Potential-Dependent Double Layer Capacitance at Room Temperature Ionic Liquids - Carbon Electrode Interfaces Studied By Electrochemical Impedance Spectroscopy
Authors: Swain Greg M. , Bhardwaj Kirti 
Published: February 2020

The electrochemical double layer (EDL) at the electrode-electrolyte interface is the near-surface region which governs the electrochemical phenomena (e.g., electrodeposition, corrosion, and heterogeneous catalysis). Energy storage technologies (e.g., supercapacitors, batteries, solar cells etc.) rely on the structural and electrical organization of the interface for the storage capabilities. Improvement in performance and efficiency of these devices seeks a comprehensive understanding of how electrode properties (e.g., its bulk electrical properties, microstructure, surface functional groups) and the type and structure of electrolyte affect the interfacial organization.

Room temperature ionic liquids (RTILs) are a unique class of electrolytes that are significantly different from conventional organic and aqueous electrolytes. RTILs have no solvent (i.e. are solely made up of ions) and exist in the liquid state at room temperature. This unique physical, chemical and molecular make-up results in remarkable properties like wide potential window (> 4 V), environmentally-benign characteristics, and excellent thermal and electrochemical stability. To optimally harness these properties and engineer RTIL-based devices, a fundamental understanding of their electrochemical behavior is critical. This critical knowledge gap is impeding the systematic progress toward innovative and more efficient energy storage technologies. Addressing this longstanding challenge requires a systematic study of structure, distal extent, and dynamics of the interfacial organization.

The shape of capacitance-potential (C-E) trend can suggest the broad molecular architecture of interfacial organization. Traditional models of interfacial organization that are based on dilute-solution approximation cannot be applied to RTILs because of absence of solvent, high concentration (3-7 M), asymmetric charge distribution, and strong inter-ionic interactions. Theoretical studies based on mean-field theories predict a bell- or a camel-shaped C-E curvature depending on the nature of the RTIL. However, experimental capacitive studies exhibit a wide range of C-E trends such as a U-shaped, bell-shaped, or relatively featureless. Rigorous justification of these seemingly contradictory observations remains a challenge due to difficulties in reconciling theoretical and experimental data.

In this presentation, capacitance of carbon electrode-RTIL interface will be discussed as a function of the applied potential, RTIL type, carbon electrode bulk material (sp2-, sp3- and hybrid sp2/sp3-bonded carbon), and the surface functional groups (hydrogen- and oxygen-terminated). Cyclic voltammetry and broadband electrochemical impedance spectroscopy were employed to investigate the C-E trends for nanostructured carbon electrodes in 1-alkyl-3-methylimidazolium-based RTILs. Comparison measurements were made using a planar gold electrode in the same RTILs. Two approaches for analyzing the impedance data were compared by fitting a) an electrical equivalent circuit in Nyquist plane b) complex capacitance using Cole-Cole function to deconvolute the effect of two relaxation processes observed over different timeframes. In addition, several ‘user-defined’ variables (such as potential window probed, potential scan direction, impedance data analyses methods etc.) will be discussed which can affect the resultant C-E trends. This work provides insights into the structure and dynamics of RTILs-electrode interfaces, the role of experimental variables, and impedance analyses methods for such electrochemical measurements.