Sensing electrode double layers

The double-layer capacitance is composed of several contributions. In a geometrical sense the double layer in "supported" systems is represented by the compact "Helmholtz" or "Stem" layer. The electrostatically attracted solvated species reside in the "outer Helmholtz plane" (OHP), and specifically adsorbed species reside closer to the electrode in the "inner Helmholtz plane" (IHP). The double-layer structure is completed by a "diffuse" layer, composed of electrostatically attracted species at some distance from the electrode surface. The fuU thickness of the double layer can be defined as the external boundary of the diffuse layer separating it from the bulk solution, where the measured potential becomes equal to that of the bulk solution and no local potential gradient driven by the difference between the electrode potential ( )j and the solution potential can be determined (Figure 5-4). 

In a broad sense similar effective double layers can be formed via gaseous adsorption or evaporation (e.g. Na evaporated on Pt electrodes deposited on fT-A Ch has been shown to behave similarly to electrochemi-cally supplied Na). In other cases, such as the effective double layer formed upon anodic polarization of Pt deposited on YSZ, the electrochemically created effective double layer appears to be unique and cannot be formed via gaseous oxygen adsorption at least under realistic (<300 bar) oxygen pressure conditions. 

The history of the observation of anomalous voltammetry is reviewed and an experimental consensus on the relation between the anomalous behavior and the conditions of measurement (e.g., surface preparation, electrolyte composition) is presented. The behavior is anomalous in the sense that features appear in the voltammetry of well-ordered Pt(lll) surfaces that had never before been observed on any other type of Ft surface, and these features are not easily understood in terms of current theory of electrode processes. A number of possible interpretations for the anomalous features are discussed. A new model for the processes is presented which is based on the observation of long-period icelike structures in the low temperature states of water on metals, including Pt(lll). It is shown that this model can account for the extreme structure sensitivity of the anomalous behavior, and shows that the most probable explanation of the anomalous behavior is based on capacitive processes involving ordered phases in the double-layer, i.e., no new chemistry is required. 

If, on the other hand, the initial rate of deposition is higher than the initial rate of dissolution, the electrical double layer will be formed just in the opposite sense, and as a result the metal becomes positive with respect to the solution. This is the case with the copper electrode in the Daniell cell. 

The solvent also acts as a dielectric medium, which determines the field diji/dx and the energy of Interaction between charges. Now, the dielectric constant e depends on the inherent properties of the molecules (mainly their permanent dipole moment and polarizability) and on the structure of the solvent as a whole. Water is unique in this sense. It is highly associated in the liquid phase and so has a dielectric constant of 78 (at 25 C), which is much higher than that expected from the properties of the individual molecules. When it is adsorbed on the surface of an electrode, inside the compact double layer, the structure of bulk water is destroyed and the molecules are essentially immobilized... 

FIGURE 2.4 Schematic diagram of the electrical double layer. (Reprinted from Zhuiykov, S., Mathematical modelling of YSZ-based potentiometric gas sensors with oxide sensing electrodes part II Complete and numerical models for analysis of sensor characteristics. Sensors and Actuators B, Chem. 120 (2007) 645-656, with permission from Elsevier Science.)... 

The pxjtential measured between WE and RE may be substantially influenced by em IR drop, in particular if high current densities are applied or an electrolyte of low conductivity is used. This is due to the fact that the reference electrode is connected with a point in the solution some distance away from the electrochemical double layer. Thus an ohmic resistance - the so-called uncompensated resistance (R j) - is included in the solution between the tip of the reference electrode and the surface of the working electrode (Fig. 6.1). As a result an error will be introduced in the measurement of the potential difference in such a way that the potential difference between the working and reference electrodes is not as large in an absolute sense as indicated by the potentiostat or an auxiliary voltmeter. 

In electrochemistry an electrode is an electronic conductor in contact with an ionic conductor. The electronic conductor can be a metal, or a semiconductor, or a mixed electronic and ionic conductor. The ionic conductor is usually an electrolyte solution however, solid electrolytes and ionic melts can be used as well. The term electrode is also used in a technical sense, meaning the electronic conductor only. If not specified otherwise, this meaning of the term electrode is the subject of the present chapter. In the simplest case the electrode is a metallic conductor immersed in an electrolyte solution. At the surface of the electrode, dissolved electroactive ions change their charges by exchanging one or more electrons with the conductor. In this electrochemical reaction both the reduced and oxidized ions remain in solution, while the conductor is chemically inert and serves only as a source and sink of electrons. The technical term electrode usually also includes all mechanical parts supporting the conductor (e.g., a rotating disk electrode or a static mercury drop electrode). Furthermore, it includes all chemical and physical modifications of the conductor, or its surface (e.g., a mercury film electrode, an enzyme electrode, and a carbon paste electrode). However, this term does not cover the electrolyte solution and the ionic part of a double layer at the electrode/solution interface. Ion-selective electrodes, which are used in potentiometry, will not be considered in this chapter. Theoretical and practical aspects of electrodes are covered in various books and reviews [1-9]. 

Finally, to conclude this introduction, and to avoid any possible confusion in the terminology, we wish to define briefly what is an ultramicroelectrode (at least in our sense ). When their interfacial properties are to be considered identical with those of any other electrode of a larger dimension, ultramicroelectrodes must remain much larger than the double layer thickness. This sets a lower dimension of a few tens of A for ultramicroelectrodes.On the other hand, if diffusional steady state voltammetry has to be observed without significant interference of convection, they must be smaller than convective layers, which sets an upper limit of a few tens of /im. Between these limits, all ultramicroelectrodes possess identical intrinsic physico-chemical properties. However, their behavior (viz. ohmic drop, steady state or transient currents, etc) obviously depends on the medium and the time-scale considered. ... 

These electrochemical double layer phenomena are equivalent to a genuine capacitor in the electrostatic sense of the term. We shall model them by way of a single equivalent capacitor (denoted as Cdi), because we shall not separate out the contribution made by each electrode. 

If EIS measurements were carried out with a constant electrolyte composition, Qj would be mainly dependent on the electrode area. So, any electrode modifier of insulating features decreases the double-layer capacitance as compared to the pure metal electrode. Thus, the double-layer capacitance usually arises from the series combination of several elements, such as analyte binding (Qnai) to a sensing layer (Qens) on an Au electrode (Cau)-... 

Impedance sensing is most useful for large species that significantly perturb the sensing interface. Generally, decreases in the double-layer capacitance and increases in charge-transfer resistance consistent with the addition of insulating layers of proteins on the electrode surface were observed. Many of the examples of impedance sensors that we will discuss later in this chapter monitor R as a measure of analyte concentration. 

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