Balancing sinterability and ionic-selectivity: Low-temperature densification and enhanced ionic transference number of BaCe0.7Sn0.1Yb0.2O3-δ electrolytes for protonic ceramic electrochemical cells

Protonic ceramic electrochemical cells (PCECs) require electrolytes combining chemical stability, high proton conductivity, good sinterability, and strong ionic selectivity. Here, we investigate the effects of NiO, ZnO, and CuO sintering aids on the densification, microstructure, and charge transport of BaCe 0.7 Sn 0.1 Yb 0.2 O 3-δ (BCSYb). Adding only 1 wt% NiO enables dense electrolyte formation at 1400 °C and improves ionic transference numbers to 88.0%, 82.8%, and 65.1% at 500, 600, and 700 °C, respectively, exceeding those of undoped BCSYb. In contrast, ZnO- and CuO-containing samples show higher total conductivity but increased electronic conduction, reducing ionic selectivity and protonic fuel cell efficiency. Electromotive force measurements and microstructural characterization indicate that NiO promotes grain growth while suppressing electronic carriers. These results demonstrate an effective strategy to simultaneously improve sinterability and ionic selectivity, addressing a key limitation of intermediate-temperature PCECs and supporting their practical application while maintaining electrolyte performance under operating conditions relevant to practical electrochemical devices.