The importance of the hydrophobic treatment in our system was evident when a comparison was made between SVG-PC and a sheet consisting of PC deposited on a bare SVG (denoted untreated SVG-PC) as shown in Fig. . The seawater splitting rate obtained with SVG-PC for was ~2.8 times greater than that of untreated SVG-PC. These results imply that without the hydrophobic treatment, some harmful inorganic ions such as Cl -and Mg 2+ may still be able to reach the PC, allowing side reactions, leading to a lower photocatalytic rate compared to using pure water as a feedstock (Supplementary Tables ). Note that overly silanized SVG (>2.5 vol%) may inhibit water evaporation by the SVG and have an adverse effect on the H2 evolution rate (Supplementary Fig. ). ICP-OES analysis of the condensed desalinated seawater, collected from the reactor wall after each experiment, showed that the concentrations of Na, K, S and Si were 9.11, 0.09, 0.14, and 9.41 mg L -1 , respectively, while the concentrations of Ca and Mg were below the detectable threshold (Fig. , Supplementary Table ). These numbers are well below the salinity standards of drinking water set by World Health Organization (WHO) and the US Environmental Protection Agency (EPA). Our design yields one of the highest activities among photocatalytic seawater splitting systems while also providing purified water as a secondary product (Supplementary Table ).