Connection between Proton Abnormal Conductivity in Water and Dielectric Relaxation Time

Structural changes in colloid solutions of nanodiamond

Segregation of particles in a nanodiamond aqueous colloid due to Stokes’ law leads to re-ordering of the lattice of particles.

NMR and Electrochemical Investigation of the Transport Properties of Methanol and Water in Nafion and Clay-Nanocomposites Membranes for DMFCs

Water and methanol transport behavior, solvents adsorption and electrochemical properties of filler-free Nafion and nanocomposites based on two smectite clays, were investigated using impedance spectroscopy, DMFC tests and NMR methods, including spin-lattice relaxation and pulsed-gradient spin-echo (PGSE) diffusion under variable temperature conditions. Synthetic (Laponite) and natural (Swy-2) smectite clays, with different structural and physical parameters, were incorporated into the Nafion for the creation of exfoliated nanocomposites. Transport mechanism of water and methanol appears to be influenced from the dimensions of the dispersed platelike silicate layers as well as from their cation exchange capacity (CEC). The details of the NMR results and the effect of the methanol solution concentration are discussed. Clays particles, and in particular Swy-2, demonstrate to be a potential physical barrier for methanol cross-over, reducing the methanol diffusion with an evident blocking effect yet nevertheless ensuring a high water mobility up to 130 °C and for several hours, proving the exceptional water retention property of these materials and their possible use in the DMFCs applications. Electrochemical behavior is investigated by cell resistance and polarization measurements. From these analyses it is derived that the addition of clay materials to recast Nafion decreases the ohmic losses at high temperatures extending in this way the operating range of a direct methanol fuel cell.

Effective improvement of water-retention in nanocomposite membranes using novel organo-modified clays as fillers for high temperature PEMFCs

Toward an enhanced water-retention of polymer electrolyte membranes at high temperatures, novel organo-modified clays were prepared and tested as fillers for the creation of hybrid Nafion nanocomposites. Two smectite clays (Laponite and montmorillonite), with different structural and physical parameters, were loaded with various cationic organic molecules bearing several hydrophilic functional groups (-NH(2), -OH, -SO(3)H) and incorporated in Nafion by solution intercalation. The resulted hybrid membranes were characterized by a combination of powder X-ray diffraction, FTIR spectroscopy, and thermal analysis (DTA/TGA) showing that highly homogeneous exfoliated nanocomposites were created where the individual organoclay layers are uniformly dispersed in the continuous polymeric matrix. In this paper, water-transport properties were investigated by NMR spectroscopy, including pulsed-field-gradient spin-echo diffusion and spectral measurements conducted under variable temperature. Organo-montmorillonite nanofillers demonstrate a considerable effect on the Nafion polymer in terms both of water absorption/retention and water mobility with a remarkable behavior in the region of high temperatures (100-130 °C), denoting that the surface modifications of this clay with acid organic molecules significantly improve the performance of the final composite membrane. (1)H NMR spectral analysis allowed a general description of the water distribution in the system and an estimation of the number of water molecules involved in the hydration shell of the sulfonic groups as well as that absorbed on the organoclay particles.

NMR investigation of the dynamics of confined water in nafion-based electrolyte membranes at subfreezing temperatures

The dynamical characteristics and the thermal analysis of water absorbed in filler-free Nafion and in silica or zirconia phosphate Nafion composites, between 20 and -50 degrees C, were investigated by NMR and DSC techniques. Self-diffusion coefficients and longitudinal NMR relaxation times (T(1)) put in evidence a fraction of water freezing at subzero temperatures. The complementary water fraction remains in the liquid state at least down to -50 degrees C. The freezing point (T(f)) depends on the initial water uptake of the electrolyte membrane and, for similar uptake values, water mobility is favorite in composites systems respect to the filler-free Nafion. By DSC thermograms the hydration water molecules number per sulfonic group in the filler-free Nafion was estimated, obtaining 8 molecules/SO(3)(-) group. In the Nafion/Zr(HPO(4))(2) composite, instead, the number of hydration water is about 20 molecules/ionic group, because of the acid nature of the zirconia particles. Below T(f), the presence of this nonfreezable water fraction allows proton transport, and therefore ensures ionic conductivity also at subzero temperatures.

NMR studies of structure and dynamics of liquid molecules confined in extended nanospaces

We fabricated an NMR cell equipped with 10-100 nm scale spaces on a glass substrate (called extended nanospaces), and investigated molecular structure and dynamics of water confined in the extended nanospaces by (1)H NMR chemical shift (delta(H)) and (1)H and (2)H NMR spin-lattice relaxation rate ((1)H- and (2)H-1/T(1)), (1)H NMR spin-spin relaxation rate ((1)H-1/T(2)), and (1)H NMR rotating-frame spin-lattice relaxation rate ((1)H-1/T(1rho)) measurements of H(2)O and (2)H(2)O. The delta(H) and (1)H- and (2)H-1/T(1) results showed that size-confinement produces slower translational motions and higher proton mobility of water, but does not affect the hydrogen-bonding structure and rotational motions. Such unique phenomena appeared in the space size of 40 to 800 nm. However, the (1)H-1/T(1) value at 40 nm was still different from that in 4 nm porous nanomaterial, because translational and rotational motions were inhibited for H(2)O molecules in the nanomaterial. By examining temperature- and deuterium-dependence of the (1)H-1/T(1) values, the molecular translational motions of the confined water were found to be controlled by protonic diffusion invoking a proton hopping pathway between adjacent water rather than hydrodynamic translational diffusion. Furthermore, we clarified that proton exchange between adjacent water molecules in extended nanospaces could be enhanced by the chemical exchange of protons between water and SiOH groups on glass surfaces, ( identical with SiO(-)...H(+)...H(2)O) + H(2)O --> triple bond SiO(-) + (H(3)O(+) + H(2)O) --> triple bond SiO(-) + (H(2)O + H(3)O(+)), based on (1)H-1/T(2) measurements. An enhancement of proton exchange rate of water due to the reduction of space sizes was verified from the results of (1)H-1/T(1rho) values, and the rate of water in the 100 nm sized spaces is larger by a factor of more than ten from that of bulk water. Such size-confinement effects were distinctly observed for hydrogen-bond solvents with strong proton-donating ability, while they did not appear for aprotic and nonpolar solvent cases. Based on these NMR results, we suggested that an intermediate phase, in which protons migrate through a hydrogen-bonding network and the water molecules are loosely coupled within 50 nm from the surface, exists mainly in extended nanospaces. This model could be supported by a three-phase theory based on the weight average of three phases invoking the bulk, adsorbed, and intermediate phases.

Protons in Supercritical Water: A Multistate Empirical Valence Bond Study

Molecular dynamics simulations have been performed to analyze microscopic details related to aqueous solvation of excess protons along the supercritical T = 673 K isotherm, spanning a density interval from a typical liquid down to vapor environments. The simulation methodology relies on a multistate empirical valence bond Hamiltonian model that includes a proton translocation mechanism. Our results predict a gradual stabilization of the solvated Eigen cation [H(3)O.(H(2)O)(3)](+) at lower densities, in detriment of the symmetric Zundel dimer [H.(H(2)O)(2)](+). At all densities, the average solvation structure in the close vicinity of the hydronium is characterized by three hydrogen bond acceptor water molecules and presents minor changes in the solute water distances. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. Compared to room temperature results, the rates at high densities are 4 times faster and become progressively slower in steamlike environments. Diffusion coefficients for the excess proton have also been computed. In agreement with conductometric data, our results show that contributions from the Grotthus mechanism to the overall proton transport diminish at lower densities and predict that in steamlike environments, the proton diffusion is almost 1 order of magnitude slower than that for pure water. Spectroscopic information for the solvated proton is accordant to the gradual prevalence of proton localization in Eigen-like structures at lower densities.