ELECTROCHEMICAL INTERCALATION OF CATIONS INTO AN AMORPHOUS WO3 FILM AND ACCOMPANYING CHANGES IN MASS AND SURFACE-PROPERTIES

The electrochemistry of electrodes coated with amorphous WO3 film in propylene carbonate containing 0.1 M LiClO4, NaClO4, KPF6 or tetraethylammonium (TEA) perchlorate was examined. The surface density of redox-active sites on the WO3 film coated electrode is larger for smaller cations (Li+ > Na+ > K+). The charge transfer rate was also faster for smaller cations. In the KPF6 solution, the solvophilicity of the film surface increased with potential cycling, while no significant changes were observed in the LiClO4 solution and NaClO4 solution. In the LiClO4 solution, an apparent mass increase during cation intercalation, estimated using an AT-cut quartz crystal resonator, was roughly in agreement with the mass number of Li+. However, in the NaClO4 solution, an apparent mass increase was significantly larger than the mass number of Na+; this may be caused by stress generated by intercalation of the massive cation. Electrochemistry was not observed in the TEAClO(4) solution. Proton intercalation was not observed in the TEAClO(4) solution even in the presence of water (1.5 wt%), but was observed in the presence of CF3COOH (10 mM).