Amphoteric Behavior of Hydrogen in Lanthanum Oxyhydrides: Correlation with Electrochemical Properties

Hydrogen incorporated into solids can adopt either protonic (H⁺) or hydridic (H^-) characteristics depending on the position of the Fermi level of the host material. In recent years, not only proton-conducting materials but also hydride-ion-conducting materials have been actively developed, enabling the development of novel electrochemical devices that exploit the amphoteric nature of hydrogen. At present, however, the relationship between the electrochemical properties of hydrogen-ion-conducting materials and the amphoteric nature of hydrogen remains scarcely explored. Herein, we investigate the electrochemical window (ECW) of the H^- conductor LaH_3-2xO_x and elucidate the underlying amphoteric behavior of hydrogen. We determine the band alignment of LaH_3-2xO_x and map the ECW onto an electronic energy scale referenced to the vacuum level. Based on this band alignment, we examine the conduction behavior of LaH_3-2xO_x in a H₂ atmosphere via electromotive force measurements of a hydrogen concentration cell. The reducing limit of the ECW was governed by hydrogen extraction from LaH_3-2xO_x, whereas its oxidizing limit was governed by the capture of positive charge by interstitial H^-, leading to H⁺ formation and electronic conduction. Electromotive force measurements of a hydrogen concentration cell revealed that, in addition to H^- conduction, a finite contribution from H⁺ conduction emerges near a hydrogen partial pressure of 1 atm. By capturing the amphoteric nature of hydrogen on an electronic energy scale, this paper provides a unified perspective across different classes of hydrogen-ion conductors.