Calcium nitrate (Ca(NO3)2)-based inorganic salt electrode for supercapacitor with long-cycle life performance

Current Applied Physics 17 (2017) 1189e1193

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Calcium nitrate (Ca(NO3)2)-based inorganic salt electrode for supercapacitor with long-cycle life performance Sangeun Cho a, 1, Jaeseok Han a, 1, Jongmin Kim a, Yongcheol Jo a, Hyeonseok Woo a, Seongwoo Lee a, Abu Talha Aqueel Ahmed a, Harish C. Chavan a, S.M. Pawar a, Jayavant L. Gunjakar a, Jungwon Kwak b, Youngsin Park c, Akbar I. Inamdar a, **, Hyunjeong Kim a, ***, Hyungsang Kim a, ****, Hyunsik Im a, * a

Division of Physics & Semiconductor Science, Dongguk University, Seoul 04620, South Korea Medical Physics Department, Asan Medical Center, Seoul 05505, South Korea c School of Natural Science, UNIST, Ulsan 44919, South Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 January 2017 Received in revised form 18 April 2017 Accepted 22 May 2017 Available online 25 May 2017

A novel water-soluble inorganic Ca(NO3)2 salt electrode is investigated for its pseudocapacitance in an aqueous KOH electrolyte. Commercially available Ca(NO3)2 salt is directly used as the key electrode material. The supercapacitor electrode contains Ca(NO3)2 salt, carbon black, and polyvinylidene fluoride (PVDF) in a ratio of 80:10:10. The Ca(NO3)2-based electrode demonstrates an exceptionally long life cycling stability, and a reasonably sound specific capacitance of 234 F/g is obtained at a current density of 3 A/g. Via chemical and electrochemical reactions, the in-situ activation of the Ca(NO3)2 forms an intermediate CaO which contributes to the pseudocapacitance of the electrode. The electrode undergoes a reversible redox reaction between Cu2þ 4 Cuþ during the charge-discharge process. Superior rate capability and excellent specific capacitance retention of ~120% over 2000 cycles are achieved compared with other inorganic salt electrodes. © 2017 Elsevier B.V. All rights reserved.

Keywords: Supercapacitor Inorganic salt electrode Ca(NO3)2 Energy storage

1. Introduction Electrochemical energy storage devices (rechargeable batteries and electrochemical capacitors) are in highly demand due to their applicability to portable electronic devices and electric vehicles [1,2]. There are two types of electrochemical supercapacitors, depending on the nature of the charge storage mechanism. There are electrical double-layer capacitors and pseudocapacitors [3e5]. Considerable efforts have been made to increase the specific power and energy densities with the use in environment-friendly materials. Carbon materials, conducting polymers, and transition metal oxides are the most widely used electrode materials for

* Corresponding author. ** Corresponding author. *** Corresponding author. **** Corresponding author. E-mail addresses: [email protected] (A.I. Inamdar), hyunkim@ dongguk.edu (H. Kim), [email protected] (H. Kim), [email protected] (H. Im). 1 Theses authors contribute equally. http://dx.doi.org/10.1016/j.cap.2017.05.013 1567-1739/© 2017 Elsevier B.V. All rights reserved.

supercapacitors [6,7]. However, these materials are limited by their own drawbacks. Carbon materials have low specific capacitances, and conducting polymers have a poor cyclability [8]. Although transition metal oxides or hydroxides show high energy density and good electrochemical stability, their synthesis processes are rather complex and expensive. Recently, the pseudocapacitance properties of water-soluble inorganic salt materials such as Ce(NO3)3 [9], CoCl2 [10], NiCl2 [11] CuCl2 [12] and ErCl3 [13] have been studied. A very high specific capacitance of 2060 F/g has been obtained from a Ce(NO3)3 electrode material where the oxidation state reversibly changes between Ce3þ and Ce4þ [9]. The reversible redox reaction Co2þ 4 Co3þ 4 Co4þ of a CoCl2 salt electrode delivers a specific capacitance of ~1962 F/g [10]. Similarly, other water soluble materials, such as NiCl2 (Ni3þ 4 Ni), CuCl2 (Cu2þ 4 Cuþ), and ErCl3 (Er2þ 4 Erþ) show high specific capacitance values [11e13]. In order to enhance specific capacitance, a common strategy is the use of a Ni-foam substrate. However, aside from their high specific capacitance, a generic drawback of water-soluble inorganic salt materials is unsatisfactory capacity retention and electrochemical instability. In generally, inorganic salt materials are

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easily dissolved in aqueous solution producing free active ions, and this makes it difficult to fabricate inorganic salt based electrodes with good retention performance. In this work, we demonstrate the utilization of another watersoluble inorganic salt electrode based on Ca(NO3)2 for its electrochemical supercapacitor characteristics. A Cu plate is used as the substrate instead of the commonly used Ni-foam for the practical application to a Ca(NO3)2 electrode. The electrode shows a fast redox reaction of Cu2þ 4 Cuþ and delivers a specific capacitance of 234 F/g. Most importantly, the Ca(NO3)2-based electrode shows superior rate reversibility and excellent long-cycle life electrochemical stability compared with other inorganic salt electrodes. 2. Experimental The supercapacitor electrodes were prepared on copper (Cu) substrates by mixing commercially available inorganic Ca(NO3)2$4H2O salt, acetylene black, and poly-vinylidene fluoride (PDVF) in a weight ratio of 80:10:10 with N-methyl-2-pyrroldone as the solvent. All of the chemicals were analytical grade (purchased from Sigma Aldrich Co.), and were used directly without any further purification. The Cu substrate was polished and then ringed with acetone, ethanol, methanol, and deionized water. The mixed slurry was pasted onto the Cu foil substrate and dried at 80  C for 12 h, as shown in Fig. 1. Electrochemical measurement of the salt electrode was performed using a potentiostat (Princeton Applied research, VersaSTAT) with a conventional three electrode electrochemical system. Galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) techniques were used in an aqueous 3 M potassium hydroxide (KOH) electrolyte. The system contained a saturated calomel electrode (SCE) as the reference electrode, a Pt wire as the counter electrode, and Ca(NO3)2 as the working electrode. The surface morphology of electrodes was obtained using field emission scanning electron microscopy (FE-SEM, Model: JSM-6701F, Japan). The structural

characteristics of the electrode before and after the electrochemical measurement were investigated by X-ray diffraction (XRD) with CuKa radiation (l ¼ 1.54056 Å). 3. Results and discussion Fig. 2a shows the CV curves of the inorganic Ca(NO3)2 salt electrode at different scan rates in the 3 M KOH electrolyte. The potential window of the CV curve was fixed between 0 V and 0.5 V (vs. the SCE). Broad cathodic and anodic peaks, due to the oxidation and reduction processes of the Ca cation, near at around 0.2 V, and the observed CV shape is typical of the pseudocapacitive behavior. This is different from electrical double layer capacitors (EDLCs) [3,14,15]. This indicates that the capacitance behavior of this electrode is mainly governed by reversible Faradaic redox reactions. When the electrode is dipped in the 3 M KOH electrolyte, Ca(NO3)2 is transformed into CaO at the surface of the electrode due to an insitu spontaneous chemical reaction where a strong bonding between the OH ions and the Ca2þ occurs. The cathodic peak at around 0.30 V (vs. the SCE) corresponds to the reduction of Ca2þ /Caþ. The anodic peak around 0.34 V (vs. the SCE) corresponds to the oxidation of the Caþ /Ca2þ. As the scan rate increases, the cathodic and anodic peak currents increase without a significant shift of the peak positions. This indicates that the diffusion controlled reactions at the electrode surface form the main mechanism [16]. The Galvanostatic charge/discharge (GCD) curves of the electrode at different current densities ranging from 3 A/g to 20 A/g are shown in Fig. 2b. The non-linearity of the charge-discharge curves is an indication of pseudocapacitor behavior and the existence of Faradaic reactions in the Ca(NO3)2 electrode. The specific capacitance of the electrode at different current densities is calculated using the following equation [17]:

CS ¼ I Dt=mDV

Fig. 1. Synthesis process of the Ca(NO3)2 salt electrode on a Cu plate.

(1)

S. Cho et al. / Current Applied Physics 17 (2017) 1189e1193

9

100 mV/s 50 mV/s 20 mV/s 10 mV/s 5 mV/s

I (mA)

6 3 0 -3

(a)

-6

0.0

0.2

0.5

Potential (V)

0.4

Potential (V)

3 A/g 5 A/g 7 A/g 12 A/g 15 A/g 20 A/g

0.4 0.3 0.2 0.1

(b) 0

50

100

150

Time (s)

200

250

3 A/g

Csp (F/g)

220 200

7 A/g 12 A/g

160

15 A/g

(c)

140 120

3 A/g

5 A/g

180

0

50

is evaluated using CV tests for up to 2000 cycles at a scan rate of 100 mV/s, as shown in Fig. 3a. The CV shape remains unchanged without any apparent deviations regarding each cycle, thereby confirming a sound electrochemical stability for up to 2000 cycles. Fig. 3b shows the capacitance and the capacitance retention that were calculated from the CV curves over 2000 cycles. The capacitance increase after the initial few-hundred cycles is presumably associated with the electrochemical activation of the electrode [18]. The specific capacitance after 2000 cycles is ~128 F/g, and showing a capacity retention of 117%. The increased specific capacitance and the excellent capacity retention are due to the gradual penetration of the electrolyte into the electrode as CV cycling progresses [19]. Most of the inorganic salt electrode supercapacitors that are based on CoCl2, NiCl2, CuCl2, (Ce(NO3)3, and ErCl3 show a relatively poor cycling stability [9e13]. Even though the specific capacity of the Ca(NO3)2 electrode is low compared with the other inorganic salt electrodes, the electrode shows outstanding electrochemical cycling performance over 2000 cycles and high capacity retention. The reason behind the lower specific capacitance of our Ca(NO3)2 electrode might be linked to the use of the Cu foil substrate with a small active surface area compared with the other salt electrodes fabricated on Ni foam substrates. The cathodic peak current increases with the square root of scan rate (s1/2), confirming that the pseudocapacitive nature of the electrode is governed by the diffusion of the (OH) to the active sites [20]. The diffusion coefficient of the Ca(NO3)2 electrode is calculated using the following equation [21]: * 1/2 ip ¼ (2.69  105) n3/2 A D1/2 o Co s

240

20 A/g

100 150 200 250 300 350

Cycle Fig. 2. Electrochemical supercapacitor measurements of the inorganic Ca(NO3)2 salt electrode. (a) CV curves at different scan rates of 5, 10, 20, 50, 100 mV/s, (b) galvanostatic charge/discharge measurements at different current densities of 3, 5, 7, 12, 15, 20 A/g, and (c) rate performances.

where CS is the specific capacitance, I is the current, Dt is the discharging time, m is the mass, and DV is the potential window. The maximum specific capacitance that is measured at 3 A/g is 234 F/g. The Ca(NO3)2 electrode is cycled at different charge-discharge current densities to investigate their rate performance, as shown in Fig. 2c. As the charge-discharge rate increases from 3 A/g to 20 A/ g, the specific capacitance decreases gradually from 231 F/g to 146 F/g. This behavior is explained in terms of the redox reaction that is governed by the diffusion of the ions into the electrolyte. As the applied current density is reduced back to 3 A/g, the reversible capacity could be maintained at ~230 F/g. The Ca(NO3)2 electrode exhibits both an outstanding reversible capacity and a high rate long-cycle life performance. The long-term electrochemical cycling stability of the electrode

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(2)

where n indicates the number of transferred electrons, A is the area of electrode, Do is the diffusion coefficient, and C*o is the concentration of the reactants. Fig. 3c shows the CV curves before and after cycling, which are used to estimate the diffusion coefficient. The relative diffusion coefficient of the electrode the 1st and after the 2000th cycle (Dafter/Dbefore)1/2 is ~130%. This suggests that the enhanced diffusion coefficient during cycling contributes to the increased specific capacitance and this is consistent with the observed retention in Fig. 3b. To evaluate the overall performance of the electrode, the energy and power densities are calculated using the following equations [22]:

Energy density ðEÞ ¼ 0:5  Csp  DV 2

(3)

Power density ðPÞ ¼ E=t

(4)

where Csp is the specific capacitance, DV is the potential window, and t is the discharge time. Fig. 3d shows the Ragone plot of the inorganic Ca(NO3)2 salt electrode. The obtained energy and power densities are comparable to those of conventional supercapacitors. To further evaluate the electrochemical performance of the inorganic Ca(NO3)2 salt electrode, EIS was performed. Fig. 4 shows the EIS spectra of the electrode recorded in the same electrolyte the 1st and after the 2000th charge-discharge cycles. The inset shows the equivalent circuit diagram. The precycling Nyquist plot of the sample shows a semi-circle in the high frequency region and a straight line portion in the low frequency region. These two phenomena are associated with the Warburg impedance and the charge transfer resistance of the electrode. On the other hand, the post cycling sample shows the straight line portion that is mainly governed by the diffusion controlled reaction. The CPE represents a constant phase element instead of a pure capacitive element. The estimated initial surface resistance (Rs) and charge transfer

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Fig. 3. Long-term electrochemical cycling stability measurements of the inorganic Ca(NO3)2 salt electrode. (a) CV curves of Ca(NO3)2 in the 3M KOH electrolyte after each of the 100 cycles, (b) specific capacitance and capacity retention as a function of the cycle number for up to 2000 cycles, (c) CV curves the 1st cycle and after the 2000th cycle for the estimation of the diffusion coefficient, and (d) Ragone plot of the inorganic Ca(NO3)2 salt electrode, comparing the energy density and power density.

resistance (Rct) of the electrode before the cycling are 1.76 U and 14.3 U, respectively. The post cycling sample shows a very small Rct value. The negligible post-cycling Rct value suggests a considerably increased electrode conductivity, contributing to the enhanced capacitive characteristics. This could be the possible reason behind the increased specific capacitance after the cycling process. Overall, the EIS results are consistent with the observed capacity retention and enhanced capacitance. To elucidate the electrochemical reaction mechanism at the electrode's surface, the microstructures and compositions of the electrode were studied before and after electrochemical measurements. Fig. 5a and b shows the XRD patterns of the Ca(NO3)2

electrode before and after the electrochemical reactions. In the XRD patterns, only the Cu and Ca metal peaks are observable. These belong to the substrate and the Ca(NO3)2 electrode, respectively. Well defined Ca-based compounds are not formed after the cycling process. The insert figure shows the SEM images of the Ca(NO3)2 electrode before and after the electrochemical reactions. Here, the microstructures are compact before cycling and remain unchanged after cycling, suggesting the favorable microstructure for a stable cycling performance. This finding is supportive of the in-situ formation of CaO in the alkaline solution when the electrode is dipped in the electrolyte [9e13]. Furthermore, this in-situ formation is activated electrochemically and contributes to the pseudocapacitive behavior of the electrode during the electrochemical process. 4. Conclusions

Fig. 4. (a) Nyquist plot of the inorganic Ca(NO3)2 salt electrode recorded the 1st and after the 2000th electrochemical cycles, and equivalent circuit diagram.

A novel inorganic Ca(NO3)2 salt electrode was fabricated and investigated for long-cycle life electrochemical supercapacitor applications. With a reasonably sound capacitance of 234 F/g, the electrode exhibited the highest reported long-cycle life performance compared with other inorganic salt electrodes. The pseudocapacitance behavior of this electrode originates from the in-situ chemical and electrochemical activations of Ca(NO3)2 through the intermediate formation of Ca(OH)2 and CaO during the electrochemical measurement process. The increased diffusion coefficient and the decreased charge transfer resistance during cycling enhance the retention properties of the electrode. We believe that this work can pave the way for further studying the performance of the Ca(NO3)2 salt as a promising candidate electrode material for supercapacitors.

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[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

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[17] Fig. 5. Morphological and compositional analyses of the Ca(NO3)2 electrode. (a) XRD graph before, and (b) after the electrochemical measurements. The insets show the SEM images.

[18]

[19]

Acknowledgements Hyunjeong Kim acknowledges that this work was supported by the research program of Dongguk University in 2006.

[20]

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