Effect of pH on Electrochemical Dissolution of Iridium
Authors: Zlatar Matej , Escalera López Daniel , Stojanovski Kevin , Briega Martos Valentin , Cherevko Serhiy 
Published: November 2022

Replacing fossil fuels with renewable energy sources is one of the most challenging tasks in today's society. As renewable sources are intermittent in nature, a promising solution is to store the produced energy in the form of hydrogen, which can be produced electrochemically by using proton exchange membrane water electrolyzers (PEMWEs). A limiting factor of PEMWEs is that the harsh conditions and sluggish kinetics of oxygen evolution reaction (OER) at the PEMWE anode require that the state-of-the-art catalysts employed are based mostly on Iridium (Ir). (1) Given its scarcity, the use of Ir in OER electrocatalysis is under increasing scrutiny, where current efforts have focused on maximizing activity while decreasing its loading. This is crucial in order to implement PEMWEs in the giga-to-terawatt scale, one of the goals set by the U.S. DoE "Hydrogen Shot". (2)
The benchmarking of Ir catalysts for OER electrocatalysis is mostly performed in aqueous model systems (AMS) using acidic electrolytes, but recent works directly comparing AMS and PEMWE showed a clear discrepancy in the catalyst lifetimes. (3) Indeed, a higher pH value under PEMWE operation (ca. 3) was proposed as the main factor responsible for the extended lifetimes. (4) Consequently, both activity and stability of Ir need to be carefully reassessed in a wider pH window. Several works have already evaluated the influence of pH on Ir OER activity over the entire pH scale with buffered (5) and unbuffered electrolytes, (6) but so far, very few works have looked into its stability. Ir stabilities have been reported either in highly acidic (1) or alkaline conditions, (7) where Ir stability significantly changed between such pHs. In this context, studying the Iridium stability over the entire pH scale is essential for understanding and designing more stable and active catalysts.
The presented work aims to fill the gap in the current literature regarding Ir activity-stability relationships at different pHs. To do so, we tested a polycrystalline iridium electrode with scanning flow cell coupled to an inductively coupled plasma mass spectrometer setup (SFC-ICP-MS) in pH range from 1 to 12.7. For pH 1 and 12.7, 0.1 M HClO4 and 0.05 M KOH were used, respectively, while pH range of 3-11 was achieved by the addition of phosphate buffer. Using this approach, we distinguished the clear influence of pH on the stability of Iridium, which decreased by shifting the pH value from 1 to 12.7. Lastly, the lower stability of Ir in near-neutral pH compared to acidic, raises questions about the applicability of OER at near-neutral pHs in buffered conditions, which recently gained increased attention.

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