Nitrogen-, Oxygen- and Sulfur-Doped Carbon-Encapsulated Ni3S2

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N-, O- and S-doped Carbon-Encapsulated Ni3S2 and NiS Core-Shell Architectures: Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Reduction Reactions Yangfei Cao, Yuying Meng, Senchuan Huang, Shiman He, Xiaohui Li, Shengfu Tong, and Mingmei Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04029 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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ACS Sustainable Chemistry & Engineering

N-, O- and S-doped Carbon-Encapsulated Ni3S2 and NiS Core-Shell Architectures: Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Reduction Reactions Yangfei Cao†, Yuying Meng*, †, Senchuan Huang†, Shiman He†, Xiaohui Li†, Shengfu Tong†, and Mingmei Wu*, † †

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-

Sen University, No. 135, Xingang Xi Road, Guangzhou 510275, P. R. China *Corresponding Author: [email protected] (Dr. Y. Meng); [email protected] (Prof. M. Wu) KEYWORDS: N-, O- and S-doped carbon-encapsulated, Ni3S2/NiS core-shell nanostructures, bifunctional, hydrogen evolution reaction, oxygen reduction reaction.

ABSTRACT: Developing cost-effective and energy-efficient noble metal-free bifunctional electrocatalysts for clean and renewable energy conversion systems, such as fuel cells and water splitting devices, has been highly desirable nowadays. Herein, we report the successful synthesis of N-, O- and S-doped carbon-encapsulated Ni3S2 and NiS core-shell architectures (Ni3S2/NiS/NOSCs) by pyrolysis of S- and Ni(II)-containing polypyrrole solid precursors,

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producing carbon-encapsulated Ni3S2 composites (Ni3S2/NOSCs), followed by the conversion of the Ni3S2 core’s surface into NiS shell with concentrated HCl solution. The materials are proven to serve as bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) in alkaline media. Notably, Ni3S2/NiS/NOSC-900, pyrolyzed at 900 °C, exhibits remarkable electrocatalytic performance toward HER with a low overpotential of 180 mV at a current density of 10 mA cm-2, a small Tafel slope of 83 mV dec-1, and a good longterm stability of 15 h. Moreover, it can also efficiently electrolyze ORR with good performance, affording positive onset and half-wave potentials, high electron transfer number (~4), as well as robust stability and methanol crossover tolerance. The materials’ excellent catalytic activities might be attributed to the synergistic effect between NOSC layers and Ni3S2/NiS core-shell nanostructures, as well as the interface effect between the NiS and Ni3S2 phases.

INTRODUCTION Nowadays, increasing energy crisis and environmental pollution caused by overconsumption of fossil fuel have driven researchers to explore clean and renewable energy. Among variety of candidates, hydrogen (H2), due to its high gravimetric energy density and no harmful products after combustion, has long been considered as the most promising energy carrier.1,2 Electrochemical water splitting for sustainable hydrogen production from renewable sources (e.g., sunlight and wind) is a prospective and appealing approach to address the aforementioned global energy-related issues.3,4 However, the sustainable electrocatalytic water splitting process, including cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER), has been largely hampered, mainly because of the high cost and scarcity of the noble-metal-based (i.e., platinum) electrocatalysts for the sluggish HER.5-7 Hence,


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numerous efforts have been devoted to finding the decent and stable HER electrocatalysts that composed exclusively of low-cost and earth-abundant elements.6,8 These efforts have fortunately borne some transition-metal-based HER electrocatalysts with good activity such as sulfides,9-14 borides,15,16 carbides,17-19 selenides,20-22 nitrides23,24 and phosphides,25,26 etc. For instance, Chhowalla et al. reported that the chemically exfoliated WS2 nanosheets can efficiently electrolyze hydrogen evolution with very low overpotentials and Tafel slopes in acidic media.27 Wang’s group successfully synthesized V-doped Co4N nanosheets that have been proven to serve as efficient HER electrocatalysts, displaying an overpotential of 37 mV at the current density of 10 mA cm-2.28 However, these HER electrocatalysts easily suffer from leaching, oxidation, sintering, and/or agglomeration, which gradually lose their catalytic activity and shelf-life over time during their electrocatalytic process.29 Over the last decades, another class of materials comprising metal/metal-based nanoparticles encapsulated by heteroatoms-doped (i.e., N, S and P et al.) carbon materials have been intensively investigated as robust and stable electrocatalysts for HER. Zou and co-workers found that the nanomaterials consisting of ultra-small molybdenum carbide (Mo2C) nanoparticles embedded within nitrogen-rich carbon nanolayers showed remarkable catalytic activity toward HER with outstanding stability over a wide pH range.30 Bao’s group reported that nonprecious metal nanoparticles encapsulated with N-doped carbon materials could efficiently improve the electrocatalytic performance of HER over a wide pH range with excellent stability.31 On the other hand, heteroatoms-doped carbon-based materials have been previously reported to serve as efficient electrocatalysts for oxygen reduction reaction (ORR), the sluggish cathodic reaction in the fuel cells or metal-air batteries.32-36 Xing and co-workers fabricated meso/macroporous nitrogen-doped carbon encapsulated Fe3C architectures, which were found to display outstanding


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activity and stability toward the ORR in both acidic and alkaline solutions.37 In virtue of their encapsulation structure and the facile electron transport existing in their large interfaces, the metallic nanoparticles cores and the carbon shells in these materials synergistically increase the electrocatalytic performance. Heteroatoms-doping can effectively increase the conductivity and modulate the work function of the carbon, both of which can enhance the electrocatalytic activity.38,39 Besides, the doped carbon layers enable protection of the nanoparticle cores from undergoing dissolution and aggregation, and thereby significantly improving the stability of the material during electrocatalytic reactions.40 In this work, we synthesized the N-, O- and S-doped carbon-encapsulated Ni3S2 and NiS core-shell architectures (Ni3S2/NiS/NOSCs) via pyrolysis of S- and Ni(II)-containing polypyrrole solid precursors, followed by transformation of the out layer of Ni3S2 core into NiS shell with concentrated HCl solution. Thanks to the synergistic effect between N-, O- and S-doped carbon layers (NOSCs) and Ni3S2/NiS core-shell nanostructures, as well as the interface effect between the NiS and Ni3S2 phases, the materials showed bifunctional electrocatalytic performance for HER and ORR in basic media, along with robust stability. EXPERIMENTAL SECTION Synthesis of Ni3S2/NiS/NOSCs S-containing polypyrrole (denoted as S-PPY) was first prepared using our previously reported method.41,42 Then, the as-prepared S-PPY (4.8 g) was uniformly dispersed in deionized water (10 mL), followed by dropwise adding nickel (II) chloride solution (10 mL, 0.46 M). After continuous stirring for 6 h, the Ni2+ ions were completely chelated with the N-sites on S-PPY. Then the dispersion was directly dried at 80 °C for 24 h in an oven without centrifugation or


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filtration, allowing water to evaporate completely. The collected black powder, containing Sspecies, Ni-species and PPY (named as S-PPY-Ni), was subsequently placed into a temperatureprogrammable tube furnace and underwent a pyrolysis treatment under Ar with a flow of 30 mL min-1, which involved two steps: 1) The furnace temperature was increased to 300 °C at a rate of 1 °C min-1 and maintained for 180 min, forming amorphous carbon from polypyrrole; 2) The temperature was raised to a final temperature (e.g., 900 °C) at a ramp of 10 °C min-1 and kept there for 120 min, where enabled the conversion of amorphous carbon to graphitic one. After letting the furnace cool down to the room temperature naturally, the as-prepared carbon-based materials (named as Ni3S2/NOSCs) were collected and then subjected with 1.0 M aqueous HCl solution for 48 h to convert the surface of Ni3S2 core to NiS, where 50 mg carbon-based material was dispersed in and stirred with 10 mL of 1.0 M HCl solution. Finally, the mixture was thoroughly washed and completely dried in an oven overnight, resulting in N-, O- and S-doped carbon-encapsulated Ni3S2 and NiS core-shell architectures (labeled here as Ni3S2/NiS/NOSC). In order to investigate the influence of pyrolysis temperature and nickel sulfides amount on the electrocatalytic performance, other Ni3S2/NiS/NOSCs were also synthesized, including Ni3S2/NiS/NOSC-T and Ni3S2/NiS/NOSC-R series of materials, where T represents the final pyrolysis temperature (T = 600, 700, 800, 900 or 1000 °C) and R represents the starting molar ratio of Ni(II) to pyrrole (R = 1/6, 1/3, 1/2, 3/4 and 1/1). The different synthetic parameters/conditions used to synthesize all the materials are all compiled in Table S1 (see the Supporting Information, SI, for details). Fabrication of electrodes


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The working electrode for HER measurement was prepared according to the following procedure: 1) 2.00 mg of the electrocatalyst was mixed with 200 µL of isopropanol and then sonicated at least 30 min to form a homogeneous ink suspension; 2) 2.0 µL of the suspension was dropped onto the GCE with the loading of 0.283 mg cm-2 and let it to dry under air; 3) 2.0 µL of 0.5 wt. % Nafion solution was casted onto the electrode to protect the electrocatalyst film and allowed to completely dry under ambient conditions. To fabricate the working electrode for ORR, 4 µL of the electrocatalyst suspension prepared above for HER was dropped onto the surface of a freshly polished RDE, with the loading of 0.204 mg cm-2. After being dried under atmospheric conditions, the electrode was covered by 4 µL of Nafion solution (0.5 wt. %) for protecting the electrocatalyst film, and then letting 2-propanol vaporize. RESULTS AND DISCUSSION The Ni3S2/NiS/NOSCs were synthesized via pyrolysis of S- and Ni(II)-containing polypyrrole solid precursors at high temperatures under inert atmosphere (Ar), followed by transformation of the out layer of Ni3S2 core into NiS shell with concentrated HCl solution, as illustrated in Scheme 1. Typically, three steps mainly occurred in the synthesis procedure, involving: 1) polymerization of pyrrole (PY) in the presence of ammonia persulfate, and chelation of Ni(II) ions with the resulting S-PPY; 2) pyrolysis of the as-obtained S-PPY-Ni solid precursors at different high temperatures (i.e., 600, 700, 800 900 and 1000 °C) and formation of the N-, O- and S-doped carbon-encapsulated Ni3S2 nanomaterials (Ni3S2/NOSCs); 3) transformation of the surface of Ni3S2 core into NiS shell using aqueous HCl solution (see SI for details), resulting in Ni3S2/NiS/NOSCs. Besides serving as the initiator for pyrrole polymerization, ammonium persulfate played an important role in the formation of S-doped carbon characteristic and the encapsulated Ni3S2/NiS core-shell material. Meanwhile, during the


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thermal treatment, PPY underwent carbonization and atomic rearrangement to enable the conversion of carbon framework with N-, O- and S-doping feature. By tuning the final pyrolysis temperature and the initial molar ratio of Ni(II) to pyrrole (Ni(II)-to-pyrrole ratio) in the precursor, two series materials, namely Ni3S2/NiS/NOSC-T and Ni3S2/NiS/NOSC-R were obtained, where T represented the final pyrolysis temperature (T = 600, 700, 800, 900 and 1000 °C) and R referred to the Ni(II)-to-pyrrole ratio (R = 1/6, 1/3, 1/2, 3/4 and 1/1), respectively (Table S1 in SI). Here, we denoted Ni3S2/NiS/NOSC-900 and Ni3S2/NiS/NOSC-1/2 as the same material, which was pyrolyzed at 900 °C with the R value of 1/2. In addition, for comparison, a control material, N-, O- and S-doped carbon black (labeled as NOSCB) with no Ni3S2/NiS coreshell embedding, was synthesized solely from S-PPY precursor in the absence of nickel species. The phase composition and crystallographic structures of the synthesized Ni3S2/NiS/NOSCs were confirmed by X-ray diffraction (XRD). The XRD patterns of all materials obtained directly from pyrolysis of S-PPY-Ni precursor before being treated with concentrated HCl solution (Figure S1) illustrated that all the peaks matched well with heazlewoodite Ni3S2 (PDF#44-1418) and no any other impurities were observed. After immersing in 1.0 M HCl solution for 48 h, some peaks associated with millerite NiS (PDF#12-0041) appeared, indicating the co-existence of Ni3S2 and NiS (Figure 1a and S2) in the obtained materials. The appearance of NiS phase after acidic treatment might be attributed to the decomposition of the out layer of the metallic Ni3S2 core and the accompanying precipitation of the dissolved Ni2+ and S2- species, which can be explained by the following total reaction: 2‫ ܪ‬ା + ܰ݅ଷ ܵଶ → ‫ܪ‬ଶ + 2ܰ݅ܵ + ܰ݅ ଶା .43 The sharp and narrow characteristic of all the peaks manifested a good crystallization of Ni3S2 and NiS. Moreover, no noticeable carbon peaks were obtained in the XRD patterns because of their relatively weak intensities compared with those of the crystalline Ni3S2 and NiS phases. Different


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with Ni3S2/NiS/NOSCs, the control sample NOSCB, derived solely from S-PPY precursor, only showed two characteristic peaks of carbon (PDF#41-1487) at 2θ of 26.6 and 43.4 ° (Figure S3). The morphology and microstructure of Ni3S2/NiS/NOSCs were acquired by scanning electron microscopy (SEM). All the materials had similar morphology with interconnected solidlike particles (Figure S4 and S5) and the particle size ranged from 100 to 200 nm. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were further carried out to confirm the structure and composition of Ni3S2/NOSC-900 and Ni3S2/NiS/NOSC-900. TEM images of Ni3S2/NOSC-900 (pyrolyzed at 900 °C without HCl etching) showed that dark particles with the size of 100 - 200 nm were surrounded by heteroatoms-doped carbon layers (Figure S6a-c), which presumably were Ni3S2 species. HRTEM images (Figure S6d-f) revealed the presence of crystalline Ni3S2 with lattice fringes of 2.04, 2.87 and 4.08 Å, corresponding to the (202), (110) and (101) crystallographic planes, respectively. Moreover, as shown in Figure 1b and S7a-c, Ni3S2/NiS/NOSC-900 also consisted of several interconnected solid particles with the size in the range of 100 - 200 nm, which was consistent with the SEM results. HRTEM images (Figure 1c-f and S7d-f) showed that lighter regions with the thickness of 1 - 10 nm and darker spots with clear lattice fringes constituted the interconnected solid particles, which were referred to the NOSC layer and nickel sulfides nanoparticles, respectively. This suggested nickel sulfides had been successfully surrounded by NOSC materials. Moreover, two phases, which were Ni3S2 and NiS, co-existed in the nickel sulfides materials. Three sets of fringes inside the cores with the lattice spacing of 1.88, 2.04 and 2.38 Å was indexed to the (120), (022) and (003) crystallographic planes of hexagonal Ni3S2, respectively. The angle between the (120) facet and the (022) facet obtained along the [2-11] crystallographic direction of Ni3S2 was found to be about 39.2 °, and the angle between (022) and (003) facets was given to be 55.1 °, which were all


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in consistent with the theoretical values in crystalline Ni3S2. However, other sets of lattice fringes outside with the interplanar spacing of 1.63, 2.63, 2.78 and 2.95 Å were assigned to the (23-1), (111), (300) and (0-11) planes of hexagonal NiS. The angle between the two facets of (111) and (0-11) was calculated to be about 52.1 °, and this agrees well with the theoretical value in crystalline NiS. It was worth noting that Ni3S2 was completely surrounded by NiS phase and the whole materials showed core-shell-shell hierarchical structure. Thanks to the intimate contact between the NOSC shell and NiS interlayer, as well as the interface between the two nickel sulfides phases (NiS and Ni3S2), the electrons can be easily transferred during the electrochemical reaction, and thereby enhance the electrocatalytic performance. Additionally, the NOSC shells should be able to protect Ni3S2/NiS core-shell nanoparticles from undergoing possible oxidation/dissolution or agglomeration during the electrocatalytic reaction, leading to a good durability of Ni3S2/NiS/NOSCs. Elemental mapping with scanning TEM energy dispersive X-ray spectroscopy (STEMEDX) was performed to explore the elemental distribution of C, N, O, S and Ni in the materials. The elemental mapping images were taken from the region of the high angle annular dark field (HAADF) images (Figure 1g). As shown in Figure 1h-l, C, N, O and S elements uniformly distributed throughout the material, while Ni atoms were inclined to be localized in a few isolated regions rather than distributed in the whole area, revealing that Ni3S2/NiS core-shell nanoparticles were completely embedded in NOSC materials. On the other hand, the control material NOSCB showed no encapsulation-like structure, but rather the typical morphology of the graphitic carbon with onion-like feature (Figure S8a). However, there was no major difference in morphology between the Ni3S2/NiS/NOSC-900 and NOSCB in SEM images. The


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interconnected nanoparticles with diameters of 100 - 200 nm were observed in both cases (Figure S5d and 8b). Raman spectroscopy was carried out to analyze the graphitization degree of the carbonbased materials. All the Raman spectra of Ni3S2/NiS/NOSCs showed two predominant peaks at the wavenumber of 1365 and 1585 cm-1 (Figure 2a and S9a), corresponding to the characteristic D and G bands, respectively.44,45 The D band, associated with the disordered carbon and structural defects, was presumably due to the substitutional doping of N, O, and/or S atoms into the carbon framework and the concomitant absence of some graphitic carbons in the Ni3S2/NiS/NOSCs. While the G band, related to tangential vibrations of sp2 carbon atoms, was suggestive of the existence of graphitic structure in Ni3S2/NiS/NOSCs.46 The ID/IG ratio, reflecting the relative degree of disorder/order carbon in the materials, was further obtained. When the pyrolysis temperature used to make the materials was raised from 600 to 1000 °C, the ID/IG ratio decreased from 1.03 to 0.80 for Ni3S2/NiS/NOSC-600 and Ni3S2/NiS/NOSC-1000, respectively (Figure 2b). This revealed the relative amount of ordered, graphitic structure in the Ni3S2/NiS/NOSCs increased as higher pyrolysis temperature was used for the synthesis of the materials. In the case of Ni3S2/NiS/NOSC-R materials, with increasing the amount of Ni(II) used for the synthesis of the materials or increasing R from 1/6 to 1/1, the ID/IG decreased from 0.84 to 0.72 (Figure S9b). The result could be explained by the fact that the higher amount of Ni-species formed during the pyrolysis process was favorable for the formation of more ordered carbon or graphitic structure on the surface of Ni3S2 nanoparticles, which, in turn, was beneficial for improving the electrical conductivity, and possibly also the electrocatalytic properties.47-49 Moreover, the Ni3S2/NiS/NOSC-900 had a smaller ID/IG ratio (≈ 0.81) than that of NOSCB (≈


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0.88, Figure S10), once again confirming the presence of Ni-species facilitate the formation of graphitic carbon. The elements composition and chemical states of Ni3S2/NiS/NOSCs were determined by Xray photoelectron spectroscopy (XPS) and high-resolution XPS. In the XPS survey spectra (Figure 2c and S11a), three main peaks centered at 284.8, 399.5 and 532.2 eV, corresponding to C 1s, N 1s, O 1s were detected for all Ni3S2/NiS/NOSCs. Besides, there were two minor peaks located at 163.4 and 855.2 eV, which were assigned to S 2p and Ni 2p, respectively. The XPS results indicated the Ni3S2/NiS/NOSCs primarily contained C, N and O, as well as a minor amount of S and Ni. While the N dopants predominantly came from the polypyrrole precursor, the O atoms might come from the oxygen-containing species (i.e., persulfate ions).41,42,50 Moreover, Ni and S atoms exclusively derived from nickel salts and ammonia persulfates. It was found that the N/C atomic ratio for Ni3S2/NiS/NOSC-T significantly decreased when the pyrolysis temperature was increased from 600 °C to 1000 °C, e.g., from 23.7 % for Ni3S2/NiS/NOSC-600 to 5.3 % for Ni3S2/NiS/NOSC-1000 (Figure 2d), suggesting that more N atoms were lost during higher temperature pyrolysis of PPY precursor. On the other hand, Ni/C atomic ratio had a slight decrease as the pyrolysis temperature increased, while S/C ratio showed a gentle increase. In other words, more S atoms were incorporated into the carbon skeleton when higher pyrolysis temperature was used. The decrease in the content of Ni over the Ni3S2/NiS/NOSC-T materials might be because Ni3S2/NOSCs, obtained from higher pyrolysis temperature and possessed lower content of carbon protection layer (Figure S12), tended to more easily dissolve Ni3S2 phase and leach out Ni2+ when treated with HCl solution. In addition, increasing the initial Ni(II) amount lead to the increase in the content of encapsulated nickel sulfides (Figure S11b).


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In the high-resolution XPS spectra of N 1s for Ni3S2/NiS/NOSC materials (Figure S13), the N 1s peaks were mainly deconvoluted into two different states, including pyridinic-N and graphitic-N with the peaks at 398.9 and 401.1 eV, respectively.51 When the pyrolysis temperature was increased, the amount of graphitic-N was found to increase while that of pyridinic-N gave an opposite tendency (Figure S14), which might be caused by the more stable of graphitic-N than pyridinic-N did when higher temperature was employed.52,53 The highresolution S 2p peaks were mainly deconvoluted into five peaks (Figure S15) associated with SNi (162.1/163.2 eV), C-S-C (163.9/165.0 eV) and SOxn- (168.9 eV) species.54 The relatively high intensity of C-S-C species in all deconvoluted S 2p spectra suggested that S atoms must have been incorporated into the carbon shells as dopants besides forming the encapsulated Ni3S2/NiS core-shell materials. The peaks located at 856.3 and 874.3 eV (Figure S16), along with two satellite peaks at 862.1 and 880.1 eV, in Ni 2p deconvoluted spectra, were assigned to the Ni-S bond in Ni3S2/NiS/NOSCs.55 In Figure S17, the deconvolution of the XPS spectra for C 1s for the Ni3S2/NiS/NOSCs showed five different characteristic peaks, which were ascribed to C=C-C (284.8 eV), C-S (285.6 eV), C-N/C-O (286.5 eV), C(=O)-O (288.1 eV), and π-π* (289.5 eV), respectively.56,57 The existence of C-S and C-N bonds was suggestive of the successful doping of S and N atoms into the carbon framework. The deconvolution of the XPS spectra for N 1s, S 2p and C 1s for the control sample NOSCB was also given in Figure S18 and no Ni-S bonds were observed in S 2p spectrum, conforming this material was nickel sulfides free. The electrocatalytic activities of Ni3S2/NiS/NOSCs toward HER were then systemically measured in 1.0 M KOH solution on a standard three-electrode configuration. For comparison, the electrocatalytic activity of control materials NOSCB, NixSy (Ni3S2/NiS core-shell nanomaterials), NixSy + NOSCB (mixture of NixSy and NOSCB with the weight ratio of 1:4), as


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well as commercially available Pt/C (20 wt. %) were also examined under otherwise identical conditions. The linear sweep voltammetry (LSV) curves with iR-correction (compensation level: 90 %) in Figure 3a and S19 displayed that Ni3S2/NiS/NOSC-900 endowed an excellent activity for HER with a small onset potential of 73 mV (vs. RHE) and a small overpotential of 180 mV delivering a current density of 10 mA cm-2. This remarkable HER performance can be closed to that of Pt/C (20 wt. %) and comparable with those of some carbon-based noble metal-free alkaline HER electrocatalysts (Table S2). It was worth mentioning that although NixSy and NOSCB could electrocatalyze HER solely, they gave inferior activities, with relatively larger overpotentials to afford the current density of 10 mA cm-2, i.e., 354 and 390 mV for NixSy and NOSCB, respectively (Figure S20). Besides, the NixSy + NOSCB also required a large overpotential of 350 mV to obtain a current density of 10 mA cm-2. Hence, we can conclude that the excellent HER performance of Ni3S2/NiS/NOSC-900 should be attributed to the core-shellshell architecture and the synergistic effect between the NOSC shells and Ni3S2/NiS core-shell nanoparticles, as well as the electron transport between the interface of Ni3S2 and NiS phases. In order to investigate the effect of pyrolysis temperature on catalytic performance, the electrocatalytic activities toward HER over different Ni3S2/NiS/NOSC-T materials were studied. As shown in Figure 3a and b, all the Ni3S2/NiS/NOSC-T materials could electrocatalyze HER in alkaline media and Ni3S2/NiS/NOSC-900 gave the best performance with the most positive onset potential and smallest overpotential at a given current density. With raising the pyrolysis temperature from 600 to 900 °C, the HER performance gradually improved with the current density increasing greatly and the onset potential shifting positively. However, the current density inclined to become weakened and onset potential negatively shifted when the pyrolysis temperature was further raised from 900 to 1000 °C. The first increase can be ascribed to the


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relatively high degree of graphitization, whereas the following decay might result from the low content of N dopants and Ni3S2/NiS core-shell nanoparticles, as discussed in the above XPS results. The best HER performance of Ni3S2/NiS/NOSC-900 can be further confirmed by the smallest Tafel slope of 83 mV dec-1 among Ni3S2/NiS/NOSC-T materials (Figure 3c), which indicates the fastest HER kinetic process. The chronoamperometry experiment was carried out to study the stability of the Ni3S2/NiS/NOSC materials (Figure 3d and S21). After 15 h continuous reaction in 1.0 M KOH electrolyte at an overpotential of 200 mV, only a slight decrease in current density (~10 %) was observed, and LSV curve after stability test showed a dinky difference with the original one. Moreover, inductively coupled plasma atomic emission spectrometer (ICP-AES) results illustrate there was no Ni2+ species in the electrolyte after 15 h long-term stability test (Table S3). In addition, XPS results demonstrated no obvious difference in the composition of electrocatalyst after stability test in comparison with the original one (Figure S22). All the above suggested a good stability over Ni3S2/NiS/NOSC-900 electrode for electrolyzing HER reaction. Actually, it can keep a good electrocatalytic stability even after 100 h continuous reaction (Figure S21b). However, the current density of commercial Pt/C (20 wt. %), NixSy and the NixSy + NOSCB decayed significantly during the same period (15 h, Figure S21). Thanks to the protection of NOSC shells, Ni3S2/NiS core-shell nanoparticles were prohibited from the possible agglomeration, dissolution or oxidation during reaction, and thereby resulted in good structure and activity stability (Figure S23). In addition, the Faradic efficiency of Ni3S2/NiS/NOSC-900, evaluated by chronoamperometry method in 1.0 M KOH electrolyte over a period of 100 min at an overpotential of 320 mV, could reach up to a high value of 99 % (Figure 3e).


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In order to further explore the electrocatalytic activity of Ni3S2/NiS/NOSCs, the electrochemical impedance spectra (EIS) and electrochemically active surface areas (ECSA) of the electrocatalyst were obtained. As shown in Nyquist plots in Figure 3f, Ni3S2/NiS/NOSC-900 possessed the smallest radius among all the Ni3S2/NiS/NOSC-T materials, manifesting the smallest charge transfer resistance (Rct) and the fastest electron transfer rate in the material during HER. The ECSA of various catalysts can be determined by the double-layer capacitance (Cdl) using the equation of ECSA ≈ Cdl / Cs in a non-Faradic reaction region (Figure S24). The Cdl of Ni3S2/NiS/NOSC-900 was found to be much larger than those of other Ni3S2/NiS/NOSC-T materials, indicating Ni3S2/NiS/NOSC-900 catalyst had the largest ECSA (109 cm2) and most accessible active sites on the catalyst’s surface, which were favorable for the enhancement of the electrocatalytic performance (Figure S25). In order to investigate the effect of nickel sulfides (Ni3S2/NiS core-shell particles) content on the electrocatalytic performance toward HER, the electrocatalytic properties of Ni3S2/NiS/NOSC-R materials obtained with different ratio of Ni(II)-to-pyrrole were all evaluated. As shown in Figure S26a, the first increase in R from 1/6 to 1/2 gave rise to the improvement in HER performance, while the electrocatalytic activity decreased when further increased R to 1/1, resulting in Ni3S2/NiS/NOSC-1/2 endowed the best performance. This suggested Ni3S2/NiS/NOSC material with the moderate amount of Ni3S2/NiS core-shell particles encapsulated in heteroatoms-doped carbons gave the best activity toward HER in alkaline medium. Moreover, Ni3S2/NiS/NOSC-1/2 (i.e., Ni3S2/NiS/NOSC-900) had the smallest Tafel slope and electrochemical impedance, as well as the largest ECSA (Figure S26b-d and Table S4) among Ni3S2/NiS/NOSC-R. This, in turn, indicated the Ni3S2/NiS/NOSC-1/2 electrode gave the


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fastest kinetic reaction, smallest charge transfer resistance and the most accessible active sites, all of which could contribute to a remarkable HER activity. Besides serving as remarkable noble metal-free HER electrocatalysts in alkaline media, the Ni3S2/NiS/NOSCs were also found to have good electrocatalytic ORR performance. Cyclic voltammetry (CV) curves acquiring at a scan rate of 100 mV s-1 for Ni3S2/NiS/NOSC-900 electrode were firstly investigated in N2- or O2-saturated 0.1 M KOH electrolyte. As shown in Figure 4a, there was a significant cathodic peak at potential of 0.69 V (vs. RHE) over Ni3S2/NiS/NOSC-900 electrocatalyst in O2-saturated 0.1 M KOH solution, while no reductive response was observed in the case of N2-saturated medium, showing the material’s good electrocatalytic activity toward ORR,58 which can be comparable with some other carbon-based noble metal-free alkaline ORR electrocatalysts (Table S5). Linear sweep voltammetry (LSV) was further studied to evaluate the electrocatalytic activity using rotating disk electrode (RDE) with different rotation rates, from 400 to 2400 rpm, in an O2-saturated 0.1 M KOH solution at a sweep rate of 10 mV s-1 (Figure 4b). Ni3S2/NiS/NOSC-900 displayed a positive onset potential (Eonset) of 0.89 V and a high limiting current (jL) of 5.3 mA cm-2, which is close to that of commercial Pt/C (20 wt. %) catalyst of 5.0 mA cm-2 (Figure 4c). Although NixSy and NOSCB could also electrolyze ORR in alkaline media, the onset potentials were more negative and the current densities were smaller. Hence, the as-achieved ORR improvement of Ni3S2/NiS/NOSC900 can be attributed to the good conductivity of heteroatoms doped carbon layer, the synergistic catalytic effect of carbon shells and inner nickel sulfides, as well as the electron transfer between NiS and Ni3S2 phases in the Ni3S2/NiS core-shell architecture. Koutecky-Levich plots (K-L or j-1 vs. ω-1/2) at different overpotentials were obtained from the polarization curves and its linearity suggested the first-order reaction kinetics with regard to the concentration of dissolved oxygen


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(Figure 4d).49 The electron transfer number (n) per oxygen molecule, calculated from K-L plots, was around 4, indicating a direct four-electron process over the material for ORR. The peroxide (H2O2) yield of Ni3S2/NiS/NOSC-900 electrode, measured from rotating ring disk electrode (RRDE) technique, was below 20 % over a wide potential range of 0.15 - 0.7 V, along with the n value ranging from 3.5 to 3.7 (Figure S27), demonstrating Ni3S2/NiS/NOSC-900 underwent a dominant four-electron pathway and yielded a high selectivity of H2O production. The long-term stability of Ni3S2/NiS/NOSC-900 was studied by chronoamperometry at 0.7 V with a RDE rotating at 1600 rpm (Figure 4e). For a fair comparison, Pt/C (20 wt. %), NixSy and NixSy + NOSCB at the potential of 0.85, 0.70 and 0.68 V were also measured in the identical way, respectively. The current density of Ni3S2/NiS/NOSC-900 retained 90 % after 12 h stability test, while that of NixSy, NixSy + NOSCB and Pt/C (20 wt. %) gave a notable decay, showing Ni3S2/NiS/NOSC-900 material had much better stability than those of NixSy, NixSy + NOSCB and Pt/C (20 wt. %). Ni3S2/NiS/NOSC-900’s good stability can be derived from the protection of NOSC shells in the hierarchical core-shell-shell material. Moreover, methanol crossover measurement of Ni3S2/NiS/NOSC-900 revealed that the original cathodic ORR current remained almost unchanged after methanol being injected in the target 0.1 M KOH solution with the concentration of 1.0 M (Figure S28), suggesting its high electrocatalytic selectivity for ORR. However, a sharp decrease in current was observed when the same volume of methanol was added in the case of Pt/C (20 wt. %), and methanol oxidation reaction occurred in the electrolyte. The Ni3S2/NiS/NOSC-900’s good stability and methanol crossover tolerance, once again, verified that the material is a promising alternative noble metal-free electrocatalyst for ORR in alkaline media.


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The ORR examination results of other Ni3S2/NiS/NOSCs (i.e., Ni3S2/NiS/NOSC-T and Ni3S2/NiS/NOSC-R), obtained with different pyrolysis temperature and different Ni(II)-to-PPY ratio, were all given. With increasing the pyrolysis temperature from 600 to 900 °C, the current density gradually increased, while further increased the pyrolysis temperature to 1000 °C, the current density decayed, confirming again Ni3S2/NiS/NOSC-900 endowed the best ORR performance (Figure 4f, S29-33 and Table S6). In addition, among Ni3S2/NiS/NOSC-R materials, Ni3S2/NiS/NOSC-1/2 (Ni3S2/NiS/NOSC-900) afforded the largest current density, smallest Tafel slope and lowest H2O2 yield, as well as highest electron transfer number (Figure S34-38 and Table S6). As a result, the Ni3S2/NiS/NOSC-900’s good activity toward ORR can be ascribed to the heteroatoms doping feature, easier mass transfer property, and more exposed catalytically active sites. CONCLUSION In summary, N-, O-, and S-doped carbon-encapsulated Ni3S2/NiS core-shell architectures (Ni3S2/NiS/NOSCs) were successfully synthesized, which exhibited excellent electrocatalytic HER and ORR performance with good stability in alkaline media. Specifically, Ni3S2/NiS/NOSC-900, pyrolyzed at 900 °C, gave a low overpotential of 180 mV at a current density of 10 mA cm-2, a small Tafel slope of 83 mV dec-1 and a good stability of 15 h toward HER. Regarding for electrolyzing ORR, Ni3S2/NiS/NOSC-900 displayed a positive onset potential of 0.89 V and half-wave potential of 0.72 V, as well as good stability and methanol tolerance. The materials’ excellent catalytic activities might be attributed to the synergistic effect between NOSC layer and Ni3S2/NiS core-shell structure, and the interface effect between NiS and Ni3S2 two phases.


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Figure 1. The structure and morphology characterization of Ni3S2/NiS/NOSC-900. a) XRD pattern, b) TEM image, c- f) HRTEM images, d) and f) are the yellow squares in the corresponding c) and e), g) HAADF image, and h-l) the elemental mapping images for g).


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Figure 2. Physical characterizations for Ni3S2/NiS/NOSC-T materials obtained with different pyrolysis temperatures. a) Raman spectra, b) ID/IG ratios as a function of pyrolysis temperature, c) XPS survey spectra, and d) the content of Pyridinic-N and Graphitic-N as a function of pyrolysis temperature.


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Figure 3. Electrochemical performance toward HER in 1.0 M KOH solution over different Ni3S2/NiS/NOSC-T electrocatalysts. a) LSV curves with iR-compensation (compensation level: 90 %) at a scan rate of 5 mV s-1, b) The overpotentials at specific current densities, c) Tafel plots from the corresponding LSV curves, d) Chronoamperometric responses over Ni3S2/NiS/NOSC900 electrocatalyst. The insert is the LSV curves before and after i-t test. e) The H2 amount of theoretically calculated and experimentally determined over Ni3S2/NiS/NOSC-900 as a function of reaction time. Inset shows chronoamperometric curves at an overpotential of 320 mV for 100


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min, f) Electrochemical impedance spectra (EIS) of the materials. The loading is ~0.283 mg cm2

.

Figure 4. Electrocatalytic properties toward ORR over different Ni3S2/NiS/NOSCs. a) CVs of Ni3S2/NiS/NOSC-900 in O2- or N2-saturated 0.1 M KOH solutions at a sweep rate of 100 mV s-1, b) polarization curves over Ni3S2/NiS/NOSC-900 on RDE rotating at diﬀerent speeds, c) polarization curves over Pt/C (20 wt. %), NixSy, NOSCB, NixSy + NOSCB, and Ni3S2/NiS/NOSC-900 on RDE rotating at 1600 rpm, d) electron transfer number (n) calculating


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from K-L plots of Ni3S2/NiS/NOSC-900, e) chronoamperometric curves of Pt/C (20 wt. %) , NixSy, NixSy + NOSCB, and Ni3S2/NiS/NOSC-900 with 1600 rpm at potentials of 0.85, 0.70, 0.68 and 0.70 V, respectively, f) polarization curves of ORR over different Ni3S2/NiS/NOSC-T on RDE rotating at 1600 rpm.

Scheme 1. Schematic illustration for synthesizing Ni3S2/NiS/NOSCs hybrid electrocatalysts.


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ASSOCIATED CONTENT Supporting Information. Detailed experimental sections, instrumental characterization results (extra SEM, TEM images, XRD and XPS results and Raman results), electrochemical performances (PDF). ACKNOWLEDGMENT This work was supported by Natural Science Foundation of China (NSFC) (No. 21701199 and 51672315), Starting Project for Doctoral Fellows Sponsored by Natural Science Foundation of Guangdong Province (2017A030310503), Fundamental Research Funds for the Central Universities of China (17lgpy84), Science and Technology Planning Project of Guangdong Province for Industrial Applications (2016B090930001), and Science and Technology Planning Project of Guangzhou City for International Cooperation Program (201704030020).


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TABLE OF CONTENTS

Synopsis. N-, O-, and S-doped carbon-encapsulated Ni3S2/NiS core-shell architectures (Ni3S2/NiS/NOSCs) have been facilely synthesized. These materials can serve as bifunctional electrocatalysts in basic media for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).


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Nitrogen-, Oxygen- and Sulfur-Doped Carbon-Encapsulated Ni3S2

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