Different manganese oxide phases were prepared as thin films to elucidate their structure–function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnO x films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were tested in neutral and alkaline electrolytes. Differential electrochemical mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of crystalline α-Mn2O3 films was determined to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnO x films heated at 573 K (MnO x -573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnO x -573 K films was identified at η = 230 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE (1 M KOH). The least active of the investigated manganese oxides was Mn3O4 with an onset at η = 290 mVRHE (at J = 0.1 mA/cm2) and current densities of ca. 10 mA/cm2 at η = 570 mVRHE (1 M KOH). In neutral solution (1 M KPi), a similar tendency was observed with the lowest overpotential found for α-Mn2O3 followed by MnO x -573 K and Mn3O4. X-ray photoelectron spectroscopy revealed that after electrochemical treatment, the surfaces of the manganese oxide electrodes exhibited oxidation of Mn II and Mn III toward Mn IV under oxygen evolving conditions. In the case of α-Mn2O3 and MnO x -573 K, the manganese oxidation was found to be reversible in KPi when switching the potential above and below the oxygen evolution reaction (OER) threshold potential. Furthermore, scanning electron microscopy (SEM) images displayed the presence of an amorphous phase on top of all manganese oxide films here tested after oxygen evolution. The results indicate that structural changes played an important role in the catalytic activity of the manganese oxides, in addition to oxidation states, a large variety of Mn–O bond lengths and a high concentration of oxygen point defects. Thus, compared to Mn3O4, crystalline α-Mn2O3 and MnO x -573 K are the most efficient catalyst for water oxidation in the manganese–oxygen system.

Atomic layer deposition was used to coat CdS photoanodes with 7 nm thick TiO2films to protect them from photocorrosion during photoelectrochemical water splitting. Photoelectrochemical measurements indicate that the TiO2coating does not provide full protection against photocorrosion. The degradation of the film initiates from small pinholes and shows oscillatory behavior that can be explained by an Avrami-type model for photocorrosion that is halfway between 2D and 3D etching. XPS analysis of corroded films indicates that a thin layer of CdS remains present on the surface of the corroded photoanode that is more resilient towards photocorrosion.

This contribution shows the potential of cold gas spraying for the production of photoelectrodes employing photoelectrocatalysts for the water oxidation reaction. Conventional methods of coating usually employ sol-gel methods and calcination to obtain a good binding of the coating to the substrate. In cold gas spraying, particles are accelerated to high velocities by a pressurized gas. Nitrogen is used as process gas, preheated and then expanded in a De Laval type nozzle. On impact with the substrate the particles deform, break up and build an efficient interface to the back contact (as revealed, for example, by scanning electron microscopy). Cold gas spraying is a method for the direct bonding of particles to a substrate and does not require additives that have to be removed e.g. by a calcination step. Thereby it allows the direct fabrication of a working electrode ensemble.In our initial experiments, the state-of-the-art photocatalyst titanium dioxide (TiO 2 ) was explored. The cold-gas-sprayed coatings revealed significantly higher activities for the oxygen evolution reaction (OER), as compared to films derived from wet-chemical processes. Due to the demand for photocatalysts with band gap suitable for visible light absorption, this approach was extended to the promising catalyst material hematite. In correlation with photoelectrochemical measurements, the operating parameters of the cold gas spray process are discussed in terms of their influence on the photocatalytic properties of the semiconductor.

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In the effort to develop an efficient and cost effective photoelectrochemical device for water splitting driven by sunlight only, transition metal oxides are promising candidates to catalyze the oxygen evolution reaction (OER) at the anode. We used X-ray photoelectron spectroscopy (XPS) to characterize very active manganese and cobalt oxide thin films deposited on FTO substrates before and after the application of different anodic potentials, in order to investigate the bias potential dependent changes on the catalysts’ surfaces. α-Mn2O3 undergoes a reversible partial oxidation from Mn3+ to Mn4+ under high anodic potentials, while the transition from Co2+ to Co3+ in amorphous CoOx samples occurs already at a potential well below the OER onset potential. This Co3+ state is then stable throughout the investigated potential range and no clear evidence for a Co4+ state at or above the OER onset potential could be found. We conclude that the OER reaction mechanism on the surface of these oxide films might be significantly different.