Cobalt-Modified Covalent Organic Framework as a Robust Water

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Cobalt modified Covalent Organic Framework as a robust water oxidation electrocatalyst Harshitha Barike Aiyappa, Jayshri Thote, Digambar Balaji Shinde, Rahul Banerjee, and Sreekumar Kurungot Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01370 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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Chemistry of Materials

Cobalt modified Covalent Organic Framework as a robust water oxidation electrocatalyst Harshitha Barike Aiyappa,a,b Jayshri Thote,a Digamber Balaji Shinde,a Rahul Banerjee*,a,b and Sreekumar Kurungot *,a, c a

Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India.

b

Academy of Scientific and Innovative Research (AcSIR), New Delhi, India.

ABSTRACT: The development of stable, efficient Oxygen Evolution Reaction (OER) catalyst capable of oxidising water is one of the premier challenges in the conversion of solar energy to electrical energy, owing to its poor kinetics. Herein, a bipyridine containing covalent organic framework (TpBpy) is utilized as an OER catalyst by way of engineering active Co (II) ions into its porous framework. The as obtained Co-TpBpy retains high accessible surface area (450 m2/g) with exceptional stability even after 1000 cycles and 24 h of OER activity in phosphate buffer at neutral pH conditions with an overpotential of 400 mV at a current density of 1 mA /cm2. The unusual catalytic stability of CoTpBpy arises from the synergetic effect of the inherent porosity and presence of coordinating units in the COF skeleton.

Electrochemical reaction is one of the cushy pathways for effective inter-conversion of chemical and electrical energy by means of bond breaking and formation. It is successfully realized in storing energy reserves like water via its splitting into hydrogen and oxygen and their eventual regeneration into water itself, making the entire process renewable, clean and green.1,2 Of the two half reactions, the reductive Hydrogen Evolution Reaction (HER) is faster while the Oxygen Evolution Reaction (OER) is hampered by the complexity involved in the process (i) breaking of four O-H bonds, (ii) removal of four electrons from two water molecules and finally (iii) the energy intensive O-O bond formation.3 As OER unavoidably involves the intervention of multiple protons and electrons, transition metal ions with variable oxidation states are effectively tried as Oxygen Evolving Catalysts (OECs). Among the many systems explored so far, RuO2 and IrO2 are found to be the most effective OECs.4,5 But, their high cost and poor availability make the entire process highly expensive and, thereby, limits their everyday use. Thus, there is an upsurge interest to develop low cost catalysts that can effectively oxidize water using minimum energy. It is important to note that water splitting invariably involves two half cell reactions: proton reduction (favorable at low pH) and water oxidation (favorable at higher pH). Thus, attempts are on to develop integrated water splitting devices capable of operating in neutral pH medium in order to balance the effects of these countering reaction conditions.6,7 Hitherto, due to the poor kinetics, very few catalysts are reported to oxidize water at neutral pH conditions. Recently, earth abundant cobalt based molecular OECs, especially their phosphonate, phthalocyanine, salophen, porphyrin complexes have been tried for the water oxidation.8-11 One of the main limitations of molecular WOCs is their eventual dissociation into soluble homogeneous species during the catalytic operation. 12, 13 Co-Fe Prussian blue polymers, electrodeposited CoPi films are some of the cobalt based OECs which have been

Figure 1. Schematic representation of the synthesis of TpBpy via proton tautomerised Schiff base condensation and Co-TpBpy via Co (II) impregnation.

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Figure 2. Comparitive (a) PXRD, (b) N2 adsorption isotherms, (c) UV- Visible DRS spectra and(d) deconvoluted XPS N1s spectra of the cobalt impregnated TpBpy; (e) Co 2p XPS spectra before and after the stability test and (f) SEM and TEM images of Co-TpBpy.

demonstrated to catalyze water oxidation in neutral pH. 14,15 These belong to the category of heterogeneous OECs which carry advantages over their homogeneous counterparts, mainly, recovery, recycle and reuse, besides rendering an additional benefit of directly implementing them for device making. 16, 17 However, catalysts demand an imperative need to control their structure at the molecular level to achieve optimal activity. In the present case, as OER involves liberation of molecular oxygen, the long term performance of such catalysts is often suboptimal. It results in the leaching of the catalyst which curtails their stability during prolong operation time and cycles. 18 This necessitates a need to develop high surface area catalysts which can be used as a porous substrate to assist easy passage of such evolved O2 molecules thereby improving their long term stability. Among the recognized porous materials, Covalent Organic Frameworks (COFs) belong to the recent class of crystalline organic polymers, known for being mechanically robust, with low density and high accessible surface area. 19-22 They can be judiciously custom-made by choosing appropriate building units with desired functionality and topology. This allows a precise control in the nature, density and spatial arrangement of the active centers in the porous backbone. By the virtue of their inherent high surface area and freedom to be structurally and electronically tuned, we believe that the COFs can be potentially designed to hold the integrity of both heterogeneous as well as molecular OECs. The COFs derived via modified Schiff base reaction undergo proton tautomerism and are chemically stable thereby providing a distinctive platform to perform catalysis in harsh conditions.25-27 In the present work, cobalt modified bipyridine based covalent organic framework (Co-TpBpy) is designed and examined for its performance as a heterogeneous OEC under neutral pH conditions (Scheme 1). The synthesis of TpBpy was carried out using the previously reported procedure. 19 The as synthesised TpBpy was found to pos-

sess a high surface area of ~1660 m2/g .The cobalt modified TpBpy (Co-TpBpy) was prepared by soaking a known amount of TpBpy in methanolic cobalt acetate solution kept under stirring for 4 h (details in Section 2, ESI). The material was then washed using copious amount of dry methanol, followed by overnight vacuum drying at 60 oC. The PXRD pattern of Co-TpBby indicated the presence of a highly intense first peak at 2θ of 3.6° corresponding to the reflections from 100 plane of the COF along with the retention of the other peaks as found in the pure TpBpy matrix. The matching PXRD pattern with no extra peaks indicates the retention of the robust COF framework structure after cobalt modification besides confirming the absence of any undesired moieties (viz., starting metal precursor, metallic cobalt or Co-oxide residues) in the cobalt-modified TpBpy (Figure 2a). In conjunction to it, the 13C NMR of TpBpy remained unchanged after the cobalt modification, thereby indicating that the TpBpy structure was undisturbed after the treatment (Figure S1, ESI). The FTIR spectra of Co-TpBpy shows retention of chemical functionalities present in the pristineTpBpy except for the apparent red shift and pronounced broadening of the ν C-N peak, indicating the coordination of cobalt ions to the bipyridinic nitrogen atoms in the COF framework. (Figure S2, ESI). The N2 isotherm evaluation of Co-TpBpy estimated that the COF retained a BET surface area of 450 m2/g after the cobalt impregnation (Figure 2b). The presence of cobalt was also confirmed using solid-state UV-Vis spectroscopy, XPS and local energy-dispersive X-ray (EDX) spectra. The solid-state UVVisible spectra of TpBpy showed the presence of peaks at 276 and 323 nm characteristic to the bipyridine π-π* and n- π* transitions, while the broad band at 520 nm arises due to delocalised π electron cloud of TpBpy. On complexation, two new, intense absorption bands at 470 nm and 510 nm (overlapping with 520 nm TpBpy peak) were observed (Figure 2c, S3, ESI). These correspond to the charge transfer transition, usually observed in the Co (II)bipyridine complexes. 23,24 The thermogravimetric analysis of


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Figure 3. (a) Comparative cyclic voltammagram profiles of Co-TpBpy (red line) and as synthesised TpBpy (black line) in phosphate buffer (pH 7.0) at 50 mV/s (inset: zoomed in CV profile); (b) LSV stability test profile of Co-TpBpy before and after 1000 cycles (inset: zoomed in LSV profile); (c)Tafel plot of Co-TpBpy at neutral pH; (d) chronoamperometric stability profile of Co-TpBpy at 1.74 V (vs. RHE) for 24 h; (e) plot of the anodic and cathodic current vs. scan rate at 0.95 V (vs. RHE); (f) gas chromatography analysis of the evolved oxygen and evaluation of the cyclic stability.

Co-TpBpy in air revealed ~12% cobalt content in the framework (Figure S5, ESI), which matched well with the Inductively Coupled Plasma (ICP) analysis result (13% cobalt content). The XPS spectra of pure TpBpy showed the presence of nitrogen, oxygen and carbon only (Figure S6, ESI). The deconvoluted N 1s spectra indicated the presence of two distinct peaks: 400.92 eV corresponding to the secondary nitrogen and 399.82 eV associated with the pyridinic nitrogen in the TpBpy matrix (Figure S8, ESI). 25, 26 On cobalt impregnation, two broad sets of signals corresponding to 2p3/2 (781.7 eV) and 2p1/2 (797.1 eV) core levels of cobalt were observed.12, 25-27 The presence of the shake-up satellite signal in the Co 2p spectra clearly specifies the existence of Co (II) ions in the matrix (Figure 2e). A shift in the binding energy corresponding to that of pyridinic nitrogen (from 399.82 to 400.04 eV) was also observed on cobalt impregnation (Figure S8, ESI). Interestingly, there was no change in the peak position of secondary nitrogen (400.92 eV), thereby hinting the complexation of cobalt to the TpBpy matrix by its bipyridinic units only. 26 Furthermore, the shift in the Co 2p3/2 binding energy (781.7 eV, corresponding to the CoN moieties) compared to that of pure cobalt acetate (784.3 eV) also evidences the interaction between Co (II) and nitrogen in the COF matrix (Figure S7, ESI). No distinct changes in morphology were observed in the cobalt modified TpBpy when compared to that of the pristine TpBpy (Figure S10, ESI). Moreover, the Transmission Electron Microscopic (TEM) images did not show the presence of any metal nanoparticles/metal oxide particles in the COF matrix (Figure 1f). However, the EDX mapping revealed uniform distribution of Co and N content in Co-TpBpy (Figure S9, ESI).

The electrochemical property of Co-TpBpy was studied by performing Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV) with Co-TpBpy coated glassy carbon working electrode in 0.1 M aqueous phosphate buffer at pH 7. All the measurements were carried out on a rotating disk electrode (RDE), rotating at a rate of 1600 rpm, to preserve uniform ionic concentration around the working electrode and simultaneously prevent the accumulation of O2 bubbles. The CV showed clear onset of an intense anodic wave at ca. 1.63 V (vs. RHE), with a marked difference in its OER activity compared to that of the plain TpBpy (without any cobalt modification) (Figure 3a). The CV (inset) of Co-TpBpy also showed the presence of a quasi reversible Co (II)/ (III) couple at a peak potential at ca. 1.57 V vs. RHE. It was observed that an overpotential of 400 mV was needed to generate an anodic current density of 1 mA/cm2, which could be found to be in good agreement with the other well documented Co-based OER catalysts (see Table S1, ESI). Co-TpBpy showed a similar LSV with remarkable stability even after 1000 scans from 0.6 to 1.8 V (vs. RHE), with ~94 % retention in the OER current (Figure 3b). The Tafel plot relating the kinetic catalytic current with the overpotential showed a linear behavior (Figure 3c). The Tafel slope of 59 mV/decade herein obtained hints at the possible reversible one-electron transfer mechanism followed for OER. 28, 29 Apart from the cyclic stability, the chronoamperometry measurement (current density, j− t, time) was also carried out as a function of the applied overpotential (1.74 V vs. RHE). A steady-state current of 15 ± 3 mA/cm2 was obtained up to 24 h (Figure 3d). The initial increase in the current density is due to the activation of the catalyst surface. During the course of OER activity, the gas bubbles were observed to cover the



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electrode surface. This non-conducting gas phase apparently blocks the active sites thereby decreasing the current density, an event normally observed in case of gas evolution reactions. 30,31 However, by the virtue of the high surface area of Co-TpBpy, the decrease in the current density was found to be < 10% at the end of 24 h of operation, which elucidates the high stability of the material. The ICP analysis of the electrolyte after the stability test did not show detectable cobalt traces thereby ruling out any possible leaching of Co (II) ions from the framework under the water oxidation condition. Conversely, the analysis of Co-TpBpy revealed the retention of 10.5% of Co2+ after the stability test. The sustained OER activity of Co-TpBpy even after 24 h could be thus mainly attributed to the stability instilled by the high surface area COF support which resists the material leaching during the course of O2 evolution, apart from holding the Co (II) centers via coordination to its bipyridinic units. The roughness factor of Co-TpBpy catalyst was determined by estimating the electrochemically accessible surface area (EASAb) of Co-TpBpy. The double layer capacitance (C) was calculated using current vs. scan rate plots in case of both bare glassy carbon (GC, Ca) and Co-TpBpy coated GC (Cb) by means of electrochemical capacitive measurements (Figure 3e).30-34 The EASAb of Co-TpBpy coated GC was further calculated by estimating the EASAa of the bare glassy carbon electrode using standard potassium ferricyanide solution (for details see ESI, Section 3.1).32 The roughness factor (Rf) of 1.44 was derived by dividing the EASAb with the electrode’s geometrical area. The catalyst was found to retain the Rf value (1.46) even after 1000 cycles stability evidencing the pronounced catalytic stability of the material (Figure S 15, ESI).34 The catalyst was found to show a Turn-over Frequency (TOF) of 0.23/s, which was calculated by estimating the surface coverage of active cobalt atoms in Co-TpBpy (details in Section S 3.1, ESI). 32 The TOF value here is the lower limit for the catalyst calculated based on the number of surface active cobalt atoms involved in the catalytic reaction. The Transmission (TEM) and Scanning Electron Microscopic (SEM) study of Co-TpBpy after the OER stability test revealed that the matrix was intact with slight increase in the surface coarseness (Figure S10). However, the Co 2p XPS spectral inspection confirmed that the chemical state of the cobalt remained unchanged after the tests (Figure 2e). The slight shift observed in the Co 2p peak positions could be due to the possible formation of small CoOx domains during the prolonged water oxidation cycle. 35,36 Nevertheless, the broad shoulder found in the spectra substantiates that most of the Co ions in CoTpBpy are in the Co (II) oxidation state confirming the pronounced chemical stability of the material. The faradaic efficiency (ε) of Co-TpBpy was determined by performing the rotating-ring disk electrode (RRDE) studies (for details see ESI).37-39 The CoTpBpy coated disk electrode (at 1600 rpm) was subjected to a series of current steps varying from 2 to 15 mA /cm2 (Figure S 17, ESI). The oxygen molecules evolving during the reaction was in turn monitored by reducing it on the platinum ring electrode, whose potential was held constant at 1.2 V (vs. RHE). The faradaic efficiency (ε) was estimated to be 0.95, which substantiates the significance of Co-TpBpy as an efficient water oxidation catalyst. The water oxidation product was detected using gas chromatography (GC) and the amount of O2 evolved was monitored by sampling out the gas from the reaction vessel headspace after definite time interval. The cyclic stability was also confirmed using the O2

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evolution study, during which the headspace was evacuated at the end of each cycle (Figure 3f, for details see Section 4, ESI). In conclusion, a highly porous, bipyridine functionalized covalent organic framework, TpBpy has been modified using cobalt ions and the resulting Co-TpBpy is examined as a OER catalyst in neutral pH buffer solution. Co-TpBpy is found to show fast and stable performance retaining 94% of its OER activity even after 1000 cycles with a TOF of 0.23/s and faradaic efficiency of 0.95. This study highlights the need for utilizing high surface catalyst support towards designing rugged and competent catalysts in order to stabilize and improve the longetivity of electrochemical evolution reactions namely OER, HER etc .

ASSOCIATED CONTENT Supporting Information Synthetic procedures for TpBpy and Co-TpBpy, FT-IR, 13C solid state NMR, TGA in N2 and O2, TEM, SEM along with details of the electrochemical study. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email: [email protected], *Email: [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT HBA acknowledges UGC, New Delhi, India for the SRF. JT and DS acknowledge CSIR, New Delhi, India, for CSIR-Scientist’s Pool Scheme and CSIR-Nehru Post-doctoral Fellowship respectively. KS acknowledges CSIR’s Five Year Plan (CSC0122) for funding. Financial assistance from DST (SB/S1/IC-32/2013) is also acknowledged.

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SYNOPSIS TOC


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Cobalt-Modified Covalent Organic Framework as a Robust Water

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