Eighty-nine percent of the total solar power in the world was installed in the last seven years. While that statistic shows how solar cells have consistently become more efficient and cheaper over time, energy storage technologies are still trying to catch up.
A solar cell only harvests energy when the sun is shining. Current home and commercial systems overcome this limitation either by storing additional energy in batteries for later self-consumption or by hooking up to the existing power grid.
Here, a barter-like contract enables the user to contribute to the grid the excess energy that they generate during the day whilst allowing them to draw back any additional energy they might require during the night. This, consequently, means that some of the energy consumed by a solar-powered home can still come from non-renewable sources.
Batteries are a more appealing alternative for the future of self-sustaining solar-powered homes, however, they are far too clunky and have a relatively short life compared to the solar panels, meaning they often end up being expensive to maintain and use.
Lithium-ion cells have the highest energy density at 150 watt-hours per kilogram when compared to nickel-metal hydride (NiMH) and lead-acid batteries and while they are better in every other respect, Li-ion batteries still leave a lot to be desired.
Governments and industry representatives still haven’t reached a price point where energy storage is a more economically feasible choice compared to hooking up to the power grid.
Hydrogen fuel cells might be the solution. Hydrogen is by far the most abundant element in the universe and the only end products in a fuel cell are water and heat. Conventionally, the main limitation of hydrogen production is the amount of energy it consumes.
Solar energy can be used to split water into hydrogen and oxygen through a process called photocatalysis. This process requires a material with specific properties, although no commercial photocatalytic materials are available yet.
A recent paper published in Applied Physics Letters claims to have discovered a new class of photocatalytic materials that could prove to be a breakthrough for solar energy storage.
A pair of researchers, George Volonakis and Feliciano Giustino, from the University of Oxford and Cornell University, used first-principle calculations to investigate the electronic properties of the surfaces of four halide double perovskite compounds.
It was found that Cs2BiAgCl6 and Cs2BiAgBr6 were potentially suitable materials for the photocatalytic splitting of water.
Titanium dioxide (TiO2) has been known to be a potential photocatalytic material, however, these new materials absorb visible light much better than TiO2 and hence could be far more efficient.
They were also observed to generate sufficient energy to split water into hydrogen and oxygen. Two other compounds Cs2SbAgCl6 and Cs2InAgCl6 would require special synthesis conditions to achieve suitable properties for water splitting.
The past few years have seen halide perovskites become a prominent class of material for many optoelectronic applications. They have also been used in tandem with silicon-based solar cells to boost their efficiency.
However, these halide perovskites contain lead and their environment-friendliness remains questionable. Halide double perovskites, on the other hand, have since been proposed as stable lead-free alternatives for similar applications. This latest research adds photocatalysis to the list of their potential applications.
The analysis is still only theoretical and it remains to be seen if the material behaves just as well as predicted in real-world experiments. If it does, it might just transform the future of solar energy storage.
George Volonakis, Feliciano Giustino. “Surface properties of lead-free halide double perovskites: Possible visible-light photo-catalysts for water splitting.” Applied Physics Letters, 2018; 112 (24): 243901.