Double layer region

For (ideally) polarizable metals with a sufficiently broad double-layer region, such as Hg, Ag, Au, Bi, Sn, Pb, Cd, H, and others, Ea=to can be obtained from measurements of the double-layer capacitance in dilute... 

The value obtained by Hamm et alm directly by the immersion method is strikingly different and much more positive than others reported. It is in the right direction with respect to a polycrystalline surface, even though it is an extrapolated value that does not correspond to an existing surface state. In other words, the pzc corresponds to the state of a bare surface in the double-layer region, whereas in reality at that potential the actual surface is oxidized. Thus, such a pzc realizes to some extent the concept of ideal reference state, as in the case of ions in infinitely dilute solution. 

Figure 4. The structure and orientation of (a) weakly bound hydrogen and its associated water on a Pt or Rh electrode (b) water on the electrode surface at potentials in the double layer region.

Activation of Water Outside the Double-Layer Region... 

Outside of the double-layer region, water itself may be oxidized or reduced, leaving stable hydride, hydroxyl, or oxide layers on the electrode surface. These species may adsorb strongly and block sites from participating in electrocatalysis, as for example, hydroxyl species present at the polymer electrolyte membrane fuel cell... 

Fig. 2.20 The Gierke model of a cluster network in Nafion. Dimensions are expressed in nm. The shaded area is the double layer region, containing the immobilized —SO3 groups with corresponding number of counterions M+. Anions are expelled from this region electrostatically...

Figure 3.22 Reflection X-ray dilTractogram from platinised Pt in 1 M H2S04 in the double layer region. Data collection time 3 x 104 seconds. From Fleischmann and Mao (1987).

On the other hand, Bagotzky and Vassilyev (1967) also measured the charge passed during methanol adsorption at a potential in the double-layer region and the adsorbate oxidation. The results they obtained are given in Table 3.2 and strongly suggested that the adsorbate was COH (see equations (3.45) and (3.47)). 

Fig. 8 Cyclic voltammograms of Au(lll) in 0.1 M HC104 (dotted line), 0.1 M H2S04 green), 0.1 M HC104 + 1 mM KC1 red), 0.1 M HC104 + 1 mM KBr blue), and 0.1 M HC104 + 1 mM KI black) in the double layer region. Scan rate 50 mV s 1...

For comparison we also show a cyclic voltammogram of a Au(lll) electrode (see Fig. 13.4). There is no detectable hydrogen adsorption region the hydrogen evolution reaction is kinetically hindered, and sets in with a measurable rate only at potentials well below the thermodynamic value. There is a much wider double-layer region in which other... 

Two other small peaks appear at E=0.45 and 0.7 V on the Pt(lll) surface. They are shifted to more negative potentials with increasing H SO concentration, suggesting their link with anion adsorption. A peak at 0.35 V for the Pt(100) surface, where one would expect the so-called double-layer region for Pt, is also intriguing. It remains to be seen whether or not it is due to sulphate adsorption on the (111)-oriented imperfections. 

The curve for Pt(110) surface shows a wide double layer region, indicating that the anion adsorption is completed concurrently with hydrogen desorption. The anions are strongly held in the "troughs" of the Pt(110) surface, i.e. in the 2(111)-(111) terrace-step sites. This point will be touched upon connection with the stepped surfaces. The C-E curves for Au(110) in clearly show a completion of anion adsorption on that... 

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