Capacitance of MnO2 Micro-Flowers Decorated CNFs in Alkaline Electrolyte and Its Bi-Functional Electrocatalytic Activity toward Hydrazine Oxidation

1 Department of Bin Fusion Technology, Chonbuk National University, Jeonju 561-756, Republic of Korea 2 Central Laboratory Unit, Qatar University, P. O. Box: 2713, Doha, Qatar 3 Chemical Engineering Program, Texas A&M University at Qatar, P.O. 23874, Doha, Qatar 4 Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Jeonbuk 565-905, Republic of Korea * E-mail: khy@jbnu.ac.kr


INTRODUCTION
Depletion of global fossil oil resources and environmental concerns has been the theme topics of the world economic and political circles over the past decades. It has been reported that a significant portion of the total energy supply comes from fossil fuels, such as coal, oil, and natural gas, causing a dramatic buildup of greenhouse gasses GHGs in the atmosphere. In order to fulfill the growing demand for clean and high-efficient energy, an alternative energy resource that independent of fossil fuels must be developed [1][2][3][4][5].
Supercapacitors (also known as electrochemical capacitors) and direct liquid fuel cells are the most promising devices in the field of energy conversion and storage [6,7]. Comparing with other types of direct liquid fuel cells, direct hydrazine fuel cell (DHFC) achieves a zero pollution emission; only nitrogen and water are produced during the oxidation process. Moreover, there is no catalyst poisoning in direct hydrazine fuel cells. Similarly, supercapacitors are likely to show equal importance to batteries and fuel cells for future energy storage system due to their high power characteristics compared to batteries, high energy density compared to conventional capacitors, long cycling life and short charging time [8,9].
The energy storage/conversion processes for supercapacitors be defined in two ways and commonly classified as electrochemical double layer capacitors (EDLCs) and pseudocapacitors [10,11]. In EDLCs energy is stored electrostatically while pseudocapacitors utilize fast and reversible Faradaic reactions between the electrolyte and electro-active materials [1]. Electrode /catalyst materials are the key elements to determine device performance; thus they have received great attention in recent years [6,[12][13][14].
In particular, cheap transition metal oxide based catalysts are desirable for both supercapacitors and fuel cells; because they are electrochemically more active. Therefore, recent work has been focused on the synthesis and use of non-precious transition metal oxides. Among the auspicious substitute electrode material, MnO 2 has been of specific attention for the past decade because of its abundance, low cost, high catalytic activity and non-toxicity, but their poor electrical conductivity and low structural stability constrain their practical application [15].
To address these problems, a template-free design of 3D open architecture process has been developed [16][17][18]. MnO 2 in the shape of micro-flower have been considered as one of the optimal nanostructures because of the fully exposed structure/high surface area which can facilitate the transportation of electrons and cations. But, the synthesis of flower-like architecture still faces a challenge owing to the fast and complex growing process.
Therefore, it is desirable to develop facile, environment-friendly, low-cost, and template-free synthetic methods for the fabrication of MnO 2 micro-flowers with rich and proper porosity. Herein, we present a facile self-assembled synthesis of MnO 2 micro-flowers decorated carbon nanofibers, which have extensively identified in terms of morphology and crystallinity. Moreover, the electrochemical properties of the material have been investigated for application as supercapacitor and electrocatalyst for hydrazine oxidation in an alkaline medium.

Preparation of MnO 2 micro-flowers decorated CNFs
The procedures used for carbon nanofibers synthesis in this study have followed those previously developed and described by the group [2,3]. In a typical synthesis of MnO 2 micro-flowers decorated CNFs, 0.01 mol/L manganese nitrate was added to 120 mL deionized water under vigorous stirring. 0.05mol/L urea, CO(NH 2 ) 2 was then added to the solution, and the solution was transferred into a 150 mL Teflon-lined stainless steel autoclave. A piece of CNFs mat was subsequently soaked in the solution, followed by autoclave heating in an oven at 90 ℃ for 10h. The synthesized sample was then taken out, cleaned several times with DI water and ethanol, dried at 80 ℃ for about an hour and annealed at 350 ℃ in air for 2 hours.

Electrode Preparation and electrochemical measurement
Based on our previous work [2,3], working electrode was prepared with 2mg of the active electrocatalyst, 20 μL of Nafion solution, and 400 μL of isopropanol while the electrochemical measurements were carried out in the conventional three-electrode electrochemical cell (VersaSTAT 4, USA) at room temperature in 1M KOH solution.

Sample characterization
The morphology of the MnO 2 micro-flowers decorated carbon nanofibers was observed by field-emission scanning electron microscopy (FESEM, Hitachi S-7400, japan), coupled with rapid energy dispersive analysis of X-Ray (EDX) system. The phase and crystallinity of the composite were characterized by X-ray diffractometer (XRD, Rigaku, Japan) with Cu-Kα (=1.54056 Å) radiation operating at 45kV and 100mA over a range of 2θ angle from 10 o to 80 o , scanning at a rate of 4 o / min. The phase composition and phase structure of as-obtained nanostructure were examined by Xray powder diffraction (XRD). Fig. 1 displays the XRD spectrogram of MnO 2 micro-flowers decorated CNFs. Comparing XRD spectrogram of the synthesized materials with the standard diffraction spectra, the synthesized product is crystalline MnO 2 . correlated to (310) hkl indices of pure tetragonal structure [space group: I4/m (87)] according to the JCPDS no.: 00-044-0141 with lattice constants a=b=9.7847 Å, c=2.8630 Å, α=β=γ=90, suggesting MnO 2 structure mainly grows along the (310) face. The main challenge in developing MnO 2 microflowers decorated carbon nanofibers is to incorporate MnO2 micro-flowers attach onto carbon nanofibers uniformly and this attachment is mainly dependent on the synthesis route. In order to crop MnO 2 outgrown on the carbon nanofibers, the nanofibers produced after calcination was allowed to go through a hydrothermal treatment as mentioned in the experimental section. Fig. 2 represents FESEM images for the product obtained by the hydrothermal treatment. As revealed, the uniform micro-flowers with an average size of about 75 μm outgrown on the surface of nanofibers due to hydrothermal treatment, as shown in Fig. 2.A. At a higher magnification (inset of Fig. 2.A), these micro-flowers show hierarchical structures consisting of nanorods with a diameter of about 15~20 nm and a length of about 1 μm. It is also noticed that the micro-flowers are evenly distributed in the network of carbon nanofibers. The diameter of the carbon nanofibers varies from 400 to 700 nm as shown in Fig. 2.B. From the EDX spectra, as shown in Fig. 2  Raman spectroscopy was executed as well to examine the local structure of MnO 2 microflowers decorated CNFs. As shown in Fig. 3, two broad overlapping peaks centered on 1321 and 1590 cm −1 that correspond to the disordered (D band ) and graphite (G band), arising from the disordered structure of carbon [19]. Concerning MnO 2 , the Raman peaks observed at 368 (marked by red circles & arrow along with high magnification), and 646 cm −1 demonstrate MnO 2 microstructures.

RESULT AND DISCUSSION
An electrochemical analysis of MnO 2 micro-flowers decorated CNFs performance was carried out using cyclic voltammetry CV. The specific capacitance of the composite can be calculated with CV results according to the following equation [20]: Where, C sp is the specific capacitance, m is the mass of the grafted composite, ʋ is the potential scan rate, ΔV is the sweep potential window, and I (V) is the voltammetric current on CV curves. Fig. 4 shows the rate-dependent cyclic voltammetry (CV) curves of MnO 2 micro-flowers decorated CNFs at various scan rates (5,10, 25, 50, and 75 mVs −1 ) between -0.2 V and 1.0 V (vs. Ag/AgCl) in 1 M KOH electrolyte. The CV curves were fairly rectangular in shape, which is a typical capacitive behavior of carbon materials [21]. In addition, the current densities increase along with increasing scan rates, suggesting good rate performance [22]. The specific capacitances of nanostructure at different scan rates are shown in Fig. 5. The specific capacitance decreases with the increase of scan rates from 5 to 75 mVs −1 . At low scan rates (5 mVs −1 ), both active areas, internal and external, of the electrode surfaces could take part in reaction while at high scan rate (75 mVs −1 ) the diffusion of the electrolyte ions was limited thus only the external surface area of electrode could participate in ion transfer reaction [23]. As shown in Fig. 5 the maximum specific capacitance obtained at a scan rate of 5 mVs −1 of was 120 Fg −1 .

Figure 5. Effect of scan rate on specific capacitance
Comparison of the capacitive performance of MnO 2 micro-flowers @CNFs based electrode and other reported CNFs and the graphene-based electrode was conducted, as shown in table 1. At low scan rate 5mVs -1 , the specific capacitance can reach 120 Fg -1 for MnO 2 micro-flowers decorated CNFs, showing a much superior capacitive performance to other reported CNFs and graphene-based electrodes. This result could not only be accredited to the capacitance input of MnO 2 but benefit from the distinctive structure of MnO 2 micro-flowers which could supply more active sites and internal space which electrolyte ions could access easily. Moreover, the support of carbon nanofibers as conducting material could improve conductivity and convenient pathway for ion transfer [26,27]. Such synergistic effect between carbon nanofibers and manganese oxides micro-flowers had a great effect on improving the capacitive performance of the electrode [28,29].   Fig. 6 showing the CV that indicates an excellent electrocatalytic activity of nanostructures by showing effective hydrazine oxidation with high current density in the presence of hydrazine (Red). At the potential of 1 V, the current density of the nanostructure reached 38 mAcm −2 , which almost double the current density in case of 1 M KOH .
Overall, it is hypothesized that the uniformly embedded electronic conducting network of carbon nanofibers can improve the high-rate capability as well as the specific capacitance of the materials. Moreover, carbon nanofibers network with hierarchical MnO 2 micro-flowers can shorten the diffusion path for charge-carrier ions, whereas the large liquid-solid interface simplifies ion exchange between the electrode and electrolyte [30][31][32][33].

CONCLUSION
MnO 2 micro-flowers decorated carbons nanofibers hierarchical structure have been prepared by calcination of electrospun nanofibers mats composed of polyvinyl acetate followed by hydrothermal treatment and were demonstrated to be superior bi-functional electrode material for energy conversion and storage devices. MnO 2 micro-flowers decorated carbon nanofibers electrode exhibited a superior capacitance of 120 Fg −1 at low scan rate. Interestingly, the MnO 2 micro-flowers decorated carbon nanofibers showed hydrazine electro-oxidation with high current density (38 mA/cm 2 ). The MnO 2 micro-flowers decorated carbon nanofibers electrode can be a hopeful catalyst for energy conversion and storage devices thanks to its low cost and satisfactory electrocatalytic performance.