The electrochemical stabilities of nanofilms of Ni oxides and hydroxides are of special importance to the diverse fields of catalysis, energy storage and conversion, and alloy corrosion resistance. Many coexisting intrinsic and environmental factors may simultaneously become significant when the material size is reduced to ultrathin dimensions, making it challenging to unravel the multiple interacting mechanisms active in complex nanoscale structural architectures. Here we establish a comparative theory-experiment approach to accurately study the stabilities of nanoscale Ni-based compounds against oxidation under various electrochemical conditions and use it to quantitatively reveal the roles of surface termination, thickness, water adsorption, and supporting substrate on phase stability. We use density functional theory to calculate the energies of Ni-based nanofilms at different thicknesses subjected to various boundary conditions and environments, including free-standing, suspended in water, and substrate-supported nanofilm geometries. We use this data to simulate the corresponding nanofilm electrochemical phase diagrams and comprehensively explain various reported electrochemical phenomena. Our theoretical findings are further validated by an electrochemical experiment designed here, where the potential-driven growth of (hydr)oxide nanofilms on Ni substrates in different solutions is precisely characterized using in situ polarized neutron reflectometry. The obtained quantitative results and insights into the microscopic corrosion mechanisms will be useful for the design, synthesis, and application of other nanoscale transition-metal compounds; in addition, the comparative theory-experiment approach can be readily translated to accurately study the electrochemical properties of other complex nanoscale systems.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films