Abstract
Lowering the densification temperature of ceria-based electrolytes is pivotal for enabling low-cost, metal-supported solid oxide electrolysis cells (SOECs). Here, we report a Li-Ga-Bi multi-doped gadolinia-doped ceria (GDC) polycrystalline composite that achieves a relative density of 97.51% at 750 °C under inert atmosphere conditions, representing the lowest sintering temperature reported for near-fully dense, multi-doped GDC for SOEC/SOFC devices. Thermodilatometry reveals a sintering onset near ∼500 °C and a complete shrinkage plateau by ∼730 °C. Raman spectroscopy reveals that multi-doping promotes extrinsic oxygen-vacancy defects (D2) while adjusting the D1/D3 populations, which correlates with reduced grain-boundary resistance. XRD and XPS identify a dopant quasi-stability window (750–950 °C), during which a single-phase fluorite lattice is preserved, and high conductivity is maintained. Beyond this range, Ga5Gd3O12 formation and dopant evaporation degrade the cell performance. The best composition (sintered at 850 °C) exhibits a total ionic conductivity of 1.15 × 10-3 to 2.40 × 10-2 S cm-1 (450–750 °C), which is 7.1 times that of co-processed commercial GDC at 450 °C, with a low activation energy of 0.28 eV. The 850 °C-sintered 3LGB-GDC (3LGB-2) was selected for full-cell validation with YSZ. In a symmetric cell (3LGB-GDC|YSZ|3LGB-GDC), the electrolyte layer enhances electrochemical performance, reaching ∼415 mW cm-2 at 850 °C (SOFC mode, non-optimised electrodes). This work presents the first mechanistic processing–structure–property map for multi-doped GDC composites sintered in an inert atmosphere, establishing a new strategy for fabricating ultra-low-temperature electrolytes compatible with metal-supported SOEC stacks, while offering a transferable framework for designing other low-temperature solid-state electrolytes.
[Display omitted]
•Li–Ga–Bi multi-doped GDC achieved 97.51% (750 °C) and 97.91% (850 °C) relative density under an inert atmosphere, representing the lowest sintering temperature reported for near-dense multi-doped GDC.•A post-densification evolution effect was revealed, and a dopant quasi-stability window (750–950 °C) was identified, linking phase stability, defect chemistry and conductivity.•Multi-doping enhanced ionic conductivity up to 7 × compared to undoped GDC, with reduced activation energy (0.28 eV), enabling low-temperature electrolyte fabrication.