Ph.D Thesis


Ph.D StudentBasila Kattouf
SubjectComposite Electrode for Proton-Based Electrochromic
Devices
DepartmentDepartment of Materials Science and Engineering
Supervisors Full Professors Frey Gitti
Professor Emeritus Siegmann Arnon (Deceased)
Full Professors Ein-Eli Yair
Full Thesis textFull thesis text - English Version


Abstract

Electrochromic (EC) materials change their absorption spectrum under applied bias and hence are promising materials for a variety of energy-saving applications including smart windows, sunroofs and displays. The operation of an electrochromic device is based on the injection of electrons into the electrochromic material, usually a multivalent transition metal oxide, followed by diffusion of cations into the material to balance the negative charge. The switching rate is therefore limited by the slowest process, i.e. cation insertion into the metal oxide. In this work we have devised two methodologies to induce fast switching in tungsten oxide-based electrochromic devices. The first approach enhances the introduction of protons into the metal oxide, while the second significantly improves proton diffusion in the material. In the latter we have found that the capacity value and charge-exchange quantities of WO3?H2O can be dramatically increased upon conversion of WO3?H2O to its di-hydrate WO3?2H2O by electrochemical cycling. The phase transformation, evident from X-ray diffraction measurements, results in the formation of additional spacious water-paved pathways for cation insertion/extraction. The transformation showed a dramatic increase of ~50% in the overall charge during the first 150 cycles. Enhancing proton insertion into WO3 was achieved by increasing the electrolyte/WO3 interfacial area through porosity. Porous amorphous WO3 films were prepared by sol-gel processing WO3 in presence of an organic surfactant which is then thermally removed. The highly porous amorphous WO3 network, evident from scanning electron microscopy analysis, is then infiltrated with Nafion, a polymer proton conductor, forming a novel hybrid electrochromic material characterized using scanning and Energy Filtered Transmission Electron Microscopy (EFTEM). The new material allows the fabrication of an all solid-state electrochromic device with rapid colour switching rates (in the order of 2 seconds).