Electro-optic spectroscopy of electrochromic processes in tungsten trioxide

The nature of the electrochromic processes in solid-state electrochromic devices (ECD) ITO/a-WO₃/a-SiO₂/Au is investigated by analysing their electro-optical behavior during the current pulses and the linear sweeping of the bias voltage. It is shown that the character of the current-coltage curve of the ECD is determined by passivation, by hydrogen/oxygen evolution on the counter electrode (Au) and by redox reactions on the electrochronic electrode (amorphous WO₃ film: a-WO₃). The light transmittance versus voltage and the high-frequency differential impedance of the active component versus voltage curves are dependent mainly on electrochromic reactions.     

Microstructural Influence on Electrochemical Properties of LiFePO4/C/Reduced Graphene Oxide Composite Cathode

Russian Journal of Electrochemistry - LiFePO4/C/reduced graphene oxide (rGO) composites with different morphologies were synthesized, allowing evaluation of the electrochemical performance as a...     

AC impedance behavior of the Ti4Ni2Oy and Ti3.5Zr0.5Ni2Oy type metal hydride electrodes

The hydrogen storage alloy electrodes of the type Ti₄Ni₂O_y (y=0, 0.3 and 0.6) and Ti_3.5Zr_0.5Ni₂O_y (y=0.15 and 0.3) were investigated by impedance spectroscopy for potential application as negative electrode in alkaline secondary nickel-metal hydride (MH) batteries. The phase Ti₄Ni₂O_0.30 was found to be electrochemically more stable during the cycling. The addition of copper or nickel powder as current collector improved the electrochemical behavior of the electrodes. It was possible in this way to decrease the charge transfer resistance. These additions have a negligible influence on the stability of electrode material during cycling. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Electrochemical hydrogen storage and usage aspects: Nickel electrode in acidic electrolyte

The conditions of electrochemical formation of nickel hydride are investigated. The electrochemical properties of different nickel-based materials (bulk, porous, foamed tapes) are examined to state the hydrogen adsorption/absorption phenomena and possibility to use them as a negative electrode in hydrogen devices (electrolyzers, rechargeable batteries, fuel cells). Surface activation of nickel materials are made by electroplating and etching methods. Thin palladium coating is used to prove the formation of nickel hydride during cathodic charging. Volt-amperometric and kinetic measurements show that not only palladium, but also activated nickel plays important role in the surface activation of electrode materials and promotion of hydrogen absorption in nickel substrate materials. Published in Russian in Elektrokhimiya, 2007, vol. 43, No. 5, pp. 624–629. Based on the paper delivered at the 8th Meeting “Fundamental Problems of Solid-State Ionics, Chernogolovka (Russia), 2006. The text was submitted by the authors in English.     

Sulfonated poly(ether-ether-ketone) and Nafion composite membrane with aluminium oxide additive for fuel cell applications

The changes of physical properties and proton conductivity due to combining two polymers with inorganic compound are studied. Sulfonated poly(ether-ether-ketone) and Nafion were mixed in proportion 9 to 1; the amount of aluminium oxide nanopowder was 2 wt % counted to polymer part. Also, two-component membranes were prepared for comparison. Resulting membranes were tested for water uptake and impedance spectra were measured at low relative humidity and 70°C temperature. The highest conductivity measured in this work was obtained for ternary membrane with the value of 7.58 mS/cm.     

Electrochromic colour centres in amorphous tungsten trioxide thin films

Amorphous tungsten trioxide films, investigated by the Raman scattering method, are shown to be composed of a spatial network of tightly bound (WO₆)_n·mH₂O clusters with a large number of terminal oxygen W=O and W-O-W bonds between clusters. The injected electrons in an amorphous tungsten trioxide film are localized in the tungsten 5d orbitals in an axially distorted octahedron, as is shown by ESR analysis. The optical absorption of a coloured amorphous tungsten trioxide film, as has previously been proposed, can be satisfactorily described by an intervalence charge-transfer transition between localized W⁵⁺ and W⁶⁺ states.     

Model of electrochromic and related phenomena in tungsten oxide thin films

We have developed a model of electrochromic and related phenomena in tungsten oxide thin films based on the assumption that the constitution of such films is heterogeneous and built up of nanosized particles, pores and adsorbed substances (mainly water). It is discussed why a high-efficiency reversible blue colour is observed in amorphous tungsten oxide films (α-WO₃ films) as well as why such porous thin films with polycrystalline or amorphous constitution and with a variety of particle properties can be easily obtained by a physical vapour deposition process in a low-pressure atmosphere in the presence of water. A substrate temperature in the range 450–550 K corresponds to some plateau on the water desorption curves which divided physically adsorbed water from chemically adsorbed water. Two types of structural units based on tetrahedrally and octahedrally coordinated tungsten ions have the main role in the formation of the film constitution. The tetrahedral structural units have a glass-forming function, but the octahedral ones have a modification function. From the electrochemistry point of view, the internal multiphase interfaces in such films are distributed multiphase electrodes. The adsorbed water together with defects of the oxide particles provide reagents for reversible coloration reactions in the film. The colour centres can be induced thermally (oxygen nonstoichiometry) or electrically (injected ions) or by radiation (photoinjected hydrogen). The electrochromism and related phenomenon of α-WO₃ films can be directly related to ion insertion/extraction processes controlled by external forces. Electronic Publication     

Conductivity measurement of different polymer membranes for fuel cells

In our work the through-plane and in-plane conductivity measurements were performed for commercial Nafion115 and original sulphonated poly(ether-ether-ketone) (SPEEK) membranes. It was found, that in-plane conductivity measurement method can be successively used for both types of membranes at maximal humidity (saturated water vapours) and temperatures till 100°C. The proton conductivities of home-made SPEEK membranes were found to be excellent in the order of 10⁻² S/cm in the fully hydrated condition at temperatures 25−90°C. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 6, pp. 699–704. The article was translated by the authors. Published by report at IX Conference “Fundamental Problems of Solid State Ionics”, Chernogolovka, 2008.     

Electrochemical behavior of metal hydrides

Metal hydride electrodes are of particular interest owing to their potential and practical application in batteries. A large number of hydrogen storage materials has been characterized so far. This paper deals with the effect of the chemical nature and stoichiometry of specific alloy families (AB₅, A₂B, AB/AB₂ and AB₂) on the hydride stability, hydrogen storage capacity and kinetics of hydrogen sorption-desorption in the solid phase/gas and solid phase/electrolyte solution systems. Special attention has been paid towards the electrochemical properties of metal hydrides in terms of their performance in Ni-MH rechargeable alkaline cells. Electronic Publication     

From electrochromic phenomena to energy harvesting and storage—an overview of solid state ionics research at the Institute of Solid State Physics,University of Latvia

Journal of Solid State Electrochemistry - Solid state ionics is one of the key research topics of the Institute of Solid State Physics, University of Latvia since its establishment. The research...