(345e) Applying DFT+U and the Self-Consistent Linear Response Approach to Establish a Thermodynamic Framework for Comparing Redox Reaction Energies in Perovskites
The successful identification of suitable materials for power storage and generation applications ranging from oxygen evolution in rechargeable metal-air batteries to hydrocarbon oxidation in solid oxide fuel cells and chemical looping is strongly reliant on the ability to accurately predict the energetics of the reduction and oxidation of these materials. A potential set of tunable materials able to serve in these applications are the perovskite (ABO3) metal oxides with La or Sr A-site and 3d transition metal (Sc – Cu) B-site compositions. In hydrocarbon oxidation, the redox energetics of perovskites are highly related to their abilities to form oxygen vacancies in their bulk structures, as the thermodynamic barriers to the migration of oxygen from bulk to surface are consistent and low in oxygen carrying perovskites. Prior to investigating the tunability of oxygen carrier candidates, a methodology capable of accurately computing their relevant reaction energies must be developed. There are known deficiencies in the ability of standard DFT GGA functionals to accurately calculate the electronic structures and reaction energies of 3d metal containing oxides. Thus, we have utilized DFT+U to address these deficiencies, employing the self-consistent extension of the linear response method to select an appropriate U for each system.
In this work, we evaluate the effects of U on the trends in oxygen vacancy formation energies of the perovskite systems discussed above. Our analysis of the effects of adding on-site Hubbard U parameters to all ABO3 B-sites – in order to account for their 3d electron self-interaction – revealed that the trends in oxygen vacancy formation energies can change while varying U in both LaBO3 and SrBO3 materials. In particular, at low values of U the trends are consistent, but for values of U between 6.5 and 7.5 eV we observed a reversal in the energetic ordering between Mn and Fe containing perovskites. Since the correct value of U for these materials is not a priori evident, the self-consistent extension for evaluating Hubbard U parameters in linear response theory is essential to determining a theoretically rigorous basis for comparing trends in sets of oxygen carriers. In applying this extension of the DFT+U method to study trends in oxygen vacancy formation energies, our results indicate that significant differences can exist between the trends observed when the 3d self-interaction error is addressed or ignored.