Co₃O₄/RGO/Co₃O₄ pseudocomposite grown in situ on a Co foil for high-performance supercapacitors

Abstract

A Co₃O₄/RGO/Co₃O₄ (CRC) pseudocomposite on a cobalt foil was prepared using a one-pot in situ hydrothermal method. Carrying out a chemical redox reaction between the Co foil and graphene oxide (GO), which involved an electrostatic attraction between GO (negative charge) and Co²⁺ (positive charge), allowed for the design and construction of a novel sandwich structure including Co₃O₄ (from Co²⁺ ions), RGO, and Co₃O₄ (from the Co foil). The CRC@Co nanocomposite was directly utilized as a binder-free supercapacitor electrode. When this composite was prepared at 200 °C for 24 h, it exhibited superior electrochemical performances: a specific capacitance of 2272.7 mF cm⁻² at 5 mA cm⁻², and a capacity retention of 81.6% after 2000 charge–discharge cycles.

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1. Introduction

Energy and environmental problems have recently become increasingly acute, and have triggered tremendous efforts at developing new clean energy resources using efficient techniques. In order to store such energy and to meet the requirements of electricity consumers, the development of energy storage devices has become necessary. Among the energy storage devices, supercapacitors have attracted a particularly large amount of attention due to their unique merits including high power density, cycle efficiency and charge/discharge rates,¹ and have, therefore, filled the gap between batteries and traditional capacitors.²

Generally speaking, the performance and charge storage mechanism of supercapacitors intimately depend on the used electrode materials. Therefore, an increasing number of researchers have been focusing on developing various kinds of electroactive materials with excellent properties for serving as supercapacitor electrode materials. Electrode materials used to date as supercapacitors can be divided into three categories: carbon-based materials, conductive polymers and transition metal compounds. Carbon-based materials are particularly widely used as electrode materials in electrical double layer capacitors because of their relatively large surface areas and good electrical conductivity levels. Although they exhibit relatively large power density values, their relatively small specific capacitance levels result in low energy density levels, which limit their applications. Regarding conductive polymers, they exhibit relatively poor cycle stability due to the large extents of volume contraction and expansion that they display during continuous charging and discharging. In contrast, supercapacitors based on conductive polymers and transition metal compounds can store energy using fast reversible oxidation reactions because these electrode materials have typical Faraday capacitance characteristics. Thus, they can achieve relatively high specific capacitance and energy density levels. Although transition metal compounds have some shortcomings, such as poor conductivity levels, they can provide a higher energy density on average than conventional carbon-based materials, and they yield on average better electrochemical stability than do the polymer materials. Therefore, transition metal compounds constitute the most researched kind of supercapacitor electrode material.

Co₃O₄, a typical transition metal compound, has garnered great interest for the development of capacitors due to its being a significant magnetic p-type semiconductor with unique energy storage properties.^4,12,17 Its high theoretical capacitance (3560 F g⁻¹), excellent electrochemical performance and environmental friendliness make it quite attractive.^5,13–16 However, the observed specific capacitance for Co₃O₄ is much lower than theoretical values due to its low electrical conductivity, small specific surface area or large volume expansion during cycling, which limits the supercapacitor performance, in particular by hindering the transfer of electrons.^6–10 Recently, various strategies have been probed to enhance the capacitance performances of Co₃O₄, and one effective method involves synthesizing various Co₃O₄ nanostructures, such as nanocubes, nanosheets, or nanospheres. Some progress has been reported in the synthesis of Co₃O₄ as electrode materials for supercapacitors, and these materials have shown excellent properties. Jiang et al. synthesized thin (2D) Co₃O₄ sheets assembled in 3D in an interconnected nanoflake array framework structure, and this product has shown a high specific capacity of 1500 F g⁻¹ at 1 A g⁻¹.¹¹ Wang and co-workers prepared mesoporous Co₃O₄@carbon composites and obtained a specific capacitance of 205.4 F g⁻¹ at a current density of 0.2 A g⁻¹.³

In this paper, a hierarchical Co₃O₄/RGO/Co₃O₄ (CRC) composite was synthesized on a cobalt foil through a one-step hydrothermal process, in which the cobalt foil served not only as the support providing in situ growth sites but also as a source of Co for the lower Co₃O₄ layer. Graphene oxide (GO) acted as an oxidizing agent to promote the synthetic redox reaction, and combined with Co²⁺ through coulombic interactions. The resultant reduced graphene oxide (RGO) effectively increased the conductivity, enlarged the specific surface area, and improved the structural strength of the electrode materials. The as-prepared CRC@Co electrode was directly utilized as a binder-free electrode and exhibited a superior specific capacitance of 2272.7 mF cm⁻² (227.3 F g⁻¹) at a current density of 5 mA cm⁻² (0.5 A g⁻¹), and of 1042.9 mF cm⁻² (104.3 F g⁻¹) at 50 mA cm⁻² (5 A g⁻¹). Meanwhile, the CRC@Co electrode retained 81.6% of its initial capacity after 2000 cycles.

2. Experimental section

2.1 Synthesis of the Co₃O₄/RGO/Co₃O₄ (CRC)@Co composite

The synthesis of the CRC@Co composite was carried out using a one-step hydrothermal process.^21,24 All chemicals were of analytical grade and used without any further purification. GO was prepared by using a modified Hummers' method.²⁵ Typically, a cobalt foil (approximately 1 cm × 2 cm) was carefully polished using sandpaper and washed with acetone and ethanol for 15 min each, and then cleaned with deionized (DI) water. A mass of 20 mg of a GO solution and 0.5 mmol Co(NO₃)₂·6H₂O were added into DI water under stirring for 30 min to form a 50 mL solution.^18,19 This solution was transferred to a 100 mL Teflon-lined stainless steel autoclave and then heated at 200 °C for 24 h to complete the hydrothermal process. Finally, oxide-loaded Co foils were washed with ethanol and DI water, and then dried at 80 °C for 12 h. For comparison, Co₃O₄/Co₃O₄@Co, RGO/Co₃O₄@Co and Co₃O₄@Co were also prepared under the same conditions except in the absence of GO or Co²⁺ or both of them, respectively. The mass loadings of Co₃O₄@Co (4.4 mg), Co₃O₄/Co₃O₄@Co (9.9 mg), RGO/Co₃O₄@Co (5.6 mg), and CRC@Co (10 mg) were determined by the weight difference method.^20,34

2.2 Characterizations

The crystal phase of each sample was characterized by collecting wide-angle (10–80°, 40 kV/200 mA) powder X-ray diffraction (XRD) data using an X-ray diffractometer with CuKα radiation (λ = 0.15406 nm). The morphology and microstructures of the samples were investigated using a field-emission scanning electron microscope (FESEM, Hitachi S-7800) and transmission electron microscope (TEM, JEOL JEM-2100), respectively. Raman spectra were recorded on an INVIA Raman microprobe (Renishaw Instruments, England) with a 514 nm wavelength laser excitation. The chemical states of the products were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi).

2.3 Electrochemical measurements

The electrochemical properties were determined by using a 1 M KOH aqueous solution as the electrolyte and a typical three-electrode system, in which the as-prepared CRC@Co composite was used as the working electrode, and a Pt foil and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) as well as galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) investigations were carried out using a CHI 660e electrochemical workstation.^20,22,23 The CV measurements were taken at various scan rates, specifically 5, 10, 20, 50, and 100 mV s⁻¹, in the potential window of −0.2 to 0.5 V. GCD experiments were carried out at various current densities, specifically 5, 10, 20, 50, and 100 mA cm⁻² (0.5, 1, 2, 5 and 10 A g⁻¹). EIS tests were carried out in the frequency range from 100 kHz to 0.01 Hz with an AC signal of 5 mV.

3. Results and discussion

3.1 XRD patterns, Raman spectra and XPS of the CRC@Co composite

XRD patterns of the CRC@Co composite and other related samples are shown in Fig. 1. Fig. 1b–d show, respectively, the XRD patterns of Co₃O₄/Co₃O₄@Co, RGO/Co₃O₄@Co and CRC@Co composites, each prepared at 200 °C for 24 h, and Fig. 1a shows that of a bare Co foil. The peaks at 41.7°, 44.8°, 47.6°, 62.7° and 75.9° in Fig. 1a arose from the Co foil substrate, specifically from the (100), (002), (101), (102), (110) planes (JCPDS no. 05-0727), and Co foil peaks can also be seen in the XRD patterns of the other three samples. The well-defined diffraction peaks at 31.3°, 36.8°, 38.5°, 44.8°, 55.7°, 59.4°, 65.2° and 77.3° in Fig. 1d derived from the (220), (311), (222), (400), (422), (511), (440) and (533) planes of the spinel Co₃O₄ crystal (JCPDS no. 42-1467).^26,27,33

Fig. 1 XRD patterns of (a) a Co foil, (b) Co₃O₄/Co₃O₄@Co, (c) RGO/Co₃O₄@Co and (d) CRC@Co.

Fig. 2a and b show the Raman spectra of GO and the CRC@Co composite, and the two distinct peaks at 1356 and 1596 cm⁻¹ were attributed to sp³ (D band) and sp² (G band) hybridized carbon atoms, respectively.²⁸ The D/G ratio of the CRC@Co composite (1.06) was found to be higher than that of GO (0.81), which confirmed that GO was reduced. The peaks located at 190, 480, 522 and 691 cm⁻¹ corresponded to the E_g, F¹_2g, F²_2g and A_1g vibration modes of Co₃O₄, respectively.²⁹

Fig. 2 Raman spectra of (a) GO and (b) CRC@Co.

The chemical composition and valence state of the CRC@Co composite was further characterized by using X-ray photoelectron spectroscopy (XPS), as shown in Fig. 3. The wide-scan spectrum (0–1000 eV) confirmed the presence of the elements Co, C and O in the composite (Fig. 3a), and the peaks at about 285, 530 and 790 eV indicated C 1s, O 1s and Co 2p hybridized orbitals. As shown in Fig. 3b, distinct Co 2p_1/2 (795.3 eV) and Co 2p_3/2 (779.7 eV) peaks were discerned, and were accompanied by two shake-up satellite peaks, which are characteristic of the Co₃O₄ phase.²⁹ The observed C 1s peak (Fig. 3c) verified the presence of carbon atoms in various functional groups, and the highest component of this peak at 284.6 eV corresponding to C–C bonds indicated the presence of large amounts of aromatic carbon, while the low-intensity component peaks at 287.0 eV and 288.2 eV, corresponding to C–O bonds and C=O bonds, respectively, indicated that the GO was reduced. As for the O 1s region of the spectrum (Fig. 3d), the strongest component of the observed peak was determined to be located at 530.0 eV, corresponding to the lattice oxygen from Co₃O₄.^26,30,31

Fig. 3 XPS spectrum of CRC@Co, including (a) the entire spectrum and (b–d) magnified views of the regions of the spectrum containing the (b) Co 2p, (c) C 1s and (d) O 1s peaks.

3.2 FESEM and TEM images of CRC@Co composite

Fig. 4 shows FESEM images of the CRC@Co composite resulting from various reaction durations. RGO nanosheets were observed to be attached to the Co foil substrate with a layer of Co₃O₄ nanoparticles for the CRC@Co-12 composite, as shown in Fig. 4a and b, and several hierarchical spheres consisting of Co₃O₄ nanoflakes were observed to be dispersed on the surface. As the hydrothermal reaction continued, the spheres of Co₃O₄ nanosheets continued to grow and expand outward, and they finally connected to each other to form a complete surface as shown for the CRC@Co-24 composite in Fig. 4c and d. This composite was observed to have a 3D porous structure with a high specific surface area and consisting of 2D nanosheets linked to each other, which was found with excellent structural strength. This nanoporous structure apparently facilitated adequate contacts between the CRC@Co composite and electrolytes, hence improving the specific capacity and rate performance of the composite. The formation of the CRC@Co composite up to a hydrothermal reaction time of 24 h is depicted in Scheme 1. When the duration of the reaction was further increased to 36 h to form the CRC@Co-36 composite, the lamellar structure became large and thick, and grew to form randomly oriented nanowalls, as shown in Fig. 4e and f, while the original 3D porous structure became covered. The results revealed an outstanding morphology for the CRC@Co composite produced with a reaction duration of 24 h, which as described below also yielded an excellent electrochemical performance of the corresponding supercapacitor.

Fig. 4 FESEM images of (a, b) CRC@Co-12, (c, d) CRC@Co-24 and (e, f) CRC@Co-36.

Scheme 1 Schematic diagram of the formation of the CRC@Co composite.

Fig. 5 shows the results of the TEM investigation of the CRC@Co composite. RGO nanosheets were clearly observed to be evenly distributed on the surface of the Co₃O₄ nanosheets (NSs) shown in Fig. 5a. Co₃O₄ nanocubes (NCs) were also clearly observed to be uniformly anchored on the Co₃O₄ nanosheets. A fringe spacing of 0.243 nm was measured from the lattice observed in the HR-TEM image (Fig. 5b), and was assigned to the (311) plane of Co₃O₄; these results were consistent with the XRD patterns.³² An SAED pattern further confirmed the polycrystalline nature of the Co₃O₄ (inset of Fig. 5b).

Fig. 5 (a) TEM image of CRC@Co. (b) HR-TEM image and SAED pattern (insert) of CRC@Co.

3.3 Electrochemical performances of CRC@Co composite

Electrochemical properties of the CRC@Co composite were determined by taking cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements using a three-electrode system in 1 M KOH.^34,35 The CV test was performed at scan rates of 5–100 mV s⁻¹ and in a potential window from −0.2 to 0.5 V. Fig. 6a and b show the CV and GCD curves of CRC@Co composites prepared using the hydrothermal process at various temperatures (180, 200, 220 °C) and durations (12, 24, 36 h) at 10 mV s⁻¹. The enveloped area of the CV curve of CRC@Co composite prepared at 200 °C for 24 h was observed to be much greater than those of the other composites (Fig. 6a). The superior capability performance of this CRC@Co composite (i.e., 200 °C, 24 h) was also indicated by the GCD data shown in Fig. 6b. Fig. 6c shows the CV curves of the samples prepared for comparison. The electrochemical performance of the CRC@Co composite electrode was observed to be much better than the performances of the Co₃O₄/Co₃O₄@Co and RGO/Co₃O₄@Co electrodes, not to mention the Co after hydrothermal treatment (Co₃O₄@Co).

Fig. 6 (a and b) CV and GCD curves of CRC@Co at various reaction temperatures and durations. (c) CV curves of various electrodes. (d) CV curves of CRC@Co at various scan rates. (e) GCD curves of CRC@Co at various current density values. (f) Discharge rate curves of CRC@Co. (g) Cyclability curves of Co₃O₄/Co₃O₄@Co, RGO/Co₃O₄@Co and CRC@Co. (h) Nyquist plots of CRC@Co at various reaction temperatures.

The performances of the CRC@Co composite (200 °C, 24 h) electrode are shown in Fig. 6d–g. Fig. 6d shows CV curves at scan rates of 5–100 mV s⁻¹. The obvious redox peaks indicated that the electrochemical performance of the CRC@Co composite was due to its extrinsic pseudocapacitive behaviors. The proposed reaction mechanism is shown in eqn (1) and (2). The CoOOH product of eqn (1) quickly underwent the reaction in eqn (2) to form CoO₂, and these two anodic peaks (in theory) were too close to be separately observed.⁷ The peak currents increased with scan rate (Fig. 6d), suggesting that the CRC@Co composite materials have good reversibility under rapid charge–discharge response.³⁶ As the scan rate was increased, the anodic peaks shifted to higher potentials and the cathodic peaks shifted to lower potentials, which were attributed to the ion diffusion rate being too slow to provide electrical neutralization during the redox reaction.³⁷

Co₃O₄ + OH⁻ + H₂O ↔ 3CoOOH + e⁻ (1)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (2)

GCD curves of the CRC@Co composite in the potential window of −0.2 to 0.5 V with current density values of 5, 10, 20, 50 and 100 mA cm⁻² (0.5, 1, 2, 5, 10 A g⁻¹) are shown in Fig. 6e. The specific capacitance values C_s and C_m were calculated by using eqn (3) and (4), respectively. In these equations, C_s (mF cm⁻²) and C_m (F g⁻¹) are both specific capacitance, ΔV (V) is the potential window, I (A) is the discharge current, t (s) is the discharge time, S (cm²) is the geometric surface area of the as-prepared electrode, and m (g) is the quality of the active substance. As shown in Fig. 6e, all of the GCD curves were nonlinear and consisted of two apparent plateaus: −0.2 to 0.02 V and 0.02–0.5 V. We ascribed the short charge/discharge duration of the first stage to the EDLC (Electronic Double Layer Capacitor) of the electrode. The combination of EDLC and faradic capacitance was responsible for the longer charge/discharge duration of the latter stage, which was in agreement with the results of the CV test.⁴⁴

The specific capacitance of the CRC@Co composite was determined to be 2272.7 mF cm⁻² (227.3 F g⁻¹) at a discharge current density of 5 mA cm⁻² (0.5 A g⁻¹), and 1907.1, 1585.7, 1042.9 and 528.6 mF cm⁻² (190.7, 158.6, 104.3 and 52.9 F g⁻¹) at 10, 20, 50 and 100 mA cm⁻² (1, 2, 5 and 10 A g⁻¹) (Fig. 6e). That is, as the current density was increased from 5 to 50 mA cm⁻², 46% of the capacitance was retained, which indicated an outstanding rate ability (Fig. 6f). Fig. 6g shows the cyclability curves of the Co₃O₄/Co₃O₄@Co, RGO/Co₃O₄@Co, CRC@Co composite electrodes. Here, the Co₃O₄/Co₃O₄@Co electrode retained its superior capacitance after 2000 cycles. Its poor storage capacitance, however, limits its application. Furthermore, the RGO/Co₃O₄@Co electrode retained only 60% of its initial capacitance after 2000 cycles. Compared with the Co₃O₄/Co₃O₄@Co and RGO/Co₃O₄@Co electrodes, the CRC@Co electrode displayed an obviously better capacity, and exhibited good overall cycle stability. It retained 84.2% of its initial specific capacitance after 1000 cycles, and 81.6% after 2000 cycles, i.e., its capacity decreased by only 3% during the second 1000 cycles. The good cyclability of the CRC@Co electrode may have been due in part to its porous structure being well retained after 2000 cycles, as shown in Fig. S2.† A comparison of electrochemical properties of this electrode with those of related works is listed in Table 1.^37,40–43

Table 1 A comparison of electrochemical properties of the CRC@Co electrode with those of related works

Samples	Potential	Cs (F g^−1)	Cyclability	Ref.
Biomorphic Co3O4	0–0.45 V	130.5 (0.5 A g^−1)	91.7% (3000 cycles)	37
Co3O4 NPs@Ni sheet	−0.2 to 0.4 V	928 (1.2 A g^−1)	93% (2200 cycles)	40
Co3O4/RGO/CNTs	−0.2 to 0.45 V	378 (2 A g^−1)	96% (2300 cycles)	41
Mesoporous Co(OH)2	−0.3 to 0.45 V	421 (1.33 A g^−1)	96.4% (1000 cycles)	42
Co3O4 microflowers	0–0.45 V	240.2 (0.625 A g^−1)	96.3% (2000 cycles)	43
CRC@Co	−0.2 to 0.5 V	227.3 (0.5 A g^−1)	81.6% (2000 cycles)	This work

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Fig. 6h shows the Nyquist plots of CRC@Co composite prepared at various reaction temperatures (180, 200, 220 °C). These plots are composed of a semicircle and a straight line, located in the high-frequency and low-frequency regions, respectively. Such EIS curves can be fitted with the solution resistance (internal resistances) and finite-length Warburg diffusion element.³⁸ The equivalent series resistance (R_s) was obtained from the X-intercept, and the CRC@Co composite (200 °C) electrode was found to have the smallest R_s of 1.7 Ω, which indicated its good electrical conductivity. In addition, the nearly vertical line in the low-frequency region verified the excellent electron-transfer performance of the CRC@Co composite (200 °C).³⁹

4. Conclusions

In summary, a Co₃O₄/RGO/Co₃O₄ (CRC) composite was grown in situ on a Co foil through a one-step hydrothermal method. The CRC@Co composite directly acted as a binder-free electrode and exhibited superior electrochemical performances. In particular, it displayed a specific capacitance of 2272.7 mF cm⁻² (227.3 F g⁻¹) at 5 mA cm⁻² (0.5 A g⁻¹), and the capacitance only fell to 1852.7 mF cm⁻² (185.3 F g⁻¹) after 2000 cycles. Moreover, it retained 46% of its capacitance when the current density was increased ten-fold, indicative of its outstanding capacity performance and rate ability.

Acknowledgements

We are grateful for the support of the National Natural Science Foundation of China (No. 20504026), Shanghai Natural Science Foundation (No. 13ZR1411900), Shanghai Leading Academic Discipline Project (B502), and Shanghai Key Laboratory Project (08DZ2230500).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17543k

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