Symmetrical dication with two imidazolium head groups tends to be parallel on negatively charged graphite electrode; whereas asymmetrical dication with one imidazolium and one trimethylammonium head groups tends to stand up on graphene. © Guang Feng 2016. All rights reserved.

Invited to our special issue on the physics of supercapacitors and electroactuators, Guang Feng and his team explain their work into asymmetrical dications.

Conventional electrolytes of energy storage devices (e.g. supercapacitors, batteries) are limited by either the narrow electrochemical window (1.23 V for aqueous electrolytes) or the toxicity due to their high volatility (organic electrolytes). The emergence of room temperature ionic liquids (RTILs) consisting of organic cations and organic/inorganic anions provides a solution to this problem. RTILs are well-known highly efficient and green electrolytes in supercapacitors with wide electrochemical windows and are environmently friendly. Monocationic ionic liquids (MILs) with one cation carrying one unit positive charge are the most frequently investigated RTILs. Recently, dicationic ionic liquids (DILs) with one cation carrying two unit positive charges are attractring more and more attention due to their high thermal stability, as well as higher capacitance of supercapacitors than their counterpart MILs according to our previous studies.

In supercapacitors, electric energy is stored in the form of an electrical double layer consisting of counter- and co-ions near a charged electrode surface, highlighting the importance of ion distribution in EDLs. As the most popular cation, imidazolium–based cations have parallel tendency with a graphene electrode surface, especially on negatively charged graphene. Thus, it is not surprising to observe the symmetrical dications consisting of two imidazolium head groups are parallelized onto graphene surfaces. However, what if an asymmetrical cation is used (one imidazolium replaced with an alternative head group)? In this work, it is of great interest to see the tendency of a standing dication on negatively charged graphene when replacing one head group. Specifically, the two distinct density peaks for imidazolium and trimethylammonium were identified in ion density profiles near graphite surfaces. Although remarkably different orientations, as well as EDL structures of symmetrical and asymmetrical DILs are observed near the graphene electrode, their capacitances are quite similar. Nevertheless, it is too rash to draw a general conclusion without systematic studies over a broad range of asymmetrical DILs, which is currently ongoing work in our group.

Our group focuses on the exploration of the molecular mechanism of ionic liquid electrolytes-based supercapacitors and development of high-performance supercapacitors by rationally designing ionic liquid electrolytes and electrode materials. Our study covers a variety of ionic liquid electrolytes including pure MILs, pure DILs, RTILs mixture, MILs/DILs-organic solvent mixture integrated with different-shaped carbon electrodes.

About the authors

Song Li received her PhD from Vanderbilt University, United States in 2014. After postdoctoral work at Northwestern University, she was appointed as an associate professor at the School of Energy and Power Engineering in Huazhong University of Science and Technology (HUST), China in 2015. Her research interests focus on electrolytes design and high-throughput screening of metal-organic frameworks for gas separation.

Mengyang Zhu is a master student in HUST.

Guang Feng is a professor in HUST, awarded by the Hubei Provincial 100 Talents Program. He received his Ph.D. from Clemson University in 2010. From 2010 to 2013 he worked as a postdoctoral research associate and then a research assistant professor in Vanderbilt University and the Fluid Interface Reactions, Structures and Transport (FIRST) Energy Frontier Research Center. His research focuses on micro-/nano-scale interface and transport phenomena in capacitive energy storage, gas storage, and biological nanochannel.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image copyright Guang Feng 2016. All rights reserved.

Categories: Journal of Physics: Condensed Matter

Tags: Condensed matter physics, Dications, Electrode surface, Electrodes, Graphene, Special Issue, Supercapacitors