Toward Resolving the RuO2/IrO2 Oxygen Evolution Activity Puzzle: Critical Roles of Spin Polarization and Exchange-Correlation Functional Accuracy

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The oxygen evolution reaction (OER) activity ordering of RuO2 and IrO2 is well established experimentally, yet density functional theory calculations have persistently failed to reproduce this trend, undermining computational catalyst design. Here, we investigate this discrepancy by evaluating OER thermodynamic and kinetic activities of IrO2 and RuO2 using four exchange-correlation (XC) functionals (PBE, RPBE, r2SCAN, and MS2-rVV10) under spin-polarized and spin-unpolarized conditions. Spin polarization selectively stabilizes the O* intermediate on IrO2 while leaving RuO2 comparatively unaffected, and neglecting spin effects yields unphysically low kinetic barriers for IrO2. Among the four functionals, only MS2-rVV10 reproduces the experimentally observed thermodynamic activity ordering; however, all functionals fail to capture the correct kinetic barrier ordering. We trace these failures to a systematic correlation between adsorption-energy errors and deviations in the predicted activity trend. Notably, the OOH*-OH* scaling offset and theoretical limiting potential are themselves XC-functional dependent, cautioning against cross-functional comparison of overpotentials. Cross-code benchmarks rule out numerical implementation as the error source, while higher-rung functionals progressively overestimate Ru local magnetic moments in RuO2, introducing additional errors. This work provides a diagnostic framework for developing XC functionals that simultaneously capture surface reaction energetics and local magnetic properties for reliable OER catalyst screening.

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