Electrochemical phase diagrams of Ni from ab initio simulations: role of exchange interactions on accuracy

The stabilities of Ni metal and its derived compounds, including oxides, hydroxides, and oxyhydroxides under electrochemical conditions, can be readily predicted from the Ni Pourbaix diagram, where the formation free energies of the involved species are utilized to construct the phase stability map with respect to electrode potential and pH. We calculate and analyze the crystal structures, electronic structures, and thermodynamic energies of Ni metal and its compounds using different exchange-correlation functionals to density-functional-theory (DFT), including the semilocal LDA and GGA density functionals, the nonlocal metaGGA, and the hybrid density functionals. Next, we simulate the corresponding Ni Pourbaix diagrams to compare systematically the performance of the functional to each other and to experimental observations. We show that the structures and energies obtained from experimental databases may not be sufficiently accurate to describe direct electrochemical observations, and we explain how the electronic exchange within the density functionals plays a key role in determining the accuracy of the DFT calculated electronic, thermodynamic, and electrochemical properties. We find that only the hybrid density functional produces reliable results owing to the fractional contribution of exact Fock exchange included therein. Last, based on our accurate Ni Pourbaix diagram, we construct band-gap and magnetic electrochemical maps which can facilitate more experimental measurements and property assessments under variable potential and pH in the future.