Interface electrocatalytic

Figure 4.7. Schematic representation of the location of electrocatalytically and catalytically active sites in a section perpendicular to the catalyst film-solid electrolyte interface.

The reference electrode-solid electrolyte interface must also be non-polarizable, so that rapid equilibration is established for the electrocatalytic charge-transfer reaction. Thus it is generally advisable to sinter the counter and reference electrodes at a temperature which is lower than that used for the catalyst film. Porous Pt and Ag films exposed to ambient air have been employed in most previous NEMCA studies.1,19... 

Again the extent to which such parallel reactions contribute to the measured current is not very easy to quantify. However, fortunately, such a quantification is not necessary for the description of NEMCA. What is needed is only a measure of the overall electrocatalytic activity of the metal-solid electrolyte interface or, equivalently, of the tpb, and this can be obtained by determining the value of a single electrochemical parameter, the exchange current I0, which is related to the exchange current density i0 via ... 

We start by considering a schematic representation of a porous metal film deposited on a solid electrolyte, e.g., on Y203-stabilized-Zr02 (Fig. 5.17). The catalyst surface is divided in two distinct parts One part, with a surface area AE is in contact with the electrolyte. The other with a surface area Aq is not in contact with the electrolyte. It constitutes the gas-exposed, i.e., catalytically active film surface area. Catalytic reactions take place on this surface only. In the subsequent discussion we will use the subscripts E (for electrolyte) and G (for gas), respectively, to denote these two distinct parts of the catalyst film surface. Regions E and G are separated by the three-phase-boundaries (tpb) where electrocatalytic reactions take place. Since, as previously discussed, electrocatalytic reactions can also take place to, usually,a minor extent on region E, one may consider the tpb to be part of region E as well. It will become apparent below that the essence of NEMCA is the following One uses electrochemistry (i.e. a slow electrocatalytic reaction) to alter the electronic properties of the metal-solid electrolyte interface E. 

In this section we treat some electrochemical reactions at interfaces with solid electrolytes that have been chosen for both their technological relevance and their scientific relevance. The understanding of the pecularities of these reactions is needed for the technological development of fuel cells and other devices. Investigation of hydrogen or oxygen evolution reactions in some systems is very important to understand deeply complex electrocatalytic reactions, on the one hand, and to develop promising electrocatalysts, on the other. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

Figure 4.1 Schematic of the atomic structure of the active three-phase interface between the metal particle that catalyzes the reaction, the carbon support necessary to conduct electrons, and the polymer electrolyte and solution necessary to conduct protons for electrocatalytic systems.

In this chapter, we will focus on some of the recent developments in understanding the influence of solution and electrochemical conditions over model single-crystal surfaces. Specifically, we will review work applying electronic structure methods to probe electrocatalytic mechanisms occurring at this complex interface. 

ELECTRONIC STRUCTURE METHODS AND MODELS FOR THE ELECTROCATALYTIC INTERFACE... 

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. 

The evolving structural characteristics of CLs are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2), and water as well as for the distribution of electrocatalytic activity at Pt-water interfaces. In principle, the mesoscale simulations allow relating these properties to the choices of solvent, ionomer, carbon particles (sizes and wettability), catalyst loading, and hydration level. Explicit experimental data with which these results could be compared are still lacking. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

The last part of the polarization curve is dominated by mass-transfer limitations (i.e., concentration overpotential). These limitations arise from conditions wherein the necessary reactants (products) cannot reach (leave) the electrocatalytic site. Thus, for fuel cells, these limitations arise either from diffusive resistances that do not allow hydrogen and oxygen to reach the sites or from conductive resistances that do not allow protons or electrons to reach or leave the sites. For general models, a limiting current density can be used to describe the mass-transport limitations. For this review, the limiting current density is defined as the current density at which a reactant concentration becomes zero at the diffusion medium/catalyst layer interface. 

Electrochemical Reaction/Transport. Electrochemical reactions occur at the electrode/electrolyte interface when gas is brought to the electrode surface using a small pump. Gas diffuses through the electrode structure to the electrode/electrolyte interface, where it is electrochemically reacted. Some parasitic chemical reactions can also occur on the electrocatalytic surface between the reactant gas and air. To achieve maximum response and reproducibility, the chemical combination must be minimized and controlled by proper selection of catalyst sensor potential and cell configuration. For CO, water is required to complete the anodic reaction at the sensing electrode according to the following reaction ... 

In this chapter, we will first discuss thermodynamic and kinetic concepts of electrified interfaces and point out some distinct features of electrochemical reaction processes. Subsequently, we will relate these concepts to chemical bonding of adsorbates on electrode surfaces. Finally, a discussion of the surface electrocatalytic mechanism of some important technological electrochemical reactions will highlight the importance of understanding chemical bonding at electrified surfaces. 

In this chapter, we have described modern electrochemical surface science and catalysis and shown fundamental correlations between the structure, bonding, composition and electrocatalytic reactivity at the interface. 

From a description of the geometric structure of electrified interfaces we moved to a description of models for electrochemical electron transfer across an electrode interface. The science of atomic scale electrochemistry was presented with an emphasis on the bonding of water molecules and anions on electrode surfaces. Subsequently, we presented an in-depth description of the role of surface bonding in a number of important electrocatalytic processes for energy conversion. We have attempted to illustrate how closely surface bonding and catalytic activity are related. 

In the future, we need to develop ways to apply novel and more sophisticated surface science techniques, such as the soft X-ray spectroscopies outlined in Chapter 2, to electrified interfaces under realistic conditions. For example, atom-specific probing of the occupied and unoccupied electronic states of electrochemical surface intermediates during the electrocatalytic reactions described in the present chapter has not been achieved to date. Modern synchrotron-based soft X-ray methods will aid these efforts to obtain more insight into the bonding of electrochemical intermediates. 

The electrochemical behavior of molecules attached to conducting surfaces (i.e., electrode surfaces) forming electro-active monolayers is discussed in the following sections. This situation has frequently been called modified electrodes in the literature [39]. The electro-active character of these monolayers arises from the presence of redox molecules on them which are susceptible to transfer to or receive charge from the supporting electrode as well as from species in solution (in this last situation, the attached molecules act as redox mediators between the electrode and the solution and are responsible for the appearance of electrocatalytic processes [39, 40]). Multipulse and Sweep Electrochemical techniques like SCV and CV have proven to be very necessary tools for understanding the behavior of these interfaces and the processes taking place at them. 

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