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Facile route to NiO nanostructured electrode grown by oblique angle deposition technique for supercapacitors Vasudevan Kannan, Akbar I. Inamdar, Sambhaji M. Pawar, Hyun-Seok Kim, Hyun-Chang Park, Hyung Sang Kim, Hyunsik Im, and Yeon Sik Chae ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03714 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 21, 2016

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ACS Applied Materials & Interfaces

Facile route to NiO nanostructured electrode grown by oblique angle deposition technique for supercapacitors Vasudevan Kannan*1, Akbar. I. Inamdar 3, Sambaji M. Pawar3, Hyun-Seok Kim#2, Hyun-Chang Park2, Hyungsang Kim3, Hyunsik Im3, Yeon Sik Chae4 1

Millimeter-wave INnovation Technology Research Center (MINT), Dongguk University, Seoul 04620, Korea 2 Division of Electronics and Electrical Engineering, Dongguk University, Seoul 04620, Korea 3 Division of Physics and Semiconductor Science, Dongguk University, Seoul 04620, Korea 4 Department of Computer Application, Seoil University, Seoul, Korea

ABSTRACT We report an efficient method for growing NiO nanostructures by oblique angle deposition (OAD) technique in an e-beam evaporator for supercapacitor applications. This facile physical vapor deposition technique combined with OAD presents a unique, direct, and economical route for obtaining high width-to-height ratio nanorods for supercapacitor electrodes. The NiO nanostructure essentially consists of nanorods with varying dimensions. The sample deposited at OAD 75o showed highest supercapacitance value of 344 F/g. NiO nanorod electrodes exhibits excellent electrochemical stability with no degradation in capacitance after 5000 charge-discharge cycles. The nanostructured film adhered well to the substrate and had 131 % capacity retention. Peak energy density and power density of the NiO nanorods were 8.78 Wh/kg and 2.5 kW/kg respectively. This technique has potential to be expanded for growing nanostructured films of other interesting metal/metal oxide candidates for supercapacitor applications. Keywords: Electrochemical supercapacitor; NiO; Oblique angle deposition; nanostructures; ebeam evaporation *Corresponding author: E-mail: [email protected], Phone: +82-2-2260-8696 1

ACS Paragon Plus Environment


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#Co-corresponding author: E-mail: [email protected], Phone: +82-2-2260-3996

INTRODUCTION The quest for clean and renewable energy is more than ever, primarily due to fast depletion of fossil fuels and environmental concerns. Green energy generation and efficient storage are the most critical components for future energy solutions. Fuel cells and rechargeable battery systems have attracted enormous attention for energy generation and storage systems respectively1, 2. Fuel cells alone are insufficient to meet the requirements of a vehicle, in particular the response of fuel cell is inadequate when there is a need for short-burst of peak power. A potential solution to this problem can be achieved by hybridizing the fuel cells with energy reservoir with elevated power and densities3. More specifically, super-capacitor also known as electro chemical capacitor or ultra-capacitors have been explored extensively in the recent past. Supercapacitor hybridized systems have higher fuel economy as well as the efficiency is more than the fuel cell/battery operating alone. It has been reported that supercapacitors are capable of delivering superior power density and higher number of charge-discharge operations than batteries as well as superior energy density compared to traditional capacitors4-6. Typically, transition metal oxides7-8 and electrically conducting polymers 9, 10 are broadly used as electrodes for pseudocapacitors. However, practical supercapacitance value and power performances are limited by high electrical resistivity and compactness of the structure. Nanostructured materials have been utilized as electrodes due to the availability of larger active specific surface area as well as providing channels for direct transport of electrons11-13 and are anticipated to assist the charge/discharge kinetics in supercapacitors14-16. Along with configuration of the electrode structure, the choice of electrode material is vital in the ultimate performance of 2


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pseudocapacitors. Nickel oxide and nickel hydroxide are promising candidates as electrode for supercapacitors, due to their low-price and environment-friendliness. The theoretical capacitances of NiO and nickel hydroxide are approximately 2100 and 2600 F/g respectively in a potential window of 0.5 V 17. Nanostructured columnar thin films can be obtained by oblique angle deposition (OAD), where the vapor deposition direction is not normal to the surface. Seeding and self-shadowing mechanism at the substrate surface have been attributed to such columnar morphology18, 19. In OAD, typically at oblique angles > 70o to the substrate normal, metal islands act as shadow masks for the succeeding vapor flux, blocking deposition on the shadowed regions of the surface20. Hence, the metal islands preferentially continue to grow inclined towards the vapor direction. An important factor for columnar growth is a well-collimated vapor source. The level of collimation will directly correspond to the homogeneity in the diameter of the growing columns21. We report on fabrication of NiO nanocolumnar electrode films for supercapacitor applications using a combination of OAD technique and thermal annealing. To our best knowledge, we demonstrate for the first time electrochemical energy storage characteristics of NiO nanocolumnar electrode films prepared by OAD. This technique offers important benefits such as convenient control of radius, ordering, density and structural form at the nanoscale. These parameters can be controlled by varying the growth conditions such as angle of deposition, duration and rate of growth, and temperature of the substrate. Since this is a physical deposition technique, it has distinct advantages over chemical methods, where there is often nonnegligible electrode material degradation during large number of charge/discharge operation.

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The fabricated samples displayed no degradation after 5000 cycles of operation and showed 131% capacity retention.

RESULTS AND DISCUSSION Field emission scanning electron micrographs (FE-SEMs) of nanostructured NiO OAD films deposited at of 65o, 75o and 85o are shown in Figures 1(a)-(e). NiO nanorods are formed at all oblique angles, but the 75o sample (Fig.1b) shows the highest nanorod density with good uniformity. The diameter of the NiO nanorods (Fig.1d) ranges between 10 nm and 100 nm, while the length of the nanorods is in the order of micro meter. Fig. 1e shows enlarged view of a NiO nanorods and they are multi-faceted. Electron micrograph of the as-deposited samples at oblique and normal deposition angles are presented in Fig. S1 (see Supporting Information). Figure 2 shows the x-ray diffraction (XRD) patterns of the 75° sample in the as-grown (Ni metal) state and annealed at 600oC in air for 2 h. The ‘hkl’ values were assigned to the observed reflection peaks using “Mercury” software (free version) and simulated XRD pattern for NiO from crystallographic data reported by Glemser et al22. XRD analysis shows that the as-grown nickel film is oxidized to nickel oxide, with oxide phases predominantly NiO along with Ni2O3. Electrochemical supercapacitive characteristics of the NiO nanostructure electrodes were studied with 2M KOH electrolyte. Figure 3 shows the cyclic voltammograms (CV) for NiO deposited normally and at different angles (100 mV/s scan rate). The CVs were cycled over 0 to 0.5 V vs SCE (Saturated calomel electrode). The mechanism of the CV plots can be attributed to the redox reaction23. Broad peaks during cathodic and anodic scans within of 0.2 and 0.3 V (vs SCE) could be attributed to reversible reduction and oxidation of the NiO electrode, according to the charge-discharge reaction 4


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NiO + OH-

NiOOH + e-

(1)

The observed pseudocapacitance can be attributed to reversible reduction-oxidation reactions between NiO and NiOOH at the surface of the electrode during cycling. The 75° sample exhibits maximum area under the CV curve in comparison with other samples, indicating a relatively larger surface area for the redox reaction. Figure 4(a) shows the CVs of the 75o sample at scan rates 5, 10, 20, 50, 80, and 100 mV/s. CVs for the 65o and 85o samples are shown in Fig.S2 and S3 (see Supporting Information). CV areas increases with scan rate, but retain its shape, indicating high efficiency in capacitive characteristics, fast reversible faradaic reaction and good rate capability of the NiO electrode material. Specific capacitance CS can be calculated from CV plots,

= ...∆

(2)

where ν is the scan rate, m is the mass of the active electrode material, and ∆V is the potential window of single scanning section. The specific capacitance calculated for normal, 65o, 75o and 85o OAD samples at 100 mV/s were 11, 61, 230 and 78.5 F/g respectively and 29, 103, 344 and 150 F/g at 5mV/s respectively. Figure 4(b) shows the specific capacitances for all the samples at different scan rates. As the scan rate increases, specific capacitance decreases, because of the extended diffusion span of reactive ions and decrease in conductivity brought about by the increased scan rates. The OAD 75o electrode shows significantly superior electrochemical properties compared to the other three electrodes, and this can be associated to a larger active surface area, which is anticipated and consistent with existing literature24-27. The surface area available for the electrochemical process can be estimated from the electrochemically active surface area (ECSA)28. Electrochemical capacitance was obtained from 5



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the non-Faradaic current (capacitive) in the linear charging region from the plot of CVs at different scan rates, and ESCA was estimated for each oblique angle deposition 65o, 75o and 85o. The non-Faradic region 0.05 - 0.15 V is considered. The capacitive current (iDL) for OAD 75o sample was estimated from non-Faradic region (0.1 V) for each scan rate (ν = 0.005, 0.01, 0.02, 0.05, 0.08, 0.1 V/s) from Fig. 5(a). Similar plots for OAD 65o and OAD 85o samples are presented in inset of Fig.S2 and S3 (see Supporting Information). This current represents charge accumulation and not redox reaction or transfer of charges and is given by iDL = CDL * ν

(3)

where CDL is the double-layer region specific capacitance. Figure 5(b) shows scan rate vs iDL, for all the samples deposited at different oblique angles, exhibiting a linear relation with the slope providing CDL. The ESCA for each system was calculated as ECSA = CDL/Cs

(4)

where Cs is the specific capacitance in alkaline electrolyte. Here we use the value of Cs is 0.040 mF cm-2 as reported for KOH28. Estimated ECSA for 65o, 75o and 85o OAD samples were 209, 496 and 99 cm-2, respectively. The ECSA values validate that surface area is largest for sample OAD75o, which is in agreement with the calculated maximum specific capacitance value. Thus, the angle of deposition plays a critical role in obtaining the largest surface area available for electrochemical redox reactions. Electrochemical stability is a critical parameter for practical applications. OAD 75o NiO electrode was subjected to 5000 CV cycles within a potential window of 0 - +0.5 V at ν = 100 mV/s. Figure 6(a) shows the CV curves in three dimentions. Almost identical CV curves indicate 6


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exceptional stability during electrochemical cycling process. Figure 6(b) presents the calculated specific capacitance against cycle number up to 5,000 cycles for 75o OAD electrode. Specific capacitance increases from 229 to 237 F/g for initial 1500 cycles, and which could be, associated with the process of activation of NiO electrodes29, then remains stable. The activation of NiO allows ensnared ions (ions trapped among the NiO structures) to diffuse out30. The capacity retention of was 131% after 5000 cycles. This increased specific capacitance can be attributed to gradual penetration of electrolyte into the nanostructure within the NiO film as CV cycling process progressed31. This confirms the stability during large cycling process along with excellent coulombic efficiency. Figures S4, S5, and S6 (see Supporting Information) show CV stability and capacitance for other samples over 1,000 cycles.

We further assess the electrochemical characteristics of the nanostructured NiO films by measuring the galvanometric charging/discharging properties. galvanostatic charge/discharge plots at 0.25 mA/cm2.

Figure 7(a) shows the

The characteristic contour of the

charge/discharge plots indicate pseudocapacitor behavior, and is in agreement with the calculated CV values. Notably, there was no voltage change (IR drop) during charge to discharge shift indicating that the nanostructured NiO films have little or no contact resistance. The OAD 75o sample has the longest discharge time (152 s at current density 0.25 mA/cm2), exhibiting superior electrochemical performance to the other samples. The inset in Fig. 7(a) shows the galvanostatic charge/discharge plots of OAD 75o sample at current densities 0.25 - 2 mA/cm2. Corresponding charge/discharge plots of OAD 65o and OAD 85o are presented in Fig. S7 and S8 (see Supporting Information). The specific capacitance for electrodes were calculated from the charge/discharge curves using, 7



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.∆

= .∆

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--------------- (5)

where I is the applied current density and ∆t is the discharge time. The highest specific capacitances (25, 65, 254, and 106 F/g for normal, 65o, 75o, and 85o OAD samples, respectively) are comparable with those obtained from the CV curves. The NiO nanostructures apart from exhibiting high supercapacitances, they also cope well at larger current density values. In particular, they retain atleast 87% of their supercapacitance as the current density surges from 0.25mA/s to 2 mA/s (from 253 to 222 F/g), as shown in Fig. 7(b). The specific capacitance value of OAD 75o electrode is larger than previously reported nanostructured NiO films, e.g. NiNiO core-shell nanowire (220 F/g)32, NiO thin film33-34, and NiO/Ni Form (235 F/g)35. These higher supercapacitive values may be due to its large active surface area and low electrical resistivity. The nanostructure could also facilitate numerous pathways for transport of charges leading to rapid Faraday reactions. Low resistance (~ 1 Ohm) were observed in electrochemical impedance spectroscopy (EIS). Figure S9 (see Supporting Information) shows the Nyquist plots of EIS spectra for the OAD 75o NiO electrode before and after 5000 cycles at frequency ranging from 1Hz to 10 KHz ( (Z│) and (Z║) are the real and imaginary parts of the impedance). From the specific capacitance value and discharge time (∆t) from the charge/discharge curve the energy density (E) and power density (P) were determined from,

= . ∆

(6)

and

=

(7)

∆

8


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Figure 8 shows the energy density and power density at various current densities (Ragone plot). The peak value of energy density was observed for NiO sample OAD 75o as 8.78 Wh/kg, while maximum power density value of 2.5 kW/kg was associated with sample OAD 85o. Energy density decreases with increasing current density, whereas the power density increases with current density (see Fig.S10, Supplementary Information). As the redox reaction between NiO and OH- is highly diffusion controlled36, it is anticipated that energy density will decrease with increasing charge/discharge rates37-39.

CONCLUSIONS We report a novel and facile way to prepare nanostructured NiO thin film electrodes for supercapacitor application by oblique deposition technique. The 75o OAD sample exhibited maximum supercapacitance (344 F/g), which could be attributed to the high surface area, as estimated from ECSA, arising from the nanostructured morphology. Enhanced energy density and power density values of 8.78 Wh/kg and 2.5 kW/kg were also observed. The electrodes showed excellent endurance with specific capacitance retention characteristic of 131 % post 5000 cycles of continuous CV operations. OAD fabricated NiO nanostructures are a good candidate for electroactive material, providing high power density and sustained favorable energy density at fast charging and discharging rates. OAD technique appears generic and is a promising approach for realizing other metal/metal oxide nanostructures by physical vapor deposition suitable for supercapacitor and related applications.

METHODS 9



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Thin films of nanostructure nickel oxide were achieved by OAD in an e-beam evaporator

as shown diagrammatically Fig. S11 (see Supporting Information). Inset in Fig. S11 shows the growth mechanism during OAD process. Nickel metal, purity 99.99%, was the source material. The distance between substrate and source was always maintained at 90 cm. Deposition rate was ~ 2 Å/s at 1 x 10-5 torr. Copper sheets, 5x1cm, were used as substrates. The substrates were first rinsed in acetone, isopropanol, de-ioniszed water, and well dried with nitrogen gas. Since the instrument was not equipped with a sample tilting stage, the samples had to be mounted on a suitably angled surface and placed on the sample stage. Therefore samples were taped (using vacuum tapes) on to metal strips which had been bent to the required angle and were placed on the stage. The uncertainty in the mounting angle can be estimated to be 0.5o based on the uncertainty of the protractor that was used. Initially a range of angles from 45o to 85o in 10o steps were tested. Based on the results the range was narrowed to 65o, 75o and 85o. Normal deposition was also carried out for comparison. The deposition processes were all conducted in a clean room (Class 1000) at room temperature. Thickness was maintained to approximately 300 nm. Finally, the samples were annealed in air at 600°C for 2 h. The samples were analyzed using x-ray diffractometer (XRD-Rigaku-Ultima IV), and field emission scanning electron microscope (FE-SEM-Hitachi-S-4800). A standard three electrode electrochemical cell was utilized for electrochemical characterization. NiO/Cu was the working electrode, while SCE served as the reference electrode and graphite rod functioned as the counter electrode. The electrochemical and galvanostatic charge–discharge measurements were carried out in 2.0 M KOH aqueous solution in a conventional three electrode configuration integrating

an

electrochemical

workstation

Versa-stat-3

10


(Princeton

applied

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research). Electrochemical impedance spectroscopy (EIS) measurement was also performed using in the same instrument.

Conflict of Interest: The authors declare no competing financial interest.

Supporting Information: FE-SEM images of the as-deposited normal and OAD samples. CV, charge-discharge and cycling performance of the supercapacitor deposited at OAD 65o and OAD 85o. Nyquist plots of the EIS spectra. Schematic of the OAD system. ACKNOWLEDGEMENT

This work was supported by Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

the

Ministry

of

Education

(2014R1A1A2058814, 2015R1D1A1A01058851 and 2015R1A2A1A15054906).

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35. Wu, C.; Deng, S.; Wang, H.; Sun, Y.; Liu, J.; Yan, H. Preparation of Novel ThreeDimensional NiO/Ultrathin Derived Graphene Hybrid for Supercapacitor Applications, ACS Appl. Mater. Interfaces 2014, 6, 1106-1112. 36. Yu, P. C.; Nazri, G.; Lampert, C. M. Spectroscopic and Electrochemical Studies of Electrochromic Hydrated Nickel Oxide Films, Sol. Energy Mater. 1987, 16, 1-17. 37. Prasad, K. R.; Miura, N. Electrochemically Deposited Nanowhiskers of Nickel Oxide as a High-Power Pseudocapacitive Electrode, Appl. Phys. Lett. 2004, 85, 4199. 38. Brezesinski, T.; Wang, J.; Polleux, J.; Dunn, B.; Tolbert, S. H. Templated NanocrystalBased Porous TiO2 Films for Next-Generation Electrochemical Capacitors, J. Am. Chem. Soc. 2009, 131, 1802-1809. 39. Zheng, J. P.; Jow, T. R. High Energy and High Power Density Electrochemical Capacitors, J. Power Sources 1996, 62, 155-159.

FIGURE CAPTIONS: Figure 1: FE-SEM images of nickel oxide nanorods grown by oblique angle deposition technique at oblique angles (a) 65o, (b) 75o, (c) 85o, (d) thickness of a typical NiO nanorod, (e) facetted morphology.

Figure 2: X-ray diffraction pattern of the as-grown and sample annealed at 600oC.

Figure 3: Cyclic voltammograms of the NiO electrodes grown at normal and oblique deposition angles 65o, 75o and 85o within potential window 0- +0.5V at a 100 mV/s scan rate. 16


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Figure 4: (a) Cyclic voltammograms for NiO grown at OAD 75o at various scan rates. (b) specific capacitance as a function of scan rate of NiO electrodes grown at normal, 65o, 75o, and 85o oblique angle deposition.

Figure 5: (a) Cyclic voltammograms of NiO grown at 75o oblique angle deposition at various scan rates in the non-Faradic voltage region from 0.05 V to 0.15 V. (b) The currents measured at 0.1 V vs Potential as a function of scan rate for 75o oblique angle deposition.

Figure 6: Cycle test (a) Cyclic voltammograms of NiO grown at 75o oblique angle deposition for 5000 cycles at a 100 mV/s scan rate, (b) specific capacitance and capacity retention (right y-axis) as a function of cycle number for 75o oblique angle deposition at a 100 mV/s scan rate.

Figure 7: (a) Galvanostatic charging/discharging profiles of NiO grown at normal, 65o, 75o, and 85o oblique angle deposition at current density 0.25 mA/cm2. The inset shows the charging/discharging profiles for the sample 75o OAD sample at current density 0.25 - 2 mA/cm2 (b) Response of specific capacitance with current density for normal, OAD 65o, 75o and 85o.

Figure 8: Calculated energy and power densities (Ragone plot) for NiO electrodes deposited at 65o, 75o and 85o oblique angle deposition at various charge/discharge current densities.

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Facile Route to NiO Nanostructured Electrode ... - ACS Publications

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