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MnOx/carbon nanotube/reduced graphene oxide nanohybrids as high-performance supercapacitor electrodes

Abstract

Nanohybrids consisting of both carbon and pseudocapacitive metal oxides are promising as high-performance electrodes to meet the key energy and power requirements of supercapacitors. However, the development of high-performance nanohybrids with controllable size, density, composition and morphology remains a formidable challenge. Here, we present a simple and robust approach to integrating manganese oxide (MnOx) nanoparticles onto flexible graphite paper using an ultrathin carbon nanotube/reduced graphene oxide (CNT/RGO) supporting layer. Supercapacitor electrodes employing the MnOx/CNT/RGO nanohybrids without any conductive additives or binders yield a specific capacitance of 1070 F g−1 at 10 mV s−1, which is among the highest values reported for a range of hybrid structures and is close to the theoretical capacity of MnOx. Moreover, atmospheric-pressure plasmas are used to functionalize the CNT/RGO supporting layer to improve the adhesion of MnOx nanoparticles, which results in theimproved cycling stability of the nanohybrid electrodes. These results provide information for the utilization of nanohybrids and plasma-related effects to synergistically enhance the performance of supercapacitors and may create new opportunities in areas such as catalysts, photosynthesis and electrochemical sensors.

Introduction

Supercapacitors are promising energy storage systems for diverse applications such as portable electronics, roll-up displays, hybrid electric vehicles, self-powered sensors, artificial muscles and biomedical implants.1, 2 The performance of supercapacitors fills the gap between batteries and conventional electrolytic capacitors, with such advantages as fast dynamic response, high power density, high rate capability, safe operation and long lifespan. The most common materials for current supercapacitor electrodes are high-surface-area carbon-based nanostructures, including activated carbons, porous/templated graphite, carbon nanotubes (CNTs) and graphene.3, 4, 5, 6 However, the relatively low specific capacitance and energy density of these carbon-based electrodes have so far hampered their widespread applications in real devices.

To address this challenge, hybrid materials that can synergistically combine the attributes of both carbon and pseudocapacitive materials such as metal oxides and conductive polymers have been actively pursued.7, 8, 9 Pseudocapacitive materials store electrochemical energy in a similar manner as carbon-based materials but have a specific capacitance that is usually one order of magnitude larger. Among various pseudocapacitive materials, manganese oxides (MnOx) have attracted the strongest interest because of their natural abundance, low cost and environmental friendliness.10 Numerous studies have reported the hybrid structures of carbon materials with electrochemically active manganese oxide (MnOx), most commonly MnO2.7 However, the maximum specific capacitance of MnO2 in these reports remains low, at approximately 300–400 F g−1, and the capacitance of overall hybrids is even lower, typically <200 F g−1;11 MnO2 still displays low electrical conductivity, limited stability and poor integration in hybrid electrode systems.

Mn can assume multiple oxidation states (Mn2+, Mn3+ and Mn4+) and diverse phases during both oxidation and reduction processes. According to the Pourbaix diagram, Mn2O3, Mn3O4 and MnO2 can all be formed from a Mn2+ salt solution at room temperature.12 Compared to single-phased MnO2, MnOx compounds (i.e., Mn atoms with multiple oxidation states and phases) usually contain both donor and acceptor sites in their microstructures as well as defects and mismatch induced by different phases, which can enable a higher charge storage capacity that is beneficial for supercapacitor applications.13 However, despite a few examples of MnOx reported recently,14 most previous studies simply denoted the synthesized Mn oxides as MnO2 with few or no structural investigations.15, 16, 17 The integration of MnOx into supercapacitor electrodes with binders is also challenging because of the non-uniform particle size, poor electrical conductivity and structural instability of MnOx. In addition, MnOx was mainly synthesized by methods involving hazardous chemicals, strong acids and high-temperature thermal annealing, with the controllability, scalability and reliability of these methods remaining unclear. Therefore, control of the size, morphology and density of MnOx in a simple and environmentally benign way is highly desirable to realize the true potential of MnOx in energy storage devices.

Here, we present a robust approach to integrating a high density of MnOx nanoparticles onto flexible graphite paper using an ultrathin CNT and reduced graphene oxide (CNT/RGO) layer as the supporting layer. The ultrathin CNT/RGO supporting layer not only enables good electrical conductivity by forming web-like percolating networks but also improves the efficiency of MnOx deposition. The electrodeposited MnOx nanoparticles have a porous nanostructure and a narrow particle size distribution. Supercapacitor electrodes utilizing this MnOx/CNT/RGO nanohybrid demonstrate good charge transfer conductance and high specific capacitance of 1070 F g−1. This specific capacitance is substantially larger than the values (300–400 F g−1) commonly obtained from MnO2 electrode material with similar thickness and mass loading. Moreover, atmospheric-pressure dielectric-barrier discharge (DBD) plasmas are used to functionalize the CNT/RGO supporting layer prior to MnOx deposition. The resulting nanohybrid displays improved cycling stability without compromising specific capacitance. These results demonstrate the promise of the synergistic use of carbon nanomaterials and plasma-related effects in the development of high-performance energy storage devices and may create new directions for functional devices in the fields of catalysis, photosynthesis and electrochemical sensing.

Materials and methods

Preparation of MnOx/CNT/RGO nanohybrids

First, 50 mg CNTs (~1.0 nm diameter; Sigma, Sydney, Australia) and 50 mg RGO monolayer (0.7–1.2 nm thick; NanoInnova Technologies, Madrid, Spain) were dissolved in 100 ml N-methyl-2-pyrrolidone. A stable solution was obtained after horn sonication (Sonics VCX130, John Morris Scientific, Sydney, Australia) at 100 W for 30 min. Graphite foil (0.25 mm thick; Alfa Aesar, Ward Hill, MA, USA) was used as the flexible substrate. An ultrathin layer of CNT/RGO was then formed on the graphite foil by spray coating, followed by annealing at 80 °C on a hot plate in air.

MnOx nanoparticles were synthesized by galvanostatic electrodeposition in a two-electrode cell configuration, with the CNT/RGO-coated graphite foil as the working electrode, a Pt plate as the counter/reference electrode and MnSO4 as the precursor solution, as shown by Route I in Figure 1a. MnOx nanoparticles were deposited at a constant current of 1 mA cm−2 for 0.5–20 min in 0.2 M MnSO4 aqueous solution. The MnOx/CNT/RGO/ electrode was then washed with ethanol and water and dried at 80 °C for 3 h in air. The mass of MnOx nanoparticles was measured by an ultrasensitive microbalance with a sensitivity of 0.1 μg (Mettler Toledo UMT2, Port Melbourne, VIC, Australia). The loading of MnOx was 30±5 μg and ~300 μg for 2 and 20 min deposition, respectively. Finally, supercapacitor electrodes were fabricated from the CNT/RGO/MnOx hybrid structure without any conductive additives or polymeric binders.

Figure 1
figure1

(a) Schematics for MnOx/CNT/RGO nanohybrid fabrication via Routes I and II. Route II includes a plasma functionalization step prior to MnOx deposition. (b) SEM image of the CNT/RGO supporting layer, with RGO highlighted by dotted lines. (c) Low- and (d) high-magnification SEM images of MnOx nanoparticles deposited on a CNT/RGO layer.

To improve the stability of MnOx nanoparticles in the nanohybrid, an atmospheric-pressure DBD plasma (see Supplementary Figure S1) was used to functionalize the ultrathin CNT/RGO layer prior to MnOx deposition, as illustrated by Route II in Figure 1a. The plasma discharge was powered by a high-frequency pulse generator (Corona Lab CTP-2000K, Nanjing, China) at 100 V and 0.16 A (i.e., at power of ~16 W). Helium was used as a working gas. The CNT/RGO layer on graphite foil was treated for 5 min, followed by the electrodeposition of MnOx nanoparticles as described above.

Electrochemical measurements

The electrochemical performance of MnOx/CNT/RGO hybrid electrodes was conducted in a three-electrode cell configuration using a potentiostat/galvanostat (Bio-Logic VSP 300, Claix, France). The nanohybrid was used as the working electrode, with Ag/AgCl as the reference electrode, a Pt plate as the counter electrode and 1 M Na2SO4 aqueous solution as the neutral electrolyte. Cyclic voltammetry (CV) was performed at scan rates from 10 to 500 mV s−1, and galvanostatic charge/discharge measurements were conducted at current densities from 150 to 800 μA cm−2. Electrochemical impedance spectroscopy using a 10 mV sinusoidal signal was measured at open-circuit voltage, with frequency scanned from 0.01 Hz to 1 MHz. Specific capacitance based on the CV curves was calculated by , where I is the current, V is the potential window, m is the mass loading of MnOx and v is the scan rate, while specific capacitance based on the charge/discharge curves was calculated by Cs= i/(−dV/dt), where i is the discharge current and dV/dt is the slope of the discharge curve. Capacitance contributed from the graphite foil was subtracted in both cases.

Materials characterization

Scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy was performed using a Zeiss Auriga microscope (Carl Zeiss, Sydney, Australia) operating at 1 keV electron beam energy with an InLens detector. Transmission electron microscopy (JEOL 2100, JEOL, Sydney, Australia) was operated at an electron beam energy of 200 keV. Samples for transmission electron microscopy characterization were prepared by shaving the surface material using a scalpel, ultrasonicating in ethanol, dropping onto holey carbon-coated copper grids and drying naturally in air. Raman spectroscopy utilized a Renishaw inVia spectroscope (Renishaw, Sandringham, VIC, Australia) with laser excitation of 514 nm and a spot of ~1 μm2. Furthermore, X-ray photoelectron spectroscopy (XPS) used a SAGE 150 SPECS system (SPECS GmbH, Berlin, Germany) with a Mg Kα source at 1253.6 eV. Peak analyses were performed using CasaXPS (Casa Software, http://www.casaxps.com).

Results and Discussion

Fabrication of MnOx/CNT/RGO nanohybrids

The morphology of the sprayed CNT/RGO supporting layer on the graphite foil and the deposited MnOx nanoparticles are shown in Figures 1b–d. Entangled CNTs and RGO with a thickness of 10–20 nm cover the entire graphite foil (Figure 1b). After galvanostatic deposition, flower-shaped MnOx nanoparticles grow on the CNT/RGO supporting layer, with sizes ranging from 100 to 200 nm (Figure 1c). MnOx nanoparticles form a near-continuous monolayer with a highly porous structure after only 2 min deposition (Figure 1d). These nanoparticles firmly attach to the CNT/RGO supporting layer, thereby avoiding the polymeric binders commonly used in the integration of supercapacitor electrodes. The mass loading of MnOx nanoparticles was ~30 μg. Moreover, energy-dispersive X-ray spectroscopy measurements indicate that the molar ratio of O and Mn atoms is approximately 1.38 (see Supplementary Figure S2), suggesting that MnOx most likely consists of Mn3O4 and Mn2O3. The formation of a mixed-valent compound rather than single-phased MnO2 is attributed to oxygen deficiency in the electrodeposition process. Compared to pristine graphite foil, the CNT/RGO supporting layer not only increases the density of MnOx nanoparticles but also leads to a smaller size and narrower size distribution (see Supplementary Figure S3).

Both CNT and RGO are promising candidates for the fabrication of electrical double-layer capacitor type supercapacitors.3, 4, 5 However, the specific capacitance and energy density of electrical double-layer capacitor-type supercapacitors are still below the requirements of most real applications. In the current hybrid structure, the ultrathin CNT/RGO supporting layer promotes MnOx electrodeposition and maintains strong electrical contact between the graphite foil and the electrochemically active but poorly conductive MnOx nanoparticles.8 The latter is apparent as CNTs form web-like conductive percolating networks and RGO further reduces the interfacial resistance in the percolating networks through its two-dimensional structure (Figure 1b).18

We have performed a series of experiments to control the structure of MnOx nanoparticles. By reducing the MnSO4 salt concentration, a broad particle size distribution is formed (see Supplementary Figure S4). At the present concentration of 0.2 M MnSO4 and a constant current of 1 mA cm−2, the voltage approaches a steady state of 2.2 V after ~100 s (see Supplementary Figure S5). With prolonged deposition at these conditions, we can readily deposit a thicker layer of MnOx while maintaining the porous structure. As shown in Figure 2, a 1.2-μm-thick film with a mass loading of ~300 μg cm−2 is obtained after 20 min, indicating that the packing density of MnOx increases proportionally with deposition time (scanning electron microscopy images of MnOx deposited for 5 and 10 min are also shown in Supplementary Figure S6). Therefore, by combining CNTs and RGO in the supporting layer, the thickness and morphology of MnOx nanoparticles can be effectively controlled.

Figure 2
figure2

(a) Top-view and (b) cross-sectional SEM images of MnOx nanoparticles with a thickness of approximately 1.2 μm deposited on a CNT/RGO supporting layer.

Hybrid structures containing Mn oxides have been previously prepared by a number of methods, such as physical mixing, vacuum filtration, direct redox precipitation and hydrothermal processes.19 Nevertheless, control of the size, density, morphology and thickness of Mn oxides remains a challenge. The galvanostatic electrodeposition method employed here is a simple and effective way to control the deposition of MnOx nanoparticles and involves only hazard-free chemicals. The method also has such advantages as being single-step, environmental friendliness, reliability, scalability, low cost and versatility.

The chemical bonding states of MnOx nanoparticles were analyzed in detail by XPS and Raman spectroscopy. The XPS survey scans of CNT/RGO and MnOx/CNT/RGO are shown in Figure 3a, which clearly indicates the introduction of Mn atoms by the electrodeposition process. The C 1s spectra show similar carbon bonds in both samples (see Supplementary Figure S7), suggesting that the CNT/RGO supporting layer remains largely intact after MnOx deposition. Figure 3b shows the O 1s spectrum of the CNT/RGO/MnOx nanohybrid, in which three deconvoluted peaks can be assigned to the chemical bonds of Mn-O-Mn, Mn-O-H and H-O-H at binding energies (BE) of 529.8, 531 and 532 eV, respectively. This O 1s spectrum is typical for compounds with Mn ions in the 2+ or 3+ oxidation states.20, 21 The Mn 2p spectrum of CNT/RGO/MnOx also shows two peaks at binding energies of ~654 and ~642 eV (Figure 3c), which can be attributed to the Mn 2p3/2 and Mn 2p1/2 spin-orbital doublets, respectively.21 Moreover, the Mn 3s spectrum displays a splitting width of ~5.6 eV, implying the presence of Mn3+ and Mn2+ states (see Supplementary Figure S8).20

Figure 3
figure3

(a) XPS survey scans of CNT/RGO and MnOx/CNT/RGO nanohybrid. (b) O 1s and (c) Mn 2p spectra of MnOx/CNT/RGO nanohybrid. (d) Raman spectrum of MnOx/CNT/RGO nanohybrid. The peak at ~838 cm−1 is from the vibrations in CNTs.

Raman spectroscopy was also used to analyze the MnOx structural features over large areas.12 A Raman spectrum of the MnOx/CNT/RGO is shown in Figure 3d, and the spectra of pristine graphite foil, CNT and RGO are shown in Supplementary Figure S9. The peak observed in Figure 3d at Raman shift of ~640 cm−1 can be attributed to the Mn-O stretching vibration in the basal plane of MnO6 and/or the symmetric stretching vibration (Mn-O) of the MnO6 group.19 In particular, the broad peak at 550–750 cm−1 can be assigned to the δ-phase of the characteristic Mn3O4 Raman spectrum, while the peaks from 400 to 500 cm−1 are fingerprints of Mn2O3.12, 21 These peaks are broad, indicative of the small crystal sizes of MnOx nanoparticles that lack significant long-range order. These spectra thus corroborate that the MnOx is comprised of mainly Mn3O4 and Mn2O3 phases.

Figure 4 shows a transmission electron microscopy image of the MnOx nanoparticles. These flower-shaped nanoparticles firmly attach to the CNT/RGO supporting layer with multiple crystalline nano-domains, which can be assigned to different facets of the mixed-valent nanohybrid, such as the (111) plane of Mn2O3, (211) plane of Mn3O4, (101) plane of Mn3O4 and (002) plane of graphitic carbon.22, 23 In fact, a very recent report has described such flower-shaped MnOx nanoparticles to have α-Mn2O3 in the core and Mn3O4 Hausmannite in the branches.14

Figure 4
figure4

(a) Low- and (bd) high-resolution transmission electron microscopy images of MnOx/CNT/RGO nanohybrid, showing that MnOx nanoparticles attached to the CNT/RGO supporting layer possess multiple crystalline nano-domains.

Electrochemical performance of MnOx/CNT/RGO nanohybrid

When subjected to a potential sweep (0–0.8 V) in the presence of aqueous electrolyte during electrochemical tests, the initial MnOx nanoparticles may undergo oxidation to a higher valent state after the first few cycles (see Supplementary Figure S10). However, no significant morphological changes associated with the phase transformation to MnO2 were observed, indicating that the oxidation of MnO2 to a monolithic state is a gradual and long process. As a result, the defects and mismatch induced by different phases in the as-synthesized MnOx remain unchanged in the oxidation process.14 The oxidation reactions are12

and

The charge storage capability of MnO2 is then mainly determined by the surface adsorption of electrolyte cations and proton incorporation according to24

These reactions are highly reversible only on the outmost surface layer in contact with the aqueous Na2SO4 electrolyte.8, 25 Thus, to best evaluate the ultimate charge storage capacity of MnOx nanoparticles, we conducted the electrochemical measurements of a monolayer of MnOx on the CNT/RGO supporting layer. Collected using a three-electrode test configuration, the CV curves of MnOx/CNT/RGO in 1 M Na2SO4 aqueous electrolyte at scan rates of 10, 20 and 50 mV/s are shown in Figure 5a.

Figure 5
figure5

(a) Cyclic voltammetry (CV) curves of MnOx/CNT/RGO nanohybrid at scan rates of 10, 20 and 50 mV s−1. (b) Charge/discharge curves of MnOx/CNT/RGO nanohybrid at current densities of 5, 10 and 20 A g−1. (c) The capacitance retention of the MnOx/CNT/RGO nanohybrid is 74% after 1000 cycles at 100 mV s−1. (d) SEM image of MnOx nanoparticles after cycling.

The CV curves show nearly symmetric quasi-rectangular shapes, similar to the electrical double-layer capacitor-type capacitive behavior, which is attributed to the successive multiple surface redox reactions of MnOx, a charge storage mechanism that is different from that of most other metal oxides.24 Therefore, smooth CV curves rather than distinct redox peaks are observed here. Additionally, the CNT/RGO supporting layer contributes negligible capacitance to the nanohybrid electrode, as suggested by comparing the CV curves of graphite foils with and without the CNT/RGO supporting layer (see Supplementary Figure S11). The thickness of CNT/RGO is very small (<20 nm), and the estimated mass is low (<2 μg). As a result, the CNT/RGO supporting layer in the nanohybrid acts similarly to a porous current collector, while the charge storage capacity is mainly determined by the MnOx nanoparticles.26

Figure 5b shows the galvanostatic charge/discharge curves at current densities of 5, 10 and 20 A g−1 (based on the mass loading of MnOx). The nearly symmetric quasi-triangular shapes of charge/discharge curves again indicate the high and reversible charge storage capacity of the nanohybrid.25 The specific capacitance Cs of MnOx/CNT/RGO is calculated to be ~1070 F g−1 from the CV curve at a scan rate of 10 mV s−1 and ~1040 F g−1 from the charge/discharge curve at a constant current of 5 A g−1 (both excluding the capacitance of graphite foil). Notably, the specific capacitance remains at a high value of 533 F g−1 at a fast scan rate of 100 mV s−1.

The above specific capacitance is substantially higher than those reported for single-phased MnO2, Mn2O3 or Mn3O4 materials (~100–300 F g−1) at a similar loading amount (see detailed comparison in Supplementary Table S1).27, 28 This capacitance value also approaches the highest values (900–1170 F g−1) ever reported for hybrid structures, which were obtained either from the very low mass-loading (<10 μg cm−2) of active materials or with a sophisticated current collector (such as nanoporous gold thin films).26, 28, 29 In fact, this value (1070 F g−1) is close to the theoretical capacitance of MnOx (Cs~1240 F g−1) by assuming that all Mn atoms are involved in the redox reactions.

We attribute the high electrochemical performance to the open and nanoporous architecture of the nanohybrid, high electrical conductivity of the CNT/RGO supporting layer and high capacity of the MnOx compound. Specifically, the porous and flower-like structure could provide a large and accessible surface area to greatly enhance surface ion adsorption, improve the accessibility of cations and shorten the ion diffusion path.2, 17 The smaller and uniformly sized nanoparticles could also facilitate fast charge transfer on the surface or sub-surface of the active material; for example, Duay et al.12 found that the specific capacitance was much higher for small MnO2 nanofibrils (5–10 nm) than for large MnO2 nanowires (4.5 μm). In addition, although MnOx could be gradually oxidized to a higher valent state during the charge/discharge process, the structural features associated with the as-prepared MnOx, such as ionic (e.g., vacancies and misplaced ions) and electronic (electrons and holes) defects and mismatches at different phases, could still be preserved because of the slow and mild nature of the process.14 The use of MnOx nanoparticles as the starting material is thus advantageous compared with the use of MnO2 for obtaining a higher specific capacitance.

The specific capacitance of the nanohybrid reduces from 1070 to 480 F g−1 at 10 mV s−1 when the thickness of MnOx increases from ~200 nm to ~1.2 μm (see Supplementary Figure S12). The relationship between specific capacitance and MnOx thickness is also plotted based on galvanostatic charge/discharge curves at a high current density of 5 A g−1 (see Supplementary Figure S13). These values are notably higher than those obtained from MnO2-based electrodes and a range of carbon/metal oxide hybrids with similar thicknesses (see Supplementary Table S1). Owing to its higher packing density, the areal capacitance of the thick MnOx is ~144 mF cm−2, much larger than that of commercial supercapacitors made of activated carbons (~20 mF cm−2).1 To reasonably translate the measured value to real devices, the loading mass of active materials should generally exceed 1 mg cm−2, and the thickness should be larger than 1 μm.30 Because one of our aims is evaluation of the ultimate electrochemical performance of MnOx nanoparticles and demonstration of their potential for energy storage applications, we tried to avoid the effects of intrinsically poor conductivity and impaired ionic accessibility in the excessively thick MnOx films.27 Thus, the nanohybrid with a monolayer of MnOx on the CNT/RGO supporting layer was tested. We emphasize that the fabrication process of single-layer MnOx/CNT/RGO nanohybrid is simple, fast and non-hazardous, and the thickness of MnOx can be easily controlled. In the future, the fabrication of multiple-layer MnOx/CNT/RGO nanohybrids using spray and electrodeposition techniques will also be explored to increase charge storage capacity (see Supplementary Figures S14 and S15).

Improved cyclic stability through plasma treatment

The cycling stability of MnOx/CNT/RGO is plotted in Figure 5c, showing a relatively poor capacitance retention of ~74% after 1000 cycles at a scan rate of 100 mV s−1. The scanning electron microscopy image of MnOx nanoparticles after cycling is shown in Figure 5d, in which blunt branches of nanoparticles are observed compared towith the structure before cycling (Figure 1d). This relatively poor cycling stability is most likely due to the dissolution of Mn atoms into the electrolyte and/or the structural instability caused by the adsorption of cations in the redox reactions.29 A closer investigation of the electrode after cycling also found that a few flake-shaped nanosheets emerge after cycling (see Supplementary Figure S16), indicating that the structure may be affected by the ‘dissolution-redeposition’ process.31 The mechanism of this process assumes that Mn atoms at the surface are first dissolved in the electrolyte during reduction at low potentials; the dissolved Mn are then re-oxidized into insoluble MnO2 and deposit on the surface. The shape of the re-deposited MnO2 can be very different from that of the original as-prepared MnOx. As very few flake-shaped nanosheets are found, we conclude that this ‘dissolution-redeposition’ process is quite slow in the present MnOx/CNT/RGO nanohybrid. In addition, the XPS spectrum of the cycled electrode shows that Na and S atoms are present in the nanohybrid in addition to the original C, O and Mn atoms (see Supplementary Figure S17), implying that ions in the aqueous electrolyte can intercalate into the active MnOx structure and cause structural alternations. These effects can thus account for the observed reduced charge storage capacity.

To improve the cyclic stability of the MnOx/CNT/RGO nanohybrid, we utilized an atmospheric-pressure DBD plasma to functionalize the CNT/RGO supporting layer prior to MnOx electrodeposition. Plasmas have recently shown a unique ability to selectively functionalize the surface of carbon-based materials with controllable and graded intensity and depth.32, 33 In particular, the low-energy ions and electrons in atmospheric-pressure DBD plasma may be ideal for grafting a variety of functional groups at the outmost regions over a large area without damaging the structure.

We have found that atmospheric-pressure DBD plasma treatment can lead to a notable change in the surface wettability of the CNT/RGO supporting layer after only 15 s of plasma exposure. As the gas temperature is low in the plasma, we were also able to extend the treatment for a much longer time of 5 min to render a more profound plasma modification effect without damaging the structure of the CNT/RGO supporting layer. As shown in the inset of Figure 6a, an almost complete wetting is observed in the plasma-treated CNT/RGO surface, in contrast to the large contact angle of the pristine sample. XPS analyses of the plasma-treated CNT/RGO confirm that substantial numbers of oxygen-containing functional groups, such as −OH and –COOH, are successfully grafted on the surface (see Supplementary Figure S18). Figure 6a also shows an scanning electron microscopy image of MnOx deposited on a plasma-treated CNT/RGO layer. No apparent difference in the density and morphology is observed compared with the nanoparticles deposited on a pristine CNT/RGO layer; again, flower-like MnOx nanoparticles form on the surface with high porosity after the same galvanostatic deposition. The results are consistent with previous observations that a carbon surface with substantial functional groups can react easily with metal ions to form metal oxide-based compounds.34

Figure 6
figure6

(a) SEM image of MnOx nanoparticles electrodeposited on a plasma-treated CNT/RGO surface. The inset is a photo of CNT/RGO surfaces that shows the wettability change before and after plasma treatment. (b) Cyclic voltammetry (CV) curves of MnOx/CNT/RGO nanohybrid with plasma treatment at scan rates of 10, 20 and 50 mV s−1. (c) Charge/discharge curves of MnOx/CNT/RGO nanohybrid with plasma treatment at current densities of 5, 10 and 20 A g−1. (d) The capacitance retention of MnOx/CNT/RGO nanohybrid with plasma treatment is 87% after 1000 cycles at 100 mV s−1.

The electrochemical performance of MnOx/CNT/RGO with plasma treatment is shown in Figures 6b–d. The CV and charge/discharge curves are clearly similar to those of CNT/RGO/MnOx without plasma treatment. Thus, the specific capacitance and rate capability remain nearly unchanged (see Supplementary Figure S19). However, the capacitance retention is greatly improved from 74% of the MnOx/CNT/RGO without plasma treatment to 87% after plasma treatment. The improved stability is comparable or better than that of many manganese oxide-based pseudocapacitors, with a similar morphology at the same test conditions, such as Mn3O4 nanoparticles decorated on CNT arrays (77% retention after 1000 cycles)35 and graphene sheet/MnO2 (85% retention after 1000 cycles).36, 37 This improved stability can be explained by noting the two main mechanisms for the capacitance degradation of MnOx-based electrodes, namely, Mn dissolution and mechanical failure.31 The former is associated with the partial dissolution of MnOx into the electrolyte, whereas the latter is caused by mechanical failure upon cycling. With plasma treatment, the better wettability of the CNT/RGO layer could lead to better accessibility of Mn ions in the electrodeposition of MnOx. However, plasma treatment can also provide a substantial number of functional groups as anchoring sites for MnOx deposition. The stronger binding between MnOx nanoparticles and the supporting layer thus effectively improves the mechanical stability during redox reactions and leads to higher capacitance retention over a large number of cycles.38

We also measured the electrochemical impedance spectra of the graphite foil, MnOx/CNT/RGO and MnOx/CNT/RGO with plasma treatment (see Supplementary Figure S20). The vertical lines in the Nyquist plots support the capacitive behavior of all three electrodes at low frequencies. Moreover, the series resistance, as obtained from the intersections of the electrochemical impedance spectroscopy spectra with the real-Z axis at high frequencies,2 only slightly increases from 2.9 Ω for the graphite foil to 3.1–3.2 Ω for the MnOx/CNT/RGO nanohybrids. This low series resistance implies a combinational effect of fast charge transfer in the redox reactions, easy accessibility of ions, and high interfacial contact in the MnOx/CNT/RGO electrodes, which are all desirable features in the operation of supercapacitors.

Conclusion

In summary, we have demonstrated high-performance supercapacitor electrodes based on MnOx/CNT/RGO nanohybrids. With an ultrathin CNT/RGO supporting layer, the size, morphology and thickness of MnOx nanoparticles are effectively controlled. Detailed analyses reveal that the MnOx nanoparticles consist of mainly Mn3O4 and Mn2O3 phases. The MnOx/CNT/RGO nanohybrids have key structural features, including porous architecture, small and uniform size, and high electrical conductivity, which lead to high charge storage capacity in supercapacitor operations. In addition, atmospheric-pressure DBD plasmas have been utilized to enhance the binding between MnOx nanoparticles and the CNT/RGO supporting layer, resulting in further enhancement of cycling stability. Our approach of utilizing nanohybrids and plasma-related effects is therefore highly promising for the development of next-generation energy storage devices for a range of applications critical for a sustainable future.

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Acknowledgements

This work is partially supported by the Australian Research Council (ARC) and CSIRO’s OCE Science Leadership program. We acknowledge the DECRA and Future Fellowships from the ARC.

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Correspondence to Zhao Jun Han or Kostya (Ken) Ostrikov.

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Han, Z., Seo, D., Yick, S. et al. MnOx/carbon nanotube/reduced graphene oxide nanohybrids as high-performance supercapacitor electrodes. NPG Asia Mater 6, e140 (2014). https://doi.org/10.1038/am.2014.100

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