Phase engineering of a donor-doped air electrode for reversible protonic ceramic electrochemical cells

Reversible protonic ceramic electrochemical cells (R-PCECs) demonstrate great feasibility for efficient energy storage and conversion. One critical challenge for the development of R-PCECs is the design of novel air electrodes with the characteristics of high catalytic activity and acceptable durability. Here, we report a donor doping of Hf into the B-site of a cobalt-based double perovskite with a nominal formula of PrBa_0.8Ca_0.2Co_1.9Hf_0.1O_5+δ (PBCCHf_0.1), which is naturally reconfigured to a double perovskite PrBa_0.8-xCa_0.2Co_1.9Hf_0.1-xO_5+δ (PBCCHf_0.1-x) backbone and nano-sized BaHfO₃ (BHO) on the surface of PBCCHf_0.1−x. The air electrode demonstrates enhanced catalytic activity and durability (a stable polarization resistance of 0.269 ​Ω ​cm² for ∼100 ​h at 600 ​°C), due likely to the fast surface exchange process and bulk diffusion process. When employed as an air electrode of R-PCECs, a cell with PBCCHf_0.1 air electrode demonstrates encouraging performances in modes of the fuel cell (FC) and electrolysis (EL) at 600 ​°C: a peak power density of 0.998 ​W ​cm⁻² and a current density of −1.613 ​A ​cm⁻² ​at 1.3 ​V (with acceptable Faradaic efficiencies). More importantly, the single-cell with PBCCHf_0.1 air electrode demonstrates good cycling stability, switching back and forth from FC mode to EL mode ±0.5 ​A ​cm⁻² for 200 ​h and 50 cycles.