Synthesis and characterisation of novel ion conductors

Abstract

As hydrogen is a renewable energy source, the studies for materials exhibiting fast hydrogen transport can have great potential to be used as electrolytes for electrochemical devices and as strong reducing agents for electrochemical conversion reactions.

The alkaline earth metals, AeH₂, where Ae = Ca, Sr and Ba are of interest as they exhibit fast hydride ion conductivity which may enable them to be used as solid-state electrolytes in electrochemical devices. BaH₂ has a high hydride ion conductivity of 0.2 S cm⁻¹ at 630 °C, which is an order of magnitude higher than other oxide ion conductors at a similar temperature. The in-situ PDF and conductivity study of BaD₂ was performed to examine the local deviation from the average crystal structure. The technique using neutron total scattering helped to examine possible short-range correlations between atoms to understand the local structure of BaD₂.

BaH₂ was doped with the monovalent ions Na or Li and its structural and electrical properties, were investigated. LiₓBa₁₋ₓH₂₋ₓ (x = 0.05, 0.10, 0.15 and 0.20) were synthesised by the solid-state reaction in which Li₀.₁₀Ba₀.₉₀H₁.₉₀ has achieved highest conductivity of 0.38 S cm⁻¹ at 600 °C.

LiBaH₃ was synthesised by mechanochemical reaction at room temperature and solid-state reaction at high temperature, and their structural and electrical properties were analysed. LiBaH₃ has a conductivity of 0.01 S cm⁻¹ at 600 °C. NaₓLi₁₋ₓBaH₃ (x = 0.05, 0.15 and 0.20) were also synthesised by solid-state reaction and their structural and electrical properties were examined. Na₀.₁₅Li₀.₈₅BaH₃ has the highest conductivity of the Na-doped LiBaH₃ samples studied, exhibiting a conductivity of 0.03 S cm⁻¹ at 600 °C.

Nitridation in LiₓBa₁₋ₓH₂₋ₓ (x = 0.05, 0.10, 0.15 and 0.20), NaₓBa₁₋ₓH₂₋ₓ (x = 0.01, 0.05 and 0.10), LiBaH₃ and NaₓLi₁₋ₓBaH₃ (x = 0.05, 0.15 and 0.20) increases their conductivity by 0.02 S cm⁻¹ initially due to exothermic reaction, except the nitridation of BaH₂ at 600 °C, where their conductivity decreases by 0.04 S cm⁻¹ upon nitridation due to endothermic reaction.

Date of Award	27 Jul 2020
Original language	English
Awarding Institution	University of St Andrews
Supervisor	John Thomas Sirr Irvine (Supervisor)