Electrochemical Hydrogen Storage in Facile Synthesized Co@N

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Electrochemical hydrogen storage in facile synthesized Co@N-doped Carbon Nanoparticle Composites Lina Zhou, Xiaosheng Qu, Dong Zheng, Haolin Tang, Dan Liu, Deyang Qu, Zhi-Zhong Xie, Junsheng Li, and Deyu Qu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14163 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Electrochemical Hydrogen Storage in Facile Synthesized Co@N-doped Carbon Nanoparticle Composites Lina Zhou a, Xiaosheng Qu *,b, Dong Zheng c,Haolin Tang,a, Dan Liu* a,c, Deyang Qu c

, ZhiZhong Xie a, Junsheng Li a, Deyu Qu*,a

a

State Key Laboratory of Advanced Technology for Material Synthesis and

Processing, Department of Chemistry, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan, 430070 Hubei, PR China b

National Engineering Laboratory of Southwest Endangered Medicinal Resources

Development, Guangxi Botanical Garden of Medicinal Plants, Nanning, 530023, PR China c

Department of Mechanical Engineering, College of Engineering and Applied

Science, University of Wisconsin Milwaukee, 3200N. Cramer Street, Milwaukee, WI 53211, USA

KEYWORDS: Co@N-doped Carbon; Cobalt nano-particles; Nitrogen functional groups; Electrochemical hydrogen storage; High hydrogen storage stability; High cyclic stability

1

ACS Paragon Plus Environment


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Abstract A Co@Nitrogen-doped carbon nanoparticle composite was synthesized via a facile molecular self-assembling procedure. The material was used as the host for the electrochemical storage of hydrogen. The hydrogen storage capacity of the material was over 300 mAh g-1 at a rate of 100 mAg-1. It also exhibited superior stability for the storage of hydrogen, high rate capability and good cyclic life. Hybridizing metallic cobalt nanoparticle with nitrogen doped meso-porous carbon is found to be a good approach for the electrochemical storage of hydrogen. Introduction H2 has already been recognized as a clean and renewable energy carrier for decades. It is still considered to be a promising candidate to replace exhausted fossil fuels and to reduce the green-house gas emissions. But, so far, there still remains problems which haunt the researchers and limit its practical application. One of the crucial problems is to find a method to effectively, safely, reversibly and economicly store the H2 at ambient temperature and pressure. Other than the conventional H2 storage methods which operate at low temperature and high pressure, electrochemical H2 storage has been regarded as a promising technique. H2 storage takes place during the process of the electrolysis of H2O while protons become inserted into a host material.1-8 Cobalt, especially nano-sized metallic Co particles have proved to be an excellent H2 storage material and are used in alkaline secondary batteries as the negative electrode.9-20 Meanwhile, carbonaceous materials with a meso-porous structure and heteroatom doping, also demonstrated the capability of up-taking H2 through the insertion of atomic H.4-8,21-25 Therefore, hybridizing the Co nanoparticles with heteroatom doped meso-porous carbon is believed to have synergetic effects which would enhance the electrochemical H2 insertion.9 In this study, a meso-porous carbon material with Co nanoparticles highly dispersed within (Co@NMC) was synthesized through a facile self-assembly route. As shown in figure 1, hydrated 2


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cobalt ions were self-assembled with HexaMethylenetetraMine (HMT) molecules through hydrogen bonds. After pyrolysis under a nitrogen atmosphere, the HMT became a nitrogen-doped carbon. In the meantime, Co nano-particles were also formed and encapsulated within the carbon matrix. The synthesized Co@NMC nanoparticle composites were characterized and their superior electrochemical hydrogen storage performance was demonstrated.

Figure 1. Schematic illustrated the synthesis of the Co@NMC nanoparticle composites. 2. Experiments 2.1. Chemicals and Co@NMC nanoparticle composites preparation Teflon suspension was obtained from Aladdin Reagent Co., Ltd. Other chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. To prepare the Co@NMC nanoparticle composites, 5.82 g of cobalt nitrate hexahydrate was dissolved in 5 mL of distilled water and mixed with 15 mL of an aqueous solution containing 11.20 g of HMT. The formed plate-like crystals were then collected through filtration and washed with acetone several times. After being dried in air, the obtained crystals were pyrolysized 3



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at 800 °C under an argon atmosphere for 3 h with a temperature ramping rate of 2 °C min−1. The Co@NMC nanoparticle composites were then obtained. An ICP measurement revealed that 56 wt% of metallic Co was loaded into the nitrogen doped carbon materials 2.2. Characterization Transmission electron microscope (TEM) images were taken with a JEM 2100F electron microscope operating at 200 kV. Scanning electron microscope (SEM) images were taken using a Hitachi S-4800 field emission scanning electron microscope. Powder X-ray diffraction (XRD) were performed on a Bruker D8 Advance diffractometer with a Cu Kα radiation (λ = 1.5406 Å) operating at 40 kV, 40 mA. X-ray photoelectron (XP) spectra were obtained using a VG Multilab 2000 X-ray photoelectron spectrometer with Mg KR radiation. Narrow scan spectra of the C 1s, N 1s, and Co 2p regions were obtained with 15 eV pass energy, 100 W electron-beam power, and a resolution of 0.1 eV. Binding energy was calibrated with a C 1s peak at 284.6 eV. A Micromeritics ASAP 2020 porosimeter was used for the surface area and porosity measurements. Nitrogen was used as absorbent gas. Density function theory (DFT) was also used to calculate the fine pores. A Perkin Elmer 3300DV Inductively Coupled Plasma Spectrometer (ICP) was used for ICP measurements. To make the electrode, Co@NMC powder (80 wt.%) was mixed with carbon black (15 wt.%) and Teflon suspension (5 wt.% of dry material). After being thoroughly mixed, the pastes were left in air to dry. The resulting dough was rolled into a thin film and then the electrode was punched out of film with a geometric surface area of 1 cm2. The disc electrode with an overall mass of 10 ± 0.5 mg was then sandwiched between two pieces of a nickel foam current collector. Aqueous KOH solution (30 wt.%) was used as the electrolyte in all measurements. An AutoLab electrochemical workstation (PGSTAT100N) was used for

electrochemical

measurements

as

well

as

impedance

measurements. 4


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Electrochemical impedance spectroscopy (EIS) was carried out at the frequency region of 0.01 Hz to 100 KHz with amplitude of 5 mV. An Hg/HgO reference electrode was used in all measurements. 3. Result and Discussion 3.1. Structure and morphology The morphology and structure of the synthesized Co@NMC nanoparticle composites were characterized by electron microscopic methods and XRD, respectively. Figure 2 showed the XRD pattern of the synthesized Co@NMC material. Three well-defined diffraction peaks, indicated by their Miller index, were indexed to the face-centered cubic (fcc) phase of cobalt (JPCDS No.15-0806). The broad diffraction peak centered around 26º, which is the typical XRD pattern for amorphous carbon, was also detected. This indicated the formation of an amorphous carbon around metallic cobalt.

Figure 3A shows the SEM image, where the

particle-like Co@NMC material with size around 700 nm can be observed. The EDS images, shown in Figure 3B to D, clearly illustrated that the metallic Co were homogenously distributed within the nitrogen doped carbon material.

Figure 2. XRD patterns of the Co@NMC nanoparticle composites. 5



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Figure 3. SEM(A) and EDS spectra of (B: Carbon; C: Nitrogen and D: Cobalt) the Co@NMC nanoparticle composites. TEM images of the synthesized nitrogen-doped carbon encapsulated Co nano-particles are illustrated in Figure 4. As shown in Figure 4A, the metallic Co nano-particles with sizes around 200 nm were encapsulated within the carbon matrix. The HR-TEM image of Co@NMC, shown in Figure 4B, clearly revealed the lattice spacing of 0.204 nm and 0.178 nm, corresponding to the spacing of the (111) and (200) plane of fcc-Co, respectively. The results were consistent with that from the XRD. The surrounding carbon appeared to be amorphous in nature.

Figure 4. (A) TEM image and (B) HRTEM TEM image of the Co@NMC nanoparticle composites.

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The

N2

absorption-desorption

isotherm

and

the

corresponding

Barret-Joyner-Halenda (BJH) pore size distribution curve of the Co@NMC nanoparticle composites were shown in Figure 5A and 5B, respectively. The Co@NMC sample exhibited a standard type-IV isotherm along with type-H2 hysteresis loop indicating meso-porous nature as well as cage-type pores with small entrances.26 The measured Brunauer-Emmett-Teller (BET) area of the Co@NMC nanoparticle composites was 259.9 m2 g-1 and the average pore diameters of the composites is 11 nm, calculated from the absorption branch of the nitrogen isotherm with the BJH method.

Figure 5. N2 adsorption/desorption isotherm(A) and pore size distribution (B) of the Co@NMC nanoparticle composites. The material was also investigated by X-ray photoelectron spectroscopy (XPS). Figure 6A shows the full-range XPS survey, with the existence of carbon, oxygen, nitrogen and cobalt being exhibited. Figure 6B−D show the XP spectra in the region of C 1s, Co 2p, and N 1s, respectively. The high resolution C 1s spectrum shown in figure 6B can be deconvoluted into three component peaks identified as C-C (284.5 eV) and C-N (285.5 eV) and C=O (288.8 eV) bonds.27 A N 1s peak with binding energy around 401 eV was observed and shown in figure 6D. The N 1s peak can be best fit with three sub-sets of peaks. The pyridine-like structure nitrogen, amino nitrogen and pyrrolic nitrogen moieties were observed at 398.0 eV, 398.9 and 400.8 eV, respectively.28-30 The binding energy of 780.4 eV and 795.9 eV shown in figure 7



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6C were assigned to metallic Co 2p3/2 and Co 2p1/2 peaks, respectively.27 This further proved the successful synthesis of the nitrogen-doped carbon encapsulated metallic Co materials. It should be noted that the relative amount of Co in the sample was calculated to be about 18 wt% based on the XPS peak area and the atom sensitivity factor.27 The value was much smaller than that from the ICP measurement. This may due to the fact that the XPS is a surface-sensitive technique and most of formed cobalt in this study were covered by the carbon materials.

Figure 6. XPS survey spectra (A) and high-resolution spectra in the region (B) C 1s, (C) Co 2p and (D) N 1s of the Co@NMC nanoparticle composites 3.2. Electrochemical hydrogen storage The electrochemical hydrogen storage of the synthesized Co@NMC nanoparticle composites were investigated. Figure 7A shows the typical charge-discharge profile at a rate of 100 mA g-1 (based on the mass of the whole electrode) in 30 wt.% KOH 8


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aqueous solution. During the charging process, four distinct regions were observed. At first, a double layer was quickly formed upon the Co@NMC electrode. The potential plateau located around -0.93 V can be attributed to the electrochemical hydrogen adsorption on the cobalt; (Co@NMC + xH2O + xe → Hx-Co@NMC +xOH-)

9-15

as the charging time increased the metallic cobalt nano-particles reached

their hydrogen storage capacity, which negatively shifted the potential again until it reach the second plateau. In this region, H2O was electrolyzed and atomic hydrogen became electrochemically adsorbed on the surface of the carbon followed by the insertion into the carbon interlayers. (Co@NMC + xH2O + xe →Co@NMC-Hx +xOH-)

8,21-24

The flat potential starting around -1.13 V is the region of H2 evolution.

The process in which proton inserted into the carbon interlayer competed with the process of proton recombination forming H2 gas through either the Tafel or Heyrovsky reactions.8,21,22 During the course of the discharge, one plateau appeared at the average potential of -0.8 V, this agrees well with previous studies and can be assigned to the electrochemical desorption process of hydrogen out of the [email protected]

Figure 7. (A) Charge-discharge profile of the Co@NMC electrode under applied current density of 100 mA g-1 and (B) Cyclic voltammogram of the Co@NMC electrode at scan rate of 10 mV s-1 in 30% KOH aqueous solution.

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Cyclic voltammetry was also applied to the study of the electrochemical hydrogen storage behavior of a Co@NMC electrode and the result is shown in Figure 7B. At a san rate of 10 mV s-1 in 30 wt.% KOH aqueous solution, one cathodic current peak and two anodic current peaks were observed at approximately -1.05 V, -0.72 V and -0.13 V, respectively. The shoulder peak at a potential of -1.05 V and the following sharp current increase in the cathodic direction can be comparable to the two charge potential plateaus of -0.93 V and -1.13 V, which were assigned to the hydrogen adsorption on Co and hydrogen evolution, respectively. While the anodic current peak with a potential of -0.72 V agreed well with the discharge potential plateau at -0.8 V. The small anodic peak at -0.13 V, which did not appear in discharge curve will be discussed later. To study the Co@NMC electrode’s capacity for the electrochemical storage of hydrogen, the charge-discharge curves of a Co@NMC electrode at a rate of 100 mA g-1 (based on the mass of the whole electrode) in 30 wt.% KOH solution (with various periods of charging time) were plotted in Figure S1 and summarized in Table 1. As shown in Table 1, as the charging time increased, the hydrogen storage capacities (discharge capacities) increased initially. The capacity was found to reach a maximum of 300 mAh g-1 (based on the mass of the whole electrode) after 5 hours charging and no capacity increase was noted with further charging. This indicated the electrochemical hydrogen storage capacity within the Co@NMC electrode was about 300 mAh g-1. The columbic efficiency varied for the different charging times. With a charging time of 2 hours, the columbic efficiency was almost 100% indicating all the electrochemically adsorbed protons were inserted into the bulk of the composite material. The columbic efficiency began to decrease with the increase of the charging time, while the discharge capacity still increased. The trend of the columbic efficiency decline became significant with a charging time over 3 hours (illustrated in Table 1). It seemed while the protons were still inserted into the electrode matrix the majority of the electrochemically adsorbed Hs became recombined to H2 gas and escaped into 10


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the atmosphere. As discussed previously, the hydrogen insertion into the carbon interlayer competes with the hydrogen electrochemical or chemical recombination processes, and the latter processes dominated in the late region of the charging process. Therefore, when the charging time is above 3 hours, the second flat potential appears as observed in Figure S1, parts of the charging energies were used for the hydrogen evolution and was responsible for the decline of the columbic efficiency. Table 1. Charging & Discharging capacities and corresponding columbic efficiency of synthesized Co@NMC material at different charging time under a current density of 100 mA g-1. Charging time (min)

Charging Capacity

Discharging Capacity (mAh/g)

Columbic efficiency

(mAh/g) 10

16.67

16.67

100%

20

33.33

33.33

100%

30

50

50

100%

60

100

99.5

100%

120

200

194

97%

180

300

285

95%

240

400

294

73.5%

300

500

301

60%

360

600

302

50.3%

The stability of the hydrogen stored in the Co@NMC electrode was investigated. After charging the Co@NMC electrode at 100 mA g-1 for 5 hours, the electrode was left to rest under open circuit potential (OCP) for different periods of time and then discharged at a rate of 100 mA g-1. The results were reported in figure 8 and tabulated in Table S1. As shown in Figure 8, after a quick positive potential shift, which may due to the self-discharge of the double layer capacity, the potential of Co@NMC 11



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electrode almost stayed constant during the whole resting process. The following discharging process clearly indicated that the leakage of the stored hydrogen (self-discharge) increased with the rise of the resting time. But the self-discharge rate was found to be quite low. As exhibited in Table S1, there are still about 261 mA g-1 capacity left, about 87% retention, even after 48 hours storage in the open circuit condition. This demonstrated the good stability of electrochemically stored hydrogen in Co@NMC material. It is worth to mentioning that, the nitrogen functional groups on the surface of carbon also play their roles in the improvement of the hydrogen storage and stability. As aforesaid, there are three kinds of nitrogen functional groups on the carbon surface, they were pyridine-like nitrogen structure, amino nitrogen and pyrrolic nitrogen moieties. Since they are all basicity nature, those surface-bonded nitrogen functional groups have the capability of stabilized the surface-adsorbed hydrogen. Therefore they may improve the hydrogen storage as well as the stability in the synthesized Co@NMC material.

Figure 8. Open circuit potential profiles and discharge curves of a Co@NMC electrode under an applied current density of 100 mA g-1 after charging the electrode with 100 mA g-1 for 5 hours in 30 wt.% KOH solution followed by resting at various of time at open circuit potential.

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To examine the kinetic properties of the electrochemical hydrogen storage in the Co@NMC electrode, the rate capability was measured at different discharging currents of 10 to 2000 mA g-1 after charging the electrode with 100 mA g-1 for 5 hours. As depicted in Figure 9A, the discharge capacities were found to be 541, 301, 227, 190 and 49 mAh g-1 (based on the mass of the whole electrode) under the current densities of 10, 100, 500, 1000 and 2000 mA g-1, respectively. The discharge capacities dropped with the rise of the applied currents, and only 17% of the total charged capacity was obtained as the current density increased from 100 mA g-1 to 2000 mA g-1. This indicated that the desorption of hydrogen from Co may be a kinetically slow process. It was also noticed that, as the discharge current decreased to 10 mA g-1, another potential plateau was observed around - 0.15 V. This flat potential region matched well with the small anodic peak appeared at -0.13 V in the CV which can be ascribed to, based on previous studies, desorption of hydrogen adsorbed on the surface of nanocomposite or a contribution from Ni foam.9-13,31-35 Since no such phenomena were detected in the high current density conditions, the kinetics of this process was believed to be an even slower one.

Figure 9. (A) discharge curves of the Co@NMC electrode under various levels of applied current after charging the electrode with 100 mA g-1 for 5 hours and (B) Cycling performance of the Co@NMC electrode operated between 0 to -1.13 V under an applied current density of 100 mA g-1 in 30 wt.% KOH solution 13



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The cycle life for the Co@NMC electrode was studied and the results are presented in Figure 9B. When the Co@NMC electrode operated between 0 to -1.13 V under an applied current density of 100 mA g-1 in 30 wt.% KOH solution, the discharge capacities slowly decreased from 300 mAh g-1 to around 170 mAh g-1 (based on the mass of the whole electrode) in the first 100 cycles and then a stable capacitance, over 160 mAh g-1, was then demonstrated for the subsequent cycle up to 200 cycles. It should be noted that the capacity decrease in the initial 100 cycles may be resulted from the gradual decreasing of the hydrogen adsorption sites on the surface of cobalt nanoparticles. As mentioned above, the kinetics of the H2 absorbed on the surface of nanocomposites varied, some could only be discharged under very low discharge current. (～10 mA g-1). The initial capacity fade could be resulted from the loss of high kinetic electro-absorbed. Figure 10 shows the impedance and fitting of the Co@NMC electrode. Two distinct sections could be identified in the Nyquist plot. Two depressed semicircles, appeared in the high-medium frequency region, representing electrochemical hydrogen formation from electrolysis of H2O and hydrogen adsorption on the Co surface. The straight line, presented in the low frequency region, corresponded to the proton insertion into the carbon material. The equivalent circuit shown as inset in figure 10 was used for numerical fitting. Clearly, the equivalent circuit fitted the spectra well. The electrochemical hydrogen formation and adsorption on the Co surface were characterized by the charge transfer resistance, Rct,H2O and Rct,Co, respectively.

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Figure 10 AC impedance spectrum and fitting results (solid line) of the Co@NMC electrode. Inset are the equivalent circuit used for the fitting of the ac impedance spectrum and the enlarged spectrum in high-medium frequency region. The hydrogen insertion into carbon matrix was represented by a finite-length Warburg element. This may due to the fact that the diffusion path for hydrogen in the high surface area porous carbon is quite short, hydrogen would penetrate the entire thickness during the low frequency modulation and create a finite length Warburg element, which can be expressed as:23 ⁄ 1 ⁄ tanh

(1)

Where δ represents the effective diffusion thickness and D is the effective diffusion coefficient of the particle. 1 ⁄ tanh

(2)

When

15



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"

2

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(3)

Then $ !%&'

()*+,"√.)*+ " √

√/0),"√./0) " √

$ 1&23&!4

()*+,"√ )*+ "√

√/0),"√./0) "√

(4)

(5)

Both σ and K can be obtained by means of least square fitting of the ac impedance results. As demonstrated in eq.4, the value of K−2 is proportional to the diffusion coefficient (D). Table 2. The value of Rsol, Rct,H2O, Rct,Co and K−2 obtained through the numerical fitting of the ac impedance spectrum. Rsol (Ω)

Rct,H2O (Ω)

Rct,Co (Ω)

K−2 (s−1)

0.70

0.50

101.5

1.80

The numerical results are shown in Table 2. The results clearly demonstrated that the value of Rct,Co, charge transfer resistance for hydrogen adsorption on the cobalt surface, is more than 2 orders of magnitude higher than that of the electrochemical hydrogen formation, Rct,H2O, indicating hydrogen adsorption on the cobalt surface is a relatively slow process. 4. Conclusion Nano-sized metallic cobalt was hybridized with a nitrogen-doped meso-porous carbon material through a simple self-assembly synthesis route. The synthesized Co@NMC nanoparticle composites were used as an electrochemical H2 storage medium. The electrochemical H2 storage in the Co@NMC nanoparticle composites were found to proceed through four steps: double-layer construction, hydrogen adsorption on Co, hydrogen insertion into carbon and hydrogen evolution. The 16


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electrochemical hydrogen storage capacity was found to be over 300 mAh g-1 at a rate of 100 mA g-1. The superior hydrogen storage stability of this synthesized material was also demonstrated as the retention of hydrogen was 87% after being stored for 48 hours under OCP at the ambient conditions. Good rate performance and a high cyclic stability were also exhibited.

ASSOCIATED CONTENT Supporting Information One figure showing the charge-discharge curves of the Co@NMC electrode at different charging time under current density of 100 mA g-1 and a table showing the discharge capacities of the Co@NMC material under current density of 100 mA g-1 after resting at open circuit potential for different period of time and their corresponded retention, while the Co@NMC material was pre-charged with 100 mA g-1 current in 30% KOH solution for 5 hours. AUTHOR INFORMATION Corresponding Author *X. Qu, E-mail: [email protected]; *D. Liu, E-mail: [email protected]; *D. Qu, E-mail: [email protected] ACKNOWLEDGMENT This work was partially supported by the National Nature Science Foundation of China (11474226, 21401145, 51676143), Fundamental Research Funds for the Central Universities (WUT: 2017-IB-003), The authors thank Dr. Xiao-Qing Liu for his assistance conducting TEM.

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(10) Dong, H. L.; Kang, M.; Paek, S. M.; Jung, H. Electrochemical Hydrogen Storage Performance of Hierarchical Co Metal Flower-like Microspheres. Electrochimica Acta 2016, 217, 132-138. (11) Cheng, C.; Gao, P.; Bao, D.; Wang, L.; Wang, Y.; Chen, Y. Ball-milling Preparation of One-dimensional Co-carbon Nanotube and Co-carbon Nanofiber Core/shell Nanocomposites with High Electrochemical Hydrogen Storage Ability. J. Power Sources 2014, 255, 318-324. (12) Gao, P.; Yang, Y.; Bao, D.; Chen, Y.; Wang, Y.; Yang, P. Flattening Sol–gel Nanospheres

into

a

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TABLE OF CONTENTS GRAPHIC AND SYNOPSIS A metallic cobalt nano-particles hybrid with nitrogen doped meso-porous carbon material was synthesized through a self-assemblied route. This material show over 300 mAh g-1 electrochemical hydrogen storage capacity under applied current of 100 mA g-1. The superior hydrogen storage stability within this synthesized material was also demonstrated as the retention of 87% after hydrogen storage of 48 hours under OCP, room temperature and atmosphere pressure condition.

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Figure 1. Schematic illustrated the synthesis of the Co@NMC nanoparticle composites. 78x45mm (300 x 300 DPI)



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Figure 2. XRD patterns of the Co@NMC nanoparticle composites. 80x63mm (600 x 600 DPI)


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Figure 3. SEM(A) and EDS spectra of (B: Carbon; C: Nitrogen and D: Cobalt) the Co@NMC nanoparticle composites. 66x54mm (300 x 300 DPI)



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Figure 4. (A) TEM image and (B) HRTEM TEM image of the Co@NMC nanoparticle composites. 363x120mm (150 x 150 DPI)


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Figure 5. N2 adsorption/desorption isotherm(A) and pore size distribution (B) of the Co@NMC nanoparticle composites. 57x20mm (300 x 300 DPI)



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Figure 6. XPS survey spectra (A) and high-resolution spectra in the region (B) C 1s, (C) Co 2p and (D) N 1s of the Co@NMC nanoparticle composites. 287x220mm (150 x 150 DPI)


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Figure 7. (A) Charge-discharge profile of the Co@NMC electrode under applied current density of 100 mA g1 and (B) Cyclic voltammogram of the Co@NMC electrode at scan rate of 10 mV s-1 in 30% KOH aqueous solution 152x60mm (300 x 300 DPI)



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Figure 8. Open circuit potential profiles and discharge curves of a Co@NMC electrode under an applied current density of 100 mA g-1 after charging the electrode with 100 mA g-1 for 5 hours in 30 wt.% KOH solution followed by resting at various of time at open circuit potential. 287x133mm (150 x 150 DPI)


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Figure 9.(a) discharge curves of the Co@NMC electrode under various of applied current after charging the electrode with 100 mA g-1 for 5 hours and (b) Cycling performance of the Co@NMC electrode operated between 0 to -1.13 V under applied current density of 100 mA g-1 in 30 wt.% KOH solution 287x107mm (150 x 150 DPI)



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Figure 10. AC impedance spectrum and fitting results (solid line) of the Co@NMC electrode. Inset are the equivalent circuit used for the fitting of the ac impedance spectrum and the enlarged spectrum in highmedium frequency region. 287x199mm (150 x 150 DPI)


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Electrochemical Hydrogen Storage in Facile Synthesized Co@N

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