Phase Transformation Induced Capacitance Activation for 3D Graphene-CoO Nanorod Pseudocapacitor

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Phase Transformation Induced Capacitance Activation for 3D Graphene-CoO Nanorod Pseudocapacitor

Yun Guang Zhu , Ye Wang , Yumeng Shi , Zhi Xiang Huang , Lin Fu , and Hui Ying Yang *

Y. G. Zhu, Y. Wang, Y. Shi, Z. X. Huang, L. Fu, H. Y. Yang Pillar of Engineering Product Development Singapore University of Technology and Design 20 Dover Drive , Singapore 138682 E-mail: [email protected]

DOI: 10.1002/aenm.201301788

Although metal oxides based pseudo-capacitors have higher energy densities, they still suffer from the low power densi-ties and unstable cycling performance due to redox reaction at the surface of active materials. [ 19 ] Recently, several approaches have been developed to overcome these issues. The composites of carbon-based materials and metal oxides were used as hybrid electrodes, which take advantages of EDLCs and pseudo-capacitors. [ 11,26–32 ] Metal oxides (or carbon based materials) with conducting polymers nanocom-posites were also employed to provide high conductivity and large energy den-sity. [ 16,33–36 ] Moreover, the increasement of capacitance during activation process of metal oxides based pseudo-capacitors has been observed by a few research groups. [ 16,20–25 ] Zhang et al. reported NiO supercapacitor with a signifi cant 190% increasement of capacitance compared to the initial value after the activation pro-cess. [ 20 ] Similar phenomenon was also

clearly observed in graphene-nickel cobaltite nanocomposite asymmetrical supercapacitor system. [ 24 ] However, a detail study of the increased capacitance during the activation process mechanism is not revealed yet. [ 16,20–25 ] Here, we investigated the activation process mechanism which can be attributed to the phase transformation process of binder-free cobalt oxides electrode during charge and discharge process via facile ex situ methods.

Cobalt oxides or hydroxides are considered as promising high performance pseudo-capacitive active materials in energy storage technologies for their high theoretical capaci-tance (i.e. > 3000 F g −1 ). [ 16,37–41 ] However, the specifi c capaci-tance of actual cobalt oxides pseudo-capacitors is often much lower than the theoretical value due to the low conductivity and limited surface area. [ 25,42,43 ] In order to tackle these chal-lenges, three dimensional graphene (3DG) grown on nickel foam was used as the substrate and current collector. [ 44,45 ] Then, CoO nanorod clusters were anchored on 3DG to form a binder-free CoO-3DG structural electrode, which facilitates electrolyte penetration and ion diffusion. During cycles, the morphology of nanoporous CoO-3DG changed from nanorods to nanofl owers was associated with the activation process. In addition, the phase transformation was analyzed to explain the unstable nature in the performance of metal oxides based pseudo-capacitors.

Development of a pseudocapacitor over the integration of metal oxide on car-bonaceous materials is a promising step towards energy storage devices with high energy and power densities. Here, a self-assembled cobalt oxide (CoO) nanorod cluster on three-dimensional graphene (CoO-3DG) is synthesized through a facile hydrothermal method followed by heat treatment. As an additive-free electrode, CoO-3DG exhibits good electrochemical performance. Compared with CoO nanorod clusters grown on Ni foam (i.e., CoO-Ni, ≈680 F g −1 at 1 A g −1 and ≈400 F g −1 at 20 A g −1 ), CoO-3DG achieves much higher capacitance (i.e., ≈980 F g −1 at 1 A g −1 and ≈600 F g −1 at 20 A g −1 ) with excellent cycling stability of 103% retention of specifi c capacitance after 10 000 cycles. Furthermore, it shows an interesting activation process and instability with a redox reaction for CoO. In addition, the phase transforma-tion from CoO nanorods to Co 3 O 4 nanostructures was observed and inves-tigated after charge and discharge process, which suggests the activation kinetics and the phase transformable nature of CoO based nanostructure. These observations demonstrate phase transformation with morphological change induced capacitance increasement in the emergent class of metal oxide materials for electrochemical energy storage device.

1. Introduction

The current trend in energy storage systems with higher energy density and longer lifespan demands the study of highly effi cient nanomaterials based on lithium ion batteries, metal air batteries, and supercapacitors. [ 1–7 ] In particular, elec-trochemical capacitors (ECs) have attracted growing interest owing to their ultrashort charge time and high power den-sity. [ 8–11 ] There are mainly two types of EC: electric double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs deliver energy via charge accumulation on electrode mate-rials’ surface due to the electrostatic force rather than Fara-daic reaction, leading to high power density and long cycle lifetime. [ 12–14 ] Pseudocapacitors, however, utilize reversible redox reactions on the surface of active materials, including metal oxides, metal hydroxides, conducting polymers, and so on. [ 15–18 ]

Adv. Energy Mater. 2014, 1301788

http://doi.wiley.com/10.1002/aenm.201301788


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2. Results and Discussion

2.1. Characterization of the Materials

CoO nanorod clusters were grown on high quality 3DG or Ni foam through a hydrothermal reaction in an autoclave followed by heat treatment. XRD was conducted for CoO-3DG and its precursor Co(OH)F, as shown in Figure 1 a. The precursor Co(OH)F was synthesized by hydrothermal method, and its characteristic peaks correspond to the orthorhombic structure of cobalt fl uoride hydroxide (JCPDS 50–0827). After heat treat-ment, the fi nal product CoO was formed and its corresponding crystalline structure was cubic (JCPDS 48–1719). XRD peaks belong to 3DG were not observed due to the low content of 3DG covered by thick CoO layer. The Raman spectrum of the as-prepared 3DG is shown in Figure 1 b. The low ratio of the D to G band indicated the high quality of the 3DG on Ni foam.

Meanwhile, the relative low ratio of the 2D to G band suggests that 3DG only consists of a few layers of graphene, which may result in good conductivity. [ 44 ] In addition, peaks centered at 526 and 686 cm −1 belonged to CoO were found from the Raman spectra of both CoO-3DG and CoO-Ni (Figure S1, Sup-porting Information). [ 46–48 ] Meanwhile, the weak D band and G band peaks of 3D gra-phene were found in Raman spectrum of CoO coated 3DG.

The surface morphology of CoO-3DG is shown in Figure 2 a. The CoO nanorod cluster is composed of rhomboid shape CoO

nanorods (top view) as shown in the inset of Figure 2 a. A typical transmission electron microscopy (TEM) image of single CoO nanorod is shown in Figure 2 b. The length and diameter of CoO nanorod were about 2.5 μm and 430 nm, respectively. The porous structure at the surface of CoO nanorod could be attributed to the loss of F − ions during the heat treatment reported by Zhu et al. [ 49 ] For the as-prepared 3DG, graphene sheets were uniformly covered on the surface of Ni foam, as shown in Figure S2a. CoO nanorods were grown at the defects or grain boundaries of 3DG, as shown in Figure S2b. Therefore, a growth process and mecha-nism of CoO nanorods on 3DG could be proposed in a sche-matic diagram as shown in Figure 2 c. According to a previous report, the graphene wrinkles are more active than the surface plan region, and the wrinkles attract the precursor in the solu-tion and act as the fi rst nucleation center for CoO growth. [ 50,51 ] Therefore, Co + , F − , and OH − ions were easy to anchor at the wrinkles or folding domains of 3DG to grow as crystal seeds at


Figure 1. a) XRD patterns of CoO-3DG and its precusor Co(OH)F-3DG, b) Raman spectrum of as prepared 3DG on Ni foam.

Figure 2. a) SEM image of CoO nanorod cluster on 3DG. Inset in (a) is a magnifi ed SEM image of rhombic CoO nanorods. b) TEM image of a single CoO nanorod with porous structure. Inset in (b) is a magnifi ed TEM image of CoO nanorod. c) Schematic illustration of the synthesis process of CoO nanorod clusters on 3DG-Ni.


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the fi rst stage of growth. After that, the self-assembled Co(OH)F nanorods aggregated together at these wrinkles to grow clus-ters. The CoO nanorods were synthesized after heat treatment. To verify the nanorods nucleation sites on the graphene surface, a control experiment by reducing the precursor concentration during the hydrothermal growth process was performed. It was found that CoO nanoparticles grew only at the 3DG wrinkled regions, which are the grain boundaries and defects sites of 3DG (Figure S3, Supporting Information). This phenomenon suggests that the graphene wrinkles can effectively serve as the fi rst nucleation centers. Therefore, CoO nanorods were easily anchored on these wrinkle regions, as shown in Figure S2b. The same phenomenon was also found in the control sample of CoO grown on nickel foam. CoO nanorods synthesized in a shortened time were only grown at the grain boundaries of nickel foam, as clearly shown in Figure S4b. At the same time, small CoO par-ticles were also found on the surface of nickel foam. With the increase of growth time, CoO nanorods on nickel foam were also grown and aggregated as clusters. In addition, the surface morphology of CoO grown on 3DG was similar as that grown on nickel foam, as indicated by the low magnifi cation SEM images as shown in Figure S5a,b. However, the loading of CoO-3DG (≈1.30 mg cm −2 ) is lower than that of CoO grown on nickel foam (≈2.70 mg cm −2 ). The reason is that the CoO nanorod clusters only grew at the grain boundaries of 3DG, and the surface of graphene is clean as shown in the inset of Figure S5c (cross sec-tion of CoO-3DG), whereas not only CoO clusters were found at the grain boundaries of nickel foam, but CoO nanoparticles were also grown on the surface of nickel foam, as shown in the inset of Figure S5d (cross section of CoO-Ni). Figure S5c,d pro-vide the cross section images of the two samples. The thickness of CoO cluster for two samples is about 10 μm. As shown in the insets of Figure S5c,d, after growth of CoO, the surface of gra-phene remained clean while that of nickel foam became rough and was covered with a lot of nanoparticles.

2.2. Electrochemical Testing

In order to study the impact of 3DG on the electrochemical per-formance of CoO, a series of electrochemical tests—including cyclic voltammetry (CV), rate capacitance, cycling stability at a constant current of 5 A g −1 , and electrochemical impedance spectroscopy (EIS)—were carried out for both CoO-3DG and CoO-Ni samples. Figure 3 a,b depict the CV curves of CoO-Ni and CoO-3DG at various scanning rates (from 5 to 100 mV s −1 ). The obvious redox peaks in the CV curves indicate the revers-ible electrochemical reactions. It can be seen that CoO-3DG had improved response than CoO-Ni at high scan rates with clear redox peaks. With the presence of 3DG, the polarization effect of CoO-3DG is signifi cant reduced compared to the sample without 3DG. [ 52 ] The rate capability was also measured at the current densities from 1 to 20 A g −1 , as shown in Figure 3 c. CoO-3DG could deliver a high capacitance of 980 F g −1 at a cur-rent density of 1 A g −1 while CoO-Ni can only deliver a capaci-tance of 680 F g −1 at the same current density. Even at a current density of 20 A g −1 , the capacitance of CoO-3DG could still maintain about 600 F g −1 , which is higher than that of CoO-Ni by almost 200 F g −1 . The specifi c capacitance of the two samples

was decreased with increasing charge and discharge current density, indicating proton depletion during the redox transition of cobalt oxides and a signifi cant decrease of ionic conductivity in the electrolyte. [ 52 ] The proton depletion could be attributed to the existence of a large portion of inaccessible (or diffi cult-to-access) surface area for the cobalt oxides with anodizing. [ 53 ]

In order to evaluate the electrochemical performance of the two samples, Ragone plots of the energy density (E) to the power density (P) are shown in the inset of Figure 3 c. The energy density (E) and power density were calculated from the galvanostatic charge-discharge curves according to Equation ( 1) and ( 2) . [ 8,54 ]

E CV /22= (1)

P QV/2t E/t= = (2)

Where E, C, V, Q, t, and P are average energy density, specifi c capacitance based on the unit mass of the active material, the potential window of discharge, total charge delivered, discharge time and average power density, respectively. From the inset of Figure 3 c, it can be seen that the energy densities of both sam-ples were reduced slowly with the increase of the power density. CoO-3DG and CoO-Ni can deliver energy densities of 22.8 and 15.6 Wh kg −1 at the power density of 5200 W kg −1 , respectively. The high power and energy densities of CoO-3DG indicate its promising potential for the application in high power and energy devices, such as electric vehicles.

Both CoO-3DG and CoO-Ni exhibited good cycling stability, as shown in Figure 3 d. The cycling stability could be attrib-uted to the stable structure of CoO nanorod clusters on 3DG or Ni foam. The fi nal specifi c capacitances of CoO-3DG and CoO-Ni were about 103% and 112% compared to their initial values, respectively. During the cycling test, the activation pro-cess (before the 1000 th cycle) resulted in an increase of spe-cifi c capacitance, which is in good agreement with previous reports. [ 20–22,55,56 ] In both electrodes (i.e., CoO-3DG and CoO-Ni), additive-free design was applied, allowing further investiga-tion of the change in the samples via ex situ methods. In addi-tion, the specifi c capacitance of CoO-3DG is much higher than that of CoO-Ni, which could be attributed to the improved elec-tron conductivity with the assistance of 3DG, indicated by the following EIS spectra. Figure 3 e shows the change of charge-discharge curves of the two samples during the process. After 10 000 cycles, the two electrodes still maintained a good electro-chemical reversibility with coulombic effi ciency above 99%. In addition, the two pseudo-capacitors still present distinct plateau region, which could indicate the two samples still maintain good activity of redox reactions after 10 000 cycles.

In order to explore the infl uence of electrochemical imped-ance to the supercapacitor, EIS measurements were carried out in room temperature after different cycles (500, 1500, and 10 000). Figure 3 f exhibits the Nyquist plots of CoO-3DG and CoO-Ni in the whole frequency range from 10 5 to 0.01 Hz. It can be seen that the CoO-3DG has a much smaller kinetic arc at high frequencies than that of CoO-Ni, which implied that the charge transfer resistance ( R ct ) of the former is much lower than that of CoO-Ni. The lower R ct of CoO-3DG could be attributed to the contribution from 3DG, which covered on



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the Ni foam and improved the surfi cial conductivity. It was also observed that CoO nanoparticles were hardly grow on smooth graphene surface, while they preferentially covered on the sur-face of Ni foam (see Figure S5c,d). The layer of CoO nanopar-ticles on the nickel foam would increase the charge resistance between CoO and nickel foam. As a result, electrochemical performance of CoO-3DG is better than that of CoO-Ni. In addition, CoO-3DG electrode exhibited higher angle than that of CoO-Ni, which implied the CoO-3DG was more suitable for low leakage supercapacitors. [ 57 ] In order to analyze the change of R ct with cycling number, EIS measurements after different cycling number (500, 1500, and 10 000 cycles) are shown in Figure S6. R ct of CoO-Ni has increased from the 500 th cycle to the 1500 th cycle, and remained the same after 1500 cycles. On the other hand, the kinetic arcs for CoO-3DG are too small to be observed, which showed that the R ct was not signifi cantly affected by the cycling process. This result also indicates the charge resistance is increased by the presence of the CoO nano-particles on Ni foam.

2.3. Mechanism of the Phase Transformation During Cycling Process

In order to explore the mechanism of improvement of spe-cifi c capacitance for both materials, we investigated morpho-logical transformation with cycling process via ex situ methods. Figure 4 a–d exhibits morphology evolution of CoO nanorods on 3DG after different cycles (0, 500, 1500, 10000). In the Figure 4 a, CoO nanorods have sharp edges and rhomboid shape without cycles, and the surface was smooth without obvious porous struc-ture. However, CoO nanorods became blurry edges and rhom-boid shape after 500 cycles (Figure 4 b) and the surface exhibited distinct porous structure and even some tiny nanofl akes. After 1500 cycles, the nanofl akes became larger and showed a fl ower-like morphology (Figure 4 c). With the increase of cycles, CoO nanorods were hardly distinguished and became nanofl owers after 10 000 cycles. A schematic diagram of the morphology evo-lution process with cycling process is shown in Figure 4 e. The porous structure of CoO nanorods would induce OH − to react


Figure 3. Electrochemical performance of CoO-Ni and CoO-3DG: a) CoO-Ni and b) CoO-3DG CV curves with various scan rates. c) Rate capability of CoO-Ni and CoO-3DG with the current densities from 1 A g −1 to 20 A g −1 . Inset in (c) is the Ragone plots of energy density vs. power density of CoO-Ni and CoO-3DG supercapacitors. d) Cycling performance of CoO-Ni and CoO-3DG at a current density of 5 A g −1 . e) Charge and discharge curves CoO-Ni and 3DG-CoO at a current density of 5 A g −1 after 500 cycles and 10000 cycles. f) EIS spectra of CoO-Ni and 3DG-CoO after 500 cycles.


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with Co 2+ in pores, which facilitated the pore to become bigger and bigger until cobalt oxides reshaped as nanofl owers. In order to investigate the function of 3DG on morphological transforma-tion, SEM images of morphology evolution of CoO-Ni before cycling process and after 10 000 cycles are shown in Figure S7. The same morphology was also found from CoO-Ni, indicating that 3DG had no effects on morphological transformation.

It is well known that pseudo-supercapacitors store energy by utilizing redox reactions on the surface area of active mate-rials. Thus, the morphological change from nanorods to nano-fl owers would facilitate the utilization of active materials for specifi c capacitance. There are several factors infl uence the specifi c capacitance for metal oxides based pseudo-capacitor: (1) the effective specifi c surface area; (2) the valence state of metal

ions; (3) the conductivity of active materials; (4) the stability of active materials on the substrate. From the above results and discus-sion, the reason for the activation process is the increase of surface area (from nanorods to nanofl owers) with redox reaction process. Further study showed that the whole cycling process was also involved with the chemical reaction induced phase transformation.

In order to identify the chemical reaction with morphological transformation during charge and discharge process, CoO-3DG after different cycles were investigated via high-resolution transmission electron microscope (HRTEM) and Raman spectra, as shown in Figure 5 . Figure 5 a shows a typical HRTEM image of a porous CoO nanorod (without cycling process). The lattice fringes with dis-tance of 0.15 and 0.25 nm are corresponding to (220) and (111) crystal plane of CoO, respectively. The inset in Figure 5 a shows the corresponding selected area electronic dif-fraction (SAED), indicating that CoO nanorod is a single crystal. After 10 000 cycles, many petaloid nanofl akes on the surface of CoO nanorod were observed (Figure 5 b). It is inter-esting to fi nd that the distance of the lattice spacing along with nanofl akes is 0.23 nm, which is corresponding to (222) plane of Co 3 O 4 according to the HRTEM image in the


Figure 4. SEM images of CoO nanorods on 3DG-Ni with various electrochemical cycles a) 0, b) 500, c) 1500, and d) 10 000 cycles. e) Schematic dia-gram of the morphologic transformation of CoO nanorods with cycles.

Figure 5. a) Low-magnifi caition TEM image of porous CoO nanorod (without cycling process). Inset in (a) is the HRTEM image and SAED pattern of the CoO nanorod. b) TEM image of CoO nanorod with nanofl akes at the surface after 10000 cycles. Inset in (b) is the HRTEM image and SAED pattern of the CoO nanofl akes. c) Raman spectra of CoO nanorod cluster with the increase of cycles (0, 1500, and 10 000).


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inset of Figure 5 b. The enlarged lattice spacing exhibited that the new products was generated together with nanofl ower mor-phology after redox reaction process. The SAED pattern of the nanofl owers exhibited that CoO single crystal had transformed to polycrystalline structure.

In order to identify what product had been produced after the cycling process, Raman spectra of the CoO-3DG after dif-ferent cycles (0, 1500, 10 000) were measured and shown in Figure 5 c. For the pristine CoO, the broaden peak centered at 526 cm −1 can be attributed to the CoO (F 2g ) symmetric stretching mode. [ 46,47 ] In addition, the peak at 686 cm −1 corresponds to A 1g mode. [ 48 ] After 1500 cycles, four peaks (481, 526, 617, and 686 cm −1 ) were found, which are the typical Raman peaks of Co 3 O 4 . This result shows the evidence of the phase transformation during charge and discharge process. [ 45 ] In the Raman spectra of CoO-3DG after 10 000 cycles, the posi-tions of these peaks were the same as these after 1500 cycles, but the intensity of the peaks became stronger. Thus, more and more CoO was transformed into Co 3 O 4 with the of the cycles. The equations of the redox reactions during charge and dis-charge process are: [ 58 ]

CoO OH CoOOH e+ = +− − (3)

CoOOH OH CoO H O e2 2+ = + +− − (4)

For the Equation ( 3) and ( 4) , if the redox reactions were com-pletely reversible, there would be no other phases after cycling process. According to the density functional theory (DFT) calcu-lation reported by Chen et al, both CoOOH and CoO 2 are stable under oxidation conditions. [ 59 ] In other words, the reaction ( 4) is a reversible process. Therefore, the side products are mainly from CoOOH. However, during the discharge process, CoOOH partially transform into Co 3 O 4 according the following reaction Equation ( 5) :

3CoOOH + e Co O OH H O3 4 2= + +− − (5)

The morphological transformation from porous nanorods to nanofl owers can be attributed to the crystalline transformation from CoO or Co 3 O 4 to CoOOH. CoOOH usually represented as layered structure. [ 59,60 ] The layered CoOOH would promote the transformation from nanorods to nano-fl akes and fi nal nanofl owers. In order to investigate the role of CoOOH for phase transformation, following control experi-ments and corresponding explanations were conducted in sequence. First, synthesized CoO nanorods were immersed in 5 M KOH solution for 48 h, the morphology of CoO did not change (see Figure S8). The situa-tion of CoO nanorods dissolved in 5 M KOH solution can be excluded. Secondly, the fi rst redox reaction (Equation ( 3) ) was carried out below 0.3 V in the cycling process as shown in Figure 3 a,b. [ 61,62 ] CoOOH was gener-ated during this process. In other words, if

the morphology of the sample became layered structure after cycling between 0–0.3 V, the phase transformation is induced by the present of CoOOH. The CV curve of CoO-3DG sample sweep between 0–0.3 V is shown in Figure S9a. After 1500 cycles, fl ower-like nanorods were found as shown in the Figure S9b. Thus, the layered CoOOH promoted the phase transfor-mation process. In addition, according to thermodynamics and the Pourbaix diagram for the Co-H 2 O system, CoO cannot dis-solve into 5 M KOH (pH value is about 13.3) when cycled from 0 to 0.52 V vs. Hg/HgO (i.e. –0.098 to 0.5102 V vs. SHE. [ 63 ] The Pourbaix diagram indicates Co 3 O 4 can be easily generated from CoOOH. In the reverse process, CoOOH was reduced to Co 3 O 4 or CoO during discharge process, while the nanofl ower-like morphology was maintained.

In order to further confi rm the phase transformation, X-ray photoelectron spectroscopy (XPS) was measured shown in Figure 6 . Figure 6 a,b show the high resolution spectra of Co 2p for CoO-3DG before and after cycling process. In Figure 6 a, the peak at 782.3 eV is corresponding to Co 2p 3/2 of CoO, while the shake-up peak at 787.8 eV would also indicate the exist-ence of CoO. On the other hand, the peaks shifted to a relative lower binding energy, indicating the presence of Co 3 O 4 . [ 62 ] In addition, the shake-up peak at 787.8 eV became weak due to the surfi cial content of CoO become much less after 10 000 cycles than that before cycles. [ 64,65 ] For the original CoO-3DG, two peaks located at 532.1 and 530.2 eV from the fi tted curves of O 1s spectra were found in Figure 6 c. The peaks at 532.1 and 530.2 were corresponding to O from the OH and oxide ions, respectively. After 10 000 cycles, a new peak at 534.6 eV could be attributed to the existence of Co 3 O 4 . [ 46 ] In addition, the peak at 530.4 was obviously increased, indicating the exist-ence of residual CoOOH or Co(OH) 2 . [ 46 ] Through the above analysis, phase transformation is confi rmed after cycling process.


Figure 6. XPS analysis of CoO-3DG with different cycles. Co 3d states of CoO-3DG sample. a) before and b) after 10 000 cycles. O 1s states of CoO-3DG. c) before and d) after 10 000 cycles


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Received: November 22, 2013 Revised: January 27, 2014

Published online:


3. Conclusion

In this study, 3DG templates provided stable substrates to grow CoO nanorod clusters, which exhibited good cycling per-formance (keep 103% retention of specifi c capacitance after 10 000 cycles) and high specifi c capacitance (980 and 680 F g −1 at current densities of 1 and 20 A g −1 , respectively). The stable 3D electrodes without additives facilitated the investigation of activation process and instability of CoO during charge and discharge process via an ex situ method. During the cycling process, CoO porous nanorods transformed to nanofl owers, improving the effective surface area and specifi c capaci-tance. The activation process detailed study provides deeper insight to understand the mechanism of activation process and transformation nature of cobalt oxides based pseudo-capacitors. In view of these promising results in the redox reaction and activation process, CoO nanostructures have the great potential for engineering and design of next-generation pseudo-capacitors.

4. Experimental Section

4.1. Modifi cation of Ni foam with Three Dimensional Graphene (3DG)

The method to grow 3DG on Ni foam can be found from previous method with further modifi cation. [ 45 ] In short, the temperature of tube furnace was heated to 1000 °C under Ar gas at a rate of 50 °C min −1 ; and then ethanol was bubbled by Ar fl ow for 10 min for 3DG growth with the presence of Ni foam. Subsequently, the tube furnace was cooled down to room temperature with a fast rate (≈100 °C min −1 ). Finally, the Ni foam with a layer of 3DG was obtained named as 3DG-Ni.

4.2. Synthesis of Co Based Composites

Co(CH 3 COO) 2 ·4H 2 O (0.498 g) and NH 4 F (0.185 g) were dispersed in deionized (DI) water (35 mL) via stirring for 10 min. Hexamethylenetetramine (HMT) (0.75 g) was added into about prepared solution and dispersed by vigorously stirring for 10 min. Then, the solution was transferred into a Tefl on-lined autoclave (50 mL) with a piece of Ni foam or 3DG-Ni foam (30 × 60 mm). The autoclave was sealed in a stainless steel tank and heated at 95 °C for 24 h. The red precipitate was uniformly grown on Ni or 3DG-Ni foam. The product was washed by DI water to remove the excess reactants and surfactants, followed by drying at 60 °C for 12 h. Finally, the obtained products were heated in tube furnace at 450 °C for 2 h under Ar gas. The loading densities were about 1.30 and 2.70 mg cm −2 for CoO-3DG and CoO-Ni, respectively.

4.3. Characterization

Crystal structure was characterized by a powder X-ray diffraction (XRD) (Siemens, D5005) with Cu K radiation (1 ≈ 1.54 Å). The morphology of the products was carried out by fi eld-emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F) and transmission electron microscopy (HRTEM, JEM-2100F). Raman spectroscopy was performed on WITec Instruments Corp. (Germany). The elemental composition of the samples surfaces was characterized by an X-ray photoelectron spectroscopy (XPS) spectrometer (Specs-XPS, mode XP-50 High Performance Twin Anode with Focus 500 Ellipsoidal Crystal Monochromator and PHOIBOS 150 MCD-9 analyzer).

4.4. Electrochemical Measurements

The electrochemical testing was performed with a standard three-electrode testing system composed of Hg/HgO as the reference electrode, Pt as the counter electrode, CoO-3DG-Ni as the working electrode, and 5 M KOH aqueous solution as the electrolyte. Cyclic voltammetry (CV) measurements were carried out at 5, 10, 20, 50, and 100 mV s −1 between 0 and 0.52 V. The electrochemical impedance spectra (EIS) measurements were measured with the amplitude of 5 mV from 10 5 to 0.01 Hz. The galvanostatic charge-discharge test was performed in the voltage range of 0–0.52 V at varied current densities. All of the electrochemical measurements were conducted on an electrochemical workstation (VMP3, Bio-logic) at room temperature.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work is supported by NRFPOC research grant: 2012NRF-POC001–057.

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