Nanocomposites of gold (Au) and iridium (Ir) oxide with various compositions (denoted as Au x Ir 1−x O y , x = 0.05, 0.10, or 0.33, Au precursor molar ratio to Ir precursor) were synthesized via electrospinning and subsequent calcination method with two different solvent composition ratios of ethanol to N,N-dimethylformamide (DMF) in the electrospinning solution (ethanol/DMF = 70:30 or 50:50% v/v). Simple single-step electrospinning successfully fabricated a hierarchical nanostructure having Au nanoparticles formed on fibrous main frames of Ir/IrO 2 . Different solvent composition in the electrospinning solution induced the formation of main frames with distinct nanostructures; nanoribbons (Au x Ir 1−x O y -70) with ethanol/DMF = 70:30; and nanofibers (Au x Ir 1−x O y -50) with ethanol/DMF = 50:50. The pure Ir or Au counterparts (IrO y and Au) were also prepared by the same synthetic procedure as Au x Ir 1−x O y . Oxygen evolution reaction (OER) activities of as-synthesized Au x Ir 1−x O y were investigated in 0.5 M H 2 SO 4 and compared to those of IrO y , Au, and commercial iridium (Ir/C, 20% Ir loading on Vulcan carbon). Among them, Au 0.10 Ir 0.90 O y -50 exhibited the best OER activity, even better than previously reported catalysts containing both Ir and Au. The high OER activity of Au 0.10 Ir 0.90 O y -50 was mainly attributed to the fiber frame structure and the optimal interfacial areas between Au and Ir/IrO 2 , which are electrophilic OER active sites. The stability of Au 0.10 Ir 0.90 O y -50 was also evaluated to be much higher than that of Ir/C during OER. The current study suggests that the presence of Au on the Ir/IrO 2 surface improves the OER activity of Ir/IrO 2 .

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