Work Package 9, Energy, looks at energy conversion and storage using graphene and other 2d materials in devices such as photovoltaic cells, fuel cells and batteries. Work package leader is Etienne Quesnel of the CEA French Alternative Energies and Atomic Energy Commission in Grenoble, and deputy is Vittorio Pellegrini of the Italian Institute of Technology Graphene Labs in Genova.
Perovskite and dye-sensitised solar cells
Graphene-enhanced perovskite photovoltaics is a key area of interest for WP9. Perovskite solar cells contain a perovskite-structured compound which acts as the light-harvesting layer. The name perovskite derives from the ABX3 crystal structure of the light-absorbing material. In perovskite photovoltaic cells, graphene and other 2d materials can replace commonly used compounds such as indium tin oxide as the transparent electrode, improving device efficiency and reducing cost. Suitable 2d materials other than graphene include molybdenum disulphide, tungsten disulphide, and tungsten diselenide.
Another advantage of perovskite technology is that it can be processed at least partially with low-cost wet processes. It is a promising technology, provided such issues as solar cell stability – i.e., ageing under irradiation – can be solved.
WP9 researchers led by Aldo di Carlo from Tor Vergata University in Rome have revisited all the steps of perovskite solar cell manufacturing, and attempted to integrate graphene and related materials into the different elements of the cells. "Their approach is not spectacular," says Quesnel, "but the results go beyond expectations, given that they observed a clear performance improvement with graphene-functionalised building blocks of the solar cells."
Di Carlo and his colleagues have demonstrated the effect of graphene and related material additives not only in terms of improved photo-conversion efficiency via better photo-carrier transport at the cell anode and cathode, but also a much improved cell stability. Moreover, they have shown that it is possible to replace the commonly used but highly expensive platinum catalytic cathode in dye-sensitised solar cells. This will make the manufacture of such solar cells much easier and more sustainable.
Flagship chemists in Cambridge recently announced to great media fanfare a breakthrough in the development of lithium-oxygen batteries. In a paper published in the journal Science1, the first author of which is Tao Liu, the group led by Clare Grey showed how graphene can improve the energy capacity, efficiency and stability of lithium-oxygen batteries, which have a theoretical energy density some 10 times that achievable with the established lithium-ion chemistry used in today's mobile devices.
Lithium-oxygen batteries have a distinct advantage over state-of-the-art rechargeable cells, but there are several practical challenges that must be addressed before they can present a viable alternative current technologies in high power applications. The advance made by the Cambridge researchers, in the form of a lithium-oxygen demonstrator cell, is in their use of a porous, ‘fluffy’ electrode made from graphene, in place of the usual graphite.
"What we've achieved is a significant advance for this technology," says Grey. "It suggests whole new areas for research. We haven't solved all the problems inherent to this chemistry, but our results do show routes forward towards a practical device."
What Liu, Grey and their colleagues have developed relies on a very different chemistry than earlier attempts at a non-aqueous lithium-air battery. With the addition of water, and the use of a lithium iodide mediator, the Cambridge battery shows fewer of the chemical reactions that degrade battery cells, making it far more stable following multiple charge-discharge cycles. By precisely engineering the structure of the electrode, the researchers were able to reduce the voltage gap between charge and discharge to just 0.2 volts. The smaller this voltage gap, the more efficient the battery. Previous attempts at a lithium-air battery have only managed to reduce the gap to 0.5–1.0 volts, whereas 0.2 volts is closer to that of a lithium-ion cell, and equates to an energy efficiency of 93%.
Grey considers the path ahead: "While there are still plenty of fundamental studies that remain to be done, to iron out some of the mechanistic details, the current results are extremely exciting. We are still very much at the development stage, but we've shown that there are solutions to some of the tough problems associated with this technology."
The Grey group's lithium-oxygen battery technology has been patented, and will be promoted through Cambridge Enterprise, the commercialisation arm of the University of Cambridge.
Image copyright: © 2015 Simone Casaluci/University of Rome "Tor Vergata".
1. Liu et al., Cycling Li-O2 batteries via LiOH formation and decomposition, Science 350, 530 (2015); doi:10.1126/science.aac7730.