Highly Pseudocapacitive NiO Nanoflakes through Surfactant-Free

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Highly Pseudocapacitive NiO Nanoflakes through Surfactant-Free Facile Microwave-Assisted Route Shubhra Goel, Anuj Kumar Tomar, Gurmeet Singh, and Raj Kishore Sharma ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00343 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Energy Materials

Highly Pseudocapacitive NiO Nanoflakes through Surfactant-Free Facile Microwave-Assisted Route Shubhra Goel, Anuj Kumar Tomar, Raj Kishore Sharma and Gurmeet Singh Department of Chemistry, University of Delhi, Delhi 110 007, INDIA

Address for Correspondence: R.K. Sharma, Email: [email protected] G. Singh, [email protected]

ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract A facile and rapid surfactant-free microwave-assisted route is developed to synthesize 10 nm sized NiO nanoflakes with high pseudocapacitive performance for supercapacitor cells. The NiO nanoflakes exhibit mesoporous channels and the surface area as high as 206 m2 g-1 as revealed under BET study, while the structural identity verified by XRD & IR confirm phase purity of NiO. NiO nanoflakes maintain ~85% of thermal stability at temperature 900 °C which can be related to strong intermolecular forces in-between the NiO nanoparticles held in the molecular matrix. Electrochemical performance investigated in 6 M KOH solution suggests maximum specific capacitance of 307 F g-1 for NiO║NiO cell at 0.5 A g-1 sustaining about 96% capacitance after being successfully cycled upto 3000 cycles. The NiO nanoflakes reveal high conductivity of 33.87 S cm-1 at the room temperature. Precisely, nanosized NiO bearing ‘flake’ morphology is of particular interest due to high surface to volume aspect and porosity features - the determining factors for swift ion diffusion into the electrode and improved redox reaction. The illustrated microwave-assisted route unfolds as a direct synthesis method to obtain nanosized NiO flakes with high surface area facilitating excellent device performance characteristics without involving any surface-capping agents.

Keywords: Nickel oxide; solution combustion microwave-assisted synthesis; nanoflakes; surface area; specific capacitance; cycling stability; supercapacitors


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1. INTRODUCTION Developing nanoparticles of transition metal oxides e.g. ZnO, MnO2, V2O5, NiO, SnO2 etc. has received tremendous attention among researchers since a decade.1-9 Thanks to the outstanding material properties arising from their dimensional anisotropy that pave way for different applications.9,10 Keeping in view the rising energy demands and recognizing the global issues,11 several attempts have been scrutinized to utilize these metal oxides as electrode material in supercapacitors (or the energy storage devices).10,12 So far, a broad range of reports illustrate synthesis of nano-dimensional transition metal oxides in this regard via diverse means to meet supercapacitor technologies.13-15 Among the various metal oxides, nickel oxide (NiO) has lately emerged as a promising candidate for magnetics, catalysis, electrochromics, sensors and more particularly supercapacitors or the electrochemical capacitors having ultrahigh theoretical capacity (2584 F g-1 within a potential window of 0.5 V).15-17 NiO is known for its fast charge/discharge processes and long cyclic life which render it worthwhile for pseudocapacitors. Likewise, its cost-effective, low toxicity and environmental benign behavior are additional lucrative attributes.18-20 Several approaches such as sol-gel,21 homogeneous precipitation,22 microemulsion,23 anodic aluminium oxide template,18 hydrothermal,24,25 plasma method,26 microwave-assisted route19 etc. have been reported to obtain NiO nanoparticles in varied morphologies.18,19,25,27,28 The drives behind utterly being to enhance the material properties29 (e.g. surface area, porosity) of NiO & underline the structure-property relationship between its dimensional anisotropy and charge



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storage characteristics for supercapacitor applications. Nevertheless, developing nanodimensional NiO as nanoflakes has been of particular interest since the ‘flake’ morphology acts as a firm platform with improved surface area for better intercalation of electrolyte ions into the electrode material.19,30,31 In other words, the ‘flake form’ of NiO encourages better electroactive sites for redox reaction to take place between the electrode and electrolyte species. This leads to (a) reduction in the ion diffusion path lengths and (b) lowering in the charge-transfer resistance,

which

overall

augment

the

specific

capacitance

of

NiO

nanoparticles.30,31 However, despite several reports on characterization of NiO nanomaterial as supercapacitor electrode, so far no reports are available that discuss practical applications of NiO electrode in fabrication of a supercapacitor device. Recently, microwave-assisted solution combustion synthesis has evolved as a popular route for developing nanoparticles of metal oxides including NiO, serving a range of technological applications.19,30,32 By definition, this method is a modified combustion synthesis interceded via microwave irradiation (MWI) involving aqueous mixture of an oxidizer (e.g. metal nitrate/nitrite) and a fuel (e.g. urea, glycine, citric acid) which ensures uniform mixing of reactants at molecular level.33 In principle, the energy transfer as MWI to the reactant molecules occurs through dipole rotation and ionic conduction mechanisms which results in instant increase in temperature. The decreased activation energy of reactants increases the reaction kinetics and fastens the rapid decomposition of the precursors forming highly supersaturated solutions wherein nucleation and/or growth takes


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place via Ostwald ripening mechanism.32 Growth of these nanoparticles can thus be controlled by use of organic surfactants or additives that stabilize and passivate their surfaces.34 In present work, we describe a facile solution combustion microwaveassisted route to synthesize NiO nanoflakes without involving use of any surfacecapping

agents

and

investigate

their

electrochemical

performance

for

pseudocapacitive applications. Synthesis of nano NiO is generally reported wherein aid of a surfactant or capping agent like cetyl trimethyl ammonium bromide (CTAB) or ethylene glycol (EG) is used to limit the growth to certain crystal planes for desired morphology, paving way for enhanced charge storage characteristics.19,34,35 In contrast, herein, the illustrated microwave-assisted synthesis unfolds as a direct and rapid approach to obtain NiO bearing ‘nanoflake’ morphology with dimensions cut down to 10 nm and surface area as high

as

206 m2 g-1. Additionally, a symmetric NiO supercapacitor device is fabricated which shows excellent capacitance of 307 F g-1 at current density 0.5 A g-1.

2. EXPERIMENTAL SECTION 2.1 Synthesis of NiO Nanoflakes All the chemicals were of AR grade and used as received. NiO nanoflakes were synthesized through solution combustion microwave-assisted route.19,36 In a set of synthesis (Scheme 1), 2.9 g Ni(NO3)26H2O and 0.1 g C2H5NO2 were altogether taken in a beaker to which 20 mL of H2O was added and thoroughly



stirred. Separately, 3 g CO(NH2)2 was mixed with 1.59 g NaOH dissolved in 20 mL of H2O. This solution was gently added to the above nickel nitrate solution. The purpose of adding NaOH was to increase the pH to 12. An extra amount of 10 mL H2O was added to the reaction mixture for homogeneity and the soformed green colored suspension was stirred continuously for half an hour at a temperature ~70 °C to form a clear solution. The reaction mixture was then placed in a household microwave oven (SAMSUNG) at 800W of radiation for 15 min. On subsequent cooling, the green colored solid (Ni(OH)2) was washed vigorously with H2O followed by ethanol and finally, vacuum dried at 110 °C for 6 hrs. Further it was calcinated at 300 oC for 2 hrs. and obtained as a black colored product. Reactions involved in the synthesis work are as follows: CO(NH2)2 + H2O → 2NH3 + CO2 ↑ Ni2+ + 6NH3 → [Ni(NH3)6]2+ NH3 + H2O → NH4+ + OHNi2+ + 2OH- → Ni(OH)2 ↓ Ni(NO3)2 + 2NaOH → Ni(OH)2 ↓ + 2NaNO3 Ni(OH)2

300 °C

NiO + H2O

Combustion reactions involving Ni(NO3)2 & C2H5NO2 mixtures are known to undergo self-propagating exothermic processes wherein, C2H5NO2 is used as a complexant or combustible agent.37 Studies also reveal wherein reaction of C2H5NO2 with metal ions upgrades the intermixing of reactants and enhances the possibility of nanoparticle formation.38 In work, hydrolysis of CO(NH2)2 releases


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NH3 during decomposition39 which promotes the precipitation22 of Ni(OH)2. Addition of NaOH warrants pH of reaction mixture towards highly basic medium and the NaNO3 formed during reaction of NaOH with Ni(NO3)2 itself has oxidizing properties to propagate the microwave-assisted route. Vijayakumar et al. have synthesized similar NiO nanoflakes by use of CTAB with NH3.19 Formation of alike NiO nanostructures are also conferred by Anandha Babu et al. involving NH3 alone but with enhanced dimensions.30 On the other hand, NiO reported via both C2H5NO2 and CO(NH2)2 in reaction mixture by Jung et al. have resulted in highly porous agglomerated powders with particle size below 35 nm.36 Apparently, along with the ratio of fuel (glycine & urea): oxidant (nickel nitrate), the proportion of NaOH is the modulating factor for formation of NiO nanoflakes. Under highly basic environment, the evolution of by-product gases33,39 like NO, N2, N2O, NH3, CO2 is favored across surfaces of nano matrix which is accompanied by pore formations in the nanomaterial.



Scheme 1. Diagrammatic illustration for synthesis of NiO nanoflakes.

2.2 Fabrication of NiO Electrode & Cell Assembly 20 mg of NiO sample was mixed with 20 µL Nafion binder using isopropyl alcohol as solvent and sonicated for 1 hr. The dispersed particles were coated onto surface of graphite plates (1 cm × 1 cm). Subsequently, the graphite plates were dried in oven at 70 °C for 3 hrs. The mass loading of active material over graphite plates was 0.5 mg cm-2. The symmetric supercapacitor cell was fabricated with two NiO electrodes using a separating membrane.

2.3 Characterization The morphological and microstructural details of NiO were characterized by Field emission scanning electron microscopy (FESEM, ZEISS GeminiSEM 500) and high resolution transmission electron microscopy (HRTEM, TECNAI G2 T30, UTWIN). N2 adsorption-desorption isotherm was evaluated at -195.85 °C (Micromeritics ASAP 2020) for specific surface area and pore size determination. X-ray diffraction data (XRD, D8 DISCOVER) was collected with Cu Kߙ radiation to study the crystal structure of NiO. Thermal stability of NiO was determined through thermogravimetric (TGA) and Differential thermal analysis (DTA) (SHIMADZU, TG-60) in temperature range ~40-900 °C and a heating rate of 10 °C/ min. Infra-red (IR, IRAffinity-1S SHIMADZU) spectrum was obtained in midIR range (4000-400 cm-1) to investigate the structural identity of NiO. The electrochemical properties were studied through cyclic voltammetry (CV) and


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electrochemical impedance spectroscopy (EIS) (CHI 604 D electrochemical workstation). The CV measurements were carried out at different scan rates in three electrode cell assembly with 6 M KOH electrolyte using NiO as the working, platinum as the counter and the Ag/AgCl as reference electrode. EIS spectrum was recorded in the frequency range 10 mHz-100 kHz at an amplitude of 5 mV. Galvanostatic charge-discharge (GCD, Potentiostat Galvanostat EIS Analyzer PARSTAT 4000) measurements were conducted at varying current densities in potential window 0-0.5 V. The current-voltage (I-V, Keithley 6517 B electrometer) measurements were done at the room temperature.

3.

RESULTS AND DISCUSSION

3.1

Microstructural Details A typical FESEM image of NiO thin film electrode is shown in Figure 1a. Cluster of NiO particles having flake-like morphology is indicated throughout the matrix. This clustering occurs due to rapid and uniform superheating effect during high microwaving power (800 W) which integrates the NiO particles leading to merging and oriented growth.19,32 On higher magnification (Figure 1b), these flakes are clearly visualized and aligned in a bunch form.



Figure 1. (a) FESEM (Low magnification), (b) FESEM (Higher magnification), (c) HRTEM and (d) SAED pattern of NiO. It is identified that choice of microwaving power and temperature of calcination are determining factors for framing morphology and size distribution of NiO particles.30,40 Herein, microwaving at 800W results in fast chemical reaction and homogeneous propagation of thermal energy in reaction vessel.30 Besides, calcination at 300 °C supports the optimum aggregation and uniform morphology of NiO flakes.19 These flakes in HRTEM image as depicted in Figure 1c appear to be composed of nanoparticles with enhanced surfaces having diameter ~10 nm. At places, fine folding shaping as narrow needles appear highlighting that these nanoparticles are ultrathin and highly porous.41,42 A noteworthy feature in work is the oriented growth of NiO with ‘flake’ morphology and reduced dimensions without


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the aid of any surfactant or directing molecule. Similar results have been reported earlier by researchers via surfactants.19,34,35 Selected-area electron diffraction (SAED) pattern depicted in Figure 1d marks typical crystal planes of NiO which agree with the literature.42,43 Large surface area and porosity characteristics are the requisites for high charge storage aspects of an electrode material.31 The nitrogen adsorptiondesorption measurements shown in Figure 2 suggests the typical Type-IV isotherm for NiO nanoflake structures and the H2-type hysteresis loop (inset Figure 2).44 BET (Brunauer-Emmett-Teller) and BJH (Barrett-Joyner-Halenda) methods were used to determine the specific surface area and pore size distribution of NiO nanoflakes. A sharp pore size distribution pattern is observed with the pores mostly in range 1.7-5.3 nm and the maxima positioned at ~8.8 nm. Evidently, the NiO nanoflakes possess few micropores (pore diameter < 2 nm) and chiefly mesopores (pore diameter < 50 nm) in matrix. Such pore dimensions are utmost complementary for penetration of KOH electrolyte and the OH- ion migration into the NiO electrode material to improve energy density and the power capability.34,45 Both the micro- and mesopores act as buffers for volume change during the redox cycling which facilitate increase in cycling life of material.46 Additionally, the mesopores (or ink bottle pores) are known to be associated with ‘capillary condensation’ phenomenon39 significant for reducing the diffusion path and thereby making space for swift ion flow enhancing electrochemical activity. The BET specific surface area calculated for NiO nanoflakes is 206 m2 g−1 which is quite uncommon as compared to earlier reports on NiO nanos26,34,39,43 and its



composites.47 The large surface area of NiO can be attributed to flake morphology and small-sized 10 nm particles present in its matrix.

Figure 2. N2 adsorption and desorption isotherm along with BJH pore size distribution (inset) of NiO nanoflakes. 3.2

Structural Investigations X-ray Diffraction XRD pattern given in Figure 3a represents d spacing for NiO at 2.76 Å, 2.17 Å and 1.63 Å corresponding to (111), (200) and (220) crystal planes respectively.25,26,30 These are indexed to cubic phase of NiO and correlate to JCPDS card no. 780429.19 Broad diffraction peaks suggest that NiO incorporates small particles having large surface area which is in good agreement with the microstructural details.39,40 Absence of secondary diffraction peaks in the XRD pattern indicates stoichiometric purity of NiO lattice structure and complete transformation of assynthesized Ni(OH)2 to NiO during calcination process at 300°C. Earlier reports reveal that calcination of Ni(OH)2 at temperatures ≤ 280°C is accompanied by


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presence of H2O molecules in the matrix, while Ni(OH)2 calcined at temperatures ≥ 300°C increases the crystal size as well crystallinity of NiO.19,48 Enhancement in crystal size tends to prevent swift flow of electrolyte ions during redox reactions due to disordering in morphology which hinders in the electrochemical performance.19

Figure 3. (a) X-ray Diffraction pattern and (b) IR spectrum of NiO nanoflakes. IR Spectroscopy IR spectrum was used to further analyze the structural characteristics as well as effect of calcination on NiO nanoflakes. Figure 3b represents the IR spectrum of NiO in mid-IR range (4000-400 cm-1) which supports the XRD findings. In the spectrum, a broad IR band at 3383 cm-1 is ascribed to the H-bonded hydroxyl groups in matrix as a result of physically absorbed water molecules.49 Appearance of a weak peak at 1634 cm-1 arises due to bending modes of interlayer water because of adsorption.25,49 These IR markers suggest physical presence of H2O molecules in NiO matrix owing to the small-sized particles and highly porous



nature. Two IR peaks are observed at 1352 cm-1 and 989 cm-1 which signify interlayering of NO3- molecule.19 A sharp peak in spectrum at 415 cm-1 indicates the typical stretching vibrations of metal-oxide bond.19,25 Absence of IR band in range 500-600 cm-1 ascribed to O-H vibrations suggests elimination of H2O molecules after heat treatment.19 The IR findings are clearly a signature for NiO formation and indicate complete dehydration of Ni(OH)2 to NiO. Thermal Stability Figure 4 represents the TG-DTA patterns for NiO nanoflakes illustrating high thermal stability of NiO structures. The TG curve reveals ~7% weight loss (40-110 °C) in NiO due to expulsion of physically adsorbed molecules.21,40 Interestingly, NiO shows a mere weight loss of 8% from temperature range 140-630 °C due to structural changes suggestive of single phase and high thermal stability. In third step (above 630 °C), the NiO shows consistency in the weight percent with rise in temperature upto 900 °C. In overall, there occurs a plain thermal degradation of ~15% only in the NiO sample which can be attributed to strong intermolecular attractions among the nanoflaked NiO molecular chains held in the matrix that rule high stability to the metal oxide. The DTA curve shows a small endothermic peak around 62 °C due to thermal dehydration and removal of physically adhered impurities.40 A broad endothermic band in temperature range 140-680 °C relates to insignificant decomposition as a result of physical ordering in NiO nanoflakes. This implies that least of decomposition occurs in NiO nanoflakes during differential thermal treatment. Considering the results, it appears that the thermal stability of


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NiO nanoflakes uphold ~85% thermal stability owing to synthesis parameters that govern the developed properties in NiO structures.

In work, the TGA or DTA curves are devoid of any sharp change in respective thermal patterns nearby 300 °C suggesting complete conversion of Ni(OH)2 to NiO during the heat treatment at 300 °C.19 This in-turn represents phase purity of the nanosized NiO and results coincide with XRD and IR findings. Reports illustrate that loss of surface properties in NiO intervenes above 300 °C during calcination process.48 Therefore, it can be deduced that temperature of calcination

has

a

stabilizing

role

on

the

surface

properties

of

NiO

nanostructures.19,48 Such highly stable nanoflakes are likely to offer enhanced surface redox reactivity for the NiO electrode material.42

Figure 4. TG-DTA patterns of NiO nanoflakes. 3.3 Electrochemical Properties



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Cyclic Voltammetry Figure 5a shows the CV measurements of NiO electrode at sweep rates 5-100 mV s-1 in 6 M KOH electrolyte. It is evident from the voltammograms that there exists large current separation between the forward and reverse scans, besides no peak formation as reported in previous studies.19,30,42 The appearance of a distinctive symmetrical

pattern

for

these

curves

signify

NiO

electrode

with

high

electrochemical reversibility and kinetically fast charge-discharge during redox reaction (NiO + OH- ↔ NiOOH + e-).48,50 Moreover, the voltammograms do not reveal the perfect rectangular shape indicating that the NiO nanoflakes possess considerable pseudocapacitive behavior.43,51 The symmetrical shape in cathodic and anodic sweeps can be attributed to the ‘nanoflake’ morphology of NiO that reduces the diffusion length of ionic species into the electrode which lead to excellent intercalation.19,30 Thus, the swift electrolytic ion transport into the electrode due to the large surface as well porous matrix of NiO enhances the faradaic

response

leading

to

ideal

pseudocapacitive

behavior

for

NiO

electrode.48,52 Interestingly, no significant change in the shape of these CV curves is observed with increase in the scan rates highlighting enhanced mass transportation capability in the nanosized NiO. This is possibly due to uniform flake morphology of NiO which supports swift charge propagation.


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Figure 5. (a) CV curves of NiO electrode at different scan rates and (b) Nyquist plot for NiO. Specific capacitance (Cs) values for NiO at different scan rates were calculated using the following expression.34,48

Cs = ]]]]]](1) Here, m (g) is the mass of electroactive material on electrode, v (V s-1) is scan rate, Vc & Va (V) are cathodic & anodic potentials respectively and i (V) is current response (A) at potential (V). The specific capacitance values deduced for NiO electrode at scan rates 5, 10, 25, 50, 75 and 100 mV s-1 are 308, 237, 158, 111, 89 and 76 F g-1 respectively. The Cs values increase with gradual decrease in scan rates due to the ion exchange mechanism explaining more accessibility time for the OH- ions to diffuse into electrode surface at lower scan rates.19,48 The



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values are pretty high than those reported earlier in literature for NiO nanos using microwave-assisted processes19,30,52 endorsing high rate capability. Impedance Spectroscopy Figure 5b represents the Nyquist plot for NiO nanoflakes in the frequency range 100 KHz-0.01 Hz at an amplitude of 5 mV. A semicircle is observed in the high frequency region while a straight line in the low frequency region. The intercept at high frequency on the Zꞌ axis (solution resistance, Rs) is merely 0.7 Ω due to low resistance offered by electrolytic ions, structural integrity of NiO, diffusion degree of ions into NiO electrode and contact at NiO electrode/current collector interface.19,42 The interfacial resistance (charge transfer resistance, Rct) is deduced as ~0.61 Ω from the intercept of Randle’s plot (Figure S1 in Supporting Information).53,54 Low charge transfer resistance value is ascribed to the enhanced electroactive surface area commanded by nanoflake morphology for faradaic reactions to occur as a result of expansion in the electrolyte accessibility.19

Diffusion coefficient (D) of NiO electrode is deduced as 0.441 x 10-9 cm2 s-1 using the expression as follows.48 D = R2T2/2A2n4F4C2σ2

]]]]]...(2)

In expression (2), R is gas constant (J K-1Mol-1), T is absolute temperature (K), A is surface area of electrode (cm2), n is no. of electrons, C is OHconcentration (M) and σ is Warburg resistance (ohm s-1/2). The Warburg resistance in expression (2) was calculated from slope of Randle’s plot53,54 (Figure S1 in Supporting Information). The diffusion coefficient is related to high porosity and


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mesoporous structure of NiO.48 The value obtained is comparable to earlier reports on porous NiO.48 It reveals high accessibility as well as the inter-connectivity of meso- and micropore channels for flow of electrolyte ions through the nanosized NiO matrix, wherein the surface chemistry plays a critical role for enduring electrochemical reactions.55 The conductivity of NiO nanoflakes is deduced as 33.87 S cm-1 at the room temperature.56 I-V graph (Figure S2 in Supporting Information) further justifies the low resistance observed for the nanostructures. Galvanostatic Charge-Discharge Figure 6a displays the GCD curves of NiO║NiO supercapacitor cell at different current densities in potential range 0 to 0.5 V. The profiles represent typical deviation from linear variation of voltage with time characteristic to electrochemical double layer capacitors (EDLCs). This non-linearity can be attributed to pseudocapacitive behavior of NiO owing to redox reactions.51 Strikingly, the shapes of GCD curves is same at different current densities which relates to constant charge-discharge rates. The results are in accordance with the CV details. A meagre iR-drop measuring 0.003-0.078 V is observed at diversified current densities 0.5-5 A g-1 claiming excellent charge flow in the cell.57 The NiO║NiO supercapacitor cell shows the



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Figure 6. (a) GCD curves at various current densities, (b) Retention graph, (c) Ragone Plot and (d) Cycling stability of NiO║NiO supercapacitor cell. specific capacitance (Cm) 307, 304, 292, 266 and 167 F g-1 at current densities of 0.5, 0.7, 1, 1.5 and 5 A g-1 respectively using the formula (3).30,58,56

Cm =

]]]]]...(3)

Wherein, i (A) and △t (s) are the discharged current and time respectively, m (g) is total mass of electroactive material coated on electrodes and △V (V) is potential window during discharge. Nevertheless, as the current density increases, there is decrease in specific capacitance of NiO║NiO cell.59 However, at higher current density of 5 A g-1, a retention in specific capacitance by 54.4% can be observed (Figure 6b). To best of our knowledge, this is the first report based on


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supercapacitor device fabricated with NiO electrode. The GCD curves of NiO electrode at different current densities19 (Figure S3 in Supporting Information) also show good agreement with deviation from true EDLC characteristics (Isosceles triangular). The capacitance values are comparable to NiO synthesized with conducting material like carbon spheres.60 High capacitance value for NiO cell can be related to its surface characteristics which support the electric conduction. Besides, the temperature of calcination has a crucial role in generating NiO with high specific capacitance.19,40 At lower calcination temperatures, uniform and oriented nanoflakes are formed by slow chemical reaction rates with less aggregations.19 Mesoporous NiO nanoflakes act as huge ‘ion reservoirs’ for splendid and continuous flow of OH- ions warranting faradaic reactions for energy storage.15,42 As such, the NiO nanoflakes are likely to exhibit enhanced pseudocapacitive properties. Figure 6c shows the Ragone plot for NiO║NiO supercapacitor cell and its three electrode cell compared with similar materials in literature.61-64 Energy density (E) and power density (P) for the cell were deduced from equations (4) and (5) respectively.13,65 The energy density of NiO symmetric supercapacitor is calculated as 10.53 Wh kg-1 with power density 124.31 W kg-1 at current density 0.5 A g-1. While at high current density of 5 A g-1, the energy density of the cell is 4.13 Wh kg-1 at the high power density of 1054.46 W kg-1. Thus, retaining high energy density, high power density is maintained suggesting least energy decay rate in the NiO║NiO cell.

E= ]]]]]...(4)



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P= ]]]]]...(5) Charge-discharge cycling tests were performed to examine the stability of the NiO║NiO supercapacitor cell. Figure 6d shows the specific capacitance vs. cycle number plot for NiO║NiO supercapacitor at 2 A g-1. Remarkably, the device is found to successfully run upto 3000 GCD cycles retaining about 96% capacitance in 6 M KOH at the high current density (2 A g-1). Figure S4 (Supporting Information) shows Nyquist plot of NiO electrode before and after cycling test. An initial increase in specific capacitance is observed during the first 1200 GCD cycles as a result of activation of NiO electrode.42,48 After 1200 cycles, there is fair stability with the device losing nearly 3-4% capacitance at the end of the test. More importantly, the Columbic efficiency (η) derived from equation (6)51 is almost 100% throughout the cycling test suggestive that the synthesized NiO nanoflake

structures

possess

stable

integrity

to

withstand

the

charging/discharging process even after running excessively 3000 cycles. Micrographs obtained for the electrode (Figure S5 in Supporting Information) after cycling at 5 A g-1 current density remarkably maintain the flake morphology of NiO structures. The results indicate high capacitance and excellent cycling stability of NiO nanoflake based NiO║NiO cell for supercapacitors. X 100

]]]]]...(6) Where, △tdischarging is discharging time and △tcharging is charging time measured in seconds.


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4.

CONCLUSIONS In summary, the work contributes two exciting research areas: firstly, rapid & direct method to synthesize NiO nanoflakes having splendid material properties through microwave-assisted route without use of any surface directing molecule or capping agents, and secondly, fabrication of a highly pseudocapacitive NiO║NiO cell exhibiting excellent performance and cycling stability. The NiO nanoflakes have been structurally identified through XRD and IR studies. NiO nanoflakes maintain high thermal stability of nearly 85% at 900 °C ascribed to strong intermolecular forces in-between the NiO nanoparticles as a result of enhanced surface properties. The NiO electrode exhibits high specific capacitance of 308 F g-1 (scan rate 5 mV s1

) while the fabricated symmetric supercapacitor shows good performance (specific

capacitance 307 F g-1 at current density 0.5 A g-1) and excellent cycling stability (retaining 96% after 3000 cycles). The potential charge storage characteristics can be attributed to ‘flake’ morphology of NiO that reinforces higher diffusion degree of the electrolyte ions into NiO electrode. Nevertheless, temperature of calcination seems to play an important role in the synthesis of NiO nanoflakes, thereby gaining control over the constrained structural features as well pseudocapacitive performance of the fabricated symmetrical cell in a better way. The work in overall underlines the significance of microwave-assisted solution combustion synthesis route in developing metal oxide nanoparticles equipped with enhanced material properties (e.g. reduced particle size, high surface area & porosity, thermal stability, electrochemical performance). The synthesized NiO nanoparticles can be further explored for developing nanocomposites for supercapacitor devices.



Associated Content Supporting Information Available: Randle’s plot; I-V graph; GCD curves of NiO electrode; Nyquist plot before and after cycling; FESEM micrograph of NiO electrode after cycling.

Acknowledgements S. Goel is thankful to CSIR for the award of RA fellowship (Sanction No. 9/45(1458)/2017-EMR-I) to undertake the research work. A.K. Tomar is grateful to UGC for award of JRF fellowship.


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Table of Content

A rapid & direct surfactant-free microwave-assisted route is discussed to synthesize NiO nanoflakes for fabrication of highly pseudocapacitive NiO║NiO exhibiting excellent performance and cycling stability.


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A rapid & direct surfactant-free microwave-assisted route is discussed to synthesize NiO nanoflakes for fabrication of highly pseudocapacitive NiO║NiO exhibiting excellent performance and cycling stability 180x145mm (72 x 72 DPI)


Highly Pseudocapacitive NiO Nanoflakes through Surfactant-Free

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