Repository logo

Optically detected ion insertion dynamics in hexagonal tungsten oxide




Evans, R. Colby, author
Sambur, Justin Barret, advisor
Prieto, Amy, committee member
Levinger, Nancy, committee member
Weinberger, Christopher R., committee member

Journal Title

Journal ISSN

Volume Title


Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which impacts charge storage dynamics and warrants their investigation at the single-nanoparticle level. This dissertation demonstrates a non-destructive, high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level. Electro-optical measurements relate the optical density change of a nanoparticle to redox changes of its redox-active elements under working electrochemical conditions. The technique was benchmarked by studying Li-ion insertion in hexagonal tungsten oxide (h-WO3) nanorods. Interestingly, the optically detected response revealed underlying processes that are hidden in conventional electrochemical measurements. This imaging technique may be applied to h-WO3 particles as small as 13 nm in diameter and a wide range of electrochemical materials such as electrochromic smart windows, batteries, solid oxide fuel cells, and sensors. This dissertation will focus on the impact of single particle h-WO3 on smart windows and batteries. Smart windows are devices used to modulate solar radiation into buildings and rely on the same ion insertion reaction as batteries. Electro-optical imaging showed that single nanorods exhibit a particle-dependent waiting time for optical changes (from 100 ms to 10 s) due to Li-ion insertion at optically inactive surface sites. Additionally, longer nanorods have larger optical modulation at equivalent electrochemical conditions than shorter nanorods and exhibit a Li-ion gradient that increases from the nanorod ends to the middle. The particle-dependent ion-insertion kinetics contribute to variable rate for optical density change and magnitudes across large-area smart windows. Single particles modulate optical density (undergo ion insertion reactions) 4 times faster and 20 times more reversibly than thin films made of the same particles. A smart window device architecture is proposed to maximize lifetime based on these findings. More information can be found in CHAPTER 4. Further, the role of crystalline surface facets on the role of ion insertion were investigated. Two samples of h-WO3 were synthesized with different ratios of surface facets exposed to a Li-ion containing electrolyte. The sample with unique {120} facets exhibited reversible optical switching after 500 cycles and negligible variation in interfacial charge transfer resistance. The (120) surface features an open network of square window channels that may enable reversible ion transport and reduced ion trapping, enhancing the optical switching stability. However, the {120}-dominant sample exhibited lower coloration efficiency (CE) than the {100}-dominant sample. The reduced optical density changes in the {120}-dominant sample could be due to a greater fraction of optically inactive trigonal cavity sites on the {001} endcaps. The results indicate surface facet and particle morphology engineering are viable strategies to enhance the CE and long-term stability/lifetime in electrochromic thin films for smart window applications. More information can be found in CHAPTER 5. On average, these h-WO3 particles exhibit a hybrid charge storage mechanism: both diffusion-limited (battery-like, slower) and pseudocapacitive (capacitor-like, faster) mechanisms contribute to the total charge stored. Individual particles exhibit different charge storage mechanisms at the same applied potential. Longer nanorods store more pseudocapacitive charge than shorter nanorods, presumably due to 1) a surface step edge gradient that exposes large hexagonal window Li-ion binding sites along the nanorod length and/or 2) higher structural water content that influences the Li-ion binding energetics and diffusion behavior. Importantly, penetration depth of Li-ion insertion was quantified which showed that Li ions insert as deep as two-unit cells below the surface. The methodology presented herein can be applied to a wide range of solid-state ion-insertion materials and its implications for future discoveries are discussed. More information can be found in CHAPTER 6.


Rights Access


ion insertion dynamics
tungseten oxide


Associated Publications