Heyrovsky step

Any combination of two or three elementary pathways will give the overall mechanism of the hydrogen evolution reaction. Thus, the electrochemical Volmer discharge of the proton, the electrodesorption Heyrovsky step, and the chemical Tafel recombination of the H adatoms can serve as a combination for the hydrogen evolution process. The electrochemical rate constants can be estimated through different experimental conditions, such as their exchange current densities 7o,i = 10 1, yo2 10 4 and jo 3 = 10 2 A cm-2 at V= 0 V where AGads = 0 with 0H 1/2 [7,48]. 

When the electrochemical desorption mechanism (the Horiuti-Heyrovsky step) becomes rate controlling,... 

For simplicity the surface concentrations of OH, H2O, and H2 are written as dimensionless = Ci(0)/C, where the superscripted asterisk denotes bulk or equilibrium values. In general, the standard potentials of the Volmer and Heyrovsky steps are different. Proceeding in a way similar to that described in Sect. 5.2.1 the following equations are obtained ... 

Heyrovsky step is rate determining. However, the latter view may not be justified, because the difference of Eq. (86) from Eq. (84) originates from Z, or the (1 - ) term. It seems more appropriate to say that the rate is still determined by the Volmer step but, since the free electrode surface now plays a decisive role, and its magnitude depends on the rate of the Heyrovsky step, the rate expression involves kinetic parameters of that step. ... 

In conclusion, the break of the cathodic Tafel line in the Heyrovsky-Volmer route is not due to onset of surface saturation with H(a) with the Heyrovsky step rate determining, but to the Volmer step becoming rate determining. ... 

This is followed by either the Tafel step (10.2) or the Heyrovsky step (10.3). The former step involves the combination of two adsorbed hydrogen atoms, while the latter envisages the combination of a proton in solution, an electron, and adsorbed hydrogen to give dihydrogen ... 

The Volmer and the Heyrovsky steps are coupled (coupled electroadsorption-electrocombination combination). 

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