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Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices
Nano Energy (2016) 19, 307–317
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanoenergy
COMMUNICATION
Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices Linlin Lia,b, Shengjie Pengc,n, Han-Yi Chenb,d, Xiaopeng Hane, Fangyi Chenge, Madhavi Srinivasana,b,n, Stefan Adamsf, Seeram Ramakrishnac, Jun Chene,n a
School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore TUM CREATE, 1 CREATE Way, #10-02 CREATE Tower, 138602 Singapore c Department of Mechanical Engineering, National University of Singapore, 117574 Singapore d Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, Garching 85748, Germany e Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China f Department of Materials Science and Engineering, National University of Singapore, 117546 Singapore b
Received 17 July 2015; received in revised form 19 November 2015; accepted 23 November 2015 Available online 30 November 2015
KEYWORDS
Abstract
Porous; Vanadium oxide; Flexible electrode; Solid electrolyte; Hybrid devices
Design and fabrication of advanced functional materials is essential but still a challenge for current energy storage devices. Herein, polypyrrole coated highly porous vanadium oxide (V2O5@Ppy) nanorod and nanoplate arrays with large mass loadings, have been successfully constructed on carbon felt (CF) via a facile solvothermal reaction followed by in-situ polymerization technique. Interestingly, the structure of the V2O5 thin films can be simply tuned from porous nanoplates to nanorods with controlled calcination time. In addition, MnO2 nanowires with Ppy coating were also grown on the CF substrates to form MnO2@Ppy/CF electrode through the similar method. As integrate electrodes for energy storage devices, V2O5@Ppy/CF nanorods demonstrate more superior electrochemical properties compared to V2O5@Ppy/CF nanoplates. By virtue of their intriguing structural features and uniformly Ppy coating, a solid-state flexible hybrid device (SFHD) based on V2O5@Ppy/CF and MnO2@Ppy/CF as the negative and positive electrode, respectively, manifests outstanding cycling stability (approximately 89% retention even after 20,000 cycles), excellent mechanical flexibility, and
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Corresponding authors. E-mail addresses: [email protected] (S. Peng), [email protected] (M. Srinivasan), [email protected] (J. Chen). http://dx.doi.org/10.1016/j.nanoen.2015.11.026 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
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Introduction With the rapid progress of nanomaterials and nanotechnology, wearable and protable electronics are becoming indispensable in the increasingly mobile society. Great efforts have been made to fabricate flexible devices in various fields, such as roll-up displays, artificial electronic skin, and so on. With the aim to create entirely flexible electronics, all of these devices need lightweight, flexible, and highperformance energy storage unites. In today's practical applications, electrochemical capacitors (ECs) [1,2] and batteries, as two major energy storage technologies, represent two extremes of the design space. To bridge the gap between ECs and batteries, hybrid devices (HDs), also known as hybrid combinations, provide a promising path [3,4]. Typically, such devices combine the advantages of both ECs and batteries by using a capacitor-type electrode to ensure high power density and battery-type electrode to enable high energy density. More importantly, compared to conventional ECs, HDs generally possess a wider working potential window, thereby further guaranteeing the energy storage capability. Consequently, a new avenue has been opened by HDs for the emerging high energy and power applications [5,6]. However, their role in flexible and smart portable devices barely satisfies the demands in mechanical flexibility and miniaturized in dimentions. Therefore, researchers are striving to explore advanced solid-state flexible HDs with both high power and energy densities, as well as excellent flexibility [7–9]. So far, great attentions on more safe and flexible HDs are devoted to build advanced integrate flexible hybrid electrodes with high energy and power densities for commercial implementation [10–12]. Since the hybrid electrodes largely determine the electrochemical performance of HDs, there has been extensive interest in the proper selection and hybridization of nanoarchitectures (phase composition, morphologies, size and uniformity) with precisely controlled structures [13–16]. It is well-accepted that the battery-type electrodes, usually metal oxides, are still limited by their intrinsic poor conductivity and suffer pulverization especially after long-term cycling. To address these issues, the most popular approach is to fabricate hybrid electrodes, which commonly combine metal oxides electrodes with various conductive polymer (polyaniline, Ppy) or carbon materials (CNT, graphene) [12,15,17]. Accordingly, it has been demonstrated that both the conductivity and capacitance of the hybrid configurations can be largely enhanced by coating a thin layer of conductive polymer on the surface of electrode materials, which not only possesses large capacitance and high electrical conductivity, but also can be easily polymerized [18–20]. Additionally, to meet the need for flexibile HDs, it should be taken into account about
the flexibility of current collector. An advent of new concept is to disperse functional nanostructures with controlled morphologies and sizes on carbon frameworks or scaffolds such as graphene foam, carbon nanotubes coated textile, carbon cloth, to construct flexible integrate hybrid electrode [19,21,22]. These carbon frameworks act as backbones to faciliate the transportation of ions and electrons, thus leading to improved rate capability [23– 25]. Baed on the above-mentioned aspects, a solid-state flexible HD, which consists of rationally designed hybrid nanoarchitectures, represents a new opportunity to achieve high power and energy applications. Despite the significant development in functional nanomaterials, transition metal oxides (TMOs) still govern the landscape of active materials for electrochemical devices. In particular, vanadium oxides (V2O5) and manganese dioxide (MnO2) exhibit significant predominance such as natural abundance, high theoretical capacitance, multiple oxidation states, and ease of synthesis [18,26–30]. In this regard, it is anticipated that introducing V2O5 as negative electrode and MnO2 as positive electrode in a solid-state flexible HDs will becoming a promising energy storage technology. In the present study, we make the attempt to employ both of those possible optimization pathways (Ppy coated nanostructures on carbon felt) to enhance the performance of solid-state flexible HDs. Herein, Ppy coated hierarchical porous composites nanoarchitectures (V2O5@Ppy and MnO2@Ppy) have been successfully built on carbon felt (CF) by a facile wet-chemical method followed by an insitu polymerization Ppy on the surface of nanoarchitectures. Specifically, CF serves as the flexible current collector, mainly owing to its good mechanical flexibility, excellent conductivity, and great durability to bear shape deformation [31,32]. Moreover, the uniformly polymerized Ppy layer could not only effectively prohibit active mateirals (V2O5 and MnO2) from corrosion in liquid electrolyte, but also provide an additional pathway for electron and ions transfer. Furthermore, it is found that the structure of the V2O5 thin films can be simply tuned from porous nanoplates to nanorods with controlled calcination time. Consequently, as integrate electrodes for energy storage devices, V2O5@Ppy/CF nanorods manifest more superior electrochemical properties compared to V2O5@Ppy/CF nanoplates. In addition, benefiting from the unique structural features, the as-prepared solid-state flexible HDs, which is based on V2O5@Ppy/CF anode and MnO2@Ppy/CF cathode, exhibits outstanding cycling stability (approximately 89% retention even after 20,000 cycles), excellent mechanical flexibility, and remarkable energy density (28.6 W h kg 1 at power density of around 200 W kg 1), which makes it hold great potential to be unexceptionably flexible devices for portable and wearable electronics.
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Experimental section
Materials characterization
Preparation of V2O5 on CF substrates
X-ray diffraction (XRD) patterns were collected on a Shimadzu X-ray diffractometer. Field-emission scanning electron microscopy (FE-SEM) images were collected using a JEOL JSM 7600F microscope. Transmission electron microscope (TEM) images were taken on a JEOL 2100 microscope. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos Axis Ultra DLD electron spectrometer (PHI, PHI5300 system). Fourier transform infrared (FT-IR) spectra were obtained using a Perkin Elmer Spectrum GX instrument using KBr pellet technique at a resolution of 1 cm 1. Thermogravimetric analysis (TGA, Q500) was determined from room temperature to 700 1C under a continuous air flow at 10 1C min 1.
All chemicals were used as received without further purification. In a typical synthesis, commercial carbon felt (CF) from company with a dimension of 1 cm 4 cm was placed against the wall of a Teflon-lined stainless steel autoclave that contained a homogeneous solution of vanadium oxytriisopropoxide (VOT) (0.5 mL) in 35 ml of iso-propanol alcohol (IPA). Afterwards, the autoclave was sealed and maintained at 180 1C for 12 h to synthesize vanadium precursor film. Then, the substrates were taken out and cleaned by ultrasonication for several minutes to remove the loosely attached products on the surface. After drying at 60 1C for 10 h under vacuum, a green thin film was coated on the CF substrate. In order to obtain crystallized V2O5 nanostructures, the conductive substrates with the asgrown films were calcined at 350 1C for 0.5 h or 3 h with a heating rate of 2 1C min 1 in air, which could produce light yellow V2O5 nanoplate or nanorod thin films, respectively. The mass loading of V2O5 on the textile was measured to be 6.22 mg cm 2 by a microbalance before and after the materials loading.
Preparation of Ppy coated V2O5/CF film V2O5 were coated with Ppy by an in situ polymerization method. 0.12 g p-toluenesulfonic acid (p-TSA) was firstly dissolved into 15 mL of ethanol. After stirring for 10 min, 50 mL of pyrrole monomer was dissolved into the above solution and stirred to form a mixed solution A. Meanwhile, 0.08 g ammonium peroxy disulfate (APS) was dissolved in 10 mL of distilled water to form a solution B. Afterwards, the CF coated by the V2O5 arrays was immersed into solution A for 3 min. Then, 0.2 mL of solution B was dropped onto the treated V2O5/CF substrate. The sample was subsequently left in the dark for 24 h before rinsing with methanol to remove residues. Finally, the product was dried at 60 1C for 12 h under vacuum. The loading of the Ppy amount is measured to be about 0.35 mg cm 2 by careful weighing after coating.
Electrochemical measurements A three-electrode system was used to measure the electrochemical activity of V2O5@Ppy/CF as the working electrode using 5 M LiCl aqueous solution as the electrolyte, a platinum foil as the counter electrode, and an Ag/AgCl as the reference electrode. The nominal area of V2O5@Ppy/CF immersed into the electrolyte was controlled to be around 1 cm2.
Solid-state flexible hybrid devices (SFHDs) The SFHDs typically include a piece of V2O5@Ppy/CF (0.5 4 cm2) as negative electrode, a piece of MnO2@Ppy/ CF (0.5 4 cm2) as positive electrode, and porous glassy fibrous paper as the separator. Meanwhile, PVA/LiCl was introduced as solid electrolyte to prevent the possible dissolving of V2O5 nanostructure. The PVA/LiCl gel electrolyte was simply prepared by mixing PVA powder (1 g), LiCl (2.2 g) and deionized water (10 mL) at around 85 1C under vigorous stirring. Prior to the assembling, V2O5@Ppy/CF, MnO2@Ppy/CF electrodes, and separator were soaked in the PVA/LiCl gel for about 10 min, and then allowed to solidify for 6 h. Then, the devices were assembled by sandwiching the separator between V2O5@Ppy/CF and MnO2@Ppy/CF electrodes, and further dried at 40 1C for 10 h. Finally, the assembled devices were carefully sealed using pouch cell setup to ensure the whole system at a stable state. The mass ratio of MnO2 electrode and V2O5 electrode was around 1.5:1 to balance the charge at each electrode.
Preparation of MnO2@Ppy on CF substrates
Results and discussion
The MnO2@Ppy/CF as a negative electrode for solid-state flexible hybrid device was obtained by two steps, including hydrothermal and followed by in situ polymerization. At the first step, CF with a certain size was immersed into 5 mM KMnO4 aqueous solution and went through a hydrothermal reaction at 160 1C for 5 h. The obtained CF with MnO2 loading was washed several times by water and dried at 60 1C for 10 h. The resultant loading mass of MnO2 could be adjusted by using different amount of KMnO4. Then Ppy coating was carried out through the similar process to that for the V2O5@Ppy/CF and the loading mass of Ppy is measured to be around 0.35 mg cm 2.
The preparation process for the solid-state flexible HDs has been presented in Scheme 1. The first step involves growing V2O5 on CF by solvothermal followed by thermal treatment. For the sake of improving charge-collection ability of V2O5 nanostructures, a layer of Ppy is polymerized on the surface of V2O5/CF films. Interestingly, it should be noticed that the morphologies of V2O5 nanostructures can be easily controlled from porous nanoplates to nanorods by simply prolonging the calcination time. Similar to the above procedure, MnO2 nanowires with uniform Ppy coating was also successfully decorated on CF. Due to the synergistic effect of active materials (V2O5 or MnO2) and Ppy, the
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Scheme 1 Schematic diagrams for the synthesis of V2O5@Ppy/CF and MnO2@PPy/CF, designed solid-state flexible hybrid devices.
hybrid films as integrate electrodes offer several critical features. (i) The porous hierarchical nanoarchitecture allows the easy diffusion of electrolyte to the electrodes and provides more active sites for fast redox reactions.
(ii) Compared with other substrates, CF is ideal for building nanosized active materials with large mass loading and remarkable mechanical stability towards flexible and advanced energy storage devices. In our work, the loading
Fig. 1 (a)–(c) the SEM images of V-precursor; (d)–(e) SEM images and (f) TEM images of V2O5 nanorods on CF substrates at different magnification; (g)–(h) SEM images and (i) TEM images of V2O5 nanoplates on CF substrates at different magnification. The inset in (a), (d) and (g) are corresponding SEM images with low magnification; the inset in (f) and (i) are corresponding HRTEM images, respectively.
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Fig. 2 (a) and (c) SEM and (e) TEM images of the Ppy coated V2O5 nanorods on CF substrates; (b) and (d) SEM and (f) TEM images of the Ppy coated V2O5 nanoplates on CF substrates. The inset in (a) shows the photographic image of the V2O5@Ppy/CF film.
mass of the V2O5 and MnO2 are calculated to be 6.22 and 9.51 mg cm 2, which is consistence with thermal gravimetric (TGA) analysis (Fig. S1, Supporting information). Such high mass loading makes it possible to be used for real devices. (iii) The thin homogeneous Ppy coating could not only help to alleviate the conductivity issue of electrode but also protect active materials (V2O5 and MnO2) against corrosion, in turn largely boosting the electrochemical performance of the integrate flexible electrodes. Accordingly, the related solid-state flexible HDs with remarkable energy density could be achieved. These results push the novel MnO2@Ppy//V2O5@Ppy solid-state flexible HDs an important step forward into practical applications. Hierarchical 3D Porous V2O5 nanoplates and nanorods could be easily grown on the CF substrate by using a facile solvothermal method followed by heat treatment. Fig. 1 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the V-precursor and V2O5 nanostructures after thermal treatment. The CF substrate is coated by a uniform layer of grayish green film on a large scale from the naked eye (Fig. S2, Supporting information). The SEM image in Fig. 1(a)–(c) shows that the CF substrate is fully and uniformly coated by the V-precursor nanoplates, indicating the present solvothermal system is favorable for growing hierarchical nanoplates on conductive
substrates. Even the end of the carbon microfibers is coated by the nanoplates, as show in Fig. 1(b). The coated film possesses average sizes of several micrometers and thicknesses of tens of nanometers. After calcination at 350 1C, it can be clearly observed that the uniform layer of grayish green film turned yellow, confirming the formation of pure V2O5 (Figs. S2–S4, Supporting information). Compared to the precursor arrays, the hierarchical V2O5 arrays could be perfectly maintained after thermal treatment (Fig. 1 (d) and its insets). When the calcination time is 3 h, it can be seen that each nanoplate splits into several nanorods with around 100 nm in length and around 20 nm in width (Fig. 1(e)). Notably, the nanorods are in the form of highly order along the same direction. The TEM in Fig. 1 (f) indicates a large number of mesopores with a size around 2–5 nm throughout the whole surface of nanorods, mainly deriving from the recrystallization process and release of gas during the calcination process. However, decreasing the calcination time to 0.5 h, the hierarchical network structure is constructed by well-defined nanoplates subunits with a thickness of around 10 nm (Fig. 1(g) and (h)). The rough surface of V2O5 nanoplates presents highly porous texture (Fig. 1(i)), which is similar to the porous nanorods. The high-resolution TEM (HRTEM) images shows lattice fringes with an interplanar distance of around
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0.34 nm, agreeing well with the spacing of the (110) planes of V2O5 (inset in Fig. 1(f) and (i)). While, a further increase of the calcination time to 6 h can destroy the binding force between V2O5 and CF, leading to the peeling of V2O5 from the CF substrate (Fig. S5, Supporting information). On the basis of experimental results, it can be found that the morphologies of V2O5 can be readily controlled by simply adjusting the calcination time. The formation mechanism for the ordered V2O5 nanorods can be simply depicted as a splitting process. Initially, the CF is fully covered by vanadium precursors through the solvothermal process, and no bare carbon fibers are found in the product, indicating the good compatibility between the vanadiumprecursor and CF. After calcination in a short time, the plate-like configuration can be preserved, and meanwhile a number of pores can be observed on each V2O5 nanoplates, mainly due to the release of gases during thermal treatment. With the calcination time prolonging, a “splitting” process occurs, in which plate-like structures gradually split into plenty of nanorods. It can be inferred that the long calcination time can provide enough energy to overcome the strain energy barrier in interlayers and further increase the crystal growth, resulting in the generation of ordered nanorods. Such “splitting” process has also been observed in the formation of other metal oxide with 1D nanostructures [33–35]. After in situ coating of Ppy, the morphologies of the composites are analyzed by SEM and TEM, as shown in Fig. 2. It is observed that the Ppy coating does not deteriorate the ordered structure and the obtained V2O5@Ppy/CF still preserve the 3D hierarchical structures without any agglomeration, as shown in Fig. 2(a)–(d) and Fig. S6a in Supporting information. The inset in Fig. 2(a) shows a photographic image of the V2O5@Ppy/CF film, exhibiting high flexibility. It can be readily rolled up, which renders it as flexible electrode for practical applications. Compared with the as-prepared V2O5/CF nanostructures, the Ppy coating makes
the nanoplates and nanorods surface more wrinkled, thereby further increasing the contact area between electrolyte and active materials (V2O5@Ppy). The TEM images in Fig. 2(e) and (f) and Fig. S6b, in Supporting information further verify that the porous nanoplates and nanorods are uniformly covered by an amorphous Ppy layer with a thickness of 3 nm. More evidence about the phase and composition of the products are provided by the X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) (Figs. S3, S4 and S7, in Supporting information). Therefore, it can be concluded that the integrated flexible V2O5@Ppy/ CF film have been successfully obtained through this facile strategy. MnO2@Ppy/CF films were fabricated by a facile hydrothermal technique in the presence of CF and KMnO4 solution followed by in-situ Ppy coating, which is similar to the preparation procedure of V2O5@Ppy/CF film. During the synthesis of MnO2/CF, the CF serves as not only a sacrificial reductant by converting aqueous permanganate (MnO4 ) to insoluble MnO2, but also a substrate material. This in-situ synthesis guarantees that the formed MnO2 nanowires can be directly loaded on the surface of CF substrate with strong adhesion and high loading mass. Representative SEM images in Fig. 3(a) and (b) shows that the well-defined interconnected architecture of MnO2@Ppy/CF, where the wire-like MnO2 nanostructures grow on the surface of CF uniformly. Importantly, although there is no apparent change in the morphology of the whole structure compared without Ppy coating (Fig. S8a and b, Supporting information), it is found that the diameter of MnO2 nanowires increases and their surface becomes slightly rough, as shown in Fig. 3(c) and Fig. S8c in Supporting information. TEM image (Fig. 3(d)) further confirms the wire-like structure of MnO2. Meanwhile it can be clearly observed that the entire surface of MnO2 nanowires is covered by a continuous amorphous Ppy layer with thickness of around 5 nm. Significantly, as confirmed by
Fig. 3 (a)–(c) SEM and (d) TEM images of the Ppy coated MnO2 nanowires on CF substrates.
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Fig. 4 Electrochemical characterizations of as-prepared four V2O5/CF samples as flexible electrodes in three-electrode system: (a) comparative CV curves at scan rate of 10 mV s 1; (b) the specific capacitance calculated based on CV curves as a function of scan rate; (c) Nyquist plots of the as-prepared four V2O5/CF samples (the inset is the equivalent electrical circuit); (d) cycling stability of V2O5@Ppy nanorods and V2O5@Ppy nanoplates flexible electrodes at 5 A g 1 (the inset shows galvanostatic charge/discharge curves of V2O5@Ppy nanorods electrode for the last five cycles).
the FT-IR, XRD, and XPS analysis (Figs. S7, S9, and S10, Supporting information), no impurity is introduced even after in-situ Ppy coating. Given the unique structural attributes, V2O5@Ppy/CF films are regarded as a highly potential electrode for flexible energy storage devices. To explore the advantages of these V2O5@Ppy/CF films as integrate electrodes, we first evaluated their electrochemical properties in a threeelectrode configuration with 5 M LiCl solution as electrolyte, Pt foil as counter electrode and Ag/AgCl as the reference electrode. Benefiting from its excellent conductivity and integrated structures, the free-standing V2O5@Ppy/CF film could be directly used as the working electrode. Fig. 4 (a) shows cyclic voltammograms (CVs) of the as-prepared different V2O5/CF films at a scan rate of 10 mV s 1. As expected, in comparison to pristine V2O5/CF films, the V2O5@Ppy/CF films demonstrate substantially larger current response, implying the improved electrochemical capacitance by Ppy coating. Meanwhile, it is found that the V2O5@Ppy/CF nanorod electrode yields the highest current response. Although slight peak shift can be obsearved from the CV curves, the four electrodes show similar redox couples, which illustrate the similar pseudocapacitive behavior. Therefore, it is anticipated that the uniform Ppy coating can not only enhance the charge transfer and collection, but also diminish the electrochemical polarization, thus resulting in enhanced electrochemical performance [36]. On the other hand, the prolonged calcination
time might cause the difference in crystallinity and particle size of V2O5 nanostructures, and meanwhile the in-situ Ppy coating inevitably leads to a small amount of reduction of V2O5 that renders it an electrically condutive mixed valence (V4 + /V5 + ) oxide [37] All of these could account for the slight difference in the CV curves. The specific capacitances of these electrodes are calculated based on their CV curves (Figs. S11 and S12, Supporting information) and shown in Fig. 4(b). Obviously, the specific capacitances of V2O5/CF electrodes drop quickly, indicating that the poor intrinsic conductivity of V2O5 limits its rates capability especially at high scan rates. Significantly, V2O5@Ppy/CF nanorods electrode demonstrates impressive specific capacitance (383 F g 1 at a scan rate of 1 mV s 1) and rate capability (80% capacitance retention even at high scan rate 20 mV s 1). The high specific capacitance was further confirmed by galvanostatic charge/discharge profiles (Figs. S11–S13, Supporting information). It should be mentioned that the Ppy alone shows negligible capacitance in the voltage range of 0.3–0.7 V (Fig. S14, Supporting Information). This justifies that Ppy is an excellent structural stabilizer for V2O5, and the synergistic effects between Ppy and V2O5 nanostructures promote the enhancement of electrochemical performance. Considering the high mass of V2O5@Ppy/CF nanorods electrode, such performance outperforms most previously reported V2O5-based electrodes [9,38,39]. In order to understand the excellent performance of V2O5@Ppy/CF nanorod electrode, electrochemical
314 impedance spectroscopy (EIS) was performed (Fig. 4(c)). Clearly, V2O5@Ppy/CF nanorods electrode possesses obviously lower charge transfer resistance (Rct) (Fig. S15, Supporting information) and a relatively more vertical line at low frequency compared to the other three samples, which indicates much better electrical conductivity and faster ion diffusion. Fig. 4(d) shows the long-term cycling stability of V2O5@Ppy/CF nanorods and V2O5@Ppy/CF nanoplates electrode at 5 A g 1. The capacitance of V2O5@Ppy/ CF nanorods maintains around 83% of its initial value even after 5000 cycles, which is almost 1.5 times higher of V2O5@Ppy/CF nanoplates electrode. Besides, the good symmetry of both charge and discharge profiles for the last
L. Li et al. five cycles of V2O5@Ppy/CF nanorods electrode further indicates the excellent capacitive behavior (inset of Fig. 4 (d)). The superior electrochemical performance of V2O5@Ppy/CF nanorods electrode might be attributed to the following factors. First, the coating of Ppy layer not only provides a good conductive path for electrons and ions transfer, but also plays an important role as a soft “armor” to suppress the dissolution of V2O5 in the aqueous electrolyte, thereby leading to the enhanced electrochemical kinetics and high capacitance. Second, compared with the V2O5@Ppy/CF nanoplates structure, the nanorods structure as the secondary subunit could more effectively prevent the undesirable volume variation and ensure the stability of the
Fig. 5 Electrochemical characterizations of MnO2@Ppy//V2O5@Ppy SFHDs: (a) CV curves collected in various potential window at 20 mV s 1; (b) CV curves at various scan rates ranging from 1–20 mV s 1; (c) galvanostatic charge/discharge profiles at various current densities in the voltage range of 0–2.0 V; (d) cycling stability of MnO2@Ppy//V2O5@Ppy SFHDs at a current density of 0.5 A g 1 (the inset shows the photograph of LED powered by MnO2@Ppy//V2O5@Ppy SFHDs); (e) cycling performance of MnO2@Ppy//V2O5@Ppy SFHDs at different bending states (the inset shows the CV curves at different bending states); (f) Ragone plot of MnO2@Ppy//V2O5@Ppy SFHDs.
Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices unique porous structure, thus resulting in the excellent cycling performance. Therefore, these will convincingly beneficial for the practical applications of V2O5@Ppy/CF nanorods electrode. Considering the huge potential of V2O5@Ppy/CF nanorod film as negative electrode for solid-state flexible hybrid devices (SFHDs), we selected MnO2@Ppy/CF with an optimum loading mass (9.86 mg cm 2) as the positive electrode (Fig. S16, Supporting information). It is worth mentioned that the charge stored in both positive and negative electrodes should be adjusted in SFHDs system. As the mass ratio of MnO2@Ppy/CF to V2O5@Ppy/CF up to around 1.5, a reasonably good charge balance between them could be obtained (Fig. S16d, Supporting information), thus maximizing the utilization of active materials when further assembled into SFHDs. Both electrodes were cut into 0.5 cm 4 cm each and the total mass of active materials used in the MnO2@Ppy//V2O5@Ppy SFHDs was around 32.8 mg, which is very high compared to other previous reported HDs [40,41]. More significantly, a typical LiCl/PVA gel electrolyte was used in the present study to achieve excellent mechanical and electrochemical properties. Apart from as an ion diffusion matrix, this neutral gel electrolyte can not only effectively avoid the chemical dissolution of vanadium oxides by minimizing the water content, but also serve as an elastic coating to enhance the mechanical stability of vanadium oxides. To estimate the best operating potential of the as-prepared SFHDs, a series of CV curves with different potential windows are collected at 20 mV s 1, as shown in Fig. 5(a). Encouragingly, by taking advantage of the unique device configuration combined with stable gel electrolyte, the potential window of the device can be extended to as large as 2.0 V without obvious polarization curves, indicating that the absence of water eliminates the issue of water decomposition, and makes it possible to achieve high energy/power densities. Fig. 5 (b) reveals the CV curves of the optimized MnO2@Ppy// V2O5@Ppy SFHDs at various scan rates ranging from 1 to 20 mV s 1 between 0 and 2.0 V. Apparently, the typical quasi-rectangular CV curves with feeble redox peaks can be seen at all scan rates, thus suggesting the combination of pseudocapacitive and electric double layer capacitor (EDLC) type properties. To further confirm the superior electrochemical properties of MnO2@Ppy//V2O5@Ppy SFHDs, the galvanostatic charge/discharge profiles at different current densities are displayed in Fig. 5(c). The specific capacitance of the as-prepared device, which was calculated based on the total mass of both positive and negative electrodes, reaches 51.5 F g 1 at a current density of 0.2 A g 1. Considering the very high mass of our device, such a capacitance value is worth highlighting [9,42]. For practical use consideration, the long-term cycling stability of the V2O5@Ppy//MnO2@Ppy SFHDs was measured at current density of 0.5 A g 1 up to 20,000 cycles (Fig. 5 (d)). Strikingly, the capacitance retention still maintains approximately 89% even after 20,000 cycles, indicating the outstanding electrochemical reversibility of the asassembled SFHDs. Additionally, the successful attempt to power red light-emitting diodes (LED) shows that
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MnO2@Ppy//V2O5@Ppy SFHDs holds the opportunity to be used in portable/flexible electronics (inset in Fig. 5(d)). Moreover, it is well accepted that the development of multifunctional portable electronics requires highly flexible and lightweight power sources. The flexibility of MnO2@Ppy//V2O5@Ppy SFHDs was also investigated by performing CV analysis at different bending state, corresponding to the states in Fig. S17 (Supporting information). As shown in Fig. 5(e), there is negligible performance degradation and almost completely overlapped CV curves, indicating remarkable mechanical flexibility and stability of our device. Furthermore, power density (P) is generally considered as critical factors to evaluate the performance of electrochemical devices. Ragone plots of MnO2@Ppy// V2O5@Ppy SFHDs calculated from the discharge curves are shown in Fig. 5(f). A maximum energy density of 28.6 W h kg 1 at power density of around 200 W kg 1 for our devices could be reached. Those values of our device are comparable or surpass most of the other previously reported Vbased or Mn-based hybrid systems, such as V2O5-rGO//rGO [9], MnO2//Bi2O3 [30], V2O5//PANI [39], MnO2@C//V2O5CNT [40], Ppy@V2O5//AC [41], MnO2//VOS@C [42], VOx// VN [43], MnO2-rGO//MoO3-rGO [44], and MnO2//In2O3 [45]. It is believed that these performance advancements may derive from the rational design of active materials (V2O5@Ppy) and unique architecture of SFHDs. These features may contribute the following merits. Specifically, the unique flexible V2O5@Ppy/CF configuration provides a favorable electric contact and electron transportations, and meanwhile maintains the good mechanical stability and flexibility of the whole device. Moreover, the uniformly coating of Ppy layer not only improves the conductivity of V2O5, but also acts as a “armor” to protect V2O5 core against dissolution in the electrolyte, accounting for the excellent rate performance and cycling stability. Furthermore, the hierarchical porous nanorods feature of V2O5@Ppy structure offers more active sites for redox reaction, thus ensuring the full play of high specific capacitance. Last, the selection of LiCl/PVA gel electrolyte in our SFHDs also plays an important role to suppress the irreversible oxidation reaction and structure pulverization of V2O5@Ppy/CF, as well as efficiently eliminate water decomposition in the electrolyte, thereby making it possible to realize enhanced energy/ power densities. Therefore, all of these render MnO2@Ppy// V2O5@Ppy SFHDs to be unexceptionably flexible power sources.
Conclusions In summary, highly porous V2O5@Ppy nanoplate and nanorod arrays with large mass loadings have been successfully designed on carbon felt through a facile solvothermal followed by in-situ polymerization technique. Significantly, the detailed evolution procedureas of V2O5 nanoplate to nanorod arrays have also been provided. Additionally, as integrate electrode for energy storage devices, V2O5@Ppy/ CF nanorods display much better cycling stability and large specific capacitance than V2O5@Ppy/CF nanoplates. Moreover, to address the practical application potential of
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V2O5@Ppy/CF nanorods as flexible electrodes, a SFHDs based on V2O5@Ppy/CF and MnO2@Ppy/CF as negative and positive electrode, respectively, has been fabricated. Benefiting from the novel structural features and Ppy coating, the MnO2@Ppy//V2O5@Ppy SFHDs demonstrates outstanding cycling stability (approximately 89% retention even after 20,000 cycles), and exhibits remarkable energy density of 28.6 W h kg 1 at power density of around 200 W kg 1. Furthermore, our as-assembled device shows negligible capacitance degradation at different bending state, indicating the excellent mechanical flexibility and stability. These encouraging merits definitely promote our material as potential candidate for practical application in highperformance flexible energy storage devices.
Acknowledgments This work was supported by the National Research Foundation Singapore (NRF-CRP10-2012-06).
Appendix A.
Supplementary material
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.11.026.
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Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices Self-Financed Student Abroad (2014) and Ian Ferguson Postgraduate Fellowship (2014). Her research interests focus on the synthesis of functional nanomaterials and their application in supercapacitors and batteries. Shengjie Peng is now working as a senior research fellow in Prof. Seeram's group in National University of Singapore. He received his Ph.D. degree in Nankai Univerisity (PR China) in 2010. He has coauthored over 60 peer-reviewed publications. His current research interests include the design and development of nanomaterials and their applications in energy conversion and storage devices. Han-Yi Chen is currently pursuing her Ph.D. in the Department of Chemistry at Technical University of Munich, Germanyand Joint Ph. D. in Nanyang Technological University, Singaporeas a part of the NRF-NTU-TUMCREATE program on Electromobility for tropical megacities. She obtained her Bachelors from the Department of Chemical and Materials Engineering, National Central University, Taiwan in 2008 and Masters from the Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Taiwan in 2011. Her current research involvesmetal oxide clusters and nanocarbon materials for energy storage applications. Xiaopeng Han was born in Shanxi province, PR China in 1988. He received his B.E. degree in chemical engineering and technology from TianJin University (2010). Then joined the Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education, NanKai University) and obtained his Ph.D. degree under the supervision of Prof. Jun Chen. His research focuses on functional materials for energy and catalysis. Fangyi Cheng is a Chemistry Professor at Nankai University. He received his Ph.D. from Nankai University in 2009. His research interests cover design and synthesis of functional nanostructures and composite materials for energy-related applications such as electrocatalysis, batteries, and hydrogen storage. He has coauthored over 100 scientific papers in peer-reviewed journals, including Nature Chem., Chem. Soc. Rev., Angew. Chem., Adv. Mater., etc. His publications have received over 4200 citations, with an H-index of 35. He is a recipient of Excellent Young Scholars (NSFC) and New Century Excellent Talents Project (MOE). Dr. Cheng is a board committee member of IAOEES.
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Madhavi Srinivasan is an associate professor at the School of Materials Science and Engineering, Nanyang Technological University (NTU). She graduated from Indian Institute of Technology (IIT), Chennai (India) and completed her Ph.D. from National University of Singapore. She is also one of the three women scientists awarded the L’Oreal-UNESCO for women in science national fellowships awards in Singapore (2010). Her research interest is to enhance performance of energy storage devices such as lithium ion batteries, supercapacitors and advanced batteries. Stefan Adams is an associate professor at Department of Materials Science and Engineering, National University of Singapore. He received his B.Sc. and Ph.D. degree from Saarland State University, Germany. Then he worked as a postdoctoral fellow in MaxPlanck-Institute for Solid State Research in Stuttgart from 1992 to 1994. He workded as Lecturer and Senior Lecturer at Göttingen University from 1994 to 2000. He joined National University of Singapore as an Assistant Professor in 2010 and became an associte Professor in 2011. His research interest is to search for new materials for sustainable energy applications by using both experimental and computational approaches. Seeram Ramakrishna, FREng, FNAE, FAAAS is a renowned Professor of Materials Engineering at the National University of Singapore. He is an acknowledged global leader for his pioneering work on the science and engineering of nanofibres. He has authored 5 books, 25 book chapters, 20 patents and 600 peer reviewed papers, which attracted about 43,000 citations with an h-index of 98. Various international databases including Thomson Reuters Web of Science, Elsevier Scopus and Microsoft Academic place him among the top 25 authors and most cited materials scientists in the world. Chen Jun obtained his B.Sc. and M.Sc. degrees from Nankai University in 1989 and 1992, respectively, and his Ph.D. from Wollongong University (Australia) in 1999. He held the NEDO fellowship at National Institute of AIST Kansai Center (Japan) from 1999 to 2002. He was appointed as the Chair Professor of Energy Materials Chemistry at Nankai University in 2002. His research expertize is energy conversion and storage with solar cells, fuel cells, and batteries.
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Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices Linlin Lia,b, Shengjie Pengc,n, Han-Yi Chenb,d, Xiaopeng Hane, Fangyi Chenge, Madhavi Srinivasana,b,n, Stefan Adamsf, Seeram Ramakrishnac, Jun Chene,n a
School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore TUM CREATE, 1 CREATE Way, #10-02 CREATE Tower, 138602 Singapore c Department of Mechanical Engineering, National University of Singapore, 117574 Singapore d Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, Garching 85748, Germany e Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China f Department of Materials Science and Engineering, National University of Singapore, 117546 Singapore b
Received 17 July 2015; received in revised form 19 November 2015; accepted 23 November 2015 Available online 30 November 2015
KEYWORDS
Abstract
Porous; Vanadium oxide; Flexible electrode; Solid electrolyte; Hybrid devices
Design and fabrication of advanced functional materials is essential but still a challenge for current energy storage devices. Herein, polypyrrole coated highly porous vanadium oxide (V2O5@Ppy) nanorod and nanoplate arrays with large mass loadings, have been successfully constructed on carbon felt (CF) via a facile solvothermal reaction followed by in-situ polymerization technique. Interestingly, the structure of the V2O5 thin films can be simply tuned from porous nanoplates to nanorods with controlled calcination time. In addition, MnO2 nanowires with Ppy coating were also grown on the CF substrates to form MnO2@Ppy/CF electrode through the similar method. As integrate electrodes for energy storage devices, V2O5@Ppy/CF nanorods demonstrate more superior electrochemical properties compared to V2O5@Ppy/CF nanoplates. By virtue of their intriguing structural features and uniformly Ppy coating, a solid-state flexible hybrid device (SFHD) based on V2O5@Ppy/CF and MnO2@Ppy/CF as the negative and positive electrode, respectively, manifests outstanding cycling stability (approximately 89% retention even after 20,000 cycles), excellent mechanical flexibility, and
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Corresponding authors. E-mail addresses: [email protected] (S. Peng), [email protected] (M. Srinivasan), [email protected] (J. Chen). http://dx.doi.org/10.1016/j.nanoen.2015.11.026 2211-2855/& 2015 Elsevier Ltd. All rights reserved.
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L. Li et al. remarkable energy density (28.6 W h kg 1 at power density of around 200 W kg 1), which makes it hold great potential to be unexceptionably flexible devices for portable and wearable electronics. & 2015 Elsevier Ltd. All rights reserved.
Introduction With the rapid progress of nanomaterials and nanotechnology, wearable and protable electronics are becoming indispensable in the increasingly mobile society. Great efforts have been made to fabricate flexible devices in various fields, such as roll-up displays, artificial electronic skin, and so on. With the aim to create entirely flexible electronics, all of these devices need lightweight, flexible, and highperformance energy storage unites. In today's practical applications, electrochemical capacitors (ECs) [1,2] and batteries, as two major energy storage technologies, represent two extremes of the design space. To bridge the gap between ECs and batteries, hybrid devices (HDs), also known as hybrid combinations, provide a promising path [3,4]. Typically, such devices combine the advantages of both ECs and batteries by using a capacitor-type electrode to ensure high power density and battery-type electrode to enable high energy density. More importantly, compared to conventional ECs, HDs generally possess a wider working potential window, thereby further guaranteeing the energy storage capability. Consequently, a new avenue has been opened by HDs for the emerging high energy and power applications [5,6]. However, their role in flexible and smart portable devices barely satisfies the demands in mechanical flexibility and miniaturized in dimentions. Therefore, researchers are striving to explore advanced solid-state flexible HDs with both high power and energy densities, as well as excellent flexibility [7–9]. So far, great attentions on more safe and flexible HDs are devoted to build advanced integrate flexible hybrid electrodes with high energy and power densities for commercial implementation [10–12]. Since the hybrid electrodes largely determine the electrochemical performance of HDs, there has been extensive interest in the proper selection and hybridization of nanoarchitectures (phase composition, morphologies, size and uniformity) with precisely controlled structures [13–16]. It is well-accepted that the battery-type electrodes, usually metal oxides, are still limited by their intrinsic poor conductivity and suffer pulverization especially after long-term cycling. To address these issues, the most popular approach is to fabricate hybrid electrodes, which commonly combine metal oxides electrodes with various conductive polymer (polyaniline, Ppy) or carbon materials (CNT, graphene) [12,15,17]. Accordingly, it has been demonstrated that both the conductivity and capacitance of the hybrid configurations can be largely enhanced by coating a thin layer of conductive polymer on the surface of electrode materials, which not only possesses large capacitance and high electrical conductivity, but also can be easily polymerized [18–20]. Additionally, to meet the need for flexibile HDs, it should be taken into account about
the flexibility of current collector. An advent of new concept is to disperse functional nanostructures with controlled morphologies and sizes on carbon frameworks or scaffolds such as graphene foam, carbon nanotubes coated textile, carbon cloth, to construct flexible integrate hybrid electrode [19,21,22]. These carbon frameworks act as backbones to faciliate the transportation of ions and electrons, thus leading to improved rate capability [23– 25]. Baed on the above-mentioned aspects, a solid-state flexible HD, which consists of rationally designed hybrid nanoarchitectures, represents a new opportunity to achieve high power and energy applications. Despite the significant development in functional nanomaterials, transition metal oxides (TMOs) still govern the landscape of active materials for electrochemical devices. In particular, vanadium oxides (V2O5) and manganese dioxide (MnO2) exhibit significant predominance such as natural abundance, high theoretical capacitance, multiple oxidation states, and ease of synthesis [18,26–30]. In this regard, it is anticipated that introducing V2O5 as negative electrode and MnO2 as positive electrode in a solid-state flexible HDs will becoming a promising energy storage technology. In the present study, we make the attempt to employ both of those possible optimization pathways (Ppy coated nanostructures on carbon felt) to enhance the performance of solid-state flexible HDs. Herein, Ppy coated hierarchical porous composites nanoarchitectures (V2O5@Ppy and MnO2@Ppy) have been successfully built on carbon felt (CF) by a facile wet-chemical method followed by an insitu polymerization Ppy on the surface of nanoarchitectures. Specifically, CF serves as the flexible current collector, mainly owing to its good mechanical flexibility, excellent conductivity, and great durability to bear shape deformation [31,32]. Moreover, the uniformly polymerized Ppy layer could not only effectively prohibit active mateirals (V2O5 and MnO2) from corrosion in liquid electrolyte, but also provide an additional pathway for electron and ions transfer. Furthermore, it is found that the structure of the V2O5 thin films can be simply tuned from porous nanoplates to nanorods with controlled calcination time. Consequently, as integrate electrodes for energy storage devices, V2O5@Ppy/CF nanorods manifest more superior electrochemical properties compared to V2O5@Ppy/CF nanoplates. In addition, benefiting from the unique structural features, the as-prepared solid-state flexible HDs, which is based on V2O5@Ppy/CF anode and MnO2@Ppy/CF cathode, exhibits outstanding cycling stability (approximately 89% retention even after 20,000 cycles), excellent mechanical flexibility, and remarkable energy density (28.6 W h kg 1 at power density of around 200 W kg 1), which makes it hold great potential to be unexceptionably flexible devices for portable and wearable electronics.
Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices
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Experimental section
Materials characterization
Preparation of V2O5 on CF substrates
X-ray diffraction (XRD) patterns were collected on a Shimadzu X-ray diffractometer. Field-emission scanning electron microscopy (FE-SEM) images were collected using a JEOL JSM 7600F microscope. Transmission electron microscope (TEM) images were taken on a JEOL 2100 microscope. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos Axis Ultra DLD electron spectrometer (PHI, PHI5300 system). Fourier transform infrared (FT-IR) spectra were obtained using a Perkin Elmer Spectrum GX instrument using KBr pellet technique at a resolution of 1 cm 1. Thermogravimetric analysis (TGA, Q500) was determined from room temperature to 700 1C under a continuous air flow at 10 1C min 1.
All chemicals were used as received without further purification. In a typical synthesis, commercial carbon felt (CF) from company with a dimension of 1 cm 4 cm was placed against the wall of a Teflon-lined stainless steel autoclave that contained a homogeneous solution of vanadium oxytriisopropoxide (VOT) (0.5 mL) in 35 ml of iso-propanol alcohol (IPA). Afterwards, the autoclave was sealed and maintained at 180 1C for 12 h to synthesize vanadium precursor film. Then, the substrates were taken out and cleaned by ultrasonication for several minutes to remove the loosely attached products on the surface. After drying at 60 1C for 10 h under vacuum, a green thin film was coated on the CF substrate. In order to obtain crystallized V2O5 nanostructures, the conductive substrates with the asgrown films were calcined at 350 1C for 0.5 h or 3 h with a heating rate of 2 1C min 1 in air, which could produce light yellow V2O5 nanoplate or nanorod thin films, respectively. The mass loading of V2O5 on the textile was measured to be 6.22 mg cm 2 by a microbalance before and after the materials loading.
Preparation of Ppy coated V2O5/CF film V2O5 were coated with Ppy by an in situ polymerization method. 0.12 g p-toluenesulfonic acid (p-TSA) was firstly dissolved into 15 mL of ethanol. After stirring for 10 min, 50 mL of pyrrole monomer was dissolved into the above solution and stirred to form a mixed solution A. Meanwhile, 0.08 g ammonium peroxy disulfate (APS) was dissolved in 10 mL of distilled water to form a solution B. Afterwards, the CF coated by the V2O5 arrays was immersed into solution A for 3 min. Then, 0.2 mL of solution B was dropped onto the treated V2O5/CF substrate. The sample was subsequently left in the dark for 24 h before rinsing with methanol to remove residues. Finally, the product was dried at 60 1C for 12 h under vacuum. The loading of the Ppy amount is measured to be about 0.35 mg cm 2 by careful weighing after coating.
Electrochemical measurements A three-electrode system was used to measure the electrochemical activity of V2O5@Ppy/CF as the working electrode using 5 M LiCl aqueous solution as the electrolyte, a platinum foil as the counter electrode, and an Ag/AgCl as the reference electrode. The nominal area of V2O5@Ppy/CF immersed into the electrolyte was controlled to be around 1 cm2.
Solid-state flexible hybrid devices (SFHDs) The SFHDs typically include a piece of V2O5@Ppy/CF (0.5 4 cm2) as negative electrode, a piece of MnO2@Ppy/ CF (0.5 4 cm2) as positive electrode, and porous glassy fibrous paper as the separator. Meanwhile, PVA/LiCl was introduced as solid electrolyte to prevent the possible dissolving of V2O5 nanostructure. The PVA/LiCl gel electrolyte was simply prepared by mixing PVA powder (1 g), LiCl (2.2 g) and deionized water (10 mL) at around 85 1C under vigorous stirring. Prior to the assembling, V2O5@Ppy/CF, MnO2@Ppy/CF electrodes, and separator were soaked in the PVA/LiCl gel for about 10 min, and then allowed to solidify for 6 h. Then, the devices were assembled by sandwiching the separator between V2O5@Ppy/CF and MnO2@Ppy/CF electrodes, and further dried at 40 1C for 10 h. Finally, the assembled devices were carefully sealed using pouch cell setup to ensure the whole system at a stable state. The mass ratio of MnO2 electrode and V2O5 electrode was around 1.5:1 to balance the charge at each electrode.
Preparation of MnO2@Ppy on CF substrates
Results and discussion
The MnO2@Ppy/CF as a negative electrode for solid-state flexible hybrid device was obtained by two steps, including hydrothermal and followed by in situ polymerization. At the first step, CF with a certain size was immersed into 5 mM KMnO4 aqueous solution and went through a hydrothermal reaction at 160 1C for 5 h. The obtained CF with MnO2 loading was washed several times by water and dried at 60 1C for 10 h. The resultant loading mass of MnO2 could be adjusted by using different amount of KMnO4. Then Ppy coating was carried out through the similar process to that for the V2O5@Ppy/CF and the loading mass of Ppy is measured to be around 0.35 mg cm 2.
The preparation process for the solid-state flexible HDs has been presented in Scheme 1. The first step involves growing V2O5 on CF by solvothermal followed by thermal treatment. For the sake of improving charge-collection ability of V2O5 nanostructures, a layer of Ppy is polymerized on the surface of V2O5/CF films. Interestingly, it should be noticed that the morphologies of V2O5 nanostructures can be easily controlled from porous nanoplates to nanorods by simply prolonging the calcination time. Similar to the above procedure, MnO2 nanowires with uniform Ppy coating was also successfully decorated on CF. Due to the synergistic effect of active materials (V2O5 or MnO2) and Ppy, the
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Scheme 1 Schematic diagrams for the synthesis of V2O5@Ppy/CF and MnO2@PPy/CF, designed solid-state flexible hybrid devices.
hybrid films as integrate electrodes offer several critical features. (i) The porous hierarchical nanoarchitecture allows the easy diffusion of electrolyte to the electrodes and provides more active sites for fast redox reactions.
(ii) Compared with other substrates, CF is ideal for building nanosized active materials with large mass loading and remarkable mechanical stability towards flexible and advanced energy storage devices. In our work, the loading
Fig. 1 (a)–(c) the SEM images of V-precursor; (d)–(e) SEM images and (f) TEM images of V2O5 nanorods on CF substrates at different magnification; (g)–(h) SEM images and (i) TEM images of V2O5 nanoplates on CF substrates at different magnification. The inset in (a), (d) and (g) are corresponding SEM images with low magnification; the inset in (f) and (i) are corresponding HRTEM images, respectively.
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Fig. 2 (a) and (c) SEM and (e) TEM images of the Ppy coated V2O5 nanorods on CF substrates; (b) and (d) SEM and (f) TEM images of the Ppy coated V2O5 nanoplates on CF substrates. The inset in (a) shows the photographic image of the V2O5@Ppy/CF film.
mass of the V2O5 and MnO2 are calculated to be 6.22 and 9.51 mg cm 2, which is consistence with thermal gravimetric (TGA) analysis (Fig. S1, Supporting information). Such high mass loading makes it possible to be used for real devices. (iii) The thin homogeneous Ppy coating could not only help to alleviate the conductivity issue of electrode but also protect active materials (V2O5 and MnO2) against corrosion, in turn largely boosting the electrochemical performance of the integrate flexible electrodes. Accordingly, the related solid-state flexible HDs with remarkable energy density could be achieved. These results push the novel MnO2@Ppy//V2O5@Ppy solid-state flexible HDs an important step forward into practical applications. Hierarchical 3D Porous V2O5 nanoplates and nanorods could be easily grown on the CF substrate by using a facile solvothermal method followed by heat treatment. Fig. 1 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the V-precursor and V2O5 nanostructures after thermal treatment. The CF substrate is coated by a uniform layer of grayish green film on a large scale from the naked eye (Fig. S2, Supporting information). The SEM image in Fig. 1(a)–(c) shows that the CF substrate is fully and uniformly coated by the V-precursor nanoplates, indicating the present solvothermal system is favorable for growing hierarchical nanoplates on conductive
substrates. Even the end of the carbon microfibers is coated by the nanoplates, as show in Fig. 1(b). The coated film possesses average sizes of several micrometers and thicknesses of tens of nanometers. After calcination at 350 1C, it can be clearly observed that the uniform layer of grayish green film turned yellow, confirming the formation of pure V2O5 (Figs. S2–S4, Supporting information). Compared to the precursor arrays, the hierarchical V2O5 arrays could be perfectly maintained after thermal treatment (Fig. 1 (d) and its insets). When the calcination time is 3 h, it can be seen that each nanoplate splits into several nanorods with around 100 nm in length and around 20 nm in width (Fig. 1(e)). Notably, the nanorods are in the form of highly order along the same direction. The TEM in Fig. 1 (f) indicates a large number of mesopores with a size around 2–5 nm throughout the whole surface of nanorods, mainly deriving from the recrystallization process and release of gas during the calcination process. However, decreasing the calcination time to 0.5 h, the hierarchical network structure is constructed by well-defined nanoplates subunits with a thickness of around 10 nm (Fig. 1(g) and (h)). The rough surface of V2O5 nanoplates presents highly porous texture (Fig. 1(i)), which is similar to the porous nanorods. The high-resolution TEM (HRTEM) images shows lattice fringes with an interplanar distance of around
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0.34 nm, agreeing well with the spacing of the (110) planes of V2O5 (inset in Fig. 1(f) and (i)). While, a further increase of the calcination time to 6 h can destroy the binding force between V2O5 and CF, leading to the peeling of V2O5 from the CF substrate (Fig. S5, Supporting information). On the basis of experimental results, it can be found that the morphologies of V2O5 can be readily controlled by simply adjusting the calcination time. The formation mechanism for the ordered V2O5 nanorods can be simply depicted as a splitting process. Initially, the CF is fully covered by vanadium precursors through the solvothermal process, and no bare carbon fibers are found in the product, indicating the good compatibility between the vanadiumprecursor and CF. After calcination in a short time, the plate-like configuration can be preserved, and meanwhile a number of pores can be observed on each V2O5 nanoplates, mainly due to the release of gases during thermal treatment. With the calcination time prolonging, a “splitting” process occurs, in which plate-like structures gradually split into plenty of nanorods. It can be inferred that the long calcination time can provide enough energy to overcome the strain energy barrier in interlayers and further increase the crystal growth, resulting in the generation of ordered nanorods. Such “splitting” process has also been observed in the formation of other metal oxide with 1D nanostructures [33–35]. After in situ coating of Ppy, the morphologies of the composites are analyzed by SEM and TEM, as shown in Fig. 2. It is observed that the Ppy coating does not deteriorate the ordered structure and the obtained V2O5@Ppy/CF still preserve the 3D hierarchical structures without any agglomeration, as shown in Fig. 2(a)–(d) and Fig. S6a in Supporting information. The inset in Fig. 2(a) shows a photographic image of the V2O5@Ppy/CF film, exhibiting high flexibility. It can be readily rolled up, which renders it as flexible electrode for practical applications. Compared with the as-prepared V2O5/CF nanostructures, the Ppy coating makes
the nanoplates and nanorods surface more wrinkled, thereby further increasing the contact area between electrolyte and active materials (V2O5@Ppy). The TEM images in Fig. 2(e) and (f) and Fig. S6b, in Supporting information further verify that the porous nanoplates and nanorods are uniformly covered by an amorphous Ppy layer with a thickness of 3 nm. More evidence about the phase and composition of the products are provided by the X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) (Figs. S3, S4 and S7, in Supporting information). Therefore, it can be concluded that the integrated flexible V2O5@Ppy/ CF film have been successfully obtained through this facile strategy. MnO2@Ppy/CF films were fabricated by a facile hydrothermal technique in the presence of CF and KMnO4 solution followed by in-situ Ppy coating, which is similar to the preparation procedure of V2O5@Ppy/CF film. During the synthesis of MnO2/CF, the CF serves as not only a sacrificial reductant by converting aqueous permanganate (MnO4 ) to insoluble MnO2, but also a substrate material. This in-situ synthesis guarantees that the formed MnO2 nanowires can be directly loaded on the surface of CF substrate with strong adhesion and high loading mass. Representative SEM images in Fig. 3(a) and (b) shows that the well-defined interconnected architecture of MnO2@Ppy/CF, where the wire-like MnO2 nanostructures grow on the surface of CF uniformly. Importantly, although there is no apparent change in the morphology of the whole structure compared without Ppy coating (Fig. S8a and b, Supporting information), it is found that the diameter of MnO2 nanowires increases and their surface becomes slightly rough, as shown in Fig. 3(c) and Fig. S8c in Supporting information. TEM image (Fig. 3(d)) further confirms the wire-like structure of MnO2. Meanwhile it can be clearly observed that the entire surface of MnO2 nanowires is covered by a continuous amorphous Ppy layer with thickness of around 5 nm. Significantly, as confirmed by
Fig. 3 (a)–(c) SEM and (d) TEM images of the Ppy coated MnO2 nanowires on CF substrates.
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Fig. 4 Electrochemical characterizations of as-prepared four V2O5/CF samples as flexible electrodes in three-electrode system: (a) comparative CV curves at scan rate of 10 mV s 1; (b) the specific capacitance calculated based on CV curves as a function of scan rate; (c) Nyquist plots of the as-prepared four V2O5/CF samples (the inset is the equivalent electrical circuit); (d) cycling stability of V2O5@Ppy nanorods and V2O5@Ppy nanoplates flexible electrodes at 5 A g 1 (the inset shows galvanostatic charge/discharge curves of V2O5@Ppy nanorods electrode for the last five cycles).
the FT-IR, XRD, and XPS analysis (Figs. S7, S9, and S10, Supporting information), no impurity is introduced even after in-situ Ppy coating. Given the unique structural attributes, V2O5@Ppy/CF films are regarded as a highly potential electrode for flexible energy storage devices. To explore the advantages of these V2O5@Ppy/CF films as integrate electrodes, we first evaluated their electrochemical properties in a threeelectrode configuration with 5 M LiCl solution as electrolyte, Pt foil as counter electrode and Ag/AgCl as the reference electrode. Benefiting from its excellent conductivity and integrated structures, the free-standing V2O5@Ppy/CF film could be directly used as the working electrode. Fig. 4 (a) shows cyclic voltammograms (CVs) of the as-prepared different V2O5/CF films at a scan rate of 10 mV s 1. As expected, in comparison to pristine V2O5/CF films, the V2O5@Ppy/CF films demonstrate substantially larger current response, implying the improved electrochemical capacitance by Ppy coating. Meanwhile, it is found that the V2O5@Ppy/CF nanorod electrode yields the highest current response. Although slight peak shift can be obsearved from the CV curves, the four electrodes show similar redox couples, which illustrate the similar pseudocapacitive behavior. Therefore, it is anticipated that the uniform Ppy coating can not only enhance the charge transfer and collection, but also diminish the electrochemical polarization, thus resulting in enhanced electrochemical performance [36]. On the other hand, the prolonged calcination
time might cause the difference in crystallinity and particle size of V2O5 nanostructures, and meanwhile the in-situ Ppy coating inevitably leads to a small amount of reduction of V2O5 that renders it an electrically condutive mixed valence (V4 + /V5 + ) oxide [37] All of these could account for the slight difference in the CV curves. The specific capacitances of these electrodes are calculated based on their CV curves (Figs. S11 and S12, Supporting information) and shown in Fig. 4(b). Obviously, the specific capacitances of V2O5/CF electrodes drop quickly, indicating that the poor intrinsic conductivity of V2O5 limits its rates capability especially at high scan rates. Significantly, V2O5@Ppy/CF nanorods electrode demonstrates impressive specific capacitance (383 F g 1 at a scan rate of 1 mV s 1) and rate capability (80% capacitance retention even at high scan rate 20 mV s 1). The high specific capacitance was further confirmed by galvanostatic charge/discharge profiles (Figs. S11–S13, Supporting information). It should be mentioned that the Ppy alone shows negligible capacitance in the voltage range of 0.3–0.7 V (Fig. S14, Supporting Information). This justifies that Ppy is an excellent structural stabilizer for V2O5, and the synergistic effects between Ppy and V2O5 nanostructures promote the enhancement of electrochemical performance. Considering the high mass of V2O5@Ppy/CF nanorods electrode, such performance outperforms most previously reported V2O5-based electrodes [9,38,39]. In order to understand the excellent performance of V2O5@Ppy/CF nanorod electrode, electrochemical
314 impedance spectroscopy (EIS) was performed (Fig. 4(c)). Clearly, V2O5@Ppy/CF nanorods electrode possesses obviously lower charge transfer resistance (Rct) (Fig. S15, Supporting information) and a relatively more vertical line at low frequency compared to the other three samples, which indicates much better electrical conductivity and faster ion diffusion. Fig. 4(d) shows the long-term cycling stability of V2O5@Ppy/CF nanorods and V2O5@Ppy/CF nanoplates electrode at 5 A g 1. The capacitance of V2O5@Ppy/ CF nanorods maintains around 83% of its initial value even after 5000 cycles, which is almost 1.5 times higher of V2O5@Ppy/CF nanoplates electrode. Besides, the good symmetry of both charge and discharge profiles for the last
L. Li et al. five cycles of V2O5@Ppy/CF nanorods electrode further indicates the excellent capacitive behavior (inset of Fig. 4 (d)). The superior electrochemical performance of V2O5@Ppy/CF nanorods electrode might be attributed to the following factors. First, the coating of Ppy layer not only provides a good conductive path for electrons and ions transfer, but also plays an important role as a soft “armor” to suppress the dissolution of V2O5 in the aqueous electrolyte, thereby leading to the enhanced electrochemical kinetics and high capacitance. Second, compared with the V2O5@Ppy/CF nanoplates structure, the nanorods structure as the secondary subunit could more effectively prevent the undesirable volume variation and ensure the stability of the
Fig. 5 Electrochemical characterizations of MnO2@Ppy//V2O5@Ppy SFHDs: (a) CV curves collected in various potential window at 20 mV s 1; (b) CV curves at various scan rates ranging from 1–20 mV s 1; (c) galvanostatic charge/discharge profiles at various current densities in the voltage range of 0–2.0 V; (d) cycling stability of MnO2@Ppy//V2O5@Ppy SFHDs at a current density of 0.5 A g 1 (the inset shows the photograph of LED powered by MnO2@Ppy//V2O5@Ppy SFHDs); (e) cycling performance of MnO2@Ppy//V2O5@Ppy SFHDs at different bending states (the inset shows the CV curves at different bending states); (f) Ragone plot of MnO2@Ppy//V2O5@Ppy SFHDs.
Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices unique porous structure, thus resulting in the excellent cycling performance. Therefore, these will convincingly beneficial for the practical applications of V2O5@Ppy/CF nanorods electrode. Considering the huge potential of V2O5@Ppy/CF nanorod film as negative electrode for solid-state flexible hybrid devices (SFHDs), we selected MnO2@Ppy/CF with an optimum loading mass (9.86 mg cm 2) as the positive electrode (Fig. S16, Supporting information). It is worth mentioned that the charge stored in both positive and negative electrodes should be adjusted in SFHDs system. As the mass ratio of MnO2@Ppy/CF to V2O5@Ppy/CF up to around 1.5, a reasonably good charge balance between them could be obtained (Fig. S16d, Supporting information), thus maximizing the utilization of active materials when further assembled into SFHDs. Both electrodes were cut into 0.5 cm 4 cm each and the total mass of active materials used in the MnO2@Ppy//V2O5@Ppy SFHDs was around 32.8 mg, which is very high compared to other previous reported HDs [40,41]. More significantly, a typical LiCl/PVA gel electrolyte was used in the present study to achieve excellent mechanical and electrochemical properties. Apart from as an ion diffusion matrix, this neutral gel electrolyte can not only effectively avoid the chemical dissolution of vanadium oxides by minimizing the water content, but also serve as an elastic coating to enhance the mechanical stability of vanadium oxides. To estimate the best operating potential of the as-prepared SFHDs, a series of CV curves with different potential windows are collected at 20 mV s 1, as shown in Fig. 5(a). Encouragingly, by taking advantage of the unique device configuration combined with stable gel electrolyte, the potential window of the device can be extended to as large as 2.0 V without obvious polarization curves, indicating that the absence of water eliminates the issue of water decomposition, and makes it possible to achieve high energy/power densities. Fig. 5 (b) reveals the CV curves of the optimized MnO2@Ppy// V2O5@Ppy SFHDs at various scan rates ranging from 1 to 20 mV s 1 between 0 and 2.0 V. Apparently, the typical quasi-rectangular CV curves with feeble redox peaks can be seen at all scan rates, thus suggesting the combination of pseudocapacitive and electric double layer capacitor (EDLC) type properties. To further confirm the superior electrochemical properties of MnO2@Ppy//V2O5@Ppy SFHDs, the galvanostatic charge/discharge profiles at different current densities are displayed in Fig. 5(c). The specific capacitance of the as-prepared device, which was calculated based on the total mass of both positive and negative electrodes, reaches 51.5 F g 1 at a current density of 0.2 A g 1. Considering the very high mass of our device, such a capacitance value is worth highlighting [9,42]. For practical use consideration, the long-term cycling stability of the V2O5@Ppy//MnO2@Ppy SFHDs was measured at current density of 0.5 A g 1 up to 20,000 cycles (Fig. 5 (d)). Strikingly, the capacitance retention still maintains approximately 89% even after 20,000 cycles, indicating the outstanding electrochemical reversibility of the asassembled SFHDs. Additionally, the successful attempt to power red light-emitting diodes (LED) shows that
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MnO2@Ppy//V2O5@Ppy SFHDs holds the opportunity to be used in portable/flexible electronics (inset in Fig. 5(d)). Moreover, it is well accepted that the development of multifunctional portable electronics requires highly flexible and lightweight power sources. The flexibility of MnO2@Ppy//V2O5@Ppy SFHDs was also investigated by performing CV analysis at different bending state, corresponding to the states in Fig. S17 (Supporting information). As shown in Fig. 5(e), there is negligible performance degradation and almost completely overlapped CV curves, indicating remarkable mechanical flexibility and stability of our device. Furthermore, power density (P) is generally considered as critical factors to evaluate the performance of electrochemical devices. Ragone plots of MnO2@Ppy// V2O5@Ppy SFHDs calculated from the discharge curves are shown in Fig. 5(f). A maximum energy density of 28.6 W h kg 1 at power density of around 200 W kg 1 for our devices could be reached. Those values of our device are comparable or surpass most of the other previously reported Vbased or Mn-based hybrid systems, such as V2O5-rGO//rGO [9], MnO2//Bi2O3 [30], V2O5//PANI [39], MnO2@C//V2O5CNT [40], Ppy@V2O5//AC [41], MnO2//VOS@C [42], VOx// VN [43], MnO2-rGO//MoO3-rGO [44], and MnO2//In2O3 [45]. It is believed that these performance advancements may derive from the rational design of active materials (V2O5@Ppy) and unique architecture of SFHDs. These features may contribute the following merits. Specifically, the unique flexible V2O5@Ppy/CF configuration provides a favorable electric contact and electron transportations, and meanwhile maintains the good mechanical stability and flexibility of the whole device. Moreover, the uniformly coating of Ppy layer not only improves the conductivity of V2O5, but also acts as a “armor” to protect V2O5 core against dissolution in the electrolyte, accounting for the excellent rate performance and cycling stability. Furthermore, the hierarchical porous nanorods feature of V2O5@Ppy structure offers more active sites for redox reaction, thus ensuring the full play of high specific capacitance. Last, the selection of LiCl/PVA gel electrolyte in our SFHDs also plays an important role to suppress the irreversible oxidation reaction and structure pulverization of V2O5@Ppy/CF, as well as efficiently eliminate water decomposition in the electrolyte, thereby making it possible to realize enhanced energy/ power densities. Therefore, all of these render MnO2@Ppy// V2O5@Ppy SFHDs to be unexceptionably flexible power sources.
Conclusions In summary, highly porous V2O5@Ppy nanoplate and nanorod arrays with large mass loadings have been successfully designed on carbon felt through a facile solvothermal followed by in-situ polymerization technique. Significantly, the detailed evolution procedureas of V2O5 nanoplate to nanorod arrays have also been provided. Additionally, as integrate electrode for energy storage devices, V2O5@Ppy/ CF nanorods display much better cycling stability and large specific capacitance than V2O5@Ppy/CF nanoplates. Moreover, to address the practical application potential of
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V2O5@Ppy/CF nanorods as flexible electrodes, a SFHDs based on V2O5@Ppy/CF and MnO2@Ppy/CF as negative and positive electrode, respectively, has been fabricated. Benefiting from the novel structural features and Ppy coating, the MnO2@Ppy//V2O5@Ppy SFHDs demonstrates outstanding cycling stability (approximately 89% retention even after 20,000 cycles), and exhibits remarkable energy density of 28.6 W h kg 1 at power density of around 200 W kg 1. Furthermore, our as-assembled device shows negligible capacitance degradation at different bending state, indicating the excellent mechanical flexibility and stability. These encouraging merits definitely promote our material as potential candidate for practical application in highperformance flexible energy storage devices.
Acknowledgments This work was supported by the National Research Foundation Singapore (NRF-CRP10-2012-06).
Appendix A.
Supplementary material
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.11.026.
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Polypyrrole-coated hierarchical porous composites nanoarchitectures for advanced solid-state flexible hybrid devices Self-Financed Student Abroad (2014) and Ian Ferguson Postgraduate Fellowship (2014). Her research interests focus on the synthesis of functional nanomaterials and their application in supercapacitors and batteries. Shengjie Peng is now working as a senior research fellow in Prof. Seeram's group in National University of Singapore. He received his Ph.D. degree in Nankai Univerisity (PR China) in 2010. He has coauthored over 60 peer-reviewed publications. His current research interests include the design and development of nanomaterials and their applications in energy conversion and storage devices. Han-Yi Chen is currently pursuing her Ph.D. in the Department of Chemistry at Technical University of Munich, Germanyand Joint Ph. D. in Nanyang Technological University, Singaporeas a part of the NRF-NTU-TUMCREATE program on Electromobility for tropical megacities. She obtained her Bachelors from the Department of Chemical and Materials Engineering, National Central University, Taiwan in 2008 and Masters from the Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Taiwan in 2011. Her current research involvesmetal oxide clusters and nanocarbon materials for energy storage applications. Xiaopeng Han was born in Shanxi province, PR China in 1988. He received his B.E. degree in chemical engineering and technology from TianJin University (2010). Then joined the Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education, NanKai University) and obtained his Ph.D. degree under the supervision of Prof. Jun Chen. His research focuses on functional materials for energy and catalysis. Fangyi Cheng is a Chemistry Professor at Nankai University. He received his Ph.D. from Nankai University in 2009. His research interests cover design and synthesis of functional nanostructures and composite materials for energy-related applications such as electrocatalysis, batteries, and hydrogen storage. He has coauthored over 100 scientific papers in peer-reviewed journals, including Nature Chem., Chem. Soc. Rev., Angew. Chem., Adv. Mater., etc. His publications have received over 4200 citations, with an H-index of 35. He is a recipient of Excellent Young Scholars (NSFC) and New Century Excellent Talents Project (MOE). Dr. Cheng is a board committee member of IAOEES.
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Madhavi Srinivasan is an associate professor at the School of Materials Science and Engineering, Nanyang Technological University (NTU). She graduated from Indian Institute of Technology (IIT), Chennai (India) and completed her Ph.D. from National University of Singapore. She is also one of the three women scientists awarded the L’Oreal-UNESCO for women in science national fellowships awards in Singapore (2010). Her research interest is to enhance performance of energy storage devices such as lithium ion batteries, supercapacitors and advanced batteries. Stefan Adams is an associate professor at Department of Materials Science and Engineering, National University of Singapore. He received his B.Sc. and Ph.D. degree from Saarland State University, Germany. Then he worked as a postdoctoral fellow in MaxPlanck-Institute for Solid State Research in Stuttgart from 1992 to 1994. He workded as Lecturer and Senior Lecturer at Göttingen University from 1994 to 2000. He joined National University of Singapore as an Assistant Professor in 2010 and became an associte Professor in 2011. His research interest is to search for new materials for sustainable energy applications by using both experimental and computational approaches. Seeram Ramakrishna, FREng, FNAE, FAAAS is a renowned Professor of Materials Engineering at the National University of Singapore. He is an acknowledged global leader for his pioneering work on the science and engineering of nanofibres. He has authored 5 books, 25 book chapters, 20 patents and 600 peer reviewed papers, which attracted about 43,000 citations with an h-index of 98. Various international databases including Thomson Reuters Web of Science, Elsevier Scopus and Microsoft Academic place him among the top 25 authors and most cited materials scientists in the world. Chen Jun obtained his B.Sc. and M.Sc. degrees from Nankai University in 1989 and 1992, respectively, and his Ph.D. from Wollongong University (Australia) in 1999. He held the NEDO fellowship at National Institute of AIST Kansai Center (Japan) from 1999 to 2002. He was appointed as the Chair Professor of Energy Materials Chemistry at Nankai University in 2002. His research expertize is energy conversion and storage with solar cells, fuel cells, and batteries.