Oxygen reduction and oxygen evolution on SrTi1 − xFexO3 − y (STFO) perovskite electrocatalysts

Abstract

Compositionally graduated thin films of a SrTi1−xFexO3−y (STFO) perovskite electrocatalysts were successfully prepared by High Throughput Physical Vapour Deposition (HT-PVD) using evaporative sources. X-ray diffraction confirmed a continuous solid solution of the cubic perovskite structure with an increase in the lattice parameter with increasing x from 0.392±0.001nm for SrTiO3 to 0.386±0.001nm for SrFeO3. A Raman mode corresponding to an O-stretching vibration was observed which is disallowed by symmetry in the cubic structure suggests a localised lattice distortion. The perovskites exhibited poor conductivity for low values of x (ρ<7×10−8Scm−1) but conductivity increased with increasing Fe content before reaching a plateau at ρ=0.041Scm−1 for x>0.75. Increasing electrocatalytic activity towards the oxygen evolution reaction (OER) with increasing Fe content was observed, characterised by a 100μA onset potential varying monotonically from 1.52VRHE (x=0.2) to 1.40VRHE (x=0.85). The high OER activity was however found to correlate with low electrode stability, consistent with the participation of lattice oxygen in the OER mechanism. The latter was evidenced by the redox electrochemistry associated with reversible oxygen intercalation. SrTi0.5Fe0.5O3−y exhibited the optimal composition with good OER activity and electrode stability. Low electrocatalytic activity towards the oxygen reduction reaction (ORR) was observed for all oxygen stoichiometric compositions. The ORR did not occur until after reduction of the films suggesting that surface reduction is required for the creation of the active surface sites. ORR activity on the oxygen sub-stoichiometric perovskites showed the opposite trend with compositional variation to OER activity on the oxygen stoichiometric perovskites.