Scaling the existence state of hydrogen in metal binary oxides

Abstract

Hydrogen in metal oxide is a classic but engaging topic in the scientific research and actual application; owing to the flexible property of hydrogen, however, a scaling relationship regarding the existence state of hydrogen remains in lack. Hereby, a systematic first-principles calculation is performed on the behavior of neutrally charged hydrogen atom in a broad variety of metal binary oxides. Three main incorporation sites for hydrogen (interstitial site, hydroxyl group and oxygen vacancy) and more than 100 metal oxides are considered. From the calculated defect energy, the relatively stable incorporation sites for hydrogen are identified, and the mechanism behind the stability is unravelled by the inherent redox and electronic structure properties of host oxides. Generally, hydrogen prefers to form hydroxyl group in the reducible oxides but occupy interstital site in the irreducible ones. More importantly, oxygen vacancy formation energy associated with the redox property of oxide is established as the pragmatic descriptor for scaling the existence state of incorporated hydrogen, and an intercept value of about 4.5 eV is determined for distinguishing H occupancy, coincidencent with the standard hydrogen electrode potential. The scaling rule is expected to be popularized to other oxides and solids associated with hydrogen behavior.