Publication date: 10th April 2024
Hydride ion (H−) conducting solids attracts much attention as catalysts and electrodes for various energy applications. A recent focus has been on cubic perovskite-type ABO3-δHε oxyhydrides, where oxygen sites are partly substituted with H− ions. The H− ions (HO•) must migrate via thermally-activated jump from one oxygen sites to adjacent vacancy sites (VO••) in ABO3-δHε. Theoretical calculations by Tang et al. [1] suggest that oxygen vacancy site percolation occurs in ABO3-δHε when the oxygen deficiency δ ≥ 0.75. In such cases, long-range hopping of H− ions predominantly occurs via jumps between nearest neighbor oxygen sites (NN-path), potentially resulting in high H− ion conductivity. Conversely, percolation of VO sites does not occur in perovskites with δ < 0.75, leading to lower ion conductivity as long-range hopping of H− ions necessitates jumps between second-nearest neighbor sites (2NN-path). However, experimental verification of this conduction model is challenging due to the scarcity of ABO3-δHε phases with δ ≥ 0.75. Recent discoveries in the perovskite BaSn1-xInxO3-δHε family, with δ values ≥ 0.75 and < 0.75 for x = 0.7 and 0.5 respectively, present an ideal opportunity to study H− ion conductivity in perovskite oxyhydrides. This study aims to compare the conductivity of phases with x = 0.5 and 0.7, namely BaSn0.3In0.7O2.33H0.14 (H-BSI55) and BaSn0.3In0.7O2.09H0.15 (H-BSI37) respectively.
The electrical conductivity of H-BSI37 was approximately 4.7 times higher than that of H-BSI55, with corresponding activation energies of 66.7 and 103 kJ mol-1 respectively. Notably, the conductivity of H-BSI37 displays a clear isotope effect; the value in 20%-H2/Ar was 1.1 times higher than that in 20%-D2/Ar at temperatures above 450 °C, indicating H− ions as the predominant carriers. However, the conductivity of H-BSI55 showed less sensitivity to atmospheric switching between H2 and D2, with values in H2 nearly identical to those in D2. Based on conductivity ratios, the ionic transport numbers of H− were calculated to be 0.28 for H-BSI37 and 0.06 for H-BSI55, with H− ion conductivity at 500 °C being 3.61×10-4 S cm-1 for H-BSI37 and 1.56×10-5 S cm-1 for H-BSI55. These results underscore the critical role of oxygen vacancy concentration in H− ion conduction in perovskite oxyhydrides.
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