Electrode side

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

The total charge of the compact and diffuse layers equals (and is opposite in sign to) the net charge on the electrode side. The potential-distance profile across the double-... 

The left-hand side of the cell, the working electrode, has its Pq2 fixed by the Ni + NiO equilibrium pressure, while on the right-hand side the reference electrode has Po2 given by the air atmosphere. Alternatively, a buffer may be used on the reference electrode side. The left- and right-hand side half-cell reactions are respectively... 

Fig. 5-19. Inteifadal charge at an electric double layer on metal electrodes (a) negative charge on the electrode side without contact ion adsoiption, G>) positive charge on the electrode side without contact ion adsorption, (c) positive charge on the electrode side with contact anion adsorption.

We shall see in the next chapter that a higher current usually requires a more extreme potential, V, and that a more extreme potential is likely to cause electrode side reactions such as solvent splitting. In short, forcing... 

Many analyses are performed with mesh electrodes, thereby increasing the surface area. However, there is an additional reason for using a clay-modified electrode. We saw above (in Worked Example 5.2) that electrode side reactions are a common enemy of accurate electroanalytical work. 

Polythiophene lends itself to the same routes to composites. A poly(3-methyl-thiophene)-poly(methylmethacrylate) composite has been made by electrochemical polymerization from a solution of thiophene and PMMA in methylene chloride and nitrobenzene. At high current densities the electrode side quickly became highly conducting while the outer side was less so 307). Similar composites have been prepared by chemical routes, using a Grignard reaction, firstly to couple the thiophene units in a step-reaction, then to initiate the polymerization of the methyl methacrylate 315). 

The porous platinum/Teflon electrodes separate the electrolytic cell from the gaseous reference chamber on one side and the sample chamber on the working electrode side. The applied voltage is controlled by a potentiostat. The sample enters into the electrolytic cell through the porous electrodes, the pore size of which also needs to be closely controlled in order to prevent their flooding with the solvent. An example of an electrochemical reaction of interest is oxidation of methane under conditions of humid air. 

Scheme 17.1. Schematic diagram of biosensor and wall-jet cell. (A) Screen-printed electrode front-view (1) silver ink acting as reference electrode, (2) graphite ink acting as working electrode successively modified with PB and (3) silver ink acting as counter electrode. (B) PB-modified screen-printed electrode side-view (1) polyester film as support for printing step, (2) graphite ink and (3) PB layer. (C) Wall-Jet flow cell side-view (1) inlet of the flow, (2) outlet, (3) cell made of Teflon and (4) glucose biosensor. (D) Wall-jet flow cell front-view (1) outlet, (2) inlet of the flow, (3) O-ring, (4) flow-cell and (5) glucose biosensor. Reprinted from Ref. [4] with permission from Elsevier.

In order for an emf to be completely defined, the phase rule must be obeyed for each electrode side. Cells involving ideal solid electrolytes (ti0B = 1) can be usually measured over a wider temperature range and are hence convenient means to deduce thermodynamic data such as formation enthalpies and formation entropies. The cell given in Eq. (82)... 

A capacitive current /DL, which is used to charge the double layer present at the solid/liquid interface [12], It corresponds to the charge of a capacitor (electrons in the electrode side, ions from the electrolytes on the electrolyte side) with a capacitance of about 15 pF/cm2. 

In order to reveal the influence of gas flow rate on the response behavior of the sensors, the response behavior has also been tested at two gas flow rates (40 or 80 seem) at the working electrode side. 

Note the analogy between Eqs. (4) and (5) electron transfer at the interface can be considered as a special case of intramolecular electron transfer, the integration taking into account the existence of a continuum of energy levels on the electrode side. Thus the experimental setup is somewhat intermediate between a molecular mixed-valence complex in solution and a raetal/molecule/metal nanojunction. 

Figure 3.53. IV-curve and power density for direct methanol fuel cell with electrodes of carbon black coated on a carbon paper substrate, with Pt-Ru (ratio 1 1) on the negative electrode side and Pt alone on the positive electrode side, both with Nafion intrusions and hot-pressed on a Nafion-112 membrane. The 2-mol methanol solution was fed at a rate of 21 ml min and at the other side non-humidified air at a rate of 700 ml/min. The temperature was 85°C. (From G. Lu and C. Wang (2004). Electrochemical and flow characterization of a direct methanol fuel cell. /. Power Sources, in press. Used with permission from Elsevier.)...

The crossover of methanol has caused problems in finding a suitable membrane material. On the positive electrode side, methanol combines with oxygen to form CO2. Among the alternatives to pure Nafion are Nafion filled with zirconium phosphate or grafted with styrene to inhibit methanol transport (Bauer and Willert-Porada, 2003 Sauk et al., 2004), as well as non-Nafion membrane materials such as sulfonated polyimide (Woo et al., 2003). None have achieved performance as good as the one shown in Fig. 3.53, which, however, has a substantial methanol crossover rate. 

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