Electrochemical behaviour of platinum in alkaline and acidic solutions of heavy and regular water☆

Abstract

The behaviour of Pt as an electrode for hydrogen (protium and deuterium) and oxygen evolution in both alkaline and acidic, heavy and regular water solutions has been investigated primarily by cyclic voltammetry. The main features, such as adsorption and underpotential deposition of hydrogen (both protium and deuterium), as well as the specific charge capacity for monolayer oxide growth with successive increase in oxygen content (preceding hydrogen and oxygen evolution, respectively, with characteristic desorption peaks), were more or less marked in both electrolytes. Some distinctly different behaviour, however, has been observed, revealing that heavy and regular water behave almost as different solvent ambients. The hydrogen evolution reaction (HER) particularly in alkaline heavy water occurs at substantially more negative potentials, while oxygen evolution becomes shifted to considerably more positive potential values. Hydrogen absorption, besides adsorption, of both protium and deuterium, has been marked clearly by the continuously growing charge capacity of the diffusional desorption peak, whose extent depends on the evolving rate and contact time of hydrogen evolution and distinctly exceeds both one-to-one hydrogen to platinum (H/Pt or D/Pt) atom coverage on the exposed electrode surface, and relative to the corresponding reversible adsorption wave charge area for its underpotential deposition. In addition, the hydrogen oxidation peak immediately following its desorption (in particular from acidic heavy water) has also been clearly marked on voltammograms. A distinct merging and melding together of two initial deuterium reversible desorption peaks into the diffusional desorption peak in acidic heavy water has been discernibly scanned, too. Oxide formation usually starts at more anodic potentials together with deuterium oxidation and, specifically in acidic media, proceeds vigorously with higher and continuously growing rates and merging together with evolving molecular oxygen, while the prevailing oxygen evolution thereby becomes shifted to more positive potential values. These features reveal that, owing to its distinctly different steric factor, heavy water (in particular in acidic media) behaves as a stronger oxidizing agent than regular water. Some discernible properties of the interplay between hydrogen and oxygen on the Pt electrode in both electrolytes along the potential axis have been clearly marked and pointed out. The Rowland or EDTA effect on the potentiodynamic features of Pt has also been scanned and displayed.