In Situ Activity-Durability Measurements of Cobalt during O ₂ /H ₂ Electrocatalysis: pH/Potential-Dependent Surface Transformations

Cobalt (Co) is a critical material often employed for oxygen and hydrogen electrocatalysis in neutral and alkaline media, yet durability remains a major challenge. While Pourbaix diagrams predict potential- and pH-dependent phase transformations, they do not capture the transformation kinetics or stability of in situ phase transformations. Herein, we apply on-line inductively coupled plasma mass spectrometry (ICP-MS), ex situ X-ray photoelectron spectroscopy (XPS), and in situ Raman spectroscopy to track and quantify Co phase evolution, dissolution, and activity across pH values 7−13 at electrocatalytically relevant potentials. On-line ICP-MS data show that dissolution decreases with increasing pH and, across all pH values, is moderately less near 0.8 V RHE than 0.3 V RHE and becomes heavily suppressed at ≥ 1.3 V RHE . Based on XPS and Raman data, the enhanced stability at higher potentials likely arises from in situ formation of Co 3 O 4 and CoOOH. Altogether, these results link phase transformations to stability beyond thermodynamic predictions, informing the design of more robust Co-based electrocatalysts.