On average, natural seawater contains a salinity of 3.1-3.8% and a pH of 7.5-8.4. It contains various ions such as Na + , K + , Mg 2+ , Ca 2+ , and Cl -with NaCl being the predominant component. Thus far, seawater has rarely been used for solar fuels synthesis using photovoltaic-electrolyzer, photoelectrochemical or photocatalytic devices due to the reduced performance as a result of catalyst poisoning and possible side reactions (Supplementary Table ). For liquid-phase seawater splitting using a RhCrOx-Al:SrTiO3 powder suspension or RhCrOx-Al:SrTiO3 immobilized on glass substrate (Glass-PC), we observed that the H2 evolution rate decreased more than 4 times and a significant drop in the O2 evolution rate was observed compared to pure water splitting (Fig. , Supplementary Fig. , Supplementary Tables ). This suggests that the performance of RhCrOx-Al:SrTiO3 is strongly inhibited when the particles are in a direct contact with seawater. The reduction in H2 evolution may be attributed to the poisoning of the hydrogen evolution catalyst from the direct contact with impurities present in seawater. The observed non-stoichiometric H2:O2 ratio implies that other oxidation reactions also occur. For example, the oxidation of Cl -(Cl -/Cl2, 1.36 V s NHE at pH 0) in seawater might be more favourable because its two-electron oxidation process is kinetically easier than the four-electron oxidation of water. While there already exists a physical separation between the PC layer and feedstock via the SVG (Fig. ), the SVG substrate was also hydrophobically treated prior to PC deposition to further repel any contaminants dissolved in the bulk solution that might permeate through the SVG top layer to the PC layer.