A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9

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A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor Hye-Min Lee a,b, Young-Jung Heo b, Kay-Hyeok An c, Sang-Chul Jung d, Dong Chul Chung a, Soo-Jin Park b,**, Byung-Joo Kim a,* a

R&D Division, Korea Institute of Carbon Convergence Technology, Jeonju, 54853, Republic of Korea Department of Chemistry, Inha University, Incheon, 22212, Republic of Korea c Department of Nano & Advanced Materials Engineering, Jeonju University, Jeonju, 55069, Republic of Korea d Department of Environmental Engineering, Sunchon National University, Sunchon, 57922, Republic of Korea b

article info

abstract

Article history:

In this study, activated polymer-based hard carbons were prepared using various steam

Received 4 August 2017

activation conditions in order to enhance their hydrogen storage ability. The structural

Received in revised form

characteristics of the activated carbons were observed by X-ray diffraction and Raman

14 September 2017

spectroscopy. The N2 adsorption isotherm characteristics at 77 K were confirmed by

Accepted 16 September 2017

Brunauer-Emmett-Teller, Barrett-Joyner-Halenda and non-local density functional theory

Available online xxx

equations. The hydrogen storage behaviours of the activated carbons at 298 K and 10 MPa were studied using a Pressure-Composition-Temperature apparatus. From the results,

Keywords:

specific surface areas and total pore volume of the activated carbons were determined to be

Activated hard carbon

1680e2320 m2/g and 0.78e1.39 cm3/g, respectively. It was also observed that various pore

Hydrogen storage

size distributions were found to be dependent on the functions of activation time. In the

Pore structure

observed result, the hydrogen adsorption of APHS-9-4 increased about 30% more than that

Steam activation

of as-prepared hard carbon. This indicates that hydrogen storage capacity could be a function not only of specific surface area or total pore volume, but also of micropore volume fraction in the range of 0.63e0.78 nm of adsorbents. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The depletion of fossil fuels and growing environmental awareness has created the need for more eco-friendly fuels. Hydrogen is gaining increasing attention as a solution to energy and environmental problems, mainly due to its abundance, high chemical energy, and pollution-free burning.

Hydrogen combustion does not generate pollutants such as particles, nitrogen oxides (NOx), sulphur oxides (SOx), hydrocarbons, and carbon monoxide (CO). The energy density of hydrogen is three times greater than the energy density of gasoline, and it is possible to produce it from renewable energy sources [1]. However, the storage of hydrogen is still a major challenge.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S.-J. Park), [email protected] (B.-J. Kim). https://doi.org/10.1016/j.ijhydene.2017.09.085 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085

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Many hydrogen storage methods have been proposed, involving liquid hydrogen [2], high-pressure hydrogen [3], metal hydrides [4], Physisorption [5e9], chemisorption, and spill-over [10]. Several means of hydrogen storage have not been proven to be practical technologies that could compete with fossil fuels. Hydrogen liquefaction is very expensive and consumes great amounts of energy. Compressed hydrogen gas is cheaper, but still requires higher volumes and heavier containers to get a practical hydrogen density. There are also risks associated with the use of hydrogen at high pressure [11]. Recently, research on hydrogen adsorption storage has focused on finding the ideal adsorbent that, used at room temperature, would allow storage of useful amounts of H2 [12e14]. Physisorption on porous materials has been proposed to lower the storage pressure of compressed gas fuels such as natural gas and hydrogen [13,14]. Hydrogen adsorption using porous materials has advantages in fast adsorption and desorption kinetics, long life cycle, no need for energy to release hydrogen, and high hydrogen uptake than other methods [12e14]. Among the various adsorbents like mesoporous silica, activated alumina, zeolite, metal-organic frameworks (MOFs), and others, carbon materials [5e9] offer many advantages: abundance, heat resistance, non-toxicity, good chemical stability, good recycling characteristics, reversibility, low-cost and availability [13]. In addition, lightweight carbon with high porosity is a preferred choice for achieving a gravimetric hydrogen storage target. Among the materials that can be used as adsorbents, carbon materials such as activated carbons [5e8], carbon nanofibers [9], carbon nanotubes [15], graphene [16], and fullerene [17] have been the subject of research. In previous studies the hydrogen storage capacity of microporous carbon materials was analyzed theoretically and experimentally for a selection of samples [18]. The results allowed us to conclude that there exist an optimum pore size for hydrogen adsorption [8,9]. Among them, activated carbons have been intensively researched as hydrogen adsorption materials, since it is easy to obtain and their porous structure is more developed than other carbon materials. Activated carbon can be prepared from a wide range of raw materials including coal [19], pitch [20], biomass [5e7,21], and polymeric resins [22,23]. The polymer precursor has specificity with a uniform chemical structure and can be control the crystallinity of the hard carbon during the carbonization process. Polymer-based hard carbon with controlled crystallinity can produce activated carbon with an optimized pore structure for hydrogen storage [8,9,23]. Recent hydrogen storage studies show that the hydrogen adsorption capacity of activated carbon is less than 1.0 wt% at room temperature [5,6]. Activated carbons with very high specific surface area (over 3000 m2/g) have only hydrogen adsorption capacity of about 4.5e0.6 wt% at room temperature [6]. Metals used as catalysts for hydrogen storage may play a role in the hydrogen uptake [10,24]. M. Zielinski et al. [24] clearly show that metal catalysts supported on activated carbon could store significant amounts of hydrogen at room temperature and high pressure (up to 0.53% at 30 bars against 0.1% for the activated carbon). The specific surface area and hydrogen adsorption capacity of the activated carbon without

metal catalysts are only 1000 m2/g and 0.1 wt%, respectively. It is known that at room temperature and high pressure, the hydrogen spill-over species are most probably stored on the micropore of activated carbon. This is the reason why an optimized pore structure for hydrogen adsorption of activated carbon is required. In this work, activated polymer-based hard carbons (APH) with high surface area were prepared from various steam activated contents and the effects of specific surface area and micropore fraction on hydrogen uptake were investigated. The pore characteristics of activated carbon were observed through analysis of crystal structure.

Experiment Preparation of activated hard carbon Polymeric precursors (Aekyung Petrochemical Co., Ltd.) were used as the starting materials. The polymeric precursors were heated to 900  C at 10  C/min in a self-made cylindrical furnace (SiC heater: 100  1000 mm) under N2 gas (99.999%, 300 mL/min), and kept at a target temperature for 1 h to obtain carbonised polymer-based hard carbon (HC). Then, the gas flow was switched to H2O at a rate of 0.5 mL/min while the temperature was raised to 1000  C and held for 20 min (APHS9-2), 40 min (APHS-9-4) or 60 min (APHS-9-6). The samples were allowed to cool under N2 gas (300 mL/min). The yield of activated sample was determined from the weight of the sample before and after activation.

Characterizations The structural changes that occurred after the H2O activation of the hard carbon were investigated using X-ray diffraction (XRD) spectroscopy and Raman spectroscopy. XRD patterns were collected within the diffraction angles from 5 to 60 with a speed of 2 /min with a customized automount and a Cu Ka radiation source. Raman spectra were measured at constant room temperature using 523 nm wavelength. The laser was focused through a microscope with a 100 objective. Raman spectra presented in this study corresponds to accumulation of 3 spectra's recorded from 0 to 3500 cm 1. For all the XRD and Raman spectra, a linear baseline correction was conducted, and the peak analysis was performed with the OriginLab Pro 8.5 (USA). The N2 adsorption isotherms at 77 K were measured using BELSORP-max (BEL Japan). For pore analysis, each sample was degassed for approximately 5e6 h at 573 K, with the residual pressure maintained at 10 3 torr, or less. The specific surface area was calculated in the relative pressure interval of 0.04e0.17 using the Brunauer-Emmett-Teller (BET) method [25]. Pore size and distribution were calculated by the BarrettJoyner-Halenda (BJH) method [26] and non-local density functional theory (NLDFT) integrated cylinder-shaped pore model from isotherm curves [27]. Micropore volume were calculated using the Dubinin-Radushkevitch (DR) equations to the N2 adsorption isotherm at P/P0 < 0.1 [28]. High-pressure hydrogen adsorption measurements at 298 K and 10 MPa were carried out in an automated high-

Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085

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pressure gas adsorption system BELSORP-HP (BEL Japan). Activation (degassing) of the samples was performed at 573 K under vacuum for at least 12 h. After the sample was cooled to room temperature, the samples were purged with flowing He gas (99.9999%) and stored under a He atmosphere. H2 gas (99.9999%) was supplied to the chamber containing the sample until a pressure of 10 MPa was achieved. The hydrogen storage capacity of sample was determined by a volumetric measurement method.

Results and discussions Normally, activated carbon is manufactured from sources such as polymer, coal, or biomass. The structure differs depending on the type of precursor, and the structure has the greatest influence on the pore characteristics of activated carbon [22]. This is why structure analysis is important for understanding the study results. X-ray diffraction is usually used to analyse the crystal structure of carbon materials. In Fig. 1, the X-ray diffraction profiles of the activated carbons prepared under different activation conditions. These

C(002)

C(10l)

HC APHS-9-2 APHS-9-4 APHS-9-6

10

20

30

40

50

60

2 theta (degree) Fig. 1 e X-ray diffraction patterns of activated carbon with various H2O activation conditions.

50

(a)

activated carbons exhibited very broad diffraction peaks. The absence of a sharp peak reveals a predominantly hard carbon structure. Hard carbon is usually amorphous carbon composed of random distributed fragments of microcrystalline carbon. It is widely known that activation is a process that develops pores by oxidizing carbon atoms of the precursors. Moreover, the oxidation of carbonaceous materials has a higher probability of occurring in the amorphous region and graphite edges [22]. The oxidation of graphite crystals increased with increase in activation time. Therefore, on the XRD curves, a decrease was observed in the C(002) peak and C(10l) peak as the activation time increased. The peaks of APHS-9-4 and APHS-9-6 were negligible. Random layered (graphene) structural parameters of the activated carbon such as lateral size (La) and stacking height (Lc) were determined using x-ray diffraction (XRD). In the graphite crystalline structure, the layers of the (002) plane consist of strongly hybridized sp2 bonds, and the vertical pbond on the (002) plane results in weak interlayer bonds. Therefore, the change of La was observed to be larger than that of Lc [22,23]. XRD shows the average structure of the material. Since HC is activated at the same temperature as the carbonization temperature, graphite crystals cannot grow during the activation process. Therefore, the increase of La and Lc cannot be seen by crystal growth and can be judged by the relative increase due to the oxidation of amorphous and small graphite crystals. It is well known that amorphous is oxidized more preferentially than crystalline in the activation process [22,23]. Therefore, at the beginning of activation, amorphous was preferentially oxidized, so Lc and La were increased in APHS-2 (Fig. 2). The Lc increased with increasing activation time. However, La increased with activation time only up to 20 min, and there was little change thereafter. It was found that the structural parameters like Lc increase, whereas the interlayer spacing d002 decrease, with increase in activation time. From the activation time after 20 min, it is judged that La is continuously increased because small crystals are oxidized. Through XRD, it was confirmed that small crystals were continuously oxidized until activation time reached 60 min.

3.8

Lc La

Spacing (angstrom)

La, Lc (Angstrom)

40

30

10.5

10.0

9.5

(b)

d002 d10l

3.7

3.6 2.08

2.06

2.04 0

20

40

Activation time (min)

60

0

20

40

60

Activation time (min)

Fig. 2 e Structural characteristics of the activated hard carbons as a function of different activation conditions: (a) structural parameters and (b) interplanar distance.

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Raman spectrometry is one of the most useful methods for characterizing carbon materials. Raman analysis provides information about atomic structure. All APHSs were further examined using Raman spectrometry to get more detailed information about the changes in atomic structure of the activated hard carbon, of which the results are shown in Fig. 3. It is known that the D-band at 1350 cm 1 is associated with disordered graphite, while the G-band at 1582 cm 1 is associated with ordered graphite. Shimodaira et al. report [29] a novel characterization method for the Raman spectra of activated carbon materials using a curve fitting technique, and proposed an interpretation of the microstructure. The crystal structure of activated carbon can be divided into graphite crystal and amorphous structure, and it can be expressed by each G-band and D-band. The structures of graphite crystal are denoted by D1 and G1, and the structures of amorphous or small graphite structure are denoted by D2 and G2. It is noteworthy that this fit, using four Gaussians, faithfully reproduces the experimental data, and that two relatively sharp peaks (at about 1600 cm 1 and about 1350 cm 1) and two relatively broad peaks (around 1560 cm 1 and around 1340 cm 1), namely G1, D1, G2 and D2, are observed in all the samples. The examples of Raman spectrometry with fitting results of the activated carbon samples are represented in Fig. 3 (b). Fig. 4 illustrates the plot of the Raman shift and the full width at half maximum (FWHM) in two dimensions of the activated carbons prepared under different activation conditions. All the positions of the G1 peaks were almost constant. On the other hand, G2, D1 and D2 points were dispersed. G2, D1 and D2 showed a decrease in FWHM with increasing activation time. These results are due to the oxidation of amorphous and small graphite crystal as the activation time increased. The peak ratios in all the activated carbon samples are plotted against activation time in Fig. 5. It is well known that the ratio of the intensity of the D to G-band (ID/IG) is correlated with the reciprocal of the crystallite size along the basal plane (1/La) measured from XRD. An increase in ID/IG would indicate that amorphous carbon and small crystals were removed during the activation process. In contrast, 1/La increases due to increase in the average crystal size. The ratio of IG2/IG1 decreases with increasing activation time. It can be confirmed that the amorphous G band is also oxidized. The Raman results are in good agreement with the XRD results discussed

(a)

500 HC APHS-9-2 APHS-9-4 APHS-9-6

-1

FWHM (cm )

400

D2

300 G2

200

D1

100

G1

0 1650

1600

1550

1500

1450

1400

1350

1300

-1

Raman Shift (cm ) Fig. 4 e 2D-Plots of de-convoluted peak parameters: peak position vs. FWHM.

IG2/IG1

4

ID/IG

Peak ratio

3

2

1

0 0

20

40

60

Activation time (min) Fig. 5 e Peak ratios as a function of various H2O activation conditions.

above. This suggests that the variety of the Raman spectral shape in the activated carbon materials is mainly dominated by the disordered structure. Activation by steam has the effect of removing disordered structure.

(b)

HC APHS-9-2 APHS-9-4 APHS-9-6

Intensity

Intensity

D1

2000

1800

1600

1400

1200

1

Raman shift (cm- )

1000

800

2000

G1 D2 G2

1800

1600

1400

1200

1000

800

1

Raman shift (cm- )

Fig. 3 e Raman spectra of activated hard carbons (a) under various conditions of H2O activation with fitting results (b).

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The specific surface area of the HC (10 m2/g) increased (2320 m2/g) with H2O activation. The pores of all activated carbons were mostly micropores. Until the activation time of 20 min, mainly micropores were developed. For activation time from 20 to 40 min, the micropores and mesopores developed together. At activation time greater than 40 min, only mesopores developed, but the micropores persisted. Especially, hysteresis curves were greatly increased in APHS. This means that pores in the form of jars are formed due to the oxidation of small crystals in the sample. In Fig. 4, D1 of APHS4 and APHS-6 did not change, but FWHM of D2 and G2 decreased. Therefore, it is judged that only the small crystals are oxidized without changing the large crystal between the activation time of 40 min and 60 min. Therefore, it is considered that pores in the shape of a jar were formed because pores were formed only in the interior of the activated carbon without changing the large crystal structure. In conclusion, the stage of pore development is composed of three stages: 1) Only micropores develop due to oxidation of amorphous carbon, 2) Micropores collapse while the mesopores increase and 3) Only mesopores remain in the third stage, the oxidation of small crystals continues, and the micropores continue to develop. Because small crystals oxidation continues to increase the number of micropores, the micropore volume is maintained even when the mesopores develop. Fig. 7(a) shows that most of the pores are <10 nm. Also, it can be seen that, as the activation time increases, the pore volume also increases. Fig. 7(b) shows that the pore diameter increases with increasing activation time. APHS-2 was greatly developed in pores around 1 nm by steam activation. As the activation time increased, APHS-4 exhibited two peak-like pore distribution curves. This is considered to be the process of developing into mesopores by collapsing micropores. The pore distribution of APHS-6 greatly decreased the volume of micropores and increased the volume of mesopores. It seems that the mesopore volume increased greatly due to the breakdown of the micropores by the long activation time. It can also be seen that mesopore are still produced, because the small graphite crystals continues to be oxidized. The DFT

3

Volume adsorbed (cm /g, STP)

1200 HC APHS-9-2 APHS-9-4 APHS-9-6

1000 800 600 400 200 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0) Fig. 6 e N2/77 K isotherm curves of carbon activated under various conditions of H2O activation.

Fig. 6 shows typical N2 adsorption/desorption isotherms for all the activated hard carbons prepared in this work. The curve of APHS-9-2 is classified as type I by the IUPAC classification [30]. Meanwhile, APHS-9-4 and APHS-9-6 exhibited type IV isotherms with hysteresis due to capillary condensation of N2 in the mesopores. Textural properties of the ACs with activation conditions were listed in Table 1.

Table 1 e Textural properties of activated carbons as a function of various H2O activation conditions. Sample

SBET (m2/g)

VTotal (cm3/g)

VMirco (cm3/g)

VMeso (cm3/g)

Yield (%)

HC APHS-9-2 APHS-9-4 APHS-9-6

10 1680 2160 2320

0.02 0.78 1.10 1.39

0.02 0.59 0.69 0.69

0 0.19 0.41 0.70

e 50 25 13

2.0

0.20

(a)

HC APHS-9-2 APHS-9-4 APHS-9-6

HC APHS-9-2 APHS-9-4 APHS-9-6

0.15

3

dV/dlog(D) (cm /g)

3

dV/dlog(D) (cm /g)

1.6

(b)

1.2

0.8

0.4

0.0

0.10

0.05

0.00 1

10

Pore diameter (nm)

100

0.1

1

10

100

Pore diameter (nm)

Fig. 7 e Pore size distribution of activated carbon as a function of various H2O activation conditions by BJH equation (a) and NDFT equation (b).

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at 298 K and 10 MPa. All of the APHS samples showed higher hydrogen uptake than did the HC sample in Fig. 8. Hydrogen uptake seems to have increased with activation time up to 40 min, only to decrease afterwards. In order to understand the role of porosity development in improving the room-temperature hydrogen storage performances of the APHs, the hydrogen adsorption capacities at 298 K and 10 MPa were investigated with respect to several textural properties. Fig. 9, the adsorption capacities of the APHs appear to be linearly dependent on micropore volume. But the adsorption capacities were found to be not linearly associated with the total pore volume and the mesopore volume. The data obtained indicated that the occurrence of mesopore has hardly any effect on hydrogen uptake at 298 K and 10 MPa. This confirms that the hydrogen storage capacity is not essentially based on the specific surface area and pore volume, as reported in previous work [11]. This suggests that pore size can play an important role in determining the hydrogen storage behaviour of the activated carbon. Fig. 10 shows a roughly linear correlation between hydrogen uptake and microcavity. When the hydrogen adsorption capacity was plotted against the micropore volume of various sizes, the highest value was observed in the micropore between 0.63 and 0.78 nm. This result is consistent

HC-Experimental HC-Langmuir fitted line APHS-9-2-Experimental APHS-9-2-Langmuir fitted line APHS-9-4-Experimental APHS-9-4-Langmuir fitted line APHS-9-6-Experimental APHS-9-6-Langmuir fitted line

0.4

0.3

0.2

0.1

0.0 0

2

4

6

8

10

Absolute pressure (MPa) Fig. 8 e 298 K/H2 adsorption isotherms at 10 MPa.

curve results are in good agreement with the XRD and Raman results discussed above. Fig. 8 shows the adsorption capacity of hydrogen of the samples at 298 K and pressures from atmosphere pressure to 10 MPa with various activation conditions. As shown in Fig. 8, the amount of hydrogen stored in the APHS samples increased

H2 uptake at 298K and 10 MPa (wt.%)

(a)

(b) 0.5

2

R =0.82 0.4

0.3

0.2

0.1

0.0 0

500

1000

1500

2000

2500

3000

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake (wt.%, 298K)

0.5

0.5

2

R =0.66

0.4

0.3

0.2

0.1

0.0 0.0

0.5

2

Specific surface area (m /g)

(d)

0.5

2

R =0.85

0.4

0.3

0.2

0.1

0.0 0.0

0.2

0.4

0.6 3

Micropore volume (cm /g)

1.5 3

0.8

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake at 298K and 10 MPa (wt.%)

(c)

1.0

Total pore volume (cm /g) 0.5

2

R =0.25

0.4

0.3

0.2

0.1

0.0 0.0

0.2

0.4

0.6

0.8

3

Mesopore volume (cm /g)

Fig. 9 e Correlations between the H2 storage capacities at 298 K and 10 MPa with specific surface area (a), total pore volume (b), micropore volume (c), and mesopore volume (d).

Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085

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(b)

0.42 0.41 0.40 0.39 0.38 0.37 0.36 0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake at 298K and 10 MPa (wt.%)

(a)

0.42 0.41 0.40 0.39 0.38 0.37 0.36 0.000

3

(d)

0.42

2

R =0.90

0.41 0.40 0.39 0.38 0.37

0.36 0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.40 0.39 0.38 0.37 0.36 0.00

0.40 0.39 0.38 0.37 0.36 0.4

0.5

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake at 298K and 10 MPa (wt.%)

0.41

0.3

0.40 0.39 0.38 0.37

0.75 3

Micropore(0~2.01 nm) volume (cm /g)

0.80

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake at 298K and 10 MPa (wt.%)

0.41

0.70

0.30

0.40 0.39 0.38 0.37 0.36 0.00

0.05

0.10

0.15

0.20

0.25

0.30

Micropore(0~1.05 nm) volume (cm /g)

(h)

0.65

0.25

3

0.42

0.60

0.20

0.41

Micropore(1.05~1.51 nm) volume (cm /g)

0.36 0.55

0.15

0.42

3

(g)

0.10

Micropore(0.78~1.05 nm) volume (cm /g)

(f)

0.2

0.05

3

0.42

0.1

0.020

0.41

Micropore(0.63~0.78 nm) volume (cm /g)

0.0

0.015

0.42

3

(e)

0.010

Micropore(0.41~0.63 nm) volume (cm /g)

H2 uptake at 298K and 10 MPa (wt.%)

H2 uptake at 298K and 10 MPa (wt.%)

(c)

0.005

3

Micropore(0~0.41 nm) volume (cm /g)

0.42 0.41 0.40 0.39 0.38 0.37 0.36 0.3

0.4

0.5

0.6

0.7 3

Micropore(1.05~2.01 nm) volume (cm /g)

Fig. 10 e Correlations between the H2 storage capacities at 298 K and 10 MPa with micropore (0e0.41 nm) volume (a), micropore (0.41e0.63 nm) volume (b), micropore (0.63e0.78 nm) volume (c), micropore (0.78e1.05 nm) volume (d), micropore (1.05e1.51 nm) volume (e), micropore (0e1.05 nm) volume (f), micropore (0e2.01 nm) volume (g), and micropore (1.05e2.01 nm) volume (h). Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085

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with previous studies [8,9] that smaller pores (0.6e0.9 nm) are important for hydrogen adsorption at room temperature. It was confirmed that the pore between 0.63 and 0.78 nm affects the adsorption capacity of hydrogen at high pressure.

Conclusions In this study, the development of pores in relation to the structural changes were examined in activated polymer-based hard carbon (APH) prepared by H2O activation. The APHs were prepared using different activation times in order to produce high specific surface area. The specific surface areas and total pore volume of the activated carbons surged from 1680 to 2320 m2/g and 0.78e1.39 cm3/g, respectively. The increase in La with increasing activation time, were caused by the development of pores resulting from the oxidation of amorphous carbon and small crystals. This suggests that the variety of the Raman spectroscopy in the activated carbon materials is mainly dominated by the disordered structure. Activation by steam has the effect of removing disordered structure. Increased activation time induced higher specific surface area and micropore volume, which resulted in higher hydrogen uptake at high pressure. Hydrogen adsorption by APHS-9-4 was about 30% greater than that by HC. A close relationship between hydrogen storage capacities and micropore volumes has been found. From the results, it is also inferred that the micropore volume fraction in the range of 0.63e0.78 nm primarily controls hydrogen adsorption at 298 K and 10 MPa.

Acknowledgements This research was supported by a grant (20161007-C2-003) from Jeonbuk Research & Development Program funded by Jeonbuk Province. This research was supported by the Activated Carbon Total Solution Project, funded by the Jeonju city, Republic of Korea.

references

[1] Yang SJ, Jung H, Kim T, Park CR. Recent advances in hydrogen storage technologies based on nanoporous carbon materials. Prog Nat Sci Mater Int 2012;22:631e8. [2] Sakamoto J, Nakayama J, Nakarai T, Kasai N, Shibutani T, Miyake A. Effect of gasoline pool fire on liquid hydrogen storage tank in hybrid hydrogen-gasoline fueling station. Int J Hydrogen Energy 2016;41:2096e104. [3] Ozsaban M, Midilli A. A parametric study on exergetic sustainability aspects of high-pressure hydrogen gas compression. Int J Hydrogen Energy 2016;41:5321e34. [4] Mellouli S, Abhilash E, Askri F, Ben Nasrallah S. Integration of thermal energy storage unit in a metal hydride hydrogen storage tank. Appl Therm Eng 2016;102:1185e96. [5] Heo YJ, Park SJ. Synthesis of activated carbon derived from rice husks for improving hydrogen storage capacity. J Ind Eng Chem 2015;31:330e4. [6] Liu X, Zhang C, Geng Z, Cai M. High-pressure hydrogen storage and optimizing fabrication of corncob-derived activated carbon. Microporous Mesoporous Mater 2014;194:60e5.

[7] Choi YK, Park SJ. Hydrogen storage capacity of highly porous carbons synthesized from biomass-derived aerogels. Carbon Lett 2015;16:127e31. [8] Yang SJ, Im JH, Nishihara H, Jung H, Lee K, Kyotani T, et al. General relationship between hydrogen adsorption capacities at 77 and 298 K and pore characteristics of the porous adsorbents. J Phys Chem 2012;116:10529e40. [9] Jimenez V, Ramirez-Lucas A, Sanchez P, Valverde JL, Romero A. Hydrogen storage in different carbon materials: influence of the porosity development by chemical activation. Appl Surf Sci 2012;258:2498e509. [10] Schaefer S, Fierro V, Szczurek A, Izquierdo MT, Celzard A. Physisorption, chemisorption and spill-over contributions to hydrogen storage. Int J Hydrogen Energy 2016;41:17442e52. [11] Jena P. Materials for hydrogen storage: past, present, and future. J Phys Chem Lett 2011;2:206e11.  n YJ, Fairen-Jimenez D, Wilmer CE, Snurr RQ. High[12] Colo throughput screening of porous crystalline materials for hydrogen storage capacity near room temperature. J Phys Chem C 2014;118:5383e9. [13] Marco-Lozar JP, Kunowsky M, Carruthers JD, Linares Gas storage scale-up at room temperature on high Solano A. density carbon materials. Carbon 2014;76:123e32. [14] Xia K, Hu J, Jiang J. Enhanced room-temperature hydrogen storage in super-activated carbons: the role of porosity development by activation. Appl Surf Sci 2014;315:261e7. [15] Barghi SH, Tsotsis TT, Sahimi M. Chemisorption, Physisorption and hysteresis during hydrogen storage in carbon nanotubes. Int J Hydrogen Energy 2014;39:1390e7. [16] Klechikov AG, Mercier G, Merino P, Blanco S, Merino C, Talyzin AV. Hydrogen storage in bulk graphene-related materials. Micropor Mesopor Mat 2015;210:46e51. [17] Ren HJ, Cui CX, Li XJ, Liu YJ. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerenes. Int J Hydrogen Energy 2017;42:312e21. [18] Yurum Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [19] Lee HM, Kim HG, An KH, Kim BJ. The effect of CO2 activation on the electrochemical performance of coke-based activated carbons for supercapacitors. J Nanosci Nanotechnol 2015;15:8797e802. [20] Lee HM, Kwac LK, An KH, Kim BJ. Electrochemical behavior of pitch-based activated carbon fibers for electrochemical capacitors. Energ Convers Manage 2016;125:347e52. [21] Kim D, Cheon J, Kim J, Hwang D, Hong I, Kwon OH, et al. Extraction and characterization of lignin from black liquor and preparation of biomass-based activated carbon therefrom. Carbon Lett 2017;22:81e8. [22] Baek J, Lee HM, Roh JS, Lee HS, Kang HS, Kim BJ. Studies on preparation and applications of polymeric precursor-based activated hard carbons: I. Activation mechanism and microstructure analyses. Micropor Mesopor Mat 2016;219:258e64. [23] Baek J, Shin HS, Chung DC, Kim BJ. Studies on the correlation between nanostructure and pore development of polymeric precursor-based activated hard carbons: II. Transmission electron microscopy and Raman spectroscopy studies. J Ind Eng Chem 2017;54:324e31.  ski M, Wojcieszak R, Monteverdi S, Mercy M, [24] Zielin Bettahar MM. Hydrogen storage in nickel catalysts supported on activated carbon. Int J Hydrogen Energy 2007;32:1024e32. [25] Brunauer S, Emmett PH, Teller E. Adsorption of gases in multimolecular layers. J Am Chem Soc 1938;60:309e19. [26] Barrett EP, Joyner LG, Halenda PP. The determination at pore volume and area distributions in porous substances. I.

Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9

computation from nitrogen isotherms. J Am Chem Soc 1951;73:373e80. [27] Kierlik E, Rosinberg ML. Free-energy density functional for the inhomogeneous hard-sphere fluid: application to interfacial adsorption. Phys Rev 1990;42:3382e7. [28] Dubinin MM. Generalization of the theory of volume filling of micropores to nonhomogeneous microporous structures. Carbon 1985;23:373e80.

9

[29] The American Carbon Society. Structural information from the Raman spectra of activated carbon materials. 2001. http://acs.omnibooksonline.com/data/papers/2001_P2.12. pdf. [30] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, et al. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57:603e19.

Please cite this article in press as: Lee H-M, et al., A study on optimal pore range for high pressure hydrogen storage behaviors by porous hard carbon materials prepared from a polymeric precursor, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.085