Jiří Vondrák

MnOx/C composites as electrode materials II. Reduction of oxygen on bifunctional catalysts based on manganese oxides

Abstract MnO x /C composites prepared by the reduction of KMnO 4 by active carbon black and doped with Ca(II), Mg(II), Ni(II), Bi(III), and Cr(III) ions were tested as catalysts of oxygen reduction in alkaline electrolyte. The polarisation curves were analysed by logarithmic wave analysis. MnO x doped with the transition metal salts are slightly more active than those with Ca(II) and Mg(II).

MnOx/C composites as electrode materials. I. Synthesis, XRD and cyclic voltammetric investigation

MnOx/C composite material was obtained by precipitation on suspended carbon black particles from dissolved KMnO4. Doped composites were obtained by addition of Ca, Mg, Ni and other ions to the reaction mixture. The resulting solids are XRD amorphous or very poorly crystalline, but their chemical composition, Mn valence and electrochemical behaviour are very similar to those reported for birnessite, 7-A layered manganate(III,IV). We tested the electrochemical reduction/oxidation of the composite in 2 M KOH using cyclic voltammetry with three kinds of electrodes: microparticles immobilised on a carbon electrode (voltammetry of microparticles, vmp), carbon paste electrodes (cpee) and PTFE-bonded electrodes (tbe). Each technique has been used for its own merit: vmp is the best pilot technique, cpee enables fast quantification of the charge/mass relations, and tbe is the most reliable, but the slowest and the most laborious one. All the employed techniques yield directly comparable results. The composite can be discharged/recharged several tens of times at approximately 1 e−/Mn efficiency.

Gel polymer electrolytes based on PMMA

New polymer gels (based on PMMA/PC) containing Li, Na, Mg, and Zn perchlorates were prepared. The conductivity of gels containing cations of smaller ionic radii (Li and Mg) is lower than that of the others.

The role of water in hydrogen insertion into WO3

Abstract The accelerating role of water in the insertion of hydrogen into tungsten trioxide WO 3 was studied in non-aqueous aprotic and/or protic reaction media and in aqueous solutions of the sulfuric acid. The activation energies of hydrogen insertion in various reaction media were estimated. The partial molar heat of hydrogen insertion ΔH =8.7±1.2 kJ ⧸ mole of H 0.45 WO 3 was evaluated.

Gel polymer electrolytes based on PMMA III. PMMA gels containing cadmium

Gel polymer electrolyte containing cadmium perchlorate was prepared by polymerisation of methylmethacrylate with a solution containing Cd perchlorate in propylene carbonate (PC). Electric conductivity, fundamental electrochemical properties and Cd electrodeposition were investigated. The deposition of Cd was fairly homogenous.

Study of electrochemical properties and thermal stability of the high-voltage spinel cathode material for lithium-ion accumulators

This article deals with the properties of high-voltage cathode material LiNi0.5Mn1.5O4 synthesised by a solid-state reactions method. The sample—LiNi0.5Mn1.5O4—was synthesised by two steps of annealing process. A number of electrochemical and physical methods were used to analyse the samples. The obtained LiNi0.5Mn1.5O4 powder was characterised by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and galvanostatic charge/discharge measurements at different loads and high temperature in lithium-ion cells with metal lithium as a counter electrode. All these analyses were used for confirmation of the structure of the material LiNi0.5Mn1.5O4 and for investigating its electrochemical properties. A special analysis was used for this paper to understand and confirm the function of this type of material. It is called electron paramagnetic resonance (EPR) spectroscopy, which is used in the field of lithium-ion batteries and also which is not common. This analysis is typically used to analyse free radicals. It is possible to study changes of valence in cathode materials during charging and confirming the valence change of Mn from Mn3+ to Mn4+ and of Ni from Ni2+ to Ni3+ and then to Ni4+ with EPR analysis. Thermogravimetric (TG) analysis of stability of the material LiNi0.5Mn1.5O4 with in situ observation of structural changes by SEM was used as the last analysis.

The accelerating role of water in hydrogen insertion into tungsten trioxide

Abstract Hydrogen insertion into tungsten oxide WO 3 was studied by in situ infrared (IR) and electron spin resonance (ESR) spectroscopic methods. The accelerating role of water in the reaction system is discussed.

Electrochemical absorption of hydrogen in transition metal oxides—II. Sorption of hydrogen in titanium monoxide TiO

Abstract The absorption of hydrogen in titanium “monoxide” was studied electrochemcially by potential-step and ac techniques. The absorption equilibrium obeys the Crandall—Faughnan—Armand isotherm and the chemical diffusion coefficient is a linear function of concentration. The heat of absorption is 80 kJ (g atom H) −1 and the true diffusion coefficient D 0 is 4 × 10 −13 cm 2 s −1 at 20°C. The absorption occurs in oxygen vacancies.

Insertion of hydrogen into hexagonal ammonium tungsten bronze (NH4)0.3WO3

Insertion of hydrogen into the hexagonal ammonium tungsten bronze was carried out by electrochemical and “hydrogen spill-over” techniques. The coefficient x = 0.06 in the hydrogen bronze Hx(NH4)0.3WO3 and the diffusion coefficient of hydrogen DH = 2.29 × 10−17 m2s−1 were found by the hydrogen insertion in the absence of water.

The influence of used precursors on the properties of high-voltage cathode materials

This article deals with the influence of precursors used in the high-voltage cathode material LiNi0.5Mn1.5O4 based on the LiMn2O4 material on its properties. Precursors with various sizes of particles were used for making the cathode material. Consequently, its influence on the stability during cycling, change of load, and the influence of higher temperatures during cycling were investigated. Produced materials were further analyzed to discover the influence of the change of the used precursor on the structure of the cathode material. The structure of the material deposited on Al foil was investigated by atomic force microscopy (AFM), and also X-ray photoelectron spectroscopy (XPS) analysis was performed. The materials were then observed by SEM and analyzed by the EDAX method. The results show that smaller particle size enhances the properties of cathode material both in the stability during cycling and higher capacity and also the potential under higher load.