Synthesis and Electrochemical Properties of IrO2 Nanosheets
IrO2 is known as one of the most highly active and stable OER catalysts. It is essential that high surface area materials are used, either by preparing porous architectures or dispersing nanoparticles on conducting supports since Ir is a precious metal. Two-dimensional nanosheets have recently attracted increased interest as building blocks for high surface area porous materials. Nanosheets with electronic conductivity, for example, RuO2 nanosheets find application in transparent conductive electrodes as well as electrochemical energy storage and conversion.1,2) IrO2 is analogous to RuO2in many aspects; both possess the rutile structure and are room-temperature metallic conductors. Here we report the successful synthesis of an ion exchangeable layered iridium oxide which can be exfoliated into elemental nanosheets and subsequently used as an electrocatalyst.
Layered K0.30Ir0.93O2•nH2O was prepared by solid-state reaction of a 1:2 mixture of K2CO3 and IrO2 under inert atmosphere. Proton exchange was conducted by reaction in 1 M HCl for 3 days. Chemical exfoliation was conducted by shaking H0.30Ir0.93O2•nH2O in an aqueous solution of tetrabutylammonium hydroxide for 1 week. Electrochemical properties were studied in H2SO4 and HClO4.
Results and discussion
The Ir0.93O2 nanosheets (typical AFM image given in Fig. 1 inset) have an average thickness of c.a. 1.5 nm. The sharp edges suggest that the nanosheets are highly crystalline. Cyclic voltammograms of the Ir0.93O2 nanosheet electrode in 0.5 M H2SO4 is shown in Fig. 1. Three distinct redox peaks are observed. The E1/2=0.95 and 1.19 peaks are typical of IrO2 3) and assigned to the Ir3+/Ir4+ and Ir4+/Ir5+ couple. The peaks at E1/2=0.83 is observed only for IrO2 nanosheet, and shifts in HClO4; this peak is attributed to ad/desorption of anion. The onset of OER is observed at a potential similar to IrO2 nanoparticles.4) The specific capacitance between 0.2 and 1.2 V vs RHE at 2 mV s-1 was 93 kF mol-1, or 432 F g-1, which is the highest reported value for IrO2-based materials. It also closely matches the value of 88 kF mol-1 for RuO2nanosheet. Thus, almost all of the nanosheets surface is electrochemically accessible, and thus should provide unprecedented high utilization of the precious metal.
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