An aqua regia-free chemical recovery and reprecipitation of Ir from IrOx catalysts: optimisation of the extraction efficiency using surface response methodology

Abstract

The widespread deployment of hydrogen is essential in the energy transition away from fossil fuels. There is an increasing demand for iridium (Ir) in the proton-exchange membrane water electrolysis (PEMWE) industry, acting as the catalyst for the oxygen evolution reaction (OER). However, Ir is one of the rarest platinum group metals (PGMs) and its recycling requires significant energy and has considerable environmental impacts. Therefore, developing an innovative energy-efficient recycling method is essential to enhance the use of Ir. This study focuses on the development of a microwave-assisted leaching protocol of Ir from a commercial iridium oxide (IrO_x) catalyst and its reprecipitation in the form of a salt. A Box–Behnken optimisation of the Ir leaching efficiency was performed by varying the temperature (70–200 °C), chloride concentration ([Cl⁻]) (0–4 M) and hydrogen peroxide concentration ([H₂O₂]) (0–3% v/v). An optimum of 91% extraction efficiency was predicted and then confirmed experimentally, achieving an Ir leaching efficiency of 83 ± 10%, which corresponds to an Ir mass extraction of 769 mg_Ir g_IrOx⁻¹. The precipitation of the Ir leachate solution was then performed to obtain Ir in the form of ammonium hexachloroiridate ((NH₄)₂IrCl₆) with 94.6% purity.