ISSN 1759-9962

COMMUNICATION Seung Uk Son et al. Microporous organic polymer-induced gel electrolytes for enhanced operation stability of electrochromic devices

Polymer Chemistry

rsc.li/polymers

Volume 10 Number 4 28 January 2019 Pages 427–556

PolymerChemistry

COMMUNICATION

Cite this: Polym. Chem., 2019, 10,455

Received 31st August 2018,Accepted 5th December 2018

DOI: 10.1039/c8py01277f

rsc.li/polymers

Microporous organic polymer-induced gelelectrolytes for enhanced operation stabilityof electrochromic devices†

Ju Hong Ko,‡a Hyunjae Lee,‡a Jaewon Choi,a June Young Jang,a Sang Moon Lee,b

Hae Jin Kim,b Yoon-Joo Koc and Seung Uk Son *a

Gel electrolytes were synthesized by the formation of microporous

organic polymers in the presence of 0.077–0.77 M LiClO4 and

N-methylpyrrolidone. Electrochromic devices fabricated using the

gel electrolytes showed enhanced operation stability, compared

with those fabricated using liquid electrolytes, which is attribu-

table to the efficient separation of working viologens from count-

ing molecules.

Gelation is an interesting phenomenon which results from thedilute 3D networking of polymer chains in solution.1 Theresultant gel materials contain a liquid in the networks andbehave like a solid. Thus, usually they do not flow. Also, gelmaterials can accommodate additional guest molecules. Basedon their intrinsic properties, gel materials have been appliedfor various purposes including molecular carriers. Forexample, gel electrolytes have been prepared through captur-ing appropriate salts and have been applied for various electri-cal devices.2 Although conventional liquid electrolytes showrelatively fast ion diffusion, they can suffer from the potentialleakage issue of electrical devices. In contrast, while gel elec-trolytes retard the ion diffusion kinetics, the electrical deviceswith gel electrolytes can be free from the leakage. Moreover,the operation stability of electrical devices can be improved inspecific cases. Thus, various gel electrolytes have been used forthe fabrication of practical devices.2

To induce gelation, synthetic methods of chemical network-ing are required.1 Recently, networking methods have beenextensively studied in the research field of microporousorganic polymer (MOP) materials.3 For example, the

Sonogashira coupling of tetraethynylarenes with dihaloarenesforms infinite network structures.4 Moreover, the chemicaland physical features of MOPs can be easily tuned by scanningbuilding blocks.5 One can expect that MOP chemistry can beapplied for the fabrication of gel electrolytes. However, as faras we are aware, there have been no reports on the develop-ment of gel electrolytes based on MOP chemistry. In this work,we report the preparation of MOP material-based gel electro-lytes (MOP-G) and the enhanced operating stability of electro-chromic devices with gel electrolytes, compared with conven-tional liquid electrolytes.

Fig. 1 reveals the formation of gel electrolytes based onMOP chemistry. Electrochromism is defined as a reversiblecolor change depending on applied potentials.6 Electrochromicdevices are very promising as energy saving systems becausenot only their operation requires relatively low electricalenergy, but also they maintain colored states without additionalenergy.7 Moreover, electrochromic devices have been appliedas energy-managing smart windows through controlling theabsorption of visible light.8 During the last decade, ourresearch group has studied electrochromic devices using new

Fig. 1 Formation of gel electrolytes based on MOP chemistry.

†Electronic supplementary information (ESI) available: Experimental pro-cedures, the PXRD pattern and the TGA curve of MOP material, and the rheologi-cal properties of MOP-Gs. See DOI: 10.1039/c8py01277f‡These authors contributed equally.

aDepartment of Chemistry, Sungkyunkwan University, Suwon 16419, Korea.

E-mail: sson@skku.edubKorea Basic Science Institute, Daejeon 34133, KoreacLaboratory of Nuclear Magnetic Resonance, National Center for Inter-University

Research Facilities (NCIRF), Seoul National University, Seoul 08826, Korea

This journal is © The Royal Society of Chemistry 2019 Polym. Chem., 2019, 10, 455–459 | 455

Publ

ishe

d on

06

Dec

embe

r 20

18. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

3/8/

2019

1:0

7:32

PM

.

View Article OnlineView Journal | View Issue

molecular and polymeric materials.9 A liquid electrolyte ofLiClO4/N-methylpyrrolidone (NMP) is one of the conventionalsystems for the fabrication of electrochromic devices.9

However, for more practical applications, efficient gel or solidelectrolytes need to be explored.

We induced the gelation of LiClO4/NMP solution throughthe Sonogashira-based networking of tetrakis(4-ethynylphenyl)methane10 and 1,4-diiodobenzene. To be free from volatileamine bases such as triethylamine and diisopropylamine, wechose 1,4-diazabicyclo[2.2.2]octane (DABCO) with a boilingpoint of 174 °C as a base for the Sonogashira coupling.11 Withthe fixed amount of tetrakis(4-ethynylphenyl)methane, 1,4-diiodobenzene, Pd/Cu catalysts, and DABCO, various amountsof LiClO4 and NMP were applied (Table S1 in the ESI†).Table 1 and Fig. 2 summarize the formation of MOP-G gelelectrolytes.

First, we induced the gelation of LiClO4 (33 mg) in NMP(3 mL). After 2 days, while the gel material was obtained,DABCO originally dissolved in NMP formed precipitatesduring gelation due to the reduced solubility of DABCO in gelmaterials (entry 2 in Table 1 and MOP-G-2 in Fig. 2). When thevolume of NMP was reduced to 2 mL, the DABCO precipitatesincreased (entry 1 in Table 1 and MOP-G-1 in Fig. 2). When thevolume of NMP was increased to 4 mL, the DABCO precipitateswere not observed and a transparent gel material was obtained(entry 3 in Table 1 and MOP-G-3 in Fig. 2).

Interestingly, LiClO4 showed good solubility in gelmaterials. As shown in entries 3–5 in Table 1 and Fig. 2, trans-parent gel materials (MOP-G-3–5) were obtained using0.077–0.77 M LiClO4 in NMP (4 mL). No precipitates of LiClO4

or DABCO were detected in MOP-G-3–5. Powder X-ray diffrac-tion (PXRD) analysis of MOP-G-3–5 showed completely amor-phous characteristics and no crystalline peaks correspondingto LiClO4 was detected, indicating the homogeneousdistribution of LiClO4 in gel materials (Fig. S1 in the ESI†).When we used 2.3 M LiClO4 in NMP (4 mL), the gelation wasretarded. Even after one week, gel materials were notobserved. Instead, the precipitates of LiClO4 originallydissolved in NMP formed because of the reduced solubility ofLiClO4 in the gel materials (entry 6 in Table 1 and MOP-G-6in Fig. 2).

Rheological analysis showed that the storage moduli (G′) ofMOP-G-3–5 were larger than their loss moduli (G″), indicatingthe gel characteristics (Fig. S2 in the ESI†) The G′ values ofMOP-G-3–5 were measured as 4.7 × 103, 5.9 × 103, and 3.4 ×103 Pa, respectively. The G″ values of MOP-G-3–5 weremeasured as 1.8 × 102, 3.0 × 102, and 4.3 × 103 Pa, respectively.As the amount of LiClO4 increased, the damping factor (G″/G′)of gel electrolytes increased from 0.038 (MOP-G-3) to 0.050(MOP-G-4) and 0.126 (MOP-G-5), indicating the retardation ofgelation by excess LiClO4.

The chemical components of MOP-G-3 were further ana-lyzed by various spectroscopies. Infrared absorption (IR) spec-troscopy showed the CvO vibration12a of NMP, the Cl–O vibra-tion12b of ClO4

−, and the N–H vibration13 of DABCO at 1663,1082, and 3413 cm−1, respectively (Fig. 3a). After these addi-tives were removed by washing, the remaining MOP materialwas analyzed by IR spectroscopy. The main vibration peaks ofthe CvC and C–H bonds of aromatic groups were observed at1492 and 814 cm−1, respectively, matching well with the majorpeaks of MOP materials in the literature.14 In the IR spectrumof MOP-G-3, the peaks of MOP materials were observed at 1491and 814 cm−1 (indicated as asterisks in Fig. 3a), implying thatthe MOP-G-3 was formed by the MOP materials.

Table 1 Synthetic conditions and properties of MOP-G electrolytesa

EntryLiClO4(mg)

NMP(mL)

Codename

Conductivityb

(mS cm−1)

1 33 2 MOP-G-1 —2 33 3 MOP-G-2 —3 33 4 MOP-G-3 2.564 99 4 MOP-G-4 3.945 330 4 MOP-G-5 6.336 1000 4 MOP-G-6 —

a Reaction conditions: Tetrakis(4-ethynylphenyl)methane (0.13 mmol),1,4-diiodobenzene (0.26 mmol), (PPh3)2PdCl2 (1.3 µmol), CuI(1.3 µmol), DABCO (0.26 mmol), 2 days, room temperature, no stirring.b Ionic conductivities of gel electrolytes at 25 °C.

Fig. 2 Photographs of the formation of gel materials. (Refer to Table 1for the notation of gel materials.)

Communication Polymer Chemistry

456 | Polym. Chem., 2019, 10, 455–459 This journal is © The Royal Society of Chemistry 2019

Publ

ishe

d on

06

Dec

embe

r 20

18. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

3/8/

2019

1:0

7:32

PM

. View Article Online

The MOP material in MOP-G-3 was isolated by washing andfreeze-drying. The surface area and micropore volume of theisolated MOP material were measured as 561 m2 g−1 and0.20 cm3 g−1, respectively, through the analysis of N2 sorptionisotherm curves (Fig. 3b). Pore sizes were distributed at <5 nmwith main pores at <2 nm (inset of Fig. 3b). Solid state 13Cnuclear magnetic resonance spectroscopy (NMR) of the MOPmaterial showed the 13C peaks of benzyl carbon and internalalkynes at 64 and 87–97 ppm, respectively (Fig. 3c). The 13Cpeaks of aromatic groups appeared at 146, 137, 130, and121 ppm, matching well with the 13C NMR spectra of MOPmaterials prepared by using the same building blocks in theliterature.14 PXRD analysis showed the amorphous nature ofthe MOP material, matching with the conventional MOPmaterials in the literature (Fig. S1 in the ESI†). According tothermogravimetric analysis, the MOP material was stable up to350 °C (Fig. S3 in the ESI†).

Next, we studied the MOP-G materials as gel electrolytes forelectrochromic devices. Fig. 4 summarizes the results. Variouselectrochromic compounds have been developed for the revers-ible color changes depending on the applied potentials.6 Oneof the most studied electrochromic compounds is a viologenderivative showing the color change from colorless to blue byone-electron reduction.15 Moreover, viologens were grafted onsemiconductor layers such as TiO2 to be engineered on theelectrode.15 For the reversible and reductive electrochromicaction of viologens, appropriate oxidative counting compoundsare required. For example, phenothiazine derivatives haveshown oxidative electrochromic behaviors from colorless tored.15 Thus, the performance of electrochromic devices con-sisting of the MOP-G-3–5, viologen-grafted TiO2/ITO as theworking electrode, N-butylphenothiazine in the electrolyte as acounting material, and an ITO electrode was studied. The

electrochemical cells showed a color change from the originalyellow (a color of MOP materials) to violet (an overlapped colorof blue from reduced viologens and red from oxidizedphenothiazines) with reversible redox peaks at −1.48–1.52 V,

Fig. 3 (a) IR absorption spectra of MOP-G-3 and MOP material. (b) N2

isotherm curves (77 K) and pore size distribution diagram (based on theDFT method) and (c) solid state 13C NMR spectrum of MOP.

Fig. 4 (a) UV/vis absorption spectra and photographs of the colored andbleached states of an electrochromic cell fabricated using MOP-G-4.(b) Nyquist plots of MOP-G-3–5 and 0.077 M LiClO4/NMP (inset: thephotographs of MOP-G-3 and 0.077 M LiClO4/NMP on the ITO elec-trode). Cycling performances of electrochromic cells fabricated using(c) MOP-G-3–5 and (d) liquid electrolytes. (Alternating voltages of −1.65and +0.5 V were applied for 30 s each for the coloring and de-coloringprocesses, respectively. A working electrode and a counting material werethe grafted viologens/TiO2/ITO and N-butylphenothiazine, respectively.The ΔA values at 550 and 528 nm were measured for the cells fabricatedwith gel and liquid electrolytes, respectively.) Operation illustrations ofelectrochromic cells with (e) gel and (f) liquid electrolytes.

Polymer Chemistry Communication

This journal is © The Royal Society of Chemistry 2019 Polym. Chem., 2019, 10, 455–459 | 457

Publ

ishe

d on

06

Dec

embe

r 20

18. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

3/8/

2019

1:0

7:32

PM

. View Article Online

showing a maximum absorption at 550 nm (Fig. 4a and S4 inthe ESI†).

According to electrochemical impedance spectroscopy(EIS), the ionic conductivities of MOP-G-3–5 were measured as2.56, 3.94, and 6.33 mS cm−1 at 25 °C, respectively (Fig. 4b). Itis noteworthy that ionic conductivities higher than 1 mS cm−1

are considered as a requirement for practical devices.16,17 Theionic conductivities of MOP-G-3–5 are superior or comparableto those (conductivities of 1.1–6.7 mS cm−1) of recent salt/polymer composite electrolytes used for electrochromicdevices.17,18

As shown in Fig. 4c, the electrochromic cells fabricatedwith MOP-G-3–5 showed excellent cycling stability. While thecells with MOP-G-3 and MOP-G-4 showed 98 and 99% reten-tion of the original ΔA (at 550 nm), respectively, after 150cycles (9000 s), that with MOP-G-5 showed a slight decrease ofΔA after 83 cycles (5000 s) and 75% retention of the originalΔA (at 550 nm) after 150 cycles (9000 s). In the extendedcycling tests, the cell with MOP-G-4 showed 99 and 88% reten-tion of the original ΔA (at 550 nm) after 670 cycles (40 000 s)and 1000 cycles (60 000 s), respectively (Fig. S5 in the ESI†). ΔAat 550 nm gradually increased from 0.43 (MOP-G-3) to 0.73(MOP-G-4) and 0.82 (MOP-G-5), resulting from the increasedionic conductivities of electrolytes. Coloring response times,defined as the time taken in reaching 90% of maximum ΔA,increased from 3.5 s (MOP-G-3) to 4.7 (MOP-G-4) and 5.1 s(MOP-G-5) due to the increased ΔA. De-coloring responsetimes increased from 7.3 s (MOP-G-3) to 8.3 (MOP-G-4) and 9.0s (MOP-G-5). The color efficiencies were measured as 390(MOP-G-3), 194 (MOP-G-4), and 154 cm2 C−1 (MOP-G-5),respectively (Fig. S6 in the ESI†). In control studies, the electro-chromic cells fabricated with liquid electrolytes of 0.077, 0.23,and 0.77 M LiClO4/NMP showed an increase of ΔA (at 528 nm)from 0.63 to 0.75 and 0.80 (Fig. 4d). Also, the coloringresponse times of the first cycle were measured as 2.6 (0.077M LiClO4), 3.6 (0.23 M LiClO4), and 3.7 s (0.77 M LiClO4). Thereduced coloring response times resulted from the fasterdiffusion of salts in the liquid electrolyte systems. The de-col-oring response times of the first cycle were measured as 8.8(0.077 M LiClO4), 9.2 (0.23 M LiClO4) and 11.5 s (0.77 MLiClO4). However, the de-coloring process seemed to beunstable. Moreover, the cycling stability of electrochromic cellswith liquid electrolytes was very poor (Fig. 4d). The enhancedcycling stability of electrochromic cells by MOP-G-3–5 can beinterpreted as follows. For the stable operation of electrochro-mic cells, the working viologens and the counting phenothia-zines were efficiently separated on each electrode. In the gelelectrolyte systems, the efficient separation of working violo-gens from counting phenothiazines was realized due to thelimited diffusion (Fig. 4e). In contrast, in the liquid electro-lytes, electron rich phenothiazines diffuse freely to form inter-molecular charge transfer complexes with the grafted electrondeficient viologens, blocking the reversible electrochromicoperation of cells (Fig. 4f). It is also noteworthy that the elec-trochromic cells with MOP-G-4 showed faster response timesand better cycling stability than those with the linear organic

polymer-based gels (Fig. S7 in the ESI†). Gel polymer electro-lytes with crosslinking networks rather than linear chains havebeen well studied in order to improve the mechanical strengthand cycling stability of EC devices.18

In conclusion, this work shows that the MOP chemistry canbe applied for the fabrication of gel electrolytes. The MOP-Gelectrolytes showed good conductivities in the range of2.56–6.33 mS cm−1. The electrochromic cells with MOP-G elec-trolytes showed not only a non-flow merit, but also an enhancedcycling stability due to the efficient separation of the graftedworking materials from counting materials. We believe that thechemical surrounding of gel electrolytes can be further tunedby screening the building blocks of MOP materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Basic Science ResearchProgram (2016R1E1A1A01941074) through the NationalResearch Foundation (NRF) of Korea funded by the Ministry ofScience, ICT and Future Planning. JHK thanks the BasicScience Research Program (NRF-2016R1A6A3A11931661) fundedthrough the NRF of Korea by the Ministry of Education.

Notes and references

1 Recent review: Q. Chen, H. Chen, L. Zhu and J. Zheng,J. Mater. Chem. B, 2015, 3, 3654–3676.

2 A. L. Eh, X. Lu and P. S. Lee, in Electrochromic Materials andDevices, ed. R. J. Mortimer, D. R. Rosseinsky and P. M. S.Monk, Wiley-VCH, 2015, pp. 289–310.

3 Reviews: (a) N. Chaoui, M. Trunk, R. Dawson, J. Schmidtand A. Thomas, Chem. Soc. Rev., 2017, 46, 3302–3321;(b) L. Tan and B. Tan, Chem. Soc. Rev., 2017, 46, 3322–3356;(c) Y. Xu, S. Jin, H. Xu, A. Nagai and D. Jiang, Chem. Soc.Rev., 2013, 42, 8012–8031; (d) R. Dawson, A. I. Cooper andD. J. Adams, Prog. Polym. Sci., 2012, 37, 530–563;(e) F. Vilela, K. Zhang and M. Antonietti, Energy Environ.Sci., 2012, 5, 7819–7832; (f ) N. B. McKeown andP. M. Budd, Chem. Soc. Rev., 2006, 35, 675–683.

4 (a) J.-X. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell,H. Niu, C. Dickinson, A. Y. Ganin, M. J. Rosseinsky,Y. Z. Khimyak and A. I. Cooper, Angew. Chem., Int. Ed.,2007, 46, 8574–8578; (b) K. Wu, J. Guo and C. Wang, Chem.Mater., 2014, 26, 6241–6250.

5 (a) W. Ma, L. Qin, Y. Gao, W. Zhang, Z. Xie, B. Yang, L. Liuand Y. Ma, Chem. Commun., 2016, 52, 13600–13603;(b) L. Qin, G.-J. Xu, C. Yao and Y.-H. Xu, Polym. Chem.,2016, 7, 4599–4602; (c) A. Efrem, K. Wang,P. N. Amaniampong, C. Yang, S. Gupta, H. Bohra,S. H. Mushrif and M. Wang, Polym. Chem., 2016, 7, 4862–

Communication Polymer Chemistry

458 | Polym. Chem., 2019, 10, 455–459 This journal is © The Royal Society of Chemistry 2019

Publ

ishe

d on

06

Dec

embe

r 20

18. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

3/8/

2019

1:0

7:32

PM

. View Article Online

4866; (d) G. Chang, Y. Xu, L. Zhang and L. Yang, Polym.Chem., 2018, 9, 4455–4459.

6 P. M. Beaujuge and J. R. Reynolds, Chem. Rev., 2010, 110,268–320.

7 G. Cai, J. Wang and P. S. Lee, Acc. Chem. Res., 2016, 49,1469–1476.

8 A. Llordés, G. Garcia, J. Gazquez and D. J. Milliron, Nature,2013, 500, 323–326.

9 (a) J. H. Ko, S. Yeo, J. H. Park, J. Choi, C. Noh andS. U. Son, Chem. Commun., 2012, 48, 3884–3886;(b) W. Sharmoukh, K. C. Ko, C. Noh, J. Y. Lee andS. U. Son, J. Org. Chem., 2010, 75, 6708–6711;(c) W. Sharmoukh, K. C. Ko, I. G. Jung, C. Noh, J. Y. Leeand S. U. Son, Org. Lett., 2010, 12, 3226–3229.

10 S. Yuan, S. Kirklin, B. Dorney, D.-J. Liu and L. Yu,Macromolecules, 2009, 42, 1554–1559.

11 J.-H. Li, Y. Liang and Y.-X. Xie, J. Org. Chem., 2005, 70,4393–4396.

12 (a) H. C. Yau, M. K. Bayazit, J. H. G. Steinke andM. S. P. Shaffer, Chem. Commun., 2015, 51, 16621–16624;(b) Y. Chen, Y.-H. Zhang and L.-J. Zhao, Phys. Chem. Chem.Phys., 2004, 6, 537–542.

13 O. Goli-Jolodar, F. Shirini and M. Seddighi, RSC Adv., 2016,6, 26026–26037.

14 C. W. Kang, J. Choi, Y.-J. Ko, S. M. Lee, H. J. Kim, J. P. Kimand S. U. Son, ACS Appl. Mater. Interfaces, 2017, 9, 36936–36943.

15 M. Grätzel, Nature, 2001, 409, 575–576.16 F. B. Dias, L. Plomp and J. B. J. Veldhuis, J. Power Sources,

2000, 88, 169–191.17 (a) R. Zhou, W. Liu, Y. W. Leong, J. Xu and X. Lu, ACS Appl.

Mater. Interfaces, 2015, 7, 16548–16557; (b) H. Oh,D. G. Seo, T. Y. Yun, C. Y. Kim and H. C. Moon, ACS Appl.Mater. Interfaces, 2017, 9, 7658–7665.

18 V. K. Thakur, G. Ding, J. Ma, P. S. Lee and X. Lu, Adv.Mater., 2012, 24, 4071–7096.

Polymer Chemistry Communication

This journal is © The Royal Society of Chemistry 2019 Polym. Chem., 2019, 10, 455–459 | 459

Publ

ishe

d on

06

Dec

embe

r 20

18. D

ownl

oade

d by

Sun

gkyu

nkw

an U

nive

rsity

on

3/8/

2019

1:0

7:32

PM

. View Article Online