Roberto Marassi

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.

An electrochemical impedance spectroscopic study of the transport properties of LiNi0.75Co0.25O2

Analysis of impedance spectra taken at closely spaced bias potential values on LixNi0.75Co0.25O2 have been interpreted in terms of electronic and ionic transport properties of this electrode material. In the 0.9<x<1 range the material shows semi-conductive properties and the electronic conductivity dominates the transport. For x≤0.9, the properties change into those of a metal-like material in which the ionic conductivity becomes the limiting factor. The transition between these two limiting conditions clearly appears in the impedance spectra sequence. This transition is reversible since the same behaviour is observed during the lithium intercalation process as well as in the reverse lithium deintercalation process.

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.

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.

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.

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.