Bioinspired Cobalt–Citrate Metal–Organic Framework as an Efficient

Jan 24, 2017 - Efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts are closely associated with many important energy convers...
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Bioinspired Cobalt−Citrate Metal−Organic Framework as an Efficient Electrocatalyst for Water Oxidation Jing Jiang,* Lan Huang, Xiaomin Liu, and Lunhong Ai* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, P.R. China S Supporting Information *

ABSTRACT: Efficient and cost-effective oxygen evolution reaction (OER) electrocatalysts are closely associated with many important energy conversion technologies. Herein, we first report an oxygen-evolving cobalt−citrate metal−organic framework (MOF, UTSA-16) for highly efficient electrocatalytic water oxidation. Benefiting from synergistic cooperation of intrinsic open porous structure, in situ formed high valent cobalt species, and existing Co4O4 cubane, the UTSA-16 exhibits excellent activity toward OER catalysis in alkaline medium. The UTSA-16 needs only 408 mV to offer a current density of 10 mA cm−2 for OER catalysis, which is superior to that of most MOF-based electrocatalysts and the standard Co3O4 counterpart. The present finding provides a better understanding of electroactive MOFs for water oxidation. KEYWORDS: electrocatalysis, water oxidation, metal−organic framework, cobalt, oxygen evolution reaction

1. INTRODUCTION Electricity-driven reactions that split H2O into H2 and O2 represent an attractive strategy to enable the conversion and storage of plentiful but intermittent solar and wind energies in the way of chemical fuels. However, up to now, its realization is still unclear, as this reaction is hampered by the kinetically sluggish and intrinsically efficiency-limited oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e−), an oxidative halfreaction of the water splitting. The solution of this challenging issue demands the rational utilization of the effective electrocatalysts to enhance energy conversion efficiency and reduce the operation cost.1,2 To date, the most efficient OER electrocatalysts are Ir- and Ru-based materials, but their scarcity and prohibitive price make them infeasible in practical applications.3,4 For this reason, extensive efforts have been made to develop inexpensive and efficient OER electrocatalysts based on earth-abundant elements for the replacement of precious catalysts. Among all candidates, cobalt-based compounds gain excellent performance and have received considerable attention as OER catalysts5,6 owing to their rich redox properties and distinctive ability to in situ electrochemically form high-oxidation cobalt species that are critical to OER catalysis. For example, spinel cobalt oxides have been © 2017 American Chemical Society

experimentally demonstrated to be an outstanding OER electrocatalyst.7,8 Co-phosphates as a promising OER electrocatalyst exhibit good stability in neutral media.9 Also, Cocontaining perovskites,10 olivines,11 chalcogenides,12 and hydro(oxy)oxides13 are identified as efficient OER catalysts in alkaline media. Furthermore, Co-based molecular catalysts such as cobalt porphyrin, salen, and phosphonate complexes also have been reported as a new form of electrocatalyst for water oxidation.14−16 Apart from the elemental composition, the behaviors of OER electrocatalysts strongly depend on the microscopic local structures, which are closely related to the macroscopic properties determining the intrinsic active sites of catalysts. In nature, photosystem II is comprised of the local structure of a CaMn3O4 cubane-type cluster, which serves as an essential oxygen-evolving center to efficiently promote water oxidation.17,18 Mimicking the structure and functionality of photosystem II, numerous promising OER electrocatalysts with structural analogies to the CaMn3O4 cubane have been Received: December 23, 2016 Accepted: January 24, 2017 Published: January 24, 2017 7193

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characterized as core unit for OER catalysis. To our best of knowledge, this is the first catalytically OER-active MOFsharing critical structure of Co4O4 cubane. In alkaline medium, the UTSA-16 exhibits highly efficient activity toward OER catalysis, and its performance is comparable to that of the benchmark catalyst RuO2 and even superior to that of the standard Co3O4 counterpart and most of the previously reported MOF-based OER catalysts. We attribute the excellent OER activity to the synergistic cooperation of the intrinsic open porous structure, in situ formed high valent cobalt, and existing Co4O4 cubane in the UTSA-16.

developed with the similar chemical principles governing OER catalysis because this structural component has been recognized as a critical domain for catalysis of water oxidation. For example, homogeneous molecular Co4O419 and Mn4O420 cubanes have been designed and confirmed as potential OER catalysts in terms of this theme. The Co4O4 cubane arrangements have also been recognized as the catalytic core subunit in crystalline Co3O4,21 Mn3O4,22 λ-MnO2,23 LiCoO2,24 and amorphous Co-Pi.9 In this context, it is reasonably speculated that in an ideal OER catalyst, cobalt atoms are expected to possess a high oxidation state and form Co4O4 cubanes. Metal−organic frameworks (MOFs) are a new class of crystalline and microporous materials which are considerably attractive for catalysis owing to their inherent features, including large surface area, unique porosity, and tailorable functionality.25,26 By virtue of active metal centers or prefunctionalized organic ligands, MOFs combine the favorable characteristics of both heterogeneous catalysts.27,28 Undoubtedly, MOFs offer great promise as OER electrocatalysts because the accessible pores and open channels in MOFs can provide the accommodation space for electrolytes, facilitate diffusion of the reactants, and assist the transport/evolution of generated oxygen gas. Moreover, homogeneously distributed metal cations in MOFs can serve as the catalytically active sites, while ligands in frameworks would control redox switching properties of neighboring metal cations through diversifying its coordination mode or chelating fashion.29 Unfortunately, limited examples have been established by directly using MOFs as active catalysts for electrocatalytic OER so far.30−36 Taking the above considerations, in this work, we report an oxygen-evolving cobalt−citrate metal−organic framework (UTSA-16) capable of efficiently electrocatalyzing water oxidation. This MOF presents an open framework structure built from tetranuclear cobalt citrate clusters as octahedral linkers and tetrahedral CoII atoms as trigonal nodes (Figure 1).37,38 The oxygen atoms in citrate chelate octahedral cobalt atoms to form a Co4O4 cubane arrangement which is

2. EXPERIMENTAL SECTION 2.1. Materials. Cobalt acetate (Co(Ac)2·4H2O), citric acid (C6H8O7·H2O), ethanol, and potassium hydroxide (KOH) were used as received from Chengdu Kelong Chemical Reagent Corporation. 2.2. Synthesis of Cobalt−Citrate Metal−Organic Framework (UTSA-16). The cobalt−citrate metal−organic framework (UTSA-16) was prepared by a solvothermal route in accordance with a reported protocol.37,39 Typically, Co(Ac)2·4H2O (1 mmol), C6H8O7·H2O (1 mmol), and KOH (3 mmol) were dissolved in 10 mL of a mixed ethanol/water (1/1 v/v) solution. The mixture solution was poured into a 20 mL Teflon-lined autoclave. The sealed autoclave was then heated to 120 °C and maintained at this temperature for 24 h. The violet precipitates were collected by centrifugation, rinsed with anhydrous ethanol three times, and dried at 60 °C for 12 h in air. The UTSA-16 samples were also synthesized by varying initial cobalt concentrations (abbreviated as UTSA-16-x, where x represents the initial cobalt concentration). If no further notification is given, the UTSA-16 designation in this case refers to UTSA-16-0.1 with the best OER performance among all the UTSA-16 products. The Co3O4 counterpart was obtained by calcination of the UTSA-16-0.1 sample at 450 °C for 2 h in air. 2.3. Characterization. Determination of crystalline structures for the UTSA-16 powders was performed on an X-ray diffractometer (XRD, Rigaku Dmax/Ultima IV) with Cu Kα radiation. The molecular structures of the UTSA-16 powders were tested using a Fourier transform infrared spectrometer (FTIR, Nicolet 6700) and the potassium bromide pellet method. The microstructural profiles of the UTSA-16 powders were examined using a scanning electron microscope (SEM, Hitachi S4800 and JEOL JSM-6510) combined with energy dispersive X-ray spectroscopy (EDS). A transmission electron microscope (TEM, FEI Tecnai G20) was used to obtain the detailed structural information on the UTSA-16 sample. The surface compositions of the UTSA-16 powders were analyzed using an X-ray photoelectron spectrometer (XPS, PerkinElmer PHI 5000C) with an Al Kα source. Raman spectra of the post-OER UTSA-16 samples were collected on a DXR micro-Raman spectrometer (Thermo Scientific) with a laser wavelength of 532 nm. The ASAP 2020 V4.00 G instrument (Micromeritics Instruments) was employed to characterize the textural properties of the UTSA-16 sample. 2.4. Electrochemical Measurements. The electrochemical water oxidation experiments were implemented on the CHI 660E electrochemical workstation. A typical three-electrode system was employed for all the electrochemical measurements by utilizing 3 M KCl-Ag/ AgCl as reference electrode, Pt foil as counter electrode, and UTSA-16 modified glassy carbon electrode (GCE, geometric area: 0.07 cm2)/ rotating disk electrode (RDE) as working electrode in 1.0 M KOH solution. To fabricate working electrode, 5 μL of catalyst ink (mixture of UTSA-16 (5 mg) and Nafion solution (5 wt %, 10 μL) in 1 mL of 3/1 (v/v) water/alcohol solution) was dripped onto the surface of GCE, resulting in the catalyst mass loading of ∼0.35 mg·cm−2. All of the potentials measured in this case were converted to the reversible hydrogen electrode (RHE) based on the equation ERHE = EAg/AgCl + 0.059 pH + 0.197 V. Prior to electrochemical water oxidation, the KOH solution was bubbled by O2 gas to reach the H2O/O2 equilibrium. The electrocatalytic performance of the UTSA-16 for

Figure 1. (a) Crystal structure of the UTSA-16 and (b) representative structures of Co4O4 cubane in the UTSA-16. 7194

DOI: 10.1021/acsami.6b16534 ACS Appl. Mater. Interfaces 2017, 9, 7193−7201

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ACS Applied Materials & Interfaces water oxidation was characterized by linear sweep voltammetry (LSV) with a scan rate of 5 mV·s−1. Electrochemical impedance spectra (EIS) were recorded in the frequency range from 0.1 to 105 Hz with an AC amplitude of 5 mV at an applied potential of 1.664 V (vs the RHE). Durability of the UTSA-16 sample for OER catalysis was evaluated by chronoamperometry at a potential of 1.65 V (vs the RHE).

UTSA-16 crystal consists of an enormous amount of irregular particles. These particles appear as pyramid polyhedral morphology with a rough surface and uneven sizes. The highly magnified SEM image (Figure 3b) of the UTSA-16 further suggests that the pyramid polyhedron is composed of clearly visible flat faces and straight edges. The TEM image given in Figure 3c also confirms the pyramid polyhedral morphology of the UTSA-16. Figure 3d shows EDS analysis of the UTSA-16, which reveals that C, O, K, and Co elements coexist in the sample. These elements are distributed evenly in the pyramid polyhedron (Figure 3e). Furthermore, a typical HRTEM image of the UTSA-16 (Figure 3f) reveals that no obvious lattice fringes are observed for the UTSA-16, consistent with other observations on previously reported MOFs.42,43 Additionally, the unique micropores can also be clearly observed, which are well-distributed in the UTSA-16. The surface component and electronic structure of the UTSA-16 were analyzed by XPS measurements. The XPS survey spectrum shown in Figure 4a confirms the existence of C, O, Co, and K elements in the UTSA-16 sample, matching with the chemical formula of {[KCo3(C6H4O7)(C6H5O7)(H2O)2]·8H2O}n.39 Figure 4b displays the high-resolution XPS spectrum of C 1s, which can be deconvoluted well into three surface carbon components at around 284.3 eV (nonoxygenated carbon: C−C), 286.2 eV (oxygen-containing carbon: C−O), and 288.1 eV (carboxyl carbon: OC−O).44 The high-resolution XPS spectrum of O 1s is presented in Figure 4c, which can be deconvoluted into Co−O (530.8 eV), OC−O (531.8 eV), and C−O (533.2 eV) species.45 In the high-resolution XPS spectrum of Co 2p (Figure 4d), the binding energies of Co 2p3/2 and Co 2p1/2 are located at 781.1 and 797.5 eV, respectively, which are characteristic features of CoII.46−48 Meanwhile, two distinct satellite peaks are observed at about 5 eV above their main peaks, further support that the surface CoII species existed in the UTSA-16.49,50 The textural properties of the UTSA-16 were determined by N2 adsorption−desorption isotherms at 77 K. As shown in Figure 5, the curves exhibit a typical type-I isotherm with a sharp steep N2 uptake at low relative pressure, evidencing microporous characteristics of the UTSA-16. On the basis of these plots, the Brunauer−Emmett−Teller (BET) specific surface area, Langmuir specific surface area, and t-plot micropore volume of the UTSA-16 are measured to be 924 m2 g−1, 936 m2 g−1, and 0.33 cm3 g−1, respectively. The Barrett−Joyner−Halenda (BJH) size distribution plot (inset in Figure 5) reveals that the pore size of the UTSA-16 is centered at about 1.46 nm. Such high specific surface area and abundant porosity of the UTSA-16 is of great importance for electrocatalytic application. To test the catalytic performance of the UTSA-16 for electrochemical oxidation of water to dioxygen, all of the electrochemical measurements were implemented in a standard three-electrode system at a constant rotation rate of 1600 rpm. The LSV method was utilized to examine the OER activity of the UTSA-16. As a comparison, the blank GCE, benchmark RuO2, Pt/C, and Co3O4 counterpart (coated on GCE with the same mass loading) were also tested in the same conditions. It is noted that Co3O4 was particularly included as a reference for evaluating OER activity of the UTSA-16 because it is considered as an outstanding OER catalyst in basic medium.5 Figure 6a gives the LSV curves of the different catalysts in an O2-saturated 1.0 M KOH solution with a scan rate of 5 mV s−1. The blank GCE has negligible catalytic activity, while the

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RESULTS AND DISCUSSION Figure 2a shows a typical powder XRD pattern of the asprepared UTSA-16 crystal. The main diffraction peaks are

Figure 2. XRD pattern (a) and FTIR spectrum (b) of the UTSA-16.

observed at 7.3°, 13.6°, 28.0°, 28.8°, 35.6°, and 37.3°, which are readily identical to the simulated pattern of UTSA-16 and the reported experimental one.37−39 The strong and sharp characteristics of these peaks suggest the high crystallinity of the UTSA-16. The FTIR spectrum was further recorded to reveal the molecular structure of the UTSA-16. As shown in Figure 2b, a broad peak centered at 3430 cm−1 is attributed to the hydroxyl group from absorbed water. The two distinct absorption peaks at 1395 and 1574 cm−1 refer to the symmetric and antisymmetric stretching vibrations of the carboxylate groups from the coordinated citrates in the framework, respectively,40 which seem to shift to lower frequencies with respect to the free citric acid, further indicating their coordination with cobalt.40,41 Of note, the absence of typical absorption peaks related to undissociated carboxylic acid groups in the region of 1700−1720 cm−1 further reflects that each carboxylic acid group has been deprotonated in the UTSA-16. The microstructures and morphologies of the as-synthesized UTSA-16 were examined by SEM. Figure 3a shows a typical low-magnified SEM image of the UTSA-16. It indicates that the 7195

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Figure 3. SEM images (a and b), TEM image (c), EDS spectrum (d), EDS mapping images (e), and HRTEM image (f) of the UTSA-16.

Figure 4. XPS spectrum of the UTSA-16: (a) survey, (b) C 1s, (c) O 1s, and (d) Co 2p.

microtubes57) but is significantly lower than that of UTSA-16derived Co3O4 counterpart (465 mV). Remarkably, the η10 value for the UTSA-16 is much smaller than those of the previously reported OER-active MOFs and compares favorably with other Co-containing inorganic catalysts (Table S1, Supporting Information). Figure 6b displays the Tafel plots of the UTSA-16 and reference catalysts derived from corresponding LSV curves. The UTSA-16 holds the Tafel slope of 77 mV dec−1. This value is slightly larger than that of the benchmark catalyst RuO2 (62 mV dec−1) but is much smaller than those of the commercial

commercial Pt/C performs poorly for OER, which are coincident with those observed previously in alkaline media.51−54 In sharp contrast, the UTSA-16 is electrocatalytically active and presents a high performance toward OER catalysis. The OER onset potential for the UTSA-16 is estimated to be about 1.60 V (vs RHE), which is earlier than that of the Co3O4 counterpart (1.64 V vs RHE). To gain a current density of 10 mA cm−2, the UTSA-16 demands an overpotential (η10) of 408 mV, which is observed to be larger than that of the benchmark catalyst RuO2 and nanostructured Co3O4 (e.g., mesoporous nanotubes,55 nanosheets,56 hollow 7196

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Figure 5. N2 adsorption−desorption isotherms and BJH size distribution (inset) of the UTSA-16.

Figure 7. (a) Chronoamperometric durability test of the UTSA-16 at an applied potential of 1.65 V (vs the RHE) in 1.0 M KOH solution. (b) OER polarization curves of the UTSA-16 at a scan rate of 5 mV s−1 in KOH solution with different pH values.

theoretical yield of oxygen during the OER process. The faradaic efficiency is thus estimated to be close to 95%. Furthermore, the pH dependent OER activity of the UTSA-16 is studied because the electrolyte concentration is an important factor for determining the electrocatalytic activity. Figure 7b shows the catalytic OER activity of the UTSA-16 in KOH solutions with different pH values. As observed, the electrocatalytic OER activity of the UTSA-16 significantly increases with enhancing the concentration of KOH solution, indicating that a high concentration of KOH is beneficial for the improvement of OER performance, which is in good agreement with previous observations in other electrocatalytic systems.59,60 To optimize the OER activity of the UTSA-16, we further prepared other UTSA-16 samples by varying the concentrations of cobalt ions in the initial reaction solution. These UTSA-16 samples exhibit a similar crystal phase (Figure 8a) and microstructural morphology (Figure S2, Supporting Information). Clearly, the initial cobalt cation concentration could significantly affect the OER activities of the UTSA-16 catalysts (Figure 8b). The UTSA-16-0.1 presents the best OER activity. The EIS measurement of the UTSA-16 was conducted under OER conditions to study the electrode kinetics during this process. All of the UTSA-16 catalysts exhibit two semicircles in the Nyquist plots (Figure 8c). The semicircle at low-frequency region is associated with the adsorption of a reaction intermediate such as Oad (a general intermediate in OER) on the electrode surface.61,62 The semicircle at the high frequency region corresponds to charge transfer processes. As a result, the UTSA-16-0.1 provides a charge transfer resistance smaller than those of other UTSA-16 samples, indicating the faster charge transfer of the UTSA-16-0.1 during the electrochemical OER

Figure 6. (a) OER polarization curves and (b) polarization curvederived Tafel plots of the UTSA-16, benchmark RuO2, Pt/C, and Co3O4 counterpart.

Pt/C (182 mV dec−1) and Co3O4 counterpart (91 mV dec−1), suggesting that the UTSA-16 has favorable kinetics of electrochemical water oxidation. The long-term durability of the UTSA-16 for catalyzing OER is also critical to evaluate its OER performance, which was tested by potentiostatic electrolysis in 1.0 M KOH solution. The I−t curve (Figure 7a) shows that the catalytic current density exhibits a slight decline after 7 h of continuous measurement, which could be caused by the catalyst mass loss from the work electrode during long-term OER test.58 The faradaic efficiency of the UTSA-16 was also measured in a gastight H-type electrochemical cell. As shown in Figure S1, the amount of experimentally produced O2 is comparable to the 7197

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Figure 8. (a) XRD patterns of the UTSA-16 synthesized using different initial cobalt concentrations. (b) OER polarization curves of the different UTSA-16 samples with a scan rate of 5 mV s−1 in 1.0 M KOH solution. (c) Nyquist plots of the different UTSA-16 samples at 1.664 V (vs the RHE). (d) Capacitive currents at 1.22 V against scan rates of the different UTSA-16 samples.

Figure 9. (a) High-resolution Co 2p XPS spectra. (b) XRD pattern. (c and d) SEM images. (e) Raman spectrum. (f) FTIR spectrum of the UTSA16 after chronoamperometric test, (g) optical photographs of the UTSA-16 before and after OER catalysis.

illustrated in Figure 8d. The plots yield a good linear relationship, whose slopes can be reflected in the ECSA because the ECSA of an electrocatalyst is proportional to its Cdl value. Clearly, the UTSA-16-0.1 possesses the largest Cdl value (52.4 μF cm−2), implying that it holds the largest ECSA among all of the samples for electrocatalytic OER.

process. To further shed light on the different OER activity, the electrochemical surface area (ECSA) of the UTSA-16 samples was estimated from the calculation of electrochemical double layer capacitances (Cdl) in the potential widow without obvious electrochemical reactions. On the basis of the CV curves recorded at different scan rates (Figure S3, Supporting Information), the plot of scan rate against current density is 7198

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ACS Applied Materials & Interfaces As widely accepted, the CoII species in Co-based electrocatalysts generally experience obvious oxidation processes and transform into high-oxidation cobalt species, which is critical to their catalytic performances. The CVs of the UTSA-16 in alkaline medium (Figure S4, Supporting Information) confirm such an oxidation transformation process (CoII → CoIII). To further support this result, XPS analysis of the UTSA-16 after OER catalysis was performed. In the high-resolution Co 2p spectrum (Figure 9a) of post-OER UTSA-16, two peaks observed at 781.2 and 796.2 eV are correlated to Co 2p3/2 and Co 2p1/2 of the CoIII species, respectively, owing to the absence of characteristic satellites relevant to CoII compared with that of UTSA-16 prior to OER (Figure 4d), indicating the effective transformation of CoII to catalytically active CoIII centers in the UTSA-16. The crystal structure change of the UTSA-16 during the OER process was also examined by XRD analysis. As shown in Figure 9b, the UTSA-16 indeed underwent the phase conversion and became the dominant CoOx species during the OER process. We then determined the morphological changes in the post-OER UTSA-16 by SEM observations. The SEM images (Figures 9c and d) suggest that the initial pyramid polyhedral morphology completely vanished after OER catalysis, accompanied with the appearance of larger amounts of aggregated particles constructed by intertwined nanosheets in the post-OER UTSA-16. This in situ transformation during OER catalysis in the UTSA-16 is also indirectly reflected by its color change. As is well-known, the reduced and oxidized states of CoII/CoIII redox can induce a distinguished visible color. Figure 9g presents the picture of the UTSA-16 deposited on ITO glass. The original purple UTSA-16 turns brown-black during the OER process. This phenomenon is similar to the OER process of Ni(OH)2/FTO63 and NiCo-LDH/Ni foam64 electrocatalysts. To further gain insight into this process, we employed Raman spectroscopy to characterize post-OER UTSA-16 (Figure 9e). It is clear that three prominent peaks at 686, 517, and 480 cm−1 dominate the spectrum of the UTSA-16. The peaks at 517 and 480 cm−1 are ascribed to a Co−O stretching vibration, while the peak at 686 cm−1 is attributed to the vibration of a [CoO6] unit, which is a characteristic of CoOx and consistent with the spectrum of previously reported CoOx.65,66 In addition, the FTIR spectrum (Figure 9f) of the post-OER UTSA-16 also confirms that the citrate is still anchored on the transformed CoOx-based products. On the basis of these experimental results, it demonstrates that the gradual in situ electrochemical conversion of UTSA-16 into catalytically active CoOx species during the OER process. This phenomenon is similar to the recent observations on the OER systems of Co4−POM67,68 and dinuclear cobalt(III) complexes.69 Here, we ascribe the excellent OER performance of the UTSA-16 to its advantageous structures. The UTSA-16 as a typical metal−organic framework (Figure 1) possesses the intrinsic porosity of MOFs and the periodic arrangement and homogeneous distribution of metal centers in the framework. Its large accessible surface and ordered porous structure offer open channels for efficient ion diffusion, gas transport/ evolution, and electron movement, which is in favor of the OER catalysis. Moreover, the electroactive cobalt centers are homogeneously distributed in the frameworks, which exhibit unique redox properties and can act as reactive sources to in situ supply the catalytically OER-active cobalt species to promote the electrochemical OER catalysis.

4. CONCLUSIONS In summary, we synthesized the bioinspired cobalt−citrate metal−organic framework (UTSA-16) by a simple solvothermal route and demonstrated its excellent performance for the electrocatalytic OER. The resulting UTSA-16 achieves small onset potential, large anodic current density, and excellent longterm durability in alkaline medium, which is superior to most MOF-based electrocatalysts and the standard Co3O4 counterpart. We attribute the observed OER activity to the synergistic cooperation of intrinsic open porous structure, in situ formed high-valent cobalt, and existing Co4O4 cubane in the UTSA-16. The present study highlights the potential utilization of MOFs for electrochemical water oxidation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16534. Faradaic efficiency, SEM images, CV curves of UTSA-16, and supplementary tables (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]; Tel./Fax: +86-0817-2568081. ORCID

Jing Jiang: 0000-0001-8592-5130 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Natural Science Foundation of China (Grant 51572227), the Sichuan Youth Science and Technology Foundation (Grant 2013JQ0012), the Major Cultivating Foundation of Education Department of Sichuan Province (Grant 17CZ0036), and the Research Foundation of CWNU (Grant 14E016) for financial support.

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REFERENCES

(1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (2) Li, X.; Hao, X.; Abudula, A.; Guan, G. Nanostructured Catalysts for Electrochemical Water Splitting: Current State and Prospects. J. Mater. Chem. A 2016, 4, 11973−12000. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446−6473. (4) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the Rational Design of NonPrecious Transition Metal Oxides for Oxygen Electrocatalysis. Energy Environ. Sci. 2015, 8, 1404−1427. (5) Deng, X.; Tüysüz, H. Cobalt-Oxide-Based Materials as Water Oxidation Catalyst: Recent Progress and Challenges. ACS Catal. 2014, 4, 3701−3714. (6) Li, X.; Niu, Z.; Jiang, J.; Ai, L. Cobalt Nanoparticles Embedded in Porous N-Rich Carbon as an Efficient Bifunctional Electrocatalyst for Water Splitting. J. Mater. Chem. A 2016, 4, 3204−3209. (7) Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J.-E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H. Advanced Zinc-Air Batteries Based on HighPerformance Hybrid Electrocatalysts. Nat. Commun. 2013, 4, 1805. 7199

DOI: 10.1021/acsami.6b16534 ACS Appl. Mater. Interfaces 2017, 9, 7193−7201

Research Article

ACS Applied Materials & Interfaces

(29) Ryu, J.; Jung, N.; Jang, J. H.; Kim, H.-J.; Yoo, S. J. In Situ Transformation of Hydrogenevolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-Oxo/Hydroxo Molecular Units. ACS Catal. 2015, 5, 4066− 4074. (30) Wang, L.; Wu, Y.; Cao, R.; Ren, L.; Chen, M.; Feng, X.; Zhou, J.; Wang, B. Fe/Ni Metal-Organic Frameworks and Their Binder-Free Thin Films for Efficient Oxygen Evolution with Low Overpotential. ACS Appl. Mater. Interfaces 2016, 8, 16736−16743. (31) Kung, C.-W.; Mondloch, J. E.; Wang, T. C.; Bury, W.; Hoffeditz, W.; Klahr, B. M.; Klet, R. C.; Pellin, M. J.; Farha, O. K.; Hupp, J. T. Metal-Organic Framework Thin Films as Platforms for Atomic Layer Deposition of Cobalt Ions to Enable Electrocatalytic Water Oxidation. ACS Appl. Mater. Interfaces 2015, 7, 28223−28230. (32) Aiyappa, H. B.; Thote, J.; Shinde, D. B.; Banerjee, R.; Kurungot, S. Cobalt-Modified Covalent Organic Framework as a Robust Water Oxidation Electrocatalyst. Chem. Mater. 2016, 28, 4375−4379. (33) Wang, S.; Hou, Y.; Lin, S.; Wang, X. Water Oxidation Electrocatalysis by a Zeolitic Imidazolate Framework. Nanoscale 2014, 6, 9930−9934. (34) Jahan, M.; Liu, Z.; Loh, K. P. A. Graphene Oxide and CopperCentered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER, and ORR. Adv. Funct. Mater. 2013, 23, 5363− 5372. (35) Flügel, E. A.; Lau, V. W.-h.; Schlomberg, H.; Glaum, R.; Lotsch, B. V. Homonuclear Mixed-Valent Cobalt Imidazolate Framework for Oxygen-Evolution Electrocatalysis. Chem. - Eur. J. 2016, 22, 3676− 3680. (36) Lu, X.-F.; Liao, P.-Q.; Wang, J.-W.; Wu, J.-X.; Chen, X.-W.; He, C.-T.; Zhang, J.-P.; Li, G.-R.; Chen, X.-M. An Alkaline-Stable, Metal Hydroxide Mimicking Metal-Organic Framework for Efficient Electrocatalytic Oxygen Evolution. J. Am. Chem. Soc. 2016, 138, 8336−8339. (37) Xiang, S.-C.; Wu, X.-T.; Zhang, J.-J.; Fu, R.-B.; Hu, S.-M.; Zhang, X.-D. A 3D Canted Antiferromagnetic Porous Metal-Organic Framework with Anatase Topology through Assembly of an Analogue of Polyoxometalate. J. Am. Chem. Soc. 2005, 127, 16352−16353. (38) Xiang, S.; He, Y.; Zhang, Z.; Wu, H.; Zhou, W.; Krishna, R.; Chen, B. Microporous Metal-Organic Framework with Potential for Carbon Dioxide Capture at Ambient Conditions. Nat. Commun. 2012, 3, 954. (39) Shen, Y. C.; Li, Z. Y.; Wang, L. H.; Ye, Y. X.; Liu, Q.; Ma, X. L.; Chen, Q. H.; Zhang, Z. J.; Xiang, S. C. Cobalt-Citrate Framework Armored with Graphene Oxide Exhibiting Improved Thermal Stability and Selectivity for Biogas Decarburization. J. Mater. Chem. A 2015, 3, 593−599. (40) Xie, Q. S.; Li, F.; Guo, H. Z.; Wang, L. S.; Chen, Y. Z.; Yue, G. H.; Peng, D. L. Template-Free Synthesis of Amorphous DoubleShelled Zinc-Cobalt Citrate Hollow Microspheres and Their Transformation to Crystalline ZnCo2O4 Microspheres. ACS Appl. Mater. Interfaces 2013, 5, 5508−5517. (41) Cho, S.; Jang, J. W.; Jung, A.; Lee, S. H.; Lee, J.; Lee, J. S.; Lee, K. H. Formation of Amorphous Zinc Citrate Spheres and Their Conversion to Crystalline ZnO Nanostructures. Langmuir 2011, 27, 371−378. (42) Dai, X.; Liu, M.; Li, Z.; Jin, A.; Ma, Y.; Huang, X.; Sun, H.; Wang, H.; Zhang, X. Molybdenum Polysulfide Anchored on Porous Zr-Metal Organic Framework to Enhance the Performance of Hydrogen Evolution Reaction. J. Phys. Chem. C 2016, 120, 12539− 12548. (43) Han, Y.; Qi, P.; Feng, X.; Li, S.; Fu, X.; Li, H.; Chen, Y.; Zhou, J.; Li, X.; Wang, B. In Situ Growth of MOFs on the Surface of Si Nanoparticles for Highly Efficient Lithium Storage: Si@MOF Nanocomposites as Anode Materials for Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 2178−2182. (44) Bag, S.; Roy, K.; Gopinath, C. S.; Raj, C. R. Facile Single-Step Synthesis of Nitrogen-Doped Reduced Graphene Oxide-Mn3O4 Hybrid Functional Material for the Electrocatalytic Reduction of Oxygen. ACS Appl. Mater. Interfaces 2014, 6, 2692−2699.

(8) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4 Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat. Mater. 2011, 10, 780−786. (9) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (10) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y.-L.; Risch, M.; Hong, W. T.; Zhou, J.; Yang, S.-H. Double Perovskites as a Family of Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nat. Commun. 2013, 4, 2439. (11) Lee, S. W.; Carlton, C.; Risch, M.; Surendranath, Y.; Chen, S.; Furutsuki, S.; Yamada, A.; Nocera, D. G.; Shao-Horn, Y. The Nature of Lithium Battery Materials under Oxygen Evolution Reaction Conditions. J. Am. Chem. Soc. 2012, 134, 16959−16962. (12) Gao, M. R.; Xu, Y. F.; Jiang, J.; Zheng, Y. R.; Yu, S. H. Water Oxidation Electrocatalyzed by an Efficient Mn3O4/CoSe2 Nanocomposite. J. Am. Chem. Soc. 2012, 134, 2930−2933. (13) Song, F.; Hu, X. Exfoliation of Layered Double Hydroxides for Enhanced Oxygen Evolution Catalysis. Nat. Commun. 2014, 5, 4477. (14) Blakemore, J. D.; Crabtree, R. H.; Brudvig, G. W. Molecular Catalysts for Water Oxidation. Chem. Rev. 2015, 115, 12974−13005. (15) Jia, H.; Sun, Z.; Jiang, D.; Du, P. Covalent Cobalt Porphyrin Framework on Multiwalled Carbon Nanotubes for Efficient Water Oxidation at Low Overpotential. Chem. Mater. 2015, 27, 4586−4593. (16) Zhou, T.; Wang, D.; Goh, S. C.-K.; Hong, J.; Han, J.; Mao, J.; Xu, R. Bio-Inspired Organic Cobalt(II) Phosphonates toward Water Oxidation. Energy Environ. Sci. 2015, 8, 526−534. (17) Ferreira, K. N.; Iverson, T. M.; Maghlaoui, K.; Barber, J.; Iwata, S. Architecture of the Photosynthetic Oxygen-Evolving Center. Science 2004, 303, 1831−1838. (18) Umena, Y.; Kawakami, K.; Shen, J. R.; Kamiya, N. Crystal Structure of Oxygen-evolving Photosystem II at a Resolution of 1.9 A. Nature 2011, 473, 55−60. (19) McCool, N. S.; Robinson, D. M.; Sheats, J. E.; Dismukes, G. C. A Co4O4 ″Cubane″ Water Oxidation Catalyst Inspired by Photosynthesis. J. Am. Chem. Soc. 2011, 133, 11446−11449. (20) Dismukes, G. C.; Brimblecombe, R.; Felton, G. A. N.; Pryadun, R. S.; Sheats, J. E.; Spiccia, L.; Swiegers, G. F. Development of Bioinspired Mn4O4-Cubane Water Oxidation Catalysts: Lessons from Photosynthesis. Acc. Chem. Res. 2009, 42, 1935−1943. (21) Jiao, F.; Frei, H. Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts. Angew. Chem., Int. Ed. 2009, 48, 1841−1844. (22) Najafpour, M. M.; Renger, G.; Hołyńska, M.; Moghaddam, A. N.; Aro, E.-M.; Carpentier, R.; Nishihara, H.; Eaton-Rye, J. J.; Shen, J.R.; Allakhverdiev, S. I. Manganese Compounds as Water-Oxidizing Catalysts: From the Natural Water-Oxidizing Complex to Nanosized Manganese Oxide Structures. Chem. Rev. 2016, 116, 2886−2936. (23) Robinson, D. M.; Go, Y. B.; Greenblatt, M.; Dismukes, G. C. Water Oxidation by λ-MnO2: Catalysis by the Cubical Mn4O4 Subcluster Obtained by Delithiation of Spinel LiMn2O4. J. Am. Chem. Soc. 2010, 132, 11467−11469. (24) Gardner, G. P.; Go, Y. B.; Robinson, D. M.; Smith, P. F.; Hadermann, J.; Abakumov, A.; Greenblatt, M.; Dismukes, G. C. Structural Requirements in Lithium Cobalt Oxides for the Catalytic Oxidation of Water. Angew. Chem., Int. Ed. 2012, 51, 1616−1619. (25) Corma, A.; García, H.; Llabres i Xamena, F. X. Engineering Metal Organic Frameworks for Heterogeneous Catalysis. Chem. Rev. 2010, 110, 4606−4655. (26) Gascon, J.; Corma, A.; Kapteijn, F.; Llabres i Xamena, F. X. Metal Organic Framework Catalysis: Quo Vadis? ACS Catal. 2014, 4, 361−378. (27) Wang, S.; Wang, X. Multifunctional Metal-Organic Frameworks for Photocatalysis. Small 2015, 11, 3097−3112. (28) Wang, S.; Wang, X. Imidazolium Ionic Liquids, Imidazolylidene Heterocyclic Carbenes, and Zeolitic Imidazolate Frameworks for CO2 Capture and Photochemical Reduction. Angew. Chem., Int. Ed. 2016, 55, 2308−2320. 7200

DOI: 10.1021/acsami.6b16534 ACS Appl. Mater. Interfaces 2017, 9, 7193−7201

Research Article

ACS Applied Materials & Interfaces (45) Ai, L.; Gao, X.; Jiang, J. In Situ Synthesis of Cobalt Stabilized on Macroscopic Biopolymer Hydrogel as Economical and Recyclable Catalyst for Hydrogen Generation from Sodium Borohydride Hydrolysis. J. Power Sources 2014, 257, 213−220. (46) Liu, X.; Jiang, J.; Ai, L. Non-Precious Cobalt Oxalate Microstructures as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction. J. Mater. Chem. A 2015, 3, 9707−9713. (47) Wang, S.; Hou, Y.; Wang, X. Development of a Stable MnCo2O4 Cocatalyst for Photocatalytic CO2 Reduction with Visible Light. ACS Appl. Mater. Interfaces 2015, 7, 4327−4335. (48) Tian, T.; Jiang, J.; Ai, L. In Situ Electrochemically Generated Composite-Type CoOx/WOx in Self-Activated Cobalt Tungstate Nanostructures: Implication for Highly Enhanced Electrocatalytic Oxygen Evolution. Electrochim. Acta 2017, 224, 551−560. (49) Bennici, S.; Yu, H.; Obeid, E.; Auroux, A. Highly Active Heteropolyanions Supported Co Catalysts for Fast Hydrogen Generation in NaBH4 Hydrolysis. Int. J. Hydrogen Energy 2011, 36, 7431−7442. (50) Wei, W.; Chen, W.; Ivey, D. G. Rock Salt-Spinel Structural Transformation in Anodically Electrodeposited Mn-Co-O Nanocrystals. Chem. Mater. 2008, 20, 1941−1947. (51) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured A-NickelHydroxide as an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, 7077− 7084. (52) Sa, Y. J.; Kwon, K.; Cheon, J. Y.; Kleitz, F.; Joo, S. H. Ordered Mesoporous Co3O4 Spinels as Stable, Bifunctional, Noble Metal-Free Oxygen Electrocatalysts. J. Mater. Chem. A 2013, 1, 9992−10001. (53) Tian, G.-L.; Zhao, M.-Q.; Yu, D.; Kong, X.-Y.; Huang, J.-Q.; Zhang, Q.; Wei, F. Nitrogen-Doped Graphene/Carbon Nanotube Hybrids: In Situ Formation on Bifunctional Catalysts and Their Superior Electrocatalytic Activity for Oxygen Evolution/Reduction Reaction. Small 2014, 10, 2251−2259. (54) Xu, C.; Lu, M.; Zhan, Y.; Lee, J. Y. A Bifunctional Oxygen Electrocatalyst from Monodisperse MnCo2O4 Nanoparticles on Nitrogen Enriched Carbon Nanofibers. RSC Adv. 2014, 4, 25089− 25092. (55) Wang, H.; Zhuo, S.; Liang, Y.; Han, X.; Zhang, B. General SelfTemplate Synthesis of Transition-Metal Oxide and Chalcogenide Mesoporous Nanotubes with Enhanced Electrochemical Performances. Angew. Chem., Int. Ed. 2016, 55, 9055−9059. (56) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2016, 55, 5277−5281. (57) Zhu, Y. P.; Ma, T. Y.; Jaroniec, M.; Qiao, S. Z. Self-Templating Synthesis of Hollow Co3O4 Microtube Arrays for Highly Efficient Water Electrolysis. Angew. Chem., Int. Ed. 2017, 56, 1324−1328. (58) Xu, D.; Mu, C.; Xiang, J.; Wen, F.; Su, C.; Hao, C.; Hu, W.; Tang, Y.; Liu, Z. Carbon-Encapsulated Co3O4@CoO@Co Nanocomposites for Multifunctional Applications in Enhanced Long-Life Lithium Storage, Supercapacitor and Oxygen Evolution Reaction. Electrochim. Acta 2016, 220, 322−330. (59) Zhou, X.; Shen, X.; Xia, Z.; Zhang, Z.; Li, J.; Ma, Y.; Qu, Y. Hollow Fluffy Co3O4 Cages as Efficient Electroactive Materials for Supercapacitors and Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, 20322−20331. (60) Cheng, N.; Liu, Q.; Asiri, A. M.; Xing, W.; Sun, X. A Fe-Doped Ni3S2 Particle Film as a High-Efficiency Robust Oxygen Evolution Electrode with Very High Current Density. J. Mater. Chem. A 2015, 3, 23207−23212. (61) Aaboubi, O.; Ali-Omar, A.-Y.; Dzoyem, E.; Marthe, J.; Boudifa, M. Ni-Mn Based Alloys as Versatile Catalyst for Different Electrochemical Reactions. J. Power Sources 2014, 269, 597−607. (62) Yan, X.; Tian, L.; He, M.; Chen, X. Three-Dimensional Crystalline/Amorphous Co/Co3O4 Core/Shell Nanosheets as Efficient Electrocatalysts for Hydrogen Evolution Reaction. Nano Lett. 2015, 15, 6015−6021.

(63) Stern, L.-A.; Hu, X. Enhanced Oxygen Evolution Activity by NiOx and Ni(OH)2 Nanoparticles. Faraday Discuss. 2014, 176, 363− 379. (64) Jiang, J.; Zhang, A.; Li, L.; Ai, L. Nickel-Cobalt Layered Double Hydroxide Nanosheets as High-Performance Electrocatalyst for Oxygen Evolution Reaction. J. Power Sources 2015, 278, 445−451. (65) Nurlaela, E.; Ould-Chikh, S.; Llorens, I.; Hazemann, J.-L.; Takanabe, K. Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst. Chem. Mater. 2015, 27, 5685−5694. (66) Xing, C.; Liu, Y.; Su, Y.; Chen, Y.; Hao, S.; Wu, X.; Wang, X.; Cao, H.; Li, B. Structural Evolution of Co-Based Metal Organic Frameworks in Pyrolysis for Synthesis of Core-Shells on Nanosheets: Co@CoOx@Carbon-RGO Composites for Enhanced Hydrogen Generation Activity. ACS Appl. Mater. Interfaces 2016, 8, 15430− 15438. (67) Stracke, J. J.; Finke, R. G. Electrocatalytic Water Oxidation Beginning with the Cobalt Polyoxometalate [Co4(H2O)2(PW9O34)2]10: Identification of Heterogeneous CoOx as the Dominant Catalyst. J. Am. Chem. Soc. 2011, 133, 14872−14875. (68) Stracke, J. J.; Finke, R. G. Water Oxidation Catalysis Beginning with 2.5 mM [Co4(H2O)2(PW9O34)2]10‑: Investigation of the True Electrochemically Driven Catalyst at ≥ 600 mV Overpotential at a Glassy Carbon Electrode. ACS Catal. 2013, 3, 1209−1219. (69) Wang, J.-W.; Sahoo, P.; Lu, T.-B. Reinvestigation of Water Oxidation Catalyzed by a Dinuclear Cobalt Polypyridine Complex: Identification of CoOx as a Real Heterogeneous Catalyst. ACS Catal. 2016, 6, 5062−5068.

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DOI: 10.1021/acsami.6b16534 ACS Appl. Mater. Interfaces 2017, 9, 7193−7201