Arrays of hyperbranched high aspect ratio mesostructures are grown by pulsed laser deposition, through a self-assembly process from the gas phase. These hierarchical arrays are used successfully as photoanodes for the photoelectrochemical splitting of water, exhibit ing an onset potential as low as 0.4 V vs RHE and saturation current densities up to 1.8 5 mA/cm 2 at 0.8 V vs RHE. While the latter is similar to the state of the art values for WO 3 nanostructured photoanodes, both the onset and saturation voltages are significantly lower than any other report so far. This peculiar behavior is attributed to the hyperbranched structure and to its excellent optical and electronic properties.
A typical EC device presents a sandwich architecture in which an ionic conductor is embedded between two EC electrodes (or between an EC electrode and a transparent conductor). Among the most widely studied EC materials, bipyridilium salts (known as viologen), conducting polymers (e.g., polythiophene, polyaniline), and inorganic metal oxides (WO 3 , IrO 2 , NiO) deserve to be mentioned. [ 5 ] Tungsten oxide is probably the most popular one: it can be reversibly switched between a colorless and a blue state by alternately applying a small positive and negative voltage. WO 3 popularity stems from its strong color contrast with a relatively low production cost. It can be subjected to optical transitions both in the near-infrared and visible region of the light spectrum, fi ltering thus an important part of the solar light and potentially reducing air-conditioning costs in buildings (thus justifying its application in intelligent windows).Optical modulation is generally obtained by reversible electrochemical double injection of small alkali metal ions (such as H + , Li + , Na + ) and electrons into the host lattice, with positiveion insertion required to satisfy charge neutrality. [ 6,7 ] Ion insertion kinetics is limited by both the diffusion coeffi cient and the diffusion length. While the former depends on the crystal structure of the metal oxide, the latter can be governed by the device and the electrode architectures. [ 8,9 ] For this reason some efforts
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