Research Hierarchical NiCo2O4@GO@PPy Nanorods are Grown … · via electrodeposition at a constant...

pg. 116 North American Academic Research , Volume 2, Issue 4 – April ; 2019, 2(4) : 116-128 ©TWASP, USA

North American Academic Research

Contents lists available at : www.twasp.info Journal homepage: http://twasp.info/journal/home

Research

Hierarchical NiCo2O4@GO@PPy Nanorods are Grown on Carbon Cloth

as Supercapacitor Electrodes

Dr. Moyinul Islam, K M Faridul Hasan, Xing Lu

State Key Laboratory of Materials Processing and Die & Mold Technology, School of

Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan

430074, P. R. China

*Corresponding Author :

M Islam

Email: [email protected]

Published online : 15 April, 2019

Abstract: Supercapacitors are one of the promising energy storage devices, which have been

widely explored for energy storage applications owing to their unique characteristics like

high power density, long cycle life (>100,000 cycles), and environmental friendliness. The

electrode materials play a key role as the performance of supercapacitors is primarily

depended on the types of the electrode materials. Therefore, it is imperative to explore new

electrode materials that simultaneously exhibit a high-capacitance, high cycle life as well as

high conductivity. Herein, we have successfully fabricated NiCo2O4@GO@PPy nanorods on

flexible carbon cloth using a facile hydrothermal and electrodeposition method. The as-

fabricated NiCo2O4@GO@PPy nanorods electrode present an excellent areal capacitance

of 405 mF cm−2

at 1 mA cm−2

and outstanding capacitance retention of ∼78% after 5000

cycles at 10 A g-1

. The improved electrochemical performance ascribed the complementary

contributions of both componential structures in the hybrid electrode design.

Keywords: NiCo2O4@GO@PPy Nanorods, Carbon Cloth, Supercapacitor.

1. Introduction

A supercapacitor usually constitutes with two electrodes, electrolyte, and separator,

where electrodes are immersed in the electrolyte and play a critical role in the property of a

supercapacitor(Ates et al., 2015; Cai et al., 2015; Devaraj & Munichandraiah, 2005;

Fernández et al., 2008; Lin et al., 2018; Masashi Ishikawa, 1994; Sevilla et al., 2007; Shen et

al., 2015; Tang et al., 2014; Yan Wang & and Yongsheng Chen*, 2009; Zheng et al.,

2015).Electrochemical energy storage is an outstanding part of sustainable energy

development has become one of the most important issues during the last few decades (He et

http://www.twasp.info/http://twasp.info/journal/homemailto:[email protected]


al., 2015). Supercapacitors are one of the promising energy storage devices among all

electrochemical energy storage systems due to their comparatively higher power density and

lower cost than lithium-ion batteries, fast charge-discharge, and excellent cycling stability

(Simon & Gogotsi, 2008). In modern society, energy storage is a focal but difficult problem

that the world is facing our growing thirst for energy must either match by supply or else

adapted to meet it (Qu et al., 2018). The rapid growth of population and the global economy

has increased the demand for energy consumption since the beginning of the 21st century. It

is necessary to develop efficient energy storage systems due to the severe impact of fossil

fuels on the environment and the urgent need for energy storage electronics(A.S.Arico &

Schalkwijk, 2005; Bordjiba & Bélanger, 2010; Burke, 2008a; Burke, 2008b; L.F.Nazar,

2001; Park et al., 2002; Patil et al., 2014; R. Ko ¨tz a*, 2000).

PPy has a lot of unique advantages such as easy synthesis, flexibility, high pseudo-

capacitance which are useful to solve challenges in the improvement of electrochemical

capacitors. To improve the electrochemical properties of CP-based supercapacitors, it is

initially important to enhance their crystallinity, control their microstructure and surface

morphology through polymerization method. Besides, other critical electrochemical

properties include thermal stability, processing abilities, and mechanical properties are crucial

factors to meet practical applications(Alabadi et al., 2016; Feng et al., 2013; Gupta & Miura,

2005; Patil et al., 2014). Another most crucial problem of PPy is its poor cycling stability.

Many kinds of research have focused on the investigations of PPy-carbon nanocomposites to

improve its cyclic stability of supercapacitors. However, carbon materials are suitable for PPy

to improve cycling stability and conductivity (Jari Keskinen; Wenjing Ji & and Qiang Fua,

2012). PPy-carbon composites can combine pseudocapacitance of PPy and fast charge

storage ability of carbon materials to enhance the energy and power density. The

electrochemical performance of PPy-carbon material composites has been developed

significantly compared with pure PPy and carbon materials (Ali Asghar Ensafi*; Chen et al.,

2015; Tang et al., 2015). The composite materials exhibit better electrochemical and

mechanical properties compared with pure PPy(Ji et al., 2015; Wang et al., 2015a; Yuan et

al., 2013). Although, energy density and power density of the composites are still lower than

lithium-ion batteries due to the limitation of charge accumulation ability, thus leading to the

supercapacitors unable to meet practical application requirements (An et al., 2002; Long et

al., 2011; Wang et al., 2015b).


Carbon cloth is a network of graphite fibers with diameters of several micrometers

produced by weaving process (Yang et al., 2014; Zhai et al., 2014). The interest in carbon

cloth has been increased extensively due to their high conductivity, high porosity and

mechanical flexibility (Estaline Amitha et al., 2008; Heo et al., 2013). Carbon cloth is more

favorable for various types of deposition techniques, as well as electrode materials for

electrochemical capacitors (Chen et al., 2010). The material has a significant advantage that it

does not require any binder or conductive agent like PVDF or carbon black during the

fabrication of the electrodes. It can be a promising substrate for wearable energy storage

devices (Yu et al., 2013; Zhang et al., 2014).

In recent year, a lot of attention has been paid to the rational design and synthesis of

unique core-shell nanostructures for high-performance supercapacitors. Hierarchical core-

shell structures have been regarded as a key strategy to enhance the electrochemical

performance of the supercapacitors. In this work, we have successfully fabricated

NiCo2O4@GO@PPy nanorods on flexible carbon cloth using a facile hydrothermal and

electrodeposition method. The as-fabricated NiCo2O4@GO@PPy nanorods electrode present

an excellent areal capacitance of 405 mF cm−2

at 1 mA cm−2

and outstanding capacitance

retention of ∼78% after 5000 cycles at 10 A g-1.

2. Experimental section

2.1 Chemicals

Carbon cloth (CC) WOS1002, Taiwan, China, with the hydrophilic surface (2.5 cm × 0.7 cm

× 360 μm) was ultra-sonicated in acetone and ethanol to remove impurities on the surfaces.

All reagent and chemical were of analytical grade and used as received, except pyrrole which

was distilled before to use.

2.2 Preparation of NiCo2O4 nanorod arrays on carbon cloth by hydrothermal method

To enhance the hydrophilicity, carbon cloth (1*1 cm2) was cleaned by ultra-sonication in 1 M

HCl, ethanol, and HNO3 each for 20 min. Then CC was washed several times with de-ionized

water and dried at 60 °C for overnight. To achieve Ni–Co carbonate hydroxide nanowires,

Ni(NO3)2·6H2O (4 mmol) and Co(NO3)2·6H2O (8 mmol) and Urea (15 mmol) were dissolved

in 75 mL of de-ionized water. Then the solution was stirred for 15 min to form a pink color.

Well-cleaned carbon cloth was placed in the solution. Then the total solution was transferred

into a 100 mL stainless autoclave. The autoclave was sealed and heated at 120 °C for 6 h.

After the reaction, the hydroxide nanowire electrode was taken out from the autoclave, and

washed with ethanol and de-ionized water and dried in air. The loading mass of nanowires in


Scheme 1.Schematic illustration of the synthesis procedure of NiCo2O4@GO@PPy nanorods.

the CC substrate was around 4.0 mg cm-2

. Generally, the mass loading on the CC could be

simply tuned by changing the concentrations of Ni and Co salt. Finally, CC substrate was

annealed at 380 °C in air for 2 h for further characterization.

2.3 Fabrication of NiCo2O4 with GO@PPy by electrochemical deposition

Electrochemical deposition was performed at room temperature in a three-electrode system.

Then added 0.1 M PPy and 0.2 M GO with 6 M KOH aqueous solution. Platinum (Pt) foil

and SCE were used as counter and reference electrodes, andNiCo2O4 nanorod arrays were

used as the working electrode. During electrodeposition, the voltage range was -1 to 0 V and

the deposition time was 1 min. Finally, the as-obtained samples were washed several times

with distilled water and ethanol to remove the impurities. For comparison, GO was

electrodeposited separately following the same processes.

2.4 Materials characterization

The morphology of the obtained carbon cloth was analyzed by scanning electron microscopy,

SEM (NOVA Nano SEM 450). X-ray photoelectron spectroscopy (XPS, PHI Quantera II)

was applied to characterize the structures and compositions of the materials.A CHI 660E

electrochemical workstation was used to measure Cyclic voltammetry (CV), galvanostatic

charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). Saturated

calomel electrodes (SCE) and platinum (Pt.) electrodes were used as a reference electrode

and counter electrode, respectively. All the electrochemical tests were measured by using a 6

M KOH aqueous solution as an electrolyte.

3. Results and discussion


Herein, we have reported an effective approach to synthesis NiCo2O4@GO@PPy

nanorod electrodes, as illustrated in Scheme 1. At first, NiCo2O4 nanorods were grown on the

carbon cloth by a hydrothermal method, followed by a uniform coating of PPy onto the

NiCo2O4 via electrodeposition at a constant voltage. Fig. (1a) shows the morphology of pure

carbon cloth, the surface of this cloth is very smooth and clean. In contrast, we can notice that

NiCo2O4 nanorod uniformly grown on the carbon cloth through the hydrothermal method

with a diameter of about 50nm shown in Fig. 1b. After electrochemical deposition, it is found

that carbon fibers are uniformly covered by GO, as shown in Fig. (1c). NiCo2O4@GO@PPy

nanorods reveal similar morphology with NiCo2O4@GO nanorods because both are treated

with the electrochemical deposition process. Fig. 1d exhibits that the NiCo2O4 nanorods are

coated with GO@PPy to form hierarchical core-shell nanostructures.

To further investigate chemical composition and oxidation state of the as-prepared

electrode, X-ray photoelectron spectroscopy (XPS) tests are carried out, and the

corresponding results are shown in Fig. 2a–c. A full survey scan spectrum in Fig. 2a indicates

the existence of Ni, Co, O and C elements. C signal corresponds to a carbon cloth substrate.

As illustrated in Figure 2b, the binding energies at 855.1 and 873 eV are ascribed to Ni2+

and

Fig. 1 SEM images of (a) Carbon cloth, (b) NiCo2O4 nanorods after hydrothermal, (c-d)

NiCo2O4@GO, and NiCo2O4@GO@PPy nanorod composite electrodes after electrodeposition.


Ni3+

. The satellite peak at around 861.7 and 879.7 eV are two shakeup type peaks of nickel at

the high binding energy side of the Ni 2p3/2 and Ni 2p1/2 edge. So, the Ni 2p spectra consist of

two kinds of nickel species that are characteristic of Ni2+

and Ni3+

and two satellite peaks

(indicated as “Sat.”). In the Co 2p spectra (Fig. 2c), a low energy band ( Co 2p3/2) and a high

energy band (Co 2p1/2) at binding energies of 780.2 and 795.2 eV indicating two kinds of

cobalt species containing Co2+

and Co3+

.

To investigate the electrochemical properties of different samples, the three-electrode

measurements were carried out with 6 M KOH aqueous solution as an electrolyte. Fig. 3a

shows the comparison of CV curves of NiCo2O4, NiCo2O4@GO, and NiCo2O4@GO@PPy

nanorod electrodes at scanrate of 5 mV s-1

. Redox peaks are visible in all CV curves, which

Fig. 2 XPS spectra of the as-obtained NiCo2O4 nanorod electrode. (a) XPS survey scan; (b-c) high-resolution

XPS spectra for Ni, Co.


Fig. 3 (a) CV curves of NiCo2O4, NiCo2O4@GO, and NiCo2O4@GO@PPy nanorods at a scan rate of 5 mVs-1

,

(b) CV curves of NiCo2O4@GO@PPy at different scan rates from 5 to 100 mVs-1

.

Fig. 4 (a) GCD curves of NiCo2O4, NiCo2O4@GO, and NiCo2O4@GO@PPy nanorods at a current density of

0.5 A g-1

, (b) GCD curves of NiCo2O4@GO@PPy nanorods at different current densities from 0.5 to 10 A g-1

.

indicates that the pseudocapacitive reaction processes, which are mainly governed by faradaic

redox reactions. It is noticed that the integrated CV area of NiCo2O4@GO@PPy nanorod are

much larger than NiCo2O4 and NiCo2O4@GO nanorods, indicating that the composite of

GO@PPy possesses large areal capacitance and NiCo2O4 nanorods have a very little

contribution to the capacitance. The CV curve of NiCo2O4@GO@PPy nanorod electrode

shows redox peaks similar to NiCo2O4 and NiCo2O4@GO but with higher areal capacitance

(405 mF cm-2

at 1 mA cm-2

), suggesting that the PPy layer in the heterostructure can deliver

much higher capacitance. Fig. 3b shows the CV curves of the NiCo2O4@GO@PPy nanorod


electrode at different scan rates from 5−100 mV s-1

. The CV curve retains well-defined redox

peaks even at a high scan rate of 100 mV s-1

, exhibiting excellent electrode kinetics and good

rate capability.

To further evaluate the application potential of the as-synthesized electrodes for

electrochemical capacitors, galvanostatic charge-discharge (GCD) measurements were

investigated in a 6M KOH electrolyte between -1 and 0 V at a current density of 0.5 A g-1

.

Fig. 4a displays the GCD curves of NiCo2O4, NiCo2O4@GO, and NiCo2O4@GO@PPy

nanorod electrodes at 0.5 A g-1

. It is found that NiCo2O4, NiCo2O4@GO nanorod electrodes

show shorter discharge time than NiCo2O4@GO@PPy nanorod. Fig. 4b shows GCD curves

of NiCo2O4@GO@PPy nanorod electrode at different current densities of 0.5, 1, 2, 5, and 10

A g-1

.

Electrochemical impedance spectroscopy (EIS) measurements were further carried

out to investigate the electrochemical behavior of the NiCo2O4@GO@PPy nanorod

electrode. The Nyquist plots achieved in the frequency range from 0.01 to 100 kHz at open

circuit potential are shown in Fig. 5a. The inset shows the blowup image at a high frequency.

In the high-frequency area, the intersection of the curve at the real part Z' indicates the bulk

resistance of the electrochemical system, and the semicircle represents the faradaic reaction

during the charge-discharge processes. From the inset picture in Fig. 5a, the

NiCo2O4@GO@PPy nanorod reveals a low bulk resistance and charge-transfer resistance. In

Fig. 5 (a) Nyquist plots of NiCo2O4, NiCo2O4@GO, and NiCo2O4@GO@PPy nanorod, (b) Cycling performance

of NiCo2O4@GO@PPy nanorod at a current density of 10 A g-1

.


the low-frequency region, where the slope of the curve represents the Warburg impedance, all

three electrodes show ideal straight lines, indicating efficient electrolyte and proton diffusion.

The EIS results imply that NiCo2O4@GO@PPy nanorod electrode displays high electrical

conductivity, leading to excellent electrochemical properties.

The long-term cycling stability is considered as another crucial factor for

supercapacitors. Fig. 5b presents the cycling charge-discharge testing of NiCo2O4@GO@PPy

nanorod at a high current density of 10 mA cm-2

with 6 M KOH electrolyte for 5000 cycles.

The capacity has reduced slightly in the initial cycles. After that, the specific capacitance

increases a little with the increase of charge-discharge cycles due to the long time soaking in

KOH electrolyte. And it remains stable of 78% capacitance even after 5000 cycles. This

superior stability is suggesting that the as-obtained NiCo2O4@GO@PPy nanorod electrode

could undergo the long-term cycling test and keep steady electrochemical performance for

supercapacitors.

4. Conclusion

CPs possesses new flexibility and pseudo-capacitive properties, leading them to become more

feasible for fabrication of high-performance supercapacitors. Therefore, the application fields

of flexible and stretchable as well as high electrochemical performance supercapacitors

deserve to pay more attention. The manufacturing cost is also a vital issue to gain

commercialization of supercapacitors. Hence, further technological advances in cost-effective

manufacturing of supercapacitor materials and devices will be highly desirable. However, the

rational design and unique core-shell nanostructures have attracted huge attention to

improving the performance of supercapacitors. In this work, we have successfully fabricated

NiCo2O4@GO@PPy nanorods on flexible carbon cloth using a facile hydrothermal and

electrodeposition method. The as-fabricated NiCo2O4@GO@PPy nanorods electrode reveal

an excellent areal capacitance of 405 mF cm−2

at 1 mA cm−2

and outstanding capacitance

retention of ∼78% after 5000 cycles at 10 A g-1. The improved electrochemical performance

ascribed the complementary contributions of both componential structures in the hybrid

electrode design.

Acknowledgement

This work was financially supported by The National Natural Science Foundation of China

(Nos. 51472095, 51672093, 51602112, and 51602097). The authors thank the Analytical and


Testing Center in Huazhong University of Science and Technology for related measurements

and instrumentation supports.

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