Enhanced Catalytic Performance for Oxygen Reduction Reaction

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Enhanced Catalytic Performance for Oxygen Reduction Reaction Derived from Nitrogen-Rich Tetrazolatebased Heterometallic Metal-Organic Frameworks Li-Qian Ji, Jing Yang, Zi-You Zhang, Yong Qian, Zhi Su, Min Han, and Hong-Ke Liu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00190 • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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Crystal Growth & Design

Enhanced Catalytic Performance for Oxygen Reduction Reaction Derived from Nitrogen-Rich Tetrazolate-based Heterometallic Metal-Organic Frameworks

Li-Qian Ji‡, Jing Yang‡, Zi-You Zhang, Yong Qian, Zhi Su*, Min Han* and Hong-Ke Liu* Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210046, China

Abstract

Efficient non-noble metal catalysts for oxygen reduction reaction (ORR) have been highly attractive for the fabrication of cost-effective fuel cells. Metal–organic frameworks (MOFs) derived heteroatom-doped carbon-based (NC) electrocatalysts have exhibited comparable electrocatalytic activity for ORR as commercial Pt/C catalyst, which were highly dependent on their MOF precursor/template structures. In this particular work, two NC composites, NC-1100 and MnO@NC-1100 (NC, nitrogen-doped carbon), have been successfully synthesized, which were derived from novel nitrogen-rich tetrazolate-based metal-organic frameworks, [Cd2(L)(OH)(H2O)] (1)

and

[Cd3Mn(L)2(OH)2(H2O)2]

(2)

(L,

5’-(4-(1H-tetrazol-5-yl)

(benzamido)-benzene-1,3-dioic acid), respectively. The structural differences between complexes 1 and 2 arised from the introduction of Mn(II) to complex 2, where the Cd(II) in 1 was partially replaced by Mn(II). Both complexes 1 and 2 possessed three-dimensional (3D) structures with one-dimensional (1D) open channels. The resultant MnO@NC-1100 from the pyrolysis of complex 2 at 1100 oC under Ar atmosphere has indicated much better electrocatalystic behavior for ORR over the NC-1100 from complex 1, due to the existence of additional active sites of cubic phase MnO particles. The presence of initial Cd(II) benefits the spatial isolation of Mn(II) and prevents the sintering of MnO during the pyrolysis for MnO@NC-1100. The evaporation of Cd(II) and the explosion of nitrogen-rich tetrazolate groups would also promote the surface area of the resulting catalysts. Our study further demonstrated that the multiple metal centers of MOF could be the rational strategy to

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enhance

the

electrocatalytical

performance.

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Compared

with

NC-1100,

MnO@NC-1100 exhibited the enhanced electrocatalytic activity with an onset potential (Eonset) of 0.90 V and a half-wave potential (Ehalf-wave) of 0.74 V, which provided an insight view of structure-property relationship.

Introduction

Oxygen reduction reaction (ORR) is an important electrode reaction for electrochemical energy conversion and energy storage, including fuel cells, metal–air batteries and water electrolysis, etc.1 Currently, Pt and Pt-based catalyst were still widely used. However, the high cost and scarce reserve significantly prohibit their large-scale application.2 Nanoporous carbon-based materials have emerged and been shown to possess efficient catalytic activity toward ORR with low cost.3 Moreover, heteroatoms-doped carbon materials (including non-metallic and metallic atoms) could avoid the electro-neutrality of the carbon matrix and create more active sites to enhance the ORR activity.2,

4-7

Metal–organic frameworks (MOFs) derived

carbon-based electrocatalysts have attracted tremendous interests and exhibited excellent electrocatalytic activity, due to its high surface area, large pore volume, orderly porous structure, tunable chemical structure and diverse composition.5,

8-14

MOF-derived carbon materials could effectively prevent the aggregation of the metal atoms due to homogeneous distribution of the metal centers in the framework, which could benefit the catalytic efficiency.8, 9 On the other hand, the decorated ligand in MOF (such as nitrogen-, phosphorus-, polyoxometalate-including and so on) could provide multi-components into the pyrolysis resultant to compensate the deficiency of a single component.15, 16 Zeolitic imidazolate frameworks (ZIFs), such as ZIF-8 and ZIF-67, were the most extensively-used precursors for carbon-based electrocatalysts, which could provide M–N–C active sites after carbonization to promote ORR catalytic activity.8,

11, 17, 18

Inspired by this idea, the ligand 5’-(4-(1H-tetrazol-5-yl)

benzamido)-benzene-1,3-dioic acid (L) was designed and synthesized to construct MOF precursors with the following considerations: first, the tetrazolate-based

MOF

could convert into nitrogen-dope composite carbon materials, which may possess


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promising electrocatalytic property;19 second, MOF materials based on this π-conjugated rigid backbone and energetic tetrazolate groups could own high porosity, which may benefit the generation of the resultant with high porosity after pyrolysis; third, both tetrazolate and carboxylate group could offer variable coordination modes, which could increase the diversity of the resulting MOF structures. Herein, two isostructural MOFs, [Cd2(L)(OH)(H2O)] (1) and bimetallic [Cd3Mn(L)2(OH)2(H2O)2] (2), were designed and successfully synthesized as the ORR catalyst precursors, where both complexes exhibited porous structures. It is worthy to note that the structure differences in 1 and 2 were induced by the introduction of Mn(II) to complex 2, where Cd(II) in complex 1 was partially replaced by Mn(II) in complex 2. MnO@NC-1100 (NC, nitrogen-doped carbon) derived from pyrolysis of complex 2 at 1100 oC under Ar atmosphere has indicated much better electrocatalystic behavior for ORR over the NC-1100 derived from complex 1, due to the presence of additional active sites of cubic MnO particles. The diffusion of Cd(II) and the explosion of the energetic tetrazolate group favored the generation of the porosity in the resultant after pyrolysis, which could also promote their electrocatalytic performances. This work has provided a great example for rational MOF structural design with enhanced functionalities. Experimental Section Materials and Measurements All solvents and starting materials were purchased commercially and were used as received without further purification. The ligand (L) was synthesized according to previously reported literature with minor modification.20 The FT-IR spectra were obtained on a Bruker Vector 22 FTIR in the range of 400-4000 cm−1 using KBr pellets. Powder X-ray diffraction (PXRD) measurements were recorded on a Bruker D8 Advance X-ray diffractometer with Cu Kα (λ = 1.5418 Å) radiation.

Elemental

analyses (EA) for C, H, and N were determined with a Perkin-Elmer 240C elemental analyzer. Thermogravimetric analyses (TGA) were taken on a Mettler-Toledo (TGA/DSC1) thermal analyzer under a N2 atmosphere at a heating rate 10 0C/min.



Inductively coupled plasma spectroscopy (ICP) was collected on a Optima 5300DV. The X-ray crystallographic data were collected on a Bruker Smart Apex II CCD diffractometer using graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å) from a rotating anode generator. Surface area and pore distribution were assessed from nitrogen sorption isotherms measured at 77 K with a Micromeritics ASAP 2050.The Raman spectra were acquired on a JY HR 800 (France) instrument with an optical multichannel spectrometer Microdil 28 (Dilor) equipped with a microscope. An objective with 100× magnification was used both for focusing the excitation light (Ar+ laser, 488 nm) and for collecting the scattered light. The SEM images were acquired by JSM-4800 at an acceleration voltage of 10 kV. The related transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images as well as the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and elemental mapping analysis were carried out on JEOL-2100F and probe aberration-corrected JEM ARM 200F apparatuses, respectively. The X-ray photoelectron spectra (XPS) were recorded on a scanning X-ray microprobe (PHI 5000 Versa, ULACPHI, Inc.) that uses Al Kα radiation. XPS spectra were calibrated by referencing C 1s peak to 284.8 eV. Preparation of [Cd2(L)(OH)(H2O)] (1) A mixture of L (35.3 mg, 0.1 mmol), CdCl2 (18.4 mg, 0.1 mmol), H2O (7.0 ml) and NaOH (8.0 mg, 0.2 mmol) were sealed in a Teflon-lined stainless steel container and heated at 140 oC for 3 days. After being cooled to room temperature, colorless prismatic crystals of 1 were obtained in 54% yield based on the consumed L. Elemental analysis (%): C, 31.02; H, 2.37; N, 11.10. IR (KBr pellet, cm-1): 3295(m), 1660(m), 1614(m), 1548(s), 1422(m), 1371(m), 1292(m), 1244(w), 1170(w), 1110(w), 1011(m), 968(w), 906(w), 885(s), 860(m), 838(w), 777(m), 730(m), 673(w), 607(m), 548(w), 536(w), 509(w), 441(w). Preparation of [Cd3Mn(L)2(OH)2(H2O)2] (2) A mixture of L (17.7 mg, 0.05 mmol), CdCl2 (9.2 mg, 0.05 mmol), MnCl2·4H2O (9.9 mg,0.05 mmol), H2O (7.0 ml) and NaOH (4.0 mg, 0.1 mmol) were sealed in a Teflon-lined stainless steel container and heated at 180 oC for 3 days. After the


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reaction mixture cooled to room temperature, colorless octahedral crystals of 2 were obtained in 51% yield based on the consumed L.

Elemental analysis (%): C, 30.83;

H, 2.95; N, 10.96. IR(KBr pellet, cm-1): 3307(m), 1660(w), 1618(m), 1551(s), 1420(m), 1370(s), 1295(w),

1279(w), 1257(w), 1242(w), 1183(w), 1145(w),

1106(w), 1012(m), 967(w), 909(m), 887(m), 858(w), 810(w), 778(m), 729(m), 691(w), 672(w), 609(m), 555(w), 523(w), 445(m). Single Crystal X-ray Crystallography The single-crystal diffraction data for 1 and 2 were collected on Bruker Smart Apex II CCD detector at 296(2) K. The X-ray generator was operated at 50 kV and 35 mA using Mo-Kα (λ = 0.71073 Å) radiation. The structures of 1 and 2 were determined by direct methods and refined by full matrix least-squares refinements based on F2 with the SHELXL Program.21, 22 All non-hydrogen atoms were refined with anisotropic temperature parameters. The hydrogen atoms except for those of water molecules were generated geometrically and refined isotropically using the riding model. In 1, free water molecules were highly disordered and difficult to model. The obtained crystal data of 1 and 2 were treated by the SQUEEZE routine in PLATON, and the results are attached to the CIF file.23, 24 Relevant crystallographic data and structure refinement results were listed in Table 1 and selected bond lengths and angles were listed in Table S1. CCDC No. 1880539 and 1880540. Syntheses of MnO@NC and NC Complexes 1 and 2 were used as the template to synthesize the carbon-based catalysts. In detail, complexes 1 and 2 were placed at the tube furnace and heated to 1100 °C under Ar atmosphere with a heating rate of 3 oC min-1 and kept at 1100 °C for 2 h. The products after heating were defined as NC-1100 and MnO@NC-1100 for complexes 1 and 2, respectively. For comparison, the MnO@NC-1000 and the MnO@NC-900 from complex 2 were also prepared with similar annealing procedures except the annealing temperature switching from 1100 oC to 1000 °C or 900 °C, respectively. Electrocatalytic Tests All electrochemical measurements were carried out in a three-electrode system on



Page 6 of 24

the electrochemical workstation (CHI 660E) that equipped a Gamry’s rotating disk electrode with the diameter of 5 mm. A platinum foil and Ag/AgCl were used as the counter and reference electrodes, respectively. The RDE modified by the as-prepared catalysts were used as the work electrodes, and the work electrodes was prepared according to a previous report.25 For ORR tests, the 0.1 M KOH solution was used as the electrolyte that was continually bubbled with pure O2 to guarantee an O2-saturated medium during the whole experiment. The LSV plots were recorded at potential range between -0.8 and 0.2 V (vs Ag/AgCl) with some rotating rates, such as 2500 rpm, 1600 rpm, 1200 rpm, 800 rpm and 400 rpm. For comparison, all the potential was calibrated to reversible hydrogen electrode (RHE) values based on the equation: E (RHE) = E (Ag/AgCl) + 0.197 + 0.0591*pH. The transferred electron number (n) during the ORR process can be calculated according to the Koutecky-Levich (K-L) equations: 1 𝐽

1

1

(1)

= 𝐽𝐾 + 𝐵𝜔0.5 2 3 ―1 6

B = 0.62nF(𝐷𝑂2)

v

𝐶𝑂2

(2)

Where, J, Jk and ω is the current density, the kinetic current density and the rotating rate of the electrode, respectively. In addition, F, DO2, v and CO2 is the Faraday constant, the diffusion coefficient of O2, the kinetic viscosity and the bulk concentration of O2, respectively.26 Results and Discussion Characterization of complexes 1 and 2 Complexes 1 and 2 were isostructural, which were synthesized with single component Cd(II) ions and bimetallic Cd(II)/Mn(II) ions, respectively (Table 1). Due to the different requirements of the coordination geometry for Cd(II) and Mn(II), even with similar tetranuclear units [Cd4(3-OH)2 in 1 and Cd3Mn(3-OH)2 in 2] and same coordination modes for the ligand (Scheme S1), the resulting structures for complexes 1 and 2 are completely different. Strong broad vibrations around 3300 cm-1 in the IR spectra have been observed for both complexes 1 and 2, which suggested the


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existence of bridging hydroxyl groups (Fig. S1). Single crystal X-ray structural analysis reveals that 1 crystallizes in monoclinic C2/c space group and exhibits a 3D framework. There are two Cd(II) atoms, one L, one hydroxy (3-OH) and one coordinated water molecule in the asymmetric unit. As shown in Fig. 1A, both Cd1 and Cd2 could be described as six-coordinated. Cd1 was coordinated by one carboxylate oxygen atom (O4), two hydroxyl oxygen atoms (O6, O6B), one coordinated water molecule (OW1) and two nitrogen atoms (N2C, N3D). Cd2 was ligated by three carboxylate oxygen atoms (O1A, O2A, O3B), one hydroxy oxygen atom (O6) and two nitrogen atoms (N1C, N4E) (Fig. 1A). It is worthy to note that the four nitrogen atoms in tetrazolate group all participated in the coordination (Scheme S1), which has been rarely reported due to the steric hindrance.27, 28 The two carboxylate groups adopted different coordination modes, chelate and bridging modes (Scheme S1), respectively. The dihedral angle between the tetrazolate group and the isophthalate group was 65.6, which would facilitate the formation of the resulting 3D structure. The symmetric tetranuclear units was formed via two 3-OH linking four Cd(II) ions, named Cd4(3-OH)2, which was further connected through the tetrazolate groups to the 1D strip-shaped chain the c axis (Fig. 1B). A typical 2D (4,4)-network was constructed along the bc plane by the isophthalate groups and tetranuclear units (Fig. S2). The interaction of the 1D chain and 2D plane through the phenyl group resulting in the final 3D structure of complex 1(Fig. 1C). An open 1D oval-shape channel with a diameter around 7.6 Å was simultaneously formed after removing the coordination water molecules (Fig. 1C). To better insight into the 3D structure of complex 1, topological analysis was performed, where each [Cd4(µ3-OH)2] unit and each ligand could be regarded as 8- and 4-connected nodes, respectively. Thus, the resulting structure of 1 could be simplified as a (4,8)-connected bi-nodal 3D net, as shown in Fig. 1D, and the point (Schläfl) symbol is (412·610·86)(46)2. Complex 2 also crystallizes in the monoclinic space group C2/c and exhibits a 3D framework as complex 1. The only difference was the introduction of the Mn(II) to complex 2, which partially replaced the Cd(II) ions. The asymmetric unit of 2



Page 8 of 24

consists of two independent Cd(II) ions (Cd1 and Cd2) and one Mn(II) ion (Mn1), where both Cd1 and Mn1 were with the 50% occupancy (Fig. 2A). As in complex 1, all metal centers in complex 2 were six-coordinated with distorted octahedral coordination geometries by the carboxylate and tetrazolate groups from the ligand, the bridging 3-OH group and the water molecules (Fig. 2A). The ligand in complex 2 adopted the exactly same coordination modes as that in complex 1, except the replacement of Cd(II) by Mn(II) (Scheme S1). To satisfy the coordination requirement of Cd(II) and Mn(II), the ligand was further twisted and the dihedral angles between the tetrazolate group and the isophthalate group shrank from 65.6 in 1 to 60.2 in 2. Similar 1D chain has also been observed in complex 2, the detail structure, however, is completely different from that in complex 1 (Figs. 2B and S3). The [Cd3Mn(µ3-OH)2] units were inter-connected through the common vertex µ3-OH and combined with the tetrazolate groups to the resulting 1D chain in complex 2 (Fig. 2B), whereas the [Cd4(µ3-OH)2] units in complex 1 were separated and linked through the tetrazolate groups to the 1D chain (Fig. 1B). Due to the existence of the Mn(II) breaking the symmetric structure of [Cd4(µ3-OH)2] unit, the 1D chain in complex 2 twisted to a S-shape with an amplitude of 4.0 Å, comparing to nearly flat in complex 1 (Fig. S3). 3D structure of complex 2 was formed with the further support of the isophthalate part from the ligand, and two 1D open rectangular channels was found with the size of 4.5×7.6 Å and 4.2×5.1 Å after removing the coordinated water molecules (Fig. 2C). Similar topological analysis was also done for complex 2, which is

a

bi-nodal

(4,10)-connected

framework

with

the

point

symbol

of

(3·45)2(34·412·58·614·75·82) (Fig. 2D). The morphology of the particles of complex 2 was observed by SEM analysis (Fig. S4). Complex 2 has shown a nanorod morphology with an average ~2 m length, and the phased purity has been confirmed as the bulk crystal sample by PXRD, where the most diffraction patterns are well matched the simulated pattern results (Fig. S5). TGA were also performed to check the thermal stability of the frameworks, and


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the TGA curves of 1 and 2 were shown in Fig. S6. Both complexes have been shown an obvious weight loss during the temperature range of 30 – 150 oC (11.4% for complex 1 and 11.7% for complex 2), which resulted from the liberation of the coordination water molecules and the undefined solvent molecules in the channels. Both complexes could maintain the framework integrity over 300 oC, showing relative high thermal stability. Characterization of MnO@NC-1100 The development of electrode materials with suitable physicochemical properties and microstructures is essential for the ORR performance. MOF-derived nano- and micro-structure materials have been widely studied recently due to their promising electrocatalytical behaviors.9,

15

In this case, complexes 1 and 2 were placed at the

tube furnace and heated to 1100 °C under Ar atmosphere for 2 h, and the resulting products for 1 and 2 were defined as NC-1100 and MnO@NC-1100 (NC, nitrogen-doped carbon), respectively. The tetrazolate group would explode during the heating procedure and the Cd(II) ions would diffuse after the temperature over its boiling point (765 C), which both could benefit the generation of the porosity and its electrocatalytic properties. Thus, the annealing products at 1100 °C would be metal-free for 1 and manganese-containing only for 2, and better electrocatalytic property of MnO@NC-1100 from complex 2 could be expected. The composition and phase structures of the typical MnO@NC-1100, annealing product from complex 2, were characterized by PXRD, Raman spectra and XPS. PXRD patterns for typical MnO@NC-1100 have shown in Fig.3A. The sharp diffraction patterns at 34.9°, 40.5°, 58.6°, 70.1°, 73.7°, 87.7° could be indexed to the (111), (200), (220), (311), (222) and (400) facet of the cubic phase MnO (JCPDS 75-1090) and the broad patterns at 26° could be assigned to the (002) facet of the hexagonal graphite structure.29 The ID/IG value of MnO@NC-1100 (ID/IG=1.38) is relatively higher than that of NC-1100 (ID/IG=0.97), which indicates the presence of more defects in MnO@NC-1100 (Fig. 3B).1 The porous structure in the obtained MnO@NC-1100 and NC-1100 was further confirmed by nitrogen sorption measurement. The N2 sorption isotherm of



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MnO@NC-1100 displayed a clear curve hysteresis loop and the BET surface area was calculated to be 123.9 m2 g−1 (Fig. S7A).6 The pore size distribution of MnO@NC-1100 indicated the existence of mesoporosity

with

the range of 2 - 50

nm and total volume in pores is 0.2 cm3 g-1 based on the NL-DFT (non-local density functional theory) model.17 The large hysteresis loop at high relative pressure (P/P0  0.4) suggesting the presence of intrinsic micropore, which could be beneficial for efficient mass transport and smooth diffusion of electrolyte and oxygen during the ORR.17

In contrast, NC-1100 showed a typical type I isotherms with a BET surface

area of 358.3 m2 g−1 (Fig. S7B). The microporous size distribution ranges from 0.7 to 2 nm and the pore volume is 0.26 cm3 g-1 for the NC-1100 based on NL-DFT model (Fig. S7B).17 The dramatic differences of pore size and BET surface area in the MnO@NC-1100 and NC-1100 were resulting from the introduction of the second metal source(Mn) and the structural distinction from precursors 1 and 2. To obtain the insight into the elmental composition and surface chemistry of MnO@NC-1100, XPS and ICP were

also performed. XPS survey spectra confirms

the presence of C, N, Mn, and O elements. The high-resolution C 1s spectrum (Fig. 3C) could be fitted into three parts of C=C (284.8 eV), C-N (285.8 eV) and O=C-O (289.9 eV).30, 31 The high-resolution N 1s spectrum (Fig. 3D) of MnO@NC-1100 was assigned to four types: pyridinic N (398.6 eV), pyrrolic N (399.8 eV), (400.1eV), and quaternary N (402.5 eV).31,

32

graphitic N

The high-resolution Mn 2p spectrum

with two major peaks corresponded to Mn 2p3/2 and 2p1/2, respectively, (Fig. 3E) association with Mn2+ (641.5 eV and 653.1 eV), the satellite peak of Mn2+ (646.2 eV) and Mn3+ (642.8 eV and 654.3 eV). The peak fitting analysis of O 1s could be deconvoluted into M−O (530.4 eV), oxygen defects (531.6 eV), H2O (532.6 eV) and C=O (533.6 eV), respectively.26,33 ICP analysis reveals the Mn content in MnO@NC-1100 is 5.45 wt%.The results indicated MnO@NC-1100 was the nitrogen-doped graphite structure including cubic phase MnO nanoparticles. The morphology and structure evolution of typical MnO@NC-1100 are characterized by TEM, high-resolution TEM (HRTEM), high-angle annular dark field-scanning transmission (HAADF-STEM) analysis. TEM images shown that


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nanorods with a size of ~ 80 nm wide were interconnected to each other, which maintained the morphology of the precursor particles of complex 2. Few MnO particles could be found on the nanorods (Fig. 4A). Abundant porous structures could be clearly observed in the high-magnified TEM images (Fig. 4B). The HRTEM images (Figs. 4C-4D) indicated that complex 2 was converted into graphene-like carbon layer containing the MnO nanoparticles through the thermal annealing. The porous structure could be defined in the HRTEM images (Fig. 4C) and the lattice parameter of 0.253 nm was attributed to the (111) facet of MnO nanoparticles (Fig. 4D). The elemental mapping images further revealed the existence of the C, N, Mn and O elements in the MnO@NC-1100 in agreement with XPS analysis (Fig. 4E). Heteroatom-doped carbon materials could destroy the electro-neutrality of the carbon matrix and create more active sites to enhance the ORR activity.34 The presence of initial Cd(II) benefits the spatial isolation of Mn(II) and prevents the sintering of MnO during the pyrolysis for MnO@NC-1100. The evaporation of Cd(II) and the explosion of nitrogen-rich tetrazolate groups would also promote the surface area of the resulted catalysts. ORR performance evaluation of MnO@NC-1100 The electrocatalytic ORR performances for the MnO@NC-1100 were evaluated in N2- and O2-saturated 0.1 M KOH electrolyte using cyclic voltammetry (CV) at room temperature. Quasi-rectangular voltammograms with featureless voltammetric currents within the potential range from 0.2 to 1.2 V (vs. RHE) were observed in the N2-saturated solution. In contrast, the occurrence of a pronounced oxygen reduction peak in O2-saturated solution was observed (Fig. 5A), implies that the MnO@NC-1100 possess ORR activity. To obtain the ORR electron-transfer mechanism, rotating ring electrode (RDE) line sweep voltammetry (LSV) measurements have been conducted under variable rotation rates, where the current density increased as the increasing of the rotation rates. The transfer electron number (n) could be computed from the polarization plots at different rotational rates according to the Koutechy-Levich (K-L) equation (Fig. 5B), the value of n was estimated to be around 4, suggesting a 4e- reduction



Page 12 of 24

pathways.1 To understand the influence of pyrolysis temperature and component on the electrocatalytic

property,

a

series

of

control

catalysts

(MnO@NC-1000,

MnO@NC-900 and NC-1100) were prepared. The morphology and structure of MnO@NC-1000 and MnO@NC-900 were similar to that of the typical MnO@NC-1100, which were obtained from complex 2 at 1000 oC and 900 oC, respectively (Fig. S8). PXRD for the NC-1100 has shown that NC-1100 was a morphous carbon-based material (Fig.S8A). XPS analyses for the MnO@NC-1000, MnO@NC-900 and NC-1100 were also employed to compare the chemical states, and the results were listed in Table S3 and S4. The contents of pyridinic N and graphitic N in MnO@NC-1100 were the highest comparing to MnO@NC-1000 and MnO@NC-900, which were considered to be beneficial for the ORR (Fig. S9).35,36 The content of oxygen defects were also increased in the MnO@NC-1100 with the temperature increasing, which illustrated that the increasing temperature would enhance the activity of the resulting catalysts for the ORR. The value of ID/IG was also investigated from Raman analysis for MnO@NC-1000 (0.94) and MnO@NC-900 (0.65), which was much lower than the value of MnO@NC-1100 (1.38) (Fig. S10). The higher value of ID/IG indicated the more defects in the catalyst, which is more beneficial for catalytic activity. Similar analyses were conducted for the NC-1100 (Fig. 6), and the results were listed in Tables S3 and S4. The contents of pyridinic N and oxygen defects in MnO@NC-1100 were much higher than that in NC-1100, which were in favor of the catalytic performance for the ORR.37 Thus, the increasing temperature and the introduction of hetero component would enhance the cataytic performance of the catalyst for the ORR. The ORR performances for the synthesized catalysts as well as the Pt/C were plotted in Fig. 5C, where MnO@NC-1100 exhibited the best electrocatalytic activity with an onset potential of 0.90 V and a half-wave potential of 0.74 V. The onset potential and half-wave potential for MnO@NC-1100 were comparable to other previously reported nitrogen-doped carbon catalysts derived from MOFs based on the


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nitrogen-containing ligands (Table S2).In addition, RDE measurements were also conducted to reveal the ORR mechanism. The smallest Tafel slope for MnO@NC-1100 (88.5 mV dec-1) to other referred catalysts (114.3 – 158.9 mV dec-1) further demonstrated its superb ORR activity (Fig. 5D). The results hereby suggested the ORR on the MnO@NC-1100 abided by a high-efficiency 4e- process and the existence of MnO was in favor of the electrocatalytic property. Moreover, the MnO@NC-1100 also exhibited excellent stability for ORR, as shown the performance was not much decreased after 2000 cycles (Fig. S11). Conclusion In summary, two isostructural nitrogen-rich tetrazolate-based metal-organic frameworks, [Cd2(L)(OH)(H2O)] (1) and bimetallic [Cd3Mn(L)2(OH)2(H2O)2] (2), (L = 5’-(4-(1H-tetrazol-5-yl) benzamido)-benzene-1,3-dioic acid) have been designed and successfully synthesized as the template/precursor. Both complexes 1 and 2 have exhibited porous structures. The resulting catalysts MnO@NC-1100 and NC-1100 derived from complex 2 and 1 by pyrolysis have been characterized to be porous nitrogen-dope graphite materials. The dramatic differences of the pore size in MnO@NC-1100 and NC-1100 further indicated that the derived catalysts were highly dependent on the structure of MOF precursors. MnO@NC-1100 has shown much better ORR catalytic behaviors (Eonset=0.90 V, Ehalf-wave=0.74 V) than that of NC-1100 due to the inclusion of the cubic phase MnO nanoparticles to provide more active sites. This particular work has provided a perfect example of the enhanced functional performances of MOF materials under rational MOF structural design.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.XXXXXX. Tables for selected bond lengths and angles, detailed characterization with varied spectra and images (PDF).

AUTHOR INFORMATION



Corresponding Authors *Email: [email protected] (Z.S.); [email protected] (M.H.); [email protected] (H.K.L.)

Author Contributions ‡L.Q.J.

and J.Y. contributed equally. All authors have given approval to the final version of

the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDFEMENTS We appreciate the financial support from the Key International (Regional) Joint Research Program of NSFC (Grant No. 21420102002), NSFC (No 21601088, 21771109, 21778033, 21671106), NSF of Jiangsu Province (No. BK20171472).


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Table1 Crystallographic data for complexes 1-2

aR 1

Complex 1

Complex 2

Formula

C16H11Cd2N5O7

C32H22Cd3MnN10O14

Mr

610.11

1162.77

T(K)

296(2)

296(2)

Crystal system

Monoclinic

Monoclinic

Space group

C2/c

C2/c

a(Å)

22.713 (4)

21.7859 (10)

b(Å)

17.390 (3)

17.0116 (7)

c(Å)

13.581 (3)

13.1203 (6)

α(°)

90

90

β(°)

117.187 (2)

112.240 (1)

γ(°)

90

90

V(Å3)

4771.7 (16)

4500.8 (3)

Z

8

4

Dc(g·cm-3)

1.699

1.716

µ(mm-1)

1.82

1.74

F(000)

2352

2260

Data collected

33398

19869

Unique reflections

5449

5195

R(int)

0.066

0.026

GOF

1.025

1.064

R1,wR2[I > 2σ(I)]a,b

0.0482，0.1196

0.0361，0.1010

R1,wR2(all data)

0.0654，0.1290

0.0393，0.1032

= Σ||Fo| - |Fc||/Σ|Fo|.

bwR 2

= |Σw(|Fo|2 - |Fc|2)|/Σ|w(Fo)2|1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP]. P = (Fo2 + 2Fc2)/3.



Figure 1. (A) Coordination environment of Cd(II) in 1 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms were omitted for clarity. (Symmetric Code: A, 1/2+x, 3/2-y, 1/2+z, B, 1-x, 2-y, 1-z, C, 3/2-x, 1/2+y, 3/2-z, D, -1/2+x, 1/2+y, z, E, 3/2-x, 3/2-y, 1-z) (B) 1D chain in 1 with the tetrazolate groups and tetranuclear units along the c axis. (C) 3D structure of 1 with 1D open channels. (D) (4,8)-connected bimodal topology of 1 (dark blue, L; turquoise, tetranuclear unit).


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Figure 2. (A) Coordination environment of Cd(II) and Mn(II) in 2 with the ellipsoids drawn at the 30% probability level. Hydrogen atoms were omitted for clarity. (Symmetric Code: A, 1-x, -y, 1-z, B, 3/2-x, -1/2+y, 3/2-z, C, 1-x, y, 1/2-z, D, 1/2+x, 1/2-y, 1/2+z, E, 1/2+x, -1/2+y, z, F, 1/2-x, -1/2+y, 1/2-z, G, 1/2-x, 1/2-y, 1-z). (B) 1D chain in 2.(C) 3D structure of 2 and the 1D open channels after removing the coordination water molecules. (D) Topology of 2 (dark blue, L; bright green, cluster).



Figure 3. PXRD pattern (A) and Raman spectrum (B) and the deconvoluted C 1s (C), N 1s (D), Mn 2p (E) and O 1s (F) core-level XPS spectra for the typical MnO@NC-1100.


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Figure 4. Low (A) and high (B) magnification TEM images for the MnO@NC-1100. (C-D) HRTEM images for the MnO@NC-1100, the circle area indicated the porous structures. (E) HAADF-STEM image as well as elemental mapping of C, N, Mn and O atoms for the MnO@NC-1100.



Figure 5. (A) CV curves of the MnO@NC-1100 in the N2 (a) or O2 (b) saturated 0.1 M KOH electrolyte. (B) LSV curves of the MnO@NC-1100 at various rotation rates and the inset exhibited the related K-L plots from LSVs at different potentials. (C) LSV curves at 1600 rpm and (D) Tafel plots for MnO@NC-1100, MnO@NC-1000, MnO@NC-900 and NC-1100, respectively.


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Figure 6. Comparison of N 1s fine XPS spectra (A) and O 1s fine XPS spectra (B) of MnO@NC-1100 and NC-1100.

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For Table of Contents Use Only

Enhanced Catalytic Performance for Oxygen Reduction Reaction Derived from Nitrogen-Rich Tetrazolate-based Heterometallic Metal-Organic Frameworks Li-Qian Ji‡, Jing Yang‡, Zi-You Zhang, Yong Qian, Zhi Su*, Min Han* and Hong-Ke Liu* Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biofunctional Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210046, China

The catalyst MnO@NC-1100 (NC, nitrogen-doped carbon) derived from tetrazolate-based bimetallic Cd-Mn-MOF has indicated much better catalytic performance for oxygen reduction reaction (ORR) than the catalyst NC-1100 derived from corresponding single Cd-MOF.


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Enhanced Catalytic Performance for Oxygen Reduction Reaction

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