Page 46 - ILL Annual Report 2019

SOFT CONDENSED MATTER
Xianwen Mao. Chinese
Department of Chemistry and Chemical Biology, Cornell University, USA Twitter: @XianwenMao
‘ I obtained my PhD in Chemical Engineering at MIT (USA) with a research focus on polymer and interface science. I am currently a
postdoctoral associate at Cornell University (USA), working on single-molecule techniques. Recently, I discovered that the self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces, as a result of a collaboration with the ILL D33 facility.’
Self-assembled nanostructures in ionic liquids facilitate charge storage at electrified interfaces
Small-angle instrument D33
High-temperature, small-angle neutron scattering (SANS) experiments have helped to elucidate the nanostructuring of new amphiphilic surface-active
ionic liquids (SAILs) based on common surfactants. At an electrified surface, this structuring leads to enhanced charge storage performance by suppressing undesired over-screening effects found in normal, electrical double layers.
As a result, this class of ILs may be used as high-energy-density electrolytes suitable for many emerging electrochemical technologies.
AUTHORS
X. Mao, P. Brown, Y. Ren, A. A. H. Padua, M. F. Costa Gomes and T. A. Hatton (Massachusetts Institute of Technology, USA) C. Červinka (Ecole Normale Supérieure de Lyon, France)
G. Hazell and J. Eastoe (University of Bristol, UK)
H. Li and R. Atkin (The University of Western Australia, Australia) D. Chen (Stanford University, USA)
I. Grillo (ILL)
ARTICLE FROM
Nat. Mater. (2019)—doi: 10.1038/s41563-019-0449-6
Currently, aqueous and organic electrolytes are used in supercapacitors. More recently, however, ionic liquids (ILs) have been used as performance boosters, as they often have large electrochemical windows and are non-volatile. Although ILs are salts, at room temperature they are liquids and not crystalline solids. In a recent Nature Materials paper, new, highly efficient, detergent-like liquid electrolytes have been studied, leading to both a better understanding of electrolyte design and structuring and the generation of more efficient devices for storing electrical energy.
This study required the input of scientists with diverse skill sets ranging from chemical synthesis and electrical techniques to computational methods and neutron scattering. We discovered that SAILs exhibit significant and competing van der Waals interactions owing to the non-polar surfactant tails, resulting
in unusual interfacial ion distributions. This sets them apart from conventional non-amphiphilic ILs, whose ion distribution is dominated by Coulombic interactions. Neutron scattering experiments performed on D33 at the ILL revealed the bulk structures of these SAILs, complementing results from atomic force microscopy (AFM) which have revealed the associated interfacial structures. Importantly, the sandwich-like bilayer structures generated by the SAILs can suppress undesired over- screening effects and thus impart much higher capacitances at elevated temperatures than those previously seen with traditional electrolytes.
This new class of electrolyte materials may be suitable
for challenging operations, such as oil drilling and space exploration, but may also pave the way to improved supercapacitors in hybrid cars. These devices are essential components in modern hybrid cars and can outperform batteries in terms of higher power and offer better efficiency. This is particularly the case during regenerative braking; here, rather than being lost, mechanical work is turned into electrical energy that can be stored quickly, ready to be released, in supercapacitors. This reduces energy consumption and is much more environmentally friendly. Importantly, by using these new electrolytes future supercapacitors may even be able to store more energy than batteries can, potentially replacing batteries in applications such as electrical vehicles, personal electronics and grid-level energy storage facilities.
The design principles of these new electrolytes will continue
to be improved using neutron scattering and reflectivity, as
the ability to selectively contrast match individual ions is the principal advantage of neutron beams. Thus, only neutron techniques can provide molecular level insight into the packing and structures of the electrolytes at charged surfaces and in the bulk medium.
ANNUAL REPORT 2019