MnCo2O4@Co(OH)2 coupled with N-doped carbon [email protected] graphene oxide nanosheets as electrodes for solid-state asymmetric supercapacitors

Journal Pre-proofs MnCo2O4@Co(OH)2 coupled with N-doped carbon nanotubes@reducedgraphene oxide nanosheets as electrodes for solid-state asymmetric supercapacitors Hongwei Che, Yamei Lv, Aifeng Liu, Hougui Li, Zengcai Guo, Jingbo Mu, Yanming Wang, Xiaoliang Zhang PII: DOI: Reference:

S1385-8947(19)32785-8 https://doi.org/10.1016/j.cej.2019.123372 CEJ 123372

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

27 June 2019 20 September 2019 2 November 2019

Please cite this article as: H. Che, Y. Lv, A. Liu, H. Li, Z. Guo, J. Mu, Y. Wang, X. Zhang, MnCo2O4@Co(OH)2 coupled with N-doped carbon nanotubes@reducedgraphene oxide nanosheets as electrodes for solid-state asymmetric supercapacitors, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.123372

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MnCo2O4@Co(OH)2 coupled with N-doped carbon nanotubes@reduced graphene oxide nanosheets as electrodes for solid-state asymmetric supercapacitors Hongwei Che, Yamei Lv, Aifeng Liu*, Hougui Li, Zengcai Guo, Jingbo Mu, Yanming Wang and Xiaoliang Zhang College of Materials Science and Engineering, Hebei University of Engineering, Handan, 056038, P. R. China *To whom correspondence should be addressed Email: [email protected] Telephone: +86-0310-8577973 Fax: +86-0310-8577966

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Abstract In this work, we report an effective approach to synthesize self-supported MnCo2O4@Co(OH)2 core-shell heterostructures (CSHs) as the positive electrode and N-doped carbon nanotubes (N-CNTs)@reduced graphene oxide (rGO) CSHs as the negative electrode. Benefiting from the unique CSHs by wrapping urchin-like MnCo2O4 microflowers assembled from nanorods with Co(OH)2 nanosheets, the prepared MnCo2O4@Co(OH)2

CSHs

electrode

demonstrates

a

higher

electrochemcial

performance (1185 C g-1 at 1 A g-1, 78% capacity retention at 20 A g-1, and 96% capacity retention after 5000 cycles at 5 A g-1) than the pristine MnCo2O4 and Co(OH)2 electrodes. Likewise, the prepared N-CNTs@rGO CSHs electrode also exhibits an improved capacitive performance compared to the pristine N-CNTs electrode, such as a specific capacitance of as high as 393 F g-1 at 2 A g-1, good capability with 71% capacitance retention at 20 A g-1 as well as excellent cycle stability with 96% capacitance retention after 5000 cycles at 10 A g-1. With good matching of these two electrodes

in

microstructures

and

electrochemical

properties,

the

fabricated

MnCo2O4@Co(OH)2//N-CNTs@rGO solid-state ASCs can deliver a high energy density of 67.2 W h kg-1 at a power density of 800 W kg-1 and good cycle stability with 94% capacitance retention over 5000 cycles at 10 A g-1. This work not only provides an attractive strategy for rationally constructing high-performance CSHs electrodes materials for ASCs, but also offers an available way to develop next-generation advanced energy storage devices. Keywords: MnCo2O4; Core-shell heterostructures; Carbon nanotubes; Graphene;

2

Asymmetric supercapacitors. 1. Introduction As the promising energy storage devices, supercapacitors (SCs) have been attracting tremendous intention in recent years owing to their long cycle life, high power density, fast charge/discharge process, and especially the ability to bridge the gap between rechargeable batteries and traditional capacitors [1-3]. Nevertheless, most SCs are suffering from the unsatisfactory energy density, which seriously limits their further applications in high-performance energy charge devices. According to the equation of energy density (E), E = 1/2×C×ΔV2, a feasible strategy to accomplish higher energy density of SCs can be used by extending the charge-discharge voltage and enhancing the specific capacitances of the electrodes [4, 5]. In this regard, constructing the ASCs has emerged as a promising strategy to achieve high energy density. Generally, ASCs are designed by coupling a battery-type faradic positive electrode as the energy source with a capacitor-type negative electrode as the power source [6, 7]. In this configuration, high operating voltage would be gained by two opposite potential windows of two different electrodes, thus increasing the energy density. Another effective way is to increase the total specific capacitances [8-10]. Therefore, considerable research interest has been devoted to developing highly capacitive positive and negative electrode materials to meet the demands for high-performance ASCs. Currently, transition metal oxides and hydroxides have been extensively studied as the positive electrode materials for ASCs in view of their high theoretical specific capacitances, abundant resources, and low cost [11-15]. Among them, cobalt manganese

3

bimetal oxide (MnCo2O4) has become one of the most promising positive electrode materials in ASCs applications by virtue of the complementarity and synergetic effect of both Co and Mn species in the redox reaction [16-20]. For instance, Co can show a higher oxidation potential than Mn, and Mn can transport more electrons [17]. So, MnCo2O4, that has a high theoretical specific capacitance of up to 3620 F g-1, demonstrates higher electrochemical activity than the single-component cobalt oxide (Co3O4) or manganese oxide (MnO2) during the redox reaction [19]. Furthermore, cobalt hydroxide (Co(OH)2) has also been considered as a very competitive positive electrode of ASCs in virtue of its layered structure, low lost, especially high theoretical specific capacitance (3460 F g-1) [21-24]. Despite these appealing features, both MnCo2O4 and Co(OH)2 positive electrodes are still subjected to lower practical capacitances than the theoretical values, poor rate capability, and cycle stability, which greatly limit their real applications in ASCs. In order to address these drawbacks, constructing appropriate CSHs has been believed to be a feasible approach toward high-performance electrodes for ASCs. Until now, both MnCo2O4-based and Co(OH)2-based CSHs with different components and micro-/nanostructures have been reported to demonstrate enhanced electrochemical performance [25-30]. Wang et al. prepared MnCo2O4@CoS CSHs through wrapping MnCo2O4 nanosheets arrays (NAs) in Ni foam with CoS nanosheets, which exhibited a capacitive value that was more than 1.9 times that of the individual MnCo2O4 NAs [25]. MnCo2O4@MnO2 CSHs were reported by Hui group, having a 0.4 time enhancement over the pristine MnCo2O4 electrode [26]. Yin et al. reported the

4

construction of Co(OH)2@Ni(OH)2 CSHs with a specific capacitance of more than 1.7 times of that of the single Co(OH)2 NAs [30]. Inspired by these research results in significantly improving the electrochemical performance via building up the CSHs with different components and microstructures, the immediate challenge motivates us to combine the advantages of MnCo2O4 and Co(OH)2 to construct a MnCo2O4@Co(OH)2 CSHs electrode with the aim of boosting the electrochemical performance. To the best of our knowledge, there are no study done on the integrated MnCo2O4@Co(OH)2 CSHs electrode as a high-performance positive electrode for ASCs. The electrochemical performance of the negative electrode materials is another factor that affects the energy density of ASCs. For the negative electrodes, carbonaceous materials, such as activated carbon (AC) [31], CNTs [32, 33], and graphene [34, 35], have been extensively studied because of their low cost, high specific surface area, and good electrical conductivity. However, they often suffer from unsatisfactory low specific capacitances. Recently, nitrogen-doped carbon materials have been demonstrated to significantly improve the electrochemical performance of the carbonaceous materials because the doping of nitrogen atom contributes to increasing available active sites and effectively modulating their electronic and chemical structure [36-38]. However, a significant decrease is observed for nitrogen-doped carbon materials at high current densities, impeding their further applications. In addition, graphene-based materials are considered to be the most promising candidate as the carbonaceous negative electrodes owing to its high electrical conductivity and ultrahigh surface area. Whereas the irreversible agglomeration of graphene nanosheets due to the

5

strong π-π stacking interactions greatly reduces their accessible surface area, and thereby diminishes their electrochemical performance [39]. Hence, a huge challenge is how to smartly integrate nitrogen-doped carbon with graphene materials towards fully utilizing their merits and overcoming their disadvantages for realizing the optimized energy storage behavior. Herein,

we

attempt

the

design

and

preparation

of

self-supported

MnCo2O4@Co(OH)2 CSHs as the positive electrode and N-CNTs@rGO CSHs as the negative electrode for high-performance solid-state ASCs. For the positive electrode, as shown in Fig. 1, self-supported MnCo2O4@Co(OH)2 CSHs were synthesized via a facile two-step process, in which urchin-like MnCo2O4 microflowers that were assembled by 1D nanorods, were initially grown directly on Ni foam using a hydrothermal reaction followed by calcination, and subsequently wrapped by Co(OH)2 nanosheets through the chemical bath deposition (CBD) method. For the negative electrode, polypyrrole (PPy) nanotubes were carbonized in the presence of KOH to generate N-CNTs. Finally, the prepared N-CNTs were wrapped by rGO nanosheets to form N-CNTs@rGO CSHs via a facile hydrothermal reaction followed by calcination in an inert atmosphere. Finally, the solid-state ASCs are fabricated comprising MnCo2O4@Co(OH)2 CSHs as the positive electrode and N-CNTs@rGO CSHs as the negative electrode, and PVA/KOH gel as the solid-state electrolyte. The device exhibits a remarkable electrochemical performance with an energy density of 67.2 W h kg-1 and good cycle stability with 94% capacitance retention over 5000 cycles at 10 A g-1. These results suggest the potential application of the prepared CSHs electrodes in ASCs devices with excellent electrochemical

6

performances. 2. Experimental section 2.1 Synthesis of urchin-like MnCo2O4 microflowers directly growing on Ni foam Typically, 0.5 mmol MnCl2∙4H2O, 1 mmol CoCl2∙6H2O, 5 mmol urea, and 2.5 mmol NH4F were dissolved into 50 ml deionized water under magnetic stirring. Then, the solution was transferred into a 100 ml Teflon-lined stainless steel autoclave with a pre-treated Ni foam (10 × 20 ×1 mm, 320 g m-2 (areal density)), then heated at 120 °C for 6 h. Finally, the Ni foam with precursors was further calcined at 350 °C for 2 h to achieve urchin-like MnCo2O4 microflowers. The mass load of MnCo2O4 was 2.11 mg cm-2. 2.2 Synthesis of urchin-like MnCo2O4@Co(OH)2 CSHs on Ni foam The obtained Ni foam loaded with urchin-like MnCo2O4 microflowers was immersed into a chemical bath by mixing 20 ml of 0.25 M K2S2O8, 16 ml of 1 M CoSO4, and 3 ml of aqueous ammonia (28 wt%) at room temperature. The CBD time was kept for 10, 30, and 60 min. The samples were accordingly denoted as MnCo2O4@Co(OH)2-10, MnCo2O4@Co(OH)2-30, and MnCo2O4@Co(OH)2-60. The corresponding mass loading was 2.56, 3.31, and 4.23 mg cm-2, respectively. For comparison, the Co(OH)2 NAs were prepared via the same CBD procedure, except that the bare Ni foam was employed. The mass loading of Co(OH)2 NAs was 1.67 mg cm−2. 2.3 Synthesis of N-CNTs@rGO CSHs PPy nanotubes were initially synthesized by a reactive self-degraded template

7

method [40]. Then, PPy nanotubes/KOH mixture with a KOH/PPy weight ratio of 1:1 was carbonized at 800 °C for 2 h under nitrogen atmosphere. The obtained N-CNTs were thoroughly washed with 0.1 M HCl, then washed with deionized water until neutral pH and finally dried. For the synthesis of N-CNTs@rGO CSHs, 30 mg N-CNTs powders were dispersed in 30 ml GO solution (0.5 mg ml-1) under ultrasonication for 30 min, and then was transferred into a 50-ml Teflon-lined stainless steel autoclave for the hydrothermal treatment at 180 °C for 10 h. The obtained composites were further annealed at 500 °C for 2 h under nitrogen atmosphere to form the resulting N-CNTs@rGO CSHs. 2.4 Materials characterization The surface analysis and crystal structures of the samples were measured by a X-ray photoelectron spectroscopy (XPS, Kratos Amicus spectrometer) and X-ray diffractometer (XRD, Rigaku Smartlab). The morphology and microstructures were examined by field emission scanning electron microscopy (FESEM, Hitachi SU8200) and transmission electron microscopy (TEM, FEI Tecnai F20). The specific surface area and pore size distribution of the samples was analyzed by a Quantachrome NOVA3200e sorption analyzer. Electrochemical measurements were conducted on a CHI 660E electrochemical workstation (Shanghai, China). 2.5 Electrochemical measurement The electrochemical performance of individual electrodes was tested in a three-electrode system in 6 M KOH aqueous solution, where MnCo2O4@Co(OH)2 and N-CNTs@rGO on Ni foam, Pt foil (10 × 10 × 0.1 mm), saturated calomel electrode

8

(SCE) served as the working electrode, counter electrode, and reference electrode, respectively. 3. Results and discussion Fig. 2a shows the morphology of Ni foam loaded with the prepared MnCo2O4 products, which indicates that the entire Ni foam is uniformly covered with numerous urchin-like MnCo2O4 microflowers with sizes of 5–10 µm. The enlarged SEM images in Figs. 2b, c reveal that these microflowers are assembled from 1D nanorods that project outwards along the radial direction. These nanorods are 40–100 nm in diameter and 2–5 µm in length. The element mapping spectra in Fig. S1 confirm that the prepared urchin-like microflowers are composed of Mn, Co, and O elements. After that, a CBD method was adopted to accomplish the wrapping of Co(OH)2 nanosheets as the shell on the surface of urchin-like MnCo2O4 microflowers as the core to form the MnCo2O4@Co(OH)2 CSHs. Fig. 2d shows that the urchin-like microflowers are still retained for the prepared MnCo2O4@Co(OH)2-30 CSHs with a Co(OH)2 CBD time of 30 min. The enlarged SEM image from an individual microsflower shows that the nanorods in the CSHs become thicker than those in the above prepared MnCo2O4 microflowers (Fig. 2e). Further close observation reveals that interconnected Co(OH)2 nanosheets are fully wrapped on the MnCo2O4 nanorods (Fig. 2f). The corresponding EDS mapping images demonstrate the co-existence of Mn, Co, and O in the entire CSHs (Fig. 2g). It is believed that such CSHs will possess the two advantages: 1) A highly porous network is formed by the interconnected Co(OH)2 nanosheets, providing a large number of extra electroactive sites and allowing the infiltration of electrolyte

9

ions into the inner MnCo2O4 nanorods core; 2) MnCo2O4 nanorods serve as the backbone to support the growth of Co(OH)2 nanosheets, which could endow the entire CSHs with richer conductive pathways and better mechanical stability. The microstructures of Co(OH)2 shell growing on the surface of MnCo2O4 nanorods were also found to strongly depend on the Co(OH)2 CBD time. When the Co(OH)2 CBD time was 10 min, Co(OH)2 nanoparticles rather than nanosheets were observed to be decorated on the surface of MnCo2O4 nanorods, as shown in Figs. S2a, b. For the Co(OH)2 CBD time of 60 min, similar to that with Co(OH)2 CBD time of 30 min, Co(OH)2 nanosheets were distributed on MnCo2O4 nanorods (Figs. S2c, d). But the difference is that Co(OH)2 nanosheets got thicker, and the open spaces between the neighboring Co(OH)2 nanosheets became narrower than those with CBD time of 30 min, which is not conducive to the infiltration of electrolytes into the inner MnCo2O4 nanorods core, thus decreasing the electrochemical performance. In addition, the morphology of the pristine Co(OH)2 on Ni foam prepared by the CBD method was also investigated and presented in Fig. S3, which exhibits densely interconnecting nanosheets with abundant porous network. The detailed microstructures of the pristine urchin-like MnCo2O4 microflowers and MnCo2O4@Co(OH)2-30 CSHs were further examined by TEM. Low-resolution TEM in Fig. 3a confirms that MnCo2O4 nanorods are indeed projected along the radial direction from the center to constitute the urchin-like microflowers, which agrees well with the above SEM results. Magnified TEM image shows that these MnCo2O4 nanorods have a diameter of 40–100 nm, and they are composed of substantial nanoparticles with sizes

10

of 10–20 nm (Fig. 3b). High-resolution TEM image in Fig. 3c reveals a (111) crystal plane of cubic spinel MnCo2O4 phase, which has a lattice spacing of 0.47 nm [41]. Furthermore, Fig. S4 shows the elements mapping image of the individual MnCo2O4 nanorod, where Mn, Co, and O elements are uniformly distributed in the entire nanorod. The atomic ratio of Co to Mn is 1.96, further confirming that these nanorods are composed of MnCo2O4. After the CBD of Co(OH)2, the MnCo2O4 nanorod is fully wrapped by Co(OH)2 nanosheets with the overall diameter being increased to 150–200 nm (Fig. 3d). These Co(OH)2 nanosheets are 5–10 nm in thickness (Fig. 3e), offering more electroactive sites and short ion diffusion distance for the CSHs. Fig. 3f presents the high-resolution TEM image, which reveals a (001) plane of Co(OH)2 nanosheet with the lattice spacing of 0.46 nm together with a (220) plane of MnCo2O4 nanorod with the lattice spacing of 0.28 nm, confirming the formation of MnCo2O4@Co(OH)2 CSHs. Fig. 3g shows the corresponding elements mapping image of the MnCo2O4@Co(OH)2-30 CSHs, revealing that Mn element is mainly located in the center core, whereas Co element is mostly distributed in both the core and shell regions. Moreover, O element is uniformly distributed in the core and shell regions. This result further evidences the formation of MnCo2O4@Co(OH)2 CSHs. The crystal structure and phase composition of the MnCo2O4@Co(OH)2-30 CSHs were analyzed by XRD. As seen from Fig. 4a, four diffraction peaks at 2θ = 30.4º, 36.1º, 58.0º, and 63.5º match well to the (220), (311), (511), and (440) planes of the cubic 一

MnCo2O4 spinel phase (JCPDS card no. 23-1237, a=8.269 Ǻ, space group Fd3m) (Fig. 4b) [41, 42]. The characteristic reflections at 2θ = 19.1º, 32.4º, and 37.9º are attributed

11

to the (001), (110), and (101) planes of the hexagonal cell of brucite-like β-Co(OH)2 一

with space group of P3ml (JCPDS card no. 30-0443, a=3.183 Ǻ, b=3.183 Ǻ, c=4.652 Ǻ) (Fig.

4b)

[22].

The

elemental

composition

and

oxidation

state

of

the

MnCo2O4@Co(OH)2-30 CSHs were further investigated by XPS. As shown in Fig. 4c, characteristic peaks for Mn, Co, and O elements were found in the survey spectrum without other impurity peaks except C signal from the reference specimen. From the high-resolution Mn 2p spectrum in Fig. 4d, two fitted peaks at 641.3 and 653.4 eV belong to Mn2+, while other two fitted peaks at 643.2 and 654.7 eV correspond to Mn3+ [19, 26]. The high-resolution Co 2p spectrum in Fig. 4e reveals two peaks of Co 2p3/2 (781.2 eV) and Co 2p1/2 (797.1 eV), which are ascribed to the Co2+ from Co(OH)2 [30, 43]. Additionally, the fitted O 1s peaks at 529.7, 531.5 and 532.6 eV correspond to the metal-O, metal-OH, and -OH species adsorbed on the surface, respectively (Fig. 4f) [44]. The above XRD and XPS results evidence the formation of the MnCo2O4@Co(OH)2 CSHs. The specific surface area of the pristine MnCo2O4 and MnCo2O4@Co(OH)2-30 CSHs were also analyzed. As shown in Fig. S5, type IV curves with distinct hysteresis loops are exhibited for these two samples, indicating the presence of mesopores. The specific surface area of the pristine MnCo2O4 is measured to be 40.1 m2 g-1, which is lower than that of the MnCo2O4@Co(OH)2 CSHs (67.4 m2 g−1). This result confirms that the construction of the MnCo2O4@Co(OH)2 CSHs endows the composites with larger specific surface area, thus generating more electroactive sites. Fig. 5a shows the XRD pattern of the prepared N-CNTs@rGO CSHs as the

12

negative electrode, where two broad peaks at 2θ = 24.7º and 43.4º were observed, suggesting a low degree of graphitic crystallinity [45]. These two peaks are accordingly attributed to the (002) and (101) planes of hexagonal carbon material (JCPDS, Card No. 75-1621) [46]. Moreover, no peak was found at 2θ = 10.6º, confirming the reduction of GO to rGO [47]. The elemental compositions and bonding configurations of the N-CNTs@rGO CSHs were also analyzed by XPS. Fig. 5b exhibits the signals of C, N, and O elements for the N-CNTs@rGO CSHs. The deconvoluted C 1s spectrum reveals four kinds of carbonaceous functional groups: C=C (284.7 eV), C-O (286.4 eV), C=O (287.7 eV), and O-C=O (289.2 eV) (Fig. 5c) [48]. The C=C peak exhibits much higher intensity than the C-O, C=O, and O-C=O peaks, which indicates considerable deoxygenation occurring in GO, agreeing well with those reported for rGO [49, 50]. The high-resolution N 1s spectrum are fitted by three peaks at 398.3 eV, 399.6 eV, and 401.5 eV, which belong to three different N environments: pyridinic N (N-6), pyrrolic/pyridine N (N-5), and quaternary N (N-Q), respectively (Fig. 5d) [45, 51]. The morphology of the N-CNTs@rGO CSHs is revealed by SEM, in which 1D nanotubes with a microscale length and 2D nanosheets co-exist in the CSHs (Fig. 5e). The enlarged SEM image shows that these nanotubes have a diameter of 100–150 nm, which are wrapped by flexible nanosheets (Fig. 5f). The N-CNTs@rGO CSHs were further characterized by TEM. It can be seen that N-CNTs are wrapped by flexible and thin rGO nanosheets to form the N-CNTs@rGO CSHs (Fig. 5g). It is noted that rGO nanosheets are well dispersed on the N-CNTs, which allows the full contact of rGO nanosheets with electrolytes. The magnified TEM image shows that these rGO

13

nanosheets have a thickness of 5–10 nm (Fig. 5h). HRTEM image displays the (002) plane of the graphite with a lattice spacing of 0.34 nm (Fig. 5i). In addition, the morphology and microstructure of the prepared N-CNTs were also characterized. SEM images in Figs. S6a, b reveal the existence of entangled 1D nanostructures with lengths of several to tens of micrometers for the N-CNTs. TEM images in Figs. S6c, d reveal a hollow tube-like structure with a wall thickness of 20–50 nm. Moreover, numerous nanopores are observed to be homogeneously distributed within N-CNTs (Fig. S6e). The specific surface and pore size distribution of the N-CNTs@rGO CSHs were also measured and presented in Fig. S7. H4-type hysteresis loops appear at the relative pressure range of 0.4–1.0 (Fig. S7a), indicating the presence of meospores in the N-CNTs@rGO CSHs. It is worth noting that the steep rise in adsorption quantity was also found at a low relative pressure area (<0.1), which was caused by the filling of micropores. The specific surface area of the N-CNTs@rGO CSHs is 1211 m2 g-1. The pore size distribution was calculated from nonlocal density functional theory (NLDFT) model and shown in (Fig. S7b). As seen, the pore diameter is mainly distributed within 1–10 nm, confirming the existence of both micropores (<2 nm) and mesopores (2–10 nm). These nanopores could provide an easy penetration and transportation of electrolyte ions within the N-CNTs@rGO CSHs. Moreover, the specific surface area and pore diameter of the pristine N-CNTs were also determined, which exhibit a lower specific surface area of 977 m2 g-1 and a pore size distribution of 1–15 nm (Figs. S7c, d). To evaluate the potential application of the above prepared MnCo2O4@Co(OH)2

14

CSHs and N-CNTs@rGO CSHs as SCs electrodes, a three-electrode system was employed to investigate their electrochemical behaviors. Fig. 6a compares the cyclic voltammetry (CV) curves of different electrodes in a potential range of 0.0–0.6 V (vs. SCE) at the same scan rate of 50 mV s-1. As seen, CV curves of the MnCo2O4@Co(OH)2 CSHs electrodes demonstrate higher current intensity and enclosed area than the pristine MnCo2O4 and Co(OH)2 electrodes. The result verifies that integrating Co(OH)2 nanosheets with MnCo2O4 microflowers in our work is feasible to remarkably enhance the capacitive performance of the resulting MnCo2O4@Co(OH)2 CSHs. It was also found that the MnCo2O4@Co(OH)2-30 CSHs electrode revealed the maximum peak current and enclosed area in those CSHs electrodes. These results are also further evidenced by comparing their galvanostatic charge-discharge (GCD) curves at 1 A g-1 (Fig. 6b), in which the MnCo2O4@Co(OH)2 CSHs electrodes exhibit longer discharge time than the pristine MnCo2O4 and Co(OH)2 electrodes. And also, the MnCo2O4@Co(OH)2-30 CSHs electrode possesses the longest discharge time. Fig. 6c shows a series of CV curves of the MnCo2O4@Co(OH)2-30 CSHs electrode at different scan rates, where redox peaks were caused by the following faradaic redox reactions [19-21]: MnCo2O4 + H2O + OH- ↔ MnOOH + 2CoOOH + eCo(OH)2

+

OH-

↔

CoOOH

+

OH-

↔

CoO2

(1) +

H2O

+

e-

+

e-

(2) CoOOH

+

H2O

15

(3) It is noted that the shapes of these CV curves almost do not change as the scan rates increase from 5 to 50 mV s-1, suggesting good rate capability for the CSHs electrode. Furthermore, as shown in Fig. 6d, a linear relationship is demonstrated between the square root of scanning rates and peak current densities of both anodic and cathodic peaks. This result reveals that the charge storage behavior of the CSHs electrodes are mainly originated from the OH- diffusion-controlled redox reactions rather than the electric double-layer physioadsorption [26]. In order to further clarify the charge storage mechanism, the power law is also adopted as follows [6]: i = a vb

(4)

where i is the current, v is the scan rate, and a and b are adjustable parameters. The b value was determined by the slope of log(i) vs log(v). The b values of 0.5 and 1.0 represent a diffusion-controlled and capacitive process, respectively. As shown in Fig. S8, the b value was determined to be 0.507 from the anodic peaks of the CSHs electrode, further confirming battery-type diffusion controlled kinetic behavior [52]. Fig. 6e presents GCD curves of the MnCo2O4@Co(OH)2-30 CSHs electrode at 1‒20 A g−1. As shown, nonlinar GCD curves with two plateaus exist, further confirming the faradaic redox behavior in the CSHs electrode. Based on these GCD curves, the corresponding specific capacities were calculated according to the corresponding formulas presented in Supporting information and shown in Fig. 6f. For comparison, GCD curves of the pristine MnCo2O4 and Co(OH)2 electrodes as well as the MnCo2O4@Co(OH)2-10 and MnCo2O4@Co(OH)2-60 CSHs electrodes are also

16

presented in Fig. S9. The corresponding specific capacities are shown in Fig. 6f. The result indicates that the MnCo2O4@Co(OH)2-30 CSHs electrode demonstrates the highest specific capacity and best rate performance among them. Specifically, the MnCo2O4@Co(OH)2-30 CSHs electrode possesses a specific capacity of up to 1185 C g-1 at 1 A g−1. This capacitive value is higher than that of the MnCo2O4@Co(OH)2-10 (811 C g-1) and MnCo2O4@Co(OH)2-60 CSHs electrode (901 C g-1) at 1 A g−1. In particular, it is is almost five times that of the pristine MnCo2O4 (254 C g-1) and more than two times that of the pristine Co(OH)2 (503 C g-1) electrodes at 1 A g−1. These results verify the synergetic effect of the Co(OH)2 and MnCo2O4 components in increasing the electrochemical performance of the resulting MnCo2O4@Co(OH)2 CSHs electrodes. The rate capability is also one of key factors for evaluating the practical application. The MnCo2O4@Co(OH)2-30 CSHs electrode demonstrates a capacity retention of 78% as the current density increases from 1 A g−1 to 20 A g−1. The capacity retention of the MnCo2O4@Co(OH)2-10 and MnCo2O4@Co(OH)2-60 CSHs electrodes was found to be 71% and 73%, respectively. The pristine MnCo2O4 and Co(OH)2 electrodes, however, exhibited 65% and 67% capacity retention, respectively, both of which were lower than those of the MnCo2O4@Co(OH)2 CSHs electrodes. To further highlight the good electrochemical performance of the prepared MnCo2O4@Co(OH)2 CSHs electrodes in our work, a comparison was also made from previously reported MnCo2O4-based and Co(OH)2-based CSHs electrodes, as presented in Table S1. Cycle stability is another important critical parameter to evaluate the practical application of the CSHs electrodes in high-performance SCs. As shown in Fig. 6g, the

17

MnCo2O4@Co(OH)2-10, MnCo2O4@Co(OH)2-30, and MnCo2O4@Co(OH)2-60 CSHs electrodes preserve 92%, 96%, and 94% of their initial capacities, respectively, after 5000 cycles at 5 A g−1. Nevertheless, the pristine MnCo2O4 and Co(OH)2 electrodes retain only 86% and 89% of their initial capacities. Enhanced cycle stability for the CSHs electrodes could be ascribed to their unique 3D core-shell architecture with good strain accommodation and robust mechanical stability during the charge-discharge cycles. The deduce is also supported by SEM images of the MnCo2O4@Co(OH)2-30 CSHs electrode after 5000 cycles, in which the morphology of the CSHs electrode has little change (Fig. S10). Electrochemical impedance spectroscopy (EIS) studies were performed to clarify the electrochemical kinetic behavior occurring within the electrodes. Fig. 6h shows the Nyquist plots of different electrodes according to the corresponding equivalent circuit (inset in Fig. 6h), which is composed of the series resistance (Rs), charge transfer resistance (Rct), Warburg impedance (W), and a constant phase element (CPE). It can be seen that all the Rs values of the MnCo2O4@Co(OH)2 CSHs electrodes are lower than that of the pristine MnCo2O4 electrode, which is attributed to the introduction of Co(OH)2 nanosheets with lower Rs value than MnCo2O4 microflowers. So, this result indicates that the wrapping of Co(OH)2 nanosheets on the surface of MnCo2O4 microflowers is beneficial to providing more electron transport pathways for the CSHs electrode. In addition, the wrapping of Co(OH)2 nanosheets was also found to decrease the Rct of the CSHs electrodes compared to that of MnCo2O4 microflowers. Low Rct for the CSHs electrodes manifests an easier and faster charge transport occurring at the

18

CSHs electrode-electrolyte interface. The negative electrode with good electrochemical performance is another decisive factor to achieve high energy density. Fig. 6a shows CV curves of the N-CNTs and N-CNTs@rGO CSHs electrodes at a scan rate of 50 mV s-1, both of which exhibit a rectangular-like shape, indicating the charge storage from the electric double-layer capacitor (EDLC) that can be described as follows [53]: C*

+

zK+

+

ze-

→

C*/ze-//zK+

(5) It is noted that these CV curves are accompanied by Faradaic humps from redox reactions, which can be described as follows [54]: C*=NH + 2H2O + 2e- ↔ C*H-NH2 + 2OH-

(6)

C*-NHOH + 2H2O + 2e- ↔ C*-NH2 + 2OH-

(7)

The enclosed area of CV curve from the N-CNTs@rGO CSHs electrode is larger than that of the pristine N-CNTs electrode, suggesting a higher specific capacitance for the former. This result is further confirmed by GCD curves of these two electrodes, in which the N-CNTs@rGO CSHs electrode exhibits longer discharge time than the pristine N-CNTs electrode. These results verify that it is feasible to wrap the N-CNTs with rGO nanosheets to increase the total capacitance of the constructed N-CNTs@rGO CSHs electrode. Fig. 7c shows CV curves of the N-CNTs@rGO CSHs electrode at 5‒50 mV s-1. As seen, all the CV curves maintain the same appearance of rectangular-like shape, suggesting a good rate capability. CV curves of the pristine N-CNTs electrode also show similar characteristics (Fig. S11). Moreover, as shown in

19

Fig. S12, the calculated b value is 0.986 for the N-CNTs@rGO CSHs electrode, reflecting the capacitor-type charge storage process [52]. The specific capacitance of the N-CNTs@rGO CSHs electrode and pristine N-CNTs electrode is calculated from GCD curves (Fig. 7d and Fig. S13) and plotted as a function of current density (Fig. 7e). The N-CNTs@rGO CSHs electrode demonstrates a specific capacitance of up to 393 F g-1 at 2 A g-1, which is higher than that of the pristine N-CNTs electrode (287 F g-1 at 2 A g-1). This capacitance value is also compared with other reported carbon-based electrodes, as shown in Table S2. Further, the capacitance value still reaches 279 F g-1 at 20 A g-1, corresponding to 71% capacitance retention with respect to that at 2 A g-1. However, the capacitance value of the pristine N-CNTs electrode decreases to 181 F g-1 at 20 A g-1 with 63% capacitive retention. To quantify the capacitive and diffusive contributions to the the total charge storage for the N-CNTs@rGO CSHs electrode, the power-law equation is applied according to the following equations [6]: i = k1v + k2v1/2 or i/v1/2 = k1 v1/2 + k2

(8) (9)

In eq. (8), i is the current response, k1v and k2v1/2 are ascribed to the capacitive and diffusive contributions, respectively. According to eq. (9), the individual component contribution can be determined and presented in Fig. 7f. As seen, the capacitive contributions rise from 95.5% to 98.5% with increasing the scan rates from 5 mV s-1 to 50 mV s-1, manifesting the surface-limited process from EDLC dominates the total charge storage. Moreover, the cycle stability of these two electrodes at 10 A g-1 is compared to further evaluate the advantage of the electrochemical performance of the

20

CSHs electrode. As seen from Fig. 7g, the capacitance retention of the N-CNTs@rGO CSHs electrode remains 97% after 5000 cycles, which is higher than that of the pristine N-CNTs electrode (93%). Fig. 7h shows the Nyquist plots of the pristine N-CNTs and N-CNTs@rGO CSHs, where the latter exhibits lower Rs and Rct than the former. The result indicates that integrating rGO nanosheets with N-CNTs contributes to providing richer conductive pathways and faster charge transport for the constructed N-CNTs@rGO CSHs, thereby improving the capacitive performance of the CSHs. A

solid-state

ASCs

device

was

fabricated

by

employing

the

MnCo2O4@Co(OH)2-30 as the positive electrode and the N-CNTs@rGO as the negative electrode together with PVA-KOH gel as the solid-state electrolyte (Supporting Information). To determine its practically operating voltage, CV curves of the positive and negative electrodes scanned at 10 mV s-1 are shown in Fig. 8a. As seen, an operating voltage of 1.6 V can be selected for the device. This voltage value can be further confirmed by the CV curves measured in different voltage windows from 1.0 to 1.7 V (Fig. 8b), which exhibit a stable capacitive characteristic until the voltage window of 1.6 V. After the voltage window is further extended to 1.7 V, a sharp peak is observed, which is possibly related to the oxygen evolution at the positive electrode [55]. Fig. 8c displays CV curves of the device recorded within 0‒1.6 V at 5‒50 mV s-1, where all the curves reveal the electrochemical characteristics of the combination of EDLC and pseudo-capacitance. This result is further verified by the GCD curves that do not exhibit the charge/discharge plateau (Fig. 8d). The specific capacitance of the device as a function of current density is also presented in Fig. 8e, which exhibits a high

21

specific capacitance of 189 F g-1 at 1 A g-1, and still remains a capacitance value of 103 F g-1 at 20 A g-1. To make out more details on charge storage mechanism of the solid-state ASCs device, the capacitive and diffusive contributions to the device can be quantified according to eqs. (8) and (9). As seen from Fig. 8f, the diffusive contributions to the device are 65.5% and 57.3%, respectively, at the corresponding scan rates of 5 mV s-1 and 10 mV s-1, whereas these contribution values are further decreased from 48.7% to 38.6% with increasing the scan rates from 20 mV s-1 to 50 mV s-1. These results indicate that the diffusion-controlled process from the Faradaic reaction dominates the total charge storage of the device at low scan rates, whereas the surface-limited process from the electric double-layer effect dominates the total capacitance at high scan rates. Cycle stability of the device were tested at 10 A g-1 for 5000 cycles, which demonstrates 94% capacitance retention of the initial value and 91% Coulombic efficiency (Fig. 9a). Ragone plot of the solid-state ASCs device are presented in Fig. 9b. An energy density of up to 67.2 W h kg-1 is achieved at a power density of 800 W kg-1 for our experimental device. And it still maintains 36.6 W h kg-1 at a power density of 16 kW kg-1. The energy and power densities in this present work are superior to those of previously reported solid-state ASC devices, as compared in Table S3. To further evaluate the practical application of the solid-state ASCs device, a fish-shaped pattern assembled by 29 LEDs can be lightened for 6 minutes using two series solid-state ASCs devices (inset in Fig. 9b). The electrochemical behaviors of our designed ASCs device can be schematically

22

described in Figure 10. During charging, the MnCo2O4@Co(OH)2 positive electrode is oxidized following the redox reactions equations (1-3), and the N-CNTs@rGO negative electrode predominantly undergoes the charge storage from EDLC according to eq. (5). As for discharging process, the above reactions occur backward. Based on the aforementioned results, the superior capacitive performance of our ASCs device can be attributed to the rational design and construction of both MnCo2O4@Co(OH)2 and N-CNTs@rGO

CSHs,

which

are

highlighted

as

follows:

1)

For

the

MnCo2O4@Co(OH)2 CSHs positive electrode, the unique CSHs are built by wrapping MnCo2O4 microflowers assembled by nanorods with Co(OH)2 nanosheets. On one hand, the MnCo2O4 nanorods serve as the backbone to immobilize the interconnected Co(OH)2 nanosheets, which is not only conducive to reinforce the mechanical strength of the 3D nanorods@nanosheets CSHs, preserving the integrated CSHs during the cycling process, but also beneficial to generating the rich electron transport pathways. On the other hand, Co(OH)2 nanosheets grown on MnCo2O4 nanorods provides more electrochemical sites, extra conductive pathways, and short charge transport distance. Meanwhile, enough voids are formed between the adjacent Co(OH)2 nanosheets, facilitating fast penetration of electrolyte into the inner MnCo2O4 nanorods and fully utilizing both the core and shell active materials. Therefore, the synergistic effect between the core and shell can ensure a full play based on the above described structural advantages of the nanorods@nanosheets CSHs. 2) For the N-CNTs@rGO CSHs negative electrode, 1D N-CNTs with porous structures are conducive to offering enhanced absorption/desorption of more active ions for EDLC. Meanwhile, 1D N-CNTs

23

are interconnected, constituting rich conductive networks for rapid electrical transport [56]. Moreover, Nitrogen doping in CNTs boosts the electrochemical activity and the pseudo-capacitance behavior via increasing the conductivity of carbon material. On this basis, 2D rGO nanosheets with high specific surface area and electric conductivity are introduced and well dispersed on the surface of N-CNTs, preventing the re-stacking of rGO nanosheets [57, 58]. As a result, extra electroactive sites and conductive pathways are accomplished. Benefiting from the respective advantages of N-CNTs and rGO nanosheets, superior capacitive performance is achieved for the N-CNTs@rGO CSHs. Given the above results and discussion, it is reasonable to believe that the good matching of the MnCo2O4@Co(OH)2 positive electrode and N-CNTs@rGO negative electrode, both of which have similar CSHs characterized by 1D nanostructures@2D nanostructures, good electroactivity in KOH electrolyte together with complementary cell voltage windows, accounts for superior energy storage performance of the assembled ASCs device. 4. Conclusions In

summary,

self-supported

urchin-like

MnCo2O4@Co(OH)2 CSHs

and

N-CNTs@rGO CSHs have been successfully synthesized, both of which can be served as a pair of well-suited composite electrodes for solid-state ASC device. Based on the full play of the synergetic effect between the cores and shells, larger specific capacitance, higher rate capability, and better cycle stability are achieved for these two CSHs electrodes. More importantly, the assembled solid-state ASCs device, using MnCo2O4@Co(OH)2 CSHs as the positive electrode and N-CNTs@rGO CSHs as the

24

negative electrode, demonstrate an energy density of 67.2 W h kg-1 and cycle stability with 94% capacitance retention over 5000 cycles at 10 A g-1. This study not only provides an available strategy for the construction of high-performance CSHs electrodes materials for ASCs, but also opens up a new avenue for exploiting next-generation high-energy storage devices. Acknowledgments This work was supported by Youth top talent support program of Hebei Province and the Natural Science Foundation of Hebei Province (Grant no. B2018402111 and B2017402110). Conflicts of interest There are no conflicts to declare. References: [1] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Q. Zhou, Y. Wu, W. Huang, Latest advances in supercapacitors: from new electrode materials to novel device designs. Chem. Soc. Rev. 46 (2017) 6816–6854. [2] W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumare, A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor technology, Nano Energy 52 (2018) 441–473. [3] M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.L. Taberna, C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for building better supercapacitors, Nat. Energy 1 (2016) 16070. [4] W. Zuo, R. Li, C. Zhou, Y. Li, J. Xia, J. Liu, Battery-supercapacitor hybrid devices:

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α-MnO2@NiCo2O4 core-shell heterostructure and 3D-nanocage N-doped porous carbon, ACS Sustainable Chem. Eng. 5 (2017) 4856–4868. [56] B. Wang, T. Liu, A. Liu, G. Liu, L. Wang, T. Gao, D. Wang, X.S. Zhao, A hierarchical porous C@LiFePO4/carbon nanotubes microsphere composite for high-rate lithium-ion batteries: Combined experimental and theoretical study, Adv. Energy Mater. 6 (2016) 1600426. [57] B. Wang, W.A. Abdulla, D. Wang, X.S. Zhao, Three-dimensional porous LiFePO4 cathode material modified with nitrogen-doped graphene aerogel for high-power lithium ion batteries, Energy Environ. Sci. 8 (2015) 869–875. [58] B. Wang, T. Ruan, Y. Chen, F. Jin, L. Peng, Y. Zhou, D. Wang, S. Dou, Graphene-based composites for electrochemical energy storage, Energy Storage Mater. (2019) https://doi.org/10.1016/j.ensm.2019.08.004. Figure cations: Fig. 1. Schematic illustration of the fabrication process of the MnCo2O4@Co(OH)2 CSHs positive electrode and the N-CNTs@rGO CSHs negative electrode toward the high-performance solid-stat ASCs device. Fig. 2. SEM images of the prepared MnCo2O4 (a-c) and MnCo2O4@Co(OH)2-30 CSHs on Ni foam (d-f); EDS mapping images of the MnCo2O4@Co(OH)2-30 CSHs (g). Fig. 3. TEM and HRTEM images of the MnCo2O4 microflowers (a-c) and MnCo2O4@Co(OH)2-30 CSHs (d-f); TEM elemental mapping images of the MnCo2O4@Co(OH)2-30 CSHs (g). Fig. 4. XRD pattern of the MnCo2O4@Co(OH)2-30 CSHs (a); Crystal structures of the

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MnCo2O4 and Co(OH)2 (b); XPS spectra of the MnCo2O4@Co(OH)2-30 CSHs: survey spectrum (c), Mn2p (d), Co2p (e), and O1s (f). Fig. 5. XRD pattern of the N-CNTs@rGO CSHs (a); XPS spectra of the N-CNTs@rGO CSHs: survey spectrum (b), C 1s (c), and N 1s (d); SEM (e, f), TEM (g, h), and HRTEM (i) images of the the N-CNTs@rGO CSHs. Fig. 6. CV (a) at 50 mV s-1 and GCD (b) curves at 1 A g-1 for the pristine MnCo2O4, Co(OH)2,

and

MnCo2O4@Co(OH)2

CSHs

electrodes;

CV

curves

of

the

MnCo2O4@Co(OH)2-30 CSHs electrode at different scan rates (c); Linear plots from ip vs.

v1/2

for

the

MnCo2O4@Co(OH)2-30

CSHs

(d);

GCD

curves

of

the

MnCo2O4@Co(OH)2-30 CSHs electrode at different current densities (e); The specific capacities (f) at different current densities, cycle stability (g) at 5 A g-1, and EIS spectra (h) for the pristine MnCo2O4, Co(OH)2, and MnCo2O4@Co(OH)2 CSHs electrodes. Fig. 7. CV (a) at 50 mV s-1 and GCD (b) curves at 1 A g-1 for the pristine N-CNTs and N-CNTs@rGO CSHs electrodes; CV (c) at different scan rates and GCD (d) curves at different current densities for the N-CNTs@rGO CSHs electrode; The specific capacitances (e) of the pristine N-CNTs and N-CNTs@rGO CSHs electrodes at different current densities; Linear plot (f) from ip/v1/2 vs. v1/2 for the N-CNTs@rGO CSHs electrode; Cycle stability (g) at 10 A g-1, and EIS spectra (h) for the pristine N-CNTs and N-CNTs@rGO CSHs electrodes. The inset in Fig. 7f is the contributions of capacitive and diffusion controlled processes in the N-CNTs@rGO CSHs electrode at different scan rates. Fig. 8. CV curves of the MnCo2O4@Co(OH)2-30 and N-CNTs@rGO CSHs electrodes

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at 10 mV s-1 (a); CV curves of the assembled solid-state ASCs device within different potential windows at 10 mV s-1 (b); CV curves (c) at different scan rates, GCD curves (d), and the specific capacitances (e) at different current densities for the ASCs device; Linear plot from ip/v1/2 vs. v1/2 for the ASCs device (f). The inset in Fig. 8f is the contributions of capacitive and diffusion controlled processes in the ASCs device at different scan rates. Fig. 9. Cycle stability of the ASCs device at 10 A g-1 (a); Ragone plots (b) of our ASCs device and reported ASCs devices in literature (the references are listed in the Supporting Information). The inset in Fig. 9b is a photograph of a fish-shaped pattern comprising 29 LEDs powered by the two connected ASCs devices. Fig.

10.

Schematic

of

the

energy

storage

mechanism

in

the

MnCo2O4@Co(OH)2//N-CNTs@rGO solid-state ASCs device.

35

Figures:

Fig. 1

36

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Fig. 2 37

(a)

(b)

(d)

(e)

(c)

(f)

(g)

38

Fig. 3

(a)

(d)

(b)

(c)

(f)

(e)

Fig. 4

39

(a)

(d)

(b)

(e)

(c)

(f) rGO N-CNTs

(g)

(h)

(i)

40

Fig. 5

(a)

(b)

(c)

(d)

(e)

(f)

(h)

(g)

Fig. 6

41

Fig. 6

(a)

(b)

(d)

(e)

(g)

(c)

Fig. 7

(f)

(h)

42

Fig. 7

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 8

43

(a)

(b)

(a)

2 min 1 min

3 min

5 min

6 min

Fig. 9

44

Fig. 10

45

Declaration of interests √ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

46

Graphic Abstract

47

Highlights  MnCo2O4

nanorods

are

wrapped

by

Co(OH)2

nanosheets

to

form

MnCo2O4@Co(OH)2 CSHs.  N-CNTs@rGO CSHs are constructed by wrapping N-CNTs with rGO nanosheets.  MnCo2O4@Co(OH)2//N-CNTs@rGO solid-state ASCs are fabricated.  An energy density of 67.2 Wh kg-1 are achieved for the assembled ASCs device.

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