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Facile Synthesis of 3D Ternary ZnCo2O4/Reduced Graphene Oxide/NiO Composite Film on Nickel Foam for Next Generation Supercapacitor Electrodes Sumanta Sahoo, and Jae-Jin Shim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01367 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Sustainable Chemistry & Engineering

Facile Synthesis of 3D Ternary ZnCo2O4/Reduced Graphene Oxide/NiO Composite Film on Nickel Foam for Next Generation Supercapacitor Electrodes Sumanta Sahoo and Jae-Jin Shim*

School of Chemical Engineering, Yeungnam University, 280 Daehak-ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea

ABSTRACT Ternary ZnCo2O4/reduced graphene oxide/NiO (ZCGNO) nanowire arrays were grown directly on a piece of Ni foam using a simple, facile, cost effective hydrothermally assisted thermal annealing process without the addition of any Ni precursor salt and used as a binderfree supercapacitor electrode. Ni foam was utilized successively as the NiO precursor, binder, and current collector. The resulting 3D ternary composite possessed an ultrahigh specific capacitance of 1,256 F/g at a current density of 3 A/g in 6 M KOH solution. Moreover, the three dimensional electrode exhibited superior electrochemical performance, such as excellent cyclic stability (~ 80% capacitance retention after 3,000 cycles), maximum energy density of 62.8 Wh/kg, maximum power density of 7,492.5 W/kg, and low equivalent series resistance (0.58 Ω). The effects of the electrolyte concentration on the electrochemical performance of ZCGNO were also examined. ZCGNO with this remarkable electrochemical performance may be considered a prospective candidate for high performance supercapacitor applications.

KEYWORDS:

Supercapacitor, reduced graphene oxide, ZnCo2O4, NiO, 3D ternary

1

ACS Paragon Plus Environment


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composite

INTRODUCTION Supercapacitors with high energy density and long cycle life have become one of the most promising sustainable energy storage devices of this century. The performances of supercapacitor electrodes are dependent mainly on the electrode materials. To achieve improved electrochemical properties, a combination of two types of supercapacitors, the electric double layer capacitor (EDLC) and pseudocapacitor, which results in the formation of hybrid capacitors, have been studied widely.1-3 Three main types of single compound components have been utilized for the fabrication of supercapacitor electrodes: (i) metal oxide/metal hydroxides (MO/MOH), (ii) conducting polymers (CP), and (iii) carbonaceous materials (CM). Among these, the first two follow a faradaic charge-storage mechanism, whereas the last one follows a non-faradaic mechanism. On the other hand, these single compound components have failed to deliver simultaneously a high specific capacitance and long-term cyclic stability. In this context, binary composites with different combinations, such as MO/MOH-CP,4 MO/MOH-CM,5 CP-CP,6 CP-CM,7 and CM-CM,8 have been studied widely as supercapacitor electrodes. Although binary composites exhibit superior capacitive properties, they also suffer from subsequent volumetric swelling and shrinking during charge/discharge.

Rationally

designed

ternary

nanocomposites

with

a

porous

nanoarchitecture are highly desirable for high performance supercapacitors. In recent years, ternary nanocomposites with different combinations, such as CP-MO-CP,9 MO-CM-CP,10 MO-MOH-CM,11 MO-CM-CM,12 MO-CM-MO,13 were reported to exhibit superior electrochemical performance, which also produces a new trend for the fabrication of the next generation supercapacitor electrodes.

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Mixed transition metal oxides (MTMOs) with the proper stoichiometric ratio have been utilized extensively as one of the prime components in the fabrication of ternary supercapacitor electrodes because of the multiple valances of cations, which allows a low activation energy for electron transfer and enhances the electrochemical properties.14-16 ZnCo2O4 is one of the most promising functional materials for a range of electrochemical applications, including Li-ion batteries, electrocatalysis, and supercapacitor, on account of their high electrochemical activity and rich redox reactions.17-19 NiO has been considered as one of the supreme candidate for the construction of supercapacitor electrodes owing to its high theoretical capacitance (3750 F/g),1 low cost, high thermal stability, and ecofriendly nature.20,21 ZnCo2O4 is utilized widely for supercapacitor applications on account of its enhanced electrochemical activity (theoretical capacitance of 2604 F/g).22 However, it suffers from poor electrical conductivity, low surface area, and large decrease in capacity during cycling. On the other hand, NiO has enhanced electrical conductivity compared to other metal oxides and mixed metal oxides. Therefore, high electrical conductivity and high surface area can be achieved by combining these two.23 In addition, NiO can alleviate the internal stress and prevent the large decrease in capacity during cycling.24 Owing to these advantages, combining ZnCo2O4 with NiO would be favorable but these metal oxides will need to be combined with other CMs to further improve the electrical conductivity. Among the various nanostructured carbon materials, graphene or reduced graphene oxide (rGO) has been utilized for supercapacitor applications because of its outstanding electrical properties, high capacitive nature, good mechanical properties, excellent chemical stability, and high specific surface area.25-27 To achieve a high energy density and high power density with a limited surface area, modern research has focused on binder-free electrodes. In

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this context, Ni foam has been commonly utilized as a 3D scaffold for the uniform growth of electrode materials and is also used as the support of a current collector. Binary composites based on graphene and NiO have been investigated as supercapacitor electrodes. Li et al. reported a flower like NiO/rGO composite that exhibited a specific capacitance (Csp) of 428 F/g at a discharge current density of 0.38 A/g in 6 M KOH electrolyte.28 In another report, Xia et al. prepared a graphene sheet/porous NiO hybrid film that showed a Csp of 400 and 324 F/g at a current density of 2 and 40 A/g, respectively.29 In another study, Bai et al. reported a ternary nanocomposite based on rGO, carbon nanotube (CNT) and NiO with a high Csp of 1180 F/g at 1 A/g current density.30 Sun et al. recently reported the facile fabrication of hierarchical ZnCo2O4/NiO core/shell nanowire arrays on Ni foam, which exhibited excellent Li-ion battery performance.31 Few reports are available on the fabrication of supercapacitor electrodes based on ZnCo2O4 individually.19,32,33 On the other hand, unique combination of ZnCo2O4, rGO and NiO has not been explored anywhere, till date. As discussed above, although several combinations have been assessed for the fabrication of ternary nanocomposites, the exceptional combination of MTMO-CM-MO has not been investigated as a supercapacitor electrode. Moreover, most of the fabrication processes of binary/ternary nanocomposites are based on a two-step hydrothermal process, which can be expensive. Therefore, a simple, facile, cost effective process for the fabrication of porous ternary hybrid supercapacitor electrodes is highly desirable. Motivated by these scientific findings, this study synthesized a free-standing porous ZnCo2O4/rGO/NiO (ZCGNO) composite on Ni foam using a facile, cost effective one-step hydrothermal process combined with a short post-annealing treatment. The as-synthesized ternary composite was investigated directly as a supercapacitor electrode without the addition of any binder, conductive additive or current collector. The synthesis process and the 4


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electrode fabrication process, both are environmentally benign. Importantly, the ternary nanohybrid was prepared without the use of any Ni salt precursors. This distinctive combination has been adopted based on the idea that the synergistic contribution from individual components (ZnCo2O4, NiO: faradaic electrode materials, rGO: double layer capacitive material) will enhance the overall electrochemical performance of ZCGNO. Further, the utilization of binder-free electrodes avoids the usage of hazardous binders and reduces the production cost. It is significant to note that, the electrochemical performance of supercapacitors depends not only on the electrodes, but also on the nature and concentration of the electrolyte. In the present study, the effects of the electrolyte (KOH) concentration on the electrochemical performance of ZCGNO were also investigated. To the best of the author’s knowledge, this is the first report of the fabrication of supercapacitor electrodes based on ZnCo2O4, NiO, and rGO.

EXPERIMENTAL Materials. All chemicals were of analytical grade and used as received. Zn(NO3)2, 6H2O and urea were supplied by Sigma Aldrich. Co(NO3)2, 6H2O, graphite powder (natural, briquetting grade, -100 mesh, 99.9995%, ultra “F” purity) were obtained from Alfa Aesar. Other chemicals, such as HCl, H2O2, ethanol, H2SO4, KMnO4, and H3PO4, were purchased from Duksan Pure Chemicals Co., Ltd. Preparation of Electrode Material (ZCGNO). The synthesis of ZCGNO was carried out using a two-step process: the hydrothermal process with GO solution and metal oxide precursors, and a thermal annealing process in atmospheric air. Prior to these chemical processes, nickel foam (1 cm x 3 cm) was cleaned consecutively in 6 M HCl, DI water, and ethanol with ultrasonication. First of all, 1 mM of Zn(NO3)2, 6H2O and 2 mM of Co(NO3)2, 5



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6H2O were mixed properly with 40 ml of GO suspension (5 mg/ml) with stirring for 30 min. Subsequently, 4 mM of urea was added to the solution and the entire solution was stirred vigorously with a magnetic stirrer for another 30 min. The entire solution was then transferred to a 50 ml Teflon-lined autoclave and a piece of cleaned Ni foam was dipped into the solution. The autoclave was heated to 210ºC for 24 h. After the reaction was complete, the autoclave was cooled to room temperature. Finally, the Ni foam was washed successively with DI water and ethanol and dried at 60ºC for 6 h. The metal oxide precursor coated Ni foam was then annealed thermally at 400ºC for 2 h. For a comparative study, other electrode materials, such as rGO@Ni foam (rGO), ZnCo2O4/NiO@Ni foam (ZCNO), and NiO/Ni foam (NO) were prepared using a similar synthesis process. A detailed description of the procedure is included in the supporting information section. Characterizations. The morphology of the electrode were analyzed by field emission scanning electron microscopy (FESEM, Hitachi, S-4800) and high resolution transmission electron (HRTEM, Philips, CM-200, at an acceleration voltage of 200 kV). Xray diffraction (XRD, PANalytical, X'Pert-PRO MPD) was performed over the 2θ range, 1080°. X-ray photoelectron spectroscopy (XPS, Thermo Scientific) was conducted using monochromatic AlKα radiation. Raman spectroscopy was performed using a model XploRA plus (HORIBA) spectrometer. All the electrochemical characterizations were carried out on a potentiostat /galvanostat (Autolab PGSTAT 302N) instrument using a conventional three-electrode cell without any additive/binder. The ZCGNO on the Ni foam acted as the working electrode, and Ag/AgCl and Pt electrodes were utilized as the reference and counter electrodes, respectively.

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The mass density of ZCGNO was 1.5 mg/cm2. All the equations used to evaluate the electrochemical properties are included in the supporting information section.

RESULTS AND DISCUSSION Morphology and Structural Analysis. FESEM was performed to analyze the morphology and structural characteristics of the as-obtained electrode material (Figure 1). The low and high magnification SEM images of the precursors showed that the precursors were covered uniformly and densely over the Ni foam substrate, indicating the large scale synthesis of ZCGNO (Figure 1a, b). On the other hand, the low magnification image of ZCGNO (Figure 1c) showed that the surface of the 3D framework was covered uniformly by nanowires, which enhance the surface area of the material significantly. In addition, the pores present between the nanowires can act as operational transport channels for the electrolyte during the charging/discharging process. Further, the high magnification image (Figure 1d) clearly reveals the uniform distribution of porous nanowires with diameters and lengths of 20-30 nm and 300-400 nm, respectively. The inset in Figure 1d shows the SEM image of a single nanowire. These types of ultra-long nanowire arrays can facilitate easy and fast ion/electron transport between the electrode and electrolyte, which are essential for achieving enhanced electrochemical performance. Morphological analysis with the corresponding SEM images of the samples rGO, NO, and ZCGO are included in the supporting information (Figure S2).

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Figure 1. SEM Analysis: (a, b) SEM images of ZCGNO precursors at low and high magnification; (c, d) ZCGNO at low and high magnifications. Figure 2a presents the TEM image of ZCGNO, which demonstrates random aggregation of MTMO/MO particles in the presence of a large number of mesopores. The mesopores are believed to be formed due to the release of gases during the thermal annealing process. Figure 2b represents the HRTEM image of ZCGNO, which indicates two sets of lattice fringes of ~0.21 nm and ~0.24 nm, corresponding to the (200) plane of NiO and the (311) plane of ZnCo2O4, respectively.19,34 In addition, the lattice fringes of rGO and the presence of nanopores are also observed. The corresponding elemental mapping confirms the good distribution of Zn, Co, Ni, O, and C in ZCGNO. Moreover, the EDX spectrum of ZCGNO also confirms the existence of individual elements (Figure S3).

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Figure 2. HRTEM Analysis: HRTEM images of ZCGNO (a) at low magnification, (b) at high magnification (inset: lattice fringes of ZCGNO), (c) corresponding elemental mapping of ZCGNO (detached from the Ni foam). The formation of ZnCo2O4 and NiO in the presence of rGO on Ni foam was further confirmed by SEM elemental mapping analysis, as shown in Figure 3. A uniform distribution of individual components, such as Zn, Co, O, Ni, and C are clearly visible from the mapping image. The corresponding EDX spectrum is included in the supporting information (Figure S4). From EDX analysis, the atomic % of the individual components was calculated, which confirmed the presence of ZnCo2O4 and NiO in ZCGNO (Figure S4).

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Figure 3. SEM elemental mapping of ZCGNO The structure of the Ni foam-supported ZCGNO nanowire arrays was characterized by XPS. Figure 4 presents the XP spectra of ZCGNO. The full wide-scan spectrum of ZCGNO showed characteristic peaks of C 1s, O 1s, Co 2p, Zn 2p, and Ni 2p, confirming the presence of C, O, Co, Zn, and Ni (Figure 4a). Figure 3b represents a high resolution Zn 2p spectrum. The peaks at 1021.6 and 1044.6 eV corresponds to Zn 2p3/2 and Zn 2p1/2, respectively, indicating the Zn(II) oxidation state of ZnCo2O4 in ZCGNO. On the other hand, the high resolution Co 2p spectrum reveals two peaks at the binding energy of 780 and 796 eV, which were assigned to Co 2p3/2 and Co 2p1/2, respectively (Figure 4c). This confirmed the Co(III) oxidation state of ZnCo2O4 in GZCNO. Furthermore, the high resolution Ni 2p spectrum displayed two distinct peaks at 854.3 and 873. 1 eV, which correspond to Ni 2p3/2 and Ni 2p1/2, respectively (Figure 4d). The Ni 2p spectrum can also be deconvoluted to two spin-orbit doublets and two shake up satellite peaks, which could appear due to the presence of both Ni2+ and Ni3+. A similar trend was observed in a previous study.35 Moreover, the high 10


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resolution O 1s spectrum showed two peaks at 529.6 and 531.5 eV, which corresponds to the MO/MTMO peak and defective sites within the MO/MTMO structure or adsorbed oxygen,36 respectively, confirming the presence of oxygen species (Figure S5a). In addition, the C 1s spectrum displayed characteristic peaks of C-C, C-OH, and O-C=O bonds, corresponding to the partial reduction of GO, which confirms the presence of rGO in ZCGNO (Figure S5b).

Figure 4. XPS Analysis: (a) XPS survey spectrum of ZCGNO confirming the presence of C 1s, N 1s, O 1s, Co 2p, Ni 2p, and Zn 2p levels; XPS spectrum of (b) Zn 2p, (c) Co 2p, and (d) Ni 2p of ZCGNO. XRD was performed to provide further structural information, such as the crystal structure and phase composition. Figure 5 shows the characteristic XRD pattern of ZCGNO. Apart from three characteristic peaks from the Ni substrate, the other XRD peaks were indexed to the (111), (220), (311), (400), (422), (522), (620) planes of ZnCo2O4 (JCPDS Card No. 23-1390), and the (111), (200), (220) planes of NiO (JCPDS Card No. 47-1049),

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respectively. The sharp XRD peaks indicate good crystallinity of ZCGNO. On the other hand, no significant XRD peak for graphene was observed, which might be due to the overlap of strong peaks from the Ni foam. This XRD result is in good agreement with the TEM images. Figure S6 presents the XRD pattern of ZCGNO powder (detached from the Ni foam), which confirms the coexistence of rGO, NiO and ZnCo2O4.

Figure 5. XRD Analysis: XRD pattern of ZCGNO. To confirm the presence of rGO in ZCGNO, the Raman spectra were recorded for rGO, ZCNO, and ZCGNO (detached from Ni foam), as shown in Figure 6. For ZCNO, the Raman active peaks at 484 cm-1 and 525 cm-1 corresponds to the Eg and F2g modes of ZnCo2O4, which are associated with the stretching of the bonds between Co-O and Zn-O.37 In addition, the peak at 700 cm-1 can be assigned to the A1g mode of ZnCo2O4. In the case of ZCGNO, broadening of peaks occurred within the Raman shift range of 450-700 cm-1, indicating the interaction between mixed metal oxide and rGO. The Raman spectrum of rGO showed the characteristic D band (1348 cm-1) and G band (1576 cm-1), which are associated with the breathing mode of A1g symmetry and the mode of E2g symmetry, respectively. The 12


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Raman spectrum of ZCGNO showed these two peaks, confirming the presence of graphene in the ternary nanocomposite. The ratio of the intensity of the D band to the G band (ID/IG) for rGO was calculated to be 0.96. In contrast, the ID/IG ratio was increased to 1.1 for ZCGNO, which was attributed to the defects created by the removal of oxygen moieties of GO. A similar trend was observed in a previous study.38

Figure 6. Raman spectra of rGO, ZCNO and ZCGNO. Based on the above morphological and structural analysis as well as previous reports, a probable growth mechanism of the ZCGNO heterostructures is proposed and a schematic diagram is shown in Figure 7. As shown in the scheme, initially, the precursor ions were located on the GO supported Ni foam due to the strong electrostatic interaction between the ions and GO (step-1). Under hydrothermal conditions, when the reaction temperature was increased to 210°C, the elemental Ni (from Ni foam) was converted to Ni2+ by a reaction with “high temperature water (HTW)” (liquid water above 200ºC) through free radical chemistry (Step-2). Further, these metal precursor ions formed primary particles and were situated on the rGO-supported Ni foam (Step-3). As the reaction proceeded, the metal ion precursors 13



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(Zinc hydroxyl carbonate and cobalt hydroxyl carbonate) and Ni(OH)2 formed primary nanosheets to minimize the surface area and aggregate randomly. Immediately after, these nanosheets were grown along with rGO to form hybrid nanosheets (Step-4). This unidirectional self-assembly of the hybrid nanosheets was driven by the intercalation of graphene sheets and the aggregation tendency of the metal ions. Finally, the porous ZCGNO were produced after the release of gases through a short thermal annealing treatment (Step-5).

+

Figure 7. Schematic diagram of the formation mechanism of ZCGNO. The probable reactions involved in the hydrothermal process and thermal annealing treatment are discussed here. During the hydrothermal reaction, elemental Ni reacted with HTW to form hydrogen radicals (H*) and hydroxide ions (OH-). OH- reacted with Ni2+ to form Ni(OH)2 on the surface of the Ni foam, whereas H* converted GO to rGO.39,40 14


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H2O + M → OH*+ H*+ M

(M - Collision Partner, primarily water)

(1)

Ni + OH*→ Ni2++ OH-

(2)

Ni2+ + 2OH- → Ni(OH)2

(3)

GO + H*→ rGO + H2O

(4)

At the same time, urea reacted with water and formed zinc hydroxyl carbonate and cobalt hydroxyl carbonate precursors.41

NH2CONH2 + H2O → 2NH3 + CO2

(5)

CO2 + H2O → H2CO3

(6)

H2CO3 → 2H+ + CO32-

(7)

4Zn2+

+

6

OH-

+

CO32-

+

H2O

→

Zn4(OH)6CO3.H2O

→

Co(OH)CO3.H2O

(8) Co2++

CO32-

+

OH-

+

H2O

(9) Finally, thermal annealing led to the conversion of zinc hydroxyl carbonate and cobalt hydroxyl carbonate precursors to ZnCo2O4 and Ni(OH)2 to NiO.41

Zn4(OH)6CO3.H2O + Co(OH)CO3.H2O → ZnCo2O4 + CO2 + H2O

(10)

Ni(OH)2 → NiO + H2O

(11)

Electrochemical Properties Effect of electrolyte concentration. Apart from the electrodes, the electrolyte is also 15



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an important parameter for supercapacitors. Among the different aqueous electrolytes, KOH is considered one of the most useful electrolytes because of its high ionic concentration and low resistance. On the other hand, the concentration of electrolytes plays an important role in determining the electrochemical properties. To understand the effects of the electrolyte concentration on the electrochemical properties of ZCGNO, electrochemical measurements were carried out in 1 M KOH and 6 M KOH. Figure 8 (a-d) compares the electrochemical properties of ZCGNO in KOH with two different concentrations. As shown in the figure, the area under the CV curve of ZCGNO was much higher in 6 M KOH than in 1 M KOH at a scan rate of 5 mV/s (Figure 8a). The presence of redox peaks within the potential range of 0 to 0.2, corresponds to the redox reactions of MTMO and MO. Figure 8b presents the chargedischarge profiles of ZCGNO at a current density of 3 A/g, which indicates that the duration of charging/discharging increased with increasing electrolyte concentration.

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Figure 8. Electrochemical Analysis of ZCGNO in 1M KOH and 6M KOH: (a) CV curves at 5 mV/s scan rate (b) Cyclic charge-discharge profiles at the current density of 3 A/g (c) Specific capacitance vs. current density curves (d) Nyquist plots. The Csp of ZCGNO was calculated from the charge/discharge curves at different current densities using equation 1 (supporting information) and Figure 8c presents the variation of Csp as a function of the current density. ZCGNO exhibited Csp of 485, 458, 433, 350, and 291.6 F/g at current densities of 3, 5, 10, 15, and 25 A/g, respectively, in 1 M KOH. In contrast, ZCGNO exhibited significantly higher Csp in 6 M KOH, as high as 1256, 1115, 716.6, 685, and 666.6 F/g at current densities of 3, 5, 10, 15, and 25 A/g, respectively. These results suggest that ZCGNO exhibits higher Csp in the highly concentrated KOH electrolyte, indicating an enhanced charge storage mechanism.42,43 In addition, with increasing concentration, the ionic conductivity of KOH increased, which not only facilitated high charge transfer in both the bulk electrolyte and electrode, but also decreased the diffusion resistance, resulting enhanced electrochemical performance. Although ZCGNO in 1 M KOH has a lower Csp, it has a higher rate stability (~ 60%) than ZCGNO in 6 M KOH (~ 53%). This indicates that at low concentration of KOH electrolyte, the electrode material provides superior charge storage performance, which is consistent with a previous report.44 Figure 8d shows the typical Nyquist plots of ZCGNO in 1 M and 6 M KOH. The x axis intercept at the high frequency region represents the equivalent series resistance (ESR), which is generally associated with the contact resistance between the active electrode materials and substrate, intrinsic resistance of the electrode material, and the ionic resistance of the electrolyte solution. The straight line at the low frequency zone corresponds to the Warburg impedance (W), which is related to the diffusion of ions in the electrode. ZCGNO possesses a relatively lower ESR in 6 M KOH (0.58 Ω) than in 1 M KOH (0.77 Ω). In addition, it exhibits a smaller 17



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slope at the low frequency region in a dense solution (6 M KOH), indicating the suitability of the active electrode material for ion diffusion of the electrolyte. The lowering of the ESR and W value with increasing electrolyte concentration can be explained by the enhanced electrolyte conductivity and the decrease in diffusion resistance.

45, 46

The superior

electrochemical performance of ZCGNO in 6 M KOH may be due to the following reasons: (i) increase in the ionic conductivity of the electrolyte with increasing concentration; (ii) increase in the ion concentration resulting in superior ion accessibility through the surface of the electrode, which allowed the electrode material to store high charge; and (iii) lowering of the diffusion resistance with increasing electrolyte concentration. Other electrochemical properties. Further electrochemical characterization of ZCGNO was carried out in 6 M KOH, as shown in Figure 9 (a-d). Figure 7a presents CV curves of ZCGNO at different scan rates within the potential range of -0.6 to 0.2 V. At all scan rates, the shape of the CV curves deviates from a rectangular nature, suggesting the presence of redox reaction and the contribution of MTMO and MO in the charge storage mechanism.

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Figure 9. Electrochemical analysis of ZCGNO in 6 M KOH: (a) CV curves at different scan rates within the potential range of -0.2 to 0.4 V, (b) Charge-discharge profiles at different current densities, (c) Ragone plot and (d) Nyquist plots (before and after cycling). Figure 9b presents the typical charge-discharge profiles of ZCGNO at different current densities. The maximum specific capacitance of 1,256 F/g for ZCGNO in 6 M KOH at a current density of 3 A/g was found to be higher than that with other promising ternary supercapacitor electrode materials (Table S1). Figure 9c shows the energy density vs. power density plot (also known as Ragone plot) of ZCGNO. The electrode exhibited a maximum energy density of 62.8 Wh/kg at a power density of 895 W/kg. The electrode achieved the highest power density of 7,492.5 W/Kg with the corresponding energy density of 33.3 Wh/kg. The maximum energy density of ZCGNO was higher than several binder free supercapacitance electrodes, such as Ni3S2/Ni foam (44.89 Wh/kg), CuCo2O4/MnO2 (43.3 Wh/kg), NiO-NFs/Ni (22.7 Wh/kg), ZnCo2O4 nanowire cluster array/Ni foam (12.5 Wh/kg), 19



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and ZnCo2O4 nanowire array/Ni foam (41 Wh/kg).47-51 For practical supercapacitor applications, apart from the high specific capacitance, the long term cyclic stability of the electrodes is also an important requirement. The cyclic stability of ZCGNO was measured by repeating the charge-discharge measurements for 1500 cycles at a current density of 25 A/g. Figure 9d shows the Nyquist plots obtained before and after cycling, revealing no significant change in the nature of the curves, indicating the stable electrochemical properties of ZCGNO. Nevertheless, the ESR value increased from 0.58 to 0.75 Ω after 1500 cycles, suggesting a lowering of the electrode conductivity during cycling. In addition, at lower frequencies, the diffusive resistance (W) increased after cycling, which suggests that the electrolyte penetration and ion diffusion in the active electrodes was not deceased significantly.52 Figure 10a presents the cycling performance of ZCGNO at a constant current density of 25 A/g. The electrode material achieved ~80% retention of its initial capacitance after 3000 cycles. The capacitance retention of ZCGNO is better than the other supercapacitor electrode materials (Table S2). Figure 10b presents last few charge-discharge cycles. The curves showed identical shape, which indicates the favorable electrochemical reversibility and charge-discharge properties of ZCGNO. After cycling, the color of the electrolyte showed no visible change. Figure S8 shows the morphology of ZCGNO after 3000 cycles. The SEM image indicates the structural deformation during cycling, which is responsible for the decrease in capacitance after 3000 cycles.

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Figure 10. Cyclic stability analysis of ZCGNO in 6M KOH: (a) Specific capacitance vs. cycle no. curve (up to 3000 cycles). (b) Last few charge-discharge cycles. To examine the effects of the Ni foam substrate on the electrochemical performance of ZCGNO, cyclic voltammetry of the bare Ni foam was performed at a 5 mV/s scan rate, as shown in Figure 11a. Compared to ZCGNO, the CV integrated area of the Ni foam was quite small, indicating a minute contribution of the Ni foam to the overall capacitance of ZCGNO. Interestingly, the direct growth of ZCGNO on the conductive Ni foam substrate utilizes majority of the electrode materials in the ultrafast electrochemical reaction, which is shown schematically in Figure 11b.

Figure 11. Electrochemical properties of ZCGNO in 6 M KOH: (a) CV curves of bare Ni foam and ZCGNO at 5 mV/s scan rate within the potential range of -0.2 to 0.4 V, (b) Schematic diagram showing the benefits of the ZCGNO electrode for supercapacitors. 21



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To understand the synergistic contribution of the individual components in ZCGNO, the electrochemical performance of rGO, NO, ZCNO were measured in 6 M KOH and compared with ZCGNO. As observed from Figure S9a, ZCGNO shows longest chargingdischarging time among all the electrode materials at a current density of 3 A/g. The ternary nanohybrid also exhibited a relatively high current response in the cyclic voltammogram at a scan rate of 5 mV/s, indicating superior supercapacitive properties (Figure S9b). The electrode materials, rGO, NO, and ZCNO, exhibited a specific capacitance of 86, 156, and 627 F/g, respectively at a current density of 3 A/g (Figure S9c). Note that a single metal oxide like NiO exhibited relatively low specific capacitance, but when it was combined with ZnCo2O4, a relatively high capacitance was achieved. In addition, by combining the three components, a very high specific capacitance was accomplished, which is double that of the binary composite. A similar trend was observed in the case of the rate capability (for current density between 3 and 25 A/g) of the electrode materials (Figure S9d). Among all the electrode materials, rGO showed lowest rate capability of 38.7%, whereas NO showed a moderate rate capability of 42.7%. Eventually, after adding ZnCo2O4, the rate stability of NiO was increased to 46.5% (for ZCNO). ZCGNO showed highest rate capability (53%) among all the electrodes examined due to the better electrostatic charge storage with the addition of graphene. The electrochemical activities of rGO and NO were limited by the low areal mass loading of the active materials. The mass loadings for rGO and NO were calculated to be 0.4 and 0.7 mg/cm2, respectively. The low mass loading of rGO was attributed to its microporous structure.53,54 Compared to rGO, the mass loading of the active material increased for NO due to the decrease in porosity, as observed from the SEM images. ZCNO showed a high mass loading of 1.3 mg/cm2, which was increased further to 1.5 mg/cm2 after the addition of graphene (for ZCGNO), owing to its mesoporous structure. Figure S10a presents Nyquist 22


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plots of all the electrodes. Among all the electrode materials examined, NO showed the highest ESR of 0.85 Ω. On the other hand, after the addition of ZnCo2O4, the conductivity increased, which resulted a lowering of the ESR value to 0.62 Ω (for ZCNO). Owing to the high conductivity of graphene, rGO showed a relatively low ESR of 0.59 Ω. Finally, through the synergistic contribution from the individual components including metal oxides and graphene, ZCGNO showed the lowest ESR value of 0.58 Ω. The Nyquist plot of ZCGNO was fitted with an equivalent electrical circuit. Figure S10b presents the experimental impedance data and the impedance data fitted with the equivalent circuit; ZCGNO showed a low solution resistance of 0.58 Ω. Finally, the admirable electrochemical performance of ZCGNO could be attributed to the following features: (i) unique nanoarchitectures (porous nanowire arrays) of ZGCNO can facilitate easy and fast electrolyte accessibility, intimate contact between the electrode and electrolytes, and endorse rapid ion diffusion; (ii) enhanced electrical conductivity of the electrode due to the presence of rGO; (iii) synergistic contribution of the individual components, such as ZnCo2O4, NiO, and rGO; and (iv) direct growth of the active electrode material on the current collector enhances the mechanical adhesion and electrical contact with the conductive substrate.

CONCLUSIONS A ternary nanohybrid based on ZnCo2O4, rGO and NiO was grown directly on Ni foam through a facile hydrothermal process followed by a short post annealing treatment. Because of the well-defined porous morphology, high active surface area, and synergistic contributions of the individual components, the as-synthesized active material exhibited high specific capacitance and energy density. Both were significantly higher than those previously reported for binary and ternary composites. In addition, the material demonstrated good long23



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term cyclic stability and low ESR. These results suggest that the as-prepared ZCGNO could be a superior candidate for the fabrication of binder free electrodes. It is also suggested that, the proposed synthetic strategy can be extended to the fabrication of other ternary binder free electrodes for next generation supercapacitor applications.

ASSOCIATED CONTENT Supporting Information Preparation method of graphene oxide, reduced graphene oxide@Ni foam, NiO@Ni foam, ZnCo2O4/NiO@Ni foam, Equations used for the electrochemical data calculation, Low magnification SEM image, EDX spectrum, XPS analysis of ZCGNO, Additional SEM images, Charge-discharge profiles of ZCGNO in 1 M KOH, Electrochemical properties of other electrode materials, EIS fitting data for ZCGNO, Comparison of the specific capacitance and cyclic stability with previously published works.

AUTHOR INFORMATION Corresponding Author

* JAE-JIN SHIM. E-mail: ([email protected]) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This

study

was

supported

by

the

Priority

Research

Centers

Program

(NRF-

2014R1A6A1031189) supported by the Ministry of Education, the Republic of Korea, and 24


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the Korea-China International Cooperation Program (NRF-2015K2A2A7053101) through the National Research Foundation of Korea (NRF).

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Interfaces 2016, 8, 2297-2305.

For Table of Contents Use Only

Facile Synthesis of 3D Ternary ZnCo2O4/Reduced Graphene Oxide/NiO Composite Film on Nickel Foam for Next Generation Supercapacitor Electrodes 31



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Sumanta Sahoo and Jae-Jin Shim*

Synopsis A 3D ternary binder-free supercapacitor electrode based on ZnCo2O4, Reduced Graphene Oxide and NiO is developed.

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A 3D ternary binder-free supercapacitor electrode based on ZnCo2O4, Reduced Graphene Oxide and NiO is developed. 232x145mm (150 x 150 DPI)


Reduced

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