Evolution of Hydrogen at a Silver Cathode

Loshkarev and 0zerov[140] were the first to find a region with a low value b - 60 mV of the Tafel constant for the evolution of hydrogen at silver. Its existence was later confirmed by several authors[118, 141-146]. However, a satisfactory explanation for this fact has not been available until recently. 

Antoniou and Wetmore[143] considered the slow stage as the surface diffusion of the adsorbed atoms. They rejected the assumption about a slow discharge on the basis of the value v = 1 for the stoichiometric number. It has been shown in [150] that this contention is completely unsubstantiated since the value v = 1 can actually correspond to a slow discharge. Moreover, the assumption about a slow surface diffusion, coupled with the requirement that a first-order reaction take place for the adsorbed hydrogen atoms (otherwise, we do not get the value b = 60 mV), leads to the value v = 2[150]. Hence, the value v = 1 actually refutes the slow surface diffusion mechanism. 

Available data on the effect of the solution composition on the overpotential[118,142] show that wherever this effect has been observed, it is stronger in the region with b - 120 mV than in the region with b - 60 mV. However, it has not been possible to draw any firm conclusions from these data on the mechanism of the process. The observed effects are small and only slightly exceed the discrepancy in the data for different electrodes in the same solution (sometimes, they are even smaller). This situation is frequently encountered in the case of solid electrodes. Besides, the surface of an electrode may change with time. Hence it was necessary to carry out measurements in a number of solutions for the same electrode. Moreover, after measurements in each new solution, the behavior of the electrode was observed in the same solution each time to ensure that its condition remained unaltered. Such measurem oats were carried out in [151-154], and the experimental technique is described in [151]. 

As in the case of a mercury electrode, a vigorous stirring during the electrolytic purification of the solution ensured a good reproducibility of results right up to current densities of 10  

Similar results have also been obtained for acidified solutions of K2SO1+ and KCl, as well as for strongly acidic phosphate buffer solutionsIn absolute magnitude, the overpotential is in good agreement with the values available in the literature. The shape of the curves is also similar to those observed by other authors the upper line or part has a slope of 100-120 mV (its value varies slightly in different solutions), while the lower part has nearly 

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