(689j) Investigation of Undoped and Strontium-Aluminum Doped Neodymium Based Perovskite with Manganese and Iron As B-Site Cation for 2-Step Thermochemical Splitting of CO2/H2O

Solar energy is considered a green alternative to mitigate the effects of fossil fuels on the environment. Among many currently accessible applications of solar energy, concentrated solar power (CSP) has advantages over solar photovoltaic (PV) for utilizing full solar spectrum & being available interminably, unlike photovoltaics which only utilize partial solar spectrum & are available only during the daytime. However, thermal energy storage systems (TES) are required in CSP’s to absorb surplus solar thermal energy during the period when sunlight is available and utilize the stored energy during the night or bad weather. Various sensible, latent & thermochemical systems have been proposed for storing surplus solar thermal energy. Splitting of H₂O/CO₂ to generate the H₂/CO fuel via two step non-stoichiometric solids using cyclic thermochemical route is a promising area.

Perovskites are materials having ABO₃ type structures where A is large 12-fold coordinated alkali or alkaline earth metal cation & B is relatively small 6-fold coordinated transition metal cation forming oxygen octahedral within the perovskite crystal system. Ideal undistorted perovskite follows cubic geometry with pm3m space group. However, Orthorhombic, and rhombohedral geometries are also common in the distorted perovskite structures. Goldschmidt tolerance factor (t) relates the ionic size of the cations & anions with the possible space group of the perovskite. Doped perovskites contain structure where & are dopants. In a 2-step thermochemical cycle, the perovskites are first reduced at high temperatures using concentrated solar heat to produce non-stoichiometric oxygen deficient material ( where is the oxygen non-stoichiometry. The reduced perovskites are subsequently reacted with H₂O/CO₂ to produce H₂/CO fuel simultaneously re-oxidizing the perovskite back to ABO₃ structure.

The 1st reaction (i.e. thermal reduction) is endothermic and favored at higher temperatures whereas, the 2nd reaction (i.e., re-oxidation) is exothermic and favored at low oxygen partial pressure & lower temperatures. The H₂/CO produced can be further converted to liquid hydrocarbon fuels using Fischer-Tropsch synthesis.

This research intends to report amongst the various novel perovskite compositions based on doped and undoped neodymium-manganite based perovskite the perovskite with highest yield for solar fuel (H₂/CO) production via thermochemical splitting of H₂O/CO₂. Various A site dopants are tested from Group II metals (i.e., with varying quantity except Mg where due to solubility issues. Mn and Fe were used as b-site cations doped with Aluminum where y=0, 0.2. Two-step thermochemical splitting of CO₂/H₂O is studied via redox experiments using NETZSCH thermogravimetric analyzer (TGA). Thermal reduction of perovskites was carried out at 1400 ℃ in 100 mL/min Argon for 45 minutes whereas re-oxidation of perovskite was carried out 800 ℃ in 40 %CO₂ in Argon for 60 minutes.

Perovskites are synthesized by citrate Sol-gel based modified Pechini method. The metal nitrates of the precursor cation are mixed in the desired stoichiometric molar quantities with anhydrous citric acid in 1:1.5 ratio and dissolved in 100 mL de-ionized water. Ethylene glycol is added into the mixture for polymerizing. The obtained resulting gel is dried and crushed using mortar pestle. The powder is burned at 300 ^oC on a hot plate and air calcined further in muffle furnace at 1400 ^oC for 6 hours using a ramp of 5 ^oC/min. Powder XRD is used to characterize the perovskite for its crystal pattern before & after Redox study using EMPYREAN PANalytical x-ray diffractometer with . ICDD database was used to compare the XRD pattrens obtained from the xrd in identification of various peak properties. Thermos scientific iCAP6000 ICP-OES is used to re-assure the metal cation ratio with the as-synthesized sample. JEOL JSM-7610F Scanning electron microscope (SEM) is used to study the pre & post redox changes in morphology of the perovskite to account for any sintering effect. The weight changes (gain or loss) of the perovskite sample are related to the oxygen non-stoichiometry of the perovskite. The oxygen non-stoichiometry data obtained through experimentation at different oxygen partial pressures is used for defect modelling of the perovskite. The research is ongoing and soon the results of the redox study will be available for publishing.

The author is interested in postdoctoral positions, any teaching or research-based opportunities and collaborations.