Incorporating electrospun nanofibers of TEMPO-grafted PVDF-HFP polymer matrix in viologen-based electrochromic devices

https://doi.org/10.1016/j.solmat.2019.110375Get rights and content

Highlights

  • 4-amino-TEMPO is grafted onto PVDF-HFP and electrospun into nanofiber (TP-10 NF) .

  • TP-10 NF polymer matrix improves the stability and response time of ECD.

  • TP-10 ECD offers a high transmittance change (ΔT) of 72.3% at 605 nm.

  • TP-10 ECD shows write-erase response times of 2.3/1.5 s for bleaching/coloring.

  • TP-10 ECD retains 91.3/83.5% of its initial ΔT after 10,000 cycles at 399/605 nm.

Abstract

A novel polymer, denoted as TP-X, is synthesized by grafting 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) onto the poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) backbone. TP-X was electrospun into a nanofibrous polymer matrix and introduced into viologen-based electrochromic devices (ECDs). Heptyl viologen (HV) was chosen as the electrochromic material and TEMPO was added as the stable redox couple in the ECDs in this study. The results show that the addition of TP-X nanofibers (TP-X NF) effectively reduced the aggregation of HV and the grafted TEMPO moieties provide the extended surface area for electron transfer. The reduction in viologen aggregation led to outstanding stability and the extended electron transfer surface area resulted in fast write-erase response for ECDs with TP-X NF polymer matrix. The optimized ECD containing the TP-10 NF showed fast write-erase response times of 2.3 and 1.5 s for bleaching and coloring at 605 nm, respectively. More importantly, the incorporation of TP-10 NF gave improved stability of the ECDs, which retained more than 83% of its initial transmittance change at 605 nm after 10,000 operation cycles, as compared to the conventional solution-type ECD (27.3%). This study provides a potential polymer matrix to be utilized in various electrochemical applications for pursuing decent performance and reliable stability.

Introduction

Recent years, the worldwide energy crisis has drawn the attention of researchers to energy saving, energy storing, and energy converting devices. Among them, intensive studies were done on electrochromic devices (ECDs) with the aim to reduce energy consumption. Electrochromism refers to a phenomenon of which the variation in applied potential drives the redox reactions of electrochromic materials, which lead to color change. The most significant advantage of utilizing electrochromism is that high optical contrast can be achieved within a small potential window. To make the most of this attractive trait, many researchers and companies strived to extend the applications of electrochromics. Over time, electrochromism has been proposed for many applications, including anti-glare rear-view mirrors [1], smart windows [[2], [3], [4]], electrochromic displays [5], sensors [6], eyewears [7], and airplane windows with controllable light inlet [8].

Electrochromic (EC) materials serve as a key part of the ECDs. One of the most popular EC materials is viologen (or 1,1′-disubstituted-4,4′-bipyridinium salts) [9]. The family of viologens is attractive because of their remarkable transmittance change in the visible range (> 70%) [10], low driving voltage (some of which are < 1 V) [11], and fast response (< 1 s) [12]. Viologens have three oxidation states, which include V2+ (di-cation state, colorless), V+ (radical cation state, colored), and V0 (di-reduced state, colored). The redox reaction between V2+ and V+ is fairly reversible, while the re-oxidation of V0 is a less reversible process. Therefore, the applied potential is preferred to trigger the viologen in between di-cation and radical cation states. Aside from the appealing characteristics of viologens, several side effects of the viologen-based electrochromic devices (ECDs) seriously harm the EC performance: (1) formation of radical cation dimer, (2) aggregation of viologens on electrode, and (3) comproportionation of V2+ and V0 which leads to dimerization. The reduction of radical cation viologen dimers needs a larger potential or a longer re-oxidation time [9]. Aggregates of viologens hinder the write-erase ability. All of the above side effects result in poor stability of the ECDs. To solve the problems of viologen-based ECDs, recently many studies focused on developing alternative electrolyte layers. For example, Yun et al. proposed a stable ion gel-based ECD with monoheptyl viologen (MHV+) as EC material which gave a transmittance change of 65% at 545 nm and retained 77% of its initial transmittance change after 1,000 cycles [13]. Alesanco et al. presented a stable ethyl viologen-based ECD containing PVA-borax slime as quasi-solid-state electrolyte that showed no decay in the transmittance change after 10,000 cycles [14]. In addition, typical viologen-based ECDs are solution-type, which often suffer from electrolyte leakage. Therefore, developing alternative electrolyte layers could partially mitigate the leakage of solution.

The electrolyte layer is a crucial part of an ECD which strongly affects the electrochromic performance. An ideal electrolyte layer should possess the following properties: (1) high mechanical, chemical, and thermal stability, (2) easy fabrication with low cost, (3) negligible optical absorbance, (4) no color change upon applying voltage, (5) prevention of possible side reactions of viologen, and (6) good encapsulation in the device. Considering the criteria listed above, poly(vinylidene fluoride-cohexafluoropropylene), namely, PVDF-HFP, would serve as a promising material to be incorporated in the electrolyte layer of viologen-based ECDs. PVDF-HFP is a co-polymer with high flexibility, stability, and mechanical strength [15]. Also, it is fairly processible and its relatively amorphous structure favors ionic flow [16,17]. In addition, the fluorine atoms on PVDF-HFP are susceptible to attracting viologen cations and retarding aggregation [18,19].

The technique of electrospinning was first applied in ECDs in 2009, when Shim et al. fabricated WO3 nanowires by electrospinning [20]. The electrospinning process was further researched for the fabrication of both electrochromic thin film materials [21,22] and electrolytes [16,19]. This technique was also widely used in other electrochemical devices, such as lithium batteries [23], fuel cells [24], and dye-sensitized solar cells [25]. Electrospinning is a simple process which facilitates the production of porous film with controllable pore size and fiber thickness. A porous medium is ideal for the electrolyte layer of ECDs since it helps to hold the liquid without severely obstructing the ionic diffusion and migration [16]. For instance, we previously reported a phenyl viologen-based ECD utilizing composite electrospun nanofibers of PVDF-HFP and poly(oxyethylene)-imide imidazolium tetrafluoroborate (POEI-IBF4) as the solid-state electrolyte [19]. This ECD exhibited good ionic conductivity and retained 95.5% of its initial transmittance change after 1,000 cycles.

In this work, we introduced a novel electrospun polymer matrix into the electrolyte layer of viologen-based ECDs to improve the EC performance. 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl(4-amino-TEMPO) was partially grafted onto the PVDF-HFP backbone at various molar ratios to form TP-X polymers. Afterward, TP-X was made into a nanofibrous polymer matrix via electrospinning. The TP-X polymers were characterized with nuclear magnetic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FTIR). The morphologies of electrospun nanofibers were investigated with field-emission scanning electron microscopy (FE-SEM). In the ECDs, heptyl viologen (HV) served as the EC material and TEMPO was added as the stable redox couple. HV is a commercially available chemical which is more stable than other widely studied viologens, like methyl viologen (MV) and phenyl viologen (PV). The long, substituted alkyl chains are less likely to form aggregates, which results from π-π stacking, as compared to MV and PV. The ECDs containing the optimized TP-10 NF gave a transmittance change of around 70% at both 399 and 605 nm and showed a fast response time (< 2.5 s). More importantly, with the addition of TP-10 NF into the ECD, the retention of the initial transmittance change after 10,000 cycles is greatly improved by 56.2% and 38.7% at 399 and 605 nm, respectively, as compared to the solution-type ECD.

Section snippets

Chemicals

N,N-dimethylformamide (DMF, 99.8%), 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO, 97%), PVDF-HFP (with a weight-average molecular weight ~400,000), magnesium oxide (MgO), tetrabutylammonium tetrafluoroborate (TBABF4, 99%), 1,1′-diheptyl-4,4′-bipyridinium dibromide (heptyl viologen; HVBr2, 97%), and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%) were purchased from Sigma-Aldrich. The bromide anions in heptyl viologen were substituted with tetrafluoroborate (BF4) anions to

Characterization of TP-X and nanofibers

In this study, 4-amino-TEMPO was partially grafted onto the VDF section of the PVDF-HFP polymer chain to give TP-X. The chemical reaction involves the elimination of F atoms and the formation of new C=N bond. The chemical structure of the TP-X polymer was studied with 13C nuclear magnetic resonance spectroscopy (NMR) and Fourier-transform infrared spectroscopy (FTIR). Fig. 1 shows the FTIR spectra of PVDF-HFP, TEMPO grafted PVDF-HFP, and 4-amino-TEMPO blended with PVDF-HFP. The three results

Conclusions

In this study, novel polymer matrices composed of TP-X electrospun nanofiber were incorporated in the heptyl viologen-based ECDs. The introduced TP-10 NF significantly improved the ECD stability compared to conventional S-ECDs while remaining a fast switching time and high transmittance change. The TP-10 ECD gave ΔT of 68.3 and 72.3% at 399 and 605 nm respectively with fast response (2.3 s for bleaching and <2 s for coloring). Furthermore, TP-10 ECD retained 91.3 and 83.5% of its initial ΔT at

Author contribution statement

Wan-Ni Wu: Conceptualization, Methodology, Formal analysis, Investigation, Writing - Original Draft, Writing - Review & Editing.

Hsin-Fu Yu: Conceptualization, Investigation, Writing - Review & Editing

Min-Hsin Yeh: Conceptualization, Methodology, Investigation.

Kuo-Chuan Ho: Funding acquisition, Supervision, Resources, Writing - Original Draft, Writing - Review & Editing.

Funding sources

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the “Advanced Research Center for Green Materials Science and Technology” from The Featured Area Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (107L9006) and the Ministry of Science and Technology in Taiwan (MOST 105-2221-E-002-229-MY3, 106-2218-E-002-038, 107-3017-F-002-001, 107-2218-E-011-022-MY2).

References (37)

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