Electrode processes, steps

The measurement of a from the experimental slope of the Tafel equation may help to decide between rate-determining steps in an electrode process. Thus in the reduction water to evolve H2 gas, if the slow step is the reaction of with the metal M to form surface hydrogen atoms, M—H, a is expected to be about If, on the other hand, the slow step is the surface combination of two hydrogen atoms to form H2, a second-order process, then a should be 2 (see Ref. 150). 

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

The electrode process at -500 mV on this potential scale is correlated to the growth of 250 20 pm high islands. They grow immediately upon a potential step from the open circuit potential to -500 mV (arrow in Figure 6.2-13). 

In contrast to the influence of velocity, whose primary effect is to increase the corrosion rates of electrode processes that are controlled by the diffusion of reactants, temperature changes have the greatest effect when the rate determining step is the activation process. In general, if diffusion rates are doubled for a certain increase in temperature, activation processes may be increased by 10-100 times, depending on the magnitude of the activation energy. 

Chronoamperometry is often used for measuring the diffusion coefficient of electroactive species or the surface area of the working electrode. Analytical applications of chronoamperometry (e.g., in-vivo bioanalysis) rely on pulsing of the potential of the working electrode repetitively at fixed tune intervals. Chronoamperometry can also be applied to the study of mechanisms of electrode processes. Particularly attractive for this task are reversal double-step chronoamperometric experiments (where the second step is used to probe the fate of a species generated in the first step). 

The rate of each of the steps in the overall electrode process has a simple dependence on the concentration of the electroactive species in the bulk of the solution, as in the following examples. 

It has been seen from the above simple examples that the concentration of the substrate has a profound effect on the rate of the electrode process. It must be remembered, however, that the process may show different reaction orders in the different potential regions of the i-E curve. Thus, electron transfer is commonly the slow step in the Tafel region and diffusion control in the plateau region and these processes may have different reaction orders. Even at one potential the reaction order may vary with the substrate concentration as, for example, in the case discussed above where the electrode reaction requires adsorption of the starting material. 

Although the effect of temperature on each of the steps in an overall electrode process is readily predictable, it is surprising to find in the literature very few systematic studies of this variable or attempts to use it to change the rate, products or selectivity of an organic electrosynthetic process. A recent paper has, however, discussed equipment and suitable solvents for low-temperature electrochemistry (Van Dyne and Reilley, 1972a). 

While it is widely realized that pressure is a useful variable for increasing the solubility of the electroactive species and hence the rate of the electrode process, it is mostly forgotten that it is also a variable which affects several of the steps in the overall process. In fact these more subtle effects of pressure on organic electrode reactions do not seem to have been investigated although it is possible to estimate their importance by considering the known effects of pressure on chemical systems (Hamann, 1957). 

Anodic stripping voltammetry (ASV) has been used extensively for the determination of heavy metals in samples of biological origin, such as lead in blood. ASV has the lowest detection limit of the commonly used electroanalytical techniques. Analyte concentrations as low as 10 M have been determined. Figure 16 illustrates ASV for the determination of Pb at a mercury electrode. The technique consists of two steps. The potential of the electrode is first held at a negative value for several minutes to concentrate some of the Pb " from the solution into the mercury electrode as Pb. The electrode process is... 

The TFTs are made on transparent glass substrates, onto which gate electrodes are patterned. Typically, the gate electrode is made of chromium. This substrate is introduced in a PECVD reactor, in which silane and ammonia are used for plasma deposition of SiN as the gate material. After subsequent deposition of the a-Si H active layer and the heavily doped n-type a-Si H for the contacts, the devices are taken out of the reactor. Cr contacts are evaporated on top of the structure. The transistor channel is then defined by etching away the top metal and n-type a-Si H. Special care must be taken in that the etchant used for the n-type a-Si H also etches the intrinsic a-Si H. Finally the top passivation SiN, is deposited in a separate run. This passivation layer is needed to protect the TFT during additional processing steps. 

The flow of electric current through the electrolytic cell is connected with chemical, electrochemical and physical processes which, as a whole, are termed the electrode process. The main electrochemical step in the electrode process is the actual exchange of charged species between the electrode and the electrolyte, which will be termed the electrode reaction (charge transfer reaction). Substances participating directly in the charge transfer reaction are termed electroactive. These substances can be either soluble or insoluble in the electrolyte or electrode material. Common basic types of electrode reactions are as follows ... 

Electrochemical kinetics is part of general chemical kinetics and has a similar purpose to determine the mechanism of the electrode process and quantitatively describe its time dependence. Mostly the study involves several stages. Firstly it is necessary to determine the reaction path, i.e. to determine the mechanism of the actual electrode reaction (for more detail, see Section 5.3.1), and the partial steps forming the overall electrode... 

Polarization is produced by the slow rate of at least one of the partial processes in the overall electrode process. If this rate-controlling step is a transport process, then concentration polarization is involved if it is the charge transfer reaction, then it is termed charge transfer polarization, etc. Electrode processes are often classified on this basis. 

If it is known which of the reactions determine the rate of the overall complex electrode process, then the concept of the stoichiometric number of the electrode process v is often introduced. This number is equal to the number of identical partial reactions required to realize the overall electrode process, as written in an equation of type (5.2.2).t If the rate constant of this partial rate-determining reaction is ka, then ka = /ca/v. Thus, for example, if the first of reactions (5.1.7) is the rate-determining step in the overall electrode process (5.1.4) then the stoichiometric number has the value v = 2. 

Case (a) If the chemical reaction preceding the electrode reaction, C(a), and the electrode reaction itself, E(a), are sufficiently fast compared to the transport processes, then both of these reactions can be considered as equilibrium processes and the overall electrode process is reversible (see page 290). If reaction C(a) is sufficiently fast and E(a) is slow, then C(a) affects the electrode reaction as an equilibrium process. If C(a) is slow, then it becomes the rate-controlling step (a detailed discussion is given in Section 5.6.3). 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

In order to understand the methodology in some detail, we first consider homogeneous processes, where the electrochemical techniques used are well-established. Such processes are not central to this book, which is primarily concerned with electrode processes, but they do serve to illustrate the manner in which mechanisms can be explored. As indicated above, any step in the electrochemical mechanism must be either chemical (denoted by C) or electro-chemical (denoted by E) in nature. It is not normally the case that more than one electron is transferred simultaneously, so possible sequences may be written down straightforwardly. 

To demonstrate an application of 3- and 4-a-Si H TFTs pixel electrode circuits in 100 and 200 dpi AM-PLEDs, we have fabricated a small size (0.5 x 0.5 in.2) engineering demonstration displays for both voltage- and current-driving pixel electrode methods, described above [24]. The processing steps of such units are described below. 

A novel microhotplate design was proposed to overcome the CMOS operating temperature limit and to avoid polysilicon-induced drift problems. A cross-sectional schematic of the device is shovm in Fig. 4.11. Instead of using a polysilicon resistor as temperature sensor, a platinum temperature sensor is patterned on the microhotplate. The Pt-metallization process step was used to simultaneously fabricate the electrodes and the temperature sensor. The CMOS-Al/Pt contacts are located off the membrane... 

In order to establish good electrical contact to the sensitive layer, it was necessary to coat the electrodes with a metal stack of Ti/W (diffusion barrier and adhesion layer) and Pt. The usage of a shadow mask during the metal deposition ensures full compatibility with other MEMS processing steps so that it is possible to fabricate various CMOS-MEMS devices on the same wafer. 

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