Preparation and electrochemical characteristics of a new Li-Mn-V-O system formed from heat-treatment of a MnO2, NH4VO3 and LiNO3 mixture

New quarternary oxides (Li₂O) _x · MnO₂ · yV₂O₅ (x = 0.125 0.25, y = 0.125 0.25), formed by heating mixtures of MnO₂, NH₄VO₃ and LiNO₃ at various Li/Mn and V/Mn atomic ratios and at different temperatures (300 400 °C in air, have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy (ESCA) and infrared spectroscopy. The quarternary oxide with x = 0.25 and y = 0.25 showed a discharge capacity of 220 A h (kg oxide)^–1 and an energy density of ca. 600 W h (kg oxide)^–1 at a current density of 0.20 mA cm^–1 in 1 m LiClO₄-propylene carbonate at 25 °C. When charge-discharge cycling with the (Li₂O) · MnO₂ · 0.25V₂O₅ electrode was performed at a constant capacity of 30 A h (kg oxide)^–1 and at a constant current density of 0.10 0.20 mA cm^–2, the electrode sustained over 100 cycles at a high mean discharge potential of ca. 3 V vs Li/Li⁺.This paper was originally presented at the 183rd meeting of the Electrochemical Society (Honolulu, Hawaii, 1993).     

Properties and initial discharge behaviour of graphite fluorides decomposed under chlorine

Two types of graphite fluoride, (CF)_n and (C₂F)_n, were partially decomposed under a chlorine atmosphere at several temperatures between 250 and 450°C. The main reaction was the C-F bond rupture to form graphite-like carbon around the particle surface. Graphite fluoride itself is an electrical insulator, but the formation of a carbon film around the surface of the graphite fluoride particles decreased the initial overpotential without a significant decrease in the capacity.     

Thermogravimetric investigation of the chemical compatibility of graphite and metal oxides

Methods of thermodynamic analysis and gravimetric measurement are used to study graphite and its mixtures with metal oxides heated to 1000°C under nonisothermal conditions. The chemical compatibility of graphite with Al₂O₃, CaO, SiO₂, MgO, Fe₂O₃, TiO₂, Na₂O, and K₂O is investigated. It is shown experimentally that under the given conditions the loss of mass (m /m) in specimens with these additives as reflected by the gravimetric curves does not exceed m /m in a purely graphite specimen.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 12 – 15, December, 1996.     

Synthesis, Characterization and Anti-microbial Activity of Oligo-N-2-aminopyridinylsalicylaldimine and Some Oligomer-metal Complexes

The oxidative polycondensation and optimum reaction conditions of N-2-aminopyridinylsalicylaldimine using air oxygen, H₂O₂ and NaOCl were determined in an aqueous alkaline solution between 40–90°C. Oligo-N-2-aminopyridinylsalicylaldimine (OAPSA) was characterized by using ¹H-NMR, FT-IR, UV-vis and elemental analysis. N-2-aminopyridinylsalicylaldimine was converted to oligomer by oxidizing in an aqueous alkaline medium. The number average molecular weight (M _n), weight average molecular weight (M _w) and polydispersity index (PDI) values were found to be 7487 gmol^–1, 7901 gmol^–1 and 1.06, respectively. According to these values, 70% of N-2-aminopyridinylsalicylaldimine turned into oligo-N-2-aminopyridinylsalicylaldimine. During the polycondensation reaction, a part of the azomethine (–CH=N–) groups oxidized to carboxylic (–COOH) group. Besides, the structure and properties of oligomer-metal complexes of oligo-N-2-aminopyridinyl salicylaldimine (OAPSA) with Cu (II), Ni (II), and Co (II) were studied by FT-IR, UV-vis DTA, TG and elemental analysis. Anti-microbial activities of the oligomer and its oligomer-metal complexes have been tested against C. albicans, L. monocytogenes, B. megaterium, E. coli, M. smegmatis, E. aeroginesa, P. fluorescen and B. jeoreseens. Also, according to the TG and DTA analyses, oligo-N-2-aminopyridinylsalicylaldimine and its oligomer-metal complexes were found to be stable thermo-oxidative decomposition. The weight loss of OAPSA found to be 20%, 50% and 98% at 350°C, 535°C and 1000°C, respectively.     

A study of electrolyte-phobic molten carbonate fuel cell electrodes

The incorporation of electrolyte-phobic material into gas diffusion electrodes enhances their performance. The principles governing the choice of electrolyte-phobic materials in molten carbonate electrolyte are discussed. Contact angle measurements of molten NaLiCO₃ on materials which have hexagonal layer-like structures confirmed that provided _{LV SV}, the contact angle will be greater than 90°. Of the materials tested (C, BN, BaO.6.Fe₂O₃, PbO.6Fe₂O₃), only C possessed adequate corrosion resistance. Electrolyte-phobic anodes, prepared by incorporation of graphite fibres into the electrocatalyst mixture, significantly reduced concentration polarization and paste electrolyte cells using such electrodes gave 160 mA/cm² at 0.6 V on 80% H₂/20% CO₂ fuel at 600°C, as opposed to 80 mA/cm² for conventional cells.     

Recent developments in the technology of sulphur dioxide depolarized electrolysis

The use of sulphur dioxide as an anode depolarizer in the electrolytic production of hydrogen can considerably reduce the electrical energy input to the electrolyzer. The present work deals with developments in the technology of SO₂-depolarized electrolysis. Recent achievements in electrode fabrication techniques and optimization of cell configuration have resulted in substantial improvements in both cell potential and performance stability. While operating in 50 wt% sulphuric acid at 50° C and 1 atm, the measured cell potentials at 200 and 400mA cm^–2 were 0.77 and 1.05V (including ohmic losses), respectively. A cell endurance test, performed at a constant current density of 100mA cm^–2, indicated that a stabilized cell potential of 675 mV was achieved after 80 hours of continuous operation. The resulting gas from the test cell contained 98.7 vol% hydrogen. The effect of acid concentration in the range 10–60 wt% on the performance characteristics of an SO₂-depolarized electrolyzer was also investigated. Experimental results revealed that the optimum acid concentration for operating SO₂-depolarized electrolyzers is approximately 30 wt%. The observed cell potential was only 0.71 V at 200mA cm^–2.     

Galvanostatic cycling of graphite intercalation electrodes with anions in aqueous acids

Natural graphite flakes (80 wt%), with polypropylene (20 wt%) as a binder, constitute a practical and non-expensive graphite electrode of high crystallinity CPP. Galvanostatic cycling of these electrodes with current densities in the range 0.3–30 mA cm^–2 (charging time 5–120 min) has been investigated in aqueous acids (12, 20 and 36 mol dm^–3 HF, 6 and 12 mol dm^–3 H₂SO₄, 4 mol dm^–3 HClO₄). The anion of the acid is anodically intercalated and cathodically de-intercalated. In spite of the high water concentrations, quantitative current efficiencies have been obtained. From variation of the rest times after charging, a corrosion current density of less than 0.03 mA cm^–2 (j _ch=3 mA cm^–2) has been derived. The overvoltage during charge and discharge is typically about 0.1 V. The potential at the start of the charging process coincides with the intercalation potential defined previously. A strong electrode formation effect is observed upon cycling. The electrode is initially smooth and non-porous; it acquires a high surface roughness after a few cycles, which is then stable. The initial charging curves increase witht ^1/2, while the charging curve after electrode formation is linear. Both clearly indicate a linear relationship between surface concentration of intercalated anions and potential. This agrees with our previous finding of linear dependence with respect to acid concentration in the solution.     

Living carbocationic polymerization

The living continuous polymerization of isobutylene initiated by a bifunctional initiator, i.e., 2,4,4,6-tetramethyl-heptane-2,6-diacetate·BCl₃ complex, in CH₂Cl₂ and C₂H₅Cl diluent in the –12 to –20°C range is described. Experimental conditions have been found under which rather narrow molecular weight distribution ,-tert.-chloro-ended polyisobutylenes of theoretical M_n and I_eff=100% can be continuously prepared in a tubular reactor in a homogeneous system (in C₂H₅Cl at –12°C). System heterogeneity tends to increase the molecular weights, decrease the I_eff, and increase the ¯M_w/¯M_n.     

Selective Modification of Pt Active Sites on Pt–Au/C Catalysts Prepared by Surface Redox Reactions

This study focused on the selective deposition of Au⁰ onto (111), (100) faces and (111)/(100) edges of cuboctahedral Pt particles present on the Pt/C(graphite) model system. The Pt–Au/C catalysts were prepared by novel surface redox methods involving the direct reduction (DR) of AuCl ₄ ^– species onto the Pt particles or reducing these species on the Pt–H interface, i.e., the refilling (RE) method. The presence of Au on the Pt particles was verified by means of high-resolution energy dispersive spectroscopy (EDS), and, after treatment at 300°C in H₂, the formation of crystalline Au⁰ aggregates was verified by X-ray wide-angle diffraction; further treatments at 500°C in H₂ led to a true Pt–Au solid solution. The Monte Carlo simulation methods indicated the selective deposition of Au⁰ onto the (111)/(100) edges of the Pt cuboctahedral particles when the relative Au concentration varied from 10 to 50 wt% Au. The catalytic conversion of n-heptane on the Pt–Au/C (DR and RE solids) catalysts presented an oscillatory behavior with respect to Pt/C, indicating modification of the active Pt ensembles, driven by the energy released during the exothermic n-C₇ dehydrogenation and cracking reactions, which should enhance the Au⁰ mobility at the Pt particle surface level.     

1,4-pentadien-3-one-1-p-hydroxyphenyl-5-p-phenyl Acrylate: Crosslinking Studies and Reactivity Ratios with Glycidyl Methacrylate

1,4-pentadien-3-one-1,5-bis(p-hydroxyphenyl) (PBHP) was synthesized by reacting p-hydroxybenzaldehyde and acetone in the presence of HCl gas. 1,4-pentadien-3-one-1-p-hydroxyphenyl-5-p-phenyl acrylate (HPA) was prepared by reacting PBHP with acryloyl chloride in ethyl methyl ketone (EMK) medium at 0°C. Copolymerization of different feed compositions of HPA with glycidyl methacrylate (GMA) was carried out using benzoyl peroxide (BPO) as initiator in EMK solvent under nitrogen atmosphere at 70±1°C. Polymers thus synthesized were characterized by IR and NMR (¹H/¹³C) spectroscopic techniques. Reactivity ratios of the monomers were calculated from the ¹H NMR data by applying linearization methods such as Fineman–Ross, Kelen–Tudos and extended Kelen–Tudos methods. Photocrosslinking property of the polymer samples was studied using the solvent method. Thermal stability of the polymers were measured using thermogravimetric analysis. Molecular weights (M _w and M _n) and polydispersity value of the polymer were determined using gel permeation chromatographic technique.     

An NMR study of acid sites on chlorided alumina catalysts using trimethylphosphine as a probe

The MAS-NMR spectra of adsorbed trimethylphosphine (TMP) were used to determine the concentration of Brønsted and Lewis acid sites on pure and chlorinated-Al₂O₃ samples. Chlorination with CHCl₃,CCl₄ or AlCl₃ promoted the formation of Brønsted acid centers, which are characterized by the protonated adduct of TMP. This adduct has a³¹P chemical shift of ca. –3.8 ppm and a J_P–H scalar coupling of 517 Hz. Additional resonances in the –44 to –54 ppm range are attributed to Lewis acid-base pairs. In some cases a partially resolved J_P–Al coupling is observed, which confirms the assignment. Upon thermal treatment of a chlorinated sample at temperatures > 200°C, the concentration of Brønsted acid centers decreased; the concentration of one type of Lewis acid increased and another remained almost constant. In a parallel set of experiments the initial conversion ofn-hexane at 150°C and the yields of cracking and isomerization products were determined. Comparable functional relationships were observed between the loss of Brønsted acid sites and the decrease in yields of both cracking and isomerization products. These results suggest that Bransted acidity is responsible for the cracking and isomerization ofn-hexane over chlorided aluminas at 150°C.     

Electrode heat balances of electrochemical cells: application to NaCl electrolysis

This paper deals with a method of estimating single electrode heat balances during the electrolysis of molten NaCl-ZnCl₂ in a cell using a-alumina diaphragm. By measuring the thermoelectric power of the thermogalvanic cells: (T) Na/-alumina/NaCl-ZnCl₂/-alumina/Na(T+dT) and (T) C,Cl₂/NaCl-ZnCl₂/Cl₂,C(T+dT) the single electrode Peltier heat for sodium deposition and for chlorine evolution at 370° C were estimated to be –0.026±0.001 JC^–1 and+0.614±0.096 J C^–1, respectively.     

Behaviour of tungsten carbide hydrogen-diffusion electrodes in chloride etching solutions

Studies were performed of tungsten carbide hydrogen-diffusion electrodes operating as anodes in electrolytic baths for regeneration of etching solutions of CuCl₂ and FeCl₃. Under conditions of electrolytic regeneration of copper chloride solutions (i = 40 mA cm^–2, 40° C) after 1500 h operation the electrode polarization increased by about 200 mV. Maximum current efficiency of 60–65% was obtained at I _k = 80 mA cm^–2. It is demonstrated that the replacement of the standard carbon anodes with tungsten carbide hydrogen-diffusion electrodes and the elimination of the ion exchange membrane separating the anodic from the cathodic space leads to a 2–4 V decrease of the electrolytic bath voltage. The regenerated solutions of CuCl₂ and FeCl₃ can be reused as etching agents after adding 7–10 ml 30% solution of hydrogen peroxide per litre.     

Electrochemical properties of bare and Ta-substituted Nb2O5 nanostructures

In this work, bare and Ta-substituted Nb₂O₅ nanofibers are prepared by electrospinning followed by sintering at temperatures in the 800–1100 °C range for 1 h in air. Obtained bare and Ta-substituted Nb₂O₅ polymorphs are characterized by X-ray diffraction, scanning electron microscopy, density measurement, and Brunauer, Emmett and Teller surface area. Electrochemical properties are evaluated by cyclic voltammetry and galvanostatic techniques. Cycling performance of Nb₂O₅ structures prepared at temperature 800 °C, 900 °C, and 1100 °C shows following discharge capacity at the end of 10th cycle: 123, 140, and 164 (±3) mAh g⁻¹, respectively, in the voltage range 1.2–3.0 V and at current rate of 150 mA g⁻¹ (1.5 C rate). Heat treated composite electrode based on M-Nb₂O₅ (1100 °C) in argon atmosphere at 220 °C, shows an improved discharge capacity of 192 (±3) mAh g⁻¹ at the end of 10th cycle. The discharge capacity of Ta-substituted Nb₂O₅ prepared at 900 °C and 1100 °C showed a reversible capacity of 150, 202 (±3) mAh g⁻¹, respectively, in the voltage range 1.2–3.0 V and at current rate of 150 mA g⁻¹. Anodic electrochemical properties of M-Nb₂O₅ deliver a reversible capacity of 382 (±5) mAh g⁻¹ at the end of 25th cycle and Ta-substituted Nb₂O₅ prepared at 900 °C, 1000 °C and 1100 °C shows a reversible capacity of 205, 130 and 200 (±3) mAh g⁻¹ (at 25th cycle) in the range, 0.005–2.6 V, at current rate of 100 mA g⁻¹.     

Effects of electrochemically incorporated bismuth on the discharge and recharge of electrodeposited manganese dioxide films in 9m aqueous KOH

Previous reported work has demonstrated that MnO₂ can be made multiple rechargeable over the two-electron capacity by chemical modification through incorporation of a small concentration of Bi(iii) species. In the present work, conditions required for inclusion of bismuth species in electrolytically produced MnO₂ deposits on porous graphite are reported together with resulting electrochemical effects of the bismuth species on rechargeability of the electrodeposited MnO₂. The optimum conditions for deposition were found to be: temperature 85–90°C; bath composition 0.5 to 2m H₂SO₄, 0.5m MnSO₄, 0.005 to 0.01m Bi³⁺ and current density 5 to 20 mA cm^–2 (apparent). The mechanism proposed for the inclusion of bismuth species is by continuous precipitation caused by high local acidity generated at the electrode by the reaction of anodic deposition of MnO₂. With respect to the mechanism of reduction and reoxidation of MnO₂ in 9m KOH with bismuth species present, a previously suggested role of soluble intermediates is confirmed. It is proposed that bismuth may aid in the nucleation and growth process associated with formation of Mn(OH)₂ or MnO₂ from a soluble Mn(iii) intermediate. Such a process must take place in order for completion of either discharge or recharge to take place at the electrode. It seems that the role of the included Bi species is to promote a discharge and recharge mechanism of the so-called heterogeneous kind involving a soluble Mn(iii) intermediate over an alternative, solid-state, homogeneous pathway.     

Low-temperature SOFCs using biomass-produced gases as fuels

The electromotive force (e.m.f) is calculated for solid oxide fuel cells (SOFCs) based on doped ceria electrolytes using biomass-produced gases (BPG, 14.7% CO, 14.2% CO₂, 15.3% H₂, 4.2% CH₄, and 51% N₂) as fuels and air as oxidant. It reveals that the BPG derived e.m.f. is very close to hydrogen when doped ceria is used as the electrolyte. A 35-m-thick samaria-doped ceria based single cell was tested between 450 and 650°C using BPG as fuel. Maximum power density of about 700 mW cm^–2 was achieved at 650 °C. The open-circuit voltage at 450 °C was 0.96 V, close to the calculated value. However, the cell power density using BPG as fuel was relatively lower than that using humidified hydrogen (3% H₂O), and close to that using humidified methane (3% H₂O). Impedance measurements indicate that the relatively lower power output may be attributed to the high anode--electrolyte interfacial polarization resistance when BPG is used as fuel.     

Effect of copper additive on Zr0.9Ti0.1V0.2Mn0.6Cr0.05Co0.05Ni1.2 alloy anode for nickel–metal hydride batteries

Zr_1–x Ti _x V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 (0 x 0.3) alloys have been characterized as metal–hydride electrodes for nickel–metal hydride batteries. Although the alloy electrodes with no Ti substitution in place of Zr exhibit a specific capacity value of 375 mA h g^–1, it has been possible to enhance the specific capacity of the electrodes to 395 mA h g^–1 by substituting 10% Ti in place of Zr, that is, with Zr_0.9Ti_0.1V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 alloy. The specific capacity value of Zr_0.9Ti_0.1V_0.2Mn_0.6Cr_0.05Co_0.05Ni_1.2 alloy was further enhanced to 415 mA h g^–1 on copper powder addition. Interestingly, the discharge curves for the latter electrode are quite flat thus providing an advantage of constant specific energy output over the entire regime of electrode discharge. Both a.c. impedance and d.c. linear polarization studies conducted on these electrodes lead to a lower charge-transfer resistance value for the metal-hydride electrode with copper additive suggesting the electrode with copper powder additive to have a higher catalytic activity than those without copper. The electrode with the copper additive also exhibits little change in its capacity over about 100 charge–discharge cycles.     

Investigations in platinum metal group electrochemistry: II The Pd(II)-Pd° reduction

The Norbide boron carbide electrode has been satisfactorily applied to polarographic studies of Pd(II)–Pd° and some other systems involving deposition of metal. By its means the following thermodynamic and kinetic data have been established: standard oxidation-reduction potentials, Pd²⁺–Pd°, 0.91 V; Ag⁺–Ag°, 0.805 V; stability constants, PdCl ₄ ^2– , log ₄, 9·38; logK ₄, 1·44; Pd(SO₄) ₂ ^2– , log ₂, 3·16; activation energies, Pd²⁺–Pd°:Q _D, 18·6; Q°, 188 kJ mole^–1. Analytical applications have been briefly examined.List of symbols A Area of the working electrode - (A°) Apparent frequency factor of the Arrhenius relationship - n Nominally the product of the transfer coefficient, , and the number of electrons,n, involved in an electrochemical process. In practice it is the value obtained from the slopeRT/anF of the lineE v. ln(i ₁–i)/i orv. ln(i ₁–i) - _j Product of dissociation constants of successive complexes:K ₁×K ₂×...×K _j - C ₀ Bulk concentration in the aqueous phase of species undergoing electrochemical reduction or oxidation - D ₀ Diffusivity of that species in the aqueous phase immediately adjacent to the electrode surface - Thickness of a diffusion layer - E _1/2 Half-wave potential, at whichi=i ₁/2 in a polarographic wave of the formE=E _1/2+RT/anF ln(i ₁–i)/i - E _mid Potential at whichi=i ₁/2 in a wave of the formE=E _mid+RT/anF ln(i ₁–i)/i - E _1/2 Displacement of half-wave potential caused by complexing of reducing species - _1/2 Overpotential at the half-wave potentialE _1/2 - _mid Overpotential atE _mid - f Activity coefficient, e.g.f _Pd ^2+(x=0) the activity coefficient of Pd²⁺ species in the aqueous phase at the electrode surface - i ₁ Limiting current - i Current at any stage of the electro-chemical processes governed byE v. ln(i ₁–i)/i relationships - j Number of complexing ligands associated with a cation—e.g. for PdCl ₃ ^– =3 - Q Arrhenius activation energy of the electrochemical process of a reduction at a working electrode [8] - Q _D Arrhenius activation energy of the diffusion stage of an electrochmical reduction [8]     

Identification of optimum conditions for zinc electrowinning in high-purity synthetic electrolytes

The optimum conditions for zinc electrowinning in synthetic acidic zinc sulphate electrolytes (0.8 M ZnSO₄+1.07 M H₂SO₄) were determined using response surface statistical analysis. The coulombic efficiency (QE) was optimized with respect to temperature (T), current density (J) and electrode rotation rate (n). For an electrolyte prepared from AR zinc sulphate and Aristar sulphuric acid, containing trace lead and nickel, QE reached a maximum of 98.8% on a zinc substrate under the following conditions:T=50°C,J=500 A m^–2,n=35s^–1. For a very-high-purity electrolyte, prepared by dissolution of 99.9999% zinc in Aristar sulphuric acid, a maximum QE of 98.4% was predicted and obtained at:T61°C,J890 A m^–2,n38s^–1. Using a statistical response surface model calculated during the optimization process, QE contours giving an overall view of electrolyte performance were constructed. The QE responses were determined principally byT andJ, with significant interaction betweenn andJ orT andJ, depending on the impurity composition of the electrolyte. The model was also used to predict the QE response of the above electrolytes under conditions similar to industrial practice.     

An attempt to bridge the pressure and material gap with a disperse model catalyst for low-temperature alkane reforming

Pt black was exposed to n>-hexane/H₂ mixtures between 483 and 663 K followed by O₂ and H₂ treatments at 603 K. XP and UP spectra were measured without exposing the samples to air. 20–30% carbon accumulated after hydrocarbon exposures. O₂ removed most carbon. The surface C content increased after a subsequent contact with H₂, C 1s showing more atomic carbon as opposed to graphite after n-hexane exposure. Anisotropic recrystallization of Pt black favoring (220) and (311) lattice planes occurred under hydrogen-rich conditions. Both findings were attributed to a H₂-induced solid-state rearrangement; H atoms penetrating into the crystal lattice, force subsurface carbon and oxygen atoms to the surface and a concomitant restructuring would occur. Thus another hydrogen effect has been recognized, leading to structures favorable for skeletal reactions of alkanes.