Reversible protonic ceramic electrochemical cells (R-PCECs) are a promising option for efficient and low-cost generation of electricity and hydrogen. Commercialization of R-PCECs, however, hinges on the development of highly active and robust air electrodes. Here, we report an air electrode consisting of PrBa 0.8 Ca 0.2 Co 2 O 5+δ and in situ exsolved BaCoO 3−δ nanoparticles (PBCC−BCO) that shows minimal polarization resistance (∼0.24 Ω cm 2 at 600 °C) and high stability when exposed to humidified air with 3−50% H 2 O. An R-PCEC utilizing PBCC-BCO demonstrates remarkable performances at 600 °C: achieving a peak power density of 1.06 W cm −2 in the fuel cell mode and a current density of 1.51 A cm −2 at 1.3 V in an electrolysis mode. More importantly, the R-PCECs demonstrate an exceptionally high durability over 1833 h of continuous operation in the electrolysis mode. This work offers an efficient approach to design of high-performance and durable electrodes for R-PCECs.

One of the main bottlenecks that limit the performance of reversible protonic ceramic electrochemical cells (R‐PCECs) is the sluggish kinetics of the oxygen reduction and evolution reactions (ORR and OER). Here, the significantly enhanced ORR and OER kinetics and stability of a conventional La0.6Sr0.4Co0.2Fe0.8O3–δ (LSCF) air electrode by an efficient catalyst coating of barium cobaltite (BCO) is reported. The polarization resistance of a BCO‐coated LSCF air electrode at 600 °C is 0.16 Ω cm2, about 30% of that of the bare LSCF air electrode under the same conditions. Further, an R‐PCEC with the BCO‐coated LSCF air electrode shows exceptional performance in both fuel cell (peak power density of 1.16 W cm−2 at 600 °C) and electrolysis (current density of 1.80 A cm−2 at 600 °C at 1.3 V) modes. The performance enhancement is attributed mainly to the facilitated rate of oxygen surface exchange.

The commercialization of reversible protonic ceramic electrochemical cells is hindered by the lack of highly active and durable air electrodes exposed to high concentration of steam under operating conditions. Here, findings that dramatically enhance the electrocatalytic activity and stability of a conventional (La0.6Sr0.4)0.95Co0.2Fe0.8O3−δ (LSCF) air electrode by a multiphase catalyst coating composed of a conformal Pr1−xBaxCoO3−δ thin film and exsolved BaCoO3−δ nanoparticles, are reported. At 600 °C, the catalyst coating decreases the polarization resistance of the LSCF air electrode by a factor of 25 (from 1.09 to 0.043 Ω cm2) in air and the degradation rate by two orders of magnitude (from 1.0 × 10−2 to 1.8 × 10−4 Ω cm2 h−1 in humidified air with 30 vol% H2O). Further, a single cell with the catalyst‐coated LSCF air electrode at 600 °C demonstrates a high peak power density of 1.04 W cm−2 in the fuel cell mode and a high current density of 1.82 A cm−2 at 1.3 V in the electrolysis mode. The significantly enhanced performance of the LSCF air electrode is attributed mainly to the high rate of surface oxygen exchange, fast surface proton diffusion, and the rapid H2O and O2 dissociation on the catalysts.

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Intermediate temperature solid oxide fuel cells (IT‐SOFCs) are cost‐effective and efficient energy conversion systems. The sluggish oxygen reduction reaction (ORR) and the degradation of cathodes are critical challenges to the commercialization of IT‐SOFCs. Here, a highly efficient multiphase (MP) catalyst coating, consisting of Ba1−xCo0.7Fe0.2Nb0.1O3−δ (BCFN) and BaCO3, to enhance the ORR activity and durability of the state‐of‐the‐art lanthanum strontium cobalt ferrite (La0.6Sr0.4Co0.2Fe0.8O3−δ, LSCF) cathode is reported. The conformal MP catalyst‐coated LSCF cathode shows a polarization resistance (Rp) of 0.048 Ω cm2 at 650 °C, about one order of magnitude smaller than that of the bare LSCF. In an accelerated Cr‐poisoning test, the degradation rate of the catalyst‐coated LSCF electrode is 10−3 Ω cm2 h−1 (0.59% h−1) over 200 h, only one fifth of the degradation rate of the bare LSCF electrode at 750 °C. In addition, anode‐supported single cells with the MP catalyst‐coated LSCF cathode show a dramatically enhanced peak power density (1.4 W cm−2 vs 0.67 W cm−2 at 750 °C) and increased durability against Cr and H2O. Both experimental results and density functional theory‐based calculations indicate that the BCFN phase improves the ORR activity while the BaCO3 phase enhances the stability of the LSCF cathode.

Efficient electrocatalysts for oxygen reduction reaction (ORR) are critical to high-performance energy conversion and storage devices. As an important family of functional materials, alkaline earth metal oxides are generally considered inert toward electrochemical reactions. Here we report the remarkable enhancement of ORR activity and durability of electrodes enabled by surface modification with a precursor of barium nitrate. During cell start-up process, an ORR active cobaltite catalyst was in situ formed on the electrode surface, as confirmed by scanning transmission electron microscopy (STEM) analysis. A combination of in situ/operando Raman and electrochemical impedance study suggests that the transition from nitrate to cobaltite may occur at ∼565 °C. The peak power density of a single cell at 750 °C is increased from ∼0.85 to ∼1.15 W cm–2 (35% increase) by the surface modification, demonstrating the remarkable enhancement of electrocatalytic activity under realistic operating conditions.

BaHf0.1Ce0.7Yb0.2O3−δ proton-conducting electrolyte exhibits high conductivity, excellent ionic transference number, remarkable chemical stability, and great compatibility with NiO, as confirmed by experimental and computational results.

Proton-conducting reversible solid oxide cells (P-ReSOCs) are receiving increasing attention because they have potential to efficiently operate at intermediate temperatures to reduce cost and prolong operational life. Here we report our findings in the rational design of a new series of donor- and acceptor-codoped proton conductors through careful manipulation of defect chemistry. Specifically, BaNb­(Ta)0.05Ce0.7Yb0.25O3‑δ exhibits high ionic conductivity (0.012 S cm–1) while maintaining exceptional stability when exposed to Ar with 30% H2O at 500 °C for 500 h. In contrast, the resistance of BaZr0.1Ce0.7Y0.1Yb0.1O3‑δ increases continuously with time under the same condition due to reaction with H2O, as rationalized using density functional theory (DFT)-based computations. In addition, single cells based on BaNb0.05Ce0.7Yb0.25O3‑δ achieve a high peak power density of 1.12 W cm–2 in the fuel cell mode and a high current density of 2.24 A cm–2 at 1.3 V in the electrolysis mode at 600 °C. Overall, this work provides new insights for the development of highly conductive and stable proton conductors for P-ReSOCs.