Long-term stability of oxygen surface exchange kinetics of Pr_0.8Ca_0.2FeO_3-δ against SO₂-poisoning

The long-term stability of the oxygen exchange kinetics of the Sr- and Co-free solid oxide fuel cell (SOFC) and solid oxide electrolyser cell (SOEC) air electrode material Pr_0.8Ca_0.2FeO_3-δ (PCF82) is investigated by in-situ dc-conductivity relaxation measurements. The chemical oxygen surface exchange coefficient k_chem is determined as a function of time at 700 °C under ideal (O₂-Ar atmosphere) and accelerated ageing conditions (O₂-Ar with 2 ppm SO₂). In pure O₂-Ar PCF82 shows fast oxygen surface exchange kinetics with k_chem = 6 × 10⁻⁴ cm s⁻¹ and excellent stability for 1000 h. Due to the addition of 2 ppm SO₂ to the test gas only a moderate degradation occurs with k_chem decreasing to 8 × 10⁻⁵ cm s⁻¹ during further 1000 h. Post-test analyses by scanning (transmission) electron microscopy and X-ray photoelectron spectroscopy show S-rich secondary phases in the near-surface region of the SO₂-poisoned sample. These inactive phases occur in an island-like arrangement of relatively large crystals, whereas significant amounts of the surface remain unaffected. This effect explains the high stability of the oxygen exchange kinetics of PCF82 against SO₂-poisoning in comparison to state-of-the art materials like La_0.6Sr_0.4CoO_3-δ (LSC64).