Achieving Active and Stable Amorphous Ir^VOOH for Water Splitting

Evaluating the structural and electronic-state characteristics of long-range disordered amorphous iridium (Ir)-based oxides is still unsatisfying. Compared with the benchmark IrO₂, the higher oxygen evolution reaction (OER) performance brought by IrOOH was normally considered to be associated with the pristine Ir^III-containing species. However, such a conclusion conflicts with the opinion that high-valence metals can create excellent OER activity. To resolve such contradictions, we synthesized a pure amorphous Lu_1.25IrOOH (Lu = lutetium) catalyst in this work. In combination with the comprehensive electrochemical evaluation in alkaline and acidic media, ex situ Ir L3-edge and O K-edge X-ray absorption spectroscopy and theoretical calculations revealed that the ultrahigh OER performance of reconstructed IrO/Lu_1.25IrOOH in acidic media was identified to be driven by the more d-hole-containing electronic state of Ir^V created by cationic vacancies. The pristine properties of Ir^III-containing Lu_1.25IrOOH conversely inhibit the OER activity in alkaline media. Additionally, the high edge-shared [IrO]-[IrO] motif proportion structure in amorphous Lu_1.25IrOOH achieves a stable OER process, which exhibits a high S-number stability index similar to IrO₂. We demonstrate that the key factor of the edge-shared [IrO]-[IrO] motif with cationic vacancies in Ir^VOOH could rationally reveal the source for most of the high-performance Ir-based materials.