Highly porous CNTs knotted cerium oxide hollow tubes with exalted energy storage performance for hybrid supercapacitors

Journal Pre-proof Highly porous CNTs knotted cerium oxide hollow tubes with exalted energy storage performance for hybrid supercapacitors Bhimanaboina Ramulu, Goli Nagaraju, S. Chandra Sekhar, Jae Su Yu PII:

S0925-8388(19)34188-X

DOI:

https://doi.org/10.1016/j.jallcom.2019.152942

Reference:

JALCOM 152942

To appear in:

Journal of Alloys and Compounds

Received Date: 16 September 2019 Revised Date:

5 November 2019

Accepted Date: 6 November 2019

Please cite this article as: B. Ramulu, G. Nagaraju, S.C. Sekhar, J.S. Yu, Highly porous CNTs knotted cerium oxide hollow tubes with exalted energy storage performance for hybrid supercapacitors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152942. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Highly porous CNTs knotted cerium oxide hollow tubes with exalted energy storage performance for hybrid supercapacitors Bhimanaboina Ramulu,a Goli Nagaraju,a,b S. Chandra Sekhar,a and Jae Su Yua,* a

Department of Electronic Engineering, Institute of Wearable Convergence Electronics, Kyung

Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea b

Department of Chemical Engineering, College of Engineering, Kyung Hee University, 1732

Deogyeong-daero, Gihung-gu, Yongin-si, Gyeonggi-do 446-701, Republic of Korea.

*Address correspondence to [email protected] Tel: +82-31-201-3820; Fax: +82-31-206-2820

Abstract: Development of porous transition metal oxide nanostructures and their conductive compounds have attracted widespread attention in hybrid supercapacitors (SCs) because of their large surface area, mesoporosity and good electrochemical activity, which significantly enhance the energy storage performance. In this work, we reported highly porous cerium oxide hollow tubes (CeO2 HTs) by a facile biomorphic process with waste cotton fibers as template. The waste cotton fibers were initially infiltrated with aqueous cerium nitrate solution, followed by solution evaporation and thermal treatment led to the formation of highly porous CeO2 HTs. To enhance the electrical conductivity, the carbon nanotubes (CNTs) were subsequently decorated on CeO2 HTs via ultrasonication and they were used as cathode material (CNTs@CeO2 HTs) for hybrid SCs. Owing to large surface area, high porosity, and electrochemical conductivity, the prepared CNTs@CeO2 HTs composite showed a higher capacity of 70.7 µAh cm-2 at 1 mA cm-2 than bare CeO2 HTs

(41.9 µAh cm-2) in alkaline electrolyte. A hybrid SC assembled with the

CNTs@CeO2 HTs and activated carbon materials as positive and negative electrodes, respectively exhibited an enlarged voltage window of 1.55 V. The device demonstrated maximum energy density of 0.041 mWh cm-2 at 1 mA cm-2 with superior cycling stability (86.4%). Utilizing the high energy storage properties, the hybrid SCs in series could light up commercial LEDs for several minutes, indicating its potency for energy storage applications. This facile convincing method may offer a cost-effective approach for the development of high capacity cathode materials for promising hybrid SCs on a large scale. Keywords: Cotton fibers; porous cerium oxides; CNTs; areal capacity; hybrid supercapacitors

1. Introduction With the rapid degradation of conventional fossil fuels and ever-growing energy requirements, development of eco-friendly energy storage systems is highly desirable to this fast-growing society.[1-4] Accordingly, supercapacitors (SCs) have received prominent attention among various energy storage systems due to their large power density, extraordinary cycling stability, rapid charge-discharge ability, high flexibility, and eco-friendly nature.[5-9] As such, they have been employed in multiple fields including hybrid electric vehicles, power backup systems for portable electronics, wearable devices, medical applications, military applications, etc.[10-13] Compared with batteries, the low energy density of SCs restricts its further wide range applications.[14, 15] However, from the equation

=

, the energy density can be enhanced

by enlarging the potential window (V) and capacitance (C) of the SC device.[16-19] Coalescing the carbon-based materials (electric double layer capacitive-type) and transition metal oxide (pseudocapacitive/battery-type)-based electrodes in a single cell configuration, i.e., hybrid SCs, could be expected to enhance the energy density.[20] The carbonaceous materials generally showed high stability and large potentials, where transition metal oxides provide high capacitance/capacity in the electrolyte solution. Thus, the hybrid SC composed of carbonaceous materials and transition metal oxides is a promising approach to achieve high energy density without scarifying its power density and cycling life.[21-25] So far, several transition metal oxides such as NiCo2O4, Co3O4, CeO2, Ni(OH) 2, Ni3Se2, and NiWO4 have been broadly employed as electrode material in hybrid/ asymmetric SCs by the reason of having versatile oxidation states, natural benignity, cheap in cost and high theoretical capacity.[26-34] Among these materials, cerium oxide (CeO2) is a novel and exciting material for use in hybrid SCs, owing to their abundance, facile synthesis processes and fast

electrochemical redox chemistry of Ce3+ and Ce4+ ions.[35-37] However, only a handful of investigation has been carried out for the preparation of CeO2 nanostructures.[38-40] Generally, the materials with high porosity and large surface area are important aspects to enable the facile penetration of solvated electrolyte ions through their pores into the inner parts of the material. As such, the corresponding material could be effectively involved in redox chemistry to improve energy storage properties.[41] Also, the intrinsic poor electrical conductivity of CeO2 could limit the charge storage performance, where the outer surface of the material can only participate in the electrochemistry process and the underneath portion of the material could be partially involved in the electrochemical process, which ultimately lowers the energy storage performance.[42, 43] Therefore, fabrication of porous CeO2 with the large surface area and encapsulation of conductive coatings is highly desirable to employ the CeO2-based materials for hybrid SCs. In this work, a simple biomorphic process was used for the synthesis of porous CeO2 HTs with the use of waste cotton fibers as a template. To improve the conductivity, carbon nanotubes (CNTs) were uniformly decorated on CeO2 HTs to form the porous CNTs@CeO2 HTs composite for cathode material in hybrid SCs. Herein, the porous CeO2 HTs provide the beneficial paths for electrolyte penetration into the electrode material, which effectively enables the redox chemistry. The encapsulated CNTs on CeO2 HTs offer enriched ion transportation and further increase the energy storage properties in aqueous electrolyte. The electrochemical behavior of the pristine CeO2 HTs and CNTs@CeO2 HTs electrode materials were investigated and compared. Furthermore, hybrid SCs were also fabricated by CNTs@CeO2 HTs and activated carbon as positive and negative electrode materials, respectively with a piece of cellulose filter paper as a

separator using alkaline electrolyte. Moreover, the practical applicability of the assembled device was also effectively studied by operating various electronic components. 2. Experimental section 2.1. Chemicals All the obtained chemicals were of analytical grade purity and used without further purification after received. Cerium (III) nitrate (Ce(NO3)3) was obtained from Sigma-Aldrich Co., South Korea. Polyvinylidene fluoride (PVdF), N-methyl-2-pyrrolidone (NMP), potassium hydroxide (KOH), hydrochloric acid (HCl) and absolute ethanol (EtOH) were purchased from Daejung chemicals Ltd., South Korea. Super p carbon black (C65, TIMCAL) and nickel foam (Ni foam) were collected from MTI Korea, South Korea. Carbon nanotubes (CNTs, OD: 5-15 nm, length: 50 µm) were received from Cetech Co., Ltd. South Korea. The waste cotton was used in the present work was obtained from a working laboratory, Kyung Hee University, South Korea. 2.2. Synthesis of honeycomb-like cerium oxide hollow tubes (CeO2 HTs) Highly porous CeO2 HTs were successfully synthesized via a facile and low-cost biomorphic method. Herein, the waste cotton fibers were used as a template. Prior to the synthesis, Ce(NO3)3.6H2O (1.80 g) was dissolved in the beaker containing 30 mL of de-ionized (DI) water. Afterward, waste cotton fibers (1.0 g) were immersed in the aqueous cerium nitrate solution. Then, the mixture was heated at 80 oC until the evaporation of water has been done inside the beaker. The resultant cerium nitrate coated precursor cotton fibers were calcined at 400 oC for 4 h to eliminate the cotton fibers. The obtained porous sample was washed with DI water and dried in the oven at 80 oC for 12 h. 2.3. Preparation of CNTs@CeO2 HTs

To prepare the CNTs@CeO2 HTs, 0.2 g of functionalized CNTs was added to 15 mL of ethanol under sonication. After that, 0.8 g of porous CeO2 HTs was dissolved into the above solution under stirring for 2 h. Then, the composite sample (i.e., CNTs@CeO2 HTs) was centrifuged and dried in a vacuum oven at 80 oC for 12 h. 2.4. Characterizations The surface morphologies and structural properties of the samples were characterized by fieldemission scanning microscope (FE-SEM, Carl Zeiss, LEO SUPRA 55, 5 kV) equipped with an energy-dispersive X-ray (EDX) spectroscopy attachment and a transmission electron microscope (TEM, JEM 200CX, JEOL, 200 kV). The crystallinity and vibrational modes present in the synthesized samples were evaluated by X-ray diffraction (XRD, M18XHF-SRA, Mac Science Ltd. (with a Cu Kα radiation λ = 0.15406)) and a high-resolution Raman (HR-Raman) spectroscopy. Specific surface area, average pore volume, and pore size were obtained with Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) analyses, respectively. 2.5. Preparation of electrodes and electrochemical measurements An easy slurry coating method was used for the fabrication of working electrodes. Briefly, the prepared materials (80 wt%), carbon black (10 wt% ) and PVdF as a binder (10 wt%) were ground uniformly in an agate motor about 30 min. Afterward, the NMP solvent was added to the above mixture, forming a slurry. After that, the obtained slurry was loaded on Ni foam substrate using a brush and kept in oven at 75 oC for 5 h. The electrochemical properties were measured using a three-electrode configuration in a glass beaker at RT. Herein, active material loaded on Ni foam, Ag/AgCl and a platinum wire as working, reference and counter electrodes, respectively.

The

cyclic

voltammetry

(CV),

galvanic

charge-discharge

(GCD)

and

electrochemical impedance spectroscopy (EIS) measurements were performed in 1 M KOH with an IviumStat electrochemical workstation (IVIUM Technologies). For fabricating the device, the prepared CNTs@CeO2 HTs composite as a positive electrode, cellulose paper as a separator and activated carbon as a negative electrode were used in 1 M KOH. The areal capacity (Qa) and areal capacitance (Cac) values were estimated from the charge-discharge cycles using the following equations:[44, 45] =

×∆

(1)

×∆

=

×∆

(2) The areal energy and power densities were estimated using the following equations:[46, 47]

where ‘



=



=



∆

×∆

(3)

(4)

’ is the areal capacity (Ah cm-2), ‘Cac’ is the areal capacitance (F cm-2), ‘I’ is the discharge

current (A), ‘∆t’ is the discharge time (s), ‘A’ is the area of the active electrode (1×1 cm2) , ‘∆V’ is voltage window (V), ‘Ed’ is areal energy density (Wh cm-2) and ‘Pd’ is areal power density (W cm-2), respectively. 3. Results and discussions The schematic diagram for the preparation process of porous CeO2 HTs and CNTs@CeO2 HTs by a cost-effective method with an aid of waste cotton fibers as a template is shown in Fig. 1. The formation of porous CeO2 HTs templated from cotton fibers was involved in the following

steps. During the initial stage of cerium nitrate infiltration, the Ce3+ cations were absorbed on the polar groups-rich (hydroxyl, carboxylic and ester radicals) cotton fibers via van der Waals forces, hydrogen, and coordination bonds.[48, 49] Under constant heating (at 80 oC), the surface of the cotton fibers were fully covered with a cerium precursor by the evaporation of water molecules. At this moment, Ce3+ ions were strongly bounded to the anions of cotton fibers. As the reaction proceeds under thermal treatment, the nucleation and precipitation of ions on the surface of cotton fibers led to the formation of CeO2. To improve the electrical conductivity, the functionalized CNTs were coated on the surface of CeO2 HTs by a solution immersion method under the sonication process. Finally, the obtained CNTs@CeO2 HTs composite was subjected to the centrifugation and dried for further use. The surface morphologies of the CeO2 HTs and CNTs@CeO2 HTs were observed using FESEM images. The low-magnification FE-SEM image of the synthesized CeO2 derived from the waste cotton fibers is shown in Fig. 2(a). From this image, it can be seen that the replicas mimicked the structure of fiber-like cotton. The randomly arranged CeO2 fibrous structures had diameters and lengths of 5-10 µm and 50-100 µm, respectively. From the high magnification (Fig. 2(b)), the prepared material exhibited a tube-like structure (i.e., CeO2 HTs) after eliminating the cotton fibers template. Moreover, the surface of the CeO2 HTs was composed of several voids and pores on their entire surface (Fig. 2(c)). Such a porous and open void structure of CeO2 HTs is beneficial for the easy diffusion of electrolyte ions on their entire surface for better electrochemical reactions. However, the CeO2 HTs had a microscale length and they showed poor electrical conductivity properties. Thus, to further increase the electrical conductivity of CeO2 HTs, the functionalized CNTs were decorated on the surface of CeO2 HTs. Fig. 2(d-f) demonstrate the FE-SEM images of the CNTs@CeO2 HTs under different

magnifications. From these FE-SEM images, it is observed that the CNTs were entirely distributed on the surface of CeO2 HTs. Moreover, the porous nature and void gaps of the CNTs@CeO2 HTs were kept intact even after the sonication process, revealing the good stability of the material. TEM measurement of the sample was also carried out to analyze its structural properties. The TEM image in Fig. 2(g) illustrates the vast cell structure of CeO2 HTs was encapsulated with CNTs. The EDX analysis and elemental mapping images of the prepared composite are shown in Fig. 2(h) and 2(i). The EDX spectrum of CNTs@CeO2 HTs showed the characteristic peaks of Ce, O, and C, where no other elements can be found. Moreover, the elemental mapping images presented in Fig. 2(i) disclosed the homogeneous distribution of Ce, O and C elements over the entire region of CNTs@CeO2 HTs composite. The XRD patterns showed the identification of phase purity and crystalline structure of the prepared CNTs@CeO2 HTs. In Fig. 3(a), all the originated diffraction peaks at 2θ values of 28.5, 33, 47.3, 53.3, 59, 69.3, 76.7 and 79.9° were indexed to the crystal planes of (111), (200), (220), (311), (222), (400), (331), and (420), respectively. These diffraction peaks were in good agreement with the JCPDS #34-0394 of CeO2 without any crystalline impurities. The CNTs peaks were not predominant in the XRD pattern due to the less amount of dispersion over the CeO2 HTs. Further confirmation of the synthesized CNTs@CeO2 HTs was made by HR-Raman analysis, as shown in Fig. 3(b). At 457 cm-1, a sharp peak represents the Eg vibration mode of CeO2 HTs. The peaks observed at 1350 and 1580 cm-1 are related to D- and G-bands, respectively, which are associated with the disordered carbon and in-plane vibrational mode of the C-C band. The Raman analysis verifies the successful formation of CNTs@CeO2 HTs composite. The surface area and porosity analyses of the prepared material were carried out by BET measurement. Fig. 3(c) shows the BET analysis of the prepared CNTs@CeO2 HTs. The N2

adsorption-desorption isotherm shape of the CNTs@CeO2 HTs indicates the type-IV isotherm with an H3-type hysteresis loop, demonstrating the mesoporous behavior of the material. The estimated specific surface area of CNTs@CeO2 HTs was about 66.5 m2 g-1 with a mean pore size of 8.7 nm, which offers large electrochemical active sites for redox reactions. The corresponding BJH plot of the CNTs@CeO2 HTs is displayed in Fig. 3(d). This result shows the multi-pore diameter distribution in the range of 2-200 nm. Such high porosity with a large surface area of CNTs@CeO2 HTs was more favorable for effective electrolyte penetration, accordingly, improved electrochemical properties could be predicted. The electrochemical properties of the prepared CeO2 HTs and CNTs@CeO2 HTs samples were measured using a three-electrode configuration in 1 M KOH solution. Fig. 4(a) displays the comparative CV analysis of the synthesized CeO2 HTs and CNTs@CeO2 HTs materials performed at a constant scan rate of 20 mV s-1. Both the CeO2 HTs and CNTs@CeO2 HTs samples showed mimicked redox peaks within the fixed potential span of 0-0.55 V, which indicates a battery-type electrochemical behavior of the materials. Meanwhile, the CNTs@CeO2 HTs exhibited superior current density response as well as enclosed CV area to the CeO2 HTs, indicating the high capacity response of the composite material. The comparative GCD curves of CNTs@CeO2 HTs and bare CeO2 HTs materials measured at a constant current density of 1 mA cm-2 are presented in Fig. 4(b). The curves showed a non-linear charge-discharge response. This further evidences the battery-type behavior and is well-correlated with the CV curves of the materials. Also, it can be clearly noticeable that the charging and discharging times of the CeO2 HTs were shorter than those of the CNTs@CeO2 HTs, which indicates the superior energy storage performance of the CNTs@CeO2 HTs. Fig. 4(c) shows the EIS analysis of the CeO2 HTs and CNTs@CeO2 HTs samples characterized by the characterized within the frequency range of

100 kHz-0.01 Hz at an open circuit potential. The Nyquist plots of CNTs@CeO2 HTs and CeO2 electrodes were fitted using Zview software. Both Nyquist plots were well-fitted with the given equivalent circuit, which is shown in the inset of Fig. 4(c). The equivalent circuit consists of four components such as Rs, Rct, CPE, and WR, which represent equivalent series resistance, chargetransfer resistance, constant phase element, and Warburg resistance, respectively. From the Nyquist plots, a small semicircle shape obtained in the high-frequency range represents the charge transfer resistance (Rct) followed by the sloped line in the high-frequency region related to the Warburg impedance resulting from the diffusion/transport in the electrolyte. The internal resistance (Rs) of these materials can be estimated from the first intercept of a semicircle with an X-axis. The EIS fitted parameter values of CNTs@CeO2 HTs and CeO2 electrodes are presented in the table (inset of Fig. 4(c)). From the equivalent circuit diagram, the Rs values were estimated to be 0.35 and 1.996 Ω for CNTs@CeO2 HTs and CeO2 HTs, respectively. Although, the Rct values were calculated to be 2.395 and 3.5 Ω for CNTs@CeO2 HTs and CeO2 HTs, respectively. Compared with CeO2 HTs electrode material CNTs@CeO2 HTs electrode exhibited lower Rct value (2.395 Ω). The corresponding equivalent circuit diagram and the obtained all component values table are represented in the inset of Fig. 4(c). Considering the high electrochemical performance of CNTs@CeO2 HTs, we chose this as an optimal sample to further characterize the other energy storage properties. The typical CV curves of the CNTs@CeO2 HTs measured at the scan rates of 10 to 100 mV/s are shown in Fig. 4(d). All CV curves of the CNTs@CeO2 HTs revealed a pair of redox peaks, signifying the stable battery-type electrochemical behavior of the active material. Meanwhile, with an increasing the scan rate from 10 to 100 mV s-1, a slight shift in the potential of anodic and cathodic peaks, which reveal the charge storage mechanism in CNTs@CeO2 HTs mainly involves the diffusion-controlled process by OH- ions.[50] The GCD

measurement of the CNTs@CeO2 HTs electrode was performed at diverse current densities in the range of 1 to 15 mA cm-2, as displayed in Fig. 4(e). The non-linear and symmetric shaped GCD curves were in good agreement with the CV curves, which is due to the good faradaic reversibility of CNTs@CeO2 HTs. The calculated areal capacities of the prepared material are presented in Fig. 4(f). The composite material showed maximum capacities of 70.7, 60.3, 54, 43.1, 32.8 23.7 and 17.45 µAh cm-2 at 1, 2, 3, 5, 7, 10 and 15 mA cm-2, respectively. The capacity values were comparatively higher than those values of the pristine CeO2 HTs, as displayed in Fig. 4(f). The obtained electrochemical capacity of our composite is higher/comparable to the previously reported pseudocapacitive/battery-type materials such as Co3O4, Carbon/CeO2, CuO, β-Co(OH)2, CeO2–RGO and CeO2, as shown in Table S1. The electrochemical characterization of the pristine CeO2 HTs is included in Fig. S1. The cycling durability of CNTs@CeO2 HTs was further investigated by repeating the GCD cycles at 5 mA cm-2. After 2000 cycles, the CNTs@CeO2 HTs composite exhibited high capacity retention of 86.1% than that of the pristine CeO2 HTs (62.9%). The FE-SEM analysis for the CNTs@CeO2 HTs and CeO2 HTs electrodes was also performed after cycling stability tests. The low- and high-magnification FE-SEM images of CeO2 electrode are presented in Fig. S3. Due to the continuous charging and discharging, the outer skeleton of CeO2 microstructures was slightly decomposed into nanoparticles (Fig. S3(a)). However, several pores can still be observed from the high-magnification FE-SEM image presented in Fig. S3(b). On the other hand, the skeleton of microtube can be seen from the low-magnification FE-SEM image of CNTs@CeO2 electrode (Fig. S3(c)). This might be attributed to the knotting of CNTs with microtube skeleton. Moreover, several CNTs without destruction and wide-open pores are also observed on the surface of microtube skeleton even after cycling tests (Fig. S3(d)). Therefore, the CNTs@CeO2

electrode demonstrated superior cycling stability to the CeO2 electrode. The higher electrochemical performance of the CNTs@CeO2 HTs could be mainly due to the following advantages, as demonstrated in Fig. 4(h). Firstly, the porous CeO2 HTs provide convenient diffusion paths for electrolyte ions, which offers extra reaction active sites for redox reactions. Secondly, an encapsulated CNTs on CeO2 HTs improve the electrical conductivity of the material, accelerate the electron transfer phenomenon and allow the entire portion of the material to be available for electrochemical reactions. Thirdly, the high surface area of the composite can effectively relieve the volume expansion during the repetitive cycling process, which leads to decent cycling stability. Additionally, the performance of the CNTs@CeO2 HTs composite material as a positive electrode was tested in a two-electrode system by using an activated carbon-based negative electrode. The schematic diagram was shown in Fig. 5(a) illustrates a simple fabrication of hybrid SC in a pouch-type configuration. The fabricated device was composed of porous CNTs@CeO2 HTs composite and activated carbon-based electrodes with cellulose filter paper as separator and 1 M KOH as an electrolyte. The sandwiching arrangement of these electrodes was inserted and packed using a commercial sealer in a pouch-type bag by adding a few mL of KOH and they were sealed carefully. Prior to the device assembly, the mass of activated carbon (AC) loaded on Ni foam (AC/Ni foam) was estimated using the following equation in order to balance the charges between both electrodes using following equation:[41] =

where ‘

’ and ‘

capacitance, ‘∆

×∆

’is are the mass and areal capacity of a positive electrode, ‘ ’ and ‘

(5)

’ is the areal

’ are the voltage and mass of the negative electrode, respectively.

The electrochemical properties of AC/Ni foam were characterized in three-electrode configuration and the corresponding graphs are presented in Fig. S2. Considering the potential windows of both positive (0 to 0.55 V) and negative (-1 to 0 V) electrode materials, the optimum potential window of fabricated hybrid SC could be predicted to be 1.55 V. Fig. 5(b) displays the CV plots of the hybrid SC device measured with the optimized voltage i.e., 0-1.55 V at diverse scan rates of 10-100 mV s-1, respectively. As shown in Fig. 5(c), it is revealed that the assembled device exhibited both battery-type and EDLC behaviors, indicating the good balancing of charges on both the electrodes. The enclosed CV area and current response under increased scan rates further confirm the good reversibility of the device without any evaluation. Also, the device can even show a dual capacitive nature at a high scan rate, representing the superior rate capability. The GCD plateaus inspected at diverse current densities (from 1 to 20 mA cm-2) are presented in Fig. 5(d) and the charge storage behavior of these curves gives good accordance with the CV results. The calculated areal capacitance values of hybrid SC using Eq. (2) were plotted against the various current densities, as displayed in Fig. (S2). The hybrid SC exhibited maximum areal capacitances of 123.1, 110.3, 101.2, 91.1, 83, 76.7, 69.4, and 58.2 mF cm-2 at 1, 2, 3, 5, 7, 10, 15 and 20 mA cm-2, respectively. Moreover, the areal energy density and areal power density of the device were estimated using Eqs. (3) and (4) and plotted in the Ragone plot as presented in Fig. 5(e). Utilizing the combined features of both the electrodes, the hybrid SC delivered a maximum areal energy density of 0.041 mWh/cm-2 at a power density of 0.77 mW cm-2. Even at a high power density (14.32 mW cm-2), the device demonstrated a maximum energy density of 0.016 mWh cm-2, respectively. The cycling stability of the fabricated device is an essential factor for a practical demonstration. Accordingly, the long-life stability test of the fabricated hybrid SC device was examined by a continuous charge-discharge analysis at a fixed

current density of 5 mA cm-2, as displayed in Fig. 5(f). After 2000 cycles, the hybrid SC device exhibited high capacitance retention of 86.4%, indicating superior cycling stability. The first 10 charge-discharge cycles with almost symmetric nature during cycling stability tests were illustrated in the inset of Fig. 5(f). Furthermore, the real-time practical applicability of the fabricated device was also tested by connecting two devices in series connection, as presented in the inset of Fig. 5(e). The as-connected two devices effectively glowed the commercially available truck boat fog light-emitting diode (LED) and blue LED for several minutes, as displayed in Fig. 5(g).

4. Conclusion In summary, the CNTs@CeO2 HTs were prepared via a facile and cost-effective biomorphic solgel method. Herein, the waste cotton fibers were used as a template, and CNTs were used to improve the electrical conductivity of the CeO2 HTs. The prepared materials demonstrated a battery-type behavior in 1 M KOH electrolyte. Particularly, the porous CNTs@CeO2 HTs composite exhibited a higher areal capacity of 70.7 µAh cm-2 than that of the CeO2 HTs 41.9 µAh cm-2 at 1 mA cm-2 with good cycling stability. Moreover, the hybrid SC was assembled with the CNTs@CeO2 HTs and activated carbon as positive and negative electrode materials, respectively using a cellulose filter paper as a separator in 1 M KOH. The constructed hybrid SC delivered maximum areal energy and power densities of 0.041 mWh cm-2 and 14.32 mW cm-2, respectively. Also, the device exhibited superior cycling stability with an excellent capacity retention of 86.4% after 2000 cycles. From these results, the improved electrochemical properties in the present work demonstrated a new potency for the fabrication of cost-effective and highly conductive composites for positive electroactive materials in energy storage systems.

Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2017R1A2B4011998 and No. 2018R1A6A1A03025708). Supporting information The Supporting information contains electrochemical properties of CeO2 HTs, FE-SEM image and electrochemical measurements of activated carbon, FE-SEM images of both electrodes after cycling and comparison table of electrochemical performances of the CNTs@CeO2 HTs with the previously reported literature.

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Fig. 1. Schematic illustration for the preparation of the CeO2 HTs and CNTs@CeO2 HTs.

Fig. 2. FE-SEM images of the prepared materials: (a-c) CeO2 HTs and (d-f) CNTs@CeO2 HTs. (g) TEM image, (h) EDX and (i) elemental mapping images of the CNTs@CeO2 HTs.

Fig. 3. (a) XRD pattern, (b) HR-Raman spectrum, (c) surface area and (d) pore distribution analysis of the CNTs@CeO2 HTs composite.

Fig. 4. Electrochemical properties of the CeO2 HTs and CNTs@CeO2 HTs in 1 M KOH electrolyte. Comparative (a) CV, (b) GCD and (c) EIS curves of the CeO2 HTs and CNTs@CeO2 HTs electrodes at a constant scan rate and current density, respectively. (d) CV and (e) GCD curves of the prepared CNTs@CeO2 HTs at different scan rates and current densities. (f) Calculated areal capacity values as a function of current density, (g) cycling stability and (h) schematic representation of electrochemical merits of the CeO2 HTs and CNTs@CeO2 HTs composites.

Fig. 5. (a) Schematic diagram of the constructed hybrid SC in a pouch-type configuration. (b) CV and (c) GCD curves of the hybrid SC measured at different scan rates and current densities under the constant cell potential of 0-1.55 V. (d) Calculated areal capacitance values vs. various applied current densities of the device. (e) Ragone plot, (f) cycling stability and (g) photographs of two hybrid SCs connected in series for practical applications.

Table of Contents Entry

Synopsis. Highly porous CNTs@CeO2 HTs as a cost-effective composite battery-type materials have been employed for hybrid supercapacitor.

Highlights CeO2 hollow tubes (HTs) are facilely prepared using waste cotton fibers as template The obtained HTs consist of numerous pores on their surface CNTs are decorated on CeO2 HTs (CNTs@CeO2 HTs) to improve electrical conductivity CNTs@CeO2 HTs delivered the areal capacity of 70.7 µAh/cm2 at 1 mA/cm2 Hybrid supercapacitor demonstrated the superior energy storage performance

Declaration of Interest statement The authors declared no conflicts of interest