ww.sciencedirect.com

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 3

Available online at w

journal homepage: www.elsevier .com/locate/he

Hydrogen production from water-splitting reaction basedon RE-doped ceriaezirconia solid-solutions

Hari Prasad Dasari a, Kiyong Ahn a,b, Sun-Young Park a, Ho-Il Ji a, Kyung Joong Yoon a,Byung-Kook Kim a, Hae-June Je a, Hae-Weon Lee a, Jong-Ho Lee a,*aHigh-Temperature Energy Materials Center, Future Convergence Research Division, Korea Institute of Science and Technology, 39-1,

Haweolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of KoreabDepartment of Materials Science and Engineering, Hanyang University, Seoul, Republic of Korea

a r t i c l e i n f o

Article history:

Received 13 September 2012

Received in revised form

18 January 2013

Accepted 21 January 2013

Available online 7 March 2013

Keywords:

Hydrogen production

SOEC

Catalytic activity

Ceriaezirconia

Oxygen storage capacity

* Corresponding author. Tel.: þ82 2958 5532;E-mail address: jongho@kist.re.kr (J.-H. L

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.01.1

a b s t r a c t

The effect of rare earth (RE ¼ Tb, Pr and La) dopant on the catalytic performance of

RE-doped ceriaezirconia (CZRE) solid-solutions for oxygen storage capacity and hydrogen

production activity has been successfully investigated. The sustainability of the solid-

solutions even after the reduction was confirmed by XRD. Raman analysis showed that

the addition of RE element in CZ system significantly decreased the intensity of the

characteristic fluorite peak (462e474 cm�1) indicating a highly deformed structure than CZ

system which can enhance the oxygen mobility and redox property of these materials and

the order of the intensity decrease was Pr > Tb > La. The XPS measurements revealed that

the CZPr sample has a homogeneous distribution of Ce/Zr and also showed a high

enrichment of Pr on the particle surface than the others. Among the CZRE solid-solution

catalysts tested, CZPr catalyst showed the best catalytic performance for high OSC and

hydrogen production from water-splitting reaction.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction A typical SOEC consists of an oxygen ion conducting solid

Hydrogen (H2), the promising energy carrier, is an environ-

mentally attractive and sustainable transportation fuel and

has the potential to displace the fossil fuels. In recent years,

high-temperature nuclear reactors are being used success-

fully for H2-production from water with substantially

increased efficiency and without consuming fossil fuels,

green house gas emissions and other forms of air pollution.

Water-splitting reaction for H2-production can be done by

high-temperature electrolysis and thermo-chemical pro-

cesses at high temperatures (>850 �C) in order to achieve

competitive efficiencies. High-temperature electrolysis can

be done using a Solid oxide electrolysis cell (SOEC) which is

a reversely operated Solid oxide fuel cell (SOFC) [1].

fax: þ82 2958 5529.ee).2013, Hydrogen Energy P45

electrolyte sandwiched between H2O-H2-electrode (which is

a cathode in SOEC) and O2-electrode (which is an anode in

SOEC). In SOECs, it is typically noticed that the degradation

rate is much greater than SOFCs. Delamination of the O2-

electrode (due to micro-structural changes in bond layer,

chromium poisoning and dissociation of bond layer), loss of

electrical/ionic conductivity of electrolyte and the adsorption

of impurities on the H2-electrode are the main reasons for the

degradation of SOECs [1e4]. Virkar et al. [2] predicted a model

for the condition where delamination of the O2-electrode

could be avoided and was successful in qualitative compari-

son for some reports [1,3,5,6]. Hauch et al. [5] demonstrated

that the origin of degradation at H2-electrode was due to the

segregation of impurities (Si and Al) on the electrode from the

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 36098

sealant due to high partial pressure of steam and this segre-

gation was successfully avoided by using a gold sealant

instead of normally used albite glass sealant. From the above

observations it can be forecasted that the degradation in

SOECs can be minimized or avoided by developing new ma-

terial systems for O2-electrode, using proper gold sealants and

proper SOEC operating conditions. Development of H2-elec-

trode materials is fundamentally very important in order to

enhance the H2-production and thereby further decrease the

H2-production costs from SOECs than compared to the H2-

production costs from other fuel cell systems. Bae et al. [7]

reported that the gadolinium doped ceria (GDC) impregnated

H2-electrode showed an enhanced performance which was

correlated to the oxygen storage capacity (OSC).

Among the ceriaezirconia (CZ) solid-solutions, ceria-rich

oxides show high oxygen storage capacity (OSC), reduction

properties and phase stability. Along with these properties,

the mixed ionic and electronic conductivity of thesematerials

makes them suitable candidates as a catalyst and/or support

for SOEC applications [8]. Doping rare earth (RE) with CZ solid-

solutions shows further improvements in OSC, redox prop-

erty, and thermal resistance compared to CZ solid-solutions

itself. In this work, the glycineenitrate process (GNP) has been

used to synthesis CZRE (RE ¼ Tb, Pr and La) samples since it is

the most suitable synthesis method for producing fairly fine,

homogeneous, and complex compositional metal oxide pow-

ders [9].

In the present study, the influence of RE dopants on the

OSC property that affects the H2-production from the water-

splitting reaction has been investigated. The role of RE dop-

ant on the homogeneous distribution of Ce/Zr and surface

enrichment of RE in CZRE solid-solutions and its influence in

the improvement of the oxygen vacancies was also studied.

Characterization of the samples was performed using X-ray

diffraction (XRD), Raman spectroscopy (RS), BET surface area,

X-ray photoelectron spectroscopy (XPS). The catalytic perfor-

mance was evaluated for OSC and H2-production from water-

splitting reaction.

2. Experimental

The Ce0.65Zr0.25RE0.1O2 � d (RE ¼ Pr, Tb and La) powders were

successfully prepared by GNP and the synthesis procedure

was reported elsewhere [10,11]. The XRD patterns were

obtained by an X-ray generator (Phillips PW 3830) using

Ni-filtered Cu Ka radiation. Raman spectra were measured

with a Raman spectrometer (BRUKER RFS 100/S FT-Raman

Spectrometer). The excitation source was Nd-YAG laser

(l ¼ 1064 nm) and the laser power was 20 mW at the sample

point. The calibration for Raman spectroscopy was achieved

by measuring silicon wafer as reference at 520 cm�1. The XPS

analysis was performed in ultra-high vacuum using PHI 5800

Versa probe instrument (Ulvac-PHI, Physical Electronics) with

a background pressure of 6.7� 10�8 Pa andmonochromator Al

Ka (1486.66 eV) anode (25 W, 15 kV). The spot size was

100 mm � 100 mm and the recorded spectra were calibrated by

the characteristic binding energy (BE) peak at 284.6 eV

belonging to the contaminant carbon in 1s region. The BET

surface area measurements were made on a Quantachrome

instrument (Quadrasorb SI). Prior to the analysis, samples

were degassed at 200 �C under vacuum for 3 h to remove any

residual moisture and other volatiles.

The oxygen release characteristics of the samples were

observed in the temperature range of 300e800 �C. The change

in the weight of the sample was monitored by thermog-

ravimetry method (TG) under cyclic heat treatments in flow-

ing nitrogen or dry air. A commercial Q-600 TG-DTA analyzer

was used for this purpose. The heat cycle consisted of heating

the sample to 800 �C, cooling to 150 �C, and again heating to

800 �C. All heating and cooling rates were 5 �C min�1. The

weight loss of the sample during the second heating cycle was

used tomeasure the OSC of the sample. This technique for the

evaluation of oxygen release characteristics is essentially

similar to that described previously [12,13].

The H2-production from water-splitting reaction on CZRE

samples was obtained in a flow-through type fixed-bed

quartz-tube reactor. Approximately 0.5 g of catalyst particles

were placed into the middle of the reactor. The reactor was

heated in an electric furnace equipped with a K-type ther-

mocouple. The temperature of the reactor bed was monitored

and controlled by a temperature controller (Model UT 150,

Yokogawa). The sample was reduced in-situ in flowing

hydrogen at 800 �C for 1 h, flushed with N2 for 30 min and the

temperature is increased to 900 �C. The carrier gas was

switched to reactant gas mixtures (steam and N2) for H2-pro-

duction by water-splitting reaction. N2 was used as a carrier

gas with a flow rate of 150 mL min�1. H2O (5 mL min�1) was

obtained by controlling the evaporator temperature and the

temperature of the heating bands were kept at 120 �C in order

to avoid condensation of steam. A cold trap at the outlet of the

reactor was used to condense water from the product gas

stream. The effluent gas mixture was analyzed for H2 by

means of an on-line H2-gas analyzer (K6050 gas analyzer, Hi-

Tech Instruments).

3. Results and discussion

3.1. Catalyst characterization

3.1.1. XRD analysisThe X-ray diffraction patterns of reduced CZRE samples were

obtained in order to find out the phase stability and the results

were illustrated in Fig.1. For comparison, pure ceria is also

included. From Fig. 1(a), single crystalline phases with cubic

fluorite structure were noticed for all the samples from the

XRD patterns [14]. Fig. 1(b) shows an interesting observation

from XRD measurements and was that the XRD peaks of CZ

sample were shifted to higher 2q values with respect to pure

ceria and the XRD peaks of CZRE samples were shifted to

lower 2q values with respect to CZ samples. This can be due to

the difference in the ionic radius of respective dopant ions

(Zr4þ ¼ 0.84�A, Tb3þ ¼ 1.04�A, Pr3þ ¼ 1.13�A and La3þ ¼ 1.16�A) in

relation to Ce4þ (0.97 �A). These observations confirm the sus-

tainability of solid-solutions even after reduction. In Table 1,

average crystallite size, specific surface area and primary

particle size calculated from BET data, are summarized. It can

be seen that all the doped samples exhibited small average

crystallite size. The CZRE samples showed highest values of

Fig. 1 e (a) XRD patterns (b) (111) peak of CZRE samples after

reduction.

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 3 6099

specific surface area, while the lowest values were obtained

for CZ and ceria samples. The primary particle size is slightly

higher than the crystallite size of the samples indicating a low

degree of agglomeration.

Table 1 e Specific surface area (SSA), calculated primaryparticle size (dBET) and crystallite average size (dXRD) forCZRE samples obtained after reduction.

Sample Latticeparameter (�A)

SSA(m2 g�1)

dBET(nm)

dXRD(nm)

CeO2 � d 5.3784 11.1 75.1 62.1

CZ 5.3480 32.1 30.1 22.1

CZTb 5.3652 29.3 33.0 24.5

CZPr 5.3692 31.7 30.5 23.5

CZLa 5.4004 53.2 18.2 16.1

3.1.2. Raman spectra analysisIn the present study, an excitation laser line of 1073 nm has

been employed during Raman spectroscopymeasurements so

that the laser line can penetrate into deeper layers of the

sample and thus all the information of the sample can be

obtained. Fig. 2 demonstrates the Raman spectra of reduced

CZRE samples. All the samples exhibited typical structure of

ceria, with the main peak at 462e474 cm�1 attributed to the

Raman mode (F2g) of the fluorite-type structure [15]. The

Raman spectra of the fluorite-type structures are dominated

by oxygen lattice vibrations and are sensitive to the crystalline

symmetry [16]. By comparing the Raman spectra of pure ceria

andCZ samples it can be noticed that the incorporation of Zr4þ

in the ceria lattice deformed the structure and the intensity of

the characteristic fluorite peak decreased significantly [16].

This deformation has been reported to favor the oxygen

mobility, affecting the redox property of the material. When

compared to peak position of pure ceria (464.5 cm�1), the CZ

sample showed an increased shift (472.6 cm�1) and this can be

due to the doping of lighter atoms such as Zr in ceria which

results in subsequent contraction of the ceria unit cell [13].

When compared to CZ sample, doping of RE elements in CZ

system resulted in decrease of the peak position. The shift of

the characteristic peak is attributed to change of M-O vibra-

tion frequency after incorporation of the dopants which ac-

count for the difference in the ionic radius [17]. The inserted

figure in Fig. 2 shows the enlarged view of the characteristic

fluorite peak of CZRE samples. It can be clearly manifested

from this figure that the addition of RE element in CZ system

has significantly further decreased the intensity of the char-

acteristic fluorite peak. This shows that the addition of RE in

CZ system can further deform the structure and this defor-

mation further enhances the oxygen mobility and redox

behavior than compared to CZ system itself. The order of the

intensity decrease with the effect of RE doping was

Pr > Tb > La. Generally, a strong decrease in the relative in-

tensity of the peak at 465 cm�1 is attributed to the improve-

ment in the reduction at low temperatures to an increased

displacement of the oxygen anions from the tetrahedral sites

Fig. 2 e Raman spectra of CZRE samples after reduction.

Fig. 3 e Ce 3d XPS spectra of CZRE samples after reduction.

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 36100

of the CZ system under reducing conditions. The oxygen dis-

placement favors higher oxygen mobility in the bulk ac-

counting for the modification of the redox behavior [18]. The

structural properties of ceria-based solid-solution systems

play important roles in the catalytic activity and other prop-

erties, such as redox behavior and enhanced lattice oxygen

mobility [16]. This clearly points out that the incorporation of

Pr in CZ system may enhance the oxygen mobility and redox

property which in turn may show an enhancement in the

catalytic property.

3.1.3. XPS analysisThe impact of the RE doping on the reducibility of CZ system

was further investigated by X-ray photoelectron spectroscopy

analysis. Fig. 3 shows the XPS spectra of Ce 3d spectra of CZRE

samples. There are ten peak assignments in the spectra,which

are labeled according to the convection established in the lit-

erature [13]. ThepeaksU0,U,U0,U00,U0 00 andV0,V,V0,V00,V0 00 referto 3d3/2 and 3d5/2, respectively. The peaks V/U and V00/U00 aredue to the mixture of 3d94f2O2p4 and 3d94f1O2p5 configura-

tions, and V0 00/U0 00 is a 3d94f0O2p6 final state. The peaksV/U, V00/U00 andV0 00/U0 00 are attributed to Ce4þ state. The peaksV0/U0 and

V0/U0 are attributed to Ce3þ state and are due to 3d94f1O2p6 and

3d94f2O2p5 configurations [19]. As it can be noticed from the

Table 2 e XPS elementary surface concentration of CZRE samp

Sample Surface atomic concentratio

Ce (at. %) Zr (at.%) RE (at. %

CZ 12.09 4.25 e

CZTb 12.70 5.25 0.61

CZPr 13.78 5.31 2.58

CZLa 11.20 5.20 1.36

a Nominal ratios were indicated in the parentheses.

figure that the CZPr sample showed high intensity of U0/V0

peaks which reveals a high concentration of Ce3þ ions on the

surface of this sample than compared to others. This is well

supported by the smaller relative intensity ofU0 00 peak for CZPr

sample,which iswell separated from the remaining peaks and

is often used to assess the reduction degree of the Ce ions in

the surface region. Table 2 presents the surface atomic con-

centrations and ratios of the CZRE samples. Valuable infor-

mation canbeobtained fromtheanalysis ofCe/Zr ratio. Taking

into account that the nominal ratio for the Ce0.75Zr0.25O2

composition is 3, if the solid-solution obtains a very homoge-

nous atomic distribution, this would also be the surface ratio.

Even though the synthesis method for CZRE samples was

same, the change in the RE element had a significant effect in

Ce/Zr ratio. TheCZPr samplepresentsaCe/Zr valueof 2.59(2.6),

reflecting a reflecting a homogeneous distribution of Ce/Zr on

the particle surface than compared to CZTb andCZLa samples,

with Ce/Zr value of 2.41(2.6) and 2.15(2.6), respectively (nomi-

nal ratios in the parentheses), indicting a Ce-depletion on the

particle surface [20].

Table 3 represents the O 1s core level XPS profiles of CZRE

samples along with the Ce3þ concentration. The band at lower

binding energy (Peak 1) was attributed to characteristic of

lattice oxygen corresponding to metal oxides [21]. The peaks

at higher binding energy side were attributed to surface car-

bonates, hydroxyl groups, surface oxygen ions and water [22].

Fig. 4 illustrates the XPS spectra of Tb, Pr, La elements of

corresponding CZRE samples. Fig. 4(a) shows the Tb 4d core

level XPS spectra of CZRE samplewhere a strong peak at below

150 eV (148 eV) is observed and corresponds to Tbþ3. A part

from this main peak, some small peaks were also noticed at

above 150 eV (152.8 eV, 155.5 eV and 161.1 eV) which may

correspond to Tbþ4. A Pr 3d5/2 core level spectrum of CZRE

sample is shown in Fig. 4(b) and it consists of two peaks at

binding energies 931.9 eV and 928.6 eV. The peaks at the

higher and lower binding energies can be assigned to Pr4þ and

Pr3þ, respectively. The splitting pattern of La 3d core level

spectra is shown in Fig. 4(c). The splitting is due to spin orbit

interaction and charge transfer from O 2p to La 4f. Splitting

energy observed from the spectra is w4.35 eV, which is well

supported by the previous reports [23]. Fig. 4 shows that the Tb

and Pr are in 3þ and 4þ states whereas Lawas in only 3þ state.

From Tables 2 and 3 it can be clearly observed that the surface

concentration of Pr is higher than that of the La and Tb in the

corresponding CZRE samples and also the Ce3þ concentration

is higher in CZPr sample. This indicates that among the CZRE

samples, CZPr sample have the high enrichment of surface by

les obtained after reduction.

n Surface atomic ratios

) O (at. %) Ce/Zra RE/(Ce þ Zr)a

53.01 2.88(3.0) e

52.51 2.41(2.6) 0.03(0.11)

54.84 2.59(2.6) 0.13(0.11)

58.36 2.15(2.6) 0.08(0.11)

Table 3 e O 1s binding energy of CZRE samples and Ce3D

concentration (%).

Sample O 1s binding energy/eV Ce3þ conc. (%)

Peak 1 Peak 2 Peak 3

CZ 528.80 530.30 532.57 37.35

CZTb 528.79 531.41 535.02 41.85

CZPr 528.80 530.33 531.86 47.39

CZLa 528.87 531.22 534.76 42.91

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 3 6101

Pr than compared to other samples. From XPS analysis it can

be interpreted that the CZPr sample may show high catalytic

activity since surface enrichment of both Ce and Pr are higher

which indicates that the reducibility of this sample is much

easier than the other sample.

Fig. 4 e (a) Tb (b) Pr (c) La XPS spectra

3.2. Catalyst performance

3.2.1. Oxygen storage capacity (OSC)The OSC property was tested by oxygen release characteristics

of the calcined samples under dry air atmosphere in the tem-

perature range of 300e800 �C. Theweight change of the sample

was monitored by thermo-gravimetric (TG) method under cy-

clic heat treatments in flowing air. During this method the

sample is subjected to consecutive cycles of heating and cool-

ing. In the first heat treatment a large decrease of weight is

noticed and it corresponds to the release of both water mole-

cules (from surface) and oxygen (from the sample). The re-

covery of weight is seen in cooling back stage. During the

second heating cycle a small decrease of weight is observed

and it corresponds to the potential oxygen release capacity of

the sample [24]. Fig. 5 illustrates the OSC values of the samples

of CZRE samples after reduction.

0

50

100

150

200

CZ LaCZPrCZTbCZ

To

tal O

SC

(m

ol/g)

Fig. 5 e Oxygen storage capacity of calcined CZRE samples

measured from thermo-gravimetric method.

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 36102

and can be noted that CZPr sample exhibits the highest OSC

(155 mmol/g), which is followed by CZLa (109 mmol/g) and CZTb

(82mmol/g) samples.This is an interestingobservation fromthe

practical viewpoint as the total OSC is a crucial parameter for

many technological applications.

3.2.2. H2-productionFig. 6 shows the time dependence of the H2-production during

the water-splitting reaction on various CZRE samples. Since

the H2-production step is considered after the reduction step

the total time for the water-splitting reaction took place

within 10e12 min and is in accordance with the literature

[25,26]. The CZPr sample showed the fastest response for the

H2-production than the other CZRE samples. It can be noticed

from the figure that the amount of H2-produced at the max-

imum point for the CZPr sample is 205 mmol/g and was

Fig. 6 e Time dependence of the hydrogen production

during water-splitting reaction on CZRE samples.

followed by CZLa, CZTb and CZ samples with 178, 165 and

134 mmol/g of sample, respectively. This order corroborates

with the total OSC of the samples measured by thermo-

gravimetric method. Furthermore, Raman spectra distinctly

shows that a large decrease in the intensity of the character-

istic fluorite peak for the CZPr sample indicating a highly

deformed structure which can result in high oxygen mobility

and redox propertywhich can enhance its catalytic activity. At

the same time, for CZPr sample, XPS analysis demonstrates

a very homogeneous distribution of Ce/Zr along with the

enrichment of Pr on the particle surface and a high Ce3þ

concentration. All these factors are responsible for showing

such a high OSC and H2-production for CZPr sample. Electro-

chemical properties of these materials will be evaluated by

hall-cell measurements under SOEC conditions and would be

the topic of the forthcoming paper.

4. Conclusion

The effect of rare earth (RE ¼ Tb, Pr and La) dopant on the

catalytic performance of CZRE solid-solutions for oxygen

storage capacity and hydrogen production has been success-

fully investigated. The solid-solutions were prepared by gly-

cineenitrate process. The sustainability of the solid-solutions

even after the reduction was confirmed by XRD showing

a single crystalline phase with cubic fluorite structure. From

Raman spectroscopy measurements, with an excitation laser

of 1064 nm, the order of the intensity decrease with the effect

of RE doping was Pr > Tb > La. This clearly points out that the

incorporation of Pr in CZ system can improve the oxygen

mobility and therefore can further enhance the catalytic ac-

tivity than the La or Tb doped CZ samples. The XPS mea-

surements of the reduced samples revealed that the CZPr

sample has a homogeneous distribution of Ce/Zr along with

high Ce3þ concentration whereas CZTb and CZLa samples

showed a Ce-depletion on the particle surface. Apart from

this, among the CZRE samples, CZPr sample showed high

enrichment of Pr on the particle surface than the others.

Among the CZRE solid-solution samples tested, CZPr sample

showed the best catalytic performance for high OSC and

hydrogen production from water-splitting reaction.

Acknowledgments

This research was supported by a grant from the Fundamental

R&D Program for Core Technology of Materials funded by the

Ministry of Knowledge Economy, Republic of Korea and Insti-

tutional Research Program of Korea Institute of Science and

Technology (KIST) (2E22802). One of the authors (D.H.P.) ac-

knowledges KIST for the award of a STAR Post-Doc Fellowship.

r e f e r e n c e s

[1] Mawdsley JR, David Carter J, Jeremy Kropf A, Yildiz B,Maroni VA. Post-test evaluation of oxygen electrodes fromsolid oxide electrolysis stacks. Int J Hydrogen Energy 2009;34(9):4198e207.

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 8 ( 2 0 1 3 ) 6 0 9 7e6 1 0 3 6103

[2] Virkar A. Mechanism of oxygen electrode delamination insolid oxide electrolyzer cells. Int J Hydrogen Energy 2010;35(18):9527e43.

[3] Herring JS, O’brien JE, Stoots CM, Hawkes GL, Hartvigsen JJ,Shahnam M. Progress in high-temperature electrolysis forhydrogen production using planar SOFC technology. Int JHydrogen Energy 2007;32(4):440e50.

[4] Ebbesen SD, Mogensen M. Exceptional durability of solidoxide cells. Electrochem Solid State Lett 2010;13(9):B106e8.

[5] Hauch A, Ebbesen SD, Jensen SH, Mogensen M. Solid oxideelectrolysis cells: microstructure and degradation of the Ni/yttria-stabilized zirconia electrode. J Electrochem Soc 2008;155(11):B1184e93.

[6] Brisse A, Schefold J, Zahid M. High temperature waterelectrolysis in solid oxide cells. Int J Hydrogen Energy 2008;33(20):5375e82.

[7] Kim-Lohsoontorn P, Kim Y-M, Laosiripojana N, Bae J.Gadolinium doped ceria-impregnated nickeleyttriastabilised zirconia cathode for solid oxide electrolysis cell. IntJ Hydrogen Energy 2011;36(16):9420e7.

[8] Prasad DH, Park SY, Ji H, Kim HR, Son JW, Kim BK, et-al.Effect of steam content on nickel nano-particle sintering andmethane reforming activity of NieCZO anode cermets forinternal reforming SOFCs. Appl Catal A Gen 2012;411e412:160e9.

[9] Chick LA, Pederson LR, Maupin GD, Bates JL, Thomas LE,Exarhos GJ. Glycine-nitrate combustion synthesis of oxideceramic powders. Mater Lett 1990;10(1e2):6e12.

[10] Prasad DH, Park SY, Ji HI, Kim HR, Son JW, Kim BK, et-al.Structural characterization and catalytic activity ofCe0.65Zr0.25RE0.1O2 � d Nanocrystalline powders synthesizedby the glycine-nitrate process. J Phys Chem C 2012;116(5):3467e76.

[11] Prasad DH, Jung HY, Jung HG, Kim BK, Lee HW, Lee JH. Singlestep synthesis of nano-sized NiOeCe0.75Zr0.25O2 compositepowders by glycine nitrate process. Mater Lett 2008;62(4e5):587e90.

[12] Ozawa M, Loong CK. In situ X-ray and neutron powderdiffraction studies of redox behavior in CeO2-containingoxide catalysts. Catal Today 1999;50(2):329e42.

[13] Reddy BM, Bharali P, Saikia P, Park S-E, Van Den Berg MWE,Muhler M, et-al. Structural characterization and catalyticactivity of nanosized CexM1 e xO2 (M ¼ Zr and Hf) mixedoxides. J Phys Chem C 2008;112(31):11729e37.

[14] Hari Prasad D, Kim HR, Son JW, Kim BK, Lee HW, Lee JH.Superior compositional homogeneity and long-term catalytic

stability of Ni-Ce0.75Zr0.25O2 cermets prepared via glycinenitrate process. Catal Commun 2009;10(9):1334e8.

[15] Mcbride JR, Hass KC, Poindexter BD, Weber WH. Raman andX-ray studies of Ce1 � xRExO2 � y, where RE ¼ La, Pr, Nd, Eu,Gd, and Tb. J Appl Phys 1994;76(4):2435.

[16] Atribak I, Bueno-Lopez A, Garcıa-Garcıa A. Combinedremoval of diesel soot particulates and NOx over CeO2eZrO2

mixed oxides. J Catal 2008;259(1):123e32.[17] Luo M-F, Yan Z-L, Jin L-Y, He M. Raman Spectroscopic study

on the structure in the surface and the bulk Shell of CexPr1 � x

O2 � d mixed oxides. J Phys Chem B 2006;110(26):13068e71.[18] Fornasiero P, Ka�spar J, Graziani M. Redox behavior of high

surface area Rh-loaded Ce0.5Zr0.5O2 mixed oxide. J Catal 1997;167(2):576e80.

[19] ReddyBM,Katta L, ThrimurthuluG.Novel nanosizedCexZr1 � x

O2, CexHf1 � xO2 and CexTb1 � xO2 � d solid solutions: structuralcharacteristics and catalytic performance. Catal Today 2011;175(1):585e92.

[20] Guillen-Hurtado N, Bueno-Lopez A, Garcıa-Garcia A. Surfaceand structural characterisation of coprecipitatedCexZr1 � xO2

mixed oxides. J Mater Sci 2012;47(7):3204e13.[21] Liu L, Yao Z, Liu B, Dong L. Correlation of structural

characteristics with catalytic performance of CuO/CexZr1 � x

O2 catalysts for NO reduction by CO. J Catal 2010;275(1):45e60.

[22] Kumar A, Babu S, Karakoti AS, Schulte A, Seal S.Luminescence properties of europium-doped cerium oxidenanoparticles: role of vacancy and oxidation states.Langmuir 2009;25(18):10998e1007.

[23] Borca CN, Canulescu S, Loviat F, Lippert T, Grolimund D,Dobeli M, et-al. Analysis of the electronic configuration of thepulsed laser deposited La0.7Ca0.3MnO3 thin films. Appl SurfSci 2007;254(4):1352e5.

[24] Reddy BM, Lakshmanan P, Bharali P, Saikia P,Thrimurthulu G, Muhler M, et-al. Influence of alumina, silica,and titania supports on the structure and CO oxidationactivity of CexZr1 � xO2 nanocomposite oxides. J Phys Chem C2007;111(28):10478e83.

[25] Meng Q-L, Lee C-I, Kaneko H, Tamaura Y. Solarthermochemical process for hydrogen production via two-step water splitting cycle based on Ce1 � xPrxO2 � d redoxreaction. Thermochim Acta 2012;532(0):134e8.

[26] Abanades S, Flamant G. Thermochemical hydrogenproduction from a two-step solar-driven water-splittingcycle based on cerium oxides. Sol Energy 2006;80(12):1611e23.