ARSENIC POISONING EFFECTS ON CATHODIC POLARIZATION AND HYDROGEN ADSORPTION AT PLATINUM AND STEEL ELECTRODES IN KF-CENTER-DOT-2HF MELTS

It is demonstrated how the presence of arsenic species in the KF . 2HF melt employed as the electrolyte in fluorine production cells greatly affects the electrocatalytic behaviour of the steel electrodes that a re used as cathodes. Effects of the introduction of As2O3, AsF3 and As F5 as the sources of catalyst poison species have been studied. Such s pecies influence the electrode kinetics of the cathodic hydrogen evolu tion reaction (h.e.r.) and the adsorption of the overpotential-deposit ed (o.p.d.) H intermediate. Using Pt, as a model electrocatalyst surfa ce, these effects were quantitatively studied by means of potential-re laxation transients, Tafel relations and cyclic voltammetry, the latte r enabling changes of underpotential-deposited C (u.p.d.) H coverage d ue to the presence of As species to be evaluated. By means of simulati on of the potential-relaxation behaviour, information on rate constant s, coverages by o.p.d. H and As species were also derived. The presenc e of As at Pt cathodes suppresses the u.p.d. H adsorption and modifies the o.p.d. H behaviour and the associated pseudocapacitance. The pres ence and generation of As(-III) species, as AsH3, in the evolved hydro gen gas was demonstrated by means of Marsh's test. The contact-angles of hydrogen bubbles generated in the h.e.r. at mild-steel electrode su rfaces were also determined comparatively in relation to observed slug gish hydrogen bubble detachment from the electrode surfaces when As sp ecies were present. Ex situ XPS analysis confirmed the presence of As species on the electrode surface.