Carbon-Coated Co3+-Rich Cobalt Selenide Derived from ZIF-67 for

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Carbon-coated Co3+-rich Cobalt Selenide Derived from ZIF-67 for Efficient Electrochemical Water Oxidation Siwen Li, Sijia Peng, Linsong Huang, Xiaoqi Cui, Abdullah M. Al-Enizi, and Gengfeng Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07986 • Publication Date (Web): 04 Aug 2016 Downloaded from http://pubs.acs.org on August 5, 2016

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ACS Applied Materials & Interfaces

Carbon-coated Co3+-rich Cobalt Selenide Derived from ZIF-67 for Efficient Electrochemical Water Oxidation Siwen Li,1,† Sijia Peng,1,† Linsong Huang,1 Xiaoqi Cui,1 Abdullah M. Alenizi,2 Gengfeng Zheng1,* 1

Laboratory of Advanced Materials, Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai, China. 2

Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia.

*E-mail: [email protected] (G.Z.) †

S.L. and S.P. contributed equally to the work.

Abstract: Oxygen evolution reaction (OER) electrocatalysts are confronted with challenges such as sluggish kinetics, low conductivity, and instability, restricting the development of water splitting. In this study, we report an efficient Co3+-rich cobalt selenide (Co0.85Se) nanoparticles coated with carbon shell as OER electrocatalyst, which are derived from zeolitic imidazolate framework (ZIF-67) precursor. It is proposed that the organic ligands in the ZIF-67 can effectively enrich and stabilize the Co3+ ions in the inorganic-organic frameworks and subsequent carbon-coated nanoparticles. In alkaline media, the catalyst exhibits excellent OER performances, which are attributed to its abundant active sites, high conductivity and superior kinetics.

Keywords: electrocatalyst, cobalt selenide, ZIF-67, oxygen evolution reaction, overpotential 1 ACS Paragon Plus Environment


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Water splitting is considered to be an effective method to obtain hydrogen gas from aqueous solutions.1 Oxygen evolution reaction (OER) [4OH- → 2H2O + O2 + 4e- in alkaline media], as one half reaction of water electrolysis, involves a four-electron transfer associated with O-H bond breaking and O-O bond formation, which usually requires a large overpotential and thus results in inherent sluggish kinetics.2 It is generally regarded to hinder efficient electrolysis of water for large-scale hydrogen production.3 Therefore, exploiting OER catalysts with excellent activities, good stability and abundance has important implications in energy conversion. A variety of earth-abundant, noble metal-free materials have been extensively investigated in recent years for OER catalysts, such as oxides4,5 and hydroxides,6 nitrides,7 borides,8 carbides,9 sulfides,10 phosphides.11 Among them, cobalt selenides have recently arouse wide concern owing to their low cost and environmental benignity,

12-14

while they are still limited to wide

applications, due to the complex synthesis, low electrical conductivity, inefficient catalytic performance and poor long-term stability. Notably, the valence of cobalt ion plays a crucial role in OER catalytic activities as well as electrical conductivity.15 It has been reported that Co3+ atoms with intermediate spin in the octahedral and square pyramidal symmetry,16,17 in fact, serve as the active sites to catalyze the OER for its lower coordination number and larger H2O molecules adsorption energy,3 thus promoting to de-protonation of the OOH species to form O2.15,18 As a result, developing OER

2 ACS Paragon Plus Environment

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catalysts with high Co3+ content is highly promising to enhance water oxidation. Diverse methods have been investigated to achieve this goal, such as high temperature calcination,5 coating with oxidant,12,14 metal ion insertion,16,17 and introducing a highly electronegative support.19 In particular, it has been demonstrated that different organic ligands can be utilized to stabilize Co3+ atoms in cobalt complexes for obtaining high activity in electrocatalytic systems.20 Similar to molecular cobalt complexes, metal organic frameworks (MOFs) assembled from metal ions and organic ligands,21 including zeolitic imidazolate frameworks (ZIFs) and prussian blue analogues (PBAs), have engaged widespread research interest, owing to their tunable structures controlled by modifying their own ligands, versatile functionalities and attractive properties.22 The metal sites in MOFs can be stabilized and reinforced through ligands surface coating strategy, which has important implications for many subsequent applications including compounds sensing, energy storage, and electrocatalysis.22,23 Further thermal treatment of MOFs can pyrolyze their carbon and/or nitrogen-containing ligands and result in the formation of carbon and/or nitrogen-coated metal or metal oxide nanoparticles.22,23 Thus, MOFs have emerged as excellent precursors or sacrificial templates for preparing metal-based with high specific surface area and carbon or and nitrogen-doped nanocomposites, improving the catalytic activity electrocatalysts.24 Metal oxides, sulfides, nitrides, phosphides, carbides derived from MOFs by thermal decomposition were recently reported.9,22,23,25 Nonetheless, to the best of our knowledge,



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metal selenides derived from MOFs as highly efficient electrocatalysts for OER have seldom been reported. Inspired by the strategy of stabilizing Co3+ atoms through organic ligands, we proposed that the Co-based MOFs may provide a unique precursor to prepare Co3+-rich materials as electrocatalysts. Herein, we developed a facile method to synthesize Co3+-rich cobalt selenide nanoparticles coated with a thin carbon shell derived from MOFs, as efficient water oxidation electrocatalysts. As shown in Figure 1, Co-based ZIF-67 is chosen to be both cobalt and carbon precursors and then converted into Co0.85Se nanoparticles (designated as ZIF-Co0.85Se) by selenium sources (i.e., NaHSe solution) and subsequent calcination. The obtained ZIF-Co0.85Se is coated with a thin continuous carbon shell, which is beneficial for high conductivity and catalytic activities.4,26 Moreover, the high content of Co3+ in ZIF-Co0.85Se offers more active sites as excellent OER electrocatalysts with low overpotentials, small Tafel slopes, high current densities and good stability. ZIF-67 was synthesized via a co-precipitation method using cobalt nitrate hexahydrate and 2-methylimidazole as precursors (Methods in the Supporting Information).27 Scanning electron microscopy (SEM) images show the uniform dodecahedron morphologies of ZIF-67 (Fig. 2a). X-ray diffraction (XRD) pattern exhibits distinctive diffraction peaks (Fig. S1), which are in accord with ZIF-67,27 confirming its high purity. After the solvothermal reaction of ZIF-67 with NaHSe, ZIF-67 with regular morphological form was converted into irregular nanoparticles (Fig.


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S2). Subsequent calcination of the as-synthesized particles, the XRD pattern shows peaks at 2θ at 33.3°, 45.2°, 51.4°, 61.3° and 62.6° (Fig. 2b), which can be well indexed to the 101, 102, 110, 112 and 103 planes of Co0.85Se phrase (JCPDS No. 52-1008). Extra peaks appeared at 34.0° and 36.4° are assigned to the minor Co3O4 phase caused by ambient air oxidation, consistent with literature reported.5 The obtained nanoparticles (designated as ZIF-Co0.85Se) present average sizes similar to the ZIF-67 precursor (Fig. 2c). High-resolution transmission electron microscopy (HRTEM) images exhibit the ZIF-Co0.85Se nanoparticles have a crystalline nature with interlayer spacings of 0.55 nm. In addition, a thin, continuous carbon shell was observed to coat on the surface of the nanoparticles (Fig. 2d and S3). Energy dispersive X-ray spectroscopy (EDS) and scanning TEM (STEM) mode elemental mappings were conducted (Fig. S4, S5), proving the uniform presence of Co, Se, C, O, and N elements. The actual atomic ratio of Co/Se obtained from EDS spectra is 0.86: 1 (Fig. S4), in agreement with the XRD results. For comparison, another subclass of MOFs, prussian blue analogues (PBAs) Co3[Co(CN)6]2 (Co-Co PBA) was also selected as cobalt and carbon precursors to synthesize cobalt selenide under the same reaction conditions as ZIF-67. SEM images show the uniform nanocubic morphologies of Co-Co PBA with an edge length of 1 µm (Fig. S6a). The diffraction peaks from XRD pattern well match with the Co-Co PBA phrase (JCPDS No. 77-1161) (Fig. S6b). Similar to ZIF-67, the nanocubic Co-Co PBA was converted into irregular nanoparticles (designated as PBA-Co0.85Se) with interlayer spacings of 0.27 nm (Fig. S7). Meanwhile, Co0.85Se nanoparticles



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without coated carbon (denoted as Co0.85Se) were produced by choosing CoCl2·6H2O as cobalt precursor.28 XRD patterns verify that both obtained PBA-Co0.85Se and Co0.85Se also belong to a Co0.85Se phrase (Fig. 2b), and the extra peak at 34.0° in the XRD pattern of PBA-Co0.85Se is due to the minor Co3O4 phase.5 X-ray photoelectron spectroscopy (XPS) was carried out to study the polyvalent states of metal ions on the catalyst surface. As XPS spectrums of the three samples display (Fig. 3a), the binding energies at 778.4, 780.6, 793.3 and 797.0 eV correspond to Co3+ 2p3/2, Co2+ 2p3/2, Co3+ 2p1/2 and Co2+ 2p1/2, respectively (Fig. 3a), suggesting the coexistence of Co2+ and Co3+.29 High-resolution XPS spectra of Co 2p show that the ratio of Co2+/Co3+ varies from 13:11 for ZIF-Co0.85Se (Fig. 3b), 13:10 for PBA-Co0.85Se (Fig. 3c) to 13:7 for Co0.85Se (Fig. 3d). The ratio of Co2+/Co3+ (13:7) for Co0.85Se is in accord with the stoichiometric value of Co0.85Se phrase, while a clear decrease of the Co2+/Co3+ ratios is observed for ZIF-Co0.85Se and PBA-Co0.85Se, suggesting much higher contents of Co3+ in these MOF-originated products. In Figure S8, The binding energies of Se 3d5/2, 3d3/2 and SeOx are located at 54.3, 55.0 and 59.2 eV, respectively , also consistent with literature.30 The N2 isotherm was further carried out to characterize the microcosmic structure of the three samples. The Brunauer-Emmett-Teller (BET) surface areas of the ZIF-Co0.85Se, PBA-Co0.85Se and Co0.85Se samples are 13.56, 4.92 and 23.89 m2g−1, respectively (Fig. S9).


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Electrochemical measurements were tested to investigate the electrocatalytic behaviors of the three catalysts. All the electrochemical tests were performed by a typical three-electrode system in 1 M KOH (Methods in the Supporting Information), and the potentials reported in this paper were all versus reversible hydrogen electrode (RHE). Cyclic voltammograms (CVs) were first conducted with the voltage ranging from 0.80 to 1.55 V (Fig. 4a). The pair of redox peaks located at 1.1−1.2 V are attributed to the Co2+/Co3+ redox couple.6 The electrocatalytic OER activity were then evaluated by linear sweep voltammetry (LSV). Among these three electrocatalysts (Fig. 4b), the polarization curve from the ZIF-Co0.85Se catalyst exhibits the smallest onset potential of ~1.52 V and highest current density, and the current density of the ZIF-Co0.85Se reaches 10 mA·cm-2 at an overpotential (η) of 0.36 V. In contrast, the PBA-Co0.85Se and Co0.85Se have much higher overpotentials (0.52 and 0.45 V, respectively). Corresponding Tafel plots were drawn to further study the OER kinetics. Remarkably, the ZIF-Co0.85Se possesses a Tafel slope of only 62 mV·dec-1, prominently smaller than PBA-Co0.85Se (115 mV·dec-1) and Co0.85Se (133 mV·dec-1) (Fig. 4b). Based on the literature (Table S1 in the Supporting Information), the best cobalt selenide-based OER catalysts previously reported were CoSe2 nanosheets2 and CoSe2 nanobelts,14 which were made from CoSe2-DETA (DETA = diethylenetriamine) intermediates.12 The obtained CoSe2 nanosheets2 and CoSe2 nanobelts14 showed onset potential of 1.49 and 1.59 V and Tafel slopes of 64 and 66 mV·dec-1, at

mass loadings of 0.17 and 0.20 mg·cm-2, respectively. In comparison, our



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ZIF-Co0.85Se catalyst, with a similar mass loading (0.14 mg·cm-2), demonstrates comparable performances, including a low onset potential (1.52 V), small Tafel slopes (62 mV·dec-1), and low overpotential (0.36 V at a current density of 10 mA·cm-2), suggesting its potential as an excellent OER catalyst. The electrochemical impendence spectroscopy (EIS) measurements were then conducted to probe the electrode kinetics under OER circumstances (Fig. 4c). Based on the Nyquist plots and the equivalent circuit model, the charge transfer resistances (Rct) were calculated as 4.5 and 1.6 Ω for ZIF-Co0.85Se and PBA-Co0.85Se, respectively, which are similar but substantially lower than that of Co0.85Se (172.1 Ω). The obvious improvement can be owed to their coated carbon shell and high content of Co3+. For ZIF-Co0.85Se and PBA-Co0.85Se, the OER catalytic activities of ZIF-Co0.85Se with larger Rct (4.5 Ω) were better than that of PBA-Co0.85Se with smaller Rct (1.6 Ω), suggesting that in addition to conductivity, the Co3+ content is another dominant contributor to exceptional OER activity. Additionally, N atoms (3.44 atom%) were doped into the carbon shell (5.62 atom%) of ZIF-Co0.85Se (Fig. S4, S5), which also helped to enhance its OER performance.4 Thus, this superior OER activity of the ZIF-Co0.85Se nanoparticles can be attributed to high content of Co3+ and enhanced electrical conductivity of the carbon shell with N doping.4,15,18 Moreover, the chronoamperometry test at the potential of 1.60 V show that the ZIF-Co0.85Se catalyst retains 92.8% of its initial activity after testing for 10,000 s, compared to PBA-Co0.85Se (85.1% of retaining) and Co0.85Se (89.3% of retaining),


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illustrating its excellent stability(Fig. 4d). Taken together, these results demonstrate the excellent potentials of ZIF-Co0.85Se as an effective and stable OER catalyst. To investigate the OER catalytic temperature-dependent performance, the as-synthesized ZIF-Co0.85Se was calcined at 400, 600 and 800°C for 3 h, respectively. As LSV curves stated (Fig. S10), the sample with 600°C calcination treatment displays the highest catalytic activity for OER. The result suggests that the calcination treatment can result in the structural difference of the as-synthesized ZIF-Co0.85Se, leading to different OER catalytic activities. As stated above, ZIF-Co0.85Se nanoparticles do not possess a large specific surface area, which means specific surface area is not the main contributor to its exceptional OER activity. Furthermore, the thin carbon shell of the ZIF-Co0.85Se nanoparticles enables high conductivity, which helps to improve the OER catalytic activity. In addition, although the PBA-Co0.85Se catalyst has the same carbon coated structure as ZIF-Co0.85Se, its OER performance is still inferior to ZIF-Co0.85Se, which can be attributed to both its lower surface area and lower Co3+ content than ZIF-Co0.85Se. It has been prevalently believed that Co3+ is able to enhance the electrophilicity of materials, promoting the oxygen evolution under the OER process.15,19 As expected, the ZIF-Co0.85Se nanoparticles with abundant Co3+ display the best OER catalytic property among the three samples. The results above demonstrate that both coated carbon structure and Co3+-rich composition are attributed to the marvelous OER activity of ZIF-Co0.85Se.



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Furthermore, it has been reported that cobalt selenides (i.e., Co0.85Se, CoSe2) made from different approaches have great potentials as bifunctional OER/HER electrocatalysts for overall water splitting,4,6-8,11,24 which is of great importance for future renewable energy systems. Thus, the potential of our ZIF-Co0.85Se sample for catalyzing hydrogen evolution reaction (HER) was also briefly evaluated. Compared to Co3O4@C-derived from ZIF-67 reported (Methods in the Supporting Information),5 our ZIF-Co0.85Se possesses better HER electrocatalytic performances in both alkaline and acid medias (Fig. S11). In summary, we have developed a facile method to synthesize a type of Co3+-rich ZIF-Co0.85Se nanoparticles coated with carbon shell, which were derived from ZIF-67 via a solvothermal reaction and calcination process. The Co3+-rich composition and the carbon-coated structure enable adequate active site, good conductivity and efficient charge transport during electrolysis, leading to an outstanding catalytic property. The obtained ZIF-Co0.85Se electrocatalyst displays a low overpotential, high current density and good durability, and is thus promising as an efficient and stable OER electrocatalyst. Our work suggests a further insight into exploring new efficient material as alternative of noble metal-free electrocatalysts for clean energy utilization.


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Acknowledgements We thank the following funding agencies for supporting this work: the National Key Basic Research Program of China (2013CB934104), the Natural Science Foundation of China (21322311, 21473038), the Science and Technology Commission of Shanghai Municipality (14JC1490500), and the Collaborative Innovation Center of Chemistry for Energy Materials (2011-iChem). The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP# 0017.

AUTHOR INFORMATION Corresponding Authors * Email: [email protected] (G.Z.) Author Contributions † S.L. and S.P. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.



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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Detailed experimental section, XRD patterns, SEM and TEM images, STEM pattern and EDS elemental maps, XPS spectra, LSV curves, HER polarization curves, and Table of comparison of OER performances of this work with other literatures.


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(26) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S. Hydrogen Evolution by a Metal-free Electrocatalyst. Nat. Commun. 2014, 5, 3783. (27) Hu, H.; Guan, B.; Xia, B.; Lou, X. W. Designed Formation of Co3O4/NiCo2O4 Double-shelled Nanocages with Enhanced Pseudocapacitive and Electrocatalytic Properties. J. Am. Chem. Soc. 2015, 137, 5590-5595. (28) Wang, Z.; Sha, Q.; Zhang, F.; Pu, J.; Zhang, W. Synthesis of Polycrystalline Cobalt Selenide Nanotubes and Their Catalytic and Capacitive Behaviors. Crystengcomm. 2013, 15, 5928-5934. (29) Zhang, Z.; Wang, X.; Wu, K.; Yue, Y.; Zhao, M.; Cheng, J.; Ming, J.; Yu, C.; Wei, X. Co0.85Se Bundle-like Nanostructure Catalysts for Hydrogenation of 4-nitrophenol to 4-aminophenol. New J. Chem. 2014, 38, 6147-6151. (30) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: an Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, 4897-4900.



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Figures

Figure 1. Schematic of the synthesis of Co3+-rich cobalt selenide nanoparticles coated with a carbon shell derived from ZIF-67 as an OER catalyst. ZIF-67 is used as both cobalt and carbon precursor and converted into Co0.85Se@C nanoparticles with high Co3+ content, which plays a vital role in the electrochemical water oxidation.


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Figure 2. (a) SEM image of ZIF-67. (b) XRD patterns of ZIF-Co0.85Se (red curve), PBA-Co0.85Se (blue curve) and Co0.85Se (black curve), and JCPDS No. 52-1008 (red bars). (c) SEM image and (d) HRTEM image of ZIF-Co0.85Se. Inset: low-resolution TEM image.



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Figure 3. XPS spectra of Co 2p for (a) the three samples. The red curve: ZIF-Co0.85Se, the blue curve: PBA-Co0.85Se, and the black curve: Co0.85Se. (b-d) High-resolution XPS spectra of Co 2p for (b) ZIF-Co0.85Se, (c) PBA-Co0.85Se, (d) Co0.85Se. In b-d, the fitted peaks correspond to Co3+ 2p (blue curves) and Co2+ 2p (magenta curves).


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Figure 4. Electrochemical tests of ZIF-Co0.85Se (red curves), PBA-Co0.85Se (blue curves) and Co0.85Se (black curves) nanoparticles. (a) Cyclic voltammograms. (b) OER polarization curves. Inset: corresponding Tafel plots. (c) Nyquist plots at an applied potential of 1.55 V. Upper left inset: equivalent circuit model. Upper right inset: zoom-in of the Nyquist plots. (d) Time-dependent current density retaining curves at 1.60 V. All test were measured in 1 M KOH with a scan rate of 5 mV·s-1 and a catalyst loading of 0.136 mg·cm−2.



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Table of Contents Graphic


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Carbon-Coated Co3+-Rich Cobalt Selenide Derived from ZIF-67 for

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