Investigating the interfacial process and bulk electrode chemistry in tungsten oxide electrochromic materials

Hu, Anyang

Investigating the interfacial process and bulk electrode chemistry in tungsten oxide electrochromic materials

dc.contributor.author	Hu, Anyang	en
dc.contributor.committeechair	Lin, Feng	en
dc.contributor.committeemember	Zhou, Wei	en
dc.contributor.committeemember	Tissue, Brian M.	en
dc.contributor.committeemember	Madsen, Louis A.	en
dc.contributor.department	Chemistry	en
dc.date.accessioned	2020-10-19T14:12:31Z	en
dc.date.available	2020-10-19T14:12:31Z	en
dc.date.issued	2020	en
dc.description.abstract	The growing need for high-performance electrode materials in electrochemical conversion and storage applications requires further fundamental investigation on the working and degradation mechanisms of these materials. Among various functional materials, transition metal oxides are still one of the main choices due to their tunable chemical compositions and diverse crystal structures in most aqueous and organic electrolytes. The charge transfer process mainly occurs at the electrode-electrolyte interface, and controlling the electrochemical interfacial stability represents a key challenge in developing sustainable and cost-effective electrochromic materials. The present thesis focuses on classical tungsten trioxide (WO3) materials as the platform to uncover the previously unknown interrelationship between phase transformation, morphological evolution, nanoscale color heterogeneity, and performance degradation in these materials during 3,000 cyclic voltammetry cycles. Through the application of novel cell design, synchrotron/electron spectroscopic, and imaging analyses, we observe that the interface between the WO3 electrode and 0.5 M sulfuric acid electrolyte undergoes constant changes due to the tungsten oxide dissolution and redeposition. The redeposition of dissolved tungsten species provokes in situ crystal growth, which ultimately leads to phase transformation from the semicrystalline WO3 to a nanoflake-shaped, proton-trapped tungsten trioxide dihydrate (HxWO3·2H2O). The multidimensional (surface and bulk) quantification of the electronic structure with X-ray measurements reveals that the tungsten reduction caused by proton trapping is heterogeneous at the nanometric scale and is responsible for the nanoscale color heterogeneity. The Coulombic efficiency, optical modulation, apparent diffusion coefficients, and switching kinetics are gradually diminished during 3,000 cyclic voltammetry cycles, resulting from the structural and chemical changes of the WO3 electrode. We hypothesize that the high interfacial reactivity in the electrode-electrolyte interfacial region could be the universal underlying mechanism leading to undesired bulk structural changes of inorganic electrochromic materials.	en
dc.description.abstractgeneral	With the rapid development of human society, the research of new energy-saving materials has become a focus of attention. Among them, electrochromic devices can effectively adjust their color through a controllable electrochemical reaction and have a wide range of uses in our daily life. For example, smart windows can reduce glare and heat without blocking the natural light, thereby providing buildings and vehicles with better thermal and visual comfort. Electrochromic optical displays can lower energy consumption. Variable reflectance mirrors such as anti-glare car rear-view mirrors can ensure the safety of driving. Lastly, wearable apparel such as electrochromic lenses for spectacles and sunglasses can protect users from ultraviolet radiation. Although electrochromic materials and devices have not expanded from the niche market, the enormous potential that they hold cannot be ignored and wide-scale commercialization should be sought after. Tungsten oxides electrochromic devices have proved to utilize the full spectrum of the incident light through structure design. These devices can also be configured with solar cells as a state-of-art integrated self-powered system with satisfactory optical modulation that can be obtained without any external electrical energy input. Moreover, WO3-based devices have also been combined with electrodeposition technology to achieve fast color-switching kinetics. However, the long-term durability in the acidic electrolyte under electrochemical cycling conditions needs to be further improved, and the road of full commercialization is still unpaved. To design high-performance electrochromic materials, it is imperative to study the degradation mechanism under long-term electrochemical cycling conditions. In the present thesis, the performance degradation of the WO3 electrode in acid electrolytes involves chemical changes. Through a better understanding of the fundamental degradation process, the design of high- performance electrochromic metal oxides can be developed.	en
dc.description.degree	M.S.	en
dc.format.medium	ETD	en
dc.identifier.uri	http://hdl.handle.net/10919/100613	en
dc.language.iso	en	en
dc.publisher	Virginia Tech	en
dc.rights	Creative Commons Attribution-ShareAlike 4.0 International	en
dc.rights.uri	http://creativecommons.org/licenses/by-sa/4.0/	en
dc.subject	electrochromism	en
dc.subject	morphological evolution	en
dc.subject	phase transformation	en
dc.subject	charge heterogeneity	en
dc.subject	durability	en
dc.title	Investigating the interfacial process and bulk electrode chemistry in tungsten oxide electrochromic materials	en
dc.type	Thesis	en
thesis.degree.discipline	Inorganic chemistry	en
thesis.degree.grantor	Virginia Polytechnic Institute and State University	en
thesis.degree.level	masters	en
thesis.degree.name	M.S.	en