This study focuses on the development of advanced multi-doped gadolinium-doped ceria (GDC) electrolytes tailored for application in low-temperature solid oxide electrochemical cells (SOECs) and solid oxide fuel cells (SOFCs). Conventional GDC-based electrolytes, despite their relatively high oxygen ion conductivity, require sintering temperatures exceeding 1300 °C to achieve sufficient densification and mechanical integrity. Such high processing temperatures are not compatible with emerging fabrication routes for metal-supported devices and significantly increase material and manufacturing costs. Moreover, undoped or singly doped GDC often suffers from limited performance at intermediate operating temperatures (450–750 °C), hindering its wider deployment in practical energy systems. This research addresses these challenges by introducing a multi-doping strategy that enables significant reductions in sintering temperature while enhancing both structural and electrochemical properties.

The innovation of this work lies in the deliberate selection and combination of multiple dopants (Li, Bi, Cu, Ga, and Co) to simultaneously influence sintering behaviour, defect chemistry, and lattice dynamics. These dopants were chosen for their known roles in promoting oxygen vacancy formation, modifying grain boundary structures, and facilitating liquid-phase sintering at lower temperatures. The co-doping strategy effectively reduced the onset of densification to ~500 °C and achieved near-full densification (≥95%) of the GDC electrolyte at sintering temperatures as low as 750–850 °C. This represents a reduction of approximately 400-500 °C compared to traditional GDC processing routes. Advanced microstructural and crystallographic analyses (XRD, SEM, Raman spectroscopy, and ToF-SIMS) confirmed the successful incorporation of dopants into the ceria lattice, the absence of deleterious secondary phases within optimal sintering windows, and the formation of uniformly dense microstructures with refined grain sizes.

A key conceptual contribution of this research is the identification of a "dopant quasi-stability window," typically between 750 and 950 °C, in which optimal phase stability and ionic conductivity are achieved without the onset of dopant evaporation or phase decomposition. Within this window, the materials exhibit enhanced lattice disorder and a high concentration of oxygen vacancies, leading to markedly improved ionic transport. The best-performing composition, Li, Bi, Cu-doped GDC, sintered at 950 °C, achieved a total ionic conductivity of 29.6 mS·cm^-1 at 750 °C with an activation energy of 0.28 eV. Other formulations, such as those based on Li-Ga-Bi and Bi-Cu systems, also demonstrated superior conductivity compared to commercial GDC and strong thermal stability under prolonged high-temperature exposure.

Furthermore, several of the optimised electrolyte compositions were successfully integrated into full-cell or symmetric cell configurations using YSZ supports. Electrochemical performance testing revealed excellent electrolyte–electrode compatibility and power densities exceeding 400 mW· cm^-2 at 850 °C without extensive performance optimisation. The materials also showed promising structural stability and open-circuit voltages, suggesting strong potential for real-world implementation in hydrogen production via high-temperature electrolysis.

In summary, this work offers a comprehensive approach to engineering ceria-based electrolytes through rational multi-doping and thermal processing design. It demonstrates that significant improvements in sinterability and electrochemical performance can be achieved through synergistic dopant interactions. Beyond the specific compositions studied, the broader processing–structure–property relationships uncovered in this research provide a valuable framework for the design of next-generation, low-cost, metal-supported solid oxide electrochemical devices aimed at sustainable energy conversion and green hydrogen generation.