Journal of Colloid and Interface Science 424 (2014) 84–89
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
YbCl3 electrode in alkaline aqueous electrolyte with high pseudocapacitance Kunfeng Chen, Dongfeng Xue ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 January 2014 Accepted 7 March 2014 Available online 14 March 2014 Keywords: Crystallization transformation YbCl3 Colloid Pseudocapacitor YbOOH
a b s t r a c t Inorganic pseudocapacitors often select synthetic solid materials as electrode materials, which show low utilization of pseudocapacitive metal cations. We reported the crystallization transformation of YbCl3 pseudocapacitor electrodes in alkaline electrolytes, which can show high cation utilization ratio. The electrochemical reactive YbOOH colloids were crystallized through the chemical coprecipitation and Faradaic redox reactions. The effect of crystallization kinetics on electrochemical performances of YbCl3 pseudocapacitor was studied. YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g, where the commercial YbCl3 salts were used directly as pseudocapacitor electrodes in an aqueous electrolyte neglecting the complex synthesis procedures. The present strategy provides a novel route to crystallize electrochemical active compounds with unusual reactivity toward Faradaic redox reaction. The development of ion-based pseudocapacitors can advance the understating of the redox mechanism of active cations. Ó 2014 Elsevier Inc. All rights reserved.
Introduction One of the key challenges in main globule problems is energy crisis, which stimulates more and more researchers to deeply think about energy generation and storage [1,2]. With the increasing demand of portable and clean energy, electrochemical capacitors become more attractive due to their much greater power density and cyclability relative to Li-ion batteries [3,4]. Inorganic pseudocapacitors also named as redox electrochemical capacitors, in which the capacitance is mainly due to the redox reaction of active cations, can show higher energy density than those traditional electrical double-layer capacitors [5]. In order to highly utilize the reactive cations during the proposed redox reactions in various electrolytes, chemists gave many efforts to focus on the existing status of active cations, for example, different oxidation states, various condensed phases (solid, liquid), etc. [6,7]. Inorganic solid materials were broadly selected as electrode materials when searching for novel supercapacitors, due to the fact that most oxides and hydroxides possess high reactivity [8,9]. Therefore, chemists and materials scientists have designed various reaction and crystallization routes to synthesize inorganic solid materials with different morphologies, sizes, phases, and structures to improve the electrochemical performances of pseudocapacitors [10–12]. However, these materials
⇑ Corresponding author. E-mail address: [email protected] (D. Xue). http://dx.doi.org/10.1016/j.jcis.2014.03.022 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
as pseudocapacitor electrodes were limited by the low capacitance due to the low utilization ratio of the active cations, in which the cations in solid materials cannot be fully utilized in the Faradaic redox reaction [7]. To solve this limitation, free cations are the most promising candidates because that the active cation can be utilized to its maximum extent possible in this existing status. With the use of aqueous electrolytes in most pseudocapacitors, water-soluble inorganic salts are the best choice because they can be directly used as electrodes materials and supply a large amount of free cations [13]. There are many challenges in designing novel inorganic pseudocapacitors because much fundamental knowledge on the origin of pseudocapacitance is not yet clear enough to general materials scientists. For example, some pseudocapacitor electrode materials showed higher specific capacitance than their theoretical capacitance values, pseudocapacitors usually suffer from lower energy densities and shorter cyclic lifetimes [14–16]. We have addressed the problem from the ionic level by designing the water-soluble inorganic salts pseudocapacitors with the direct use of the commercial inorganic salts as electrode materials in alkaline electrolyte [13,17,18]. Our previous works have confirmed that reactive cations can transform into colloids in water-soluble inorganic salts pseudocapacitors [17]. The as-formed colloids were shown high reactivity toward electrochemical redox reactions, which contribute to the ultrahigh specific capacitance. The reactive colloids were formed by the chemical coprecipitation in alkaline
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K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Scheme 1. Schematic drawing of YbCl3 salts supercapacitor electrodes. The crystallization processes of Yb3+ during electrochemical redox reaction in alkaline electrolytes. The coupled chemical and electrochemical reactions favor the formation of reactive species with the unusual reactivity toward pseudocapacitive reactions. During this reaction system, YbOOH compounds were crystallized in alkaline conditions under the effect of extra electric field.
electrolyte, through which the metal cations were fixed at the colloids within the electrode. Coprecipitation is an important route in the synthesis of micro- and nanomaterials with various sizes, morphologies structures and properties [19,20]. In addition,
a
0.3
1A/g
3A/g
5A/g
7A/g
a
10A/g
20
10mV/s 20mV/s
Current (A/g)
Potential (V vs. SCE)
0.4
coprecipitation methods are also broadly applied to the crystallization of morphology-controlled solid materials [21]. In the designed system, metal cations can form colloids that can keep the high reactivity of cations. In this work, we reported the crystallization transformation of Yb3+ in pseudocapacitors during the electrochemical redox
0.2
0.1
10
0
-10 0.0
0
100
200
300
400
Time (s)
-20 0.0
0.1
0.3
0.4
YbCl3⋅6H2O
2000
Yb3+
b
1600
40 10mV/s
30
20mV/s 30mV/s
20
Current (A/g)
Specific capacitance (F/g)
0.2
Potential (V vs. SCE)
b
900
50mV/s 10
70mV/s
0 -10
600 2
4
6
8
10
-20
Current density (A/g) -30 Fig. 1. Electrochemical performance of YbCl36H2O supercapacitors. (a) The discharge curves (time versus potential) measured at various current densities and potential range of 0–0.41 V. (b) The specific capacitance upon current density, which can be calculated according to the equation: Sc = IDt/mDV, where I is the current used for discharge in A, Dt is the time elapsed for the discharge cycle in s, m is the mass of the active electrode material in g, and DV is the voltage interval of the discharge. All data are taken in a 2 M KOH solution at room temperature.
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) Fig. 2. (a and b) CV curves (current density versus potential) of YbCl36H2O salts electrodes obtained at potential range of 0–0.45 V and various scan rates. A pair of redox peaks is present at the CV curves, showing the pseudocapacitive characteristic. All data are taken in a 2 M KOH solution at room temperature.
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K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Energy density (Wh/kg)
55
50
45
40
35
30 1
2
3
4
5
Power density (kW/kg) Fig. 3. Power and energy densities of the YbCl36H2O salts electrodes. The power and energy densities were calculated according to the data measured from the three-electrode cell in 2 M KOH electrolytes.
reaction. With the assistance of the coprecipitation and external electric field, Yb3+ cations were transformed into YbOOH with higher reactivity toward redox reaction. The present results prove that the crystallization kinetics played a key role on the electrochemical performance of YbCl36H2O salts pseudocapacitors. The YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g in alkaline electrolyte. The present strategy provides a novel route to crystallize electrochemical active compounds with unusual reactivity toward Faradaic redox reaction. The development of ion-based pseudocapacitors can advance the understating of the redox mechanism of active cations.
Experimental The supercapacitor electrodes were prepared by mixing YbCl3 6H2O salts, acetylene black, and polyvinylidene fluoride (PVDF)
in a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone (NMP). Briefly, the resulting slurry was pasted onto a sheet of nickel foam. The above nickel foams were dried at 80 °C for 24 h. Then, the sheet of nickel foam with Ni salts was pressed at 10 MPa and served as working electrode. Afterward, the as-prepared electrodes were placed in air for the designed time. The electrode loading was between 2 and 4 mg for each of electrodes. Cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were obtained using an electrochemical workstation (CHI 660D) at designed potential range, scan rate and current density. All electrochemical experiments were carried out using a classical three-electrode configuration in 2 M KOH electrolytes. The saturated calomel electrode (SCE) was used as the reference electrode and Pt wire electrode as a counter electrode. The electrodes were characterized by field-emission scanning electron microscopy (FESEM, Hitachi-S4800), powder X-ray diffraction with Cu Ka radiation (k = 0.15418 nm) on a Bruker D8 Focus.
Results and discussion In the present reaction system, both crystallization and electrochemical redox processes were occurred in alkaline electrolyte simultaneously. With the help of the alkaline electrolytes and external electric field, electrochemical reactive YbOOH compounds were crystallized. Scheme 1 shows the reaction and crystallization processes of YbCl3 salts supercapacitor electrodes during the electrochemical redox reactions in alkaline electrolytes. The YbCl3 salts underwent crystallization in air and electrolyte and then delivered high capacitance through the redox reactions. The coupled chemical coprecipitation and electrochemical reactions favor the formation of reactive materials with the unusual reactivity toward pseudocapacitive reactions. Fig. 1 shows the discharge curves of YbCl36H2O salts electrodes at various current densities. The non-linear discharge curves are the reflection of Faradaic redox reactions of pseudocapacitors
a
YbOOH
Intensity (a.u.)
Intensity (a.u.)
b
YbOOH
Ni
10
20
30
40
2θ (degree)
50
Ni
60
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 4. XRD patterns of YbCl36H2O electrodes after electrochemical measurements. (a) The blending electrodes including YbCl36H2O, conductive carbon, and binder. (b) The electrodes only including YbCl36H2O. The standard JCPDS No.19-1432 for YbOOH and JCPDS No. 1-1258 for Ni current collector are also indicated in the figures.
K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
87
Fig. 5. SEM images of YbCl36H2O electrodes after electrochemical measurements. (a and b) The blending electrodes include YbCl36H2O, conductive carbon, and binder. (c) The electrodes only include YbCl36H2O.
[10,12]. Specific capacitances obtained from the discharge curves are shown in Fig. 1b. The ultrahigh specific capacitance of 2210 F/g at current density of 1 A/g and potential window of 0.41 V is obtained based on the weight of Yb3+. The specific capacitances are 2210, 1859, 1635, 1449, and 1299 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V. The specific capacitances are 987, 830, 730, 647, and 580 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V based on the weight of YbCl36H2O. The specific capacitances are decreased with the increase in the current density. With the increase in the current density, the electrons and ions cannot efficiently transfer into the electrodes due to the diffusion limitation. The present results proved that the commercial YbCl36H2O salts showed electrochemical reactivity in alkaline electrolyte. CV curves of YbCl36H2O electrodes with different scan rates are shown in Fig. 2. A pair of cathodic and anodic peaks is clearly observed with the scan rate of 10 and 20 mV/s (Fig. 2a). The anodic peaks (positive current) and cathodic peaks (negative current) in the CV curves originate from the oxidation and reduction processes of the Yb cation, indicated that the capacitance was originated from Faradaic redox reactions. The anodic peak potential is 0.4 V, while the cathodic peak is present at 0.14 V at scan rate of 10 mV/s (Fig. 2a). With the increase in scan rate, cathodic peak potentials shift to negative direction and the peak currents are increased (Fig. 2b). The nonrectangular shapes of the CV curves reveal that the charge storage is a characteristic of the pseudocapacitance process originating from the reversible redox reactions of cation [12,22]. The energy and power densities are two important parameters that characterize the electrochemical performances of supercapacitors [14–16]. The energy density was calculated using the following equation:
E ¼ CV 2 =2
ð1Þ
where C is the specific capacitance and V is the potential interval of the discharge. The average power density during discharge was calculated according to the equation, P = E/t (W/g), where t (h) is the
discharge time [14]. The power and energy densities of the YbCl36H2O salts electrodes based on the single electrode are shown in Fig. 3. The energy density decreases from 51.6 to 30.3 Wh/kg as the power density increase from 0.46 to 4.59 kW/kg. Noticeably, an energy density of 51.6 Wh/kg at a power density of 0.46 W/kg was obtained. To find the reaction mechanism of YbCl36H2O salts electrodes, we studied phases and microstructures of the electrode after electrochemical measurement. Fig. 4 shows the XRD patterns of YbCl36H2O salts electrodes after electrochemical characterization. The XRD pattern of the blending electrodes including YbCl36H2O, conductive carbon, and binder is shown in Fig. 4a. Except the peaks of Ni current collector, the peak with low intensity was ascribed to the YbOOH phase. The following reaction can be occurred: 3þ
Yb
þ 3OH ! YbOOH þ H2 O
ð2Þ
In order to prove the native role of YbCl36H2O salts, we prepared the electrodes only including YbCl36H2O. The XRD pattern also proves that the YbOOH phases were formed after electrochemical measurement. The weak peaks indicated that the YbOOH colloids were poorly crystallized. Fig. 5 shows SEM images of YbCl36H2O salts electrodes after electrochemical measurement. After the chemical coprecipitation and electrochemical reactions, colloids were formed on the electrode (Fig. 5). The colloids were also present in the pure YbCl36H2O salts electrodes as shown in Fig. 5c. The above results confirmed that the chemical coprecipitation of YbOOH colloids was occurred in alkaline electrolyte. The crystallization kinetics in air was also studied to show its influence on the electrochemical performance of YbCl36H2O salts electrodes. A pair of cathodic and anodic peaks is also present in the CV curves with aging time of 12 h (Fig. 6a). However, the peak potentials are different with cathodic peak at 0.33 V and anodic peak at 0.18 V. The specific capacitances are 658, 511, 381, 340, and 289 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V (Fig. 6b and c). The specific capacitances are lower than that with aging time of 72 h. With the increase in the aging time, peak potentials shift to negative and positive direction, while
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K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89 Table 1 Pseudocapacitance of Yb3+ cations in our work.
a 20
Current (A/g)
10mV/s 10
20mV/s
Active species
Faradic redox reaction
Theoretical capacitance at 0.41 V (F/g)
Measured capacitance at 0.41 V (F/g)
50mV/s
Yb3+
One electron transfer Two electron transfer
1358
2210
70mV/s 3+
Yb
0
2716
-10
a -20 0.0
0.1
0.2
0.3
30 25
0.4
12h 24h 36h 48h 72h
20
Potential (V vs. SCE)
b
0.4 1A/g
Potential (V vs. SCE)
3A/g 5A/g
0.3
Current (A/g)
15 10 5 0 -5
7A/g
-10
10A/g
-15 -20
0.2
-25 0.0
0.1
0.1
0
20
40
60
80
100
120
Specific capacitance (F/g)
Time (s)
600
YbCl 3⋅6H2 O 3+
Yb
Potential (V vs. SCE)
0.0
700
0.3
0.4
0.4
b
c
0.2
Potential (V vs. SCE)
12h 24h 36h 48h 72h
0.3
0.2
0.1
500 0.0
400
0
50
100
150
200
250
300
350
400
Time (s) 300
c
2500
100
0
2
4
6
8
10
Current density (A/g) Fig. 6. Electrochemical performances of YbCl36H2O electrodes after aging for 12 h. (a) CV curves obtained at different scan rates and potential range of 0–0.45 V. (b) Discharge curves at different current densities and potential range of 0–0.41 V. (c) Specific capacitance upon the current density based on the weight of YbCl36H2O and Yb3+.
the peak intensities also increase (Fig. 7a). When the aging times were increased to 36 h, the peak potentials show little change. The specific capacitances increase with the increase of the aging time (Fig. 7b and c). The specific capacitances are 658, 1218, 1312, 1959, and 2210 F/g at aging time of 12, 24, 36, 48 and 72 h with the current density of 1 A/g. The present results proved that the crystallization in air also influenced the electrochemical performance of YbCl36H2O salts electrodes. In our present system, the crystallization of electrochemical active materials is important for the improvement of the specific capacitance.
Specific capacitance (F/g)
200
YbCl 3⋅6H 2O 3+ Yb
2000
1500
1000
500
0 10
20
30
40
50
60
70
80
Aging time (h) Fig. 7. Electrochemical performances of YbCl36H2O electrodes after aging different time. (a) CV curves obtained at scan rate of 20 mV/s and potential range of 0–0.45 V. (b) Discharge curves at current density of 1 A/g and potential range of 0–0.41 V. (c) Specific capacitance upon the aging time at current density of 1 A/g and potential range of 0–0.41 V based on the weight of YbCl36H2O and Yb3+.
K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Theoretical specific capacitance of active cation Cm = Q/(V M), where Q = 9.632 104 C for the transfer of one electrons during the redox reaction, M is molecular weight of Yb3+ ion (M = 173.04 g/mol), V is the operating voltage window [13]. The theoretical and measured specific capacitances of active Yb3+ are shown in Table 1. The theoretical specific capacitances with the transfer of one and two electrons are 1358 and 2716 F/g, respectively. The highest measured value is 2210 F/g at 0.41 V, which is higher than that of one electron transferred reaction. The present results confirmed that the transferred electron number of the redox reaction in our YbCl36H2O electrodes is between one and two. In traditional pseudocapacitor electrode, the specific capacitances are calculated according to the weight of solid materials, which are valuable for the practical application of supercapacitor. However, from the viewpoint of intrinsic pseudocapacitance mechanism, the specific capacitances including theoretical and measured capacitances, calculated according to the weight of ions, can reflect the essential redox mechanism. The utilization ratios of reactive cations can be conveniently calculated from the capacitance of cations. Therefore, we use the cation-based specific capacitance in this work to explore the mechanism of Faradaic redox in inorganic pseudocapacitors. Conclusion In summary, the crystallization transformation of YbCl3 pseudocapacitor electrodes in alkaline electrolytes was reported. The electrochemical reactive YbOOH colloids were crystallized through the chemical coprecipitation and Faradaic redox reactions. The capacitance of YbCl3 pseudocapacitors increased with the increase in crystallization times. The YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g, and the commercial YbCl3 salts can be directly used as pseudocapacitor electrodes in an aqueous electrolyte neglecting the complex synthesis procedures. The transferred electron number of the redox reaction in our YbCl36H2O electrodes is between one and two. From the viewpoint of intrinsic pseudocapacitance mechanism, the specific capacitances including theoretical and measured capacitances, calculated according to the weight of ions, can reflect the essential redox
89
mechanism. This method provides one solution to the problem of exploiting intrinsic mechanism of Faradaic reaction of pseudocapacitor from ionic level. Acknowledgments Financial support from the National Natural Science Foundation of China (Grant Nos. 50872016, 20973033 and 51125009) and National Natural Science Foundation for Creative Research Group (Grant No. 21221061), and Hundred Talents Program of Chinese Academy of Science is acknowledged. References [1] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Science 342 (2013) 716. [2] M.R. Lukatskaya, O. Mashtalir, C.E. Ren, Y. Dall’Agnese, P. Rozier, P.L. Taberna, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Science 341 (2013) 1502. [3] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [4] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [5] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Academic, New York, 1999. [6] M. Song, S. Cheng, H. Chen, W. Qin, K. Nam, S. Xu, X. Yang, A. Bongiorno, J. Lee, J. Bai, T.A. Tyson, J. Cho, M. Liu, Nano Lett. 12 (2012) 3483. [7] Q. Lu, J.G. Chen, J.Q. Xiao, Angew. Chem. Int. Ed. 52 (2013) 1882. [8] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [9] K. Chen, Y.D. Noh, K. Li, S. Komarneni, D. Xue, J. Phys. Chem. C 117 (2013) 10770. [10] C. Guan, X. Li, Z. Wang, X. Cao, C. Soci, H. Zhang, H. Fan, Adv. Mater. 24 (2012) 4186. [11] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 25 (2013) 3302. [12] P. Lu, F. Liu, D. Xue, H. Yang, Y. Liu, Electrochim. Acta 78 (2012) 1. [13] K. Chen, S. Song, K. Li, D. Xue, CrystEngComm 15 (2013) 10367. [14] H.B. Li, M.H. Yu, F.X. Wang, P. Liu, Y. Liang, J. Xiao, C.X. Wang, Y.X. Tong, G.W. Yang, Nature Commun. 4 (2013) 1894. [15] X.Y. Lang, A. Hirata, T. Fujita, M.W. Chen, Nat. Nanotechnol. 6 (2011) 232. [16] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P. Taberna, S.H. Tolbert, H.D. Abruña, P. Simon, B. Dunn, Nat. Mater. 12 (2013) 518. [17] K. Chen, D. Xue, J. Colloid Interf. Sci. 416 (2014) 172. [18] K. Chen, Y. Yang, K. Li, Z. Ma, Y. Zhou, D. Xue, ACS Sustain. Chem. Eng. 2 (2014) 440. [19] S. Suh, K. Yuet, D.K. Hwang, K.W. Bong, P.S. Doyle, T. Alan, J. Am. Chem. Soc. 134 (2012) 7337. [20] G.R. Patzke, Y. Zhou, R. Kontic, F. Conrad, Angew. Chem. Int. Ed. 50 (2011) 826. [21] P.M. Rørvik, T. Grande, M. Einarsrud, Adv. Mater. 23 (2011) 4007. [22] Y. Zhang, C. Sun, P. Lu, K. Li, S. Song, D. Xue, CrystEngComm 14 (2012) 5892.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
YbCl3 electrode in alkaline aqueous electrolyte with high pseudocapacitance Kunfeng Chen, Dongfeng Xue ⇑ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China
a r t i c l e
i n f o
Article history: Received 21 January 2014 Accepted 7 March 2014 Available online 14 March 2014 Keywords: Crystallization transformation YbCl3 Colloid Pseudocapacitor YbOOH
a b s t r a c t Inorganic pseudocapacitors often select synthetic solid materials as electrode materials, which show low utilization of pseudocapacitive metal cations. We reported the crystallization transformation of YbCl3 pseudocapacitor electrodes in alkaline electrolytes, which can show high cation utilization ratio. The electrochemical reactive YbOOH colloids were crystallized through the chemical coprecipitation and Faradaic redox reactions. The effect of crystallization kinetics on electrochemical performances of YbCl3 pseudocapacitor was studied. YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g, where the commercial YbCl3 salts were used directly as pseudocapacitor electrodes in an aqueous electrolyte neglecting the complex synthesis procedures. The present strategy provides a novel route to crystallize electrochemical active compounds with unusual reactivity toward Faradaic redox reaction. The development of ion-based pseudocapacitors can advance the understating of the redox mechanism of active cations. Ó 2014 Elsevier Inc. All rights reserved.
Introduction One of the key challenges in main globule problems is energy crisis, which stimulates more and more researchers to deeply think about energy generation and storage [1,2]. With the increasing demand of portable and clean energy, electrochemical capacitors become more attractive due to their much greater power density and cyclability relative to Li-ion batteries [3,4]. Inorganic pseudocapacitors also named as redox electrochemical capacitors, in which the capacitance is mainly due to the redox reaction of active cations, can show higher energy density than those traditional electrical double-layer capacitors [5]. In order to highly utilize the reactive cations during the proposed redox reactions in various electrolytes, chemists gave many efforts to focus on the existing status of active cations, for example, different oxidation states, various condensed phases (solid, liquid), etc. [6,7]. Inorganic solid materials were broadly selected as electrode materials when searching for novel supercapacitors, due to the fact that most oxides and hydroxides possess high reactivity [8,9]. Therefore, chemists and materials scientists have designed various reaction and crystallization routes to synthesize inorganic solid materials with different morphologies, sizes, phases, and structures to improve the electrochemical performances of pseudocapacitors [10–12]. However, these materials
⇑ Corresponding author. E-mail address: [email protected] (D. Xue). http://dx.doi.org/10.1016/j.jcis.2014.03.022 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
as pseudocapacitor electrodes were limited by the low capacitance due to the low utilization ratio of the active cations, in which the cations in solid materials cannot be fully utilized in the Faradaic redox reaction [7]. To solve this limitation, free cations are the most promising candidates because that the active cation can be utilized to its maximum extent possible in this existing status. With the use of aqueous electrolytes in most pseudocapacitors, water-soluble inorganic salts are the best choice because they can be directly used as electrodes materials and supply a large amount of free cations [13]. There are many challenges in designing novel inorganic pseudocapacitors because much fundamental knowledge on the origin of pseudocapacitance is not yet clear enough to general materials scientists. For example, some pseudocapacitor electrode materials showed higher specific capacitance than their theoretical capacitance values, pseudocapacitors usually suffer from lower energy densities and shorter cyclic lifetimes [14–16]. We have addressed the problem from the ionic level by designing the water-soluble inorganic salts pseudocapacitors with the direct use of the commercial inorganic salts as electrode materials in alkaline electrolyte [13,17,18]. Our previous works have confirmed that reactive cations can transform into colloids in water-soluble inorganic salts pseudocapacitors [17]. The as-formed colloids were shown high reactivity toward electrochemical redox reactions, which contribute to the ultrahigh specific capacitance. The reactive colloids were formed by the chemical coprecipitation in alkaline
85
K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Scheme 1. Schematic drawing of YbCl3 salts supercapacitor electrodes. The crystallization processes of Yb3+ during electrochemical redox reaction in alkaline electrolytes. The coupled chemical and electrochemical reactions favor the formation of reactive species with the unusual reactivity toward pseudocapacitive reactions. During this reaction system, YbOOH compounds were crystallized in alkaline conditions under the effect of extra electric field.
electrolyte, through which the metal cations were fixed at the colloids within the electrode. Coprecipitation is an important route in the synthesis of micro- and nanomaterials with various sizes, morphologies structures and properties [19,20]. In addition,
a
0.3
1A/g
3A/g
5A/g
7A/g
a
10A/g
20
10mV/s 20mV/s
Current (A/g)
Potential (V vs. SCE)
0.4
coprecipitation methods are also broadly applied to the crystallization of morphology-controlled solid materials [21]. In the designed system, metal cations can form colloids that can keep the high reactivity of cations. In this work, we reported the crystallization transformation of Yb3+ in pseudocapacitors during the electrochemical redox
0.2
0.1
10
0
-10 0.0
0
100
200
300
400
Time (s)
-20 0.0
0.1
0.3
0.4
YbCl3⋅6H2O
2000
Yb3+
b
1600
40 10mV/s
30
20mV/s 30mV/s
20
Current (A/g)
Specific capacitance (F/g)
0.2
Potential (V vs. SCE)
b
900
50mV/s 10
70mV/s
0 -10
600 2
4
6
8
10
-20
Current density (A/g) -30 Fig. 1. Electrochemical performance of YbCl36H2O supercapacitors. (a) The discharge curves (time versus potential) measured at various current densities and potential range of 0–0.41 V. (b) The specific capacitance upon current density, which can be calculated according to the equation: Sc = IDt/mDV, where I is the current used for discharge in A, Dt is the time elapsed for the discharge cycle in s, m is the mass of the active electrode material in g, and DV is the voltage interval of the discharge. All data are taken in a 2 M KOH solution at room temperature.
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) Fig. 2. (a and b) CV curves (current density versus potential) of YbCl36H2O salts electrodes obtained at potential range of 0–0.45 V and various scan rates. A pair of redox peaks is present at the CV curves, showing the pseudocapacitive characteristic. All data are taken in a 2 M KOH solution at room temperature.
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K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Energy density (Wh/kg)
55
50
45
40
35
30 1
2
3
4
5
Power density (kW/kg) Fig. 3. Power and energy densities of the YbCl36H2O salts electrodes. The power and energy densities were calculated according to the data measured from the three-electrode cell in 2 M KOH electrolytes.
reaction. With the assistance of the coprecipitation and external electric field, Yb3+ cations were transformed into YbOOH with higher reactivity toward redox reaction. The present results prove that the crystallization kinetics played a key role on the electrochemical performance of YbCl36H2O salts pseudocapacitors. The YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g in alkaline electrolyte. The present strategy provides a novel route to crystallize electrochemical active compounds with unusual reactivity toward Faradaic redox reaction. The development of ion-based pseudocapacitors can advance the understating of the redox mechanism of active cations.
Experimental The supercapacitor electrodes were prepared by mixing YbCl3 6H2O salts, acetylene black, and polyvinylidene fluoride (PVDF)
in a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone (NMP). Briefly, the resulting slurry was pasted onto a sheet of nickel foam. The above nickel foams were dried at 80 °C for 24 h. Then, the sheet of nickel foam with Ni salts was pressed at 10 MPa and served as working electrode. Afterward, the as-prepared electrodes were placed in air for the designed time. The electrode loading was between 2 and 4 mg for each of electrodes. Cyclic voltammetry (CV) and galvanostatic charge–discharge measurements were obtained using an electrochemical workstation (CHI 660D) at designed potential range, scan rate and current density. All electrochemical experiments were carried out using a classical three-electrode configuration in 2 M KOH electrolytes. The saturated calomel electrode (SCE) was used as the reference electrode and Pt wire electrode as a counter electrode. The electrodes were characterized by field-emission scanning electron microscopy (FESEM, Hitachi-S4800), powder X-ray diffraction with Cu Ka radiation (k = 0.15418 nm) on a Bruker D8 Focus.
Results and discussion In the present reaction system, both crystallization and electrochemical redox processes were occurred in alkaline electrolyte simultaneously. With the help of the alkaline electrolytes and external electric field, electrochemical reactive YbOOH compounds were crystallized. Scheme 1 shows the reaction and crystallization processes of YbCl3 salts supercapacitor electrodes during the electrochemical redox reactions in alkaline electrolytes. The YbCl3 salts underwent crystallization in air and electrolyte and then delivered high capacitance through the redox reactions. The coupled chemical coprecipitation and electrochemical reactions favor the formation of reactive materials with the unusual reactivity toward pseudocapacitive reactions. Fig. 1 shows the discharge curves of YbCl36H2O salts electrodes at various current densities. The non-linear discharge curves are the reflection of Faradaic redox reactions of pseudocapacitors
a
YbOOH
Intensity (a.u.)
Intensity (a.u.)
b
YbOOH
Ni
10
20
30
40
2θ (degree)
50
Ni
60
10
20
30
40
50
60
70
80
2θ (degree)
Fig. 4. XRD patterns of YbCl36H2O electrodes after electrochemical measurements. (a) The blending electrodes including YbCl36H2O, conductive carbon, and binder. (b) The electrodes only including YbCl36H2O. The standard JCPDS No.19-1432 for YbOOH and JCPDS No. 1-1258 for Ni current collector are also indicated in the figures.
K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
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Fig. 5. SEM images of YbCl36H2O electrodes after electrochemical measurements. (a and b) The blending electrodes include YbCl36H2O, conductive carbon, and binder. (c) The electrodes only include YbCl36H2O.
[10,12]. Specific capacitances obtained from the discharge curves are shown in Fig. 1b. The ultrahigh specific capacitance of 2210 F/g at current density of 1 A/g and potential window of 0.41 V is obtained based on the weight of Yb3+. The specific capacitances are 2210, 1859, 1635, 1449, and 1299 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V. The specific capacitances are 987, 830, 730, 647, and 580 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V based on the weight of YbCl36H2O. The specific capacitances are decreased with the increase in the current density. With the increase in the current density, the electrons and ions cannot efficiently transfer into the electrodes due to the diffusion limitation. The present results proved that the commercial YbCl36H2O salts showed electrochemical reactivity in alkaline electrolyte. CV curves of YbCl36H2O electrodes with different scan rates are shown in Fig. 2. A pair of cathodic and anodic peaks is clearly observed with the scan rate of 10 and 20 mV/s (Fig. 2a). The anodic peaks (positive current) and cathodic peaks (negative current) in the CV curves originate from the oxidation and reduction processes of the Yb cation, indicated that the capacitance was originated from Faradaic redox reactions. The anodic peak potential is 0.4 V, while the cathodic peak is present at 0.14 V at scan rate of 10 mV/s (Fig. 2a). With the increase in scan rate, cathodic peak potentials shift to negative direction and the peak currents are increased (Fig. 2b). The nonrectangular shapes of the CV curves reveal that the charge storage is a characteristic of the pseudocapacitance process originating from the reversible redox reactions of cation [12,22]. The energy and power densities are two important parameters that characterize the electrochemical performances of supercapacitors [14–16]. The energy density was calculated using the following equation:
E ¼ CV 2 =2
ð1Þ
where C is the specific capacitance and V is the potential interval of the discharge. The average power density during discharge was calculated according to the equation, P = E/t (W/g), where t (h) is the
discharge time [14]. The power and energy densities of the YbCl36H2O salts electrodes based on the single electrode are shown in Fig. 3. The energy density decreases from 51.6 to 30.3 Wh/kg as the power density increase from 0.46 to 4.59 kW/kg. Noticeably, an energy density of 51.6 Wh/kg at a power density of 0.46 W/kg was obtained. To find the reaction mechanism of YbCl36H2O salts electrodes, we studied phases and microstructures of the electrode after electrochemical measurement. Fig. 4 shows the XRD patterns of YbCl36H2O salts electrodes after electrochemical characterization. The XRD pattern of the blending electrodes including YbCl36H2O, conductive carbon, and binder is shown in Fig. 4a. Except the peaks of Ni current collector, the peak with low intensity was ascribed to the YbOOH phase. The following reaction can be occurred: 3þ
Yb
þ 3OH ! YbOOH þ H2 O
ð2Þ
In order to prove the native role of YbCl36H2O salts, we prepared the electrodes only including YbCl36H2O. The XRD pattern also proves that the YbOOH phases were formed after electrochemical measurement. The weak peaks indicated that the YbOOH colloids were poorly crystallized. Fig. 5 shows SEM images of YbCl36H2O salts electrodes after electrochemical measurement. After the chemical coprecipitation and electrochemical reactions, colloids were formed on the electrode (Fig. 5). The colloids were also present in the pure YbCl36H2O salts electrodes as shown in Fig. 5c. The above results confirmed that the chemical coprecipitation of YbOOH colloids was occurred in alkaline electrolyte. The crystallization kinetics in air was also studied to show its influence on the electrochemical performance of YbCl36H2O salts electrodes. A pair of cathodic and anodic peaks is also present in the CV curves with aging time of 12 h (Fig. 6a). However, the peak potentials are different with cathodic peak at 0.33 V and anodic peak at 0.18 V. The specific capacitances are 658, 511, 381, 340, and 289 F/g at current densities of 1, 3, 5, 7, 10 A/g and potential range of 0–0.41 V (Fig. 6b and c). The specific capacitances are lower than that with aging time of 72 h. With the increase in the aging time, peak potentials shift to negative and positive direction, while
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K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89 Table 1 Pseudocapacitance of Yb3+ cations in our work.
a 20
Current (A/g)
10mV/s 10
20mV/s
Active species
Faradic redox reaction
Theoretical capacitance at 0.41 V (F/g)
Measured capacitance at 0.41 V (F/g)
50mV/s
Yb3+
One electron transfer Two electron transfer
1358
2210
70mV/s 3+
Yb
0
2716
-10
a -20 0.0
0.1
0.2
0.3
30 25
0.4
12h 24h 36h 48h 72h
20
Potential (V vs. SCE)
b
0.4 1A/g
Potential (V vs. SCE)
3A/g 5A/g
0.3
Current (A/g)
15 10 5 0 -5
7A/g
-10
10A/g
-15 -20
0.2
-25 0.0
0.1
0.1
0
20
40
60
80
100
120
Specific capacitance (F/g)
Time (s)
600
YbCl 3⋅6H2 O 3+
Yb
Potential (V vs. SCE)
0.0
700
0.3
0.4
0.4
b
c
0.2
Potential (V vs. SCE)
12h 24h 36h 48h 72h
0.3
0.2
0.1
500 0.0
400
0
50
100
150
200
250
300
350
400
Time (s) 300
c
2500
100
0
2
4
6
8
10
Current density (A/g) Fig. 6. Electrochemical performances of YbCl36H2O electrodes after aging for 12 h. (a) CV curves obtained at different scan rates and potential range of 0–0.45 V. (b) Discharge curves at different current densities and potential range of 0–0.41 V. (c) Specific capacitance upon the current density based on the weight of YbCl36H2O and Yb3+.
the peak intensities also increase (Fig. 7a). When the aging times were increased to 36 h, the peak potentials show little change. The specific capacitances increase with the increase of the aging time (Fig. 7b and c). The specific capacitances are 658, 1218, 1312, 1959, and 2210 F/g at aging time of 12, 24, 36, 48 and 72 h with the current density of 1 A/g. The present results proved that the crystallization in air also influenced the electrochemical performance of YbCl36H2O salts electrodes. In our present system, the crystallization of electrochemical active materials is important for the improvement of the specific capacitance.
Specific capacitance (F/g)
200
YbCl 3⋅6H 2O 3+ Yb
2000
1500
1000
500
0 10
20
30
40
50
60
70
80
Aging time (h) Fig. 7. Electrochemical performances of YbCl36H2O electrodes after aging different time. (a) CV curves obtained at scan rate of 20 mV/s and potential range of 0–0.45 V. (b) Discharge curves at current density of 1 A/g and potential range of 0–0.41 V. (c) Specific capacitance upon the aging time at current density of 1 A/g and potential range of 0–0.41 V based on the weight of YbCl36H2O and Yb3+.
K. Chen, D. Xue / Journal of Colloid and Interface Science 424 (2014) 84–89
Theoretical specific capacitance of active cation Cm = Q/(V M), where Q = 9.632 104 C for the transfer of one electrons during the redox reaction, M is molecular weight of Yb3+ ion (M = 173.04 g/mol), V is the operating voltage window [13]. The theoretical and measured specific capacitances of active Yb3+ are shown in Table 1. The theoretical specific capacitances with the transfer of one and two electrons are 1358 and 2716 F/g, respectively. The highest measured value is 2210 F/g at 0.41 V, which is higher than that of one electron transferred reaction. The present results confirmed that the transferred electron number of the redox reaction in our YbCl36H2O electrodes is between one and two. In traditional pseudocapacitor electrode, the specific capacitances are calculated according to the weight of solid materials, which are valuable for the practical application of supercapacitor. However, from the viewpoint of intrinsic pseudocapacitance mechanism, the specific capacitances including theoretical and measured capacitances, calculated according to the weight of ions, can reflect the essential redox mechanism. The utilization ratios of reactive cations can be conveniently calculated from the capacitance of cations. Therefore, we use the cation-based specific capacitance in this work to explore the mechanism of Faradaic redox in inorganic pseudocapacitors. Conclusion In summary, the crystallization transformation of YbCl3 pseudocapacitor electrodes in alkaline electrolytes was reported. The electrochemical reactive YbOOH colloids were crystallized through the chemical coprecipitation and Faradaic redox reactions. The capacitance of YbCl3 pseudocapacitors increased with the increase in crystallization times. The YbCl3 pseudocapacitor can show ultrahigh specific capacitance of 2210 F/g, and the commercial YbCl3 salts can be directly used as pseudocapacitor electrodes in an aqueous electrolyte neglecting the complex synthesis procedures. The transferred electron number of the redox reaction in our YbCl36H2O electrodes is between one and two. From the viewpoint of intrinsic pseudocapacitance mechanism, the specific capacitances including theoretical and measured capacitances, calculated according to the weight of ions, can reflect the essential redox
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mechanism. This method provides one solution to the problem of exploiting intrinsic mechanism of Faradaic reaction of pseudocapacitor from ionic level. Acknowledgments Financial support from the National Natural Science Foundation of China (Grant Nos. 50872016, 20973033 and 51125009) and National Natural Science Foundation for Creative Research Group (Grant No. 21221061), and Hundred Talents Program of Chinese Academy of Science is acknowledged. References [1] M. Ebner, F. Marone, M. Stampanoni, V. Wood, Science 342 (2013) 716. [2] M.R. Lukatskaya, O. Mashtalir, C.E. Ren, Y. Dall’Agnese, P. Rozier, P.L. Taberna, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Science 341 (2013) 1502. [3] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [4] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [5] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Academic, New York, 1999. [6] M. Song, S. Cheng, H. Chen, W. Qin, K. Nam, S. Xu, X. Yang, A. Bongiorno, J. Lee, J. Bai, T.A. Tyson, J. Cho, M. Liu, Nano Lett. 12 (2012) 3483. [7] Q. Lu, J.G. Chen, J.Q. Xiao, Angew. Chem. Int. Ed. 52 (2013) 1882. [8] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [9] K. Chen, Y.D. Noh, K. Li, S. Komarneni, D. Xue, J. Phys. Chem. C 117 (2013) 10770. [10] C. Guan, X. Li, Z. Wang, X. Cao, C. Soci, H. Zhang, H. Fan, Adv. Mater. 24 (2012) 4186. [11] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 25 (2013) 3302. [12] P. Lu, F. Liu, D. Xue, H. Yang, Y. Liu, Electrochim. Acta 78 (2012) 1. [13] K. Chen, S. Song, K. Li, D. Xue, CrystEngComm 15 (2013) 10367. [14] H.B. Li, M.H. Yu, F.X. Wang, P. Liu, Y. Liang, J. Xiao, C.X. Wang, Y.X. Tong, G.W. Yang, Nature Commun. 4 (2013) 1894. [15] X.Y. Lang, A. Hirata, T. Fujita, M.W. Chen, Nat. Nanotechnol. 6 (2011) 232. [16] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P. Taberna, S.H. Tolbert, H.D. Abruña, P. Simon, B. Dunn, Nat. Mater. 12 (2013) 518. [17] K. Chen, D. Xue, J. Colloid Interf. Sci. 416 (2014) 172. [18] K. Chen, Y. Yang, K. Li, Z. Ma, Y. Zhou, D. Xue, ACS Sustain. Chem. Eng. 2 (2014) 440. [19] S. Suh, K. Yuet, D.K. Hwang, K.W. Bong, P.S. Doyle, T. Alan, J. Am. Chem. Soc. 134 (2012) 7337. [20] G.R. Patzke, Y. Zhou, R. Kontic, F. Conrad, Angew. Chem. Int. Ed. 50 (2011) 826. [21] P.M. Rørvik, T. Grande, M. Einarsrud, Adv. Mater. 23 (2011) 4007. [22] Y. Zhang, C. Sun, P. Lu, K. Li, S. Song, D. Xue, CrystEngComm 14 (2012) 5892.
