pH dependence of noble metals dissolution: Iridium

Iridium's stability and catalytic properties make it indispensable in a wide range of electrochemical applications including water electrolysis, yet its dissolution behavior across varying pH conditions remains poorly understood. This study addresses this gap by employing online inductively coupled plasma mass spectrometry (ICP-MS) to monitor iridium dissolution across a pH range of 1 to 12.7 under both pre-OER and OER conditions. An electrochemical protocol consisting of potential scans and holds is designed to track how potential and time affect Ir dissolution. In the pre-OER region, iridium shows consistent qualitative dissolution behavior across pH levels, although the extent of anodic and cathodic dissolution depends on the electrolyte. To explain this dependence, the thermodynamics of Ir in aqueous media, as revealed by the Pourbaix diagram, electrochemical profiles representing oxidation / reduction processes, and adsorption of electrolyte species are considered. During potential scans, kinetic effects from native oxides and phosphate adsorption lead to the absence of anodic dissolution, while prolonged holds at higher pH result in increased anodic dissolution, likely due to IrO 4 2− formation. Iridium's cathodic dissolution is observed in all studied protocols. It decreases from acidic to neutral pH, in line with the stability window of Ir 3+ ionic species in the Pourbaix diagram, and surprisingly increases in highly alkaline conditions again, suggesting the existence of species not reflected on the diagram. In the OER region, iridium remains stable in acidic to neutral conditions, with stability decreasing under alkaline conditions. Two distinct dissolution mechanisms are proposed based on pH: in acidic environments, H 2 O primarily acts as the reactant, leading to Ir³⁺ formation, while above pH 9, OH − becomes the main reactant, contributing to IrO 4 2− formation and reduced stability. Under neutral conditions, buffers play a key role in maintaining local pH and supporting consistent OER rates, especially when the buffer's pK a aligns with the solution's pH. These findings improve our understanding of iridium dissolution mechanisms and support the development of more stable and cost-effective iridium-based catalysts for various electrochemical technologies.