Eduardo Enciso

A high voltage solid state symmetric supercapacitor based on graphene–polyoxometalate hybrid electrodes with a hydroquinone doped hybrid gel-electrolyte

In pursuit of high capacitance and high energy density storage devices, hybrid materials have quickly garnered well-deserved attention based on their power to merge complementary components and properties. Here, we report the fabrication of all-solid state symmetric supercapacitors (ASSSC) based on a double hybrid approach combining a hybrid electrode (reduced graphene oxide-phoshomolybdate, rGO-PMo12) and a hybrid electrolyte (hydroquinone doped gel-electrolyte). To begin with, a high-performance hybrid electrode based on H3PMo12O40 nanodots anchored onto rGO was prepared (rGO-PMo12). Later, an all-solid state symmetric cell based on these rGO-PMo12 electrodes, and making use of a polymer gel-electrolyte was assembled. This symmetric cell showed a significant improvement in cell performance. Indeed, it allowed for an extended potential window by 0.3 V that led to an energy density of 1.07 mW h cm-3. Finally, we combined these hybrid electrodes with a hybrid electrolyte incorporating an electroactive species. This is the first proof-of-design where a redox-active solid-state gel-electrolyte is applied to rGO-PMo12 hybrid supercapacitors to accomplish a significant enhancement in the capacitance. Strikingly, a further excellent increase in the device performance (energy density of 1.7 mW h cm-3) was realized with the hybrid electrode-hybrid electrolyte combination cell as compared to that of the conventional electrolyte cell. Thus, this unique symmetric device outclasses the high-voltage asymmetric counterparts under the same power and represents a noteworthy advance towards high energy density supercapacitors.

Monte Carlo simulations of liquid n-butane

We have carried out Monte Carlo (MC) simulations for a model of liquid n-butane in three thermodynamic states. In each case, both the flexible model in the limit when bond-length and bond-angle forces become infinite and the constrained model were studied. The different trajectories followed by each model lead to different ensemble averages for the intramolecular properties, such as the conformational distribution and intramolecular energy. The changes observed in the external configurational properties, such as the intermolecular energy and pressure are less than the statistical error. In all the cases we have found a weak shift of the trans conformational population with respect to the ideal-gas distribution (i.e. less than 3%). In addition, we have explored the possibilities of a purely repulsive model as a reference system for this liquid. As in the case of simple atomic liquids, the standard hard-core theoretical approaches are good starting points for obtaining the contribution of the external degre...

Macroporous silica and titania obtained using poly[styrene-co-(2-hydroxyethyl methacrylate)] as template

Ordered macroporous silica and titania have been prepared using poly[styrene-co-(2-hydroxyethyl methacrylate)] (PS-HEMA) latex arrays as templates. Polystyrene (PS) particles have also been used to template macroporous titania. Arrays of spherical latex particles, with average diameters of 260 nm (PS-HEMA) and 390 nm (PS), have been obtained by either evaporation of the solvent at room temperature or by deposition on filtration membranes. In both cases, 3D extended regions with cubic close packing of microspheres are observed, but in the samples obtained after evaporation of the solvent the self-assembled particles show a higher degree of order. The oxides were synthesized by hydrolysis of sodium silicate or titanium isopropoxide (diluted in n-propanol or hexane) which were infused into the cavities of the latex arrays. Two different methods have been used to fill the cavities with the precursor: filtration with suction and capillary forces. When solidification of the ceramic matrix took place, the template was eliminated by heating at 500 °C. As replica of the latex assemblies, periodic porous silica and titania were obtained. Sheets of amorphous silica, as thin as 4–7 nm, form the wall of the interconnected pores whose diameters range from 200 to 220 nm. In macroporous titania obtained from titanium isopropoxide diluted in n-propanol and imprinted on PS-HEMA particles, the pore diameters range from 160–180 nm. The porous network is shaped by walls that are 8–12 nm thick and are formed from anatase nanocrystals with a major dimension of 8–20 nm. Pore diameters, crystallite sizes and thicknesses are similar when hexane is used as the solvent of the inorganic precursor. However, in macroporous titania imprinted on PS particles, nanocrystals with a major dimension of 6–10 nm form the pore wall with a thickness of 6 to 8 nm. Interactions between the titania precursor and the latex particles seem to affect the microstructural parameters. The porous skeleton of titania is preserved after treatment at 600 °C for 16 hours but the pore wall thickness slightly increases with crystallite growth and in some particles disordered areas appear.

Monte Carlo simulation of liquid n‐alkanes. I. Intramolecular structure and thermodynamics

The conformational properties of liquid n‐alkanes (ranging from n‐pentane to n‐decane) have been investigated using Monte Carlo computer simulation techniques. The method of simulation combines the ‘‘reptation’’ method with a scheme of preferential sampling, which leads to an improvement of the simulation efficiency. The change of internal properties and structure as an effect of the density is studied.

Conventional unidirectional laser action enhanced by dye confined in nanoparticle scatters.

The synthesis, structural characterization, and lasing properties of new dye-sensitized organic scattering gain medium based on Rhodamine 6G (Rh6G) confined in polymeric nanoparticles are reported. We have demonstrated coherent laser action from amplifying random media using dye confined into polymeric nanoparticles as scattering centers and gain media. Lasing efficiency and photostability were significantly enhanced by nonresonant feedback of the emission by multiple scattering.

Ethanol force fields: A molecular dynamics study of polarization effects on different phases

We have studied two simple approaches to consider thermodynamic state dependent electrostatic interactions for molecular modeling of different phases of ethanol: the addition of an isotropic point polarizability located in the oxygen, and a self-consistent calculation of the effective dipole moment at each thermodynamic state. We have performed molecular dynamics simulations in order to investigate the thermodynamic properties, structure and dynamics of the liquid phase at three experimental densities, as well as in the monoclinic crystal and at critical conditions. In order to rationalize the effects of changing the dipole moment of the molecules, simulations with a nonpolarizable model for ethanol were also performed. The results show that a nonpolarizable model with an effective dipole moment is able to reproduce most of the static and dynamic properties of the condensed phases of ethanol, while the need to take into account the real dipole moment of the isolated molecule by using a polarizable model i...

Diffusional dynamics of hydrogen-bonded liquids: methanol

Medium-resolution ( Delta E approximately=0.1 meV) quasielastic neutron scattering spectra have been measured for partially (CD3OH) and fully deuterated methanol, (CD3OD) in the temperature range between 200 and 300 K. The spectra are analysed in terms of a simplified model which takes into account both coherent and incoherent contributions, and includes translational and rotational motions of the molecular frame as well as the rapid reorientation of the CD3 group. Such a procedure enabled the separation of the translational and rotational contributions to the measured intensity. The apparent lack of saturation of the translational linewidths versus the momentum transfer is finally discussed in terms of a phenomenological stochastic model.

Low density equation of state of asymmetric hard sphere mixtures

The equation of state of highly asymmetric hard sphere mixtures at low and moderate densities is investigated using NpT Monte Carlo simulations. The discrepancies of the estimated fifth virial coefficient with the reported values of Saija et al. (Saija, F., Fiumara, G. and Giaquinta, P. V., 1996, Molec. Phys., 89, 1181) led us to compute such a coefficient. The new results are fully compatible with the compressibility factors estimated by Monte Carlo simulation and show excellent agreement with the predicted coefficients from the Boublik and Mansoori et al. equation of state.

Statistical mechanics of small chain molecular liquids. I. Conformational properties of modeled n‐butane

The density functional formalism can also be profitably applied to the statistical mechanics of molecular fluids with internal degrees of freedom. The change of the intrinsic chemical potential for different conformers of nonpolar molecules is given by a zeroth order perturbation approach. This approach is applied to study the isomerization of n‐butane in liquids, which models the neat liquid and CCl4 solutions. We find that a nonpolar solvent medium produces a shift in the conformational equilibria of model liquid n‐butane from that found in the gas phase. The theoretical predictions show a good agreement with recent Monte Carlo and molecular dynamics simulations results. Discrepancies with Jorgensen et al.’s data are explained in terms of the attractive intermolecular forces used in their MC codes.

Collective excitations in liquid methanol studied by coherent inelastic neutron scattering

Inelastic spectra of deuteromethanol measured by triple-axis neutron spectroscopy at several temperatures within the liquid range are reported. The observed scattering intensities are analysed in terms of reconstructions of the dynamical structure factor after deconvolution from instrumental effects, as well as model fits to the measured intensities. The data indicate the existence of a propagating high-frequency excitation, which at T=200 K becomes overdamped for wavevectors Q>0.6 AA-1. An insight into the nature of the observed excitation is given by the temperature behaviour of the damping factors. Such a trend gives additional support to the assignment of the excitation to closely packed patches of hydrogen-bonded molecules.