In two new studies, Stanford scientist Yi Cui and colleagues have developed technologies that could overcome a couple of the major energy challenges of the 21st century, that of cleaner fuel for transportation using hydrogen fuel from water as well as improved large-scale energy storage from zinc batteries.
Hydrogen fuel has been intensively researched as a more environmentally friendly alternative to gasoline, and hydrogen-powered cars – which are emission-free – are already being produced for the mass market, although sales remain limited. However, producing hydrogen fuel for the cars is not emission-free, as it tends to involve natural gas in a process that releases carbon dioxide. To resolve this, the team looked at photovoltaic water splitting, which uses a solar-powered electrode immersed in water.
On sunlight hitting the electrode, it generates an electric current that separate the water into its constituent parts of hydrogen and oxygen. To produce clean hydrogen from water is not easy, as silicon solar electrodes corrode when exposed to oxygen as a byproduct of the water splitting. In this study, published in the journal Science Advances [Qiu et al. Sci. Adv. (2016) DOI: 10.1126/sciadv.1501764], the researchers used bismuth vanadate as a coating, as it is cheap, absorbs sunlight and produces modest amounts of electricity.
“The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability”Yi Cui
However, the material is not an effective conductor of electricity – to carry a current, solar cells made from bismuth vanadate have to be sliced so thinly that it is nearly transparent. This allows visible light that can be used to generate electricity to pass through the cell. To capture the sunlight, microscopic arrays containing thousands of silicon nanocones were developed. The arrays were deposited on a film of bismuth vanadate, and both layers were then positioned on a solar cell made of the photovoltaic material perovskite. On being submerged, the device immediately started to split water at a useful solar-to-hydrogen conversion efficiency. As Cui revealed, “The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability”.
In a second paper, which appeared in Nature Communications [Higashi et al. Nat. Commun. (2016) DOI:10.1038/ncomms11801], Yi Cui, Shougo Higashi from Toyota Central R&D Labs and colleagues showed a new battery design with electrodes made of zinc and nickel. Although there is already a range of zinc metal batteries available, not many are rechargeable due to the dendrites that form on the zinc electrode during charging. The dendrites can keep growing until they reach the nickel electrode, resulting in the battery short-circuiting.
They overcame this challenge by redesigning the battery so that, instead of the zinc and nickel electrodes facing each other as in a traditional battery, they were separated by a plastic insulator and a carbon insulator was wrapped around the edges of the zinc electrode. Zinc ions are reduced and deposited on the exposed back surface of the zinc electrode during charging so that if zinc dendrites do form, they will grow away from the nickel electrode and not short the battery.
They showed the stability of the battery by successfully charging and discharging it over 800 times without shorting. The design is also straightforward and could be applied to a wide range of metal batteries.