Hydrogen adsorption-desorption peaks

Figure 12.5 CO stripping voltammogram with a CO- tee 0.1 M H2SO4 electrolyte. Compare the data in Fig. 12.4 the CO oxidation region begins at V = 0.43 V. After CO stripping, hydrogen adsorption/desorption peaks and the beginning of the Pt oxidation range are shown.

The second most widely used noble metal for preparation of electrodes is gold. Similar to Pt, the gold electrode, contacted with aqueous electrolyte, is covered in a broad range of anodic potentials with an oxide film. On the other hand, the hydrogen adsorption/desorption peaks are absent on the cyclic voltammogram of a gold electrode in aqueous electrolytes, and the electrocatalytic activity for most charge transfer reactions is considerably lower in comparison with that of platinum. 

However the aore iaportant question which can be solved with such saaples is related to characterization of surface sites because each basal orientation shows well-characterized hydrogen adsorption-desorption peaks which eight be helpfully used for this purpose. 

The electrochemical active surface area (ECSA) reflects the total catalyst surface that has the potential to participate in the fuel cell reaction. It is typically measured by the hydrogen adsorption/desorption peak area or the CO oxidative stripping peak area. A larger ECSA normally gives better fuel cell performance. The ratio of ECSA to the mass of the catalyst is an indication of how effectively the precious metal catalyst is used. The ratio of ECSA to the total geometrical surface area of the catalyst estimated by the particle size is an indication of how effectively the catalyst surface is used. The latter ratio can be used to gauge how well (high ratio) or bad (low ratio) a catalyst layer is made. 

Electrochemical Active Surface Area (ESCA) of Pt-based catalysts through the hydrogen adsorption/desorption peak area, CO stripping, and underpotential deposition of copper (Cu-UPD) methods. 

Organic compounds were added to 0.05mol dm at 1, 10, and lOOmmol dm , and the change of the ORR current was evaluated as the measure of the ORR degradation in the presence of organic impurities. Cyclic voltammetry was performed to obtain the platinum active surface from the hydrogen adsorption/ desorption peaks. 

Cyclic voltammetry studies of single-crystal platinum electrodes in acidic aqueous electrolytes showed that the two characteristic peaks of hydrogen adsorption/desorption on platinum (see Fig. 5.40) correspond in fact to reactions at two different crystal faces the peak at lower potential to Pt(100) and the other one to Pt(lll). 

The sharp and narrow peak at -0.15 V on the Pt(110) plane indicates on a coupling of hydrogen adsorption/desorption and HSO - and S0 2- desorption/adsorption. This will be further discussed in connection with the data for stepped surfaces. 

The cyclic voitammogram for Pt (111) in 5 M sulfuric acid is shown in Fig. 2-21. Compared with that in 0.5 M sulfuric acid (Fig. 2-15), the anodic part of the two split hydrogen adsorption-desorption areas was compressed in the cathodic direction and became two sharp peaks while the cathodic part did not change its shape very much. The asymmetric peak at 700 mV shifted cathodicly and became more symmetric and sharp. The oxidation of platinum shifted about 100 mV in the anodic direction. All these changes could be attributed to the increase in specific adsorption of anions or the decrease of the activity of water as well as the pH change. 

Voltammograms of Ptdll) with and without COad adsorption in 0.5 M perchloric add are shown in Fig. 2-25. The voltammogram without CO was considerably different from those in sulfuric acid. The symmetric features in the range from 600 to 800 mV correspond to the anodic portion of the two split area of hydrogen adsorption-desorption in sulfuric add. Hydrogen adsorption-desorption features did not change after the oxidation peak at 1050 mV and its reduction while the further oxidation removes the feature irreversibly. Therefore the peak at 1050 mV is considered as a formation of a weak interaction with water. 

In the hydrogen region (50 - 350 mV) in the first cycle, the hydrogen adsorption-desorption ciirrents were depressed because the surface was covered with COad COad oxidized to CO2 in the anodic peak between 700 mV and 1000 mV. This peak overlapped with the platiniim oxidation whose voltammogram is shown as the dashed line. After this peak the voltammogram became identical with the voltammogram without CO. The h rdrogen adsorption-desorption peaks were fully recovered. This shows the COad completely oxidized and there was no CO in the liquid phase. 

The CV and MSCV plots, started from the double-layer region and swept in the cathodic direction, for benzene at Au(332) modified with 0.82 ML of Pd [hereafter designated as Au(332)-0.82ML-Pd] are shown in Figure 12. The reversible peaks at ca. 0.25 V in subsequent cycles (at which most of the benzene have already been desorbed) are due to the hydrogen adsorption-desorption reactions that transpire only on ultrathin Pd films. The prominent peak at 1.15 V on the first (reverse) anodic scan corres-... 

Figure 7. (A). Cyclic voltammetric profile of the hydrogen adsorption/desorption and (B). the reduction peak of platinum oxide observed at GC/Nf/Ptnano electrode in 0.1 M H2S04. Scan rate = 10 mV s 1. (Reprinted from Ref. 36 with permission from Elsevier)...

Fig. l.S (a) Cyclic voltammetry from the Pt(OOl) and Pt(001)-Pd surface in 0.05 M H2SO4 at a sweep rate of 20 mV s L Subsequent to Pd deposition, sharp peaks arise at 0.2 and 0.29 V which correspond to hydrogen adsorption/desorption accompanied by (bi)sulfate desorption/... 

---