The Role of Ni in Stabilizing BaZrO₃ Surfaces at Ni/BaZrO₃ Interfaces: A Density Functional Theory Analysis

Using previous computational findings on the stability of surface terminations of BaZrO₃ as a starting point, we discuss how adsorbed Ni could introduce additional stable terminations of BaZrO₃. As a benchmark, we show that BaO(001) is indeed the most stable termination of pure BaZrO₃, whereas (011) and (111) terminations have higher surface energies, in agreement with previous studies. We also discuss the underlying reasons for the greater stability of (001) versus (011) and (111) terminations and show that the high coordination numbers of Zr and Ba on (001) surfaces are key factors responsible for the lower free energy of (001) terminations. With regard to Ni cluster adsorption, we demonstrate that the (011) termination could be substantially stabilized, particularly due to the presence of additional oxygens at the Ni/BaZrO₃ interface, which could occupy otherwise empty sites of a stoichiometric slab of BaZrO₃ with (011) terminations. In contrast, the Ni/BaO(001) termination tends to be less stable (depending on coverage and the amount of adsorbed Ni) due to the less favorable introduction of oxygens at the interface with Ni. Using calculated phase diagrams and Wulff constructions, we comment on the percentage of (011) terminations in crystal shapes. We also discuss the impact of Y doping on the relative stabilization of BaZrO₃ surfaces with adsorbed Ni, proposing that conclusions similar to those for undoped Ni/BaZrO₃ could be drawn. Our work shows that, in addition to the models of Ni/BaO(001) interfaces, the models of Ni mounted on the (011) termination of BaZrO₃ should be considered for adequate modeling of reactions at the metal/oxide boundaries, which are essential for electrochemical processes in fuel cells and electrolyzers.