*3.1. Material Synthesis*

The flame spray synthesis setup is described by Heel et al [54]. For the preparation of the PrBaCo2O5+x (PBC) and PrBaCo2-yFeyO5+x (y = 0.4 and 1.0) precursor solutions, stoichiometric amounts of praseodymium oxide (Pr6O11, 99.9%, Auer Remy, Hamburg, Germany), barium carbonate (BaCO3, ≥99%, Sigma-Aldrich, Darmstadt, Germany), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99.9%, Sigma-Aldrich, Darmstadt, Germany) and iron nitrate nonahydrate (Fe(NO3)3·9H2O, ≥98%, Sigma-Aldrich, Darmstadt, Germany) were dissolved in a mixture of solvents composed by N,N-Dimethylformamide (DMF, ≥99.8%, Roth, Frankfurt, Germany), acetic acid (HAc, ≥99.0%, Sigma-Aldrich, Darmstadt, Germany), nitric acid (HNO3, 70%, Sigma-Aldrich, Darmstadt, Germany) and water in 45:25:5:25 volume ratio, respectively. Firstly, the Pr6O11 was dissolved in the mixture of water and nitric acid at 80 ◦C; when a clear green solution was obtained, the BaCO3 was added and then, when no more CO2 bubbles were observed, the Co and Fe (only for PBCF solutions) metal precursors. When all dissolved, the HAc and the DMF were added, obtaining a final total metal concentration of 0.1 M. The precursor solutions were pumped into the flame by using a three piston pump (C-601, Büchi, Flawil, Switzerland) with a flow controller (C-610, Büchi, Flawil, Switzerland), using a constant flow free of pulsations of 20 mL min−1. Pure oxygen (99.95%, Carbagas, Bern, Switzerland) was used as dispersion gas with a flow rate of 35 L min−1. The combustion gas was formed by acetylene (99.6%, Carbagas, Bern, Switzerland), with a flow rate of 13 L min−1, and pure oxygen (17 L min−1). Finally, the powders were collected in a baghouse filter on four ashless paper filters (Whatman, Buckinghamshire, UK).
