Understanding the mechanism of water oxidation to dioxygen represents the bottleneck towards the design of efficient energy storage schemes based on water splitting. The investigation of kinetic isotope effects has long been established for mechanistic studies of various such reactions. However, so far natural isotope abundance determination of O2 produced at solid electrode surfaces has not been applied. Here, we demonstrate that such measurements are possible. Moreover, they are experimentally simple and sufficiently accurate to observe significant effects. Our measured kinetic isotope effects depend strongly on the electrode material and on the applied electrode potential. They suggest that in the case of iron oxide as the electrode material, the oxygen evolution reaction occurs via a rate-determining O−O bond formation via nucleophilic water attack on a ferryl unit.

Nanoporous iridium electrodes are prepared and electrochemically investigated towards the water oxidation (oxygen evolution) reaction. The preparation is based on ‘anodic’ aluminum oxide templates, which provide straight, cylindrical nanopores. Their walls are coated using atomic layer deposition (ALD) with a newly developed reaction which results in a metallic iridium layer. The ALD film growth is quantified by spectroscopic ellipsometry and X‐ray reflectometry. The morphology and composition of the electrodes are characterized by scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, and X‐ray diffraction. Their catalytic activity is quantified for various pore geometries by cyclic voltammetry, steady‐state electrolysis, and electrochemical impedance spectroscopy. With an optimal pore length of L≈17–20 μm, we achieve current densities of J=0.28 mA cm−2 at pH 5 and J=2.4 mA cm−2 at pH 1. This platform is particularly competitive for achieving moderate current densities at very low overpotentials, that is, for a high degree of reversibility in energy storage.

Nanostructured Ir/TiO 2 electrodes are investigated toward the oxygen evolution reaction (OER) from water. The electrodes are prepared based on highly ordered TiO 2 nanotubes grown from Ti foils with full geometric control. The tube walls are coated with iridium using atomic layer deposition (ALD), which allows for an accurate tuning of the amount deposited. The electrocatalytic performance of electrodes with different TiO 2 tube lengths and iridium catalyst loadings toward OER is quantified by cyclic voltammetry and steady-state electrolysis. This study enables us to minimize the catalyst loading, and we reach a current density of 31.3 mA cm −2 at an overpotential η = 0.34 V for a tube length of L = 12 μm and a Ir coating thickness of t = 6 nm. The benchmark of 10 mA cm −2 is already achieved at a lower overpotential of η = 0.24 V.

A systematic study into the effects of metal substitution on the visible-light photocatalytic activity of prototype metal oxide cluster anions is presented. When comparing the reactivity under aerated vs. de-aerated conditions, it was found that molybdate-based clusters show significantly increased reaction rates in the absence of oxygen; in contrast, marginally reduced reaction rates were observed for the tungstate-based species under de-aerated conditions. Wavelength-dependent quantum efficiency studies provide insight into the visible-light reactivity of all four species. Radical scavenging experiments suggest that the photocatalysis proceeds via formation of hydroxyl radicals. Cluster recycling studies demonstrate the robust nature of the homogeneous photocatalysts.