Fig. 1. Temperature evolution of the most important crystalline phases of niobium oxide (amorphous, TT, T, M, and H) for different preparation methods according to Refs. [4] (a, b) and [3] (c).

Fig. 2. DTA and TG curves measured at 10 K/min of a Nb₂O₅ xerogel dried at 100°C prepared by the chloroalkoxide route [29 and 33].

Fig. 3. C_xH_y fractions mass spectrometry analysis of the Nb₂O₅ xerogel of Fig. 2 [29 and 33].

Fig. 4. HCl mass spectrometry analysis of the Nb₂O₅ xerogel of Fig. 2 [29 and 33].

Fig. 5. X-ray diffraction of a two layers undoped Nb₂O₅ coatings (thickness approx. 120 nm) deposited on ITO coated glass sintered between 400°C and 600°C. (o: ITO-peaks. Vertical lines JCPDS 28-317 data (hexagonal structure of Nb₂O₅)) [29 and 33].

Fig. 6. 50th cycle voltammetry curves of two layers Nb₂O₅ coatings (thickness approx. 120 nm) sintered at 450°C (amorphous) and 500°C and 600°C (crystalline) [29 and 33].

Fig. 7. 50th chronoamperometric cycle (−2.2 ; +1.0 V vs. Ag/AgClO₄₎) of two layers Nb₂O₅ coatings (thickness approx. 120 nm) sintered at 450°C (amorphous) and 500°C and 600°C (crystalline) [29 and 33].

Fig. 8. Spectrum of the optical density change ΔOD of two layers Nb₂O₅ coatings (thickness approx. 120 nm) sintered at 450°C (amorphous) and 500°C and 600°C (crystalline) measured during the 50th chronoamperometric cycle (−2.2 V,120 s; +1.0 V,120 s vs. Ag/AgClO₄₎) [29 and 33].

Fig. 9. Typical optical spectra (ΔOD vs. λ) of pure and Ti doped niobia EC layers after Li⁺ ions insertion. Well ordered layers with crystallite size >30 nm show a blue color, weakly ordered layers with crystallite size <25 nm show a gray color while all amorphous layer have a brown color [29 and 33].

Fig. 10. Fermi potential curves determined from coulometric titration after the 50th chronoamperometric cycle for different pure and doped niobium oxides as well as for the ion storage layer TiO₂–CeO₂ [33 and 38].

Fig. 11. Change of light transmittance Δτ calculated according to DIN EN 410 (weighted with the relative light function D₆₅ and the human eye sensibility for day-light V(λ)) vs. the number of cycles of 6×8 cm² devices made with a 180 nm thick Nb₂O₅·0.4 Mo EC electrode and a TiO₂–CeO₂ IS electrode having a thickness varying between 240 nm (1 layer) to approx. 950 nm (4 layers). Applied voltage : −2.5; +2.5 V each during 120 s [33 and 38].

Fig. 12. Second cycle discharge–charge curves of a 6 μm thick Nb₂O₅ coating sintered at 520°C. Current density 0.1 mA/cm² [51].

Fig. 13. Photocurrent–voltage characteristics of 0.2 cm² solar cells made with Ru(II)-complex sensitized sol–gel niobia films measured under 10 mW/cm² solar irradiation (0.1 sun) [60 and 61].

Fig. 14. Solar-to-electric conversion efficiency of 0.2 cm² solar cells made with niobia electrodes [60 and 61] and a 0.4 cm² TiO₂ cell [63].

Table 1. Structural parameters for two different niobium oxide layers [60 and 61] and a TiO₂ layer [62] and electric parameters of solar cells made with such layers measured under various irradiance