G.H.J. Broers

Bounded diffusion in solid solution electrode powder compacts. Part II. The simultaneous measurement of the chemical diffusion coefficient and the thermodynamic factor in LixTiS2 and LixCoO2

Abstract Two electrochemical methods - involving the application of a long-time galvanostatic current pulse and a small potentiostatic voltage step to a M/M x SSE cell - are presented. From the overvoltage, respectively current response the chemical diffusion coefficient ( D M + ) and the thermodynamic factor (∂ ln a /∂ ln c ) are obtained. The methods have been applied to the cells: Li/1M·LiClO 4 in propylenecarbonate/Li x Ti 1.03 S 2 0.05 x T = 20°C; and Li x CoO 2 0.10 T = 20°C. From the application of the current pulse/voltage decay method it followed: D Li + ( Li x Ti 1.03 S 2 ) = 1−4 × 10 −8 cm 2 s −1 , with a slight tendency to increase with decreasing x ; D Li C ( Li x CoO 2 ) = 2−40 × 10 −9 cm 2 s −1 , decreasing with decreasing x . These values are among the highest found for solid state Li + -ion diffusion, and will be closely evaluated and compared with data reported by other workers. The x -dependence of the thermodynamic factor, determined from kinetic data, for Li x Ti 1.03 S 2 ( x = 0.05-0.95) and Li x CoO 2 ( x = 0.60-1.00) is in accordance with a simple thermodynamic model. Unlike for Li x Ti 1.03 S 2 , the thermodynamic factor for Li x CoO 2 , determined from the EMF- x relation, cannot be accounted for by this model. Furthermore, a fast, but crude method to determine the average chemical diffusion coefficient in Li x Ti 1.03 S 2 and Li x CoO 2 is discussed.

The electrical conductivity of δ-Bi2O3 stabilized by isovalent rare-earth oxides R2O3

Abstract The phenomenon of δ-phase stabilization in the systems (Bi 2 O 3 ) 0.75 (R 2 O 3 ) 0.25 where R = lanthanide metal (including La, but Pm excepted) was investigated. The electrical conductivity of those systems, which did show δ-stabilization, was investigated; the system (Bi 2 O 3 ) 1−x (Yb 2 O 3 ) x was studied in more detail. A Vegard-like relation between the cubic cell-axis and cationic radius of the substituted trivalent metal ion was found. A model is presented which accounts for results of electrical conductivity measurements and thermogalvanic measurements as well.

Bounded diffusion in solid solution electrode powder compacts. Part I. The interfacial impedance of a solid solution electrode (MxSSE) in contact with a m+-ion conducting electrolyte

Abstract The transient and ac electrical response of electrochemical systems containing a solid solution electrode is derived using Laplace transform methods. First, the operational impedance of the electrolyte/solid solution electrode interface is derived, for both asymmetrical and symmetrical cell systems. From the operational impedance the current, voltage and ac impedance response is calculated. By combination of semi-infinite and bounded diffusive behaviour, both the chemical diffusion coefficient of the inserted ions and the thermodynamic factor (∂ ln a /∂ ln c ) can be obtained from kinetic data only.

The oxygen evolution on La0.5Ba0.5CoO3: Theoretical impedance behaviour for a multi-step mechanism involving two adsorbates

An alternative mechanism with two adsorbed intermediates is proposed for the oxygen evolution on La0.5Ba0.5CoO3 in 1–6 M potassium hydroxide solution, in connection with previously reported experimental results. The dc and ac behaviour is derived, the latter both by a simple and a more rigorous method. The rigorous method gives an analytical description of the impedance behaviour for Langmuirtype adsorption only. Numerical analysis based on the simpler method has been applied to the more general Frumkin-type adsorption case and to an extension with three adsorbed intermediates. Also, the reaction order towards hydroxyl activity at constant overpotential has been derived for all cases considered. The peroxide mechanism with two adsorbed intermediates fits the experimental results on oxygen evolution reasonably well.

The oxygen evolution on La0.5Ba0.5CoO3: Overpotential decay behaviour : Theory and experimental results

In section (I) the overpotential decay behaviour of an electrode during oxygen evolution is described on the basis of calculations for two mechanisms with a chemical rate-determining step (rds). Quasi-equilibrium in the steps preceding the rds and Frumkin-type adsorption of intermediates is assumed. In section (II) decay experiments are reported for oxygen evolution on La0.5Ba0.5CoO3 in 6M KOH solution at room temperature. Both aged and renewed oxide electrodes were used. Measured decay rates at constant overpotential were found to vary as a function of starting overpotential, while the theory requires equal rates. Also, an increased capacitance for the aged electrode with respect to the renewed one was not accompanied by an expected increase in current density. Neither phenomenon can be explained by assuming non-equilibrium in the mechanism, but can be understood on the basis of a growing passive multilayer with a presumably enhanced CoIV/CoIII-ratio. The peroxide mechanism with a chemical rds and low coverage of the second intermediate is the most likely of the alternatives considered. Capacitances calculated from the initial overpotential decay were approximately equal to capacitances derived from ac impedances measured prior to the interruption.

Properties of La1−xSrxBO3−y (B=Co or Fe) compounds as oxygen electrodes in alkaline solution: General aspects

Abstract La 0.50 Sr 0.50 BO 3−y (B=Co or Fe) were tentatively tested as oxygen electrodes in alkaline solution. Earlier reported reversible behaviour of La 0.50 Sr 0.50 CoO 3−y could be explained by non-equilibrium phenomena. Supported by measurements of electrical conductivity and thermogravimetry evidence exists that very slow oxygen ion diffusion inside the electrode material is involved in the equilibration process. Observable oxygen evolution starts at comparatively low potentials (≈1400 mV vs. a Pt/H 2 (1 atm) electrode in the same solution).

The electrochemical determination of oxygen ion diffusion coefficients in La0.50Sr0.50CoO3−y: Experimental results and related properties

Abstract Results of an electrochemical method for the determination of oxygen ion diffusion coefficients D O 2− in porous pellets of La 0.50 Sr 0.50 CoO 3−y are presented. Log D O 2− can be expressed as log D O 2− =−(2.3±0.2) 10 3 / T −(6.6±0.6) cm 2 s −1 . The activation energy E a for the diffusion process equals 44 kJ mol −1 . Further the related electrochemical measurement and the adjustment of the oxygen deficiency y are described. At 75°C the following empirical Nernst relation between the electrode potential E (vs. a Pt/H 2 (1 atm) electrode in the same solution) and Δ y is found: E =627–108 ln (Δ y + 0.0054) mV. (Δ y = y−y 0 ; y 0 =mole fraction of vacancies at the reversible O 2 potential of 1190 mV). The use of La 0.50 Sr 0.50 CoO 3−y as a solid solution electrode in practical storage cells seems to be excluded for thermodynamic and kinetic reasons.

The oxygen evolution on La0.5Ba0.5CoO3 : Passivation processes

Earlier publications on the oxygen evolution of La0.5Ba0.5CoO3 have left a number of observations involving passivation effects of the electrode unexplained. Therefore, the electrode surface used was studied by means of X-ray powder diffraction and electron microscopy (with diffraction and elemental analysis). It could be concluded that the surface had partly been changed into (hydrated) cobalt oxides. From this and earlier experiments it was concluded that the La0.5Ba0.5CoO3 electrode is activated by a (possibly diffusion-controlled) oxidation, but a further anodization at high overpotentials leads to passivation with loss of activity for the oxygen evolution. The activity is regained by lowering the overpotential.

An electrochemical method for the determination of oxygen ion diffusion coefficients in La1−xSrxCoO3−y compounds: Theoretical aspects

Abstract Theoretical aspects of a novel electrochemical method for the determination of O 2− diffusion coefficients in porous pellets of La 1− x Sr x CoO 3− y are discussed. This method is based on a bounded 3-dimensional diffusion model.

The oxygen evolution reaction on cobalt: Part II. Transient measurements

Abstract Open-circuit overpotential decays on an aged cobalt electrode in the oxygen evolution range in 6 M KOH show different slopes for two overpotential regions. These slopes are lower than the Tafel slope in the same region: Tafel slopes of ∼100 and ∼40 mV/dec, at high and low overpotentials, respectively. compared to decay slopes of ∼−60 and ∼−20 to −30 mV/dec. For a fresh cobalt electrode a decay slope of ∼−40 mV/dec is found at high overpotentials. From impedance measurements during a decay it is concluded that the electrode capacitance cannot account for the decay curves obsered. By means of steady-state potentiostatic impedance measurements (with stabilization times > 24 h) it is found that the differential Tafel slope remains constant at ∼40–50 mV/dec and differs considerably from the Tafel slope at high overpotentials, ∼100 mV/dec. Galvanostatic pulse experiments give evidence of the presence of CoO 2 in the oxide layer. Two models which may explain the observed experimental results are analysed. Both include a potential-dependent (extra) process which is fixed by the amount of CoO 2 at the surface. In one model, CoO 2 is responsible for partial surface blockage (parallel process); in the other model, CoO 2 controls the conductivity of the top layer of the oxide layer on the electrode.