MOF-derived Co3O4 nanosheets rich in oxygen vacancies for efficient all-solid-state symmetric supercapacitors

Journal Pre-proof MOF-derived Co3O4 nanosheets rich in oxygen vacancies for efficient all-solid-state symmetric supercapacitors Simeng Dai, Fenfen Han, Jian Tang, Weihua Tang PII:

S0013-4686(19)31974-7

DOI:

https://doi.org/10.1016/j.electacta.2019.135103

Reference:

EA 135103

To appear in:

Electrochimica Acta

Received Date: 11 July 2019 Revised Date:

14 October 2019

Accepted Date: 15 October 2019

Please cite this article as: S. Dai, F. Han, J. Tang, W. Tang, MOF-derived Co3O4 nanosheets rich in oxygen vacancies for efficient all-solid-state symmetric supercapacitors, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135103. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

MOF-derived Co3O4 nanosheets rich in oxygen vacancies for efficient all-solid-state symmetric supercapacitors Simeng Dai†, Fenfen Han†, Jian Tang*, Weihua Tang*

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China.

*

Corresponding author.

Tel: (86)-25-8431-7311. E-mail: [email protected] and [email protected] †

These authors contributed equally to this work.

1

Abstract: Co3O4 is a promising pseudocapacitive material with a high theoretical capacity. We demonstrate herein a facile interfacial engineering toward Co-metal organic frameworks (Co-MOFs) derived Co3O4 nanosheets for improving the overall capacitive performance. Firstly, a thin hydrophilic carbon layer was coated onto carbon cloth (CC) to stablize the interfacial coordination effect between Co3O4 and carbon fibres. Secondly, oxygen vacancies by reducing Co3O4 was introduced to improve the electron and ions transfer on the interface between Co3O4 and electrolyte. The unique Co3O4 structure and composition endow the optimal v-Co3O4/CC composite with significantly improved specific capacity (414 C g-1 at 1 A g-1) and excellent stability (0.00174% capacity loss per cycle for 15000 cycles). Asymmetric supercapacitor by assembling v-Co3O4/CC composite as positive electrode with the same MOF derived carbon nanosheets as negative electrode demonstrates a high volumetric and gravimetric energy density of 0.74 mWh cm-3 (14.7 mW cm-3) and 45.3 Wh kg-1 (915 W kg-1), respectively. This work might provide a simple but efficient approach for boosting the pseudocapacitive performance of transition metal oxides.

Keywords: oxygen vacancy, metal-organic framework, cobalt oxide, interfacial engineering, symmetric supercapacitors

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1. Introductions Metal organic frameworks (MOFs) have recently attracted ever-increasing interest due to their unique features including tailorable structure, large surface area, uniform cavity, abundant accessible active sites [1]. They have found wide applications such as gas storage and separation, drug delivery, catalysis sensing and energy storage [2-6]. Importantly, MOFs-based materials have been explored as excellent sacrificial templates or the precursors for a variety of metal compounds, porous carbon or their composites through pyrolysis or reaction with certain reagents [7-11]. The MOFs derived materials can thus heritage the unique morphology of MOF to promote their performance. Especially for supercapacitors (SCs), bulk electrode materials often exhibit sluggish reaction kinetics, poor electronic conductivity and volume expansion, leading to low specific capacitance, poor rate capability and short lifespan. The unsatisfactory performance. Co-based MOFs (Co-MOFs) have emerged as fascinating templates for the thermal conversion to generate porous Co3O4 structures [12-14]. The mixed-valence cobalt(II,III) oxide (Co3O4) is good electrode candidate for SCs due to the high theoretical capacity, easy preparation and rich redox reactions [15]. For example, Yamauchi’s group [12] prepared ZIF-67 derived nanoporous Co3O4 cages, which displayed a high specific capacity of 252 C g-1 at 5 mV s-1 in SCs. Sun et al. [16] developed ~3.5 nm thick Co3O4 hexagonal nanosheets, which showed a specific capacity of 1625 C g-1 at 1 A g-1 and 1267 C g-1 at 25 A g-1. However, the lack of 3

good electron conductive routes endows Co3O4 with a capacity far from its theoretical value. The deposition of Co3O4 nanoseets or nanoparticles on conductive substrate like graphene or carbon cloth (CC) have been explored to enhance its electrochemical activity. In the regard, Wang’s group [17] achieved binder-free Co3O4 nanosheets on CC to afford a highest specific capacity of ~540 C g-1 at 10 A g-1. However, the specific capacity of Co3O4 needs to be greatly improved for practical applications. Similar to organic electrochemistry [18], the interface properties of Co3O4 nanosheets are crucial for the electron injection/transport and electrolyte diffusion. One challenge remains at how to strengthen the interaction between Co3O4 and current collector. The use of a thin layer of conductive nanoglue is an advisable choice [19]. Another task is to enhance the electron transport and redox reaction within Co3O4. In this regard, the chemical reduction using sodium borohydride (NaBH4) has been established to be effective to improve the electric conductivity. Co3O4 is a common catalyst for the hydrolysis of NaBH4. In return, the generated hydrogen can serve as a strong reduction agent toward Co3O4, leading to the formation of oxygen vacancies [20,21]. An additional charge storage way of ions intercalation/de-intercalation on nanosheets/electrode interface is thus supplemented to electrodes. Unfortunately, the synergetic effect of as-mentioned interfacial engineering of Co3O4 is rarely investigated. Herein, we report a facile approach of engineering the interfaces of Co3O4 that coordinated with substrate and electrolyte. With the use of Co-MOFs as self-sacrificing templates, we can obtain ordered corrugated Co3O4 nanoarrays (NAs), 4

offering an intimate electrolyte penetration and rich redox reactions sites. Our interfacial engineering involves two aspects: one is to enhance the adhesion effect for NAs with CC fibres using polydopamine derived hydrophilic carbon. Besides the improved stability, the electron injection is also enhanced. The other side is the chemical reduction of NAs to form oxygen vacancies on the surface, which can control the charge storage process. In oxygen-free ethanol solution, partial solvents are oxidized by Co3O4 nanosheets with the generation of water [22]. The hydrolysis of NaBH4 is further triggered to release hydrogen gas. Therefore, the chemical reduction of Co3O4 is allowed to proceed under a mild condition. Both interfacial treatments show a positive improvement in charge storage. As a result, optimized v-Co3O4/CC displays a high specific capacity (414 C g-1 at 1 A g-1) and excellent stability (72.6% capacity retention after 15000 cycles). The specific capacity is much higher than the counterpart electrodes without oxygen vacancies (donated as Co3O4/CC) or PDA treatment (donated as v-Co3O4/CC-wo). Asymmetric supercapacitor with Co-MOFs derived carbon nanosheets as negative electrodes also shows a high energy density.

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Fig. 1. Schematic preparation of nitrogen-doped Co3O4 nanosheets with oxygen vacancies on CC substrate.

2. Experimental Section Hydrophilic CC [23]: The CC was ultrasonicated with acetone, deionized water and ethanol for 2 h, respectively. The dopamine hydrochloride (76.59 mg, 0.5 mmol) was dissolved in deionized water (50 mL) to prepare a clear solution. Pieces of clean CC (2 cm × 5 cm) were immersed in dopamine hydrochloride solution (50 mL, 10 mM) with stirring for 2 h before standing for 12 h. The CC was transferred to Tris-Cl buffer (10 mL, 10 mM, pH = 8.5) for self-polymerization within 12 h. The PDA-coated CC was taken out and washed with DI-water. After dried in vacuum, the PDA@CC was calcined in N2 atmosphere at 900 °C for 1 h with a ramp rate of 5 oC min-1. Finally, the as as-obtained nitrogen-doped CC was super-hydrophilic in nature, which can guide the subsequent growth of Co-MOFs in aqueous medium. For comparison, the CC without nitrogen-doped carbon (denoted as CC-wo) was also prepared. ZIF-67/CC: A piece of nitrogen-doped CC (2 cm × 5 cm) was immersed in the aqueous solution of cobalt nitrate hexahydrate (Co(NO3)2·6H2O 40 mL, 0.05 M) for 15 min. Towards the cobalt solution was then quickly added with 2-methylimidazole aqueous solution (40 mL, 0.4 M). The suspension was allowed to stand for 1 h at room temperature to generate the ZIF-67. The purple CC-supported hybrid was washed with DI-water for 3 times and dried in vacuum to afford ZIF-67/CC. The mass loading of Co-MOFs is about 0.623 mg cm-2. Co3O4/CC: A piece of Co-MOFs/CC (1 cm × 2 cm) was annealed in N2 atmosphere at 500 oC for 0.5 h with a heating rate of 2 oC min-1. Then, the sample was cooled ，

down to 350 oC with a cooling rate of 5 oC min-1. Finally, the sample was further calcinated in air at 350 oC for 2 h. The mass loading of Co3O4 was about 0.561 mg cm-2. For comparison, the CC-loaded Co3O4 (denoted as Co3O4/CC-wo) without the use of PDA treatment was also prepared by the same method, which has an average mass loading of per unit area is about 0.495 mg cm-2. v-Co3O4/CC: The oxygen-deficient Co3O4/CC hybrid was achieved with one-step reduction of sodium borohydride (NaBH4). Typically, a piece of Co3O4/CC (1 cm × 2 cm) was immersed in oxygen-free sodium borohydride ethanol solution (1 mM). The chemical reduction was conducted under N2 atmosphere at room temperature for different durations (1, 2, 3 or 4 h). The as-obtained hybrids were washed with ethanol and DI-water for three times in sequence before dried at 60 oC in vacuo. The obtained binder-free electrode materials were denoted as v-Co3O4/CC-1h, v-Co3O4/CC-2h, v-Co3O4/CC-3h and v-Co3O4/CC-4h, respectively. The corresponding mass loading was calculated to be 0.564, 0.513, 0.509 and 0.496 mg cm-2. v-Co3O4/CC refers to v-Co3O4/CC-2h unless otherwise stated. The chemical reduction of Co3O4/CC-wo for 2 h afforded the resulted electrode material (v-Co3O4/CC-wo) with a mass loading of 0.48 mg cm-2. Nitrogen-doped carbon nanosheets (NC): The nitrogen-doped carbon nanosheets were prepared from the calcination of Co-MOFs. Typically, 2-methylimidazole aqueous solution (0.16 M, 25 mL) was quickly added into Co(NO3)2·6H2O solution (0.04 M, 25 mL) under vigorous stirring over 5 mins. The reaction was allowed to stand for 1 h. The purple precipitate (ZIF-67) was centrifuged and washed with water 7

for three times before dried at rt for 24 h. The ZIF-67 was then carbonized at 800 oC for 5 h in N2 atmosphere with a program-controlled heating rate of 5 oC min-1. The as-prepared black powder was immersed into diluted HCl (10 mL, 1 M) for 5 h to remove any metal element. Nanopourous carbon nanosheets were then centrifuged, washed with DI-water, and dried for device fabrication. Fabrication of solid-state ASC device: The NC was firstly coated onto CC using a slurry of carbon and polytetrafluoroethylene (weight ratio is 8:2) and used as negative electrode for ASC. It was assembled with v-Co3O4/CC positive electrode and PVA/LiOH gel electrolyte in the middle. After the removal of unnecessary water, a solid-state ASC cell was obtained for energy storage study. The geometric area and thickness are 2 cm2 and 11 µm for v-Co3O4/CC electrode, while 2 cm2 and 11.5 µm for NC electrode, respectively.

3. Results and Discussion 3.1. Morphology and physicochemical characterization The binding of Co3O4 NAs with CC fibres was strengthened by using PDA derived carbon. By calcining the pre-deposited PDA, we obtained a rougher surface for the CC fibres (Fig. S1a&b in Supplementary Information, SI). Raman spectra show typical D (~1350 cm-1) and G bands (1585 cm-1) for the CC substrates before and after surface treatment (Fig. S2). The increased intensity ratio (ID/IG from 1.00 to 1.06) can be attributed to the more structural defects resulting from the N-doped carbon layer [23]. The fibres surface turns to be hydrophilic, which is beneficial for 8

the growth of metal compounds in aqueous medium (Fig. S1c&d). As shown in Fig. 2a,b, uniform and compact Co-MOFs nanosheets are vertically grown on the CC fibres. The average thickness of these nanosheets is estimated to be 300~430 nm. The subsequent thermal treatment results in significant morphology change for NAs. As depicted in Fig. 2c&d, the compact nanosheets NAs are found to be sparse due to the collapse of Co-MOFs. Each sheet presents a corrugated surface with soft and curved leaf shape. The overall morphology of v-Co3O4/CC NAs is well preserved after 2h-chemical reduction in oxygen-free NaBH4 solution (Fig. 2e&f). The resulted NAs on CC fibres look like natural tremella attaching to tree trunks. However, we find that v-Co3O4 nanosheets turned to be more corrugated and rougher. Meanwhile, partial nanosheets peel off from the fibres, resulting to the sparser NAs. This corresponds well with the loading mass change of electroactive materials, which decreased from 0.56 mg cm-2 for Co3O4/CC to 0.50 mg cm-2 for v-Co3O4/CC.

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Fig. 2. SEM images of (a,b) Co-MOFs/CC, (c,d) Co3O4/CC and (e,f) v-Co3O4/CC. (g,h) TEM and HRTEM images of v-Co3O4/CC. (i) SAED patterns of v-Co3O4/CC.

The nanostructure of v-Co3O4/CC was further investigated by using transmission electron microscopy (TEM). It is observed that the two-dimensional flakes are composed of many small nanoparticles with an average size of ~20 nm (Fig. 2g). The presence of nanoparticles suggests the successful formation of Co3O4 due to the thermal decomposition of Co-MOFs. The high-resolution TEM (HRTEM) image reveals obvious lattice fringes with an interplanar spacing of 0.24 nm that is consistent with (311) plane of Co3O4 (Fig. 2h). The lattice fringes with an interplanar spacing of 0.47 nm correspond to the (111) crystal plane [24]. Besides, amorphous regions around the crystallographic Co3O4 are also observed, indicating the formation of disordered lattice due to the chemical reduction. According to previous works,

10

amorphous or low-crystalline TMOs can

greatly improve electrochemical

performance [25,26]. As shown in Fig. 2i, the selected area electron diffraction (SAED) image displays a set of continuous concentric circles, implying the polycrystalline structure of resulted Co3O4. We can find typical crystal planes of (311), (111), (400), (511) and (440) for our sample, which is consistent with the standard card (PDF#42-1467) [27]. We used X-ray diffraction (XRD) patterns to obtain the crystalline information of as-prepared Co3O4 NAs. Fig. 3a clearly shows typical peaks for the (011), (002), (112) and (222) crystal plane peaks of ZIF-67, indicating the successful formation of Co-MOFs [28]. For Co3O4/CC and v-Co3O4/CC, all the characteristic peaks of Co-MOFs are disappeared, suggesting the decomposition of MOF crystals. Compared to Co-MOFs/CC, two new peaks at 32.1o and 36.8o can be observed, which correspond to (220) and (311) planes of Co3O4, respectively [29,30]. Furthermore, the XRD patterns for the samples with different reduction time are similar (Fig. S3), confirming the stable crystal structure of Co3O4 during the oxygen vacancies engineering [31]. Fig. 3b shows the comparative Raman spectra of Co3O4/CC and v-Co3O4/CC. Compared to Co3O4/CC, v-Co3O4/CC demonstrates downshifted peaks of Co3O4 crystals [32]. However, the position of D (~1350 cm-1) and G (~1600 cm-1) bands retain unchanged, along with the same ID/IG value. This suggests that the chemical reduction treatment only affects the metal oxide crystals, resulting in increased surface structural turbulence and decreased metal bond strength [33,34].

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Fig. 3. (a) XRD patterns of CC, Co-MOFs/CC, Co3O4/CC and v-Co3O4/CC. (b) Raman spectra of Co3O4/CC and v-Co3O4/CC. (c,d) XPS Co 2p and O 1s spectra of Co3O4/CC and v-Co3O4/CC. (e) EPR spectra of Co3O4/CC and v-Co3O4/CC.

X-ray photoelectron spectroscopy (XPS) full survey reveals typical Co 2p, O 1s and C 1s peaks for both Co3O4/CC and v-Co3O4/CC. It is widely acknowledged that the oxygen vacancy engineering using NaBH4 can result in the generation of low-valence metal ions in the crystals. As depicted in Fig. 3c and Fig. S5a,b, characteristic peaks of Co 2p1/2 and Co 2p3/2 are found for both Co3O4/CC and v-Co3O4/CC. Impressively, the corresponding satellite peaks v-Co3O4/CC show higher peak intensity than that of Co3O4/CC, indicating an increased Co2+ concentration in reduced Co3O4 crystals. Furthermore, the chemical reduction results in shifted Co 12

2p1/2 and Co 2p3/2 peaks toward a higher binding energy, which can be attributed to the increased electron density of Co (i.e. the leave of oxygen element). Fig. S5c&5d reveal that O 1s peak can be deconvoluted to lattice oxygen at 530.2 eV (Olat, Co-O) and OI at the 531.7 eV (surface-adsorbed oxygen). The OI peak usually corresponds to the oxygen deficiency in the metal oxide crystals. We find that the intensity ratio of OI/Olat is significantly increased after the chemical reduction (Fig. 3d), indicating the introduction of oxygen vacancies in the Co3O4 crystals [35,36]. Electron paramagnetic resonance (EPR) was further used to analyze the oxygen vacancies. Both Co3O4/CC and v-Co3O4/CC show a symmetric EPR signal at g=2.003, indicating the trapping of electrons on oxygen vacancies. Because Co-MOFs are pre-calcinated in the oxygen-deficient atmosphere (i.e. N2), a spot of oxygen vacancies is thus introduced into the resulted Co3O4 crystals [37]. Based on the normalized EPR signal intensity that correlates with oxygen vacancies concentration, we can conclude that much richer oxygen vacancies are presented in the reduced Co3O4 crystals [38]. Therefore, an improved electrochemical charge storage performance is expected for v-Co3O4/CC electrode. 3.2. Electrochemical characterization To investigate the influence of two interfacial treatments on charge storage, we first compare the electrochemical properties for v-Co3O4/CC, v-Co3O4/CC-wo and Co3O4/CC. Fig. 4a shows the comparison cyclic voltammetry (CV) curves at a scan rate of 10 mV s-1 in a three-electrode cell using 2 M LiOH as electrolyte. No reverse redox peaks are observed for all electrodes, indicating the pseudocapacitive charge 13

storage [39,40]. By integrating the CV curves, we can conclude that both hydrophilic fibres and oxygen vacancies can contribute to the improved specfic capacity. This agrees well with the the galvanostatic charge/discharge (GCD) curves in a potential window of 0-0.45 v at a current density of 0.5 mA cm-2 (Fig. 4b). v-Co3O4/CC presents a 2.8-folder specific capacity value (414 C g-1) compared to Co3O4/CC (150 C g-1), while 1.6-folder higher than that for v-Co3O4/CC-wo (267 C g-1). As such, the influence of oxygen vacancies on areal specfic capacity is more significant than PDA-derived hydrophilic carbon. The rate capacbility retentions for three electrode are excellent with capacity retentions over 50% when the current density is increased to 20-folder vaule (Fig. 4c). The chemical reduction time was optimized to achieve the best electrochemical performance. By immersing Co3O4/CC electrode in oxygen-free NaBH4 solution for different time (1, 2, 3 and 4 h), we obtained four electrodes denoted as v-Co3O4/CC-1h, v-Co3O4/CC-2h, v-Co3O4/CC-3h and v-Co3O4/CC-4h, respectively. With the increased reduction time, the loading mass of electroactive Co3O4 is decreased accordingly (0.51, 0.50, 0.48 and 0.47 mg cm-2). As shown in Fig. S6, the reduction time significantly affects the morphology of Co3O4 NAs. With increased reduction time, these reduced Co3O4 NAs tend to fall off from CC fibers, resulting in more and more discrete nanosheets. The ordered nanostructure is destoried for v-Co3O4/CC-3h and v-Co3O4/CC-4h, with partially covered nanosheets on fibres surface. As a result, the loading mass of electroactive Co3O4 is accordingly decreased with longer reaction time. The influence of reduction time on electrochemical charge 14

strogae is shown in Fig. S7. We find that the charge storage mechanism is well retained as reflected by the CV and GCD behaviors (Fig. S7a&7b). In the early reduction stage, the specific capacity gradually increased from 150 C g-1 for Co3O4/CC to 189 C g-1 for v-Co3O4/CC-1h and 414 C g-1 for v-Co3O4/CC-2h (Fig. S7c). Although the electroactive materials mass is decreased, the introduced oxygen vacancy

can

contribute

to

the

charge

storage

through

the

ions

intercalation/deintercalation [41]. However, prolonging the reduction time results in decreased performance (345 and 308 C g-1 for v-Co3O4/CC-3h and v-Co3O4/CC-4h, respectively), indicating that the loading mass and nanostructure evolution dominates the

change

in

electrochemcial

results.

Nevertheless,

the

performance

of

v-Co3O4/CC-3h and v-Co3O4/CC-4h is still better than that of Co3O4/CC. The RS and RCT values also vary significantly with the reduction time, where the trend is consistent with pseudocapacitive performance (Fig. S7d). Considering the longer reduction treatment can lead to more reduced Co3O4, the overall charge storage should be determined by the balance of nanostructure and oxygen vacancy concentration.

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Fig. 4. Comparison electrochemical properties for Co3O4/CC, v-Co3O4/CC-wo and v-Co3O4/CC. (a) CV curves at a scan rate of 10 mV s-1, (b) GCD curves at a current density of 0.5 mA cm-2, (c) Specific capacities against current densities, and (d) Nyquist plots from EIS measurements.

We further investigate the intrinsic charge storage for v-Co3O4/CC, v-Co3O4/CC-wo and Co3O4/CC by the electrochemical impedance spectroscopy (EIS). As revealed in Fig. 4d, the internal resistances are descresed by the following order: v-Co3O4/CC(1.69 Ω)
line in the low frequency region, implying the most efficient ion diffusion [42]. This is consistent with diffusion resistances obtained from the fittings between Zre and the reciprocal square root of the angular frequency in the low frequency region (Fig. 5a). In the comparison bode phase plots (Fig. 5b), v-Co3O4/CC and v-Co3O4/CC-wo possess the same relaxation time constant τ (1/2πf0) of 0.12 s, which is much shorter than Co3O4/CC (2.6 s). The smaller τ again confirms the highly efficient charge (electron and ion) transport induced by oxygen vacancies. The enhanced electrochemical process can be attributed to the synergistic effect of interfacial and structure tailoring. The hydrophilic PDA-derived carbon layer can provide a coordination interaction between NAs and substrate, promoting the electron transfer and ion diffusion on the interface [19]. On the other hand, the oxygen vacancies can significantly improve the electronic conductivity of Co3O4. The artificial intercalated sites allow the efficient diffusion of lithium ions on Co3O4 surface. As such, the resulted v-Co3O4/CC shows significantly improved electrochemical performance. By increasing the scan rate from 10 to 50 mV s-1, we observe no obvious shape change in the CV curves, indicating the good rate capability. Based on the discharged curves (Fig. S8), the specific capacity is 414, 396, 362, 309 and 218 C g-1 at the corresponding current density of 1, 2, 4, 10 and 20 A g-1, respectively. The performance is superior to that of some representative Co3O4-based electrodes such as UCNG (440 C g-1 at 1 A g-1) [38], CoNW/CF (344 C g-1 at 1 A g-1) [43], Co3O4/3DGN/NF (161 C g-1 at 1 A g-1) [44], Co3O4/GF (223 C g-1 at 0.5 A g-1) [45]. The stability of v-Co3O4/CC is evaluated by a long-term charging/discharging cycles 17

at a current density. As shown in Fig. 5d, v-Co3O4/CC shows an excellent capacity retention of 73.9% after 15000 cycles, with only 0.00174% capacity loss per cycle. The columbic efficiency is higher than 96% during the cycles.

Fig. 5. (a) Plot of Z’ against the reciprocal square root of the angular frequency in the low frequency region. (b) Bode phase plots for Co3O4/CC, v-Co3O4/CC-wo and v-Co3O4/CC. Dashed line highlights the characteristic frequency f0 (1/τ) at the phase angle of −45°. (c) CV curves for v-Co3O4/CC in a scan rate of 10-50 mV s-1. (d) Cycle performances with columbic efficiency of v-Co3O4/CC at a current density of 2 mA cm-2.

3.3. Asymmetric supercapacitor In order to evaluate the potentials of as-prepared v-Co3O4/CC in asymmetric supercapacitor (ASC) cells, we also prepared nitrogen-doped nanoporous carbon nanosheets from the same Co-MOFs (ZIF-67) by calcination and HCl etching. As shown in Fig. 6a-c, the carbon nanosheets well preserve the pristine morphology of Co-MOFs, with a thickness of about 500 nm. The nanosheets exhibit fluffy surface 18

and feature plenty of nanochanels interpenetrating the sheets (Fig. 6d). Obvious these hollow interior voids were generated through the transformation of ZIF-67 into carbon nanosheets. Raman spectrum demonstrates two typical bands of carbon with a high disorder (ID/IG>1) (Fig. S9). The absence of any other peaks especially for metal-carbon bonds, indicating the complete removal of Co during acid etching. The Co-MOFs derived carbons exhibited robust electrochemical properties featuring a potential window ~1.1 V at scan rates increased from 10 to 50 mV s-1 in 2 M LiOH electrolyte, indicating a good rate capability (Fig. S10a). When the current densities increased from 1 A g-1 to 2, 4, 10 and 20 A g-1, the carbon electrode contributed a specific capacity value of 534, 349, 275, 220, and 184 C g-1, respectively (Fig. S10b). A flexible asymmetric supercapacitor (ASC) was fabricated by assembling v-Co3O4/CC positive electrode, carbon nanosheets negative electrode and PVA/LiOH gel electrolyte. The ASC (v-Co3O4/CC//NC) exhibits quasi-rectangular CV curves as shown in Fig. 6e, indicating its ideal capacitive properties. Good rate capability was observed within a voltage window of 1.6 V at increased scan rates from 10 to 50 mV s-1. Based on the discharged curves, the ASC showcases an areal specific capacitance value of 145, 140, 132, 120 and 113 mF cm-2 corresponding to increased current intensity from 1.3 to 2, 4, 10 and 20 mA cm-2 (Fig. 6f). With mass specific capacitance in hand, the volumetric specific capacitance (CV) is further calculated to be 2.07, 2.00, 1.89, 1.71 and 1.62 F cm-3 when current density increased from 1.3 to 20 mA cm-2 (the thickness is determined to be ~70 µm). Our ASC device also demonstrates good long-term stability, where the capacitance retention as high as 19

84.1% is achieved after 5000 cycling test at 2 mA cm-2 (Fig. 6g). Compared to other cobalt oxide-based ASC devices, both the specific capacitence and cycling stability of our ASC have considerable advantages (Table. S1).

Fig. 6. (a-c) SEM images and (d) TEM images of of Co-MOFs derived nitrogen-doped carbon nanosheets (NC). (e) CV curves for our ASC device at a scan rate of 10-50 mV-1. (f) Galvanostatic discharged curves of ASC device at various current densities. (g) Cycle performances of ASC device at a current density of 2 mA cm-2. (h) Ragone plots for our ASC device. (i) Ragone plots of the ASC device compared with other ASCs. 20

The Regone plots of energy density (E) versus power density (P) for the ASC are presented in Fig. 6h & 6i. The highest volumetric energy density of 0.74 mWh cm-3 is achieved at a volumetric power density of 14.70 mW cm-3. The volumetric energy density can be retained as 0.58 mWh cm-3 at the power density of 265.60 mW cm-3. When taking into account the mass of active materials, the ASC was found to contribute a maximum gravimetric energy density of 45.3 Wh kg-1 at the power density of 16.3 kW kg-1, respectively. When compared with other MOF-derived solid-state ACS [10,17,29,46-50], our v-Co3O4/CC//NC exhibited relatively better energy storage performance as shown in Figure 6i. The enhanced capacitance can be attributed to the nitrogen doping and oxygen vacancy engineering on Co3O4/CC.

4. Conclusions In summary, we have developed corrugated Co3O4 nanosheets through a simple MOF-template and oxygen vacancy engineering method. By artificially controlling the interfacial properties (electrolyte/NAs and NAs/CC), the electron and ions transport are simultaneously enhanced. An improved specific capacity and long-term stability are achieved for v-Co3O4/CC. With the well-tailored electrode as positive electrode, we can obtain an ASC device with a high energy density of 0.74 mWh cm-3 at 14.70 mW cm-3 or 45.3 Wh kg-1 (915 W kg-1). This result demonstrates that the interfacial engineering is a promising approach for improving the pesudocapacitive performance of TMOs electrodes.

Acknowledgements 21

The authors gratefully appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51861145401) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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27

Highlights


Interfacial engineering on Co-metal organic framework generates Co3O4 nanosheets;
Co3O4 nanosheets were stabilized on carbon cloth by hydrophilic carbon coating;
Abundant oxygen vacancies were introduced into Co3O4 nanosheets;
v-Co3O4/CC composite exhibits high specific capacitance (920 F g-1 at 1 A g-1) and excellent stability;
Asymmetric supercapacitor shows a maximum volumetric energy density of 0.74 mWh cm-3 at ap ower density of 14.70 mW cm-3.

Declaration of Interest Statement We herein declare no conflict interest for the submission.