Engineering the Defect Chemistry of Oxide Electrolytes for Hydrogen Production

Defects and impurities play an important role in determining the properties of materials and require careful consideration during materials design and synthesis. In semiconductors, defects can act as electron donors or acceptors, and their concentrations can be tuned by introducing species of the opposite charge. When needed, dopants must be carefully chosen to avoid undesirable side effects; in ionic conductors, these may include high binding energies with ionic carriers and the reduction of efficiency due to unwanted electrical conductivity.

Here, I will introduce two case studies under the umbrella of solid-oxide electrolyzer cells (SOECs) demonstrating how first-principles calculations can address the role of defects and impurities in improving the performance of devices for clean hydrogen production. First, I will consider certain high-performing proton conductors, such as BaZrO₃ and BaCeO₃. They are routinely doped with an acceptor species (e.g., Y) to introduce oxygen vacancies, which are precursors for proton incorporation. However, high doping concentrations also carry the risk of introducing p-type electrical carriers in the form of hole polarons, which will limit the Faradaic efficiency of devices.

Second, I will discuss work on oxide-ion conducting SOECs based on yttria-stabilized zirconia (YSZ) as the electrolyte. Due to their high temperatures of operation (> 850 °C), these materials are subject to a range of degradation phenomena, including the formation of secondary phases, such as Sr-containing SrZrO₃ and SrO. Investigating the defect chemistry and oxygen vacancy mobility of these phases makes it possible to understand their impact on device performance, as well as how their properties can be tuned through impurity incorporation. These and other case studies highlight the vital role of first-principles defect calculations for optimizing the performance of electrochemical systems for energy production.