Optically Controlled Supercapacitors: Functional Active Carbon Electrodes with Semiconductor Particles

Abstract

Supercapacitors, S-C—capacitors that take advantage of the large capacitance at the interface between an electrode and an electrolyte—have found many short-term energy applications. The parallel plate cells were made of two transparent electrodes (ITO), each covered with a semiconductor-embedded, active carbon (A-C) layer. While A-C appears black, it is not an ideal blackbody absorber that absorbs all spectral light indiscriminately. In addition to a relatively flat optical absorption background, A-C exhibits two distinct absorption bands: in the near-infrared (near-IR and in the blue. The first may be attributed to absorption by the OH− group and the latter, by scattering, possibly through surface plasmons at the pore/electrolyte interface. Here, optical and thermal effects of sub-μm SiC particles that are embedded in A-C electrodes, are presented. Similar to nano-Si particles, SiC exhibits blue band absorption, but it is less likely to oxidize. Using Charge-Discharge (CD) experiments, the relative optically related capacitance increase may be as large as ~34% (68% when the illuminated area is taken into account). Capacitance increase was noted as the illuminated samples became hotter. This thermal effect amounts to <20% of the overall relative capacitance change using CD experiments. The thermal effect was quite large when the SiC particles were replaced by CdSe/ZnS quantum dots; for the latter, the thermal effect was 35% compared to 10% for the optical effect. When analyzing the optical effect one may consider two processes: ionization of the semiconductor particles and charge displacement under the cell’s terminals—a dipole effect. A model suggests that the capacitance increase is related to an optically induced dipole effect.

Optically Controlled TiO2-Embedded Supercapacitors: The Effects of Colloidal Size, Light Wavelength, and Intensity on the Cells’ Performance

“…The normalized optical transmission for the TiO 2 colloids (small, 0.2 µm and large, 2 µm that are embedded in poly(methyl methacrylate), PMMA) are shown in Figure 2 b for dry films (see Ref. [ 20 ] regarding light absorption by wet films). Each transmission curve was first referenced to its substrate (a glass slide) in order to remove the effects of the glass (if any).…”

“…That effect persisted in the dark for over 5 min after switching off the illuminator. Unlike SiC-embedded [ 20 ] or n-Si-embedded A-C [ 21 ], one needs to remeasure the sample in the dark after each light exposure in order to maintain a better baseline.…”

“…Finally, optically enabled charge-trapping mechanisms may explain the lack of faradaic behavior, which is typical of pseudocapacitors, namely, the absence of chemical reaction peaks in the C-V curves - the effect is purely physical. Neither as-is A-C electrodes ( Figure 8 ), nor other semiconductor embedded systems [ 20 , 21 ] seem to exhibit such a lingering polarization effect.…”

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

“…On the average, the A-C particulates' dimension were ca 15 microns [15,29] and their specific surface area was rated as 1100 m 2 /g. Through mixing, the AuNPs would end up either on the A-C particulate, in the binder, or in the electrolyte.…”

“…Our intent is to gain basic knowledge on the capacitance changes when incorporating very low dispersion of gold colloids with active-carbon (A-C) electrodes; these are deposited on a current-collector, made of grafoil. The A-C electrode material is consisted of relatively large particle of size of ~15 m [15] and the colloids may find themselves on its surface or within its pores owing to an electrostatic bond between the negatively charged ligand, coating the AuNPs and the conductive (yet, neutral) A-C. The very large ratio between the A-C mass and the mass of the colloids (a ratio of more than 100 mg to 10 g, or 100,000:10) makes it plausible that the observed ca x10 amplification may be attributed to near-field polarization effects at the electrolyte-electrode interface (see also simulations at the end of this manuscript).…”