Interphase electrode-solution structure

Vibrational spectroscopy techniques are quite suitable for in situ characterization of catalysts. Especially infrared spectroscopy has been used extensively for characterization of the electrode/solution interphases, adsorbed species and their dependence on the electrode potential.33,34 Raman spectroscopy has been used to a lesser extent in characterizing non-precious metal ORR catalysts, most of the studies being related to characterization of the carbon structures.35 A review of the challenges and applications associated with in situ Raman Spectroscopy at metal electrodes has been provided by Pettinger.36... 

Electrodic reactions that underlie the processes of metal deposition, etc., cannot be understood without knowing the potential difference at the electrode/solution interface and how it varies with distance from the electrode. The ions from the solution must be electrically energized to cross the interphase region and deposit on the metal. This electrical energy must be picked up from the field at the interface, which itself depends upon the double-layer structure. Thus, control over metal deposition processes can be improved by an increased understanding of double layers at metal/solutioii interfaces. 

When solute molecules and ions are introduced into the polar liquid, a more complicated situation is encountered. The solute molecules will be either attracted or repulsed when reaching the interphase. When the solute consists of large molecules or ions, they will also be polarized in the electric field of the interphase. Thus the structure of the solution will be modified in the interphase region and some species may become adsorbed on the electrode as a result of this modification. Can this adsorption be treated in the same way as adsorption of gases on metals There is one important difference between gas adsorption and solute adsorption on metals— in the gas-metal case the reaction is... 

First let us turn our attention to the metal surface and understand the simplifications which we usually make when considering it as part of the electrode-solution interphase then we shall present a model for the structure of the solution at the interphase and discuss the expected behaviour of this model when the potential of the electrode varies. We shall continue to discuss the influence that the interphase may have on electrode reactions and conclude this section by enumerating the different variables which are important in the study of the interphase. 

This chapter dealt with the structure and properties of the electrode solution interphase. It showed the difficulties encountered when trying to understand the interphase and the behaviour of adsorbed species on it. The... 

Techniques are described which obtain the IR absorption spectra of species, either adsorbed or free In the electrode/electrolyte solution Interphase. Applications slanted towards topics relevant to electrocatalytic processes are discussed to Illustrate the capabilities of the methods In probing molecular structure, orientation and Interactions. 

In principle, therefore, these valuable techniques can provide all of the information needed to specify the molecular structure of the electrode/electrolyte solution interphase, the dynamics of adsorption/... 

The interphase between an electrode and an electrolyte solution has a very complex electrical structure (Section 10.1). In this interphase various adsorption processes take place ... 

Double layer emersion continues to allow new ways of studying the electrochemical interphase. In some cases at least, the outer potential of the emersed electrode is nearly equal to the inner potential of the electrolyte. There is an intimate relation between the work function of emersed electrodes and absolute half-cell potentials. Emersion into UHV offers special insight into the emersion process and into double layer structure, partly because absolute work functions can be determined and are found to track the emersion potential with at most a constant shift. The data clearly call for answers to questions involving the most basic aspects of double layer theory, such as the role water plays in the structure and the change in of the electrode surface as the electrode goes frcm vacuum or air to solution. 

The surface tension was stated (Section 6.4.5), on general grounds, to be related to the surface excess of species in the interphase. The surface excess in turn represents in some way the structure of the interface. It follows therefore that electrocapillaiy curves must contain many interesting messages about the double layer at the electrode/ electrolyte interface. To understand such messages, one must learn to decode the electrocapillary data. It is necessary to derive quantitative relations among surface tension, excess charge on the metal, cell potential, surface excess, and solution composition. 

The simplest model of the structure of the metal-solution interphase is the Helmholtz compact double-layer model (1879). According to this model, all the excess charge on the solution side of the interphase, qs. is lined up in the same plane at a fixed distance away from the electrode, the Helmholtz plane (Fig. 4.4). This fixed distance xH is determined by the hydration sphere of the ions. It is defined as the plane of the centers of the hydrated ions. All excess charge on the metal, qM, is located at the metal surface. 

Aside from the selectivity criterion that is essential to all ion specific electrodes, the principal objective of applied design is to physically and chemically control the phase and interphase boundaries across the multiple layers that comprise the electrode structure. The conduction path of electrical charge across all the phases including the solid conductors and external measurement circuitry, as well as the chemical charge across polymeric and solution phases, may be represented by the schematic illustrated in Figure 4. 

One such properly is the capacitance, which is observed whenever a metal-solution interphase is formed. This capacitance, called the double layer capacitance, is a result of the charge separation in the interphase. Since the interphase does not extend more than about 10 nm in a direction perpendicular to the surface (and in concentrated solutions it is limited to 1.0 nm or less), the observed capacitance depends on the structure of this very thin region, called the double layer. If the surface is rough, the double layer will follow its curvature down to atomic dimensions, and the capacitance measured under suitably chosen conditions is proportional to the real surface area of the electrode. 

An understanding of the properties of liquids and solutions at interfaces is very important for many practical reasons. Some reactions only take place at an interface, for example, at membranes, and at the electrodes of an electrochemical cell. The structural description of these systems at a molecular level can be used to control reactions at interfaces. This subject entails the important field of heterogeneous catalysis. In the discussion which follows in this chapter the terms surface and interface are used interchangeably. There is a tendency to use the term surface more often when one phase is in contact with a gas, for example, in the case of solid I gas and liquid gas systems. On the other hand, the term interface is used more often when condensed phases are involved, for example, for liquid liquid and solid liquid systems. The term interphase is used to describe the region near the interface where the structure and composition of the two phases can be different from that in the bulk. The thickness of the interphase is generally not known without microscopic information but it certainly extends distances corresponding to a few molecular diameters into each phase. 

The surface films discussed in this section reach a steady state when they are thick enough to stop electron transport. Hence, as the surface films become electrically insulating, the active electrodes reach passivation. In the case of monovalent ions such as lithium, the surface films formed in Li salt solutions (or on Li metal) can conduct Li-ions, and hence, behave in general as a solid electrolyte interphase (the SEI model ). See the basic equations 1-7 related to ion transport through surface films in section la above. The potentiodynamics of SEI electrodes such as Li or Li-C may be characterized by a Tafel-like behavior at a high electrical field and by an Ohmic behavior at the low electrical field. The non-uniform structure of the surface films leads to a non-uniform current distribution, and thereby, Li dissolution from Li electrodes may be characterized by cracks, and Li deposition may be dendritic. The morphology of these processes, directed by the surface films, is dealt with later in this chapter. When bivalent active metals are involved, their surface films cannot conduct the bivalent ions. Thereby, Mg or Ca deposition is impossible in most of the commonly used polar aprotic electrolyte solutions. Mg or Ca dissolution occurs at very high over potentials in which the surface films are broken. Hence, dissolution of multivalent active metals occurs via a breakdown and repair of the surface films. 

As was illustrated schematically in Fig. 1, the electrical situation at or near an electrode interface in solution is very complicated on a microscopic scale, as is also the metal surface itself when atomic or nuclear discreteness on the scale of 0.1 - 0.2 nm is taken into consideration in a realistic view of the structure of the interphase. 

An electrode reaction takes place between the electrode and the solution, (which may contain some neutral chemicals and electrolytes). Since the electrode surface is the only place where the solution and the electrode meet, it must be a surface reaction. Therefore it is very important to be familiar with the structure of the interphase, i.e. the region which includes the surface of the electrode and that part of the solution which is influenced by it. Unfortunately, our knowledge of the surface is far from satisfactory. The classical approach will be presented in this book. 

Thus as a rule one may think of the potential at the o.h.p. in almost all electrolyte solutions of this and higher concentrations as negative when considering the effect of the interphase structure on electrode processes. 

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