AbstractThe question of the structure of the electrical double-layer at electrodes in contact with electrolyte solutions has several aspects in common with the long and short-range structure of electrolyte solutions. Measurements of the capacitance of the double-layer have traditionally provided information on this question for the case of dilute electrolyte solutions. With proton hydrates from H3O+ through H9O4+ to 1 M aqueous H+ solution made by stoichiometric addition of water to the acid CF3SO3H, the transition between "molten-salt" (H3O+ CF3SO3 above 307 K) to "dilute-solution" behaviour of the interphasial double-layer at Hg and Au electrodes can be investigated. The H3O+ CF3SO3 is a highly conducting molten salt above 307 K. The results indicate that the double-layer capacitance at potentials negative to the RHE, both at Hg and Au, in the H3O+ molten salt has values surprisingly similar to that in 1 M aq. H+ solution. Intermediate states of hydration of the proton (H5O2+, H7O3, H9O4+) have higher values, especially at Au. However, the capacitance behaviour is quite different from that observed in high-temperature, alkali-metal molten salts at Pb. Models are proposed for the structures of the double-layers in these proton-hydrate systems, and the problem of accounting for a "normal" value (ca. 18 μF cm-2) of capacitance at Hg in the H3O+ melt is examined by means of some model calculations. Comparative electrode-kinetic measurements on proton discharge add further information on the behaviour of the proton hydrates.
