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Abstract
Previous studies have shown that a large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides, such as ceria, for two-step thermochemical water splitting cycles. In this context, the configurational entropy arising from oxygen off-stoichiometry in the oxide, has been the focus of most previous work. Here we report a different source of entropy, the onsite electronic configurational entropy, arising from coupling between orbital and spin angular momenta in lanthanide f orbitals. We find that onsite electronic configurational entropy is sizable in all lanthanides, and reaches a maximum value of ≈4.7 kB per oxygen vacancy for Ce4+/Ce3+ reduction. This unique and large positive entropy source in ceria explains its excellent performance for high-temperature catalytic redox reactions such as water splitting. Our calculations also show that terbium dioxide has a high electronic entropy and thus could also be a potential candidate for solar thermochemical reactions.
Highlights
Previous studies have shown that a large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides, such as ceria, for two-step thermochemical water splitting cycles
Ceria (CeO2) has been among the very first[1] and most widely studied[2] materials for catalytic and energy applications. It is used in three-way exhaust automotive catalysts[2,3,4,5,6], solid-state fuel cells[7,8,9,10], two-step thermochemical water splitting cycles (TWSC)[11,12,13,14], low-temperature water–gas shift reactions[15], and several other industrial catalytic applications[16,17,18,19,20]
Meredig and Wolverton[22] examined the thermodynamics of these two-step reaction cycles and showed that a key thermodynamic quantity for iÀncreaseÁd efficiency is a Stoichiometric large solid-state oxides typically entropy of reduction have ΔSsroedlid
Summary
Previous studies have shown that a large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides, such as ceria, for two-step thermochemical water splitting cycles. In this context, the configurational entropy arising from oxygen off-stoichiometry in the oxide, has been the focus of most previous work. We find that onsite electronic configurational entropy is sizable in all lanthanides, and reaches a maximum value of ≈4.7 kB per oxygen vacancy for Ce4+/Ce3+ reduction This unique and large positive entropy source in ceria explains its excellent performance for high-temperature catalytic redox reactions such as water splitting. By excluding the Àgas phaÁse entropy, we have the solid-state entropy of reduction ΔSsroedlid , which is the material dependent contribution
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