Asymmetric supercapacitors: optical and thermal effects when active carbon electrodes are embedded with nano-scale semiconductor dots

TL;DR

This study investigates how embedding nano-silicon quantum dots in active-carbon electrodes of asymmetric supercapacitors affects optical and thermal properties, leading to significant capacitance enhancements through optical and thermal effects.

Contribution

It introduces novel supercapacitor structures with nano-silicon quantum dots and demonstrates their impact on capacitance via optical and thermal mechanisms.

Findings

Absorption bands in active carbon influence capacitance.

Optical absorption in blue enhances capacitance by over 100%.

Thermal effects contribute to capacitance increase similarly to optical effects.

Abstract

Optical and thermal effects in asymmetric supercapacitors, whose active-carbon (AC) electrodes were embedded with nano-Si (n-Si) quantum dots (QD), are reported. We describe two structures: (1) p-n like, obtained by using a polyethylimine (PEI) binder for the n-like electrode and a polyvinylpyrrolidone (PVP) binder for the p-like electrode; (2) a single component binder, poly(methyl methacrylate) (PMMA). In general, AC appears black to the naked eye and one may assume that it acts as a black body absorber, namely, indiscriminately absorbing all light spectra. Yet, on top of a flat lossy spectra, AC (from two manufacturers) exhibited two distinct absorption bands: one in the blue (~ 400 nm) and the other one in the near IR (~ 840 nm). The n-Si material accentuated the absorption in the blue and bleached the IR absorption. Both bands contributed to capacitance increase: (a) when using aqueous solution and a PMMA binder, the optical related increased capacitance was 20% at low n-Si concentration and more than 100% for a high concentration dose; (b) when using IL electrolyte, the large, thermal capacitance increase (of ca 40%) was comparable to the optical effect (of ca 42%) and hence was assigned as an optically-induced thermal effect. The experimental data point to an optically induced capacitance increase even in the absence of the n-Si dots; this could be attributed to the absorption of AC in the blue.