Two-Step Sintering for Dual Enhancement of Electrolyte and Hydrogen Electrode in Protonic Ceramic Electrochemical Cells

Proton ceramic electrochemical cells (PCECs) offer significant advantages for operation at intermediate-to-low temperature operation (≤600 °C), but their development is hindered by the challenge of achieving a fully dense electrolyte without compromising the hydrogen electrode's high active surface area. The extensively studied electrolyte BaCe_0.4Zr_0.4Y_0.1Yb_0.1O_3-δ (BCZYYb4411) is widely known for its high proton conductivity and excellent chemical stability. However, it typically requires high sintering temperatures (≥1550 °C) to achieve full densification, but such high temperatures cause barium volatilization, reduced ionic conductivity, and significantly decrease the active surface area of the hydrogen electrode. Conversely, lower sintering temperatures (<1450 °C) maintain electrode activity but result in incomplete densification, hindering the formation of thin-film electrolytes. This inherent trade-off between electrolyte densification and hydrogen electrode area limits the effectiveness of conventional approaches, including cosintering with the hydrogen electrode, using additional sintering aids, or employing nanoparticles, which often lead to stoichiometric deviations, reduced conductivity, or scalability issues. To address these challenges, we optimized the PCEC fabrication approach by implementing a two-step sintering (TSS) process. This method begins with a brief, high-temperature hold to achieve rapid electrolyte densification, followed by a prolonged hold at a lower temperature to promote grain growth and minimize barium volatilization. Our results demonstrate that the TSS process simultaneously produces a fully dense, stoichiometric electrolyte and a highly porous, active hydrogen electrode. PCECs fabricated using this optimized approach exhibit 1.42-2.10 times higher electrochemical performance at 600 °C compared to those produced via conventional sintering methods. These findings highlight two-step sintering as a promising strategy for improving both electrolyte and hydrogen electrode performance in PCECs.