Voltage holding and self-discharge phenomenon in ZnO-Co3O4 core-shell heterostructure for binder-free symmetric supercapacitors

https://doi.org/10.1016/j.cej.2021.131895Get rights and content

Highlights

  • The ZC-CSH SSC exhibits a high specific capacitance of 177.0F g−1.

  • It elucidates high energy (39.3 W h kg−1) and power density (19064.3 W kg−1).

  • It reveals stability 92.8 & 96.1 % using 10,000 cycles and 16 h voltage holding and self-discharge.

  • It displays a trivial leakage current of 0.06 mA during first 2 h voltage holding tests.

  • It represents the good health of the ZC-CSH SSC during the 8 h self-discharge.

Abstract

We report an eco-friendly, in-situ, and one-step synthesis of ZnO-Co3O4 core-shell heterostructure (ZC-CSH) using the hydrothermal process as a transcendent nanomaterial for the supercapacitor applications. The ZC-CSH SSC showed a wide potential window (1.6 V), the excellent specific capacitance of 177.0F g−1 at 1.4 A g−1, high energy density (39.3 W h kg−1), and power density (19064.3 W kg−1). Further, the ZC-CSH SSC revealed excellent stability of 92.8 % after 10,000 cycles at 12.4 A g−1 using galvanostatic charging-discharging. Besides, the ZC-CSH SSC unraveled the outstanding stability of 96.1 % after the 8 h Voltage holding tests (VHT) at 1.6 V + 8 h Self-discharge tests (SDT). Moreover, the ZC-CSH SSC indicated a trivial leakage current of 0.06, 0.11, 0.15, and 0.17 mA during 2, 4, 6, and 8 h VHT, respectively. The ZC-CSH SSC demonstrated a voltage drop from 1.6 V to 0.39, 0.38, 0.37, and 0.36 V after 2, 4, 6, and 8 h VHT and SDT. To understand the ZC-CSH SSC's self-discharge behavior, this work explored the insights of the self-discharge mechanisms based on two thermodynamic processes, ionic concentration gradient (diffusion-control model) and potential difference (potential-driven model). Also, according to the tight-bonding (strong interactions) and loose-bonding (weak interactions), this work envisaged electrolyte ions' interactions with electrode materials to explore the coherent insights of the self-discharge behavior of the ZC-CSH SSC. It is concluded that this approach can lead to an unwavering performance of the ZC-CSH SSC for electronic portable futuristic gadgets.

Introduction

Recently, due to rapidly increasing tech innovations in various fields such as space exploration, the Moon missions, the Mars missions, portable electronic gadgets, electric vehicles, and military radars, there is a need for high-performance, reliable, long life, and fast chargeable energy storage systems [1], [2], [3]. On the other hand, the depletion of fossil fuels raises severe global ecological and energy concerns. Therefore, the universal energy paradigm is swiftly bending from fossil fuels to sustainable renewable energy storage sources [4]. However, the power generation from sustainable energy sources is still lagging behind the required energy in electric vehicles and portable electronic gadgets, making it challenging to find out a reliable solution to this task. Supercapacitors, a well-known electrochemical capacitor from the family of batteries, have triggered remarkable research interests because of their features cost-effectiveness, fast charging ability, high power density, and long cycle span [5], [6], [7]. As well, several factors influence the supercapacitors' electrochemical properties in which electrode materials choice is one of them and has significant importance. The metal oxide semiconductors have been extensively studied due to their richness in the earth's crust, inexpensive and non-toxic but desperately suffer from low energy density, electronic and ionic conductivity [8], [9]. To overcome these limitations, suitable and unique heterostructures need to be developed, which can fulfill insights to boost surprisingly the electrode materials properties, such as electrical, ionic conductivities, and reduce the conduction pathways during electrochemical investigations. Therefore, electrode materials have vital significance for the fabrication of high-performance electrochemical devices.

Zinc oxide (ZnO) has a wide bandgap (3.2–3.4 eV), high mobility (130–200 cm2 V−1 s−1), and the conduction band edge (−4.36 eV), which are suitable as electron transporters in various applications [10]. Moreover, ZnO is an n-type and irreducible oxide semiconductor that shows one oxidation state (+2), illustrates positive valance band position, and robust oxidizing ability [11]. The ZnO attracted vast attention due to its phase stability at high temperature (~1800 °C) and Young’s modulus (~150 GPa) [12]. Additionally, ZnO has received remarkable attention due to its wide range of applications in chemical, bio-sensors, organic light-emitting diodes, water splitting, and dye-sensitized solar cells [12], [13], [14], [15], [16], [17]. As a result, ZnO is emerging as outstanding electrode material for electrochemical capacitor applications [18]. However, ZnO-based supercapacitors suffer from low specific capacitance and energy density, and short cycle life. The pseudocapacitance of electrochemical capacitor devices arises from faradaic redox reactions on the electrode material's surface through continuous intercalation/deintercalation of electrons and ions, which results in a short life cycle due to the surface destruction [19]. Therefore, exclusive and practical approaches have been instituted to increase the electrochemical performance of supercapacitors, included doping of different metals, heteroatom doping, morphological engineering such as hierarchical, core-shell, nanocomposites, which could make them desirable candidates for electrode materials. Besides, the Co3O4 nanostructure is a premium nanomaterial due to its prominent theoretical capacitance of 3560F g−1, eco-friendly, p-type optical bandgap, and excellent electrochemical behaviour [19]. However, the experimental specific capacitance of Co3O4 does not become comparable to its theoretical capacity due to its slow electrical conduction and short-term stability during galvanostatic charging-discharging [20]. Therefore, the combination of these two nanomaterials ZnO and Co3O4 can be useful to improve the energy density and stability of supercapacitors, due to their excellent properties and vast applications. Significantly, the in-situ synthesis of a hierarchical core-shell heterostructure is favorable for growing the specific surface area and generating the nano junctions between the core (ZnO) and shell (Co3O4) nanostructures, which can lead to cultivating the properties and performance of the electrochemical capacitors. Therefore, the in-situ building of a hierarchical ZnO-Co3O4 core-shell heterostructure can be a promising candidate to improve the supercapacitor's overall electrochemical performance.

Recently, tremendous attention has been made to the ZnO-Co3O4 core-shell heterostructure for supercapacitor applications. Shaheen et al. demonstrated ZnO-Co3O4 nanocomposite for supercapacitor applications [18]. The supercapacitor demonstrated a power density (7.5 kW kg−1), energy density (4.1 W h kg−1), and specific capacitance (165F g−1). Gao et al. studied the ZnO-coated Co3O4 nanorod-based asymmetric supercapacitor for energy storage applications [21]. The ZnO-coated Co3O4 nanorod-based supercapacitor exhibited high power density (7500 W kg−1), energy density (47.7 W h kg−1), and specific capacitance (1135F g−1 at 1 A g−1). Furthermore, it clarified the long life of 83% after 5000 charging-discharging cycles at 10 A g−1. Cai et al. studied the ZnO@Co3O4 core/shell heterostructures for electrochemical supercapacitor applications [22]. It was observed that the prepared electrode reveals high specific capacitance (857.7F g−1 at 1 A g−1). It was also investigated that the ZnO@Co3O4 core/shell heterostructure retains its high specific capacitance (830F g−1) even after 6000 cycles using charging-discharging at 6 A g−1. Zhu et al. discussed the metal azolate framework-derived ZnO/Co3O4 for electrode materials [23]. The ZnO/Co3O4 electrode demonstrated high power density (1.8 kW kg−1), energy density (24.3 W h kg−1), and specific capacitance (830.2F g−1 at 1 A g−1), and excellent durability (89 % after 1000 cycles using charging-discharging at 1 A g−1). Hu et al. studied the ZnO/Co3O4 Nano-bundle arrays (NBs), and Stereotaxically constricted graphene (SCG) nanomaterials based on two-terminal configurations of Asymmetric supercapacitor (ASC) [24]. The ZnO/Co3O4 NBs-1//SCG-ASC depicted the high power density of 18820 W kg−1, the specific capacitance of 198F g−1, the energy density of 70.4 W h kg−1, and admirable durability (86.5 %) after 5000 cycles using charging-discharging.

However, the lack of study on several other significant and urgent issues [like stability using voltage holding tests (VHTs), Leakage current (LC), and Self-discharge test (SDT)] in the above-discussed supercapacitors is necessary to examine. Because, these parameters have not received much attention in the rapidly increasing literature on supercapacitors. A considerable realm of self-discharge of a supercapacitor in a relaxation state considers unreliable, which makes it inappropriate during prolonged use for specific and essential purposes. Consequently, studying the mechanism behind the phenomenon of leakage current during VHTs and SDT of supercapacitors, and after that, how it can be inhibited, is extremely important for its commercial reliability.

This work unravels the Voltage holding tests (VHTs), leakage current, and Self-discharge test (SDT) of ZnO-Co3O4 core-shell heterostructures (ZC-CSH) based symmetric supercapacitor (ZC-CSH SSC). The ZC-CSH SSC illustrated a wide potential window (1.6 V), excellent power density, high energy density, specific capacitance, and impressive stability of 92.8 % after 10,000 GCD cycles and 96.1 % after 8 h VHT + 8 h SDT. The ZC-CSH SSC shows minimum leakage currents of 0.06, 0.11, 0.15, and 0.17 mA during 2, 4, 6, and 8 h VHT. The ZC-CSH SSC represented voltage drop from 1.6 V to 0.39, 0.38, 0.37, and 0.36 V after 2, 4, 6, and 8 h VHT and SDT. The self-discharge mechanism also interpreted the insights of the self-discharge mechanisms by potential driven and diffusion-controlled processes.

Section snippets

Materials synthesis

Ni foam substrate was first cleaned with (CH3)2CO, C2H5OH, and deionized (DI)-H2O for 15 min each by using an ultrasonicator. In the first step, cobalt nitrate hexahydrate (4 mmol), zinc nitrate hexahydrate (2 mmol), ammonium fluoride (8 mmol) decomposed in 60 mL of DI-H2O under magnetic stirring to prepare a homogeneous mixture. Further, 8 mmol of urea was mixed in the prepared mixture under vigorous stirring. In the second step, the prepared homogenous solution mixture was transferred into

Morphological, elemental, and structural studies

Scheme 1 shows a schematic representation of in-situ growth on 3D Ni foam of the ZnO, Co3O4 nanostructure, and ZC-CSH. The Ni-foam image, FESEM images, and crystallographic illustrations of the ZnO, Co3O4 nanostructure, and ZC-CSH are shown in Scheme 1. A comprehensive synthesis process is described in the appropriate section of the experimental procedure.

FESEM image shows the uniform deposition of ZnO-Co3O4 core-shell heterostructure (ZC-CSH) on Ni-foam as a current collector substrate which

Conclusions

In conclusion, we studied the stability using the voltage holding tests and insights of the self-discharge mechanism of ZnO-Co3O4 core-shell heterostructures (ZC-CSH) SSC. As a result, the ZC-CSH SSC reveals a wide voltage window, high power density, specific capacitance, and energy density. More importantly, the ZC-CSH SSC unveils excellent stabilities of 92.8 % and 96.1 % by 10,000 GCD cycles and 8 h VHT, respectively. The ZC-CSH SSC discloses outstanding reproducibility, which was

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The Basic Science Research Program supported this work by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning, South Korea (Grant No. 2019R1D1A3A03103662). SDU authors acknowledge the funding by Interreg Deutschland–Denmark with money from the European Regional Development Fund, project number 096-1.1-18 (Access and Acceleration) and from the ESS lighthouse on hard materials in 3D, SOLID, funded by the Danish Agency for Science and Higher

References (62)

  • Z. Wang et al.

    Extremely low self-discharge solid-state supercapacitors via the confinement effect of ion transfer

    J. Mater. Chem. A

    (2019)
  • C. Hitz et al.

    Experimental study and modeling of impedance of the her on porous Ni electrodes

    J. Electroanal. Chem.

    (2001)
  • H. Chen et al.

    Core-shell assembly of Co3O4@NiO-ZnO nanoarrays as battery-type electrodes for high-performance supercapatteries

    Inorg. Chem. Front.

    (2019)
  • J. Xu et al.

    Co3O4/ZnO nanoheterostructure derived from core-shell ZIF-8@ZIF-67 for supercapacitors

    RSC Adv.

    (2016)
  • W.G. Pell et al.

    Electrochemical efficiency in multiple discharge/recharge cycling of supercapacitors in hybrid EV applications

    J. Power Sources

    (1999)
  • J. Black et al.

    Effects of charge redistribution on self-discharge of electrochemical capacitors

    Electrochim. Acta

    (2009)
  • C. Yu et al.

    Microstructure design of carbonaceous fibers: a promising strategy toward high-performance weaveable/wearable supercapacitors

    Small

    (2020)
  • Y. Wang et al.

    A flexible, electrochromic, rechargeable Zn-ion battery based on actiniae-like self-doped polyaniline cathode

    J. Mater. Chem. A

    (2020)
    A. Sanger et al.

    Silicon carbide nanocauliflowers for symmetric supercapacitor devices

    Ind. Eng. Chem. Res.

    (2016)
  • B. Shen et al.

    Enhanced energy-storage performance of an all-inorganic flexible bilayer-like antiferroelectric thin film via using electric field engineering

    Nanoscale

    (2020)
    J.S. Shaikh et al.

    A phosphorus integrated strategy for supercapacitor: 2D black phosphorus-doped and phosphorus-doped material

    Mater. Today Chem.

    (2021)
  • C. Ogata et al.

    All-graphene oxide device with tunable supercapacitor and battery behaviour by the working voltage

    Chem. Commun.

    (2016)
    A. Kumar et al.

    Performance of high energy density symmetric supercapacitor based on sputtered MnO2 nanorods

    ChemistrySelect

    (2016)
  • Q. Zhang et al.

    Rapid and controllable synthesis of nanocrystallized nickel-cobalt boride electrode materials via a mircoimpinging stream reaction for high performance supercapacitors

    Small

    (2020)
  • S. Kandambeth et al.

    Covalent organic frameworks as negative electrodes for high-performance asymmetric supercapacitors

    Adv. Energy Mater.

    (2020)
    R. Yuksel et al.

    Necklace-like nitrogen-doped tubular carbon 3D frameworks for electrochemical energy storage

    Adv. Funct. Mater.

    (2020)
  • M. Huang et al.

    MnO2-based nanostructures for high-performance supercapacitors

    J. Mater. Chem. A

    (2015)
  • J. Xu et al.

    Surface engineering of ZnO nanostructures for semiconductor-sensitized solar cells

    Adv. Mater.

    (2014)
  • Y. Jia et al.

    A synergistic effect between S-scheme heterojunction and noble-metal free cocatalyst to promote the hydrogen evolution of ZnO/CdS/MoS2 photocatalyst

    Chem. Eng. J.

    (2021)
  • Y.K. Mishra et al.

    ZnO tetrapod materials for functional applications

    Mater. Today

    (2018)
  • N. Luo et al.

    High-sensitive mems hydrogen sulfide sensor made from PdRh bimetal hollow nanoframe decorated metal oxides and sensitization mechanism study

    ACS Appl. Mater. Interfaces

    (2020)
  • C.-L. Hsu et al.

    Nonenzymatic glucose sensor based on Au/ZnO core−shell nanostructures decorated with au nanoparticles and enhanced with blue and green light

    J. Phys. Chem. B

    (2017)
  • D. Kim et al.

    Nanosinusoidal surface zinc oxide for optical out-coupling of inverted organic light-emitting diodes

    ACS Photonics

    (2018)
  • I. Shaheen et al.

    Green synthesis of ZnO–Co3O4 nanocomposite using facile foliar fuel and investigation of its electrochemical behaviour for supercapacitors

    New J. Chem.

    (2020)
  • S. Adhikari et al.

    Encapsulation of Co3O4 nanocone arrays via ultrathin NiO for superior performance asymmetric supercapacitors

    Small

    (2020)
  • Cited by (47)

    View all citing articles on Scopus
    View full text