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A self-rechargeable electrochromic battery based on electrodeposited polypyrrole film
Solar Energy Materials and Solar Cells 192 (2019) 1–7
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A self-rechargeable electrochromic battery based on electrodeposited polypyrrole film Bing Yanga, Dongyun Maa,b, Enming Zhenga, Jinmin Wanga,b, a b
T
⁎
School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai 201209, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochromic Self-powered Self-rechargeable Polypyrrole
Polypyrrole (PPy) film doped with p-toluenesulfonate has been grown on indium doped tin oxide (ITO) coated glass by electrochemical polymerization. The PPy film, aluminum (Al) sheet and KCl solution are assembled into a bi-functional electrochemical device exhibiting self-powered electrochromic device and self-rechargeable battery characteristics. As an electrochromic device, the maximum optical modulation of the PPy/Al device is 59.0% at the wavelength of 698.5 nm. The original black device will become yellow after connecting its two electrodes with a response time of 6.5 s. As a battery, the capacity of the PPy/Al device is 75.25 mAh g−1 when it was initially discharged at a constant current density of 1.0 A g−1. The open circuit voltage of the PPy/Al device is 0.94 V. The charging-discharging processes of the PPy/Al device accompany a clear color change. So the capacity of the PPy/Al device can be known from the color of the device. The PPy/Al device can recover its color and capacity after disconnecting its two electrodes, which is attributed to the spontaneous transformation of PPy film from its neutral state (yellow) to p-type doped state (black) by the oxidation of oxygen.
1. Introduction With huge energy consumption and environmental pollution, clean energy [1–3], energy-saving materials and devices [4–8] are attracting considerable attention. Lithium ion batteries have been widely applied in mobile phones, laptops and cars [9,10]. Electrochromic materials can be used in buildings, cars and anti-glare mirrors due to the clear color change caused by a potential [11–16]. As one of energy-saving devices, electrochromic smart windows can manage the light and heat of space [17–19]. Recently, Wang et al. [20] reported a bi-functional device that has characteristics of self-rechargeable battery and self-powered electrochromic device. Prussian blue (PB) is used as the electrochromic material and as the cathode of battery with an open circuit voltage of 1.26 V. The developed PB/Al device can be bleached to colorless state by connecting the two electrodes of the device. And the bleached state of the device can be recovered to its coloration state by employing the spontaneously oxidation of oxygen to Prussian white (PW), realizing the PB/Al device capable of self-powering and self-recharging. Zhao et al. [21] and Hui et al. [22] use H2O2 and NaClO as oxidizing agents in electrolytes to accelerate the recovery speed of the electrochromic batteries. However, H2O2 and NaClO are strong oxidizing agents that are very active to react with other chemicals in the devices. Chang [23] et al. use sunlight irradiation to improve the recovery speed of ⁎
electrochromic battery. The sunlight irradiation is not controllable because it strongly depends on weather conditions. Bi et al. [24] reported a bi-functional electrochromic and energy-storage device based on tungsten trioxide and zinc oxide nanocomposites. From the angle of materials selection, the materials used in electrochromic batteries should perform good properties for both electrochromic devices and batteries. Furthermore, the reduced states of electrochromic materials should be unstable in air so that oxygen can easily oxidize them to their original states. To meet these requirements, polypyrrole (PPy) is regarded as one of the ideal candidates. As one of polymer-based energy-storage materials [25–29], PPy has been extensively studied because of easy preparation, low cost of monomer, and water solubility [29]. PPy doped with dopants can easily acquire electrons to convert to its neutral state after being discharged in a battery [30]. Sultana et al. [31] prepared PPy film by electrochemical polymerization and used it as cathode of lithium ion batteries. Tavoli et al. [32] prepared dye doped PPy film and used it in an electrochromic device. Though the above-mentioned work reports the energystorage or electrochromic properties of PPy films, they don’t combine the energy-storage and electrochromic properties of PPy films to design a bifunctional device and study the self-recharging properties and selfpowering properties of PPy-based device. In this work, we report a PPy/Al device exhibiting a bifunctional
Corresponding author at: School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.solmat.2018.12.011 Received 29 September 2018; Received in revised form 30 November 2018; Accepted 6 December 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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was used as the counter electrode to fabricate an electrochromic device. The left, right and bottom sides of conductive surface of the ITO glass were covered with adhesive tapes with sizes of 5 × 35 mm2, 5 × 35 mm2 and 5 × 25 mm2. An Al sheet with dimensions of 10 × 70 mm2 was adhered to the conductive surface of the ITO glass. A sandwich-like structure was adopted by assembling the ITO glass deposited with PPy film, ITO glass adhered with Al sheet, and tapes with sizes of 5 × 35 mm2. Therefore, the distance between two electrodes (PPy film and Al sheet) is determined by the tapes’ thickness. KCl aqueous solution (3.0 mol L−1) was used as electrolyte and was injected to the space between two electrodes of the device. 2.4. Characterizations The morphology of PPy film was obtained by scanning electron microscope (Hitachi S4800). Transmittance spectra of the device before and after switching were measured by an ultraviolet-visible (UV–Vis) spectrophotometer (SHIMADZU UV-2600). The cyclic voltammetry (CV) curves of PPy film were measured by electrochemical workstation (CHI 660D) with a three-electrode system. ITO glass deposited by PPy film, Al sheet and SCE were used as working electrode, counter electrode and reference electrode, and 3.0 mol L−1 of KCl solution was used as electrolyte. The charging-discharging curves and recovery-discharging curves of the device were measured by electrochemical workstation. The in situ transmittance spectrum of the PPy/Al device was measured by a UV–Vis spectrophotometer at wavelength of 698.5 nm and from 300 to 800 nm when it was charged-discharged at a constant current density of 1.0 A g−1.
Fig. 1. Schematic diagram for the structure of the PPy/Al device.
characteristic of self-powered electrochromic device and self-rechargeable battery. 2. Experimental 2.1. Materials Pyrrole was purchased from Shanghai Macklin Biochemical Co., Ltd (China). It was distilled and stored in refrigerator prior to use. Sodium p-toluenesulfonate was purchased from Shanghai Macklin Biochemical Co., Ltd (China). Potassium chloride was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Deionized water was used throughout.
3. Results and discussion Fig. 2 shows the surface morphology and cross-sectional SEM images of the as-deposited PPy film. It can be seen that the PPy film consists of compact nanoparticles (Fig. 2a). The thickness of the PPy film is 13 µm (Fig. 2b). To study the electrochemical performance of the as-prepared PPy film, CV curves were acquired in the potential range of −1.5–0.5 V at different scanning rates. As shown in Fig. 3a, well-defined redox peaks were obtained. The reduction and oxidization peaks are centered at −0.76 and 0.073 V, respectively, when the scanning rate is fixed at 100 mV s−1. With the decrease of scanning rate, the current densities of oxidization and reduction peaks are decreased linearly, as shown in Fig. 3b. This result indicates that PPy film is electroactive and welladhered to the surface of ITO glass [33]. The original color of the PPy/Al device is black, as shown in Fig. 4a. After connecting the ITO glass coated by PPy film and Al sheet, the color of the PPy/Al device became transparent yellow (Fig. 4b). The corresponding transmittance spectra of colored and bleached states of the PPy/Al device are shown in Fig. 4c. The maximum transmittance modulation of the device is up to 59.0% at the wavelength of 698.5 nm, which is higher than that of the previously reported PB/Al device [20].
2.2. Preparation of PPy film Indium doped tin oxide (ITO) glasses with dimensions of 25 × 50 mm2 were successively washed by acetone, isopropanol and deionized water. The electrochemical polymerization was carried out by a three-electrode system using ITO glass as working electrode, Pt sheet as counter electrode and saturated calomel electrode (SCE) as reference electrode. PPy film was electrodeposited on ITO glass with a constant current density of 0.4 mA cm−2 for 300 s in an aqueous solution containing 0.1 mol L−1 of pyrrole and 0.1 mol L−1 of sodium ptoluenesulfonate at room temperature. PPy film was obtained by washing with deionized water to remove the adsorptive electrolyte, oligomers and unreacted monomers. 2.3. Device fabrication The schematic diagram for the structure of the PPy/Al device is shown in Fig. 1. Another ITO glass with dimensions of 25 × 40 mm2
Fig. 2. (a) Top view and (b) cross-sectional view of SEM images of the as-deposited PPy film. 2
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Fig. 3. (a) CV curves of the as-prepared PPy film at scan rates of 25, 50, 75 and 100 mV s−1, (b) relationship between current densities of anodic and cathodic peaks and scan rates.
Fig. 4. Digital photos of the PPy/Al device (a) before and (b) after connecting the two electrodes, (c) transmittance spectra of the bleached and colored states of the device.
Fig. 5. Digital photos of the disconnected PPy/Al device after recovery for (a) 5, (b) 10, (c) 30 min and (d) 1 h, (e) corresponding transmittance spectra of the disconnected device, (f) in situ transmittance measurement of the device by connecting for 65.9 s and then disconnecting for next 926.5 s.
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Fig. 6. Photos of two PPy/Al devices connected in series as self-rechargeable batteries. (a) The original (black) state (before connecting) of the devices, and (b) a red LED is lighted up after it is connected with the devices, (c) the bleached (yellow) state of the devices after exhausting the quantity of electricity, (d) the LED cannot be lighted up by the bleached devices, (e) the recovered state (black) of the devices after disconnecting their electrodes in air for 4 h, and (f) the red LED is lighted up again by the recovered devices. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
light up a red light emitting diode (LED), as shown in Fig. 6a and b. The process of lighting up a red LED accompanies color change of the PPy/ Al devices from black to transparent yellow. As shown in Fig. 6c and d, the PPy/Al devices can’t light up the red LED after the quantity of electricity of the devices is exhausted. However, the PPy/Al devices can light up the red LED again after disconnecting their electrodes for 4 h (Fig. 6e and f). We further characterize the charging-discharging property of the PPy/Al device and the transmittance modulation at a wavelength of 698.5 nm. As shown in Fig. 7a, the capacity of the PPy/Al device is 75.25 mAh g−1 when it is initially discharged and charged in a potential range of 0–1.70 V with a constant current density of 1.0 A g−1. Fig. 7b shows the cyclic stability of the PPy/Al device. It can be noted that the capacity of the device becomes stable from the third cycle. After 50 cycles, the capacity of the device can retain 92.0% of that measured from the first cycle. The charging-discharging process of the PPy/Al device accompanies with the transmittance change. As shown in Fig. 7c, the original transmittance of the PPy/Al device at 698.5 nm is 3.0%. When the PPy/Al device is discharged from 0.94 V to 0 V with a constant current density of 1.0 A g−1, the capacity of the device is 75.25 mAh g−1 and the transmittance of the device arises to 56.6%. As the battery is charged from 0 V to 1.70 V with the same current density, the capacity of the device is 54.27 mAh g−1 and the transmittance of the device drops to 4.0%. The synchronous transmittance spectra (Fig. 7d) of the PPy/Al device were measured when the device was discharged
The color of the PPy/Al device will be spontaneously recovered to black when the two electrodes of the device were disconnected (Fig. 5ad), which indicates that the device is a self-powered electrochromic device. The corresponding transmittance spectra of the PPy/Al device with different recovery time are shown in Fig. 5e. At 698.5 nm, the transmittance of the device is decreased by 31.1%, 34.2%, 38.4%, and 48.3% after recovery for 5, 10, 30 min and 1 h, respectively. The decreased transmittance of the device within 1 h amounts to the PB/Al device within 4 h [20]. To measure the bleaching and recovery (coloration) time of the PPy/Al device, the two electrodes of the device were connected and disconnected in a UV–Vis spectrophotometer. As shown in Fig. 5f, the transmittance modulation of the device is rapidly increased by 53.1% (90% of the maximum transmittance modulation) in the first 6.5 s after connecting the two electrodes. However, it needs 59.4 s if the transmittance modulation reaches 59.0% (the original value of the device). After disconnecting the two electrodes, the transmittance is decreased by 29.42% in 926.5 s. The as-prepared device is different from traditional electrochromic device that needs an applied external voltage to change the color or transmittance of the electrochromic device. However, the PPy/Al device belongs to a type of battery that can change its color by connecting its two electrodes. So it is necessary to characterize the battery properties of the PPy/Al device. The open circuit potential of the PPy/Al device is 0.94 V, which is lower than that of PB/Al device [20]. Two PPy/Al devices were connected in series to supply a voltage of 1.88 V to 4
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Fig. 7. (a) Charging-discharging curves of the PPy/Al device with a constant current density of 1.0 A g−1, (b) discharge capacity of the PPy/Al device after different cycles showing the cyclic stability of the device, (c) transmittance change at 698.5 nm accompanying with charging-discharging process with a constant current density of 1.0 A g−1, (d) synchronous transmittance spectra as the PPy/Al device was discharged and charged with a constant current density of 1.0 A g−1, (e) the discharging curves of the PPy/Al device with constant current densities of 0.2, 0.5 1.0 and 1.0 A g−1.
and charged with a constant current density of 1.0 A g−1. When the PPy/Al device was discharged to 0 V, the transmittance of the device corresponds to its bleached state (yellow). While the PPy/Al device was charged to 1.7 V, the transmittance of the device nearly reaches that of its original state. Therefore, the capacity of the PPy/Al device is related to its transmittance. We can generally know the capacity of the device from its transmittance or color. Initial discharging curves of the PPy/Al device measured at different current densities are shown in Fig. 7e. With the increase of the discharge current densities from 0.2, 0.5, 1.0 to 2.0A g−1, the measured capacities of the PPy/Al device decrease from 111.43, 101.57, 75.25 to 67.78 mAh g−1, respectively. After exhausting the quantity of electricity of the PPy/Al device, not only the transmittance but also the capacity and open circuit voltage of the device can be recovered. Fig. 8a shows the discharging curves of the PPy/Al device after different recovery time measured at constant
current density of 1 A g−1. It can be seen from Fig. 8b, the capacity of the PPy/Al device is 30.64 mAh g−1 after recovery for 1 h. With the increase of the recovery time, the capacity of the PPy/Al device is also increased. After 24 h, the capacity reaches 67.01 mAh g−1, reaching 89.0% of its original capacity. The recovered capacity of the device will be closer to its original capacity if more recovery time is used. The recovered open circuit voltage of the PPy/Al device reaches to 0.42, 0.47, 0.59, 0.74, 0.85 and 0.87 V after recovery for 1, 2, 3, 4, 12 and 24 h, respectively. The mechanism of the PPy/Al device acting as an electrochromic device and a battery is discussed and schematically illustrated in Fig. 9. As anode of the battery, Al sheet can be easily oxidized to release electrons to form Al3+ ions. As cathode of the battery, p-type doped PPy film can be reduced to neutral PPy film by acquiring electrons from the Al sheet and K+ ions from the electrolyte, accompanying the color
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Fig. 8. (a) Discharging curves of the PPy/Al device after different recovery time of 1, 2, 3, 4, 12 and 24 h measured at a constant current density of 1.0 A g−1, (b) the change of the recovered open circuit voltage and capacity of the PPy/Al device with the recovery time.
Fig. 9. Schematic diagrams of working mechanism for the PPy/Al device (a-c) the bleaching and recovery process, (d-g) the discharging and self-recharging process of the device.
film acting as an electrochromic device and a battery has been fabricated. The device consists of PPy film electrodeposited on ITO glass, Al sheet attached on another ITO glass and KCl electrolyte. As an electrochromic device, the color of the PPy/Al device can be changed from black to yellow by connecting the two electrodes of the device. As a battery, the capacity of the PPy/Al device can reach 75.25 mAh g−1 when it is discharged at a constant current density of 1.0 A g−1. The open circuit voltage of the PPy/Al device is 0.94 V. By combining both of the electrochromic and battery characteristics, the PPy/Al device can exhibit its capacity through its color. More interestingly, the PPy/Al device can recover its color and capacity after disconnecting the two electrodes of the device, which is attributed to the spontaneous transformation of PPy film from the neutral state (yellow) to p-type doped state (black) by the oxidation of oxygen. The PPy/Al device exhibits promising applications in self-powered electrochromic windows and self-rechargeable batteries.
change of the film from black to yellow. So the half reaction of the PPy/ Al device (battery) can be expressed by the following reactions: Anode: Al ⇌Al3+ + 3e+
-
(1) +
Cathode: PPy PTS (Black) + K
+ e ⇌ PPy PTS K -
0
- +
(Yellow)
(2)
The overall reaction of the battery can be expressed as: Al + 3PPy+PTS- (Black) + 3K+ ⇌ Al3+ + 3PPy0PTS-K+ (Yellow) (3) After the quantity of electricity of the PPy/Al device is exhausted, the p-type doped PPy film starts to transform to the neutral state of PPy film. The neutral state of PPy film can be oxidized to its oxidizing state when it reacts with oxygen [34,35], as expressed in the following reaction:
PPy 0 (Yellow) +
1 1 O2 + H2 O ⇌ PPy+OH− (Black) 4 2
(4) Acknowledgements
So the PPy/Al device can not only be charged by external power source but also be charged by oxygen. Accompanying the recovery of the device’ color, its energy is also recovered.
We would like to thank support from National Natural Science Foundation of China (NSFC) (Nos. 61775131, 61376009), Gaoyuan Discipline of Shanghai – Environmental Science and Engineering (Resource Recycling Science and Engineering), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. 2013-70), “Shu Guang” project
4. Conclusions In conclusion, a bi-functional device based on electrodeposited PPy 6
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supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG55), Graduate Program Fund supported by Shanghai Polytechnic University (EGD17YJ0032).
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
A self-rechargeable electrochromic battery based on electrodeposited polypyrrole film Bing Yanga, Dongyun Maa,b, Enming Zhenga, Jinmin Wanga,b, a b
T
⁎
School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China Research Center of Resource Recycling Science and Engineering, Shanghai Polytechnic University, Shanghai 201209, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochromic Self-powered Self-rechargeable Polypyrrole
Polypyrrole (PPy) film doped with p-toluenesulfonate has been grown on indium doped tin oxide (ITO) coated glass by electrochemical polymerization. The PPy film, aluminum (Al) sheet and KCl solution are assembled into a bi-functional electrochemical device exhibiting self-powered electrochromic device and self-rechargeable battery characteristics. As an electrochromic device, the maximum optical modulation of the PPy/Al device is 59.0% at the wavelength of 698.5 nm. The original black device will become yellow after connecting its two electrodes with a response time of 6.5 s. As a battery, the capacity of the PPy/Al device is 75.25 mAh g−1 when it was initially discharged at a constant current density of 1.0 A g−1. The open circuit voltage of the PPy/Al device is 0.94 V. The charging-discharging processes of the PPy/Al device accompany a clear color change. So the capacity of the PPy/Al device can be known from the color of the device. The PPy/Al device can recover its color and capacity after disconnecting its two electrodes, which is attributed to the spontaneous transformation of PPy film from its neutral state (yellow) to p-type doped state (black) by the oxidation of oxygen.
1. Introduction With huge energy consumption and environmental pollution, clean energy [1–3], energy-saving materials and devices [4–8] are attracting considerable attention. Lithium ion batteries have been widely applied in mobile phones, laptops and cars [9,10]. Electrochromic materials can be used in buildings, cars and anti-glare mirrors due to the clear color change caused by a potential [11–16]. As one of energy-saving devices, electrochromic smart windows can manage the light and heat of space [17–19]. Recently, Wang et al. [20] reported a bi-functional device that has characteristics of self-rechargeable battery and self-powered electrochromic device. Prussian blue (PB) is used as the electrochromic material and as the cathode of battery with an open circuit voltage of 1.26 V. The developed PB/Al device can be bleached to colorless state by connecting the two electrodes of the device. And the bleached state of the device can be recovered to its coloration state by employing the spontaneously oxidation of oxygen to Prussian white (PW), realizing the PB/Al device capable of self-powering and self-recharging. Zhao et al. [21] and Hui et al. [22] use H2O2 and NaClO as oxidizing agents in electrolytes to accelerate the recovery speed of the electrochromic batteries. However, H2O2 and NaClO are strong oxidizing agents that are very active to react with other chemicals in the devices. Chang [23] et al. use sunlight irradiation to improve the recovery speed of ⁎
electrochromic battery. The sunlight irradiation is not controllable because it strongly depends on weather conditions. Bi et al. [24] reported a bi-functional electrochromic and energy-storage device based on tungsten trioxide and zinc oxide nanocomposites. From the angle of materials selection, the materials used in electrochromic batteries should perform good properties for both electrochromic devices and batteries. Furthermore, the reduced states of electrochromic materials should be unstable in air so that oxygen can easily oxidize them to their original states. To meet these requirements, polypyrrole (PPy) is regarded as one of the ideal candidates. As one of polymer-based energy-storage materials [25–29], PPy has been extensively studied because of easy preparation, low cost of monomer, and water solubility [29]. PPy doped with dopants can easily acquire electrons to convert to its neutral state after being discharged in a battery [30]. Sultana et al. [31] prepared PPy film by electrochemical polymerization and used it as cathode of lithium ion batteries. Tavoli et al. [32] prepared dye doped PPy film and used it in an electrochromic device. Though the above-mentioned work reports the energystorage or electrochromic properties of PPy films, they don’t combine the energy-storage and electrochromic properties of PPy films to design a bifunctional device and study the self-recharging properties and selfpowering properties of PPy-based device. In this work, we report a PPy/Al device exhibiting a bifunctional
Corresponding author at: School of Environmental and Materials Engineering, College of Engineering, Shanghai Polytechnic University, Shanghai 201209, China. E-mail address: [email protected] (J. Wang).
https://doi.org/10.1016/j.solmat.2018.12.011 Received 29 September 2018; Received in revised form 30 November 2018; Accepted 6 December 2018 0927-0248/ © 2018 Elsevier B.V. All rights reserved.
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was used as the counter electrode to fabricate an electrochromic device. The left, right and bottom sides of conductive surface of the ITO glass were covered with adhesive tapes with sizes of 5 × 35 mm2, 5 × 35 mm2 and 5 × 25 mm2. An Al sheet with dimensions of 10 × 70 mm2 was adhered to the conductive surface of the ITO glass. A sandwich-like structure was adopted by assembling the ITO glass deposited with PPy film, ITO glass adhered with Al sheet, and tapes with sizes of 5 × 35 mm2. Therefore, the distance between two electrodes (PPy film and Al sheet) is determined by the tapes’ thickness. KCl aqueous solution (3.0 mol L−1) was used as electrolyte and was injected to the space between two electrodes of the device. 2.4. Characterizations The morphology of PPy film was obtained by scanning electron microscope (Hitachi S4800). Transmittance spectra of the device before and after switching were measured by an ultraviolet-visible (UV–Vis) spectrophotometer (SHIMADZU UV-2600). The cyclic voltammetry (CV) curves of PPy film were measured by electrochemical workstation (CHI 660D) with a three-electrode system. ITO glass deposited by PPy film, Al sheet and SCE were used as working electrode, counter electrode and reference electrode, and 3.0 mol L−1 of KCl solution was used as electrolyte. The charging-discharging curves and recovery-discharging curves of the device were measured by electrochemical workstation. The in situ transmittance spectrum of the PPy/Al device was measured by a UV–Vis spectrophotometer at wavelength of 698.5 nm and from 300 to 800 nm when it was charged-discharged at a constant current density of 1.0 A g−1.
Fig. 1. Schematic diagram for the structure of the PPy/Al device.
characteristic of self-powered electrochromic device and self-rechargeable battery. 2. Experimental 2.1. Materials Pyrrole was purchased from Shanghai Macklin Biochemical Co., Ltd (China). It was distilled and stored in refrigerator prior to use. Sodium p-toluenesulfonate was purchased from Shanghai Macklin Biochemical Co., Ltd (China). Potassium chloride was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Deionized water was used throughout.
3. Results and discussion Fig. 2 shows the surface morphology and cross-sectional SEM images of the as-deposited PPy film. It can be seen that the PPy film consists of compact nanoparticles (Fig. 2a). The thickness of the PPy film is 13 µm (Fig. 2b). To study the electrochemical performance of the as-prepared PPy film, CV curves were acquired in the potential range of −1.5–0.5 V at different scanning rates. As shown in Fig. 3a, well-defined redox peaks were obtained. The reduction and oxidization peaks are centered at −0.76 and 0.073 V, respectively, when the scanning rate is fixed at 100 mV s−1. With the decrease of scanning rate, the current densities of oxidization and reduction peaks are decreased linearly, as shown in Fig. 3b. This result indicates that PPy film is electroactive and welladhered to the surface of ITO glass [33]. The original color of the PPy/Al device is black, as shown in Fig. 4a. After connecting the ITO glass coated by PPy film and Al sheet, the color of the PPy/Al device became transparent yellow (Fig. 4b). The corresponding transmittance spectra of colored and bleached states of the PPy/Al device are shown in Fig. 4c. The maximum transmittance modulation of the device is up to 59.0% at the wavelength of 698.5 nm, which is higher than that of the previously reported PB/Al device [20].
2.2. Preparation of PPy film Indium doped tin oxide (ITO) glasses with dimensions of 25 × 50 mm2 were successively washed by acetone, isopropanol and deionized water. The electrochemical polymerization was carried out by a three-electrode system using ITO glass as working electrode, Pt sheet as counter electrode and saturated calomel electrode (SCE) as reference electrode. PPy film was electrodeposited on ITO glass with a constant current density of 0.4 mA cm−2 for 300 s in an aqueous solution containing 0.1 mol L−1 of pyrrole and 0.1 mol L−1 of sodium ptoluenesulfonate at room temperature. PPy film was obtained by washing with deionized water to remove the adsorptive electrolyte, oligomers and unreacted monomers. 2.3. Device fabrication The schematic diagram for the structure of the PPy/Al device is shown in Fig. 1. Another ITO glass with dimensions of 25 × 40 mm2
Fig. 2. (a) Top view and (b) cross-sectional view of SEM images of the as-deposited PPy film. 2
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Fig. 3. (a) CV curves of the as-prepared PPy film at scan rates of 25, 50, 75 and 100 mV s−1, (b) relationship between current densities of anodic and cathodic peaks and scan rates.
Fig. 4. Digital photos of the PPy/Al device (a) before and (b) after connecting the two electrodes, (c) transmittance spectra of the bleached and colored states of the device.
Fig. 5. Digital photos of the disconnected PPy/Al device after recovery for (a) 5, (b) 10, (c) 30 min and (d) 1 h, (e) corresponding transmittance spectra of the disconnected device, (f) in situ transmittance measurement of the device by connecting for 65.9 s and then disconnecting for next 926.5 s.
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Fig. 6. Photos of two PPy/Al devices connected in series as self-rechargeable batteries. (a) The original (black) state (before connecting) of the devices, and (b) a red LED is lighted up after it is connected with the devices, (c) the bleached (yellow) state of the devices after exhausting the quantity of electricity, (d) the LED cannot be lighted up by the bleached devices, (e) the recovered state (black) of the devices after disconnecting their electrodes in air for 4 h, and (f) the red LED is lighted up again by the recovered devices. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
light up a red light emitting diode (LED), as shown in Fig. 6a and b. The process of lighting up a red LED accompanies color change of the PPy/ Al devices from black to transparent yellow. As shown in Fig. 6c and d, the PPy/Al devices can’t light up the red LED after the quantity of electricity of the devices is exhausted. However, the PPy/Al devices can light up the red LED again after disconnecting their electrodes for 4 h (Fig. 6e and f). We further characterize the charging-discharging property of the PPy/Al device and the transmittance modulation at a wavelength of 698.5 nm. As shown in Fig. 7a, the capacity of the PPy/Al device is 75.25 mAh g−1 when it is initially discharged and charged in a potential range of 0–1.70 V with a constant current density of 1.0 A g−1. Fig. 7b shows the cyclic stability of the PPy/Al device. It can be noted that the capacity of the device becomes stable from the third cycle. After 50 cycles, the capacity of the device can retain 92.0% of that measured from the first cycle. The charging-discharging process of the PPy/Al device accompanies with the transmittance change. As shown in Fig. 7c, the original transmittance of the PPy/Al device at 698.5 nm is 3.0%. When the PPy/Al device is discharged from 0.94 V to 0 V with a constant current density of 1.0 A g−1, the capacity of the device is 75.25 mAh g−1 and the transmittance of the device arises to 56.6%. As the battery is charged from 0 V to 1.70 V with the same current density, the capacity of the device is 54.27 mAh g−1 and the transmittance of the device drops to 4.0%. The synchronous transmittance spectra (Fig. 7d) of the PPy/Al device were measured when the device was discharged
The color of the PPy/Al device will be spontaneously recovered to black when the two electrodes of the device were disconnected (Fig. 5ad), which indicates that the device is a self-powered electrochromic device. The corresponding transmittance spectra of the PPy/Al device with different recovery time are shown in Fig. 5e. At 698.5 nm, the transmittance of the device is decreased by 31.1%, 34.2%, 38.4%, and 48.3% after recovery for 5, 10, 30 min and 1 h, respectively. The decreased transmittance of the device within 1 h amounts to the PB/Al device within 4 h [20]. To measure the bleaching and recovery (coloration) time of the PPy/Al device, the two electrodes of the device were connected and disconnected in a UV–Vis spectrophotometer. As shown in Fig. 5f, the transmittance modulation of the device is rapidly increased by 53.1% (90% of the maximum transmittance modulation) in the first 6.5 s after connecting the two electrodes. However, it needs 59.4 s if the transmittance modulation reaches 59.0% (the original value of the device). After disconnecting the two electrodes, the transmittance is decreased by 29.42% in 926.5 s. The as-prepared device is different from traditional electrochromic device that needs an applied external voltage to change the color or transmittance of the electrochromic device. However, the PPy/Al device belongs to a type of battery that can change its color by connecting its two electrodes. So it is necessary to characterize the battery properties of the PPy/Al device. The open circuit potential of the PPy/Al device is 0.94 V, which is lower than that of PB/Al device [20]. Two PPy/Al devices were connected in series to supply a voltage of 1.88 V to 4
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Fig. 7. (a) Charging-discharging curves of the PPy/Al device with a constant current density of 1.0 A g−1, (b) discharge capacity of the PPy/Al device after different cycles showing the cyclic stability of the device, (c) transmittance change at 698.5 nm accompanying with charging-discharging process with a constant current density of 1.0 A g−1, (d) synchronous transmittance spectra as the PPy/Al device was discharged and charged with a constant current density of 1.0 A g−1, (e) the discharging curves of the PPy/Al device with constant current densities of 0.2, 0.5 1.0 and 1.0 A g−1.
and charged with a constant current density of 1.0 A g−1. When the PPy/Al device was discharged to 0 V, the transmittance of the device corresponds to its bleached state (yellow). While the PPy/Al device was charged to 1.7 V, the transmittance of the device nearly reaches that of its original state. Therefore, the capacity of the PPy/Al device is related to its transmittance. We can generally know the capacity of the device from its transmittance or color. Initial discharging curves of the PPy/Al device measured at different current densities are shown in Fig. 7e. With the increase of the discharge current densities from 0.2, 0.5, 1.0 to 2.0A g−1, the measured capacities of the PPy/Al device decrease from 111.43, 101.57, 75.25 to 67.78 mAh g−1, respectively. After exhausting the quantity of electricity of the PPy/Al device, not only the transmittance but also the capacity and open circuit voltage of the device can be recovered. Fig. 8a shows the discharging curves of the PPy/Al device after different recovery time measured at constant
current density of 1 A g−1. It can be seen from Fig. 8b, the capacity of the PPy/Al device is 30.64 mAh g−1 after recovery for 1 h. With the increase of the recovery time, the capacity of the PPy/Al device is also increased. After 24 h, the capacity reaches 67.01 mAh g−1, reaching 89.0% of its original capacity. The recovered capacity of the device will be closer to its original capacity if more recovery time is used. The recovered open circuit voltage of the PPy/Al device reaches to 0.42, 0.47, 0.59, 0.74, 0.85 and 0.87 V after recovery for 1, 2, 3, 4, 12 and 24 h, respectively. The mechanism of the PPy/Al device acting as an electrochromic device and a battery is discussed and schematically illustrated in Fig. 9. As anode of the battery, Al sheet can be easily oxidized to release electrons to form Al3+ ions. As cathode of the battery, p-type doped PPy film can be reduced to neutral PPy film by acquiring electrons from the Al sheet and K+ ions from the electrolyte, accompanying the color
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Fig. 8. (a) Discharging curves of the PPy/Al device after different recovery time of 1, 2, 3, 4, 12 and 24 h measured at a constant current density of 1.0 A g−1, (b) the change of the recovered open circuit voltage and capacity of the PPy/Al device with the recovery time.
Fig. 9. Schematic diagrams of working mechanism for the PPy/Al device (a-c) the bleaching and recovery process, (d-g) the discharging and self-recharging process of the device.
film acting as an electrochromic device and a battery has been fabricated. The device consists of PPy film electrodeposited on ITO glass, Al sheet attached on another ITO glass and KCl electrolyte. As an electrochromic device, the color of the PPy/Al device can be changed from black to yellow by connecting the two electrodes of the device. As a battery, the capacity of the PPy/Al device can reach 75.25 mAh g−1 when it is discharged at a constant current density of 1.0 A g−1. The open circuit voltage of the PPy/Al device is 0.94 V. By combining both of the electrochromic and battery characteristics, the PPy/Al device can exhibit its capacity through its color. More interestingly, the PPy/Al device can recover its color and capacity after disconnecting the two electrodes of the device, which is attributed to the spontaneous transformation of PPy film from the neutral state (yellow) to p-type doped state (black) by the oxidation of oxygen. The PPy/Al device exhibits promising applications in self-powered electrochromic windows and self-rechargeable batteries.
change of the film from black to yellow. So the half reaction of the PPy/ Al device (battery) can be expressed by the following reactions: Anode: Al ⇌Al3+ + 3e+
-
(1) +
Cathode: PPy PTS (Black) + K
+ e ⇌ PPy PTS K -
0
- +
(Yellow)
(2)
The overall reaction of the battery can be expressed as: Al + 3PPy+PTS- (Black) + 3K+ ⇌ Al3+ + 3PPy0PTS-K+ (Yellow) (3) After the quantity of electricity of the PPy/Al device is exhausted, the p-type doped PPy film starts to transform to the neutral state of PPy film. The neutral state of PPy film can be oxidized to its oxidizing state when it reacts with oxygen [34,35], as expressed in the following reaction:
PPy 0 (Yellow) +
1 1 O2 + H2 O ⇌ PPy+OH− (Black) 4 2
(4) Acknowledgements
So the PPy/Al device can not only be charged by external power source but also be charged by oxygen. Accompanying the recovery of the device’ color, its energy is also recovered.
We would like to thank support from National Natural Science Foundation of China (NSFC) (Nos. 61775131, 61376009), Gaoyuan Discipline of Shanghai – Environmental Science and Engineering (Resource Recycling Science and Engineering), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (No. 2013-70), “Shu Guang” project
4. Conclusions In conclusion, a bi-functional device based on electrodeposited PPy 6
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supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (No. 13SG55), Graduate Program Fund supported by Shanghai Polytechnic University (EGD17YJ0032).
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