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Feb 26, 2018 - College of Chemistry, Sichuan University, Chengdu 610064, China. #. Institute of Fundamental and Frontier Science, University of Electr...
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Hierarchical CoTe2 Nanowire Array: An Effective Oxygen Evolution Catalyst in Alkaline Media Lei Ji, Zhichao Wang, Hui Wang, Xifeng Shi, Abdullah M. Asiri, and Xuping Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04309 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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Hierarchical CoTe2 Nanowire Array: An Effective Oxygen Evolution Catalyst in Alkaline Media Lei Ji,† Zhichao Wang,† Hui Wang,‡,* Xifeng Shi,§ Abdullah M. Asiri,ʃ and Xuping Sun†,#,* †

College of Chemistry, Sichuan University, Chengdu 610064, China, #Institute of Fundamental and Frontier Science, University of Electronic Science and Technology of China, Chengdu 610054, China, ‡Analysis and Test Center, Sichuan University, Chengdu 610064, China, §College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, Shandong, China, and ʃChemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia. * E-mail: [email protected] (H.W.); [email protected] (X.S.) ABSTRACT: In this Letter, we report the development of hierarchical CoTe2 nanowire array on Ti mesh (CoTe2 NA/TM) via topotactical conversion from its Co(OH)F precursor through hydrothermal tellurization reaction. As a 3D oxygen evolution electrocatalyst, such CoTe2 NA/TM exhibits excellent catalytic activity with an overpotential of 340 mV to attain 50 mA cm–2 in 1.0 M KOH. Notably, it also exhibits high durability for 25 h.

To reduce the dependency on fossil fuels and alleviate the alarming emissions of greenhouse gases, considerable recent effort has been put to search for clean and renewable alternatives.1,2 Hydrogen has been deemed as a promising alternative clean and renewable energy source for meeting future energy demands, owing to its outstanding gravimetric energy density, high energy conversion efficiency, and environmental benignity.3,4 Alkaline water electrolysis, involving anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER), is an environmentally friendly technique for commercial hydrogen production.5 The OER however suffers from slow kinetics and still remains the key to improvement of water-splitting technologies because of the activation energy barriers of O-H bond breaking and O-O bond formation.6,7 To lower overpotential, efficient OER electrocatalysts must be implemented.8 RuO2 and IrO2 are the most active OER catalysts but their extensive commercialization has been restricted by their scarcity and high cost,9 pushing the researchers to design and develop efficient high-abundant alternatives. Co has emerged as an attractive transition metal owing to its catalytic power toward oxygen evolution,8,10–13 and great efforts have been devoted to developing cobalt oxides,14–18 sulfides19,20 and selenides21,22 as OER catalysts in the past years. As is well documented, the declination of electronegativity from O to Te is connected with enhanced metallic properties and thus optimized electron states/transmittability in metal dichalcogenides.23,24 Besides, tellurium has less toxicity than selenium25 and there are a few of researches about the cobalt tellurides toward the OER.24,26 Yu and co-workers prepared cobalt telluride (CoTe) and cobalt ditelluride (CoTe2) nanofleeces powder derived from Te nanowires for OER electrocatalysis with overpotential of 365 mV@10 mA cm–2 and 357 mV@10 mA cm–2, respectively.24 Han and co-workers prepared CoTe film for OER electrocatalysis with overpotential of 370 mV@10 mA cm–2.26 Compared to powder and film

catalysts, nanoarray counterpart are beneficial to enhanced activity because of more exposed active sites and facilitated diffusion of electrolyte and gas evolved.14,27,28 Until now, however, no work reports on CoTe2 nanoarray for alkaline water oxidation electrocatalysis. Here, we report hierarchical CoTe2 nanowire array on Ti mesh (CoTe2 NA/TM) derived from Co(OH)F nanoarray on Ti mesh (Co(OH)F NA/TM) via hydrothermal tellurization. Such CoTe2 NA/TM acts as an efficient 3D OER electrocatalyst with overpotential of 340 mV to drive 50 mA cm–2 in 1.0 M KOH. Notably, it also manifests strong long-term electrochemical durability to retain catalytic activity for at least 25 h.

Figure 1. (a) A schematic to show the whole fabrication process of CoTe2 NA/TM. (b) XRD pattern for CoTe2 NA/TM. SEM images for (c) Co(OH)F NA/TM and (d) CoTe2 NA/TM. (e) TEM image, (f) HRTEM image and (g) the electron diffraction pattern taken from one single CoTe2 nanowire.

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CoTe2 NA/TM was derived from Co(OH)F NA/TM precursor through hydrothermal tellurization reaction, and Figure 1a shows a diagrammatic drawing to expound the fabrication process (see SI for preparation details). The X-ray diffraction (XRD) patterns of CoTe2 NA/TM are shown in Figure 1b. As observed, CoTe2 NA/TM presents six diffraction peaks at 31.8°, 33.0°, 43.7°, 46.7°, 49.3°, and 58.4° indexed to the (111), (012), (121), (200), (103), and (212) facets of CoTe2 (JCPDS No. 74-0245), respectively, and other peaks belong to TM substrate (JCPDS No. 44-1294). The scanning electron microscopy (SEM) diagrams of Co(OH)F NA/TM (Figure S1b and Figure 1c) indicate that Co(OH)F nanowire array is anchored on the entire surface of TM (Figure S1a). Following tellurization, the product still maintains the nanowire array feature (Figure S1c and 1d). Very interesting, such nanowire is hierarchical in nature. Transmission electron microscope (TEM) analysis further suggests that hierarchical CoTe2 nanowires possess hollow structure due to a difference in the diffusion rate of the core atoms outward, which exceeds the rate of the added secondary species inward into the core.29 The high-resolution TEM (HRTEM) diagram of the CoTe2 nanotube (Figure 1f) provides well-defined lattice fringes with dspacing of 0.280 nm matching with the (111) plane of CoTe2. The electron diffraction pattern (Figure 1g) shows welldefined spots assigned to the (111), (121) and (200) planes of CoTe2. The energy dispersive X-ray (EDX) spectrum shows the presence of Co and Te elements, as shown in Figure S2a. The SEM and corresponding elemental maps (Figure S2bS2d) confirm the uniform distribution of Co and Te elements.

Figure 2. (a) XPS survey spectrum of CoTe2. XPS spectra in the (b) Te 3d, (c) Co 2p, and O 1s regions for CoTe2.

The X-ray photoelectron spectroscopy (XPS) spectrum (Figure 2a) of CoTe2 shows the peaks of Co and Te with signals of C and O elements due to absorbed CO2/surface oxidation of the product.30,31 For Co 2p region (Figure 2b), two dominant peaks at 796.6 and 780.9 eV can be ascribed to the binding energies (BEs) of Co 2p1/2 and Co 2p3/2, respectively, manifesting the existence of Co with high oxidation states.31,32 The peaks at 785.9 and 802.5 eV are well fitted with two shake-up satellites.32 Figure 2c shows the XPS spectrum in Te 3d region. The BEs at 572.4 and 582.7 eV can be assigned to Te–.33 The BEs at 575.9 and 586.3 eV belong to TeO2, and those at 577.0 and 587.5 eV correspond to TeO3 on the sur-

face.33,34 The two peaks at 572.8 and 583.2 eV can be assigned to the BEs of metallic Te.34 The O 1s region (Figure 2d) shows two obvious peaks at 530.3 and 531.4 eV ascribed to the lattice O and adsorbed O, respectively.35,36

Figure 3. (a) LSV curves of RuO2/TM, Co(OH)F NA/TM, CoTe2 NA/TM, and blank TM for OER. (b) Tafel plots. (c) Multi-step chronopotentiometric of CoTe2 NA/TM with the current density range of 40 – 480 mA cm–2 (an incremental quantity of 40 mA cm–2 per 500 s) without iR correction. (d) Chronopotentiometry curve under 1.55 V for 25 h.

The electrocatalytic OER activity of CoTe2 NA/TM (CoTe2 loading: 1.45 mg cm–2) was characterized under a threeelectrode cell. Blank TM, Co(OH)F NA/TM, and RuO2 on TM (RuO2/TM) with the same catalyst loading were also examined for comparison. Because as-measured catalytic current densities cannot directly reflect the intrinsic behavior of catalysts due to the solution resistance, an iR correction was applied to all experimental data,37-39 and all potentials were reported on a reversible hydrogen electrode (RHE) scale except specifically explained.Figure 3a shows their linear sweep voltammetry (LSV) curves. As expected, RuO2/TM shows excellent OER activity with low overpotential of 250 mV@50 mA cm–2, however, blank TM has poor OER activity. Comparing with pure CoTe2 powder, the CoTe2 NA/TM has a similar onset potential but its current density is much higher at the same overpotential (Figure S3). CoTe2 NA/TM is highly efficient for the OER and only needs overpotential of 340 mV@50 mA cm–2, which is 64 mV less than that for Co(OH)F NA/TM. Note that the overpotential of CoTe2 NA/TM catalyst is comparable to and even smaller than those of many reported Co-based OER catalysts (Table S1). Figure 3b shows the Tafel plots of Co(OH)F NA/TM, CoTe2 NA/TM and RuO2/TM. Tafel slope of 67 mV dec–1 for CoTe2 NA/TM is much smaller than that of Co(OH)F NA/TM (139 mV dec–1), indicating its more superior catalytic kinetics toward OER. Moreover, the RuO2/TM and CoTe2 NA/TM own the similar Tafel slope. Figure 3c represents a multi-step chronopotentiometric curve for CoTe2 NA/TM. The potential immediately levels off and remains constant in each constant current process, manifesting the excellent mass transport properties and mechanical robustness of CoTe2 NA/TM.40,41 To assess the durability of CoTe2 NA/TM electrode, linear sweeps were processed repeatedly upon the anode (Figure S4). After 1000 cycles,

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only a slight OER current loss is observed for CoTe2 NA/TM electrode at a 100 mV s−1 scanning rate. The chronopotentiometry curve further reveals a good OER activity retention over 25 h testing in alkaline solution (Figure 3d), suggesting its high stability of the CoTe2 NA/TM. Notably, although the XRD (Figure S5) and XPS analyses (Figure S6) after durability test show the formation of oxides, such electrode still preserves hierarchical nanoarray feature (Figure S7), revealing its excellent mechanical robustness.

S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] (H.W.); [email protected] (X.S.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 21575137).

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Figure 4. CVs of (a) Co(OH)F NA/TM and (b) CoTe2 NA/TM under different scan rates. (c) The capacitive currents at 0.3 V (vs. Hg/HgO) (∆j = ja – jc). (d) Nyquist plots.

Electrochemical active surface areas (ECSA) of Co(OH)F NA/TM and CoTe2 NA/TM were estimated by measuring the capacitance of the electric double layer.42,43 Figure 4a and 4b show the cyclic voltammograms (CVs) collected in nonfaradaic and OER potential regions (0.25 to 0.35 V vs. Hg/HgO). The capacitances are 92.4 and 247.8 mF cm–2 for Co(OH)F NA/TM and CoTe2 NA/TM (Figure 4c), respectively, suggesting that CoTe2 NA/TM has a higher active surface area and thus more exposed active sites.44,45 Furthermore, the Nyquist plots (Figure 4d) uncover a smaller charge-transfer resistance (Rct) of this CoTe2 NA/TM than that of Co(OH)F NA/TM, indicating a better charge transfer capability. The Faradic efficiency was determined as nearly 100% (Figure S8),46 implying no side reaction occurred during catalytic process.47 In summary, hierarchical CoTe2 nanowire array us proposed as an efficient and durable water oxidation electrocatalyst with the need of overpotential of 340 mV@50 mA cm–2 in 1.0 M KOH. This work is of significance because it not only offers us a high-active and durable 3D catalyst toward OER under alkaline conditions, but would exploit an exciting new avenue to develop metal telluride nanoarrays for catalytic applications.

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ASSOCIATED CONTENT Supporting Information Experimental section; SEM and EDX elemental mapping images; EDX and XPS spectra; LSV curves; XRD pattern; FE data; Table

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Hierarchical CoTe2 nanoarray on Ti mesh (CoTe2 NA/TM) derived from Co(OH)F NA/TM shows high activity and durability for oxygen evolution reduction in alkaline solution, offering 50 mA cm–2 at overpotential of 340 mV in 1.0 M KOH.

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