Bulk electrodes

Regarding the electrode/electrolyte interface, it is important to distinguish between two types of electrochemical systems thermodynamically closed (and in equilibrium) and open systems. While the former can be understood by knowing the equilibrium atomic structure of the interface and the electrochemical potentials of all components, open systems require more information, since the electrochemical potentials within the interface are not necessarily constant. Variations could be caused by electrocatalytic reactions locally changing the concentration of the various species. In this chapter, we will focus on the former situation, i.e., interfaces in equilibrium with a bulk electrode and a multicomponent bulk electrolyte, which are both influenced by temperature and pressures/activities, and constrained by a finite voltage between electrode and electrolyte. 

Now having specified the bulk electrode, the bulk electrolyte, and the interface between them, our aim in this section is to quantify the atomistic structure of the interface and derive an expression that allows us to evaluate its stabUity. Based on (5.5), we wUl extend the ab initio atomistic thermodynamics approach to electrochemical systems. 

For an electrode/electrolyte interface in equUibrium with the bulk electrode and the electrolyte reservoir, and constrained by the potential difference A, the most relevant structures are those with low interfacial free energies... 

Before we can apply the extended ab initio atomistic thermodynamics approach to the oxygen-covered surface or the surface/bulk oxide, we have to investigate the structure of the bulk electrode. 

While at low electrode potentials the bulk electrode will be purely platinum, at high positive electrode potentials the cyclic voltammogram shows that it becomes an... 

For the non-electrochemical system of a Pt surface in contact with an O2 atmosphere, the Pt cOj, bulk electrode is only stable if... 

In the Pt bulk oxide range of the phase diagram, which is relevant provided that (5.26) is fulfilled, the bulk electrode is Pt oxide and no longer pure Pt. Therefore the corresponding term in (5.28) that accounts for the bulk electrode reservoir now has to involve Since the bulk electrode should be in thermodynamic equihbrium with the surroundings. 

The bulk electrode has to have very good elastic properties, which can be created by proper optimization of conductive matrix. 

Electropolishing is well established as a simple, in situ method to separate porous silicon layers from the silicon electrode. By switching the anodic current density from values below JPS to a value above JPS, the PS film is separated at its interface to the bulk electrode. The flatness of a PS surface separated by electropolishing is sufficient for optical applications, as shown in Fig. 10.10. 

The evaluation of catalysts typically uses two techniques. The first is evaluation as a thin layer on a bulk electrode (e.g., glassy carbon) in dilute liquid electrolyte (e.g., H2 4) either as a static electrode or an RDE. In the study of oxygen reduction, there has been much discussion as to the most appropriate electrolyte to use. In general, dilute perchloric acid (HCIOJ is preferred because of its noncoordinating nature, it is thus closest to the environment foxmd within a FEM catalyst layer with perfluorosulfonic acid ionomer. A possible alternative is trifluoromethylsulfonic acid (CF3SO3H), which mimics perfluorosulfonic acids closely, but there are relatively few studies with this acid. Rotating... 

UPD process has also been studied on screen-printed silver electrodes using voltammetric techniques and scanning electron microscope analysis [293]. The relative occurrence of UPD and bulk Pb process has been dependent on the scan rate, with increasing role of UPD process in higher rates. Studies on Pb deposition on silver colloids have pointed to its similarity to bulk electrode [283]. 

Finally, one additional observation should be of interest to this group, namely the use of stable interference-colored membranes (ca. 2000 A thick) to enable thicknesses to be studied intermediate between that of the above bilayer (60 A) and Simon s bulk electrodes. This is illustrated in Fig. 3 where the steady-state conductance of stable interference-colored membranes made from GMO/hexadecane (bottom) could be compared with that in black bilayers of GMO/decane (filled circles). At the highest carrier concentration the interference-colored membrane was caused to go black with an applied voltage, giving the 500x increase in conductance indicated by the open circle, which is nicely in agreement with that of the GMO/decane bilayers. 

The special properties of OTEs that permit the use of transmission spectro-electrochemical techniques are often at cross purposes with the acquisition of reliable electrochemical data. The desire to have the superior electrical properties of bulk conducting materials, and thereby reliable electrochemical data, together with the power of a coupled optical probe led groups to develop various diffraction and reflection approaches to spectroelectrochemistry. Light diffracted by a laser beam passing parallel to a planar bulk electrode can be used to significantly increase the effective path length and sensitivity in spectroelectrochemistry [66]. 

Normal reflection optics have been used to advantage with bulk electrode materials. Examples of this type of spectroelectrochemical cell are shown in Figure 9.11 [67]. Simple bifurcated fiber-optic waveguides are used to direct source light onto reflective bulk electrode surfaces and to collect the reflected light for transmission to a detector. This is a simple means for performing spectroelectrochemical experiments at bulk metal electrodes that cannot be as... 

Electrodes or arrays of electrodes can be produced by these techniques with complex or fine patterns that would be very difficult to fabricate with conventional bulk electrode materials. Films can commonly be reproducibly formed in quite pure chemical form (with a caveat based upon interfering chemical effects that may arise from diffusion of adhesion-promoting intermediate layers through the thin film to the exposed surface). Film electrodes lend themselves to inexpensive fabrication of disposable electrodes, cells, or eventually complete integrated measurement devices in large quantities that would be economically infeasible with conventional materials, due to either the complexities of fabrication or the cost of materials. 

Heterogeneous semiconductor systems involve either suspensions or slurries of larger-sized semiconductor powders, or smaller colloids in solution. In principle, these semiconductor particles may act as tiny photoelectrolysis cells, similar to the photoelectrochemical systems discussed above. However, as many of the materials used for bulk electrodes are also described here in particulate form, both similar and new problems may arise, most notably irreproducibility in particle preparation, stability issues, and low C02 reduction rates. 

The use of films as electrodes makes possible numerous experiments that would be difficult or impractical to implement with the conventional bulk electrodes. Discussion here emphasises either thin (< 5 pm thick usually quite a bit thinner) or thick (> 5 pm usually quite a bit thicker) film electrode materials, consisting of a conductor, either a continuous or a spatially patterned film, most commonly deposited on a suitably prepared insulating substrate. Films consisting primarily of insulators are not considered here, except to the extent that they may be used to form patterned arrays or electrodes with special geometries. A view of applications and properties of film... 

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