CHEMIJA. 2020. Vol. 31. No. 2. P. 87–94 © Lietuvos mokslų akademija, 2020

Carbon supported manganese(IV)–cobalt(II/III) oxides nanoparticles for high-performance electrochemical supercapacitors

Jolita Jablonskienė*,

Dijana Šimkūnaitė,

Jūratė Vaičiūnienė,

Giedrius Stalnionis,

Audrius Drabavičius,

Loreta Tamašauskaitė-Tamašiūnaitė,

Eugenijus Norkus

Center for Physical Sciences and Technology, 3 Saulėtekio Avenue, 10257 Vilnius, Lithuania

The  carbon supported manganese(IV)–cobalt (II/III) oxides nanoparticles labelled as MnO2–Co3O4/C nanocomposites have been prepared by a simple one-step microwave-assisted heating method using different precursor materials. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES) have been used for the  characterization of morphology, structure and composition of the synthesized nanocomposites, whereas the electrochemical performance of the prepared nanocomposites has been evaluated by using cyclic voltammetry (CV).

It was determined that the use of different precursor materials for the synthesis of the carbon supported MnO2 and Co3O4 nanocomposites results in a different morphology of the prepared substances. A high specific capacitance (Cs) of 658.8 F g−1 at a scan rate of 10 mV s−1 in a 1 M Na2SO4 solution has been obtained for the MnO2–Co3O4/C-2 nanocomposite that has a spherical shape of nanoparticles. Moreover, it significantly outperforms the  MnO2–Co3O4/C-1 nanocomposite that has a lamellar shape of nanoparticles.

Keywords: manganese, cobalt, carbon, nanocomposites, supercapacitors

INTRODUCTION

Supercapacitors (SCs) have gained increasing attention due to their high power density, long cycle life, fast charge/discharge rate, and in recent years are seriously viewed as potential candidates of next-generation energy storage devices [1, 2]. They are especially valued for lightness and flexibility determining their large-scale of possible applications ranging from consumer electronic or portable devices like mobile phones, computers and memory back-up systems to hybrid electric vehicles or even large industrial machinery, for defense and military or space equipment [3, 4].

The key issues in the  commercial success of SCs application are directly linked to the selection and fabrication of the new, low-cost and efficient electrode materials offering a  high potential for the  substantially enhanced energy densities [3, 5]. Based on different energy storage principles, supercapacitors are generally categorized into electrochemical double-layer capacitors that use large surface-area carbon materials [6–8] and pseudocapacitors that use transition metal oxides (TMOs) as active materials. Among the  emerging electrode materials for pseudocapacitors, nanoscaled or mixed TMOs such as RuO2, MnO2, Co3O4, NiO, Fe3O4 and V2O5 are the most attractive materials for their high theoretical specific capacitances (Cs), originating from fast and reversible redox reactions with the electrolyte ions [3].

Nevertheless, Ru-based materials dominate. They are most extensively explored due to the ultrahigh theoretical specific capacitance of RuO2 (reaching over 1300  F  g–1), exceptional properties, including metallic-type conductivity, wide potential window and highly reversible redox reaction [9, 10]. But the relatively high cost, toxicity and limited environmental distribution of RuO2 restrict its large-scale commercial application in SCs. On the contrary, TMOs, specially Co3O4 [11–13] and MnO2 [14, 15] are considered rather promising electrode materials for SCs, since both of them are low priced, abundant, environment- friendly materials that are rich in oxidation states for efficient charge transfer, have various morphologies and exceptionally high theoretical specific capacitances (Cs) of 3560 or 1370 F g−1, respectively. However, regardless the fascinating characteristics typically TMOs suffer from their low actual specific capacitance due to poor conductivity and electrochemical stability, low contacting area with the  electrolytes and structural degradation during the charge-discharge process [16, 17]. In order to improve the capacitive performance of TMOs nanostructured electrode material with various morphologies that have a  high specific surface area and a  large surface-to-volume ratio for more effective contact with electrolyte ions, such as mesoporous MnO2 nanotubes/nanosheets [18], nanowires [19] or flowerlike, urchin-like and nanorod-like structures [20] or ultra-small Co3O4 nanocubes [21], hollow coral-shaped nanostructures [12] or nanosheets [22] have been created. Incorporation of foreign conductive metals including Au, Al, Cu, Fe, Mg and Co [23–30] into TMOs lattices has been successfully applied and revealed the increase in Cs, for example, in the presence of Co the achieved Cs value was of 350 F g−1 at a current density of 0.1 A g−1 [29]. Analogously, creating binary, ternary or multy-mixed TMOs allowed improving the  capacitive performance of those electrode materials, compared to that of the  single metal compounds [31–35]. Among them, for example, are ultrathin amorphous Co-Fe-B nanosheets J. Jablonskienė, D. Šimkūnaitė, J. Vaičiūnienė, G. Stalnionis, A. Drabavičius, L. Tamašauskaitė-Tamašiūnaitė, E. Norkus deposited on the 3-dimensional nickel foam substrate with Cs of ca. 981 F g−1 at 1 A g−1 [31]; Mn–Co–Fe ternary hydroxide nanoplatelets directly settled on Ni foam delivering the  maximum Cs of 1200 F g−1 at a scan rate of 5 mV s−1 [32]; the Co3O4@PPy@MnO2 ‘core-shell-shell’ nanowire arrays on the nickel foam substrate exhibited a  prominent electrochemical performance with a high energy density of 34.3 Wh kg–1 at a power density of 80.0 W kg–1 [33]; MnO2/Ni(OH)2/NF composite with high Cs of 506 F g−1 at 16.7 A g−1 [34]; tremella-like NiO@Co3O4@MnO2 particles offered high Cs of 792.6 F g−1 at 2 A g−1 [35]. The  flower-like Co3O4@MnO2 core-shell microspheres coated on nikel foam exhibited a significantly enhanced Cs of 671  F  g−1 at 1  A  g−1 [36]. Furthermore, in order to overcome the  above-mentioned drawbacks, combining TMOs with high conductive substrates such as graphene, porous carbon, carbon nanotubes, activated carbon or carbon fiber is regarded not only an effective way to improve the electrochemical performance of SCs, but makes those materials rather attractive for fabricating flexible electrode materials with the aim to apply them as flexible, light and inexpensive energy storage devices. Recently, due to the  synergic effect Co-Co3O4@carbon-nanotube (CNT)-incorporated N-doped carbon (Cs of 671 F g−1 at 1 A g−1) [37] or in situ fabricated MnO2 and its derived FeOOH nanostructures on mesoporous carbon [38] further enhance the electrochemical performance of those TMOs stuctures and showed a  great potential in application for energy storage devices. The core-shell nanostructured TMOs have shown to be more stable, and able for providing an outstanding cycling stability at a  high current density. Among them are the  hierarchical core-shell NiCo2O4@NiMoO4 nanowires grown on carbon cloth [39] or manganese–nickel–cobalt sulfide (MNCS) multi-tripod nanotube arrays (NTAs) on a carbon nanotube fibers (CNTFs) surface [40], which could serve as flexible binder-free electrode materials for supercapacitors.

Herein, we demonstrate a facile microwave-assisted heating method for the synthesis of carbon supported MnO2–Co3O4 nanocomposites using different precursor materials with the aim to apply them as material for electrochemical supercapacitors.

EXPERIMENTAL

Preparation of nanocomposites

The MnO2–Co3O4 nanocomposite supported on carbon was prepared in two different ways by the  microwave-assisted heating. In the  first way, 0.2  g of KMnO4, 0.737  g of Co(NO3)2∙7H2O and 0.1  g of carbon powder were dispersed in 20  ml of deionized water. The  obtained reaction mixture was put into a microwave reactor Monowave 300 (Anton Paar) for 5 min at 150°C temperature. The precipitate was filtered out, washed with water and dried in a vacuum oven at 80°C temperature for 4 h. The prepared nanocomposite was labelled as MnO2–Co3O4/C-1. In the second way, 0.2 g of KMnO4, 0.01 g of Co3O4 and 0.1 g of carbon powder were dispersed in 20  ml of deionized water. The synthesis of nanocomposite was carried out at the same conditions. The prepared nanocomposite was labelled as MnO2–Co3O4/C-2. Notably, Co3O4 was prepared by annealing of Co(NO3)2∙7H2O in air atmosphere in a muffle furnace at 400°C temperature. The  formation of pure Co3O4 was confirmed by XRD analysis (not shown).

For comparison, the  carbon supported Co3O4 nanocomposite labelled as Co3O4/C was prepared by mixing 0.01 g of Co3O4 and 0.1 g of carbon powder in 20 ml of deionized water. The synthesis of nanocomposite was also carried out at the  same conditions.

Characterization of nanocomposites

The morphology and composition of the  prepared catalysts were characterized using a  SEM-focused ion beam  facility (Helios Nanolab 650) equipped with an EDX spectrometer (INCA Energy 350 X-Max 20). A shape and size of catalyst particles were examined using a transmission electron microscope Tecnai G2 F20 X-TWIN equipped with an EDX spectrometer with an r-TEM detector. For microscopic examinations, 10 mg of the sample was first sonicated in 1 ml of ethanol for 1 h and then deposited on the Ni grid covered with a continuous carbon film. Mn and Co loadings in the prepared samples were estimated using an ICP optical emission spectrometer Optima 7000DV (Perkin Elmer).

Electrochemical measurements

The  electrochemical performance of MnO2–Co3O4/C and Co3O4/C nanocomposites was tested using a  Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co. KG). The prepared nanocomposites coated on the glassy carbon electrode (GCE) with a  geometric surface area of 0.07 cm2 were employed as a working electrode, a Pt sheet as a counter electrode, and an Ag/AgCl/KCl electrode was used as a  reference. The  working electrodes were prepared as follows: 10  mg of the  prepared nanocomposites were dispersed ultrasonically in 0.1 ml of 2% of polyvinylidene fluoride (PVDF) in an N-methyl-2-pyrrolidinone (NMP) solution for 1 h. Then, 5 μL of the prepared suspension mixture was pipetted onto the polished surface of GCE and dried in an oven for 2 h at 80°C.

Cyclic voltammograms (CVs) were recorded in a 1 M Na2SO4 solution at different scan rates between 10 and 200 mV s–1 at ambient temperature. The  measuring potential range was from 0.05 to 1.10 V. All solutions were deaerated by argon for 15 min prior to measurements.

RESULTS AND DISCUSSION

The surface morphology of the  prepared MnO2–Co3O4/C nanocomposites was investigated by SEM analyses. Figure 1 shows the SEM images of MnO2–Co3O4/C-1 (a, b) and MnO2–Co3O4/C-2 (c, d) under different magnification. The use of different precursor materials for the synthesis of the carbon supported MnO2 and Co3O4 nanocomposites results in a different morphology of the prepared ones. As evident from Fig. 1a and b, the MnO2–Co3O4/C-1 nanocomposite has a  spatial layer of irregularly shaped lamellar nanostructures of ca. 10–50 nm in size. Meanwhile, in the case of MnO2–Co3O4/C-2 nanocomposite, the  spongy and porous frost-like three-dimensional surface is observed (Fig. 1c, d). The widely sprayed branches range from several to hundred nm in size.

More detailed microstructural information of the  synthesized MnO2–Co3O4/C nanocomposites has been provided by TEM analysis and are shown in Fig. 1e–h. The obtained data are in line with those determined by SEM analysis. In the case of MnO2–Co3O4/C-1 nanocomposite, tapered nanowires and nanosheets are almost uniformly distributed on the surface (Fig. 1e, f). Moreover, the aggregated spherical nanostructures composed of oblong branches are seen in the prepared MnO2–Co3O4/C-2 nanocomposite (Fig. 1g, h).

Fig. 1. SEM and TEM images of MnO2–Co3O4/C-1 (a, b, e, f) and MnO2–Co3O4/C-2 (c, d, g, h) under different magnifications

The electrochemical performance of the MnO2–Co3O4/C nanocomposites was evaluated from the cyclic voltammetry using a three-electrode system in an aqueous 1 M Na2SO4 electrolyte. Figure 2 presents the CVs of MnO2–Co3O4/C-1 (a), MnO2–Co3O4/C-2 (b) and Co3O4/C (c) at scan rates of 10, 50, 100 and 200 mV s−1 in a potential window of 0.05 to 1.1 V vs SHE. The CVs of all the nanocomposites at a scan rate of 10 mV s–1 are shown in Fig. 2d. As evident, the all prepared nanocomposites show a  quasi-rectangular and symmetric voltammetry curves at a low scan rate, indicating their good capacitive behaviour (Fig. 2d). Notably, with increasing scan rate up to 200 mV s−1, the current density values increase gradually, but CVs do not always maintain a rectangular shape, especially at a high scan rate (Fig. 2a–c). The CV area of MnO2–Co3O4/C-2 nanocomposite is larger than that of MnO2–Co3O4/C-1 and Co3O4/C (Fig.  2d). This result suggests that the  specific capacitance of MnO2–Co3O4/C-2 nanocomposite is higher as compared with that for MnO2–Co3O4/C-1 and Co3O4/C nanocomposites. Cs (F  g−1) of the  electrode material was calculated from the CV test according to the following equation (Eq. 1) [41]:

C s = 1 m⋅v⋅ΔV ∫ idv .                                       (1)

Here Cs is the  specific capacitance (F  g−1), m is the mass of the active material (g), v is the scan rate of potential (V s−1), ∆V is the range of scan potential (V), and i is the current (A). Plots of Cs versus scan rate for MnO2–Co3O4/C-1, MnO2–Co3O4/C-2 and Co3O4/C are presented in Fig. 3. It is clearly seen that the Cs values of all the nanocomposites decrease with the growth of scan rate from 10 to 200 mV s−1, nevertheless the  Cs values of the  nanocomposites are still higher. The  calculated Cs value for the  MnO2–Co3O4/C-2 nanocomposite is highest at a  scan rate of 10  mV  s−1 among the  investigated nanocomposites and equal to 658.8 F g−1 (Fig. 3). This value is much larger than that of MnO2–Co3O4/C-1 (335.0 F g−1) and pure Co3O4/C (144.1  F  g−1). When the  scan rate increases to 200 mV s−1, the MnO2–Co3O4/C-1, MnO2–Co3O4/C-2 and Co3O4/C nanocomposites still exhibit Cs of 91.6, 384.2 and 36.7  F  g−1, respectively (Fig.  3). The  MnO2–Co3O4/C-1, MnO2–Co3O4/C-2 and Co3O4/C nanocomposites preserve 27.3, 58.3 and 25.5%, respectively, of their specific capacitance (from 335.0 F g−1 to 91.6 F g−1, from 658.8 F g−1 to 384.2 F g−1, and from 144.1 F g−1 to 36.7 F g−1) as the  scan rate increases from 10  mV  s−1 to 200 mV s−1.

Fig. 2. CVs of MnO2–Co3O4/C-1 (a), MnO2–Co3O4/C-2 (b) and Co3O4/C (c). (d) CVs of the same nanocomposites at a scan rate of 10 mV s−1
Fig. 3. Plots of the  specific capacitance versus scan rate of MnO2–Co3O4/C-1, MnO2–Co3O4/C-2 and Co3O4/C nanocomposites
Table. Comparison of Cs for various MnO2-based electrode materials
Materials Scan rate, current density Specific capacitance, F g−1 Ref.
MnO2–Co3O4/C-1 10 mV s−1 335.0 This work
MnO2–Co3O4/C-2 10 mV s−1 658.8 This work
Co-doped MnO2 10 mV s−1 287.0 [29]
Co3O4@PPy@MnO2 2 mV s−1 518.0 [33]
Biomorphic Co3O4 nanocrystal/mesoporous carbon microtube composites 1 A g−1 284.2 [11]
Hollow coral-shaped Co3O4 nanostructures 5 mV s−1 626.5 [12]
Co3O4 core-shell microspheres 0.5 A g−1 261.1 [13]
Au-doped MnO2 5 mV s−1 626.0 [24]
Cobalt-doped MnO2 hierarchical yolk-shell spheres 0.1 A g−1 350 [29]
Flower-like Co3O4@MnO2 core-shell composite 1 A g−1 671 [36]

Comparison of the supercapacitive behaviour of MnO2 nanocomposites reported in literature and the present work are listed in the Table, exhibiting the high specific capacitance of our prepared electrode materials.

CONCLUSIONS

In this study, we report a rapid synthesis of the carbon supported MnO2 and Co3O4 nanocomposites via a  simple and facile microwave-assisted heating method without any complicated extra-post-treatment procedures. The application of different precursor materials for the  preparation of carbon supported MnO2–Co3O4 determines directly the formation of nanocomposites having different morphology such as irregularly shaped lamellar or spherical nanostructures that have a decisive impact on the electrochemical performance. The  highest specific capacitance of 658.8  F  g−1 at a scan rate of 10 mV s−1 has been achieved at the  MnO2–Co3O4/C-2 nanocomposite that has a  spherically shaped nanoparticle architecture in comparison with that of MnO2–Co3O4/C-1 with a  lamellar shape structure. The  prepared MnO2–Co3O4 nanocomposites are expected to be a promising electrode material for supercapacitor applications.

ACKNOWLEDGEMENTS

This research is funded by the European Social Fund under Measure No.  09.3.3-LMT-K-712-02-0142 ‘Development of Competences of Scientists, other Researchers and Students through Practical Research Activities’.

Received 17 March 2020

Accepted 19 March 2020

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Jolita Jablonskienė, Dijana Šimkūnaitė, Jūratė Vaičiūnienė, Giedrius Stalnionis, Audrius Drabavičius, Loreta Tamašauskaitė-Tamašiūnaitė, Eugenijus Norkus

MANGANO(IV)-KOBALTO(II/III) OKSIDŲ, SUFORMUOTŲ ANT ANGLIES PAGRINDO, TAIKYMAS ELEKTROCHEMINIAMS SUPERKONDENSATORIAMS

Santrauka

Naudojant iš skirtingų prekursorių susintetintus MnO2 ir Co3O4, MnO2-Co3O4/C nanokompozitai buvo formuojami ant anglies pagrindo taikant mikrobangų sintezės metodą. Skirtingais būdais suformuotų nanokompozitų morfologija ir sudėtis buvo tiriami naudojant skenuojančiąją elektronų mikroskopiją (SEM), peršviečiamąją elektroninę mikroskopiją (TEM) ir indukciškai susietos plazmos optinės emisijos spektroskopiją (ICP-OES). MnO2-Co3O4/C nanokompozitų elektrocheminė elgsena buvo tiriama taikant ciklinę voltamperometriją (CV).

Nustatyta, kad sintezės metu susidarančių nanokompozitų paviršiaus morfologija priklauso nuo naudotų prekursorių prigimties. Kai sintezėje buvo naudojamas Co3O4, gaunamas MnO2-Co3O4/C-2 nanokompozitas, kuriam yra būdingos sferinės formos nanodalelės. 1 M Na2SO4 tirpale, kai potencialo skleidimo greitis buvo 10  mV  s−1, išmatuotoji didžiausia specifinės talpos (Cs) vertė siekė 658,8  F  g−1. Be to, pastarasis nanokompozitas pasižymi žymiai geresnėmis katalizinėmis savybėmis nei MnO2-Co3O4/C-1 nanokompozitas (šiuo atveju kaip prekursorius buvo naudojamas Co(NO3)2), kuriam būdingos plokštelinės formos nanodalelės.

* Corresponding author. Email: jolita.jablonskiene@ftmc.lt