3(OH)2 Reinforced by Ethyl Carbamate

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Functional Nanostructured Materials (including low-D carbon)

Hydrangea-Like #-Ni1/3Co2/3(OH)2 Reinforced by Ethyl Carbamate “Rivet” for All-Solid-State Supercapacitor with Outstanding Comprehensive Performance Wutao Wei, Wanyu Ye, Jing Wang, Chao Huang, Jia-Bin Xiong, Huijie Qiao, Shizhong Cui, Weihua Chen, Liwei Mi, and Pengfei Yan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09555 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Hydrangea-Like α-Ni1/3Co2/3(OH)2 Reinforced by Ethyl Carbamate “Rivet” for All-Solid-State Supercapacitor with Outstanding Comprehensive Performance Wutao Wei,† Wanyu Ye,† Jing Wang,† Chao Huang,† Jia-Bin Xiong,† Huijie Qiao,† Shizhong Cui,† Weihua Chen,*, ‡ Liwei Mi,*, † Pengfei Yan,*, § †

Center for Advanced Materials Research, Zhongyuan University of Technology, Henan 450007, China

‡ College

of Chemistry and Molecular Engineering, Zhengzhou University, Henan 450001, China

§ Institute

China

of Microstructure and Properties of Advanced Materials, Beijing University of Technology, Beijing 100124,

KEYWORDS. High ionic and electronic transmission performance · Ethyl carbamate as the rivet · Excellent structural stability · Ultrahigh rate performance · All-solid-state supercapacitor. ABSTRACT: Improving the self-conductivity and structural stability of electrode materials is a key strategy to improve the energy density, rate performance and cycle life of supercapacitors. Controlled intercalation of ethyl carbamate (CH3CH2OCONH2) as rivet between Ni-Co hydroxide layers can be used to obtain sufficient ion transport channels and robust structural stability of hydrangea-like α-Ni1/3Co2/3(OH)2 (NC). Combing the improved electronic conductivity offered by the coexistence of Ni2+ and Co2+ optimizing itself electronic conductivity and the addition of carbon nanotubes (CNTs) as the electron transport bridge between active material and current collector and the large specific surface area (296 m2 g-1) reducing concentration polarization, the capacitance retention ratio of NC-CNT from 0.2 to 20 A g-1 is up to 93.4 % and its specific capacitance is as high as 1228.7 F g-1 at 20 A g-1. The large total hole volume (0.40 cm3 g-1) and wide crystal plane spacing (0.71 nm) provide an adequate space to withstand structure deformation during charge/discharge processes and enhance the structural stability of NC material. The capacitance fading ratio of NC-CNT is only 4.5 % at 10 A g-1 for 10000 cycles. The aqueous supercapacitor (NC-CNT//AC) and all-solid-state supercapacitor (PVA-NCCNT//PVA-AC) exhibits high energy density (35.2 Wh kg-1 at 100.0 W kg-1 and 35.4 Wh kg-1 at 100.7 W kg-1), ultrahigh rate performance (the specific capacitances at 20 A g-1 are 92.8 % and 87.2 % of that at 0.5 A g-1) and long cycling lifespan (the specific capacitances after 100000 cycles at 10 A g-1 are 91.5 % and 90.8 % of their initial specific capacitances), respectively. Therefore, hydrangea-like α-Ni1/3Co2/3(OH)2 could be a promising for advanced next-generation supercapacitors.

Introduction Energy technology innovation has increased the demand for energy storage devices with high comprehensive performance that can efficiently utilize renewable energy.1 Supercapacitors offer the advantages of batteries and conventional capacitors and are thus a suitable innovation for energy storage devices.2 The construction of abundant ionic and electronic transport channels to improve the utilization rate of active materials has emerged as an effective method used to assemble supercapacitors with outstanding performance.3 Previous studies attempted to reduce the concentration polarization by micro/nano preparation methods and accelerate the electronic transmission between electrode materials and current collector by using high-conductive materials.4-6 However, there is still no solution to the problem that rapid redox reaction only occurs on the surface of active materials. The key to address this challenge is to improve the ionic and electronic

transmission performance of electrode materials. Therefore, the development of a novel electrode material with high ionic and electronic transmission performance is of great interest. Ni(OH)2 has been extensively investigated as an electrode material because of its well-defined redox behavior, high redox activity, low cost and environmental friendliness. In particular, α-Ni(OH)2 displays not only a characteristic layered crystal structure but also an adjustable layer spacing depending on different intercalated ions.7-8 This structure offers a breakthrough in the design of electrode materials with high ionic conductivity. However, α-Ni(OH)2 is easily converted into β-Ni(OH)2 because the interlayer ions can easily escape in alkaline environment.8 Thus, Gunjakar et al. built αNi(OH)2 intercalated with polyoxovanadate anions as pillar to improve ionic conductivity and structural stability and proposed a novel strategy to improve the structure stability of α-Ni(OH)2.9 The improved ionic

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conductivity changed the speed control step of reversible redox reaction into the electronic conduction process. Therefore, improving the electronic conductivity of αNi(OH)2 has become an urgent challenge. Binary metal compounds (such as Mn−Mo Mixed Oxide, NiCo2S4, NiMn LDH, NiCoSe2, and so on) exhibit fascinating properties in photoelectrocatalysis, energy storage and conversion because of their more rapid charge transportation than their single-metal compounds, as a result of the cationic substitutions by any other metal cations more or less affecting the crystal structure and electron cloud density of parent material and further optimized their electronic conductivity.10-12 In addition, binary metal compounds have multiple oxidation states for reversible redox reactions as well as the synergistic effects and complementary advantages among different cations also can significantly improve the cycling stability of electrode materials.13 On the other hand, Ni2+ and Co2+, which have excellent reversible redox activities, possess similar ionic radii. Co(OH)2 possesses higher electronic conductivity than Ni(OH)2.6 Thus, the introduction of Co2+ into α-Ni(OH)2 is expected to enhance the electronic conductivity and cycling stability of α-Ni(OH)2. According to previous studies, α-NixCo1-x(OH)2 with suitable intercalation anions as rivets to fix the different layer crystal structure layers is expected to display high ionic and electronic conductivity and excellent cycling stability.3, 9 These findings have been rarely reported. Besides, structural deformation and even collapse of electrode materials because of volume change during charge/discharge processes are major factors that weaken the lifespan of supercapacitors and severely impede their industrialization. One of the major reasons is that the electrode materials have not enough porosity for their volume change during reversible redox reaction. The stress caused by volume change gradually destroyed the structure of electrode materials and diminishes their usefulness. Micro/nano preparation technology can endow electrode materials with abundant pore channels and a large specific surface area and is considered an effective method for optimizing the structure stability of electrode materials. Poor conductivity is another important factor that affects structural deformation, which results in only external active materials to participate in the electrochemical reaction. The different deformation degree of internal and external active materials during charge/discharge processes causes the external active materials to gradually fall off, thereby gradually destroying their structure. In general, αNi(OH)2 crystal easily forms micro/nano sheets because of the edge-sharing octahedral MO6 layers.14-16 Thus, the assembly of α-NixCo1-x(OH)2 with a three-dimensional (3D) hierarchical structure and suitable intercalation anions and ultrathin nanosheets as building blocks will result in high ionic and electronic conductivity and excellent structure stability, and further present high specific capacitance, excellent rate performance and long service life.

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This study proposes a novel strategy for skillfully constructing unique electrode materials with high-self ionic and electronic conductivity and excellent structural stability. 3D hierarchical hydrangea-like α-Ni1/3Co2/3(OH)2 with the ultrathin nanosheets of ~15 nm in thickness as building block was synthesized. CH3CH2OCONH2 with a chain length of 0.63 nm was introduced as rivet in the interlayers of Ni-Co hydroxides with a layer spacing of 0.71 nm as the result of the formation of hydrogen bonds between CH3CH2OCONH2 and Ni-Co hydroxide layers, leading to the high ionic conductivity and excellent structure stability. The synergistic effect of Ni2+ and Co2+ in crystal lattice provides multiplex oxidation states and enhances the electronic conductivity. In addition, the large specific surface area (296 m2 g-1), high total hole volume (0.40 cm3 g-1) and wide spacing between Ni-Co hydroxide layers provide an adequate space for volume deformation during electrochemical reaction, thereby ensuring its structure stability and exposing abundant active sites in electrolyte. The introduction of CNTs builds 3D conductivity bridges between NC material and current collectors and guarantees the fast transfer of electron outside active material. The electrochemical measurements reveal the high activity and stability of the NC material. The as-prepared NC-CNT electrode display a specific capacitance of 1228.7 F g-1 at 20 A g-1 and an ultrahigh rate performance and a high capacitance retention ratio of 95.5 % at 10 A g-1 for 10000 cycles. The performance of the as-assembled aqueous (NC-CNT//AC) and all-solid-state (PVA-NC-CNT//PVA-AC) devices were also evaluated. There devices exhibit the outstanding comprehensive performance.

Result and Discussion The direct synthesis of α-nickel cobalt bimetallic hydroxide remains challenging because all commonly used methods result in the production of β-hydroxide. We successfully designed a one-step green solvothermal method, as illustrated in Equation 1-4. CH3CH2OH and CO(NH2)2 were selected as the solvent and precipitant, respectively. The slow nucleophilic substitution reaction between CH3CH2OH and CO(NH2)2 produced CH3CH2OCONH2 with a chain length of 0.63 nm and NH3 (Equation 1),17 and CO(NH2)2 was hydrolyzed to produce CO32- and NH4+ (Equation 2). In the closed system, the generated NH3 was combined with the crystalline water of nitrate to obtain ammonium ion (NH4+) and hydroxide ion (OH-) (Equation 3). Hydroxide precipitation was synthesized by the reaction of metal ions (Ni2+ and Co2+) with hydroxide ions (Equation 4). The slow nucleophilic substitution reaction limited the formation rate of hydroxides, resulting in the intercalation of some ions (CH3CH2OCONH2, H2O, OH-, CO32-, NO3-) between Ni-Co hydroxide layers and the preparation of α-nickel cobalt bimetallic hydroxide. Both ends of CH3CH2OCONH2 were connected to Ni-Co hydroxide layers by hydrogen bonding, thereby effectively strengthening its structure stability. CH3CH2OH + CO(NH2)2 = CH3CH2OCONH2 + NH3↑


(1)

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CO(NH2)2 + 2H2O = CO32- + 2NH4+

(2)

NH3 + H2O = NH4+ + OH-

(3)

M2+ + 2OH- = α-M(OH)2 (M = Co2+ or Ni2+)

(4)

By adopting this design, a uniform hydrangea-like NC material was successfully synthesized for the first time (Figure 1a). The material consists of curved sheets of ~500 nm in thickness as the venation of hydrangea interconnecting with one another (Figure S1), providing many micropores to accommodate the electrolyte (Figure 1b). The morphology of the as-obtained NC material is similar to that of the red-silk hydrangea representing celebration. The ~15 nm thick nanosheets as the building units and the petals of hydrangea grow together in the form of a cross along the venation of hydrangea, which effectively shorten the electron and ion transmission paths and split the existing micropores into many mesoporous, resulting in

large specific surface area and abundant redox reactive active sites (Figure 1c). The TEM image of NC material illustrates the venations and petals of hydrangea and the porous internal structure (Figure 1d). The interior of the NC material also consists of crossed nanosheets with abundant micro/nano-pores inside the NC material (Figure S2). Beyond that, many nanopores were homogeneously distributed on the surface of the nanosheets (Figures 1e and 1f). Most of the nanopores are about 4.2 nm in diameter. Hence, the NC material exhibited high porosity, which allows the intimate electrolyte penetration and fast ion transport.18, 19

Figure 1. Structure and physical characterization of hydrangea-like NC material. (a) Low-magnification field emission scanning electron microscope (FESEM) image of NC material. (b) FESEM image of the single hydrangea-like NC material. (c) High-magnification FESEM image of the nanosheets as building blocks and high resolution transmission electron microscope (HRTEM) image of cross section of a single nanosheet (inset). (d) Transmission electron microscope (TEM) of the NC material. (e) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of the NC material. (f) HRTEM image of a single layer nanosheet. (g) N2 adsorption–desorption isotherms of the hydrangea-like NC material, the



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corresponding pore size distribution curves (inset). (h) 1H NMR spectra of product extracting with methanol from NC material under continuous magnetic stirring for one week.

The nitrogen adsorption isotherms exhibits type IV isotherm hysteresis loops within 0.45~1.0 P/P0, indicating the presence of a mesoporous structure (Figure 1g). Its BET surface area is calculated to be 296 m2 g-1, which is higher than the previous reports values.20-21 The pore-size distribution plots (the inset of Figure 1g) indicates that a uniform pore size distribution centered at 4.2 nm for NC material echoing Figure 1e and 1f, and its total pore volume is 0.40 cm3 g-1. The hydrangea-like NC material presented a high specific surface area and large total pore volume because of the 3D hierarchical hydrangea-like structure with crossed porous nanosheets as the building units. This porous structure provides the hierarchical space for the storage of electrolytes, exposes the large liquid junction surface area, supplies adequate space for structural deformation during electrochemical reaction, and enhances the stability and effectively utilization of active material. Figure S3a shows the typical XRD patterns of hydrangea-like α-Ni1/3Co2/3(OH)2, which indicates a good crystallinity. Several sharp diffraction peaks at 12.3o, 25.0o, 32.9o, 36.1o and 58.6o are belong to the lattice planes of (0 0 1), (0 0 2), (1 1 0), (1 1 1) and (3 0 0) planes of α-Ni(OH)2 (JCPDS NO. 22-0444) with interplanar crystal spacing of 0.71, 0.36, 0.27, 0.25 and 0.16 nm, respectively.22, 23 The value of d(001) represents the thickness of the brucite-like layer.24 Thus, the interlayer spacing of Ni-Co hydroxide is 0.71 nm, which is slightly narrower than that (0.76 nm) of standard card. This finding might be due to the fact that intercalated CH3CH2OCONH2 with chain length of 0.63 nm narrowed down the interlayer spacing under the action of hydrogen bonds. To capture direct evidence of the presence of CH3CH2OCONH2, Figure 1h shows the 1H NMR spectra obtained for the product extracting with methanol from NC material under continuous magnetic stirring for one week. Three characteristic signals are available in the 1H NMR spectrum of CH3CH2OCONH2. A broad singlet at 6.41 ppm, a quartet at 3.92 ppm and a triplet at 1.15 ppm correspond to the amide (-NH2), methyleneoxy (-CH2-) and methyl (-CH3) protons, respectively.25 The molar ratio of protons corresponding to these three signals can be obtained by integration as 2:2:3, which is completely responsive to the molecular formula for CH3CH2OCONH2. It is important to note here that the chemical shifts at 2.51 and 3.33 ppm in 1H NMR spectra are due to residual H atoms present in DMSO-d6.26 These results reveal that the conjecture of CH3CH2OCONH2 as rivet is correct. The intercalated CH3CH2OCONH2 as rivet further strengthens the structure stability of the NC material. In addition, the in-situ replacement of Co2+ affects the crystal structure of α-Ni(OH)2 and slightly shifted the position of the diffraction peaks, which realizes the multiple oxidation states and complementary performance and synergistic enhancement of Ni-Co bimetallic ions, leading to excellent conductivity and good stability.

Figure S3b shows the elemental linear sweep EDS signals, as recorded by FESEM-EDS linear sweeping along the yellow line (in the inset). The element signals with diameters of 3 to 7 μm reflected the true element distribution of the NC material because of its microsphere structure. The NC material contains Co, Ni, C, O and N. The EDS curve in Figure S3c further confirms the coexistence of Co, Ni, C, O and N in the NC material. The mole ratio of Co and Ni elements is approximately 10.7:5.19, which matches the results of inductively coupled plasma mass spectrometry (ICP-MS) (~69.1:30.9, Table S1), and is close to 2:1, thereby confirming that the hydrangealike material is α-Ni1/3Co2/3(OH)2. The HAADF-EDS mappings display the even distribution of these elements (Figure S3d), proving that the Co ions in situ replaced part of the Ni ions because of their similar ionic radius and the anions of OH-, CO32- and NO3- and the molecule of CH3CH2OCONH2 and H2O are evenly interspersed in NiCo hydroxide layers. The HRTEM image (Figure 2a) displays clear lattice fringes with an interplanar spacing of 0.27 nm that were indexed to the (1 1 0) plane of the NC material. The interplanar spacing of the lattice fringes is slightly wider than that of α-Ni(OH)2 (0.267 nm) because of the introduction of Co2+. The corresponding FFT pattern implies that the as-synthesized electrode material is made of a highly crystalline NC material with a hexagonal structure. The clear and regular atomic arrangement in the IFFT image indicates that Co2+ in situ replaces part of the Ni2+ in the lattice. Figure 2b shows the model of the atomic arrangement of (1 1 0) crystal plane in the hexagonal structure viewed along the [0 0 1] direction, which displays the typical atomic arrangement of the hcp phase (ABAB…stacking sequence of closed packed planes) and the arrangement of metal and oxygen ions in the NiCo hydroxide layers. The legible diffraction spots can be indexed to the lattice planes (1 1 1) and (1 1 0) with an interplanar spacing of 0.25 nm and 0.27 nm for α-Ni(OH)2 (JCPDS NO. 22-0444), respectively (Figure 2c). These findings are consistent with the previous XRD results. Furthermore, XPS was conducted to determine the valence states of the hydrangea-like NC electrode material (Figure 2d-2f). The coexistence of Ni (Ni 2p) and Co (Co 2p) confirms that the NC material contains Ni and Co, consistent with the above results. Three peaks for Co 2p3/2 at 779-792 eV and three peaks for Co 2p1/2 at 792-810 eV appear in the high-resolution XPS spectrum of Co 2p (Figure 2d). The peak at 780.6 eV for Co 2p3/2 and 796.5 eV for Co 2p1/2 are assigned to Co2+ for Co-O bonds, whereas the abreast peaks at 782.2 and 797.7 eV are attributed to the Co(OH)2.27, 28, 29 The peaks at 786.3 and 803.0 eV are satellite peaks attributed to the Co 2p3/2 and Co 2p1/2 spin orbit levels of Co(OH)2.24 The Ni XPS (Figure 2e) shows two major peaks centered at 855.9 and 873.5 eV with a spin-energy separation of 17.6 eV, which corresponded to Ni 2p3/2 and Ni 2p1/2, respectively, and are assigned to Ni2+.7, 28, 29 Satellite peaks (Ni 2p1/2, satellite:


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861.3 eV; Ni 2p3/2, satellite: 879.5 eV) are also observed and are associated with the Ni(OH)2 phase according to previous reports.28 The high-resolution XPS spectrum of O 1s (Figure 2f) suggests two kinds of oxygen contributions. The signal at 531.1 eV originates from the M-OH bonds.7, 31, 32 The peak at 532.6 eV is assigned to OC=O from CH3CH2OCONH2 existing between the Ni-Co hydroxide layers to improve the ionic conductivity and structure stability.32 Thus, the as-obtained NC material is composed of Ni-Co hydroxide layers and rich intercalated anions and molecular and maintains the hexagonal crystalline phase of α-Ni(OH)2, which contributes to an ideal electronic and ionic conductivity.

Figure 2. Crystal structure analysis of hydrangea-like NC material. (a) HRTEM image of the NC nanosheet, the relevant fast Fourier transform (FFT), and inverse FFT (IFFT) images in the HRTEM image shown in the right. (b) Model of the atomic arrangement of (1 1 0) crystal plane in the hexagonal structure viewed along the [0 0 1] direction. (c) The corresponding selected area electron diffraction (SAED) pattern. X-ray photoelectron spectroscopy (XPS) spectra of (d) Co 2p, (e) Ni 2p and (f) O 1s for the NC material. (g) HRTEM image of the NC material. (h) The enlarged HRTEM image of red box in g. (i) HRTEM image of (0 0 1) crystal surface in the NC material. (j) Fourier transform infrared (FTIR) spectra of the hydrangea-like NC material. (k) Schematic illustration and layered crystal structure of the hydrangea-like NC material reinforced by ethyl carbamate as rivet.



For α-Ni(OH)2, the types and valence relations of intercalated ions significantly affect its performance, and so does NC material. HRTEM and FTIR characterizations were conducted to determine the types and existing forms of interlayer ions between the Ni-Co hydroxide layers in detail. The HRTEM image (Figure 2g) shows a clear layered crystal structure with a spacing of 0.71 nm. The enlarged HRTEM image (Figure 2h) of the red box in Figure 2g shows the high crystallinity of the NC material and several layers of parallel Ni-Co hydroxide layers. A (0 0 1) crystal surface with an interlayer spacing of 0.71 nm was also observed (Figure 2i), which is the enlarged drawing of the corresponding selection in Figure 2h. There are some small free ions and long-chain ions as rivet linking upper and lower layer crystals interspersing between Ni-Co hydroxide layers. These results are consistent with the above results. Moreover, FTIR was also conducted on the NC material to further analyze vibrations at the molecular level (Figure 2j). The vibrational bands at approximately 3647, 3499, 3416, 2987, 2921, 2230, 1655, 1490, 1385, 1372, 1291, 1076, 995, 641, 522, 485 and 469 cm-1 are typical features of α-type hydroxides.8, 22, 33 A narrow band at ~3647 cm-1 corresponds to the OH groups in the brucite-like structure.34 The band located at ~3499 cm-1 could be attributed to the O-H vibration of a hydrogen-bonded water molecule of the interlayer water molecule.35 The absorption band observed at 3416 cm-1 originates from the stretching of amidogen and the C-H stretching of methyl groups are located at 2987 and 2921 cm-1, which denote the incorporation of ethyl carbamate.25, 35, 36 The absorption band at 2230 cm-1 could be attributed to the existence of intercalated N-C=O species from CH3CH2OCONH2.15, 37 The peak around 1655 cm-1 is due to bending vibration of CH3CH2OCONH2 and H2O molecules adsorbed onto the Ni(OH)2 by hydrogen bonding, which hold the CH3CH2OCONH2 between Ni-Co hydroxide layers.34 CH3CH2OCONH2 as a rivet further strengthens the crystal structure stability of the NC material. The band at 1490 cm-1 could be due to the C=O vibration in CO32-.37 The sharp band observed at 1385 cm-1 could be attributed to υ3 vibration modes of NO3- in D3h symmetry.35, 38 The bands at 1372 and 1076 cm-1 are assigned to υa (-COO-) of the ester group from CH3CH2OCONH2, which validates the existence of interbedded CH3CH2OCONH2.22, 35, 39 The single strong absorption at 1291 cm-1 is the characteristic of chelate nitrato-group.8, 36 The band at 995 cm-1 is assigned to the υ1 mode of CO32- that remains inactive in the free ion, and the mode becomes active in hydroxide due to the reduction of its symmetry.8, 15 The bands located at 641 and 485 cm-1 are assigned to the δOH and υNi-OH vibrations.8, 15 The absorption bands at 522 and 469 cm-1 are associated with υ(NiO) vibrations.35, 38 These findings are identical with previous reports, proving that the asobtained hydrangea-like NC material has the crystal structure of α-Ni(OH)2. The abundant anions of OH-, OCN-, CO32- and NO3- and the molecule of CH3CH2OCONH2 and H2O exist in the inter-lamellar

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space of α-Ni(OH)2, which provides the travel path of OHand increases the effective contact area of the active material and the electrolyte. The wide lattice plane spacing between the layered crystal structures provides an enough structural deformation space for the NC material during the redox process, thereby conferring the target material with excellent structural stability. What is more interesting is that the presence of interbedded CH3CH2OCONH2 as rivet further effectively enhances the structure stability of the NC material. These features endow the NC material with high specific capacitance, excellent rate performance and long cycle life. Basing on the above phase and structure characterization results, a structure of the hydrangea-like α-Ni1/3Co2/3(OH)2 was proposed (Figure 2k), where porous nanosheets were as building blocks and which has a large specific surface area and total pore volume. In addition, the parallel layers of Ni-Co hydroxide are interspersed with OH-, OCN-, CO32-, NO3- and H2O, and connected by CH3CH2OCONH2 as rivet. The ion intercalation offers abundant ion transport paths. The in situ substitution of Co2+ realized the complementary advantages and synergistic effect of Ni2+ and Co2+ results in NC material with good electronic conductivity and excellent stability. These properties maximizes the utilization ratio of the active material and ensures that the active material inside and outside of the NC material have the similar deformation degrees during electrochemical reaction. The interlayered CH3CH2OCONH2 as the rivet reinforces the crystal structure of the parallel layered Ni-Co hydroxide. The wide lattice plane spacing and large total pore volume provide a sufficient space for the structure deformation of the NC material caused by the charge/discharge processes. In addition to the abundant ion transmission channels and ideal electronic conductivity, the stability of NC material is greatly enhanced, which was conducive for improving the rate performance and the cycle life. Time-dependent parallel experiments were performed to explore the morphology evolution mechanism of the hydrangea-like NC electrode material. The generated product is insufficient to collect under less than 2 h. Flower-like microspheres are quickly constructed using crossed nanosheets as the building block at 2 h (Figure 3aI and S4a). In addition, the obtained nanosheets as the venation present two different states, namely, a stable state with CH3CH2OCONH2 as the rivet and an unstable state without CH3CH2OCONH2 between the Ni-Co hydroxide layers (Figure 3aII). The latter dissolves again because of the coordination between metal ions and NH4+ (Figure 3aIII).27 The resulting gaps with a high surface energy become the new growth point of hydroxide (Figure 3aIV), which changes the direction of the crystal growth. As the reaction time increased to 4 h, small bumps appear on the surface of the nanosheets and cover the gaps (Figure 3bI-II and S4b). Figure 3bIII shows the curved and crossed layered crystal structure of the NC material and further supports this growth mechanism of redissolve and secondary growth, resulting in the


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porously nanosheets (Figure 3bIV). When the reaction time reaches to 8 h, some nanosheets grow on the surface of the venation of hydrangea as petals, which effectively improve its specific surface area and total pore volume (Figure 3c and S4c). As the reaction proceed, the nanosheets as the petals grow increasingly densely on the surface of the nanosheets as the venation, leading to the formation of the hydrangea-like NC material with the porous nanosheets as building block (Figure 3d and S4d). The young nanosheets are evenly arrayed on the surface of the old nanosheets, and no lamination phenomenon occurred, further exposing the surface area of the materials in the electrolyte and improving the total pore volume. The former effectively reduce the concentration polarization, and the latter provides adequate space for structure deformation of the NC electrode material during electrochemical reaction. With the extension of reaction time, the morphology and structure of the material did not change, and the hydrangea structure was maintained (Figure S5). Therefore, the optimal reaction time is 12 h.

redox peak and the fractions of current contributed by capacitive and diffusion-controlled processes, respectively.40, 41 The quantified results (Figure S6 and S7) shows that NC and NC-CNT electrodes displayed the high diffusion capacity, indicating a diffusion-controlled process. This may be due to the typical battery-type characteristic of the two-dimensional layered crystal structure of NC material, which performs charge storage through adsorption/desorption; another reason is that the hydrangea-like and porous structure enlarges the surface area and thus provides more opportunity for a pseudocapacitive reaction.41

The pesudocapacitance properties of the hydrangealike NC material are evaluated by using a three-electrode cell configuration in 2 M KOH solution, and the results are shown in Figure 4. Figure 4a and 4b show the CV curves of the as-assembled electrodes at scan rates of 1 to 10 mV s-1. The strong redox peaks reveals the good redox activity of the as-prepared electrode based on hydrangealike NC material, which is due to the special crystal structure and the complementary performance and synergistic enhancement of nickel and cobalt ions. A pair of redox peaks is detected. The probable reason is that the redox reactions of Ni2+↔Ni3+ and Co2+↔Co3+ have similar redox potentials, resulting in their overlap.3, 27 The peak current obviously increases with the construction of a 3D conductive network through the addition of CNTs, which can further greatly increases the specific capacitance of the original electrode (Figure 4c). The capacitive contribution of capacitor-like and diffusion-controlled behaviors at a certain scan rate (v) were calculated from CV curves of the as-assembled two electrodes according to the formula ic = k1v + k2v0.5, where ic, k1, and k2 are the current response of the



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Figure 3. Exploration of growth mechanism. (a) I is the FESEM image of the material under 2 h. II is the corresponding simulation diagrams. III depicts the redissolution of the unstable regions. IV illustrates the secondary growth process of the target product along the edge of nanopores. (b) I is the FESEM image of the material under 4 h. II is the corresponding simulation diagrams. III is the HRTEM images of the NC material. IV shows the schematic diagram of the formed nanopores after secondary growth. (c) I is the FESEM image of the material under 8 h. II is the corresponding simulation diagrams. (d) I is the FESEM image of the material under 12 h. II is the corresponding simulation diagrams. III shows the corresponding HRTEM images and proves the porous nanosheets, which are vividly depicted in IV. V and VI are the HAADF-STEM and HRTEM images of the cross section of porous nanosheets, which also displays the porous structure and graphically shows in VII.

The galvanostatic discharge curves of the as-assembled two electrodes at current densities from 0.2 to 20 A g-1 (Figure 4d and 4e) present the obvious voltage platform, which sufficient reveals the Faraday energy storage model. The addition of CNTs further increases all the discharge times at the corresponding current densities, and the specific capacitance was also enhanced. Figure 4f shows the rate performance of the hydrangea-like NC and NCCNT electrodes. The specific capacitances of the hydrangea-like NC electrode are 1139.9, 1122.8, 1094.3, 1067.3, 1057.6, 1046.2, 1035.1, 1016.6, 997.3, 983.7 and 943.2 F g-1 at 0.2, 0.5, 1, 2, 3, 5, 8, 10, 15, 18 and 20 A g-1, respectively. When the current density increases by 100 times, the specific capacity decreases by only 17.3 %. For the NC-CNT electrode, the specific capacitances are 1314.9, 1296.7, 1287.0, 1276.8, 1265.4, 1258.3, 1251.1, 1246.8, 1244.9, 1237.0 and 1228.7 F g-1 from 0.2 to 20 A g-1, respectively.42, 43 The corresponding capacity decay ratio was cut to 6.6 %. These results prove all these two electrodes with an excellent rate performance and high specific capacitance because of the anionic intercalation crystal structure and

the in situ substitution of cobalt ion in crystal lattice. The addition of CNTs constructed the 3D conductive network, which ensures the rapid transmission of electron between the


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electrode material and the current collector and further endows the NC-CNT electrode with outstanding comprehensive energy storage performance. The specific capacitances of these two electrodes at 0.5 A g-1 before and after the large current cycle are similar, further verifying that the crystal structure of the as-obtained electrode material have an excellent stability.

Figure 4. Electrochemical performance evaluation of NC and NC-CNT electrodes in three-electrode mode. (a-b) CV curves of NC and NC-CNT electrodes at 1 to 10 mV s-1. (c) The contrast CV curves of electrodes without or with CNTs at 1 mV s-1. (d-e) The discharged curves of the as-assembled electrodes at 02 to 20 A g-1. (f) The contrast rate performance curves. (g) Coulomb efficiency and cyclic stability curves of the as-assembled electrodes at 10 A g-1 for 10000 cycles.

The cycle stability of the as-assembled electrodes is also recorded at 10 A g-1 for 10000 cycles (Figure 4g). The capacitance retention ratio of NC reaches up to 86.7 % because of the stable crystal structure of the Ni-Co bimetallic material. The capacitance retention ratio of NC-CNT is up to 95.5 % because of the addition of CNTs. The corresponding coulomb efficiencies are close to 100 % (Figure 4g), exhibiting that the as-assembled electrodes have good reversibility. Thus, the as-obtained hydrangealike α-Ni1/3Co2/3(OH)2 electrode material exhibits an excellent rate performance, a long cycle life and a high specific capacitance, all of which are further improved by the addition of CNTs. A comparison of NC-CNT with a previously reported transition metal compound electrode material is presented in Table S2.7, 11, 12, 44-56 The NC-CNT electrode has a more excellent or comparable rate

performance and cycle life, demonstrating its superior electrochemical performances. The NC or NC-CNT-based asymmetric supercapacitors were assembled to further assess the practicability of the hydrangea-like α-Ni1/3Co2/3(OH)2 material. The CV curves of the as-assembled aqueous devices at 10 to 100 mV s-1 (Figure S8a and S8b) have a pair of obvious redox peaks with a small potential difference, which confirms the ideal reversible redox activity of the as-obtained electrode material. The anion intercalated crystal structure and the lattice coexistence of Ni2+ and Co2+ endows the NC material with a fast ion and electron transfer performance, which effectively reduces the concentration and electrochemical polarization, resulting in a small shift of the redox peak potential as the scan rates increased (Figure S8a). The 3D conductive network of CNTs greatly



enriches the electron transport channels, further improves the utilization rate of the electrode material and weakens the electrochemical polarization. Consequently, the CV curves of the aqueous (NC-CNT//AC) device have a higher peak potential and a smaller redox potential difference than those of the NC//AC device at same scan rate (Figure S8b). This phenomenon is also shown in Figure S8c, proving that the electrochemical performance of the NC//AC device was significantly improved by the addition of CNTs. An obvious discharge platform is also observed on the discharged curves of the as-assembled aqueous devices (Figure S8d and S8e), which reflects the redox peaks on the corresponding CV curves. In addition, the corresponding partial enlargement images in the inset illustrate the discharge time. It is obvious that the construction of 3D conductive network can effectively prolong the discharging time of the device. Figure S8f shows the corresponding rate performances of the asassembled aqueous devices on the basis of the specific capacitances of the working electrodes. The electrode specific capacitances based on aqueous NC//AC device are 398.7, 386.8, 372.5, 361.1, 350.1, 345.7, 339.5, 335.0, 328.9 and 320.8 F g-1 at 0.5 to 20 A g-1 (the device specific capacitances are 99.7, 96.7, 93.1, 90.3, 87.5, 86.4, 84.9, 83.7, 82.2 and 80.2 F g-1 at corresponding current densities (Figure S9), and the capacitance retention ratio is 80.4 %), respectively. These values implied that the NC//AC device exhibited an ideal specific capacitance and excellent rate performance because of the fast ion and electron transfer performance of the as-obtained NC material. The addition of CNTs further enhances the electronic conductivity of original electrode, resulting in the higher specific capacitance and rate performance of the aqueous NCCNT//AC device. The specific capacitances of the corresponding NC-CNT electrode based on the NCCNT//AC device are 568.7, 564.3, 561.9, 556.7, 550.1, 547.3, 542.2, 537.3, 531.3 and 527.8 F g-1 and those of NCCNT//AC device are 142.2, 141.1, 140.5, 139.2, 137.5, 136.8, 135.5, 134.3, 132.8 and 132.0 F g-1 at the similar current densities of the NC//AC device (Figure S8f and S9). The capacitance retention ratio is increased to 92.8 %. The asassembled aqueous devices have also withstood the large current cycles, as confirmed by the similar specific capacitance at 1 A g-1 before and after the charge and discharge at large currents. Figure S8g displays the cycle life and Coulombic efficiency curves of the as-assembled aqueous devices. After 100000 cycles, the aqueous devices (NC//AC and NC-CNT//AC) still the initial specific capacitances of 80% 86.2 % and 91.5 %, respectively. The Coulombic efficiencies of both as-assembled aqueous devices are close to 100 %. So the redox reaction is highly reversible. The FESEM images of the NC, NC-CNT electrodes before and after 100000 cycles at 10 A g-1 are presented in Figure S10, which shows that the hydrangea-like morphology was nearly unchanged after ultra-long cyclic testing at a high current density. This finding proves that the NC material has an excellent structural stability because of the

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interlayered CH3CH2OCONH2 as rivet, the wide lattice plane spacing and the large total pore volume. The high specific capacitance and capacitance retention ratio should be due to the as-obtained hydrangea-like αNi1/3Co2/3(OH)2 with a novel crystal structure, which contributes to the abundance of ion and electron transport pathways. Thus, the as-assembled aqueous NCCNT//AC device based on the hydrangea-like αNi1/3Co2/3(OH)2 with a high conductivity, large specific surface area and excellent structural stability exhibits an outstanding comprehensive energy storage performance, further proving that the aqueous NC-CNT//AC device has a potential for industrial application. To expand the application fields of the as-obtained NC material, an all-solid-state PVA-NC-CNT//PVA-AC device was also assembled by using PVA/KOH/H2O gel as the electrolyte and separator and a NC-CNT electrode as working electrode. There are also obvious redox peaks with weak peak displacement as the enhancement of scan rates on the CV curves (Figure 5a) owing to the hydrangea-like α-Ni1/3Co2/3(OH)2 with novel and beneficial crystal structure and the 3D conductivity network constructed by CNTs. Hence, the PVA-NCCNT//PVA-AC device also has excellent rate performance and high reversible redox reaction activity. Figure 5b shows the discharge curves of the PVA-NCCNT//PVA-AC device at the current densities of 0.5 to 20 A g-1. To illustrate the discharge time at large current densities, the partial discharge curves are enlarged and displayed in the corresponding insets. All the discharge curves show a long discharge platform similar to the discharge platform potential of the NC-CNT//AC device, further illustrating that the PVA-NC-CNT//PVA-AC device depends on the redox reaction for energy storage and high specific capacitance. The corresponding specific capacitances at difference current densities were calculated (Figure 5c and Figure S11). The specific capacitances of the NC-CNT electrode based on all-solidstate supercapacitor are 395.8, 389.6, 379.7, 374.2, 369.3, 361.8, 358.9, 353.5, 349.8 and 345.2 F g-1 and the specific capacitances of the all-solid-state (PVA-NC//PVA-AC) device are 99.0, 97.4, 94.9, 93.6, 92.3, 90.5, 89.7, 88.4, 87.5 and 86.3 F g-1 at different current densities, respectively. Although the transmission rate of OH- in the gel is much slower than that in aqueous solution, the PVA-NCCNT//PVA-AC device still exhibits high specific capacitance and excellent rate performance, which possess a capacitance retention ratio of 87.2 % from 0.5 A g-1 to 20 A g-1. The specific capacitances at 1 A g-1 are similar before and after the charge and discharge at large current densities, further proving that the PVA-NCCNT//PVA-AC device has an excellent stability to withstand large currents. Figure 5d is the cycle performance and Coulombic efficiency curves of the asassembled all-solid-state device at 10 A g-1 for 100000 cycles. The capacitance retention ratio is 90.8 %, and the Coulombic efficiency is almost 100 %, demonstrating that the as-assembled all-solid-state supercapacitor device has a long cycle life, excellent stability and high reversible


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redox activity. Thus, we successfully designed and assembled an all-solid-state PVA-NC-CNT//PVA-AC device with high specific capacitance, excellent rate performance, and long cycle life because of the high ionic and electronic conductivity, large specific surface area, wide lattice plane spacing and large total pore volume of the hydrangea-like α-Ni1/3Co2/3(OH)2 electrode material, the interlayered ethyl carbamate as rivet, and the 3D conductive network constructed by CNTs.

the as-assembled aqueous (NC-CNT//AC) and all-solidstate (PVA-NC-CNT//PVA-AC) devices using the hydrangea-like α-Ni1/3Co2/3(OH)2 as electrode material are comparable or superior to previously reported devices (Table S3).7, 11, 46, 48-52, 54, 55, 57-63. The as-assembled supercapacitor devices display an excellent rate performance and long cycle stability.

To our knowledge, the rate and cycle performance of

Figure 5. Electrochemical performance evaluation of the all-solid-state PVA-NC-CNT//PVA-AC device. (a) CV curves of PVA-NC-CNT//PVA-AC device at different scan rates. (b) The discharged curves of the PVA-NC-CNT//PVA-AC device at different current densities (the inset is the corresponding enlargement curves). (c) The corresponding rate performance curves (The data are the specific capacitances of working electrode). (d) Coulombic efficiency and cycle performance of the all-solidstate device at 10 A g-1 for 100000 cycles. (e) Nyquist plots of all the supercapacitor devices. (f) Ragone plots of the aqueous (NCCNT//AC) and all-solid-state (PVA-NC-CNT//PVA-AC) devices. (g) Photographs of the red LED, which powered by two fully charged as-assembled all-solid-state devices in series. Device numbers 1 and 2 are marked on the left side of the device. (h)



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Schematic diagram of the as-assembled all-solid-state (PVA-NC-CNT//PVA-AC) device with excellence electrochemical performance.

Nyquist impedance spectroscopy is applied to investigate the ion and electron transport performance of all the as-assembled supercapacitor devices. The corresponding Nyquist plots are shown in Figure 5e. All curves display a small X-intercept position and diameter of semicircle at the high-frequency region and a large slope of the straight line at the low frequency region, indicating that all the as-assembled supercapacitor devices have a low charge-transfer resistance (Rs) and diffusive resistance (Rct). These properties are due to the NC material with a high specific surface area, the ion intercalation crystal structure, and the in situ substitution of Co2+. In contrast to the curves of the aqueous devices, the addition of CNTs improved the slope of the straight line and reduced the diameter of the semicircle, which means that both Rs and Rct were reduced. Compared with the curve of the aqueous (NC-CNT//AC) device, the ionic and electronic transmission rates are limited by the PVA/KOH electrolyte and the Rs and Rct of the all-solidstate (PVA-NC-CNT//PVA-AC) device are slightly improved, further confirming the obtained results. Thus, the hydrangea-like α-Ni1/3Co2/3(OH)2 electrode material with a novel crystal structure and Ni-Co bimetal composition and the addition of CNTs endow the asassembled all-solid-state (PVA-NC-CNT//PVA-AC) device with outstanding comprehensive performance. The energy density and power density of the aqueous (NC-CNT//AC) and all-solid-state (PVA-NC-CNT//PVAAC) devices were estimated. The corresponding Ragone plots are displayed in figure 5f. All the NC-CNT-based aqueous and all-solid-state devices display the remarkable energy and power densities. The fabricated aqueous (NCCNT//AC) device exhibits an energy density of 35.2 Wh kg-1 at a power density of 100.0 W kg-1, and retains 30.7 Wh kg-1 at 3999.8 W kg-1. Benefiting from the excellent rate performance, the PVA-NC-CNT//PVA-AC device can deliver a maximum energy density of 35.4 Wh kg-1 at a power density of 100.7 W kg-1 and a maximum power density of 4781.3 W kg-1 at an energy density of 28.5 Wh kg-1. These values are substantially larger than the values of previously reported supercapacitor devices, including H-NiCoSe2//AC (25.5 Wh kg-1 at 3750 W kg-1),50 CoSe//AC (18.6 Wh kg-1 at 750 W kg-1),64 NiFe2O4//NiFe2O4 (20 Wh kg-1 at 1666 W kg-1),65 PB@MnO2//PG (16.5 Wh kg-1 at 550 W kg-1),66 Ni0.85Se@MoSe2//GNS (25.5 Wh kg-1 at 420 W kg-1),67 suggesting that this all-solid-state device has a prominent energy storage performance. To test the practical application of the as-assembled device, a commercial red LED was lit using two PVA-NCCNT//PVA-AC devices connected in series. The glowing intensity of the LED for various time intervals is shown in Figure 5g. After 80 min, the LED is still very bright, indicating that the PVA-NC-CNT//PVA-AC device have a high specific capacitance. Figure S12 displays the photographic images of a stopwatch driven by a PVA-NCCNT//PVA-AC device. The stopwatch steadily works for more than 30 minutes, once again proving that the PVA-

NC-CNT//PVA-AC device has a promising practical application potential in the field of energy storage. These results indicate that the as-assembled NC-CNTbased devices possess an outstanding comprehensive energy storage performance. These are very relevant to the hydrangea-like α-Ni1/3Co2/3(OH)2 electrode material with a high ion and electron transfer performance and a high crystal structure stability. Herein, the schematic diagram of the as-assembled all-solid-state (PVA-NCCNT//PVA-AC) device is shown in Figure 5h. The hydrangea-like α-Ni1/3Co2/3(OH)2 crystal consists of twodimensional layered structure with many intercalated ions, which constructs the rich ionic transport channel inside the electrode material and further improve its ionic conductivity. The complementary advantages and synergistic effect of Ni2+ and Co2+ effectively improve the electronic conductivity and the structure stability of NC material. These properties ensure the rapid transmission of ion and electron inside electrode material and promote the utilization of electrode material. The interlayered CH3CH2OCONH2 as the rivet reinforces the crystal structure of the Ni-Co hydroxide layers. Moreover, the wide lattice plane spacing and large total pore volume provide an ample space for structural deformation of hydrangea-like α-Ni1/3Co2/3(OH)2 material during charge and discharge processes, thereby ensuring that the asobtained NC material has an excellent structural stability. The hydrangea-like structure assembled by ultrathin porous nanosheets offers a high specific surface area, which provides the rich liquid interface, further weakens the concentration polarization and ensures the rapid transport of OH- during the electrochemical reaction process. The added CNTs serve as the bridges that link current collector and hydrangea-like α-Ni1/3Co2/3(OH)2 and facilitate the electronic transmission outside electrode material. All of these features contribute to the outstanding comprehensive performance of the asassembled supercapacitor devices.

Conclusion We demonstrated a novel strategy for controlling the morphology and structure of hydrangea-like αNi1/3Co2/3(OH)2. The complementary performance and synergistic enhancement of Ni-Co bimetallic ions owing to their coexistence in the crystal lattice realizes endows the hydrangea-like α-Ni1/3Co2/3(OH)2 material with a high electronic conductivity, and ensures that the electrons can be transferred quickly within the electrode material. The abundant intercalated anions and molecules provide sufficient channels for the rapid transfer of OH- in the hydrangea-like α-Ni1/3Co2/3(OH)2. The large specific surface area and total hole volume guarantee the prompt diffusion of the OH- between the active material and the electrolyte and effectively reduce the concentration polarization. The addition of CNTs as the bridge between the active material and the current collector allows electrons to flow unimpededly outside the active material.


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Consequently, 3D electronic and ionic transport channels are co-constructed inside and outside the hydrangea-like α-Ni1/3Co2/3(OH)2 resulting in high specific capacitance, excellent rate performance and high energy density of both NC-CNT-based aqueous and all-solid-state devices. With the introduction of CH3CH2OCONH2 as the rivet between Ni-Co hydroxide layers, the average capacitance fading rate of the NC-CNT-based aqueous and all-solidstate devices are 0.000085 % and 0.000092 %, respectively, per cycle for 100000 cycle at 10 A g-1. These findings offer new opportunities for the use of the hydrangea-like αNi1/3Co2/3(OH)2 as a promising positive material for high performance supercapacitors.

ASSOCIATED CONTENT Supporting Information. Additional experimental section; The FESEM image of hydrangea-like NC electrode materials confirms that the as-obtained material consists of curved sheets of ~500 nm in thickness as the venation of hydrangea interconnecting with one another; The cross section FESEM image of hydrangea-like NC electrode materials, revealing that the internal of material also consists of crossed nanosheets leading to the high specific surface area; Phase and element composition characterization of hydrangea-like NC material; The ICP-MS results of hydrangea-like NC material; The FESEM images of the parallel-experimental materials with different reaction time and the corresponding simulation diagrams; Electrochemical performance comparison between this work and the previous reports at three-electrode system; Capacitive and diffusioncontrolled contributions of NC and NC-CNT electrodes; Electrochemical performance evaluation of the as-assembled aqueous devices; The rate performance of the as-assembled aqueous devices; The FESEM image of NC electrode, NCCNT electrode before the electrochemical test and after the 100000 cycles; The rate performance of PVA-NC-CNT//PVAAC device; Electrochemical performance comparison between this work and the previous reports at two-electrode system; Photographic images of a stopwatch driven by a PVA-NC-CNT//PVA-AC device. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]. * Email: [email protected]. * Email: [email protected].

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. U1804126, U1804129, 21671205, 21771164 and U1407103), the Program for Interdisciplinary Direction Team in Zhongyuan University of Technology, the Collaborative Innovation Centre of Henan Textile and Clothing Industry, the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No. 164100510007 and CXTD2015018).

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SYNOPSIS TOC



Figure 1. Structure and physical characterization of hydrangea-like NC material. (a) Low-magnification field emission scanning electron microscope (FESEM) image of NC material. (b) FESEM image of the single hydrangea-like NC material. (c) High-magnification FESEM image of the nanosheets as building blocks and high resolution transmission electron microscope (HRTEM) image of cross section of a single nanosheet (inset). (d) Transmission electron microscope (TEM) of the NC material. (e) High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of the NC material. (f) HRTEM image of a single layer nanosheet. (g) Nitrogen adsorption–desorption isotherms of the hydrangea-like NC material, the corresponding Barrett-Halenda-Joyner pore size (BHJ) distribution (inset). (h) 1H NMR spectra of product extracting with methanol from NC material under continuous magnetic stirring for one week.


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Figure 2. Crystal structure analysis of hydrangea-like NC material. (a) HRTEM image of the NC nanosheet, the relevant fast Fourier transform (FFT), and inverse FFT (IFFT) images in the HRTEM image shown in the right. (b) Model of the atomic arrangement of (1 1 0) crystal plane in the hexagonal structure viewed along the [0 0 1] direction. (c) The corresponding selected area electron diffraction (SAED) pattern. X-ray photoelectron spectroscopy (XPS) spectra of (d) Co 2p, (e) Ni 2p and (f) O 1s for the NC material. (g) HRTEM image of the NC material. (h) The enlarged HRTEM image of red box in g. (i) HRTEM image of (0 0 1) crystal surface in the NC material. (j) Fourier transform infrared (FTIR) spectra of the hydrangea-like NC material. (k) Schematic illustration and layered crystal structure of the hydrangea-like NC material reinforced by ethyl carbamate as rivet.



Figure 3. Exploration of growth mechanism. (a) I is the FESEM image of the material under 2 h. II is the corresponding simu-lation diagrams. III depicts the redissolution of the unstable regions. IV illustrates the secondary growth process of the target product along the edge of nanopores. (b) I is the FESEM image of the material under 4 h. II is the corresponding simulation diagrams. III is the HRTEM images of the NC material. IV shows the schematic diagram of the formed nanopores after secondary growth. (c) I is the FESEM image of the material under 8 h. II is the corresponding simulation diagrams. (d) I is the FESEM image of the material under 12 h. II is the corresponding simulation diagrams. III shows the corresponding HRTEM images and proves the porous nanosheets, which are vividly depicted in IV. V and VI are the HAADFSTEM and HRTEM images of the cross section of porous nanosheets, which also displays the porous structure and graphically shows in VII.


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Figure 4. Electrochemical performance evaluation of NC and NC-CNT electrodes in three-electrode mode. (ab) CV curves of NC and NC-CNT electrodes at scan rates from 1 to 10 mV s-1. (c) The contrast CV curves of NC and NC-CNT electrodes at 1 mV s-1. (d-e) The discharged curves of the as-assembled electrodes at 02 to 20 A g-1. (f) The contrast rate performance curves. (g) Coulomb efficiency and cyclic stability curves of the as-assembled electrodes at 10 A g-1 for 10000 cycles.



Figure 5. Electrochemical performance evaluation of the all-solid-state PVA-NC-CNT//PVA-AC device. (a) CV curves of PVA-NC-CNT//PVA-AC device at different scan rates. (b) The discharged curves of the PVA-NCCNT//PVA-AC device at different current densities (the inset is the corresponding enlargement curves). (c) The corresponding rate performance curves (The data are the specific capacitances of working electrode). (d) Coulombic efficiency and cycle performance of the all-solid-state device at 10 A g-1 for 100000 cycles. (e) Nyquist plots of all the supercapacitor devices. (f) Ragone plots of the aqueous (NC-CNT//AC) and allsolid-state (PVA-NC-CNT//PVA-AC) devices. (g) Photographs of the red LED, which powered by two fully charged as-assembled all-solid-state devices in series. Device numbers 1 and 2 are marked on the left side of the device. (h) Schematic illustration of the PVA-NC-CNT//PVA-AC device with excellence electrochemical performance.


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3(OH)2 Reinforced by Ethyl Carbamate

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