HE-D-10-00896 Revised version
N. Comisso a*
, L.E.A. Berlouis b
, J. Morrow b
and C. Pagura a a National Research Council, Institute for Energetics and Interphases,
Corso Stati Uniti 4, 35127 Padova, Italy b
WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde,
295 Cathedral Street, Glasgow G1 1XL, United Kingdom
Abstract
The effect of thermal oxidation on the hydrogen storage properties of carbon nano-horns was investigated by gravimetric and electrochemical methods. The pristine nano-horn sample was oxidised at 673 K in air for different periods (15, 30 and 60 min) and the resulting materials were characterised. The N
2
adsorption experiments reveal a marked increase in the surface area, from
267 m 2 g -1 , for the pristine sample, up to 1360 m 2 g -1 for the sample oxidised for the 60 min period, and a reduction in the average pore diameter. The gravimetric investigation, conducted at low temperature (77 K) showed an increase in the hydrogen storage, from 0.75 wt% for the pristine sample up to 2.60 wt% for the oxidised material. Reproducible and stable hydrogen storage was found for all the samples examined apart from the sample oxidised for 60 min. For the latter, a decrease in the amount of hydrogen stored between the first and second cycles was found.
Electrochemical loading of hydrogen in the samples was performed at room temperature (298 K) in alkaline solution by the galvanostatic charge/ discharge technique. The results obtained here however show a much lower hydrogen storage level by the samples as compared to the gas storage method, with a maximum value of 0.124 wt% H
2
and with very little dependence on the thermal oxidation treatment.
Key words :
Hydrogen storage, electrochemical loading, carbon nano-horns.
Corresponding author. Tel.: +39 049 8295854; fax: +39 049 8295853.
E-mail address: nicola.comisso@ieni.cnr.it (N. Comisso)

HE-D-10-00896 Revised version
1. Introduction
Single Wall Carbon Nano Horns [1, 2] (SWNH) represent one of the most interesting carbon nanostructures of the thriving nanotube family. A key characteristic behaviour is their tendency to group together and form aggregates (spherical clusters or bundles) like dahlia flowers or buds, with overall diameters of tens to hundreds of nanometers. One advantage of this “self assembling” characteristic is not only the very large surface area, but also the easy permeation of gases and liquids inside their structure. In spite of their tiny tubular structure, SWNHs maintain many of the typical properties of carbon nanotubes (CNTs), such as ease of functionalization and good electrical and thermal conductivities. SWNHs present a very wide range of potential uses, from methanol fuel cells support [3] and super-capacitors [4] to generation of hydrogen by decomposition or steam-reforming of methane [5] and as composites for lubrication [6]. Although not as yet commercially available, SWNHs can nevertheless be produced in laboratory-scale quantities without the need of metal catalysts and can thus be obtained in high purity by laser ablation/vaporization of graphite, by arc discharge or other techniques [1, 7-10].
A comparison, of the hydrogen storage capability of SWNHs with others carbon nano-structures
(CNTs), shows that apart from the absence of any trace of catalytic metal used in the formation of
SWNHs, these materials are essentially similar. Several experimental techniques have been employed, by various research groups, to determine the hydrogen loading of the different carbon nano-structures and large variations in the loading found have been reported. Dillon et al.
[11,12] found a hydrogen content between 5-10 wt% could be obtained on a single-walled carbon nanotubes (SWNTs) sample at 300 K and at low pressure (0.04 MPa) though the use of the
Temperature Programmed Desorption method. Ye et al.
[13], using the Sieverts' apparatus on a very pure SWNT sample, measured a hydrogen content of 8 wt% at low temperature (80 K) and high pressure (8 MPa). Liu et al.
[14] examined SWNT samples which had undergone equilibration
2

HE-D-10-00896 Revised version for 300 min at 298 K and 12 MPa pressure and from the decrease in pressure in the sample chamber, they deduced a hydrogen storage value of 4.2 wt%, of which only 80% was reversible.
An investigation of the hydrogen storage mechanism in these carbon materials has been carried out by Texier-Mandoki et al.
[15], who studied the porous textures of activated carbon (AC) samples using gas adsorption experiments. A full analysis of the microporosity of the different carbon samples using both N
2
and CO
2
as adsorbents, was performed and the values of specific surface area (SSA, using the Brunauer-Emmett-Teller (BET) method), V t
(total volume), V
DR
(N
2
)
(microporous volume calculated from N
2
adsorption at 77 K) and V
DR
(CO
2
) (from CO
2
adsorption at 273 K) were obtained. The results were compared with the hydrogen volume released from the
AC samples following exposure to H
2
at 77 K, at pressures ranging from 1 to 10 bar. A strong correlation between the hydrogen loading and the homogeneous narrow microporosity (pore diameters < 0.7 nm) was found from their data. The authors also underlined the importance of the micropore volume determination at 273 K by CO
2
adsorption, because the same analysis conducted at 77 K by N
2
adsorption did not give a similar relationship. On the other hand, Panella et al.
[16] were able to find a linear correlation between the hydrogen loading of different carbonaceous materials and the SSA. Their SSA analysis was conducted at 77 K using N
2
adsorption at different pressures and the determination of the hydrogen loading was at room temperature (298 K) using a
Sieverts' apparatus. Hydrogen monolayer adsorption at 77 K on the surface of all the nanostructures investigated was confirmed through analysis which showed that all the isotherms fitted Langmuir-type behaviour. A surface coverage of 84% was obtained at 77 K but this value decreased to 10% at room temperature and 65 bar pressure. The maximum hydrogen uptake found was 4.5 wt% at 77 K on a sample with a SSA of 2560 m
2
g

1
.
Xu et al.
[17] compared the hydrogen storage capacity measured by Sieverts' apparatus at 303 K with that at 77 K, over the pressure range 1 to 10 MPa on different carbon nano-structures, viz.
,
SWNH, SWNT, graphite nonofibers (GNF) and activated carbon (AC). For the SWNH and SWNT samples, their surface and porosity were modified by thermal oxidation at different temperatures.
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HE-D-10-00896 Revised version
For the SWNH, the SSA increased from 296 to 1536 m
2
g

1
following oxidation at 773 K but decreased to 1330 m 2 g -1 on oxidation at 823 K. A similar trend was found in the volume of the micropores (V micro
), which initially increased from 0.14 mL g

1
to 0.53 mL g

1
upon oxidation at
773 K but then decreased to 0.41 mL g

1 at 823 K. The hydrogen loading changed in a parallel fashion, with a maximum value of 0.6 wt% at 303 K (10 MPa) for the oxidized sample at 773 K, ca
6 times the value (0.1 wt %) of the pristine SWNH sample. Up to 4 wt% hydrogen could be achieved at 77 K at this same pressure.. Chemical oxidation through treatment with HNO
3
was also able to improve both the SSA and V micro
of the SWNH sample as well as the hydrogen loading capacity, but the same effect could not be obtained using KOH.
Very different hydrogen loading conditions are employed in the electrolysis method. Here, the sample is connected as the cathode in the electrochemical cell, with an aqueous solution of KOH as the electrolyte at 298 K. Using this method Nützenadel et al.
[18] measured a value of 0.41 wt% for hydrogen loading in SWNT. The much lower value in the latter is consistent with a study carried out by Züttel et al.
[19] where the hydrogen content of a large number of carbon nanostructures, including SWNT and high surface area graphite, were compared following electrochemical and gas phase loading. The gas phase method showed a maximum loading of 5.5 wt% at 77 K and 2 MPa, but at room temperature, the value found was only 0.6 wt%.
Electrochemical measurements on the other hand showed results ranging from 0.04 wt% for a fullerene sample C
60
of laboratory grade purity and up to 2.0 wt% for various SWNTs. However, as underlined by these same authors, the presence of not insignificant quantities of the metal catalyst
( e.g.
Ni, Fe, Y and Co) in most of the samples studied was an important source of error in these measurements, not only for the redox processes taking place on the metals at the low cut-off potential (0.0 V vs Hg/HgO reference electrode) used in the discharge process but also for the possible formation of metallic hydrides.
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HE-D-10-00896 Revised version
In a recent article Wang et al.
[20] used ball-milling techniques to modify the structure of multiwall carbon nanotubes (MWNTs). They found that after a milling time of 12 hours, V micro
and SSA reached a maximum value as did the electrochemical hydrogen loading amount.
Other modifications methods, such as the alkali metal doping or metal decorating system, have also been employed to increase the hydrogen loading properties of carbon nano-structures.
Summaries of the methodologies and of the effects of these modifications can be found in reviews by Darkrim et al.
[21], Züttel et al.
[22], Ströbel et al.
[23] and Yürüm et al . [24].
In this article we investigate the use of a simple modification, viz.
thermal oxidation, to alter the structure of SWNHs, thus avoiding any treatment which could introduce in the carbon nanostructures metallic particles able to modify the hydrogen storage capacity. The aim is thus an accurate comparison between the original sample and the modified products obtained. The key techniques employed for the textures characterization of carbonaceous materials obtained was the
BET analysis to obtain information about the SSA and the pore volume from adsorption isotherm plots (pore diameter ≥ 2 nm), using N
2
adsorption at 77 K. Hydrogen loading was investigated both at low (77 K and 5 bar) and room temperature (298 K and atmospheric pressure) using the gravimetric and electrochemical charging/ discharging techniques respectively. The results obtained are then compared with similar work in the literature.
2. Materials and methods
2.1. Chemicals and procedures for the thermal oxidation
The single wall carbon nano-horns (SWNHs) used in this work were produced [10] and kindly provided by Carbonium Srl (Padova, Italy, info@carbonium.it). The morphological characterization was performed by transmission electron microscopy (TEM, FEI Tecnai at 100 keV) and high resolution scanning electron microscopy (HR SEM, FEI Quanta 200 FEG ESEM Instruments) and are reported in Fig.’s 1A and B. Typical the nano-horns assemblies exhibit a mixture of bud-like and dahlia-like patterns. The average dimensions of aggregate structures were found to be about 80
5

HE-D-10-00896 Revised version nm for the bud-like form and 120 nm for the dahlia-like form, in good agreement with the findings of other authors [1,2]. The thermogravimetric analysis of the sample carried out in air is reported in
Fig. 2 which shows the weight loss plot (continuous line, refer to left axis) and the derivative weight plot (dotted line, refer to right axis). The TGA data suggests that the purity was not less than 98% by weight, based on the low residual amount (< 1%) left at 1173 K. The high purity and homogeneity of the nano-horns sample was confirmed by the presence of only one weight loss event, as evident from the derivative TGA plot, with a maximum in the derivative signal at a temperature of 852 K.
For sample modification, the thermal oxidation of the samples was performed in a furnace at
673 K in air, a region up to which, from the TGA data, no significant weight losses occur in the nano-horn samples. Typically, a weighed amount of pristine nano-horns (~ 40

50 mg) was placed in a crucible (no lid) and inserted in the hot furnace for different times. The crucible was then removed from the furnace, covered by a lid and allowed to cool to room temperature in air. The samples thus obtained are referred to as nH (nano-horns with no thermal oxidation), nH-15 (after 15 min thermal oxidation), nH-30 (after 30 min thermal oxidation) and, finally, nH-60 (after 60 min thermal oxidation). The data on these samples are given in Table 1.
The chemicals used for preparing the electrolytic solutions were reagent grade KOH and LiOH, obtained from Sigma-Aldrich. All solutions were prepared with Millipore grade water.
2.2. Adsorption apparatus and procedure
The determination of the surface area was performed on the Micromeritics ASAP-2020 porosity analyser. Prior to analysis the samples were degassed at 100°C for 4 h on the degassing port of the analyser. The specific surface areas of the samples were calculated by the BET method and, for the pore size distribution, (pore diameters ≥ 2) the BJH (Barrett, Joyner and Halenda) method was employed [25].
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HE-D-10-00896 Revised version
2.3. Intelligent Gravimetric Analyser (IGA) apparatus and procedure
A Hiden Isochema Intelligent Gravimetric Analyser (IGA) was employed to determine the hydrogen uptake by the materials. Typically, a 5 mg sample was loaded into the sample bucket and degassed under vacuum (~1.5

10

4
mbar) at room temperature for 20 h. The temperature was then increased to 573 K at a rate of 3 K min
-1
followed by an isotherm of 6 h duration, to ensure that the sample was perfectly dry and devoid of all volatile species prior to exposure to hydrogen. Once cooled back to ambient temperature, hydrogen was introduced into the system at a rate of 50 mbar min
-1
to a final pressure of 5 bar and the sample was allowed to stabilise for 3 h under these conditions. The furnace was then removed and replaced with a thermally insulating jacket around the reactor which was filled with liquid nitrogen. Once the temperature reached 77 K, the reactor was slowly allowed to warm up back to room temperature whilst recording the weight changes.
During all the steps described above, the following parameters were measured as a function of time: sample weight, temperature of the sample and pressure inside the sample chamber. All experiments reported here were carried out in duplicate. Control experiments were carried using exactly the same procedures as above but using only the empty bucket.
N
2
and H
2
used in nitrogen adsorption and hydrogen uptake experiments (IGA) respectively were both of 99.9995 % purity from Air Products.
2.4. Electrochemical procedure and apparatus
For the electrochemical investigations, a weighed amount of sample was inserted into a strip of
Ni foam (Inco Europe Ltd) of known mass, followed by pressing at 100-300 bar in a mechanical
(hydraulic) press and finally, fixing by a diluted solution of a two-component epoxy resin. In this way a self-sustaining fabric was obtained which, upon pressure-bonding to a Ni wire (Johnson
Mattley Company) contact, constituted a working electrode capable of prolonged electrochemical investigation. Each sample electrode was then assembled in a cylindrical undivided cell constructed in polyethylene and equipped with a Teflon lid. As well as the working electrode, the lid also
7

HE-D-10-00896 Revised version supported a Ni coil (

= 1.0 mm, length = 1000 mm) counter electrode wrapped around the working electrode and a Hg|HgO|KOH (5.0 M) reference electrode, manufactured by Amel-Milan. All the potentials values are reported with respect to this electrode. The cell was usually filled with 200 ml of electrolyte consisting of 6.0 M KOH. The body of the cell was immersed in the water bath of a
Haake F3 thermostat and the working temperature was 25.
 °C ( 
0.1

C). Electrochemical instrumentation consisted of an EG&G Princeton Applied Research model 273A apparatus interfaced to a personal computer, running custom written software.
Electrolytic charging/ discharging of hydrogen were carried out galvanostatically to ensure a better determination of the charge involved in the process. The charging step was carried out using one of two methodologies, viz.
(i) high cathodic current of 100 mA applied for a period of 1.0 h and
(ii) low cathodic current of 50 mA applied for a long period (overnight, ~ 16.5 h).
The electrodes were then immediately discharged by applying an anodic current density of 100 mA g
-1
, (weight based only on that of the nano-horn sample contained within the electrode) and the discharge curve ( E vs.
t ) was recorded. The cut-off potential was selected as

0.400 V vs the
Hg|HgO|OH

electrode. Usually all the samples showed their maximum hydrogen capacity within a few cycles but the number of charging/ discharging cycles performed was designed to check the electrochemical stability of the electrode material in relation to the discharge rate and to the cut-off potential. The electrochemical method is thus a precise and rapid method for estimating the hydrogen storage capacity.
3. Results and Discussion
3.1. Surface area and porosity determinations
The N
2
adsorption isotherm data of the sample nH (no thermal oxidation) is reported in Fig. 3A as the plot of the amount of gas adsorbed vs.
the relative pressure. The type of adsorption exhibited by the nH sample appears to correspond closely to a Type II isotherm, consistent with a non-porous or macroporous adsorbent. The hysteresis which occurs beyond p/p
0
> 0.8 gives a H3 loop,
8

HE-D-10-00896 Revised version typically observed with aggregates of plate-like particles giving slit-shaped pores [25]. The oxidised samples, nH-15, nH-30 and nH-60 show a similar but accentuated behaviour (Fig. 3B-D).
Table 2 reports the results obtained from the N
2
adsorption experiments, and of significance here is the large increase in the surface area obtained from the thermally oxidised samples. These results are in accordance with the reported effect of the thermal oxidation, viz.
, the opening up of the surface of the horns so that the internal nanopores as well as the external surfaces are now accessible to the probing gas used to determinate the surface area. The treatment also led to an increase in the pore volume and a decrease in the average diameter of the mesopores. The highest
SSA obtained for our samples was 1360 m
2
g

1
. In Fig. 4, the BET surface area is plotted against the pore volume. It can be seen that there is a very good linear relationship (gradient = 1182 m
2
cm
-
3
and R
2
= 0.996) between these two physical properties and is good evidence that increasing access to the pore structure will also increase the surface area. The data here is in good agreement with those of Xu et al [17] on SWNHs where the maximum SSA obtained was 1536 m
2
g
-1
. Their data also exhibited a good linear relationship between the SSA and V micro
, with a gradient of 2725 m
2 cm

3
. Similar results were also obtained by Texier-Mandoki [15] where for an activated carbon of
SSA 3000 m
2
g

1
, the slope of the linear fit obtained was 1913 m
2
cm

3
. Although the maximum
SSA obtained from our data was lower compared to the latter, the excellent linear relationship between SSA and pore volume found shows that the changes in the pore structures of the nH samples after the thermal oxidation treatment at 673 K are in line with those from previous studies
[15, 17].
3.2. Gravimetric determinations
All the gravimetric determinations, including the control experiments with the empty sample basket, were very carefully carried out so as to account for all the weight changes occurring during the uptake and desorption of hydrogen, as a function of gas pressure and temperature. This enabled the weight associated solely with H
2
desorption to be derived from the measurements. Indeed, the
9

HE-D-10-00896 Revised version raw data from the nH samples showed that as the sample was warmed up from liquid N
2 temperature (77 K) to room temperature, an increase in the apparent weight of the sample during H
2 desorption was observed. There are two main reasons for this. The first is the buoyancy effect whereby on cooling, the density of the gas present in the sample chamber increases and so the buoyancy force increases and as a net result, the total weight of the sample decreases. The second is associated with the very low apparent density of the nH samples. As a consequence of this, the small volume of the sample holder (a quartz basket) of the IGA could only accommodate a small weight of the nH sample. Thus, the low weight decrease on hydrogen desorption was more than masked by the buoyancy force effect as the temperature increased.
This can be readily observed from the following calculations:
W f

W i
 
V sh

  f
 i

(1) where W f and W i
are the final and initial weight measured,
ρ f and ρ i
the density of gas at the temperatures where W f and W i
, respectively, are measured and V sh the sample holder volume. The latter can be obtained from Equation (1) as:
V sh

W i

W f
 f
  i
(2)
Using now the ideal gas equation applied for the hydrogen gas: p
H
2
V
H
2
= n
H
2
R ⋅ T
or p
H
2
V
H
2
= w
H
2 mw
H
2
R ⋅ T
(3) where all the symbols have their usual meanings. The density of the gas is from Eq. (3):

H
2
 w
H
2
V
H
2
 p
H
2 mw
H
2
RT
(4) and by substitution of Eq. (4) in Eq. (2) we obtain:
V sh
=
R p
H
2
⋅ mw
H
2
W
1 / T i f
−
W f
−
1 / T i
or
V sh
= p
H
R
2
⋅ mw
H
2
⋅ W i
−
W f
T
T f
⋅ T i
−
T i f
(5)
This allows the sample holder volume V sh
to be evaluated from the weight change at the two extreme temperatures of the measurements, for the IGA experiment with the empty bucket.
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HE-D-10-00896 Revised version
By using for the calculation the following experimental values: T i
= 77 K, T f
= 298 K, p
H2
= 5.0 atm, (W i
-W f
) = 1.8

10

4
mg and the appropriate value of the gas constant R = 8.206

10

2
dm
3 atm K

1
mol

1
, we obtain V sh
= 1.53

10
-4
dm
3
or 0.153 cm
3
. The calculated volume is in excellent agreement with the geometrical dimensions of the sample holder. More rigorous calculations performed using both the virial and Van der Waals [26] equations, have verified that the difference obtained from the above value is less than 1%.
Taking thus the solid density of graphite as 2.2 g cm
-3
[26] or the more accurate particle density as 1.25 g cm
-3
[27], the calculated true volume of the samples employed in this study is between
0.0023 cm
3
and 0.0040 cm
3
, i.e.
, 1/67 or 1/38 of the volume of the sample holder. Hence the change in the buoyancy force due to the presence of the nano-horns samples is negligible and it is that due to the quartz bucket which so dominates the weight measurements.
Following from the above discussion, and in order to be absolutely certain of the evaluation of the hydrogen loading from the IGA measurements, the following experimental procedure was adopted and the data collected was analysed as follows.
Blank experiments were conducted with the empty basket to determine the change in the buoyancy force during cooling and heating in a system with no possible hydrogen uptake. Fig. 5A shows the weight change measured by the IGA during the cooling of the empty bucket to liquid nitrogen temperatures (
 196 ºC) and on subsequent warming back to ambient temperature. Of particular interest here are the stable weights recorded at the two extremes, viz.
 196 ºC and at ambient temperature. These values are shown in the insets, Fig.’s 5B and 5C, respectively. Clearly, for the blank (empty bucket) experiments the weight change can only be attributed to the change in the buoyancy force but in the experiments involving the nH samples, the observed weight changes will also contain a contribution from hydrogen loading/desorption by the nH samples. Any difference between these two absolute weight changes can thus be attributed to hydrogen loading in the samples.
11

HE-D-10-00896 Revised version
Fig. 6A shows the buoyancy weight vs.
temperature change patterns during the warming to room temperature for all the blank experiments. For simplicity, all the patterns were normalized to the final reference value of zero. The inset in Fig. 6B shows an enlarged section of the previous graph, to emphasize the good reproducibility of the experiment conducted on the empty basket. A polynomial equation (4 th
degree, R
2
= 0.9999) was fitted to the blank buoyancy data and this result was used for the correction of the weight change observed during the IGA experiments on the nH samples. The hydrogen loading, as wt% vs . temperature evaluated for all the samples (nH unannealed, nH, nH-15, nH-30 and finally nH-60, 1 st
and 2 nd
cycle) are shown in Fig. 7. The data in the figure reveal several interesting trends. Firstly, there is a clear difference in the weight loss between the as-prepared sample (unannealed) and the sample annealed at 573 K in vacuum for 6 hours in the IGA sample chamber. Secondly, the oxidation time for the samples has a remarkable effect on the hydrogen uptake. Thirdly, the annealed sample exhibits a plateau between
 120ºC to
 90°C, as indicated by the arrow in the figure; and finally, the sample nH-60 shows a remarkable difference in the hydrogen uptake between the first and the second thermal cycles.
In Table 3 the results from the IGA determinations are presented. Note that all the values reported here are the average of two determinations on the same sample, except for sample nH-60 where there was a notable difference between the 1 st
and 2 nd
cycles, as discussed below. The reported error in the values is thus the half-difference between these two determinations. The last column reports the hydrogen loading (as wt%) calculated using the data from the previous two columns. As indicated in Fig. 7, and reported in Table 3 there are two rows for the nH sample. The first refers to the as-prepared sample and the second to the sample annealed at 573 K in vacuum for
6 hours in the IGA sample chamber. (The latter procedure was adopted for all subsequent analyses of the nH samples.) The resulting difference in hydrogen uptake (0.75

0.45 = 0.30 wt%), viz., an increment in the hydrogen loading on annealing, is in accordance with the expectation of a complete elimination of the traces of atmospheric gases adsorbed on the surface of the nano-horns so that all the pores are now available for hydrogen adsorption. It can be seen that all the results obtained
12

HE-D-10-00896 Revised version unambiguously show that even a very simple change to the duration of the thermal oxidation treatment of the nH samples leads to an increase in the hydrogen loading capacity, from a starting value of 0.70 wt% for the un-oxidised sample up to 2.68 wt% for the nH sample oxidized at 673 K for 60 min.
Fig. 8A illustrates the impact of the increased surface area on the hydrogen uptake by the nanohorns. A strong linear relationship (R 2 = 0.998) is found between these two quantities suggesting that the process here is the straightforward adsorption of hydrogen onto the exposed surface of the porous material. The slope calculated for the linear relationship, 1.34 × 10 -3
wt% m
-2
g is lower than that estimated from purely geometrical considerations by other authors, e.g.
2.27 × 10
-3
wt% m
-2
g for a single-side graphene sheet or 3.0 mass% for SWNTs with an area of 1315 m
2
g
-1
at 77 K
[28], but higher than the experimental slope calculated by Nijkamp et al.
[29], of 0.1369 ml H
2
(STP) m
-2
g or 1.23 × 10
-3
wt% m
-2
g. Panella et al [16] found a value of 1.91 × 10
-3
wt% m
-2
g at 1 bar whereas Texier-Mandoki et al [15] reported values ranging from 1.01 × 10
-3
to 1.67 × 10
-3
wt% m
-2
g for pressures between 1 to 10 bar. In these studies, linear relationships between H
2
uptake and surface area were obtained from a range of carbon nanomaterials. More in line to the work carried out here, it is possible to estimate from the data of Xu et al.
[17] a value of 3.0 × 10 -3 wt% m -2 g for
SWNHs. Thus, our measured value of 1.34 ×10 -3
wt% m
-2
g for the SWNH is entirely consistent with previous work on carbon nanomaterials.
Fig. 8B shows the variation in the hydrogen uptake by the SWNH samples as a function of the evaluated pore volume. Again a strong linear relationship (R
2
= 0.998) is found, with a gradient of
1.59 wt% cm
-3
g. As before, it is useful to compare this to the values obtained by other authors.
Nijkamp et al.
[29], in their survey of hydrogen physisorption on carbon materials established a lower limit of 1.80 wt% cm
-3
g (or 216.66 ml H
2
(STP) ml
-1
) and an upper limit of 4.29 wt% cm
-3
g
(or 514.81 ml H
2
(STP) ml -1 ) at 77 K and 1 bar. In their article Texier-Mandoki et al.
[15] reported values ranging from 1.92 to 4.41 wt% cm
-3
g calculated at 1 bar using V t
or V
DR
(CO
2
). This increases to between 3.20 and 7.26 wt% cm
-3
g at 10 bar, not far from the value reported by Panella
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HE-D-10-00896 Revised version et al.
[16], of 5.9 wt% cm -3 g at 77 K for pores with diameter < 1.3 nm. Xu et al . [17] estimated a value of 7.5 wt% cm
-3
g for SWNH samples with pore diameters < 0.7 nm.
The data obtained in this study are therefore consistent with the as-prepared nH samples having limited open intra-horn spacing [27]. Upon treatment by thermal oxidation, the intra-particle pores become progressively opened, so enabling increased hydrogen adsorption at these new sites.
More difficult to explain though are the observed plateau in the hydrogen loading vs.
temperature for the nH sample not subjected to thermal oxidation in air and also, the difference in the hydrogen loading for the nH-60 sample between the 1 st
and 2 nd
cycle.
In the former, where the plateau is observed at around
 100°C, a possible explanation could be that in the nano-horn sample annealed in the IGA sample chamber under vacuum, there are more than one type of adsorption sites presented to the hydrogen gas. The interaction energy for physisorption between H
2
and graphite is reported to be of the order of 4

5 kJ mol
-1
[23]. The higher temperature site may thus be associated with the stacking of the nH’s since it has been noted in the literature [27,30-32] that there exists an optimum pore size, of ~0.7 nm, which can enhance adsorbent-adsorbate interaction and so increase the energy required for the desorption of the H
2
.
For the oxidised samples (nH-15, nH-30 and nH-60) no plateau was observed suggesting that this type of site has been modified or eliminated by the thermal oxidation in air.
The second unresolved issue with the data is concerned with the large difference in the hydrogen loading for the nH-60 sample between the 1 st
and 2 nd
cycles. The data here indicates that there is a change in the nature and/or magnitude of the sites for H
2
physisorption on the nH sample following H
2
treatment over the temperature range
 196ºC to ~25ºC. It may well be that for this sample which had undergone prolonged oxidation at 400ºC in air, the surface concentration of oxygenated carbon species ( e.g.
aldehydes, ketones and quinones) would be quite substantial, compared to the other samples. On subsequent exposure to H
2
at 5 bar pressure at 25ºC, reduction of these groups would inevitably occur, leading to species which would increase the interaction, for example, through hydrogen bonding, between these groups. The data here suggests that the nature
14

HE-D-10-00896 Revised version of these interactions and their impact on hydrogen gas interaction with the exposed surface of the carbon nano-material appears to be sensitive to the thermal treatment. Indeed, it is only in this experiment that the difference between the initial and final weight of the sample, following exposure to hydrogen, was observed to be significant (~0.9 wt%).
3.3. Electrochemical results
The galvanostatic patterns of charge (hydrogen) extraction are reported in Fig. 9 for all the electrodes, including both blanks 1 and 2 and the nH-containing samples. The electrodes were charged for a prolonged period (16.50 h) at

50 mA and discharged at 100 mA g
-1
of nH sample
(ranging from 0.44 mA for the nH-30 to 0.62 mA for the nH electrodes) and 0.50 mA for the blank electrodes. For the latter, the curves (a) and (b) do not show any plateau except for a small shoulder
(for blank 2) at c.a.

0.75 V. On the other hand, all the curves (c) to (f), relating to the nHcontaining samples show plateaus at

0.89 V (nH),

0.86 V (nH-15),

0.925 V (nH-30) and

0.90
V (nH-60), respectively. These plots, and the plateau values at ca.

0.90 V are similar to the potentials observed from metal hydride electrodes [33] and give clear evidence that electrochemical hydrogen uptake and consequently, desorption has occurred on the nano-horns, as has been reported by other authors [34, 35].
The very low apparent density of the nH samples though, as noted above, has a consequence that only a small sample weight can be inserted in the electrodes. Thus a careful evaluation of the charge extracted from the electrode supporting material has to be conducted. In this case the limiting extraction potential (

0.40 V) remains cathodic enough to avoid any oxidation of Ni to form Ni(OH)
2
in the basic solution used for the electrolytic experiments [36]. However, it has to be noted that a small quantity of hydrogen can also be adsorbed/absorbed into the Ni foam after prolonged cathodic polarization [37, 38]. The determination of the latter quantity is readily done by using the same charge/discharge experimental conditions on the nickel foam electrodes containing no nH sample.
15

HE-D-10-00896 Revised version
Table 4 shows the results obtained from two different blank electrodes using the full range of discharge currents as applied to the electrodes containing the nH samples. As mentioned above, the charging mode adopted was either (i)

100 mA applied for 1.00 hour or (ii)

50 mA for 16.50 hours (overnight). For the first blank electrode (referred to as Blank 1, 0.2048 g) the current values employed for the discharge process were 0.25 mA, 0.50 mA, 1.0 mA and 1.5 mA and for the second
(Blank 2, 0.2353 g), 0.25 mA, 0.50 mA and 0.75 mA. All the experiments were repeated several times to check for reproducibility of the data. The two last columns in the table report the extracted charge data and the capacity of the blank electrodes, calculated at the different discharging currents.
In Table 5 the results for the nH-containing samples electrodes are reported. The second column shows the weight of the Ni foam used for the electrode manufacture while the net weight of the nH sample of each electrode is reported in the third column. The discharging current used was
100 mA g -1 calculated on the net weight for all the samples, ranging from 0.44 mA (nH-30 sample) to 0.62 mA (nH sample). The charge extracted results are reported in the second last column of the table. As before, the last column reports the capacity of the sample electrodes. Here, this was called the “raw capacity” because it was calculated from the extracted charge with respect to the weight of the Ni foam (only) present in the electrodes, without any correction for the Ni foam hydrogen storage capacity.
Fig. 10 displays the “raw capacity” (mA h g -1
) data for all the electrodes, including the blanks and nH samples, charged for 1.0 hour at

100 mA (graph A) or for 16.5 hours at

50 mA (graph B).
The increase in hydrogen uptake by the nH-containing electrodes can be clearly seen from the graphs but in order to calculate the absolute capacity of the nH samples, the contribution due to the
Ni foam at the different charging conditions and at the different extraction currents must be accounted for. Table 6 reports the “net charge” (sixth column) with the subtracted Ni foam charge contribution and, finally, the “net capacity” due to nH hydrogen storage ability only (penultimate column) and the results are reported in Fig. 11, where the “net capacity” for each sample is plotted.
The non negligible experimental error on each data point is attributed to the unfortunate
16

HE-D-10-00896 Revised version combination of the small weight of the nH sample present within each electrode and the associated low hydrogen loading. Significant variations were found in the data, and the maximum hydrogen loading capacity reached was 33.1 mA h g
-1
(or 119 C g
-1
) for the nH-30 sample. To aid comparison with the pressurised hydrogen storage results obtained at low temperature (77 K), the last column of Table 6 shows the electrochemical hydrogen loading as weight %. The values obtained here are indeed rather low, ranging from 0.08 wt% for the pristine nH sample to 0.125 wt% for the nH-30 sample ( c.f
1.98 wt% in Table 3).
The hydrogen loading values of SWNHs obtained here through electrochemical loading, are quite low when compared with those achieved with other types of carbon nanostructures. Chen et al.
[39] found a capacity of 64 mA h g
-1
and 267 mA h g
-1
for the carbon nanofibres and carbon nanotubes, respectively. Li et al [34] reported a discharge capacity of 70 mA h g -1 for an unmodified activated carbon sample. On mixing the activated carbon with Cu powder, this value increased to 520 mA h g

1
but only after repeated cycling. Wang et al [20] obtained values between ca.
125 and 175 mA h g
-1
for different unmodified MWNTs. Following ball milling, the samples showed a strong increase in the hydrogen storage with a maximum capacity of 741.1 mA h g
-1
or
2.77 wt%, Much higher values, equivalent to 12 to 14 wt% of H
2
have been reported by Lipson et al [40] for SWNTs encapsulated by Pd. It is clear therefore that the hydrogen loading of the present
SWNH samples achieved through electrochemical method is poor by comparison. Furthermore, the impact of thermal oxidation here is negligible. It may well be therefore that the compacting process to make the electrode severely restricts the access of the electrolyte to the active surfaces of the
SWNH and so limits the hydrogen storage capability via the electrochemical route.
Furthermore, the cut-off potential employed for the electrochemical experiments,

0.40 V (vs.
Hg/HgO electrode) could have had an impact on the estimation of the electrochemical hydrogen storage capacity. This value is commonly used to indicate the point of total discharge of hydrogen stored as hydrides [35], but others use more negative limits, such as

0.50 V [34], or more positive limits, 0.00 V [20], and even 0.10 V [41]. A more in depth investigation conducted on the
17

HE-D-10-00896 Revised version discharge plots indicate that the underestimated discharge capacity due to the limit of

0.40 V being employed is at most between 10 to 20%, certainly not enough to explain the low hydrogen storage capacity values obtained for the nH samples. A possible explanation for this low H
2 electrochemical uptake has been proposed by Bleda-Martinez et al [42] who indicated that the presence of oxygen groups at the surface of the carbon structures inhibit the hydrogen storage, in effect obscuring the active sites of the material. Further studies are in progress in order to better understand and resolve this large difference in hydrogen loading levels obtained between pressure loading at low temperatures and electrochemical loading so as to improve the hydrogen storage capacity.
4. Conclusions
The thermal oxidation method employed here to modify the nano-horns was simple and straightforward but was nevertheless capable of bringing about substantial and controlled changes to the morphological properties of the samples. Clear evidence for this was shown by the BET surface area and porosity analysis as well as through the hydrogen loading experiments performed by the IGA instrument at low temperature. From the former, a large increase in the surface area accompanied by a marked change in the porosity distribution of the carbon material was observed, with a very good correlation between surface area and pore volume. From the IGA experiments, an increase in the hydrogen uptake from 0.5 wt% for the pristine nH samples to 2.7 wt% for the nH sample submitted to thermal oxidation for 60 min was found. Hydrogen loading via the electrochemical route at room temperature yielded quite poor uptake values when compared to those obtained by gas loading at 77 K. A more quantitative analysis of the data here was limited however by the relatively low amount of the nH samples that could be incorporated within the electrode matrix for the electrochemical study.
18

HE-D-10-00896 Revised version
Acknowledgements
The authors gratefully acknowledge the instrumental support from ITI Energy (Scotland) for this work. Special thanks to Dr. Cecila Mortalò (CNR – IENI, Padova) for the thermal analysis reported in Fig. 2 and to Dr. Paolo Guerriero (CNR -. ICIS, Padova) for the HR-SEM images.
19

HE-D-10-00896 Revised version
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Carbon 2008; 46(7):1053-9.

HE-D-10-00896 Revised version
Table 1 - Sample preparation by thermal oxidation of nH samples at 673 K in air.
Oxidation time
(min)
Initial weight
(mg)
Sample
15 39.4 nH-15 nH-30 nH-60
30
60
49.7
54.5
Final weight
(mg)
36.2
44.9
47.3
Weight change on oxidation
(wt %)
8.1
9.7
13.2

HE-D-10-00896 Revised version
Table 2 - Surface area and porosity results from N
2
adsorption experiments.
Surface area a
(m
2
g
-1
)
Pore volume b
(cm³ g -1
)
Sample nH nH-15 nH-30 nH-60
267
947
1193
1360
0.622
1.18
1.44
1.52
Pore size b
, adsorption
(nm)
10.6
6.08
5.86
5.32 a Calculated according to BET method b Calculated according to BJH method
Pore size b
, desorption
(nm)
11.0
7.57
6.90
6.26

HE-D-10-00896 Revised version
Table 3 - Hydrogen loading results for nH samples by IGA.
Weight a
(µg)
Sample nH, unannealed nH nH-15 nH-30 nH-60, 1 st cycle nH-60, 2 nd
cycle
4504.991 ± 0.018
4767.651 ± 0.036
4651.68 ± 0.49
5733.0 ± 2.2
5137.567 ± 0.077
5136.826 ± 0.050
Weight change due to hydrogen loading b
(µg)
20.3 ± 2.4
33.5 ± 2.4
73.5 ± 2.4
113.1 ± 2.4
137.8 ± 2.4
110.6 ± 2.4
Hydrogen loading
(wt %)
0.45 ± 0.05
0.70 ± 0.05
1.58 ± 0.05
1.98 ± 0.04
2.68 ± 0.05
2.15 ± 0.05 c a
All the values reported are the average of two determinations on the same sample, except for nH-60, 1 st
and 2 nd
cycle, where the s.d. is reported. b The reported error for each data is the difference between the results from two different blank experiments. c
The error reported is the sum of the previously reported errors for each data.

HE-D-10-00896 Revised version
Table 4 - Electrochemical results for blank Ni foam electrodes charging-discharging experiments.
Sample Sample weight
(g)
Charging mode
(mA × h)
Discharging current
(mA)
Extracted charge
(C)
Blank 1
Blank 1
Blank 1
Blank 1
Blank 1
Blank 1
Blank 1
Blank 2
Blank 2
Blank 2
Blank 2
Blank 2
Blank 2
0.2048
0.2048
0.2048
0.2048
0.2048
0.2048
0.2048
0.2353
0.2353
0.2353
0.2353
0.2353
0.2353
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
-
 50 × 16.50
 100 × 1.00
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
1.000
1.500
0.250
0.250
0.250
0.250
0.500
0.500
1.000
0.500
0.500
0.750
0.750
0.333 ± 0.022
0.424
0.237 ± 0.015
0.338
0.166 ± 0.008
0.224
0.140 ± 0.004
0.444 ± 0.025
0.506 ± 0.010
0.318 ± 0.013
0.428 ± 0.008
0.233 ± 0.016
0.332 ± 0.006
Capacity
(mA h g
-1
)
0.452 ± 0.030
0.575
0.321 ± 0.020
0.458
0.225 ± 0.011
0.304
0.190 ± 0.005
0.524 ± 0.030
0.597 ± 0.012
0.375 ± 0.015
0.505 ± 0.009
0.275 ± 0.015
0.392 ± 0.007

HE-D-10-00896 Revised version
Table 5 - Electrochemical results for nH samples electrodes charging-discharging experiments.
Sample nH
Ni foam weight
(g)
0.2230 nH weight
(mg)
6.2
Charging mode
(mA × h)
Discharging current
(mA)
0.620 nH nH-15 nH-15 nH-30 nH-30 nH-60 nH-60
0.2230
0.2150
0.2150
0.2368
0.2368
0.2429
0.2429
6.2
5.2
5.2
4.4
4.4
5.1
5.1
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
0.620
0.520
0.520
0.440
0.440
0.510
0.510
Extracted charge
(C)
0.738 ± 0.020
0.918 ± 0.013
0.728 ± 0.034
0.968 ± 0.015
0.760 ± 0.033
0.958 ± 0.023
0.763 ± 0.018
0.915 ± 0.040
Raw capacity
(mA h g
-1
)
0.919 ± 0.025
1.143 ± 0.016
0.941 ± 0.044
1.251 ± 0.019
0.892 ± 0.039
1.124 ± 0.027
0.873 ± 0.021
1.046 ± 0.046

HE-D-10-00896 Revised version
Table 6 - Corrected electrochemical results for nH samples electrodes.
Sample Ni weight
(g) nH weight
(mg)
Charging mode
(mA × h)
Extracted charge
(C) nH nH nH-15 nH-15 nH-30 nH-30 nH-60 nH-60
0.2368
0.2368
0.2429
0.2429
0.2230
0.2230
0.2150
0.2150
6.2
6.2
5.2
5.2
4.4
4.4
5.1
5.1
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
 100 × 1.00
 50 × 16.50
0.738 ± 0.020
0.918 ± 0.013
0.728 ± 0.034
0.968 ± 0.015
0.760 ± 0.033
0.958 ± 0.023
0.763 ± 0.018
0.915 ± 0.040
Net charge
(C)
0.486 ± 0.031
0.568 ± 0.026
0.464 ± 0.053
0.600 ± 0.032
0.433 ± 0.057
0.525 ± 0.042
0.462 ± 0.040
0.497 ± 0.059
Net capacity
(mA h g
-1
)
21.8 ± 1.4
25.4 ± 1.2
24.8 ± 2.8
32.1 ± 1.7
27.3 ± 3.6
33.1 ± 2.7
25.2 ± 2.2
27.1 ± 3.2
Hydrogen storage
(wt %)
0.082 ± 0.005
0.096 ± 0.004
0.093 ± 0.011
0.121 ± 0.006
0.103 ± 0.014
0.125 ± 0.010
0.095 ± 0.008
0.102 ± 0.012

HE-D-10-00896 Revised version
Captions to figures
Fig. 1 TEM micrograph (A) and HR-SEM image (B) of SWNH aggregates.
Fig. 2 Thermogravimetric analysis of SWNH aggregates in air. The continuous line is the weight loss data plot (left axis); and the dotted line, the derivative plot (right axis).
Fig. 3 Nitrogen adsorption isotherms at 77 K on samples: (A) nH; (B) nH-15; (C) nH-30; (D) nH-
60. Filled symbols (adsorption); open symbols (desorption).
Fig. 4 Plot of BET surface area vs.
pore volume for the SWNH samples.
Fig 5 Weight and temperature changes recorded during the initial cooling to 77 K and subsequent warm up to room temperature for the blank experiment in the IGA. Graph (A) indicates the weight (continuous line, left axis) and sample temperature (dotted line, right axis) changes vs. time. Graph (B) shows the initial cooling stage and graph (C), the final warm up stage to room temperature. Filled symbols (weight); open symbols (temperature).
Fig 6 Buoyancy weight as a function of temperature for the blank experiments over the temperature range from
 196ºC to 0°C. Symbols (●) blank 1, first cycle; (○) blank 1, second cycle; (■) blank 2, first cycle; (□) blank 2, second cycle.
Fig 7 Hydrogen desorption from the SWNH samples, evaluated from the IGA measurements: (

) nH-unannealed; (

) nH; (  ) nH-15; (

) nH-30; (  ) nH-60 first cycle; (  ) nH-60 second cycle. The arrow indicates the plateau observed for the nH sample.

HE-D-10-00896 Revised version
Fig. 8 Graph (A): hydrogen loading vs. SSA and graph (B, ) hydrogen loading vs.
the pore volume for the different nH samples. Best fit line performed only on the filled symbols. Open symbols shows the hydrogen loading obtained from the first cycle on sample nH-60.
Fig 9 Discharge curves recorded in 6.0 M KOH for the blank and nH electrodes at different discharge currents: (a) Ni blank 1, 0.50 mA; (b) Ni blank 2, 0.50 mA, dotted line; (c) nH,
0.62 mA; (d) nH-15, 0.52 mA; (e) nH-30, 0.44 mA; (f) nH-60, 0.51 mA, dotted line.
Fig. 10 Recorded discharge capacity as a function of the discharge current for the samples: (●) blank
1; (■); blank 2 and (♦) nH modified electrodes. Graph (A) after charging at 
100 mA for
1.0 h; Graph (B) after charging at

50 mA for 16.5 h.
Fig. 11 Calculated net capacity for the pristine and modified nH samples for the different charging conditions:
 100 mA for 1.0 h (●) and  50 mA for 16.5 h (○).

(A)
HE-D-10-00896 Revised version
Figure 1
(B)
20 nm

HE-D-10-00896 Revised version
Figure 2
2.00
1.60
1.20
0.80
0.40
0.00
200
0.015
0.010
0.005
0.000
400 600
T / K
800 1000 1200
-0.005

HE-D-10-00896 Revised version
50
40
30
20
10
0
50
40
30
20
10
0
0.0
(A)
0.2
(C)
0.0
0.2
0.4
0.6
0.4
P/P 0
0.6
Figure 3
50
40
30
20
10
0
0.8
1.0
0.0
(B)
0.2
50
40
30
20
10
0
(D)
0.8
1.0
0.0
0.2
0.4
0.4
0.6
P/P 0
0.6
0 .8
0 .8
1.0
1.0

HE-D-10-00896 Revised version
Figure 4
1600
1200
800
400
0
0.00
0.40
0.80
1.20
Pore Volume / cm 3 g -1
1.60

HE-D-10-00896 Revised version
0.15
0.10
0.05
0.00
-0.05
-0.10
0
0.15
0.10
0.05
0.00
-0.05
-0.10
0
Figure 5
(A)
400
(B)
800 t / min
1200
(C)
1600
-200
20
18
-192
1230 1245 t / min
1260
16
-194
15 t / min
30
-196
50
0
-50
-100
-150

(A)
HE-D-10-00896 Revised version
Figure 6
0.0 00
-0.040
-0.080
-0.120
-0.160
-0.200
-200 -160 -120
T / °C
-80
-0.092
(B)
-0.096
-0.100
-0.104
-40
-0.108
-160 -158 -156 -154 -152 -150
T / °C
0

HE-D-10-00896 Revised version
Figure 7
3.00
2.00
1.00
0.00
-200 -160 -120
T / °C
-80 -40 0

3.00
(A)
HE-D-10-00896 Revised version
Figure 8
2.00
1.00
0.00
0
3.00
(B)
400 800
Surface Area / m 2
1200 g -1
2.00
1600
1.00
0.00
0.00
0.40
0.80
1.20
Pore Volume / cm 3 g -1
1.60

HE-D-10-00896 Revised version
Figure 9
-1.20
-1.00
-0.80
-0.60
-0.40
0 a b
500 c d f e
100 0 t / s
1500 2000 2500

HE-D-10-00896 Revised version
Figure 10
1.50
1.00
(A)
0.50
0.00
0.00
1.50
0.50
1.00
1.50
(B)
2.00
1.00
0.50
0.00
0.00
0.50
1.00
i / mA
1 .50
2.00

HE-D-10-00896 Revised version
Figure 11
40
30
20
10
0
0 nH nH-15 nH-30 nH-60
1 2 3
Sample sequence
4 5