- Email: [email protected]
One-pot synthesis of Pd nanoparticles on ultrahigh surface area 3D porous carbon as hydrogen storage materials
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
One-pot synthesis of Pd nanoparticles on ultrahigh surface area 3D porous carbon as hydrogen storage materials Jinliang Zhu a,1, Juhong Cheng a,1, Anne Dailly b, Mei Cai b, Matthew Beckner c, Pei Kang Shen a,* a
The State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou, 510275, PR China b General Motors Research and Development Center, Warren, MI, 48090-9055, USA c Optimal CAE Inc., Plymouth, MI, 48107, USA
article info
abstract
Article history:
Pd on three dimensional (3D) porous carbon (Pd/PC) has been one-pot synthesized via in
Received 12 February 2014
situ potassium hydroxide activation of PdCl2 6 -exchanged resin for the first time. The
Received in revised form
ions to obtain uniformly dispersed Pd anion-exchange resin not only anchors PdCl2 6
27 June 2014
nanoparticles but also acts as a carbon source. The Pd/PC composites exhibit an ultrahigh
Accepted 1 July 2014
surface areas (2734e3316 m2 g1) and large pore volumes (1.789e1.991 cm3 g1). The excess
Available online 2 August 2014
hydrogen uptake of Pd/PC-850 reaches 4.6 wt% at 77 K and 20 bar. At room temperature, the hydrogen spillover process from Pd nanoparticles to the 3D activated porous carbon has
Keywords:
been clearly observed. The 1.86 wt% Pd-containing composite (Pd/PC-850) displays a high
Pd nanoparticles
hydrogen spillover enhancement (154%) at 298 K.
One-pot synthesis
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
3D porous carbon
reserved.
Ultrahigh surface area Hydrogen storage
Introduction In recent years, hydrogen produced from many resources has attracted much attention as an ideal pollution-free energy medium for replacing fossil fuels. Efficient hydrogen storage and transportation is crucial for a future hydrogen economy. Until now, approaches to hydrogen storage have been explored using various systems such as compression, liquefaction, solid state storage into materials (absorption in
metallic hydrides and adsorption onto high surface area sorbents) [1]. In particularly, hydrogen physisorption has been considered as the most promising hydrogen storage method [2e4]. Carbon materials such as carbon nanotubes (CNTs) [5], activated porous carbons [6,7], template-produced carbons [8], and carbon nanofibers [9] were widely studied as hydrogen storage media. Among the numerous possible carbon materials, activated porous carbons have received significant attention as potential hydrogen storage media due to their low density, high surface area, large pore volume, good chemical
* Corresponding author. Tel.: þ86 20 84036736; fax: þ86 20 84113369. E-mail address: [email protected] (P.K. Shen). 1 The authors contributed equally. http://dx.doi.org/10.1016/j.ijhydene.2014.07.002 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
14844
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
stability, and high storage capacity. High-temperature treatment of a mixture of raw carbonaceous material and KOH is regarded as an effective method to synthesize highly porous carbons [10,11]. The following prominent reaction occurs during activation of the carbonaceous material by KOH [12]. 6KOH þ 2C 4 2K þ 3H2 þ 2K2CO3 Porous carbons produced using KOH as an activating agent present attractive properties of high surface area and a large porosity made up of super-micropores and small mesopores. Significant hydrogen adsorption uptake on activated carbons can be achieved at low temperature (77 K) and moderated pressures; however, the capacity will be significantly reduced at ambient temperatures [13,14]. Experimental results and molecular orbital calculations showed that the hydrogen spillover mechanism for hydrogen storage at ambient temperature is effective at increasing the hydrogen adsorption capacity [14,15]. Parambhath et al. reported that Pd/f-HEG (functionalized hydrogen-exfoliated-graphene) had a hydrogen storage capacity of 1.76 wt% at 25 C and 2 MPa, while the hydrogen storage capacity of HEG was only 0.5 wt% under the same conditions [16]. Palladium has been widely used to enhance the room temperature hydrogen storage capacity in carbon materials by acting as a good catalyst for hydrogen spillover [17,18]. Hydrogen spillover is defined as the dissociative adsorption of hydrogen molecules onto metal catalyst particles in the form of atomic hydrogen, followed by migration of these hydrogen atoms to the porous materials via surface diffusion [16]. Herein, we originally report a facile technique for the synthesis of palladium nanoparticles with controllable size less than 5 nm embedded into 3D porous carbon (PC) networks via direct activation of the ion-exchange resin (Scheme 1). In this work, the anion-exchange resin not only anchors PdCl2 6 ions by ion-exchanging to obtain uniformly dispersed Pd nanoparticles with an average size down to 5 nm, but it also acts as a carbon source. Compared with traditional porous carbon loaded with Pd on its surface [14,18], Pd embedded into 3D porous carbon does not decrease the specific surface area or the pore volume of the composites in this strategy. Moreover, the Pd nanopaticles embedded into PC are well stabilized for swelling/shrinking of Pd during hydridingedehydriding cycles [19].
Experimental Synthesis of Pd/3D porous carbon Typically, the pretreated anion-exchange resin (20 g, Shanghai Hualing Resin Co., Ltd, China) was impregnated with 50 mL K2PdCl6 solution (0.01 mol/L) at 50 C for 5 h. The Pdexchanged resin was washed with deionized water and dried at 80 C for 24 h. Then, the dried resin (10 g) was transferred into 300 mL KOH (40 g) solution and heated under magnetic stirring. The pale white mixture was transferred into a tube furnace and heated in a nitrogen atmosphere at 750, 850, or 950 C for 2 h with a heating rate of 2 C min1 (the final products were denoted as Pd/PC-750, Pd/PC-850 and Pd/ PC-950, respectively). After cooling to room temperature, the samples were thoroughly washed with dilute hydrochloric acid and deionized water to remove the excess alkali and impurities. Finally, the products were dried in an oven at 80 C.
Characterization of Pd/3D porous carbon The X-Ray powder diffraction (XRD) patterns were recorded on a D/Max-III (Rigaku Co., Japan) with Cu Ka radiation source (30 kV and 30 mA) at a scan rate of 10 (2q) min1. The X-ray photoelectron spectroscopy (XPS) measurements were performed under vacuum (about 2 109 mbar) on an ESCALAB 250 spectrometer. Monochromatic Al Ka (150 W, 1486.6 eV) was used as the excitation source. All the binding energies were calibrated with respect to the C1s peak at 284.8 eV. The Raman spectroscopic measurements were carried out on a Raman spectrometer (Renishaw Corp., UK) using a HeeNe laser with a wavelength of 514.5 nm. The morphology characterizations were performed on a scanning electron microscopy (SEM) (Quanta 400 FEG, FEI Company). The transmission electron microscopy (TEM) investigations were carried out on a JEOL JEM-2010 (JEOL Ltd.) operating at 200 kV. The Pd contents of the samples were analyzed using both X-ray photoelectron spectroscopy (XPS) and inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The pore structure of the Pd/PC samples and the Pd-free PC-850 was characterized by analyzing 77 K nitrogen adsorption/desorption isotherms measured on a Micromeritics ASAP 2420 in the relative
Scheme 1 e Schematic illustration for the synthesis of Pd/3D porous carbon.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
pressure range from 106 to 0.995 P/P0. Prior to measurement, the samples were degassed at 150 C for 6 h.
Hydrogen storage tests High-pressure volumetric hydrogen adsorption measurements were performed on the carbon based materials using a custom-made high-pressure volumetric Sieverts's apparatus operating at ambient temperature and pressures up to 250 bar. The reservoir volume is instrumented with three high-accuracy pressure transducers of 70, 200, and 700 bar ranges (all Druck PMP4060 type transducers, 0.04% FS accuracy), as well as an internal T-type thermocouple to measure the reservoir gas temperature. The multiple-transducer configuration is necessary to reduce the uncertainty in the final gas adsorption measurement at high-pressures. Low temperature hydrogen adsorption measurements were performed at 77 K over a pressure range 0e60 bar using an automatically controlled Sieverts's apparatus (PCT-Pro 2000 from Hy-Energy LLC). The mass of adsorbent used to determine the adsorption/desorption isotherms ranged from 150 mg to 500 mg. The volumetric measurements were carried out at 77 K by submerging the sample holder in a liquid nitrogen bath whose level was maintained constant throughout each experiment. All samples were activated or dehydrated by heating under high vacuum (~105 Torr) prior to sorption experiments.
14845
Results and discussion The XRD patterns of the Pd/PC composites are shown in Fig. 1(a). Three samples exhibited similar XRD patterns. The broad peaks around 2q ¼ 25 can be attributed to two separated forms of carbon referred to as amorphous carbon and graphitic carbon [20]. The peaks at 2q ¼ 40.4 , 46.9 , 68.6 and 82.7 are assigned to the (111), (200), (220) and (311) facets of Pd (PDF#87-0645), respectively. Fig. 1(b) presents Raman spectra of the Pd/PC composites. The band observed at 1360 cm1 is assigned to the D-band. The D-band originates from the disordered and imperfect structures of the carbon materials. The emerging band at 1580 cm1 is assigned to the G-band. The G-band corresponds to the vibrations of carbon atoms with an sp2 electronic configuration in graphene sheet structures [21,22]. The intensity ratios of the G-band to the D-band (IG/ID) of Pd/PC-750, Pd/PC-850 and Pd/PC-950 are 1.035, 1.037 and 2.06, respectively. The XPS spectra of the Pd/PC composites are shown in Fig. 1(c). The C1s peak and O1s peak were centered at about 284.8 eV and 533 eV, respectively. The peak located at about 340 eV is assigned to the Pd3d peak. Fine scans of the Pd3d peak (Fig. 1(d)) were used to analyze the chemical state of palladium. The Pd3d5/2 peak at 335.0 eV and the Pd3d3/2 peak at 340.4 eV belong to Pd(0), while the Pd3d5/2 peak at 336.6 eV and the Pd3d3/2 peak at 342.3 eV are assigned to Pd(2) in PdO
Fig. 1 e (a) XRD patterns, (b) Raman spectra and (c) XPS spectra of the Pd/PC composites and (d) Pd3d spectra of the Pd/PC-850 and Pd/PC-950.
14846
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
[23,24]. The percentage of Pd(0) in Pd/PC-950 was 36.89%. This value was calculated from the relative areas of these peaks. The percentage of the Pd(0) in Pd/PC-850 was 60.18%. However, Pd(0) was not detected in Pd/PC-750 as the Pd particles were partly coated by the porous carbon and the Pd content in the composite was very low. Fig. 2(a) shows the scanning electron micrograph of Pd/PC850, which exhibited a 3D interconnected porous structure over a large area and well-developed open macropores. Meso and macroporous carbon networks were observed in the magnified image of Pd/PC-850 (Fig. 2(b)). Fig. 2(d and e) shows the elemental dispersion of C and Pd in Pd/PC-850 (Fig. 2(c)). In this randomly selected area, we can find that the Pd element was uniformly distributed in the porous carbon.
From the TEM images of Pd/PC-850 (Fig. 3(a and b)), we can find that a large area of Pd/PC-850 consists of a 3D interconnected network of macropores of hundreds of nanometers in diameter. It is also shown that Pd/PC-850 has a large number of micropores and certain amount of mesopores. This result agrees with the SEM analysis. The high-resolution TEM (HRTEM) images of Pd/PC-850 (Fig. 3(c and d)) further revealed the presence of dark contrast crystals of 5 nm in size located in the carbon's micropores. The Pd nanoparticles were highly crystallized with well-resolved lattice spacing of about 0.22 nm which is assigned to the d-spacing of the (111) plane. The nitrogen adsorptionedesorption isotherms at 77 K and corresponding pore size distribution curves of the Pd/PC composites and the Pd-free PC-850 are shown in Fig. 4(a and
Fig. 2 e (aec) SEM images of the Pd/PC-850 and (d and e) elemental mapping images of Pd/PC-850, (d) carbon and (e) palladium.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
b), respectively. The Pd/PC composites appear to have a characteristic Type I isotherm with an H4 hysteresis loop [25]. The composites exhibit very high adsorption in the low relative pressure range (smaller than P/P0 ¼ 0.1), which is ascribed to micropore filling. The ‘knee’ and the almost linear increase in adsorption at higher relative pressures indicate an increasing contribution of the small mesoporosity, mainly for the composites activated with KOH [3,10,26]. The pore size distribution curves (Fig. 4(b)), determined using the DFT method, show more detailed information about the pore structure of the Pd/PC composites and PC-850. All of the samples investigated show a pore size distribution with a dominant micropore feature and less prominent mesopore features. Pd/PC-850, PC-850, and Pd/PC-950 have a micropore peak at about 0.7e1.2 nm and mesopore peaks in the range of 2e3.4 nm. However, Pd/PC-750 has a micropore peak at 0.5 nm and a very broad mesopore peak from 2 to 4 nm. It is worth noting that Pd/PC-850 possesses a dominant proportion of 0.6e0.7 nm micropores and that 0.6e0.7 nm diameter pores have been shown to be the most effective in hydrogen storage [27,28]. The porous surface characterizations and the Pd contents of Pd/PC samples and PC-850 are listed in Table 1. The results indicated that the PC and the three Pd/PC composites
14847
have ultrahigh surface areas ranging from 2734 to 3316 m2 g1. The surface area increases to a maximum for the Pd/PC activated at 850 C then decreases for Pd/PC-950. Pd/PC-850 also displayed a large pore volume of 1.991 cm3 g1. The Pd/PC composites are highly microporous, with a micropore volume between 1.187 and 1.466 cm3 g1. The Pd content and textural properties of the Pd/PC products can be easily tuned by changing the concentration of PdCl2 6 and by modifying the activating conditions. It has been noted that, according to the pore structure characterization of Pd/PC-850 and PC-850, there is hardly any decrease in specific surface area or pore volume when incorporating Pd nanoparticles into the porous carbon using the newly developed synthesis method. However, an obvious reduction in the specific surface area and pore volume was observed in traditional loading of Pd nanoparticles on porous carbons [14,18]. The excess hydrogen storage uptakes of PC-850 and the Pd/ PC materials were measured at 77 K over the pressure range of 0e60 bar. As shown in Fig. 5(a), PC-850 shows the highest excess hydrogen uptake at 77 K. Also, PC-850 shows the fastest diffusivity of H2 in relative to Pd/PC-850 at 77 K, which can be attributed to a bigger adsorption capacity and bigger pore size of PC-850. The excess hydrogen uptakes of Pd/PC-750 and Pd/
Fig. 3 e (a and b) TEM and (c and d) HRTEM images of the Pd/PC-850.
14848
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
affected by the porous structure/texture parameters such as specific surface area, pore volume, and especially the micropore volume and pore size [31e33]. Many studies [27,28,34] showed that 0.6e0.7 nm diameter pores play a key role in hydrogen adsorption. Excitingly, Pd/PC-850 showed pores with a diameter of 0.6e0.7 nm. The Pd/PC-850 has a higher surface area and a very high pore volume of 1.991 cm3 g1. The ultrahigh surface area and unique pore structure of Pd/PC-850 contributed to its better hydrogen storage performance than Pd/PC-750 and Pd/PC-950. As shown in Fig. 5(b), hydrogen uptake was significantly enhanced by the addition of the Pd nanoparticles at 298 K for Pd/PC samples. The excess hydrogen uptakes of Pd/PC-750 and Pd/PC-850 were 0.7 wt% at 100 bar, which is an obvious increase over PC-850 and the commercially available superactivated carbon AX-21 [15] (with a BET surface area about 2600e2800) at 100 atm and 298 K. Because the system is close to saturation at 77 K, the specific surface area has a larger impact on the hydrogen uptake than that of the Pd nanoparticles. The effect of the Pd nanoparticles is clearer at room temperature where adsorption uptake is more susceptible to changes in surface energy. Also, the presence of Pd nanoparticles may facilitate dissociation of hydrogen molecules over Pd surfaces due to the
Fig. 4 e (a) Nitrogen adsorptionedesorption isotherms and pore size distribution curves of a set of Pd/PC composites and PC-850.
PC-850 were 3.2 wt% and 4.3 wt% at 10 bar, respectively, which are higher than that of commercially available activated carbon (G212) reported by Gao's group [11]. From the experimental results, the hydrogen uptake of Pd/PC-850 at 10 bar is close to previously reported activated carbons [27,29,30]. It is notable that the hydrogen adsorption capacity was greatly
Table 1 e Texture properties and Pd contents of the samples. Samples
SBET (m2/g)
Pd/PC-750 Pd/PC-850 PC-850 Pd/PC-950
2935.328 3316.123 3197.523 2734.042
V0.98ta Vmib Vmeac Pd contentICP 3 3 (cm /g) (cm /g) (cm3/g) (wt%) 1.923 1.991 1.936 1.789
1.297 1.466 1.412 1.187
0.626 0.525 0.524 0.602
1.65 1.86 0 2.90
a
Single point desorption to total pore volume of pores at P/ P0 ¼ 0.98. b t-Plot micropore volume. c Mesopore and macropore volume obtained by subtracting Vmi from V0.98t.
Fig. 5 e (a) Hydrogen uptake curves of the Pd/PC composites and PC-850 at 77 K over the pressure range 1e60 bar and (b) hydrogen uptake curves of the PC-850 and Pd/PC composites from 0 to 250 bar at 298 K.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
incompletely occupied d-orbitals [17]. Dissociated hydrogen atoms can also migrate to the porous carbon. Hydrogen atoms can diffuse into smaller pores more easily than hydrogen molecules. Therefore, the Pd/PC-850 shows faster diffusivity of H2 and increased hydrogen uptake compared with PC-850 at room temperature.
Conclusions Ultrahigh surface area Pd/3D activated porous carbon (Pd/PC) has been synthesized by a novel one-pot ion-exchange method. In this simple strategy, the anion-exchange resin not ions to obtain uniformly dispersed Pd only anchors PdCl2 6 nanoparticles with an average size down to 5 nm but also acts as a carbon source. The Pd/PC materials exhibited ultrahigh surface areas up to 3316 m2 g1 and large pore volume up to 1.991 cm3 g1. The Pd/PC-850 possesses a dominating proportion of 0.6e0.7 nm micropores and small mesopore in the size of 2e3.4 nm. The Pd/PC-850 composite showed better hydrogen uptake properties at room temperature because of the spillover effect. The Pd/PC-850 composite with ultrahigh surface area and unique pore texture achieved good excess hydrogen storage uptake up to 4.6 wt% at 77 K and 20 bar. In summary, the obtained Pd/PC composites showed good ‘hydrogen spillover’ at room temperature and displayed a good hydrogen storage capacity.
Acknowledgments This work was supported by the Major International (Regional) Joint Research Project (51210002), the Link Project of the National Natural Science Foundation of China (21073241, 21201058) and Guangdong Province (U1034003) and General Motors Corporation (Project No. RD-07-295-NV508).
references
[1] Qu D. Investigation of hydrogen physisorption active sites on the surface of porous carbonaceous materials. Chem Eur J 2008;14:1040e6. [2] Zhou L. Progress and problems in hydrogen storage methods. Renew Sust Energ Rev 2005;9:395e408. nchez A, Dela Torre[3] Zyzlila Figueroa-Torres M, Robau-Sa enz L, Aguilar-Elgue zabal A. Hydrogen adsorption by Sa nanostructured carbons synthesized by chemical activation. Micropor Mesopor Mat 2007;98:89e93. [4] Panella B, Hirscher M, Roth S. Hydrogen adsorption in different carbon nanostructures. Carbon 2005;43:2209e14. [5] Dillon A, Jones K, Bekkedahl T, Kiang C, Bethune D, Heben M. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386:377e9. [6] Sevilla M, Fuertes A, Mokaya R. High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials. Energy Environ Sci 2011;4:1400e10.
14849
[7] Ahluwalia R, Peng J. Automotive hydrogen storage system using cryo-adsorption on activated carbon. Int J Hydrogen Energy 2009;34:5476e87. [8] Alam N, Mokaya R. Evolution of optimal porosity for improved hydrogen storage in templated zeolite-like carbons. Energy Environ Sci 2010;3:1773e81. [9] Im JS, Park SJ, Kim T, Lee YS. Hydrogen storage evaluation based on investigations of the catalytic properties of metal/ metal oxides in electrospun carbon fibers. Int J Hydrogen Energy 2009;34:3382e8. [10] Guo H, Gao Q. Cryogenic hydrogen uptake of high surface area porous carbon materials activated by potassium hydroxide. Int J Hydrogen Energy 2010;35:7547e54. [11] Wang H, Gao Q, Hu J. High hydrogen storage capacity of porous carbons prepared by using activated carbon. J Am Chem Soc 2009;131:7016e22. denas M, Juan-Juan J, Cazorla-Amoro s D, [12] Lillo-Ro Linares-Solano A. About reactions occurring during chemical activation with hydroxides. Carbon 2004;42:1371e5. [13] Yu¨ru¨m Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [14] Adams BD, Ostrom CK, Chen S, Chen A. High-performance Pd-based hydrogen spillover catalysts for hydrogen storage. J Phys Chem C 2010;114:19875e82. [15] Wang L, Yang RT. New sorbents for hydrogen storage by hydrogen spillover e a review. Energy Environ Sci 2008;1:268e79. [16] Parambhath V, Nagar R, Sethupathi K, Ramaprabhu S. Investigation of spillover mechanism in palladium decorated hydrogen exfoliated functionalized graphene. J Phys Chem C 2011;115:15679e85. [17] Kim H, Lee D, Moon J. Co-electrospun Pd-coated porous carbon nanofibers for hydrogen storage applications. Int J Hydrogen Energy 2011;36:3566e73. [18] Campesi R, Cuevas F, Gadiou R, Leroy E, Hirscher M, VixGuterl C, et al. Hydrogen storage properties of Pd nanoparticle/carbon template composites. Carbon 2008;46:206e14. [19] Sachs C, Pundt A, Kirchheim R, Winter M, Reetz M, Fritsch D. Solubility of hydrogen in single-sized palladium clusters. Phys Rev B 2001;64:075408. [20] Wu G, Nelson M, Ma S, Meng H, Cui G, Shen PK. Synthesis of nitrogen-doped onion-like carbon and its use in carbonbased CoFe binary non-precious-metal catalysts for oxygenreduction. Carbon 2011;49:3972e82. [21] Wang Y, Alsmeyer DC, McCreery RL. Raman spectroscopy of carbon materials: structural basis of observed spectra. Chem Mater 1990;2:557e63. [22] Ghosh K, Kumar M, Maruyama T, Ando Y. Micro-structural, electron-spectroscopic and field-emission studies of carbon nitride nanotubes grown from cage-like and linear carbon sources. Carbon 2009;47:1565e75. [23] Vasilyeva SV, Vorotyntsev MA, Bezverkhyy I, Lesniewska E, Heintz O, Chassagnon R. Synthesis and characterization of palladium nanoparticle/polypyrrole composites. J Phys Chem C 2008;112:19878e85. [24] Cui Z, Kulesza PJ, Li CM, Xing W, Jiang SP. Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells. Int J Hydrogen Energy 2011;36:8508e17. [25] Everett D, Haul R, Moscou L, Pierrotti R, Rouquerol J, Siemienieswka T. Reporting physisorption data for gas/solid systems. Pure Appl Chem 1982;54:2201e18. [26] Yang Z, Xia Y, Sun X, Mokaya R. Preparation and hydrogen storage properties of zeolite-templated carbon materials
14850
[27]
[28]
[29]
[30]
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
nanocast via chemical vapor deposition: effect of the zeolite template and nitrogen doping. J Phys Chem C 2006;110:18424e31. Sevilla M, Mokaya R, Fuertes AB. Ultrahigh surface area polypyrrole-based carbons with superior performance for hydrogen storage. Energy Environ Sci 2011;4:2930e6. Gogotsi Y, Portet C, Osswald S, Simmons JM, Yildirim T, Laudisio G, et al. Importance of pore size in high-pressure hydrogen storage by porous carbons. Int J Hydrogen Energy 2009;34:6314e9. ndez-Huerta N, Izquierdo M, Zhao W, Fierro V, Ferna Celzard A. Impact of synthesis conditions of KOH activated carbons on their hydrogen storage capacities. Int J Hydrogen Energy 2012;37:14278e84. Masika E, Mokaya R. Hydrogen storage in high surface area carbons with identical surface areas but different pore sizes:
[31]
[32]
[33]
[34]
direct demonstration of the effects of pore size. J Phys Chem C 2012;116:25734e40. -Beneyto M, Sua rez-Garcı´a F, Lozano-Castello D, Jorda s D, Linares-Solano A. Hydrogen storage on Cazorla-Amoro chemically activated carbons and carbon nanomaterials at high pressures. Carbon 2007;45:293e303. Yushin G, Dash R, Jagiello J, Fischer JE, Gogotsi Y. Carbidederived carbons: effect of pore size on hydrogen uptake and heat of adsorption. Adv Funct Mater 2006;16:2288e93. Pacula A, Mokaya R. Synthesis and high hydrogen storage capacity of zeolite-like carbons nanocast using assynthesized zeolite templates. J Phys Chem C 2008;112:2764e9. Xia K, Gao Q, Wu C, Song S, Ruan M. Activation, characterization and hydrogen storage properties of the mesoporous carbon CMK-3. Carbon 2007;45:1989e96.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
One-pot synthesis of Pd nanoparticles on ultrahigh surface area 3D porous carbon as hydrogen storage materials Jinliang Zhu a,1, Juhong Cheng a,1, Anne Dailly b, Mei Cai b, Matthew Beckner c, Pei Kang Shen a,* a
The State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou, 510275, PR China b General Motors Research and Development Center, Warren, MI, 48090-9055, USA c Optimal CAE Inc., Plymouth, MI, 48107, USA
article info
abstract
Article history:
Pd on three dimensional (3D) porous carbon (Pd/PC) has been one-pot synthesized via in
Received 12 February 2014
situ potassium hydroxide activation of PdCl2 6 -exchanged resin for the first time. The
Received in revised form
ions to obtain uniformly dispersed Pd anion-exchange resin not only anchors PdCl2 6
27 June 2014
nanoparticles but also acts as a carbon source. The Pd/PC composites exhibit an ultrahigh
Accepted 1 July 2014
surface areas (2734e3316 m2 g1) and large pore volumes (1.789e1.991 cm3 g1). The excess
Available online 2 August 2014
hydrogen uptake of Pd/PC-850 reaches 4.6 wt% at 77 K and 20 bar. At room temperature, the hydrogen spillover process from Pd nanoparticles to the 3D activated porous carbon has
Keywords:
been clearly observed. The 1.86 wt% Pd-containing composite (Pd/PC-850) displays a high
Pd nanoparticles
hydrogen spillover enhancement (154%) at 298 K.
One-pot synthesis
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
3D porous carbon
reserved.
Ultrahigh surface area Hydrogen storage
Introduction In recent years, hydrogen produced from many resources has attracted much attention as an ideal pollution-free energy medium for replacing fossil fuels. Efficient hydrogen storage and transportation is crucial for a future hydrogen economy. Until now, approaches to hydrogen storage have been explored using various systems such as compression, liquefaction, solid state storage into materials (absorption in
metallic hydrides and adsorption onto high surface area sorbents) [1]. In particularly, hydrogen physisorption has been considered as the most promising hydrogen storage method [2e4]. Carbon materials such as carbon nanotubes (CNTs) [5], activated porous carbons [6,7], template-produced carbons [8], and carbon nanofibers [9] were widely studied as hydrogen storage media. Among the numerous possible carbon materials, activated porous carbons have received significant attention as potential hydrogen storage media due to their low density, high surface area, large pore volume, good chemical
* Corresponding author. Tel.: þ86 20 84036736; fax: þ86 20 84113369. E-mail address: [email protected] (P.K. Shen). 1 The authors contributed equally. http://dx.doi.org/10.1016/j.ijhydene.2014.07.002 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
14844
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
stability, and high storage capacity. High-temperature treatment of a mixture of raw carbonaceous material and KOH is regarded as an effective method to synthesize highly porous carbons [10,11]. The following prominent reaction occurs during activation of the carbonaceous material by KOH [12]. 6KOH þ 2C 4 2K þ 3H2 þ 2K2CO3 Porous carbons produced using KOH as an activating agent present attractive properties of high surface area and a large porosity made up of super-micropores and small mesopores. Significant hydrogen adsorption uptake on activated carbons can be achieved at low temperature (77 K) and moderated pressures; however, the capacity will be significantly reduced at ambient temperatures [13,14]. Experimental results and molecular orbital calculations showed that the hydrogen spillover mechanism for hydrogen storage at ambient temperature is effective at increasing the hydrogen adsorption capacity [14,15]. Parambhath et al. reported that Pd/f-HEG (functionalized hydrogen-exfoliated-graphene) had a hydrogen storage capacity of 1.76 wt% at 25 C and 2 MPa, while the hydrogen storage capacity of HEG was only 0.5 wt% under the same conditions [16]. Palladium has been widely used to enhance the room temperature hydrogen storage capacity in carbon materials by acting as a good catalyst for hydrogen spillover [17,18]. Hydrogen spillover is defined as the dissociative adsorption of hydrogen molecules onto metal catalyst particles in the form of atomic hydrogen, followed by migration of these hydrogen atoms to the porous materials via surface diffusion [16]. Herein, we originally report a facile technique for the synthesis of palladium nanoparticles with controllable size less than 5 nm embedded into 3D porous carbon (PC) networks via direct activation of the ion-exchange resin (Scheme 1). In this work, the anion-exchange resin not only anchors PdCl2 6 ions by ion-exchanging to obtain uniformly dispersed Pd nanoparticles with an average size down to 5 nm, but it also acts as a carbon source. Compared with traditional porous carbon loaded with Pd on its surface [14,18], Pd embedded into 3D porous carbon does not decrease the specific surface area or the pore volume of the composites in this strategy. Moreover, the Pd nanopaticles embedded into PC are well stabilized for swelling/shrinking of Pd during hydridingedehydriding cycles [19].
Experimental Synthesis of Pd/3D porous carbon Typically, the pretreated anion-exchange resin (20 g, Shanghai Hualing Resin Co., Ltd, China) was impregnated with 50 mL K2PdCl6 solution (0.01 mol/L) at 50 C for 5 h. The Pdexchanged resin was washed with deionized water and dried at 80 C for 24 h. Then, the dried resin (10 g) was transferred into 300 mL KOH (40 g) solution and heated under magnetic stirring. The pale white mixture was transferred into a tube furnace and heated in a nitrogen atmosphere at 750, 850, or 950 C for 2 h with a heating rate of 2 C min1 (the final products were denoted as Pd/PC-750, Pd/PC-850 and Pd/ PC-950, respectively). After cooling to room temperature, the samples were thoroughly washed with dilute hydrochloric acid and deionized water to remove the excess alkali and impurities. Finally, the products were dried in an oven at 80 C.
Characterization of Pd/3D porous carbon The X-Ray powder diffraction (XRD) patterns were recorded on a D/Max-III (Rigaku Co., Japan) with Cu Ka radiation source (30 kV and 30 mA) at a scan rate of 10 (2q) min1. The X-ray photoelectron spectroscopy (XPS) measurements were performed under vacuum (about 2 109 mbar) on an ESCALAB 250 spectrometer. Monochromatic Al Ka (150 W, 1486.6 eV) was used as the excitation source. All the binding energies were calibrated with respect to the C1s peak at 284.8 eV. The Raman spectroscopic measurements were carried out on a Raman spectrometer (Renishaw Corp., UK) using a HeeNe laser with a wavelength of 514.5 nm. The morphology characterizations were performed on a scanning electron microscopy (SEM) (Quanta 400 FEG, FEI Company). The transmission electron microscopy (TEM) investigations were carried out on a JEOL JEM-2010 (JEOL Ltd.) operating at 200 kV. The Pd contents of the samples were analyzed using both X-ray photoelectron spectroscopy (XPS) and inductively coupled plasmaatomic emission spectroscopy (ICP-AES). The pore structure of the Pd/PC samples and the Pd-free PC-850 was characterized by analyzing 77 K nitrogen adsorption/desorption isotherms measured on a Micromeritics ASAP 2420 in the relative
Scheme 1 e Schematic illustration for the synthesis of Pd/3D porous carbon.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
pressure range from 106 to 0.995 P/P0. Prior to measurement, the samples were degassed at 150 C for 6 h.
Hydrogen storage tests High-pressure volumetric hydrogen adsorption measurements were performed on the carbon based materials using a custom-made high-pressure volumetric Sieverts's apparatus operating at ambient temperature and pressures up to 250 bar. The reservoir volume is instrumented with three high-accuracy pressure transducers of 70, 200, and 700 bar ranges (all Druck PMP4060 type transducers, 0.04% FS accuracy), as well as an internal T-type thermocouple to measure the reservoir gas temperature. The multiple-transducer configuration is necessary to reduce the uncertainty in the final gas adsorption measurement at high-pressures. Low temperature hydrogen adsorption measurements were performed at 77 K over a pressure range 0e60 bar using an automatically controlled Sieverts's apparatus (PCT-Pro 2000 from Hy-Energy LLC). The mass of adsorbent used to determine the adsorption/desorption isotherms ranged from 150 mg to 500 mg. The volumetric measurements were carried out at 77 K by submerging the sample holder in a liquid nitrogen bath whose level was maintained constant throughout each experiment. All samples were activated or dehydrated by heating under high vacuum (~105 Torr) prior to sorption experiments.
14845
Results and discussion The XRD patterns of the Pd/PC composites are shown in Fig. 1(a). Three samples exhibited similar XRD patterns. The broad peaks around 2q ¼ 25 can be attributed to two separated forms of carbon referred to as amorphous carbon and graphitic carbon [20]. The peaks at 2q ¼ 40.4 , 46.9 , 68.6 and 82.7 are assigned to the (111), (200), (220) and (311) facets of Pd (PDF#87-0645), respectively. Fig. 1(b) presents Raman spectra of the Pd/PC composites. The band observed at 1360 cm1 is assigned to the D-band. The D-band originates from the disordered and imperfect structures of the carbon materials. The emerging band at 1580 cm1 is assigned to the G-band. The G-band corresponds to the vibrations of carbon atoms with an sp2 electronic configuration in graphene sheet structures [21,22]. The intensity ratios of the G-band to the D-band (IG/ID) of Pd/PC-750, Pd/PC-850 and Pd/PC-950 are 1.035, 1.037 and 2.06, respectively. The XPS spectra of the Pd/PC composites are shown in Fig. 1(c). The C1s peak and O1s peak were centered at about 284.8 eV and 533 eV, respectively. The peak located at about 340 eV is assigned to the Pd3d peak. Fine scans of the Pd3d peak (Fig. 1(d)) were used to analyze the chemical state of palladium. The Pd3d5/2 peak at 335.0 eV and the Pd3d3/2 peak at 340.4 eV belong to Pd(0), while the Pd3d5/2 peak at 336.6 eV and the Pd3d3/2 peak at 342.3 eV are assigned to Pd(2) in PdO
Fig. 1 e (a) XRD patterns, (b) Raman spectra and (c) XPS spectra of the Pd/PC composites and (d) Pd3d spectra of the Pd/PC-850 and Pd/PC-950.
14846
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
[23,24]. The percentage of Pd(0) in Pd/PC-950 was 36.89%. This value was calculated from the relative areas of these peaks. The percentage of the Pd(0) in Pd/PC-850 was 60.18%. However, Pd(0) was not detected in Pd/PC-750 as the Pd particles were partly coated by the porous carbon and the Pd content in the composite was very low. Fig. 2(a) shows the scanning electron micrograph of Pd/PC850, which exhibited a 3D interconnected porous structure over a large area and well-developed open macropores. Meso and macroporous carbon networks were observed in the magnified image of Pd/PC-850 (Fig. 2(b)). Fig. 2(d and e) shows the elemental dispersion of C and Pd in Pd/PC-850 (Fig. 2(c)). In this randomly selected area, we can find that the Pd element was uniformly distributed in the porous carbon.
From the TEM images of Pd/PC-850 (Fig. 3(a and b)), we can find that a large area of Pd/PC-850 consists of a 3D interconnected network of macropores of hundreds of nanometers in diameter. It is also shown that Pd/PC-850 has a large number of micropores and certain amount of mesopores. This result agrees with the SEM analysis. The high-resolution TEM (HRTEM) images of Pd/PC-850 (Fig. 3(c and d)) further revealed the presence of dark contrast crystals of 5 nm in size located in the carbon's micropores. The Pd nanoparticles were highly crystallized with well-resolved lattice spacing of about 0.22 nm which is assigned to the d-spacing of the (111) plane. The nitrogen adsorptionedesorption isotherms at 77 K and corresponding pore size distribution curves of the Pd/PC composites and the Pd-free PC-850 are shown in Fig. 4(a and
Fig. 2 e (aec) SEM images of the Pd/PC-850 and (d and e) elemental mapping images of Pd/PC-850, (d) carbon and (e) palladium.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
b), respectively. The Pd/PC composites appear to have a characteristic Type I isotherm with an H4 hysteresis loop [25]. The composites exhibit very high adsorption in the low relative pressure range (smaller than P/P0 ¼ 0.1), which is ascribed to micropore filling. The ‘knee’ and the almost linear increase in adsorption at higher relative pressures indicate an increasing contribution of the small mesoporosity, mainly for the composites activated with KOH [3,10,26]. The pore size distribution curves (Fig. 4(b)), determined using the DFT method, show more detailed information about the pore structure of the Pd/PC composites and PC-850. All of the samples investigated show a pore size distribution with a dominant micropore feature and less prominent mesopore features. Pd/PC-850, PC-850, and Pd/PC-950 have a micropore peak at about 0.7e1.2 nm and mesopore peaks in the range of 2e3.4 nm. However, Pd/PC-750 has a micropore peak at 0.5 nm and a very broad mesopore peak from 2 to 4 nm. It is worth noting that Pd/PC-850 possesses a dominant proportion of 0.6e0.7 nm micropores and that 0.6e0.7 nm diameter pores have been shown to be the most effective in hydrogen storage [27,28]. The porous surface characterizations and the Pd contents of Pd/PC samples and PC-850 are listed in Table 1. The results indicated that the PC and the three Pd/PC composites
14847
have ultrahigh surface areas ranging from 2734 to 3316 m2 g1. The surface area increases to a maximum for the Pd/PC activated at 850 C then decreases for Pd/PC-950. Pd/PC-850 also displayed a large pore volume of 1.991 cm3 g1. The Pd/PC composites are highly microporous, with a micropore volume between 1.187 and 1.466 cm3 g1. The Pd content and textural properties of the Pd/PC products can be easily tuned by changing the concentration of PdCl2 6 and by modifying the activating conditions. It has been noted that, according to the pore structure characterization of Pd/PC-850 and PC-850, there is hardly any decrease in specific surface area or pore volume when incorporating Pd nanoparticles into the porous carbon using the newly developed synthesis method. However, an obvious reduction in the specific surface area and pore volume was observed in traditional loading of Pd nanoparticles on porous carbons [14,18]. The excess hydrogen storage uptakes of PC-850 and the Pd/ PC materials were measured at 77 K over the pressure range of 0e60 bar. As shown in Fig. 5(a), PC-850 shows the highest excess hydrogen uptake at 77 K. Also, PC-850 shows the fastest diffusivity of H2 in relative to Pd/PC-850 at 77 K, which can be attributed to a bigger adsorption capacity and bigger pore size of PC-850. The excess hydrogen uptakes of Pd/PC-750 and Pd/
Fig. 3 e (a and b) TEM and (c and d) HRTEM images of the Pd/PC-850.
14848
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
affected by the porous structure/texture parameters such as specific surface area, pore volume, and especially the micropore volume and pore size [31e33]. Many studies [27,28,34] showed that 0.6e0.7 nm diameter pores play a key role in hydrogen adsorption. Excitingly, Pd/PC-850 showed pores with a diameter of 0.6e0.7 nm. The Pd/PC-850 has a higher surface area and a very high pore volume of 1.991 cm3 g1. The ultrahigh surface area and unique pore structure of Pd/PC-850 contributed to its better hydrogen storage performance than Pd/PC-750 and Pd/PC-950. As shown in Fig. 5(b), hydrogen uptake was significantly enhanced by the addition of the Pd nanoparticles at 298 K for Pd/PC samples. The excess hydrogen uptakes of Pd/PC-750 and Pd/PC-850 were 0.7 wt% at 100 bar, which is an obvious increase over PC-850 and the commercially available superactivated carbon AX-21 [15] (with a BET surface area about 2600e2800) at 100 atm and 298 K. Because the system is close to saturation at 77 K, the specific surface area has a larger impact on the hydrogen uptake than that of the Pd nanoparticles. The effect of the Pd nanoparticles is clearer at room temperature where adsorption uptake is more susceptible to changes in surface energy. Also, the presence of Pd nanoparticles may facilitate dissociation of hydrogen molecules over Pd surfaces due to the
Fig. 4 e (a) Nitrogen adsorptionedesorption isotherms and pore size distribution curves of a set of Pd/PC composites and PC-850.
PC-850 were 3.2 wt% and 4.3 wt% at 10 bar, respectively, which are higher than that of commercially available activated carbon (G212) reported by Gao's group [11]. From the experimental results, the hydrogen uptake of Pd/PC-850 at 10 bar is close to previously reported activated carbons [27,29,30]. It is notable that the hydrogen adsorption capacity was greatly
Table 1 e Texture properties and Pd contents of the samples. Samples
SBET (m2/g)
Pd/PC-750 Pd/PC-850 PC-850 Pd/PC-950
2935.328 3316.123 3197.523 2734.042
V0.98ta Vmib Vmeac Pd contentICP 3 3 (cm /g) (cm /g) (cm3/g) (wt%) 1.923 1.991 1.936 1.789
1.297 1.466 1.412 1.187
0.626 0.525 0.524 0.602
1.65 1.86 0 2.90
a
Single point desorption to total pore volume of pores at P/ P0 ¼ 0.98. b t-Plot micropore volume. c Mesopore and macropore volume obtained by subtracting Vmi from V0.98t.
Fig. 5 e (a) Hydrogen uptake curves of the Pd/PC composites and PC-850 at 77 K over the pressure range 1e60 bar and (b) hydrogen uptake curves of the PC-850 and Pd/PC composites from 0 to 250 bar at 298 K.
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
incompletely occupied d-orbitals [17]. Dissociated hydrogen atoms can also migrate to the porous carbon. Hydrogen atoms can diffuse into smaller pores more easily than hydrogen molecules. Therefore, the Pd/PC-850 shows faster diffusivity of H2 and increased hydrogen uptake compared with PC-850 at room temperature.
Conclusions Ultrahigh surface area Pd/3D activated porous carbon (Pd/PC) has been synthesized by a novel one-pot ion-exchange method. In this simple strategy, the anion-exchange resin not ions to obtain uniformly dispersed Pd only anchors PdCl2 6 nanoparticles with an average size down to 5 nm but also acts as a carbon source. The Pd/PC materials exhibited ultrahigh surface areas up to 3316 m2 g1 and large pore volume up to 1.991 cm3 g1. The Pd/PC-850 possesses a dominating proportion of 0.6e0.7 nm micropores and small mesopore in the size of 2e3.4 nm. The Pd/PC-850 composite showed better hydrogen uptake properties at room temperature because of the spillover effect. The Pd/PC-850 composite with ultrahigh surface area and unique pore texture achieved good excess hydrogen storage uptake up to 4.6 wt% at 77 K and 20 bar. In summary, the obtained Pd/PC composites showed good ‘hydrogen spillover’ at room temperature and displayed a good hydrogen storage capacity.
Acknowledgments This work was supported by the Major International (Regional) Joint Research Project (51210002), the Link Project of the National Natural Science Foundation of China (21073241, 21201058) and Guangdong Province (U1034003) and General Motors Corporation (Project No. RD-07-295-NV508).
references
[1] Qu D. Investigation of hydrogen physisorption active sites on the surface of porous carbonaceous materials. Chem Eur J 2008;14:1040e6. [2] Zhou L. Progress and problems in hydrogen storage methods. Renew Sust Energ Rev 2005;9:395e408. nchez A, Dela Torre[3] Zyzlila Figueroa-Torres M, Robau-Sa enz L, Aguilar-Elgue zabal A. Hydrogen adsorption by Sa nanostructured carbons synthesized by chemical activation. Micropor Mesopor Mat 2007;98:89e93. [4] Panella B, Hirscher M, Roth S. Hydrogen adsorption in different carbon nanostructures. Carbon 2005;43:2209e14. [5] Dillon A, Jones K, Bekkedahl T, Kiang C, Bethune D, Heben M. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386:377e9. [6] Sevilla M, Fuertes A, Mokaya R. High density hydrogen storage in superactivated carbons from hydrothermally carbonized renewable organic materials. Energy Environ Sci 2011;4:1400e10.
14849
[7] Ahluwalia R, Peng J. Automotive hydrogen storage system using cryo-adsorption on activated carbon. Int J Hydrogen Energy 2009;34:5476e87. [8] Alam N, Mokaya R. Evolution of optimal porosity for improved hydrogen storage in templated zeolite-like carbons. Energy Environ Sci 2010;3:1773e81. [9] Im JS, Park SJ, Kim T, Lee YS. Hydrogen storage evaluation based on investigations of the catalytic properties of metal/ metal oxides in electrospun carbon fibers. Int J Hydrogen Energy 2009;34:3382e8. [10] Guo H, Gao Q. Cryogenic hydrogen uptake of high surface area porous carbon materials activated by potassium hydroxide. Int J Hydrogen Energy 2010;35:7547e54. [11] Wang H, Gao Q, Hu J. High hydrogen storage capacity of porous carbons prepared by using activated carbon. J Am Chem Soc 2009;131:7016e22. denas M, Juan-Juan J, Cazorla-Amoro s D, [12] Lillo-Ro Linares-Solano A. About reactions occurring during chemical activation with hydroxides. Carbon 2004;42:1371e5. [13] Yu¨ru¨m Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [14] Adams BD, Ostrom CK, Chen S, Chen A. High-performance Pd-based hydrogen spillover catalysts for hydrogen storage. J Phys Chem C 2010;114:19875e82. [15] Wang L, Yang RT. New sorbents for hydrogen storage by hydrogen spillover e a review. Energy Environ Sci 2008;1:268e79. [16] Parambhath V, Nagar R, Sethupathi K, Ramaprabhu S. Investigation of spillover mechanism in palladium decorated hydrogen exfoliated functionalized graphene. J Phys Chem C 2011;115:15679e85. [17] Kim H, Lee D, Moon J. Co-electrospun Pd-coated porous carbon nanofibers for hydrogen storage applications. Int J Hydrogen Energy 2011;36:3566e73. [18] Campesi R, Cuevas F, Gadiou R, Leroy E, Hirscher M, VixGuterl C, et al. Hydrogen storage properties of Pd nanoparticle/carbon template composites. Carbon 2008;46:206e14. [19] Sachs C, Pundt A, Kirchheim R, Winter M, Reetz M, Fritsch D. Solubility of hydrogen in single-sized palladium clusters. Phys Rev B 2001;64:075408. [20] Wu G, Nelson M, Ma S, Meng H, Cui G, Shen PK. Synthesis of nitrogen-doped onion-like carbon and its use in carbonbased CoFe binary non-precious-metal catalysts for oxygenreduction. Carbon 2011;49:3972e82. [21] Wang Y, Alsmeyer DC, McCreery RL. Raman spectroscopy of carbon materials: structural basis of observed spectra. Chem Mater 1990;2:557e63. [22] Ghosh K, Kumar M, Maruyama T, Ando Y. Micro-structural, electron-spectroscopic and field-emission studies of carbon nitride nanotubes grown from cage-like and linear carbon sources. Carbon 2009;47:1565e75. [23] Vasilyeva SV, Vorotyntsev MA, Bezverkhyy I, Lesniewska E, Heintz O, Chassagnon R. Synthesis and characterization of palladium nanoparticle/polypyrrole composites. J Phys Chem C 2008;112:19878e85. [24] Cui Z, Kulesza PJ, Li CM, Xing W, Jiang SP. Pd nanoparticles supported on HPMo-PDDA-MWCNT and their activity for formic acid oxidation reaction of fuel cells. Int J Hydrogen Energy 2011;36:8508e17. [25] Everett D, Haul R, Moscou L, Pierrotti R, Rouquerol J, Siemienieswka T. Reporting physisorption data for gas/solid systems. Pure Appl Chem 1982;54:2201e18. [26] Yang Z, Xia Y, Sun X, Mokaya R. Preparation and hydrogen storage properties of zeolite-templated carbon materials
14850
[27]
[28]
[29]
[30]
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 3 9 ( 2 0 1 4 ) 1 4 8 4 3 e1 4 8 5 0
nanocast via chemical vapor deposition: effect of the zeolite template and nitrogen doping. J Phys Chem C 2006;110:18424e31. Sevilla M, Mokaya R, Fuertes AB. Ultrahigh surface area polypyrrole-based carbons with superior performance for hydrogen storage. Energy Environ Sci 2011;4:2930e6. Gogotsi Y, Portet C, Osswald S, Simmons JM, Yildirim T, Laudisio G, et al. Importance of pore size in high-pressure hydrogen storage by porous carbons. Int J Hydrogen Energy 2009;34:6314e9. ndez-Huerta N, Izquierdo M, Zhao W, Fierro V, Ferna Celzard A. Impact of synthesis conditions of KOH activated carbons on their hydrogen storage capacities. Int J Hydrogen Energy 2012;37:14278e84. Masika E, Mokaya R. Hydrogen storage in high surface area carbons with identical surface areas but different pore sizes:
[31]
[32]
[33]
[34]
direct demonstration of the effects of pore size. J Phys Chem C 2012;116:25734e40. -Beneyto M, Sua rez-Garcı´a F, Lozano-Castello D, Jorda s D, Linares-Solano A. Hydrogen storage on Cazorla-Amoro chemically activated carbons and carbon nanomaterials at high pressures. Carbon 2007;45:293e303. Yushin G, Dash R, Jagiello J, Fischer JE, Gogotsi Y. Carbidederived carbons: effect of pore size on hydrogen uptake and heat of adsorption. Adv Funct Mater 2006;16:2288e93. Pacula A, Mokaya R. Synthesis and high hydrogen storage capacity of zeolite-like carbons nanocast using assynthesized zeolite templates. J Phys Chem C 2008;112:2764e9. Xia K, Gao Q, Wu C, Song S, Ruan M. Activation, characterization and hydrogen storage properties of the mesoporous carbon CMK-3. Carbon 2007;45:1989e96.