Hydrogen production from water-splitting reaction based on RE-doped ceria–zirconia solid-solutions

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 e n e r g y 3 8 ( 2 0 1 3 ) 6 0 9 7 e6 1 0 3

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Hydrogen production from water-splitting reaction based on 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,* a

High-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 Korea b Department of Materials Science and Engineering, Hanyang University, Seoul, Republic of Korea

article info

abstract

Article history:

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

Received 13 September 2012

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

Received in revised form

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

18 January 2013

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

Accepted 21 January 2013

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

Available online 7 March 2013

characteristic fluorite peak (462e474 cm1) indicating a highly deformed structure than CZ system which can enhance the oxygen mobility and redox property of these materials and

Keywords:

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

Hydrogen production

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

SOEC

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

Catalytic activity

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

Ceriaezirconia

hydrogen production from water-splitting reaction.

Oxygen storage capacity

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

1.

Introduction

Hydrogen (H2), the promising energy carrier, is an environmentally attractive and sustainable transportation fuel and has the potential to displace the fossil fuels. In recent years, high-temperature nuclear reactors are being used successfully 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 processes 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].

A typical SOEC consists of an oxygen ion conducting solid 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 O2electrode (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 comparison 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

* Corresponding author. Tel.: þ82 2958 5532; fax: þ82 2958 5529. E-mail address: [email protected] (J.-H. Lee). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.01.145

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sealant due to high partial pressure of steam and this segregation 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 material systems for O2-electrode, using proper gold sealants and proper SOEC operating conditions. Development of H2-electrode 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 H2production 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 these materials makes them suitable candidates as a catalyst and/or support for SOEC applications [8]. Doping rare earth (RE) with CZ solidsolutions shows further improvements in OSC, redox property, 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 powders [9]. In the present study, the influence of RE dopants on the OSC property that affects the H2-production from the watersplitting reaction has been investigated. The role of RE dopant 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 performance was evaluated for OSC and H2-production from watersplitting reaction.

2.

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 thermogravimetry method (TG) under cyclic heat treatments in flowing 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 min1. The weight loss of the sample during the second heating cycle was used to measure 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 thermocouple. 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-production by water-splitting reaction. N2 was used as a carrier gas with a flow rate of 150 mL min1. H2O (5 mL min1) 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, HiTech Instruments).

3.

Results and discussion

3.1.

Catalyst characterization

3.1.1.

XRD analysis

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 cm1. 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  108 Pa and monochromator 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

The 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 A, Tb3þ ¼ 1.04  A, Pr3þ ¼ 1.13  A and La3þ ¼ 1.16  A) in (Zr4þ ¼ 0.84  A). These observations confirm the susrelation to Ce4þ (0.97  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

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 e n e r g y 3 8 ( 2 0 1 3 ) 6 0 9 7 e6 1 0 3

3.1.2.

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

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Raman spectra analysis

In the present study, an excitation laser line of 1073 nm has been employed during Raman spectroscopy measurements 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 cm1 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 and CZ 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 cm1), the CZ sample showed an increased shift (472.6 cm1) 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 vibration frequency after incorporation of the dopants which account 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 characteristic fluorite peak. This shows that the addition of RE in CZ system can further deform the structure and this deformation 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 intensity of the peak at 465 cm1 is attributed to the improvement in the reduction at low temperatures to an increased displacement of the oxygen anions from the tetrahedral sites

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 primary particle size (dBET) and crystallite average size (dXRD) for CZRE samples obtained after reduction. Sample CeO2  CZ CZTb CZPr CZLa

d

Lattice parameter ( A)

SSA (m2 g1)

dBET (nm)

dXRD (nm)

5.3784 5.3480 5.3652 5.3692 5.4004

11.1 32.1 29.3 31.7 53.2

75.1 30.1 33.0 30.5 18.2

62.1 22.1 24.5 23.5 16.1

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

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

of the CZ system under reducing conditions. The oxygen displacement favors higher oxygen mobility in the bulk accounting 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 properties, 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 analysis

The 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 literature [13]. The peaks U0, U, U0 , U00 , U000 and V0, V, V0 , V00 , V000 refer to 3d3/2 and 3d5/2, respectively. The peaks V/U and V00 /U00 are due to the mixture of 3d94f2O2p4 and 3d94f1O2p5 configurations, and V000 /U000 is a 3d94f0O2p6 final state. The peaks V/U, V00 / U00 and V000 /U000 are attributed to Ce4þ state. The peaks V0/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

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 of U000 peak for CZPr sample, which is well 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 concentrations and ratios of the CZRE samples. Valuable information can be obtained from the analysis of Ce/Zr ratio. Taking into account that the nominal ratio for the Ce0.75Zr0.25O2 composition is 3, if the solid-solution obtains a very homogenous 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. The CZPr sample presents a Ce/Zr value of 2.59(2.6), reflecting a reflecting a homogeneous distribution of Ce/Zr on the particle surface than compared to CZTb and CZLa samples, with Ce/Zr value of 2.41(2.6) and 2.15(2.6), respectively (nominal 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 carbonates, 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 sample where 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 La was 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

Table 2 e XPS elementary surface concentration of CZRE samples obtained after reduction. Sample

CZ CZTb CZPr CZLa

Surface atomic concentration

Surface atomic ratios

Ce (at. %)

Zr (at.%)

RE (at. %)

O (at. %)

Ce/Zra

RE/(Ce þ Zr)a

12.09 12.70 13.78 11.20

4.25 5.25 5.31 5.20

e 0.61 2.58 1.36

53.01 52.51 54.84 58.36

2.88(3.0) 2.41(2.6) 2.59(2.6) 2.15(2.6)

e 0.03(0.11) 0.13(0.11) 0.08(0.11)

a Nominal ratios were indicated in the parentheses.

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Table 3 e O 1s binding energy of CZRE samples and Ce3D concentration (%). Sample

CZ CZTb CZPr CZLa

O 1s binding energy/eV Peak 1

Peak 2

Peak 3

528.80 528.79 528.80 528.87

530.30 531.41 530.33 531.22

532.57 535.02 531.86 534.76

Ce3þ conc. (%)

37.35 41.85 47.39 42.91

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.

3.2.

Catalyst performance

3.2.1.

Oxygen storage capacity (OSC)

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The OSC property was tested by oxygen release characteristics of the calcined samples under dry air atmosphere in the temperature range of 300e800  C. The weight change of the sample was monitored by thermo-gravimetric (TG) method under cyclic heat treatments in flowing air. During this method the sample is subjected to consecutive cycles of heating and cooling. In the first heat treatment a large decrease of weight is noticed and it corresponds to the release of both water molecules (from surface) and oxygen (from the sample). The recovery 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

Fig. 4 e (a) Tb (b) Pr (c) La XPS spectra of CZRE samples after reduction.

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200

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 thermogravimetric method. Furthermore, Raman spectra distinctly shows that a large decrease in the intensity of the characteristic fluorite peak for the CZPr sample indicating a highly deformed structure which can result in high oxygen mobility and redox property which 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. Electrochemical properties of these materials will be evaluated by hall-cell measurements under SOEC conditions and would be the topic of the forthcoming paper.

Total OSC ( mol/g)

150

100

50

0

CZ

CZTb

CZPr

CZ La

Fig. 5 e Oxygen storage capacity of calcined CZRE samples measured from thermo-gravimetric method.

and can be noted that CZPr sample exhibits the highest OSC (155 mmol/g), which is followed by CZLa (109 mmol/g) and CZTb (82 mmol/g) samples. This is an interesting observation from the practical viewpoint as the total OSC is a crucial parameter for many technological applications.

3.2.2.

H2-production

Fig. 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 maximum point for the CZPr sample is 205 mmol/g and was

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 successfully investigated. The solid-solutions were prepared by glycineenitrate 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 activity than the La or Tb doped CZ samples. The XPS measurements 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 Institutional Research Program of Korea Institute of Science and Technology (KIST) (2E22802). One of the authors (D.H.P.) acknowledges KIST for the award of a STAR Post-Doc Fellowship.

references

Fig. 6 e Time dependence of the hydrogen production during water-splitting reaction on CZRE samples.

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