High‐Temperature Protonic Conduction in La₂NiO_4+δ‐Based Ruddlesden–Popper Type Oxides: Correlation with Concentration of Interstitial Oxide Ions

Abstract Oxygen‐excess La₂NiO_4+δ (LNO) conducts oxide ions, electron holes, and hydroxide ions simultaneously on exposing to wet oxygen, exhibiting the potential as a cathode material in protonic ceramic fuel cells. Since the incorporation of protons in oxygen‐excess LNO is via the hydration reaction assisted by interstitial oxide ions, in this work, the concentration of interstitial oxide ions is reduced and increased by substituting Ni with Cu and Co, respectively. A higher concentration of interstitial oxide ions leads to a high proton concentration, indicating the predominant role of interstitial oxide ions in the hydration reaction, different from that in the oxygen‐deficient oxides, where protons are introduced by dissociative absorption of water molecules by oxygen vacancies. The theoretical calculation indicates that protons in Co‐doped LNO prefer to locate between the interstitial oxide ions and unshared apical oxide ions. A trapping effect is found between protons and the oxide ions near Cu, leading to decreased proton mobility. Protonic conductivity at 400–575 °C is then directly measured by a Hebb–Wagner direct current polarization method with La_0.99Ca_0.01NbO_4−δ as the blocking electrode, enabling the observation that Co‐doped LNO has the highest protonic conductivity among the samples studied in this work.