First-principles design of solid-state hydrogen electrolytes*

Defects and impurities play an important role in determining the properties of materials and must be carefully considered as part of materials design and synthesis. This applies to semiconductors and materials used for energy generation and storage. Defects can act as electron donors or acceptors, implying that their concentrations can be tuned by introducing oppositely charged species. These dopants must be carefully chosen to avoid undesirable side effects, such as autocompensation or, in the case of ionic conductors, high mobile carrier binding energies.

We discuss how first-principles calculations can address these considerations for two promising solid-state hydrogen electrolyte materials. The alkaline-earth zirconates (AeZrO₃; Ae = Sr, Ca, Ba) are solid-state hydrogen conductors in which protons move interstitially. Oxygen vacancies, which act as donors, act as precursors to proton incorporation. To maximize oxygen vacancy concentrations, acceptor dopants are introduced. We identify the alkali metal dopants that are most ideal for incorporating oxygen vacancies, while avoiding harmful compensation effects.¹ The alkaline-earth hydrides (AeH₂) conduct hydrogen via a vacancy-mediated mechanism. Optimizing the vacancy concentration again requires the introduction of acceptor dopants, and alkali metal dopants are most effective to this end. We estimate an increase in ionic conductivity by roughly two orders of magnitude in the case of BaH₂ doped with potassium.² We discuss implications of our results for studies of other novel hydrogen-conducting materials and, in general, how to use defect engineering to optimize material performance.

¹A. J. E. Rowberg, L. Weston, C. G. Van de Walle, APS Appl. Ener. Mater. 2, 2611 (2019).
²A. J. E. Rowberg, L. Weston, C. G. Van de Walle, Chem. Mater. 30, 5878 (2018).