Supported iridium oxide films in aqueous electrolytes: Charge injection dynamics as monitored by time-resolved differential reflectance spectroscopy

Dynamic aspects of the electrochemical charge-discharge of hydrated Ir oxide, IrOx (hyd), films in aqueous 0.5 M H2 SO4 have been investigated by simultaneous chronocoulometry and time-resolved, normal incidence (λ=633 nm) differential reflectance spectroscopy, ΔR/R. Experiments were performed by applying potential steps between two judiciously selected values to IrOx (hyd) films electrodeposited on smooth Au disk microelectrodes to reduce the overall time constant of the cell. The analysis of the results obtained revealed that the rate at which charge is injected into or released from the film, as determined from the chronocoulometric response, is faster than the rate at which Ir sites in the lattice undergo redox transitions, as monitored by ΔR/R. This behavior appears consistent with the relatively high electronic conductivity of IrOx (hyd), which allows for charge to be stored on the highly convoluted surface of this structurally disorganized material, i.e., strictly capacitive, a process that occurs in parallel and at much higher rates than changes in the oxidation state of the Ir sites, i.e., pseudocapacitive.