Electrochemical Stabilities of Ni-Based Compounds from Bulk to Nanoscale Dimensions

Ni-based compounds (e.g., oxides, hydroxides, oxyhydroxides) are important materials in various fields (e.g., energy, catalysis, corrosion, and oxidation), and exhibit spatial dimensions ranging from bulk down to the nanoscale. The electrochemical stability is a critical factor determining applications of Ni-based compounds in aqueous conditions, and can be described by phase diagrams consisting of electrode potential and solution pH, i.e., Pourbaix diagrams. Using density functional theory, we calculated the thermodynamic energies of bulk and nanoscale Ni-based compounds and examine the effects of water adsorption and substrate adhesion. The Pourbaix diagrams simulated using these thermodynamic energies are consistent with various direct electrochemical observations as well as recent electrochemical measurements by our experimental collaborators (the Scully Group at University of Virginia). This work both quantitatively reveals the roles of many factors in nanoscale electrochemistry and develops a first-principles approach for studying material electrochemistry at various conditions.