3D Nitrogen, Sulfur-Codoped Carbon Nanomaterial-Supported Cobalt

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3D nitrogen, sulfur co-doped carbon nanomaterials supported cobalt oxides with polyhedron-like particles grafted onto graphene layers as highly active bicatalysts for oxygen evolving reactions. Xiaobo Huang, Jian-Qiang Wang, Hongliang Bao, Xiangkun Zhang, and Yongmin Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00504 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Applied Materials & Interfaces

3D nitrogen, sulfur co-doped carbon nanomaterials supported cobalt oxides with polyhedron-like particles grafted onto graphene layers as highly active bicatalysts for oxygen evolving reactions. Xiaobo Huang 1, Jianqiang Wang 2, Hongliang Bao 2, Xiangkun Zhang 1, and Yongmin Huang 1, * 1

Key Laboratory of Specially Functional Polymeric Materials and Related Technology, School of Chemistry and Molecular Engineering, East China University of Science and Technology, No. 130 Meilong Road, Xuhui District, Shanghai, 200237, P.R.China, E-mail: [email protected] 2 Shanghai Institute of Applied Physics Chinese Academy of Sciences, No. 2019 Jialuo Road, Jiading District, Shanghai, 201800, P.R.China. KEYWORDS oxygen electrode reactions, ZIF-67, 3D hierarchical porous structure, bicatalyst, carbon nanomaterials

ABSTRACT: The extensive researches and developments of highly efficient oxygen electrode electrocatalysts to get rid of the kinetic barriers for ORR and OER are very important in energy conversion and storage devices. Specially, exploring non-precious metal alternatives to replace traditional noble metal catalysts with high cost and poor durability is the paramount mission. In this paper, we utilize property flexible ZIF-67 and sulfur functionalized GO to obtain a cobalt, nitrogen and sulfur co-doped nanomaterial with 3D hierarchical porous structure, owning both rich dopant species and good conductivity. The cross-linked structures of polyhedron particles throughout the whole carbon framework speeds up the mass transportation and charge delivering processes during oxygen evolving reactions. Also, by exploring the location and coordination type of sulfur dopants, we emphasize the effects of sulfone and sulfide functional groups anchored into graphitic structure on enhancing the catalytic abilities for ORR and OER. To note, compared to the noble metal electrocatalysts, the best-performing CoO@Co3O4/NSG-650 (0.79 V) suffers 40 mV less active than commercial Pt/C catalyst (0.83 V) for ORR, and merely 10 mV behind IrO 2 (1.68 V) for OER. Besides, the metric between ORR and OER difference for CoO@Co3O4/NSG-650 to evaluate its overall electrocatalytic activity is 0.90 V, surpassing 290 and 430 mV over Pt/C (1.19 V) and IrO2 (1.33 V). Comprehensively, the as-prepared CoO@Co3O4/NSG-650 indicates excellent bifunctional catalytic activities for ORR and OER, which shows great potential for replacing noble metal catalysts in application of fuel cells and metal-air batteries.

INTRODUCTION. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial reactions for electrochemical energy storage and conversion devices, including fuel cells, metal-air batteries and water electrolysis.1-4 Often, these electrochemical reactions kinetically need to be catalyzed by noble metal catalysts, which exhibit excellent activities, famous for Pt-based catalysts for ORR and Ir-based and Ru-based compounds for OER.5-7 Whereas, the scarcity, prohibitive cost and poor stability of precious metals along with their unsatisfying bifunctional catalytic activities for both ORR and OER limit their commercial implementation to a certain extent.8-9 Besides, the reaction mechanisms and intermediates of ORR and OER are totally different, magnify the difficulty to design effective electrocatalysts to accelerate both processes.10 Therefore, it is highly imperative to develop non-precious electrocatalysts with earth-abundance, low cost, desirable activity and decent stability for ORR and OER. In general, the electro-reduction of O2 in alkaline medium consists of rather complicated mechanisms with several elementary steps,11-12 as summarized in Equations (1)-(4). The oxygen reduction reaction can proceed mainly through two kinds of pathways. Reaction (1) is a direct reduction pathway of O2 with a four electron transfer, while the reaction (2)-(4)

indicate the indirect reduction of O2 by a two-step, two electron process with the formation of *16? as the intermediate specie. Since the two electron transfer pathway of O2 provides only about half of the energy compared to the four electron pathway, reaction (1) is often desired in fuel cell devices.13 However, the kinetic investigations suggest that the direct reduction of oxygen is inherently insufficient in practice. Therefore, the electrocatalysts are needed to address these drawbacks. 6

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Heteroatoms (e.g., N, S, B, P) doped in carbon lattice are able to change the charge and spin densities of carbon near the dopant atoms. Since the doping-induced charge redistribution is efficacious to weaken the O-O bonding, it makes the O2 adsorption and reduction processes accessible at the nearsurface of catalysts.14-20 Recently, pure graphitic materials, especially graphene-based derivations have drawn extensive attentions for catalyzing oxygen electrode reactions. Nitrogen-

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doped and sulfur-doped graphene materials have been proved to be efficient catalysts for ORR and OER. 21-27 In particular, transitional metals (Fe, Co, Ni and Mn, etc.) and heteroatoms (N, S, P and B, etc.) co-doped graphitic materials display tremendous potentials for replacing noble metal catalysts due to the strong synergetic effects between heteroatom dopants and transitional metals. 7, 19, 28-33 Metal-organic frameworks (MOFs), composed of wellorganized metal centers and organic linkers, have become new platforms for the synthesis of porous carbon-based electrocatalysts.34-40 Among them, zeolitic imidazolate frameworks (ZIFs) exhibit considerable electrocatalytic abilities due to the existence of abundant metal and nitrogen species severing as possible active sites.10, 14, 41-45 However, the direct carbonization of such ZIFs would mostly lead to poor electronic conductivity of the as-synthesized catalysts, which is sinister for ORR.14 GO has been widely acknowledged as an efficient carbon support for electrocatalysts due to its excellent electronic conductivity.46-47 Herein, we apply property flexible ZIF-67 combing with sulfur functionalized GO to obtain cobalt, nitrogen and sulfur co-doped nanomaterials, owning both rich dopant species and good conductivity. Therefore, the as-fabricated products show comparable catalytic performance with precious metal electrocatalysts for oxygen evolving reactions. For detail, ZIF-67 was chosen as the nitrogen and cobalt sources since it provided plenty of nitrogen and cobalt species in its framework structure. And GO was utilized as a binder and support for the growth of polyhedral nanocrystals of ZIF-67 with an average particle size of 300 nm. What worth a mention was that the addition of Na2S·9H2O for decorating the GO structure truly influenced the catalytic performance of ultimate products. Moreover, ZIF-67 nanoparticles were converted into interconnected polyhedron carbon architecture with hierarchical porosity through final carbonization process, which was beneficial for reactant transportation and active sites exposure. By adjusting the pyrolyzing temperatures, we explored the optimum method for the most active bicatalyst with favorable reactive crystalline facets and proper sulfur contents. Eventually, the as-fabricated CoO@Co3O4/NSG-650 displayed best performance with halfwave potential of ORR and OER being 0.79 and 1.69 V at -3 and 10 mA cm-2, respectively. Compared with Pt/C (1.19 V) and IrO 2 (1.33 V), the total oxygen electrode activity of CoO@Co 3O4/NSG-650 is 0.90 V, surpassing 290 and 430 mV in potential than the asmentioned noble metals. The ultimate product exhibited excellent activities, superior durability and strong tolerance to methanol, which provided gigantic prospects for the replacement of precious metal electrocatalysts in future energy conversion and storage system. Combined with our previous work,7 we credit the excellent catalytic performance of CoO@Co3O4/NSG-650 to the suitable doping states of nitrogen and sulfur atoms along with the uniformly dispersed cobalt oxides on graphitic layers acting synergistically together. As well, this work emphasizes the improving effects of sulfone and sulfite functional groups anchored into graphitic structure for catalyzing electrocatalytic processes.

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Scheme 1. The synthesis procedure of CoO@Co3O4/NSG electrocatalysts. EXPERIMENTAL SECTION. Synthesis of Graphite Oxide (GO). Graphite Oxide (GO) was prepared by a modified Hummers method.48-49 Typically, a mixture of concentrated H2SO4 (12 mL), K2S2O8 (2.5 g), and P2O5 (2.5 g) was heated to 80 oC. Then graphite powder (3 g, 325 mesh) was added to the above solution. The mixture was placed still under 80 oC for 4.5 h. After the blend was cooled down to 25 oC, de-ionized water (0.5 L) was added into the solution and left overnight. Then, the mixture was filtered and washed with deionized (DI) water for several times to remove the residual acid. The pre-oxidized graphite was dried under room temperature overnight. The pre-oxidized graphite was further oxidized with +XPPHUV¶ PHWKRG ,Q GHWDLO SUHWUHDWHG JUDSKLWH SRZGHU g) was added to concentrated H2SO4 (40 mL, 0 oC). Afterwards, 5 g of KMnO4 was added under stirring. The whole process was operated being 20 oC below. The mixture was stirred for 2 h before 85 mL of DI water were injected for dilution with the temperature under 50 oC. After stirring for another 2 h, more DI water (250 mL) was added, shortly after which, 7 mL of H2O2 were impregnated. After filtering, 2 M HCl (350 mL) was used for removing the residual metal. The resulting mixture was centrifuged several times to netrual and purified by dialysis for one week to fully remove residual metallic impurity. The as-synthesized GO was dispersed in DI water to obtain the concentration of ca. 5 mg mL-1 for future experimental exploration. Synthesis of ZIF@S-GO composite. Co(NO3)2·6H2O (1.44 g) was dispersed in 10 mL of methanol to form solution A. 2Methylimidazole (1.70 g) was dispersed in another 10 mL of methanol to form solution B. Meanwhile, 5 mL of GO solution was poured into 80 mL of methanol containing 100 mg of PVP to form solution C. Then, solution A and B were dropped in turn to solution C under vigorous stirring. After 10 min, stirring was stopped and the mixture was kept for 24 h at 25 oC, the asprepared product was separated by centrifugation and washed with methanol for three times, which was then dispersed in 120 mL of ethanol to form solution D. Hereafter, 10 mL of Na2S·9H2O aqueous solution was mingled with solution D under stirring at the temperature of 40 oC. After reacting for 5 h, the purple powder was obtained by filtering and washed with DI water, followed by vacuum drying at 40 oC for 12 h. Synthesis of CoO@Co3O4/NSG. The as-fabricated ZIF@S-GO compounds were placed at a quartz boat depositing in the tube furnace for terminal pyrolysis under N2 flow. The carbonization temperature of the furnace was directly raised to set temperature (550, 650 and 750 oC) with the ramp rate of 2 oC min-1. There was a heat preservation of 2 h inside the tube furnace at the set temperature, whose heat would subside naturally afterwards. The obtained products were denoted as CoO@Co3O4/NSG-x (x represents for the set temperature during the carbonization process). Synthesis of CoO@Co3O4/NG and CoO@Co3O4/NSC. CoO@Co3O4/NG and CoO@Co3O4/NSC were prepared by similar synthesis procedures to CoO@Co3O4/NSG, except that Na2S·9H2O or GO was not added, respectively. Electrochemical Tests. All the electrochemical tests were conducted on CHI 760D (CH Instruments, Chen Hua Co., China) electrochemical workstation. The experiment utilized a conversational three-electrode system. The glassy carbon rotating disk was used as working electrode, while a platinum wire and an Ag/AgCl with saturated KCl were chosen as counter and reference elec-


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Koutecky-Levich equations are employing to calculate the electron transfer numbers during ORR. The equations are as follows;

Physical characterization. Field-emission scanning electron microscopy (FESEM) was applied on Nova NanoSEM 450. Transmission electron microscope (TEM) was conducted at JEM 2100. Elemental mappings were obtained at X-ray spectroscopy (EDS, EDAX Falcon). The nitrogen adsorption-desorption isotherms were determined at the liquid nitrogen temperature (77 K), on Quantachrome Autosorb iQ. The specific surface area was acquired with Brunauer-Emmett-Teller (BET) method and the pore size distribution was calculated from the desorption branch according to the Density Functional Theory (DFT) model. X-ray diffraction (XRD) was performed on the RINT2000 vertical goniometer. Fourier transform infrared spectroscopy (FTIR) was obtained on Nicolet 380 instrument. Raman spectroscopy was collected by Renishaw inVia Reflex with a 785 nm laser excitation source. X-ray photoelectron spectroscopy (XPS) was FDUULHG RXW RQ 7KHUPR (6&$/$% ZLWK $O .. K eV) as the source. The XAFS data was collected at the Hard X-ray Micro-Analysis (HXMA) beamline 06ID-1 at the Canadian Light Source (CLS). The electron storage ring was operated at 2.9 GeV with a beam current from 250 to 180 mA.

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RESULTS AND DISCUSSION.

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The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) are utilized to investigate the morphology and structure of the as-prepared CoO@Co3O4/NSGs. Figure 1a displays the FESEM image of ZIF@S-GO before carbonization. The ZIF nanoparticles which are grown on GO aggregate together, indicating that GO acts as a binder and support for the nanocrystals growth. After pyrolysis process, the as-fabricated CoO@Co3O4/NSG-650 shows a 3D hierarchical porous structure (Figure 1b), with the polyhedron shape derived from ZIF-67 distributing uniformly on the graphene layer. Also, it can be observed that the polyhedron particles link to each other throughout the whole carbon framework, facilitating both mass transportation and charge delivering processes. In this condition, the oxygen evolving reactions can be largely promoted by catalyst with such a 3D hierarchical architecture. The TEM image (Figure 1c) suggests that the cobalt oxide nanoparticles, with a size about 40 nm, are deposited on graphene layer homogenously, which is further confirmed by the corresponding elemental mapping results (Figure S4). The HRTEM and XRD pattern of CoO@Co3O4/NSG-650 are shown in Figure 1d. From the HRTEM image, the CoO@Co3O4/NSG-650 shows interplanar distances of 2.43 Å and 2.45 Å, which can be associated to CoO (111) and Co3O4 (311), besides, the 3.42 Å and 3.44 Å refer to the graphitic carbon (002). However, there is no lattice fringe of Co nanoparticles, and no detectable signals of those particles by XPS, indicating that these Co nanoparticles are entirely encapsulated by the graphitic layer and exist in the inner structure. The XRD analysis of CoO@Co3O4/NSG-650 suggests three kinds of cobalt species. In specific, peaks at 44.2 o and 51.5 o correspond to the crystalline facets of (111) and (200) of Co (JCPDS card no. 150806), the peaks locating at 19 o, 31.2 o, 36.9 o, 38.5 o, 44.8 o, 55.7 o , 59.3 o and 65.2 o are indexed to the crystalline planes of (111), (220), (311), (222), (400), (422), (511) and (400) of Co 3O4 (JCPDS card no. 42-1467), while the signals sitting at 36.5 o, 42.4 o and 61.5 o refer to the crystalline planes of (111), (200) and (220) of CoO (JCPDS card no. 48-1719), respectively. The CoO@Co3O4/NSGs exhibit typical II adsorption isotherms with H3 hysteresis loop (Figrue S5), demonstrating both microporous and mesoporous features.50-51 The pore size distribution of samples ranges from 9 Å to 140 Å. In general, the micropores (< 20 Å) offer adequate surface area and abundant active sites for O2 adsorption and interfacial reactions, while the mesopores (20-500 Å) facilitate the mass transportation process. The cobalt species are generally confined in the micropores. This structure prevents

trode, respectively. The preparation of catalyst ink was as follows; PJ RI FDWDO\VW ZDV VXVSHQGHG LQ P/ HWKDQRO DQG / wt. % Nafion aqueous solution under ultrasonication for 30 min. 7KHQ / RI WKH DERYH dispersion was dropped onto a polished glassy carbon electrode (RDE, 5 mm diameter, RRDE, 5.61 mm diameter, Pine) and dried at room temperature. The catalyst loading is 0.24 or 0.19 mg cm-2 respectively. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) curves were recorded with a scan rate of 10 mV s-1 in the potential range of -1.0 V to +0.2 V (vs. Ag/AgCl) for ORR and +0.2 V to +1.0 V (vs. Ag/AgCl) for OER. Besides, the rotating speeds for ORR were set as 400, 625, 900, 1225 and 1600 rpm, while the value for OER was fixed to 1600 rpm in order to repel the oxygen bubble released during the reacting process. Unless otherwise mentioned, 20 CV cycles were conducted to obtain stable experimental data before actual measurements. For fully comparison, the commercial 20 wt. % Pt/C and IrO2 were also inspected utilizing the same method.

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J is the measured current density, JL, JK are the diffusion and kinetic limiting current densitiHV UHVSHFWLYHO\ & LV DQJXODU YHORFLW\ J is electron transfer numbers, ( is the Faraday constant (96485 C molí1), %4 is the bulk concentration of oxygen (1.26 × 10í6 mol cmí3), &4 is the diffusion coefficient of oxygen in the electrolyte (1.9 × 10í5 cm2 sí1) and R is kinetic viscosity of 0.1 M KOH (0.01 cm2 sí1). Basically, $ is the slope of a straight line obtained from linear fitting of plots from measured current density between -0.8 V to -0.3 V of LSV curves at different disk rotating speeds. With further exploration of the mechanism of ORR, rotating ringdisk electrode measurement was carried out through recording LSVs in O2-saturated alkaline media. A glassy carbon disk (5.61 mm diameter, Pine) with a Pt ring (6.25 mm inner-diameter and 7.92 mm outer-diameter, Pine) was used as the working electrode. The *16? % yield is an evaluation index to four-electron selectivity of catalysts, which could be calculated from the following equation: *6 16 :¨; L trr H

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+½ is the disk current and +Ë is the ring current, N=0.37 is the ring collection efficiency of Pt ring. Calibration of Ag/AgCl/saturated KCl reference electrode and conversion to RHE. The calibration of Ag/AgCl/saturated KCl reference electrode was performed in a standard three-electrode system with polished Pt wires as the working and counter electrodes, and Ag/AgCl/saturated KCl as the reference electrode. Before tests, the electrolyte (0.1 M KOH) was pre-purged with high purity H2 for at least 20 min to ensure H2 saturation. After that, Cyclic voltammetry were carried out at a scan rate of 1 mV s1 . The average of the two potentials at which the current crossed zero was chosen as the thermodynamic potential.



the metal species from dissolution and agglomeration, which can explain the favorable electrochemical stability of as-fabricated CoO@Co3O4/NSGs. The summary of specific surface area (SSA) and pore volume of CoO@Co3O4/NSG-550, 650 and 750 are displayed in Table S2, where the fractions of each pore category are specifically calculated. Notably, with the largest micropores SSA/ mesopores SSA ratio at 650 oC, the CoO@Co3O4/NSG-650 behaves best for oxygen evolving reactions among all samples synthesized at different carbonization temperatures. By solely adjusting the pyrolysis temperatures, this work suggests the possibility of designing a good catalyst with coexistence of micropores and mesopores at optimum combination. The FTIR spectra of CoO@Co3O4/NSG-550, 650, 750 and ZIF@S-GO (Figure S6) exhibits almost identical patterns, elucidating that the S\URO\VLV WHPSHUDWXUH GRHVQ¶W KDYH VLJQLILFDQW LQIOXHQFH RQ WKH fundamental bonding structures of the as-prepared catalysts. The wide peaks at 3460 cm-1 and 3145 cm-1 are due to the hydroxyl (OH) and unsaturated hydrocarbon (C-H) stretching vibration, while the narrow peak appears at 1398 cm-1 stems from C-H bending vibration. The weak peaks emerge at 1628 cm-1 and 1101 cm-1 can be ascribed to C=N and C-S-C streching vibration, respectively, which can be in consistence with the XPS results. Adsorption peaks around 660 cm-1 and 570 cm-1 can be associated with Co-O vibration, among which the band at 570 cm-1 refers to Co3+ and the one at 660 cm-1 refers to Co2+ .52

Figure 1. The FESEM images of (a) ZIF@S-GO composites, (b) CoO@Co3O4/NSG-650, (c) TEM and (d) HRTEM of CoO@Co3O4/NSG-650 (insert d is the XRD curve of CoO@Co3O4/NSG-650). ID/IG ratios of CoO@Co3O4/NSG-550, 650 and 750 are displayed in the Raman spectra (Figure S7). The CoO@Co3O4/NSG-750 shows a ratio of 0.95, the lowest value of these catalysts, suggesting the largest graphitization degree. This proposal could also be confirmed by XPS results (Table S1), in which the CoO@Co3O4/NSG-750 shows the lowest ratio of (N+O+S)/C (0.19). Figure 2 shows the Co K-edge X-ray absorption near-edge structure (XANES) spectra of Co3O4, CoO, CoO@Co3O4/NSG-550, 650 and 750. The spectral shape of CoO@Co3O4/NSG-550 is similar to that of Co3O4 compound, rather than that of CoO compound, revealing the CoO@Co3O4/NSG-550 catalyst has an analogous structure to Co3O4. However, the XANES spectral of CoO@Co3O4/NSG-650 gradually moves closer to that of CoO, indicating an increasing amount of CoO compound in CoO@Co3O4/NSG-650, which is in consistent with the XPS data. In addition, observing from the FT of extended X-ray absorption fine structure (EXAFS) curves. The average bond length of the CoO@Co3O4/NSGs lies in the range

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from CoO to Co3O4, confirming the co-existence of CoO and Co3O4 in the CoO@Co3O4/NSGs.

Figure 2. Cobalt K-edge (a) XANES and (b) EXAFS curves of CoO, Co3O4, CoO@Co3O4/NSG-550, 650 and 750. The surface properties are of great significance to the activity and durability of the as-synthesized CoO@Co3O4/NSGs, for which we apply the reliable X-ray photoelectron spectroscopy (XPS) measurement to investigate. Figure 3a displays the XPS survey of CoO@Co3O4/NSG-650, containing C (66.2 at. %), N (7.27 at.%), O (19.2 at. %), Co (5.76 at. %) and S (1.57 at. %) elements. The C 1s high resolution XPS profiles (Figure 3b) of CoO@Co3O4/NSG-650 demonstrate three types of carbon binding configurations, among which the peak sits at 284.8 eV as a standard peak corresponding to the presence of C=C. Besides, the signal at 285.5 eV comfirms the existence of saturated carbon in form of C-S, C-O and C-N, while the one at 287.7 eV relates to the unsaturated carbon species in C=O and C=N.53-54 Figure 3c reveals the N 1s high resolution XPS envelopes of CoO@Co3O4/NSG-650. It is obvious that the pyrrolic N (400.5 eV) derives from primitive imidazole precursor, while the pyridinic N (398.6 eV) generates during annealing process. 55 To note, the sharp decrease of total nitrogen content (Table S1) from 12.96 at. % to 7.27 at. % does not lead to the commensurate drop in either ORR or OER activity. In turn, CoO@Co 3O4/NSG-650 possesses even better catalytic performance than CoO@Co3O4/NSG-550. Consistent to our previous work, this phenomenon suggests that it is not the quantity of nitrogen dopants that links to the catalytic abilities for oxygen evolving process, but how these atoms incorporate. Given the pyrrolic N/ pyridinic N ratios listed in Table S1, it could be observed this value increases as the carbonization temperature raises. To note, the electrocatalytic activities of CoO@Co3O4/NSGs represent a volcanic change with the increasing pyrrolic N/ pyridinic N ratio. Thereafter, it hints the possibility of designing excellent catalysts by exploring the best pyrrolic N and pyridinic N incorporation state. Figure 3d collects the oxygen binding information of CoO@Co3O4/NSG-650. Specifically, O1 at 529.9 eV refers to the oxygen in the lattice, usually combining with metals. At the meantime, O2 at 531.2 eV mostly corresponds to unsaturated oxygen type, and O3 at 532.7 eV can be ascribed to the oxygen groups at near-surface region of GO or from physi-/chemi-sorbed water under ambient condition.7, 56 Also, the XPS deconvoluted spectra of S 2p peak (Figure 3e) elucidate the sulfur atoms with binding energies of 163.5, 168.5 and 169.5 eV. The peak observed at relative low binding energy (163.5 eV) refers to the sulfur species in C-S-C, while the other two signals at 168.5 and 169.5 eV can be allocated to the functional sulfone group (C-SO2-C) and sulfite group (C-SO3-C), respectively.38, 57-58 Combining with the XAFS analysis, no evidence for the existence of cobalt sulfides can be noticed in the XPS spectra (Figure S10). These results underline the importance of sulfur dopants incorporated into the carbon plane for improving the oxygen electrode abilities. As for Co 2p spectra (Figure 3f), the deconvoluted profile accounts for CoO and Co3O4, along with their satellite peaks. The 2p spectra are split into 2p1/2 and 2p3/2 doublets owing to the orbitspin coupling. In specific, one doublet locating at 779.9 and 794.9


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eV agree to the existence of Co3O4, and the other pair at 781.4 and 796.4 eV associate with CoO, leading to an average 2p level splitting energy of 15.0 eV.59-60 Figure S11 lists the Co 2p XPS spectra of as-fabricated catalysts carbonized at 550, 650 and 750 o C, it can roughly tell that all samples consist of same types of cobalt species (CoO and Co3O4). Noticeably, the ratios of Co3O4/CoO (Table S1) of CoO@Co3O4/NSG-550, 650 and 750 are 1.07, 0.72 and 1.40, respectively. Given that the CoO@Co3O4/NSG-650 owns the best performance in boosting both ORR and OER rates, it can be proposed that there is equilibrium between CoO and Co3O4 contents. As for as current experimental results concerned, the electrocatalytic activity increases as the ratio of Co3O4/CoO declines. It brings us the prospect for deep investigating the nature of cobalt oxide sites for oxygen involving processes through merely adjusting the ratio of Co3O4 and CoO incorporated in nitrogen and sulfur co-doped carbon frameworks.

sulfur atoms are preferred in catalyzing ORR process. The halfwave potential at -3 mA cm-2 of CoO@Co3O4/NSG-550, 650, 750, CoO@Co3O4/NG-650, CoO@Co3O4/NSC-650, commercial Pt/C and IrO2 are +0.77 V, +0.79 V, +0.76 V, +0.75 V, +0.70 V, +0.83 V and +0.35 V respectively. To note, it could be simply gained that CoO@Co3O4/NSG-650 is the best-performing electrocatalyst among all CoO@Co3O4/NSG composites for ORR, falling merely 40 mV in value behind noble catalyst Pt/C. In addition, the tafel slopes of CoO@Co3O4/NSG-550, 650, 750 and Pt/C lied in Figure 5b are 57, 63, 59 and 67 mV dec-1, respectively, which signifies CoO@Co3O4/NSGs hold better intrinsic catalytic activities even than commercial Pt/C. We ascribe the good intrinsic catalytic abilities of CoO@Co3O4/NSGx to the unique 3D structure with self-linked polyhedron-like particles despoiting uniformly on GO, speeding up both mass transportation and electron delivering processes. To further figure out the ORR pathway, rotating ring-disk electrode (RRDE) tests are adopted to monitor the formation of *16? during ORR. Hereafter, the current density of disk and ring of CoO@Co3O4/NSG-650 are shown in Figure 6a. Meanwhile, the corresponding *16? yield and electron transfer numbers, calculated from Figure 6a, are displayed in Figure 6b. Thus, It can be concluded the *16? % yield is below 13 %, indicating a one-step reduction of O2 to H2O. The electron transfer numbers varies from 3.77 to 3.92, leading to 3.83 on average. Comprehensively, CoO@Co3O4/NSG-650 has been proved to be an excellent electrocatalyst for promoting ORR, with favorable catalytic activity through nearly absolute four-electron pathway, owing to the complicated mutual effects between active metal species CoO@Co3O4 and nitrogen, sulfur functionalized graphene layer.

Figure 3. (a) XPS survey and the HR-XPS spectra of (b) C 1s, (c) N 1s, (d) O 1s, (e) S 2p and (f) Co 2p of CoO@Co3O4/NSG -650, respectively. For ORR, the electrocatalytic activities of all catalysts are evaluated by cyclic voltammetry and linear sweep voltammogram measurements in 0.1 M KOH electrolyte. The CV tests are carried out in both N2- and O2- saturated 0.1 M KOH. As shown in Figure 4a, b, no obvious redox peak is observed in N2- saturated electrolyte. In contrast, a distinct cathodic reduction peak appears around +0.75 V, standing for the oxygen reduction process, which suggests the decent ability of the as-prepared materials for catalyzing ORR. Figure 4c illustrates the rotating disk electrode (RDE) polarization curves of CoO@Co3O4/NSG-650 at varied rotating rates ranging from 400 rpm to 1600 rpm. The shortened diffusion layer at high speed results in a noticeable increase of limiting current density along with the growing of the rotating speed.61-62 The corresponding Koutecky-Levich (K-L) plots obtained from LSVs at different rotating speeds show good linearity ranged from +0.2 V to +0.6 V, suggesting the similar electron transfer numbers (n) even at different potentials (Figure 4d). Compared with commercial 20 wt. % Pt/C (n=4.11), CoO@Co3O4/NSG-550, 650 and 750 represent almost equivalent electron transfer numbers, which are successively 3.98, 4.15 and 3.99, during ORR catalyzing process. This phenomenon confirms that CoO@Co3O4/NSG-550, 650 and 750 boost ORR via a favorable four-electron pathway. Furthermore, Figure 5a presents the electrocatalytic activities and kinetic parameters of CoO@Co3O4/NSG-550, 650, 750, CoO@Co3O4/NG-650, CoO@Co3O4/NSC-650, Pt/C and IrO2 at 1600 rpm. Here, CoO@Co3O4/NSC-650, fabricated without addition of GO, displays the worst catalytic performance among all the assynthesized samples. Thereafter, it could be simply assumed that the introduction of GO seems indispensable for enhancing the catalytic ability through the improvement of the catalyst¶s conductivity. As well, CoO@Co3O4/NG-650 without any sulfur functional groups shows poor ORR catalytic ability, indicating that the

Figure 4. CV curves of CoO@Co3O4/NSG-550, 650 and 750 in (a) N2-saturated and (b) O2-saturated alkaline media, (c) LSV curves and (d) K-L plots of CoO@Co3O4/NSG-650.

Figure 5. (a) LSV curves of CoO@Co3O4/NSG-550, 650, 750, CoO@Co3O4/NG-650, CoO@Co3O4/NSC-650, Pt/C and IrO2, and (b) Tafel slopes of CoO@Co3O4/NSG-550, 650, 750 and Pt/C.



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Figure 6. (a) RRDE voltammogram of CoO@Co3O4/NSG-650, and (b) the *16? % yield and the electron transfer numbers. Apart from ORR, we as well employ three-electrode system for investigating the catalytic performance for OER of the asfabricated materials. Figure 7a contains the polarization curves of CoO@Co3O4/NSG-550, 650, 750, CoO@Co3O4/NG-650, CoO@Co3O4/NSC-650, Pt/C and IrO2 at 1600 rpm from +1.2 to +2.2 V (vs. RHE). The potentials of CoO@Co3O4/NSG-550, 650 and 750 turn out to be 1.83, 1.69 and 1.73 V at the current density of 10 mA cm-2, respectively. The sample pyrolyzed at 650 oC requires the lowest overpotential to reach the same current density, which proves the best catalytic activity for OER. To note, there is merely 10 mV in potential at 10 mA cm-2 between CoO@Co3O4/NSG-650 and best-performing precious metal catalyst IrO2 (1.68 V) for OER. Furthermore, the alike tafel slopes of CoO@Co3O4/NSG-650 (366.7 mV dec-1) and IrO2 (396.4 mV dec-1) imply that they undergo almost the same reaction mechanism in catalyzing OER process (Figure 7b). Generally, the total oxygen electrocatalytic activity is evaluated by the discrepancy between the OER current density at 10 mA cm-2 and the ORR current density at -3 mA cm-2. As shown in Figure 8, the CoO@Co3O4/NSG-550, 650 and 750 exhibit a difference of 1.06, 0.90 and 0.96 V, respectively, much smaller than Pt/C (1.19 V), IrO2 (1.33 V) and so forth. Without sulfur element dopant, the catalyst shows a poorer activity, indicating that co-doped by nitrogen and sulfur elements which possess different electronegativities can achieve an extraordinary electron distribution that further improves the activity towards electrochemical reactions. The results indicate that we have synthesized a serious of excellent bifunctional catalysts for ORR and OER.

Figure 7. (a) Oxygen evolution activities of CoO@Co3O4/NSG550, 650, 750, CoO@Co3O4/NG-650, CoO@Co3O4/NSC-650, Pt/C and IrO2, and (b) Tafel slopes of CoO@Co3O4/NSG-550, 650, 750, Pt/C and IrO2.

Figure 8. The overall oxygen electrode CoO@Co3O4/NSG-550, 650, 750, Pt/C and IrO2.

activities

of

Figure 9. (a) RDE polarization curves of CoO@Co3O4/NSG-650 before and after 10000 potential cycles raging from 0.6 to 1.0 V (vs. RHE) in O2-saturated 0.1 M KOH and (b) methanol crossover resistance test at +0.57 V (vs. RHE) for CoO@Co3O4/NSG-650 and commercial 20 wt. % Pt/C catalyst. Remarkably, As displayed in Figure 9a, the best CoO@Co3O4/NSG-650 exhibits a robust long-term stability even after 10000 cycles in rotating disk electrode (RDE) test. The similar phenomenon could also be observed by i-t test in Figure S16, even better than Pt/C, the CoO@Co3O4/NSG-650 suffers negligible current loss with 95 % retention after reacting 40000 s at the rotating speed of 1600 rpm in 0.1 M KOH. The good stability may be due to the unique structural and electronic properties of CoO@Co3O4/NSG-650. The methanol tolerance of catalysts is one important index for methanol fuel cell applications. The current of CoO@Co3O4/NSG-650 changes insignificantly and returns to stable quickly, while the Pt/C shows a sharp current decrease with the injection of methanol, indicating the CoO@Co3O4/NSG-650 has higher fuel selectivity for ORR with strong methanol tolerance against crossover effect than commercial Pt/C catalyst. CONCLUSION In summary, 3D nitrogen, sulfur co-doped carbon architectures supported cobalt oxides with polyhedron-like particles grafted onto the graphene layers have been developed. The unique structural and electronic properties endow the as-synthesized nanomaterials with excellent catalytic activity and durability for both ORR and OER. The facile two-step synthesis only associates with cheap and earth-abundant precursors, which is crucial for the renewable development of future energy conversion and storage system. To note, the sulfone and sulfite groups with high binding energies decorated in the graphene layers evidently play indelible roles for enhancing the electrocatalytic performance toward oxygen electrode reactions. Besides, the synergistic effect


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between nitrogen, sulfur species and cobalt oxides as well improves the catalytic abilities of the as-synthesized carbon materials. Along with its high stability and durability, we would expect that CoO@Co3O4/NSG-650 could be used as an available bicatalyst for the application of fuel cells and metal-air batteries.

$662&,$7(' &217(17 Supporting Information. Ag/AgCl/saturated KCl reference electrode calibration; FESEM and TEM images of CoO@Co3O4/NSG-550 and 750; EDS mappings of CoO@Co3O4/NSG-650; N2-adsorption/desorption isotherms, FTIR profiles and Raman spectra of all samples; HR-XPS images; Elemental information; Polarization curves of CoO@Co3O4/NSG550 and 750 by RRDE; Table of the performance of other bifunctional electrocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Tel: +86-21-64250924.

ORCID Xiaobo Huang: 0000-0002-3235-4837 Jianqiang Wang: 0000-0003-4123-7592 Hongliang Bao: 0000-0001-8657-5355 Xiangkun Zhang: 0000-0002-1237-3678 Yongmin Huang: 0000-0002-4005-2150

Author Contributions All authors have given approval to the final version of the manuscript.

Funding Sources Dr. Xiaobo Huang, Dr. Xiangkun Zhang and Prof. Yongmin Huang received funding from the National Natural Science Foundation of China (No. 21476071).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was provided by the National Natural Science Foundation of China (No. 21476071) and Research Centre of Analysis and Test of ECUST.

ABBREVIATIONS ORR, oxygen reduction reaction; OER, oxygen evolution reaction; MOFs, metal-organic frameworks; ZIFs, zeolitic imidazolate frameworks; GO, graphene oxide; FESEM, field-emission scanning electron microscopy; TEM, transmission electron microscopy; BET, Brunauer-Emmett-Teller; XRD, X-ray diffraction; FTIR, Fourier transform infrared spectroscopy; XPS, X-ray photoelectron spectroscopy; XAFS, X-ray absorption fine structure; XANES, X-ray absorption near-edge structure; ESAFS, extended X-ray absorption fine structure.

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Sputtering Deposition Cu@Onion-like N-C as HighPerformance Electrocatalysts for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2017, 9 (48), 41945-41954.



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3D nitrogen, sulfur co-doped carbon nanomaterials supported cobalt oxides with polyhedron-like particles grafted onto graphene layers as highly active bicatalysts for oxygen evolving reactions


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3D nitrogen, sulfur co-doped carbon nanomaterials supported cobalt oxides with polyhedron-like particles grafted onto graphene layers as highly active bicatalysts for oxygen evolving reactions 35x15mm (300 x 300 DPI)


3D Nitrogen, Sulfur-Codoped Carbon Nanomaterial-Supported Cobalt

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