Conformal Coating of Cobalt-Nickel Layered Double

Electrochimica Acta 135 (2014) 513–518

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Conformal Coating of Cobalt-Nickel Layered Double Hydroxides Nanoflakes on Carbon Fibers for High-performance Electrochemical Energy Storage Supercapacitor Devices Muhammad Farooq Warsi a , Imran Shakir b,∗ , Muhammad Shahid c , Mansoor Sarfraz b , Muhammad Nadeem d , Zaheer Abbas Gilani e a

Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur-63100, Pakistan Deanship of scientific research, College of Engineering, King Saud University, PO-BOX 800, Riyadh c Material Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia d Production chemistry department, Petroleum a Development Oman (PDO) LLC, Muscat-100, Sultanate of Oman e Department of Physics, Balochistan University of Information Technology, Engineering and Management Sciences, Quetta-87300, Pakistan b

a r t i c l e

i n f o

Article history: Received 25 February 2014 Received in revised form 27 April 2014 Accepted 5 May 2014 Available online 14 May 2014 Keywords: Nickel-cobalt layered double hydroxides Nanoflakes Conformal Coating supercapacitor

a b s t r a c t High specific capacitance coupled with the ease of large scale production is two desirable characteristics of a potential pseudo-supercapacitor material. In the current study, the uniform and conformal coating of nickel-cobalt layered double hydroxides (CoNi0.5 LDH,) nanoflakes on fibrous carbon (FC) cloth has been achieved through cost-effective and scalable chemical precipitation method, followed by a simple heat treatment step. The conformally coated CoNi0.5 LDH/FC electrode showed 1.5 times greater specific capacitance compared to the electrodes prepared by conventional non-conformal (drop casting) method of depositing CoNi0.5 LDH powder on the carbon microfibers (1938 Fg−1 vs 1292 Fg−1 ). Further comparison of conformally and non-conformally coated CoNi0.5 LDH electrodes showed the rate capability of 79%: 43% capacity retention at 50 Ag−1 and cycling stability 4.6%: 27.9% loss after 3000 cycles respectively. The superior performance of the conformally coated CoNi0.5 LDH is mainly due to the reduced internal resistance and fast ionic mobility between electrodes as compared to non-conformally coated electrodes which is evidenced by EIS and CV studies. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction From last few decades, electrochemical supercapacitors have attracted substantial attention as they can provide higher power density than batteries, higher energy density than conventional electrostatic capacitors, fast charging and long cycle life [1–5]. There are three major types of electrode materials widely utilized for electrochemical supercapacitors: metal oxides/hydroxides, carbon materials and conducting polymers [6–14]. As to the electrode material, layered double hydroxides (LDHs) with a general formula[15] of [M1−x 2+ Mx3+ (OH)2 ](Ax/n n− )·mH2 O where M2+ and M3+ are divalent and trivalent cations and An− have drawn extensive and intensive research attention in recent years due to their low cost, high redox activity, flexible ion exchangeability, tunable composition and environmentally friendly nature [16–20]. Among

∗ Corresponding author. E-mail address: [email protected] (I. Shakir). http://dx.doi.org/10.1016/j.electacta.2014.05.020 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

them, CoNiLDH has been widely studied as a high-performance electrode material for supercapacitors due to its ability to offer richer redox reactions with better electrochemical activity and electrical conductivity as compared to the to the both corresponding single component hydroxides (Ni(OH)2 and Co(OH)2 ) [21–25]. However, CoNiLDH often suffers from low power performance and cycle life because redox kinetics is limited by the rate of mass diffusion and electron transfer. The high mass loading of active CoNiLDH materials usually leads to the increased electrode resistance and the decreased specific capacitance, because CoNiLDH becomes densely packed with limited electrochemically active surface area. Therefore, it is necessary to design CoNiLDH based electrodes in such a way which allows high mass loading, prevent the agglomeration and maximize the number of active sites for capacitance. To solve these critical problems, we developed a novel and simple method for obtaining a conformal coating of CoNi0.5 LDH nanoflakes with 3D nanonetwork on commonly available fibrous carbon cloth using cost-effective and scalable chemical precipitation method combined with a simple thermal treatment. The results show that conformally coated

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M.F. Warsi et al. / Electrochimica Acta 135 (2014) 513–518

CoNi0.5 LDH with 3D nanonetwork electrode exhibits a higher specific capacitance by increasing electrolyte penetration, giving easy access to the active CoNi0.5 LDH sites on the electrode. 2. Experimental Section 2.1. Synthesis of Conformal coating of CoNi0.5 LDH Nanoflakes on Carbon Cloth All the reagents Co(NO3 )2 .6H2 O, Ni(NO3 )2 .6H2 O, NH3 ·H2 O used in the experiment were of analytical grade (Sigma-Aldrich) and were used without further purification. In a typical procedure, 10 mmol of Co(NO3 )2 .6H2 O and 5 mmol of Ni(NO3 )2 .6H2 O were dissolved in 35 ml of H2 O to form a solution. After being magnetically stirred for 30 min in air at room temperature, a piece of a commercial available carbon fabric (carbon cloth, Fuel Cell Store) was immersed and NH3 ·H2 O was added drop wise and resulting suspension was kept in autoclave at 150 ◦ C for 10 hrs. The chemically coated carbon cloth was collected, washed several times with deionized water and absolute ethanol, and finally dried on hot plate at 120 ◦ C for 10 hrs. The mass loading of CoNi0.5 LDH on the carbon cloth (CC) was found to be 1.16 ± 0.02 mg/cm2 which were calculated by weighing it before and after the deposition using a microbalance. The electrochemical measurements of the electrode material was performed in a three-electrode cell consisting of CoNi0.5 LDH nanoflakes coated CC electrodes as a working electrode, Pt wire and Ag/AgCl electrodes as counter and reference electrodes, respectively in 2 M KOH electrolyte at room temperature. 2.2. Synthesis of Conformal coating of Ni(OH)2 Nanoflakes on Carbon Cloth All the reagents Ni(NO3 )2 .6H2 O, NH3 ·H2 O used in the experiment were of analytical grade (Sigma-Aldrich) and were used without further purification. In a typical procedure, 5 mmol of Ni(NO3 )2 .6H2 O were dissolved in 35 ml of H2 O to form a solution. After being magnetically stirred for 30 min in air at room temperature, a piece of a commercial available carbon fabric (carbon cloth, Fuel Cell Store) was immersed and NH3 ·H2 O was added drop wise and resulting suspension was kept in autoclave at 150 ◦ C for 10 hrs. The chemically coated carbon cloth was collected, washed several times with deionized water and absolute ethanol, and finally dried on hot plate at 120 ◦ C for 10 hrs. The electrochemical measurements of the electrode material was performed in a three-electrode cell consisting of Ni(OH)2 , nanoflakes coated CC electrode as a working electrode, Pt wire and Ag/AgCl electrodes as counter and reference electrodes, respectively in 2 M KOH electrolyte at room temperature. 2.3. Synthesis of Conformal coating of Co(OH)2 Nanoflakes on Carbon Cloth All the reagents Co(NO3 )2 .6H2 O, NH3 ·H2 O used in the experiment were of analytical grade (Sigma-Aldrich) and were used without further purification. In a typical procedure, 5 mmol of Co(NO3 )2 .6H2 O were dissolved in 35 ml of H2 O to form a solution. After being magnetically stirred for 30 min in air at room temperature, a piece of a commercial available carbon fabric (carbon cloth, Fuel Cell Store) was immersed and NH3 ·H2 O was added drop wise and resulting suspension was kept in autoclave at 150 ◦ C for 10 hrs. The chemically coated carbon cloth was collected, washed several times with deionized water and absolute ethanol, and finally dried on hot plate at 120 ◦ C for 10 hrs. The electrochemical measurements of the electrode material was performed in a three-electrode cell consisting of Co(OH)2 , nanoflakes coated CC electrode as a working electrode, Pt wire and Ag/AgCl electrodes as counter and reference

Fig. 1. Schematic illustration for the conformal coating of CoNi0.5 LDH nanoflakes on carbon cloth substrate.

electrodes, respectively in 2 M KOH electrolyte at room temperature. 2.4. Synthesis of Non-Conformal coating of CoNi0.5 LDH Nanoflakes on Carbon Cloth All the reagents Co(NO3 )2 .6H2 O, Ni(NO3 )2 .6H2 O, NH3 ·H2 O used in the experiment were of analytical grade (Sigma-Aldrich) and were used without further purification. In a typical procedure, 10 mmol of Co(NO3 )2 .6H2 O and 5 mmol of Ni(NO3 )2 .6H2 O were dissolved in 35 ml of H2 O to form a solution. After being magnetically stirred for 30 min in air at room temperature, NH3 ·H2 O was added drop wise and resulting suspension was kept stirring for few minutes in air. The precipitates were collected by filtration, washed several times with deionized water and absolute ethanol, and finally dried on hot plate at 120 ◦ C for 10 hrs. For the nonconformal coating 10 mg of CoNi0.5 LDH nanoflakes was dispersed in 500 ␮l of ethanol by ultrasonication for 20 minutes. From this appropriate dispersion was drop casted on a carbon electrode cloth to achieve mass loading of 1.16 ± 0.02 mg/cm2 and the solvent was slowly evaporated by placing the electrode 3. Results and Discussion The schematic illustration for the conformal coating of CoNi0.5 LDH nanoflakes on carbon cloth by cost-effective and scalable chemical precipitation method is shown in the Fig. 1. The morphology of conformal and non-conformal coating of CoNi0.5 LDH nanoflakes on carbon cloth was examined with field emission scanning electron microscopy (FE-SEM) and the results are presented in the Fig. 2. Fig. 2(a) shows a low-magnification field emission scanning electron microscopy image of carbon cloth substrate without coating of CoNi0.5 LDH nanoflakes and Fig. 2(b-c) shows the FE-SEM images of carbon cloth substrate after the conformal coating of CoNi0.5 LDH nanoflakes. It can be clearly seen that a high-density of CoNi0.5 LDH nanoflakes are uniformly coated on the carbon cloth substrate. The CoNi0.5 LDH nanoflakes have thin walls with a thickness of ∼ 5 nm, and a height of approximately 1 ␮m indicates a high lateral aspect ratio. The ultrathin CoNi0.5 LDH nanoflakes are interconnected with each other and create pores and crevices which ensure large surface area for fast diffusion rate within the redox phase, an easier electrolyte ion transport and more superficial electroactive species. The non-conformal coating of CoNi0.5 LDH nanoflakes is shown in the Fig. 2(d) and it can be seen that drop casting covers the carbon cloth non-uniformly and hence limits the penetration of electrolyte ions. The crystallographic information of conformally coated CoNi0.5 LDH nanoflakes was examined by X-ray diffraction and the XRD pattern is shown in Fig. 3(a). It can be observed that the spinel CoNi0.5 LDH phase is completely formed at 120 ◦ C as the XRD patterns are in good


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Fig. 2. (a) FE-SEM images of (a) carbon cloth (CC), (b-c) conformally coated CoNi0.5 LDH nanoflakes and (d) non-conformally coated CoNi0.5 LDH nanoflakes.

agreement with the standard peaks for the CoNi0.5 LDH as reported in the literature [21]. The surface area of conformally and nonconformally coated CoNi0.5 LDH nanoflakes CC was determined from BET measurements (Coulter SA3100) using nitrogen adsorption. The nonconformally coated CoNi0.5 LDH nanoflakes show a surface area of 136 m2 g−1 and pore size distribution with maximum at around 6 nm whereas conformally coated CoNi0.5 LDH nanoflakes show a surface area of 212.5 m2 g−1 and pore size distribution with maximum at 3 nm as shown in the inset of Fig. 3(a). The high BET surface area and porous features of conformally coated CoNi0.5 LDH nanoflakes as compared to nonconformally coated CoNi0.5 LDH nanoflakes provide the possibility of efficient transport of electrons and ions within the pores for fast redox reactions, greatly improving the electrode–electrolyte contact area which leads to the excellent electrochemical properties of conformally coated CoNi0.5 LDH nanoflakes. To examine the capacitive performance of the CoNi0.5 LDH nanoflakes coated CC electrodes, cycle voltammetry (CV), galvanostatic charge–discharge (CD) and electrochemical impedance spectroscopy (EIS) measurements were carried out using the threeelectrode cell consisting of Ni(OH)2 , Co(OH)2 and CoNi0.5 LDH nanoflakes coated CC electrodes as a working electrode, Pt wire and Ag/AgCl electrodes as counter and reference electrodes, respectively in 2 M KOH electrolyte. The CV of conformally and nonconformally coated CoNi0.5 LDH nanoflakes CC electrode at a scan rate of 5 mVs−1 is shown in Fig. 3(b). The CV of both electrodes consists of well-defined pair of strong redox peaks within the potential

range from 0 to 0.6 V, which reveals that the capacitive characteristics of CoNi0.5 LDH nanoflakes are governed by faradaic redox reactions which mainly originated from the faradaic reactions of surface oxycation species. It can be seen from Fig. 3(b) that conformally coated CoNi0.5 LDH nanoflakes CC electrode exhibit large area under the CV curve as compared to that of non-conformally coated electrode, implying that the conformal coating have a higher capacitance. This enhancement in the electrochemical properties of conformally coated CoNi0.5 LDH nanoflakes as compared to nonconformally electrodes is mainly attributed to its unique porous and uniform features which enable the faster permeation process of the KOH electrolyte by significantly reducing the diffusion time of OH− ions. To further quantify the rate performance and redox processes, the CV experiments were carried out at various sweep rates for conformally and non-conformally coated CoNi0.5 LDH CC electrode as shown in Fig. 3(c). Both conformally and non-conformally coated electrodes exhibit well-defined quasi-reversible reduction and oxidation process in the sweep rate from 5 to 50 mV s−1 with slight shift in both cathodic and anodic peak potentials with respect to scan rate. This slight shift in the peaks is mainly due to the fact that at lower scan rate electrolyte ions are fully utilized by both outer and inner active sites of CoNi0.5 LDH nanoflakes while at higher scan rate only outer active sites can participate in the redox reactions[5,26–35]. The variation of specific capacitance of both conformally and non-conformally coated CoNi0.5 LDH nanoflakes with increasing scan rates is shown in Fig. 3(d). It is imperative to note that the decrease in the specific capacitance of

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Fig. 3. (a) XRD patterns of conformally coated CoNi0.5 LDH nanoflakes on CC before and after heat treatment (inset showing the N2 adsorption–desorption isotherm and pore-size distributions of conformally coated CoNi0.5 LDH nanoflakes), (b) Comparison of CV of conformally and non-conformally coated CoNi0.5 LDH nanoflakes on CC at a scan rate of 5 mVs−1 in 2 M aqueous KOH electrolyte (c) CV of conformally coated CoNi0.5 LDH nanoflakes on CC electrode at various scan rates from 5 to 50 mV s−1 in 2 M aqueous KOH electrolyte and (d) Specific capacitance variation of conformally and non-conformally coated CoNi0.5 LDH nanoflakes on CC electrodes at different scan rates in 2 M aqueous KOH electrolyte.

conformally coated CoNi0.5 LDH nanoflakes CC electrode at higher scan rate 50 mVs−1 is only 26% indicating that conformal coating have excellent capacitive characteristics. This comparative decrease in specific capacitance at higher scan rates than lower scan rates is due to increase in ionic resistivity and the inaccessibility of the conformally coated CoNi0.5 LDH nanoflakes electrode surface leading to either depletion or saturation of protons in the electrolytes inside the electrode during the redox process. On the other hand non-conformally coated CoNi0.5 LDH nanoflakes suffer from 53% decrease in specific capacitance at higher scan rate (50 mVs−1 ) as compared to low scan rate (5 mVs−1 ) which confirms that non-conformally coated CoNi0.5 LDH nanoflakes have higher ionic resistivity as compared to conformally coated CoNi0.5 LDH nanoflakes CC electrodes. Inset of Fig. 4(a) shows the CD curves of conformally and non-conformally coated CoNi0.5 LDH nanoflakes CC electrodes at a current density of 1 Ag−1 . The CD curve of conformally coated CoNi0.5 LDH nanoflakes CC electrodes indicates that CoNi0.5 LDH have pseudocapacitive behavior with a maximum specific capacitance of 1938 Fg−1 which is 1.6 times higher than that of 1292 Fg−1 at 1Ag−1 for non-conformal coated CoNi0.5 LDH nanoflakes electrode. These results are comparable to the results of the CV tests and the electrode retain 1551 Fg−1 (∼79%retention) even at a current density as high as 50 Ag−1 as shown in Fig. 4(a) which is significantly higher specific than non-conformal coated CoNi0.5 LDH nanoflakes electrode (∼43%retention). For comparison we also studies the electrochemical performance of conformally coated Ni(OH)2 and Co(OH)2 on CC electrode as shown in Fig. 4(a). We believed that such high rate performance of conformally coated CoNi0.5 LDH nanoflakes compared to the non-conformally

coated CoNi0.5 LDH nanoflakes and conformally coated Ni(OH)2 and Co(OH)2 on CC is mainly due to easy and better access of the electrolyte solution, shortening of the OH− diffusion distance, rapid ion and electron transportation process which results in sufficient Faradaic reactions even at very high current density conditions. The long-term electrochemical stability of supercapacitor was studied at 20 Ag−1 for continuous 3000 cycles and the results are presented in Fig. 4(b). A total capacitance loss of 4.6% and 27.9% was observed for conformally and non-conformally coated CoNi0.5 LDH nanoflakes, indicating the excellent stability of the conformally coated CoNi0.5 LDH nanoflakes in energy storage applications. The energy density versus power density of conformally and nonconformally coated CoNi0.5 LDH nanoflakes electrodes was plotted on a Ragone chart as shown in Fig. 4(c). The energy density for conformally coated CoNi0.5 LDH nanoflakes was 48.6 Whkg−1 at a power density of 3 kWkg−1 , and more significantly it still remains 11.5 Whkg−1 at a power density of 35.6 kWkg−1 , suggesting that conformally coated CoNi0.5 LDH nanoflakes are very promising electrode material for high-rate charge/discharge operations in supercapacitors. The enhanced electrochemical performance of the conformally coated CoNi0.5 LDH nanoflakes as compared to the non-conformally coated CoNi0.5 LDH nanoflakes was further confirmed by EIS in the frequency range of 100 kHz to 0.01 Hz. Fig. 4 (d) shows Nyquist plots of EIS spectra of both electrodes which indicate that the semicircle in the plot was much shorter for conformally coated CoNi0.5 LDH nanoflakes as compared to nonconformally coated CoNi0.5 LDH nanoflakes. The electrode with conformal coating of CoNi0.5 LDH nanoflakes shows an internalcharge transfer resistance of around 23.6 which is much smaller than smaller than non-conformal coated CoNi0.5 LDH nanoflakes.


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Fig. 4. (a) Variation in specific capacitance of both electrodes at various current densities (b) Cycling stability at 20 Ag−1 (inset CD profile of conformally coated CoNi0.5 LDH nanoflakes), (c) Ragone chart and (d) Nyquist plots for conformally and non-conformally coated CoNi0.5 LDH nanoflakes.

This low charge-transfer resistances implying that the conformally coated CoNi0.5 LDH nanoflakes favors fast electron transport and high diffusion rate of the electrolyte ions leads to better electrochemical performance. 4. Conclusions In summary, a cost-effective and scalable chemical precipitation method was used for the uniform and conformal coating of CoNi0.5 LDH nanoflakes on carbon cloth. The synthesized structure exhibited significantly enhanced capacitive properties as compared to non-conformal coated electrodes as each nanoflake has its own electrical contact with the electrode which results in fast electrolyte ion diffusion and faradaic redox reaction, reduces the internal resistance and ensures that all CoNi0.5 LDH nanoflakes participate in the electrochemical reaction in order to enhance the utilization of CoNi0.5 LDH nanoflakes. Acknowledgements The authors would like extend their sincere appreciation to the Deanship of Scientific Research at King Saudi University for its funding of this research through the Research Group Project no RGP-VPP-312. References [1] W. Gao, N. Singh, L. Song, Z. Liu, A.L.M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei, P.M. Ajayan, Direct laser writing of micro-supercapacitors on hydrated graphite oxide films, Nature Nanotechnology 6 (2011) 496–500. [2] L.Q. Mai, F. Yang, Y.L. Zhao, X. Xu, L. Xu, Y.Z. Luo, Hierarchical MnMoO4/CoMoO4 heterostructured nanowires with enhanced supercapacitor performance, Nature Communications 2 (2011).

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