Electrical Properties of Hollandite-Type Ba_1.33Ga_2.67Ti_5.33O₁₆, K_1.33Ga_1.33Ti_6.67O₁₆, and K_1.54Mg_0.77Ti_7.23O₁₆

Abstract

Electrical\nconductivity and electrochemical catalytic activity for H₂ oxidation of Ti-based hollandite-type Ba_1.33Ga_2.67Ti_5.33O₁₆ (BGT), K_1.33Ga_1.33Ti_6.67O₁₆ (KGT), and K_1.54Mg_0.77Ti_7.23O₁₆ (KMT) were investigated,\nalong with the chemical stability of KMT under H₂ at elevated\ntemperature. BGT, KGT, and KMT crystallized in a tetragonal structure\nwith the space-group <i>I</i>4/<i>m</i>. The electrical\nconductivity in H₂ increases with increasing Ti content,\nand the highest total electrical conductivity of 2 S/cm at 800 °C\nin H₂ was observed for KMT. KGT:Fe (1:1) + 20% LSGM + 30%\nporosity composite electrode showed the lowest area specific resistance\nof ca. 1.6 Ω cm² at 800 °C for hydrogen oxidation\nreaction (HOR) under the open circuit condition. Moderate catalytic\nactivity for HOR could be attributed to poor oxide ion conductivity\nand exclusion of potassium and hydrogen uptake in H₂ at\nelevated temperature. Bond valence sum mismatch map calculation showed\nthat the ionic transport happens along the 1D channel of <i>c</i>-axis in the hollandite oxides.