Subscriber access provided by GEORGE WASHINGTON UNIV
Article
Non-stoichiometry in oxide thin films operating under anodic conditions: A chemical capacitance study of the praseodymium-cerium oxide system Di Chen, Sean R. Bishop, and Harry L Tuller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503440v • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 26, 2014
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 7
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Non-stoichiometry in oxide thin films operating under anodic conditions: A chemical capacitance study of the praseodymium-cerium oxide system Di Chen a, Sean R. Bishop a,b , Harry L. Tuller a,b,* a
Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
b
International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Nishi-ku Fukuoka 819-0395, Japan KEYWORDS: thin film, non-stoichiometry, chemical capacitance, SOFCs
ABSTRACT: PrxCe1-xO2-δ can potentially serve as both the cathode and anode in solid oxide fuel cells given that it exhibits significant mixed ionic-electronic conductivity (MIEC) under both cathodic and anodic conditions. While MIEC depends strongly on oxygen non-stoichiometry δ, there have been few ways to extract this information reliably and in situ, particularly for thin films. In this work, this is achieved by analysis of chemical capacitance values extracted from impedance spectroscopy data obtained on electrochemical cells of the form Pr0.1Ce0.9O2−δ / Y0.16Zr0.84O1.92 / Pr0.1Ce0.9O2−δ operating at temperatures between 450 to 700 oC and over the pO2 range of 10-33-10-14 atm. In combination with our prior investigation, the oxygen nonstoichiometry over a very wide pO2 range is derived. We found that δ in thin films differs from that of the bulk, with δfilm> δbulk. This follows from a decreased enthalpy of reduction of 4.12 ± 0.25 eV for the film as compared to 4.52 ± 0.37 eV for the bulk.
1.
Introduction
As a result of their high ionic or mixed ionic electronic conductivities (MIEC), and high thermal and chemical stability, oxides are commonly used in solid oxide fuel cells (SOFCs) for efficient chemical to electrical energy conversion,1 chemical sensors for environmental monitoring,2 or three way catalyst supports for emissions reduction,3 and thermal barrier coatings (TBCs) for turbine engines.4 A key determining factor controlling both the magnitude and type of electrical conductivity (i.e., ionic vs. electronic) and therefore performance of SOFC electrodes and electrolytes, for example, is the concentration of mobile point defects, typically inferred from changes in oxygen stoichiometry. Likewise, the operation of the automotive three-way catalyst depends critically on the ability of the oxygen storage material, e.g. Ce1-xZrxO2−δ, to buffer the pO2 of the exhaust gas by rapidly exchanging oxygen between the oxygen storage material and the gas phase. 3 Here the key parameters include the accessible range of non-stoichiometry δ and the rate of exchange, which again depends on the nature and degree of MIEC induced by changes in pO2 or temperature, normally analysed with the aid of thermogravimetric and coulometric titration methods.5,6 As devices are scaled down, with materials prepared on the nano- to micron-scale, their mass and charge transport properties are often reported to change dramatically.7–10 Due to small sample volumes,
it becomes difficult to quantify defect concentrations by traditional techniques. Recently, our group and others have demonstrated the use of an electrochemical based technique to probe the chemical capacitance (Cchem) of thin films (~100-1000 nm thick), allowing quantification, of the oxygen vacancy concentration, with high precision, in non-stoichiometric oxides.11–14 Part of the high precision, originates from the ability to reproducibly prepare films with well-defined microstructure, thickness, orientation, crystallinity, and composition. Nevertheless, discrepancies in reported stoichiometry values calculated from Cchem are found in the literature, e.g. for La1-xSrxCoO3δ, when comparing the results of Kawada et. al. with those of Jose la O’ et. al..15,16 In this study, we examine oxygen non-stoichiometry with the aid of chemical capacitance in the praseodymium-ceria solid solution system, Pr0.1Ce0.9O2−δ (PCO), a model MIEC material with potential as a SOFC electrode, chemically and structurally compatible with both zirconia and ceria based SOFC solid electrolytes.17 In an earlier study, we demonstrated that the ability of Pr to reduce from the 4+ to the 3+ valence state, even in air at SOFC operating temperatures, renders the material a MIEC with area specific resistances (ASR) comparable to those of popular perovskite based cathodes such as (La,Sr)CoO3−δ.18 At the same time, related acceptor doped ceria solid solutions such as Sm0.1Ce0.9O2−δ and
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Gd0.1Ce0.9O2−δ have also been reported to be excellent anode materials,19,20 thereby offering the possibility of constructing an all ceria-based SOFC with PCO serving as both cathode and anode.21 In this work, Cchem values are extracted from impedance spectroscopy data obtained on electrochemical cells of the form Pr0.1Ce0.9O2−δ/Y0.16Zr0.84O1.92/Pr0.1Ce0.9O2−δ, for temperatures from 450 to 700 oC, and over the pO2 range of 10-33-10-14 atm O2. δ is calculated directly from Cchem with the aid of a defect equilibria model developed previously in our group.22,23 The calculated non-stoichiometry allows extraction of the thermodynamic constants defining defect generation (e.g., H ro,Ce , the ceria reduction enthalpy). In combination with our earlier high pO2 chemical capacitance study of this system, the ionization energy of elec-
where brackets denote concentration. Kr, Hr, k ro and n are the equilibrium constant, reduction enthalpy, entropy pre-exponential containing vibrational entropy, and electron concentration, respectively. The quasi-free electrons are now retained in the conduction band, made up of Ce 4f levels, and move between Ce ions via a small polaron hopping process.24 In order to maintain charge neutrality, the following equation applies ' n + [PrCe ] = 2[VO•• ]
2. 2.1.
Defect chemistry of Pr0.1Ce0.9O2−δ system
In a recent paper published by the authors, measurements of oxygen non-stoichiometry and electrical conductivity performed on bulk Pr0.1Ce0.9O2−δ were presented, and on this basis, a defect equilibrium model was developed.5,14 In PCO, oxygen vacancies are introduced in the material through the following defect reactions written in Kröger-Vink notation. (1)
× •• where OO , VO , and e' are oxide ions on oxygen sites, doubly positive charged (with respect to the lattice) oxygen vacancies, and electrons, respectively.
Electrons in the conduction band can drop down in en× ergy and occupy deep empty PrCe acceptor sites available at high pO2 through the following reaction × ' e' + PrCe ↔ PrCe
(2)
× ' where PrCe and PrCe are Pr4+ and Pr3+, respectively. As the pO2 is decreased, oxygen vacancies are generated through reaction 1 with the electrons thereby generated trapped on Pr sites via reaction 2 until all the Pr is reduced to the trivalent state. At that point, PCO begins to behave like a conventional acceptor doped material, such as Gd or Sm doped ceria. At sufficiently low pO2 (as shown in the results section), Ce cations themselves begin to reduce with oxygen vacancies generated, consistent with Equation (1) and the corresponding mass action relation given below in Equation (3) •• 2 1/2 H [V ]n pO K r = kroexp - r ≈ O × 2 [O O ] kT
n≈
2Kr [O×O ] ' [PrCe ]
pO2−1/4
(5)
Substituting Equation (5) into Equation (4) results in the equation below for oxygen vacancy concentration, later used to interpret chemical capacitance: 1 ' 1 2 K r [O×O ] −1/4 [VO•• ] ≈ [PrCe ]+ pO2 ' 2 2 [PrCe ]
Theory
O×O ↔ VO•• + 2e' + 1/ 2O2 ( g )
(4)
' ' •• At intermediate pO2, n δbulk. This follows from a decreased enthalpy of reduction of
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
4.12 ± 0.25 eV for the film vs 4.52 ± 0.37 eV for bulk PCO10. These differences need to be taken into account when considering the use of thin film electrodes in the design of, for example, thin film based micro-SOFC devices and likely impact on nonstoichiometric excursions in nanocrystalline oxygen storage materials. For example, a lower reduction enthalpy for acceptor doped thin film ceria would result in a narrower electrolytic domain boundary under reducing conditions, leading to a reduction in the open circuit voltage of the cell. On the other hand, thin film ceria, operating under anodic conditions, would be expected to exhibit a larger electronic conductivity, and thereby improved electrode performance. 34 The source of the decreased enthalpy of reduction in the thin films remains of interest, and may be related to substrate induced constraints and/or surface enhanced oxygen vacancy concentrations.15,31 Further studies are needed to establish the controlling factors in this phenomenon.
AUTHOR INFORMATION
Page 6 of 7
(9)
Knauth, P.; Tuller, H. L. J. Eur. Ceram. Soc. 1999, 19, 831–836.
(10)
Vayssieres, L.; Persson, C.; Guo, J.-H. Appl. Phys. Lett. 2011, 99, 183101.
(11)
Chueh, W. C.; Haile, S. M. Phys. Chem. Chem. Phys. 2009, 11, 8144–8148.
(12)
Lai, W.; Haile, S. M. J. Am. Ceram. Soc. 2005, 88, 2979–2997.
(13)
Jamnik, J.; Maier, J. Phys. Chem. Chem. Phys. 2001, 3, 1668–1678.
(14)
Chen, D.; Bishop, S. R.; Tuller, H. L. Adv. Funct. Mater. 2013, 23, 2168–2174.
(15)
Kawada, T.; Suzuki, J.; Sase, M.; Kaimai, A.; Yashiro, K.; Nigara, Y.; Mizusaki, J.; Kawamura, K.; Yugami, H. J. Electrochem. Soc. 2002, 149, E252.
(16)
La O’, G. J.; Ahn, S.-J.; Crumlin, E.; Orikasa, Y.; Biegalski, M. D.; Christen, H. M.; Shao-Horn, Y. Angew. Chem. Int. Ed. Engl. 2010, 49, 5344–5347.
(17)
Tuller, H. L.; Bishop, S. R. Annu. Rev. Mater. Res. 2011, 41, 369.
(18)
Chen, D.; Bishop, S. R.; Tuller, H. L. J. Electroceramics 2012, 28, 62–69.
(19)
Jung, W.; Dereux, J. O.; Chueh, W. C.; Hao, Y.; Haile, S. M. Energy Environ. Sci. 2012, 5, 8682.
(20)
Fu, C.; Chan, S. H.; Liu, Q.; Ge, X.; Pasciak, G. Int. J. Hydrogen Energy 2010, 35, 301–307.
(21)
Kramer, S. A.; Spears, M. A.; Tuller, H. L. Solid electrolyte-electrode system for an electrochemical cell. U.S. Patent No. 5,403,461, April 04, 1995.
(22)
Bishop, S. R.; Stefanik, T. S.; Tuller, H. L. Phys. Chem. Chem. Phys. 2011, 13, 10165–10173.
(23)
Bishop, S. R.; Stefanik, T. S.; Tuller, H. L. J. Mater. Res. 2012, 27, 2009–2016.
(24)
Tuller, H.; Nowick, A. J. Phys. Chem. Solids 1977, 38, 859–867.
(25)
Adler, S. B. Chem. Rev. 2004, 104, 4791–4843.
(26)
Chueh, W. C. Electrochemical & Thermochemical Behavior of CeO2, Ph.D thesis, California Institute of Technology, 2011.
Corresponding Author *
[email protected];
[email protected] ACKNOWLEDGMENT This work was supported by the Basic Energy Sciences, U.S. Department of Energy under award DE-SC0002633. The authors thank Jae Jin Kim (MIT) for preparation of the PLD targets. Sean R. Bishop gratefully recognizes support from I2CNER, established and supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan.
REFERENCES (1)
Tsuchiya, M.; Lai, B.-K.; Ramanathan, S. Nat. Nanotechnol. 2011, 6, 282–286.
(2)
Min, Y.; Tuller, H. L.; Palzer, S.; Wöllenstein, J.; Böttner, H. Sensors Actuators B Chem. 2003, 93, 435– 441.
(3)
Boaro, M.; Leitenburg, C. De; Dolcetti, G.; Trovarelli, A. J. Catal. 2000, 193, 338–347.
(4)
Padture, N. P.; Gell, M.; Jordan, E. H. Science 2002, 296, 280–284.
(5)
Bishop, S. R.; Stefanik, T. S.; Tuller, H. L. Phys. Chem. Chem. Phys. 2011, 13, 10165–10173.
(6)
Shah, P.; Kim, T.; Zhou, G. Chem. Mater. 2006, 18, 5363–5369.
(7)
Chiang, Y.-M.; Lavik, E. B.; Kosacki, I.; Tuller, H. L.; Ying, J. Y. Appl. Phys. Lett. 1996, 69, 185.
(8)
Maier, J. Nat. Mater. 2005, 4, 805–815.
ACS Paragon Plus Environment
Page 7 of 7 (27)
Bishop, S.; Duncan, K.; Wachsman, E. Electrochim. Acta 2009, 54, 1436–1443.
(32)
Weppner, W.; Huggins, R. A. J. Electrochem. Soc. 1977, 124, 1569–1578.
(28)
Otake, T. Solid State Ionics 2003, 161, 181–186.
(33)
Maier, J. Solid State Ionics 2000, 135, 575–588.
(29)
Bishop, S. R.; Duncan, K. L.; Wachsman, E. D. Acta Mater. 2009, 57, 3596–3605.
(34)
(30)
Kim, J. J.; Bishop, S. R.; Thompson, N.; Chen, D.; Tuller, H. L. Chem. Mater. 2014, 26, 1374–1379.
Tuller, H.L. 2000. Defects and Transport: Implications for Solid Oxide Electrolytes and Mixed Conductors, in: H.L. Tuller, J. Schoonman, and I. Riess (Eds.), Oxygen Ion and Mixed Conductors and their Technological Applications. Kluwer Academic Publishers, the Netherlands, 57-74.
(31)
Chueh, W. C.; McDaniel, A. H.; Grass, M. E.; Hao, Y.; Jabeen, N.; Liu, Z.; Haile, S. M.; McCarty, K. F.; Bluhm, H.; El Gabaly, F. Chem. Mater. 2012, 24, 1876– 1882.
Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manuscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equation, concept, or theorem, etc., that is discussed in the manuscript. Consult the journal’s Instructions for Authors for TOC graphic specifications. -1
-1.5
-1
500oC
-2
o
550 C 600oC 650oC
-2.5
log( 0.05)
450oC
log
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
-2 -3 -4
o
700 C
-5
-30
-20
log(pO 2[atm])
-3
-30
-25
-20
-15
-10
logpO log (pO 2/ atm)
ACS Paragon Plus Environment
-5
0
