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Water oxidation electrocatalysis by a zeolitic imidazolate framework† Sibo Wang, Yidong Hou, Sen Lin and Xinchen Wang*

Received 4th May 2014 Accepted 24th June 2014 DOI: 10.1039/c4nr02399d www.rsc.org/nanoscale

The search for efficient water oxidation catalysts (WOCs) is of paramount importance in energy and environmental fields, but there exists no good non-noble catalyst that works under acidic and alkaline conditions. Intensive investigations have recently focused on cobalt based complex/solid catalysts. Here, we have introduced a new type of cobalt-based WOC made of metal–organic frameworks where the redox function of cobalt centres was modified by imidazolate linkers for facilitating the proton transfer process. This cobalt-containing zeolitic imidazolate framework (Co-ZIF-9) has been demonstrated for the first time to electrocatalyze the oxygen evolution reaction in a wide pH range. The catalyst was found by theoretical calculation to be capable of activating the water molecule via binding the OH-group to the metal sites with low activation barriers, while the eliminated proton was accepted by the nearby benzimidazolate motifs. This allows Co-ZIF-9 to work effectively for the electrochemical oxygenevolution reaction.

With increasing energy demand and growing environmental concerns, it is of vital importance to exploit energy from renewable sources, independent of fossil fuels.1 Solar energy is a vast and inexhaustible resource, which has long been envisioned as the most promising renewable energy to run endergonic fuel generation reactions, such as water splitting to produce hydrogen and oxygen fuels (eqn (1)).2,3 The overall water splitting reaction consists of two half reactions, namely, water oxidation to liberate O2 and water reduction to produce H2 (eqn (2) and (3), respectively). There are known materials that can catalyze the reduction step efficiently, but the oxidation half reaction is still considered the most challenging step because it involves the coupling of four electron and proton transfers and the formation of two O–O bonds.4 This oxygen evolution reaction (OER) always suffers from overpotentials that State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, People's Republic of China. E-mail: [email protected] † Electronic supplementary 10.1039/c4nr02399d

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arise from activation barriers, surface defects/impurities, electrolyte concentrations, voltage drops due to resistance, and so on. The OER process naturally occurs in plant photosystem II (PSII) providing electrons and protons which are ultimately used to store energy in the phosphoanhydride bond of ATP and to reduce CO2 to carbohydrate, while releasing dioxygen gas in the atmosphere. The Mn4CaO5 cluster in PSII is the active site for water oxidation and it is linked by protein subunits to ensure a harmonic cooperation of the inorganic core, membraneproteins, and cofactors to construct a self-repairing waterevolving complex that is the only biological blueprint in all oxygenic phototrophs for creating biomimetic water oxidation catalysts (WOCs).5–7 Two applications for which catalyst are sought are the direct splitting of water for hydrogen production8–10 and the reversible reduction of O2 to water in fuel cell cathodes.11–14 2H2O / 2H2 + O2 (DG0 ¼ 113 kcal mol1)

(1)

2H2O / O2 + 4H+ + 4e (oxidation)

(2)

4H+ + 4e / 2H2 (reduction)

(3)

Articial WOCs have been explored extensively both in homogeneous and heterogeneous systems. A number of molecular homogeneous catalysts, including Ru,15 Ir,16 Mn,17 Fe,18 Co19 and Cu20 complexes, have been developed for water oxidation reaction. The homogeneous catalysts are benecial for mechanistic studies, characterization of catalytic species and performance tunability. But, the homogenous systems are complicated by the problem of catalyst retrieval from the reaction solution together with separation from the products, thus restricting their advance in large-sale applications. By contrast, heterogeneous WOCs mostly based on oxides of Pt, Ru and Ir are more favorable because of their robustness and efficiency.21 However, the scarcity and the high price of these noble-metalbased WOCs severely limit their application on a large scale. Thus, there is an urgent need for developing new heterogeneous

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WOCs made of abundant elements. Although Co- and Ni-oxide based catalysts have been tested as WOCs since 1950's,22 great interest in these abundant catalysts has been stimulated by recent advances that Co23 and Ni24 based clusters that form in solution spontaneously can act as efficient O2-evolving catalysts under benign conditions. More recently, Berlinguette and coworkers developed an amorphous metal oxide material deposited from organometallic precursors for water oxidation.25 These studies opened up new opportunities for water splitting chemistry using inexpensive and earth-abundant materials. A recent study suggested that the attachment of benzimidazolate motifs to the metal centers facilitates proton-coupled electron transfer and the resultant WOCs can react with water to form high-valent metal-oxo species at low potentials, avoiding high-energy intermediates.26 These features are demonstrated to be favorable for water oxidation reaction both thermodynamically and kinetically, highlighting the importance of a synergistic catalysis of inorganic catalysts and ligands in articial oxygen-evolving systems. Indeed, such a promotional effect of organic motifs is rather reected by the amino-acid residues (involving either a carboxylate or an imidazole ring) in the PSII–WOC complex being postulated to act as the redox and proton transfer mediator motifs to promote natural water oxidation reaction.5–7 For all the above reasons, it is our interest to design welldened heterogeneous WOCs that contain abundant metal centers in combination with benzimidazole ligands as both redox and proton transfer mediator motifs for catalyzing water oxidation reaction. A model case of such desired WOCs is metal–organic frameworks (MOFs)27 that contain redox-active metals and organic linkers. MOFs have been identied as an intriguing class of crystalline porous materials that show diverse potential applications, such as gas storage and separation,28 ion exchange,29 chemical sensors,30 imaging,31 catalysis32 and drug delivery.33 Zeolitic imidazolate frameworks34 (ZIFs) are a subclass of MOFs with tetrahedral networks structurally similar to the aluminosilicate zeolites. Several of these materials, particularly ZIF-8, -9, -10, exhibit high thermal and chemical stability.35 Due to their striking features, ZIFs have been developed for use in hydrogen storage,36 CO2 capture/conversion37,38 and heterogeneous catalysis.39 However, to the best of our knowledge, the applications of ZIFs as electrocatalysts for water oxidation is much less covered. Herein, we introduce a cobalt-containing zeolitic imidazolate framework (termed Co-ZIF-9) as a sustainable heterogeneous water-oxidizing catalyst. Quantum chemical and electrochemical studies were conducted to illustrate the cooperative catalysis of cobalt centers and benzimidazole ligands in conned nanodomains for efficient water oxidation. The Co-ZIF-9 electrocatalyst was synthesized following a previously reported literature35 and was subjected to full physical characterizations with power X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), eld emission electron scanning microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy and simultaneous thermal analysis (STA). Both the XRD (Fig. S1†) and the FTIR (Fig. S2†) characterizations strongly

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conrmed the successful synthesis of Co-ZIF-9. The crystalline morphology of the catalyst was examined by FESEM and TEM analyses as shown in Fig. S3 and S4,† respectively. The elemental analysis of the catalyst was identied by the EDX spectrum (Fig. S5†). The STA (Fig. S6†) measurement revealed that the Co-ZIF-9 shows a high thermal stability up to 500  C. The Co-ZIF-9 has an open-framework structure adopting sodalite topology with hexagonal symmetry, and is composed of cobalt ions solely coordinated by nitrogen atoms in the 1,3positions of benzimidazolate (bIm) linkers forming a tetrahedral CoN4 building unit, where the bond between the bIm linkers and Co(II) is one of the most stable N-donor ligands35 (Scheme 1). Apparently, the Co-ZIF-9 features both cobalt(II) and imidazolate functionalities in a microporous chemical environment, being a ne model to study single-site catalysis. Firstly, DFT calculations were performed to nd out whether Co-ZIF-9 can catalyze water splitting by abstracting the hydrogen atom in water. This initial dehydrogenation of the H2O molecule is an important and necessary step during the water oxidation process. The optimized initial state, transition state and nal state along the reaction pathway for water molecule dissociation is shown in Fig. 1. In the initial state, the H2O molecule is found to adsorb on the Co-ZIF-9 catalyst, pointing its hydrogen atom to the carbon atom of ˚ At the transition state, the distance of Co-ZIF-9 (dC–H ¼ 2.11 A). ˚ longer than that (0.99 A) ˚ of the dissociated H–O is about 0.26 A initial state, while the distance of C–H becomes shorter to be

Scheme 1 The topological view of Co-ZIF-9 (left) and the tetrahedral coordination of benzimidazolate linkers to the cobalt ion (right). Color: Co (light blue), N (blue), C (white). Hydrogen atoms are omitted for clarity.

DFT calculated energetic and geometries of the initial dehydrogenation of the water molecule. Color: H (white), C (grey), N (dark blue), Co (light blue), O (red).

Fig. 1

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˚ Surprisingly, the activation barrier for this important 1.39 A. step is very low with a value of 4.38 kcal mol1, indicating its facility at low temperatures. On the other hand, this step is also exothermic with a reaction heat of 15.20 kcal mol1. Aer decomposition, the C–H bond is completely formed with a ˚ and the OH species is observed to adsorb on the length of 1.12 A ˚ The produced cobalt atom with the Co–O bond length of 1.86 A. OH might continue to break up and the relevant studies will be carried out in future. Our DFT calculation however suggests that, on this Co-ZIF-9 catalyst, the benzimidazole ligands indeed facilitate direct abstraction of a H atom from substrate water bound to the Co active site, indicating the supportive catalysis functions of benzimidazolate motifs as proton acceptors in assisting the proton-coupled electron transfer process in water oxidation reaction. Such ligand-promoted water decomposition by inorganic species is indeed visible via the quantum chemistry study, consistent with the current experimental observations as will be investigated later on. The electrochemical behaviour of Co-ZIF-9 was investigated (Fig. 2a). The cyclic voltammetry (CV) scans demonstrated that Co-ZIF-9 has greater current density and an earlier onset of catalytic current density as compared to its background FTO glass. The onset of the catalytic current (1.6 V vs. the reversible hydrogen electrode, RHE) occurs 0.4 V beyond the thermodynamic potential for water oxidation (1.23 V vs. RHE); this value approximates to that of a recently reported molecular cobalt catalyst.40 The inset in Fig. 2a shows that the catalyst has a Tafel slope value of 193 mV per decade. The curvature of the

Fig. 2 (a) Cyclic voltammograms of the FTO glass background (black line) and 1.7 mmol cm2 of the Co-ZIF-9/FTO electrode (blue line) in 0.1 M potassium phosphate buffer (pH ¼ 7.0). Inset: Tafel plot of the Co-ZIF-9/FTO electrode. (b) Chronoamperometric current density and the amount of O2 produced as a function of time at the Co-ZIF-9/ FTO electrode under 1.8 V vs. the RHE, O2 production measured by a fluorescent sensor (black line) and the theoretical amount of O2 produced (red line), assuming a Faradic efficiency of 100%.

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Tafel plot is likely due to the mediation of electron and proton transfer by the Naon lm rather than the mechanistic details related to the intrinsic activity of the catalyst.41 To assess the effect of the electrolyte environment on the catalytic activity of Co-ZIF-9 towards water oxidation reaction under neutral conditions, controlled experiments were performed in K2SO4, KNO3 and KClO4 solutions (Fig. S7†). Results displayed that sustained water oxidation catalysis also occurred in these electrolytes, but the catalytic current density was much smaller than that in the potassium phosphate buffer solution, which is attributed to the poor proton-accepting capacity of the species (SO42, ClO4, NO3) in the electrolytes.42 The concentration of the catalyst on the FTO electrode was also optimized for effective water oxidation catalysis (Fig. S8†). The catalytic current density enhanced gradually with the increase in the concentration of Co-ZIF-9, achieving a maximum value with a Co-ZIF-9 concentration of 1.7 mmol cm2 under the same reaction conditions. Further increasing the amount of Co-ZIF-9, the catalytic current density reduced obviously due to the higher impediment of the effective electron transfer caused by the excessive catalyst loaded. The Faradaic efficiency of Co-ZIF-9 for oxygen evolution reaction (OER) was tested with a uorescence-based O2 sensor. Electrolysis was performed in a gas-tight electrochemical cell under an argon atmosphere with a sensor placed in the headspace. The measured amount of O2 approximately matches onefourth of the number of electrons passed through the circuit (Fig. 2b). Aer reaction for 3 hours, the amount of the produced oxygen (8 mmol) greatly exceeds the amount of catalyst (0.42 mmol), indicating that the current is indeed due to the electrocatalytic oxygen evolution rather than oxidation of the Co-ZIF-9 catalyst. The turnover frequency (TOF) with respect to the deposited Co-ZIF-9 is 1.76  103 s1, which is similar to the previously reported cobalt-based electrocatalyst.43 The pH dependence of the OER reaction reveals that the overpotential for OER decreases with increasing pH (Fig. 3a). A plot of the potential as measured at constant current density (1 mA cm2) versus pH shows a steady decrease from pH ¼ 13.4 to pH ¼ 2.3 with a slope of 93 mV per pH unit (Fig. 3b), which is close to the value of 88 mV per pH unit reported in the literature.26 This reveals that the catalyst is effective in a wide pH

(a) pH dependence of CVs measured at different pH values (2.3–13.4) over the Co-ZIF-9/FTO electrode, the return waves are omitted for clarity; (b) pH dependence of steady-state electrode potential at constant current density (Ianodic ¼ 1.0 mA cm2) over the Co-ZIF-9/FTO electrode. Fig. 3

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range and preferably works under basic solution. The decomposition of the catalyst to cobalt oxide is unlikely to occur, because such oxide species are typically unstable in very acidic solutions.41 This is indeed further conrmed by the Co2p and N1s XPS spectrum obtained on the Co-ZIF-9 sample aer the CV scans (Fig. S9†). The catalytic current density for water oxidation was found to be almost independent of the scan rate in the range of 10–100 mV s1, which indicated the rate limiting water oxidation at the electrode (Fig. S10a and b†).44 However, on subsequent scans at all scan rates, a broad anodic wave that is attributed to the adsorption of an electroactive species was observed,42 and the height of the anodic wave grew almost linearly with the scan rate (Fig. S10c†). Furthermore, the stability of the Co-ZIF-9/FTO electrode was also tested, and no obvious deactivation was observed aer over 25 hours operation (Fig. 4). Further improvements in both efficiency and stability of such MOF based electrodes will be pursued by devising methods, for example, by improving catalyst adhesion onto the FTO substrate by engineering MOF membranes. In summary, we have introduced a new type of heterogeneous WOC by assembling cobalt ions and benzimidazolate ligands into a crystalline microporous metal–organic framework (Co-ZIF-9) that electrocatalyzes the oxygen evolution reaction in a wide pH range. Co-ZIF-9 was found by DFT calculation to be thermodynamically feasible to activate the water molecule via binding the OH-group to the metal sites with low activation barriers, while the eliminated proton was accepted by the nearby benzimidazolate motifs. This allows CoZIF-9 to work effectively for the electrochemical oxygen-evolution reaction. We believe that this work makes one important step in water splitting chemistry by translating synergic catalysis of redox-active metal centres and organic motifs into a dened MOF structure. Considering the exible tunabilities of

Fig. 4 Dependence of current density on time over the Co-ZIF-9/FTO electrode in 0.1 M potassium phosphate buffer (pH ¼ 7.0). The turbulence in current density is due to the adsorption/desorption of the produced O2 bubbles over the electrode surface. Inset: a photo taken from the stability testing.

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both metals and ligands in MOFs, it is envisioned to stimulate intensive research on MOF-based WOCs by framework engineering in the chemical, electronic and crystal structures, together with the post-modication of MOFs.

Acknowledgements This work was supported by the National Basic Research Program of China (973 Project, no. 2013CB632405) and the National Natural Science Foundation of China (Grant no. 21033003 and 21173043).

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Water oxidation electrocatalysis by a zeolitic imidazolate framework.

The search for efficient water oxidation catalysts (WOCs) is of paramount importance in energy and environmental fields, but there exists no good non-...
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