Pure Nanoscale Morphology Effect Enhancing the Energy Storage

Oct 14, 2015 - We report a new synthesis approach for the precise control of wall morphologies of colloidal polypyrrole microparticles (PPyMPs) based ...
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Pure Nanoscale Morphology Effect Enhancing the Energy Storage Characteristics of Processable Hierarchical Polypyrrole Rodtichoti Wannapob,†,‡ Mikhail Yu. Vagin,*,† Itthipon Jeerapan,†,‡ and Wing Cheung Mak*,† †

Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden Department of Chemistry, Faculty of Science, Prince of Songkla University, Hat Yai, Songkla 90112, Thailand

‡

S Supporting Information *

ABSTRACT: We report a new synthesis approach for the precise control of wall morphologies of colloidal polypyrrole microparticles (PPyMPs) based on a timedependent template-assisted polymerization technique. The resulting PPyMPs are water processable, allowing the simple and direct fabrication of multilevel hierarchical PPyMPs films for energy storage via a self-assembly process, whereas convention methods creating hierarchical conducting films based on electrochemical polymerization are complicated and tedious. This approach allows the rational design and fabrication of PPyMPs with well-defined size and tunable wall morphology, while the chemical composition, zeta potential, and microdiameter of the PPyMPs are well characterized. By precisely controlling the wall morphology of the PPyMPs, we observed a pure nanoscale morphological effect of the materials on the energy storage performance. We demonstrated by controlling purely the wall morphology of PPyMPs to around 100 nm (i.e., thinwalled PPyMPs) that the thin-walled PPyMPs exhibit typical supercapacitor characteristics with a significant enhancement of charge storage performance of up to 290% compared to that of thick-walled PPyMPs confirmed by cyclic voltametry, galvanostatic charge−discharge, and electrochemical impedance spectroscopy. We envision that the present design concept could be extended to different conducting polymers as well as other functional organic and inorganic dopants, which provides an innovative model for future study and understanding of the complex physicochemical phenomena of energy-related materials.

1. INTRODUCTION The energy deficiency due to the growing development of a global economy and industry has spurred research on low-cost, environmentally friendly, distributed, and renewable energy resources. A great deal of attention has been focused on the development of low-cost, stable, and efficient electrode materials for energy storage.1−7 The general challenge with these technologies is how to achieve optimized electrode reaction kinetics at multiphase boundaries synchronized with controllable mass and charge transport under the operating conditions of a device. Most of these limiting features are sensitive to the material morphology and might be optimized by the control of nanodimensional structures and porosity. In particular, the enhancement performance of nanostructured and porous materials is observed not only because of the higher surface-to-volume ratios but also because of real size effects reflecting the material property changes.8,9 The development of multilayered nanostructures or, alternatively, 3D architectures built from blocks with nanodimensional features can be a possible way to overcome the drawback of 2D materials as a limitation of the amount of stored or delivered energy.10,11 One of the possible paths toward the 3D hierarchical organization of materials is a combination of a host material with appropriate macrostructure with a guest material micro/nanoscale substructure in order to obtain the advantages of both.12 © XXXX American Chemical Society

Supercapacitors as devices bridging the gap between batteries and capacitors by fast charge and discharge capabilities of intermediate specific energy are considered to have a potential area of application in hybrid and pure electric vehicles.13 As a complement to the electrochemical double-layer supercapacitors operating as a result of the capacitance of the electric double layer, the faradaic supercapacitors (pseudocapacitors) store the charge by means of redox reactions in the bulk material. Both approaches compromise the higher density of stored energy achievable in pseudocapacitors with a faster charge/discharge kinetics of double-layer supercapacitors. Among the variety of materials available for charge storage due to the bulk redox reactions (e.g., metal oxides), conducing polymers combine a variety of unique features such as exceptional electrical properties that cover the whole insulator−semiconductor−metal range, a reversible doping/ dedoping process, controllable physical−chemical properties, mechanical flexibility, possibilities for processing, lightness, biocompatibility, and cost advantages.14,15 To explore the feasibility of conducting polymers used as a guest material with nanoscale substructure, several approaches have been develReceived: September 3, 2015 Revised: October 13, 2015

A

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Langmuir oped for the fabrication of thin films, nanotubes, nanowires, and spherical nanoparticles.16−20 Processable conducting polymers are more important in the fabrication of devices for energy applications, allowing the simple deposition of conducting polymer materials onto various substrates for the fabrication of organic solar cells, supercapacitors, and flexible batteries. However, nowadays the availability of processable conducting polymers is limited, and there is no existing model for controlled synthesis allowing us to study the relationship between the performance of the conducing polymer and its morphology. Colloidal chemistry provides a solution by allowing the fabrication of polymer materials in the form of microparticles, thus opening an easy path toward solution-processable materials. Therefore, it is important to develop new method for the fabrication of processable conducting polymers (i.e., microparticles) and simultaneously allow control of the nanoscale hierarchal morphologies of the resulting microparticles for optimizing performance. The spherical shape of microparticles offers the simplest path toward solution-processable materials. The hollow spherical microparticles possess the advantage of carrying materials inside the cavity, which is valuable for drug delivery in combination with biocompatibility.21,22 Hollow sphere micromorphology is beneficial for energy applications due to the better accessibility of structured material, ensuring faster charge/discharge kinetics and higher capacitance.23−28 The most commonly used method includes the formation of shells of the conducting polymer on template beads, followed by selective dissolution of the template. This approach has been applied for polyaniline,29 polypyrrole (PPy),30−35 and poly(3,4ethylenedioxythiophene).36 Alternatively, hollow microspheres can be fabricated via template-free synthesis based on the selfassembly of monomers in a micellar microenvironment.37−41 In the present study, we report an innovative green chemistry approach to the rational design and fabrication of waterprocessable polypyrrole microparticles (PPyMPs) with tunable nanoscale morphologies based on calcium carbonate (CaCO3) template-assisted polymerization. We demonstrate the systematic control of the structural morphologies of conducting materials using PPy as a model and observe the pure nanoscale morphological effect of zero-dimensional conducting materials influencing the energy storage behavior. This new fabrication method and design strategy has the following unique advantages: (i) it allows the rational design and fabrication of PPyMPs will well-defined size and surface properties; (ii) it is a time-dependent template-directed synthesis for precise control of the nanoscale morphologies of the PPyMPs (i.e., wall thickness); (iii) it is a facile solvent- and surfactant-free bulkphase synthetic process; and (iv) the water-processable PPyMP building blocks allow the creation of a hierarchically structured PPyMP layer via a self-assembly process. Our developed technology enables us to design and fabricate processable zerodimensional PPyMPs with tunable nanoachitectures as building blocks, which is a significant element for the future design of high-level hierarchically organized materials for improving the performance of electrodes for energy storage.

Sigma−Aldrich and purified before use by passing through a neutral column of alumina (0.05 μm) to obtain a colorless liquid. Water was purified using a Milli-Q water purification system. 2.2. Synthesis of PPy Microparticles. The polypyrrole microparticles (PPyMP) with tunable morphology were prepared with timedependent template polymerization. In brief, 1 mL of CaCl2 (0.5 M) and 1 mL of Na2CO3 (0.5 M) were rapidly mixed at 600 rpm and stirred for 30 s to produce calcium carbonate (CaCO3) microparticles as a removable template. The CaCO3 microparticles were sequentially washed twice with water, ethanol, and 2-propanol by centrifugation (2000 rpm, 1 min) and redispersion cycles. The purified pyrrole as a monomer was loaded by mixing with CaCO3 microparticles followed by incubation for 30 min. The pyrrole-loaded CaCO3 microparticles were centrifuged (2000 rpm, 1 min), and supernatant was discarded to remove the unabsorbed monomer. To carry out the polymerization, the resulting pyrrole-loaded CaCO3 microparticles were mixed with 1 mL of copper(II) perchlorate solution (1 M) and incubated for 1, 3, and 6 h. The PPy-loaded CaCO3 microparticles were then washed twice sequentially with 2-propanol, ethanol, and water by centrifugation (2000 rpm, 1 min) and redispersion cycles. Subsequently, the CaCO3 template was removed by the addition of 1 mL of EDTA solution (0.2 M) and incubated for 1 h at room temperature. Finally, pure PPyMPs were washed twice with water by centrifugation (2000 rpm, 1 min) and redispersion cycles. 2.3. Morphological Characterizations. Optical microscopy images were recorded using a Nikon Eclipse Ti (Nikon, Japan). Images were captured by using an NIS-Elements AR (version 4.1, Nikon, Japan). Scanning electron microscopy (SEM) images of the microparticle film were recorded with PHENOM PRO (FEI, Netherlands). High-resolution SME images of the microparticles were recorded with a LEO 1550 Gemini (Zeiss, Germany). Microparticle samples suspended in Milli-Q water were applied on a flat silicon surface, air-dried at ambient temperature, and coated with platinum. 2.4. Particle Size and Zeta Potentials Analysis. The particle size and zeta potentials (surface charge) of the microparticles were measured using a Zetasizer Nano ZS90 (Malvern Instruments Ltd., Worcestershire, U.K.). Particle size was measured on the basis of the dynamic light scattering technique, measuring the Brownian motion of the particles and converting the data into a size distribution graph using the Stokes−Einstein relationship. The zeta potential is measured on the basis of the laser Doppler microelectrophoresis principle. The electrophoretic mobility (μ) was converted to the zeta potential (ζ) by using the Smoluchowski relation ζ = μη/ε, where η and ε are the viscosity and permittivity of the solution, respectively. One milliliter of suspended microparticles in 1 mM sodium chloride solution was loaded into a zeta potential measuring cell. The measurements were performed at 25 °C, and the mean zeta potential values were calculated by taking an average of three repeated measurements. 2.5. Fourier Transform Infrared Spectroscopy and Spectrophotometry. Fourier transform infrared (FTIR) spectroscopy was performed with VERTEX 70 (Bruker, USA) equipped with a germanium attenuated total reflectance (ATR) sample cell. Microparticle suspensions were dropped onto the surface of the ATR cell, and FTIR spectra were recorded in the frequency region of 800−1800 cm−1 with a resolution of 4 cm−1 and run for 100 cycles. Absorption spectra of microparticles were recorded with a NanoDrop ND-1000 (Thermo) in the range of 250−700 nm. 2.6. Electrochemical Measurements. All electrochemical experiments and simulations were performed with a PGSTAT30 potentiostat (Autolab, Nertherland) under NOVA or GPES software control employing a conventional three-electrode electrochemical cell. A glassy carbon electrode (GCE, 3 mm diameter, surface area 0.0707 cm2) was used as the working electrode, a platinum wire was used as the auxiliary electrode, and a silver/silver chloride was used as the reference electrode (3 M KCl) in aqueous media in all experiments unless otherwise stated. Prior to use the working electrode was successively polished with 1.0, 0.3, and 0.05 μm alumina powders and sonicated in water for 10 min after each polishing step. Finally, the electrode was washed with ethanol and then dried with a high-purity

2. EXPERIMENTAL SECTION 2.1. Materials. Calcium chloride, copper(II) perchlorate, pyrrole, sodium carbonate, ethylenediaminetetracetic acid (EDTA), lithium perchlorate, ethanol, and 2-propanol were purchased from SigmaAldrich and were used as received. Pyrrole (99%) was received from B

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Langmuir argon stream. Electrochemical impedance spectroscopy has been performed with an applied potential frequency in the range 1 Hz-10 kHz with an amplitude of 5 mV. The GCE were modified with PPyMPs by drop-casting of aqueous solutions followed by curing at 60 °C for 30 min. 2.7. Conductivity Measurements. The conductivity of different PPyMPs films was measured using four-point probe dc conductivity analyzer (Keithley 4200 Semiconductor Characterization System, Keithley Instruments, Inc., Cleveland, OH, USA). Thin-, medium-, and thick-hollow PPyMP films with dimensions of 4 mm × 10 mm were prepared by drop casting onto a mask, followed by mask lift off. The average thicknesses of the PPyMP films were measured with a Dektak 6 M stylus profiler (Veeco Instruments, Tucson, AZ, USA).

ization times of 1, 3, and 6 h have positive zeta potential values of 23.4, 20.9, and 18.9 mV, respectively (Figure 2B). The positively surface charge is mainly contributed by the protonated nitrogen atoms in the PPy at pH 7.2.42 The polymerization kinetics of PPyMPs were monitored by spectroscopy. The absorbance at 600 nm corresponding to PPy increased linearly with polymerization time from 1 to 5 h and reached a slight plateau at 6 h (Figure 2C). The possibility to control the polymerization time within the dynamic window of the growth kinetics provided us a simple approach to tailor the nanostructure of the PPyMPs. The morphologies of the PPyMPs were further examined with a high-resolution SEM. The morphology of the thin- and medium-hollow PPyMPs appeared flattened with folds and creases resulted from the collapsed PPyMPs (Figure 1D,E) due to the high-vacuum condition during SEM imaging, while the thick-hollow PPyMP shows an intact spherical shape (Figure 1F). The wall thickness of thin- and thick-hollow PPyMPs can be deduced by measuring the thickness of the folds, which is equivalent to twice the capsule wall thickness (assuming the capsules were fully dried and collapsed under vacuum conditions of SEM). It was calculated that the capsule thicknesses of the thin- and medium-hollow PPyMPs obtained from polymerization times of 1 and 3 h were 116.9. ± 10.1 and 200.4 ± 11.7 nm, respectively (Figure S1). The capsule thickness of the thick-hollow PPyMPs was estimated on the basis of the calculation from the polymerization kinetics curve (Figure 2A) to be 303.05 nm. The SEM images of broken PPyMPs confirmed the hollow interior structure obtained with various polymerization times (Figure 1G,H). The cross-section of the broken hollow shell shows an estimated shell thickness of the thin-, medium-, and thick-hollow PPyMPs obtained from 1 to 6 h of polymerizations, ranging from 100 to 300 nm, which is consistent with the above measured and calculated values. These results demonstrated the possibility to fabricate and tune the morphologies of the PPyMPs with a simple time-dependent interfacial polymerization technique. The evolution of the PPyMPs from thin- to thick-hollow morphologies is controlled by the diffusion of oxidant (copper(II) perchlorate) from the bulk solution, causing polymerization starting from the interface of the CaCO3 template to the deeper region into the interior of the CaCO3 core. Unlike the conventional interfacial polymerization performed in the solution phase, the diffusion barrier created by the CaCO3 template regulating the polymerization process allows high-precision control of the growth and the subsequent thickness and morphology of the PPyMPs. It is important to notice that although the structural morphologies of the PPyMPs were different, the size (i.e., diameter) of the PPyMPs remained similar due to the CaCO3-template-assisted polymerization. This enables us to design and study the effect of nanomorphology and internal structural organization of zerodimensional materials in the electrode, which is important for understanding the complex physicochemical phenomena occurring at the nanoscale level of the material. Moreover, the high-magnification SEM images revealed that the PPyMPs was composed of assembled granular structure with a size of tens of nanometers (Figure 1D,E), likely created by the porous structure of the CaCO3 template that consisted of a vaterite crystal structure having interconnected pores with a uniform size of about 10 to 40 nm42 (Figure S2). During the PPyMP fabrication process, pyrrole monomers were entrapped within the pores of the CaCO3 template, followed by template-

3. RESULTS AND DISCUSSION 3.1. PPy Microparticles with Tunable Nanoarchitecture. PPyMPs with tunable nanoarchitecture from thin- to thick-hollow morphologies were fabricated with a CaCO3 template-assisted time-dependent polymerization technique. The CaCO3 templating method allows the fabrication of polymer microparticles with well-defined sizes as well as controlled internal structures. The fabrication process is performed in aqueous media at room temperature, which provide a green strategy for the development of novel conducting and processable materials. Optical microscopy was used to examine the macroscopic morphologies of PPyMPs obtained at different polymerization times of 1, 3, and 6 h, respectively (Figure 1A−C). It is seen that the whole set of

Figure 1. Optical images (A−C) and SEM images (D−F) of PPyMPs obtains after 1, 3, and 6 h of polymerization. SEM images of broken PPyMPs showing the hollow interior structure of the microparticles with increased wall thickness obtained with various polymerization times (G−I); insets show the corresponding low-magnification images.

PPyMPs has a fairly uniform spherical shape and homogeneous narrow size distribution of average diameter of 2.57 ± 0.45 μm (Figure 2A). PPyMPs obtained from polymerization times of 1 and 3 h had capsulelike (hollow) morphology with a slightly transparent center region, while PPyMP obtained from a polymerization time of 6 h appeared dark in color as a result of the increase in capsule thickness. The increased polymerization time led to a decrease in microparticle transparency, suggesting the evolution of the PPyMP morphologies from thin-hollow to thick-hollow structures. The surface charge of the PPyMPs was characterized with a zeta potential measurement at pH 7.2. The result shows that PPyMPs prepared with different polymerC

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Figure 2. Size distribution curve (A) and zeta potentials (B) of thin-, medium-, and thick-hollow PPyMP. (C) Curve showing the polymerization kinetics of PPyMP measured by spectroscopy at 600 nm. (D) FTIR spectra of PPyMP recorded in the frequency region of 800−1800 cm−1 with a resolution of 4 cm−1, which was run for 100 cycles.

assisted polymerization forming the PPyMPs. The granular structure could further increase the effective surface area of the PPyMPs for ion storage. The molecular structure of the PPyMPs was characterized by FTIR spectroscopy (Figure 2D). The bands observed in the spectrum at 1556.5 and 1303 cm−1 are assigned to CC stretching and C−N stretching vibrations of PPy, respectively. The peaks at 1188 and 916.2 cm−1 reflected the doping state of PPy, while the peaks located at 1286 and 1043 cm−1 are ascribed to the C−N stretching vibration and C−H out-ofplane deformation vibration, respectively. All of these characteristic bands are consistent with the previous reports,43 proving the formation of conducting PPy. Moreover, studies from Geng et al. show that the chemical structure of PPy obtained by chemical polymerization was not changed by extending the polymerization time to 96 h.44 In addition, the chemical polymerization reaction will produce PPyMPs in an oxidized (conductive) form, which is important to creating a conductive hierarchical PPyMP film for energy storage applications. 3.2. Multilevel Hierarchical PPy Structure with SelfAssembled Microparticles. PPyMPs can be drop-cast/ printed on a masked substrate, followed by a mask lift-off process to create PPy films (Figure 3A). SEM images show the (bulk) morphology of the PPy film with a hierarchical organized structure (Figure 3B−D). The macrostructured PPy film created by the three-dimensional organization of the highly homogeneous PPyMPs as building blocks takes intrinsic structural advantage of the PPyMPs in combination with a secondary mesopore (spaces between microparticles) built from the assembly process of the PPyMPs, creating a multilevel hierarchically organized structure. It is suggested that the

Figure 3. (A) PPy film created by drop-casting/printing of PPyMPs onto a masked substrate, followed by a mask lift-off process. (B−D) SEM images showing the (bulk) morphologies of the PPy films with multilevel hierarchical structures created by self-assembled thin-, medium-, and thick-hollow PPyMPs, respectively.

mesopores resulting from the microparticle assemblies allow effective diffusion and facilitate the charge transfer reaction, while the nanoarchitecture on the guest PPyMPs allows high absolute capacities.12 The whole electrochemical process in the hierarchical PPyMPs film involves the Faradaic transfer, which should be accompanied by the ionic contribution combining both the interfacial transfer of the ions in order to retain the electroneutrality of the polymer film and mass transfer by diffusion of ions from the bulk. In general, the rate-determining step is the interfacial transfer of the ions. Therefore, the facilitation of this step by the increase in the accessible area of the hierarchical (mesoporous) polymer| solution interface facilitates the whole electrochemical process. This technique provides a facile approach to fabricating highly homogeneous zero-dimensional PPyMPs with tunable nanoarchitectures as building blocks, which is a critical element in designing high-level hierarchically organized materials for improving the performance of electrodes for energy storage and conversion.12 D

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Langmuir 3.3. Electrochemical Characterization of PPyMP Films. Electrochemical methods have been utilized for the characterization of PPyMP films formed on a GCE surface from the aqueous suspension of thick-hollow PPyMP as a model system. The deposition of material has been carried out via drop casting followed by thermal curing (60 °C for 30 min). The curing process promotes the assembly of PPyMPs with close contacts to ensure the fabrication of an intact hierarchical film structure by the guest microparticle building blocks. The PPyMP filmmodified electrode was used as a working electrode for cyclic voltammetry measurements in aqueous electrolyte (0.05 M LiClO4). No peeling of active material from the electrode substrate was observed during the electrochemical measurement. In spite of the blank electrode, the presence of PPyMPs was attributed with a 2 order of magnitude increase in capacitive currents in comparison to the blank electrode (Figure 4A), which demonstrates the presence of materials of

of the ferro/ferricyanide couple obtained at a PPyMP-modified electrode showed a high permeability of film accompanied by increases in peak currents as well as capacitive currents in comparison with blank GCE. The simulation of the obtained voltammetric responses showed an approximate 20% increase in the effective electrode surface due to the PPy modification while the electron-transfer rate constant (3 × 10−3 cm s−1) and the diffusion coefficients of the ferro/ferricyanide couple (6.3 × 10−6 and 7.6 × 10−6 cm2 s−1, respectively45) remained the same. The conductivities of the samples were measured using the four-point probe dc method. The conductivities of the thin-, medium-, and thick-hollow PPyMPs films were measured to be 1.06, 1.15, and 1.29 S cm−1, respectively. The conductivity of the Cl-doped PPy microparticle film is comparable to the reported value for the electrochemically polymerized Cl-doped PPy film.46 The conductivity of the resulting PPyMPs films shows an increasing trend from thin- to thick-hollow PPyMPs. In general, the conductivity of the polymer is affected by the monomer and oxidant concentration, types of dopants, solvents, and polymerization time.47 In our studies to fabricate PPyMPs with different wall thicknesses, we control only the polymerization time, while the reagent concentrations and reaction conditions remain the same. Therefore, the increases in conductivity of the thick-hollow PPyMPs film could be explained by the longer polymerization time resulting in the formation of a thicker and denser conducting polymer shell.48 3.4. Morphology-Dependent Energy Storage Characteristics. The net effect resulting from the nanoscale morphology and structural organization of the host materials on energy storage characteristics was investigated by comparing the capacitive behavior of hierarchical films composed of host PPyMPs with different morphologies ranging from thin- to medium- to thick-hollow structure while having similar chemical compositions, surface zeta potentials, and physical diameters. In order to investigate the effect of potential on the electrocapacitive behavior of the developed materials, the voltammetric responses were examined (Figure 5A). The scan rate- and mass-normalized cyclic voltammograms obtained at an electrode modified with thin-hollow PPyMPs as the primary guest material showed a box-shape typical of the electrochemical pseudocapacitive response. In contrast, the cyclic voltammogram obtained at an electrode modified with

Figure 4. Electrochemical responses of blank and thick-hollow PPyMP-modified electrodes (black and red curves respectively). (A) Voltammetric response obtained in 0.05 M LiClO4, scan rate 50 mV/s. (B) Cyclic voltammograms obtained in 5 mM Fe(CN)63−/4− in 0.1 M PBS with a pH of 7.4 at a scan rate of 50 mV/s; (---) simulated curves.

high conductivity at the electrode surface. The well-defined transition between a low-conductivity state and highconductivity states of conducting polymer is observed at −0.4 V. The low conductivity of materials at high cathodic potentials led to the disappearance of peak currents of oxygen reduction observed at a blank electrode at potentials around −0.6 V. The permeability of the multilevel hierarchical PPyMP film was investigated by studying the voltammetric response using a well-known standard anionic redox probe−ferro/ferricyanide redox couple, which undergoes reversible monoelectronic innersphere redox processes (Figure 4B). The redox response

Figure 5. Voltammetric characterization of energy storage properties of PPyMPs. Cyclic voltammograms normalized by the scan rate and by the mass of active material (500 mV/s, A) and the dependences of the apparent capacitance on the scan rate (at 300 mV, B) were obtained at electrodes modified with thin-, medium-, and thick-hollow PPyMPs (0.05 M LiClO4). E

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Figure 6. Galvanostatic characterization of energy storage properties of PPyMPs. Galvanostatic charge−discharge curves were obtained in aqueous electrolyte (0.05 M LiClO4) at electrodes modified with thin-, medium-, and thick-hollow PPyMPs (A−C, respectively). (D) Dependence of specific capacitance on a current density obtained at electrodes modified with thin-, medium-, and thick-hollow PPyMPs (black, red, and blue respectively).

Table 1. Specific Capacitances of Ppy-Based Materials Measured in Aqueous Electrolytes electrode material

material structure

electrolyte

thin-hollow particle PPyMP

0.05 M LiClO4(aq)

medium-hollow particle thick-hollow particle

current density, A G−1

specific capacitance, F G−1

0.4 4 0.3 3.4 0.7 2.9

413.8 390.3 209.7 196.1 209.5 132.7 620 480 240 78−137 240 144−200 495 230−290 420

Ppy, theoretical Ppy

Ppy-CNT Ppy-PEDOT Ppy-Fe2O3

electropolymerized film with an average particle size of 50 nm electropolymerized film electropolymerized film electropolymerized film electropolymerized film pressed pellets from chemical polymerization sequentially electropolymerized film chemically polymerized composite

1 M KCl(aq) 0.5 M LiCl(aq) TEAClO4 (EC-PC)a 0.1 M KCl(aq) 1 M KCl(aq) 1 M H2SO4(aq) 1 M KCl(aq) EMITFSIb

a

Tetraethylammonium perchlorate in a mixture of ethylene carbonate and propylene carbonate. ((trifluoromethyl)sulfonyl) imide.

b

references this work

49 50 51 52 53 54 55 56 57

1-Ethyl-3-methylimidazolium bis-

to the medium- and thick-hollow PPyMPs, respectively. The scan rate dependences showed the decreases in the apparent capacitance on the increases in scan rate for all PPyMPs due to the reduced diffusion time at faster potential ramping limiting the accessibility of ions to the active materials. These results clearly demonstrated that the effect of the nanodimensional architecture of the thin-hollow PPyMPs with the decrease in diffusion length and larger accessible surface area facilitates the ions’ insertion processes.

medium- and thick-hollow PPyMPs showed a significant decrease in appearance capacitances. We further compared the kinetics of electrocapactive performance of different PPyMPs at various scan rates and observed a nanodimensional morphology effect of the guest PPyMPs on the capacitive behavior (Figure 5B). The apparent capacitive value of the thin-hollow PPyMPs shows significant increases in the charge storage performance of 6.9 times and 44.8 times (assessed at 5 mV s−1) and significant increases of 17.9 times and 102.2 times (assessed at 500 mV s−1) compared F

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Langmuir The galvanostatic charge−discharge curves (Figure 6A−C) obtained at electrodes modified with PPyMPs showed a typical triangular shape illustrating electrocapacitive behavior of the material. As in voltammetry, nanoscale morphology revealed a significant effect on the galvanostatic assessment of performance. The thin- and medium-hollow PPyMPs showed slower discharge kinetics (i.e., a longer times of charge or discharge) in comparison with a thick-hollow MP of the same microscopic dimensions, charge, and current density. The specific capacitance representing the ion storage ability of the guest PPyMP was calculated from discharge slopes at various current densities according to the following equation Cspec =

I Δt mΔV

(1)

where I is the current, Δt and ΔV are the discharge time and the discharge potential window, respectively, and m is the mass of the material. The dependence of specific capacitance on the discharge current density (Figure 6D and Table 1) shows the achievement of specific capacitance values of 400.3, 205.5, and 209.5 F g−1 for thin-, medium-, and thick-hollow PPyMPs at a similar low current density of 0.7−0.8 A g−1, respectively. A retention of specific capacitances at similar high current density of 3 to 4 A g−1 of 94.3, 93.5, and 63.3% has been observed for thin-, medium-, and thick-hollow PPyMPs, respectively. The high retention of the specific capacitance values at high current densities illustrates the nanoscale morphological advantage of thin- and medium-hollow PPyMPs on the energy storage performance. Comparing further, the specific capacitances of the thin- and medium-hollow PPyMPs show approximately 290- and 150-fold increases in comparison with the thickhollow PPyMP at a current densities of around 3 A g−1 (Figure 6D). The specific capacitance observed for the purely chloridedoped PPyMP is higher than those of most PPy-based electrocapacitive systems (Table 1). The coherence of nanodimensional morphology with the electrocapacitive properties of the PPyMP films has been confirmed with impedance spectroscopy as an independent electrochemical technique. The dramatic increase in chargetransfer resistance as the diameter of a high-frequency arc illustrating the electrode reaction kinetics is seen with the morphology change from thin- to thick-hollow PPyMPs (Figure 7A). The spectrum obtained for thin-hollow PPyMP in the Nyquist plot is characterized with a nearly vertical line in the low-frequency region derived from the ideal polarizable capacitance with a decline from vertical due to the resistive element of nonideal capacitance. Medium- and thick-hollow morphologies are attributed to slower kinetics at high frequencies and less prominent capacitive behavior at low frequencies. In order to characterize the impedance spectra quantitatively, the equivalent circuit analysis of experimental data has been carried out. First, the equivalent circuit proposed for the supercapacitors58,59 (inset II in Figure 7A) has been utilized, where RS is the solution resistance, CDL is the double-layer capacitance, RCT is the charge-transfer resistance as a measure of electrode reaction kinetics, W is the Warburg element corresponding to the diffusion contribution, and CF is the capacitance raised due to the faradaic process. The good matches between fitted and experimental data have been obtained only for electrodes modified with thin- and mediumhollow PPyMPs. The faradaic capacitance (Table S1) assessed by fitting for a thin-hollow PPyMP-modified electrode was 9.8

Figure 7. Impedance analysis of PPyMPs. (A) Nyquist plot of impedance spectra obtained with electrodes modified with thin-, medium-, and thick-hollow PPyMPs (black, red, and blue, respectively). Insets: I, high-frequency domain of impedance spectra; II and III, equivalent circuits. Solid lines represent the fitted spectra. (B) Frequency dependence of the mass-normalized total capacitance (0.05 M LiClO4, 0 V, 5 mV amplitude).

F g−1, which is almost 3 orders of magnitude larger than the double-layer capacitance. The thin-hollow PPyMP-modified electrode had a twice-larger specific capacitance (from 4.4 to 9.8 F g−1) and a smaller double-layer capacitance (from 0.5 to 0.14 μF). A 2-fold decrease in the charge-transfer resistance (from 59 to 23 Ω) accompanied by a 7-fold increase in the Warburg coefficient (from 0.2 to 1.4 mΩ s−1) for the thinhollow material in comparison to the medium-hollow material illustrates faster electrode reaction kinetics and faster diffusion, which are due to better accessibility for ion insertion achieved with PPyMP having thin-hollow morphology. In order to obtain a unified quantitative analysis of impedance data for all developed PPyMP materials, the transmission line model has been utilized for the fitting of experimental data (inset III at Figure 7A), where B represents the Bisquert element60−62 derived from the classical model of the porous/hierarchical electrode (Figure S4). It consists of two parallel rails of repeating elements, which represent the solution and porous material phases along the pore. The rails are repeatedly connected through the elements representing the hierarchical material/solution interface. The resistances for both phases (R1 and R3 in Table S2) and the electrode thickness have been considered to be constants for all three materials. The good matches between fitted and experimental data have been obtained for all PPyMP-modified electrodes. G

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Langmuir The significant decrease in C2 (from 15 to 0.13 μF) assigned to the capacitance inside the hierarchical material due to the faradaic process is observed with a trend from thin- to thickhollow PPyMP. A more than 2 orders of magnitude increase in R3 (from 11 to 481 Ω) assigned to the charge transfer across the polymer/solution interface is obtained with a trend from thin- to thick-hollow PPyMPs, which illustrates the decrease in the electrode reaction rate due to the lack of accessibility of polymer as a pure effect of the nanodimensional morphology of the guest material. The total capacitance analysis of impedance data as an additional approach to investigate the penetration of the alternating current into the hierarchical electrode material illustrates the actual number of solvated ions accessing the hierarchical material/solution interface at a specific frequency (Figure 7B). The low-frequency region is attributed to the increase in specific capacitance due to the involvement of a larger accessible interface available for charge storage. The thinhollow material showed more than a 3-fold larger specific capacitance in comparison to medium- and thick-hollow PPyMP (from 1.46 and 1.92 to 5.75 F g−1, respectively at the 2 Hz frequency), illustrating the larger accessibility of conducting polymer due the larger accessible interface available for the hierarchical material. Nevertheless, the increase in the charge storage capability observed here originates purely from the hierarchical architecture of material controlled by the nanoscale morphology of the guest material. A further advantage demonstrated by the nanoarchitecture is that a significant retention of the charge storage capability at high current densities is attained with the hierarchical PPyMP film fabricated with thin-hollow and thickhollow guest microparticles, contrary to thick-hollow PPyMP. The high surface area offered by the hollow microparticle morphology with a thin nanoscale wall allows effective ion diffusion by increasing the material accessibility via a larger effective surface leading to the faster charge/discharge kinetics. It is important to notice that the effects of dopants such as polyelectrolytes and various organic and inorganic nanoparticles, which make significant contributions to the enhancement of the overall electrocapacitive performance,13 are excluded from this study. This enables us to understand the pure effect of nanoarchitecture and the structural organization of materials influencing the capacitive behavior. Nevertheless, the addition of organic and inorganic dopants as well as nanoparticles and a variety of functionalities as a way toward hybrid materials of advanced characteristics is feasible for CaCO3 template polymerization63 for the potential fabrication of composited PPyMPs.

and electrochemical impedance spectroscopy. As a result of the intrinsic structural advantages, the multilevel hierarchical PPy film showed good storage performance with a high specific capacitance of up to 413.7 F g−1. Our developed technology enables the design and fabricate of processable zero-dimensional PPyMPs with tunable nanoarchitectures as building blocks, which is a critical element for the future design of highlevel hierarchically organized materials for improving the performance of electrodes for eneryg storage. In future work, the synergy effect of combining chemical dopants and physical morphologies toward multifactorial design on composited conducting materials will be investigated, which can provide a deeper understanding of the complex physicochemical phenomena of energy-related material

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03318. Capsule thickness measured by SEM images for different polymerization times, the transmission line equivalent circuit, and the fitted parameters obtained with the use of supercapacitor and transmission line equivalent circuits (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +46(0)702753087. *E-mail: [email protected]. Phone: +46(0)762674445. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a scholarship for an overseas thesis research study supported by the Graduate School of Prince of Songkla University. Financial support was received from the Higher Education Research Promotion and National Research University Project of Thailand (NRU), Office of the Higher Education Commission, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education, Thailand.

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REFERENCES

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DOI: 10.1021/acs.langmuir.5b03318 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.5b03318 Langmuir XXXX, XXX, XXX−XXX