Enhanced Pseudocapacitive Performance of α-MnO2 by Cation

Nov 23, 2016 - During cyclic voltammetry cycling in a broad potential window, K+ ions can be replaced by Na+ ions, which determines the pseudocapacita...
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Enhanced Pseudocapacitive Performance of α‑MnO2 by Cation Preinsertion Nawishta Jabeen,†,‡ Qiuying Xia,†,‡ Serguei V. Savilov,§ Sergey M. Aldoshin,∥ Yan Yu,*,⊥ and Hui Xia*,†,‡ †

School of Materials Science and Engineering and ‡Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing 210094, China § Department of Chemistry and ∥Department of Physical Chemistry Engineering, M. V. Lomonosov Moscow State University, Moscow 119991, Russia ⊥ Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *

ABSTRACT: Although the theoretical capacitance of MnO2 is 1370 F g−1 based on the Mn3+/Mn4+ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K+-inserted α-MnO2 (or KxMnO2) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of KxMnO2 is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K+ ions can be replaced by Na+ ions, which determines the pseudocapacitance of the electrode. The K+ or Na+ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0−1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K+-free α-MnO2 nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO2 and new strategy to further improve the specific capacitance of MnO2-based electrodes. KEYWORDS: MnO2, pseudocapacitance, charge storage mechanism, nanorod, supercapacitors

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high theoretical specific capacitance of 1370 F g−1.20,21 MnO2 has different crystallographic forms, including α, β, γ, and δ, whereas α-MnO2 and δ-MnO2 (birnessite) possess more open structures, thus exhibiting better supercapacitive performance.22−25 The experimentally obtained specific capacitance of MnO2, however, is far below the theoretical value, except for some special cases with extremely low loading of active material. The discrepancy has been attributed to the poor electrical conductivity (10−5− 10−6 S cm−1) and limited surface area of MnO2.26 Various strategies, including designing nanoarchitectures and developing MnO2-based composites with conductive components, have been used to enhance the utilization and specific capacitance of MnO2.27−31 The improvement is still limited, and to achieve a large specific capacitance of MnO2-based electrode with a practical mass loading is still a challenge. On the other hand, the electrochemical behavior of MnO2 cannot be fully understood based on the previously proposed charge storage mechanism.32,33 Generally, two mechanisms have been proposed to

INTRODUCTION Supercapacitors, possessing higher energy density than traditional capacitors and higher power density than batteries, have drawn considerable attention because of their potential applications in power tools, electric vehicles, and stand-by power systems.1−5 However, the energy density of current commercialized supercapacitors is still low and cannot satisfy the fast growing power requirement of the next-generation electronic devices.3,6 To solve this problem, developing pseudocapacitive electrode materials with fast and reversible redox reactions at the electrode surface is the key because the specific capacitance delivered by pseudocapacitive materials could be multiple times larger than that of the electric double-layer capacitive materials, which can only physically store charges via reversible ion adsorption/desorption at the electrode/electrolyte interface.7−10 Tremendous efforts have been devoted to the redox-active transition metal oxides (such as NiO, RuO2, Co3O4, Fe2O3, SnO2, WO3, TiO2, and MnO2) to develop high performance pseudocapacitive electrode materials with improved specific capacitance.11−19 Among these materials, MnO2 stands out as one of the most promising electrode materials because of its natural abundance, low cost, environmental friendliness, and © 2016 American Chemical Society

Received: October 1, 2016 Accepted: November 23, 2016 Published: November 23, 2016 33732

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

Research Article

ACS Applied Materials & Interfaces explain the charge storage behavior of MnO2. The first one based on the surface adsorption of cations from electrolytes on MnO2 is described as follows:32

The morphology and microstructure of the samples were investigated by field emission scanning electron microscopy (FESEM, FEI Quanta, 250F), transmission electron microscopy (TEM, FEI Philips CM300 UT/FEG), and high-resolution transmission electron microscopy (HRTEM). The electronic structure and compositional information on the samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) and energy-dispersive X-ray spectroscopy (EDS). The elemental analyses of the samples were performed by scanning transmission electron microscope (STEM) mode with EDS mapping. Electrochemical Measurements. The carbon cloth supported MnO2 nanorod arrays were directly used as electrodes for electrochemical measurements without any binders or conductive additives. The electrochemical measurements of the samples were measured in a standard three-electrode setup with a Pt as counter electrode and Ag/AgCl as reference electrode. CHI760C electrochemical workstation (Chenhua, Shanghai) was used to perform cyclic voltammetry (CV) and galvanostatic charge−discharge measurements in 1 M Na2SO4 aqueous electrolyte. CV and galvanostatic charge−discharge measurements were carried out in different potential windows of 0−0.8, 0−1.0 and 0−1.2 V (vs Ag/AgCl).

(MnO2 )surface + M− + e ⇔ (MnOOM)surface (M+ = Li+, Na +, K+)

(1)

The second one involves the intercalation and deintercalation of electrolyte cations in the bulk of MnO2.33 MnO2 + M− + e ⇔ MnOOM

(2)

The theoretical capacitance of MnO2 is calculated based on the Mn4+/Mn3+ redox couple associated with cation adsorption or intercalation. MnO2-based electrodes were usually tested in a positive potential window of 0−0.8 or 0−1.0 V (vs Ag/AgCl) in neutral aqueous electrolytes (such as 1 M Na2SO4). In the following text, all electrode potentials are referred to the Ag/AgCl reference electrode. With an open circuit potential of about 0 V, the initial electrochemical process of the MnO2 electrode is a charge or oxidation process, which is contradict to the forward reactions of equations 1 and 2. Recently, several groups reported the improved supercapacitive performance of cation predoped MnO2 and speculated that the incorporation of cation in MnO2 can facilitate the ion diffusion.25,34,35 However, the improved specific capacitance even at a slow current rate cannot be solely explained by the improved ion diffusion. Therefore, the charge storage mechanism of MnO2 and the influence of cation preinsertion require further investigation. α-MnO2 is body-centered tetragonal with 1 × 1 and 2 × 2 tunnels, where the 2 × 2 tunnels are suitable for cation intercalation and deintercalation. The 2 × 2 tunnels are generally stabilized by preintercalated cations introduced during the synthesis process.36 In this work, K+-stabilized α-MnO2 (or KxMnO2) nanorod arrays were prepared on carbon cloth and investigated as electrodes for supercapacitors in 1 M Na2SO4 electrolyte. To understand the charge storage mechanism of KxMnO2 and the function of preinserted K+, we explored the electrochemical behaviors of the electrodes in different potential windows. It is interesting to find that the pseudocapacitive performance of the KxMnO2 electrodes is highly dependent on the potential window. Extending the upper cutoff potential to 1.2 V can significantly enhance the specific capacitance with high reversibility. Combining with the structural characterizations, the charge storage mechanism of KxMnO2 has been proposed and the origin of pseudocapacitance of MnO2 has been discussed.

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RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of the carbon cloth supported KxMnO2 nanorod arrays. Except for the diffraction peaks from carbon cloth, all other diffraction peaks can be assigned to α-MnO2 with tetragonal symmetry (I4/m, JCPDS No. 44− 0141). Figure 1b−d show the core-level XPS spectra of Mn 2p, K 2p, and O 1s for the as-prepared KxMnO2 nanorod arrays. Two peaks located at 642.5 and 653.6 eV (Figure 1b) can be observed in the Mn 2p XPS spectrum, corresponding to the spin orbit doublet of Mn 2p3/2 and Mn 2p1/2.32 Both Mn 2p3/2 and Mn 2p1/2 peaks can be best fitted with two components. The major components located at 642.3 and 654.1 eV can be attributed to Mn4+, while the minor components located at 641.2 and 653.2 eV indicate the existence of Mn3+ in KxMnO2.38 Figure 1c reveals the notable presence of K+ (K 2p1/2 = 295.43 eV and K 2p3/2 = 292.7 eV) within the sample, ensuring that the structure is stabilized by K+ ions located at the center of 2 × 2 tunnels. The O 1s core-level XPS spectrum (Figure 1d) can be deconvoluted into three components at 529.9, 531.4, and 532.3 eV, corresponding to the oxygen species in the MnO2 (Mn−O−Mn), hydroxyl groups (Mn−O−H), and absorbed water (H−O−H), respectively.36,37 The morphology of the as-prepared KxMnO2 nanorod arrays was investigated by FESEM. Figure 2a−c show the FESEM images of the KxMnO2 nanorods grown on a single carbon fiber at different magnifications. It can be seen that KxMnO2 nanorods are uniformly aligned on the carbon fiber with an average diameter of 100−200 nm. The low-magnification FESEM images of the as-prepared KxMnO2 nanorod arrays grown on the carbon cloth show uniform distribution of nanorods on every carbon fibers (Figure S1). Figure 2d presents a TEM image of a single KxMnO2 nanorod at low magnification, revealing one-dimensional morphology with a flat end of the nanorod. The HRTEM image (Figure 2e) reveals the single crystalline feature of the nanorod, and the interplanar spacing of 0.5 nm of the wellresolved lattice fringes can be attributed to the (200) planes of α-MnO2. The fast Fourier transform (FFT) pattern and HRTEM analysis suggest that the nanorods grow along the [001] direction (c-axis). A representative STEM image of a single KxMnO2 nanorod and corresponding element mappings for Mn, O, and K elements are shown in Figure 2f. It can be clearly observed that all the elements are distributed throughout the whole nanorod area homogenuosly, indicating the existence of certain amount of K+ ions in the 2 × 2 tunnels of the α-MnO2. Without using K source,

EXPERIMENTAL SECTION

Synthesis of KxMnO2 Nanorod Arrays. KxMnO2 nanorod arrays were grown on carbon cloth by a facile hydrothemal method. In a typical synthesis, 4 mmol of KMnO4 and 1 mL of concentrated HCl (38 wt %) were mixed in 45 mL deionized water to make a solution. After stirring for 10 min, the solution was transferred into a 60 mL Teflon-lined stainless steel autoclave, along with a piece of carbon fabric immersed in the solution. The autoclave was heated at 140 °C for 12 h in an electric oven. After the hydrothermal treatment, the sample was taken out and rinsed several times in deionized water, and then was annealed at 400 °C for 2 h in air. The mass loading of MnO2 on carbon cloth was about 2−3 mg cm−2, which was measured by a microbalance (ModelCPA225D with a resolution of 0.01 mg). For comparison, the K+-free α-MnO2 nanorod arrays were also prepared on carbon cloth according to literature and the experimental details can be found in the Supporting Information.37 Materials Characterization. The crystallographic information and phase purity of the samples were investigated by X-ray diffraction (XRD, Bruker-AXS D8 Advance with monochromatized Cu Kα radiation). 33733

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

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Figure 1. (a) XRD pattern and high-resolution XPS spectra of the (b) Mn 2p, (c) K 2p, and (d) O 1s for the KxMnO2 nanorod arrays.

Figure 2. (a−c) FESEM images of the KxMnO2 nanorod arrays at different magnifications. (d) TEM image of a single KxMnO2 nanorod. (e) HRTEM image of the KxMnO2 nanorod (inset is the corresponding FFT pattern). (f) STEM image of the single KxMnO2 nanorod with corresponding elemental mappings of Mn, O, and K.

the K+-free α-MnO2 nanorod arrays can also be prepared on the carbon cloth substrate but with much finer size (Figure S2). The electrochemical measurements were all conducted in three-electrode cells using 1 M Na2SO4 electrolyte. Figure 3a shows the typical CV curves of the KxMnO2 electrode at a scan rate of 5 mV s−1 in different potential windows of 0−0.8, 0−1.0, and 0−1.2 V, respectively. It is noted that the CV curves exhibit distinct behaviors and current densities in different potential

windows. The CV curve of 0−0.8 V displays nearly ideal rectangular shape, which is characteristic CV behavior for the doublelayer capacitive electrodes. As the upper cutoff potential is increased to 1.0 V, the CV curve exhibits a pair of redox peaks at about 0.4 V. In addition, an obvious current leap emerges for the charge process at about 1.0 V, which was previously assumed as water decomposition with oxygen evolution. In most of previous works on MnO2-based electrodes for supercapacitors, the 33734

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

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ACS Applied Materials & Interfaces

Figure 3. (a) Typical CV curves of the KxMnO2 electrode in different potential windows of 0−0.8, 0−1.0 and 0−1.2 V at 5 mV s−1. (b) CV curve of KxMnO2 electrode between 0 and 1.2 V with shadowed area representing the surface capacitive contribution. (c) Separation of diffusion-controlled and capacitive charge at different scan rates for the KxMnO2 electrode. (d) Charge and discharge curves of the KxMnO2 electrode in different potential windows of 0−0.8, 0−1.0 and 0−1.2 V. (e) Specific capacitances of xMnO2 electrode as a function of current density in different potential windows. (f) Ragone plots and (g) cycle performances of the KxMnO2 electrode in different potential windows. (h) Typical CV curves of the K+-free MnO2 electrode in different potential windows of 0−0.8, 0−1.0 and 0−1.2 V. (i) Specific capacitiances of KxMnO2 and K+-free MnO2 electrodes between 0 and 1.2 V as a function of current density.

upper cutoff potential for electrochemical measurement was set below 1.0 V to avoid the assumed water decomposition.22−24,27−29 In the present work, when the upper cutoff potential is further extended to 1.2 V, it is interesting to observe another pair of redox peaks between 1.0 and 1.2 V, indicating the current leap at about 1.0 V is not due to water decomposition but due to reversible redox reaction. It is known that the current contributions of the CV curve include surface-controlled process and diffusion-controlled process. The capacitive behavior of the KxMnO2 electrode can be characterized by analyzing the CV curves at different scan rates (Figure S3). By using the current separation method developed by Dunn et al.,40 the surfacecontrolled capacitance contribution can be separated in the shadowed region in the CV curve (Figure 3b). The surface capacitive and diffusion controlled contributions to the total stored charge of the KxMnO2 electrode at different scan rates are shown in Figure 3c. At a scan rate of 2 mV s−1, the estimated surface capacitive contribution accounts for 71% of the total charge storage, which is much larger than the diffusion-controlled contribution. As the scan rate increases, the diffusioncontrolled contribution becomes smaller and smaller, indicating the redox reactions are limited at the surface region at high scan rates. Figure 3d shows the typical charge/discharge curves of the KxMnO2. electrode in different potential windows of 0−0.8, 0−1.0, and 0−1.2 V, respectively, at a current density of 4 A g−1. Agreeing well with the CV results, the charge/discharge curves display two pairs of voltage plateaus and enhanced charge storage

capability in the extended potential window of 0−1.2 V. To investigate the rate performance of the KxMnO2 electrode, CV curves were carried out at different scan rates varying from 5 to 800 mV s−1 and charge/discharge measurements were also carried out at various current densities from 1 to 32 A g−1 in different potential windows of 0−0.8, 0−1.0 and 0−1.2 V, respectively (Figure S4). The specific capacitances as a function of current density in different potential windows are compared in Figure 3e. At a small current density of 1 A g−1, the specific capacitance of the KxMnO2 nanorod array electrode between 0 and 1.2 V can reach 260 F g−1, which is much larger than 195 F g−1 between 0 and 1 V and 152 F g−1 between 0 and 0.8 V. The KxMnO2 electrode exhibits good rate capability between 0 and 1.2 V and can still deliver a large specific capacitance of 115 F g−1 at a high current density of 32 A g−1. In addition to the increased specific capacitance, extension of upper cutoff potential to 1.2 V results in significant enhancement in energy density for the KxMnO2 nanorod array electrode (Figure 3f). At a current density of 1 A g−1, the KxMnO2 nanorod array electrode can deliver an energy density of 52 Wh kg−1 between 0 and 1.2 V, which is much larger than 27 Wh kg−1 between 0 and 1 V and 13 Wh kg−1 between 0 and 0.8 V. To investigate the stability of the KxMnO2 electrode working in different potential windows, Figure 3g compares the cycle performances of the electrode cycled in 0−0.8, 0−1.0 and 0−1.2 V at a current density of 4 A g−1 for 5000 consecutive cycles. The capacitance retentions of the KxMnO2 electrode in 0−0.8, 0−1.0 and 0−1.2 V are 96, 96, and 33735

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

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α-MnO2 electrode exhibits remarkably decreased capacitance retention of 88% between 0 and 1.2 V for 5000 continuous charge/discharge cycles at a current density of 4 A g−1 as compared to 95% for the KxMnO2 electrode. The impaired cycle performance for the K+-free α-MnO2 electrode is probably caused by water decomposition with oxygen evolution at high electrode potentials. Therefore, the KxMnO2 electrode is highly promising to be used in the broad potential windows to achieve large specific capacitance while maintain good cycling stability and high Coulombic efficiency. To investigate the charge storage mechanisms of KxMnO2 and K+-free MnO2, we carried out XPS measurements and analysis on both electrodes at different charge states. Figure 4a, b shows the K 2p and Na 1s core-level XPS spectra of the KxMnO2 electrode at as-prepared state, first charge to 1.2 V, first discharge to 0 V, and discharge to 0 V after 5000 cycles. The K 2p peaks are clear for the as-prepared KxMnO2 sample and they are significantly reduced after the first charge to 1.2 V, indicating the extraction of K+ ions from the host during the first charge process. When the electrode is discharged to 0 V, Na+ ions, instead of K+ ions, insert into the host as confirmed by the emerged Na 1s peak for the KxMnO2 sample. After 5000 cycles, no more K 2p peaks can be observed while well-defined Na 1s peak forms, suggesting K+ ions in the host have been replaced by Na+ ions. The K content for the pristine KxMnO2 was measured to be 0.26 and the Na content for the final NaxMnO2 was measured to be 0.29 by inductively coupled plasma mass spectrometry (ICP-MS) measurements, agreeing well with the XPS results. The evolution of K 2p and Na 1s signals clearly suggest that the charge and discharge processes of the KxMnO2 electrode are accompanied by extracting and inserting of cations. In comparison, the Na 1s core-level XPS spectrum for the K+-free MnO2 sample only shows a very weak peak when discharged to 0 V after 5000 cycles (Figure S8), indicating much less Na+ ions are incorporated in the charge storage process. The Mn 3s and Mn 2p core-level spectra can be used to evaluate the change of Mn oxidation state for the sample surface at different charge and discharge states. The obtained analysis data for both KxMnO2 and K+-free MnO2 samples are presented in Table 2. For Mn 3s core-level XPS spectra, the energy separation between the two peaks is sensitive and correlated to the mean Mn oxidation state. Figure 4c shows the Mn 3s corelevel XPS spectra at different states of as-prepared, 1.2 and 0 V, respectively. It was previously reported that a linear relation exists between the energy separation of Mn 3s peaks and the Mn oxidation state in the oxides.39,41,42 The energy separations of the Mn 3s peaks are 4.92, 4.79, and 5.32 eV for the KxMnO2 electrode at as-prepared, 1.2 and 0 V states, respectively, corresponding to the mean Mn oxidation states of 3.7, 3.9, and 3.0. The Mn oxidation state is less than 4 due to the preinserted K+ ions in KxMnO2 and the followed redox reaction at the electrode surface is associated with Mn3+/Mn4+. In comparison, the energy

95%, respectively. It is noted that extending the upper cutoff potential for KxMnO2 will not deteriorate the cycling stability, which makes the use of KxMnO2 in larger potential window with higher energy density feasible in the future. To understand the influence of preinserted K+ ions on the electrochemical performance of MnO2, K+-free α-MnO2 nanorod arrays were also prepared according to literature37 for comparison. XRD result confirms the pure phase of α-MnO2 without any impurity (Figure S2). The FESEM image reveals that α-MnO2 nanorods have similar morphology as KxMnO2 nanorods but with much smaller diameter (Figure S2). Figure 3h shows the typical CV curves of the α-MnO2 nanorod array electrode in different potential windows of 0−0.8, 0−1.0, and 0−1.2 V, respectively, at a scan rate of 5 mV s−1. Although the current density of the CV curve increases as the potential window extends, no obvious redox peaks can be observed from the CV curves for all potential windows. At a scan rate of 5 mV s−1, the estimated surface capacitance contribution of the K+-free α-MnO2 electrode is about 81% (Figure S5), which is larger than that of the KxMnO2 electrode. To investigate the rate performance of the K+-free α-MnO2 electrode, we carried out CV curves at different scan rates varying from 5 to 800 mV s−1 and charge/discharge measurements were also carried out at various current densities from 1 to 32 A g−1 in potential window of 0−1.2 V (Figure S6). It is speculated that the redox peaks detected in the CV curves of the KxMnO2 naonrod array electrode are originated from the interstitial cations in the 2 × 2 tunnels. For a detailed comparison, the specific capacitance, energy density, cycle performance, and Coulombic efficiency of KxMnO2 and K+-free α-MnO2 in different potential windows are summarized and compared in Table 1. Although the K+-free α-MnO2 nanorod arrays possess much finer nanorods, the specific capacitances of the KxMnO2 electrode are much higher compared to those of the K+-free α-MnO2 electrode in all different potential windows. As shown in Figure 3i (also see in Figure S7), the KxMnO2 electrode can deliver much larger specific capacitance than the K+-free α-MnO2 electrode in the potential window of 0−1.2 V at different current densities, indicating improved charge storage capability of the α-MnO2 with interstitial K+ ions. In addition to the improved specific capacitance, the preinserted K+ ions in the α-MnO2 also have profound influence on the Coulombic efficiency and cycling stability. When the upper cutoff potential was increased to 1.2 V, the Coulombic efficiencies of the KxMnO2 and K+-free α-MnO2 electrodes were decreased to 96 and 89%, respectively. It is speculated that the K+ ion-stabilized α-MnO2 has a higher overpotential for oxygen evolution in the electrolyte compared to the K+-free α-MnO2, thus resulting in better reversibility and higher Coulombic efficiency for KxMnO2. The cycling stability of α-MnO2 can also be greatly improved in the large potential window of 0−1.2 V with K+ ion incorporation. The K+-free

Table 1. Comparison of Electrochemical Performance for the KxMnO2 and K+-Free MnO2 Electrodes in Different Potential Windows in 1 M Na2SO4 Electrolyte comparison of materials

potential window (V vs Ag/AgCl)

specific capacitance (F g−1) at 1 A g‑1

capacitance retention after 5000 cycles (%)

energy density (Wh kg−1) at 1 A g−1

Coulombic efficiency at 4 A g−1 (%)

KxMnO2 MnO2 KxMnO2 MnO2 KxMnO2 MnO2

0−0.8 V

152 110 195 145 260 184

96 95 96 92 95 88

13 9.7 27 20 52 37

99 100 97 94 96 89

0−1.0 V 0−1.2 V

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Figure 4. (a) K 2p and (b) Na 1s core-level XPS spectra for the KxMnO2 at different charge states. Mn 3s core-level XPS spectra of (c) the KxMnO2 electrode and (d) the K+-free MnO2 electrode at different charge states. (e) TEM image of a single KxMnO2 nanorod after 5000 cycles. (f) HRTEM image of the KxMnO2 nanorod (inset shows the corresponding SAED pattern). (g) STEM image of the KxMnO2 nanorod with corresponding elemental mappings of Mn, O, Na, and K after 5000 cycles.

is much less for K+-free MnO2 sample and Mn stays at nearly 4+ for all charge and discharge states. Although there is no K+ in the α-MnO2 for the K+-free MnO2 sample, it is possible that a small amount of protons (H+) exist in the 2 × 2 tunnels to stabilize the structure, which explains that the oxidation state of the as-prepared sample is slightly less than 4+ and a small amount of Na+ can be inserted into the host to replace the protons. Therefore, the K+-free MnO2 sample is still not the stoichiometric MnO2. Nevertheless, the K+-free MnO2 sample is not favorable for incorporating the Na+ ions during the charge and discharge process because XPS result shows very weak Na 1s peak for the electrode after 5000 CV cycles (Figure S8). Figure 4e, f shows the TEM and HRTEM images for a single KxMnO2 nanorod after 5000 CV cycles. The nanorod retains its integrity without any pores or defects being observed after 5000 cycles, indicating good structural stability for long-time cycling. The corresponding STEM and energy-dispersive X-ray spectroscopy (EDS) element

Table 2. Obtained XPS Analysis Data for the KxMnO2 and K+-Free MnO2 Samples at Different Charge States Mn 3s (eV) materials KxMnO2 MnO2 KxMnO2 MnO2 KxMnO2 MnO2

potential (V) as-prepared 1.2 V 0V

Mn 2P

peak 1

peak 2

e ΔV

3/2

oxidation state

88.67 88.18 88.89 88.21 88.94 88.24

83.75 83.37 84.10 83.49 83.62 83.32

4.92 4.81 4.79 4.72 5.32 4.92

642.00 642.30 642.10 642.39 641.35 642.29

3.7 3.9 3.9 4.0 3.0 3.7

separations of Mn 3s peaks are 4.81, 4.72, and 4.92 eV for the K+-free MnO2 electrode at as-prepared, 1.2 and 0 V states, respectively, corresponding to the mean Mn oxidation states of 3.9, 4.0, and 3.7 (Figure 4d). It is obvious that Mn valence change 33737

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

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ACS Applied Materials & Interfaces mapping result (Figure 4g) demonstrate that K+ ions have been almost replaced by Na+ ions in MnO2 after the cycling, agreeing well with the XPS result. Given that the KxMnO2 and K+-free MnO2 samples showed different electrochemical behaviors and performances, charge storage mechanisms for these two samples could be different. From the XPS analysis, it is noted that the preinserted K+ ions in α-MnO2 play an important role in determining the pseudocapacitance. As illustrated in Figure 5, K+ ions can be nearly fully

positive potential window, Mn3+ is required to exist in the MnO2 (like KxMnO2), which then can be further oxidized during the charge process. The previously proposed redox reaction of MnO2 should be revised as the following equation MxMnO2 ⇔ MnO2 + x M+ + x e−(M+ = H+, Na +, K+) (3)

As MxMnO2 or MnO2 is used as cathode material for supercapacitors, the initial charge process requires low valence Mn3+. If it is stoichiometic MnO2, theoretically, the charge storage can only proceed through double-layer capacitance because Mn4+ is difficult to be further oxidized. Nevertheless, certain amount of cations always exist in MnO2 during the synthesis process and a small amount of Na+ ions can be inserted into MnO2 during the discharge process. Therefore, the preinserted cations in MnO2 are critical to achieve large pseudocapacitance because they determine the amount of Mn3+ in MxMnO2. In order to achieve the theoretical value of 1370 F g−1, MMnO2 is required to be prepared and it is possible for NaMnO2.43 In the present study, the surface area of KxMnO2 nanorod arrays is believed to be much smaller compared to that of the K+-free nanorod arrays due to the much larger size of KxMnO2 nanorods. The KxMnO2 electrode delivered much larger specific capacitance than the K+-free MnO2 electrode, indicating pseudocapacitance is the major capacitance contribution for KxMnO2 while double-layer capacitance is the major capacitance contribution for K+-free MnO2.

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CONCLUSIONS In summary, K+-stabilized and K+-free α-MnO2 nanorod arrays were prepared via a facile hydrothermal route as electrodes for supercapacitors. It has been found that the preinserted K+ ions in α-MnO2 can extend the electrode potential window to 1.2 V with significantly improved pseudocapacitance. A pair of redox peaks was observed between 1.0 and 1.2 V from the CV curve for the KxMnO2 electrode, indicating full utilization of cation insertion and extraction in α-MnO2 can only be achieved at high potential beyond 1.0 V. With a potential window being extended to 1.2 V, the KxMnO2 electrode can deliver a large energy density of 52 Wh kg−1, which is about three times larger than that obtained between 0 and 0.8 V. In addition to the enhanced pseudocapacitance, the preinsertion of K+ ions in α-MnO2 can increase the overpotential for oxygen evolution and enables good Coulombic efficiency and cycling stability in such a large potential window. The charge storage mechanism of KxMnO2 was further investigated and a revised redox reaction was proposed in this work. It is believed that the preinserted cations in α-MnO2 will induce Mn3+ ions that account for the pseudocapacitance associated with the Mn3+/Mn4+ redox couple. As for K+-free α-MnO2, the electrode shows much lower specific capacitance compared to the KxMnO2 electrode in the same potential window, indicating the major capacitance contribution is from double-layer capacitance. The present work provides an effective strategy to develop high-performance MnO2-based electrodes with large specific capacitance and energy density.

Figure 5. Schematic illustration for the electrochemical reaction process of the KxMnO2 electrode and proposed charge storage mechanisms for MxMnO2 and MnO2.

extracted from MnO2 at about 1.2 V and Na+ ions will be inserted back into MnO2 during the following discharge process. The preinserted K+ ions determine the amount of Na+ ions that can be involved in the redox process of MnO2. The Mn oxidation state changes between 3+ and 4+ at the sample surface upon the redox switching, effectively contributing to the pseudocapacitance for the KxMnO2 electrode. In contrast, the K+-free MnO2 sample did not show obvious redox peaks in CV curves and the surface Mn oxidation state only changes between 3.7 and 4 indicating much smaller pseudocapacitance contribution from this electrode. For ideally stoichiometric MnO2, all Mn exist in 4+ oxidation state, the previously proposed Mn3+/Mn4+ redox couple is not applicable, which probably explains that the reported specific capacitances for MnO2-based electrodes are far less than the theoretical value of 1370 F g−1 based on one electron transfer per unit formula of MnO2. Therefore, if researchers want to utilize the Mn4+/Mn3+ redox couple for stoichiometric MnO2, the electrode needs to be first discharged and it will work in a negative potential window such as −1.0−0 V (vs Ag/AgCl). In this way, it becomes a negative electrode for asymmetric supercapacitor. If people want to use MnO2 as cathode in a

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12518. XRD and SEM results for the K+-free α-MnO2 nanorod arrays, b-value calculation for the KxMnO2 electrode using 33738

DOI: 10.1021/acsami.6b12518 ACS Appl. Mater. Interfaces 2016, 8, 33732−33740

Research Article

ACS Applied Materials & Interfaces

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CV method, CV curves, and charge/discharge curves of the KxMnO2 electrode and the K+-free MnO2 electrode, XPS spectra for O 1s, Mn 2p, and Na 1s of the as-prepared, charged, and discharged K+-free MnO2 electrode (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: (86) 25 84303408. Fax: (86) 25 84303408. ORCID

Hui Xia: 0000-0002-2517-2410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51572129, U1407106), Natural Science Foundation of Jiangsu Province (No. BK20131349), QingLan Project of Jiangsu Province, A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Fundamental Research Funds for the Central Universities (30915011204), and the Russian Science Fundation (Project 14-43-00072).

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