Synthesis and Electrochemical Properties of IrO2 Nanosheets

1. Introduction

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.

2. Experimental

Layered K_0.30Ir_0.93O₂•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 H_0.30Ir_0.93O₂•nH₂O in an aqueous solution of tetrabutylammonium hydroxide for 1 week. Electrochemical properties were studied in H₂SO₄ and HClO₄.

Results and discussion

The Ir_0.93O₂ 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 Ir_0.93O₂ nanosheet electrode in 0.5 M H₂SO₄ is shown in Fig. 1. Three distinct redox peaks are observed. The E_1/2=0.95 and 1.19 peaks are typical of IrO₂ ³⁾ and assigned to the Ir³⁺/Ir⁴⁺ and Ir⁴⁺/Ir⁵⁺ couple. The peaks at E_1/2=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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Figure 1. Cyclic voltammograms of IrO₂ nanosheet electrodes in 0.5 M H₂SO₄ at 50 mV -1 (inset shows a typical AFM image of nanosheets on Si).