Jānis Kleperis

Electrochromic colour centres in amorphous tungsten trioxide thin films

Abstract 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.

Proton conducting composite electrolytes based on antimonic acid

Abstract This report concerns a composite proton electrolyte suitable for use in electrochromic devices. The electrolyte consists of nanosize pyrochlore antimonic acid particles suspended in a poly(vinyl acetate) matrix by a gel route. It was found possible to substitute the antimonic acid by inert oxides of aluminum and silicon, thus making the electrolyte less harmful to the oxide electrodes of the electrochromic devices without considerably decreasing the conductivity. The proton conductivity of the antimonic acid electrolyte was ∼10 −4 S/cm at room temperature, practically independent of its amount of absorbed water.

Antimonic acid hydrate xerogels as proton electrolytes

Abstract Two high stability types of protonic solid electrolytes based on antimonic acid hydrate xerogels have been obtained by the sol-gel technique: Sb 2 O 5 ·(3–4)H 2 O (colloidal) and Sb 2 O 5 ·5H 2 O (polymeric). The first one is a white compact material with conductivity 0.40 mS/cm (298 K) for n =3.7. The last one is transparent for visible light with a conductivity of 0.80 mS/cm. An electrochromic system based on the colloidal gel electrolyte in the form of paste has good performance-more than 10 7 cycles and a lifetime of more than five years.

Photocatalytic activity of non-stoichiometric ZnFe2O4 under visible light irradiation

Nanostructured zinc ferrites with different excess iron contents (ZnFe2+zO4, where z = 0.00, 0.05, 0.10 and 0.15) have been synthesized using the sol–gel auto-combustion method. The effect of excess iron on the structural, optical and visible light photocatalytic activity of zinc ferrite samples has been investigated. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray magnetic circular dichroism (XMCD), Brunauer–Emmett–Teller theory, scanning electron microscopy (SEM), diffuse reflectance spectroscopy (UV–Vis) and photoluminescence spectroscopy were used to characterize the synthesized non-stoichiometric ZnFe2O4 powders. The XRD patterns demonstrated that the samples consist of single phase spinel structure with crystallite sizes of ~25 nm. SEM analysis indicated that the nanosized particles grow together in porous clusters with a size of several microns. The XPS and XMCD analyses revealed that the excess iron ion substitutes Zn2+ in tetrahedral sites and in octahedral sites Fe2+ can be found in addition to Fe3+, which could be created to restore the overall charge balance in the crystal lattice. Stoichiometric zinc ferrite (ZnFe2+zO4, z = 0.00) exhibited higher photocatalytic activity (40%) than the excess iron sample (ZnFe2+zO4, z = 0.15, i.e. 4%) under visible light irradiation for 3 h. This was explained by the formation of Fe2+ in the octahedral sites of excess iron ZnFe2O4; these species act as recombination centres.

Gaseous sensors based on solid proton conductors

Abstract he chemical sensors for different gaseous (alcohol, acetone, ammonia, water vapour) detection at room temperature are developed by using polycrystalline β-alumina and xerogel of antimonic acid hydrate (AAH). The sensitivity and selectivity of sensors depend on the ion-exchange and preparative methods. The possibility of producing different types of potentiometric, amperometric, voltammetric and resistance sensors on the basis of these ion-conducting materials is shown. More success is achieved by producing ammonia-sensitive devices on β-alumina as well as on AAH xerogel.

Characterization of LiFePO4/C Composite Thin Films Using Electrochemical Impedance Spectroscopy

The composite LiFePO4/C thin films were prepared on steel substrate by radio frequency (RF) magnetron sputtering. Electrochemical properties of the obtained thin films were investigated by cyclic voltammetry charge-discharge measurements and electrochemical impedance spectroscopy (EIS). The films annealed at 550 °C exhibited a couple of redox peaks at 3.45 V vs. Li/Li + characteristic for the electrochemical lithium insertion/extraction in LiFePO4. At low current rate such composite thin film showed a discharge capacity of over 110 mAh g -1 . The dependence of charge transfer resistance, double layer capacitance and lithium diffusion coefficients on applied electrode potential were calculated from EIS data. Determined values of lithium diffusion coefficient were in the range from 8.3·10 -13 cm 2 s -1 to 1.2·10 -13 cm 2 s -1 at 3.4 V and 3.7 V, respectively.

Solid proton conductors as room-temperature gas sensors

Ammonia-exchanged ceramic samples of beta alumina have been obtained from plasma-dispersed powders. The ionic conductivity is slightly affected by ion exchange, but the surface conductivity for the amonia-exchanged sample drastically changes in the presence of the water and ammonia vapours. The ammonia-doped xerogel of antimonic acid hydrate in the form of a thick film has been tested as a potentiometric ammonia sensor.

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.

Application of micro Raman spectroscopy to industrial FC membranes

Raman spectra of as-received and protonated membranes (Nafion® NRE-212, Fumapem® F-14100 and Fumasep® FAA) were measured with He-Cd and Ar laser. For the first time the Raman and IR spectra are reported of Fumasep membranes. Most of peaks in vibration spectra active in Raman and IR of membranes are interpreted with C-F, C-S, C-O-C, SO3, C-C bonds. The vibration region connected with protons and H-O bond in both types of membranes is found in Raman and IR spectra.

Influence of thin film coatings on the gas sensitivity properties of narrow laser cut gap in In2O3 on glass substrate

Abstract New type resistive/capacitive gas sensitive structures were obtained by using a specially prepared substrate, glass, covered by a thin conducting In2O3 layer, cut into comb teeth-type electrodes by help of a laser beam. Such laser processed gap in the layer of conducting material was a prototype of an excellent humidity sensor, especially at high values of relative humidity (RH). Different additional coatings onto a laser processed gap were examined for humidity sensing. Sol-gel, vacuum thermal evaporation and laser evaporation methods were used to obtain thin layers of different materials.