Reactivity of Scratched Cadmium Electrodes in Alkaline Solutions

Abstract

The kinetics of reaction of scratched cadmium electrodes in alkaline solution have been measured. The anodic reaction rate of the freshly generated metal surface follows Tafel's law with a slope aE/a log i = 63 mV and a reaction order a log/D log (OH-) = 1.1. The equilibrium parameters are defined by Eo ~ = 18 mV (NHE) and io = 0.002 A cm -2. The parameters are interpreted in terms of simultaneous transfer of two electrons per cadmium atom oxidized from the bare metal surface.The behavior of cadmium electrodes in alkaline solution is still the subject of some debate (1, 2), despite its widespread use as a reversible battery electrode material. Even in acidic sulfate and perchlorate solutions in which the metal does not equilibrate with any of it~ oxides (3), there is no agreement regarding its mechanism of dissolution (4-10). Lorenz (4-6) originally showed that the reaction Cd ~ Cd 2+ -t-2e proceeds in a single step with both electrons being transferred simultaneously; this process has been cited (7) as typifying a two-electron transfer reaction. The work was, however, criticized by Heusler and Gaiser (8) for the limited potential range over which it was studied. By extending the potential range to more positive values, Heusler and Gaiser showed that the anodic behavior deviates from the 60 mV Tafel law given by Lorenz (4-6) and gave a Tafel slope of 108 mV where the anodic current density i is between ~60 and 500 mA-cm -s, The idea that both mechanisms operate in different potential ranges was supported by Hampson et al, (9,10).The mechanism of the initial stages of oxidation in alkaline solution also involves the question of whether each electron is transferred consecutively or whether both electrons are transferred simultaneously (1, 2).The former mechanism would involve the formation of a surface intermediate of Cd(I), probably as adsorbed CdOH, allowing film growth to occur by a solid-state mechanism which does not involve the formation of a dissolved intermediate, as proposed by Armstrong et al. (11)(12)(13). This is not in agreement with the dissolution-precipitation model (14-16) in which metal ions enter solution and are then precipitated to form the film. While there is general agreement that the steady-state film consists primarily of Cd(OH)2 (2), in both the ~ and -y structures (17, 18), the formation of CdO, either anhydrous or hydrated, cannot be ruled out (16,17). Indeed, it is claimed that a thin amorphotis film of CdO may well exist (17) and that this film provides the passivity of the Cd electrode (16). The dissolved ion in alkaline solutions has been identified as Cd(OH)~ 2-(11, 12) or Cd(OH)~-(t5). Species of this kind would probably result from dissolution of Cd(OH)~ rather than acting as dissolved ions prior to precipitation of Cd(OH)2 since it is improbable that three or four hydroxide ligands can react in one step. Thus, measurements of dissolved ion concentra-9 Electrochemical Society Active Member. tions in ring disk electrode studies (12, 16) probably detect dissolution of the films, Mi...

Non-stationary electrochemical response of polycrystalline cadmium in sodium hydroxide solutions

“…Likewise, pseudocapacitance measurements suggest that the adsorption of OH species precedes the potential range of the possible formation of Cd(OH)i[40J . It should be noticed, however, that in contrast the reactivity of scratched cadmium electrodes in alkaline solutions has been interpreted in terms of simultaneous transfer of two electrons [41], but it appears that the kinetic data resulting from the transient electrochemical behaviour of freshly generated surfaces by the scratching tech-appreciable activation polarization, one can only nique are insufficient to disregard the formation of a admit that the rate of reaction (Ia) is at least as fast as Cd(I) intermediate if the electrodissolution reaction is that of reaction (I b). Then, for describing the kinetics a rather complex process .…”

Depolarization of the hydrogen evolution reaction on cadmium electrodes in alkaline media

“…The analysis of the transient electrochemical behavour of freshly generated cadmium surfaces created by :~cratehing shows that the maximum cathodic current density at fixed potentials under bare surface polarization is approximately independent of pH, although :he bare surface mixed potential obeys a Nernst type ~elationship [6]. It should also be noted that kinetic naeasurements of HER on continuously renewed 0021-891X/88 $03.00 + .…”

The Electrochemical Behavior of Cadmium in Sulfide Solutions: I . The Initial Stages of Film Growth

“…CdO formation is predicted to occur at a potential of about +65 mV vs. RHE (21) and although the anodic current observed in Fig. Also, Burstein (23) found that no underpotentially deposited oxides form at a Cd anode in alkaline solutions, using the scratched electrode technique. This is consistent with other results reported recently (22), where it was also indicated that Cd(OH)2 deposition precedes CdO film growth.…”

“…This is consistent with other results reported recently (22), where it was also indicated that Cd(OH)2 deposition precedes CdO film growth. Also, Burstein (23) found that no underpotentially deposited oxides form at a Cd anode in alkaline solutions, using the scratched electrode technique.…”