Hydration and transport properties of the Pr2−xSrxNiO4+δ compounds as H+-SOFC cathodes
Résumé
The result of the substitution of Pr3+ by Sr2+ in the 214 Ruddlesden-Popper Pr2NiO4+δ material was studied with regard to its electrochemical properties as a H+-SOFC cathode. Structural characterizations as well as physical properties of the Pr2−xSrxNiO4+δ compounds (x ≤ 0.50), in particular hydration as a function of water partial pressure, have shown that oxygen over-stoichiometry and oxygen exchange with atmosphere decrease with increasing x, which has been correlated with the stabilization of the 214 structure by Sr2+ substitution. Electrochemical studies on the oxygen reduction versus hydration have allowed determination of the rate determining steps of the formation of water and evidence the role of protons in Pr2NiO4+δ in contrast to Pr2−xSrxNiO4+δ oxides. It has been concluded that triple mixed conductivity (i.e. protonic, ionic as well as electronic conductivities) exists in this nickelate Pr2NiO4+δ. In addition, there was evidence for strong correlation between the insertion of protonic defects and additional oxygen in the interstitial position of Pr2NiO4+δ.