Hydrogen overpotential, time effect

Recent measurements of the diffusion of electrolytically generated hydrogen into platinum and the associated change of overpotential with time lend support to the view that the time effects observed on Pt (and possibly on other metals which absorb hydrogen, e.g., Ni, Fe) are due to slow saturation of the bulk of the metal with atomic hydrogen, associated with possible phase formation. 

For a long time, the effect of the solvent on the overpotential was explained by the change in the solvation energy of the discharging ion. As we have stated above, however, this idea is not correct. It also does not agree with the available experimental data on hydrogen overpotential in various solvents (Table 1)[31]. 

Interesting results have been obtained for an Fe electrode in the presence of adsorbed iodide[416]. It was found that the hydrogen overpotential increases considerably and at the same time, the isotope effect remains unaltered. This phenomenon confirms the above statement that the rate-determining stage is not associated with the formation of H2 molecules. 

Figure 25 shows the evolution of cell voltage with time of Raney-nickel anodes that are deliberately operated at too high current densities so that the effectively applied overpotential was above the threshhold for nickel oxidation, which amounts to +80 mV vs the reversible hydrogen electrode. Evidently at a current density of 400 mA/cm2 and at 80°C the oxidation of Raney nickel proceeds within hours and at 300 mA/cm2 still within a week. 

Copper and copper alloys exhibit special catalytic effects in the electroreduction of carbon dioxide. They represent unique cathode materials, which can electrocatalytically convert CO2 and water into hydrocarbons and alcohols, at ambient temperature and atmospheric pressure [2]. So far, copper metal is the only electrode material able to produce significant amounts of hydrocarbons at high reaction rates and over 50% Faradaic yield, over a sustained period of time. Its drawbacks are that a copper electrode can operate only at high overpotential (of almost 1 V), and a mixture of major and minor products are obtained, which contains hydrogen, ethylene and methane [43,47,88]. In these reactions, carbon monoxide appears to be a key intermediate, and its further reduction yields a series of reaction products [2,89]. Copper cathodes can be operated at high current density in aqueous... 

Ag /Ag electrode in the same solution. This intermediate would have to survive a very long time, compared to the time taken to form it. However, at the potential of + 0.80 V there would be a very high positive overpotential of 2.55 V to oxidize it. Moreover, at this potential (corresponding to —1.75 V vs. SHE), it would in effect corrode, that is, reduce water to molecular hydrogen, while it is oxidized to Ag jj that will form the stable hydrated cation in solution. 

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