Coverage by Adsorbed H in Hydrogen Evolution Reaction at Transition Metals

Determination of Coverage by Adsorbed H in Hydrogen Evolution Reaction at Transition Metals 

Until recently, surprisingly little work had been done experimentally on the important aspect of coverage by adsorbed H in the kinetic and catalytic behavior of the cathodic H2 evolution reaction. Theoretically, the relation between potential dependence of coverage, 0 , of the H intermediate [see Eqs. (65) and (81)] and the mechanism and kinetics of the HER had been treated extensively, but experimentally evaluated On data to which kinetic behavior could be related remained mostly lacking until recently. It is obviously a very important aspect of electrocatalysis behavior that should be experimentally determined. 

Early work by Gerischer and Mehl (106) employed impedance analysis at Ag and Cu electrodes. However, these metals are not of major interest as H adsorption is weak, and these materials are not attractive as water electrolyzer cathodes. Bockris et al. (121) and Selvaratnam and Devanathan (122) employed the double-pulse method (see Section VI,B,1) for Ag and Ni, but the results did not seem to be very meaningful. 

The most satisfactory experimental methods are (a) analysis of potential relaxation after current interruption from a prior steady-state potentials and (b) ac impedance spectroscopy at steady-state potentials. These methods have been referred to in Section VI. They both have the advantages that no H2 reoxidation occurs and no surface oxidation of the electrode takes place, as can arise in the current pulse method (121). The principal applications of the potential-relaxation method to determination of OPD H have been in the work of Bai and Conway (75) on H adsorption in the HER at Ni, Ni-Mo composites, and Pt (136), and by Conway and Brousseau (162) at bulk, single-phase Ni-Mo alloys (Mo 0 to 19 at%). 

The 0H results are recovered in the form of profiles of H adsorption pseudocapacitance, Q [Eqs. (66) and (67)], as a function of overpotential, t), which can be integrated to give changes of coverage by H with increasing overpotential (shown below in Figs. 19 and 20). 

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