Metal cathodes and hydrogen : part 1, lead disintegration ; part 2, hydrogen adsorption on silver

Rostami, Abbas Ali (1979) Metal cathodes and hydrogen : part 1, lead disintegration ; part 2, hydrogen adsorption on silver. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In Part 1 of the thesis the disintegration of lead cathodes in aqueous perchloric and sulphuric acids at room temperature is considered. Electrochemical methods of analysis (differential pulse anodic stripping voltammetry and differential pulse polarography) of the Pb²⁺ concentrations generated by lead disintegration has enabled it to be detected at lower rates and hence at lower current densities than hitherto. Periodic sampling has enabled observations to continue during several hours of cathodisation. -- Lead disintegration rates in aqueous H₂SO₄ were found to be 10-25 times those observed by Gastwirt and Salzberg in 1957, probably because of the exclusion of air, other precautions against impurities and because of the use of more sensitive techniques for measuring disintegration. Disintegration rates in aqueous HC10₄ are comparable to those in H₂SO₄, in both cases decreasing with increase in [H₃O⁺] because of the inhibiting effects of hydronium ions on disintegration. -- As found by Gastwirt and Salzberg, threshold current densities also increase with [H₃O⁺], continuous disintegration being unobservable at lower current densities. Nevertheless, disintegration of a different type does occur below threshold current densities, in this case being a discontinuous process. This previously unknown phenomenon is attributed to accumulation of atomic hydrogen in the lead, combining within the lattice to H₂ and exerting such pressures that particles of lead are ejected. -- Cathodic disintegration of lead is thought to involve chemical and mechanical processes simultaneously. The chemical process is proposed to be formation and decomposition of surface PbHx where x ≤ 2, while the mechanical process involves bulk diffusion of H atoms and their subsequent combination to H₂ resulting in Pb ejection. -- In Part 2 of the thesis, studies of the diffusion of electro-lytically generated hydrogen through silver are described, together with the results of overvoltage and capacitance measurements at silver cathodes in aqueous perchloric acid at room temperature. Capacitances were deduced from open-circuit potential decay measurements. The increase of overvoltage and the growth of the measured capacitances in the course of electrolysis are attributed to a growing pseudocapacitance component. This is analysed in terms of an increasing coverage, ⊖H, of the cathode by adsorbed hydrogen atoms. The increase in surface coverage from the beginning up to several days of cathodisation ranged up to 2% of a monolayer for silver in 0.1 mol ℓ⁻¹ HC10₄ and up to 0.9% of a monolayer in 1 mol ℓ⁻¹ HC10₄. -- The increase of overvoltage at constant current density and the increase of electrode capacitance at constant overvoltage with the logarithm of the cathodic charge (-Q) were both linear up to Q values of about -60 C cm⁻². This increase of overvoltage and pseudo-capacitance might be attributed to the adsorption and absorption of hydrogen atoms by silver cathodes.

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