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A flexible, transparent and super-long-life supercapacitor based on ultrafine Co3O4 nanocrystal electrodes X. Y. Liu, Y. Q. Gao and G. W. Yang* Flexible and transparent supercapacitors, as advanced energy storage devices, are essential for the development of innovative wearable electronics because of their unique optical and mechanical qualities. However, all previous designs are based on carbon-based nanostructures like carbon nanotubes and graphene, and these devices usually have poor or short cycling lives. Here, we demonstrate a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using a novel process involving laser ablation in liquid. The fabricated flexible and transparent pseudocapacitor exhibits a high capacitance of 177 F g−1 on a mass basis and 6.03 mF cm−2 based on the area of the active material at a scan rate of 1 mV s−1, as well as a super-long cycling life with 100% retention rate after 20 000 cycles. An optical transmittance of up to 51% at a wavelength of 550 nm is achieved, and there are not any obvious changes in the specific capacitance after bending from 0° to 150°, even
Received 23rd December 2015, Accepted 25th January 2016
after bending over 100 times. The integrated electrochemical performance of the Co3O4-based super-
DOI: 10.1039/c5nr09145d
capacitor is greatly superior to that of the carbon-based ones reported to date. These findings open the door to applications of transition metal oxides as advanced electrode materials in flexible and transparent
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pseudocapacitors.
Introduction Flexible and transparent portable electronic products may greatly affect people in the near future. Energy storage devices are the key component of portable electronic products. As very promising energy storage devices because of their excellent characteristics, such as high power density, fast charge– discharge rate, and long cycling life, supercapacitors are an ideal choice for implementing flexible and transparent characteristics.1–5 The performance of active materials is the most vital factor affecting supercapacitor operation. According to the charge storage mechanism, supercapacitors can be divided into double layer capacitors and pseudocapacitors. To date, several examples of flexible and transparent supercapacitor have been demonstrated, and these devices all use carbonbased nanostructures like carbon nanotubes and graphene as their active material.6–8 However, most of these flexible and transparent supercapacitors made from carbon-based materials exhibit a very poor or short cycling life, making them difficult to apply in practice. Typically, Jung et al. fabricated a flexible and transparent supercapacitor based on nano-engin-
State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. E-mail: [email protected]
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eered carbon films with an 84% retention rate after 10 000 cycles.6 Gao et al. demonstrated a CNF–[RGO]n hybrid paperbased supercapacitor with an 81% retention rate after 5000 cycles.7 Yuksel et al. presented a flexible and transparent supercapacitor using single-walled carbon nanotubes with a 94% retention rate after 500 cycles.8 Therefore, increasing the cycling life of flexible and transparent supercapacitors remains an important challenge for their practical application. Transition metal oxides have been widely used to build supercapacitors.9–14 Therefore, we focus our attention on transition metal oxides, expecting to solve the problem of the cycling life. In specific electrolytes, Faradaic reactions can occur on the surface of transition metal oxide electrodes; thus, pseudocapacitance is produced. Among these transition metal oxides, cobalt oxide(II,III) (Co3O4) has the advantages of high electrochemical performance and environmental friendliness.15,16 Therefore, many kinds of Co3O4 nanostructure, such as nanobelts, nanosheets, hollow-structured nanoparticles, and flower-like nanostructures, have been synthesized using methods such as the solid phase method,17 sol–gel method,18 electrochemical method,19 and hydrothermal method20 for energy storage devices. Therefore, it is an interesting challenge to fabricate a flexible and transparent Co3O4-based supercapacitor with a green and facile method because there have not been any reports of high-performance, flexible, and transparent supercapacitors based on transition metal oxides so far.
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Here, for the first time, we experimentally demonstrate a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using a novel process involving laser ablation in liquid (LAL).21 Amazingly, our measurements show that the ultrafine Co3O4 nanocrystals exhibit excellent electrochemical performance in the as-fabricated pseudocapacitors, which have high capacitance, energy density, and super-long cycling life. Importantly, the integrated electrochemical performances of the flexible and transparent Co3O4-based supercapacitor are far superior to those of carbon-based ones.6–8 For example, the cycling stability reaches a 100% retention rate after 20 000 cycles. Therefore, these investigations open the door to applications of transition metal oxides as advanced electrode materials in flexible and transparent pseudocapacitors.
Experimental
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300 kV. Powder X-ray diffraction (XRD) was performed using a RigakuD-Max2200 VPC with Cu Kα radiation (1.54056 Å, 40 kV, 20 mA), and the scanning rate was 1° s−1. X-ray photoelectron spectroscopy (XPS) patterns were obtained on a Thermo Fisher ESCALab250 spectrometer using an Al Kα radiation X-ray source with respect to the position of the C 1s peak at 284.9 eV. The Raman spectrum was acquired using a Renishaw inVia laser micro-Raman spectrometer using an Ar+ laser with an excitation wavelength of 514.5 nm. The diffuse reflectance ultraviolet visible spectrum was acquired on a UV-2501PC spectrophotometer. The mass of the as-synthesized Co3O4 was measured using inductively coupled plasma–atomic emission spectrometry using a TJA, IRIS (HR) spectrometer. The samples were dissolved in dilute nitric acid for 48 h to ensure that Co3O4 had completely reacted with the dilute nitric acid and then the concentration of the cobalt element was measured. Electrochemical measurements were carried out using a CHI660D electrochemical workstation.
Synthesis of Co3O4 nanocrystal dispersions Co3O4 nanocrystals were synthesized using LAL. In this case, a cobalt target (99.99% purity) was fixed on the bottom of a square groove with 20 ml of deionized water. Then, the cobalt target was irradiated using a Q-switched Nd:YAG laser device with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency of 10 Hz, and a laser pulse power of 400 mJ. The focused spot size was 1 cm in diameter on the surface of the target. The whole ablation lasted for 20 min at ambient temperature and pressure. As a result, a brownish colloid solution was synthesized. Fabrication of Co3O4 supercapacitor The substrate was made of indium tin oxide (ITO)-coated polyethylene terephthalate (PET) (15 Ω per square) on one surface. The substrate was sonicated in a mixture of deionized water, ethanol, and acetone for 20 min to remove the impurities on the surface and make it hydrophilic, it was then washed with deionized water three times. Then, 400 µl of the liquid synthesized in the previous step was dropped onto the surface of the PET/ITO dropwise, and the electrode was then dried at a temperature of 55 °C for 3 h to vaporize the deionized water. As a result, a Co3O4 electrode was obtained with an area of active material of 1 × 1 cm2. A polyvinyl alcohol/phosphoric acid (PVA/H3PO4) gel electrolyte was prepared by mixing 1 g of PVA powder, 1 g of H3PO4, and 10 ml of deionized water under violent magnetic stirring at a constant temperature (85 °C) until the mixed solution became clear. The electrolyte was coated onto the electrodes to serve as the ionic electrolyte and the separator; then, the electrode was dried overnight. After the excess water was vaporized, two such electrodes were pressed against each other, forming a transparent and flexible supercapacitor. Characterization and electrochemical measurements Transmission electron microscopy (TEM) images were obtained using an FEI Tecnai G2 F30 microscope operated at
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Results and discussion Structure and morphology A Co3O4-based colloid solution was synthesized using laser ablation of a cobalt target in deionized water. Fig. 1a shows the powder XRD pattern of the as-synthesized Co3O4 nanoparticles. All the diffraction peaks can be indexed to the standard PDF card corresponding to Co3O4 with a cubic structure (JCPDS no. 43-1003). The results indicate that the as-synthesized Co3O4 nanoparticles are of high purity. XPS was used to confirm the chemical states of the bonded elements by measuring the bonding energy. Fig. 1b presents the full survey XPS spectrum of the as-synthesized Co3O4 nanocrystals. Clearly, the peaks corresponding to the characteristic peaks of Co 2p, C 1s, and O 1s can prove the existence of the three elements (Co, C, O). Fig. 1c shows the high-resolution XPS spectrum of Co 2p; there are two major peaks, at 780.6 and 795.8 eV, which should be assigned to the Co 2p3/2 and Co 2p1/2 spin–orbit peaks, respectively. The spin–orbit splitting (15.2 eV) between Co 2p3/2 and Co 2p1/2, along with the two satellite peaks at 789.1 and 804.5 eV, is a typical sign of a pure Co3O4 XPS spectrum.22 Because Raman scattering is very sensitive to the microstructure of nanocrystalline materials, it was also used here to clarify the structure of the Co3O4 nanoparticles. To further confirm the XPS analysis, the Raman spectrum was measured at room temperature. As shown in Fig. 1d, there are clearly five peaks, at 188, 468, 513, 603, and 670 cm−1, which can be assigned to Co3O4. The peaks at 468 and 670 cm−1 can be attributed to the Eg and A1g modes of Co3O4, and the peaks at 188, 513, and 603 cm−1 can be attributed to the F2g mode of Co3O4.23,24 From these results, we can conclude that the synthesized nanoparticles are Co3O4 nanocrystals. A small drop of the as-synthesized suspension liquid was dropped onto a copper grid coated with a carbon membrane for the transmission electron microscopy (TEM). Fig. 2a shows
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fraction ring pattern from the crystallographic planes (111), (220), (311), (400), (511), and (440) of Co3O4. Fig. 2c shows a high-resolution TEM (HRTEM) image. The inset in Fig. 2c shows the corresponding nanoparticle size distribution histogram based on HRTEM data. It can be seen that these nanoparticles have a uniform size distribution, with an average size of approximately 3.5 nm. The growth time of the synthesized particles, namely the plasma quenching time, is very short, which is one reason why the size of the Co3O4 nanoparticles is so small. Fig. 2d shows details of the HRTEM image in Fig. 2c; the interplanar spacings of 0.24 nm correspond to the (311) crystallographic plane of a Co3O4 nanocrystal. Formation mechanism In LAL, a plasma plume containing cobalt atoms, ions, and radicals is produced as soon as the laser irradiates the surface of the cobalt target, and the plasma plume is in a thermodynamic state of high temperature and high pressure because of the high laser power density and the effect of the confinement of the liquid.21 Simultaneously, the Co species in the plasma plume continuously reacts with the ions and radicals, which are produced by the water. Finally, the Co3O4 nanoparticles form. This process can be described as follows: CO þ 2H2 O ! CoðOHÞ2 þ H2 "
ð1Þ
2CO þ 4H2 O ! 2CoOOH þ 3H2 "
ð2Þ
CoðOHÞ2 þ 2CoOOH ! Co3 O4 þ 2H2 O
ð3Þ
Electrochemical characterization
Fig. 1 Spectroscopy analyses of the as-synthesized Co3O4 nanoparticles. (a) XRD pattern of the as-synthesized Co3O4 nanoparticles, (b) XPS survey spectrum of the sample, and (c) the high-resolution XPS spectrum of the Co 2p. (d) Raman spectrum of the sample.
Fig. 3 describes the fabrication process of the flexible and transparent supercapacitor. First, the synthesized Co3O4 nanocrystals were dispersed on ITO-coated PET as the active material. Second, the transparent and flexible polymer electrolyte consisting of PVA and H3PO4 was coated onto the two electrodes working as a separator and electrolyte. Finally, the two fabricated electrodes were assembled into a flexible and transparent supercapacitor with an electrolyte. Fig. 4a presents the cyclic voltammetry (CV) curves of the as-synthesized Co3O4 electrode in a 0.5 M H3PO4 electrolyte in a three-electrode system at scan rates from 1 mV s−1 to 20 mV s−1. The potential window is from 0.25 V to 0.6 V, and the reference electrode is Ag/AgCl. There is one cathodic peak and one anodic peak in each curve because of the Faradaic redox reactions.25 For Co3O4 electrode materials in acid solutions, the charge–storage mechanism can be described using the following chemical equation:26,27 Co3 O4 þ xHþ $ Co3 O4x ðOHÞx
ð4Þ
+
a TEM image of the as-synthesized Co3O4 nanoparticles. The inset of Fig. 2a shows a photograph of the as-synthesized colloid solution. The corresponding selected area electronic diffraction pattern in Fig. 2b demonstrates that the polycrystalline Co3O4 nanocrystals are consistent with the strong dif-
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When charging, Co3O4 combines with H and charges in the electrolyte, so some of the Co3+ is reduced to Co2+. When Co3O4 is discharging, the H+ and charges that are combined in Co3O4 return to the electrolyte, and the reduced Co2+ is oxidized to Co3+. It can be seen that these CV curves are similar in shape, but the peak currents increase with increasing scan
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Fig. 2 The morphology and structure of the Co3O4 nanocrystals. (a) TEM bright-field image of the Co3O4 nanocrystals and the inset is the colloidal solution of the synthesized nanocrystals. (b) The corresponding SAED pattern of the synthesized nanocrystals. (c) The corresponding HRTEM image and the distribution histogram of the Co3O4 nanocrystals. (d) The detailed HRTEM image of the synthesized nanocrystals in (c).
rate because the corresponding electrochemical reaction rates also increase with increasing scan rate. From the CV curves, the specific capacitance can be calculated using the following equation:28 C¼
Fig. 3 Schematic illustration of the fabrication process for the transparent and flexible supercapacitor based on the as-synthesized Co3O4 nanocrystals.
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1 mvΔV
ðV
I ðV ÞdV
ð5Þ
V0
where m is the mass (g) of the active material, v is the potential scan rate (V s−1), I is the response current (A), and ΔV is the potential window (V). The loading mass of the Co3O4 is 17 µg cm−2. The calculated specific capacitances are 1049, 1001, 907, 787 and 600 F g−1 at scan rates of 1, 2, 5, 10, and 20 mV s−1, respectively, as shown in Fig. 4b. The specific capacitances decrease with increasing scan rate because the diffusion effect limits the ion migration in the electrolyte.25
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Fig. 4 Electrochemical characterization of the Co3O4 electrode and blank substrate. (a) CV curves of the Co3O4 electrode at various scan rates. (b) Specific capacitance of the Co3O4 electrode at various scan rates. (c) Galvanostatic charging and discharging curves of the Co3O4 electrode at various current densities. (d) Specific capacitance of the Co3O4 electrode at various current densities. (e) The contrast of CV curves of the blank substrate and the Co3O4 electrode at a scan rate of 10 mV s−1.
To further study the electrochemical performance of the electrode, galvanostatic charging and discharging curves were measured at a series of current densities, as shown in Fig. 4c. The specific capacitance can be calculated using the following equation:29 C¼
IΔt mΔV
ð6Þ
where Δt is the discharge time (s), I is the discharge current (A), ΔV is the potential window (V), and m is the mass (g) of the active material. The specific capacitances are 762, 722, 688, and 630 F g−1 at current densities of 6, 12, 18, and 36 A g−1, respectively, as shown in Fig. 4d. The specific capacitances
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decrease with the current because as the current increases, more active materials take part in the reaction in a shorter amount of time.30 To estimate the influence of the substrate on the specific capacitance of the electrode, CV curves of the Co3O4-based electrode and the ITO-coated PET substrate were measured at a scan rate of 10 mV s−1, as shown in Fig. 4e. Compared with the Co3O4-based electrode, the ITO-coated PET substrate shows a very low current; thus, the contribution to the capacitance from the substrate can be neglected. The electrochemical performance of the fabricated supercapacitor was tested in a two-electrode sandwich structure. Electrochemical measurements of the supercapacitor were conducted on a CHI660D electrochemical workstation. Fig. 5a
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presents the CV curves of the supercapacitor measured at scan rates from 1 to 200 mV s−1. There are two strong redox peaks in each curve, indicating the pseudocapacitive performance of the supercapacitor.31 The nearly symmetrical peaks show the excellent reversibility of the response to charge and discharge.32 Fig. 5b shows the specific capacitances of the supercapacitor at various scan rates calculated by choosing the potential window of 0.35 V. A specific capacitance of 177 F g−1 is achieved at the scan rate of 1 mV s−1. In a symmetric supercapacitor, the theoretical capacitances of the cell can be obtained by dividing the capacitances of the single electrodes by four.33 The difference between the actual capacitances and
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theoretical capacitances calculated for a single electrode occurs primarily because the PVA in the gel electrolyte greatly limits ionic migration. The specific capacitance is 6.03 mF cm−2, based on the area of the active material, at 1 mV s−1. Fig. 5c shows the galvanostatic charging and discharging curves for the fabricated supercapacitor measured at current densities from 0.9 to 1.8 A g−1, calculated by choosing the positive potential part. The supercapacitor delivers a specific capacitance value of 172 F g−1 at a current density of 0.9 A g−1 (Fig. 5d). The specific capacitances decrease with the scan rate and the currents increase for the same reason as in the threeelectrode test. These galvanostatic charging and discharging
Fig. 5 Electrochemical characterization of the fabricated transparent and flexible supercapacitor. (a) CV curves of the supercapacitor at various scan rates. (b) Specific capacitance of the supercapacitor at various scan rates. (c) Galvanostatic charging and discharging curves of the supercapacitor at various current densities. (d) Specific capacitance of the supercapacitor at various current densities. (e) Power and energy densities of the supercapacitor. (f ) Cycling performance of the supercapacitor for 30 000 cycles, the inset presents the CV curves of the supercapacitor at 1, 10 000, 20 000 and 30 000 cycles.
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curves display nonlinearities, which confirm the CV results: the capacitance of the supercapacitors is primarily contributed by the Faradaic redox reactions. For the symmetric supercapacitor, the energy densities and power densities were calculated from the CV curves and plotted on the Ragone diagram shown in Fig. 5e. The energy densities of the supercapacitor decrease from 3.01 to 0.56 W h kg−1 as the power densities increase from 0.031 to 1.152 kW kg−1. The energy density and power density were calculated using the following equations, respectively:29 E¼
CðΔV Þ2 1000 2 3600
ð7Þ
E 3600 Δt 1000
ð8Þ
P¼
where C is the specific capacitance (F g−1) of the supercapacitor, ΔV is the potential window (0.35 V), and Δt is the discharge time (s). The units of energy density (E) and power density (P) are W h kg−1 and kW kg−1, respectively. As a critical parameter, determining the energy storage performance for practical values, the cycling stability of the supercapacitor was tested through a CV process, as shown in Fig. 5f.
A specific capacitance retention ratio of 93% was obtained after 30 000 CV tests, illustrating the excellent electrochemical stability of the supercapacitor. The capacitance retention reaches 100% after 20 000 cycles. The supercapacitor is capable of retaining 106% and 100% of the initial specific capacitance after 10 000 and 20 000 cycles, respectively. It is worth noting that after 20 000 cycles, the specific capacitance decreases relatively quickly at first, and then decreases more and more slowly. During the first 6000 cycles, the capacitance retentions increase from 100% to 107%. This enhancement in specific capacitance can be interpreted as showing that the electrochemical activity of the active material is activated gradually at the beginning. Therefore, these results demonstrate the good electrochemical performances and excellent cycling stability of the Co3O4-based supercapacitor. Note that the integrated electrochemical performance of the flexible and transparent Co3O4-based supercapacitor is far superior to that of carbon-based ones.6–8 Fig. 6a shows that the CV curves measured are not affected after bending the supercapacitor at different angles, demonstrating that the structural integrity of the device is not destroyed when bending. For example, the device was bent to 60° more than 100 times, and the performance of the device
Fig. 6 The transparency and bending properties of the fabricated supercapacitor. (a) CV curves of the supercapacitor under different bending angles from 0° to 150° at a scan rate of 100 mV s−1, the inset shows how to compute the bending angles. (b) Bending cycling performance of the supercapacitor. (c) Transmittance spectra of the blank substrate, the CO3O4 electrode and the supercapacitor. (d) The photograph of the supercapacitor showing the transparent performance. (e) The photograph of the supercapacitor showing the flexible performance. (f ) A LED is turned on when the supercapacitor is in a bent state.
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was measured every 10 times. The result shows that the capacitance retention of the supercapacitor fluctuated between 99.7% and 100.4%, as shown in Fig. 6b, demonstrating the excellent mechanical flexibility of the supercapacitor under harsh bending conditions. The transmittances of the substrate, electrode, and resultant supercapacitor were measured, and the results are shown in Fig. 6c. At a wavelength of 550 nm, the ITO-coated PET substrate shows a transmittance of 76%, and the transmittance of a single electrode is approximately 58%, which decreases to 51% for the corresponding assembled supercapacitor. For example, our university logo is clearly visible when it is placed under the supercapacitor (Fig. 6d). The flexibility of the supercapacitor can be seen in Fig. 6e. A blue LED with an operating voltage of 1.5 V was lit up using a tandem supercapacitor with an active area of 14 cm2 made up of six small supercapacitors, as shown in Fig. 6f. We suggest one reasonable explanation for the poor cycling life of flexible and transparent supercapacitors made from carbon-based materials. For an ideal double-layer capacitor, no electrochemical reaction occurs on the surface of the active materials. However, in practical charging and discharging processes, the electrode process will destroy the ideality of the double layer; thus, there is charge transfer, resulting in leakage current.34 An ultrathin film composed of carbon nanotubes or graphene usually has a highly oriented structure, which results in a strong orientation direction of the charge transfer. As a result, the surface structure of the active material will gradually be destroyed. As for our flexible and transparent Co3O4-based supercapacitor, we consider four factors that contribute to the good electrochemical performance and excellent cycling stability. First, the Co3O4 nanocrystals synthesized using LAL have a very clean surface. The synthesis method for the active materials is laser ablation of a cobalt target in deionized water. There are no chemical reagents remaining on the surface of the products, which means the active materials contact the electrolyte directly and effectively. Additionally, LAL is a method that can improve the activity of nanomaterials in certain applications.35,36 Second, the average size of the Co3O4 nanocrystals is approximately 3.5 nm. For electrochemical energy storage materials, an ultrafine particle size can reduce the ion transport length.37 This greatly enhances the electrochemical activity and electrochemical efficiency of materials, which improves the electrochemical performance and the cycling stability. Third, the ultrafine size of the nanomaterials would cause a considerable surface-to-volume ratio, making the electrochemically active area larger. Generally, the surface plays a critical role in the transportation of ions, which means the insertion and extraction of the ions in the electrolyte primarily occur on the surface of the active materials, reducing the damage to the lattice structure. This can contribute to the excellent cycling stability of supercapacitors. Fourth, the uniform size distribution makes the resistance, current density, and reactive state of each part in the electrode stable, improving the cycling stability.
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Conclusions In summary, we have, for the first time, proposed using transition metal oxides as advanced active materials for flexible and transparent pseudocapacitors to overcome the poor cycling life of carbon-based devices. We have experimentally demonstrated a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using LAL. Our measurements have shown that the integrated electrochemical performance of the flexible and transparent Co3O4-based supercapacitor is far superior to the performances of carbon-based ones. In particular, the cycling stability reaches a 100% retention rate after 20 000 cycles, much higher than the capacitance retentions of the transparent and flexible supercapacitors reported to date. Additionally, the supercapacitor showed a transmittance of 51% at a wavelength of 550 nm, and the bending angle has no influence on the specific capacitance, presenting great mechanical flexibility. Therefore, these results suggest that ultrafine transition metal oxide nanocrystals like Co3O4 nanocrystals can be used as advanced active materials for flexible and transparent supercapacitors.
Acknowledgements The National Basic Research Program of China (2014CB931700), the National Natural Science Foundation of China (91233203) and the State Key Laboratory of Optoelectronic Materials and Technologies supported this work.
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A flexible, transparent and super-long-life supercapacitor based on ultrafine Co3O4 nanocrystal electrodes X. Y. Liu, Y. Q. Gao and G. W. Yang* Flexible and transparent supercapacitors, as advanced energy storage devices, are essential for the development of innovative wearable electronics because of their unique optical and mechanical qualities. However, all previous designs are based on carbon-based nanostructures like carbon nanotubes and graphene, and these devices usually have poor or short cycling lives. Here, we demonstrate a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using a novel process involving laser ablation in liquid. The fabricated flexible and transparent pseudocapacitor exhibits a high capacitance of 177 F g−1 on a mass basis and 6.03 mF cm−2 based on the area of the active material at a scan rate of 1 mV s−1, as well as a super-long cycling life with 100% retention rate after 20 000 cycles. An optical transmittance of up to 51% at a wavelength of 550 nm is achieved, and there are not any obvious changes in the specific capacitance after bending from 0° to 150°, even
Received 23rd December 2015, Accepted 25th January 2016
after bending over 100 times. The integrated electrochemical performance of the Co3O4-based super-
DOI: 10.1039/c5nr09145d
capacitor is greatly superior to that of the carbon-based ones reported to date. These findings open the door to applications of transition metal oxides as advanced electrode materials in flexible and transparent
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pseudocapacitors.
Introduction Flexible and transparent portable electronic products may greatly affect people in the near future. Energy storage devices are the key component of portable electronic products. As very promising energy storage devices because of their excellent characteristics, such as high power density, fast charge– discharge rate, and long cycling life, supercapacitors are an ideal choice for implementing flexible and transparent characteristics.1–5 The performance of active materials is the most vital factor affecting supercapacitor operation. According to the charge storage mechanism, supercapacitors can be divided into double layer capacitors and pseudocapacitors. To date, several examples of flexible and transparent supercapacitor have been demonstrated, and these devices all use carbonbased nanostructures like carbon nanotubes and graphene as their active material.6–8 However, most of these flexible and transparent supercapacitors made from carbon-based materials exhibit a very poor or short cycling life, making them difficult to apply in practice. Typically, Jung et al. fabricated a flexible and transparent supercapacitor based on nano-engin-
State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. E-mail: [email protected]
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eered carbon films with an 84% retention rate after 10 000 cycles.6 Gao et al. demonstrated a CNF–[RGO]n hybrid paperbased supercapacitor with an 81% retention rate after 5000 cycles.7 Yuksel et al. presented a flexible and transparent supercapacitor using single-walled carbon nanotubes with a 94% retention rate after 500 cycles.8 Therefore, increasing the cycling life of flexible and transparent supercapacitors remains an important challenge for their practical application. Transition metal oxides have been widely used to build supercapacitors.9–14 Therefore, we focus our attention on transition metal oxides, expecting to solve the problem of the cycling life. In specific electrolytes, Faradaic reactions can occur on the surface of transition metal oxide electrodes; thus, pseudocapacitance is produced. Among these transition metal oxides, cobalt oxide(II,III) (Co3O4) has the advantages of high electrochemical performance and environmental friendliness.15,16 Therefore, many kinds of Co3O4 nanostructure, such as nanobelts, nanosheets, hollow-structured nanoparticles, and flower-like nanostructures, have been synthesized using methods such as the solid phase method,17 sol–gel method,18 electrochemical method,19 and hydrothermal method20 for energy storage devices. Therefore, it is an interesting challenge to fabricate a flexible and transparent Co3O4-based supercapacitor with a green and facile method because there have not been any reports of high-performance, flexible, and transparent supercapacitors based on transition metal oxides so far.
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Here, for the first time, we experimentally demonstrate a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using a novel process involving laser ablation in liquid (LAL).21 Amazingly, our measurements show that the ultrafine Co3O4 nanocrystals exhibit excellent electrochemical performance in the as-fabricated pseudocapacitors, which have high capacitance, energy density, and super-long cycling life. Importantly, the integrated electrochemical performances of the flexible and transparent Co3O4-based supercapacitor are far superior to those of carbon-based ones.6–8 For example, the cycling stability reaches a 100% retention rate after 20 000 cycles. Therefore, these investigations open the door to applications of transition metal oxides as advanced electrode materials in flexible and transparent pseudocapacitors.
Experimental
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300 kV. Powder X-ray diffraction (XRD) was performed using a RigakuD-Max2200 VPC with Cu Kα radiation (1.54056 Å, 40 kV, 20 mA), and the scanning rate was 1° s−1. X-ray photoelectron spectroscopy (XPS) patterns were obtained on a Thermo Fisher ESCALab250 spectrometer using an Al Kα radiation X-ray source with respect to the position of the C 1s peak at 284.9 eV. The Raman spectrum was acquired using a Renishaw inVia laser micro-Raman spectrometer using an Ar+ laser with an excitation wavelength of 514.5 nm. The diffuse reflectance ultraviolet visible spectrum was acquired on a UV-2501PC spectrophotometer. The mass of the as-synthesized Co3O4 was measured using inductively coupled plasma–atomic emission spectrometry using a TJA, IRIS (HR) spectrometer. The samples were dissolved in dilute nitric acid for 48 h to ensure that Co3O4 had completely reacted with the dilute nitric acid and then the concentration of the cobalt element was measured. Electrochemical measurements were carried out using a CHI660D electrochemical workstation.
Synthesis of Co3O4 nanocrystal dispersions Co3O4 nanocrystals were synthesized using LAL. In this case, a cobalt target (99.99% purity) was fixed on the bottom of a square groove with 20 ml of deionized water. Then, the cobalt target was irradiated using a Q-switched Nd:YAG laser device with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency of 10 Hz, and a laser pulse power of 400 mJ. The focused spot size was 1 cm in diameter on the surface of the target. The whole ablation lasted for 20 min at ambient temperature and pressure. As a result, a brownish colloid solution was synthesized. Fabrication of Co3O4 supercapacitor The substrate was made of indium tin oxide (ITO)-coated polyethylene terephthalate (PET) (15 Ω per square) on one surface. The substrate was sonicated in a mixture of deionized water, ethanol, and acetone for 20 min to remove the impurities on the surface and make it hydrophilic, it was then washed with deionized water three times. Then, 400 µl of the liquid synthesized in the previous step was dropped onto the surface of the PET/ITO dropwise, and the electrode was then dried at a temperature of 55 °C for 3 h to vaporize the deionized water. As a result, a Co3O4 electrode was obtained with an area of active material of 1 × 1 cm2. A polyvinyl alcohol/phosphoric acid (PVA/H3PO4) gel electrolyte was prepared by mixing 1 g of PVA powder, 1 g of H3PO4, and 10 ml of deionized water under violent magnetic stirring at a constant temperature (85 °C) until the mixed solution became clear. The electrolyte was coated onto the electrodes to serve as the ionic electrolyte and the separator; then, the electrode was dried overnight. After the excess water was vaporized, two such electrodes were pressed against each other, forming a transparent and flexible supercapacitor. Characterization and electrochemical measurements Transmission electron microscopy (TEM) images were obtained using an FEI Tecnai G2 F30 microscope operated at
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Results and discussion Structure and morphology A Co3O4-based colloid solution was synthesized using laser ablation of a cobalt target in deionized water. Fig. 1a shows the powder XRD pattern of the as-synthesized Co3O4 nanoparticles. All the diffraction peaks can be indexed to the standard PDF card corresponding to Co3O4 with a cubic structure (JCPDS no. 43-1003). The results indicate that the as-synthesized Co3O4 nanoparticles are of high purity. XPS was used to confirm the chemical states of the bonded elements by measuring the bonding energy. Fig. 1b presents the full survey XPS spectrum of the as-synthesized Co3O4 nanocrystals. Clearly, the peaks corresponding to the characteristic peaks of Co 2p, C 1s, and O 1s can prove the existence of the three elements (Co, C, O). Fig. 1c shows the high-resolution XPS spectrum of Co 2p; there are two major peaks, at 780.6 and 795.8 eV, which should be assigned to the Co 2p3/2 and Co 2p1/2 spin–orbit peaks, respectively. The spin–orbit splitting (15.2 eV) between Co 2p3/2 and Co 2p1/2, along with the two satellite peaks at 789.1 and 804.5 eV, is a typical sign of a pure Co3O4 XPS spectrum.22 Because Raman scattering is very sensitive to the microstructure of nanocrystalline materials, it was also used here to clarify the structure of the Co3O4 nanoparticles. To further confirm the XPS analysis, the Raman spectrum was measured at room temperature. As shown in Fig. 1d, there are clearly five peaks, at 188, 468, 513, 603, and 670 cm−1, which can be assigned to Co3O4. The peaks at 468 and 670 cm−1 can be attributed to the Eg and A1g modes of Co3O4, and the peaks at 188, 513, and 603 cm−1 can be attributed to the F2g mode of Co3O4.23,24 From these results, we can conclude that the synthesized nanoparticles are Co3O4 nanocrystals. A small drop of the as-synthesized suspension liquid was dropped onto a copper grid coated with a carbon membrane for the transmission electron microscopy (TEM). Fig. 2a shows
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fraction ring pattern from the crystallographic planes (111), (220), (311), (400), (511), and (440) of Co3O4. Fig. 2c shows a high-resolution TEM (HRTEM) image. The inset in Fig. 2c shows the corresponding nanoparticle size distribution histogram based on HRTEM data. It can be seen that these nanoparticles have a uniform size distribution, with an average size of approximately 3.5 nm. The growth time of the synthesized particles, namely the plasma quenching time, is very short, which is one reason why the size of the Co3O4 nanoparticles is so small. Fig. 2d shows details of the HRTEM image in Fig. 2c; the interplanar spacings of 0.24 nm correspond to the (311) crystallographic plane of a Co3O4 nanocrystal. Formation mechanism In LAL, a plasma plume containing cobalt atoms, ions, and radicals is produced as soon as the laser irradiates the surface of the cobalt target, and the plasma plume is in a thermodynamic state of high temperature and high pressure because of the high laser power density and the effect of the confinement of the liquid.21 Simultaneously, the Co species in the plasma plume continuously reacts with the ions and radicals, which are produced by the water. Finally, the Co3O4 nanoparticles form. This process can be described as follows: CO þ 2H2 O ! CoðOHÞ2 þ H2 "
ð1Þ
2CO þ 4H2 O ! 2CoOOH þ 3H2 "
ð2Þ
CoðOHÞ2 þ 2CoOOH ! Co3 O4 þ 2H2 O
ð3Þ
Electrochemical characterization
Fig. 1 Spectroscopy analyses of the as-synthesized Co3O4 nanoparticles. (a) XRD pattern of the as-synthesized Co3O4 nanoparticles, (b) XPS survey spectrum of the sample, and (c) the high-resolution XPS spectrum of the Co 2p. (d) Raman spectrum of the sample.
Fig. 3 describes the fabrication process of the flexible and transparent supercapacitor. First, the synthesized Co3O4 nanocrystals were dispersed on ITO-coated PET as the active material. Second, the transparent and flexible polymer electrolyte consisting of PVA and H3PO4 was coated onto the two electrodes working as a separator and electrolyte. Finally, the two fabricated electrodes were assembled into a flexible and transparent supercapacitor with an electrolyte. Fig. 4a presents the cyclic voltammetry (CV) curves of the as-synthesized Co3O4 electrode in a 0.5 M H3PO4 electrolyte in a three-electrode system at scan rates from 1 mV s−1 to 20 mV s−1. The potential window is from 0.25 V to 0.6 V, and the reference electrode is Ag/AgCl. There is one cathodic peak and one anodic peak in each curve because of the Faradaic redox reactions.25 For Co3O4 electrode materials in acid solutions, the charge–storage mechanism can be described using the following chemical equation:26,27 Co3 O4 þ xHþ $ Co3 O4x ðOHÞx
ð4Þ
+
a TEM image of the as-synthesized Co3O4 nanoparticles. The inset of Fig. 2a shows a photograph of the as-synthesized colloid solution. The corresponding selected area electronic diffraction pattern in Fig. 2b demonstrates that the polycrystalline Co3O4 nanocrystals are consistent with the strong dif-
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When charging, Co3O4 combines with H and charges in the electrolyte, so some of the Co3+ is reduced to Co2+. When Co3O4 is discharging, the H+ and charges that are combined in Co3O4 return to the electrolyte, and the reduced Co2+ is oxidized to Co3+. It can be seen that these CV curves are similar in shape, but the peak currents increase with increasing scan
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Fig. 2 The morphology and structure of the Co3O4 nanocrystals. (a) TEM bright-field image of the Co3O4 nanocrystals and the inset is the colloidal solution of the synthesized nanocrystals. (b) The corresponding SAED pattern of the synthesized nanocrystals. (c) The corresponding HRTEM image and the distribution histogram of the Co3O4 nanocrystals. (d) The detailed HRTEM image of the synthesized nanocrystals in (c).
rate because the corresponding electrochemical reaction rates also increase with increasing scan rate. From the CV curves, the specific capacitance can be calculated using the following equation:28 C¼
Fig. 3 Schematic illustration of the fabrication process for the transparent and flexible supercapacitor based on the as-synthesized Co3O4 nanocrystals.
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1 mvΔV
ðV
I ðV ÞdV
ð5Þ
V0
where m is the mass (g) of the active material, v is the potential scan rate (V s−1), I is the response current (A), and ΔV is the potential window (V). The loading mass of the Co3O4 is 17 µg cm−2. The calculated specific capacitances are 1049, 1001, 907, 787 and 600 F g−1 at scan rates of 1, 2, 5, 10, and 20 mV s−1, respectively, as shown in Fig. 4b. The specific capacitances decrease with increasing scan rate because the diffusion effect limits the ion migration in the electrolyte.25
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Fig. 4 Electrochemical characterization of the Co3O4 electrode and blank substrate. (a) CV curves of the Co3O4 electrode at various scan rates. (b) Specific capacitance of the Co3O4 electrode at various scan rates. (c) Galvanostatic charging and discharging curves of the Co3O4 electrode at various current densities. (d) Specific capacitance of the Co3O4 electrode at various current densities. (e) The contrast of CV curves of the blank substrate and the Co3O4 electrode at a scan rate of 10 mV s−1.
To further study the electrochemical performance of the electrode, galvanostatic charging and discharging curves were measured at a series of current densities, as shown in Fig. 4c. The specific capacitance can be calculated using the following equation:29 C¼
IΔt mΔV
ð6Þ
where Δt is the discharge time (s), I is the discharge current (A), ΔV is the potential window (V), and m is the mass (g) of the active material. The specific capacitances are 762, 722, 688, and 630 F g−1 at current densities of 6, 12, 18, and 36 A g−1, respectively, as shown in Fig. 4d. The specific capacitances
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decrease with the current because as the current increases, more active materials take part in the reaction in a shorter amount of time.30 To estimate the influence of the substrate on the specific capacitance of the electrode, CV curves of the Co3O4-based electrode and the ITO-coated PET substrate were measured at a scan rate of 10 mV s−1, as shown in Fig. 4e. Compared with the Co3O4-based electrode, the ITO-coated PET substrate shows a very low current; thus, the contribution to the capacitance from the substrate can be neglected. The electrochemical performance of the fabricated supercapacitor was tested in a two-electrode sandwich structure. Electrochemical measurements of the supercapacitor were conducted on a CHI660D electrochemical workstation. Fig. 5a
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presents the CV curves of the supercapacitor measured at scan rates from 1 to 200 mV s−1. There are two strong redox peaks in each curve, indicating the pseudocapacitive performance of the supercapacitor.31 The nearly symmetrical peaks show the excellent reversibility of the response to charge and discharge.32 Fig. 5b shows the specific capacitances of the supercapacitor at various scan rates calculated by choosing the potential window of 0.35 V. A specific capacitance of 177 F g−1 is achieved at the scan rate of 1 mV s−1. In a symmetric supercapacitor, the theoretical capacitances of the cell can be obtained by dividing the capacitances of the single electrodes by four.33 The difference between the actual capacitances and
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theoretical capacitances calculated for a single electrode occurs primarily because the PVA in the gel electrolyte greatly limits ionic migration. The specific capacitance is 6.03 mF cm−2, based on the area of the active material, at 1 mV s−1. Fig. 5c shows the galvanostatic charging and discharging curves for the fabricated supercapacitor measured at current densities from 0.9 to 1.8 A g−1, calculated by choosing the positive potential part. The supercapacitor delivers a specific capacitance value of 172 F g−1 at a current density of 0.9 A g−1 (Fig. 5d). The specific capacitances decrease with the scan rate and the currents increase for the same reason as in the threeelectrode test. These galvanostatic charging and discharging
Fig. 5 Electrochemical characterization of the fabricated transparent and flexible supercapacitor. (a) CV curves of the supercapacitor at various scan rates. (b) Specific capacitance of the supercapacitor at various scan rates. (c) Galvanostatic charging and discharging curves of the supercapacitor at various current densities. (d) Specific capacitance of the supercapacitor at various current densities. (e) Power and energy densities of the supercapacitor. (f ) Cycling performance of the supercapacitor for 30 000 cycles, the inset presents the CV curves of the supercapacitor at 1, 10 000, 20 000 and 30 000 cycles.
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curves display nonlinearities, which confirm the CV results: the capacitance of the supercapacitors is primarily contributed by the Faradaic redox reactions. For the symmetric supercapacitor, the energy densities and power densities were calculated from the CV curves and plotted on the Ragone diagram shown in Fig. 5e. The energy densities of the supercapacitor decrease from 3.01 to 0.56 W h kg−1 as the power densities increase from 0.031 to 1.152 kW kg−1. The energy density and power density were calculated using the following equations, respectively:29 E¼
CðΔV Þ2 1000 2 3600
ð7Þ
E 3600 Δt 1000
ð8Þ
P¼
where C is the specific capacitance (F g−1) of the supercapacitor, ΔV is the potential window (0.35 V), and Δt is the discharge time (s). The units of energy density (E) and power density (P) are W h kg−1 and kW kg−1, respectively. As a critical parameter, determining the energy storage performance for practical values, the cycling stability of the supercapacitor was tested through a CV process, as shown in Fig. 5f.
A specific capacitance retention ratio of 93% was obtained after 30 000 CV tests, illustrating the excellent electrochemical stability of the supercapacitor. The capacitance retention reaches 100% after 20 000 cycles. The supercapacitor is capable of retaining 106% and 100% of the initial specific capacitance after 10 000 and 20 000 cycles, respectively. It is worth noting that after 20 000 cycles, the specific capacitance decreases relatively quickly at first, and then decreases more and more slowly. During the first 6000 cycles, the capacitance retentions increase from 100% to 107%. This enhancement in specific capacitance can be interpreted as showing that the electrochemical activity of the active material is activated gradually at the beginning. Therefore, these results demonstrate the good electrochemical performances and excellent cycling stability of the Co3O4-based supercapacitor. Note that the integrated electrochemical performance of the flexible and transparent Co3O4-based supercapacitor is far superior to that of carbon-based ones.6–8 Fig. 6a shows that the CV curves measured are not affected after bending the supercapacitor at different angles, demonstrating that the structural integrity of the device is not destroyed when bending. For example, the device was bent to 60° more than 100 times, and the performance of the device
Fig. 6 The transparency and bending properties of the fabricated supercapacitor. (a) CV curves of the supercapacitor under different bending angles from 0° to 150° at a scan rate of 100 mV s−1, the inset shows how to compute the bending angles. (b) Bending cycling performance of the supercapacitor. (c) Transmittance spectra of the blank substrate, the CO3O4 electrode and the supercapacitor. (d) The photograph of the supercapacitor showing the transparent performance. (e) The photograph of the supercapacitor showing the flexible performance. (f ) A LED is turned on when the supercapacitor is in a bent state.
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was measured every 10 times. The result shows that the capacitance retention of the supercapacitor fluctuated between 99.7% and 100.4%, as shown in Fig. 6b, demonstrating the excellent mechanical flexibility of the supercapacitor under harsh bending conditions. The transmittances of the substrate, electrode, and resultant supercapacitor were measured, and the results are shown in Fig. 6c. At a wavelength of 550 nm, the ITO-coated PET substrate shows a transmittance of 76%, and the transmittance of a single electrode is approximately 58%, which decreases to 51% for the corresponding assembled supercapacitor. For example, our university logo is clearly visible when it is placed under the supercapacitor (Fig. 6d). The flexibility of the supercapacitor can be seen in Fig. 6e. A blue LED with an operating voltage of 1.5 V was lit up using a tandem supercapacitor with an active area of 14 cm2 made up of six small supercapacitors, as shown in Fig. 6f. We suggest one reasonable explanation for the poor cycling life of flexible and transparent supercapacitors made from carbon-based materials. For an ideal double-layer capacitor, no electrochemical reaction occurs on the surface of the active materials. However, in practical charging and discharging processes, the electrode process will destroy the ideality of the double layer; thus, there is charge transfer, resulting in leakage current.34 An ultrathin film composed of carbon nanotubes or graphene usually has a highly oriented structure, which results in a strong orientation direction of the charge transfer. As a result, the surface structure of the active material will gradually be destroyed. As for our flexible and transparent Co3O4-based supercapacitor, we consider four factors that contribute to the good electrochemical performance and excellent cycling stability. First, the Co3O4 nanocrystals synthesized using LAL have a very clean surface. The synthesis method for the active materials is laser ablation of a cobalt target in deionized water. There are no chemical reagents remaining on the surface of the products, which means the active materials contact the electrolyte directly and effectively. Additionally, LAL is a method that can improve the activity of nanomaterials in certain applications.35,36 Second, the average size of the Co3O4 nanocrystals is approximately 3.5 nm. For electrochemical energy storage materials, an ultrafine particle size can reduce the ion transport length.37 This greatly enhances the electrochemical activity and electrochemical efficiency of materials, which improves the electrochemical performance and the cycling stability. Third, the ultrafine size of the nanomaterials would cause a considerable surface-to-volume ratio, making the electrochemically active area larger. Generally, the surface plays a critical role in the transportation of ions, which means the insertion and extraction of the ions in the electrolyte primarily occur on the surface of the active materials, reducing the damage to the lattice structure. This can contribute to the excellent cycling stability of supercapacitors. Fourth, the uniform size distribution makes the resistance, current density, and reactive state of each part in the electrode stable, improving the cycling stability.
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Conclusions In summary, we have, for the first time, proposed using transition metal oxides as advanced active materials for flexible and transparent pseudocapacitors to overcome the poor cycling life of carbon-based devices. We have experimentally demonstrated a high-performance, flexible, transparent, and super-long-life supercapacitor made from ultrafine Co3O4 nanocrystals synthesized using LAL. Our measurements have shown that the integrated electrochemical performance of the flexible and transparent Co3O4-based supercapacitor is far superior to the performances of carbon-based ones. In particular, the cycling stability reaches a 100% retention rate after 20 000 cycles, much higher than the capacitance retentions of the transparent and flexible supercapacitors reported to date. Additionally, the supercapacitor showed a transmittance of 51% at a wavelength of 550 nm, and the bending angle has no influence on the specific capacitance, presenting great mechanical flexibility. Therefore, these results suggest that ultrafine transition metal oxide nanocrystals like Co3O4 nanocrystals can be used as advanced active materials for flexible and transparent supercapacitors.
Acknowledgements The National Basic Research Program of China (2014CB931700), the National Natural Science Foundation of China (91233203) and the State Key Laboratory of Optoelectronic Materials and Technologies supported this work.
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