- Email: [email protected]
Electrochemical hydrogen storage of expanded graphite decorated with TiO2 nanoparticles
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical hydrogen storage of expanded graphite decorated with TiO2 nanoparticles Yanmin Yu a,b, Naiqin Zhao a,b,c, Chunsheng Shi a,b,*, Chunnian He a,b, Enzuo Liu a,b, Jiajun Li a,b a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China c Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin 300072, China b
article info
abstract
Article history:
The electrochemical hydrogen storage of expanded graphite (EG) decorated with TiO2
Received 17 November 2011
nanoparticles (NPs) calcined at different temperatures has been investigated with the
Received in revised form
galvanostatic charge and discharge method. The TiO2 NPs are deposited on and between
24 December 2011
the graphene-like nanosheets of EG by a sol-gel method. The morphology, structure,
Accepted 28 December 2011
composition, and specific surface area of the samples were characterized. The electro-
Available online 25 January 2012
chemical measurement reveals that the EG decorated with TiO2 NPs calcined at 500 C has a discharge capacity of 373.5 mAh/g which is 20 times higher than that of pure EG and quite
Keywords:
appealing for the battery applications. The mechanism of enhancement of the electro-
Expanded graphite
chemical activity for the TiO2-decorated EG could be attributed to the preferable redox
TiO2 nanoparticles
ability and photocatalytic property of TiO2 NPs.
Electrochemical hydrogen
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
storage capacity
1.
Introduction
Nowadays, hydrogen is recognized as an ideal fuel for energy conversion because of its high efficiency and its role in the reduction of air pollution [1]. However, due to the existence of major scientific challenges such as the lack of a safe, effective and cheap hydrogen storage system [2,3], it has not been so far used in great extent. To date, different methods of hydrogen storage have been applied but none of them have completely satisfied the proposed standard by the US Department of Energy (DOE) [2]. Hydrogen sorption in hydrogen storage materials can be performed by physical, chemical and electrochemical methods [2,4,5]. In the last case, there is no need to use high pressure, and the hydrogen sorption process occurs directly in
reserved.
electrode material at ambient conditions. Despite some discrepancies in the mechanism of hydrogen storage in various materials, it is without any doubt that hydrogen storage capacity in the electrochemical processes is strongly influenced by the properties of the electrode material used as hydrogen reservoir, such as chemical composition, crystalline, porous structure, and catalytic features in the reaction of hydrogen sorption [2]. Various forms of carbon, such as activated carbons [2,6], carbon nanofibres, and carbon nanotubes have been concerned as candidates for electrochemical hydrogen storage in last decade [2,7e21]. In order to enhance the hydrogen storage capacity for carbon materials, some transition metals (mainly Ni, Pt, Pd), transition metal alloys and metal oxides exhibiting low overpotentials in the hydrogen adsorption/absorption
* Corresponding author. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel./fax: þ86 22 27891371. E-mail address: [email protected] (C. Shi). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.151
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
reaction, are admixed to or incorporated in the carbon matrices [8,13,14,22,23]. Recently, various theoretical calculations revealed that graphene is an ideal hydrogen storage material [24e27]. In order to obtain a high efficient hydrogen storage material, in this work, we first investigated the electrochemical hydrogen storage properties of EG and TiO2-decorated EG which had not been reported in the literatures as far as we know, the electrochemical hydrogen adsorption/desorption properties of electrodes made of graphite, expanded graphite (EG), TiO2 and EG decorated with TiO2 nanoparticles (NPs) were studied. The electrochemical investigations including charge-discharge characteristics test, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) experiment were carried out. Also, we gave a detailed explanation about the function of TiO2 nanoparticles as catalyst and Cu powder as conductive agent, and proposed a model for the electrochemical hydrogen storage process of TiO2/EG. It was shown that the anatase TiO2 NPs enhanced the electrochemical activity of EG significantly.
2.
Experimental details
Expand graphite (EG) was obtained from graphite intercalation compounds (GICs), which was prepared by a two-step intercalation method. In the synthesis of GICs, the mixture of graphite, H2SO4, and K2Cr2O7 was put in water bath at 30 C in the weight ratio of 1: 4.29: 0.1. Then Fe(NO3)3 with 1.28 times the weight of the graphite was added in the mixture and kept in water bath. The resultant product was filtered with distilled water until neutral pH reached, and dried successively at 120 C for 5 h to obtain GICs. EG was acquired by heating the GICs to 800 C very quickly and holding for 30 s. In the preparation of EG decorated with TiO2 NPs, tetrabutyl titanate was added into ethanol, and the solution was marked as the base solution. Titrant, composed of de-ionized water, ethanol and HCl in the volume ratio of 1:1:2, was slowly dropped into the rapidly stirred base solution. Ethanol, as well as EG, was added into the mixed solution. The mixed solution was intensively stirred for 6 h, let stand in the atmosphere for 24 h, and then heated to 80 C until all liquid is evaporated. The resultants were calcined at 300 C, 400 C, and 500 C respectively for 2 h in air, which were marked as EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500. As a comparison, pure TiO2 NPs calcined at 500 C was prepared in the same conditions and marked as TiO2-500. SEM (Hitachi S-4800), TEM (Philips TECNAI G2 F20), XPS (PHI1600), and XRD (Rigaku D/MAX-2500) were used to characterize the samples. The specific surface area of the graphite and EG was determined by the Brunauer Emmett Teller (BET) method using nitrogen gas as an adsorbent. The nitrogen adsorption/desorption isotherm has been determined at 195.5 C. The electrochemical measurement was conducted by employing a three-electrode cell in an aqueous 6 M KOH medium with a sintered nickel electrode as the counter electrode and Hg/HgO as the reference electrode in the chargedischarge characteristics test. In the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments, the reference electrode was replaced by saturated
5763
calomel. All the experiments were conducted at 30 C. The work electrode was prepared as follows. The mixture of 20 mg as-prepared sample and 180 mg Cu powders was filled in a porous nickel substrate and pressed in a die into a platelet with 10 mm in diameter under 400 MPa pressure. The negative electrodes made of EG, TiO2-500, EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 are marked as EG/Cu, TiO2-500/Cu, EG-TiO2300/Cu, EG-TiO2-400/Cu, and EG-TiO2-500/Cu, respectively. For comparison, Cu electrode with porous nickel substrate filled only with copper powders, Ni electrode with porous nickel substrate filled only with nickel powders, and TiO2-500/Ni electrode with porous nickel substrate filled with 20 mg TiO2-500 and 180 mg nickel powders were prepared under the same conditions. The prepared working electrodes were charged at a current density of 100 mA/g for 30 min, and then discharged at a current density of 100 mA/g until the cutoff potential vs. Hg/ HgO was 0 V. When the discharge capacity tended to be stable after several cycles, the working electrodes were employed for the EIS and CV experiments after discharged fully in the last cycle. The EIS measurements were conducted in the range of 10 mHze100 kHz at the cut off potential. The cyclic voltammograms (CVs) were measured in a potential range of 1500 to 500 mV with the scan rate of 20 mV/s. Galvanostatic charge and discharge, EIS, and CV experiments were conducted in DC-5 battery testing instrument, advanced electrochemical system-PARSTAT2273, and Shanghai Zhengfang ZF-10, respectively.
3.
Results and discussion
As shown in Fig. 1(a) and (b), the typical lamellar structure of graphite disappears after intercalation and expansion. The pore structure in EG distributes uniformly with thin walls. As a result, the specific surface area is enlarged, and the average pore size decreases dramatically, as listed in Table 1. From the XPS spectra of EG shown in Fig. 2, it can be inferred that the groups inserted in the gap of GICs are fully removed in the expansion process, except for a little oxygen. As shown in Fig. 1(c) and (d), TiO2 NPs calcined at 500 C with 5e10 nm in diameter disperse uniformly on the nanosheets of EG. Fig. 3 shows the discharge capacity of EG, EG-TiO2-300, EGTiO2-400, and EG-TiO2-500 as a function of cycle number. The discharge capacities of different samples are listed in Table 2. It can be noticed that the discharge capacity of EG-TiO2 is much higher than that of EG, which implies TiO2 NPs plays important role for the electrochemical hydrogen storage capacity of EG. Furthermore, the discharge capacity of EG-TiO2 increases in a certain degree, as the calcined temperature increases from 300 to 500 C. When the calcined temperature is 500 C, the discharge capacity of 373.5 mAh/g is obtained which is 21.4 times as high as that of EG. The XRD spectra in Fig. 4 indicate the TiO2 NPs with an anatase structure. The degree of crystallinity of TiO2 NPs increases with increasing the calcined temperature. Since anatase TiO2 NPs have catalytic ability and can enhance the reduction of Hþ and the oxidation of H atoms in a sunlight adequate circumstance [28], the higher degree of crystallinity indicates the stronger catalytic ability of TiO2 NPs, which is
5764
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 1 e SEM images of graphite (a) and EG (b), and TEM images of TiO2-500 (c) and EG-TiO2-500 (d).
consistent to the tendency of the discharge capacity change shown in Fig. 3. Furthermore, the specific capacity increases with increasing cycle number for the TiO2-decorated EG, and after a certain number of activation cycles, the discharge capacity tend to stabilization. We think that the activation mechanism may be related to the structure change of EG induced by impressed current and catalysis of TiO2 nanoparticles, and it takes some time to complete the structure change. As a result, the activation positions increase and the electrochemical activity is enhanced. But this is just an assumption, the related studies are ongoing in our research group with both experiments and numerical simulations. As a comparison, the discharge capacity of Cu, TiO2-500/ Cu, Ni, and TiO2-500/Ni electrodes were also investigated. The discharge capacity of pure Cu powder is 3.4 mAh/g as listed in Table 2, indicating that the pure Cu has little hydrogen oxidation in the discharge process. The TiO2 NPs can improve the electrochemical activity of Cu. However, in comparison of the discharge capacity of Ni and TiO2-500/Ni electrodes, the TiO2 NPs give just a little contribution to the discharge
capacity of Ni electrodes. Therefore, the high discharge capacity of EG-TiO2/Cu electrode mainly results from the TiO2catalyzed electrochemical activity of both Cu and EG. Previous researches [11,29,30] also revealed that Cu powders can improve not only the conductivity, stability, oxidation resistance and discharge performance, but also the reaction surface area of electro-catalytic reaction and reactivity of the electrodes, which are helpful for the spread of hydrogen to increase hydrogen storage capacity of the electrode.
Table 1 e The average pore size (S ) and specific surface area (A) of graphite and EG.
Graphite EG
S (nm)
A (m2$g1)
32.9 11.8
3.4 30.0
Fig. 2 e XPS spectra of as-prepared EG.
5765
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 3 e Discharge capacity of EG-TiO2-300, EG-TiO2-400, EG-TiO2-500 and EG as a function of cycle number.
The impedance data in our study was analyzed with the simple model shown in Fig. 5(a), and the obtained results by electrochemical impedance spectroscopy measurements are shown in Fig. 5(b)e(d) in the form of a Nyquist plot. Each square marker in Fig. 5 represents experimental data and the hollow box marks are theoretical impedance spectra derived from the equivalent circuit proposed in Fig. 5(a). Experimental data have a good agreement with the theoretical impedance spectra, thus proposed equivalent circuit is validated in this reaction. Proposed circuit is designed with serial combination of solution resistance R1, contact resistance R2, reaction resistance R3, and adsorption resistance R4. Inner surface charge transfer impedance consists of space charge capacitance (CPE1), interfacial reaction impedance is modeled as a parallel combination of electrochemical double-layer capacitance (CPE2), and Warburg impedance (CPE3). In this circuit, the constant phase element CPEi ¼ [Yi( ju)]1 (i ¼ 1,2,3,.), where Yi is the admittance magnitude of CPE [31]. It is found that the reaction resistances of the TiO2 calcined 300, 400, and 500 C are 1.101, 0.1923, and 0.01 U, respectively. Thus, the reactive activity of the working electrode increases with the increase of the calcined temperature, which agrees with the results of the discharge capacity measurements. The CV experiment was conducted to analyze the electrochemical behavior about hydrogen adsorption and desorption of the EG and the TiO2-decorated EG electrodes. As can be seen in Fig. 6, the voltammetry loop curves are mainly composed of several fundamental hump regions. The hydrogen adsorption peak of EG is at around 1000 mV vs. SCE, while for the EG decorated with TiO2 the peak is at around 850 mV vs. SCE.
Fig. 4 e XRD spectra at 30 kV, 15 mA for Cu Ka (l [ 0.1543 nm) radiation, with a step size of 0.02 in 2q of EG-TiO2-300 (a), EG-TiO2-400 (b), and EG-TiO2-500 (c).
The less overvoltage for the TiO2-decorated samples indicates the easily reduction of Hþ which could be contributed to the preferable redox ability of TiO2 NPs. In the TiO2-decorated EG, negative charged centers may form on the TiO2 NPs which are profitable for the decomposition of water molecules and the formation of hydroxyl radical (H2O þ e / H þ OH). Thus, TiO2 NPs could improve the adsorption of hydrogen atoms on EG, the schematic diagram of this process is shown in stage (1)e(4) of Fig. 7. There are two anodic oxidation humps of hydrogen for both EG and TiO2-decorated EG, as shown in Fig. 6. The first one is within a potential domain comprised from 450 and 350 mV for the EG and the TiO2-decorated EGs, respectively, which corresponds to the desorption of hydrogen atom stored in EGs via the reaction CHad / C þ H. The second one is within a potential domain comprised from 200 and 100 mV for the EGs without and with TiO2 NPs decorated, respectively, corresponding to the oxidation of the absorbed hydrogen atoms in EGs which have strong chemical bonds with C atoms. When EG is decorated with TiO2 NPs, the two anodic oxidation humps both move along the positive potential direction compared with the as-received EG, which suggests stronger chemical sorption of hydrogen on TiO2-decorated EGs [32]. Furthermore, the oxidation peaks are significantly enhanced with the presence of TiO2 NPs and the increase of the calcined temperature. Thus, TiO2 NPs with high degree of crystallinity could improve the discharge capacity of EG.
Table 2 e Discharge capacity (C, mAh/g) of different negative electrodes. Electrode
Cu
Ni
TiO2 e 500/Cu
TiO2 e 500/Ni
EG/Cu
EG-TiO2 e 300/Cu
EG-TiO2 e 400/Cu
EG-TiO2 e 500/Cu
C
3.4
1.7
155.8
17.3
17.5
203.2
349.9
373.5
5766
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 5 e (a) schematic representation of the equivalent circuit for the EIS of electrode process. R1: Solution resistance, R2: Contact resistance, R3: Reaction resistance, R4: Adsorption resistance, and CPEi [ [Yi ( ju)]L1 (i [ 1,2,3,4.): Constant phase element; (b)e(d) Nyquist plots corresponding to EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 electrodes, respectively.
Based on the above discussion, we consider that the enhancement effect of TiO2 NPs on the electrochemical discharge capacity of EG results from the following mechanism. First, the photocatalytic property of TiO2 NPs benefits the reduction of Hþ in the charging process [28]. Due to preferable redox ability of TiO2 NPs, the negatively charged centers may form on the TiO2 NPs which benefit the decomposition of water molecules and the reduction of Hþ. On the other hand,
since the TiO2 NPs exist between the graphene-like layers of EG, the reduced H atoms can diffuse from TiO2 NPs to EG easily, like the spillover mechanism. The mechanism of the electrochemical hydrogen storage process with the presence of TiO2 NPs can be expressed as H2O þ e þ TiO2 / TiO2-Had þ OH
Fig. 6 e CVs of as-received EG, EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 electrodes.
Fig. 7 e Schematic diagram of electrochemical hydrogen storage process of TiO2/EG.
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
and TiO2-Had þ C $ C-Had þ TiO2. Second, electrochemical activity of Cu increases which benefits the increase of the discharge capacity of the electrode because of the presence of TiO2 NPs. It should be noted that the effect of TiO2 NPs on the electrochemical hydrogen storage behavior of carbon nanostructures is still unclear. For example, the hydrogen atoms adsorbed in multiwalled carbon nanotubes are oxidated more easily [28] if the multiwalled carbon nanotubes decorated with TiO2 NPs, while in EG there is a stronger chemical sorption of hydrogen, resulted in difficulty for oxidation of the hydrogen atoms adsorbed in EG. First-principles calculations may provide an in-depth understanding on the effect of TiO2 NPs in microscopic scale, and the related study is underway in our research group.
4.
Conclusions
In summary, the deposition of TiO2 NPs on the surface of EG by sol-gel method can improve the electrochemical hydrogen storage capacity of EG. The crystallinity of TiO2 NPs plays an important role and EG modified with the TiO2 NPs calcined at 500 C has the highest discharge capacity of 373.5 mAh/g, which indicates the great application potential of EG decorated with TiO2 NPs in the secondary cell and hydrogen storage materials. The high discharge capacity of EG-TiO2/Cu electrode is mainly resulted from the TiO2-catalyzed electrochemical activity of both Cu and EG, due to the preferable redox ability of TiO2 NPs.
Acknowledgments The authors acknowledge the financial support by the National Basic Research Program of China (2010CB934700), National Natural Science Foundation of China (No. 51071107), and the Innovation Foundation of Tianjin University.
references
[1] Thomas KM. Hydrogen adsorption and storage on porous materials. Catal Today 2007;120:389e98. [2] Stro¨bel R, Garche J, Moseley PT, Jo¨rissen L, Wolf G. Hydrogen storage by carbon materials. J Power Sources 2006;159: 781e801. [3] Hermosilla-Lara G, Momen G, Marty PH, Le Neindre B, Hassouni K. Hydrogen storage by adsorption on activated carbon: investigation of the thermal effects during the charging process. Int J Hydrogen Energy 2007;32:1542e53. [4] Li S, Pan W, Mao Z. A comparative study of the electrochemical hydrogen storage properties of activated carbon and well-aligned carbon nanotubes mixed with copper. Int J Hydrogen Energy 2005;30:643e8. [5] Lueking AD, Pan L, Narayanan DL, Clifford CEB. Effect of expanded graphite lattice in exfoliated graphite nanofibres on hydrogen storage. J Phys Chem B 2005;109:12710e7. [6] Jurewicz K. Influence of charging parameters on the effectiveness of electrochemical hydrogen storage in activated carbon. Int J Hydrogen Energy 2009;34:9431e5.
5767
[7] Ferna´ndez PS, Castro EB, Real SG, Martins ME. Electrochemical behaviour of single walled carbon nanotubes e hydrogen storage and hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:8115e26. [8] Wu MS, Hsu HL, Chiu HH, Lin YP. Fabrication of nickel boride-coated carbon nanotube films by electrophoresis and electroless deposition for electrochemical hydrogen storage. Int J Hydrogen Energy 2010;35:8993e9001. [9] Khoshnevisan B, Behpour M, Kaveh D. Optimization of hydrogen uptake in Ag-CNTs electrodes with chargedischarge cyclic currents. Physica B 2009;404:1733e6. [10] Hsieh CT, Chou YW, Lin JY. Fabrication and electrochemical activity of Ni-attached carbon nanotube electrodes for hydrogen storage in alkali electrolyte. Int J Hydrogen Energy 2007;32:3457e64. [11] Rajalakshmi N, Dhathathreyan KS, Govindaraj A, Satishkumar BC. Electrochemical investigation of singlewalled carbon nanotubes for hydrogen storage. Electrochim Acta 2000;45:4511e5. [12] Lee SM, An KH, Lee YH, Seifert G, Frauenheim TA. Hydrogen storage mechanism in single-walled carbon nanotubes. J Amer Chem Soc 2001;123:5059e63. [13] Gao XP, Lan Y, Pan GL, Wu F, Qu JQ, Song DY, et al. Electrochemical hydrogen storage by carbon nanotubes decorated with metallic nickel. Electrochem Solid-State Lett 2001;4:173e5. [14] Hirscher M, Becher M, Haluska M, Dettlaff-Weglikowska U, Quintel A, Duesberg GS, et al. Hydrogen storage in sonicated carbon materials. Appl Phys A 2001;72:129e32. ski JM, Scharff P, Pfa¨nder N, Cui S. Room [15] Skowron temperature electrochemical opening of carbon nanotubes followed by hydrogen storage. Adv Mater 2003;15:55e7. [16] Chen X, Zhang Y, Gao XP, Pan GL, Jiang XY, Qu JQ, et al. Electrochemical hydrogen storage of carbon nanotubes and carbon nanofibers. Int J Hydrogen Energy 2004;29:743e8. [17] Zu¨ttel A, Wenger P, Sudan P, Mauron P, Orimo S. Hydrogen density in nanostructured carbon, metals and complex materials. Mater Sci Eng B 2004;108:9e18. [18] Skowronski JM, Rozmanowski T. The CVD carbon fiber modification of nickel foam’s electrochemical properties of hydrogen sorption. Przem Chem 2006;85:1224e6. [19] Guo ZP, Ng SH, Wang JZ, Huang ZG, Liu HK, Too CO, et al. Electrochemical hydrogen storage in single-walled carbon nanotube paper. J Nanosci Nanotechnol 2006;6:713e8. [20] Feng H, Wei Y, Shao C, Lai Y, Feng S, Dong Z. Study on overpotential of the electrochemical hydrogen storage of multiwall carbon nanotubes. Int J Hydrogen Energy 2007;32: 1294e8. [21] Skowronski JM, Czerwinski A, Rozmanowski T, Rogulski Z, Krawczyk P. The study of hydrogen electrosorption in layered nickel foam/palladium/carbon nanofibers composite electrodes. Electrochim Acta 2007;52:5677e84. [22] Skowronski JM, Krawczyk P, Rozmanowski T, Urbaniak J. Electrochemical behavior of exfoliated NiCl2-graphite intercalation compound affected by hydrogen sorption. Energy Convers Manage 2008;49:2440e6. [23] Skowronski JM, Krawczyk P. Improved hydrogen sorption/ desorption capacity of exfoliated NiCl2-graphite intercalation compound effected by thermal treatment. Solid State Ionics 2010;181:653e8. [24] Reunchan P, Jhi SH. Metal-dispersed porous graphene for hydrogen storage. Appl Phys Lett 2011;98:093103. [25] Park HL, Yoo DS, Chung YC. Adsorption and diffusion of Li and Ni on graphene with boron substitution for hydrogen storage: ab-initio method. Jpn J Appl Phys 2011;50:06GJ2. [26] Lu Y, Feng YP. Adsorptions of hydrogen on graphene and other forms of carbon structures: first principle calculations. Nanoscale 2011;3:2444e53.
5768
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
[27] Lo´pez-Corral I, Germa´n E, Juan A, Volpe MA, Brizuela GP. DFT study of hydrogen adsorption on palladium decorated graphene. J Phys Chem C 2011;115:4315e23. [28] Liu EZ, Wang J, Li JJ, Shi CS, He CN, Du XW, et al. Enhanced electrochemical hydrogen storage capacity of multi-walled carbon nanotubes by TiO2 decoration. Int J Hydrogen Energy 2011;36:6739e43. [29] Nu¨tzenadel C, Zu¨ttel A, Chartouni D, Schlapbach L. Electrochemical storage of hydrogen in nanotube materials. Electrochem Solid-State Lett 1999;2:30e2.
[30] Liu J, Mao ZQ, Hao DH, Zhang XF, Xu CL, Wu DH. Electrochemical storage hydrogen of the aligned multiwalled carbon nanotubes. Chem J Chinese U 2004;2: 334e7. [31] Yun HJ, Lee H, Kim ND, Yi J. Characterization of photocatalytic performance of silver deposited TiO2 nanorods. Electrochem Commun 2009;11:363e6. [32] Chen X, Gao XP, Zhang H, Zhou Z, Hu WK, Pan GL, et al. Preparation and electrochemical hydrogen storage of boron nitride nanotubes. J Phys Chem B 2005;109:11525e9.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical hydrogen storage of expanded graphite decorated with TiO2 nanoparticles Yanmin Yu a,b, Naiqin Zhao a,b,c, Chunsheng Shi a,b,*, Chunnian He a,b, Enzuo Liu a,b, Jiajun Li a,b a
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China c Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin 300072, China b
article info
abstract
Article history:
The electrochemical hydrogen storage of expanded graphite (EG) decorated with TiO2
Received 17 November 2011
nanoparticles (NPs) calcined at different temperatures has been investigated with the
Received in revised form
galvanostatic charge and discharge method. The TiO2 NPs are deposited on and between
24 December 2011
the graphene-like nanosheets of EG by a sol-gel method. The morphology, structure,
Accepted 28 December 2011
composition, and specific surface area of the samples were characterized. The electro-
Available online 25 January 2012
chemical measurement reveals that the EG decorated with TiO2 NPs calcined at 500 C has a discharge capacity of 373.5 mAh/g which is 20 times higher than that of pure EG and quite
Keywords:
appealing for the battery applications. The mechanism of enhancement of the electro-
Expanded graphite
chemical activity for the TiO2-decorated EG could be attributed to the preferable redox
TiO2 nanoparticles
ability and photocatalytic property of TiO2 NPs.
Electrochemical hydrogen
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
storage capacity
1.
Introduction
Nowadays, hydrogen is recognized as an ideal fuel for energy conversion because of its high efficiency and its role in the reduction of air pollution [1]. However, due to the existence of major scientific challenges such as the lack of a safe, effective and cheap hydrogen storage system [2,3], it has not been so far used in great extent. To date, different methods of hydrogen storage have been applied but none of them have completely satisfied the proposed standard by the US Department of Energy (DOE) [2]. Hydrogen sorption in hydrogen storage materials can be performed by physical, chemical and electrochemical methods [2,4,5]. In the last case, there is no need to use high pressure, and the hydrogen sorption process occurs directly in
reserved.
electrode material at ambient conditions. Despite some discrepancies in the mechanism of hydrogen storage in various materials, it is without any doubt that hydrogen storage capacity in the electrochemical processes is strongly influenced by the properties of the electrode material used as hydrogen reservoir, such as chemical composition, crystalline, porous structure, and catalytic features in the reaction of hydrogen sorption [2]. Various forms of carbon, such as activated carbons [2,6], carbon nanofibres, and carbon nanotubes have been concerned as candidates for electrochemical hydrogen storage in last decade [2,7e21]. In order to enhance the hydrogen storage capacity for carbon materials, some transition metals (mainly Ni, Pt, Pd), transition metal alloys and metal oxides exhibiting low overpotentials in the hydrogen adsorption/absorption
* Corresponding author. School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China. Tel./fax: þ86 22 27891371. E-mail address: [email protected] (C. Shi). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.151
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
reaction, are admixed to or incorporated in the carbon matrices [8,13,14,22,23]. Recently, various theoretical calculations revealed that graphene is an ideal hydrogen storage material [24e27]. In order to obtain a high efficient hydrogen storage material, in this work, we first investigated the electrochemical hydrogen storage properties of EG and TiO2-decorated EG which had not been reported in the literatures as far as we know, the electrochemical hydrogen adsorption/desorption properties of electrodes made of graphite, expanded graphite (EG), TiO2 and EG decorated with TiO2 nanoparticles (NPs) were studied. The electrochemical investigations including charge-discharge characteristics test, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) experiment were carried out. Also, we gave a detailed explanation about the function of TiO2 nanoparticles as catalyst and Cu powder as conductive agent, and proposed a model for the electrochemical hydrogen storage process of TiO2/EG. It was shown that the anatase TiO2 NPs enhanced the electrochemical activity of EG significantly.
2.
Experimental details
Expand graphite (EG) was obtained from graphite intercalation compounds (GICs), which was prepared by a two-step intercalation method. In the synthesis of GICs, the mixture of graphite, H2SO4, and K2Cr2O7 was put in water bath at 30 C in the weight ratio of 1: 4.29: 0.1. Then Fe(NO3)3 with 1.28 times the weight of the graphite was added in the mixture and kept in water bath. The resultant product was filtered with distilled water until neutral pH reached, and dried successively at 120 C for 5 h to obtain GICs. EG was acquired by heating the GICs to 800 C very quickly and holding for 30 s. In the preparation of EG decorated with TiO2 NPs, tetrabutyl titanate was added into ethanol, and the solution was marked as the base solution. Titrant, composed of de-ionized water, ethanol and HCl in the volume ratio of 1:1:2, was slowly dropped into the rapidly stirred base solution. Ethanol, as well as EG, was added into the mixed solution. The mixed solution was intensively stirred for 6 h, let stand in the atmosphere for 24 h, and then heated to 80 C until all liquid is evaporated. The resultants were calcined at 300 C, 400 C, and 500 C respectively for 2 h in air, which were marked as EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500. As a comparison, pure TiO2 NPs calcined at 500 C was prepared in the same conditions and marked as TiO2-500. SEM (Hitachi S-4800), TEM (Philips TECNAI G2 F20), XPS (PHI1600), and XRD (Rigaku D/MAX-2500) were used to characterize the samples. The specific surface area of the graphite and EG was determined by the Brunauer Emmett Teller (BET) method using nitrogen gas as an adsorbent. The nitrogen adsorption/desorption isotherm has been determined at 195.5 C. The electrochemical measurement was conducted by employing a three-electrode cell in an aqueous 6 M KOH medium with a sintered nickel electrode as the counter electrode and Hg/HgO as the reference electrode in the chargedischarge characteristics test. In the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments, the reference electrode was replaced by saturated
5763
calomel. All the experiments were conducted at 30 C. The work electrode was prepared as follows. The mixture of 20 mg as-prepared sample and 180 mg Cu powders was filled in a porous nickel substrate and pressed in a die into a platelet with 10 mm in diameter under 400 MPa pressure. The negative electrodes made of EG, TiO2-500, EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 are marked as EG/Cu, TiO2-500/Cu, EG-TiO2300/Cu, EG-TiO2-400/Cu, and EG-TiO2-500/Cu, respectively. For comparison, Cu electrode with porous nickel substrate filled only with copper powders, Ni electrode with porous nickel substrate filled only with nickel powders, and TiO2-500/Ni electrode with porous nickel substrate filled with 20 mg TiO2-500 and 180 mg nickel powders were prepared under the same conditions. The prepared working electrodes were charged at a current density of 100 mA/g for 30 min, and then discharged at a current density of 100 mA/g until the cutoff potential vs. Hg/ HgO was 0 V. When the discharge capacity tended to be stable after several cycles, the working electrodes were employed for the EIS and CV experiments after discharged fully in the last cycle. The EIS measurements were conducted in the range of 10 mHze100 kHz at the cut off potential. The cyclic voltammograms (CVs) were measured in a potential range of 1500 to 500 mV with the scan rate of 20 mV/s. Galvanostatic charge and discharge, EIS, and CV experiments were conducted in DC-5 battery testing instrument, advanced electrochemical system-PARSTAT2273, and Shanghai Zhengfang ZF-10, respectively.
3.
Results and discussion
As shown in Fig. 1(a) and (b), the typical lamellar structure of graphite disappears after intercalation and expansion. The pore structure in EG distributes uniformly with thin walls. As a result, the specific surface area is enlarged, and the average pore size decreases dramatically, as listed in Table 1. From the XPS spectra of EG shown in Fig. 2, it can be inferred that the groups inserted in the gap of GICs are fully removed in the expansion process, except for a little oxygen. As shown in Fig. 1(c) and (d), TiO2 NPs calcined at 500 C with 5e10 nm in diameter disperse uniformly on the nanosheets of EG. Fig. 3 shows the discharge capacity of EG, EG-TiO2-300, EGTiO2-400, and EG-TiO2-500 as a function of cycle number. The discharge capacities of different samples are listed in Table 2. It can be noticed that the discharge capacity of EG-TiO2 is much higher than that of EG, which implies TiO2 NPs plays important role for the electrochemical hydrogen storage capacity of EG. Furthermore, the discharge capacity of EG-TiO2 increases in a certain degree, as the calcined temperature increases from 300 to 500 C. When the calcined temperature is 500 C, the discharge capacity of 373.5 mAh/g is obtained which is 21.4 times as high as that of EG. The XRD spectra in Fig. 4 indicate the TiO2 NPs with an anatase structure. The degree of crystallinity of TiO2 NPs increases with increasing the calcined temperature. Since anatase TiO2 NPs have catalytic ability and can enhance the reduction of Hþ and the oxidation of H atoms in a sunlight adequate circumstance [28], the higher degree of crystallinity indicates the stronger catalytic ability of TiO2 NPs, which is
5764
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 1 e SEM images of graphite (a) and EG (b), and TEM images of TiO2-500 (c) and EG-TiO2-500 (d).
consistent to the tendency of the discharge capacity change shown in Fig. 3. Furthermore, the specific capacity increases with increasing cycle number for the TiO2-decorated EG, and after a certain number of activation cycles, the discharge capacity tend to stabilization. We think that the activation mechanism may be related to the structure change of EG induced by impressed current and catalysis of TiO2 nanoparticles, and it takes some time to complete the structure change. As a result, the activation positions increase and the electrochemical activity is enhanced. But this is just an assumption, the related studies are ongoing in our research group with both experiments and numerical simulations. As a comparison, the discharge capacity of Cu, TiO2-500/ Cu, Ni, and TiO2-500/Ni electrodes were also investigated. The discharge capacity of pure Cu powder is 3.4 mAh/g as listed in Table 2, indicating that the pure Cu has little hydrogen oxidation in the discharge process. The TiO2 NPs can improve the electrochemical activity of Cu. However, in comparison of the discharge capacity of Ni and TiO2-500/Ni electrodes, the TiO2 NPs give just a little contribution to the discharge
capacity of Ni electrodes. Therefore, the high discharge capacity of EG-TiO2/Cu electrode mainly results from the TiO2catalyzed electrochemical activity of both Cu and EG. Previous researches [11,29,30] also revealed that Cu powders can improve not only the conductivity, stability, oxidation resistance and discharge performance, but also the reaction surface area of electro-catalytic reaction and reactivity of the electrodes, which are helpful for the spread of hydrogen to increase hydrogen storage capacity of the electrode.
Table 1 e The average pore size (S ) and specific surface area (A) of graphite and EG.
Graphite EG
S (nm)
A (m2$g1)
32.9 11.8
3.4 30.0
Fig. 2 e XPS spectra of as-prepared EG.
5765
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 3 e Discharge capacity of EG-TiO2-300, EG-TiO2-400, EG-TiO2-500 and EG as a function of cycle number.
The impedance data in our study was analyzed with the simple model shown in Fig. 5(a), and the obtained results by electrochemical impedance spectroscopy measurements are shown in Fig. 5(b)e(d) in the form of a Nyquist plot. Each square marker in Fig. 5 represents experimental data and the hollow box marks are theoretical impedance spectra derived from the equivalent circuit proposed in Fig. 5(a). Experimental data have a good agreement with the theoretical impedance spectra, thus proposed equivalent circuit is validated in this reaction. Proposed circuit is designed with serial combination of solution resistance R1, contact resistance R2, reaction resistance R3, and adsorption resistance R4. Inner surface charge transfer impedance consists of space charge capacitance (CPE1), interfacial reaction impedance is modeled as a parallel combination of electrochemical double-layer capacitance (CPE2), and Warburg impedance (CPE3). In this circuit, the constant phase element CPEi ¼ [Yi( ju)]1 (i ¼ 1,2,3,.), where Yi is the admittance magnitude of CPE [31]. It is found that the reaction resistances of the TiO2 calcined 300, 400, and 500 C are 1.101, 0.1923, and 0.01 U, respectively. Thus, the reactive activity of the working electrode increases with the increase of the calcined temperature, which agrees with the results of the discharge capacity measurements. The CV experiment was conducted to analyze the electrochemical behavior about hydrogen adsorption and desorption of the EG and the TiO2-decorated EG electrodes. As can be seen in Fig. 6, the voltammetry loop curves are mainly composed of several fundamental hump regions. The hydrogen adsorption peak of EG is at around 1000 mV vs. SCE, while for the EG decorated with TiO2 the peak is at around 850 mV vs. SCE.
Fig. 4 e XRD spectra at 30 kV, 15 mA for Cu Ka (l [ 0.1543 nm) radiation, with a step size of 0.02 in 2q of EG-TiO2-300 (a), EG-TiO2-400 (b), and EG-TiO2-500 (c).
The less overvoltage for the TiO2-decorated samples indicates the easily reduction of Hþ which could be contributed to the preferable redox ability of TiO2 NPs. In the TiO2-decorated EG, negative charged centers may form on the TiO2 NPs which are profitable for the decomposition of water molecules and the formation of hydroxyl radical (H2O þ e / H þ OH). Thus, TiO2 NPs could improve the adsorption of hydrogen atoms on EG, the schematic diagram of this process is shown in stage (1)e(4) of Fig. 7. There are two anodic oxidation humps of hydrogen for both EG and TiO2-decorated EG, as shown in Fig. 6. The first one is within a potential domain comprised from 450 and 350 mV for the EG and the TiO2-decorated EGs, respectively, which corresponds to the desorption of hydrogen atom stored in EGs via the reaction CHad / C þ H. The second one is within a potential domain comprised from 200 and 100 mV for the EGs without and with TiO2 NPs decorated, respectively, corresponding to the oxidation of the absorbed hydrogen atoms in EGs which have strong chemical bonds with C atoms. When EG is decorated with TiO2 NPs, the two anodic oxidation humps both move along the positive potential direction compared with the as-received EG, which suggests stronger chemical sorption of hydrogen on TiO2-decorated EGs [32]. Furthermore, the oxidation peaks are significantly enhanced with the presence of TiO2 NPs and the increase of the calcined temperature. Thus, TiO2 NPs with high degree of crystallinity could improve the discharge capacity of EG.
Table 2 e Discharge capacity (C, mAh/g) of different negative electrodes. Electrode
Cu
Ni
TiO2 e 500/Cu
TiO2 e 500/Ni
EG/Cu
EG-TiO2 e 300/Cu
EG-TiO2 e 400/Cu
EG-TiO2 e 500/Cu
C
3.4
1.7
155.8
17.3
17.5
203.2
349.9
373.5
5766
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
Fig. 5 e (a) schematic representation of the equivalent circuit for the EIS of electrode process. R1: Solution resistance, R2: Contact resistance, R3: Reaction resistance, R4: Adsorption resistance, and CPEi [ [Yi ( ju)]L1 (i [ 1,2,3,4.): Constant phase element; (b)e(d) Nyquist plots corresponding to EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 electrodes, respectively.
Based on the above discussion, we consider that the enhancement effect of TiO2 NPs on the electrochemical discharge capacity of EG results from the following mechanism. First, the photocatalytic property of TiO2 NPs benefits the reduction of Hþ in the charging process [28]. Due to preferable redox ability of TiO2 NPs, the negatively charged centers may form on the TiO2 NPs which benefit the decomposition of water molecules and the reduction of Hþ. On the other hand,
since the TiO2 NPs exist between the graphene-like layers of EG, the reduced H atoms can diffuse from TiO2 NPs to EG easily, like the spillover mechanism. The mechanism of the electrochemical hydrogen storage process with the presence of TiO2 NPs can be expressed as H2O þ e þ TiO2 / TiO2-Had þ OH
Fig. 6 e CVs of as-received EG, EG-TiO2-300, EG-TiO2-400, and EG-TiO2-500 electrodes.
Fig. 7 e Schematic diagram of electrochemical hydrogen storage process of TiO2/EG.
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
and TiO2-Had þ C $ C-Had þ TiO2. Second, electrochemical activity of Cu increases which benefits the increase of the discharge capacity of the electrode because of the presence of TiO2 NPs. It should be noted that the effect of TiO2 NPs on the electrochemical hydrogen storage behavior of carbon nanostructures is still unclear. For example, the hydrogen atoms adsorbed in multiwalled carbon nanotubes are oxidated more easily [28] if the multiwalled carbon nanotubes decorated with TiO2 NPs, while in EG there is a stronger chemical sorption of hydrogen, resulted in difficulty for oxidation of the hydrogen atoms adsorbed in EG. First-principles calculations may provide an in-depth understanding on the effect of TiO2 NPs in microscopic scale, and the related study is underway in our research group.
4.
Conclusions
In summary, the deposition of TiO2 NPs on the surface of EG by sol-gel method can improve the electrochemical hydrogen storage capacity of EG. The crystallinity of TiO2 NPs plays an important role and EG modified with the TiO2 NPs calcined at 500 C has the highest discharge capacity of 373.5 mAh/g, which indicates the great application potential of EG decorated with TiO2 NPs in the secondary cell and hydrogen storage materials. The high discharge capacity of EG-TiO2/Cu electrode is mainly resulted from the TiO2-catalyzed electrochemical activity of both Cu and EG, due to the preferable redox ability of TiO2 NPs.
Acknowledgments The authors acknowledge the financial support by the National Basic Research Program of China (2010CB934700), National Natural Science Foundation of China (No. 51071107), and the Innovation Foundation of Tianjin University.
references
[1] Thomas KM. Hydrogen adsorption and storage on porous materials. Catal Today 2007;120:389e98. [2] Stro¨bel R, Garche J, Moseley PT, Jo¨rissen L, Wolf G. Hydrogen storage by carbon materials. J Power Sources 2006;159: 781e801. [3] Hermosilla-Lara G, Momen G, Marty PH, Le Neindre B, Hassouni K. Hydrogen storage by adsorption on activated carbon: investigation of the thermal effects during the charging process. Int J Hydrogen Energy 2007;32:1542e53. [4] Li S, Pan W, Mao Z. A comparative study of the electrochemical hydrogen storage properties of activated carbon and well-aligned carbon nanotubes mixed with copper. Int J Hydrogen Energy 2005;30:643e8. [5] Lueking AD, Pan L, Narayanan DL, Clifford CEB. Effect of expanded graphite lattice in exfoliated graphite nanofibres on hydrogen storage. J Phys Chem B 2005;109:12710e7. [6] Jurewicz K. Influence of charging parameters on the effectiveness of electrochemical hydrogen storage in activated carbon. Int J Hydrogen Energy 2009;34:9431e5.
5767
[7] Ferna´ndez PS, Castro EB, Real SG, Martins ME. Electrochemical behaviour of single walled carbon nanotubes e hydrogen storage and hydrogen evolution reaction. Int J Hydrogen Energy 2009;34:8115e26. [8] Wu MS, Hsu HL, Chiu HH, Lin YP. Fabrication of nickel boride-coated carbon nanotube films by electrophoresis and electroless deposition for electrochemical hydrogen storage. Int J Hydrogen Energy 2010;35:8993e9001. [9] Khoshnevisan B, Behpour M, Kaveh D. Optimization of hydrogen uptake in Ag-CNTs electrodes with chargedischarge cyclic currents. Physica B 2009;404:1733e6. [10] Hsieh CT, Chou YW, Lin JY. Fabrication and electrochemical activity of Ni-attached carbon nanotube electrodes for hydrogen storage in alkali electrolyte. Int J Hydrogen Energy 2007;32:3457e64. [11] Rajalakshmi N, Dhathathreyan KS, Govindaraj A, Satishkumar BC. Electrochemical investigation of singlewalled carbon nanotubes for hydrogen storage. Electrochim Acta 2000;45:4511e5. [12] Lee SM, An KH, Lee YH, Seifert G, Frauenheim TA. Hydrogen storage mechanism in single-walled carbon nanotubes. J Amer Chem Soc 2001;123:5059e63. [13] Gao XP, Lan Y, Pan GL, Wu F, Qu JQ, Song DY, et al. Electrochemical hydrogen storage by carbon nanotubes decorated with metallic nickel. Electrochem Solid-State Lett 2001;4:173e5. [14] Hirscher M, Becher M, Haluska M, Dettlaff-Weglikowska U, Quintel A, Duesberg GS, et al. Hydrogen storage in sonicated carbon materials. Appl Phys A 2001;72:129e32. ski JM, Scharff P, Pfa¨nder N, Cui S. Room [15] Skowron temperature electrochemical opening of carbon nanotubes followed by hydrogen storage. Adv Mater 2003;15:55e7. [16] Chen X, Zhang Y, Gao XP, Pan GL, Jiang XY, Qu JQ, et al. Electrochemical hydrogen storage of carbon nanotubes and carbon nanofibers. Int J Hydrogen Energy 2004;29:743e8. [17] Zu¨ttel A, Wenger P, Sudan P, Mauron P, Orimo S. Hydrogen density in nanostructured carbon, metals and complex materials. Mater Sci Eng B 2004;108:9e18. [18] Skowronski JM, Rozmanowski T. The CVD carbon fiber modification of nickel foam’s electrochemical properties of hydrogen sorption. Przem Chem 2006;85:1224e6. [19] Guo ZP, Ng SH, Wang JZ, Huang ZG, Liu HK, Too CO, et al. Electrochemical hydrogen storage in single-walled carbon nanotube paper. J Nanosci Nanotechnol 2006;6:713e8. [20] Feng H, Wei Y, Shao C, Lai Y, Feng S, Dong Z. Study on overpotential of the electrochemical hydrogen storage of multiwall carbon nanotubes. Int J Hydrogen Energy 2007;32: 1294e8. [21] Skowronski JM, Czerwinski A, Rozmanowski T, Rogulski Z, Krawczyk P. The study of hydrogen electrosorption in layered nickel foam/palladium/carbon nanofibers composite electrodes. Electrochim Acta 2007;52:5677e84. [22] Skowronski JM, Krawczyk P, Rozmanowski T, Urbaniak J. Electrochemical behavior of exfoliated NiCl2-graphite intercalation compound affected by hydrogen sorption. Energy Convers Manage 2008;49:2440e6. [23] Skowronski JM, Krawczyk P. Improved hydrogen sorption/ desorption capacity of exfoliated NiCl2-graphite intercalation compound effected by thermal treatment. Solid State Ionics 2010;181:653e8. [24] Reunchan P, Jhi SH. Metal-dispersed porous graphene for hydrogen storage. Appl Phys Lett 2011;98:093103. [25] Park HL, Yoo DS, Chung YC. Adsorption and diffusion of Li and Ni on graphene with boron substitution for hydrogen storage: ab-initio method. Jpn J Appl Phys 2011;50:06GJ2. [26] Lu Y, Feng YP. Adsorptions of hydrogen on graphene and other forms of carbon structures: first principle calculations. Nanoscale 2011;3:2444e53.
5768
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 7 ( 2 0 1 2 ) 5 7 6 2 e5 7 6 8
[27] Lo´pez-Corral I, Germa´n E, Juan A, Volpe MA, Brizuela GP. DFT study of hydrogen adsorption on palladium decorated graphene. J Phys Chem C 2011;115:4315e23. [28] Liu EZ, Wang J, Li JJ, Shi CS, He CN, Du XW, et al. Enhanced electrochemical hydrogen storage capacity of multi-walled carbon nanotubes by TiO2 decoration. Int J Hydrogen Energy 2011;36:6739e43. [29] Nu¨tzenadel C, Zu¨ttel A, Chartouni D, Schlapbach L. Electrochemical storage of hydrogen in nanotube materials. Electrochem Solid-State Lett 1999;2:30e2.
[30] Liu J, Mao ZQ, Hao DH, Zhang XF, Xu CL, Wu DH. Electrochemical storage hydrogen of the aligned multiwalled carbon nanotubes. Chem J Chinese U 2004;2: 334e7. [31] Yun HJ, Lee H, Kim ND, Yi J. Characterization of photocatalytic performance of silver deposited TiO2 nanorods. Electrochem Commun 2009;11:363e6. [32] Chen X, Gao XP, Zhang H, Zhou Z, Hu WK, Pan GL, et al. Preparation and electrochemical hydrogen storage of boron nitride nanotubes. J Phys Chem B 2005;109:11525e9.