Rare Cobalt-Based Phosphate Nanoribbons with Unique 5

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Rare Cobalt-Based Phosphate Nanoribbons with Unique 5-coordination for Electrocatalytic Water Oxidation Hao Wan, Renzhi Ma, Xiaohe Liu, Jiangling Pan, Haidong Wang, Shuquan Liang, Guanzhou Qiu, and Takayoshi Sasaki ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00621 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 1, 2018

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ACS Energy Letters

Rare Cobalt-Based Phosphate Nanoribbons with Unique 5-coordination for Electrocatalytic Water Oxidation Hao Wan,1,2,3 Renzhi Ma,1* Xiaohe Liu,2* Jiangling Pan,2 Haidong Wang,3* Shuquan Liang,2 Guanzhou Qiu,3 Takayoshi Sasaki1 1

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for

Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan 2

State Key Laboratory of Powder Metallurgy and School of Materials Science and Engineering,

Central South University, Changsha, Hunan 410083, P. R. China 3

School of Minerals Processing and Bioengineering, Central South University, Changsha,

Hunan 410083, P. R. China AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]; [email protected]

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ABSTRACT. Coordinatively asymmetric or unsaturated metal centers may serve as accessible active sites and allow strong interaction with incoming guests in catalysis, sensing and separation applications. Herein, a peculiar Co-based phosphate NaCo4(PO4)3, in which all Co atoms were in very rare 5-coordinations, was firstly synthesized in nanoribbon morphology. As a combined merit of unique Co coordination, intrinsic half-metallicity and nanostructured morphology, NaCo4(PO4)3 nanoribbons were characterized with high efficiency for water oxidation in neutral electrolyte, which was remarkably outperformed congeneric phosphates such as Na2CoP2O7 with 4-coordinated Co configurations and even comparable to benchmarking RuO2 nanoparticles. The high catalytic performance validates a great potential in exploiting inorganic nanostructures with coordinatively asymmetric or unsaturated metal centers for energy storage and conversion applications.

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Increasing energy demands and severe pollution problems have driven us to explore alternative clean and sustainable energy storage and conversion systems. Hydrogen fuel has been greatly evaluated due to high energy density and clean combustion.1,2 Overall water splitting is a simple way for hydrogen generation. But it is usually hindered by a kinetically sluggish oxygen evolution reaction (OER) because of the multiple proton-coupled electron transfer.3 Traditional electrocatalysts for oxygen evolution are based on precious metals, e.g. IrO2 and RuO2, suffering from high cost and rigorous scarcity.4 Therefore, developing earth-abundant and low-cost catalysts for efficient oxygen evolution is becoming a critical target. Moreover, conventional oxygen evolution is generally catalyzed in highly alkaline solution, which may bring about serious corrosion issues and is neither environmentally friendly nor economic.5,6 On the contrary, water oxidation in neutral media is considered benign and rarely do harm to electrolysis devices. However, OER activity is pH dependent due to the competition of two different electrocatalytic mechanisms, as shown in Figure S1.7,8 In a neutral system, since there exist only few hydroxyls (OH–), a higher overpotential than that in alkaline media is needed to drive the oxygen evolution. Exploiting efficient electrocatalysts for water oxidation under neutral conditions is therefore a more immense challenge. Coordinatively unsaturated metal configurations, such as pyramidal 5-coordinated sites, have been generated in metal-organic frameworks (MOFs) by either the peculiar bridging connection between metal centers and organic ligands9 or the removal and/or desorption of ligands, such as water and methanol molecules, and demonstrated to possess strong adsorption ability beneficial for the application in hydrogen storage as well as high-performance in catalysis, etc..10-12 However, for inorganic materials, these 5-coordinated configurations are so far created by the artificial surface engineering.13,14 Few literatures have investigated the physicochemical


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implication and potential of inorganic materials with all metal sites in 5-coordinated configurations. Cobalt-based phosphates, containing electrochemically redoxable Co atoms, have been widely used as electrode materials for supercapacitors and secondary batteries.15-17 Furthermore, they can also be applied to electrocatalytic water oxidation. It has been reported that LiCoPO4 with 6coordinated Co atoms yielded a current density of 0.5 mA cm–2 at a relatively low overpotential of 0.58 V for oxygen evolution in 0.1 M phosphate buffer solution (PBS).18 However, this Cobased phosphate doesn’t show a stable durability in long-term water oxidation process. Kim et al. have demonstrated that Na2CoP2O7, containing 4-coordinated Co atoms, exhibited higher electrocatalytic activity than 6-coordinated NaCoPO4 for water oxidation, drawing a conclusion that coordination tuning brings about strong influence on the electrocatalytic activity.19 Co-based phosphate materials, in which there exist diverse Co coordination configurations derived from the flexible connecting and coordinating ability of phosphate groups, can provide an ideal platform for the investigation on the relationship between electrocatalytic efficiency and coordination geometry. However, Co-based phosphates were routinely synthesized by a solid-state calcination method at high temperature. The products were usually in irregular bulk morphology of several to tens of micrometers,20,21 easily contaminated with the formation of by-products or impurities.22 Compared with bulk structures, nanostructures including ultra-thin nanoplates,23 nanoparticles (NPs),24 and single-atom clusters,25 can conduce to better electrochemical kinetics and larger active surface area, resulting in higher electrocatalytic efficiency. In this aspect, nanostructures of Co-based phosphates are highly expected for enhancing electrocatalytic activity. NaCo4(PO4)3 is considered as a unique compound, in the regard that all Co atoms are in very


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rare 5-coordinations. Figure 1 shows the crystallographic structure of NaCo4(PO4)3 in comparison with Na2CoP2O7.17,26 It can be seen that all Co atoms in NaCo4(PO4)3 phase are in [CoO5] form and each [CoO5] polyhedron connects with neighboring ones through edge-sharing and/or corner-sharing, rather different from the isolated [CoO4] in Na2CoP2O7. These lowsymmetrical 5-coordinated Co configurations may possess strong adsorption ability on electronegative oxygen atoms from water molecules, beneficial for the promotion of electrocatalytic water oxidation.

Figure 1. Crystal structure models of (a) NaCo4(PO4)3 and (b) Na2CoP2O7 phases. The yellow, red, purple and blue balls refer to Na, O, P and Co atoms, respectively, while the yellow-white ones are deficient-occupied Na sites. In this work, a new synthetic approach was developed for the synthesis of NaCo4(PO4)3 nanoribbons by calcining a hydrothermal treated precursor. Density of states (DOS) calculations revealed that NaCo4(PO4)3 was a half-metallic phase. As-prepared NaCo4(PO4)3 nanoribbons were applied to electrocatalytic water oxidation in neutral PBS, achieving a comparably high efficiency with noble-metal electrocatalyst such as RuO2 NPs, and considerably outperforming


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Na2CoP2O7 material with 4-coordinated Co atoms. The high electrocatalytic efficiency of NaCo4(PO4)3 nanoribbons was elucidated through a proposed water oxidation mechanism based on coordination geometry.

Figure 2. (a, b) SEM and (c, d) TEM images for the hydrothermal product. The red arrow in (b) refers to a crink of the nanoribbon. NaCo4(PO4)3 nanoribbons were synthesized by calcining the corresponding precursor obtained from a hydrothermal process at 120 °C by using hydrated cobalt acetate (Co(CH3COO)2·4H2O), urea (CO(NH2)2) and hydrated disodium hydrogen phosphate (Na2HPO4·12H2O) as starting materials (see Supporting Information for more details). Figure 2a and b show scanning electron microscopy (SEM) images of the hydrothermal product, i.e., precursor. As can be seen in Figure


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2a, the precursor was uniform nanoribbons with an average width of ~250 nm. From a magnified SEM image in Figure 2b, the nanoribbons were found to possess a clean and smooth surface. Meanwhile, a crinkle was occasionally observed at the labeled area in Figure 2b, suggesting a very flexible nature of these nanoribbons. Figure 2c depicts a bright-field transmission electron microscopy (TEM) image of the nanoribbons. In accordance with SEM observations, uniform nanoribbons with a smooth profile were also apparent. Likewise, the light contrast suggested that the nanoribbons were endowed with a thin thickness. The thickness was estimated to be ~10 nm, as shown by a magnified TEM observation on the lateral section of a nanoribbon in Figure 2d. Figure S2 shows a typical X-ray powder diffraction (XRPD) pattern collected on the precursor. The major peaks in the pattern were rather similar with those of Co3(PO4)2·4H2O phase (PDF file No. 34-0844). Nevertheless, some characteristic peaks, such as those marked by red dashed lines, didn’t match well with the standard card. Typical energy dispersive X-ray spectrum (EDS), as depicted in the inset of Figure S2, confirmed the coexistence of Co, P, O and Na elements (C element was from the conductive carbon tape), revealing the successful incorporation of Na into the nanoribbons. An estimated molar ratio of Na:Co:P at 1:4:3 was very close to the stoichiometric composition of NaCo4(PO4)3. In terms of the XRPD pattern and elemental analysis results, the hydrothermal product might be an intermediate phase between Co3(PO4)2·4H2O and NaCo4(PO4)3, e.g. Co3(PO4)2·4H2O framework with some ions and groups, such as Na+ and/or HPO42–, either absorbed on or embedded in. The high-resolution transmission electron microscopy (HRTEM) image, as shown in Figure S2b, further reveals the poor crystallinity of the hydrothermal product. For a better understanding of the formation of nanoribbons precursor, similar experiments were designed at room temperature. As evidenced by Figure S3, no regular morphology, such as


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nanoribbon, could be seen and a notable absence of Na element was confirmed for the products obtained at room temperature, therefore it’s reasonable to infer that a hydrothermal condition at an elevated temperature, e.g. 120 °C, was critical for the formation of nanoribbon morphology, as well as for the incorporation of Na element into the precursor. Figure 3a shows the XRPD pattern of calcined product at 550 °C (determined by thermogravimetric-differential scanning calorimetry (TG-DSC) measurements, as decipted in Figure S4), matching well with the simulated card of monoclinic NaCo4(PO4)3 (a=6.339 Å, b=9.867 Å, c=15.300 Å and β=91.049 °). No peaks of any other impurities appeared, indicating that a pure NaCo4(PO4)3 phase was obtained. Previous synthesis of NaCo4(PO4)3 using solid state sintering method usually needed to raise the temperature to as high as 900 °C.26 Current synthetic strategy substantially decreased the temperature to 550 °C. Figure 3b depicts a SEM image of calcined NaCo4(PO4)3 product. Regular nanoribbons could be clearly seen, indicating that both size and morphology were almost maintained during the calcination process. Figure 3c displays a TEM image of calcined product with nanostructured ribbons observed. Figure 3d shows a typical selected area electron diffraction (SAED) pattern, which was collected from the circled area in Figure 3c. The regular spots matrix revealed a single-crystalline nature of the nanoribbons. Based on the distances measured from the diffraction spots and an intersection angle of 89 °, the pattern could be indexed to be monoclinic NaCo4(PO4)3 under the incident electron beam along [010] direction. The representative spots were indexed to be (100), (004) and (104) planes, respectively. Figure 3e depicts the corresponding HRTEM image. Homogeneous lattice fringes with separation of 0.383 nm were discerned as interplanar spacings of (004) plane, indicating that the nanoribbons have a preferred elongation direction along the c axis. Figure 3f shows the in-plane X-ray diffraction (XRD) pattern of NaCo4(PO4)3 nanoribbons


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deposited on a Si substrate, in which two strong diffraction peaks were indexed as (200) and (0012) planes, respectively, further confirming the nanoribbons lying on the substrate with a vertical orientation along the b axis. In view of the above results, a geometrical model was established, as shown in the inset of Figure 3f, clearly revealing the crystallographic orientation of the nanoribbons and the crystal structure along the b axis.


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Figure 3. (a) XRPD pattern, (b) SEM, (c) TEM, (d) SAED pattern collected from the marked area in (c), (e) HRTEM images and (f) in-plane XRD pattern for calcined NaCo4(PO4)3 product, the inset of (f) depicts a geometrical model showing the crystallographic orientation of NaCo4(PO4)3 nanoribbons, and the crystal structure along the b axis. Since few literatures have reported the properties characterizations of NaCo4(PO4)3 phase, DOS calculations were carried out to gain a deep insight into inherent properties, as evidenced by Figure S5, revealing that NaCo4(PO4)3 was a half-metallic phase and might show metallic conductivity.27 It has been well demonstrated that superior electrical conductivity of electrocatalysts, induced by the inherent electron configurations and nanostructures, would facilitate electron transfer and enhance electrocatalytic reactions.28 This is regarded as an apparent advantage of as-prepared NaCo4(PO4)3 nanoribbons in electrochemical applications.


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Figure 4. OER activity tests. (a) Polarization curves at a scan rate of 5 mV s–1, (b) Tafel plots, (c) Nyquist plots and the fitting equivalent circuit model for NaCo4(PO4)3 nanoribbons, Na2CoP2O7 powder and RuO2 NPs; (d) OER activity comparison between NaCo4(PO4)3 nanoribbons and recently advanced electrocatalysts. The electrocatalytic OER activity, 2H2O → 4H+ + O2 + 4e–, was evaluated in neutral PBS system (pH=6.86±0.02). Figure 4a shows the linear sweep voltammetry (LSV) curve of NaCo4(PO4)3, in comparison with Na2Co2P2O7 product, wherein the latter was prepared by a conventional solid-state sintering method and the phase purity was evidenced by the XRPD pattern in Figure S6. It could be seen that NaCo4(PO4)3 nanoribbons markedly outperformed Na2Co2P2O7 in the potential range of 1.2 ~ 1.8 V vs RHE. Even in low-concentration PBS of 0.05 M, NaCo4(PO4)3 nanoribbons showed a current density of 1 and 11.02 mA cm–2 at a low overpotential of 0.373 and 0.570 V, respectively, indicating high OER activity in neutral electrolyte. On the other hand, a comparable polarization curve was observed for the RuO2 NPs which were prepared following a previous report.29 On account of the similar anodic scan of conventional cobalt phosphate (Co-Pi) material in neutral electrolytes,19 the half wave at the potential of ~1.41 V vs RHE for NaCo4(PO4)3 nanoribbons, as shown in the inset of Figure 4a, could be referred to the oxidation of Co(II) to Co(III). The oxidation was further evidenced by the X-ray photoelectron spectroscopy (XPS) analysis as displayed in Figure S7. On the contrary, no obvious oxidation peak was observed below the onset of electrocatalysis for Na2Co2P2O7 product. Furthermore, the polarization curves after 1st and 50th cycle, as depicted in Figure S8, showed a negligible shift, revealing excellent structure stability of NaCo4(PO4)3 nanoribbons. Figure 4b displays the Tafel slope of 121 mV dec–1 for NaCo4(PO4)3 nanoribbons, which is superior to 134 mV dec–1 for Na2Co2P2O7 and 152 mV dec–1 for RuO2, suggesting more


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favorable electrocatalytic reaction kinetics for NaCo4(PO4)3 nanoribbons. This could also be implied by the electrochemical impedance spectroscopy (EIS) data, as depicted in Figure 4c. In view of the EIS spectra, an equivalent circuit model was established as the inset of Figure 4c. The charge transfer resistance, represented by Rct, was evaluated to be 197.5 Ω, far lower than 651.4 Ω for Na2Co2P2O7 powder and 739.4 Ω for RuO2 NPs (wherein the intersection points of the three curves with the real axis, ~240 Ω, was referred to the electrolyte resistance value, corresponding to Rs in the circuit diagram31). The low impedance of NaCo4(PO4)3 nanoribbons might be derived from the inherent half-metallicity as well as the nanostructured morphology. According to the polarization curves, turnover frequency (TOF) was calculated to be 0.0791 s–1 for NaCo4(PO4)3 nanoribbons, remarkably exceeding that for Na2Co2P2O7 (0.0086 s–1) and RuO2 NPs (0.0171 s–1) by over 9 and 4 times at the overpotential of 0.55V, respectively. Such a high TOF value means a very high activity of each electrocatalytic site in NaCo4(PO4)3.32 As indicated in Figure S9a-c, the ECSA of NaCo4(PO4)3 nanoribbons was calculated to be 0.336 mF cm–2, ~2.7 times larger than that of Na2Co2P2O7 (0.124 mF cm–2), suggesting more catalytic sites available for water oxidation reaction. The chronopotentiometry result of NaCo4(PO4)3 nanoribbons at a constant current density of 3 mA cm-2, as shown in Figure S9d, indicated that the overpotential remained stable at ~0.44V in a water oxidation process up to 10 h, revealing a stable durability for NaCo4(PO4)3 nanoribbons. Meanwhile, the structural characterizations, as displayed in Figure S10, further indicate the structural stability. Figure 4d and Table S1 list the OER activity comparison of recently reported OER electrocatalysts in neutral PBS electrolytes. It can be found that NaCo4(PO4)3 nanoribbons stand out with a low overpotential, outperforming most congeneric phosphates, such as Co-Pi/GO and Mn3(PO4)2·3H2O, as well as other advanced OER catalysts, e.g. atomically-thin Co3S4 nanosheets, Ni2P/carbon cloth and amorphous


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CoS4.6O0.6 porous cubes. It is noteworthy that such a high activity for NaCo4(PO4)3 nanoribbons was achieved at a very low loading mass of 0.177 mg cm–2 and a low PBS concentration of 0.05 M. The high OER activity of NaCo4(PO4)3 nanoribbons might be derived from the following factors. Firstly, the unique 5-coordination will contribute to the higher OER activity of NaCo4(PO4)3 nanoribbons. The ECSA ratio was lower than the current density ratio of NaCo4(PO4)3 nanoribbons to Na2CoP2O7 powder, e.g., ~4.5 at the potential of 1.8 V vs RHE, suggesting higher electrocatalytic activity of each [CoO5] configuration. Figure 5 shows possible OER mechanism comparison for NaCo4(PO4)3 and Na2CoP2O7 materials. Generally, Co atoms were favorably coordinated in the forms of [CoO4] and/or [CoO6] in inorganic materials.33,34 On the contrary, all the Co atoms in NaCo4(PO4)3 phase were of asymmetrical 5-coordination. More importantly, different from previous [CoO5] in trigonal bipyramidal shape with Co atoms located at the center site,35 the [CoO5] configuration in this case was either severely distorted trigonal bipyramid or even tetragonal pyramidal structure, i.e., all Co atoms were on the bottom surface of [CoO5] pyramids. This seriously unbalanced electrostatic interaction might result in a more favorable adsorption for accepting electronegative oxygen atoms from water molecules (Step I) to form more stable octahedral (Oh) coordination environment accompanied by the oxidation of Co(II). An oxidation process could be evidenced by the anodic scan in the inset of Figure 4a. After shifting the potential positive to the oxidation peak of Co(II)/Co(III), i.e., 1.51 V vs RHE, the current density exponentially increased, revealing the initiation of apparent water oxidation. The valence analysis also clearly indicated the oxidation of Co(II) to Co(III) as shown in Figure S7. Moreover, as can be seen in Figure S7c, no Co(IV) signal was detected after an electrolysis process even at 1.70 V vs RHE, where efficient oxygen evolution proceeded. This fact might


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imply that the Co(IV) state was intermediate and metastable. In other words, the formation of -O (referring to Step II) with the oxidation of Co(III) might be considered sluggish, and became the rate determining step for OER. After another rapid two-step proton-coupled electron transfer (corresponding to Step III and IV), O2 was produced. In contrast, no such obvious oxidation and/or current density increase were observed below the potential of 1.50 V vs RHE in the LSV curve of Na2CoP2O7 product, implying that it’s rather difficult to proceed on Step I, Co(II)/Co(III), because of the weak adsorption to water molecules for 4-coordination configurations. In addition, the better electrical conductivity of NaCo4(PO4)3 nanoribbons, originating from nanostructured morphology and half-metallicity evidenced by the EIS data in Figure 4c, might accelerate the electron transfer and conduce to highly efficient electrocatalytic water oxidation. Meanwhile, the nanostructured morphology could also provide more accessible catalytic sites. As depicted by the crystal structure of NaCo4(PO4)3 compound in the inset of Figure 3f, profiting from the thick thickness feature along the b axis, deficient occupancy of Na sites could provide a more convenient approach for the entrance and diffusion of water molecules along the tunnels inside the nanoribbons, resulting in more accessible active sites. The ECSA ratio of NaCo4(PO4)3 nanoribbons to Na2CoP2O7 powder, ~2.7, was ~35% higher than the weight ratio of Co content in these two electrocatalyts, revealing that the nanostructured morphology of NaCo4(PO4)3 nanoribbons should be more advantageous for the access of Co catalytic sites. Hence, the higher OER catalytic activity for NaCo4(PO4)3 nanoribbons might be derived from the unique [CoO5] coordination, as well as the peculiar nanoribbon morphology, the higher Co content and better electrical conductivity.


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Figure 5. Possible OER mechanisms for (a) NaCo4(PO4)3 and (b) Na2CoP2O7 products under neutral PBS system. Green, yellow and white balls refer to electrons, absorbed oxygen and hydrogen atoms, respectively. In summary, a new strategy was proposed for the preparation of NaCo4(PO4)3 nanoribbons, which significantly reduced the synthesis temperature from 900 °C of conventional solid-state sintering method to 550 °C. DOS calculations revealed that NaCo4(PO4)3 with peculiar 5coordinated Co configuration was a type of half-metallic materials, which was also confirmed by the electrochemical impedance test. As a result of unique 5-coordinated Co configurations and half-metallicity as well as the nanoribbon morphology, NaCo4(PO4)3 nanoribbons were capable of yielding a current density of 1 and 11.02 mA cm–2 at a low overpotential of 0.373 and 0.570 V for electrocatalytic OER with a little loading mass in neutral PBS, respectively, which not only markedly surpassed Na2Co2P2O7 with 4-coordinated Co configuration, but also was comparable to precious metal electrocatalyst such as RuO2 NPs. This work has disclosed the excellent electrocatalytic activity of unique 5-coordinated configurations in inorganic materials, offering a new approach for greatly improving the electrochemical and catalytic properties by rationally designing unique coordination structures, which might endow a great potential for developing


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high-performance energy storage and conversion systems. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental sections, OER mechanism schemes, XRD, SEM, EDS, TGDSC, XPS data and additionally electrochemical measurements for Co-based phosphates. AUTHOR INFORMATION Corresponding Authors *E-mail: (R. M.) [email protected] *E-mail: (X. L.) [email protected]; *E-mail: (H. D. W.) [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported in part by the WPI-MANA, Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors acknowledge the financial support by National Natural Science Foundation of China (51372279), Hunan Provincial Natural Science Foundation of China (13JJ1005). X. L. acknowledges support from Shenghua Scholar Program of Central South University. R. M. acknowledges support from JSPS KAKENNHI (15H03534, 15K13296). Drs. Nobuyuki Sakai and Yasuo Ebina were gratefully acknowledged for their help in in-plane XRD measurements.


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REFERENCES (1) Wang, D. Y.; Gong, M.; Chou, H. L.; Pan, C. J.; Chen, H. A.; Wu, Y.; Lin, M. C.; Guan, M.; Yang, J.; Chen, C. W. et al. Highly active and stable hybrid catalyst of cobalt-doped FeS2 nanosheets-carbon nanotubes for hydrogen evolution reaction. J. Am. Chem. Soc. 2015, 137, 1587. (2) Zhang, W.; Lai, W.; Cao, R. Energy-related small molecule activation reactions: oxygen reduction and hydrogen and oxygen evolution reactions catalyzed by porphyrin- and corrole-based systems. Chem. Rev. 2017, 117, 3717. (3) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A perovskite oxide optimized for oxygen evolution catalysis from molecular orbital principles. Science 2011, 334, 1383-1385. (4) Ren, X.; Ge, R.; Zhang, Y.; Liu, D.; Wu, D.; Sun, X.; Du, B.; Wei, Q. Cobalt-borate nanowire array as a high-performance catalyst for oxygen evolution reaction in nearneutral media. J. Mater. Chem. A 2017, 5, 7291. (5) Roger, I.; Symes, M. D. Efficient electrocatalytic water oxidation at neutral and high pH by adventitious nickel at nanomolar concentrations. J. Am. Chem. Soc. 2015, 137, 13980. (6) Xie, L.; Zhang, R.; Cui, L.; Liu, D.; Hao, S.; Ma, Y.; Du, G.; Asiri, A. M.; Sun, X. High-performance electrolytic oxygen evolution in neutral media catalyzed by a cobalt phosphate nanoarray. Angew. Chem. Int. Ed. 2017, 56, 1064. (7) Trotochaud, L.; Boettcher, S. W. Precise oxygen evolution catalysts: status and opportunities. Scripta Mater. 2014, 74, 25. (8) Huynh, M.; Bediako, D. K.; Nocera, D. G. A functionally stable manganese oxide


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oxygen evolution catalyst in acid. J. Am. Chem. Soc. 2014, 136, 6002. (9) Dincǎ, M.; Long, J. R. High-enthalpy hydrogen adsorption in cation-exchanged variants

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metal−organic

framework

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Rare Cobalt-Based Phosphate Nanoribbons with Unique 5

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