Enhancing cycling stability of transition metal-based layered double hydroxides through a self-sacrificial strategy for hybrid supercapacitors

Journal Pre-proof Enhancing cycling stability of transition metal-based layered double hydroxides through a self-sacrificial strategy for hybrid supercapacitors Teng Wang, Feng Yu, Xiaoxiang Wang, Shibo Xi, Kai-Jie Chen, Hongxia Wang PII:

S0013-4686(19)32458-2

DOI:

https://doi.org/10.1016/j.electacta.2019.135586

Reference:

EA 135586

To appear in:

Electrochimica Acta

Received Date: 27 October 2019 Revised Date:

20 December 2019

Accepted Date: 28 December 2019

Please cite this article as: T. Wang, F. Yu, X. Wang, S. Xi, K.-J. Chen, H. Wang, Enhancing cycling stability of transition metal-based layered double hydroxides through a self-sacrificial strategy for hybrid supercapacitors, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135586. 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.

Credit Author Statement

Teng

Wang: Conceptualization,

Methodology,

Writing-

Original

draft

preparation, Investigation, Funding acquisition. Feng Yu: Data curation, Formal analysis, Investigation. Xiaoxiang Wang: Visualization, Investigation. Shibo Xi: Data curation, Investigation. Kai-Jie Chen: Writing- Reviewing and Editing, Validation, Supervision, Project administration, Funding acquisition. Hongxia

Wang: Writing-

Reviewing

administration, Funding acquisition.

and

Editing,

Supervision,

Project

Enhancing Cycling Stability of Transition Metal-Based Layered Double Hydroxides Through a Self-Sacrificial Strategy for Hybrid Supercapacitors

Teng Wang,a, b Feng Yu,b Xiaoxiang Wang,b Shibo Xi,c Kai-Jie Chen,*, a and Hongxia Wang *, b a Department of Applied Chemistry, School of Natural and Applied Sciences, Northwestern Polytechnical University, Xi’ an, Shaanxi 710072, PR China. Email: [email protected]. b School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia. E-mail: [email protected]. c Institute of Chemical & Engineering Sciences, Agency for Science, Technology and Research (A *Star), Singapore.

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ABSTRACT Transition metal-based layered double hydroxides (LDH) are attractive electrode materials for supercapacitors (SCs) owing to their advantage of high theoretical specific capacity. However, the material structure of most LDHs fails to sustain long charge/discharge cycling of the device, leading to a short lifespan. Herein, we demonstrate a self-sacrificial strategy to boost the cyclability of Ni-Co LDH nanosheet arrays for SCs by using electrochemically inert Zn cation as a sacrificial agent (Zn-Ni-Co LDH). At an optimal content of Zn incorporation, a maximum specific capacity of 231.7 mAh g-1 (at 1 A g-1) and a capacity increment of over 500% after 20, 000 charge/discharge cycling test at 20 A g-1 have been obtained. For practical application, a hybrid SC based on Zn-Ni-Co LDH material demonstrated a high energy density of 40.3 Wh kg-1 and a high power density of 16.1 kW kg-1, along with extraordinary cycling stability of over 20, 000 cycles. Measurements by ex-situ synchrotron X-ray absorption spectroscopy (XAS) and other characterisation techniques like EDS and TEM have shown a gradual loss of Zn from the electrode during the charge/discharge process, which not only helps to create free space to maintain the material microstructure but also exposes more active sites for electrochemical reaction. The findings in this work provide new avenues towards the fabrication of robust electrode materials for advanced SCs with both high energy density and cycling stability. Keywords: Supercapacitor, Layered double hydroxides, Cycling stability, Self-sacrificial strategy, XAS

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1. Introduction Supercapacitors (SCs, also called electrochemical capacitors) have been recognized as one of the most important electric energy storage devices owing to their advantages of high power density, high safety, ultra-long cycling stability, and broad working temperature range.[1-3] SCs can be classified into electric double layer capacitors (EDLCs) and pseudocapacitors. Currently commercial SCs are mainly EDLCs that use activated carbon as electrode materials owing to their outstanding cycling life. Nevertheless, carbon based SCs generally deliver a low energy density (< 10 Wh kg-1) due to the limitation of its physical charge storage mechanism. In contrast, pseudocapacitors store electricity through surface redox reaction mechanism, thus possess much higher energy storage capability. However, typical pseudocapacitive electrode material such as RuO2 suffers from issues such as material scarcity and high cost,[4] which prevents their large-scale practical applications. To address the issue, a variety of substitute electrode materials that use abundant and inexpensive transition metal compounds such as Ni, Co, Mn, V, and Fe[5-11] have been explored in the past three decades for SC applications. Among them, transition metal based layered double hydroxides (LDHs) show great potential due to its layered structure, tunable interlayer distance, easy fabrication, and ultra-high theoretical specific capacity.[12, 13] LDH adopts a chemical formula of [MII1-xMIIIx (OH)2]x+ [An−x/n]x-·mH2O, where MII (divalent metal cations) and MIII (trivalent metal cations) form octahedral brucite-like host layers, which are balanced by charge-balancing anions An− such as CO32-, SO42-, and Cl- in the interlayers. The laminar structure and hydrophilic property of LDHs can facilitate the fast diffusion of alkaline electrolyte ions into the materials, resulting in a high charge storage ability. Nevertheless, their poor electric conductivity, fragile crystalline structures, slow dissolution in alkaline electrolyte often result in compromised electrochemical performance such as low specific capacity, poor cycling stability and inferior rate capability. 3

One common strategy to alleviate the problems is to fabricate binder-free electrode by growing array structure directly on the current collectors.[14-16] For example, we recently reported a solvothermal reaction method to synthesize Ni-Co LDH nanosheet arrays on carbon fiber cloth (CFC).[17] The good contact of LDH materials with CFC substrate, enlarged interlayer distance, and ultrathin nanosheet thickness resulted in a high electrochemical performance including a high specific capacity of 1009.3 C g-1 at 1 A g-1 and a super high rate capability of 61% capacity retention at 60 A g-1. Nevertheless, the cyclability of the material is still unsatisfactory. After 5000 cycling test, the performance of the SCs dropped to 80% of its initial value. Clearly it is of high importance to develop strategies that can overcome the relatively poor stability of transition metal based SCs electrode material to meet the needs for practical applications. To tackle the issues of LDH materials, other methods such as nanostructure engineering, composites designing, composition optimization and interface engineering have also been reported.[18-20] Among these strategies, the design and fabrication of bimetallic or multiplemetallic LDHs such as Ni-Co, Co-Mn, Co-Fe, Ni-Co-Al and Ni-Co-Fe LDHs have attracted great interests considering the improved electrical conductivity, microstructure stability, and the increased number of surface active sites of the materials compared with single metalbased counterparts.[21-29] Despite the reports of various LDH materials for SC applications, most of them can hardly possess a high charge storage capability as well as a long cyclability of over 10, 000 cycles at a large discharge current density (>10 A g-1), which however is critical for the practical application of SCs. Herein, we firstly demonstrate a self-sacrificial strategy to boost the cyclability of Ni-Co LDH by using electrochemically inert zinc cation. The Zn-Ni-Co LDH nanosheet arrays have been grown on a carbon fiber cloth (CFC) by a facile solvothermal method. Through tuning the proportion of Zn, the optimized electrode materials exhibited an outstanding specific capacity and ultrahigh cycling stability (> 20, 000 cycles) in SC applications. In addition, a 4

dramatic increase in the specific capacity of ternary Zn-Ni-Co LDH materials after cycling stability test was observed. A systematic study of the electrode material after the cycling test showed self-etching of Zn cations during the charge storage process was responsible for the extraordinary electrochemical performance of the material. The loss of Zn cations not only stabilizes the material structure and transforms the material to amorphous phase, but also exposes more active metal sites. As a consequence, the device demonstrated significant enhancement of cycling stability. The hybrid SC based on optimized ternary Zn-Ni-Co LDH showed a high energy density of 40.3 Wh kg-1 (at the power density of 403.3 W kg-1) and a high power density of 16.1 kW kg-1 (at the energy density of 15.1 Wh kg-1), as well as extraordinary cycling stability of over 20, 000 cycles. The findings in this work provide a new pathway to solve the stability issue of transition metal-based electrode materials for SCs that can meet practical needs with both high energy density and high cycling stability. 2. Experimental section 2.1 Materials All the chemicals of analytical grade used in this work were purchased from SigmaAldrich and used as received without any further purification. Ultrapure water with a resistivity of 18.25 MΩ.cm was used in the experiment. Carbon fibre cloth was purchased from ElectroChem Inc (USA). 2.2 Synthesis of Zn-Ni-Co LDH on carbon fiber cloth (CFC) Zn-Ni-Co LDH nanosheets grown on carbon fiber cloth (CFC) were fabricated through the modified solvothermal method reported by us previously.[17, 28] In a typical experimental procedure, Ni(NO3)2·6H2O, Co(NO3)2·6H2O and Zn(NO3)2·6H2O were dissolved in absolute methanol solvent with the concentration of 10 mM for both Ni and Co respectively and variable amount of Zn (0 ∼ 15 mM). Then 20 mM 2-methylimidazole (MIM) 5

and 20 mM urea dissolved in absolute methanol were added into the above mixed metal cation solution under vigorous stirring. After reaction for 20 min at room temperature under stirring, the reaction solution was transferred into a Teflon-lined autoclave with a piece of CFC (1 x 4 cm2) leaning in the container. The CFC used for the reaction was pre-cleaned with acetone, ethanol, and pure water under ultrasonication for 15 min in sequence. The autoclave was then put in an electric oven for 10 h at 120 oC. After the reaction was completed, the CFC with grafted product was taken out followed by rinsing with a large amount of methanol to remove the loose particles and residues on the surface. The electrode was dried in an electric oven at 70 oC before being used for characterization and cell assembly. For clarity, the products synthesized from 5mM, 8mM, 10mM, and 15mM Zn precursors are named as 5mMZn-Ni-Co LDH, 8mM-Zn-Ni-Co LDH, 10mM-Zn-Ni-Co LDH, and 15mM-Zn-Ni-Co LDH, respectively. In the control experiment, Ni-Co LDH materials made from the solution containing only 10 mM Ni(NO3)2·6H2O and 10 mM Co(NO3)2·6H2O were fabricated under the otherwise same reaction conditions. 2.3 Material characterization The information on micro morphology and elemental composition of the as-synthesized materials were characterized by Field emission scanning electron microscope (FESEM, Verios G4, FEI) with an energy dispersive spectrometer (EDS) and transmission electron microscope (TEM, 2100, JEOL). X-ray diffraction (XRD, PANaytical) using Co Kα source was employed to study the crystal structure of the products while X-ray photoelectron spectroscopy (XPS, Axis Supra, Kratos) with Al (Kα =1486.6eV) X-ray as excitation source was used to analyze the surface chemical composition. Ex-situ X-ray absorption spectroscopy (XAS) measurements were carried out at the Australian Synchrotron facility centre.[30] The XAS data were recorded in transmission mode 6

using a 100-element Ge fluorescence detector at 25 °C. All specimens’ spectra were calibrated by recording the corresponding metal foil simultaneously. NiO, CoO, Ni(OH)2, Co(OH)2, LiNiO2, and LiCoO2 were used as the standard materials for different valence states of Ni and Co. XAS data were processed using Demeter software.[31] X-ray absorption nearedge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) were obtained for analysis. Ab initio phases and amplitudes were used in the EXAFS equation (1) to fit experimental data:

χ (k ) = ∑ Γ [

N Γ S02 FΓ (k ) −2 k 2σ Γ2 −2 RΓ / λ ( k ) e e × sin(2kRΓ + φΓ (k ))] 2kRΓ2

(1)

Where Γ is the summation over the individual scattering pathways and k is the photoelectron wavevector. F(k), λ(k) and Φ(k) are the scattering amplitude, mean free path of inelastically-scattered photoelectrons and the phase shift, respectively. Amplitude reduction factor (S02), half-path length (R), degeneracy (N), energy shift parameter (E0), and meansquared disorder (σ2, set as Debye-Waller factor), were fitted in order to extract the local structural information. 2.4 Electrochemical Measurements The electrochemical properties of as-prepared Zn-Ni-Co LDHs and Ni-Co LDH grown on CFC substrate were measured with an electrochemical workstation (VSP, BioLogic) through a three-electrode test system using Pt foil (1*2 cm2), Ag/AgCl electrode (0.222 V vs RHE, reversible hydrogen electrode), and active materials as counter electrode, reference electrode, and working electrode, respectively. Electrochemical impedance spectra (EIS) of the samples were collected in the range of 0.01 Hz to 100 kHz with an AC potential amplitude of 5 mV at open circuit voltage. The cycling stability results were recorded by a programcontrolled automatic charge/discharge testing system (CT-4008, NEWARE). 7

The specific capacity of the electrode materials was calculated based on the corresponding galvanostatic charge/discharge (GCD) plots according to equation (2):


=

 (2) 3.6

Where C (mAh g-1) is the specific capacity while I (A) is the discharge current. ∆t (s) is discharge time and m (g) is the mass loading of active materials. For practical application, hybrid supercapacitors using the as-prepared Zn-Ni-Co LDH as cathode and commercial active carbon as anode were assembled. The mass ratio of the active materials in the two electrodes was determined based on equation (3)

m+ m-

=

C- ×V-

(3)

C+ ×V+

Where C+ and C- are the specific capacity (mAh g-1) of positive and negative electrode materials; m+ and m- (g) stand for mass loading of positive and negative electrodes while V+ and V- (V) represent the voltage window of the positive and negative electrodes respectively. The specific capacitance (CT, F g-1) of the hybrid SC was obtained based on equation (4):


 =

∆ (4) ∆

Where I (A) is the discharge current density, m (g) is the total mass of active material, ∆t (s) is the discharge time, and ∆U (in V) is the potential window. The energy density (E, Wh kg-1) and power density (P, W kg-1) of the hybrid SC device were calculated according to equation (5) and (6):
 ∆  = (5) 7.2 8

 =

3600 (6) ∆

Where CT (F g-1) is specific capacitance, ∆U (V) is discharge voltage, and ∆t (s) is discharge time.

3. Results and Discussion A modified solvothermal reaction method has been developed to fabricate the Ni-Co LDHs incorporated with different content of Zn. For simplicity, the material that contains different amounts of Zn is named as xmM Zn-Ni-Co (x denotes the concentration of Zn2+ in the precursor for synthesis of the LDH). It is found that all the materials have very similar morphology and crystal structure regardless of the amount of Zn. Therefore, material characterization based on a representative sample of 8mM-Zn-Ni-Co LDH was shown in the following. A control sample of Ni-Co LDH without Zn was also made under the same reaction condition and characterized. The SEM image of the bare carbon fibre cloth (CFC) substrate shows a smooth surface with the diameter of each carbon fiber around 5 - 10 µm (Figure S1). After the solvothermal reaction for the growth of 8mM-Zn-Ni-Co LDH, it can be seen that the carbon fibers are coated with a dense layer of nanosheets with a thickness of each nanosheet around 10 - 20 nm (Figure 1a and b). A porous structure with a relatively large pore size (> 0.5 µm) is formed among the nanosheets which is beneficial for diffusion of electrolyte ions in the electrode materials. The abundant wrinkles and edges of the nanosheet may provide a high density of active sites for charge storage. The as-prepared Ni-Co LDH control sample possesses a similar porous nanostructure composed of nanosheets with a slightly larger thickness (20 - 30 nm) that are firmly attached to the CFC substrate (Figure 1c and d). It is noted that the average pore size of the Ni-Co LDH is below 0.5 µm, which is much smaller than that of the 8mM-Zn-Ni-Co LDH. Comparison of the SEM images of ZnNi-Co LDHs containing different amounts of Zn (5mM-Zn-Ni-Co LDH, 10mM-Zn-Ni-Co 9

LDH, and 15mM-Zn-Ni-Co LDH) (Figure S2) reveals that the average pore size of the porous structure of the Zn-Ni-Co LDH increases with the increase of Zn content. This implies that the Zn-Ni-Co LDH materials may have fewer surface edges and smaller surface area at a very high content of Zn, which can affect the electrochemical performance of the material. Clearly control the amount of Zn in the LDH is important to obtain the optimum device performance.

Figure 1. SEM images of 8mM-Zn-Ni-Co LDH (a and b) and Ni-Co LDH (c and d) nanosheet arrays grown on CFC substrate with different resolutions. The inset in (b and d) are the corresponding enlarged SEM image showing the nanosheet thickness of both materials. Figure 2 shows the XRD patterns of bare CFC substrate, 8mM-Zn-Ni-Co LDH and NiCo LDH. It is clear that CFC has two broad diffraction peaks at about 30.0° and 50.8°, which belong to amorphous carbon. Besides the characteristic peaks of the CFC substrate, the Ni-Co LDH control sample grown on CFC possesses additional distinctive diffraction peaks at 12.1°, 24.4°, 39.7°, and 44.2°, which can be indexed to (003), (006), (012), and (015) facets of hydrotalcite-like layered crystalline structure of LDH material.[17, 32] The dominated diffraction peak of (003) facet at 12.1° indicates the Ni-Co LDH has an interlayer sheet 10

distance (d003) of 8.5 Å. The tunable layered structure has been reported to be beneficial for fast electron transport and diffusion of electrolyte ions.[33] Compared to the binary Ni-Co LDH control sample, the 8mM-Zn-Ni-Co LDH has a nearly identical XRD pattern, suggesting that they share the same crystal structure. However, the intensity of the characteristic diffraction peaks of the 8mM-Zn-Ni-Co LDH is much reduced and its peak width is broadened, indicating that introduction of Zn into the material decreases the crystallinity of the material. The structural disorder should be attributed to the complex metal cation interactions during synthesis procedure since Zn ion has a different ionic size and electronegativity from Ni and Co cations.

Figure 2. XRD patterns of bare CFC, Ni-Co LDH and 8mM-Zn-Ni-Co LDH. The TEM image of 8mM-Zn-Ni-Co LDH (Figure 3a) further confirms its nanosheet morphology while the Scanning Transmission Electron Microscopy (STEM, Figure 3b) image of the material reveals the thickness of the nanosheet is a. 10.4 nm. This is in good agreement with the SEM results of the sample shown in Figure 1b above. The element mapping by EDS 11

of the sample shown in Figure 3(c-e) proves the uniform distribution of Ni, Co, and Zn elements on the nanosheets. Note that a bright spot in Figure 3b is probably related to the existence of ZnO impurities as a high concentration of Zn is detected in this area (Figure 3e). However, we did not detect any XRD diffraction peaks of ZnO in LDH sample (Figure 2), which suggests that the amount of ZnO in the LDH should be very low. The EDS spectrum of the 8mM-Zn-Ni-Co LDH (Figure S3) confirms the as-prepared product is composed of Ni, Co, Zn, O and C. Based on the EDS spectrum, the molar ratio of transition metals Ni, Co, and Zn in the 8mM-Zn-Ni-Co LDH is Ni: Co: Zn = 9: 7: 5. In order to obtain the relatively accurate atomic ratios of Zn, Ni, and Co in Zn-Ni-Co LDHs, ICP-MS analysis has been carried out (Table S1). It can be calculated that the atomic ratios of Zn/Ni/Co are about 0.100: 0.514: 0.386 for 5mM-Zn-Ni-Co LDH, 0.169: 0.505: 0.326 for 8mM-Zn-Ni-Co LDH, 0.223: 0.492: 0.285 for 10mM-Zn-Ni-Co LDH, and 0.382: 0.410: 0.208 for 15mM-Zn-Ni-Co LDH, respectively. Figure S4 depicts the change of Zn, Ni, and Co in all four samples. Clearly, the percentage of Zn in Zn-Ni-Co LDHs keeps growing as the concentration of Zn(NO3)2·6H2O increases, while the amount of Ni and Co keep decreasing. It indicates that the adding percentage of Zn can be well tuned through simply changing the precursor concentration.

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Figure 3. TEM (a) and STEM (b) images of 8mM-Zn-Ni-Co LDH and its EDS element mapping of Ni (c), Co (d), and Zn (e) elements. Since surface properties are critical to the electrochemical energy storage performance of the material, we investigated the elemental composition on the surface of the 8mM-Zn-Ni-Co LDH by XPS. Figure 4a shows the as-prepared LDH material contains Zn, Ni, Co, O, C, and N elements, which is consistent with the above EDS results. The trace amount of N should originate from the nitrate anions absorbed on the surface during the material synthesis process. The high resolution XPS (HRXPS) spectrum of Zn 2p (Figure 4b) presents two main peaks located at 1043.8 eV and 1020.7 eV, which are ascribed to the characteristic peaks of Zn 2p1/2 and Zn 2p3/2 of Zn2+,[34, 35] respectively. The HRXPS spectra of Co 2p and Ni 2p were deconvoluted in order to analyse the elemental valence states. Specifically, the HRXPS spectra of Co 2p (Figure 4c) can be fitted with four satellite peaks at 784.5 eV, 789.0 eV, 801.6 eV, and 804.9 eV and other four characteristic peaks. The two main peaks positioned at 780.1 eV and 795.8 eV belong to the characteristic peaks of Co3+ while those located at 781.4 eV and 797.2 eV are the characteristic peaks of Co2+.[36] Similarly, the deconvoluted HRXPS spectrum of Ni 2p shown in Figure 4d contains three satellite peaks at 861.1 eV, 866.2 eV, and 879.3 eV, along with the characteristic peaks of Ni3+ (located at 856.1 eV and 873.8 eV) and Ni2+ (at 854.8 eV and 872.4 eV)[37, 38]. The HRXPS spectra of Ni 2p and Co 2p confirm that both elements have a mixed valence state of 2+ and 3+.

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Figure 4. (a) XPS survey of 8mM-Zn-Ni-Co LDH and its HRXPS spectra of Zn 2p (b), Co 2p (c), and Ni 2p (d). Due to the severe overlaps of the XPS characteristic peaks of 2+ and 3+ of Ni and Co elements, the deconvolution of their XPS spectra is tricky which sometimes can lead to controversial results.[36, 39-42] This poses an obstacle for revealing the nature of Ni and Co cations in the material. To resolve this issue, we have employed synchrotron X-ray absorption spectroscopy (XAS) to detect the chemical state of Ni and Co elements in the as-prepared binary Ni-Co LDH and ternary Zn-Ni-Co LDH materials. XAS technique is a powerful tool for distinguishing the valence state of transition metals such as Ni and Co which normally present very complex XPS results.[30, 31] Moreover, XAS can provide more detailed information on chemical environment at atomic level. The Co K-edge XANES spectra (Figure 5a) clearly show that the absorption edge position of Co in both the 8mM-Zn-Ni-Co LDH and Ni-Co LDH sits between standard samples CoO (2+) and LiCoO2 (3+). This directly proves that the Co has a mixed oxidation state of 2+ and 3+. The weak pre-edge intensity of the Co 14

XANES indicates that the Co atoms are located in octahedral sites of the crystal structure.[43] Compared to the standard sample Co(OH)2 and control sample Ni-Co LDH, Co in 8mM-ZnNi-Co LDH has a larger absorption edge, indicating a higher oxidation state. On the other hand, the Ni K-edge XANES spectra (Figure 5b) reveal that the Ni in both 8mM-Zn-Ni-Co LDH and Ni-Co LDH has the same absorption edge with the standard sample Ni(OH)2, which is in the range of the Ni K-edge XANES spectra of standard samples NiO (2+) and LiNiO2 (3+). It means the Ni in all three hydroxide materials have identical valence state which is between 2+ and 3+. The XANES results provide direct evidence on the valence state of Ni and Co, which is consistent with the above XPS results.

Figure 5. K-edge XANES spectra of Co (a) and Ni (b) in as-synthesized products (8mM-ZnNi-Co LDH (referred as Zn-Ni-Co LDH) and Ni-Co LDH) and standard samples (NiO, CoO, Ni(OH)2, Co(OH)2, LiNiO2, and LiCoO2). Fourier transform (FT) magnitudes of k2-weighted EXAFS spectra of Co (Figure 6a) and Ni (Figure 6b) of the as-prepared samples were obtained to analyze their local chemical structural variations. Generally, the first two peaks of both elements below 3.10 Å can be ascribed to the single scattering paths of the closest oxygens (TM-O, TM = Ni or Co) and the second neighbouring transition metals within the same a-b plane (i.e. TM-TM) surrounding the absorbing atoms Co or Ni.[43] The peak position and peak intensity of the FT magnitudes of the k2-weighted EXAFS spectra are strongly correlated to the interatomic distance (R) and the coordination numbers (N).[44] The experimental data were fitted to acquire the local atomic structural information. Table S2 and Table S3 present the fitted pathway parameters 15

for the 8mM-Zn-Ni-Co LDH and the Ni-Co LDH respectively. The coordination number (N) of Co-O bond (with a bond length of 1.88 Å) and Co-TM bond (3.09 Å) in the 8mM-Zn-NiCo LDH are 4.2 and 5.8 respectively while the N of Co-O bond (1.92 Å) and Co-TM bond (3.10 Å) in the Ni-Co LDH are 3.7 and 6.7 respectively. The shorter Co-O bond length and its higher N further confirm a higher oxidation state of Co in the 8mM-Zn-Ni-Co LDH sample. Note that the fitted results indicate that there is an additional Co-O bond with the bond length of 2.09 Å and an N of 3.3, suggesting a mixed atomic structural environment. Figure 6b gives similar FT magnitudes of Ni in both products,[45] which is in accordance with the XANES results. The N of Ni-O bond (at 2.05 Å) and Ni-TM bond (at 3.09 Å) in the 8mM-Zn-Ni-Co LDH are 7.7 and 7.6 respectively while the N of Ni-O bond (at 2.05 Å) and Ni-TM bond (at 3.10 Å) in the Ni-Co LDH are 8.0 and 8.7 respectively. The nearly identical bond length of Ni-O bond and Ni-TM bond proves the same oxidation state of Ni in the two samples.

Figure 6. Fourier transform (FT) magnitudes of the k2-weighted EXAFS spectra of Co (a) and Ni (b) of 8mM-Zn-Ni-Co LDH (referred as Zn-Ni-Co LDH) and Ni-Co LDH. The electrochemical property of the Zn-Ni-Co LDH materials and the Ni-Co LDH control sample was investigated through a three-electrode configuration system by using the LDH active materials, Pt foil (1*2 cm2), and Ag/AgCl electrode as working electrode, counter electrode, and reference electrode, respectively in 2.0 M KOH aqueous electrolyte solution. It is found that the specific capacity of Zn-Ni-Co LDH and Ni-Co LDH experienced a sharp surge in the initial galvanostatic charge/discharge (GCD) test before they reached a relatively 16

stable value. Therefore, the electrochemical performance of both Zn-Ni-Co LDH materials and Ni-Co LDH after 500 GCD cycling tests at the current density of 20 A g-1 was compared in order to obtain reliable and practically important conclusion. The electrochemical pre-cycling results of all samples are shown in the supporting information (Figure S5 and Figure S6). The CV plots of stabilized Zn-Ni-Co LDHs with different Zn proportion and Ni-Co LDH at the scanning rate of 2 mV s-1 are shown in Figure 7a. The CV curves of all Zn-Ni-Co LDH materials exhibit a pair of distinctive oxidation and reduction peaks while the Ni-Co LDH possesses two pairs of merged oxidation (at 0.30 V and 0.27 V) and reduction peaks (at 0.18 V and 0.14 V). The single pair of redox peaks in the ZnNi-Co LDH is probably due to a stronger interaction of Co and Ni in the presence of Zn cations, resulting in the merging of the two redox reactions. The non-rectangular CV shape and significant redox reaction peaks of all materials demonstrate a typical battery-like charge storage mechanism of these materials. The charge storage process involves the following redox reaction as shown in equation (1-3)[17]: Ni(OH)2 + OH- ⇔ NiOOH + H2O + e

(1)

Co(OH)2 + OH- ⇔ CoOOH + H2O + e

(2)

CoOOH + OH- ⇔ CoO2 + H2O + e

(3)

The narrower width of the redox peak and the smaller potential distance of reduction and oxidation peaks of 5mM-Zn-Ni-Co LDH than Ni-Co LDH indicate its higher material reversibility during charge/discharge process. A clear positive shift of the redox peaks along with decreasing CV integration area of the Zn-Ni-Co LDH from low Zn content (5 mM) to high Zn content (15 mM) indicates a tendency of increased overpotential, therefore leading to decreased charge storage capability. This is consistent with the observation of the enlarged pore sizes, thus decreased surface edges available for electrochemical reaction by SEM (Figure 1d).

17

Figure 7. (a) CV curves (under the scanning rate of 2 mV s-1), GCD curves (at the current density of 2 A g-1, b), specific capacity (under different charge/discharge current densities, c), and EIS spectra (d) of binary Ni-Co LDH and ternary Zn-Ni-Co LDH materials. The inset in (d) is the equivalent electrical circuit used for plot fitting. The GCD curves of the Zn-Ni-Co with different amounts of Zn at 2 A g-1 are shown in Figure 7b. The Ni-Co LDH presents a notable discharge plateau at 0.19 V vs Ag/AgCl (Figure 7b), further confirming its battery-type charge storage mechanism. Similar GCD discharge behaviour is also observed with the Zn-Ni-Co LDHs except that the potential of their discharge plateaus rises steadily with the increase of Zn content. Based on the GCD curves of all LDH materials, their specific capacity was calculated and is presented in Figure 7c. Clearly the 5mM-Zn-Ni-Co LDH shows the best electrochemical property including a high specific capacity of 231.7 mAh g-1 at 0.5 A g-1 and a good rate capability of 154.8 mAh g-1 at 30 A g-1 (66.8% capacity retention). Note that the electrochemical property of the 5mM-ZnNi-Co LDH is slightly higher than the binary Ni-Co LDH. Further increase of Zn results in 18

the gradual decrease of the specific capacity of Zn-Ni-Co LDHs. Especially, the 15mM-ZnNi-Co LDH shows the lowest specific capacity of 151.2 mAh g-1 at 0.5 A g-1. The above results confirm that the addition of a low percentage of Zn (5 mM) can slightly increase the specific capacity of pristine Ni-Co LDH while a higher Zn content damages the charge storage capability of the materials. This is probably related to the electrochemically inert nature of Zn2+. The electrochemical impedance spectra (EIS) of both Ni-Co LDH and Zn-NiCo LDHs were obtained to gain a deep understanding of the effect of Zn on the electrochemical property of Zn-Ni-Co LDHs (Figure 7d). All the EIS data were fitted with an equivalent electrical circuit (the inset of Figure 7d) consisting of four electronic components: series resistance (Rs), charge transfer resistance (Rct), and two constant phase elements (CPE1 and CPE2). The semi-circle in the EIS plot represents the charge transfer process of the electrode material while the cross point on X-axis corresponds to the series resistance of the electrode and electrolyte. The linear part of the EIS normally reveals the dynamics of electrolyte diffusion of the electrode material. Table 1 shows the fitted value of Rs and Rct of all the LDH materials. It can be clearly seen that the 5mM-Zn-Ni-Co LDH has a smaller Rs and Rct than the Ni-Co LDH, indicating a higher electric conductivity and faster charge transfer process at the interface of electrode/electrolyte. Nevertheless, both the series resistance and charge transfer resistance of the Zn-Ni-Co LDH materials that contain higher percentage of Zn increase slightly. In particular, the Rct value of the 10 mM-Zn-Ni-Co LDH and 15 mM-Zn-Ni-Co LDH is comparable to the Rct of the control Ni-Co LDH. Table 1. The fitted series resistance (Rs) and charge transfer resistance (Rct) of Ni-Co LDH and Zn-Ni-Co LDH materials.

Resistance

5mM-Zn-

8mM-Zn-

10mM-Zn-

15mM-Zn-

Ni-Co LDH

Ni-Co LDH

Ni-Co LDH

Ni-Co LDH

Ni-Co LDH

19

Rs (Ω cm-2)

2.56

1.89

2.16

1.99

2.60

Rct (Ω cm-2)

0.62

0.31

0.34

0.79

0.55

Based on the characterization of morphology and electrochemical performance of Zn-NiCo with different Zn content, it is clear that the addition of Zn influences the morphology and electrochemical properties of the Ni-Zo LDH including enlarged pore sizes, enhanced electric conductivity. Meanwhile, the electrochemically inert nature of Zn2+ limits its usage. The optimum performance of the Zn-Ni-Co was obtained at 5 mM Zn in the precursor. Since the unsatisfactory cycling stability is the main challenge for transition metal based electrode materials for supercapacitors, we further investigated cycling performance of asprepared Zn-Ni-Co LDH materials with different amounts of Zn. As shown in Figure 8, all the Zn-Ni-Co LDH materials show much higher capacity retention than the Ni-Co LDH after 20,000 cycling tests. Specifically, 5mM-Zn-Ni-Co LDH, 8mM-Zn-Ni-Co LDH, 10mM-ZnNi-Co LDH, 15mM-Zn-Ni-Co LDH, and Ni-Co LDH show 102.7%, 179.3%, 316.5%, 504.9%, and 72.4% capacity retention rate after 20,000 GCD cycles under the current density of 20 A g-1, respectively. It is worth noting that all the samples experienced a capacity enhancement after the initial few hundreds of GCD cycles. Compared to the initial value, the specific capacity of 5mM-Zn-Ni-Co LDH, 8mM-Zn-Ni-Co LDH, 10mM-Zn-Ni-Co LDH, 15mM-Zn-Ni-Co LDH, and Ni-Co LDH was 1.84, 2.64, 4.47, 7.14, and 1.57 times higher after around 2000 cycles, respectively. This is ascribed to the benefit from the activation process which results in improved interfacial contact between the electrode and the electrolyte. After the maximum specific capacity was reached, decrease of the performance in the course of cycling test is noted for all the electrodes. However, the Zn-contained sample showed a much higher retention rate. 5mM-Zn-Ni-Co LDH, 8mM-Zn-Ni-Co LDH, 10mM-Zn-Ni-Co LDH, and 15mM-Zn-Ni-Co LDH demonstrate capacity retention rate of 55.7%, 68.0%, 20

70.8%, and 72.4% after 20, 000 cycles respectively compared to their maximum specific capacity, whereas the Ni-Co LDH control sample only showed 46.0% retention rate. The extraordinary cycling stability of ternary Zn-Ni-Co LDH is comparable or even superior to most

state-of-the-art

Ni-based

materials

reported

previously

such

as

α-

(Ni/Co)(OH)2/graphene//AC (83% capacity retention after 10, 000 cycles under 5 A g-1),[42] Zn-Ni-Co TOH-150 (110.6% retention in 10,000 CV cycles at 100 mV s-1),[29] NiCo-LDHgraphene (80% retention in 10,000 cycles at 6 A g-1),[21] Ni-Zn-Co oxide nanowire arrays (113.3% retention in 10,000 cycles at 15 mA cm-2),[46] NiCo2O4-CNT@DNA (96.2% after 5000 cycles at 5 A g-1),[47] and Ni3S2/CoNi2S4//AC (92.8% after 6000 cycles at 10 A g-1).[48] The above results demonstrate that the introduction of Zn to the Ni-Co LDH can significantly improve the cycling stability of the pristine Ni-Co LDH.

Figure 8. GCD cycling stability comparison of Ni-Co LDH and Zn-Ni-Co LDH materials under the current density of 20 A g-1. In order to get a profound understanding of the effect of Zn incorporation on the cycling stability of the ternary Zn-Ni-Co LDHs electrode, 8mM-Zn-Ni-Co LDH after 20, 000 GCD cycling stability test was characterized comprehensively. The EDS measurement of the sample (Figure S7) reveals that there is no Zn element detected after the long-term stability test. The failure detection of Zn from the LDH electrode material suggests Zn was probably dissolved in the electrolyte during the cycling process. The K-edge XANES spectra of both Ni 21

and Co of the 8mM-Zn-Ni-Co LDH after cycling test (Figure 9a and b) show similar absorption edges of Ni and Co compared with the original sample, indicating that both transition metal cations remained the same oxidation state. The EXAFS spectra (Figure 9c and d) and their fitting results (Table S4) reveal that the coordination number (N) of Co-O (with the bond length: 1.88 Å), Co-TM (bond length: 3.05 Å), Ni-O (bond length: 2.05 Å), Ni-TM (bond length: 3.05 Å) are 4.7, 5.5, 6.2, and 6.7 respectively. Compared to the original 8mM-Zn-Ni-Co LDH, the N of Co-TM, Ni-O, and Ni-TM are reduced while the N of Co-O is increased, suggesting a more relaxed local structure, which is more tolerable for volume change of crystal structure during ion insertion/extraction in the charge/discharge process. The shorten bond length of Co-TM and Ni-TM indicating that Ni and Co have stronger interactions as a result of release of Zn from the crystal structure in the electrochemical cycling process.

Figure 9. Comparison of XANES spectra of Co (a) and Ni (b) of 8mM-Zn-Ni-Co LDH before (named Zn-Ni-Co LDH) and after 20, 000 GCD cycling test (named Zn-Ni-Co LDHcycle). (c and d) Fourier transform (FT) magnitudes of the k2-weighted EXAFS spectra of Co (c) and Ni (d) of 8mM-Zn-Ni-Co LDH after 20, 000 GCD cycling test. 22

Moreover, the SEM image of the cycled 8mM-Zn-Ni-Co LDH (Figure S8) confirms that the nanosheet morphology was well maintained after the cycling test. The TEM images of the Zn-contained LDH material (Figure 10a and S9) after the cycling test further reveals that the nanosheets possess uniform nanopores on the surface. These nanopores are probably formed as a result of Zn release from the LDH material during the charge/discharge test. No obvious lattice fringes were observed in the high resolution TEM image, revealing an amorphous structure. The XRD patterns of the Ni-Co LDH and the 8mM-Zn-Ni-Co LDH after the 20,000 GCD cycling test in Figure 10b show the (003) facet of the 8mM-Zn-Ni-Co LDH is greatly reduced compared to the original sample. Two extra weak peaks in the XRD at 35.1° and 36.4° are also observed in the cycled 8mM-Zn-Ni-Co LDH products. For the Ni-Co LDH after cycling test, the characteristic peaks belong to LDH structure are also much reduced and only a weak peak of (003) facet can be identified. In addition, multiple new strong peaks were detected. The XRD results indicate that the layered structure of the Ni-Co LDH material with and without Zn was nearly destroyed after the cycling stability measurement. Nevertheless the 8mM-Zn-Ni-Co LDH experienced less crystal phase transformation and tend to remain an amorphous structure, which makes the material better tolerant against structure change at a large current charge/discharge process.[23] This in turn contributes to the much higher cycling stability of the ternary LDH.

Figure 10. (a) TEM images of 8mM-Zn-Ni-Co LDH after 20, 000 GCD cycling test. (b) XRD patterns of bare CFC, Ni-Co LDH and 8mM-Zn-Ni-Co LDH after 20,000 GCD cycling test. 23

The red hearts mean the (003) facets of LDH structure while the blue diamonds represent the new crystal phase formed during the cycling test. Based on the above results, we believe that the outstanding cycling stability of Zn-Ni-Co LDH should be mainly attributed to the gradually self-etching, sacrificial loss of Zn cation during the charge/discharge process. As illustrated in Figure 11, the self-etching strategy has distinctive benefits: 1) It can protect the nanosheet morphology from fierce phase change during GCD test at large current density, stabilizing material structure; 2) the etching of Zn produces massive nanopores, which not only increases the surface area but also exposes more Ni and Co active sites for surface redox reaction with diffused electrolyte ions, resulting in a higher specific capacity. 3) The transformation of the ternary material into an amorphous structure instead of forming electrochemically less active phases makes it more tolerable against

the

expansion/shrinkage

of

the

material

structure

during

large

current

charge/discharge process, leading to much higher cycling stability than Ni-Co LDH.

Figure 11. Schematic illustration of the self-etching process of Zn-Ni-Co LDH materials during GCD cycling test.

To further evaluate the capacitive property of Zn-Ni-Co LDHs for practical applications, we assembled a hybrid supercapacitor (HSC) based on a two-electrode system using 8mMZn-Ni-Co LDH as the positive electrode and activated carbon as the negative electrode to take advantage of the high energy density of the LDH electrode material and high power density of 24

the activated carbon. The activated carbon-based electrode (Figure S10) exhibited a typical double-layer capacitive behaviour with considerable charge storage capability (specific capacitance: 161.9 F g-1 at 1 A g-1). Figure 12a shows the CV curves of as-prepared HSC under different voltage windows. It is clear that the device has a safety voltage window until 1.6 V without decomposition of water. Therefore, it was set as the device working voltage window in the performance evaluation below. The quasi-rectangular CV shape shown in Figure 12b under all scanning rates demonstrates that the charge storage mechanism of the cell is dominated by capacitive behaviour. The mild CV shape distortion at a high scanning rate of 200 mV s-1 suggests a promising rate capability. The GCD curves of the HSC under different current density (Figure 12c) exhibit a quasi-triangle charge/discharge profile, suggesting a capacitive behaviour which is in good accordance with the CV results. Based on the GCD curves, the specific capacitance of the HSC at different discharge current density was calculated and is presented in Figure 12d. The maximum specific capacitance of asprepared HSC reaches 113.4 F g-1 at 0.5 A g-1. When the current density increases 40 times (20 A g-1), the specific capacitance of 42.5 F g-1 was obtained, suggesting its considerable electric storage ability and good rate capability. The Ragone plot of as-prepared HSC (Figure 12f) based on the 8mM-Zn-Ni-Co LDH reveals the cell exhibits the highest energy density of 40.3 Wh kg-1 (at the power density of 403.3 W kg-1) and the highest power density of 16.1 kW kg-1 (at the energy density of 15.1 Wh kg-1). This charge storage capability is much higher than commercial electric double layer capacitors and is comparable to many other transition metal based SCs reported in the literature.[49-52] Note that the cycling stability of the material in this work is superior to nearly all previously reported transition metal based SCs electrode. The deconvoluted EIS spectrum of the HSC shows it has a series resistance (Rs) of 2.68 Ω cm-2 and a charge transfer resistance (Rct) of 2.66 Ω cm-2 (Figure 12e). The nearly vertical orientational EIS curve at low frequency indicates fast electrolyte diffusion and ideal capacitive behaviours. To further 25

demonstrate the practical application of the material, two coin cells (CR2032) based on the 8mM-Zn-Ni-Co LDH and activated carbon were fabricated, which could easily light up a LED light successfully after being charged at 1 A g-1 (inset in Figure 12f).

Figure 12. (a) CV curves (with different voltage windows at the scanning rate of 20 mV s-1), (b) CV curves under the scanning rate of 2 - 200 mV s-1, (c) GCD curves (at the discharge current density of 0.5 - 20 A g-1), (d) specific capacitance results (at the discharge current density of 0.5 - 20 A g-1), (e) EIS spectrum, and (f) Ragone plot of the HSC using 8mM-ZnNi-Co LDH as positive electrode and activated carbon as negative electrode. The inset in (e) is the equivalent electrical circuit used for plot fitting. The inset in (f) is the LED light powered by two coin cells in series based on 8mM-Zn-Ni-Co LDH. (g) Cycling stability results of the HSC under the discharge current density of 10 A g-1. One of the crucial properties of a supercapacitor for practical application is the long-term cycling stability. Figure 12g depicts that the specific capacitance of as-obtained HSC experienced a relatively fast capacity decay in the first 800 GCD cycles which dropped to 88.6% of its initial specific capacitance at the current density of 10 A g-1. After this, the HSC 26

became very stable and has a negligible drop in its specific capacitance in the rest 20,100 charge/discharge cycling test. Moreover, the HSC also exhibits a very high coulombic efficiency of over 98%, proving its excellent charge transfer efficiency. This significantly high cycling stability outperforms the majority of SCs based on transition metal based electrode materials reported previously. Table 2 compares the electric energy storage ability of the Zn-Ni-Co LDH material to other representative Ni-based electrode materials for SCs reported previously. It is clear that the HSC based on 8mM-Zn-Ni-Co LDH in this work possesses a much superior energy density and longer cycling stability, demonstrating the great potential of the ternary Zn-Ni-Co LDH materials in this work for practical applications. Table 2. The supercapacitive performance comparison of different supercapacitors based on Ni-based active materials. Voltage window (V)

Specific

Energy density

Power density

Capacitance

capacitance

(E, Wh kg-1)

(P, W kg-1)

retention rate

CC/ZnO@C@NiO CSNAs//commercial graphene

1.5

--

35.7 at the P of 380.9

2704.2 at the E of 16.0

87.5% after 10, 000 cycles at 4 A g-1

[53]

NixCoyMozO//activated carbon

1.8

126 mF cm-2

22.02 at the P of 3.5

--

--

[49]

ZNCO//AC

1.5

113.9 F g-1 at 1 A g-1

35.6 at the P of 187.6

938.1 at the E of 19.1

94 % after 3000 cycles at 3 A g-1

[52]

NiCoP/CNF// NiCoP/CNF

1.6

161 C g-1 at 1.5 A g-1

36 at the P of 1200

4000 at the E of 26

~75% after 25,000 cycles at 5 A g-1

[54]

CC/NiCo2O4-N@ NiO//graphene

1.5

108.2 F g-1 at 0.5 A g-1

33.8 at the P of 375

6000 at the E of 28

95.2 % after 10,000 cycles at 4 A g-1

[55]

CoMoO4-NiMoO4 //active carbon

1.6

105 F g-1 at 0.5 A g-1

33 at the P of 375

6000 at the E of 16.3

91.8 % after 1000 cycles at 50 mA cm-2

[56]

AC//CQDs/ NiCo2O4

1.5

88.9 F g-1 at 0.5 A g-1

27.8 at the P of 128

10240 at the E of 13.1

101.9 % after 5000 cycles at 3 A g-1

[57]

Supercapacitors

27

Ref.

NiCo2O4@NiCo2S4@PPy12//AC

8mM-Zn-Ni-Co LDH//AC

1.6

--

--

1.6

113.4 F g-1 at 0.5 A g-1

40.3 at the P of 403.3

5945.1 at the E

75.9% after

of 23.75

10,000 cycles at 10 mA cm-2

16100 at the E of 15.1

80.1% after 20,100 cycles at 10 A g-1

[58]

This work

4. Conclusion In summary, we have developed a facile method to synthesize highly stable Zn-Ni-Co LDH materials as the electrode for supercapacitors. The materials demonstrated high specific capacity, ultralong cycling stability and excellent capacitance retention over 20, 000 cycles. The in-depth investigation reveals that the electrochemically inert Zn plays a critical role in stabilizing the nanosheet microstructure, preventing new crystal phase transformation, and exposing more Ni and Co active sites for the electrochemical reactions during the electrochemical charge/discharge process. The HSC assembled by Zn-Ni-Co LDH and activated carbon demonstrated high charge storage ability with the highest energy density of 40.3 Wh kg-1 and the highest power density of 16.1 kW kg-1, as well as ultra-high cycling stability over 20, 000 cycles. The method of using electrochemically inert and low-cost selfsacrificial elements as a buffer to stabilize the structure of Ni-Co LDH electrode materials for SCs sheds a light for the fabrication of high performances SCs by using transition metal-based materials. Supporting Information. The following files are available free of charge: The SEM, TEM, ICP, and EDS results of asprepared samples. The electrochemical properties of the materials. Notes The authors declare no competing financial interest. Acknowledgment

28

K.-J.C is thankful for the support from the National Natural Science Foundation of China (No. 21805227) and Fundamental Research Funds for the Central Universities (Grant No. 3102017jc01001). T. W thank the support from the National Natural Science Foundation of China (No. 21905229) and the Natural Science Basic Research Plan in Shaanxi Province of China (No. 2019JQ627). H.W and T. W thank the financial support of Queensland University of Technology (QUT) through QUT-GDSTC strategic grant. We would like to thank the Central Analytical Research Facility (CARF) operated by the Institute for Future Environments, QUT and the Analytical & Testing Center of Northwestern Polytechnical University (NPU) for collecting the characterization data for us. Access to CARF is supported by generous funding from the Science and Engineering Faculty (QUT). This research was undertaken on the XAS beamline at the Australian Synchrotron, part of ANSTO. We acknowledge the travel funding provided by the Australian Synchrotron. References [1] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater., 7 (2008) 845854. [2] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin?, Science, 343 (2014) 1210-1211. [3] T. Wang, H.C. Chen, F. Yu, X.S. Zhao, H. Wang, Boosting the cycling stability of transition metal compounds-based supercapacitors, Energy Storage Mater., 16 (2019) 545-573. [4] H. Ma, D. Kong, Y. Xu, X. Xie, Y. Tao, Z. Xiao, W. Lv, H.D. Jang, J. Huang, Q.H. Yang, Disassembly-reassembly approach to RuO2/graphene composites for ultrahigh volumetric capacitance supercapacitor, Small, 13 (2017) 1701026. [5] S. Zhu, L. Li, J. Liu, H. Wang, T. Wang, Y. Zhang, L. Zhang, R.S. Ruoff, F. Dong, Structural directed growth of ultrathin parallel birnessite on beta-MnO2 for high-performance asymmetric supercapacitors, ACS Nano, 12 (2018) 1033-1042.

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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: