Electrochemical half-cells evaluation

Electrochemical Half-Cells for Evaluating PEM Fuel Cell Catalysts and Catalyst Layers... 

It must be realized that because of kinetic limitations, most half-cells that can be written cannot be the basis of a practical cell which will display the appropriate emf. It has however proved convenient to include such halfequations in tables of redox potentials if their emf could be evaluated in some other way. In a large number of cases electrochemical data are not used at all. Rather, partial molar heats and entropies of the species involved are determined by calorimetric methods and these are used to derive AG°for the cell reactions. ceii values can then be calculated. 

Two types of EIS diagnosis exist for PEM fuel cells material/component evaluation before assembly into a fuel cell and material/component evaluation after assembly into a fuel cell. Normally, the former is evaluated using a half-cell (or an electrochemical cell), while the latter is evaluated in a fuel cell or stack. We define the former as an ex situ diagnosis, and the latter as an in situ diagnosis. In the following sections, we will describe them separately. 

Rapid screening of the electrochemical performance. DuPont has developed rapid half-cell screening techniques to evaluate catalyst activity for methanol oxidation, CO tolerance and oxygen reduction. These measurements are performed in liquid sulfuric acid electrolytes, and up to 32 electrodes can be screened in parallel using a variety of electrochemical techniques in a 3-electrode... 

Corrosion potential. Any embedded reference electrode allows the electrochemical potential of the adjacent rebars to be measured. This allows depassivation of the rebars or of any other steel sensor element put at different depths to be detected by a drop in half-cell potential. The corrosion potential will be influenced by concrete humidity and oxygen content (Chapter 7). The depassivation of the steel probe located in the outermost cover concrete will present an early warning and suitable in-depth distribution of a set of steel probes allows the corrosion risk to be evaluated or the time of depassivation of the rebars to be calculated. 

The activity and performance of fuel cell electrocatalysts need to be evaluated in terms of electrochemieal parameters, including current density and electrode potential. Electrochemieal sereening methods have been identified as ideal direct approaches for combinatorial studies of fuel cell catalysts. Two types of electrochemical measurement systems have been developed for combinatorial screening of fuel cell catalysts the array half-cell system and the array single-cell system. 

Since the final and initial states are identical in both experiences, the free enthalpy change AGgyst is the same in the two processes. This result is general and applies to all redox reactions. A galvanic cell discharge is equivalent to a chemical process that can be decomposed into two electrochemical half-reactions that take place simultaneously but separately, the thermodynamic evaluations of both processes being identical. 

RDF and RRDE techniques have been widely used for studying the kinetics of the ORR [1-10] and HOR [11-17] in PEM fuel cells. Particularly in ex situ evaluations of catalysts and catalyst layers, the conventional half-cell is used to measure the electrochemical Pt surface area (EPSA), ORR mass activity, catalyst stability, as well as non-noble metal catalyst activity and stability, as described in later sections of this chapter. 

Tables of this sort are extremely useful, because they feature much chemical and electrical information condensed into quite a small space. A few electrode potentials can characterize quite a number of cells and reactions. Since the potentials are really indices of free energies, they are also ready means for evaluating equilibrium constants, complex-ation constants, and solubility products. Also, they can be taken in linear combinations to supply electrochemical information about additional half-reactions. One can tell from a glance at an ordered list of potentials whether or not a given redox process will proceed spontaneously.

Kinetic Currents. In kinetic currents, the limiting current is determined by the rate of a chemical reaction in the vicinity of the electrode, provided this precedes the cell reaction. Electrochem-ically inactive compounds are converted into reducible or oxidizable forms (time-dependent protonation and deprotonation processes, formation and decomposition of complexes, etc.). Conversely, during a chemical reaction after the cell reaction, the product of the electrode reaction is converted to an electrochemically inactive form without influence on the current. However, owing to the changed equilibria between the concentrations of the oxidized and reduced forms at the electrode surface, the half-wave and peak potentials are shifted (Section 25.2.1). In evaluating kinetic effects, cyclic voltammetry can be helpful (Section 25.2.4). 

---