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Manganese oxides have received much attention over the years among the widerange of electrocatalysts for the oxygen evolution reaction (OER) due to theirlow toxicity, high abundance and rich redox chemistry. While many previousstudies focused on the activity of these materials, a better understanding ofthe material transformations relating to activation or degradation is highlydesirable, both from a scientific perspective and for applications. Weelectrodeposited Na-containing MnOx without long-range order from an alkalinesolution to investigate these aspects by cyclic voltammetry, scanning electronmicroscopy and x-ray absorption spectroscopy at the Mn-K and Mn-L edges. Thepristine film was assigned to a layered edge-sharing Mn3+/4+ oxide with Mn-Obond lengths of mainly 1.87 {\AA} and some at 2.30 {\AA} as well as Mn-Mn bondlengths of 2.87 {\AA} based on fits to the extended x-ray fine structure. Thedecrease of the currents at voltages before the onset of the OER followed powerlaws with three different exponents depending on the number of cycles and theTafel 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 unreactedintermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edgeshowed no change of the microstructure, bulk local structure or bulk Mnvalence. Yet, the surface region of MnOx oxidized toward Mn4+, which explainsthe reduction of the currents in agreement with literature. Surprisingly, wefind that MnOx reactivates after 30 min at open-circuit (OC), where thecurrents and also the Tafel slope increase. Reactivation processes during OCare crucial because OC is unavoidable when coupling the electrocatalysts tointermittent power sources such as solar energy for sustainable energyproduction.

Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction