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Highlights A novel A/B-sites co-substitution strategy was introduced to enhance the performance and durability of Ruddlesden–Popper perovskite Sr3Fe2O7−δ (SF)-based air electrodes for reversible protonic ceramic cells (RePCCs). Simultaneous Sr-deficiency and Nb-substitution in SF result in Sr2.8Fe1.8Nb0.2O7−δ (D-SFN), offering improved structural stability under RePCC conditions by suppressing the formation of Sr3Fe2(OH)12 phase. The introduction of Sr-deficiency enhances oxygen vacancy concentration in D-SFN, promoting efficient oxygen transport within the material and contributing to excellent activity in RePCCs.

Highlights: A novel A/B-sites co-substitution strategy was introduced to enhance the performance and durability of Ruddlesden–Popper perovskite Sr3Fe2O7−δ (SF)-based air electrodes for reversible protonic ceramic cells (RePCCs). Simultaneous Sr-deficiency and Nb-substitution in SF result in Sr2.8Fe1.8Nb0.2O7−δ (D-SFN), offering improved structural stability under RePCC conditions by suppressing the formation of Sr3Fe2(OH)12 phase. The introduction of Sr-deficiency enhances oxygen vacancy concentration in D-SFN, promoting efficient oxygen transport within the material and contributing to excellent activity in RePCCs. Reversible protonic ceramic cells (RePCCs) hold promise for efficient energy storage, but their practicality is hindered by a lack of high-performance air electrode materials. Ruddlesden–Popper perovskite Sr3Fe2O7−δ (SF) exhibits superior proton uptake and rapid ionic conduction, boosting activity. However, excessive proton uptake during RePCC operation degrades SF's crystal structure, impacting durability. This study introduces a novel A/B-sites co-substitution strategy for modifying air electrodes, incorporating Sr-deficiency and Nb-substitution to create Sr2.8Fe1.8Nb0.2O7−δ (D-SFN). Nb stabilizes SF's crystal, curbing excessive phase formation, and Sr-deficiency boosts oxygen vacancy concentration, optimizing oxygen transport. The D-SFN electrode demonstrates outstanding activity and durability, achieving a peak power density of 596 mW cm−2 in fuel cell mode and a current density of − 1.19 A cm−2 in electrolysis mode at 1.3 V, 650 °C, with excellent cycling durability. This approach holds the potential for advancing robust and efficient air electrodes in RePCCs for renewable energy storage. [ABSTRACT FROM AUTHOR]

Reversible protonic ceramic cells (r-PCCs) can operate alternately in fuel cell and electrolysis cell modes, while their practical applications are limited due to the lack of air electrodes with high oxygen reduction/evolution reaction (ORR/OER) activities. A nanocomposite Ba 0.95 (Co 0.4 Fe 0.4 Zr 0.1 Y 0.1) 0.95 Ni 0.05 O 3-δ (BCFZYN), consisting of a major perovskite phase (D -BCFZYN) and a minor NiO phase, has demonstrated outstanding ORR activity in protonic ceramic fuel cells [1]. Herein, we experimentally and theoretically demonstrate that BCFZYN possesses excellent OER activity. Density functional theory calculations indicate that NiO nanoparticles enhance water adsorption while D -BCFZYN accelerates oxygen desorption and proton conduction, thus promoting OER kinetics. A cell with BCFZYN air electrode achieved a current density of − 1267 mA cm-2 at 1.3 V at 600 oC, while maintaining favorable durability of 372 h. The corresponding cell demonstrated stable operation during cycling mode between fuel cell and electrolysis modes, suggesting the material has great potential as an air electrode for r-PCCs. [Display omitted] • A nanocomposite BCFZYN powders are synthetized by selective cation exsolution strategy. • BCFZYN has good steam adsorption and high ion diffusion capacities, thereby resulting in prominent ORR/OER activities. • The enhanced ORR/OER activities of BCFZYN were demonstrated by DFT calculations. • BCFZYN composite as an air electrode shows high performance in r-PCCs. • BCFZYN air electrode exhibits excellent phase stability and operational stability in r-PCCs test. [ABSTRACT FROM AUTHOR]

Reversible protonic ceramic cell (RePCC) is an efficient, scalable, and fuel-flexible energy conversion and storage technology. However, finding single-phase triple conducting (H+/O2-/e-) electrodes with high electrochemical activity, structural and thermomechanical stability still faces great challenges. Herein, we propose a thermodynamically quasi-stable triple conducting perovskite-perovskite nanocomposite as bifunctional RePCC air electrode with exceptional performance, which is composed of a Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ -based mixed oxygen ion and electronic conductor and a BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3-δ -based protonic conductor and synthesized through one-pot method. Some unique structure introduces increased number of active sites and rich two-phase boundary, and tailored two phase composition, which contributes to the superior performance for both oxygen reduction and evolution reactions. In addition, the intimate connection of the two phases, their cation interexchange characteristics and the thermodynamically quasi-stable composition in the nanocomposite brings the electrode matchable thermal compatibility to the electrolyte and high structural stability that accounts for the superior durability. [Display omitted] • A quasi-stable triple conducting nanocomposite is derived through one-pot method. • Unique structure introduces abundant active sites and two-phase boundary. • Cation-exchange characteristic accelerates the ORR and OER kinetic rates of the air electrode. • High peak power density and electrolysis current density are achieved at 650 oC. • The nanocomposite electrode achieves a superior stability in dual modes. [ABSTRACT FROM AUTHOR]