We
present results on the thermochemical redox performance and
analytical characterization of Hf4+, Zr4+, and
Sc3+ doped ceria solutions synthesized via a sol–gel
technique, all of which have recently been shown to be promising for
splitting CO2. Dopant concentrations ranging from 5 to
15 mol % have been investigated and thermally cycled at reduction
temperatures of 1773 K and oxidation temperatures ranging from 873
to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr
doped materials is substantially higher than those of pure ceria and
Sc doped ceria and increases with dopant concentration. Overall, 10
mol % Hf doped ceria results in the largest CO yields per mole of
oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based
on measured mass changes during oxidation. However, these yields were
largely influenced by their rate of reoxidation, not necessarily thermodynamic
limitations, as equilibrium was not achieved for either Hf or Zr doped
samples after 45 min exposure to CO2 at all oxidation temperatures.
Additionally, sample preparation and grain size strongly affected
the oxidation rates and subsequent yields, resulting in slightly decreasing
yields as the samples were cycled up to 10 times. X-ray diffraction,
Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX
have been applied to characterize the elemental, crystalline, and
morphological attributes before and after redox reactions.
Perovskite oxides have recently been proposed as promising redox intermediates for solar thermochemical splitting of H2O and CO2, offering the benefit of significantly reduced operating temperatures. We present a systematic experimental screening of doped lanthanum manganites within the composition space La1−x(Ca,Sr)xMn1−yAlyO3 and identify several promising redox materials. In particular, La0.6Sr0.4Mn0.6Al0.4O3 and La0.6Ca0.4Mn0.6Al0.4O3 boast a five‐ to thirteen‐fold improvement in the reduction extent compared to the state‐of‐the‐art material CeO2 in the temperature range 1200–1400 °C. The materials are shown to be capable of splitting CO2 into CO fuel when isothermally cycled between low‐pO2 and high‐pCO2 environments at 1240 °C and to approach full reoxidation in CO2 with temperature swings as low as 200 °C, with mass‐specific fuel yields up to ten times that of CeO2. The underlying material thermodynamics are investigated and used to explain the favorable redox behavior.
Thermochemical CO2-splitting via redox cycling of Ca, Sr and Al-doped La-Mn perovskites induces irreversible changes in the texture and chemical composition of these oxides. Though the crystal structure is mostly preserved after high-temperature redox cycling, the chemical stability is detrimentally affected by sintering and by the formation and eventual segregation of a carbonate phase during oxidation by CO2. Carbonation of the Ca and Sr phase was diminished by Al-substitution of the Mn-cation in the B-position.
This work reports an improved and stable oxygen exchange capacity (OEC) of optimized doped ceria Ce1−xMxO2−δ (M = Zr, Hf, Nb) materials for two-step thermochemical CO2 splitting over 50 consecutive redox cycles (7 days).
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