Study on Redox, Structural and Electrical Properties of Ce[sub x]Zr[sub 1−x]O[sub 2] for Applications in SOFC Anodes

Abstract

The effect of redox cycles on the reducibility, structural, and electrical properties of dense pellets of ceria-zirconia solid solutions has been investigated by temperature programmed reduction studies, X-ray diffraction technique, Raman spectroscopy, and electrochemical impedance spectroscopy. Similarities and differences among the compositions with high ͑Ce 0.80 Zr 0.20 O 2 ͒, intermediate ͑Ce 0.50 Zr 0.50 O 2 ͒, and low ͑Ce 0.20 Zr 0.80 O 2 ͒ amounts of ceria were evaluated in view of their application as active components for solid oxide fuel cell ͑SOFC͒ anodes. Cycles of reduction and reoxidation, respectively, at 1273 and 873 K, promote the reduction of all the compositions investigated, shifting it at a lower temperature. Moreover, redox treatments cause a significant change in the crystal structure of Ce 0.50 Zr 0.50 O 2 , ultimately leading to the formation of cubic pyrochlore phases as the reduction temperature increases. The correlations among structural changes, redox, and electrical properties of these materials are discussed.

New Insights into the Dynamics That Control the Activity of Ceria–Zirconia Solid Solutions in Thermochemical Water Splitting Cycles

“…Only a few studies investigated the correlations between water splitting performance and structural/compositional modification undergone during the redox steps, 23,25 despite that it is well-known that the redox properties of the materials are markedly affected by the thermal and redox history. 26 In a recent communication, we showed that also zirconia-rich ceria−zirconia mixed oxides can be suitable for the WS process. 27 In that study, it appeared clear that phase segregation and nitridation processes, which were induced by the thermal redox cycles, might play a major role in determining the high activity.…”

“…Most of the research has been focused on ceria-rich compositions, which were supposed to be structurally stable to the high temperatures adopted in the reduction step, and the attention of scientists was mainly drawn on kinetic and thermodynamic issues of the cycle. Only a few studies investigated the correlations between water splitting performance and structural/compositional modification undergone during the redox steps, , despite that it is well-known that the redox properties of the materials are markedly affected by the thermal and redox history . In a recent communication, we showed that also zirconia-rich ceria–zirconia mixed oxides can be suitable for the WS process .…”

Enhanced activities of nano-CeO2−δ@430L composites by zirconium doping for hydrogen electro-oxidation in solid oxide fuel cells

“…For example, Ce 0.75 Zr 0.25 O 2Àd was used as the ceramic component of the Ni-cermet anodes to enhance the gasesolid interaction via H 2 adsorption on the ceramic components, promote hydrogen electro-oxidation via an oxygen spillover mechanism and thereby reduce the anode polarization resistances [19]. Boaro et al demonstrated that redox cycling at 973 K conduced to activation and stabilization of Ce 0.5 Zr 0.5 O 2Àd -Cu anodes, resulting from a rearrangement of the morphology and microstructure of Ce 0.5 Zr 0.5 O 2Àd oxides at the electrode/electrolyte interface and diffusion of more copper into the porous electrode matrix [18]. Recently, we have proposed a dual-scale hierarchical anode with nanoporous catalysts coated inside the macroporous 430L stainless steel scaffolds [23e25].…”

Kinetic Relevance of Surface Reactions and Lattice Diffusion in the Dynamics of Ce–Zr Oxides Reduction–Oxidation Cycles

“…The relative kinetic relevance of these two processes depends not only on the identity and concentration of the reductant/oxidant and the CZO surface-to-volume ratio but also on the extent of reduction (ξ red ). CZO (and CeO 2 ) exhibit highly nonideal thermodynamics [10][11][12][20][21][22][23]28,41,42 with Gibbs free energies for O-removal that increase strongly as O-atoms are removed, and this nonideal behavior plausibly also affects the mobility and the reactivity of remaining O-atoms/O-vacancies such that they also depend strongly on ξ red , as predicted by (linear) free energy relationships. 43 These effects become discernible and "separable" only by considering chemical and transport dynamics driven by gradients in chemical potential that are pertinent for thermodynamically nonideal systems, for which the mobility and reactivity of O-atoms depend sensitively on the extent of reduction.…”