In Situ L-Edge XAS Study of a Manganese Oxide Water Oxidation Catalyst

A new emerging water oxidation catalyst (WOC) generated via electrodeposition from aqueous solutions containing methylphosphate (MePi) and Mn2+ has been studied by in situ X-ray absorption spectroscopy (XAS) in transmission mode and in situ extended X-ray absorption fine-structure (EXAFS) in fluorescence mode. XA spectra were obtained for freshly prepared and activated MnPi films under different potentials and after different potential durations. Via linear combination fitting the contribution of different Mn species was revealed from the L-edge spectra. The XAS results show that the freshly prepared film at OCP contains a dominant contribution of MnO2 (∼75%) and a contribution from a birnessite-like material (∼25%). No or only a neglectable percentage of MnO, Mn3O4, or Mn2O3-like Mn species was found in the freshly prepared sample. After 51 min of in situ activation at 1.2 V at pH 7.0, the birnessite contribution increased to 75% in the spectrum. We correlate these changes to the material conversion into an efficient WOC. After activation, the film behavior is reversible in potentials above and below water oxidation regime. EXAFS results are found to be consistent with our L-edge spectra fitting results.