Manganese-based systems are considered as candidate electrocatalysts for the electrochemical oxygen evolution reaction (OER), because of their abundance in biochemical oxygen producing catalyst systems. In this work, the surface of metallic manganese was investigated in situ and operando in potentiodynamic cyclic voltammetry (CV) experiments and potentiostatic chronoamperometry (CA) experiments in NaOH. In both cases, the surfaces were initially reduced. At corresponding potentials, no oxide species can be detected by Raman spectroscopy, though electrochemical data and the absence of dissolution above the reversible potential for reactions of type Mn → Mn^II indicate that the material is passive. The CV shows anodic peaks at potentials in line with expectations on the basis of thermodynamic data for the oxidation to Mn₃O₄ and Mn₂O₃; the thickness of the surface layer increases by a few nm during these peaks, as evidenced by spectroscopic ellipsometry. Dissolution of Mn as evidenced by downstream electrolyte analysis by inductively coupled plasma mass spectrometry in a scanning flow cell (SFC-ICP-MS) of the electrolyte is negligible in the range of electrode potential vs. Ag|AgCl|3 M KCl, E_Ag|AgCl, up to 0.3 V. Remarkably, Raman spectra already show the occurrence of α-MnO₂ at E_Ag|AgCl > −0.25 V, which is ca. 0.5 V below the potential at which oxidation to MnO₂ is expected. This observation is attributed to disproportionation above a certain level of Mn^III. For E_Ag|AgCl > 0.4 V, dissolution sets in, at a constant layer thickness. Above the onset potential of the OER, at E_Ag|AgCl ≈ 0.6 V, SFC-ICP-MS analysis shows fast dissolution, and the oxide layer thickness is constant or increases. CA experiments during the OER show strong dissolution, and the re-formation of a strongly disordered, β-MnO₂-like oxide, which exists in a quasi-stationary state at the interface. Several CV cycles increase the dissolution per cycle and the fraction of α-MnO₂ on the surface which cannot be reduced. The high dissolution currents show that metallic Mn is hardly suitable as an OER catalyst, however, at least the Mn^IV oxides remain stationarily present in the system.

High entropy oxides (HEOs) offer great potential as catalysts for oxygen electrocatalytic reactions in alkaline environments. Herein, a novel synthesis approach to prepare (FeCrCoMnZn)₃O_4‐δ high entropy spinel oxide in a vacuum atmosphere, with the primary objective of introducing oxygen vacancies into the crystal structure, is presented. As compared to the air‐synthesized counterpart, the (FeCrCoMnZn)₃O_4‐δ with abundant oxygen vacancies demonstrates a low (better) bifunctional (BI) index of 0.89 V in alkaline media, indicating enhanced electrocatalytic oxygen catalytic activity. Importantly, (FeCrCoMnZn)₃O_4‐δ demonstrates outstanding long‐term electrochemical and structural stability. When utilized as electrocatalysts in the air cathode of Zn‐air batteries, the vacuum atmosphere synthesized (FeCrCoMnZn)₃O_4‐δ catalysts outperform the samples treated in an air atmosphere, displaying superior peak power density, specific capacity, and cycling stability. These findings provide compelling evidence that manipulating the synthesis atmosphere of multi‐component oxides can serve as a novel approach to tailor their electrochemical performance.

Perovskite oxides have been considered promising oxygen evolution reaction (OER) electrocatalysts due to their high intrinsic activity. Yet, their poor long-term electrochemical and structural stability is still controversial. In this work, we apply an A-site management strategy to tune the activity and stability of a new hexagonal double perovskite oxide. We synthesized the previously inaccessible 2H-Ba₂CoMnO_6-δ (BCM) perovskite oxide via the universal sol-gel method followed by a novel air-quench method. The new 2H-BCM perovskite oxide exhibits outstanding OER activity with an overpotential of 288 mV at 10 mA cm^-2 and excellent long-term stability without segregation or structural change. To understand the origin of outstanding OER performance of BCM, we substitute divalent Ba with trivalent La at the A-site and investigate crystal and electronic structure change. Fermi level and valence band analysis presents a decline in the work function with the Ba amount, suggesting a structure-oxygen vacancy-work function-activity relationship for Ba _x La_2-x CoMnO_6-δ (x = 0, 0.5, 1, 1.5, 2) electrocatalysts. Our work suggests a novel production strategy to explore the single-phase new structures and develop enhanced OER catalysts.