Pure H ^– conduction in oxyhydrides

Abstract

A variety of proton (H(+))-conducting oxides are known, including those used in electrochemical devices such as fuel cells. In contrast, pure H(-) conduction, not mixed with electron conduction, has not been demonstrated for oxide-based materials. Considering that hydride ions have an ionic size appropriate for fast transport and also a strong reducing ability suitable for high-energy storage and conversion devices, we prepared a series of K2NiF4-type oxyhydrides, La(2-x-y)Sr(x + y)LiH(1-x + y)O(3-y), in the hope of observing such H(-) conductors. The performance of an all-solid-state TiH2/o-La2LiHO3 (x = y = 0, o: orthorhombic)/Ti cell provided conclusive evidence of pure H(-) conduction.

Ambient Pressure Synthesis and H^− Conductivity of LaSrLiH₂O₂

“…Recently, the conduction of H ¹ in solids has been verified for a BaH 2 and La 2¹x¹y Sr x+y LiH 1¹x+y O 3¹y oxyhydride system (LSLHO), and the results showed that H ¹ is a new charge carrier for the transport of hydrogen in solids. 4,5 Particularly, realization of a battery reaction using LSLHO as a solid electrolyte in our previous work have confirmed not only the capability of the LSLHO to act as an H ¹ solid electrolyte but also the possibility of developing electrochemical devices based on H ¹ conduction. 5 Hydride ions are attractive as charge carriers because they have an ionic radius similar to those of oxide and fluoride ions in crystal lattices, which are suitable for rapid ionic conduction and also exhibit strong reducing properties with a standard redox potential of H ¹ /H 2 (¹2.3 V) comparable to that of Mg/Mg 2+ (¹2.4 V).…”

“…The Rietveld analysis was carried out on the basis of previously reported structural models of LaSrLiH 2 O 2 synthesized at high pressure. 5 A total of four sites were assigned to La/Sr, Li, H, and O, respectively, for the refinement of the tetragonal phase: 4e ð0; 0; zÞ site for La and Sr, 2a ð0; 0; 0Þ site for Li, 4c ð0: 1=2; 0Þ site for H, and 4e ð0; 0; zÞ site for O in I4/mmm (space group 139). Figures 4-5 show the synchrotron X-ray and neutron Rietveld refinement patterns, and Tables 1-2 summarizes the refinement structural parameters for the sample.…”

“…For the sample synthesized at ambient pressure, the activation energy for ionic conduction was calculated as 80.7 kJ mol ¹1 , which is nearly the same as that observed for the sample synthesized at high pressure. 5 The total conductivity (bulk + grain boundary) for the sample was observed to be 3.2 © 10 ¹6 S cm ¹1 at 300°C, which was slightly less than that of the high-pressure sample. 5 From the viewpoint of the crystal structure of LaSrLiH 2 O 2 in which tetragonal (LiH 2 )…”

“…The conductivity of LaSrLiH 2 O 2 synthesized at high pressure in our previous study is also plotted for comparison. 5 The typical behavior of the impedance plot was described by the combination of two components, semicircle in the high-frequency range and a spike in the low-frequency region, which correspond to contributions from the mix with bulk and grain boundary and the electrode, respectively. Impedance spectra were fitted using the equivalent circuit to estimate the bulk and grain boundary resistance.…”

“…La 0.6 Sr 1.4 LiH 1.6 O 2 (x = 0.4, y = 1 in LSLHO) with H ¹ vacancies exhibits a high conductivity of 1.2 © 10 ¹4 S cm ¹1 at 300°C. 5 Typically, the control of the mobile ion concentration and lattice structure are crucial factors for rapid ionic conduction and for the creation of smooth diffusion pathways, respectively. Hence, oxyhydrides possessing the structural flexibility as typified by LSLHO system have potential for further improvement of the H ¹ conductivity.…”