Bamboo-Structured Nitrogen-Doped Carbon Nanotube

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Bamboo-structured Nitrogen-doped Carbon Nanotube Coencapsulating Cobalt and Molybdenum Carbide Nanoparticles: An Efficient Bifunctional Electrocatalyst for Overall Water Splitting Lunhong Ai, Jinfeng Su, Mei Wang, and Jing Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01120 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Sustainable Chemistry & Engineering

Bamboo-structured Nitrogen-doped Carbon Nanotube Co-encapsulating Cobalt and Molybdenum Carbide Nanoparticles: An Efficient Bifunctional Electrocatalyst for Overall Water Splitting Lunhong Ai, Jinfeng Su, Mei Wang, and Jing Jiang* Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, 1 Shida Road, Nanchong 637002, P.R. China

*Corresponding Author E-mail: [email protected] (J. Jiang) Tel/Fax: +86-817-2568081

ACS Paragon Plus Environment

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ABSTRACT

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Developing efficient bifunctional electrocatalysts based on inexpensive and

earth-abundant materials for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential for large-scale renewable energy storage and conversion processes but remains a major challenge. In this study, a bamboo-structured nitrogen-doped carbon nanotube co-encapsulated with metallic cobalt and Mo2C nanoparticles (Co-Mo2C@NCNT) is designed and synthesized by a successive pyrolysis approach, and demonstrated to be an efficient and stable bifunctional electrocatalyst for overall water splitting in alkaline medium. Attributing to favorable synergy interaction in composition and structure, the resultant Co-Mo2C@NCNT presents the superior performances towards HER, OER and even overall water splitting in alkaline medium. To drive a current density of 10 mA cm-2, it needs only an overpotential of ~186 and ~377 mV for the electrocatalytic HER and OER, respectively, and a relatively low cell voltage (~1.628 V) for overall water electrolysis. The present finding would open a new avenue to design and develop electocatalytically active multicomponent architectures for overall water splitting.

KEYWORDS

Oxygen evolution; Hydrogen evolution; Electrocatalyst; Bamboo structure;

Nanotubes

INTRODUCTION

Splitting water into molecular hydrogen and oxygen by electrochemical pathway represents a promising manner for sustainable production of clean renewable energy.1,

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However, its

implementation is still a mirage, as two half-cell reactions of water electrolysis including oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) encounter the inevitable dynamic overpotentials, resulting in low energy conversion efficiency.3 In order to realize these


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reactions proceeded smoothly, the efficient electrocatalysts are highly desired. It is well witnessed the good performances of HER-active platinum-based materials4 and OER-active ruthenium/iridium-based compounds.5 However, the high price and scarcity significantly hinder their widespread applications. A number of cost-effective alternatives have been recently developed as potential candidates for above noble-metal catalysts.6-11 Unfortunately, actual overall water splitting device demands electrocatalysts simultaneously work in the same electrolyte,12 but most available catalysts fail to retain high performance under such conditions. Undoubtedly, developing bifunctional electrocatalysts would endow water splitting device with more economical efficiency and easy operation. Molybdenum carbide (Mo2C) has been considered as the new-generation electrocatalysts for boosting HER, because of their similar electronic structure to that of Pt, high electrical conductivity, good chemical stability and low cost.13-15 However, single Mo2C materials still own drawback of the negative hydrogen-binding energy, which can greatly restricts adsorbed H (Hads) desorption during HER process and thus affect the HER kinetics.16, 17 To overcome this bottleneck, recent significant advances have moved to construct Mo2C-based heterostructures to optimize the hydrogen-binding energy for boosting HER.18-22 For example, Gao et al. introduced the cobalt doping in Mo2C nanowires and demonstrated that the rich electrons in dopants can efficiently lower the unoccupied d-orbitals of Mo, thus improving the HER performance of parent Mo2C.18 Similar observations have been reported in other Ni-containing molybdenum carbide structures, which also exhibited the enhanced activity in the electrocatalytic HER.23-25 Meanwhile, extensive studies have recently shown that embedding Mo2C into carbon matrix are feasible strategy toward promoting HER,26-30 because these carbon-supported structures would


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help these catalytic systems with the improved electrical conductivity, highly dispersed distribution of molybdenum carbide and the favorable H* absorption-desorption property. Considering that bifunctional and robust molybdenum carbide catalysts for the electrocatalytic both HER and OER, and even for overall water splitting are scarce,31, 32 we herein design a robust multicomponent architectures comprised of Mo2C and metallic cobalt, where carbides and metallic cobalt can effectively mutually stabilize and complement each other's functions via synergistic effects.33 By a successive pyrolysis approach, a porous bamboo-structured architecture based on the nitrogen-doped carbon nanotube co-encapsulating metallic cobalt and molybdenum carbide nanoparticles (Co-Mo2C@NCNT) is facilely synthesized for the first time, which are demonstrated to be highly active and show long-term stability for electrocatalyzing both the HER and OER process. As expected, the Co-Mo2C@NCNT presents good performance toward overall water splitting in alkaline solution. The observed catalytic feature would be attributed to the synergy interaction in the Co-Mo2C@NCNT. EXPERIMENTAL SECTIONS Materials Ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), cobalt chloride hexahydrate (CoCl2·6H2O), potassium hydroxide (KOH), and melamine purchased from Aladin Ltd (Shanghai, China). All chemicals used in this study were of commercially available analytical grade and used without further purification. Synthesis of bamboo structured nitrogen-doped carbon nanotube encapsulating metallic cobalt nanoparticles (Co@NCNT)


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Typically, CoCl2·6H2O (4 mmol) and melamine (16 mmol) were mixed thoroughly in an agate mortar, which were then annealed at 700 °C for 2 h in tubular furnace under a Ar flow with a heating rate of 10 °C min-1. In order to remove the metallic Co on the outer wall of carbon nanotubes, the annealed products were ultrasonically soaked into 3.0 M HCl for 12 h, then filtered and washed with distilled water several times and dried at 60° C overnight in air. Synthesis of porous bamboo-structured N-doped carbon nanotubes co-encapsulating metallic Co and Mo2C nanoparticles (Co-Mo2C@NCNT) Typically, Co@NCNT (0.2 g) and (NH4)6Mo7O24·4H2O (0.4 g) were added into distilled water (20 mL) and mixed by ultrasound to obtain a homogeneous dispersion. The resulting mixture was dried at 60 °C, ground to obtain a fine powder, and annealed at 850 °C for 2 h under a Ar flow in tubular furnace with a heating rate of 10 °C min-1. For comparison, other samples treated at 700, 750, 800, and 950 °C were prepared. Meanwhile, the samples with 0.4, 0.6, 0.8 and 1.0 g of (NH4)6Mo7O24·4H2O added were also prepared under other conditions unchanged. In addition, the Mo2C@N-doped carbon (NC) and Mo2C@CNT references were also prepared by using melamine and commercial CNT instead of Co@NCNT under other conditions unchanged, respectively. Characterization The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku-Dmax/Ultima-IV diffractometer with Cu Kα radiation (λ = 0.15418 nm). The microstructures were determined by using a Hitachi-S4800 field scanning electron microscope (SEM). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and mapping images were recorded by an FEI Tecnai F20 TEM with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy


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(XPS) measurements were recorded on a Perkin-Elmer PHI 5000C spectrometer using monochromatized Al Kα excitation. All binding energies were calibrated by using the contaminant carbon (C1S = 284.6 eV) as a reference. Electrochemical measurements 5 mg of Co-Mo2C@NCNT powder and 10 µL of Nafion (5 wt%) were added in 1 mL of 3/1(v/v) water/ethanol solution, which was sonication treatment for 30 min to get homogeneous inks. 5 µL of catalyst inks were then deposited on the glassy carbon electrode (GCE, diameter: 3 mm) prior to electrochemical tests. Electrocatalytic water splitting experiments were conducted in a typical three-electrode setup controlled by a CHI 660E electrochemical workstation in 1.0 M KOH solutions, using a KCl-saturated Ag/AgCl electrode as the reference electrode, a Pt wire as the auxiliary electrode, and catalyst-modified GCE as the working electrode. All potentials measured were reported as a form of reversible hydrogen electrode (RHE) according to Nernst equation: ERHE=EAg/AgCl+0.197+0.059pH

(1)

OER and HER activities of the catalysts were evaluated by linear sweep voltammetry at a scan rate of 5 mV s-1. The current density was calculated based on the geometric area of GCE (0.07 cm-2). RESULTS AND DISCUSSION The Co-Mo2C@NCNT was synthesized by annealing process under flowing nitrogen using Co@NCNT as effective director and carbon source. Figure 1a shows a typical scanning electron microscopy (SEM) image of the as-prepared Co@NCNT, which illustrates that the Co@NCNT consists of numerous nanotubes with an average diameter of 250 nm and the length ranging from


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hundreds of nanometers to several micrometers. The transmission electron microscopy (TEM) image (Figure 1b) clearly displays that these nanotubes are actually made of bamboo-like structure, where the cobalt particles with size of 40-80 nm are encapsulated integrally inside the nanotubes. In the high-resolution TEM images (HRTEM, Figure 1c), the clear lattice fringes with 0.34 and 0.20 nm interspaces correspond to (002) plane of graphitic carbon and (111) plane of metallic Co, respectively. The XRD pattern (Figure S1, Supporting Information) shows that the Co@NCNT contain only crystalline metallic Co phase with face centered cubic (fcc) structure (JCPDS file No. 01-1255) and crystalline graphitic carbon phase.

Figure 1. SEM (a), TEM (b) and HRTEM (c) images of Co@NCNT. When cosintering with ammonium molybdate tetrahydrate (AMT), the resulting CoMo2C@NCNT well inherits the bamboo-like shape (Figure 2a). The Co-Mo2C@NCNT bamboos present the ultralong length and flexible features. The fine nanoparticles regularly and firmly


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encapsulated in the nanotubes (Figure 2b). Furthermore, the magnified TEM images suggest two types of nanoparticles within carbon shell (Figure S2, Supporting Information), that is, spherelike particles (Figure 2c) and cube-like particles (Figure 2e). They are well covered by graphitic carbon layers (Figure 2d and Figure 2f). The cube-like particles with lattice fringes of 0.25 nm (Figure 2g) correspond to the Mo2C (100), while the sphere-like particles with lattice fringes of 0.20 nm (Figure 2i) are identified to Co(111). Moreover, the detailed elemental distribution in resulting Co-Mo2C@NCNT was observed by a typical high-angle annular dark-field scanning TEM (HAADF-STEM) image. As shown in Figure 3, the Co, Mo, N, C, and O atoms appear to be uniform spatial distribution across the selected bamboo-like nanotubes.


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Figure 2. TEM images (a,b) of the Co-Mo2C@NCNT. Enlarged TEM image (c) of the Co-Mo2C@NCNT. TEM image (d) taken from region of Mo2C@NCNT. TEM image (e) of the one single nanotube of Co-Mo2C@NCNT. TEM image (f) taken from region of Co@NCNT. HRTEM image (g) taken from region of Mo2C@NCNT. HRTEM image (h) taken from region of Co@NCNT.

Figure 3. HAADF-STEM element mapping images (h) of the Co-Mo2C@NCNT. In addition to the morphology preservation, the phase structure change is confirmed by the Xray diffraction (XRD). Compared with the strong signal on Co (111) peak of the Co@NCNT, the Co-Mo2C@NCNT exhibits main peaks indexed well to hexagonal phase β-Mo2C (JCPDS card no.65-8766), along with a greatly weakened Co (111) peak and a prominent (002) peak of graphitic carbon (Figure 4a), indicating the coexistence of each component in the CoMo2C@NCNT. Figure 4b displays typical N2 adsorption-desorption isotherms of the CoMo2C@NCNT. The isotherms present a characteristic type IV along with a distinct hysteresis loop, confirming the mesopores exist in samples. In term of the Barrett-Joyer-Halenda (BJH) model (inset in Figure 4b), the Co-Mo2C@NCNT has an average mesopore diameters of 4.8 nm.


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Additionally, the specific surface area calculated from Brunauer-Emmett-Teller (BET) method is 28.7 m2 g-1.

Figure 4. XRD pattern (a) and N2 adsorption-desorption isotherms (b) of the CoMo2C@NCNT. X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the valence state and composition of Co-Mo2C@NCNT. The Mo 3d region in the XPS spectra (Figure 5a) suggests the coexistence of C-bound Mo and O-bound Mo in the [email protected] The formation of high content of high valent Mo species could be attributed to the ready surface oxidation of Mo2C when exposed to air, especially the porous NCNT enabling oxygen transport and diffusion. The similar results were also observed in previously reported porous carbon encapsulated-molybdenum carbides.35-37 The Co 2p core level (Figure 5b) confirms the presence of metallic Co in the Co-Mo2C@NCNT. The presence of a certain amount of superficial oxides is attributed to unavoidable oxidation at the surface upon exposure to air for carbide and metallic Co. The C 1s XPS spectrum (Figure 5c) reveals three types of carbon species of Mo-bound carbon, C–C and C–N/C=N in [email protected], 39 The deconvolution


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of N 1s XPS spectrum illustrates the presence of pyridinic-N, pyrrolic-N and quaternary-N in the NCNT (Figure 5d).40

Figure 5. XPS spectra of the Co-Mo2C@NCNT: Mo 3d (a), Co 2p (b), C 1s (c) and N 1s (d). To probe the phase transformation with the annealing process, XRD analysis was conducted on samples obtained at different annealing temperatures (Figure S3, Supporting Information). At relative low annealing temperature, the product is mainly composed of weak metallic Co and dominate MoO2 (JCPDS file no. 32-0671). Furthermore, the Mo2C is formed and completely instead of MoO2 at annealing temperature of 800 oC through the carbothermal reaction between


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MoO2 and NCNT. During this process, the Mo2C can be in situ embedded into excess carbon matrix. This phenomenon is similar to the Mo2C/carbon nanotube composite synthesized by using the carbon nanotube and MoO3 as starting materials.41 However, when the annealing temperature is increased to 950 oC, the Mo2C phase in the product is disappeared and changed into η-MoC phase (Figure S4, Supporting Information), similar to the previous reported results.42, 43 In addition, rationally tuning the mass ratio of Co@NCNT and AMT can effectively control the Co/Mo2C ratio but well retain the phase structure in the resulting Co-Mo2C@NCNT (Figure S5, Supporting Information). We first studied the performance of the Co-Mo2C@NCNT in 1.0 M KOH solution towards the electrocatalytic HER, along with Co@NCNT, Mo2C@NC (Figure S6, Supporting Information), Mo2C@CNT and commercial Pt/C as comparison, and the results were illustrated in Figure 6a. Clearly, the Pt/C exhibits the expected HER activity. Both Co@NCNT, Mo2C@NC and Mo2C@CNT (Figure S7, Supporting Information) display poor activity toward HER. Instead, the Co-Mo2C@NCNT presents a giant enhancement in HER activity, which holds earlier onset potential and merely demands an overpotential of ∼186 and ∼242 mV to produce a catalytic current of 10 and 50 mA cm-2, respectively. More specifically, the current density value driven at overpotential of ∼242 mV for Co-Mo2C@NCNT (~50 mA cm-2) is about seven times higher than the sum of the current densities for Co@NCNT (~2 mA cm-2) and Mo2C@NC (~5 mA cm2

), suggesting a synergic corporation between each component. First, the metallic Co at heterostructure interface could increase the electron density around

Fermi level of Mo2C and reduce the strength of Mo–H, thus facilitating HER kinetics,18, 19 which could be further optimized after covered by NCNT.44 Second, high percentage of pyridinic N (Figure 5d) doped into the CNT can enhance hydrophilicity strengthening the electrolyte-


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electrode interaction.45 Third, the bamboo-like NCNT can efficiently protect the encapsulated nanoparticles and prevents the aggregation of particles. Simultaneously, the presence of NCNT can also improve the electroconductivity of the composites and benefit the electron transfer of encapsulated particles, thus boosting electrocatalytic HER on the surface of carbon arising from the electron penetration by the encapsulated nanoparticles.46-50 In addition, by varying the annealing temperature and mass ratio of precursor, the optimized HER activity of the CoMo2C@NCNT is accomplished (Figure S8 and S9, Supporting Information).


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Figure 6. (a) Polarization curves and (b) Tafel plots of the Co-Mo2C@NCNT, Co@NCNT, Mo2C@NC, and commercial Pt/C at a scan rate of 5 mV s-1 in 1.0 M KOH solution for electrocatalytic HER. (c) Nyquist plots of the Co-Mo2C@NCNT, Co@NCNT, and Mo2C@NC for electrocatalytic HER at a potential of -257 mV. (d) Capacitive currents at 0.83 V against scan rates of the Co-Mo2C@NCNT, Co@NCNT, Mo2C@NC. (e) Chronopotentiometric durability test for the Co-Mo2C@NCNT at a constant current density of ~10 mA cm-2 Of special note, the HER activity of the Co-Mo2C@NCNT is still not as good as Pt/C, but its economic cost and high abundance composition highlight its promise as cost-effective electrocatalyst, which compares favorably most of the reported molybdenum carbide-based electrocatalysts and Co-based electrocatalysts in 1.0 M KOH solutions for HER catalysis (Table S1, Supporting Information). The Tafel plots (Figure 6b) gives the slope value of 79 mV dec-1 for Co-Mo2C@NCNT, according to the Tafel equation: η = b log j + a

(2)

where η is overpotential, j is the measured current density, and b is the Tafel slope. It is very close to that of Pt/C (75 mV dec-1) but is significantly smaller than that of the Mo2C@NC (129 mV dec-1) and Co@NCNT (134 mV dec-1), suggesting outstanding intrinsic HER kinetics of CoMo2C@NCNT. Figure 6c shows the Nyquist plots of the Co@NCNT, Mo2C@NC and CoMo2C@NCNT, which clearly indicates the most efficient charge transport of the CoMo2C@NCNT for electrocatalytic HER. The electrochemical surface area (ECSA) of the CoMo2C@NCNT was estimated to further shed light on the intrinsic HER activity, which mainly determined by measuring the electrochemical double layer capacitances (Cdl). The Co-


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Mo2C@NCNT yields the remarkably largest Cdl values (Figure 6d and Figure S10, Supporting Information), implying it bears a largest ECSA for HER. Also, the chronopotentiometric curve (Figure 6e) illustrates that the Co-Mo2C@NCNT maintains a quite stable overpotential by a long period of 45 h tests to yield a current density of 10 mA cm-2, indicating the high durability for the electrocatalytic HER. Impressively, we found that the Co-Mo2C@NCNT also behaves as an excellent OER electrocatalyst. As clearly given in Figure 7a, the as-prepared Co-Mo2C@NCNT still exhibits much better OER performance than that of Co@NCNT and Mo2C@NC in 1.0 M KOH solutions, which can deliver an earlier OER onset overpotential and smaller overpotential. Strikingly, the HER-active Mo2C@NC is inactive in OER catalysis, while the Co@NCNT presents the electrocatalytic OER activity as well. Of special interest, the Co@NCNT meeting Mo2C to form Co-Mo2C@NCNT could induce a favourable synergy to boost a great enhancement of intrinsic OER kinetics (Figure 7b). To confirm the synergic effect between Co@NCNT and Mo2C, we performed the CV tests on the Co@NCNT and Co-Mo2C@NCNT (Figure S11, Supporting Information). In general, the Co-based OER electrocatalysts usually experienced gradual oxidization and then served as actual active sites for electrocatalysis. As for Co@NCNT, the surface oxidization occurred at 1.06 V vs RHE. Interestingly, the corresponding oxidation peak of Co-Mo2C@NCNT tended to positively shift and present the enhanced peak area, confirming that the synergic effect between Co@NCNT and Mo2C contributes the enhanced OER performance. We also employ a potential to obtain current density of 10 mA cm-2 as criterion to evaluate the OER performance. The Co-Mo2C@NCNT requires an overpotential of ~377 mV to achieve current density of 10 mA cm-2, which is significantly lower than most of the Co-based electrocatalyts (Table S2, Supporting Information). Stimulated by the excellent HER and OER


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activity, we further fabricate a two-electrode cell based on the Co-Mo2C@NCNT by casting it onto a Ti plate. This electrode can be directly used as both anode and cathode to realize the actual electrolysis. As shown in Figure 7c, the Co-Mo2C@NCNT/Ti affords a current density of 10 mA cm2 at a cell voltage of 1.628 V, which compares favorably with those of recently reported bifunctional eletrocatalysts (Table S3, Supporting Information), including Ni-Mo nitride nanotubes,51 nickel-iron nitride nanosheets,52 CoxB,53 hollow CoP nanopaticle/N-doped graphene hybrids,54 N-Doped graphene-supported Co@CoO,55 MoSe2 nanosheets,56 and Ni-Co phosphides.57 Meanwhile, the Co-Mo2C@NCNT/Ti electrodes can maintain continuous electrolysis (Figure 7d). The cell voltage of electrolysis is only increased 6% of initial value in 10 h.


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Figure 7. (a) Polarization curves of the Co-Mo2C@NCNT, Co@NCNT, and Mo2C@NC at a scan rate of 5 mV s-1 in 1.0 M KOH solution for electrocatalytic OER. (b) Nyquist plots of the Co-Mo2C@NCNT, Co@NCNT, and Mo2C@NC for electrocatalytic OER at a potential of 1.683 V. (c) Polarization curves for overall water splitting of CoMo2C@NCNT/Ti||Co-Mo2C@NCNT /Ti in a two electrode configuration at a scan rate of 5 mV s-1 in 1.0 M KOH. (d) Chronopotentiometric curve of water electrolysis for CoMo2C@NCNT/Ti||Co-Mo2C@NCNT/ Ti with constant current density of 10 mA cm-2 in 1.0 M KOH. CONCLUSIONS In summary, we developed an effective synthetic strategy for the construction of a bamboolike architecture based on nitrogen-doped carbon nanotube encapsulating metallic cobalt and Mo2C nanoparticles by using Co@NCNT as both the template and the precursor. The resulting Co-Mo2C@NCNT as a bifunctional electrocatalyst exhibits the excellent activity and stability towards both HER and OER in alkaline medium, making it one of the best-performing nonnoble-metal electrocatalysts. The present finding would open a new avenue to design and develop electocatalytically active multicomponent architectures for full water splitting. ASSOCIATED CONTENT Supporting Information: XRD patterns, SEM images, TEM images, EDS spectra, polarization curves, CVs, and summaries of electrocatalytic activities of HER, OER and overall water splitting. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * Tel/Fax: +86-817-256808. E-mail: [email protected] (J. Jiang)


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51572227 and 21771147), Sichuan Youth Science and Technology Foundation (2013JQ0012), Major Cultivating Foundation of Education Department of Sichuan Province (17CZ0036), Meritocracy Research Funds of CWNU (17YC007 and 17YC017) and Innovative Research Team of CWNU (CXTD2017-1). REFERENCES (1)

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Table of Contents A bamboo-like architecture consisted of nitrogen-doped carbon nanotube co-encapsulating metallic cobalt and Mo2C nanoparticles was fabricated, which presents superior performances towards both the HER and OER in alkaline medium.


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Bamboo-Structured Nitrogen-Doped Carbon Nanotube

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