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Organic/Inorganic Electrochromic Films and Devices with Tunable Structure and Properties for Smart Window Applications

Author: MaDongYun
Tutor: WangHongZhi
School: Donghua University
Course: Materials Science and Engineering
Keywords: electrochromism micro/nanostructured film smart windows nickel oxide tungsten oxide electrochromic device
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Type: PhD thesis
Year: 2013
Downloads: 90
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Abstract


With the emergence of environmental pollution and energy crisis, effective energy conservation and environmental protection for achieving sustainable development are currently important issues in scientific community. Electrochromic (EC) materials, as one of the energy-saving materials, have attracted vast attention owing to the potential application in smart windows. Although the research is very wide for the organic/inorganic EC materials, there are many problems in the practical applications, such as the poor cycle stability, slow switching responses, low optical modulation and high power consumption, which make them difficult to meet the needs of practical applications. This thesis focused on the liquid-phase fabrication of inorganic and organic/inorganic composite EC films composed of micro/nanomaterials. The effects of the various nanostructures on the EC performance of these films were examined in detail. Finally, an all-solid-state electrochromic device based on NiO/WO3complementary structure and gel electrolyte was fabricated and its EC properties were also systematically investigated.Vertically aligned 1D WO3nanostructure and self-weaving WO3nanoflake films were grown on fluorine-doped tin oxide (FTO) coated glass substrate using a crystal-seed-assisted hydrothermal technique, respectively. Detailed mechanistic studies revealed that the composition of the precursor solution played important roles in determining the final shape and size of the WO3nanostructures. The rectangular nanorod array film can be obtained in the presence of1.2mmol urea with water as the solvent. While the cylindrical nanorods were grown by tuning the solvent composition with the urea content fixed at1.2mmol. Due to the large tunnels in the hexagonally structured WO3, and the large active surface area available for electrochemical reactions, a large optical modulation of66% at632.8nm and a potential of -2.0V, fast switching speeds of6.7s and3.4s for coloration and bleaching, respectively, and a high coloration efficiency of106.8cm2/C were achieved for the cylindrical nanorod array film. Interestingly, with addition of oxalic acid and urea to a mixed solvent of de-ionized water and acetonitrile, WO3nanoflakes woven from nanowires were grown. This unique nanostructure gives a coarse surface, making a larger surface area available for reactions in electrochemical processes and therefore a higher coloration efficiency of134.4cm2/C was achieved. Moreover, the braided structure is more solid than that of the previously reported nanowire arrays and may enhance the long-term cycling stability, which is a crucial parameter for practical devices. However, its coloration and bleaching times were prolonged because the intertwined structure would hinder the ion intercalation/deintercalation.Thin films of hexagonal NiO nanoplates and hierarchical NiO micro flakes assembled from nanoleaves were grown directly on FTO-coated glass substrates using a facile and template-free hydrothermal technique. As compared to conventional nanoporous structures or other1D nanostructures, the hierarchically assembled microflakes produced a larger active surface area, which is not only limited to the large open-pore voids between the microflakes, but also to the pore structure formed by the oriented assembly of the nanoleaves. It is noteworthy that the microflakes are made of atomically thin nanoleaves, which is similar to the layered WO3and MoO3nanosheets reported in the previous literatures. Such a layered structure that made of atomically thin nanoleaves would facilitate ion intercalation/deintercalation by reducing their diffusion path lengths. Thus, the films exhibited a high optical modulation (62.5%at550nm), large coloration efficiency (89.3cm2/C at550nm by applying a low coloration voltage of-1.0V) and fast switching responses with a coloring time of4.6s and a bleaching time of7.2s. Moreover, for the hierarchically assembled NiO films, the nanoleaves were assembled into microflake through the noncovalent bonds and gave an orderly integration architecture. This structure exhibited better structural stability and mechanical properties than the randomly distributed nanoplates. Therefore, it would reduce the dissolution caused by volumetric changes and lattice stresses during the repeated Li+ insertion and extraction process and result in a good cycling durability.The soaking method and electrochemical deposition method were used to fabricate micro/nanostructured PANI EC films. Films of PANI nanoparticles were obtained by soaking the FTO glass in a solution containing0.5mol/L aniline and0.5mol/L aqueous H2SO4with oxalic acid as a dopant.When acrylic acid was used as the dopant, the resulting films were composed of PANI microrods. For the electrochemical oxidation polymerization of PANI film, the mode of deposition had a significant impact on the morphologies of the films. The dense amorphous PANI films were prepared through the constant current or voltage mode. While the nanorods film was grown by potentiodynamic cycle at a sweep rate of100mV/s. Therefore, the potentiodynamic cycle method was used to deposit PANI nanostructures on the WO3nanorods array film. The resulting WO3/PANI composite films exhibited double electrochromic effect of WO3and PANI. Compared with the single WO3films, the composite films showed tuning-color and fast switching responses with a coloring time of800ms and a bleaching time of500ms, which is attributed to the formation of donor-acceptor systems. The composite films also exhibited a high optical modulation (42% at632.8nm), large coloration efficiency (76cm2/C by applying a low coloration voltage of0.8V) as compare to the single PANI film.The highly porous NiO/polyaniline composite films were prepared by combining hydrothermal process and electro-polymerization. Firstly, three types of porous NiO films have been synthesized by tuning the composition of the precursor solution in hydrothermal reaction vessel. Secondly, PANI nanostructures were deposited onto the as-prepared NiO porous structures by potentiodynamic cycle method. Since NiO and PANI are both colored under positive voltage and bleached under negative voltage, the NiO/PANI composite films showed highly enhanced EC performance as compared to the WO3/PANI composite films. A large optical modulation of62% at550nm, fast switching speeds of90and120ms for coloration and bleaching, respectively, and a high coloration efficiency of121cm2/C were achieved for the composite film assembled from NiO nanoplates and polyaniline pleated structures. An all-solid-state EC device for modulating the optical transmittance was fabricated based on NiO/W03complementary structure and gel electrolyte. For comparation, a corresponding device based on liquid electrolyte was also manufactured. Due to the fast ions migration velocity, the device based on liquid electrolyte showed fast switching speeds of1.8and3.2s for coloration and bleaching, respectively, and a high coloration efficiency of146.9cm2/C. However, the device based on gel electrolyte gave poor EC performance because the migration of ions in the gel electrolyte was hindered by the non-conductive polymer frame, which led to a low switching kinetics. Moreover, due to the poor chemical stability and mechanical strength of the gel electrolyte, the polymer frame would be destroyed or collapsed during the coloration/bleaching process, and led to an irreversible cycle. Thus, it is necessary to improve the EC performance of all-solid-state device in our following research work and make it feasible for practical applications.

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