Benedikt Huber

Hysteresis Effects in the Potential-Dependent Double Layer Capacitance of Room Temperature Ionic Liquids at a Polycrystalline Platinum Interface

This paper has been withdrawn by the author since it had been submitted to Journal of Physical Chemistry C.

Slow and fast capacitive process taking place at the ionic liquid/electrode interface

Electrochemical impedance spectroscopy was used to characterise the interface between the ultrapure room temperature ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and a Au(111) working electrode at electrode potentials more positive than the open circuit potential (-0.14 V vs. Pt pseudo-reference). Plots of the potential-dependent data in the complex capacitance plane reveal the existence of a fast and a slow capacitive process. In order to derive the contribution of both processes to the overall capacitance, the complex capacitance data were fitted using an empirical Cole-Cole equation. The differential capacitance of the fast process is almost constant between -0.14 V and +0.2 V (vs. Pt pseudo-reference) and decreases at more positive potentials, while the differential capacitance of the slower process exhibits a maximum at +0.2 V. This maximum leads to a maximum in the overall differential capacitance. We attribute the slow process to charge redistributions in the innermost ion layer, which require an activation energy in excess of that for ion transport in the room temperature ionic liquid. The differential capacitance maximum of the slow process at +0.2 V is most likely caused by reorientations of the 1-butyl-1l-methylpyrrolidinium cations in the innermost layer with the positively charged ring moving away from the Au(111) surface and leaving behind voids which are then occupied by anions. In a recent Monte Carlo simulation by Federov, Georgi and Kornyshev (Electrochem. Commun. 2010, 12, 296), such a process was identified as the origin of a differential capacitance maximum in the anodic regime. Our results suggest that the time scales of capacitive processes at the ionic liquid/metal interface are an important piece of information and should be considered in more detail in future experimental and theoretical studies.

The Differential Capacitance of Ionic Liquid / Metal Electrode Interfaces – A Critical Comparison of Experimental Results with Theoretical Predictions

Results of potential-dependent differential capacitance measurements on the interface between six different ionic liquids and the (111) surface of single-crystalline gold are presented. The measurements were done by means of broadband impedance spectroscopy in a frequency range from 10 mHz to 1 MHz. We discuss the influence of the IL cation, the IL anion and the cations’ alkyl chain length on the interfacial capacitance. Our results suggest that (i) there is no simple relationship between the cation size and the value of the differential capacitance, (ii) the general shape of the potentialdependent differential capacitance curve is more strongly influenced by the IL anion, and (iii) experimental differential capacitance curves do not exhibit a simple “camel-” or “bell-shaped” curvature as predicted by mean-field theories. Furthermore, the broadband measurements show that two capacitive processes can be distinguished, which take place on millisecond and second time scales, respectively. While a millisecond time scale is expected for double-layer charging governed by the bulk conductivity of the IL, the existence of a slow process points to additional barriers for charge transport at the interface. The capacitance contribution of the slow process is most pronounced for ILs based on the N-butyl-N-methyl-pyrrolidinium ([Pyr1,4]) cation. A comparison of capacitance data with insitu STM data from previous studies suggests that the slow process is connected to herringbone-type structures at the interface. While the herringbone superstructure of the Au(111) surface is well known in aqueous electrochemistry, a herringbone-type structure of adsorbed ions was described in a recent MD simulation paper by Federov and coworkers (K. Kirchner, T. Kirchner, V. Ivaništĕv, M. V. Fedorov, Electrochim. Acta 2013, in press: doi: 10.1016/j.electacta.2013.05.049). Graphical Abstract The Differential Capacitance of Ionic Liquid / Metal Electrode Interfaces – A Critical Comparison of Experimental Results with Theoretical Predictions

Synthesis of Novel Lithium Salts containing Pentafluorophenylamido-based Anions and Investigation of their Thermal and Electrochemical Properties

Abstract Three novel lithium salts, lithium bis(pentafluorophenyl)amide LiN(Pfp)2, lithium pentafluorphenyl(trifluormethylsulfonyl)imide LiN(Pfp)(Tf) and lithium pentafluorphenyl(nonafluorbutylsulfonyl)imide LiN(Pfp)(Nf) were synthesized and characterized with respect to their thermal and electrochemical properties. LiN(Pfp)2 decomposes at 108 ºC, whereas Li-N(Pfp)(Tf) and Li-N(Pfp)(Nf) show a much higher thermal stability of 307 ºC and 316 ºC, respectively. The ionic conductivity at 100 ºC measured by means of impedance spectroscopy decreases in the order LiN(Pfp)(Tf) > LiN(Tf)2> LiN(Pfp)(Nf). Both, the activation energy and entropy for ion conduction in the new salts are lower than in LiN(Tf)2 (LiTFSI), most likely due to the lower symmetry of the new anions. The electrochemical stability and ionic conductivity of LiN(Pfp)(Tf) and LiN(Pfp)(Nf) solutions (0.1 mol/l) in ethylene carbonate/dimethyl carbonate (1:3 w/w) are slightly lower than those of the LiTFSI solution, but still sufficient for application in lithium ion batteries. The high thermal stability of the novel salts and their stability towards hydrolysis makes them attractive candidates for overcoming the drawbacks of LiPF6-based electrolytes at elevated temperatures.

Synthesis and characterization of 5-cyanotetrazolide-based ionic liquids.

New salts based on imidazolium, pyrrolidinium, phosphonium, guanidinium, and ammonium cations together with the 5-cyanotetrazolide anion [C2 N5 ](-) are reported. Depending on the nature of cation-anion interactions, characterized by XRD, the ionic liquids (ILs) have a low viscosity and are liquid at room temperature or have higher melting temperatures. Thermogravimetric analysis, cyclic voltammetry, viscosimetry, and impedance spectroscopy display a thermal stability up to 230 °C, an electrochemical window of 4.5 V, a viscosity of 25 mPa s at 20 °C, and an ionic conductivity of 5.4 mS cm(-1) at 20 °C for the IL 1-butyl-1-methylpyrrolidinium 5-cyanotetrazolide [BMPyr][C2 N5 ]. On the basis of these results, the synthesized compounds are promising electrolytes for lithium-ion batteries.