Platinum Tafel reaction

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 ... 

For the HOR, exhibiting an exchange current density in the range of io = 10 A/cm with platinum as electrode material [9], only a small overvoltage arises even by drawing a current density of technical interest, e.g., 1 A/cm, due to the fact that the H2 molecule is easily split into two adsorbed H atoms (Tafel reaction), followed by the transfer of one electron per H atom (Volmer reaction) to the electrode generating two hydrated H". Alternatively, one electron is transferred to an adsorbed H2 molecule, followed by a split into a hydrated and an adsorbed H atom. In a consecutive step, the second H atom is oxidized (Heyrovsky reaction). 

The electrooxidation reaction of hydrogen consists of two steps [27]. The rate-limiting step is the dissociation of the hydrogen molecule, which requires two adjacent bare platinum (Pt) sites. This reaction is also sometimes referred fo as the Tafel reaction and is given as follows [27] ... 

In particular, Yeager and co-workers (42) have described a = a 4- j3 T for the reduction of oxygen on platinum in concentrated phosphoric acid in the temperature range 25—250°C with cathodic Tafel slopes nearly independent of temperature (a — 0.08 and j3 = 0.0012/K). The effect of electrode potential on the reaction kinetics is mainly through the entropy of activation [43]. 

This leads to a Tafel slope of b = 2.3RT/PF = - 0.12 V for P = 0.5, and a reaction order (at constant potential) of unity. 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 assuming essenti illy full coverage, whereas that on mercury was obtained assuming a very low value of the coverage. The Tafel plot observed on platinum in acid solutions is shown in Fig. 5F. 

The portions of the curves, which describe the hydrogen evolution have Tafel slopes equal to 60 and 140 mV for platinum, 120-140 mV for copper, and 150 mV for cadmium. The reaction rate depends on the nature of the cathode and the acid concentration. Overvoltage at which hydrogen is evolved on the copper electrode equals 0.5 V at pH 4 and a current density of 10 This value... 

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