Dynamical Observation of the Intercalation Process
of H2SO4 into Pyrographite by X-Ray Microscopy
D. Erre, E. Bourelle, B. Claude-Montigny,
A. Métrot, J. Cazaux
LASSI/DTI EP 120, BP 1039, Faculté des Sciences,
51687 REIMS Cedex 2, France
E-mail : lassi@univ-reims.fr
Abstract. A time series of radiographic images with a lateral resolution in
the micron range has been recorded to follow, in situ, the intercalation
process of H2SO4 into pyrographite. With respect to the conventional use of
electrochemical equilibrum curves, the use of X-ray microscopy permits,
for the first time, to detect very high intercalation stages and to establish the
co-existence in the specimen of various progression fronts. Moreover, the
intercalation velocity can be estimated. This experiment demonstrates the
ability of X-ray microscopy in studying the dynamics of liquid/solid
diffusion processes.
1 Introduction
It is possible to intercalate a large variety of partly ionized elements or molecules
between the layers of lamellar crystals such as graphite, molybdenum disulphide,
etc. These compounds open new fields of applications in catalysis, electrical
conductors or more recently in stocking electrical energy for instance in lithiumion secondary batteries [1]. One problem for such in-situ characterization is the
lack of lateral resolution : X-ray diffractometry provides only global analysis, at
best on areas in the square millimeter range and moreover the chemical
composition is only obtainable by ex situ sequential microanalyses of samples
obtained from quasi-equilibrium electrochemical curves [2]. In fact the most
suitable experimental method to follow "in situ" intercalation processes seems
presently to be X-ray microscopy if one considers the intrinsic advantage of this
technique to observe the inner of rather thick specimens even set in liquids and its
sensitivity for the detection of medium elements embedded in light matrices [3].
Combined to the speed of the image acquisition brought by a CCD camera used as
detector, this technique seems particularly convenient to follow the intercalation
processes of H2SO4 into pyrographite. Its usefulness has been recently
demonstrated for following the Zn2+ ion diffusion in aqueous media [4].
Electrochemical intercalation of H2SO4 into pyrographite is a well-known process
which occurs via stage formation, i.e. dense layers of solvated hydrogenosulphate
ions successively invade interlamellar spaces of graphite ordered along the c-axis.
The equilibrium potential versus charge curve E=f(Q=It) is characterized by twophase plateaux alternating with capacitive linear slopes for pure stages [2]. Such
co-existing intercalation fronts will be investigated here by X-ray microscopy.
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D. Erre et al.
2 Experimental
In the reported experiment the specimen is made of a parallelipipedic Highly
Oriented PyroGraphite block (HOPG : 5,05 x 4,15 x 0,80 mm3) inserted into a
PTFE thermo retractable cell whose external walls are 70 µm thick. The cell is
filled with 96% H2SO4 (18 mol.l-1) before being hermetically closed. The
electrochemical set-up includes classically three electrodes. A pseudo reference
electrode is realised by polarizing a platinum wire with an electric circuit between
this wire and the counter electrode CE. Two platinum wires crossing the cell wall
ensure an electric contact respectively with the solution (CE) and the graphite
(working electrode WE). A constant current is applied between WE and CE by
means of an external power supply standing outside the microscope (experimental
insertion current: 13 mA/mg). Our shadow X-ray microscope is derived from a
conventional scanning electron microscope equipped with a cooled CCD camera
[4,5]. The X-ray beam is generated by the bombardment of a metallic target with a
primary electron beam (1 to 30 keV). In the vacuum chamber, a target holder
allows to change the energy of incoming photons by simply using its x-movement
and a specimen holder permits to visualise different parts of a sample by moving it
in the x-y-z directions. Depending upon the position of the specimen, the lateral
resolution is situated between the dimension of the X-ray source (about 1 µm) and
the pixel size (about 25 µm) of each detecting element of the CCD camera. In the
present work, the lateral resolution is around 10 µm. The detection limit of heavy
atoms embedded in a weak absorbing medium reach 10-4 at/at, even 10-5 at/at in
particular cases. The advantage of the experimental arrangement is related to the
speed, the good linearity and the wide dynamic (up to 3.104) of the 2D detector.
Moreover because these images are acquired and digitally stored, they lead
themselves naturally to direct measurement and quantitative analysis of the
specimen absorptivity at each pixel. Following the well known Beer's law ( I = Ioeµt
with µ : linear absorption coefficient and t : thickness of the specimen) a
logarithmic compression of each images permits to obtain µt maps [5]. After, the
atomic concentration c of the intercalated species (i.s.) in the HOPG block can be
obtained from the following expression, considering that the thickness is constant
for the two concerned zones in the image ( ti.s.+HOPG = tHOPG ) and that the radiation is
monochromatic: c = ∆µt/µt (σi.s./σHOPG - 1)-1 where ∆µt = µti.s.+HOPG - µtHOPG and σ
designates the photoabsorption cross-sections. In addition, during the intercalation
process, the µt image acquired at each time of the intercalation process (µt1, µt2, ...,
µtn) includes the absorption of the wall cell and of the HOPG specimen. But this
spurious contribution can be easily removed by subtracting pixel by pixel each
image µtn from the first one µt0 (acquired at the beginning of the process) in order
to obtain (from µtn - µt0) the µt value of only the intercalated species. Also the
subtraction pixel by pixel of two successive images, µtn-µtn-1, allows to visualize the
spatial change of the intercalated species between the corresponding acquisition
period.
In the present experiment, 80 microradiographies were acquired; their
acquisition time was 1 minute and the interval time between two successive
Dynamical Observation of the Intercalation Process of H2SO4 into HOPG
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images 5 minutes. The operating conditions for the X-ray microscope were Io = 1
µA ; Eo = 30 keV (primary beam intensity and energy respectively) and the used
radiation was produced with a cobalt target foil, 3 µm thick.
3 Results and Discussion
Figure 1 shows six images resulting from the difference µtn - µt0 where µtn with the
following acquisition instants τ = 10 minutes (a) , τ = 40 minutes (b), τ = 125
minutes (c), τ = 195 minutes (d), τ = 205 minutes (e) and τ = 280 minutes (f).
Fig.1a shows the beginning of the intercalation process with the insertion of a
poor stage (in black) into pristine HOPG (in gray).
a
b
c
500 µm
10 min
d
195 min
40 min
e
205 min
125 min
f
280 min
Fig. 1. Exemples of microradiographic images taken at different instants of the
intercalation process (on the top of each images, there is the shadow image of the Pt working electrode). See the scale in image 1c.
From the electrochemical equilibrium curve E=f(t) the poor stages cannot be
assigned. However this analysis is feasible by X-ray microscopy using figures 1d
and 1e as references where stages 1 and 2 coexist. From their absorption
measurement it is possible to deduce that the initial stage is 8 (Fig.1a).
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D. Erre et al.
St 4
a
b
c
St 3
St 2
St 2
80-75
d
175-170
85-80
e
f
St 2
St 1
St 1
200-195
210-205
345-340
Fig. 2. Difference between two successive µt images (µtn-µtn-1) in order to outline the
progression of the H2SO4 fronts (in black). Three different stages coexist in the images a
and b; two in d and e. The stage is pure in each images c and f.
The subtraction, µtn-µtn-1 is illustrated in Fig.2. In the electrochemical
conditions of Fig.2a and 2b, a mixture of stages poorer than stage 2 was expected.
In fact X-ray microscopy demonstrates that three distinct phases coexist with stage
2, stage 3 and stage 4. Furthermore the progression of stage 2 into stage 3 is
similar to the progression of stage 3 into stage 4, giving then a new experimental
information of this process. The absence of front on Fig.2c is consistent with the
E=f(t) curve where only a pure stage 2 was expected. In fact, during a pure stage,
the acid concentration remains constant, only its ionicity changes. In the same
way, Fig.2d and 2e show that the stages 2 and 1 coexist and also Fig.2f proves that
stage 1 is pure for t ≥ 340 minutes. The direct view of the process also allows to
observe the front bending which is certainly due to the fact that the HOPG crystal
was more compressed at its borders than at its central part because of the thermoretractable cell, preventing thus the acid access.
Moreover, the intercalation velocity can be easely estimated from images µtn-µtn-1
for each stage by deducing the penetration length di (of stage i) versus time curves
(Fig.3). It is demonstrated therefore that there is a mixture of quasi-diffusionnal
and linear processes in our operating conditions.
Thus, the penetration velocity of stage 3 into poor stages n deduced from Fig.3,
is v3->n =
d(d 3 )
= 120 τ3-1/2 mm.min-1 because d3 = 240 τ31/2 and the process appears
dτ
consequently as quasi-diffusionnal. In the same way, the penetration velocity of
stage 2 into stage 3 can be calculated, but for two periods: before τ2 = 40 min and
after. Indeed, for τ2 < 40 min, (d2)1 = 175 τ2-1/2 consequently (v2->3)1 = 87.5 τ2-1/2
mm.min-1 and besides, for τ2 ≥ 40 min, (d2)2 = 30 τ2 therefore (v2->3)2 = 30 mm.min1
. In that case the process is firstly quasi-diffusionnal and secondly linear
Dynamical Observation of the Intercalation Process of H2SO4 into HOPG
stage 2 ---> stage 3
stage 3 ---> poor stages
2000
d (µm)
1000
2000
(d2) 1
1000
0
stage 1 ---> stage 2
(d 2)2
2000
0
0
20
40
60
τ 3(min) = τ
80
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1000
0
0
20
40
60
τ 2(min) = τ − 65
80
0
20
40
60
τ 1 (min) = τ − 185
80
Fig. 3. H2SO4 penetration length d for each stages 1, 2 and 3 versus time with τι = τ − τ
(start of stage i).
(Faradayís law). Finally, the stage 1 penetrates into stage 2 with linear process
which regression gives d1 = 30 τ1 and v1->2 = 30 mm.min-1.
4 Conclusion
The experiment reported here illustrates the advantage of X-ray microscopy to
follow the in-situ intercalation of H2SO4 into pyrographite. More generally the
experiment demonstrates that X-ray microscopy is presently a very convenient
technique to investigate diffusion and migration studies in liquids or at solid/liquid
interfaces including the diffusion of liquids into porous materials [5]. In the near
future we plan to perform similar intercalation experiments but with two different
types of intercalated acceptors by using various characteristic radiations [3,4,5,6].
Another possible experiment is to perform X-ray microtomography during the
intercalation process in order to visualize in 3 dimensions the migration front of
the intercalated species. This last experiment is feasible but it requires the building
of a rotating cell. These examples show that X-ray microscopy as operated with
our instrument may open new fields of investigation in chemistry and
electrochemistry in condensed media, a field where the lack of techniques for
microinvestigation is obvious. Its application to electrointercalation may be usefull
for a better understanding of kinetics mechanisms.
Acknowledgements
This work has been funded by the Commission of the European Communities
(Human Capital and Mobility : X-ray Microscopy Network) under the contract
number ENAA 168 388.
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D. Erre et al.
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