Hydrogen transport in single-walled carbon nanotubes encapsulated by palladium

international journal of hydrogen energy 37 (2012) 5676–5685

Available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Hydrogen transport in single-walled carbon nanotubes encapsulated by palladium Andrei G. Lipson*, Boris F. Lyakhov, Eugenyi I. Saunin, Lyudmila N. Solodkova, Aslan Y. Tsivadze A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31 Leninsky prospect, Bldg. 4, 119991 Moscow, Russian Federation

article info

abstract

Article history:

Hydrogen transport and loading into single-walled carbon nanotubes (SWCNT) encapsu-

Received 15 October 2009

lated by thin Pd layers onto a massive Pd substrate were studied using a complex of

Accepted 28 December 2009

vacuum thermal desorption, cyclic voltammetry and ESR methods. By adding SWCNT the

Available online 9 February 2012

hydrogen capacity of the Pd–SWCNT composite under electrochemical loading increases as much as 25% relative to Palladium metal alone. This provides moderate growth in the

Keywords:

gravimetric capacity of the total composite based on a massive Pd substrate. The hydrogen

Hydrogen transport

binding energy in the SWCNT (eH ¼ 0.075 eV/H-atom), estimated by studies of hydrogen

Hydrogen loading

transport in the Pd–SWCNT composite was lower than predicted for the Pd–SWCNT

Single-walled carbon nanotubes

complex, but higher than the physisorption on the bare SWCNT. Using ESR we established

Palladium coating

that the Pd–Cx e-complexes formed at the wall of nanotube could be considered as hydrogen adsorption site, providing both high net gravimetric capacity and low hydrogen binding energy in the Pd encapsulated SWCNT. The results obtained provide an opportunity to probe a condensed hydrogen phase of nanometer scale confined in SWCNT, encapsulated by transition metals. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The potential use of single-walled carbon nanotubes (SWCNT) for effective hydrogen storage [1] has been actively explored during the last decade. Previous studies demonstrate the hydrogen capacity of SWCNT range from 0.25 to 20-wt.% [1–9], but the reproducibility and the reversibility of H2 storage in SWCNT still remains insufficient. Nanotube samples contain impurities (i.e., amorphous carbon, catalyst, hydrocarbons, water, etc.), which influences hydrogen adsorption in an uncontrolled manner, leading to large systematic errors. Moreover, a variation in the SWCNT diameter and length may affect the hydrogen capacity [5]. Reports suggest that physisorption of hydrogen molecules into the SWCNT should be

strictly limited by steric factors representing impurities blocking the SWCNT opening [6,7]. Atomic hydrogen impingement allows a better hydrogen uptake due to enhanced H-permeation into the SWCNT. The result of loading SWCNT with an atomic hydrogen beam was reported by Nikitin et al. [9]. A 5.5% wt. H2 gravimetric capacity was achieved using this technique, indicating hydrogen chemisorption into the SWCNT. Atomic hydrogen creates C–H chemical bonds which bind with the carbon atom at the SWCNT wall. However, the C–H bonds can be broken only at t ¼ 600  C [8,9]. In general, both physisorption and chemisorption could be a potential hydrogen storage mechanism in SWCNT. According to existing theoretical prediction and experimental studies

* Corresponding author. E-mail address: [email protected] (A.G. Lipson). 0360-3199/$ – see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.173

international journal of hydrogen energy 37 (2012) 5676–5685

the strength of hydrogen binding in the nanotubes was found in the range of 0.03 (for pure physisorption case) to 2.5 eV/Hatom (for pure chemisorption case), depending on the adsorption type. Note that pure atomic hydrogen chemisorption on SWCNT cannot provide hydrogen loading with H/C ratio exceeding 1.0 [10]. On the other hand, the ability to enhance hydrogen physisorption in bare SWCNT using high hydrogen pressure, except the preceding mentioned steric factor problem would be limited by the small binding energy between the hydrogen molecule and the nanotube. Adsorption and dissociation of hydrogen molecules on bare and transition metals (including Pd) in functionalized SWCNT has recently been studied in details by Dag et al. using first principle plane wave method [11]. In fact the binding energy of hydrogen molecule physisorbed at the outer surface of the bare SWCNT were weak (3 w 60 meV/H2 molecule), and could not be enhanced significantly by, increasing the curvature of the surface (e.g., decreasing SWCNT diameter) through radial deformation. The interaction between the H2 molecule at ambient pressure within the inner surface of a small radius bare SWCNT is repulsive, preventing the H2 molecules from entering the SWCNT interior. The character of hydrogen bonding changes drastically when SWCNT or organic compounds are functionalized by the adsorption of transition metal atoms. So, recently it was theoretically predicted [12] and experimentally shown that some organic complexes of transition metals (in particular Ti-ethylene) are able to store up to 14% wt. of hydrogen at relatively low energy of C–H binding [13]. A single H2 molecule can be chemisorbed in the Pt/Pd atom at the SWCNT wall either dissociatively or molecularly. The dissociative adsorption was energetically favorable and was followed by the weakening of metal SWCNT bonds. Palladium’s importance is that it promotes dissociative hydrogen adsorption with a relatively small binding energy (which is in the range between physisorption and chemisorption of hydrogen [11]). There is, however, another approach to provide massive atomic hydrogen permeation and adsorption into the SWCNT using encapsulation of SWCNT by transition metal with high hydrogen solubility, transparency and diffusivity. Palladium metals possess the unique advantage of dissociating molecular hydrogen into an atomic form while simultaneously exhibiting high H-diffusivity of a quantum character [14]. Thus, if SWCNT were embedded in the Pdmatrix, the result would be an atomic hydrogen/proton flux permeating into the nanotubes under a high kbar pressure range. Using this approach we demonstrated that the hydrogen loading of the composite material representing SWCNT encapsulated by electroplating Pd on top of the massive Pd substrate with respect to the Pd matrix can reach x ¼ H/Pd w 0.9 [15]. This suggests a high ‘‘net’’ H2 capacity of SWCNT, assuming that whole excessive hydrogen above the loading ratio H/Pd w 0.7 is absorbed by nanotubes. Simultaneously, we found that activation energy of hydrogen desorption from Pd–SWCNT composite is 3H ¼ 0.08  0.02 eV per H-atom, slightly exceeding physisorption energy and indicating a weak hydrogen binding in SWCNT. In present paper we explored a new synergistic approach based upon combining the unique Pd transport properties

5677

(high H-flux diffusivity and pressure) with SWCNT (high potential capacity) [15]. Here a hydrogen transport in SingleWalled Carbon Nanotubes (SWCNT) encapsulated by thin Pd layers onto a massive Pd substrate using vacuum thermal desorption and cyclic voltammetry techniques has been studied. We showed that by adding SWCNT the hydrogen capacity of the Pd–SWCNT composite increases under electrochemical loading up to 25% relative to the Pd metal alone. This provides a moderate growth in the gravimetric capacity of the total composite based on a massive Pd substrate. However, with regards to the added SWCNT, the ‘‘net’’ nanotube storage capacity could exceed the ratio H/C w 1.0. The net SWCNT capacity in the Pd þ SWCNT system depends on the volumetric ratio between the Pd and the SWCNT components (V (Pd)/V (SWCNT)) and reaches the limit value at V (Pd)/V (SWCNT)  10. The hydrogen binding energy in the SWCNT (3H ¼ 0.075 eV/H-atom), estimated by studies of hydrogen transport in the Pd–SWCNT composite was lower than predicted for the Pd–SWCNT complex, but higher than physisorption on the bare SWCNT [11]. It was established that the Pd–Cx p-complexes formed at the SWCNT walls could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT. These obtained results open a possibility to probe a dense hydrogen condensed phase within the nanometer scale confined into SWCNT, encapsulated by the transition metal and also suggest some special applications of the Pd–SWCNT–Pd composite material.

2.

Experimental technique

Two types of SWCNT with the same diameter (1.3 nm) and lengths (l w 1.0 mm), but various purities were evaluated: (1) HiPco Bucky tubes (CNI), lot #79, manufactured by Carbon Technologies Inc., (length range 0.5–1.5 mm, purity 95%); and (2) Carbon-C Inc., (l w 1.0 mm, purity > 98%;). The 25–100 mm thick cold-rolled Pd foils (Johnson and Mathew, 99.95%), with an area of S ¼ 1.0–2.0 cm2 were used as the substrates. Prior to deposition, the SWCNT (1) were etched in concentrated nitric acid, for 3 h at T ¼ 300 K. The high purity SWCNT (2) have not been etched at all. Preparation of the Pd foils is described in [16]. A thin Pd layer of w0.6 mm thick was electroplated onto both sides of the Pd foil (Pd0 layer) using the PdCl2 electrolyte [17]. The gel of the SWCNT was extracted from the water suspension with (pH ¼ 6.5), and applied by dipping the top of the Pd0 /Pd/Pd0 sample into the gel (the specific mass of deposited SWCNT ms w 0.4 mg/cm2). To encapsulate the SWCNT in the Pd matrix, an additional thin layer of Pd was electroplated onto the SWCNT, coating both sides. The synthesized Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 composite foil for vacuum thermal desorption measurements was subjected to a final annealing in a high vacuum at t ¼ 500  C for 2 h. The electrochemical hydrogen loading procedures and vacuum thermal desorption measurements of hydrogen content in the samples are described elsewhere [15]. In our special experiments for independent in situ electrochemical analysis of the hydrogen gravimetric capacity in Pd–SWCNT–Pd composite the chronoamperometry (CA) technique [18] was applied. The same types of samples used

5678

international journal of hydrogen energy 37 (2012) 5676–5685

for isothermic vacuum desorption measurements were employed. The hydrogen content was determined by integration of the current–time (I–t) curve representing the charge (Q) spent on the hydrogen oxidation in the sample that underwent anodic polarization. The anodic polarization (i.e., hydrogen oxidation process) was installed automatically, immediately (in 2 ms) after the cathode loading of the Pd, the reference or the cathodes (using galvanostatic mode at j ¼ 10 mA/cm2, in 1 M NaOH) was completed. After the cathode loading, the hydrogen oxidation process was carried out either at constant potential (E ¼ 0.65 V vs. Ag/AgCl) or at the potentiodynamic condition (the sweep scanning rate of 0.3 mV s1), both methods showed similar results. The low scanning rate in the range from 1.0 to 0.2 V provided full oxidation of the hydrogen absorbed by the cathode. The net hydrogen capacity (CH) of SWCNT in the composite was determined from the simple relation CH ¼ (DQ  mp)/(e  mSWCNT), where DQ – is the charge spent on hydrogen oxidation in the cathode polarized composite sample by subtracting the charge spent for the reference Pd sample oxidation (the reference Pd of the same mass as the was cathode charged under the same condition as the composite cathode); mp – is the proton mass, e – is the electron charge and mSWCNT – is the weight of the SWCNT in the composite sample. Cyclic voltammetry (CVA) was used to study the kinetics of hydrogen reduction and oxidation in the reference Pd as well as in the and the Pd–SWCNT–Pd composites. The working electrode was either a carbon disk with the area of (S ¼ 3  102 cm2) (spectral grade, supplied by Metrohm company, Swiss) or a glassy carbon disk (S ¼ 7  102 cm2) covered with palladium. The palladium layers of 1–4 mm thick were deposited electrochemically at a constant current density ( j ¼ 5 mA cm2) from an acidic (4 M HCl) PdCl2 solution. Next, the microscopic amount of etched SWCNT (type (2)) was dipped into the water suspension on the top of the Pd film

(the specific mass of SWCNT ms ¼ 0.40  0.15 mg/cm2of the Pd coating). After drying the SWCNT another layer of Pd metal was added on top of the SWCNT in order to encapsulate the nanotubes. Notice, that all the cathodes prepared for voltammetric measurements were not subjected to thermal treatment above room temperature. The electrode was then placed in the 1 M NaOH solution to record the CVA in the range of the hydrogen potential consistent with its reduction and oxidation. CVA was performed using a platinum wire as a counter electrode and a silver chloride (Ag/AgCl) as the reference electrode. The potential range of electrode polarization (E ) varied from –0.2 V to –1.3 V (Ag/AgCl), with a linear sweep of 0.5 mV s1. Shown in [19], the presence of the air atmosphere did not affect the measurement of hydrogen concentration absorbed by the Pd. In order to test the effect of high surface area carbon on the Pd loading in the Pd matrix, the composite containing a high surface area (SC ¼ 200 m2/g) of carbon powder, instead of SWCNT was also probed by CVA technique. The reference composite was prepared at the surface of the carbon-working electrode, in the same manner as the samples. The ESR spectra of the Pd–SWCNT composite materials as well as the spectra of their individual components (including original SWCNT powders and the Pd0 –Pd–Pd0 or [Pd0 –Pd–Pd0 ]:Hx reference foils) were detected with the use of a Radiopan SE/X 2547 ESR-spectrometer at the frequency of 9.4 GHz and microwave power of 8–10 mW in the temperature range of 77–293 K.

3.

Experimental results

3.1.

Vacuum thermal desorption

The results on measurements of hydrogen loading of SWCNT encapsulated by Pd in the Pd/SWCNT/Pd composite are shown in Table 1. The hydrogen volume released in the composite

Table 1 – Volumetric hydrogen concentration in the Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd composite and the Pd0 /Pd/Pd0 reference samples loaded by electrolysis at a constant temperature, T [ 290 K. The electrolysis charge transferred through the cathode is Q [ 45 C/cm2. #

Sample type

Mass Pd[g]

Desorbed H2 volume, [cm3]

¼[H]/[Pd]

1 2

Pd0 /Pd/Pd0 Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 -(1a), M(SWCNT) ¼ 3.2 mg Pd0 /Pd/Pd0 Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 -(1*), M(SWCNT) ¼ 1.8 mg Pd0 /Pd/Pd0 Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 -(1a), M(SWCNT) ¼ 5.5 mg Pd0 /Pd/Pd0 Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 -(1a), M(SWCNT) ¼ 3.2 mg Pt-Pd0 -SWCNT-Pd0 (1a), M(SWCNT) ¼ 1.7 mg Cu-Pd0 -SWCNT-Pd0 (1a), M(SWCNT) ¼ 3.6 mg

0.275 0.274

19.75  0.21 24.88  0.39

0.68  0.02 0.86  0.03

0.145 0.148

10.13  0.24 10.04  0.20

0.66  0.02 0.65  0.02

0.405 0.403

28.26  0.48 34.91  0.57

0.66  0.01 0.82  0.03

0.203 0.204

14.40  0.20 17.04  0.29

0.67  0.02 0.79  0.03

0.011 0.032

0.48  0.05 1.80  0.06

0.51  0.02 0.54  0.02

3 4 5 6 7 8 9 10

The samples of Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 were synthesized with the SWCNT of types (1) – CNI. The sample of Pd0 /SWCNT/Pd0 /Pd/Pd0 / SWCNT/Pd0 -(1*) is prepared with non-etched SWCNT of type (1); ¼ [H]/[Pd] ¼ [2 NLV]/(mNA/MA), where NL – is the Loschmidt number; V – is the desorbed H2 volume; NA – is the Avogadro number, MA –is the atomic weight of Pd; m- is the Pd mass. No hydrogen above the normal H/Pd electrochemical loading ratio (x < 0.7) was found in the Pd–SWCNT–Pd composites deposited onto Pt and Cu substrates (lines 7 and 8).

international journal of hydrogen energy 37 (2012) 5676–5685

cathode with etched SWCNT of CNI type encapsulated by Pd onto the Pd foil substrate at various volumetric Pd/SWCNT ratio (0 < VPd/VSWCNT < 8) – (Table 1, lines 2,6,8) exhibits approximately 20%–25% excess hydrogen, compared to the reference samples (Table 1, lines 1,5 and 7). The total H/Pd loading ratio corresponding to high net H2 value is always H/Pd  0.8. Note that the magnitude of the H/Pd ratio of x  0.7 is normally characterized as the highest Pd loading (PdHx b-phase) that can be achieved electrochemically at a relatively low current density ( j < 100 mA/cm2) in an alkaline solution [20,21]. The samples with high Pd/SWCNT volumetric ratio (VPd/ VSWCNT > 5) demonstrate rather larger hydrogen loading in the range of 0.8–0.9 H/Pd. In contrast, the original non-etched SWCNT even at the VPd/VSWCNT z 7.0 show no H2 permeation (Table 1, line 4). This effect is probably referred to the fact that the etching process clears blocked tube openings and allows permeation of the hydrogen atoms inside the SWCNT, which facilitates high net loading process in the SWCNT. At the same time, the samples with low volumetric Pd/SWCNT ratio (VPd/ VSWCNT  1), representing Pd encapsulated SWCNT deposited onto a ‘‘neutral’’ substrates of Cu and Pt (that do not absorb hydrogen) did not show excessive hydrogen loading in SWCNT (Table 1, lines 8 and 10). The loading ratio x ¼ H/Pd w 0.5 shows that the real density of electroplated Pd is well below those of crystalline Pd (r < 12 g/cm3 due to the porous structure of the electroplated Pd layer). The last experiments with the Pd layers deposited on Cu and Pt bases showed that SWCNT encapsulated by Pd on top of inert (with respect to hydrogen absorption) substrate cannot be loaded with hydrogen.

3.2.

Voltammetric measurements

The cyclic voltammograms, for the carbon (curve 1) and for the Pd coatings of various thickness (curves 2 and 3) are 0.4

3

0.2

2

J,Am

4

0

4 2 1 3 -0.2

-0.4 -1.6

-1.2

-0.8

-0.4

0

E,V

Fig. 1 – Cyclic voltammograms of hydrogen reduction and oxidation at the surface of carbon (1) and palladium (2 and 3); Pd–C–Pd composite (4), respectively. The Pd thickness: 1.1 mm (2); and 2.3 mm (3). The potential scanning rate is 0.5 mV sL1. The base is a carbon disk (S [ 3 3 10L2 cm2).

5679

presented in (Fig. 1). The hydrogen reduction in the Pd occurred near the potential E w 1.0 V. Meanwhile, the hydrogen reduction potential at the surface of the carbon (glassy carbon) substrate was observed in the range of E ¼ 1.45 O 1.6, which is w250 mV shifted to the negative sign compared to those associated with the Pd coating. No hydrogen oxidation peak were observed for the carbon electrode substrate during reverse scanning of the potential compared to the Pd, which confirmed that carbon substrates do not absorb hydrogen. The presence of peaks at 1.2 O 1.25 V indicate saturation of the Pd with the hydrogen (Fig. 1, curves 2 and 3). The hydrogen peak increases due to the increased thickness of the Pd coating. The anodic part of the voltammograms between 0.80 and 0.40 V shows the peaks of hydrogen oxidation in the Pd. The hydrogen oxidation peak (anodic peak) is in direct proportion to the Pd mass (Fig. 1 curves 2 and 3). For carbon (glassy carbon) no oxidation peaks were found, which indicates the absence of hydrogen adsoprtion at the surface of the carbon (glassy carbon) in the range of the used potential. In order to make sure that high surface area carbon in the Pd cannot be a source of hydrogen storage, we examined the reference. As seen in (Fig. 1, curve 4), the reference composite shows one oxidation peak, which is located at the same potential range as the hydrogen oxidation for pure Pd (Fig. 1, curve 3). Our observation shows that the hydrogen content in the Pd–C–Pd sample is exactly the same as it was for the pure palladium coating. This demonstrates that excessive hydrogen adsorption occur neither in the carbon powder nor at the interface between the Pd and the carbon nanoparticles. In contrast to the pure Pd, and composite, hydrogen reduction for the Pd–SWCNT–Pd system begins at E w 100 mV, while the signature for hydrogen oxidation occurs at 250–300 mV earlier than that in the pure Pd (Fig. 2, curves 1–3). This indicates that the Pd–SWCNT–Pd system provides higher hydrogen mobility than that of the pure Pd and the . As a result two oxidation peaks were observed in Pd þ SWCNT composite, in contrast to one peak demonstrated by pure Pd. The occurrence of the double peak structure in the Pd–SWCNT composite reflects the splitting of hydrogen localization between the SWCNT and the Pd matrix. What is remarkable about the double peak structure is that it occurs only when the Pd matrix is saturated with hydrogen (e.g., Pd loading is x  H/Pd w 0.65). The saturation is characterized by a peak current in the cathode area of E–I curves. The first oxidation peak at NT ¼ 0.89  0.04 V refers to the SWCNT discharge, while the second peak at Pd ¼ 0.65  0.03 V (Fig. 2) is a signature of hydrogen oxidation for the Pd, respectively. As seen in (Fig. 2) the peak of hydrogen oxidation for SWCNT (corresponding to the amount of absorbed hydrogen) increases due to the thickness of the Pd coating; and the Pd oxidation peak is reduced when the Pd coating is decreased. The observed effects are probably associated with the redistribution of the mobile hydrogen atoms between the SWCNT and the encapsulating Pd layers in the Pd–SWCNT–Pd system. The magnitude of the potentials NT and Pd allowed us to estimate the hydrogen binding energy 3H(NT) in the SWCNT because the hydrogen oxidation potential

5680

international journal of hydrogen energy 37 (2012) 5676–5685

a

16

1

3

14 12

0.5

CH, [% wt.]

2

J,mA

2 3

1

10 8 6

0 1

b

100

4

J, mA

2

60

0

40 0

20

-1

3

-0.8

-0.6

-0.4

-0.2

-0.8

4

6

8

10

12

14

16

0

E.V

-1

2

V(Pd)/V(SWCNT)

0 -1

electrochemical data vacuum desorption data Boltzmann fit of electrochemical data

2

80 -0.5

-0.6

-0.4

E,V

Fig. 2 – Cyclic voltammograms of hydrogen reduction and oxidation in the composite cathode (1–3) with the SWCNT of CNI type and the Pd coatings of various thicknesses: 0.5 mm (1); 2.6 mm (2); 4.0 mm (3). The base is a glassy carbon (S [ 7 3 10L2 cm2). Inset: Cyclic voltammogram of hydrogen oxidation in the composite cathode with SWCNT of Carbon-C. The base is the 50 mm thick foil of Pd (S [ 2 cm2). Sweep rate 0.5 mV sL1. Solution is the 1 M NaOH.

Pd ¼ 0.65 V should be associated with the well known H-diffusion activation energy in Pd: 3H(Pd) ¼ 0.20 eV/H-atom [14], which is consistent with the activation energy of H-atom escape from Pd into electrolyte. Following this consideration, the higher potential value NT ¼ 0.886 V would be associated with the activation energy 3H(Pd þ SWCNT), which characterizes a two step H-atom escaping from the SWCNT via the Pd matrix into the media (electrolyte). The hydrogen binding energy in SWCNT is thus expressed as: 3H(NT) ¼ 3H(Pd)[(NT/Pd)  1] ¼ 0.073  0.05 eV/H-atom. This value is close to the activation energy of hydrogen desorption from the SWCNT to the vacuum via the Pd matrix (<3H> ¼ 0.072  0.080 eV/H-atom) which was obtained by thermal desorption technique. Qualitative results on net SWCNT hydrogen capacity were detected using nanostructure deposited on a massive Pd foils similarly to those used in vacuum thermal desorption measurements and these results are presented in (Fig. 2, inset). The Pd0 /Pd/Pd0 reference sample demonstrate one large oxidation peak with the same position (E ¼ 0.65 V) as shown by the small area electrode with the deposited Pd layers (Fig. 2). The Pd/ sample show a very broad peak reaching a maximum at E ¼ 0.65 V, with a pronounced shoulder corresponding to E ¼ 0.886 V (Fig. 2, inset), in compliance with hydrogen oxidation for SWCNT, obtained with a small electrode area. The ‘‘net’’ SWCNT capacity was derived by comparing the hydrogen oxidation peak in the reference Pd0 /Pd/Pd0 with that showed by the Pd cathode.

Fig. 3 – Net gravimetric hydrogen capacity of SWCNT in the composite vs. volumetric Pd/SWCNT ratio. The black squares represent data obtained within situ chronoamperometry technique, using high purity Carbon-C SWCNT. Empty circles show the results of vacuum thermal desorption measurements with CNI (1a type) SWCNT. The CH dependence on y [ VPd/VSWCNT can be satisfactorily fitted with the Boltzmann function (thick line): CH [ a D (a L b)/{1 D exp[( y L c)/d]}, where a, b, c and d – are numerical constants.

We performed special experimental series to study the ‘‘net’’ SWCNT gravimetric capacity vs. Pd/SWCNT volumetric ratio (including encapsulating layers and the substrate of the Pd) in the broad interval of 0.5 < VPd/VSWCNT  15.0. The net gravimetric H2 capacity of the SWCNT in the Pd–SWCNT composite obtained by two independent methods (using two types of SWCNT of different purity – CNI and Carbon-C) vs. volumetric Pd/SWCNT ratio is plotted in (Fig. 6). Despite the different types of SWCNT used both the Voltammetry and the Vacuum thermals desorption (the last results partly shown in Table 1) show very consistent values for CH. This observation supports the reliability of our measurements and indicates an absence of net H-loading dependence on the purity of SWCNT. The net gravimetric hydrogen capacity of SWCNT vs. Pd/ SWCNT volume ratio is described by Boltzmann function with saturation at CH ¼ 11.8% wt. consistent with a large VPd/ VSWCNT ratio. As seen in (Fig. 3), at low Pd/SWCNT ratios (VPd/ VSWCNT  8.0) the net amount of absorbed hydrogen in the SWCNT deposited and encapsulated by Pd onto a massive Pd substrate increased proportionally to the increase in the VPd/ VSWCNT. At higher volume ratio ranging from 8.0 to 12, the growth of CH slowly reaches 11.5% wt. and then at VPd/ VSWCNT > 12 does not change significantly (CH w 12% wt.). Thus the value CH z 12% wt. can be considered as a ‘‘net’’ H2 gravimetric capacity limit of the SWCNT confined within the Pd–SWCNT composite. Using voltammetry we here in situ estimated a gravimetric hydrogen capacity in the SWCNT, encapsulated in the Pd matrix. These results show good agreement with the thermal desorption measurements [15] and suggest applicability of the electrochemical method for quantitative splitting of the Pd matrix and the SWCNT contributions into the hydrogen

5681

international journal of hydrogen energy 37 (2012) 5676–5685

Table 2 – Parameters of ESR signal detected in the Pd–SWCNT–Pd system and in its individual components. ESR signal

T ¼ 293 K

Sample type

I (spin/mg) 18

SWCNT (1): CNI, non-etched SWCNT (1): CNI -etched SWCNT (2) – carbon-C, non-etched Pd0 –Pd–Pd0 Pd–OUH(1) Pd–OUH(2)

1.5  10 1.0  1018 2.1  1017 – 8.2  1017 1.6  1017

T ¼ 77 K

DH (G)

g-factor

I (spin/mg)

DH (G)

g-factor

75  3 73  3 73  2 – 73  3 73  3

2.066 2.065 2.065 – 2.067 2.065

– – – – 3.0  1014 2.5  1014

– – – – 6.0  0.5 6.0  0.5

– – – – 2.0017 2.0018

Where I – is the integrated signal intensity reduced to the SWCNT weight in mg (estimated by using of DPPH standard); DH – is the line width at half height; the g-factor was evaluated by comparing with the standard; and the (–) sign denotes an absence of measurable signal.

loading in the Pd–SWCNT–Pd composite. It was of importance that the Pd/SWCNT/Pd cathodes used for voltammetry measurements were not subjected to thermal annealing above T ¼ 300 K. This fact completely rules out possible effect of Pd carbide formation (requiring, at least, T w 573 K for Pd–carbon compound [22] and T w 470 K for fullerene C60/Pd multilayer films [23]) on hydrogen storage in the Pd–SWCNT.

3.3.

ESR study

ESR studies were performed using Pd0 /SWCNT/Pd0 /Pd/Pd0 / SWCNT/Pd0 samples (the same sample types were studied by thermal desorption technique) based on deposited SWCNT either of (1) type – CNI or pure SWCNT (purity > 98%) supplied by Carbon-C Inc. The summaries for the ESR results are shown in (Table 2). We found the composite samples with both types of SWCNT (Fig. 4, curves 1 and 2) shows broad impurity resonance (DH ¼ 75 G, g ¼ 2.065 at T ¼ 293 K). This broad signal possessed a zero intensity while measured at T ¼ 77 K. The presence of the identical broad resonance in the original SWCNT of both types at T ¼ 293 K, and the dependence of signal intensity on the purity of SWCNT as well as, the comparison for the parameters DH and g with the reference literature [24,25], allowed us to identify the broad resonance lines as impurity centers, representing Fe3O4 – nanoparticles localized mainly inside the SWCNT near their openings [25].

Drastic reduction of the broad signal intensity is referred to a Verwey transition in Fe3O4 nanoparticles. This transition occurred below 120 K and was associated with charge ordering in the Fe3O4 structure due to reversible transformation of ferromagnetic iron oxide into non-magnetic oxide form (Fe2O3 / FeO) [26,27]. The ESR measurements indicated no signal were observed in the individual components of the Pd0 /SWCNT/Pd0 /Pd/Pd0 / SWCNT/Pd0 structure, including the SWCNT of both types and the Pd0 –Pd–Pd0 reference samples, at T ¼ 77 K (Table 2). samples However, Pd0 /SWCNT/Pd0 /Pd/Pd0 /SWCNT/Pd0 demonstrated a narrow resonance with the parameters DH w 6.0 G and g w 2.002 (Fig. 5). The absence of this resonance in the individual components of the Pd–SWCNT composite and the appearance of a single narrow signal with the g-factor is close to a free electron, indicating a strong p-complex (charge transfer complex) formation [28] between the system of conjugated bonds of C-ring in SWCNT and the d-orbital of the Pd atom adsorbed at the SWCNT. During hydrogen loading of the Pd–SWCNT composite samples at T ¼ 293 K followed by a rapid cooling to 77 K (the procedure normally used for vacuum thermal desorption measurements), a growth of narrow signal intensities was

30

3

ESR Signal, [arb.unit]

25 20

ESR Signal, [arb.unit]

0

2 -20 -40 -60

2

20 15 10

1

5 0 -5 -10 -15

-80

1

-20

-100

3290

3300

3310

3320

3330

Magnetic Field, [G]

-120 2900

3000

3100

3200

3300

3400

3500

3600

Magnetic field, [G]

Fig. 4 – Broad ESR signal at T [ 293 K caused by Fe3O4 nanoparticle impurities in the Pd–SWCNT (CNI) – curve 1 and the Pd–SWCNT (Carbon-C) – curve 2 samples.

Fig. 5 – Narrow ESR signal in the Pd–SWCNT (Carbon-C) sample at T [ 77 K: the sample without hydrogen (prior and 60 min after hydrogen loading) – curve 1; H-loaded sample at T [ 77 K – curve 2; H-loaded sample at T [ 77 K after its at T [ 293 K during 5 min – curve 3.

5682

international journal of hydrogen energy 37 (2012) 5676–5685

observed, though showing no changes in DH and g parameters of the narrow resonance (Fig. 5, curve 2). The signal remained stable during several hours of exposure at T ¼ 77 K. After heating and ‘‘annealing’’ during 5–10 min at T ¼ 293 K (when the H-desorption rate is most intense) followed by cooling to T ¼ 77 K, further expansion of the narrow signal intensity occurred, accompanied by its broadening from 6 to 9 G. During further cycles of heating and annealing at T ¼ 293 K (up to t ¼ 60 min, corresponding to full H-desorption from the Pd–SWCNT composite) followed by cooling to T ¼ 77 K a gradual recovery was observed for the narrow resonance parameters to those detected prior to hydrogenation of the Pd–SWCNT–Pd sample (Fig. 6). This behavior of the narrow resonance reflects a high gravimetric capacity of hydrogen in SWCNT (H/C > 1.0) and is dramatically distinctive from that of the paramagnetic centers in low capacity SWCNT (H/C < 0.2) loaded with molecular hydrogen [24,25]. Indeed, the physisorption of low density molecular hydrogen in the SWCNT results in the strict reduction of the intensity of the narrow ESR signal originated by the dangling C bonds in SWCNT. The paramagnetic response of SWCNT at low temperature after H2 physisorption is reduced compared to that of initial SWCNT due to the diamagnetic contribution of the H2 molecules adsorbed on paramagnetic centers [24]. On the contrary, the detected effect of the narrow resonance intensity increase in the Pd–SWCNT composite at high loading (H/C > 1.0) is quite similar to observed increase in the dangling bond signal due to the atomic hydrogen (MeV proton) impingement into the graphite [29]. In this connection we assume that high gravimetric capacity of hydrogen can be achieved as a result of dissociative H-adsorption at the SWCNT sites containing active centers of H-adsorption (e.g., Pd–Cx p-complexes). At the same time, significant increase in the intensity of the narrow signal during thermal cycling of the H-loaded Pd–SWCNT composite could be associated with an increase in the number of Pd–C bonds. This increase is referred to a strong compressing deformation in the encapsulating Pd film upon partial

Signal Intensity, T=77K, [arb. unit]

10 9

2

8 7 6 5 4

1

3

H-desorption which is accompanied by a b / a transition (producing a miscellaneous gap [16]) and a change in the lattice parameter. During deformation the Pd coating volume surrounding the SWCNT increases, resulting in an increase of the area of adhesion contact between the Pd film and the SWCNT wall. This process must be accompanied by electron transfer from SWCNT to the d-orbital of the decorating Pd atoms, resulting in formation of additional p-complexes [30]. In turn, it is likely that the narrow signal broadening under the thermal cycling was due to the spin–exchange interaction, which expands with an increasing concentration of the pcomplexes.

4.

We showed, therefore, that the net SWCNT capacity in the Pd þ SWCNT system depends on the volume ratio between the Pd and the SWCNT components (V (Pd)/V (SWCNT)) and reaches its limit value of w12% wt. at V (Pd)/V (SWCNT)  12. Here we consider the physical reasons for the net H2 capacity limit of SWCNT in the Pd–SWCNT nanocomposite shedding light on a possible model of hydrogen impingement and storage in SWCNT encapsulated by Pd onto a massive Pd substrate. In order to clarify hydrogen impingement in SWCNT we will consider the atomic hydrogen permeation into the nanotube openings through encapsulating Pd membrane. This permeation would only be possible above the threshold hydrogen pressure created in the Pd lattice under loading. Indeed, according to our data, the measurable net SWCNT loading was observed only when the Pd loading ratio x ¼ H/Pd exceeds the value x ¼ 0.65. This loading can be achieved if the hydrogen occupies all of the octahedral (O)- sites in the lattice [14]. At higher Pd loading (x > 0.65) the mobile (free) hydrogen appears in the lattice. This mobile hydrogen flux interacts with SWCNT, thus penetrating into the nanotube inner space. So, SWCNT loading in the Pd þ SWCNT composite become possible when effective hydrogen hydrostatic pressure in the Pd lattice exceeds P0 ¼ 180 MPa (corresponding to x ¼ H/ Pd ¼ 0.65). At H2 pressure P > P0 in the Pd matrix the effective hydrogen pressure inside the nanotube (SWCNT volume is VSWCNT) encapsulated by Pd on a massive Pd substrate (total Pd volume is VPd) would be determined as P ¼ a[P0  (VPd/ VSWCNT)], where a is the Poisson coefficient (a w 0.3), reflecting the ratio between the relative lateral and the longitudinal deformations of nanotube under hydrogen loading. Taking into account that H2 concentration in SWCNT could be expressed via the pressure inside the nanotube as NH ¼ P/kBT we derive the gravimetric H2 capacity in SWCNT (CH) as: CH ¼

2 1 0

10

20

30

40

50

60

Elapsed time at T=293K, [min] Fig. 6 – Kinetics of narrow ESR signal upon the annealing of the Pd–SWCNT (CNI) – curve 1 and the Pd–SWCNT (Carbon-C) – curve 2 samples at T [ 293 K during various time intervals, with the following ESR measurements at T [ 77 K.

Discussions

aP0 mp VPd ; rSWCNT kB TVSWCNT

(1)

where mp – is the proton mass, rSWCNT ¼ 1.33 g/cm3 is the density of the SWCNT. The Eq. (1) satisfactorily describes the CH value as a function of VPd/VSWCNT at lower Pd/SWCNT volumetric ratio VPd/VSWCNT < 8 (Fig. 7). On the contrary, in case of VPd/VSWCNT  1.0 the Eq. (1) does not work because the effective pressure P in SWCNT is too low to provide hydrogen loading into SWCNT. (Eq. (1).) also shows that SWCNT gravimetric capacity linearly increases when temperature

international journal of hydrogen energy 37 (2012) 5676–5685

Fig. 7 – Energy diagram of hydrogen transport in the composite.

decreases. This satisfactorily reflects the temperature dependence of Pd þ SWCNT hydrogen loading vs. loading temperature in the range of 285–315 K [15]. At higher hydrogen pressure (P > 500 MPa) inside the SWCNT consistent with VPd/VSWCNT > 8, the (Eq. (1)) is invalid because the net gravimetric capacity in SWCNT become independent of volumetric Pd/SWCNT ratio (Fig. 7). The plateau of the CH function at VPd/VSWCNT > 8 is determined by the balance between the hydrogen binding inside the SWCNT (with the binding energy 3H) and the energy of hydrogen leakage (WH) from the nanotube (which leads to breaking of H–C bonds in SWCNT) due to a large gradient between H2 pressures inside the SWCNT and in the surrounding Pd lattice. Indeed, in adiabatic approximation the work of hydrogen gas escaping from the nanotubes into the Pd lattice is expressed as: h i WH ¼ ðmH =mÞRT=ðg  1Þ 1  ðVSWCNT =VPd Þg1 ;

(2)

where mH ¼ CH mNT – is the hydrogen mass in the SWCNT (the mNT – is the mass of SWCNT), m ¼ 1 for atomic H, R – is the universal gas constant and g ¼ Cp/CV – is the characteristic constant of hydrogen species; depending on the hydrogen form (atomic or molecular one) the g number is located in the range of 1.40  g  1.67. At VPd > VSWCNT the energy of hydrogen extraction from the SWCNT, reflecting the breaking of C–H bonds is WH ¼ CHmSWCNTRT/(g  1). Thus, the balancing equation 3H  Nc ¼ WH, (where Nc – is the number of carbon atoms in the SWCNT) can be written in following form: 3H ¼ 12CH kB T=ðg  1Þ:

(3)

At room temperature and CH ¼ 0.118 (consistent with highest net gravimetric capacity), the hydrogen binding energy varies in the range of 3H ¼ 0.053  0.089 eV/H-atom, depending on g value. Notice that the range of H-binding energy with SWCNT derived from Eq. (3) is in agreement with the activation energy of hydrogen desorption from SWCNT obtained experimentally (<3H> ¼ 0.075 eV/H-atom). Thus, we come to the conclusion that the ‘‘net’’ gravimetric capacity of SWCNT in the Pd–SWCNT composite is defined by the balance between the hydrogen binding energy inside the SWCNT and the energy of hydrogen escaping from the nanotube (resulting in breaking H–C bonds in SWCNT). The performed analysis of the SWCNT H-loading in Pd–SWCNT–Pd composite also supports our preliminary assumption on the preferential hydrogen binding on the inner sites of the SWCNT, based on the absence of non-etched SWCNT loading (Table 1). Otherwise, in case of hydrogen storage at the interface between

5683

SWCNT and Pd coating, the net SWCNT loading would not be proportional to the hydrogen pressure in the Pd, and must be a function of SWCNT surface area (e.g., CH should increase with a decrease in Pd/SWCNT volumetric ratio). Moreover, in the case of outer SWCNT hydrogen adsorption sites, the character of hydrogen binding with the nanotubes must show preferential chemisorption origins, demonstrating a significantly higher binding energy <3H> [11], than were measured in our experiments. Experimental studies of hydrogen transport in composite allows us to propose an energy diagram of hydrogen transport into SWCNT, encapsulated inside a massive Pd matrix (Fig. 7). In pure Pd, hydrogen atoms occupy potential wells created by the Pd lattice. The presence of nanotubes fully encapsulated into the Pd matrix is caused by the appearance of shallow hydrogen traps with an effective depth of w3kBT ¼ 0.075 eV. At a lower hydrogen concentration in the Pd matrix (H/Pd  0.65) the diffusion of hydrogen atoms in the Pd has an activation hopping character, accompanied also by H-atom tunneling between the Pd potential wells [14]. Thus, at H/Pd  0.65 the H-atoms moving through the Pd matrix can permeate SWCNT traps and escape back into the Pd lattice by quantum tunneling. As a result, the hydrogen loading in SWCNT traps remain low, especially at higher concentration of SWCNT traps. At high H-loading and pressure in the Pd (H/Pd > 0.65), when O-site in the Pd lattice is already filled, the hydrogen flux would move over the potential barrier and fill the SWCNT traps. Because all hydrogen states in encapsulating Pd lattice (surrounding the nanotubes) at ambient temperature are already occupied, the hydrogen atoms from SWCNT cannot tunnel back into the Pd matrix. The additional (with respect to the Pd matrix) H-atoms trapped in the SWCNT escaping from the SWCNT by thermal activation at elevated T would then diffuse via the Pd matrix and finally escape the Pd–SWCNT–Pd sample. The presence of the separate SWCNT hydrogen band localized in the Pd matrix in the composite is supported by the observation of two hydrogen oxidation peaks (Fig. 2) in CVA measurements. Our experimental data on H-binding energy and identification of paramagnetic centers in the composite (in particular, observation of charge–transfer complexes between the conjugated C bonds of SWCNT and the d-orbital of Pd atom) can be used to clarify a possible mechanism of hydrogen adsorption, thus providing a large net H2 capacity in SWCNT. The averaged activation energy of hydrogen desorption (consistent with the H-binding energy with nanotubes) was found to be <3H> ¼ 0.075  0.05 eV/H-atom. This binding energy is larger than that ascribed to H2 physisorption on SWCNT (3H ¼ 0.057 eV/H2-molecule [11]), but lower than the energy of H-chemisorption into SWCNT (3H ¼ 2.5 eV/H-atom [9]). According to theoretical study a possible way exist to achieve high hydrogen storage and relatively low H-binding energy in SWCNT [31]. This approach involves the formation of chemical complex at the interface between the transition metal atom and the nanotube or organic molecules, thus allowing for the absorption of several hydrogen molecules. Single H2 molecule can be chemisorbed on the Pd atom at the SWCNT outer wall either dissociatively or molecularly [11,31].

5684

international journal of hydrogen energy 37 (2012) 5676–5685

However, this is not exactly the case with respect to composite, because the calculated hydrogen binding energy in the Pd–C complex (3H ¼ 0.58 eV/H2) formed at the outer surface of the nanotube [31] would be rather higher than our average experimental value (<3H> ¼ 0.075 eV). On the other hand, the results of our ESR study showed that the p-complexes of Pd–Cx type play a specific role of the active centers for atomic hydrogen adsorption, resulting in low hydrogen binding energy (3H  0.1 eV/H-atom) and high net gravimetric capacity (H/C > 1.0) in SWCNT. So, the high net loading of the nanotubes could be achieved via dissociative hydrogen adsorption on the SWCNT inner sites containing active centers of H-adsorption (e.g., Pd–Cx p-complexes). In this consideration, formation of strong charge transfer complexes of PdC4 configuration would occur at the interface between the Pd and the carbon (similarly to mechanism suggested in [31]) during the interaction of 2p2 C bonds in SWCNT’s C-ring with 4d (hybridized with 5s) orbital of the palladium atom. This interaction, accompanied by an electron (hole) transfer through the potential barrier of the Pd–C [32] results in a weak polarization of the nanotube’s inner wall. Thus, the atomic hydrogen moving through the PdHx to the SWCNT openings permeate the interior of the nanotube, resulting in the adsorption preferentially, at the Pd–Cx centers. The, atomic hydrogen would form weak hydrogen bonds with the p-complexes, which do not affect the parameters of ESR signals. Thus, the final atomic configuration of hydrogen storage in SWCNT could be expressed as CxPdHy ( y > x).

p-complexes could be considered as hydrogen adsorption sites, providing both high gravimetric capacity (H/C > 1) and low hydrogen binding energy in the Pd encapsulated SWCNT. Finally, we note that the magnitude of hydrogen adsorption related to the total weight of the composite achieved in our experiments cannot meet 6% wt. DOE criterion for effective hydrogen storage medium. However, the large ‘‘net’’ H2 loading in nanotubes encapsulated by Pd suggests some special applications of the Pd–SWCNT–Pd composite and also provides a unique possibility to probe a dense condensed hydrogen nano-phase confined inside the SWCNT. Principally, the fact that at H/C w 1.5 the mean distance between the hydrogen atoms is the same as the de Broglie wavelength (lD ¼ 0.215 nm at T ¼ 205 K) makes it possible to consider a hydrogen phase flowing through the SWCNT as a sort of quantum ‘‘nanofluid’’, existing at high temperatures (T w 200 K). One possible application of the nanostructure could be directed to design high deuterium/ tritium density ‘‘nano-pellets’’ for inertial laser confinement [33] because the deuterium density in the nanotube would be twice as higher than that of liquid D2 and does not require extremely low temperature for DT storage. The other potential use of the Pd–SWCNT–Pd composite (in terms of our voltammetry results) could be related to applications in fuel cells as a hydrogen transparent anode material with a strictly negative (w0.9 V) potential of hydrogen oxidation required for Pt electrode substitution [1,34].

references

5.

Conclusions

In summary, we report a study of hydrogen storage and transport in a material, representing Single-Walled Carbon Nanotubes (SWCNT) encapsulated by thin Pd layers onto a massive Pd substrate. A synergistic effect resulted in the combination of the Pd and the SWCNT properties with regards to hydrogen were achieved. Using vacuum thermal desorption and cyclic voltammetry techniques we showed that adding SWCNT increases the H2 capacity of the Pd–SWCNT composite under electrochemical loading up to 25% relative to the Pd metal alone. This provides modest growth in the gravimetric capacity of the total composite based on a massive Pd substrate. However, with regards to the added SWCNT, the ‘‘net’’ nanotube loading capacity could exceed the ratio H/C w 1.0. The SWCNT capacity in the Pd þ SWCNT system depends on the volumetric ratio between the Pd and SWCNT components (VPd/VSWCNT). Our analysis showed that at VPd/VSWCNT < 8, the net gravimetric capacity increases linearly with an increase in the Pd/SWCNT volumetric ratio resulting in hydrogen pressure increase in the SWCNT. The ‘‘net’’ capacity limit of SWCNT in the Pd–SWCNT composite is defined by balancing the hydrogen binding energy inside SWCNT and the work of hydrogen escaping from the nanotube (resulting in H–C bonds breaking in SWCNT). The hydrogen binding energy in the SWCNT (3H ¼ 0.075 eV/H-atom), derived from studies of hydrogen transport in the Pd–SWCNT composite was found to be lower than predicted for the hydrogen adsorption at the Pd–SWCNT complex formed on the outer SWCNT wall, but higher than those related to the bare SWCNT [13]. Using ESR we have established that the Pd–Cx

[1] Schlapbach L. Hydrogen fuelled vehicles. Nature 2009;460:809–11. [2] Zuttel A. Materials for hydrogen storage. Mater Today 2003;6: 24–33. [3] Becher M, Haluska M, Hirscher M, et al. Hydrogen storage in carbon nanotubes. Physique 2003;4:1055–62. [4] Lamari Darkrim F, Malbrunot L, Tartaglia GP. Review of hydrogen storage by adsorption in carbon nanotubes. Int J Hydrogen Energy 2002;27:193–202. [5]. Zuttel A, Sudan P, Mauron P, Wenger P. Model for the hydrogen adsorption on carbon nanostructures. Appl Phys 2004;A78:941–6. [6] Pradhan BK, Harutyunyan A, Stojkovic D, et al. Large cryogenic storage of hydrogen in carbon nanotubes at low pressures. J Mater Res 2002;17:2209–16. [7] Matranga C, Bockrath B. Controlled confinement and release of gases in single-walled carbon nanotube bundles. J Phys Chem B 2005;109(19):9209–15. [8] Park S, Srivastava D, Cho K. Generalized chemical reactivity of curved surfaces: carbon nanotubes. Nano Lett 2003;3: 1273–6. [9] Nikitin A, Ogasawara H, Mann D, et al. Hydrogenation of single-walled carbon nanotubes. Phys Rev Lett 2005;95: 225507. [10] Ye Y, Ahn CC, Witham C, et al. Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes. Appl Phys Lett 1999;74:2307–10. [11] Dag S, Ozturk Y, Ciraci S, Yildrim T. Adsorption and dissociation of hydrogen molecules on bare and functionalized carbon nanotubes. Phys Rev B 2005;72:155404. [12] Durgun E, Ciraci S, Zhou W, et al. Transition-metal-ethylene complexes as high-capacity hydrogen-storage media. Phys Rev Lett 2006;97:226102.

international journal of hydrogen energy 37 (2012) 5676–5685

[13] Phillips AB, Shivaram BS. High capacity hydrogen absorption in transition metal–ethylene complexes observed via nanogravimetry. Phys Rev Lett 2008;100:105505. [14] Fukai Y, Sugimoto H. Hydrogen diffusion in metals. Adv Phys 1985;32:263–302. [15] Lipson AG, Lyakhov BF, Saunin EI, Tsivadze AYu. Evidence for large hydrogen storage capacity in single-walled carbon nanotubes encapsulated by electroplating Pd onto a Pd substrate. Phys Rev B 2008;77:081405R. [16] Lipson AG, Heuser BJ, Castano C, et al. Transport and magnetic anomalies below 70 K in a hydrogen-cycled Pd foil with a thermally grown oxide. Phys Rev B 2005;72: 212507. [17] Schlesinger M, Paunovic M, editors. Modern electroplating. 4th ed. New York: Wiley; 2000. [18] Vetter KJ. Elektrochemische kinetik. Berlin-Go¨ttingenHeidelberg: Springer-Verlag; 1961. [19] Lawson DR, Tierney MJ, Cheng IF, et al. Use of a coulometric assay technique to study the variables affecting deuterium loading levels within palladium electrodes. Electrochim Acta 1991;36:1515–21. [20] Strizker B, Wuhl H. Hydrogen in metals II. In: Alefeld G, Volkel J, editors. Topics in applied physics, vol. 29. Berlin: Springer-Verlag; 1978. [21] Tripodi P, McKubre MCH, Tanzella F, et al. Temperature coefficient of resistivity at compositions approaching PdH. Phys Lett A 2000;276:122–7. [22] McCaulley JA. Temperature dependence of the Pd K-edge extended x-ray-absorption fine structure of PdCx (x w 0.13). Phys Rev B 1993;47:4873–9. [23] Wang B, Wang HQ, Li YQ, et al. Formation of palladium carbide in fullerene-C60/Pd multilayers. Mat Res Bull 2000;35:551–7.

5685

[24] Shen K, Tierney DL, Pietraß T. Electron spin resonance of carbon nanotubes under hydrogen adsorption. Phys Rev B 2003;68:165418. [25] Musso S, Porro S, Rovere M, et al. Low temperature electron spin investigation on SWNT after hydrogen treatment. Diam Relat Mater 2006;15:1085–9. [26] Tiwari S, Choudhary RJ, Phase DM. Effect of growth temperature on the structural and transport properties of magnetite thin films prepared by pulse laser deposition on single crystal Si substrate. Mater Today 2008;517: 3253–6. [27] Mi WB, Shen JJ, Hou DL, et al. Structure, magnetic and transport properties of polycrystalline Fe3O4–Ge nanocomposite films. J Phys D Appl Phys 2009;41:055009. [28] Poole CP. Electron spin resonance. New York: Wiley; 1983. [29] Lee KW, Lee CE. Electron spin resonance of proton-irradiated graphite. Phys Rev Lett 2006;97:137206. [30] Palacios JJ, Tarakeshwar P, Kim DM. Metal contacts in carbon nanotube field-effect transistors: beyond the Schottky barrier paradigm. Phys Rev B 2008;77:113403. [31] Yildirim T, Ciraci S. Titanium-decorated carbon nanotubes as a potential high-capacity hydrogen storage medium. Phys Rev Lett 2005;94:175501. [32] Reihert H, Denk M, Okasinski J, et al. Giant metal compression at liquid-solid (Pb–Si, In–Si) Schottky junctions. Phys Rev Lett 2007;98:116101. [33] Naumova N, Schlegel T, Tikhonchuk VT, et al. Hole boring in a DT pellet and fast-ion ignition with ultraintense laser pulses. Phys Rev Lett 2009;102:025002. [34] Gong K, Du F, Xia Z, et al. Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction. Science 2009;323:760–4.