Ion insertion in tungsten trioxide electrodes

The role of ion insertion into the electrochromic metal oxide WO3 has been studied using a combination of electrochemical, gravimetric and spectroscopic techniques. The electrochemical quartz crystal microbalance (EQCM) was used to investigate the effect of electrolyte composition by studying the identity of the cations and anions involved and the concentration of the electrolyte used. It was found that the effect of electrolyte concentration and choice of anion did not affect film reduction and re-oxidation. The two cations studied, Li- and Na+, however cause marked differences in film redox behaviour.;The effect of film history was studied using a combination of EQCM and spectrophotometry. The initial cycle at a slow scan rate (5 mV s-1) showed responses different to those of subsequent cycles. Results indicated that at completion of the first cycle there is both injected charge and mass remaining in the WO3 film. This effect has been termed the "break-in" effect and has been seen for all experiments initially cycled at this slow scan rate, irrespective of anion or cation, or the electrolyte concentration. It is possible to correlate current (or charge), mass change and absorbance data together in a novel way, to give information regarding electrons, ions and solvent and electron and ion distribution (via the extinction coefficient, ). Also, the effect of long term cycling (>10,000 cycles) has been investigated with respect to cation insertion. This shows that Li+ insertion yields stable WO3 films after many cycles, whereas Na+ insertion does not.;The effect of experimental time scale also has been studied using cyclic voltammetry and chronoamperometry, together with the EQCM. Again, the "break-in" effect was recorded using chronoamperometry, indicating that the effect is not only seen for cyclic voltammetric experiments. Under all experimental conditions used, there is a second, neutral species being inserted and extracted alongside the cations.