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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.

The air electrode with strontium and cobalt elements has the elemental diffusion or delamination issue on the air electrode/electrolyte interface in reversible protonic ceramic cells (RPCCs). Hence the development of strontium and cobalt-free triple conducting (H+/O2-/e-) air electrodes with high electrochemical activity and stability is urgent. Here, we report a strontium and cobalt-free Ba 0.8 Ca 0.2 FeO 3-δ (BCF82) air electrode with high catalytic activity and electrochemical stability for oxygen reduction/evolution reactions. A RPCC with the BCF82 air electrode shows a high peak power density of 1.14 W cm−2 in the fuel cell mode and a current density of 3.49 A cm−2 under 1.3 V in the electrolysis mode at 700 °C. Furthermore, the cell demonstrates suitable stability in the fuel cell, electrolysis and reversible modes at 600 °C for hundreds of hours without elemental segregation and delamination issue. This work offers an efficient approach to develop low-cost and durable air electrodes for RPCCs. [Display omitted] • Ba 0.8 Ca 0.2 FeO 3-δ (BCF82) shows good steam adsorption and ion diffusion capacities. • BCF82 air electrode shows high activity and stability. • Reversible cells with the BCF82 air electrode show suitable performance and stability. [ABSTRACT FROM AUTHOR]

Reversible protonic ceramic cells (RePCCs) are limited by several factors, including high cost, poor stability, and insufficient fuel electrode activity toward fuel oxidization/generation reactions. Herein, a novel Ni−Fe metal‐supported RePCC (MS‐RePCC) to address these issues simultaneously is proposed. Specifically, the Ni−Fe support possesses good mechanical strength and thermal compatibility with cermet‐based electrodes/electrolytes, ensuring a facile cell fabrication and robust durability. Density functional theory calculations suggest that Fe in the Ni−Fe support enhances the fuel electrode functional layer by providing additional and more active sites for the electrocatalytic reactions. The as‐fabricated MS‐RePCC at 700 °C achieves an excellent peak power density (PPD) of 586 mW cm−2 and an electrolysis current of −428 mA cm−2 (at 1.3 V). Furthermore, the cell is exceptionally stable, as evidenced by 930 h of fuel cell operation with ultralow degradation (≈0.78% kh−1), and much better than an analogous anode‐supported cell (≈17.78% kh−1). In addition, the cell is stable for 50 h of reversible fuel cell/electrolyzer cycling, further demonstrating the potential of this MS‐RePCC. This article proposes a simple and new approach to enhance the electrochemical activity and durability of RePCC, thereby accelerating the commercialization of this technology.

Reversible protonic ceramic cells (RePCCs) can facilitate the global transition to renewable energy sources by providing high efficiency, scalable, and fuel‐flexible energy generation and storage at the grid level. However, RePCC technology is limited by the lack of durable air electrode materials with high activity toward the oxygen reduction/evolution reaction and water formation/water‐splitting reaction. Herein, a novel nanocomposites concept for developing bifunctional RePCC electrodes with exceptional performance is reported. By harnessing the unique functionalities of nanoscale particles, nanocomposites can produce electrodes that simultaneously optimize reaction activity in both fuel cell/electrolysis operations. In this work, a nanocomposite electrode composed of tetragonal and Ruddlesden–Popper (RP) perovskite phases with a surface enriched by CeO2 and NiO nanoparticles is synthesized. Experiments and calculations identify that the RP phase promotes hydration and proton transfer, while NiO and CeO2 nanoparticles facilitate O2 surface exchange and O2‐ transfer from the surface to the major perovskite. This composite also ensures fast (H+/O2‐/e‐) triple‐conduction, thereby promoting oxygen reduction/evolution reaction activities. The as‐fabricated RePCC achieves an excellent peak power density of 531 mW cm‐2 and an electrolysis current of −364 mA cm‐2 at 1.3 V at 600 °C, while demonstrating exceptional reversible operation stability of 120 h at 550 °C. [ABSTRACT FROM AUTHOR]

To enhance the electrocatalytic activity in the oxygen reduction and the evolution reactions of air electrode materials, we fabricate a double perovskite structured NdBa 0.5 Sr 0.5 Co 1.5 Fe 0.5- x Ni x O 5+δ with triple-conducting properties for reversible protonic ceramic cells (RPCCs). In particular, Ni doping into B-site (of ABO 3 perovskite structure) is used to tailor the surface properties and bulk diffusion of NdBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+δ perovskites. Among the various amounts of dopants, doping with 5 mol% Ni significantly increases the activity of oxygen electrodes in moist air, including the conductivity of protons and oxygen-ions. The area specific resistance (ASR) values of the NdBa 0.5 Sr 0.5 Co 1.5 Fe 0.45 Ni 0.05 O 5+δ (NBSCFN5) electrode in the BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3-δ electrolyte are 0.70 Ω·cm2 at 650 ℃. The electrical conductivity of NBSCFN5 is 423–714 S·cm−1 at the RPCCs operating temperature (500–700 ℃), which is significantly higher than that of the conventional perovskite air electrode materials. Furthermore, the ASR of the NBSCFN5 symmetrical cell decreases considerably from 9.02 to 6.86 Ω·cm2 in dry air to p H 2 O = 0.6 atm at 550 ℃, implying its excellent proton conduction. This suggests that NBSCFN5 is a promising air electrode material for next-generation high-performance RPCCs at lower temperatures. • A double perovskite NBSCFN5 is developed as a highly active oxygen electrode. • The electrical conductivity of NBSCFN5 is 423–714 S·cm−1 at 500–700 ℃. • NBSCFN5 achieves a high performance of 1.937 W·cm−2 at 700 ℃ in PCFCs. • NBSCFN5 demonstrates an exceptional current of − 3.602 A·cm−2 at 650 °C, 1.4 V. • Outstanding performance of NBSCFN5 is due to the increased δ by Ni doping. [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]

Due to the well-matched thermal expansion coefficient and small volume change, air electrode supported reversible protonic ceramic cells (AS-RePCCs) look promising in cyclic stability. In this work, AS-RePCCs are successfully fabricated at 1220 °C for the first time using the single-step co-firing technique. A tightly integrated interface and enriched proton migration network are created between the hydrogen electrode and electrolyte, which effectively promote the proton transfer and alleviate hydrogen partial pressure at the interface, enhancing the performance and stability of the cell. To further optimize the performance, nano La 0.6 Sr 0.4 CoO 3 (LSC) particles are deposited into the air electrode matrix via infiltration. At 700 °C, the cell with 1.2 wt% LSC shows the maximum power density of 227 mW cm−2 and the current density of 543 mA cm−2 and 1113 mA cm−2 at 1.3 V and 1.5 V, respectively. More significantly, the performance remains stable during more than 60 rapid reversible cycles with a rate of one cycle every 2 h, followed by 30 deep thermal cycles (between 700 and 300 °C), demonstrating the excellent cyclic capability of single-step co-firing air electrode supported structure. This paper proposes a feasible scheme for developing AS-RePCCs with high performance, stability, and affordable manufacturing. [Display omitted] • AS-RePCCs are prepared by single-step co-firing and infiltration technique. • The S-cell has intimate interfaces and enriched proton migration network. • The 1.2 wt% LSC-cell has a current density of 1113 mA cm−2 at 1.5 V, 700 °C. • The cells show excellent reversible and thermal cycling performance. [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]

The sluggish kinetics of oxygen electrode reactions represent one of the most significant barriers to realizing effective reversible protonic ceramic cells (RPCCs) at intermediate temperatures. Maximization of oxygen ion and proton conduction characteristics through hydration of electron-conducting solid oxides is a key technology that can solve this issue. We report on the exceptional performance at the oxygen electrode in RPCCs achieved using a highly defective material with excessive oxygen nonstoichiometry, Nd 1.5 Ba 1.5 CoFeMnO 9− δ (NBCFM). Use of this material enables a superior reaction rate and conductivity during oxygen electrode reactions, maximizing the concentration of protons (with a high degree of hydration) as a guest ion as well as intrinsic oxygen vacancies. The peak power densities of this NBCFM cell are quite high, and a 1.4 V electrolyzing potential is achieved by NBCFM at −2.34 A‧cm−2 at 600 °C. Furthermore, these NBCFM cells are stable under prolonged (960 h) continuous operation at 600 °C. • Highly hydrated triple-conducting electrode with triple perovskite structure. • High oxygen defective NBCFM with the rapid reaction rates for oxygen electrodes. • Outstanding performance of NBCFM for steam and air electrodes of RPCCs. • Mechanistic study of the superior performance of NBCFM oxygen electrode. [ABSTRACT FROM AUTHOR]