Fig. 1: Preparation of hierarchically porous Ir hydroxide.

a Arrangement of IrO₆ octahedra (isolated by Sr) in Sr₄IrO₆. b SEM image of the as-prepared Sr₄IrO₆ particles. c SEM images of acid-treated Sr₄IrO₆ particles and a surface slit. d XPS of Sr_3d from the as-prepared and acid-treated Sr₄IrO₆. e, f SEM images and corresponding sketch maps of the cross-sectional specimen (acid-treated Sr₄IrO₆) prepared by FIB milling. g TEM images of local structure of pores/channels within the bulk of acid-treated Sr₄IrO₆. h The surface areas for the as-prepared and acid-treated Sr₄IrO₆. The upper limit of the surface area for the as-prepared Sr₄IrO₆ is estimated using a spherical model, with a diameter >10 μm (marked in the green circle). The orange circle represents the equivalent dense IrO₂ sphere for the acid-treated Sr₄IrO₆, with a calculated diameter of ~12.8 nm based on the BET surface area of the acid-treated Sr₄IrO₆.