Serguei N. Lvov

The Ionization Constant of Water over Wide Ranges of Temperature and Density

A semitheoretical approach for the ionization constant of water, KW, is used to fit the available experimental data over wide ranges of density and temperature. Statistical thermodynamics is employed to formulate a number of contributions to the standard state chemical potential of the ionic hydration process. A sorption model is developed for calculating the inner-shell term, which accounts for the ion–water interactions in the immediate ion vicinity. A new analytical expression is derived using the Bragg–Williams approximation that reproduces the dependence of a mean ion solvation number on the solvent chemical potential. The proposed model was found to be correct at the zero-density limit. The final formulation has a simple analytical form, includes seven adjustable parameters, and provides good fitting of the collected KW data, within experimental uncertainties, for a temperature range of 0–800 °C and densities of 0–1.2 g cm−3.

High temperature transport properties of polyphosphazene membranes for direct methanol fuel cells

Experimental methods for studying the conductivity and methanol permeability of proton conductive polymers over a wide range of temperatures have been developed. The proton conductivity and methanol permeability of several polymer electrolyte membranes including sulfonated and phosphonated poly[(aryloxy)phosphazenes] was determined at temperatures up to 120 °C. Nafion 117 membranes were tested using the same methods in order to determine the reliability of the methods. Although the conductivities of the polyphosphazene membranes were either similar to or lower than that of the Nafion 117 membranes, they continue to hold promise for fuel cell applications. We observed similar activation energies of proton conduction for Nafion 117, and for sulfonated and phosphonated polyphosphazene membranes. However, the methanol permeability of a sulfonated membrane was about 8 times lower than that of the Nafion 117 membrane at room temperature although the values were comparable at 120 °C. The permeability of a phosphonated phosphazene derivative was about 40 times lower than that of the Nafion 117 membrane at room temperature and about 9 times lower at 120 °C. This is a significant improvement over the behavior of Nafion 117.

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.

Effect of TiO2 Surface Properties on Performance of Nafion-Based Composite Membranes in High Temperature and Low Relative Humidity PEM Fuel Cells

Nafion/10% (mass % is used in this paper) TiO 2 composite membranes were studied in a H 2 /O 2 proton exchange membrane (PEM) fuel cell over a range of relative humidity (RH) from 26 to 50% at temperatures of 80 and 120°C.According to the scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) structural analysis, the composite Nafion/TiO 2 membranes had a two-layer structure, one layer enriched with TiO 2 particles, and the other dominated by the Nafion polymer. Although the TiO 2 particles were mainly concentrated on one side of the composite membrane, sufficient hydration was apparently achieved for the whole membrane. Two TiO 2 (rutile) powders used for the preparation of the composite membranes differed in specific surface area (SSA), surface zeta potential, and particle morphology. A TiO 2 powder with five times higher SSA, 22 mV higher zeta potential (at the low-pH limit), and distinctly different individual particle and particle aggregate morphologies resulted in a four-times increase of current density (at 0.6 V) when the composite membranes were made and tested in PEM fuel cell at temperature of 120°C and relative humidity of 26%. We speculate that a greater number of protonated sites per unit mass of powder in the membrane and a higher density of the protonated sites contribute to the enhanced fuel cell performance.

Quantitative evaluation of general corrosion of Type 304 stainless steel in subcritical and supercritical aqueous solutions via electrochemical noise analysis

Abstract Electrochemical noise (EN) sensors have been developed to measure the corrosion rate of Type 304 stainless steel ( SS ) in subcritical and supercritical environments. The EN sensors were tested in flowing aqueous solutions containing NaCl and HCl at temperatures from 150°C to 390°C, a pressure of 25 MPa, and flow rates from 0.375 to 1.00 ml/min. The potential and coupling current noise were recorded simultaneously and the noise resistance ( R n ) was calculated from the standard deviations in the potential and current records. We found that the inverse noise resistance correlated very well with the corrosion rate evaluated from separate mass loss experiments, and that both the inverse noise resistance and the average corrosion rate were functions of temperature and flow rate. In the temperature range from 200°C to 390°C, the corrosion rate was found to be proportional to the inverse noise resistance and hence the Stern–Geary relationship can be used to evaluate the corrosion rate. However, at 150°C, the relation between inverse noise resistance and corrosion rate significantly deviated from the Stern–Geary relationship. It was found that the deviation was related to the low corrosion rate of Type 304 SS and 150°C.

Progress on yttria-stabilized zirconia sensors for hydrothermal pH measurements

Electrochemical cells are reviewed and a new design is evaluated for potentiometric pH measurements to above 300 jC. The new design system minimizes the effects of metal corrosion on measured pH. In addition, a recently developed [Zhou, X.Y., Lvov, S.N., Ulyanov, S.M., 2003. Yttria-Stabilized Zirconia Membrane Electrode, US Patent #6, S17, 694] flow-through, yttriastabilized zirconia (YSZ) pH sensor has been further tested. The Nernstian behavior and precision of the YSZ electrode were evaluated by measuring the potentials vs. H2–Pt electrode at 320 and 350 jC. Also, using the YSZ electrode, the association constants of HCl(aq) at 320 and 350 jC have been determined from the potentials of a HCl(aq) solutions at 0.01 to 0.001 mol kg � 1 . The results, pK320= � 1.46F0.46 and pK350= � 2.35F0.25, in good agreement with literature data, both demonstrate the effective use of the cell and YSZ electrode for pH measurements to about F0.05 pH units, and confirm the Nernstian behavior of the YSZ electrode in acidic HCl solutions up to 350 jC. Commercial YSZ tubes available for high-temperature pH sensing are, however, far from ideal because of irregular compositions, phase structures, and interstitial materials. A consequence is the premature structural decay of YSZ tubes in acidic solutions at elevated temperatures. In spite of the longterm decay, YSZ sensors respond rapidly to changes in pH, apparently limited only by the rate of mixing of solutions within the cell. This system makes the measurement feasible above 300 jC of mineral hydrolysis equilibrium constants and their free energy changes within uncertainties of about F1.0 kJ. D 2003 Elsevier Science B.V. All rights reserved.

Nafion/Zirconium Phosphate Composite Membranes for PEMFC Operating at up to 120 ° C and down to 13 % RH

Nafion/zirconium phosphate (ZP) composite membranes of different structures containing in situ precipitated ZP, crystalline layer-structured α-zirconium hydrogen phosphate α-Zr(HPO 4 ) 2 ·H 2 O (a-ZPL), and three-dimensional-network zirconium hydrogen phosphate H 3 OZr 2 (PO 4 ) 3 (ZPTD) were synthesized. The ZP materials used have different structure and surface properties (specific surface area, zeta potential, and particle morphology). The composite membranes were studied in an H 2 /O 2 proton exchange membrane fuel cell (PEMFC) over a range of relative humidity (RH) from 13 to 50% at temperatures of 80 and 120°C. At the operating temperature of 120°C, all studied composite membranes demonstrated significant improvement in performance under dehydrating conditions compared to unmodified Nafion membranes. Among the tested membranes, the Nafion/a-ZPL composite membrane demonstrated the highest performance at 120°C at all RH values, especially at 13% RH. At 26 and 50% RH, the performance of Nafion/a-ZPL and Nafion/19% ZP in situ membranes were close, and the performance of the Nafion/ZPTD membrane was the lowest. The effects of filled polymer pore constriction, scaffold formation, membrane structure, and surface properties of the ZP particles on the membrane performance in PEMFCs are discussed in the paper.

CuCl Electrolysis for Hydrogen Production in the Cu–Cl Thermochemical Cycle

The Cu―Cl thermochemical cycle is among the most attractive technologies proposed for hydrogen production due to moderate temperature requirements and high efficiency. In this study, the key step of the cycle, H 2 gas evolution via oxidation of CuCl(s) dissolved in high concentrated HCl(aq), was experimentally investigated. The electrolysis parameters and system performance were studied by linear sweep voltammetry and electrochemical impedance spectroscopy at ambient temperature. Promising performance of the electrolyzer was obtained when pure water was used as catholyte. A thermodynamic model previously developed for speciation of the CuCl―CuCl 2 ―HCl aqueous solutions was used to speculate on the effects of reagent concentration, flow rate, and temperature on electrolysis kinetics. The experimental decomposition potential necessary to initiate the hydrogen evolution reaction was more than 3 times lower than the potential necessary for water electrolysis at the same conditions. Close correspondence of the hydrogen production rate to Faradays law of electrolysis indicated the current efficiency of about 98%, while the voltage efficiency was estimated at 80% at 0.5 V and 0.1 A/cm 2 .

Advanced flow-through external pressure-balanced reference electrode for potentiometric and pH studies in high temperature aqueous solutions

Abstract A new flow-through external pressure-balanced reference electrode (FTEPBRE) has been developed for potentiometric and pH measurements in high temperature aqueous solutions. The unique feature of this advanced FTEPBRE is that the reference solution flows through the electrode so that the concentration of solution across the thermal liquid junction is well defined. Because the electrolyte concentration profile in the reference electrode is maintained constant, uncertainty in the thermal liquid junction potential (TLJP) can be eliminated at a given temperature and pressure. We have employed the silver⊢silver chloride |platinum|hydrogen non-isothermal electrochemical system for potentiometric measurements of potentials between HCI (aq) solutions of 0.01 and 0.001 mol kg −1 at temperatures up to 633 K and at pressures of 275 and 338 bar. The results of the measurements have been compared with calculations made using the available thermodynamic data, and good agreement, within ±3 to 10 mV, between the calculated and the measured potentials is obtained. We concluded that the advanced reference electrode can be used to make accurate potentiometric measurements (within a few mV) in aqueous systems over wide ranges of temperatures and pressures. Also we demonstrated an ability of the thermocell to measure pH with high accuracy of ± 0.04 to 0.09 over wide ranges of temperature and pressure.