The effect of local thermal non-equilibrium on impulsive conduction in porous media

We examine the effect of local thermal non-equilibrium on the evolution of the stagnant temperature field in a semi-infinite porous medium. When local thermal equilibrium pertains, the temperature field which is induced by a step change in the temperature of a plane boundary is given by the classical conduction solution involving the complementary error function. When thermal local equilibrium does not apply, then conduction takes place more rapidly in one phase than in the other, although local thermal equilibrium is always approached as time increases. This note examines the evolution of the temperature field in each phase in detail using numerical methods, and the numerical solutions are supplemented by asymptotic solutions valid for both small and large times.     

Surface film effect on the electrochemical behaviour of active metals

It has been suggested that a film on the surface of an active metal should be a dielectric with low ionic and electronic conductivity. The film-electrolyte boundary can be regarded either as an interface between two electrolytes or as the surface of an insulating electrode. In this zone a potential difference is set up and redox reactions proceed. These processes, controlled by the concentration and by the transfer of charged particles through the film, determine the anode-cathode behavior of metals under consideration.Corrosion and anodic dissolution of lithium and magnesium have been investigated in dilute solutions of water in propylene carbonate. The potential of the lithium electrode reveals the properties of the equilibrium potential and does not depend on either the content of water in the solution or the lithium corrosion rate. The observed orders of interaction between lithium and water, however, does depend on the water concentration, on the electrolyte concentration, and on the thickness of the initial film.At the beginning of the process the rate of the interaction between lithium and water is described by the equations of topochemical kinetics, but when the maximum is reached, by the Tamman equation. A stationary potential is realized on magnesium; its value depends on the water content in the solution. The potential shifts towards negative values with increased water concentration.     

Influence of perfluorinated surfactants on the electrochemical behaviour of a lead electrode in sulfuric acid solution

The influence of various types of perfluorinated surfactants and their analogous hydrocarbon chain surfactants on the electrochemical behaviour of a lead electrode in sulfuric acid is studied by linear sweep voltammetry and cyclic voltammetry. The hydrogen evolution reaction is affected mainly by the type of the hydrophobic chain of the added surfactant (perfluorinated or hydrogenated). Nevertheless, these surfactants exert little expander acticn on the negative electrode, or even show an adverse effect on the discharge capacity of the lead electrode. A possible explanation is given in terms of the adsorption properties of different surfactants on the electrode surface during charge and discharge.     

The effect of dispersion of nano-carbon on electrochemical behavior of Fe/nano-carbon composite electrode

In order to estimate the effect of the dispersion of nano-carbon species into a composite electrode, ultrasonic pretreatments under various conditions have been carried out for two kinds of nano-carbons, vapor grown carbon fiber (VGCF) and acetylene black, before preparing a composite electrode with iron powder. The cyclic voltammetry behavior of the resulting Fe/nano-carbon composite electrodes is significantly influenced by the kind of nano-carbon and the pretreatment conditions in spite of as a low content of nano-carbon as 20 wt.%. The largest redox current at initial cycle is obtained for Fe/VGCF composite electrode with the 30 min pretreatment of VGCF in ethanol. The dispersion of nano-carbon verifies the apparent density of the composite, and then affects the feature of three-phase interface among the ion-conducting electrolyte, the redox reaction surface, and the electron conductor.     

The effect of stable thermal stratification on the onset of double-diffusive convection in electrochemical systems

The onset time of double-diffusive convection in time-dependent, nonlinear concentration fields was investigated theoretically and experimentally. The stability analysis was conducted on the basis of the propagation theory. Under the linear stability theory, the linearized perturbation equations were transformed similarly by using the concentration penetration depth as a length-scaling factor. The newly derived stability equations were solved numerically. Also the onset time was determined experimentally by employing an electrochemical technique where 0.03–0.3 M CuSO₄ + 1.5 M H₂SO₄ aqueous solutions were adopted as electrolyte. The onset time of double-diffusive convection was delayed or shortened depending on the degree of stable stratification.     

The electrochemical behaviour of the carbon-coated Ni0.5TiOPO4 electrode material

Ni_0.5TiOPO₄ oxyphosphate exhibits good electrochemical properties as an anode material in lithium ion batteries but suffers from its low conductivity. We present here the electrochemical performances of the synthesized Ni_0.5TiOPO₄/carbon composite by using sucrose as the carbon source. X-ray diffraction study confirms that this phosphate crystallizes in the monoclinic system (S.G. P2₁/c). The use of the Ni_0.5TiOPO₄/C composite in lithium batteries shows enhanced electrochemical performances compared with the uncoated material. Capacities up to 200 mAh g⁻¹ could be reached during cycling of this electrode. Furthermore, an acceptable rate capability was obtained with very low capacity fading even at 0.5C rate. Nevertheless, a considerable irreversible capacity was evidenced during the first discharge. In situ synchrotron X-ray radiation was utilized to study the structural change during the first discharge in order to evidence the origin of this irreversible capacity. Lithium insertion during the first discharge induces an amorphization of the crystal structure of the parent material accompanied by an irreversible formation of a new phase.     

Analysis of thermal dispersion effect on vertical-plate natural convection in porous media

The effect of transverse thermal dispersion on natural convection from a vertical, heated plate in a porous medium is examined analytically. The results show that due to the better mixing of the thermal dispersion effect, the heat transfer rate is greatly increased. By taking the no-slip boundary effect into account, it is found that the flow has dual-layer structure. For the inner region, the velocity gradient is very large and the viscous resistance due to the presence of the wall is important. For the outer, much wider, region the velocity gradient is rather small and the viscous resistance term can be neglected. A singular perturbation method is employed to get the higher-order corrections due to the inclusion of the no-slip boundary effect.     

The influences of some additives on electrochemical behaviour of nickel electrodes

Nickel hydroxide is used as an active material in positive electrodes of rechargeable alkaline batteries. Since the nickel hydroxide electrode exhibits a poor performance which results not only from the competitive reactions of the oxidation of the active material but also from the evolution of oxygen. Its reduced charge acceptance is suspected to be related to a relatively long distance between nickel hydroxide particles and the nearest portion of the substrate. The practical capacity of the positive nickel electrode depends on the efficiency of the conductive network connecting the Ni(OH)₂ particle with the current collector.     

Enhancement of hydrogen evolution reaction by Pt nanopillar-array electrode in alkaline media and the effect of nanopillar length on the electrode efficiency

Pt nanopillar-array 3D electrodes with nanopillar length of 150, 450 and 900 nm and nanopillar density of ~10⁹ cm⁻² were fabricated. Their catalytic activity for hydrogen evolution reaction (HER) was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy. In comparison with straightly electrodeposited black Pt film and forged Pt sheet electrodes, the HER current density has been significantly improved by the nanopillar-array architecture. The overpotential of HER at current density of 10 mA cm-2 at 26 °C is as low as 78 mV, lower than the black Pt film of 107 mV and the Pt sheet of 174 mV. The improvement of HER is ascribed to the low charge transfer resistance of the 3D electrode and the high desorption capability of hydrogen bubbles at the nanotips. Interestingly, the nanopillar-array 3D electrode has an optimal nanopillar length for HER. The mechanisms for the optimal nanopillar length were investigated here.     

The influence of calcium compounds on the behaviour of the nickel electrode

The nickel hydroxide electrode normally exhibits a poor change-acceptance which results from the competitive reactions of the oxidation of active material and the evolution of oxygen. The combined addition of cobalt and a calcium compound to the nickel hydroxide can improve appreciably the utilization and discharge potential of the active material. The action of the calcium additive is to increase the oxygen evolution potential, while the cobalt imparts good conductivity to the active material for both electrons and protons.     

The effect of local thermal non-equilibrium on forced convection boundary layer flow from a heated surface in porous media

A steady two-dimensional forced convective thermal boundary layer flow in a porous medium is studied. It is assumed that the solid matrix and fluid phase which comprise the porous medium are subject to local thermal non-equilibrium conditions, and therefore two heat transport equations are adopted, one for each phase. When the basic flow velocity is sufficiently high, the thermal fields may be described accurately using the boundary layer approximation, and the resulting parabolic system is analysed both analytically and numerically. Local thermal non-equilibrium effects are found to be at their strongest near the leading edge, but these decrease with distance from the leading edge and local thermal equilibrium is attained at large distances.     

Cresyl diphenyl phosphate effect on the thermal stabilities and electrochemical performances of electrodes in lithium ion battery

To improve the safety of lithium ion battery, cresyl diphenyl phosphate (CDP) is used as a flame-retardant additive in a LiPF₆ based electrolyte. The electrochemical performances of LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C half cells are evaluated. The thermal behaviors of Li_0.5CoO₂ and Li_0.5CoO₂-CDP-electrolyte, and Li_xC₆ and Li_xC₆-CDP-electrolyte are examined using a C80 micro-calorimeter. For the LiCoO₂/CDP-electrolyte/Li cells, the onset temperature of single Li_0.5CoO₂ is put off and the heat generation is decreased greatly except the one corresponding to 5% CDP-containing electrolyte. When Li_0.5CoO₂ coexists with CDP-electrolyte, the thermal stability is enhanced. CDP improves the thermal stability of lithiated graphite anode effectively and the addition of 5% CDP inhibits the decomposition of solid electrolyte interphase (SEI) films significantly. The electrochemical tests on LiCoO₂/CDP-electrolyte/Li and Li/CDP-electrolyte/C cells show that when less than 15% CDP is added to the electrolyte, the electrochemical performances are not worsen too much. Therefore, the addition of 5-15% CDP to the electrolyte almost does not worsen the electrochemical performance of LiCoO₂ cathode and graphite anode, and improves theirs thermal stability significantly; thus, it is a possible choice for electrolyte additive.     

Effects of heat treatment time on electrochemical properties and electrode structure of polytetrafluoroethylene-bonded membrane electrode assemblies for polybenzimidazole-based high-temperature proton exchange membrane fuel cells

To improve cell performance, the effects of heat treatment time on the electrochemical properties and electrode structure of PTFE-bonded membrane electrode assemblies for PBI-based high-temperature proton exchange membrane fuel cells are investigated. The cell performance is observed to decrease in the high-current-density region rather than in the low-current-density region with increasing heat treatment time at 350 °C from 1 to 30 min. Microscopic studies reveal remarkable differences in the electrode structure by the agglomeration of dispersed PTFE and adjacent catalyst particles, depending on the heat treatment time. As the heat treatment time increases, only the large pore (secondary pore) volume in the electrode decreases, resulting in increase in mass transport resistance and concentration overpotential in the high-current-density region. Cell performance is not measured without heat treatment because the electrodes are not formed. When the electrodes are heat treated for 1 min at 350 °C, the best cell performance is obtained, 0.67 V at 200 mA cm⁻².     

Investigations on the electrochemical behaviour of prussian blue films in acetonitrile

The differences noticed in the order of preference for alkali metal cations through the channels of Prussian blue surface modified electrodes during redox reaction in aqueous versus acetonitrile media are presented. Since intercalation/deintercalation processes by alkali metal cations like K⁺, Na⁺, Li⁺, etc., help in deciding the efficacy of the PB film in device fabrications, the associated columbic charges and diffusion coefficient values computed from chronoamperometric and chronocoulometric studies are reported.     

The effect of various acids treatment on the purification and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of HCl, HNO₃, H₂SO₄ and HF acids on the purification and the electrochemical hydrogen storage of multi-walled carbon nanotubes (MWCNTs) were studied. The MWCNTs were synthesized on Fe–Ni catalyst by thermal chemical vapor deposition method. The X-ray diffraction and thermal gravimetric analysis results indicated that the MWCNTs purified by HF acid had the highest impurities as compared with the other acids. The N₂ adsorption results at 77 K indicated that all the samples were mainly mesoporous and the purified MWCNTs by HF acid had the highest surface area as compared with the other acids. The hydrogen storage capacities of the purified MWCNTs by the following acids were in ascending order as: H₂SO₄, HCl, HNO₃ and HF. It was found that the 1–2 nm micropores in the MWCNTs are very important for hydrogen storage. Further, the presences of catalyst and defective sites in MWCNTs influence the hydrogen storage capacity.     

Grafting effect on the wetting and electrochemical performance of carbon cloth electrode and polypropylene separator in electric double layer capacitor

Activated carbon (AC) fiber cloths and hydrophobic microporous polypropylene (PP) membrane, both modified by plasma-induced graft polymerization of acrylic acid (AAc) under UV irradiation, and filled with saturated lithium hydroxide solution were used as electrodes, a separator and electrolyte in electric double layer capacitors (EDLCs). The modification process changed the hydrophobic character of AC and PP materials to hydrophilic, made them wettable and serviceable as components of an electrochemical capacitor. The presence of poly(acrylic acid) on the AC and PP surface was confirmed by SEM and XPS methods. Electrochemical characteristics of EDLCs were investigated by cyclic voltammetry and galvanostatic charge-discharge cycle tests and also by impedance spectroscopy. At the 1000th cycle of potential cycling (1 A g⁻¹) the specific capacitance of 110 F g⁻¹ was obtained with a specific energy of 11 Wh kg⁻¹ at power density of 1 kW kg⁻¹. The above results provide valuable information which may be used when developing novel compositions of EDLCs.     

The influence of some additives on the electrochemical behaviour of sintered nickel electrodes in alkaline electrolyte

Sintered nickel electrodes containing cobalt and cadmium hydroxides as additives were prepared by anodic polarization in 42 wt.% KOH solution of sintered nickel supports impregnated with mixed metal nitrate solutions. The structural and electrochemical characteristics of these electrodes were investigated by scanning electron microscopy, X-ray diffraction, cyclic voltammetry and charge–discharge curves. The addition of Co(OH)₂ and Cd(OH)₂ in active material have been shown to increase the discharge capacity and coulombic efficiency of nickel electrode by improvement of the electrode processes reversibility and by minimizing of the parasitic oxygen evolution reaction.     

The effect of functionalized group concentration on the stability and thermal conductivity of carbon nanotube fluid as heat transfer media

In this research, the pristine CNTs sample synthesized by the CCVD method contains catalytic particles and the carbonaceous impurities, and then the special purification procedure was done. By different methods CNT functionalized with various concentration of COOH was prepared. The carboxylated CNTs were analyzed by back titration method for determining the COOH concentrations on the surface of the oxidized CNTs. Thermal conductivity of difference carbon nanotube fluid has been measured under the stable condition by KD₂ prob. For the first time, we have compared the effect of difference COOH concentration as important parameter in stability and heat transfer behavior of nanofluid. The results show that increasing the functionalized group causes better stability and higher thermal conductivity if the surface of MWNT does not damage in functionalize process.