Mehul N. Patel

Hybrid MnO2–disordered mesoporous carbon nanocomposites: synthesis and characterization as electrochemical pseudocapacitor electrodes

MnO2–mesoporous carbon hybrid nanocomposites were synthesized to achieve high values of redox pseudocapacitance at scan rates of 100 mV s−1. High-resolution transmission electron microscopy (HRTEM) along with energy dispersive X-ray spectroscopy (EDX) demonstrated that ∼1 nm thick MnO2 nanodomains, resembling a conformal coating, were uniformly distributed throughout the mesoporous carbon structure. HRTEM and X-ray diffraction (XRD) showed formation of MnO2 nanocrystals with lattice planes corresponding to birnessite. The electrochemical redox pseudocapacitance of these composite materials in aqueous 1 M Na2SO4 electrolyte containing as little as 2 wt% MnO2 exhibited a high gravimetric MnO2 pseudocapacitance (CMnO2) of 560 F gMnO2−1. Even for 30 wt% MnO2, a high CMnO2 of 137 F gMnO2−1 was observed at 100 mV s−1. Sodium ion diffusion coefficients on the order of 10−9 to 10−10 cm2 s−1 were measured using chronoamperometry. The controlled growth and conformal coating of redox-active MnO2–mesoporous carbon composites offer the potential for achieving high power energy storage with low cost materials.

High pseudocapacitance of MnO2 nanoparticles in graphitic disordered mesoporous carbon at high scan rates

Nanocomposites composed of MnO2 and graphitic disordered mesoporous carbon (MnO2/C) were synthesized for high total specific capacitance and redox pseudocapacitance (CMnO2) at high scan rates up to 200 mV s−1. High resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy (EDX) demonstrated that MnO2 nanodomains were highly dispersed throughout the mesoporous carbon structure. According to HRTEM and X-ray diffraction (XRD), the MnO2 domains are shown to be primarily amorphous and less than 5 nm in size. For these composites in aqueous 1 M Na2SO4 electrolyte, CMnO2 reached 500 F/gMnO2 at 2 mV s−1 for 8.8 wt% MnO2. A capacitance fade of only 20% over a 100-fold change in scan rate was observed for a high loading of 35 wt% MnO2 with a CMnO2 of 310 F/gMnO2 at the highest scan rate of 200 mV s−1. The high electronic conductivity of the graphitic 3D disordered mesoporous carbon support in conjunction with the thin MnO2 nanodomains facilitate rapid electron and ion transport offering the potential of improved high power density energy storage pseudocapacitors.

Flow-Based Multiadsorbate Ellipsometric Porosimetry for the Characterization of Mesoporous Pt−TiO2 and Au−TiO2 Nanocomposites

Au and Pt nanoparticle distributions within hierarchically ordered mesoporous TiO2 were explored using a combination of techniques including ellipsometric porosimetry (EP) and X-ray photoelectron spectroscopy (XPS). EP studies were used to examine adsorbate-TiO2 interactions and the influence of adsorbate polarity upon adsorption isotherms for mesoporous TiO2 films with and without Pt and Au nanoparticles. In particular, methods are described for modeling EP data to estimate the surface area and porosity of mesoporous TiO2 films and for estimating the pore size distribution (PSD) directly from the ellipsometry parameters Psi and Delta when fitting parameters alone are unable to extract reliable optical constants from the ellipsometry data. This approach reveals that mesoporous TiO2 films of approximately 200 nm thickness and approximately 10 nm pore diameter can be loaded with 1.7 nm diameter Pt and 3.9 nm diameter Au nanoparticles up to 26 and 21 wt %, respectively. The BET surface area of a representative mesoporous TiO2 sample using toluene as the adsorbate was found to be 44 m2/g with a mean pore diameter of 8.8 nm. EP and XPS depth profiling experiments indicate that 1.7 nm diameter Pt nanoparticles are well dispersed through the mesoporous TiO2 film, while 3.9 nm diameter Au nanoparticles are concentrated at the top of the film, blocking a significant portion of the available TiO2 pore volume. UV irradiation of the TiO2 films indicates that adsorbate-TiO2 interactions and surface wetting effects can play a critical role in the resulting isotherm and in evaluation of PSD.

Electrophoretic mobility of concentrated carbon black dispersions in a low-permittivity solvent by optical coherence tomography

Electrophoretic mobilities of concentrated dispersions of carbon black particles in a low-permittivity solvent were measured using differential-phase optical coherence tomography (DP-OCT). An electrode spacing of only 0.18 mm enables measurement of highly concentrated dispersions up to 1 wt.% of highly absorbing carbon black particles with high electric fields at low potentials. The capabilities of this DP-OCT method, including high sensitivity, high spatial resolution, and strong electric fields, enable enhanced measurement of low electrophoretic mobilities encountered in low-permittivity solvents. The zeta potential of carbon black particles ranged from -24 mV to -12 mV as the concentration of surfactant sodium bis(2-ethyl-1-hexyl)sulfosuccinate (AOT) was increased from 1 mM to 100 mM. A mechanism is presented to explain the electrostatic charging of carbon black particles in terms of the partitioning of the ions between the reverse micelles in the double layer and the surfactant adsorbed on the particle surface, as AOT concentration is varied.