nanoparticles synthesized via a mild ionic liquid-assisted route

Journal of Power Sources 303 (2016) 49e56

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Capacitive behaviour of MnF2 and CoF2 submicro/nanoparticles synthesized via a mild ionic liquid-assisted route Ruguang Ma a, b, Yao Zhou a, b, Lin Yao c, Guanghui Liu a, b, Zhenzhen Zhou a, b, Jong-Min Lee c, Jiacheng Wang a, b, *, Qian Liu a, b, ** a

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China Shanghai Institute of Materials Genome, Shanghai, China c School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore b

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


MnF2 rods and hierarchical CoF2 cuboids were synthesized by the assistance of ionic liquid.
The as-prepared submicro-/nanosized MnF2 and CoF2 particles exhibit remarkable capacitance.
The cycled electrodes were investigated by different characterization techniques.
The mechanism can be ascribed to the redox reactions between MnF2/ CoF2 and MnOOH/CoOOH.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 September 2015 Accepted 27 October 2015 Available online xxx

Submicro-/nano-sized MnF2 rods and hierarchical CoF2 cuboids are respectively synthesized via a facile precipitation method assisted by ionic liquid under a mild condition. The as-prepared MF2 (M ¼ Mn, Co) submicro/nanoparticles exhibit impressive specific capacitance in 1.0 M KOH aqueous solution, especially at relatively high current densities, e.g. 91.2, 68.7 and 56.4 F g1 for MnF2, and 81.7, 70.6 and 63.0 F g1 for CoF2 at 5, 8 and 10 A g1, respectively. The mechanism of striking capacitance of MF2 is clarified on the basis of analysing the cycled electrodes by different characterization techniques. Such remarkable capacitance is ascribed to the redox reactions between MF2 and MOOH in aqueous alkaline electrolytes, which can not be obtained in aqueous neutral electrolytes. This study for the first time provides direct evidences on the pseudocapacitance mechanism of MF2 in alkaline electrolytes and paves the way of application of transition metal fluorides as electrodes in supercapacitors. © 2015 Elsevier B.V. All rights reserved.

Keywords: Transition metal fluoride Ionic liquid Supercapacitor

1. Introduction * Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China. ** Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China. E-mail addresses: [email protected] (J. Wang), [email protected]. ac.cn (Q. Liu). http://dx.doi.org/10.1016/j.jpowsour.2015.10.102 0378-7753/© 2015 Elsevier B.V. All rights reserved.

With the ever-increasing demand on sustainable and renewable energy, massive studies have been triggered or diverted to environmental friendly energy storage systems [1]. Supercapacitors (SCs) or ultracapacitors, as energy storage devices holding the advantages of high power density, long cycle life, and rapid

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chargingedischarging rate, present great promising application in portable electronic devices, electric vehicles, and other devices requiring burst power [2,3]. Based on the charge storage mechanism, SCs are classified into electrical double layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, energy storage arises from the accumulation of electronic and ionic charges at the interface between electrode material and electrolyte solution, while in pseudocapacitors, the charges are stored via the Faradaic reactions [4]. Compared with EDLCs, pseudocapacitors exhibit much higher specific capacitance and energy density due to their reversible multielectron redox reactions. Among various electrode materials, transition metal (TM) compounds and conducting polymers have been extensively studied for pseudocapacitor applications. For examples, MnO2 nanowire/graphene [5] and a-Fe2O3/MnO2 coreeshell nanowires [6] were reported to exhibit high power density and long-term cycling stability after 1000 cycles. Nanoporous nickel compounds, such as a-Ni(OH)2 and NiO [7,8], also present high rate chargeedischarge performance and good electrochemical stability. To the best of our knowledge, till now there have been few reports about the study on TM fluorides as electrodes for SCs ranging from fundamental understanding to application trial except for two samples as follows. In Jin's work, they synthesized sponge-like porous Ni(OH)2eNiF2 composite film by the anodization of nickel in an electrolyte containing NH4F and H3PO4, which shows a superior rate capability (>1200 F g1 at 100 A g1 after 2000 cycles), and is proposed to mainly result from the contribution of Ni(OH)2 [9]. Yang et al. demonstrated a flexible SC composed of NiF2 as active material and proposed that NiF2 converts to NiOOH through Ni(OH)2, thereby contributing the capacitance during the electrochemical testing [10]. However, in the studies mentioned above, both active materials exist as anodized thin film, which possess low dimensional structure as well as high activity. Therefore, the universality of TM fluorides powder not only film used as electrodes for SCs needs to be further studied and elucidated. On the other hand, the conventional synthesis of TM fluorides is mainly achieved by utilizing the highly dangerous and volatile HF solution [11,12]. Considering the safety of processing procedure, it is greatly necessary to synthesize TM fluorides by utilizing other fluorine sources instead of HF solution. Ionic liquids (ILs) are good candidates to cater for such requirement, because of its environmental friendliness, low inflammability as well as large electrochemical window [13,14]. In particular, some reports have already demonstrated that ILs can not only reproduce conventional inorganic nanomaterials, but also provide the pathways towards new inorganic nanomaterials with unique properties that are difficult to be made via the conventional processes [15e18]. Therefore, it is highly attractive to synthesize TM fluorides under a mild experimental condition and further investigate their electrochemical performance. Herein, for the first time, we report the facile synthesis of submicro-/nano-sized MF2 (M ¼ Mn, Co) particles by utilizing IL (Bmim [BF4]) as a fluoride source and their good rate capability as the electrode materials for SCs in aqueous alkaline electrolyte. The mechanism on the remarkable capacitive performance of MnF2 and CoF2 powders was also investigated and discussed based on the analysis of cycled electrodes by utilizing various characterization techniques. 2. Experimental 2.1. Synthesis of MnF2 and CoF2 In a typical procedure, 3.0 g 50 wt.% Mn(NO3)2 solution was mixed with 5 mL of IL 1-butyl-3-methylimidazolium

tetrafluoroborate (Bmim[BF4]). The Bmim[BF4] serves as a co-solvent and fluorine source, while Mn(NO3)2 provides Mn2þ ions for the target material. The mixed solution was then heated and maintained at 80  C overnight until pink precipitates were formed in the IL medium. The products were washed with ethanol and distilled water, and centrifuged at 6000 rpm several times to remove the residual IL and other organic impurities, followed by subsequent drying under vacuum at 80  C for 12 h. At last, about 0.7 g of MnF2 powder was obtained, which is close to the theoretical value (0.78 g). The synthesis of CoF2 follows the same procedure as that of MnF2, except that 1.8 g of Co(NO3)2$6H2O was used as Co2þ source instead of Mn(NO3)2 solution. Finally, 0.53 g of CoF2 was obtained after washing and drying. 2.2. Physical characterization Structure and crystallinity were characterized by X-ray powder diffraction (XRD) on a Rigaku D/MAX-2250 V diffractometer with Cu Ka radiation. Scanning electron microscopy (SEM) images were recorded using a JEOL JSM-6700F field emission scanning electron microscopy. Transmission electron microscopy (TEM) images were observed using a JEOL 2010F electron microscope with energydispersive X-ray (EDX) spectroscopy operating at 200 kV. The asprepared powders were glued onto indium (In) metal particles by pressing for X-ray photoelectron spectroscopy (XPS) measurements with an ESCALAB 250 X-ray photoelectron spectrometer with Al Ka (hy ¼ 1486.6 eV) radiation. All spectra were calibrated using 284.5 eV as the line position of adventitious carbon. The cycled electrode materials were rinsed by alcohol and acetone for several times and then used for XRD, SEM, and XPS investigation. 2.3. Electrochemical characterization The electrodes were prepared by mixing 80 wt.% MF2 as active materials, 10 wt.% carbon black and 10 wt.% Nafion as binder in ethanol to form uniform slurry. The resulting uniform slurry was then coated onto Ni foam serving as current collector. The electrode area and mass of active material were 1.0 cm2 and 1.5 mg, respectively. The coated MF2 electrodes were dried at 80  C in vacuum oven for 6 h. The electrochemical performance of the asprepared electrode materials for SCs was evaluated in a threeelectrode electrochemical cell containing 1.0 M KOH (or Na2SO4) aqueous solution as electrolyte at room temperature. The asprepared MF2 electrodes were used as working electrode, Pt wire as counter electrode and saturated calomel electrode (SCE) as reference electrode. Cyclic voltammetry (CV) and galvanostatic chargeedischarge (GCD) tests were carried out with an electrochemical analyser (CHI 760D, CH Instruments). 3. Results and discussion The phase and composition of the as-prepared samples were investigated by XRD and XPS analysis. As shown in Fig. 1a, all diffraction peaks agree well with the standard pattern of MnF2 (JCPDS card No. 24-0727), thus indicating high purity of the asprepared sample. XRD pattern of CoF2 also indicates the successful synthesis of target material with high purity (Fig. S1). The XPS survey further confirms the stoichiometry of the as-prepared MnF2, as shown in Fig. 1bed. In Fig. 1b, the survey spectrum of MnF2 displays the dominant peaks of Mn and F with an atomic ratio of 1:2.12, indicating the formation of MnF2. The tiny peaks of O 1s and C 1s could arise from a negligible amount of IL residual. The magnitude of peak splitting of Mn 3s doublet (DE3s) are usually used for diagnostic of oxidation state according to a linear equation (vMn ¼ 9.67  1.27DE3s/eV) [19]. As shown in the inset of Fig. 1b,

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Fig. 1. (a) XRD pattern, (b) survey XPS spectrum (inset: enlarged XPS spectrum of Mn 3s), high-resolution XPS spectra of (c) Mn 2p, and (d) F 1s of MnF2 powder.

DE3s in the as-prepared MnF2 powder is about 6.1 eV, confirming the Mn valence of 2þ. Fig. 1c shows the high-resolution XPS spectrum of Mn 2p doublet, which can be deconvoluted into three components: Mn 2p1/2 at 655.9 eV, shake-up satellite at 650.6 eV, and Mn 2p3/2 at 643.6 eV [20]. The unambiguous shake-up satellite is a good indication of Mn2þ, because the formation of shake-up satellites at 5 eV from the main peak with higher binding energy [19]. The high-resolution F 1s spectrum in Fig. 1d is fitted by only one peak at 686.3 eV [21,22]. SEM and TEM images reveal the morphology and microstructure of the as-prepared MnF2, as shown in Fig. 2. The MnF2 powder mainly consists of submicro- or nano-rods with a broad length range of hundreds of nanometres as displayed in Fig. 2a. A zoom-in SEM image in Fig. 2b presents a typical morphology of MnF2 rods with a length of ca. 400 nm and a diameter of ca. 80 nm. TEM image with low magnification confirms the rod-like morphology of MnF2, which is not perfectly uniform. The enlarged TEM images (Fig. 2d and e) show a close-up of two individual areas, which exhibit dendritic aggregation and rough surface with pores. Highresolution TEM (HRTEM) image of the area highlighted in Fig. 2e by the white rectangle is illustrated in Fig. 2f. The well-resolved lattice fringes marked by the parallel lines are indexed to the (101) plane with a spacing of 0.2749 nm and the (110) plane with a spacing of 0.3475 nm, respectively, matching well with the XRD results. The selected-area electron diffraction (SAED) pattern (Fig. 2g) is indexed to a tetragonal MnF2 crystallite along the [111] zone axis, indicating the good crystallinity of MnF2 in nature and the preferential growth direction. The as-prepared CoF2 powder by utilizing the same method exhibits different morphology from that of MnF2. In Fig. 3a, it is observed that some cuboids, which are governed by the inherently tetragonal structure of CoF2, gather together to form a large aggregate with a diameter of ca. 1 mm. Fig. 3b clearly presents the morphology of CoF2 with a cross section that ultrafine particles assembles into a cuboid with some pores, while the cuboids further coalesce with each other to form a stable spherical structure in

order to minimize the surface energy [23,24]. TEM image also identifies the presence of the pores with high brightness (Fig. 3c), especially displayed by the zoom-out TEM image in the inset of Fig. 3c. Such pores will greatly increase ion diffusion, thus leading to good electrochemical performance [24,25]. HRTEM image, enlarged from the marked white frame of Fig. 3c, reveals welldefined lattice fringes of plan (110) with a spacing of 0.3397 nm, in good agreement with the XRD result. The capacitive performance of MnF2 and CoF2 was evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests. Fig. 4a shows CV curves of MnF2 in the range from 0.4 to 0.55 V (vs. saturated calomel electrode (SCE)) at various scan rates in 1.0 M KOH electrolyte. Redox current peaks are observed in CV curves of MnF2 electrode as highlighted by the rectangle, especially in the curves at a low scan rate, signifying a pseudocapacitive behaviour. The specific capacitance calculated from the CV curves at 20, 100, 200 and 500 mV s1 is about 107.4, 71.9, 63.2 and 30.0 F g1 in sequence, based on Eq. (1)

C¼

1 mvDV

Z iðVÞdV

(1)

where m is the mass of active materials, v is the potential scan rate, DV is the potential window in the CV curves, i(V) is the voltammetric current. The GCD curves of the as-prepared MnF2 rods at different current densities in 1.0 M KOH electrolyte are shown in Fig. 4b. The specific capacitance is about 170, 130.5, 116.3, 91.2, 68.7 and 56.4 F g1 at the current density of 1, 2, 3, 5, 8 and 10 A g1, based on the calculation according to Eq. (2)

C¼

IDt mDU

(2)

where I is the applied current, DU is potential window in the discharging process and Dt is the discharging time. It is noteworthy that MnF2 electrode exhibits impressive specific

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Fig. 2. (a) SEM, (b) enlarged SEM, (c) TEM, (dee) enlarged TEM, (f) HRTEM, and (g) SAED images of MnF2 rods.

Fig. 3. (a) SEM, (b) enlarged SEM, (c) TEM, and (d) HRTEM images of CoF2, inset of (c) zoom-out TEM image of CoF2.

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Fig. 4. (a) CV curves, (b) GCD curves at different current densities, (c) cycling stability of the MnF2 electrode at a current density of 2 A g1 in 1.0 M KOH electrolyte, the inset shows the GCD curves after 3000 cycles, (d) GCD curves of MnF2 electrode at different current densities in 1.0 M Na2SO4 electrolyte, (e) CV curves, and (f) GCD curves of CoF2 electrode at different current densities in 1.0 M KOH electrolyte.

capacitance, especially at high current rates. To evaluate the cycling stability, the MnF2 electrode was cycled at 1 A g1 for 1000 cycles (Fig. S2) and 2 A g1 for 3000 cycles (Fig. 4c), respectively. The specific capacitance of 139.4 F g1, i.e. 82.4% of the initial specific capacitance, is maintained after 1000 cycles at 1 A g1. Moreover, 86.1% of the initial specific capacitance (113.8 F g1) is remained after 3000 cycles at 2 A g1, indicating a good cycling stability. The insets of Fig. S2 and Fig. 4c present the GCD curves after 1000 cycles at 1 A g1 and 3000 cycles at 2 A g1, respectively. It is also noted that the humps on the chargeedischarge curves become more obvious compared with those in Fig. 4b, suggesting the occurrence of redox reactions. The electrochemical performance of MnF2 electrode in neutral electrolyte (1.0 M Na2SO4 solution) was also investigated, as shown in Fig. 4d. In contrast, the discharge capacitance is only 33.4, 12, 7.2 and 3.5 F g1 at 1, 2, 3 and 5 A g1 in sequence, showing negligible electrochemical activity. CoF2 electrode shows similar electrochemical performance to MnF2 electrode. Tiny peaks of redox reactions on the CV curves can be observed in 1.0 M KOH solution as shown in Fig. 4e. The specific capacitance calculated from the CV curves at 20, 100, 200 and 500 mV s1 is about 260.9, 165.5, 108.6 and 50.0 F g1 in order. And the discharge capacitance of CoF2 electrode is 122.6, 105.6, 95.4,

81.7, 70.6 and 63.0 at 1, 2, 3, 5, 8 and 10 A g1 in sequence (Fig. 4f). The capacitances estimated from the CV curves and those at high current rates (e.g. 5, 8 and 10 A g1) are higher than the corresponding values of MnF2 electrode. This could be ascribed to rich pores in CoF2 cuboids with hierarchical structure, which is favourable for the ion diffusion at high scan rates and current densities [26]. To rule out the contribution of Ni foam substrate and carbon black, the GCD curves of bare Ni foam and carbon black were also tested at the same current density as that of MnF2 in 1.0 M KOH solution as shown in Figs. S3 and S4. The bare Ni foam only exhibits 5.4 F g1 at 1 A g1, while carbon black on Ni foam shows 13.7 F g1 at 1 A g1. Such small values indicate that there is a negligible contribution of Ni foam and carbon black to the overall capacitance of the electrode. In this work, the specific capacitance of MnF2 and CoF2 is comparable to those of Mn3O4 [27], Co3O4 [28], and better than those of reported graphene/SnO2 [29], graphene/ZnO [29], and activated carbon/Fe3O4 [30]. But it is not as high as those of graphene/MnO2 [31,32], and Co(OH)2 [33], which could be attributed to two factors. One is the large size of MnF2 and CoF2 particles, because the bulk volume slows the current response to voltage reversal at each end potential and causes a large initial potential

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drop (iR drop) at high current densities [34]. The other is the low electronic conductivity of TM fluorides originating from strong ionicity of TM-F bonds, because electron transport is necessary for the redox transitions [12,35,36]. As a result, it is reasonable to deduce that the specific capacitance could be highly improved by either downsizing the particles of TM fluorides [37] or incorporating with electrically conductive materials [11,38,39], which is similar to the application of TM fluorides in lithium ion batteries. To gain insight into the mechanism on capacitive behaviour of submicro-/nano-sized MnF2 rods and CoF2 cuboids, the cycled MnF2 and CoF2 electrodes in 1.0 M KOH electrolyte were investigated by SEM, XRD and XPS. The overview SEM image of cycled MnF2 electrode consists of binder, carbon black, and MnF2 rods is presented in the inset of Fig. 5a. From the enlarged SEM image (Fig. 5a), it is noted that the surface of cycled MnF2 rods becomes rough compared with that of fresh MnF2 rods. A large amount of flakes form on the rods, indicating a chemical reaction occurs during the GCD process. XRD pattern of the cycled MnF2 electrode suggests that a phase of MnOOH (JCPDS card No. 41-1379) appears accompanying MnF2 as shown in Fig. 5b. This signifies that MnF2 converts to MnOOH in 1.0 M KOH electrolyte, during the GCD process. Therefore, a pseudocapacitive behaviour occurs for MnF2 in 1.0 M KOH electrolyte. TEM image of cycled MnF2 rod (Fig. S4a) also confirms the change of morphology, because there is a thin layer with the thickness of ca. 40 nm as marked by the dash line, which is consistent with flakes observed by SEM. The EDX spectrum of the cycled MnF2 rod (Fig. S4b) reveals the existence of Mn, F, O, in good agreement with the XRD result. The elements of C and Cu come from carbon black and Cu grid, respectively. Furthermore, XPS survey of the cycled MnF2 (Fig. 5c) shows a distinct increase of O 1s peak compared with that of fresh MnF2 (Fig. 1b), which also indicates the occurrence of an oxidation reaction. The splitting width of the Mn 3s doublet peaks decreases to 5.2 eV, corresponding to a valence of 3þ, which indicates the oxidation of Mn2þ.

The high-resolution O 1s spectrum of cycled MnF2 electrode is shown in Fig. 5d. After cycling, an extra peak locating at 530.4 eV appears, which can be assigned to Mne(OH)O band [34], in contrast to the absence in the O1s spectrum of MnF2 before cycling (inset of Fig. 5d). The cycled CoF2 electrode shows similar change as shown in Fig. 6. Fig. 6a and b compare the evolution of CoF2 electrode before and after cycling. It is also noted that the surface becomes dendritelike structure, rougher than that before cycling. In Fig. 6c, XRD pattern of the cycled CoF2 electrode also clearly presents remarkable peaks assigned to CoOOH (JCPDS card No. 07-0169) accompanying weak peaks of CoF2, indicating an mutual conversion of Co2þ and Co3þ. XPS survey of cycled CoF2 in Fig. 6d also shows the presence of O 1s, confirming the conversion by the redox reactions. Based on the analysis of cycled MnF2 and CoF2 electrodes, it is rational to propose that the capacitive behaviour of MF2 (M ¼ Mn, Co) arises from the mutual conversion of MF2 and MOOH in alkaline electrolytes, such as 1.0 M KOH. The electrochemical reaction can be expressed as Eq. (3): Mn2þþ3OH4MnOOH þ H2O þ e

(3)

This is consistent with the result of NiF2 film reported by Yang et al., except that no intermediates i.e. M(OH)2 were detected in this work. In contrast to alkaline electrolytes, it could be difficult for such kind of conversion to take place in neutral electrolytes due to the scarceness of OH ions, so no remarkable capacitive behaviour appears in 1.0 M Na2SO4 solution. 4. Conclusions In summary, submicro-/nano-sized MnF2 rods and hierarchical CoF2 cuboids were prepared with the assistance of ionic liquid by a facile precipitation method. The as-prepared MF2 powders exhibit

Fig. 5. (a) SEM, (b) XRD pattern, (c) survey XPS spectrum, (d) high-resolution O 1s XPS spectrum of the cycled MnF2 electrode; inset of (a): overview SEM image, inset of (c): enlarged Mn 3s XPS spectrum of the cycled MnF2 electrode; inset of (d) high-resolution O 1s XPS spectrum of the fresh MnF2 electrode.

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Fig. 6. (a) SEM image of fresh CoF2, (b) SEM image, (c) XRD pattern, (d) XPS survey of cycled CoF2 electrode.

remarkable pseudocapacitive performance in 1.0 M KOH aqueous electrolyte rather than in 1.0 M Na2SO4 aqueous electrolyte. The mechanism is elucidated that the capacitance stems from the redox reaction between MF2 to MOOH in aqueous alkaline electrolytes based on the detailed analysis of cycled electrode materials. This study definitely extends the application range of metal fluorides in energy storage systems. It can be envisaged that optimizing the microstructure and combining fluorides with electrically conductive materials via rational design will significantly improve their electrochemical performance, thus making them promising electrodes for supercapacitors. Acknowledgements This work is financially supported by the Youth Science and Technology Talents “Sail” Program of Shanghai Municipal Science and Technology Commission (15YF1413800), Shanghai Institute of Ceramics, the One Hundred Talent Plan of Chinese Academy of Sciences, National Natural Science Foundation of China (21307145), and the research grant (No. 14DZ2261200) from Shanghai government. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.10.102. References [1] [2] [3] [4] [5]

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