Fast ionic conductors have considerable potential to enable technological development for energy storage and conversion. Hydride (H − ) ions are a unique species because of their natural abundance, light mass, and large polarizability. Herein, we investigate characteristic H − conduction, i.e., fast ionic conduction controlled by a pre-exponential factor. Oxygen-doped LaH 3 (LaH 3 −2 x O x ) has an optimum ionic conductivity of 2.6 × 10 −2 S cm −1 , which to the best of our knowledge is the highest H − conductivity reported to date at intermediate temperatures. With increasing oxygen content, the relatively high activation energy remains unchanged, whereas the pre-exponential factor decreases dramatically. This extraordinarily large pre-exponential factor is explained by introducing temperature-dependent enthalpy, derived from H − trapped by lanthanum ions bonded to oxygen ions. Consequently, light mass and large polarizability of H − , and the framework comprising densely packed H − in LaH 3 − 2 x O x are crucial factors that impose significant temperature dependence on the potential energy and implement characteristic fast H − conduction.

Haber-Bosch process requires harsh condition, that is, high temperature (400-500 °C) and high pressure (10-30 MPa) to dissociate strong NN bonds (945 kJ mol −1) over Fe-based catalyst effectively, [2] resulting in the enlargement of ammonia synthesis plant to meet the production cost. The ruthenium (Ru) catalysts are known to work as efficient catalysts under milder conditions than iron-based catalyst used in the industrial process, because Ru has an optimum N 2 adsorption energy. [3] However, hydrogen atoms tend to be adsorbed strongly on the active sites of Ru surface at low reaction temperatures (<350 °C), preventing nitrogen adsorption on the Ru surface which is requisite for N 2 dissociation. [4] This is known as a hydrogen poisoning effect. For this reason, high N 2 activation ability and suppression of hydrogen poisoning are two major requirements for Ru catalysts especially for lowtemperature ammonia synthesis. We have previously reported that electride and hydride materials such as C12A7:e − , Ca 2 NH, CaH 2 , LnH 2+x (Ln = La, Ce, Y), and Ca(NH 2) 2 promote the catalytic activity of Ru catalysts significantly at low reaction temperatures when these materials are used as catalyst supports. [5] In these catalyst systems, the high catalytic performance is realized by two unique properties of the support material. First, electrons are located at crystallographic interstitial void Lanthanum oxyhydrides were recently reported to be fast hydride ion conductors with the highest conductivity at 100-400 °C. Here, the relationship between the hydride-ion conduction and the ammonia synthesis activity of ruthenium-loaded lanthanum oxyhydrides (Ru/LaH 3−2x O x) is investigated. The onset ammonia formation temperature by the Ru/LaH 3−2x O x is lower by 100 °C when compared to the Ru-loaded lanthanum oxides. The apparent activation energy of ammonia synthesis over Ru/LaH 3−2x O x is 64 kJ mol −1 , which is much smaller than that of hydride-ion conductivity (≈100 kJ mol −1), indicating no direct relationship between the catalytic activity and the bulk hydride-ion conductivity. However, the catalytic performance is strongly correlated with the surface H − ion mobility of Ru/LaH 3−2x O x , which gives rise to the formation of low work function electrons at H − ion vacancies near the Ru-support interface and high resistance for H 2 poisoning on the Ru catalyst. Moreover, LaH 3−2x O x has high nitridation resistance as compared with lanthanum hydride (LaH 3) under ammonia synthesis condition. As a result, the high surface H − concentration of Ru/LaH 3−2x O x is preserved during ammonia synthesis, exhibiting more robust activity than Ru/LaH 3. Almost the same results are obtained for Ru/CeH 3−2x O x implicating the common characteristics of rare-earth oxyhydride support.

The hydride ion (H–) is a unique anionic species that exhibits high reactivity and chemical energy. H– conductors are key materials to utilize advantages of H– for applications, such as chemical reactors and energy storage systems. However, low H– conductivity at room temperature (RT) in current H– conductors limit their applications. In this study, we report a H– conductivity of ∼1 mS cm–1 at RT, which is higher by 3 orders of magnitude than that of the best conductor, in lightly oxygen-doped lanthanum hydride, LaH3–2x O x with x < 0.25. The oxygen concentration (x) is crucial in achieving fast H– conduction near RT; the low activation barrier of 0.3–0.4 eV is attained for x < 0.25, above which it increases to 1.2–1.3 eV. Molecular dynamics simulations using neural-network potential successfully reproduced the observed activation energy, revealing the presence of mobile and immobile H–.

Anion-excess fluorite is a unique structure type of inorganic crystals and is well known as an appropriate crystal structure for fast anion conduction. In particular, the introduction of excess anion and charged defect by chemical doping significantly enhances the conductivity. However, the clustering of dopants and defects is the main obstacle for further enhancement of conductivity. We investigated the pressure−chemical composition phase diagram of the LaHO−LaH 3 system, in which the highly H − conducting LaH 1+2x O 1−x phase with the anion-excess fluorite structure appears. The sample at x = 0 crystallizes in a distorted fluorite structure with a monoclinic symmetry. For 0 < x ≤ 1, the fluorite lattice is maintained while H − and O 2− ions are disordered at the regular anion position of fluorite, and an excess H − is distributed at the interstitial site. Comparing the phase diagram and crystal structure with those of the La−F−O, Y− H−O, and Y−F−O systems, we found that the large radius ratio of cations and anions in LaH 1+2x O 1−x alleviates the intrinsic Coulomb repulsion between the anions at regular positions in fluorite and the interstitial atoms. This is crucial in stabilizing the anion-excess fluorite structure without forming defect clustering and enabling fast anion conduction. These results provided guidelines for avoiding cluster formation and achieving higher conductivity in the fluorite structure.

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