In operando observation system for electrochemical reaction by soft X-ray absorption
spectroscopy with potential modulation method
Masanari Nagasaka, Hayato Yuzawa, Toshio Horigome, and Nobuhiro Kosugi
Citation: Review of Scientific Instruments 85, 104105 (2014); doi: 10.1063/1.4898054
View online: http://dx.doi.org/10.1063/1.4898054
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REVIEW OF SCIENTIFIC INSTRUMENTS 85, 104105 (2014)

In operando observation system for electrochemical reaction by soft X-ray
absorption spectroscopy with potential modulation method
Masanari Nagasaka,1,2,a) Hayato Yuzawa,1 Toshio Horigome,1 and Nobuhiro Kosugi1,2
1
2

Institute for Molecular Science, Myodaiji, Okazaki 444-8585, JapanThe Graduate University for Advanced Studies, Myodaiji, Okazaki 444-8585, Japan(Received 22 August 2014; accepted 2 October 2014; published online 17 October 2014)
In order to investigate local structures of electrolytes in electrochemical reactions under the samescan rate as a typical value 100 mV/s in cyclic voltammetry (CV), we have developed an in operando
observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with apotential modulation method. XAS spectra of electrolytes are measured by using a transmission-type
liquid flow cell with built-in electrodes. The electrode potential is swept with a scan rate of 100 mV/s
at a fixed photon energy, and soft X-ray absorption coefficients at different potentials are measured
at the same time. By repeating the potential modulation at each fixed photon energy, it is possible
to measure XAS of electrochemical reaction at the same scan rate as in CV. We have demonstrated
successful measurement of the Fe L-edge XAS spectra of aqueous iron sulfate solutions and of thechange in valence of Fe ions at different potentials in the Fe redox reaction. The mechanism of theseFe redox processes is discussed by correlating the XAS results with those at different scan rates.
© 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4898054]
I. INTRODUCTION

The electrochemical reaction occurs at the solid-liquidinterface between the electrode surface and the liquid electrolyte. The mechanism of the electrochemical reaction has
been mainly investigated by cyclic voltammetry (CV) measurements. In order to understand the electrochemical reaction more precisely, however, it is necessary to investigate
local structural changes of electrolytes including electric
double layers in the electrochemical reaction by using
in operando spectroscopic techniques that can measure the reaction at the same condition as in CV. X-ray absorption spectroscopy (XAS) of core electron regions is an element specificmethod to study local electronic structures and is applicableto liquid samples as well as gas and solid samples. Especially,
the soft X-ray region below 1 keV has many chemically important core levels such as C, N, and O K-edges. Recently,
the structure of liquid water has been extensively studied by
the O K-edge XAS.1–3 The transmission mode is a quantitative method to measure the absolute photoabsorption crosssection. XAS spectra of liquid samples can be measured byvarious methods, not only transmission mode4, 5 but also fluorescence yield,2 nonresonant Raman scattering process,3 total electron yield of liquid microjet,1 and inverse partial fluorescence yield of liquid microjet.6–9 The latter methods are
based on secondary processes, proportional to the probability of the core hole creation following the X-ray absorption.
Their compatibility with the transmission measurement has
been discussed as regards background subtraction, normalization, and saturation correction. Note that XAS in bulk liquid
phase measured by the nonresonant Raman scattering and theinverse partial fluorescence yield is nearly the same as XAS
in transmission mode. For the XAS measurement in transa) Electronic mail: nagasaka@ims.ac.jp0034-6748/2014/85(10)/104105/7/$30.00

mission mode, however, it is necessary to keep the thicknessof the liquid layer below a few micrometers because soft Xrays are strongly absorbed by solvent molecules.5, 10 Recently,
we have successfully developed a liquid flow cell for XAS intransmission mode.4 The liquid layer is sandwiched between
two Si3 N4 membranes under atmospheric condition and thethickness of the liquid layer is controllable between 20 nm
and 2000 nm. This liquid cell is also applicable to the XAS
measurement of solid-liquid interface by depositing a solid
substrate on the membrane and adjusting the thickness of theliquid layer as small as possible.
Furthermore, we have developed an in situ XAS measurement system to study local structures of electrolytes in electrochemical reaction by using a transmission-type liquid cellwith built-in electrodes. Change in valence of Fe ions in an
aqueous iron sulfate solution at different potentials is investigated by Fe L-edge XAS spectra.11 Each Fe L-edge XAS
spectrum is measured at a constant potential and the potential
is changed to the next one after finishing each XAS measurement. As a result, the scan rate of the potential corresponds
to 0.08 mV/s and is quite slower than a typical scan rate of
100 mV/s in CV. CV is not applicable in such slow scan rate,
and the valence of Fe ions reaches the equilibrium at eachpotential. Therefore, this XAS measurement is not able to investigate kinetics of the electrochemical reaction. In order toinvestigate the kinetics of electrochemical reaction under realistic conditions, it is necessary to develop an in operando observation system that is able to measure XAS of electrolytes
at the same scan rate as in CV.
The local structures of electrolytes in electrochemical
reactions have been investigated by several spectroscopic
techniques. Endo et al. studied adsorption structures of Br
ions on Ag(100) surfaces at different potentials in aqueous
NaBr solutions by measuring Br K-edge XAS in the hard Xray region.12 Bora et al. developed an in situ electrochemical

85, 104105-1© 2014 AIP Publishing LLC

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reaction for XAS in fluorescence yield.13 Masuda et al.
developed an hard X-ray photoelectron spectroscopy apparatus for in situ electrochemical reaction of solid-liquid interface and investigated potential-induced Si oxide growthin water at the Si membrane surface.14 The structures ofsolvent water molecules at different potentials were determined from the OH stretching mode in Fourier transform infrared spectroscopy.15, 16 The adsorption structures of watermolecules were studied by methods focusing on solid-liquidinterfaces, such as sum frequency generation17 and surfaceenhanced Raman spectroscopy.18 X-ray scattering study revealed that the orientation of water molecules at the first layerof Ag(111) electrodes is changed at different potentials in
0.1 M NaF.19 Wu et al. used Cu K-edge XAS to investigate
the effect of different anions on the structure of underpotential deposition of Cu on Au.20, 21 The in situ spatial measurements by scanning transmission X-ray microscope also
applied to the electrochemical reaction.22, 23 These spectroscopic methods revealed the local structures of electrolytes
and solid-liquid interfaces at electrodes in electrochemical reactions. However, these measurements were performed at aconstant potential and the potential is stepwise changed inelectrochemical reactions. The scan rate of the potential is
too slow to study the electrochemical reaction under the samescan rate as in CV.
In order to solve the above problem, it is necessary todevelop the spectroscopic technique that is able to sweep theelectrode potential at the same scan rate as in CV. Several
groups tried to study the electrochemical reaction under thesame scan rate as in CV. Ataka et al. studied the structuresand orientations of water molecules adsorbed on Au(111)electrodes at different potentials in perchloric acid solutions by surface-enhanced infrared absorption spectroscopy at
5 mV/s.24 Kunimatsu et al. studied the adsorption and oxidation of a methanol molecule on a platinum electrode inperchloric acid solutions containing methanol during electrochemical reaction by attenuated total reflectance Fouriertransform infrared reflection absorption spectroscopy at
5 mV/s.25 The structural changes of electrode surfaces in electrochemical reactions were studied by in situ surface X-ray
diffraction at several mV/s.26, 27 Braunschweig et al. studied the surface transformation of sulfate anions on Pt(111)
surfaces during the electrochemical reaction of sulfuric acidsolutions by sum-frequency generation at 5 mV/s.28 As described above, there are several spectroscopic studies to measure electrochemical reactions at several mV/s, but it is stillslower than a typical scan rate of 100 mV/s in CV. There arefew studies to observe the electrochemical reaction at 100
mV/s and to study the local structures of electrolytes andsolid-liquid interfaces at electrodes in electrochemical reaction by element-specific spectroscopy.
In the present study, we have developed an in operando
observation system for electrochemical reactions under thesame scan rate as in CV by XAS with a potential modulation method. This method is based on in situ XAS measurement system for electrochemical reaction recently developedby the present author group.11 By using the present system,
we have measured the potential-modulated Fe L-edge XAS
spectra of aqueous iron sulfate solutions in electrochemical

Rev. Sci. Instrum. 85, 104105 (2014)

reactions at 50 mV/s and 100 mV/s and discussed the change
in valence of Fe ions in the Fe redox reaction at the different
scan rates. The Fe L-edge (700 eV) is in the soft X-ray region
below 1 keV and is more sensitive to the 3d valence and spinstates of Fe ions than the Fe K-edge.11, 29–31 Since XAS is anelement-selective method to study local structures of liquid,in operando XAS with potential modulation has an advantage to investigate not only change in valence of ions but also
the local structural changes of molecules, such as adsorption
structures of water in electric double layers, in electrochemical reaction.
II. DESIGN OF INSTRUMENTATION

Figure 1 shows a schematic of the present in operando
observation system for electrochemical reactions by potentialmodulated XAS. The experiments were performed on the soft
X-ray undulator beamline BL3U at the UVSOR-III facility.32
Section II A describes some details of the electrochemical cellfor XAS in transmission mode. Section II B explains the inoperando observation system for the electrochemical reaction
by XAS with a potential modulation method.
A. Electrochemical cell

Figure 1(a) shows schematics of the liquid flow cell for
XAS in transmission mode. As previously described,4, 11, 33
the liquid flow cell consists of four regions (I), (II), (III), and(IV), separated by 100 nm-thick Si3 N4 membranes (NTT AT).
SiC membranes (NTT AT) are used for the N K-edge measurement. Region (I) is connected to the beamline under vacuum. Regions (II) and (IV) are under atmospheric pressure ofhelium buffer gas. The size of the Si3 N4 membrane window
between regions (I) and (II) is 0.2 mm × 0.2 mm, which issmall enough to endure a large difference in pressure, evenlarger than 1 atm. The size of the soft X-ray beam on the sample is determined by this orifice. The flow rate of the heliumgas (and other gases) in the regions (II) and (IV) is changeable by a mass flow controller (Kofloc), and the pressure (ifnecessary larger than 1 atm) is adjusted by a needle valve at
the gas outlet.
The thin liquid layer in the region (III) is sandwiched between two 100 nm-thick Si3 N4 membranes with a window
size of 2 mm × 2 mm. The liquid flow is under atmosphericcondition. Two 100 μm-thick spacers are set between thewindow frames of the membranes and the membranes arecompressed by sealing o-rings to keep the thin liquid layer below 2000 nm. Liquid samples can be exchanged in situ witha tubing pumping system (Cole-Parmer Masterflex L/S). The
thickness of the liquid layer is controllable between 20 nm
and 2000 nm by changing the He pressure in the regions (II)and (IV).4, 11 Although the thickness of the liquid layer varies
at different positions over the 2 mm × 2 mm membranes,
it is possible to measure XAS of the liquid layer at a constant thickness because of the small beam size. In order toinvestigate electrochemical reactions, three electrodes are included in the liquid layer (III).11 The working electrode (WE)
is a gold deposited Si3 N4 membranes, which consists of Au(20 nm), Cr (5 nm), and Si3 N4 (100 nm) multilayer films. One

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Rev. Sci. Instrum. 85, 104105 (2014)

FIG. 1. Schematic of the in operando observation system for the electrochemical reaction by XAS with a potential modulation method. (a) The electrochemicalcell for XAS in transmission mode. (b) The XAS measurement system with potential modulation method. (c) The data acquisition system for the reference XAS
spectra. (d) The data acquisition system for the XAS spectra in electrochemical reaction at same scan rate as in CV.

side of this membrane faces the liquid layer (III) and is connected with the Au tab for electrical conduction. The Teflon
spacer is placed on the Si frame, opposite the Au contact. The
counter electrode (CE) is a Pt mesh, which is immersed ina sample electrolyte solution. The reference electrode (RE)is Ag/AgCl immersed in a saturated KCl solution and isolated from the liquid sample by a Teflon cover. The sample
electrolyte solution is completely isolated from the ground.
The potential is controlled by using a potentiostat (Solartron
1287).
Soft X-rays pass through the regions (I), (II), and (III) andare detected by a photodiode (IRD AXUV100) in the region(IV). The intensity of soft X-ray is normalized by an electric
current of a gold mesh in the region (I), where the current
is proportional to the photon flux and is measured by a picoammeter (Keithley 6514/J). The absorption of soft X-raysin regions (II) and (IV) is small due to the high transmittanceof helium.10 The XAS spectra are obtained from the measured
transmission signal at different photon energies, by using theLambert-Beer formula ln(I0 /I), in which the current I0 is measured for the blank and the current I is for the liquid sample.
B. System for potential modulation method

In the present potential-modulated XAS, the electrodepotential is swept at a fixed photon energy and the soft X-ray
absorption coefficients at different potentials are measured at
the same time and is able to measure XAS of electrochemical
reactions at the same scan rate as in CV. Figure 1(b) showsa schematic of this system. In order to perform time-resolved
measurements of soft X-ray absorption coefficients, the electric currents obtained by the photodiode detector are amplified as voltages by a preamplifier (NF LI-76), and the voltages are converted to TTL signals by using a V-F converter(Tsujicon SN2VF-01). Figure 1(c) shows the data acquisition system for the reference XAS spectrum that is not de-pendent on the reaction time. When measuring the reference
XAS spectrum, the TTL signals are converted to NIM signals
by a logic level adapter (Kaizu Works KN200), and the NIM
signals are counted by a NIM counter (Ortec 974). Figure
1(d) shows the data acquisition system for XAS in the electrochemical reaction. When measuring XAS spectra in theelectrochemical reaction, the TTL signals at different elapsed
times are accumulated in order with a constant dwell time byusing a multichannel scaler (Ortec EASY-MCS). It is necessary to synchronize the start of the soft X-ray absorption measurement with that of the potential modulation. The measurement by the multichannel scaler is started by receiving a TTL
trigger signal generated by an analog I/O (NI USB-6259). The
trigger signal is received at the same time by a function generator (NF WF1974), which generates the potential curve of
CV. By entering the generated potential curve to the externalpotential input in the potentiostat, the electrode potential iscontrolled in the electrochemical cell.
When the measurement of soft X-ray absorption coefficients during the potential modulation at a fixed photon
energy is finished, the server PC controlling the potentialmodulated measurements communicates with the beamline
PC via a TCP/IP network. Then, the beamline PC changes
the photon energy to the next one. The liquid sample is replaced by the fresh one at the same time by starting the tubing
pump. The bottle of the liquid sample is bubbled by helium
gas in order to remove dissolved oxygen. After setting thenext photon energy, the measurement of soft X-ray absorption coefficients with the potential modulation is started bycommunicating with the server PC via a TCP/IP network.
III. RESULTS AND DISCUSSION
A. XAS with potential modulation method

The in operando XAS measurement of the electrochemical reaction has been performed by the potential modulation

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method as above described. The sample electrolyte is 0.5 M
aqueous iron sulfate at pH = 2.2. The temperature of theelectrolyte is 27 ◦ C. The change in valence of Fe ions in theelectrochemical reaction of aqueous iron sulfate solution ismeasured by the Fe L-edge XAS. The photon energy resolution was set to 0.7 eV. The oxidation of Fe(II) to Fe(III) ionsis occurred when the potential is increased, and the reduction of Fe(III) to Fe(II) ions is occurred when the potential isdecreased.11, 34–38
Figure 2 shows the changes of the potential and the soft
X-ray transmission signals, which are measured in the potential modulation at a fixed photon energy. The scan rate of thepotential is 100 mV/s, the same as in CV. Figure 2(a) showsthe electrode potential versus Ag/AgCl with saturated KCl solutions. The potential is increased from 0.0 V to 1.0 V after
receiving the trigger signal, then decreased to −0.4 V, andincreased to 0.0 V again. The one cycle of the potential modulation takes 28 s. Figure 2(b) shows the changes of the soft
X-ray transmission during the potential modulation at the different photon energy measured by the multichannel scaler.
The dwell time was set to 0.5 s, which corresponds to the potential step of 0.05 V. The soft X-ray transmission at 705 eV is
not changed at different potentials because it is the region before the Fe L-edge. The absorption of Fe(II) ions is maximum
at the photon energy of 708 eV. The transmission is increased
when the potential is increased, and shows a maximum value
at 10 s, which corresponds to the potential of 1.0 V. Then, it
is decreased again in the downward potential after 10 s. On
the other hand, the transmission at 710 eV is decreased in the
upward potential, and is increased in the downward potential,
because the absorption of Fe(III) ions is maximum at 710 eV.
The photon energy of 713 eV is the region after the edgejump. The transmission is slightly decreased in the upward
potential due to the formation of Fe(III) ions.

FIG. 2. Changes in soft X-ray transmission observed by modulating the potential of the electrodes at a fixed photon energy. One cycle of the potential
modulation takes 28 s at 100 mV/s. Both the modulation of potentials andthe measurement of soft X-ray transmission are started at 0 s. (a) The potential versus Ag/AgCl with saturated KCl solutions and (b) the soft X-ray
transmission at several photon energies as a function of time.

Rev. Sci. Instrum. 85, 104105 (2014)

FIG. 3. (a) Soft X-ray transmission spectra at different potentials at a scanrate of 100 mV/s. (b) Difference spectra at different potentials from the Fe(II)spectra obtained at −0.4 V.

Figure 3(a) shows the changes of soft X-ray transmission with potential modulation as a function of photon energy, in which several changes of transmission signals areshown in Fig. 2(b). The intensities at the peaks of Fe(II) ions(708 eV) and Fe(III) ions (710 eV) are changed at different
potentials. Figure 3(b) shows the difference spectra of soft Xray transmission at different potentials from that at −0.4 V,
where Fe(II) ions only exist by the reduction of Fe(III). The
intensities of the Fe(II) and Fe(III) ions are changed continuously at different potentials. The isosbestic points are foundat 709.2 eV, which is consistent with the previous study.11 It
means that only Fe(II) and Fe(III) ions are involved in the Fe
redox reaction.
In the soft X-ray absorption coefficients ln(I0 /I), the intensity I0 is for pure liquid water and is measured at the dwell
time of 5 s by the NIM counter and is averaged over several measurements. The reference Fe L-edge XAS spectrumof Fe(II) ions was measured at no applied potential, whereasthat of Fe(III) ions was obtained at an applied potential of
0.8 V after inducing 1.0 V during 1 min. The intensities I for
the reference spectra are measured at the dwell time of 5 s
by the NIM counter and are averaged over several measurements. Figure 4 shows the reference XAS spectra of Fe(II)and Fe(III) ions. These spectra are consistent with the previous studies.11, 29–31
For in operando XAS spectra of the electrochemical reaction, the intensities I at different potentials are obtainedby summation of the reference soft X-ray transmission of
Fe(II) ions and the difference spectra at different potentials
from Fe(II) shown in Fig. 3(b), which are normalized for theThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:
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FIG. 4. Reference Fe L-edge XAS spectra for Fe(II) and Fe(III) ions in aqueous iron sulfate solutions.

same dwell time. The intensities I0 is for pure liquid water.
Figure 5(a) shows three-dimensional plots of Fe L-edge XAS
spectra for the electrochemical reaction of iron sulfate solution at 100 mV/s. By increasing the potential from 0.0 V to
1.0 V, the peak intensity of Fe(II) is decreased and that ofFe(III) is increased. When the potential is decreased from
1.0 V to −0.4 V, the peak intensity of Fe(II) is increased

FIG. 5. (a) Three-dimensional plots of the Fe L-edge XAS spectra in theelectrochemical reaction of aqueous iron sulfate solution at 100 mV/s. One
axis is photon energy, and the other is the applied potential versus Ag/AgCl.
(b) Fe L-edge XAS spectra at the potential step of 0.05 V in the electrochemical reaction shown in (a).

Rev. Sci. Instrum. 85, 104105 (2014)

and that of Fe(III) is instead decreased. Figure 5(b) shows
the Fe L-edge XAS spectra during the electrochemical reaction at the potential step of 0.05 V taken from Fig. 5(a). The
isosbestic point is found at 709.2 eV, and the intensities ofFe(II) and Fe(III) ions are changed continuously at different
potentials. Previously, the XAS spectra at constant potentials
need long acquisition time and it is difficult to observe small
spectral changes in electrochemical reactions. The potentialmodulated XAS measurement enables us to investigate a subtle spectral change in the electrochemical reaction.
To obtain the fraction of Fe(II) and Fe(III) ions, we fit the
Fe L-edge XAS spectra at different potentials to a superposition of the reference spectra of Fe(II) and Fe(III) ions shownin Fig. 4. Figure 6 shows an example of the least-squares fitting for the Fe L-edge XAS spectrum measured at a potentialof 0.95 V in the downward scan direction and it is in good
agreement with a simple superposition of the two reference
spectra.

B. Rate dependence of Fe redox reaction

Figure 7(a) shows the CV spectra of aqueous iron sulfate
solution at 100 mV/s. The oxidation of Fe(II) to Fe(III) ionswith increasing potential is observed as the peak at 0.76 V.
The reduction of Fe(III) to Fe(II) ions with decreasing potential is observed as the peak at 0.19 V.
In order to compare the CV spectra with the XAS spectra,
we obtain the fraction of Fe(II) and Fe(III) ions by fitting the
XAS spectra as shown in Fig. 6. Figure 7(b) shows change invalence of Fe ions in aqueous iron sulfate solution during theelectrochemical reaction at 100 mV/s. The error of the fraction at 100 mV/s is within ±2%. By increasing the potentialfrom 0.0 V to 1.0 V, the fraction of Fe(II) ions decreases andthat of Fe(III) ions increases. The fraction of Fe(III) at 1.0 V
is 0.26. The Fe(II) ions are partially changed to the Fe(III)ions by the oxidation process. The central potentials of the
Fe redox reaction are obtained by fitting the fraction changeof Fe(III) ions with sigmoid profiles. The central potential of
the oxidation process by increasing the potential is 0.59 V
and that of the reduction process by decreasing the potential
is 0.39 V. These potentials are different from that obtained

FIG. 6. Example of the fitting of the Fe L-edge XAS spectrum at the potential of 0.95 V in the downward scan direction by superposition of the two
reference spectra.

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Rev. Sci. Instrum. 85, 104105 (2014)

electrolyte is slower than that at the solid-liquid interface.
That is why the central potentials in the upward scan direction obtained by XAS are lower than those obtained by CV,
and the central potentials in the downward potentials obtained
by XAS are higher than those obtained by CV. The diffusionof Fe ions also explains that the maximum fraction of Fe(III)ions at 50 mV/s is larger than that at 100 mV/s.
Recently, we have obtained the central potential of Fe redox reaction by measuring XAS at 0.08 mV/s, in which each
XAS spectrum is measured at a constant potential.11 In thiscondition, all the Fe(II) ions are changed to the Fe(III) ionsby the oxidation process. The central potentials of both thesimple oxidation and reduction processes obtained by XAS
are consistent with those obtained by CV at 20 mV/s. Because of the slow scan rate, the diffusion of Fe ions does
not affect the Fe redox reaction in the bulk electrolytes at
0.08 mV/s.

IV. SUMMARY

FIG. 7. (a) CV spectra of aqueous iron sulfate solution at 100 mV/s. Fractions of Fe(II) and Fe(III) ions as a function of potential versus Ag/AgCl in
the electrochemical reaction of iron sulfate solutions at the scan rates of (b)100 mV/s and (c) 50 mV/s. The inset shows that the liquid layer consistsof the solid-liquid interface of the gold electrode and the bulk electrolyte ofFe(II) ions.

by CV at 100 mV/s: The central potential of the oxidation
process obtained by XAS is lower than that obtained by CV,
and the central potential of the reduction process obtained by
XAS is higher than that obtained by CV.
Figure 7(c) shows change in valence of Fe ions at
50 mV/s obtained by the potential-modulated XAS. The error of the fraction at 50 mV/s is within ±4%. The fraction of
Fe(III) at 1.0 V is 0.44, and is larger than that at 100 mV/s.
The central potentials of the oxidation and the reduction processes at 50 mV/s are 0.62 V and 0.43 V, respectively. These
central potentials are not different from those at 100 mV/s.
The current peaks in CV reflect changes in valence of Fe
ions at the solid-liquid interface of a gold electrode. As shownin the inset of Fig. 7, on the other hand, the XAS spectra include both the solid-liquid interface and the bulk electrolyte.
The change in valence of Fe ions in the bulk electrolyte is influenced by the diffusion of Fe ions in the solution. When thescan rate is fast, the fraction change of Fe ions in the bulk

The local structures of liquid electrolytes and solid-liquidinterfaces at electrodes in the electrochemical reaction areinvestigated by a newly developed in operando XAS measurement system at the scan rate of 100 mV/s, which is thesame as in CV. The XAS measurement is performed by using a transmission-type liquid flow cell with built-in electrodes. The electrode potential is swept with the scan rate of
100 mV/s at a fixed photon energy and soft X-ray absorption
coefficients at different potentials are measured at the sametime. By repeating the potential modulation at every photon
energy, it is possible to obtain the XAS spectra of electrochemical reaction at 100 mV/s.
By measuring the Fe L-edge XAS spectra at 100 mV/s,
we have investigated change in valence of Fe ions in the electrochemical reaction of aqueous iron sulfate solutions. In theoxidation process by increasing the potential, the maximum
fraction of the Fe(III) ions is only 0.26. The central potentials of both the oxidation and reduction processes obtained
by XAS are different from those obtained by CV. The Fe redox reaction at 50 mV/s also shows the same feature. The
current peaks in CV reflect the Fe redox reaction at the solidliquid interface; on the other hand, the XAS spectra include
both the solid-liquid interface and the bulk electrolyte. Thechange in valence of Fe ions in the bulk electrolyte is influenced by the diffusion of Fe ions, and the central potentials are dependent with the scan rate. The XAS for different
thickness of the liquid layer may distinguish the change invalence of Fe ions at the solid-liquid interface and in the bulk
electrolyte.
The liquid cell for XAS has given a chance to reveal the
local structures of liquid and aqueous solution, such as hydrogen bond of water in liquid water1–3 and hydrophobic interaction of methanol in aqueous methanol solution.33 The present
in operando electrochemical cell opens a possibility to reveal
the adsorption structural change of water molecules in electric
double layers at different potentials by using O K-edge XAS
for electrochemical reactions, such as aqueous sulfuric acidsolutions on platinum electrodes.

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ACKNOWLEDGMENTS

16 M. Nakamura, H. Kato, and N. Hoshi, J. Phys. Chem. C 112, 9458 (2008).

104105-7This work is supported by JSPS Grant-in-Aid for Scientific Research (Grant Nos. 23685006 and 23245007). We
acknowledge the staff members of the UVSOR-III facility for
their kind support.
1 J. D. Smith, C. D. Cappa, K. R. Wilson, B. M. Messer, R. C. Cohen, and

R. J. Saykally, Science 306, 851 (2004).
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