be considered a simple mixture of hexacyanoferrates of nickel and cobalt, and it shows different voltammetric and electrochromic characteristics in comparison to the respective single-metal hexacyanoferrates. On the basis of X-ray absorption, elemental analysis, and comparative voltammetric measurements, the following approximate formulas, K 2 Ni 0.5 II Co 0.5 II [Fe II (CN)] 6 and KNi 0.5 II Co 0.5 II [Fe III (CN) 6 ], have been proposed for the predominant reduced and oxidized forms of hybrid Ni/CoHCNFe. The results are consistent with the existence of linear units -Fe-CN-Co-NC-Fe-CN-Ni- in the structure. Hybrid nickel/cobalt hexacyanoferrate film has unique electrochromic properties, and it shows long-term stability and promising charge storage/transport capabilities during voltammetric potential cycling.

Evidence of four-body contributions in the EXAFS spectrum of Na2Co[Fe(CN)6]

Abstract This contribution deals with an EXAFS investigation of Na 2 Co[Fe(CN) 6 ] at the K-edges of both Fe and Co. Na 2 Co[Fe(CN) 6 ], a well known and easily prepared mixed hexacyanoferrate, is characterised by a unit cell containing linear “FeCNCo” chains where a super-focusing effect may be observed at the K-edge of both transition metals. The multiple scattering (MS) signal intensity associated with the linear chain (four body contribution) is comparable with the first shell contributions. The combined fit over two edges, the intensity of the MS signal and the large number of experimental points included in the fit, permit an accurate extraction of the structural parameters at the limit of the EXAFS sensitivity.