Reactivity of CeO2-based ceramics for solar hydrogen production via a two-step water-splitting cycle with concentrated solar energy

Meng, Qing-Long; Lee, Chong-il; Ishihara, Toshihiko; Kaneko, Hiroshi; Tamaura, Yutaka

doi:10.1016/j.ijhydene.2011.07.089

Cited by 145 publications

(109 citation statements)

References 25 publications

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“…Among the dopants, we rst discuss Zr and Hf tetravalent dopants which aid the redox performance, 24,30,[32][33][34] before discussing those dopants which do not, i.e. di-, tri-, Ti-, and pentavalent dopants, 16,20,[23][24][25][26][27][28][29][30][31] in that order. In each case, we investigated the elements from periodic table rows 4, 5, and 6, and detail the row 5 elements and then compare the row 4 and 6 elements to the exemplar.…”

Section: Resultsmentioning

confidence: 99%

“…Di-, tri-and pentavalent dopants, including Li, Mg, Cu, Zn, Sr, Al, Sc, Cr, Mn, Fe, Co, Ni, Y, La, Pr, Sm, Gd, and Ta provide little, if any, improvement in the H 2 production capacity of ceria, although some prevent sintering and grain growth. 16,20,[23][24][25][26][27][28][29][30][31] Tetravalent Zr and Hf, however, have been shown to signicantly improve the redox capacity of ceria. 24,30,[32][33][34] To date the selection of dopants for STWS is mostly based on trial and error; therefore, we attempt to bring an understanding of ceria dopants to the STWS community.…”

Section: Oxidationmentioning

confidence: 99%

“…Currently, Zr and Hf dopants are thought to decrease the reduction energy of ceria, and thereby increase the reduction extent and the resulting H 2 production, 24,30,[32][33][34] by compensating for the expansion that ceria undergoes upon reduction. 33,[35][36][37][38] The rational for this hypothesis is explained as follows.…”

Section: Oxidationmentioning

confidence: 99%

“…This explains the improved behavior or Zr-CeO 2 for STWS. 24,30,[32][33][34] Additionally, there is no attraction between Zr-dopants and therefore no driving force for the formation of O-sites with two Zr neighbors which have reduction energies too low for water splitting. 30,36 Ti, however, behaves differently than Hf and Zr.…”

Section: Performance Improving Materials: Tetravalent Dopantsmentioning

confidence: 99%

“…Additionally, because the second O-vacancy has a formation energy similar to that of undoped ceria, they should have similar STWS activities, as is seen experimentally. 24,31,64 3.3.4 Trivalent Sc and La doped ceria. Because they are isovalent with Y, Sc-and La-CeO 2 behaves very similarly to Y-CeO 2 , and only major differences are discussed here while calculation results are summarized in Table 1 When a second +3 dopant is present, the O-vacancy formation becomes exothermic for both La (À0.96 eV) and Sc (À1.52 eV) and are incapable of splitting water.…”

Section: Non-performance Improving Materialsmentioning

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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