Interfacial impedance behaviour of polished and paint platinum electrodes at Na2WO4-Na2MoO4 solid electrolytes

Interfacial impedances of the cell systems polished Pt/Na₂WO₄-Na₂MoO₄ and painted Pt/Na₂WO₄-Na₂MoO₄ were studied as a function of temperature and oxygen partial pressure by a.c. and pulse methods. The impedances are probably related to rate determining surface reactions of oxygen atoms and molecules. With Pt-paint, a particular type of impedance behaviour characterized by a constant phase angle, CPA, is observed: Z_p=K_p(jω)^?p (K_p and p independent of ω). No simple physical models were found to explain this behaviour, which is probably due to highly inhomogeneous current distribution effects.     

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

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 CoO₂ 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 CoO₂ at the surface. In one model, CoO₂ is responsible for partial surface blockage (parallel process); in the other model, CoO₂ controls the conductivity of the top layer of the oxide layer on the electrode.     

Impedance measurements in different alkaline solutions on an oxygen-evolving La0.5Sr0.5CoO3 electrode

It was found that the oxygen evolution reaction on La_0.5Sr_0.5CoO₃ in strong alkaline solutions has a Tafel slope of ～ 60 mV/dec and a reaction order at constant overpotential with respect to the KOH activity of 0.6–0.8. From impedance measurements differential Tafel slopes were calculated and were found to be ～ 60 mV/dec at overpontentials > 250 mV. Effective capacitances having a broad maximum at an overpotential of about 200 mV in all alkaline solutions were calculated. The effective capacitance increased with increasing KOH concentration. Furthermore, the material decomposed at the surface when exposed to strong oxygen evolution. From the results a modified Krasil'shchikov reaction path is analysed.     

Interface formation and intergrain contact growth in some Ag/M2XO4 solid cell systems

The effect of annealing upon the isothermally measured complex impedance of some Ag/M₂XO₄ symmetrical cell systems (with M=Na, Ag and X=S, W) is studied. One can distinguish between four effects: (1) The growth of intergrain contact in the electrolyte pressed pellet. (2) Plastic deformation of the electrolyte pellet. (3) Mechanical formation of the interface. (4) Chemical corrosion at the Ag/Na₂XO₄ interface in air. The correlation between these phenomena is discussed.     

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

Results of an electrochemical method for the determination of oxygen ion diffusion coefficients D_O^2? in porous pellets of La_0.50Sr_0.50CoO_3?y are presented. Log D_O^2? can be expressed as log D_O^2?=?(2.3±0.2) 10³/T?(6.6±0.6) cm² 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/H₂ (1 atm) electrode in the same solution) and Δy is found: E=627–108 ln (Δy + 0.0054) mV. (Δy=y?y₀; y₀=mole fraction of vacancies at the reversible O₂ potential of 1190 mV). The use of La_0.50Sr_0.50CoO_3?y as a solid solution electrode in practical storage cells seems to be excluded for thermodynamic and kinetic reasons.