Protonic Conduction in La2NiO4+δ and La2-xAxNiO4+δ (A = Ca, Sr, Ba) Ruddlesden-Popper Type Oxides

A significant hydration and protonic conduction in La2NiO4+delta-based Ruddlesden-Popper (RP) oxides will enable their use as positrode materials in proton ceramic electrochemical cells (PCECs). Unlike perovskite electrolytes and positrode materials, where protons reside and jump on regular oxide ions, RP oxides such as La2NiO4+delta may contain oxygen interstitials and offer the possibility that protons locate on and migrate by help of these, suggesting a mechanism by which positrodes may operate by triple conduction. Here, theoretical calculations that support proton migration between interstitial oxide ions in the rock-salt layer via unshared oxide ions in the perovskite layer are reported. Furthermore, water contents of pristine La2NiO4+delta and La(2-)(x)A(x)NiO(4+)(delta) (A = Ca, Sr, Ba) are reported, showing that hydration increases with the level and basicity of the dopant. Hebb-Wagner blocking electrode measurements show significant partial protonic conductivity in all samples. The effects of doping on hydration and of pH2O on the p-type conductivity suggest that protons reside primarily on structural oxide ion, but a role of interstitial oxide ions for dissolution and migration of protons cannot be ruled out in cases with oxygen excess. The results are instructive for further studies and optimization of RP oxides as potential positrode materials for PCECs.