Oxygen vacancy ordering within anion-deficient Ceria

Hull, S.; Norberg, Stefan T.; Ahmed, Istaq; Eriksson, Sten; Marrocchelli, Dario; Madden, Paul A.

doi:10.1016/j.jssc.2009.07.044

Cited by 114 publications

(106 citation statements)

References 44 publications

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“…33,37,[39][40][41][42] The presence of the smaller dopant cations, i.e. 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.…”

Section: Oxidationmentioning

confidence: 99%

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: Oxidationmentioning

confidence: 99%

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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“…where a is the lattice constant and L is the sample length [42]. On the other hand, δ can be calculated from gravimetric data (orange curve in Figure 1a)…”

Section: Degradation Of (Cezr)o 2 Redox Ceramics By Selective Sublimmentioning

confidence: 99%

Ceria: Recent Results on Dopant-Induced Surface Phenomena

et al. 2017

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Abstract:Redox studies on dense zirconia-doped ceria pellets were carried out by thermogravimetric investigations and dilatometry. Up to 1600 K reduction parameters determined by both methods correspond to each other. At higher temperatures, however, thermogravimetry overestimates the degree of reduction since mass loss is not only due to oxygen exsolution but also to selective evaporation of CeO 2 whose vapour pressure is considerably higher than that of ZrO 2 . As a consequence surface segregation of zirconia occurs in (Ce,Zr)O 2−δ pellets leading to a porous surface zone of Ce 2 Zr 2 O 7 pyrochlore which gradually grows in thickness. Surface enrichment of zirconia is detrimental for splitting CO 2 or H 2 O since re-oxidation temperatures of (Ce,Zr)O 2−δ are known to be shifted towards lower temperatures with increasing ZrO 2 content. Thus, very harsh reduction conditions should be avoided for the (Ce,Zr)O 2−δ redox system. The kinetics investigations comprised the high temperature reduction step (T ∼ = 1600 K) and the "low" temperature oxidation reaction with a carbon dioxide atmosphere (T ∼ = 1000 K). The reduction kinetics (at around 1600 K and an oxygen activity of 7 × 10 −4 in the gas phase) directly yield the (reduction) equilibrium exchange rate of oxygen in the order of 10 −7 mol·O/(cm 3 ·s) as the kinetics are surface controlled. The oxidation step at around 1000 K, however, occurs in the mixed control or in the diffusion control regime, respectively. From oxygen isotope exchange in combination with SIMS depth profiling oxygen exchange coefficients, K, and oxygen diffusivities, D, were determined for so-called equilibrium experiments as well as for non-equilibrium measurements. From the obtained values for K and D the (oxidation) equilibrium exchange rates for differently doped ceria samples were determined. Their dependency on the oxygen activity and the nature and the concentrations of a tetravalent dopant (Zr) and trivalent dopants (La, Y, Sm) could be semi-quantitatively rationalised on the basis of a master equation for the equilibrium surface exchange rate.

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“…2,3 As an example, the high ionic conductivity SOFC electrolyte and anode material, Gd x Ce 1Àx O 2Àx/2Àd , is known to become highly oxygen decient ($4% at 800 C) under anode conditions with isothermal chemical expansion on the order of 1-2%. [4][5][6] In order to avoid mechanical failure, such behavior places constraints on the operational range and design of SOFCs.…”

mentioning

confidence: 99%

Reducing the chemical expansion coefficient in ceria by addition of zirconia

Bishop

Marrocchelli

Fang

et al. 2013

Energy Environ. Sci.

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Oxygen vacancy ordering within anion-deficient Ceria

Cited by 114 publications

References 44 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

Ceria: Recent Results on Dopant-Induced Surface Phenomena

Reducing the chemical expansion coefficient in ceria by addition of zirconia

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Oxygen vacancy ordering within anion-deficient Ceria

Cited by 114 publications

References 44 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

Ceria: Recent Results on Dopant-Induced Surface Phenomena

Reducing the chemical expansion coefficient in ceria by addition of zirconia

Contact Info

Product

Resources

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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