Protonic Conduction in La₂NiO_4+δ and La_2‐xA_xNiO_4+δ (A = Ca, Sr, Ba) Ruddlesden–Popper Type Oxides

Abstract

A significant hydration and protonic conduction in La2NiO4+δ‐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+δ 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+δ and La2‐xAxNiO4+δ (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.

Tuning proton kinetics in BaCo0.4Fe0.4Zr0.2–XYXO3–δ triple ionic-electronic conductors via aliovalent substitution

“…[22][23][24] Zhong, et al recently utilized Hebb-Wagner polarization with blocking electrodes for estimation of proton conductivity of La 2 NiO 4 -based Ruddlesden-Popper oxides, though only at temperatures below 300 °C where protonic blocking electrodes could be realized. 25 The BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3−d (BCFZY0.1) class of materials is widely studied as a model single-phase cubic perovskite and air-electrode for ceramic fuel cells and electrolysis applications. Beginning from BaCo 0.4 Fe 0.4 Zr 0.2 O 3−d (BCFZ), the substitution of Y 3+ for Zr 4+ was expected to impart greater protonic conductivity and improve oxygen kinetics.…”

“…22–24 Zhong, et al recently utilized Hebb-Wagner polarization with blocking electrodes for estimation of proton conductivity of La 2 NiO 4 -based Ruddlesden–Popper oxides, though only at temperatures below 300 °C where protonic blocking electrodes could be realized. 25…”

Electrochemically Assisted Construction of a La2NiO4+δ@Pt Core–Shell Structure for Enhancing the Performance and Durability of La2NiO4+δ Cathodes

“…The Ruddlesden–Popper (RP) series of A n +1 B n O 3 n +1 oxides have attracted much attention, with La 2 NiO 4+δ (LNO) being a promising candidate cathode material. − LNO has a superb oxygen bulk diffusion coefficient (2 × 10 –7 cm 2 s –1 at 800 °C), a high surface exchange coefficient (2 × 10 –6 cm s –1 at 800 °C), and a moderate thermal expansion coefficient (13.0 × 10 –6 K –1 ). , Different from the transport of oxygen ions through oxygen vacancies in the perovskite oxides, the transport of oxygen ions in LNO is initiated by an interstitial mechanism that involves migrating interstitial oxygen and adjacent apical oxygen. − As LaNiO 3 in the chalcocite layer and LaO in the rock salt layer are alternately stacked along the c -axis, the mismatch between the bond lengths of La–O and Ni–O bonds allows the LaO rock salt layer to accommodate overstoichiometric interstitial oxygen with reduced internal stress in the lattice. − The high overstoichiometric oxygen content in LNO leads to a high ionic conductivity …”

(La1‐xMx)2(Nb0.45Yb0.55)2O7‐δ (M=Ca, Sr, Ba) Ionic Conductors Promoted by Foreign/Domestic Dual Acceptor‐Doping Strategies

“…[15][16][17] The conduction of protons in the oxide ceramics may follow a vehicle mechanism, asking for protons moving along with oxide ions (e. g., as hydroxide ions), such as the case reported recently in the rocksalt layers in oxygen-excess La 2 NiO 4 + δ with a Ruddlesden-Popper structure. [18] However, the vehicle mechanism is rarely observed in oxide ceramics, and most of the proton conductors show a Grotthuss conduction mechanism, where protons are introduced by dissociative hydration of oxygen vacancies following Equation ( 1):…”