Nanoparticles in mesoporous materials : optical and electrochemical properties for energy storage applications
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The design of nanoparticles in mesoporous supports is explored through synthetic strategies of electrophoretic deposition and electroless deposition with application towards energy storage. Electrophoretic deposition of nanoparticles into a mesoporous thin film is examined using charged nanocrystals in a low-permittivity solvent. To provide a basis for the deposition, the mechanism of particle charging in a low-permittivity solvent was studied. Dispersions of carbon black particles in toluene with an anionic surfactant were characterized using differential-phase optical coherence tomography with close electrode spacing to measure the electrophoretic mobility. The particle charge in concentrated dispersions was found to decrease as a function of increasing surfactant concentration. Partitioning of cations between the surfactant-laden particle surface and micelle cores in the double-layer was found to govern the dynamics of particle charging. Subsequently, charged Au nanocrystals were deposited by electrophoresis within perpendicular mesochannels of a TiO2 support. High loadings of 21 wt% Au with good dispersion were achieved within the mesoporous TiO2 support using electrophoretic deposition, which would otherwise be inhibited by the weak nanocrystal-support interaction. According to a modified Fokker-Planck equation, the mean penetration depth of a single nanocrystal inside of the perpendicular pores was found to be dependent on the electric field strength, electrophoretic mobility, pore diameter, nanocrystal size, and local deposition rate constant. Nanocomposites for electrochemical capacitors were designed via electroless deposition of redox-active MnO2 in a high surface area mesoporous carbon support. Disordered mesoporous carbon supports with a pore size of ~8 nm were used both in amorphous (AMC) and graphitic (GMC) form, with a ~1000-fold larger conductivity for GMC. High loadings of 30 wt% MnO2 were achieved in the AMC in the form of ~1 nm thick domains, which were highly dispersed throughout the support. Oxidation of the GMC was necessary to facilitate wetting and deposition of the MnO2 precursor in order to achieve high loadings of 35 wt% MnO2 with ~1 nm thickness. High gravimetric MnO2 pseudocapacitances of >500 F/gMnO2 were achieved at low loadings and low scan rate of 2 mV/s for both carbon supports. However, at high scan rates ≥100 mV/s, the MnO2 pseudocapacitance is twofold larger for MnO2/GMC, relative to MnO2/AMC. Sodium ion diffusion throughout both MnO2/AMC and MnO2/GMC was shown to be facile. For the GMC versus AMC support, the higher MnO2 pseudocapacitance is attributed to the higher electronic conductivity, which facilitates electron transport to the MnO2 domains.