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[en] Forced convection nucleate boiling is encountered in heat exchangers during normal and non-nominal modes of operation in pressurized water or boiling water reactors (PWRs or BWRs). If the wall temperature of the piping is higher than the saturation temperature of the nearby liquid, nucleate boiling occurs. In this regime, bubbles are formed at the wall. Their growth is promoted by the wall superheat (the difference between the wall and saturation temperatures), and they depart from the wall as a result of gravitational and liquid inertia forces. If the bulk liquid is subcooled, condensation at the bubble-liquid interface takes place and the bubble may collapse. This convection nucleate boiling is called as a subcooled nucleate boiling. As for the fuel channel of a CANDU 6 reactor, forced convection nucleate boiling models for flows along fuel elements enclosed inside typical CANDU-6 fuel channel has encountered difficulties due to the modeling of local effects along the horizontal channel. Therefore, the subcooled nucleate boiling has been modeled through temperature driven boiling heat and mass transfer, using a model developed at Rensselaer Polytechnic Institute. Therefore, the objectives of this study are: (i) to investigate a proposed sub-cooled boiling model developed at Rensselaer Polytechnic Institute and (ii) to apply against a experiment and (iii) to predict local distributions of flow fields for the actual fuel channel geometries of CANDU-6 reactors. The numerical implementation is conducted using by the FLUENT 6.2 CFD computer code. The RPI model has been implemented in FLUENT 6.2 via user-defined functions (UDFs) in conjunction with the Eulerian multiphase model
[en] Highlights: • Enhancement of decal-transfer rate by controlling hydrophilicity of the CL. • Well-distributed CL microstructures by using NMP-containing ink. • Improved cell polarization by achieving highly connected Pt/C agglomerates. • Effect of different solvents on the electrochemical performance of the CL. -- Abstract: Both the catalytic and ohmic polarization of a proton exchange membrane fuel cell must be improved to achieve commercialization. Herein, high-performance membrane electrode assemblies (MEAs) were prepared using an amide-type chemical, i.e., N-methyl-2-pyrrolidinone (NMP) as a solvent for the catalyst ink. Influence of different solvent on the catalyst-layer (CL) surface microstructure was clarified to achieve high fuel-cell performance. The electrochemical performance of the MEAs made from different ink formulation was examined in detail using electrochemical impedance spectroscopy and cyclic voltammetry. As a result, the NMP–glycerol ink with a 10 wt.% glycerol was used to form a dense and well-connected Pt/C–Nafion agglomerated CL, followed by enhanced catalytic and ohmic polarizations relative to the glycerol and NMP CLs
[en] Graphical abstract: Schematic diagram for (a) the preparation of Pt/MWCNT functionalized with PEI and (b) TEM of Pt/PEI–MWCNT. Highlights: ► We prepared Pt/PEI/MWCNT where MWCNT was first functionalized by PEI (polyethyleneimine) followed by Pt deposit onto it. ► PEI functionalization provided high density homogeneous functional groups on MWCNT's sidewall. ► Cationic PEI leads to homogeneous dispersion in solutions such as water and organic solvents. ► Pt/PEI/MWCNT catalyst exhibits excellent electrocatalytic activity compared to that of Pt/MWCNT catalyst obtained with polyvinylpyrrolidine (PVP). -- Abstract: Composite materials with highly dispersed platinum (Pt) nanoparticles on multiwalled carbon nanotubes (MWCNTs), functionalized with polyethyleneimine (PEI) by a noncovalent method were prepared. The PEI-functionalization provided high density homogeneous functional groups on the MWCNTs’ sidewalls for binding Pt nanoparticles. Cationic PEI leads to homogeneous dispersion in solutions such as water and organic solvents. The effects of a reducing agent on the Pt nanoparticles that form on the surfaces of the MWCNT were studied by varying the molar ratio of NaOH to H2PtCl6. These composite materials were characterized with transmission electron micrograph (TEM), X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS). The Pt/PEI–MWCNT catalyst exhibits excellent electrocatalytic activity and compared with Pt/PVP–MWCNT catalysts obtained with polyvinylpyrrolidone (PVP). Finally, the cyclic voltammogram of methanol electrooxidation for Pt/PEI–MWCNT shows better tolerance to CO and methanol oxidation to CO2 than of Pt/PVP–MWCNT.
[en] Using ab initio ultrasoft pseudopotential plane wave method, the effect of doping concentration of Mn on the magnetic properties of β-SiC (SiC:Mn) was quantitatively investigated. It is found that the SiC:Mn shows stable ferromagnetism, and the total magnetic moment of SiC:Mn depends on the substitution site of Mn in the SiC lattice. Using the density of states calculation, it is shown that SiC:Mn has half-metallic properties for selected doping concentrations of 1.56, 3.13 and 6.25%, irrespective of substitution site. The conduction electron mobility of SiC:MnC was expected to be higher than that of SiC:MnSi. On the contrary, SiC:MnSi has a wider spin band gap compared to SiC:MnC. It is predicted that SiC with 12.5% Mn doping represents desirable characteristics for realizing spintronic devices, which include stable ferromagnetism, half-metallic properties, fast electron mobility and a wide spin band gap
[en] The performance of a PEMFC (proton exchange membrane fuel cell) severely decreases as the relative humidity decreases. Herein, we present size-controlled SiO2 (silica) nanoparticles in the CLs (catalyst layers) to provide sufficient water to the Nafion ionomer. It is found that the microstructure of the agglomerated CL is notably improved using the SiO2 particles with smaller diameter. In addition, as the SiO2 particle diameter decreases, both the electrochemical surface area and ohmic performance are improved, as well as the wettability, for the PEMFC application. The highest performance is achieved for the CL with the 8 nm SiO2 particle, which results in 2.93 times increased current density at 0.5 V relative to the 80 nm SiO2-containing CL, when SiO2-to-carbon ratio was fixed to 0.20. Consequently, it is more effective to improve the electrode morphology of the SiO2 CL than simply increase the SiO2 content, in order to enhance the fuel-cell performance under low relative humidity. - Highlights: • The SiO2 CL MEA is presented with different particle diameter and SiO2 content. • Electrode morphology of SiO2 CL is verified using FE-SEM. • Dependence of the contact angle of SiO2 CL on SiO2 particle diameter is studied. • Influence of SiO2 particle diameter on cell performance is elucidated
[en] Highlights: • A highly phase-separated catalyst layer is proposed and fabricated for PEMFCs. • Drastic increases in the water uptake and contact angle were observed. • The MEA composed of the highly phase-separated catalyst layer is analyzed. • The proposed dual-layered catalyst layer gives an improved cell polarization. - Abstract: A novel design of the dual-layered catalyst layer is performed using catalytic layers with different degrees of phase separation of Nafion. Empirical equations of the water uptake and the proton conductivity are introduced for highly phase-separated catalyst layer. Simulation results are in good agreement with experimental data using catalyst layers prepared from different solvents, which are carefully designed to have different degrees of phase separation. The effect of highly phase-separated catalyst layer is further investigated with different electrode configurations. As a result, the highly phase-separated catalyst layer shows promising aspects as an efficient water protective layer with catalytic activity. Consequently, the proposed electrode, composed of a highly phase-separated external layer and a lowly phase-separated inner layer, results in an increase of the cell performance in the high-current region
[en] Increasing global energy demands have been accelerating the research and development of reversible electrochemical systems that can realize an efficient use of the intermittent renewable energy resources. This paper thus describes a numerical investigation of reversible solid oxide cells (RSOCs), for their high energy efficiency delivered from the high operating temperatures ranging from 600 to 1000 °C. Unlike the previous studies, a model-based strategy is applied for the simultaneous integration of different operating modes (namely, fuel cell and electrolysis cell modes) to enable more realistic predictions on the trade-off behavior of the effects of electrode design parameters on the cell performance. This approach was taken to investigate the effects of various geometric designs and operating parameters (electrode backing layer thickness; interconnector rib size; fuel gas composition) on the current-potential characteristic and the round-trip efficiency. The cell performance was significantly affected by the rib size, particularly when the backing layer was thin, because of the uneven distribution of the reactant species. Overall, this study provides insights into key geometric design parameters that dominate the performance of dual-mode RSOCs.
[en] Nano sized Pt particles were successfully immobilized onto SiO2 and polystyrene-encapsulated silica core shell (SiO2@PS). To make the immobilization of Pt onto both silica and polystyrene-encapsulated silica core shell, SiO2 was first functionalized with -NH2 using 3-amino propyl trimethoxysilane (APTMS) while for core shell, the negatively charged surface of polystyrene (PS) was changed with positive charge by cationic surfactant such as cetyltrimethylammonium chloride (CTACl) to make the formation of SiO2 shell on preformed PS sphere. Transmission electron micrograph (TEM) images shows that Pt nanoparticles immobilized onto SiO2 and SiO2@PS were to be 3-4 nm without agglomeraiton. The energy dispersive spectroscope (EDS) shows that Pt contents on both SiO2 and SiO2@PS were to be 21.45% and 20.28%, respectively. In case of Pt-SiO2@PS, it is believed that Pt should have been immobilized onto PS surface and pore within SiO2 shell as well as SiO2 surface. The MEA fabricated with Pt-SiO2@PS shows better cell performance than of Pt-SiO2.
[en] Pt nanoparticles with an average size of ∼3 nm were successfully immobilized on the surface of SiO2 functionalized with -NH2 and -SH groups through chemical reduction process using polyvinylpyrrolidone as a stabilizer and different reducing agents. The effects of molecular weight of polyvinylpyrrolidone, molar ratio of reducing agent to Pt salt, type of reducing agent on the size and degree of agglomeration of Pt nanoparticles on the SiO2 surface were investigated. The X-ray diffraction and transmission electron micrograph analyses were performed to identify the product phase, size and morphology of immobilized Pt onto SiO2. UV-vis analysis was also conducted to identify the degree of reduction of Pt ions. The Pt-SiO2 nanocomposite prepared from both NH2- and SH-functionalized SiO2 exhibited similar behavior. The number of immobilized Pt nanoparticles and their average size was increased with polyvinylpyrrolidone concentration while the number of immobilized Pt was decreased with its molecular weight.
[en] A two-dimensional, non-isothermal model of a proton exchange membrane fuel cell was implemented to elucidate heat balance through the membrane electrode assembly (MEA). To take local utilization of platinum catalyst into account, the model was presented by considering the formation of agglomerated catalyst structure in the electrodes. To estimate energy balance through the MEA, various modes of heat generation and depletion by reversible/irreversible heat release, ohmic heating and phase change of water were included in the present model. In addition, dual-pathway kinetics, that is a combination of Heyrovsky–Volmer and Tafel–Volmer kinetics, were employed to precisely describe the hydrogen oxidation reaction. The proposed model was validated with experimental cell polarization, resulting in excellent fit. The temperature distribution inside the MEA was analyzed by the model. Consequently, a thorough investigation was made of the relation between membrane thickness and the temperature distribution inside the MEA.