  Manganese oxides have received much attention over the years among the wide
range of electrocatalysts for the oxygen evolution reaction (OER) due to their
low toxicity, high abundance and rich redox chemistry. While many previous
studies focused on the activity of these materials, a better understanding of
the material transformations relating to activation or degradation is highly
desirable, both from a scientific perspective and for applications. We
electrodeposited Na-containing MnOx without long-range order from an alkaline
solution to investigate these aspects by cyclic voltammetry, scanning electron
microscopy and x-ray absorption spectroscopy at the Mn-K and Mn-L edges. The
pristine film was assigned to a layered edge-sharing Mn3+/4+ oxide with Mn-O
bond lengths of mainly 1.87 {\AA} and some at 2.30 {\AA} as well as Mn-Mn bond
lengths of 2.87 {\AA} based on fits to the extended x-ray fine structure. The
decrease of the currents at voltages before the onset of the OER followed power
laws with three different exponents depending on the number of cycles and the
Tafel slope decreases from 186 \pm 48 to 114 \pm 18 mV dec-1 after 100 cycles,
which we interpret in the context of surface coverage with unreacted
intermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edge
showed no change of the microstructure, bulk local structure or bulk Mn
valence. Yet, the surface region of MnOx oxidized toward Mn4+, which explains
the reduction of the currents in agreement with literature. Surprisingly, we
find that MnOx reactivates after 30 min at open-circuit (OC), where the
currents and also the Tafel slope increase. Reactivation processes during OC
are crucial because OC is unavoidable when coupling the electrocatalysts to
intermittent power sources such as solar energy for sustainable energy
production.
