Insights into the Path-Dependent Charge of Iridium Dissolution Products and Stability of Electrocatalytic Water Splitting

Abstract

The electrocatalytic stability of the oxygen evolution reaction (OER) is challenging for the storage of fluctuating renewable energies using polymer electrolyte membrane water electrolyzers (PEMWEs). Investigations are commonly conducted in so-called half-cell setups and different OER-related dissolution pathways have been proposed. However, the orders of magnitude difference in dissolution rate between half-cells and PEMWE using membrane electrode assemblies (MEA) is not well understood. In this work, the charge-related absorption affinity of Iridium (Ir) dissolution products, from both half-cell and MEA setups, is investigated, using cation and anion exchange materials. In the half-cell, a roughly constant ratio of cationic to anionic dissolution species is indicative of a single, dominant OER-related Ir dissolution pathway. While Ir dissolved in half-cells is mainly cationic, the Ir species from the MEA appear mainly in anionic form. This can be explained by the transport conditions of different Ir ions inside the catalyst layer, influenced by their ionomer absorption affinity and the migration driving force. Based on this understanding, key influences of electrocatalytic stability of MEAs, the effect of confinement of dissolved Ir species and the stability discrepancy to half-cells are discussed.

Quantification of Iridium Dissolution at Water Electrolysis Relevant Conditions Using a Gas Diffusion Electrode Half-Cell Setup

“…26 Since we do not observe such a dependence, we assume that either the measured Ir concentration does not have a stabilizing effect, or the true Ir concentration inside the CL is similarly high for both current densities but not adequately reflected by the Ir concentration in the liquid electrolyte, or both concentration and potential differences must be considered simultaneously. A higher Ir concentration in the CL is supported by absorption measurements (Supporting Information, Section S8) and in line with previously modeled Ir concentrations 21 in the anodic ionomer of up to ∼25 mg Ir L −1 . The high observed and modeled Ir concentrations in the ionomer are also in line with the apparent absence of this stabilization at low Ir concentrations.…”

“…The high observed and modeled Ir concentrations in the ionomer are also in line with the apparent absence of this stabilization at low Ir concentrations. 16 On a side note, if the concentration in the anodic ionomer was as high as calculated by Dam et al, 21 due to the small ionomer volume, it does not significantly contribute to the total amount of dissolved Ir, see Supporting Information, Section S9.…”

“…Hence, the stability increases with increasing pH, as shown for AMSs and single-cell electrolyzers. 16 The crossover of perchlorates in the GDE setup could also induce lower concentrations of Ir ions in the ionomer (one model indicates up to ∼25 mg Ir L −121 ) and membrane, e.g., by complexation 21,33 and as shown by absorption experiments (Supporting Information, Section S8). A lower Ir ion concentration would lead to a Nernst shift facilitating dissolution 26 while simultaneously reducing Ir redeposition.…”

“…A lower Ir ion concentration would lead to a Nernst shift facilitating dissolution 26 while simultaneously reducing Ir redeposition. 21 Additionally, the samples for the measurements in the GDE setup were prepared as PTEs, in contrast to the CCMs investigated in the single-cell electrolyzers. The inhomogeneous catalyst distribution observed for PTEs 44,45 might lead to heterogeneities in the local Ir concentration, thus enhancing dissolution.…”

“…17 In real industrial devices, this degradation is typically masked by the ∼40 times higher loading of conventional anodic CLs and emphasizes the importance of understanding dissolution in electrolyzer systems with ultralow Ir loadings. 20 A thorough evaluation of the distribution of Ir 19 showed that low amounts of mainly anionic Ir species are found in the anodic water cycle, 21 while more than 90% of the dissolved Ir remains inside the CCM. The applied current density and, thus, voltage 18 influence the degradation of the anodic CL.…”