The rotating ring-disk electrode has been applied in this study to the related phenomena of dissolution, current oscillation, and passivation of zinc in concentrated aqueous NaOH. The results of potentiostatic and potentiodynamic experiments are compared with those of previous workers, and a physical model is proposed to describe the anodic behavior of zinc in four successive potential regions: active dissolution, prepassivation, pseudopassivation, and true passivation.The requirements for high specific energy, high specific power, and low cost make zinc an attractive negative electrode material for aqueous alkaline batteries in electric vehicular applications (1). However, a number of specific limitations await satisfactory resolution. Recent studies have been concerned with the limitations of cycle life associated with dendritic (2-5) and nonadherent (6-9) zinc deposition during charging, and with the problems of electrode shape change with cycling (1, 2, 10). The shelf life and cycle life of zinc battery electrodes is also limited by the corrosion of zinc in concentrated alkali (11)(12)(13).In the present study we focus on the related processes of anodic dissolution and passivation. Rapid discharge of the zinc electrode is associated largely with the high solubility of Zn(II) as a zincate ion (14, 15) in alkaline solutions. At high currents, however, zinc discharge is often inhibited by the formation of a passivating zinc oxide or hydroxide layer by a dissolution-precipitation mechanism (16-28). Before the onset of passivation, the discharge of zinc is generally agreed (21) to proceed via the formation of complex hydroxyl zincate anion. The nature of this anion has been the subject of considerable thermodynamic (14, 29-31) and spectroscopic (15, 32-34) investiga.tion, and is thought to have the form Zn (OI-I)4 ~+ in strongly alkaline solutions. However, the spectroscopic investigations of Jackovitz and Langer (32) suggest that "only a small fraction, about one-sixth of the anodically dissolved zinc, was converted to Zn (OH)42-"
Experimental ProcedureExperiments were performed using a rotating ringdisk electrode (RRDE). The importance of adequate hydrodynamic control in studies of zinc dissolution and passivation phenomena has been stressed by several authors (20,26,35,36).The RRDE was constructed using a vitreous carbon~ ring with a 99.999% purity zinc disk, as shown in Fig. 1. Vitreous carbon constitutes a nearly ideal, nonporous, inert ring material for current collection at large cathodic potentials. The radius ratios rs/r~ and re~r1 exceed those tabulated by Albery et al. (37) andPleskov (38), and the electrode collection efficiency was determined experimentally in O.IM K3Fe(CN)e with 1.0M NaOH supporting electrolyte. This calibration procedure has been described previously (39). At 20~ the measured collection efficiency was 70.9 • 0.2%. Details of the rotating ring-disk electrode construction are shown in Fig. 1. " Electrochemical Society Active Member. Present address: The Experiments were conducted in a...
