Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide

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Energy, Environmental, and Catalysis Applications

Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide/Heteroatom (N, S)-Doped Double-Layered Carbon Tubes as an Efficient Bifunctional Oxygen Electrocatalyst Chencheng Hu, Jin Liu, Juan Wang, Wanxin She, Junwu Xiao, Jiangbo Xi, Zheng-Wu Bai, and Shuai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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ACS Applied Materials & Interfaces

Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide/Heteroatom (N, S)-Doped Double-Layered Carbon Tubes as an Efficient Bifunctional Oxygen Electrocatalyst

Chencheng Hu,a,ǁ Jin Liu,b,ǁ Juan Wang,a Wanxin She,a Junwu Xiao,a,* Jiangbo Xi,b,* Zhengwu Bai,b and Shuai Wanga,* a

Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education,

Hubei Key Laboratory of Material Chemistry and Service Failure, Department of Chemistry and Chemical Engineering, Huazhong University of Science & Technology, Wuhan 430074, PR China

b

School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan

430205, PR China

*E-mail: [email protected], [email protected], chmsamuel@mail. hust.edu.cn ǁ

They contribute equally to this work.

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Abstract: It’s a critical challenge of constructing efficient precious-metal-free bifunctional oxygen electrocatalysts for fuel cell and metal-air batteries via structural and component engineering. Herein, one-dimensional mesoporous double-layered tubular structure, where Co9S8 nanocrystals are incorporated into nitrogen, sulfur co-doped carbon, is successfully synthesized via the coordinated-assisted polymerization and scarifical template methods. The double-layered tubular structure provides for large electrochemically active surface area and promotes fast mass transfer. Cobalt oxides/oxyhydroxides, which are evolved from the sulfides during the catalytic processes, as the main active sites efficiently catalyze the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), in cooperation with the Co-N-C and heteroatom-induced active sites. Hence, it demonstrates excellent bifunctional electrocatalytic activity with the overvoltage between the OER potential at 10 mA cm-2 (E10) and ORR half-wave potential (E1/2) of 0.707 V, which is superior to most of precious-metal-free bifunctional oxygen electrocatalysts reported recently, as well as the state-of-art Pt/C and RuO2 catalysts. Keywords: coordination-assisted polymerization, transition metal chalcogenides, doping carbon, double-layered tubular structure, oxygen electrocatalysis


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Introduction

The major challenge in developing fuel cells and metal-air batteries lies in the requirement of high overpotential for catalyzing the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).1-2 To date, platinum (Pt)-based catalysts are thought as the state-of-art ORR catalysts, while they are easily oxidized into the insulating platinum oxides at high anodic potential, resulting in exhibiting poor OER activity.3-4 Ruthenium and iridium oxides manifest colossal promotion towards OER but inconspicuous effect towards ORR.5-6 Besides that, natural scarcity and high cost terribly impede the widespread applications. Therefore, extensive efforts are devoted to developing efficient precious-metal-free OER/ORR bifunctional electrocatalysts, including transition metal oxides/hydroxides,7-12 sulfides,13-14 carbides,15 nitrides,16 phosphides,17-18 and doping carbon.19-21 Cobalt sulfides with rich valence state and exceptional 3d electronic configurations have the structural and compositional semblance to the active centre of the oxygen-evolving complex, resulting in showing the OER/ORR catalytic activities.22-28 The significant progress has made in recent years, while the catalytic activity was still severely limited due to the self-accumulation and intrinsically poor electric conductivity. In response, a common strategy is adopted for fully exposing catalytic active sites and facilitating the ion diffusion via optimizing the structures, and enhancing the electric conductivity and structural stability via dispersing it on conducting supports (carbon nanotubes and graphene). However, cobalt sulfide nanocrystals, which are loaded on graphene sheets via the weak interactions,24, 29 may be self-aggregated and easily detached from the supports during the catalytic process,



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especially for the gas bubbling/evolution process. By contrast, it can be conquered via the direct sulfidation of metal-organic frameworks (MOFs), since cobalt sulfide nanocrystals are incorporated into carbon supports.30 Despite numerous studies, the catalytic activities aren’t yet comparable to that of precious-metal catalysts, since the pore structure of MOFs is collapsed during the pyrolysis process to severely influence the mass transfer ability and limit the exposed active site number. It’s proposed to be improved via two approaches. One is to construct the promising carbon supports, which have large surface area, rich pore structure, and high crystallinity degree to provide for facile conductive network and facilitate fast mass transfer. Another is to decrease the crystal sizes of cobalt sulfides for exposing efficient catalytic sites as much as possible, and enhance the intrinsic activity via bridging the electron cloud migration between carbon supports and cobalt sulfides. Herein, as a proof-of-concept experiment, we have proposed coordinated-assisted polymerization

and

scarifical

template

methods

to

successfully

prepare

cobalt

sulfides/nitrogen, sulfur co-doped carbon composite with one-dimensional mesoporous double-layered tubular structure. Owing to large electrochemically active surface area, rich mesopores, good electric conductivity, and efficient catalytic active sites, it demonstrates outstanding bifunctional catalytic activities with the ORR half-wave potential of 0.890 V vs. RHE and OER overpotential of 0.367 V at 10 mA cm-2.

Experimental section Synthesis of the double-layered silica tube templates


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In a typical synthesis, 0.32 g of N-myristoyl-D-alanine sodium salt (C14-D-AlaS) was dissolved in 24 mL of de-ionized water. Then, 8 ml of HCl aqueous solution (0.1 M) was added to C14-D-AlaS solution under vigorous stirring for 1 h at 5 oC. 1.44 g of tetraethyl orthosilicate (TEOS) and 0.23 g of 3-aminopropyl-triethoxysilane (APES) were subsequently added into above solution with slow stirring for 15 min. After 24 h reaction at 5 oC without stirring, the products were collected by the filtration and dried at 80 °C. Finally, the double-layered silica tubes are obtained after the exhaustive solid-liquid extraction, which was carried out overnight using 1% 2-Aminoethanol in 15% H2O/ethanol mixture (v/v) at the boiling temperature to remove the C14-D-AlaS. Preparation of Co9S8/nitrogen, sulfur co-doped double-layered carbon tubes (Co9S8/N, S-DLCTs) 0.05 g of the double-layered silica tubes were ultrasonically dispersed in dopamine hydrochloride aqueous solution (100 mL, 2 mg mL-1) to form the solution A. 0.12 g of CoCl2·6H2O was dissolved in 5 ml of ammonia to form a brown transparent solution B. Solution B was subsequently added into solution A with vigorous stirring for 12 h at room temperature. The black products, namely Co-PDA/SiO2, were washed by de-ionized water for three times, and dried up using the freeze-drying technique. Subsequently, it’s pyrolyzed at 500 oC for 1 h with a rate of 5 oC min-1 under flowing Ar (100 sccm), forming the cobalt/carbon/silica (Co/C/SiO2). The Co/C/SiO2 was then etched in HF aqueous solution (5.0 %) at room temperature or NaOH aqueous solution (6.0 M) at 45 oC for 4 h with continuous stirring, forming the cobalt/double-layered carbon tubes (Co/DLCTs)X, where the X indicated



the chemical reagents (HF or NaOH) for etching the silica templates. The precipitates were collected by centrifugation, and dried up via using the freeze-drying method. Subsequently, ~ 16 g of thiourea were placed at the intake end of the tube furnace as sulfur sources, and ~ 0.1 g of the Co/DLCTs were sulfurized at 700 ~ 1000 oC for 1 h with a heating rate of 5 oC min-1 and 100 sccm of Ar flow, forming the Co9S8/nitrogen and sulfur co-doped double-layered carbon tubes (Co9S8/N, S-DLCTs). The Co9S8/nitrogen, sulfur co-doped double-layered carbon tubes/silica (Co9S8/N, S-DLCTs/SiO2) was formed as the Co/C/SiO2 replaced of the Co/DLCTs were pyrolyzed at 900

o

C under the similar experimental condition. The

cobalt/nitrogen-doped double-layered carbon tubes (Co/N-DLCTs) were obtained as the Co/DLCTs was pyrolyzed at 900 oC under the mixed atmosphere of NH3 (50 sccm) and Ar (100 sccm). Synthesis of metal-free nitrogen, sulfur co-doped double-layered carbon tubes (N, S-DLCTs) 0.05 g of the double-layered silica tubes were dispersed in dopamine hydrochloride solution (100 mL, 2 mg mL-1). Then, 5.0 ml of ammonia was added into above solution with vigorous stirring for 12 h. The products were collected via the centrifugation, and dried up via the freeze-drying. Finally, the products were sulfurized at 900 oC for 1 h with a heating rate of 5 oC min-1 under flowing Ar (100 sccm) using thiourea (~ 16 g) as sulfur sources, forming the nitrogen, sulfur co-doped double-layered carbon tubes (N, S-DLCTs). Preparation of Co9S8/nitrogen, sulfur co-doped single-layered carbon tubes (Co9S8/N, S-SLCTs)


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0.24 g of CoCl2·6H2O and 0.20 g of urea were dissolved in 80 ml of deionized water, and then transferred into a 100ml Teflon-lined stainless-steel autoclave and heated at 100 oC for 12h. The samples were then collected by the centrifugation, and dried up at 60 oC. Finally, the samples replaced of the double-layered silica tube templates were added during the preparation process of the Co9S8/N, S-DLCTs to form the Co9S8/nitrogen, sulfur co-doped single-layered carbon tubes (Co9S8/N, S-SLCTs). Preparation of Co9S8/nitrogen, sulfur co-doped hollow carbon spheres (Co9S8/N, S-HCSs) 10 mL of tetraethylorthosilicate (TEOS) was slowly dropped into the mixed solution composed of anhydrous ethanol (100 mL), de-ionized water (30 mL), and ammonia solution (10 mL, 25 wt.%) with continuously stirring for 4 h, to form the silica sphere templates. The products were then washed with anhydrous ethanol, and dried up. Finally, the silica sphere templates replaced of the double-layered silica tubes were added during the preparation process of the Co9S8/N, S-DLCTs to form the Co9S8/nitrogen, sulfur co-doped hollow carbon spheres (Co9S8/N, S-HCSs). Materials Characterization Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed on Tecnai G2 20 and Nova NanoSEM 450 microscopes, respectively. Scanning transmission electron microscopy (STEM) images coupled with electron energy loss spectroscopy (EELS) analysis were measured on Titan G2 60-300 Probe Cs Corrector UHR-STEM. X-ray diffraction (XRD) was carried out on a Philips PW-1830 X-ray diffractometer with Cu kα irradiation (λ = 1.5406 Å) and step size of 0.05 o. N2 sorption



isotherms were measured on Micrometrics ASAP 2020 instrument at 77 K for analyzing the specific surface area and pore volume. X-ray photoelectron spectroscopy (XPS) was investigated on a Perkin-Elmer model PHI 5600 XPS system with Kα radiation (1486.6 eV) and a resolution of 0.3-0.5 eV. The microwave plasma-atom emission spectrometer (MP-AES, Agilent 4100) is used for determining the cobalt content. The C, H, N, and S contents are obtained from the element analyser (Vario Micro cube, Elementar). Electrochemical Characterizations Electrochemical characterizations were carried out on the rotating ring-disk electrode (RRDE-3A) system and potentiostat (IVIUM Verter) in a three-electrode configuration, where the reference electrode (RE) and counter electrode (CE) were Hg/HgO (1.0 M KOH) and carbon nanorod , respectively, and the electrolyte was 0.1 M KOH aqueous solution. The working electrode (WE) with the mass loading of ~ 0.40 mg cm-2 was prepared via dropping 5 µL of the catalyst ink, which were preapred via dispersing 5 mg of the catalysts into 10 µL of Nafion (5 wt%) and 0.49 mL of N, N-Dimethylformamide (DMF), on the glassy carbon electrode (GCE). For comparison, Pt/C and RuO2 electrodes with the mass loading of ~ 0.40 mg cm-2 were prepared via the similar procedures using commercial Pt/C (20 wt%) and RuO2 catalysts. The electrode potential is reported relative to the reversible hydrogen electrode (RHE) potential based on the following equation: ERHE = E(Hg/HgO) + φ(Hg/HgO) + 0.0591 × pH V. The ORR catalytic activity was measured by using linear sweep voltammograms (LSVs) and cycle voltammetry (CV) techniques, which were conducted at N2 or O2-saturated 0.1 M


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KOH solution, corrected by iR-compensation. The currents were normalized in terms of geometric area (mA cm-2). The number of electron transferred (n) at the electrode potentials of 0.6-0.8 V vs. RHE were obtained from the Koutecky-Levich (J-1 vs. ω-1/2) plots according to the following equation:

1/ J = 1/ J k + 1/ J L = 1/ J k + 1/( Bω 1/ 2 )

(1)

B = 0.62nFCo Do 2 / 3ν −1/ 6

(2)

J k = nFkCo

(3)

Where J, JL, and Jk are the measured current density, diffusion-limiting current density, and kinetic current density (mA cm-2), respectively, F is the Faraday constant (96485 C mol-1), ω is the angular velocity, n is the electron transfer number, Co is the O2 bulk concentration (1.2 × 10-6 mol mL-1), ν is the kinematic viscosity of the electrolyte (0.01 cm2 s-1), Do is the O2 diffusion coefficient (1.9 × 10-5 cm2 s-1), and k is the electron-transfer rate constant. Tafel plots (E vs. Log (Jk)) were obtained from the LSV curves at 1600 rpm, where the Jk was obtained from the mass-transport correction of RDE:

J k = ( J × J L ) /( J L − J )

(4)

For the RRDE measurement, the disk potential was scanned from 1.1 to 0.3 V vs. RHE at 5 mV s-1, and the ring potential was set as 1.5 V vs. RHE. The HO2-% and electron transfer number (n) were calcuated according to the followed equations:

HO2- %=(200 × Ir / N )/(Id + Ir / N )

(5)

n = (4 × Id ) /(Id + Ir / N )

(6)

Where Id and Ir are the disk and ring currents (mA), respectively. N is the current collection efficiency (0.36) of the Pt ring.



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The OER catalytic activities were evaluated in O2-saturated 0.1 M KOH solution by using LSVs techniques, corrected by iR-compensation. The overpotential at 10 mA cm-2 (η10) was calculated as follow:

η=E10 -1.23

(7)

where E10 is OER polarization potential relative to the RHE at 10 mA cm-2, and the O2/H2O equilibrium potential is suggested as 1.23 V. Long-term durability was measured using the chronoamperometry (CA) method polarized at 1.60 V vs. RHE without iR-compensation. The electrical impedance spectroscopy (EIS) was measured at open-circuit voltage in a frequency range from 100 kHz to 0.01 Hz at 5 mV. CV curves were measured in quiescent solution by sweeping the potential across the non-faradaic region (0.913-1.013 V vs. RHE) at the scan rates of 2, 4, 6, 8, and 10 mV s-1 for determining the Electrochemical double-layer capacitance (Cdl) and electrochemically active surface area (ECSA), since the measured current in the non-Faradaic potential region is supposed to be ascribed to double-layer charging. The Cdl was obtained from the double-layer charging current (ic, mA cm-2) and scan rate (ν, mV s-1) on the basis of the following equation: Cdl = ic / v

(8)

Results and discussion Scheme 1 shows the fabrication route of the Co9S8/N, S-DLCTs. Firstly, the double-layered silica tube template is synthesized using the modified surfactant-templated


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method.31 Then, dopamine hydrochloride (DA) molecules are polymerized and encapsulated at the silica templates via the coordination-assisted polymerization process, to form the cobalt-polydopamine/silica (Co-PDA/SiO2) composite. Subsequently, it’s pyrolyzed into the cobalt/carbon/silica (Co/C/SiO2), followed by the removal of silica templates to form the cobalt/double-layered carbon tubes (Co/DLCTs). The Co/DLCTs is finally evolved into the Co9S8/N, S-DLCTs after the sulfidation process. The structures and crystal phases of the silica templates, Co-PDA/SiO2, Co/C/SiO2, and Co/DLCTs are characterized by using electron microscopy and XRD techniques. As revealed by Figure 1a and 1b, the silica templates exhibit porous double-layered tubular structure. After the coordination-assisted polymerization process in the presence of [Co(NH3)6]2+, the double-layered tubular structure is conserved, and an amorphous Co-PDA layer is uniformly attached at the silica templates (Figure S1). By contrast, in the absence of DA molecules, the hydrolysis products (Co(OH)2) of the [Co(NH3)6]2+ are randomly deposited on and out of the double-layered silica tube templates (Figure S2). Hence, [Co(NH3)6]2+ ions have strong interaction with PDA to form the organic-inorganic (Co-PDA) composites during the coordination-assisted polymerization process.32 When the Co-PDA/SiO2 is thermally decomposed into the Co/C/SiO2 under inert atmosphere, a few black dots ascribed to cobalt nanocrystals are appeared in the TEM images (Figure S3). It’s further illustrated by the appearance of the diffraction peaks of metallic cobalt (2θ = 44.3o, 51.6 o, and 75.9 o) in the XRD pattern (Figure S4). After the template removal process using HF aqueous solution, the double-layered tubular structure and metallic cobalt phases are remained (Figure S4 and S5).



However, high-resolution Co 2p XPS spectrum in Figure S6 shows that, besides the Co0 (778.9 eV), Co3+ (780.8 eV) and Co2+ (782.4 eV) species are observed in the (Co/DLCTs)HF, which should be due to the surface oxidation of cobalt nanocrystals under air atmosphere and/or during the template removal process. During the subsequent sulfidation process, the Co/DLCTs are evoloved into cobalt sulfides/nitrogen, sulfur co-doped double-layered carbon tubes. As seen from the XRD pattern (Figure S7a), the cobalt sulfide phases are closely related to the sulfidation temperature. Cobalt sulfide is existed in the formula of CoS (PDF: 75-0605) at 700 oC, and is transformed into Co9S8 (PDF: 86-2273) with increasing the temperature to 800-1000 oC. The morphology of the (Co9S8/N, S-DLCTs)HF formed at the sulfidation temperature of 900 oC can be clearly seen from SEM, TEM and STEM images. It shows the double-layered tube structure with the wall thickness of ~ 20 and 50 nm, where Co9S8 nanocrystals with average particle size of ~ 9 nm are almost embedded into carbon matrix (Figure 1c and 1d). HRTEM image in Figure 1e shows that the lattice fringes with the distances of 0.299 and 0.339 nm are corresponding to the (311) crystal face of Co9S8 and (002) plane of graphite, respectively. The STEM and EELS images in Figure 1f-1j reveal that nitrogen, sulfur, and cobalt elements are uniformly distributed in the carbon matrix, revealing that, cobalt nanocrystals are transformed into cobalt sulfides, and meanwhile nitrogen and sulfur species are successfully introduced into carbon structure during the sulfidation process. The C, N and S contents determined by the elemental analyser are estimated as 65.7 wt.%, 7.5 wt.%, and 5.5 wt.%, respectively (Table S1), and the cobalt content measured by MP-AES is 11.7 wt.%.


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As comparison, the (Co9S8/N, S-DLCTs)NaOH is obtained by using NaOH aqueous solution to etch the silica templates. It has the similar crystal phase and structure with the (Co9S8/N, S-DLCTs)HF, while more Co9S8 nanocrystals are appeared into carbon matrix (Figure S7b and S8). It’s further illustrated by that the cobalt content (18.7 wt.%) of the (Co9S8/N, S-DLCTs)NaOH is higher than that of the (Co9S8/N, S-DLCTs)HF, which can be ascribed by that HF can partially dissolve cobalt species durig the template removal process. Difference from that in the (Co9S8/N, S-DLCTs)HF and (Co9S8/N, S-DLCTs)NaOH, Co9S8 nanocrystals are partially located at the surface of the Co9S8/nitrogen, sulfur co-doped double-layered carbon tubes/silica (Co9S8/N, S-DLCTs/SiO2) as the silica templates aren’t etched (Figure S9), suggesting that the template removal not only increases the exposed surface area, but also provides free-space for incorporating cobalt sulfide nanocrystals. Moreover, the coordination-assisted polymerization method can also be explored for synthesizing Co9S8/nitrogen, sulfur co-doped hollow carbon spheres (Co9S8/N, S-HCSs) using the silica sphere templates (Figure S10), and nickel sulfide/nitrogen, sulfur co-doped double-layered carbon tubes as [Ni(NH3)6]2+ is replaced of [Co(NH3)6]2+ during the preparation process (Figure S11). The surface area and pore volume of the (Co9S8/N, S-DLCTs)HF and (Co9S8/N, S-DLCTs)NaOH are obtained from N2 sorption isotherms using Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) models, as shown in Figure 2a. They exhibit the Type II physisorption isotherm, and the BET surface area and pore volume are 404.9 m2 g-1 and 0.391 cm3 g-1 for the (Co9S8/N, S-DLCTs)HF, much larger than that of (253.3 m2 g-1 and 0.177 cm3



g-1) for the (Co9S8/N, S-DLCTs)NaOH. This is because that, compared with NaOH aqueous solution, HF can partially etch cobalt species during the template removal process to increase the surface area and pore volume, resulting in facilitating fast mass transpor and exposing the active sites. The hysteresis loop is clearly appeared at a relative pressure of 0.45 ~ 1.0, suggesting the existence of the mesopores, and the pore size distribution curves demonstrate that the pore size is centred at ~ 6.4 nm (Inset of Figure 2a), which are proposed to be originated from the removal of the template and cobalt species. Figure 2b shows Raman spectra of the Co/DLCTs and Co9S8/N, S-DLCTs. Two intense peaks at the wavenumber of approximately 1350 and 1590 cm-1 are observed, which are corresponding to the graphitic D and G bands, respectively. The ratio of D and G intensities (ID/IG) of the (Co9S8/N, S-DLCTs)HF and (Co9S8/N, S-DLCTs)NaOH is achieved to 1.01 and 0.99, respectively, higher than 0.93 for the (Co/DLCTs)HF and 0.91 for the (Co/DLCTs)NaOH, owing to the successful doping of N and S atoms in the carbon structure after the sulfidation process. Besides the D and G peak, a weak peak at the wavenumber of ~ 670 cm-1 is attributed to Co9S8,33 confirming the successful transformation into cobalt sulfides during the sulfidation process. The ORR catalytic activity is first evaluated using cyclic voltammograms (CVs) method. Notably, compared with that in N2-saturated electrolyte, a distinct reduction peak is observed in O2-saturated electrolyte (Figure S12), demonstrating that the catalysts possess the ORR activity. The influence of the sulfidation temperature on the catalytic activity is measured, and the results are shown in Figure 3a. When the sulfidation temperature increases from 700


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to 800 oC, the doping content determined by the XPS spectra decreases from 15.5 to 6.6 at.% for nitrogen species and from 1.9 to 1.3 at.% for sulfur species (Figure S13), while the ORR activity is obviously improved, indicating that heteroatom doping-induced active sites aren’t proposed as the main ORR active sites in the (Co9S8/N, S-DLCTs)HF. With further increasing the sulfidation temperature to 900 oC, the ORR catalytic activity is somewhat enhanced, in accompany with decreasing the nitrogen and sulfur contents to 4.1 and 0.8 at.% (Figure S13), confirming that cobalt sulfides, especially for Co9S8, have higher intrinsic activity towards the ORR than heteroatom doping-induced active sites. When the sulfidation temperature is achieved to 1000 oC, although the crystal phase of cobalt sulfide is still Co9S8 and the graphitization degree is increased for improving the electric conductivity, the catalytic activity is slightly deteriorated due to the decrease of the defect-induced active site number, as seen from that the ID/IG value decreases from 1.01 at 900 oC to 0.95 at 1000 oC. The most optimized (Co9S8/N, S-DLCTs)HF catalyst formed at the sulfidation temperature of 900 oC exhibits the half-wave potential (E1/2) of 0.890 V vs. RHE, and diffusion-limited current density of 6.20 mA cm-2 at + 0.40 V vs. RHE (J+0.40), which are superior to that of the state-of-art Pt/C (0.871 V vs. RHE for E1/2, 5.13 mA cm-2 for J+0.40) and other precious-metal-free electrocatalysts reported recently (Table S2).10, 12-14, 23, 25, 27, 34-37 Hence, in the following discussion, the sulfidation temperature is fixed at 900 oC for all catalysts. Cobalt sulfide and heteroatom-doped carbon are reported to be efficiently catalyzed the ORR process, and thus it’s essential to give insight into their catalytic roles in the (Co9S8/N, S-DLCTs)HF. Cobalt/nitrogen-doped double-layered carbon tubes (Co/N-DLCTs) pyrolyzed



from the (Co/DLCTs)HF under NH3 atmosphere, and cobalt-free N, S-DLCTs show the similar structure with the (Co9S8/N, S-DLCTs)HF (Figure S4 and S14), and are referred for comparison. The (Co9S8/N, S-DLCTs)HF exhibits much better catalytic activity than the N, S-DLCTs and Co/N-DLCTs with the Co-N-C and heteroatom (N and/or S)-induced active sites (Figure 3b), suggesting that, in the (Co9S8/N, S-DLCTs)HF, Co9S8 species play a vital role in determining the catalytic activity instead of Co-N-C and heteroatom doping. Moreover, the ORR activity is obviously deteriorated as the (Co9S8/N, S-DLCTs)HF are etched in HCl aqueous solution to form the (Co9S8/N, S-DLCTs)HF-HCl (Figure S15), while it’s still superior to that of the N, S-DLCTs, confirming that the ORR activity of the (Co9S8/N, S-DLCTs)HF is mainly determined by Co9S8 active sites, and a small contribution from Co-N-C and heteroatom doping. To explore the influence of the structural characteristics on the ORR catalytic activity, the Co9S8/nitrogen, sulfur co-doped single-layered carbon tubes (Co9S8/N, S-SLCTs) with the wall thickness of ~ 140 nm and cobalt content of 13.7 wt.% is formed via the similar fabrication route of the (Co9S8/N, S-DLCTs)HF using cobalt carbonate hydroxide nanorod templates for comparison (Figure S7b and S16). The ORR polarization curves in Figure 3c show that the ORR activity of the Co9S8/N, S-SLCTs is much inferior to that the (Co9S8/N, S-DLCTs)HF, although they both have the similar composition. Hence, in comparison with the single-layer one, the double-layered tubular structure has larger electroactive area and higher pore volume for being favourable for exposing catalytic active sites and improving the mass transfer ability. Moreover, It’s gleaned from the Tafel plots that the double-layered tubular


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structure exhibits smaller Tafel slope than the single-layered one (Figure 3d), illustrating better ORR catalytic kinetics. More importantly, the Tafel slope of the (Co9S8/N, S-DLCTs)HF is even smaller than that of the Pt/C catalyst. The number of electron transferred and H2O2 yield under the catalysis of the (Co9S8/N, S-DLCTs)HF are investigated using the RRDE and RDE techniques. Calculating from the disk and ring currents (Figure 3c), the H2O2 yield under the catalysis of the (Co9S8/N, S-DLCTs)HF is 8.9 ~ 13.2 %, and the electron transfer number is 3.78 ~ 3.92 as the potential is scanned negatively from 0.80 to 0.40 V vs. RHE (Figure 3e). The electron transfer number also can be obtained from the RDE curves according to the Koutecky- Levich (K-L) equations, and is achieved to 3.86 ~ 3.98 at the potentials of 0.80 ~ 0.60 V vs. RHE (Figure 3f). Hence, the RRDE results and K-L plots both demonstrate a four-electron-dominant ORR process under the catalysis of the (Co9S8/N, S-DLCTs)HF. Moreover, the Jk obtained from the intercepts of the K-L plots is 26.2 mA cm-2 at 0.80 V vs. RHE for the (Co9S8/N, S-DLCTs)HF, even higher than that of the Pt/C catalyst (17.9 mA cm-2), confirming excellent ORR catalytic kinetics. The catalytic stability and methanol crossover effect are measured using the chronoamperometry and LSV methods. As revealed by Figure S17a and S17b, after 50000 s continuous operation at 0.50 V vs. RHE, 93.4 % of the initial current is retained, and the E1/2 just shifts negatively for 11 mV for the (Co9S8/N, S-DLCTs)HF, while the retention ratio decreases to 77.6 % for the Pt/C catalyst. With regard to the methanol crossover effect, ~ 94.8 % of the initial current is retained, and the E1/2 shifts negatively for 10 mV for the (Co9S8/N, S-DLCTs)HF as 4.0 M methanol is injected at the polarizaition potential of 0.5 V vs. RHE,



while the retention ratio is as low as ~ 69.9 % for the Pt/C (Figure S17c and S17d). Hence, compared with the state-of-art Pt/C catalyst, the (Co9S8/N, S-DLCTs)HF shows more excellent ORR catalytic performance including the catalytic activity, durability, and methanol tolerance. The OER catalytic activity is also examined using RDE technique in O2-statured 0.1 M KOH electrolyte. Notably, it’s seen from the OER polarization curves in Figure 4a that, the OER activities of the N, S-DLCTs and Co/N-DLCTs are much worse than that of the composite catalysts, such as, (Co9S8/N, S-DLCTs)HF, (Co9S8/N, S-DLCTs)NaOH, and Co9S8/N, S-SLCTs, revealing that the intrinsic OER actvity of Co9S8 species is superior to that of the Co-N-C and heteroatom doping, in accordance with that for the ORR. Besides the intrinsic activity, the structural characteristics also play an important role in influencing the OER catalytic activity. Figure 4b and 4c reveal that the (Co9S8/N, S-DLCTs)HF demonstrates much lower the required over-potential at 10 mA cm-2 (η10) and Tafel slope than the Co9S8/N, S-SLCTs, which is due to the unique features of the double-layered tubular structure. The structural characteristics are further investigated via the ECSA and EIS. In comparison with that (59.7 mF cm-2) of the single-layered one, the double-layered tubular structure has much higher double layer capacitance (Cdl, 93.7 mF cm-2) in the non-Faradic region (Figure 4d and S18), and thus provides for larger ECSA. The (Co9S8/N, S-DLCTs)HF and (Co9S8/N, S-DLCTs)NaOH both have the double-layered tubular structure, while the HF etching process increases the surface area and pore volume due to the partial dissolution of cobalt species, resulting in exposing larger ECSA and more active sites


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(Figure 4d and S18), in comparison with using NaOH aqueous solution. Hence, the OER activitiy of the (Co9S8/N, S-DLCTs)NaOH is worsen than that of the (Co9S8/N, S-DLCTs)HF, even the Co9S8/N, S-SLCTs. Figure S19 shows the EIS and corresponding equivalent circuit composed of the solution resistance (Rs), charge-transfer resistance (Rct), double-layer capacitance (Cdl), as well as Warburg resistance (W). The Rct is as low as 23.2 ohm for the (Co9S8/N, S-DLCTs)HF, which is smaller than 25.4 ohm for the (Co9S8/N, S-DLCTs)NaOH and 32.0 ohm for the Co9S8/N, S-SLCTs, suggesting good charge transfer ability. More importantly, the Warburg coefficient (σw) is 108.4 for the (Co9S8/N, S-DLCTs)HF, much smaller than that (527.3) of the Co9S8/N, S-SLCTs, highlighting fast ion diffusion characteristic of the double-layered tubular structure, since the ion diffusion coefficient (D) is inversely proportional to the square of the σw.38 Hence, owing to high intrinisc activity of Co9S8 sites and the characteristics of the double-layered tubular structure, the (Co9S8/N, S-DLCTs)HF exhibits excellent OER catalytic activity with the η10 of 367 mV and Tafel slope of 95 mV dec-1, which is almostly comparable to the state-of-art RuO2 catalyst with the η10 of 352 mV and the Tafel slope of 139 mV dec-1 and previously reported precious-metal-free catalysts (Table S3).10, 12-14, 25, 27, 34-37, 39-44 Moreover, after 10 h continous operation at the potential of 1.6 V vs. RHE, only 5.6 % of the initial current is lost for the (Co9S8/N, S-DLCTs)HF, and the the anode potential with a current density of 10 mA cm-2 (E10) shifts positively for 5 mV (Figure 4e), demonstrating good OER stability. The overvoltage between ORR and OER suggests the loss in efficiency and is, therefore, a crucial parameter for evaluating the bifunctional catalytic activities of a given



electrocatalyst towards oxygen electrochemistry. The OER activity is compared at the E10, which is proposed as a figure-of-merit for the viability of a given catalyst in solar fuel synthesis,45 and the ORR activity is derived at E1/2. Notably, as seen from Figure 4f and S20, the overvoltage between ORR and OER is low to 0.707 V under the catalysis of the (Co9S8/N, S-DLCTs)HF, which is even smaller than 0.711 V if instead of the Pt/C as the ORR catalyst and RuO2 catalyst as the OER catalyst. More importantly, to the best of our knowledge, the OER/ORR bifunctional activity of the (Co9S8/N, S-DLCTs)HF is superior to than that of most of precious-metal-free oxygen electrocatalysts reported recently (Figure 4f).10, 12-13, 27, 34-37 The structure and chemical composites of the catalysts obtained after the OER/ORR stability tests, namely the (Co9S8/N, S-DLCTs)HF-ORR and (Co9S8/N, S-DLCTs)HF-OER, are examined using TEM and XPS techniques. Figure S21 shows that, after the OER/ORR measurements, the double-layered tubular structure is well conserved, and cobalt species showed by black dots are embedded into carbon matrix, and aren’t obviously aggregated, demonstrating good structural stability. The XPS surveys in Figure S22 reveal that, Co, N, C, O and S species are existed in the catalysts before and after the OER and ORR measurements. High-resolution N 1s XPS spectrum of the (Co9S8/N, S-DLCTs)HF is fitted into Pyridinic N (398.7 eV), Co-Nx (399.4 eV), pyrrolic N (400.6 eV), graphitic N (401.3 eV), as well as the oxidized N (402.7 eV),14, 46 which are similar to that of the (Co9S8/N, S-DLCTs)HF-ORR and (Co9S8/N, S-DLCTs)HF-OER (Figure 5a and Table S4), revealing that the types of nitrogen dopants are unchanged during the catalytic process. In the deconvoluted Co 2p XPS spectrum


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of the (Co9S8/N, S-DLCTs)HF, besides the shake-up satellite peaks (784.5, 787.8, 801.8, and 804.1 eV), the peaks at the binding energies of 780.2 and 796.6 eV are ascribed to the Co 2p3/2 and 2p1/2 of Co (III), respectively,47-48 and the peaks located at the binding energies of 781.7 and 798.8 eV are corresponding to Co 2p3/2 and 2p1/2 of Co(II), respectively (Figure 5b and Table S4).42, 49 However, the binding energies of the Co 2p3/2 and 2p1/2 peaks are 779.7 eV and 794.8 eV after the ORR catalytic process, in agreement with that of Co3O4,50 and are 780.3 eV and 795.4 eV after the OER measurement, in accordance with that of CoOOH.51 High-resolution O 1s XPS spectra in Figure 5c show that, besides the C-O-C/C-OH (531.0 eV) and C=O species (532.2 eV), the peaks centred at 528.7 eV and 529.2 eV appeared in the (Co9S8/N, S-DLCTs)HF-ORR and (Co9S8/N, S-DLCTs)HF-OER are corresponding to the lattice oxygen species of Co3O4 and CoOOH, respectively.52-53 Moreover, the peak located at 534.4 eV is corresponding to the adsorbed water molecules.51 High-resolution S 2p XPS spectra in Figure 5d show that sulfur species including Co-S, C-S-C, and SOx are existed in the (Co9S8/N, S-DLCTs)HF,54 while the Co-S species are disappeared after the OER/ORR process. Hence, above results demonstrate that, during the catalytic process, the double-layered tubular structure is almost retained, while cobalt sulfide is evolved into Co3O4 and CoOOH, which are proposed as efficient ORR and OER catalytic sites, respectively.50, 55-57 The rational design of the structure and chemical composition of the (Co9S8/N, S-DLCTs)HF renders the unique features for enhancing electron and mass transfer abilities, and constructing efficient ORR/OER catalytic sites discussed as follows, resulting in demonstrating excellent



bifunctional catalytic performance. First of all, highly conducting double-layered tubular structure with rich mesopore and high pore volume provides for large ECSA, and constructs fast electron and ion transport network.49 Secondly, cobalt sulfides-derived catalytic active sites, such as, Co3O4 and CoOOH, promote the adsorption of water molecules and hydroxide ions, and efficiently catalyze the ORR and OER processes, respectively. Thirdly, Co-N-C and heteroatom doping (pyridinic N, graphitic N, and C-S-C species) are in favor of the adsorption of oxygen molecules and the formation of peroxide due to the charge polarization and electron spin effects, and thus synergistically improve the catalytic activities,58-60 in cooperation with cobalt oxide/oxyhydroxide catalytic sites. The last but not the least, the good structural stability of the double-layered tubular structure inhibits the self-aggregation of cobalt species during the long-term catalytic process, resulting in showing good durability.

Conclusions In sum, we have successfully synthesized mesoporous double-layered tubular structure via the coordinated-assisted polymerization and sacrificial template methods, where Co9S8 nanocrystals are incorporated into nitrogen, sulfur co-doped carbon matrix. The well-defined double-layered tubular structure with large specific surface area, rich mesopore, and high graphitization degree can give rise to many appealing features, especially for a large exposed active surface, and a facilitated charge and mass transfer network. By virtue of the structural advantages, as well as efficient cobalt oxide/oxyhydroxide active sites in-situ transformed from cobalt sulfide during the catalytic process, the resulting catalyst demonstrated excellent


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OER/ORR bifunctional electrocatalytic activities with considerable stability. The remarkably high oxygen electrode activity with the low overvoltage of ~ 0.707 V is better than that of previously reported precious-metal-free oxygen catalysts. This work would inspire the development of structure-controllable precious-metal-free bifunctional electrocatalysts in the applications of renewable energy storage and conversion devices.

Supporting Information SEM images, TEM images and XRD patterns of the Co-PDA/SiO2, Co/C/SiO2, (Co/DLCTs)HF, (Co9S8/N, S-DLCTs)NaOH, Co9S8/N, S-DLCTs/SiO2, Co9S8/N, S-HCSs, nickel sulfide/ nitrogen, sulfur co-doped double-layered carbon tubes, Co/N-DLCTs, N, S-DLCTs, (Co9S8/N, S-DLCTs)HF-HCl, Co9S8/N, S-SLCTs, (Co9S8/N, S-DLCTs)HF-ORR, as well as (Co9S8/N, S-DLCTs)HF-OER; High-resolution Co 2p XPS spectrum of the (Co/DLCTs)HF; CV curves of the (Co9S8/N, S-DLCTs)HF in O2 or N2-saturated 0.1 M KOH solution; The nitrogen and sulfur contents in the (Co9S8/N, S-DLCTs)HF determined from the XPS spectra; The stability and methanol crossover effect performance of the (Co9S8/N, S-DLCTs)HF; The OER/ORR catalytic activities of the (Co9S8/N, S-DLCTs)HF and previously reported precious metal-free catalysts

Acknowledgments



This work was sponsored by National Natural Science Foundation of China (Grant No. 21771069 and 51772110), and the Innovation Research Funds of Huazhong University of Science & Technology (No. 2017KFXJJ164).


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Scheme 1. Schematic representation of the fabrication process of the Co9S8/N, S-DLCTs and N, S-DLCTs.


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Figure 1 a) SEM and b) TEM images of the double-layered silica tube templates. c) SEM image, d) TEM, e) HRTEM, f) STEM, and g-j) EELS images of the (Co9S8/N, S-DLCTs)HF. (Inset of d: the particle size distribution curves of Co9S8 nanocrystals in the (Co9S8/N, S-DLCTs)HF)



Figure 1. a) The N2 adsorption-desorption isotherms of the (Co9S8/N, S-DLCTs)HF and (Co9S8/N, S-DLCTs)NaOH (Inset: the pore size distribution). b) Raman spectra of the (Co/DLCTs)NaOH, (Co/DLCTs)HF, (Co9S8/N, S-DLCTs)NaOH, and (Co9S8/N,S-DLCTs)HF.


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Figure 2. The ORR polarization curves of a) the (Co9S8/N, S-DLCTs)HF formed at the sulfidation temperatures of 700 ~ 1000 oC and b) the (Co9S8/N, S-DLCTs)HF, Co/N-DLNCTs, and N, S-DLCTs in O2-saturated 0.1 M KOH solution at 1600 rpm. c) Rotating disk ring electrode (RRDE) voltammograms, d) Tafel plots, and e) the H2O2 yield and electron transfer number curves of the Pt/C, (Co9S8/N, S-DLCTs)HF, (Co9S8/N, S-DLCTs)NaOH, and Co9S8/N, S-SLCTs. f) RDE voltammograms of the (Co9S8/N,S-DLCTs)HF at the rotation rates of 400 ~ 2400 rpm (Inset: the corresponding K-L plots at the potentials of 0.80 ~ 0.60 V vs. RHE).



Figure 4. a) The OER polarization curves, b) Tafel plots, and c) over-potentials at 10 mA cm-2 (η10, black) and Tafel slopes (blue) of the catalysts in O2-saturated 0.1 M KOH solution at a scan rate of 5 mV s-1. d) The non-Faradic current densities at the potential of 0.963 V vs. RHE versus scan rates measured in 0.1 M KOH aqueous solution. e) The OER polarization curves of the (Co9S8/N, S-DLCTs)HF before and after the chronoamperometry test (Inset: the chronoamperometric responses (i-t curves) at 1.60 V vs. RHE for 10 h). f) The oxygen electrode activity (∆E) of the (Co9S8/N, S-DLCTs)HF and efficient precious metal-free bifunctional electrocatalysts reported recently.


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Figure 5. High-resolution a) N 1s, b) Co 2p, c) O 1s, and d) S 2p XPS spectra of the (Co9S8/N, S-DLCTs)HF before and after the OER/ORR catalytic process.



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Coordination-Assisted Polymerization of Mesoporous Cobalt Sulfide

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