Nanoporous CuCo2S4 microspheres: A novel positive electrode for

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Nanoporous CuCoS microspheres: A novel positive electrode for high-performance hybrid energy storage devices Abdolkhaled Mohammadi, Seyyed Ebrahim Moosavifard, Amin Goljanian Tabrizi, Mahnaz M Abdi, and Gholamreza Karimi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01651 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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ACS Applied Energy Materials

Nanoporous CuCo2S4 Microspheres: A Novel Positive Electrode for High-performance Hybrid Energy Storage Devices Abdolkhaled Mohammadi ƺa, Seyyed Ebrahim Moosavifard ƺb, c, Amin Goljanian Tabrizi a, Mahnaz M. Abdi *d, Gholamreza Karimi *a a

Department of Chemical Engineering, Shiraz University, Shiraz 71348-51154, Iran.

Department of Advanced Medical Sciences & Technologies, School of Medicine, Jahrom University of Medical Sciences (JUMS), Jahrom 46199-74148, Iran. b

Research Center for Noncommunicable Diseases, School of Medicine, Jahrom University of Medical Sciences (JUMS), Jahrom 46199-74148, Iran. c

Young Researchers and Elite Club, Islamshahr Branch, Islamic Azad University, Islamshahr 33147-67653, Iran. d

ƺ

both authors contributed equally to this manuscript.

Abstract Demands for safer, faster and more efficient energy storage systems have motivated researchers to design and develop new electrode materials. Ternary transition metal oxides are the electrode materials commonly investigated for application in energy storage systems. Nevertheless, the low active surface area and poor conductivity limit their electrochemical performances. In the present study, nanoporous CuCo2S4 microspheres (CCS) electrode has been successfully prepared via a facile self-template method for high-performance hybrid energy storage devices. An ultrahigh specific capacitance of 1566 F g−1 at 2 A g−1 and a superb cycling performance of 95.7% retention after 5000 GCD cycles have been obtained for this electrode. In addition, when this CCS electrode is assembled with an activated carbon (AC) electrode, the as-fabricated asymmetric supercapacitor delivers outstanding performance with maximum power and energy densities of 16 kW kg−1 and 43.65 W h kg−1, respectively, which is superior to conventional supercapacitors. The fabricated supercapacitor can light two green LEDs for more than 15 minutes. This work sheds a further light on the design of efficient electrodes which can be utilized in the next generation of highperformance hybrid energy storage devices. Keywords: Hollow spheres, nanoporous, metal sulfide, self-template, CuCo2S4, supercapacitor 1 ACS Paragon Plus Environment

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1 Introduction There is a growing demand for the safe, green and high-performance energy storage and conversion systems as an alternative to fossil fuels. The developed systems should provide an effective approach to overcome the challenges of global warming, energy crisis, and environment pollutions. In the last decade, supercapacitors (SCs) have attracted the attention of many researchers due to their unique properties such as high power density, fast rate, pollution-free operation, and long cycle life 1-2. Since the performance of SCs is strongly affected by the type, design, and morphology of their electrode materials, a tremendous amount of researches have been conducted on the design and synthesis of more efficient electrode materials with unique architectures to achieve higher energy storage performance 3. Advanced materials with highly porous and accessible nanostructures, low density and reduced transport lengths (for ions/electrons transfer) are not only a promising candidate in energy storage applications but also have attracted great attention in other application fields, such as drug storage and delivery, adsorption and separation, catalysis and biosensors 4-5. It is generally accepted that hollow cavities enhance the active sites for chemical/electrochemical reactions by mitigating the agglomeration of nanoparticles 6. In addition, the diffusion distance between the interior surfaces and the external electrolyte ions can be shortened by cavities which play the role of an “ion buffering reservoir” 7. Therefore, porous hollow nanostructures can effectively enhance the electrochemical performance of electrodes by providing a competent link between the electrolyte and electrode materials. In recent years, extensive research efforts have been devoted to designing and fabrication of many types of metal oxides hollow structures via soft- or hard- or self-template methods 8. However, the soft- and hard-templating methods require tedious operating procedures and expensive surfactants or harmful etching agents 9. Therefore, the self-templating methods are more applicable due to the elimination of hard/soft templates, ease of scaling up, lower production cost and simpler synthesis procedure 10. Besides the electrode morphology, the type of metal oxides strongly affects the electrode performance 11. Although researchers have examined the energy storage applications of different types of transition metal oxides in the last decade 12-13, recent researches have demonstrated that the transition metal sulfides (TMS) show greater performance as compared to their corresponding oxides owing to their higher electrical conductivity 14-16. It is also revealed that the mixed metal sulfides such as thiospinels display electrochemical 2 ACS Paragon Plus Environment

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performances enhancement in energy storage devices compared to the single-component metal sulfides due to their higher electrical conductivity. Until now, several NiCo2S4 hollow structures of different shapes such as nanotubes, spheres, ellipsoids, and prisms have been fabricated for various applications 17-21. However, the hollow structures of the new kinds of ternary transition metal sulfides such as CuCo2S4 (CCS) have been rarely reported 22-24. In the present work, we designed and developed nanoporous CuCo2S4 hollow spheres by a simple self-templated method to be used in efficient asymmetric supercapacitors. As far as the authors know, our work is the first report on CuCo2S4 hollow spheres asymmetric supercapacitors. In addition, the activated carbon (AC) and nanoporous CCS electrodes are employed respectively as negative and positive electrodes, to prepare the asymmetric supercapacitor. The fabricated supercapacitor delivered an excellent performance in terms of power density of up to 16 kW kg−1 and energy density of 43.65 W h kg−1. Finally, the lightemitting diodes (LEDs) were powered by two of the fabricated supercapacitors, assembled in series, as a proof of concept for practical applications.

2 Experimental 2.1 Materials Synthesis The chemicals used in this research are of analytical grade and applied without additional purification. The whole process for the synthesis of the hollow nanoporous CuCo2S4 (CCS) microspheres by a facile self-templated method is depicted in Scheme 1. In a typical synthesis, 36.9 mg of Cu(NO3)2·6H2O and 72.7 mg of Co(NO3)2·6H2O were added to a stirred solution of 40 mL isopropanol and 8 mL of glycerol to achieve a clear solution. The resulting pinkish solution was then maintained in a sealed Teflon-lined stainless-steel autoclave at 180 °C for 6 h. Next, the as-prepared CuCo-glycerate precursors were separated and dried at 80 °C. To prepare CCS hollow spheres, 100 mg of thioacetamide (TAA) was added to a 60 ml of CuCo-glycerate precursor/ethanol suspension (0.5 mg ml-1) and the mixture was kept in the Teflon-lined stainless steel autoclave at 180 °C for 6h. The hollow CuCo2O4 microspheres were also synthesized by calcination of CuCo-glycerate precursors at 350 °C in air with a heating rate of 1 °C min−1 for 2 h for comparison purpose 25.

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Scheme 1: Schematic illustration of the formation of CCS and CCO hollow structures by the selftemplate method.

2.2 Electrochemical Measurements The electrochemical properties of the electrodes were investigated in three- and twoelectrode cells. In the former, platinum and saturated calomel electrodes were employed as the counter and reference electrodes, respectively and an aqueous 3 M KOH solution was used as the electrolyte. A slurry of electrode materials was prepared by mixing the active material, polyvinylidenefluoride, and carbon black with a mass ratio of 85:5:1 in the Nmethylpyrrolidone solvent. Subsequently, the slurry was coated onto a piece of Ni foam and dried at 80 °C for 10 h. The mass loadings on electrodes were adjusted to around 3 mg cm-2. Cyclic voltammetry (CV), Galvanostatic charge/discharge (GCD) and electrochemical impedance spectroscopy (EIS) were carried out by a BioLogic VSP 300 potentiostat/galvanostat device. The specific capacitance (C, F g−1), energy density (E, Wh kg−1) and power density (P, W kg−1) were calculated, respectively as follows 26: 𝐶=

𝐼 × ∆𝑡 (F g ―1) 𝑚 × ∆𝑉

(1)

𝐸=

1 𝐶∆𝑉2 (Wh kg ―1) 2 × (3.6)

(2)

𝑃=

𝐸 × 3600 (W kg ―1) ∆𝑡

(3)


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where m is the total mass of the active material (g), ∆V is the potential window (V), ∆t is the discharge time (s), and I is the discharge current (A)

2.3 Characterization The morphological observation of the samples was conducted using transmission electron microscopy (TEM) (Hitachi H-7100 TEM) and ﬁeld-emission scanning electron microscopy (FESEM, JEOL JSM-7600F). The X-ray diffraction (XRD) analysis was performed on a PAN analytical EMPYREAN diffractometer equipped with a monochromator and using Cu Kα radiation (λ=0.154 nm). The pore size distribution and specific surface area of the samples were measured by a Micrometrics instrument (3Flex Version 1.02). A VG ESCALAB MKII spectrometer (Mg Ka X-ray source, 120 W) was used for X-ray photoelectron spectroscopy (XPS).

3 Results and Discussion The crystallinities and phase purity of the as-prepared CCS and CCO samples were investigated by XRD analysis. As shown in Fig. 1a, the CCO diffraction peaks are in good agreement with the pattern of the pure CuCo2O4 (JCPDS File No. 001-1155). The CCS diffraction peaks of are also perfectly matched with the standard pattern of the cubic thiospinel of CuCo2S4 (JCPDS File No. 42-1450) 27-28. No characteristic peaks are assigned to the possible impurities such as CoS, CoS2, Co3S4, Co9S8, CuS or Cu9S8, demonstrating the successful formation of pure CuCo2S4. The appearance of broad diffraction peaks is also the evidence of nano-crystalline structures. To gain further insights into the valence states of the near-surface elements, XPS analysis was conducted. As shown in Fig. 1b, the high-resolution XPS spectra of Co 2p, Cu 2p, and S 2p were sued to study the valance state of cobalt, copper and sulfur elements in the CCS sample. The appearance of Co 2p3/2 and Co 2p1/2 peaks with two shake-up satellites (marked as ‘‘Sat.”) demonstrates the presence of mixed Co3+ and Co2+ in CCS sample 29. In the Cu 2p spectrum, the two peaks at 931.62 eV and 951.63 eV (with a splitting of 20 eV), assigned to Cu 2p3/2 and Cu 2p1/2, are characteristic peaks of Cu+. In the S 2p spectrum, the S 2p3/2 and S 2p1/2 peaks appeared at the binding energy of 161.7 and 162.4 eV, respectively and they are demonstrating the typical presence of S2– 30-31. The specific surface area of the CCS sample and its pore size distribution were measured using BET and BJH methods, respectively. The N2 adsorption/desorption isotherm of the 5 ACS Paragon Plus Environment


CCS sample presented the hysteresis loop of type IV, verifying its mesoporous structure, as shown in Fig. 1c 32. The BET specific surface area was also calculated to be around 47 m2 g– 1.

The BJH curve of the sample (inset of Fig. 1c) further revealed the nanoporous structure of

the sample where the pore size distribution was mainly centered at around 8 nm. Since the large specific surface area and the high porosity play a key role in facilitating ion transportation and providing more electroactive sites for electrochemical reactions, it can be predicted that the CCS sample will exhibit an excellent electrochemical performance 33-34.

Fig. 1: (a) XRD patterns of CCS and CCO, (b) high-resolution XPS spectra of S 2p, Co 2p and Cu 2p (c) N2 isotherms and BJH curve of nanoporous CCS sample (d) Low magnification FESEM image of CCO (scale bars, 100 nm), (e, f) high and low magnification FESEM images of CCS (scale bars, 500 and 100 nm), (g) Low magnification TEM image of CCO (scale bars, 200 nm), and (h, i) high and low magnification TEM images of CCS (scale bars, 500 and 200 nm, respectively).

The morphology and detailed structure of the as-prepared CCS, CCO, and Cu–Co precursor samples were investigated by FESEM and TEM. As presented in Fig. S1a, the CuCo-gly precursors, synthesized through the first solvothermal step, displayed a uniform spherical shape with a diameter of around 500-600 nm without any by-products. As seen in 6 ACS Paragon Plus Environment

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the wide FESEM images (Figs. S1b and 1e) the CCO and CCS samples successfully retained their spherical morphology after calcination (for CCO) and second solvothermal transformation (for CCS). The rough surface of the CCO and CCS spheres (Fig. 1 d, f) suggests a highly porous texture composed of numerous small nanoparticles for both structures. As illustrated in Fig. 1 (g-i), TEM was used to investigate the interior structure of the microspheres. The porous shell and the hollow interior of the microspheres are clearly distinguished by comparing the dark edges and pale parts. The shell thicknesses were estimated to be less than 100 nm. Such a unique hierarchical nanoporous and hollow nanostructures not only increase the electroactive sites effectively but also strongly enhance electrolyte penetration; therefore lead to improved electrochemical performance 6-7.

3.1 Electrochemical Evaluation Cyclic voltammetry (CV), galvanostatic charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) were conducted in a 3 M KOH solution as the electrolyte for evaluating the electrochemical performance of the electrodes. Fig. 2a presents the CV curves of the CCS and CCO electrodes at a constant scan rate of 20 mV s−1. By comparing the larger integrated CV area of the CCS electrode with that of the CCO electrode, it can be found that the CCS electrode can store much more charge on its surface leading to much more specific capacitance. Fig. 2b shows typical CV curves of the CCS electrode at different scan rates of 5, 10, 20, 50 and 100 mV s−1 in a potential window of −0.1 to 0.5 V. The strong redox peaks in the CV curves, indicate the faradaic redox reactions of cobalt and copper redox pairs related with OH– during the electrochemical process 35. The possible reaction equations for the CCS electrode can be described as follows 24: CuCo2S4 + OH ― + H2O↔CuSOH + 2CoSOH + e ―

(4)

CoSOH + OH ― ↔CoSO + H2O + e ―

(5)

CuSOH + OH ― ↔CuSO + H2O + e ―

(6)

Furthermore, another important observation is that no significant change occurs in the shape and position of the peaks during the increase of scan rate suggesting excellent electrochemical reversibility and promoted mass transport for the CCS electrode 36-37.



Fig. 2: (a) CV curves of CCO and CCS electrodes at 20 mV s−1(b, c) CV and GCD curves of the CCS electrode at various scan rates and current densities, (d) the specify capacitance of the CCO and CCS electrodes at various current densities, (e) Retention of specific capacitance of the CCS electrode at 10 A g−1, and (f) Nyquist plots of CCO and CCS electrodes.

The GCD curves for the CCS electrode measured at different current densities ranging from 2 to 20 A g−1 are illustrated in Fig. 2c. Consistent with the CV results, the nonlinear shape in the GCD curves relates to the Faradaic redox reactions of Co4+/Co3+ and Cu2+/Cu+ couples 30. The small iR drop, which is even observed at ultrahigh currents, indicates an excellent rate capability and a high conductivity for electrode 38. The Nyquist plot (Fig. 2f) further confirms the insignificant internal resistance of the electrode. Low internal resistance and high conductivity are known as crucial parameters in high rate supercapacitors. In order to compare, the rate capability of the CCS and CCO electrodes, their capacitance at various current densities were calculated from the GCD curves and illustrated in Fig. 2d. As seen, the specific capacitance of the CCS electrode (1566 F g−1 at 2 A g−1) is much larger than that of CCO electrode (1216 F g−1). The specific capacitance of the electrodes gradually decreased with the increase of current density. This stems from the fact that the active material is not sufficiently involved in the electrochemical reactions 39. Compared with CCO electrode, not only did the CCS electrode deliver a higher capacitance but also its capacitance performance was retained better as the current density was increased. The CCS electrode maintained 59% of its capacitance after increasing the current density to 60 A g−1 (923 F g−1), against 50% for CCO electrode (609 F g−1).


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The cycling stability of the CCS electrode was further investigated at a current density of 10 A g−1. As shown in Fig. 2e, a capacitance loss of only 4.3% (after 5000 continuous GCD cycles) was observed for CCS electrode indicating excellent long-term cycling stability for this electrode. EIS was further used to study the electrochemical behavior of the electrodes 40-41 and the Nyquist plots of the CCS and CCO electrodes are shown in Fig. 2f. Generally, both CCS and CCO electrodes exhibit semicircle behavior at the high-frequency region followed by a straight line at low-frequencies that are related to the charge transfer resistance (Rct) and mass transfer resistances controlled by a diffusion process, respectively 42. The impedance plot of the CCS modified electrode exhibited a smaller semicircle with a real axis intercept (Rct=0.27 Ω) compared to that of CCO (Rct=0.46 Ω), verifying its lower internal resistance. The linear line for both CCS and CCO electrodes displayed almost the same slope with an ideal behavior for diﬀusion of the analyte into the surface of electrode 43. All of these results have clearly confirmed that the fabricated hollow nanoporous CCS sample is a superior electrode material as a positive electrode in asymmetric supercapacitors.

3.2 Electrochemical evaluation of the CCS//AC device Low energy density is the main drawback of the commercial development of supercapacitors. Therefore, tremendous research efforts have been focused to improve the energy density of supercapacitors while keeping their high power density. Generally, and based on the type of electrode materials performing as the negative and positive electrodes, supercapacitors are classified as symmetric and asymmetric devices 44. Compared with symmetric devices, asymmetric devices can store much more charge over wider working potentials. This is due to the fact that with different electrode materials, the device’s operating voltage can be expanded beyond the thermodynamic decomposition voltage of water (1.2 V) 45. Accordingly, we used the activated carbon (AC) electrode as the negative power source electrode and CCS electrode as the positive energy source electrode (signified as CCS//AC) to assemble the asymmetric supercapacitor. To achieve the optimal performance in asymmetric supercapacitors, the charges stored on the negative and the positive electrodes have to be balanced based on the charge balanced theory (q+=q−). The charge on each electrode depends on its mass loading (m), potential window (ΔE), and specific capacitance (C) as represented by 46-47: 𝑄 = 𝐶 × 𝑚 × ∆𝐸

(7) 9

ACS Paragon Plus Environment


𝑚+ 𝑚―

=

𝐶 ― × ∆𝐸 ―

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(8)

𝐶 + × ∆𝐸 +

Accordingly, the mass ratio between the CCS electrode and AC electrode was calculated to be around 1:4.5. Hence, the total mass loading was 5.5 mg cm-2.

Fig. 3: Electrochemical performance of CCS//AC asymmetric device: (a) CV curves of the CCS and AC electrodes at a scan rate of 20 mV−1 after charge balancing, (b) Potential window variation for the upper potential windows ranging for CCS//AC device, (c) CV curves of the device at different scan rates from 5–100 mV s−1, (d) GCD curves of the device at various current densities, (e) Dependence of specific capacitance on the applied current density, (f) Cycling stability over 10 000 cycles at a current density of 5 A g−1, (g) Nyquist plot, and (h) Ragone plot of the CCS//AC asymmetric device.

Fig. 3a represents the CV curves of the CCS and AC electrodes at a scan rate of 20 mV s−1 after charge balancing. As shown in Fig. 3b, the CV was recorded at different voltage windows to detect the stable working voltage of the as-prepared CCS//AC asymmetric device. As seen, the working voltage of the CCS//AC asymmetric cell can be expanded to 1.6 V in aqueous KOH electrolyte (higher than that of commercial aqueous electrolyte AC supercapacitors) 45. Fig. 3c shows the typical CV curves of the device at different scan rates in a voltage window of 0 to 1.6 V. All the CV curves have almost similar shape indicating an excellent rate capability, high reversibility, and low internal resistance for the device 48. Fig. 3d illustrates the GCD curves of the CCS//AC device at various current densities. As shown, the curves have a symmetrical and quasi-triangular shape with negligible iR drops at various current densities, demonstrating excellent rate capability and low internal resistance for the ACS device. The specific capacitances of the CCS//AC asymmetric device at the current density range of 1 to 20 A g−1 based on the total mass of active material is shown in Fig. 3e. As shown, the 10 ACS Paragon Plus Environment

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maximum specific capacitance of 123 F g−1 was achieved at the current density of 1 A g−1. Moreover, the assembled device delivered an excellent rate performance with 57% capacitance retention when the current density increased from 1 to 20 A g−1. The GCD test was also used to evaluate the long-term cycling stability of the device by repeating at a stable current density. As shown in Fig. 3f, the fabricated device has retained 91.7% of its specific capacitance after 10000 cycles at a current density of 5 A g−1, confirming its excellent cycling stability. EIS measurements (Fig. 3g) also confirmed the low internal resistance, and small charge/mass transfer resistance of the CCS//AC asymmetric device which are responsible for the excellent performance of the device as is evident from both GCD and CV tests 49. Power density and energy density are the critical factors in assessing the operational performance/efficiency of energy storage devices. The relationship between power density and energy density is described through the Ragone plot. Fig. 3h shows the Ragone plot of the CCS//AC device in comparison to other energy storage systems. As it can be observed, our device has delivered a power density of up to 16 kW kg−1 and a maximum energy density of 43.65 W h kg−1, which are more than most of the asymmetric/hybrid devices previously reported (see Table 1). As shown in scheme 2, the excellent performance of CCS electrode can be attributed to the following reasons 8, 10, 33: 1) The thiospinels not only have higher conductivity than spinels but also possess richer redox reactions than single metal/sulfides. 2) The hollow microsphere morphology improves structural and cycling stability by shortening diffusion pathways for electrolyte ions. 3) Not only does the hollow porous structures lead to large electroactive sites for the electrochemical reactions, but also it enhances the use of the active material. 4) Nanoscale shell thickness provides faster kinetics and higher conductivity by shortening the electron transport pathways. Finally, for demonstrating the real application, we connected two of asymmetric devices in series and lit up two green round light-emitting diodes (LEDs) assembled in parallel for more than 15 minutes as shown in Scheme 2.



Scheme 2: Illustration of the ionic transport process in the CCS electrode and LEDs powered by the CCS//AC ACS device.


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Table 1: Comparison of CCS electrode (two and three electrode system) performance with previous works. Cell (conf.)

Specific capacitance (F g−1), Cycles, Retention Current density (A g−1)

3E

1137.5, 2

6000, 94.9 %

3E

752, 2

CuCo2S4/CNT/graphene

3E

CuCo2S4/rGO CuCo2S4 nanospheres

Electrode Hierarchical CuCo2S4 hollow spheres Mesoporous CuCo2S4

CuCo2S4 NRAs

ED (Wh kg−1)

ΔV (V)

Ref.

-

0.5

24

5000, 98.1 %

-

0.5

50

504, 10

2000, 92.3 %

-

0.4

51

3E

525, 1

1000, 83 %

-

0.5

52

2E, Sym

53.5 mF cm−2, 0.17 mA cm−2 5500, 86 %

7.29 μWh cm−2

1

30

3E

1536.9, 1

10000, 94.9 %

-

0.45

5000, 88 %

56.96

1.6

cm−2,

29

2E, AC

0.4 F

3E

908.9, 5 mA cm−2

2000, 91.1 %

-

0.5

2E, AC

93.5, 1 mA cm−2

2000, 126.4 %

29.2

1.5

Oriented CuCo2S4 nanograss arrays/Ni foam

3E

1852, 2

4000, 96 %

-

0.5

2E, Sym

81.2, 18.94

5000, 99 %

31.88

1.5

CuCo2O4@C core-shell

3E

1432, 1

3000, 98 %

-

0.5

55

CuCo2O4/CuO

2E, AC

57, 1 mA cm−2

5000, 79 %

18

1.5

56

CuCo2O4 NSs@G

3E

1331, 1

5000, 80%

-

0.6

57

CuCo2O4 nanobelts

3E

809, 10 mV s−1

1800, 127%

-

0.45

58

CuCo2O4 nanograss

3E

796, 2

5000, 94.7%

-

0.6

59

3E

982, 1.5

3000, 101%

-

0.45

2E, Sym

118.5, 1

2000, 82%

16.9

3E

1210, 1

-

-

0.5

2E, AC

137, 1

5000, 86%

42.8

1.5

3E

416, 1

4200, 92%

-

1

2E, AG

78, 1

-

43.3

2

3E

889, 2 mA cm−2

2000, 102%

-

0.45

2E, AC

57.6, 2 mA cm−2

2000, 101%

18

1.5

3E

327, 1.25 A g−1

5000, 90%

-

0.5

-

-

1

Flower-like CuCo2S4/Ni foam

CuCo2O4 nanowires Ordered CuCo2O4 CuCo2O4@MnO2 nanoflakes CuCo2O4@ CuCo2O4 nanowire CuCo2O4@MnO2 on carbon fibers Double-shell CuCo2O4 CuCo2O4/CuO nanowire CuCo2O4/MnCo2O4 on graphite paper CuCo2O4@Co(OH)2 core/shell Leaf-like CuCo2O4 Corn-like CuCo2O4 NiCo2S4 nanotube array CCS

1 mA

cm−2

cm−2

0.71, 1 mA

3E

1472, 4 mA cm−2

5000, 93.8%

-

0.5

2E, AC

119, 20 mA cm−2

6000, 92.5%

37.3

1.5

3E

642, 1

5000, 95%

-

0.6

2E, Fe2O3 93, 0.25

5000, 83%

33

1.6

3E

1434, 0.5

5000, 81.4%

-

0.5

2E

118.4, 0.5

10000, 88.4%

42.1

1.6

3E

424, 0.5

10000, 86%

-

0.4

2E, AG

70, 0.5

-

19.2

1.4

3E

1223, 1

2000, 88%

-

0.4

2E, Sym

158, 1.35

5000, 83.5%

26.5

1.1

3E

820, 2 mA cm−2

1500, 94%

-

0.4

2E, Sym

101, 5 mA cm−2

-

17

1.1

3E

578 C g−1, 0.5 A g−1

6000, 71.1%

-

0.5

5000, 131.3%

24.8

1.5

5000, 95.7%

-

0.5

2E, AC

48 C

3E

1566, 2

5 mA

cm−2


54

60

1

2E, Sym

g−1,

53

61

62

63

64

25

65

66

67

68

69

70

This


2E, AC

123, 1

10000, 91.7%


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43.65

1.6

Work

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4 Conclusions In summary, we developed nanoporous CuCo2S4 hollow microspheres via a facile selftemplated method for high-performance asymmetric supercapacitors. The nanoporous hollow spheres morphology with unique structural features is a promising candidate for electrochemical energy storage devices. For comparison purposes, the electrochemical performance of the nanoporous CCO microspheres was also studied. The electrochemical results in the three-electrode system showed that the as-prepared nanoporous CCS microspheres electrode exhibits better electrochemical performances over nanoporous CCO microspheres electrode. The asymmetric supercapacitor assembled by CCS and AC electrodes delivered a power density of up to of 16 kW kg−1 and a high energy density of 43.65 Wh kg−1, which are largely more than those of CuCo2S4 electrodes previously. Lighting two green LEDs connected in parallel for more than 15 minutes has confirmed the practical application of the CCS electrodes in electrochemical energy storage devices.

ASSOCIATED CONTENT Supporting Information available: The supporting information consists of FESEM image of CuCo-gly and high magnification FESEM images of CCO samples.

AUTHOR INFORMATION Corresponding authors * E-mail: [email protected] * E-mail: [email protected] ORCID Abdolkhaled Mohammadi: 0000-0002-7936-3407 Seyyed Ebrahim Moosavifard: 0000-0002-9706-2560 Amin Goljanian Tabrizi: 0000-0003-0362-5463 Mahnaz M. Abdi: 0000-0003-0844-9884 Gholamreza Karimi: 0000-0002-1954-6818 Notes There are no conflicts to declare.

ACKNOWLEDGMENTS 15 ACS Paragon Plus Environment


The financial support provided by “Iran Nanotechnology Initiative Council” is hereby greatly acknowledged by the authors.

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