Near Equilibrium Bulk Response

The NEB (Near Equilibrium Bulk) theme aims to undertake comprehensive studies assessing hydrogen (H⁺, H⁰, and H) content and transport through bulk oxide materials of relevance to energy and computing applications. The transport characteristics of hydrogen are distinct from those of other elements owing to its small mass, which is comparable to the electronic effective mass of some heavy-Fermion solids, and its ambipolar nature. Dominant transport via tunneling at ambient temperatures is possible for protons, with appreciable kinetic isotope effects. Its redox flexibility, occurring as either a cation or an anion and potentially transforming between states within a given host material, is accompanied by a dramatic change in ionic/atomic radius, from effectively 0 in H⁺, to 0.6 Å in H⁰, and 1.1 Å in H^–. Furthermore, the hydride ion is far more polarizable than other anions of comparable ionic radius such as O^2- and F^–. Traps, in the form of point or extended defects (for instance, vacancies, interstitials, substitutional alloying elements/dopants, internal interfaces, dislocations, grain boundaries, edge and corner sites on particles), therefore play outsized roles in dictating hydrogen transport and incorporation rates. Yet the definitive characteristics that determine transport dynamics and charge-transfer reaction rates remain to be categorized and quantified in a manner that enables predictive materials design or even enhancement of hydrogen mobility in known materials. Here, we seek to advance our understanding of near-equilibrium hydrogen transport in bulk oxides by addressing critical open questions:

• What are the rate-limiting steps in hydrogen transport and incorporation mechanisms, and the electronic, structural, and dynamic descriptors thereof?
• Can we vary the hydrogen oxidation state within a given material? Is the oxidation state fixed at the point of incorporation, or can it be dynamically tuned?
• What is the nature of traps that inevitably display affinity for hydrogen species? Traps typically retard bulk transport, but can extended defects serve as high mobility highways?
• What underlying transport principles apply across multiple classes of oxide materials (wide band-gap electrolytes, small band-gap semiconductors, mixed ionic-electronic conductors), and which transport features are unique to a particular class?

To effectively coordinate efforts and leverage the interdisciplinary expertise of the HEISs team, we have identified three model materials systems of focus for our initial studies:

• BaTiO₃-BaZrO₃-SrTiO₃ H⁺/H^– conducting oxide system, which provides a perovskite-structured solid-solution compositional space in which either H⁺ or H^– conduction is possible, depending on composition and environment (oxidizing versus reducing conditions).
• BaCo_xFe_yZr₁Y_0.1O₃ mixed H⁺/O^2-/h conducting oxide system, which provides a perovskite-structured solid-solution compositional space where the tradeoff between three different mobile charge-carrying species (H⁺, O^2-, and h) can be studied as a function of composition, defect structure, and environment.
• WO₃-type binary metal oxide systems which can host simultaneous H⁺ and e^– conduction, and where the H⁺ and electronic conductivity are intimately linked and tunable by orders of magnitude with small changes in bulk hydrogen content.

In all three systems, we leverage experts and capabilities from across the HEISs team in synthesis, theory/computation, and fundamental characterization of oxide materials to delineate the nature and transport of bulk hydrogenic defects as well as their dependence on oxide composition, defect structure, and external environment.