A solar rechargeable battery based on polymeric charge storage electrodes

A solar rechargeable battery based on polymeric charge storage electrodes

Electrochemistry Communications 16 (2012) 69–72 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage:...

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Electrochemistry Communications 16 (2012) 69–72

Contents lists available at SciVerse ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

A solar rechargeable battery based on polymeric charge storage electrodes P. Liu a, H.X. Yang a,⁎, X.P. Ai a, G.R. Li b, X.P. Gao b,⁎⁎ a b

Hubei Key Laboratory of Electrochemical Power Sources, Department of Chemistry, Wuhan University, Wuhan 430072, China Institute of New Energy Material Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 27 October 2011 Received in revised form 28 November 2011 Accepted 28 November 2011 Available online 20 December 2011 Keywords: Solar cell Rechargeable battery Charge storage Hole-transporting material Polypyrrole

a b s t r a c t A solar rechargeable battery is constructed by use of a hybrid TiO2/poly(3,4-ethylenedioxythiophene, PEDOT) photo-anode and a ClO4− doped polypyrrole counter electrode. Here, the dye-sensitized TiO2/PEDOT photoanode serves for positive charge storage and a p-doped PPy counter electrode acts for electron storage in LiClO4 electrolyte. The proposed device demonstrates a rapid photo-charge at light illumination and a stable electrochemical discharge in the dark, realizing an in situ solar-to-electric conversion and storage. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical rechargeable batteries are actively investigated as a key technology of effective electric storage from renewable energy resources [1,2]. Such batteries can only convert electrical energy to chemical energy and vice versa, but usually not be able to store solar energy. Though various photovoltaic and photo-electrochemical cells can convert solar light to electricity, they are not suitable for electric storage. Conventionally, photovoltaic energy storage is performed by use of photovoltaic solar cells in connection with external rechargeable batteries [3,4]. If solar-to-electric energy conversion and storage can be carried out in situ in a photo-electrochemical cell, this would offer a simple and efficient way for the utilization of solar energy. In the past decades, photo-rechargeable electrochemical cells have attracted considerable attention as a technology of choice for the development of high efficient solar energy storage devices [5– 11]. Photovoltaically self-charging batteries were proposed by combining a dye-sensitized solar cell (DSSC) with a rechargeable battery by introducing a third accessorial electrode in-between the photoanode and the counter electrode, where the Pt counter electrode in cells was replaced with a charge storage electrode for photogenerated electron storage, such as WO3 [6,7] and polypyrrole (PPy) [8,9], or carbon capacitors [10]. To ensure effective storage of the solar-generated electricity, photo-chargeable cells must be able to ⁎ Corresponding author. ⁎⁎ Corresponding author. Tel./fax: + 86 22 23500876. E-mail addresses: [email protected] (H.X. Yang), [email protected] (X.P. Gao). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.11.035

store photo-generated holes in the photo-anode and electrons in the counter electrode. Recently, organic hole-transporting materials (HTMs), such as PEDOT, have been successfully applied for the construction of solid-state DSSCs due to their strong ability of hole transport and storage [12–15]. With this in mind, we tried to fabricate a bifunctional photoelectrode by hybridizing a dye-sensitized TiO2 photo-anode with a hole-storage electrode for in situ storage of photo-generated positive charges, so as to build up a solar rechargeable battery (SRB) using an electron-storage electrode as counter electrode. In this communication, we describe a novel SRB system, in which a dye-sensitized TiO2/PEDOT photo-anode serves for positive charge storage and a p-doped PPy counter electrode acts for electron storage in LiClO4 electrolyte. This new configuration of SRB demonstrates feasible solar-to-electric conversion and storage. 2. Experimental The photo-anode was fabricated through a photo-electrochemical polymerization as follows [13]. First, a TiO2 photo-anode was made by coating a TiO2 paste (0.6 g TiO2 powder (P25, Degussa), 0.12 g ethyl cellulose, 1 mL terpineol, and 9 mL ethanol) on a F-doped tin oxide glass (FTO, 15 Ω cm − 2, Nippon Sheet Glass), followed by sintering the anode at 500 °C for 30 min to form a mesoporous TiO2 film of ~7 μm thickness. Then, the photo-anode was immerged in the solution overnight in an acetonitrile solution with 2,2′-bis(3,4-ethylenedioxythiophene (bis-EDOT, 0.01 M) and lithium perchlorate (0.1 M) and then rinsed with anhydrous ethanol and dried again. A filtered (500–1100 nm) Xe lamp with 100 mW cm − 2 irradiation was used as the light source, and the polymerization was lasted for 30 min

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under 10 μA cm − 2 with the Pt foil as the counter electrode and the Ag/ Ag + electrode as reference. The PPy electrode was prepared by the electrochemical polymerization in solution with pyrrole (0.01 M), and lithium perchlorate (0.1 M) in acetonitrile. The PPy was directly deposited on FTO substrate. The electrochemical polymerization was lasted for 120 min at the potential of 0.4 V (vs Ag/Ag +) with a Pt foil as the counter electrode. The cyclic voltammetry (CV) was conducted using a LK2005 electrochemical workstation (Tianjin). It can be easily represented as FTO/dye-sensitized TiO2/PEDOT/carbon felt/absorbent paper (0.2 M LiClO4 and 4-tertiary butylpyridine (TBP, 0.2 M) in propylene carbonate (PC)/PPy/FTO. Both photo-charge and discharge processes were recorded using an IM6ex electrochemical workstation (Zahner). During the photo-charge, the photo-anode was illuminated by a solar simulator (CHF-XM500, Beijing Trusttech) under 100 mW cm − 2 irradiation. 3. Results and discussion The configuration and operating mechanism of the SRB cell are illustrated in Fig. 1. The red arrows represent the photo-charge process, including the photo-generated electron transfer and hole transport on the hybrid TiO2/PEDOT photo-anode, as well as the electron storage on the PPy counter electrode. The green arrows indicate the discharge process of the battery in the dark. During the photocharge process, the photo-anode (A) and the electron-storage electrode (B) are short-circuited while the hole-storage electrode (C) and the electron-storage electrode (B) are disconnected (open circuit). When the photo-anode is illuminated, the dye is firstly excited to produce a pair of photo-generated electrons and holes, and then photo-generated electrons are injected from the excited state of dye into the conduction band of TiO2, leaving strong oxidizing holes on the excited Dye (D* +), which are rapidly reduced to its ground state through transferring the positively charged holes onto the attached PEDOT polymer, i.e., oxidizing the PEDOT into PEDOT +. In the meantime, the photo-generated electrons pass through the external circuit to the PPy counter electrode, where the doped ClO4− anions are simultaneously de-doped into electrolyte for counterbalancing the injected negative charges. For counterbalancing the positive

charges accumulated in the PEDOT, ClO4− anions in electrolyte have to move into the PEDOT chains to form the PEDOT +ClO4−. As a result, the photo-generated electrons and holes are continuously produced and separately stored in the PEDOT hole-storage electrode and in the PPy counter electrode, which actually constitutes an electric storage battery with PEDOT and PPy electrodes functioning as positive and negative electrodes, respectively. The discharge reaction of the battery takes place just in an opposite direction to the above-described photo-charging process. Since the Fermi level of the electron-storage PPy electrode (B) is higher than that of the PEDOT electrode (C) after the photo-charge, electrons can spontaneously flow from the PPy electrode to the PEDOT electrode once the external B to C circuit is closed. At the same time, ClO4− anions in electrolyte are doped into the PPy skeleton to form a steady state complex of PPy +ClO4−, whereas ClO4− anions in the PEDOT +ClO4− matrix are de-doped to reconvert the un-doped PEDOT, forming an internal ionic current flow in the battery. The photo-charge and electrochemical discharge reactions of the solar rechargeable battery can be described as follows: During photo-charging process, the reactions occurring at the photo-anode: þ − Dye þ hv→Dyeðh ; e Þ

ð1Þ

þ − − þ Dyeðh ; e Þ þ TiO2 →e ðTiO2 Þ þ Dye

ð2Þ

þ − xþ − Dye þ PEDOT þ xClO4 →PEDOT ·xClO4 þ Dye

ð3Þ

While the electron-storage reaction proceeding at the counter electrode: ·xþ

PPy







·xClO4 þ e ðTiO2 Þ→PPy þ xClO4

Thus; the overall charge reaction : PEDOT − xþ − þ xClO4 →PEDOT ·xClO4

ð4Þ

ð5Þ

During discharge in the dark, at the electron-storage electrode (battery anode): −







PPy þ xClO4 →PPy ·xClO4 þ xe

ð7Þ

At the hole-storage electrode (battery cathode): xþ







PEDOT ·xClO4 þ xe →PEDOT þ xClO4





Overall discharge reaction : PEDOT ·xClO4 þ PPy→PEDOT xþ − þ PPy ·xClO4

Fig. 1. Schematic demonstration of the configuration and working mechanism of the solar rechargeable battery. A porous carbon sheet was used as the current collector of PEDOT and a piece of absorbent paper impregnated with electrolyte was used as the separator and electrolyte reservoir.

ð8Þ

ð9Þ

Theoretically, the open circuit voltage of the SRB cell depends on the potential difference between the Fermi levels of the doped PEDOT and PPy materials, which are closely related to and can be determined by their redox potentials from CVs. As shown in Fig. 2a, the PEDOT electrode gives a pair of symmetric redox bands with its anodic and cathodic branches centered at + 0.49 V and + 0.02 V, respectively, indicating reversible doping/de-doping reactions of ClO4anion on PEDOT chains. In particularly, the oxidation potential peak of the dye appears at +0.39 V, considerably higher than the reduction peak potential (+0.02 V) of PEDOT. This means that PEDOT can be

P. Liu et al. / Electrochemistry Communications 16 (2012) 69–72

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Cycle number Fig. 2. CVs of (a) PEDOT and (b) PPy on Pt disc (100 μm diameter) microelectrode in 1.0 M LiClO4/PC solution. CV of Z907 dye is inserted into panel a.

sufficiently oxidized to reach a quite high potential of ~+0.39 V during the photo-charge process, which is probably the highest cathode potential of the SRB cell. Fig. 2b shows the CV curve of reversible ClO4− anion doping/de-doping reactions on the PPy electrode. The main feature of the CV curve is a pair of redox bands with its cathodic peak at − 0.55 V and anodic peak at −0.32 V. The potential of the conduction band edge of TiO2 in aprotic solvents (about − 0.68 V vs. Ag/Ag +, or −0.46 V vs NHE) [16] is more negative than the reduction potential (−0.55 V) of the ClO4−-doped PPy, suggesting a feasible transport of photo-generated electrons from the TiO2 photo-anode to the PPy electrode during the photo-charge reaction. In order to justify the reaction mechanism, we photo-charged the SRB cell by short-circuiting the hybrid photo-anode and the PPy electrode (A and B are electrically connected) under illumination for 120 s, and then discharged the cell in the dark by disconnecting the A–B circuit and connecting the C–B circuit to allow a constant current of 8 μA cm − 2 to flow through. Fig. 3 shows the potential profiles measured between the photo-anode and the PPy electrode at photocharge, and the subsequent discharge curve measured between PEDOT and PPy electrodes in the dark. It can be seen that once the hybrid photo-anode was illuminated, the voltage between the PEDOT and PPy electrodes rose very rapidly from a rest potential

Fig. 3. (a) Potential profiles of the SRB cell at photo-charge under irradiation and electrochemical discharge at 8 μA cm− 2. (b) Discharge capacity vs cycle number during 10 cycles. The discharge capacity was calculated based on the loading amount of PEDOT.

of + 0.25 V to a stable value of + 0.76 V, demonstrating an effective hole injection from the TiO2 photo-anode into the PEDOT electrode and a simultaneous electron transfer into the PPy electrode through external circuit (A–B). After the photo-charge completed, the SRB cell can discharge galvanostatically in the dark with its working voltage slowly down to + 0.27 V. The discharge capacity calculated from the amount of the loaded PEDOT on the hybrid photo-anode is about 8.3 mAh g − 1. Although the loading amount and realized capacity of the PEDOT material are not high enough, these data given in Fig. 3 do prove the feasibility of the SRB cell for in situ storage of solar energy. In addition, the SRB cell exhibits almost a negligible capacity fade during 10 cycles of the photo-charge/discharge, revealing a good working stability for solar-to-electric conversion. Based on the oxidation peak potential (+0.39 V) of the dye and the reduction peak potential of the PPy (−0.55 V) for the ClO4− anion dedoping reaction, the maximum open circuit voltage calculated for the SRB cell is about 0.94 V after a full photo-charge. However, the open circuit voltage of the SRB cell is experimentally observed to be 0.76 V, which is 0.18 V lower than the calculated value. This difference may arise from an incomplete oxidation of the PEDOT because of the insufficient oxidizability of the dye employed in the present work.

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It has to be pointed out that the SRB cell has a very poor solar-toelectricity conversion efficiency of ~0.1% at the present state of the art, primarily due to the low light-to-electricity efficiency (0.5%) of the TiO2/ PEDOT anode [17] and also the storage capacity of the SBR system is much lower in terms of gravimetric energy density as compared with conventional batteries because of the low specific capacities of the polymer materials used in this work. Nevertheless, the experimental results described above do reveal the possibility to construct a single SRB cell for a direct solar-to-electric energy conversion and storage as long as the dye, hole-transport and electron-storage materials are selected to be photo-electrochemically compatible. It is also interesting to note that the photo-charge and electrochemical discharge reactions of this SRB cell proceed through reversible doping/de-doping (insertion/extraction) of ClO4− anions between PEDOT and PPy electrodes, seeming as a “rocking chair” battery of ClO4− anions in analogy to lithium-ion batteries.

Acknowledgements This work is supported by the 973 Program (2009CB220100). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

4. Conclusion In summary, we developed a new type of solar rechargeable battery by use of a bifunctional TiO2/PEDOT photo-anode and a PPy counter electrode in LiClO4 electrolyte. This SRB cell demonstrates a direct solar-to-electric conversion and storage with rapid photo-charge efficiency at light illumination and certain discharge ability in the dark. Since the photo-electrochemical performance of this SRB cell depends mainly on the polymeric hole-transport and electron-storage electrodes, further exploration of new polymer materials and optimization of the TiO2/Dye/HTM interfaces may bring about better SRB cells for wider applications of solar energy conversion and storage.

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[15] [16] [17]

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