Nanoporous water oxidation electrodes with a low loading of laser-deposited Ru/C exhibit enhanced corrosion stability.

For the oxidation of water to dioxygen, oxide-covered ruthenium metal is known as the most efficient catalyst, however, with limited stability. Herein, we present a strategy for incorporating a Ru/C composite onto a novel nanoporous electrode surface with low noble metal loading and improved stability. The Ru/C is coated on the pore walls of anodic alumina templates in a one-step laser-induced deposition method from Ru3(CO)12 solutions. Scanning electron microscopy proves the presence of a continuous Ru/C layer along the inner pore walls. The amorphous material consists of metallic Ru incorporated in a carbonaceous C matrix as shown by X-ray diffraction combined with Raman and X-ray photoelectron spectroscopies. These porous electrodes reveal enhanced stability during water oxidation as compared to planar samples at pH 4. Finally, their electrocatalytic performance depends on the geometric parameters and is optimized with 13 µm pore length, which yields 2.6 mA cm-2, or 49 A g-1, at η = 0.20 V.

Direct oxygen isotope effect identifies the rate-determining step of electrocatalytic OER at an oxidic surface.

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.

Highly Reversible Water Oxidation at Ordered Nanoporous Iridium Electrodes Based on an Original Atomic Layer Deposition.

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.

Design Rules for Oxygen Evolution Catalysis at Porous Iron Oxide Electrodes: A 1000-Fold Current Density Increase.

Nanotubular iron(III) oxide electrodes are optimized for catalytic efficiency in the water oxidation reaction at neutral pH. The nanostructured electrodes are prepared from anodic alumina templates, which are coated with Fe2 O3 by atomic layer deposition. Scanning helium ion microscopy, X-ray diffraction, and Raman spectroscopy are used to characterize the morphologies and phases of samples submitted to various treatments. These methods demonstrate the contrasting effects of thermal annealing and electrochemical treatment. The electrochemical performances of the corresponding electrodes under dark conditions are quantified by steady-state electrolysis and electrochemical impedance spectroscopy. A rough and amorphous Fe2 O3 with phosphate incorporation is critical for the optimization of the water oxidation reaction. For the ideal pore length of 17 µm, the maximum catalytic turnover is reached with an effective current density of 140 µA cm-2 at an applied overpotential of 0.49 V.