High Mass Loading MnO2 with Hierarchical Nanostructures for

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High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors Zi-Hang Huang, Yu Song, Dong-Yang Feng, Zhen Sun, Xiaoqi Sun, and Xiao-Xia Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00621 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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High Mass Loading MnO2 with Hierarchical Nanostructures for Supercapacitors

Zi-Hang Huang, Yu Song, Dong-Yang Feng, Zhen Sun, Xiaoqi Sun*, Xiao-Xia Liu*

Department of Chemistry, Northeastern University, Shenyang, 110819, China

* Corresponding authors: Xiaoqi Sun, [email protected] Xiao-Xia Liu, [email protected]

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Abstract Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates seriously with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase controlled electro-deposition of MnO2 with ultra high mass loading of 10 mg cm-2 on carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum condition, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO2 nanosheets and secondary one-dimensional α-MnO2 nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode as well as maintain the structure stability during cycling. The hierarchically structured MnO2 electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm-2 (or a specific capacitance of 304 F g-1) at 3 mA cm-2 and an excellent rate capability comparable to those of low mass loading MnO2 electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO2 cathode exhibit extremely high volumetric energy densities (8.3 mWh cm-3 at the power density of 0.28 W cm-3 for aqueous ASC and 8.0 mWh cm-3 at 0.65 W cm-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.

Keywords: manganese dioxide, high mass loading, hierarchical structures, multiple phases, high energy density, supercapacitors

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Electrochemical energy storage continues to be a highly active research field due to its wide application areas ranging from portable devices and wearable electronics to electric vehicles and smart grids.1-3 Supercapacitors have attracted great interest for their high power density and stable cycling.4-7 However, the rather moderate energy density has limited their applications. Traditional carbon based materials function on the double layer capacitance, which can only store limited energy. Recent cutting-edge research has been focusing on the pseudocapacitive type electrode materials, with the charge stored through not only ion adsorption but also near surface redox reactions. Among various candidates, manganese oxide shows significant predominance because of the abundant resources, low fabrication cost and high theoretical capacitance.8-11 More importantly, it provides a wide voltage window in neutral aqueous electrolytes, resulting in higher energy density than other cathode material candidates (e.g. NiO, Ni(OH)2, Co3O4, PANI, etc.). Nevertheless, a small active material loading (usually less than 0.5 mg cm-2) or the utilization of rather thin film structure is generally required for the MnO2 electrodes to achieve satisfactory electrochemical performance.12,13 The total capacitance and amount of energy stored in the low mass loading materials are in fact small, which limits their practical application for high energy systems.8,14,15 Usually a typical active material mass loading of 8-10 mg cm-2 is required to provide feasible energy for commercial devices.16,17 However, increasing the mass loading or film thickness seriously declines the charge-storage capability, including specific capacitance and rate capability. This is mainly due to the moderate electrical conductivity, slow ion diffusion and poor mechanical stability of the oxide active materials. One of the efficient solutions to enhance the electrochemical performance of high mass loading materials is to construct electrodes with 3D hierarchical structures and establish conductive linkages. This can not only improve the charge and mass transport in the electrodes but also help to release the strain created during electrochemical cycling to ensure the long-term stability. For example, Xu et al. demonstrated a Ni nanowire array supported MnO2 electrode with the mass loading of 16.8 mg cm-2. It yielded an excellent areal capacitance of 1.85 mF cm-2 at 1 mV s-1.18 MnO2-coated 3D graphene network electrode with the mass loading of 9.8 mg cm-2 was also developed, exhibiting a high areal capacitance of 1.42 F cm-2 at 2 mV s-1.19 Lee and co-workers reported an electrochemically grown multiscale architecture composed of ~ 30 nm diameter MnO2 nanofibers with a high mass loading of 16 mg cm-2.20 Attributed to the large amount of nano-sized active materials and pores in the microstructure, the electrode was able to deliver 7.34 F cm-2 3


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capacitance at a fast scan rate of 200 mV s-1 in alkali systems. The MnO2 electrodes composed of heterojunctions of α- and δ- phases MnO2 were also reported to show improved electrochemical performance due to the synergistic effect.21-23 Inspired by those works, we expect that the combination of hierarchical structures and hetero-phases would help to improve the electric conductivity and ionic diffusion in the MnO2 electrodes. As a result, high mass loading can be applied without the sacrifice of electrochemical performance and electrodes with high energy storage capacity can be obtained. Herein, we perform an in-depth investigation on the hierarchical nanostructured manganese oxide with hetero- morphologies and crystal structures, both of which playing key roles in determining the final electrochemical performance. With a facile electrodeposition technique and careful control of the deposition temperature, the optimized hierarchical architecture composed of primary two-dimensional ε-MnO2 nanosheets and secondary one-dimensional α-MnO2 nanorod arrays was grown on the surface of carbon fiber, with a large MnO2 mass loading of 10 mg cm-2. The obtained electrode shows an outstanding areal capacitance of 3.04 F cm-2 at 3 mA cm-2, which is among the highest values reported for manganese oxide nanostructures in neutral aqueous electrolytes. The high specific capacitance (304 F g-1 @ 10 mg cm-2) indicates the high utilization of the active material. The electrode also exhibits superior rate performance which is comparable to low loading manganese oxide electrodes. At the same time, it is able to avoid the normal mechanical peeling problem of composite materials upon long-term cycling, allowing a good cycle life. Subsequently, we assemble asymmetric supercapacitors (ASCs) by using the as prepared MnO2 electrode as the cathode and vanadium oxide nanoflowers as the anode, with 1 M Na2SO4 aqueous or Na2SO4/PVA electrolytes. Both ASCs show high volumetric capacitance of 14.85 F cm-3 (aqueous) or 14.41 F cm-3 (all-solid), and high energy density of 8.3 mWh cm-3 at the power density of 0.28 W cm-3 (aqueous) or 8.0 mWh cm-3 at 0.65 W cm-3 (all-solid). The performance is superior to most of the state-of-the-art supercapacitors.

Results and discussions Material characterization and growing mechanism of MnO2 We performed electro-deposition of nanostructured MnO2 on the carbon fibers of carbon cloth at different temperatures. Morphologies of the obtained samples presented in Figure 1 show strong correlation with the deposition temperature. At 25 °C, inter-connected MnO2 nanosheets (MnO2-25) 4


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are grown on the surface of the carbon fiber substrate (Figure 1a and b). Similar sheets are observed in the MnO2 deposited at 40 °C (MnO2-40) as well, with some small particles shown on top (Figure 1c and d). At more elevated temperatures, the nanosheets mostly present as the primary structure for the samples, on which a secondary structure – either aligned nanorods at 60 °C (MnO2-60, Figure 1e and f) or random particles at 80 °C (MnO2-80, Figure 1g and h) – are grown.

Figure 1 SEM images of MnO2 electrodes electro-deposited at temperatures of 25 oC (a, b), 40 oC (c, d), 60 oC (e, f) and 80 oC (g, h). Cross-sectional SEM images of MnO2 synthesized at different temperatures (i-l) and elemental mapping (C Kα1, Mn Kα1 and O Kα1) images of MnO2-60 (m).

The detailed growing process of MnO2 was further characterized by a series of time-dependent experiments at the representative temperatures of 25 °C and 60 °C. Figures 2a-e show the SEM images of MnO2 obtained from different electro-deposition periods (20, 120, 300, 600, 1200, and 2400 s) at the two temperatures, which discloses the morphological evolution of MnO2. At 25 °C, dense and small MnO2 nanosheets were grown on the surface of the carbon fiber substrate 5


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continuously without apparent local morphology change up to 1200 s (Figure 2a-e); however, cracks started to form at 300 s (Figure S1). During 1200s to 2400 s, the nanosheets started to grow larger and resulted in the final morphology of the MnO2-25 sample (Figure 1a and b). At 60 °C, on the contrary, the nanosheets started to grow larger at a much earlier stage. The morphology of the sample deposited for 20 s at 60 °C already resembled the one for 1200 s at 25 °C, while the MnO2 deposited for 120 s at 60 °C showed the loose and porous nanostructure constructed by the large nanosheets. Upon further deposition (600 and 1200s), the nanosheets continued to grow and the surface started to be covered by some nanoparticles (Figure 2j and k). This now resembled the morphology of the sample deposited at 40 °C. Finally at 2400 s, secondary MnO2 nanorods were grown upwardly on both sides of the primary nanosheets, resulting in the final hierarchical structure (Figure 2l). High magnification SEM (Figure 2l) and transmission electron microscopy (TEM) images (Figure 2m and n) suggest that the average diameter and length of the nanorods are around 30 and 100 nm, respectively.

Figure 2 SEM images of MnO2-25 (a-f) and MnO2-60 (g-l) afforded from different electro-deposition periods: 20 s, 120 s, 300 s, 600 s, 1200 s, and 2400 s. TEM images of the nanorods from the MnO2-60 material (m-n). Chronopotentiometric plots of MnO2 electro-deposition at 25 °C and 60° C (o). 6


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The results demonstrate a temperature dependent stepwise growing mechanism of the MnO2. Despite the final morphology varieties, nanosheets are preferred grown on the carbon fiber substrate at the beginning stage of electro-deposition, similar to previous reports;24,25 however, the specific growing process depends on the deposition temperature. Fig. 2o shows the chronopotentiometric plots of MnO2 electro-deposition at different temperatures. The initial high overpotential corresponds to the nucleation process.26 Since nucleation requires more energy than the growth, it results in a higher overpotential and longer duration (~ 600 s) under the lower temperature condition (25 °C). The extended period of nucleation at lower temperature results in the dense nanosheets, which creates internal strain and causes cracks. At higher temperature (60 °C), on the other hand, higher energy is provided so that the nucleation overpotential is lower and duration is shorter. Within 100 s, the majority of primary nucleation finishes and the nanosheets start to grow larger to form the loose MnO2 layer. With the growth of larger MnO2 nanosheets, more available nucleation sites are created on the surface. Higher temperature allows new nucleation and the secondary growth of lower dimensional oxide is observed with the increased temperature (D = 0, 1 at 80 °C and 60° C, respectively). Similar temperature influenced growth has also been observed for the MnO2 deposition on the Au-coated glass substrate.25 At the low temperature of 25 °C, on the contrary, the continuous growth of primary nanosheets dominates and no secondary morphology is observed. The intermediate 40 °C condition leads to a combination of continuous growth of primary nanosheets and the nucleation of secondary structures, resulting in the present morphology. Figures 1i-l show the cross-sectional SEM images of MnO2 deposited at different temperatures. A dense layer of MnO2 covers the carbon fiber at lower temperature depositions (25 °C and 40 °C), while a porous structure is observed for MnO2-60 and MnO2-80 (Figure 1k and l). The primary nanosheets of the MnO2-60 and MnO2-80 samples are larger and their secondary nanorods or particles may further separate out the primary nanosheets, leaving pores behind. The loose porous nanostructure would enhance the ionic transfer during electrochemical reactions, which is especially important for electrodes with ultrahigh mass loading of active materials. In addition, the porous structure, in combination with the hetero-structures, allows better flexibility of the deposited layer to buffer the internal strain and potentially suppress mechanical peeling during electrochemical cycling. This is in contrast to the MnO2-25 and MnO2-40 samples where strong internal strains are created within the dense layer and cracks are resulted. Elemental dispersive X-ray spectroscopy (EDS) reveals the uniform distribution of manganese and oxygen elements in the thick oxide layer 7


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(Figure 1m). The crystal structures of the electro-deposited MnO2 were characterized by X-ray diffraction (XRD, Figure 3a). For the samples obtained at lower temperatures (25 °C and 40 °C), the XRD peaks correspond to the ε-MnO2 phase (JCPDS 30-0820).27 When the deposition temperature increases to 60 °C and 80 °C, the α-MnO2 phase (JCPDS 44-0141) 21 shows up, in addition to the presence of the ε-MnO2 phase. The XRD pattern of the MnO2-60 sample deposited for 300 s, where only nanosheets exist (Figure 2i), shows the ε-phase diffractions (Figure S2). It suggests that the initially grown MnO2 nanosheets belong to the ε-MnO2 phase, while the secondarily grown nanorods or particles have the α-structure. The 2×2 MnO6 octahedra in α-MnO2 construct large 1D tunnels that allow facile ion migration (Figure S3), while the crystal structure of ε-MnO2 is denser (Figure S4) so that the ion migration in the structure could be sluggish. The broad XRD peaks are a result of the nano-sized material and/or the lattice defects created during electro-deposition during which the two phases may diffuse into each other at the interphases.28-30 The latter would further enhance the flexibility of the deposited layer as well as help with the formation of the porous structure in MnO2 film. The MnO2 electrode was also studied by X-ray photoelectron spectroscopy (XPS). Figures 3b and 3c show the Mn and O elemental spectra of the example MnO2-60 material, while the rest are present in Figures S5-S8. In the Mn 2p spectrum, the characteristic Mn 2p1/2 and Mn 2p3/2 spin-orbit peaks locate at 653.9 and 642.0 eV with a spin-energy separation of 11.9 eV, similar to the previous reported values for nanostructured MnO2 materials.31-34 The detailed valence states of Mn were estimated by the Mn 3s XPS. According Toupin et al., the peak separation energy (∆E) of the Mn 3s component is linearly related to the mean oxidation state of Mn in MnOx,34 with the values of 4.7 and 5.4 eV corresponding to Mn4+ and Mn3+, respectively.33-35 The MnO2-25, MnO2-40, MnO2-60 and MnO2-80 samples show ∆E values of 5.18 eV, 5.15 eV, 5.09 eV and 5.09 eV, respectively, corresponding to the average Mn valence states of 3.31, 3.35, 3.44 and 3.44 (Figure 3b and S7). The deviation from 4 is a result of the defects created during the facile electro-deposition process. Finally, the O 1s peak is fitted with three components, representing the O2- component at 530.2 eV, OH- component at 531.5 eV, and the structural or physisorbed water at 532.6 eV (Figure 3c and S8).8,33

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Figure 3 XRD patterns of the as-prepared MnO2-based electrodes (a). Mn 2p and Mn 3s XPS spectra (b), and the specific fitting of the O 1s XPS peaks of the MnO2-60 electrode (c).

Electrochemical performance of MnO2-based electrodes The electrochemical studies were conducted for the MnO2-based electrodes (MnO2-25, MnO2-40, MnO2-60 and MnO2-80) in three-electrode cells containing 1 M Na2SO4 electrolyte with saturated calomel electrode (SCE) and graphite foil as the reference and the counter electrode, respectively (Figure S9a). Figure 4a shows the cyclic voltammetry (CV) curves of the MnO2 electrodes deposited at different temperatures as well as the blank carbon cloth. A significant increase of current density is observed after the carbon cloth is covered by MnO2, due to the additional surface area and pseudo capacitance provided by the high loading MnO2 (10 mg cm-2). For the MnO2–based electrodes, the capacitance increases along with the increase of electro-deposition temperature up to 60 °C. Further increase of deposition temperature, however, leads to the decrease in capacitance for the fabricated electrode as shown with the MnO2-80 sample. The same trend is observed when the electrodes were tested by galvanostatic charge-discharge (GCD) experiments. Figure 4b provides the GCD profiles of various electrodes at the current density of 3 mA cm-2, and those measured at other current densities between 1 and 30 mA cm-2 are shown in Figure S10. The calculated capacitances are summarized in Figure 4c, S11 and Table S1. Among all the electrodes, the MnO2-60 sample exhibits the largest capacity under all tested conditions. For example at 3 mA cm-2, MnO2-60 yields a high capacitance of 3.04 F cm-2 (equivalent to a gravimetric capacitance of 304 F g-1 normalized to the mass loading of 10 mg cm-2), larger than the others (MnO2-25: 2.71 F cm-2, MnO2-40: 2.19 F cm-2 and MnO2-80: 1.64 F cm-2). The MnO2-60 electrode also exhibits excellent rate capability performance, with 57.2% capacitance retaining at the high current density of 30 mA cm-2 (1.9 F cm-2) in comparison to 1 mA cm-2. In the 9


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contrast, the capacitances of other samples decrease much faster with the increase of current density, with only 41.2% (1.24 F cm-2 for MnO2-25), 49.5% (1.57 F cm-2 for MnO2-40) and 37.6% (0.8 F cm-2 for MnO2-80) areal capacitance remaining when the current density increases from 1 to 30 mA cm-2. Similar results were also obtained by CV measurements with scan rates normalized with the total charge-discharge time of the GCD processes (Figure S12, S13). For example, the capacitance is calculated to be 3.87 F cm-2 for the MnO2-60 electrode based on the data measured at 0.3 mV s-1 scan rate, close to the 3.32 F cm-2 capacitance measured by GCD at the current density of 1 mA cm-2, both with similar discharging time. The results reveal the important roles that morphology and crystal structure play on the charge storage capability of the MnO2 material. The optimized material demonstrates a hierarchical structure which is constructed of α-phase nanorods interconnected with ε-phase nanosheets. The electrochemical performance of the MnO2-60 electrode is substantially better than the reported manganese oxide based electrodes in neutral electrolyte (Table S2 and S3). To understand the promising electrochemical performance of the MnO2-based electrodes, electrochemical impedance spectroscopy (EIS) studies were conducted. In the Nyquist plots (Figure 4d and e), MnO2-60 presents a considerably smaller semi-circle (correlated to the charge transfer resistance, Rct) compared to those of MnO2-25, MnO2-40 and MnO2-80, confirming that the electrochemical reaction was the most efficient in the former. The magnitude of the Z’-intercept correlates to the equivalent series resistance Rs, which includes the electrical resistance of electrodes. It shows the lowest value of 1.477 Ω for MnO2-60, while the MnO2-25, 40, 80 electrode each presents 1.736, 1.639 and 1.948 Ω respectively. The low charge transfer and electrical resistances of the MnO2-60 material contribute to its large capacitance and good rate capability. The detailed charge storage mechanisms and electrode kinetics were further investigated by Dunn’s method, which provides the quantitative separation of the capacitive charge storage (electrical double-layer effect and fast faradaic contribution from the surface/subsurface) and the diffusion-controlled insertion processes (capacitance arising from slow ion de/intercalation in the bulk structure).36-38 For our MnO2-based electrodes, the capacitive-controlled capacitance increases drastically with the increase of deposition temperature up to 60 °C, followed by a decrease for the MnO2-80 material (Figure 4f). The capacitive-controlled capacitance of MnO2-60 electrode accounts for around 74.4% of its total capacitance, corresponding to 2.12 F cm-2. This high value ensures a faster charge storage process in the electrode and results in its excellent rate capability. The higher contribution from the diffusion-controlled process in the other three electrodes 10


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demonstrates their slower kinetics, leading to the poorer rate capability. EIS and capacitance contribution separation experiments suggest a low charge-transfer resistance, small electrical resistance and high contribution of capacitance-controlled process with the MnO2-60 electrode, essentially establishing its excellent electrochemical performance. We then took a closer look at the material itself. The morphology plays one of the key roles in determining the final electrochemical performance. The MnO2-60 material contains hetero-structures with secondary nanorods interconnected with primary nanosheets. The multiple connection points create a “super highway” for fast electron transportation. At the same time, the porous structure gives rise to more thorough immersion of electrolyte into electrode and allows faster ionic transportation. The charge-transfer process in the MnO2-60 electrode is thus fast enough to ensure the large capacity and good rate capability. In addition, the MnO2-60 material is composed of a combination of α- and ε-phase MnO2 as demonstrated earlier. This would cause lattice defects at the intersection of the two phases and create more electrochemical active sites. More importantly, the α-phase contains large 1D tunnels (Figure S3) for facile ion migration. Such process could be further enhanced with structural water entering the tunnel. Thus, fast faradaic reaction could process into deeper subsurface of the material which results in a larger contribution of capacitive charge storage. The ε-structure, on the other hand, does not contain clear ion migration pathway (Figure S4) so that the ion diffusion in the structure is sluggish. The fast capacitive charge storage could only occur at the top surface of electrode and the rest contribution from the diffusion-controlled process is low. Although α-MnO2 is present in the 80 °C deposited electrode as well, the large randomly aligned particle is the major drawback for its ionic and electronic conductivity in comparison to the highly ordered morphologies. The excellent electrochemistry of MnO2-60 is also attributed to the self-standing nature of the electrode where non-active species are eliminated. For comparison, the MnO2 powder were scraped off from the carbon cloth and casted on Ni foam with the addition of carbon and binder (see Methods for details). The casted electrode shows slightly smaller capacitance based on both CV and GCD measurements (Figure S14a and b) and demonstrates a poorer rate capability (48% with 30 times increase of current density, Figure S14c). The overall electrochemical performance did not degrade a lot thanks to the intrinsic nature of the hetero-phases and hetero-morphologies of the MnO2-60 active material. The slightly poorer performance is a result of the additions of inactive carbon and binder, which disturbs the transportation of ions or electrons. The self-standing 11


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MnO2-60 electrode, on the other hand, does not require these additives and shows the superior electrochemical performance.

Figure 4 Electrochemistry of the four MnO2-based electrodes: CV curves measured at a scan rate of 10 mV s-1 (a); GCD profiles measured at 3 mA cm-2 (b); areal specific capacitance measured at different current densities in the potential range of 0∼0.9 V (c); Nyquist plots (d) and high frequency domain (e) collected at open circuit potential with a perturbation of 10 mV; histogram illustration of capacitive and diffusive capacitance contribution at a scan rate of 2 mV s-1 (f).

Figure 5 compares the capacitance retention of the MnO2-based electrodes tested at a current density of 40 mA cm-2. The MnO2-60 material exhibits the most superior cycle stability. An increase of capacitance from 1.69 F cm-2 to 1.93 F cm-2 is observed during the first 200 cycles, due to the gradual diffusion of electrolyte into the entire electrode which allows a more thorough interaction. A stable capacitance retention is shown afterwards, and a slightly increased capacitance (108%) in comparison to cycle 1 is obtained upon 2000 cycles. The MnO2-80 electrode shows the similar trend with a slightly smaller increase of 105% capacitance at the 2000th cycle; however, the actual capacitance is lower due to the low initial value. On the contrary, the MnO2-25 and MnO2-40 materials show a decay of capacitance, with 77.4% and 84.7% remaining at the 2000th cycle. At the end of the 2000th cycle, we collected the SEM images of the tested electrodes (Figure 5b-e). The 12


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MnO2-25 and MnO2-40 samples show apparent peeling off of the deposited layer from the carbon fiber substrate. This is likely to start from the cracks which exist in the as-deposited electrodes, leading to its poorer cycling behavior. On the other hand, no morphological difference is observed for the MnO2-60 and MnO2-80 electrodes before and after cycling, i.e. the homogeneous MnO2 coverage on the carbon fiber as well as the hierarchical structure is retained. The porous nanostructures of MnO2-60 and MnO2-80 electrodes resulted from their large primary nanosheets and hetero-morphologies ensure the good flexibility, which helps to buffer the internal strain created during cycling. In addition, their α-phase component allows stable long-term cycling due to the large ion tunnels,39, 40 and the multiple connection joints between the α- and ε-phases help to further buffer the internal crystal strain.21, 23 These factors overall ensure the good mechanical stability of the deposited layer, which effectively suppresses electrode degradation and results in the enhanced cycling stability.

Figure 5 Cycling stability (a) and SEM images of MnO2-25 (b), MnO2-40 (c), MnO2-60 (d) and MnO2-80 (e) before and after the cycling test. The inset shows high magnification SEM images of MnO2-60 and MnO2-80.

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Electrochemical performance of asymmetric supercapacitor Asymmetric supercapacitors (ASC) were assembled with the as prepared MnO2-60 as cathode and vanadium oxide nanoflowers (V2O5NF) as anode (Figure S9b and S15), denoted as MnO2-60//V2O5NF. The 1 M Na2SO4 solution was used as the electrolyte for the aqueous ASC, while Na2SO4/PVA was used for the all-solid ASC. The fabrication details are described in the Methods section. The mass loading of the cathode and anode were adjusted to balance the charge on both sides (see Supporting Information for detailed calculation). The CV curves of both electrodes tested with a three-electrode cell in Na2SO4 aqueous electrolyte demonstrate the potential windows of 0 to 0.9 V for the MnO2-60 cathode and -1.1 to 0 V for the V2O5NF anode, which in combination gives rise to a large stable voltage window up to 2 V for the ASC (Figure 6a). Figures 6b and 6c show the GCD curves of the two ASC devices collected at different current densities (1 mA cm-2 to 20 mA cm-2) and similar results were obtained. The symmetric and linear profiles demonstrate an ideal capacitive behavior. The ASCs exhibit excellent volumetric capacitance of 14.85 F cm-3 (aqueous) and 14.41 F cm-3 (all-solid) at the current density of 1 mA cm-2 (equivalent to 70.2 F g-1 and 69.1 F g-1 based on the total mass of active materials, 21.6 mg cm-2 and 21.7 mg cm-2 in the two devices, respectively), which is substantially higher than many representative ASCs assembled with nanostructured electrodes, such as MnO2/graphene//V6O13-X/C NWs

ASC

(1.92

F

cm-3

at

1

mA

cm-2),41

MnO2//Ti-Fe2O3@PEDOT

(poly(3,4-ethylenedioxythiophene)) ASC (2.40 F cm-3 at 1 mA cm-2),42 CNT/MnO2//CNT/PPy (polypyrrole) ASC (2.20 F cm-3 at 2 mA cm-2),43 mesoporous VN/CNT//VN/CNT symmetric supercapacitor ( 7.9 F cm-3 at 0.025 A cm-3),44 graphene/MnO2//graphene/PPy ASC (2.67 F cm-3 at 1 mA cm-2),45 CNT//CNT/WO3 ASC (2.60 F cm-3 at 0.3 mA cm-2).46 More importantly, as the current density increases 20 times from 1 to 20 mA cm-2, the ASC devices still maintain 66.1% (aqueous) and 65.7% (all-solid) of the volumetric capacitances, reaching 9.79 F cm-3 (aqueous) and 9.47 F cm-3 (all-solid), respectively (Figure 6d). A highly stable capacitance retention is also obtained with our ASCs. At a current density of 40 mA cm-2, 90.7% (aqueous) and 90.1% (all-solid) capacitances are retained after 8000 charge-discharge cycles (Figure 6e). Energy and power densities are two important factors to evaluate the energy storage performance of supercapacitors. Figure 6f shows the Ragone plots of our MnO2-60//V2O5NF ASCs compared with the representatives of recently reported supercapacitor devices.41-62 Our devices 14


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exhibit the most superior behavior, with the volumetric energy density reaching 8.25 mWh cm-3 at a power density of 0.28 W cm-3 and 5.44 mWh cm-3 at 1.12 W cm-3 for the aqueous ASC, or 8.01 mWh cm-3 at 0.65 W cm-3 and 5.26 mWh cm-3 at 0.98 W cm-3 for the all-solid one. In addition, the all-solid ASC can effectively operate a red-light-emitting diode (LED) or an electrical watch in its flat (Figure 6g and S16a) and folded form (Figure 6h and S16b), demonstrating the flexibility and stability of the device and highlighting its potential usage for portable and wearable energy-storage systems. The outstanding electrochemical behavior of the ASC devices are attributed to the nice electrochemical performance of both the MnO2-60 cathode and the V2O5NF anode demonstrated in this work, as well as the flowing key aspects of: 1) the large loading of the nano-structure metal oxide (the total mass of the active materials on the two electrodes is 21.6 mg cm-2) which contributes to large pseudocapacitance; 2) the stable high operating voltage of 2.0 V (in aqueous electrolyte) that drastically increases the energy density; 3) the direct growth of the nano-structured materials on carbon fiber without the need of binders, resulting in small internal resistance of the ASC (negligible IR drop was observed in the GCD profiles at all current densities as shown in Figure 6b).

15


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Figure 6 CV curves of the cathode (MnO2-60) and anode (V2O5NF) in a three-electrode cell (a). GCD profiles of the aqueous (b) and all-solid (c) ASC devices collected at various current densities. Volumetric capacitance of the aqueous and all-solid ASC devices measured at different current densities (d). Cyclic stability of the aqueous and all-solid ASC devices tested at a current density of 40 mA cm-2, with the inset showing the profiles of the first and the 8 000th galvanostatic charge-discharge cycles (e). Ragone plots of the aqueous and all-solid ASC devices compared with other recently reported ASCs (f). The fully charged solitary all-solid ASC operates a red LED in its flat (g) and folded (h) forms, demonstrating the potential application for wearable electronics.

Conclusion In summary, a high mass loading of 10 mg cm-2 and hierarchically structured MnO2 was grown on the conductive carbon cloth by a facile electro-deposition technique. The morphologies and 16


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crystal phases were carefully controlled by the deposition temperature. The optimized material (MnO2-60, deposited at 60 °C) is composed of the interconnected primary ε-MnO2 nanosheets and secondary α-MnO2 nanorods. Thanks to the hetero-phases and morphologies, the structural strain in the electrode can be buffered and a highly porous and uniform coverage of MnO2 on the carbon fiber is resulted. This hierarchical structure ensures the good ionic and electric conductivities as well as the mechanical stability of the material. The MnO2-60 electrode provides an ultra high areal capacitance of 3.04 F cm-2 at 3 mA cm-2 in a neutral aqueous electrolyte, which is superior to recently reported nanostructured MnO2 electrodes tested under similar conditions. The high specific capacitance of 304 F g-1 demonstrates the thorough utilization of the active material despite the large loading. The ASCs assembled with the MnO2-60 cathode coupled with vanadium oxide anode exhibit high volumetric capacitance of 14.85 F cm-3 (aqueous) or 14.41 F cm-3 (all-solid), and an excellent capacitance rate retention of 66% when the current density increases from 1 to 20 mA cm-2. Significantly, our ASC devices are able to reversibly cycle with a high operating voltage of 2.0 V in aqueous electrolyte and delivers an extremely high volumetric energy density of 8.3 mWh cm-3 at the power density of 0.28 W cm-3. At the same time, both devices show excellent cycle lives with > 90% capacitance retained after 8000 charge/discharge cycles. Our results suggest the important role that hetero-structures plays in enhancing the electrochemical performance of high loading electrodes, which is essential to bring them one step towards practical applications.

Methods Materials All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. and used as received. Carbon cloths were purchased from Fuel Cell Earth (United States), graphite foils manufactured from natural expanded graphite were from SGL group (Germany). Cellulose separators were purchased from Nippon Kodoshi Corporation (Japan).

Electro-deposition of high mass loading manganese oxide on carbon cloth Electro-deposition of manganese oxide (MnO2) was carried out at 10 mA cm-2 on a piece of carbon cloth (1×1 cm2, Fuel Cell Earth, United States) in a 0.1 M manganese acetate aqueous solution, using SCE as the reference electrode and graphite foil (SGL group, Germany) as the counter electrode. MnO2 was electro-deposited at various temperatures of 25 °C, 40 °C, 60 °C and 80 °C to investigate the influence of temperature on MnO2 growth. The as-prepared samples were 17


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denoted as MnO2-25, MnO2-40, MnO2-60 and MnO2-80, respectively. The mass loading of MnO2 was controlled to be 10.0 ± 0.01 mg cm-2 by adjusting the electro-deposition time (MnO2-25: 43 min, MnO2-40: 41 min, MnO2-60: 40 min and MnO2-80: 38 min). After electro-deposition, the working electrode was washed thoroughly with deionized water and dried at 60 °C under vacuum for 24 h. The casted MnO2-60 electrode was prepared by scraping off the powder from the carbon cloth substrate, mixing with acetylene black and polyvinylidene fluoride binder in 8:1:1 weight ratio in N-methyl-2-pyrrolidinone, and casting the slurry on Ni foam. The electrode was dried in air and the active material loading was maintained at around 10 mg cm-2.

Fabrication of asymmetric supercapacitor device A model asymmetric supercapacitor was assembled by using the as prepared MnO2 (MnO2-60) as the cathode and vanadium oxide nanoflower (V2O5NF) as the anode, respectively. The V2O5NF anode was fabricated as follows: the carbon cloth was cleaned with ethanol, acetone and distilled water in sequence. V2O5NF was electrochemically deposited on a piece of 1.0×1.0 cm2 carbon cloth in a solution of 0.5 M VOSO4 and 0.5 M ammonium acetate at a constant potential of 0.7 mA vs. SCE for 80 min. The product was washed thoroughly with deionized water and dried at 60 °C under vacuum for 24 h. The mass loading of V2O5NF was 11.6 mg cm-2 (to balance the charge with the cathode, see Calculation part in the Supporting Information). The total mass loading of active materials on both electrodes was 21.6 mg cm-2. Subsequently, the cathode and anode, as well as a piece of cellulose paper (NKK separator, Japan) which was used as the separator, were soaked in a 1 M Na2SO4 aqueous solution for 3 h to absorb electrolyte. The cathode, separator and anode were then assembled in sequence to fabricate the aqueous asymmetric supercapacitor (ASC) device, which was wrapped and sealed by parafilm. The all-solid ASC was assembled in a similar manner with Na2SO4/PVA replacing the separator and liquid electrolyte. The working area of the ASC was 1.0 cm2. The volume of aqueous and all-solid ASC devices was 102 mm3 [10 mm (L)×10 mm (W)×1.02 mm (H)] and 104 mm3 [10 mm (L)×10 mm (W)×1.04 mm (H)].

Characterization and electrochemical measurements Surface morphologies and elemental mappings of the materials were studied with a field emission scanning electron microscope (Ultra Plus, Carl Zeiss, Germany) equipped with an energy dispersive X-ray spectroscopy (EDX) detector. Transmission electron microscopy (TEM) images were collected by Tecnai G2 F20 S-TWIN TEM (FEI, USA). The mass loading of active materials 18


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was measured by the weight difference of the electrode before and after electro-deposition, using a Sartorius BT 25 S semi-microbalance with a sensitivity of 0.01 mg. The crystal structures of the materials were studied by X-ray diffraction (X’Pert Pro, PANalytical B.V.). X-ray photoelectron spectroscopy (XPS) analyses were carried out on an XPS spectrometer (ESCALAB 250Xi, Thermo Scientific Escalab, USA). All core level XPS spectra were calibrated using C 1s photoelectron peak at 284.6 eV as the reference. XPS peak de-convolution was performed with a XPSPEAK software. Electrochemical properties of manganese oxide were investigated in three-electrode cells with SCE and graphite foil as the reference and counter electrode, respectively, in 1 M Na2SO4 aqueous electrolyte. Electrochemical impedance spectroscopy (EIS) was carried out at open-circuit potential in a frequency range from 0.05 Hz to 40 kHz with a perturbation of 10 mV. Electrochemical performance of the ASC was evaluated by a two-electrode testing system. All electrochemical measurements were carried out on a multichannel electrochemical analyzer (VMP3, Bio-Logic-Science Instruments, France).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Electrochemistry calculations, SEM images of MnO2-25 and MnO2-60 at different electro-deposition periods, XRD patterns of MnO2-60 obtained by 300 s and 2400 s electro-deposition, the crystal structures of α-MnO2 and ε-MnO2, the survey, Mn 2p, Mn 3s and O 1s XPS data of MnO2-25, MnO2-40 and MnO2-80 electrodes, schematic illustration of the three-electrode cell for electrochemical measurements and the device architecture of the MnO2-60//V2O5NF asymmetric supercapacitor, GCD profiles of MnO2-25, MnO2-40, MnO2-60 and MnO2-80 electrodes collected at various current densities, gravimetric capacitance of four MnO2-based electrodes measured at different current densities, CV curves, areal capacitance and specific capacitance of MnO2-25, MnO2-40, MnO2-60, and MnO2-80 measured at different scan

rates,

electrochemistry

of

the

casted

MnO2

electrode,

characterization

and

electrochemistry of the V2O5NF, the fully charged solitary ASC operating an electrical watch (PDF). The authors declare no competing financial interest. 19


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AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] ORCID Xiaoqi Sun: 0000-0003-2324-7631 Xiao-Xia Liu: 0000-0002-0172-5826

Acknowledgements We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21673035).

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ACS Nano

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High Mass Loading MnO2 with Hierarchical Nanostructures for

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