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ELECTROCHEMICAL KINETICS 583tion is reduced when the dimer is formed. Thus, the destabilization of the EC anion radical would be reflected in the larger observed equilibrium constant. ConclusionThis work may represent the first successful attempt at fitting two experimental parameters to digitally simulated working curves. The preequilibrium mechanism that has been invoked explains concentration results that were hitherto ambiguous. A mechanism of this type is necessary to explain • log t~/A log C data obtained experimentally. Of course, other mechanisms may be invoked that result in fractional reaction orders (5). However, this mechanism accounts for the experimental data better than any pure first or second order mechanism. This work also provides an operational definition of reaction order in the study of homogeneous electrochemical kinetics (Eq. [17]). This may prove to be useful in future kinetic studies.

Display devices based on electrochromic effects on metal oxides are currently being investigated intensively. For example a number of studies on the electroreduction of WO 3 to form the blue hydrogen tungsten bronze, HxWO 3, which can be reversibly bleached upon oxidation have been reported (I-4). However the slow dissolution of WO 3 in the aqueous sulfuric acid solutions used as the electrolytes in most cells has prevented application of this system to practical devices. Systems with WO 3 based on nonaqueous solvents, such as glycerol and acetonitrile, with formation of H-or Li-bronzes have also been described (5-8), but these also have problems (dissolution, irreversible coloring, or slow response). We report here preliminary experiments demonstrating electrochromic behavior of Nb205 in both aqueous and acetonitrile (AN) solutions. This oxide, which is insoluble in many media, shows reasonableresponse times and reversible coloring and bleaching, and so appears to be a promising system for electrochromic devices. Moreover the excellent stability of Nb towards corrosion in mineral acids because of the protective Nb205 layer (9) suggests that this material should form stable systems.The Nb205 electrodes were prepared by heating a niobium metal disk (12.7 mm dia., cut from a rod of 99.8% Nb obtained from Alfa Ventron) in air to ~500 ~ C for about I0 min which produced a white layer of Nb205 about 15 ~m thick. Electrical contact was made to the Nb metal by scraping off par t of the oxide and connecting a Cu wire to the Nb with silver epoxy cement. A typical currentpotential curve recorded with this electrode in 1M H2SO 4 with a Pt wire as a counter electrode and vs. a Hg/Hg2S04/I M H2SO 4 *Electrochemical Society Active Member. Key words: semiconductors, displays, voltammetry.reference electrode, is shown in Fig. I. The cathodic current which started at tO V vs. NHE is associated with coloration of the Nb205 layer and the anodic currens which appeared after reversal of the potential scan direction, is associated with the bleaching process. Upon reducing this electrode in the aqueous 1MH2SO 4 with a potential step to -0.6 V vs. NHE, a dark blue color appeared on the electrode surface. The blue color disappeared upon oxidation by a step to + 1 V vs. NHE. The electrochromic process was reversible with a response time of less than 1 sec. Electrochemical reduction and coloration of the Nb205 electrode could also be carried out in AN solution containing 0.8 M LiCIO 4.All solutions were prepared in an inert atmosphere glove-box by dissolving the LiCIO 4 (dried by heating under vacuum at II0 ~ C for three days), in AN (distilled before use by a previously described procedure (I0)). The electrochemical cell containing a Pt counter electrode and bleached Nb205 quasi-reference electrode was filled in the glove box, then sealed with vacuum grease and removed for the electrochemical experiments. A currentpotential curve of the Nb205 electrode in the AN/LiCIO 4 solution is shown in Fig. 2. The cathodic current which started at ab...

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