Compressive Strain-Induced Uphill Hydrogen Distribution in Strontium Ferrite Films

Hydrogen incorporation into metal oxides enhances their electrochemical properties, making them highly suitable for various energy conversion applications. The controlled distribution of hydrogen ions in material systems and their conduction at elevated temperatures have garnered significant attention for various energy storage and environmental monitoring applications, including fuel cells, smart windows, and sensor technologies. In this work, cost-effective, high-concentration hydrogen-doped SrFeO_3-δ (HSrFeO_3-δ) films were prepared under ambient conditions by treating Al(s)|SrFeO_3-δ(s) films with KOH(aq), utilizing electron-proton codoping to investigate hydrogen distribution. The uphill hydrogen distributions in SrFeO_3-δ films with compressive strain, in contrast to the density gradient behavior under tensile strain, suggest the fundamental role of the strain states in the hydrogen accommodation. Compressively strained films with a rich Al source follow an anomalous uphill feature of hydrogen distribution, highlighting their potential use as electrolyte for fuel cells. The strain significantly influences the structure, chemical lattice coupling, and consequently the ionic transport in SrFeO_3-δ. Ionic conductivity measurements reveal that compressively strained HSrFeO_3-δ films with uphill hydrogen distributions exhibit a significant ionic conductivity of 0.189 S/cm at 413 K, with an activation energy of approximately 0.29 eV, making them suitable for low-temperature electrochemical applications. These findings provide a promising approach for tuning material properties and valuable insights for building iontronic devices.