New approaches to enhancing the performance of ceria-based electrolytes: A study on the microstructure, ionic conductivity, and mechanical properties of Cu-Gd Co-doping

This study addresses the performance limitations of traditional SOFC electrolytes at intermediate temperatures by synthesizing Cu-Gd co-doped ceria-based electrolytes, Gd0.1Ce0.9-xCuxO2-delta (x=0.04, 0.07, 0.10), using the citric acid-EDTA combustion method. This aims to enhance the ionic conductivity and mechanical stability of solid oxide fuel cells (SOFC) at intermediate to high temperatures. XRD and Rietveld refinement indicate that the introduction of Cu ions has not disrupted the original single-phase cubic fluorite structure of Gd0.1Ce0.9O1.95. SEM and EDS analyses indicate that Cu incorporation increased grain size and improved elemental distribution. XPS analysis confirmed that Cu-Gd co-doping increased the concentration of oxygen vacancies and hindered the reduction of Ce4+. Density functional theory calculations suggest that Cu addition effectively suppressed electronic conductivity in the Gd0.1Ce0.9O1.95 system and significantly lowered the formation, binding, and migration energy barriers of oxygen vacancies. EIS analysis revealed that the sample with x=0.07 had a 20 % reduction in ionic conduction activation energy and exhibited the highest ionic conductivity of 0.013 S/cm at 750 degrees C, two orders of magnitude higher than that of singly doped Gd0.1Ce0.9O1.95. This enhancement is attributed to a certain extent to the scavenging rate of Gd0.1Ce0.83Cu0.07O2-delta reaching 22.30, while the blockage factor has decreased by nearly 60 % compared to Gd0.1Ce0.9O1.95. Additionally, Cu incorporation resulted in a nearly tenfold decrease in the extent of oxygen vacancy concentration loss in the space charge layer, reducing the potential from 0.41 V (Gd0.1Ce0.9O1.95) to 0.34 V (Gd0.1Ce0.83Cu0.07O2-delta), which benefits the concentration of oxygen vacancies in the space charge layer. Moreover, in terms of mechanical properties, compared to singly doped samples, co-doped samples exhibited increases of approximately 20.2 % in Vickers hardness, 78.3 % in fracture toughness, and 54.8 % in flexural strength. Thus, Cu-Gd co-doping can be considered an effective strategy for enhancing the performance of ceria-based electrolytes in IT-SOFCs.