Phase Transformation of Ce3+-Doped MnO2 for Pseudocapacitive

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Phase Transformation of Ce Doped MnO for Pseudocapacitive Electrode Materials Kunfeng Chen, Wei Pan, and Dongfeng Xue J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07708 • Publication Date (Web): 25 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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The Journal of Physical Chemistry

Phase Transformation of Ce3+ Doped MnO2 for Pseudocapacitive Electrode Materials Kunfeng Chen, Wei Pan and 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]; Tel: +86 431 85262294

ABSTRACT: Doping is one of important methods to modify the physical and chemical properties of functional materials, which can be used to synthesize mixed ionic and electronic conducting metal oxides. Herein, the phase transformation of MnO2 from βto α-phase has been proven by doping Ce3+ ions. With the increase of the amount of Ce3+ ions, the sizes of MnO2 nanorods were first decreased to 10-20 nm, then increased to 70 nm. The capacitive performance indicated that specific capacitance of Ce-doped MnO2 electrode materials increased 10-fold compared with undoped MnO2, while the charge transfer resistance of Ce-doped MnO2 decreased. The present results show that rare earth ions can be used as promising dopant to modify the crystallization behavior and electrochemical performance of MnO2 electrode materials.

INTRODUCTION Supercapacitor is one of important energy storage devices, which possesses high power density and long cycling life compared with batteries.1,2 The often used electrode materials for supercapacitors are electric double-layer type (activated carbon, graphene) 1

ACS Paragon Plus Environment


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and pseudocapacitive type materials (metal oxides, hydroxides, sulfides, nitrides).3 Although they show high specific capacitance, transition metal oxides, i.e., RuO2, MnO2, NiO, V2O5, often suffer from low conductivity, and slow ion diffusion rate, leading to the reduction of electrochemical performance.4 To improve their conductivities, transition metal oxides were blended with conductive additives, such as graphene, carbon nanotube, graphite, other conductive carbons, or were grown on the current collectors, i.e., Ni foam, Ti foil, Fe foil, graphene film/foam.5,6 However, these methods can only increase the apparent conductivities of electrode materials. The inherent conductivity can only be modified by ion doping or defects in electrode materials.7,8 For example, the hole concentration of Na doped SnSe material increased from 3.2 × 1017 to 4.4 × 1019 cm−3 at room temperature, which can enhance the electronic conductivity.9 It is reported that V-doped MnO2 nanosheets displayed a high specific capacitance of 439 F/g, due to the shortened ion diffusion distance and increased electronic conductivity.10 Because the electron transfer and ion diffusion simultaneously occurred in pseudocapacitive reaction, the most ideal electrode materials should be the mixed ionic and electronic conducting metal oxides.11 It is reported that doping is an important method to control the ionic and electronic conductivities.12,13 As a typical pseudocapacitive material, MnO2 possesses multiple crystal structures, i.e., α, β, γ, δ-phases, etc., which can show different electrochemical performances.14 For example, Faradaic reactivity sequence of MnO2 electrode materials was δ >α- > γ- > β-MnO2 due to their different tunnel and layer structures, which can enable the ion intercalation reactions.15,16 The control of doping of MnO2 materials can also favor the 2


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increase of their ionic and electronic conductivities.17 The resistance of MnO2 material is high, 105−106 Ω cm, leading to low rate performance. It is reported that K+ ions can stabilize MnO2 with large tunnel or layer structures, for example in birnessite K0.17MnO2 (interlayer space of 0.7 nm), K+ ions was settled in the interlayer.18 The ionic radius of Ce3+/Ce4+ is about 0.1 nm approaching the radius of K+ 0.133 nm. In addition, Ce ion has two oxidation states (Ce3+ and Ce4+), which can show pseudocapacitive activity.19 Recently, MnO2/CeO2 composites showed high electrochemical performance, for example, the CeO2 nanowire@MnO2 nanostructures exhibited pseudocapacitance of 255 F g−1 with good rate capability, owing to the synergistic effect between CeO2 and MnO2.20 However, the study of the doping effect of Ce ions on MnO2 electrode materials was rarely reported. Considering the high cost of rare earth materials, we studied the doping of rare earth ions on MnO2 electrode materials to increase their pseudocapacitance performances. Herein, we used rare earth ions (Ce3+) doping to modify the pseudocapacitive performance of MnO2 electrode materials. Firstly, Ce-doping changed the phase of MnO2 from β- to α-phase, while the α-MnO2 electrode materials show high capacitance than that of β-MnO2. Secondly, Ce-doped MnO2 showed increased conductivity compared with undoped MnO2. The present results confirmed that rare earth ions doping in electrode materials can be one of promising methods to synthesize high performance mixed ionic and electronic conducting metal oxide electrode materials. EXPERIMENTAL SECTION Materials synthesis: In a typical procedure, 1 mmol MnSO4, 2 mmol (NH4)2SO4 and 3



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1 mmol (NH4)2S2O8 were dissolved in 20 ml deionized water. After the complete dissolution of reagents, different amounts of Ce(NO3)3⋅6H2O, 0.0, 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 mmol, were added into above solution with stirring. Then the above solution was transferred into an autoclave and maintained at a temperature of 140 °C for 12 h. After cooling down to room temperature, the black precipitates at the bottom of kettle were collected by filtering and washing with deionized water. The as-obtained precipitates were dried at 70 °C for 5 h, which can be used for further characterization. Characterization: The morphologies and phases of as-obtained samples were measured with FESEM (Hitachi- S4800), TEM (FEI Tecnai G2 F20) and XRD (Bruker D8 Focus). For electrochemical test, electrochemical workstation (CHI 660E) was used to conduct cyclic voltammetry (CV) and galvanostatic discharging-charging tests. Work electrode was prepared by firstly mixing as-obtained sample, carbon black and PTFE (weight ratio of 6:3:1), then pressing mixed samples on Ni foam. Pt wire and saturated calomel electrode (SCE) were served as counter and reference electrodes. The electrolyte was 1 M Li2SO4 solution. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of MnO2 materials with doping different amounts of Ce3+ ions. Without doping, β-MnO2 materials were formed in consistent with standard JCPDS No. 1-799. In β-MnO2, octahedral [MnO6] units by corner-sharing forms are to form 1×1 tunnel structure with the size of 0.23 × 0.23 nm.21 With adding 0.05 mmol Ce3+ to the precursor solutions, the as-obtained products included both αand β-MnO2. With further increasing the Ce3+ amount, only α-MnO2 can be formed in 4


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consistent with standard JCPDS No. 44-141. α-MnO2 has 2×2 tunnel structure with the size of 0.46 × 0.46 nm, which can accommodate large-size ions, such as K+ (0.133 nm). It should be indicated that the XRD peak intensity of α-MnO2 sample with adding 0.2 mmol Ce3+ is weak, showing the formation of small-size of MnO2 nanomaterials (Figure 1). SEM and TEM images of Ce-doped MnO2 materials are shown in Figures S1 and 2. All MnO2 samples display nanorod-like morphologies, but with different sizes. β-MnO2 nanorods have the diameter of 120 nm and the length of several micrometers (Figure S2). With the increase of the Ce-doped amount, the diameter of α-MnO2 rods decreases from 50 to 10-20 nm, then increase to 70 nm, while the length decreases from 500 to 100 nm, then increases to 400 nm (Figure S1). α-MnO2 doped with 0.2 mmol Ce3+ has the smallest size (10-20 nm in diameter and 100 nm in length). As shown in Figure 2b, some nanorods are grown together at the end. HRTEM image shows that the d space of 0.49 nm corresponds to (200) facet of α-MnO2 (Figure 2c). Electron diffraction pattern also confirm the formation of α-MnO2 (Figure 2d). Figure S3 shows IR spectrum of Ce-doped MnO2 materials. It is reported that the split bands between 400 cm−1 and 800 cm−1 were attributed to the Mn–O band. After doped with Ce3+ ions, peaks shift toward high wavenumber, indicating that Ce3+ ions have intercalated into MnO2 lattices.22 ICP data of Ce and Mn ratio in final production are shown in Table 1. The highest Ce concentration in MnO2 is 5.6 % with adding 0.2 mmol Ce3+ ions. When excess Ce3+ ions were added, the Ce concentrations in MnO2 were decreased to about 3%. Excess Ce3+ ions disturbed the crystallization of Ce-doped MnO2, leading to the formation of 5



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different Ce existing forms. It should be noted that Ce-doped amount in final MnO2 materials can significantly affect the size of MnO2 materials. According to above results, scheme 1 shows the phase transformation mechanism of Ce-doped MnO2. Large size of Ce3+ can stabilize the 2×2 tunnel structure, thus favor the formation of α-MnO2 by Ce3+ entering into MnO2 tunnel. During hydrothermal reaction, MnO2 can be formed by the following chemical reaction. Mn2+ + S2O82− + 2H2O → MnO2 + 4H+ + 2SO42−

(1)

The existence of K+ ions in solution can change the thermodynamic stable phase of β-MnO2 to α-MnO2. Ce3+ ions possess strong interaction with O2− ions than K+, because they have empty 5d and 4f orbits. During the crystallization process, Ce3+ ions favored the formation of 2×2 tunnel structures by inserting Ce3+ ions into the 2×2 tunnels to avoiding their collapse. Therefore, α-MnO2 were formed at the presence of Ce3+ ions. In addition, Ce3+ ions can control the sizes of α-MnO2 nanorods. The thermodynamic morphology of α-MnO2 material is rod-like according to our previous calculation.14 The continuous extension of MnO6 octahedra along the c-axis direction can lead to the preferred growth of α-MnO2 along the [001] direction.23 When the concentration of Ce3+ ions less than 0.2 mmol, Ce3+ ions were inserted into the tunnel structure of α-MnO2 at the surface of nanorods, inhibiting their growth along and perpendicular the [001] direction. However, excess Ce3+ ions cannot further play the role, because the doping amount in MnO2 materials was decreased. The doped Ce ions in MnO2 materials should inhibit the continuous extension of MnO6 octahedra along the c-axis direction. Using as-obtained MnO2 as electrode materials for supercapacitors, all samples 6


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display typical pseudocapacitance characterization with rectangle CV curves and straight discharge-charge curves (Figure 3). The specific capacitance of β-MnO2 electrode materials is only 13.1 F g−1, while all specific capacitances of α-MnO2 samples are larger than β-MnO2 (Table 1). The smallest sized α-MnO2 electrode materials show the highest value of 101.1 F g−1. Small sized nanorods can shorten the ion diffusion and electron transfer length leading to high electrochemical performance.24 Table S1 shows the comparison of specific capacitances of the reported MnO2 electrodes and the present work. α-MnO2 electrode materials can show specific capacitance of 100-300 F g−1 at low current densities. It is reported that poor-crystallized supercapacitor electrode materials often showed high specific capacitance than well-crystallized materials due to the latter had more active sites and larger specific surface area. However, Ce-doped MnO2 materials were well-crystallized materials. With the pure α-phase MnO2 materials, Ce-doped MnO2 electrode materials showed relatively high specific capacitance at high current density of 1 A g−1. Furthermore, we studied the conductivity of Ce-doped MnO2 from electrochemical impedance spectrum (Figure 4). From the Nyquist images, the charge transfer resistances decreased from 2 Ω for β-MnO2 to 1.3 Ω for Ce-doped α-MnO2 with adding 0.2 mmol Ce3+ ions (Figure 4a and 4b). Low-frequency straight lines almost perpendicular the Z’ axis indicated the typical capacitive performance (Figure 4a). When the concentration of Ce3+ ions further increased, the charge transfer resistances also increased to 2 Ω. The doped Ce ions in MnO2 materials can indeed increase conductivity of MnO2 electrode materials. From Bode images, phase angles of all 7



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samples can approaching 80°, showing the typical capacitive performance (Figure 4c). The curves shift to low frequency with the increase of doped Ce3+ ions. The largest shift can be found in Ce-doped α-MnO2 with adding 0.2 mmol Ce3+ ions, which corresponds to high specific capacitance. As shown in Figure 4d, the peaks of Ce-doped MnO2 shift to high frequency. After Ce-doped, the time constant was decrease (τ = 1/ω*), indicating fast surface redox reaction.25,26 According to above results, we concluded that Ce-doped MnO2 not only increased the specific capacitance, but also optimized the electron and ion transfer/diffusion. Figure 5 shows the rate capability and cycling performance of Ce-doped MnO2 electrode materials with adding 0.2 mmol Ce3+ ions. When the scan rates changed from 10 to 100 mV/s, CV curves show rectangle outlines, indicating the typical capacitive characterization (Figure 5a). The specific capacitances of Ce-doped MnO2 electrode materials are 80.8 and 66.5 F/g at the current densities of 5 and 10 A/g (Figure 5b). The specific capacitance can keep 65.8 % changing from 1 to 10 A/g. The cycling performance is shown in Figure 5c tested at the current density of 5 A/g. After 1000 charge-discharge cycles, Ce-doped MnO2 electrode materials show 99.5 % of capacitance retention. The results proved that Ce-doped MnO2 materials can be served as promising electrode materials for supercapacitors. CONCLUSION In this work, we proved that Ce-doping not only induced the phase transformation from β to α-MnO2, but also decreased the size of nanorods from both longitude and latitude. Ce3+ ions can significantly adjust the crystallization behavior of MnO2 owing 8


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to they possessing 5d4f orbit structures. Ce3+ ions can insert into the tunnel structure of α-MnO2 at the surface of nanorods, inhibiting their growth along and perpendicular the [001] direction. Served as supercapacitor electrode materials, Ce-doped MnO2 materials made the 10-fold increase in specific capacitance and the charge transfer resistance was decreased. Ce-doping is an important route to optimize electron and ion conductivity of MnO2 electrode materials. Rare earth ions doped electrode materials can be one of promising method to synthesize high performance mixed ionic and electronic conducting metal oxides.

Supporting Information Available SEM images and IR patterns of Ce:MnO2 materials. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (grant nos. 51125009, 91434118, 21601176), the National Natural Science Foundation for Creative Research Group (grant no. 21521092), the External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121522KYS820150009), the Hundred Talents Program of the Chinese Academy of Sciences, and Jilin Provincial Science and Technology Development Program of China (Grant No. 20160520002JH) is acknowledged. REFERENCES: 9



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0.30mmol Ce 0.20mmol Ce 0.10mmol Ce 0.05mmol Ce 0.00mmol Ce β-MnO2

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2 theta (degree) Figure 1 XRD patterns of β-MnO2 and Ce-doped α-MnO2 materials with different Ce3+ amounts. The standard JCPDS Nos. 44-141 for α-MnO2 and 1-799 for β-MnO2.

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Scheme 1. Schematic drawing of the phase transformation and size change of Ce-doped MnO2 nanorods.

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c

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80

100 120 140 160

Time (s)

110

Capacitance retention (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


105 100 95 90 85 80

0

200

400

600

800

1000

Cycle number (n)

Figure 5 Pseudocapacitive performances of Ce-doped MnO2 electrode materials with adding 0.2 mmol Ce3+ ions. (a) CV curves at different scan rates, (b) charge-discharge curves at different current densities. (c) cycling performance at the current density of 5 A/g.

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Table 1 Structure, Ce doping amount and specific capacitance of Ce-doped MnO2 electrode materials Sample No. 1 2 3 4 5 6 7

Phase of MnO2 β β+α α α α α α

Added Ce3+ amount (mmol) 0.00 0.05 0.10 0.20 0.30 0.40 0.50

Ce:Mn ratio from ICP (%) 0.0 1.2 2.1 5.6 3.0 3.1 3.4

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Specific capacitance (F/g at 1A g−1) 13.1 59.6 57.0 101.1 65.8 52.5 53.1

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TOC Ce3+

β-MnO2

Ce doped α-MnO2

Increase of Ce3+ amount

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Phase Transformation of Ce3+-Doped MnO2 for Pseudocapacitive

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