Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties

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Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties Xiaozhi Guo*, Guozheng Liang**, Aijuan Gu*** College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, PR China

highlights

graphical abstract

 The CoS2 NBs are constructed by employing

ZIF-67

NCs

as

the

starting template.  The unique nanobox structure endows the CoS2 with enlarged surface area.  Such CoS2 nanoboxes display high electrocatalytic performance for OER.

article info

abstract

Article history:

Hollow nanostructures with their intriguing structural features are attractive for efficient

Received 19 July 2019

energy conversion and storage technology. Herein we report the designed construction of

Received in revised form

novel CoS2 nanoboxes by employing the Co-based zeolitic imidazolate (ZIF-67) nanocubes

10 September 2019

(NCs) as the starting template. Delicate manipulation of the template-engaged reaction

Accepted 30 September 2019

time between thioacetamide (TAA) and ZIF-67 NCs leads to the formation of hollow CoS2

Available online xxx

nanostructures. As a result of the unique nanobox structure, the optimized CoS2 nanoboxes (NBs) have an enlarged electrolyte-accessible surface, abundant mass diffusion 2

at

Keywords:

pathways, and high structural integrity, which afford the current density of 10 mA cm

CoS2 nanoboxes

a low overpotential of 290 mV for oxygen evolution reaction (OER) in 1 M KOH solution.

Hollow nanostructure

Remarkably, these CoS2 NBs could also display excellent long-term stability for more than

Oxygen evolution reaction

40 h electrochemical test in an alkaline medium, showing a class of advanced electro-

High electrocatalytic performance

catalysts for OER and beyond. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (X. Guo), [email protected] (G. Liang), [email protected] (A. Gu). https://doi.org/10.1016/j.ijhydene.2019.09.219 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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Introduction The electrolysis of water offers a promising strategy to produce clean hydrogen energy [1e5]. The electrochemical water splitting involves two half reactions, anodic oxygen evolution reaction (OER) and cathodic hydrogen energy reaction (HER), needs high cell voltage to overcome the activation barrier and boost the reaction process [6,7]. Particularly, due to the sluggish reaction kinetics, a high overpotential is required to lower the activation energy barriers of OeH bond breaking and OeO bond formation [8e10]. To this end, high-performance electrocatalysts are urgently sought for OER. Generally speaking, precious metal oxides (e.g., RuO2 and IrO2) are widely used as commercial OER catalysts. However, their high cost and scarcity have seriously impeded their practical application. In view of this situation, many researchers have dedicated to designing and developing high-performance yet nonprecious electrocatalysts for OER. Among various potential candidates, transition metal sulfides (TMSs), such as CoS2 [11,12], NiS2 [13], and MoS2 [6], have attracted increasing attention, due to their favorable electrochemical properties. Unfortunately, their limited electrochemical active sites and unstable properties in alkaline media seriously restricted their OER activity [14,15]. Therefore, it is necessary for researchers to design and develop efficient catalysts enriched with electroactive sites [16,17]. Considering that the electrochemical reactions commonly take place on the surface or interface, the morphology, electronic structure, and surface defects are thus playing key role in affecting their electrocatalytic performance [18e24]. Therefore, by appropriately tuning the geometric and electronic structure of TMSs can greatly increase the active sites and realize the OER activity enhancement of TMSs [25,26]. As for tuning the geometric structure, constructing hollow nanostructures can generate abundant surface active sites because of their large specific surface area and large void space [27,28]. Moreover, the hollow nanostructures not only facilitate the contact the between electrolyte and reaction active sites, but also reduce the transport lengths for mass and charge transport [29e32], leading to the large promotion of electrocatalytic performance. For example, Nai et al. reported the synthesis of CoeFeeO frames employing a template method, where the asobtained CoeFeeO frames with large void space and surface areas display much higher electrocatalytic OER performance than the bulk counterpart [33]. Shao and co-worker designed the NiFe layer double hydroxides hollow nanospheres by in situ growth method, and the NiFe layer double hydroxides hollow nanospheres display preferable electrocatalytic activity and stability [18]. These previous works have further demonstrated that the design of hollow nanostructures is favorable for promoting their electrocatalytic performance [34]. Taking above factors into considerations, we herein developed a template-engaged method for the construction of CoS2 nanoboxes (NBs) as advanced electrocatalysts for OER by taking advantage of the unique reactivity of ZIF-67. The template-engaged synthesis offers the additional possibility for engineering the CoS2 nanoboxes with tunable chemical composition and hollow interior by tailoring the sulfurization

time. Impressively, the optimized CoS2 NBs manifest high active surface areas with improved OER activity and stability, showing bright promise for serving as efficient electrode materials for OER.

Experimental section Synthesis of ZIF-67 NCs The ZIF-67 NCs were synthesized according to a reference with a few modifications [22]. In a typical synthesis, 908 mg of 2-methylimidazole was dissolved in 14 mL of deionized water. Then 2 mL of aqueous solution containing 58 mg of Co(NO3)2$6H2O and 1 mg of cetyltrimethylammonium bromide (CTAB) was rapidly injected into above solution and stirred at room temperature for 20 min. The solutions were then centrifugalized at 9000 rpm for 3 min and washed with ethanol for six times.

Synthesis of CoS2 nanoboxes 22 mg of the freshly prepared ZIF-67 NCs was dispersed in 15 mL of ethanol by ultrasound. Then, 5 mL of aqueous solution with 100 mg thioacetamide (TAA) was poured into the ZIF-67 dispersion to heat and reflux at 95  C for another 12 min, the products (denoted as CoS2 NBs-12) were then collected after centrifugation and washing with ethanol for six times. For comparison, CoS2 NBs-6 and CoS2 NBs-30 were also prepared by changing the sulfurization time to 6 min and 30 min, respectively.

Characterizations The transmission electron microscopy (TEM) was performed utilizing FEI Tecnai T20 microscope (200 kV), and highresolution TEM (HRTEM) and scanning TEM (STEM) images and elemental mapping analyses results were obtained on an FEI Tecnai F20 microscope (200 kV) with an Energy Dispersive Spectrometer (EDS). Scanning electron microscopy (SEM) images were taken on a XL30 ESEM FEG scanning electron microscope at the voltage of 20 kV. The crystal structures of the samples were analyzed by X-ray diffraction (XRD). The surface chemical compositions and chemical valences of electrocatalysts were characterized by X-ray spectroscopy (XPS).

Electrochemical measurements We used a three-electrode electrochemical cell controlled by CHI 660E electrochemistry workstation for the electrochemical measurements. We used the Ag/AgCl as the reference electrode (3 M KCl), as-prepared sample modified glassy carbon electrode as the working electrode, and carbon rod as the counter electrode. The OER activity was evaluated by conducting linear sweep voltammetry (LSV; at 5 mV s 1) from 1.0 to 1.7 V (vs. RHE). The cyclic voltammetry (CV) was performed at the scan rate of 5 mV s 1 for 50 cycles to activate the electrocatalysts. The electrochemical impedance spectra (EIS) measurement was performed in the same configuration at room temperature over a frequency range from 0.1 Hz to

Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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1000 kHz. Long-term stability is evaluated by I-t test at 1.5 V (vs. RHE) and prolonged chronopotentiometry (CP) test at the current density of 10 mA cm 2 for long time.

Results and discussion The synthetic route for the CoS2 NBs was displayed in Fig. 1, where the highly uniform ZIF-67 NCs was obtained as the solid precursors (Fig. S1), using TAA as the sulfur source, the solid NCs could be successfully transformed into CoS2 NBs with well-defined hollow interior due to the diffusion effect. Specifically, the sulfide ions could be released from the TAA, and then react with the metal ions to generate the CoS2 thin layer [35]. The morphology of the CoS2 NBs was examined by transmission electron microscopy (TEM). The representative TEM images revealed that the morphology of CoS2 was well featured with the hollow NBs with a rough surface (Fig. 2aed). HAADF-STEM study also provides further insights into the hollow interior and detailed morphology of the as-prepared NBs (Fig. S2). In agreement with TEM observations, the welldefined inner cavities of highly uniform boxes are clearly elucidated by the sharp contrast between the center and the edge, further confirming the successful construction of NBs [36]. The crystal structure of the as-obtained CoS2 NBs was evaluated by XRD (Fig. S3). As shown in the XRD pattern, the as-generated CoS2 NBs were amorphous [12]. In accordance with XRD analysis, the HRTEM image also confirmed the amorphous nature of CoS2 NBs (Fig. 2e). The Co/S molar ratio of CoS2 NBs was 40.4:59.6 (Fig. S4), as illustrated by SEM-EDS. The elemental mapping images also confirmed that Co and S elements were distributing throughout the whole nanobox (Fig. 2fej), being consistent with the SEM-EDS analysis. The surface chemical compositions and valences of the CoS2 NBs-12 were evaluated by XPS. As shown in Fig. 3a, the

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presence of S 2p and Co 2p in the surface spectrum indicated the formation of cobalt sulfide. Fig. 3b showed the highresolution XPS spectrum of Co 2p. As seen, the Co 2p core level consisted of two peaks at 796.9 eV and 780.4 eV, which were corresponding to the Co 2p1/2 and Co 2p3/2, respectively, suggesting the existence of CoeS bond in the CoS2 NBs [37]. Besides, the other two weak peaks at around 801.9 eV and 784.1 eV implied the existence of CoeN bond, which was ascribed to the insufficient sulfurization of ZIF-67 NCs [12]. In regard to S 2p, the two well-fitted peaks at around 162.1 eV and 163.2 eV were corresponding to the disulfide S22 , while another peak at around 168.3 eV is associated with the SeO resulting from the surface oxidation [38]. These results have further confirmed the formation of CoS2. For comparison, the detailed morphological information and chemical compositions of the CoS2 NBs-6 and CoS2 NBs-30 were also analyzed by TEM and SEM-EDS. As shown in Fig. 4a and b, the CoS2 NBs-6 obtained by sulfurizing for 6 min displayed the typical nanocube structure with rough surface, indicating the occurrence of sulfurization on the surface of ZIF-67 NCs. The molar ratio of Co/S was 42.4:57.6 (Fig. 4c), where the content of S was less than that of CoS2 NBs-12, implying that the sulfurization reaction was an ion exchange process. When the sulfurization time was further extended to 30 min, the CoS2 NBs-30 also displayed the typical nanobox structure (Fig. 4d and e), while the content of S further increased to the 63.8% (Fig. 4f). This phenomenon further demonstrated that the sulfurization reaction was a typical ion exchange process [39]. Taking advantages of intrinsic electrochemical properties of TMSs and the unique hollow nanostructures, we here employed the CoS2 NBs as the electrode material for the OER in alkaline medium (1 M KOH solution). For comparison, the electrochemical properties of ZIF-67 NCs are also studied. Fig. 5a showed the polarization curves recorded by LSV of CoS2

Fig. 1 e Schematic illustration of the synthesis of hollow CoS2 NBs. Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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Fig. 2 e Morphological and structural characterizations of CoS2 NBs-12. (aed) TEM images of CoS2 NBs-12. (e) HRTEM image, (f) STEM image, elemental mapping of (g) Co K, (h) Co L, (i) S, and (j) overlap. NBs-6, CoS2 NBs-12, CoS2 NBs-30, ZIF-67 NCs. It was clearly observed that the as-obtained CoS2 NBs displayed the remarkably lower onset potential and higher current density in comparison with ZIF-67 NCs. As shown in Fig. 5b, to reach to the current density of 10 mA cm 2, the optimized CoS2 NBs-12

required the overpotential of only 290 mV. On the other hand, 124 mV higher overpotential is required for ZIF-67 NCs to reach the same current density. With respect to the overpotential required at 10 mA cm 2, the electrocatalytic activity of CoS2 NBs-12 is also comparable with those of commercial

Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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IrO2 and RuO2 catalysts. Besides the CoS2 NBs-12, both of CoS2 NBs-6 and CoS2 NBs-30 also displayed relatively low overpotential of 317 mV and 340 mV, respectively, suggesting the high OER activity of CoS2 NBs regardless of the variation chemical compositions. Moreover, the optimized CoS2 NBs-12 also displayed superior OER activity in comparison with similar systems reported in the literature (Table S1). The kinetics parameter of these electrocatalysts were calculated by plotting overpotential against log(j). As seen in Fig. 5c, the Tafel slopes of CoS2 NBs-12, CoS NBs-6, and CoS2 NBs-30 are 72.2, 78.4, and 88.3 mV dec 1, respectively, much lower than that of ZIF-67 NCs (128.2 mV dec 1), suggesting the faster reaction kinetics of CoS NBs. The higher charge transfer coefficient of CoS2 NBs was also confirmed by electrochemical impedance spectra (EIS) (Fig.5d and Fig. S5) [40]. These results have demonstrated the excellent OER activity of CoS2 NBs. To gain a better understanding of the CoS2 NBs towards electrocatalytic OER process and explore the origin of the OER activity improvement, the electrochemical surface areas (ECSAs) of four electrocatalysts were first measured since the OER process was a typical surface-sensitive reaction (Fig. S6). As seen in Fig. 6a and b, the ECSA investigated by double-layer capacitance (Cdl) displayed that the Cdl and ECSA of CoS2 NBs12 (7.7 mF cm 2,154 cm2) were larger than that of ZIF-67 NCs (2.3 mF cm 2, 46 cm2), CoS2 NBs-30 (2.6 mF cm 2, 52 cm2) and CoS2 NBs-30 (6.7 mF cm 2, 134 cm2), indicating higher density

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of catalytically active sites in CoS2 NBs-12. The higher ECSA greatly contributed to the largely improved electrocatalytic OER activity of CoS2 NBs-12 in alkaline medium [41,42]. The stability is another significant indicator for evaluating the properties of catalysts. Thus, the I-t test at the potential of 1.5 V was operated for 10000 s to assess the CoS2 NBs-12 stability. As depicted in Fig. 7a, the CoS2 NBs-12 could retain the high current density after 10000 s. More importantly, the LSV polarization curve after the continuous 10000 s of I-t test displayed less activity degradation in alkaline medium, which required the overpotential of 323 mV to achieve the current density of 10 mA cm 2, demonstrating the excellent long-term stability. Moreover, the prolonged CP test at the fixed current density of 10 mA cm 2 was also conducted. As shown in Fig. 7b, the CoS2 NBs-12 maintained good durability with limited potential variation after continuous 24 h. This phenomenon demonstrated the good stability of CoS2 NBs-12 and further depicted that constructing hollow nanobox structure is an effective route to improve the longterm stability of catalysts due to the structure robustness [43e45]. More importantly, the CoS2 could also maintain the typical nanobox structure after OER test (Fig. S7), which is similar with the nanobox structure before OER test, suggesting the excellent morphology and structure stability [46,47].

Fig. 3 e XPS spectra of (a) surface scan, (b) Co 2p, and (c) S 2p. Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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Fig. 4 e TEM images of (a and b) CoS2 NBs-6 and (d and e) CoS2 NBs-30 with different magnifications. SEM-EDS spectra of (c) CoS2 NBs-6 and (f) CoS2 NBs-30.

Fig. 5 e (a) Polarization curves, (b) histograms of the overpotentials at 10 mA cm¡2, (c) Tafel slopes, (d) Nyquist plots (potential ¼ 1.5 V) of CoS2 NBs-6, CoS2 NBs-12, CoS2 NBs-30, and ZIF-67 NCs. Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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Fig. 6 e (a) Double layer currents and (b) ECSA values of CoS2 NBs-6, CoS2 NBs-12, CoS2 NBs-30, and ZIF-67 NCs.

Fig. 7 e (a) I-t curve of CoS2 NBs-12 at 1.5 V for 10000 s, (b) LSV polarization curves of CoS2 NBs-12 before and after i-t test, (c) prolonged CP test at the current density of 10 mA cm¡2.

Conclusions In summary, a novel template-engaged synthetic strategy has been developed for the designed formation of hollow CoS2 nanostructures. The synthesis process involves the generation of cube-like ZIF-67 and subsequent template-engaged

reaction between TAA and ZIF-67. The synthesis strategy in this work can be easily extended to the fabrication of other nanobox structures. By precisely manipulating the reaction time, three kinds of hollow CoS2 nanostructures were obtained, where the optimized CoS2 nanoboxes could display the largely enhanced OER electrocatalytic activity and stability in alkaline medium, outperforming ZIF-67 and many other

Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219

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related materials. Specifically, the CoS2 nanoboxes can deliver a low overpotential of 290 mV to reach to the current density of 10 mA cm 2, and outstanding stability for long-term electrochemical tests. This work will inspire rational design of hollow nanostructure with largely promoted electrocatalytic properties for various applications.

Acknowledgements This project was supported by the National Natural Science Foundation of China (Grant No. 51273135), the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD).

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.219.

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Please cite this article as: Guo X et al., Designed formation of CoS2 nanoboxes with enhanced oxygen evolution reaction electrocatalytic properties, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.219