Theoretical insights into nonprecious oxygen-evolution active sites in Ti–Ir-Based perovskite solid solution electrocatalysts

Abstract

Deep understanding of the properties of catalytically active sites on complex solid surfaces will benefit the rational design of catalysts with desirable performances. Here, we present DFT calculations that demonstrate the unexpected ability of nonprecious titanium sites on SrTi_x(Ir)_1−xO₃ solid solution (STIO) surfaces to electrocatalyze the oxygen evolution reaction (OER), and the crucial roles of iridium content (defined by the atomic ratio Ir/(Ir + Ti) in STIO) and spatial distribution in generating highly active titanium catalytic sites. The surface iridium sites for STIO with iridium content from 12% to 50% show higher activity than those on IrO₂ surfaces. The surface titanium sites for STIO with iridium content from 25% to 75% show high catalytic activity similar to iridium catalytic sites, while the surface titanium sites for SrTiO₃ (STO) are intrinsically catalytically inert for the OER. These results suggest that for an optimum catalytic activity to be realized, STIO should have an iridium content between 25% and 50% due to the existence of efficient, dual metallic catalytic sites, in agreement with our previous experimental observation. Additionally, the e_g-filling of surface titanium sites for STIO is found to be reduced with the increase of the iridium content and strongly correlates with the surface adsorption ability of surface titanium sites towards the key reaction intermediates and thereby the catalytic activity for the OER. Besides the iridium content, the spatial distribution of iridium in STIO is also found to be vital to activate surface titanium sites. The results show that the surface iridium atoms play a dominant role in the activation of surface titanium sites, while the inner iridium atoms have a minor impact on this activation. As presented in this work, a novel core/shell-type material model, where an iridium-free STO core is covered with a single-layered-thick STIO shell, is overall suggested to achieve highly active oxygen evolution electrocatalysts with an ultralow iridium content.