Colloidal pseudocapacitor: Nanoscale aggregation of Mn colloids from MnCl2 under alkaline condition

Journal of Power Sources 279 (2015) 365e371

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Colloidal pseudocapacitor: Nanoscale aggregation of Mn colloids from MnCl2 under alkaline condition Kunfeng Chen a, Dongfeng Xue a, *, Sridhar Komarneni b, * a

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b Materials Research Institute, Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, United States

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Novel colloidal pseudocapacitors were designed and tested.
Highly electroactive Mn7O13$5H2O colloids were formed from MnCl2 in KOH solution.
High specific capacitance of 2518 F/g based on active Mn cations was obtained.
The charge reactions were Mn2þ 4 Mn3þ and Mn3þ 4 Mn4þ in alkaline MnCl2 salt electrodes.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 October 2014 Received in revised form 24 December 2014 Accepted 4 January 2015 Available online 5 January 2015

Novel colloidal pseudocapacitors are designed using commercially available MnCl2 salts as starting materials and KOH as electrolyte, where the colloids synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale. Highly electroactive Mn7O13$5H2O colloids are formed in-situ by electric field assisted chemical coprecipitation in KOH solution. The highly efficient Faradaic redox reactions involving Mn3þ 4 Mn4þ and Mn2þ 4 Mn3þ are confirmed in electroactive Mn7O13$5H2O pseudocapacitors, which can deliver high specific capacitance of 2518 F/g based on active Mn cations at current density of 5 A/g. The present results show that instead of one-electron Faradaic reaction, Mn cations in our designed system can lead to two-electron Faradaic reactions. The colloidal pseudocapacitor system involving Mn-based colloids is a novel route to engineer electrochemical performances of inorganic pseudocapacitors. © 2015 Elsevier B.V. All rights reserved.

Keywords: Colloidal pseudocapacitor Energy storage MnCl2 electrode Coprecipitation KOH electrolyte

1. Introduction Electrical energy can be more effectively utilized if we have the ability to generate new powerful energy storage devices, which depend on developing new electrode materials for electrochemical

* Corresponding authors. E-mail addresses: [email protected] (S. Komarneni).

(D.

http://dx.doi.org/10.1016/j.jpowsour.2015.01.017 0378-7753/© 2015 Elsevier B.V. All rights reserved.

Xue),

[email protected]

energy storage systems such as supercapacitors, and lithium-ion batteries [1,2]. Supercapacitors exhibit the desirable properties of high power density, fast charging rate, excellent cycling stability, which make them one of the most promising candidates for electrochemical energy storage devices [3,4]. Redox-type pseudocapacitors can deliver much higher energy densities than electrochemical double layer capacitors [5,6]. However, pseudocapacitors still suffer from the low energy density compared with battery systems [7]. Because the capacitance value of a supercapacitor is largely determined by its electrode materials, research

366

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

to improve the performance of supercapacitors by blueprinting electrode materials has dramatically increased [8e10]. Electrode materials were often designed to form some specific nanostructures, which can guarantee shorter path for ion diffusion and electron transport [11,12]. The poor electronic conductivity of inorganic pseudocapacitor materials as well as only their surface layers participating in redox reactions often limit the utilization of these electrode materials [13,14]. To solve these problems, we need to retrieve the charge storage mechanism of pseudocapacitor. The pseudocapacitive behavior is associated primarily with the redox reactions of transition metal cations in electrode materials during operation [5,14]. However, the manipulation of electrochemically active metal cations with multiple oxidation states is often neglected, but it is intrinsically important for improving inorganic pseudocapacitive efficiency. Therefore, the preparation of electrode materials with more available active cations can significantly improve the capacitance of inorganic pseudocapacitors [15e17]. With several oxidation states and a wide range of phases, manganese is a vital component in electrochemical energy storage systems due to its unique ability to cycle between various oxidation states [14,18]. Mn-based materials including MnO2, Mn3O4, and Mn2O3, have been broadly investigated for pseudocapacitors due to their low cost, high specific capacitance, environmental amicability, and natural abundance [18e20]. To optimize the electrochemical performance of MnO2, considerable efforts have been made to design state-of-the-art electrode materials and electrodes [18]. Specific capacitance of ~1145 F/g in MnO2 supercapacitors has been obtained with the increase of conductivity of MnO2 by adding Au to electrodes [6]. The electrodeposited MnO2 film can exhibit a significantly high capacitance over 2000 F/g, using an activation controlled potential deposition rather than diffusion controlled potential route [21]. However, most of these research efforts only focused on manganese oxide compounds with the aim to achieve high capacitance and cyclability. The fundamental understanding of the charge storage of manganese cations is still essential in optimizing the performance of Mn-based pseudocapacitors. Recently, mixed-valence manganese oxide film, containing Mn3þ and Mn4þ, exhibited anomalously high specific capacitance of ~2530 F/g, indicating the important effect of Mn cations on promoting pseudocapacitive reactions [14]. Therefore, to deeply understand the intrinsic relationship between metal cations and supercapacitor performances, we designed colloidal pseudocapacitor to display the relationship of cation and pseudocapacitance. With MnCl2 salts serving as starting materials and KOH as electrolyte, electroactive Mn7O13$5H2O colloids were formed by electric field assisted chemical coprecipitation, which can provide more available Mn cations for Faradaic reaction. These Mn cations in colloidal electrode underwent the confirmed phase transformations from MnCl2$5H2O, through Mn(OH)2, to Mn7O13$5H2O colloid which can be controlled by external electric field within currently designed electrode system. The MnCl2 colloidal pseudocapacitors can deliver high specific capacitance of 2518 F/g on the basis of electrochemically active Mn cations with the potential interval of 0.8 V and the current density of 5 A/g. The redox reactions of Mn2þ 4 Mn3þ and Mn3þ 4 Mn4þ within colloidal electrodes were confirmed through cyclic voltammetry (CV) measurements, indicating that these electroactive Mn2þ, Mn3þ and Mn4þ cations are responsible for the electric energy storage. 2. Experimental section 2.1. Materials MnCl2$5H2O KOH were purchased from Beijing Chemical Corp. The polyvinylidene fluoride, N-methyl-2-pyrrolidone and carbon

black were purchased from Hefei Kejing Materials Technology Co., Ltd.. Nickel foam was purchased from Changsha Liyuan New material Co., Ltd. The water was purified through a Millipore system. 2.2. Electrode fabrication and electrochemical testing Commercial MnCl2 salts were directly used without further purification. In order to prepare work electrodes, the following procedures can be described: firstly, MnCl2, acetylene black, and polyvinylidene fluoride (PVDF) were mixed with weight ratio of 70:20:10; then, N-methyl-2-pyrrolidone (NMP) solution was added into above mixture to form slurry; after that, the resulting slurry was pasted onto a nickel foam with size of 1  1 cm2 and the nickel foams were dried at 80  C for 24 h; finally, the nickel foam with MnCl2 salts was pressed to form working electrode. The loading of each electrode is about ~2e3 mg. In addition, the working electrodes were aged in air at room temperature with the given time for further characterization and electrochemical testing. Three-electrode electrochemical set was used. The electrolyte, reference electrode and counter electrode were 2 M KOH solution, the saturated calomel electrode (SCE), and Pt wire electrode, respectively. In order to obtain active electrode, about 50 cyclic voltammetry (CV) cycles should be conducted to obtain stable behavior. An electrochemical workstation (CHI 660D) was used to conduct these electrochemical reactions and tests. For supercapacitor testing, CV and galvanostatic chargeedischarge measurements were performed at the given potential range, scan rate and current density. 2.3. Fabrication of asymmetry supercapacitor Asymmetry supercapacitor was constructed with MnCl2 and graphene as positive and negative electrodes. Graphene materials were synthesized according to previous work [3]. Graphene negative electrodes were fabricated by directly pressing graphene on nickel foams without using any binder and additional conducting agent. The electrolyte was 2 M KOH solution. 2.4. Characterization The morphology and composition of these working electrodes were examined using a scanning electron microscope (SEM, Hitachi-S4800), a FEI Tecnai F20 transmission electron microscope (TEM) operated at 200 kV and powder X-ray diffraction (XRD) with CuKa radiation (l ¼ 0.15418 nm) on a Bruker D8 Focus. 3. Results and discussion Scheme 1 shows the formation of highly electroactive Mn7O13$5H2O colloids by electric field assisted chemical coprecipitation in KOH aqueous electrolyte. After MnCl2 electrodes were inserted into KOH electrolyte, CV measurements were immediately performed. The chemical reactions leading to the formation of manganese hydroxide occurred fast under electric field (CV testing) after electrode contacting with KOH. Then, various chemical reactions, phase transformations and electrochemical reactions were occurring simultaneously. Highly reactive Mn7O13$5H2O colloids were formed by electric field assisted chemical coprecipitation, wherein Mn cations can be efficiently utilized toward Faradaic reactions. More intriguingly, the formation of electroactive Mn7O13$5H2O colloids and the occurrence of Faradaic reactions take place at the same spatial and temporal scale. The enhanced specific capacitance mainly originates from the in-situ electric field assisted activation of Mn7O13$5H2O media. Fig. 1 shows the electrochemical performances of MnCl2

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

367

Scheme 1. Schematic drawing shows chemical processes of colloidal electrode in KOH aqueous electrolyte. After MnCl2 electrode was inserted into KOH electrolyte, electrochemical measurement was performed to this electrode immediately (a). After undergoing chemical and electrochemical reactions, electroactive Mn7O13$5H2O colloids were formed in electrode (b). Electric field and coprecipitation were performed at the same electrode and time (c). Mn(OH)2 and highly electroactive Mn7O13$5H2O colloids were formed by electric field assisted chemical coprecipitation. Faradaic reactions of electroactive Mn7O13$5H2O colloids occurred at the same time and electrode, with the corresponding cation reactions of Mn2þ 4 Mn3þ and Mn3þ 4 Mn4þ (d).

electrode in 2 M KOH electrolyte. The specific capacitance values of MnCl2 electrodes were measured by the galvanostatic charge/ discharge method. The discharge curves are not linear, reflecting the occurrence of Faradaic redox reactions [12]. As the current density is one of the important parameters that control the capacitive performance of electrode, the charge/discharge curves with different current densities are shown in Fig. 1a, b and Fig. S1. The specific capacitance can be calculated according to the equation: Sc ¼ IDt/mDV, where I is the current used for chargeedischarge in A, Dt is the time elapsed for the discharge cycle in s, m is the mass of manganese cations in g, and DV is the voltage interval of the discharge. Specific capacitances obtained from the discharge curves are shown in Fig. 1c. The highest specific capacitance of 2518 F/g at the current density of 5 A/g and the potential interval of 0.8 V is obtained on the basis of the weight of Mn cations. These results confirm that the highly electroactive colloid was formed in our currently designed MnCl2eKOH system.

The traditional specific capacitance of a supercapacitor is calculated according to the weight of active materials, such as the weight of MnO2 and MnCl2 herein. However, from the physical origin of electrochemical energy storage, the specific capacitance calculated from Mn cations can more deeply reflect their real redox mechanism in pseudocapacitors. In addition, MnCl2 salts with free Mn2þ cations were directly used in fabricating electrodes and the pseudo-ion-state Mn7O13$5H2O colloids were formed after electrochemical operation. The use of specific capacitance of Mn cations herein, can more properly evaluate the performance of MnCl2 salt pseudocapacitors. The specific capacitance values of Mn cations are 2082, 1610, 1174, 720, and 382 F/g at the current densities of 10, 20, 30, 40, 50 A/ g and the potential interval of 0.8 V. The typical loading weight of each electrode is ~2e3 mg, and the weight of manganese ions is about 0.56e0.83 mg. The high capacitance behavior of MnCl2 electrodes is thus not limited to their low loading weight compared

368

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

Fig. 1. Electrochemical performance of colloidal supercapacitors. (a) The discharge and (b) charge curves measured at various current densities and potential interval of 0.8 V. (c) The specific capacitance upon current density. (d) CV curves obtained at potential range of 0.6e0.45 V and scan rate of 5 mV/s. A1 and A2 indicate anodic peaks, while C1 and C2 represent cathodic peaks. A1 and C1 peaks represent oxidation and reduction of Mn3þ 4 Mn4þ, while A2 and C2 peaks represent oxidation and reduction of Mn2þ 4 Mn3þ. (e) CV curves obtained at different scan rates and potential range of 0.6e0.45 V. All data were taken in a 2 M KOH solution at room temperature. (f) The long-term cycling stability of MnCl2$4H2O colloidal supercapacitors was measured at a current density of 30 A/g.

to thin-film electrodes with high capacitance [21]. 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 [22,23]. The charge storage mechanism was further characterized by CV curves (Fig. 1d and e). Two pairs of cathodic and anodic peaks were clearly observed with the scan rate of 5 mV/s (Fig. 1d and Fig. S1). The anodic peak potentials are 0.26 V (A2) and 0.38 V (A1), while the cathodic peaks are present at 0.35 V (C2) and 0.10 V (C1). Both anodic and cathodic peaks in these CV curves can be ascribed to the oxidation and reduction processes of active Mn cations. The current of electrode responds rapidly to the switching potential, particularly at the potential switching point of 0.45 V, representing smaller equivalent series resistance [23]. In neutral electrolyte, e.g., Na2SO4, CV curve of MnO2 is often rectangle [10]. However, in alkaline electrolyte MnO2 electrodes often display significant redox peaks [23,24]. The standard redox potentials of Mn compounds in alkaline conditions are shown in Fig. S2. The redox potential of Mn3þ 4 Mn4þ is 0.12 V vs. SCE, while the potential of Mn2þ 4 Mn3þ is 0.52 V vs. SCE. Due to the special chemical conditions, the practical potentials clearly differ from the standard redox potentials. Because the reduction potential of metal Mn is more negative, the formation of metal Mn in the present potential interval is not possible. Therefore, these two pairs of redox peaks can be assigned to the redox reactions of Mn2þ 4 Mn3þ and Mn3þ 4 Mn4þ. A1 and C1 peaks represent the oxidation and reduction of Mn3þ 4 Mn4þ, while A2 and C2 peaks represent the oxidation and reduction of Mn2þ 4 Mn3þ. The larger the area of reduction and oxidation peaks, the higher is the capacitance of electrode. The peak intensity of A2 and C2 peaks is smaller than that of A1 and C1 peaks, indicating that the redox reaction of Mn3þ 4 Mn4þ is dominant in the pseudocapacitive reaction. In our current measurements, MnCl2 electrodes showed

the pseudocapacitive characteristics, which originate from Faradaic redox reactions of Mn cations. We have provided electrochemical impedance spectroscopy as shown in Fig. S3. The charge transfer resistance of MnCl2 electrode is only about 1 U, which confirms high electrochemical activity of our designed electrode. Theoretical specific capacitance of active Mn cations can be calculated according to the equation Cm ¼ Q/(V  M), where Q ¼ 9.632  104 C for the transfer of one electron during the redox reaction, M is molecular weight of Mn ion (M ¼ 54.938 g/mol), V is the operating potential window. Table S1 shows both theoretical and measured specific capacitance values of active Mn cations. Mn cations have the high theoretical capacitance of ~2192 F/g as the oxidation state of Mn ion is changed from þ4 to þ3 over a potential window of 0.8 V (Table S1). When the oxidation state of Mn ions is changed from þ4 to þ2 over a potential window of 0.8 V, the specific capacitance is ~4383 F/g. Compared with the measured value of 2518 F/g, the involved Faradaic redox reactions in the present pseudocapacitors can transfer more than one electron. Table S2 shows specific capacitance of MnO2 supercapacitors reported by others. These results confirm that our designed electrode has the highest specific capacitance. As shown in Table S1, for oneelectron Faradaic reaction, the utilization ratio of Mn cation is 115%. For two-electron reactions, the utilization ratio of Mn cation can reach 57%. The present results show that Mn cations in our designed system can lead to two-electron and one-electron Faradaic reactions. Besides the reaction of Mn4þ 4 Mn3þ, Faradaic reaction of Mn3þ 4 Mn2þ was also occurring, which can be proven by the measured CV curves (Fig. 1d). The long-term cycling stability of MnCl2 electrodes was measured with galvanostatic chargeedischarge at the current density of 30 A/g for 5000 cycles (Fig. 1d). The capacitance retention can reach 67% after 5000 cycles. A decline occurs in the first three thousand cycles, after that the curve retains a smooth plateau.

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

To identify the physical origin of high electrochemical performance of these MnCl2 electrodes, we performed XRD, SEM and TEM measurements. XRD patterns of MnCl2 electrodes before and after electrochemical measurements showed the formation of Mn7O13$5H2O colloids via electric field assisted chemical coprecipitation of MnCl2$4H2O (Fig. 2). The very weak characteristic peaks in these XRD patterns indicate that the as-obtained Mn7O13$5H2O colloid is in poorly crystallized state, which can provide more active cations for Faradaic reaction (Fig. 2a) similar to the amorphous phase of MnO2, which has been reported to be favorable as electrode material of pseudocapacitors [18]. These results are consistent with our assumption that the formation of pseudo-ion-state Mn-based colloids, which can show high electrochemical reactivity toward high-efficiency redox reactions. The morphologies and microstructures of MnCl2 electrode before and after electrochemical measurements were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images. The pristine MnCl2 electrode shows particle-like morphology with diameter of 100 nm (Fig. 2b). Fig. 2c shows that after electrochemical reaction, the nanoscale sheetlike structures were formed within the electrode. TEM image confirms that the formed Mn7O13$5H2O colloid is spatially close to conductive carbon and binder matrix at nanoscale (Fig. 2d). Weak electron diffraction circle also confirms the formation of poorly crystallized Mn7O13$5H2O colloids (inset of Fig. 2d). HRTEM images prove the formation of Mn7O13$5H2O colloids (Fig. 2e and f). The lattice fringes of 0.66 and 0.21 nm correspond to (110) and (128) crystal planes, respectively of Mn7O13$5H2O phase. More interestingly, Mn7O13$5H2O colloids showed high reactivity. When exposed to high-energy electron beams in HRTEM, the well-defined lattice fringes disappeared after tens of seconds, because of the instability of highly active Mn7O13$5H2O colloids. This specific structure can shorten the electron/ion transportation

369

length and increase the number of electrochemical active sites for the redox reaction. It should be noted that MnCl2 electrode in KOH electrolyte can form highly electroactive Mn7O13$5H2O colloids at nanoscale in-situ and these can greatly improve Mn utilization efficiency. The cations in electroactive Mn7O13$5H2O colloids can be possibly utilized to maximum extent in this state. In traditional electrode materials, only the surface cations can be utilized for redox reaction. In fact, active Mn cations, ions of electrolyte and electrons take part in the redox reaction of Mn7O13$5H2O colloids during electrochemical operation. The transport of electrons to the current collector and the diffusion of ions to the electrode materials can influence the pseudocapacitive performance of electrodes. Therefore, many researchers designed electrode materials with specific structures, with the aim to shorten the diffusion path for ions and the transport path for electron [4,11,12]. In our currently designed system, highly active Mn7O13$5H2O colloids possess a large amount of available active cations that can facilely participate in the redox reaction for charge storage. The following chemical reactions could be occurring during the chemical coprecipitation and electrochemical operation: MnCl2$4H2O þ 2OH / Mn(OH)2 þ 4H2O þ 2Cl

(1)

7Mn(OH)2 þ 12OH / Mn7O13$5H2O þ 8H2O þ 12e

(2)

When the electrode was inserted into KOH electrolyte, Mn(OH)2 colloids can be produced by electrically assisted coprecipitation. Then, Mn(OH)2 colloids can be easily oxidized to Mn7O13$5H2O colloids with the help of the external electric field. The external electric field was applied by CV and constant current discharge/ charge tests. The initial chemical reactions (Eqs. (1) and (2)) and Faradaic redox reactions can be controlled by external electric field.

Fig. 2. Physical characterization of colloidal electrodes. (a) XRD patterns of MnCl2 electrodes after (I) and before (II) electrochemical measurements. The formed phases can be indexed with the standard JCPDS No. 23-1239 for Mn7O13$5H2O and JCPDS No. 73-1451 for MnCl2$4H2O. Peaks of Ni current collector are also indicated in the figures with JCPDS No. 87-712 for Ni. (b) SEM images of MnCl2 electrodes before electrochemical measurements. (c) SEM of MnCl2 electrodes after electrochemical measurements. (d) TEM images of MnCl2 electrodes after electrochemical measurements. Inset of d is electron diffraction pattern. Scale bar of inset is 5 1/nm. (e and f) HRTEM images show the presence of Mn7O13$5H2O phase after electrochemical measurements.

370

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

Simultaneously, electric field further directs the formation of Mn7O13$5H2O colloids with high electroactivity. To study the phase transformation and its effect on the electrochemical performance of MnCl2 electrode, we measured our designed MnCl2 electrodes with different aging times in air. Comparisons of CV curves, discharge curves and specific capacitances of MnCl2 electrodes with different aging times are shown in Fig. 3a, b and Fig. S4. The relative peak intensity of A1-C1 and A2-C2 changed with the increase of aging times. When increasing aging time, the intensity of A1-C1 became larger, indicating that the redox reaction of Mn3þ 4 Mn4þ dominated compared to that of Mn2þ 4 Mn3þ (Fig. S4). The aging time affects both chemical and electrochemical reactions during operation. The larger area of CV curves at a high scan rate indicates the higher capacitive capability of electrodes. Increasing aging time enhances the capacitive performance (Fig. S4). The specific capacitances of Mn cations are 133, 832, 936, 1988 and 2082 F/g at the aging times of 0, 1, 2, 3 and 4 days, respectively with the current density of 10 A/g and the potential interval of 0.8 V (Fig. 3b). Our starting chemical compound is MnCl2$4H2O, which changed during reaction. The phase changes were detected by XRD patterns during these aging processes (Fig. S5). Only Mn2(OH)3Cl phase can be found in electrode with aging time of 0 day, however, this electrode shows the lowest specific capacitance. By prolonging the aging time, MnCl2$4H2O phase was re-formed. The present results prove that MnCl2$4H2O is the most suitable starting chemical. In addition, the deliquescence of MnCl2$4H2O salts can distribute manganese salts on conductive carbon black and favor

the formation of tiny salt particles, which is important for the phase transformation in alkaline electrolyte during electrochemical operation. With prolonging aging time, MnCl2$4H2O could be more dispersed into carbon and binder matrix due to the deliquescence effect. During electrode fabrication process, MnCl2$4H2O was dissolved. During the aging time, it can undergo dissolution and recrystallization processes, thus nanoparticles can be formed. XRD pattern (Fig. S5) also proves the formation of Mn2(OH)3Cl phase after deliquescence of MnCl2$4H2O salts. After undergoing 50 CV cycles, Mn7O13$5H2O colloids were formed by electric field assisted chemical coprecipitation (Fig. 3c). SEM images were used to characterize the morphologies of electrode. Before electrochemical measurement, MnCl2 electrode is composed of nanoparticles (Fig. S6). According to SEM images, nanosheet structures were formed in all MnCl2 electrodes after electrochemical measurement (Fig. 3d). The above results showed that the crystallization or phase transformation in air also influenced the electrochemical performance of MnCl2 salt electrodes. In our present system, the formation of highly electroactive colloids was found to be important for the improvement of specific capacitance. To evaluate the application potential of MnCl2 electrode, we fabricated asymmetric supercapacitor with MnCl2 and rGO as positive and negative electrodes. Fig. 4 shows the energy density and power density of MnCl2//rGO asymmetric supercapacitor operated at 1.4 V in 2 M KOH electrolyte. The electrochemical performance of graphene materials have been studied in our previous work [9]. In order to compare with other works, in twoelectrode system, the specific capacitance, energy density and

Fig. 3. Comparison of electrochemical performances and physical characterization of MnCl2 colloidal electrodes with different aging times. (a) Charge/discharge curves at current density of 10 A/g and potential range of 0.35e0.45 V. (b) Specific capacitance upon the current density calculated from the weight of Mn cations. All data are taken in a 2 M KOH solution at room temperature. (c) XRD patterns of MnCl2 electrodes after electrochemical measurements with different aging times. The formed phases can be indexed with the standard JCPDS No. 23-1239 for Mn7O13$5H2O. Peaks of Ni current collector are also indicated in the figures with JCPDS No. 87-712 for Ni. (d) SEM images of MnCl2 electrodes after electrochemical measurements with different aging times: (i) 0 day, (ii) 1 day, (iii) 2 day, and (iv) 3 day.

K. Chen et al. / Journal of Power Sources 279 (2015) 365e371

371

Mn3þ 4 Mn4þ occurring. The present results show that instead of one-electron Faradaic reaction, Mn cations in our designed system can lead to two-electron Faradaic reactions. The MnCl2-based colloidal pseudocapacitor system provides a novel route to engineer electrochemical performances of inorganic colloidal pseudocapacitors. Acknowledgments Financial support from the National Natural Science Foundation of China (grant nos. 51125009, and 91434118), the National Natural Science Foundation for Creative Research Group (grant no. 21221061) and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged. Appendix A. Supplementary data Fig. 4. Energy density and power density of MnCl2//rGO asymmetric supercapacitor compared with commercially available energy-storage system. MnCl2//rGO asymmetric supercapacitor operated at 1.4 V in 2 M KOH electrolyte.

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.01.017. References

power density are calculated here based on the total mass of two electrodes. The specific energy density (E, Wh/kg) and power density (P, W/kg) MnCl2 electrode can be calculated using the following equations: E ¼ 1/2CV2 and P ¼ E/t, where C is the specific capacitance, V is voltage change during the discharge process, and t is the discharge time. The energy density was 19.6 Wh/kg at the power density of 1.4 kW/kg. When the power density was increased to 7 kW/kg, energy density of 10.5 Wh/kg could still be reached. Energy densities and power densities reported by others for Graphene/MnO2//Graphene/MoO3, MnO2//Mesoporous CNT, were 42.6 Wh/kg-276 W/kg and 47.4 Wh/kg-200 W/kg [25,26]. Here, our MnCl2//rGO asymmetric supercapacitor showed relatively higher energy density than conventional capacitors and normal supercapacitors, maintaining their power density considerably higher than the conventional batteries and fuel cells [27,28]. Fig. S7 shows the Ragone plot for energy density and power density of MnCl2 electrodes based on three-electrode system. MnCl2 pseudocapacitors can exhibit the energy density of 224 Wh/kg at the power density of 2 kW/kg. At the power density of 24 kW/kg, an energy density of 12 Wh/kg was obtained. 4. Conclusion In summary, novel colloidal pseudocapacitors in alkaline electrolyte were reported with MnCl2 salts serving as starting materials and KOH as electrolyte. Highly electroactive Mn7O13$5H2O colloids were formed by electric field assisted chemical coprecipitation of MnCl2 in KOH solution. The designed MnCl2 colloidal pseudocapacitor can deliver high specific capacitance of 2518 F/g based on active Mn cations with the potential interval of 0.8 V and current density of 5 A/g, because the cations in electroactive Mn7O13$5H2O colloidal can be utilized possibly to maximum extent. The occurrence of Faradaic redox reactions of Mn3þ 4 Mn4þ and Mn2þ 4 Mn3þ were confirmed in our designed MnCl2 electrodealkaline electrolyte system with the preponderant reaction of

[1] 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) 1502e1505. [2] M.V. Reddy, G.V. Subba Rao, B.V.R. Chowdari, Chem. Rev. 113 (2013) 5364e5457. [3] F. Liu, S. Song, D. Xue, H. Zhang, Adv. Mater. 24 (2012) 1089e1094. [4] V. Augustyn, J. Come, M.A. Lowe, J.W. Kim, P. Taberna, S.H. Tolbert, ~ a, P. Simon, B. Dunn, Nat. Mater. 12 (2013) 518e522. H.D. Abrun [5] B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer-Academic, New York, 1999. [6] X.Y. Lang, A. Hirata, T. Fujita, M.W. Chen, Nat. Nanotechnol. 6 (2011) 232e236. [7] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845e854. [8] I.E. Rauda, V. Augustyn, B. Dunn, S.H. Tolbert, Acc. Chem. Res. 46 (2013) 1113e1124. [9] F. Liu, D. Xue, Chem. Eur. J. 19 (2013) 10716e10722. [10] Y. Zhang, C. Sun, P. Lu, K. Li, S. Song, D. Xue, CrystEngComm 14 (2012) 5892e5897. [11] Q. Lu, J.G. Chen, J.Q. Xiao, Angew. Chem. Int. Ed. 52 (2013) 1882e1889. [12] P. Lu, F. Liu, D. Xue, H. Yang, Y. Liu, Electrochim. Acta 78 (2012) 1e10. [13] T. Brezesinski, J. Wang, S.H. Tolbert, B. Dunn, Nat. Mater. 9 (2010) 146e151. [14] M. Song, S. Cheng, H. Chen, W. Qin, K. Nam, S. Xu, X. Yang, A. Bongiorno, J. Lee, J. Bai, Nano Lett. 12 (2012) 3483e3490. [15] K. Chen, S. Song, K. Li, D. Xue, CrystEngComm 15 (2013) 10367e10373. [16] K. Chen, Y. Yang, K. Li, Z. Ma, Y. Zhou, D. Xue, ACS Sustainable Chem. Eng. 2 (2014) 440e444. [17] K. Chen, D. Xue, J. Colloid Interface Sci. 416 (2014) 172e176. [18] V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 7 (2014) 1597e1614. [19] K. Chen, Y.D. Noh, K. Li, S. Komarneni, D. Xue, J. Phys. Chem. C 117 (2013) 10770e10779. [20] J.W. Lee, A.S. Hall, J. Kim, T.E. Mallouk, Chem. Mater. 24 (2012) 1158e1164. [21] A. Cross, A. Morel, A. Cormie, T. Hollenkamp, S. Donne, J. Power Sources 196 (2011) 7847e7853. [22] J. Lee, M. Shin, S. Kim, H.U. Cho, G.M. Spinks, G.G. Wallace, M.D. Lima, X. Lepro, M.E. Kozlov, R.H. Baughman, S.J. Kim, Nat. Commun. 4 (2013) 1970. [23] D. Sarkar, G.G. Khan, A.K. Singh, K. Mandal, J. Phys. Chem. C 117 (2013) 15523e15531. [24] L. Benhaddad, L. Makhloufi, B. Messaoudi, K. Rahmouni, H. Takenouti, J. Mater. Sci. Technol. 27 (2011) 585e593. [25] J. Chang, M. Jin, F. Yao, T.H. Kim, V.T. Le, H. Yue, F. Gunes, B. Li, A. Ghosh, S. Xie, Y.H. Lee, Adv. Funct. Mater. 23 (2013) 5074e5083. [26] H. Jiang, C. Li, T. Sun, J. Ma, Nanoscale 4 (2012) 807e812. [27] B.C. Jones, A.F. Hollenkamp, S.W. Donne, J. Electrochem. Soc. 157 (2010) A551eA557. [28] S.K. Nataraj, Q. Song, S.A. Al-Muhtaseb, S.E. Dutton, Q. Zhang, E. Sivaniah, J. Nanomater 2013 (2013) 272093.