A universal interfacial-engineering strategy for the air electrodes of reversible protonic ceramic electrochemical cells†

The main obstacles to developing reversible protonic ceramic electrochemical cells (R-PCECs) are the sluggish kinetics of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) in air electrodes and weak interface contact between the air electrode and the electrolyte. Here, we report a universal interfacial engineering strategy that lowers the polarization resistance (R_p) and ohmic resistances (R_ohm) by adding an appropriate amount of Ag into an air electrode (PrBaCo_1.8Fe_0.1Y_0.1O_5+δ, PBCFY) and depositing a thin nano-structured PBCFY interlayer between the air electrode and electrolyte via a cost-effective drop coating method. Similar performance enhancement is found in the other two state-of-the-art air electrodes (PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ and PrBa_0.8Ca_0.2Co₂O_5+δ). The designed electrodes with the interlayer display low R_p and R_ohm values of 0.062 and 0.088 Ω cm² on a single cell at 600 °C, much lower than that of a PBCFY electrode without the interlayer. The R-PCECs with such an electrode and interlayer can stably operate for hundreds of hours with excellent performance (1.278 W cm⁻² in the fuel cell mode; −1.133 A cm⁻² at 1.3 V in the electrolysis mode) at 550 °C. Moreover, the symmetrical cells and single cells with the PBCFY interlayer both demonstrate excellent thermal cycling stability, even at a fast heating/cooling rate of 10 °C min⁻¹. This work provides a new interfacial design of active electrode materials and nano-structures for high-performance R-PCECs.