On Spatial Behavior of the Harmonic Vibrations in Thermoviscoelastic Mixtures

This paper concerns the study of time–harmonic vibrations for homogeneous and anisotropic thermoviscoelastic mixtures. The dissipative effects are used to introduce an appropriate measure associated with the amplitude of the steady–state vibrations and to establish an exponential decay estimate of Saint–Venant type, which holds for every value of the frequency of vibrations and for arbitrary values of the elastic coefficients.     

Exponential stability of a thermoviscoelastic mixture with second sound

In this work, we consider a one-dimensional model of a thermoviscoelastic mixture with second sound. Under suitable assumption on the constitutive constants of the system, we prove, using the theory of semigroup of linear operators and results obtained by Prüss (1984) and Borichev and Tomilov (2009), that the damping effect through heat conduction given by the Cattaneo law is strong enough to stabilize the system.     

On Spectrum of a General Petrovsky Type Equation and Riesz Basis of N-Connected Beams with Linear Feedback at Joints

A framework of a general type of Petrovsky equation is formulated. The characteristic equation for eigenvalues of the system is derived and the associated eigenfunctions are found. For N-connected beams with linear feedbacks at joint points, the asymptotic expansions of eigenvalues and eigenfunctions are further developed, and the Riesz basis property and the exponential stability are then concluded.     

Improved electrolytes for Li-ion batteries: Mixtures of ionic liquid and organic electrolyte with enhanced safety and electrochemical performance

Physical and electrochemical characteristics of Li-ion battery systems based on LiFePO₄ cathodes and graphite anodes with mixture electrolytes were investigated. The mixed electrolytes are based on an ionic liquid (IL), and organic solvents used in commercial batteries. We investigated a range of compositions to determine an optimum conductivity and non-flammability of the mixed electrolyte. This led us to examine mixtures of ILs with the organic electrolyte usually employed in commercial Li-ion batteries, i.e., ethylene carbonate (EC) and diethylene carbonate (DEC). The IL electrolyte consisted of (trifluoromethyl sulfonylimide) (TFSI) as anion and 1-ethyl-3-methyleimidazolium (EMI) as the cation. The physical and electrochemical properties of some of these mixtures showed an improvement characteristics compared to the constituents alone. The safety was improved with electrolyte mixtures; when IL content in the mixture is ≥40%, no flammability is observed. A stable SEI layer was obtained on the MCMB graphite anode in these mixed electrolytes, which is not obtained with IL containing the TFSI-anion. The high-rate capability of LiFePO₄ is similar in the organic electrolyte and the mixture with a composition of 1:1. The interface resistance of the LiFePO₄ cathode is stabilized when the IL is added to the electrolyte. A reversible capacity of 155 mAh g⁻¹ at C/12 is obtained with cells having at least some organic electrolyte compared to only 124 mAh g⁻¹ with pure IL. With increasing discharge rate, the capacity is maintained close to that in the organic solvent up to 2 C rate. At higher rates, the results with mixture electrolytes start to deviate from the pure organic electrolyte cell. The evaluation of the Li-ion cells; LiFePO₄//Li₄Ti₅O₁₂ with organic and, 40% mixture electrolytes showed good 1st CE at 98.7 and 93.0%, respectively. The power performance of both cell configurations is comparable up to 2 C rate. This study indicates that safety and electrochemical performance of the Li-ion battery can be improved by using mixed IL and organic solvents.     

Synthesis of LiFePO4/C composite with high-rate performance by starch sol assisted rheological phase method

LiFePO₄/C composite was synthesized at 600 °C in an Ar atmosphere by a soluble starch sol assisted rheological phase method using home-made amorphous nano-FePO₄ as the iron source. XRD, SEM and TEM observations show that the LiFePO₄/C composite has good crystallinity, ultrafine sphere-like particles of 100-200 nm size and in situ carbon. The synthesized LiFePO₄ could inherit the morphology of FePO₄ precursor. The electrochemical performance of the LiFePO₄ by galvanostatic cycling studies demonstrates excellent high-rate cycle stability. The Li/LiFePO₄ cell displays a high initial discharge capacity of more than 157 mAh g⁻¹ at 0.2C and a little discharge capacity decreases from the first to the 80th cycle (>98.3%). Remarkably, even at a high current density of 30C, the cell still presents good cycle retention.     

Synthesis and characterization of macroporous tin oxide composite as an anode material for Li-ion batteries

A macroporous SnO₂/C composite anode material was synthesized using an organic template-assisted method. Polystyrene spheres were synthesized and used as template and lead to macroporous morphology with pores of 300-500 nm in diameter and a surface area of 54.7 m² g⁻¹. X-ray diffraction showed that the SnO₂ nanoparticles are crystallized in a rutile P42/mnm lattice with the presence of Sn metal traces. The synthesized macroporous SnO₂/C composite provided promising performance in lithium half cells showing a discharge capacity of 607 mAh g⁻¹ after 55 cycles. It was found that the macroporous SnO₂/C composite is stable and resistant to pulverization upon cycling.     

Effect of synthesis temperature on the properties of LiFePO4/C composites prepared by carbothermal reduction

LiFePO₄/C composite cathode materials were synthesized by carbothermal reduction method using inexpensive FePO₄ as raw materials and glucose as conductive additive and reducing agent. The precursor of LiFePO₄/C was characterized by differential thermal analysis and thermogravimetry. The microstructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and particle size analysis. Cyclic voltammetry (CV) and charge/discharge cycling performance were used to characterize their electrochemical properties. The results showed that the LiFePO₄/C composite synthesized at 650 °C for 9 h exhibited the most homogeneous particle size distribution. Residual carbon during processing was coated on LiFePO₄, resulting in the enhancement of the material's electronic properties. Electrochemical measurements showed that the discharge capacity first increased and then decreased with the increase of synthesis temperature. The optimal sample synthesized at 650 °C for 9 h exhibited a highest initial discharge capacity of 151.2 mA h g⁻¹ at 0.2 C rate and 144.1 mA h g⁻¹ at 1 C rate with satisfactory capacity retention rate.     

Photocatalytic reduction of CO2 with H2O on CuO/TiO2 catalysts

Carbon dioxide was photocatalytically reduced to produce methanol and ethanol in the presence of CuO-loaded titania powders suspended in water containing Na₂SO₃ as the hole scavenger. The photocatalysts were synthesized by an impregnation method using P25 (Degussa) as support. At the optimum amount of copper oxide loading (3 wt%), the methanol and ethanol yields were 12.5 and 27.1 μmol/g-catal., respectively, following 6 h of UV illumination. The redistribution of photogenerated charge carriers in CuO/TiO₂ facilitates electron trapping and prohibits the recombination of electrons and holes, which significantly increases photoefficiency. The addition of Na₂SO₃ promotes the formation of ethanol.     

Investigation of single SOEC with BSCF anode and SDC barrier layer

In this paper, Sm_0.2Ce_0.8O_1.9 (SDC) is used as a barrier interlayer between Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) anode and 8YSZ electrolyte to avoid solid state interaction of solid oxide electrolysis cells (SOEC) for high temperature application. The crystal structure and surface morphologies of BSCF and SDC powders were characterized, respectively. BSCF-SDC/YSZ/SDC-BSCF symmetric cells and BSCF-SDC/YSZ/Ni-YSZ single button cells were prepared and the related electrochemical performances were tested at 850 °C. The results showed that ASR data of BSCF-SDC/YSZ is 0.42 Ω cm² at 850 °C. The hydrogen production rate of the single SOEC using BSCF/SDC anode can be up to 177.4 mL cm⁻² h⁻¹, also the cell exhibits excellent stability, which indicates that it could be a potential candidate for the future application of SOEC technology.     

Photocatalytic overall water splitting with dual-bed system under visible light irradiation

Photocatalytic overall water splitting has been demonstrated with WO₃ for oxygen producing photocatalyst (OPC), and Rh-doped SrTiO₃ for hydrogen producing photocatalyst (HPC) in a simulated dual-bed system under visible light irradiation (λ ≥ 400 nm). The Fe³⁺/Fe²⁺ redox couple was chosen as the most effective electron mediator between OPC and HPC. The overall performance of the dual-bed system was limited by the low activity of HPC, and thus the activity of HPC should be increased to improve the overall performance. For overall water splitting reaction in a dual-bed system, the conduction band of OPC must be more negative than the redox potential of the electron acceptors and the valence band of HPC must be more positive than the redox potential of the electron donors.     

Physical and electrochemical properties of LiMnPO4/C composite cathode prepared with different conductive carbons

The olivine structured LiMnPO₄/C composites were prepared by a combination of spray pyrolysis and wet ballmilling using different conductive carbons: acetylene black and two types of ketjen black. The ketjen black with a larger specific surface area and dibutyl phthalate absorption number was found to be more preferable compared with other conductive carbons studied in this work. The LiMnPO₄/C composite cathode with ketjen black, which has the largest specific surface area, exhibited the largest discharge capacity compared with other LiMnPO₄/C composites. The largest discharge capacity delivered by this composite cathode was 166 mAh g⁻¹ at 0.05 C, which is about 97% of the theoretical value for LiMnPO₄. The performance improvement by using this conductive carbon was attributed to its extremely large specific surface area and high ability to absorb the electrolyte, which provide enhanced charge transfer and lithium ion transport in the composite cathode structure.     

Direct synthesis of nanocrystalline titanium dioxide/carbon composite and its catalytic effect on NaAlH4 for hydrogen storage

Nanocrystalline titanium dioxide/carbon composite (TiO₂/C) was synthesized through a direct solution-phase carburization using tetrabutyl titanate (Ti(OBu)₄) and resol as precursors. The prepared TiO₂/C composite was mainly in the anatase structure with an average particle size under 20 nm, which was then introduced in NaAlH₄ as a catalyst through ball milling. The desorption curves show that both nanocrystalline TiO₂/C and TiO₂ can obviously improve the kinetics of NaAlH₄, while NaAlH₄ with 3 mol% TiO₂/C exhibits better cycling stability than NaAlH₄ with 3 mol%TiO₂. The hydrogen storage capacity of NaAlH₄ with TiO₂/C remains stable after 5th cycle, and about 94% of initial hydrogen is released, while the capacity of NaAlH₄ with TiO₂ decreases continuously during cycling, and only 88% of initial hydrogen is released after 10th cycle. Furthermore, NaAlH₄ with 3 mol%TiO₂/C exhibits good reversibility at relatively low hydrogen pressures, and it can reload 4.16 and 1.63wt% hydrogen at 50 and 30 bar hydrogen pressures, respectively.     

Electrocatalysis of carbon black- or chitosan-functionalized activated carbon nanotubes-supported Pd with a small amount of La2O3 towards methanol oxidation in alkaline media

The Pd/C catalysts with and without a small amount of La₂O₃ were synthesized by a simple reduction reaction with sodium borohydride in aqueous solution. The structure and morphology of these catalysts were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. The electrocatalytic performance of these catalysts for methanol oxidation in alkaline media was investigated using cyclic voltammetry, chronoamperometry and CO stripping experiments. The results show that the Pd–La₂O₃/C catalyst has a higher catalytic activity than the Pd/C catalyst, but the effect of La₂O₃ cannot be explained by a bi-functional mechanism. X-Ray photoelectron spectroscopy analyses suggest that the higher content of metallic Pd caused by the addition of La₂O₃ contributes to the better catalytic activity of Pd–La₂O₃/C. Based on the good electrocatalytic performance of Pd–La₂O₃/C, the Pd–La₂O₃ catalyst supported on chitosan (CS)-functionalized activated carbon nanotubes was prepared, and it exhibited a better catalytic activity. The improvement is attributed to the good dispersion status of metal particles and the further increase of metallic Pd due to the presence of CS.     

Electrochemical behavior of Li3−xM′xV2−yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries

Composites of monoclinic Li_3−xM′_xV_2−yM″_2y(PO₄)₃ (M′ = K, M″ = Sc, Mg + Ti) with carbon were synthesized by solid-state reaction using oxalic acid or 6% H₂/Ar gas mixture as reducing agents at sintering temperature of 850 °C. The samples were characterized by X-ray diffraction (XRD), voltammetry and electrochemical galvanostatic cycling. The capacity of Li₃V₂(PO₄)₃ synthesized using hydrogen as the reducing agent was 127 mA h g⁻¹ and decreased to 120 mA h g⁻¹ after 20 charge-discharge cycles. The substitution of lithium and vanadium for other ions did not result in the improvement of the electrochemical characteristics of the samples.     

Enhanced H2 production activity under solar irradiation over N-doped TiO2 prepared using pyridine as a precursor: a typical sample of N-doped TiO2 series

A series of N-doped TiO₂ photocatalysts were synthesized by simple hydrolysis method using titanium isopropoxide and pyridine precusors. XRD studies of all the catalysts showed a single anatase phase and TEM results confirmed that the particles obtained by this procedure are of nano size and in the range of 3–5 nm. UV–vis DRS of N-doped TiO₂ catalysts clearly show an enhanced absorption in the visible light region, and the band gaps are seen decreasing as the content of doped nitrogen increases. The formation of oxynitride linkages is evidenced by the FTIR results. Further the XPS analysis confirms the presence of Ti-O-N linkage, and the binding energy values indicate that the doped nitrogen is either interstitial or chemisorbed. The photocatalytic activity of N-doped TiO₂ catalysts is evaluated for hydrogen production in methanol: water mixture under solar light irradiation and 0.5 wt% N-doped TiO₂ has shown a stable and high activity, 3500 μ mol/h/g.cat. The enhanced hydrogen production activity of N-doped TiO₂ catalysts is attributed to the high surface area of the catalysts containing doped nitrogen mostly at the interstitial positions and enhanced visible light absorption.     

PVTxy properties of CO2 mixtures relevant for CO2 capture,transport and storage: Review of available experimental data and theoretical models

The knowledge about pressure–volume–temperature–composition (PVTxy) properties plays an important role in the design and operation of many processes involved in CO₂ capture and storage (CCS) systems. A literature survey was conducted on both the available experimental data and the theoretical models associated with the thermodynamic properties of CO₂ mixtures within the operation window of CCS. Some gaps were identified between available experimental data and requirements of the system design and operation. The major concerns are: for the vapour–liquid equilibrium, there are no data about CO₂/COS and few data about the CO₂/N₂O₄ mixture. For the volume property, there are no published experimental data for CO₂/O₂, CO₂/CO, CO₂/N₂O₄, CO₂/COS and CO₂/NH₃ and the liquid volume of CO₂/H₂. The experimental data available for multi-component CO₂ mixtures are also scarce. Many equations of state are available for thermodynamic calculations of CO₂ mixtures. The cubic equations of state have the simplest structure and are capable of giving reasonable results for the PVTxy properties. More complex equations of state such as Lee–Kesler, SAFT and GERG typically give better results for the volume property, but not necessarily for the vapour–liquid equilibrium. None of the equations of state evaluated in the literature show any clear advantage in CCS applications for the calculation of all PVTxy properties. A reference equation of state for CCS should, thus, be a future goal.     

Carbonates formed during BSCF preparation and their effects on performance of SOFCs with BSCF cathode

Series of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) samples have been prepared by modified citrate-nitrate combustion method from the precursor solutions with different pH values and citrate/metal ion ratios. The XRD results reveal that BSCF oxide free of impurity phases can be obtained from a precursor solution with a suitable pH value and a suitable C/M value, whereas CO₂-TPD profiles show that there are minor carbonates species present in all BSCF samples, but the amount of these carbonates varies with the pH and C/M values of precursor solutions. The current density–voltage characteristics indicate that carbonates in the BSCF samples reduce the cell performance. The electrochemical impedance spectra (EIS) show that carbonates in BSCF lead to increases in ohmic and polarization resistances. High performance is achieved on the cell with a cathode using a pure BSCF calcined under O₂ flow at 900 °C.     

Effect and siting of Nafion in a Pt/C proton exchange membrane fuel cell catalyst

This paper explores the effect and siting (location) of Nafion on Pt/C as exists in a PEM fuel cell catalyst layer. The addition of 30 wt% Nafion on Pt/C (Nfn-Pt/C) resulted in a severe loss of BET surface area by filling/blocking the smaller pore structures in the carbon support. Surprisingly, the presence of this much Nafion appeared to have only a minimal effect on the adsorption capability of either hydrogen or CO on Pt. However, the presence of Nafion doubled the amount of time required to purge most of the gas-phase and weakly-adsorbed hydrogen molecules away from the catalyst during hydrogen surface concentration measurements. This strongly chemisorbed surface hydrogen was determined by a H₂/D₂ switch and exchange procedure. Nafion had an even more pronounced effect on the reaction of a larger molecule like cyclopropane. Results from the modeling of cyclopropane hydrogenolysis in an idealized pores suggest that partial blockage of only the pore openings by the Nafion for the meso-macropores is sufficient to induce diffusion limitations on the reaction. The facts suggest that most of the Pt particles are in the meso-macropores of the C support, whereas Nafion is present primarily on the external surface of the C where it blocks significantly the micropores but only partially the meso-macropores.     

Chromium doping as a new approach to improve the cycling performance at high temperature of 5 V LiNi0.5Mn1.5O4-based positive electrode

LiCr_2YNi_0.5−YMn_1.5−YO₄ (0 < Y ≤ 0.2) spinels have been synthesized by a sucrose-aided combustion method. Two sets of Cr-doped samples have been obtained by heating the “as-prepared” samples at 700 and 900 °C for 1 h. X-ray diffraction and thermogravimetric data show that pure and single phase spinels with similar lattice parameter have been synthesized. The homogeneity and the sub-micrometric particle size of the spinels have been shown by SEM and TEM. The main effect of the temperature is to increase the particle size from ≈50 to ≈500 nm, on heating from 700 to 900 °C. The study of the influence of Cr-dopant content and thermal treatment on the electrochemical properties at 25 °C and at 55 °C has been carried out by galvanostatic cycling in Li-cells. The discharge capacity (≈130 mAh g⁻¹) does not noticeably change with the synthesis conditions; but the cycling performances are strongly modified. Key factors that control the cycling performances have been determined. The most highlighted result is that spinels heated at 900 °C with Y ≤ 0.1 have very high capacity retention at 55 °C (>96% after 40 cycles, cyclability >99.9% by cycle) indicating that metal doping is a new approach to prepare 5 V LiNi_0.5Mn_1.5O₅-based cathodes with excellent cycling performances at high temperature.     

Solar photocatalytic decomposition of Probenazole in water with TiO2 in the presence of H2O2

The photocatalytic decomposition of Probenazole in water using TiO₂/H₂O₂ under sunlight illumination is studied. The addition of H₂O₂ is effective for the improvement of photocatalytic decomposition of Probenazole with TiO₂. Furthermore, the operating conditions, such as photocatalyst dosage, temperature, pH, sunlight intensity and illumination time are also optimized. The kinetics of photocatalytic decomposition follow a pseudo–first–order kinetic law, and the rate constant is 0.129 min^?1. The activation energy (E_a) is 11.34 kJ/mol. The photocatalytic decomposition mechanism is discussed on the basis of molecular orbital (MO) simulation for frontier electron density.