Direct current measurements, solid electrolytes

It is conceivable that direct-current measurement, which is a perfectly valid method for solid electrolytes associated with non-polarizable electrodes, would only allow us to access qualitative information if the electrode is ideally or partially polarizable. 

The interface between two immiscible solutions (e.g. water and nitrobenzene) containing dissolved species is a site of an electric potential. By measuring this potential difference at the aqueous elec-trolyte/solid electrolyte phase boundary, the phenomena taking place at the interface between two immiscible solutions or the membranes of ion-selective electrode have been studied. Changing the composition of the solutions in contact can alter this potential or applied current can alter the composition of the solutions. Thus, judicious choice of applied potential or current can be used to study the structure of the interface. Since the interface is ul-trathin (< cl nm), it cannot be observed directly. It can be, however, investigated by electrochemical or optical methods [14,... 

With the advent of solid electrolytes, such as the stabilized forms of zirconia, the field of solid-state electrochemistry has grown. Galvanic cells utilizing this material as an electrolyte for anionic (0 ) conduction have been used in conjunction with the Nernst equation to measure within various ceramic systems (1) the Gibbs free energy of formation, (2) the activity of, and (3) the kinetics of solid-state reactions. Electrolytic cells can be used to drive reactions in the non-equilibrium direction by the application of an electrical current. The reader is again referred to Schmalzried. ... 

A very frequently used technique for the study of electrode reactions is measuring the impedance of an electrode at variable frequency. This technique can be applied to electrodes at equihbrium where the external ac current causes concentration changes of both components of the redox reaction in opposite directions. The ac current can also be superimposed upon a constant current, provided a steady state can be reached for this dc current. This requires the presence of convection in the transport process. Since in solid electrolytes convection is impossible, such cases will not be discussed here. 

Strictly speaking, /q is a measure of the electrocatalytic activity of the tpb for a given electrocatalytic reaction. It expresses the rates of the forward (anodic) and reverse (cathodic) directions of the electrocatalytic reaction under consideration, for example, Eq. (12), when there is no net current crossing the metal/solid electrolyte interface or, equivalently, the tpb. In this case, the rates of the forward and the reverse reaction are obviously equal. It has been recently shown that, in most cases, as one would intuitively expect, /q is proportional to the length / of the tpb. The measurement of /q, or /q, is based on the classical Butler-Vol-... 

In practice, for a ternary system, the decomposition voltage of the solid electrolyte may be readily measured with the help of a galvanic cell which makes use of the solid electrolyte under investigation and the adjacent equilibrium phase in the phase diagram as an electrode. A convenient technique is the formation of these phases electrochemically by decomposition of the electrolyte. The sample is polarized between a reversible electrode and an inert electrode such as Pt or Mo in the case of a lithium-ion conductor, in the same direction as in polarization experiments. The decomposition causes a sharp linear increase in the direct current. 

Fig. 4 Examples of current density-voltage curves for a solid oxide (see Sect. 8.1.3.2.2) fuel cell stack as a function of temperatures. These curves are measured directly. Parameter list interconnector CrFesY anode Ni cermet (Ni +ZrC>2) cathode lanthan strontium i manganite (LSM) electrolyte YSZ 800 (ZrC>2 — Y2O3) fuel H2 (1 bar) oxidant 02/air (1 bar) stacks of 80 cells in serial connection.

The Direct Methanol Fuel Cell, DMFC, (see Fig. 7-6 in section 7.2.2.4.) is another low temperature fuel cell enjoying a renaissance after significant improvements in current density. The DMFC runs on either liquid or, with better performance but higher system complexity, on gaseous methanol and is normally based on a solid polymer electrolyte (SPFC). R-Ru catalysts were found to produce best oxidation results at the anode, still the power density is relatively low [5, 29]. Conversion rates up to 34 % of the energy content into electricity were measured, an efficiency of 45 % is expected to be feasible in the future. SPFC in the power order of several kW to be used in automobile applications are currently in the development phase. 

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