Understanding the Role of Charge Storage Mechanisms in the Electrochromic Switching Kinetics of Metal Oxide Nanocrystals

The development of electrochromic metal oxide nanocrystals holds promise for improving the sluggish switching kinetics of conventional electrochromic smart windows. Nevertheless, the microscopic processes controlling switching kinetics in nanocrystals may differ from those in traditional bulk materials where ion diffusion following intercalation is often rate limiting. Herein, by systematically investigating the electrochromic response of Sn-doped In2O3 nanoparticles, ortho-rhombic Nb2O5 nanorods, and monoclinic Nb12O29 nanoplatelets, we elucidate how different charge storage mechanisms, including capacitive charging, surface redox, and intercalation, affect the switching kinetics of electrochromic nanocrystals. The nanocrystals were reduced in both lithium- and tetrabutylammonium-based electrolytes at various potentials to determine which charge storage mechanism governs their electrochromic response, and the optical switching kinetics at a reducing potential were quantified by fitted with an exponential-growth model based on the charging behavior of capacitors. For the surface-dominated capacitive charging and surface redox mechanisms, dual-stage switching kinetics were observed regardless of the materials, switching rapidly at the early stage of reduction and becoming slower over time as charge accumulates in the electric double layer. As for the intercalation mechanism, single-stage switching kinetics con-trolled by the reaction rate of ion intercalation were observed. By using spectroelectrochemical methods, we demonstrated approaches to define the charge storage mechanisms in electrochromic metal oxide nanocrystals and investigated how these mechanisms affect the switching kinetics of the electrochromic response.