Electrolytic Hydrogen Evolution Kinetics and Its Relation to the Electronic and Adsorptive Properties of the Metal

Abstract

A significant correlation between the logarithm of the exchange current density i0 of the electrolytic hydrogen evolution reaction and the electronic work function φ is found for one group of metals. Another group of metals probably having a different relationship is distinguished. The relationship between logi0 and φ is explained as arising from the dependence of heat of adsorption upon φ. The bond strength DMH of the adsorbed hydrogen calculated from Pauling's equation is less than the experimental value, owing to the neglect of the image potential of H at the metal, but parallels the experimental values. DMH is dependent on the work function since the parameters DMM (the metal-metal dissociation energy) and XM (the electronegativity of the metal) are statistically significant linear functions of φ. For the metals Ta, Mo, W, Cu, Ni, Fe, Rh, Pd, and Pt (at high current densities) logi0 increases as the heat of adsorption of H decreases. The heats of adsorption of H at Hg, Cd, Pb and Tl are low compared with the values for other metals having respectively similar work functions and the distinct relation, having a different direction, between logi0 and φ for these metals is explained. Logi0 increases linearly with increase of d character for the metals Mo, W, Cu, Ni, Fe, Pd, and Rh. The correlation coefficient is 0.62. The relation to d character arises since the d character determines the heat of adsorption of H. The usually observed Tafel slope of 0.12 is consistent either with the rate determining reactionH3O·+e0−+M→MH+H2Oor H3O++MH+e0−→H2+M+H2O.If A is rate determining, i0 should increase with increase of the heat of adsorption of H; if B, the converse should occur. Hence, B is rate determining in acid solution for the metals, Cu, Ni, W, Mo, Ta, Fe, Pd, Rh, Pt (high current density) and A for the metals Hg, Tl, and Pb. The available stoichiometric number data support this conclusion, which implies that during steady evolution of hydrogen, a maximum coverage of the metal by adsorbed H occurs under the condition that the electrode is in contact with liquid water. A general theory of electrolytic reduction and electrode poisoning is given. A mathematical analysis of the dependence of the rate of the proton discharge reaction in terms of the heat of adsorption of H, MH bond force constants, and internuclear distance, is given.