mC@MoS2 composite loaded on g-C3N4

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Fabrication of porous CoOx/mC@MoS2 composite loaded on g-C3N4 nanosheets as a high-efficient dual electrocatalyst for oxygen reduction and hydrogen evolution reactions Linghao He, Bingbing Cui, Jiameng Liu, Minghua Wang, Zhihong Zhang, and Hong-Zhong Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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ACS Sustainable Chemistry & Engineering

Fabrication of porous CoOx/mC@MoS2 composite loaded on g-C3N4 nanosheets as a high-efficient dual electrocatalyst for oxygen reduction and hydrogen evolution reactions

Linghao He*, Bingbing Cui, Jiameng Liu, Minghua Wang, Zhihong Zhang*, Hongzhong Zhang

Henan Provincial Key Laboratory of Surface & Interface Science, Zhengzhou University of Light Industry, No. 136 Science Avenue, High-tech Development Zone, Zhengzhou 450002, People’s Republic of China.

*Corresponding authors: Tel. Fax +8637186609676, e-mail: [email protected] (L. -H. He), [email protected] (Z.-H. Zhang)

1

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Abstract Mesoporous cobalt oxide/carbon@molybdenum disulfide@graphitic carbon nitride (CoOx/mC@MoS2@g-C3N4) composite, derived from cobalt-based metal-inorganic framework (ZIF-67) embedded with MoS2 and melamine, was synthesized as a novel dual electrocatalyst for both oxygen reduction reaction (ORR) and hydrogen evolution reaction

(HER).

It

demonstrates

that

the

g-C3N4

nanosheets

in

the

CoOx/mC@MoS2@g-C3N4 composite can accelerate electron transport, and simultaneously result in abundant N-dopants and Co–N complex formation, which could be further utilized as the new catalytically active sites. The synergistic effect among different components endows the as-synthesized composite with dual electrocatalytic activity for both ORR and HER with highly catalytic efficiency. It shows the developed CoOx/mC@MoS2@g-C3N4 comosite exhibits an onset overpotential of 0.89 V (vs. reversible hydrogen electrode) and a Tafel slope of 84 mV dec−1 in KOH (0.1 M) for ORR. Concurrently, this electrocatalyst shows a low onset overpotential of 0.031 V (vs. reversible hydrogen electrode) and a Tafel slope of 66 mV dec−1 in H2SO4 (0.5 M) for HER. The present finding may inspire the development of remarkably excellent electrochemical catalysts and devices and broaden the applications of metal organic framework-derived composites as efficient, economical, stable, and non-precious metal electrocatalyst with bifunctionality toward ORR and HER. Keywords: Metal organic framework, Abundant N-dopant, Hydrogen evolution reaction, Oxygen reduction reaction, Non-precious metal; Electrocatalyst. 2


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INTRODUCTION

With increasing demand in global energy production and the highly negative impact of fossil fuels on the environment, environment-friendly renewable energy technologies, i.e., metal-air batteries, water splitting, and fuel cells, have increasingly attracted researchers’ attention; these technologies are also highly expected to solve the current energy crisis.1-3 In these fields, the main challenge for water splitting and metal-air batteries is the rational design of a highly efficient and low-cost catalysis for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). To date, precious metal-based electrocatalysts, including platinum, ruthenium, rhodium, iridium, and palladium, are often applied as the highly efficient ORR and HER electrocatalysts.4 However, given the high costs of these noble metals and their global scarcity, their applications in the water-splitting system have been limited5,6. Therefore, the development of non-precious metal,7-9 metal composites,10-12 and metal-free electrocatalysts is highly desirable.13,14 Among different metal composites, metal Co-based nanocomposites have been considered as one kind of the most promising catalysts for HER due to low energy barrier for H adsorption.15,16 Many Co-based

catalysts,

including,

CoSe2CNT,17

CoP4N2,18

CuCo@NC19

and

Co-coordinated compounds,20,21 were designed to catalyze the HER. Nevertheless, these catalysts are limited with unsatisfactory catalytic properties because achieving homogeneous carburization and preventing the aggregation of nanoparticles at high reaction temperatures are difficulty.22 Considerable amount of work is still needed for non-precious bifunctional catalyst developments to understand the precise reaction 3



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mechanisms and degradation pathways of mixed spinel oxides, such as NiCo2O4,23 MnCo2O4,24 and FeCo2O4.25 Liu et al. reported a Co-Mo-C/graphene composite as an efficient bifunctional electrocatalyst.26 Additionally, transition metal carbides demonstrate superior electrocatalytic activities. Notably, transition-metal sulfides, especially

2D

MoS2-based materials,

form a

group

of economical and

environment-friendly HER catalysts because of the high structural stability, high electrocatalytic performance, and cost effectiveness.11,27,28 Furthermore, the conduction band (0.12 eV vs. normal hydrogen electrode) and valance band (1.78 eV vs. normal hydrogen electrode) of MoS2 endow it application in electrochemical water splitting.29 It illustates that the combination of Mo- and Co-based nanomaterials exhibits the bifunctionalized catalysis activity. Recently, transition metal (e.g., Fe, Co, Ni, Mn and Cu) active species containing carbon and nitrogen materials have been widely investigated as promising electrocatalysts to promote the slow OER and ORR in alkaline solution and enhance the HER activity under acidic environment due to their abundance and considerable catalytic activity.30 Furthermore, the electrocatalytic activities depend closely on the structure and morphology of catalysts because of the formation of various active sites. For instance, Fe3C@Fe/N-doped graphene composite exhibited a half-wave potential of 0.84 V and limiting current density of 5.35 mA cm-2 for ORR in 0.1 M KOH.31 The nickel–iron NPs-supported the nitrogen-doped carbon (NiFe@NC) composites as electrocatalyst show bifunctional catalytic performances for OER and HER.32 N-doped carbon nanocapsules containing Co NPs 4


displayed highly dual

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electrocatalytic activities for ORR and OER; these nanocapsules were prepared through a solid-state reaction using graphitic carbon nitride (g-C3N4) as the carbon and nitrogen resources and Co(CH3COO)2·4H2O as the Co resource.33 Co3O4 nanocrystals embedded in N-doped mesoporous graphitic carbon layer/MWCNT hybrids showed an onset potential of 1.50 V (vs. reversible hydrogen electrode (RHE)) and an overpotential of only 320 mV (vs. RHE) at 10 mA cm−2 for OER and also exhibited superior activity for ORR.34 On the basis of this discussion, the most bifunctional electrocatalysts are explored and utilized in the combination of HER and OER or OER and ORR. Few reports about the bifunctional electrocatalysts toward HER and ORR are available. Currently, the direct pyrolysis of covalent organic frameworks and MOFs have been applied as a new strategy to prepare various metal/heteroatom-doped carbon materials without additional carbon supports and reducing agents are generated.17 For example, ZIF-8 and ZIF-67, with a high specific surface area and tenability on the structure and composition, were exploited as new precursors and successfully transformed into NC-based materials for ORR.35 Given the intrinsic feature of micropores for MOFs, most-derived

NCs

prepared

by

direct

carbonization

usually

possess

a

micropore-dominant structure that will significantly weaken the reaction kinetics by limiting the mass transport and the use of active sites in the catalysis reactions. Therefore, the MOF-derived NC can be combined with conventional nanomaterials to enhance the bifunctional electrocatalysis activity. For instance, the CoSe2 NPs embedded

in

defective

CNTs,

which

were

5


obtained

by

a


carbonization-oxidation-selenylation

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treatment of Co-based

MOFs,

exhibited

excellent HER activity with a low onset potential of −40 mV and a small Tafel slope of 82 mV dec−1.17 The MoS2@TiO2 nanocomposites derived from Ti-MOF showed prominent electrocatalytic performance with an onset overpotential of −300 mV and a Tafel slope of 81 mV dec−1.36 Simple impregnation of melamine into MOF-5 resulted in mesoporous carbon, thereby enriching the nitrogen content to 7.0%–8.3%; additionally, maintaining a high surface area exhibits an enhanced ORR activity.37 Despite these efforts, much work is still needed to develop non-precious bifunctional catalysts with simultaneous catalytic performances for HER and ORR. In this study, for the first time, combining the high ORR capacity of Co-based nanomaterials and the excellent HER catalysis activity of MoS2 and introducing N-rich g-C3N4 nanosheets, we prepared Co3O4/mC@MoS2@g-C3N4 composite as a novel

dual

electrocatalyst

Co3O4/mC@MoS2@g-C3N4

for

both

ORR

composite

was

and

HER.

obtained

Notably,

the

from

the

ZIF-67@MoS2@melamine and the synthesis procedure is depicted schematically in Scheme 1. Comparison with the routine electrocatalysts showed that the as-presented Co3O4/mC@MoS2@g-C3N4 composite exhibits two advantages, namely, (1) the hierarchical porous nanostructure caused by the well-organized MOF structure and pyrolysis procedure and (2) the combination of ORR and HER abilities that originated from the individual component. We found that the synergistic effects among different components endow the as-synthesized Co3O4/mC@MoS2@g-C3N4 composite with highly efficient dual-electrocatalytic activity for ORR and HER. 6


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Scheme 1

RESULTS AND DISCUSSION

Chemical structure and components of all samples. The chemical structure and composition

of

the

series

of

CoOx/mC,

CoOx/mC@MoS2,

and

CoOx/mC@MoS2@g-C3N4 composites were first analyzed by X-ray diffraction data (XRD) and Raman spectra (Figure 1), and the characterization results of the MoS2 nanosheets were provided as a comparison. The XRD pattern of MoS2 (curve i, Figure 1a) exhibits four characteristic peaks at 2θ = 13.9°, 33.1°, 39.3°, and 58.6°, which are attributed to the hexagonal MoS2 phase (JCPDS no. 37-1492).38 Simultaneously, the XRD spectrum and FE-SEM images of ZIF-67 are illustrated in Figure S1. After calcination at 550 °C, the as-obtained CoOx/mC composite displays four diffraction peaks at 2θ = 36.5°, 42.3°, 44.2°, and 61.5°. The first two and the last peaks match well with the planes of CoO at (111), (200), and (220) (JCPDS no. 43-1004), respectively, and the peak at 2θ = 44.2° corresponds to the (111) crystal facet of metallic Co (JCPDS no. 15-0806), thereby indicating the existence of Co and CoO.39,40 For the CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 composites, the peaks assigned to MoS2 at 2θ = 13.9°, 33.1°, and 58.6° are still obtained. When the graph is enlarged, two weak peaks at 2θ = 44.2° and 51.9° are observed, which are ascribed to the (111) and (200) reflections of metallic Co (JCPDS no. 15-0806), respectively. Within the detectable limitation, CoO and Co3O4 shows no characteristic peak possibly because of low crystallinity and/or the small size of the Co oxides. The 7



diffraction peak of g-C3N4 nanosheets at 27° is also not observed. This absence may be explained by that g-C3N4 was obtained by in situ calcination of melamine, without being stripped.

Here Figure 1. The comparative Raman spectra of MoS2, CoOx/mC, CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4 composites are shown in Figure 1b. The MoS2 nanosheets display two dominant peaks at 376 and 406 cm−1, which correspond to the E12g and A1g modes of the hexagonal MoS2 crystal, respectively.41,42 The E12g mode originates from the opposite vibration of two S atoms with one Mo atom. The A1g mode is associated with the out-plane vibration of only S atoms. Comparison with the Raman spectrum of the pristine MoS2 shows the relatively larger peak width and weaker intensity of E12g peak of the CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 composites, indicating that the crystal structure of MoS2 is unideal, and in-layer disorders or defects exist between the Mo and S atoms (the inset of Figure 1b). The certain amount of defect sites in composites may provide many active site to favor the HER activity of the samples.43 Moreover, the CoOx/mC, CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4 composites showtwo characteristic peaks at ~1345 and ~1586 cm−1, which correspond to D-band (C–C, the disordered graphite structure) and G-band (C=C, sp2-hybridized carbon), respectively.44,45 In addition, the ID/IG ratio is defined as the intensity ratio of the D-band to the G-band of carbon materials; this ratio also directly indicates the structural changes in samples.46 As illustrated in 8


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Figure 1b, the ID/IG value of the CoOx/mC@MoS2@g-C3N4 composite is higher (1.09) than those of CoOx/mC@MoS2 (1.04) and CoOx/mC (1.01), hinting the presence of many defects in the CoOx/mC@MoS2@g-C3N4 composite. These defects result from the incorporation of heterogeneous N atoms into the CoOx/mC composite. The X-ray photoelectron spectroscopy (XPS) analysis was employed to further characterize the chemical structure and compositions of all samples, including MoS2 nanosheets and CoOx/mC,

CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4

composites (Figure S2). The substantial signals of C 1s, O 1s, and N 1s are observed in all samples. The Co 2p signal is present in CoOx/mC composite. The strong Mo 3d and S 2p also appeare in MoS2 nanosheets. Additionally, C 1s, N 1s, O 1s, Mo 3d, S 2p, and Co 2p can be found in the CoOx/mC@MoS2@g-C3N4 composite. Simultaneously, the atomic% of each element in the samples is summarized in Table S1. For the MoS2 nanosheets, 9.86% C 1s, 35.78% N 1s, and 6.32% O 1s coexist with Mo 1s and S 1s, which indicated the carboxylation of the as-synthesized MoS2 nanosheets. In case of the CoOx/mC composite, the atomic% of Co 2p, C 1s, N 1s, and O 1s is 15.98%, 47.74%, 30.83%, and 5.45%, respectively. After the combination with MoS2 nanosheets, the atomic% of Co 2p, C 1s, and O 1s decreases, and 3.09% Mo 3d and 8.01% S 2p appeare in the CoOx/mC@MoS2 composite. It hints the presence of MoS2 in the as-prepared CoOx/mC@MoS2 composite. For comparison, the increase in the atomic% of C 1s and N 1s from 44.99% to 47.87% and from 15.98% to 18.54%, respectively, in CoOx/mC@MoS2@g-C3N4 composite implies the combination of the g-C3N4 nanosheets with the CoOx/mC@MoS2 composite. 9



To evaluate the chemical environments of each element contained in all samples, the core-level XPS spectra were fitted and analyzed by using the software of XPSPEAK1. The high-resolution XPS spectra of Mo 1s, S 2p, C 1s, N 1s, and O 1s contained in the MoS2 nanosheets are illustrated in Figure S3. The Mo 3d spectrum reveals the +4 oxidation state of Mo (Mo 3d3/2 at 232.1 eV and Mo 3d5/2 at 228.5 eV) (Figure S3a), which are assigned with the formation of Mo-based carbide, sulfide, or nitride.47 The +6 oxidation state of Mo observed at 235.7 eV (Mo 3d3/2) and 232.8 eV (Mo 3d5/2) may be attributed to the oxidation of the samples due to air exposure.48 Furthermore, an additional binding energy at 225.76 eV in the Mo 3d XPS spectrum belongs to S 2s.49 In Figure S3b, the major peaks at 161.64 and 162.82 eV belong to the S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions (S2+), respectively, which are also considered the active sites in HER.50 For C 1s core-level XPS spectrum, two peaks at 283.82 and 285.12 eV are observed due to the C–C/C–H and C–N/C–S groups (Figure S3c), respectively. In case of the N 1s core-level XPS spectrum (Figure S3d), the weak peak at 397.16 eV is the Mo–N group, and the binding energy at 394.6 eV corresponds to Mo 3p3/2. Literature shows that the presence of MoN can supply the vital active sites for HER.51 As shown in Figure S3e, the O1s core-level XPS spectrum is deconvoluted into the main peak at 530.5 eV due to the value typical for Mo–O bond, while the two other peaks at 531.4 and 532.3 eV are for C–O and N–C–O bonds, respectively.52 All of these results display the multiple nanostructure of the MoS2 nanosheets.

10


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Concurrently, the Co 2p, C 1s, N 1s, and O 1s core-level XPS spectra of ZIF-67 and CoOx/mC composite are summarized in Figures S4 and S5, respectively. For the Co 2p core-level XPS spectrum of ZIF-67, five components are deconvoluted at 780.46, 781.86, 786.02, 796.63, and 802.21 eV. Among them, two prominent peaks at around 780.46 and 796.63 eV are ascribed to Co 2p3/2 and Co 2p1/2, respectively, which are the typical characteristics of Co3+ species. The peaks at 786.02 and 802.21 eV are their satellites. In the C 1s core-level XPS spectrum, two main groups of C–C/C–H and C–N/C–O at the binding energies of 284.6 and 285.7 eV are fitted out, and two weak peaks at 287.4 and 290.7 eV are due to the presence of C=O and π−π*, respectively, which originate from the organic ligands contained in the framework. Two peaks of C–N/N–H and N–C=O are also present in the N 1s core-level spectrum. The O 1s spectrum can be separated into three components, including 530.55, 531.36, and 532.37 eV, which are attributed to the lattice oxygen anions (O2−), –O–H, and C–O/C=O, respectively.53 However, after calcination, i.e., the CoOx/mC composite, a new peak at 778.7 eV appeared; this peak is ascribed to the presence of metallic Co. The original peak at 796.63 eV, which is the Co 2p1/2 of the Co3+ species of ZIF-67, shifts to 795.42 eV, together with the peak at 793.84 eV, which is due to the Co 2p1/2 of Co2+ species. These results further identify the coexistence of metallic Co and Co oxides with the thermal treatment, thereby indicating that some Co ions are reduced during calcination. In the C 1s core-level XPS spectrum of the CoOx/mC composite, the carbon-related groups are oxidized to multiple functional groups, especially the appearance of O–C=O group. The same kinds of nitrogen-related groups, i.e., 11



N–H/C-N and N–C=O, are obtained but with low intensity due to the pyrolysis of the corresponding functional moieties at high temperature. In the O 1s core-level XPS spectrum of CoOx/mC composite, the peak at 590 eV corresponds to the surface lattice oxygen in the formed metallic oxide, i.e., CoOx. During the pyrolysis, the –OH at the binding energy of 530.4 eV disappeares. All of these results suggest that the new nanostructure of the metallic oxides is formed. As shown in Figure S6, the Co 2p, Mo 3d, C 1s, N 1s, O 1s, and S 2p core-level XPS spectra of ZIF-67@MoS2 composite were analyzed and summarized. Evidently, their corresponding deconvoluted peaks are similar to those of the individual ZIF-67 and MoS2. Nevertheless, the Mo 3p3/2 is obtained at the 394.3 eV in the N 1s core-level XPS spectrum, hinting the combination of ZIF-67 and MoS2. The high-resolution XPS spectra of each element of the resultant CoOx/mC@MoS2 after the calcination of the ZIF-67@MoS2 composite are shown in Figures 1c and S7. In addition to the peaks in the Co 2p XPS spectrum of the CoOx/mC composite, an additional peak at 782.0 eV, which is ascribed to the Co-Nx group, is fitted out.54 These results show that the simulated peaks in the C 1s and O 1s XPS spectra of CoOx/mC@MoS2 are consistent with those of the CoOx/mC, whereas the Mo 3d is the same as that of the MoS2 nanosheets. In the S 2p core-level XPS spectrum of CoOx/mC@MoS2, the main peaks at 167.5 and 168 eV are obtained, possibly due to the presence of S–N/S–O groups.47,55 In addition to the peaks in the ZIF-67@MoS2, which are caused by the Mo-N group, an additional peak at 397.8 eV in the N 1s core-level XPS spectrum is also fitted out. As reported by Liu et al., the presence of Mo-N bonds can supply the 12


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vital active sites for electrocatalytic reactions.51 All of these results indicate that the large number of complicated composite produced is favorable to the enhancement of the electrocatalytic activity of the as-synthesized CoOx@mC@MoS2. After the combination of the CoOx/mC@MoS2 with g-C3N4 nanosheets, i.e., CoOx/mC@MoS2@g-C3N4 composite, no substantial change is observed for each element, which is similar to that of the individual components of CoOx/mC@MoS2 composite (Figure 1c). Nonetheless, in the C 1s core-level XPS spectrum of these composites, the relatively high intensity of the C–N increases when combined with the g-C3N4 nanosheets.56,57 Comparison of the N 1s core-level XPS spectra of the CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 composite shows the enhanced intensity of the N–C group at the binding energy of 398.4 eV (Figure S7). Evidently, the increased intensity of the C–N group in the CoOx/mC@MoS2@g-C3N4 composite is caused by the addition of the g-C3N4 nanosheets. Surface morphologies of the samples. The surface morphologies of the as-obtained CoOx/mC, CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4 composites were characterized by field emission scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM). The FE-SEM and TEM images of the as-prepared MoS2 nanosheets are shown in Figure S8. MoS2 nanosheets exhibit a uniform nanosheets structure with a relatively smooth surface. The ZIF-67 shows polyhedral crystals with a size of ~750 nm (Figure S1b). After introducing MoS2 into ZIF-67, the ZIF-67@MoS2 hybrid retains the polyhedral structure of the ZIF-67, except for some nanosheets between the particles (Figure S9a). The TEM image of 13



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ZIF-67@MoS2 binary hybrid (Figure S9b) also shows MoS2 nanosheets inside the ZIF-67 crystals. In the ZIF-67@MoS2@melamine ternary hybrids, the particles are not considerably independent; they adhered to one another in FE-SEM and TEM images (Figures S9c and S9d). After calcination at 550 °C, the CoOx/mC composite displayed blocky structure, which appeares as small particles stacked together (Figures 2a and 2b). Regarding the CoOx/mC@MoS2 composite, the polyhedral morphology of ZIF-67 can be clearly observed,

despite

their

surface

collapse

(Figures

2c

and

2d).

In

the

CoOx/mC@MoS2@g-C3N4 composite, the ZIF-67 skeleton is also reserved, and the sizes are smaller than those of the CoOx/mC@MoS2 composites. The white flakes between particles should be g-C3N4. To investigate the distribution of the main elements,

the

energy-dispersive

X-ray

spectroscopy

was

conducted

on

CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 composites. Results show that Mo, S, and Co were uniformly dispersed in the composite (Figure S10), which is desirable for high catalytic efficiency.

Here Figure 2. Detailed information about NPs was further investigated by TEM and high-resolution TEM (HR-TEM) images. The CoOx/mC surface is considerably rougher than that of ZIF-67 (Figure 3a). The HR-TEM image of CoOx/mC (Figure 3b) shows interplanar distances of 0.204 and 0.244 nm, corresponding to the lattice spacing of Co (111) plane and Co oxide (311) plane, respectively.12 In the CoOx/mC@MoS2 composite, 14


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the NPs are irregularly shaped (Figure 3c) and embedded with small NPs; this observation is confirmed by the distinct lattice fringes for Co (111)and Co oxide (311) planes, as indicated in the HR-TEM image (Figure 3d). As shown in Figures 3e and 3f, the morphology of the CoOx/mC@MoS2@g-C3N4 composite is similar to that of CoOx/mC@MoS2 composite. However, many lattices assign to C (002) with the interplanar distance of 0.341 nm surrounded the particles, which originate from g-C3N4 nanosheets.

Here Figure 3. Electrocatalytic

performances

of

as-prepared

composites.

Initially,

we

investigated the electrocatalytic activity of the MoS2 nanosheets and the CoOx/mC, CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4 composites for the ORR in KOH solution (0.1 M). The commercial Pt/C catalysts loaded with 20% Pt were also investigated under the same experimental conditions. The potentials were referenced to RHE (see Supplementary Information). As shown in Figure 4a, in O2-saturated KOH

solution

(0.1

M),

the

cyclic

voltammetry

(CV)

curve

of

CoOx/mC@MoS2@g-C3N4 hybrid exhibits an evident ORR peak at 0.68 V, whereas the N2-the saturated electrolyte shows no peak, thereby indicating the electrocatalytic activity of CoOx/mC@MoS2@g-C3N4 for ORR. Afterward, the ORR performances of all samples were measured by linear scanning polarization curves on a RDE in alkaline (O2-saturated KOH solution (0.1 M)) electrolytes. As shown in Figure 4b, Pt/C exhibits the highest ORR electrocatalytic activity with an onset potential of 1.01 15



V and a high kinetic-limiting current density (Jk) of 5.89 mA cm−2. The pristine MoS2 nanosheets present poor ORR activity with a low onset potential of 0.65 V and Jk of 0.77 mA cm−2, which confirms that MoS2 is not an excellent ORR catalyst. Combining with the CoOx/mC composite, the remarkable improvement is obtained for the CoOx/mC@MoS2, which clearly benefits from high ORR activity of CoOx/mC. With further doping with the g-C3N4 nanosheets, the CoOx/mC@MoS2@g-C3N4 composite exhibits improved performance with an onset potential of 0.89 V and a high Jk (4.53 mA cm−2). The Jk and onset potentials of all samples are displayed in Figure 4c. The present CoOx/mC@MoS2@g-C3N4 composite possesses more efficient catalytic activity toward ORR than those of other composites reported in literature (Table S2), such as N-doped nanoporous carbon, nitrogen-doped graphene/carbon nanotube, phosphorus-doped graphene, Co/C-700, and so on.

Here Figure 4. Results signify the significant effect of the g-C3N4 nanosheets in enhancing the ORR performance. Nevertheless, the pure g-C3N4 nanosheets show only a poor ORR activity with the onset potential at 0.688 V in Figure S11. Actually, the poor electroconductivity is a serious problem limiting the electrocatalytic activity of g-C3N4-based materials. Zheng et al. theoretically clarify the major barrier of ORR on g-C3N4 by first-principle calculations.58 They demonstrated that the limited electron transfer ability of g-C3N4 is conducive for the accumulation of OOH intermediate products via an inefficient two-electron ORR pathway, one of the main reasons for its 16


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relatively low ORR catalytic activity. Based on this theoretical prediction, they incorporated the g-C3N4 nanosheets into the framework of a ordered mesoporous carbon to promote transfer ability in the composite and the nanoporous g-C3N4@carbon showed an excellent electrocatalytic ORR activity. Liu et al. proposed that the g-C3N4 contains so-called “nitrogen pots” with abundant melon moieties, which are potential ideal sites for the modification of molecular electronic structures.59 In our study, the hierarchical porous nanostructure with a large amount of carbon material was generated by the well-organized MOF structure and pyrolysis procedure. Accordingly, a cooperative and/or synergistic interaction between the CoOx/mC@MoS2 composite and the g-C3N4 in CoOx/mC@MoS2@g-C3N4 can be proposed to account for the high ORR performance. Electrochemical impedance spectroscopic (EIS) measurements were used to further investigate the electron transfer kinetics, in which the semicircle represents the electron transfer process and the straight line corresponds to the diffusion process.60 As shown in Figure S12, the charge-transfer resistance (Rct) is 20.4 ohm for CoOx/mC@MoS2@g-C3N4 composites; this value is much lower than those of other samples (659.9, 655, and 335.5 ohm for MoS2, CoOx/mC, and CoOx/mC@MoS2, respectively). This result indicate that the incorporation of the g-C3N4 nanosheets can significantly improve the electrochemical activity of the as-developed composite. Fan et al. reported that the excellent electrocatalytic activity of the metal carbide-graphene nanoribbon hybrid catalyst resulted from the synergistic effect between the metal carbide and graphene nanoribbon sheets, in which graphene played an important role in electron 17



Page 18 of 42

transmission; furthermore, the close interconnection between the metal carbide and carbon species was convenient for the formation of highly active sites on the surfaces.61 In the present work, the ORR activity of the CoOx/mC@MoS2@g-C3N4 composite is evidently improved compared with that of the CoOx/mC@MoS2. This high activity toward ORR could be attributed to the many available interfaces with C3N4. The Co oxides also considerably accelerate the rapid electron transport due to the highways of the g-C3N4 nanosheets. The specific area of the three samples were measured by N2 adsorption-desorption isotherms, as shown in Figure S13. The type IV isotherms with distinct hysteresis loops in the range of 0.5-1.0 P/P0 are observed, which are characteristic of mesoporous

materials

related

to

capillarity

condensation.62

The

Brunauer−Emmett−Teller (BET) specific surface area of CoOx/mC, CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 are 391.68, 51.79 and 94.59 m2 g-1, respectively. In contrast to the CoOx/mC, the smaller BET surface area of CoOx/mC@MoS2 and CoOx/mC@MoS2@g-C3N4 composite should be due to the reduction of pores and channels caused by in situ reaction. We also compare the electrochemically active surface area (ECSA) using a simple CV method (Figure S14).63 As expected, the double layer capacitance of CoOx/mC@MoS2@g-C3N4 (12.6 mF cm-2) is slightly higher than that of CoOx/mC@MoS2 (11.3 mF cm-2) and CoOx/mC (12.0 mF cm-2). The high surface area of CoOx/mC@MoS2@g-C3N4 would provide a large contact area and rich active sites for the electrocatalytic process.

18


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To further evaluate the ORR kinetics of the CoOx/mC@MoS2@g-C3N4 composite, linear sweep voltammetry (LSV) curves with different rotation rates were recorded at a scanning rate of 10 mV s−1 (Figure 4d). Evidently, the current densities (J) gradually increase with the increase of the rotation rates from 500 rpm to 2000 rpm. The corresponding Koutecky – Levich (K–L) plots (J−1 vs. ω −1/2) from −0.1 to +0.6 V are shown in Figure 4d (inset). The good linearity of CoOx/mC@MoS2@g-C3N4 shows first-order reaction kinetics for ORR regarding oxygen concentration in the solution.64 By caculation, the numbers of electron transfer are between 3.50 and 3.80 in the potential ranging from −0.1 to +0.6 V. It indicates that the oxygen is directly reduced to OH− via a four-electron oxygen reduction process in alkaline solution, which is similar to that of Pt/C (3.48–4.00).65 By contrast, the n values at MoS2, CoOx/mC, and CoOx/mC@MoS2 are significantly low at 1.56–2.71, 2.91–3.62, and 2.68–3.25, respectively. Moreover, according to the LSVs curves at 1600 rpm, the Tafel plots of all the composites and the commercial Pt/C are given (Figure 4e). In ternary composites, the CoOx/mC@MoS2@g-C3N4 composite shows a small Tafel slope of 84 mV dec−1 at low overpotential; this slope is considerably close to that for the Pt/C (77 mV dec−1). Furthermore, to assess the immunity of CoOx/mC@MoS2@g-C3N4 to methanol, antimethanol tests were performed (Figure 4f). The current density of the Pt/C catalyst drastically decreases upon addition of 3.0 mL of methanol, whereas the current density of CoOx/mC@MoS2@g-C3N4 changes slightly. Results demonstrate that the CoOx/mC@MoS2@g-C3N4 composite possesses high selectivity for ORR 19



Page 20 of 42

with strong tolerance to the crossover effect, which is remarkably important for practical

applications

in

fuel

cells

and

metal-air

batteries.

The

CoOx/mC@MoS2@g-C3N4 composite also exhibits excellent ORR stability, which was evaluated in an O2-saturated KOH solution (0.1 M). The current density generally remains stable over 40 h with little degradation (Figure S15), thereby resulting in the strong stability in alkaline medium. All these results indicate that the CoOx/mC@MoS2@g-C3N4 composite is a highly efficient ORR electrocatalyst. In addition to excellent ORR activity, the HER catalytic performances of all samples were also investigated in N2-saturated H2SO4 solution (0.5 M). Figure 5a shows the HER polarization curves of Pt/C, MoS2, CoOx/mC, CoOx/mC@MoS2, and CoOx/mC@MoS2@g-C3N4 composites at 5 mV s−1 in H2SO4 solution (0.5 M). The Pt/C exhibits the superior performance with near-zero onset potential. The CoOx/mC@MoS2@g-C3N4 composite exhibits better electrocatalytic activity with an onset potential of −0.031 V than that of the CoOx/mC@MoS2 composite; this potential is more positive than that of the pure MoS2 nanosheets (−0.19 V). The kinetic current density can reach approximately 10 mA cm−2 at an overpotential of −0.098 V. However, the g-C3N4 alone shows considerably poor HER performance with an onset potential of −0.31 V (Figure S16), which further confirms the synergistic contribution of the g-C3N4 nanosheets and the CoOx/mC@MoS2 composite. These results are much better than those reported for non-precious HER electrocatalysts measured in acidic medium, such as MoO3-MoS2 nanowires,

20


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electrodeposited amorphous MoS2, Co9S8@C, NCo@G, MoS2 NA/CC, and so on (Table S3).

Here Figure 5. The corresponding Tafel slopes of all composites were calculated from their polarization plots (Figure 5b). The small onset of catalytic activity and small Tafel slope suggest that the hydrogen adsorption is close to equilibrium. In general, three possible reaction steps are suggested for the HER in acidic media: a primary discharge step (Volmer reaction, 120 mV dec−1), which is followed by either the electrochemical desorption step (Heyrovsky reaction, 40 mV dec−1) or the recombination step (Tafel reaction, 30 mV dec−1).11 Therefore, the mechanism by which HER occurs on any electrode material may be determined from the Tafel slope value. The CoOx/mC@MoS2@g-C3N4 composite displays smaller Tafel slopes of 66 mV dec−1 than that of CoOx/mC@MoS2 composite (155 mV dec−1). This slope is in between the Volmer mechanism and the Heyrovsky mechanism. Thus, the CoOx/mC@MoS2@g-C3N4 composite probably follows a Volmer–Heyrovsky mechanism under acidic medium. In this mechanism, both reactions influence the HER rate, in which the fast discharge of a proton is followed with a rate-limiting electrochemical

recombination

with

an

additional

proton.42,49

The

CoOx/mC@MoS2@g-C3N4 stability was determined by continuous CV scanning between 0.05 and −0.5 V at 50 mV s−1 in H2SO4 (0.5 M). As shown in Figure 5c, the current density generally remains stable over 40 h with little degradation, which 21



Page 22 of 42

further demonstrates that CoOx/mC@MoS2@g-C3N4 possesses a strong stability in acidic medium. Moreover, after long-term electrochemical tests, the XRD and SEM characterizations were conducted on the catalyst to investigate its morphology and structure. The results were compared with those before durability test. In the XRD pattern (Figure S17), the peaks attributed to MoS2 at 2θ = 13.9°, 33.1°, and 58.6° can still be clearly seen. Simultaneously, the peak assigned to metallic Co at 2θ = 44.2° was still observed. From the FE-SEM image (Figure S18), the polyhedral morphology with surface collapse was clearly observed. Therefore, it can be concluded the structure of CoOx/mC@MoS2@g-C3N4 was almost unchanged before and after the stability test, suggesting a more stable structure originated from the chemical coordination or electron coupling between the elements. To obtain further insight into the electrocatalytic activity of the as-synthesized CoOx/mC@MoS2@g-C3N4 composite, the ORR and HER activities of a simplely physical

mixture

of

CoOx/mC@MoS2

and

g-C3N4

(denoted

as

CoOx/mC@MoS2/g-C3N4) were also examined (Figure S19). The results show the CoOx/mC@MoS2/g-C3N4 hybrid exhibits a much lower catalytic activity for both ORR and HER than those of CoOx/mC@MoS2@g-C3N4. To further investigate the weak electrocatalytic ability of the CoOx/mC@MoS2/g-C3N4 hybrid, the chemical structure were probed by XRD, SEM and XPS. As shown in Figure S20, the XRD pattern of CoOx/mC@MoS2/g-C3N4 hybrid exhibits five characteristic peaks at 13.9°, 27.2°, 33.2°, 44.2° and 58.9°. The three peaks at 13.9°, 33.2° and 58.9° are originated from the MoS2 nanosheets and the peak at 44.2° is corresponded to the metallic Co 22


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phase in CoOx/mC composite. The peak at 27.2° is indexed to the (002) planes of g-C3N4 nanosheets,66 which is not observed in CoOx/mC@MoS2@g-C3N4 composite. It suggests that there is not interaction between components in physical mixture of CoOx/mC@MoS2 and g-C3N4, which is further confirmed by SEM and XPS characterizations. As shown in Figure S21, the polyhedral morphology of ZIF-67 can be clearly observed, which is almost identical to CoOx/mC@MoS2 composite (Figures 2c and 2d), indicating that the g-C3N4 nanosheets cannot be well compounded with CoOx/mC@MoS2 composite only by physical mixing. The Co 2p, Mo 3d, S 2p, C 1s, N 1s and O 1s XPS core-level spectra of CoOx/mC@MoS2/g-C3N4 hybrid are also presented in Figure S22 and compared with those of the CoOx/mC@MoS2@g-C3N4

composite

(Figures

1c

and

S7b).

For

the

CoOx/mC@MoS2/g-C3N4 hybrid, no peak ascribed to the Co-Nx group is not observed in the Co 2p core level spectrum, as well as no peak assigned to the Mo-N group in the N 1s core level spectra. Moreover, the EIS of CoOx/mC@MoS2/g-C3N4 hybrid were performed (Figure S23), giving the Rct value of 97.6 ohm, which is much higher than that of CoOx/mC@MoS2@g-C3N4 (20.4 ohm, Figure S12). It indicates that the physically mixed CoOx/mC@MoS2/g-C3N4 hybrid exhibits the low electrochemical activity. Therefore, these results signify that the g-C3N4 nanosheets embedded within CoOx/mC@MoS2 obtained

by

in

situ

reaction

can

facilitate

remarkably

electromigration at the catalyst interface. Therefore, the presence of the g-C3N4 nanosheets in the CoOx/mC@MoS2@g-C3N4 composite can result in abundant N-dopants and Co–N complex formation, which 23



could be further utilized as the new catalytically active sites.53 The synergetic chemical coupling effects among the MoS2 nanosheets, Co oxides, and C3N4 nanosheets due to their intimate electronic interactions may provide an efficient electron transport network in the CoOx/mC@MoS2@g-C3N4 composite.56,57 Consequently, the composite exhibits excellent electrocatalytic activity and stability toward to ORR and HER.

CONCLUSIONS

A new core-shell nanostructured and nitrogen-containing CoOx/mC@MoS2@g-C3N4 composite was applied as a novel advanced dual electrocatalyst for the ORR and HER. The composite was obtained by the introduction of MoS2 nanosheets and melamine in ZIF-67 and subsequent pyrolysis at high temperature. The present findings demonstrate that the g-C3N4 incorporation can increase the percentage of carbon and nitrogen, which further improved the comprehensive catalytic activity of the composites. The developed CoOx/mC@MoS2@g-C3N4 composite exhibits an onset overpotential of 0.89 V, a Tafel slope of 84 mV dec−1, an excellent stability, and a good tolerance to methanol crossover in KOH solution (0.1 M) for ORR. Moreover, the as-synthesized electrocatalyst shows a relatively low onset potential of only 0.031 V and excellent durability in H2SO4 solution (0.5 M) toward HER. This work opens up new opportunities for designing bifunctional electrocatalysts by combining MoS2 and versatile MOF precursors and further extends potential application in energy conversion applications.

ASSOCIATER CONTENT 24


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Supporting Information The Supporting Information is is available free of charge on the ACS Publications website at DOI: SEM images, XPS spectra, EDS elemental mapping, BET, cyclic voltammograms and part of electrochemical performance of ZIF-67, MoS2, ZIF-67@MoS2, CoOx/mC, CoOx/mC@MoS2 , CoOx/mC@MoS2@g-C3N4 and CoOx/mC@MoS2/g-C3N4 composites, and tables of comparison of the electrochemical performance of different catalysts. (PDF)

AUTHOR INFORMATION

Corresponding Author Tel.: +86-37186609676. E-mail: [email protected] (L. -H. He) and [email protected] (Z.-H. Zhang), ORCID Zhihong Zhang: 0000-0002-5888-4107 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We are grateful to the support from the National Natural Science Foundation of China (21601161 and U1604127) and Innovative Technology Team of Henan Province (CXTD2014042).

25



Page 26 of 42

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polyphthalocyanine to N-doped carbon supported Ni-Fe nanoparticles for highly efficient

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Figures

Scheme 1 Schematic of the preparation for CoOx/mC@MoS2@g-C3N4 composite, including i) preparation of MoS2 nanosheets, ii) synthesis of ZIF-67@MoS2 and adsorption of melamine, and iii) observation of Co3O4/mC@MoS2@g-C3N4 composite.

35



(a)

iv

15

30 45 2θ (degree)

60

MoS2 PDF#37-1492

(c)

20

30 40 50 2θ (degree)

Co3+

Co-Nx Co2+ Satellite, b Satellite, a Co3+ Co0

CoO x/mC@MoS2@g-C3N4 Co 2p

800

790

360

780

380 400 420 440 Raman Shift (cm -1)

ii) CoOx/mC iii) CoOx/mC@MoS2

60

500

70

1000

1500

2000

-1

Raman Shift (cm )

CoO x /m C@ M oS 2 M o 3d

CoOx/mC@MoS2 Co 2p

810

i) MoS2

i ii iii iv

iv) CoOx/mC@MoS2@g-C3N4

CoO PDF#43-1004 Co PDF#15-0806

10

Intensity (a. u.)

iii

Intensity (a. u.)

Intensity (a. u.)

@g-C3N4

Intensity (a. u.)

(b)

i) MoS2 ii) CoOx/mC iii) CoOx/mC@MoS2 iv) CoOx/mC@MoS2

Intensity (a. u.)

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 36 of 42

M o 3d 5/2 M o 4+ M o 3d 3/2 M o 4+

M o 6+

S 2s

232

228

C-C/C-H C-N/C-S C-O C=O -O-C=O π -π* CoOx/mC@MoS2@g-C3N4 C 1s

C oO x /m C @ M oS 2 @ g-C 3 N 4 M o 3 d

236

CoO x/mC@MoS 2 C 1s

224

292

288

284

280

Binding Energy (eV)

Figure 1. (a) X-ray diffraction patterns and (b) Raman spectra of (i) the MoS2 nanosheets

and

(ii)

CoOx/mC,

(iii)

CoOx/mC@MoS2,

and

(iv)

CoOx/mC@MoS2@g-C3N4 composites (Inset: partial magnification). (c) The Co 2p, Mo 3d, and C 1s core-level XPS spectra of (iii) the CoOx/mC@MoS2 and (iv) CoOx/mC@MoS2@g-C3N4 composites.

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CoOx/mC

CoOx/mC

CoOx/mC@MoS2

CoOx/mC@MoS2

CoOx/mC@MoS2@g-C3N4

CoOx/mC@MoS2@g-C3N4

Figure 2. Low- and high-magnitude FE-SEM images of (a, b) CoOx/mC, (c, d) CoOx/mC@MoS2, and (e, f) CoOx/mC@MoS2@g-C3N4 composites.

37



Figure 3. TEM and high-resolution TEM images of (a, b) CoOx/mC, (c, d) CoOx/mC@MoS2, and (e, f) CoOx/mC@MoS2@g-C3N4 composites.

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Page 39 of 42

(a)

(b) ii

O2 -2

J (mA cm )

0.5 0.0 -0.5

-2 iv

-4

iii

i) Pt/C ii) MoS2

v

iii) CoOx/mC

-1.0 -1.5 -0.4

0.6

4 0.3

2 i

ii

(e)

iii iv Samples

v

-2

0.4V n=3.6 0.5V n=3.5 0.6V n=3.5

0.2 0.02

0.03

x

v

2

v) CoO /mC@MoS @g-C N x

2

3

4

iii

iv

-2

(f)

0.6 0.8 Potential (V vs. RHE)

ii

40

i

20

i) Pt/C ii) CoOx/mC@MoS2@g-C3N4

0.5 -2.5

1.0

60

0

-3.0

500 rpm 1000 rpm 1500 rpm 2000 rpm

Methanol

ii -4.5 -4.0 -3.5 -2 Log [J (mA cm )]

0.04

ω-1/2( rad-1/2•s-1/2)

80

iv) CoO /mC@MoS

-5.0

-0.1V n=3.8 0.1V n=3.7 0.2V n=3.7 0.3V n=3.6

i x

0.6

0.3

0.4

2

iii) CoO /mC

0.7

0.4

0

100

ii) MoS

0.8

2

1.0

(d)

-6

0.0

i) Pt/C

0.9

4

0.6 0.8 Potential (V vs. RHE)

-4

Retention (%)

-2

0.9

J (mA cm )

6

6

1.0

8

1.2

8 Jk (mA cm )

0.4

(c)

0

v) CoOx/mC@MoS2@g-C3N4

1.2

Onset Potential (V)

10

0.0 0.4 0.8 Potential (V vs. RHE)

iv) CoOx/mC@MoS2

i

-6

J-1 (mA-1•cm2)

-2 J (mA cm )

0

N2

1.0

Potential (V vs. RHE)

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


0

200

400 Time (s)

600

800

Figure 4. (a) The CV curves for ORR on the electrode of CoOx/mC@MoS2@g-C3N4 in KOH solution (0.1 M) at a scanning rate of 5 mV s−1. (b) The polarization curves obtained with (i) Pt/C, (ii) MoS2, (iii) CoOx/mC, (iv) CoOx/mC@MoS2, and (v) CoOx/mC@MoS2@g-C3N4 composites on glassy carbon working electrode (GCE) in a KOH solution (0.1 M) with a sweep rate of 10 mV s−1 at 1600 rpm and (c) the corresponding Jk and onset potentials. (d) The polarization curves obtained for 39



CoOx/mC@MoS2@g-C3N4 in a KOH solution (0.1 M) with a sweep rate of 10 mV s−1 at different rotating rates (Inset: the corresponding Koutecky–Levich plots at potentials ranging from −0.1 V to 0.6 V). (e) Tafel plots corresponding to (b). (f) I–t responses for (i) Pt/C and (v) CoOx/mC@MoS2@g-C3N4 at a potential of 0.7 V vs. RHE and a rotation rate of 1600 rpm with methanol addition (2.4 mL, 3 M) at approximately 300 s.

40


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(b)

(a) i) Pt/C

0.6

iii

ii) MoS

Potential (V vs. RHE)

0

-40

iii) CoO /mC x

-2

J (mA cm )

2

iv) CoO /mC@MoS x

2

v) CoO /mC@MoS @g-C N

-80

x

-120

iv

2

ii

3

4

i

v

i v

0.4 ii i) Pt/C

0.2

ii) MoS

iii

2

iii) CoO /mC x

0.0

iv

iv) CoO /mC@MoS x

2

v) CoO /mC@MoS @g-C N x

-0.6

-0.4 -0.2 0.0 Potential (V vs. RHE)

-4.0

2

3

4

-3.6 -3.2 -2.8 -2.4 -2 Log [J (mA cm )]

-2.0

(c)

)

20 0

J (mA

cm-2

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


-20 -40 -60 0

8

16 24 Time (h)

32

40

Figure 5. (a) Polarization curves obtained with (i) Pt/C, (ii) MoS2, (iii) CoOx/mC, (iv) CoOx/mC@MoS2, and (v) CoOx/mC@MoS2@g-C3N4 on GCE at 5 mV s−1 in H2SO4 solution (0.5 M) and (b) their corresponding Tafel plots. (c) I–t curve for CoOx/mC@MoS2@g-C3N4 at an overpotential of 70 mV for 4 h in N2-saturated H2SO4 solution (0.5 M).

41



For Table of Contents Use Only The synergistic effect among different components endows the nanocomposite with dual electrocatalytic activity for both ORR and HER, which is of high significance in the renewable energy technologies.

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mC@MoS2 composite loaded on g-C3N4

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