TY - THES

T1 - In-Situ SAXS study on the ion dynamics in microporous carbon based supercapacitors

AU - Prehal, Christian

N1 - embargoed until 01-10-2014

PY - 2014

Y1 - 2014

N2 - Supercapacitors or electrical double-layer capacitors (EDLCs) are energy storage devices consisting of two electrodes immersed in a liquid electrolyte and contacted by metal current collectors. The energy is stored electrostatically by an electrical double-layer that forms upon applying a potential between the two electrodes. In contrast to conventional battery technologies no charge transfer across the electrode-electrolyte interface takes place. Hence the charge on the electrode side (electrons or holes) is situated across from the countercharge on the electrolyte side formed by the ions (corresponding to a plate capacitor). Due to the high surface area of the electrode materials (up to 2000m2/g in activated carbons) and the small distance between the opposed charges on the interface, extremely high capacitance values are achieved. In such electrode materials the average pore width is typically below one nanometer inhibiting the formation of the conventional electrical double layer. Furthermore it was found that in the smallest accessible pores ions lose partially their solvation shell, leading to an anomalous increase of the normalized capacitance (F/m2). There is still a lack of understanding on the appearance of the equilibrium ion structure and in particular the ion rearrangement and kinetics within such micropores. In order to study the kinetics of ions with a time-resolution in the sub-second regime, in-situ Small Angle X-ray Scattering (SAXS) experiments were performed at the Austrian SAXS beamline at the Synchrotron ELETTRA in Trieste, Italy. While the working supercapacitor-device was irradiated with the Synchrotron X-ray beam, various potential signals via a potentiostat were applied to the cell. The scattering intensity (SAXS and WAXS with position sensitive detectors) as well as the sample transmission (with a photodiode) were recorded simultaneously. Beside the construction of the in-situ sample holder, in particular the data-analysis and interpretation bear a big challenge since a variety of processes take place simultaneously. The in-Situ SAXS measurements revealed a detailed analysis of the ion transport within different hierarchical levels (such as micro-, meso-, macropores, corresponding to pore sizes 50nm, respectively) depending on the cell anatomy and the investigated spot on the electrode. The actual electrosorption corresponds to a local ion transport from the macropores (serving as ion reservoirs) into the meso- and micropore volume, exhibiting a lower time constant than the macroscopic ion transport from one electrode to the other taking place within the macropores. Furthermore the expansion of micro- and mesopores in dependence of applied potential could be tracked by analyzing the SAXS correlation length. Combining the measured transmission and electrical current signals, the absolute ion concentration changes within the irradiated pores could be evaluated. The total ion concentration was found to be constant for all potentials. Hence, a mechanical pressure due to the increased amount of ions within the pores is not the reason for pore swelling. This effect requires further systematic investigations beyond this work. In contrast to conventional electrochemical and other In-situ techniques (e.g. In-situ dilatometry) , In-situ scattering not only provides integral information of the entire cell. It enables on the one hand a distinction between processes in different pore size regimes, on the other hand the study of the local properties on the investigated electrode, with a time resolution in the sub-second regime. The present work exhibits for the first time a detailed In-situ SAXS/WAXS study on the ion transport in microporous carbon based supercapacitors offering a distinction between the kinetics within different pore size levels.

AB - Supercapacitors or electrical double-layer capacitors (EDLCs) are energy storage devices consisting of two electrodes immersed in a liquid electrolyte and contacted by metal current collectors. The energy is stored electrostatically by an electrical double-layer that forms upon applying a potential between the two electrodes. In contrast to conventional battery technologies no charge transfer across the electrode-electrolyte interface takes place. Hence the charge on the electrode side (electrons or holes) is situated across from the countercharge on the electrolyte side formed by the ions (corresponding to a plate capacitor). Due to the high surface area of the electrode materials (up to 2000m2/g in activated carbons) and the small distance between the opposed charges on the interface, extremely high capacitance values are achieved. In such electrode materials the average pore width is typically below one nanometer inhibiting the formation of the conventional electrical double layer. Furthermore it was found that in the smallest accessible pores ions lose partially their solvation shell, leading to an anomalous increase of the normalized capacitance (F/m2). There is still a lack of understanding on the appearance of the equilibrium ion structure and in particular the ion rearrangement and kinetics within such micropores. In order to study the kinetics of ions with a time-resolution in the sub-second regime, in-situ Small Angle X-ray Scattering (SAXS) experiments were performed at the Austrian SAXS beamline at the Synchrotron ELETTRA in Trieste, Italy. While the working supercapacitor-device was irradiated with the Synchrotron X-ray beam, various potential signals via a potentiostat were applied to the cell. The scattering intensity (SAXS and WAXS with position sensitive detectors) as well as the sample transmission (with a photodiode) were recorded simultaneously. Beside the construction of the in-situ sample holder, in particular the data-analysis and interpretation bear a big challenge since a variety of processes take place simultaneously. The in-Situ SAXS measurements revealed a detailed analysis of the ion transport within different hierarchical levels (such as micro-, meso-, macropores, corresponding to pore sizes 50nm, respectively) depending on the cell anatomy and the investigated spot on the electrode. The actual electrosorption corresponds to a local ion transport from the macropores (serving as ion reservoirs) into the meso- and micropore volume, exhibiting a lower time constant than the macroscopic ion transport from one electrode to the other taking place within the macropores. Furthermore the expansion of micro- and mesopores in dependence of applied potential could be tracked by analyzing the SAXS correlation length. Combining the measured transmission and electrical current signals, the absolute ion concentration changes within the irradiated pores could be evaluated. The total ion concentration was found to be constant for all potentials. Hence, a mechanical pressure due to the increased amount of ions within the pores is not the reason for pore swelling. This effect requires further systematic investigations beyond this work. In contrast to conventional electrochemical and other In-situ techniques (e.g. In-situ dilatometry) , In-situ scattering not only provides integral information of the entire cell. It enables on the one hand a distinction between processes in different pore size regimes, on the other hand the study of the local properties on the investigated electrode, with a time resolution in the sub-second regime. The present work exhibits for the first time a detailed In-situ SAXS/WAXS study on the ion transport in microporous carbon based supercapacitors offering a distinction between the kinetics within different pore size levels.

KW - in-situ

KW - SAXS

KW - ion dynamics

KW - micropores

KW - carbon

KW - supercapacitor

KW - EDLC

KW - Synchrotron

KW - ion kinetics

KW - in-situ

KW - SAXS

KW - ion dynamics

KW - ion kinetics

KW - carbon

KW - supercapacitor

KW - EDLC

KW - Synchrotron

KW - micropores

M3 - Diploma Thesis

ER -