High-Performance Asymmetric Electrochromic-Supercapacitor Device

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Applications of Polymer, Composite, and Coating Materials

High performance asymmetric electrochromic-supercapacitor device based on poly(indole-6-carboxylicacid)/TiO2 nanocomposite Qingfu Guo, Jingjing Li, Bin Zhang, Guangming Nie, and Debao Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19505 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Applied Materials & Interfaces

High

Performance

Electrochromic-supercapacitor

Asymmetric Device

Based

on

Poly(indole-6-carboxylicacid)/TiO2 Nanocomposite Qingfu Guo, Jingjing Li, Bin Zhang, Guangming Nie*, Debao Wang* Key Laboratory of Optic-electric Sensing and Analytical Chemistry for Life Science, MOE; Shandong Key Laboratory of Biochemical Analysis; Key Laboratory of Analyti cal Chemistry for Life Science in Universities of Shandong; College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China. *Corresponding author. Tel. +86-532-88959058, Fax. +86-532-88957187. E-mail address: [email protected] (G. N.), [email protected] (D. W.) Abstract A

difunctional

porous

network

poly(indole-6-carboxylicacid)

(PICA)/TiO2

nanocomposite is firstly prepared using TiO2 nanorod arrays as scaffold. Because of the synergistic effect of PICA and TiO2, the nanocomposite shows good electrochemical performance, high specific capacitance value (23.34 mF cm-2) and excellent galvanostatic charge and discharge stability. Meanwhile, this nanocomposite can be reversibly switched among yellow, green, brown with high coloration efficiency (124 cm2 C-1). An asymmetric electrochromic-supercapacitor device (ESD) is also constructed using PICA/TiO2 nanocomposite as anode material and poly(3,4-ethylenedioxythiophene) (PEDOT) as cathode material. This ESD has robust cycle stability and high specific capacitance value (9.65 mF cm-2), which can be 1

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switched from light green to dark blue. After charged, the device can light up a single LED for 108 s and the energy storage level can also be monitored by the corresponding color changes. This constructed ESD will have great potential application in intelligent energy storage and other smart electronics fields. Key words: Poly(indole-6-carboxylic acid); TiO2; Nanocomposite; Electrochromism; Supercapacitor Introduction Supercapacitor as a newly developed energy storage device between battery and conventional capacitor has shown great promise in many fields in recent years because of the high power density, rapid charging and discharging capacity, long service life etc.1. However, with the continuous development of intelligent electronic products, there is an urgent need to develop multifunctional supercapacitor, such as flexible supercapacitor2-4, self-healing supercapacitor, piezoelectric supercapacitor and electrochromic supercapacitor etc.5. Electrochromism is caused by the change of optical properties of the material (reflectivity, transmittance, absorption, etc.) under the external voltage, and the visual representation is a reversible change in color and transparency6. Commercial electrochromic smart window can be used to effectively adjust the indoor light through reversible color changes, thus enabling effective energy utilization7. Because supercapacitor and electrochromic smart windows have the same working mechanism and similar device structure, it will be highly attractive to integrate energy storage and electrochromic function in one device, realizing the multifunctional application of the device. The integrated device can not only realize 2


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efficient storage and utilization of energy, but also can monitor the energy storage status of the supercapacitor through the electrochromic performance of the device8. It is well-known that the materials with both electrochromic and supercapacitive properties mainly include metal oxide and conducting polymer. However, each kind of material has its own pros and cons. Metal oxide has high energy storage density and good electrochromic cycle stability9, but the application is limited by the single color change, low color efficiency, long response time and poor electrical conductivity.

For

example,

tungsten

oxide

(WO3)

is

a

widely

used

electrochromic-supercapacitive material which has good electrochemical activity and stability. However, its application in electrochromic-supercapacitor has not been fully developed due to its low electrochemical energy storage capacity in some degree. In addition, it has only one color change from colorless to blue (single color change) compared with conducting polymer10,11.

Conducting polymer has high electron

conduction ability, large specific capacitance, rich color change, nonetheless, its stability is poor compared with metal oxides. In order to make up for the deficiency of these two materials, it is an effective method to composite the metal oxide and conducting polymer to improve electrochemical performance via synergistic effects to make use of their respective performance advantages. Recently, some researches about electrochromic-supercapacitor based on composite materials have been reported. For example, Reddy et al. used poly(3,4-ethylenedioxypyrrole)-Au@WO3 composite electrode to realize both optical modulation and energy storage12. Wei et al. successfully prepared the PANI/graphene oxide (GO) nanocomposite film, which had 3



multi-color electrochromism and good capacitance performance13. Changing the surface morphology of electrode materials is also an effective method to enhance the electrochemical performance and cycling stability of the materials. Especially, electrode materials with porous network structure have many advantages for the performance of supercapacitor and electrochromic device. The porous network structure can effectively transfer the doped ions on the electrode surface and significantly reduce the ions-transport resistance, thus improving the charging and discharging rate of the device and shorting the response time of electrochromism. Moreover, the porous network structure can significantly reduce the damage to the material surface structure caused by continuous doping and dedoping during the redox process, enhancing the cycling stability of materials. In addition, the porous network structure of the material also can reduce the influence of the strain by changing the section structure, thus improving the mechanical properties of the material14. In recent years, conducting polyindole and its derivatives are more and more concerned by researchers. Compared with WO3 and other metal oxide electrochromic-supercapacitive materials, there are some pros of polyindoles. Firstly, in terms of electrochromic performance, the polyindoles have faster switching time, higher coloration efficiency and rich color change. Secondly, the supercapacitors based on metal oxide usually show narrow potential and limited specific capacitance. Polyindoles can store and release charge through redox reaction of polymer conjugated chain, which makes it have wide potential window. Furthermore, the 4


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doped and dedoped processes of polyindoles occur on the whole electrode, which makes it possible to obtain high specific capacitance15. These advantages make it have potential application in smart supercapacitor. On the other hand, during the doped and dedoped processes polyindoles have volume change, thus bringing poor cyclic stability compared with transition metal oxide. Until now, a series of polyindole derivatives with good electrochromic properties have been reported by our group16-19. Xu’s group has also reported the supercapacitive property of polyindole derivatives20-22. As a kind of semiconductor material with wide gap, TiO2 has good supercapacitive properties23,24. However, low electrical conductivity, imperceptible color change and slow color conversion speed limit its electrochromic application to a certain extent25. In recent years, TiO2 with one-dimensional nanostructure has attracted wide concern by researchers because this kind of structure can provide shorter pathways for electron transfer, reduce the electron transfer resistance, and then improve electron transfer rate and the electrical conductivity26. Alsawat et al. demonstrated that TiO2 nanotube arrays had good photocatalytic activity under UV light27. Shao et al. applied PANI nanowire/TiO2 nanotube array composite to supercapacitor28. Xia et al. developed high performance lithium-ion microbatteries based on lithium titanate nanoarrays and TiO2-B nanowire, respectively29,30. It would be very interesting to composite TiO2 and polyindoles to collectively improve the electrochromic and capacitive properties of TiO2 and polyindoles. More importantly, when composited with p-type conducting polymers, a donor-acceptor (D-A) pair can be formed between TiO2 (as electron acceptor) and conducting polymers (as electron 5



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donor). The D-A structure has controllable molecular energy gap, excellent optical physics and carrier transmission properties, which has attracted more and more attention by researchers in the field of photoelectric materials31-33. Tu et al. successfully prepared TiO2/PANI nanocomposite, its electrochromic performance had been significantly enhanced compared with PANI due to the introduction of TiO234. Based on the considerations mentioned above, our group constructed an asymmetric

electrochromic-supercapacitor

using

porous

network

poly(indole-6-carboxylicacid) (PICA)/TiO2 nanocomposite. This porous network PICA/TiO2 nanocomposite was successfully prepared using TiO2 nanorod arrays as scaffold and showed good electrochromic and capacitive performance. The constructed asymmetric electrochromic-supercapacitor device (ESD) with robust cycle stability can be switched from light green to dark blue and the energy storage level of the device can be monitored by the corresponding color changes. After charged, the device could light up a single LED for 108 s.

2. Experimental section 2.1 Materials and Apparatus Indole-6-carboxylicacid

(ICA,

98%),

propylene

carbonate

(PC,

98%),

3,4-ethylenedioxythiophene (EDOT, 98%), LiClO4 · 3H2O and poly(methyl methacrylate) (PMMA, 98%) were all bought from Aldrich. Tetrabutyl titanate (TBT, 98%) was received from J&Kchemical. Acetonitrile (ACN, AR) was bought from Tianjin Guangcheng Chemical Reagent Co., Ltd. Hydrochloric acid (HCl, AR) was obtained from Yantai Sanhe Chemical Reagent Co., Ltd. Ethanol absolute (AR) was 6


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got from Tianjin Basifu Chemical Co., Ltd. Electro-thermostatic blast oven (DHG-9623A Shanghai Jinghong) was used to control

the

preparation

temperature

of

TiO2.

Cyclic

voltammetry

(CV),

electrochemical impedance spectroscopy (EIS), galvanostatic charge and discharge measurements were all carried out by CHI 660E electrochemical workstation (Shanghai Chenhua Ins., China). Cary 500 UV-Vis-NIR spectrophotometer (Varian, USA) and Model 263 electrochemical workstation (EG&G Princeton Applied Research, USA) were used to study the spectroelectrochemical properties of the nanocomposite under computer control. The scanning electron microscope (SEM) measurement was studied by JEOL JSM-6700F scanning electron microscope (JEOL Ltd., Japan). Raman spectroscopy were acquired using Renishaw InVia Raman microscope (Renishaw Ltd., UK). Infrared spectra were recorded using Nicolet 510P FTIR spectrometer (Nicolet Ltd., USA) with KBr pellets. Transmission electron microscope (TEM) measurements were taken by using JEM-2100 electron microscope (JEOL Ltd., Japan). The X-ray photoelectron spectroscopy (XPS) were recorded by Omicron ESCA spectrometer (Omicron Ltd., Germany) using Al Kα irradiation as the excitation source. XPSPEAK41 software was used to analyze the XPS peaks.

2.2 Preparation of PICA/TiO2 nanocomposite and asymmetric ESD The preparation procedure of TiO2 nanorod arrays, PICA/TiO2 nanocomposite and asymmetric ESD based on PICA/TiO2 nanocomposite and PEDOT were illustrated in Fig. 1(A). TiO2 nanorod arrays were prepared on fluorine doped SnO2 7



conductive glass (FTO) substrate by a previously reported hydrothermal technique35. HCl and TBT were dissolved in deionized water. After stirred for 30 min, the resulting solution was transferred to Teflon-lined stainless steel autoclave. After the reactor was sealed, it was kept at 150 oC for 2 hours. After that, the prepared TiO2 nanorod arrays were rinsed several times by deionized water and dried at room temperature. PICA was deposited directly on TiO2 nanorod surface by electrochemical polymerization. The TiO2 nanorod arrays covered FTO, stainless steel and Ag/AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. The PICA layer was prepared in ACN solution containing 0.05 M ICA by galvanostatic method and 0.1 M LiClO4.3H2O was added to the solution as supporting electrolyte. All the experimental steps were carried out in a dry argon flow, which was kept under a slight overpressure during the experiment. The current density 0.08 mA cm−2 was carried out in the polymerization process. After that, the working electrode was subsequently washed by ultrapure water and ethanol, and finally dried under the air for further characterization. The asymmetric ESD had a sandwich structure. PICA/TiO2 nanocomposite and PEDOT were coated separately on FTO electrodes. PEDOT was prepared in ACN/TBATFB using the constant potential method. In order to maintain the injected/ejected charge balance, the two electrodes need the same amount of electric quantity during the redox process. PICA/TiO2 nanocomposite electrode and PEDOT electrode were separated by gel electrolyte which was prepared by LiClO4·3H2O, 8


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PMMA, CH3CN and PC.

2.3 Coloration efficiency and capacitance calculation. Coloration efficiency (CE) was calculated according to the following equation: CE = ΔOD/ Qd

(1)

ΔOD = log (Tb /Tc )

(2)

where Qd refers to the injected/ejected charge during the redox process; Tb and Tc refer to the transmittances of the material in its bleached and colored states, respectively. The areal specific capacitance can be calculated by means of equation (3) when used the charge-discharge method: (3)

C=It∕VS

Where C stands for areal capacitance (F cm-2), S represents the surface area of the active materials (cm2), V is the potential window (V), I is the discharge current (mA), t stands for the discharge time (s).

3. Results and discussion 3.1 Morphology and structure characterizations

9



Fig.1 The preparation procedure of TiO2 nanorod arrays, PICA/TiO2 nanocomposite and asymmetric ESD (A). SEM images of TiO2 nanorod arrays (B), PICA/TiO2 nanocomposite (C) and TEM images of TiO2 nanorod arrays (D) and PICA/TiO2 nanocomposite (E), respectively.

SEM images of TiO2 nanorod arrays and PICA/TiO2 nanocomposite were shown in Fig.1. TiO2 nanorod arrays were evenly distributed and arranged vertically on the FTO substrate (Fig.1B). The diameters of the nanorod arrays were about 50 nm. This unique nanostructure provided a larger surface area for polymerization of PICA in the electrochemical process. The morphology of PICA/TiO2 nanocomposite was completely different from PICA with a planar structure and a small number of pores 10


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on the surface (Fig.S1). PICA/TiO2 composite material formed a porous network structure, in which nanorod arrays were used as the crosslinking center (Fig.1C). This porous network structure of PICA/TiO2 nanocomposite had higher electrical conductivity compared with TiO2 due to the effect of PICA on the composites surface. Moreover, the porous structure on the surface of the material could be used as an ion/electron transport channel to facilitate the entry and exit of doped ions. Hence, this structure will significantly reduce the ion diffusion resistance and then enhance the electrochromic and capacitive performances of the material because of the synergistic effect of PICA and TiO2. TEM was also used to analyze the structural characteristics of TiO2 nanorod arrays and PICA/TiO2 nanocomposite. As can be seen from Fig.1D, TiO2 had a nano-rod-like structure with a diameter of about 50 nm, which was consistent with its SEM. After electrochemical polymerization of PICA, porous network of PICA/TiO2 can be also observed (Fig.1E), which confirmed the successful preparation of PICA/TiO2 nanocomposite.

11



Fig.2 (A) FTIR spectra of TiO2 nanorod array (a), PICA (b) and PICA/TiO2 nanocomposite (c). (B) Raman spectra of TiO2 nanorod array (a), PICA (b) and PICA/TiO2 nanocomposite (c). (C) Ti 2p, (D) N 1s and (E) O 1s XPS spectra of PICA/TiO2 nanocomposite.

The structure of PICA, TiO2 nanorod arrays and PICA/TiO2 nanocomposite were characterized by FTIR spectra (Fig.2A). As shown in curve (a), the bands at 3443 and 1633 cm-1 were ascribed to the deformation vibrations of water molecule adsorbed on the surface of TiO2. The band at 1123 cm-1 was assigned to Ti-OH deformation vibrations of hydroxyl groups on the TiO2 surface36. The bands at 625 cm-1 were assigned to the stretching vibration of Ti-O-Ti37. The main FTIR peak positions of TiO2 were listed in Table S1. For the PICA film (curve c), the intense band at 1692 was attributed to the characteristic absorption of –C=O and 1379 cm-1 was the C–O stretching vibration of the carboxylic group. The broad peak at 2560 cm-1 was the -OH stretching vibration of PICA. The bands at 768 and 835 cm-1 were ascribed to out-of-plane C–H vibrations of 1,2,4-trisubstituted benzene. In addition, the strong and wide peak at 3399 cm-1 and 1621 cm-1 was the N-H stretching vibration and 12


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deformation vibration (the main FTIR peak positions of PICA were listed in Table S2). It can be seen from curve (b), the corresponding characteristic absorption peaks of PICA and TiO2 were all appeared in the nanocomposite with slight shift (the main FTIR peak positions of PICA/TiO2 were listed in Table S3). By comparing the infrared spectra of PICA and PICA/TiO2, it can be seen that the N-H band shifted to the lower wavenumbers (3335 cm-1) direction after PICA composited with TiO2. These shifts may be caused by the chemical bonding (ionic or covalent bonds) between the nitrogen atom of PICA and the exposed titanium atom of TiO234. Inversely, the C=O band was shifted to the higher wavenumbers (1730 cm-1). The shift was probably due to the formation of ester-like binding between the carboxyl groups of some PICA molecules and the hydroxyl groups on the TiO2 surface38. Fig. 2B showed the Raman spectra of the TiO2, PICA and PICA/TiO2 nanocomposite. For the TiO2 (curve a), the bands at 243, 432, and 603 cm-1 may be attributed to the TiO2 single crystal, which was consistent with previous reports34. For PICA (curve b), the two bands at about 1326 and 1582 cm-1 may be attributed to the stretching vibrations of pyrrole ring and phenyl ring39. When PICA was successfully polymerized on TiO2, the absorption bands of PICA and TiO2 both appeared in the Raman spectrum of the nanocomposite (curve c). However, the absorption band of TiO2 was shifted to the higher wavenumbers. This shift may be caused by the interaction between TiO2 and PICA mentioned above. These results further proved that the PICA/TiO2 nanocomposite was successfully prepared. XPS was also used to analyze the surface composition of PICA/TiO2 13



nanocomposite. Ti 2p, N1s and O1s XPS spectra of the PICA/TiO2 nanocomposite were shown in Fig.2(C, D, E). For the Ti 2p XPS spectrum (Fig.2C), the peaks at 458.7 eV and 464.7 eV were ascribed to the characteristic peak of Ti 2p 3/2 and Ti 2p 1/2 for Ti4+, respectively40. The peak at about 400.2 eV may be attributed to pyrrolic-N of PICA (Fig.2D)41. Fig.2E presented the O1s XPS spectrum of the PICA/TiO2 nanocomposite. The peak at 531.8 eV may be assigned to the Ti-O and the peak at ~529.8 eV may be attributed to the Ti–O–C=O42, which may be caused by the esterification of the carboxyl group of PICA and the hydroxyl group on the surface of TiO2. XPS further proved that PICA was deposited successfully on TiO2 nanorod arrays by ester-like binding.

3.2 Electrochemical properties of PICA/TiO2 nanocomposite

Fig.3 (A) CVs of TiO2 nanorod arrays (a), PICA/TiO2 nanocomposite (b) and PICA (c) measured in ACN/LiClO4 solution with a scan rate 100 mV s-1. (B) CVs of PICA/TiO2 nanocomposite with 14


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scan rates of 50 (a), 100 (b), 150 (c), 200 (d), 250 mV s-1 (e). CVs of (C) PICA/TiO2 nanocomposite and (D) PICA film for 1000 cycles with a scan rate of 100 mV s-1.

Cyclic voltammetry (CV) was a useful electrochemical research method, which can be used to investigate the properties, mechanism and kinetic parameters of electrode reaction. The CV curves of pure TiO2, PICA and PICA/TiO2 nanocomposite were shown in Fig.3A. Because of the redox reaction of TiO2 under the potential, the TiO2 array had a pair of typical redox peaks at -0.48 and -0.15 V (Fig.3A(a)). The reaction mechanism can be described as TiO2+xe−+xLi+ → LixTiO2 (x can vary between 0 and 1)43. We can see from Fig.3A(c), the CV of PICA represented the broad redox peaks at 1.32 V and 0.26 V, respectively, indicating good redox activity of the polymer. In Fig.3A(b), the PICA/TiO2 nanocomposite had typical redox peaks of TiO2 and PICA, which indicated that the PICA/TiO2 composite film was obtained successfully. Moreover, PICA/TiO2 nanocomposite showed wider redox peaks and larger peak current densities compared with pure TiO2 and PICA. This result indicated that PICA/TiO2 nanocomposite had higher redox activity and much larger capacitance. To further investigate the electrochemical behavior of PICA/TiO2 nanocomposite film, CV curves of the PICA/TiO2 nanocomposite film were recorded in monomer free ACN solution (LiClO4•3H2O as supporting electrolyte) with scan rates of 50 (a), 100 (b), 150 (c), 200 (d), 250 mV s-1 (e). As can be seen from Fig.3B, the anodic and cathodic peak current density of the film were linearly proportional to the scanning rates, indicating that the electrochemical behavior of the electrode was adsorption 15



control process. At the same time, the anodic and cathode peak potential had slight shift as the scan rate was increased. This peak potential shift may be caused by slow electron transfer due to heterogeneous electrode surface, local rearrangement of polymer chains at a specific potential, and the electron exchange occurred at the metal/polymer and polymer/solution interfaces, etc44. Moreover, the broad redox peak meant that the composite had a large capacitance. The good redox behavior and the wide potential window indicated that the nanocomposite material was an ideal electrochemical supercapacitor material. To further confirm the good electrochemical property of the PICA/TiO2 nanocomposite, the long-term CV cycling of the composite was studied in ACN/LiClO4 solution. As shown in Fig.3C, there was only little decrease in maximal peak current densities of PICA after 1000 cycles. In this process, the current density of the oxidation peak of TiO2 did not change significantly, indicating good electrochemical stability of PICA/TiO2 nanocomposite. For better comparison, the cyclic stability of PICA was also studied (Fig.3D). Obviously, the stability of PICA was poor. After 1000 cycles, the material had basically lost its redox activity. This may be ascribed to the continuous swelling/shrinkage of the material due to the constant doping/dedoping of external ions when the redox reaction occurred45. The enhanced electrochemical stability of PICA/TiO2 nanocomposite was probably attributed to the nanoarray structure of TiO2, which served as the crosslinking center for PICA to have a network structure. The special network nanostructure reduced the damage caused by the deformation of materials during the doping-dedoping process. 16


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3.3 Supercapacitive properties of PICA/TiO2 nanocomposite

Fig.4 (A) Galvanostatic charge/discharge curves of PICA/TiO2 nanocomposite. (B) Areal specific capacitance of PICA/TiO2 nanocomposite as a function of current density. (C) EIS of PICA/TiO2 nanocomposite and PICA film. (D) Stability testing of PICA/TiO2 nanocomposite at a current density of 0.25 mA cm-2 for 5000 cycles.

The supercapacitive performance of PICA/TiO2 nanocomposite was evaluated by the galvanostatic charging and discharging measurement in ACN/LiClO4 solution at different current densities using a potential window from -0.5 to 2.0 V. The charge-discharge curves of PICA/TiO2 nanocomposite with good symmetry showed a non-ideal triangular shape due to redox process, suggesting that the nanocomposite had the typical pseudocapacitance characteristic and the fast charging and discharging ability (Fig.4A). The areal specific capacitance of the PICA/TiO2 nanocomposite was calculated to be 23.34 mF cm-2 when the current density was 0.25 mA cm-2. This 17



specific

capacitance

values

were

roughly

Page 18 of 39

the

same

as

the

electrochromic-supercapacitor electrode (PANI) reported by Mai’s group (23.2 mF cm−2)46 and higher than other reported composite materials (Table.1). The areal specific capacitance values of the PICA/TiO2 nanocomposite corresponding to different charge-discharge current densities were presented in Fig.4B. It was obvious that the areal specific capacitance gradually decreased with the charge-discharge current density gradually increased. The reason for this phenomenon can be explained as follows: under the condition of higher current density, the slow transfer process of doping ions produced a dynamic resistance, which affected the charge transfer process on the electrode/electrolyte interface so that some polymer chains can’t fully participate in the redox reaction. Table 1 Comparison of specific capacitance values of other reported composite materials Type of composite material

Specific capacitance maximum

References

PICA/TiO2 nanocomposite rGO/TiO2NR/rGO electrode e-WO3/Ppy hybrid electrodes C/TiO2/rGO H-TiO2 Air -TiO2 W18O49/PANI (PEDOT:PSS)/PANI

23.34 mF cm-2 13.74 mF cm-2 11.38 mF cm-2 22.6 mF cm-2 3.24 mF cm-2 0.08 mF cm-2 10 mF cm-2 17 mF cm-2

This work 47 48 49 50 50 51 52

In order to investigate the reason for the enhanced capacitive performance of PICA/TiO2 nanocomposite, EIS was applied to study the electrical conductivity and ion transfer behavior of the PICA/TiO2 nanocomposite and PICA film. The Nyquist plot was consisted by a semicircle in high-frequency region, which represented the charge transfer resistance on electrode/electrolyte interface (Rct). The intercept on the real axis in the high frequency range provided the equivalent series resistance (Rs). An 18


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inclined line in low-frequency region was related to the diffusion behavior of ions in electrode holes (Rw)53. Fig.4C showed the Nyquist plot of the PICA/TiO2 nanocomposite and PICA film (the electrochemical polymerization charge of materials was both 300 mC). Obviously, the Rs of PICA/TiO2 nanocomposite (13.9 Ω) was smaller than PICA film (26.2 Ω) and poly(indole-5-carboxylic acid) in ACN/LiClO4 electrolyte (149 Ω)20. The enhanced capacitive performance firstly can be attributed to the smaller equivalent series resistance (Rs) of the PICA/TiO2 nanocomposite compared with PICA. In addition, PICA increased the electrical conductivity of composite material and the well contact between TiO2 and FTO electrode leading to the maximum utilization of TiO2 during the redox reaction. Thus, the synergistic effect of the electrode materials (TiO2 and PICA) and the special porous network structure made PICA/TiO2 electrode have high capacitance performance. This synergistic effect also enhanced the stability of the nanocomposite. As shown in Fig. 4D, the nanocomposite exhibited remarkably high stability. After 5000 cycles galvanostatic charge and discharge, the electrode material can retain 91% of its initial capacitance. This extraordinary cycling performance further proved the stability of the redox activity and the reversibility of the nanocomposite on the above discussion.

3.4 Electrochromic properties of PICA/TiO2 nanocomposite

19



Fig.5 (A) Spectroelectrochemistry of PICA/TiO2 nanocomposite, applied potentials: -0.5V to 2.3V with 0.2V step (the inset were the digital photos of PICA/TiO2 nanocomposite). (B) Transmittance-time profile of PICA and PICA/TiO2 nanocomposite at 720 nm between -0.5V and 2.3V.

Spectroelectrochemical method is usually used to investigate the electrochromic performance of conducting polymer materials because the polarons and bipolarons generated in the oxidation process will directly cause the changes in visible light absorption. Fig.5A showed the corresponding optical changes of the PICA/TiO2 nanocomposite which was recorded by the UV-Vis spectra for different applied voltages from -0.5 to 2.3 V. There was an absorption band at 400 nm when -0.5 V potential was applied to the nanocomposite electrode. This absorption band was resulted from the π-π* transition of the conjugated structure in the composite material. And the corresponding color of the composite material was yellow at this state (reduced state) (Fig.5A, inset). With the gradual increase of voltage, the nanocomposite material was gradually oxidized, the absorption band at 400 nm gradually weakened, and a new absorption band began to appear at 720 nm. The reason was that the polaron was generated and caused a new electronic transition during the oxidation process of the material. When the potential reached 1.1 V, the 20


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absorption band at 720 nm reached its maximum, and the color of the composite material became green (Fig.5A inset). As the potential increased, a new absorption band was generated at 500 nm. In this process, the composite material continued to be oxidized, producing bipolaron. When the potential reached to 2.1 V, the color of the nanocomposite material turned to brown (Fig.5A inset). It should be noted that the nanocomposite material can be switched among the three colors in a stable manner, indicating good multicolor conversion reversibility of the nanocomposite. In the practical application, the switching time, optical contrast (ΔT%) and coloration efficiency (CE) of the electrochromic materials were important evaluation indexes. The ΔT% of PICA/TiO2 nanocomposite at 720 nm was 36%, higher than pure PICA film (21%) (Fig. 5B). However, it’s worth noting that the lowest and highest transmittance of PICA/TiO2 nanoarrays were lower than the PICA. This phenomenon was consistent with the results of the literature34. This might be ascribed to the introduction of TiO2 which had some effect on the transmission of light. The response time was defined as the time required to achieve 95% of the total transmittance, and the response time of the PICA/TiO2 nanocomposite was 1.2 s from oxidation state to reduced state, 2.4 s from reduced state to oxidized state at 720 nm (the response time of PICA was 1.45 s and 3 s, respectively). The high CE value indicated that a larger optical modulation can be achieved with less charge or energy54. The CE value of PICA/TiO2 nanocomposite was calculated to be 124 cm2 C-1 at 720 nm according to equation (1) and (2), which was higher than pure PICA (102 cm2 C-1 at 410 nm)19. The comparation of the electrochromic parameters of 21



Page 22 of 39

PICA/TiO2 nanocomposite with other reported electrochromic-supercapacitive difunctional materials were shown in Table S4. As can be seen in Table S4, the response

time

of

this

material

was

fastest

among

the

reported

electrochromic-supercapacitive materials. The CE of PICA/TiO2 nanocomposite was similar to WO3 on the silver grid/PEDOT:PSS substrate, and much higher than other reported materials. The optical contrast was also comparable to PANI and higher than MoO3/Ni(OH)2 composite,

carbon

nanotubes/PBDTC

composite,

WO3·H2O,

g-C3N4/WO3·H2O composite, graphene/WO3·H2O composite. The shorter response time, higher optical contrast and CE achieved with PICA/TiO2 nanocomposite can be attributed to the porous network morphology of the material which facilitated the ion transport during the redox reaction.

3.5 Capacitive property of PICA/TiO2//PEDOT ESD

22


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Fig.6 (A) CVs of the device with scan rates of 25 (a), 50 (b), 100 (c),150 (d), 200 (e), 250 mV s-1 (f). (B) Galvanostatic charge/discharge curves of the device under different current densities. (C) Specific capacitance of the device as a function of current density. (D) EIS of the device.

Supercapacitors based on conducting polymer can be classified into three types. The type Ⅰ device used the same p-doped polymer as electrode material. When charging a type Ⅰ supercapacitor, the positive electrode was completely oxidized and the negative electrode remained neutral. When fully discharged, both electrodes were in a half-oxidation state, only 50% capacitance was used. Hence, this capacitor usually had low energy density55. To further improve the capacitive performance of the device, we fabricated a type Ⅱ device (using two different p-doped polymers as electrode material) using PICA/TiO2 nanocomposite and PEDOT as the positive and negative material, respectively. The structure and morphology of PEDOT were also characterized by SEM (Fig.S2), FTIR and Raman spectra (Fig.S3), respectively. When type II device was fully discharged, the anode oxidation degree is less than 50% and the cathode is more than 50%. Therefore, 75% of the doping capacity of the polymer can be utilized. Moreover, type II device avoided the use of unstable n-doped polymers compared with type III/IV device (two electrodes use the same/different conducting polymer, which can be either p-doped or n-doped), so the cycling stability of the device can be improved. In this work, a type II device based on PICA/TiO2 and PEDOT (PICA/TiO2//PEDOT) was successfully constructed. During device construction, PEDOT was prepared in ACN/LiClO4 solution at a constant potential of 1.1 V vs. Ag/AgCl. Its morphology and structure characterizations were studied by SEM, FTIR and Raman spectrum which were all illustrated in the supporting

23



information (Fig.S2-S3). For comparison with PICA, PICA/TiO2 nanocomposite and PEDOT, the infrared spectrum of PICA/TiO2/PEDOT (Fig.S4) was also illustrated in the supporting information. In order to gain the optimized device performance, we investigated the effect of the amount of PICA in the composite on the EIS results (Fig.S5). By contrast, when the polymerization charge of PICA was 300 mC, PICA/TiO2 nanocomposite had the smallest Rs (13.9 Ω). When the polymerization charge was lower than this value, the resistance value of the material became high due to the large proportion of TiO2 in the composite material. However, when the polymerization charge was higher than 300 mC, excessive PICA covered the surface of TiO2 and destroyed the porous network structure of the composite material, also leading to the increased resistance of the composite material. Therefore, PICA/TiO2 nanocomposite with polymerization charge of 300 mC was used for device assembly. Meanwhile, in order to maintain the balance of charge injection and removal during the operation of the ESD, the redox charges of the PICA/TiO2 nanocomposite and PEDOT must be matched. The electrochemical property of the PICA/TiO2//PEDOT ESD was studied by CV under different scan rates. We can see from Fig.6A, the CV curves of the ESD had a near-ideal rectangular shape, indicating good capacitance performance and charge-discharge symmetry. As illustrated in Fig. 6B, when charged to 1.8 V, the charge-discharge curves of the device had good symmetry, which indicated a good Coulombic efficiency. And when the current density was 0.1 mA cm-2, the areal specific capacitance of the device was 9.65 mF cm-2. This capacitance value was 24


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much lower than that of a single PICA/TiO2 nanocomposite. This was due to the fact that the device was composed of two electrodes sticking together, equivalenting to connecting two capacitors in series. As a result, the theoretical capacitance value of the device was lower than that of a single electrode. Moreover, imperfect contact between the components of the device resulted in the increase of the equivalent resistance, this maybe also reduce the capacitance value of the device56. This capacitance value of the device was higher than the specific capacity of the asymmetric electrochromic-energy storage device based on WO3·H2O and Prussian white film (5.12 mF cm-2)57 and electrochromic-supercapacitor based on WO3 materials (5.3 mF cm-2)58. The high capacitance value may be ascribed to the good capacitance performance of the PICA/TiO2 nanocomposite and PEDOT used as the cathode electrode in the device. In addition, IR drop of the ESD in charge-discharge curves were also investigated (Fig.S6). Generally, IR drop was caused by the contact resistance, electrolyte resistance and it was also depended on charge-discharge current density. From the discharge curves in Fig.S6, the IR drop of the device increased from 40 mV at 0.1 mA cm-2 to 300 mV at 1.5 mA cm-2, respectively. Fig.6C showed the areal specific capacitance values of the device at different charge-discharge current density. It was obvious that the areal specific capacitance of the device gradually decreased as the charge-discharge current density gradually increased. When the current density increased to 1.5 mA cm-2, the areal specific capacitance was still 4.06 mF cm-2. The results showed that the device had good rate capability. The internal resistance

of

the

PICA/TiO2//PEDOT

electrochromic-supercapacitor

25


was


characterized by EIS (Fig.6D). The equivalent resistance (Rs) and the charge transfer resistance (Rct) of the device was about 34.5 Ω and 0.8 Ω respectively. The value of the Rs of this device was a little higher than the supercapacitor based on the poly(indole-5-carboxylic acid) in 1.0 M HClO4 aqueous electrolyte (24.6 Ω) and the Rct of these two devices were basically the same (the Rct of the device based on poly(indole-5-carboxylic acid) was 0.83 Ω)20. The increased Rs of the device maybe mainly attribute to the resistance of the gel electrolyte. However, the Rs was much lower than PANI/WO3 composite (196 Ω)59 and PANI/GO composite (246 Ω)13. Moreover, it’s worth noting that the Rs of the device was higher than the single electrode, this may be caused by the resistance of the gel electrolyte and the polarization occurred at the electrode of device60. The slope of the straight line in the low frequency region was about 84°, which meant that the device had a very high ion transmission rate and low transmission resistance.

3.6 Stability of PICA/TiO2//PEDOT ESD

Fig.7 Stability testing of the device by CV (A) and galvanostatic charge/discharge method (B) for 5000 cycles.

The stability of the ESD was also an important parameter to determine whether it 26


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can last long and stable work in practical application. Generally, the supercapacitor based on conducting polymers usually had poor cycle stability. This was due to the fact that the polymer electrodes were subjected to volume changes repeatedly during the process of doping and dedoping, resulting in the mechanical destruction of polymer materials. The long-term CV scan of this device was applied the voltages from -0.5 to 2.0 V with a scan rate of 100 mV s-1. After 5000 cycles, there was only a small amount of degradation of the electroactivity, and 91% of the original peak current density was retained (Fig.7A). This meant that the device had good long-term cyclic voltammetry stability. The cycle stability of the capacitive performance was also studied by galvanostatic charge and discharge method. It can be seen from Fig.7B, after 5,000 cycles of charging and discharging between 0V and 1.8V, the areal specific capacitance of the device remained 92% of its initial value. The good stability of the device may be caused by the special surface morphology and the synergistic effect between PICA and TiO2 nanorod arrays. The good stability can satisfactorily meet the requirement of practical application of the device.

3.7 Application of PICA/TiO2//PEDOT ESD

27



Fig.8 (A) The color photos of the device under different charging potential. (B) The working photo of this electrochromic-supercapacitor difunctional device.

The actual application process of the difunctional device was shown in Fig.8. As can be seen from Fig.8A, during the charging process, as the charging voltage increased, the color of the device changed gradually. Before charging, the color of the device was light green (0 V). As the charging process proceeds, the device gradually turned to greyish-green (1.0 V), dark blue (1.8 V). Thus, the energy storage level of the device can be monitored by the corresponding color changes. In order to demonstrate the device’s energy storage capacity, two electrochromic-supercapacitor were charged by the solar cell panel and then used to power a red LED. From the working photo of this electrochromic-supercapacitor difunctional device (Fig.8B), the device can light up a single LED for 108 s and then gradually faded. During this process, the color of the device gradually changed from dark blue to light green. This indicated that the color change of the device had good invertibility. Herein, a smart device with both electrochromic and energy storage functions had been achieved. Such device may have a wide range of applications, such as intelligent supercapacitor, energy-storage smart window or sunglasses, and other smart wearable field.

4. Conclusions A novel bifunctional PICA/TiO2 nanocomposite material with electrochromic and supercapacitance properties was successfully prepared. Compared with the PICA, the nanocomposite showed reversible color changes among yellow, green and brown, larger optical contrast (36% at 720 nm), higher CE (124 cm2 C-1), outstanding specific capacitance value (23.34 mF cm-2) and good cycling stability. The high performance 28


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asymmetric ESD based on PICA/TiO2 nanocomposite and PEDOT also showed good electrochromic and energy storage capacity, which can be switched from light green to dark blue and had high areal specific capacitance value (9.65 mF cm-2). After 5000 cycles charge and discharge, the device can retain 92% of the original initial capacitance value. The device also can light up a single LED for 108 s after charged and the energy storage level of the device can also be monitored by the corresponding color

changes.

This

work

provided

a

new

approach

to

design

electrochromic-supercapacitive bifunctional nanomaterials and these nanomaterials might be applied in other intelligent energy storage field.

Supporting Information SEM images of PICA and PEDOT; FTIR and Raman spectra of PEDOT; FTIR spectrum of PICA/TiO2/PEDOT; EIS spectra of PICA/TiO2 nanocomposite with different polymerization charge of PICA; Assignments of infrared spectra of TiO2 nanorod arrays, PICA and PICA/TiO2 nanocomposite; The comparation of the electrochromic parameters of PICA/TiO2 nanocomposite with other reported electrochromic-supercapacitive materials.

ORCID Qingfu Guo: 0000-0002-0844-738X Jingjing Li: 0000-0001-7101-685X Bin Zhang: 0000-0001-9711-1476 Guangming Nie: 0000-0002-1413-2587 Debao Wang: 0000-0003-1992-7570 29



Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (51373089, 51872152), Natural Science Foundation of Shandong (ZR2011BM003) and Talent Fund of QUST(2018).

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Graphical Abstract

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High-Performance Asymmetric Electrochromic-Supercapacitor Device

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