Supercapacitive Properties of 3D-Arrayed Polyaniline Hollow

Feb 7, 2017 - A major limitation of polyaniline (PANi) electrodes for supercapacitors is the slow rate of ion transport during redox reactions and the...
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Supercapacitive Properties of 3D-Arrayed Polyaniline Hollow Nanospheres Encaging RuO2 Nanoparticles Hyemin Kwon,† Dajung Hong,† Ilhwan Ryu, and Sanggyu Yim* Department of Chemistry, Kookmin University, Seoul, 02707, South Korea S Supporting Information *

ABSTRACT: A major limitation of polyaniline (PANi) electrodes for supercapacitors is the slow rate of ion transport during redox reactions and the resultant easy saturation of areal capacitance with film thickness. In this study, threedimensionally (3D)-arrayed PANi nanospheres with highly roughened surface nanomorphology were fabricated to overcome this limitation. A hierarchical nanostructure was obtained by polymerizing aniline monomers on a template of 3D-arrayed polystyrene (PS) nanospheres and appropriate oxidative acid doping. The structure provided dramatically increased surface area and porosity that led to the efficient diffusion of ions. Thus, the specific capacitance (Csp) reached 1570 F g−1, thereby approaching a theoretical capacitance of PANi. In addition, the retention at a high scan rate of 100 mV s−1 was 77.6% of the Csp at a scan rate of 10 mV s−1. Furthermore, 3D-arrayed hollow PANi (H-PANi) nanospheres could be obtained by dissolving the inner PS part of the PS/PANi core/shell nanospheres with tetrahydrofuran. The ruthenium oxide (RuO2) nanoparticles (NPs) were also encaged in the H-PANi nanospheres by embedding RuO2 NPs on the PS nanospheres prior to polymerization of PANi. The combination of the two active electrode materials indicated synergetic effects. The areal capacitance of the RuO2-encaged PANi electrode was significantly larger than that of the RuO2-free PANi electrode and could be controlled by varying the amount of encaged RuO2 nanoparticles. The encagement could also solve the problem of detachment of RuO2 electrodes from the current collector. The effects of the nanostructuring and RuO2 encagement were also quantitatively analyzed by deconvoluting the total capacitance into the surface capacitive and insertion elements. KEYWORDS: supercapacitors, polyaniline, hollow nanospheres, RuO2 nanoparticles, encagement, capacitive elements

1. INTRODUCTION Recently, pseudocapacitors using fast Faradaic charge-transfer reactions have attracted growing attention due to their superior capacitive performances when compared with other types of supercapacitors such as electrochemical double-layer capacitors (EDLCs).1−3 Typical electrode materials used for pseudocapacitors include conductive polymers (CPs) and transition metal oxides (TMOs).4−6 Among CPs, polyaniline (PANi) is the most widely studied because of its high theoretical specific capacitance (Csp) given multiple redox states and inexpensive monomers.7−9 However, the major limitation of PANi involves the slow rate of ion transport during the redox reactions,7,10 which leads to the saturation of areal capacitance of the PANi electrode with increase in the film thickness, because it is difficult for the bottom part of the thick film to contribute to the capacitive performance. To overcome this limitation, a variety of PANi nanostructures such as nanowires,9,11−14 nanofibers,15−17 nanocones,18 and nanocomposites with graphene oxide11,16−21 have been prepared and studied. Enhanced surface area of the nanostructures and consequently improved ion transport between the electrolyte and electrode increase the capacitive properties of the PANi-based pseudocapacitors. However, the aggregation and consequent © XXXX American Chemical Society

nonhomogeneity of the nanostructures often hamper the optimal use of the nanostructures and precise control of the capacitance. Therefore, the reported Csp values in the range 406−1126 F g−1 are considerably below the theoretical PANi value of 2000 F g−1.9 Although a few studies have reported Csp values even larger than 3400 F g−1, which are higher than the theoretical value,22,23 these high values contradict the charge storage stoichiometry of PANi.24 This could potentially be due to a misreading of extremely small amounts of the PANi (0.1−5 μg) used.22,23 Thus, this study introduced a new method to fabricate threedimensionally (3D)-arrayed hierarchical hollow PANi (HPANi) nanospheres to maximize the benefits of the nanostructures and enhance capacitive properties of the PANi electrode. The hierarchical structure consisted of hollow nanospheres with a scale of several hundred nanometers and rambutan-like nanofibers on the surface of the nanospheres within a scale of few nanometers. The thin-film PANi hollowsphere structure has recently attracted growing attention due to Received: November 9, 2016 Accepted: February 7, 2017 Published: February 7, 2017 A

DOI: 10.1021/acsami.6b14331 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic illustration of the fabrication of the 3D-arrayed PS/PANi, H-PANi, PS/RuO2/PANi, and RuO2/H-PANi electrodes.

2. EXPERIMENTAL SECTION

its large specific area, enhanced permeation characteristics, and good electrical conductivity.11,25,26 Composites of PANi hollow spheres with graphenes and/or graphene oxides have also been investigated in order to improve electrical, mechanical, and capacitive properties of the electrodes.27−30 However, study on encaging metal oxide nanoparticles in the PANi hollows to pursue synergetic improvement by combining two different types of pseudocapacitive materials has not been reported to the best of our knowledge. The PANi hollow nanospheres in this work were obtained by polymerizing aniline monomers on a template of 3D-arrayed polystyrene (PS) nanospheres and subsequently employing PS extraction, as shown in the upper part of Figure 1. The 3D-arrayed PANi nanostructure showed considerable improvements in supercapacitive properties such as significant increase in Csp and rapid voltammetric response. Another advantage of the H-PANi nanostructure is the additional enhancement of the areal capacitance obtained by encaging TMO nanoparticles in the hollow. In this study, ruthenium oxide (RuO2) was selected because of advantages such as high reported capacitance reported (900−1400 F g−1), excellent proton transfer characteristics, and superior reversible redox transition.31−33 Despite these advantages, RuO2 is not suitable for commercially viable supercapacitors due to its high cost. In addition, the RuO2 film easily detaches from current collectors and only a very thin surface layer can participate in the charge storage process because of its dense morphology.34,35 To solve these problems, RuO2 nanoparticles were synthesized and encaged in the hollows of the PANi nanospheres. The areal capacitance of this RuO2/H-PANi electrode gradually increased with the amount of the RuO2 nanoparticles encaged. This implies that supercapacitors with controllable areal capacitance can be successfully fabricated with consideration of both performance and cost. In addition to the capacitance enhancement, the encaged RuO2 nanoparticles in the hollow could be barely removed and hence led to an improvement in the cycle stability. Further quantitative analyses on the enhancement of capacitive properties were performed by deconvoluting the total capacitance into the surface capacitive and insertion elements.

2.1. Fabrication of 3D-Arrayed H-PANi Nanospheres. The upper part of Figure 1 illustrates the process used in the study to fabricate 3D-arrayed H-PANi nanospheres. First, 2D close-packed PS nanospheres with a diameter of 580 nm (Interfacial Dynamics Co.) were transferred onto a thoroughly cleaned ITO-coated glass substrate using the scooping transfer technique.36 The fabrication of 3D-arrayed PS nanospheres was performed layer-by-layer by repeating the scooping transfer process. The PS-arrayed substrate was then placed in a solution of 0.2 g of aniline (Sigma-Aldrich, 99%) dispersed in 8.0 mL of distilled water, and sonicated for 1 h. Another solution with 0.48 g of ammoniumpersulfate (APS) (Sigma-Aldrich) and 40 mL of various concentrations of doping acid such as hydrochloric acid (Daejung, 35 wt %) and perchloric acid (Daejung, 70 wt %) was poured into the above PS/aniline solution and stirred at 0 °C for 24 h. The fabricated PS/PANi core−shell nanospheres were thoroughly rinsed with distilled water several times and dried at 60 °C for 2 h. The fabricated PS/PANi sample was put into THF (Daejung, 99%) solution and stirred at ambient temperature for 5 h to remove PS inside and hence obtain H-PANi nanospheres. For comparison, a planar PANi film electrode was also prepared by a doctor blade coating method. The slurry composed of 1.0 g of PANi and 1 mL of Nafion (1 wt % in isopropyl alcohol, Aldrich) was loaded in an applicator (Lab-Q P100). It was cast onto the ITO-coated glass substrate, followed by drying under vacuum at 120 °C for 24 h. 2.2. Encaging RuO2 Nanoparticles into the H-PANi Nanospheres. Hydrous ruthenium oxide (RuO2·xH2O) nanoparticles were synthesized by slowly adding 0.1 M sodium bicarbonate solution into 300 mL of 8 × 10−3 M aqueous RuCl3·xH2O (Sigma-Aldrich Regent Plus) solution using a syringe pump until the solution pH reached 5, followed by additional stirring for 24 h. The synthesized RuO2 nanoparticles were separated using a centrifuge, rinsed with distilled water three times, and vacuum-dried for 24 h. The crystalline structure of the synthesized nanoparticles were examined by X-ray diffraction (XRD, Philips PW1827). The XRD pattern is shown in Supporting Information Figure S1, which is in good agreement with the previously reported disordered structure of RuO2·xH2O.35 The lower part of Figure 1 shows a process of encaging RuO2 nanoparticles into the hollow of PANi nanospheres. First, a 30-μL solution with various concentrations of RuO2 nanoparticles dispersed in ethanol was spincoated on the arrayed PS nanosphere layer twice, followed by heating the layer to 100 °C to stick the nanoparticles to the PS nanospheres. The cycle of the PS transfer and RuO2 fixation was repeated to obtain 3D-arrayed RuO2-incorporated PS multilayers. Aniline polymerization and subsequent PS extraction were performed using the same procedure described in the previous section and yielded 3D-arrayed B

DOI: 10.1021/acsami.6b14331 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. FE-SEM images of periodically arrayed 1Lyr PS nanospheres (a and b) and PANi polymerized on these nanospheres with HClO4 doping at concentrations of (c) 0.2 M, (d) 0.5 M, (e) 1.0 M, and (f) 1.5 M. H-PANi nanospheres encaging RuO2 nanoparticles. For comparison, a planar RuO2 film electrode was also prepared by electrodeposition technique. The electrodeposition was performed in a three-electrode electrochemical cell in which RuO2-deposited ITO substrate was used as a working electrode, a platinum plate was used a counter electrode, and Ag/AgCl (in 3.0 M KCl) served as a reference electrode. A cyclic voltammetric technique was used to deposit RuO2·xH2O nanoparticles onto the ITO substrate by repeating the 200 cycles of potential between −0.2 and 1.2 V in 0.01 M aqueous RuCl3·xH2O solution at a scan rate of 30 mV s−1. 2.3. Characterization. The surface morphology of the fabricated electrodes was characterized by field emission scanning electron microscopy (FE-SEM, JEOL JSM-7410F, JEOL Ltd.). A UV−vis spectrophotometer (S-3000, Scinco) and a four-point probe (CMT-SR 2000N) were used to measure the optical transmittance and sheet resistance of the electrodes, respectively. The electrochemical properties were evaluated by cyclic voltammetry and galvanostatic charge/discharge (GCD) in 1.0 M aqueous HClO4 solution at room temperature using a cyclic voltammeter (ZIVE SP2, WonATech). The specific surface area and crystalline structure of the synthesized PANi were estimated by Brunauer−Emmett−Teller (BET) surface area analyzer (QuadraSorb Station 2, Quadrachrome Instrument) and XRD (Philips PW1827), respectively.

roughens the surface and enhances the electrical conductivity of the PANi film.37,38 As shown in Figure 2c−2f, the surfaces of PANi layers fabricated on the PS nanospheres gradually roughened as the concentration of aqueous perchloric acid (HClO4) increased. The surface roughening was not apparent when a low concentration of 0.2 M HClO4 (aq) was used (Figure 2c). However, highly roughened surface and rambutanlike morphology were clearly observed for the cases with acid concentration exceeding 1.0 M (Figure 2e and 2f). The same tendency in the morphology change of the PS/PANi surface was observed when a different doping acid, aqueous hydrochloric acid (HCl), was used (Figure S2). The electrical conductivity and electronic absorption property of the PANi film were strongly dependent on the type and concentration of the doping acids. Figure 3a shows sheet resistances for the HClO4- and HCl-doped 1Lyr PS/PANi films at various doping concentrations. Whereas the resistance of the HClO4-doped film at very low (0.1 M) and high (1.5 M) concentrations exceeded that of the HCl-doped film, it was smaller at middle concentrations (0.5 and 1.0 M). The smallest resistance corresponding to 3.1 kΩ □−1, was achieved for the 1.0 M HClO4-doped film, and this was ∼10 times smaller than the value for the HCl-doped film, 32.8 kΩ □−1, as shown in the inset of Figure 3a. The relatively smaller resistance of the HClO4-doped film compared to that of the HCl-doped film implies that a larger amount of conductive PANi emeraldine salts existed in the HClO4-doped film. This is consistent with

3. RESULTS AND DISCUSSION Figure 2a and 2b show the top-view and side-view FE-SEM images of one layer (1Lyr) of periodically arrayed PS nanospheres. Previous studies indicate that appropriate acid doping during the polymerization of aniline monomers C

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Figure 3. (a) Sheet resistances as a function of the concentration of the doping acids, (b) electronic absorption spectra, (c) X-ray diffraction pattern, and (d) cyclic voltammograms of the 1Lyr PS/PANi electrodes doped with 1.0 M aqueous HClO4 (blue) or HCl (red).

Figure 4. Surface FE-SEM images of 3Lyr (a) PS, (b) PS/PANi, and (c) H-PANi electrodes, and 7Lyr (d) PS, (e) PS/PANi, and (f) H-PANi electrodes. The insets indicate cross-sectional FE-SEM images of the electrodes.

pattern of the HClO4-doped film reveals four peaks centered at 2θ = 10°, 16°, 20°, and 25° as shown in Figure 3c. These peaks are consistent with the characteristic peaks of PANi emeraldine salt in a pseudo-orthorhombic unit cell.41 As shown in Figure 3d, the areal capacitance of the 1.0 M HClO4-doped 1Lyr PS/ PANi film was also superior to that of the 1.0 M HCl-doped

the electronic absorption measurements shown in Figure 3b. The intensities of the absorption bands at ∼400 and 800 nm for the HClO4-doped film were significantly larger than that for the HCl-doped film. These two bands are indicative of the emeraldine salt form23,39 and are respectively attributed to a polaron−π* and π−polaron transition.40 The X-ray diffraction D

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Figure 5. (a)Deposit weight of PS/PANi as a function of the number of layers. Cyclic voltammograms of the (b) PS/PANi and (c) H-PANi electrodes given various numbers of layers. The scan rate is fixed at 10 mV s−1. Plots of the (d) areal and (e) specific capacitances for the PS/PANi (black) and H-PANi (red) electrodes as a function of the number of layers. (f) Galvanostatic charge/discharge curves of H-PANi electrodes with various numbers of layers at a current density of 5 mA cm−2.

film. The results indicated that the doping with 1.0 M HClO4 was the most effective on an overall basis, and hence, this doping condition was adopted for the fabrication of all the PANi films in this study unless stated otherwise. A similar evolution of surface morphology was observed for the fabrication of multilayered PANi nanospheres. Figure 4 shows representative surface FE-SEM images of 3D-arrayed three layer (3Lyr) and seven layer (7Lyr) nanostructures. They correspond to (a) 3Lyr PS, (b) 3Lyr PS/PANi, (c) 3Lyr HPANi, (d) 7Lyr PS, (e) 7Lyr PS/PANi, and (f) 7Lyr H-PANi nanostructures. Hierarchical nanostructures comprising rambutan-like surface nanowires formed on periodically arrayed nanospheres were clearly observed for the 3Lyr and 7Lyr PS/ PANi (Figure 4b and 4e, respectively). During the PS extraction by THF, the PANi surface appeared to slightly dissolve, and hence, the boundary between the nanospheres of the H-PANi became less apparent (Figure 4c and 4f) compared with those of the PS/PANi. The surface FE-SEM images for five layer (5Lyr) and nine layer (9Lyr) nanostructures also show similar morphologies (Figure S3). The cross-sectional FESEM images shown in the insets of Figure 4 and Figure S4 indicate that 3D-arrayed nanostructures can be successfully fabricated up to at least nine layers without any structural collapse. The cross-sectional FE-SEM images for the H-PANi electrodes also show that the inner PS nanospheres were clearly removed. The enlarged cross-sectional images are presented in Figure S4. The deposit weight of the PS/PANi layers as measured by quartz crystal microbalance (QCM) was almost linearly proportional to the number of layers as shown in Figure 5a, and this is consistent with the successful fabrication of multilayer nanostructures in the FE-SEM images. The average deposit weight of PANi was calculated as 19.0 μg cm−2 per layer by subtracting the weight of PS spheres from the PS/

PANi weight. Little deviation from the linear increase with the electrode thickness and significantly larger amounts of deposit weight up to ∼190 μg (7 layers and a substrate area of 1.43 cm2) compared with the previously reported weights of PANi nanostructure electrodes that ranged 0.1−5 μg22,23 indicate that the following measurements on the mass-based electrochemical properties such as specific capacitances are fairly reliable. Cyclovoltammetric (CV) measurements for half-cell supercapacitors with the PS/PANi and H-PANi electrodes given various numbers of layers were performed in a 1.0 M HClO4 aqueous solution at a potential range from 0.0 to 0.7 V. The results are shown in Figure 5b and 5c, respectively. The scan rate for all the measurements was fixed at 10 mV s−1. The areal capacitances of the PS/PANi and H-PANi electrodes were calculated by the following equation and plotted as a function of the number of layers (Figure 5d): Careal =

∫ J dV ΔV (dV /dt )

(1)

−2

where J (mA cm ) is the current density in the CV curve, ΔV (V) is the potential window, and dV/dt (mV s−1) is the scan rate. The capacitances for both electrodes increased almost linearly with the number of layers, implying that this technology could be applied to exact control of the device capacitance with ease. As shown in Figure 5e, the specific capacitance, Csp, values were also calculated by the following equation and plotted as a function of the number of layers:

Csp =

Careal m

(2)

where m (g cm−2) is the mass of the deposited electrode materials on the unit area. The Csp value was largest for the E

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Figure 6. Cyclic voltammograms of the 3Lyr (a) PS/PANi and (b) H-PANi electrodes at various scan rates. Plots of the (c) specific capacitances (Csp) of the PS/PANi (black), H-PANi (red), and PANi film (green) electrode as a function of the scan rate, (d) galvanostatic charge/discharge curves of 3Lyr H-PANi electrode measured at various current densities, (e) Nyquist plot for the 3Lyr H-PANi electrode, and (f) Csp retention for the 3Lyr H-PANi electrode as a function of the number of galvanostatic charge/discharge cycles at a current density of 10 mA cm−2.

1Lyr film, which was 1570 F g−1 and 1426 F g−1 for the PS/ PANi and H-PANi electrodes, respectively. These, to the best of our knowledge, correspond to the largest specific capacitance value of PANi nanostructure-based electrodes and approach the theoretical value of 2000 F g−1.9 Generally, the specific capacitance of the pseudocapacitor electrodes gradually decreases as the electrode thickness increases because the distant part of the electrode from the electrolyte becomes difficult to contribute to the capacitance. However, the specific

capacitances of the 3D-nanostructured PANi electrodes in this study did not change significantly with the electrode thickness (i.e., number of layers), which also indicates that the nanostructures were effective even for the 7Lyr-thick electrodes. The Csp values of the 7Lyr PS/PANi and H-PANi electrodes were 1454 F g−1 and 1288 F g−1, respectively, which were 92.6% and 90.3% of the values of the 1 Lyr electrodes, indicating that the nanostructures were successfully fabricated and almost the entire PANi contributed to the capacitance of F

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Figure 7. Surface FE-SEM images of the 3Lyr (a) PS/RuO2, (b) PS/RuO2/PANi, and (c) RuO2/H-PANi electrodes. (d) Surface SEM-mapping image for Ru atoms of the 3Lyr RuO2/H-PANi electrode. The concentration of RuO2 nanoparticles used was 2.0 wt % in ethanol. (e) Plots of the areal capacitance for the electrodes as a function of the scan rate. (f) Capacitance retention for the RuO2/H-PANi (blue) and RuO2 film (pink) electrodes as a function of the number of galvanostatic charge/discharge cycles at a current density of 10 mA cm−2.

already participated in the electrochemical process even prior to the PS extraction because the thickness of the PANi shell was significantly small. Simple calculations using the nanosphere size and the deposit weight and density of PANi estimated the average thickness of the PANi shell as 44 nm, which is also confirmed by the estimation from the cross-sectional FE-SEM image (Figure S5). The areal capacitances of the H-PANi electrodes for various numbers of layers were also examined by GCD cycles at a current density of 5 mA cm−2 (Figure 5f). As

the electrodes. Interestingly, the capacitances of the PS/PANi electrode slightly exceeded those of the H-PANi electrode for every film thickness. This differs from expectations involving superior capacitances of the hollow structures because of the additionally exposed inner surfaces of PANi nanospheres. This discrepancy could potentially be because the benefits of the increase in the contact area were offset by the deterioration of the surface nanomorphology during the PS extraction, as shown in Figure 4. In addition, the inner PANi molecules could have G

DOI: 10.1021/acsami.6b14331 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces shown in the inset of the figure, an almost linearly increasing tendency of the capacitance is observed again, although the capacitance values slightly exceeded those obtained by the CV measurements. Figure 6a and 6b show the CV curves at various scan rates ranging from 10 to 100 mV s−1 for the 3Lyr PS/PANi and HPANi electrodes, respectively. Although the shape and area of the CV contours for both electrodes were similar at low scan rates, the area of the H-PANi electrode apparently exceeded that of the PS/PANi electrode at high scan rates such as 50 and 100 mV s−1. This could be because the advantages of the additional surfaces formed by the PS extraction are more effective at high scan rates. Figure 6c shows the Csp of the 3Lyr PS/PANi (black squares) and H-PANi (red circles) electrodes at various scan rates. Whereas the largest Csp value of 1444 F g−1 at 10 mV s−1 for the PS/PANi electrode dropped to 855 F g−1 at 100 mV s−1, the Csp value for the H-PANi electrode decreased from 1304 to 890 F g−1 at the same scan rate conditions. The large voltammetric response retentions (59.2% and 77.6% for the PS/PANi and H-PANi electrodes, respectively) could potentially be attributed to the efficient access of electrons and ions through the maximized surface active sites of the hierarchical 3D nanostructures. This was more effective in the case of the H-PANi electrode because the hollow structure led to a more rapid and effective contact between the inner part PANi and the electrolyte. The Csp values at a scan rate of 100 mV s−1 were 890 and 855 F g−1, which are significantly larger than the values reported by extant research (92−441 F g−1) for PANi-based supercapacitor electrodes at scan rates ≥100 mV s−1.9,19,42−45 This implies that the nanostructured electrode obtained in the present study could be a plausible candidate for manufacturing rapidly responsive supercapacitors even at high charge/discharge rates. To clarify the advantages of the PANi nanostructures, a planar PANi film electrode was also fabricated and its specific capacitances were measured at various scan rates as shown by the green triangles in Figure 6c. A maximum specific capacitance of 278 F g−1 was obtained at a scan rate of 10 mV s−1. However, as the scan rate increased, the Csp value decreased rapidly to 21.3 F g−1 at 100 mV s−1. This small Csp retention at high scan rates can be attributed to significant decreases in electrochemical accessibility in the lower part of the planar film compared with that of 3D-arrayed nanostructures. The rapid voltammetric response of the 3D-nanostructured electrode was also observed by the GCD measurements. Figure 6d shows the GCD curves of the 3Lyr H-PANi electrode measured at various current densities from 0.3 to 3.0 mA cm−2. The curves are nearly triangular and the capacitance retention at the scan rate of 3.0 mA cm−2 is 88.0% of the value at 0.3 mA cm−2, indicating that the electrode is rapidly responsive to the charge/discharge process. The GCD results of the 3Lyr PS/PANi and 2.0 wt %-RuO2/HPANi electrodes are also presented in the Figure S7. The superior capacitance and voltammetric response of the 3Dnanostructured PANi electrodes were also observed when the electrodes were applied to the full-cell device. Figure S8 shows the GCD curves of the asymmetric full-cell supercapacitor unit consisting of two half-cells based on the 3Lyr H-PANi and activated carbon electrodes. As shown in the inset of the figure, the capacitance was hardly affected by the current density. The superior voltammetric response of the nanostructured electrode is probably attributed to the efficient access of ions to the electrode surface. A Nyquist plot of the 3Lyr H-PANi electrode obtained in the frequency range from 10−2 to 105 Hz is shown

in Figure 6e. The Z′ and Z″ are the real and imaginary part of the impedance, respectively. The series resistance, Rs, and charge transfer resistance, Rct, obtained from the x intercept and diameter of the semicircle in the high-frequency region were estimated to be 3.3 and 4.8 Ω, respectively. The smaller Rct value compared to that of planar PANi films reported46−48 indicates the superior charge-transfer characteristics of the nanostructured electrode. The steep line in the low-frequency region indicates small diffusive resistance, Rd. The long-term stability of the nanostructured 3Lyr H-PANi electrode was also examined by continuous GCD cycles at a current density of 10 mA cm−2 (Figure 6f). The capacitance of the H-PANi electrode retained 76.2% of the initial value even after 5000 cycles, indicating that the nanostructured PANi electrode possessed long-term electrochemical stability and could firmly bind to the substrate surface. Another important advantage of the hollow structured electrode was the capability to encage other materials such as RuO2 nanoparticles into the hollows and consequently modify capacitive properties. The bottom part of Figure 1 shows the basic strategy to encage nanoparticles in the hollows. Figure 7a shows the FE-SEM images of RuO2 nanoparticles that were stuck to the surface of the 3Lyr PS nanospheres. The aniline polymerization on these PS/RuO2 nanospheres and subsequent extraction of the PS by THF resulted in 3Lyr PS/RuO2/PANi and RuO2/H-PANi nanostructured electrodes, respectively. The surface morphologies of the PS/RuO2/PANi (Figure 7b) and RuO2/H-PANi (Figure 7c) electrodes were less distinct than those of the RuO2-free nanostructures (Figure 4b and 4c); this is probably because the precise surface periodicity of the arrayed PS nanosphere layer was partly lost due to the RuO2 nanoparticles attached on the surface. The SEM mapping for Ru atoms (Figure 7d) also confirmed the encagement of the RuO2 nanoparticles over the entire substrate surface. The areal capacitance of the electrode increased with the amount of RuO2 nanoparticles encaged across all scan rates, as shown in Figure 7c. This is obviously attributed to the additional capacitance of the RuO2 nanoparticles, which was also confirmed by the gradual change of CV contours from characteristic PANi shape to rectangular shape as the amount of RuO2 nanoparticles encaged increased (Figure S9). The increment in the areal capacitance was less apparent at a higher scan rate as shown in Figure 7e. As a result, increases in the amount of RuO2 encaged led to decreased retention at high scan rates. The retentions at 100 mV s−1 for the 0.5 wt %-, 1.0 wt %-, and 2.0 wt %-RuO2/HPANi electrodes were 63.2%, 49.4%, and 53.7% of the capacitance at 10 mV s−1, respectively. These values are smaller than that of the RuO2-free H-PANi electrode, 77.6%. This is probably because a certain amount of time was involved in the transfer of charge carriers between the current collector and RuO2 nanoparticles lying in the hollow. In addition to the control of areal capacitance, encaging the nanoparticles could provide a solution to the detachment problem of RuO2 films. As shown in Figure 7f, the capacitance of the RuO2-film-based electrode (pink squares) decreased rapidly at the early stage of the repeating potential sweep, which is attributed to the easy detachment of the RuO2 film from the current collector. Although polymeric binders such as polytetrafluoroethylene (PTFE),49,50 polyvinylidene fluoride (PVDF),31,51,52 and Nafion53,54 were often used to tightly bind RuO2 electrodes on the current collectors, the addition of the binders resulted in the inevitable deterioration of device properties such as specific capacitance and electrical conductivity. In contrast, the H

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ACS Applied Materials & Interfaces

Figure 8. Deconvolution of the total capacitance into the surface pseudocapacity (shaded part) and insertion (solid part) elements for the RuO2-free H-PANi electrode at scan rates of (a) 10 mV s−1 and (b) 100 mV s−1 and for the 2.0 wt %-RuO2/H-PANi electrode at scan rates of (d) 10 mV s−1 and (e) 100 mV s−1. Bar graphs represent the areal capacitances of the two capacitive elements for the (c) RuO2-free H-PANi and (f) 2.0 wt %-RuO2/H-PANi electrodes at various scan rates.

the 3Lyr H-PANi electrode was 48% at a scan rate of 10 mV s−1 and 23% at a scan rate of 100 mV s−1. These values are slightly lower than the values previously reported for nanostructured electrodes at similar scan rates, i.e. 60% at 5 mV s−1 and 25% at 100 mV s−1 for mesoporous MnO2 nanowires59 and 66% at 20 mV s−1 and 55% at 100 mV s−1 for Mn3O4 nanoparticles.57 The relatively smaller insertion elements of the H-PANi electrodes obtained in the present study indicate that the surface capacitive elements were more dominant in the total capacitance, and hence, the nanostructures were effectively well fabricated. In contrast, in the case of the RuO2-encaged HPANi electrodes, the contribution of the insertion element became considerably larger, as shown in Figure 8d−8f. The contributions of the surface capacitive and insertion elements for the 3Lyr 2.0 wt %-RuO2/H-PANi electrode are indicated by the green shaded and solid parts, respectively. The contribution of the surface capacitive element (green shaded part) was 72.2 F cm−2, and it was also independent of the scan rate. This value is 1.3 times larger than the value for the equivalent RuO2-free H-PANi electrode. Moreover, the increase in the insertion element due to encagement of RuO2 nanoparticles was significantly greater. The contributions of the insertion element (solid green part) were 155.5 F cm−2 at 10 mV s−1 and 50.0 F cm−2 at 100 mV s−1, which are approximately three times that of the corresponding values obtained for the RuO2-free HPANi electrode. As a result, the contribution of the insertion element in the total capacitance of the 3Lyr 2.0 wt %-RuO2/HPANi electrode was 68.4% and 40.9% at scan rates of 10 mV s−1 and 100 mV s−1, respectively. Figure S10 presents additional deconvolution data for the electrodes at other scan rates. The results indicate that the capacitive process of the RuO2 nanoparticles inside the PANi hollow was mainly diffusion-

capacitance retention of the 1.0 wt %-RuO2/H-PANi electrode (blue circles) was 90.8% of the initial value even after 5000 cycles, indicating that the RuO2 nanoparticles were well preserved in the PANi hollows. To quantitatively estimate the effect of the 3D hollow nanostructures and RuO2 encagement, a separation of two capacitive elements was performed using the deconvolution method proposed by Conway55 and Dunn.56 The deconvolution indicated that the current at a given voltage can be divided into two elements, namely, the surface capacitive element (k1v) and Faradaic insertion element (k2v1/2) by using the following equation: i(V ) = k1v + k 2v1/2

(3)

where i(V) is the current, v is the scan rate, and k1 and k2 are constants. The details of the process to extract the elements are described elsewhere.57 Representative k1v plots for the 3Lyr HPANi electrode at low (10 mV s−1) and high (100 mV s−1) scan rates are represented by shaded areas in Figure 8a and 8b, respectively. The other part of the total CV graph (red solid part) corresponds to the Faradaic insertion element. The contribution of each of the two elements at various scan rates was calculated and is represented in Figure 8c. The surface capacitive element (red shaded part) denotes the sum of double-layer charging and surface Faradaic pseudocapacity and was 55.4 F cm−2. This value was virtually independent of the scan rate.58 In contrast, the contribution of the Faradaic insertion capacity (red solid part) gradually decreased with the scan rate, increasing from 51.0 F cm−2 at 10 mV s−1 to 16.4 F cm−2 at 100 mV s−1. This decrease is potentially because the diffusion-controlled insertion was less accessible at higher scan rates, and this is consistent with previous reports.57−59 The contribution of the insertion element in the total capacitance of I

DOI: 10.1021/acsami.6b14331 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces limited and contributed more toward the insertion capacity than to the surface capacitive element. Large-scale fabrication of 3D-arrayed PS/PANi and H-PANi electrodes on various current collector substrates was also possible. The 3Lyr HPANi electrodes fabricated on a silicon wafer with a diameter of 2 in. and an ITO substrate (2.5 cm × 2.5 cm) are shown in Figure S11. The apparent iridescence of the surfaces is due to the photonic crystal effect and indicative of the well-arrayed nanostructures. This implies that the long-range order of PANi nanostructures was successfully fabricated and that the new technique proposed in the present study could be applied to manufacturing large-area supercapacitor electrodes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 82-2-910-4734.

4. CONCLUSION The fabrication of 3D-arrayed PANi nanospheres and their surface roughening were successfully carried out by polymerizing aniline monomers on the surface of layer-by-layer stacked PS 2D arrays and subsequent doping of acid oxidants. The areal capacitance of this 3D-arrayed PS/PANi nanosphere electrode increased almost linearly with the number of layers, indicating that the merits of nanostructures, i.e., large contact area and short diffusion path for charge carriers, were effectively retained at least up to the thickness we investigated in this work, ∼4 μm. The maximum specific capacitance, Csp, of 1570 F g−1 was obtained for the 1Lyr PS/PANi electrode at a scan rate of 10 mV s−1. The Csp value little changed with the electrode thickness, which also indicates the successful formation of 3D nanostructures. By dissolving inner PS spheres, 3D-arrayed PANi hollow nanospheres could be obtained. Despite additionally exposed inner surfaces, the capacitance of the electrode slightly decreased, which is because the PANi layer is intrinsically thin enough and the surface nanomorphology is slightly deteriorated during the PS extraction. However, the additional surfaces are more effective at higher scan rates. The voltammetric response retentions at the scan rate of 100 mV s−1 from the Csp values measured at 10 mV s−1 were 59.2% and 77.6% for the 3Lyr PS/PANi and H-PANi electrode, respectively. These retention values are much larger than that for the planar PANi film electrode, 7.7%, indicating that the electrochemical accessibility of the lower part of the 3D nanostructure electrode is much more superior to that of the film electrode. The capacitive properties of the 3D-arrayed HPANi electrode could be further enhanced by encaging RuO2 nanoparticles in the hollows. The areal capacitance increased with the amount of RuO2 encaged, although the synergetic effect became less apparent at higher scan rates. Encaging the nanoparticles in the hollow could also solve the detachment problem of the RuO2 films from the current collector substrates. The effect of the 3D nanostructuring and RuO2 encagement was analyzed by deconvoluting the capacitive elements of the electrodes. The relatively large contribution of the surface capacitive elements for the 3D-arrayed H-PANi electrode implies the successful fabrication of the nanostructures, and increased contribution of the insertion elements for the RuO2-encaged electrode indicates the diffusion-limited characteristics of the RuO2 nanoparticles inside the PANi hollows.

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XRD of RuO2·xH2O nanoparticles, FE-SEM images of 1Lyr PS/PANi electrode at various HCl doping concentrations, surface FE-SEM images of 5Lyr and 9Lyr electrodes, cross-sectional FE-SEM images of electrodes, BET plots of H-PANi nanospheres, GCD curves of PS/PANi and RuO2/H-PANi half-cells and fullcells, CV curves of RuO2/H-PANi electrodes, deconvolution graphs at scan rates of 30 mV s−1 and 50 mV s−1, and photographs of large-scale electrodes (PDF)

ORCID

Sanggyu Yim: 0000-0003-1322-2713 Author Contributions †

H.K. and D.H. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Part & Material Technology Development Program of the Korea Evaluation Institute of Industrial Technology (KEIT) Grant (10046671) funded by the Ministry of Trade, Industry & Energy, and National Research Foundation of Korea (NRF) Grants (2016R1A5A1012966 and NRF-2013R1A1A2A10012336) funded by the Korean Government.

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* Supporting Information S

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