Oxygen storage–release behavior of delafossite-type CuCr_1−x M _x O₂ (M = Fe, Ga)

The oxygen storage capacity (OSC) and oxidation–reduction properties of delafossite-type CuCr_1−x M _x O₂ (M = Fe and Ga) were investigated. The solid solutions having 3R-type delafossite-type structure were synthesized by a solid-state reaction method at 860–1100 °C in N₂ atmosphere. Substitution of Fe³⁺ and Ga³⁺ into Cr³⁺ site of CuCrO₂ improved their OSC and stability under cyclic oxidative/reductive atmosphere. CuCr_0.5Fe_0.5O₂ and CuCr_0.3Ga_0.7O₂ exhibited the largest OSC for M = Fe and Ga, respectively. The thermogravimetry in an oxidative atmosphere and XRD analyses revealed that CuCr_1−x M _x O₂ oxidized to Cu(Cr_1−x M _x )₂O₄ spinel and CuO. Under 5 % H₂/He atmosphere, CuCr_1−xFe_xO₂ was reduced to Cu and (Cr, Fe)₃O₄ spinel phase, which was eventually reduced to Fe and Cr₂O₃. In the M = Ga system, the delafossite phase was reduced to Cu + Cr₂O₃ solid solution (x = 0–0.1) or Cu + Ga₂O₃ solid solution (x = 0.9–1.0). In the region x = 0.3–0.7, metallic Cu was formed, coexisting with the delafossite phase, without the formation of Cr and Ga oxides. These results suggest that oxygen release behaviors were attributed to reductive decomposition for CuCr_1−x Fe _x O₂ and deposition of Cu from the delafossite phase maintaining the structure for CuCr_1−x Ga _x O₂ (x = 0.3–0.7).