Silvia Zamponi

The absorption of hydrogen and deuterium in thin palladium electrodes - Part I: acidic solutions

Abstract A comparative study of hydrogen and deuterium sorption in Pd from basic solutions (0.1 M NaOH, NaOD, LiOH and LiOD) has been performed, using electrodes obtained by electrochemical deposition of palladium on gold. The amount of absorbed hydrogen or deuterium has been found to depend on the electrode potential. The shape of H(D)/Pd vs. E plots and the rate of hydrogen (or deuterium) absorption are strongly influenced by the composition of the solution. Lithium appears to have a marked influence on the α to β phase transition. The maximum H(D)/Pd ratios were about 0.8 for all studied solutions. The greater isotopic effect has been found during absorption and desorption in lithium solutions. 1. 1. The amount of absorbed hydrogen-deuterium in the bulk of palladium depends on the electrode potential. 2. 2. The shape of the plots of absorbed hydrogen amount from basic solutions vs. absorption potential strongly depends on composition of solution and significantly differs from the shape of the same plots in acidic solution (2). The results show that alkali metal cations markedly influence hydrogen and deuterium sorption processes. 3. 3. Lithium ions seem to affect the α-β transition more than sodium ions. 4. 4. The isotopic effect is much stronger during oxidation of absorbed hydrogen than during absorption. This effect is greater in lithium solutions. 5. 5. No significant influence of solution composition on the maximum values of the H(D)/Pd ratios has been found. 6. 6. The charging curves in all the solutions studied are not reversible, as was the case in acidic solutions. This means that the sorption or desorption causes irreversible changes in the Pd lattice.

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.

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.

Electrochromic features of hybrid films composed of polyaniline and metal hexacyanoferrate

Abstract Hybrid organic/inorganic films, composed of polyaniline (PANI) matrix and Prussian blue-like nickel hexacyanoferrate redox centers, showed reversible electrochromic behavior in acidic potassium salt electrolytes. The systems coloration properties were assessed from various spectroelectrochemical measurements including voltabsorptometry that involved monitoring of the time-derivative signal of absorbance at 700 and 410 nm as a function of linearly scanned potential. Gold-covered foil was used as a conductive, optically transparent, substrate onto which the composite film was electrodeposited by potential cycling in the mixture for modification consisting of aniline monomer, Ni2+, Fe(CN)63− and electrolyte containing K+ and H+ ions. An important feature of hybrid (composite) material was that its electrochromic properties were dominated by color changes occurring in the PANI component. Coloration originating from nickel hexacyanoferrate barely affected the systems electrochromic characteristics. But the cyanometallate redox centers distributed in the PANI matrix behaved reversibly as expected for a system capable of fast charge transport.

The influence of carbon monoxide on hydrogen absorption by thin films of Palladium

Abstract (1) Adsorbed carbon monoxide inhibits the hydrogen absorption and desorption reactions in a thin-layer palladium electrode. A“blocking” effect of adsorbed hydrogen inside the palladium by adsorbed carbon monoxide is observed. (2) During CO adsorption at 0.00 V an anodic current appears, probably due to the oxidation of adsorbed hydrogen during the exchange with carbon monoxide molecules. (3) In addition to “bridged” and “linearly” bonded CO molecules as the predominant surface species, another adsorption product may exist on the palladium surface. (4) Electrodes constructed by depositing a thin layer of Pd on gold allow the ratio between absorbed and adsorbed hydrogen to be decreased. This creates new possibilities for electrochemical studies of this metal.

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.

Multivariate Curve Resolution Analysis for Interpretation of Dynamic Cu K-Edge X-ray Absorption Spectroscopy Spectra for a Cu Doped V2O5 Lithium Battery

Vanadium pentoxide materials prepared through sol-gel processes act as excellent intercalation hosts for lithium as well as polyvalent cations. A chemometric approach has been applied to study the X-ray absorption near-edge structure (XANES) evolution during in situ scanning of the Cu(0.1)V(2)O(5) xerogel/Li ions battery. Among the more common techniques, the fixed size windows evolving factor analysis (FSWEFA) permits the number of species involved in the experiment to be determined and the range of existence of each of them. This result, combined with the constraints of the invariance of the total concentration and non-negativity of both concentrations and spectra, enabled us to obtain the spectra of the pure components using a multivariate curve resolution refined by an alternate least squares fitting procedure. This allowed the normalized concentration profile to be understood. This data treatment evidenced the occurrence, for the first time, of three species during the battery charging. This fact finds confirmation by comparison of the pure spectra with the experimental ones. Extended X-ray absorption fine structure (EXAFS) analysis confirms the occurrence of three different chemical environments of Cu during battery charging.

In-situ X-ray absorption spectroelectrochemical study of hydroxocobalamin

Abstract An in situ X-ray absorption spectroscopy (XAS) spectroelectrochemical study of aquocobalamin (system B12a-B12r-B12s) has been carried out in aqueous solutions buffered at different pH values. To the best of our knowledge, this is the first structural study of aquocobalamin at room temperature under controlled oxidation conditions. Most of the previous work was in fact performed using frozen samples chemically treated to produce the species. The spectroelectrochemical approach offers several advantages: (1) the reduction products may be studied without poisoning the system with chemical reductive reagents and (2) any possible variation of the oxidation state owing to the electrons produced by the incident beam is avoided as the electrode, under potentiostatic control, acts as a scavenger. The spectroelectrochemical approach, together with more careful data analysis, has led to an improved interpretation of the XAS data. These conditions were not met in previous works where the oxidation state was not controlled and multiple scattering contributions were not taken into account. The general shape of the XAS spectra of the different species is not greatly affected by pH. A signature for the base-off square-planar coordination has been evidenced for the Co(II) compound at basic pH. A new signature for Co(I), indicating square-planar coordination, has been identified on the experimental spectra and simulated in theoretical X-ray absorption near-edge structure (XANES) studies. The flexibility of the electrochemical approach, that permits to unambiguously establish the formal oxidation state, has led to very reliable values for energy shift and peak intensity variations. The experimental XANES and extended X-ray absorption fine structure (EXAFS) spectra with a very good signal-to-noise ratio have been processed using the GNXAS package that takes into account multiple scattering contributions. EXAFS and XANES independent analysis result in the same structural model. The reduction from Co(III) to Co(II) produces the most significant structural changes: the cobalt coordination number decreases from six to five, and the edge position shifts by 2.4±0.3 eV. In addition, the XANES spectra are strongly modified. The reduction from Co(II) to Co(I) produces mainly electronic effects with no apparent change of the coordination number. A discussion of the limits and potentialities of EXAFS in this type of study has also been included.