Hydrogen Evolution on Platinum

This leads to a Tafel slope of 1 = 23RT/p F = llSmVforp = 0.5, and a reaction order (at constant potential) of pj = 1. The transfer coefficient is equal to the symmetry factor p., as in the case of mercury, but we recall that the Tafel slope is calculated here on the assumption of essentially full coverage, whereas that on mercury was obtained for very low coverage. 

The region of micro-polarization, where the j/r) plot is linear, can extend to about il/i) 0.2 whereas the linear Tafel region starts at about r]/b 1. As a result, the intermediate region of 0.2 t]/b 1 would be left unused, as far as the evaluation of kinetic parameters is concerned. For fast reactions, such as the HER on platinum, this represents a loss of crucial data, since it may be difficult to extend the measurements to overpotentials much above rj/h = 1, because of mass-transport limitations. Fortunately, modern microcomputers allow us to make use of this intermediate region. To do this, we write the full equation for an activation-controlled electrode reaction as follows  

The adsorbed hydrogen atom can then follow two parallel paths form molecular hydrogen or be absorbed in the metal. Penetration of hydrogen into the metal is a side reaction for the HER. If it is substantial, as in the case of palladium, the j /E plot cannot be analyzed properly, unless the relative rates of the two parallel reactions are determined as a function of potential and time. This could be performed, for example, by employ a ring-disc electrode. Molecular hydrogen would be formed on the disc, and its rate of formation could be determined by oxidizing it back to HsO on the ring, but such measurements have rarely, if ever, been reported. 

It should be noted here that Pt and some of its alloys are of great importance in the area of fuel cells and water electrolyzers and in the general context of the so-called 

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Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have ... 

Maier CU, Specht M, Bilger G (1996) Hydrogen evolution on platinum-coated p-silicon photocathodes. Int J Hydrogen Energy 21 859-864... 

Fig. 15.9. Depth profiles of the Pt Af and 2s photoelectron signal of the electron beam-evaporated Pt layer on Si and the concentration profiles of Pt, Si, O and C. (Reprinted from C. U. Maier, Hydrogen Evolution on Platinum-Coated p-Silicon Photocathodes, Int. J. Hydrogen Energy21 840,1996. Reproduced with permission of the International Association for Hydrogen Energy.)...

Therefore, it is quite rational to say that ja is the (equal and opposite) current density in each direction at equilibrium. Thus, /, is a real finite number, for example, 10-3 A cm 2 for hydrogen evolution on platinum at 25°C. The dependence of the current on temperature would be helpful, too, but there is little of it as yet. 

A cathodic polarization curve for hydrogen evolution on platinum in 2.8 M sulfuric acid is shown in Fig. 6.4. 

According to mixed potential theory, the hydrogen evolution exchange current density on the cathode is the sum of the hydrogen evolution on platinum and hydrogen evolution... 

A. N. Frumkin, P. Dolin, and B. V. Ershler [1940a] Kinetics of Processes on the Platinum Electrode. II. The Rate of Discharge of H-Ions and the Rate of the Over-All Process of Hydrogen Evolution on Platinum, Acta Physicochimica URSS 13, 779-792. 

Nichols, R.J. and Bewick, A. (1988) Spectroscopic identification of the adsorbed intermediate in hydrogen evolution on platinum. Journal of Electroanalytical Chemistry, 243, 445 53. 

Table 1.3 contains the approximate exchange current density for the reduction of hydrogen ions on a range of materials. Note that the value for the exchange current density of hydrogen evolution on platinum is approximately 10 A/cm, whereas that on mercury is 10 A/cm. ... 

According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

One of the most important reasons for the application of mercury to the construction of working electrodes is the very high overpotential for hydrogen evolution on such electrodes. Relative to a platinum electrode, the overpotential of hydrogen evolution under comparable conditions on mercury will be -0.8 to -1.0 V. It is therefore possible in neutral or (better) alkaline aqueous solutions... 

Fio. 24. Effect of ultrasonic vibrations on the overpotential-current density relation for hydrogen evolution on a platinum electrode in 1 H28O4 at 26° 104). 

Hydrogen evolution rate on the tin surface increases when tin is coupled with inert platinum. The observed increase in Fig. 6.6 results from the exchange current density difference of the coupled metals. The intersection between the tin dissolution polarization curve and the polarization curve for hydrogen evolution on tin results in fcorr.Sn- When equal surface area of tin (1 cm and platinum (1 cm are coupled, the sum of the rates of hydrogen evolution reactions on both metals is equal to the total rate of hydrogen evolution. 

Estimate the effect of 1% impurity of platinum on the corrosion rate of zinc in acidic and alkaline solutions. Use the values of Tafel constants for the hydrogen evolution on the respective metals from Table E6.1. 

Tafel slope for tin dissolution is fca = 0.1 V/decade. The Tafel slopes for hydrogen evolution reaction on both tin and platinum is bc = — 0A/decade. Exchange current density for hydrogen evolution on Sn, sn 10 A/cm and on Pt,... 

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