TY - BOOK

T1 - Performance evaluation and in situ X-ray scattering of ordered mesoporous carbons for electrochemical energy storage applications

AU - Koczwara, Christian

N1 - no embargo

PY - 2019

Y1 - 2019

N2 - Ongoing climate change is emerging to a global crisis. It is necessary to develop new environmentally friendly ways to satisfy the need for energy to reach the ambiguous goal of keeping global warming below 2°C. Not only the production is a big challenge, but also the storage systems need to be improved in order to reach a society that only relies on renewable energy sources. Electrical double layer capacitors (EDLCs, also known as supercapacitors) can deliver a high power density and have a long lifetime, but suffer from a comparably low energy density. They are well suited for applications demanding very short charging and discharging times (e.g. frequency regulations in power grids). EDLCs usually consist of two porous (carbon) electrodes immersed in a liquid electrolyte. If a voltage is applied, an electrical double layer forms at each electrode, which causes the capacitive behavior. While the basic concept of the double-layer formation is well known, details of the ion rearrangement in microporous electrodes is still not fully understood. The main goal of this thesis is to provide new insights on the ion movement (re-arrangement) and the volumetric change of the electrode during charging and discharging, as well as to study the influence of the pore structure on the charging process. To reach this goal, two different model materials with a hierarchically ordered mesopore structure were used. These materials provide a well-defined pore structure that can be adjusted by additional CO2 activation treatment. This results in a hierarchically ordered macro/meso/microporous system. Additionally, the ordered structure results in Bragg peaks in the small-angle X-ray scattering regime, which allows additional analysis compared to (disordered) activated carbon materials. Three different topics were investigated: •Influence of the pore structure on the capacitance for elevated charging rates: An extensive structural and electrochemical characterization of nanocast (consisting of hexagonally ordered carbon nanorods) and soft-templated carbons (hexagonally ordered cylindrical pores) in order to study the influence of the pore structure on the electrochemical performance. It was shown that a hierarchical structure is beneficial, if they guarantee a three-dimensional access for the ions to the micropore network. •Electrode swelling: The well-resolved Bragg peaks (in the small-angle scattering regime) originating from hexagonally ordered structure allow new measurement concepts. It is shown that these materials allow us to track the deformation of a carbon based supercapacitor cell on the mesopore length scale, which is not possible with other (disordered) carbon materials. •Proof of concept: Is in situ anomalous small-angle X-ray scattering suitable to study the rearrangement of individual ion species? Conventional small-angle X-ray scattering suffers from a major drawback: it is not element sensitive. These experiments rely on a simplification of the system and on data from additional measurements (e.g. overall charge accumulated) to link the measured changes to a single element or component. Anomalous small-angle X-ray scattering was evaluated as a tool to directly analyze the behavior of a single ion species without the need of additional data from other experiments.

AB - Ongoing climate change is emerging to a global crisis. It is necessary to develop new environmentally friendly ways to satisfy the need for energy to reach the ambiguous goal of keeping global warming below 2°C. Not only the production is a big challenge, but also the storage systems need to be improved in order to reach a society that only relies on renewable energy sources. Electrical double layer capacitors (EDLCs, also known as supercapacitors) can deliver a high power density and have a long lifetime, but suffer from a comparably low energy density. They are well suited for applications demanding very short charging and discharging times (e.g. frequency regulations in power grids). EDLCs usually consist of two porous (carbon) electrodes immersed in a liquid electrolyte. If a voltage is applied, an electrical double layer forms at each electrode, which causes the capacitive behavior. While the basic concept of the double-layer formation is well known, details of the ion rearrangement in microporous electrodes is still not fully understood. The main goal of this thesis is to provide new insights on the ion movement (re-arrangement) and the volumetric change of the electrode during charging and discharging, as well as to study the influence of the pore structure on the charging process. To reach this goal, two different model materials with a hierarchically ordered mesopore structure were used. These materials provide a well-defined pore structure that can be adjusted by additional CO2 activation treatment. This results in a hierarchically ordered macro/meso/microporous system. Additionally, the ordered structure results in Bragg peaks in the small-angle X-ray scattering regime, which allows additional analysis compared to (disordered) activated carbon materials. Three different topics were investigated: •Influence of the pore structure on the capacitance for elevated charging rates: An extensive structural and electrochemical characterization of nanocast (consisting of hexagonally ordered carbon nanorods) and soft-templated carbons (hexagonally ordered cylindrical pores) in order to study the influence of the pore structure on the electrochemical performance. It was shown that a hierarchical structure is beneficial, if they guarantee a three-dimensional access for the ions to the micropore network. •Electrode swelling: The well-resolved Bragg peaks (in the small-angle scattering regime) originating from hexagonally ordered structure allow new measurement concepts. It is shown that these materials allow us to track the deformation of a carbon based supercapacitor cell on the mesopore length scale, which is not possible with other (disordered) carbon materials. •Proof of concept: Is in situ anomalous small-angle X-ray scattering suitable to study the rearrangement of individual ion species? Conventional small-angle X-ray scattering suffers from a major drawback: it is not element sensitive. These experiments rely on a simplification of the system and on data from additional measurements (e.g. overall charge accumulated) to link the measured changes to a single element or component. Anomalous small-angle X-ray scattering was evaluated as a tool to directly analyze the behavior of a single ion species without the need of additional data from other experiments.

KW - Kleinwinkelstreuung

KW - Superkondensator

KW - Doppelschichtkonensator

KW - in situ

KW - anomrmale Röntgenkleinwinkelstreuung

KW - supercapacitor

KW - electric double-layer capacitor

KW - small angle X-ray scattering

KW - in situ

KW - anomalous small angle X-ray scattering

M3 - Doctoral Thesis

ER -