Unraveling the evolution of exsolved Fe–Ni alloy nanoparticles in Ni-doped La0.3Ca0.7Fe0.7Cr0.3O3−δ and their role in enhancing CO2–CO electrocatalysis

Abstract

The growth and phase evolution characteristics of exsolved metal nanoparticles (NPs) in a Ni-doped La_0.3Ca_0.70Fe_0.7Cr_0.3O_3−δ (LCFCrN) perovskite is investigated in H₂–N₂ and CO–CO₂ environments. Exsolution kinetics are rapid in H₂–N₂ while those in CO–CO₂ atmospheres are sluggish, possibly due to a combination of pO₂ difference in excess of three orders of magnitude as compared to that in H₂–N₂, and a different reaction pathway in the two atmospheres. NPs grown in H₂–N₂ exhibit a compositional and structural progression from an initially Ni-rich phase to an Fe-rich phase at short and long heat treatment durations, respectively. Once the subsurface Ni depletes, the NPs seem to coarsen via a combination of addition of Fe from the parent perovskite and Ostwald ripening. For longer heat treatment durations, CaO particles are observed to be appended to the Fe–Ni NPs. Exsolution also occurred in CO₂–CO atmospheres exhibiting similar trends, although the composition of the NPs was Ni-rich even after a 25 h reduction treatment in 70 : 30 CO : CO₂ at 800 °C, indicating that the NPs are resistant to coarsening and stable for use in highly reducing CO–CO₂ environments. In reversible solid oxide cells (RSOC) applications, the CO oxidation kinetics are typically sluggish on single phase perovskite electrodes. However, for Fe–Ni alloy NP-decorated LCFCrN (Fe–Ni@LCFCrN), the NPs are shown to enhance the CO oxidation kinetics (by ca. 75%) and the CO₂ reduction reaction (CO₂-RR) kinetics (by ca. 15%) as compared to the parent material, LCFCr. This makes the Fe–Ni@LCFCrN catalyst equally active for both reactions, hence significantly enhancing its potential for use in reversible solid oxide cell applications.