Reducing the chemical expansion coefficient in ceria by addition of zirconia

Bishop, Sean R.; Marrocchelli, Dario; Fang, Wen‐Hui; Amezawa, Koji; Yashiro, Keiji; Watson, Graeme W.

doi:10.1039/c3ee23917a

Cited by 50 publications

(51 citation statements)

References 35 publications

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“…The fact that strained bonds reduces the ceria reduction energy corrects a long held misunderstanding that they compensate for ceria expansion. 33,[35][36][37][38] Additionally, this explanation provides the fundamental understanding for why the geometric relaxation of Zr-CeO 2 during reduction is associated with a decreased Ovacancy formation energy. 59 While the energy released is sizable, it does not decrease the reduction energy to the same extent as the presence of pre-broken bonds or decreased electron promotion energy.…”

Section: Straining O-bondsmentioning

confidence: 99%

“…33,[35][36][37][38] This nding builds on the work of Wang et al, who suggested that structural relaxation is the driver for the decreased reduction energy, 59 by explaining that the energy released during structural relaxation stems from strain release of the Zr-O bonds. The promotion of two electrons into the Ce forbitals and breakage of four O-cation bonds results in an Ovacancy formation energy that is high enough to drive water splitting when the vacancy is lled.…”

Section: Performance Improving Materials: Tetravalent Dopantsmentioning

confidence: 99%

“…33,[35][36][37][38] The rational for this hypothesis is explained as follows. When O-vacancies form in ceria during reduction, each removed O 2À anion nominally releases two electrons to the ceria bulk; these electrons localize on two Ce 4+ reducing them to Ce 3+ cations, as shown in Kroger-Vink notation:…”

Section: Oxidationmentioning

confidence: 99%

“…Zr and Hf, are believed to alleviate the expansive strain caused by the larger Ce 3+ cations via the overlap of tensile and compressive strain elds; simultaneously decreasing the extent of expansion and decreasing the reduction energy. 33,[35][36][37][38] This is very logical based on the cationic radius explanation for the expansion of ceria upon reduction. However, we have recently shown that the formation of Ce 3+ cations is not responsible for ceria expansion, but rather ceria expands because of outward relaxation from the O-vacancy due to non-counterpoised forces.…”

Section: Oxidationmentioning

confidence: 99%

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Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Muhich

Steinfeld

2017

J. Mater. Chem. A

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Ceria-based metal oxides are promising redox materials for solar H 2 O/CO 2 -splitting thermochemical cycles. Density functional theory (DFT) computations are applied to elucidate the underlying mechanism of the role of dopants in facilitating ceria based redox cycles; specifically, we explain why some dopants increase performance, while others do not. Firstly, we find that Zr and Hf dopants increase the oxygen exchange capacity of ceria because they store energy in tensilely strained Zr-or Hf-O bonds which is released upon O-vacancy formation. This finding corrects a long held assumption that Zr and Hf decrease the O-vacancy formation energy by compensating for ceria expansion upon reduction.Although the released strain energy decreases the O-vacancy formation energy, O-vacancy formation remains sufficiently endothermic to split H 2 O and CO 2 . Secondly, we show that two electrons must be promoted into the high energy Ce f-band during reduction if the O-vacancies are to store sufficient energy to drive the oxidative gas splitting step. This means that di-and trivalent dopants are not suitable for this process. Lastly, we show that dopants which break O bonds due to their small size or strongly covalent character, such as Ti and the pentavalent dopants, substantially decrease the O-vacancy formation energy because only three O bonds must break during reduction. These vacancies, therefore, are too low in energy to drive gas splitting. Based on these findings, we develop guidelines for new ceria doping strategies to facilitate solar thermochemical gas splitting cycles.

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Section: Straining O-bondsmentioning

confidence: 99%

Section: Performance Improving Materials: Tetravalent Dopantsmentioning

confidence: 99%

Section: Oxidationmentioning

confidence: 99%

Section: Oxidationmentioning

confidence: 99%

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Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Muhich

Steinfeld

2017

J. Mater. Chem. A

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“…In addition, to increase fracture strength and to avoid Figure 4a and e do not correspond to any previous samples cracking of GDC during co-firing in a reducing atmosphere, GDC20 should be sintered to result in low porosity and small grain size. Reduction of chemical-expansion coefficient by adding ZrO 2 in the CeO 2 is also a feasible way [15,43,44]. To avoid cracking, we suggest sintering GDC20 at T < 1088°C or at Po 2 >10 −15 atm or both, or sintering GDC20 to result in little porosity with small grain size.…”

Section: Resultsmentioning

confidence: 96%

Phase stability and oxygen non-stoichiometry of Gd-doped ceria during sintering in reducing atmosphere

Kim

2015

J Electroceram

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Firing of Gd-doped ceria (GDC) powder compact in reducing atmosphere leads to reduction and subsequent decomposition of GDC, and cracking of the pellet upon cooling, depending upon the firing temperature T and oxygen partial pressure Po 2 . In this study, GDC powder compacts were fired in various reducing atmosphere by controlling the ratio of H 2 and H 2 O mixing (H 2 O content: 0.28, 1.3, 2.7, 3 %) at temperatures T=703, 835, 975, 1088, 1180, 1262 and 1354°C. The phase and oxygen non-stoichiometry of GDC, respectively, were examined by using X-ray diffraction (XRD) and measuring the weight change of GDC pellet after cooling with fixed cooling rate to room temperature. Po 2 below which GDC decomposes was determined as a function of temperature for a fixed cooling rate (5°C/min). GDC pellets were stable at T≤1088°C regardless of Po 2 , and decomposed at T=1354°C in all tested Po 2 . A single-phase GDC without decomposition may be obtained at T=1180 or 1262°C with appropriate Po 2 . Both the degree of nonstoichiometry and the temperature are important factors to determine the stability of GDC pellet. To avoid cracking of GDC pellets due to chemical expansion during sintering in reducing atmosphere, GDC may be sintered at low T or high Po 2 or both, or it may be sintered to result in low porosity with small grain size for increased fracture strength.

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Introduction

Bishop¹,

Perry²

2017

Electronic Materials: Science &Amp; Technology

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Reducing the chemical expansion coefficient in ceria by addition of zirconia

Cited by 50 publications

References 35 publications

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Phase stability and oxygen non-stoichiometry of Gd-doped ceria during sintering in reducing atmosphere

Introduction

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Reducing the chemical expansion coefficient in ceria by addition of zirconia

Cited by 50 publications

References 35 publications

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

Phase stability and oxygen non-stoichiometry of Gd-doped ceria during sintering in reducing atmosphere

Introduction

Contact Info

Product

Resources

About

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂

Principles of doping ceria for the solar thermochemical redox splitting of H₂O and CO₂