Beschreibung:
<jats:title>Abstract</jats:title><jats:p>WO<jats:sub>3</jats:sub>, an abundant transition metal semiconductor, is one of the most discussed materials to be used as a photoanode in photoelectrochemical water‐splitting devices. The photoelectrochemical properties, such as photoactivity and selectivity of WO<jats:sub>3</jats:sub> in different electrolytes, are already well understood. However, the understanding of stability, one of the most important properties for utilization in a commercial device, is still in the early stages. In this work, a photoelectrochemical scanning flow cell coupled to an inductively coupled plasma mass spectrometer is applied to determine the influence of co‐catalyst overlayers on photoanode stability. Spray‐coated WO<jats:sub>3</jats:sub> photoanodes are used as a model system. Iridium is applied to the electrodes by atomic layer deposition in controlled layer thickness, as determined by ellipsometry and x‐ray photoelectron spectroscopy. Photoactivity of the iridium‐modified WO<jats:sub>3</jats:sub> photoanodes decreases with increasing iridium layer thickness. Partial blocking of the WO<jats:sub>3</jats:sub> surface by iridium is proposed as the main cause of the decreased photoelectrochemical performance. On the other hand, the stability of WO<jats:sub>3</jats:sub> is notably increased even in the presence of the thinnest investigated iridium overlayer. Based on our findings, we provide a set of strategies to synthesize nanocomposite photoelectrodes simultaneously possessing high photoelectrochemical activity and photostability.</jats:p>