A Nitrogen Doping Method for CoS2 Electrocatalysts with Enhanced

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A Nitrogen Doping Method for CoS2 Electrocatalysts with Enhanced Water Oxidation Performance Jinhui Hao, Wenshu Yang, Zhen Peng, Chi Zhang, Zhipeng Huang, and Weidong Shi ACS Catal., Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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A Nitrogen Doping Method for CoS2 Electrocatalysts with Enhanced Water Oxidation Performance Jinhui Hao,† Wenshu Yang, ‡ Zhen Peng,ξ Chi Zhang, *,§ Zhipeng Huang, *,§ and Weidong Shi*, †

† School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R.

‡ Jingjiang College of Jiangsu University, Jiangsu University, Zhenjiang 212013, P. R. China

ξSchool of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China

§China-Australia Joint Research Center for Functional Materials, School of Chemistry Science and Engineering, Tongji University, Shanghai, 200092, P. R. China.

ABSTRACT: The thorough understanding the effects of N doping on oxygen evolution reaction (OER) are of great significance for constructing next generation electrocatalysts with optimal configuration and high efficiency for fuel cell. Herein, we reported the synthesis of N doped CoS2 through a facile method using ammonium 1 ACS Paragon Plus Environment

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hydroxide as N source, the subjection of N doped CoS2 as efficient electrocatalysts for OER, and the identification of intrinsic activities by exploring the composition and electronic configurations and their correlations with the electrochemical performance. The DFT studies evidenced that N doping could alter the electronic density of the adjacent Co atoms and thus forming well-defined electronic configurations for intermediates adsorption. Specifically, the N enriched CoS2 afford a small overpotential of 240 mV at the current density of 10 mA cm−2 and long-term durability, endowing this N doped materials the ideal, but not limited to, OER electrocatalysts.

Keywords: Nitrogen doping, Cobalt sulfide, Electrocatalysts, Oxygen evolution raction, DFT calculations 1. INTRODUCTION

To meet the increasing demand for sustainable energy, developing alternative and renewable energy sources and carriers are still ongoing challenge.1-2 Solar and wind are important sources of renewable energy and their technologies are booming worldwide.3-4 However, the intermittency and unpredictability of solar and wind energy make them difficult to stored durably. Electrochemical splitting water into hydrogen and oxygen is an appealing way for solar and wind energy conversion and storage.5-6 The electrochemical water splitting consists of two half-cell reaction: the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER).7 Of the two half-cell reaction, OER is a key determinant for overall viability of water 2 ACS Paragon Plus Environment

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splitting, which is still challenging to overcome high reaction barriers, high oxidation potential, couple of multiple proton and electron transfers, and formation of two oxygen-oxygen bonds.8-10 For OER, the state-of-the-art electrocatalysts are Iridium and Ruthenium group materials. These noble materials are capable of overcoming the thermodynamic potential with low overpotential and exhibit high electrocatalytic activities, versatility, high conductivity, and chemical inertness.11 However, the scarce nature of noble materials limits their practical application for global energy demand. To this end, the development of non-noble or metal-free electrocatalysts, which tend to lower the intrinsic activation barriers present on anode, is drawing great enthusiasm.12-13

Transition metal dichalcogenides have been thought to be promising alternative OER electrocatalysts, and intensely pursued lately. Zou et al.14 reported high-index faceted Ni3S2 nanosheet arrays for OER, the high-index faceted feature endowed the excellent performance and remarkable catalytic stability. Xie et al.15 synthesized ultrathin Co3S4 nanosheets, which served as superior electrocatalysts for OER performance under neutral conditions due to the exposed octahedral planes and self-adapted spin states. Zhang et al.16 reported a Ni3S2 nanorods/Ni foam composite and achieved an onset overpotential of 157 mV for a current density of 1 mA cm-2. These studies provided alternative ways for high performance OER electrocatalysts construction and, more importantly, parts of works verified the experimental results through theoretical calculations. However, it still remains difficult to achieve OER

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systems that are both low-cost and efficient enough to generate fuel at a price that is competitive with fossil fuels.17

N doping is considered as an efficient mean to optimize electrochemical performance by overcoming the intrinsic activation barriers during catalytic reaction. Among the effort made currently, N doped carbon based materials have been widely investigated for electrochemical applications due to their unique electronic properties and structural features. From the electronic conformations point of view, the N has a lone-pair electron in the extra nuclear electron, which prospects to be beneficial for better interaction with reactants.18 On the other hand, N atom bears the relatively higher electronegativity than that of C atom. The higher electronegativity means the increased electron occupation. The excess electrons locate near the N atom, which then process stronger electron-donating ability and thus becoming preferable sites for proton discharge.19 For C atom adjoined N atom, it renders higher positive charge density and then becomes catalytic active site for oxygen reduction reaction.20 Ascribed to the excellent electronic properties and structural features, doping with N into carbon based materials offers a promising approach for enhancing the electrocatalysts properties and has been widely served for supercapacitors, oxygen reduction reaction, HER, and OER.21-23 However, compared with carbon, the transition metal based materials are expected more efficient for water splitting, as their d bands have valence electron, which close to the Fermi level, with respect to overcome the intrinsic activation barriers and reaction kinetics.24 On the basis of this scenario, the development of N doped transition metal based materials is more 4 ACS Paragon Plus Environment

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appealing for constructing high performance water splitting systems. Previous efforts made on N related transition metal materials have been predominantly concerned on the development of transition metal nitride by using high corrosive NH3 as N source,25-26 whereas the preparation route through a mild condition has been rarely reported. Moreover, the roles of N, especially its influence on the electronic density and intermediates adsorption for OER, have yet to be further explored.

Herein, we synthesized the N doped CoS2 through a facile approach with ammonium hydroxide as both N source and precipitant and thus eliminating the need of high corrosive NH3 during the post annealing process. The functions of N doping in OER were investigated by density functional theory (DFT) calculations, including the electronic density change and the adsorption free energy variation of different intermediates. We also showed some initial results on the roles of N doping and demonstrated the N doping changes the electronic density and minimizes the adsorption free energy. In line with the results, the N enriched CoS2 exhibits a low overpotential of 240 mV for a current density of 10 mA cm-2 and long-term durability in an alkaline electrolyte, which compares favorably to other leading non-precious metal OER elctrocatalysts, rendering such advanced N doped CoS2 promising for practical applications.

2. EXPERIMENTAL SECTION

Catalyst fabrication and characterization. The NXXX-Co3O4 were synthesized through a simple hydrothermal reaction. Briefly, 58 mg Co(NO)3•H2O and 4 mg 5 ACS Paragon Plus Environment

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NaNO3 were dissolved in 10 mL water to form a homogeneous solution. A piece of carbon paper (CP) (1 cm×4 cm) was immersed in the above solution and then added 9 mL ammonium hydroxide. The mixture was transferred to a Teflon-lined autoclave and kept at 140 oC for 12 h. After being washed with deionized water and ethanol several times, the product was dried at room temperature. This NXXX- Co3O4 product was designated as N1- Co3O4. When the amount of ammonium hydroxide were 18 and 27 mL, the Co3O4 products were donated as N2- Co3O4 and N3- Co3O4, respectively.

In a typical synthesis of N doped CoS2, the NXXX- Co3O4 and sulphur powder (200 mg) were placed separately in a fused silica tube, and sulphur powder was at the upstream side. Then the fused silica tube was heated to YYY oC for 1 h with a heating rate of 10 °C/min under N2. The obtained product was donated as NXXX-CoS2-YYY. For comparison, pure CoS2 (donated as N0-CoS2) was synthesized using Co(OH)2 as procedure.27 N-CP was prepared without using Co(NO)3•H2O and NaNO3 in the hydrothermal step.

XRD spectra were obtained using a D8 ADVANCE diffractometer (Bruker, Germany) using Cu Kα (1.5406 Å) radiation. FESEM images were inspected on a Hitachi S-4800. XPS measurements were performed on an ESCALAB MKII spectrometer (VG Co., United Kingdom) with Al Kα X-ray radiation as the X-ray source for excitation.

Electrochemical measurements. Electrochemical measurements were performed with a CHI 614D electrochemical workstation. All measurements were carried out in 6 ACS Paragon Plus Environment

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a three-electrode electrochemical cell at room temperature. The obtained N doped CoS2-CP was directly used as the working electrode and a Pt electrode as the counter electrode, a mercury/mercury oxide electrode (MOE) as the reference electrode. For preparation of the RuO2 on CP, the catalyst was dispersed in ethanol to achieve a concentration of 1 mg mL-1 with 4 wt% polytetrafluoroethylene. After sonication for 30 minutes, 250 µL of the catalyst ink was drop-dried onto a 1 cm×1 cm CP (loading 0.25 mg cm-2). The potential was calibrated with respect to reversible hydrogen electrode potential (RHE), which was determined by a Pt wire as the working electrode in electrolyte saturated with the high purity hydrogen. Before the electrochemical measurement, the electrolyte (1.0 M KOH) was degassed by bubbling argon for 30 min. EIS spectra were performed at 1.53 V vs. RHE in the frequency range of 10-2–106 Hz. The volume of O2 during a potentiostatic electrolysis experiment was monitored by the water displacement method.28 The TOF value was calculated from the equation29:

TOF =

J × A

(1)

4 × F × m

J is the current density at overpotential of 300 mV in A cm-2. A is the area of the carbon fiber paper electrode. F is the faraday constant (a value of 96485 C/mol). m is the number of moles of the active materials that are deposited onto the carbon fiber paper.


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DFT calculations. The electronic properties of N doped CoS2 were investigated by DFT calculations using the CASTEP (Cambridge Serial Total Energy Package) package.30 The wave functions of the valence electrons were expanded in a plane wave basis set with k-vectors within a specified energy cutoff (300 eV). A 2×3×1 Monkhorst-Pack k-point mesh was employed. The unit cell with a slab along the (001) direction was applied in the calculations. All of the structures were fully optimized and relaxed to the ground state.

3. RESULTS

To obtain the N doped CoS2, a series of Co3O4 were first obtained through simple hydrothermal reaction by varying the amount of ammonium hydroxide, and the corresponding products were denoted as NXXX- Co3O4 hereafter, where xxx was the different percentage of ammonium hydroxide used (see details in Experimental section). The compositions of NXXX- Co3O4 were determined by standard X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. XRD patterns of the three products (Figure S1A) all exhibit negative shifts compared with Co3O4 spinel phase (JCPDS No. 42-1467). The shifts are caused by the doping of N atom, which has a bigger diameter than that of O atom. The peak positions at 2θ = 18.7o, 30.9o, 36.5o, 38.3o, 58.9o, and 64.9o, are corresponding to the (001), (100), (101), (002), (110), and (220) planes, respectively. The strong and symmetrical features of diffraction peaks suggest well crystallinity of the products.


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XPS measurements were performed to analyze the surface compositions of NXXXCo3O4. The survey XPS patterns (Figure S2) indicate the presence of Co, O, and N species. Figure S3A reveals the electron binding energies (BE) of Co 2p3/2 at 779.8 eV and Co 2p1/2 at 795.3 eV, which spin–orbit splitting value is over 15 eV, suggesting the coexistence of the Co2+ and Co3+.31-32 In Figure S3B, the O 1s peaks indicates the presence Co-O species (529.7 eV). The N 1s peaks (Figure S3C) located at 401.6, 399.9, and 397.7 eV demonstrate the presence of NH4+, NH3,33 and N-Co species,34 respectively. The morphology and structure of as-prepared Co3O4 were further examined by field emission scanning electron microscopy (FESEM) analysis. As shown in Figure S4, the N1- Co3O4, N2- Co3O4, and N3- Co3O4 nanosphere are uniformly distributed on the carbon paper (CP), interconnected with each other, with average size of 200, 400, and 500 nm, respectively. The effect of reaction time for N1-Co3O4 was also studied (Figure S5). As observed, the size of Co3O4 nanosphere is about 80 nm in diameter when reacting for 3 h. As the reaction time proceed, the particle size gradually growing bigger.


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Figure 1. Co 2p (A), S 2p (B), and N 1s (C) high-resolution XPS spectrum of N1-CoS2-400 (a), N2-CoS2-400 (b), and N3-CoS2-400 (c). After sulfidation treatment, the products were denoted as NXXX-CoS2-YYY hereafter, where XXX was the different percentage of ammonium hydroxide and YYY was the sulfidation temperature (see details in Experimental section). The XRD patterns of the three products (Figure S1B) all exhibit positive shifts compared with the CoS2 cubic phase (JCPDS No. 41-1471). The peak positions around 2θ = 32.3o, 36.3o, 39.9o, and 46.4o, are corresponding to the (200), (210), (211), and (220) planes, respectively. Wide XPS patterns in Figure S6 indicate the presence of C, S, and N elements. High resolution XPS results were showed in Figure 1. For Co 2p, the electron-binding energies gap of Co 2p3/2 (782.1 eV) and Co 2p1/2 (798.5 eV) indicates the coexistence of the Co2+ and Co3+. The satellite peaks located at 785.6 and 803.9 eV indicate the oxidation state of Co, which might be caused by oxidation due to air contact. For S 2p (Figure 1B), there are two kinds of S species and donated as S2- and S22-. Compared with pure CoS2,35 the BE of both Co 2p (Co2+) and S 2p of N doped CoS2 exhibit a positive shift, implying the successfully incorporation of N atom (has higher electronegativity than those of Co and S atom) into CoS2. As reported by Song, the XPS peaks of both Mo 3d and S 2p shifted to higher BE after incorporation of Cl into MoS2.36 Similarity, Wang reported the lower BE of both Co 2p and S 2p peaks after incorporating the P atom with different electronegativity.37 The XPS patterns of N 1s show a single peak, indicating the successfully conversion of N-H bond (NH3 or NH4+) into Co-N bond. Novelty, the percentage of N in the 10 ACS Paragon Plus Environment

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N2-CoS2-400 is as high as 13.25%. FESEM results (Figure S7) indicate no obvious change in morphology and structure after sulfidation. The microstructure of N2-CoS2-400 was further examined by high-resolution transmission electron microscopy (HRTEM) analysis. As shown in Figure S8A, there is no mesopore structure in the obtianed product. Figure S8B shows that the N2-CoS2-400 is crystalline. The lattice fringes with an average spacing of 0.246 nm can be indexed to the (210) plane.

Figure 2. (A) Polarization curves of N doped CoS2, CP, commercial RuO2, N0-CoS2-400 and N-CP in 1.0 M KOH at a scan rate of 2 mV s-1. (B) Tafel analysis of N doped CoS2 in 1.0 M KOH. (C) Polarization curves of N2-CoS2-400 in 1.0 M KOH


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at a scan rate of 2 mV s-1 before and after 1000 cycles. (D) Long-term stability measurements for N2-CoS2-400 in 1.0 M KOH.

The obtained N doped CoS2 (supported on CP) were directly used as working electrodes for OER in an Ar-saturated alkaline solution (1.0 M KOH) at room temperature using a three-electrode configuration. All the polarization curves were obtained at a scan rate of 2 mV s-1 against iR-corrected. Figure 2A shows the polarization curves of N doped CoS2 prepared with different amount of ammonium hydroxide. All the N doped CoS2 exhibit high current response, which afford a current density of 10 mA cm-2 at low overpotentials of 285, 240, and 270 mV for the N1-CoS2-400, N2-CoS2-400, and N3-CoS2-400, respectively. The N2-CoS2-400 achieves a large anodic current density of 200 mA cm-2 at the overpotential of 360 mV and a larger current density of 400 mA cm-2 at the overpotential of 410 mV. Significantly, the overpotential of the N2-CoS2-400 is much lower than the values reported for most earth-abundant OER electrocatalysts (Table S1). The OER performance of RuO2 and pure CoS2 (obtained by electrodeposition method, see details in Experimental section, donated as N0-CoS2-400) were evaluated, which show null activity in comparison with N doped CoS2. The support material CP shows negligible activity towards OER. In order to exclude the effect of N doped CP for OER performance, the N-CP product was prepared without using Co(NO)3•H2O and NaNO3. The N-CP product show null activity in comparison with N doped CoS2. Moreover, the N2-CoS2-400 is also found to perform better performance than the Co3O4 (Figure S9) and thus indicating the importance of sulfidation process. 12 ACS Paragon Plus Environment

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The Tafel plots of N doped CoS2 were shown in Figure 2B, which exhibit the Tafel slope values of 93, 115, and 113 mV decade-1 for N1-CoS2-400, N2-CoS2-400, and N3-CoS2-400, respectively. These Tafel slope values suggest different rate determining steps of N doped CoS2.38 The durability of the N2-CoS2-400 is a critical factor for its practical OER applications, which was further studied. As shown in Figure 2C, the polarization curves show negligible change between 1st and 1000th cycle. The long-term durable of the N2-CoS2-400 was investigated through the amperometric i-t measurements at the constant overpotential of 240 mV and the results were depicted in Figure 2D, which shows no obvious decay of overall current density after 38 h, affirming the excellent stability of the N2-CoS2-400 under strong alkaline solution. The OER performance of N2-CoS2-400 was confirmed by the Faradaic efficiency, which evaluated by the comparison of detected volume and theoretical volume of generated gases during potentiostatic electrolysis experiments in 1.0 M KOH. As shown in Figure S10, the detected volume of gases matches the theoretical one, implying the high water splitting efficiency for oxygen generation.

Turnover frequency (TOF) is the intrinsic quality elucidating the activities of electrocatalysts, since the current density lose insight of the difference in electrocatalysts loading amount (about 84 µg cm-2). Upon assuming all the metal sites are involved in the electrochemical reaction, the total number of active sites is the upper limit value in this scenario. Figure S11 show the calculated TOF values versus the applied potentials in the 1 M KOH solution. The N2-CoS2-400 shows a high TOF


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of 0.18 and 0.53 s-1 at 1.53 and 1.58 V versus RHE, which is much higher than those of N0-CoS2-400, demonstrating its excellent intrinsic catalytic activity for OER.

We also investigated the influence of temperature on the structure and water oxidation abilities. XRD patterns (Figure S12) indicate the high-crystallinity of N2-CoS2-400 and N2-CoS2-500, while the low-crystallinity of N2-CoS2-300 (with wide and weak XRD peaks). The survey XPS patterns (Figure S13) evidence the presence of Co, S, and N species. The spectra of the Co, S, and N windows (Figure S14) are analogous to those shown in Figure 1, shifting to higher BE compared with pure CoS2. It is therefore suggested that the successful incorporation of N atom into CoS2 for all five products. The FESEM results (Figure S15) indicate that the differences in sulfidation temperature give no rise to the morphology properties. The OER activities of N doped CoS2 prepared at different temperatures were explored and the results were showed in Figure S16. Obviously, N2-CoS2-400 exhibits the best OER performance, whereas the N2-CoS2-300 shows lowest activity among the three samples, implying the well-defined crystal structure are beneficial for reducing overpotential and promoting reaction kinetics.39 Figure S17 shows the high TOF values versus the applied potentials in the 1 M KOH solution, demonstrating the excellent intrinsic catalytic activities of different temperature N doped CoS2.

4. DISCUSSION

In general, the electrocatalysts are always inclined to exhibit excellent OER performance when they render low electrical conductivity, high electrochemical 14 ACS Paragon Plus Environment

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active area, or/and well-defined electronic and crystal structure. The intrinsic conductivity of electrocatalysts, together with contact between the electrode and electrolyte, influence the transport of electrons upon OER, determining the catalytic efficiency. Therefore, electrochemical impedance spectroscopy (EIS) accompanying an interpretation according to an equivalent circuit model was first employed to evaluate the electrical conductivity of N doped CoS2. Figure S18 and S19 show the Nyquist plots of N doped CoS2, the semicircles in the high frequency range reflect to the charge transfer resistance (Rct) at the interface between the electrocatalysts and the electrolyte, which are correlated to the kinetics of reaction, with a smaller value rising to fast charge transfer kinetics. The CPE2 is the capacitance phase element for double layer at electrocatalysts/electrolyte interface, which is proportional to the surface area of the solid–liquid interface.40 The equivalent circuit model parameters were listed in Table S2, as can be seen, the N2-CoS2-500 has the lowest Rct, while N2-CoS2-400 exhibits the largest CPE2. Obviously, the variation trends of Rct and CPE2 are inconsistent with the overpotential difference (Table S3), implies that the OER performance of N doped CoS2 are insensitive to the electrical conductivity.

Then the electrochemical active area of N doped CoS2 was investigated by the measurements of double-layer capacitance (Cdl), which were obtained by cyclic voltammetry (CV) tests within the region of no Faraday reaction occurred. As shown in Figure S20 and S21, ∆j is the difference between anodic and cathodic current density at potential indicated by the black dash lines, where no redox current peaks are observed. The linear slopes are equivalent to twice of the electrochemical 15 ACS Paragon Plus Environment

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double-layer capacitances (Cdl), with a larger value rising to bigger electrochemical active area. The N2-CoS2-400 shows the largest slope value, while the N2-CoS2-300 has the smallest. This result seems to be in accordance with the OER activities. However, the variation trends are conflicting with OER performance when take all the five N doped CoS2 data into accounts (Table S3). In addition, the OER performance of N2-CoS2-400 is superior to N0-CoS2-400, whereas the electrochemical active area of the latter is larger (Figure S22).

We calculated the percentage of N in the N doped CoS2 (Table S3), the N contents are in accordance with the overpotential differences, indicating the strong influence of nitrogen doping for OER performance. The N percentage was also confirmed by elemental analyzer (EA) analysis, which show a highest N doping percentage for N2-CoS2-400, a medium value for N2-CoS2-500, and a lowest value for N2-CoS2-300. The percentage trend of five N doped CoS2 products was similar to the XPS results.


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Figure 3. Models of the structures of pure CoS2 (A) and N doped CoS2 (B, C), and their electronic density differences (D, E, and F corresponding to A, B and C structure, respectively).

In order to verify the roles of N doping theoretically, DFT calculations were conducted to investigate the characteristic structural features and the Gibbs free energy (∆G) profiles of N doped CoS2 by constructing the correlative theoretical models. The electronic density differences were first investigated. Figure 3 shows the transfer of electron from Co atom to the adjacent N atom. The transfer reveals the strong electron-withdrawing features of N in the material, whereas electron-donating role of the adjacent Co atom. The electron-deficient Co atom tends to form higher oxidation state, which becomes the active site during the OER. This result reinforces the inducement of N doping on the electron-transfer process, which then render the high catalytic activity of adjacent Co atom, making N doped CoS2 the highly efficient OER electrocatalysts.

Figure 4. (A) Free energy profiles of different sites. (B) Free-energy diagram for the four steps of the OER at the different applied potentials.


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The difference in electronic configurations and active sites might induce the change of ∆G. In this scenario, the calculations of different steps ∆G were carried out to illustrate the origin of the catalytic activities. For the overall OER reaction, the largest ∆G of a reaction step means the key determinant role among four reaction steps, which is always considered as the overall viability of the reaction and defined as the rate-limiting step. We sampled five active sites on the (001) surface, involving Co-0, Co-1, Co-2, Co-3, and Co-4 site (Figure S23). As depicted in Figure 4A, the rate-limiting step corresponds to the step c for Co-0, Co-1, Co-2, and Co-3 site, while the step b for Co-4 site. Specifically, the lower ∆G value of the rate-limiting step accrues at Co-0 and Co-1 site for pure CoS2 and N doped CoS2 (Table S4). Moreover, the active sites of N doped CoS2 process relative lower reaction barriers (smaller ∆G value) than those of pure CoS2, indicating their intrinsic high activities for OER performance.

The ∆G of the intermediates adsorption for Co-0 and Co-1 site were plotted in Figure 4B and the effects of a potential bias on all states involving one electron in the electrode were investigated. The Co-1 site shows lower ∆G of each intermediate adsorption than Co-0 site, accompanying with the ∆G uphill when no electrode potential was applied (U = 0 V). When the electrode potential (U = 1.23 V) was applied, step a and b moved downhill while the rate-limiting step (step c) still uphill. In order to have every step downhill, a larger electrode potential is needed. For Co-0 site, it is at potentials above 1.60 V that all steps become downhill in free energy, while the potential above 1.55 V make all reaction steps of oxygen evolution on Co-1 18 ACS Paragon Plus Environment

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site exothermic. Therefore, the N doping in the CoS2 are more favorable for reducing reaction barriers of OER.

5. CONCLUSIONS

In summary, a facile ammonium hydroxide assisted method for synthesis of N enriched CoS2 was developed. The N doping could alter the electronic density and configurations, which minimize the reaction barriers and thus forming the superior OER performance. As the results, the as-prepared N doped CoS2 exhibit low overpotential along with high stability in alkaline media. Possible active sites for the OER were investigated by DFT calculations, which indicate the N doping is beneficial for excitation of the adjacent Co atoms, forming well-defined electronic configurations, reducing ∆G of the rate-limiting step and the intermediates adsorption, suggesting the potential candidates of N doped CoS2 for various energy-related practical applications.

ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author * E-mail: swd1978@ ujs.edu.cn

* E-mail: [email protected]

* E-mail: [email protected]


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Notes The authors declare no competing financial interests.

Supporting Information. DFT calculations, XRD patterns, FESEM images, XPS patterns, polarization curves, Faraday efficiency plot, TOF curves, Nyquist plots, CV curves, possible absorption sites.

ACKNOWLEDGMENT This research was financially supported by the National Natural Science Foundation of China (21522603), the Chinese-German Cooperation Research Project (GZ1091), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), the Research Foundation of Jiangsu University (16JDG040) and the Jiangsu Postdoctoral Foundation (1601091B).

REFERENCES (1) Gao, M.-R.; Liang, J.-X.; Zheng, Y.-R.; Xu, Y.-F.; Jiang, J.; Gao, Q.; Li, J.; Yu, S.-H., Nat. Commun. 2015, 6, 5982. (2) Wang, H.; Lee, H.-W.; Deng, Y.; Lu, Z.; Hsu, P.-C.; Liu, Y.; Lin, D.; Cui, Y.,

Nat. Commun. 2015, 6, 7261. (3) Trancik, J. E., Nature 2014, 507, 300-302. (4) Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z., Science 2015, 347, 970-974. (5) Chen, L.; Wang, M.; Han, K.; Zhang, P.; Gloaguen, F.; Sun, L., Energy Environ.

Sci. 2014, 7, 329-334. 20 ACS Paragon Plus Environment

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(6) Wang, J.; Cui, W.; Liu, Q.; Xing, Z.; Asiri, A. M.; Sun, X., Adv. Mater. 2016, 28, 215-230. (7) de Levie, R., J. Electroanal. Chem. 1999, 476, 92-93. (8) Huynh, M. H. V.; Meyer, T. J., Chem. Rev. 2007, 107, 5004-5064. (9) Siegbahn, P. E. M., J. Am. Chem. Soc. 2009, 131, 18238-18239. (10) Yang, X.; Hall, M. B., J. Am. Chem. Soc. 2010, 132, 120-130. (11) Huang, X.; Zeng, Z.; Bao, S.; Wang, M.; Qi, X.; Fan, Z.; Zhang, H., Nat.

Commun. 2013, 4, 1444. (12) Ren, J. W.; Antonietti, M.; Fellinger, T. P., Adv. Energy Mater. 2015, 5, 1401660. (13) Zou, X.; Zhang, Y., Chem. Soc. Rev. 2015, 44, 5148-5180. (14) Feng, L.-L.; Yu, G.; Wu, Y.; Li, G.-D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., J. Am. Chem. Soc. 2015, 137, 14023–14026. (15) Liu, Y.; Xiao, C.; Lyu, M.; Lin, Y.; Cai, W.; Huang, P.; Tong, W.; Zou, Y.; Xie, Y., Angew. Chem. 2015, 127, 11383–11387. (16) Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.; Zhang, H., Energy Environ. Sci. 2013, 6, 2921-2924. (17) Pinaud, B. A.; Benck, J. D.; Seitz, L. C.; Forman, A. J.; Chen, Z.; Deutsch, T. G.; James, B. D.; Baum, K. N.; Baum, G. N.; Ardo, S.; Wang, H.; Miller, E.; Jaramillo, T. F., Energy Environ. Sci. 2013, 6, 1983-2002. (18) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmeková, E.; Asefa, T., Angew. Chem. 2014, 126, 4461-4465. 21 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 24

(19) Liu, Y.; Zou, X., Nanoscale 2015, 7, 3130-3136. (20) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L., Science 2009, 323, 760-764. (21) Wang, Q.; Yan, J.; Fan, Z., Energy Environ. Sci. 2016, 9, 729-762. (22) Dai, L.; Xue, Y.; Qu, L.; Choi, H.-J.; Baek, J.-B., Chem. Rev. 2015, 115, 4823-4892. (23) Xu, Y.; Kraft, M.; Xu, R., Chem. Soc. Rev. 2016, 45, 3039-3052. (24) Hammer, B.; Norskov, J. K., Nature 1995, 376, 238-240. (25) Chen, P.; Xu, K.; Fang, Z.; Tong, Y.; Wu, J.; Lu, X.; Peng, X.; Ding, H.; Wu, C.; Xie, Y., Angew. Chem. 2015, 127, 14923–14927. (26) Zheng, Y.; Jiao, Y.; Ge, L.; Jaroniec, M.; Qiao, S. Z., Angew. Chem. Int. Ed. 2013, 52, 3110-3116. (27) Zhang, Z.; Hao, J.; Yang, W.; Tang, J., RSC Adv. 2016, 6, 9647-9655. (28) Huang, Z.; Lv, C.; Chen, Z.; Chen, Z.; Tian, F.; Zhang, C., Nano Energy 2015,

12, 666-674. (29) Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H., J. Am. Chem. Soc. 2013, 135, 8452-8455. (30) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M.; Refson, K.; Payne, M. C., Z. Kristallogr. 2005, 220, 567-570. (31) Xiao, J.; Zeng, X.; Chen, W.; Xiao, F.; Wang, S., Chem. Commun. 2013, 49, 11734-11736. (32) Xu, J.; Gao, P.; Zhao, T. S., Energy Environ. Sci. 2012, 5, 5333-5339.


Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(33) Anto Jeffery, A.; Nethravathi, C.; Rajamathi, M., J. Phys. Chem. C 2014, 118, 1386-1396. (34) Liu, J.; Tang, S.; Lu, Y.; Cai, G.; Liang, S.; Wang, W.; Chen, X., Energy

Environ. Sci. 2013, 6, 2691-2697. (35) Peng, S.; Li, L.; Han, X.; Sun, W.; Srinivasan, M.; Mhaisalkar, S. G.; Cheng, F.; Yan, Q.; Chen, J.; Ramakrishna, S., Angew. Chem. 2014, 126, 12802–12807 (36) Zhang, X.; Meng, F.; Mao, S.; Ding, Q.; Shearer, M. J.; Faber, M. S.; Chen, J.; Hamers, R. J.; Jin, S., Energy Environ. Sci. 2015, 8, 862-868 (37) Wang, S.; ouyang, c.; Wang, X., Chem. Commun. 2015, 51, 14160-14163 (38) Bediako, D. K.; Surendranath, Y.; Nocera, D. G., J. Am. Chem. Soc. 2013, 135, 3662-3674. (39) Oh, S. H.; Black, R.; Pomerantseva, E.; Lee, J.-H.; Nazar, L. F., Nat. Chem. 2012, 4, 1004-1010. (40) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro, S.; Hu, X., Chem. Sci. 2012, 3, 2515-2525.


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TABLE OF CONTENTS The N doping is beneficial for excitation of the adjacent Co atoms, forming well-defined electronic configurations, reducing ∆G of the rate-limiting step and the intermediates adsorption for OER.


A Nitrogen Doping Method for CoS2 Electrocatalysts with Enhanced

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