NiCo2O4-based nanostructured composites for high-performance pseudocapacitor electrodes

Journal Pre-proof NiCo2 O4 -based Nanostructured Composites for High-Performance Pseudocapacitor Electrodes Mehdi Eskandari, Rasoul Malekfar, David Buceta, Pablo Taboada

PII:

S0927-7757(19)31030-1

DOI:

https://doi.org/10.1016/j.colsurfa.2019.124039

Reference:

COLSUA 124039

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

24 June 2019

Revised Date:

26 September 2019

Accepted Date:

27 September 2019

Please cite this article as: Eskandari M, Malekfar R, Buceta D, Taboada P, NiCo2 O4 -based Nanostructured Composites for High-Performance Pseudocapacitor Electrodes, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), doi: https://doi.org/10.1016/j.colsurfa.2019.124039

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

NiCo2O4-based Nanostructured Composites for High-Performance Pseudocapacitor Electrodes Mehdi Eskandari a, b, Rasoul Malekfar a, David Buceta c, Pablo Taboada b,* a

Atomic and Molecular Group, Physics Department, Faculty of Basic Sciences, Tarbiat Modares

University, 14115-111 Tehran, I.R. Iran; Colloids and Polymer Physics Group, Department of Particle Physics, Faculty of Physics,

of

b

c

ro

Campus Vida, Universidade de Santiago de Compostela, 15782 Santiago de Compostela Spain NANOMAG Laboratory, Departments of Physical Chemistry and Applied Physics, IIT,

Correspondence author: [email protected]; Tel.: +34-881-814-111 (F.L.)

Jo

ur na

Graphical abstract

lP

re

*

-p

Campus Vida, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain

1

Abstract: Nanostructured electrodes for supercapacitors made by combination of electrical double layer and pseudocapacitive based-nanomaterials within a single hybrid composite has a great potential on expanding the range of use of these devices and increase their electrochemical performance. In this work, we developed several nanostructured composites with combinations of such types of materials with potential applicability as electrodes in supercapacitors. In particular, these composites were obtained by easy, cost-effective and scalable procedures, and

of

were composed by NiCo2O4 cores as the main reversible fast faradaic-based nanomaterial and

ro

either the conductive polymer polyaniline (PANI), multiwall carbon nanotubes (MWCNTs), or reduced graphene oxide (r-GO) as the electrical double layer-based carbonaceous-based

-p

nanomaterials in order to enable the combination of both type of energy storage processes within

re

a single nanostructured electrode. These constructions allowed us to obtain specific capacitance as large as 1760 F/g, 900 F/g and 734 F/g at a current density of 1 A/g for NiCo 2O4/PANI,

lP

NiCo2O4/MWCNT, and NiCo2O4/r-GO nanostructured composite electrodes, respectively. Besides, the stability of NiCo2O4/MWCNTs and NiCo2O4/r-GO-based electrodes was

ur na

outstanding, with capacity losses below 10% after the periods of operation (> 1000 cycles). Keywords: NiCo2O4 cores, nanostructured composite, carbon-based materials, electrochemical capacitance, pseudocapacitor

Jo

1. Introduction

In the last decades, energy has become one of the most important bottlenecks to further improve human wellbeing. In order to break the excessive energy dependence from fossil fuels, significant progresses on renewable energy technologies such as solar and wind power, hydroelectricity, and biomass have been extensively done[1]. Nevertheless, the dependence of renewable sources and related energy production on environmental conditions together with the 2

increasing use of these “clean” energies effectively require a strong development of energy devices to store and deliver the produced energy on demand. Although different types of storage devices to harvest renewable energy such as fuel cells, lithium-ion batteries, and solar cells to cite a few, are already available even in the market this type of energy devices still need further development to fulfill a plethora of suitable operation conditions as large efficiencies, storage

of

capacities, and charge/discharge powers[2] Supercapacitors have recently attracted a great deal of interest in various applications including

ro

the development of hybrid electric vehicles, large industrial equipment, memory back-up devices

-p

and renewable energy power plants thanks to their high power density, fast charge/discharge rates, and long cycle life times. There exist two main classes of supercapacitors, which include

re

electrical double layer (EDLCs) and reversible fast faradaic supercapacitors (FSs) or pseudocapacitors[3], respectively. As mentioned by Brousse et.al pseudocapacitive terminology

lP

should be used for a given electrode investigated individually in a three-electrode cell [4]. EDLCs bridge the gap between batteries, which offer a high-energy storage capacity (energy

ur na

density, ED) but a slow charge−discharge, and conventional capacitors that comparatively possess larger power densities (PDs, that is, faster charge−discharge processes) but hold a much lower energy storage capacity. EDLCs typically are made of carbon-based materials bearing high surface area and porosity[5]. They store energy by forming an ionic double layer which diffuses

Jo

very quickly, hence, providing a very rapid discharge profile as well as high power densities. However, EDs and charges which can be physically stored within EDLCs are still quite low in aqueous as well as organic electrolytes. To overcome these issues, relatively new carbon-based nanomaterials such as one dimensional carbon nanotubes (CNTs) and two-dimensional graphene

3

have been increasingly used to provide higher charge−discharge rates; however, they exhibit a lower volumetric capacitance compared to that of traditional porous carbons. Conversely, pseudocapacitors use reversible fast faradaic reactions to store electrical charges, which allow to achieving higher capacitance by at least one order of magnitude than those obtained by EDLCs. Materials sustaining such redox reactions on their surfaces include, for example, conducting polymers and transition metal oxides[6,7]. Conducting polymers have been

of

also used in pseudocapacitors as a result of their highly π-conjugated polymeric chains,

ro

environmental stability and outstanding electrochemical properties [8]. In this regard, polyaniline (PANI) is one of the most significant conductive fillers because of its easy synthesis, low cost,

-p

high electrical conductivity and reduced ecotoxicity [9,10]. On the other hand, ternary metal

re

oxides are another class of electrode materials for supercapacitors with excellent electrochemical performances, which provide additional free space to enhance the mobility of active species, and

lP

reduce the structural strain of electrodes [11,12]. Some metal oxides such as MnO2 and RuO2 have intrinsic capacitor-like behavior, while some constructed metal oxides in high concentration

ur na

of electrolyte show capacitor-behavior which is only due to the electrode design or architecture and not an intrinsic property such as LiCoO2, this behavior called extrinsic pseudocapacitance [4]. In particular, metal oxide-based spinel structures are able to provide much larger mobilities of the active species with outstanding electrochemical stability. Amongst spinel ternary metal

Jo

oxides, nickel cobaltite (NiCo2O4) has been considered as a very interesting, cost-effective and scalable material thanks to its low cost, abundance, environmental friendliness, and larger electrical conductivity compared to pure nickel and cobalt oxides[13]. As a consequence, nickel cobaltite is expected to display much more effective redox reactions than the respective pure

4

oxides thanks to the larger density of active sites and its open structure, which favor an enhanced mobility of charge carriers [14]. Nevertheless, the former two types of materials also display a series of drawbacks as, for example, their poor ion/electron conductivity which limits attainable high capacities at high charge/discharge rates and, in the particular case of metal oxides, the distortion and volume

of

change-induced stress of their microstructure stemming from the continuous supported redox reactions[14], which severely limit the attainable theoretical capacity at high charge/discharge

ro

rates. One strategy to improve the electrochemical performance of pseudocapacitive materials

-p

while avoiding some of the aforementioned drawbacks involves the construction of hybrid nanostructures, which combine EDLCs and pseudocapacitive materials in a single

re

nanostructured composite electrode [15,16]. This type of nanostructures not only expands the potential range of use and the effective performance of active pseudocapacitive materials as

lP

metal oxides, but also improves the electronic conductivity and charge/discharge rates of the resulting hybrid nanomaterials by using conductive EDLCs carbonaceous materials such as

ur na

porous carbon, CNTs, graphene, graphene oxide (GO) sheets, etc [17,18]. Moreover, these hybrid nanostructured composites possess the capability of preserving the structural integrity of the active metal oxide material, thereby, allowing an enhanced cyclability and rate capacity[19].

Jo

Hence, we here report the fabrication of different nanostructured composites composed of NiCo2O4 cores as the main extrinsic pseudocapacitive material and carbon allotropes. The combination of NiCo2O4 nanoparticles (NPs) with carbon-based nanomaterials gives the opportunity to exploit the excellent electrical conductivity and large activated surface areas of the latter as well as the ultrahigh specific capacitance of the former [20]; that is, carbon materials with large contact areas make possible to transfer fast electrons and allow a deeper penetration of 5

electrolyte ions into the hybrid storage nanostructured composite. Therefore, the resulting nanostructured materials including NiCo2O4 and a carbon allotrope can improve the capacitance of composite electrode by sharing the advantages of both classes of materials, which have a synergistic positive effect in the device operation and performance [21]. In addition, the nanostructuration of NiCo2O4 in the form of cores with nanoscale porosity on their surfaces should allow a better accommodation of induced mechanical stresses and cycling stability[22],

of

shorten the electron transport lengths, reduce the aggregation of active materials, and provide a

ro

larger electrode surface. Therefore, NiCo2O4 cores obtained through a solvothermal method [20,23] were combined with either PANI or multi-wall CNTs (MWCNTs) since these fillers

-p

improve the electrical conductivity and activated surface areas of the resulting composite

re

nanostructured materials. In the case of NiCo2O4/MWCNTs composite, a simple solution mixing methodology was followed to obtain the nanostructured composite, which facilitates large scale

lP

production while ensuring outstanding electrochemical performances at much lower electrolyte concentrations than commonly used [24,25]. In an alternative configuration, NiCo2O4 cores were

ur na

also deposited on the surface of a nickel foam substrate and combined with reduced-graphene oxide (r-GO) to improve the electrochemical properties of the metal oxide while avoiding the use of other carbon active materials or isolating binders as polyvinylidene fluoride (PVDF). This procedure for NiCo2O4/r-GO production is easy, binder free, affordable and more efficient than

Jo

previously developed, allowing the obtaining of high capacitances (up to ca. 1760 F/g at 1A/g) thanks to the improvement of the contact between r-GO nanosheets with the metal [26,27]. X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET) techniques were used to structurally characterize the resulting nanostructured composites. Furthermore, the

6

electrochemical performances of the prepared NiCo2O4/PANI, NiCo2O4/MWCNT, and NiCo2O4/r-GO derived electrodes were characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopies (EIS). To analyze the obtained impedance experimental data, Zview software was used to design the equivalent theoretical circuit. According to the equivalent circuit elements of NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNT and NiCo2O4/r-GO composites, the electrochemical behavior of the

ro

2. Experimental section

of

constructed supercapacitor electrodes was deciphered.

-p

2.1 Materials

re

Co (NO3)2·6H2O and Ni (NO3)2·6H2O were purchased from Sigma Aldrich. 2-isopropanol and glycerin were purchased from Merck. Multi-walled carbon nanotubes (MWCNTs, outer diameter

lP

20–30 nm, average length ~50 µm, specific surface area ~110 m2 g-1 and purity > 95 wt%) were purchased from Neunano, Iran. Aniline, sodium dodecyl sulfate (SDS) and ammonium per

ur na

sulfate (APS) were also purchased from Sigma Aldrich. 2.2 Synthesis of materials

2.2.1 Synthesis of NiCo2O4 cores

Jo

NiCo2O4 cores were prepared based on a previously reported methodology with little modifications [27]. Briefly, 70 mg of cobalt nitrate and 30 mg nickel nitrate were dispersed in 5 mL 2-isopropanol. Then, this solution was added to a mixture of isopropanol (30 mL) and glycerin (8 mL). The resulting solution was poured into a sealed Teflon stainless steel autoclave and put in the oven at 180 °C for 6 h. Then, the solution was washed with ethanol several times

7

and dried at 70 °C. The resulting powder was put in the oven at 350 °C at a heating rate of 1°C/min for 2 h. 2.2.2 Purification of MWCNTs To purify the MWCNTs, a combined heating an acidic treatment was used[28,29]. Firstly, MWCNTs were heated in an oven at 300°C for 1 h to remove amorphous carbon, and then

of

refluxed with HNO3 for 12 h at 120°C to remove other remaining impurities [30]. The resulting

2.2.3

Preparation

of

NiCo2O4/MWCNTs,

NiCo2O4/PANI

and

NiCo2O4/r-GO

-p

nanostructured composites

ro

purified MWCNTs were washed several times with DI water to reach neutral pH.

re

NiCo2O4/MWCNTs nanostructured composites were prepared based on a previously reported method[31]. Briefly, both the starting metal oxide and carbonaceous material were firstly mixed

lP

at a weight ratio of 2:1. Then, the mixed powder was dissolved in 2-isopropanol and kept in an ultrasonic bath for 6 h. Next, the dispersed solution was heated at 70 °C to evaporate the solvent.

ur na

To prepare NiCo2O4/PANI nanostructured composites, NiCo2O4 cores were dispersed in doubleionized (DI) water (50 mL). Then, SDS (0.08 g) and APS (0.01 g) were dispersed in the former solution, which was held at room temperature for 12 h. Then, aniline (8 µl) was added under

Jo

stirring at 4 °C. An amount (50 µl) of hydroxide acid (1 M) was added to the previous solution to keep the pH at 3.0. The resulting solution was stirred for 24 h at 4 °C and, then, washed several times with DI water and ethanol[32]. To prepare the NiCo2O4 cores, NiCo2O4/PANI and NiCo2O4/MWCNTs electrodes, the obtained active nanostructured composite materials (85 wt. %), carbon black (10 wt. %) and PVDF (5 wt. %) were dispersed in 2 mL acetone. The mixed

8

solution was dropped on a nickel foam substrate, and then dried at 70°C for 7 h to evaporate the solvent. For the preparation of the NiCo2O4/r-GO electrodes, NiCo2O4 cores were dispersed in acetone. Then, this solution was slowly dropped onto a nickel foam substrate using a micro-sampler and left in the oven at 70°C for 7 h to evaporate the solvent. Next, graphene oxide was synthesized

of

through a well-established methodology [33], and a solution of GO in DI water (1 mg/mL)

used at a voltage of -1.6V, as previously reported [34].

-p

2.3 Physical characterization

ro

prepared. To reduce and deposit GO onto the nickel foam the chronoamperometry method was

The crystallization of the spinel metal oxide particles and the crystalline structure of the

re

nanostructured composites were analyzed by means of an X-ray diffractometer (X’pert model

lP

1480 manufactured by Philips) with CuKα radiation. The X-ray wavelength was 1.54056 Å, and the diffraction patterns were recorded at room temperature and over a 2θ range of 10- 80°, at a

ur na

scanning speed of 2° per minute. Surface area and size distributions of the metal oxide nanocores were measured using a Micromeritics instrument 3020 under nitrogen atmosphere and determined following Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The morphology of the obtained nanostructured composites was analyzed by field-

Jo

emission scanning electron (FESEM) (ZEISS Sigma, manufactured by Germany) and transmission electron microscopies (TEM, Philips CM30). Finally, the electrochemical properties and performance of the obtained hybrid nanostructured composites-based electrodes were determined by cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves and electrochemical impedance spectroscopy (EIS) using a SP-

9

300 Bio-Logic potentiostats/galvanostats instrument at room temperature. All tests were performed using a three-electrode setup in which nanostructured composite samples, platinum, and Hg/HgO were selected as the working, counter and reference electrodes, respectively. Electrodes were immersed in 3 M KOH used as the electrolyte. CV tests were repeated for all different types of electrodes at various scan rates ranging from 5 to 50 mVs-1 within the potential window of 0.1 to 0.5 V vs. Hg/HgO. GCD measurements were performed by

of

chronopotentiometry (CP) at various current densities. EIS measurements were recorded within

ur na

3. Results and Discussion

lP

re

-p

ro

the frequency range of 100 kHz–0.1 Hz.

3.1 Structural characterization

The crystalline phases of NiCo2O4 cores, NiCo2O4/MWCNT, NiCo2O4/PANI, and NiCo2O4/rGO nanostructured composites were investigated by X-ray diffraction. As illustrated in Fig. 1a,

Jo

the diffraction peaks of the spinel phase of NiCo2O4 cores are observed at 2θ values of 22°, 36.3°, 42.8°, 44.9°, 52.3°, 69.8° and 77.1°, which can be indexed to (111), (220), (311), (222), (400), (511), and (440) planes in the face centered cubic spinel structure (JCPDS card No. 200781), and matched very well with previously reported data [23,35]. NiCo2O4 as a binary metal

10

oxide has a cubic spinel structure in which all nickel ions occupy octahedral sites, whereas cobalt ions distribute on both octahedral and tetrahedral ones[20]. Regarding NiCo2O4/PANI nanostructured composites, the presence of the conductive polymer does not affect the crystallinity of the metal oxide phase, and the soft slope observed in the XRD pattern (Fig. 1) can be assigned to the periodicity of the repeat units of PANI chains parallel to

of

the polymer backbone[36]. A similar diffraction pattern for NiCo2O4/MWCNTs nanostructured composites was also noted, but with additional peaks detected at 30º, 47º and 65º attributed to

ro

(002), (100) and (004) reflection planes of the carbonaceous phase, respectively, and related to

-p

the hexagonal graphite structures of MWCNTs (JCPDS card No. 41-1487) [35]. Finally, the XRD patterns of NiCo2O4/r-GO nanostructured composites showed some sharp peaks at 45º, 52º,

lP

are much smaller but still present [38].

re

and 76º corresponding to the nickel foam substrate [37]; those corresponding to nickel cobaltite

Raman spectroscopy was additionally used to further complete the structural characterization of the obtained nanostructured composites. The 100 to 2000 cm-1 regions of the Raman spectra of

ur na

NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNTs, NiCo2O4/r-GO nanostructured composites are shown in Fig. 1b. Vibrational modes of NiCo2O4 cores are presented at 187, 459, 509, 656, and 1096 cm-1, respectively [39,40], which correspond to F2g, Eg, LO, A1g, and 2 LO modes of

Jo

the NiCo2O4 spinel phase [27]. Raman spectra of NiCo2O4/PANI displays the PANI characteristic vibration peaks including the bands attributed to the C–N+ vibration of delocalized polaronic structures at 1330 cm-1, C=N stretching at 1486 cm-1, and C–C stretching modes at 1590 cm-1, respectively [41]. For NiCo2O4/MWCNTs nanostructured composite, bands at 191, 459, 468, 509 and 665 cm−1 attributed to the F2g, Eg, F2g, LO and A1g modes of NiCo2O4, respectively, are observed while peaks at 1352 and 1585 cm−1 correspond to D and G bands of 11

the carbon phase of MWCNTs [42]. Moreover, a single G peak at 1585 cm-1 proved the multiwall nature of the used CNTs [43]. Besides, Raman spectra of MWCNTs before and after the acidic treatment are also shown in Fig. S1. Fig. 1b also showed peaks at 1352, 1594 cm−1 corresponding to D and G, bands of grapheme in NiCo2O4/r-GO nanostructured composites. 2D and D+G bands are attributed to the substantial removal of oxygen-containing groups during the GO reduction process [44]. The reduction of GO nanosheets can improve the electrical

of

conductivity and provides better current discharge capacities [45]. Furthermore, the intensity

ro

ratio between D and G bands (ID/IG) for GO and NiCo2O4/r-GO nanostructured composites (0.84 and 0.94, respectively) confirms the reduction of GO to r-GO after the chronoamperometric

-p

treatment used for the nanostructured composite production[46]. Further, Fourier-transform

re

infrared spectroscopy (FTIR) spectra were performed on NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNTs, NiCo2O4/r-GO nanostructured composite electrodes (see Fig. S2), which

lP

additionally corroborate the results obtained by Raman.

BET specific surface areas of NiCo2O4 cores, NiCo2O4/PANI and NiCo2O4/MWCNT

ur na

nanostructured composites were determined by N2 adsorption/desorption isotherms at 77 K, and the corresponding pore size distributions calculated by the Barrette-Joynere-Halenda (BJH) method. For NiCo2O4/r-GO nanostructured composites no adsorption data were obtained due to the different procedure followed for the production of this nanostructured composite which

Jo

impedes this type of measurement, that is, r-GO nanosheets completely covered and support NiCo2O4 cores on the surface of the nickel foam substrates precluding BET analysis. As shown in Fig. 1c, the observed adsorption isotherms for both bare NiCo2O4 cores and NiCo2O4/PANI nanostructured composites can be considered as type IV; the hysteresis loop can be related to the formation of a mesoporous material with pore widths with sizes between 2 and 50 nm. Based on

12

the BET analysis, the specific surface area of NiCo2O4 cores and NiCo2O4/PANI nanostructured composites were ca. 39.78 m²/g and 44.02 m²/g, respectively. Besides, according to the BJH analysis the center of both distributions was located at ca. 3 and 3.2 nm for NiCo2O4 cores and NiCo2O4/PANI composites, respectively[25,47]. Additionally, the profile and hysteresis loop of NiCo2O4/MWCNT nanostructured composites also denote a type IV isotherm, with a specific surface area of 63.1 m2/g and pore size distribution of 3 nm and 39.7 nm, respectively, originated

of

from NiCo2O4 cores and MWCNT respectively (see Fig. S3)

ro

The surface morphology of the obtained spinel metal oxide particles and nanostructured

-p

composites were additionally investigated by means of FESEM. NiCo2O4 cores appear highly uniform, with relatively smooth surfaces and diameters between 250-300 nm (Fig. 2a). Besides,

re

NiCo2O4 surfaces become rougher and more porous upon oxidation (see inset in Fig. 2a) confirming the high surface area values obtained through nitrogen adsorption experiments. This

lP

nanostructuration should enable the formation of suitable channels for an effective ion/electron transfer. Regarding NiCo2O4/PANI nanostructured composites it is clearly observed that

ur na

NiCo2O4 cores were uniformly covered by PANI (Fig. 2b). Conversely, in the case of NiCo2O4/MWCNTs nanostructured composite NiCo2O4 cores seem to be attached to MWCNTs walls and distributed uniformly (Fig. 2c). Finally, Fig. 2d confirms that r-GO sheets completely cover NiCo2O4 cores onto the surface of the porous nickel foam in NiCo2O4/r-GO nanostructured

Jo

composites.

3.2 Electrochemical performance Cyclic voltammetry was performed to determine the electrochemical capacity and performance of NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNT and NiCo2O4/r-GO nanostructured composite electrodes in a standard three-electrode cell configuration. As observed in Fig. 3a, 13

redox peaks are very clear during the cathodic sweeps as a result of faradic redox reactions related to M-O/M-O-OH (M represents Ni or Co) [25], which are the origin of the pseudocapacitive characteristics of the nanostructured composite electrode [23,48]. As shown in of CV plots (see Fig. S4) the behavior of developed nanostructured composite materials can be pseudocapacitive as confirmed by the presence of different redox peaks of both

of

NiCo2O4 cores and nanostructured composite electrodes from redox reactions in the alkaline medium, which would correspond to the reversible reactions of Ni2+/Ni3+ and Co3+/Co4+

ro

transitions associated with OH − anions. As reported previously [13,20,23], the spinel structure of

-p

NiCo2O4 cores shows a potential window of 0 to 0.5 V in alkaline conditions, and their pseudocapacitive behavior can be described by the following equations: (1)

𝐶𝑜𝑂𝑂𝐻 + 𝑂𝐻 − ↔ 𝐶𝑜𝑂2 + 𝐻2 𝑂 + 𝑒 −

(2)

lP

re

𝑁𝑖𝐶𝑜2 𝑂4 + +𝑂𝐻 − + 𝐻2 𝑂 ↔ 2𝐶𝑜𝑂𝑂𝐻 + +𝑁𝑖𝑂𝑂𝐻 + 𝑒 −

The oxidation peak potentials of Ni2+/Ni3+ and Co3+/Co4+ transitions are so close one each other

ur na

that is difficult to split them apart[49]. Even at high scan rates, CV plots show well-defined redox peaks, which points to the capability of the porous NiCo2O4 cores to sustain these fast redox reactions. It is also observed that the current density increases as the scan rate does and, consequently, the peak potential is progressively shifted in agreement with the low polarization

Jo

and high power of the selected electrode nanomaterials [48]. As illustrated in Fig. S4a, the cathodic peak is slightly shifted from 0.26 to 0.24 V as the scan rate increases from 5 to 50 mV s1

, that is, the NiCo2O4 electrode can sustain very fast redox reactions. In addition, it is worth

mentioning that the constructed nanostructured composite electrodes have low resistivity and good electrochemical reversibility.

14

In particular, for NiCo2O4/PANI nanostructured composite electrode, cathodic and anodic peaks progressively shift from 0.25 to 0.22 V and from 0.4 to 0.45 V at increasing scan rates, respectively, and the capacitance decreases thanks to the synergistic effect of the embedded pseudo-reactions and additional conductive pathways provided by PANI (Fig. S4b). Similarity, anodic and cathodic peaks of NiCo2O4/MWCNT nanostructured composite electrode shifted from 0.41 to 0.47 V and 0.31 to 0.28 V, respectively (Fig. S4c). It has been demonstrated that

of

porous MWCNTs networks and the subsequent stability of their electrical response greatly

ro

promote the conductivity in NiCo2O4/MWCNT nanostructured composite electrode. Finally, CV plots of NiCo2O4/r-GO nanostructured composites show a quasi-rectangular shape and large

-p

enclosed areas as the scan rate increases from 5 to 50 mVs-1 due to presence of r-GO, with some

re

smooth peaks originated from NiCo2O4 nanocores (Fig. S4d). It can be inferred that the behavior of NiCo2O4 /r-GO in comparison with other developed nanostructured composite materials is so

lP

closer to extrinsic pseudocapacitive behavior than battery-like behavior The capacitances of porous NiCo2O4 cores, NiCo2O4/MWCNT, NiCo2O4/PANI and NiCo2O4/r-

ur na

GO nanostructured composite electrodes can be calculated by integration of the area under CV plots at different scan rates by following relationship: 𝐶 = 𝐴⁄(𝑚 × 𝜈 × ∆𝑉)

(3)

where A is the integrated area of CV curve in one cycle, C is the specific capacitance (F/g), ΔV

Jo

is the potential window of CV (V), ν is the scan rate (V/s) m is the mass of loaded material (g) [50]. (see Supplementary Materials for further details). The calculated specific capacitance of porous NiCo2O4 cores, NiCo2O4/MWCNT, NiCo2O4/PANI and NiCo2O4/r-GO nanostructured composite electrodes are given in Fig. 3b. As mentioned above, capacitances decrease as the scan rates increase, which can be attributed to the lower diffusion of electrolyte ions. At higher 15

scan rates only the outer surfaces can contribute to the charge-discharge processes, which result in a lower effectiveness of the electro-active materials. As shown in Fig. 3c, the potential difference of oxidation and reduction peaks (∆𝑉 = 𝑉𝑂𝑥 − 𝑉𝑅𝑒 ) [51] confirmed the pseudocapacitive behavior of the obtained nanostructured electrodes. This was additionally corroborated by analyzing the dependence of the electrical current with the scan rate as: 𝐼𝑝 = 𝐾1 𝜈 + 𝐾2 𝜈 1/2

of

(4)

ro

where 𝜈[𝑉𝑠 −1 ] is the scan rate. 𝐾1 and 𝐾2 coefficients are related to fast surface processes and diffusion limited faradic reactions, respectively. By plotting 𝐼𝑝 ⁄𝜈 1/2 versus 𝜈 1/2 , 𝐾1 and 𝐾2 can

-p

be obtained and the kinetics of the reaction determined (Fig. 3d)[51] .

re

In addition to specific capacitances (Csp) calculated from CV curves, these can be alternatively obtained from galvanostatic charge/discharge (C/D) measurements, allowing a comparison and

lP

evaluation of the quality of the measurements. Fig. 4a shows C/D plots of the different constructed nanostructured electrodes at a current density within the potential range 0-0.5 V at 1

ur na

A/g (see Fig. S5 for additional C/D plots at other current densities). The shape of C/D plots is non-linear as a consequence of the 𝐶𝑜3+ /𝐶𝑜4+ , 𝑁𝑖 2+ /𝑁𝑖 3+ ions redox reactions with 𝑂𝐻 − . 𝐶𝑠𝑝 can be calculated by means of the following equation: 𝐶𝑠𝑝 = (𝐼 × ∆𝑡)⁄(𝑚 × ∆𝑉)

Jo

(5)

where I is the current applied (A), 𝛥𝑡 the discharge time (s), m the mass of loaded material (g), and ∆𝑉 the window potential (V)[18]. The specific capacitance per surface area (F/cm2 is also illustrated in Fig. 4b. The obtained values for the different electrodes are in fair agreement with those previously obtained from CV data. From these data, a superior performance of the

16

NiCo2O4/MWCNTs nanostructured composite electrode compared to the NiCo2O4/PANI one is again noted, which is related to the advantageous structural features of MWCNTs; that is, the carbon-based material provides a faster electron transfer and a larger contact area for electrolyte ions and their subsequent penetration inside the hybrid storage system. Nevertheless, NiCo2O4 cores supported on ultrathin r-GO nanosheets electrode exhibit the largest specific capacitances. This outstanding performance might be attributed to the advantageous nanostructuration of this

of

electrode. Specifically, the ultrathin carbon nanosheets of r-GO could provide numerous electro-

ro

active sites for redox reactions thanks to its extensive surface area. Besides, the open space inside NiCo2O4 cores provided by their mesoporous structure can serve as a reservoir for ions,

-p

whereas the presence of r-GO covering the NiCo2O4 cores would enhance the diffusion kinetics

re

within the electrode; in other words, r-GO nanosheets may ensure an efficient contact between the surface of electroactive r-GO nanosheets and the electrolyte, which should allow a faster ion

lP

diffusion to the active sites for the redox reactions to proceed. Furthermore, the avoidance of binder and other conductive materials such as PVDF and carbon black powders in the

ur na

compositions of this nanostructured electrode, commonly added to prepare nickel foam-based commercial electrodes, can additionally improve the direct contact between r-GO sheets and NiCo2O4 cores, consequently, enhancing the intrinsic electrical conductivity. Additionally, cyclic tests were performed at 2 A/g to evaluate the cycling stability of the

Jo

electroactive nanomaterials constituting the electrodes during the operation periods. For NiCo2O4 cores electrode, a capacitance retention ability of 73% after 2000 cycles (Fig. 5a) was measured, that is, the specific capacitance decrease from 404 to 298 F/g as a consequence of the deintercalation of the electrochemical species. Conversely, the cycling stability of NiCo2O4/PANI nanostructured composite electrode was larger, with slow decreases in

17

capacitance after 2000 cycles of operation from 648 to 458 F/g, with a total the capacitance retention of 70%. The cycling performance of the NiCo2O4/MWCNT-based nanostructured electrode impressively showed a very stable performance with a specific capacitance of 768 F/g retained over 2000 cycles, corresponding to the capacitance retention of 88% (Fig. 5(a)). This enhanced cycling stability is probably attributed to the uniform distribution of NiCo2O4 cores and MWCNTs along the electrode. Finally, the stability curve for the NiCo2O4/r-GO nanostructured

of

electrode showed that the specific capacitance decreased from 1560 to 1378 F/g (87%

ro

capacitance retention) due to the activation process resulting from the complete intercalation

-p

between NiCo2O4 cores and r-GO nanosheets with electrolyte ions.

A comparison of the electrochemical outputs of the present NiCo2O4/MWCNT and NiCo2O4/r-

re

GO nanostructured composite electrodes with others previously obtained by different methodologies is shown in Table S1. In this regard, the method followed to obtain the

lP

NiCo2O4/MWCNT composites is faster, cost-effective and more reproducible than previous ones, and also leads to large capacitance of ca. 900 F/g at 1 A/g. For example, Liu et al.

ur na

developed NiCo2O4/CNTs composites by an electrodeposition process and subsequent annealing treatment obtaining a capacitance of ca. 694F/g at 1 A/g and cycling stabilities of 79.6% [24]. Another multistep procedure including chemical co-deposition, calcination and thermal treatment was performed by Cai et al., in which a high concentration electrolyte 6M KOH was used to

Jo

obtain a capacitance of 1038F/g at 0.5 A/g, with cycling stability of 100% [25]. On the other hand, Wang et al. developed NiCo2O4/r-GO composites by a self-assembled approach with capacitances of up to 835 F/g at 1 A/g and an outstanding cycling stability of ca. 100% but using much higher electrolyte concentrations (6 M KOH) [52]. Ma et al. reported a NiCo2O4/RGO (NCG) composite using a reflux method which deliver a capacitance of 1186 F/g at 0.5 A/g, with

18

a cycling stability of 97% [53]. In our case, using similar raw starting materials the simple and scalable manufacturing process here used allowed to obtain much larger capacitances of ca. 1760 F/g at 1 A/g with improved stability thanks to efficient contact between the electroactive material surfaces including r-GO nanosheets and the metal oxide NPs. In addition, comparison with other relevant carbon and/or metal oxide-based composites demonstrates the superior performance of our present nanostructures materials. In this regard, for example, very recently Wang et al.

of

reported manganese silicate hybridized carbon-based composites (MnSi-C) derived from natural

ro

bamboo leaves by a simple hydrothermal treatment. The single electrode based on such nanocomposite delivered a capacity of 162.2 F/g at a current density of 0.5 A/g with a

-p

capacitance retention of 85% after 10000 cycles [16]. Through a similar hydrothermal approach,

re

Zheng et al. prepared VO2(A)@C core-shell structured composite by carbonization of glucose in the presence of V2O5 nanowires [15]. The obtained composites showed good pseudocapacitative

lP

behavios and displayed a specific capacitance of 179 F/g at 1 A/g, with a cycling performance of 85% retention after 1600 cycles after substitution of the aqueous electrolyte by a LiCl/PVA gel

ur na

one.

According to the Nyquist plots (Fig. 5 b), the diameter of the semicircle is related to the charge transfer resistance (Rct). This quantity for the NiCo2O4/MWCNT electrode is much smaller than for the others, suggesting an enhanced electrical conductivity of the former electrode; that is, it

Jo

holds an enhanced electron mobility and, consequently, a better electrical conductivity in comparison with nanocomposites without MWCNT. Moreover, the NiCo2O4/r-GO electrode bears a small charge transfer resistance thanks to the conductivity of the r-GO nanosheets. A high charge transfer resistance is observed for the NiCo2O4 electrode; even in the NiCo2O4/PANI electrode, the conducting PANI improved the conductivity. Besides, based on Warburg region,

19

the steeper slope is attributed to the high mobility of ions in the electrodes, as presented in Fig S6, NiCo2O4/MWCNT and NiCo2O4/r-GO electrodes showed higher ion mobilities than the others. After 2000 C/D cycles, the surface morphology of the nanostructured nanocomposite electrodes was additionally investigated by FESEM (see Fig S6). It is observed that the structure of NiCo2O4 cores after 2000 cycles is damaged and some little fractures cracks on their surfaces can be observed. In the case of the NiCo2O4/PANI electrode, while NiCo2O4 cores kept their

of

shape, the PANI formed on the surface of NiCo2O4 cores was partially removed. For the

ro

NiCo2O4/MWCNTs composite, some NiCo2O4 cores seems to be damaged, but all MWCNTs conserved their structures without apparent deformation or break. Finally, in the NiCo2O4/r-GO

-p

composite, r-GO nanosheets are cleaved into small parts, but most of them still are covering the

re

NiCo2O4 cores maintaining the structure of the nanocomposite.

Electrochemical impedance spectroscopy measurements recorded in the frequency range from

lP

100 kHz to 0.01 Hz were also done (Fig. S7). Based on EIS data, capacitors do not display an ideal behavior, indeed, they act as a constant phase element (CPE), which is a typical behavior

ur na

shown by double layer capacitors in real cells [54]. CPE is applied in the equivalent circuit for the simulation of the experimental impedance data [55]. The impedance of a CPE is given by: 1

𝑍

= 𝑄 0 (𝑗𝜔)𝑛

(6)

Jo

where 𝑄 0 is the absolute value of the admittance (1/|Z|) at 𝜔 = 1 rad/s . When the exponent is n = 1, the latter equation describes the impedance of a capacitor, so the constant 𝑄 0 = 𝐶 is the capacitance. In this manner, the present impedance spectra can be interpreted and modeled using equivalent circuits according to the structure of the nanostructured composites and the different layers of the materials present within their structure. This modeling method was carried out by

20

means of Zview software. To do that, EIS data were fitted in a first step to a simple equivalent circuit model and, then, additional components were added until reaching the best outcome. These equivalent circuits were illustrated in Fig. S7 (left column), the so-called Nyquist impedance plots, in which R1, L1 and L2 are the solution resistance, the internal charge of electrode, and the charge transfer resistance, respectively. The value of R1 is attributed to the ionic and electronic resistance, the intrinsic resistance, and the diffusion and contact resistance of

of

the electrode/current collector interface. Nyquist plots provide information about the resistance

ro

of electrolyte, pre-stored charge in the electrode, the capacitance and resistance of each nanomaterial within the nanoelectrode, and the capacitance of the resulting supercapacitor. Thus,

-p

the resulting electrical circuit was used to evaluate the equivalent capacitor. In this manner, it is

re

possible to evaluate the capacitances of the resulting equivalent circuits in order to compare the different nanostructured composite electrodes. The fitting data based on the relevant equivalent

lP

circuits as equivalent capacitors are given in Table S2. The phase (𝜑) was calculated as:

ur na

tan(𝜑) = 𝑍"⁄𝑍′

(7)

Bode plots (Fig. S7 c, f, i, l) show that the capacitance decreases as the frequency increases for all nanostructured composite-based electrodes; each change in the slope corresponds to a layer of

Jo

material within the electrode structure. At high-frequencies all supercapacitors possess a pure resistance-like behavior indicating that electrolyte ions probably cannot penetrate into mesopores at high frequencies [6]. In addition, each maximum in the phase plots (Fig. S7 b, e, h, k) also represents a layer in the electrode. Therefore, both bode and phase plots confirm that the electrode are also composed of an electric double layer provided by the presence of the carbonbased nanomaterials. 21

Fig. S7a-c shows the existence of one slope and a smooth peak in Bode and phase plots, respectively, for the bare NiCo2O4-based electrode, that is, the electrode is formed by one layer of material. In Fig. S7d, the Nyquist plot for NiCo2O4/PANI-based electrode represents two impedances connected in parallel, and the corresponding Bode and phase plots confirmed the presence of two layers of material for this electrode (Fig. S7e, f). For the NiCo2O4/MWCNT based-electrode, the Nyquist plot is indicative of two impedances connected in series, each one

of

with a resistive and capacitive effect (Fig. S7g). It is also observed from Bode and phase plots

ro

that MWCNTs provide a high-power electrode thanks to its good electrical conductivity and readily accessible surface area (Fig. S7h, i). Finally, as shown in Fig. S7j, l, Nyquist, Bode and

-p

phase plots, and the equivalent circuit for the NiCo2O4/r-GO nanostructured composite electrode

re

confirm the existence of a double layer structure. The charge-transfer resistance was assigned nearest the solution interface and the other resistance to the binder. Another possible reason for

lP

the observed enhancement of efficiency in this nanostructure can be originated from the interfacial properties between r-GO nanosheets and NiCo2O4 cores; in particular, the r-GO

ur na

skeleton retains a high electrical conductivity even for very thick electrodes and prevents cycling-induced degradation [6].

Jo

4. Conclusions

Porous spinel NiCo2O4 cores were prepared by a simple and controlled solvothermal method. The advantages of using NiCo2O4 cores as the main electroactive material reside in their good electrical conductivity, rich electroactive behavior, stable spinel structure, and environmentally benign use. The introduction of carbon-based nanomaterials of different nature in the nanostructure of the developed electrodes allows incorporate the benefits of electrical double 22

layer conductive materials to complement the action and performance of the redox reactions supported by the porous metal oxide core nanomaterial. In this manner, the incorporation of an organic conductive polymer such as PANI via in situ polymerization to obtain a NiCo 2O4/PANI nanostructured composite resulted in an important enhancement of the capacitance up to 734 F/g at 1 A/g, in comparison with the bare NiCo2O4 core-based single electrode. When incorporating MWCNTs, the formed MWCNT network within the nanostructured composite improves its

of

electrical conductivity and consequently, the ion transfer speed, thus, giving rise to a

ro

NiCo2O4/MWCNTs composite electrode with additional enhancements in specific capacitance (up to ca. 900 F/g at 1 A/g) and stable life cycles (88% of capacitance retention) during the

-p

operation periods. Also, it was shown that the morphology of the electrode material has a

re

substantial influence on the ion transfer speed and the capacity of charge storage. This was demonstrated when performed the electrochemical deposition of r-GO onto a modified nickel

lP

foam covered with NiCo2O4 cores to obtain a NiCo2O4/r-GO composite electrode, which holds a maximum specific capacitance of 1760 F/g at 1 A/g. In this preparation two common materials

ur na

such as carbon black and PVDF regularly used in the preparation of commercial supercapacitor electrodes were not required, which involves the reduction of the associated costs of production while avoids at the same time inefficient contacts between the electroactive and conducting materials. Therefore, the obtained superior properties of the present active nanoelectrodes make

Jo

them an interesting alternative to those currently used, and the flexibility of the production methodologies should allow the fabrication of additional electro-active materials with even improved properties. Supplementary data: Supplementary data to this article can be found online at

23

Acknowledgements: This work was supported by Agencia Estatal de Investigación (AEI) through Project MAT2016-80266-R, and Xunta de Galicia (Grupo de Referencia Competitiva ED431C 2018/26; Agrupación Estratégica en Materiales-AEMAT ED431E 2018/08). FEDER funds are also acknowledged. Conflicts of Interest: The authors declare no conflict of interest.

R.R. Salunkhe, Y. V Kaneti, Y. Yamauchi, Metal−Organic Framework-Derived

ro

[1]

of

References

Nanoporous Metal Oxides toward Supercapacitor Applications: Progress and Prospects,

X. Wang, L. Chen, S. Zhang, X. Chen, Y. Li, J. Liu, F. Lu, Y. Tang, Compounding δ-

re

[2]

-p

ACS Nano. 11 (2017) 5293–5308. doi:10.1021/acsnano.7b02796.

MnO2 with modified graphene nanosheets for highly stable asymmetric supercapacitors, Surfaces

A

Physicochem.

lP

Colloids

Eng.

Asp.

573

(2019)

57–66.

doi:10.1016/j.colsurfa.2019.04.040.

M. Qorbani, A. Esfandiar, H. Mehdipour, M. Chaigneau, A. Irajizad, A.Z. Moshfegh,

ur na

[3]

Shedding Light on Pseudocapacitive Active Edges of Single-Layer Graphene Nanoribbons as High-Capacitance Supercapacitors, ACS Appl. Energy Mater. 2 (2019)

Jo

3665–3675. doi:10.1021/acsaem.9b00375. [4]

T. Brousse, B. Daniel, To Be or Not To Be Pseudocapacitive ?, J. Electrochem. Soc. 162 (2015) 5185–5189. doi:10.1149/2.0201505jes.

[5]

S.S. Jayaseelan, S. Radhakrishnan, B. Saravanakumar, M.K. Seo, M.S. Khil, H.Y. Kim, B.S. Kim, Mesoporous 3D NiCo2O4/MWCNT Nanocomposite Aerogels Prepared by a

24

Supercritical CO2 Drying Method for High Performance Hybrid Supercapacitor Electrodes, Colloids Surfaces A Physicochem. Eng. Asp. 538 (2018) 451–459. doi:10.1016/j.colsurfa.2017.11.037. [6]

G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828. doi:10.1039/C1CS15060J. K. Chen, S. Yin, D. Xue, A binary AxB1-x ionic alkaline pseudocapacitor system involving

of

[7]

ro

manganese, iron, cobalt, and nickel: Formation of electroactive colloids via in situ electric field assisted coprecipitation, Nanoscale. 7 (2015) 1161–1166. doi:10.1039/c4nr05880a. L. Ma, L. Su, J. Zhang, D. Zhao, C. Qin, Z. Jin, K. Zhao, A controllable morphology

-p

[8]

re

GO/PANI/metal hydroxide composite for supercapacitor, J. Electroanal. Chem. 777 (2016) 75–84. doi:10.1016/j.jelechem.2016.07.033.

Y. Tan, Y. Zhang, L. Kong, L. Kang, F. Ran, Nano-Au@PANI core-shell nanoparticles

lP

[9]

via in-situ polymerization as electrode for supercapacitor, J. Alloys Compd. 722 (2017) 1–

ur na

7. doi:10.1016/j.jallcom.2017.06.068.

[10] L. Tang, Z. Yang, F. Duan, M. Chen, Fabrication of graphene sheets/polyaniline nanofibers composite for enhanced supercapacitor properties, Colloids Surfaces A

Jo

Physicochem. Eng. Asp. 520 (2017) 184–192. doi:10.1016/j.colsurfa.2017.01.083. [11] M. Qorbani, N. Naseri, A.Z. Moshfegh, Hierarchical Co3O4/Co(OH)2/ nanoflakes as a supercapacitor electrode: Experimental and semi-empirical model, ACS Appl. Mater. Interfaces. 7 (2015) 11172–11179. doi:10.1021/acsami.5b00806. [12] J. Yang, Y. Yang, J. Lan, Y. Yu, X. Yang, Polyaniline-manganese dioxide-carbon

25

nanofiber

ternary composites

supercapacitors,

J.

with

enhanced

Electroanal.

electrochemical

Chem.

843

performance (2019)

for

22–30.

doi:10.1016/j.jelechem.2019.04.073. [13] Y. Zhu, X. Ji, Z. Wu, W. Song, H. Hou, Z. Wu, X. He, Q. Chen, C.E. Banks, Spinel NiCo2O4 for use as a high-performance supercapacitor electrode material: Understanding its

electrochemical

properties,

J.

Power

Sources.

267

(2014)

888–900.

of

of

ro

doi:10.1016/j.jpowsour.2014.05.134.

[14] G. Settanni, J. Zhou, T. Suo, S. Schöttler, K. Landfester, F. Schmid, V. Mailänder, Porous

Systems,

Phys.

Chem.

doi:10.1039/C5CP05936D.

Chem.

Phys.

17

(2015)

30963-30977.

re

Storage

-p

Nanoarchitectures of Spinel-Type Transition Metal Oxides for Electrochemical Energy

lP

[15] J. Zheng, Y. Zhang, Q. Wang, H. Jiang, Y. Liu, T. Lv, C. Meng, Hydrothermal encapsulation of VO2(A) nanorods in amorphous carbon by carbonization of glucose for

ur na

energy storage devices, Dalt. Trans. 47 (2018) 452–464. doi:10.1039/c7dt03853d. [16] Q. Wang, Y. Zhang, H. Jiang, C. Meng, In-situ grown manganese silicate from biomassderived heteroatom-doped porous carbon for supercapacitors with high performance, J.

Jo

Colloid Interface Sci. 534 (2019) 142–155. doi:10.1016/j.jcis.2018.09.026. [17] K. Malaie, F. Scholz, Realizing alkaline all-pseudocapacitive supercapacitors based on highly stable nanospinel oxide coatings, Colloids Surfaces A Physicochem. Eng. Asp. 577 (2019) 576–582. doi:10.1016/j.colsurfa.2019.06.024. [18] S. Rolda, D. Barreda, M. Granda, R. Mene, R. Santamarı, C. Blanco, An approach to classification and capacitance expressions in electrochemical capacitors technology, Phys. 26

Chem. Chem. Phys. 17 (2015) 1084–1092. doi:10.1039/C4CP05124F. [19] L. Jinlong, L. Tongxiang, Y. Meng, K. Suzuki, H. Miura, Comparing different microstructures of CoS formed on bare Ni foam and Ni foam coated graphene and their supercapacitors performance, Colloids Surfaces A Physicochem. Eng. Asp. 529 (2017) 57–63. doi:10.1016/j.colsurfa.2017.05.074.

ro

Mater. Chem. A. (2016) 3504–3512. doi:10.1039/c4ta00988f.

of

[20] Z. Wu, Y. Zhu, X. Ji, NiCo2O4-based Materials for Electrochemical Supercapacitor,

[21] W. Ma, S. Chen, S. Yang, W. Chen, W. Weng, Y. Cheng, M. Zhu, Flexible all-solid-state

-p

asymmetric supercapacitor based on transition metal oxide nanorods/reduced graphene

doi:10.1016/j.carbon.2016.11.051.

re

oxide hybrid fibers with high energy density, Carbon. 113 (2017) 151–158.

lP

[22] S. Mo, L. Peng, C. Yuan, C. Zhao, W. Tang, C. Ma, J. Shen, W. Yang, Y. Yu, Y. Min, A.J. Epstein, Enhanced properties of poly(vinyl alcohol) composite films with

ur na

functionalized graphene, RSC Adv. 5 (2015) 97738–97745. doi:10.1039/C5RA15984A. [23] L. Shen, L. Yu, X.Y. Yu, X. Zhang, X.W.D. Lou, Self-templated formation of uniform NiCo2O4 hollow spheres with complex interior structures for lithium-ion batteries and Angew.

Chem.

Int.

Ed.

54

(2015)

1868–1872.

Jo

supercapacitors,

doi:10.1002/anie.201409776.

[24] W.W. Liu, C. Lu, K. Liang, B.K. Tay, A three dimensional vertically aligned multiwall carbon nanotube/NiCo2O4 core/shell structure for novel high-performance supercapacitors, J. Mater. Chem. A. 2 (2014) 5100–5107. doi:10.1039/c4ta00107a.

27

[25] F. Cai, Y. Kang, H. Chen, Q. Li, Hierarchical CNT@NiCo2O4 core–shell hybrid nanostructure for high-performance supercapacitors, J. Mater. Chem. A. 2 (2014) 11509– 11515. doi:10.1039/c4ta01235f. [26] E. Mitchell, A. Jimenez, R.K. Gupta, B.K. Gupta, K. Ramasamy, M. Shahabuddin, S.R. Mishra, Ultrathin porous hierarchically textured NiCo2O4-graphene oxide flexible

of

nanosheets for high-performance supercapacitors, New J. Chem. 39 (2015) 2181–2187.

ro

doi:10.1039/c4nj02110j.

[27] D. Li, Y. Gong, M. Wang, C. Pan, Preparation of sandwich-like NiCo2O4/rGO/NiO

-p

heterostructure on nickel foam for high-performance supercapacitor electrodes, Nano-

re

Micro Lett. 9 (2017) 1–9. doi:10.1007/s40820-016-0117-1.

[28] F. Avilés, J. V. Cauich-Rodríguez, L. Moo-Tah, A. May-Pat, R. Vargas-Coronado,

lP

Evaluation of mild acid oxidation treatments for MWCNT functionalization, Carbon. 47 (2009) 2970–2975. doi:10.1016/j.carbon.2009.06.044.

ur na

[29] A.F. Ismail, P.S. Goh, J.C. Tee, S.M. Sanip, M. Aziz, A review of purification techniques for carbon nanotubes, Nano. 03 (2008) 127–143. doi:10.1142/S1793292008000927. [30] V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis, A. Siokou, I. Kallitsis, C.

Jo

Galiotis, Chemical oxidation of multiwalled carbon nanotubes, Carbon. 46 (2008) 833– 840. doi:10.1016/j.carbon.2008.02.012.

[31] D. Silambarasan, V.J. Surya, V. Vasu, K. Iyakutti, Single walled carbon nanotube − Metal oxide nanocomposites for reversible and reproducible storage of hydrogen, ACS Appl. Mater. Interfaces. 3 (2013) 11419–11426. doi.org/10.1021/am403662t

28

[32] C.L. Zhu, S.W. Chou, S.F. He, W.N. Liao, C.C. Chen, Synthesis of core/shell metal oxide/polyaniline nanocomposites and hollow polyaniline capsules, Nanotechnology. 18 (2007). doi:10.1088/0957-4484/18/27/275604. [33] D. Marcano, D. Kosynkin, J. Berlin, Improved synthesis of graphene oxide, ACS Nano. 4 (2010) 4806–4814. doi:10.1021/nn1006368.

of

[34] P. Moozarm, W. Pei, F. Lorestani, M.R. Mahmoudian, Y. Alias, Electrodeposition of

ro

copper oxide / polypyrrole / reduced graphene oxide as a nonenzymatic glucose biosensor, Sensors Actuators Chem. 209 (2015) 100–108.

-p

[35] T.H. Ko, S. Radhakrishnan, W.K. Choi, M.K. Seo, B.S. Kim, Core/shell-like NiCo2O4-

re

decorated MWCNT hybrids prepared by a dry synthesis technique and its supercapacitor applications, Mater. Lett. 166 (2016) 105–109. doi:10.1016/j.matlet.2015.12.053.

lP

[36] Y. Miao, W. Fan, D. Chen, T. Liu, High-performance supercapacitors based on hollow polyaniline nanofibers by electrospinning, ACS Appl. Mater. Interfaces. 5 (2013)

ur na

4423−4428. doi:10.1021/am4008352.

[37] M. Huang, F. Li, J.Y. Ji, Y.X. Zhang, X.L. Zhao, X. Gao, Facile synthesis of singlecrystalline NiO nanosheet arrays on Ni foam for high-performance supercapacitors,

Jo

CrystEngComm. 16 (2014) 2878–2884. doi:10.1039/c3ce42335b. [38] L. Stobinski, B. Lesiak, A. Malolepszy, M. Mazurkiewicz, B. Mierzwa, J. Zemek, P. Jiricek, I. Bieloshapka, Graphene oxide and reduced graphene oxide studied by the XRD, TEM and electron spectroscopy methods, J. Electron Spectros. Relat. Phenomena. 195 (2014) 145–154. doi:10.1016/j.elspec.2014.07.003.

29

[39] A.K. Das, R.K. Layek, N.H. Kim, D. Jung, J.H. Lee, Reduced graphene oxide (RGO)supported NiCo2O4 nanoparticles: an electrocatalyst for methanol oxidation, Nanoscale. 6 (2014) 10657–10665. doi:10.1039/C4NR02370F. [40] N. Jabeen, Q. Xia, M. Yang, H. Xia, Unique core-shell nanorod arrays with polyaniline deposited into mesoporous nico2o4 support for high-performance supercapacitor ACS

Appl.

Mater.

Interfaces.

8

(2016)

6093–6100.

of

electrodes,

ro

doi:10.1021/acsami.6b00207.

[41] A.L. Cabezas, Z. Bin Zhang, L.R. Zheng, S.L. Zhang, Morphological development of

re

664–668. doi:10.1016/j.synthmet.2009.12.023.

-p

nanofibrillar composites of polyaniline and carbon nanotubes, Synth. Met. 160 (2010)

[42] A.I. López-Lorente, B.M. Simonet, M. Valcárcel, Raman spectroscopic characterization

lP

of single walled carbon nanotubes: Influence of the sample aggregation state, Analyst. 139 (2014) 290–298. doi:10.1039/c3an00642e.

ur na

[43] M.A. Bissett, A.J. Barlow, J.G. Shapter, J.S. Quinton, Raman Characterisation of carbon nanotubes grown by plasma enhanced chemical vapour deposition, Adv. Mater. Nanotechnol. 700 (2012) 112–115. doi:DOI 10.4028/www.scientific.net/MSF.700.112.

Jo

[44] S. Muhammad Hafiz, R. Ritikos, T.J. Whitcher, N. Md. Razib, D.C.S. Bien, N. Chanlek, H. Nakajima, T. Saisopa, P. Songsiriritthigul, N.M. Huang, S.A. Rahman, A practical carbon dioxide gas sensor using room-temperature hydrogen plasma reduced graphene oxide, Sensors Actuators, B Chem. 193 (2014) 692–700. doi:10.1016/j.snb.2013.12.017. [45] A. Bo, Z. Tao, W. Li, J. Kai, Z. Matthew, NiO mesoporous nanowalls grown on RGO coated nickel foam as high performance electrodes for supercapacitors and biosensors, 30

Electrochim. Acta. 192 (2016) 205–215. doi:10.1016/j.electacta.2016.01.211. [46] G.T.S. How, A. Pandikumar, H.N. Ming, L.H. Ngee, Highly exposed {001} facets of titanium dioxide modified with reduced graphene oxide for dopamine sensing, Sci. Rep. 4 (2014) 1–8. doi:10.1038/srep05044. [47] M. Thommes, K. Kaneko, A. V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J.

of

Rouquerol, K.S.W. Sing, Physisorption of gases, with special reference to the evaluation

(2015) 1051–1069. doi:10.1515/pac-2014-1117.

ro

of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem. 87

-p

[48] S. Khalid, C. Cao, L. Wang, Y. Zhu, Microwave Assisted synthesis of porous nico2o4

re

microspheres: application as high performance asymmetric and symmetric supercapacitors with large areal capacitance, Sci. Rep. 6 (2016) 1–13. doi:10.1038/srep22699.

lP

[49] S. Sun, S. Li, S. Wang, Y. Li, L. Han, H. Kong, P. Wang, Fabrication of hollow NiCo2O4 nanoparticle/graphene composite for supercapacitor electrode, Mater. Lett. 182 (2016) 23–

ur na

26. doi:10.1016/j.matlet.2016.06.063.

[50] S. Al-Rubaye, R. Rajagopalan, S.X. Dou, Z. Cheng, Facile synthesis of a reduced graphene oxide wrapped porous NiCo2O4 composite with superior performance as an

Jo

electrode material for supercapacitors, J. Mater. Chem. A. 5 (2017) 18989–18997. doi:10.1039/c7ta03251j.

[51] M. Qorbani, T. Chou, Y. Lee, S. Samireddi, N. Naseri, A. Ganguly, A. Esfandiar, C.-H. Wang, L.-C. Chen, K.-H. Chen, A.Z. Moshfegh, layer partially reduced graphene oxide hybrids : towards highly stable asymmetric supercapacitors, J. Mater. Chem. A. 5 (2017) 12569–12577. doi:10.1039/C7TA00694B. 31

[52] H.W. Wang, Z.A. Hu, Y.Q. Chang, Y.L. Chen, H.Y. Wu, Z.Y. Zhang, Y.Y. Yang, Design and synthesis of NiCo2O4-reduced graphene oxide composites for high performance supercapacitors, J. Mater. Chem. 21 (2011) 10504–10511. doi:10.1039/c1jm10758e. [53] L. Ma, X. Shen, Z. Ji, X. Cai, G. Zhu, K. Chen, Porous NiCo2O4 nanosheets/reduced graphene oxide composite: Facile synthesis and excellent capacitive performance for J.

Colloid

Interface

Sci.

440

(2015)

211–218.

of

supercapacitors,

ro

doi:10.1016/j.jcis.2014.11.008.

[54] J.B. Jorcin, M.E. Orazem, N. Pébère, B. Tribollet, CPE analysis by local electrochemical spectroscopy,

Electrochim.

Acta.

51

(2006)

1473–1479.

-p

impedance

re

doi:10.1016/j.electacta.2005.02.128.

[55] H. Guan, L.Z. Fan, H. Zhang, X. Qu, Polyaniline nanofibers obtained by interfacial

lP

polymerization for high-rate supercapacitors, Electrochim. Acta. 56 (2010) 964–968.

Jo

ur na

doi:10.1016/j.electacta.2010.09.078.

32

of ro

Jo

ur na

lP

re

-p

Scheme 1. Graphical abstract

33

of

ro

Fig. 1: (a) XRD patterns and (b) Raman spectra of NiCo2O4, NiCo2O4/PANI, NiCo2O4/MWCNT and NiCo2O4/r-GO nanostructured composites. (c) N2 adsorption–desorption isotherms of

-p

NiCo2O4 cores, NiCo2O4/PANI, and NiCo2O4/MWCNT nanostructured composite and their

Jo

ur na

lP

re

corresponding BJH pore size distributions.

Fig. 2: FESEM images of (a) NiCo2O4 cores, (b) NiCo2O4/PANI, (c) NiCo2O4/MWCNTs, and (d) NiCo2O4/r-GO nanostructured composites. Inset in Fig. 2(a): NiCo2O4 cores after the first hour (top) and at the end of the oxidation process (down). 34

of ro -p re lP

ur na

Fig. 3: Evaluation of the electrochemical performance of the nanocomposites in a three-electrode configuration: (a) CV of NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNT and NiCo2O4/r-GO nanostructured composite electrodes at scan rates of 20 mVs-1 in an aqueous 3 M KOH solution.

(b)Specific

capacitances

of

NiCo2O4

Jo

electrolyte

cores,

NiCo2O4/PANI,

NiCo2O4/MWCNT and NiCo2O4/r-GO nanostructured composite electrodes derived from CV measurements, (c) potential difference of faradic oxidation and reduction peaks versus various scan rates. (d) Plot of 𝐾1 /𝐾2 ; the isent plot of 𝐼𝑝 ⁄𝜈 1/2 versus 𝜈 1/2

35

of

ro

Fig. 4: (a) Galvanostatic charge/discharge plots at current density 1A/g and (b) rate capability of, NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNTs, and NiCo2O4/r-GO nanostructured-based

Jo

ur na

lP

re

-p

electrodes

Fig. 5: (a) Cycling stability over 2000 C/D cycles and (b) Nyquist plot of NiCo2O4 cores, NiCo2O4/PANI, NiCo2O4/MWCNTs, and, NiCo2O4/r-GO nanostructured composite-based electrodes.

36