Step-change in high temperature steam electrolysis performance of perovskite oxide cathodes with exsolution of B-site dopants

B-site doped, A-site deficient perovskite oxide titanates with formula La0.4Sr0.4Mnxn+Ti1-xO3-gamma-delta (M = Fe3+ or Ni2+; x = 0.06; gamma = (4 - n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis at 900 degrees C. A-site deficiency provided additional driving force for the exsolution of a proportion of B-site dopants at the surface in the form of metallic nanoparticles under reducing SOEC cathode operating conditions. In the case of La0.4Sr0.4Fe0.06Ti0.94O2.97, this represents the first time that Fe-0 has been exsolved from a perovskite in such a way. Exsolution was due in part to the inability of the host lattice to accommodate vacancies (introduced (delta) oxygen vacancies (V-o(..)) and fixed A-site (V-Sr('')) and inherent (gamma) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Fe-0 or Ni-0 nanoparticles and higher V-o(..) concentrations dramatically lowered the activation barrier to steam electrolysis compared to the parent material (x = 0). The use of defect chemistry to drive the exsolution of less reducible dopant cations could conceivably be extended to produce new catalytically active perovskites with unique properties.