Co9S8 Catalysts Derived

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36755-36761

Two-Dimensional N,S-Codoped Carbon/Co9S8 Catalysts Derived from Co(OH)2 Nanosheets for Oxygen Reduction Reaction Shaofang Fu,†,∥ Chengzhou Zhu,†,∥ Junhua Song,† Shuo Feng,† Dan Du,*,†,‡ Mark H. Engelhard,§ Dongdong Xiao,§ Dongsheng Li,§ and Yuehe Lin*,† †

School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164, United States Key Laboratory of Pesticide and Chemical Biology, Ministry of Education of the PR China and College of Chemistry, Central China Normal University, Wuhan 430079, China § Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States ‡

S Supporting Information *

ABSTRACT: The development of highly active and cost-efficient electrocatalysts for the oxygen reduction reaction (ORR) is of great importance in a wide range of clean energy devices, including fuel cells and metal-air batteries. Herein, the simultaneous formation of Co9S8 and N,S-codoped carbon with high ORR catalytic activity was achieved in an efficient strategy with a dual templates system. First, Co(OH)2 nanosheets and tetraethyl orthosilicate were utilized to direct the formation of two-dimensional carbon precursors, which were then dispersed into thiourea solution. After subsequent pyrolysis and template removal, N,S-codoped porous carbon-sheet-confined Co9S8 catalysts (Co9S8/NSC) were obtained. Owing to the morphological and compositional advantages as well as the synergistic effects, the resultant Co9S8/NSC catalysts with a modified doping level and pyrolysis degree exhibit superior ORR catalytic activity and long-term stability compared with the state-of-the-art Pt/C catalysts in alkaline media. Remarkably, the as-prepared carbon composites also reveal exceptional tolerance of methanol, indicating their potential applications in fuel cells. KEYWORDS: nonprecious metal catalysts, two-dimensional composites, nitrogen/sulfur-codoped carbon, Co9S8, oxygen reduction reaction

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INTRODUCTION Because of their environmental friendliness, high efficiency, and low working temperature, polymer electrolyte membrane fuel cells (PEMFCs) have become one of the most promising technologies to address the issues associated with energy shortage and pollution.1−3 In comparison with the hydrogen oxidation reaction on the anode, the cathodic oxygen reduction reaction (ORR) is sluggish and complex. Instead of a one-step reaction, ORR involves four-electron transfer combining with complicated adsorption/desorption and dissociation/recombination of the intermediates.4 Pt nanoparticles supported on active carbon (Pt/C), state-of-the-art catalysts, are widely used on the electrode, a high ORR overpotential of 300 mV is still needed to generate satisfactory current density. Aside from this insufficient activity, the development and large scale fabrication of PEMFCs are also hindered by other issues, including high cost and scarcity of Pt, poor stability, and intolerance to fuel crossover of the catalysts.5−7 Therefore, developing nonprecious metal catalysts (NPMCs) derived from earth-abundant elements, such as Co, Fe, N, S, and C, is important to address Pt catalyst-related cost issues. Since Jasinski tested metal phthalocyanine-derived catalysts in fuel cells in 1964,8 a large group of NPMCs has emerged and become promising candidates for ORR electrocatalysts in fuel cells.9−12 Recently, © 2017 American Chemical Society

cobalt sulfides with various structures are proposed to serve as ORR electrocatalysts owing to their good catalytic activity, high abundance, and environmental friendliness.6,13−15 However, the electrocatalytic performance of these cobalt sulfide-based nanocatalysts is still far from satisfactory because of the poor conductivity and insufficient exposed active sites. To overcome these drawbacks, heteroatom-doped carbon nanostructures are introduced to increase the conductivity and number of active sites.6,16,17 Importantly, the incorporation of heteroatoms within the catalytic system can not only provide extra active sites but also efficiently modulate the electronic properties, the adsorption energy of O2, and the catalytic mechanism, resulting in an improved catalytic performance for ORR.18−20 For example, Dai’s group reported an advanced etched and doped Co9S8/graphene hybrid for ORR electrocatalysis, where NH3plasma was applied to treat graphene-supported Co9 S 8 nanoparticles under high temperature.21 The plasma treatment can introduce N atoms into both Co9S8 nanoparticles and graphene, which effectively modified the electronic structures of Co9S8/graphene. In addition, the treatment can also etch the Received: July 13, 2017 Accepted: September 26, 2017 Published: September 26, 2017 36755

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

Research Article

ACS Applied Materials & Interfaces

Figure 1. (A) Schematic synthesis of Co9S8/NSC composites. (B) SEM, (C) TEM, and HRTEM (D) images of Co9S8/NSC-900-1. (D, inset): FFT pattern from (D). (E) HAADF-STEM image of Co9S8/NSC-900-1. (F−J) Elemental distribution of C, N, Co, S, and O in Co9S8/NSC-900-1 (scale bar: 500 nm).

Co(OH)2 nanosheets are uniform and freestanding with microsized width. Upon the successful synthesis of Co(OH)2 nanosheets at large scale, the 2D porous Co9S8/NSC catalysts were prepared using a hard template method (Experimental Section). Briefly, resorcinol/formaldehyde/ethylenediamine (EDA) were used as polybenzoxazine (PB) precursors, and Co(OH)2 nanosheets were employed as templates to obtain PB/silica-coated Co(OH)2 in the presence of tetraethyl orthosilicate (TEOS).35 After further dispersion into thiourea solution and freeze-drying, the precursors were pyrolyzed under N2 atmosphere at different temperatures. The final products, Co9S8/NSC-T-n (T = temperature, n = mass ratio of Co(OH)2/thiourea), were obtained after the removal of templates. We also synthesized 2D Co/N/C-900 carbon sheets (Figure S2) for comparison using the same process except for the addition of thiourea. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out to investigate the morphology and structure of the as-prepared Co9S8/NSC composites. As revealed in Figure 1B, the 2D nanosheets in Co9S8/NSC-900-1 interconnect with each other and assemble into a hierarchical microstructure. The sheet-like structure was well maintained after the pyrolysis process regardless of the annealing temperature and doping level (Figures 1C and S3). Remarkably, no obvious particle aggregation is observed on the carbon sheets, indicating the uniform active sites in the catalysts. There is a need to point out that the introduction of TEOS during the PB coating process plays a vital role in retaining the sheet-like morphology. In the absence of TEOS, the carbon sheets were broken into small pieces after thermal treatment, as revealed in Figure S4. Under high temperature conditions, Co(OH)2 nanosheets are pyrolyzed into cobalt oxides, resulting in shrinkage of the carbon sheets. In comparison with PB, the silica/PB hybrids are robust enough to suppress the stress inside, thus effectively retaining the sheet-like structure. In this strategy, the resultant silica also serves as a pore-forming agent, which leads to the

surface of Co9S8 nanoparticles and graphene, resulting in the exposure of more active sites for electrocatalysis. Importantly, the as-prepared N-doped Co9S8/graphene showed exceptional ORR performance with a similar onset potential and current density to the commercial Pt/C. Two-dimensional (2D) nanomaterials, which usually possess a large flat open layer with high surface area and good electronic properties, are of considerable interest and show promising applications in electrocatalysis.22−27 Further chemical and structural modification of the materials are expected to offer additional properties and broaden their applications.28−31 For example, the combination of 2D sheets with porous structures can contribute significant enhancement of ORR catalytic performance because this structure not only favors the reactants diffusion from the electrolyte to the electrodes but also facilitates the electron transfer to the active sites.20,32 Herein, we realized the simultaneous formation of Co9S8 and occurrence of N,S-doping into the 2D porous carbon sheets in the presence of Co(OH)2 nanosheets and silica templates. On the one hand, the addition of thiourea can induce dopants into Co(OH)2 and carbon, leading to the construction of Co9S8 and N,S-codoped carbon. Both are effective species for ORR electrocatalysts. On the other hand, Co(OH)2 nanosheets and silica templates contribute to the evolution of 2D porous carbon sheets, which ensure fast electron transfer/mass transport as well as extensive exposure of the active sites. Because of their unique structure and heteroatom doping, the obtained N,S-codoped porous carbon-sheet-confined Co9S8 (Co9S8/NSC) catalysts with a modified doping level and pyrolysis degree exhibit an exceptional ORR catalytic performance in alkaline media.

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RESULTS AND DISCUSSIONS The Co(OH)2 template was synthesized via microwave-assisted liquid-phase growth according to reported works and as illustrated in Figure 1A.33,34 As revealed in Figure S1, the 2D 36756

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

Research Article

ACS Applied Materials & Interfaces

Figure 2. (A) XRD patterns of Co9S8/NSC. (B) XPS spectrum of Co9S8/NSC-900-1. (C) High-resolution XPS spectrum of N. (D) Content of pyridinic-N and graphitic-N in different catalysts. (E) High-resolution XPS spectrum of S and (F) Co in Co9S8/NSC-900-1.

of Co9S8 (PDF #19−0364), regardless of pyrolysis temperature. The XRD diffraction peaks confirm the successful involvement of Co9S8 in carbon sheets. The X-ray photoelectron spectroscopy (XPS) survey was then carried out to determine the surface chemical composition of Co9S8/NSC sheets. The XPS spectrum in Figures 2B and S6 show the presence of Co, N, C, S, O on the surface of Co9S8/NSC samples obtained under various pyrolysis temperatures, indicating the successful doping of N and S into the catalysts. The high-resolution XPS spectrum of N in Co9S8/NSC-900-1 (Figure 2C) can be deconvoluted into four peaks at 398.31, 399.36, 401, and 403 eV, corresponding to pyridinic-N, pyrrolic-N, graphitic-N, and oxidized-N, respectively. Among these nitrogen functional groups, 26.5 and 61.1% of them are pyridinic-N and graphitic-N. In comparison with Co9S8/NSC-800-1 and Co9S8/NSC-1000-1, Co9S8/NSC-900-1 reveals the highest concentration of pyridinic-N and graphitic-N as illustrated in Figure 2D. These two nitrogen functional groups have been demonstrated to be the active species for ORR.36,37 It should be pointed out that the atomic percentage of N in Co9S8/NSC-

large surface area after template removal. To obtain the phase information, high-resolution TEM (HRTEM) was conducted, and the results are shown in Figure 1D. The well-defined lattice fringe with a distance of 0.496 nm can be assigned to the (200) plane of cubic Co9S8. Additionally, the fast Fourier transformation (FFT) of the HRTEM image is in correspondence with that of the Co9S8 crystals as well. Instead of the formation of aggregated nanoparticles, Co9S8 in this work was uniformly distributed throughout the entire structure as indicated in the high-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 1E, inset) and the electron dispersive spectroscopy (EDS) mapping profile (Figure 1F−J). The crystalline structure of both Co9S8/NSC and Co/N/C900 carbon sheets was characterized by X-ray diffraction (XRD). As revealed in Figure S5, two diffraction peaks were found in Co/N/C-900 at 2θ = 26 and 43°, corresponding to the (002) and (101) crystal planes of carbon. One additional peak at 2θ = 37° indicates the residue Co species. In Figure 2A, the intense peaks at 2θ of 15.8, 30.2, 31.5, 47.8, and 52.3° can be ascribed to the (111), (311), (222), (511), and (440) planes 36757

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

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ACS Applied Materials & Interfaces

Figure 3. (A) Nitrogen adsorption and desorption isotherms and (B) pore size distribution curve of Co9S8/NSC-900-1.

Figure 4. (A) Polarization curves of different catalysts in O2-saturated 0.1 M KOH solution with a scan rate of 10 mV/s and a rotation rate of 1600 rpm. (B) Tafel plot of Co9S8/NSC-900-1 and commercial Pt/C. (C) Polarization curves of Co9S8/NSC-900-1 at different rotation rates and (C, inset) corresponding K−L plots. (D) Current−time curves of Co9S8/NSC-900-1 at 0.7 V in O2-saturated 0.1 M KOH with a rotation rate of 200 rpm.

oxide. The strong peaks at 778.77, 781.16, 793.68, and 796.76 eV can be assigned to Co−S species, while the peaks of cobalt oxide are located at 782.55, 796.17, 798.73, and 803.02 eV.6,38 The information on the surface area and pore size distribution of Co9S8/NSC-900-1 were studied using N2 adsorption−desorption. As shown in Figure 3, the Brunauer− Emmett−Teller surface area and pore volume of the final product were 332 m2/g and 0.33 cm3/g, respectively. The pore size distribution profile in Figure 3B indicates the abundant mesopores in Co9S8/NSC-900-1, which should be attributed to the removal of silica template. The high surface area and pore volume can not only offer a large number of active sites but also facilitate electron transfer and mass transport, leading to superior catalytic activity. The ORR activities of the as-synthesized 2D carbonaceous catalysts and commercial Pt/C catalyst were evaluated using rotating disk electrode (RDE) measurements in alkaline media. The linear scan voltammetry (LSV) curves in Figure 4A clearly

800-1, Co9S8/NSC-900-1, and Co9S8/NSC-1000-1 is 4.16 %, 3.15 % and 1.99 %, respectively, based on the XPS quantitative results. The decreased N content is ascribed to the bond breaking under high pyrolysis temperature. Figure 2E shows the high-resolution XPS spectrum of S 2p in Co9S8/NSC-900-1. The small peak at 161.69 eV can be ascribed to cobalt sulfide. Two intensive peaks at 163.97 and 165.12 eV correspond to carbon-bonded sulfur, which demonstrates the successful doping of S into C atoms. We also found two peaks at 168.15 and 169.37 eV, indicating the oxidized sulfur on the surface.6,38 Note that the weak signal of the Co−S peak indicates the low concentration of Co. During the pyrolysis process, Co(OH)2 was transferred into cobalt oxides or cobalt sulfides, while most oxides were removed by the following acid treatment. Only a tiny amount of oxides remained on the surface. The formation of cobalt sulfide is further confirmed by the XPS spectrum of Co (Figure 2F). On the surface of the Co9S8/NSC-900-1, Co atoms are in the form of sulfide and 36758

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

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ACS Applied Materials & Interfaces

Figure 5. (A) RRDE polarization curves of different catalysts in O2-saturated 0.1 M KOH solution with a scan rate of 10 mV/s and a rotation rate of 1600 rpm. (B) HO2− yield and (C) electron transfer number of various samples based on RRDE data. CV curves of (D) Co9S8/NSC-900-1 and (E) Pt/C with and without the addition of 1 M methanol.

suggests the highly intrinsic activity of Co9S8/NSC-900-1 catalyst. The LSV curves of Co9S8/NSC-900-1 with different rotation speeds ranging from 225 to 2500 rpm were recorded and used to obtain the Koutecky−Levich (K−L) plots (Figure 4C). The linear and parallel K−L plots at various potentials indicate the first-order reaction kinetics with respect to the concentration of dissolved O2 and the similar electron transfer number. Another important parameter to evaluate the performance of ORR catalysts is long-term stability. We studied the durability of both Co9S8/NSC-900-1 and Pt/C via chronoamperometric testing at 0.7 V in O2-saturated 0.1 M KOH with a rotation rate of 200 rpm. As shown in Figure 4D, the current density of Co9S8/NSC-900-1 retains more than 95% of the initial value, while Pt/C displays a substantial drop of the current after 20 000 s. Furthermore, the sheet-like structure of Co9S8/NSC-900-1 is well maintained after the durability test, whereas Pt/C shows severe aggregation of Pt nanoparticles (Figure S8). The better ORR activity and long-term stability imply the superior catalytic performance of Co9S8/NSC-900-1. Rotating ring-disk electrode (RRDE) measurements were further conducted to study the ORR mechanism on Co9S8/ NSC-900-1. According to the polarization curves in Figure 5A, we calculated HO2− yield using eq 1

demonstrate the superior ORR catalytic activity of Co9S8/NSC900-1. In specific, the Co9S8/NSC-900-1 displays an onset potential of 0.953 V and a half-wave potential (E1/2) of 0.896 V, which is much more positive than those of Co/N/C-900 (0.843 and 0.795 V) as well as Pt/C (0.934 and 0.845 V). Importantly, the ORR performance is even better than that in some reported works as listed in Table S1. The exceptional performance of Co9S8/NSC-900-1 confirms that the formation of Co9S8 and the occurrence of N,S-doping can significantly improve the ORR catalytic activity of the catalyst. We further examined the effect of both annealing temperature and doping level on the ORR activity. These two factors play a crucial role in the catalytic performance as revealed in Figure S7. Even though these Co9S8/NSC sheets possess similar morphology/structure, Co9S8/NSC-900-1 exhibits the best ORR activity compared with other materials in term of onset potential, E1/2, and diffusion-limiting current, and this should be mainly attributed to multiple active sites, the proper doping level, and the synergistic effect among the active species. Such high electrocatalytic activity of Co9S8/NSC-900-1 is closely related to each component, including N, S, and Co9S8, in the catalysts. It is clear that Co9S8 serves as an active species for ORR, and more Co9S8 active sites were exposed after the removal of templates, contributing to the enhanced activity. Also, it has been well established that a large amount of carbon atoms adjacent to dopants serve as active sites when N and S are doped into carbon materials.15,38,39 The ORR activity of such catalysts originates from the changes of spin and charge densities of carbon atoms.3 The formation of Co−N−C structures is inevitable under pyrolysis conditions, and they can also serve as efficient ORR active sites.37,40 To further evaluate the structural and compositional advantages of Co9S8/NSC900-1, Tafel plots were derived from ORR polarization curves (Figure 4B). The Tafel slope of Co9S8/NSC-900-1 and Pt/C is 74 and 92 mV/dec, respectively. The smaller Tafel slope

HO2−% = 200

n=4×

IR /N ID + IR /N

ID ID + IR /N

(1)

(2)

where ID and IR are the disk and ring current, respectively. N = 0.37 is the collection efficient. In contrast with the high HO2− yield on the Co/N/C-900 electrode, Co9S8/NSC-900-1 produces less than 10% HO2−, which is similar to the case in Pt/C (Figure 5B). Based on the RRDE results, the average electron transfer number on the Co9S8/NSC-900-1 electrode 36759

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

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ACS Applied Materials & Interfaces Notes

was also calculated from eq 2. As displayed in Figure 5C, the average electron transfer number of Co/N/C-900 and Co9S8/ NSC-900-1 is 3.4 and 3.9, which is very close to that of Pt/C (3.97). This indicates that the reaction on the surface of the catalyst undergoes an ideal four-electron pathway. We next assessed the tolerance to the methanol crossover effect for Co9S8/NSC-900-1 via the addition of 1 M methanol into the electrolyte. From Figure 5D, we can see that the CV curves of Co9S8/NSC-900-1 present negligible current change, while a significant methanol oxidation peak shows up on the Pt/C electrode (Figure 5E). Given the excellent activity, stability, and tolerance to the methanol crossover effect together, the asprepared 2D Co9S8/NSC-900-1 sheets are considered as promising NPMNs for ORR. The enhanced catalytic performance is ascribed to the unique morphology and composition of Co9S8/NSC composites. On the one hand, the hierarchical structures assembled with 2D porous carbon sheets not only facilitate electron transfer and mass transport but also offer a large surface area and high accessibility of active sites for ORR. On the other hand, for the formation of Co9S8, heteroatom doping with proper doping level results in multiple active species in the catalyst, which is important to improve the ORR performance.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a startup fund of Washington State University. We thank Franceschi Microscopy & Image Center at Washington State University for TEM measurements. The XPS analysis was performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL0183.

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CONCLUSIONS To summarize, we demonstrated an efficient strategy to prepare 2D Co9S8/NSC sheets. Silica/PB-coated Co(OH)2 nanosheets were first prepared in aqueous solution. The subsequent pyrolysis in the presence of thiourea realized the N,S-codoping and the formation of Co9S8 simultaneously. By virtue of the unique compositional and morphological advantages, the asprepared Co9S8/NSC sheets exhibit much better ORR activity and stability than commercial Pt/C in alkaline media. These 2D carbon composites might be considered as one of the most promising catalysts for ORR.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10227. Experimental section; TEM images of Co(OH)2 nanosheets, Co/N/C-900, Co9S8/NSC-T-n, and carbon sheets in the absence of TEOS; XRD spectrum of Co/ N/C; XPS spectra of Co9S8/NSC-800-1 and Co9S8/ NSC-1000-1; LSV curves of different Co9S8/NSC catalysts in 0.1 M KOH solution; TEM images of Co9S8/NSC-900-1 and Pt/C before and after stability test (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.D.) *E-mail: [email protected] (Y.L.) ORCID

Chengzhou Zhu: 0000-0003-0679-7965 Dan Du: 0000-0003-1952-4042 Dongsheng Li: 0000-0002-1030-146X Yuehe Lin: 0000-0003-3791-7587 Author Contributions ∥

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S.F. and C.Z. contributed equally to this work. 36760

DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761

Research Article

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DOI: 10.1021/acsami.7b10227 ACS Appl. Mater. Interfaces 2017, 9, 36755−36761