Being abundant but inconsistent across most of the world, solar energy is perhaps best stored in another form. This can be achieved through a process which has been termed “artificial photosynthesis”, which uses semiconductor-based systems to split water into hydrogen and oxygen.
Thanks to its high energy density, hydrogen has been suggested as an ideal replacement for fossil fuels. When burned, it releases only water. However, the manufacturing of solar hydrogen on an industrial scale has so far failed, given the inefficiency of the process compared with other approaches to energy production.
The previous solar-to-hydrogen efficiency record was 14.2 per cent; set in 2015 by a team of German and American researchers.
The new world record, recaptured by the US Department of Energy’s National Renewable Energy Laboratory, is 16.2 per cent. Their achievement was reported in Nature Energy.
This is accomplished using a semiconductor-based cell which splits water into hydrogen and oxygen gases.
Both the new device and the original device use stacks of light-absorbing semiconductors in an acid and water solution (electrolyte), in which the water splitting occurs. While the original device is made of two different semiconductors sandwiched in layers, the new cell is grown upside-down (i.e. top to bottom). This results in an ‘inverted’ device.
This inversion allowed researchers to substitute a different semiconductor for some of the original device’s layers, improving efficiency, and to deposit a thin semiconductor layer as a ‘window layer’ on top of the device, protecting the layers in the cell and eliminating defects which reduce efficiency.
Dr John Turner, a prominent spokesman for hydrogen energy, developed the original cell in the 1990s. This set a long-unmatched solar-to-hydrogen efficiency of 12.4 per cent.
While the cost of hydrogen production remains above the US Department of Energy’s target of $2 per kg or less, the technology remains commercially unavailable. Researchers are focused on improving the cell’s efficiency and lifetime. At present the cell has only a few hours of operational life.
It has been suggested that wiring the device to an external electrolyser rather than submerging it in the acid and water solution could make it more durable, but this would be likely to make the process itself more expensive.
As international pressure mounts to find alternatives to fossil fuels, hydrogen has been gradually growing in popularity as a future source of energy. Recently, German scientists built an enormous ‘artificial sun’ to develop a new method of water splitting for hydrogen.