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CoO) nanoparticles as supercapacitor electrode materials
Electrochimica Acta 132 (2014) 127–135
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Solution combustion synthesis of cobalt oxides (Co3 O4 and Co3 O4 /CoO) nanoparticles as supercapacitor electrode materials Jiachun Deng a , Litao Kang a,∗ , Gailing Bai d , Ying Li a , Peiyang Li a , Xuguang Liu b , Yongzhen Yang a , Feng Gao a,c , Wei Liang a a
Nano-Energy Inorganic Materials Laboratory, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China d College of Materials Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China b
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 26 March 2014 Accepted 29 March 2014 Available online 5 April 2014 Keywords: Solution combustion Cobalt oxide Nanoparticle Supercapacitor
a b s t r a c t Co3 O4 and Co3 O4 /CoO nanoparticles have been synthesized by a one-step solution combustion process by adjusting the molar ratio of citric acid (fuel) and Co(NO3 )2 ·6H2 O (oxidizer). The effects of citric acid/Co(NO3 )2 ·6H2 O molar ratios on phase composition and morphology of products were investigated by XRD and SEM. With the increase of the fuel dosage, the products transformed from granular aggregates of cubic Co3 O4 into a mixture of cubic Co3 O4 and tetragonal CoO with fluffy sheet morphologies. Electrochemical measurements indicated that the products (Co3 O4 ) showed a capacitance up to 179.7 F·g−1 (at 0.2 A·g−1 ) when the citric acid/Co(NO3 )2 ·6H2 O molar ratio was 7/27. Significantly, the capacitance could be further improved by 102% (362.8 F·g−1 at 0.2 A·g−1 ) after annealing at 350 ◦ C for 3 h under nitrogen atmosphere. This annealed sample also demonstrated decent rate performance (285.7 F·g−1 at 4 A·g−1 ) and cycling stability (73.5% retention after 1000 cycles). The current study suggests that this process has promise in large-scale production of electrode materials for supercapacitors. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction The increasing demand for energy has triggered tremendous research efforts for energy storage with high power and energy densities [1]. Supercapacitors (SCs), also called electrochemical capacitors, are promising energy storage devices for portable systems and hybrid electric vehicles [2], because of their high power densities (about 1-100 kW·kg−1 ), long cycling lives (105 ∼106 cycles) and abilities of fast energy storage and release [3,4]. SCs could be divided into two types according to different energy storage mechanisms: electric double-layer capacitors (EDLCs) and electrochemical pseudocapacitors (EPCs) [5,6]. The capacitance of EDLCs is based on reversible physical ion absorption at the electrode/electrolyte interface, while the capacitance of EPCs arises from fast and reversible faradic redox reactions at the surface or near-surface of electroactive materials [7]. EPCs usually show higher capacitance (typically 300-1000 F·g−1 ) than EDLCs
∗ Corresponding author. E-mail addresses: [email protected] (L. Kang), [email protected] (W. Liang). http://dx.doi.org/10.1016/j.electacta.2014.03.158 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
(typically 100-250 F·g−1 ) [4]. Among the electroactive materials used for EPCs, amorphous hydrated RuO2 has been identified as an influential candidate because of its high theoretical specific capacitance (1358 F·g−1 ), excellent electrical conductivity (300 S·cm−1 ) and high electrochemical stability [4,8,9]. The nanotubular hydrous RuO2 electrodes by AAO (anodic aluminum oxide) membranetemplates electrodeposition showed a specific capacitance up to 1300 F·g−1 [10]. However, the commercial application of RuO2 was hindered by the high cost and toxicity associated with Ru [10–12]. As one of promising replacements for RuO2 , cobalt oxides show advantages of low cost and high theoretical specific capacitance (Co3 O4 : 3560 F·g−1 , CoO: 4292 F·g−1 ) [13,14]. Many synthesis approaches have been successfully employed to prepare wellpreformed cobalt oxides nano-structures, such as microwave assisted precipitation of Co3 O4 nanoparticles (519 F·g−1 at 0.5 mA·cm−2 [15]), hydrothermal synthesis of Co3 O4 nanosheets (548 F·g−1 at 8 A·g−1 [16]) and Co3 O4 nanorods (281F·g−1 at 5 mV·s−1 [17]), solvothermal synthesis of Co3 O4 nanosheets/nanorods (258 F·g−1 at 10 mA·cm2 /563 F·g−1 at 10 mA·cm−2 [18]), chemical bath deposition of Co3 O4 nanotubes (574 F·g−1 at 0.1 A·g−1 [19]) and nanowire arrays on nickel foam (746 F·g−1 at 5 mA·cm−2 [20]). To further promote the large scale and especially continuous
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production of cobalt oxides supercapacitor electrodes, a one-step route was developed by directly spraying cobalt acetate tetrahydrate aqueous solution onto current collectors with the appearance of plasma gas [12]. The formed porous Co3 O4 coatings showed a specific capacitance of 162 F·g−1 at 2.75 A·g−1 . Solution combustion synthesis utilizes the violent, selfsustained exothermic reaction between oxidizer (usually nitrates) and fuel to achieve high temperature and intense gaseous product release within a time framework of seconds [21–24]. These features promote crystallization (high temperature) but depress the crystal growth process (limit lasting time), favoring the facile and continuous preparation of porous nanomaterials [24,25]. In principle, porous and nano-size features are highly admired for the electrodes materials of electrochemical energy storage devices, by accommodating the volume change and increasing the electrode/electrolyte interface during cycling [26]. Therefore, this one-step method has been successfully employed to produce electrode materials for lithium ion batteries (LIBs) and supercapacitors. For example, Wen et al. [27].and Rai et al. [28] reported the solution combustion synthesis of Co3 O4 (30-40 nm) and CoO nanoparticles (20-30 nm); as anode electrode materials for LIBs, they respectively exhibited specific capacities of 400 (after 100 cycles) and 565 mAh·g−1 (after 23 cycles). On the other hand, Yu et al. [29] and Jayalakshmi et al. [30] separately synthesized -MnO2 nanoparticles and Fe2 O3 /C, Fe2 O3 SnO2 /C, Fe2 O3 -ZnO/C composites via this method, which showed specific capacitances of 123 F·g−1 (at 1 A·g−1 ) and 254.9, 78.6, 122.4 F·g−1 (at 200mV·s−1 ), respectively. In this paper, nano-sized cobalt oxides (Co3 O4 and Co3 O4 /CoO) have been synthesized by combusting the mixture gel of Co(NO3 )2 ·6H2 O and citric acid. Experimental results confirmed that the molar ratios of citric acid/Co(NO3 )2 ·6H2 O effectively influenced the phase composition, morphologies and thus electrochemical measurements of products. These products showed a best capacitance of 179.7 F·g−1 (at 0.2 A·g−1 ) with a citric acid/Co(NO3 )2 ·6H2 O molar ratio of 7/27. Furthermore, after 350 ◦ C annealing under N2 for 3 h, a higher specific capacitance (362.8 F·g−1 at 0.2 A·g−1 ), an excellent rate performance (with 78.8% capacitance retention rate at 4 A·g−1 ) and decent cycling stability (73.5% retention after 1000 cycles) were successfully achieved. 2. Experimental 2.1. Material preparation In our solution combustion processes, citric acid monohydrate (C6 H8 O7 ·H2 O), cobalt nitrate hexahydrate (Co(NO3 )2 ·6H2 O), and ammonium nitrate (NH4 NO3 ) were used as fuel, oxidizer and combustion improver, respectively. Assuming that N2 , CO2 , and H2 O were evolved as the gaseous products in this citric acid–nitrate system, the combustion reaction can be generalized and expressed as follows [25,31]: Co(NO3 )2 · 6H2 O + C6 H8 O7 · H2 O + (9/2 − 7/3)O2 = 1/3Co3 O4 + 6CO2 ↑ +(6 + 5)H2 O ↑ +N2 ↑
(1)
In this reaction, = 14/27 means that the initial mixture does not require atmospheric oxygen for complete oxidation of fuel (being named as: stoichiometric combustion). It should be noted that the gaseous products of this reaction are smelly and show a weak reddish brown color, which is the typical characteristics of NO2 . Therefore, besides reaction (1), following reaction also occurred in our combustion experiments. Co(NO3 )2 · 6H2 O + C6 H8 O7 · H2 O + (9/2 − 1/3)O2 = 1/3Co3 O4 + 6CO2 ↑ +(6 + 5)H2 O ↑ +2NO2 ↑
(2)
In this reaction, stoichiometric combustion requires = 2/27. For the real combustion, the condition of stoichiometric combustion located between = 2/27 and = 14/27. The real combustion reaction may could be determined by measuring the kinds and contents of nitrogen-related gaseous products. Related results will be reported in further work. In this paper, the molar ratios of citric acid monohydrate/Co(NO3 )2 ·6H2 O were fixed to be 3.5/27, 7/27, 14/27, 21/27 in order to investigate the effects of fuel/oxidant molar ratio (i.e., the molar ratio of citric acid monohydrate/Co(NO3 )2 ·6H2 O) on microstructure and electrochemical performance of products,. The corresponding products were denoted as sample I, II, III and IV, respectively. For the synthesis of sample-II, 7.246 g Co(NO3 )2 ·6H2 O, 1.356 g citric acid monohydrate and 2 g ammonium nitrate were added in a 250 ml beaker that contained 100 ml deionized water. After 15 min magnetic stirring, the beaker was placed in a preheated muffle furnace at 350 ◦ C for 30 min. At this temperature, the violent evaporation of solvent firstly produced a voluminous and fluffy gel within 15 min, which was further ignited with a flame and combusted into corresponding oxides within 15 s. To synthesize sample-I, -III, and -IV, the dosages of citric acid monohydrate were changed into 0.678, 2.712 and 4.068 g, respectively. The ingredient lists of precursor solutions used to prepare samples I-IV have been summarized in Table 1. In order to investigate the influence of annealing, the sample with the best performance (sample-II) was further annealed at 350, 450, and 550 ◦ C for 3 h under a nitrogen flow, corresponding to sample-II-350, -II-450, and -II-550. 2.2. Material characterization. X-ray powder diffraction (XRD) patterns of the as-prepared samples were recorded using a Y-2000 X-ray diffractometer (Dan˚ After being dong, China) with a Cu − K␣ radiation ( = 1.542 A). pre-heated at 100 ◦ C for 1 h, thermogravimetric (TG) curves of samples were measured on a NETZSCH TG 209 F3 thermal analyzer from 100 to 900 ◦ C with a heating rate of 10 ◦ C·min−1 in flowing air. The morphologies of the products were observed on a Mira3 field-emission scanning electron microscope. The Brunauer-Emmett-Teller (BET) specific surface area and BJH pore size distribution were measured with a surface area and pore size analyzer (V-sorb2800P, Gold APP Instruments, China). 2.3. Electrochemical Measurements. For evaluating the electrochemical properties of samples, working electrodes were fabricated by mixing active materials (samples), acetylene black and poly (vinyldifluoride) (PVDF, in Nmethylpyrrolidone solvent with a solid content of 10 g·L−1 ) at a weight ratio of 8:1:1 to form the coating slurry. The slurry was smeared onto the current collector (Ni foam), then dried in air at 80 ◦ C overnight and pressed under a pressure of 10 MPa to serve as the working electrode. The loading mass of active material was ∼1.5 mg·cm−2 . All electrochemical measurements were carried out at room temperature in a 6.0 mol·L−1 KOH aqueous electrolyte with a working electrode (electrodes with cobalt oxides powders), a saturated calomel reference electrode (SCE, Hg/HgCl2 ), and a platinum plate counter electrode (1 × 2 cm2 ). The electrochemical behaviors of the active electrodes were characterized by cyclic voltammetry (CV, in a potential window between -0.30 and 0.45 V vs. SCE) and galvanostatic charge–discharge (GCD, in a potential window between 0 and 0.35 V vs. SCE) tests on a CHI660D electrochemical workstation (Shanghai, China). The electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 100 kHz to 10
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Table 1 Ingredient lists of precursor solutions used to synthesize sample-I, -II, -III and -IV. C6 H8 O7 ·H2 O/Co(NO3 )2 ·6H2 O (molar ratio)
Sample
3.5/27 7/27 14/27 21/27
I II III IV
mHz at open circuit voltage with a AC perturbation of 10 mV on a Cs 350 electrochemical workstation (Wuhai, China). The specific capacitance can be calculated with CV data according to equation (3) [16,32]: C=
1 mv(Va − Vc )
Vc
I(V )dV
(3)
Va
At the same time, the specific capacitance could also be calculated with GCD data via following equation (4) [33,34]: C=
i × t V × m
(4)
3. Results and discussion 3.1. Effects of fuel-to-oxidant molar ratio Fig. 1 displays the X-ray diffraction (XRD) spectra of samples I-IV synthesized by our solution combustion process. It is shown that all XRD peaks of sample-I and -II could be assigned to cubic ˚ For sample-III, Co3 O4 (JCPDS Card no.43-1003, a= b = c = 8.084 A). however, an obvious XRD peak centered at 42.4◦ appeared besides the signals of cubic Co3 O4 phase. Meanwhile, the XRD peak at ∼36.6◦ has divided into two peaks (inset (c) of Fig. 1), characterizing the appearance of tetragonal CoO phase (JCPDS Card no.65-5474, ˚ c= 4.214 A). ˚ XRD peaks of both cubic Co3 O4 and a = b = 3.015 A, tetragonal CoO have been detected in sample-IV, but they became weak and broaden, suggesting a poor crystalline degree. In solution combustion processes, the amount of gaseous products would increase with the increase of citric acid (fuel) [35]. And the intensive gaseous product release could prevent agglomeration of tiny resulting oxide crystallites and thus depress the crystal growth process. Therefore, the decrease of crystalline degree from sample-I to
Fig. 1. XRD spectra of sample-I, -II,-III and -IV. The bars at the bottom indicate the standard lines of CoO (JCPDS Card no.65-5474) and Co3 O4 (JCPDS Card no.43-1003). The enlarged peaks at ∼36.6◦ and their shape evolution for all samples have been shown in insets (a) - (d).
Co(NO3 )2 ·6H2 O(g)
C6 H8 O7 ·H2 O (g)
NH4 NO3 (g)
7.246 7.246 7.246 7.246
0.678 1.356 2.712 4.068
2 2 2 2
sample-IV could be ascribed to the increase dosage of citric acid in the precursor solution. TG curves of samples I-IV evidentially confirmed the appearance of tetragonal CoO with the increase of fuel/oxidizer molar ratios. As shown in Fig. 2, a slight weight loss was detected in 150-290 ◦ C for all samples, and it could be ascribed to desorption of chemicallyabsorbed H2 O. For sample sample-I, there was another weight loss (1.5%) located at around 450-600 ◦ C, which possibly attributed to the decomposition of residual raw materials (e.g., cobalt nitrate). With the increase of fuel dosages in precursor solution, this weight loss became very weak (sample-II) and finally evolved into weight increase (1.3% and 6.7% for sample-III and -IV, respectively). Combining with XRD results, we attributed these weight increase to the oxidization of CoO, which was shown in following Equation 5. The appearance of CoO could be attributed to the reduction of Co3+ by excess fuel, and the CoO weight ratios in sample- III and -IV were calculated to be 18.3% and 94.1% with the TG data. 6CoO + O2 = 2Co3 O4
(5)
Fig. 3 shows typical SEM images of samples I-IV, which indicated that sample-I presented in uniform granular aggregates (diameter: ∼150 nm) of tiny particles, while the aggregates in sample-II (Fig. 3b) became relatively large (diameter: 100-250 nm) and uneven. The sizes of particles in sample-I and -II were identified to be about 30 nm, in line with the grain sizes of cubic Co3 O4 estimated by Scherer formula (20 nm by XRD). However, sample -III and -IV (Fig. 3c-d) were found consisting of fluffy sheets. Compared with sample-III, the micro-pores became low in density in sampleIV. The porous sheet-like morphology should been introduced by three steps: firstly, the precursor solution concentrated into viscous sol with film-forming ability [36] with elevated temperature (350 ◦ C); secondly, numerous pores formed in the sol with further solvent evaporation to form a fluffy gel; finally, the gel combusted with intense gaseous product release, transforming the pore walls into porous sheets. XRD, TG and SEM data indicated that the fuel/oxidizer molar ratio showed a obvious influence on the composition and morphology of products. Low molar ratio of fuel/oxidizer (e.g., sample-I and -II) favored the formation of cubic Co3 O4 with the morphologies of
Fig. 2. TG curves of sample-I, -II, -III, and - IV in air.
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Fig. 3. SEM images of sample-I (a), -II (b), -III (c), and -IV (d).
granular aggregates; while with high molar ratio of fuel/oxidizer (e.g., sample-III and -IV), the excess fuel seems to reduce partially Co3+ and results in a mixture phase of cubic Co3 O4 and tetragonal CoO with a fluffy sheet morphology. With increase of molar ratio of fuel/oxidizer, the weight ratio of CoO increase while the CoO and Co3 O4 crystalline degree decrease in the mixture. Sample-III and -IV showed CoO weight ratio of 18.3% and 94.1%, respectively. Additionally, BET analyses were further performed for sample I-IV to determine their specific surface area and pore parameters (Fig. 4 and Table 2). All samples display type IV adsorptionedesorption isotherms, according to the IUPAC classification [37]. The BET specific surface area (SBET ) and the BJH desorption pore volumes (Vpore ) were 13.1, 21.3, 14.1, 17.9 m2 ·g−1 and 0.037, 0.095, 0.054, 0.095 cm3 ·g−1 for sample-I,-II, -III and -IV, respectively. Experimental results also revealed that the pore size distribution of samples was dramatically different. The pores in sample-I distributed in a narrow range of 3-10 nm. Whereas, sample-II showed a hierarchical pore structure, with pores mainly distributing in about 3.5, 6 and 32 nm. For sample-III and -IV, most of pores located in size range of 10-50 nm. These BET analysis indicated that sample-II possessed larger surface area, pore volume and mesopore volume (Vmeso ) than others, which may favor the supercapacitive performance. Fig. 5a shows CV curves of samples I-IV at the scan rate of 5 mV·s−1 in a potential range between -0.30 and 0.45 V (versus SCE). For all samples, an anodic peak (with positive current density) and three cathodic peaks (with negative current density) were visible, indicating faradic redox reaction nature of these materials. This result is in good line with CV curves of literature [16,33,38]. Generally, there are three redox couples, Co(OH)2 /Co3 O4 , Co3 O4 /CoOOH, and CoOOH/CoO2 , involved in cyclic voltammetry study, corresponding to following reversible reactions. Anodic scan (toward positive potential): 3Co(OH)2 + 2OH − → Co3 O4 + 4H2 O + e−
(6)
Co3 O4 + OH − + H2 O → 3CoOOH + e− −
CoOOH + OH → CoO2 + H2 O + e
(7)
−
(8)
Cathodic scan (toward negative potential): CoO2 + H2 O + e− → CoOOH + OH − −
(9)
−
3CoOH + e → Co3 O4 + OH + H2 O −
Co3 O4 + H2 O + 2e → 3Co(OH)2 + 2OH
(10) −
(11)
In our samples, the anodic peak (e.g., peak A in CV curves of sample-II) was believed to be an overlapping peak by three oxidation reactions of Equation.(6), (7), and (8) [16,38]. On the other hand, the three cathodic peaks for samples I-IV (i.e., peak C1, C2, and C3 in the CV curves) were ascribed to the three reduction processes as shown in Equation.(9), (10), and (11), respectively. The sharp current density increases above 0.4 V were attributed to an oxygen evolution reaction as Equation.(12) 4OH − → O2 + 2H2 O + 4e−
(12)
This CV results indicated that our cobalt oxides underwent electrochemical charge transfer reactions Co(II)-Co(III)-Co(IV) in basic electrolyte, making it a potential candidate for pseudocapacitor application. Specific capacitance is one of the most important indices for evaluation of electrochemical performance. According to Equation.(3) and CV curves in Fig. 5a, the specific capacitance of sample-I, -II, -III and -IV were determined to be 68.3, 155.3, 146.8 and 135.7 F·g−1 , respectively,. In agreement with above CV results, the GCD data (Fig. 5b) determined that specific capacitances of sample-I, -II, -III and -IV were 73.1, 179.7, 141.6 and 130.5 F·g−1 at a current density of 0.2 A·g−1 , respectively. Both measurements indicated that sample II showed the best capacitor performance among samples. Fig. 6 shows Nyquist plots of experimental impedance data for samples I-IV. On the whole, all the plots were composed of an arc in the high frequency region and inclined line in the low
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Fig. 4. Nitrogen adsorption/desorption isotherm and pore size distribution (inset) of sample-I, -II, -III, and -IV.
Table 2 Microstructure parameters obtained by BET analyses for sample-I, -II, -III and -IV. sample
SBET (m2 ·g−1 )
Vpore(cm3 ·g−1 )
Vmeso(cm3 ·g−1 )
Vmacro(cm3 ·g−1 )
I II III IV
13.1 21.3 14.1 17.9
0.037 0.095 0.054 0.095
0.029 0.067 0.035 0.052
0.008 0.028 0.019 0.043
frequency region. The arc usually attributes to the charge transfer impedance, while the linear part of the impedance spectrum corresponds to the Warburg impedance [39,40], which is a result of the frequency dependence of ionic diffusion/transport in the electrolyte and to the surface of the electrode [40,41]. From the intersection with X-axis, one can find that the internal resistances of electrodes of samples I-IV were almost constant (∼2.0 ), which included the total resistances of the ionic resistance of electrolyte, intrinsic resistance of the active material, and contact resistance
at the active material/current collector interface [19]. The linear region (Zw) of the plots exhibit slops of ∼60◦ for samples II-IV and ∼45◦ for sample I. The ∼45◦ straight line corresponds to a Warburg impedance related to the diffusion of electrolyte within the electrode. While the ∼60◦ straight line shows the capacitance nature (it would be a vertical line for an ideal capacitor) [42], implying the good accessibility of OH− ions to the electrode surface and good supercapacitive performance of samples II-IV. From microstructure point of view, mesopores and high specific surface area can provide
Fig. 5. (a) CV curves of sample-I, -II, -III and -IV at the scan rate of 5 mV·s−1 . (b) galvanostatic charge–discharge curves of sample-I, -II, -III, and -IV at a constant discharge current density of 0.2 A·g−1 .
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Fig. 6. Nyquist plots of experimental impedance data for sample-I, -II, -III and -IV electrodes.
short ion-transport pathways for electrochemical reactions [43] to decrease the Warburg impedance [44]. As revealed by BET analyses, sample-II exhibited largest specific surface area and mesopore volume among samples. Therefore, it shows the lowest Warburg impedance and best supercapacitive performance. 3.2. Effect of annealing temperature on electrochemical properties. To further optimize the specific capacitance of sample-II, we annealed sample-II at 350, 450 and 550 ◦ C for 3 hours in nitrogen atmosphere, which were denoted as sample-II, -II-350, -II-450 and -II-550, respectively. Fig. 7 shows the XRD spectra of these samples, all of which could be well indexed to cubic Co3 O4 phase
Fig. 7. XRD spectra of sample-II, -II-350, -II-450 and -II-550. The bars at the bottom indicate the standard lines of Co3 O4 (JCPDS Card no.43-1003).
(JCPDS no. 43-1003), with no signals of other phases being detected. Further analyses indicated that the primary crystal size of Co3 O4 kept almost constant up to 450 ◦ C (crystal sizes estimated with Scherer formula: 17.2, 17.3, and 19.1 nm for sample-II, -II-350, and -II-450, respectively), and then increased by ∼50% (26.1 nm) when annealed at 550 ◦ C. Fig. 8 shows typical SEM image-II-350 (a), -II-450 (b) and -II550 (c). The particle sizes and size ranges of sample-II, -II-350, -II-450, and -II-550 were plotted in Fig. 8d by randomly measuring the dimensions of 100 particles in a given micrograph. Comparing Fig. 7a with Fig. 3b, one can find that the particle shapes of sampleII kept almost constant with only slightly increase of particle size
Fig. 8. SEM images of sample -II-350 (a), -II-450 (b) and -II-550 (c). Part (d) shows the average particle size and particle size range of sample-II, -II-350, -II-450 and -II-550 measured with part (a-c).
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Fig. 9. (a) CV curves of sample-II, -II-350, -II-450 and -II-550 at the scan rate of 5 mV·s−1 . (b) galvanostatic charge–discharge curves of sample-II, -II-350, -II-450 and -II-550 at a constant discharge current density of 0.2 A·g−1 .
during 350 ◦ C annealing. When the annealing temperature increased up to 450 ◦ C, the particles became obviously aggregated (Fig. 8b) besides the elevation of particle size (Inset of Fig. 8b). After 550 ◦ C annealing, most of the aggregates of small particles have transformed into large ones with smooth surfaces. Fig. 9a exhibits CV curves of sample-II-350, -II-450, and-II-550, as well as that of sample-II for comparison, at a scan rate of 5 mV·s−1 . All curves show obvious pseudocapacitance features with distinct anodic and cathodic peaks. It is evident that the CV curve of the sample-II-350 displays a much higher redox peak current density and larger enclosed area than the others, implying better capacitor performance of this sample. The calculated specific capacitance of sample-II, -II-350, -II-450 and -II-550 was 155.3, 391.4, 108.4 and 65.1 F·g−1 , respectively. Fig. 9b shows galvanostatic charge–discharge curves of sample-II, -II-350, -II-450 and -II-550 at a constant discharge current density of 0.2 A·g−1 . All curves show decay stage with obviously similar line-type. The specific capacitance calculated with GCD data was 179.9, 362.8, 121.3 and 63.1 F·g−1 , respectively. Experimental results (Fig. 9a-b) showed that the electrochemical performances of sample-II were effective improved by 350 ◦ C, 3 hours annealing under nitrogen. With further increases of annealing temperatures, the electrochemical performances deteriorated rapidly. Fig. 10 shows Nyquist plots for electrodes of sample-II,-II-350, -II-450 and -II-550. It is shown that the internal resistances of the electrodes (the intersection of curves with X-axis) are respectively 1.9, 1.6, 2.1 and 2.5 , for sample-II, -II-350, -II-450 and -II-550
Fig. 10. Nyquist plots of experimental impedance data for sample-II, -II-350, -II-450 and -II-550 electrodes.
electrodes, which suggests that sample-II-350 demonstrated the highest electrical conductivity. It was also obvious that the Co3 O4 electrode of sample-II-350 exhibited lower charge-transfer resistance and slope straight line with higher slop than other electrodes, demonstrating the lower diffusion resistance of ions and improved electrolyte ions diffusion in the nanostructure of corresponding electrode [18].
3.3. Rate and cycling performance of the optimal performance sample Fig. 11a shows the CV curves of sample-II-350 at different scan rates from 5-50 mV·s−1 in the potential range between -0.3 and 0.35 V in a 6.0 mol·L−1 KOH aqueous electrolyte. The nonrectangular shapes of the CV curves revealed that the charge storage was a pseudocapacitance process originating from reversible redox reactions. With an increase in the scan rate, the anodic and cathodic peaks shift to higher and lower potentials, respectively, possibly due to polarization of the electrode [45,46]. The almost linear (quasi-linear) relationship between anodic peak current density and scan rate (inset in Fig. 11b) was suggestive of surface redox reactions associated with the pseudocapacitance behavior of the electrode. Excellent cyclic performance and high rate discharge capability are crucial for an electrode material used in supercapacitors. The rate performance of Co3 O4 has been investigated by recording GCD curves at different current densities at the potential range between 0-0.35 V, and the results are shown in Fig. 10c. The nonlinearity in the discharge curves shows the pseudocapacitance behavior of Co3 O4 , which corresponds well with the CV test. At the current densities between 0.2-4 A·g−1 , the specific capacitances of sample-II-350 Co3 O4 electrode ranged from 362.8 to 285.7 F·g−1 (Fig. 11d). It is also found that even at a high current density of 4 A·g−1 , nearly 78.7% of the initial capacitance value remains, demonstrating excellent rate performance of the sample-II-350 Co3 O4 electrode. Fig. 12 shows the cycling performance of the four 350 ◦ C annealed samples at current density of 1 A·g−1 . For all samples, clear increases in specific capacitance are observed during the first 50∼200 cycles, which derive possibly from the structural activation and pore opening [16]. In further cycling processes, the specific capacitances manifest gradual but continuous decreases. After 1000 cycles, the capacity retention of sample-I-350, -II-350, -III-350 and -IV-350 were 67.2%, 73.5%, 70.5% and 71.8%, respectively. As expected, sample-II-350 showed the highest specific capacitance and the best cycling performance among samples.
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Fig. 11. (a) CV curves of sample-II-350 at various scan rates. (b) the linearity of anodic current density with scan rates. (c) GCD curves of sample-II-350 at different current densities. (d) the specific capacitances of sample-II-350 calculated from GCD curves at different current densities.
Acknowledgments The authors thank the China Postdoctoral Science Foundation (2012M520605), Research Foundation of Taiyuan University of Technology (tyut-rc201369a, 2013Z040), the Open Foundation of State Key Laboratory of Coal Conversion (09-102), and the International S&T Co-operation Program of Shanxi Province (2010081017) for financial support. We are also deeply grateful for the characterization assistance from Dr. Lingpeng Yan and Xinghai Liu.
References
Fig. 12. The cycling performance of sample-I-350, -II-350, -III-350 and -IV-350 electrodes tested for 1000 cycles at a current density of 1 A·g−1 in 6.0 mol·L−1 KOH solution.
4. Conclusions Cobalt oxides (Co3 O4 and Co3 O4 /CoO) have been synthesized by combusting the mixture of Co(NO3 )2 ·6H2 O and citric acid. Experimental results confirmed that with the increase of citric acid/Co(NO3 )2 ·6H2 O molar ratios, the products transformed from granular aggregates of cubic Co3 O4 into a mixture of cubic Co3 O4 and tetragonal CoO with fluffy sheet morphologies. Electrochemical measurements indicated that the products (Co3 O4 ) showed a capacitance up to 179.7 F·g−1 (at 0.2 A·g−1 ) when the citric acid/Co(NO3 )2 ·6H2 O molar ratio was 7:27. Then, the capacitance could be further improved to 362.8 F·g−1 at 0.2 A·g−1 by annealing at 350 ◦ C for 3 h under nitrogen atmosphere. This annealed sample also demonstrated decent rate performance (285.7 F·g−1 at 4 A·g−1 ) and cycling stability (73.5% retention after 1000 cycles).
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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Solution combustion synthesis of cobalt oxides (Co3 O4 and Co3 O4 /CoO) nanoparticles as supercapacitor electrode materials Jiachun Deng a , Litao Kang a,∗ , Gailing Bai d , Ying Li a , Peiyang Li a , Xuguang Liu b , Yongzhen Yang a , Feng Gao a,c , Wei Liang a a
Nano-Energy Inorganic Materials Laboratory, College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan 030024, China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China d College of Materials Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China b
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 26 March 2014 Accepted 29 March 2014 Available online 5 April 2014 Keywords: Solution combustion Cobalt oxide Nanoparticle Supercapacitor
a b s t r a c t Co3 O4 and Co3 O4 /CoO nanoparticles have been synthesized by a one-step solution combustion process by adjusting the molar ratio of citric acid (fuel) and Co(NO3 )2 ·6H2 O (oxidizer). The effects of citric acid/Co(NO3 )2 ·6H2 O molar ratios on phase composition and morphology of products were investigated by XRD and SEM. With the increase of the fuel dosage, the products transformed from granular aggregates of cubic Co3 O4 into a mixture of cubic Co3 O4 and tetragonal CoO with fluffy sheet morphologies. Electrochemical measurements indicated that the products (Co3 O4 ) showed a capacitance up to 179.7 F·g−1 (at 0.2 A·g−1 ) when the citric acid/Co(NO3 )2 ·6H2 O molar ratio was 7/27. Significantly, the capacitance could be further improved by 102% (362.8 F·g−1 at 0.2 A·g−1 ) after annealing at 350 ◦ C for 3 h under nitrogen atmosphere. This annealed sample also demonstrated decent rate performance (285.7 F·g−1 at 4 A·g−1 ) and cycling stability (73.5% retention after 1000 cycles). The current study suggests that this process has promise in large-scale production of electrode materials for supercapacitors. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction The increasing demand for energy has triggered tremendous research efforts for energy storage with high power and energy densities [1]. Supercapacitors (SCs), also called electrochemical capacitors, are promising energy storage devices for portable systems and hybrid electric vehicles [2], because of their high power densities (about 1-100 kW·kg−1 ), long cycling lives (105 ∼106 cycles) and abilities of fast energy storage and release [3,4]. SCs could be divided into two types according to different energy storage mechanisms: electric double-layer capacitors (EDLCs) and electrochemical pseudocapacitors (EPCs) [5,6]. The capacitance of EDLCs is based on reversible physical ion absorption at the electrode/electrolyte interface, while the capacitance of EPCs arises from fast and reversible faradic redox reactions at the surface or near-surface of electroactive materials [7]. EPCs usually show higher capacitance (typically 300-1000 F·g−1 ) than EDLCs
∗ Corresponding author. E-mail addresses: [email protected] (L. Kang), [email protected] (W. Liang). http://dx.doi.org/10.1016/j.electacta.2014.03.158 0013-4686/© 2014 Elsevier Ltd. All rights reserved.
(typically 100-250 F·g−1 ) [4]. Among the electroactive materials used for EPCs, amorphous hydrated RuO2 has been identified as an influential candidate because of its high theoretical specific capacitance (1358 F·g−1 ), excellent electrical conductivity (300 S·cm−1 ) and high electrochemical stability [4,8,9]. The nanotubular hydrous RuO2 electrodes by AAO (anodic aluminum oxide) membranetemplates electrodeposition showed a specific capacitance up to 1300 F·g−1 [10]. However, the commercial application of RuO2 was hindered by the high cost and toxicity associated with Ru [10–12]. As one of promising replacements for RuO2 , cobalt oxides show advantages of low cost and high theoretical specific capacitance (Co3 O4 : 3560 F·g−1 , CoO: 4292 F·g−1 ) [13,14]. Many synthesis approaches have been successfully employed to prepare wellpreformed cobalt oxides nano-structures, such as microwave assisted precipitation of Co3 O4 nanoparticles (519 F·g−1 at 0.5 mA·cm−2 [15]), hydrothermal synthesis of Co3 O4 nanosheets (548 F·g−1 at 8 A·g−1 [16]) and Co3 O4 nanorods (281F·g−1 at 5 mV·s−1 [17]), solvothermal synthesis of Co3 O4 nanosheets/nanorods (258 F·g−1 at 10 mA·cm2 /563 F·g−1 at 10 mA·cm−2 [18]), chemical bath deposition of Co3 O4 nanotubes (574 F·g−1 at 0.1 A·g−1 [19]) and nanowire arrays on nickel foam (746 F·g−1 at 5 mA·cm−2 [20]). To further promote the large scale and especially continuous
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production of cobalt oxides supercapacitor electrodes, a one-step route was developed by directly spraying cobalt acetate tetrahydrate aqueous solution onto current collectors with the appearance of plasma gas [12]. The formed porous Co3 O4 coatings showed a specific capacitance of 162 F·g−1 at 2.75 A·g−1 . Solution combustion synthesis utilizes the violent, selfsustained exothermic reaction between oxidizer (usually nitrates) and fuel to achieve high temperature and intense gaseous product release within a time framework of seconds [21–24]. These features promote crystallization (high temperature) but depress the crystal growth process (limit lasting time), favoring the facile and continuous preparation of porous nanomaterials [24,25]. In principle, porous and nano-size features are highly admired for the electrodes materials of electrochemical energy storage devices, by accommodating the volume change and increasing the electrode/electrolyte interface during cycling [26]. Therefore, this one-step method has been successfully employed to produce electrode materials for lithium ion batteries (LIBs) and supercapacitors. For example, Wen et al. [27].and Rai et al. [28] reported the solution combustion synthesis of Co3 O4 (30-40 nm) and CoO nanoparticles (20-30 nm); as anode electrode materials for LIBs, they respectively exhibited specific capacities of 400 (after 100 cycles) and 565 mAh·g−1 (after 23 cycles). On the other hand, Yu et al. [29] and Jayalakshmi et al. [30] separately synthesized -MnO2 nanoparticles and Fe2 O3 /C, Fe2 O3 SnO2 /C, Fe2 O3 -ZnO/C composites via this method, which showed specific capacitances of 123 F·g−1 (at 1 A·g−1 ) and 254.9, 78.6, 122.4 F·g−1 (at 200mV·s−1 ), respectively. In this paper, nano-sized cobalt oxides (Co3 O4 and Co3 O4 /CoO) have been synthesized by combusting the mixture gel of Co(NO3 )2 ·6H2 O and citric acid. Experimental results confirmed that the molar ratios of citric acid/Co(NO3 )2 ·6H2 O effectively influenced the phase composition, morphologies and thus electrochemical measurements of products. These products showed a best capacitance of 179.7 F·g−1 (at 0.2 A·g−1 ) with a citric acid/Co(NO3 )2 ·6H2 O molar ratio of 7/27. Furthermore, after 350 ◦ C annealing under N2 for 3 h, a higher specific capacitance (362.8 F·g−1 at 0.2 A·g−1 ), an excellent rate performance (with 78.8% capacitance retention rate at 4 A·g−1 ) and decent cycling stability (73.5% retention after 1000 cycles) were successfully achieved. 2. Experimental 2.1. Material preparation In our solution combustion processes, citric acid monohydrate (C6 H8 O7 ·H2 O), cobalt nitrate hexahydrate (Co(NO3 )2 ·6H2 O), and ammonium nitrate (NH4 NO3 ) were used as fuel, oxidizer and combustion improver, respectively. Assuming that N2 , CO2 , and H2 O were evolved as the gaseous products in this citric acid–nitrate system, the combustion reaction can be generalized and expressed as follows [25,31]: Co(NO3 )2 · 6H2 O + C6 H8 O7 · H2 O + (9/2 − 7/3)O2 = 1/3Co3 O4 + 6CO2 ↑ +(6 + 5)H2 O ↑ +N2 ↑
(1)
In this reaction, = 14/27 means that the initial mixture does not require atmospheric oxygen for complete oxidation of fuel (being named as: stoichiometric combustion). It should be noted that the gaseous products of this reaction are smelly and show a weak reddish brown color, which is the typical characteristics of NO2 . Therefore, besides reaction (1), following reaction also occurred in our combustion experiments. Co(NO3 )2 · 6H2 O + C6 H8 O7 · H2 O + (9/2 − 1/3)O2 = 1/3Co3 O4 + 6CO2 ↑ +(6 + 5)H2 O ↑ +2NO2 ↑
(2)
In this reaction, stoichiometric combustion requires = 2/27. For the real combustion, the condition of stoichiometric combustion located between = 2/27 and = 14/27. The real combustion reaction may could be determined by measuring the kinds and contents of nitrogen-related gaseous products. Related results will be reported in further work. In this paper, the molar ratios of citric acid monohydrate/Co(NO3 )2 ·6H2 O were fixed to be 3.5/27, 7/27, 14/27, 21/27 in order to investigate the effects of fuel/oxidant molar ratio (i.e., the molar ratio of citric acid monohydrate/Co(NO3 )2 ·6H2 O) on microstructure and electrochemical performance of products,. The corresponding products were denoted as sample I, II, III and IV, respectively. For the synthesis of sample-II, 7.246 g Co(NO3 )2 ·6H2 O, 1.356 g citric acid monohydrate and 2 g ammonium nitrate were added in a 250 ml beaker that contained 100 ml deionized water. After 15 min magnetic stirring, the beaker was placed in a preheated muffle furnace at 350 ◦ C for 30 min. At this temperature, the violent evaporation of solvent firstly produced a voluminous and fluffy gel within 15 min, which was further ignited with a flame and combusted into corresponding oxides within 15 s. To synthesize sample-I, -III, and -IV, the dosages of citric acid monohydrate were changed into 0.678, 2.712 and 4.068 g, respectively. The ingredient lists of precursor solutions used to prepare samples I-IV have been summarized in Table 1. In order to investigate the influence of annealing, the sample with the best performance (sample-II) was further annealed at 350, 450, and 550 ◦ C for 3 h under a nitrogen flow, corresponding to sample-II-350, -II-450, and -II-550. 2.2. Material characterization. X-ray powder diffraction (XRD) patterns of the as-prepared samples were recorded using a Y-2000 X-ray diffractometer (Dan˚ After being dong, China) with a Cu − K␣ radiation ( = 1.542 A). pre-heated at 100 ◦ C for 1 h, thermogravimetric (TG) curves of samples were measured on a NETZSCH TG 209 F3 thermal analyzer from 100 to 900 ◦ C with a heating rate of 10 ◦ C·min−1 in flowing air. The morphologies of the products were observed on a Mira3 field-emission scanning electron microscope. The Brunauer-Emmett-Teller (BET) specific surface area and BJH pore size distribution were measured with a surface area and pore size analyzer (V-sorb2800P, Gold APP Instruments, China). 2.3. Electrochemical Measurements. For evaluating the electrochemical properties of samples, working electrodes were fabricated by mixing active materials (samples), acetylene black and poly (vinyldifluoride) (PVDF, in Nmethylpyrrolidone solvent with a solid content of 10 g·L−1 ) at a weight ratio of 8:1:1 to form the coating slurry. The slurry was smeared onto the current collector (Ni foam), then dried in air at 80 ◦ C overnight and pressed under a pressure of 10 MPa to serve as the working electrode. The loading mass of active material was ∼1.5 mg·cm−2 . All electrochemical measurements were carried out at room temperature in a 6.0 mol·L−1 KOH aqueous electrolyte with a working electrode (electrodes with cobalt oxides powders), a saturated calomel reference electrode (SCE, Hg/HgCl2 ), and a platinum plate counter electrode (1 × 2 cm2 ). The electrochemical behaviors of the active electrodes were characterized by cyclic voltammetry (CV, in a potential window between -0.30 and 0.45 V vs. SCE) and galvanostatic charge–discharge (GCD, in a potential window between 0 and 0.35 V vs. SCE) tests on a CHI660D electrochemical workstation (Shanghai, China). The electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 100 kHz to 10
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Table 1 Ingredient lists of precursor solutions used to synthesize sample-I, -II, -III and -IV. C6 H8 O7 ·H2 O/Co(NO3 )2 ·6H2 O (molar ratio)
Sample
3.5/27 7/27 14/27 21/27
I II III IV
mHz at open circuit voltage with a AC perturbation of 10 mV on a Cs 350 electrochemical workstation (Wuhai, China). The specific capacitance can be calculated with CV data according to equation (3) [16,32]: C=
1 mv(Va − Vc )
Vc
I(V )dV
(3)
Va
At the same time, the specific capacitance could also be calculated with GCD data via following equation (4) [33,34]: C=
i × t V × m
(4)
3. Results and discussion 3.1. Effects of fuel-to-oxidant molar ratio Fig. 1 displays the X-ray diffraction (XRD) spectra of samples I-IV synthesized by our solution combustion process. It is shown that all XRD peaks of sample-I and -II could be assigned to cubic ˚ For sample-III, Co3 O4 (JCPDS Card no.43-1003, a= b = c = 8.084 A). however, an obvious XRD peak centered at 42.4◦ appeared besides the signals of cubic Co3 O4 phase. Meanwhile, the XRD peak at ∼36.6◦ has divided into two peaks (inset (c) of Fig. 1), characterizing the appearance of tetragonal CoO phase (JCPDS Card no.65-5474, ˚ c= 4.214 A). ˚ XRD peaks of both cubic Co3 O4 and a = b = 3.015 A, tetragonal CoO have been detected in sample-IV, but they became weak and broaden, suggesting a poor crystalline degree. In solution combustion processes, the amount of gaseous products would increase with the increase of citric acid (fuel) [35]. And the intensive gaseous product release could prevent agglomeration of tiny resulting oxide crystallites and thus depress the crystal growth process. Therefore, the decrease of crystalline degree from sample-I to
Fig. 1. XRD spectra of sample-I, -II,-III and -IV. The bars at the bottom indicate the standard lines of CoO (JCPDS Card no.65-5474) and Co3 O4 (JCPDS Card no.43-1003). The enlarged peaks at ∼36.6◦ and their shape evolution for all samples have been shown in insets (a) - (d).
Co(NO3 )2 ·6H2 O(g)
C6 H8 O7 ·H2 O (g)
NH4 NO3 (g)
7.246 7.246 7.246 7.246
0.678 1.356 2.712 4.068
2 2 2 2
sample-IV could be ascribed to the increase dosage of citric acid in the precursor solution. TG curves of samples I-IV evidentially confirmed the appearance of tetragonal CoO with the increase of fuel/oxidizer molar ratios. As shown in Fig. 2, a slight weight loss was detected in 150-290 ◦ C for all samples, and it could be ascribed to desorption of chemicallyabsorbed H2 O. For sample sample-I, there was another weight loss (1.5%) located at around 450-600 ◦ C, which possibly attributed to the decomposition of residual raw materials (e.g., cobalt nitrate). With the increase of fuel dosages in precursor solution, this weight loss became very weak (sample-II) and finally evolved into weight increase (1.3% and 6.7% for sample-III and -IV, respectively). Combining with XRD results, we attributed these weight increase to the oxidization of CoO, which was shown in following Equation 5. The appearance of CoO could be attributed to the reduction of Co3+ by excess fuel, and the CoO weight ratios in sample- III and -IV were calculated to be 18.3% and 94.1% with the TG data. 6CoO + O2 = 2Co3 O4
(5)
Fig. 3 shows typical SEM images of samples I-IV, which indicated that sample-I presented in uniform granular aggregates (diameter: ∼150 nm) of tiny particles, while the aggregates in sample-II (Fig. 3b) became relatively large (diameter: 100-250 nm) and uneven. The sizes of particles in sample-I and -II were identified to be about 30 nm, in line with the grain sizes of cubic Co3 O4 estimated by Scherer formula (20 nm by XRD). However, sample -III and -IV (Fig. 3c-d) were found consisting of fluffy sheets. Compared with sample-III, the micro-pores became low in density in sampleIV. The porous sheet-like morphology should been introduced by three steps: firstly, the precursor solution concentrated into viscous sol with film-forming ability [36] with elevated temperature (350 ◦ C); secondly, numerous pores formed in the sol with further solvent evaporation to form a fluffy gel; finally, the gel combusted with intense gaseous product release, transforming the pore walls into porous sheets. XRD, TG and SEM data indicated that the fuel/oxidizer molar ratio showed a obvious influence on the composition and morphology of products. Low molar ratio of fuel/oxidizer (e.g., sample-I and -II) favored the formation of cubic Co3 O4 with the morphologies of
Fig. 2. TG curves of sample-I, -II, -III, and - IV in air.
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Fig. 3. SEM images of sample-I (a), -II (b), -III (c), and -IV (d).
granular aggregates; while with high molar ratio of fuel/oxidizer (e.g., sample-III and -IV), the excess fuel seems to reduce partially Co3+ and results in a mixture phase of cubic Co3 O4 and tetragonal CoO with a fluffy sheet morphology. With increase of molar ratio of fuel/oxidizer, the weight ratio of CoO increase while the CoO and Co3 O4 crystalline degree decrease in the mixture. Sample-III and -IV showed CoO weight ratio of 18.3% and 94.1%, respectively. Additionally, BET analyses were further performed for sample I-IV to determine their specific surface area and pore parameters (Fig. 4 and Table 2). All samples display type IV adsorptionedesorption isotherms, according to the IUPAC classification [37]. The BET specific surface area (SBET ) and the BJH desorption pore volumes (Vpore ) were 13.1, 21.3, 14.1, 17.9 m2 ·g−1 and 0.037, 0.095, 0.054, 0.095 cm3 ·g−1 for sample-I,-II, -III and -IV, respectively. Experimental results also revealed that the pore size distribution of samples was dramatically different. The pores in sample-I distributed in a narrow range of 3-10 nm. Whereas, sample-II showed a hierarchical pore structure, with pores mainly distributing in about 3.5, 6 and 32 nm. For sample-III and -IV, most of pores located in size range of 10-50 nm. These BET analysis indicated that sample-II possessed larger surface area, pore volume and mesopore volume (Vmeso ) than others, which may favor the supercapacitive performance. Fig. 5a shows CV curves of samples I-IV at the scan rate of 5 mV·s−1 in a potential range between -0.30 and 0.45 V (versus SCE). For all samples, an anodic peak (with positive current density) and three cathodic peaks (with negative current density) were visible, indicating faradic redox reaction nature of these materials. This result is in good line with CV curves of literature [16,33,38]. Generally, there are three redox couples, Co(OH)2 /Co3 O4 , Co3 O4 /CoOOH, and CoOOH/CoO2 , involved in cyclic voltammetry study, corresponding to following reversible reactions. Anodic scan (toward positive potential): 3Co(OH)2 + 2OH − → Co3 O4 + 4H2 O + e−
(6)
Co3 O4 + OH − + H2 O → 3CoOOH + e− −
CoOOH + OH → CoO2 + H2 O + e
(7)
−
(8)
Cathodic scan (toward negative potential): CoO2 + H2 O + e− → CoOOH + OH − −
(9)
−
3CoOH + e → Co3 O4 + OH + H2 O −
Co3 O4 + H2 O + 2e → 3Co(OH)2 + 2OH
(10) −
(11)
In our samples, the anodic peak (e.g., peak A in CV curves of sample-II) was believed to be an overlapping peak by three oxidation reactions of Equation.(6), (7), and (8) [16,38]. On the other hand, the three cathodic peaks for samples I-IV (i.e., peak C1, C2, and C3 in the CV curves) were ascribed to the three reduction processes as shown in Equation.(9), (10), and (11), respectively. The sharp current density increases above 0.4 V were attributed to an oxygen evolution reaction as Equation.(12) 4OH − → O2 + 2H2 O + 4e−
(12)
This CV results indicated that our cobalt oxides underwent electrochemical charge transfer reactions Co(II)-Co(III)-Co(IV) in basic electrolyte, making it a potential candidate for pseudocapacitor application. Specific capacitance is one of the most important indices for evaluation of electrochemical performance. According to Equation.(3) and CV curves in Fig. 5a, the specific capacitance of sample-I, -II, -III and -IV were determined to be 68.3, 155.3, 146.8 and 135.7 F·g−1 , respectively,. In agreement with above CV results, the GCD data (Fig. 5b) determined that specific capacitances of sample-I, -II, -III and -IV were 73.1, 179.7, 141.6 and 130.5 F·g−1 at a current density of 0.2 A·g−1 , respectively. Both measurements indicated that sample II showed the best capacitor performance among samples. Fig. 6 shows Nyquist plots of experimental impedance data for samples I-IV. On the whole, all the plots were composed of an arc in the high frequency region and inclined line in the low
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Fig. 4. Nitrogen adsorption/desorption isotherm and pore size distribution (inset) of sample-I, -II, -III, and -IV.
Table 2 Microstructure parameters obtained by BET analyses for sample-I, -II, -III and -IV. sample
SBET (m2 ·g−1 )
Vpore(cm3 ·g−1 )
Vmeso(cm3 ·g−1 )
Vmacro(cm3 ·g−1 )
I II III IV
13.1 21.3 14.1 17.9
0.037 0.095 0.054 0.095
0.029 0.067 0.035 0.052
0.008 0.028 0.019 0.043
frequency region. The arc usually attributes to the charge transfer impedance, while the linear part of the impedance spectrum corresponds to the Warburg impedance [39,40], which is a result of the frequency dependence of ionic diffusion/transport in the electrolyte and to the surface of the electrode [40,41]. From the intersection with X-axis, one can find that the internal resistances of electrodes of samples I-IV were almost constant (∼2.0 ), which included the total resistances of the ionic resistance of electrolyte, intrinsic resistance of the active material, and contact resistance
at the active material/current collector interface [19]. The linear region (Zw) of the plots exhibit slops of ∼60◦ for samples II-IV and ∼45◦ for sample I. The ∼45◦ straight line corresponds to a Warburg impedance related to the diffusion of electrolyte within the electrode. While the ∼60◦ straight line shows the capacitance nature (it would be a vertical line for an ideal capacitor) [42], implying the good accessibility of OH− ions to the electrode surface and good supercapacitive performance of samples II-IV. From microstructure point of view, mesopores and high specific surface area can provide
Fig. 5. (a) CV curves of sample-I, -II, -III and -IV at the scan rate of 5 mV·s−1 . (b) galvanostatic charge–discharge curves of sample-I, -II, -III, and -IV at a constant discharge current density of 0.2 A·g−1 .
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Fig. 6. Nyquist plots of experimental impedance data for sample-I, -II, -III and -IV electrodes.
short ion-transport pathways for electrochemical reactions [43] to decrease the Warburg impedance [44]. As revealed by BET analyses, sample-II exhibited largest specific surface area and mesopore volume among samples. Therefore, it shows the lowest Warburg impedance and best supercapacitive performance. 3.2. Effect of annealing temperature on electrochemical properties. To further optimize the specific capacitance of sample-II, we annealed sample-II at 350, 450 and 550 ◦ C for 3 hours in nitrogen atmosphere, which were denoted as sample-II, -II-350, -II-450 and -II-550, respectively. Fig. 7 shows the XRD spectra of these samples, all of which could be well indexed to cubic Co3 O4 phase
Fig. 7. XRD spectra of sample-II, -II-350, -II-450 and -II-550. The bars at the bottom indicate the standard lines of Co3 O4 (JCPDS Card no.43-1003).
(JCPDS no. 43-1003), with no signals of other phases being detected. Further analyses indicated that the primary crystal size of Co3 O4 kept almost constant up to 450 ◦ C (crystal sizes estimated with Scherer formula: 17.2, 17.3, and 19.1 nm for sample-II, -II-350, and -II-450, respectively), and then increased by ∼50% (26.1 nm) when annealed at 550 ◦ C. Fig. 8 shows typical SEM image-II-350 (a), -II-450 (b) and -II550 (c). The particle sizes and size ranges of sample-II, -II-350, -II-450, and -II-550 were plotted in Fig. 8d by randomly measuring the dimensions of 100 particles in a given micrograph. Comparing Fig. 7a with Fig. 3b, one can find that the particle shapes of sampleII kept almost constant with only slightly increase of particle size
Fig. 8. SEM images of sample -II-350 (a), -II-450 (b) and -II-550 (c). Part (d) shows the average particle size and particle size range of sample-II, -II-350, -II-450 and -II-550 measured with part (a-c).
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Fig. 9. (a) CV curves of sample-II, -II-350, -II-450 and -II-550 at the scan rate of 5 mV·s−1 . (b) galvanostatic charge–discharge curves of sample-II, -II-350, -II-450 and -II-550 at a constant discharge current density of 0.2 A·g−1 .
during 350 ◦ C annealing. When the annealing temperature increased up to 450 ◦ C, the particles became obviously aggregated (Fig. 8b) besides the elevation of particle size (Inset of Fig. 8b). After 550 ◦ C annealing, most of the aggregates of small particles have transformed into large ones with smooth surfaces. Fig. 9a exhibits CV curves of sample-II-350, -II-450, and-II-550, as well as that of sample-II for comparison, at a scan rate of 5 mV·s−1 . All curves show obvious pseudocapacitance features with distinct anodic and cathodic peaks. It is evident that the CV curve of the sample-II-350 displays a much higher redox peak current density and larger enclosed area than the others, implying better capacitor performance of this sample. The calculated specific capacitance of sample-II, -II-350, -II-450 and -II-550 was 155.3, 391.4, 108.4 and 65.1 F·g−1 , respectively. Fig. 9b shows galvanostatic charge–discharge curves of sample-II, -II-350, -II-450 and -II-550 at a constant discharge current density of 0.2 A·g−1 . All curves show decay stage with obviously similar line-type. The specific capacitance calculated with GCD data was 179.9, 362.8, 121.3 and 63.1 F·g−1 , respectively. Experimental results (Fig. 9a-b) showed that the electrochemical performances of sample-II were effective improved by 350 ◦ C, 3 hours annealing under nitrogen. With further increases of annealing temperatures, the electrochemical performances deteriorated rapidly. Fig. 10 shows Nyquist plots for electrodes of sample-II,-II-350, -II-450 and -II-550. It is shown that the internal resistances of the electrodes (the intersection of curves with X-axis) are respectively 1.9, 1.6, 2.1 and 2.5 , for sample-II, -II-350, -II-450 and -II-550
Fig. 10. Nyquist plots of experimental impedance data for sample-II, -II-350, -II-450 and -II-550 electrodes.
electrodes, which suggests that sample-II-350 demonstrated the highest electrical conductivity. It was also obvious that the Co3 O4 electrode of sample-II-350 exhibited lower charge-transfer resistance and slope straight line with higher slop than other electrodes, demonstrating the lower diffusion resistance of ions and improved electrolyte ions diffusion in the nanostructure of corresponding electrode [18].
3.3. Rate and cycling performance of the optimal performance sample Fig. 11a shows the CV curves of sample-II-350 at different scan rates from 5-50 mV·s−1 in the potential range between -0.3 and 0.35 V in a 6.0 mol·L−1 KOH aqueous electrolyte. The nonrectangular shapes of the CV curves revealed that the charge storage was a pseudocapacitance process originating from reversible redox reactions. With an increase in the scan rate, the anodic and cathodic peaks shift to higher and lower potentials, respectively, possibly due to polarization of the electrode [45,46]. The almost linear (quasi-linear) relationship between anodic peak current density and scan rate (inset in Fig. 11b) was suggestive of surface redox reactions associated with the pseudocapacitance behavior of the electrode. Excellent cyclic performance and high rate discharge capability are crucial for an electrode material used in supercapacitors. The rate performance of Co3 O4 has been investigated by recording GCD curves at different current densities at the potential range between 0-0.35 V, and the results are shown in Fig. 10c. The nonlinearity in the discharge curves shows the pseudocapacitance behavior of Co3 O4 , which corresponds well with the CV test. At the current densities between 0.2-4 A·g−1 , the specific capacitances of sample-II-350 Co3 O4 electrode ranged from 362.8 to 285.7 F·g−1 (Fig. 11d). It is also found that even at a high current density of 4 A·g−1 , nearly 78.7% of the initial capacitance value remains, demonstrating excellent rate performance of the sample-II-350 Co3 O4 electrode. Fig. 12 shows the cycling performance of the four 350 ◦ C annealed samples at current density of 1 A·g−1 . For all samples, clear increases in specific capacitance are observed during the first 50∼200 cycles, which derive possibly from the structural activation and pore opening [16]. In further cycling processes, the specific capacitances manifest gradual but continuous decreases. After 1000 cycles, the capacity retention of sample-I-350, -II-350, -III-350 and -IV-350 were 67.2%, 73.5%, 70.5% and 71.8%, respectively. As expected, sample-II-350 showed the highest specific capacitance and the best cycling performance among samples.
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Fig. 11. (a) CV curves of sample-II-350 at various scan rates. (b) the linearity of anodic current density with scan rates. (c) GCD curves of sample-II-350 at different current densities. (d) the specific capacitances of sample-II-350 calculated from GCD curves at different current densities.
Acknowledgments The authors thank the China Postdoctoral Science Foundation (2012M520605), Research Foundation of Taiyuan University of Technology (tyut-rc201369a, 2013Z040), the Open Foundation of State Key Laboratory of Coal Conversion (09-102), and the International S&T Co-operation Program of Shanxi Province (2010081017) for financial support. We are also deeply grateful for the characterization assistance from Dr. Lingpeng Yan and Xinghai Liu.
References
Fig. 12. The cycling performance of sample-I-350, -II-350, -III-350 and -IV-350 electrodes tested for 1000 cycles at a current density of 1 A·g−1 in 6.0 mol·L−1 KOH solution.
4. Conclusions Cobalt oxides (Co3 O4 and Co3 O4 /CoO) have been synthesized by combusting the mixture of Co(NO3 )2 ·6H2 O and citric acid. Experimental results confirmed that with the increase of citric acid/Co(NO3 )2 ·6H2 O molar ratios, the products transformed from granular aggregates of cubic Co3 O4 into a mixture of cubic Co3 O4 and tetragonal CoO with fluffy sheet morphologies. Electrochemical measurements indicated that the products (Co3 O4 ) showed a capacitance up to 179.7 F·g−1 (at 0.2 A·g−1 ) when the citric acid/Co(NO3 )2 ·6H2 O molar ratio was 7:27. Then, the capacitance could be further improved to 362.8 F·g−1 at 0.2 A·g−1 by annealing at 350 ◦ C for 3 h under nitrogen atmosphere. This annealed sample also demonstrated decent rate performance (285.7 F·g−1 at 4 A·g−1 ) and cycling stability (73.5% retention after 1000 cycles).
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