Presence Of Saturated Water Vapor Research Articles

Transient Characteristics of the Low-Temperature PEMFC: A Comparative Analysis Using a Physics-Based Model

Low-temperature proton exchange membrane fuel cells (LT-PEMFCs) have experienced a considerable technological leap in the last few decades for transportation and stationary applications. Specifically, the precious metal loading in the catalyst layers (CLs) is notably decreased while enhancing the overall cell performance.1 However, the durability and cost remain major factors that affect the widespread commercialization of the LT-PEMFC. Though this can be achieved with further material development, some existing issues may be resolved by implementing effective design and control strategies. A good understanding of fuel cell water dynamics is required to improve durability and performance. Although some dynamic characteristics can be observed from the experiments, a physics-based model of the LT-PEMFC is necessary to study in-depth transient responses under various conditions like current profiles, operating temperatures, pressure, and humidification levels. Lately, the transient analysis of the fuel cell has gained more attention to elucidate the complex transport phenomena.2 Several transient models of the LT-PEMFC are reported to capture the two-phase behavior inside the cell. Usually, these models are 1D, 2D, or single channel 3D with assumptions like isothermal operation, vapor-equilibrated membrane, simplified reaction kinetics, and more.3–5 Hence, they do not provide spatio-temporal insights into the cell performance containing water dynamics with various flow configurations.The current study focuses on a transient physics-based model of the LT-PEMFC for dynamic analysis across its thickness and flow channels. The membrane used in LT-PEMFC requires humidification to enhance the ionic conductivity and function properly.6 This study considers Schroeder’s paradox for the membrane, which exhibits a different water uptake behavior in the presence of saturated water vapor and liquid water.7 The water uptake property is strongly dependent on the temperature. Therefore, we have developed a single polynomial expression for the membrane water uptake, including temperature and water activity dependency. Using the governing equations & empirical correlations from the numerical and experimental studies, we develop a comprehensive 3D, multiphase, non-isothermal, transient physics-based model of the LT-PEMFC that can closely approximate the liquid water transport under various operating conditions. It is a Multiphysics model that couples electrochemical reactions, porous media flow, transport of reactant species, liquid water transport, heat generation, charge transfer, and ionomer water transport. COMSOL Multiphysics is used to solve the model. It provides a spatio-temporal water distribution inside the membrane, CL, microporous layer, gas diffusion layer, and flow channels. Experiments are performed on 32 cm2 LT-PEMFC with the hybrid flow configuration at 75 °C and 80% RH to validate the physics-based model in the steady state condition. The validation study suggests that the present multiphase model estimates the LT-PEMFC performance with an acceptable RMS current density error of 15 mA/cm2. The simulations are operated in the Galvano-dynamic mode, and the corresponding LT-PEMFC response for the hybrid configuration is shown in Figure 1(a) at various operating pressures. Also, the preliminary results of the liquid water saturation under the channel and rib are incorporated in Figure 1(b). It shows that the water accumulation under the rib is more than the channel due to lower local temperature and weaker convection under the rib, promoting water vapor condensation. Further, the comparison between hybrid and parallel configurations is performed and illustrated in this presentation. Another objective is to study how quickly the liquid water saturation and membrane water content react upon changing the load cycle, RH, operating temperature, and flow configuration. This work is an important step toward understanding the impact of various flow configurations and operating conditions on the water dynamics and membrane water content inside the LT-PEMFC. The developed physics-based model would be extended further by considering the membrane deformation due to the shrinking-swelling under the humidification and thermal cycling, which is critical for the performance and degradation.

INFLUENCE OF MOISTURE ON THE SHORT-RANGE ORDERING OF CROSSLINKED GLYCOPOLYMERS BASED ON PLANT POLYSACCHARIDE KONJAC GLUCOMANNAN

Using X-ray diffraction data the short-range ordering was analyzed of the plant polysaccharide konjac glucomannan (KGM) and glycopolymers of different composition based on KGM and ε-caprolactam blocked isocyanates in the presence of saturated water vapor both for moistened and dried systems. The decrease in the values of the Breg’s period corresponding to the average distances between atoms and atomic groups in saturated water vapor is shown for glycopolymers placed under normal conditions. It was found that when the crosslinked KGM based glycopolymers are moistened, a secondary maximum is observed on the diffractograms, which corresponds to the ordering of the polysaccharide chains in the transverse direction. This secondary maximum was observed on the diffractograms of uncrosslinked KGM, but was absent on the initial diffractograms of crosslinked samples. This secondary maximum does not appear again on the diffractograms of dried glycopolymer samples. The influence of moisturizing-drying processes on the structuring and moisture content in polysaccharide and glycopolymers is analyzed. The results of WAXS correlate with thermogravimetry data on changes in the ability of KGM and KGM-based glicopolymers to retain moisture in humidified and dried systems. Using a nitroxyl paramagnetic probe, it has been demonstrated that the increase in molecular mobility of glycomolymers based on konjac glucomannan under the influence of low molecular weight plasticizer does not depend on isocyanate used. This effect allows correlating the irreversible effect of swelling and subsequent drying on the characteristics of these KGM-based systems with the achievement of macrochains of the polymer more equilibrium conformations due to increasing molecular mobility in the presence of moisture.

Trace Ammonia Removal from Air by Selective Adsorbents Reusable with Water

Ammonia adsorbents effective even in trace concentration are keys of the countermeasure for air pollution of particulate matter caused by ammonia emission from agriculture sectors. We revealed that Prussian blue (PB) and its analogues (PBAs), one of the porous coordination polymers, have higher ammonia adsorption capacity in 10 ppmv of ammonia (part per mil-lion in volume, 10 ppmv = 0.0001 volume percent), ≥ 8 times that of conventional adsorbents. Moreover, these compounds can be recycled only through water flushing. The adsorption capacity of PBA was restricted to 10 cycles of adsorption/desorption, and the air sample for the experiment was collected from the composting equipment present in a swine farm. Despite the presence of saturated water vapor in the exhaust gas, the adsorbents showed excellent selectivity in the removal of the ammonia from the sample.

Reduced Cu/Pt–HCa2Ta3O10 Perovskite Nanosheets for Sunlight‐Driven Conversion of CO2 into Valuable Fuels

Reduced perovskite HCa2Ta3O10 nanosheets loaded with Pt and Cu are synthesized for sunlight‐driven conversion of CO2 with water vapor into valuable fuels. Perovskite nanosheets are prepared by exfoliating layered perovskite CsCa2Ta3O10 via tetra butyl ammonium ion exchange, followed by liquid ultrasonic exfoliation. The obtained nanosheets exhibits a high specific surface area (>200 m2 g−1). The photocatalytic performance of the resulting reduced perovskite nanosheets is evaluated for CO2 photoreduction under sunlight in the presence of saturated water vapor. The reduced nanosheets exhibit much higher photoactivity than the nonreduced ones. This can be ascribed to their unique structure. The hydrogen treatment in the presence of platinum induces a considerable amount of Ta+4 and oxygen vacancies, which apparently improves the visible light absorption of perovskite nanosheets. Moreover, the introduction of CuO nanoparticles significantly improves the electron–hole separation through the formation of a p–n junction. It also enhances the adsorption of CO2 and stabilizes C1 intermediates which are favorable for CC coupling to form C2 products (e.g., ethanol). The formation rates of ethanol and methanol are 113 and 7.4 µmol g−1 h−1, respectively, while only methanol is obtained at the rate of 125.9 µmol g−1 h−1 in the absence of CuO nanoparticles.

CO2 capture with amine-loaded carbon nanotubes via a dual-column temperature/vacuum swing adsorption

A dual-column temperature/vacuum swing adsorption (TVSA) with 3-aminopropyltriethoxysilane-loaded carbon nanotubes (CNT(APTS)) was built to study cyclic CO2 capture from gas streams. The working CO2 capacities and the characteristics of CNT(APTS) were preserved through 100 TVSA cycles under dry or wet conditions, displaying the multi-cycle stability of CO2 capture with CNT(APTS). The cyclic working CO2 capacity of CNT(APTS) was notably enhanced in the presence of saturated water vapor in the gas stream at 25°C which gives relatively high desorbed CO2 concentrations (∼67%). These results suggest that a dual-column TVSA with solid CNT(APTS) has the possibility to be a promising CO2 capture technology, especially in the post-flue gas desulfurization in which saturated water vapor is present in the flue gas.

Room-Temperature Reactor Packed with Hydrophobic Catalysts for the Oxidation of Hydrogen Isotopes Released in a Nuclear Facility

A room-temperature reactor packed with hydrophobic catalysts for the oxidation of hydrogen isotopes released in a nuclear facility will contribute to nuclear safety. The inorganic-based hydrophobic Pt catalyst named H1P has been developed especially for efficient oxidation over a wide concentration range of hydrogen isotopes at room temperature, even in the presence of saturated water vapor. The overall reaction rate constant for hydrogen oxidation with the H1P catalyst in a flow-through system using a tritium tracer was determined as a function of space velocity, hydrogen concentration in carriers, temperature of the catalyst, and water vapor concentration in carriers. The overall reaction rate constant for the H1P catalyst in the range near room temperature was considerably larger than that for the traditionally applied Pt/Al2O3 catalyst. Moreover, the decrease in reaction rate for H1P in the presence of saturated water vapor was slight compared with the reaction rate in the absence of water vapor due to the excellent hydrophobic performance of H1P. Oxidation reaction on the catalyst surface is the rate-controlling step in the range near room temperature and the rate-controlling step is shifted to diffusion in a catalyst substratum above 313K due to its fine porosity. The overall reaction rate constant in the range near room temperature was dependent on the space velocity and hydrogen concentration in carriers. The overall reaction rate constants in the range of 1;000=T greater than 3.2 correlated to k overall[s−1] = 5.59 × 107 × SV[h−1] × exp (−67.7 [kJ/mol]/RgT), where the space velocity range was from 600 to 7,200 h−1.

Development of Pt/ASDBC catalyst for room temperature recombiner of atmosphere detritiation system

A combined catalytic reactor and water absorber system has been applied in the field of nuclear fusion to control the tritium release into the environment as low as reasonably achievable. Efficient tritium oxidation performance at room temperature has been strongly required taking a severe accident in a facility into consideration. We have developed hydrophobic Pt catalysts applicable for tritium oxidation in the presence of saturated water vapor at room temperature. A new type of hydrophobic catalyst, Pt/ASDBC, has been prepared by dipositting platinum on alkyl-styrene diviyl-benzene copolymer (ASDBC). Tritium oxidation tests of the catalysts using 3 GBq/m 3 of tritium were performed in the absence/presence of saturated water vapor at room temperature. The overall reaction rate constant for Pt/ASDBC catalyst in the ambient temperature range was considerably larger than that for the traditionally applied Pt/Al 2O 3 catalyst. Oxidation reaction on the catalyst surface is the rate-controlling step in the ambient temperature range. The overall reaction rate constant in the ambient temperature range was dependent on the space velocity and hydrogen concentration in carrier. Values of the overall reaction rate constant in the ambient temperature range were correlated to k overall = 1.60 × 10 5exp(−33.0 (kJ/mol)/ R g/T) (s −1) at the space velocity of 5556 h −1 under the dry condition.

On shortwave radiation absorption in overcast atmospheres

Using a numerical model of solar radiative transfer that is calibrated against benchmark computations, it is shown that atmospheric water vapor, together with the microphysical characteristics of water drops (liquid water path and effective radius), plays an important role in the total solar spectrum reflection and absorption in overcast skies. For any specific cloud type, the water vapor column above the cloud and the presence of saturated water vapor inside the cloud contribute significantly to atmospheric absorption. These factors also affect the relationship between the net shortwave fluxes at the top and bottom of overcast atmospheres, in particular, inhibiting a general universal linkage between these two quantities. Thus neglect of details concerning the vertical location, extent, and microphysical aspects of clouds can lead to biases in the inference of surface irradiance using top‐of‐the‐atmosphere measurements.

Analytical electron microscopy study of surface layers formed on the French SON68 nuclear waste glass during vapor hydration at 200°C

Extensive solid-state characterization (AEM/SEM/HRTEM) was completed on six SON68 (inactive R7T7) waste glasses which were altered in the presence of saturated water vapor (200°C) for 22, 91, 241, 908, 1000, 1013, and 1021 days. The samples were examined by AEM in cross-section (lattice-fringe imaging, micro-diffraction, and quantitative thin-film EDS analysis). The glass monoliths were invariably covered by a thin altered rind, and the surface layer thickness increased with increasing time of reaction, ranging from 0.5 to 30 μm in thickness. Six distinctive zones, based on phase chemistry and microstructure, were distinguished within the well-developed surface layers. Numerous crystalline phases such as analcime, gyrolite, tobermorite, apatite, and weeksite were identified on the surfaces of the reacted glasses as precipitates. The majority of the surface layer volume was composed of two basic structures that are morphologically and chemically distinct: The A-domain consisted of well-crystallized fibrous smectite aggregates; and the B-domain consisted of poorly-crystallized regions containing smectite, possibly montmorillonite, crystallites and a ZrO 2-rich amorphous silica matrix. The retention of the rare-earth elements, Mo, and Zr mostly occurred within the B-domain; while transition metal elements, such as Zn, Cr, Ni, Mn, and Fe, were retained in the A-domain. The element partitioning among A-domains and B-domains and recrystallization of the earlier-formed B-domains into the A-domain smectites were the basic processes which have controlled the chemical and structural evolution of the surface layer. The mechanism of surface layer formation during vapor hydration are discussed based on these cross-sectional AEM results.

Influence of the aerosol formed in the fast evaporation of superheated water on the combustion of methane-air mixtures in a closed vessel

The influence of the aerosol formed in the fast evaporation of superheated water at 150°C on the combustion of a methane-air mixture in a near-spherical closed vessel (volume 20 dm3) is investigated experimentally. The concentrational limits of flame propagation and the normal combustion rate are determined as a function of the mass concentration of the water aerosol. It is found that the flegmatizing ability of the aerosol is relatively low; it is not due to the formation of fairly large water drops but practically completely to the presence of saturated water vapor. Exponential reduction in normal combustion rate of the mixture with increase in concentration of the superheated-water aerosol is observed.

Secondary Phase Formation and the Microstructural Evolution of Surface Layers During Vapor Phase Alteration of the French Son68 Nuclear Waste Glass at 200 °C

AbstractThe SON68 inactive “R7T7” composition is the French reference glass for the LWR nuclear waste glass. Vapor phase alteration was used to accelerate the reaction progress of glass corrosion and to develop the characteristic suite of secondary, alteration phases. Extensive solid-state characterization (AEM/SEM/HRTEM) was completed on six inactive R7T7 waste glasses which were altered in the presence of saturated water vapor (200 °C) for 91, 241, 908, 1000, 1013, and 1021 days. The AEM samples were examined in cross-section (lattice-fringe imaging, microdiffraction, and quantitative thin-film EDS analysis). The glass monoliths were invariably covered with a thin altered rind. The layer became thicker with time: 0.5μm for 22 days; 4 μm for 91 days; 6 μm for 241 days; 10 μm for 908 days; 26 μm for 1013 days; and <35μm for 1021 days. The composite alteration layer of the SON68 samples is at least four time less thick than that of the SRL 131 glass composition.Six distinctive zones, based on phase chemistry and microstructure, were distinguished within the well-developed surface layers. Numerous crystalline phases such as analcime, tobermorite, apatite, and weeksite were identified on the surfaces of the reacted glasses as precipitates. Two crystalline phases, Ag2TeO3 and (Ca,Sr)Mo3O9(OH)2, were found within the inner zones of surface layers, and they must have nucleated in situ, indicating that Ag, Te, Sr, and Mo can be retained within the surface layer. The majority of the surface layer volume is composed of two morphologically and chemically different structures: one consists of well-crystallized fibrous smectite aggregates occurring along with cavities, the A-domain; and the other consists of poorlycrystallized regions containing needle-like smectite (montmorillonite) crystallites, a silica-rich amorphous matrix, and possibly ZrO2 particles, the B-domain. The retention of rare-earths and Zr mostly occurred within B-domains and that of transition elements, such as Zn, Cr, Ni, and Mn, in A-domains. The recrystallization of poorly-crystallized B-domains into well-crystallized Adomains may influence the long-term behavior of rare-earths, Zr, and transition elements. The mechanism of surface layer formation during vapor phase alteration is discussed based on the cross-sectional AEM studies of surface layers of the SON68 waste glasses.

Photocatalytic Degradation of Gaseous Pyridine over Zeolite-Supported Titanium Dioxide

The photocatalyzed degradation of pyridine in the gas phase was investigated using titanium dioxide semiconductor supported on mordenite. The complete mineralization was found to occur over a catalyst containing 75 wt% TiO2 and 25 wt% mordenite in about 180 min in the presence of saturated water vapor at 0°C (4.6 Torr). Low water vapor pressure of 0.096 Torr was found to be sufficient to achieve a reasonably high percentage and rate of degradation. Diffusion of pyridine within the catalyst adversely affects the activity when the thickness of the catalyst coating exceeds the penetration depth of illumination. The activity of the zeolite-supported catalysts was higher than that of TiO2. The photonic efficiency for the complete mineralization of pyridine to CO2 was determined to be 0.48.

Characterization of polymer electrolytes for fuel cell applications

We review here our recent work on polymer electrolyte fuel cells emphasizing membrane transport issues. Transport parameters measured at 30°C for several available perfluorosulfonic acid membranes are compared. The diffusion coefficient and conductivity of each of these membranes were determined as functions of membrane water content. Data on water sorption and conductivity are reported for an experimental membrane which is a modified form of NAFION®. Contact angle measurements indicate that the surface of a perfluorosulfonic acid membrane exposed to water vapor is quite hydrophobic, even in the presence of saturated water vapor. Modeling of water distribution in PEFCs based on the uptake and transport data shows that membrane thickness contributes in a nonlinear fashion to performance in PEM fuel cells.