Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn₄CaO₅ cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn₃O₄ nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm⁻¹ and assigned to the active Mn^IV=O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal Mn^IV=O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.