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.
“The vessel contained the necessary sensors to analyse the atmosphere to detect any produced hydrogen. In this case we allowed the hydrogen to simply diffuse out of the printed film. Obviously this is not an option when considering using the material for practical applications” he conceded.
“The good news is that there are two ways to operate the developed molybdenum catalyst. The first one is described in the paper, where light absorbing nanoparticles are intermixed with the catalyst. These particles absorb light and convert the energy into an electrochemical potential that will then drive the water splitting reaction, producing hydrogen. This system can be applied as a paint, but will need to be incorporated within membrane technology that allows collecting the hydrogen while still allowing moisture and oxygen to pass in and out. As you have identified it yourself, this might be challenging but could overall be achieved”, the researcher continued.
“The second operation mechanism which is not discussed in the publication is a two component system, one that absorbs the light and the second that absorbs and splits the moisture. This is possible since the developed catalysts can also be used as an electro-catalyst. In this scenario we would build a system similar to a wheel dehumidifier. Wheel dehumidifiers are already widely used in order to provide air-conditioning to larger buildings. Instead of the usual desiccant we would use our catalyst which we have demonstrated to be highly hygroscopic and electrically conductive when exposed to moisture.
We would then apply a voltage across the catalyst which has been loaded with moisture. This will then lead to the production of hydrogen and oxygen. In this scenario the light may be absorbed by a standard silicon solar cell, the electricity would then be used to drive the catalysis. The produced hydrogen would be in a separated gas stream that can be easily separated and stored. In this case the hydrogen production would serve as an energy storage technology instead of a battery. Since lithium batteries are quite expensive and very harmful to the environment in production this may well be a suitable solution to an existing technological challenge”.
“Overall we are currently looking into both pathways, and we’ll decide which one is more feasible after further investigations” concluded Daeneke.
RMIT University – www.rmit.edu.au