Supercapacitors with Gold Colloids That Are Placed at a Short Distance from Active-Carbon Electrodes; The Case for a Plasmonic Effect at the Electrolyte/Electrode Interface

Abstract

Supercapacitors (S-C) are fast charge/discharge elements. These short-term energy storage found many applications; among which are, powering electronic devices, boosters of fast charging batteries and possibly, as buffers between highly fluctuating sustainable sources and the rather stable electrical grid. For double-layer S-C, one takes advantage of the very narrow interface between an electrolyte and a conductive porous electrode. In contrast, ordinary capacitors are made of a relatively thicker dielectric material, sandwiched between two electrodes.In the past, polarity increase in capacitors was achieved by dispersing conductive colloids in the entire volume of the dielectric material. This approach was primarily used for microwave and optical devices – a field known as plasmonics. For S-C it means that the conductive colloids should be dispersed within the double-layer region, or fairly close to it. The impact of such an approach on the low frequency supercapacitors is not fully yet known. In order to maintain a full polarization effect (as opposed to simply increasing the electrode conductivity), we kept the AuNP at a short distance from the electrode. This is achieved by coating the colloids with a negatively charged ligand - the same approach used to keep a colloidal suspension intact.Here we explore the addition of nano-size Au particles (AuNP) to active-carbon electrodes to find a large amplification in the specific capacitance. For example, C-V data at a scan rate of 20 mV/s indicated a specific capacitance amplification of more than 10 (!) (with respect to a reference sample without the colloids) when 30 micro-g of AuNPs were incorporated with 200 milli-g of active carbon while using a 1 M Na2SO4 electrolyte and a 5% cellulose acetate butyrate binder. These experiments were corroborated by simulations: the Au colloids are indeed to be placed at close proximity but not in contact with the electrodes.