(603f) Electrochromic Transition Metal Oxide Nanocrystals

Transition metal oxide thin films are the active elements of electrochromic smart windows used to dynamically control daylighting and enhance energy efficiency. These devices operate on the same principles as lithium-ion batteries, but mechanistic understanding connecting their electrochemical and optical properties is underdeveloped for all but the most common electrochromic material, tungsten oxide. We use colloidal metal oxide nanocrystal synthesis to control composition and structure at both the atomic and nanoscale, which enables systematic investigation of how capacitive charging and ion intercalation processes activate the absorption of solar infrared and visible light. Nanocrystal inks are readily coated to make optical quality films, facilitating spectroelectrochemical analysis and potentially providing a pathway for low-cost manufacturing compatible with flexible form factors. We discovered that the crystal structure and morphology of niobium oxide nanocrystals play a dominant role in controlling their spectral response upon electrochemical reduction. Voltage-selective infrared and visible coloration in this material make it highly attractive for smart windows to control solar heating and daylighting on demand. Combining experimental and computational analysis of niobium, titanium, and tungsten oxides, we rationalize their distinctive electrochromic responses based on the nature of the electronic states populated when ions are electrochemically inserted. The mechanistic insights made possible by combining optical and electrochemical analyses may further be leveraged to understand and design next generation materials for battery electrodes.