Researchers reveal charge storage mechanisms of single-layer graphene in ionic liquid

(Nanowerk News) Electrochemical double layer capacitors (EDLCs), also known as super capacitors, store energy by reversible electrostatic attraction of electrolyte ions onto high surface area carbon electrodes. Since the limitation of battery-like charge transfer kinetics is not involved, super capacitors can operate at very high charge and discharge rates within a few seconds with excellent cyclability of over a million cycles, making them useful in a broad range of energy applications.
Graphene with its high theoretical specific surface area of 2630 m2/g and theoretical capacity of up to 550 F/g has already attracted great attention for super capacitor applications. However, for graphene-based materials, the dynamic charge separation mechanism of the graphene/electrolyte interface has not been well understood, which hinders the further development of high-performance two-dimensional or three-dimensional graphene electrodes.
Recently, the team led by Prof. ZHU Yanwu from the University of Science and Technology of China of the Chinese Academy of Sciences found that a positively charged ion-species desorption and an ion re-organization dominate the double layer charging during positive and negative polarizations, respectively, leading to the increase in electrical double-layer (EDL) capacitance with applied potential.
The study was published in Journal of the American Chemical Society ("Charge Storage Mechanisms of Single-Layer Graphene in Ionic Liquid").
The researchers used electrochemical impedance spectroscopy (EIS) and electrochemical quartz crystal microbalance (EQCM) to characterize the ion fluxes and adsorption on single layer graphene in neat ionic liquid (EMI-TFSI) electrolyte. They delivered a qualitative demonstration of the combination of EQCM and single layer graphene (SLG) in neat EMI-TFSI. Scanning from potential of zero charge (PZC) towards positive charge, positively charged ion-species were expelled. Yet, from PZC to negative charge, the orientation of ions changed, resulting in more compact packing.
These results suggested a surface-bound ions dominated charging process on SLG in neat IL, which is completely different from that of porous carbons, and highlighted the important role of electrostatic correlations on ion fluxes and ion separation at a non-porous carbon interface, introduced by long-range Coulombic force.
Collaborating with the French Patrice Simon group, the team also studied the kinetic response of ionic liquid (EMI-TFSI) electrolyte on the surface of single-layer graphene in situ by electrochemical impedance spectroscopy combined with electrochemical quartz crystal microbalance system.
They found that in the graphene anodization interval, the charge storage is dominated by the desorption of positively charged cluster ions, and in the negative polarization region, the surface quality of graphene changes little. The results showed the surface ion rearrangement effect. As the applied potential increases, the two types of interface responses dominate the changes in the electric double layer, resulting in an increase in the electric double layer capacitance.
This study provides a basis for further understanding of the graphene-electrolyte interface structure and graphene double layer energy storage. Further investigations are needed to enable a better understanding of the detailed capacitance response at graphene/ionic liquid interface.
Source: Chinese Academy of Sciences
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