Two Novel Polyoxometalate-Encapsulated Metal-Organic Nanotube

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Functional Inorganic Materials and Devices

Two Novel Polyoxometalate-Encapsulated MetalOrganic Nanotube Frameworks as Stable and Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction Li Zhang, Shaobin Li, Carlos J. Gomez-Garcia, Huiyuan Ma, Chunjing Zhang, Haijun Pang, and Bonan Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10447 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Applied Materials & Interfaces

Two

Novel

Polyoxometalate-Encapsulated

Metal-Organic

Nanotube

Frameworks as Stable and Highly Efficient Electrocatalysts for Hydrogen Evolution Reaction Li Zhang,†,ǁ Shaobin Li,†,‡,ǁ Carlos J. Gómez-García,§ Huiyuan Ma,*,† Chunjing Zhang,† Haijun Pang,*,† and Bonan Li† †

School of Materials Science and Engineering, College of Chemical and

Environmental Engineering, Harbin University of Science and Technology (HUST) No.4, Linyuan Road, Harbin, 150040, China. ‡

College of Materials Science and Engineering, Heilongjiang Provincial Key

Laboratory of Polymeric Composite Materials, Qiqihar University No. 42, Wenhua Street, Qiqihar, 161006, China. §

Instituto de Ciencia Molecular, Departamento de Química Inorgánica Universidad de

Valencia. C/Catedrático José Beltrán, 2. 46980 Paterna, Spain.

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:

Two

novel

polyoxometalate-encapsulated

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metal-organic

nanotube

frameworks crystalline materials with unprecedented copper-mixed ligands nanotubes, HUST-200 and HUST-201 have been successfully synthesized by an effective synthesis strategy. The encapsulation not only provides a shield to increase the chemical stability, but also does not affect its catalytic activity and, therefore, the crystalline materials are very active for HER (H+ can diffuse easily through the pores of the MONTs). Remarkably, HUST-200 displays low overpotential of 131 mV (catalytic current density is equals to 10 mA·cm−2). This work thus offers a new way for devising HER electrocatalysts with low cost using POM-encapsulated metal-organic nanotube frameworks.

Keywords: polyoxometalate, metal-organic nanotube frameworks, crystalline materials, high stability, high catalytic activity

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INTRODUCTION

As well known, hydrogen (H2) is a renewable fuel of friendly environment, which have overall trend to substitute for the fossil fuels .1,2 Electrocatalytic splitting of water by the hydrogen evolution reaction (HER) to produce H2 is a hot topic in recent years.3-6 Platinum (Pt) is the conventional HER electrocatalyst, howbeit the high-priced and limited reserves may restrict its wide-ranging application.7 Therefore, design and preparation of low-cost, high stable and efficient HER electrocatalysts is of great necessity.8-16

Metal-organic frameworks (MOFs) as a kind of solid-state porous materials, have become a very hot topic in materials chemistry in the last 20 years.17-24 Metal-organic nanotubes (MONTs) with well-defined cylindrical nanostructures can be considered as a special kind of MOFs.25-29 MONTs possesses the advantages of MOFs, such as an open nanoporous structure, large specific surface area and crystallinity.30 These structural features make MONTs promising as non-noble-metal-based catalysts toward HER.31,32 However, it is difficult to purposefully construct MONTs since this requires a control of the growing direction of the coordination bonds, in most cases, these structures are thermodinamically less stable and kinetically more difficult to obtain that the classical extended 2D or 3D lattices. Accordingly, the applications of MONTs are limited due to their low chemical stability. Therefore, the control of the assembly processes for constructing MONTs with chemical stability is distinctly an urgent conundrum. An impactful method is using anions, organic molecules and 3



metal-organic

compounds

as

templates

to

form

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MONTs

directly.33-35

Polyoxometalates (POMs) are outstanding inorganic nanoclusters that could provide many different models to guide the construction of stable MONTs. POMs are particularly attractive for the following reasons: (i) POMs may contain from 4 to 368 metal ions in a single molecule, and their monodisperse size could be regulated from several angstroms up to 10 nm.36,37 Thus, POMs offer the opportunity to control and adjust the pore sizes/shapes of MONTs. (ii) POMs possess many surface oxygen atoms with high negative charges that increase the template-scaffold interactions and the stability of the MONTs.38 (iii) Besides their structural role, POMs can undergo reversible multi-electron transfer processes, since they possess electron and proton transfer and/or storage abilities, which make them show beneficial applications in advanced electrochemical electrode materials and magnetic materials.39-46 Albeit, their electrocatalytic activity is limited by major shortcomings, for example, low surface area and high solubility.47 To solve this problem, we intend to adopt a synthetic strategy consisting in using POMs as a template and insert it into cavities of MOFs to form insoluble POMOFs. Unfortunately, no example of POM inserted into MONTs has been reported to date.

Therefore, an interesting question is whether POM can be encapsulated into the MONTs hosts. If so, such materials should be able to combine the virtues of both matrices, such as catalytic, magnetic properties, the template effect of the POM and the new performances arising from the synergistic reaction. To achieve this aim, the crucial problem is the construction of suitable nanotube frameworks with the adequate 4


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POMs as templates. Two possible strategies may be developed: (i) metal-organic multiple helical chains that form hollow, channel-shaped structures during their extension27 or (ii) the use of metal ions connected by organic ligands generating squares or hexagons that can be connected (via their vertex) by a second organic ligand to generate square or hexagonal nanotubular frameworks. This second strategy allows the preparation of coordination polymers with mixed ligands forming porous nanotubular frameworks.29 Herein, we adopted the second strategy with copper ions, a multidentate ligand as 5-(2-pyrazinyl) tetrazole (pzta) and a rigid secondary ligand as 4,4′-bipyridine (bpy). We have selected the pzta ligand given its high ability to form loops when coordinated to different metal ions and the bpy ligand given its capacity to act as a rigid bridge connecting metal atoms acting as pillars linking the metallic rings to construct the open tubular framework (Scheme S1).48 Here we present two isostructural POM-encapsulated MONTs, [CuII6(pzta)6(bpy)3(X2W18O62)]·2H2O with X = P (HUST-200) and As (HUST-201). HUST-200 and HUST-201 are the first kind of POM-encapsulated copper-mixed ligands MONTs crystalline material. More importantly, this work evidences that POM clusters may serve as a template to guide the construction of MONTs. Unlike most MONTs with limited stability,49 HUST-200 and

HUST-201

exhibit

exceptional

chemical

stability.

Furthermore,

the

as-synthesized HUST-200 and HUST-201 show highly efficient activity in hydrogen evolution reaction under acidic aqueous medium.

5



RESULTS AND DISCUSSION

Syntheses and Characterizations

Synthesis of HUST-200. The mixture of α-K6P2W18O62·15H2O (480 mg, 0.1 mmol), Cu(NO3)2·3H2O (150 mg, 0.6 mmol), pzta (59 mg, 0.4 mmol), bpy (57 mg, 0.3 mmol) deionized water (15 mL) was intensive mixed for 60 minutes. Then mixture was loaded in 23 mL hydrothermal reactor and heated at 160 ºC for 96 hours with a starting pH = 3.4 adjusted with 6 M HCl. The final pH was ca. 3.6. Green block crystals of HUST-200 were obtained (53 % yield based on W) (Figure S1). The reproducibility of HUST-200 is good. Elemental analysis calcd. (%) for C60H46N42Cu6P2W18O64: C 11.75, H 0.76, N 9.59, P 1.01, Cu 6.22, W 53.97, found: C 11.66, H 0.69, N 9.65, P 0.96, Cu 6.33, W 53.86. The CCDC Number: 1588734.

Synthesis of HUST-201. The synthetic method of HUST-201 was similar to that of HUST-200, except that the H6P2W18O62 was replaced by H6As2W18O62. The final pH was ca. 3.5. Yield: 47 % (based on W). The reproducibility of HUST-201 is good. Elemental analysis calcd. (%) for C60H46N42Cu6As2W18O64: C 11.58, H 0.75, N 9.45, As 2.41, Cu 6.13, W 53.21 %; Found: C 11.52, H 0.69, N 9.61, As 2.35, Cu 6.21, W 53.11 % (the details of experimental section in the Supporting Information). The CCDC Number: 1588962.

Single Crystal Structure. Single crystal structure analysis indicates that two compounds are isostructural. Therefore, we only describe the structure of HUST-200.

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The asymmetric unit is comprised by one [P2W18O62]6- polyanion (abbreviated as P2W18), six cooper ions, six pzta ligands and three bpy ligands (Figure S2). The lengths of Cu-O and Cu-N bonds are in ranges of 2.220(7)-2.426(8) Å and 1.950(9)-2.092(10) Å, respectively. The bond lengths of title compounds are within the normal values (Table S2).50 Bond valence sum (BVS) calculations show that tungsten and copper ions are in the +II and +VI oxidation states, respectively. (Table S3).51 The structure of HUST-200 exist three crystallographically independent Cu ions (Cu1, Cu2 and Cu3) (Figure S3). Cu1 and Cu2 present six-coordinated modes. The coordination environment of Cu1 is two O atoms from P2W18 polyoxoanions and four N atoms from pzta ligands. The coordination environment of Cu2 is two O atoms from two P2W18 polyoxoanions and four N atoms from two pzta ligands and two bpy ligands. Cu3 is penta-coordinated, with trigonal biyramidal coordination mode, which formed by one O atom from one P2W18 polyoxoanion and four N atoms from three pzta ligands and one bpy ligand. Copper ions are linked by six pzta bridging ligands to generate two kinds of Cu6(pzta)6 macrocycles (A and B). Macrocycle A is a 33-membered ring with a diameter of 14.431 Å whereas macrocycle B is a 36-membered ring with a diameter of 14.871 Å. Additionally, two pzta ligands chelate one copper ion to generate Cu(pzta)2 fragments (Figure S4). One of the intriguing features of HUST-200 is the quasi-capsule subunit, which is formed by six bridging [Cu(pzta)2] fragments and six bridging bpy ligands connecting two A-type and one B-type macrocycles (Figure 1a). A prominent structural feature of HUST-200

7



is the 1D metal-organic nanotube formed by six bpy ligands along the c axis bridging the quasi-capsule subunits (Figure 1b).

Figure 1. (a) The formation of the quasi-capsule subunit. (b) The 1D metal-organic nanotube in HUST-200.

Additionally, each MONT is connected to its six surrounding MONTs through Cu1 and Cu2 ions to produce a rare 3D nanotubular aggregate (Figure 2a). POM-encapsulated MONTs assembled by covalent bonds have not been reported so far. It is noteworthy that the metal-organic nanotube has a suitable cross section to contain the Wells-Dawson POM with an exterior wall diameter of 1.4 nm and an interior channel diameter of 1.1 nm (Figure S5). The P2W18 polyoxoanions are linked to the internal wall of the MONT through a total of nine bonds between the Cu(II) atoms of the nanotube and the oxygen atoms of the POM (Figure 2b).

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Figure 2. (a) 3D POM-encapsulated MONTs architecture. (b) Diagram of POM linkage mode in 1D metal-organic nanotube.

It should be noted that HUST-200 represents the first copper-mixed ligands metal-organic nanotube encapsulating a POM. The overall 3D structure formed by the nanotubes connected in a honeycomb like structure can be described as a (4,5,6,9)-connected

network

with

point

symbol

(32·64)(33·44·51·61·71)

(32·46·64·83)(39·411·57·64·72·82), if Cu1 Cu2, Cu3 cations and Dawson POM clusters are successively regarded as 4,5,6,9-connected nodes. (Figures S6, S7 and S8). All the attempts to construct these frameworks using other POMs have failed because the MONT could not be stabilized with other types of POMs. This proves that Dawson POM (suited elongated shape and structure size) is an efficient template in this building-up process for construction of MONT. SEM, EDS, PXRD, IR, and ICP Spectroscopic Characterizations. Scanning electron micrographs (SEM) with energy dispersive spectroscopy (EDS) was employed to further confirm the structure of HUST-200 and HUST-201. The SEM images of HUST-200 and HUST-201 shows polyhedron shape and micron-size of 9



single crystal (Figure S9). EDS analysis of HUST-200 and HUST-201 are shown in Figure S10. The formation of HUST-200 and HUST-201 were found to consist of only O, C, P, N, Cu, W and O, C, As, N, Cu, W elements, respectively. Moreover, it can be seen from elemental mapping images that all the elemental compositions are uniformly distributed (Figure 3 and Figure S11).

Figure 3. The EDS elemental mapping images of HUST-200.

Study and Discussion on Stability. As is known, large structural voids are unstable and therefore most of MOFs are easy to be collapsed in water/acidic/basic solutions.9,52-54 Also, polyoxometalates (POMs) are often decomposed in basic solutions.55 However, the HUST-200 and HUST-201 are two extra-stable POM-encapsulated metal-organic nanotube framework materials. The HUST-200 and HUST-201 are air-stable, they still maintain their crystallinities without efflorescence after several months. PXRD and IR test is an effective method to prove the stability of the material and thus it is widely employed in many reports.11,38,56,57 Therefore, the stability of the title materials was firstly investigated by PXRD and IR tests. The 10


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HUST-200 and HUST-201 are stable into various organic solvents (DMF, acetone, CH3OH, CH2Cl2, and CH3CN solutions) (Figure 4). The HUST-200 and HUST-201 are also stable in acidic and basic solutions (pH = 1-13) (Figures S12, S13), confirmed by PXRD and IR measurements. Furthermore, the results of ICP measurements (Table S4) indicate that the content of Cu and W in the filtrate are negligible, further confirming the stability of both compounds at pH = 13. It is rare that POM hybrid materials are stable into acidic and basic solutions.11,58 Additionally, according to a very recent review,59 there are two methods to characterize the electrochemical stability of the electrodes: (1) cycling experiments were tested by using cyclic voltammetry (CV) and linear sweep voltammetry (LSV), and (2) the current variation with time (I-t curve). Figure 5c shows that after continuous 2000 cycles CV scanning, the LSV curve is close to the initial one, indicating HUST-200 and HUST-201-modified electrodes are stable in the electrochemical process under these conditions. The second method (I-t curve) for characterizing the electrochemical stability of the electrodes further confirms the stability of the title compounds (Figure S14). In all, all of the above results suggest that the HUST-200 and HUST-201 are highly stable materials. This high stability thanks to the protection provided by the MONT surrounding the Dawson POMs. The protective shield provided by the MOF improves the stability of in water/acidic/basic media, as observed in single molecule magnets inserted in carbon nanotubes.60 Meanwhile, POMs possess many surface oxygen atoms with high negative charges that increase the template-scaffold interactions and the stability of 11



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the MOFs. This encapsulation of title compounds provides a shield to increase the chemical stability (until very acid pH media and different organic solvents). More important, the encapsulation does not affect its catalytic activity and, therefore, these materials are very active for hydrogen evolution reaction (H+ can diffuse easily through the pores of the MONTs).

Figure 4. PXRD patterns of HUST-200 a) and HUST-201 b) after soaked in organic solvents.

Hydrogen Evolution Reaction Performance. The high stability of HUST-200 and HUST-201 prompted us to study their catalytic activity for the HER (the details experiment process of HER in the Supporting Information). The results of the experiments are shown in Figure 5. As well known, overpotential, Tafel slope and exchange current density are reliable points to evaluate the HER activities of materials. Both HUST-200 and HUST-201 doped carbon black composite electrodes show high activity toward HER, especially for HUST-200. Namely, HUST-200 (loading amounts of 50 wt %) demands overpotential of 131 mV (current density=10 mA·cm−2).

These

values

improved

those

observed

12


in

some

excellent

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(POM-)MOF-based HER catalysts (Table S5).11 Obviously, our composite electrodes exhibit lower overpotentials than these reported materials, suggesting that the electron transfer on modified electrode surface is faster and more efficient. Furthermore, a comparative study of the electrocatalytic activity of different materials (including two related POMs and a Cu/ptza/bpy coordination network have been performed and presented in supporting information. As can be seen in Figure S15, Cu/ptza/bpy has the poorest HER performance because it is difficult to obtain 3D coordination networks without POMs as templates. As well known, POMs could be used as outstanding electrocatalysts, since they possess a multi-step electron and proton transfer and/or storage abilities.41,46 Albeit, their electrocatalytic activity is limited due to high solubility and low surface area. Using POMs as a template and insert it into cavities of MOFs to form POMOFs may induce a huge enhancement of their electrocatalytic activity because MOFs possess large specific surface area, ordered porous structure, tunable space structures and crystallinity and, very important, the resulting POMOFs are insoluble.47,59 Therefore, the encapsulation of POMs into 3D framework show a synergistic activity and overcome the disadvantages of POMs alone.

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Figure. 5 HER performance of crystalline materials. (a) Polarization curves of materials. (b) Tafel plots of corresponding materials. (c) Cycling stability tests of crystalline materials. (d) EIS Nyquist plots of crystalline materials.

As shown in Figure 5b, the Tafel plots are fitted by the Tafel equation.9 Our measurement of Pt/C shows a Tafel slope of approximately 30 mV·dec−1.61 The measurements reveal that Tafel slopes of HUST-200 and HUST-201 are lower than some excellent (POM-)MOF-based HER catalysts (Table S5). The exchange current density (j0) of HUST-200 is equal to 0.53 mA·cm−2. HUST-200 presents lowest overpotential, highest exchange current density and lowest Tafel slope than above mentioned reported (POM-)MOF materials. HUST-200 represents a promising high-performance non-noble-metal HER electrocatalyst. According to the above series of tests and previously reported relevant literature, a potential mechanism for the HER was deduced as follows: HER must undergo a 14


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multi-step electrochemical process occurring on the surface of a work electrode. In the acid electrolyte, three main steps may be involved.62,63 In first step, a hydrated proton and electron chemically attaches on the catalyst (cat), namely, a Volmer reaction [Eq. (1)]. In second step, either an electrochemical desorption step (Heyrovsky reaction) [Eq. (2)] or a recombination step (Tafel reaction) [Eq. (3)] takes place. In the Heyrovsky route, the generated cat-H combines with a hydrated proton, and then couples with an electron from the catalyst surface forming H2 [Eq. (2)]. Instead, in the Tafel route, two cat-H atoms combine directly [Eq. (3)]. Volmer reaction: H3O+ + e- + cat ⇌ cat-H + H2O

(1)

Heyrovsky reaction: cat-H + H3O+ + e- ⇌ cat + H2 + H2O

(2)

Tafel reaction: cat-H + cat-H ⇌ 2cat + H2

(3)

Tafel slope is an inherent property of the electrocatalysts and commonly used to elucidate the predominant HER mechanism.64 Generally, a slope of 120 mV dec-1 indicates that the Volmer reaction is the rate-determining step of the HER. When the Heyrovsky or Tafel reactions are the rate-determining step, the slope is 40 and 30 mV dec-1, respectively.65 A complete HER process usually involves the combination of steps: the Volmer-Heyrovsky mechanism or the Volmer-Tafel mechanism. In this work, HUST-200 and HUST-201 exhibit Tafel slope of 51 and 79 mV dec-1, respectively. This HER process could attribute to a Volmer-Heyrovsky mechanism. Figure 5d exhibits the ESI Nyquist plots of electrodes, which were fitted by semicircle features to achieve a charge transfer resistance. HUST-200 exhibits the lower charge transfer impedance, indicating the reason for excellent HER performance. 15



In summary, the encapsulation of POMs in HUST-200 and HUST-200 does not affect their catalytic activities and, therefore, these materials are very active for HER (H+ can diffuse easily through the pores of the MONTs). Also, this encapsulation provides a shield to increase the chemical stability (until very acid pH media). As far as we know, HUST-200 has superior HER activity compared to other MOFs and POMOFs electrocatalysts till now. Magnetic Properties. The magnetic susceptibility (χm) of HUST-200 and HUST-201 were measured in range of 2-300 K for both crystalline materials (the detailed magnetism discussion of HUST-200 and HUST-201 in section 4 of the Supporting Information). The thermal variation (χmT) per six Cu centres is equals to 2.4 cm3 K mol-1, approximate to value of six independent S = ½ centres (Figure 6a). When the temperature is lowered, χmT shows a progressive decrease to reach a value close to zero at low temperatures. This behaviour indicates the presence of moderate antiferromagnetic Cu-Cu interactions. An additional proof of this behaviour is provided by the thermal variation of χm that reveals a maximum value at ca. 40-50 K, authenticating an antiferromagnetic Cu-Cu interaction into MONT by this mode (Figure 6b). Since the structure of HUST-200 and HUST-201 shows the presence of Cu(pzta)2 units that generate chains through -N-N-N- bridges of a pzta ligand, and a simple S = ½ antiferromagnetic chain pattern was employed to reproduce magnetic properties of both materials. This pattern reproduces very satisfactorily the magnetic properties of both compounds with the following set of parameters: g = 2.128, J = -40.9 cm-1 and a paramagnetic impurity c = 5.1 % for HUST-200 and g = 2.153, J 16


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= -41.8 cm-1 and c = 4.5 % for HUST-201 (solid lines in Figure 6, the hamiltonian is written as H = -JSiSi+1). These J values (Figure S14) are similar to those found in other Cu(II) compounds with the same type of bridge (-43.4 and -78 cm-1) 54,66 and confirm the presence of moderate antiferromagnetic Cu-Cu interactions in HUST-200 and HUST-201.

Figure 6. Magnetic properties of HUST-200: (a) Thermal variation of χmT. (b) Solid line is fitted to the model. Inset shows structure of the Cu(II) chain.

CONCLUSION

In summary, two unprecedented POM-encapsulated metal-organic nanotube frameworks crystalline materials, HUST-200 and HUST-201 have been successfully synthesized. These compounds represent the first examples of Dawson-type POM-encapsulated MONTs. Furthermore, HUST-200 and HUST-201 exhibit exceptional chemical stability due to the encapsulation. Remarkably, HUST-200 is a stable and highly efficient electrocatalyst in HER under acidic aqueous medium. Successful synthesis of title compounds constitutes an actual case of stable

17



POM-encapsulated MONTs, and provides highly efficient electrocatalysts for HER. Further investigation is currently underway using other metals (including cobalt) to construct the MONTs, and other POMs to be inserted in the MONTs.

ASSOCIATED CONTENT

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, supplementary structural information and physical characterizations, and detailed magnetism discussion of HUST-200 and HUST-201. AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected] ORCID Huiyuan Ma: 0000-0002-2695-3243 Haijun Pang: 0000-0002-0405-8352 Shaobin Li: 0000-0002-8818-3547 Author Contributions ǁ

Z. Li and S. B. Li contributed equally to this work.

Notes The authors declare no competing financial interest. 18


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ACKNOWLEDGEMENTS

This work was financially supported by the National Science Foundation of China (21671049, 51572063, 21371041 and 21603113), the National Science Foundation of Heilongjiang Province (QC2016014). We thank the Generalitat Valenciana (Prometeo2014/II/076 project) and the Spanish MINECO (Project CTQ2017-87201-P) for financial support.

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Table of contents:

The first examples of Dawson-type polyoxometalate (POM)-encapsulated in metal-organic nanotubes (MONTs) have been synthesized, which exhibit the highly active in hydrogen evolution reaction (HER) as electrocatalysts.

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Two Novel Polyoxometalate-Encapsulated Metal-Organic Nanotube

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