In-Situ Electrodeposition of Cobalt Sulfide Nanosheet Arrays on

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In-Situ Electrodeposition of Cobalt Sulfide Nanosheet Arrays on Carbon Cloth as a Highly Efficient Bifunctional Electrocatalyst for Oxygen Evolution and Reduction Reactions Bin Liu, Shengxiang Qu, Yue Kou, Zhi Liu, Xu Chen, Yating Wu, Xiaopeng Han, Yida Deng, Wenbin Hu, and Cheng Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10645 • Publication Date (Web): 27 Jul 2018 Downloaded from http://pubs.acs.org on July 28, 2018

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ACS Applied Materials & Interfaces

In-Situ Electrodeposition of Cobalt Sulfide Nanosheet Arrays on Carbon Cloth as a Highly Efficient Bifunctional Electrocatalyst for

Oxygen

Evolution

and

Reduction

Reactions Bin Liu,a Shengxiang Qu,b Yue Kou,b Zhi Liu,a Xu Chen,a Yating Wu,a Xiaopeng Han,c Yida Deng,c Wenbin Hub,c and Cheng Zhongb,c* a

State Key Laboratory of Metal Matrix Composites, Department of Materials Science

and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b

Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Ed

ucation), School of Materials Science and Engineering, Tianjin University, Tianjin 30 0072, China c

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials

Science and Engineering, Tianjin University, Tianjin 300072, China * Corresponding author. E-mail address: [email protected] (C. Zhong) KEYWORDS: cobalt sulfides, bifunctional electrocatalyst, electrodeposition, oxygen evolution reaction, oxygen reduction reaction.

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ABSTRACT: As one of the advanced cobalt-based materials, cobalt sulfides with the novel architecture have attracted numerous interests due to the low cost, easy availability, and promising bifunctional activity for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which is essential for next-generation energy storage devices. Herein, we demonstrated a facile and clean electrochemical technique to directly synthesize CoS nanosheets with high purity onto the surface of carbon cloth, and a quick thermal treatment was performed to further improve the catalytic performance (CoS-A). This novel electrochemical technique avoids the use of the binder, surfactant and other organic additives which may cause poor electric conductivity as well as undesirable surface wettability, exhibiting great potential of the large-scale applications. The obtained CoS-A exhibits a superior electrocatalytic performance for the OER and ORR, with high ORR current density (– 1.51 mA cm−2 at 0.2 V), considerable OER current density (148 mA cm−2 at 1.9 V), and excellent durability in continuous measurement for over 12 h. The approach offers a powerful while simple method to control the phase, composition, and morphology of highly active CoS catalyst, which provides a new idea for the design of high-performance catalysts. 1. Introduction With the ever-increasing concerns about the urgent global energy crisis and environmental problem, the pursuing for renewable and clean energy supplements are unprecedented in past few decades.1-5 To meet the demands for sustainable power sources, numerous efforts have been devoted to the development of energy storage devices such as metal–air batteries and fuel cells 6-8, of which active and regenerative conversion associates with chemical energy as well as electric energy could be realized.9 One of their commercialization blocks lies in the intrinsic sluggish kinetics


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and the large overpotentials of OER (oxygen evolution reaction) and ORR (oxygen reduction reaction), which significantly restrains the energy density, efficiencies and cycle lifetimes.1,10,11 To alleviate the undesirable issues, noble metals, famous for Pt and its alloys for ORR and Ir and Ru contained composites for OER, are needed.12,13 Unfortunately, the rareness, high cost, and unsatisfactory durability of these widely used catalysts as well as their inadequate bifunctional activity towards both OER and ORR serve as road-blocks hindering their commercial implementation as well as large-scale applications.7,12,14 Therefore, it is highly imperative to search earth-abundant and noble-metal free electrocatalysts with long-term durability and decent activity for OER and ORR.15,16 Hitherto, transitional-metal based catalysts, especially those of cobalt, have received tremendous interest owing to their attractive features including low cost, easy availability, and promising bifunctional activity.17,18 Although cobalt hydroxide (Co(OH)2) and cobalt oxide (Co3O4) have emerged as front-runners serving as cheap and efficient alternatives for OER and ORR in alkaline solution,19,20 there is still substantial room for the improvement in both preparation techniques and electrochemical performance of the catalysts. One of the most eye-catching strategies rests in discovering other advanced cobalt-based materials. Recently, cobalt sulfides, including Co9S8,9 CoS2,21 CoS22 and Co1-xS,23 have been considered as other candidates for OER and ORR by exhibiting remarkable bifunctional catalytic activities as well as low overpotential and high recyclability. During the oxygen electrochemical reaction, the surface of cobalt sulfides would convert to Co(OH)2/Co3O4, of which the catalytic performance usually surpasses that of bulk Co(OH)2 and Co3O4, which accounts for their superior performance.12,24 In addition to the continuous search for new catalyst compositions, a practical consideration to



further enhance their catalytic properties lies in optimizing the morphology architectures of these catalysts.9,25 Since electrocatalysis is a surface-sensitive process, it is well accepted that the hierarchical rough nanostructure enables the catalysts with improved performance by means of enlarging the specific surface area as well as increasing the number of catalytic sites.26 For instance, Liu et al.24 successfully prepared Co9S8 hollow nanoplates via sulfuration and calcination method. The unique hollow structure enables Co9S8 with high rate performance as well as excellent long-term durability for water oxidation.24 Co1-xS porous microplates for OER were prepared by a solvothermal method. The porous architecture with high specific area offers efficient ion and gas transfer pathway, exhibiting low overpotential and durable stability.23 However, the catalysts mentioned above were generally synthesized via multi-step time-consuming approaches such as solvothermal and hydrothermal methods, which require high-pressure condition and the use of expensive organic ingredients, thus making their practical applications problematic.12,27,28 Even worse, among these preparation processes, polymeric binders are usually needed to prepare robust coatings, which may result in unsatisfactory catalytic performance owing to the underutilization of accessible catalytic sites, poor electric conductivity, and undesirable surface wettability.11,30 On the other hand, the abundant forms of cobalt sulfides also bring difficulties to the precise phase and composition control of these materials.31 Therefore, it is highly demanded while remain challenging to develop straightforward yet reliable methods to synthesize cobalt sulfides with the novel structure, controlled phase, and composition to investigate their bifunctional catalytic activity for the OER and ORR. In this work, we demonstrated a facile electrochemical deposition method to in-situ fabricate CoS nanosheets on carbon cloth (CoS) and a quick annealing treatment in air


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atmosphere were adopted to further enhance its performance (designated as CoS-A). This controlled synthesis strategy is favored by following two advantageous features. First, the simple and clean electrochemical deposition process avoids the use of the binder, organic surfactant and conductive materials mentioned above, fundamentally averting inferior conductivity, debilitate performance and high cost. Second, by carefully adjusting the electrochemical parameters, the precise structure and composition design of catalysts are unprecedently convenient. The as-prepared nanosheets exhibit rough surface, forming a hierarchical network which could not only maximize the specific surface area but also effectively facilitate the ion and gas transfer at the catalyst-electrolyte interface. Benefiting from the novel nanostructure and the excellent intrinsic properties of cobalt sulfides, the CoS-A electrode displays superior activity as well as remarkable long-term stability towards OER and ORR with low onset potential, high reaction current density, and negligible activity degradation, promising its large-scale use as stable, high-performance and low-cost catalyst for next-generation energy supplements.

2. Experimental section 2.1. Materials Synthesis. Synthesis of CoS and CoS-A. CoS nanosheets were synthesized via a clean electrodeposition technique with a three-electrode system at 25 °C. The carbon cloth (WOS1009, Taiwan) was adopted to be working electrode while the counter electrode was the platinum sheet (1 × 1 cm2) and the calomel electrode (SCE) was the reference electrode. Before electrodeposition, the carbon cloth was cut into small pieces (~ 1 × 2 cm2) and ultrasonically washed with acetone for 10 min at first and then ethanol and deionized



(DI) water were used in turn in the same way. After thoroughly washing, the carbon cloth was dried in the drying oven at 50 °C under air atmosphere for 2 h. To prepare the electrodeposition electrolyte, 5 mM Co(NO3)2 (Macklin Biochemical Co., Ltd, Shanghai) and 0.5 M CH4N2S (TU, Macklin Biochemical Co., Ltd, Shanghai) were mixed. The electrodeposition was performed by the CHI660D workstation, using a cyclic voltammetry (CV) technique. The scan was performed ranging from –1.2 V to 0.2 V for 20 cycles and the scan rate was 10 mV s–1. The as-prepared catalyst was washed with DI water and dried at 50 °C for 2 h, yielding CoS nanosheets (~1.8 mg, designated as CoS). To obtain CoS-A sample (~1.6 mg), the CoS sample was annealed at 300 °C in the furnace for 1 h at air atmosphere at the heating rate of 5 °C min–1 and then cool down in the furnace. Synthesis of Co(OH)2. To synthesize Co(OH)2, the same condition as mentioned above except the absence of TU was adopted. The sample was designated as Co(OH)2 (~2.0 mg) and stored for the following measurements. 2.2. Materials Characterization To acquire the phase information, X-ray diffraction was performed by a diffractometer (XRD, Bruker D8 Advanced, German) ranging from 10° to 70° at the scan rate of 4° min−1 with the Cu Kα radiation. A scanning electron microscope (SEM, Hitachi S-4800, Japan) was adopted to investigate the surface morphology of as-prepared samples. To obtain the elemental distribution information, the energy-dispersive X-ray spectrometer (EDX) was used. To further verify the microstructure of the catalysts, a transmission electron microscope (TEM, JEM-2100F, Japan) was used and the evaluation of the surface chemistry of the as-prepared materials was performed with the X-ray photoelectron spectroscopy


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(XPS, Escalab 250XI, ThermoFisher Scientific). The full scan energy was 100 eV and the high-resolution study was performed with energy of 20 eV. 2.3. Electrochemical Measurements. A typical three-electrode system was adopted to conduct the electrochemical measurements with the as-prepared catalysts to be the working electrode, and the other electrodes were as same as mentioned above. The electrochemical measurements were performed in KOH solution, which was bubbled with oxygen (purity of 99.99%, Air Liquide) for 30 min before all the test and the oxygen bubble was maintained during the experiment. As for the solution concentration, 0.1 M KOH was used for the ORR test while 1 M KOH was used for the OER test. For the convenience of comparation, all the potentials were adjusted to the reversible hydrogen electrode, also known as RHE. The calibration was performed according to the formula: E(RHE) = E(SCE) + 0.0591 × pH + 0.24. The OER and ORR performance of the as-prepared catalysts were acquired through linear sweep voltammetry (LSV) technique, ranging from 1.2 V to 2.0 V and 1.1 V to 0.2 V, respectively, and the scan rate was 5 mV s–1. The electrochemical impedance spectra test, also known as EIS, was tested at 1.6 V and the range was between 100 MHz and 10 kHz. The stability tests for OER and ORR were performed in 1 M KOH solution at 1.76 V and in 0.1 M KOH solution at 0.56 V for 12 h, respectively, using chronopotentiometry technique. The applied potential was chosen from the LSV curves.

3. Results and Discussion The CoS nanosheets in situ grown on carbon fiber were synthesized using a facile and clean electrochemical method with the electrolyte consisted of Co(NO3)2 and



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CH4N2S (TU). During the electrochemical process, TU is reacted with the OH– ions generated from reduced NO3– ions,11 forming S2– ions,32 which subsequently produce CoS with Co2+ ions. The reactions could be described as following: NO3– + H2O + 2e– → NO2– + 2OH–

(1)

SC(NH2)2 + 2OH– → S2– + OC(NH2)2 + H2O

(2)

Co2+ + S2– → CoS

(3)

The Co(OH)2 was synthesized as the same method as CoS except for the absence of TU. The reactions could be described as following: NO3– + H2O + 2e– → NO2– + 2OH–

(4)

Co2+ + 2OH– → Co(OH)2

(5)

The XRD patterns revealing crystal phase and purity of the two samples and pristine carbon cloth are presented in Figure 1. Both samples display a broad XRD peak at around 2θ=26.5°, which is the typical signal of the carbon cloth substrate. Before the annealing treatment, a series of diffraction peaks could be observed at 2θ = 29.9°, 43.5°and 53.2°, which are attributed to the (100), (102) and (110) planes of the CoS phase (JCPDS 65-0407). Besides, there are no peaks of other phases are seen, indicating the high purity of the sample. After the thermal treatment, the peaks of cobalt sulfides (CoS-A) become sharper and more intense without changing their positions, indicating the crystallinity improvement of the catalyst.


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Figure 1. X-ray diffraction (XRD) pattern of the cobalt sulfides before (CoS) and after thermal treatment (CoS-A) and the pristine carbon cloth. The surface morphologies of CoS and CoS-A were characterized by the SEM and are displayed in Figure 2. It is clearly shown that large amounts of CoS nanosheets are formed and vertically aligned onto the carbon cloth (Figure 2a and 2b). Importantly, the nanosheets are uniformly covered over the surface of carbon fibers in the carbon cloth, which plays a key role in ensuring the overall performance of electrode for electrochemical reactions. After the thermal treatment, the thickness of CoS-A nanosheets increases while the basic structure of the sample remains the same, indicating the excellent structural stability of the CoS-A nanosheets. It is worth noticing that lots of wrinkles appear on the surface of nanosheets owing to the rearrangement of CoS nanosheets during annealing,33 with which the interconnected nanosheets form a 3D hierarchical network (Figure 2d).



Figure 2. (a) SEM images of CoS and CoS-A (c) and high-resolution images of (b) CoS, (d) CoS-A. In order to further testify the presence of CoS on the surface of carbon cloth, the SEM elemental mapping was performed on the CoS-A sample. As shown in Figure 3, the 3D hierarchical network is composed of Co and S with a uniform distribution, which further confirming the successful synthesis of cobalt sulfide.

Figure 3. Elemental mapping of CoS-A on a single carbon fiber. To further verify the structure of CoS-A nanosheets, the CoS-A nanosheets were scraped from the carbon cloth by a scalpel and dispersed in the alcohol through


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ultrasonication, after which TEM characterization was performed. Under the TEM images, a 2D CoS-A nanosheet with a transparent feature could be seen, suggesting it has the relatively low thickness (Figure 4a). Besides, the nanosheet exhibits rough surface with some cracks, which could offer numerous high energetic defects for OER and ORR and thus facilitate the reaction kinetics.10,24 In addition, some nanosheets are seen to grow on the others, which also reveals the interconnected feature of CoS-A nanosheets, being consistent with SEM results depicted in Figure 2d. To further study the elemental distribution of Co and S in CoS nanosheets, the EDX elemental mapping was done for the CoS-A sample. As shown in Figure 4b and 4c, the EDX elemental mapping images of CoS-A also identify the presence of Co and S in the nanosheets, which are uniformly distributed all around the nanosheets, showing an almost coincident image. Besides, the Co and S mapping images show more intense points in the perpendicular area, indicating the net-like structure of CoS-A nanosheets. When studied under high-resolution TEM (HRTEM), clear lattice fringes are seen and the d-spacing values are measured to be 0.29 nm and 0.25 nm, which could be indexed to the (110) and the (102) lattice planes of CoS (Figure 4d), being consistent with the XRD results. On account of the XRD results shown in Figure 1 and HRTEM results of the nanosheets, CoS is the only existing crystalline material on the surface of the carbon cloth.

Figure 4. TEM image of (a) CoS-A and the corresponding EDX elemental mapping of (b) Co and (c) S in CoS-A, and the HRTEM image of (d) CoS-A.



Further investigation of the surface chemical information of CoS after annealing was performed with X-ray photoelectron spectroscopy (XPS) characterization. The full scan spectrum suggests the presence of elements including C, O, Co and S, of which the C should be ascribed to the carbon cloth substrate and O might be introduced because of the exposure to the air atmosphere. As shown in Figure 5, the Co 2p spectrum of the CoS-A sample displays two spin–orbit doublets and the main peaks are at the 780.6 and 796.7 eV, which demonstrates the existence of Co2+.15, 34 At the same time, two satellite peaks of Co 2p at the binding energy of 785.7 and 802.6 eV are also captured, with the shift of 4−5 eV from the main peaks. These two peaks are ascribed to the configuration 2d53d7 (ground state of Co2+) and 2d53d8L, which exhibits the charge transfer between the p-band of ligands and the d-band of metal.22,35 All of these testify the Co exists in the state of Co2+. As for the XPS image of S 2p, a doublet centered at 169.4 and 168.2 eV are attributed to the S 2p1/2 and S 2p3/2, respectively, in which the peak at 168.2 eV indicates the S exists in the valence state of −2.22,35 In summary, the XPS results reveal the coexistence of Co2+ and S2− in CoS-A sample, depicting the cobalt sulfide only exist in the form of CoS on the carbon cloth, which is well consistent with XRD and HRTEM results. Benefiting from the high purity of the as-prepared cobalt sulfide, a true measurement of the OER and ORR activity of CoS becomes possible, without being disturbed by the various forms of cobalt sulfides.31


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Figure 5. (a) XPS survey spectra and high-resolution spectra of (b) Co 2p, (c) S 2p of CoS after the heat treatment. The evaluation of the ORR performance of the as-prepared samples were done for CoS and CoS-A coated carbon cloth, and the pristine carbon cloth and electrodeposited Co(OH)2 were chosen as references. To acquire stable performance, cyclic voltammetry (CV) scans were performed for all the samples before tests, ranging from 1.1 V and 0.2 V for 20 cycles and the scan rate was 20 mV s−1. Typical CVs of CoS-A are shown in Figure 6a. In the linear sweep voltammogram (LSV) image, compared with Co(OH)2, CoS shows a positively shifted onset potential and higher current density, exhibiting its better electrocatalytic performance (Figure 6b). After the annealing, the catalytic activity of cobalt sulfide is further improved and the CoS-A displays the onset potential at 0.85 V, which is higher than that of CoS (0.79V) and Co(OH)2 (0.7 V). Moreover, the reduction current density of CoS-A reaches –



1.51 mA cm−2 at 0.2 V, surpassing that of CoS (–0.95 mA cm−2) and Co(OH)2 (–0.76 mA cm−2). Besides, the bare carbon cloth exhibits poor ORR activity, indicating the superb ability mainly rest with the CoS-A rather than the substrate. All the results mentioned above demonstrate that CoS-A has the superior ORR performance. As shown in Figure 6c and d, the OER electrocatalytic activities of the samples were tested in 1.0 M KOH electrolyte. As well as mentioned in ORR test, the catalysts were cycled for 20 times as the working electrode in the electrolyte to acquire the surface stability (Figure 6c). The LSV curve of the bare carbon cloth suggests its poor OER activity (Figure 6d). The OER potential of CoS-A to afford 10 mA cm−2 is 390 mV, which is apparently smaller than that of CoS (420 mV), indicating the performance improvement benefited from the thermal treatment. Although the overpotential of CoS-A at 10 mA cm−2 is slightly larger (10 mV larger) than that of Co(OH)2, its OER current density at 1.9 V clearly surpasses CoS (133 mA cm−2) and Co(OH)2 (89 mA cm−2), reaching a high value of 148 mA cm−2. Conclusively, benefiting from the high crystallization, excellent intrinsic OER activity, and 3D hierarchical structure, CoS-A exhibits superior OER performance in the evaluation.


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Figure 6. (a) CVs of CoS-A before the ORR test, (b) LSV curves for ORR tests of the bare carbon cloth, Co(OH)2, CoS and CoS-A, (c) CVs of CoS-A before the OER test and (d) LSV curves for OER tests of carbon cloth, Co(OH)2, CoS and CoS-A.

As a powerful tool to investigate reaction kinetics, electrical impedance spectroscopy (EIS) was performed for both CoS and CoS-A to study the electrode/electrolyte interface activity. As shown in the inset of Figure 7, the EIS results could be interpreted by means of the equivalent circuit,7 in which Rs, CPE, and Rct stands for solution resistance, constant phase element, and charge transfer resistance, respectively. As shown in Figure 7, Rct of samples could be directly reflected by the diameter of the semicircle in the Nyquist plots.1 The carbon cloth shows the biggest Rct of 14 Ω while the Rct considerably reduces to 4.5 Ω after the coupling with CoS, implying a higher electrical conductivity as well as quicker faradaic process at the CoS/electrolyte interface. As for the CoS-A, it shows a smaller



charge transfer resistance (3.5 Ω) than the CoS, indicating a better electrical performance which is ascribed to higher crystallization after the thermal treatment (Figure 1). The EIS result fits well with the ORR and OER results, demonstrating that the superior catalytic activity of CoS-A is attributed to the quicker catalytic kinetics.

Figure 7. EIS measured on CoS and CoS-A at 1.66 V and the equivalent circuit is shown in the inset. As another important feature of the catalyst, the operational stability of CoS-A was investigated through chronopotentiometry measurement for 12 h. As shown in Figure 8, after the measurement, the CoS-A displays almost 100% of the initial current density for both the OER and ORR, indicating the excellent long-term stability of CoS-A for two different half-reactions. Besides, the OER current density is increasing during the measurement, which is probably due to the surface wetting and the activation of the electrode.36 Such a stable long-term performance of CoS-A is ascribed to the enhanced crystallinity of CoS-A after thermal treatment, which may play a role to offer strong electrochemical coupling for the Co(OH)2 and Co3O4 derivatives formed during the OER and ORR. In addition, the clean and in-situ synthesis electrodeposition technique avoids the interference of binder, surfactant,


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conductive carbon and other additives, ensuring the robust adherence and effective contact of CoS nanosheets on the carbon cloth, further improving the durability.

Figure 8. Chronopotentiometry measurement of CoS-A for ORR at 0.56 V (vs. RHE) and OER at 1.76 V (vs. RHE). 4. Conclusion Conclusively, we successfully synthesized CoS nanosheets with 3D hierarchical structure in-situ grown on the carbon cloth, which could directly serve as an integrated electrode while avoiding the use of the surfactant, binder, and other additives. Besides, the high purity of the electrodeposited materials allows a true performance measurement of the catalysts, which highly increases the credibility of the results. The CoS nanosheets are interconnected with each other, with wrinkles appear on the surface of nanosheets due to the post thermal treatment. Moreover, compared with Co(OH)2 and CoS, the CoS-A exhibits superior catalytic activity with higher OER and ORR current density response (148 mA cm–2 at 1.9 V, –1.51 mA cm–



2

at 0.2 V) owing to its excellent intrinsic electrocatalytic activity as well as the

enhanced crystallinity, which could be verified in the electrochemical measurements and XRD results. The high specific surface area, as well as the stable structure of the CoS-A, bring about excellent bifunctional activity and considerable durability, which provides new ideas for the design of other catalysts. Corresponding Author E-mail: [email protected] (C.Z.). ORCID Cheng Zhong: 0000-0003-1852-5860 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation for Excellent Young Scholar (No. 51722403), National Natural Science Foundation of China (Nos. 51771134 and 51571151), National Natural Science Foundation of China and Guangdong Province (No. U1601216), the National Youth Talent Support Program, and Tianjin Natural Science Foundation (No.16JCYBJC17600). REFERENCES (1) Shen, M.; Ruan, C.; Chen, Y.; Jiang, C.; Ai, K.; Lu, L., Covalent Entrapment of Cobalt-Iron Sulfides in N-Doped Mesoporous Carbon: Extraordinary Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions. ACS Appl. Mater.

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Table of Contents 245x177mm (300 x 300 DPI)



Figure 1. X-ray diffraction (XRD) pattern of the cobalt sulfides before (CoS) and after thermal treatment (CoS-A) and the pristine carbon cloth. 289x202mm (150 x 150 DPI)


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Figure 2. (a) SEM images of CoS and CoS-A (c) and high-resolution images of (b) CoS, (d) CoS-A. 228x160mm (300 x 300 DPI)



Figure 3. Elemental mapping of CoS-A on a single carbon fiber. 289x78mm (300 x 300 DPI)


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Figure 4. TEM image of (a) CoS-A and the corresponding EDX elemental mapping of (b) Co and (c) S in CoSA, and the HRTEM image of (d) CoS-A. 288x66mm (300 x 300 DPI)



Figure 5. (a) XPS survey spectra and high-resolution spectra of (b) Co 2p, (c) S 2p of CoS after the heat treatment. 289x220mm (300 x 300 DPI)


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Figure 6. (a) CVs of CoS-A before the ORR test, (b) LSV curves for ORR tests of the bare carbon cloth, Co(OH)2, CoS and CoS-A, (c) CVs of CoS-A before the OER test and (d) LSV curves for OER tests of carbon cloth, Co(OH)2, CoS and CoS-A. 289x209mm (300 x 300 DPI)



Figure 7. EIS measured on CoS and CoS-A at 1.66 V and the equivalent circuit is shown in the inset. 289x202mm (300 x 300 DPI)


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Figure 8. Chronopotentiometry measurement of CoS-A for ORR at 0.56 V (vs. RHE) and OER at 1.76 V (vs. RHE). 289x202mm (300 x 300 DPI)


In-Situ Electrodeposition of Cobalt Sulfide Nanosheet Arrays on

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