A mild screen-printing method was developed to coat conductive oxide surfaces (here: fluorine-doped tin oxide) with micrometer-thick layers of presynthesized calcium manganese oxide (Ca-birnessite) particles. After optimization steps concerning the printing process and layer thickness, electrodes were obtained that could be used as corrosion-stable water-oxidizing anodes at pH 7 to yield current densities of 1 mA cm(-2) at an overpotential of less than 500 mV. Analyses of the electrode coatings of optimal thickness (≈10 μm) indicated that composition, oxide phase, and morphology of the synthetic Ca-birnessite particles were hardly affected by the screen-printing procedure. However, a more detailed analysis by X-ray absorption spectroscopy revealed small modifications of both the Mn redox state and the structure at the atomic level, which could affect functional properties such as proton conductivity. Furthermore, the versatile new screen-printing method was used for a comparative study of various transition-metal oxides concerning electrochemical water oxidation under "artificial leaf conditions" (neutral pH, fairly low overpotential and current density), for which a general activity ranking of RuO2 >Co3 O4 ≈(Ca)MnOx ≈NiO was observed. Within the group of screened manganese oxides, Ca-birnessite performed better than "Mn-only materials" such as Mn2 O3 and MnO2 .

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Physical vapour deposition of Mn metal followed by annealing in air is a promising route to prepare MnOx-anodes for water-oxidation.

Calcium manganese oxide films were prepared by cosputter deposition from Mn and CaMnO targets and evaluated for their suitability as catalysts for the oxygen evolution reaction (OER). Scanning electron microscopy (SEM) revealed a compact morphology for the as-deposited films and the formation of nanorodlike features on the surfaces after annealing at 600 °C. X-ray-photoelectron-spectroscopy analysis showed that the surface oxidation state is close to +III (as in MnO) for the as-deposited films and increases slightly to a mixture of III and IV after annealing occurs in dry air at 400-600 °C. Glancing-incidence X-ray diffraction (GIXRD) suggested that the CaMnO films are amorphous even when heated to 600 °C. However, transmission electron microscopy (TEM) showed that there is actually a polycrystalline component of the film, which best matches MnO (hausmannite with the average Mn oxidation state of ∼+2.7) but may have a slightly expanded unit cell because of the incorporation of Ca. Electrochemical analyses revealed that the as-deposited CaMnO films were OER-inactive. In contrast, annealing at 400 or 600 °C resulted in an increase of ∼15-fold in the current densities, which reached j ≅ 1.5 mA·cm at OER overpotentials of η ≈ 550 mV in cyclic voltammetry (CV) sweeps. For the same η, annealed CaMnO electrodes also showed good electrochemical stabilities during 2 h of electrolysis, as rather constant steady-state current densities of j ≅ 0.4-0.5 mA·cm were observed. The thicknesses and surface morphologies of the CaMnO films did not change during the electrochemical measurements, indicating that corrosion was negligible. In comparison with a previous study in which Ca-free thin layers of MnO were evaluated, the results demonstrate that Ca incorporation can enhance the OER activity of MnO electrocatalysts prepared by sputter deposition. This work provides guidance for designing new electrodes for water oxidation on the basis of the abundant and nontoxic elements manganese and calcium.

Hydrogen production by electrochemical water splitting is limited by the sluggish oxygen evolution reaction (OER). In order to improve our understanding of the underlying mechanisms, information about the atomic surface structure of the active state of the electrode is required. Here, we present environmental transmission electron microscopy studies of Ca-birnessite (K0.20Ca0.21MnO2.21·1.4H2O) electrodes under conditions close to those of the OER. Remarkably, in H2O vapor, a highly dynamic state of the surface and subsurface develops with a thickness of the formed dynamic layer of up to 0.6 nm, which is absent in O2 and inert gases. Electron beam-induced effects are carefully studied, showing high stability of the material against radiation damage in high vacuum until a dose rate of 42,000 e–/(Å2 s). In contrast, in H2O, the dynamic surface layer develops and forms a stationary state even at low dose rates, down to 5000 e–/(Å2 s). Electron energy-loss spectroscopy reveals an increase in the Mn oxidation state in H2O and in O2 ambient. Our results are interpreted as the formation of a few-angstrom-thick, dynamic, and hydrated surface layer of birnessite in H2O, with an increased Mn valence state. Such a dynamic surface layer with a flexible Mn coordination and valence state might be optimal for oxygen evolution due to the higher effective interaction volume beyond the surface area and a flexible bond coordination of partially hydrated Mn species.