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Development and Characterization of Glassy Carbon Electrodes for a Bipolar Electrochemical DoubleLayer Capacitor
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The thermal gasphase oxidation (activation) of Glassy Carbon (GC) and bipolar monolithic electrodes as prepared from thermally activated GC were investigated. The goal of these investigations was to understand the thermal activation process and to find a correlation between process parameters plus GC materials properties and the electrode performance. With this knowledge it should be possible to optimize the performance of GC electrodes.
Thermal activation creates a porous film on the surface of GC. With proceeding activation the film grows into the GC.
The film thickness of various GC plates and sheets was determined with scanning electron microscopy.
The microstructure and mesostructure of the samples were investigated with Xray diffraction and Small Angle X-ray scattering.
The internal surface area of activated GC samples were determined with gas adsorption measurements and Small Angle X-ray scattering.
The electrochemical double layer capacitance and the diffusion resistance of GC samples were determined with electrochemical impedance spectroscopy.
Studies on GC with different temperature of pyrolysis reveal a correlation between pyrolysis temperature and asymptotic active film thickness: The higher the pyrolysis temperature, the smaller the asymptotic film thickness.
X-ray diffraction on GC with various pyrolysis temperatures exhibits a correlation between temperature of pyrolysis as well as intensity and full width at half maximum of prominent diffraction peaks: The higher the pyrolysis temperature, the smaller the defect density in GC.
From the two latter findings, a correlation between asymptotic film thickness and defect density is found. The defect density is a qualitative, macroscopic and global measure for the effective diffusion coefficient Deff., which is an essential for the film growth, but experimentally difficult to measure.
Results from the small angle X-ray scattering reveal that the micropores in GC have a diameter of about 1 ° A to 2 ° A and that the distribution of the diameter is not very sharp.
Considerable part of the micropores has diameters smaller than 7 ° A to 9 ° A, which cannot be wetted by electrolytes and therefore do not contribute to the capacitance. The diffusive resistance has a minimum (at about 50 m.cm2) after a specific activation time (around 30 to 60 minutes). For larger activation times the resistance increases monotonously.
8 Due to the activation the pore size […]
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