Hydrogen evolution, catalytic overvoltage

Historically, electrocatalytic science developed from investigations into cathodic hydrogen evolution, a reaction that can be reahzed at many metals. It was found in a number of studies toward the end of the nineteenth century that at a given potential, the rate of this reaction differs by severaf orders of magnitude between metals. In one of the first theories of hydrogen evofution, the recombination theory of hydrogen overvoltage, the rate of this reaction was finked directfy to the rate of the catalytic... 

Cathode The hydrogen overvoltage of carbon is relatively high. Thus, if the cathode is the working electrode, a lot of reduction reactions is enabled. On the other hand, the catalytic activity for hydrogenation reactions is low. This is advantageous if the cathode is the counter electrode cathodic side-reactions are avoided besides hydrogen evolution. 

Microscopic and spectroscopic investigations (SEM and XPS) reveal the relatively fast change of the chemical composition of nickel sulfide coatings upon the onset of cathodic hydrogen evolution (74). Indeed, at 90°C all nickel sulfide phases are reduced to porous nickel within several days to a week s time. They lose some catalytic activity with time with an increase in overvoltage between 0.15 and 0.3 V after continuous operation for 1 year. It is clear that the catalyst after I week is already no longer nickel sulfide but some type of Raney nickel. Thus far the initial catalytic activity of the NiS, coating is of little relevance. The respective results and data are due to be published by the present authors (73). 

Electrocatalytic reduction may be important at electrodes with low hydrogen overvoltage and high catalytic activity, and its mechanism is closely related to the mechanism of the hydrogen evolution. 

H. Yamashita, T. Yamamura and K. Yoshimoto, The relation between catalytic ability for hydrogen evolution reaction and characteristics of nickel-tin alloys, J. Electrochem. Soc., 1993,140, 2242-2243 K. Yoshimoto, H. Yamashita and M. Miyashita, Development of Ni-Sn alloy plating active catholyte and application in industrial electro-dialyzer, Soda Enso (Soda Chlorine), 1994, 45, 418-427 Yoshida and T. Morimoto, A new low hydrogen overvoltage cathode for chlor-alkali cell, Electrochim. Acta, 1994, 39, 1733-1737. 

Perhaps the most common example of overvoltage encountered in electrochemistry is that needed to reduce H+ ions at a mercury electrode. On a catalytic Pt surface (platinized Pt, which is a large-surface-area, black Pt deposit on Pt metal), the H2/H+ ion couple is said to behave reversibly. This means that one can oxidize hydrogen gas, or reduce H+ ions, at the standard reduction potential, 0.00 V, under standard conditions. At a mercury cathode, however, it takes about -1 V versus SHE to reduce H+ ions, because hydrogen evolution is kinetically slow. This phenomenon has great importance in polarography, because it enables the analysis of many of the metals whose standard potentials are negative versus the SHE. 

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