An IrO₂ catalyst was prepared using a colloidal method followed by a thermal treatment. The catalyst was later mixed with Pt-Black and supported on the Sb-doped SnO₂ (ATO), synthesized through the same colloidal method. ATO was investigated as a possible catalyst support in an electrode of a regenerative fuel cell (URFC), where Pt–IrO₂ was used as the catalyst for the oxygen evolution and reduction reactions. The morphology and composition of the ATO support was investigated through transmission electron microscopy, X-ray diffraction (including Rietveld Refinement), BET analysis, and X-ray fluorescence. An ATO support was obtained with a highly homogeneous distribution and crystal sizes, measuring approximately 4–6 nm. The Pt–IrO₂/ATO material was deposited on a Nafion 115 membrane with 0.5 mg cm⁻² of catalyst loading. Pt/Vulcan XC-72 (30 wt. %, E-TEK) was used as the catalyst in the H₂ compartment with a Pt loading of 0.4 mg cm⁻². The electrochemical activity of the Pt–IrO₂/ATO for oxygen evolution/reduction in the URFC system was investigated by AC-impedance spectroscopy, linear voltammetry, and chronoamperometry techniques. The maximum mass current activity was 1118 A g⁻¹ at 1.8 V in proton-exchange membrane water electrolyser mode (PEMWE) and 565 A g⁻¹ at 0.3 V in proton-exchange membrane fuel cell mode (PEMFC), both at 80 °C. The value of the round-trip energy efficiency was approximately 48% at 50 A g⁻¹.