The effect of dopants on the redox performance, microstructure and phase formation of ceria

• Sintering, microstructure and redox performance of doped ceria are investigated.
• Solid state reaction and Pechini synthesis yield qualitatively equal samples.
• Grain growth of CeO2 is decreased by doping with HfO2, ZrO2, Pr6O11 and Li2O-HfO2.
• Pores can be effectively retained at high temperatures Li by co-doping.
• Oxygen exchange capacity of ceria is dramatically increased with Zr and Hf doping.

A solid state reaction is employed to investigate the influence of ZrO2, HfO2, Pr6O11, TiO2 and Li2O doping on CeO2 for a possible use in solar thermochemical redox cycles. Ceramics with a macroscopic interconnected porosity, necessary for high mass transport during redox cycles, are produced by the addition of spherical carbon particles prior to sintering. Partial closure of porosity is detectable when CeO2 is doped with Pr, Zr or Hf, while Li co-doping retains interconnected porosity more effectively than other doped or pristine ceria samples. In dense ceramics, microstructures reveal a reduction of the average grain size of pristine CeO2 with increasing Zr and Hf dopant concentration. These trends are validated using Pechini synthesized materials of the same composition. The reduction in grain size is even more pronounced for Pr doped CeO2 and Li doped Hf0.1Ce0.9O2, while TiO2 doping induces softening of samples under operating conditions (>1500 °C) limiting its use for high temperature applications. The redox performance of MxCe1−xO2−δ (M = Zr, Hf; 0 ≤ x ≤ 0.2) can be increased significantly with increasing Zr and Hf dopant concentration. At x = 0.2 (Zr, Hf) the fuel production rates are doubled as compared to pristine CeO2. The redox performance of Hf doped CeO2 remains stable upon co-doping with Li+.