| Summary: | Chromium-oxyhydroxide (Cr<i><sub>x</sub></i>O<i><sub>y</sub></i>H<i><sub>z</sub></i>)-based thin films have previously been shown in photocatalysis and industrial chlorate production to prevent unwanted reduction reactions to occur, thereby enhancing the selectivity for hydrogen evolution and thus the overall process efficiency. Here, a highly reproducible synthesis protocol was developed to allow for the electrodeposition of Cr<i><sub>x</sub></i>O<i><sub>y</sub></i>H<i><sub>z</sub></i>-based thin films with controlled thickness in the range of the sub-monolayer up to (>4) multilayer coverage. Electrodeposited Cr<i><sub>x</sub></i>O<i><sub>y</sub></i>H<i><sub>z</sub></i> coatings were electrochemically characterized using voltammetry and stripping experiments, allowing thickness-dependent film selectivity to be deduced in detail. The results are discussed in terms of mass transport properties and structure of the electrodeposited chromium oxyhydroxide films. It is shown that the permeation of diatomic probe molecules, such as O<sub>2</sub> and CO, was significantly reduced by films as thin as four monolayers. Importantly, it is shown that the prepared thin film coatings enabled prolonged hydrogen oxidation in the presence of CO (up to 5 vol.%), demonstrating the benefits of thin-film-protected electrocatalysts. In general, this study provides insight into the synthesis and use of thin-film-protected electrodes leading to improvements in (electro)catalyst selectivity and durability.
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