Evaporated manganese films as a starting point for the preparation of thin-layer MnO x water-oxidation anodes

A novel method to prepare anodes for water electrolysis cells has been developed, which starts from layers of elemental manganese deposited by physical vapour deposition (PVD) on indium-doped tin oxide (ITO). Oxidation in dry air at 300 °C transforms this metallic Mn layer into a manganese(II)-rich MnOx coating (x = 1–1.3), which also contains a buried layer of an In–Sn alloy originating from reactions with the ITO support. The MnOx films are well connected to the underlying substrate and act as efficient catalysts for water-oxidation catalysis (WOC) at neutral pH. Detailed post-operando analyses using XRD, SEM, TEM and XAS revealed that the dense MnO/Mn3O4 film is virtually not affected by 2 h of electrochemical WOC at E ≈ +1.8 V vs. RHE, corresponding well to the observed good stability of catalytic currents, which is unusual for such thin layers of a MnOx catalyst. The current densities during electrolyses are so far low (i ≈ 50–100 μA cm−2 at pH 7), but optimization of the preparation process may allow for significant improvements. This new, rather easy, and adaptable preparation method for stable, thin-layer MnOx water-oxidation anodes could thus prove to be very useful for a variety of applications

Water Oxidation Catalysis by Birnessite@Iron Oxide Core–Shell Nanocomposites

In this work, magnetic nanocomposite particles were prepared for water oxidation reactions. The studied catalysts consist of maghemite (γ-Fe₂O₃), magnetite (Fe₃O₄), and manganese ferrite (MnFe₂O₄) nanoparticles as cores coated in situ with birnessite-type manganese oxide shells and were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermal, chemical, and surface analyses, and magnetic measurements. The particles were found to be of nearly spherical core–shell architectures with average diameter of 150 nm. Water oxidation catalysis was examined using Ce⁴⁺ as the sacrificial oxidant. All core–shell particles were found to be active water oxidation catalysts. However, the activity was found to depend on a variety of factors like the type of iron oxide core, the structure and composition of the shell, the coating characteristics, and the surface properties. Catalysts containing magnetite and manganese ferrite as core materials displayed higher catalytic activities per manganese ion (2650 or 3150 mmol_O₂ mol_Mn^–1 h^–1) or per mass than nanoiron oxides (no activity) or birnessite alone (1850 mmol_O₂ mol_Mn^–1 h^–1). This indicates synergistic effects between the MnO_<i>x</i> shell and the FeO_<i>x</i> core of the composites and proves the potential of the presented core–shell approach for further catalyst optimization. Additionally, the FeO_<i>x</i> cores of the particles allow magnetic recovery of the catalyst and might also be beneficial for applications in water-oxidizing anodes because the incorporation of iron might enhance the overall conductivity of the material