Nickel Cobaltite Nanostructures with Enhanced Supercapacitance

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Nickel Cobaltite Nanostructures with Enhanced Supercapacitance Activity Neha Garg, Mrinmoyee Basu, and Ashok Kumar Ganguli J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5039738 • Publication Date (Web): 08 Jul 2014 Downloaded from http://pubs.acs.org on July 13, 2014

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The Journal of Physical Chemistry

Nickel Cobaltite Nanostructures with Enhanced Supercapacitance Activity Neha Garga, Mrinmoyee Basua, Ashok Kumar Gangulia,b* a Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b Institute of Nano Science and Technology, Mohali, Punjab 160062, India

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Abstract Herein, we report a strategy for controlled synthesis of functional nanomaterials desired for energy conversion and power storage applications. NiCo2O4 nanostructures with square sheets, hexagonal sheets and spherical form have been synthesized using a solvothermal route by tuning of reaction conditions as well as selection of hydrolyzing agents. The synthesized nanostructures exhibited significant shape dependent electrochemical behavior with improved supercapacitance as well as good electrocatalytic properties towards oxygen evolution reaction. Among all the three morphologies, the square sheets, assembled from nanoparticles ~ 5 nm diameter, exhibited higher specific capacitance with good stability. Due to high surface area (~100 m2/g) and mesoporous nature of the square sheets, NiCo2O4 reveals better pseudocapacitance.

Keywords: Solvothermal, Supercapacitance, Pseudocapacitance


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Introduction The key technology challenges for 21st century scientists are global warming, climate change and decreasing the availability of fossil fuels for last few decades. To solve these critical issues, there is a need for urgent development of clean alternative energy sources and to fabricate devices with advanced energy storage and power density; like batteries and electrochemical capacitors (EC) or supercapacitors.1 Generally, supercapacitors are considered as one of the most promising candidates for high power energy storage devices and have attracted significant attention due to their high power potential, long cycle life and low maintenance cost.2,3 Portable electronic devices with high power energy sources have led to considerable interest towards nanostructured supercapacitor materials which show high performance in energy storage using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudocapacitors). In electrochemical double layer capacitors (EDLS), carbon based materials and for pseudocapacitors, transition metal oxides and conductive polymers are commonly used as an electrode.4, 5Pseudocapacitors with electrochemically active materials show higher energy density compared to the electrical double layer capacitors.6To enhance the specific capacitance as well as energy density, the specific surface area of the electrode material needs to be as high as possible. 6Nanostructured materials with specific morphology such as nanosheets, nanowires are significant for enhanced supercapacitance due to their high specific surface areas and availability of short electron and ion transport pathways.7These nanostructures help the electrical double-layer capacitances and accommodate a large amount of superficial electroactive species to participate in faradaic redox reactions. Nanostructured transition metal oxides like RuO2, Co3O4, NiO, Mn3O4 have been widely studied as electrodes for flexible supercapacitors.8-10Among all these, RuO2 is the best candidate for supercapacitance behavior but due to its high cost and toxic effect, it is not commercially applicable.11,12Therefore, researchers have made efforts to find alternative, inexpensive, electrode materials. Compared to the single-component oxides, binary metal oxides like spinel cobaltites (MCo2O4; M = Mn, Ni, Zn, Cu, Mg, etc.) offer more affluent redox chemistry with combining the contributions from both the Co and M ions.13,14Among all these oxides, spinel nickel cobaltite (NiCo2O4) is one of the most promising materials with higher 3 ACS Paragon Plus Environment


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electronic conductivity by two orders greater than nickel oxides or cobalt oxides.15,16The improved behavior of nickel cobaltite is because of its enormous capability to offer richer redox reactions with contribution of both the ions (nickel and cobalt), instead of the monometallic nickel oxide (NiO) or cobalt oxide (Co3O4).17These attractive features make this material more useful in high performance electrochemical supercapacitor applications. In earlier reports, for synthesis of nickel cobaltite, various strategies including coprecipitation, 18 spray pyrolysis, 19 hydroxide decomposition route,

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combustion methods,

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pulsed laser depositions,

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reverse

micellar route23 are known. Along with this, there are some other methods where different growth controlling agent like PVP has been applied to get uniformity in shape and morphology.24Among all these methods, soft chemistry routes have been widely used for the synthesis of nanostructured materials with controlled shape and size. In particular, hydrothermal technique is a facile solution based technique, without using of any template or surfactant. Since the shape, size and dimensionality of nanostructured electrodes are vital parameters for their properties, developing a facile method to synthesize ternary metal oxides with various morphologies is of interest and important for their further applications. Because of the mesoporous behavior, nanostructures can afford more active sites on the surface and improve their electrochemical performance. Hence, here we report on a shape-controlled synthesis of mesoporous NiCo2O4 through a simple hydrothermal route followed by calcination process. NiCo2O4 nanostructures with three distinct and well-defined morphologies, including square sheets, hexagonal sheets and spheres have been synthesized by controlling the reaction conditions without applying any growth controlling agent. The present solution-based route for the preparation of nanostructures is simple, controllable and scalable for industrial production. The electrochemical behavior of these nanostructures show a strong dependency on the morphology and confirms that square sheets of NiCo2O4 show higher specific capacitance as well as enhanced electrocatalysis towards oxygen evolution reaction compared to the other shapes. Experimental details Chemicals: Ni(NO3)2.6H2O (Fisher Scientific, 98%), Co(NO3)2.6H2O (Fisher Scientific, 98%), NH4OH(Fisher Scientific, 25 % NH3), urea(Sigma Aldrich, 98%), NaOH (Fisher Scientific,


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98%), ethylenediamine (Acros, 99%), ethanol(Merck, 96%)and KOH (Fisher Scientific, 85 %) were used in the synthesis of NiCo2O4 nanostructures. Synthesis of NiCo2O4 nanostructures: All the chemical reagents were of analytical grade and used without further purification. In a typical synthesis of NiCo2O4 square sheets (NC-1), mixture of Ni(NO3)2.6H2O (0.1 M), Co(NO3)2.6H2O (0.1 M), and urea (0.1 M) were loaded into a 100 ml Teflon-lined autoclave. After 1 h of magnetic stirring, the autoclave was sealed and maintained at 180 ˚C for 24 h and then allowed to cool to room temperature. The product was collected by centrifugation, washed several times with absolute ethanol and distilled water, and finally dried at room temperature. The precursor obtained was calcined at 350 ˚C for 3 h with heating/cooling rate 25˚C/h. Similarly, NiCo2O4 hexagonal sheets (NC-2) were prepared under the same conditions, except that the nitrate salts were exchanged with ethylenediamine and the hydrolyzing agent used was NaOH. Synthesis of NiCo2O4 spheres (NC-3) follows the procedure as described: Nickel nitrate (0.1 M) with cobalt nitrate (0.1 M) and hydrolyzing agent NH4OH were mixed, stirred and then the mixture was transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity. Finally, the autoclave was sealed and maintained at 180 °C for 24 h and then cooled to room temperature naturally. Then after washing with ethanol and deionized water and drying of the product at room temperature, NiCo2O4 spheres obtained. Characterization Powder x-ray diffraction data was collected using a Bruker D8 Advance diffractometer with Nifiltered Cu Kα radiation (λ = 1.5418 Å) source with 2-theta range from 10˚ to 70˚ at a scanning speed of 0.02˚ sec-1 while tube voltage and current were set at 40 kV and 30 mA respectively. Raw data were subjected to background correction and Kα2 lines were removed. Least squares fit method was used for the refinement of lattice parameters of the observed‘d’ values by Powder Cell software.25Thermogravimetric analysis (TGA), differential thermal analysis (DTA) was carried out using a Perkin-Elmer TGA-DTA-DSC system on well ground samples in a flowing nitrogen atmosphere with a heating rate of 10˚C per min. FTIR studies were carried out on a Nicolet Protege 460 spectrometer. The data of FTIR were recorded with KBr disk in the range of 500−4000 cm−1. The morphology of the samples was studied using a Field emission scanning electron microscope (FESEM, JEOL and JSM-6700 F). Transmission electron microscope (Tecnai G2 20) was used for the investigation of structure and morphology of the 5 ACS Paragon Plus Environment


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samples using at 200 kV. Compositional analysis of the samples was completed from the Energy dispersive x-ray spectroscopy (EDX) attached with TEM- EDX. For TEM analysis, the samples were prepared by dispersing the oxide powder in ethanol by ultrasonic treatment and one or two drops of the sample were poured onto a porous carbon film supported copper grid and dried in air. X-ray photoelectron spectroscopy measurements have been carried out on an ESCALAB MkII (VG Scientific) electron spectrometer at a base pressure in the analysis chamber of 5x10-10 mbar using twin anode MgKα/AlKα x-ray source with excitation energies of 1253.6 and 1486.6 eV, respectively. The spectra were recorded at the total instrumental resolution (as it was measured with the FWHM of Ag3d5/2peak) of 1.06 and 1.18 eV for MgKα and AlKα excitation sources, respectively. The energy scale has been calibrated by normalizing the C1s line of adsorbed adventitious hydrocarbons to 285.0 eV. The processing of the measured spectra includes a subtraction of X-ray satellites and Shirley-type background.26 The peak positions and area were evaluated by a symmetrical Gaussian-Lorentzian curve fitting. The relative concentrations of the different chemical species were determined based on normalization of the peak areas to their photoionization cross-sections, calculated by Scofield. 27 Nitrogen adsorption-desorption isotherms were recorded at liquid nitrogen temperature (77 K) using a Nova 2000e (Quantachrome Corp.) equipment and the specific area was determined by the Brunauer–Emmett–Teller (BET) method. The samples were degassed at 150 ºC for 4 h prior to the surface area measurements.Magnetic measurement was carried out with a SQUID(Superconducting quantum interference device) vibrating sample magnetometer (VSM) under field-cooled (FC) and zero-field-cooled (ZFC) conditions in the temperature range of 5300 K in presence of an applied magnetic field of 500 Oe. Cyclic voltammetry (CV) measurements were performed with a computer-controlled electrochemical work-station (Metrohm Autolab 302/PGSTAT). The three-electrode cell consists of Ag/AgCl as reference electrode, Pt wire as counter electrode and glassy carbon as working electrode. Glassy carbon electrode (GCE) was polished using (0.05 µm) alumina paste, ultrasonication treatment was done in distilled water and in ethanol for the electrocatalytic (OER) studies. Fabrication of the electrode for electrocatalysis measurement: A mixture of 2 mg of the catalyst (NiCo2O4) with 5 μl of isopropyl alcohol was prepared by ultrasonication for 30 min and 5 μl of Nafion was added as a binder. Nafion acts as a proton conducting binder for 6 ACS Paragon Plus Environment

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nanoparticles which forms a membrane MEA (membrane electrode assembly) over the surface of electrode.28From the above solution, 2 drops were taken and placed on the glassy carbon electrode and air-dried for 1 h. 1 M KOH was used as an electrolyte and the glassy carbon electrode was placed in the cell containing 40 ml of 1 M KOH solution. For each experiment freshly prepared solutions of KOH was used. The experiment was carried out at room temperature with a scan rate of 0.05 V/s with a peak window between 0 and 0.9 V vs. Ag/AgCl electrode. All potentials were referred to the reference electrode. All electrochemical measurements were performed at 25 ˚C. 1 M aqueous KOH solution served as electrolyte and Nafion 115 membrane was employed as the separator. The CV and GCD measurements were performed in a potential window of 0 to 0.5 V. Results and discussion NiCo2O4 was successfully synthesized through a simple one pot approach employing hydrothermal method. The influence of synthesis conditions on the morphology of the oxide was studied by altering hydrolyzing agents and ligands. Powder x-ray diffraction study of the precursors confirms the mixture of nickel hydroxide and cobalt hydroxide (ESI†, Figure S1). After calcination of these precursors at 350˚C for 3 h at a very slow scan rate 25˚C/min, NiCo2O4 nanostructures obtained. Purity, phase and crystallinity of the prepared nanostructures were confirmed using powder x-ray diffraction study. As shown in Figure 1, all the three types of nanoparticles showed the pronounced Bragg peaks corresponding to the reflections from (111), (220), (311), (222), (400), (422), (511) and (440) planes, clearly demonstrates a face centered cubic (FCC) lattice with Fd3m space group (JCPDS card no-731702). X-ray diffraction data for NiCo2O4 nanoparticles revealed the lattice parameters increase with reduction in particle size, resulting in noticeable expansion of the unit cell, shown in Figure 1. Refined lattice parameters are 8.142(2), 8.141(1) and 8.139(2) Å for the NiCo2O4 nanostructures synthesized using hydrolyzing agent Urea (NC1), NaOH (NC-2) andNH4OH (NC-3),respectively. In NC-1, peaks are slightly broader than the others because square sheets in NC-1 are assembled with smaller nanoparticles. These size induced changes in the lattice parameters are caused by the surface effects and is proved through the linear dependency of the unit cell volume on surface/volume ratio.29These lattice



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parameters are slightly higher than previous reports

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which can be attributed to the

nanocrystalline nature of these nanostructures. Thermogravimetric analysis (TGA) of the Ni–Co hydroxide precipitate (Figure2) shows a net weight loss of 16 % in the temperature range between 180˚C and 440˚C. The observed weight loss indicates that the precipitate is a mixture of Ni(OH)2 and Co(OH)2.31The overall reaction is as following: 2Co(OH)2 + Ni(OH)2 + 1/2 O2→ NiCo2O4 + 3H2O (l)

Equation (1)

The net weight loss is ~13.6 %from the above reaction which is close to the observed experimental value. The weight loss can also be partly ascribed to the decomposition of the NiCo2O4starts at about 400˚C.32 At 800˚Ca further weight loss observed, corresponding to the complete decomposition of the spinel structure.30 Figure S2 (ESI†) shows the characteristic FTIR spectrum of NiCo2O4. The region below 1000 cm-1show characteristic bands due to metal oxide (Ni–O band at ~560 cm-1, Co–O at ~652 cm-1) vibrations which match well with the literature. 33 FESEM micrographs (Figure S3(a) (ESI†)) of the NC-1 precursor confirms the flake like morphology with length ~ 600-700 nm and thickness is ~54 nm while in Figure S3(b) (ESI†) hexagonal plates like morphology is confirmed for NC-2 precursor in which the length of the hexagonal edges is ~ 400 nm. After the calcination of these precursors at 350˚C for 3 h at a very slow heating/cooling rate 25˚C/min, NiCo2O4 nanostructures (NC-1, NC-2) were obtained with controlled size and morphology. Calcination of the precursor NC-1 provided square sheets (inset of Figure S4(a) (ESI†)) with micron size edges. The thickness of these sheets was ~ 50-100 nm and these sheets were uniform in shape and size (Figure S4(a) (ESI†)). Decomposition of the NC-2 precursor at 350˚C (3 h) led to NiCo2O4 hexagonal sheets (inset of Figure S4(b) (ESI†)) with edge length of ~ 150 to 180 nm (Figure S4(b) (ESI†)). Morphology of the NiCo2O4 nanostructures has been tuned by using the variable hydrolyzing agents (Urea/NaOH/NH4OH) and ligands like ethylenediamine etc. Figure S4(c) (ESI†) shows spherical shape of NiCo2O4 nanoparticles with diameter ~ 400-500 nm, obtained using NH4OH hydrolyzing agent without post heating treatment. Hydrolysing agent plays an important role in controlling the morphology of nanostructures. Urea is slowly hydrolysed into CO3-2 and OH8 ACS Paragon Plus Environment

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ions. Then, Co+2 and Ni+2 cations react with these anions to start the process of nucleation. From the previous reports,

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it has been proved that the formation of nanoplates morphology is

related to the intrinsic square coordination structure of Ni+2 (d8) and due to the lower interfacial energy preferential growth occurs, which validate the reason of forming sheet like morphology.35 while using NH4OH as a hydrolyzing agent, due to fast hydrolysis, we are getting spherical particles. FT-IR spectroscopy confirmed the presence of ethylenediamine, (Figure S5(ESI†)) and the band at ~1060 cm-1 signified the presence of –C-N stretching vibrations. 36 From FESEM and TEM micrographs of the NC-2 precursors, hexagonal sheet like morphology is confirmed. Then, NiCo2O4 nanoparticles in situ nucleate and preferentially grow (interface nucleation to generate sheets because of the lowest activation energy of nucleation 35, 37

which results in the formation of the sheet like porous structure. Uniform distribution along

with well defined morphology of the precursors as well as the oxides nanostructures have been confirmed with Transmission electron micrographs (Figure 5, 6). Figure 3(a) shows the side view of the precursor obtained by using urea as hydrolyzing agent. From Figure 3(a) (side view), flakes of NC-1 precursor confirms with length of ~400 nm and width of ~ 40 nm while using NaOH (along with ethylenediamine) as a hydrolyzing agent, hexagonal sheets with edge length ~ 250 nm were obtained (Figure 3(b)). After calcination of these hydroxide precursors at 350 ˚C, 3h (25˚C/h), NiCo2O4 nanostructures with various morphologies were obtained. Figure 4(a) confirms the square sheet morphology of NC-1 nanostructures (NiCo2O4) as also observed in FESEM micrographs. As in the inset, we can see that these sheets (micron-size edges) are made up of small nanoparticles (particles with diameter ~ 5 nm). Figure 4(b) shows the oxide from NC-2 having hexagonal sheets of NiCo2O4 nanostructures assembled from particles ~ 5-10 nm (in the inset).Using NH4OH as a hydrolyzing agent, agglomerated spherical particles with diameter of ~500 nm were obtained. After heating the precursor NC-3, spherical particles with small diameter (~ 20-30 nm) (Figure S6(ESI†)) obtained. HRTEM (Figure5) analyses of the nanostructures obtained reveal that these are high quality crystalline NiCo2O4 nanostructures with cubic symmetry. The interplanar distance is measured to be 2.44 Å which corresponds to the (311) crystal plane of the cubic NiCo2O4 structure. The selected area of electron diffraction (SAED) pattern (Figure S7(ESI†)) demonstrates a polycrystalline structure of the NiCo2O4nanostructures. Diffraction rings obtained in SAED pattern (Figure 5 (a,b,c)) recorded from the nanostructures in Figure S5 (a), (b) and (c) respectively, can be uniquely indexed to the 9 ACS Paragon Plus Environment


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spinel NiCo2O4 crystal structure (JCPDS No.73−1702, a = b = c = 0.814 nm).To know the elemental composition, Energy dispersive analysis x-ray study has been done and shown in Figure S8 (ESI†)which confirm that the ratio of Ni: Co is ~1:2 in both the precursors (NC-1 and NC-2 precursor) as well as in all the three oxides (NC-1, NC-2 and NC-3). We have performed detailed study of N2 adsorption-desorption (Figure 6) to find out the available surface area as well as pore size and pore volume on the electrode material. From Figure 9a, we can see that the curves exhibited a typical Langmuir type IV characteristic with an hysteresis loop, in the range of 0.4−1.0 P/Po (Figure 6 a, b, c), which suggests the presence of a mesoporous structure for the square sheet, hexagonal sheet and spherical particles.34,

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The

Brunauer−Emmett−Teller (BET) surface area value of the nanostructures with square sheet, hexagonal sheet and spherical particles is calculated to be 100, 86 and 40 m2 g−1, respectively. Surface area of square sheet is much higher fromNiCo2O4 nanowires and also the nanosheets synthesized on highly conductive carbon cloth reported by Wang et al.36Mesoporous structure was further supported by the Barrett–Joyner–Halenda (BJH) pore size distribution data shown in inset of Figure9a.NiCo2O4square sheets show narrow and ordered distributions of pores at ~ 5 nm (Figure 9a). The pore volume of square sheet is up to 0.227 cm3 g−1, hexagonal sheets have pore volume of 0.125 cm3 g−1while for spherical particles, the pore volume was found to be 0.61 cm3 g-1 respectively. Due to high surface area, large pore volume, and ordered pore distribution, it is expected that the square sheets of NiCo2O4 may have potential applications in sensors and as electrochemical supercapacitors. To evaluate the chemical bonding state and composition of the as-synthesized nickel cobaltite materials (NCs), XPS measurements were conducted and the spectra are shown in Figure 7. Typical signals of O1s, Co2p and Ni2p core levels are detected based on the full scan survey spectra (Figure 7a). Ni2p spectra given in Figure 7b are composed of two spin–orbit doublets at 853.6 eV and 855.5 eV characteristic of Ni2+ and Ni3+ and two shakeup satellite peaks occurred at 859.7 and 862 eV.37The Co2p spectra, as shown in Figure 7c, consist of two spin–orbit doublets at 779.1 eV and 780.6 eV, attributed to Co3+ and Co2+ with shakeup satellite (identified as “Sat.”).38O1s spectra, as presented in Figure 10d, show four oxygen contributions, at 529 eV(denoted as “O1”) (associated with typical metal–oxygen bonds), 529.7 eV (denoted as “O2”) (due to the oxygen in hydroxyl groups), 531 eV (denoted as “O3”)(the high number of 10 ACS Paragon Plus Environment

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defect sites with low oxygen coordination in the material with small particle size) and 532.5 eV (denoted as “O4”) which corresponds to the multiplicity of physi-/chemisorbed water at and within thesurface.39These results show that the surface of as-synthesized nickel cobaltite nanostructures contain Co2+, Co3+ and Ni2+ as well as Ni3+species. The formula, therefore can be generally expressed as, Co1-x2+Cox3+[Co3+Nix2+Ni1-x3+]O4 (0 < x < 1) (the cations within the square bracket are in octahedral sites and the outside ones occupy the tetrahedral sites.38In addition, the quantitative results of the sample NC-1, based on XPS data reveals that the O/Co/Ni atomic ratio is 4.1/1.25/1 (Table S1, ESI†), which shows the enrichment in Ni species at the surface of the NC materials and possibly enhances the electrochemical behavior.40 Electrochemical Analysis: To confirm the potential applications of these (NiCo2O4) nanostructures as electrocatalysts towards oxygen evolution reaction (OER) as well as for supercapacitors, cyclic voltammetry (CV) and galvanostatic charge-discharge tests were performed. Representative cyclic voltammogram measurements for the square sheets (NC-1), hexagonal sheets (NC-2) and the spherical particles (NC-3) are shown in Figure 8a which has been carried out in a three electrode cell at slow scan rate of 10 mV/sec at a potential sweep window of 0 to 0.9 V. In the water electrolysis following reaction takes place: At anode:

4 OH-

At cathode:

4 H2O + 4 e-

O2 + 4 e- + 2 H2O

Equation (2)

2 H2 + 4 OH-Equation (3)

As from the previous studies, 41 it is well known that shape and size of the nanostructures play an important role in electrochemical studies and NiCo2O4 is an efficient electrode towards OER. From Figure 8a, it can be seen that NC-1 shows the highest current density ~ 280 mA/cm2 among all the three nanostructures (Table 1) and it is also significantly higher than previous studies as earlier reported, Baydi et al. have synthesized much larger (µm size) NiCo2O4 particles and obtained a low current density of 5–10 mA/cm2,42 Chang et al. have synthesized NiCo2O4– graphene composite, which shows a current density of 4–6 mA/cm2.43 The shape of the plots deviates slightly from ideal rectangular voltammogram indicating pseudocapacitance behavior of nickel cobaltite nanostructures .44As from the cyclic voltammogram curve, it can be seen that, square sheets of NiCo2O4 cover large area to the voltage and current plot, indicating the high 11 ACS Paragon Plus Environment


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charge storage among the three nanostructures which can be attributed to the higher surface area of the NC-1 sample since it favors faster ionic transport in between the electrodes. The area under the curve of the CV plot is directly proportional to the specific capacitance.45, 46 From the cyclic voltammogram curve of NC-1 at various scan rates (10 mV/s, 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s and 100 mV/s), it can be seen that the anodic peaks shift towards positive potential and the cathodic peaks shift towards negative potential. The peak current increases almost linearly with the increased scan rate (Figure 8b) and confirm the pseudocapacitive behavior of the NiCo2O4 nanostructures. Along with it, well - defined redox peak also attests to the capacitance characteristic of these nanostructures being governed by Faradaic reactions at the surface.36 These redox peaks correspond to the conversion between different cobalt and nickel oxidation states (Ni+2 to Ni+3, Co+2 to Co+3 and Co+4). The most plausible elementary processes associated with the capacitive behavior of NiCo2O4 can be described as follows47, 48 NiCo2O4 + OH- + H2O CoOOH + OH-

NiOOH + 2CoOOH + eCoO2 + H2O + e-

(4) (5)

To further quantify the specific capacitance of these electrochemical capacitors, galvanostatic charge-discharge experiments were conducted in the three-electrode cell configuration at various charge-discharge current densities with the potential window between 0 to 0.5 V, as shown in Figure 9. The specific capacitance could be calculated from the following equation49: C = IΔt/ (mΔV)

Equation (6)

Where I refers to the current density used for the charge/discharge measurements, Δt refers to the time elapsed for the discharge cycle, m denotes the mass of the active material in a single electrode and ΔV represents the voltage interval of the discharge measurement. The specific capacitance of NC-1 (square sheet) is shown as a function of the current density in Figure 9a. From the Eq. (6), specific capacitance of the NC-1nanostructures could be calculated as 980, 926, 600 and 384 Fg−1 at different current density of 0.5, 1, 5, and 10 A g−1 respectively. Figure 9b, depicts the comparative galvanostatic charge–discharge measurements of all the three NiCo2O4 nanostructures (NC-1 square sheet, NC-2 hexagonal sheet and NC-3 spherical particles) that were performed with an applied constant current density of 0.01 A g -1. We observe that both 12 ACS Paragon Plus Environment

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the charge and the discharge time of the square sheet based nanostructures are much longer than those of the others, which is in accordance with the CV results shown in Figure 8a.The enhanced electrochemical properties of the mesoporous NiCo2O4 particles is attributed to the layered platelet like arrangement and the presence of exceptionally high number of regularly ordered pores.12 Electrochemical capacitance of the square sheet based NiCo2O4 nanostructures at various current densities are listed in Table 2. Compared with the previous literature, specific capacitance of NC-1 is quite higher. At a particular constant current density of 1 Ag-1, NiCo2O4 nanowires synthesized by Lei Jiang et al. shows specific capacitance 760 F g-1,45Yang et al. have synthesized urchin shaped NiCo2O4 via sequential crystallization process which shows specific capacitance of 658 Fg-1 at 1 Ag-1 current density.47In another report, Wang et al. have synthesized NiCo2O4 nanowires and nanosheets on carbon cloth via hydrothermal route with 245 and 123 Fg-1 specific capacitance at 1 Ag-1 current density.36 A long cycling performance along with stability of an electrode is the most important criteria for a supercapacitor.50,

51

So to prove the performance of NiCo2O4 electrode towards

electrochemical applications, an endurance test (specific capacitance vs. cycle number) is conducted using galvanostatic charge−discharge cycles at 12 Ag−1 (Figure 9c). The specific capacitance of the NiCo2O4 square sheet increases with cycling numbers. After a 1000-cycle test, the specific capacitance reaches a high value of 310 F g−1, which is higher than its initial value (200 F g−1). As for the NiCo2O4nanosheets, the decay in specific capacitance after a 1000cycletest is ~ 9% (Figure 9c). These results indicate that the NiCo2O4 square sheet exhibit excellent cycling stability. Conclusions In summary, mesoporous NiCo2O4 with controlled morphology (square sheets, hexagonal sheets and spherical) have been successfully synthesized using a facile hydrothermal route without using any capping agent or catalyst under mild conditions. This nickel cobaltite can be used as an electrode material towards oxygen evolution reaction and in electrochemical supercapacitors. Due to having significant difference in surface area and pore volume, three crystals present different pseudocapacitive performance. Square sheet NC-1 shows significantly higher specific 13 ACS Paragon Plus Environment


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capacitance. This cobaltite can be used for supercapacitor electrodes and in alternative energy generation and storage devices. Acknowledgement A.K.G. thanks DST, CSIR and DeitY Government of India for financial support. NG is thankful to UGC, Government of India for a fellowship. We thank our collaborator Dr. Hristo Kolev (Institute of Catalysis, Bulgarian Academy of Sciences) (Indo- Bulgaria project (RP 02803) supported by DST), for their contribution in XPS measurement.

Supporting Information Powder x-ray diffraction pattern of the precursors, FT-IR spectra of NiCo2O4 (NC-1, NC-2 and NC-3), FESEM micrographs of the precursors NC-1, NC-2 and NC-3 and also their calcined NiCo2O4 nanostructures. TEM micrograph of the calcined NiCo2O4 spheres obtained using ammonium hydroxide. EDX spectrum of NiCo2O4 nanostructures and quantitative analysis of nickel cobaltite (NC-1). This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author E- mail: [email protected] References: 1. Miller, J. R.; Burke, A. F. Electrochemical Capacitors: Challenges and Opportunities for Real-World Applications. ECS Interface 2008, 17, 53-57. 2. Miller, J. R.; Simon, P. Electrochemical Capacitors for Energy Management. Science 2008, 321, 651-652. 3. Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater 2008, 7, 845-854.


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Table 1: Electrocatalytic properties of NiCo2O4nanostructures as an electrocatalysts towards the Oxygen evolution reaction

S.N.

NiCo2O4

Surface area (m2/g)

Current density (mA/cm2)

Onset potential (V)

1.

NC-1

100

280

0.38

2. 2.

NC-2 NC-3

82 40

210 120

0.31 0.60

Table 2: Specific capacitance of NC-1at various current densities

Current density (Ag-1)

Specific capacitance (NC-1) (Fg-1)

0.5 1 5 10

980 926 600 384


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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


Figure Captions Figure 1. Powder x-ray diffraction patterns of NiCo2O4 Figure 2. Thermogravimetric analysis of the hydroxide precursor synthesized by urea as a hydrolyzing agent. Figure 3. TEM micrographs of the hydroxide precursors obtained using (a) urea (NC-1) (b) ethylenediamine (NC-2). Figure 4. TEM micrographs of NiCo2O4 nanostructures using (a) urea (NC-1) (b) ethylenediamine (NC-2) and (c) ammonium hydroxide (NC-3). Figure 5. HRTEM micrographs of the NiCo2O4 nanostructures obtained using (a) urea (NC-1) (b) ethylenediamine (NC-2) and (c) ammonium hydroxide (NC-3). Figure 6. Nitrogen adsorption and desorption isotherms measured at 77 K for (a) NC-1, (2) NC-2 and (c) NC-3. The inset shows the corresponding BJH pore size distributions. Figure 7. XPS spectra of NiCo2O4 (NC-1) (a) survey scan, (b) Ni2p core level, (c) Co2p core level and (D) O1s core level. Figure 8 a. Electrocatalytic study of NiCo2O4 nanostructures (NC-1, NC-2 and NC-3) as catalyst for oxygen evolution reaction, (b) Scan rate vs. current density plot. Figure 9 (a) Charge-discharge curves at different current densities of NC-1 electrode; (b) Galvanostatic charge-discharge at a current density of 1 A g-1for the three electrodes (NC-1, NC2 & NC-3) (c)Variation of specific capacitance with cycle number at 12 A g−1 of the NC-1 (square sheet) electrode.



50000

(311)

NC-3 NC-2 NC-1

40000

Intensity (cps)

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

Page 22 of 31

30000 20000

(111)

10000

(220)

(511) (400) (222) (422)

(440)

0 10

20

30

40

50

60

70

2-Theta (degree) Figure 1. Powder x-ray diffraction patterns of NiCo2O4


Page 23 of 31

100 95

Weight(%)

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


90 85 80 75 100

200

300

400

500

600

700

800

900

o

Temperature ( C) Figure 2. Thermogravimetric analysis of the hydroxide precursor synthesized by urea as a hydrolyzing agent.



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

bbb

a

Figure 3. TEM micrographs of the hydroxide precursors obtained using (a) urea (NC-1) (b) ethylenediamine (NC-2).


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Page 25 of 31

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


a

b

0.5 µm

c

0.5 μm

Figure 4. TEM micrographs of NiCo2O4 nanostructures using (a) urea (NC-1) (b) ethylenediamine (NC-2) and (c) ammonium hydroxide (NC-3).



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

Figure 5. HRTEM micrographs of the NiCo2O4 nanostructures obtained using (a) urea (NC-1) (b) ethylenediamine (NC-2) and (c) ammonium hydroxide (NC-3).


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100

100 80

0.03

Volume of gas (cc/g)

120

Pore volume (cm3/g.nm)

140

0.040

a

0.04

0.02

0.01

0.00 0

10

20

30

40

50

60

70

Pore diameter (nm)

60 40 20

80

60


160

b

0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000 -0.005 0

10

20

30

40

50

60

70

Pore diameter (nm)

40

20

0 0

0.0

0.2

0.4

0.6

0.8

1.0

0.0

Relative pressure (P/Po)

0.2

0.4

0.6

0.8

1.0


60

c

50 40 30


0.035

Volume of gas (cc/g)

Volume of gas(cc/g)

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


0.030 0.025 0.020 0.015 0.010 0.005 0.000 -0.005 0

10

20

30

40

50

Pore diameter (nm)

20 10 0 -10 0.0

0.2

0.4

0.6

0.8

1.0


Figure 6. Nitrogen adsorption and desorption isotherms measured at 77 K for (a) NC-1, (2) NC-2 and (c) NC-3. The inset shows the corresponding BJH pore size distributions.



300000

(b) Ni 2p

Ni 2p 3/2

6000

200000

Counts (s)

Co3s

100000

CoLMM

Ni3s

1400

1200

1000

+2

Ni 2p1/2 3000

+3

2000

+2

1000

NiLMM

0

4000

C1s

50000

Co3p

O1s

Co2p

Ni2p

+3 Co2s

150000

Ni2s OKLL

5000 CKLL

Counts (s)

7000

Survey AlKα Survey MgKα

(a)

250000

800

600

400

Ni3p

200

0 870

0

868

Binding energy (eV)

866

864

862

860

858

856

854

852

850

Binding Energy (eV) 10000

10000

(c) Co 2p

(d) O1s

8000

Co 2p1/2

6000

+2

8000

Counts (s)

Counts (s)

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

Page 28 of 31

+3

4000

O3

4000

2000

2000

Sat.

O1

6000

O4

0

0

534

784

782

O2

780

778

776

532

530

528

Binding Energy (eV)

Binding Energy (eV)

Figure 7. XPS spectra of NiCo2O4 (NC-1) (a) survey scan, (b) Ni2p core level, (c) Co2p core level and (D) O1s core level.


526

300

28

Blank GC NC-1 NC-2 NC-3

250 200

(a)

26

Current density (A)

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


Current density (mA/cm2)

Page 29 of 31

150 100 50 0 -50 -100

(b)

24 22 20 18 16 14 12 0

0.0

0.2

0.4

0.6

0.8

20

1.0

40

60

80

100

Scan rate (mV/s)

E vs Ag/AgCl (V)

Figure 8 a. Electrocatalytic study of NiCo2O4 nanostructures (NC-1, NC-2 and NC-3) as catalyst for oxygen evolution reaction, (b) Scan rate vs. current density plot.



0.45

10 Ag-1 5 Ag-1

(a)

Potential vs. Ag/AgCl (V)

0.5

Potential vs. Ag/AgCl (V)

1 Ag-1 0.5 Ag-1

0.4

0.3

0.2

0.1

0.0 0

500

1000

1500

2000

NC-1 at 0.01 Ag-1 NC-2 at 0.01 Ag-1 NC-3 at 0.01 Ag-1

(b)

0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 0

200 400 600 800 1000 1200 1400 1600 1800

Time (sec)

Time (Sec)

400

Specific capacitance (Fg-1)

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

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(c)

350 300 250 200 150 100 50 0 0

200

400

600

800

1000

Cycle number

Figure 9 (a) Charge-discharge curves at different current densities of NC-1 electrode; (b) Galvanostatic charge-discharge at a current density of 1 A g-1for the three electrodes (NC-1, NC2 & NC-3) (c)Variation of specific capacitance with cycle number at 12 A g−1 of the NC-1 (square sheet) electrode.


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Table of contents

Nickel Cobaltite Nanostructures with Enhanced Supercapacitance Activity Neha Garga, Mrinmoyee Basua, Ashok Kumar Gangulia,b*

0.5 µm


Nickel Cobaltite Nanostructures with Enhanced Supercapacitance

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