Abstract
Electrochemical double layer capacitors (EDLCs) employing ionic liquid electrolytes are the subject of much research as they promise increased operating potentials, and hence energy densities, when compared with currently available devices. Herein we report on the influence of the particle size distribution of activated carbon material on the performance of ionic liquid based EDLCs. Mesoporous activated carbon was ball-milled for increasing durations and the resultant powders characterized physically (using laser diffraction, nitrogen sorption and SEM) and investigated electrochemically in the form of composite EDLC electrodes. A bi-modal particle size distribution was found for all materials demonstrating an increasing fraction of smaller particles with increased milling duration. In general, cell capacitance decreased with increased milling duration over a wide range of rates using CV and galvanostatic cycling. Reduced coulombic efficiency is observed at low rates (<25 mVs−1) and the efficiency decreases as the volume fraction of the smaller particles increases. Efficiency loss was attributed to side reactions, particularly electrolyte decomposition, arising from interactions with the smaller particles. The effect of reduced efficiency is confirmed by cycling for over 15,000 cycles, which has the important implication that diminished performance and reduced cycle life is caused by the presence of submicron-sized particles.
Highlights
The large capacitance of Electrochemical double layer capacitors (EDLCs) arises from the reversible adsorption of electrolyte ions at the interface with high surface area electrode and as a result carbon materials are usually employed as the main electrode component as they couple relatively high specific surface areas with good electrical conductivity and electrochemical stability
It has been reported that when using different particle size classifications of the same carbon material, an increase in resistivity with decreasing particle size is encountered which arises from a greater number of contacts per unit length[12]
In this report we investigate the influence of particle size distribution on the performance of EDLC electrodes using an ionic liquid electrolyte
Summary
The large capacitance of EDLCs arises from the reversible adsorption of electrolyte ions at the interface with high surface area electrode and as a result carbon materials are usually employed as the main electrode component as they couple relatively high specific surface areas with good electrical conductivity and electrochemical stability. EDLC electrodes typically consist of a composite layer of activated carbon particles with an organic polymer (such as PTFE or PVdF) or carboxymethyl cellulose bound to a metallic current collector[11] Each of these components contributes to the equivalent series resistance (ESR) of devices as well as the resistance offered by the electrolyte and the interface between the electrode and electrolyte. EDLC electrodes comprising a composite coating can be considered as conductive particles contained within an insulating matrix and the contributions to the ESR arise from the binder material and interparticle contact resistances. In addition these coatings include void space from the intrinsic porosity of the carbon particles but arising from the interstitial space between particles. Such voids are necessary to facilitate electrolyte access and contribute significantly to device resistance
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