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

Abstract

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.

Thermodynamics of paired charge-compensating doped ceria with superior redox performance for solar thermochemical splitting of H2O and CO2

“…This result is not surprising, given that singly trivalent and pentavalent dopants are expected to behave similarly to ceria. 17,30,46,47 The trends for the partial molar entropy are very similar to those of the partial molar enthalpy and are shown in Fig. 8b.…”

“…30 No single element other than Zr and Hf exists which meets the criteria for improving ceria performance. 31 Interestingly, by using paired charge-compensating dopants (PCCD), aliovalent cations can be made to mimic tetravalent dopant behavior.…”

Assessment of kinetic model for ceria oxidation for chemical-looping CO2 dissociation

“…In all the cases, an almost linear rise of total CO production is observed from 9 ml/g at 700°C to 33 ml/g at 1000 o C. Figure 4 also shows that the effect of CO2 concentration on the total production of CO is sensibly lower than temperature variation. The strong temperature dependence is evident from the earlier studies [ 14,16,25,29 ].…”

Heat Transfer Model of a 50 kW Solar Receiver–Reactor for Thermochemical Redox Cycling Using Cerium Dioxide

“…If volumetric absorption and uniform heating is achieved inside the ceria RPC, mass loading could also be increased to obtain higher efficiencies, provided the latter criterion of uniform heating is not compromised in the process. While the numerical model indicates the potential of this solar receiver-reactor technology to achieve high efficiency, critical issues remain: (i) stable ceria structures with optimized volumetric absorption characteristics (i.e., ordered structures) must be fabricated and demonstrated to survive in the solar reactor environment, and (ii) with increased power, the directly irradiated surface area of the redox active material will always be at risk of sublimation; the search for new redox active materials which can be reduced at lower temperatures while maintaining favorable oxidation properties is critically important [51,61,62].…”