Supercapacitor Behavior of Cerium Oxide ... - ACS Publications

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Supercapacitor Behaviour of Cerium Oxide Nanoparticles in Neutral Aqueous Electrolytes Nallappan Maheswari, and Gopalan Muralidharan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02144 • Publication Date (Web): 01 Dec 2015 Downloaded from http://pubs.acs.org on December 4, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Supercapacitor Behaviour of Cerium Oxide Nanoparticles in Neutral Aqueous Electrolytes Nallappan Maheswari and Gopalan Muralidharan * Department of Physics, Gandhigram Rural Institute - Deemed University, Dindigul, Tamilnadu, India *Corresponding author e-mail: [email protected]

1 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 24

ABSTRACT: Hexagonal CeO2 nanoparticles have been prepared through hydrothermal method using CTAB (Cetyl trimethyl ammonium bromide) as the surfactant.

The structural and

morphological studies have been made using XRD, FTIR, SEM and TEM analysis. The electrochemical behaviour of CeO2 nanoparticles was investigated using cyclicvotammetry (CV), charge- discharge studies (CHDH) and ac impedance spectroscopy in different neutral electrolytes such as NaCl, KCl, Na2SO4 and K2SO4. Maximum Specicific capacitance of 523 F g-1 was attained with the NaCl electrolyte at 2 mV s-1. The capacitance values obtained with various electrolytes are in the order of NaCl > Na2SO4 > KCl > K2SO4. Charge- discharge and impedance analysis further confirms this behaviour.

After 2000 cycles of charging and

discharging only 18% degradation in the specific capacitance could be observed.

All the

electrochemical studies indicate the NaCl aqueous electrolyte to be most suited electrolyte for CeO2 supercapacitor electrodes.

Keywords: Neutral aqueous electrolytes, hexagonal CeO2, Hydrothermal method, Cyclic stability, Hydration number


Page 3 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1. INTRODUCTION With the fear that the fossil fuels may not last another couple of decades, the need to develop environmentally friendly, non polluting, renewable energy systems and energy storage devices is the order of the day.1 As energy storage devices, supercapacitors have drawn increasing attention in addressing the emerging energy demands on account of their trademark features like high power delivery, long cycle life, and fast charge /discharge characteristics.2, 3 Supercapacitors can be divided into two categories according to the energy storage principle, namely, electrochemical double layer capacitors (EDLC’s) and psuedocapacitors.4 The former is usually made of mesoscopic materials like activated carbon, carbon nanotube, graphene that are associated with high specific surface area. Separation of charges at the interface between the electrode and the electrolyte is the mechanism of charge storage in EDLCs. Transition metal oxides with several oxidation states and conducting polymers permit insertion and de-insertion of charges through redox reactions. These are labeled as pseudocapacitor materials.5, 6, 7 In contrast to EDLCs, pseudocapacitors exhibit larger specific capacitance owing to their redox properties. 8 RuO2 is accepted as the best electrode material for supercapacitors. Its commercial exploitation is hindered by the fact that it is a costly material while its limited availability on the earth’s crust makes it worse.

9

As an alternative to ruthenium oxide MnO2,10 NiO,11 Co3O412 have been

extensively studied as pseudocapacitor electrode material. Recently new types of electrode materials have been explored to further enhance the energy density of these pseudocapacitors. Due to 4f electronic structure, rare earth elements exhibit unique spectroscopic properties. In particular, CeO2 has been seen as a potential electrode material for supercapacitors due to the following reasons: abundance, excellent redox properties.13, 14 From the literature, it can be seen that there are only few reports on the supercapacitor behaviour of the rare earth oxides. 3 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 24

Proper selection of electrode materials and electrolytes is one of the means to achieve greater charge storage capacities associated with large energy densities.

Non aqueous organic

electrolytes with higher decomposition voltage have been identified for use in a larger electrochemical window. The high cell voltage of supercapacitors using organic electrolytes allows them to deliver a higher specific energy than with aqueous electrolytes, but it suffers from the fact that these are flammable, high cost, low ionic conductivity and exhibit bulging on continuous cycles of charge- discharge.15 Aqueous electrolytes that are capable of higher ionic conductivity deliver higher power densities. The added advantage of the aqueous electrolytes is that they are environment friendly.

The non-aqueous electrolytes require an anhydrous

atmosphere for cell assembly while this difficulty is circumvented with the aqueous electrolytes.16 The high mobility of protons due to its smaller size and weight, and the associated high ionic conductivity makes aqueous acidic electrolytes an attractive option. In addition to the degradation of the electrode material, corrosion of current collectors and environmental concerns need to be considered with strong acid electrolytes.17 Large number of research groups are putting efforts at identifying safe, environment friendly power sources towards use in flexible, wearable devices.18

These issues have created interest in the use of neutral electrolytes.

Generally a neutral electrolyte exhibits low H+ and OH- concentration, resulting in the use of a higher potential window without gas evolution.16,

19

For the first time we have examined the

supercapacitive properties of hydrothermally synthesized cerium oxide nanoparticle in different neutral aqueous electrolytes such as KCl, NaCl, K2SO4, Na2SO4. To the best our knowledge, very little literature is available on the optimization of an electrolyte for CeO2 nanoparticles with respect to their electrochemical behaviour. The pseudocapacitive performance of CeO2 electrode with these electrolytes has been investigated and reported in this paper.


Page 5 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2. MATERIALS AND ELECTRODE PREPARATION 2.1. Materials and methods Analytical grade reagents have been employed in the present work. 0.2 M of Ce(NO3)3. 6H2O was dissolved in 40 mL of water.

Appropriate amount of CTAB (Cetyl trimethyl

ammonium bromide) was added to get 0.05 M of CTAB in the solution. The pH of the solution was maintained at 10 by the drop wise addition of 1 M NaOH. This solution was subjected to continuous stirring for 2 h. The hydrothermal reaction of this solution was achieved by placing this solution at 160 ̊C in a Teflon-lined stainless steel autoclave for 24 h. The autoclave was allowed to cool to room temperature. The precipitate formed in this process was washed with deionized water and ethanol several times. The final product was dried in an atmosphere of air at 80 ̊C for 24 h and then calcined at 500 ̊C for 4 h in air. The X-ray diffraction patterns of the samples were recorded using a PANalytical XPERT-PRO X-ray diffractometer with Cu Kα radiation. FTIR Spectra have been recorded using a Perkin-Elmer Spectrum BX-II spectrophotometer. The surface morphology of the CeO2 was studied using Tescan VEGA-3 LMU Instrument. Transmission electron microscopy (TEM) images were obtained with JEOL JEM 2100 (200 kV) system to study the morphology of the samples. 2.2. Electrode preparation and electrochemical measurements CHI-660D electrochemical workstation was used to make electrochemical measurements. 80 % CeO2 as the active material, 15 % of activated carbon and 5 % PTFE as the binder were used to make the working electrode. Few drops of ethanol were used as the solvent to aid the formation of slurry. Pretreated graphite sheet (1cm x 1cm) was used as the current collector. 1 mg of the active material was used to prepare the working electrode. The drying of the prepared 5 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrode was carried out in an atmosphere of at 70 ̊C for 12 h. The electrode prepared employing this methodology was used as the working electrode. 1 M aqueous solutions of KCl, NaCl, Na2SO4 and K2SO4 were used as the electrolyte. The three electrode cell configuration was employed for all the electrochemical measurements. Here, the working electrode was made up of CeO2 prepared in the present work, a platinum wire as the counter electrode and Ag/AgCl as the reference electrode.

A potential window of 0 to 0.8 V was chosen for the

cyclicvoltammetry measurements.

Scan rates of 2 mV s-1 to 100 mV s-1 were employed in

making CV measurements. The same potential window of 0 to 0.8 V was used in carrying out charge-discharge characterization at different current densities. AC amplitude of 5 mV and bias potential of 0.3 V were applied to the electrodes in making electrochemical impedance measurements. The EIS data were obtained in the frequency range of 0.01 Hz and 100 kHz. RESULTS AND DISCUSSION 3.1. Structural and morphological analysis X-ray diffraction pattern of CeO2 nanoparticles is presented in figure 1. The diffraction peaks could be associated with (111), (200), (220), (311), (400), (331) planes of cubic CeO2. The XRD patterns agree well with the JCPDS card (81-0792) indicating the cubic phase of CeO2. The average crystallite size of 11.7 nm could be obtained through Debye-Scherrer formula.


Page 6 of 24

Page 7 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 1. XRD pattern of CeO2 nanoparticles

The FTIR spectrum of CeO2 is shown in figure 2.

The C–H stretching mode of

hydrocarbons attached to CTAB could be identified through absorption band in the region 2800– 2900 cm−1. The absorption band at 1026 cm−1 is assigned to the stretching mode of C=O vibration.20,

21

The Ce–O stretching band around 450 cm−1 reveals the formation of CeO2

nanoparticles.22, 23 OH related vibrations were found through the bands around 3400 and 1625 cm−1.24

Figure 2. FTIR spectrum of CeO2 nanoparticles


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 24

The SEM micrographs of the CeO2 nanoparticles are shown in figure 3(a, b). It reveals the uniform distribution of CeO2 nanoparticles.

a

b

1 nm

2 nm

c

d

50 nm

20 nm

e

10 nm

f

2 nm

Figure 3.

SEM ( a,b) and TEM (c-f) images of CeO2 nanoparticles


Page 9 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

The TEM images of the CeO2 nanoparticles (figure 3c-f) reveal the hexagonal morphology of the nanoparticles. The well distributed particles can shorten the diffusion path and facilitate the easy diffusion of the ions. From the TEM images the mean particle size was found to be 14 nm. This agrees well with the crystallite size obtained from the XRD pattern. Scheme 1 represents the formation mechanism of CeO2 hexagonal nanostructures. CTAB controls the hydrolysis rate during nucleation process and ensures the slow release of OHto form Ce(OH)4-. During the protracted hydrothermal treatment, the Ce(OH)4- converted to CeO2.25 After calcining at 500 ̊C the small size hexagonal nanostructure were obtained.

Scheme 1. Schematic representation of CeO2 nanoparticle formation

3.2. Electrochemical Studies The cyclic voltammetry traces of CeO2 electrode in different electrolytes (NaCl, KCl, Na2SO4 and K2SO4) corresponding to various sweep rates are shown in figure 4a-d. In all the 9 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 24

electrolytes CeO2 electrodes exhibit pseudocapacitance nature, no obvious redox peaks could be observed. The mechanism proposed for charge storage of CeO2 based electrodes are the intercalation of the monovalent alkali cations (Na+, K+). The chemical equation for the same is given by CeO2 + M+ + e-

CeOOM

Where M= Na+, K+. It should be noticed that the proposed mechanism involved a redox reaction between the III and IV oxidation states of CeO2.26 From the CV curves, specific capacitance was estimated using the formula cited elsewhere.27 The maximum specific capacitance of 523±5, 488±5, 502±5, 222±2 F g-1 were obtained with NaCl, KCl, Na2SO4, K2SO4 electrolytes respectively, at the lowest scan rate of 2 mV s-1.

(a)

(b)


Page 11 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(c)

(d)

(e)

Figure 4. CV curves of CeO2 nanoparticles with different electrolytes at different scan rates (a) NaCl (b) KCl, (c) Na2SO4 and (d) K2SO4 , (e) Dependence of specific capacitance on scan rate

The shape of the CV curve indicates the capacitive behaviour.

It is well known that the

voltammetric current is always directly proportional to scan rate.28 At lower scan rates, the electrolyte ions completely diffuse into the material and hence they have enough time to utilize all the active sites of the electrode material for charge storage. At higher sweep rates, the movement of electrolyte ions is limited by time constraints. Only the outer active surface is utilized for charge storage.29 From figure 4e, it can be seen that the supercapacitor behaviour is 11 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 24

of the trend NaCl > Na2SO4 > KCl > K2SO4. This can be ascribed to the size difference among the hydrated electrolyte ions and transference numbers of anion and cation in the electrolytes. Generally K+ ions have a small hydration sphere.30 But compared with NaCl , both Na+ and K+ ions have similar hydration numbers, but ionic radius of Na+ (0.95 Å) is far lower than K+ (1.33 Å) ion

31

which permits better diffusion of Na+ ions and it leads to a larger capacitance (7 %)

with the NaCl electrolyte. Compared to Na2SO4, NaCl electrolyte provides larger specific capacitance (4 %).

The SO42- counterions of Na2SO4 (258 pm) are bigger than Cl- (190 pm)

counterions of NaCl. The size difference is the most probable cause for the decrease in the capacitance values with sodium sulphate.32 Similar results were obtained by Jeiong et al.32 for MnO2 nanostructures and Reddy et al.33 for MnO2 prepared through sol-gel method. They reported the highest capacitance with NaCl electrolyte than KCl and Na2SO4 electrolytes. The observed specific capacitance values are nearly twice that of the values reported in the literature, 289 F g-1 (5 mV s-1 in 0.5 M Na2SO4) reported by Kalubarme et al.11 while Wang et al.34 reported 208 F g-1 for CeO2 / graphene electrode in a potential window 0.6 V (in 3 M KOH). The poor crystallinity and smaller hexagonal nanoparticles are found to enhance the mobility of the charge carriers and increase the capacitance. The galvanostatic charge- discharge measurements of CeO2 electrode carried out in the four electrolytes at current densities of 2, 5, 10 A g-1 are depicted in figure 5. The symmetric structure of the charge- discharge curves indicates excellent electrochemical reversibility of CeO2 electrode. The specific capacitance of the electrodes was calculated using the chargedischarge data using the equation mentioned elsewhere.27


Page 13 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

(b)

(d)

(c)

Figure 5. Charge- discharge curves of CeO2 electrodes (a) NaCl (b) KCl, (c) Na2SO4 and (d) K2SO4

The maximum capacitance was obtained at a current density of 2 A g-1. The specific capcacitance values in different electrolytes are: 457±5 (NaCl), 395±4 (KCl), 400±4 (Na2SO4), and 320±3 F g-1 (K2SO4). These values agree very well with those obtained through CV. Maximum specific capacitance of 644 F g-1 was reported by Padmanathan et al.25 They used 3 M KOH as the electrolyte while the current density was maintained at 0.5 A g-1. It is to be noted that the present results are obtained with neutral electrolytes and at four times the current density employed by Padmanathan and his co-workers.

Scheme 2 explains the electrochemical

mechanism of CeO2 electrode. Hexagonal nanostructures act as an ion buffering reservoir and 13 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 24

hence enhance the Na+ diffusion rate within the bulk of the material. The nanosized hexagonal structures of the CeO2 particles help in a significant reduction in the diffusion length of the cations.

Scheme 2. Schematic representation of the electrochemical performance of CeO2 electrode

Conductivity and mobility of the cations are important factors that determine the behaviour of the electrolytes. The hydration radius of Na+ is larger than that of K+. Hence Na+ ions have a larger hydration sphere than K+ ions as a result of the strong interaction between Naδ+ and H2Oδ-. But, it is interesting to note that the overall radius of hydration for all ions is about the same, from 3.3 to 3.8 Å.32, 33 Jeiong et al.32 and Reddy et al.33 reported that the size of the hydration sphere is not a major deciding factor. In the case of KCl and NaCl the Na+ yields higher specific capacitance inspite of the fact that the potassium ions exhibit better conductivity. Jeiong et al.32 and Reddy et al.33 reported similar results with MnO2 nanostructures. The values of discharge capacitance and available active sites against current density are presented in Figure


Page 15 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

6a–d. The available active sites (Z) are determined from the specific capacitance of the CeO2 electrode using the following equation 35

Ζ= Where

Cs ∆VW F

(3)

Cs is the specific capacitance, ∆V , the potential window, W , the molecular

weight of CeO2 (172.115 g) and F , the Faradic constant.

(a)

(b)

(d)

(c)

Figure 6. Specific capacitance and active site against current density in (a) NaCl (b) KCl, (c) Na2SO4 and (d) K2SO4 electrolytes


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 24

Figure 6 testifies that the electrode is endowed with higher number of active sites and higher specific capacitance at low-current density irrespective of the kind of electrolyte employed. It is reasoned that the number of active sites accessed by the ions is quite high at low current densities due to large amounts of time available for diffusion. The reduction in the access to inner sites of the electrode leads to a reduced capacitance at higher rates of charge – discharge.36 Electrochemical impedance spectroscopy (EIS) studies were carried out for the CeO2 electrode with the four (1 M NaCl, KCl, Na2SO4 and K2SO4) at conditions mentioned in section 2.2. These measurements are carried out as the charge transfer resistance and the solution resistances are essential parameters that characterize the performance of a supercapacitor The typical Nyquist plots of CeO2 electrode are shown in Figure 7a. The solution resistance (Rs) is obtained from the intersection of the high frequency side with the horizontal axis of the Nyquist plot. The semicircle observed at high-to-medium frequency range yields the charge transfer resistance (Rct) associated with the CeO2 electrode. A spike associated with the low-frequency region is indicative of the ideal capacitive behaviour of the electrode. Warburg impedance (ZW) of the electrode, i.e., the diffusive resistance of ions Na+ and K+ into electrode is obtained through this spike.36, 37 Of the four electrolytes employed in the present study, NaCl (0.84 Ω) is the one that is endowed with the lowest charge transfer resistance. The other electrolytes exhibited considerably larger resistance: Na2SO4 (0.9 Ω), KCl (1.1 Ω) and K2SO4 (1.4 Ω). Table 1. Shows the impedance data fitted to an equivalent circuit. All the impedance values suggest that NaCl is the most suitable electrolyte for CeO2 electrode.


Page 17 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(a)

(b)

Figure 7. (a) Nyquist plot of CeO2 electrode, (b) Equivalent circuit

Table 1. The components of the equivalent circuits obtained by fitting the impedance data. Electrolytes

Rct (Ω)

Cdl (mF)

Rs (Ω)

ZW

Cp(F)

NaCl

0.84

0.795

2.55

0.0816

0.9732

KCl

1.1

0.420

4.22

0.0795

0.9437

Na2SO4

0.9

0.584

3.30

0.0795

0.9521

K2SO4

1.4

0.335

4.32

0.0713

0.8336

Table 1. shows the parameters obtained by fitting the impedance data to an equivalent circuit. The fitted equivalent circuit model is shown in Figure 7b. The model circuit comprised of Rs, which includes bulk electrolyte solution resistance and electron transfer resistance at the


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 24

electrode/ electrolyte boundary. Cp is the pseudocapacitance component and Rct is the either resistance associated with the transfer of charges at the boundary of the electrolyte and electrode. The Rct values are around 0.84- 1.4 Ω for all the electrolytes. ZW is the semi infinite diffusion of cations in the electrode.

The Warburg impedance values for CeO2 electrode in various

electrolytes due to electrolytes’ cation diffusion within the electrode. The higher Warburg impedance in the case of NaCl electrolyte compared to other electrolytes may be attributed to the more affordable diffusion of electrolyte cations (Na+) in the electrode.38 The observed higher capacitance in NaCl electrolyte associated with the decrease in charge transfer resistance and an increase in Warburg impedance. Cyclic stability is another important aspect of a supercapacitor operation. Figure 8 shows the effect of continuous cycles of charge-discharge on the specific capacitance and Columbic efficiency of the prepared electrode in 1M NaCl. From the cyclic test on CeO2 electrode, it is noted that even after 2000 continuous charge discharge cycles, the electrode could retain 82 % of the initial value. It is essential to note that the cyclic stability test has been conducted at a far larger current density of 10 A g-1. Figure 8 illustrates the excellent long term cyclic stability of the CeO2 electrode after 2000 cycles. The Columbic efficiency was calculated from the relation 27

η=

td ×100% tc

(4)

Where η is the Columbic efficiency; t d is the discharging time and t c is the charging time. Even after 2000 cycles, 100 % of Columbic efficiency could be observed.


Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 8. Cyclic stability and Columbic efficiency of CeO2 electrode in NaCl electrolyte at a current density of 10 A g-1

This is an indication of excellent kinetic reversibility associated with the hexagonal nanoparticles of cerium oxide. The overall electrochemical studies strongly suggest that I M NaCl electrolyte is the most suited electrolyte for working with CeO2 electrodes prepared via hydrothermal method using CTAB. 4. CONCLUSION The CeO2 nanoparticles have been prepared by hydrothermal method. XRD pattern reveals the cubic phases of CeO2.

FTIR spectrum confirms the presence of CeO2.

The

performances of electrochemical studies on CeO2 electrode were investigated with various aqueous electrolytes such as NaCl, KCl, Na2SO4 and K2SO4. The CeO2 electrode yielded the maximum specific capacitance in NaCl electrolyte.

The measurements of the same from

cyclicvoltammetry, charge- discharge analysis and impedance spectroscopy confirm the aqueous NaCl to be best candidate.

The cyclic test on CeO2 electrode showed the good capacity

rentention about 82 % over 2000 cycles. All the electrochemical studies suggest that NaCl is the best suited electrolyte for supercapacitor applications employing cerium oxide. 19 ACS Paragon Plus Environment

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

AUTHOR INFORMATION Corresponding Author E.mail: [email protected] Phone: +91 451 2452371.

Fax: +91 451 2454466


Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

REFERENCES (1) Xu, Y.; Wang, X.; An, C.; Wang, Y.; Jiao, L.; Yuan, H. Facile synthesis route of porous MnCo2O4 and CoMn2O4 nanowires and their excellent electrochemical properties in supercapacitors. J. Mater. Chem. A, 2014, 2, 16480-16488. (2) Yuan, C.; Li, J.; Hou, L.; Zhang, X.; Shen, L.; Wen, X.; (David) Lou. Ultrathin Mesoporous NiCo2O4 nanosheets supported on Ni foam as advanced electrodes for supercapacitors. Adv. Funct. Mater. 2012, 22, 4592-4597. (3) Gao, Z.; Yang, W.; Wang, J.; Song, N.; Li, X. Flexible all-solid-state Hierarchical NiCo2O4/porous graphene paper asymmetric supercapacitors with an exceptional combination of electrochemical properties. Nano Energy, 2015, 13, 306-317. (4) Hsu, C.; Hu, C. Synthesis and characterization of mesoporous spinel NiCo2O4 using surfactant-assembled dispersion for asymmetric supercapacitors. J.Power Sources, 2013, 242 , 662 – 671. (5) Niu, Z.Q.; Luan, P.S.; Shao, Q.; Dong, H.B.; Li, J.Z.; Chen, J.; Zhao, D.; Cai, L.; Zhou, W.Y.; Chen,X.D.; Xiea, S.S. A “skeleton/skin” strategy for preparing ultrathin freestanding single- walled carbon nanotube/polyaniline films for high performance supercapacitor electrodes. Energy Environ. Sci. 2012, 5 8726 - 8733. (6) Choi, B.G.; Yang, M.H.; Hong, W.H.; Choi, J.W.; Huh, Y.S. 3D macroporous graphene framework for supercapacitors with high energy and power densities. ACS Nano, 2012, 6, 4020 – 4028. (7) Yang, W.; Gao, Z.; Ma, J.; Zhang, X.; Wang, J.; Liu, J. Hierarchical NiCo2O4@NiO core– shell heterostructured nanowire arrays on carbon cloth for a high-performance flexible all-solidstate electrochemical capacitor. J. Mater. Chem. A, 2014, 2, 1448-1457. (8) Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors, Nat. Mater. 2008, 7, 845−854. (9) Xue, T.; Xu, C.L.; Zhao, D.D.; Li, X.H.; Li, H.L. Electrodeposition of mesoporous manganese dioxide supercapacitor electrodes through self-assembled triblock copolymer templates. J. Power Sources, 2007, 164, 953–958. (10) Jiang, H.; Zhao, T.; Ma, J.; Yan, C.; Li, C.Z. Ultrafine manganese dioxide nanowire network for high-performance supercapacitors. Chem. Commun., 2011, 47, 1264–1266.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(11) Liu, M.; Chang, J.; Sun, J.; Gao, L. A facile preparation of NiO/Ni composites as high performance pseudocapacitor materials, RSC Adv., 3 (2013) 8003– 8008. (12) Xia, X.H.; Tu, J.P.; Mai, Y.J.; Wang, X.L.; Gu, C.D.;

Zhao, X.B. Self-supported

hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercapacitor capacitance. J. Mater. Chem. 2011, 21, 9319–9325. (13) Kalubarme, R.S.; Kim, Y.H.; Park, C.J. One step hydrothermal synthesis of a carbon nanotube/cerium oxide nanocomposite and its electrochemical properties. Nanotechnology, 2013, 24, 365401–365408. (14) Zec, S.; Boskovic, S.; Kaluperovic, B.; Bogdanov, Z.; Popovic, N. Chemical reduction of nanocrystalline CeO2, Ceram Int. 2009, 35, 195–198. (15) Tian,L.; Yuan, A. Electrochemical performance of nanostructured spinel LiMn2O4 in different aqueous electrolytes. J. Power Sources, 2009, 192, 693–697. (16) Qu, Q.; Zhang, P.; Wang, B.; Chen, Y.; Tian, S.; Wu, Y.; Holze, R. Electrochemical performance of MnO2 nanorods in neutral aqueous electrolytes as a cathode for asymmetric supercapacitors. J. Phys. Chem. C. 2009, 113, 14020–14027. (17) Jeong, Y.U.; Manthiram, A. Nanocrystalline manganese oxides for electrochemical capacitors with neutral electrolytes. J. Electrochem. Soc., 2002, 149 (11), A1419-A1422. (18) Gao, Z.; Song, N.; Li, X. Microstructural design of hybrid CoO@NiO and graphene nanoarchitectures for flexible high performance supercapacitors. J. Mater. Chem. A, 2015, 3, 14833-14844. (19) Wang, Y.; Jianyun Cao, Z.; Zhou, Y.; Ouyang, J.H.; Jia, D.; Guo, L. Ball milled graphite as an electrode material for high voltage supercapacitor in neutral aqueous electrolyte. J. Electrochem. Soc., 2012, 159 (5), A579-A583. (20) Zhu, T.; Chen, J.S.; (David) Lou, X.W. Glucose-assisted onepot synthesis of FeOOH nanorods and their transformation to Fe3O4@Carbon nanorods for application in lithium ion batteries, J. Phys. Chem. C, 2011, 115, 9814–9820. (21) Yang, D.; Liu, P.; Gao, Y.; Wu, H.; Cao, Y.; Xiao, Q.; Li, H. Synthesis, characterization, and electrochemical performances of core-shell Ni(SO4)0.3(OH)1.4/C and NiO/C nanobelts, J. Mater. Chem., 2012, 22, 7224–7231. (22) Goharshadi, EK.; Samiee, S.; Nancarrow, P. Fabrication of cerium oxide nanoparticles : Characterization and optical properties, J. Colloid and Interface Science, 2011, 356, 473–480. 22 ACS Paragon Plus Environment

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(23) Padmanathan, N.; Selladurai, S. Electrochemical capacitance of porous NiO–CeO2 binary oxide synthesized via sol–gel technique for supercapacitor. Ionics, 2014, 20, 409–420. (24) Santos, M. L. D.; Lima, R. C.; Riccardi, C. S.; Tranquilin, R. L.; Bueno, P. R. Varela, J. A.; Longo, E. Preparation and characterization of ceria nanospheres by microwave hydrothermal method. Mater. Lett., 2008, 62, 4509–4511. (25) Padmanathan, N.; Selladurai, S. Shape controlled synthesis of CeO2 nanostructures for high performance supercapacitor electrodes, RSC Adv., 2014, 4, 6527–6534. (26) Maiti, S.; Pramanik, A.; Mahanty, S. Extraordinarily high pseudocapacitance of metal organic framework derived nanostructured cerium oxide, RSC. Chem. Commun., 2014, 50, 11717–11720. (27) Nagamuthu, S.; Vijayakumar, S.;

Muralidharan, G. Synthesis of Mn3O4/amorphous

carbon nanoparticles as electrode material for high performance supercapacitor applications, Energy & Fuel, 2013, 27, 3508−3515. (28) Kalai Selvan, R.; Perelshtein, I.; Perkas, N.; Gedanken, A. Synthesis of hexagonal-shaped SnO2 nanocrystals and SnO2@C nanocomposites for electrochemical redox supercapacitors. J. Phys. Chem. C, 2008, 112, 1825-1830. (29) Meher, S.K.; Justin, P.; Rao, G.R. Microwave-mediated synthesis for improved morphology and pseudocapacitance performance of nickel oxide, Appl. Mater. Interfaces. 2011, 3, 2063-2073. (30) Lee, H.Y. Goodenough, J.B. Supercapacitor behaviour with KCl electrolyte. J. Solid State Chem., 1999, 144, 220 -223. (31) Conway, B.E, Electrochemical Supercapacitors; Kluwer Academic/Plenum: New York, 1999. (32) Jeong, Y.U.; Manthiram, A. Nanocrystalline manganese oxides for electrochemical capacitors with neutral electrolytes, J. Electrochem. Soc. 2002, 149, 1419- 1422. (33) Reddy, R.N.; Reddy, R.G, Sol–gel MnO2 as an electrode material for electrochemical capacitors, J. Power Sources, 2003, 124, 330–337. (34) Wang, Y.; Xian Guo, C.; Liu, J.; Chen, T.; Yang, H.; Ming Li, C. CeO2 nanoparticles / graphene nano composite-based high performance supercapacitor, Dalton Trans., 2011, 40, 6388–6391.


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35) Chang, J.; Lee, W.; Mane, S.R.; Cho, B.W.; Hanb, S.H.

Page 24 of 24

Morphology-dependent

electrochemical supercapacitor properties of indium oxide, Electrochem. Solid- State Lett., 2008, 11 (1), A9-A11. (36) Vijayakumar, S.; Nagamuthu, S.; Muralidharan, G. Supercapacitor studies on NiO nanoflakes synthesized through microwave route, ACS Appl. Mater. Interfaces, 2013, 5(6), 21882196. (37) Saravanakumar, B.; Purushothaman, K.K.; Muralidharan, G.

Interconnected V2O5

nanoporous network for high-performance supercapacitors, ACS Appl. Mater. Interfaces, 2012, 49, 4484−4490. (38) Yan, J.; Fan, Z.; Wei, T.; Qian, W.; Zhang, M.; Wei, F. Fast and reversible surface redox reaction of graphene–MnO2 composites as supercapacitor electrodes, Carbon, 2010, 48, 38253833.


Supercapacitor Behavior of Cerium Oxide ... - ACS Publications

Recommend Documents