We present different approaches to electrochemical in-situ measurements on an active catalyst for water splitting using synchrotron radiation. The main aim of our work is to identify the changes in the oxidation state of manganese under electrochemical conditions and at the same time elucidate the chemical role of oxygen.

In-situ obtained O K- and Mn L-edges were recorded in total electron yield (TEY) or partial fluorescence yield (PFY) where electrons and photons were detected, respectively. For fluorescence detection, a SiNx membrane was used as separating interface between 1 bar pressure at the electrolyte and vacuum on the spectrometer side. The studied MnOxelectrode was prepared by electrodeposition on the SiNx membrane covered with a thin gold layer.

The development of the manganese oxidation state was monitored under applied potential. It was established that as deposited manganese oxide contains mainly Mn⁺⁴. For lower applied potentials the lower oxidation states up to Mn⁺² were observed. At the potentials where oxygen evolution is expected the Mn L-edge typical for MnO₂ was obtained. The reduction followed by a re-oxidation led always to the same MnO₂ spectrum. The Birnessite layered kind of structure of the manganese oxide after the oxygen evolution reaction was confirmed by TEM studies.

A Nafion cell was used for more surface sensitive detection in TEY consisting of a flow cell with a Nafion proton exchange membrane (PEM) as a barrier between electrolyte (1 bar) and spectrometer (1 mbar). Birnessite, which is predominantly Mn⁺⁴, was drop-casted on the Nafion membrane with the catalyst layer facing the spectrometer side.

For anodic potential applied to the working electrode in KOH solution the Mn L-edges reveal that the oxidation state of manganese remains unchanged at Mn⁺⁴. The O K-edge spectra, however, change continuously with increasing potential. In particular, a resonance feature at 529 eV can be seen to increase with potential. This resonance is due to transitions into locally unoccupied O 2p states hybridized with the Mn spin-majority egand spin-minority t2gstates. The intensity increase is a direct measure of the growth of O 2p hole-character showing holes are introduced into the O 2p states hybridized with Mn egstates. This may lead to the conclusion that manganese oxide with increasing applied potential gains more covalent over ionic character.