Enhanced Electrocatalytic Oxygen Evolution by Exfoliation of a Metal

the vast majority of them has been on employing them as a precursor for carbon materials or as a host material for encapsulating catalytically act...
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Letter Cite This: ACS Appl. Energy Mater. 2018, 1, 2446−2451

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Enhanced Electrocatalytic Oxygen Evolution by Exfoliation of a Metal−Organic Framework Containing Cationic One-Dimensional [Co4(OH)2]6+ Chains Xueling Song, Chengdong Peng, and Honghan Fei* Shanghai Key Laboratory of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, Shanghai 200092, People’s Republic of China

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S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are an emerging class of heterogeneous electrocatalyst, though the focus for the vast majority of them has been on employing them as a precursor for carbon materials or as a host material for encapsulating catalytically active species. Herein, we report the preparation of a metal−organic nanosheet with onedimensional (1-D) [Co4(OH)2]6+ chains via delamination of a 3-D MOF. The resultant atomically thin nanosheets are highly active, robust, and recyclable oxygen evolution reaction (OER) electrocatalysts with a low overpotential 318 mV (without iR compensation) to achieve a current density of 10 mA cm−2. These values along with the small Tafel slope (54 mV dec−1) exhibit a superior performance to the bulk MOF precursor and the benchmark RuO2 catalyst under the identical condition. The electrochemical studies ascribe the excellent OER activity to the high surface area, accessible CoII sites, and good charge transfer of the nanosheets. KEYWORDS: metal−organic frameworks, exfoliation, metal−organic nanosheets, electrocatalysis, oxygen evolution reaction

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conductive and coordinatively saturated, thus limiting their efficiencies in electrocatalysis.19 Exfoliation of the bulk materials into atomically thin nanosheets is an advantageous approach to expose the catalytically active sites, thus significantly improving the electrocatalytic performance.4,20−22 The vast majority of the 2D ultrathin nanosheets are prepared from layered inorganic materials with strong intralayer covalent bonds.23 In contrast, exfoliation of MOFs to realize ultrathin metal−organic nanosheets with less robust coordination bonds are rather challenging, and only a few examples were reported.21,22 Although less explored in the literature, the exfoliated MOF nanosheets with an ordered array of the open metal catalytic sites as well as infinite one-dimensional metal−oxo chains for electron transport will be an excellent platform for electrocatalysis. Co-based OER electrocatalysts are regarded as one of the most promising alternatives to noble-metal-based OER catalysts in view of their rich oxidation states and excellent electron conductivity.23−25 Herein, we reported a rational “top-down” synthetic strategy to afford ultrathin CoII−organic nanosheets consisting of one-dimensional [Co4(OH)2]6+ chains as an active, recyclable water oxidation electrocatalyst. In contrast to

lectrochemical water splitting provides a promising alternative to produce hydrogen fuels and to store the electricity from sustainable sources, which include hydrogen evolution reaction (HER)1 and oxygen evolution reaction (OER).2 Particularly, the oxidative half-reaction of the OER process is an inherently sluggish reaction, which involves a fourelectron transfer process coupled with breaking of the O−H bonds and the O−O bond formation.3,4 Although ruthenium (Ru) and iridium (Ir) oxides have been widely used to be the state-of-the-art efficient OER electrocatalysts,5,6 endowing highly active electrocatalysts based on earth-abundant elements is greatly needed.7,8 Metal−organic frameworks (MOFs) are a class of crystalline microporous materials with exceptionally high surface area and tunable functionality,9−12 which are essentially desirable in OER electrocatalysis.13,14 The vast majority of MOF-based electrocatalysts studies focused on preparing the porous carbon materials via pyrolysis as well as encapsulation of catalytically active species into the MOF hosts.15−18 However, these MOFderived materials often lead to partial decomposition of the open MOF framework and/or a significant sacrifice in the longrange well-ordered porosity, thus undesirable for diffusion of catalytic substrates. An efficient earth-abundant OER catalyst often requires all three properties of (a) high surface area, (b) unsaturated metal catalytic sites, and (c) high electrical conductivity.3,4 Meanwhile, the typical metal−carboxylate structural motifs residing in MOFs are electrically non© 2018 American Chemical Society

Received: April 24, 2018 Accepted: June 5, 2018 Published: June 5, 2018 2446

DOI: 10.1021/acsaem.8b00659 ACS Appl. Energy Mater. 2018, 1, 2446−2451

Letter

ACS Applied Energy Materials

Figure 1. (a) Schematic illustration of the synthesis process for TMOF-4 and the corresponding naonsheets, (b) crystallographic view of TMOF-4 along the a-axis, and (c) crystallographic view of a single [Co4(OH)2]6+ along the b-axis (Co, orange; O, red; C, black).

Figure 2. (a) SEM image, (b) low-magnification TEM image, (c) high-magnification TEM image, (d) tapping mode AFM image, and (e) height profile of the TMOF-4 nanosheets. Inset: Tyndall effect of a colloidal suspension of exfoliated TMOF-4 nanosheets.

proposed nomenclature of Cheetham et al. (Figure 1b,c).26 Each cationic cobaltate chain propagates along the a axis and is connected by six glutarate ligands in the bc plane. All of the four crystallographic independent CoII centers occupy the slightly distorted octahedral coordination geometry. The CoII centers residing in the [Co4(OH)2]6+ chains are covalently bonded to one or two intrachain O atoms, while the rest of the coordination spheres are completed by carboxylate oxygens from glutarate. In addition, both of the crystallgraphically independent O atoms are protonated and triply bridged to metal centers, lowering the Co−Co distance and probably enhancing the charge transfer along the inorganic chain. In order to enhance the accessibility of the CoII catalytically active sites, we continue to treat TMOF-4 with aqueous

the bulk MOF prior to exfoliation, the as-synthesized MOF nanosheets effectively triggered OER at a low overpotential of 318 mV at 10 mA cm−2 with a small Tafel slope of 54 mV dec−1. All of these values outperformed most of the other Cobased (single-metal) catalysts as well as the commercial RuO2, which was mainly ascribed to the ultrathin 2D MOF nanostructure with high surface area, good electron conductivity, and high accessibility of catalytic metal sites. Hydrothermal reactions of Co(OH)2 and glutaric acid afforded dark purple block-shaped crystals of Co4(OH)2(O2C(CH2)3CO2)3 (which we denote TMOF-4, Tongji MOF, structure no. 4; Figure 1a). X-ray crystallography reveals TMOF-4 consists of one-dimensional, cationic [Co4(OH)2]6+ chains bridged by glutarate into an “I1O2” framework, using the 2447

DOI: 10.1021/acsaem.8b00659 ACS Appl. Energy Mater. 2018, 1, 2446−2451

Letter

ACS Applied Energy Materials

S, 6.0%. Calc: C, 27.8%; H, 4.5%; S, 4.4%.). The slight differences between the experimental and theoretical values are reasonable, owing to the surface-absorbed solvent molecules and weakly coordinating ligands from the high surface area of the ultrathin nanosheets. Fourier transform infrared spectra (FTIR) of TMOF-4 nanosheets exhibited prominent bands at ∼1560 cm−1 that are characteristic of glutarate after the exfoliation treatment, confirming retention of the metal−organic nature throughout the exfoliation process (Figure 3b). In addition, the sulfate adsorption bands at ∼1150 cm−1 were observed in TMOF-4 nanosheets as well (Figure 3b and Figure S4), suggesting partial replacement of glutarate with SDS anions during the exfoliation process. Indeed, C 1s X-ray photoelectron spectroscopy (XPS) further confirmed the ligand exchange process during delamination, presenting as an obvious enhancement in alkane carbon components (C−C) and concomitant reduced intensity of carboxylate-based carbon components (O−CO and C− O). The Co 2p peaks of exfoliated TMOF-4 are slightly blueshifted from 781.1 to 781.5 eV, ascribed to the weaker interactions between [Co4(OH)2]6+ chains and the incoming SDS ligands. With a dense array of highly accessible CoII sites on the highly conductive cobaltate chains residing in the TMOF-4 nanosheets, we sought to investigate the OER activity using a standard three-electrode system at a slow scan rate of 10 mV s−1 in 1.0 M KOH. Importantly, the linear sweep voltammetry (LSV) curves without iR correction exhibited a low overpotential (η) of 318 mV to drive a catalytic current density of 10 mA cm−2, which is known to be a key evaluation criterion for a photoelectrochemical device with a 12.3% solar-tohydrogen efficiency (Figure 4a).2 Meanwhile, bulk TMOF-4 and the commercial RuO2 required a much higher overpotential of 370 and 380 mV, respectively, to achieve a current density of 10 mA cm−2 under the identical condition. In addition, the superior OER performance of TMOF-4 nanosheets was further evidenced by a steep enhancement in current density with increased overpotential, requiring an overpotential of 350 mV to reach a high current density of 40 mA cm−2 (80 and 140 mV lower than bulk TMOF-4 and RuO2). The intrinsic catalytic kinetics of TMOF-4 nanosheets is evaluated by Tafel measurement, which gives a Tafel slope via a linear fit of the polarization curve based on an equation of η = b log|J| (η is the overpotantial, J denotes the current density, and b was calculated as the Tafel slope). Importantly, the Tafel slope of TMOF-4 nanosheets (54 mV dec−1) is substantially lower than those of bulk TMOF-4 (83 mV dec−1) and RuO2 (114 mV dec−1) under the identical measurement condition (Figure 4b). Thus, the TMOF-4 nanosheets exhibit both low overpotential and small Tafel slope, superior to the vast majority of the Co-based and MOF-based catalysts (Figure 4f and Table S2).27−37 In order to further investigate the OER activity of TMOF-4 nanosheets, we performed the electrochemical impedance spectroscopy (EIS) at the overpotential of 300 mV (Figure 4c). The solution resistances (RS) in the highfrequency range for the three catalysts are nearly identical, which is closely related to the electrolyte.38 An obvious semicircle at low-frequency region that is related to the massdiffusion process can be observed in the TMOF-4 nanosheets EIS plot, suggesting a stronger adsorption of a reaction intermediate (such as Oad) on the surface of the working electrode. Moreover, the high-frequency semicircle is mainly associated with the charge transfer resistances (Rct) of different

solution containing excess amount of sodium dodecyl sulfate (SDS) at elevated temperature for 12 h. After extensive washing of the free or weakly bound SDS species, the resultant transparent colloidal solution exhibited a clear Tyndall light scattering effect, as shown in the inset of Figure 2a. After centrifugation and vacuum drying, the morphology of green aggregates was studied by using field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). In contrast to the block-shaped TMOF-4 crystals (Supporting Information Figure S1), the TMOF-4 materials after SDS treatment exhibited ultrathin sheet-like morphology (Figure 2a). In addition, TEM images revealed the folded, curved edges of the nanosheets, further confirming the formation of ultrathin 2D nanostructure (Figure 2b,c). Tapping-mode atomic force microscopy (AFM) suggested the nanosheet thickness of ∼0.6 nm, though partial aggregation was observed largely owing to the high surface energy of the nanosheets. In addition, the high phase purity of as-synthesized TMOF-4 (before exfoliation) was supported by the match of the experimental powder X-ray diffraction (PXRD) pattern to the simulated pattern from single-crystal data (Figure 3a). After

Figure 3. (a) PXRD patterns of TMOF-4 (red) and TMOF-4 nanosheets (blue) (theoretical PXRD simulated from single-crystal data given at the bottom); (b) FTIR of disodium glutarate (black), TMOF-4 (red), and TMOF-4 nanosheets (blue); (c) C 1s XPS spectrum; and (d) Co 2p XPS spectrum of the TMOF-4 and TMOF-4 nanosheets.

SDS treatment, the intensity of the low-angle (001) diffraction peak that is characteristic of bulk TMOF-4 was completely diminished in the as-synthesized nanosheets, demonstrating the collapse of the 3-D framework (Figure 3a). To note, thermogravimetric analysis (TGA) and PXRD results suggest high thermal and chemical stabilities of TMOF-4, which are stable up to 250 °C as well as in 0.1 M KOH aqueous solution for 8 h (Figures S2 and S3). The exfoliation process is strongly induced by the ligandexchange process in which the excess amount of incoming SDS anions partially replace the glutarate. Meanwhile, the monodentate SDS gives rise to the missing connectivity between adjacent metal−organic layers, resulting in the formation of nanosheets. The elemental analysis indicates the nanosheets have a molecular formula of Na3[Co4(OH)2(glutarate)(SDS)] (Obs: C, 24.4%; H, 5.8%; 2448

DOI: 10.1021/acsaem.8b00659 ACS Appl. Energy Mater. 2018, 1, 2446−2451

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Figure 4. (a) LSV polarization curves, (b) Tafel plots, and (c) EIS plots of the three electrocatalysts of TMOF-4 (red), TMOF-4 nanosheets (blue), and RuO2 (black); (d) linear relationship of Δj = ja − jc vs different scan rates to evaluate the ESCA of TMOF-4 (black) and TMOF-4 (red); (e) Chronopotentiometry curves of TMOF-4 nanosheets and RuO2 based on a constant potential mode corresponding to the current density of 20 mA cm−2; (f) OER performance comparison between the catalysts in our work and those of previous reports, with the top left of the graph (low overpotential and small Tafel slope) exhibiting superior performances.

electrodes. The value of Rct for bulk TMOF-4 (11.69 Ω) and TMOF-4 nanosheets (10.66 Ω) are comparable to the benchmark RuO2 electrocatalyst (13.18 Ω), confirming the role of 1-D [Co4(OH)2]6+ significantly contributing the high charge transfer in the extended framework. Compared to bulk TMOF-4 crystals, the slightly lower Rct value of TMOF-4 nanosheets is probably owing to the effect of 2D quantum confinement.4,23 Other than the charge transfer in OER, the number of accessible catalytic sites on the catalyst surface is also the key contribution to the catalyst performance. We determine the double-layer capacitance (Cdl) and the electrochemical surface area (ECSA) of catalysts both bulk TMOF-4 and TMOF-4 nanosheets using cyclic voltammetry (CV) in a small potential range of 1.15−1.27 V (vs RHE) without redox processes at different scan rates (Figure S6). The plot of Δj = ja − jc at 1.2 V vs the scan rates exhibits a good linear relationship, which provides the calculated Cdl values to be 22.14 mF cm−2 for TMOF-4 nanosheets and 6.08 mF cm−2 for the bulk counterpart, respectively (Figure 4d). These values suggest the enhanced presence of the highly exposed catalytic sites on the surfaces of nanosheets, which contribute to the superior OER performance. Equally important to high catalytic activity in OER is the robustness and durability. The long-term stability of nanosheets and RuO2 was studied using chronopotentiometry curves at their constant potential corresponding to the current density of 20 mA cm−2. A mere 5.32% decay in anodic current is observed after 15 h, while RuO2 exhibits a more significant decay of 11.75% under the identical condition (Figure 4e). Moreover, the ultrathin 2D nanosheet morphology was well-maintained throughout the long-term electrochemical test, demonstrating the excellent stability of TMOF-4 nanosheets (Figure S7). Overall, the high OER activity of our metal−organic nanosheets can be ascribed to the following reasons: (1) the lower Tafel slope favoring the fast OER kinetics; (2) the impedance studies

confirming the enhanced charge transfer derived from 1-D cobaltate chain; (3) the increased catalytic sites on the high exposed facet of nanosheets reaching a large Cdl value and high electrochemical surface area. In summary, a metal−organic nanosheet with 1-D cobalt hydroxide chains was successfully achieved using the top-down exfoliation strategy. The atomically thin nanostructure demonstrates excellent OER catalytic activity with a low overpotential and a small Tafel slope, presenting one of the best OER performances not only in Co-based electrocatalysts but also in intrinsic MOF materials. The electrochemical studies indicate the superior OER performance is ascribed to the highly accessible CoII catalytic sites on the exposed facet of nanosheets as well as the high charge transfer from the 1-D Co−OH−Co connectivity. We believe the work opens up the fabrication of metal−organic nanosheets in heterogeneous electrochemical catalysis.

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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/acsaem.8b00659.

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Detailed experimental procedures and additional characterization figures and tables (PDF) Crystallographic data (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Honghan Fei: 0000-0003-1353-9921 Notes

The authors declare no competing financial interest. 2449

DOI: 10.1021/acsaem.8b00659 ACS Appl. Energy Mater. 2018, 1, 2446−2451

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ACS Applied Energy Materials

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an Advanced Bifunctional Oxygen Electrode. Angew. Chem., Int. Ed. 2016, 55, 4087−4091. (18) Xu, Y.; Tu, W.; Zhang, B.; Yin, S.; Huang, Y.; Kraft, M.; Xu, R. Nickel Nanoparticles Encapsulated in Few-Layer Nitrogen-Doped Graphene Derived from Metal-Organic Frameworks as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Mater. 2017, 29, 1605957. (19) Stavila, V.; Talin, A. A.; Allendorf, M. D. MOF-Based Electronic and Opto-Electronic Devices. Chem. Soc. Rev. 2014, 43, 5994−6010. (20) Hai, G.; Jia, X.; Zhang, K.; Liu, X.; Wu, Z.; Wang, G. HighPerformance Oxygen Evolution Catalyst Using Two-Dimensional Ultrathin Metal-Organic Frameworks Nanosheets. Nano Energy 2018, 44, 345−352. (21) Huang, J.; Li, Y.; Huang, R.-K.; He, C.-T.; Gong, L.; Hu, Q.; Wang, L.; Xu, Y.-T.; Tian, X.-Y.; Liu, S.-Y.; Ye, Z.-M.; Wang, F.; Zhou, D.-D.; Zhang, W.-X.; Zhang, J.-P. Electrochemical Exfoliation of Pillared-Layer Metal−Organic Framework to Boost the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2018, 57, 4632−4634. (22) Zhao, S.; Wang, Y.; Dong, J.; He, C. T.; Yin, H.; An, P.; Zhao, K.; Zhang, X.; Gao, C.; Zhang, L.; Lv, J.; Wang, J.; Zhang, J.; Khattak, A. M.; Khan, N. A.; Wei, Z.; Zhang, J.; Liu, S.; Zhao, H.; Tang, Z. Ultrathin Metal−Organic Framework Nanosheets for Electrocatalytic Oxygen Evolution. Nat. Energy. 2016, 1, 16184. (23) Tan, C.; Cao, X.; Wu, X. J.; He, Q.; Yang, J.; Zhang, X.; Chen, J.; Zhao, W.; Han, S.; Nam, G. H.; Sindoro, M.; Zhang, H. Recent Advances in Ultrathin Two-Dimensional Nanomaterials. Chem. Rev. 2017, 117, 6225−6331. (24) Zhang, Y.; Ouyang, B.; Xu, J.; Jia, G.; Chen, S.; Rawat, R. S.; Fan, H. J. Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low-Cost Catalysts for Oxygen Evolution. Angew. Chem., Int. Ed. 2016, 55, 8670−8674. (25) Wang, H. Y.; Hung, S. F.; Chen, H. Y.; Chan, T. S.; Chen, H. M.; Liu, B. In Operando Identification of Geometrical-Site-Dependent Water Oxidation Activity of Spinel Co3O4. J. Am. Chem. Soc. 2016, 138, 36−39. (26) Cheetham, A. K.; Rao, C. N. R.; Feller, R. K. Structural Diversity and Chemical Trends in Hybrid Inorganic−Organic Framework Materials. Chem. Commun. 2006, 4780−4795. (27) Jiang, J.; Huang, L.; Liu, X.; Ai, L. Bioinspired Cobalt-Citrate Metal-Organic Framework as an Efficient Electrocatalyst for Water Oxidation. ACS Appl. Mater. Interfaces 2017, 9, 7193−7201. (28) Xu, W.; Lyu, F.; Bai, Y.; Gao, A.; Feng, J.; Cai, Z.; Yin, Y. Porous Cobalt Oxide Nanoplates Enriched with Oxygen Vacancies for Oxygen Evolution Reaction. Nano Energy 2018, 43, 110−116. (29) Yin, H.; Zhao, S.; Zhao, K.; Muqsit, A.; Tang, H.; Chang, L.; Zhao, H.; Gao, Y.; Tang, Z. Ultrathin Platinum Nanowires Grown on Single-Layered Nickel Hydroxide with High Hydrogen Evolution Activity. Nat. Commun. 2015, 6, 6430. (30) Han, X.; Yu, C.; Zhou, S.; Zhao, C.; Huang, H.; Yang, J.; Liu, Z.; Zhao, J.; Qiu, J. Ultrasensitive Iron-Triggered Nanosized Fe-CoOOH Integrated with Graphene for Highly Efficient Oxygen Evolution. Adv. Energy Mater. 2017, 7, 1602148. (31) Wei, Y.; Ren, X.; Ma, H.; Sun, X.; Zhang, Y.; Kuang, X.; Yan, T.; Wu, D.; Wei, Q. In Situ Formed Co(TCNQ)2 Metal-Organic Framework Array as a High-Efficiency Catalyst for Oxygen Evolution Reactions. Chem. - Eur. J. 2018, 24, 2075−2079. (32) Shen, J. Q.; Liao, P. Q.; Zhou, D. D.; He, C. T.; Wu, J. X.; Zhang, W. X.; Zhang, J. P.; Chen, X. M. Modular and Stepwise Synthesis of a Hybrid Metal-Organic Framework for Efficient Electrocatalytic Oxygen Evolution. J. Am. Chem. Soc. 2017, 139, 1778−1781. (33) Manna, P.; Debgupta, J.; Bose, S.; Das, S. K. A Mononuclear Coii Coordination Complex Locked in a Confined Space and Acting as an Electrochemical Water-Oxidation Catalyst: A “Ship-in-a-Bottle” Approach. Angew. Chem., Int. Ed. 2016, 55, 2425−2430. (34) Xue, Z. H.; Su, H.; Yu, Q. Y.; Zhang, B.; Wang, H. H.; Li, X. H.; Chen, J. S. Janus Co/CoP Nanoparticles as Efficient Mott-Schottky Electrocatalysts for Overall Water Splitting in Wide Ph Range. Adv. Energy Mater. 2017, 7, 1602355.

ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (Grant Nos. 51772217 and 21501136), the Recruitment of Global Youth Experts by China, and the Fundamental Research Funds for the Central Universities, and the Science & Technology Commission of Shanghai Municipality (Grant No. 14DZ2261100).

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REFERENCES

(1) Zheng, Y.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory. Angew. Chem., Int. Ed. 2015, 54, 52−65. (2) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 16977−16987. (3) 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. (4) Zhang, W.; Zhou, K. Ultrathin Two-Dimensional Nanostructured Materials for Highly Efficient Water Oxidation. Small 2017, 13, 1700806. (5) Kim, J.; Shih, P. C.; Tsao, K. C.; Pan, Y. T.; Yin, X.; Sun, C. J.; Yang, H. High-Performance Pyrochlore-Type Yttrium Ruthenate Electrocatalyst for Oxygen Evolution Reaction in Acidic Media. J. Am. Chem. Soc. 2017, 139, 12076−12083. (6) Audichon, T.; Napporn, T. W.; Canaff, C.; Morais, C.; Comminges, C.; Kokoh, K. B. IrO2 Coated on RuO2 as Efficient and Stable Electroactive Nanocatalysts for Electrochemical Water Splitting. J. Phys. Chem. C 2016, 120, 2562−2573. (7) McAlpin, J. G.; Stich, T. A.; Casey, W. H.; Britt, R. D. Comparison of Cobalt and Manganese in the Chemistry of Water Oxidation. Coord. Chem. Rev. 2012, 256, 2445−2452. (8) Hunter, B. M.; Gray, H. B.; Muller, A. M. Earth-Abundant Heterogeneous Water Oxidation Catalysts. Chem. Rev. 2016, 116, 14120−14136. (9) Zhang, G.; Yang, H.; Fei, H. Unusual Missing Linkers in an Organosulfonate-Based Primitive−Cubic (pcu)-Type Metal−Organic Framework for CO2 Capture and Conversion under Ambient Conditions. ACS Catal. 2018, 8, 2519−2525. (10) Wang, Y.; Huang, N. Y.; Shen, J. Q.; Liao, P. Q.; Chen, X. M.; Zhang, J. P. Hydroxide Ligands Cooperate with Catalytic Centers in Metal-Organic Frameworks for Efficient Photocatalytic CO2 Reduction. J. Am. Chem. Soc. 2018, 140, 38−41. (11) Peng, C.; Zhuang, Z.; Yang, H.; Zhang, G.; Fei, H. Ultrastable, Cationic Three-Dimensional Lead Bromide Frameworks That Intrinsically Emit Broadband White-Light. Chem. Sci. 2018, 9, 1627−1633. (12) Zhuang, Z.; Peng, C.; Zhang, G.; Yang, H.; Yin, J.; Fei, H. Intrinsic Broadband White-Light Emission from Ultrastable, Cationic Lead Halide Layered Materials. Angew. Chem., Int. Ed. 2017, 56, 14411−14416. (13) Wang, C.; Liu, X.; Keser Demir, N.; Chen, J. P.; Li, K. Applications of Water Stable Metal-Organic Frameworks. Chem. Soc. Rev. 2016, 45, 5107−5134. (14) Li, F. L.; Shao, Q.; Huang, X.; Lang, J. P. Nanoscale Trimetallic Metal-Organic Frameworks Enable Efficient Oxygen Evolution Electrocatalysis. Angew. Chem., Int. Ed. 2018, 57, 1888−1892. (15) Liao, P. Q.; Shen, J. Q.; Zhang, J. P. Metal−Organic Frameworks for Electrocatalysis. Coord. Chem. Rev. 2017, DOI: 10.1016/j.ccr.2017.09.001. (16) Zhou, T.; Du, Y.; Wang, D.; Yin, S.; Tu, W.; Chen, Z.; Borgna, A.; Xu, R. Phosphonate-Based Metal−Organic Framework Derived Co−P−C Hybrid as an Efficient Electrocatalyst for Oxygen Evolution Reaction. ACS Catal. 2017, 7, 6000−6007. (17) Aijaz, A.; Masa, J.; Rosler, C.; Xia, W.; Weide, P.; Botz, A. J.; Fischer, R. A.; Schuhmann, W.; Muhler, M. Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as 2450

DOI: 10.1021/acsaem.8b00659 ACS Appl. Energy Mater. 2018, 1, 2446−2451

Letter

ACS Applied Energy Materials (35) Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X.; Mao, J.; Du, X. W.; Hu, Z.; Jaroniec, M.; Qiao, S. Z. Engineering Surface Atomic Structure of Single-Crystal Cobalt (II) Oxide Nanorods for Superior Electrocatalysis. Nat. Commun. 2016, 7, 12876. (36) Wan, S.; Qi, J.; Zhang, W.; Wang, W.; Zhang, S.; Liu, K.; Zheng, H.; Sun, J.; Wang, S.; Cao, R. Hierarchical Co(OH)F Superstructure Built by Low-Dimensional Substructures for Electrocatalytic Water Oxidation. Adv. Mater. 2017, 29, 1700286. (37) 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. (38) Yan, L.; Cao, L.; Dai, P.; Gu, X.; Liu, D.; Li, L.; Wang, Y.; Zhao, X. Metal-Organic Frameworks Derived Nanotube of Nickel-Cobalt Bimetal Phosphides as Highly Efficient Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater. 2017, 27, 1703455.

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