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

Abstract

Paired charge-compensating doped ceria (PCCD) using trivalent and pentavalent cations are evaluated as redox materials for the thermochemical splitting of H₂O and CO₂. The oxygen nonstoichiometries of PCCD materials with formulas of Ce_0.9A_0.05Nb_0.05O₂ (A = Y, La, Sc) and Ce_xLa_(1−x)/2Nb_(1−x)/2O₂ (x = 0.75, 0.95) were measured in a thermogravimetric analyzer over a range of temperatures (T = 1173–1773 K) and oxygen partial pressures (p_O2 = 10⁻¹⁵–10⁻¹ atm). Undoped and single element doped ceria (Ce_0.9B_0.1O₂ where B = Y, La, Nb, Hf) served as a reference. At any given set of T and p_O2, the relative reduction extent follows Ce_0.9Hf_0.1O₂ > Ce_0.9Sc_0.05Nb_0.05O₂ > Ce_0.9Y_0.05Nb_0.05O₂ > Ce_xLa_(1−x)/2Nb_(1−x)/2O₂ > CeO₂ > solely trivalent or pentavalent doped ceria. The partial molar reduction enthalpies were determined using Van't Hoff analysis coupled to defect modeling and range from 360 to 410 kJ mol⁻¹. A system efficiency model predicts that these PCCD materials have the potential of achieving high solar-to-fuel energy conversion efficiencies because of their balanced reduction and oxidation properties. Ce_0.9Y_0.05Nb_0.05O₂ in particular can outperform undoped ceria and reach efficiency values of 31% and 28% for H₂ and CO production, respectively.