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Self-standing Polypyrrole/Black Phosphorus Laminated Film: Promising Electrode for Flexible Supercapacitor with Enhanced Capacitance and Cycling Stability Shaojuan Luo, Jinlai Zhao, Jifei Zou, Zhiliang He, Changwen Xu, Fuwei Liu, Yang Huang, Lei Dong, Lei Wang, and Han Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15458 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Self-standing Polypyrrole/Black Phosphorus Laminated Film: Promising Electrode for Flexible Supercapacitor with Enhanced Capacitance and Cycling Stability Shaojuan Luo,†,‡ Jinlai Zhao,§ Jifei Zou,†,‡ Zhiliang He,†,‡ Changwen Xu,‡ Fuwei Liu,§ Yang Huang,*,§ Lei Dong,⊥ Lei Wang,§ and Han Zhang*,†,‡ †

Shenzhen Engineering Laboratory of Phosphorene and Optoelectronics, International

Collaborative Laboratory of 2D Materials for Optoelectronic Science and Technology, Shenzhen University, Shenzhen 518060, China ‡

Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and

Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China §

Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science

and Engineering, Shenzhen University, Shenzhen 518060, China ⊥

Department of Physics, Southern University of Science and Technology, Shenzhen 518055,

China Email: [email protected]; [email protected] 1 ACS Paragon Plus Environment

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ABSTRACT With rapid development of portable electronics, solid-state flexible supercapacitors (SCs) are considered as one of the promising energy devices in powering electronics because of their intrinsic advantages. Polypyrrole (PPy) is an ideal electrode material in constructing flexible SCs owing to its high electrochemical activity and inherent flexibility, though its relative low capacitance and poor cycling stability are still worthy to be improved. Herein, through ingenious introduction of black phosphorous (BP) nanosheets, we developed a laminated PPy/BP selfstanding film with enhanced capacitance and cycling stability via a facile one-step electrochemical deposition method. The film exhibits a high capacitance of 497.5 F g-1 (551.7 F cm-3) and outstanding cycling stability of 10000 charging/discharging cycles, thanks to BP nanosheets inducing laminated assembly which hinders dense and disordered stacking of PPy during electrodeposition, consequently provides a precise pathway for ions diffusion and electrons

transportation

together

with

alleviation

of

structural

deterioration

during

charging/discharging. The flexible SC fabricated by laminated films delivers a high capacitance of 452.8 F g-1 (7.7 F cm-3) besides its remarkable mechanical flexibility and cycling stability. Our facile strategy paves the way to improve electrochemical performance of PPy-based SC that could serve as promising flexible energy device for portable electronics.

KEYWORDS: black phosphorus, polypyrrole, self-standing, flexible, supercapacitor

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INTRODUCTION Supercapacitors (SCs), mainly including double layer capacitors and pseudocapacitors, show great potential in energy storage and draw enormous attention owing to their intrinsic advantages, such as fast charging/discharging rates, high power density and long cycle lifetimes.1-5 To meet with the blooming of wearable electronics, it is highly desirable to develop SCs with flexible and/or compact configuration besides outstanding electrochemical performance. Thus, flexible pseudocapacitors should be desired candidates because they could normally provide much higher capacitance than double-layered capacitors due to their fast and reversible redox reactions, which have attracted much scientific and technical interest.6-10 As one major electrode materials for pseudocapacitor, conducting polymers possess significant advantages over other contenders because of their inherent flexibility and high conductivity when compared with other rigid pseudocapacitive materials, for example, metal oxides, nitrides and sulphides, making them perfect candidates for high-performance flexible SCs.11-18 Polypyrrole (PPy) is widely used in fabricating flexible SCs on account of its high electrochemical activity and intrinsic flexibility as aforementioned.19-29 However, most of these reported flexible PPy-based electrode materials present dense and/or disordered structure. This not only impedes electrolyte penetration and ion diffusion, but also aggravates structural pulverization of PPy backbone during the charging/discharging, resulting in low capacitance and poor cycling stability.30-32 One emerging solution to overcome the above challenges is to hybridize other nanostructured materials with PPy during polymerization in order to increase contact area and create buffer space. Accordingly, various materials (e.g. graphene and carbon nanotube) were selected to reinforce PPy-based materials and hinder the formation of dense and disordered structure.7,30,33-36 2D materials nanosheets, offering uniform and large surface area 3 ACS Paragon Plus Environment


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that could immobilize PPy for charge storage, have been currently studied and used for PPy based SC.7,36 Similar to the classical 2D materials (e.g. graphene), p-type direct-bandgap semiconducting layered black phosphorous (BP) can be delaminated into single- and few-layer nanoflakes or nanosheets through mechanical and liquid phase exfoliation. These nanosheets have attracted substantial attention in virtue of its versatility in different fields, such as field-effect transistors (FETs),37-38 Li- and Na-ion batteries,39-42 SCs,43-44 to name a few. Remarkably, BP has shown potential as electrode material in electrochemical energy-storage devices because of its large spacing (5.3 Å) between adjacent puckered layers, which is larger than that of graphene (3.6 Å) and comparable to 1T phase MoS2 (6.15 Å).13 Such large spacing allows facile intercalation and fast diffusion of cations (e.g. H+, Li+ or Na+) as indicated by the theoretical investigations.45-46 As an anode material of batteries, the theoretical specific capacity of BP can reach 2596 mAh g-1, much higher than that of commercial graphite (372 mAh g-1).40 Recently, liquid-exfoliated BP nanoflakes were utilized for all-solid-state SC that delivered a capacitance of 45.8 F g-1 at the scan rate of 10 mV s-1.43 Besides, another study reported the BP/PANI nanocomposite with a high capacitance of 354 F g-1 at a current density of 0.3 A g-1.44 Apparently, it would be a brilliant concept to take advantage of this relatively new 2D nanosheet as efficient supporter to construct PPy based hybrid electrode, which should exhibit enhanced capacitance and cycling stability concurrently. Herein, for the first time, we fabricated a PPy/BP self-standing film with laminated structure via a facile one-step electrochemical polymerization method, which enhances capacitance and cycling stability of the flexible SC significantly. The existent BP nanosheets lead the ingenious laminated assembly and thus hinder dense and disordered stacking of PPy 4 ACS Paragon Plus Environment

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films, resulting in a high capacitance of 497 F g-1 (552 F cm-3), while cycling stability shows no obvious deterioration even after 10000 charging/discharging. This innovative laminated structure not only provides a precise pathway for ions diffusion and electrons transportation, but also reinforces structural stability of PPy framework because of BP acting as buffer layers to alleviate volume change caused by common counterion drain effect. Similarly, solid-state flexible SC assembled by the PPy/BP laminated film exhibits a high capacitance of 452.8 F g-1 (7.7 F cm-3), outstanding cycling stability after 10000 charge/discharge cycles and superior mechanical flexibility after 3000 bending. Our work demonstrated a facile strategy to improve electrochemical performance of PPy-based SC by the ingenious introduction of BP 2D nanosheets, inspiring application of BP in optimizing other conductive polymers with remarkable energy storage performance. EXPERIMENTAL SECTION Preparation of black phosphorus nanosheets: Bulk BP (15 mg) was firstly ground to fine powder by an agate mortar in argon filled glove box, and then added to 30 mL of saturated NaOH/NMP solution in a 50 mL sealed centrifuge tube. The mixture was sonicated with a sonic tip for 3 h at the power of 1200 W. The ultrasound probe worked for 2 s with an interval of 4 s at an ultrasonic frequency ranged from 20 to 25 kHz. Following that the dispersion was continuously sonicated in ultrasonic bath for another 8 h at the power of 300 W, which was kept at 277 K using ice bath. The above two steps were repeated twice to achieve the exfoliation of bulk BP. The obtained solution was centrifuged at 2000 rpm for 20 min to remove non-exfoliated bulk BP, the supernatant containing BP nanosheets was decanted gently and then centrifuged at 8000 rpm for 30 min to exact BP nanosheets. The resulting precipitate was rinsed by DI water



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and re-dispersed in the deaerated aqueous solution with a concentration of 1.0 mg mL-1, which was stored in a desiccator for further use. Preparation of Gel Electrolyte: According to previous studies, H3PO4-PVA was chosen as the gel electrolyte for our solid-state SC.26,43,47 Typically, 3 g of H3PO4 was mixed with 30 mL deionized water, and 3 g of PVA powder was subsequently added into the acid solution. Then the mixture was heated to ~90 °C with vigorous stirring until the solution became transparent. Fabrication of electrodes: Three-electrode configuration was applied in the electrochemical polymerization process with ITO coated PET film as working electrode, platinum gauze as counter electrode, and saturated calomel electrode (SCE) as reference electrode. The PPy/BP film was electrodeposited on the PET by using a constant voltage of 0.8 V. In brief, 300 µL of distilled pyrrole monomer added to 45 mL electrodeposition solution containing sodium dodecylbenzenesulfonate (0.3M), p-toluenesulfonic acid (0.1 M) with stirring. After that, a certain volume (300, 600 and 1200 µL) of as-prepared BP solution was added into above solution, followed by 30 min bath sonication to ensure homogeneity of solution. The electrochemical deposition was carried out under a constant voltage of 0.8 V, and the whole process kept at 0 °C (ice bath) for 400 s. Pristine PPy film was electrodeposited in a similar process only without adding the BP nanosheets. Then the as-obtained PET film was washed with mixture solvent of DI water and ethanol (v:v = 1:1), and dried in a vacuum oven at 60 °C overnight prior to the use. Afterwards, the self-standing PPy/BP film can be easily peeled off from the ITO-coated PET substrate. The solid-state supercapacitors (SCs) were assembled as follow: a thin layer of H3PO4-PVA gel electrolyte was spread onto PPy/BP laminated film electrodes, and then dried in air. After 2 h


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solidification of electrolyte, two piece of the electrodes were pressed together to generate a flexible device with classical sandwich structure. Electrochemical

measurement:

Electrochemical

measurements

including

cyclic

voltammetry (CV) curves, galvanostatic charging/discharging (GCD) curves and electrochemical impedance spectroscopy (EIS) (100 kHz-0.01 Hz) were conducted by an electrochemical workstation (CHI 660e). The performance of solid-state flexible SC was measured by using a two-electrode method. RESULTS AND DISCUSSION The self-standing PPy/BP laminated film was fabricated by a facile one-step electrochemical deposition process, during which pyrrole monomer and BP nanosheets mixture directly “attached” onto indium-tin oxide (ITO) coated PET and self-assembled into a flexible conductive film. Figure 1a schematically depicted the electrochemical deposition process, pyrrole monomer and BP nanosheets were dispersed into electrolyte via vigorous stirring and sonication at the very beginning, and then BP nanosheets were captured by PPy chains during electrochemical polymerization process. Afterwards, these PPy wrapped (OH– protected) BP hybrids were driven onto the surface of ITO substrate by the constant-voltage deposition. After a continued electrodeposition process, a flexible conductive thin film was subsequently formed, which can be readily peeled off from PET substrate, as shown in Figure 1b. The duration of this electrodeposition process was optimized to be 400 s, and detailed information is described in the supporting information (Figure S1). As a demonstration, a large-area self-standing PPy/BP film (5 cm × 5 cm) was prepared, which was robust enough and could be readily folded into a paper plane, presenting its excellent mechanical strength and flexibility (Figure 1c). Areal density of



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this film is only about 0.336 mg cm-2 (Figure S2a), indicating its great potential in fabricating flexible and lightweight electronic devices with compact configuration. BP nanosheets is an indispensable part for forming the laminated film, which was prepared by a modified solvent exfoliation method as reported before.48-50 The exfoliated BP nanosheets exhibited an excellent stability in water because zeta potential of BP nanosheets solution was about -31.2 mV (Figure S2b), and this negative charges come from a large number of surface anchored OH– ions originating from exfoliation process. Therefore, due to the strong electrostatic interaction, these BP nanosheets could not only disperse homogeneously in the electrolyte, but also readily interact with pyrrole monomer, key of the special laminated structure. The Raman peaks of as-prepared BP nanosheets solution exhibited a blue shift comparing to bulk BP and moved to 361.7 cm-1, 437.6 cm-1 and 466.2 cm-1, respectively, which could be attributed to enhancement of P atoms oscillation, as well as decreasing influence of interlayer van der Waals force,51 as shown in Figure 2a. The most layer-sensitive Ag2 mode shows a 2.4 cm1

blue-shift deviation to the bulk BP and average thickness of BP nanosheets in the solution

could be determined to about 4-6 layers, which was consistent with Atomic force microscopy (AFM) results that clearly depicted the nanosheets with a thickness of ~2 nm (4-5 layers) (Figure 2b). By embedding in such super thin flexible BP nanosheets, it is reasonable to believe that electrochemical performance (e.g. capacitance) of this hybrid film will be significantly improved since BP nanosheets can not only contribute additional charge storage capacity by providing facile intercalation and fast diffusion of ions, but also act as effective flexible buffer layers to alleviate structural pulverization and suppress counterion drain effect of PPy, leading to a synergistic effect as other studies.40-41


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The main diffraction peaks of bulk BP as shown in the XRD pattern (Figure 2c) can be assigned to (020), (040), (060) and (080) reflections of orthorhombic BP (JCPDS No.73-1358), whereas PPy/BP hybrid film only exhibited a broad peak at 20-35° that is attributed to PPy chains, as characteristic peak of PPy located at around 27° (Figure S3a).47 Besides, no peaks of discernable BP or phosphate compounds were detected, elucidating that BP nanosheets were successfully embedded in the backbone of PPy chains and thus forming a homogeneous PPy/BP hybrid film, which was further proved by the FT-IR and transmission electron microscopy (TEM) results. The FT-IR spectrum of PPy/BP laminated film is almost identical to that of pristine PPy film, indicating BP nanosheets are wrapped intimately by PPy chains (Figure S3b). The TEM sample was prepared by in-situ electro-depositing the hybrid onto a copper grid. As shown in Figure 2d, a thin layer of PPy wrapped BP nanosheets forming a hybrid composite, marginal region of which was rolled up because high energy electron beam projected on the polymer during data acquisition. However, it is hard to obtain lattice fingers of embedded BP nanosheets due to amorphous PPy coating, though HRTEM of as-prepared BP nanosheets showed a lattice spacing of 2.247 Å corresponding to interplanar spacing of (041) plane of orthorhombic BP (JCPDS: 73-1358), indicating three neighbors atoms at that distance,52 which reveals that the liquid exfoliation BP sheets are still in original crystalline state (Figure S4) and thus provide a relatively large spacing for facile intercalation and fast diffusion of ions. Once electro-deposition process is finished, the as-obtained PET film samples were washed and dried. Afterwards, flexible PPy/BP hybrid film could be readily peeled off from the substrate (as shown in Figure 1b) and be directly served as a free-standing supercapacitor electrode. Thanks to the introduction of BP nanosheets, this film electrode, with a relative smooth surface, presented a special laminated ordered structure, whereas pristine PPy film shows a dense and 9 ACS Paragon Plus Environment


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disordered stacking architecture (Figure 2e, f and Figure S5). The formation of such laminated structure can be attributed to the existence of BP nanosheets that served as template and induced layer by layer ordered assembly of PPy. This special structure effectively impeded dense stacking and created wrinkles and/or pores (Figure S5d), which should be propitious for electrolyte diffusion, and consequently enhanced energy storage of film electrode.53 In addition, efficient intercalation and fast diffusion of ions between the laminated layers would also improve capacitance, which is confirmed by the following electrochemical measurement results (Figure S6).54-55 Owing to the intentional introduction of BP nanosheets, PPy/BP laminated film showed better flexibility than pristine PPy film. As shown in the load-displacement graph of nanoidentation testing (Figure S7), the calculated results indicated that pristine PPy film was with the modulus of 1.5 ± 0.4 GPa and the hardness of 0.02 ± 0.01 GPa, while PPy/BP laminated film was with the modulus of 0.9 ± 0.2 GPa and the hardness of 0.01 ± 0.005 GPa. Due to its dense and disordered structure (as shown in Figure S5), pristine PPy film possesses relative higher modulus and hardness than PPy/BP laminated film; in other words, the PPy/BP film exhibited enhanced flexibility because of its special lamellar ordered architecture, which would be an advantage for the fabrication of flexible electronic devices. The corresponding electrochemical measurement results confirmed the advantages of special laminated structure: the enclosed area of PPy/BP laminated film in cyclic voltammetry (CV) scan is obviously much larger than that of pristine PPy film and/or BP nanosheets, suggesting an optimized capacitance arising from the synergistic effect between PPy and BP (Figure 3a and Figure S8). As shown in Figure 3b, the capacitance of PPy/BP film achieved a high value of 431.4 F g-1 (483.1 F cm-3) at a current density of 1 A g-1, 25% higher than that of 10 ACS Paragon Plus Environment

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pristine PPy film (344.6 F g-1 and 385.9 F cm-3); particularly, this laminated electrode could further provide a remarkable capacitive value of 497.5 F g-1 (551.7 F cm-3) at 0.5 A g-1, which are better than or comparable to most of the other PPy hybrid electrodes (Table S1). The specific capacitance of BP nanosheets was about 41.75 F g-1 (Figure S8), which is comparable to other studies of BP electrodes,43 graphene-based electrodes54,56 and multi-walled carbon nanotubes electrodes.57 It is certain that these embedded BP nanosheets in film could contribute capacitance to the hybrid flexible electrode. However, considering their limited addition in the film (Table S2), it should attribute the significant boost (~25%) in capacitance to a synergistic effect between PPy and BP nanosheets rather than a simple sum of capacitances contributed by individual components. This is evidenced by adding different amount of BP starting solution, since capacitance of laminated film gradually decreased with an increase of BP nanosheets, on account of reduced conductivity (Figure S9): at the beginning, emergence of BP engenders an ordered laminated structure that results in several advantages, for instance, better ion intercalation and diffusion, leading to an enhanced capacitance; while with continuously increased BP, series resistance of film increases because excessive semiconductive BP nanosheets hampers effective connection of conductive PPy network, bringing about deterioration of electrical conductivity. From the equivalent series resistance (ESR) values of Figure S9c, it is obvious that PPy/BP film with 600 µL BP starting solution is with smaller resistance, one of the reasons for its better electrochemical performance (Figure S9a). Relative CV curves of the optimized laminated electrode showed pseudo-rectangular shape (Figure 3c), demonstrating low internal resistance and good reversibility of charging/discharging, which leads to a satisfactory rate ability: remaining 65% when scan rate was amplified by 15 times (Figure S10a); nevertheless, slight deviation occurred at a high scan rate that can be ascribed to limited time constant (8.26 s) 11 ACS Paragon Plus Environment


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calculated from Nyquist plot at the phase angle of 45° (Figure 3f). The corresponding galvanostatic charging/discharging (GCD) curves presented close to ideal triangular shapes as shown

in

Figure

3d,

indicating

excellent

coulombic

efficiency

with

symmetric

charging/discharging process, which could maintain about 60% of capacitance under a high current density of 10 A g-1 (20 times amplified, Figure S10b). Besides significant improvement in capacitance, PPy/BP laminated film also possesses enhanced cycling stability, which is another advantage benefits from the synergistic effect, as illustrated in Figure 3e. For PPy/BP laminated film, no obvious deterioration was observed even after 10000 charging/discharging cycles, whereas pristine PPy film could only maintain about 65% of its capacitance, indicating effective improvements of cycling stability via the introduction of BP nanosheets. The Nyquist plots of laminated film obtained before and after cycling test showed similar pure capacitive behavior since only vertical line could be seen at low frequency region (Figure 3f); meanwhile, charge transfer resistance did not increase obviously after 10000 charging/discharging cycles, indicating that kinetic of charge migration process was retained. We assume that PPy backbones are effectively enhanced by the introduction of BP nanosheets which function as interlinks and buffer layers between PPy chains, while forming a laminated ordered structure, and hence impedes structural pulverization and counterion drain effect, leading to better cycling stability. The morphology of PPy/BP film after 10000 charging/discharging cycles did confirm our speculation to some extent, since it is almost the same as that before cycling test (Figure S11a, e); on the contrary, pristine PPy film exhibited certain aggregations after repeated charging/discharging hampering internal connection channels, and thus affected cycling performance (Figure S11b, f).


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To further investigate the effects of BP nanosheets in enhancing cycling stability, X-ray photoelectron spectroscopy (XPS) was applied to characterize core levels of atoms and reveal chemical bonds in the PPy/BP laminated film before and after repeated charging/discharging, as shown in Figure S12. In the P2p spectrum of fresh PPy/BP film, besides characteristic peak of pure P–P at 131 eV, another two peaks located at 132.9 eV and 133.8 eV could be assigned to P– C bonding and P–Ox bonding, respectively. The P–C bonding indicated strong interactions between P atom and C atoms in PPy framework which comes from the integrated structure of BP nanosheets and PPy chains (Figure 1a, Figure 2d and Figure S6); while to P–Ox bonding, it was ascribed to residual protecting groups OH– on the surface of BP nanosheets deriving from exfoliation

process

which

could

provide

additional

capacitance.58-59

After

10000

charging/discharging cycles, the laminated electrode showed a peak at 131 eV with decreased intensity, indicating BP nanosheets embedded in PPy networks without serious degradation, yet the enhanced P–Ox peak arose from remnant H3PO4 attached on the surface and/or partial oxidation of BP during cycling.43,60 The presence of oxygen functional groups on surfaces would be beneficial for capacitance, since they can serve as additional trapping sites of charge ions, for example H+ ions.61 As for C1s spectra (Figures S12b and e), beyond strong C–C peak located at 284.4 eV, additional five deconvoluted Gaussian peaks could be recognized. The C–N bonding of pyrrole corresponds to the peak located at 285.2 eV, while the peak of C–P bonding between pyrrole and BP at 288.2 eV showed highest binding energy.62 The intensity of C–P signal was strengthened after cycling test as well as the bonds of carbonyl carbons (C=O) at 287.7 eV and hydroxyl carbons (C–OH) at 286.5 eV, indicating presence of H3PO4 or P–Ox. The peak with lowest binding energy of 283.7 eV was assigned to the carbon atoms bonded with sulfur (C–S) as 13 ACS Paragon Plus Environment


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dopant (from p-TSA), which is normally used to estimate doping level of PPy. The C–S signal of pristine film would disappear after repeated charging/discharging cycles stating serious structure distortion in pristine PPy film,33 but in the PPy/BP laminated film electrode, C–S signal was still observed (Figure S12e), revealing its structural stability, which proved the function of BP nanosheets in improving cycling stability of PPy. As for N1s spectrum (Figures S12c), it can be deconvoluted into three Gaussian peaks centered at 399.6, 400.4, and 402.4 eV at initial stage, which can be assigned to benzenoid amine (–NH–), protonation benzenoid amine (–N+H–), and protonation quinonoid imine (–N+=), respectively.30,63 The benzenoid amine (–NH–) belongs to nitrogen atoms in the backbone of PPy chain, while the latter two cations are usually regarded as PPy polarons, providing electrochemical activity. A new shoulder peak of quinonoid imine (–N=) with lower binding energy at 398.5 eV occurred after cycling test which was usually used to estimate density of defects in PPy matrix, because the amount of quinonoid imine significantly affects energy storage ability of PPy. The increased amount of –N= was in accordance with previous report that PPy polarons (–N+=) can be reduced to –N= during charging/discharging cycles.30 The polaron ratio (defined as the areal ratio between polaron signals (–N+= and –N+H– peaks) and total nitrogen signal (–N+=, –N+H–, –NH–, and –N=)) slightly decreased from 34.0% to 33.6% after 10000 charging/discharging cycles, suggesting that our PPy/BP film possesses stable electrochemical activity with little distortion after repeated charging/discharging. Therefore, the introduction of BP nanosheets, leading to special laminated PPy film, are considered as the key factor of outstanding cyclic stability. Considering its outstanding capacitive performance and remarkable cycling stability, this free-standing laminated PPy/BP film is ideal thin electrode for flexible SC, which is favorable 14 ACS Paragon Plus Environment

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for fabricating compact electronic devices. The solid-state SC was assembled by casting a thin layer of H3PO4-PVA gel electrolyte onto one piece of PPy/BP film electrode and subsequently sandwiching another one. As shown in Figure 4a, CV curves of the as-prepared device tended to remain pseudo-rectangular shape even at very high scan rate (150 mV s-1), revealing low resistance and good capacitive behavior; besides, GCD curves showed nearly triangular shape, illustrating excellent reversibility in charging/discharging process, which is consistent with CV curves (Figure 4b). As demonstrated above, this PPy/BP laminated film is mechanical flexible enough that can be readily folded into arbitrary shape; thus, it is expected that the SC device as assembled by these electrodes should also possess similar outstanding mechanical flexibility. Indeed, the SC could retain more than 92% of its original capacitance under different bending angles (0-180o) as shown in Figure 4c; moreover, even after 3000 severe bending cycles (from 0 to 135o), CV curves of the device showed no obvious degradation and corresponding capacitance was almost unchanged, demonstrating superior mechanical flexibility and excellent structural integrity (Figure 4d). Cycling stability of this PPy/BP based SC reached a satisfactory level due to the advantages of laminated structure in flexible electrodes as discussed previously: nearly no performance decay occurred after 10000 charging/discharging cycles (Figure 4e). Benefit from light weight thin electrodes, the flexible SC device, with a compact configuration (Figure 4f), showed exceptional gravimetric (based on the total mass of active material) and volumetric (combined volume) capacitance, achieving high values of 452.8 F g-1 and 7.7 F cm-3 at a current density of 0.5 A g-1, respectively, which is comparable or superior to a number of previously reported symmetric SCs.33,64-66 Even though a higher capacitance could be obtained at a lower current density of 0.25 A g-1 (Figure S13), the corresponding GCD curves deteriorated to a certain degree that could not maintain a symmetric quasi-triangular shape, suggesting that side 15 ACS Paragon Plus Environment


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reactions occurred and contributed certain capacitance. Thus, we assume this value can not accurately reflect the capacitance of our SC device. As shown in Figure S14, over 50% and 35% capacitance were maintained when scan rate and current density were increased by 15 times and 10 times, suggesting a good rate ability. In addition to significantly enhanced capacitance and cycling stability, the solid-state flexible SC based on our PPy/BP laminated film could deliver a high energy density of 30.8 W h kg-1 at a power density of 700 W kg-1, and maintain 10.5 W h kg-1 even at 7000 W kg-1 as showed in the Ragone plot (Figure 4g), which are superior to a number of previously reported symmetric systems, for instance, layered PPy/rGO composite (27.3 W h kg-1 at 305 W kg-1),64 3D PPy/GO composite (15.1 W h kg-1 at 80 W kg-1),65 PPy/MXene hybrid (20 W h kg-1 at 1000 W kg-1),33 flower-like PPy/rGO hybrid (7 W h kg-1 at 90 W kg-1),66 paper-like PANI/CNT composite (7.1 W h kg-1 at 700 W kg-1),67 and MnO2 coated Au stem device (20 W h kg-1 at 400 W kg-1).68 By connecting these high-performance PPy/BP laminated film based solid-state SCs in series or in parallel, different voltage or current needs for practical applications could be perfectly met. As is vividly shown in Figure S15a-d, three of the as-prepared SC devices in series by charging to 2.1 V are able to light red and yellow LEDs easily, which worked in the voltage ranges of 1.7-1.9 V and 1.8-2.0 V, respectively. The electrochemical performances of three connected devices in series and in parallel were also tested and displayed in Figure 4h and i. Compared to a single device, the output voltage of three ones in series would increase from 0.7 to 2.1 V, exhibiting the same operating time. The Ohmic resistance increases from 32 to 238 Ω while iR drop increases from 0.0363 to 0.267 V, this significant increase may attribute to the contact resistances among connected devices. Likewise, the running time of three ones in parallel reached 3 times longer than the single device while charging to the same voltage of 0.7 V. Ohmic resistance of this parallel circuit slightly increases 16 ACS Paragon Plus Environment

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from 32 to 34Ω in corresponding to the iR drop increases from 0.0363 to 0.0386 V, revealing almost no performance decay. As demonstrated above, PPy/BP laminated film electrode exhibited significantly improved electrochemical performance, thanks to the brilliant introduction of BP nanosheets, and their great contributions can be concluded as follows: first, BP nanosheets serve as templates and induce a layer by layer self-assembly of PPy, effectively hinder its dense and disordered stacking and thus form a self-standing lamellar film with wrinkle and pores inside. This will facilitate electrolyte filtration, resulting in facile intercalation and fast diffusion of ions; second, oxygen functional

groups

originated

from

reaction

between

BP

and

electrolyte

during

charging/discharging, can serve as additional trapping sites of charge ions;43,61 third, PPy is strongly bonded with BP nanosheets through C–P bonds and hydrogen bonds between N–H groups in pyrrole rings and hydroxyl anchored to BP nanosheets, which guarantees entire conductive and inter-connections of BP and PPy chains, providing a straightforward pathway for electrons transportation,69 and consequently enhance capacitance; last, BP nanosheets can act as efficient supporter for constructing laminated PPy film as well as effective buffers for volume changes (swelling and shrinkage) during charging/discharging, leading to the outstanding cyclic stability. CONCLUSIONS In summary, our work demonstrated successful preparation of self-standing PPy/BP laminated film via a facile one-step electrodeposition method, which can be directly used as high-performance electrodes for flexible SC with enhanced capacitance and cycling stability. The ingenious introduction of BP nanosheets is the origin of laminated polymerization of PPy that suppressed its dense and disordered stacking during electrochemical deposition, which not 17 ACS Paragon Plus Environment


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only provided a straightforward pathway for electrons transportation, but also enhanced structural stability of PPy framework, leading to a high capacitive value of 497.5 F g-1 (551.7 F cm-3) and almost perfect capacitance retention after 10000 charging/discharging cycles. Thanks to its outstanding electrochemical performance and excellent mechanical flexibility, this laminated PPy/BP film is an ideal electrode for solid-state SC. The as-prepared flexible device achieved a high gravimetric capacitance of 452.8 F g-1 corresponding to a volumetric capacitance of 7.7 F cm-3 at a current density of 0.5 A g-1, together with outstanding energy and power densities, let alone its excellent mechanical and cycling stability. It is believed that the highperformance flexible SC on the basis of our laminated PPy/BP film would be a promising energy storage device for a variety of portable electronics, inspiring other ingenious combinations of conductive polymer and 2D materials with enhanced capacitance and cyclic life.

ASSOCIATED CONTENT Supporting Information Experimental section (includes reagents and materials, details of pure BP electrode preparation process, characterization and calculation), SEM images, TEM images, XPS spectra, XRD spectrum, FTIR spectra, mechanical testing curves, digital photos of the BP nanosheets, PPy/BP film and three connected devices series, electrochemical performance (CV, CD, ESR, etc.) of various PPy/BP laminated films. This material is available free of charge via the ACS Publication website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors 18 ACS Paragon Plus Environment

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*E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial supports from Natural Science Foundation of Guangdong Province (Grant No. 2016A030310048), China Postdoctoral Science Foundation (Grant No. 2016M592530), Guangdong Research Center for Interfacial Engineering of Functional Materials, Natural Science Foundation of Shenzhen University (SZU) (Grant No. 2017004), Science and Technology Innovation Commission of Shenzhen (Grant Nos. KQTD2015032416270385

and

JCYJ20150625103619275),

National

Natural

Science

Foundation of China (Grant Nos. 61435010, 61575089 and 61704112) and Student Innovation Development Foundation of Shenzhen University (PIDFP-ZR2017023). The authors would also like to thank Mr. WEI Wei in the School of Materials Science and Engineering of Southwest Jiaotong University for their valuable advice on preparing the manuscript.

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FIGURES



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Figure 1. (a) The design and manufacturing process flow of self-standing PPy/black phosphorus (BP) laminated film. BP nanosheets were dispersed into electrolyte at the very beginning, and subsequently captured by PPy under electrodeposition process. Afterwards, this hybrid film can be readily peeled off from substrate. (b) The demo of peeling off process of laminated PPy/BP film from PET substrate. (c) Photograph of a large-area self-standing PPy/BP laminated film with a size of 5 cm × 5 cm, which is robust enough and can be folded into a paper plane.


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Figure 2. (a) Raman spectra of bulk BP and BP nanosheets in solution. (b) AFM image and height profile of exfoliated BP nanosheets. (c) XRD patterns of bulk BP crystal and PPy/BP



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laminated film. (d) TEM image of PPy/BP film and SAED pattern of BP (inset). (e) Surface and (f) cross-section SEM images of PPy/BP laminated film.


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Figure 3. Electrochemical measurements for PPy/BP laminated film. (a) Comparison of CVs between laminated PPy/BP and pristine PPy films, obtained at a scan rate of 50 mV s-1. (b) Comparison of GCDs between laminated PPy/BP and pristine PPy films, obtained at a current density of 1 A g-1. (c) CVs and (d) GCDs of laminated PPy/BP film obtained at different scan rates and current densities. (e) Capacitance retention of laminated and pure films after 10000 cycles under a current density of 5 A g-1. (f) Nyquist plots and fitted plots of PPy/BP films before and after 10000 cycles.



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(g)

Figure 4. (a) CVs and (b) GCDs of as-prepared flexible solid-state device obtained at different scan rates and current densities. (c) Performance stability of flexible SC under different bending angles. (d) Capacitance stability of flexible SC device under bending test, And (e) flexible SC cycling stability of 10000 charging/discharging cycles at 3 A g-1. (f) Digital photographs of flexible device and its thickness (~400 µm). (g) Ragone plot of as-prepared flexible SC and its comparison with some reported devices.[33,64-68] GCD curves for a single as-prepared PPy/BP



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flexible solid-state SC device and three connected devices in series (h) and parallel (i) at a current density of 0.7 A g-1.


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Promising Electrode for Flexible Supercapacitor ... - ACS Publications

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