Co(II)-Doped Cd-MOF as an Efficient Water Oxidation Catalyst: Doubly

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Co(II) Doped Cd-MOF as an Efficient Water Oxidation Catalyst: Doubly Interpenetrated Boron Nitride Network with the Encapsulation of Free Ligand Containing Pyridine Moieties Kartik Maity, Kousik Bhunia, Debabrata Pradhan, and Kumar Biradha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12926 • Publication Date (Web): 20 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Co(II) Doped Cd-MOF as an Efficient Water Oxidation Catalyst: Doubly Interpenetrated Boron Nitride Network with the Encapsulation of Free Ligand Containing Pyridine Moieties Kartik Maity,† Kousik Bhunia,‡ Debabrata Pradhan‡ and Kumar Biradha*† †

Department of Chemistry and ‡Materials Science Centre, Indian Institute of Technology,

Kharagpur 721302, India

KEYWORDS: metal-organic framework, boron nitride network, doping, cobalt, water oxidation, oxygen evolution reaction.

ABSTRACT: Development of an efficient and inexpensive water oxidation electrocatalyst using the earth-abundant elements is still far to go. Herein, a novel strategy has been demonstrated for developing the OER electrocatalyst by doping Co(II) in to a three dimensional Cd-based MOF that contains a naked pyridine moieties in the form of uncoordinated ligand. Electrochemically active CoCd-MOF was resulted through the doping of Co(II) into the inactive Cd-MOF. CoCdMOF exhibited very high catalytic activity in water oxidation reaction. An overpotential of 353

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mV is required to produce of 1 mA/cm2 under alkaline conditions. Further, the CoCd-MOF exhibits remarkable recyclability over 1000 cycles.

Sustainable energy generation from renewable sources is one of the greatest challenges to the scientific world. In this endower, the electrochemical splitting of water molecules to produce hydrogen and oxygen is considered as a viable green path. The water splitting reaction occurs through two half-cell reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The OER is most challenging reaction as it has to occur through oxidation of OH− ion via multiple proton-coupled electron transfer steps.1 However, the entire process is sluggish in nature as the electron transfer deals with a high kinetic barrier.2 Therefore there is a high demand for an efficient electrocatalyst to suppress such huge potential barriers in acidic or alkaline conditions.3 . To date the metal oxides IrOn and RuOn are considered as the best OER electrocatalysts.4 However, these oxides are economically less viable as Ir and Ru are expensive and less abundant. Accordingly, the current challenge is to develop an efficient and inexpensive OER electrocatalyst using the earth-abundant elements.5,6,7 The materials containing transition metals such as cobalt, nickel and manganese have been considered as an alternative option.8,9,10 Specifically the cobalt–based molecular complexes11 and nanoparticles have been explored as OER electro catalysts given the remarkable redox properties of cobalt and its ability to exist in higher oxidation states under electrochemical conditions. For example, cobalt oxides (Co3O4) were found to be one of the highly efficient OER electrocatalysts.12 In alkaline media cobalt containing chalcogenides, olivines, hydro(oxy)oxides and perovskites were also shown to be


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effective for OER catalysis.13,14 Even in neutral media, amorphous cobalt-phosphate was shown to exhibit productive OER activity with high stability.3 It has been a long journey in the design and synthesis of metal-organic frameworks (MOFs) and exploration of their functional properties such as storage, separation, catalysis etc.1517

Currently, MOFs are being widely explored in various heterogeneous catalysis as they contain

homogeneous distribution of the metal ions which act as active sites for catalysis while the organic ligands help in fine tuning the catalytic abilities of the metal ions.18 Given these aspects, the custom designed MOFs could be promising candidates for OER catalytic activity as they possess significant crystallinity as well as robustness which allow the excellent recyclability of the catalyst. However, very limited number of examples have been explored in which the MOFs were directly used as active OER electrocatalyst.19-22 On the other hand, in number of cases the MOF-derived materials have been shown to act as an effective and stable water-oxidation electrocatalysts.23-29 One such example is the Co3O4-carbon porous material that is derived through carbonization of MOFs.30,31 Recently, a Co-MOF that encapsulates {Co(H2O)4(DMF)2}2+ was shown as an effective electro catalyst for water splitting reaction in alkaline medium.18 The hetero bi-metallic Fe-CoMOF was also shown to exhibit remarkable OER activity with very low over potentials.21 Further, a 2D-Co MOF derived from 3D-Co-MOF through chemical etching process was shown to exhibit significantly enhanced OER activity over the parent Co-MOF.22 In this manuscript we wish to present a novel strategy for developing a OER electrocatalyst by doping Co(II) in to a Cd-based MOF that contains a naked pyridine moieties in the form of uncoordinated ligand. The Cd-MOF possesses doubly interpenetrated infrequent boron nitride topology (BNN) in which the channels are filled by uncoordinated ligand (L) (Scheme S1, SI) that contains pyridine moieties.


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Interestingly, the presence of free pyridyl unit within the network promotes the absorption of Co(II) that makes the MOF electrochemically active and significantly lowers the overpotentials of water oxidation process. Colourless block shaped crystals of Cd-BNN, {[Cd2(L)3(fumarate)2](L)(H2O)2)}n were obtained upon the reaction of L (34.64 mg, 0.1 mmol) with Cd(NO3)2•4H2O (15.42 mg, 0.05 mmol) and sodium fumarate (8.0 mg, 0.05 mmol) under hydrothermal condition. Single-crystal X-ray diffraction analysis revealed that Cd-BNN crystallised in P-1 space group and the asymmetric unit contains one unit of Cd(II), three half units of coordinated L and half unit of uncoordinated L, one unit of coordinated fumarate and one free water molecule. The Cd(II) exhibits a distorted octahedral geometry with the equatorial positions occupied by three pyridine units of L (Cd-N: 2.347(4), 2.330(4), 2.361(4) Å) and one O-atom of carboxylate (Cd-O: 2.473(4) Å) whereas the apical positions are occupied by the two O-atoms of carboxylates (CdO: 2.235(4), 2.370(4) Å). The coordination of Cd(II) to three L units resulted in a trigonal node that propagates honeycomb layer of M2L3 composition. These 2D-layers are interconnected by the coordination of fumarate ions to Cd(II) such that the resulted 3D-network contains huge channels of hexagonal shape. On a whole the network topology can be described as inorganic boron nitride network (BNN) (Figure 1a) with five connected Cd(II) centers. The huge empty space in the network is filled by the self-interpenetration and also by the inclusion of one free uncoordinated L. The interpenetrated networks interact with each other via N-H···O hydrogen bonds between the N-H of amide and O-atom of carboxylate of fumarate (Figure 1b,c). The pyridine units of uncoordinated L do not participate in any strong hydrogen bonds. However, the amide groups of the free L involve in strong hydrogen bonding with the walls via N-H···O hydrogen bonds (Table S2, SI).


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a)

+ 5-connected node

linker

3-connected node

3D-hexagonal network (Cd-BNN) b)

c)

Figure 1. Illustrations for the crystal structure of Cd-BNN: a) schematic description of formation of 5-connected node (trigonal bipyramidal) that forms 3D-hexagonal network (Cd-BNN); b) encapsulation of free L molecules in the 1D-channels that are formed by the double interpenetrated Cd-BN networks. c) 2-fold self-interpenetrated network of Cd-BNN, the L and fumarate were shown as node connecters and Cd(II) as nodes.


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The presence of uncoordinated L-units prompted us to explore the above material (CdBNN) for the absorption of transition metal such as Co(II). In case of successful absorption of Co(II) by Cd-BNN, it was anticipated that the presence of electrochemically active Co(II) within the Cd-BNN may lead to an efficient heterogeneous electrocatalysis for water oxidation process. In anticipation of Co(II) absorption, the Cd-BNN was dipped in to the 0.1 M DMF solution of Co(NO3)2. After 24 hours the dipped material of Cd-BNN was found to loose single crystalline nature and turned to pink from colourless. The solid material (CoCd-BNN) was taken out of the solution, washed thoroughly with DMF and dried in vacuum. The XRPD patterns of the CoCdBNN were found to be almost identical with that of as synthesized material of Cd-BNN indicating the structural integrity of Cd-BNN network after doping with Co(II) (Figure S1, SI). The IR-spectra of these two solids are also found to be identical indicating that there is no change in functionality of the organic moieties (Figure S2, SI).

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Figure 2. a) solid-state UV-vis spectra (diffuse reflectance spectra) of Cd-BNN and CoCd-BNN; b) solid state luminescence spectra of Cd-BNN and CoCd-BNN. Further, the doping of Co(II) into Cd-BNN was also characterised by solid-state UV-vis spectroscopy, solid-state luminescence spectroscopy, local energy-dispersive X-ray (EDX)


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spectra and inductively coupled plasma-mass spectrometry (ICP-MS) analysis. The solid-state UV-vis spectra of Cd-BNN exhibited two characteristic peaks at 240 nm and 285 nm which are due to the π-π* and n-π* transitions respectively for the amide carbonyl group. However, the solid-state UV-vis spectrum of CoCd-BNN exhibits three peaks, an intense peak at 520 nm along with the peaks at 240 nm and 285 nm corresponding to amide carbonyl groups (Figure 2a). The resulted new peak at 520 nm can be attributed to the charge transfer interaction between the uncoordinated ligand L and Co(II). Further, the solid state luminescence studies indicate significant quenching of luminescence intensity in CoCd-BNN due to the presence of Co(II) ion (Figure 2b). a)

cps/eV 7

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6 5 4 Cd

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Figure 3. a) Elemental mapping of CoCd-BNN showing uniform distribution of cobalt and cadmium throughout the surface; b) EDX spectra of CoCd-BNN indicating the presence of cobalt and cadmium. The EDX spectra of CoCd-BNN confirms the presence of Cd(II) as well as Co(II) in the CoCd-BNN as shown in Figure 3b. While the elemental mapping revealed that Cd(II) and Co(II)


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were suffused uniformly throughout the surface of the modified metal organic materials i.e, CoCd-BNN (Figure 3a). Moreover, the ICP-MS analysis for CoCd-BNN was carried out which revealed that it contained Cd and Co at a ratio of 2.54:1. The TGA studies indicate that the overall % of weight loss is lower for CdCo-BNN compared to Cd-BNN which further supports the presence of Co(II) ion in the material (Figure S4, SI). The electrocatalytic activities of Cd-BNN and CoCd-BNN for water oxidation reaction were studied in alkaline solution (0.1 M, pH = 13) using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) with standard three-electrode system in which rotating disk electrode (RDE) acts as a working electrode. The detailed procedure of electrochemical experiments and calculations are given in the ESI. At pH = 13, a profound anodic signal at 1.17 V (vs reversible hydrogen electrode (RHE)) was observed for CoCd-BNN in CV experiments which was not observed for the parent material under identical conditions indicating its inactiveness (Figure 4a). Further, the CV of CoCd-BNN exhibited a clear quasi reversible couple at pH = 13 which was found to be more prominent at higher pH (pH=13.6, S14, SI). The quasi reversible couple can be assigned to the Co(II)/Co(III) redox couple. The LSV curve for CoCd-BNN revealed that an overpotential of 353 mV was required to produce an anodic current density of 1 mA/cm2 (Figure 4b). We note here that the required overpotential (353 mV) is lower than the some of the recently reported literature8,9,19,20,32 (Table S3, SI). The overpotential was found to increase with increase in pH. The electro catalysis cycle of CoCd-BNN can be described as follows: Co(II) was first oxidized to Co(III) and then to Co(IV) via proton coupled electron transfer process and subsequently, the Co(II) state is being regenerated by releasing O2 (Scheme S2:, SI). These higher oxidation states of cobalt i.e. Co(III or IV) are well balanced by the OH- ion (from KOH


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solution) and thus the CoCd-BNN acts as highly efficient OER electrocatalyst in alkaline solution. Further, the CoCd-BNN was found to have a remarkable stability, as it exhibited a similar kind of LSV curve even after 1000 scans in the range of 0.8 to 1.5 V (vs RHE) (Figure 4b). Further, the ICP-MS analysis of electrolyte, after these catalytic cycles, shows no traces of

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Co(II) or Cd(II) reaffirming the stability of CoCd-BNN.

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Figure 4. a) Relative cyclic voltammogram profile for Cd-BNN and CoCd-BNN in alkaline solution (pH = 13); b) linear sweep voltammetry study for CoCd-BNN initially and after 1000 cycles; c) Tafel plot of IrO2 and CoCd-BNN at pH = 13; d) chronoamperometry study: plots at 1.7 V vs RHE.


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The reaction kinetics for water oxidation by CoCd-BNN are analyzed by Tafel plot that correlates the catalytic current density (i) with overpotential (ƞ). The i value is measured from the LSV plot under different potentials. At pH = 13 (0.1 M KOH) the Tafel plot exhibited linear behaviour for a broad range of potential values. The slope of the Tafel plot is found to be 110 mV/decade (Figure 4c), indicating the high efficiency of catalyst and this value is comparable to some of the Co-based water oxidation catalysts reported recently.19 Further the Tafel plot for the highly efficient but precious water oxidation catalyst IrO2 is also studied at pH = 13 for the sake of comparison. The Tafel slope for IrO2 was found to be 100 mV/decade. These results reveal that CoCd-BNN can serve as an efficient electrocatalyst for the water oxidation process. In addition, the chronoamperometry measurement was also performed to evaluate the variance of current density with time as a function of overpotential (1.7 V vs RHE). These studies reveal that a constant current with a range of 9 to 7.8 mA/cm2 found to occur up to 5 hrs (Figure 4d). Initially the current density was found to be somewhat higher as the catalyst surface was active enough but with time it is found to reduce. The reduction with time is due to the blocking of electrode surface by the gas bubbles that are produced by OER activity. Further, at pH = 13, the value of turnover frequency (TOF) was evaluated with the assumption that only the Co(II) centres that are present on the surface of CoCd-BNN coated electrode are involved in the OER catalysis. The resulted TOF value was found to be 3.314×10-2/s (S13, SI). The electrochemical assessable surface area (EASA) of the electrocatalyst CoCdBNN has been estimated by calculating the electrochemical double layer capacitance (Cdl) for CoCd-BNN and specific electrochemical double layer capacitance (Cs) of an atomically smooth surface (Figure S7, SI). The value of EASA was determined to be 0.14 cm2. The active Co atoms/cm2 on the CoCd-BNN coated GC surface was calculated by measuring the surface


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coverage (τo). The Co2+/3+ redox current (i) corresponding to different scan rates were measured and the surface coverage was eventually determined from the slope of the anodic peak current (ip) vs. scan rate (Figure S8, SI). The roughness factor (Rf) is found to be 2 which is evaluated from the ratio of EASA to the geometric surface area of the electrode. It has been well known that in case of OER process only the surface atoms of the catalyst play a crucial role. Therefore the observed high Rf value indicates that the CoCd-BNN coated electrode provides more surface area for electrolytes to promote better OER activity. In summary, the results illustrate a novel strategy for developing the MOF-based OER electrocatalyst. The unusual inclusion of uncoordinated pyridine containing ligand in the double interpenetrated 3D-MOF paves the way for the absorbing transition metal atoms.

Such

absorption of Co(II) into Cd-BNN resulted in the generation of efficient electrocatalyst for OER activity. The CV and LSV studies reveal that the CoCd-BNN is very efficient elctro catalyst for OER process although the parent Cd-BNN is inactive. The overpotential required for the generation of 1 mA/cm2 using CoCd-BNN is found to be quite low (353 mV) for these kind of materials indicating its importance as a catalyst for OER activity. It also exhibited exceptional stability even after 1000 cycles. This study highlights one of the easy, economically cheap and green routes of preparation for the generation of water oxidation electrocatalyst. Moreover the study also reveals that the catalytic activity of CoCd-BNN is comparable with that of efficient but precious metal oxide IrO2.


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ASSOCIATED CONTENT Supporting Information: Experimental details, crystallographic parameters (CCDC1568662), electrochemical data and calculations, IR spectra, XRPD patterns, and other experimental data.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +91-3222-282252. Tel.: +91-3222-283346. ORCID: Kumar Biradha: 0000-0001-5464-1952

ACKNOWLEDGMENT We acknowledge DST (SERB), New Delhi, India for financial support and DST-FIST for the single crystal X-ray diffractometer, and K.M. thanks UGC for a research fellowship.

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Co(II)-Doped Cd-MOF as an Efficient Water Oxidation Catalyst: Doubly

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