High-Performance Asymmetric Supercapacitors of MnCo2O4

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High-performance asymmetric supercapacitors of MnCo2O4 nanofibers and N-doped reduced graphene oxide aerogel Tanut Pettong, Pawin Iamprasertkun, Atiweena Krittayavathananon, Phansiri Suktha, Pichamon Sirisinudomkit, Anusorn Seubsai, Metta Chareonpanich, Paisan Kongkachuichay, Jumras Limtrakul, and Montree Sawangphruk ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09440 • Publication Date (Web): 17 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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ACS Applied Materials & Interfaces

High-Performance Asymmetric Supercapacitors of MnCo2O4 Nanofibers

and

N-doped

Reduced

Graphene Oxide Aerogel Tanut Pettonga,b, Pawin Iamprasertkuna,b, Atiweena Krittayavathananona, Phansiri Sukhaa,b, Pichamon Sirisinudomkita,b, Anusorn Seubsaib, Metta Chareonpanichb, Paisan Kongkachuichayb, Jumras Limtrakulc, and Montree Sawangphruka *

a

Department of Chemical and Biomolecular Engineering, School of Energy Science and

Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand b

Department of Chemical Engineering, Center for Advanced Studies in Nanotechnology for

Chemical, Food and Agricultural Industries, and NANOTEC-KU-Centre of Excellence on Nanoscale Materials Design for Green Nanotechnology, Kasetsart University, Bangkok 10900, Thailand c

Department of Materials Science and Engineering, School of Molecular Science and

Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand

KEYWORDS: MnCo2O4 nanofibers, N-doped reduced graphene oxide aerogel, asymmetrical supercapacitor, the charge storage mechanism, X-ray absorption spectroscopy

ACS Paragon Plus Environment

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ABSTRACT

The working potential of symmetric supercapacitors is not so wide because one-type material used for the supercapacitor electrodes prefers either positive or negative charge to both charges. To address this problem, a novel asymmetrical supercapacitor (ASC) of battery-type MnCo2O4 nanofibers (NFs)//N-doped reduced graphene oxide aerogel (N-rGOAE) was fabricated in this work. The MnCo2O4 NFs at the positive electrode store the negative charges i.e., solvated OH-. Whilst, the N-rGOAE at the negative electrode stores the positive charges i.e., solvated K+. An asfabricated aqueous-based MnCo2O4//N-rGOAE ASC device can provide a wide operating potential of 1.8 V, high energy density and power density at 54 Wh kg-1 and 9851 W kg-1, respectively with 85.2 % capacity retention over 3000 cycles. To understand the charge storage reaction mechanism of the MnCo2O4, the synchrotron-based X-ray absorption spectroscopy (XAS) technique was also used to determine the oxidation states of the Co and Mn at the MnCo2O4 electrode after electrochemically tested. The oxidation number of Co is oxidised from +2.76 to +2.85 after charged and reduced back to +2.75 after discharged. On the other hand, the oxidation state of Mn is reduced from +3.62 to +3.44 after charged and oxidized to +3.58 after discharged. Understanding in the oxidation states of Co and Mn at the MnCo2O4 electrode here leads to the awareness of the uncertain charge storage mechanism of the spinel-type oxide materials. High-performance ASC here in this work may be practically used in high power applications.

INTRODUCTION


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The supercapacitors (SCs) are of interest and widely used in electric vehicles (EVs) and hybrid EVs and energy backup and storage systems due to their high power density up to 10 kW kg-1, long cycle life (300,000 cycles), low-cost, and environmentally friendly1-3. As compared to Li-ion batteries and fuel cells, the supercapacitors have lower energy density but higher power density4. Normally, the SCs are principally divided into two types including electric double layer capacitors (EDLCs) and pseudocapacitors (PCs). Carbon-based materials (i.e., activated carbon, carbon nanotubes, graphene, and carbon aerogel) are normally applied as the active materials in the EDLCs due to their high specific surface area and electrical conductivity5-7, which can store the ionic charges by a physical adsorption at the solid-liquid interfaces leading to high charge/discharge rate, high power density, and long cycle life. In contrast, the EDLCs have in principle lower specific capacitance and energy density than those of the PCs. This is because the charge storage mechanisms of PCs are based on surface redox reactions at the electrode/electrolyte interface, which can in principle store more electronic charges8-11. Recently, spinel-type oxides with a formula of AB2O4 where A and B are transition metals e.g., MnCo2O4, CuCo2O4, NiCo2O4, and ZnCo2O4 are of interest as the active materials of the PCs owing to their high theoretical capacitances12-13. Among these ternary cobaltites, MnCo2O4 has a high theoretical specific capacitance of 3,620 F g-1 14. This is rather high when compared with other supercapacitor materials i.e., graphene (550 F g-1), MnO2 (ca. 1370 F g-1), and RuO2.xH2O (ca. 1300-2200 F g-1)15-17. In addition, these spinel-type MnCo2O4 has lower activation energy level for transporting electron between cations than those of monometallic oxides (such as Co3O4, MnOx, NiO, and Fe3O4) leading to higher rate capability18.Besides, MnCo2O4 has excellent physicochemical and electrochemical properties19-20. This is because Co exhibits high oxidation potential and Mn can store and transport more electrons leading to high


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capacity21.Although the MnCo2O4 is appropriated to be used as the active material of the PCs, its ionic and electrical conductivities are rather poor. As the result, MnCo2O4 nanofibers (NFs), which can reduce the diffusion path of ionic electrolytes and accelerate the charge transfer, are produced by an electrospinning technique in this work. It is also necessary to note here that other cheap materials with the general formula of AB2O4 e.g., MnFe2O4 with a cubic-type structure22 are also of interest since they are cobalt-free compounds and involve a charge transfer at Mnand Fe-ion sites23. However, they provide low specific capacitances22-24. Although the symmetric supercapacitors have many outstanding properties, their working potentials are in principle limited by one-type material used at both positive and negative electrodes. This is because a material basically prefers either positive or negative charge to both charges. In order to address this issue, the asymmetric supercapacitors (ASC) using different electrode materials are of interest25. Over the past decade, graphene-based materials have been extensively studied as the negative electrodes of the supercapacitors because of their wide negative potential ranges, high theoretical surface area (2,630 m2g-1), and outstanding electrical conductivity26-28. However, the restacking of graphene sheets is a drawback limiting the charge storage capacity of the graphene-based supercapacitors having low specific capacitance (< 550 F g-1). Hence, the interconnected porous structure of graphene with diluted nitrogen-containing groups known as N-doped reduced graphene aerogel (N-rGOAE) is currently of interest because the framework structure of graphene aerogel can avoid the restacking and agglomerating issues of the individual graphene sheet and also help to significantly increase the ionic conductivity owing to the nitrogen-containing groups29-31. In this work, the MnCo2O4 NFs produced by an electrospinning technique followed by an optimized calcination process are used as the positive electrodes basically storing the solvated


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anions (OH-)13,

32

. Whilst, the negative electrode is N-rGOAE, which is prepared by a simple

hydrothermal reduction process of graphene oxide (GO) using hydrazine hydrate as a nitrogen source compound (reducing agent), for storing the solvated cations (K+)31. Note, the ASC of MnCo2O4 NFs//N-rGOAE has not yet been investigated previously. Ex situ X-ray adsorption spectroscopy (XAS), a synchrotron-based technique, was also used to probe the oxidation states of Mn and Co after charged/discharged. The results showed that the ASC device can provide a wide working potential (1.8 V), a high-energy density of 54 Wh kg-1 and a high-power density of 9851 W kg-1. Interestingly, the XAS showed that the oxidation number of Co is oxidized from +2.76 to +2.85 after the 1st charged and reduced to +2.75 after the 1st discharged. The oxidation number of Mn is reduced from +3.62 to +3.44 after the 1st charged. It is then oxidized to +3.58 after the 1st discharged. EXPERIMENTAL SECTION Synthesis of MnCo2O4 NFs. Firstly, 1 g polyacryonitrile (PAN, Mw = 150,000 g mol-1, Sigma Aldrich) was dissolved in 10 ml of N,N-dimethylformamide (grade AR, Qrec) solution under a successive stirring speed 100 rpm at 60 °C for 3 h. Then, 0.90 mmol of Co(CH3COO)2·4H2O (UNILAB) and 0.45 mmol of Mn(CH3COO)2·4H2O (ACROS) were slowly added into the as-prepared polymer matrix and kept stirring at same speed for 2 h at room temperature. The mixture was delivered into a plastic syringe (10 ml) by metallic needle (NIPRO, 22Gx1 int.). During the electrospinning process, the syringe was positioned on the syringe pump and the metallic needle was connected with a high-voltage generator. The electrospun fibers were collected on a rotating metal drum collector, which is covered by aluminium foil. The distance between the syringe tip and the drum collector was maintained at about 12 cm. The direct current (DC) voltage was applied at 15 kV and the feeding rate was


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fixed at 2 ml h-1. The bright pink fiber was collected and followed by the calcination process at 500 °C for 3 h with a heating rate of 1 °C min-1 to remove the polymer matrix resulting in the MnCo2O4 NFs. Synthesis of N-rGOAE. To prepare the N-rGOAE, the graphene oxide (GO) from our development of modified Hummers method10,

27, 28, 33

was reduced by a hydrothermal process

using hydrazine as a strong reducing agent. Briefly, 200 mg of GO was dispersed in 80 ml of Milli-Q water (pH 6.5). The dispersion of GO is significantly required. In this work, the container of GO in Milli-Q water was placed into an ultrasonic bath for 1 h. After adding 0.5 M N2H4 (80% hydrazine hydrate, Merck), the brown suspension solution was transferred into a 100 ml of Teflon autoclave. The hydrothermal process was controlled at 80 °C in oven (SLN, POLEKO APARATURA) for three days. Then, the autoclave was cooled down to room temperature at a natural cooling rate. The black cylindrical sponge, known as nitrogen doped reduced graphene oxide hydrogel, was soaked in Milli-Q water for two days to remove some residual hydrazine inside the hydrogel pores. The sponge was immediately frozen via liquid nitrogen before placed in a freezing dryer machine (Labconco, 2.5 Liter Benchtop Freeze Dry Systems) at -50 °C for two days. Morphological and structural characterizations. The morphologies of MnCo2O4 NFs and N-rGOAE were characterized by Field-emission scanning electron microscopy (FE-SEM, JSM7001F (JEOL Ltd., Japan)) and transmission electron microscopy (TEM, JEM 1220 (JEOL Ltd., Japan)). The temperature profile of as-spun fibers was observed via thermogravimetric analysis (TGA) using a thermogravimetric analyzer (LINSEIS, STA PT1600) at a heating rate of 5°C·min-1 under air atmosphere. To study the crystal structure of MnCo2O4 NFs, X-ray diffraction (XRD) from Bruker optics, Germany was carried out using monochromatic Cu Kα


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radiation (λ = 0.15405 nm) on the powder of MnCo2O4 NFs. The functional groups of MnCo2O4 NFs were investigated by Fourier Transform Infrared Spectrometer (FT-IR, PerkinElmer Paragon 1000). The characteristic and defect information of N-rGOAE was studied by Raman spectroscopy (Senterra Dispersive Raman Microscope, Bruker) with an excitation wavelength of 532 nm. N2 sorption/desorption isotherm of N-rGOAE was performed at 77 K (Autosorp 1 MP, Quantachrome) using Brunauer-Emmett-Teller (BET) technique to determine the specific surface areas. The functional groups and bonding on the surface of MnCo2O4 NFs and N-rGOAE were characterized by X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos Analytical Ltd., with Al-K alpha radiation, hʋ = 14,866 eV). The changed oxidation numbers of Mn and Co on MnCo2O4 NFs electrode after charged/discharged were also investigated by Ex situ XAS on the V beamline at the Synchrotron Light Research Institute (Public Organization), Thailand using a double-crystal of Ge (220) in Fluorescence mode.

Fabrication of MnCo2O4//N-rGOAE ASCs and the electrochemical evaluation. To fabricate the electrodes of ASCs, the as-prepared material (8 mg), conductive carbon black (1 mg) and polyvinylidene difluoride (1 mg) (PVDF, Sigma–Aldrich, Mw~534,000) were mixed in a proportion of 8:1:1 and dispersed in n-methyl-2-pyrolidone (3 ml) and spray-coated on the circle carbon fiber paper (CFP, SGL Carbon SE (Germany)), with a diameter of 1.58 cm using an airbrush (Paasche Airbrush Company, USA) with 0.3 mm of brush nozzle at 20 psi and 25 °C. The electrode was dried under vacuum at 60 °C for 1 day. Each electrode contains about 2 mg of the active material. The cellulose separator was cut in a diameter of 2.0 cm and soaked in 1 M KOH. The separator was then put in between the positive and the negative electrodes. Also, the spacer was placed on the negative side. Then, the supercapacitor cell with the CR-2016 size was


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pressed with the crimper machine at a pressure of 100 psi. The electrochemical properties of the as-fabricated coin cell were characterized using the Metrohm AUTOLAB potentiostat (PGSTAT 302N) using a NOVA software (1.1.0). RESULTS AND DISCUSSION

The as-prepared solution between CoAc and MnAc at a mole ratio of 2:1, was electrospun to form the composite fibers with a rough surface and a diameter of about 500 nm (see Figure S1) due to the effect of humidity during the electrospinning process34. To make a binary transition metal oxide for the ASC, the calcination process in air was used for which it can remove the polymeric component (PAN) at the temperature over 480°C as shown in thermogravimetric analysis (TGA) result (see Figure S2). After annealing process at 500 °C for 3 h, the MnCo2O4 NFs as shown in Figure 1a are formed and the diameter of the fibers is ca. 150-200 nm. The reaction mechanism in the calcination process is as following reaction (1);

Mn(CH3COO)2 + 2Co(CH3COO)2 + 25/2 O2 → MnCo2O4 +12CO2 + 9H2O

(1)

The smaller diameter here is because of the removal of the PAN matrix and the crystallization of MnCo2O4. The high magnification of the fibers is displayed in the inset image of Figure 1a. It can be observed that these fibers consist of polygonal-shaped crystallites with the size of ca. 2050 nm connected each other to form the fibers with the rough surface35-36. The morphology of the MnCo2O4 NFs also investigated by transmission electron microscopy (TEM) shows a single fiber with a diameter of ca. 159 nm (see Figure 1c), which is in good agreement with that carried out by FE-SEM. The high resolution TEM image of the MnCo2O4 NFs (see inset image in Figure. 1c) displays clear lattice fringes with interplanar spacing value of 0.287 nm attributing to the


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(220) plane of the spinel phase. Figure 1b illustrates the morphology of N-rGOAE, which has a 3D porous network containing interconnected pores of ca. 0.5-2 µm and ultrahigh porosity, which can improve the electrolyte diffusion via a capillary force8, 37. The TEM image of NrGOAE is shown in Figure 1d showing almost transparent sheets with wrinkled paper-like structure.

Figure 1. FE-SEM images of (a) MnCo2O4 NFs and (b) N-rGOAE as well as TEM images of (c) a MnCo2O4 NF and (d) N-rGOAE. Figure 2a shows an XRD pattern of the as-obtained binary transition metal oxide. The peaks at 2θ of 18.58, 30.52, 36.02, 43.82, 57.90 and 63.53o are assigned to (111), (220), (311), (400), (511) and (400) planes, respectively. The diffraction patterns are related to a cubic spinel


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structure of MnCo2O4 phase (JCPDS no. 23-1237) with a space group of Fd3m13, 38. Moreover, the crystallite size of MnCo2O4 nanostructures calculated using Scherrer’s equation is ca. 17.31 nm confirming that the as-prepared fibers consist of nanocrystalline subunits39. The functional groups of the MnCo2O4 fibers characterized by FT-IR spectroscopy are shown in Figure 2b. Two main peaks at 539 and 636 cm-1 are attributable to the vibrational bending modes of Co-O and Mn-O, respectively. The peaks around 3,412 and 1,647 cm-1 are related to the -OH stretching vibration mode and the angular deformation of absorbed water, respectively. The peak at 1,121 cm-1 is owing to a C-O bond40. The N2 adsorption/desorption analysis was conducted to examine the specific surface area and pore size distribution. Figure S4a shows a type IV isotherm indicating that the MnCo2O4 NFs mainly consist of mesopores. In addition, a hysteresis loop exhibits the type H1 referring that the cylindrical pores were formed from the aggregation of the particles on the cylinder wall41. The BET surface area of the as-prepared nanofibers is 29.4 m2 g1

which is higher than the previous report36. The Raman spectra of N-rGOAE as shown in Figure

2c presents two district bands at 1,353 and 1592 cm-1 owning to the D and G bands, respectively. Generally, the peak intensity of D band indicates the disorder properties of carbon structure, while the peak intensity on G band position normally displays the C-C bond stretching vibration on the graphitic backbone. The peak intensity ratio between D and G bands is 1.10 indicative of the quality of the graphitic lattice, which agrees well with other previous work31. Besides, the gas adsorption of the as-prepared N-rGOAE was investigated using a nitrogen adsorption technique as shown in Figure 2d. The N-rGOAE displays a type IV isotherm, suggesting that it mainly comprises of mesoporous structure. This is attributed to the inter-layer void between N-rGOAE sheets42. A hysteresis loop of the relative pressure from 0.4 to 0.99 can be assigned to type H2, which comes from the interconnected pore networks. The BET surface area and the total pore


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volume of the N-rGOAE are 368.25 m2 g-1 and 0.55 cm3 g-1, respectively, which are higher than other previous report31, 43. The pore size distribution curve (see an inset image in Figure 2d) derived from the desorption using the DFT method indicates that it exhibits a predominant peak at 3.67 nm thanks to the existing mesopores within the N-rGOAE.

Figure 2. Structural analyses of the as-synthesized materials: (a) XRD pattern and (b) FTIR of MnCo2O4 as well as (c) Raman spectra and (d) N2 sorption isotherm of N-rGOAE. To confirm the chemical composition of the as-prepared materials, the Co2p spectrum of the MnCo2O4 in Figure 3a appears two main peaks at 780.8 and 796.1 eV, representing the two main spin-orbit lines of Co2p3/2 and Co2p1/2, respectively. Likewise, the satellite peak at 785.2 eV refers to the existence of Co2+, whilst the satellite peak at 790.1 eV is attributed to the presence


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of Co3+. Thus, the as-synthesized MnCo2O4 is the mixed valances of Co2+/3+ at the surface of the sample44. The Mn2p spectrum of the MnCo2O4 is shown in Figure 3b for which two main peaks are found at 642.4 and 653.9 eV, indicating the Mn2p3/2 and Mn2p/2, respectively. The energy pairs at 642.2 and 653.5 eV are defined as the existence of Mn3+ and the other two peaks at 644.6 and 655.1 eV are designated as the presence of Mn4+45-47. The as-calcined MnCo2O4 fibers also contain the mixed valences of Mn3+/4+. The C1s and N1s XPS spectra of the N-rGOAE are presented in Figure 3c and Figure 3d, respectively. The C1s XPS spectrum can be deconvoluted into 284.1, 284.95, 285.5, 286.3 and 288.2 eV, attributing to C=C, C-C, C-N, C-O and C=O, respectively48-49. Besides, the N1s spectrum with the nitrogen content of ca. 4.14 % is shown in Figure 3d. The N1s spectrum can be separated into three main components of pyridinic N, pyrolic N, and graphitic N structures at 398.9, 400.0 and 401.4 eV, respectively50. The presence of the graphitic-N content on the rGO sheets can increase ionic conductivity while the pyridinicN and pyrolic-N can improve the charge storage via increasing surface redox reaction30.


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Figure 3. XPS spectra of MnCo2O4 and N-rGOAE: (a) Co2p and (b) Mn2p of the MnCo2O4 as well as (c) C1s and (d) N1s of N-rGOAE. To further study the electrochemical properties of the as-prepared materials before applied as the active material in the positive and negative electrodes of the asymmetric supercapacitor, the cyclic voltammetry (CV) of the half-cell electrodes were carried out in 1 M KOH. A threeelectrode system was set up using a Pt wire as a counter electrode, Ag/AgCl as a reference electrode, and as-synthesized material as a working electrode. The CVs of the MnCo2O4 storing the solvated anions i.e., OH- at an anodic potential region from 0.0 V to 0.8 V (vs. Ag/AgCl) are shown in Figure S6a. The CVs reveal that the capacitive characteristics mostly rely on the redox reaction behavior. The anodic peak appearing at about 0.35 V (vs. Ag/AgCl) corresponds to the


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oxidation process of Co2+/Co3+, while the cathodic peak at about 0.24 V (vs. Ag/AgCl) represents the reduction process of Mn3+/Mn4+ 51-52. In addition, Figure S6c shows the CVs of the N-rGOAE negative electrode at the cathodic potential range from 0.0 to -1.0 V (vs. Ag/AgCl). The CVs exhibit the typical rectangular-like shape of EDLC indicating a fast diffusion of the solvated K+ into the porous electrode of the N-rGOAE due to a small equivalent series resistance (ESR) of the electrode. In addition, the observation of the small redox peak presents the pseudocapacitive behavior of N-containing groups of the N-rGOAE. To improve the cell efficiency of the ASC device, the mass ratio of the active materials should be determined by following the previous reports53-54. Herein, the proportion of mass loading on the positive electrode over the negative electrode is about 1.03. An ASC device with a coil cell size (CR 2016) was assembled using the MnCo2O4 NFs for the positive electrode and the N-rGOAE for the negative electrode (assigned as the MnCo2O4//NrGOAE) for which 1 M KOH soaked cellulose membrane was used as the separator. Figure 4a shows the CVs of MnCo2O4//N-rGOAE ASC carried out at various scan rates (10, 25, 50, 75 and 100 mV s-1) from 0 to 1.8 V. The CVs indicate the contributions from both EDLC and pseudocapacitance. In addition, it can be observed that the scan rate has influenced to CVs shape and redox potential. Owing to increasing the scan rate, the slight change in the CV profiles is observed for which both cathodic and anodic peaks are moved gradually to more cathodic and anodic directions due to the electrical polarization in the active layer on the electrode during a fast scan rate55. The specific capacitances of a supercapacitor cell are 108 F g-1 at 10 mV s-1 and 77.5 F g-1 at 100 mV s-1 (∼72 % retention) (Figure 4b). The capacitive effects were also investigated by using Power’s law (see the calculation detail in the supporting information), the calculated b-value of this device are shown in Figure S8b. It implies that the capacitance consists


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of both the redox intercalation process (battery-like behavior, b = 0.5) and ions adsorption (capacitive effect, b = 1). Besides, we further determined the contribution percentage of intercalation and capacitive effects at different scan rates (Figure 4c). The results showed that the capacitive contribution is significantly increased at slow scan rates while the capacitive contribution is slightly increased at faster scan rates, indicating the underused inner bulk of the active materials and the

diffusion limit of the electrolytes at fast scan rate56. To find an

appropriate operating potential of the device excluding iR drop, the galvanostatic charge/discharge (GCD) was used57. Figure 4d shows GCD curves of an optimized operating window potential at various voltage windows (from 1 to 1.8 V) at a current density of 1 A g-1. The GCD curves exhibit a symmetric triangular shape up to 1.8 V, which is a stable electrochemical voltage of this device. This is an extremely wide working voltage of 1.8 V for the aqueous-based supercapacitors. The GCD curves of MnCo2O4//N-rGOAE at different current densities are shown in Figure 4e. All curves are symmetrical shape with small iR drop representing low internal resistance due to 1D structure of the MnCo2O4 NFs on the positive electrode. This structure can reduce the diffusion time of solvated OH- ions. In addition, all discharge lines are almost linear to their representing charge counterparts because of good electrical conductivity property and high electrochemical reversible behavior of N-rGOAE at the negative electrode. The MnCo2O4//N-rGOAE device provides the specific capacitances of 128.9, 110.4, 95.1, 88.5 and 84.0 F g-1 at the current densities of 0.5, 1, 2, 3 and 4 A g-1, respectively (Figure 4f).


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Figure 4. The electrochemical properties of the MnCo2O4//N-rGOAE ACS device: (a) CVs and (b) the specific capacitance at different scan rates, (c) bar chart showing the percentage contribution from capacitive and intercalation at different scan rates, (d) GCD curves at different


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operating potentials at 1 A/g, (e) GCD curves and (f) the specific capacitance at different current densities. The electrochemical impedance spectroscopy (EIS) of MnCo2O4//N-rGOAE ASC was also studied by applying a small amplitude of 5 mV over a range of frequencies from 100 kHz to 1 mHz. In Figure 5a, the Nyquist plot of MnCo2O4//N-rGOAE ASC device displays a semicircle shape at a high frequency region (bottom left of Figure 5a) owing to the redox reaction of MnCo2O4 NFs, and a linear line at low frequency region attributing to an ideal capacitance property. The intercept on the x-axis at high frequency implies the small ESR, which is only 1.6 Ω, which is well agreeable with other previous work58. Also, the charge transfer resistance (Rct) value of the as-fabricated device is 6.5 Ω, indicating a good conductivity and low internal resistance. We further investigated the relaxation time (τ0) using the complex power analysis59. The result is plotted in Figure 5b for which the τ0 of the MnCo2O4//N-rGOAE ASC device is ∼1.2 s. The smaller τ0 implies that the as-fabricated devices can provide a quick access to the discharging process, representing the high power density of the device60. The capacitance retention of MnCo2O4//N-rGOAE ASC device remained at 85.2% after the charged/discharged over 3000 cycles at a current density of 1 A g-1, as presented in Figure 5c. This is rather high since the MnCo2O4 is basically classified as the battery-type material, which is in principle poor in the capacity retention. As compared with recent related work, this capacity retention is comparatively higher than 60.0% of the MnO2/MnCo2O4 electrode32. This is due to the 1D structure of nanofiber that can prevent self-aggregation after the long period of charged/discharged processes61. The as-fabricated device provides a maximum energy density of 54.11 Wh kg-1 at 0.5 A g-1 and a maximum power density of 9851.02 W kg-1 at 4 A g-1, which are higher than those of other previous work (Figure 5d)62-66.


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Figure 5. (a) Nyquist plot of the MnCo2O4//N-rGOAE ACS device, (b) the complex power analysis of the as-fabricated ASC, (c) the capacitance retention, (d) Ragone plots of the asfabricated ASC compared with other previous work. It is well known that the binary transition metal oxides such as NiCo2O4, CuCo2O4, MnCo2O4 have many valances leading to various oxidation states, which play a significant role to faradic redox reaction during the electrochemical processes. The ex situ Co and Mn K-edge X-ray absorption spectroscropy (XAS) at the MnCo2O4 electrode was carried out after the charged/discharged.


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Figure 6. Ex situ high resolution XAS: (a) Co K-edge spectra and (c) Mn K-edge spectra of the MnCo2O4 NFs after charged/discharged. The oxidation numbers of (b) Co and (d) Mn elements in the MnCo2O4 NFs using the edge energy shift of the reference compounds. Figure 6a shows the XANES spectra of Co atoms in the MnCo2O4 electrode after the electrochemical process compared with the CoO (Co2+) and Co2O3 (Co3+) standards. The oxidation state of Co in each sample was calculated by an empirical eq. (2)67.

Co oxidation state = 3×

∆E of sample ∆E of Co2+ and Co3+

+ 2× 1-

∆E of sample ∆E of Co2+ and Co3+

(2)


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At the beginning, the Co edge energy of the fresh electrode is 7720.58 eV corresponding to the combining oxidations state of Co2+/3+ with the average oxidation number of +2.76, which is well agreeable with the XPS result. After applied a static current at 1 A/g for a charging process, the position of the edge is significantly increased to 7721.19 eV (+2.85) due to a lost electron in p orbital character of the 1s → 4p orbital due to the d-p mixing structure68. The oxidation processes for the charge storage mechanism of the MnCo2O4 are proposed as following reactions (3-5);

MnCo2 O4 + H2 O + OH- ↔ 2CoOOH- + MnOOH- + e-

(3)

CoOOH- + OH- ↔ CoO2 + H2 O + e-

(4)

MnOOH- + OH- ↔ MnO2 + H2 O + e-

(5)

After discharged, the Co edge energy is almost fully recovered to the initial state at about 7720.50 eV (+2.75). This behavior shows the complete reversible capacity of Co in the MnCo2O4 structure. Interestingly after 3000 cycles, it reveals the same Co edge energy as the 1st cycle. Alternatively, Figure 6c shows the Mn K-edge XAS spectra of MnCo2O4 during the electrochemical process as compared with Mn2O3 (Mn3+) and MnO2 (Mn4+) standards. The oxidation number of Mn can be determined by the following eq. (6).

Mn oxidation state = 4×

∆E of sample ∆E of Mn3+ and Mn4+

+ 3× 1-

∆E of sample ∆E of Mn3+ and Mn4+

(6)


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Before charged, the Mn edge energy of the electrode is 6551.22 eV reflecting to the combination oxidation number of Mn3+/4+ with the average oxidation state of +3.62 in good agreement with the XPS result. After fully charged, the edge energy of Mn is reduced to 6550.46 eV (+3.44) since the edge energy is shifted to the lower energy in its K-edge spectra. This shows the reduced oxidation state of Mn in the MnCo2O4 electrode. As noticed, after the 1st discharged, the edge energy of Mn is increased to 6551.04 eV corresponding to the Mn oxidation state of +3.58, which is lower than that at the initial state before tested. This is because of the limit of electronic and ionic conductivities of the MnCo2O469. After 3000 cycles, the oxidation state of Mn nearly returns to the initial state at +3.57. The understanding in the oxidation states of Co and Mn in the MnCo2O4 electrode leads to the proposed charge storage mechanisms of the MnCo2O4 proposed in the reactions (3-5) above.

CONCLUSIONS MnCo2O4 NFs were prepared via the electrospinning technique together with the subsequent calcination process and used as the positive electrode of the ASC. On the other hand, N-rGOAE was synthesized by a hydrothermal process of GO with hydrazine (a nitrogen source) and used as the negative electrode of the ASC. As-fabricated MnCo2O4//N-rGOAE ASC device with an optimum mass loading ratio (m+/m-) of 1.03 provides a wide working potential of 1.8 V, high energy density of 54.11 Wh kg-1 and high power density of 9851.02 W kg-1. This device has 85.2 % retention after 3000 cycles, which is rather high since battery-type MnCo2O4 is basically poor in capacity retention. Furthermore, the ex situ XAS technique was further used to study the oxidation state of Co and Mn in the MnCo2O4 electrode after electrochemically tested. The oxidation number of Co is oxidized from +2.76 to +2.85 after charged and reduced back to +2.75


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after discharged. Interestingly for Mn in the MnCo2O4 electrode, its oxidation state is reduced from +3.62 to +3.44 after charged. It is then oxidized to +3.58 after discharged. The understanding in the oxidation states of Co and Mn in the MnCo2O4 electrode leads to the clearer charge storage mechanisms of this material. The ASC of MnCo2O4//N-rGOAE may also be practically used as the energy storage device for high power applications.

ASSOCIATED CONTENT Supporting Information FE-SEM images of the as-electrospun composite fibers and of MnCo2O4 NFs and N-rGOAE coated on CFP, TGA analysis of the as-synthesized composite fibers, BET and the pore size distribution results of MnCo2O4 NFs, XPS spectra of MnCo2O4 NFs and N-rGOAE including survey spectrum and high-resolution spectra of O1s, CV and GCD curves for MnCo2O4 NFs and N-rGOAE of the three electrode system, Power’s law analysis of the MnCo2O4 NF//N-rGOAE ASC device, and the calculation details of the asymmetric supercapacitor performances. The Supporting Information is available free of charge on the http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Corresponding author. Tel: 66(0)33-01-4251. Fax: 66(0)33-01-4445. E-mail address: [email protected] (M. Sawangphruk).

Author Contributions


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M.S. conceived and designed this work and wrote the paper; T.P. carried out the experiments (synthesis, electrochemical evaluation, TEM, and SEM), and P.I., A. K. and T.P. performed Raman, FTIR, and XRD. All authors participated in the analysis and discussion of the results. The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the Thailand Research Fund and Vidyasirimedhi Institute of Science and Technology (RSA5880043). The scholarship supported to T. Pettong by Faculty of Engineering, Kasetsart University was acknowledged.

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High-Performance Asymmetric Supercapacitors of MnCo2O4

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