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A Facile Green Route to Ni/Co Oxide Nanoparticle Embedded 3D Graphitic Carbon Nanosheets for High Performance Hybrid Supercapacitor Device Cuili Xiang, Yin Liu, Ying Yin, Pengru Huang, Yongjin Zou, Marcus Fehse, Zhe She, Fen Xu, Dipanjan Banerjee, Daniel Hermida-Merino, Alessandro Longo, Heinz-Bernhard Kraatz, Dermot F. Brougham, Bing WU, and Lixian Sun ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00202 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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ACS Applied Energy Materials

A Facile Green Route to Ni/Co Oxide Nanoparticle Embedded 3D Graphitic Carbon Nanosheets for High Performance Hybrid Supercapacitor Device Cuili Xiang,† Yin Liu,† Ying Yin,† Pengru Huang,† Yongjin Zou,*,†,‡ Marcus Fehse,,§, Zhe She‡, Fen Xu†, Dipanjan Banerjee,║, Daniel Hermida Merino, Alessandro Longo, Heinz-Bernhard Kraatz‡, Dermot F. Brougham∏, Bing Wu*,‡,, Lixian Sun†

†Guangxi

Key Laboratory of Information Materials, Guilin University of Electronic Technology,

Guilin 541004, P.R. China ‡Department

of Physical and Environmental Science, University of Toronto, 1265 Military Trail,

Toronto, M1C 1A4, Canada, Dutch-Belgian

Beamline (DUBBLE), ESRF- The European Synchrotron Radiation Facility, CS

40220, 38043 Grenoble Cedex 9, France §Faculty

of Applied Sciences, Delft University of Technology, Mekelweg 5, 2628 CD Delft, the

Netherlands ║Department ∏School

of Chemistry, KU Leuven, Celestijnenlaan 200F box 2404, 3001 Leuven, Belgium

of Chemistry, University College Dublin, Belfield, Dublin 4, D04 V1W8, Ireland

1 ACS Paragon Plus Environment

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ABSTRACT: The demand for energy storage systems with superior performance has led to the creation of hybrid supercapacitor device. With proper designs, the hybrid supercapacitive materials can achieve high performance while reducing the overall cost. Herein, a novel method is developed for preparing three-dimensional hierarchical graphitic carbon nanocomposites with highly dispersed mixed Co-Ni oxide nanoparticles (Co–Ni–O/3DG) by a facile one-pot process involving carbonization and subsequent oxidation of metal ion doped biopolymer precursors. The mixed metal nanoparticles produced during carbonization enabled a top-down preparation of 3D graphitic carbon nanosheets. The nanocomposites were fully characterized and showed excellent electrochemical performance supported by the DFT calculation. Specific capacitance of 1586 F∙g–1 was achieved (current density 1.0 A∙g–1), with capacitance retention of 94.5% after 10,000 cycles demonstrating exceptional cycling stability. In an asymmetric full-cell system using a Co–Ni– O/3DG positive electrode, high energy densities of 32.8 to 54.7 Wh kg–1 associated with very high power densities of 11358 W kg–1 to 748.6 W kg–1 were obtained, comparable to the most advanced contemporary supercapacitive materials while also possesses an improved cyclability as well as using bio-sourced staring materials, underlining its potential application in hybrid supercapacitor devices. KEYWORDS: graphitic carbon nanosheets; hybrid supercapacitor; cobalt oxide; nickel oxide; pseudocapacitance


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INTRODUCTION Rising global demands for new energy storage solutions has led to a wave of innovations in the materials and design of electrochemical energy storage devices. Hybrid supercapacitors (HSCs), consisting of carbon and pseudocapacitive materials, have attracted significant attention.16

These hybrid devices possess battery-level energy density together with a long cycle life and

short charging time as supercapacitors. Impressive progress has been achieved in the fundamental research of these hybrid devices in recent years.7-16 A milestone was the implementation/discovery of pseudocapacitive storage into carbon-based HSC systems which may provide a route to greatly enhanced electrochemical storage performance.17-20 Carbon-based materials doped with conducting polymers, transition metals, or heteroatoms which have multiple accessible oxidation states, have all demonstrated promising pseudocapacitance with an increase in capacitance on doping of typically of an order of magnitude, as compared to the undoped analogues.21,

22

However, poor conductivity and volume changes

during charge–discharge cycles limit their applications. A fully controlled synthesis of nanocomposite utilizing both electric double-layer capacitors (EDLC) and pseudocapacitor components would provide a new generation of hybrid supercapacitors holding the prospect of enhanced electrochemical performance. However, doping porous carbonaceous materials with pseudocapacitive components in a controlled manner remains a significant challenge.23, 24 Graphene is a layered graphitic material with extraordinary properties due to the high electronic charge mobility tailored by n- or p-type doping. The high electrochemical performance of this layered graphitic material strongly relies on the structural properties of the material. These materials can have a large number of applications including energy storage materials given that the irreversible aggregation of graphene nanosheets can be inhibited.25 In this regard, the formation



of three-dimensional (3D) graphitic networks is aspired as it can prevent the stacking between neighboring layers, thereby ensuring a large electrode-electrolyte interface that effectively reduce the charge-transfer resistance.26 Multiple synthetic routes to 3D graphitic carbon based electrodes for energy storage materials have been reported in the literature.27-29 Through procedures like templated chemical vapor deposition or controlled self-assembly, 3D graphitic carbon with hierarchical porous structures have been prepared in the form of sponges,30-32 aerogels,33, 34 frameworks35, 36 and hydrogels37-39. In other studies, spacers such as carbon nanotubes,40-42 metal oxides,27 and conducting polymers28, 29

were introduced to increase the inter-planar spacing and render both sides of the graphene

nanosheets accessible for charge storage. However, most of these synthesis routes are rather complex and expensive, often encompassing the preparation of graphene sheets followed by multiple modifications and processing steps. Furthermore, a common weakness in these materials is that the self-assembled materials are mostly derived from chemically exfoliated graphene, which has a partly restored graphitic structure and a relatively large O/C atomic ratio, resulting in poor conductivity and limited structural stability.25 One strategy to overcome these drawbacks is to produce conductive graphene materials from molecular carbon precursors by transition-metalbased catalytic routes such as the use of Ni substrates by templated chemical vapor deposition method.43 Compared to chemical exfoliation of graphite, the catalytic routes have distinct advantages such as increasing the electrochemical stability and conductivity of the final product, as chemically derived graphene sheets and inter-sheets have minimal contact resistance.43 However, removing the catalytic substrate from the 3D graphitic carbon is time-consuming and usually leads to loss of the 3D structure. Herein, a simple, scalable approach was reported to prepare metal-oxide nanoparticle-


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embedded carbon nanosheets HSC materials. A 3D carbon nanosheets doped with highly dispersed Co-Ni nanoparticles was synthesized from environmental benign biopolymers persimmon tannin (PT) (Figure S1a) and chitosan (Figure S1b).44 Characterization of this composite material demonstrates that during the carbonization process, the doped chelating complex decomposed into a metal nanoparticle embedded carbon form, while these newly-formed nanoparticles simultaneously catalyzed the formation of 3D graphitic carbon with significant reduction in restacking. Subsequent treatment with H2O2 oxidized the metal nanoparticles imparting a very high pseudocapacitance to the nanocomposite arising from the highly dispersed metal oxide NPs (M– O). Furthermore, due to the beneficial combined effects of the porous carbon nanosheets structure45 and pseudocapacitive properties of mixed metal-oxide system46, the as-prepared CoNi-O/3DG has shown remarkable energy and power density, while retaining excellent cycling stability and rate performance.

MATERIALS AND METHODS Materials preparation. Persimmon tannin (extracts of astringent persimmon) was purchased from Huikun Company of Agricultural Products (China); cobalt (II) nitrate hexahydrate and chitosan extracted from crab shells (85% deacetylation, 200,000 Da) were obtained from commercial source and used as received. Reduced graphene oxide (rGO) was synthesized using Hummers’ method.47 Since persimmon tannin (PT) is water-soluble, it was cured before use. To cure the PT, 10 g of material was initially dissolved in 100 mL of water. Afterwards, the mixture was added with chitosan (5 g), and heated to 60 ºC under stirring for 1 h. The solution was later added with 25 mL of glutaraldehyde. This mixed solution was further reacted for 6 h under stirring



at 60 °C. Finally, the hydrogel was dried and milled into fine powder for use in the study; the mixed polymer powder is hereafter denoted as PTC. To prepare the precursor for carbonization, Co(NO3)2 (2 mmol), of Ni(NO3)2 (2 mmol), and PTC (1 g) were added to 100 mL of water. After 6 h of stirring to reach adsorption equilibrium, the suspension was filtered and dried at 80 ºC. The dried powder was then calcined at 800 ºC under a nitrogen atmosphere and for 6 h. The as-synthesized sample was further treated with a 30 wt% H2O2 solution for 12 h; hereafter it is denoted as Co–Ni–O/3DG. For comparison, we prepared two separate batches of the PTC biopolymer; Co ions alone were adsorbed on one batch of PTC, while only Ni ions (at the same metal concentration as for the mixed metal material) were adsorbed on the other. The carbonized samples of these two biopolymer complexes are denoted as CoO/3DG and NiO/3DG. Bare PTC was also carbonized under identical conditions, but was not treated with H2O2, this sample is hereafter denoted as PTC–C. Characterization of prepared carbon. The instrumental information can be found in our previous work5 including Fourier-transform infrared (FTIR) spectrometer, Thermogravimetric (TG) analysis, Brunauer−Emmett−Teller (BET) measurements, Powder X-ray diffraction (XRD), Field-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), select-area electron diffraction (SAED) and Raman spectra. The average conductivity of the samples was measured through the four-probe method with a Keithley 2602 digital electrometer. Before testing, the powder was pressed into pellet with a diameter of 0.5 cm. Each sample was measured for five times from different position. The X-ray absorption near-edge structure (XANES) analyses and the Extended X-ray Absorption Fine Structure (EXAFS) spectra of Co and Ni K edges (7709 eV and 8333 eV respectively) were collected for each sample at the Dutch-Belgian Beamline (DUBBLE, BM26A)


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48

at the European Synchrotron Radiation Facility (ESRF). The detailed information can be found

in our previous work.49, 50 Electrochemical measurements. For the electrochemical tests, a three-electrode system was set up and connected to a potentiostat (IM6e, Zahner-Elektrik, Germany). Platinum foil was used as the counter and Ag/AgCl electrode was used as reference electrode. The working electrodes were prepared according our previous paper17 except replacing the active materials with the material prepared in this work. The active electrode was prepared as a uniform film with a typical areal mass of approximately 4 mg/cm2. All the electrochemical tests were carried out in a 6 M KOH electrolyte solution at room temperature (25 ± 1 ºC). An asymmetric two-electrode hybrid cell system was assembled by using Co–Ni–O/3DG as the positive electrode. RGO was employed as the negative electrode; hereafter, this cell is denoted as (Co–Ni–O/3DG)//rGO. The energy density (E) and power density (P) were calculated according to Eqs. (1) and (2), respectively: 1 𝐸 = ( 𝐶s∆𝑉2)/3.6 2

𝑃=

3600 × 𝐸 ∆𝑡

(1)

(2)

where 𝐸 is in units of Wh kg−1 and 𝑃 is in units of W kg−1, 𝐶𝑠 (F g−1) is the specific capacitance, ∆𝑉 is the potential window, and ∆𝑡 (s) is the discharging time. DFT calculations. Spin-polarized Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP) code with the projector augmented wave (PAW) pseudopotentials.51, 52 The configurations of the valence electrons of Co, Ni, H and O were (3d84s1), (3d94s1), (s1) and (2s2p4), respectively. Total energies and electron densities were



computed using the DFT+U approach of Dudarev et al., in which a Hubbard U-like term describing the onsite Coulomb interactions is added to the PBE functional.53, 54 We used an effective U-J value of 3.3 eV for Co atoms and 3.8 eV for Ni atoms. The energy cut-off for the plane-wave basis set was 500 eV. A (15× 15 × 3) Monkhorst–Pack k-point grid was used for the Brillouin-zone sampling. The unit cell of CoOOH containing two formula units was optimized to be (a=b=2.895 Å; c=8.781 Å). Similar cells were used for the calculations of NiOOH and CoNi(OOH)2 with lattice constants of (a=b=2.2.973 Å; c=8.732 Å), and (a=b=2.939 Å; c=8.794 Å), respectively. All considered structures were fully optimized with the force on each atom set to CoO/3DG > NiO/3DG > PTC–C, indicating that the Co–Ni–O/3DG electrode has a larger capacitance compared to single metal oxide electrode materials. While theoretically extremely high capacities can be obtained for CoO and NiO, experimental capacities are usually much lower and strongly depend on doping and morphology as shown in Table S2 and S3. The capacitances calculated from the galvanostatic charge–discharge (GCD) curves in Figure 6b were found to be 1538, 1028, 850, and 210 F g–1 at the current density of 1 A g–1 for Co–Ni–O/3DG, CoO/3DG, NiO/3DG, and PTC–C, respectively. Analogous to CV measurements, the electrode doped with the metal oxide composite showed higher capacitance than the electrodes doped with a single metal oxide. This superior storage performance of Co–Ni–O/3DG electrode as compared to the single metal oxide equivalents may arise from the mixed dual-transition metal redox reaction. Furthermore the 3D interconnected carbon nanosheets which shows highest degree of graphitization for the metal oxide composite provides an electronic wiring for fast charge.



Moreover, the intimate confinement of the two transition-metal oxides in the composite is expected to yield a beneficial electron coupling effect, as demonstrated in the DFT calculation. This adequate tailoring of electronic structure in the metal oxide composite system results in unique properties and superior performance compared to single metal oxide system.73 The hetero-nanostructure can also effectively decrease the surface energy of the active nanomaterials that may lead to a high level of electrochemical reversibility.74

Figure 6. (a) CV curves of Co–Ni–O/3DG, CoO/3DG, NiO/3DG, and PTC–C electrodes at scan rates of 5 mV s–1. (b) GCD curves of Co–Ni–O/3DG, CoO/3DG, NiO/3DG and PTC–C 20 ACS Paragon Plus Environment

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electrodes at a current density of 1 A g–1. (c) Nyquist plots obtained from electrochemical impedance spectroscopy (EIS) of Co–Ni–O/3DG, CoO/3DG, NiO/3DG and PTC–C electrodes; (d) CV curves of Co–Ni–O/3DG at various scan rates. (e) GCD curves of Co–Ni–O/3DG electrode at current densities ranging from 1 to 15 A g–1. (f) Plots of current density vs. specific capacitance of Co–Ni–O/3DG, obtained from GCD curves.

Figure 6c shows the results of the impedance study of the composite materials. The corresponding equivalent circuit of the electrode is shown in Figure S9 in SI, which consists of an electrolyte resistance (Rs) (the intercept at the real axis), a double layer capacitance (Cd), a Warburg impedance (Zw), and a charge-transfer resistance (Rct) (radius of the high frequency semicircle)75. The fitted Rct values for Co–Ni–O/3DG, CoO/3DG, NiO/3DG, and PTC–C electrodes were 0.2, 0.4, 0.5, and 0.7 Ω, respectively. Co–Ni– O/3DG shows lowest Rct, indicating a more efficient electron transfer. The Nyquist plots are nearly vertical for the Co–Ni–O/3DG electrode at low frequencies, revealing that the carbon exhibited ideal capacitive behavior and improved ion diffusion properties in the electrode. In any case it is clearly that Co–Ni–O/3DG electrode possesses higher capacitance and conductivity than those formed using single metal oxide. This enhanced conductivity of Co–Ni–O/3DG nanocomposite can be attributed to the electrical wiring of the 3DG network. The intimate contact between Co–Ni–O and 3DG facilitates the electron flow, whereas the channels formed by the 3D open scaffold structure provide short diffusion paths for rapid ionic transport. Compared with single phase oxide, metal oxides with complex hetero-nanostructure possesses several competitive advantages such as rich accessible electroactive sites, superior electronic conduction, higher degree of



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graphitization as well as beneficial electronic structural properties, all of which improve its electrochemical performance. CV curves of the Co–Ni–O/3DG electrode were shown in Figure 6d. Oxidation and reduction peaks appear in the CV curves, and the peak currents increased with increasing scan rate. The shape of the CV curve is largely preserved upon 30 fold increase of scan rate and only a minor increase in overpotential (c. 1mV) was observed. These findings prove that Co–Ni–O/3DG has excellent transport properties which are the foundation for the highrate capability and hence elevated power density. This was further confirmed by the capacitance calculated from the GCD curves of Co–Ni–O/3DG (shown in Figure 6e) which decreases by only 40% from 1586 to 940 F g–1 upon a 15-fold increase in current density from 1 to 15 A g–1 (Figure 6f). This demonstrates an excellent rate capability along with a high specific capacitance outperforming many previously reported Co- and/or Nicompound-based electrodes, as shown in Table 1. Table 1 Comparison of specific capacitance of Co–Ni–O/3DG with reported data. Electrode material

Current

Specific

Electrolyte

Ref.

(loading mass)

density

capacitance

(A g–1)

(F g–1)

Ni/NiO/RGO (3 mg)

2

1027.27

6 M KOH

76

D-NiCo2O4 (0.8 mg)

5

667

2 M KOH

77

Co3O4 @MnO2 (4.3 mg)

2.67

480

1 MLiOH

78

NixCo3-xO4 (3 mg)

1

797

6 M KOH

79

Co3O4 tube (2 mg)

0.5

1498

6 M KOH

80

CoMoO4@Co3O4 (8 mg)

0.6

1168

2 M KOH

81

Co3O4@SrGO (0.12-2.27 mg)

1

406

1 M KOH

82

NCA/Co3O4 (4 mg)

1

616

6 M KOH

83

Co3O4@PPy@MnO2 (0.7 mg)

0.5

782

1 M KOH

84

MoS2–NiO (1 mg)

1

1080.6

6 M KOH

85

Co–Ni–O/3DG (4 mg)

1

1586

6 M KOH

This work


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Finally, the cycling stability of the hybrid Co–Ni–O/3DG electrodes was tested at 1 A g–1 over 10000 cycles as shown in Figure S10 in SI, in which this hybrid electrode can retain 93.6% of its initial capacitance even after 10000 cycles, demonstrating outstanding long-term stability. Based on these results, we suggest that the graphitic carbon nanosheets provide a good matrix for the loading of the metal oxides. Besides preventing nanoparticles’ aggregation during extended use, this matrix also can preserve electrode integrity to mitigate the detrimental effects of volume changes, thus significantly enhancing the electrochemical performance stability of this nanocomposite.

Performance in asymmetric two-electrode hybrid supercapacitor cells. In order to explore the potential application of the Co–Ni–O/3DG in a complete cell and compare our material to previously reported devices, an asymmetric hybrid supercapacitor cell was assembled. The optimal mass loading for the Co–Ni–O/3DG and rGO electrodes was estimated to be to be 32.1 and 8 mg, respectively based on CV curves (Figure S11) and the equation reported elsewhere17. The GCD curves of the (Co–Ni–O/3DG)//rGO asymmetric cell have typical shapes of pseudocapacitor in different potential windows from 1 to 1.5 V (Figure 7a). Since no significant polarization effect was observed in the asymmetric hybrid supercapacitor cell up to 1.5 V, this value was chosen as upper voltage limit for further electrochemical investigation. The current density evolved within the above mentioned potential window at different scanning rates ranging from 5 to 150 mV s–1, are shown in Figure 7b. The general shape of the CV curves was maintained even at high scan rate of 150 mV s–1, indicating stable electrochemical cycling under high current load.



a

01.0 V 01.1 V 01.2 V 01.3 V 01.4 V 01.5 V

0.9 0.6

0

c

Voltage (V)

1.5

1 A g 2 A g 5 A g 8 A g 10 A g 15 A g

1.2 0.9 0.6 0.3 0.0

50 40

0

100 200 300 400 500 600 Time (s)

e

30 20

This work CoMoO4@Co3O4/OMEP//AC MnO/NCA//Fe2O3/NCA

10

b

5 mV s1 10 mV s1 20 mV s1 50 mV s1 100 mV s1 150 mV s1

20 0

-40

100 200 300 400 500 600 Time (s)

Specific capacitance (F g1)

0.0

40

-20

0.3

2D-CMO//CNT CMMN-4//AC GF–CNT@NiO//G–CNT, CoO/ACT//ACT/graphene CoOx//graphene

100 1000 10000 Power density (W kg1)

Capacitance retention (%)

Voltage (V)

1.2

Current density (A g1)

1.5

Energy density (Wh kg1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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180

0.0

0.4 0.8 1.2 Potential (V)

1.6

d

160 140 120 100 0 100

3 6 9 12 15 Current density (A g1)

f

90 80 70 60 50

0

1500 3000 4500 Number of cycles

Figure 7. (a) GCD curves of the (Co–Ni–O/3DG)//rGO asymmetric hybrid supercapacitor cell in different potential ranges at current density of 1 A g–1. (b) CV curves of the (Co–Ni– O/3DG)//rGO asymmetric hybrid supercapacitor cell at different scan rates. (c) GCD curves of the (Co–Ni–O/3DG)//rGO asymmetric hybrid supercapacitor cell at current densities from 1 to 15 A g–1. (d) Specific capacitance as a function of current density of the asymmetric cell. (e) Ragone plot of (Co–Ni–O/3DG)//rGO asymmetric hybrid supercapacitor cell and comparison with the reported data. (f) Cycling stability of (Co–Ni–O/3DG)//rGO asymmetric hybrid supercapacitor cell at current density of 1 A g–1. 24 ACS Paragon Plus Environment

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Figure 7c shows the GCD curves of (Co–Ni–O/3DG)//rGO asymmetric cell at different current rates. It exhibits a high specific capacitance of 175.2 F g−1 (1 A g−1) (Figure 7d). The Ragone plot of (Co–Ni–O/3DG)//rGO asymmetric hybrid supercapacitor cell was shown in Figure 7e. The nanocomposite mixed metal electrode delivers the highest energy density of 54.7 Wh kg–1 at a corresponding power density of 748.6 W kg–1, while it can stilll retain an elevated energy density of 32.8 Wh kg–1 at an impressive 11358 W kg–1. Such high values of power density are far outperforming those of previously reported two-cell supercapacitors67,

74, 80, 81, 83.

Moreover, the (Co–Ni–O/3DG)//rGO

asymmetric cell demonstrated outstanding cycling stability. A gradual capacitance drop is observed, resulting in 86.3% of the original value retained after 5000 cycles (Figure 7f). This impressive electrochemical performances is again attributed to the open hybrid nanostructure of the Co–Ni–O/3DG electrode which provides an excellent electrolyteelectrode interface, enhanced the conductivity based on the 3D carbon electrical wiring network, and optimized electronic structure tailoring.

CONCLUSIONS In summary, three-dimensional graphitic carbon nanosheets embedded with highly dispersed Co–Ni oxides were obtained in a simple process from homogeneous biopolymer–metal complex monoliths. This approach represents a significant step forward in producing carbon-based batterysupercapacitor-hybrid materials using green resources. The resulting Co–Ni–O/3DG nanocomposite exhibits excellent electrochemical performance. A high capacitance of 1586 F g-1 was obtained at a current density of 1 A g-1 as well as remarkable cycling stability over 10000 cycles. By combining the Co–Ni–O/3DG positive electrode with a rGO



negative electrode, the resulting asymmetric hybrid supercapacitor delivered an energy density of 32.8 to 54.7 Wh kg–1 associated with very high power densities of 11358 to 748.6 W kg–1, which is beyond the recent state of the art and represents an important advancement in the HSC research field. Furthermore, the asymmetric hybrid supercapacitor showed a very stable cycling over thousands of cycles. Since the preparation of Co–Ni–O/3DG is simple, inexpensive and easy to implement in an industrial setup, the material has great potential to fill demands in future electrochemical storage applications.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX XXX XXX-XXX [Figure S1 depicts the structure of PT and C; Figure S2 TG curve; Figure S3-S4 Fourier transformed EXAFS; Figure S5-S6 SEM/TEM images; Figure S7 XPS Spectrum; Figure S8 DFT model structure; Figure S9 Circuit illustration; Figure S10 Cycle stability test; Figure S11 additional CV curves; Table S1 lattice calculation; Table S2-S3 literature capacitance value comparison (PDF)] AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] ORCID Cuili Xiang: 0000-0001-8225-6568 Yongjin Zou: 0000-0002-9012-2639 Marcus Fehse: 0000-0001-8650-6974 Notes The authors declare no competing financial interest. ACKNOWLEGDMENTS The authors gratefully express their thanks to National Science Foundation of China (Grant No. 27 ACS Paragon Plus Environment


51861004), Guangxi Natural Science Foundation (Grant No. 2016GXNSFFA380012, 2017AD23029 and 2017GXNSFDA198018), the EU Horizon2020 Marie Curie Cofund project (Grant No. 713279) and NSERC (RGPIN-2016-06122), as well as the synchrotron beamtime granted on DUBBLE through the ESRF proposal (MA 4396).


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