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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 7
Available online at w
ScienceDirect
journal homepage: www.elsevier .com/locate/he
Graphene coating as a protective barrier againsthydrogen embrittlement
Tae-Heum Nam a, Jung-Hun Lee a, Seok-Ryul Choi a, Ji-Beom Yoo a,b,Jung-Gu Kim a,*
a School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300, Chunchun-dong,
Jangan-gu, Suwon 440746, Republic of Koreab SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 300, Chunchun-dong,
Jangan-gu, Suwon 440746, Republic of Korea
a r t i c l e i n f o
Article history:
Received 20 March 2014
Received in revised form
19 May 2014
Accepted 20 May 2014
Available online 14 June 2014
Keywords:
Graphene
Protective barrier
Hydrogen embrittlement
Slow strain rate tests
* Corresponding author. Tel.: þ82 31 290 736E-mail address: [email protected] (J.-G. Kim
http://dx.doi.org/10.1016/j.ijhydene.2014.05.10360-3199/Copyright © 2014, Hydrogen Energ
a b s t r a c t
The applicability of a graphene coating as a protective barrier against hydrogen embrit-
tlement was studied. To simulate the hydrogen embrittlement, complex environment of
tensile stress with simultaneous hydrogen charging was applied. The strain at fracture,
ductility and ultimate tensile strength of graphene-coated copper under the charged
condition were preserved above 95% comparing uncharged bare copper. After hydrogen
charging for 12 h, the hydrogen content in graphene-coated copper was lower than that in
bare copper. Using attenuated total reflectance infrared spectroscopy and Raman spec-
troscopy, it was verified that graphene can interrupt the hydrogen penetration by the
formation of CeH sp3 bonds. Unfortunately, it induced a distortion of graphene structure,
which increased the defects in the graphene. Nevertheless, the graphene coating is ex-
pected to decrease the hydrogen embrittlement susceptibility of metal substrate.
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
Introduction
The effect of hydrogen on themechanical properties ofmetals
is known as hydrogen embrittlement (HE), which can cause
catastrophic failures [1]. HE has been an important issue in
various applications, such as reactor vessels in nuclear plants
[2,3], high-pressure gaseous hydrogen (HPGH2) storage tanks
[4,5], and pipelines for natural gas and petroleum [6,7]. There
are two mechanisms of HE in metals. One is that the atomic
hydrogen is attracted to crack tips, and reduces the fracture
energy while encouraging cleavage-like failure [8,9]. The other
mechanism involves atomic hydrogen enhancing themobility
of dislocations through an elastic shielding effect, causing
0; fax: þ82 31 290 7371.).
32y Publications, LLC. Publ
locally reduced shear strength, and eventually enhancing the
local plasticity [10]. Although there is a difference in reaction
process of hydrogen, it is distinct that HE is induced by
interaction between internal hydrogen and external tensile
stress. Therefore, in order to reduce the potential risk of
hydrogen embrittlement, or to protect metal from hydrogen
permeation, several methods have been proposed, including
heat treatment [11], neon/helium glow discharge [12], in-
hibitors of hydrogen permeation [13], and protective barriers
of zirconium dioxide [14]. However, the limitations of these
methods include bulky thickness, toxic substance usage,
relatively low inhibition efficiency, and detrimental effects on
the matrix materials. Recently, we report new methods for
eliminating the hydrogen inside low alloy steel by
ished by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 7 11811
electrochemical discharge [15] and electrical field [16]. Devel-
opment of new method, which can fundamentally protect
material from hydrogen permeation, is still required.
Graphene possesses various unique properties that are
suitable for protective barriers of metal against hydrogen
permeation. Graphene is impermeable to gas molecules [17].
Hydrogen is also adsorbed on graphene surfaces as sp3 CeH
bonds [18e20], which enables the viability of graphene for
hydrogen storage [21,22]. Additionally, graphene is chemically
stable in ambient atmosphere at up to 400 �C [23], and can
protect the underlying metal substrate from corrosion [24,25]
and oxidation [26]. Also, growth techniques for large-area
graphene film are being developed, which will enable the
commercial applicability of graphene [27]. However, graphene
has not been studied as a protective barrier against HE.
Especially, under applied stress and hydrogen charge simu-
lating HE condition, the effect of graphene on the mechanical
properties of bulky metal has not also been investigated.
In this study, the applicability of graphene as a protective
barrier against HE is investigated. Graphenewas coated onto a
copper substrate by chemical vapor deposition (CVD). The
mechanical properties of specimens were assessed by slow
strain rate tests (SSRTs) under tensile stress with simulta-
neous hydrogen charging. In order to quantitatively evaluate
the susceptibility of specimen to the HE, the hydrogen
embrittlement ratio (HER, %) was calculated. The actual pro-
tective efficiency of graphene for hydrogen permeation was
evaluated by measuring the content of hydrogen inside of a
specimen using gas chromatography (GC). The effect of gra-
phene on the hydrogen evolution reaction on the specimen
surfacewas investigated by cathodic polarization tests. Lastly,
a protective mechanism of graphene against HE is proposed
and verified by attenuated total reflectance infrared spec-
troscopy (ATR-IR) and Raman spectroscopy.
Material and methods
Graphene preparation
For the synthesis of graphene by chemical vapor deposition
(CVD), copper foil (70 mm, purity 99.9%) was placed into a
quartz tube. The specimen was annealed at 1050 �C under H2.
A gas mixture of CH4 (20 sccm) and H2 (10 sccm) was flowed
into the quartz tube for the growth of graphene under 10 Torr.
After 30 min, the chamber was cooled down to room tem-
perature under H2.
Hydrogen charging
Cathodic charging is used to introduce hydrogen into a spec-
imen. The use of so-called poisons in the electrolyte is applied
to hinder the H2 formation and to consequently improve the
hydrogen absorption. Arsenic-based poisons such as As2O3
are used in sulfuric acid during hydrogen charging [28]. Before
the experiment, nitrogen gaswas bubbled into the solution for
2 h to exclude the influence of oxygen. To charge the
hydrogen, the galvanostatic test is performed at �1.0 mA/cm2
for 12 h. A solution of 0.5 M sulfuric acid (H2SO4) þ 250 mg/L
arsenic trioxide (As2O3) (pH ¼ 1.0) was used to control the
hydrogen formation and to ease the percolation and diffusion
of the hydrogen atoms into the specimen.
Slow strain rate tests (SSRTs)
To identify the effect of graphene as a hydrogen diffusion
barrier on the mechanical properties of a specimen, slow
strain rate tests (SSRTs) are conducted at a constant strain rate
of 1.0 � 10�6/s. This test is extensively used to evaluate the
resistance of a material to HE and stress corrosion cracking
[29,30]. A schematic picture of the test setup was presented
(Fig. 1a). Cylindrical tensile specimens are fabricated accord-
ing to NACE Standard TM0177 and have threaded ends with a
diameter of 6.35mmand a gauge length of 25.4mm. Except for
the gauge length, the specimens are coated with insulating
lacquer to make the exposed surface areas identical. The
specimen is connected to pull-rods, and the load and elon-
gation are monitored continuously by a load cell and a linear
variable differential transformer (LVDT) until fracture occurs.
During the SSRT, hydrogen is charged until failure occurs. By
applying complex conditions of hydrogen charge and tensile
stress, the susceptibility of the metal to HE can be evaluated.
Hydrogen content measurement
Hydrogen is electrochemically charged for 12 h, and then the
amount of hydrogen accumulated inside the specimen is
measured by gas chromatography in the temperature range of
50e500 �C. The hydrogen charged specimens are introduced to
a thermostat-controlled furnace and held under inert argon
purge. A constant heating rate of 10 �C/min is applied, and the
amount of hydrogen evolved is measured.
Cathodic polarization
An EG&G PAR Model 273A potentiostat was used for electro-
chemical polarization tests which were conducted using a
conventional three-electrode system. The test specimen was
used as the working electrode, a saturated calomel electrode
(SCE) was used as the reference electrode, and graphite was
used as an auxiliary electrode. A solution of 0.5 M sulfuric acid
(H2SO4) þ 250 mg/L arsenic trioxide (As2O3) (pH ¼ 1.0) was
used. Before the experiment, nitrogen gas was bubbled into
the solution for 2 h to exclude the influence of oxygen.
Cathodic polarization tests were performed at a scanning rate
of 0.166 mVSCE from þ50 mVOCP to �1200 mVSCE.
Characterization of graphene
The hydrogen charged and uncharged graphene was trans-
ferred onto Si wafer (SiO2(300 nm)/Si) by the polymethyl
methacrylate (PMMA) for Raman spectroscopy and atomic
force microscopy (AFM) analyses. First, the PMMA was coated
on graphene/Cu samples as a supporting layer. The copper foil
was etched by copper etchant (SigmaeAldrich Korea Ltd.,
3e4% of HCl, 30% of FeCl3, 66e67% of water) and the PMMA/
graphene transferred onto the wafer. Finally, the PMMA was
removed by acetone. Attenuated total reflectance infrared
(ATR-IR) spectra were recorded with an IFS 66/S FTIR spec-
trometer (Bruker) equipped with a Harrick Scientific
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 711812
horizontal reflection Ge-attenuated total reflection accessory
and the spectrum was collected for 64 scans. Raman spec-
troscopy and mapping (WiTec, Alpha 300 M) with an excita-
tion wavelength of 532 nm were used to measure the
crystallinity and binding with hydrogen.
Results and Discussion
Slow strain rate tests (SSRTs)
The stressestrain curve measured in the SSRT is shown
(Fig. 1b). The behavior of charged bare copper is distinctly
different, but that of graphene-coated copper is similar to that
of uncharged bare copper. Under hydrogen charging, all of
mechanical properties of graphene-coated copper are higher
than those of bare copper. Detailed results of SSRT, such as
time to fracture, ductility and ultimate tensile strength, are
presented (Table S1). The strain at failure is used for assessing
the HE resistance, and is presented (Fig. 1c). It was clearly
Fig. 1 e The results of specimen by slow strain rate tests. (a) sc
presented and it is composed of (1) a saturated calomel referen
tensile specimens (with a diameter of 6.35 mm and a gauge len
corrosion cell containing an electrolyte. (b) the strain-stress cur
embrittlement ratio (HER, %).
declined as a result of the adverse effect of the hydrogen
accumulated inside the metal. The fracture strain of the
hydrogen-charged copper (45.5%) decreased compared to that
of uncharged copper (50.6%). However, the fracture strain of
hydrogen-charged graphene-coated copper (49.1%) is similar
to that of uncharged copper. In the study of graphene fracture,
monolayer graphene canbe elongated at the strain of 25%until
fracture [31]. From these results, it is supposed that hydrogen
permeation can be effectively blocked by graphene coating
until the strain of 25%. After that the cracked graphene coating
reduces the surface area of underlying copper exposed to
hydrogen, which may decrease the amount of hydrogen
entering. Therefore, the strain at fracture of graphene-coated
copper is slightly lower than that of uncharged bare copper.
In order to quantitatively evaluate the susceptibility of a
specimen to the HE, the hydrogen embrittlement ratio (HER,
%) is calculated by the following Eq. (1) [32]:
Hydrogeneembrittlement ratio ðHER;%Þ¼�
3O� 3H
3O
��100 (1)
hematic of hydrogen embrittlement test equipment is
ce electrode, (2) a graphite counter electrode, (3) cylindrical
gth of 25.4 mm), (4) an SSRT extending specimen, and (5) a
ves (c) the strain (%) at failure and (d) hydrogen
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 7 11813
where 3o is the strain of the uncharged specimen at failure,
and 3H is the strain of the hydrogen-charged specimen at
failure. The hydrogen embrittlement ratio (HER) is presented
(Fig. 1d), where a higher HER signifies higher susceptibility of
the metal to HE. The HER of hydrogen-charged graphene-
coated copper (3.0%) is much lower than that of hydrogen-
charged copper (10.1%). In other words, the susceptibility of
copper to HE is reduced by using graphene coating. Mean-
while, fracture type of all specimens is not brittle. Due to the
low diffusivity of hydrogen in face-centered cubic (FCC) crys-
tal structure, hydrogen can be only trapped near the charging
surface in case of room temperature charging [33]. Therefore,
fracture type of copper is not changed from ductile to brittle
until HER of 30% due to the low susceptibility of copper to HE
[32]. However, it is obvious that the susceptibility of copper is
distinctly declined due to the graphene barrier. The results of
fracture strain and HER confirm that graphene acts as an
excellent hydrogen permeation barrier under tensile stress.
Hydrogen content measurement
The actual protective efficiency of graphene for hydrogen
permeation is evaluated by measuring the amount of
hydrogen inside of the specimen. Hydrogen is electrochemi-
cally charged for 12 h, and then the amount of hydrogen
accumulated in the specimen is measured by gas chroma-
tography in the temperature range from 50 to 500 �C. The
accumulated hydrogen is easily released by heat treatment,
which is generally used for eliminating hydrogen from ma-
terials [34]. In a recent study on graphene as a hydrogen
storage material, the hydrogen absorbed by the graphene was
dehydrogenated in the temperature range from 200 to 500 �C[18,35].
The hydrogen content released with temperature is pre-
sented (Fig. 2a). In the whole temperature range, the hydrogen
content of bare copper was higher than that of graphene-
coated copper. The maximum peak of the hydrogen content
appeared at temperatures of 200 and 450 �C, corresponding to
the diffusional (reversibly) and trapped (irreversibly) hydrogen
Fig. 2 e Hydrogen is electrochemically charged for 12 h, and then
is measured by gas chromatography in the temperature range
temperature and (b) the total amount of hydrogen released.
in the metal, respectively. The absorbed hydrogen diffuses
toward the interior of the material and a portion of hydrogen
was retained in various preferential locations. For example,
the lattice defects (grain boundaries, vacancies, dislocations)
and precipitates (carbide) provide a variety of trapping sites
[36]. The diffusional hydrogen is easily released by relatively
low thermal energy, but comparatively high thermal energy is
required to release the trapped hydrogen from the material.
Therefore, the diffusional hydrogen is released at relatively
low temperature, but the trapped hydrogen can be evolved
after the critical temperature reached. The total amount of
hydrogen released from graphene-coated copper (0.25 mmol/g)
was much lower than that from bare copper (0.59 mmol/g)
(Fig. 2b). Unfortunately, the protective efficiency of the
monolayer graphene is not perfect, but it is expected that the
efficiencymay be enhanced bymultiple layers of graphene, or
optimization of graphene growth. The results of hydrogen
measurement confirm that graphene is a proper barrier for
hydrogen penetration, and effectively declines the suscepti-
bility of copper to HE.
Cathodic polarization
Hydrogen can be electrochemically generated during the
cathodic reaction of corrosion, electroplating, and cathodic
protection, which increase the potential risk of HE. Hydrogen
is produced by cathodic reduction reactions of water and acid
by reactions:
H2Oþ e�/HþOH� (2)
Hþ þ e�/H (3)
Most of the hydrogen atoms are recombined as the mo-
lecular hydrogen (H2), but a portion of atomic hydrogen can
diffuse into the lattice and retained in the material. These
reactions can be occurred below the equilibrium potential ðEOHÞ
given by the Nernst Equation [28,37]:
EOHðmVSHEÞ ¼ �59 pH� 29:5 log PH2
(4)
the amount of hydrogen accumulated inside the specimen
from 50 to 500 �C. (a) the released hydrogen content with
Fig. 3 e The effect of graphene on the hydrogen evolution
reaction is investigated by cathodic polarization tests. (a)
the measured cathodic polarization curve and (b)
schematic cathodic polarization curve of hydrogen
evolution reaction.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 711814
At pH ¼ 1, corresponding to the electrolyte pH in the pre-
sent study, the hydrogen equilibrium potential is �59 mVSHE
(�300 mVSCE) for a hydrogen partial pressure PH2of 1 atm. The
amount of hydrogen (N) produced per second per unit area
(cm2) is given by Faraday's law:
N ¼ ired;H�F (5)
ired;H ¼ iO;H exp�� 2:3
�E� EO
H
��bred;H
�(6)
where ired,H is the current density induced by cathodic
reduction reaction, iO,H is the exchange current density (cur-
rent density at equilibrium potential Ecorr), bred,H is the Tafel
constant (slope of cathodic polarization curve), E is any po-
tential lower than EOH, and F is the Faraday constant
(96487 Coulomb/mole). This equation indicates that hydrogen
production at the specimen surface occurs at higher rates as
the current density (ired,H) increases and the potential (E) be-
comes progressively less than EOH.
The measured cathodic polarization curve and the sche-
matic cathodic reduction behavior in a deaerated acid solution
(pH ¼ 1) is shown (Fig. 3). Based on the cathodic polarization
curves (Fig. 3b), the hydrogen reduction reactiondominated the
cathodic reactions below the potential of �300 mVSCE. In the
whole potential range (Fig. 3a), the hydrogen production rate
(current density) on graphene-coated copper is much lower
than that on bare copper. In a recent study on graphene as
corrosion-inhibiting coating, it is reported that graphene acts
primarily as an inhibitor of the cathodic reactions [25]. Also, it
was revealed that a graphene monolayer forms a charge
transfer barrier between the solution and the graphene mono-
layer on the copper surface, which suppresses the corrosion
reaction [24]. The charge transfer resistance of specimen is
evaluated by electrochemical impedance spectroscopy (Fig. S1
and Table S2). The charge transfer resistance of graphene
coated-copper is much higher than that of bare copper. From
these results, it is confirmed that graphene acts as a charge
transfer barrier for the hydrogen evolution reaction. Therefore,
the graphene coating can improve the resistance against HE.
Characterization of graphene
In a recent study on graphene as a hydrogen storage material,
it was revealed that hydrogen is absorbed by graphene as CeH
sp3 bonds, which enables the hydrogen storage [18e20]. As a
hydrogen barrier, it may be proposed that graphene interrupts
hydrogen permeation by the interaction with hydrogen (for-
mation of CeH sp3 bonds). This proposed barrier mechanism
of graphene was verified through the CeH stretching bands of
ATR-IR spectra and the bands of Raman spectra. The IR
spectra of the hydrogenated (hydrogen charged) graphene
clearly show aliphatic CeH stretching bands in the region of
2850e2950 cm�1, but they disappear for the dehydrogenated
graphene [18]. ATR-IR spectra of pristine graphene and
hydrogen-charged graphene were shown (Fig. 4a). There was
no CeH stretching band (region of 2750e3000 cm�1) for pris-
tine graphene, but it was distinctly appeared for hydrogen-
charged graphene.
The pristine graphene synthesized on copper foil by
chemical vapor deposition is predominantly single-layer
graphene, which is confirmed by the ratio of the G band peak
(1580 cm�1) and the 2D band peak (2680 cm�1) in Fig. 4b. The D
peak represents the disorder in the graphene and the D0 peakoccurs due to the presence of defects in the graphene [38e40].
The D peak (1350 cm�1), D0 peak (1620 cm�1), and a combina-
tionmode of these peaks (Dþ D0 lie at 2950 cm�1) appeared for
hydrogen-charged graphene. When hydrogen absorbed onto
graphene, geometrical deformation of the hexagonal struc-
ture was induced, which increase disorders and defects in the
graphene [38,41]. During hydrogen charging, most of the
hydrogen exists as a molecule and is physisorbed above the
graphene. However, a portion of the hydrogen is retained as
atomic hydrogen, and absorbed hydrogen on top of the gra-
phene breaks p bonds and produces additional s bonds, which
induces transition from C]C to CeH and CeC bonds by
forming sp3 hybridization bonds [41e43]. In order to under-
stand why the atomic hydrogen breaks p bonds on the gra-
phene sheet and produces additional s bonds, we simply
calculated the bonding energy (BE) by comparing which is
more stable between graphene and hydrogenated graphene.
Fig. 4 e Hydrogen adsorption on graphene was verified through the CeH stretching bands of ATR-IR spectra and the bands
of Raman spectra. (a) the region of CeH stretching bands (2750e3000 cm¡1 ATR-IR spectra) (b) the Raman spectra of the
pristine and hydrogen-absorbed graphene and (c) the change of energy depending on reaction processes.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 7 11815
For the simple comparison, graphene was assumed that the
hexagonal system consists of three CeC bonds (s bond, BE
3.596 eV) and three C]C bonds (s bond and p bond, BE
6.239 eV). The BE of hydrogenated graphene system consists of
six CeC bonds and three CeH bonds (BE 4.280 eV) [44]. The
change of free energy is calculated by:
DG¼X
BEðgrapheneÞ�X
BEðhydrogenated�grapheneÞ (7)
DG¼ ð3�3:596þ3�6:239Þ� ð6�3:596þ3�4:280Þ ¼�4:911eV
(8)
Based on this result, the hydrogenated graphene is ener-
getically favorable and thus atomic hydrogen can be chemi-
cally absorbed on graphene. Recent studies on adsorption of
hydrogen onto graphene have reported that absorbed
hydrogen forms hydrogen clusters [45,46] and cover gra-
phene up to about 75% of surface [46,47]. The energy barrier
depending on reaction process (penetration through gra-
phene or absorption on graphene) is schematically shown
(Fig. 4c). Passing through a center of hexagonal structure in
graphene, above 2.89 eV is required because there is a
repulsive force induced by strong electron cloud of graphene
[48,49]. It is much larger than the energy barrier of CeH
forming (0.18 eV) [50]. Hence, hydrogen penetration through
graphene is much harder than CeH sp3 formation. From
these experimental results and theoretical calculation, it can
be proposed that the charged hydrogen is combined with
graphene as CeH bonds, which interrupts the hydrogen
penetration into copper.
Conclusion
In this study, the applicability of graphene as a protective
barrier against HE was verified. To simulate the hydrogen
embrittlement (HE), complex condition of tensile stress with
simultaneous hydrogen charging was applied. The suscepti-
bility of copper to HE is drastically depleted due to graphene
protective coating, but protection effect was disappeared after
graphene have been strained above 25%. The actual protective
efficiency of graphene for hydrogen permeation is evaluated
bymeasuring the amount of hydrogen inside of the specimen.
The hydrogen permeation is sufficiently suppressed, but not
perfect due to the defects of graphene. Through the charac-
terization of hydrogen absorbed graphene, it was identified
that graphene can effectively protect the hydrogen penetra-
tion by formation of CeH bonds.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 1 8 1 0e1 1 8 1 711816
Acknowledgments
This work was supported by the National Research Founda-
tion of Korea (NRF) grant funded by the Korea government
(MEST) (No.NRF-2012R1A2A2A03046671).
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.ijhydene.2014.05.132.
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