Accepted Manuscript Formation of electroactive colloids via in-situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor Kunfeng Chen, Dongfeng Xue PII: DOI: Reference:
S0021-9797(14)00367-1 http://dx.doi.org/10.1016/j.jcis.2014.05.053 YJCIS 19605
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
9 April 2014 23 May 2014
Please cite this article as: K. Chen, D. Xue, Formation of electroactive colloids via in-situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.053
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Formation of electroactive colloids via in-situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor Kunfeng Chen and Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. *Corresponding author at: State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street, No. 5625, Changchun 130022, China. Tel: +86 431 85262294. E-mail: [email protected].
Abstract For the first time, a new ErCl3 alkaline aqueous pseudocapacitor system was demonstrated by designing commercial ErCl3 salt electrode in alkaline aqueous electrolyte, where the materials synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale. Highly electroactive ErOOH colloids were in-situ crystallized via electric field assisted chemical coprecipitation of ErCl3 in KOH aqueous electrolyte. These electroactive ErOOH colloids absorbed by carbon black and PVDF matrix were highly redox-reactive with higher cation utilization ratio of 86 % and specific capacitance values of 1811 F/g, Er2+). We believe that exceeding the one-electron redox theoretical capacitance (Er3+ ↔ additional two-electron (Er2+ ↔Er) or three-electron (Er3+ ↔Er) reactions can occur in our designed ErCl3 alkaline aqueous pseudocapacitor system. The specific electrode configuration with ErOOH colloids grown among the carbon black/PVDF matrix can create short ion diffusion and electron transfer length to enable the fast and reversible Faradaic reactions. This work shows promising for finding high-performance electrical energy storage systems via designing the colloidal state of electroactive cations with the utilization of in-situ crystallization route.
Keywords: ErCl3, inorganic salt pseudocapacitor, electroactive colloid, chemical coprecipitation, supercapacitor 1
1. Introduction Energy storage is vital for the development of numerous fields including portable electronic devices, medical devices, transport vehicles, and stationary energy resources [1,2]. The two major charge storage systems that impact the growth of electrical energy storage field are batteries and supercapacitors. Compared to lithium ion batteries, supercapacitors offer unique advantages of high power density and exceptional cycle life [3,4]. The current main electrode materials for supercapacitors include carbon-based materials, conductive polymer and metal oxides [5,6]. However, the main bottleneck that hinders the practical applications of supercapacitors is their low energy density. To solve this problem, maximizing the specific capacitance and/or the cell voltage can achieve the increase in energy density [7]. One of the promising approaches for the improvement of energy density is to develop transition metal oxides or hydroxides as electrode materials, which can exhibit typical pseudocapacitive behavior. Because that their fast and reversible surface Faradaic redox reaction, transition metal oxides or hydroxides can significantly enhance their specific capacitances [8]. It has been demonstrated that the pseudocapacitance of transition metal oxides or hydroxides is much higher than that of carbon-based materials behaving electric double-layer capacitance [8]. To extract the maximum pseudocapacitance functionality of metal oxides/hydroxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures [9]. Expressing the metal oxide/hydroxides in a nanoscale form often 2
enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge [9, 10]. For example, the hybrid electrode with MnO2 nanocrystal deposited on nanoporous gold structures displayed a specific capacitance of ∼1145 F/g that is close to its theoretical value (∼1370 F/g) due to the enhanced conductivity [11]. However, most synthetic pseudocapacitive materials often show low specific capacitances compared with their theoretical value and display much low material utilization ratio. In order to increase the specific capacitance of pseudocapacitor, redox-based metal cations of pseudocapacitive electrode materials must be fully utilized in Faradaic reaction. It is because that the pseudocapacitive behavior is associated primarily with the redox reactions of the cations or changes in oxidation states of the cations in electrode materials during operation [12]. Recently, we have explored the pseudocapacitance of commercial water-soluble inorganic salts with multiple valence state metal ions in aqueous alkaline electrolyte [13-16]. It is interesting that the commercial metal salts electrode displayed highly electrochemical activity in KOH electrolyte and showed ultrahigh specific capacitance. Water-soluble inorganic salts including many free active cations are promising candidates to study the pseudocapacitance of cations. Another significant advantage of water-soluble salts pseudocapacitors is that the use of commercial inorganic salt as pseudocapacitor does not need complex synthesis procedures [17-19]. During the study of the inorganic salt pseudocapacitors, the in-situ crystallized colloids during the electrochemical reaction can deliver high specific capacitance. However, the detail mechanism of high 3
capacitance was not fully understood. Erbium compound materials, sun as ErOOH and Er2O3, are recognized as one of the most important rare earth oxides for sensing applications in the lasers and optical amplifiers, due to intra Er3+ 4f shell transition from its first excited state (4I3/2) to the ground state (4I5/2) [20]. Recently, flaky rare earth oxide including Er2O3 was used as additives on nickel electrodes and their effects on high temperature performance of nickel electrodes were investigated [21]. The result proved that flaky rare earth oxides increased the oxygen evolution potential and improved the reversibility of nickel electrodes. To the best of our knowledge, Er-based supercapacitors have not been reported. In one case, Er(OH)3 was used to increase structural and electrochemical stability of Co-Al layered double hydroxide pseudocapacitor [22]. Less soluble of Er(OH)3 can adhere to the Co-Al layered double hydroxide more strongly, stop the directly contact of OH−, and thus inhibit leaking process of Al3+. Herein, we designed an in-situ crystallization route for ErCl3 salts pseudocapacitors in KOH aqueous electrolyte. In our designed system, ErOOH colloids
were
in-situ
crystallized
through
electric-field
assisted
chemical
coprecipitation. These electroactive ErOOH colloids absorbed by carbon black and PVDF matrix were highly redox-reactive and delivered high specific capacitance values of 1811 F/g exceeding the one-electron redox theoretical capacitance (Er3+ ↔ Er2+). This work demonstrated that ErCl3 alkaline aqueous pseudocapacitors can Er2+), two-electron (Er2+ ↔ Er) or three-electron (Er3+ ↔ Er) produce one-electron (Er3+ ↔ reactions. 4
2. Experimental Electrode preparation and test: The commercial ErCl3⋅6H2O salts were directly used without further purification. The supercapacitor electrodes were prepared by mixing 70 wt% ErCl3⋅6H2O salts, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) and dissolving in N-methyl-2-pyrrolidone (NMP) solution. Briefly, the resulting slurry was spread on nickel foam current collector with an area of 1×1 cm2. The electrodes were dried at 80 °C for 24 h, and finally pressed at 10 MPa and served as working electrode. The loading of each electrode is ∼4-5 mg. 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) electrode was used as the reference electrode, and Pt wire electrode as a counter electrode. Characterization. To observe surface morphologies of ErCl3⋅6H2O salts electrodes before and after electrochemical tests, scanning electron microscope was performed by a field-emission scanning electron microscope (FESEM, Hitachi-S4800) at acceleration voltage of 10kV. Chemical compositions of integrated electrodes were recorded using powder X-ray diffractometer (XRD, Rigaku-D/max 2500 V).
3. Results and discussion 5
Scheme 1 shows the fabrication process of ErCl3 electrode. ErCl3 electrodes were prepared by pasting a slurry mixture of commercial ErCl3⋅6H2O salts, carbon black, and poly(vinylidene fluoride) (PVDF) on a Ni foam, which did not need the additional complex synthesis procedures for active materials. The present results challenge the view that the synthesis of advanced materials is the key step to increase the specific capacitance of inorganic pseudocapacitors. When the working electrodes were measured in 2 M KOH electrolyte, electroactive ErOOH colloids were in-situ formed by electric-field assisted chemical coprecipitation. At the same time, pseudocapacitive Faradaic reaction was occurred at the same electrode. The in-situ crystallized ErOOH colloids, which subsequently integrated into practical electrode structures, can enhance electrochemical utilization ratio of active Er3+ cations. Therefore, the electrode can show significantly high electrochemical reactivity and can deliver ultrahigh specific capacitance of 1811 F/g behaving the pseudocapacitive mechanism (redox reaction). Furthermore, in our designed inorganic salt-alkaline electrolyte system, the highly reactive ErOOH colloids interlaced with carbon spheres were formed by the coupled in-situ chemical/electrochemical crystallizations and electrochemical Faradaic reactions. The specific electrode configuration can shorten diffusion length of ion/electron and can efficiently utilize the active Er cations in salt electrode, thus high specific capacitance was obtained. To evaluate the electrochemical performances of ErCl3 electrodes, galvanostatic charge-discharge and CV tests were performed in 2 M KOH electrolyte. The galvanostatic charge-discharge curves at different current densities of 3-30 A/g are 6
shown in Fig. 1a. According to the discharge curves under galvanostatic conditions, the high specific capacitance of 1811 F/g at the current density of 3 A/g and the potential window of 0.55 V was obtained on the basis of the weight of Er3+ ions. The non-linear charge-discharge curves indicate the pseudocapacitive mechanism (Fig. 1a) [23]. The present results proved that our designed inorganic salt-alkaline electrolyte system can display highly electrochemical activity toward pseudocapacitive reaction. Traditionally, the specific capacitance of an electrode material is calculated on the basis of the weight of active materials. According to the physical origin of electrochemical energy storage, the pseudocapacitive behavior is associated primarily with the redox reactions of the cations or changes in oxidation states of the cations in electrode materials during operation [12]. Therefore, specific capacitance calculated from Er3+ cations can more deeply reflect their real redox mechanism. The specific capacitance (Cs) values can be theoretically calculated by the integration of discharge curve, Cs =
2 I ⋅ ∫ Vdt m ⋅ ΔV 2
where I is the current used for charge-discharge in A,
(1)
∫ Vdt
is the intergration area of
discharge curve, m is the mass of Er3+ in g, and ΔV is the potential interval of the discharge. Generally, the above equation can also be simplified as the following equation: Cs = IΔt/mΔV
(2)
where I (A) is the current used for discharge, Δt (s) is the time elapsed for the discharge cycle, m (g) is the mass of Er3+ ions or ErCl3⋅6H2O salts, and ΔV is the 7
voltage interval of the discharge. The mass of Er3+ ions can be calculated according to the mass of ErCl3⋅6H2O salts from equation: m(Er3+) = m(ErCl3⋅6H2O) ×M(Er3+) / M (ErCl3⋅6H2O), where M(Er3+) and M(ErCl3⋅6H2O) represent the molar mass of Er3+ and ErCl3⋅6H2O salt. Fig. 1b shows the specific capacitances vs. current density and the weight of ErCl3⋅6H2O salts or Er3+ ions. The specific capacitance values of Er cations are 1811, 1620, 1449, 1202, 858, 572, and 130 F/g at the current densities of 3, 5, 7, 10, 15, 20, 30 A/g and a potential interval of 0.55 V. The specific capacitances decrease with the increase of current densities, which is mainly due to the limited accessible regions for ion diffusion with increasing current densities [24]. Because the loading of each electrode is ∼4-5 mg, the high capacitance values of ErCl3 electrodes are thus not limited to their low loading weight compared to thin-film electrodes with high capacitance. In lanthanides, element with empty (4f0), half-full (4f7), full (4f14) 4f electron can form stable +3 oxidation state. Er has electron configuration of 4f126s2, and Er3+ has electron configuration of 4f11. Therefore, except for Er3+, Er can form different valences (Er2+, Er+) by redox reaction (Table 1). Recently, Er2+ was reported and had 4f115d1 ground-state configurations in molecular complexes [25]. Table 1 shows the Faradaic reactions, reduction potentials and theoretical capacitances of Er cations. Theoretical specific capacitance of active metal cations can be calculated according to the equation of Cm = Q/(V×M), where Q= 9.632 ×104 C for transferring one electron, M is molecular weight of Er ion (M = 167.3 g/mol), V is the operating voltage 8
window (0.55 V). Er cations have the theoretical capacitance of ∼1047 F/g as the transfer of one electron (Er3+ ↔Er2+) over a potential window of 0.55 V. The two-electron redox reaction (Er3+ ↔Er+) can deliver capacitance of ∼2094 F/g. The obtained practical capacitance of 1811 F/g exceeds one-electron theoretical reaction value of 1047 F/g. The utilization ratio of Er cations in two-electron redox reaction is calculated as 86 %. The results proved that the present Faradaic reaction in our designed ErCl3 pseudocapacitor can transfer more than one electron. The one-electron Er2+, two-electron Er2+ ↔ Er, or three-electron Er3+ ↔ Er redox reactions, can take Er3+ ↔ place in our designed ErCl3 pseudocapacitor (Table 1). These results demonstrated that specific capacitance calculated from Er3+ cations can more deeply reflect their real redox mechanism. To further demonstrate the charge storage mechanism of ErCl3 electrodes, CV measurement were performed. Fig. 2a shows CV curves of ErCl3 electrodes at a scan rate of 5 mV/s and a potential range of −0.1-0.45V. One pair of cathodic and anodic peaks was clearly observed in CV curves, suggesting that the capacitance of ErCl3 electrode mainly originates from the Faradaic reactions. The standard reduction potentials of Er in acid solution with respect to the standard hydrogen electrode are shown as following [26]: Er3+ + e− ↔Er
E0= −2.0 V
(3)
Er2+ + e− ↔ Er
E0= −2.33 V
(4)
Er3+ + e− ↔Er2+
E0= −3.0 V
(5)
In alkaline solution, the following reaction can take place. 9
Er(OH)3 + 3e− →Er + 3OH−
E0= −2.75V
(6)
However, the reduction reaction of Er3+ to Er is irreversible. Actually, the redox potential differs from standard potential values due to the existence of polarization. In addition, different chemical environments, such as electrolyte concentration, pH value, ligand concentration, can affect redox potential of electrode. In our ErCl3 alkaline aqueous electrolyte system, we believed that the redox reactions, including one-electron Er3+ ↔Er2+, two-electron Er2+ ↔Er, or three-electron Er3+ ↔Er reactions, can take place in the ErCl3 pseudocapacitor. Fig. 2b shows CV curves of ErCl3 salt electrodes at different scan rates. With the increase of scan rate, the intensities of redox peaks were increased. The reduction peak potentials shift to low potential direction from 0.14 V at 5 mV/s to 0.1 V at 50 mV/s, while the oxidation peak potentials shift to high potential from 0.4 to 0.45 V (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 [27]. Good cycling stability is another important characteristic for high performance supercapacitor. Fig. 3 shows cycling performance of the inorganic ErCl3 salt electrodes at a current density of 10 A/g and a potential interval of 0.55V for 1000 continuing charging/discharging cycles. After 1000 cycles, 85% of the initial capacitance can be retained, indicating long-term lifetime of ErCl3 salt pseudocapacitors. To confirm the physical and chemical changes of the ErCl3 electrodes, we performed XRD and SEM characterizations. XRD pattern of ErCl3 electrode after 10
electrochemical measurement displays the formation of ErOOH colloids via electric field assisted chemical coprecipitation of ErCl3 (Fig. 4). The very weak characteristic peaks in the XRD pattern indicate that the as-obtained ErOOH colloids are in poorly crystallized state. Recent report has proven that the colloidal Er(OH)3 precipitates were formed at room temperature, which also shows poor-crystalline state [28]. It has been reported that the poor-crystallized materials can show high specific capacitance compared to their crystallized materials [29]. The synthesized ErOOH colloids showed highly reactive and can provide more active cations towards Faradaic reaction. Fig. 5 shows SEM images of ErCl3 electrode before and after electrochemical measurements. Carbon black used as conductive agent in electrode preparation has spherical structures with size of about 50 nm (Fig. 5e and f). These carbon black spheres were loosely aggregated with porous structures. During the electrode preparation process, ErCl3 salts were mixed with carbon black and PVDF to form slurry. Then, slurry was dispersed on Ni current collector. SEM images of ErCl3 electrode before electrochemical measurements showed that ErCl3 salts were uniformly mixed with carbon and PVDF matrix (Fig. 5c and d). The loosely aggregated carbon spherical structures disappeared. Instead, ErCl3 salts were filled in interspace between carbon particles and adsorbed at the surface of carbon spheres. After electrochemical measurements, the size of spherical structures increases due to the adsorbed ErOOH colloids (Fig. 5a). During the phase transformation from ErCl3 to ErOOH, the volume change was occurred. Therefore, some porous can be found at 11
the surface of electrode (Fig. 5b). Thus, carbon spheres with ErOOH colloids at their surfaces were overlapped and formed link-like structures. The specific electrode configuration can shorten the electron transfer length and ion diffusion pathway, which can significant enhance the electrochemical performance of ErCl3 electrodes. Scheme 2 shows schematic drawing of the reaction process and electrode configuration of ErCl3 pseudocapacitor. When the as-prepared ErCl3 electrode was dipped into KOH electrolyte, ErCl3 can be firstly reacted with KOH to form Er(OH)3 within the electrode due to the large Ksp value (1.3 ×10−23). Er(OH)3 ↔ Er3+ + 3OH
−
−
−
Ksp = [Er3+][ OH ]3 = 1.3 ×10 23
Er(OH)3 ↔ErOOH + H2O
(7) (8)
Then, Er(OH)3 were transformed to ErOOH by dehydrated reactions occurred in the strong alkaline condition (Scheme 2a). At the same time, the chemical coprecipitation underwent electrochemical reactions. Two-layer ErCl3/ErOOH structures were further formed by electric-field assisted chemical coprecipitation (Scheme 2b). The in-situ formed ErOOH colloids showed highly electrochemical activity in alkaline electrolyte. Finally, ErOOH colloids were grown within the carbon black/PVDF matrix (Scheme 2c, d). It is most intriguing that the formation of electroactive ErOOH colloids and the production of pseudocapacitance (Faradaic reaction) occurred at the same time and electrode. The small size of ErOOH colloids and the specific configuration can assure the fast electron and ion transfer, thus the whole ErOOH colloid can be utilized in redox reaction. Furthermore, we studied the effect of crystallization kinetics in air on the 12
electrochemical performance of ErCl3 salts electrodes. Comparisons of CV curves, charge/discharge curves and specific capacitances of ErCl3 electrodes with different aging times are shown in Fig. 6. A pair of redox peaks can be found in all CV curves (Fig. 6a). The larger area of CV curves at a particular scan rate indicates the higher capacitance; the significant enhancement in capacitive performances was obtained with the increase of aging time. The similar outlines of charge/discharge curves indicate the same pseudocapacitive mechanism in these electrodes with different aging times (Fig. 6b). The specific capacitances of Er cations are 80, 940, 1024 and 1278 F/g at the aging time of 1, 3, 5 and 7 days (Fig. 5c). The specific capacitances are 35, 412, 449 and 560 F/g at the aging time of 1, 3, 5 and 7 days according to the weight of ErCl3 salts. The aging time affects both chemical and electrochemical reactions during operation. Fig. 7 shows SEM images of ErCl3 electrodes with different aging times after electrochemical measurements. SEM images show that ErCl3 electrode with aging time of 1 day has loose structure and the particles are separated (Fig. 7a and b). With aging time increased to 3-7 day, these particles were closely contacted with each other, which indicated the active ErOOH colloids having firm connection with conductive carbon. The specific electrode configuration can shorten electron transfer length between ErOOH colloids and conductive carbon, which can favor the increase of electrochemical performance. Therefore, the electrode with aging time of 1 day only shows low specific capacitance. In our designed inorganic salt-alkaline electrolyte system, the highly reactive 13
ErOOH colloids were in-situ crystallized within carbon/PVDF matrix by electric-field assisted chemical coprecipitation. However, the active materials obtained through traditional material synthesis process need to blend with conductive carbon and binder to fabricate electrodes, which cannot form the similar electrode configuration in our work. Therefore, our inorganic salt pseudocapacitors can efficiently utilize the active Er cations and can exhibit high capacitance.
4. Conclusions In summary, the results reported herein highlight a new ErCl3 alkaline aqueous pseudocapacitor system for the development of easily prepared and high-performance supercapacitors. The electrode materials, which were highly redox-reactive ErOOH colloids, were in-situ crystallized through electric field assisted chemical coprecipitation by designing commercial ErCl3 salt electrode in alkaline aqueous electrolyte. It showed significant predominance that the electrode materials synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale, which combines separate material-synthesis and electrode-preparation processes used in traditional procedure. The in-situ formed ErOOH colloids can show higher Er cations utilization ratio of 86 % and deliver high specific capacitance of 1811 F/g, which exceeds one-electron capacitance value (Er3+ ↔ Er2+). Thus, two-electron (Er2+ ↔ Er) or three-electron (Er3+ ↔ Er) reactions can occur in our
designed
ErCl3
alkaline
aqueous
pseudocapacitor
system.
The
high
electrochemical activity of ErCl3 alkaline aqueous pseudocapacitor system 14
contributed from the in-situ formed specific electrode configuration, ErOOH colloids grown among the carbon black/PVDF matrix, which can create short ion diffusion and electron transfer paths to enable the fast and reversible Faradaic reactions. This work demonstrated a new supercapacitor system, including in-situ formed reactive electrode materials and the coupled alakline electrolyte, which can bring new direction for designing high-performance electric energy storage systems through a low-cost, convenient way.
Acknowledgements 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. 20921002 and 21221061), and Hundred Talents Program of Chinese Academy of Science is acknowledged. References [1] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 25 (2013) 3302. [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] S. Kondrat, P. Wu, R. Qiao, A. A. Kornyshev, Nat. Mater. 13 (2014) 387. [4] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (2014) 1210. [5] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [6] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [7] Y. Wang, Y. Xia, Adv. Mater. 25 (2013) 5336. 15
[8] Q. Lu, J.G. Chen, J.Q. Xiao, Angew. Chem. Int. Ed. 52 (2013) 1882. [9] M. B. Sassin, C. N. Chervin, D. R. Rolison, J. W. Long, Acc. Chem. Res. 46 (2013) 1062. [10] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [11] X. Lang, A. Hirata, T. Fujita, M. Chen, Nat. Nanotechnol. 6 (2011) 232. [12] B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Academic, New York, 1999. [13] K. Chen, D. Xue, J. Colloid Interface. Sci. 416 (2014) 172. [14] K. Chen, Y. Yang, K. Li, Z. Ma, Y. Zhou, D. Xue, ACS Sustain. Chem. Eng. 2 (2014) 440. [15] K. Chen, D. Xue, J. Colloid Interface. Sci. 424 (2014) 84. [16] K. Chen, S. Song, K. Li, D. Xue, CrystEngComm 15 (2013) 10367. [17] K. Chen, D. Xue, CrystEngComm 16 (2014) 4610. [18] K. Chen, S. Song, D. Xue, RSC Adv. 2014, DOI: 10.1039/c4ra03037k. [19] X. Chen, K. Chen, H. Wang, S. Song, D. Xue, CrystEngComm 16 (2014) DOI: 10.1039/C4CE00660G. [20] T. Nguyen, C. Dinh, T. Do, ACS Nano 4 (2010) 2263. [21] Q. Fang, Y. Cheng, X. Jian, L. Zhu, H. Yu, Z. Wang, L. Jiang, J. Rare Earths 28 (2010) 72. [22] M. Hua, X. Ji, L. Lei, X. Lu, Electrochim. Acta 105 (2013) 261. [23] D. Sarkar, G. G. Khan, A. K. Singh, K. Mandal, J. Phys. Chem. C 117 (2013) 16
15523. [24] L. Cao, F. Xu, Y. Liang, H. Li, Adv. Mater. 16 (2004) 1853. [25] M. R. MacDonald, J. E. Bates, M. E. Fieser, J. W. Ziller, F. Furche, W. J. Evans, J. Am. Chem. Soc. 134 (2012) 8420. [26] G. Milazzo, S. Caroli, and V. K. Sharma, Tables of Standard Electrode Potentials, Wiley, Chichester, 1978. [27] P. Lu, F. Liu, D. Xue, H. Yang, Y. Liu, Electrochim. Acta 78 (2012) 1. [28] H. Assaaoudi, Z. Fang, J. E. Barralet, A. J. Wright, I. S. Butler, J. A. Kozinski, Nanotechnology 18 (2007) 445606. [29] W. Wei, X. Cui, W. Chen, D. G. Ivey, Chem. Soc. Rev. 40 (2011) 1697.
17
Scheme 1. Schematic drawing shows in-situ fabrication of ErCl3 pseudocapacitor. Firstly, the electrode was fabricated with the use of commercial ErCl3⋅6H2O salts by slurry-coating manufacturing. Then, ErOOH colloids were in-situ crystallized by electric-field assisted chemical coprecipitation, and subsequently integrated into practical electrode structures. At the same time, pseudocapacitive Faradaic reaction 18
was occurred at the same electrode. In-situ crystallized ErOOH colloids can enhance electrochemical utilization of active Er cations.
19
a
Potential (V vs. SCE)
0.4 0.3 0.2
3A/g 5A/g 7A/g 10A/g 15A/g 20A/g 30A/g
0.1 0.0 -0.1
0
50
Specific capacitance (F/g)
b
100 150 200 250 300 350 Time (s)
1500
3+
Weight of Er ion Weight of ErCl3 salt
1000
500
0 0
5
10
15
20
25
30
Current density (A/g) Fig.
1
Electrochemical
performance
of
ErCl3
pseudocapacitor.
(a)
The
charge/discharge curves (time versus potential) measured at various current densities and a potential range of 0.55 V. (b) Specific capacitance of inorganic ErCl3 salt electrode versus discharge current density at potential window of 0.55 V on the basis of the weight of ErCl3⋅6H2O salt and Er3+ ion. All data are taken in a 2 M KOH electrolyte at room temperature.
20
Table 1 Electron configuration, Faradaic reaction, reduction potential and theoretical capacitance of Er cations Active
Electron
ion
configuration
Faradaic reaction
(V vs. SHE)[26]
capacitance (F/g)
Er3+ + e ↔ Er2+
−3.0
1047 (0.55V)
Er3+ + 2e− ↔ Er+
-
2094 (0.55V)
Er3+ + 3e ↔ Er
−2.33
3141 (0.55V)
−2.0
2094 (0.55V)
-
1047 (0.55V)
−
Er3+
[Xe]4f12
Reduction potential Theoretical
−
Er2+
[Xe]4f115d1 [25]
Er2+ + 2e− ↔ Er
Er+
[Xe]4f126s1
Er+ + e ↔ Er
−
SHE = standard hydrogen electrode
21
a Oxidation
Current (A/g)
10
Er2+ Er3+ + e− ↔ 0
-10
Reduction -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) b
Current (A/g)
20 5mV/s 10mV/s 30mV/s 50mV/s
10 0 -10 -20 -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) Fig. 2 (a) CV curves (current density versus potential) of the inorganic ErCl3 salt electrodes at a scan rate of 5 mV/s. (b) CV curves of the inorganic ErCl3 salt electrodes at different scan rates. The present of the redox peaks in the CV curves confirms the occurrence of pseudocapacitive reaction in this electrode. The CV curve was obtained at the potential range of −0.1-0.45 V. All data are taken in 2M KOH electrolyte at room temperature. 22
400 350 Potential (V vs. SCE)
Specific capacitance (F/g)
450
300 250 200 150 100 50
0.4 0.3 0.2 0.1 0.0 -0.1
0
20
40
60
80
Time (s)
0
200
400
600
800
1000
Cycle number
Fig. 3 Cycling stability of the inorganic ErCl3 salt electrodes at a current density of 10 A/g and a potential interval of 0.55V. The inset shows the last five charge/discharge curves. After 1000 continuing charging-discharging cycles, 85% of initial capacitance can be retained.
23
Intensity (a.u.)
ErOOH
10
15
20
25
30
35
2θ (degree)
40
45
Ni
50
55
Fig. 4 XRD patterns of ErCl3 electrodes after electrochemical measurements. The standard JCPDS No.76-700 for ErOOH and JCPDS No. 1-1258 for Ni current collector are also indicated in the graph.
24
a
b
200 nm
500 nm
c
d
200 nm
500 nm
e
f
200 nm
500 nm
Fig. 5 (a-d) SEM images of ErCl3 electrodes after and before electrochemical measurements. (a and b) high-magnification and low-magnification SEM images after electrochemical measurements. (c and d) SEM images of ErCl3 electrode before electrochemical measurements. (e and f) SEM images of carbon black used as conductive agent in electrode preparation. After electrochemical reaction, carbon matrix-constrained ErOOH colloids electrode configuration can be confirmed.
25
Scheme 2 Schematic drawing shows the reaction process and electrode configuration of ErCl3 pseudocapacitor. (a) Initial electric field assisted chemical coprecipitation. (b) Two-layer ErCl3/ErOOH structures were further formed by electric field assisted chemical coprecipitation. (c) Final formed ErOOH colloids, which can take part in Faradaic redox reaction immediately. (d) Carbon matrix-constrained ErCl3 electrodes. The small size of colloids and specific configuration can assure the fast electron and ion transfer, thus the whole ErOOH colloid can be utilized to occur redox reaction.
26
a
Current (A/g)
20 1 day 3 day 5 day 7 day
10
0
-10
-20 -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE)
b Potential (V vs. SCE)
0.4 0.3 0.2 1 day 3 day 5 day 7 day
0.1 0.0 -0.1
0
20 40 60 80 100 120 140 160 180 200 220
c
Time (s)
Specific capacitance (F/g)
1400 1200 1000 800 600 400 3+
Weight of Er ion Weight of ErCl3 salt
200 0 1
2
3
4
5
6
7
Aging time (day)
Fig. 6 Electrochemical performance of ErCl3 pseudocapacitors with different aging times. (a) CV curves of the inorganic ErCl3 salt electrodes at a scan rate of 30 mV/s. (b) The charge/discharge curves measured at a current density of 3A/g and the potential range of 0.55 V. The charge/discharge curves of one day measured at a current density of 1A/g. (c) Specific capacitances of inorganic ErCl3 salt electrodes versus discharge current density at a potential window of 0.55V according to the weight of ErCl3⋅6H2O salt and Er3+ ion. The specific capacitance of electrode with aging time of one day was measured at current density of 1A/g. All data are taken in a 27
2 M KOH electrolyte at room temperature.
a2
a1
200 nm
200 nm b2
b1
500 nm
200 nm c2
c1
200 nm
500 nm d2
d1
200 nm
500 nm
Fig. 7 SEM images of ErCl3 electrodes with different aging times after electrochemical measurements: (a1 and a2) 1 day, (b1 and b2) 3 day, c1 and c2) 5 day, and (d1 and d2) 7 day. ErCl3 electrode with aging time of 1 day has loose structure and the particles are separated, while particles in electrodes with aging time of 3-7 days are closely contacted with each other.
28
Graphical abstract
Formation of electroactive colloids via in-situ coprecipitation under electric field: erbium chloride alkaline aqueous pseudocapacitor Kunfeng Chen, Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China *E-mail: [email protected].
Highlights
Novel ErCl3-KOH aqueous pseudocapacitor system was demonstrated. In-situ crystallization of ErCl3 in alkaline electrolyte leads to highly electroactive ErOOH colloids. Electric field assisted chemical coprecipitation and Faradaic redox reactions occur at same time and same electrode. ErCl3 pseudocapacitors can show ultrahigh capacitance of 1811 F/g.
29
S0021-9797(14)00367-1 http://dx.doi.org/10.1016/j.jcis.2014.05.053 YJCIS 19605
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
9 April 2014 23 May 2014
Please cite this article as: K. Chen, D. Xue, Formation of electroactive colloids via in-situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.053
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Formation of electroactive colloids via in-situ coprecipitation under electric field: Erbium chloride alkaline aqueous pseudocapacitor Kunfeng Chen and Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. *Corresponding author at: State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street, No. 5625, Changchun 130022, China. Tel: +86 431 85262294. E-mail: [email protected].
Abstract For the first time, a new ErCl3 alkaline aqueous pseudocapacitor system was demonstrated by designing commercial ErCl3 salt electrode in alkaline aqueous electrolyte, where the materials synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale. Highly electroactive ErOOH colloids were in-situ crystallized via electric field assisted chemical coprecipitation of ErCl3 in KOH aqueous electrolyte. These electroactive ErOOH colloids absorbed by carbon black and PVDF matrix were highly redox-reactive with higher cation utilization ratio of 86 % and specific capacitance values of 1811 F/g, Er2+). We believe that exceeding the one-electron redox theoretical capacitance (Er3+ ↔ additional two-electron (Er2+ ↔Er) or three-electron (Er3+ ↔Er) reactions can occur in our designed ErCl3 alkaline aqueous pseudocapacitor system. The specific electrode configuration with ErOOH colloids grown among the carbon black/PVDF matrix can create short ion diffusion and electron transfer length to enable the fast and reversible Faradaic reactions. This work shows promising for finding high-performance electrical energy storage systems via designing the colloidal state of electroactive cations with the utilization of in-situ crystallization route.
Keywords: ErCl3, inorganic salt pseudocapacitor, electroactive colloid, chemical coprecipitation, supercapacitor 1
1. Introduction Energy storage is vital for the development of numerous fields including portable electronic devices, medical devices, transport vehicles, and stationary energy resources [1,2]. The two major charge storage systems that impact the growth of electrical energy storage field are batteries and supercapacitors. Compared to lithium ion batteries, supercapacitors offer unique advantages of high power density and exceptional cycle life [3,4]. The current main electrode materials for supercapacitors include carbon-based materials, conductive polymer and metal oxides [5,6]. However, the main bottleneck that hinders the practical applications of supercapacitors is their low energy density. To solve this problem, maximizing the specific capacitance and/or the cell voltage can achieve the increase in energy density [7]. One of the promising approaches for the improvement of energy density is to develop transition metal oxides or hydroxides as electrode materials, which can exhibit typical pseudocapacitive behavior. Because that their fast and reversible surface Faradaic redox reaction, transition metal oxides or hydroxides can significantly enhance their specific capacitances [8]. It has been demonstrated that the pseudocapacitance of transition metal oxides or hydroxides is much higher than that of carbon-based materials behaving electric double-layer capacitance [8]. To extract the maximum pseudocapacitance functionality of metal oxides/hydroxides, one must carefully consider how they are synthesized and subsequently integrated into practical electrode structures [9]. Expressing the metal oxide/hydroxides in a nanoscale form often 2
enhances electrochemical utilization (maximizing specific capacitance) and facilitates high-rate operation for both charge and discharge [9, 10]. For example, the hybrid electrode with MnO2 nanocrystal deposited on nanoporous gold structures displayed a specific capacitance of ∼1145 F/g that is close to its theoretical value (∼1370 F/g) due to the enhanced conductivity [11]. However, most synthetic pseudocapacitive materials often show low specific capacitances compared with their theoretical value and display much low material utilization ratio. In order to increase the specific capacitance of pseudocapacitor, redox-based metal cations of pseudocapacitive electrode materials must be fully utilized in Faradaic reaction. It is because that the pseudocapacitive behavior is associated primarily with the redox reactions of the cations or changes in oxidation states of the cations in electrode materials during operation [12]. Recently, we have explored the pseudocapacitance of commercial water-soluble inorganic salts with multiple valence state metal ions in aqueous alkaline electrolyte [13-16]. It is interesting that the commercial metal salts electrode displayed highly electrochemical activity in KOH electrolyte and showed ultrahigh specific capacitance. Water-soluble inorganic salts including many free active cations are promising candidates to study the pseudocapacitance of cations. Another significant advantage of water-soluble salts pseudocapacitors is that the use of commercial inorganic salt as pseudocapacitor does not need complex synthesis procedures [17-19]. During the study of the inorganic salt pseudocapacitors, the in-situ crystallized colloids during the electrochemical reaction can deliver high specific capacitance. However, the detail mechanism of high 3
capacitance was not fully understood. Erbium compound materials, sun as ErOOH and Er2O3, are recognized as one of the most important rare earth oxides for sensing applications in the lasers and optical amplifiers, due to intra Er3+ 4f shell transition from its first excited state (4I3/2) to the ground state (4I5/2) [20]. Recently, flaky rare earth oxide including Er2O3 was used as additives on nickel electrodes and their effects on high temperature performance of nickel electrodes were investigated [21]. The result proved that flaky rare earth oxides increased the oxygen evolution potential and improved the reversibility of nickel electrodes. To the best of our knowledge, Er-based supercapacitors have not been reported. In one case, Er(OH)3 was used to increase structural and electrochemical stability of Co-Al layered double hydroxide pseudocapacitor [22]. Less soluble of Er(OH)3 can adhere to the Co-Al layered double hydroxide more strongly, stop the directly contact of OH−, and thus inhibit leaking process of Al3+. Herein, we designed an in-situ crystallization route for ErCl3 salts pseudocapacitors in KOH aqueous electrolyte. In our designed system, ErOOH colloids
were
in-situ
crystallized
through
electric-field
assisted
chemical
coprecipitation. These electroactive ErOOH colloids absorbed by carbon black and PVDF matrix were highly redox-reactive and delivered high specific capacitance values of 1811 F/g exceeding the one-electron redox theoretical capacitance (Er3+ ↔ Er2+). This work demonstrated that ErCl3 alkaline aqueous pseudocapacitors can Er2+), two-electron (Er2+ ↔ Er) or three-electron (Er3+ ↔ Er) produce one-electron (Er3+ ↔ reactions. 4
2. Experimental Electrode preparation and test: The commercial ErCl3⋅6H2O salts were directly used without further purification. The supercapacitor electrodes were prepared by mixing 70 wt% ErCl3⋅6H2O salts, 20 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) and dissolving in N-methyl-2-pyrrolidone (NMP) solution. Briefly, the resulting slurry was spread on nickel foam current collector with an area of 1×1 cm2. The electrodes were dried at 80 °C for 24 h, and finally pressed at 10 MPa and served as working electrode. The loading of each electrode is ∼4-5 mg. 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) electrode was used as the reference electrode, and Pt wire electrode as a counter electrode. Characterization. To observe surface morphologies of ErCl3⋅6H2O salts electrodes before and after electrochemical tests, scanning electron microscope was performed by a field-emission scanning electron microscope (FESEM, Hitachi-S4800) at acceleration voltage of 10kV. Chemical compositions of integrated electrodes were recorded using powder X-ray diffractometer (XRD, Rigaku-D/max 2500 V).
3. Results and discussion 5
Scheme 1 shows the fabrication process of ErCl3 electrode. ErCl3 electrodes were prepared by pasting a slurry mixture of commercial ErCl3⋅6H2O salts, carbon black, and poly(vinylidene fluoride) (PVDF) on a Ni foam, which did not need the additional complex synthesis procedures for active materials. The present results challenge the view that the synthesis of advanced materials is the key step to increase the specific capacitance of inorganic pseudocapacitors. When the working electrodes were measured in 2 M KOH electrolyte, electroactive ErOOH colloids were in-situ formed by electric-field assisted chemical coprecipitation. At the same time, pseudocapacitive Faradaic reaction was occurred at the same electrode. The in-situ crystallized ErOOH colloids, which subsequently integrated into practical electrode structures, can enhance electrochemical utilization ratio of active Er3+ cations. Therefore, the electrode can show significantly high electrochemical reactivity and can deliver ultrahigh specific capacitance of 1811 F/g behaving the pseudocapacitive mechanism (redox reaction). Furthermore, in our designed inorganic salt-alkaline electrolyte system, the highly reactive ErOOH colloids interlaced with carbon spheres were formed by the coupled in-situ chemical/electrochemical crystallizations and electrochemical Faradaic reactions. The specific electrode configuration can shorten diffusion length of ion/electron and can efficiently utilize the active Er cations in salt electrode, thus high specific capacitance was obtained. To evaluate the electrochemical performances of ErCl3 electrodes, galvanostatic charge-discharge and CV tests were performed in 2 M KOH electrolyte. The galvanostatic charge-discharge curves at different current densities of 3-30 A/g are 6
shown in Fig. 1a. According to the discharge curves under galvanostatic conditions, the high specific capacitance of 1811 F/g at the current density of 3 A/g and the potential window of 0.55 V was obtained on the basis of the weight of Er3+ ions. The non-linear charge-discharge curves indicate the pseudocapacitive mechanism (Fig. 1a) [23]. The present results proved that our designed inorganic salt-alkaline electrolyte system can display highly electrochemical activity toward pseudocapacitive reaction. Traditionally, the specific capacitance of an electrode material is calculated on the basis of the weight of active materials. According to the physical origin of electrochemical energy storage, the pseudocapacitive behavior is associated primarily with the redox reactions of the cations or changes in oxidation states of the cations in electrode materials during operation [12]. Therefore, specific capacitance calculated from Er3+ cations can more deeply reflect their real redox mechanism. The specific capacitance (Cs) values can be theoretically calculated by the integration of discharge curve, Cs =
2 I ⋅ ∫ Vdt m ⋅ ΔV 2
where I is the current used for charge-discharge in A,
(1)
∫ Vdt
is the intergration area of
discharge curve, m is the mass of Er3+ in g, and ΔV is the potential interval of the discharge. Generally, the above equation can also be simplified as the following equation: Cs = IΔt/mΔV
(2)
where I (A) is the current used for discharge, Δt (s) is the time elapsed for the discharge cycle, m (g) is the mass of Er3+ ions or ErCl3⋅6H2O salts, and ΔV is the 7
voltage interval of the discharge. The mass of Er3+ ions can be calculated according to the mass of ErCl3⋅6H2O salts from equation: m(Er3+) = m(ErCl3⋅6H2O) ×M(Er3+) / M (ErCl3⋅6H2O), where M(Er3+) and M(ErCl3⋅6H2O) represent the molar mass of Er3+ and ErCl3⋅6H2O salt. Fig. 1b shows the specific capacitances vs. current density and the weight of ErCl3⋅6H2O salts or Er3+ ions. The specific capacitance values of Er cations are 1811, 1620, 1449, 1202, 858, 572, and 130 F/g at the current densities of 3, 5, 7, 10, 15, 20, 30 A/g and a potential interval of 0.55 V. The specific capacitances decrease with the increase of current densities, which is mainly due to the limited accessible regions for ion diffusion with increasing current densities [24]. Because the loading of each electrode is ∼4-5 mg, the high capacitance values of ErCl3 electrodes are thus not limited to their low loading weight compared to thin-film electrodes with high capacitance. In lanthanides, element with empty (4f0), half-full (4f7), full (4f14) 4f electron can form stable +3 oxidation state. Er has electron configuration of 4f126s2, and Er3+ has electron configuration of 4f11. Therefore, except for Er3+, Er can form different valences (Er2+, Er+) by redox reaction (Table 1). Recently, Er2+ was reported and had 4f115d1 ground-state configurations in molecular complexes [25]. Table 1 shows the Faradaic reactions, reduction potentials and theoretical capacitances of Er cations. Theoretical specific capacitance of active metal cations can be calculated according to the equation of Cm = Q/(V×M), where Q= 9.632 ×104 C for transferring one electron, M is molecular weight of Er ion (M = 167.3 g/mol), V is the operating voltage 8
window (0.55 V). Er cations have the theoretical capacitance of ∼1047 F/g as the transfer of one electron (Er3+ ↔Er2+) over a potential window of 0.55 V. The two-electron redox reaction (Er3+ ↔Er+) can deliver capacitance of ∼2094 F/g. The obtained practical capacitance of 1811 F/g exceeds one-electron theoretical reaction value of 1047 F/g. The utilization ratio of Er cations in two-electron redox reaction is calculated as 86 %. The results proved that the present Faradaic reaction in our designed ErCl3 pseudocapacitor can transfer more than one electron. The one-electron Er2+, two-electron Er2+ ↔ Er, or three-electron Er3+ ↔ Er redox reactions, can take Er3+ ↔ place in our designed ErCl3 pseudocapacitor (Table 1). These results demonstrated that specific capacitance calculated from Er3+ cations can more deeply reflect their real redox mechanism. To further demonstrate the charge storage mechanism of ErCl3 electrodes, CV measurement were performed. Fig. 2a shows CV curves of ErCl3 electrodes at a scan rate of 5 mV/s and a potential range of −0.1-0.45V. One pair of cathodic and anodic peaks was clearly observed in CV curves, suggesting that the capacitance of ErCl3 electrode mainly originates from the Faradaic reactions. The standard reduction potentials of Er in acid solution with respect to the standard hydrogen electrode are shown as following [26]: Er3+ + e− ↔Er
E0= −2.0 V
(3)
Er2+ + e− ↔ Er
E0= −2.33 V
(4)
Er3+ + e− ↔Er2+
E0= −3.0 V
(5)
In alkaline solution, the following reaction can take place. 9
Er(OH)3 + 3e− →Er + 3OH−
E0= −2.75V
(6)
However, the reduction reaction of Er3+ to Er is irreversible. Actually, the redox potential differs from standard potential values due to the existence of polarization. In addition, different chemical environments, such as electrolyte concentration, pH value, ligand concentration, can affect redox potential of electrode. In our ErCl3 alkaline aqueous electrolyte system, we believed that the redox reactions, including one-electron Er3+ ↔Er2+, two-electron Er2+ ↔Er, or three-electron Er3+ ↔Er reactions, can take place in the ErCl3 pseudocapacitor. Fig. 2b shows CV curves of ErCl3 salt electrodes at different scan rates. With the increase of scan rate, the intensities of redox peaks were increased. The reduction peak potentials shift to low potential direction from 0.14 V at 5 mV/s to 0.1 V at 50 mV/s, while the oxidation peak potentials shift to high potential from 0.4 to 0.45 V (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 [27]. Good cycling stability is another important characteristic for high performance supercapacitor. Fig. 3 shows cycling performance of the inorganic ErCl3 salt electrodes at a current density of 10 A/g and a potential interval of 0.55V for 1000 continuing charging/discharging cycles. After 1000 cycles, 85% of the initial capacitance can be retained, indicating long-term lifetime of ErCl3 salt pseudocapacitors. To confirm the physical and chemical changes of the ErCl3 electrodes, we performed XRD and SEM characterizations. XRD pattern of ErCl3 electrode after 10
electrochemical measurement displays the formation of ErOOH colloids via electric field assisted chemical coprecipitation of ErCl3 (Fig. 4). The very weak characteristic peaks in the XRD pattern indicate that the as-obtained ErOOH colloids are in poorly crystallized state. Recent report has proven that the colloidal Er(OH)3 precipitates were formed at room temperature, which also shows poor-crystalline state [28]. It has been reported that the poor-crystallized materials can show high specific capacitance compared to their crystallized materials [29]. The synthesized ErOOH colloids showed highly reactive and can provide more active cations towards Faradaic reaction. Fig. 5 shows SEM images of ErCl3 electrode before and after electrochemical measurements. Carbon black used as conductive agent in electrode preparation has spherical structures with size of about 50 nm (Fig. 5e and f). These carbon black spheres were loosely aggregated with porous structures. During the electrode preparation process, ErCl3 salts were mixed with carbon black and PVDF to form slurry. Then, slurry was dispersed on Ni current collector. SEM images of ErCl3 electrode before electrochemical measurements showed that ErCl3 salts were uniformly mixed with carbon and PVDF matrix (Fig. 5c and d). The loosely aggregated carbon spherical structures disappeared. Instead, ErCl3 salts were filled in interspace between carbon particles and adsorbed at the surface of carbon spheres. After electrochemical measurements, the size of spherical structures increases due to the adsorbed ErOOH colloids (Fig. 5a). During the phase transformation from ErCl3 to ErOOH, the volume change was occurred. Therefore, some porous can be found at 11
the surface of electrode (Fig. 5b). Thus, carbon spheres with ErOOH colloids at their surfaces were overlapped and formed link-like structures. The specific electrode configuration can shorten the electron transfer length and ion diffusion pathway, which can significant enhance the electrochemical performance of ErCl3 electrodes. Scheme 2 shows schematic drawing of the reaction process and electrode configuration of ErCl3 pseudocapacitor. When the as-prepared ErCl3 electrode was dipped into KOH electrolyte, ErCl3 can be firstly reacted with KOH to form Er(OH)3 within the electrode due to the large Ksp value (1.3 ×10−23). Er(OH)3 ↔ Er3+ + 3OH
−
−
−
Ksp = [Er3+][ OH ]3 = 1.3 ×10 23
Er(OH)3 ↔ErOOH + H2O
(7) (8)
Then, Er(OH)3 were transformed to ErOOH by dehydrated reactions occurred in the strong alkaline condition (Scheme 2a). At the same time, the chemical coprecipitation underwent electrochemical reactions. Two-layer ErCl3/ErOOH structures were further formed by electric-field assisted chemical coprecipitation (Scheme 2b). The in-situ formed ErOOH colloids showed highly electrochemical activity in alkaline electrolyte. Finally, ErOOH colloids were grown within the carbon black/PVDF matrix (Scheme 2c, d). It is most intriguing that the formation of electroactive ErOOH colloids and the production of pseudocapacitance (Faradaic reaction) occurred at the same time and electrode. The small size of ErOOH colloids and the specific configuration can assure the fast electron and ion transfer, thus the whole ErOOH colloid can be utilized in redox reaction. Furthermore, we studied the effect of crystallization kinetics in air on the 12
electrochemical performance of ErCl3 salts electrodes. Comparisons of CV curves, charge/discharge curves and specific capacitances of ErCl3 electrodes with different aging times are shown in Fig. 6. A pair of redox peaks can be found in all CV curves (Fig. 6a). The larger area of CV curves at a particular scan rate indicates the higher capacitance; the significant enhancement in capacitive performances was obtained with the increase of aging time. The similar outlines of charge/discharge curves indicate the same pseudocapacitive mechanism in these electrodes with different aging times (Fig. 6b). The specific capacitances of Er cations are 80, 940, 1024 and 1278 F/g at the aging time of 1, 3, 5 and 7 days (Fig. 5c). The specific capacitances are 35, 412, 449 and 560 F/g at the aging time of 1, 3, 5 and 7 days according to the weight of ErCl3 salts. The aging time affects both chemical and electrochemical reactions during operation. Fig. 7 shows SEM images of ErCl3 electrodes with different aging times after electrochemical measurements. SEM images show that ErCl3 electrode with aging time of 1 day has loose structure and the particles are separated (Fig. 7a and b). With aging time increased to 3-7 day, these particles were closely contacted with each other, which indicated the active ErOOH colloids having firm connection with conductive carbon. The specific electrode configuration can shorten electron transfer length between ErOOH colloids and conductive carbon, which can favor the increase of electrochemical performance. Therefore, the electrode with aging time of 1 day only shows low specific capacitance. In our designed inorganic salt-alkaline electrolyte system, the highly reactive 13
ErOOH colloids were in-situ crystallized within carbon/PVDF matrix by electric-field assisted chemical coprecipitation. However, the active materials obtained through traditional material synthesis process need to blend with conductive carbon and binder to fabricate electrodes, which cannot form the similar electrode configuration in our work. Therefore, our inorganic salt pseudocapacitors can efficiently utilize the active Er cations and can exhibit high capacitance.
4. Conclusions In summary, the results reported herein highlight a new ErCl3 alkaline aqueous pseudocapacitor system for the development of easily prepared and high-performance supercapacitors. The electrode materials, which were highly redox-reactive ErOOH colloids, were in-situ crystallized through electric field assisted chemical coprecipitation by designing commercial ErCl3 salt electrode in alkaline aqueous electrolyte. It showed significant predominance that the electrode materials synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale, which combines separate material-synthesis and electrode-preparation processes used in traditional procedure. The in-situ formed ErOOH colloids can show higher Er cations utilization ratio of 86 % and deliver high specific capacitance of 1811 F/g, which exceeds one-electron capacitance value (Er3+ ↔ Er2+). Thus, two-electron (Er2+ ↔ Er) or three-electron (Er3+ ↔ Er) reactions can occur in our
designed
ErCl3
alkaline
aqueous
pseudocapacitor
system.
The
high
electrochemical activity of ErCl3 alkaline aqueous pseudocapacitor system 14
contributed from the in-situ formed specific electrode configuration, ErOOH colloids grown among the carbon black/PVDF matrix, which can create short ion diffusion and electron transfer paths to enable the fast and reversible Faradaic reactions. This work demonstrated a new supercapacitor system, including in-situ formed reactive electrode materials and the coupled alakline electrolyte, which can bring new direction for designing high-performance electric energy storage systems through a low-cost, convenient way.
Acknowledgements 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. 20921002 and 21221061), and Hundred Talents Program of Chinese Academy of Science is acknowledged. References [1] Z. Yu, B. Duong, D. Abbitt, J. Thomas, Adv. Mater. 25 (2013) 3302. [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] S. Kondrat, P. Wu, R. Qiao, A. A. Kornyshev, Nat. Mater. 13 (2014) 387. [4] P. Simon, Y. Gogotsi, B. Dunn, Science 343 (2014) 1210. [5] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845. [6] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089. [7] Y. Wang, Y. Xia, Adv. Mater. 25 (2013) 5336. 15
[8] Q. Lu, J.G. Chen, J.Q. Xiao, Angew. Chem. Int. Ed. 52 (2013) 1882. [9] M. B. Sassin, C. N. Chervin, D. R. Rolison, J. W. Long, Acc. Chem. Res. 46 (2013) 1062. [10] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, Y. Takasu, Angew. Chem. Int. Ed. 42 (2003) 4092. [11] X. Lang, A. Hirata, T. Fujita, M. Chen, Nat. Nanotechnol. 6 (2011) 232. [12] B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Academic, New York, 1999. [13] K. Chen, D. Xue, J. Colloid Interface. Sci. 416 (2014) 172. [14] K. Chen, Y. Yang, K. Li, Z. Ma, Y. Zhou, D. Xue, ACS Sustain. Chem. Eng. 2 (2014) 440. [15] K. Chen, D. Xue, J. Colloid Interface. Sci. 424 (2014) 84. [16] K. Chen, S. Song, K. Li, D. Xue, CrystEngComm 15 (2013) 10367. [17] K. Chen, D. Xue, CrystEngComm 16 (2014) 4610. [18] K. Chen, S. Song, D. Xue, RSC Adv. 2014, DOI: 10.1039/c4ra03037k. [19] X. Chen, K. Chen, H. Wang, S. Song, D. Xue, CrystEngComm 16 (2014) DOI: 10.1039/C4CE00660G. [20] T. Nguyen, C. Dinh, T. Do, ACS Nano 4 (2010) 2263. [21] Q. Fang, Y. Cheng, X. Jian, L. Zhu, H. Yu, Z. Wang, L. Jiang, J. Rare Earths 28 (2010) 72. [22] M. Hua, X. Ji, L. Lei, X. Lu, Electrochim. Acta 105 (2013) 261. [23] D. Sarkar, G. G. Khan, A. K. Singh, K. Mandal, J. Phys. Chem. C 117 (2013) 16
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17
Scheme 1. Schematic drawing shows in-situ fabrication of ErCl3 pseudocapacitor. Firstly, the electrode was fabricated with the use of commercial ErCl3⋅6H2O salts by slurry-coating manufacturing. Then, ErOOH colloids were in-situ crystallized by electric-field assisted chemical coprecipitation, and subsequently integrated into practical electrode structures. At the same time, pseudocapacitive Faradaic reaction 18
was occurred at the same electrode. In-situ crystallized ErOOH colloids can enhance electrochemical utilization of active Er cations.
19
a
Potential (V vs. SCE)
0.4 0.3 0.2
3A/g 5A/g 7A/g 10A/g 15A/g 20A/g 30A/g
0.1 0.0 -0.1
0
50
Specific capacitance (F/g)
b
100 150 200 250 300 350 Time (s)
1500
3+
Weight of Er ion Weight of ErCl3 salt
1000
500
0 0
5
10
15
20
25
30
Current density (A/g) Fig.
1
Electrochemical
performance
of
ErCl3
pseudocapacitor.
(a)
The
charge/discharge curves (time versus potential) measured at various current densities and a potential range of 0.55 V. (b) Specific capacitance of inorganic ErCl3 salt electrode versus discharge current density at potential window of 0.55 V on the basis of the weight of ErCl3⋅6H2O salt and Er3+ ion. All data are taken in a 2 M KOH electrolyte at room temperature.
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Table 1 Electron configuration, Faradaic reaction, reduction potential and theoretical capacitance of Er cations Active
Electron
ion
configuration
Faradaic reaction
(V vs. SHE)[26]
capacitance (F/g)
Er3+ + e ↔ Er2+
−3.0
1047 (0.55V)
Er3+ + 2e− ↔ Er+
-
2094 (0.55V)
Er3+ + 3e ↔ Er
−2.33
3141 (0.55V)
−2.0
2094 (0.55V)
-
1047 (0.55V)
−
Er3+
[Xe]4f12
Reduction potential Theoretical
−
Er2+
[Xe]4f115d1 [25]
Er2+ + 2e− ↔ Er
Er+
[Xe]4f126s1
Er+ + e ↔ Er
−
SHE = standard hydrogen electrode
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a Oxidation
Current (A/g)
10
Er2+ Er3+ + e− ↔ 0
-10
Reduction -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) b
Current (A/g)
20 5mV/s 10mV/s 30mV/s 50mV/s
10 0 -10 -20 -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE) Fig. 2 (a) CV curves (current density versus potential) of the inorganic ErCl3 salt electrodes at a scan rate of 5 mV/s. (b) CV curves of the inorganic ErCl3 salt electrodes at different scan rates. The present of the redox peaks in the CV curves confirms the occurrence of pseudocapacitive reaction in this electrode. The CV curve was obtained at the potential range of −0.1-0.45 V. All data are taken in 2M KOH electrolyte at room temperature. 22
400 350 Potential (V vs. SCE)
Specific capacitance (F/g)
450
300 250 200 150 100 50
0.4 0.3 0.2 0.1 0.0 -0.1
0
20
40
60
80
Time (s)
0
200
400
600
800
1000
Cycle number
Fig. 3 Cycling stability of the inorganic ErCl3 salt electrodes at a current density of 10 A/g and a potential interval of 0.55V. The inset shows the last five charge/discharge curves. After 1000 continuing charging-discharging cycles, 85% of initial capacitance can be retained.
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Intensity (a.u.)
ErOOH
10
15
20
25
30
35
2θ (degree)
40
45
Ni
50
55
Fig. 4 XRD patterns of ErCl3 electrodes after electrochemical measurements. The standard JCPDS No.76-700 for ErOOH and JCPDS No. 1-1258 for Ni current collector are also indicated in the graph.
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a
b
200 nm
500 nm
c
d
200 nm
500 nm
e
f
200 nm
500 nm
Fig. 5 (a-d) SEM images of ErCl3 electrodes after and before electrochemical measurements. (a and b) high-magnification and low-magnification SEM images after electrochemical measurements. (c and d) SEM images of ErCl3 electrode before electrochemical measurements. (e and f) SEM images of carbon black used as conductive agent in electrode preparation. After electrochemical reaction, carbon matrix-constrained ErOOH colloids electrode configuration can be confirmed.
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Scheme 2 Schematic drawing shows the reaction process and electrode configuration of ErCl3 pseudocapacitor. (a) Initial electric field assisted chemical coprecipitation. (b) Two-layer ErCl3/ErOOH structures were further formed by electric field assisted chemical coprecipitation. (c) Final formed ErOOH colloids, which can take part in Faradaic redox reaction immediately. (d) Carbon matrix-constrained ErCl3 electrodes. The small size of colloids and specific configuration can assure the fast electron and ion transfer, thus the whole ErOOH colloid can be utilized to occur redox reaction.
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a
Current (A/g)
20 1 day 3 day 5 day 7 day
10
0
-10
-20 -0.1
0.0
0.1
0.2
0.3
0.4
Potential (V vs. SCE)
b Potential (V vs. SCE)
0.4 0.3 0.2 1 day 3 day 5 day 7 day
0.1 0.0 -0.1
0
20 40 60 80 100 120 140 160 180 200 220
c
Time (s)
Specific capacitance (F/g)
1400 1200 1000 800 600 400 3+
Weight of Er ion Weight of ErCl3 salt
200 0 1
2
3
4
5
6
7
Aging time (day)
Fig. 6 Electrochemical performance of ErCl3 pseudocapacitors with different aging times. (a) CV curves of the inorganic ErCl3 salt electrodes at a scan rate of 30 mV/s. (b) The charge/discharge curves measured at a current density of 3A/g and the potential range of 0.55 V. The charge/discharge curves of one day measured at a current density of 1A/g. (c) Specific capacitances of inorganic ErCl3 salt electrodes versus discharge current density at a potential window of 0.55V according to the weight of ErCl3⋅6H2O salt and Er3+ ion. The specific capacitance of electrode with aging time of one day was measured at current density of 1A/g. All data are taken in a 27
2 M KOH electrolyte at room temperature.
a2
a1
200 nm
200 nm b2
b1
500 nm
200 nm c2
c1
200 nm
500 nm d2
d1
200 nm
500 nm
Fig. 7 SEM images of ErCl3 electrodes with different aging times after electrochemical measurements: (a1 and a2) 1 day, (b1 and b2) 3 day, c1 and c2) 5 day, and (d1 and d2) 7 day. ErCl3 electrode with aging time of 1 day has loose structure and the particles are separated, while particles in electrodes with aging time of 3-7 days are closely contacted with each other.
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Graphical abstract
Formation of electroactive colloids via in-situ coprecipitation under electric field: erbium chloride alkaline aqueous pseudocapacitor Kunfeng Chen, Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China *E-mail: [email protected].
Highlights
Novel ErCl3-KOH aqueous pseudocapacitor system was demonstrated. In-situ crystallization of ErCl3 in alkaline electrolyte leads to highly electroactive ErOOH colloids. Electric field assisted chemical coprecipitation and Faradaic redox reactions occur at same time and same electrode. ErCl3 pseudocapacitors can show ultrahigh capacitance of 1811 F/g.
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