Metal-solution interfaces that approach electrodes

More recently, the curvature at air/solution interfaces has been accounted for by Nikitas and Pappa-Louisi98 in terms of a specific molecular model that predicts a linear dependence of (lM/ ) on (1/0). The same model also reproduces the behavior at metal/solution interfaces, specifically Hg electrodes, for which most of the experimental data exist. Nikitas treatment provides a method for an unambiguous extrapolation of the adsorption potential shift to 0= 1. However, the interpretation of the results is subject to the difficulties outlined above. Nikitas approach does provide... 

Infrared spectroscopy is frequently applied to investigate CO adsorption on electrodes, because CO is important as an intermediate and surface poison in many electrocatalytic reactions and the C-O stretching vibrational modes of the adlayer are sensitive to the chemical environment at the metal/solution interface. Infrared spectra of CO adsorbed on low-index surface planes of Pt single-crystal electrodes have become a benchmark for use in understanding the behavior of CO on other surfaces. Related approaches have been extended to bulk single-crystal metal electrodes that include Pd [66, 67], Ir [68-71], Rh [13, 70], Ru [72-74], Ni [75, 76] and Au [77]. 

Alternatively, the second barrel can be filled with the electrolyte and used in the same manner as in an ion conductance microscope [50]. It is possible to relate the solution conductance between tip and counter electrode to the normalized tip-to-substrate distance L. If the conductance is measured by a dc technique and the surface is impermeable to ions, a negative feedback effect (decrease of conductance as the tip approaches the surface) is observed. If an ac technique is used and the substrate is metallic, a positive feedback effect can be observed irrespective of whether the substrate is biased or not because the lowest impedance pathway for the current is through the metal via the metal-solution interface. In fact, after normalizing the conductance G(L) by the value with the tip far from the surface G , the distance dependence of the conductance is identical to that for faradaic currents in feedback SECM with a redox mediator ... 

In work by Bockris and his co-workers [1, 10, 119-123, 137, 138] the dependence of reversible adsorption of organic substances on the potential at platinum metals was interpreted on the basis of the assumption that there is competition between the organic molecules and the water molecules for sites on the surface and that the standard free energy of adsorption of water depends on the electric field at the electrode — solution interface. This approach has been criticized [9,139] from the point of view of the analysis made... 

Another progress in our understanding of the ideally polarizable electrode came from theoretical works showing that the metal side of the interface cannot be considered just as an ideal charged plane. A simple quantum-mechanical approach shows that the distribution of the electron gas depends both on the charge of the electrode and on the metal-solution coupling [12,13]. 

Ideal polarizable interfaces are critical for the interpretation of electrochemical kinetic data. Ideality has been approached for certain metal electrode-solution interfaces, such as mercury-water, allowing for the collection of data that can be subjected to rigorous theoretical analysis. 

The contribution of the metal to the double layer was discussed in Sections 6.6.7 to 6.6.9. However, we have said little about the ions in solution adsorbed on the electrode and how they affect the properties of the double layer. For example, when presenting the Stem model of the double layer (Section 6.6.6), we talked about ions sticking to the electrode. How does an interface look with ions stuck on the metal What is the distance of closest approach Are hydrated ions held on a hydrated electrode i.e., is an electrode covered with a sheet of water molecules Or are ions stripped of their solvent sheaths and in intimate contact with a bare electrode What are the forces that influence the sticking of ions to electrodes ... 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105],... 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

The use of galvanostatic transients enabled the measurement of the poten-tiodynamic behavior of Li electrodes in a nearly steady state condition of the Li/film/solution system [21,81], It appeared that Li electrodes behave potentio-dynamically, as predicted by Eqs. (5)—(12), Section III.C a linear, Tafel-like, log i versus T dependence was observed [Eq. (8)], and the Tafel slope [Eq. (10)] could be correlated to the thickness of the surface films (calculated from the overall surface film capacitance [21,81]). From measurements at low overpotentials, /o, and thus the average surface film resistivity, could be measured according to Eq. (11), Section m.C [21,81], Another useful approach is the fast measurement of open circuit potentials of Li electrodes prepared fresh in solution versus a normal Li/Li+ reference electrode [90,91,235], While lithium reference electrodes are usually denoted as Li/Li+, the potential of these electrodes at steady state depends on the metal/film and film/solution interfaces, as well as on the Li+ concentration in both film and solution phases [236], However, since Li electrodes in many solutions reach a steady state stability, their potential may be regarded as quite stable within reasonable time tables (hours —> days, depending on the system s surface chemistry and related aging processes). 

For studies involving air/water interface monolayers or Langmuir-Blodgett techniques, a submarine UME is required. Such an electrode is immersed in the solution and approached to the layer from below (110, 160). The submarine electrode consists simply of a conventional UME of the desired metal and size that is fixed to a glass J tube by Teflon tape or Epoxy (Figure 12.34). 

From the experimental results and theoretical approaches we learn that even the simplest interface investigated in electrochemistry is still a very complicated system. To describe the structure of this interface we have to tackle several difficulties. It is a many-component system. Between the components there are different kinds of interactions. Some of them have a long range while others are short ranged but very strong. In addition, if the solution side can be treated by using classical statistical mechanics the description of the metal side requires the use of quantum methods. The main feature of the experimental quantities, e.g., differential capacitance, is their nonlinear dependence on the polarization of the electrode. There are such sophisticated phenomena as ionic solvation and electrostriction invoked in the attempts of interpretation of this nonlinear behavior [2]. 

When a metal electrode is placed in an electrolyte solution, an equilibrium difference usually becomes established between the metal and solution. Equilibrium is reached when the electrons left in the metal contribute to the formation of a layer of ions whose charge is equal and opposite to that of the cations in solution at the interface. The positive charges of cations in the solution and the negative charges of electrons in the metal electrode form the electrical double layer [4]. The solution side of the double layer is made up of several layers as shown in Fig. 2.7. The inner layer, which is closest to the electrode, consists of solvent and other ions, which are called specifically adsorbed ions. This inner layer is called the compact Helmholtz layer, and the locus of the electrical centers of this inner layer is called the inner Helmholtz plane, which is at a distance of di from the metal electrode surface. The solvated ion can approach the electrode only to a distance d2. The locus of the centers of the nearest solvated ion is called the outer Helmholtz plane. The interaction of the solvated ion with metal electrode only involves electrostatic force and is independent of the chemical properties of the ions. These ions are called non-specifically adsorbed ions. These ions are distributed in the 3D region called diffusion layer whose thickness depends on the ionic concentration in the electrolyte. The structure of the double layer affects the rate of electrode reactions. 

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