Defect-induced instability of single-crystal IrO₂ electrocatalyst for water-splitting

Ir dissolution is observed in the state-of-the-art IrO₂ electrocatalyst during oxygen evolution reactions (OER). However, atomistic level understanding of the mechanism causing this stability issue is lacking. The correlation between Ir dissolution and lattice O evolution is still unclear. This is largely due to the challenge of fabricating well-defined model surface experimentally, as well as a model describing the atomic-level process under the real electrochemical condition. Here we carried out a first-principles based defect calculations combined with thermodynamic models considering the real electrochemical conditions. From there we constructed so-called "defect" Pourbaix diagrams for the bulk and (110) surface of IrO₂. Counter-intuitively, lattice point defects within the bulk is thermodynamically favorable under OER conditions, indicating a kinetically stabilized mechanism for IrO₂ electrocatalyst under working conditions. However, point defect on surface should be more readily to leave the lattice without overcoming multiple kinetic barriers. Our works shows easier Ir dissolution than lattice O evolution under high anodic bias (~2V w.r.t SHE), although lattice O evolution already takes place below 1.23 V. On the (110) surface, O on CUS is easier to peel off the lattice than O on BRG, which promotes the dissolution of Ir underneath on CUS with higher anoidc bias. However, Ir on BRG dissolution becomes dominant under high anodic bias (~2V w.r.t SHE) without a prior lattice O evolution on BRG. Overall, IrO₂ should be more stable in acid than in base.