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Pseudocapacitive Charge Storage in Thin Nanobelts

  • Ria Kunwar
  • Midhun Harilal
  • Syam G. Krishnan
  • Bhupender Pal
  • Izan Izwan Misnon
  • C. R. Mariappan
  • Fabian I. Ezema
  • Hendry Izaac Elim
  • Chun-Chen Yang
  • Rajan JoseEmail author
Research Article
  • 85 Downloads

Abstract

This article reports that extremely thin nanobelts (thickness ~ 10 nm) exhibit pseudocapacitive (PC) charge storage in the asymmetric supercapacitor (ASC) configuration, while show battery-type charge storage in their single electrodes. Two types of nanobelts, viz. NiO–Co3O4 hybrid and spinal-type NiCo2O4, developed by electrospinning technique are used in this work. The charge storage behaviour of the nanobelts is benchmarked against their binary metal oxide nanowires, i.e., NiO and Co3O4, as well as a hybrid of similar chemistry, CuO–Co3O4. The nanobelts have thickness of ~ 10 nm and width ~ 200 nm, whereas the nanowires have diameter of ~ 100 nm. Clear differences in charge storage behaviours are observed in NiO–Co3O4 hybrid nanobelts based ASCs compared to those fabricated using the other materials—the former showed capacitive behaviour whereas the others revealed battery-type discharge behaviour. Origin of pseudocapacitance in nanobelts based ASCs is shown to arise from their nanobelts morphology with thickness less than typical electron diffusion lengths (~ 20 nm). Among all the five type of devices fabricated, the NiO–Co3O4 hybrid ASCs exhibited the highest specific energy, specific power and cycling stability.

Keywords

Battery-supercapacitor hybrid devices Energy storage materials Hybrid materials 

Introduction

Supercapacitors (SC), which are broadly classified into symmetric and asymmetric supercapacitors, show promising energy storage properties due to their high specific power (PS) and cycle life compared to batteries [1]. Among them, asymmetric supercapacitors (ASCs) (synonymously called battery-supercapacitor hybrid, BSH) are characterized by higher specific energy (ES) than symmetric supercapacitors (SSCs). This is because one of the electrodes of ASCs is pseudocapacitive (PC) or battery-type of larger charge storability, whereas both are electric double layer capacitive (EDLC) in the SSCs [2]. The charge storage mechanism in EDLC, PC, and battery-type can be simply described as charge adsorption, charge intercalation in a surface monolayer, and deep intercalation to the bulk of the electrode, respectively [3, 4]. Consequently, they show an ascending order of specific capacitance (CS) and ES although their PS and cycle life follow a descending order. The CS of the electrodes, and ES of the device thereby, could be further increased by increasing the specific surface area and electrical conductivity of the electrode materials [5]. Due to their surface related charge storage, PC electrodes have gained considerable attention recently [6, 7, 8, 9]. The most widely investigated PC materials for SC applications are MnO2, RuO2, and conducting polymers, which give equal CS values at equal potential intervals from cyclic voltammetry data [10, 11, 12]. We have recently reported hybrid nanowires of CoO–MnO2–MnCo2O4 by appropriately increasing the electrical conductivity of ternary metal cobaltites, which otherwise show battery-type charge storage [7].

There has been considerable debate recently in SC research on whether an electrode is PC or battery-type [7, 10, 13, 14, 15]. Electrodes showing nonlinear voltammetric current with potential and galvanostatic potential with time are routinely characterized as a battery-type material, whereas PC electrode materials do not show such nonlinearities [10]. The charge storage in battery-type electrodes is through deep intercalation, whereas the intercalation is limited to the surface layer in PC electrodes. ASCs can be fabricated either using a battery-type//EDLC or PC//EDLC combination of electrodes. By analysing whether the shape of discharge curve is linear (capacitive) or with a plateau (battery-type), the charge storage mechanism of device could be differentiated. Similarly, the redox peaks in cyclic voltammetry curves characterize the battery-type charge storage. The charge storage is expressed differently in each case, the CS is expressed in F g−1 for capacitive devices and in C g−1 or mA h g−1 for battery-type. The ASC devices utilising the battery-type electrodes have the disadvantages of low power density and cycle life [6, 16]. There have been numerous reports of ASC devices based on battery-type electrode materials expressed in F g−1, and very few reports on ASCs fabricated using PC electrodes. Detailed analysis of ASCs based on battery-type electrodes is reported in a recent article [17].

This article demonstrates that thin nanobelts, which show battery-type behaviour in single electrodes, offer PC charge storage in ASCs. Nanobelts of two materials, viz. NiO–Co3O4 hybrid (HNBs) and spinal-type NiCo2O4, developed by electrospinning technique were used in this work. On the other hand, the component electrodes of HNBs (i.e., NiO and Co3O4 nanowires) as well as a similar hybrid but nanowires instead of nanobelts (CuO–Co3O4 hybrid nanowires [18]) showed nonlinear voltammetric currents and galvanostatic potentials. The HNBs showed clearly different charge storage behaviour compared with the others. The PC properties are attributed to a lower thickness of the HNBs (10 nm), which is lower than typical electron diffusion lengths (~ 20 nm) in metal oxide electrodes.

Experimental Details

The NiO–Co3O4 hybrid and NiCo2O4 nanobelts as well as CuO–Co3O4 hybrid, NiO, CuO, and Co3O4 nanowires were prepared by electrospinning as reported before [18, 19]. The hybrid materials were synthesized from an equal molar ratio of their component precursors and stoichiometric ratios were used for the synthesis of NiCo2O4. Crystal structure, phase, morphology, and surface property of the materials have been reported in previous publications [17, 18].

For electrochemical energy storage studies, the electrodes were prepared as explained in the previous publications [17, 18]. Briefly, the slurry was prepared by mixing the electrode material with polyvinylidenefluoride (PVDF) (Sigma Aldrich, USA) and carbon black (Super P conductive, Alfa Aesar, UK) in the ratio of 75:10:15. The slurry was homogenized by adding N-methyl-2-pyrrolidinone (NMP) and stirred for 24 h, coated on pre-cleaned nickel substrate (area ~ 1 cm2) and dried at 60 °C and finally compacted at a pressure of 5 ton using a hydraulic press. The electrode material loaded on the electrode was ~ 2.4 mg. The metal oxide electrodes were used as the positive electrode and activated carbons (AC) synthesized from palm kernel shells [20, 21] were used as the negative electrodes in the ASCs. The coin type cells were fabricated as explained previously [18].

The charge storage properties were evaluated in 6 M KOH by means of cyclic voltammetry (CV), charge–discharge cycles (CDC), electrochemical impedance spectroscopy (EIS) measurements employing a potentiostat–galvanostat (PGSTAT M101, Metrohm Autolab B.V., The Netherlands). The impedance spectra were recorded in the frequency range between 100 kHz–0.01 Hz at the open circuit potential of the electrolyte.

Results and Discussion

Detailed characterizations of the NiO–Co3O4, CuO–Co3O4, NiCo2O4, NiO, CuO, and Co3O4 are reported elsewhere [18, 19]. The purpose of the present paper is to highlight the difference in their charge storage behaviours when these materials are used as the positive electrode in ASCs. For the sake of simplicity, Fig. 1 displays the FESEM images of NiO–Co3O4, NiCo2O4, and CuO–Co3O4. Obviously, the NiO–Co3O4 and NiCo2O4 display nanobelt morphology whereas the other show conventional nanowire morphology. The nanobelts are of average thickness ~ 10 nm and average width ~ 200 nm as seen in Fig. 1a–d. The nanowires are of average diameter ~ 60 nm as observed in Fig. 1e, f. The morphologies of hybrids and parent compound are further examined using TEM and the images shown in Fig. 2a–d. These images confirm the diameter of nanostructures and show that they are composed of densely packed particles with size ranging from 10 to 20 nm. Figure 2b is a high resolution transmission electron micrograph of NiO–Co3O4 hybrid nanobelts in which the grains are identified for NiO and Co3O4, which is further confirmed using selected area electron diffraction pattern. Furthermore, the crystal structures of all three materials are analysed using XRD (Fig. 3). XRD peaks in Fig. 3a are indexed to face centred cubic of NiO (PDF card No. 10711179) and spinel Co3O4 (PDF card No. 653103) confirming the NiO–Co3O4 hybrid formation. Figure 3b shows the XRD peaks of NiCo2O4, which follow the JCPDF card No. 20-0781. Similarly Fig. 3c confirms the CuO–Co3O4 hybrid formation.
Fig. 1

FESEM images of a, b NiO–Co3O4 hybrid nanobelts; c, d NiCo2O4 nanobelts; e, f CuO–Co3O4 hybrid nanowires

Fig. 2

a Bright field image of NiO–Co3O4 hybrid nanobelts; b lattice of NiO–Co3O4 hybrid nanobelts showing Co3O4 and NiO grains. Inset: corresponding electron diffraction pattern indexed for Co3O4 and NiO; c bright field image of NiCo2O4 nanobelts; d lattice of NiCo2O4 nanobelts. Inset: corresponding electron diffraction pattern; e bright field image of CuO–Co3O4 hybrid nanowires; f lattice of CuO–Co3O4 hybrid nanowires. Inset: corresponding electron diffraction pattern indexed for Co3O4 and CuO

Fig. 3

XRD patterns of a NiO–Co3O4 hybrid nanobelts showing peaks of NiO (#) and Co3O4 (*); b NiCo2O4 (filled square); and c CuO–Co3O4 hybrid nanowires peaks of CuO (open circles) and Co3O4 (*)

Detailed electrochemical characteristics of the NiO–Co3O4, CuO–Co3O4, and NiCo2O4 electrodes are reported in recent publications [18, 19]. For a quick comparison of the electrochemical properties of the electrodes (area of ~ 1 cm2 and active material loading with ~ 2.5 mg cm−2) under the present discussion, their CDC and CV data in 6 M KOH electrolyte are presented in Fig. 4a, b. Figure 4a shows the discharge curves of the electrodes. Obviously, all of them showed battery-type behaviour in this electrolyte irrespective of their morphology [10, 13]. Furthermore, the NiO–Co3O4 hybrid nanobelts showed longer discharge time than the others, thereby indicating its superiority in the charge storage ability. Similar observations can be made from CV curves (Fig. 4b) also: (1) presence of redox peaks make all of them battery-type electrodes in the chosen electrolyte and (2) the area under CV curve is higher for the hybrid nanobelts. Additional advantages of higher surface area and shorter ionic diffusion path due to the belt morphology are shown to be the reason behind the better electrochemical performance of nanobelts over nanowires [18, 22, 23, 24, 25].
Fig. 4

a Discharge curves of NiO–Co3O4, NiCo2O4 and CuO–Co3O4 at a current density of ~ 1 A g−1; b CV curves of NiO–Co3O4, NiCo2O4 and CuO–Co3O4 at a scan rate of 2 mV s−1

Dramatic difference in the charge storage mechanisms were observed when ASCs were fabricated using the above battery-type electrodes as positive electrodes and palm kernel shell AC as negative electrodes. The ASCs are generally considered as two capacitors in series; therefore, both electrodes should have similar capacitance for the maximum equivalent capacitance. This demands a charge balance in the two electrodes, which is usually achieved by adjusting the mass of the electrodes such that q+ = q, where q+ refers to the charge stored at the positive electrode and q- to that at the negative electrode. The accumulated charge over each electrode is given by \(q = {\text{C }}_{s} \times \Delta {\text{V }} \times {\text{m,}}\) where ΔV is the potential window. Following these considerations, the electrode mass loading for optimal performance could be obtained from the Eq. (1) [26]:

$$\frac{{{\text{m}}^{ + } }}{{{\text{m}}^{ - } }} = \frac{{{\text{C}}_{\text{s }} \left( {\text{EDLC}} \right)\; \times \;\Delta {\text{V}}^{ - } }}{{{\text{C}}_{\text{s }} \left( {\text{PC}} \right)\; \times \;\Delta {\text{V}}^{ + } }}.$$
(1)
Figure 5a shows the discharge curves of NiO–Co3O4//AC, NiCo2O4//AC, NiO//AC, Co3O4//AC, and CuO–Co3O4//AC devices. Interestingly, the devices employed the nanobelts, i.e., NiO–Co3O4//AC and NiCo2O4//AC showed a linear potential drop unlike other devices, suggesting PC behaviour [10, 13]. On the other hand, discharge curves of NiO//AC, Co3O4//AC, and CuO–Co3O4//AC devices, whose morphologies of the positive electrodes were solid cylindrical nanowires, displayed clear battery-type behaviour (nonlinear potential decay). Interestingly, the discharge curve of the NiCo2O4//AC agreed only on average with that of the NiO–Co3O4//AC, and the former showed a slightly different discharge behaviour. The NiO//AC, Co3O4//AC, and CuO–Co3O4//AC devices showed a clear two-stage discharge behaviour whereas the discharge was smoothed out in NiCo2O4//AC and a clear linear behaviour was observed in the device based on the hybrid device. The Cs of the ASCs determined from the CDC curves are shown in Fig. 5b. The two devices fabricated using the composite electrodes (NiO–Co3O4 and CuO–Co3O4) show higher values than the others. This superiority could be attributed to the synergistic combination of highly conducting materials (NiO and CuO) and high electrochemical charge storability (Co3O4) [18, 19]. Similar strategy of combining properties of two materials in a single electrospun nanowire for obtaining enhanced performance has recently been successfully demonstrated in dye-sensitized solar cells, perovskite solar cells, and catalysis [27, 28, 29, 30]. In Fig. 5c, the CV curves of the ASCs devices developed in this study are compared. Evidently, the CV curves of the devices fabricated using the solid nanowires showed an asymmetry in the positive and negative electrode regions despite adjusting the masses for equal charge storability. We have recently addressed this issue on equating the charge storage by an unequal mass-loading on both electrodes through a trial and error method[26]. However, in the present case even with the conventional charge balance method, a near-symmetrical of charge storage has been observed in devices fabricated using the nanobelts. Similar to CDC results, a potential window of 1.6 V was shown by all ASC devices, while the cathodic and anodic currents and total area of the CV curves (Fig. 5c) were much higher for the devices fabricated using the hybrid materials, thereby indicating the superiority over the others. Variations of capacitance with scan rate for all ASC devices in the present study are shown in Fig. 5d. The NiO–Co3O4//AC shows superior values than the other devices.
Fig. 5

a Discharge curves of ASCs at a current density of ~ 1 A g−1; b variation of CS with current density calculated from discharge curves; c CV curves of ASCs at a scan rate of 2 mV s−1; d variation of CS with scan rate calculated from CV curves

Thus obviously, despite similar battery-type behaviour in a single electrode characteristic, the nanobelts morphology offers PC behaviour in ASCs. This topic of whether an electrode is PC or not has been under intense discussion recently [7, 10, 13, 14, 15]. It has been suggested that the best avenue to decide whether or not an electrode is PC is fabricating ASCs and analysing their CDC and CV curves. In the ASCs, if the CDC shows linear decay with time or CV shows linear current with voltage, an electrode can be considered PC [7, 10, 13, 14, 15]. Thus, the nanobelts electrodes offer pseudocapacitance whereas the nanowire electrodes are still battery-type. Although the present results agree with the above assignment, the origin of pseudocapacitance in this case is the difference in morphology of the materials. The nanobelts have a thickness much lower than typical electrolyte ionic diffusion lengths (~ 20 nm), which would spatially hinder a charge diffusion whereas such hindrances do not appear in the case of nanowires. It is noteworthy that no appreciable difference in the ion diffusion coefficients was determined in the NiO–Co3O4, NiCo2O4, and CuO–Co3O4, which were ~ 4.6 × 10−13, ~ 3.4 × 10−13 cm2 s−1 and ~ 2.5 × 10−13 cm2 s−1 [18, 19], respectively, i.e., all these three electrodes accommodate charges almost similarly. While we did not prove this hypothesis of morphology driven pseudocapacitance using a unique choice of electrode material with different morphologies (primarily because only one report has been published recently on the composite electrodes discussed here), the close agreement between the ion diffusion coefficients and the discharge capacitances ensures that similar electrochemical charge storage mechanism prevails in these electrodes [18, 19]. Nevertheless, a literature survey on ASCs fabricated using NiCo2O4 as the positive electrode with different morphologies supports this argument that the pseudocapacitance in the nanobelts electrode is morphology driven. For example, the ASCs fabricated using the NiCo2O4 microspheres as positive electrode and activated carbon as negative electrode showed nonlinear currents in the CV and voltage in the CDC curves with Es of 19.1 W h kg−1 [31]. Similar observations were made on the yolk-shelled NiCo2O4 based ASCs despite two-fold increase in the Es than that obtained using the microspheres [32]. On the other hand, Xu et al. [33] recently reported hollow NiCo2O4 nanospheres assembled from ultrathin NiCo2O4 nanosheets with thicknesses of several nanometers, which showed similar CV and CDC to that in Fig. 5 when used as a positive electrode in ASCs. Therefore, from the present results and those published in literature, it is reasonable to conclude that electrodes with thickness less than the ion diffusion coefficient (< 20 nm) support PC charge storage.

This idea of morphology driven pseudocapacitance in thin materials structures could be drawn from the fundamental mechanism governing the charge storage in electrochemical cells [3] as well as by recently published literatures [31, 32, 33, 34]. Fundamentally, the extend of charge diffusion in the electrode differentiates batteries and pseudocapacitors despite their similar faradaic processes. The rate-limited diffusion in electrodes of thickness ~ 300 μm was recently demonstrated in lithium ion batteries [34], which is due to the diffusion of charge carriers into the bulk of the electrode in batteries. On the other hand, the charge diffusion is limited to few surface atomic layers in pseudocapacitors, which is characterized by typical charge diffusion length of ~ 20 nm, whereas no faradaic reaction occurs and charges are only absorbed at the surface in pure SCs. In the present case as well as in the experiments of Xu et al. [33], electrodes of thickness less than typical charge diffusion length was used (~ 10 nm). The charge diffusion from the opposite side of the electrodes and subsequent charge screening is expected to limit the charge diffusion within few monolayers of the thin electrodes, thereby exhibiting PC charge storage.

The superiority of nanobelts and hybrid electrodes over the others are displayed in the energy storage parameters such as ES and PS as shown in Fig. 6 and the key performance indicators of the devices are summarized in Table 1. The NiO–Co3O4//AC device delivered ES of 65, 56, 50 and 47 W h kg−1 at PS values of 1650, 4200, 9345 and 12700 W kg−1, respectively. Comparable performance was observed in the NiCo2O4//AC and CuO–Co3O4//AC devices. The NiO//AC, Co3O4//AC, and the controlled carbon-based EDLC exhibited comparatively lower (Fig. 6) performances. Besides, the nanobelts based ASCs showed enhanced operational stability also. The operation stability of the NiO–Co3O4//AC device was examined by CDC testing at (1) a constant current density (5 A g−1) and (2) varying current densities (2–10 A g−1). Capacitive retention and coulombic efficiency of ~ 99% were exhibited by NiO–Co3O4//AC device at the end of the 5000-cycle test (Fig. 6a) at 5 A g−1. The rate capability of the device is shown in the inset of Fig. 7a, which was evaluated by CDC at current densities of 2, 5, and then 10 A g−1. As observed from the figure, the device maintains a steady capacitance at different current densities. Cycling was continued for 1500 cycles at these different current densities and was brought back to 2 A g−1 for the final 500 cycles. The device CS was found to decrease by only 1%. In comparison, the nanowire based ASC, i.e., CuO–Co3O4//AC device, showed inferior capacitive retention (97%) and coulombic efficiency (98%). Figure 7b shows the Nyquist plots of the ASCs determined by EIS in the frequency range 0.01 Hz–10 kHz at an open circuit potential in 6 M KOH electrolyte. Inset of Fig. 6b shows the high frequency region of Nyquist plot of NiO–Co3O4//AC device, from which the charge-transfer resistance (RCT) was obtained as 0.65 Ω. Charge relaxation time (τ), which indicates the responsiveness of the SCs was calculated from the characteristic frequency (2πfo = 1/τ) and summarized in Table 1. Table 1 also details the charge transport parameters of the devices under study. Clearly, the ASCs fabricated using the composite electrodes show superior properties in all aspects.
Fig. 6

Ragone plot of all ASC devices under study

Table 1

The specific capacitance (CS), coulombic efficiency (η), specific energy (ES), and specific power (Ps) of the chosen devices

Device

CS (F g−1)

η (%)

ES (Wh kg−1)

PS (W kg−1)

fo (Hz)

τ (s)

RCT (Ω)

NiO–Co3O4//AC

194

99

62

1405

25.8

0.04

0.65

NiCo2O4//AC

148

98

43

1250

10.3

0.09

0.78

CuO–Co3O4//AC

178

98

44

1400

18.6

0.05

0.71

The charge kinetic parameters extracted from EIS such as peak frequency, relaxation time, and charge transfer resistance of the ASCs are also shown

Fig. 7

a Dependence of CS and coulombic efficiency as a function of charge–discharge cycle numbers (inset shows capacitance variation at progressively varying current densities); b Nyquist plot of all the devices at open circuit potential (inset shows high frequency region of NiO–Co3O4//AC)

Conclusions

In conclusion, we show that thin nanobelts of metal oxides would offer PC charge storage in ASC configuration even if the corresponding electrode show battery-type storage in a three-electrode system measurement. This conclusion has been drawn using asymmetric SCs in the NiCo2O4//AC, NiO–Co3O4//AC, NiO//AC, Co3O4//AC, and CuO–Co3O4//AC device configurations. Among them, NiCo2O4 and NiO–Co3O4 are thin nanobelts of thickness ~ 10 nm whereas the others formed as nanowires of cylindrical cross-section. The PC behaviour of the nanobelts has been evaluated from the charge–discharge cycling and the cyclic voltammetry experiments, which showed linear discharge and nearly equal distribution of charges in the positive and negative electrodes, respectively. On the other hand, the other devices showed non-linear discharge and unequal distribution of charges, thereby display a battery type behaviour. Among the devices which are studied here, NiO–Co3O4//AC device showed the best charge storage capability and operational stability. The best performing device displayed an ES of ~ 65 W h kg−1 at PS of ~ 1650 W kg−1, superior rate capability, and could be recharged up to 5000 cycles with 99% retention.

Notes

Acknowledgements

This work is supported by the Research and Innovation Department of University Malaysia Pahang (http://ump.edu.my) under the Flagship Leap 3 Program (RDU172201).

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Copyright information

© Donghua University, Shanghai, China 2019

Authors and Affiliations

  • Ria Kunwar
    • 1
  • Midhun Harilal
    • 1
  • Syam G. Krishnan
    • 1
  • Bhupender Pal
    • 1
  • Izan Izwan Misnon
    • 1
  • C. R. Mariappan
    • 2
  • Fabian I. Ezema
    • 3
  • Hendry Izaac Elim
    • 4
  • Chun-Chen Yang
    • 5
  • Rajan Jose
    • 1
    Email author
  1. 1.Nanostructured Renewable Energy Materials Laboratory, Faculty of Industrial Sciences and TechnologyUniversiti Malaysia PahangKuantanMalaysia
  2. 2.Department of PhysicsNational Institute of TechnologyKurukshetraIndia
  3. 3.Department of Physics and AstronomyUniversity of NigeriaNsukkaNigeria
  4. 4.Department of PhysicsUniversity of PattimuraAmbonIndonesia
  5. 5.Battery Research Centre of Green EnergyMing Chi University of TechnologyNew TaipeiTaiwan

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