Facile Synthesis of Sphere-Like NiO-CuO Composites and Their Supercapacitor Properties

-, -, , -, ,

(College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Facile Synthesis of Sphere-Like NiO-CuO Composites and Their Supercapacitor Properties

ZHONGJian-jian,LIUKai-yu*,SUGeng,LVMei-yu,LIYan,WEILai

(College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Split sphere-like NiO-CuO composites were successfully prepared by thermal decomposition of mixed oxalate precursors in air. The synthesized NiO-CuO composites were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM). The capacitive behavior of NiO-CuO composites was studied by galvanostatic charge-discharge studies in 6 mol·L-1KOH solution at different current density. The results show that the split sphere-like composites at 350 ℃ show good capacitive behavior with a specific capacitance of 735 F·g-1and the capacitance loss after 580 cycles is only 2%, which was far higher than pure NiO(351 F·g-1) and pure CuO(262 F·g-1).

NiO-CuO composites; supercapacitor; split sphere-like; specific capacitance

Because of global warming issues and the consumption of fossil fuels, numerous efforts have been made to develop renewable energies, as well as electric vehicles (EVs) or hybridelectric vehicles (HEVs) with low CO2emission. The exploitation of other energy conversion storage resources with high power and large energy is very important[1-2]. Among them, supercapacitors, have been extensively applied in many fields, ranging from portable consumer electronics and computer memory backup systems, to hybrid electric vehicles (HEVs) and EVs, due to their pulse power supply and long cycle life[1-2]. Supercapacitors, which are also known as electrochemical capacitors and are divided into electrical double-layer capacitors and pseudocapacitors according to the charge-storage mechanism, have drawn worldwide research attention as the most promising candidate for next-generation high-capacitance energy storage devices[1-4]. EDLCs store energy based on charge separation at the electrode-electrolyte interface, while pseudocapacitors rely on fast and reversible redox reactions occurring on the surfaces of the active materials. Supercapacitors made of metal oxides bearing pseudocapacitance attract much interest due to fast and reversible surface redox reactions (faradaic reactions). Various transition metal oxides, such as RuO2[5], Co3O4[6], Fe2O3[7], MnO2[8], MoO2[9], CuO[10]are being studied for the supercapacitor applications. Hydrous ruthenium oxides have high specific capacitance and excellent reversibility. However, the high cost and toxic nature of RuO2limit its applications. The best alternative materials are other metal oxides such as MnO2, Fe3O4and V2O5[11], whose main unresolved issues include poor electrical conductivity and low electrochemical cycle ability. Hence, the work of finding alternative cheap and environmentally friendly metal oxide materials attach much importance to the development of supercapacitors.

NiO draw much interest owing to its high theoretical specific capacitance of 2 573 F·g-1with high chemical/thermal stability, ready availability, environmentally benign nature and lower cost as compared to RuO2[12-16], however, In the previous reports, NiO were fabricated using the template assisted deposition, the electrochemical deposition, and the solvothermal methods. CuO is also a potential electrode material as supercapacitor due to its low cost, and it is chemically stable and environmental-friendly. But the specific capacitance of CuO is still low due to their low electrical conductivity and unstable cycling performances[17-18].

Further research is needed to develop low cost metal oxide electrodes combining suitable electrical conductivity and electrochemical stability. Studying combining metal oxides is an effective way to improve electrochemical performance. Synergistic effects between two different metal oxides could result in a composite material with improved properties.

Oxalate is extensively used as precipitant to prepare metal oxides, Guo et al. fabricated various kinds of mesporous metal oxides with good crystallinity, such as magneticγ-Fe2O3, CoFe2O4, and NiFe2O4, Co3O4, and NiO using sodium oxalate as a precipitant[19]. Zhang et al. synthesized mono-disperse porous bread-like CuO via thermal decomposition of copper oxalate precursors using sodium oxalate as a precipitant[20]. Zhang et al. prepared mesoporous CuO-NiO micropolyhedrons with a specific capacitance of 370 F·g-1[21]. Huang et al. synthesized hierarchical NiO nanoflake coated CuO flower core-shell nanostructures with a specific capacitance[22]of 280 F·g-1. Unfortunately, in these cases the observed specific capacitances are still low.

Herein, we prepared split sphere-like NiO-CuO composites as a supercapacitor by thermal decomposition of mixed oxalate precursor in air using oxalic acid as a precipitant. The split structure is different from NiO and CuO prepared by thermal decomposition of corresponding precursors. Electrochemical measurements are made on the split sphere-like NiO-CuO composites obtained under different thermal decompositions. The maximum specific capacitance of 735 F·g-1by thermal decomposition of 350 ℃ in 6.0 mol·L-1KOH solution at a current density of 1 A·g-1, whose capacitance is much larger than pure NiO(351 F·g-1) and pure CuO(262 F·g-1).

1 Experimental

1.1 Materials and reagents

NiO-CuO composites: Chemically analytical grade Ni(CH3COO)2·4H2O, Cu(CH3COO)2·H2O, H2C2O4·2H2O, were used as starting materials without further purification. 50 mL 0.2 mol·L-1H2C2O4aqueous solution, metal salts (5 mmol Ni(CH3COO)2·4H2O and 5 mmol Cu(CH3COO)2·H2O) were dissolved into 50 mL of water and then poured into the above-described oxalic acid aqueous solution for 24 h. The green precipitate was filtered and heated in air for 0.5 h at a rate of 2 ℃·min-1at 350, 450, 500 ℃.

NiO:5 mmol Ni(CH3COO)2·4H2O was dissolved into 50mL of water and then poured into the above-described oxalic acid aqueous solution for 24 h. The precipitate was filtered and heated at different temperatures in air for 0.5 h at a rate of 2 ℃·min-1at 350 ℃.

CuO:5 mmol Cu(CH3COO)2·H2O was dissolved into 50mL of water and then poured into the above-described oxalic acid aqueous solution for 24 h. The precipitate was filtered and heated at different temperatures in air for 0.5 h at a rate of 2 ℃·min-1at 350 ℃.

1.2 Characterization of split sphere-like NiO-CuO composites

Thermogravimetric analysis was conducted on a SAT449C simultaneous thermal analysis. X-ray diffraction (XRD) patterns was obtained by an X-ray diffraction (XRD) (Rigaku D/max 2500). Scanning electron microscopy (SEM) was performed by a FET-Quanta-200 scanning electron microscope (FE-SEM). Transmission electron microscopy (TEM) was conducted on a JEM-2100F transmission electron microscope.

1.3 Electrode preparation and electrochemical measurements

The working electrodes were prepared as follows: a mixture containing 80 wt% active material, 15 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) was well mixed, and then was pressed onto a foam nickel that serves as a current collector. The electrode assembly was dried at 80 ℃ in a vacuum oven for 12 h.

Electrochemical measurements were conducted in 6 mol·L-1aqueous KOH. A platinum electrode and an Hg/HgO electrode served as the counter electrode and the reference electrode, respectively. Cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) measurements were conducted on electrochemical workstation (RST-5000). Galvanostatic charge-discharge studies were measured using a LAND-CT2001A between the potential of 0.1～0.55 V.

2 Results and discussion

Thermogravimetric analysis of NiO-CuO composites precursors from 35 to 600 ℃ under air atmosphere is shown in Fig.1. From 35 to 300 ℃ about 22% weight loss can be seen. This can be explained by the removal of chemically adsorbed H2O. A weight loss of 31% is present between 120 and 360 ℃ on the TG curve, it is believed that the metal oxalates produce much gas in this region. From this graph we can know at 350 ℃ the NiO-CuO composites precursors have been transformed into metal oxide which corresponds to the following XRD results.

The XRD patterns of NiO, CuO and NiO-CuO composites are shown in Fig.2. The NiO and CuO pattern corresponds to NiO (JCPDS card no.75-0629) and CuO (JCPDS card no.48-1548), and no other peaks arising from impurities can be found, indicating that NiO and CuO crystals were successfully synthesized. NiO and CuO peaks can be seen for the XRD patterns of NiO-CuO composites, indicating that the precursor has completely transformed into CuO and NiO by 350 ℃. As annealing temperature increases, the composite crystalline has been enhanced as shown in Fig.2.

Fig.1 Thermogravimetric analysis (TGA) of NiO-CuO composites precursors in air atmosphere

Fig.2 X-ray diffraction pattern (XRD) of NiO-CuO composites at 350, 450, and 500 ℃; NiO and CuO at 350 ℃

The morphology of NiO, CuO, composites and their corresponding precursors were observed by SEM. The SEM images for the CuO do not show obvious differences compared with CuO precursors and CuO microspheres. The images for NiO precursors and NiO are composed of nanoflakes, which pile up densely. The images for the NiO do not show obvious differences compared with NiO precursors. NiO-CuO composites precursors microspheres were obtained as shown in Fig.3(e). After calcination in air, the composites show gap.

Fig.3 (a)SEM image of CuO precursors; (b) SEM image of CuO; (c) SEM image of NiO precursors; (d) SEM image of NiO; (e) SEM image of NiO-CuO composites precursors; (f) SEM image of NiO-CuO composites

Fig.4 (a)(b)(c) TEM of CuO; (d)(e)(f)TEM of NiO; (g)(h)(k)TEM of NiO-CuO composites

Porous structures are expected to be generated after gas release. To more clearly show the porous properties of the NiO-CuO composites, TEM images of all samples are shown in Fig.4. Pores of all samples are distributed homogeneously among the surface of all samples. HRTEM results of CuO and NiO are shown in Fig.4(c, f). The high-resolution TEM (HRTEM) images of CuO and NiO shown in Fig.4(c,f) reveal an interplanar spacing of ～0.271 nm and ～0.243 nm, close to that of 2.76 Å(110)(CuO) given in the JCPDS card no.48-1548 and 2.415 Å(111)(NiO) given in the JCPDS card no.75-0629. Fig.4(k) show the HRTEM images of nanoflakes outside the NiO-CuO composites and reveal an interplanar spacing of ～0.238 nm. Based on the HRTEM images and the XRD results, we can speculate that the nanoflakes outside the composites can be NiO. It is believed that the spilt-like NiO-CuO composites have been successfully prepared.

The electrochemical performances of composites as electrode materials for supercapacitors were further evaluated. Fig.5(a) shows the CV curves of the composites obtained at 350 ℃. The shapes of the CV reveal that the capacitance characteristic is very distinct from that of electric double-layer capacitance whose shape is a rectangular. We can see a pair of redox peaks at a scan rate of 5 mV·s-1, indicating that the capacitance mainly resulted from faradaic reactions. As the sweep rate increases, the shape of the CV changes, the anodic peak potential and cathodic peak potential shift in more anodic and cathodic direction, respectively.

Fig.5(b) demonstrates the CV curves of pristine NiO, CuO and composites at a scan rate of 5 mV·s-1. For the CV curve of composites, redox peaks shift to a cathodic potential compared with the curves of NiO and CuO. As clearly demonstrated in the CV curves, the integral area of the composites is larger than that of pristine NiO and CuO. Fig.5(c) shows the results obtained for the composites (calcined at 350 ℃ ) electrode at various current density (1, 5, 10, 15 A·g-1). The specific capacitance has been calculated from the following equation:

Whereiis the discharge current in ampere, Δtis the discharge time in second,mis the weight of active electrode in gram and ΔVis the voltage interval of the discharge.

The specific capacitances are calculated from the galvanostatic discharge curves to be 735, 533, 555 and 533 F·g-1at 1, 5, 10 and 15 A·g-1. The drop might be explained by an ion-exchange mechanism. The OH-needs enough time to transfer between the solutions onto the surface of electrode materials in order to be intercalated/extracted into/out of activated materials when charging/discharging. The pure NiO and CuO calcined at 350 ℃ are 351 and 262 F·g-1at 1 A·g-1which is far lower than that of composites. This result may be explained by the unique structure of the NiO-CuO composites. The split sphere-like structures ensure enough electrolyte ions to rapidly contact much larger surfaces of NiO-CuO composites. The shapes of the discharge curves do not show the characteristics of a pure double-layer capacitor, but mainly pseudo-capacitance, which are in accordance with the result of the CV curves. Moreover, at the highest current density (15 A·g-1), the specific capacitance of the composites(533 F·g-1) is maintained at up to 72% of that measured at 1 A·g-1.

Fig.5 (a) CV curves of the NiO-CuO composites at sweep rates of 5,10,20 and 50 mV·s-1 ; (b) CV curves of the NiO, CuO and NiO-CuO composites at sweep rate of 5 mV·s-1; (c)the discharge curves of NiO-CuO composites at current density of 1, 5, 10 and 15 A·g-1;(d) discharge curves of the NiO-CuO composites, NiO and CuO.

Fig.6 Discharge curves of the composites obtained at different temperatures

We also measure the electrochemical capacitance of the composites samples obtained at different thermal decomposition, as shown in Fig.6, with increasing annealing temperature, the decline of the specific capacitance can be found. The specific capacitance is 735, 531 and 482 F·g-1. Calcining at higher temperature may lead to an increasing in crystalline structures, which could be proven by XRD may lead to an increase in crystal size, and the specific area will decrease, thereby leading to a decreased capacitance.

The cyclability is another important quality required for application in supercapacitors. Therefore, a cycle charge/discharge measurement was employed to examine the service life of the NiO-CuO composites electrode. A charge-discharge cycling measurement is performed (obtained at an annealing temperature of 350, 450, 500 ℃) electrode at a current density (1 A·g-1) between 0.1～0.55 V (vs. Hg/HgO) in 6 mol·L-1KOH, as shown in Fig.7. Interestingly, the specific capacitance of the composites at 350 and 450 ℃ have an increase in the first several cycles, and can reach a maximum of 735 and 531F·g-1, respectively. Then, both decrease slightly compared with the maximum value and can remain at 722 and 517 F·g-1, respectively, in the last cycles. The penetration of electrolyte ions and the gradual activation of the active materials may be responsible for the increase of the specific capacitance in the first several hundred cycles. The capacitance loss after 580 cycles is only 2% for sample at 350 ℃ and 1% for sample at 450 ℃. Moreover, the capacitance maintained almost 100% for sample at 500 ℃.

The electrochemical performance of the composites was further confirmed by the electrochemical impedance spectroscopy (EIS) measurements. Fig.8 shows the Nyquist plots of the EIS spectra for NiO-CuO composites, NiO, CuO. It is well accepted that the semicircle are corresponds to equals the electron transfer resistance.The semicircle-like shape for the EIS spectra of these samples can be seen with the following order: CuO, NiO and NiO-CuO composites. The results are in high agreement with their CV and GV behaviors, demonstrating that the split sphere-like structures possesses lower resistance and thus allows for much faster electron transfer. The lower resistance of the split sphere-like structures indicates a faster electron transfer between the active material and the charge collector.

Fig.7 Cycle life of as-prepared NiO-CuO composites electrode at 1A·g-1 at 350, 450 and 500 ℃

Fig.8 Nyquist plots of the EIS for the CuO, NiO and NiO-CuO composites

3 Conclusions

We have firstly synthesized split sphere-like composites materials using a facile chemical precipitation method without any template or surfactant. The split sphere-like structures are in favor of OH-transferring between electrodes and electrolytes and enhance the electrochemical performance. The specific capacitances of the composites material at 1, 5, 10 and 15 A·g-1were 735,533,555 and 533 F·g-1, whose performance was much higher than pure NiO and pure CuO and the capacitance loss after 580 cycles is only 2% for sample at 350 ℃, which shows better rate capability and great potential for electrode materials for supercapacitors.

[1] TOLLEFSON J. Car industry: charging up the future [J]. Nature, 2008,456(7221):436-440.

[2] SIMON P , GOGOTSI Y. Materials for electrochemical capacitors [J]. Nat Mater, 2008,7(11):845-854.

[3] WANG G P, ZHANG L, ZHANG J J. A review of electrode materials for electrochemical supercapacitors [J]. Chem Soc Rev, 2012,41(2):797-828.

[4] LIU C, LI F, MA L P,etal. Advanced materials for energy storage [J]. Adv Mater, 2010,22(8):E28-E62.

[5] HU C C, CHANG K H, LIN M C,etal. Design and tailoring of the nanotubular arrayed architecture of hydrous RuO2for next generation supercapacitors [J]. Nano Lett, 2006,6(12):2690-2695.

[6] XIONG S, YUAN C, ZHANG X,etal. Controllable synthesis of mesoporous Co3O4nanostructures with tunable morphology for application in supercapacitors [J]. Chemistry, 2009,15(21):5320-5326.

[7] KULAL P M, DUBAL D P, LOKHANDE C D,etal. Chemical synthesis of Fe2O3thin films for supercapacitor application [J]. J Alloys Compd, 2011,509(5):2567-2571.

[8] ZHANG Y, LI G Y, LV Y,etal. Electrochemical investigation of MnO2electrode material for supercapacitors [J]. Int J Hydrogen Energy, 2011,36(18):11760-11766.

[9] RAJESWARI J, KISHORE P S, VISWANATHAN B,etal. One-dimensional MoO2nanorods for supercapacitor applications [J]. Electrochem Commun, 2009,11(3):572-575.

[10] ZHANG H X, FENG J, ZHANG M L. Preparation of flower-like CuO by a simple chemical precipitation method and their application as electrode materials for capacitor [J]. Mate Res Bull, 2008,43(12):3221-3226.

[11] COTTINEAU T, TOUPIN M, DELAHAYE T,etal. Nanostructured transition metal oxides for aqueous hybrid electrochemical supercapacitors [J]. Appl Phys A, 2006, 82(4):599-606.

[12] ZHANG X, SHI W, ZHU J,etal. Synthesis of porous NiO nanocrystals with controllable surface area and their application as supercapacitor electrodes [J]. Nano Res, 2010, 3(9):643-652.

[13] XIA X, TU J, WANG X,etal. Hierarchically porous NiO film grown by chemical bath deposition via a colloidal crystal template as an electrochemical pseudocapacitor material [J]. J Mater Chem, 2011,21(3):671-679.

[14] WU M S, HUANG Y A, JOW J J,etal. Anodically potentiostatic deposition of flaky nickel oxide nanostructures and their electrochemical performances [J]. Int J Hydrogen Energy, 2008,33(12):2921-2926.

[15] WANG D C, NI W B, HANG P,etal. Preparation of mesoporous NiO with a bimodal pore size distribution and application in electrochemical capacitors [J], Electrochim Acta, 2010,55(22):6830-6835.

[16] REN Y, GAO L. From three-dimensional flower-likeα-Ni(OH)2nanostructures to hierarchical porous NiO nanoflowers: microwave-assisted fabrication and supercapacitor properties [J].J Am Ceram Soc, 2010, 93(11):3560-3564.

[17] DUBAL D P, DHAWALE D S, SALUNKHE R R,etal. Fabrication of copper oxide multilayer nanosheets for supercapacitor application [J]. J. Alloys Compd, 2010, 492(1-2):26-30.

[18] PATAKEA V D, JOSHI S S, LOKHANDE C D,etal. Electrodeposited porous and amorphous copper oxide film for application in supercapacitor [J]. Mater Chem Phys, 2009,114(1):6-9.

[19] GUO L, ARAFUNE H, TERAMAE N. Synthesis of mesoporous metal oxide by the thermal decomposition of oxalate precursor [J]. Langmuir, 2013,29(13):4404-4412.

[20] JIA Z, YUE L, ZHENG Y,etal. The convenient preparation of porous CuO via copper oxalate precursor [J]. Mater Res Bull, 2008,43(8-9):2434-2440.

[21] ZHANG Y X, KUANG M, WANG J J. Mesoporous CuO-NiO micropolyhedrons: facile synthesis, morphological evolution and pseudocapcitive performance [J]. Cryst Eng Comm, 2014,16(3):492-498.

[22] HUANG M, LI F, ZHANG Y X,etal. Hierarchical NiO nanoflake coated CuO flower core-shell nanostructures for supercapacitor [J]. Ceram Int, 2014,40(4):5533-5538.

(编辑 杨春明)

2014-03-28

国家自然科学基金资助项目(21071153)

O646.21

A

1000-2537(2014)05-0037-06

裂开球形氧化镍氧化铜复合氧化物的简单制备及其超级电容器性能

钟剑剑，刘开宇*, 苏 耿，吕美玉，李 艳，魏 来

(中南大学化学化工学院,中国 长沙 410083)

通过液相共沉淀法及高温热解法制备了裂开球形氧化镍氧化铜复合物．采用了X射线衍射光谱(XRD)、扫描电子显微镜(SEM)及透射电子显微镜(TEM)表征了该材料的结构.采用恒流充放电法研究了制备的NiO-CuO复合物在6 mol·L-1KOH溶液中的电化学行为．实验结果表明：这种裂开球形复合氧化物由氧化镍、氧化铜组成．该材料在1 A·g-1的电流密度下所得复合氧化物单电级比电容为735 F·g-1,并且在580次充放电循环后，容量保持率为98%，远远高于氧化镍(351 F·g-1)和氧化铜(262 F·g-1)的比容量．

NiO-CuO 复合物; 超级电容器; 开口球形; 比电容

*

,E-mail：kaiyuliu@263.net