High Area Platinum Electrode

Chemically and electrochemically platinized platinum electrode (180 as roiighness factors for both electrodes) were used as high area platinum electrodes. While they showed slightly different COad oxidation characteristics, their methanol oxidation characteristics were virtually the same. Therefore, the results will be shown for only chemically platinized platinum electrodes. 

The step functions were appUed to the high area platinum electrode. The results are shown and compared with the smooth electrode in Fig. 3-34. Although the high area platinum showed slightly large current most of the time, the difference is siuprisingly small except after 20 s for 400 mV. 

High area platinum electrodes were made by painting platinum powders as described earlier. The electrodes were electrochemically cleaned by scanning the potential 50 to 1500 mV for 3 h in 3 M sulfuric add. After that the electrolyte was exchanged with 3 M sulfuric acid with 1 M methanol. 

CO was introduced into the electrolste while the potential was held at 50 mV. After the full coverage was achieved, the potential was swept in the anodic direction. The voltammograms are compared with ones without ruthenium in Fig. 4-5 and Fig. 4—6 for the smooth and the high area platinum electrodes, respectively. 

The voltammograms are shown and compared with that of pure platinum in Fig. 4-14 and Fig. 4-15 for smooth and high area platinum electrodes, respectively. Since the COad oxidation current overlaps with the current related to tin, the net current of the COad oxidation is taken as the difference of with CO and without CO . 

Despite the fact that iron has been the subject of the large majority of Mossbauer studies reported in the literature, it is rather interesting to note that the first in situ application of this technique involved tin as the active nuclide. In that work, Bowles and Cranshaw examined the emission spectra of adsorbed on a high-area platinum electrode. Although some of the conclusions made by the authors may be open to question, the results obtained clearly indicated that the tin, in whatever form was present at the interface, was bound sufficiently strongly to the host lattice to give a Mossbauer spectrum. Thus, these types of measurements were indeed feasible. 

In both cases, the COad oxidation peaks shifted significantly to more cathodic potentials with the Pt-Ru electrode. On high area platinum without ruthenium as discussed in Copter 2, the oxidation curve was made up of two peaks, i. e. peaks for easily oxidized COad (eoCOad) and hard-to-... 

Ruthenium was first deposited on platinum in the same condition as described in the section for Pt-Ru electrodes. For high area platinum, tin was deposited after washing the electrode with hydrogen-saturated water. For smooth platinum, before that, ruthenium was removed at 1100 mV for 30 s to obtain higher activities as described before. 

Scavenging of residual impurities in the solution may be necessary ifth e rate-potential relation betrays the effects of impurities in the solution by some kind of aberrant behavior. One introduces a large auxiliary electrode of high area platinum black and changes the potential on it slowly and cyclically over the range of potentials of the intended experiment (Bockris and Conway, 1949). A day or so of this cycling may be necessary and the platinum black sheet (which now contains deposited or adsorbed impurities) must be removed carefully with the potential still on (so that the impurities do not desorb back into the solution). 

Further, the cathode Tafel slope of -—0.12 V/decade at Eh = 115 to 1.20 V vs. RHE in Figure 25a differs from the usual low-current-density Tafel slope of -0.06 V/decade on smooth platinum listed in Table 7. It is quite possible that the rate-controlling step and even the mechanism differ at the more positive potentials involved with the Teflon-bonded high-area Pt electrode in Figure 25. Thus the question of the stoichiometric number for the four-electron reduction remains unresolved. 

Figure 25. Polarization curves for the O2 electrode reaction on Teflon-bonded high-area platinum in O2 saturated 0.1 Af a, log i vs. Eh 1, cathodic 2, anodic, b, i vs. t/h 1, cathodic ...

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

In this chapter, oxidation characteristics of COad on platinum electrode were investigated focusing on this issue using various types of platinum electrodes such as single cnrstals, smooth and high area... 

The chronoamperometry curves at 500 mV in 3 M sulfuric acid with 1M methanol are shown in Fig. 4-9 and Fig. 4-10 for smooth and high area Pt-Ru electrodes, respectively. Although Pt-Ru electrodes showed less current than the pure platinum at first, they showed much less decay and higher sustained current. Even the electrode with higer coverage of ruthenium (I.IV for 15 s), which showed more than ten fold smaller current than pure platinum at first, gave higher ciirrent after 40s. In the case of... 

Tin was electrochemically deposited on smooth and high area (chemically platinized) platinum electrodes at -300 mV for 10 s. The thermodynamic potentials for tin metal and its ions are ... 

For both the smooth and high area electrodes, the methanol oxidation current was small at first and most of the current was made up of the oxidation of tin. Then the methanol oxidation current increased. The high area Pt-Sn electrode maintained the constant oxidation cxirrent afrer that. In contrast, the smooth Pt-Sn electrode showed a decay although the final current (after 2 min.) was slightly larger than that of pure platimun. Somehow tin seems more easily desorbed from the surface of smooth platinum than of hi area platimun. 

It is desirable to measure R (and thus G) at a high frequency in order to reduce Xc to a negligible value compared to R. Unfortunately, other complications arise if the frequency is increased above a few kilohertz, and therefore other means must be devised to decrease Xc. A commonly used remedy is to increase the surface area and thus the capacitance of the electrodes as much as possible. A 100-fold area increase is obtained by platinizing the electrodes, that is, electrodepositing a layer of platinum black onto the platinum electrodes, usually from a solution of chloroplatinic acid [8]. 

The time (t) dependences of the charging current (Ic) of a cell where the capacitance of the electrode investigated is much smaller than that of the reference electrode (e.g., saturated calomel electrode) and the counter electrode (e.g., high surface area of platinum electrode) as well as the geometrical capacitance of the cell are neglected, therefore the total capacitance of the cell is equal to the capacitance of the electrode (Q) under study, and - ohmic (solution) resistance is Rs> - are as follows. 

The use of such techniques for radiochemical work has been discussed In detail (203,250,251). In these studies a platinum electrode of large surface area (52 mesh, 30 mm high, 10 mm i.d.) coated with silver chloride is placed in contact with 10 ml of solution containing a very low concentration of radioactive silver. The solution Is stirred magnetically. The electrode is then removed from the solution, washed with nitric acid, rinsed with acetone, and the radioactivity counted. 

For both these reference electrode types, the potentials are accurately known relative to the standard hydrogen electrode (SHE) for which the chemical reaction is (5). The SHE is based upon a high surface area platinum black-coated electrode in contact with hydrogen gas (one atmosphere... 

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