The catalyst for this solar-powered hydrolysis comes in the shape of molybdenum sulfides that could readily be mixed to the titanium oxide particles typically used in white paint.
In a recent paper titled "Surface Water Dependent Properties of Sulfur-Rich Molybdenum Sulfides: Electrolyteless Gas Phase Water Splitting" published in the ACS Nano journal, the researchers report that sulfur-rich MoSx (x = 32/3) is a highly hygroscopic semiconductor which can reversibly bind up to 0.9 H2O molecule per Mo. On that basis, they developed an electrolyteless water splitting photocatalyst (formulated as an ink) that relies entirely on the hygroscopic nature of MoSx as the water source, and which could be coated onto insulating substrates, such as glass, to obtain hydrogen and oxygen from water vapour.
Sharing their story on the RMIT University's news feed, the researchers were keen to put the emphasis on the potential for cheap paint-based hydrogen fuel production, though collecting the useful gas would not be as simple as spraying the paint on a brick wall, as most media reported.
So this got me thinking. To design a practical hydrogen harvesting solution based on this type of paint, you'd need a way to concentrate the generated hydrogen and store it away (from oxygen among other things to prevent counterproductive recombination).
Necessarily, that would mean using some sort of encapsulation, maybe enclosing the water splitting paint within a double-glazed panel. But then, encapsulation means no gas renewal, which defeats the whole concept. As for hydrogen collection, I imagine some specifically designed H2 adsorbing materials could store it and make it available for a purpose-made integrated fuel cell.
I sent out my questions to lead author Dr Torben Daeneke to understand where the research was heading for a practical implementation.
"You are correct to say that the produced oxygen and hydrogen will need to be removed from the surface. In our lab experiment we placed the catalyst inside a glass vessel that was sealed" Daeneke replied.