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 La_0.4Sr_0.4Mn+xTi_1-xO _3-γ-δ (M = Fe³⁺ or Ni²⁺; x = 0.06; γ = (4 - n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis at 900 °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 La_0.4Sr_0.4Fe_0.06Ti_0.94O _2.97, this represents the first time that Fe⁰ 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 (δ) oxygen vacancies () and fixed A-site () and inherent (γ) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Fe ⁰ or Ni⁰ nanoparticles and higher 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.