Highly Efficient Electrocatalysis and Mechanistic Investigation of Intermediate IrOx(OH)y Nanoparticle Films for Water Oxidation

A new transparent iridium oxide (IrO x ) film on fluorine-doped tin oxide (FTO) electrodes were achieved from a homogeneous precursor complex solution by employing a facile spin-coating technique. The composition of the nanostructure and crystallinity of the IrO x film is tunable by a simple annealing treatment of a compact complex layer, which is responsible for their significantly different electrocatalytic performances for water oxidation. Transmission electron microscopy (TEM) observations showed uniformly dispersed small IrO x nanoparticles of dimensions ca. 2–5 nm for the film annealed at 300 °C, and the nanoparticles gradually agglomerated to form relatively large particles at higher temperatures (400 and 500 °C). The IrO x films prepared at different annealing temperatures are characterized by Raman spectroscopic data to reveal intermediate IrO x (OH) y nanoparticles with two oxygen binding motifs: terminal hydroxo and bridging oxo at 300 and 350 °C annealing, via amorphous IrO x at 400 °C, transforming ultimately to crystalline IrO 2 nanoparticles at 500 °C. Cyclic voltammetry suggests that the intrinsic activity of catalytic Ir sites in intermediate IrO x (OH) y nanoparticles formed at 300 °C annealing is higher in comparison with amorphous and crystalline IrO x nanoparticles. Electrochemical impedance data showed that the charge transfer resistance ( R ct = 232 Ω) for the IrO x (OH) y film annealed at 300 °C is lower relative to that of films annealed at higher temperatures. This is ascribable to the facilitated electron transfer in grain boundaries between smaller IrO x particles to lead the efficient electron transport in the film. The high intrinsic activity of catalytic Ir sites and efficient electron transport are responsible for the high electrocatalytic performance observed for the intermediate IrO x (OH) y film annealed at 300 °C; it provides the lowest overpotential (η) of 0.24 V and Tafel slope of 42 mV dec –1 for water oxidation at neutral pH, which are comparable with values for amorphous IrO x · n H 2 O nanoparticle films (40–50 mV dec –1 ) reported as some of the most efficient electrocatalysts so far.