OER activity manipulated by IrO₆ coordination geometry: an insight from pyrochlore iridates

The anodic reaction of oxygen evolution reaction (OER), an important point for electrolysis, however, remains the obstacle due to its complicated reaction at electrochemical interfaces. Iridium oxide (IrO₂) is the only currently known 5d transition metal oxide possessing admirable OER activity. Tremendous efforts have been carried out to enhance the activity of iridium oxides. Unfortunately there lies a gap in understanding what factors responsible for the activity in doped IrO₂ or the novel crystal structure. Based on two metallic pyrochlores (Bi₂Ir₂O₇ and Pb₂Ir₂O_6.5) and IrO₂. It has been found that there exists a strong correlation between the specific OER activity and IrO₆ coordination geometry. The more distortion in IrO₆ geometry ascends the activity of Ir sites, and generates activity order of Pb-Ir > IrO₂ > Bi-Ir. Our characterizations reveal that distorted IrO₆ in Pb-Ir induces a disappearance of J = 1/2 subbands in valence band, while Bi-Ir and IrO₂ resist this nature probe. The performed DFT calculations indicated the distortion in IrO₆ geometry can optimize binding strength between Ir-5d and O-2p due to broader d band width. Based on this insight, enhancement in OER activity is obtained by effects that change IrO₆ octahedral geometry through doping or utilizing structural manipulation with nature of distorted octahedral coordination.