Layer-by-layer assembled (high-energy carbon nanotube/conductive carbon nanotube)_n nanocomposites for high volumetric capacitance supercapacitor electrodes

Abstract

We introduce high-performance ultrathin electrochemical electrodes based on multi-stacking of high-energy multiwall carbon nanotube (MWCNT) hybrids and conductive MWCNTs. The MWCNT hybrids coated with oleic acid-stabilized pseudocapacitive nanoparticles (i.e., OA-PC–MWCNTs) were assembled via a sequential covalent-bonded layer-by-layer (LbL) approach with amine-functionalized MWCNTs (NH₂–MWCNT) in organic media, generating a highly porous structure and allowing for precise nanoscale control of the electrode thickness. The resultant NH₂–MWCNT/OA-PC–MWCNT multilayer electrodes exhibited a high energy capacity and remarkable operational stability, considerably higher than the capacity and stability of conventional blended nanocomposite or electrostatic LbL-assembled electrodes. The volumetric capacitances of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ and (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ were approximately 394 ± 10, and 674 ± 13 F cm⁻³ at 1 A cm⁻³, respectively. Additionally, these electrodes maintained their high volumetric capacitances without loss of initial capacitance even after 10 000 cycles; this cycling stability stemmed from the formation of chemically stable covalent bonds between the MWCNT hybrids and NH₂–MWCNTs and between the PC NPs and NH₂–MWCNTs. Given that a variety of PC NPs can be used to prepare MWCNT hybrids and that this approach can be further expanded to nanocomposite films including LbL-assembled multilayers, our approach may provide a promising platform for designing electrodes for use as thin film-type energy storage devices.

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Introduction

The rapid growth in the use of sustainable and renewable energy sources such as the sun, wind and sea tide, as well as the development of various electronics, including electric vehicles and mobile electronics, has increased the need for efficient energy storage systems that can supply energy continuously.^1–3 Among the various energy storage systems, electrochemical capacitors based on an electrochemical double-layer mechanism have high power density and low energy density, which makes them suitable for high-power applications such as load leveling devices for heavy machinery or integrated power supplies for microscale devices.^4,5 Batteries have high energy density because of the full usage of their electrodes, but additional time is required for the stored charge to enter and leave the electrodes, resulting in a lower power density and a substantial volume change of the electrodes during charge and discharge.^6–8 Although supercapacitors, which can fill the gap between conventional electrochemical capacitors and lithium-ion batteries, have attracted considerable attention as an excellent candidate for energy storage systems, their performance should be improved further for electronics requiring power sources with higher energy storage and longer lifetimes.^9–13

Recent research efforts have focused on the development of promising supercapacitor electrode materials composed of pseudocapacitive nanoparticles (PC NPs) with high specific capacitance and electrically conductive materials with large surface areas such as multiwall carbon nanotubes (MWCNTs), reduced graphene oxides or mesoporous carbon.^14–17 Although ruthenium oxide (RuO₂) NPs exhibit a theoretically high specific mass capacitance (>1000 F g⁻¹) among the various electrode materials, the high cost of RuO₂ has limited its commercial viability for supercapacitor applications. In this regard, magnetite (Fe₃O₄) and manganese oxide (MnO₂ or MnO), which have different valence states, have emerged as promising electrode materials because of their low cost and environmentally benign nature.^18–23 However, in most cases, these PC materials have been simply solution-blended or electrostatically bonded with conductive materials without serious consideration of the amount of PC NPs adsorbed onto conductive materials and/or the interfacial stability between PC NPs and conductive materials, which has limited the improvement of the energy storage capacity and the cycling stability. More recently, the high demand for a variety of microscale or miniaturized energy storage devices with high efficiency has prompted the development of ultrathin supercapacitor electrodes with high volumetric capacitance in a limited electrode volume instead of specific mass capacitance.^24–26

Recently, electrostatic layer-by-layer (LbL) assembly methods allowing for the nanoscale blending of several functional electrode materials in aqueous media have demonstrated that various charged NPs can be easily incorporated into oppositely charged conductive matrices by electrostatic interaction in aqueous media. This approach can produce binder-free ultrathin films with highly controllable thickness up to the micrometer scale, irrespective of electrode size and shape.^27–29 However, this electrostatic LbL assembly method has difficulty in significantly increasing the total film thickness as well as the amount of NPs adsorbed because of long-range electrostatic repulsion occurring among NPs with the same charge.³⁰ Additionally, the relatively weak electrostatic bonding between NPs and conductive materials causes desorption of the NPs onto the conductive materials during repetitive charge/discharge operations, as previously mentioned. As a result, despite the various advantages of electrostatic LbL assembly, these phenomena act as drawbacks that restrict the high volumetric capacitance and long-term stability of thin film-type energy storage devices. To our knowledge, the various electrostatic LbL-assembled electrodes such as (MWCNT/MnO₂)_n and (MWCNT/TiO₂)_n electrodes display a volumetric capacitance of approximately 200–250 F cm⁻³ (at 1 mV s⁻¹) with approximately 8–10% loss of initial capacitance after 1000 cycles.

Herein, we report ultrathin film-type supercapacitor electrodes via the covalently bonded LbL assembly of high-energy MWCNT hybrids and conductive MWCNTs; the resulting electrodes display excellent volumetric capacitance and long-term cycling stability. First, for the preparation of these high-energy MWCNT hybrids, the well-defined OA-PC NPs (i.e., OA–Fe₃O₄ or OA–MnO NPs) synthesized in toluene were densely deposited onto amine-functionalized MWCNTs (i.e., NH₂–MWCNTs) via a ligand exchange reaction³¹ between the OA ligands of the NPs and the NH₂ groups of the MWCNTs. The mass density of these MWCNT hybrids were 1.65 ± 0.02 g cm⁻³.

The formed OA-PC–MWCNT hybrids were highly dispersible in nonpolar media without the aid of an additional surfactant. As previously noted,^31,32 the OA-PC–coated MWCNT hybrids were further LbL-assembled with NH₂–MWCNTs via the same ligand-exchange reaction for the preparation of ultrathin film electrodes with a highly uniform and chemically stable structure, allowing for precise thickness control. That is, these MWCNT hybrids can be used as a high-energy storage component with a large surface area and simultaneously as a building block for the LbL-assembled electrodes. Additionally, our ligand-exchange LbL assembly approach for the formation of OA-PC–MWCNT hybrids/MWCNT film in organic media does not require complex assembly conditions (for instance, the pH value and ionic strength of the deposition solution that should be considered in traditional electrostatic LbL assembly), and furthermore allows for more facile and scalable thickness control than traditional LbL assembly methods (based on the repetitive deposition of PC NPs and MWCNTs) including electrostatic LbL assembly. To our knowledge, the LbL assembly of films (or electrodes) using high-energy MWCNT hybrids (i.e., MWCNT hybrids densely coated with pseudocapacitive or metal oxide NPs) has not been previously reported.

The volumetric capacitances of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrodes were approximately 394 ± 10 F cm⁻³ at 1 A cm⁻³ in neutral electrolyte solution instead of a harsh acidic solution. This volumetric capacitance was significantly (more than three-fold) higher than the volumetric capacitance of conventional supercapacitor electrodes based on carbon-based materials and iron oxide NPs. The volumetric capacitances of the (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ samples were further increased up to 674 ± 13 F cm⁻³ at 1 A cm⁻³, which is also superior to the volumetric capacitances of other manganese oxide-based supercapacitor electrode.²⁹ Additionally, these electrodes maintained high electrochemical performance without loss of their initial capacitance, even after 10 000 cycles. Furthermore, given that many researchers have attempted to combine MWCNTs with metal oxide NPs, including PC NPs, to prepare materials for various other applications, including microwave absorbers, gas sensors, and catalysts, in addition to materials for energy storage devices, we believe that our approach of using ligand-exchange-induced MWCNT hybrids may provide a basis for exploiting and designing hybrid nanocomposites with tailored functionalities and high performance.

Experimental

Experimental details are shown in ESI.†

Results and discussion

Our strategy for ultrathin multilayered electrode composed of PC NP-coated MWCNTs and MWCNTs is shown in Fig. 1. First, the NH₂–MWCNTs^33–35 dispersed in ethanol and the well-defined OA–Fe₃O₄ NPs^31,36 with a diameter of approximately 8 ± 1 nm in toluene were prepared as described in Experimental.† For the preparation of OA–Fe₃O₄ NP-coated NH₂–MWCNT (i.e., OA–Fe₃O₄–MWCNT) hybrids, 10 mg mL⁻¹ OA–Fe₃O₄ NP toluene solution was mixed with NH₂–MWCNT ethanol solution with a concentration of 1 mg mL⁻¹. In this case, the NH₂–MWCNTs were phase-transferred from an alcohol to a toluene solution because of the high affinity between the surface of the Fe₃O₄ NP and the NH₂ groups of the NH₂–MWCNT. These OA–Fe₃O₄–MWCNT hybrids with high dispersion stability in toluene also have a high affinity for NH₂–MWCNTs.

Fig. 1 Schematic diagram showing the preparation of OA-PC–MWCNT hybrids and (NH₂–MWCNT/OA-PC–MWCNT)_n multilayer film using ligand exchange reaction. In the case of OA–Fe₃O₄–MWCNTs with high dispersion stability in toluene, they displayed the strong magnetic properties due to the densely coated Fe₃O₄ NPs onto MWCNTs.

This high affinity was confirmed by FTIR in transmission and attenuated total reflectance (ATR) modes. First, the C–H stretching (2854 and 2925 cm⁻¹) and COO⁻ stretching (1411 and 1540 cm⁻¹) modes originated from carboxylate ion groups and the long aliphatic chains of the OA ligands loosely bound to Fe₃O₄ NPs, respectively. Additionally, the N–H bending (1640 cm⁻¹) and N–CH₃ stretching (1372 cm⁻¹) modes corresponded to the amine groups of NH₂–MWCNTs.^35,37–39 Therefore, OA–Fe₃O₄–MWCNT hybrids exhibited the characteristic absorption peaks (1372, 1411, 1540, and 1640 cm⁻¹) assigned to the OA ligands and NH₂ groups (Fig. 2a). To better understand this adsorption mechanism, we investigated the traces of the C–H stretching peaks of OA ligands through the additional adsorption of the NH₂–MWCNTs onto the outermost OA–Fe₃O₄ NP layer-coated substrate (i.e., (NH₂–MWCNT/OA–Fe₃O₄–MWCNT/NH₂–MWCNT)-coated Si substrate) (Fig. 2b). In this case, the peak intensity of the C–H stretching modes gradually decreased, and the intensity of the N–H bending and N–CH₃ stretching peaks intensified as the adsorption time for the NH₂–MWCNTs was increased from 0 min to 1 day. These phenomena imply that the OA ligands loosely bound to the surface of Fe₃O₄ NPs were replaced by the NH₂ groups of NH₂–MWCNTs. Furthermore, they imply that the interfaces of the Fe₃O₄ NPs and MWCNTs were bridged to the –NH₂ groups. However, the outermost surfaces of the hybrids were still stabilized by hydrophobic OA ligands bound to the surface of Fe₃O₄ NPs. These phenomena were also confirmed by the fact that water contact angles for the outermost NH₂–MWCNT and OA–Fe₃O₄–MWCNT hybrids were approximately 88 ± 4° and 132 ± 6°, respectively (Fig. S1†).

Fig. 2 (a) FTIR spectra of NH₂–MWCNTs, OA–Fe₃O₄ NPs, and OA–Fe₃O₄–MWCNTs. (b) FT-IR spectra of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)-coated Si substrate as a function of adsorption time of NH₂–MWCNT. (c) HR-TEM images of the OA–Fe₃O₄–MWCNT hybrids.

On the basis of the aforementioned results, we investigated the surface morphology of the OA–Fe₃O₄–MWCNT hybrids using high-resolution transmission electron microscopy (HR-TEM). As shown in Fig. 2c, the well-defined OA–Fe₃O₄ NPs were densely and uniformly adsorbed onto the NH₂–MWCNTs, constituting a loading amount of approximately 57.3 wt% (Fig. S2†). These hybrids also displayed superparamagnetic properties with a relatively high saturated magnetism of 50 emu g⁻¹ at room temperature (Fig. S3†). Although various approaches, including electrostatic interactions and the ion-reduction method, have been used to prepare metal oxide NP-coated MWCNT (or single-walled CNT) hybrids,^40–43 the degree of adsorption of metal oxide NPs onto the MWCNTs has typically been limited to a decoration level with a low packing density (<30%) in the lateral dimension (Fig. S4†). Cao et al. have reported that Fe₃O₄–MWCNT hybrids prepared by the ion reduction process in aqueous solution exhibit a saturation magnetization of 30 emu g⁻¹.⁴⁴ Additionally, other metal oxide nanoparticles (i.e., OA–TiO₂ and OA–BaTiO₃ NPs) as well as OA–Fe₃O₄ NPs could also be densely deposited onto the NH₂–MWCNTs via a ligand exchange mechanism (Fig. S5†).

An additional benefit of our approach is that the OA–Fe₃O₄–MWCNT hybrids could be consecutively LbL-assembled with the NH₂–MWCNTs, generating a high loading amount (or layer thickness) per layer and a porous structure (by the formation of voids between OA-PC–MWCNTs and NH₂–MWCNTs), which is effective for the preparation of LbL-assembled supercapacitor electrodes requiring high energy density, facile charge transfer and large surface area (see Fig. 1). To demonstrate this possibility, the adsorbed amount and film thickness of LbL-assembled NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer films were quantitatively investigated using a quartz crystal microbalance (QCM)⁴⁵ and cross-sectional SEM images. The frequency changes, ΔF (or mass changes, Δm), per layer measured from alternating depositions of OA–Fe₃O₄–MWCNT hybrids (concentration of approximately 11 mg mL⁻¹) in toluene and NH₂–MWCNTs (1 mg mL⁻¹) in ethanol were 111.8 ± 10 Hz (Δm ≈ 1975 ng cm⁻²) and 16.4 ± 5 Hz (Δm ≈ 289 ng cm⁻²), respectively (Fig. 3a). The total thickness and mass density of the 20-bilayered film with a porous structure was approximately 241 nm (i.e., a thickness per bilayer of approximately 12 nm) (Fig. 3b) and 1.65 ± 0.02 g cm⁻³, respectively (Experimental† and Fig. 3c). However, in the case of electrostatically LbL-assembled NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT multilayer films, their mass changes per layer and film thicknesses were notably lower than those of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer films and resulted in a mass density of 1.14 ± 0.03 g cm⁻³ (Fig. 3d). This electrostatic LbL assembly places an upper limit on the loading amount of charged components because of the long-range electrostatic repulsion between components with the same charges during deposition. Additionally, the high loading amount shown in the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers could be further increased by controlling the solution concentration of the OA–Fe₃O₄–MWCNT hybrids. As shown in Fig. 3e, the bilayer thickness of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer films was increased from 12 to 35 nm when the solution concentration of the OA–Fe₃O₄–MWCNT hybrids was increased from 11 to 22 mg mL⁻¹. Additionally, the 1.37 μm-thick (NH₂–MWCNT/22 mg mL⁻¹ OA–Fe₃O₄–MWCNT)₄₀ film exhibited a highly porous but uniform structure, with no agglomeration. These results show that our approach has excellent processing efficiency, allowing for a high loading amount, and creates a uniform surface/internal structure.

Fig. 3 (a) Frequency (ΔF) and mass change (Δm) of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n, and (NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT)_n films as a function of the layer number. (b) FE-SEM images of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ multilayers. (c) The average mass density of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers that was calculated from the linear slope of mass per unit area vs. film thickness curve. (d) Total film thicknesses of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n, and (NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT)_n multilayers measured from the cross-sectional FE-SEM images. In addition, the solution concentrations of the OA–Fe₃O₄–MWCNTs and anionic octakis–Fe₃O₄–MWCNTs were adjusted to be approximately 11 mg mL⁻¹. (e) The film thicknesses of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n multilayers as the solution concentration of the OA–Fe₃O₄–MWCNT hybrids was changed from 11 to 22 mg mL⁻¹.

The electrical properties of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers were examined by measuring their sheet resistance and electrical conductivity using the four-point probe method. As shown in Fig. 4a, the sheet resistance and electrical conductivity of the multilayer films prepared from the hybrid solution of 11 mg mL⁻¹ decreased from 7.1 × 10⁸ to 9.0 × 10⁷ Ω sq⁻¹ and increased from 0.01 to 0.04 S m⁻¹ as the bilayer number (n) increased from 5 (thickness of approximately 70 nm) to 20 (thickness of approximately 241 nm). Although the sheet resistance of the hybrid-based multilayers was higher than the 6.8 × 10⁶ Ω sq⁻¹ value for the (NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT)₂₀ films, the increased percolation of the NH₂–MWCNTs enhanced the electrical conductivity of the hybrid-based films by increasing the number of bilayers despite the dense packing of the OA–Fe₃O₄ NPs with high resistivity (>10¹² Ω sq⁻¹) onto the MWCNTs. In the case of the multilayer film composed of OA–Fe₃O₄ NPs and insulating poly(ethylene imine) (PEI), the sheet resistance was not measured because its high resistivity exceeded the gauge range. These phenomena were also confirmed by EIS measurements of the 241 nm-thick NH₂–MWCNT/OA–Fe₃O₄–MWCNT and 237 nm-thick PEI/OA–Fe₃O₄–MWCNT multilayer electrodes (Fig. S6†). In this case, the equivalent series resistance values of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT and PEI/OA–Fe₃O₄–MWCNT multilayer films were 39 and 45 Ω, respectively. These results suggest that the NH₂–MWCNTs can operate as continuous conductive pathways within the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers, facilitating the infiltration of MWCNTs within the porous multilayers as well as the removal of insulating OA ligands bound to the surface of the Fe₃O₄ NPs during LbL deposition. Additionally, porous NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers were more effective at facilitating ion and electron transfer compared to the insulating PEI/OA–Fe₃O₄–MWCNT multilayers with the same film thickness.

Fig. 4 (a) Sheet resistance and electrical conductivity of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n multilayer films as a function of the bilayer number (n). These films were prepared using a solution concentration of 11 mg mL⁻¹ OA–Fe₃O₄–MWCNT in toluene and 1 mg mL⁻¹ NH₂–MWCNT in ethanol. (b) N₂ adsorption–desorption isotherm and the pore size distribution (inset) of NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer.

The porosity of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers was also investigated by N₂ adsorption–desorption (Fig. 4b). In this case, the BET specific surface area and the pore volume of NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers were calculated to be 67.21 m² g⁻¹ and 0.29 cm³ g⁻¹, respectively. The surface area of pristine MWCNTs has been reported to be approximately 10–20 m² g⁻¹.⁴⁶ Additionally, the multilayer sample had a small diameter of approximately 3 nm and a relatively large diameter ranging from 16 to 44 nm (obtained by Barrett–Joyner–Halenda (BJH) analysis).

Furthermore, given that these Fe₃O₄ NPs are high-energy-density materials with pseudocapacitive properties, postulating that these nanocomposite multilayers, including densely packed Fe₃O₄ NPs, can be used as high-performance supercapacitor electrodes^30,47–53 with excellent volumetric capacitance in a limited electrode volume is counterintuitive. To confirm this possibility, we first investigated the charge storage capacities of NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers deposited onto ITO electrodes using a three-electrode cell. CV curves of multilayer electrodes were measured at various scan rates ranging from 5 to 100 mV s⁻¹ in 1 M Na₂SO₃ electrolyte instead of a harsh acidic electrolyte. These measurements were also performed in a potential window between −0.9 V and +0.1 V to prevent degradation of the electrode and electrolyte at higher potentials. Fig. 5a displays the CV data for the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n=5–20 electrodes at a scan rate of 5 mV s⁻¹ as a function of the bilayer number (n). The geometric current density level of the asymmetric CV curve with a redox peak increased with increasing bilayer number (or total film thickness) of the multilayer electrode. According to previous reports,^19,54 the redox behavior of Fe₃O₄ NPs in Na₂SO₃ solution originates from multi-electron redox reactions, corresponding to both the surface redox reactions of sulfur in the form of sulfate and sulfite anions adsorbed onto the surface of Fe₃O₄ NPs and the redox reaction between Fe^II and Fe^III. For a more detailed analysis, the charge storage capacity per unit area (or the total integrated charge density (in mC cm⁻²)) was calculated using the integrated area of the bilayer number-dependent CV curves shown in Fig. 5a. In this case, the charge storage density of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrodes increased nearly linearly with respect to the bilayer number of the multilayer electrodes, suggesting that the charge storage capacity of the multilayer electrodes could be further enhanced and precisely controlled by adjusting the bilayer number (Fig. 5b). The CV curves of the NH₂–MWCNT/COOH–MWCNT multilayer electrode without pseudocapacitive Fe₃O₄ NPs and the NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT multilayer electrode with a low packing density (<30%) of Fe₃O₄ NPs exhibited a relatively symmetrical and rectangular shape without evident redox peaks, similar to the characteristics of double-layer capacitance (Fig. S7†). By contrast, the NH₂–MWCNT/OA–Fe₃O₄–MWCNT electrode with double-layer capacitance and pseudocapacitance behavior displayed asymmetrical CV curves with evident redox peaks due to the formation of densely packed Fe₃O₄ NPs within electrodes. The mass densities of the NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT and NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrodes were approximately 1.14 ± 0.03 and 1.65 ± 0.02 g cm⁻³, respectively. These results explain why the geometric current level of the 241 nm-thick NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrode is considerably higher than those of the 247 nm-thick NH₂–MWCNT/COOH–MWCNT and 246 nm-thick NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT multilayer electrodes, despite their similar film thicknesses. As a result, the volumetric capacitances of the NH₂–MWCNT/COOH–MWCNT, NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT, and NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrodes with similar film thicknesses of approximately 241 nm were approximately 63 ± 2, 192 ± 7, and 289 ± 10 F cm⁻³ (specific mass capacitance of approximately 190 ± 17 F g⁻¹) at 5 mV s⁻¹, respectively. In the case of the 1.37 μm-thick (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₄₀ electrode, its volumetric capacitance decreased to 221 F cm⁻³ at 5 mV s⁻¹ because the thick film electrode blocked the transfer paths of the electrolytic ions. In particular, the volumetric capacitance obtained from the OA–Fe₃O₄–MWCNT hybrid-based electrode was considerably higher than the volumetric capacitance of the conventionally blended nanocomposite⁵⁵ or the LbL-assembled (MWCNT/MnO₂ NP)_n electrodes,²⁸ although the specific capacitance value (60–80 F g⁻¹) of the Fe₃O₄ NPs⁵⁵ has been reported to be significantly lower than the specific capacitance values of other pseudocapacitive NPs such as MnO₂ (theoretical capacitance value of approximately 1370 F g⁻¹) and RuO₂ (720–900 F g⁻¹). These phenomena were caused mainly by the dense packing of high-quality OA–Fe₃O₄ NPs on the porous MWCNT layers with good access to electrons and ions in the electrolyte.

Fig. 5 (a) CV curves and (b) total integrated charge density of the LbL-assembled (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n electrode as a function of bilayer number. In this case, the scan rate was fixed at 5 mV s⁻¹. (c) CV curves of the LbL-assembled (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode as a function of scan rate (5 to 100 mV s⁻¹). (d) Galvanostatic charge–discharge curves and (e) volumetric and specific capacitance of the LbL-assembled (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode as a function of current density (1 to 5 A cm⁻³). (f) Cycling performance of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode collected at a scan rate of 100 mV s⁻¹ for 10 000 cycles.

The NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrodes also exhibited good pseudocapacitive behavior as the scan rate was increased from 5 to 100 mV s⁻¹, implying that the multilayer electrodes possess the properties of relatively fast charge transfer and good rate capability despite the densely packed OA–Fe₃O₄ NP layers (Fig. 5c and S8†). However, a further increase in the scan rate caused a shift in the peak current toward higher potentials during the charging step because of poor reaction kinetics. Thus, the decrease in volumetric capacitance along with the increase in scan rate implies that the charges accumulated to the inner active sites of the electrode at low scan rates can be utilized for charge storage. Conversely, at higher scan rates, only the outer active surface of the electrode materials can be used as charge storage sites, which is typical behavior for an electrochemical charge storage system.

These phenomena were also confirmed by the galvanostatic charge–discharge curves of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode in the volumetric current density range from 1 to 5 A cm⁻³. In this case, the current density of the LbL-assembled multilayer electrode was expressed as the volumetric current density (mA cm⁻³) to obtain the thickness (i.e., bilayer number)-independent galvanostatic charge–discharge curve. As shown in Fig. 5d, the slanted parts in the charging potential window in the range from −0.6 to −0.4 V and the discharging potential window in the range from −0.5 to −0.7 V displayed overlapped redox reactions, consistent with the occurrence of redox peaks in the cyclic voltammograms in Fig. 5a. That is, the linear dependence of the potential on time at potentials greater than −0.4 V means that the electrical double-layer capacitance (EDLC) behavior originates from the charge separation at the electrode–electrolyte interface. The time dependence of the potential below −0.4 V was nonlinear because of the pseudocapacitance behavior of the OA–Fe₃O₄ within the electrode. In addition, the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ multilayer electrode exhibited a longer discharge time compared to that of the conventional NH₂–MWCNT/COOH–MWCNT capacitor electrodes with similar film thicknesses (Fig. S9†), implying that the OA–Fe₃O₄–MWCNT-based electrode has good charge storage performance.⁵⁷

As previously mentioned (see Fig. 5b), the charge storage capacity per unit area of the LbL-assembled electrode increased according to the bilayer number (n) of multilayers, which was confirmed by the galvanostatic charge–discharge curves (Fig. S10†). That is, the integrated area of the galvanostatic charge–discharge curve of (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)_n multilayer electrodes (i.e., the total charge storage capacity of multilayer electrode) increased with increasing the bilayer number (n) at a fixed areal current density (i.e., 0.1 mA cm⁻²). Fig. 5e shows the capacitance of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode calculated from galvanostatic charge–discharge curves at different current densities (the equation used is shown in the Experimental). The volumetric capacitance was measured to be 394 ± 10 F cm⁻³ (specific capacitance ∼260 ± 15 F g⁻¹) at 1 A cm⁻³, respectively. Furthermore, with an increase in the discharge current density to 5 A cm⁻³, the volumetric capacitance was maintained at 297 ± 10 F cm⁻³ (196 ± 13 F g⁻¹). This good rate capability is similar to the results obtained from CV measurements (see Fig. 5c and S8†). These capacitance values were much higher than those of previously reported electrodes based on carbon-based materials and iron oxide NPs (i.e., carbon–iron oxide NP or CNT–iron oxide NP nanocomposite electrodes prepared by simple blending and/or a doctor-blade approach).^58–60

The long-term cycling performance of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode was tested over 10 000 cycles at a scan rate of 100 mV s⁻¹ (Fig. 5f). In this case, the electrode maintained 103% of its initial volumetric capacitance after 10 000 cycles. The conventional electrostatic NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT multilayer electrode exhibited a 37% loss of initial capacitance after the same number of cycles (Fig. S11†). These results demonstrate that the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrode formed via covalent bonding is more stable than the electrostatically LbL-assembled NH₃⁺–MWCNT/anionic octakis–Fe₃O₄–MWCNT multilayer electrode.

On the basis of these results obtained using the OA–Fe₃O₄–MWCNT hybrids, our approach was further extended to the NH₂–MWCNT/OA–MnO–MWCNT multilayer electrodes for the preparation of a more efficient energy storage device. First, 15 ± 1 nm-sized OA–MnO NPs synthesized in toluene were densely coated onto the NH₂–MWCNTs using a ligand exchange reaction (Fig. 6a). These hybrids were then sequentially LbL-assembled with the NH₂–MWCNTs onto ITO electrodes. The electrochemical properties of the resultant (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrodes were investigated in the potential range between 0 V and +0.8 V in 1 M Na₂SO₄ solution. As shown in Fig. 6b, these electrodes exhibited good rate capability similar to that of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrodes at scan rates in the range from 5 to 100 mV s⁻¹. Fig. 6c shows that NH₂–MWCNT/OA–MnO–MWCNT multilayer electrodes exhibit greater charge storage than the NH₂–MWCNT/COOH–MWCNT multilayer electrode without pseudocapacitive MnO NPs. In particular, the volumetric capacitance based on discharge curves of the (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode was calculated to be approximately 674 ± 13 F cm⁻³ (354 ± 9 F g⁻¹) at 1 A cm⁻³; this capacitance is greater than the values for the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer electrodes at the same current density and greater than those previously reported for electrodes based on carbon-based materials and manganese oxide NPs (Fig. 6d and e).^{21,23,28,30,61,62} Additionally, the (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode also displayed notable electrochemical stability during repeated cycling operations. An increase of approximately 30% in the initial capacitance after 1100 cycles was caused by the enhanced effective interfacial area between the electrolyte and electrode materials as well as by the full activation of the electrode with increasing reaction time.^56,63–66 These phenomena were also observed in the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ electrode (see Fig. 6f). As a result, the initial capacitance of the (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode was maintained without additional loss until 10 000 cycles, implying that the covalently bonded LbL electrodes formed after the ligand exchange reaction are highly stable electrochemically.

Fig. 6 (a) HR-TEM image of the OA–MnO–MWCNT hybrids. (b) CV curves of the LbL-assembled (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode as a function of scan rate. (c) CV curves of the 227 nm-thick NH₂–MWCNT/COOH–MWCNT, and 219 nm-thick NH₂–MWCNT/OA–MnO–MWCNT multilayer electrodes measured at a scan rate of 5 mV s⁻¹. (d) Galvanostatic charge–discharge curves and (e) volumetric and specific capacitance of the LbL-assembled (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode as a function of current density (1 to 5 A cm⁻³). (f) Cycling performance of the (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrode collected at a scan rate of 100 mV s⁻¹ for 10 000 cycles.

Conclusions

We demonstrated that the LbL-assembled multilayers using high-energy MWCNTs could be used effectively as a promising electrode material for high-performance energy storage devices without the aid of an additional polymer binder. The volumetric capacitances of the (NH₂–MWCNT/OA–Fe₃O₄–MWCNT)₂₀ and (NH₂–MWCNT/OA–MnO–MWCNT)₂₀ electrodes were 394 ± 10 and 674 ± 13 F cm⁻³ at 1 A cm⁻³, respectively. The NH₂–MWCNT/OA-PC–MWCNT multilayers maintained their initial capacitance even after 10 000 cycles. This notable operational stability was caused by chemically stable covalent bonding between OA-PC NP and NH₂–MWCNTs and between OA-PC–MWCNTs and NH₂–MWCNTs. Furthermore, given that MWCNT hybrids with a variety of high-energy PC NPs can easily be prepared in organic media, our approach provides a basis for exploiting and designing high-performance electrochemical electrodes.

Acknowledgements

This work was supported by the National Research Foundation (NRF) grant funded by the Ministry of Science, ICT & Future Planning (MSIP) (2015R1A2A1A01004354).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details for the material preparation, contact angles of the NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayers, TGA date of OA–Fe₃O₄–MWCNT hybrids, magnetic curve of OA–Fe₃O₄–MWCNT hybrids, HR-TEM images of aqueous anionic octakis–Fe₃O₄–MWCNTs, electrochemical performances of NH₂–MWCNT/OA–Fe₃O₄–MWCNT multilayer. See DOI: 10.1039/c6ra02461k

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