Soaked to the skin: tuning ionic liquids for electrochemical devices

By Tom Sharp on October 28, 2016

Researchers at the Universities of Santiago de Compostela, A Coruña, Tartu, Stratchclyde and Cambridge, shed light on the structure of the electrified interface in mixtures of contaminated ionic liquids in their recently published JPCM letter.

The design of new devices for energy harvesting and storage is of major interest for modern science and technology, and the search for new electrolytes lies at the heart of it. The combination of ionic liquids with electrochemically active salts is currently an excellent method for designing new advanced batteries, supercapacitors or fuel cells. With these new low-temperature molten salts of unrivalled electrochemical stability, a new generation of higher-voltage electrochemical cells is at hand, which could open the door to smaller, more efficient and long-lasting batteries and ultracapacitors. Of course, in order to produce safe and pure ionic-liquid-based electrolytes, many challenges still remain, including the seemingly unavoidable presence of water in the highly hydroscopic ionic environment.

Many applications of ionic liquids require their confinement in nanoscopic geometries. In a letter in Journal of Phyics: Condensed Matter new simulations of humid ionic liquids ([BMIM][BF₄]) close to a planar graphene layer -mimicking these mixtures in realistic microporous carbon electrodes- show that water molecules mainly follow the anions of the ionic liquid, and they are always present at the neutral and charged surfaces where they are more strongly confined than in the bulk.

Their results also show that nanoconfinement increases the probability of the presence of water molecules near the positively charged surface while leading to depletion at the neutral and, more markedly, at the negatively charged surfaces. Moreover, a careful analysis of the lateral structure of the interfacial layer showed a transition from ordered stripes to an ordered phase with hexagonal symmetry upon water addition, in a similar manner to increasing the electrode voltage. This must be studied for other cosolvents, and it may be useful for designing new mixed ionic-liquid-based electrolytes with ad hoc interfacial topologies that avoid the well-known high energy barriers to salt transfer to electrodes.

Borja Docampo is a PhD student at the University of Santiago de Compostela, Spain. His research focuses on the structural and dynamic properties of mixtures of ionic liquids with different cosolvents such as water, alcohol and other ionic liquids.

Víctor Gómez-González is a PhD student at the Department of Condensed Matter, Faculty of Physics, University of Santiago de Compostela, Spain. His particular research is focused on mixtures of ionic liquids with salts of electrochemical interest, via computer simulation.

Hadrián Montes Campos is a master student at the faculty of physics in the University of Santiago de Compostela. His particular research interests are focused in the behaviour of nanoconfined ionic liquids and the study of the effect of the polarization of the electronic atmosphere in molecular dynamics simulations.

José Manuel Otero Mato is a Masters Degree student at the Faculty of Physics at the University of Santiago de Compostela, Spain. The next year, he is going to start a PhD course in theoretical and computational study of nanostructures dispersed in ionic liquids and mixtures.

Trinidad Méndez Morales received her PhD from the Department of Condensed Matter Physics, Faculty of Physics, University of Santiago de Compostela (Spain) in 2015, for her work on “Static and dynamic properties of homogeneous and inhomogeneous mixtures of ionic liquids”. She is now a postdoctoral researcher at CEA (Saclay, France) and her current research focuses on the development of supercapacitors with superior performance by optimizing the combination of electrode/electrolyte

Dr Oscar Cabeza is Professor of Applied Physics from 1999. He works in Physics of Matter from his early career, trying to understand the relationship between macroscopic physical properties and microstructure, first in superconductors, later for magnetoresistive materials, and actually in ionic liquids. He has published about 100 papers in high impact journals and also he has participated in more than one hundred of international congress about matter sciences.

Luis Javier Gallego is Full Professor at the Department of Condensed Matter Physics of the University of Santiago de Compostela, Spain, since 1992. His research interests have been focused on amorphous systems, liquid metals and a wide range of nanomaterials, including free and supported clusters, fullerene clusters, nanowires, quantum dots and graphene-like honeycomb structures. The investigation has been performed using theoretical models and computer simulation techniques. He is currently the Coordinator of the Nanomaterials, Photonics and Soft Matter Group of the University of Santiago de Compostela (for a general overview of the main activities of this group, see www.usc.es/gnmmb/), and his investigation is now devoted to the study of the electronic and transport properties of nanostructures, and the properties of systems composed by nanostructures and ionic liquids.

Ruth Lynden-Bell has worked in the Universities of Sussex, Cambridge and Queen’s University Belfast. She is now an Emeritus Professor and is based in Cambridge. She has been using computer simulation since 1980.

Vladislav Ivaništšev is a research associate at the University of Tartu, Estonia. He received his PhD for his work on “Double layer structure and adsorption kinetics of ions at metal electrodes in room temperature ionic liquids” in 2012, and then worked in the group lead by Prof. M Fedorov on molecular dynamics simulations of charged interfaces. His current research focuses on electrochemistry of ionic liquids and employs computer modeling. More details of his research can be found at http://doublelayer.eu/tartu

Maxim V Fedorov is SUPA2 Professor (Chair) at the Department of Physics of the University of Strathclyde in Glasgow, UK (the Department is a part of the Scottish Universities Physics Alliance, SUPA). He received his PhD (2002, Biophysics) and DSc (2007, Physical Chemistry) degrees from the Russian Academy of Sciences. He moved to the University of Strathclyde in 2011 from the Max Planck Institute for Mathematics in the Sciences in Leipzig (Germany) where he was a research group leader. Since 2012 he is also Director of the West of Scotland Academia-Industry Supercomputer Centre (aka ARCHIEWeST) in Glasgow (located at the University of Strathclyde). In 2012 he received the Helmholtz Award from the International Association of the Properties of Water and Steam (IAWPS) for his work on theoretical physical chemistry of liquids. His research interests focus on modeling of ionic liquids at electrified interfaces, ion effects on (bio) macromolecules and nano-objects, and molecular-scale theories of solvation interfaces. More details of his research can be found athttp://bcp.phys.strath.ac.uk/ molecular-theory-and-simulations/profmaxim-fedorov. He may be reached at maxim.fedorov@strath.ac.uk.

Luis M. Varela has been at Professor of the Condensed Matter Physics Department of the University of Santiago de Compostela since 2003. He received his PhD in this University in 2000, and his research interests have been mainly focused in theory and simulations of ionic systems, specifically of ionic liquids. He is currently the Dean of the Physics Faculty of the University of Santiago de Compostela.