Active carbon based supercapacitors with Au colloids: the case of placing the colloids in close proximity to the electrode interface

Supercapacitors (SCs) are short-term energy storage elements that find many applications, e.g., electronic charging devices and suppressors of power fluctuations in grids that are interfaced with sustainable sources. The capacitance of an ordinary capacitor increases when dispersing metallic colloids in its dielectric. A similar strategy for SCs means deployment of nano-scale metal colloids (in our case, Au nanoparticles, or AuNPs) at the very narrow interface between an electrolyte and a porous electrode (here, active carbon film, AC, on a grafoil current collector). Unlike previous studies, here we placed AuNPs at a small distance from the electrode. This was achieved by coating the AuNPs with a negatively charged ligand that also enables strong adhesion to the electrode. A very large specific capacitance amplification was demonstrated: for example, C-V data at a scan rate of 20 mV s⁻¹ indicated a specific capacitance amplification of more than 10 when 30 μg of AuNPs was incorporated with 200 mg of active carbon while using a 1 M Na₂SO₄ electrolyte and a 5% cellulose acetate butyrate binder. Upon replacing the 1 M Na₂SO₄ electrolyte with 1 M KOH, and keeping the same set of electrodes, the amplification factor decreased but remained large, ∼3, as determined using C-V traces at the same scan rate. This proves that the AuNPs adhered well to the AC electrodes. Simulations indicated the importance of keeping the AuNPs in close proximity to the electrodes, but not in direct contact with them, in order to maintain a substantial amplified polarization effect. Unlike semiconductor embedded electrodes, optical effects were found to be minimal.