Defect Chemistry and Hydrogen Transport in La/Sr-Based Oxyhydrides

Abstract

Oxyhydrides in the series La 2 − y Sr y LiH 1 + y O 3 − y are attractive materials for solid-state hydride electrolytes. However, their chemical stability is poor, and thus far only La 2 LiHO 3 (y = 0) has been demonstrated in a working fuel cell. Using a first-principles approach to study defect chemistry and stability, we find that La 2 LiHO 3 can be stabilized over a range of H-rich, O-poor chemical conditions, while Sr 2 LiH 3 O has low stability. Defects are critical for ionic transport and for stability, particularly in Sr 2 LiH 3 O, in which anion antisites and H vacancies readily form. In fact, we show that the ground-state structure of Sr 2 LiH 3 O is inherently disordered. La 2 LiHO 3 has low conductivity, which we connect to the high formation energy of defects such as V O and O H that are necessary for hydride conduction. When Sr 2 LiH 3 O is grown under carefully selected conditions, V H + defects can be made prevalent; these defects give rise to much higher ionic conductivity than can be obtained in La 2 LiHO 3 . In general, we show that the choice of synthesis conditions is vital when seeking to optimize the stability and ionic conductivity of these oxyhydrides, thereby tailoring them for use in solid-state hydrogen fuel cells.

Hydride Conductivity in Nitride Hydrides

“…Two groups have synthesized and characterized Sr 2 LiH 2 N (SLHN), , though ion transport was not measured. This material has a structure very similar to La 2– x Sr x LiH 1+ x O 3– x (LSLHO), the only oxyhydride to have been demonstrated in a fuel cell. , Kobayashi et al noted that hydride conductivity increases as the Sr-to-La ratio increases in LSLHO; however, as we have previously seen, the stability of the oxyhydride decreases with increasing Sr content . Thus, it is encouraging that the first experimental reports on the analogous nitride hydride system center on the Sr-containing compound.…”

“…12,28 Kobayashi et al noted that hydride conductivity increases as the Sr-to-La ratio increases in LSLHO; 12 however, as we have previously seen, the stability of the oxyhydride decreases with increasing Sr content. 29 Thus, it is encouraging that the first experimental reports on the analogous nitride hydride system center on the Sr-containing compound. The structure of SLHN also contains free space in the form of linear cavities for hydride ions to migrate through the crystal, potentially providing pathways for hydride diffusion.…”

Combining Nitridoborates, Nitrides and Hydrides—Synthesis and Characterization of the Multianionic Sr6N[BN2]2H3

“…Further details of the measurements and calculations can be found in the Supporting Information. Compared to other hydride ion conductors such as Sr 2 LiH 2.2 O 0.2 N 0.8 (7.2×10 −6 S/cm at 300 °C) or La 2 LiHO 3 (3.85×10 −6 S/cm at 275 °C), Sr 6 N[BN 2 ] 2 H 3 shows a lower conductivity, which may be attributed to the relatively low measurement temperature of 75 °C [54–55] . However, the ionic conductivity at 300 °C can be estimated using the Arrhenius equation, yielding an ionic conductivity of 5.8×10 −7 S/cm at 300 °C (Figure S10), which is in the range of other known hydride ion conductors.…”