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Porous Co9S8/Nitrogen, Sulfur-Doped Carbon@Mo2C Dual Catalyst for Efficient Water Splitting Xiaohu Luo, Qiulan Zhou, Shuo Du, Ji Li, Jiawen Zhong, Xiulin Deng, and Yali Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06166 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Applied Materials & Interfaces

Porous

Co9S8/Nitrogen,

Sulfur-Doped

Carbon@Mo2C

Dual

Catalyst for Efficient Water Splitting Xiaohu Luo,a,b Qiulan Zhou,a Shuo Du,a Ji Li, a Jiawen Zhong,a Xiulin Deng,a and Yali Liua* a

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and

Chemical Engineering, Hunan University, Changsha, Hunan, 410082, P. R. China b

School of Chemistry and Chemical Engineering, Qiannan Normal University for

Nationalities, Duyun, Guizhou, 558000, P. R. China

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ABSTRACT: The exploration of highly efficient and stable bifunctional electrocatalysts for the over water splitting is currently of extreme interest for efficient conversion of sustainable energy sources. Herein, we provide an earth-abundant, low-cost, and highly efficient bifunctional electrocatalyst composed of cobalt sulfide (Co9S8) and molybdenum carbide (Mo2C) nanoparticles anchored to metal-organic framworks (MOFs) derived nitrogen, sulfur-co-doped graphitic carbon (Co9S8-NSC@Mo2C). The new composite mode of the electrocatalyst was realized through simple pyrolysis processes. The composite electrocatalyst shows outstanding hydrogen evolution reaction (HER) performance and excellent stability over the entire pH range. For example, it has a lower overpotential of 74, 89, and 121 mV with Tafel slopes of 69.3, 86.7, and 106.4 mV dec-1 to achieve a current density of 10 mA cm-2 in 0.5 M H2SO4, 1.0 M KOH, and 1.0 M phosphate buffer saline (PBS) solutions, respectively. Moreover, it shows a small overpotential of 293 mV with a Tafel slope of 59.7 mV dec-1 to reach 10 mA cm-2 for the oxygen evolution reaction (OER) in 1.0 M KOH. The significantly enhanced HER and OER activities of Co9S8-NSC@Mo2C are mainly attributable to electron transfer from Co to Mo2C, resulting in a lower Mo valence and a higher Co valence in Co9S8-NSC@Mo2C. Furthermore, using the Co9S8-NSC@Mo2C bifunctional electrocatalyst as both the anode for the OER and the cathode for the HER for overall water splitting, a cell voltage of only 1.61 V is needed to derive a current density of 10 mA cm-2. This interesting work offers a general method for designing and frbricating highly efficient and stable non-noble electrocatalysts for promising energy conversion. KEYWORDS: molybdenum carbide, cobalt sulfide, Co9S8-NSC@Mo2C composite electrocatalyst, bifunctional electrocatalyst, hydrogen evolution reaction

1. INTRODUCTION Continuous energy consumption and the resulting serious environmental contamination have left researchers no choice but to develop clean, renewable and high efficiency energy sources.1-2 Hydrogen (H2), as a high gravimetric energy density, carbon-free and renewable fuel, can be a potential energy candidate to replace the contaminative fossil fuels in the future.3 Electrochemical water splitting to generate O2 and H2 through the anodic OER and


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cathodic HER has widespread attention.3-4 However, H2 generated from the cathode is seriously limited by the reaction kinetics of the OER at the anode during the water splitting process, and the reaction kinetics determine the overall electrochemical water splitting efficiency.5 Therefore, a highly efficient electrocatalyst for overall water splitting is the basis for reducing the energy barrier and improving the energy efficiency. Noble metal oxide (RuO2) and the precious metal platinum (Pt) have efficient catalytic activity for the OER and HER, respectively. However, their scarcity, prohibitive price, and low chemical stability have severely restricted their large-scale

practical application.6-7 Therefore, developing

earth-abundant, low-cost and highly efficient bifunctional catalysts to replace noble metal-based catalysts for overall water splitting is very significant and imperative. To solve these challenges, recently, considerable efforts have been directed on searching for earth-abundant, low-cost transition metal-based electrocatalysts, such as transition metal-based carbides,8 sulfides,9 phosphides,10-11 nitrides,12-13 and oxides.14 Unfortunately, most of these non-noble metal-based electrocatalysts perform well in the OER or HER in an alkaline and/or acidic solution. An ideal electrocatalyst should work well in the OER and HER under different pH conditions to obtain the energy-efficient overall water splitting. However, to date, a highly efficient and stable bifunctional electrocatalyst has rarely been reported. Molybdenum carbide (Mo2C), as a transition metal-based carbide, with an excellent HER activity in both acidic and basic solution has been investigated.15-17 Mo and the noble metal Pt have similar d-band electronic structure. Recently, some reports have revealed that coupling Mo2C nanoparticles with nanocarbon can significantly improve the HER performance.18-19 which can be explained by the following reasons: (1) coupling conjugation causes a downshift in the d-band center of Mo, leading to improved H desorption; (2) the nanocarbon supports can create more electron transfer paths and effectively protect Mo2C nanoparticles against aggregation and corrosion. Morevoer, if the nanocarbon supports contain heteroatoms (i.e., N, S, P, O, and B), their synergistic effect are more remarkable.20-21 Recently, cobalt chalcogenide (S and Se) and its hybrid materials as a series of promising OER or ORR electrocatalysts have attracted intensive attention, due to their low-cost, excellent catalytic activity and stability.22-23 Gao and coworkers found that CoSe2 hybrid materials exhibited excellent catalytic activity toward the OER in an alkaline solution.24 As



another example, a nanocomposite catalyst composed of Co9S8 decorated on reduced graphene oxide by an in situ growth method showed efficient activity toward the OER with a low overpotential of 0.34 V at 10 mA cm-2.25 Unfortunately, Co9S8 exhibits sluggish ion transport kinetics in acidic solution, leading to inferior catalytic performance and stability toward the HER.26-27 Principally, coupling Mo2C with Co9S8 into rationally designed architectures can offer overall structural merits over the single-component counterpart due to the synergistic effects of two materials. To date, very few works have focused on the synthesis of a Mo2C-Co9S8 hybrid architecture as a bifunctional electrocatalyst. Furthermore, if the synthesized hybrid material exhibits a novel polyhedral morphologies with a porous structure, it can provide enhanced electrocatalytic performance. However, controlled fabrication of a hybrid material with multimetallic carbide and sulfide and a porous polyhedral architecture is still a very challenging task. Nanostructured carbons (graphene nanosheet, carbon nanotubes) have a series of unique properties, including high conductivity, large surface area, and high chemical activity, and they have been widely applied for the hybridization of other catalytic species. For example, transition metal nanoparticles embedded in N-doped carbon nanoshells could act as excellent non-noble ORR electrocatalysts both in acidic and basic solutions to produce H2O with a yield of about 99%.28 Bao and coworker reported that non-noble Co nanoparticles encapsulated in N-doped carbon can efficiently electrocatalyze the OER in basic solution and the HER over the entire pH range.28-29 Carbon nanomaterials doped with heteroatoms, such as N, S, P, and O, can provide unique electronic structures with lower local work functions, which can significantly promote the adsorption of intermediates at the material surface, leading to the improvement of electrocatalytic activity. Additionally, the doped carbon shell can effectively protect the embedded metal nanocatalysts against aggregation, corrosion, dissolution, and oxidation during the electrocatalytic reaction process, thereby resulting in a significant improvement in the stability. In recent years, with perfectly assembled organic ligands and metal ions in the crystal lattice, metal-organic frameworks (MOFs) as a favorable self-scarificial template have often been utilized to fabricate porous carbon materials.30-33 Xiao and coworkers recently reported that a zeolitic imidazolate framework (ZIF-67) was unitized to fabricate a bimetallic phosphide nanosheet for OER catalysts in a basic solution by


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an ion-assisted solvothermal method.34 Additionally, three-dimension (3D) MOF materials have a much greater ability to prevent structural / morphological damage during pyrolysis at high temperature, and they exhibit large surface-to-volume ratios. To the best of our knowledge, Mo2C combined with the N, S-co-doped carbon (NSC)-encapsulated metal chalcogenide (Co9S8) polyhedral structure with electrocatalytic activities toward the HER and OER has not been reported. Based on the above analysis, we describe a simple pyrolysis method to prepare porous nanostructured Co9S8-NSC@Mo2C polyhedron material as a highly efficient and highly stable bifunctional catalyst for the HER and OER by using well-defined and highly conductive Co9S8-NSC polyhedrons that originate from ZIF-67 as the porous carbon substrate. The as-synthesized Co9S8-NSC@Mo2C composite catalyst has a regular polyhedral morphology. The precursor Co9S8-NSC can significantly enhance the conductivity of the hybrid material, thereby promoting adsorption of the intermediate on the surface of the material and improving electron transfer between the catalyst surface and the reaction intermediate. Furthermore, poly(2-aminothiazole) (P-TA) was chosen as an all-in-one N- and S-containing precursor for the fabrication of N, S-co-doped carbon (NSC). Moreover, the poly(2-aminothiazole) precursor can provide a sulfur source to synthesize Co9S8 nanoparticles embedded in the NSC shell by the pyrolysis treatment. As expected, the synthesized Co9S8-NSC@Mo2C hybrid material as a bifunctional electrocatalyst exhibits highly efficient catalytic activity and stability toward the HER and OER.

2. EXPERIMENTAL SECTION Raw Material. Methanol, Cobalt nitrate hexhydrate, 2-methylimidazole (2-mIM), 1.4-dioxane, 2-aminothiazole (AT), benzoperoxide (BPO), ammonium molybdate tetrahydrate, Glucose, commercial Pt/C (20 wt %), commercial RuO2, and Nafion (5 wt %) were purchased from Adamas (Shanghai, China). Preparation of Co-ZIF-67. Typically, 0.3274 g Cobalt nitrate hexhydrate and 0.3296 g 2-mIM were added into 25 mL methanol under the stirring condition, respectively. Then the two solutions were mixed together and aged at 25-28 °C for 24 h. Subsequently, the purple



product was separated by ultracentrifugation with 500 mL ethanol and dried in a vacuum oven at 45 °C overnight. Preparation of P-AT@Co-ZIF-67. 0.1 g as-obtained Co-ZIF-67 was well dispersed in 50 mL 1.4-dioxane under the sonication condition, then 0.2 g AT and 0.06 g BPO were dissolved in the mixed solution under the sonication and vigorous stirring. The mixture solution was stirred continuously at 60 °C for 6 h with a reflux condenser and nitrogen atmosphere. After that, the yellowish product was separated by ultracentrifugation with 800 mL ethanol, and then dried at 45 °C overnight. The final product is called as P-AT@Co-ZIF-67. Preparation of Co9S8-NSC. The as-synthesized P-AT@Co-ZIF-67 was placed in a tune furnace and annealed at 600 °C for 3 h in the Ar atmosphere. It was notable that the heating rate was 2 °C min-1. Afterward, the black product was obtained. To remove metallic Co species, the product was further placed in the H2SO4 (0.5 mol/L) solution under the vigorous stirring condition. Successively, the black product was further separated by the centrifugation with 200 mL ethanol and 200 mL deionized H2O, respectively, and then dried in a vacuum oven at 45 °C overnight, which is called as Co9S8-NSC. Preparation of Co9S8-NSC@Mo2C. 0.2 g of as-synthesized Co9S8-NSC was added to 25 mL the water containing 0.06 g of (NH4)6Mo7O28·4H2O and 0.1 g of glucose. The mixture solution was vigorously stirring at 25 °C for 24 h, and then frozen in liquid nitrogen and freeze-dried in the dryer. Finally, the dried Co9S8-NSC@(NH4)6Mo7O28 was pyrolyzed at 800 °C under a following Ar and maintained at this temperature for 2 h. The heating rate was controlled at 2 °C min-1. Successively, the black Co9S8-NSC@Mo2C composite catalyst was obtained. For comparison, the Co9S8-NSC@(NH4)6Mo7O28 was also pyrolyzed at 850 °C, 900 °C, and 950 °C, which are denoted as Co9S8-NSC@Mo2C-t1, Co9S8-NSC@Mo2C-t2, and Co9S8-NSC@Mo2C-t3, respectively. Preparation of the control Co-C. The as-synthesized Co-ZIF-67 was placed in a tune furnace, then pyrolyzed at 600 °C with a heating rate of 2 °C min-1 under a following Ar and maintained at this temperature for 2 h. After that, the black product was obtained, which is called as Co-C. Preparation of the control NSC. 0.5 g of as-synthesized P-AT@Co-ZIF-67 was added into 50 mL water under the stirring condition, and the mixture solution was stirred


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continuously at 60 °C for 2 days with a reflux condenser to remove the Co atoms in P-AT@Co-ZIF-67. The yellow precipitate was separated by the centrifugation with 500 mL water and 200 mL ethanol, respectively, and dried at 45 °C overnight. The product was placed in a tune furnace, and then pyrolyzed at 600 °C with a heating rate of 2 °C min-1 under a following Ar and maintained at this temperature for 2 h. After that, the black product was obtained, which was called as NSC. Preparation of the control Mo2C-NSC. 0.2 g of as-synthesized NSC was added into 25 mL the water containing 0.06 g of (NH4)6Mo7O28·4H2O and 0.1 g of glucose, and the mixture solution was aged at the room temperature for 24 h under the vigorous stirring, then the mixture was frozen in liquid nitrogen and freeze-dried in the dryer. Finally, the dried NSC@(NH4)6Mo7O28 were pyrolyzed at 800 °C for 2 h under a following Ar atmosphere to obtain the black product, which was called as Mo2C-NSC. Structural Characterization. The phase structural analyses of the samples were obtained by PANalytical X'Pert Powder diffractometer (Holland). Raman spectrum was determined on a Renishaw inVia Reflex Micro-Raman spectroscopy (Renishaw, UK). The composition and binding energies of the samples were characterized by X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA, USA). Sample morphologies were characterized by using an emission scanning electron microscopy (SEM, Hitachi S-4800, Japan) and a transmission electron microscopy (TEM, JEOL JEM-2008, Japan). The Brunaurer-Emmett-Teller (BET) surface and nitrogen-adsorption-desorption isotherms of the samples were determined by using a tri-star 3020 (USA). Electrochemical Measurement. The electrochemical measurements were carried out on an electrochemical station (CHI660E, Shanghai, China) in a standard three-electrode system with a saturated calomel (Hg/Hg2Cl2) as the reference electrode, a graphite rod as the counter electrode, and a glass carbon electrode uniformly covered by the electrocatalyst as the working electrode in 0.5 M H2SO4, 1.0 M KOH, and 1 M PBS. In our case, 1 M PBS was obtained by addition of 0.1 M KH2PO4 into 100 mL deionized water, and 1.0 M KOH solution was used to adjust the pH. The saturated Hg/Hg2Cl2 reference electrode was calibrated with corresponding to the reversible hydrogen electrode (RHE) in hydrogen-saturated electrode solution. E (RHE) = E (Hg/Hg2Cl2) + (0.242 + 0.059 pH) V. Linear sweep voltammetry (LSV) ACS Paragon Plus Environment


curves of electrocatalyst in HER were performed in electrolyte in the potential window of 0.1 to -0.5 V vs. RHE with a scanning rate of 5 mV s-1. LSV curves of electrocatalyst in OER were obtained in the electrolyte in the potential window of 1.0 to 1.8 V vs. RHE with a scanning rate of 5 mV s-1. The electrochemical impedance spectroscopy (EIS) measurements of electrocatalyst in HER and OER were carried out at an overpotential of 120 mV and 300 mV from 0.1 to 100000 Hz with 5 mV sinusoidal perturbation, respectively. The semicircle of the Nyquist plot is assigned to the charge-transfer resistance (Rct), which can reflect the electrocatalytic kinetics. Cyclic voltammetry (CV) was performed in the nonfaradaic potential window at different scan rates, which was utilized to determine the electrochemical double layer capacitance (Cdl). Meanwhile, the electrochemical surface area (ECSA) was estimated by using the Cdl. The ECSA can be calculated by the following equation: ECSA ≈ Cdl / Cs, where the Cs is the specific capacitance of the electrocatalyst under identical electrolyte condition. All the polarization curves were obtained with iR compensation (95%). The electrocatalyst suspension was obtained by dissolving 5 mg electrocatalyst into the mixture solution with 475 µL water, 475 µL ethanol and 50 µL Nafion solution (5 wt %), following by the ultrasonication for 60 min. 6 µL of the electrocatalyst suspension was uniformly covered on the GCE surface, and dried at the room temperature. For synthesis of nickel foam (NF)-supported Co9S8-NSC@Mo2C (Co9S8-NSC@Mo2C/NF) electrode, the NF was washed in the water and ethanol with the sonication, in order to remove the impurity at the NF surface, and dried in air. The prepared electrocatalyst suspensions were drop-casted on the NF, and then the Nafion was coated on the surfaces of the electrocatalyst-modified NF electrodes.

3. RESULTS AND DISCUSSIONS The detailed fabrication of Co9S8-NSC@Mo2C dodecahedral nanocrystals is schematically illustrated in Scheme 1. First, monodisperse well-defined Co-ZIF-67 dodecahedral nanocrystals were easily synthesized by a simple precipitation approach with 2-mIM and cobalt nitrite in methanol solution. The size of the uniform Co-ZIF-67 nanocrystals can be easily controlled by the molar ratio of cobalt nitrite and 2-Mim. The as-obtained pure


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Co-ZIF-67 exhibits a typical dodecahedron shape with a size of ~ 800 nm (Figure S1a, b), which acts as a self-sacrificial template. Subsequently, P-AT was coated on the Co-ZIF-67 dodecahedron nanocrystal by polymerization of 2-aminothiazole (AT) (which can serve as a source of S and N), and the presence of P-AT was verified by the appearance of a broad N-H band (3308 cm-1) originating from P-AT and a shift in the C=N band (1621 to 1586 cm-1) in the FT-IR spectra (Figure S2). After successfully coating the Co-ZIF-67 nanocrystals, the P-AT@Co-ZIF-67 nanocrystals retain the well-defined structure and morphology of ZIF-67, but exhibit a smooth surface compared to that of the precursor (Co-ZIF-67) (Figure S1c, d). The wall thickness of P-AT coated on Co-ZIF-67 ranges from 5 to 15 nm (Figure S3). Then, P-AT@Co-ZIF-67 nanocrystals were converted to Co9S8-NSC hybrid nanocrystals by a simple calcination process, accompanied with in situ formation of Co9S8 nanoparticles wrapped with the N, S-doped carbon derived from the 2-mIM and P-AT. The shrinkage and erosion of 2-mIM and P-AT during the pyrolysis process cause a decrease in the size (~ 600 nm) and the formation of the rough surface (Figure 1a, b). Meanwhile, the polyhedral morphology of the Co9S8-NSC nanocrystals is well maintained. Finally, the Co9S8-NSC nanocrystals were immersed in a (NH4)6Mo7O24 solution with glucose at the room temperature for 24 h and then freeze-dried to obtained Co9S8-NSC@(NH4)6Mo7O24 hybrid aerogels. Subsequently, the hybrid aerogels were further calcined at 800 °C under a following Ar atmosphere to obtain Co9S8-NSC@Mo2C hybrid nanocrystals. FESEM images of the Co9S8-NSC@Mo2C hybrid nanocrystals show that the polyhedral shape is perfectly retained during the second calcination process (Figure 1c, d). Notably, the wrinkled textures attached at the polyhedron surface form Mo2C nanocrystals, leading to a surface rougher than that of Co9S8-NSC. This offers plentiful exposed active sites at the Co9S8-NSC@Mo2C surface. The crystallinity and co-occurrence of phases in Co9S8-NSC@Mo2C hybrid nanocrystals were demonstrated by X-ray diffraction (XRD). The obtained XRD patterns are shown in Figure 2 and Figure S4. First, all the characteristic peaks of the as-synthesized Co-ZIF-67 are in good agreement with high-quality ZIF-67 (Figure S4a), which coincides well with the results in recent reports.35 Similar to Co-ZIF-67, P-AT@Co-ZIF-67 exhibits the distinguished peaks, suggesting that the polymerization of AT does not cause a change in the crystallinity of the precursor (Co-ZIF-67) (Figure S4a). After calcination of P-AT@Co-ZIF-67, the main



diffraction peaks of Co9S8-NSC appeared at 30.2°, 31.3°, 36.1°, 39.5°, 47.8°, 51.9°, 54.3°, and 73.3° are indexed to the (311), (222), (400), (331), (511), (400), (531), and (731) planes of cubic Co9S8 (PDF # 19-0364) (Figure S4b), respectively, and no diffraction peaks associated with crystalline Co were obtained, indicating that the Co atoms in the P-AT@Co-ZIF-67 were completely turned into Co9S8 species. For comparison, Co-C was synthesized by the calcination of Co-ZIF-67 without P-AT. The Co-C samples exhibited three main peaks located at 43.2°, 51.1° and 75.4° corresponding to the (111), (200) and (220) faces of metallic Co (PDF # 89-4307), respectively, while the other two peaks observed at around 25.9° and 43.1° are ascribed to the characteristic peaks of graphitic carbon (Figure S4c). This result suggests that the Co nanoparticles are embedded in the graphitic carbon. Furthermore, the XRD pattern of Mo2C-NSC (the control material) (Figure S4d) shows diffraction peaks at 34.2°, 37.8°, 39.4°, 52.1°, 61.2°, 69.6°, 74.7°, and 75.4°, which are indexed to (100), (002), (101), (102), (110), (103), (112), and (103) faces of the typical β-Mo2C (PDF # 35-0787),8 respectively. The XRD pattern (Figure S4e) of NSC (the N, S-co-doped carbon material) derived from P-AT and the 2-mIM precursor exhibits two typical broad diffraction peaks appeared at 26.1° and 43.2°, resulting from the (002) and (101) faces of graphitic carbon and disordered amorphous carbon, respectively. When the Co9S8-NSC@(NH4)6Mo7O24 hybrid aerogels were further pyrolyzed, the diffraction peaks of the Co9S8-NSC@Mo2C nanocrystals (Figure 2) observed the coexistence of Mo2C, Co9S8 and graphitic carbon in the hybrid. The chemical structures and surface electronic states of samples were studied by XPS. The XPS survey spectra of both Co9S8-NSC@Mo2C and Co9S8-NSC in Figure 3a exhibit main peaks located at 161.2, 284.5, 398.3, 531.2, and 777.9 eV which can be attributable to S 2p, C 1s, N 1s, O 1s, and Co 2p, respectively. The presence of O atoms in the materials may be attributed to O atoms present in the graphitic structure of the materials or the oxygen-containing groups present on the materials, as others have reported previously.36-37 Compared with Co9S8-NSC, the appearance of Mo atoms in Co9S8-NSC@Mo2C could be due to the formation of Mo2C in Co9S8-NSC@Mo2C. For both Co9S8-NSC@Mo2C and Co9S8-NSC, quantitative analysis suggests that the Co:S molar ratio in both materials is lower than 9/8 (Table S1). This indicates that there should be some extra S atoms in the materials, besides those in the Co9S8. The extra S atoms as dopants are incorporated into graphitic ACS Paragon Plus Environment

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carbon during the calcination process. The N:C atomic ratio of Co9S8-NSC@Mo2C is found to be higher than that of Co9S8-NSC (Table S1). This is explained by the extra N atoms originating from the NH+4 of (NH4)6Mo7O24 during the second pyrolysis process. Moreover, the Co and Mo elemental contents in the Co9S8-NSC@Mo2C are ~ 17.3% and 12.5%, respectively (Table S1), thus, the mass fractions of Co9S8 and Mo2C are ~ 25.67% and 13.28% in Co9S8-NSC@Mo2C, respectively. High-resolution XPS spectra for C 1s of both Co9S8-NSC@Mo2C and Co9S8-NSC are shown in Figure 3b and Figure S5a. The C 1s peaks locate at 284.1 (C-S-C), 284.7 (C=C-C), 285.1 (C-O), 286.1 (C-N), and 289.2 eV (O=C-O).38-39 Additionally, the two other peaks related to C-S (284.3 eV) and C-N (286.3 eV) characteristic peaks reveal that the S and N elements were successfully incorporated into graphitic carbon. For Co9S8-NSC@Mo2C, the peak intensity of O=C-O is slightly deceased compared with that of Co9S8-NSC, due to the instability of oxygen groups at the high calcination temperature (800 °C).40 For N 1s spectra of the Co9S8-NSC@Mo2C (Figure 3c), three peaks located at 398.2, 400.1, and 400.8 eV are attributed to pyridinic-N, pyrrolic-N, and graphitic-N, respectively.41 In the case of Co9S8-NSC@Mo2C, the percentage of the pyridinic-N and graphitic-N peaks increases and the pyrrodic-N peak displays a decreased trend compared with Co9S8-NSC (Figure S5b), which are all helpful for enhancing the OER and HER performance.42 Figure 3d shows the S 2p spectra with four main peaks. The peaks centered at 161.6 and 163.8 eV, with an energy split of 2.2 eV, are associated with the S 2p3/2 and S 2p1/2 core levels of S2-, respectively, resulting from Co9S8.43 The peaks located at 163.3 and 168.4 eV are attributed to C-S-C and SO2-4 , respectively.44 The existence of C-S-C in S 2p spectra further confirms that the S atoms were availably doped into carbon, while the presence of SO2-4 in the hybrid materials is attributed to possible oxidation of S in the air.45 There are two shake-up satellites and two spin-orbit doublets in the Co 2p spectra (Figure 3e). The binding energies observed at 782.6 eV (Co 2p3/2) and 796.8 eV (Co 2p1/2) result from the spin-orbit characteristics of Co3+, while the binding energy centered at 786.3 eV in Co 2p3/2 and 797.8 eV in Co 2p1/2 should be assigned to Co2+.23 Most importantly, coupling Co9S8 with Mo2C in Co9S8-NSC@Mo2C results in a negative shift in the binding energies of Co 2p compared to the Co9S8-NSC (Figure S5d), suggesting that Co9S8 and Mo2C interact. The peaks appearing at 231.2 and ACS Paragon Plus Environment


227.7 eV (Figure 3f) should be attributed to the Mo0 of Mo2C, while other peaks appearing at 235.9, 234.8, 233.6, 232.1, 231.7, 230.5, and 229.1 eV are related to MoOx. MoOx may arise from slight oxidation of Mo2C by air. The detailed structure of the Co9S8-NSC@Mo2C hybrid materials was investigated through transmission electron microscopy (TEM). It is found that the dark spots, probably Mo2C and Co9S8, are surrounded by lighter regions (Figure 4). A high-resolution TEM (HRTEM) image (Figure 4b) clearly shows the presence of cubic crystalline Co9S8 with 0.29 nm of an interplanar distance, corresponding to the (311) crystallographic face of cubic Co9S8. On the other hand, the HRTEM image of Mo2C (Figure 4b) reveals the presence of hexagonal Mo2C possessing lattice fringes of 0.26 nm which is identical to the (100) crystal face of Mo2C.46 Furthermore, the Co9S8 and Mo2C particles are wrapped by the graphene-like carbon shell which plays a vital role in preventing them from oxidation, corrosion and agglomeration. Moreover, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and the corresponding element mappings exhibit a uniform distribution of these elements throughout the whole polyhedron (Figure 4c-h). Previous reports have proved that the appropriated synergistic effect originating from the excellent structure and different components is beneficial for improving the electrocatalytic activity.47-48 Raman spectroscopy was performed further to characterize the degree of graphitization and composition of the as-synthesized materials (Figure 5a). Two characteristic Raman peaks centered at ~ 1350 and 1590 cm-1 are attributable to sp3-hydridized disordered structures (D-band) and sp2-hydridized graphitic carbon (G-band), respectively.49 In the case of Co9S8-NSC@Mo2C and Mo2C-NSC, two relatively weaker peaks at around 815 and 996 cm-1 are attributed to the characteristic Raman peaks of Mo2C,50 which are absent in the spectra of Co9S8-NSC and NSC. Compared to the spectra of the NSC, the weakest peak at 663 cm-1 is also observed, which should correspond to Co9S8.27 Generally, the relative ratio of ID / IG can reflect surface defects in a graphite layer within the carbon structure of the materials. The ID / IG values of Co9S8-NSC@Mo2C, Co9S8-NSC and NSC are ~ 0.86, 0.91 and 0.97, respectively, suggesting that Co9S8-NSC@Mo2C has a higher degree of graphitization or a more ordered carbon structure after the second pyrolysis process, which is favorable for improving the electron transfer ability.51


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Here, the surface area and pore size of the samples are calculated by the N2 adsorption/desorption measurements and are shown in Figure 5b and Figure S6. The specific surface area for Co9S8-NSC@Mo2C is 202.5 cm2 g-1 (Figure 5b), which is lower than that of NSC (268.5 cm2 g-1), Co-NSC (237.2 cm2 g-1), Co9S8-NSC (223.6 cm2 g-1) and Mo2C-NSC (211.3 cm2 g-1) (Figure S6). The pore volumes of Co9S8-NSC@Mo2C is around 0.39 cm3 g-1, which is also lower than that NSC (0.58 cm3 g-1), Co-NSC (0.51 cm3 g-1), Co9S8-NSC (0.48 cm3 g-1), and Mo2C-NSC (0.43 cm3 g-1). Thus, Co9S8-NSC@Mo2C exhibits a decrease in specific surface area and pore volume relative to Co9S8-NSC and NSC, which may be attributed to a proportion of the pores of the polyhedrons being covered or occupied by Mo2C nanocrystals and collapsing during the second pyrolysis process. The high specific surface area and massive mesopores of Co9S8-NSC@Mo2C are beneficial to the rapid transportation of electrolyte and the rich active sites, leading to an enhanced electrocatalytic activity.

Electrocatalytic Performance of Co9S8-NSC@Mo2C Composite Catalyst The electrocatalytic performance of the as-synthesized materials for various reactions was then studied. The electrocatalytic performance toward the HER for all the as-synthesized materials was investigated in a 0.5 M H2SO4 solution (pH=0) by using a typical disk electrode with 0.425 mg cm-2 of mass loading. For comparison, the commercial Pt/C, NSC, Co-C, Mo2C, and Co9S8-NSC were also measured under the same conditions. The obtained HER polarization curves (j-V) are displayed in Figure 6a. Co9S8-NSC@Mo2C shows remarkable electrocatalytic activity for the HER, exhibiting a small onset potential of 4.5 mV vs. RHE at 0.1 mA cm-2. To reach 10 mA cm-2 (a metric relevant to solar fuel synthesis), Co9S8-NSC@Mo2C requires a very low overpotential of ~ 74 mV, which is lower than that of some efficient noble metal-free HER electrocatalysts previously reported (Table S2). To obtain the same current density (10 mA cm-2), the control samples, NSC, Co-NSC, Co9S8-NSC, and Mo2C need larger overpotentials of 302, 209, 158 and 119 mV, respectively, indicating that none of them match Co9S8-NSC@Mo2C in the terms of the electrocatalytic property for the HER in acidic solution. Some recent reports have demonstrated that Mo2C have an efficient activity for the HER in acidic solution.15-17, 46, 52 However, the HER activity



is significantly enhanced on Co9S8-NSC compared with that on the pure NSC and Co-NSC samples, which is associated with Co9S8 nanoparticles wrapped in the N, S-doped carbon matrices. To study the HER kinetics of the electrocatalysts, Tafel plots (log (j) vs η) were obtained from the corresponding HER polarization curves, which are presented in Figure 6b. The Pt/C electrocatalyst shows a Tafel slope of 37.6 mV dec-1 (Figure 6b). The Tafel slope of Co9S8-NSC@Mo2C is 69.3 mV dec-1, much smaller than those of the control materials, Co-NSC (196.4 mV dec-1), Co9S8-NSC (127.5 mV dec-1), and Mo2C (97.1 mV dec-1) and those of previous reports (Table S2), implying that Co9S8-NSC@Mo2C has a higher HER rate and faster catalytic kinetics. The Tafel slope value for Co9S8-NSC@Mo2C lies in the 40-120 mV dec-1 range, indicating that the corresponding HER on the Co9S8-NSC@Mo2C surface abides by a Volmer-Heyrovsky process with a rate-limiting step of electrochemical desorption.53-54 Furthermore, the exchange current density (j0) can be obtained through extrapolation of the Tafel plot to 0 V vs. RHE,

which definitely reflects the inherent

electrocatalytic rate under the equilibrium condition.55 As expected, the j0 value of Co9S8-NSC@Mo2C is 1.16 mA cm-2, which is the largest among the control samples (i.e., Mo2C-NSC (0.32 mA cm-2), Co9S8-NSC (0.23 mA cm-2) and Co-NSC (0.07 mA cm-2)). Electrochemical impedance spectroscopy (EIS) was utilized to further understand the favorable kinetics at an overpotential of 120 mV. The results are displayed in Figure 6c. Generally, the charge transfer resistance (Rct) is attributable to the catalytic kinetics at the electrolyte/electrocatalyst interface. The Rct value is smaller, and the electron transfer capacity is faster.56 The improved charge transfer results in a decrease in the corresponding resistance, which is beneficial toward enhancing the catalytic performance.57-58 The Rct value of Co9S8-NSC@Mo2C is about 70.1 Ω, which is much lower than that of Mo2C-NSC (131.2 Ω), Co9S8-NSC (261.3 Ω), Co-NSC (419.5 Ω) and NSC (656.2 Ω), implying that Co9S8-NSC@Mo2C has rapid electronic transport and low mass transport resistances. Typically, the ECSA is nearly proportional to the double-layer capacitance (Cdl), which can be carried out to evaluate the exposed catalytically active sites in the materials.59 In our case, the Cdl values of the samples were obtained from cyclic voltammetry (CV) measurements in the non-Faradaic potential window at different scan rates, which are shown in Figure S7a-e. As


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shown in Figure 6d, the Cdl value of Co9S8-NSC@Mo2C is about 17.5 mF cm-2, much larger relative to the controlled samples. These results suggest that Co9S8-NSC@Mo2C exhibits more catalytically active sites for electrolysis at the electrolyte/electrocatalyst interface during the HER process. Additionally, good agreement is obtained between the measured hydrogen amount and that predicted theoretically (Figure S7f), presenting nearly 100% Faradaic efficiency for catalytic HER. To explore the influence of different pyrolysis temperatures on the catalytic activity of these samples for HER, the electrocatalytic performance of different Co9S8-NSC@Mo2C materials with different pyrolysis temperatures was determined. As shown in Figure S8a, the onset potential shifts to more negative values with the increase of pyrolysis temperature. Among the materials we prepared and tested, the one pyrolyzed at 800 °C shows the best electrocatalytic activity. The phenomenon can be explained by the following reason: the increase of pyrolysis temperature causes the pores in the materials to collapse, resulting in a decrease in the specific surface area (Figure S8b). The durability of the electrocatalysts is another critical criterion for their practical application. Hence, the stability of Co9S8-NSC@Mo2C in the 0.5 M H2SO4 solution was determined by conducting continuous CV sweeps between -0.3 and 0 V vs. RHE with a scan rate of 50 mV s-1. Co9S8-NSC@Mo2C shows a small reduction in overpotential (~ 4 mV) after 2500 CV cycles at 10 mA cm-2 (Figure 6e), suggesting a superior cycling property. Furthermore, the long-term durability was measured by sustaining a current density of 10 (74 mV vs. RHE) and 20 mA cm-2 (86 mV vs. RHE) for 20 h in the 0.5 M H2SO4, and the results are displayed in Figure 6f. Figure 6f shows that after 20 h the current density has very little decay, implying that Co9S8-NSC@Mo2C exhibits outstanding stability in acidic solutions. The chemical stability of Co9S8-NSC@Mo2C after the long-term stability test was investigated by XPS (Figure S9). Figure S9 shows that the XPS spectra of the crucial elements remain identical to the original material without the obvious changes, suggesting that the elements show the same valence states after long-term stability measurements. Additionally, the results from the ICP measurement (Table S3) show that the concentrations of these elements in the Co9S8-NSC@Mo2C electrodes and electrolytes undergo no obvious changes before and after HER measurements, further confirming excellent stability of Co9S8-NSC@Mo2C during the ACS Paragon Plus Environment


HER reactions. The electrocatalytic activities for as-synthesized Co9S8-NSC@Mo2C hybrid materials for the HER in alkaline and neutral solutions were also determined. Notably, the Co9S8-NSC@Mo2C requires 89 mV vs. RHE with a Tafel slope of around 86.7 mV at 10 mA cm-2 in a 1.0 M KOH solution (pH=14) (Figure S10a, b), indicating that the Co9S8-NSC@Mo2C hybrid electrocatalyst also abides by the Volmer-Heyrovsky mechanism in alkaline media. This is one of the best results of previously reported non-noble metal HER catalysts in alkaline solution (Table S2). Additionally, the Co9S8-NSC@Mo2C exhibits a stable electrocatalytic performance in a 1.0 M KOH solution (Figure S10c). The results suggest that Co9S8-NSC@Mo2C also has superior HER activity in alkaline solution. Additionally, to obtain 10 mA cm-2 in 1.0 M PBS solution (pH=7), Co9S8-NSC@Mo2C needs an overpotential of 121 mV with a Tafel slope of 106.4 mV dec-1 (Figure S10d, e). Additionally, the time dependence of current density curve of Co9S8-NSC@Mo2C (Figure S10f) shows that Co9S8-NSC@Mo2C has superior durability in neutral solution According to the above experimental results, the Co9S8-NSC@Mo2C shows an excellent HER performance. To gain deep insight into the origin of the enhanced HER mechanism of the Co9S8-NSC@Mo2C composite electrocatalyst, XPS is further carried out on the catalyst of interest (Figure S11). It is found that the heteroatoms (N and S) are doped, and the Co9S8-NSC@Mo2C composite catalyst has a high specific surface area. However, the results show that the heteroatom content and high specific surface area in our as-synthesized electrocatalysts have minimal influence on the HER activity, although the heteroatom doping in the carbon-based materials and the high specific surface area are believed to be crucial to the HER activity. Therefore, it is concluded that the heteroatom content and high specific surface area are not the main reasons for the enhanced HER performance of Co9S8-NSC@Mo2C. Because the HER activity and the valence state of metal elements are closely related, the chemical states of the Mo and Co elements in Co9S8-NSC@Mo2C were investigated by XPS. Co 2p XPS shows that the majority of the surface Co for Co9S8-NSC@Mo2C is oxidized with a +3 oxidation state (Figure 3e) compared to Co9S8-NSC (Figure S5d). Notably, the Co3+ species are considered to be effective active sites for water dissociation (the Volmer step), which is the critical HER step. Most importantly, it is


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found from Figure S11a that the Mo 3d peaks in Co9S8-NSC@Mo2C shift to lower binding energy compared with those in the Mo2C-NSC, indicating that a lower Mo valence in Co9S8-NSC@Mo2C. Deconvolution of the Mo 3d XPS peaks (Figure S11b) shows differences in the Mo valence

states between Co9S8-NSC@Mo2C and Mo2C-NSC. For

example, the main valence states for Mo in Co9S8-NSC@Mo2C are 0, +2, +3, and +4, but in Mo2C-NSC, they are +2, +3 +4, and +6. Among these valence states for Mo, Mo6+ is considered to be inactive for the HER, which indicates that there are more HER active sites in Co9S8-NSC@Mo2C. Based on these analyses, the intercalation of Co9S8 may cause the electron transfer from Co9S8 to Mo2C, resulting in a higher Co valence and a lower Mo valence in Co9S8-NSC@Mo2C, which are HER active states and account for the enhanced HER activity. Generally, cobalt-based materials can possess a high OER property.22, 36, 57, 60-62 Therefore, the OER electrocatalytic performance of Co9S8-NSC@Mo2C was also tested again by using a typical disk electrode with a mass loading of 0.425 mg cm-2 in a standard three-electrode cell in a KOH solution (1.0 M, pH=10). For comparison, the electrocatalytic activities of Co9S8-NSC, Co-NSC, Mo2C-NSC and RuO2 are evaluated under the same conditions. The OER LSV curves of the as-synthesized materials are shown in Figure 7a. As shown in Figure 7a, Co9S8-NSC@Mo2C requires an overpotential of 293 mV to obtain 10 mA cm-2 in the OER, which is much smaller compared to that in the control samples, such as Co9S8-NSC (336 mV), Co-NSC (435 mV), Mo2C-NSC (590 mV) as well as the other recently reported catalysts (Table S4), revealing excellent catalytic activity of Co9S8-NSC@Mo2C for the OER in alkaline solutions. The corresponding Tafel plots for the species are presented in Figure 7b. The Tafel slope of Co9S8-NSC@Mo2C is ~ 59.7 mV dec-1, which is much lower than that of Co9S8-NSC (87.7 mV dec-1), Co-NSC (124.5 mV dec-1), and Mo2C-NSC (186.2 mV dec-1). Moreover, Co9S8-NSC@Mo2C has a low Rct value (93.1 Ω) (Figure 7c), which is much lower than that of the other as-synthesized materials, suggesting that Co9S8-NSC@Mo2C has faster OER kinetics in alkaline solution. Compared with Co9S8-NSC, the OER activity of Co9S8-NSC@Mo2C is evidently enhanced. The Co content in Co9S8-NSC@Mo2C is lower than that in Co9S8-NSC (Table S1), but Co9S8-NSC@Mo2C has a higher of Co3+ content according to the XPS analysis (Figure 3c and Figure S5d), which serve as OER active sites,



and is in good agreement with the redox peak during the OER operation (Figure S12). Therefore, the enhanced OER performance of Co9S8-NSC@Mo2C can be attributable to its higher Co valence resulting from strong electron transfer. Notably, after 2500 continuous CV sweeps between 1.0 and 1.8 V vs. RHE with 50 mV s-1 of scan rate in a 1.0 M KOH solution, the OER polarization curves of Co9S8-NSC@Mo2C has a slight change (Figure S13a). Meanwhile, the time dependence of current density curve (Figure S13b) for Co9S8-NSC@Mo2C at an overpotential of 300 mV for 20 h shows no obvious degradation. In addition, the polyhedral morphology of Co9S8-NSC@Mo2C (Figure S14) after long-term electrolysis remains similar to that before the test. Furthermore, XPS was performed to analyze the variation in Co9S8-NSC@Mo2C after cycling tests (Figure S15). The valence state of Co does not change after the OER cycling tests, suggesting the excellent stability of Co2+/Co3+ during the long-term durability tests. However, it is found that the Mo 3d peaks of Co9S8-NSC@Mo2C after the OER cycling tests shift to higher binding energies, indicating a higher Mo valence during the cycling tests. The experimental O2 productions for Co9S8-NSC@Mo2C is in good agreement with theoretical predictions (Figure 7d), indicating that Co9S8-NSC@Mo2C has a Faradaic efficiency of nearly 100%. For the practical application of these materials, we measure the possibility of utilizing Co9S8-NSC@Mo2C as a cathode and an anode (Co9S8-NSC@Mo2C/NF || Co9S8-NSC@Mo2C /NF) for overall water splitting. The results are shown in Figure 8. Continuous oxygen and hydrogen gas are obtained at the anode and cathode during the LSV measurement, respectively. It is clear found that the water splitting cell voltage is ~ 1.61 V at 10 mA cm-2 in 1.0 M KOH solution (Figure 8a), which is lower relative to that of the recently reported bifunctional catalysts, such as FeNi3N/NF ||

FeNi3N/NF (~ 1.62 V),63 Co5Mo1.0O NSs/NF

(anode) || Co5Mo1.0P NSs/NF (cathode) (~ 1.94 V),64 NiS/NF || NiS/NF (∼1.64 V),65 and commercial electrolyzers (1.8 to 2.0 V).66 Furthermore, the Co9S8-NSC@Mo2C/NF || Co9S8-NSC@Mo2C/NF couple also shows excellent durability with slight degradation, achieving a current density at 10 mA cm-2 (Figure 8b).

Conclusions


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Summarily,

a

porous

Co9S8-NSC@Mo2C

composite

catalyst

comprising

only

earth-abundant elements has been successfully prepared by a simple pyrolysis method. Co9S8-NSC@Mo2C has been found to show highly efficient electrocatalytic activity and excellent stability for the HER in acidic/neutral/alkaline solutions. In particular, Co9S8-NSC@Mo2C requires a low overpotential of 74 mV with a small Tafel slope of 69.3 mV dec-1 at a current density of 10 mA cm-2 in a 0.5 M H2SO4 solution. This excellent electrocatalytic activity toward the HER is mainly due to the synergistic effect between the anchored Mo2C and Co9S8 nanoparticles in the porous N, S-doped carbon. Additionally, Co9S8-NSC@Mo2C has been found to be a remarkable electrocatalyst for the OER with good stability in alkaline media. The optimized Co9S8-NSC@Mo2C hybrid material exhibits excellent activity toward overall water splitting. It is strongly believed that the controllable fabrication and excellent electrocatalytic performance can be expanded to gain a new insight into the potential applications of MOFs as precursor.

ASSOCIATED CONTENT Supporting

Information.

SEM

images

of

Co-ZIF-67,

P-AT@Co-ZIF-67,

and

Co9S8-NSC@Mo2C before and after OER in 1.0 M KOH. TEM images of P-AT@Co-ZIF-67. XRD patterns of Co-ZIF-67, P-AT@Co-ZIF-67, Co9S8-NSC, Co-C, Mo2C-NSC and NSC. C 1s, N 1s, S 1s and Co 1s spectra for the Co9S8-NSC. The CV curves for the Co9S8-NSC@Mo2C, Co9S8-NSC, Mo2C-NSC, Co-NSC and NSC in 0.5 M H2SO4 solution. XPS spectrum of the Co9S8-NSC@Mo2C electrocatalyst before and after 20 h durability measurements in 0.5 M H2SO4 and 1.0 M KOH solutions. HER polarization curves, the corresponding

Tafel

slopes

and

time-dependent

current

density

curves

of

the

Co9S8-NSC@Mo2C and Pt/C in 1.0 M KOH solution and 1.0 M PBS solution. OER polarization curves and the current-time curves of the Co9S8-NSC@Mo2C in 1.0 M KOH solution. Comparison of OER and HER activity data for different catalysts.

AUTHOR INFORMATION Corresponding Authors



Y. Liu ([email protected]. Tel.:+86 135 0748 8663) Notes The authors declare no conflict of interest.

ACKNOWLEDGMENTS The work was supported by the Joint Foundation of Science and Technology Department of Guizhou Province, China (Guizhou “LH” [2014]7430), the Major Science and Technology Projects of Hunan Province, China (2015GK1004), and the Research Foundation of Education Bureau of Guizhou Province, China (Guizhou [2015]402).


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Scheme 1. Schematic illustration of the synthesis of the uniform Co9S8-NSC@Mo2C polyhedrons.



Figure 1. SEM images of (a, b) the Co9S8-NSC and (c, d) the Co9S8-NSC@Mo2C


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Figure 2. XRD pattern of Co9S8-NSC@Mo2C.



Figure 3. (a) XPS survey spectra of Co9S8-NSC@Mo2C and Co9S8-NSC, XPS sepctra of (b) C 1s, (c) N 1s, (d) S 1s, (e) Co 1s, and (f) Mo 3d of the Co9S8-NSC@Mo2C.


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Figure 4. (a) TEM, (b) HRTEM, (c) HAADF-STEM, (d, e, f, g, h) elemental mapping images of Co9S8-NSC@Mo2C



Figure 5. (a) Raman spectra of Co9S8-NSC@Mo2C, Co9S8-NSC, Mo2C-NSC and NSC, (b) N2 adsorption/desorption isotherms of Co9S8-NSC@Mo2C at 77 K and the corresponding pore size distribution from the desorption curve (inset).


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Figure 6. HER polarization curves (a), the corresponding Tafel plots (b), and Nyquist plots with the corresponding equivalent circuit (inset) (c) of the Co9S8-NSC@Mo2C, Co9S8-NSC, Mo2C-NSC, Co-NSC, Pt/C and NSC. The Cdl curves (d) for the as-synthesized samples determined at 150 mV vs. RHE. HER polarization curves (e) for the Co9S8-NSC@Mo2C before and after 2500 CV scans. The chronoamperometry (i-t) curves (f) of the



Co9S8-NSC@Mo2C.

Figure 7. OER polarization curves (a), the corresponding Tafel plots (b), and Nyquist plots with the corresponding equivalent circuit (inset) (c) of the Co9S8-NSC@Mo2C, Co9S8-NSC, Co-NSC, Mo2C-NSC, RuO2 and NSC. The amount curves (d) of theoretical and experimental O2 versus time of the Co9S8-NSC@Mo2C at 1.56 V vs. RHE.


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Figure 8. Polarization curves (a) of water electrolysis for the Co9S8-NSC@Mo2C/NF || Co9S8-NSC@Mo2C/NF

in

1.0

M

KOH.

The

current-time

curve

(b)

of

the

Co9S8-NSC@Mo2C/NF || Co9S8-NSC@Mo2C/NF at a constant current density of 10 mA cm-2.

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