Michel Kleitz

Electrochemical Coloration and Redox Reactions in Solid Ionic Conductors

Simple solid ionic crystals such as alkali halides are generally described as perfect crystals in which more or less mobile point defects are dissolved. With regard to the electronic distribution, these point defects constitute traps sometimes having energy levels located in the forbidden band. When a suitable voltage is applied to a cell involving such a material, it is observed that a change in the electronic trap filling, compensated by a change in trap concentrations, can occur. The phenomenon starts at one electrode and spreads through the bulk of the material. When it starts at the anode, it is in fact an electrochemical oxidation of the material and when it starts at the cathode, an electrochemical reduction. As described in chapter 1, this corresponds to an enrichment or an impoverishment in one of the basic crystal components. The process is commonly regarded as resulting from the ionic conductivity of the crystal. Sometimes it even serves as a qualitative test for the existence of such a conductivity2,3. it is frequently accompanied by the onset of a coloration of the material and for this reason is generally called electrochemical coloration. (In one case1 the coloration was observed to suddenly occur in the whole crystal).

Determination of standard Gibbs energy change for CO + 12O2 = CO2 using an electrochemical pump and gauge

Abstract The standard Gibbs energy change of the reaction: CO + 1 2 O 2 = CO 2 was determined from an electrochemical reduction of pure carbon dioxide to carbon monoxide in the temperature range 970 to 1270 K. The equilibrium oxygen pressure in the flowing gas was controlled using an electrochemical pump and measured with a zirconia gauge. By using the Faradays-law test, the temperature and oxygen pressure ranges where no side-reactions take place were determined. The sources of error associated with the solid electrolyte cells were evaluated. The standard Gibbs energy change is ΔG o (T)/ kJ mol −1 = −283.328+0.08753( solT K ) ; the uncertainty in ΔG o is less than 250 J mol −1 . The enthalpy variation at 298.15 K obtained using the third-law method, is −(282.99±0.32) kJ mol −1 .