Influence of A-Site Deficiency and Ca Concentration on the Electrical and Crystallographic Properties of (Nd_0.2Sr_0.7-...

This study explores the impact of A-site deficiency and Sr/Ca ratio on the electrochemical and crystallographic properties of a (Nd_0.2Sr_0.7- Ca ) Ti_0.95Fe_0.05O_3-δ hydrogen electrode for solid oxide cells under reducing and air atmospheres. 5% and 10% A-site deficient (Nd_0.2Sr_0.7- Ca ) Ti_0.95Fe_0.05O_3-δ (x = 0.35-0.45, y = 1.05, 1) (referred to as 5NSCTF-x and 10NSCTF-x) materials were studied, while the ratio between A-site cations was kept the same with both deficiencies. The results demonstrate that the extent of A-site deficiency and the Ca concentration in the A-site have a significant impact on the microstructure (sinterability), conductivity, and catalytic activity of electrodes. Segregation of Nd from the lattice with 5% A-site deficiency was observed as a result of thermal treatment at low pO₂. Among the studied materials, the highest total electrical conductivity of porous electrode layer at 850 °C and in 97% H₂ + 3% H₂O atmosphere was 4.8 S cm^-1 observed for the Nd_0.2Sr_0.35Ca_0.35Ti_0.95Fe_0.05O_3-δ (10NSCTF-35). The highest electrochemical performance was observed in the case of Nd_0.2Sr_0.25Ca_0.45Ti_0.95Fe_0.05O_3-δ (10NSCTF-45), which showed a polarization resistance value equal to 0.19 Ω cm² after 100 h of stabilization at 800 °C in a humidified (1.7% H₂O) H₂ atmosphere. The best electrochemical performance with 606 mW cm^-2 power density at 850 °C in 98.3% H₂ + 1.7% H₂O atmosphere was demonstrated by a 50 wt % Nd_0.2Sr_0.25Ca_0.45Ti_0.95Fe_0.05O_3-δ + 50 wt % Ce_0.9Gd_0.1O_2-δ composite.