Alteration of the morphology and electrocatalytic activity of IrO₂ nanowires upon reduction by hydrogen gas

This paper reports the facile synthesis of highly single-crystalline IrO₂ nanowires grown on an Au microwire (IrO₂NW-Au) and its reduced form (Ir/IrO₂NW-Au); and their electrocatalytic activity for oxygen reduction reaction, H₂O₂ reduction/oxidation, and dopamine (DA) oxidation. IrO₂NW-Au, prepared by direct vapor transport process under atmospheric pressure, was reduced by H₂ gas flowing at 200°C. This additional modification resulted in the significant morphological changes from the smooth nanowire structures of IrO₂NW-Au to substantially porous structures of Ir/IrO₂NW-Au with sustaining the external nanowire frameworks. The compositions were also changed from mostly IrO₂ in IrO₂NW-Au to the mixture of IrO₂ and Ir(0) metal in Ir/IrO₂NW-Au. Ir/IrO₂NW-Au showed highly enhanced and facilitated electrochemical reaction kinetics compared to IrO₂NW-Au for ORR and H₂O₂ reduction/oxidation. The ORR limiting-like current at Ir/IrO₂NW-Au was measured to be ∼19-fold greater than that of IrO₂NW-Au. In addition, the amperometric responses to varying H₂O₂ concentration confirmed that Ir/IrO₂NW-Au exhibited ∼8-fold (for H₂O₂ oxidation) and ∼750-fold (for H₂O₂ reduction) higher sensitivity than IrO₂NW-Au. The observed enhanced activity of Ir/IrO₂NW-Au could be attributed to the enlarged active surface area as well as the inherent electroactivity of Ir/IrO₂NW material induced by co-existence of Ir oxide and metal. In contrast, more stable and decent anodic current responding to DA oxidation was measured at IrO₂NW-Au than Ir/IrO₂NW-Au, indicating IrO₂ rather than Ir(0) has higher catalytic activity for DA oxidation.