promising properties for redox applications, including gravimetric oxygen storage up to 4% by mass, high stability and rapid reversibility, with re-oxidation in less than 1 min at 400 C. Finally, the redox chemistry of Ca 0.8 Sr 0.2 MnO 3 was also investigated using in situ X-ray photoelectron spectroscopy and near-edge X-ray absorption measurements at near ambient pressure in oxygen atmospheres.

In regard to earth-abundant cobalt water oxidation catalysts, very recent findings show the reorganization of the materials to amorphous active phases under catalytic conditions. To further understand this concept, a unique cobalt-substituted crystalline zinc oxide (Co:ZnO) precatalyst has been synthesized by low-temperature solvolysis of molecular heterobimetallic Co(4-x)Zn(x) O4 (x = 1-3) precursors in benzylamine. Its electrophoretic deposition onto fluorinated tin oxide electrodes leads after oxidative conditioning to an amorphous self-supported water-oxidation electrocatalyst, which was observed by HR-TEM on FIB lamellas of the EPD layers. The Co-rich hydroxide-oxidic electrocatalyst performs at very low overpotentials (512 mV at pH 7; 330 mV at pH 12), while chronoamperometry shows a stable catalytic current over several hours.

The synthesis of monodisperse, surfactant‐free, Ni‐substituted ZnO nanocrystallites (ZnO:Ni) by the mild solvolysis of heterobimetallic Ni4−xZnxO4 cubane‐like precursors (x=1–3) in benzylamine is reported. Ni4−xZnxO4 was grafted by electrophoretic deposition onto fluorine‐doped tin oxide glass substrates and used as an active and stable working electrode for water oxidation. Upon the application of a voltage at the electrodes, the ZnO:Ni precatalyst leads to an active composite material that can oxidize water (>15 h) with an increasing catalytic current. In contrast, the performance of homometallic NiO reference materials decreases rapidly over time and is surpassed by the composite from the ZnO:Ni precatalyst in terms of both stability and activity. Extensive characterization of the as‐prepared and activated ZnO:Ni precatalyst by using hard X‐ray photoelectron spectroscopy revealed that the excellent performance of the electrode material is because of the formation of a unique self‐supported turbostratically disordered mixture of γ‐NiOOH/α‐Ni(OH)2‐like phases from the rapid dissolution of ZnII in the ZnO:Ni precatalyst into the electrolyte during activation.

Perovskite oxides with temperature and oxygen partial pressure dependent non‐stoichiometry δ, such as SrFeO3‐δ or its Cu‐doped variants, can be applied as redox materials for two‐step thermochemical processes, i. e. to reversibly store oxygen and thereby thermal energy, or separate air using concentrated solar power. We studied the redox state of Cu in SrFe1‐xCuxO3‐δ samples using in‐situ X‐ray photoelectron spectroscopy (XPS) and X‐ray absorption (XAS) measurements in oxygen atmospheres using synchrotron radiation, and characterized these materials through thermogravimetric analysis. By this means, we show how spectroscopic and thermogravimetric data are correlated, suggesting that Cu and Fe are reduced simultaneously for x=0.05, whereas the reduction of samples with x=0.15 is mainly driven by a change in the Fe oxidation state. Furthermore, we studied the re‐oxidation kinetics of reduced SrFe1‐xCuxO3‐δ, revealing very high reaction speeds with t1/2=13 min at 150 °C for SrFeO3‐δ. Our results indicate that SrFe1‐xCuxO3‐δ solid solutions can be applied for oxygen storage and air separation with high capacity at relatively low temperatures, which allows an efficient thermochemical process.

Novel thin film composite photocathodes based on device‐grade Cu(In,Ga)Se2 chalcopyrite thin film absorbers and transparent conductive oxide Pt‐implemented TiO2 layers on top are presented for an efficient and stable solar‐driven hydrogen evolution. Thin films of phase‐pure anatase TiO2 are implemented with varying Pt‐concentrations in order to optimize simultaneously i) conductivity of the films, ii) electrocatalytic activity, and iii) light‐guidance toward the chalcopyrite. Thereby, high incident‐photon‐to‐current‐efficiencies of more than 80% can be achieved over the full visible light range. In acidic electrolyte (pH 0.3), the most efficient Pt‐implemented TiO2–Cu(In,Ga)Se2 composite electrodes reveal i) photocurrent densities up to 38 mA cm−2 in the saturation region (−0.4 V RHE, reversible hydrogen electrode), ii) 15 mA cm−2 at the thermodynamic potential for H2‐evolution (0 V RHE), and iii) an anodic onset potential shift for the hydrogen evolution (+0.23 V RHE). It is shown that the gradual increase of the Pt‐concentration within the TiO2 layers passes through an efficiency‐ and stability‐maximum of the device (5 vol% of Pt precursor solution). At this maximum, optimized light‐incoupling into the device‐grade chalcopyrite light‐absorber as well as electron conductance properties within the surface layer are achieved while no degradation are observed over more than 24 h of operation.