Time-of-Flight Secondary Ion Mass Spectrometry for Characterization of Ir-Based Catalysts for PEM Water Electrolyzers

Abstract Iridium-based catalysts remain essential for enabling durable oxygen evolution in proton-exchange membrane water electrolyzers (PEMWEs), yet their surface and subsurface chemistries remain insufficiently resolved by conventional characterization methods. Here, we establish time-of-flight secondary ion mass spectrometry (ToF-SIMS), combined with multivariate analysis, as a framework for deconvoluting chemical-, structural-, and depth-dependent features across commercial IrOx powders spanning amorphous and rutile phases. Static and Ar cluster-sputtered ToF-SIMS spectra reveal distinct surface hydroxylation, oxide fragment distributions, and subsurface chemical signatures that differentiate amorphous and rutile IrOx and expose synthesis-dependent residues. Across all powders, surface regions are enriched in hydroxylated IrOx fragments, while subsurface regions show increased reduced Ir species and carbon-based ions. ToF-SIMS fragments are correlated with X-ray photoelectron spectroscopy (XPS) extracted parameters, providing complementary surface chemical information that supports species identification. Finally, integrating ToF-SIMS and XPS descriptors with electrochemical and structural metrics yields a chemically resolved picture linking hydration depth, oxidation state distribution, and structural order to catalytic behavior. This multi-technique, multivariate framework provides insight into the chemical features governing Ir-based OER performance and supports the rational design of robust, efficient catalysts.