J. Vondrák

Plastic-bonded electrodes for nickelcadmium accumulators. X. The nature of the second discharge step of nickel oxide electrodes

Abstract The second discharge step of nickel oxide accumulator electrodes is suppressed in the presence of a Teflon binder and/or cobalt, cerium, or manganese hydroxide, whereas the first discharge step increases. This behaviour is discussed in terms of ohmic resistance effects and semiconducting properties of the active material.

The electrochemical insertion of alkali metal into YBa2Cu3O7−δ superconductor

Abstract The electrochemical insertion of lithium into ceramic superconductor YBa2Cu3O7−δ was observed in potential range 3.15–2.15 V vs Li (in 1 M LiClO4/PC solution). The process is reversible. The estimated value of the diffusion coefficient, DLi = 10−12−10−11 cm2s−1, indicates a rather rigid bond between layers in the crystal lattice of the electrode material. The highest lithium concentration corresponds to the ratio Li: Y = 0.4–0.5.

Electrochemical properties of tungsten bronzes—I. Hydrogen absorption in sodium tungsten bronzes

Abstract Cathodic evolution of hydrogen on the electrodes from sodium tungsten bronzes was studied. Great dependence of cd on time and prepolarization of the electrode is caused by absorption of atomic hydrogen in the electrode; this process is controlled by hydrogen diffusion in bronze. Mechanism of hydrogen evolution in sulphuric acid solutions was proposed on the basis of polarization curves, the dependence of overvoltage on the activity of hydrogen ion and anodic chronopotentiometric curves. The rate of hydrogen evolution is controlled by the reaction of electrochemical desorption of atomic hydrogen, the adsorption of which is fast and obeys Langmuir isotherm. Exchange cd of hydrogen evolution is of the order 10 −7 to 10 −6 A cm −2 and increases with the increase of sodium content in the bronze.

Electrochemical insertion of lithium in manganese dioxide

Abstract The insertion of lithium into compact electrolytic MnO2 was studied by galvanostatic and admittance techniques. The diffusion coefficient, 3.4 × 10−14 m2/s at 20 °C, and its activation energy of 30 kJ were determined. The rate of the process is controlled mainly by the high resistivity of MnO2 and by the polarization of the electrode reaction itself. Similarly, the behaviour of MnO2C plastic bonded electrodes is controlled mainly by the high resistivity of MnO2 if the content of carbon black is not sufficient to form a conductive network, and by polarization on the grains of MnO2 at higher carbon black contents. The optimum concentration of carbon is 30 – 40% by volume.

Electrochemical absorption of hydrogen in transition metal oxides

The current response of an electrode which absorbs reactant on a potential step was computed by digital simulation and by Boltzmann substitution under assumption of diffusion coefficient being a linear function of concentration. The results were expressed as an correction factor α in Sand equation i = −αF ΔC(D/πt)12. Similar results were obtained for admittance of such electrodes. Results were tested on absorption of hydrogen in sodium tungsten bronze Na0.72WO3. The potential step method is suitable for evaluation of concentration-potential-diffusion coefficient profiles.

Influence of mercury on hydrogen overvoltage on solid metal electrodes—I. Stationary polarization curves of hydrogen deposition on pure and poisoned electrodes

Abstract Mercury deposited in amounts of the order of a few monolayers on Pt, Fe, Ni and Co raises the overvoltage of hydrogen evolution in alkaline solution. On Cr it decreases the overvoltage and on Mo it has little influence, but these metals corrode in alkali.

The increase of stability of LixCoO2 electrodes of cointercalated sodium

Abstract Cyclic electrochemical deintercalation and intercalation of LiCoO 2 was studied in LiClO 4 /propylene carbonate electrolyte. The stability of the material was markedly improved if 10 to 20% of lithium atoms were replaced by cointercalated sodium. The increase of the stability is explained by preventing of complete deintercalation in anodic range due to essentially lower mobility of sodium ions in the van der Waals gaps.

Influence of mercury on hydrogen overvoltage on solid metal electrodes—III. Adsorption and evolution of hydrogen on poisoned platinum

Abstract The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order 10 −6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2–3 active sites of the electrode surface.

Influence of mercury on hydrogen overvoltage on solid metal electrodes—II. Desorption of mercury from electrodes caused by cathodic evolution of hydrogen

Abstract The removal of mercury from smooth mercury poisoned platinum electrodes by means of simultaneous hydrogen evolution is described. It is caused by volatility of mercury and its evaporation into the stream of electrolytically discharged hydrogen. The kinetic equation for this process is derived and limitations of its validity are established and discussed. At higher values of current density and temperature, a platinum electrode is not poisoned in a medium containing traces of mercury ions. The behaviour of nickel and cobalt electrodes is similar.

The preparation and electrochemical properties of chromium oxides CrOx both in lithium and sodium aprotic electrolytes

Abstract Chromium oxides CrO x were prepared under various conditions from CrO 3 . Their capacity and reversibility as cathodic materials both in LiClO 4 /propylene carbonate and NaClO 4 electrolytes were measured. The charge density in lithium electrolyte (750 C/g) was higher than that in sodium one (400–600 C/g). Coin cells were assembled with CrO x cathode; their capacity was 0.1 A h for 2025 size, and the average voltage 3.0 V.