Polyoxometalate-Incorporated Metallacalixarene@Graphene

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Polyoxometalate-Incorporated Metallacalixarene@Graphene Composite Electrodes for High-Performance Supercapacitor Yan Hou, Dongfeng Chai, Bonan Li, Haijun Pang, Huiyuan Ma, Xinming Wang, and Lichao Tan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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ACS Applied Materials & Interfaces

Polyoxometalate-Incorporated Metallacalixarene@Graphene Composite Electrodes for High-Performance Supercapacitor Yan Hou, Dongfeng Chai, Bonan Li, Haijun Pang,* Huiyuan Ma,* Xinming Wang and Lichao Tan

School of Materials Science and Engineering, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin, 150040, P. R. China

KEYWORDS: polyoxometalate; metallacalix[6]arene framework; crystal structures; supercapacitor; graphene oxide

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT

Composites

of

polyoxometalate

(POM)/

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metallacalixarene/

graphene-based electrode materials not only integrate the superiority of the individual components perfectly, but also ameliorate the demerit to some extent, providing a promising route to approach high performance supercapacitors. Herein, firstly, we report the preparations, structures and electrochemical performance of two fascinating polyoxometalate

(POM)-incorporated

[Ag5(C2H2N3)6][H5  SiMo12O40]

(1)

and

metallacalixarene

compounds

[Ag5(C2H2N3)6][H5  SiW12O40]

(2),

(C2H2N3 = 1H-1,2,4-triazole). Single-crystal X-ray diffraction analyses illustrated that both 1 and 2 possess intriguing POM sandwiched metallacalix[6]arene frameworks. Nevertheless, our investigations, including the electrochemical cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) reveal that the oxidation ability of the Keggin ions is a primary effect in electrochemical performance of these POM-incorporated metallacalixarene compounds. Namely, the electrode containing Mo as metal atoms in the Keggin POM shows much higher capacitance than the corresponding W-containing ones. Moreover, compound 1@graphene oxide (GO) composite electrodes are fabricated and systematically explored their supercapacitor performance. Thanks to the synergetic effects

of

GO

and

POM-incorporated

metallacalixarenes,

the

compound

1@15%GO-based electrode exhibits the highest specific capacitance up to 230.2 F g-1 (current density equal to 0.5 A g-1), which is superior to the majority of the reported POM-based electrode materials.

2


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INTRODUCTION In the quest for clean and efficient energy storage systems, supercapacitors are importance for the pressing need of energy demands.1,

2

In terms of working

mechanism, supercapacitors are constructed by the pseudocapacitors and electric double-layer capacitor (EDLCs). Generally, the pseudocapacitors have more advantages of electrochemical behavior than the EDLCs, such as higher specific capacitance, durability and so on.3,

4

Electrode material is accredited to the pivotal

centre of pseudocapacitor, which directly affects the performance of the pseudocapacitor. Motivated by the aforementioned consideration, multitudinous endeavors have been efficiently catered to the tendency for searching the electrode materials of the pseudocapacitors. However, only limited kinds of electrode materials have been explored for pseudocapacitors till now, including the conducting polymers,5,

6

metal oxides,3,

7

hydroxides,8,

9

sulﬁdes10-12 and their composites.13-15

Therefore, seeking new kinds of electrode materials for preparation of the high-performance supercapacitors is still a challengeable endeavor. Polyoxometalates (POMs),16,

17

as a large library of polynuclear metal-oxygen

clusters, are famous for their high negative charge, tunable sizes (typically ranging from nano- to micrometer scale) and enormous structural diversity. Besides their fascinating structures, rapid reversible multielectron redox transformations have been explored in many areas, such as pseudocapacitors18-25 and electrocatalysts26-30. However, when pristine POMs are used as electrode materials, they are prone to dissolve in the electrolyte, and their inherently low conductivity and aggregation leads to an evident capacity degradation, which restrict their applications in pseudocapacitors.19, 31, 32 To surmount this obstacle, POMs are usually immobilized with some proper substrates. 20, 23-25, 31-38 Metallacalixarenes,39, 40 a sort of cup-shaped cavity metal-organic coordination compounds with tunable sizes and apertures, can act as host matrixs to encapsulate many class of guest molecules, and therefore have aroused great interest recently.41-49 By deliberately choosing appropriate combination of organic linkers and metal cations, 3



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a battery of cationic and neutral guest moieties as active sites have been encapsulated inside the cavities of metallacalixarenes, which have been applied in a vast range of fields, such as catalytic,50 hydrogen evolution reaction (HER).51 Nevertheless, the research area of POM-incorporated metallacalixarenes is only in its infancy. The limited examples including: in 2004, two organic-inorganic assemblies have been obtained by weak intermolecular interactions of calix[4]arene-Na+ fragments and POM clusters, which represents the first calixarene-based POM ionic material.52 In 2007, an unique calixarene-based POMs with interesting photochemical properties have been assembled from polynuclear silver clusters and phosphomolybdates.53 In 2010, a novel calixarene-based POM ionic material [Cu12(C7H12N8S2)9(HSiW12O40)4] was fabricated, showing a promising electrocatalytic performance towards the reduction of nitrite.54 Until 2017, three isostructural POM-based metallacalix[6]arene were prepared and applied in lithium ion batteries (LIBs), exhibiting an excellent efficiency.55 Inspired by the above cases, POM-incorporated metallacalixarene as electrode material towards pseudocapacitor is promising, but it has not yet been explored up to present. In

this

work,

two

isostructural

POM-incorporated

metallacalixarenes,

[Ag5(C2H2N3)6][H5  SiMo12O40] (1) and [Ag5(C2H2N3)6][H5  SiW12O40] (2) (C2H2N3 = 1H-1,2,4-triazole (abbreviate to trz)), showing intriguing 3D POM sandwiched metallacalix[6]arene frameworks, are fabricated by utilizing a facile hydrothermal synthesis method arise from trz, Keggin type [SiMo12O40]4−/ [SiW12O40]4− (abbreviated to SiMo12/ SiW12) polyoxoanions and silver nitrate. Our primary investigation shows that the POM-incorporated metallacalixarene of 1 containing Mo as metal atoms in the Keggin polyoxoanions shows higher capacitance performances than the corresponding W-containing one of 2. Meanwhile, the electrochemical performance of 1-modified electrode is confronted to its parent’ SiMo12-based electrode under the same condition. Namely, 1-modified electrode with specific capacitance and cycling stability after 1000th (155.0 F g-1 and 78.5%) are higher than parent’ SiMo12-based electrode (78.3 F g-1 and 64.5%), respectively. Moreover, for effectively boosting the electrical conductivity and finally 4


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enhancing the capacitor performance of electrodes, the compound 1@n graphene oxide (GO) composite materials have been prepared by an ultrasonic synthesis, where “n” is the weight ration between 1 and GO used in the synthesis process, 5%, 10%, 15% and 20% are used respectively). The synthetic route of 1@nGO composite materials are briefly illustrated in Scheme 1. The supercapacitor properties of 1@nGO composite electrodes have been systematically investigated. Thanks to the synergetic effects

of

GO

and

POM-incorporated

metallacalixarenes,

compound

1@15%GO-based electrode exhibits the highest specific capacitance 230.2 F g-1 (current density equal to 0.5 A g-1), which outperforms most of the reported POM-based electrode materials (Please see detailed comparison in the section of supercapacitor properties of compound 1@graphene composite electrodes). Furthermore, the cycling ability experiments of 1@15%GO-based electrode showed 92.7% of the 1st capacitance maintained after 1000th, which is much higher than 1-based electrode (78.5%), not to mention parent’ SiMo12-based electrode (64.5%). To some extent, this work verifies that composites of POM/ metallacalixarene/ graphene-based materials not only integrate the superiority of the individual components perfectly, but also ameliorate the demerit of them, providing a feasible route to approach promising POM-based supercapacitor electrodes with enhanced capacitance, improved stability, and high electrical conductivity.

Scheme 1. The synthetic route of compound 1@GO composite materials.

RESULTS AND DISCUSSION Experimental section The structures of 1 and 2 are accurately determined by Single crystal X-ray diffractions. A summary of the crystallographic parameters for them is provided in 5



Page 6 of 26

Table 1. To further confirm the structures of 1 and 2, they were also investigated by elemental analyses (EA), bond valence sum (BVS) and FI-IR spectra (FT-IR). The results of all above characterizations match well with the analyses of the single crystal X-ray diffractions. The purities of the compounds are determined by their powder X-ray diffraction (PXRD). Obviously, the diffraction peaks of experimental patterns are in accord with their simulated ones, signifying their good purities. Please see details of EA, BVS, FT-IR (Figure S4), PXRD (Figure S5) and other synthesis experiments in the Supporting Information. Table 1. Crystal data and structure refinement for compounds 1 and 2 Compounds

1

2

Empirical formula

C12H17Ag5N18O40SiMo12

C12H17Ag5N18O40SiW12

Mr

2772.06

3826.86

Color

yellow

colourless

Habit

block

block

CCDC Nos.

1893480

1893496

Crystal system

Hexagonal

Hexagonal

Space group

P-31m

P-31m

a/Å

12.157(5)

12.1796(7)

b/Å

12.157(5)

12.1796(7)

c/Å

10.457(5)

10.4736(8)

γ/°

120

120

Volume/Å3

1338.4(15)

1345.5(2)

Z

1

1

Dcalcd/g cm-3

3.442

4.706

T/K

293(2)

293(2)

μ(MoKα), mm-1

4.597

27.136

F(000)

1279.2

1663.2

Refl. measured/unique

25238/1197

7788/871

Rint

0.0352

0.0514

Data/parameters

1189/83

1222/87

GoF on F2

1.060

1.060

R1/wR2 [I≥2σ(I)]a,b

0.0800/0.1801

0.0374/0.0906

R1∕wR2 (all data)

0.0900/0.1865

0.0489/0.0977

aR 1

= ∑║Fo│─│Fc║/∑│Fo│, bwR2 = {∑[w(Fo2─Fc2)2]/∑[w(Fo2)2] }1/2 6


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Description of the Crystal Structures. Both 1 and 2 are isostructural and crystallize in the hexagonal crystal system with the P-31m (No. 162). Herein, compound 1 as a representative to demonstrate the structural characteristics in specifics. The unit cell of 1 consists of one [SiMo12O40]4− (abbreviated to SiMo12) polyoxoanion, six 1,2,4-triazole (trz) ligands, and ten Ag cations, as illustrated in Figure S1. The SiMo12 polyoxoanion exhibits an α-Keggin structure, where Si is perched on the centre of eight half-occupied disordered oxygen atoms, forming a cuboctahedral coordination environment.34, 36, 55, 56 The bond length of Si-O and Mo-O are ranging from 1.59(11) Å to 1.62(4) Å and 1.63(3) Å to 2.40(3) Å, respectively. There are two crystallographic unique Ag ion showing distinct coordination environments: the Ag1 cation is in a “seesaw-shape” geometry, coordinating with two nitrogen atoms and oxygen atoms from different trz ligands and SiMo12 polyoxoanions, respectively. The Ag2 cation is unique in that it coordinates with three carbon atoms from three different trz ligands through Ag+-π interactions (Figure S2). The bond and interaction lengths around Ag ions are 2.76(8) Å, 2.19(4) Å and 2.19(4) Å for Ag-O and Ag-N bonds as well as Ag+-π interactions, respectively, which is consistent with the reported Ag-containing compounds.57, 58 Therefore, each trz group coordinates with four Ag+ ions through two Ag-N bonds and two Ag+-π interactions (Figure S2). A fascinating structural feature of 1 is its 2D metallacalixarene framework. Specifically, initially, six trz ligands and six Ag2 cations are fused together to form a neutrally hexanuclear [Ag6(trz)6] hexagon with the side dimensions of ca. 7.02(20) Å (Figure S3a). Subsequently, one [Ag6(trz)6] hexagon superposes another one (it is rotating 30° by superstratum [Ag6(trz)6] hexagon), achieving a novel {Ag6[Ag6(trz)6]2} double metallacalix[6]arene through sharing six Ag1 ions (Figure 1). In the double metallacalix[6]arene, six Ag1 cations form an equilateral hexagon symmetrical plane with the side dimensions of ca. 4.68(49) Å (Figure S3b). The double metallacalix[6]arene consists of two regular hexagonal windows and twelve isosceles trapezoid windows (Figure S3c). Ultimately, by sharing their all edgs with adjacent six metallacalix[6]arenes, forming a 2D metallacalixarene framework (Figure 2). 7



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Another predominant structural feature of 1 is 3D POM sandwiched metallacalix[6]arene framework, which can be described as follows: firstly, each SiMo12 polyoxoanion as a twelve-dentate high-connected connector coordinates with twelve Ag1 cations through all terminal oxygen, generating a 1D Ag-POM chain (Figure 2). Subsequently, the 1D chain was inserted into cup-shaped cavities of the 2D metallacalixarene framework. The most interesting thing is that the regular hexagonal windows of 2D metallacalixarene framework are suitable to partly encapsulate the Keggin polyoxoanion (~10.5 Å) (Figure 2). Consequently, the adjacent SiMo12 polyoxoanions of Ag-POM chain are respectively incorporated into top and down windows of the double metallacalix[6]arene of the metallacalixarene framework. Thus, the double metallacalix[6]arene is sandwiched and connected by SiMo12 polyoxoanions, and meanwhile the neighboring 2D metallacalixarene frameworks are linked together. Finally, a unique 3D POM sandwiched metallacalix[6]arene framework is achieved (Figure 2).

Figure 1. The formation process of double metallacalix[6]arene.

Figure 2. The formation process of unique POM sandwiched metallacalix[6]arene framework.

Supercapacitor properties of compounds 1 and 2. The electrochemical properties of 1- and 2-based electrodes were explored by the electrochemical cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and 8


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electrochemical impedance spectroscopy (EIS). CV measurements. The CV curves of 1- and 2-based electrodes (Figure 3a and Figure S6a) are performed in a typical three-electrode configuration at scanning rates from 50 to 300 mV·s-1. Clearly, there are three pairs of redox peaks in the CV curves of both 1- and 2-based electrodes in the given potential range, evincing that the main contribution of the faradic capacitive behavior, which is absolutely diverse with normal electrical double-layer capacitor.33 At 300 mV·s-1, the mean peak potentials E1/2 = (Epa + Epc)/ 2 form I-I’, II-II’ to III-III’ are +0.332, +0.207 V, +0.008 V for 1 and -0.321, -0.443 V, -0.592 V for 2, which is corresponding to the two-, four-, and six- electron redox processes59 of [SiMo12O40]4- and three steps of one-, one-, and two- electron redox processes60 of [SiW12O40]4-, respectively. Meanwhile, with increasing of the scan rates from 50 to 300 mV·s-1, the average peak potentials are almost

unaffected,

signifying

that

the

electrodes

enable

the

outstanding

electrochemical reversibility. Figure 3b and Figure S6b show the current variation with scan rates, which indicate that both currents of the anodic and cathodic peaks increased nearly linearly with scan rates, evincing that the redox processes are surface controlled.35, 36, 57, 61 GCD measurements. The GCD measurements of 1- and 2-based electrodes at various current densities are depicted in Figure 3c and Figure S6c. Obviously, the potential plateaus of GCD curves stemming from faradaic reactions of the POM incorporated metallacalix[6]arene frameworks of 1 and 2, which is in accord with their CV curves.35, 36 The values of the specific capacitance (Cs) are 155.0, 85.4, 58.8, 53.9, 49.4, 46.5 and 45.3 F g-1 for 1-based electrode at 0.5, 1.0, 2.0, 3.0, 5.0, 8.0 and 10.0 A g-1, respectively (Figure 3d). Meanwhile, the Cs of 2-based electrode are 29.8, 23.6, 22.2, 22.3, 24.7, 22.7 and 21.6 F g-1 at 6.0, 8.0, 10.0, 12.0, 14.0, 16.0 and 20.0 A g-1, respectively. Moreover, the GCD tests of 1- and 2-based electrodes under identical current density (6.0 A g-1) also indicated that 1-based electrode possesses longer discharge time than 2-based electrode (please see Figure S7), and therefore the specific capacitance of 1-based electrode (48.2 F g-1) is larger than 2-based electrode (29.8 F g-1). The capacitance of 1-based electrode is superior to 2-based electrode, which mainly benefiting from the oxidation ability of Mo is better than W.62 From the

9



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comparison of the current value of CVs and Cs of the isostructural compounds, it is clear that 1 is more efficiently catered to the superelectrical properties.

Figure 3. The electrochemical properties of 1-based electrode: (a) CVs at various scan rates (50 to 300 mV s-1); (b) The peak currents VS scan rates; (c) GCD curves at distinct current densities (0.5, 1.0, 2.0, 3.0, 5.0, 8.0 and 10.0 A g-1); (d) Cs at distinct current densities (0.5, 1.0, 2.0, 3.0, 5.0, 8.0 and 10.0 A g-1).

The cycling stability of GCD. As observed from Figure 4a, the 1-electrode retains 78.5%, meanwhile 78.3% for 2-based electrode of the initial Cs after 1000th at 10.0 A g-1, respectively. Therefore, both 1- and 2- electrodes afford the excellent cyclic stability in a long-term GCD process. The GCD durability of 1- and 2-electrodes is higher than traditional POM-based electrode materials, possibly thanking to the unique twelve-connected structures of Keggin polyoxoanions and the POM incorporated metallacalix[6]arene frameworks in 1 and 2.

Figure 4. The comparison between 1- and 2-based electrodes: (a) Cycling performances during 1000th (current density equal to 10.0 A g-1); (b) The EIS. 10


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EIS measurements. For comprehension the relationship between the Cs and the resistance of 1 and 2, the EIS of the 1- and 2-based electrodes at 1st are investigated. As observed from Figure 4b, the charge-transfer resistance (Rct) is 1 < 2, and 1-based electrode manifests the steeper vertical line. Therefore, 1-based electrode has higher conductivity and faster electron transfer kinetics than 2-based electrode.63 The comparison of 1-based and it’s parent’ SiMo12-based electrodes are shown in Figure 5. Initially, the cathode and anode peak potential of 1-based electrode is consistent with SiMo12-based electrode (Figure S8) in operated with a potential range from -0.1 V to 0.7 V, and it can be seen that the summit current values of former is bigger than latter, which is acquired at 300 mV s-1 (Figure 5a). Subsequently, Figure 5b signifies the GCD measurements, which evidently illustrate that the Cs of 1-based electrode is twice times as the SiMo12-based electrode (current density equal to 0.5 A g-1), further testifying the superiority of the POM incorporated metallacalix[6]arene framework. Lastly, as observed from Figure 5c, the cycling stability of SiMo12-based electrode lose more than 35.5%, while 78.5% specific capacitance retention of 1-based electrode after 1000th (current density equal to 10.0 A g-1). In view of the cycling stability results, illustrating the compound 1 has the excellent cycling stability, which is consistence with the EIS trends. Obviously, the Rct of SiMo12-based electrode is significantly higher than 1-based electrode, which is consistent with the GCD measurements and cycling stability of GCD.

11



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Figure 5. The comparison of 1- and SiMo12-based electrodes: (a) CVs in 0.5 M H2SO4 solution at 300 mV s-1. (b) GCD curves at a current density of 0.5 A g-1; (c) Cycling performance during 1000th (current density equal to 10.0 A g-1). (d) The EIS.

Supercapacitor properties of compound 1@graphene composite electrodes Graphene, with the exceptionally high surface area (up to 2630 m2 g-1) and outstandingly electrical conductivity (106 S cm-1), which has been postulated to the privileged candidate for the supercapacitors.64-66 Herein, for the sake of further effectively enhancing the supercapacitor performance of 1, four different weight of 1 VS graphene oxide (GO) composite materials have been prepared by ultrasonic, defined as 1@nGO (“n” is the weight of GO utilized in the synthesis process, 5%, 10%, 15% and 20% are used respectively). The combination of GO and 1 in these composite materials are confirmed by both the FT-IR spectra and the TEM images. Taking 1@15%GO composite material as an example, the FT-IR spectra and the TEM images are illustrated in Figure 6a. In contrast to the FT-IR spectrum of 1 with 1@15%GO composite material, emerging the vibrations peaks at 1640, 1518, 1425, 1383, 1307, 1256, 1055, 943, 910, 796, 700, 628 cm-1,which assign to the characteristic bands of 1. And, the wider −OH group (3494 cm-1), emerged C=O (1737 cm-1), stronger C=C sp2 species (1529 cm-1) and C−O (1065 cm-1) vibration peaks, signifying the successful preparation of 1@15%GO composite material.22,

67

12


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Meanwhile, the TEM image shows that the fusiformis morphology of the crystals of 1 are well spread on the surface of ultrathin GO nanosheets, affirming the existence of both 1 and GO in the 1@GO composite material (Figure 6b).

Figure 6. (a) FT-IR spectra of 1 and 1@15%GO and (b) TEM image of 1@15%GO.

The supercapacitor properties of 1@nGO composite electrodes have been systematically investigated. Figures S9-S12 show the CV and GCD curves of 1@nGO-based electrodes in 0.5 M H2SO4 solution. The comparison of CV measurements of 1-based and 1@nGO-based (n = 5%, 10%, 15% and 20%, respectively) electrodes is shown in Figure 7a. It can be obviously seen that the summit current values of 1@15%GO-based electrode shows the highest current values, while the cathode and anode peak potential is consistent with each other, which is in operated acquired at 300 mV s-1 with a potential range from -0.1 V to 0.7 V. The Cs of 1@5%GO-, 1@10%GO-, 1@15%GO- and 1@20%GO-based electrodes is 147.1, 217.1, 230.2 and 104.2 F g-1 (Figure 7b) (current density equal to 0.5 A g-1), respectively. Moreover, the cycling stability performance is also corresponding to the CV and GCD measurements (Figure S13). Particularly, the cycling stability are 90.1%, 89.1%, 92.7% and 81.0% after 1000th (current density equal to 10.0 A g-1), which assign to the 1@5%GO-, 1@10%GO-, 1@15%GO- and 1@20%GO-based electrodes, respectively. Clearly, the 1@15%GO-based electrode possesses the highest current value, Cs and cycling stability among these electrodes.

13



Page 14 of 26

Figure 7. The comparison of CVs (a) and Cs (b) for 1- and 1@nGO-based electrodes in 0.5 M H2SO4 solution at 300 mV s-1.

For comparison, the GCD curves and cycling performance (Figure S14) of SiMo12@15%GO during 1000th at a current density of 10.0 A g-1 are also investigated in 0.5 M H2SO4 solution, and the Cs and cycling performance of SiMo12@15%GO are 114.5 F g-1 and 80.0%, respectively. Furthermore, to well compare 1@15%GO with SiMo12, SiMo12@15%GO and 1, the Cs and cycling stability of SiMo12, SiMo12@15%GO, 1 and 1@15%GO electrodes is summarized in Figure 8. From the Figure 8, the order of Cs is 1@15%GO > 1 > SiMo12@15%GO > SiMo12, while the order of cycling stability is 1@15%GO > SiMo12@15%GO > 1 > SiMo12. It can be distinctly seen that 1@15%GO composite electrode owns the best properties in terms of both Cs and cyclic stability. What's more, the Cs of 1@15%GO outperforms most of reported POM-based supercapacitor electrode materials, and relative literatures have been listed in Table 2. And, the cycling stability performance (Figure S15) could be comparable with that of the typical state-of-the-art MOF-based and POM-based supercapacitor electrode materials (Table S1 and Table S2). Meanwhile, the SEM images and FI-IR spectra of the 1@15%GO composite electrode material before and after electrochemical measurements are utilized to explore its morphologies and structural stability. The results of experiment show that the stability of 1@15%GO composite material is basically good (Figure S16 and S17). The excellent capacitive performance and cycling stability of 1@15%GO is briefly stemming from the following points: (i) the oxidation ability of the Keggin ion 14


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probably plays a primary role in the electrochemical performance; (ii) the host matrix of metallacalixarene not only owns unique crystal structure, but also offers a ordered electronic transfer pathway, which consequently realizes the improvement of stabilized conductivity; (iii) the 1D Ag-POM chain is propitious to electron transfer between adjacent POMs;36 (iv) the outstandingly high surface area and excellently electrical conductivity of graphene further facilitated the transmission of electrons and improved the stability of electrodes.

Figure 8. The summary of Cs (a) and cycling stability (b) of SiMo12, SiMo12@15%GO, 1 and 1@15%GO electrodes.

Table 2 Summary of POM-based composite electrodes of 3-electrode supercapacitors Electrode

Electrolyte

Cs

Current density

Reference

H3PMo12O40/CNTs

1 M H2SO4

40 F g-1

10 mA g-1

20

CNTs/PDDA/[P2W17VO62]8−

0.5 M H2SO4

82 F g-1

200 mA g-1

21

PEDOT/PMo12

H2SO4/PVA gel

130 F g-1

400 mA g-1

23

Cs-PMo12/CNT/PVA

1 M H2SO4

285 F g-1

200 mA g-1

65

RGO/PMo12O40

1 M H2SO4

218 F g-1

1 A g-1

68

Na6V10O28

1M LiClO4

189 Fg-1

0.5 A g-1

69

AC/PMo12O40

1 M H2SO4

183 F g-1

2 A g-1

70

1@15%GO-based electrode

0.5 M H2SO4

230.2 F g-1

0.5 A g-1

This work

15



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CONCLUSION In conclusion, two fascinating 3D POM sandwiched metallacalix[6]arene frameworks based on 1D Ag-POM chains and 2D metallacalix[6]arene frameworks are synthesized by modulating the hetero-atoms of Keggin anions, and their application in supercapacitor is investigated for the first time. The results of supercapacitor performance tests clearly indicate that 1 shows higher specific capacitance and cycling stability than 2 due to higher oxidation capacity of Mo-system Keggin ion. Furthermore, considering higher supercapacitor performance of 1, compound 1@graphene oxide composite electrodes with different mixing ratio have been prepared and systematically investigated. Compound 1@15% graphene oxide composite electrode exhibits exceptional specific capacitance (230.2 F g-1 at 0.5 A g-1) and cycling stability (92.7% after 1000th), which is superior to not only individual component of compound 1 or graphene oxide but also most of the reported POM-based electrode materials. Therefore, this work affords an efficient strategy to design and synthesize novel POMOF@GO-based electrode materials with excellent specific capacitance and cyclic stability and may open up an approaching avenue for high-performance supercapacitors. ASSOCIATED CONTENT Supplementary Information Syntheses of compounds 1-2 and 1@nGO (n = 5%, 10% 15% and 20%), single crystal X-ray crystallography, preparation of the modified electrodes, the durability tests of 1@15%GO, other figures, additional electrochemical measurements, additional discussion including the morphologies and stability of 1@15%GO electrodes in the Supporting Information, which are all available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], Tel./fax.: 86-0451-86688575. *E-mail: [email protected], Tel./fax: 86-0451-86392716. 16


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ORCID Yan Hou: 0000-0003-0545-6747 Dongfeng Chai: 0000-0002-7971-7391 Haijun Pang: 0000-0002-0405-8352 Huiyuan Ma: 0000-0002-2695-3243 Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (21671049, 51572063 and 21701037), the National Science Foundation of Heilongjiang Province (LH2019B009) and the Fundamental Research Foundation for Universities of Heilongjiang Province (LGYC2018JQ007). REFERENCES (1) Fu, Y. P.; Wu, H. W.; Ye, S. Y.; Cai, X.; Yu, X.; Hou, S. C.; Kafafy, H.; Zou, D. C. Integrated Power Fiber for Energy Conversion and Storage System. Energy Environ. Sci. 2013, 6, 805-812. (2) Strauss, V.; Marsh, K.; Kowal, M. D.; El-Kady, M.; Kaner, R. B. A Simple Route to Porous Graphene from Carbon Nanodots for Supercapacitor Applications. Adv. Mater. 2018, 30, 1704449. (3) Shang, Y. Y.; Gai, Y. S.; Wang, L. Q.; Hao, L.; Lv, H. J.; Dong, F. Y.; Gong, L. Y. A Facile and Effective Method for Constructing Rambutan-like NiCo2O4 Hierarchical Architectures for Supercapacitors Applications. Eur. J. Inorg. Chem. 2017, 17, 2340-2346. (4) Wang, Y. G.; Song, Y. F.; Xia, Y. Y. Electrochemical Capacitors: Mechanism, Materials, Systems, Characterization and Applications. Chem. Soc. Rev. 2016, 45, 5925-5950. (5) Ajdari, F. B.; Kowsari, E.; Ehsani, A. P-type Conductive Polymer/Zeolitic Imidazolate

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Polyoxometalate-Incorporated Metallacalixarene@Graphene

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