Transport measurements, solids polarization

Imanaka et al. (1995) demonstrated direct evidence for trivalent-ion conduction in Sc2(W04)3. They purposely chose this material due to its large tunnel size of 2.2 A, relative to the ionic size of 1.8 A for Sc ". By combining results from polarization experiments (both dc and ac) with emf measurements the authors provided confirmation for trivalent-Sc-ion conduction in a solid oxide material. At 823 K, the conductivity value was greater than 10 S/cm, and the transport munber for Sc + was estimated to be 0.92. The same laboratory reported that Y2(W04)3 also exhibits trivalent ion conduction for Y " ions (Adachi and Imanaka 1996). 

In corrosion, phenomena other than mass transport in the electrolyte can slow down the establishment of steady state conditions, including adsorption, precipitation or film growth. Especially, solid state transport processes in passive oxide films are generally slow (Chap. 6) and as a consequence the measured current density will depend on the sweep rate, even if from a solution mass transport point of view steady state prevails (t 1). Polarization curves measured under these conditions are sometimes called pseudo-steady-state polarization curves. When reporting such data one should always indicate the sweep rate used. 

The purpose of this tutorial paper is to review the theory, the method of measurement and treatment of the data for experiments on polarized solid electrolytes according to the theory of C. Wagner and the developments of H. Rickert, K. Weiss, D. 0. Raleigh, A. Joshi and others. Methods to obtain the total and electronic conductivities, transport number, number and mobility of electronic charge carriers and the double layer capacitance at the electrolyte-electrode interface are to be discussed. Examples will be chosen which exhibit good agreement with theory— primarily the silver and copper halides. 

FI G U RE 4.21 Exact numerical (points) and analytical (Equation 4.189) (solid lines) polarization curves of the cathode catalyst layer with the finite rate of oxygen transport. The indicated parameter for the curves is the ratio D/Dref, where Dref = 1.37 10 cm s is the CCL oxygen diffusivity measured in Shen et al. (2011). The bottom solid line is the curve for infinitely fast oxygen transport in the CCL (Equation 4.141). 

A deeper insight into electrochemical reaction mechanisms is possible by electrochemical studies employing solid electrolyte instead of liquid electrolyte With a solid electrolyte having preponderantly only one mobile ionic species electrode polarization can be studied under thermodynamically well-defined conditions without superimposed side effects by solvents and without the complications created by the presence of hydrated films or hydrolytic layers. Such measurements can be used, for instance, for the study of electrodeposition, formation of monolayers or of dendrites due to nucleation, for the study of polarization phenomena in ionic solids, solid-state reaction kinetics, transport phenomena, thermodynamics or constitutional diagrams, and for the development of practical devices. 

A modified version of this experiment is to measure the sum of the oxidation and reduction potential of the charge-transporting molecule in solution by cyclovoltammetry. Although reliable as far as relative level positions is concerned, it is likely to underestimate because the stabilization energy of radical cations and anions in a polar solvent, such as acetonitrile, may be up to a few tenths of an electronvolt larger than in a molecular solid in which only electronic polarization is important. 

---