Photon Factory Activity Report 2019 #37 (2020)

BL-2A/2018G009

Room Temperature Proton Conduction of (111)-Oriented Ce0.9Sm0.1O2-δ ThinFilmTohru HIGUCHI1*, Daiki NISHIOKA1, Koji HORIBA2 and Hiroshi KUMIGASHIRA2,3

1 Department of Applied Physics, Tokyo University of Science, Katsushika, Tokyo 125-8585,

Japan

2 Photon Factory, Institute of Materials Structure Science, High Energy ResearchOrganization,
Tsukuba, 305-0801, Japan
3 Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University,Sendai 980-8577, JapanThe Ce0.9Sm0.1O2-δ (SDC) electrolyte thin films with the spacing of the (111) plane (d111) expanded by 2.6% to compensate
for lattice mismatch against the Al2O3 (0001) substrate were prepared by RF magnetron sputtering. The wet-annealed SDC
thin film exhibited surface protonic conduction in the low temperature region below 100°C. The O 1s photoemission
spectrum exhibits H2O and OH- peaks on the SDC surface. These results indicate the presence of physisorbed water layers
and the generation of protons on the SDC (111) surface with oxygen vacancies. The protons generated on the SDC surface
were conducted through a physisorbed water layer by the Grotthuss mechanism.
1 Introduction
Fluorite-type CeO2-δ oxides are good solid
electrolyte candidates for solid oxide fuel cells(SOFC) due to their high oxygen ion conductivity inhigh-temperature (HT) regions above 800ºC. In
particular, the oxygen ion conduction of CeO2-δ bulk
crystal can be tuned by substituting trivalent rareearth cations, while small electron conduction alsoprevails under low-oxygen potential conditions owing
to the formation of hopping electrons on cation sites
through the propagation of oxygen nonstoichiometry.
However, the operating temperature of SOFC is toohigh and the oxide ion conduction is not practical level.
In this study, in order to reduce the operating
temperature, we prepared a preferentially oriented
Ce0.9Sm0.1O2-δ (SDC) electrolyte thin film on an Al2O3(0001) substrate and investigated its proton
conductivity at surface below 100°C.
2 Experiment
The SDC thin films were deposited on Al2O3 (0001)
substrates by RF magnetron sputtering using aceramic target. The ceramic target RF power and theAr gas flow rate were set at 50 W and 30 sccm,respectively. The deposition pressure and thesubstrate temperature were fixed at 8.0×10-3 Torr and700 ºC, respectively.
The crystalline quality of the thin films wascharacterized by X-ray diffraction (XRD) with CuKαusing a Rigaku Miniflex 600. The electricalconductivities were characterized by the AC
impedance method, using a frequency response
analyzer (Solartron 1260) and an amplifier (Solartron1296). The measurement of the electrical conductivity

frequency region was changed from 32 MHz to 100mHz.
The electronic structures were measured byphotoemission spectroscopy (PES) and X-ray
absorption spectroscopy (XAS). The spectroscopic
measurements were conducted at the Photon
Factory BL-2A MUSASHI undulator beamline. The
XAS spectrum were recorded in a total electron yieldmode. PES spectra were acquired using a VGScienta SES-2002 hemispherical analyzer. The PES
and XAS resolutions were set at approximately 100
and 80 meV, respectively.
3 Results and Discussion
Figure 1 shows the XRD patterns of the SDC
ceramic, as-deposited and as wet-annealed SDC thinfilms. The ceramic was polycrystalline and the thinfilm was preferentially grown along the [111] direction.
In this study, we prepared a nanocrystalline ceramic
which, while exhibiting admittedly poor crystallinity,
did exhibit sufficient proton conductivity to allow us todiscuss the differences between the ceramic and thinfilm. The 111 peak positions of the SDC ceramic andas-deposited thin film are at ~29.02 ° and ~28.31°,
and the calculated spacing of the (111) plane (d111)
are 3.070 and 3.151 Å, respectively. The d111 of thethin film was expanded by 2.6% from that of theceramic target, so as to compensate for the latticemismatch between SDC and Al2O3. In addition, at
3.091 Å, the d111 of the wet-annealed thin film was
1.9% less than that of the as-deposited thin film.

Photon Factory Activity Report 2019 #37 (2020)

Fig. 1: XRD patterns of the as-deposited, wetannealed SDC thin films and SDC ceramic.

Fig. 3: O 1s core level PES spectra of dry- andwet-annealed thin films. The blue, green andyellow curves are the O2- in the lattice site, andOH- and H2O on the surface, respectively,
obtained from Gaussian fitting.

Fig. 2: Arrhenius plots of the electrical conductivities
in the in-plane of the SDC thin films and bulk ceramics,
measured in dry and wet atmospheres.
Figure 3 shows the Arrhenius plots of the electricalconductivities of the SDC thin films and bulk ceramics
measured in dry and wet atmospheres. In the dryatmosphere, the SDC thin film and bulk ceramic
exhibit Arrhenius-type thermal activation behaviors
over the whole temperature range. The activationenergies (EA) of the thin film and bulk ceramic are0.70 and 1.1 eV, respectively. The conductivity of thepolycrystalline SDC ceramic was two orders ofmagnitude lower than that of the SDC thin film, due
to the influence of grain boundaries.
In contrast, due to the proton migration, theconductivities of the thin film and the bulk ceramicmeasured in a wet atmosphere gradually increase as
the temperature decreases to below 100 and 250°C,respectively. In particular, the increase in theconductivity ratio was more marked in the thin film.
Single crystals and micropolycrystalline CeO2 do notexhibit proton conductivity, but since such proton
conduction is caused by absorbed protons at thesurface, nanopolycrystals and porous CeO2 doexhibit proton conductivity.
In general, the room temperature surface proton
conduction of fluorine-type oxides such as CeO2 [1]or YSZ [2] is explained by the Grotthus-mechanism.
According to this mechanism, physisorbed H2O forms
OH- and H3O+ ions on the surface at room
temperature and the H3O+ proton transfers from one
H2O molecule to a neighboring H2O molecule, as inthe following reaction:.
H2 O+ + H2 O → H2 O + H2 O+

Figure 3 shows the O 1s PES spectra of the dryand the wet-annealed thin films. Both exhibited adouble-peak structure and a sharp peak at ~529.5 eV,
which corresponds to O2- in oxygen sites. On theother hand, the positions of the weaker peaks aredifferent, and can be interpreted as follows: (i) thebroad peak at ~532 eV in the as-deposited thin film
corresponds to the OH- absorbed at the surface
created by chemisorbed water.; and (ii) the peak at
533 eV in the wet-annealed thin film corresponds to
H2O molecules physisorbed at the surface. The peakratio of physisorbed H2O was increased from 7.8% to
24% by wet-annealing. Thus, the increase in
conductivity by wet-annealing reflects an increase in
the physically adsorbed water molecules at the SDC
surface.
A proton conductivity of 5.98×10-5 S⁄cm wasachieved at room temperature in the preferentiallyoriented thin film, which is two orders of magnitude
higher than that of polycrystalline ceramics. Most
notable was the ~10-2 S/cm proton conductivity
exhibited in a high humidity atmosphere. Such highPhoton Factory Activity Report 2019 #37 (2020)

proton conductivity is considered to derive from two
features of the preferentially oriented SDC thin filmwith oxygen vacancy. The first feature is high water
adsorbability on the SDC (111) surface. In the O1s
PES spectrum, 16.9% of the detected oxygen wasattributed to chemically adsorbed water and 24% wasattributed to physically adsorbed water. This meansthat there are layers of physisorbed water on the SDC
surface that can acts as proton conducting paths. The
second feature is the dissociation of adsorbed waterat the SDC (111) surface. The reduced CeO2-δ (111)
surface promotes the dissociation of water molecules
and the formation OH- and H+, which contribute toproton conduction. Dissociated protons can migrate
through a physically adsorbed water layer by theGrotthus-mechanism. Therefore, the preferentiallyoriented SDC thin film contributed to such high proton
conduction.
References[1] D. Nishioka et al., Nanoscale Res. Lett. 15, 42(2020).
[2] M. Takayanagi et al., Solid State Ionics 311, 46(2017).
Research Achievements
1. D. Nishioka et al., Nanoscale Res. Lett. 15 (2020)
42.
2. Ｍ. Takayanagi, et al., Jpn. J. Appl. Phys. 58 (2019)
SODG08.
* higuchi@rs.tus.ac.jp