Boosting Electrochemistry of Manganese Oxide Nanosheets by

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Boosting Electrochemistry of Manganese Oxide Nanosheets by Ostwald Ripening During Reduction for Fiber Electrochemical Energy Storage Device Dedong Jia, Xianqi Chen, Hua Tan, Fang Liu, Lijun Yue, Yiwei Zheng, Xueying Cao, Chenwei Li, Yuanyuan Sun, Hong Liu, and Jingquan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09592 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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ACS Applied Materials & Interfaces

Boosting Electrochemistry of Manganese Oxide Nanosheets by Ostwald Ripening During Reduction for Fiber Electrochemical Energy Storage Device Dedong Jia,†,* Xianqi Chen,† Hua Tan,§ Fang Liu,† Lijun Yue,† Yiwei Zheng,† Xueying Cao,† Chenwei Li,† Yuanyuan Sun,† Hong Liu,‡,§,* Jingquan Liu†,*

†

College of Material Science and Engineering, Qingdao University, Qingdao 266071, Shandong,

China ‡

State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, Shandong,

China §

Institute of Advanced Interdisciplinary Research (IAIR), University of Jinan, Jinan 250022,

Shandong, China *

email jiadedong@qdu,edu.cn (Dedong Jia), [email protected] (Hong Liu), [email protected]

(Jingquan Liu)

ACS Paragon Plus Environment 1

ACS Applied Materials & Interfaces 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

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Abstract The poor electronic conductivity of MnOx severely limits the practical application as high-performance electrode materials for faradaic pseudocapacitors. Herein，a facile vapor reduction method is demonstrated for treatment of MnOx with hydrazine hydrate (HH) to improve the electronic conductivity. The HH vapor treatment without annealing process not only introduces oxygen vacancies to form oxygen-deficient MnOx, but also leads to obvious structural transformation from highly aggregated and poorly crystallized MnOx nanorobs and nanoparticles to uniformly orientated and highly crystallized MnOx nanosheets via the Ostwald ripening process. Compared with the pristine MnOx on carbon fibers (CFs-MnOx), the reduced CFsMnOx exhibits a highly improved specific capacitance of 1130 mF cm-1 (434 F g-1) with excellent rate capability and cycling stability. Our results have shown that the moderate concentration of oxygen vacancies and highly uniform orientation of reduced MnOx endow the electrode with fast electron and ion transport, respectively. Moreover, a flexible fiber asymmetric supercapacitors (ASCs) device with high energy and power density based on the as-prepared reduced CFs- MnOx as cathode and electrochemical activated graphene oxide on carbon fibers (CFs-ArGO) as anode is fabricated. The MnOx//ArGO ASCs device delivers a high volumetric capacitance of 1.9 F cm-3, a maximum energy density of 1.06 mWh cm-3, and a volumetric power density of 371.3 mW cm-3. The present work opens a new way for oxygen vacancies introduction and structural modification of metal oxide as high performance materials for energy storage applications.

Keywords: supercapacitor, oxygen vacancies, Ostwald ripening, asymmetric, wearable


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1. Introduction

With the extensive application of portable and flexible electronics, it is quite necessary to explore high-performance energy storage devices, such as supercapacitors and metal ion batteries.1-7 The application of metal ion batteries is often limited by its low power, short cycle life, and long charge time. Therefore, the high-power electrochemical supercapacitors (SCs) have been intensively studied due to the fast charge/discharge process, light weight and excellent reliability. In order to meet the high energy density need of next generation electronic device, the energy density of SCs must be greatly enhanced. According to the equation of E = 1/2CV2,8 the energy density (E) increases with the specific capacity (C) or/and the operating potential range (V). The asymmertric supercapacitors (ASCs) have been extensively developed to enhance the energy density of SCs, combining the voltage windows of a battery-type (or pseudocapacitors) electrode and an electrochemical double layer capacitors (EDLCs) electrode. Unlike EDLCs, which store electrical energy by accumulation of ions on the electrode surface, the battery-type (or pseudocapacitive) materials make use of fast redox reactions or phase changes on the surface or subsurface of electrode,9 offering much higher energy densities than EDLCs. In pursuit of the SCs materials with high capacitance, transition-metal oxides including MnO2, Co3O4, NiO, etc., have been extensively exploited.10-15 Among these various transition-metal oxide, MnO2, an typical pseudocapacitive material, has attracted much attention due to the high theoretical specific capacitance (~1400 F g-1), low-cost, and environmental friendly.16,

17

However, the

electrochemical performance of MnO2 is greatly limited by its poor conductivity (10-5~10-6 S cm-1) , hindering its application in energy storage filed. In order to overcome the limitation and boost MnO2’ electrochemical performance, many researchers have been focusing on developing nanostructured MnO2 or MnO2 based composite materials. For instance, some nanostructured



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MnO2, such as nanoparticles,18 nanoneedles19 and nanowires,20 have been prepared to shorten the charge-transport pathway and increase the accessible surface area. Ultrathin MnO2 nanosheets with the thickness of 3-4 nm was prepared and exhibited a high capacitance of 634.5 F g-1.21 This strategy of structure controlling is somewhat complicated, which usually involves accurate manipulation of the reaction kinetics as well as synthesis parameters. Another promising approach is to combine MnO2 with highly external conductive materials such as graphene,22 carbon fibers,23 carbon nanotube24,

25

and so on. Jang et al. fabricated carbonized

polypyrrole-coated MnO2 micronodule through electrodeposition technique.26 Bao et al. synthesized graphene/MnO2/CNT and graphene/MnO2/PEDOT composites for high performance electrochemical electrodes by three dimensional conductive wrapping method.27 However, due to the restriction of the interface between MnO2 and other materials, the enhancement of MnO2 conductivity is still limited by this integration strategy.28 Recent years, in order to enhance the electronic properties and electrochemical performance of transition oxides, more and more researchers are focusing on the defect engineering (vacancy controlling) and exploiting the structure-property performance relationships.29 The electronic structure of some semiconductors can be modified significantly by defect engineering. Even more, in some extreme cases, the semiconductors that have low intrinsic electrical conductivity could be transformed into half-metallicity materials. The significent improvement in electrical conductivity for transition oxides realized by this defect engineering lead to the remarkable enhancements in catalytic and electrocatalytic performance. Annealing in reducing atmosphere (hydrogen) was reported to be an effective method for post-treatment of TiO2 , MnO2, and other metal oxide.30-32 Oxygen vacancies and lower valence ceated by annealing allowed metal oxides to have enhanced conductivity and charge transport property. However, the annealing method


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can only introduce oxygen vacancies in the surface region of metal oxides and the amount of oxygen vacancies is severely limited. In addition, the thermal annealing usally lead to the decreas of surface area and structural water, which was very improtant for promote the ion diffusion in oxide lattic. Alternatively, the thermal annealing with high temperature requires large amounts of energy and fuel. Therefore, it is not suitable for those thermally unstable materials, while it is not practical for the clean and sustainable energy development. Up to now, there are a few reports about the treatment of metal oxides with graphene oxide composite with reductant.33 Zhou et al fabricated SnO2 nanocrystal or Sn nanoparticles and nitrogen-doped reduced graphene oxide composites for Li-ion batteries anode materials through HH vapor reduction method.34,

35

However, the purpose of using reductant is to reduce the graphene oxide, and little attention has been paid to the modification of reductant on the geometric and electronic structure of metal oxides. Therefore, it is still highly desirable to develop facile and effective strategy that can simultaneously improve the electrical conductively and ion diffusion of MnO2 for achiving significant improvement of electrochemical performance. In this work, we report the synthesis of oxygen vacancies dominated manganese oxide loading on carbon fibers (denoted as CFs-MnOx) as flexible electrodes for SCs through an HH vapor reduction method for the first time. The HH vapor was utilized not only as reducing agent to introduce oxygen vacancies in MnOx, but also a dissolution recrystallization medium in Ostwald repening36 to form MnOx nanosheets with uniform orientation (Scheme 1). Owing to the oxygen vacancies and uniform orientation, the reduced MnOx nanosheets exhibited enhanced crystallinity, electrical conductivity and ion-transfer rate, and direct electrical contact to the supporting carbon fibers. Therefore, the reduced CFs-MnOx possesses a much higher specific length capacitance (1004.8 mF cm-1) compared to the pristine CFs-MnOx (437.4 mF cm-1) at the



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current density of 4 mA cm-1. Moreover, a high performance, fiber-shaped, wearable, and all-solid-state ASCs with adopting the reduced CFs-MnOx as cathode and an electrochemical activated graphene oxide on carbon fibers (denoted as CFs-ArGO) as anode was demonstrated for the first time. The fiber-shaped ASCs MnOx//ArGO exhibited a maximum operating voltage of 2 V, a high specific capacitance of 1.9 F cm-3, an exceptional energy density of 1.06 mWh cm-3 and power density of 371.3 mW cm-3. Our work sheds new light on the modification of transition metal oxide based electrodes to improve the performance of energy storage devices.

Scheme 1. Schematic showing the in situ introduction of oxygen vacancies into MnOx on carbon fiber via reduction using hydrazine hydrate (HH) vapor.

2. Results and discussion

Manganese oxide layer was directly formed on the carbon fibers surface via cyclic voltammetry (CV) technology with the potential window between 0.3 and 0.6 V at a scan rate of 100 mV s-1 for 200 cycles. The as-prepared MnOx was then subjected to reduction under HH vapor at 90 oC, aiming to introduce oxygen vacancies. To identify the possible transformation before and after


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Figure 1. (a) XRD patterns of Mn3O4 (red line) reference (JCPDS#24-0734), pristine CFs-MnOx (black line) and reduced CF-MnOx (red line); (b) SEM images of pristine CFs-MnOx; (c) SEM images of reduced CFs-MnOx; (d-f) TEM image, elements mapping and HRTEM of pristine CFs-MnOx; (g-i) TEM image, elements mapping and HRTEM of reduced CFs-MnOx; inset of (f) and (i) is the SAED of the pristine CFs-MnOx and reduced CFs-MnOx respectively.

reduction, the power X-ray diffraction (XRD) analysis was performed. From the XRD pattern of both the pristine and the reduced samples, the diffraction peak at around 25o is the characteristic peak of carbon, assigning to the (002) lattic phase.37 The black line in Figure 1a presents the XRD diffraction peak of pristine MnOx, the characteristic diffraction peaks of 2θ locating around 36.08 o, 53.97 o and 65.72 o could be ascribed to the (211) plane of Mn3O4, (312) plane of MnO2



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and (622) plane of Mn2O3, respectively. The other characteristic diffraction peaks could hardly be clearly observed due to the relatively low intensity of MnOx in carbon fibers. This result indicated that the MnOx coated on the carbon fibers using the CV electrochemical deposition technology was mixed valance oxide with poor crystallinity. The positions and relative intensities of the peaks associated with the reduced CFs-MnOx (red line in Figure 1a) are quite different with those of pristine CFs-MnOx. The reduced MnOx could be perfectly identified as a pure Mn3O4 (space group: I41/amd)) with lattice constants of a=b=5.780 Å and c=9.330 Å, which is consistent with the values given in the standard card for Mn3O4 (JCPDS#24-0734). In addition, the diffraction peak of the reduced CFs-MnOx exhibits a full width at half-maximum, which is much smaller than that of pristine CFs-MnOx, indicating the higher crystallinity.38 A clear change to a lower oxidation state was confirmed, from mixed valance containing Mn3O4, MnO2 and Mn2O3 to lower oxidation state of Mn3O4, suggesting the abundant oxygen vacancies were created during the reduction process. Additionally, there are no detected impurities in the XRD pattern, indicating that no impurities are introduced during the reduction process. Therefore, we believe that, through this reduction method, the HH vapor with strong reducing ability, acting as oxygen scavenger, is beneficial for the formation of oxygen vacancies according to the following equations: 3MnO2 + N2H4.H2O Mn3O4 + N2 + 3H2O 6Mn2O3 + N2H4.H2O 4Mn3O4 + N2 + 3H2O Moreover, the X-ray diffraction patterns of reduced MnOx at different time are shown in Figure S1 and it is observed that the crystal phase of MnOx is still Mn3O4 when the reduction time increases to 48 h. The morphology and structure difference between the pristine CFs-MnOx and


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the reduced CFs-MnOx was examined by the field-emission scanning electron microscopy (FESEM). As seen in the Figure 1b, the entire surface of the carbon fibers was covered with a uniform film of MnOx for the pristine CFs-MnOx. The high magnification SEM image (Figure 1c) shows that the film is composed of closely packed MnOx nanorobs and nanoparticles with low size uniformity and crystallinity. The high scan rate will lead to the high nucleating rate, which cause the formation of poorly crystalline manganese oxide.39 The as-prepared CFs-MnOx was reduced in a HH vapor to introduce oxygen vacancies. After being treated for 12 h, the MnOx still uniformly covered on carbon fibers surface (Figure S2a). From Figure S2b, the architecture of surface decorated MnOx changes to nanoparticles with more uniform size distribution compared with pristine MnOx. Further increasing the reduction time to 24 h lead to distinct change in the morphology of the MnOx nanosheets: the MnOx nanosheets with highly crystalline and uniformly orientation were formed on the carbon fibers surface (Figure 1c). There are many gaps between these MnOx nanosheets, forming a porous structure, which is favorable for electron transfer through the low-resistance pathway. When the reduction time increased to 48 h, the structure of MnOx finally changed to nanobelts (Figure S2 c and d). The structure modifications could be attributed to a crystal growth process named Ostwald ripening.36, 40 In the reduction process, HH vapor not only acted as the reductant, but also as a dissolution-recrystallization medium. In particular, the small MnOx nanorobs and nanoparticles would directly merge with the adjacent MnOx which possess the similar orientation to low the interfacial energy.41 In addition, the primary MnOx nanorobs and nanoparticles can aggregate in an order way to form its high crystalline. With the MnOx nanorobs and nanoparticles are dissolved and the seeds recrystallized, the MnOx seeds gradually aggregate and grow, and finally form into MnOx nanosheets with larger diameters. Furthermore, the morphology of the as-obtained reduced



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MnOx nanocrystals can be controlled by tuning the reduction time. The morphology of the pristine MnOx and reduced MnOx were also investigated using transmission electron microscopy (TEM). Figure 1d and 1f shows the TEM and HRTEM images of pristine MnOx, revealing the nanorod and nanoparticle structure with poor crystallinity, consistent with the XRD and SEM results. EDX mapping images obtained during the TEM measurements show the uniform distribution of Mn and O elements across the nanorobs (Figure 1e). The HRTEM image from the selected area of the pristine MnOx shows lattice fringe spacing of 0.408, 0.314 and 0.490 nm, corresponding to the spacing of MnO2 (101), Mn2O3 (122) and Mn3O4 (200) planes, respectively, which provide the direct evidence for the formation of mixed valence of MnOx via the CV electrodepositing technology. The poor crystallinity of pristine MnOx was also verified by the highly diffusive rings (Figure 1f inset). From the Figure 1g, it is obviously that the HH vapor treatment converted poorly crystalline nanorods and nanoparticles into a highly crystalline nanosheets and the reduced MnOx is porous in structure (Figure 1g). EDX mapping of the reduced MnOx confirmed the uniform distribution of Mn and O elements across the Mn3O4 nanosheets (Figure 1h). The lattice spacings of 0.249, 0.276, 0.285 and 0.490 nm, corresponding to the (211), (103), (200) and (101) plane of Mn3O4 were clearly observed in the HRTEM image (Figure 1i). The thickness of the nanosheets is about 10~20 nm, which is in good accord with the SEM findings, corresponding to 20-40 layers of Mn3O4 in the (101) direction with an interplaner spacing of around 0.490 nm. In the selected area electron diffraction SAED patterns of the reduced MnOx, the appearance of multiple bright electron diffraction rings indicates the polycrystalline nature of the Mn3O4 nanosheets. Well-defined diffraction rings can be indexed to the (101), (211) and (103) planes of Mn3O4 (inset of Figure 1i).


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Figure 2. N2 sorption isotherms (a) and pore size distribution (b) of pristine CFs-MnOx and reduced CFs-MnOx. XPS survey patterns (c), high-resolution Mn 2p (d), high-resolution Mn 3s (e) and O 1s (f) for pristine CFs-MnOx and reduced CFs-MnOx.

The existence of porous structure of reduced CFs-MnOx is further confirmed by N2 sorption. The pore size distribution data show that the pore size of reduced CFs-MnOx is about 6 - 8 nm and 30 - 50 nm. After reduction, the CFs-MnOx still shows low Brunauer-Emmett-Teller specific surface area due to the existence of carbon fibers. However, the BET specific surface area of 54.4 m2 g-1 for reduced sample (CFs-MnOx-24) is relatively higher than that for the pristine CFs-MnOx (30.2 m2 g-1), indicating the formation of more porous structure during the HH vapor reduction process. The higher BET specific surface area of the MnOx facilitates infiltration of the electrolyte into the porous electrode to enhance the specific capacitance. The oxidation states of the Mn ions in the pristine and reduced MnOx were investigated using X-ray photoelectron spectroscopy (XPS). The peaks of Mn 3p, Mn 3s, C 1s, O 1s, Mn 2p and Mn 2s can be observed in both CFs-MnOx ’and CFs-MnOx’ survery spectrum (Figure 2c). The peak position of reduced



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CFs-MnOx is similar to that of CFs-MnOx, and there were not around 400 eV peak related to N 1s in the spectrum of reduced CFs-MnOx, demonstrating the HH does not act as the N source for doping but instead act simply as reductant. As shown in Figure 2d, the Mn 2p XPS spectra for both of them show two peaks at the binding energy of 653 eV and 642 eV, corresponding to the Mn 2p1/2 and Mn 2p3/2 which are in accordance with those previously reported on manganese oxide.42 The Mn 2p1/2 peak of pristine MnOx was deconvoluted into three peaks, at 643.0 eV, 641.6 eV and 640.7 eV, which corresponding to the Mn4+, Mn3+ and Mn2+ respectively. Therefore, it clearly indicates the coexistence of three different phases of MnOx, including MnO2, Mn2O3 and Mn3O4,43 coinciding well with the observation of HRTEM in Figure 1f. More precisely, the spin-energy separation of Mn 2p1/2 and Mn 2p3/2 for the reduced MnOx is about 11.7 eV, which is in good harmony with the demonstrated data in spectrum of Mn3O4.44 The oxidation states in Mn 2p spectrum is complexity and is lack of standardization, therefore, it is difficult to determine the valence state of manganese from it. The Mn 3s spectrum could be used to determine the oxidation state of Mn. The lower valence of Mn will lead to the wider splitting of the Mn 3s peaks.45 Therefore, it is possible to qualitatively compare the Mn4+, Mn3+ and Mn2+ content in pristine and reduced samples by studying the Mn 3s peak energy separation as shown in Figure 2e. The △E values obtained are 5.24 for the pristine CFs-MnOx and 5.50 for the reduced CFs-MnOx-24. Chigane and Ishikawa 46 reported the peak separation of Mn 3s for MnO, Mn3O4, Mn2O3, and MnO2 as 5.79, 5.50, 5.41 and 4.78 eV, respectively. The values of the pristine CFs-MnOx is 5.24 which is between 4.78 and 5.79 eV for Mn4+ and Mn2+ respectively, suggesting the characteristic of mixed oxidation state. Compared to the pristine CFs-MnOx, the splitting energy of CFs-MnOx-24 was measured to be 5.50 eV, indicating the formation of Mn3O4. It suggests that MnOx with mixed oxidation state can be reduced to a lower oxidation


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state of Mn3O4 by HH vapor, leaving oxygen vacancies in the MnOx. The peak separation of 5.48 eV is observed for the CFs-MnOx-12, while the CFs-MnOx-48 showed a separation energy of ~5.52 eV for the Mn3s as shown in Figure S3. Based on the results of analysis of XPS spectrum, the mean valence state of Mn in MnOx is 3.7, 2.8, 2.7 and 2.6 for pristine CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48, respectively, which is in good agreement with result of the XRD analysis. This result indicated that the HH vapor treatment can lead to the reduction of Mn4+ to Mn3+ or Mn2+ and the formation of lower valence of Mn in the MnOx. Furthermore, there was no significant change (from 2.6 to 2.8) of the average oxidation state through adjusting the HH vapor treatment time. In addition, in order to further evaluate the valence of Mn in pristine CFs-MnOx and reduced CFs-MnOx, the O 1s is also analyzed. The O 1s XPs spectrum for pristine CFs-MnOx and reduced CFs-MnOx exhibited two peaks, corresponding to the anhydrous manganese oxides (Mn-O-Mn), and hydrated manganese oxides (Mn-OH) as shown in Figure 2f. From the comparison of the peak area, reduced CFs-MnOx exhibits lower content of Mn-OH than that of pristine CFs-MnOx. Based on the analysis of O 1s XPS spectrum, the valence of Mn is estimated to be 3.7 and 2.7 for pristine CFs-MnOx and reduced CFs-MnOx, respectively, which is good agreement with the result of the Mn 3s spectrum. To assess the electrochemical behavior of the reduced CFs-MnOx from the pristine CFs-MnOx, electrochemical studies have been conducted in a three-electrode cell filled with 0.5 M Na2SO4 electrolyte. The electrochemical performance of CFs was tested as reference. By comparison of CV curves of CFs and CFs-MnOx under 20 mV s-1 as shown in Figure S4, the capacitance of CFs is almost negligible. Figure 3a shows the CV curves of four different electrodes collected at a scan rate of 50 mV s-1. As expected, the reduced CFs-MnOx exhibits substantially larger current



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Figure 3. Electrodechemical performance of supercapacitor electrodes before and after HH vapor reduction for different time. (a) Cyclic voltammetry (CV) and (b) galvanostatic charge-discharge (GCD) curves of pristine CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48 at the scan rate of 20 mV s-1 and 4 mA cm-1, respectively; (c) Specific capacitances of pristine CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48 calculated from GCD curves as function of current density; (d) Nyquist plots for pristine CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48; k1, k2 analysis of (e) CFs-MnOx, and (f) CFs-MnOx-24.

density than the pristine CFs-MnOx, indicating the much larger capacitance of reduced CFs-MnOx. In comparison to the pristine CFs-MnOx, the presence of broad redox peaks in CV curves of the three reduced samples (CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48) reveals


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an obvious pseudocapacitive characteristic. The pseudocapacitive faradic redox reaction of Mn2+/3+/4+ on the surface is an active and detectable charge storage mechanism for the reduced specimens, which can be attributed to the presence of much more lower valence Mn ions (Mn2+, Mn3+) on the surface of the reduced CFs-MnOx.32 It has been reported that the low valence Mn in MnOx is beneficial for charges storing via transferring the excess negative charge onto proximate O atoms.37 In addition, we also studied the effect of the reduction time on capacitive performance. Obviously, the current density of the CV curves collected for reduced MnOx gradually increased with the increasing reduction time from 12 to 24 h, indicating that the more oxygen vacancies are helpful to the enhancement of capacitive properties. However, with increasing reduction time from 24 to 48 h, the reduced CFs-MnOx-48 exhibited decreased current density than the CFs-MnOx-24. It suggests that when the content of oxygen vacancies is too high, the electrode has poor electrochemical performance. Except for CV measurement, the comparison GCD curves of different electrodes are presented in Figure 3b. The GCD curves of both pristine and reduced CFs-MnOx demonstrate symmetrical features between the charging and discharging branches, suggesting its ideal pseudocapacitive nature.47 The discharge time are about 106.7, 147.1, 251.2 and 197.4 s for pristine CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48 at 4 mA cm-1, respectively, and the corresponding capacitance are 437.4 mF cm-1 for the pristine CFs-MnOx and 1004.8 mF cm-1 for the reduced CFs-MnOx-24, indicating a 2.3 folds increase as a result of HH vapor treatment. Furthermore, the CFs-MnOx-24 has the highest charge storage capability than the other three electrodes which is consistent with the CV results. The CV curves at various scan rates from 5 to 200 mV s-1 and GCD curves at different current densities from 2 to 20 mA cm-1 of the CFs-MnOx and CFs-MnOx-24 are shown in Figure S5. CV curves of CFs-MnOx-24 show no obvious deformation as the scan rate increased from 5 to 100



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mV s-1, which indicate its high-rate performance. The GCD curves at different densities exhibit the nearly symmetric feature with small IR drop, suggesting the excellent electrochemical capacitance. Specific length capacitance and specific gravimetric capacitance of all the electrodes are calculated based on the GCD curves and the corresponding results are summarized in Figure 3c and Figure S5a. In contrast, the pristine CFs-MnOx exhibits lower capacitance than that of the reduced CFs-MnOx at all current densities, and the CFs-MnOx-24 yields the highest capacitance. For instance, CFs-MnOx-24 delivers the length capacitances of 1130, 1004.8, 927, 840, 771 and 655.5 mF cm-1 (equivalent to gravimetric capacitance of 434.6, 386.5, 356.5, 323.1, 296.5 and 252.1 F g-1) at 2, 4, 6, 8, 10 and 15 mA cm-1, while the CFs-MnOx only delivers the length capacitances of 604, 437.4, 426.8, 379.2, 338, 250.5 and 194 mF cm-1 (equivalent to gravimetric capacitance of 232.3, 168.2, 164.2, 145.9, 130.4, 96.3 and 74.6 F g-1) at the corresponding current densities. These values are also higher than those reported for manganese oxide.20, 23, 24, 30 Even at the current density of 20 mA cm-1, the capacitance can still reach 562 mF cm-1 (216.2 F g-1), indicating excellent capacitance retention of near 50%. In addition, long cycling test is performed to explore the electrochemical stability of both pristine CFs-MnOx and reduced CFs-MnOx-24 electrodes (Figure S6b). For the reduced CFs-MnOx-24, the overall capacitance loss is only about 5.9% after 5000 cycles of charging and discharging at the current density of 15 mA cm-1, which is lower than that for the pristine CFs-MnOx (12.4%). It suggests that the oxygen vacancies formed through reduction are very stable during the electrochemical cycling test. The SEM was used to characterize the morphological change of pristine CFs-MnOx and reduced CFs-MnOx electrodes after the cycling stability test.48 As for CFs-MnOx, some MnOx layers were detached from the carbon fibers, which is the major reason resulting in the decrease of capacitance (Figure S7). Unlike the CFs-MnOx, after the cycling stability test, the


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surface of the carbon fibers was still covered with the MnOx uniform film, which is similar with the SEM images before the cycling test as shown in Figure 1c. In addition, the nanosheets structure of MnOx was still preserved after the cycling test. The strong bonding forces between the reduced MnOx and carbon fibers, as well as the well-preserved nanostructure are favorable to the long-cycle and high-performance of the reduced CFs-MnOx. We would like to attribute the tremendous differences in the electrochemical performance between the pristine CFs-MnOx and reduced CFs-MnOx electrodes to the following key factors: (1) For the pristine CFs-MnOx, the highly aggregation of MnOx nanorobs and nanoparticles on carbon fibers will cause dead volume and shrink the gap spacing, both of which would reduce the specific surface area and active pseudocapacitive sites. (2) The random orientation of pristine MnOx hinder the effective permeation of electrolytes to the surface of active electrochemical, and further hamper the faradaic charge transfer.49 (3) The introduced oxygen vacancies in reduced MnOx enable the enhanced electrical conductivity and the increased active sites, both of which intrinsically improved the electrochemical properties. (4) The highly uniform orientation of reduced CFs-MnOx is not only beneficial for exposing more redox sites and the active surface, but also facilitate the electrolyte permeation and shorten ion pathway for Na ion intercalation and de-intercalation. The specific capacitance and rate capability of reduced CFs-MnOx are far more improved. (5) The reduced CFs-MnOx has higher specific surface area, which can provide more electrochemical active sites to significantly improve the capacitive properties. Figure 3d showed the Nyquist plots of electrochemical impedance spectroscopy (EIS) spectrum for the samples and the inset in Figure 3d is the enlarged view for the high-frequency regions. All the measured EIS spectra are similar with an arc part at high frequency and a spike part at low-frequency, and it could be simulated by the equivalent circuit which is shown in Figure S8.



Page 18 of 38

At high frequency, the point of intersection on the real axis represents the internal resistance (RS), which includes the intrinsic electronic resistance of the electrode material, the ohmic resistance of the electrolyte, and the interfacial resistance between the electrode and current collector. The diameter of the semicircle represents the charge transfer resistance (Rct), while the Zw is the Warburg impedance. As for CFs-MnOx, CFs-MnOx-12, CFs-MnOx-24 and CFs-MnOx-48, the RS values are 8.5, 6.3, 5.6 and 4.9 Ω (Table S1), which is reasonable due to the electrolyte is same when the electrochemical properties are measured. In addition, the increase of RS refers to the smaller conductivity, confirming the enhanced electric conductivity of CFs-MnOx after the HH vapor reduction. The values of charge transfer resistance Rct) for this four electrodes are followed the order: CFs-MnOx (2.1 Ω) > CFs-MnOx-12 (1.2 Ω) > CFs-MnOx-48 (1.1 Ω) > CFs-MnOx-24 (0.4 Ω), suggesting the lowest charge transfer resistance of CFs-MnOx-24. In addition, more vertical line along to the imaginary axis at low frequency region suggests the more ideal capacitive performance. Among all the samples, CFs-MnOx-24 shows the more vertical plot in this region, indicating the better capacitive performance, which may result from the combined action of the improved electrical conductivity after introducing oxygen vacancies and the porous structure in the MnOx nanosheets. The analyses of quantitative and electrode kinetic using Conway and Dunn’s method,50, 51 were carried to illustrate the influence of oxygen vacancies on (1) capacitive controlled capacitance, coming from capacitor-like processes including EDLC charging and fast pseudocapacitive current associated with redox reaction of Mn2+/3+/4+ from the MnOx surface/subsurface and (2) diffusion-controlled capacitance, arising from faradic reaction of Mn3+/4+ needing Na-ion intercalation /deintercalation in MnOx bulk structure. In this method, the current under a


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particular potential is considered to contain the contributions from the surface capacitive and the diffusion controlling insertion processes: i(V)=k1v1/2+k2v1/2 Here “i(V)” is the measured current as a function of potential (V), and “v” is the scan rate. Through determining the values of k1 and k2, it is possible to calculate the percentage of current arising from the diffusion-controlled intercalation processes and the capacitor-like processes. In Figure 4c, the fraction of current coming from capacitor-like processes is quantitatively determined through separating the current response “i” from the diffusion-controlled process and from the capacitor-like process. Clearly, 65.2% of the total capacity is identified as the capacitor-like contribution for CFs-MnOx-24, which is the highest compared with the pristine CFs-MnOx (30.3%), CFs-MnOx-12 (34.7%) and CFs-MnOx-48 (47.8%) as showing in Figure 3e, Figure 3f, Figure S9a and Figure S9b. The finding indicated that MnOx with oxygen vacancies exhibits higher capacity and faster kinetic than the pristine MnOx. In addition, the excessive oxygen vacancies will lead to more diffusion-controlled contribution in CFs-MnOx-48 electrode. With the increasing scan rate, the capacitive contribution is further enlarged, while the capacitive contribution is depressed as expected for both CFs-MnOx and CFs-MnOx-24. We find that the capacitive effects contributed 50.7%, 65.2%, 73.4%, 80.9% and 92.8% of the total capacitance for CFs-MnOx-24, which are always higher than that of the CFs-MnOx electrode (26.4%, 30.3%, 41.2%, 49.3%, 61.7% and 69.5%) in the scan rate range from 5 to 200 mV s-1, respectively (Figure S9c). We believed that the enhanced electrical conductivity, combined with preferentially oriented nanocrystals of CFs-MnOx-24 produce an environment suitable for higher capacitive charge storage.



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Prior to the utilization of CFs-MnOx-24 as the positive electrode of ASCs, the partner negative electrodes need to be prepared and examined. Graphene with high electrical conductivity and large theoretical specific surface area (2630 m2 g-1) has attracted much attention as negative electrode materials for ASCs.12, 33 We prepared reduced oxide graphene (rGO) decorated on carbon fibers through electrodeposition as negative electrode (denoted as CFs-rGO). In order to boost the capacitive properties, the CFs-rGO was activated by a one-step electrochemical activation process in a mixed acid solution of H2SO4 and HNO3 under a potential of 1 V for 10 min denoting as CFs-ArGO (details in the Experimental Section). Typical SEM images and electrochemical performance of CFs-rGO and CFs-ArGO are shown in Figure S10 and S11. It can be seen that the carbon fibers were uniformly coated with activated rGO after the electrodeposition process and the electrochemical oxidation treatment. The electrochemical capacitive properties of CFs-rGO and CFs-ArGO were also characterized by CV and GCD. For CFs-rGO, the CV curves were nearly rectangular with no observed redox peak, implying the EDLC behavior. The potential window of CFs-rGO is between -1 - 0 V, while the potential window of CFs-rGO is widened to -1 - 0.8 V, and a broad redox peak is observed between -0.4 and -0.5 V for the CFs-ArGO, which can be attributed to the reversible redox reaction of oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl groups on the surface of ArGO.53 As expected, the CFs-ArGO exhibits superior charge-storage capability compared to the CFs-rGO due to the increase of the oxygen functional groups after the electrochemical activation. Owing to its superior capacitive performance, the CFs-ArGO was selected as anode to construct a simple ASCs device with a CFs-MnOx-24 as cathode and PVA/LiCl as electrolyte (both as the ionic electrolyte and the separator) was assembled. In order to obtain the optimal performance in ASCs, the charge between the cathode and the anode should


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Figure 4. (a) Scheme of the assembled all-solid-state fiber-shape ASC device; (b) CV curves of the solid-state ASC device collected in different voltage window at 50 mV s-1. (c) GDC curves of the all-solid ASC device in different scan voltage window at 4 mA cm-1. (d) CV curves of the ASC device collected at varied scan rates in voltage window of 0 - 2 V. (e) GDC curves of ASC device collected at varied current densities in voltage window of 0 - 2 V. (f) Specific volumetric capacitance and capacitance retention calculated for the ASC device based on the GDC as a function of current density.



Page 22 of 38

be balanced, and the number ratio of fibers between cathode and anode is estimated to be 1: 3 as shown in Figure 4a. The length and diameter of this fiber-shape ASC is 1 cm and 0.2 cm, respectively. The CV curves at 20 mV s-1 and GCD curves at a current density of 2 mA cm-1 collected at different voltage window are shown in Figure 4b and Figure 4c. Clearly, the stable electrochemical potential window of the ASCs can be extended to 2 V. Figure S12 shows the volumetric capacitance and energy density of the ASC device based on GCD curves. The calculated specific volumetric capacitance based on the GCD curves increases from 1.1 to 1.7 F cm-3 when the operation voltage extends from 0.8 to 2 V. Importantly, the energy density of the ASCs is improved by 956% from 0.098 to 0.94 mWh cm-3, according to the equation of E =1/2CV2. To evaluate the capacitive performance at the voltage window of 0 - 2 V, the CV and GCD curves of the fiber-shaped all-solid-state ASCs collected at various scan rates and current densities are shown in Figure 4d and 4e. CV curves of the ASCs show rectangular-like shapes, revealing

the ideal

capacitive behavior

and

fast

charge-discharge properties.

The

charge/discharge curves of the device presents a slightly asymmetry due to the increased internal resistance of CFs-ArGO. The calculated coulombic efficiency of our device is about 75%, 90%, 92%, 94.3%, 95.7%, 98% at the current density of 2, 4, 6, 8, 10, 12 mA cm-1, respectively. With the exception of coulombic efficiency under the low current density of 1mA cm-1, our device always exhibit high coulombic efficiency (higher than 90%). Additionally, the superior electrochemical performance of the ASC is further confirmed by GDC curves at 2.0 V which were reasonably symmetric and linear over time. Figure 4f shows the volumetric capacitance calculated from the GCD curves as a function of the current density, and the device exhibits high volumetric capacitance of 1.9 F cm-3 under current density of 2 mA cm-1.


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Figure 5. (a) Cycling stability performance of the ASC device under the current density of 6 mA cm-1. (b) CV curves of the ASC device obtained at a scan rate of 200 mV s-1 under different bending states. (c) The LED powered by our ASC device. (d) Ragone plots of the ASC device compared with other supercapacitor devices.

An excellent stability with 92% retention of the capacitance was obtained after 10000 charge-discharge cycles under the current density of 6 mA cm-1(Figure 5a). When designing for wearable application, the mechanical property is an important factor to consider. Clearly, the corresponding CV performance remains nearly unchanged under different bending state (Figure 5b), highlighting the excellent flexibility of the fiber-shaped ASCs. In order to further test the feasibility, we weaved our device into knitted fabric as illustrated in Figure 5C. A red light emitting diode with working voltage of 3 V could be lighted up by three ASCs devices in series for 5 min after charging for 90 s at 4 mA cm-1. The Ragone plots shown in Figure 5d compares the performance of the ASC device with other previously reported energy storage devices with



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comparable size. The device exhibits an excellent volumetric energy density of 1.06 mWh cm-3 at a power density of 371.3 mW cm-3. These values are significantly greater than other electrochemical capacitor devices, including H-MnO2//rGO (0.25 mWh cm-3),30 ASV/FO//V/CO (0.95

mWh

cm-3),54

MoS2/rGO/MWCNT//rGO/MWCNT

(0.0017

mWh

cm-3),55

MnO2//T-Fe2O3/PPy (0.22 mWh cm-3),18 carbon /MnO2 fiber (0.22 mWh cm-3),23 MnO2//N-Fe2O3 (0.55 mWh cm-3),20 and TiN-SCs (0.05 mWh cm-3).56 The excellent electrochemical performance demonstrated that the reduced CFs-MnOx might be the potential cathode material for developing advanced ASCs with high energy and power density.

3. Conclusions

In summary, we have demonstrated an in situ facile HH vapor reduction method for making the oxygen vacancies dominated MnOx on carbon fibers, where HH not only acts as the reducing agent to create oxygen vacancies, but also as Ostwald ripening-induced structure reorganization medium to regenerate MnOx nanosheets with highly uniform orientation. Thanks to the unique features including existence of oxygen vacancies, higher uniform orientation, short path for electron transport and ion diffusion, a remarkable length capacitance of 1004.8 mF cm-1 (gravimetric capacitance, 386.5 F g-1) was obtained for reduced CFs-MnOx at 4 mA cm-1, which is 2.3 times greater than that of the pristine CFs-MnOx (length capacitance, 437.4 mF cm-1; gravimetric capacitance 168.2 F g-1). Furthermore, flexible ASCs devices with high-performance were successfully assembled using the as-prepared reduced CFs-MnOx-24 as cathode and CFs-ArGO as anode. The MnOx//ArGO ASC devices could deliver an maximum volumetric energy density of 1.06 mWh cm-3 at a current density of 2 mA cm-1 and a maximum power density of 371.3 mW cm-3, which are substantially higher than those of other reported


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supercapacitors with comparable dimension. It can be anticipated that this facile, robust and without high-temperature condition HH vapor strategy can be further extended to the preparation of other metal oxide or hydroxide for oxygen vacancies introduction and structural modifications, thus opening up new opportunities for enhancing the electrochemical performance of supercapacitor electrodes.

4. Experimental section

4.1 Materials Manganese (II) acetate tetrahydrate (Mn(AC)2.4H20), sodium sulfate decahydrate (Na2SO4.10H2O), lithium perchlorate (LiClO4), poly(vinyl alcohol) (PVA, MW 85000) and lithium chloride (LiCl) were obtained from Aladdin, and used as received. Hydrazine hydrate (85%) was purchased from Sinopharm Chemical Reagent Co., Let (China) and the carbon fiber (M40-JB-12K) was obtained from Toray (Japan). 4.2 Synthesis of Reduced MnOx on Carbon Fibers The electrochemical deposition method was used to directly prepare nanostructured MnOx on carbon fiber tow substrates. Prior to electrodeposition, the carbon fibers tow were cut into 3 cm in length and cleaned by ultrasonication for 30 min in acetone, ethanol and deionized water (DI) in sequence. The mixed solution of 0.1 M Mn(CH3COO)2 and 0.1 M Na2SO4 solution was used as the electrolyte. MnOx was deposited using a three-electrode system consisting of the carbon fibers tow as the working electrode, a platinum sheet (1 × 1 cm) as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The electrodeposition was performed by CV technology between 0.3 and 0.6 V at a scan rate of 100 mV s-1 for 200 cycles. The as-prepared samples were washed carefully with DI water for several times to remove the electrolyte residue and then dried in a vacuum at 60 oC for 24 h, which is denoted as CFs-MnOx. The oxygen vacancies were



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introduced to MnOx through a vapor reduction method. In details, the obtained CFs-MnOx and 0.2 mL HH was firstly put in a 10mL glass bottle. The bottle was sealed and heated to 90 oC for different time (12, 24 and 48 h) with heat rate of 10 oC/min. When cooled to room temperature naturally, sample was taken out and washed with DI water several times and dried at 60 oC under vacuum overnight. The products are denoted as CF-MnOx-12 (12 h), CF-MnOx-24 (24 h) and CF-MnOx-48 (48 h) corresponding to the different reduction time. The loading mass of reduced MnOx on carbon fiber tow was about 2.6 mg cm-2. 4.3 Fabrication of Activated Graphene Oxide on Cabon Fibers(CFs-ArGO) Graphene oxide (GO) was successfully synthesized using a modified Hummers method as described in previous reports.12, 33 The obtained GO solution with the concentration was estimated to be 3 mg mL-1. The reduced graphene oxide on carbon fibers (CFs-rGO) was fabricated by electrodeposition method. Briefly, the electrodeposition process was carried out in 3 mg mL-1 GO aqueous suspension containing 0.1 M LiClO4 on the carbon fibers at a consistent potential of -1.2 V for 1800 s. After electrodeposition, the sample was washed with DI water and then immersed in DI water for 30 min to remove the residual GO absorbed on the carbon fibers, followed by drying at 60 oC for 10 h in air. The CFs-rGO was electrochemical activated in a three-electrode cell in a mixture acid of HNO3 and H2SO4 (V : V = 1 : 1) with a platinum sheet as count electrode and saturated calomel electrode (SCE) as reference electrode under a constant potential of 1 V. The processing time was set as 10 min. The final product was washed with DI water and denoted as CFs-ArGO. 4.4 Fabrication of All-Solid-State ASCs The ASCs were assembled by using CFs-MnOx-24 as cathode, CFs-ArGO as anode, and PVA/LiCl gel as electrolyte. The PVA/LiCl gel was prepared as follows: 3 g LiCl and 5 g PVA were dissolved in 50 mL DI water with stirring at 90 oC until it


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became clear. Subsequently, both CFs-MnOx-24 and CFs-ArGO were put in gel electrolyte for 5 min and then were taken out to solidify at 45 oC for 5 min. Finally, the device was prepared successfully by assembling two electrodes together and leaving it overnight until the electrolyte was solidified. 4.5 Materials Characterization Morphologies of as-prepared samples were investigated by scanning electron microscopy (SEM, ZEISS-Merlin), transmission electron microscopy (TEM, FEI Titan G2 60-300 microscope, 200 kV) and high-resolution transmission electron microscopy (HRTEM). X-ray diffraction (XRD, Rigaku D/MAX 2500) and X-ray photoelectron spectroscopy (XPS, AXIS ULTRA DLD, UK) were used to study the structure of the samples. The specific surface area were determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Helenda (BJH) methods at 77 K using an ASAP 2020. The electrochemical characterization methods, such as cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance measurement were carried out in a conventional three-electrode cell combined with a CHI 760E electrochemical workstation (CHI Instruments,

Shanghai

Chenhua

Instrument

Corp.,

China).

All

the

electrochemical

characterizations for the electrodes were performed in 0.5 M Na2SO4 solution at room temperature. The specific length capacitances (CL) and gravimetric capacitances (CG) of each electrode in the three-electrode configuration are calculated from the GCD curves at different current density, using the following equations: CL = [i×(dV/dt)]/L and CG=[i×(dV/dt)]/mMnOx, where i denotes discharge current (A), dV/dt is the slope of discharge curve, respectively, L is the active length (cm) of carbon fiber tows and mMnOx refers to the mass of MnOx on the surface of carbon fiber tow. For the ASCs, the charge balance will follow the relationship: q+ = q-, q = CL×V×n, and in



Page 28 of 38

order to achieve q+ = q-, the number of carbon fibers tow will follow n+/n- = (CL-V-)/(CL+)×(V+); the specific volumetric capacitance (CV) is also calculated from GCD curves according to the following equations: CV = [i×(dV/dt)]/VASC, where VASC refers to the total volume of the ASC. The power density and energy density were calculated using the following equations: E = CVV2/3600×2 and P = 3600E/t, where V is the applied voltage, CV is the measured volumetric capacitance, t is the discharge time.


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Acknowledgements

The authors are thankful to funds from Qingdao Innovation Leading Expert Program, and Qingdao Basic & Applied Research project (15-9-1-100-jch), and the National Natural Science Foundation of China (Grant No. 51372142). Supporting Information Materials characterization of reduced CFs-MnOx-12, CFs-MnOx-48, CFs-ArGO, SEM images of elelctrodes after the cycling test, and the additional electrochemical characterization have been provided. This material is freely available via the Internet at http://pubs.acs.org/.

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected]



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References 1 Yin, H.; Li, Q.; Cao, M.; Zhang, W.; Zhao, H.; Li, C.; Huo, K.; Zhu, M. Nanosized-Bismuth-Embedded 1D Carbon Nanofibers as High-Performance Anodes for Lithium-Ion and Sodium-Ion Batteries. Nano Research, 2017, 10, 2156-2167. 2 Yao, H.; Wang, P.; Gong, Y.; Zhang, J.; Yu, X.; Gu, L.; OuYang, C.; Yin, Y.; Hu, E.; Yang, X.; Stavitski, E.; Guo, Y.; Wan, L. Designing Air-Stable O3-Type Cathode Materials by Combined Structure Modulation for Na-Ion Batteries. J. Am. Chem. Soc., 2017, 139, 8440-8443. 3 Zhang, R.; Li, N.; Cheng, X.; Yin, Y.; Zhang, Q, Guo, Y. Advanced Micro/Nanostructures for Lithium Metal Anodes. Adv. Sci., 2017, 4, 1600445. 4 Li, X.; Feng, Y.; Li, M.; Li, W.; Wei, H.; Song, D. Smart Hybrids of Zn2GeO4 Nanoparticles and Ultrathin g-C3N4 Layers: Synergistic Lithium Storage and Excellent Electrochemical Performance. Adv. Funct. Mater., 2015, 25, 6858-6866. 5 Li, X.; Li, W.; Li, M.; Cui, P.; Chen, D.; Gengenbach, T.; Chu, L.; Liu, H.; Song, G. Glucose-Assisted Synthesis of The Hierarchical TiO2 Nanowire@MoS2 Nanosheet Nanocomposite and Its Synergistic Lithium Storage Performance. J. Mater. Chem. A., 2015, 3, 2762-2769. 6 Li, X.; Wu, G.; Liu, X.; Li, W.; Li, M. Orderly Integration of Porous TiO2 (B) Nanosheets Into Bunchy Hierarchical Structure For High-rate and Ultralong-Lifespan Lithium-ion Batteries. Nano Energy, 2017, 31, 1-8. 7 Wu, G.; Chen, J.; Guo, Y.; Li, X.; Luo, B.; Chu, L.; Han, Y.; Jiang, B.; Xu, L.; Li, M. Freestanding Sodium-Ion Batteries Electrode Using Graphene Foam Coaxially Integrated With TiO2 Nanosheets. Journal of the Electrochemical Society, 2017, 164, A3060-A3067.


Page 31 of 38 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


8 Xiao, X.; Peng, X.; Jin, H.; Li, T.; Zhang, C.; Gao, B.; Hu, B.; Huo, K.; Zhou, J. Freestanding Mesoporous VN/CNT Hybrid Electrodes for Flexible All-Solid-State Supercapacitors. Adv. Mater., 2013, 25, 5091-5097. 9 Xu, W.; Yan, C.; Sumboja, A.; Lee, P. High Performance Porous Nickel Cobalt Oxide Nanowires for Asymmetric Supercapacitor. Nano Energy, 2014, 3, 119-126. 10 Feng, C.; Zhang, J.; He, Y.; Zhong, C.; Hu, W.; Liu, L.; Deng, Y. Sub-3 nm Co3O4 Nanofilms With Enhanced Supercapacitor Properties. ACS Nano., 2015, 9, 1730-1739. 11 Lu, Q.; Lattanzi, M. W.; Chen, Y. P.; Kou, X. M.; Li, W. F.; Fan, X.; Unruh, K. M.; Chen, J. G.; Xiao, J. Q. Supercapacitor Electrodes With High-Energy and Power Densities Prepared From Monolithic NiO/Ni Nanocomposites. Angew. Chem., 2011, 123, 6979-6982. 12 Amir, F. Z.; Pham, V. H.; Schultheis, E. M.; Dickerson. Flexible, All-Solid-State, High-Cell Potential Supercapacitors Based on Holey Reduced Graphene Oxide/Manganese Dioxide Nanosheets. Electrochim. Acta, 2018, 260, 944-951. 13 Guan, Y.; Kushima, A.; Yu, L.; Li, S.; Li, J.; Lou, X. W. Coordination Polymers Derived General Synthesis of Multishelled Mixed Metal-Oxide Particles for Hybrid Supercapacitors. Adv. Mater., 2017, 29, 1605902. 14 Bag, S.; Raj, C. R. Hierarchical Three-Dimensional Mesoporous MnO2 Nanostructures for High Performance Aqueous Asymmetric Supercapacitors, J. Mater. Chem. A. 2016, 4, 587-595. 15 Yu, L.; Guan, B.; Xiao, W.; Lou, X. W.; Formation of Yolk-Shelled Ni-Co Mixed Oxide Nanoprisms With Enhanced Electrochemical Performance for Hybrid Supercapacitors and Lithium Ion Batteries, Adv. Energy Mater., 2015, 5, 1500981.



Page 32 of 38

16 Zeng, Y.; Zhang, X.; Meng, Y.; Yu, M.; Yi, J.; Wu, Y.; Lu, X.; Tong, Y. Achieving Ultrahigh Energy Density and Long Durability in a Flexible Rechargeable Quasi‐Solid‐ State Zn–MnO2 Battery. Adv. Mater., 2017, 29, 1700274. 17 Yu, M.; Cheng, X.; Zeng, Y.; Wang, Z,; Tong, Y.; Lu, X.; Yang, S. Dual‐Doped Molybdenum Trioxide Nanowires: A Bifunctional Anode for Fiber‐Shaped Asymmetric Supercapacitors and Microbial Fuel Cells. Angew.Chem., 2016, 128, 6874–6878. 18 Wang, L.; Yang, H.; Liu, X.; Zeng, R.; Li, M.; Huang, Y.; Hu, X. Constructing Hierarchical Tectorum-like α-Fe2O3/PPy Nanoarrays on Carbon Cloth for Solid‐State Asymmetric Supercapacitors. Angew. Chem. Int. Ed., 2017, 129, 1125-1130. 19 Kong, D.; Luo, J.; Wang, Y.; Ren, W.; Yu, T.; Luo, Y.; Yang, Y.; Cheng, C. Three-Dimensional Co3O4@MnO2 Hierarchical Nanoneedle Arrays: Morphology Control and Electrochemical Energy Storage. Adv. Funct. Mater., 2014, 24, 3815-3826. 20 Yang, P. H.; Ding, Y.; Lin, Z. Y.; Chen, Z. W.; Li, Y. Z.; Qiang, P. F.; Ebrahimi, M.; Mai, W. J.; Wong, C. P.; Wang, Z. L. Low-cost High-Performance Solid-state Asymmetric Supercapacitors Based on MnO2 Nanowires and Fe2O3 Nanotubes. Nano Lett., 2014, 14, 731-736. 21 Yu, N.; Yin, H.; Zhang, W.; Liu, Y.; Tang, Z.; Zhu, M. Q. High-Performance Fiber-Shaped All-Solid-State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics. Adv. Energy Mater., 2016, 6, 1501458. 22 Luo, Y.; Jiang, J.; Zhou, W.; Yang, H.; Luo, J.; Qi, X.; Zhang, H.; Yu, D. Y. W.; Li, C. M.; Yu, T. Self-Assembly of Well-Ordered Whisker-Like Manganese Oxide Arrays on


Page 33 of 38 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


Carbon Fiber Paper and Its Application as Electrode Material for Supercapacitors. J. Mater. Chem., 2012, 22, 8634-8640. 23 Xiao, X.; Li, T.; Yang, P.; Gao, Y.; Jin, H.; Ni, W.; Zhan, W.; Zhang, X.; Cao, Y.; Zhong, J.; Gong, L.; Yen, W.; Mai, W.; Chen, J.; Huo, K.; Chueh, Y.; Wang, Z. L.; Zhou, J. Fiber-Based All-Solid-State Flexible Supercapacitors for Self-Powered Systems. ACS Nano., 2012, 6, 9200-9206. 24 Peng, L.; Peng, X.; Liu, B.; Wu, C.; Xie, Y.; Yu, G. Ultrathin Two-Dimensional MnO2/Graphene

Hybrid

Nanostructures

for

High-Performance,

Flexible

Planar

Supercapacitors. Nano Lett., 2013, 13, 2151-2157. 25 Zhu, J.; He, J. Facile synthesis of Graphene-Wrapped Honeycomb MnO2 Nanospheres and Their Application in Supercapacitors. ACS Appl. Mater. Interfaces, 2012, 4, 1770-1776. 26 Lee, J. S.; Shin, D. H.; Jang, J. Polypyrrole-Coated Manganese Dioxide With Multiscale Architectures for Ultrahigh Capacity Energy Storage. Energy Environ. Sci., 2015, 8, 3030-3039. 27 Yu, G.; Hu, L.; Liu, N.; Wang, H.; Vosgueritchian, M.; Yang, Y.; Cui, Y.; Bao, Z. Enhancing The Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping. Nano Lett., 2011, 11, 4438-4442. 28 Kang, J.; Hirata, A.; Kang, L.; Zhang, X.; Hou, Y.; Chen, L.; Li, C.; Fujita, T.; Akagi, K.; Chen, M. Enhanced Supercapacitor Performance of MnO2 by Atomic Doping. Angew. Chem., 2013, 125, 1708-1711. 29 Wang, H.; Zhang, J.; Hang, X.; Zhang, X.; Xie, J.; Pan, B.; Xie, Y. Half-metallicity in Single-Layered Manganese Dioxide Nanosheets by Defect Engineering. Angew. Chem., Int. Ed., 2015, 54, 1195-1199.



Page 34 of 38

30 Zhai, T.; Xie, S.; Yu, M.; Fang, P.; Liang, C.; Lu, X.; Tong, Y. Oxygen Vacancies Enhancing Capacitive Properties of MnO2 Nanorods for Wearable Asymmetric Supercapacitors. Nano Energy, 2014, 8, 255-263. 31 Chen, X.; Liu, L.; Peter, Y. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis With Black Hydrogenated Titanium Dioxide Nanocrystals. Science, 2011, 331, 746-750. 32 Wang, G.; Ling, Y.; Wang, H.; Yang, X.; Wang, C.; Zhang, J. Z.; Li, Y. Hydrogen-treated WO3 Nanoflakes Show Enhanced Photostability. Energy Environ. Sci., 2012, 5, 6180-6187. 33 Wang, Y.; Lai, W.; Wang, N.; Jiang, Z.; Wang, X.; Zou, P.; Lin, Z.; Fan, H.; Kang, F.; Wong, C.; Yang, C. A Reduced Graphene Oxide/Mixed-Valence Manganese Oxide Composite Electrode for Tailorable and Surface Mountable Supercapacitors With High Capacitance and Super-Long Life. Energy Environ. Sci., 2017, 10, 941-949. 34 Zhou, X.; Bao, J.; Dai, Z, Guo, Y. Tin Nanoparticles Impregnated in Nitrogen-Doped Graphene for Lithium-Ion Battery Anodes. J. Phys. Chem. C., 2013, 117, 25367−25373. 35 Zhou, X.; Wan, L.; Guo, Y. Binding SnO2 Nanocrystals in Nitrogen-Doped Graphene Sheets as Anode Materials for Lithium-Ion Batteries. Adv. Mater., 2013, 25, 2152−2157. 36 Weng, W.; Lin, Du, Y.; Ge, X.; Zhou, X.; Bao, J. Template-Free Synthesis of Metal Oxide Hollow Micro-/Nanospheres Via Ostwald Ripening for Lithium-Ion Batteries. J. Mater. Chem. A.; 2018, 6, 10168−10175. 37 Prahas, D.; Kartika, Y.; Indraswati, N.; Ismadji, S. Activated Carbon From Jackfruit Peel Waste By H3PO4 Chemical Activation: Pore Structure and Surface Chemistry Characterization. Chem. Eng. J., 2008, 140, 32-42.


Page 35 of 38 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


38 Chuah, G. K.; S. Jaenicke, S.; Pong, B. K. The Preparation of High-Surface-Area Zirconia: II. Influence of Precipitating Agent and Digestion on the Morphology and Microstructure of Hydrous Zirconia. J. Catal., 1998, 175, 80-92. 39 Yang, M.; Zhang, T.; Schulz, P.; Li, Z.; Li, G.; Kim, D. H.; Guo, N.; Berry, J. J.; Zhu, K.; Zhao, Y. Facile Fabrication of Large-Grain CH3NH3PbI3−xBrx Films for High-Efficiency Solar Cells Via CH3NH3Br-Selective Ostwald Ripening. Nat. Commun., 2016, 7, 12305. 40 Song, Y.; Liu, T.; Yao, B.; Li, M.; Kou, T.; Huang, Z.; Feng, D.; Wang, F.; Tong, Y.; Liu, X.; Li, Y. Ostwald Ripening Improves Rate Capability of High Mass Loading Manganese Oxide for Supercapacitors. ACS Energy Lett., 2017, 2, 1752-1759. 41 Penn, R. L.; Banfield, J. F. Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals. Science, 1998, 281, 969-971. 42 Reddy, A. L. M.; Shaijumon, M. M.; Gowda, S. R.; Ajayan, P. M. Coaxial MnO2/Carbon Nanotube Array Electrodes for High-Performance Lithium Batteries. Nano Lett., 2009, 9, 1002-1006. 43 Shin, J.; Shin, D.; Hwang, H.; Yeo, T.; Park, S.; Choi, W. One-Step Transformation of MnO2 into MnO2-X@carbon Nanostructure for High-Performance Supercapacitors Using Structure-Guided Combustion Waves. J. Mater. Chem. A, 2017, 5, 13488–13498. 44 Dubal, D. P.; Holze, R. High Capacity Rechargeable Battery Electrode Based on Mesoporous Stacked Mn3O4 Nanosheets. RSC Advances, 2012, 2, 12096-12100. 45 Chigane, M.; Ishikawa, M. Manganese Oxide Thin Film Preparation By Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc., 2000, 147, 2246-2251.



Page 36 of 38

46 Wang, B.; Zhu, T.; Wu, H. B.; Xu, R.; Chen, J. S.; Lou, X. W. Porous Co3O4 Nanowires Derived From Long Co(CO3)0.5(OH)0.11H2O Nanowires With Improved Supercapacitive Properties. Nanoscale, 2012, 4, 2145-2149. 47 Yoo, J. J.; Balakrishnan, K.; Huang, J. S.; Meunier, V.; Sumpter, B. J.; Srivastava, A.; Conway, M.; Reddy, A. L. M.; Yu, J.; Vajtai, R.; Ajayan, P. M. Ultrathin Planar Graphene Supercapacitors. Nano Lett., 2011, 11, 1423-1427. 48 Liu, Y.; Yu, X.; Fang, Y.; Zhu, X.; Bao, J.; Zhou, X.; Lou, XWD. Confining SnS2 Ultrathin Nanosheets in Hollow Carbon Nanostructures for Efficient Capacitive Sodium Storage. Joule, 2018, 2, 725-735. 49 Liu, T. C.; Conway, B. E.; Roberson, S. L. Behavior of Molybdenum Nitrides as Materials for Electrochemical Capacitors Comparison With Ruthenium Oxide. J. Electrochem. Soc., 1998, 145, 1882-1888. 50 Kim, H. S.; Cook, J. B.; Lin, H.; Ko, J. S.; Tolbert, S.; Ozolins, V.; Dunn, B. Oxygen Vacancies Enhance Pseudocapacitive Charge Storage Properties of MoO3−x. Nat. Mater., 2017, 16, 454-460. 51 El-Kady, M. F.; Kaner, R. B. Scalable Fabrication of High-Power Graphene Micro-Supercapacitors for Flexible and On-Chip Energy Storage. Nat. Commun., 2013, 4, 1475. 52 Wang, W.; Liu, W.; Zeng, Y.; Han, Y.; Yu, M.; Lu, X.; Tong, Y. A Novel Exfoliation Strategy to Significantly Boost The Energy Storage Capability of Commercial Carbon Cloth. Adv. Mater., 2015, 27, 3572-3578.


Page 37 of 38 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


53 Sun, S.; Zhai, T.; Liang, C.; Savilov, S.; Xia, H. Boosted Crystalline/Amorphous Fe2O3-δ core/Shell Heterostructure for Flexible Solid-State Pseudocapacitors in Large Scale. Nano Energy, 2018, 45, 390-397. 54 Sun, G.; Zhang, X.; Lin, R.; Yang, J.; Zhang, H.; Chen, P. Hybrid Fibers Made of Molybdenum Disulfide, Reduced Graphene Oxide, and Multi-Walled Carbon Nanotubes for Solid-state, Flexible, Asymmetric Supercapacitors. Angew. Chem. Int. Ed., 2015, 54, 4651-4656. 55 Lu, X. H.; Wang, G. M.; Zhai, T.; Yu, M. H.; Xie, S. L.; Ling, Y. C.; Liang, C. L.; Tong, Y. X.; Li, Y. Stabilized TiN Nanowire Arrays for High-Performance and Flexible Supercapacitors. Nano Lett., 2012, 12, 5376-5381.



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Boosting Electrochemistry of Manganese Oxide Nanosheets by

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