Black electrochromic device enabled by reversible nickel-copper electrodeposition

• Main Author: Guo, Xiaoyu
• Other Authors: Lee Pooi See
• Format: Thesis-Master by Research
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/162745
• Institution: Nanyang Technological University

An electrochromic device (ECD) continuously controls optical properties such as optical transmittance, reflectance or emittance in a reversible manner when applying a voltage. There are several mechanisms controlling the color change of ECD, in which reversible metal electrodeposition and dissolution (RMED) is particularly attractive owing to its excellent electrochromic performance and easy processability. A black ECD that can switch its state between highly transparent and opaque black when a potential pulse is applied, is an excellent option for dynamic windows owing to outstanding color neutrality and good stability. The majority of the black ECDs are realized by reversible metal electrodeposition mechanism, nevertheless, they usually have relatively slow response time as well as poor cycling stability. Additionally, as a result of the low electrical conductivity of nickel, reversible nickel electrodeposition always results in poor homogeneity of the metallic layer and an unstable switching process. As a result, an alloying element (copper), which has excellent electrical conductivity and more positive reduction potential, is introduced to create a nucleation layer, facilitating the electrodeposition of the parent metal, and thus fulfilling good uniformity and stability. Gel polymer electrolytes, which aid in the formation of compact and uniform metallic film and maintain a fast response time, are employed to fabricate a heat-insulating black ECD. By utilizing Cu as an alloy metal and a suitable viscous electrolyte, the black ECD can realize switching stability (λ = 550 nm) for over 1500 cycles with maximum optical contrast of 55.2 % in a short respond time (coloration: 6.2 s; stripping: 13.2 s). Besides, the ECD is able to be converted from its original transparent state (visible light transparency: 56.6 %) to opaque black state (visible light transparency: 0.3 %) in less than 1 min, realizing a transmittance of less than 5 % over the visible-near-infrared regions (400 – 2000 nm) to effectively block solar heat. Additionally, the black-colored ECD at the voltage-off state can be retained for over 1 hour (at room temperature, 550 nm). Under 30-min infrared radiation (170 W/m2), the black-colored ECD prevents up to 35.0 % of infrared irradiation from entering, successfully preventing heat transmission for energy management. Solid polymer electrolytes can further enhance the transparency, stability as well as memory retention of ECD. The adoption of a co-solvent with a compatible weight fraction in the electrolyte (DMSO: water = 4: 1) can aid in the development of an optimal electrolyte with superior transparency (visible light transmittance: 83.8 %) conductivity (0.11 mS/cm), and mechanical properties (tensile strength: 11.1 kPa, breaking strain: 242.6 %). Additionally, the black ECD containing solid electrolyte can realize outstanding optical contrast from a transparent state to a colored state (visible light transmittance: 70.8 % vs 0.085 %), excellent cycling stability of 2000 cycles with minor optical contrast degradation (less than 10 %) at the wavelength of 550 nm, and superior color neutrality (c* = 1.33). Besides, due to the high viscosity of solid electrolyte (over 100 kcp), the homogeneous electrodeposition film can be electrodeposited on the working electrode with a surface roughness of approximately 11.2 nm, and the black-colored ECD can be retained for more than 350 min with transmittance increase of only 5 % when voltage is removed after the application of 3 V for 5 min, indicating the promising optical memory for smart windows, particularly where privacy protection is required. The flexible electrode is the latest development in meeting a variety of growing demands. The key to flexible ECD is to fabricate flexible transparent and conductive electrodes that are both chemically and environmentally stable. Commercial ITO-coated polyethylene naphthalate (PEN) was proved to be unsuitable for use due to the instability of the ITO layer, which potentially dissolves into the electrolyte. Therefore, post-annealing is employed to enhance the crystallization of the ITO thin layer on the flexible substrate, resulting in increased conductivity as well as stability. Flexible ECDs using annealed ITO-coated PEN as electrodes are realized excellent switching stability for 400 cycles with up to 43 % optical contrast, which is the first demonstration of flexible ECDs based on RMED.