On the formation and diffusion of oxygen vacancies in non-stoichiometric mixed Co3-x Mn x O4 spinel structures from first-principles calculations

Abstract

Using first-principles simulations, we focus on the study of Co3O4–Mn3O4 mixed oxides, which have recently shown alluring features as thermochemical heat storage materials. We provide fundamental atomistic-level insight into the thermodynamics and kinetics of a series of non-stoichiometric Co3-x Mn x O4-y (0 ⩽ x ⩽ 3 and y = 0, 0.125, 0.250) bulk systems, by examining in detail the formation and diffusion processes of oxygen vacancies as a function of Mn content. We find a preference for the formation of vacancies at x = 1.5. And we predict a significant drop of diffusion barriers for x ⩾ 1.5, when Mn atoms start to populate the spinel octahedral sites as Mn3+. Our results pave the way for better understanding the underlying mechanisms that govern oxygen vacancy dynamics in Co3-x Mn x O4 in general, and, in particular, the reversible reduction and re-oxidation reactions of these promising mixed oxides for thermal energy storage. Nevertheless, some discrepancies are found between our calculations on bulk models and recent experimental insights from the literature, which suggests that surface and finite size effects might play an important role in controlling the observed macroscopic behavior of these materials during reversible reduction and re-oxidation cycles.