Molybdenum oxide nanowires based supercapacitors with enhanced capacitance and energy density in ethylammonium nitrate electrolyte

Accepted Manuscript Molybdenum Oxide Nanowires based Supercapacitors with Enhanced Capacitance and Energy Density in Ethylammonium Nitrate Electrolyte Mansoor Sarfraz, Mohamed F.A. Aboud, Imran Shakir PII:

S0925-8388(15)30671-X

DOI:

10.1016/j.jallcom.2015.07.274

Reference:

JALCOM 34970

To appear in:

Journal of Alloys and Compounds

Received Date: 24 June 2015 Revised Date:

26 July 2015

Accepted Date: 29 July 2015

Please cite this article as: M. Sarfraz, M.F.A. Aboud, I. Shakir, Molybdenum Oxide Nanowires based Supercapacitors with Enhanced Capacitance and Energy Density in Ethylammonium Nitrate Electrolyte, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.07.274. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Molybdenum Oxide Nanowires based Supercapacitors with Enhanced Capacitance and Energy Density in

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Ethylammonium Nitrate Electrolyte

Mansoor Sarfraz, Mohamed F. A. Aboud and Imran Shakir*

Sustainable Energy Technologies Center, College of Engineering, King Saud University,

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PO-BOX 800, Riyadh 11421, Kingdom of Saudi Arabia

Orthorhombic molybdenum trioxide (α-MoO3) nanowires as an electrode for

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electrochemical supercapacitors in ethylammonium nitrate (EAN) electrolyte exhibits a high specific capacitance of 288 Fg−1, which is 8 times higher than the specific capacitance obtained from MoO3 nanowires in water based electrolyte. MoO3 nanowires in EAN electrolyte exhibit energy density of 46.32 Whkg−1 at a power density of 20.3

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kW kg−1 with outstanding cycling stability with specific capacitance retention of 96% over 3000 cycles. We believe that the superior performance of the MoO3 nanowires in EAN based electrolyte is primarily due to its relatively low viscosity (0.28 poise at

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25°C), high electrical conductivity (20 mS·cm−1 at 25 °C) and large working voltage window. The results clearly demonstrate that EAN as electrolyte is one of the most

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promising electrolyte for high performance large scale energy storage devices. Keywords: Molybdenum oxide nanowires, ethylammonium nitrate, ionic liquid, energy storage devices, supercapacitors, electrochemical capacitance

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Corresponding author: [email protected],

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1. Introduction During the last few decades, electrochemical energy storage devices such as

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supercapacitors have gained considerable attention due to their fast charge and discharge rate, high power density, extraordinary cyclic lifetime and high reliability as compared to batteries [1-3]. According to charge storage mechanisms, supercapacitors are divided into

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two categories, double layer and pseudocapacitors [4-6]. Among these, pseudocapacitors provide very high energy and power density as compared to the double-layer capacitors.

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However, their energy density is still quite low for their practical applications in electrical vehicles [6]. The energy density can be enhanced either by new electrode materials which can provide very high capacitance or by enhancing the working voltage window of the existing materials as the decomposition of aqueous electrolytes at high voltage is a major

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obstruction in achieving this goal. Therefore, usage of non-aqueous electrolytes for supercapacitor applications has recently gained immense interest among researches. And among these non-aqueous electrolytes, ionic liquids (ILs) have been used extensively to

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increase the voltage widow operability with high ionic conductivity. ILs provide unique combination of properties like high ionic conductivity, high stability, non-volatility and

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wide electrochemical stability, which make them a potential candidate to get desired results [7-10]. The idea of using ILs for higher performance of supercapacitors have recently gained popularity and researchers have tested ILs and their mixtures, most of them belonging to pyrrolidinium and imidazolium cations family, for EDLCs containing different forms of carbon based materials [9-12]. However, there are a very few studies available targeting the enhanced electrochemical window (Ew) for transition metal oxide

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pseudocapacitors. The challenge of achieving high power density and good cyclability in pseudocapacitors using room temperature ionic liquids (RTILs) is very big and our present work is an attempt to find a suitable RTIL with wide electrochemical window,

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low viscosity and high conductivity to increase the performance of transition metal oxides such as MoO3, which not only offers good potential for the intercalation of ions between the crystal layers but also provide multiple oxidation states (faradic capacitance).

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In the current work, we report for the first time, the use of RTIL (ethylammonium nitrate (EAN) as an electrolyte for MoO3 nanowires due to its relatively low viscosity (0.28

2. Experimental Methods

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poise at 25°C) and high electrical conductivity (20 mS·cm−1 at 25 °C ) [12].

Ethylammonium nitrate (EAN) with purity > 97 % was purchased from io-li-tec Germany and molybdic acid (99.99%), nitric acid (70%), carbon black and polytetrafluoroethylene

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(PTFE) binder were purchased from Sigma Aldrich and used as received. For the synthesis of MoO3 nanowires, 20 mmol solution of MoO2 powder was made in 25 mL of H2O2 which was magnetically stirrered for 30 min and pH of the solution was adjusted to

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1 by adding HNO3 with constant stirring. After that the solution was transferred into teflon-lined stainless autoclave (25 mL capacity). The autoclave was placed in the oven at

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180 °C for 12 hours. In the final step the precipitates were filtered, washed many times with distilled water and ethanol, and dried over night in the oven at 90 °C. In order to make the electrode for supercapactor, MoO3 nanowires, carbon balck and polytetrafluoroethylene (PTFE) binder in the ratio of 75:15:10 were dispersed in ethanol and stirred untill homogeneous soultion was achevied. After that solution was coated onto a conductive carbon-cloth (ELAT, Nuvant systems Inc.) followed by drying at 95 °C

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for 12 h in a vacuum oven. The morphological and structural properties of the synthesized MoO3 nanowires were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM–7401F) and X-ray diffraction (XRD, D8 FOCUS

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2200V, Bruker AXS, Cu Kα radiation (λ) 1.5418 Å), respectively. All electrochemical measurements were carried in a three-electrode assembly consisting of Ag/AgCl, Pt foil and MoO3 nanowires coated carbon cloth as a reference, counter and working electrode,

3. Results and discussion

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Biologic VMP3 potentiostat/galvanostat.

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respectively, in ethylammonium nitrate as electrolyte at room temperature using a

Field emission scanning electron microscopy (FE-SEM) analysis of MoO3 nanowires synthesized at 180 oC for 12 hours was carried out, and the corresponding results are shown in Figure 1(a). The FE-SEM analysis of MoO3 nanowires shows that MoO3

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nanowires have an average diameter of 110 nm with a length in the range of tens of micrometers. The crystallographic information of MoO3 nanowires was examined by Xray diffraction as shown in Figure 1(b) which matches with the standard peaks for the

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orthorhombic phase of MoO3 (JCPDS card 89-7112). To evaluate the electrochemical properties of MoO3 nanowires, electrochemical

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measurements were conducted in a three-electrode electrochemical cell with MoO3 nanowires coated carbon cloth, Pt wire, Ag/AgCl (satd. KCl) electrodes as working, counter and reference electrodes in ethylammonium nitrate electrolyte solution. Cyclic voltammograms (CVs) of MoO3 nanowires electrode at a scan rate of 20mVs-1 in 1M H2SO4 and ethylammonium nitrate electrolyte solution is shown in Figure 2(a). It can be seen that MoO3 nanowires has a very broad potential window of about 1.8 V in

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ethylammonium nitrate electrolyte as compared to about 1 V in 1M H2SO4 electrolyte solution.

Furthermore,

MoO3

nanowires

electrode in

both

1M

H2SO4

and

ethylammonium nitrate electrolyte solution show a faradaic reaction at the interface of

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MoO3 electrodes with electrolyte ions, which is a distinctive behavior of pseudocapacitors. The CV curve shows the redox peaks in the MoO3 nanowires electrode in both 1M H2SO4 and ethylammonium nitrate electrolyte solution suggesting a major

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contribution of redox capacitance to the overall specific capacitance [13-16]. The presence of these redox peaks in ethylammonium nitrate electrolyte system clearly

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indicates the [NO3]- anion insertion and de-insertion reactions at different energy states [10].

To further quantify the rate performance and redox processes, the cyclovoltammetry experiments were carried out for MoO3 nanowires at various sweep rates in

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ethylammonium nitrate electrolyte solution at room temperature as shown in the Figure 2(b). As the scan rates increase from 5 to 50 mV s−, the shape of all these CVs remain same indicating excellent capacitive behavior and high rate capability of MoO3

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nanowires in ethylammonium nitrate electrolyte. The change in the specific capacitance of MoO3 nanowires in ethylammonium nitrate electrolyte with increasing scan rates is

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shown in Figure 2(C) and it was observed that specific capacitance of MoO3 nanowires in ethylammonium nitrate electrolyte is significantly higher than the MoO3 nanowires in 1 M H2SO4. The MoO3 nanowires electrode achieve specific capacitance of 288 Fg-1 at 5 mVs-1 in ethylammonium nitrate electrolyte which is 8 times higher as compared to the MoO3 nanowires (36 Fg-1) in 1M H2SO4. The specific capacitance of MoO3 nanowires in ethylammonium nitrate electrolyte is also higher than MoO3 nanowires and its

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composites reported in literature [14-20]. The increase of capacity observed is usually attributed to irreversible decomposition of the electrolyte, relatively low viscosity (0.28 poise at 25°C) and high electrical conductivity (20 mS•cm−1 at 25 °C). Here, [NO3]- ions

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are stable in oxidation, as recently shown by Zhang et al. [80] in graphitic carbon electrodes. The long-term electrochemical stability of the MoO3 in ethylammonium nitrate electrolyte was examined by CV at a scan rate of 20 mVs-1 for 3000 cycles, and

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the corresponding results are presented in Figure 2(d). The capacity decay was only

for energy storage applications.

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3.4%, even after 3000 cycles, indicating the excellent stability of the electrode material

The electrochemical performance of MoO3 nanowires in ethylammonium nitrate electrolyte was further studied by galvanostatic charge/discharge measurements. Figure 3(a) show the charge/discharge curves of MoO3 nanowire electrodes collected at a current

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density of 0.5Ag-1. It is obvious from the results that MoO3 nanowires showed a nonlinear charge–discharge curve, indicating that the nanowires are pseudocapacitive with specific capacitances of 288 Fg-1 respectively.

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The rate capability is another important consideration for industrial use of supercapacitors in power applications. Therefore, the influence of current density on

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specific capacitance of MoO3 nanowires in ethylammonium nitrate electrolyte yte was also measured as shown in the Figure 3 (b). The low ohmic drop at low current densities enables the full access of the inner active sites or pores of the MoO3 nanowires electrode in ethylammonium nitrate, resulting in high specific capacitance, which decreases with the increase in current density because effective utilization of MoO3 nanowires for ion insertion is limited to the outer surface of MoO3 electrodes.

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The electrochemical performance of MoO3 nanowires electrode in ethylammonium nitrate electrolyte was further investigated by calculating the energy and power density. The MoO3 nanowires electrode in ethylammonium nitrate electrolyte showed a clear

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relationship between the power output and energy density, which is very beneficial in determining its potential industrial applications. We found that the energy density of MoO3 nanowires electrode in ethylammonium nitrate electrolyte reached maximum 46.32

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Whkg−1 at a power density of 20.3 kWkg−1, confirming that ethylammonium nitrate is a promising electrolyte for high performance supercapacitors. We compared the energy

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density of our MoO3 nanowires electrode in ethylammonium nitrate electrolyte to MoO3 nanowires electrodes reported elsewhere. To the best of our knowledge, the energy density obtained in the current study is the highest value reported to date for supercapacitors using a MoO3 nanowires [10, 11, 21-23]. This enhancement in the energy

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density of MoO3 nanowires without compromising the power density suggests that the supercapacitors made by using ethylammonium nitrate electrolyte are an ideal candidate for a wide range of viable energy storage device applications. These results suggest that

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ethylammonium nitrate is a potential electrolyte for pseudocapacitors which not only exhibit stable electrochemical behavior but can also be operated in a wider potential

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window as compared to water based electrolyte.. 4. Conclusion

In summary, this work describes the use of ethylammonium nitrate as an efficient electrolyte for supercapacitor applications. The MoO3 nanowires based electrodes achieve specific capacitance of 288 Fg-1 at a scan rate of 5 mVs-1 in ethylammonium nitrate electrolyte which is 8 fold enhancement compared to the MoO3 nanowires in water based

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electrolytes (36 Fg-1). Furthermore, MoO3 nanowires based electrode showed a very high cycling stability, an ultrafast charge discharge rate, and excellent energy and power densities in ethylammonium nitrate, making them promising as an electrode material for

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high performance large scale energy storage devices at room temperature.

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Acknowledgement

The authors would like to extend their sincere appreciation to the Deanship of Scientific

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Research at the King Saudi University for its funding of this research work through the

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Prolific Research Group PRG-1436-25.

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List of Figures

Figure 1: (a) FE-SEM image of the MoO3 nanowires synthesized at 180 oC for 12 hours,

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and (b) XRD patterns of MoO3 nanowires synthesized at 180 oC for 12 hours.

Figure 2. (a) Cyclic voltammograms (CVs) of the MoO3 nanowires electrodes at a scan

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rate of 20 mVs-1 in ethylammonium nitrate and 1 M aqueous solution of H2SO4, and (b) CV of MoO3 nanowires electrode at various scan rates from 5 to 50 mVs-1 in

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ethylammonium nitrate electrolyte, (c) Specific capacitance variation of MoO3 nanowires electrode in ethylammonium nitrate electrolyte at different scan rates, (d) Cyclic voltammetry of the first and after 3000 cycles of MoO3 nanowires electrode with scan

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rate 20 mVs-1 in ethylammonium nitrate electrolyte.

Figure 3: (a) Cycling performance of MoO3 nanowires electrode at a constant current of 0.5 A g-1. The inset shows the charge–discharge curves of the last few cycles at a constant

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current of 0.5 A g-1 and (b) Specific capacitance variation of MoO3 nanowires at different

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current densities.

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Figure 1: (a) FE-SEM image of the MoO3 nanowires synthesized at 180 oC for 12 hours,

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and (b) XRD patterns of MoO3 nanowires synthesized at 180 oC for 12 hours.

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Figure 2. (a) Cyclic voltammograms (CVs) of the MoO3 nanowires electrodes at a scan rate of 20 mVs-1 in ethylammonium nitrate and 1 M aqueous solution of H2SO4, and (b)

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CV of MoO3 nanowires electrode at various scan rates from 5 to 50 mVs-1 in ethylammonium nitrate electrolyte, (c) Specific capacitance variation of MoO3 nanowires

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electrode in ethylammonium nitrate electrolyte at different scan rates, (d) Cyclic voltammetry of the first and after 3000 cycles of MoO3 nanowires electrode with scan rate 20 mVs-1 in ethylammonium nitrate electrolyte.

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Figure 3: (a) Cycling performance of MoO3 nanowires electrode at a constant current of 0.5 A g-1. The inset shows the charge–discharge curves of the last few cycles at a constant

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current densities.

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current of 0.5 A g-1 and (b) Specific capacitance variation of MoO3 nanowires at different

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Research Highlights Synthesis of single crystalline molybdenum oxide nanowires.

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Ethylammonium Nitrate as an electrolyte for high performance large scale psuedocapacitor based energy storage devices.

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Molybdenum oxide nanowires based electrodes shows 8 fold enhancement in

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The supercapacitor devices in Ethylammonium Nitrate exhibit excellent cycling stability, retaining over 96% of its initial charge after 3000 cycles.

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Ethylammonium Nitrate electrolyte as compared to water based electrolytes.