Entropy Changes upon Double Layer Charging at a (111)-Textured Au Film in Pure 1-Butyl-1-Methylpyrrolidinium Bis[(trifluoromethyl)sulfonyl]imide Ionic Liquid

Abstract

We investigated the entropy of the electrical double layer (EDL) formation at a (111)-textured Au film in the ionic liquid 1-butyl-1methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide by electrochemical microcalorimetry. We found reversible heat exchange upon EDL charging in the potential range of −0.6 to 1 V versus Fc/Fc + . Dependent on the potential, the heat evolution continued for 10 to 200 ms after the current flow, indicating slow reorganization of the EDL on this time scale. The partial molar entropy of the formation of the EDL Δ R S form was derived from the reversibly exchanged heat upon EDL charging. It varied about linearly with the potential from 27 J K −1 mol −1 at −0.6 V to −79 J K −1 mol −1 at 1 V versus Fc/Fc + . Δ R S form crosses zero close to the potential of zero charge of the system. The entropy values are much higher than those arising from purely configurational entropy of a 2D lattice gas, thus pointing to strong, potentialdependent immobilization of the constituents of the EDL, where the immobilization is correlated with the strength of the electric field in the EDL. In addition, from our entropy values, we derive an apparent contradiction with experimentally determined temperature coefficients of the EDL capacitance, which implies a strongly varying activity of the ions participating in charging of the EDL with temperature.

Reversible heat production during electric double layer buildup depends sensitively on the electrolyte and its reservoir

“…Further, we performed MD simulations for ρ± = 1 M with 600 particles per species in a box of (10 nm) 3 . The MD simulations give access to the (equilibrium) internal energy U and the electric work W el done to the system (see Eq.…”

“…In a recent experiment [1], the reversible heat flowing into and the electric work applied to a supercapacitor during isothermal charging were measured. This experiment is one of several recent studies, both experimental [1][2][3][4] and theoretical [5][6][7][8][9][10], on the intricate interplay between the electrolyte's temperature and the properties of the electric double layer (EDL). Until now, however, no comparison has been made between the experimental findings of Ref.…”

“…As Joule heating is mainly a bulk phenomenon, while reversible heating happens only in the nanometer-wide EDL, a capacitor with a large surfaceto-volume ratio is needed to notice appreciable reversible temperature variations. Advanced "microcalorimetry" measurements near flat electrodes, however, can detect much smaller temperature variations [3]. With a setup as sketched in Fig.…”

Dataset of the electrochemical potential windows for the Au(hkl)|ionic liquid interfaces defined by the cut-off current densities

“…E dl and E pw were calculated using the following equations: where E dl-AL and E pw-AL are the electrode potentials at which the positive j cut-off values are measured, and E dl-CL and E pw-CL are the electrode potentials at which the negative j cut-off values are measured. The j cut-off values for E dl and E pw were chosen based on previous studies [7 , [12] , [13] , [14] , [15] . The potential values of LSV were referenced to the redox potential of 2 mM ferrocene (Fc) in the corresponding IL, as recommended by IUPAC [16] .…”

“… In addition, these data are useful for identifying potential regions in which ILs exhibit nearly ideal capacitive behavior. Such potential regions are essential for ensuring the accuracy of microcalorimetric measurements [7] and amperometric sensors using ILs [8] . …”