Morphological Features of SiO2 Nanofillers Address Poor Stability Issue in Gel Polymer Electrolyte-Based Electrochromic Devices

Gaurav Kumar Silori; Subashchandrabose Thoka; Kuo-Chuan Ho

doi:10.1021/acsami.3c04685

Morphological Features of SiO₂ Nanofillers Address Poor Stability Issue in Gel Polymer Electrolyte-Based Electrochromic Devices

ACS Appl Mater Interfaces. 2023 May 31;15(21):25791-25805. doi: 10.1021/acsami.3c04685. Epub 2023 May 19.

Authors

Gaurav Kumar Silori¹, Subashchandrabose Thoka¹, Kuo-Chuan Ho^{1

2

3}

Affiliations

¹ Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan.
² Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan.
³ Advanced Research Center of Green Materials Science and Technology, National Taiwan University, Taipei 10617, Taiwan.

PMID: 37205840
DOI: 10.1021/acsami.3c04685

Abstract

Nanofillers' applicability in gel polymer electrolyte (GPE)-based devices skyrocketed in the last decade as soon as their remarkable benefits were realized. However, their applicability in GPE-based electrochromic devices (ECDs) has hardly seen any development due to challenges such as optical inhomogeneity brought by incompetent nanofiller sizes, transmittance drop due to higher filler loading (usually required), and poor methodologies of electrolyte fabrication. To address such issues, herein, we demonstrate a reinforced polymer electrolyte tailored through poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP),1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF₄), and four types of mesoporous SiO₂ nanofillers, porous (distinct morphologies) and nonporous, two each. The synthesized electrochromic species 1,1'-bis(4-fluorobenzyl)-4,4'-bipyridine-1,1'-diium tetrafluoroborate (BzV, 0.05 M), counter redox species ferrocene (Fc, 0.05 M), and supporting electrolyte (TBABF₄, 0.5 M) were first dissolved in propylene carbonate (PC) and then immobilized in an electrospun PVDF-HFP/BMIMBF₄/SiO₂ host. We distinctly observed that spherical (SPHS) and hexagonal pore (MCMS) morphologies of fillers endowed higher transmittance change (ΔT) and coloration efficiency (CE) in utilized ECDs; particularly for the MCMS-incorporated ECD (GPE-MCMS/BzV-Fc ECD), ΔT reached ∼62.5% and CE soared to 276.3 cm²/C at 603 nm. The remarkable benefit of filler's hexagonal morphology was also seen in the GPE-MCMS/BzV-Fc ECD, which not only marked an astounding ionic conductivity (σ) of ∼13.5 × 10^-3 S cm^-1 at 25 °C, thus imitating the solution-type ECD's behavior, but also retained ∼77% of initial ΔT after 5000 switching cycles. The enhancement in ECD's performance resulted from merits brought by filler geometries such as the proliferation of Lewis acid-base interaction sites due to the high surface-to-volume ratio, the creation of percolating tunnels, and the emergence of capillary forces triggering facile ion transportation in the electrolyte matrix.

Keywords: PVDF-HFP nanofiber; SiO2 morphology; electrochromic devices; electrospinning; gel polymer electrolytes; ionic liquid; nanofillers; stress−strain.