Facile and sustainable fabrication of transparent mesoporous IrOx films formed by nanoparticle assembly for efficient electrocatalytic water oxidation

Abstract

An efficient electrocatalyst film of iridium oxide (IrO_x) with low iridium usage is pivotal towards improvement of the sustainability of sparse iridium-based catalysts. A new transparent mesoporous IrO_x film consisting of small interconnected nanoparticles has been developed by simple drop-casting of a homogeneous solution containing a very low concentration of a hydroxyiridate complex and a polymer surfactant Pluronic F127 followed by annealing. The intrinsic activity of catalytic Ir-sites in the mesoporous IrO_x-F127 film was higher than that of the IrO_x-untemplate film at the same annealing temperature as indicated by cyclic voltammetry. Electrochemical impedance measurements suggested faster electron transport through interconnected nanoparticles of the mesoporous network in the former film compared with the discrete nanoparticulate structure in the latter. The high intrinsic activity of Ir-sites and efficient electron transport in the novel mesoporous structure contributed to the high electrocatalytic performance observed for the IrO_x-F127 film annealed at 400 °C. The catalytic current of the IrO_x-F127 film for water oxidation with respect to the iridium amount (I_cat/Γ_cov) was 5.3 and 4.3 times higher than those of the transparent and efficient electrocatalyst films of the hitherto-reported 2D-hexagonal mesoporous IrO_x (ChemSusChem, 2015, 8, 795–799) and nanoparticle IrO_x(OH)_y (ACS Catal., 2016, 6, 3946–3954), respectively.