Evolution of Hydrogen at Other Cathodes

The facts presented above firmly establish the existence of a barrierless discharge of hydrogen ions (as well as a barrierless reduction of undissociated acid molecules or their corresponding ion pairs) at mercury in aqueous and ethylene glycol solutions, and at silver in aqueous solutions. From this point of view, let us consider the available data on the evolution of hydrogen at other cathodes, starting with mercury-like metals which poorly adsorb hydrogen. 

Tenno and Palm[164] observed a decrease in the slope of the polarization curve for bismuth in the presence of halide ions, especially iodide ions, down to 60 mV. Later, this system was investigated in greater detail in the same laboratory[165]. It was found that in the region b - 60 mV, the overpotential is independent of the solution composition for moderately acidified solutions, while a noticeable dependence was observed for concentrated solutions. In analogy with the data for a mercury cathode (cf. sections 2.1 and 2.2), these results are considered as proof of a barrierless discharge of hydrogen ions and undissociated acid molecules respectively. 

The existence of a region with a lower slope b = 50-80 mV was reported for the case of gold by Hilson[141] and Pentland et al.[149] (some decrease in slope was also observed in [167,168]). Conway[145] 

Metals with the highest adsorbability for hydrogen constitute a group with opposite properties. For tungsten[118,171,172], molyb-denum[149,173], and niobium[174-176], two Tafel regions with slopes of 60-80 mV and 100-120 mV have been reported. For W and Nb, there were no data concerning the dependence of the overpotential on the solution composition, while a stronger dependence for the upper part of the polarization curve was indicated in the case of Mo[173]. 

This data was interpreted by us[71,157] as an indication of a barrierless electrochemical desorption. This mechanism seemed to be most probable for metals with a very high energy (about 75 kcal/mol) of hydrogen adsorption bond[158]. 

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