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.

A conductive SnO2 layer and small quantities of IrO2 surface cocatalyst enhance the catalytic efficiency of nanoporous Fe2O3 electrodes in the oxygen evolution reaction at neutral pH. Anodic alumina templates are therefore coated with thin layers of SnO2, Fe2O3, and IrO2 by atomic layer deposition. In the first step, the Fe2O3 electrode is modified with a conductive SnO2 layer and submitted to different postdeposition thermal treatments in order to maximize its catalytic performance. The combination of steady‐state electrolysis, electrochemical impedance spectroscopy, X‐ray crystallography, and X‐ray photoelectron spectroscopy demonstrates that catalytic turnover and e− extraction are most efficient if both layers are amorphous in nature. In the second step, small quantities of IrO2 with extremely low iridium loading of 7.5 µg cm−2 are coated on the electrode surface. These electrodes reveal favorable long‐term stability over at least 15 h and achieve maximized steady‐state current densities of 0.57 ± 0.05 mA cm−2 at η = 0.38 V and pH 7 (1.36 ± 0.10 mA cm−2 at η = 0.48 V) in dark conditions. This architecture enables charge carrier separation and reduces the photoelectrochemical water oxidation onset by 300 mV with respect to pure Fe2O3 electrodes of identical geometry.

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We present the preparation and electrochemical analysis of a novel type of positive regenerative fuel cell electrode based on commercially available Ti felts with a Pt/Ir catalyst. Anodic oxidation of the Ti felts leads to the formation of a TiO2 nanotube layer. The high specific surface area allows for a particularly efficient utilization of the noble metal catalyst. Its loading in the nanoporous system is controlled accurately and minimized systematically by atomic layer deposition. The electrochemical activity towards water splitting of both metals is investigated in acidic media by cyclic voltammetry and steady‐state electrolysis for various catalyst loadings. An optimal oxygen evolution reaction is found for a catalyst loading of 76 μg cm−2 resulting in a mass activity of 345 A g−1 at η=0.47 V, whereas the simultaneous presence of Pt at the surface is demonstrated by X‐ray photoelectron spectroscopy and by the high activity observed for the hydrogen evolution reaction.

The development of stable, cost‐efficient and active materials is one of the main challenges in catalysis. The utilization of platinum in the electroreduction of oxygen is a salient example where the development of new material combinations has led to a drastic increase in specific activity compared to bare platinum. These material classes comprise nanostructured thin films, platinum alloys, shape‐controlled nanostructures and core–shell architectures. Excessive platinum substitution, however, leads to structural and catalytic instabilities. Herein, we introduce a catalyst concept that comprises the use of an atomically thin platinum film deposited on a potential‐triggered passivating support. The model catalyst exhibits an equal specific activity with higher atom utilization compared to bulk platinum. By using potential‐triggered passivation of titanium carbide, irregularities in the Pt film heal out via the formation of insoluble oxide species at the solid/liquid interface. The adaptation of the described catalyst design to the nanoscale and to high‐surface‐area structures highlight the potential for stable, passivating catalyst systems for various electrocatalytic reactions such as the oxygen reduction reaction.