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.