Experimental screening of perovskite oxides as efficient redox materials for solar thermochemical CO2 conversion

Abstract

Developing efficient redox materials for the thermochemical synthesis of renewable fuels from CO₂ and H₂O via two-step redox cycling by utilizing concentrated solar energy is a major challenge that needs to be overcome for the realization of the solar fuel technology. Metal oxide redox materials are receiving recent interest on this regard, among which perovskite structured mixed oxides with lattice oxygen transfer are particularly important. In this contribution, we examined a series of perovskites and related Ruddlesden–Popper (RP) structures for their effectiveness in solar thermochemical conversion of CO₂. All the materials used in this study were synthesized from the corresponding oxides by following the conventional solid state method. Substitutions were made on the cationic A and B sites in the structure in order to modify their redox properties and to enhance their stability under the reaction conditions. In conjugation, with the compositional tailoring of the perovskite materials, the temperatures of reduction and re-oxidation were varied. Issues regarding carbonate formation were identified, for some compositions so as to affect their CO₂-splitting efficiency. Also in some cases, despite the higher extent of reduction observed at low temperatures, given the presence of Co⁴⁺, Fe⁴⁺ valence states, the extent of re-oxidation and stability were comparatively lower (case of (La,Sr)(Co,Fe)O₃, (Ba,Sr)(Co,Fe)O₃). The best performance among the investigated series was observed for La(B,B′)O₃ perovskites with Mn and Ni or Mn and Co together in the B site, as well as strontium doped lanthanum manganites with perovskite (La_0.5Sr_0.5MnO₃) and parent RP structures. Ruddlesden–Popper materials with only Mn occupying the B-site (LaSrMnO₄, LaSr₂Mn₂O₇), exhibited noteworthy thermochemical stability for two-step redox cycling and the amount of produced CO was consistently higher without any microstructural or morphological optimization at this stage.