Lanthanide-Driven Li+/H+ Exchange for High Proton Conductivity in Li13.9Sr0.1Zn(GeO4+δ)4 Electrolyte for Proton-Conducting Ceramic Fuel Cells

Developing advanced electrolytes is crucial for intermediate-temperature proton-conducting ceramic fuel cells (IT-PCFCs). Li_13.9Sr_0.1Zn(GeO_4+δ)₄ is a lithium conductor exhibiting outstanding Li⁺ conductivity. Its nonframework and interstitial Li⁺ can be reversibly exchanged with H⁺, thereby creating migration pathways for H⁺ transport. This study utilizes a Li⁺/H⁺ ion-exchange strategy to convert the Li⁺ conductor Li_13.9Sr_0.1Zn(GeO_4+δ)₄ into a high-performance H⁺ electrolyte. Lanthanide doping is further employed to enhance the H⁺ conduction capability. Li_13.8La_0.1Sr_0.1Zn(GeO_4+δ)₄ exhibits a proton conductivity of 0.067 S cm^–1 at 600 °C, surpassing most state-of-the-art proton conductors reported in the literature, and enables the electrolyte-supported single cell to achieve a peak power density of 0.42 W cm^–2. Structural and electrochemical characterization reveals that larger lanthanide ions effectively promote lattice expansion and Li⁺/H⁺ exchange, which directly correlates with improved electrochemical performance. The developed Li_13.8La_0.1Sr_0.1Zn(GeO_4+δ)₄ electrolyte offers superior proton conductivity and long-term stability, highlighting its significant potential as a next-generation electrolyte for IT-PCFCs.