Mark Fedkin

Nafion ∕ TiO2 Proton Conductive Composite Membranes for PEMFCs Operating at Elevated Temperature and Reduced Relative Humidity

Nafion/TiO 2 composite membranes with different TiO 2 contents were studied in an H 2 /O 2 proton exchange membrane fuel cell (PEMFC) over a wide range of relative humidity (RH) values from 26 to 100% at temperatures of 80and 120°C. The composite membranes, which were prepared using a recast procedure, showed a pronounced improvement over unmodified Nafion membranes when operated at 120°C and reduced RH. For instance, at 50% RH, the Nafion/20% TiO 2 membrane demonstrated a performance identical to that of an unmodified Nafion membrane operated at 100% RH. This performance level was comparable to that of a bare Nafion membrane at 80°C. The high performance of the Nafion/TiO 2 composite membranes at low RH was attributed to improved water retention due to the presence of absorbed water species in the electrical double layer on the TiO 2 surface. The zeta potential and thickness of the hydrodynamically immobile water layer at the TiO 2 /water interface were discussed as parameters influencing the water balance in the membranes. The obtained experimental PEMFC performance data were fitted using an analytical equation, and calculated parameters were analyzed as functions of RH and TiO 2 content in the composite membranes.

Evaluation of methanol crossover in proton-conducting polyphosphazene membranes

Abstract A diffusion cell was developed to evaluate the methanol crossover for a novel class of polyphosphazene electrolyte membranes. It was found that the methanol diffusion coefficients of phenyl phosphonic acid functionalized poly[aryloxyphosphazene] membranes in an aqueous methanol solution (50% v/v) were ∼40 times lower than for Nafion 117, and ∼10–20 times lower than for sulfonated polyphosphazene membranes.

Comparison of cation adsorption by isostructural rutile and cassiterite.

Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.

Electrophoretic study of the SnO2/aqueous solution interface up to 260 degrees C.

An electrophoresis cell developed in our laboratory was utilized to determine the zeta potential at the SnO(2) (cassiterite)/aqueous solution (10(-3) mol kg(-1) NaCl) interface over the temperature range from 25 to 260 degrees C. Experimental techniques and methods for the calculation of zeta potential at elevated temperature are described. From the obtained zeta potential data as a function of pH, the isoelectric points (IEPs) of SnO(2) were obtained for the first time. From these IEP values, the standard thermodynamic functions were calculated for the protonation-deprotonation equilibrium at the SnO(2) surface, using the 1-pK surface complexation model. It was found that the IEP values for SnO(2) decrease with increasing temperature, and this behavior is compared to the predicted values by the multisite complexation (MUSIC) model and other semitheoretical treatments, and were found to be in excellent agreement.

Zetameter for microelectrophoresis studies of the oxide/water interface at temperatures up to 200 °C

The zeta potential (ZP) is an important and measurable parameter related to the electrical double layer structure at a solid-aqueous solution interface. A high temperature zetameter based on the microelectrophoresis technique was developed to determine the zeta potential and the isoelectric point (IEP) of the metal oxide/water interfaces at temperatures up to 200 °C and pressures up to 50 bar. Design of the microelectrophoresis cell, the main unit of the zetameter, utilized a flow-through concept and the cell internals were made from corrosion resistant materials in order to minimize materials degradation and solution contamination. Two sapphire windows were installed to the microelectrophoresis cell to enable observation of the particle movement under an imposed electrical field. A ZrO2 powder was used to test the zetameter. The ZP for the ZrO2/water system was measured over wide ranges of temperature and pH. The IEP of the ZrO2/water system was found equal to 6.05 at room temperature, 5.00 at 120 °C, an...

Proton Conductive Inorganic Materials for Temperatures Up to 120 ° C and Relative Humidity Down to 5%

The proton conductivities of inorganic conductors in a temperature range of 25-120°C and relative humidity (RH) from 1 to 100% were studied. A conductivity cell was designed and built. A conductivity measurement method was developed including inorganic sample preparation, a reliable condition-controlled experimental system, conductivity measurement protocol, and an electrochemical impedance spectroscopy equivalent circuits. Alpha zirconium phosphate was used as a reference material to test the designed experimental approach based on literature study and laboratory measurements. Solid acid materials, such as sulfated zirconia (S-ZrO 2 ), phosphosilicate gels (P-SiO 2 ), and sulfonic-functionalized silica (SBA-15) were synthesized, characterized, and their conductivities were measured. The impedance characteristics and transport mechanisms were discussed. Under a fully hydrated condition at 120°C, S-ZrO 2 presented a proton conductivity of 10 mS cm -1 . P-Si0 2 was found to be a pure proton conductor. The materials with a P:Si ratio of 1.5 reached a high proton conductivity of 100 mS cm -1 , which was higher than that of Nafion at 120°C and 70% RH. In the study, SBA-15 appeared to be a partially electron-conductive material.

Chapter 12 – Ion Adsorption into the Hydrothermal Regime: Experimental and Modeling Approaches

This chapter provides an overview of the experimental and modeling approaches that are used to study ion adsorption and zeta potential phenomena into the hydrothermal regime. There are many hydrothermal environments where the phenomena are important, including during the transport and deposition of ore-forming fluids, in the cooling circuits of fossil- and nuclear-powered steam generators, for the development and characterization of high temperature proton-exchange membrane fuel cells, and in the near-field environments of high-level nuclear waste repositories. The hydrogen electrode concentration cell (HECC) design and configuration are used in these studies. Multivalent cations and anions, and any other solution species that strongly adsorb at the mineral/water interface induce much larger release or uptake of H + than the relatively inert background electrolytes, MX, even at concentrations as low as 10 –4 to 10 –3 molal. Electrokinetic phenomena generally refer to the tangential motion of liquids with respect to charged solid surfaces, and the measurement of these phenomena provide important information on the structure of electrical double layers. Electrophoresis is the most suitable electrokinetic method to study particulate solids. A complete surface complexation model (SCM) consists of both mass-law expressions that describe ion binding to surface functional groups, and electrostatic correction terms for the mass law expressions derived from electrical double layer (EDL) theory, because the ion adsorption process results in charge development at the interface.

Composite Proton Conductive Membranes for Elevated Temperature and Reduced Relative Humidity PEMFC

Creation of new membrane materials for proton exchange membrane (PEM) fuel cells operating at elevated temperature and significantly reduced relative humidity (RH) is one of the major challenges in the implementation of the fuel cell technology. High-temperature PEMs are desirable for transportation applications as well as for stationary applications. Our approach is the development of new membrane materials using an inorganic proton conductor along with a proton conductive polymer (Nafion in this study). The inorganic material provide water-rich surfaces inside the membranes and allows holding on to water more tightly than in the ionomer and maintaining membrane conductivity at higher temperature and low RH. Inorganic proton conductors of different structural types with different functional groups were synthesized and characterized with respect to surface area, particle morphology, and conductivity. Composite membranes with the following inorganic additives were prepared: SiO2-SO3H, SBA-15, MCM-41, S-ZrO2, and phosphosilicate gels with different P:Si molar ratios. Three different techniques were used for fabricating the composite membranes:

Electrophoresis system for high temperature mobility measurements of nanosize particles

The electrophoretic mobility, which reflects the zeta potential of a solid material, is an important experimental quantity providing information about the electrical double layer at the solid/liquid interface. A new high temperature electrophoresis cell was developed suitable for electrophoretic mobility measurements of dispersed nanosize particles up to 150 degrees C and 40 bars. Amorphous silica (SiO(2)) particle size standards were used to test the particle size detection limit of the new instrument at 25, 100, and 150 degrees C and several pH values. The microscopic detection of the particles was enabled by dark-field illumination, which allowed extending the previously available capabilities and provided higher accuracy of the electrophoretic mobility data. The electrophoretic mobility measurements for SiO(2) at temperatures above 100 degrees C were reported for the first time and indicated a gradual increase in particle electrophoretic response with increasing temperature. The obtained data indicated negatively charged SiO(2) surface throughout the pH and temperature ranges studied.

The Protonation Behavior of Metal Oxide Surfaces to Hydrothermal Conditions

Metal oxide surface protonation under hydrothermal conditions is summarized. Important concepts and definitions are introduced first, followed by a brief overview of experimental methods and presentation of representative results. Finally, the modeling methods that are most useful in predicting surface protonation behavior between 0 and 300oC are presented and compared.