A Host–Guest Supercapacitor Electrode Material Based on a Mixed

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A Host−Guest Supercapacitor Electrode Material Based on a Mixed Hexa-Transition Metal Sandwiched Arsenotungstate Chain and Three-Dimensional Supramolecular Metal−Organic Networks with One-Dimensional Cavities Kun-peng Wang,†,‡ Kai Yu,*,†,‡ Jing-hua Lv,*,† Mao-lin Zhang,†,‡ Fan-xue Meng,†,‡ and Baibin Zhou*,†,‡

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Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, P. R. China ‡ Key Laboratory of Synthesis of Functional Materials and Green Catalysis, College of Heilongjiang Province, School of Chemistry and Chemical Engineering, Harbin Normal University, Harbin 150025, P. R. China S Supporting Information *

ABSTRACT: The mixed hexa-transition metal (hexa-TM) sandwiched arsenotungstate derivative, [CuI3(pz)2(phen)3]2[CuI(phen)2][{Na(H2O)2}{(VIV5CuIIO6)(AsIIIW9O33)2}]·6H2O (1) (pz = pyrazine; phen = 1,10phenanthroline), has been hydrothermally synthesized and structurally characterized. In compound 1, two {AsW9O33} clusters are connected by mixed hexa-TM ring unit {VVI5CuIIO6} to form a sandwich-type dimer, which are further bonded in “ABAB” mode by the {Na(H2O)2} linker resulting in pure inorganic chains. The unique “L-shaped” trinuclear complex {Cu3(phen)3(pz)2} is supported together via staggered π−π interactions to generate extending waveform two-dimensional supramolecular layers, which are further aggregated with their adjacent analogues by complexes {Cu(phen)2} via H-bonding interaction to yield an unprecedented three-dimensional (3D) metal−organic networks with one-dimensional (1D) cavities. The pure inorganic 1D sandwich chains are implanted in the cavities as guest units via supramolecular interactions to form a POMOF 3D framework. Compound 1, as the electrode of the supercapacitor, exhibits higher specific capacitances (825 F g−1 at a current density of 2.4 A g−1), better rate capability, more durable cyclic stability (91.4% of cycle efficiency after 3000 cycles), and improved conductivity and electroactivity compared to those of parent polyoxometalate (POM) Na9[AsW9O33]·19H2O (2) and 6-Cu-substituted POM [Cu6(imi)6{AsIIIW9O30Cl3}2]· 6H2O (3), which may be attributed to the introduction of V4+, the unique host−guest structure, and the rich π electron system. In addition, compound 1 exhibits dual-function electrocatalytic behavior in reducing inorganic salt IO3− and oxidizing the organic molecule dopamine.

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INTRODUCTION

ever, many supercapacitor materials still have the shortcomings of high cost, inferior rate performance, and low energy density. Therefore, it is necessary and urgent to look for electrode active materials with low cost and excellent supercapacitor performance. Polyoxometalates (POMs) make up a significant class of nanometallic oxygen clusters, which are built from MO6 octahedra (M = Mo, W, V, etc.) and XOn tetrahedra (X = P, As, Si, B, etc.) in edge-sharing and/or angle-sharing ways. They have traditionally been applied in catalysis, magnetism, photo/ electrochemistry, etc., due to their remarkable properties and structural diversities.12 Recently, POMs as environmentally

With the exhaustion of coal, oil, natural gas, and other nonrenewable energy sources, developing sustainable clean energy sources and advanced technology related to energy storage and conversion is an urgent need.1 A supercapacitor with advantages of long cycle life, fast energy delivery, and high power density is a promising energy storage device and has wide applications in many portable electronic products, hybrid electric vehicles, military equipment, aerospace engineering, and other fields.2,3 Pseudocapacitor materials have a higher theoretical specific capacitance and are the most promising materials for increasing the energy density of supercapacitors. So far, pseudocapacitor electrode materials mainly include RuO2,4 MnO2,5 V2O5,6 MoO3,7 WO3,8 MWO4,9 and their composites with carbon nanotubes10 and graphene.11 How© XXXX American Chemical Society

Received: March 12, 2019

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DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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dimensional (1D) channels is obtained for the first time in the reported sandwiched derivatives. The electrocatalytic behaviors and electrochemical performances of compound 1 as supercapacitor electrodes were investigated in detail.

friendly energy storage materials are attracting more attention because of their outstanding multielectron transfer ability, thermal stability, and capacity for molecular conductivity.13 They can perform a variety of reversible multielectron redox processes rapidly without any change in their framework. However, most POMs have a small specific surface area and are quite soluble in polar solvents, which limits their practical applications in aqueous supercapacitors. To overcome the disadvantages, researchers tried to introduce POMs into carbon material with good conductivity to synthesize hybrid nanocomposites. For instance, CNT/Cs-PMo12 hybrid materials,14 PMo12-PANI/GS nanocomposites,15 AC/PMo12,16 POM/PIL/rGO nanohybrids,17 and PPy-PMo12/rGO ternary nanohybrids18 have exhibited the desired composite properties and improved supercapacitor performance. However, the synthesis method described above requires the precursor of POMs, and the structure and composition of the synthesized composite are not easy to regulate or control. In fact, various transition metals, organic ligands, and metal organic complexes can be introduced in situ into POMs system to form various POM-based metal organic frames, hybrid coordination polymers, and host−guest structures via hydrothermal reaction, which can effectively improve the specific surface area, the solution stability, and the electron transport capacity of the POM.19 The materials obtained by the method are single crystals with definite structures; thus, as supercapacitor electrodes, they are more favorable for the analysis of the relationship between the material structure and the performance of supercapacitors. Trivacant Keggin POMs have been extensively studied as one of the important inorganic building blocks due to their richer surface oxygen atoms, higher charges, and sufficient coordination vacancies. They can integrate different numbers of transition metals (TMs) into the lacunary positions of the skeleton to form TM-substituted sandwich-type POM derivatives with fascinating catalytic, magnetic, and biological activity.20−32 Although various dimeric sandwich-type POMs with two to eight nuclear TM groups on the central belt have been described, most of them are sandwiched by bi- or trivalent TM ions (such as Mn2+, Fe3+, Co2+, Ni2+, and Cu2+).20−30 High-value TM vanadium (V5+ or V4+)-substituted derivatives have rarely been reported. To date, only a few di-Vand tri-V-substituted sandwich POMs have been obtained.31 It is still a challenge to build TM-substituted sandwich-type POMs incorporating more than three vanadium atoms. During the construction of sandwich POMs, we are trying to synthesize high-nuclear V-substituted derivatives with the consideration that the {VOx} sandwich unit can perform reversible multielectron transfer reactions that are more abundant than that of TM2+/TM3+, which will greatly enhance the energy storage capacity of sandwich complexes.6,13e Herein, five V4+ atoms and one Cu2+ atom were introduced into the {AsW9O33} skeleton to yield a mixed hexa-TM sandwiched POM, [CuI3(pz)2(phen)3]2[CuI(phen)2][{Na(H2O)2}{(VIV5CuIIO6)(AsIIIW9O33)2}]·6H2O (1), via onestep hydrothermal synthesis. In our in situ reaction, using common transition metal salts (sodium tungstate and cupric acetate), simple inorganic matter (sodium arsenite), and cheap organic ligands (phenanthroline and pyrazine) as raw materials reduces the cost of the electrode to a certain extent. To the best of our knowledge, such host−guest structure based on a pure inorganic ABAB chain and unique three-dimensional (3D) supramolecular metal−organic networks with one-

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EXPERIMENTAL SECTION

The details of the general methods and materials, synthesis of compound 1, control compounds 2 and 3, X-ray crystallography, and preparation of the electrodes are available in section 1 of the Supporting Information. Details of the crystallographic data and structural determination for 1 are listed in Table 1.

Table 1. Crystal Data and Structural Refinement for Compound 1 empirical formula formula weight (g) temperature (K) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (kg m−3) μ (mm−1) F(000) data/parameters Rint goodness of fit final R indices [I > 2σ(I)] R indices (all data)a

C112H96As2Cu8N24NaO80V5W18 7303.18 273(2) monoclinic C2/c 27.8592(12) 30.7780(13) 20.6299(9) 90 118.2660 90 15579.8(12) 4 3.114 15.102 13328 13874/1133 0.0865 1.031 R1 = 0.0723; wR2 = 0.1121 R1 = 0.0824; wR2 = 0.1309

R1 = ∑(|Fo| − |Fc|)/∑|Fo|; wR2 = {∑[w(|Fo|2 − |Fc|2)2]/∑|w(| Fo|2)2]}1/2. a

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RESULTS AND DISCUSSION Synthesis. During our experiment on the exploration of new sandwich units to construct sandwich-type arsenotungstate assemblies, we tried to add V2O5 or NaVO3 as a vanadium source to the reaction system to obtain a Vsubstituted sandwich structure. At the beginning, the mixture of NaAsO2·2H2O, H2WO4, V2O5/NaVO3, imi, and H2O at 140 °C for 5 days results in a series of simple zero-dimensional structures based on {AsW12O40} clusters and free ligands without any V atoms. It seems that H2WO4 is not a good starting reactant for V-substituted architectures. This led us to change the tungsten source. Thus, H2WO4 was replaced by NaWO4, and the pH of the medium was adjusted to a similar value to ensure the formation of a sandwich-type structure. In addition, NH4VO3 was chosen to allow the solubility to be better than that of V2O5. Another kind of TM2+ should be simultaneously added to the system described above, because all of our efforts to construct {AsW9O33} clusters incorporating the {VOx} sandwich unit without the aid of TM2+ have been unsuccessful so far. A possible reason is that the vanadium with a high oxidation state has difficulty forming a typical metal− organic complex linkage unit, which may induce sandwich clusters and their extended assembly.31c,d Therefore, Cu2+ ion B

DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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distributed on two asymmetric positions of ring with the V3 atom. In the {VIV5CuIIO6} unit, the Cu1 and V3 atoms share the same position, which are doped with a Cu:V occupancy ratio of 0.5:0.5 [with V−O distances of 1.93(2)−1.992(14) Å, VO distances of 1.578(17)−1.603(16) Å, and Cu1/V3− O24 distances of 1.872(11) Å]. Na1 adopts a six-coordinated octahedral geometry defined by two terminal O atoms from two adjacent sandwich clusters {(AsW9O33)2(V5CuO6)} with a Na1−O34 distance of 2.40(2) Å, two μ-O atoms from V/Cu atom of two sandwich units {VIV5CuIIO6} with a Na1−O24 distance of 2.357(19) Å, and two water molecules with a Na1−O50 distance of 2.33(2) Å. Adjacent sandwich POM clusters are connected in “ABAB...” mode by the Na(H2O)2 linker, resulting in an infinite inorganic chain {Na(H2O)2(V5CuO6)(AsW9O33)2}n (Figure S3). Besides Cu1, four crystallographically independent copper ions show two kinds of bonding geometries coexisting in compound 1. Cu5 is bonded to two phen ligands in a four-coordination manner to form isolated complexes {Cu(phen)2} (Figure S4). Cu3 and Cu4 exhibit a three-coordinated “triangle” geometry by two N atoms from one phen ligand and one N atom from a pz ligand. Cu2 takes a “tetrahedral” configuration of {Cu(2)N4} by two N atoms from one phen ligand and two N atoms from two pz ligands. The Cu−N bond lengths range from 1.91(3) to 2.14(2) Å, and the N−Cu−N bond angles vary from 82.8(9)° to 152.4(10)°. Each pz ligand acts as a binode ligand connected to two adjacent copper atoms. Each phen molecule is linked to one copper atom in the form of a terminal ligand, which make skeletons of the complex robust. In this manner, three phen and two pz ligands and three copper atoms are alternately connected to generate a unique “L-shaped” trinuclear complex unit {Cu3(phen)3(pz)2} containing two kinds of ligands (Figure S4). Two L-shaped complexes are connected in a head-to-head manner to form a complex dimer via π−π interaction between two phen planes (Figure 2a). Adjacent dimers are bonded together via similar π−π interactions in an ABAB manner to yield a supramolecular ring (Figure 2b), which is further aggregated with their adjacent analogues in the edge-sharing ways generating an infinite 1D chain (Figure 2c). Adjacent 1D chains are supported by staggered π−π interactions between phen groups resulting in an extended waveform two-dimensional (2D) supramolecular layer (Figure 2e and Figure S5). It is worth noting that such a waveform 2D layer formed by an L-shaped trinuclear complex via the π−π stacking effect is observed for the first time in the reported metal−organic assemblies. The average π−π interaction distance between adjacent phen faces is ∼3.482 Å. The complexes {Cu(phen)2} connect adjacent 2D layers via weak interaction between H and C atoms from {Cu(phen)2} and L-shaped complexes {Cu3(phen)3(pz)2} to yield unique 3D supramolecular metal−organic networks with 1D cavities along the 101 crystal plane (Figure 2d and Figure S6). The pure inorganic 1D sandwich chains are implanted into the cavities as guest units via supramolecular interactions (C19···O30, 3.053 Å; C31···O35, 3.040 Å; C19···O30, 3.088 Å; and C18···O29, 3.156 Å) between C atoms of the L-shaped complex and surface O atoms of the {AsW9O33} cluster to form an unprecedented POMOF 3D network (Figure 3 and Figure S7). The oxidation states of W, As, V, and Na atoms in compound 1 are +6, +3, +4, and +1, respectively, based on bond valence sum (BVS) calculations.33 In addition, the oxidation number of Cu1 is +2, and that of other copper is +1, which is consistent with their coordination numbers.

was added to the reaction system due to its multiple coordination geometries and bonding modes. However, this was unsuccessful in a parallel experiment using other transition metal salts (Ni2+, Zn2+, Mn2+, etc.) to replace CuAC2, which suggests that the type of TM is also one of the key factors in the formation of compound 1. At the same time, the introduction of two kinds of ligands usually tends to yield more complex organometallic networks. As a binode ligand, 1,10-phenanthroline (phen) was selected because of its roles in making a complex skeleton strong and an abundant π electron system. However, phen is not a good ligand for extending the complex skeleton. As flexible and rigid ligands, pyrazines can easily connect with dX metals to form a variety of complex connection units or multinuclear complexes. The combination of these two ligands will lead to novel metal−organic networks. It is worth noting that V5+ and part of Cu2+ were reduced during the reaction and any additional reductants were not added to the whole system, so it is possible that the excess organic ligand acted as the reducing agent. In fact, our attempts to add only phen or pz to induce crystal 1 were unsuccessful, which further shows that they are both important reducing agents and indispensable structural guides in the formation of hexa-V-substituted sandwich frameworks. Structural Descriptions. Compound 1 crystallizes in the monoclinic C2/C space group, and the basic unit of 1 is constructed from two trivacant {AsW9O33} clusters, a hexanuclear heterometallic unit {VIV5CuIIO6}, one {Na(H2O)2} linker, two trinuclear complex {Cu3(phen)3(pz)2} groups, and one {Cu(phen)2} cation (Figure S1). The {AsW9O33} cluster is B-α-type trivacant Keggin structure and derives from the {AsW12O40} polyanion upon removal of one edge-sharing {W3O13} trimer. The W and As centers display {WO6} octahedron and {AsO3} triangle pyramid coordination geometry, respectively. The W−O and As−O bond lengths are within the normal range (Table S1). Two {AsW9O33} clusters are connected by mixed metal hexanuclear unit {VIV5CuIIO6} to form a unique sandwich-type dimer. The central {VIV5CuIIO6} group consists of one Cu atom and five V atoms, which lie on the same plane. Each metal takes a tetragonal pyramid geometry, bonded with four u3-O atoms from two {AsW9O33} anions and one terminal oxygen atom, respectively. Six tetragonal pyramids are bonded together in edge-sharing mode to form hexagonal metal ring {VIV5CuIIO18} via 12 u3-O atoms (Figure 1 and Figure S2). Cu1 occupies two angles of the hexagonal ring and is

Figure 1. Polyhedral and ball-and-stick representation of {(VIV5CuIIO6)(AsIIIW9O33)2}. C

DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) L-shaped trinuclear complex dimer. (b) Ring unit containing a pair of trinuclear complex dimers. (c) 1D chain consisting of numerous edge-sharing ring units via π−π interaction between phen planes. (d) 2D layer formed by a dense π−π interaction between adjacent chains. (e) 3D supramolecular metal−organic network with 1D cavities linked by Cu(phen)2 via H-bond interaction.

six angles of a hexagonal star are not exactly the same, and two of them are longer, which is different from the case for type I29a,b and type II.29c,d Third, the vertex of the square-pyramid geometry of {VIVO5} is completed by a terminal O atom with a short VO bond length range of 1.578(17)−1.603(16) Å, which is different from the bonding atmosphere of terminal water for other TM metals (such as four Cu2+ cations of type III).29e Infrared (IR) Spectroscopy. The IR spectrum of compound 1 (Figure S9) exhibits the characteristic absorption bands of {(V5CuO6)(AsW9O33)2} at 978, 926, 837, 755, and 697 cm−1, which can be assigned to WOd, V−Od, As−Oa, W−Ob−W/V−Ob−O, and W−Oc−W bonds, respectively.13e,32a The peaks at 1571 and 1376 cm−1 are characteristic bands of the pz and phen ligands, respectively.29e,32b The broad band at 3487 cm−1 is characteristic of the water molecules. TG Analyses. The TG curve of compound 1 shows a twostep weight loss (Figure S10). The first weight loss of 2.51% (calcd 1.97%) at 150−275 °C is assigned to the loss of eight H2O molecules. The second weight loss between 275 and 550 °C of 21.48% derived from the loss of all of the phen and pz molecules (calcd 21.93%, for eight phen and two pz ligands). All of the weight losses from the TG curves are in accord with the formulas of compound 1. Powder X-ray Diffraction (PXRD) and X-ray Photoelectron Spectral Analyses. The PXRD pattern of compound 1 is depicted in Figure S11. The diffraction peaks are consistent with that of simulated patterns, indicating that compound 1 is a crystallographically pure phase. The oxidation states of W, As, Cu, and V are further validated by XPS data, which were depicted in the energy area of W 4f7/2, W 4f5/2, Cu 2p3/2, Cu 2p1/2, V 2p3/2, V 2p1/2, As 3d5/2, and As 3d3/2 (Figure S12a−d). The XPS spectrum of compound 1 exhibits two

Figure 3. 3D inorganic−organic framework comprised of supramolecular metal−organic networks and 1D inorganic chains.

The sandwich unit {VIV5CuIIO6} is different from any reported hexanuclear TM sandwich cluster. Five kinds of hexaTM clusters, trigonal-prismatic cluster (Figure S8a),25 triangle arrangement (Figure S8b),26 beltlike motif (Figure S8c),27 irregular hexa-TM unit (Figure S8d), 28 and annular hexanuclear group (Figure S8e),29 have been reported so far. The {VIV5CuIIO18} unit belongs to a novel annular hexa-TM type, which is fundamentally different from three kinds of reported hexanuclear ring groups. First, five VVI cations and one CuII cation instead of six of the same TMII cations build the hexagonal cycles in 1 (type VI). V-Substituted sandwich POM derivatives are rare species, although various transition metals have been widely introduced into the trivacant Keggin skeleton as sandwich units. Only a few cases of bi- and tri-Vsubstituted types have been reported so far.31 Sandwich derivatives containing a tetra-, penta-, or hexa-V unit have not yet emerged. Second, because CuII and VIV share the same position on two symmetrical angles of the hexagonal ring, the D

DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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ascribed to the successive oxidation−reduction process of the tungsten center of sandwich-type polyanions {(V5CuO6)(AsW9O33)2}.37 The redox peak current increases correspondingly as the sweeping rate increases from 25 to 150 mV/s. Meanwhile, the cathodic peaks migrate to the negative potentials, while the anodic peaks migrate in the positive potential direction. As depicted in Figure S15, the linear relationship of the anode and cathode currents of peak II versus the sweeping rate can be observed, illustrating that the kinetics of interfacial faradic redox reactions and the rate of electronic and ionic transport are rapid enough.19b,38 Compared with CV of the parent POM α-Na9[AsW9O33]· 19H2O-based electrodes (2-GCE) and the 6-Cu-substituted POM [Cu6(imi)6{AsIIIW9O30Cl3}2]·6H2O-based electrodes (3-GCE) (Figure 6b), the larger peak current and the wider gap between the oxidation and reduction peak current for the 1-based electrode at a scanning speed of 70 mV s −1 demonstrate that the capacitance of compound 1 is higher than that of parent POM and pure 6-Cu-substituted POM. The expanded CV area versus that of two control compounds indicates that compound 1 has better advantages in terms of mixed hexa-TM substitution and unique host−guest architecture. The galvanostatic charge−discharge (GCD) experiments for 1-GCE were conducted at different current densities in the voltage window range of −0.8 to 0.8 V (Figure 6c). The voltage plateau during the discharge process is in good agreement with the reduction peak in the CV curve and corresponds to the reduction procedure, indicating a typical pseudocapacitance performance. The specific capacitance of 1GCE is calculated to be 825, 810, 750, 729, and 696 F g−1 at different current densities of 2.4, 4.8, 9.6, 14.4, and 19.2 A g−1, respectively. When the current density is increased to 19.2 A g−1, the capacitance can be maintained at 696 F g−1. It can be seen that the specific capacitance decreases slowly with an increase in current density, which also confirms the high rate capability of 1-GCE. As depicted in Figure 6d, the specific capacitance of 1-GCE is much higher than that of two control compounds at the same current density, which further illustrates that compound 1 has an electrode activity and an electron conversion efficiency that are higher than those of parent POM and pure 6-Cu-substituted POM. In addition, when the current density is 2.4 A g−1, the specific capacitance value is as high as 825 F g−1, which is much higher than that of other reported POMs and POM-based composite materials. For example, only specific capacitance values of 285, 587, 183, 408, 297, and 168 F g−1 are reached for CNT/Cs-PMo12 hybrid materials,14 PMo12-PANI/GS,15 AC/PMo12,16 POM/ PIL/RGO nanohybrids,17 PPy-PMo12/rGO ternary nanohybrids, 1 8 and (H 2 bpe)(Hbpe) 2 {[Cu(pzta)(H 2 O)][P2W18O62]}·5H2O,19b respectively, under similar conditions. It is worth noting that the retention rates of capacitance are 84.4%, 68.3%, and 74.8% when the current density is changed from 2.4 to 19.2 A g−1 for compound 1, [Cu6(imi)6{AsIIIW9O30Cl3}2]·6H2O, and α-Na9[AsW9O33]· 19H2O, respectively. The rate capability of compound 1 is better than those of two control compounds. This may be due to the unique host−guest structure and rich π electron system in compound 1, which improves the conversion efficiency and migration rate of ions and electrons in the redox reaction. The cycle stability of the electrode directly determines the durability and service life of the supercapacitor, so it is one of the important parameters to be considered in practical

peaks at 35.2 and 37.4 eV in the W 4f domain, two peaks at 516.4 and 523.7 eV in the V 2p region, and two peaks at 39.8 and 43.9 eV in the As 3d district, which are ascribed to W6+,34 V4+,35 and As3+,36 respectively. In addition, four peaks at 934.5, 954.2, 943.3, and 962.5 eV in the Cu 2p region are attributed to Cu+ and Cu2+ ions.37 The oxidation states of these elements are in agreement with the results of valence bond calculations. Specific Surface Area Analyses. The nitrogen adsorption isotherm of compound 1 was performed for the fully activated sample at 77 K. The part of the nitrogen absorption isotherm in the P/P0 range of 0.05−0.30 was processed into a straight line by the term 1/[V(p0/p − 1)], and the slope and intercept of the straight line are obtained. The specific surface area was estimated by the BET formula. As shown in Figure S13, the BET surface area of compound 1 is 52.28 m2 g−1. The value is larger than that of parent POM (compound 2) and six-Cusubstituted sandwich-type POM representatives (compound 3) (Figure 4).

Figure 4. N2 sorption isotherm of compounds 1−3 at 77 K.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) Analyses. The structure and surface topography of compound 1 were further investigated by SEM and EDS. From the enlarged SEM diagram (Figure S14a) of compound 1, the rectangular shape with approximate dimensions of 240 μm × 90 μm × 60 μm can be detected. The EDS analysis image of 1 is shown in Figure S14b. The microstructure of 1 was discovered to be comprised of O, C, N, V, Cu, W, As, and Cu, which further confirms the elemental composition of compound 1. Moreover, the uniform distribution of these elements in compound 1 can be clearly observed from Figure 5. Capacitive Performance. The electrochemical properties of the 1-based glassy carbon electrode (1-GCE) were studied in a three-electrode system with 0.5 mol/L H2SO4 electrolyte. Cyclic voltammetry (CV) of 1-GCE was carried out in the potential range of −0.8 to 0.8 V at different sweeping speeds (Figure 6a). As shown in Figure 6a, there are five pairs of redox peaks with half-wave potentials [E1/2 = (Epa + Epc)/2], 440 mV (I−I′), 185 mV (II−II′), −38 mV (III−III′), −240 mV (IV− IV′), and −430 mV (V−V′). The first and second pairs of redox peaks (I−I′ and II−II′, respectively) should be attributed to redox reactions of Cu2+ and Cu+37 and V4+ and V3+,35a respectively. The other three reversible peaks can be E

DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. EDS elemental mappings of W, O, V, Cu, As, Na, N, and C for compound 1.

Figure 6. (a) Cyclic voltammograms of 1-GCE in a 0.5 mol/L H2SO4 solution recorded at 25, 50, 75, 100, 125, and 150 mV s−1. (b) CV curves for 1-GCE, 2-GCE, and 3-GCE at 70 mV s−1. (c) GCD curves of the 1-based electrode at different current densities. (d) Comparative diagrams of the specific capacitance of 1-GCE, 2-GCE, and 3-GCE at different current densities.

applications. The cycling behaviors of 1-GCE for 3000 cycles were tested at a current density of 2.4 A g−1 in the voltage window range from −0.8 to 0.8 (Figure S16). During the first 1000 cycles, the specific capacity decreases by ∼8%, and then

the magnitude of the decrease becomes insignificant. The inset shows that the shapes of GCD curves for the last five cycles have changed very slightly compared with the cycle curve of cycles 1501−1505 for compound 1, which shows its good F

DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) Degradation of the capacitance of 1-GCE, 2-GCE, and 3-GCE during 1000 cycles at a current density of 2.4 mA cm−2. The inset shows the GCD curves of the last cycle and the first cycle for compound 1. (b) EIS spectra of 1-GCE, 2-GCE, and 3-GCE. The inset shows a magnified part of the high-frequency range for the EIS spectra.

Figure 8. Cyclic voltammograms of 1-GCE in a 1 M H2SO4 solution containing (a) IO3− and (b) DA at different concentrations at a scan rate of 50 mV s−1.

the electrode. The charge transfer resistance (Rct, the semicircle of the high-frequency area) is also smaller than that of two control compounds, further demonstrating that the rich π electron system in the object metal−organic network of compound 1 greatly facilitates the acceleration of charge transfer. The slope of the curve in the low-frequency region indicates the Warburg resistance (Zw). Compared with two control compounds, compound 1 has a more perfect straight line with a larger gradient in the low-frequency part, which further suggests that the unique host−guest structure is more conducive to the diffusion of the electrolyte on the electrode surface. The results of EIS analysis indicate that compound 1 had higher conductivity, better electroactivity, and improved capacitance performance compared to those of parent POM and pure 6-Cu-substituted POM, which can be attributed to the introduction of V4+, the unique host−guest structure, and the richness of π electrons in the host metal−organic network composed of polynuclear complexes. The excellent electrochemical properties of 1-GCE may be attributed to the following. (1) The introduction of V4+ into the {AsW9} system enriches and enhances the multielectron redox process at the interface, which improves the electrode activity and fundamentally reduces the inherent resistance of the electrode. (2) The rich π electron system in the host metal−organic network based on two kinds of ligands accelerates charge transfer and reduces the Rct value. (3) The embedded host−guest network provides a convenient

electrochemical stability. In addition, the cycle stabilities of 1-, 2-, and 3-GCE for 1000 cycles were compared at the same current density. As shown in Figure 7a, the specific capacitance decreases 8.3%, 14%, and 19.9% for compounds 1−3, respectively, after 1000 charge/discharge cycles, which shows that compound 1 has a cycle life that is longer than that of parent POM and pure 6-Cu-substituted POM. Moreover, compared with the reported POM electrode and the POMbased composite electrode, the cycling stability of 1-GCE also exhibits outstanding advantages (Table S3), for Na6V10O28 (∼70% after 1000 cycles),13e PPy/PMA (57% after 200 cycles),13d and PMo12-PANI/GS (80% after 1000 cycles).15 As shown in the inset of Figure 7a, the curve shapes of the last circle are similar to that of the first circle. The invariability of the curve shape before and after cycling further suggests the reversible redox performance of compound 1. 1-GCE, 2-GCE, and 3-GCE were examined by electrochemical impedance spectroscopy (EIS), and the results are shown in Figure 7b. The inset figure exhibits an enlarged viewpoint in the high-frequency region, in which the intersection of the curve at the real part shows the resistance of the electrochemical system (Rs). From the figure, the Rs of compound 1 is smaller than those of two control compounds (the measured values are 3.48, 3.89, and 4.17 Ω for compounds 1−3, respectively). This comparison further illustrates that the introduction of V4+, which has better multielectron transfer ability, reduces the intrinsic resistance of G

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channel for the diffusion of the electrolyte on the electrode surface, reduces the Zw value, and improves the conductivity. In addition, there is a certain gap between the host and guest structure that can buffer the change in volume during the GCD process and improve the structural stability and contact property of the electrode material. (4) The unique 3D supramolecular metal−organic networks with 1D cavities not only increase the specific surface area of the complex to provide more electrically active centers for the Faraday reaction but also shorten the distance of ion−electron conversion, which accelerates the reaction kinetics and improves the electrochemical performance. (5) The insolubility of compound 1 obtained by the hydrothermal method may also be the key factor for maintaining a long cycle life during the charging−discharging process. Bifunctional Electrocatalytic Properties. The electrocatalytic reduction of iodate (IO3−) and the oxidation of dopamine (DA) by 1-GCE were also studied. As depicted in Figure 8a, upon addition of potassium iodate, the reduction peak currents II′, III′, IV′, and V′ progressively increase; meanwhile, the corresponding oxidation peak currents decrease, which illustrates that IO3− is reduced by the II−II′, III/III′, IV−IV′, and V−V′ processes. The electrode reaction mechanism of compound 1 with respect to IO3− is shown in the following equations:39 Electrochemical reactions:

Electrochemical reactions: [{AsIIIWVI 9O33}2 (VIV 5Cu IIO6 )]8 − + 2H+ + 2e F [H 2{As IIIWVI 9O33}2 (VIV 3VIII 2Cu IO6 )]8 −

Electrocatalytic reactions:Moreover, the nearly linear relationship between anodic/cathodic current peaks and IO3−/DA concentration indicates the stability and high efficiency of the catalytic activity of 1-GCE (Figure S17).

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CONCLUSION In conclusion, the {VIV5CuIIO6} sandwich units were first introduced into the {AsW9} system to yield a host−guest architecture based on a sandwiched inorganic chain and unique 3D supramolecular metal−organic networks with 1D cavities. Compound 1, as the electrode material of the supercapacitor, exhibits higher specific capacitances (825 F g−1 at a current density of 2.4 A g−1), better rate capability, more durable cyclic stability (91.4% of the cycle efficiency after 3000 cycles), and improved conductivity and electroactivity compared to those of parent POM, pure 6-Cu-substituted POM, which may be attributed to the introduction of V4+, the unique host−guest structure, and the rich π electron system in the host metal− organic network containing polynuclear complexes. In addition, compound 1 exhibits dual-function electrocatalytic behavior in reducing inorganic salt IO3− and oxidizing the organic molecule DA. This serves to enrich the supercapacitor electrode and provide a useful prototype for improving the capacitive performance by changing the ingredients and spatial structure of the POM in situ.

[{As IIIWVI 9O33}2 (VIV 5Cu IIO6 )]8 − + H+ + e F [H{As IIIWVI 9O33}2 (VIV 5Cu IO6 )]8 −

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[H{As IIIWVI 9O33}2 (VIV 5Cu IIO6 )]8 − + 2H+ + 2e

ASSOCIATED CONTENT

S Supporting Information *

F [H3{As IIIWVI 7WV 2O33}2 (VIV 5Cu IO6 )]8 −

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00692. Experimental Section, structural figures (Figures S1− S8), structural data (Tables S1−S3), and physical characterization (Figures S9−S17) (PDF)

[H3{As IIIWVI 7WV 2O33}2 (VIV 5Cu IIO6 )]8 − + 2H+ + 2e F [H5{As IIIWVI 5WV 4O33}2 (VIV 5Cu IO6 )]8 −

[H5{As IIIWVI 5WV 4O33}2 (VIV 5Cu IIO6 )]8 − + H+ + e F [H6{As IIIWVI 4WV 5O33}2 (VIV 5Cu IO6 )]8 −

Accession Codes

CCDC 1900677 and 1901517 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Electrocatalytic reactions: [H6{As IIIWVI 4WV 5O33}2 (VIV 5Cu IO6 )]8 − + IO3− F [{As IIIWVI 9O33}2 (VIV 5Cu IIO6 )]8 − + 3H 2O

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On the other hand, dopamine is a kind of nerve conduction substance; a change in its content can affect a person’s mood. DA deficiency in humans can cause Parkinson’s disease and heart disease,40,41 so it is important to monitor the content of DA in humans by an electrochemical method. As depicted in Figure 8b, the 1-based electrode also shows electrocatalytic oxidation activities toward DA. The anodic and cathodic peak current V(I/I′) wave increases with an increase in DA concentration. The extent of the increase in the anode peak current is much larger than that of the cathode peak current, which illustrates that DA is oxidized by the I−I′ processes of 1GCE. The electrode reaction mechanism of compound 1 with respect to DA is shown in the following equations:42

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: (+86) 0451-88060334. Fax: (+86) 451-8806-0653. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kai Yu: 0000-0002-6662-3724 Baibin Zhou: 0000-0001-8304-5092 Notes

The authors declare no competing financial interest. H

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21771046 and 21571044), the Natural Science Foundation of Heilongjiang Province (JC2016001 and B2017007), and the Opening Project of Key Laboratory of Polyoxometalate Science of Ministry of Education.

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DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.9b00692 Inorg. Chem. XXXX, XXX, XXX−XXX