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Supercapacitor electrode of nano-Co3O4 decorated with gold nanoparticles via in-situ reduction method
Journal of Power Sources 363 (2017) 1e8
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Supercapacitor electrode of nano-Co3O4 decorated with gold nanoparticles via in-situ reduction method Yongtao Tan, Ying Liu, Lingbin Kong, Long Kang, Fen Ran* State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano-Co3O4 decorated with Au nanoparticles is synthesized by insitu reduction. Au amount loaded in AuNP/nanoCo3O4 composite is mediated from 0 to 1.2 wt %. AuNP/nano-Co3O4 shows high specific capacitance of 681 F g1. AuNP/nano-Co3O4 remained 83.1% of initial specific capacitance after 13000 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2017 Received in revised form 13 July 2017 Accepted 15 July 2017
Nano-Co3O4 decorated with gold nanoparticles is synthesized by a simple method of in-situ reduction of HAuCl4 by sodium citrate for energy storage application, and the effect of gold content in the product on electrochemical performance is investigated in detail. Introducing gold nanoparticles into nano-Co3O4 bulk would contribute to reduce internal resistance of charge transmission. The results show that after in-situ reduction reaction gold nanoparticles imbed uniformly into nano-Co3O4 with irregular nanoparticles. The gold nanoparticles decorated nano-Co3O4 exhibits specific capacitance of 681 F g1 higher than that of pristine Co3O4 of 368 F g1. It is interesting that a good cycle life with the specific capacitance retention of 83.1% is obtained after 13000 cycles at 5 A g1, which recovers to initial specific capacitance value when the test current density is turned to 2 A g1. In addition, the device of asymmetric supercapacitor, assembled with gold nanoparticles decorated nano-Co3O4 as the positive electrode and activated carbon as the negative electrode, exhibits good energy density of 25 Wh kg1, which is comparable to the asymmetric device assembled with normal nano-Co3O4, or the symmetric device assembled just with activated carbon. © 2017 Elsevier B.V. All rights reserved.
Keywords: Metal oxide Electrode materials Gold nanoparticles Supercapacitors
1. Introduction Electrochemical capacitors (ECs), also called supercapacitor, have been attracted much attention in recent years for application in energy storage field due to its high power density, fast chargingdischarging rate, and long cycle stability. ECs bridge the gap
* Corresponding author. E-mail addresses: [email protected], [email protected] (F. Ran). http://dx.doi.org/10.1016/j.jpowsour.2017.07.054 0378-7753/© 2017 Elsevier B.V. All rights reserved.
between traditional electric capacitors and batteries for the application of energy management/conservation applications, day-night storage, power tools et al. [1e3]. ECs store energy on the basis of either ion adsorption double electric layer capacitors (EDLCs), which base on the charges absorbed in the surface or interface of electrode/electrolyte, or pseudocapacitors, which involve fast reversible multi-electron surface redox faradaic reaction [4]. Electrode material of ECs plays the critical role for the electrochemical performance such as specific capacitance, energy density, power
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density et al. Activated carbon is one typical commercial material in EDLCs owing to its high specific surface area, high conductivity, and long cycle life et al. Hence, carbon-based materials have been full developed as EDLCs electrode material [5e9], such as carbon nanotube (CNT) [10], hierarchical porous carbons [11,12], N-doped carbon [13,14], graphene [6] et al.; however, the specific capacitance, and especially energy density (<10 Wh kg1) still be lower than that of pseudocapacitive electrode materials. Metal oxides and conducting polymers as pseudocapacitive electrode materials have been investigated widely due to the high specific capacitance and high energy density comparing favorably with battery. Conducting polymers [15] include polyaniline [16,17], polypyrrole [18] et al. Metal oxides involve NiO [19,20], Co3O4 [21], MnO2 [22e24] and others [25,26], and metal nitrides include VN [27e29], TiN [30] et al. Among them, Co3O4 is particularly attractive for application in ECs, owing to its low cost, low environmental footprint, great redox activity, and extremely high theoretical SC (ca. 3560 F g1) [31]. In the past decade, many kinds of Co3O4 have been explored for superior performance in the field of supercapacitor, such as ultrathin mesoporous nanosheet [31], freestanding nanowire [32], hollow structure [33], 3D nanonet [34] and so on. Nevertheless, conductivity of Co3O4 is not good, which is critical to enhance the kinetics of ion and electron transport in electrode and at the electrode/electrolyte interface, and to engage sufficient electroactive species exposed on the surface for the Faradaic redox reaction, further leading to make the cycling life and performances at high current density get improvement [31,35]. Metallic nanoparticles have high electrical conductivity, high surface activity, and catalytic capability, especially gold nanoparticles [36,37], which have been widely used as conductive dopants in electrode materials for supercapacitors [38e40]. Qu reported the electrode material Au-NiO exhibited the high specific capacitance 619 F g1 at current density of 20 A g1, higher than that of pristine NiO (216 F g1) [38]. Kim reported Ni(OH)2 by simply coating gold nanoparticles on the surface showed an obvious enhancement of 41% capacitance value [39]. Zhu synthesized NiCo2O4@Au nanotubes by electrospinning method, which showed specific capacitance of 1013.5 F g1 and maintained at 85.13% after 10, 000 cycles [40]. These literatures provide new method to improve electrochemical performance of electrode materials. Zhuang [36] reported monodisperse Au@Co3O4 coreshell nanocrystals based oxidation of Au@Co core shell nanocrystals. Fang [41] prepared the CNT-Au@Co3O4 tubular hybrids involving two steps: decorating CNTs with gold nanoparticles via NaBH4 as reduction agent on CNTs surface and loading of cobalt oxide on gold nanoparticles-decorated CNTs. Mesoporous nanosheet Au/Co3O4 was reported via the method combining Co3O4 preparation and in-situ reduction by Ren et al. [42]. However, most works about Au/Co3O4 focused on catalysts field or other fields [36,41e45]. Meanwhile, there are few reports on the synthesis of nano-Co3O4 decorated with gold nanoparticles for the application in energy storage field. In this paper, the nano-Co3O4 decorated with gold nanoparticles (AuNP/nano-Co3O4) was synthesized by in-situ reduction method, which was simple, cost-effective, robust, and less time consuming. Introducing gold nanoparticles into nano–Co3O4 bulk contribute to reduce internal resistance of charge transmission. The effect of gold content in the product on electrochemical performance was also investigated in detail. Finally, an asymmetric supercapacitor device of AuNP/nano-Co3O4//AC was assembled using AuNP/nano-Co3O4 as the positive electrode and activated carbon as the negative electrode compared with the asymmetric device assembled with normal nano-Co3O4, and the symmetric device assembled with activated carbon.
2. Experimental 2.1. Chemicals Cobaltous chloride (CoCl2$6H2O), NH3$H2O, hydrogen tetrachloroaurate hydrate (HAuCl4$3H2O), NH3$H2O and sodium citrate (Na3C6H5O7$2H2O) were of analytical grade and used without further purification. 2.2. Fabrication of nano-Co3O4 Co(OH)2 was fabricated according to the method in our previous reported work [46]: cobalt chloride hydrate solution (1 M, 25 mL) was transferred to a glass beaker. After being stirred for 30 min, the solution was adjusted to pH ¼ 9 by dropwise addition of 5 wt% NH3$H2O with a constant time interval of 5 s at room temperature. The resulting suspension was further stirred at the same temperature for an additional 3 h. Then the solid was filtered and washed with a copious amount of distilled water for several times. The obtained Co(OH)2 product was heat-treated in air at 250 C for 6 h to fabricate Co3O4 powder. 2.3. Decorating nano-Co3O4 with gold nanoparticles A certain amount of Co3O4 powder (0.1, 0.15, 0.2 or 0.3 g) was mixed with HAuCl4 (2.5 mg) and dissolved in 50 mL water, to which 1 wt% sodium citrate aqueous solution (0.75 mL) was added. The mixture system was heated to 100 C and kept the temperature for 30 min; then it was cooled down naturally and dried at 80 C for 6 h to achieve gold nanoparticles decorated nano-Co3O4, the obtained product was termed as AuNP/nano-Co3O4. 2.4. Characterization The morphologies were examined by scanning electron microscopy (SEM, JSM-6701) and transmission electron microscopy (TEM, TECNAI TF20). The crystal structures and compositions of the as-prepared samples were characterized by using X-ray diffraction (XRD, Philips, X0 pert pro, Cu Ka, 0.154056 nm). X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI ESCA System. Au content for Au/Co3O4 composite was determined by electron probe microanalyzer (EPMA-1600). 2.5. Electrode preparation The prepared active material, conducting graphite, acetylene black, and poly (tetrafluoroethylene) at the weight ratio of 80: 7.5: 7.5: 5 were mixed in an agate mortar, which was coated on the Ni foam of geometric surface area of ca. 1 cm2 as current collector and pressed at 10 M Pa for ca. 15 s. Finally, the electrode was obtained by being dried at 80 C for 12 h. 2.6. Electrochemical performance evaluation The electrochemical evaluation was employed by threeelectrode system including the as-prepared active material as work electrode, a platinum foil electrode as counter electrode, and a saturated calomel electrode (SCE) as reference electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charge/ discharge (GCD) measurements were tested in 2 M KOH aqueous solution. Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency in a range of 0.01~105 Hz at an amplitude 5 mV. Cycle stability was carried out by using CT2001A (Land, China). All the electrochemical experiments were carried out at 20 ± 1 C and using CHI660E workstation.
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Specific capacitance (SC) was calculated from GCD curve according to equation:
Cs ¼ I Dt =mDV
(1)
where Cs is specific capacitance (F g1), I is discharging current (A), Dt is discharging time (s), m is active material mass (g), and DV is potential window (V). 2.7. Performance evaluation of the assembled energy device The energy device was fabricated with AuNP/nano-Co3O4 as positive electrode, and activated carbon as negative electrode. For the energy device, the charge balance follows the relationship:
Qþ ¼ Q
(2)
where Q represents the charge stored, þ and e represents positive electrode and negative electrode, respectively.
Q ¼ Cs *m*DV
(3)
where Cs means specific capacitance, m represents total mass of two electrodes, and DV is the potential window of charge/discharge tests. According to above equations (2) and (3),
mþ =m ¼ Cs = Csþ DV =DVþ
(4)
The energy density (E) and power density (P) were calculated based on the following equation:
E ¼ 0:5 Cs DV 2
(5)
P ¼ E=t
(6)
3. Results and discussion The synthesized nano-Co3O4 and AuNP/nano-Co3O4 were characterized by SEM, TEM, EDS mapping, and XRD, respectively, as shown in Fig. 1. Nano-Co3O4 exhibited flower-like morphology (Fig. 1a), consisting of stacked flat-sheet and irregular nanoparticles (Fig. 1b and c). After the in-situ reduction reaction, AuNP/nanoCo3O4 composite mainly retained sheet-like structure with closedgrained surface and irregular nanoparticles (Fig. 1d and e). The collapse of the flower-like shape was due to the reduction reaction of the added HAuCl4 and the loss of H2O molecule, which could reduce size of the nanoparticles and improve the volume density of the material (Fig. 1f). In order to more clearly observe the structure and composition, the synthesized AuNP/nano-Co3O4 was characterized by HRTEM and EDS mapping, as shown in Fig. 1g and h. As shown in the figure, Co3O4 nanoparticles were in irregular shape and gold was in regular shape with a size of ca. 15 nm. The lattice fringes with interplanar spacing of 0.467 nm and 0.243 nm corresponded to (111) and (311) planes of Co3O4, respectively, while the lattice fringe with interplanar spacing of 0.236 nm corresponded to (111) plane of Au. From the EDS mapping of SEM, one can find that the O and Co element-distribution diagrams were continuous because of Co3O4, while Au element distributed dispersedly and uniformly in nanoscale. Furthermore, the crystal structure of nanoCo3O4 and AuNP/nano-Co3O4 were characterized by XRD, as shown in Fig. 1i. All diffraction peaks both for Co3O4 and AuNP/nano-Co3O4 composite perfectly matched with the cubic spinel crystal structure (JCPDS No. 42e1467) of Co3O4 as obtained. Besides, in the curve of
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AuNP/nano-Co3O4, the diffraction peak located at ~38 was also indexed as (111) plane of metal Au (JCPDS No. 04e0784). The other diffraction peaks of metal Au were overlapped and concealed with peaks of Co3O4, to a certain extent, causing difficulty to distinguish the full diffraction peaks. XPS measurement was performed to further confirm the composition of the samples. The all-elements XPS spectra of nanoCo3O4 and AuNP/nano-Co3O4 exhibited the characteristic peaks of Co and O (Fig. S1 in the supporting information). The Co 2p XPS spectra included two characteristic peaks with binding energy at 779.8 and 795.1 eV with a spin energy separation of 15.3 eV, corresponding to Co 2p3/2 and 2p1/2 [31,47,48], respectively, indicating the distinctive traits of Co3O4 phase (Fig. 2a). Fig. 2b was the highresolution XPS spectra of Au 4f for nano-Co3O4 and AuNP/nanoCo3O4, respectively. The two characteristic peaks with binding energy at 84.0 and 87.7 eV, corresponding to Au 4f7/2 and 4f5/2 [49], respectively, conforming the existence of Au element in AuNP/ nano-Co3O4. On the basis of above results, one can conclude that in AuNP/nano-Co3O4 the nano-Co3O4 was decorated with Au nanoparticles. The electrochemical performances of nano-Co3O4 and AuNP/ nano-Co3O4 were evaluated by a three-electrode system in 2 M KOH, and the results are shown in Fig. 3. From the typical CV curves of nano-Co3O4 and AuNP/nano-Co3O4 in the range of 0.2e0.6 V at scan rate of 5e50 mV s1 (Fig. 3a, and Fig. S2a in the supporting information), one can see that two pairs of redox peaks were clearly observed due to the conversation conversion of oxidation states of Co from þ2 to þ3 to þ4 through the Co3O4/CoOOH/CoO2 transformation associated with OH anions as the following redox reactions [21,34,50,51]:
Co3 O4 þ OH þ H2 O43CoOOH þ e $$CoOOH þ OH 4CoO2 þ H2 O þ e The capacitive behaviors of both nano-Co3O4 and AuNP/nanoCo3O4 were distinct from EDLCs, which are close to rectangle shape. With scan rate increasing, the potential of the oxidation peaks shifted away from the positive potential direction and the potential of the reduction peaks shifted away the negative potential direction. The current response was found to increase proportionally with increasing scan rates, indicating a fast electron transfer rate during the diffusion controlled redox reaction [51]. The GCD curves of nano-Co3O4 and AuNP/nano-Co3O4 revealed the nonlinear potential-time relationships, reflecting the pseudocapacitive behavior in accordance with the CV results; however, the discharging time of AuNP/nano-Co3O4 was much longer than that of Co3O4 (Fig. S3 and Fig. S2b in the supporting information). The specific capacitances at various current densities were also calculated based on GCDs as shown in Fig. 3b. The specific capacitances of AuNP/nano-Co3O4 were higher than that of nano-Co3O4 at the current densities from 0.5 to 10 A g1, and AuNP/nano-Co3O4 electrode manifested higher specific capacitance values of 681, 657, 620, 585, 550, 525, and 392 F g1 than those of 368, 350, 330, 307, 300, 275, and 200 F g1 for nano-Co3O4 at current densities of 0.5, 1, 2, 3, 4, 5, and 10 A g1, respectively. The 185% enhancement of specific capacitance for AuNP/nano-Co3O4 (681 F g1 at 0.5 A g1) compared with that for nano-Co3O4 (368 F g1 at 0.5 A g1) was due to the introduction of Au nanoparticles into the active materials. When the current density increased from 0.5 to 10 A g1 (20 times of initial current density), the specific capacitance of AuNP/nanoCo3O4 composite still retained 392 F g1; while Co3O4 only retained 200 F g1, 196% enhancement of specific capacitance for AuNP/ Co3O4 compared with that for nano-Co3O4 at current density of 10 A g1. This indicated that obvious improvement of the
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Fig. 1. SEM and TEM images of (a, b, and c) nano-Co3O4 and (d, e, f, and g) AuNP/nano- Co3O4; (h) EDS mapping of AuNP/nano-Co3O4; and (i) XRD patterns.
Fig. 2. High-resolution XPS spectra of (a) Co 2p and (b) Au 4f for nano-Co3O4 and AuNP/nano-Co3O4.
electrochemical performance at high current density was achieved. Fig. 3c shows the EIS measurement of nano-Co3O4 and AuNP/nanoCo3O4 composite. All the profiles revealed a semi-circle at high frequency region and a line at low frequency region. The point of intersection between the semicircle and real axis (Z0 axis) was the intrinsic resistance (Rs) involving the resistance from electrolyte, electrode and other parts and the radius of semicircle represented charging transfer resistance (Rct). It can be seen from the figure, Rs and Rct of AuNP/nano-Co3O4 were smaller than these of nano-
Co3O4, indicating better conductivity and faster electron transfer ability for AuNP/nano-Co3O4, which agreed well with the results from CV measurements. Meanwhile, the IR drop were calculated based on GCDs and the polts are shown in Fig. 3d. The slope of the line represents internal resistance. It can be seen from the figure that the slope value of AuNP/nano-Co3O4 composite was smaller than that of nano-Co3O4, demonstrating AuNP/nano-Co3O4 had smaller internal resistance than that of Co3O4, also as higher conductivity. Meanwhile, the conductivity of nano-Co3O4 and AuNP/
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Fig. 3. Electrochemical performance of AuNP/nano-Co3O4: (a) CV, (b) SC at different current density from 0.5 to 10 A g1, (c) EIS, (d) IR drop for nano-Co3O4 and AuNP/nano-Co3O4 at different current density and (e) cycle-life.
nano-Co3O4 were 1.52 and 2.04 S m1, respectively, which were measured by four point probe method, which could be explained for the smaller IR drop for AuNP/nano-Co3O4 than that of nanoCo3O4. Further, the EIS spectra were fitted with the equivalent circuit shown in Fig. S4 and fitted data for circuit elements are listed in Table S1. Rs represents the series resistance consisting of contact resistance, electrolyte resistance and electrode resistance [52e55]. Cdl denotes capacitance representing the double layer capacitance arising from the storage of charges on the surface of electrode, Rct is the resistance associated with the charge transfer process, which indicates the rate of the redox reaction taking place at the interface, and CF is a faradic capacitor. A smaller Rs (0.724 U) was observed for AuNP/nano-Co3O4 composite compared with that of Co3O4 (0.846 U), indicating smaller IR drop, which was consistent with the results of Fig. 3d. More importantly, a smaller Rct (0.311U) for AuNP/ nano-Co3O4 composite was also observed compared with that of nano-Co3O4 (0.323 U). The fitting data and EIS plots indicated AuNP/nano-Co3O4 material owned faster ion diffusion rate and charge transport rate than those of nano-Co3O4. Fig. 3e shows the cycle stabilities of nano-Co3O4 and AuNP/nano-Co3O4 at the current density of 2 and 5 A g1 alternately. In the first 1000 cycles, the specific capacitance of AuNP/nano-Co3O4 and nano-Co3O4 increased due to the activation during the charging-discharging process; after that, with the increase of current density to 5 A g1, the SC retention for AuNP/nano-Co3O4 (96.2%) was a little bit higher than that of nano-Co3O4 (94.5%) in the next 5000 cycles. Furthermore, the cycle ability was measured at 2 A g1 once again and the SCs for both samples were recovered, and then with the current density again increased up to 5 A g1 during another 6000 cycles, the SC retention for AuNP/nano-Co3O4 of 83.1% was also higher than 79.8% of nano-Co3O4. Finally, in the additional 300 cycles, the specific capacitance tested at 2 A g1 again recovered 100% for AuNP/nano-Co3O4 and 99.2% for nano-Co3O4. In general, the SC retention of AuNP/nano-Co3O4 and nano-Co3O4 was 83.1% and 79.8% after 13000 cycles, respectively; and the SCs for both
AuNP/nano-Co3O4 and nano-Co3O4 recovered to 100% and 99.2%, respectively. The cycle ability of AuNP/nano-Co3O4 composite was better than that of nano-Co3O4 at high current density due to the introduction of Au nanoparticles, which improved the conductivity of the electrode material and shortened the diffusion length of the charges during charging and discharging process, further enhancing the cycle stability at high current density. Based on the above-obtained results that the introduction of gold nanoparticles could improve the electrochemical performance of Co3O4 as supercapacitor electrode, the effect of Au amount on the electrochemical performance of AuNP/nano-Co3O4 was investigated, as shown in Fig. 4. The Au amount loading in AuNP/nanoCo3O4 was mediated to be 0, 0.3, 0.6, 0.9, and 1.2%, the corresponding products were termed as C0, C1,C2, C3, and C4 (Fig. 4a). The theoretical values were calculated by weight ratio between HAuCl4 and Co3O4, and the real values and the related EPMA images were measured by EPMA-1600 (inset in Fig. 4a). It could be seen that the real values were very much close to these of theoretical values. The morphologies of the samples exhibited similar and irregular shape of nanoparticles. The XRD data are shown in Fig. S4 in the supporting information, reflecting the main peaks of Co3O4, overlapping the peaks by Au. The typical CV curves of C0~C4 possessed two pair of peaks redox peaks for the conversation conversion of oxidation states of Co from þ2 to þ3 to þ4 through the Co3O4/CoOOH/CoO2 transformation associated with OH anions (Fig. 4b), and the GCD curves revealed the nonlinear potentialtime relationships reflecting the pseudocapacitive behavior (Fig. 4c). The SCs calculated by discharging times at different current density for all samples are plotted in Fig. 4d. With Au amounts increased from 0.3 to 1.2 wt%, the samples showed the increased SCs from 368 to 681 F g1 (C0~C3) and then the SCs decreased to 371 F g1 (C3~C4) at the current density of 0.5 A g1 (inset in Fig. 4d). As such, C3 delivered the highest SC up to 681 F g1 when the Au amount loaded in AuNP/nano-Co3O4 was 0.9 wt%. The best Au content of 0.9 wt% maybe resulted from the synergy effect
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Fig. 4. Electrochemical performance of AuNP/nano-Co3O4 with different Au nanoparticles content: (a) Au content in AuNP/nano-Co3O4 (Inset is the related EPMA photos), (b) CV, (c) GCD, and (d) SC at different current density from 0.5 to 10 A g1 (inset is the calculated SCs measured at the current density of 0.5 A g1).
between gold nanoparticles and nano-Co3O4 and the better dispersion in nano-Co3O4 well. Lower amount of Au nanoparticles led to low conductivity of the electrode material, while high content of Au nanoparticles (>0.9 wt%) resulted in the decrease of active materials in electrode materials. In this case, there should be a best Au content (that is 0.9 wt%) in electrode materials for the electrochemical performance. When the current density increased from 0.5 to 10 A g1 (20 times of the initial current density), the SC retentions of C0, C1, C2, C3, and C4 were 54.3, 62.5, 59.7, 57.6, and 67.5%, respectively. For practical application, AuNP/nano-Co3O4 was further used as the positive electrode to assemble the asymmetric supercapacitor including activated carbon (AC) as the negative electrode. For comparison, the symmetric supercapacitor devices of AC//AC and Co3O4//AC were also assembled in the electrolyte of 2 M KOH. The electrochemical performance of AuNP/nano-Co3O4//AC, nanoCo3O4//AC, and AC//AC devices are shown in Fig. 5, and Fig. S6-9 in the Supporting Information. The CVs of AuNP/nano-Co3O4//AC device at different voltage range both in 0e1.4 V and 0e1.6 V (Fig. 5a) were different from that of AC//AC device (Fig. S6a) with the rectangle shape of CV curves and linear potential-time relationship of GCD curves, i. e., the EDLC behavior (Fig. S6). It was seen from Fig. 5b, with the scan rate increasing, the redox voltage peaks shifted away and CV shape of 50 mV s1 was similar to that of 5 mV s1, indicating a fast electron transfer rate during the diffusion controlled redox reaction. The GCD curves of AuNP/nano-Co3O4//AC device were also different from these of AC//AC device (Fig. 5c). The SC of AuNP/nano-Co3O4 was calculated from the GCD curve to be 80 F g1 at 0.5 A g1 (Fig. S10 in Supporting Information). When current density increased to 15 A g1, the SC was found to be 50 F g1 and the SC retention was 62.5% of the initial SC at the
current density of 0.5 A g1. The relationship of energy density and power density, named Ragone plots, are shown in Fig. 5d. The device based on AuNP/nano-Co3O4//AC showed the high energy density of 25 Wh kg1 and high power density of 11.25 kW kg1 with the energy density of 15.6 Wh kg1, higher than that of AC//AC device and (7.64 Wh kg1) and nano-Co3O4//AC device (19.7 Wh kg1), respectively. This performance was also comparable to the related materials in the reported literatures like NiCo2O4@Au//AC device (19.56 Wh kg1) [40], carbon//carbon device (4.4e12.6 Wh kg1) [56], AC//MnO2 device (17 Wh kg1) [57], and carbon nanotube//TiO2 nanowire device (12.5 Wh kg1) [58]. In general, these results indicated that the fabricated AuNP/nano-Co3O4 composite exhibited high electrochemical performance as electrode material for supercapacitors.
4. Conclusions The AuNP/nano-Co3O4 composite was synthesized by a simple method of in-situ reduction of HAuCl4 by sodium citrate for supercapacitors. Au nanoparticles imbedded uniformly into nanoCo3O4 with irregular nanoparticles with small amounts from 0 to 1.2 wt%. The AuNP/nano-Co3O4 composite exhibited higher specific capacitance of 681 F g1 than that of pristine Co3O4, 368 F g1. It was interesting that a good cycle life with the specific capacitance retention of 83.1% after cycling 13000 cycles at 5 A g1, which recovered to initial specific capacitance value when the test current density changed to 2 A g1. The asymmetric supercapacitor device of AuNP/nano-Co3O4//AC exhibited maximum energy density of 25 Wh kg1 and maximum power density of 11.25 kW kg1 with the energy density of 15.6 Wh kg1.
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Fig. 5. Electrochemical performance of asymmetric supercapacitors: (a) CV curves of nano-Co3O4//AC and AuNP/nano-Co3O4//AC devices at different voltage range, (b) CV curves of AuNP/nano-Co3O4//AC at different scan rate (0e1.6 V), and (c) GCD curves of AuNP/nano-Co3O4//AC at different current density (0e1.5 V) and Ragone plots of various supercapacitor devices.
Acknowledgements This work was partly supported by the National Natural Science Foundation of China (51203071, 51363014, and 51463012), China Postdoctoral Science Foundation (2014M552509, and 2015T81064), Natural Science Funds of the Gansu Province (2015GS05123), and the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology (J201402). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Supercapacitor electrode of nano-Co3O4 decorated with gold nanoparticles via in-situ reduction method Yongtao Tan, Ying Liu, Lingbin Kong, Long Kang, Fen Ran* State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Nano-Co3O4 decorated with Au nanoparticles is synthesized by insitu reduction. Au amount loaded in AuNP/nanoCo3O4 composite is mediated from 0 to 1.2 wt %. AuNP/nano-Co3O4 shows high specific capacitance of 681 F g1. AuNP/nano-Co3O4 remained 83.1% of initial specific capacitance after 13000 cycles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 March 2017 Received in revised form 13 July 2017 Accepted 15 July 2017
Nano-Co3O4 decorated with gold nanoparticles is synthesized by a simple method of in-situ reduction of HAuCl4 by sodium citrate for energy storage application, and the effect of gold content in the product on electrochemical performance is investigated in detail. Introducing gold nanoparticles into nano-Co3O4 bulk would contribute to reduce internal resistance of charge transmission. The results show that after in-situ reduction reaction gold nanoparticles imbed uniformly into nano-Co3O4 with irregular nanoparticles. The gold nanoparticles decorated nano-Co3O4 exhibits specific capacitance of 681 F g1 higher than that of pristine Co3O4 of 368 F g1. It is interesting that a good cycle life with the specific capacitance retention of 83.1% is obtained after 13000 cycles at 5 A g1, which recovers to initial specific capacitance value when the test current density is turned to 2 A g1. In addition, the device of asymmetric supercapacitor, assembled with gold nanoparticles decorated nano-Co3O4 as the positive electrode and activated carbon as the negative electrode, exhibits good energy density of 25 Wh kg1, which is comparable to the asymmetric device assembled with normal nano-Co3O4, or the symmetric device assembled just with activated carbon. © 2017 Elsevier B.V. All rights reserved.
Keywords: Metal oxide Electrode materials Gold nanoparticles Supercapacitors
1. Introduction Electrochemical capacitors (ECs), also called supercapacitor, have been attracted much attention in recent years for application in energy storage field due to its high power density, fast chargingdischarging rate, and long cycle stability. ECs bridge the gap
* Corresponding author. E-mail addresses: [email protected], [email protected] (F. Ran). http://dx.doi.org/10.1016/j.jpowsour.2017.07.054 0378-7753/© 2017 Elsevier B.V. All rights reserved.
between traditional electric capacitors and batteries for the application of energy management/conservation applications, day-night storage, power tools et al. [1e3]. ECs store energy on the basis of either ion adsorption double electric layer capacitors (EDLCs), which base on the charges absorbed in the surface or interface of electrode/electrolyte, or pseudocapacitors, which involve fast reversible multi-electron surface redox faradaic reaction [4]. Electrode material of ECs plays the critical role for the electrochemical performance such as specific capacitance, energy density, power
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density et al. Activated carbon is one typical commercial material in EDLCs owing to its high specific surface area, high conductivity, and long cycle life et al. Hence, carbon-based materials have been full developed as EDLCs electrode material [5e9], such as carbon nanotube (CNT) [10], hierarchical porous carbons [11,12], N-doped carbon [13,14], graphene [6] et al.; however, the specific capacitance, and especially energy density (<10 Wh kg1) still be lower than that of pseudocapacitive electrode materials. Metal oxides and conducting polymers as pseudocapacitive electrode materials have been investigated widely due to the high specific capacitance and high energy density comparing favorably with battery. Conducting polymers [15] include polyaniline [16,17], polypyrrole [18] et al. Metal oxides involve NiO [19,20], Co3O4 [21], MnO2 [22e24] and others [25,26], and metal nitrides include VN [27e29], TiN [30] et al. Among them, Co3O4 is particularly attractive for application in ECs, owing to its low cost, low environmental footprint, great redox activity, and extremely high theoretical SC (ca. 3560 F g1) [31]. In the past decade, many kinds of Co3O4 have been explored for superior performance in the field of supercapacitor, such as ultrathin mesoporous nanosheet [31], freestanding nanowire [32], hollow structure [33], 3D nanonet [34] and so on. Nevertheless, conductivity of Co3O4 is not good, which is critical to enhance the kinetics of ion and electron transport in electrode and at the electrode/electrolyte interface, and to engage sufficient electroactive species exposed on the surface for the Faradaic redox reaction, further leading to make the cycling life and performances at high current density get improvement [31,35]. Metallic nanoparticles have high electrical conductivity, high surface activity, and catalytic capability, especially gold nanoparticles [36,37], which have been widely used as conductive dopants in electrode materials for supercapacitors [38e40]. Qu reported the electrode material Au-NiO exhibited the high specific capacitance 619 F g1 at current density of 20 A g1, higher than that of pristine NiO (216 F g1) [38]. Kim reported Ni(OH)2 by simply coating gold nanoparticles on the surface showed an obvious enhancement of 41% capacitance value [39]. Zhu synthesized NiCo2O4@Au nanotubes by electrospinning method, which showed specific capacitance of 1013.5 F g1 and maintained at 85.13% after 10, 000 cycles [40]. These literatures provide new method to improve electrochemical performance of electrode materials. Zhuang [36] reported monodisperse Au@Co3O4 coreshell nanocrystals based oxidation of Au@Co core shell nanocrystals. Fang [41] prepared the CNT-Au@Co3O4 tubular hybrids involving two steps: decorating CNTs with gold nanoparticles via NaBH4 as reduction agent on CNTs surface and loading of cobalt oxide on gold nanoparticles-decorated CNTs. Mesoporous nanosheet Au/Co3O4 was reported via the method combining Co3O4 preparation and in-situ reduction by Ren et al. [42]. However, most works about Au/Co3O4 focused on catalysts field or other fields [36,41e45]. Meanwhile, there are few reports on the synthesis of nano-Co3O4 decorated with gold nanoparticles for the application in energy storage field. In this paper, the nano-Co3O4 decorated with gold nanoparticles (AuNP/nano-Co3O4) was synthesized by in-situ reduction method, which was simple, cost-effective, robust, and less time consuming. Introducing gold nanoparticles into nano–Co3O4 bulk contribute to reduce internal resistance of charge transmission. The effect of gold content in the product on electrochemical performance was also investigated in detail. Finally, an asymmetric supercapacitor device of AuNP/nano-Co3O4//AC was assembled using AuNP/nano-Co3O4 as the positive electrode and activated carbon as the negative electrode compared with the asymmetric device assembled with normal nano-Co3O4, and the symmetric device assembled with activated carbon.
2. Experimental 2.1. Chemicals Cobaltous chloride (CoCl2$6H2O), NH3$H2O, hydrogen tetrachloroaurate hydrate (HAuCl4$3H2O), NH3$H2O and sodium citrate (Na3C6H5O7$2H2O) were of analytical grade and used without further purification. 2.2. Fabrication of nano-Co3O4 Co(OH)2 was fabricated according to the method in our previous reported work [46]: cobalt chloride hydrate solution (1 M, 25 mL) was transferred to a glass beaker. After being stirred for 30 min, the solution was adjusted to pH ¼ 9 by dropwise addition of 5 wt% NH3$H2O with a constant time interval of 5 s at room temperature. The resulting suspension was further stirred at the same temperature for an additional 3 h. Then the solid was filtered and washed with a copious amount of distilled water for several times. The obtained Co(OH)2 product was heat-treated in air at 250 C for 6 h to fabricate Co3O4 powder. 2.3. Decorating nano-Co3O4 with gold nanoparticles A certain amount of Co3O4 powder (0.1, 0.15, 0.2 or 0.3 g) was mixed with HAuCl4 (2.5 mg) and dissolved in 50 mL water, to which 1 wt% sodium citrate aqueous solution (0.75 mL) was added. The mixture system was heated to 100 C and kept the temperature for 30 min; then it was cooled down naturally and dried at 80 C for 6 h to achieve gold nanoparticles decorated nano-Co3O4, the obtained product was termed as AuNP/nano-Co3O4. 2.4. Characterization The morphologies were examined by scanning electron microscopy (SEM, JSM-6701) and transmission electron microscopy (TEM, TECNAI TF20). The crystal structures and compositions of the as-prepared samples were characterized by using X-ray diffraction (XRD, Philips, X0 pert pro, Cu Ka, 0.154056 nm). X-ray photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI ESCA System. Au content for Au/Co3O4 composite was determined by electron probe microanalyzer (EPMA-1600). 2.5. Electrode preparation The prepared active material, conducting graphite, acetylene black, and poly (tetrafluoroethylene) at the weight ratio of 80: 7.5: 7.5: 5 were mixed in an agate mortar, which was coated on the Ni foam of geometric surface area of ca. 1 cm2 as current collector and pressed at 10 M Pa for ca. 15 s. Finally, the electrode was obtained by being dried at 80 C for 12 h. 2.6. Electrochemical performance evaluation The electrochemical evaluation was employed by threeelectrode system including the as-prepared active material as work electrode, a platinum foil electrode as counter electrode, and a saturated calomel electrode (SCE) as reference electrode, respectively. Cyclic voltammetry (CV) and galvanostatic charge/ discharge (GCD) measurements were tested in 2 M KOH aqueous solution. Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency in a range of 0.01~105 Hz at an amplitude 5 mV. Cycle stability was carried out by using CT2001A (Land, China). All the electrochemical experiments were carried out at 20 ± 1 C and using CHI660E workstation.
Y. Tan et al. / Journal of Power Sources 363 (2017) 1e8
Specific capacitance (SC) was calculated from GCD curve according to equation:
Cs ¼ I Dt =mDV
(1)
where Cs is specific capacitance (F g1), I is discharging current (A), Dt is discharging time (s), m is active material mass (g), and DV is potential window (V). 2.7. Performance evaluation of the assembled energy device The energy device was fabricated with AuNP/nano-Co3O4 as positive electrode, and activated carbon as negative electrode. For the energy device, the charge balance follows the relationship:
Qþ ¼ Q
(2)
where Q represents the charge stored, þ and e represents positive electrode and negative electrode, respectively.
Q ¼ Cs *m*DV
(3)
where Cs means specific capacitance, m represents total mass of two electrodes, and DV is the potential window of charge/discharge tests. According to above equations (2) and (3),
mþ =m ¼ Cs = Csþ DV =DVþ
(4)
The energy density (E) and power density (P) were calculated based on the following equation:
E ¼ 0:5 Cs DV 2
(5)
P ¼ E=t
(6)
3. Results and discussion The synthesized nano-Co3O4 and AuNP/nano-Co3O4 were characterized by SEM, TEM, EDS mapping, and XRD, respectively, as shown in Fig. 1. Nano-Co3O4 exhibited flower-like morphology (Fig. 1a), consisting of stacked flat-sheet and irregular nanoparticles (Fig. 1b and c). After the in-situ reduction reaction, AuNP/nanoCo3O4 composite mainly retained sheet-like structure with closedgrained surface and irregular nanoparticles (Fig. 1d and e). The collapse of the flower-like shape was due to the reduction reaction of the added HAuCl4 and the loss of H2O molecule, which could reduce size of the nanoparticles and improve the volume density of the material (Fig. 1f). In order to more clearly observe the structure and composition, the synthesized AuNP/nano-Co3O4 was characterized by HRTEM and EDS mapping, as shown in Fig. 1g and h. As shown in the figure, Co3O4 nanoparticles were in irregular shape and gold was in regular shape with a size of ca. 15 nm. The lattice fringes with interplanar spacing of 0.467 nm and 0.243 nm corresponded to (111) and (311) planes of Co3O4, respectively, while the lattice fringe with interplanar spacing of 0.236 nm corresponded to (111) plane of Au. From the EDS mapping of SEM, one can find that the O and Co element-distribution diagrams were continuous because of Co3O4, while Au element distributed dispersedly and uniformly in nanoscale. Furthermore, the crystal structure of nanoCo3O4 and AuNP/nano-Co3O4 were characterized by XRD, as shown in Fig. 1i. All diffraction peaks both for Co3O4 and AuNP/nano-Co3O4 composite perfectly matched with the cubic spinel crystal structure (JCPDS No. 42e1467) of Co3O4 as obtained. Besides, in the curve of
3
AuNP/nano-Co3O4, the diffraction peak located at ~38 was also indexed as (111) plane of metal Au (JCPDS No. 04e0784). The other diffraction peaks of metal Au were overlapped and concealed with peaks of Co3O4, to a certain extent, causing difficulty to distinguish the full diffraction peaks. XPS measurement was performed to further confirm the composition of the samples. The all-elements XPS spectra of nanoCo3O4 and AuNP/nano-Co3O4 exhibited the characteristic peaks of Co and O (Fig. S1 in the supporting information). The Co 2p XPS spectra included two characteristic peaks with binding energy at 779.8 and 795.1 eV with a spin energy separation of 15.3 eV, corresponding to Co 2p3/2 and 2p1/2 [31,47,48], respectively, indicating the distinctive traits of Co3O4 phase (Fig. 2a). Fig. 2b was the highresolution XPS spectra of Au 4f for nano-Co3O4 and AuNP/nanoCo3O4, respectively. The two characteristic peaks with binding energy at 84.0 and 87.7 eV, corresponding to Au 4f7/2 and 4f5/2 [49], respectively, conforming the existence of Au element in AuNP/ nano-Co3O4. On the basis of above results, one can conclude that in AuNP/nano-Co3O4 the nano-Co3O4 was decorated with Au nanoparticles. The electrochemical performances of nano-Co3O4 and AuNP/ nano-Co3O4 were evaluated by a three-electrode system in 2 M KOH, and the results are shown in Fig. 3. From the typical CV curves of nano-Co3O4 and AuNP/nano-Co3O4 in the range of 0.2e0.6 V at scan rate of 5e50 mV s1 (Fig. 3a, and Fig. S2a in the supporting information), one can see that two pairs of redox peaks were clearly observed due to the conversation conversion of oxidation states of Co from þ2 to þ3 to þ4 through the Co3O4/CoOOH/CoO2 transformation associated with OH anions as the following redox reactions [21,34,50,51]:
Co3 O4 þ OH þ H2 O43CoOOH þ e $$CoOOH þ OH 4CoO2 þ H2 O þ e The capacitive behaviors of both nano-Co3O4 and AuNP/nanoCo3O4 were distinct from EDLCs, which are close to rectangle shape. With scan rate increasing, the potential of the oxidation peaks shifted away from the positive potential direction and the potential of the reduction peaks shifted away the negative potential direction. The current response was found to increase proportionally with increasing scan rates, indicating a fast electron transfer rate during the diffusion controlled redox reaction [51]. The GCD curves of nano-Co3O4 and AuNP/nano-Co3O4 revealed the nonlinear potential-time relationships, reflecting the pseudocapacitive behavior in accordance with the CV results; however, the discharging time of AuNP/nano-Co3O4 was much longer than that of Co3O4 (Fig. S3 and Fig. S2b in the supporting information). The specific capacitances at various current densities were also calculated based on GCDs as shown in Fig. 3b. The specific capacitances of AuNP/nano-Co3O4 were higher than that of nano-Co3O4 at the current densities from 0.5 to 10 A g1, and AuNP/nano-Co3O4 electrode manifested higher specific capacitance values of 681, 657, 620, 585, 550, 525, and 392 F g1 than those of 368, 350, 330, 307, 300, 275, and 200 F g1 for nano-Co3O4 at current densities of 0.5, 1, 2, 3, 4, 5, and 10 A g1, respectively. The 185% enhancement of specific capacitance for AuNP/nano-Co3O4 (681 F g1 at 0.5 A g1) compared with that for nano-Co3O4 (368 F g1 at 0.5 A g1) was due to the introduction of Au nanoparticles into the active materials. When the current density increased from 0.5 to 10 A g1 (20 times of initial current density), the specific capacitance of AuNP/nanoCo3O4 composite still retained 392 F g1; while Co3O4 only retained 200 F g1, 196% enhancement of specific capacitance for AuNP/ Co3O4 compared with that for nano-Co3O4 at current density of 10 A g1. This indicated that obvious improvement of the
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Fig. 1. SEM and TEM images of (a, b, and c) nano-Co3O4 and (d, e, f, and g) AuNP/nano- Co3O4; (h) EDS mapping of AuNP/nano-Co3O4; and (i) XRD patterns.
Fig. 2. High-resolution XPS spectra of (a) Co 2p and (b) Au 4f for nano-Co3O4 and AuNP/nano-Co3O4.
electrochemical performance at high current density was achieved. Fig. 3c shows the EIS measurement of nano-Co3O4 and AuNP/nanoCo3O4 composite. All the profiles revealed a semi-circle at high frequency region and a line at low frequency region. The point of intersection between the semicircle and real axis (Z0 axis) was the intrinsic resistance (Rs) involving the resistance from electrolyte, electrode and other parts and the radius of semicircle represented charging transfer resistance (Rct). It can be seen from the figure, Rs and Rct of AuNP/nano-Co3O4 were smaller than these of nano-
Co3O4, indicating better conductivity and faster electron transfer ability for AuNP/nano-Co3O4, which agreed well with the results from CV measurements. Meanwhile, the IR drop were calculated based on GCDs and the polts are shown in Fig. 3d. The slope of the line represents internal resistance. It can be seen from the figure that the slope value of AuNP/nano-Co3O4 composite was smaller than that of nano-Co3O4, demonstrating AuNP/nano-Co3O4 had smaller internal resistance than that of Co3O4, also as higher conductivity. Meanwhile, the conductivity of nano-Co3O4 and AuNP/
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5
Fig. 3. Electrochemical performance of AuNP/nano-Co3O4: (a) CV, (b) SC at different current density from 0.5 to 10 A g1, (c) EIS, (d) IR drop for nano-Co3O4 and AuNP/nano-Co3O4 at different current density and (e) cycle-life.
nano-Co3O4 were 1.52 and 2.04 S m1, respectively, which were measured by four point probe method, which could be explained for the smaller IR drop for AuNP/nano-Co3O4 than that of nanoCo3O4. Further, the EIS spectra were fitted with the equivalent circuit shown in Fig. S4 and fitted data for circuit elements are listed in Table S1. Rs represents the series resistance consisting of contact resistance, electrolyte resistance and electrode resistance [52e55]. Cdl denotes capacitance representing the double layer capacitance arising from the storage of charges on the surface of electrode, Rct is the resistance associated with the charge transfer process, which indicates the rate of the redox reaction taking place at the interface, and CF is a faradic capacitor. A smaller Rs (0.724 U) was observed for AuNP/nano-Co3O4 composite compared with that of Co3O4 (0.846 U), indicating smaller IR drop, which was consistent with the results of Fig. 3d. More importantly, a smaller Rct (0.311U) for AuNP/ nano-Co3O4 composite was also observed compared with that of nano-Co3O4 (0.323 U). The fitting data and EIS plots indicated AuNP/nano-Co3O4 material owned faster ion diffusion rate and charge transport rate than those of nano-Co3O4. Fig. 3e shows the cycle stabilities of nano-Co3O4 and AuNP/nano-Co3O4 at the current density of 2 and 5 A g1 alternately. In the first 1000 cycles, the specific capacitance of AuNP/nano-Co3O4 and nano-Co3O4 increased due to the activation during the charging-discharging process; after that, with the increase of current density to 5 A g1, the SC retention for AuNP/nano-Co3O4 (96.2%) was a little bit higher than that of nano-Co3O4 (94.5%) in the next 5000 cycles. Furthermore, the cycle ability was measured at 2 A g1 once again and the SCs for both samples were recovered, and then with the current density again increased up to 5 A g1 during another 6000 cycles, the SC retention for AuNP/nano-Co3O4 of 83.1% was also higher than 79.8% of nano-Co3O4. Finally, in the additional 300 cycles, the specific capacitance tested at 2 A g1 again recovered 100% for AuNP/nano-Co3O4 and 99.2% for nano-Co3O4. In general, the SC retention of AuNP/nano-Co3O4 and nano-Co3O4 was 83.1% and 79.8% after 13000 cycles, respectively; and the SCs for both
AuNP/nano-Co3O4 and nano-Co3O4 recovered to 100% and 99.2%, respectively. The cycle ability of AuNP/nano-Co3O4 composite was better than that of nano-Co3O4 at high current density due to the introduction of Au nanoparticles, which improved the conductivity of the electrode material and shortened the diffusion length of the charges during charging and discharging process, further enhancing the cycle stability at high current density. Based on the above-obtained results that the introduction of gold nanoparticles could improve the electrochemical performance of Co3O4 as supercapacitor electrode, the effect of Au amount on the electrochemical performance of AuNP/nano-Co3O4 was investigated, as shown in Fig. 4. The Au amount loading in AuNP/nanoCo3O4 was mediated to be 0, 0.3, 0.6, 0.9, and 1.2%, the corresponding products were termed as C0, C1,C2, C3, and C4 (Fig. 4a). The theoretical values were calculated by weight ratio between HAuCl4 and Co3O4, and the real values and the related EPMA images were measured by EPMA-1600 (inset in Fig. 4a). It could be seen that the real values were very much close to these of theoretical values. The morphologies of the samples exhibited similar and irregular shape of nanoparticles. The XRD data are shown in Fig. S4 in the supporting information, reflecting the main peaks of Co3O4, overlapping the peaks by Au. The typical CV curves of C0~C4 possessed two pair of peaks redox peaks for the conversation conversion of oxidation states of Co from þ2 to þ3 to þ4 through the Co3O4/CoOOH/CoO2 transformation associated with OH anions (Fig. 4b), and the GCD curves revealed the nonlinear potentialtime relationships reflecting the pseudocapacitive behavior (Fig. 4c). The SCs calculated by discharging times at different current density for all samples are plotted in Fig. 4d. With Au amounts increased from 0.3 to 1.2 wt%, the samples showed the increased SCs from 368 to 681 F g1 (C0~C3) and then the SCs decreased to 371 F g1 (C3~C4) at the current density of 0.5 A g1 (inset in Fig. 4d). As such, C3 delivered the highest SC up to 681 F g1 when the Au amount loaded in AuNP/nano-Co3O4 was 0.9 wt%. The best Au content of 0.9 wt% maybe resulted from the synergy effect
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Fig. 4. Electrochemical performance of AuNP/nano-Co3O4 with different Au nanoparticles content: (a) Au content in AuNP/nano-Co3O4 (Inset is the related EPMA photos), (b) CV, (c) GCD, and (d) SC at different current density from 0.5 to 10 A g1 (inset is the calculated SCs measured at the current density of 0.5 A g1).
between gold nanoparticles and nano-Co3O4 and the better dispersion in nano-Co3O4 well. Lower amount of Au nanoparticles led to low conductivity of the electrode material, while high content of Au nanoparticles (>0.9 wt%) resulted in the decrease of active materials in electrode materials. In this case, there should be a best Au content (that is 0.9 wt%) in electrode materials for the electrochemical performance. When the current density increased from 0.5 to 10 A g1 (20 times of the initial current density), the SC retentions of C0, C1, C2, C3, and C4 were 54.3, 62.5, 59.7, 57.6, and 67.5%, respectively. For practical application, AuNP/nano-Co3O4 was further used as the positive electrode to assemble the asymmetric supercapacitor including activated carbon (AC) as the negative electrode. For comparison, the symmetric supercapacitor devices of AC//AC and Co3O4//AC were also assembled in the electrolyte of 2 M KOH. The electrochemical performance of AuNP/nano-Co3O4//AC, nanoCo3O4//AC, and AC//AC devices are shown in Fig. 5, and Fig. S6-9 in the Supporting Information. The CVs of AuNP/nano-Co3O4//AC device at different voltage range both in 0e1.4 V and 0e1.6 V (Fig. 5a) were different from that of AC//AC device (Fig. S6a) with the rectangle shape of CV curves and linear potential-time relationship of GCD curves, i. e., the EDLC behavior (Fig. S6). It was seen from Fig. 5b, with the scan rate increasing, the redox voltage peaks shifted away and CV shape of 50 mV s1 was similar to that of 5 mV s1, indicating a fast electron transfer rate during the diffusion controlled redox reaction. The GCD curves of AuNP/nano-Co3O4//AC device were also different from these of AC//AC device (Fig. 5c). The SC of AuNP/nano-Co3O4 was calculated from the GCD curve to be 80 F g1 at 0.5 A g1 (Fig. S10 in Supporting Information). When current density increased to 15 A g1, the SC was found to be 50 F g1 and the SC retention was 62.5% of the initial SC at the
current density of 0.5 A g1. The relationship of energy density and power density, named Ragone plots, are shown in Fig. 5d. The device based on AuNP/nano-Co3O4//AC showed the high energy density of 25 Wh kg1 and high power density of 11.25 kW kg1 with the energy density of 15.6 Wh kg1, higher than that of AC//AC device and (7.64 Wh kg1) and nano-Co3O4//AC device (19.7 Wh kg1), respectively. This performance was also comparable to the related materials in the reported literatures like NiCo2O4@Au//AC device (19.56 Wh kg1) [40], carbon//carbon device (4.4e12.6 Wh kg1) [56], AC//MnO2 device (17 Wh kg1) [57], and carbon nanotube//TiO2 nanowire device (12.5 Wh kg1) [58]. In general, these results indicated that the fabricated AuNP/nano-Co3O4 composite exhibited high electrochemical performance as electrode material for supercapacitors.
4. Conclusions The AuNP/nano-Co3O4 composite was synthesized by a simple method of in-situ reduction of HAuCl4 by sodium citrate for supercapacitors. Au nanoparticles imbedded uniformly into nanoCo3O4 with irregular nanoparticles with small amounts from 0 to 1.2 wt%. The AuNP/nano-Co3O4 composite exhibited higher specific capacitance of 681 F g1 than that of pristine Co3O4, 368 F g1. It was interesting that a good cycle life with the specific capacitance retention of 83.1% after cycling 13000 cycles at 5 A g1, which recovered to initial specific capacitance value when the test current density changed to 2 A g1. The asymmetric supercapacitor device of AuNP/nano-Co3O4//AC exhibited maximum energy density of 25 Wh kg1 and maximum power density of 11.25 kW kg1 with the energy density of 15.6 Wh kg1.
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Fig. 5. Electrochemical performance of asymmetric supercapacitors: (a) CV curves of nano-Co3O4//AC and AuNP/nano-Co3O4//AC devices at different voltage range, (b) CV curves of AuNP/nano-Co3O4//AC at different scan rate (0e1.6 V), and (c) GCD curves of AuNP/nano-Co3O4//AC at different current density (0e1.5 V) and Ragone plots of various supercapacitor devices.
Acknowledgements This work was partly supported by the National Natural Science Foundation of China (51203071, 51363014, and 51463012), China Postdoctoral Science Foundation (2014M552509, and 2015T81064), Natural Science Funds of the Gansu Province (2015GS05123), and the Program for Hongliu Distinguished Young Scholars in Lanzhou University of Technology (J201402). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.054. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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