Ceria Doped with Zirconium and Lanthanide Oxides to Enhance Solar Thermochemical Production of Fuels

Abstract

Word Style "BD_Abstract").Developing an efficient redox material is crucial for thermochemical cycles that produce solar fuels (e.g. H 2 and CO), enabling a sustainable energy supply. In this study, the effects of varying the rare-earth content y in Ce 0.85-y Zr 0.15 RE y O 2-0.5y with RE = Y, La, Sm and Gd on the fuel productivity and long-term stability were investigated. Compared to the none-RE-doped reference material, Ce 0.85 Zr 0.15 O 2 , none of the compositions exhibits higher performances.However, long-term cycling of more than 80 cycles reveals enhanced performance due to rareearth doping. Ce 0.85 Zr 0.15 O 2 suffers from linear degradation of the yields and of the CO:O 2 ratio r, which is attributed to declining oxidation kinetics, whereas for instance Ce 0.82 Zr 0.15 Sm 0.03 O 1.99 features stable yields and kinetics. The suggested rationale behind is found in a vacancy-depleted region that occurs in the grains of Ce 0.85 Zr 0.15 O 2 . While cycling, the specific surface decreases and the impact of these regions on the reaction rate increases which leads to declining oxidation kinetics. In contrast, Ce 0.82 Zr 0.15 Sm 0.03 O 1.99 displays structural vacancies corresponding to the Sm 3+ , which remain during oxidation. Because of these structural vacancies the oxygen bulk transport is enhanced resulting in a constant reaction rate. Due to the long-term cycling, rareearth doping is in particular beneficial for the oxidation kinetics and hence, important for the technical realization of the process.

Concentration-Dependent Solar Thermochemical CO 2 /H 2 O Splitting Performance by Vanadia–Ceria Multiphase Metal Oxide Systems

“…It shows high and stable fuel production rates via a nonstoichiometric oxygen exchange process. The fast redox kinetics and high fuel selectivity are distinct characteristics of CeO 2 as compared to other redox materials [10,11,15,[21][22][23][24][25][26][27]. Fuel selectivity indicates the percentage of product gases (CO, H 2 ) produced upon splitting of CO 2 , H 2 O, CH 4 , etc.…”

“…Doping CeO 2 with transition metals has been proposed to further improve the material chemical, thermal, mechanical, and optical characteristics. However, transition metal-doped CeO 2 has so far demonstrated inferior redox reaction rates and oxygen ion mobility as compared to pure CeO 2 [11,[26][27][28]. In addition, doped CeO 2 shows extensive sintering at high temperatures, which decreases gas-phase mass transfer and consequently decreases the overall fuel production performance [10].…”

Non-Stoichiometric Redox Active Perovskite Materials for Solar Thermochemical Fuel Production: A Review

“…The high reduction temperature induces sintering and reactor materials issues, and thus limits the practical use of ceria. Different dopants were investigated to increase fuel production while lowering the reduction temperature, like Li 2+ , Mg 2+ , Sc 3+ , Dy 3+ , La 3+ , Sm 3+ , Gd 3+ , Pr 3+ , Hf 4+ , and Zr 4+ [5][6][7][8][9][10][11][12][13][14], at the expense of lowering the oxidation rate. In spite of the numerous dopants investigated, the relatively low fuel productivity of ceria still limits its future implementation in large-scale processes [5].…”

Metal Oxides Applied to Thermochemical Water-Splitting for Hydrogen Production Using Concentrated Solar Energy

“…The addition of dopants has been shown to enhance the reduction extent at low oxygen partial pressure when compared with un-doped ceria (at the expense however of lower oxidation capability with water). Hence, the introduction of dopants such as Zr [145][146][147][148][149][150][151][152][153][154], Sm [139] and transition metals oxides (MO x with M = Mn, Ni, Fe, Cu) [135,140,141] has been extensively explored for applications in solar-driven thermochemical redox cycles, as an efficient means to enhance the thermodynamic driving force of the O 2 -releasing reduction reaction at lower temperatures. However, it should be noted that trivalent dopants do not improve significantly the thermochemical performance of ceria because they do not present any meaningful increase of the fuel production or improvement of the thermal stability.…”

“…However, the material shaping process represents an additional step required for the preparation of tailored materials and for their incorporation in suitable solar reactors, which could have an impact on the process economics. The thermodynamic and kinetic properties of ceria can be altered by doping its fluorite structure with transition metals and rare earth metal oxides [135][136][137][138][139][140][141][142][143][144][145][146][147][148][149][150][151][152][153][154]. Indeed, ceria is an active compound used in many catalytic systems, and it is attractive due to its thermal stability (good resistance to sintering) and ability to exchange oxygen via storing and releasing oxygen reversibly.…”