The adsorption of trifluoromethanesulfonate anions at the mercury/electrolyte interface

Recently, there has been much interest in the use of trifluoromethanesulfonic acid as an electrolyte in advanced fuel cell systems because of the enhanced rate of oxygen reduction in this medium. In order to elucidate this characteristic from a double layter point of view, the present work was undertaken to study the adsorption of trifluoromethanesulfonate ions on mercury electrodes. Differential capacities and potentials of zero charge were measured in constant ionic strength solutions of composition X M CF₃SO₃ K + (1-X) M KF at 25°C. Analysis of the results shows that the extent of specific adsorption of CF₃SO₃^? anions is small, and the anions even desorb from the surface at sufficiently positive electrode charges. The structure of the inner layer is elucidated, and an isotherm for the adsorption of CF₃SO₃^? based on electrostatic models of the double layer is derived. The results indicate that at the potentials for the electroreduction of oxygen CF₃SO₃^? anions will not interfere with the adsorption of the reactant and of intermediates, allowing faster rates to be achieved.