Abstract
A model study of electric double layer capacitor (EDLC)-style capacitors in which the electrodes were composed of low surface area-oriented flakes of graphite that compressed to form a paper-like morphology has suggested that ion transport rates significantly impact EDLC energy and power density. Twelve capacitors were constructed, each using the same model electrode material and the same aqueous NaCl electrolyte, but differing in relative electrode orientation, degree of electrode compression, and presence/absence of an ionic transport salt bridge. All were tested with a galvanostat over a range of discharge currents. Significant differences in energy and power density and estimated series resistance were found as a function of all the factors listed, indicating that capacitor performance is not simply a function of the electrode surface area. This simple postulation was advanced and tested against data: net ion (Na+, Cl−) ‘velocity’ during both charge and discharge significantly impacts capacitive performance.
Highlights
The development of capacitors with high energy and/or power density will help enable the proposed switch from combustion-based energy systems to ‘green’ electric systems that are recharged with renewables
It is unlikely that capacitors will ever match the energy density of batteries, but the superior power output and durability of capacitors ensures that capacitors will have a number of niche rolls, including load leveling in systems for which batteries are the primary source, battery life extension applications such as for satellites, collecting energy from high power surge sources such as regenerative braking, and providing pulsed power for inherently high power applications, as in [1,2,3]
The data for the model system showed many of the qualitative results expected for capacitors, including: (1) The higher the discharge current, the higher the power density; (2) The higher the electrode surface area/kg, the higher the energy and power density
Summary
The development of capacitors with high energy and/or power density will help enable the proposed switch from combustion-based energy systems to ‘green’ electric systems that are recharged with renewables. In ‘carbon only’ systems, as per this report, virtually all the increased research has been directed at exploring the effect of the microstructure (e.g., graphene, CNT) and the impact of various additives on carbon energy density, power density, and electric conductivity [1,2,4,5,6,7,8,9,10,11,12]. There has been relatively little work designed to quantify the impact of these factors on carbon-based capacitor performance [13,17,18]
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