Krümmungs-Kapazitäts-Beziehungen gewellter Elektroden in elektrochemischen Doppelschichtkondensatoren

Like capacitors, the storage mechanism of electrochemical double-layer capacitors is based on the formation of electrostatic fields between the electrodes and a double layer formed by the ions in the electrode-electrolyte interface. Beside cycle stability and the charging and discharging speed the performance is also characterized by the capacity of the electrodes. However the capaitance is strongly influenced by the size of the electrode surface on which it is possible for the electrolyte ions to accumulate in the form of a double layer. Due to their high surface area, amorphous carbons are considered typical electrode materials for common supercapacitors. Among others, these electrodes are characterized by convex and concave curvatures, among other things. However, the interaction of convex and concave regions and their superposition have rarely been investigated. In addition this work demonstrates a large impact of nanoscale surface curvatures, either convex or concave, on the capacitive performance of supercapacitors. Molecular dynamics simulations of trilayer graphene electrodes with convex and concave curved regions of different curvature radii are performed to describe this interaction in more detail. A 1.5 M solution of the ionic liquid 1-butyl-3-methylimidazoliumhexafluorophosphate [BMI][PF6] and acetonitil as solvent serves as the electrolyte. The definition of constant potentials at the electrodes allows the determination of the charge of each electrode atom depending on the time-dependent electrolyte configuration. The results show a variable behavior of the differential capacitance with respect to different curvature radii both for the overall electrode and when the concave and convex regions were plotted separately. Furthermore, this work describes the influence of the electrode curvature on the orientation of the cations adsorbed on the electrode surface and the composition of the ionic layer accumulated in the electrode-electrolyte-interface and correlates this influence with the differential capacitance peaks. The evaluation of more exhaustive simulations of selected systems by using the Unbinned Weighted Histogram Analysis Method (UWHAM) provides a higher statistical accuracy and allows the results to be presented as a continuous function of the applied potential. These calculations also reveals potential-dependent fluctuations of the differential capacitance, the compostion of the ionic layer and the cation orientation. If the curvature radii are of the same order of magnitude as the ions, the restricted space in the electrode-electrolyte interface causes a reduced number of possibilities for the orientation of the ions within this interface. This leads to changes in free energy compared to flat electrodes or electrodes with significantly larger curvature radii. Furthermore, the capacitance peaks of the negative electrode can be related not only to variations in the composition of the Helmholtz layer of the negative electrode, but also to variations at the positive electrode. It is found that both the applied potential and the electrode curvature determine the occurrence of capacitive peaks in the convex and concave regions and thus determine the capacitive performance of the electrode. Furthermore, by calculating a charge-charge covariance matrix from the charge fluctuations of the electrode atoms, the influence of individual atomic electrode charges or defined regions of the electrode on the distinct nontrivial features of the differential capacitance can be quantified in a sophisticated manner. This method can also be applied to more complex electrode models with different pore geometries, thus showing the influence of different morphological characteristics on the capacitance. For this reason, the method presented here can serve as a tool in defining design criteria of future electrodes.

Schlagworte

Molecular dynamics

Electrical double layer supercapacitor

Weighted histogram analysis

Ionic liquids

DDC Class

540: Chemistry

Lizenz

https://creativecommons.org/licenses/by/4.0/

Name

Seebeck_Jannes_Krümmungs-Kapazitäts-Beziehungen-gewellter-Elektroden-in-elektrochemischen-Doppelschichtkondensatoren.pdf

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10.96 MB

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Krümmungs-Kapazitäts-Beziehungen gewellter Elektroden in elektrochemischen Doppelschichtkondensatoren