  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.
