An experimental setup is presented, which provides the opportunity to perform optical transmission analysis during electrochemical ion insertion into electrochromic materials, such as WO3, with a time resolution on the second scale and local information on the micrometer scale. To conduct the experiments, WO3 thin film electrodes were synthesized by various methods and lithographically micro structured. WO3 was analyzed with respect of two main types of electrochemical experiments: (1) time resolved in situ transmission experiments during electrochemical ion insertion/extraction and (2) spatially and time resolved transmission experiments for the visualization and quantification of diffusion in WO3.(1) The correlation between the oxidation states of tungsten and the coloration efficiency of nanocrystalline WO3 was analyzed and a two-step mechanism for the coloration process is proposed. For this the lithium inserted WO3 electrodes were additionally investigated by XRD and XPS. During coloration tungsten is electrochemically reduced, and the formation of W5+ was found in the first step. However, only the formation of W4+, which takes place in the second step (at lower electrode potentials), comes along with the main coloration. XRD results show that after the formation of W4+, LixWO3 exhibits a cubic crystal structure instead of the initial tetragonal structure.(2) By patterning a transparent layer deposited on top of the WO3 thin film, local insertion of hydrogen into the thin film could be achieved and ion transport in the plane of the WO3 film could be visualized. The obtained temporal and spatial dependence of the observed coloration of WO3 was described by a simple 1D diffusion model. A diffusion coefficient for hydrogen in amorphous WO3 thin films was determined with a value of D = 1.3E(-10) cm^2/s.
