• Vegge, Tejs
• Tripkovic, Vladimir
• García Lastra, Juan Maria
• Hansen, Heine Anton

Abstract

The most common method for incorporating strong electron correlations iseither to apply the Hubbard U correction on top of standard densityfunctional theory calculations (DFT) or to use hybrid functionals. Inthis study, we elucidate the sensitivity of the Hubbard U correction inthe PBE+U functional and the amount of exact exchange, α, in the hybridHSE functional on the structural stability, catalytic activity andelectronic conductivity of pure and doped perovskite oxides, ABO3, (A =La, Ca, Sr and Ba, B = Cr, Mn, Fe, Co, Ni and Cu) for oxygen evolutionelectrocatalysis. We find a strong dependence of heat of formations andreaction overpotentials for a range of U = 0, 3 and 5 eV and α = 0,0.15, 0.25, 0.35 values investigated in this study, which we attributeprimarily to changes in the oxidation state of B cations. If the valenceof B cations in the perovskite and reference oxide is the same, thenthe U- and α dependence is very small. On the other hand, if thevalences are different then heat of formations can change by as much as 1eV. As the oxidation state of a surface metal ion depends on adsorbedintermediate and nature of the element, similar differences in energiesappear in the calculated reaction overpotentials for oxygen evolution.The large U and α dependence sets serious constraints on the use ofDFT+U and HSE methods for assessing stabilities and catalytic activitiesof perovskite oxides. In addition, the large α dependence raises thequestion whether HSE calculations can improve sufficiently the accuracyof DFT+U results for multi-step electrochemical reactions to justify theexcess computational cost. Although we have investigated only oneparticular class of catalysts and one electrochemical reaction, theresults of this study can expectedly be generalized to other stronglycorrelated systems in which the oxidation state of the surface changesduring reaction. The influence of U on the electronic conductivity issignificant only in cases where it qualitatively changes the electronicstructure, by e.g. opening the band-gap. From a combinatorial analysison pure and doped oxides, we identify electronically conductivecatalysts classified according to different electron conduction types:intrinsic conductivity (Fe4+, Co3+(intermediate spin, IS) and Ni3+),electron polaron hopping (along Mn3+-O-Mn4+ chains) and charge transportthrough holes in the valence band.

Topics

• density
• perovskite
• impedance spectroscopy
• surface
• theory
• Oxygen
• density functional theory