The role of the iridium precursor in iridium deposition by galvanic displacement of Cu.

Iridium is a costly element on which water electrolysis under acidic conditions depends for catalyzing the oxygen-evolution reaction. As a means of minimizing iridium loading, this paper reports a one-pot procedure for iridium deposition by successive galvanic displacement of Cu monolayers on a rotating-disc electrode of polycrystalline gold. The Cu monolayers were formed on Au(poly) electrodes by underpotential deposition in solutions containing H2SO4, CuSO4 and low concentrations of H2IrCl6 (Ir(IV)) or IrCl3 (Ir(III)) at 70 °C. With Ir(IV) precursors, the Cu monolayers readily reacted under open circuit conditions via galvanic displacement, resulting in submonolayers of metallic Ir at the gold surface. The process can be repeated in the same solution until the desired loading has been achieved. The results indicate that the reduction of the Ir(IV) precursor takes place via the formation of an Ir(III) intermediate. However, galvanic replacement directly from IrCl3-containing solutions, i.e. from Ir(III) precursors, did not take place to any significant extent. Density functional theory (DFT) calculations show that the Ir(III) intermediate complex in solutions of H2IrCl6 has a much higher reduction potential than that of Ir(III) precursors in equilibrated solutions of IrCl3. The latter is only marginally higher than the reduction potential for the copper monolayers, and Ir deposition is therefore insignificant from IrCl3 solutions. This demonstrates the importance of selecting appropriate precursors and bath chemistry.